distributor- trickle bed reactor

REVIEWS

Gas-Liquid Distributors for Trickle-Bed Reactors: A Review

R. N. Maiti and K. D. P. Nigam*

Department of Chemical Engineering, Indian Institute of Technology, Hauz Khas, New Delhi-110016, India

A concise review of the gas-liquid distributors used in trickle-bed reactors (TBRs) is presented. The followingtopics are considered: distributors in a large-scale reactor, quench box/redistributor, inert particle layer,application of fluid flow modeling (CFD) in distributor studies, and distributors used in a laboratory-scalereactor. Mainly four types of distributors used in a large-scale reactor (e.g., perforated plate, multiport chimney,bubble cap, and gas-lift distributors) are described along with their advantages and disadvantages. Effects ofvarious types of weep hole, such as inverted V notch and rectangular slot at the distributor tube wall andfluid distributing device at downcomer outlet, are discussed. Sizing methodology of multiport downcomer inchimney type distributors is presented. The performance of a gas-lift distributor is found to be more promisingcompared to other distributors. It provides intimate mixing of vapor and liquid, is less vulnerable to fouling,is insensitive to tray levelness, and distributes liquid uniformly at a large turndown ratio. This is also reflectedin the increasing use of gas-lift distributors with increasingly stringent product specifications. This reviewpresents all the information available in the literature to the best of the author’s knowledge and focuses theattention on enhancing the further understanding of internals toward uniform distribution of liquid in TBRs.It also focuses the future directions of work in designing of gas-liquid distributors to further facilitate theunderstanding of the design of TBRs to meet the challenges of the stringent sulfur specification in transportationfuel (10 ppmw in EURO V by 2009).

1. Introduction

Trickle-bed reactors (TBRs) are one of the important classesof multiphase flow reactors. It consists of a fixed bed of solidcatalyst particles contacted by a cocurrent downward gas-liquidflow carrying both reactants and products. In some cases,upward-flow TBRs are used but, because of flooding problems,downward flow is the most widely preferred. TBRs have beenwidely used in the petroleum industry for many years and arenow gaining widespread use in several other fields. They areemployed in petroleum, petrochemical, and chemical industries,in waste treatment, and in biochemical and electrochemicalprocessing, as well as in other applications.1-4

In general, the reaction occurs between the dissolved gas andthe liquid-phase reactant at the interior surface of the catalyst.In some cases, the liquid phase may be an inert medium forcontacting the dissolved gaseous reactant with the catalyst. Theobserved and expected reaction rates, when the particles arefully covered with liquid, are directly related to partial wettingof the catalyst.5 For this reason, it is desirable to have externalsurfaces of the catalyst fully covered with liquid (as it isperceived that the pore gets filled with liquid by capillary force)for maximum utilization of catalyst. In some cases, gas issparingly soluble (gas-limiting reactions) and incomplete particlewetting is desirable because it increase the effectiveness factor,owing to reduced gas-to-particle resistance. Obviously, there isa balance that must be maintained to avoid particle dry out,local temperature gradients, and vapor-phase reactions.

Since the introduction of the first commercial hydrotreatingunits in the 1950s, catalyst manufacturers have developed and

commercialized catalysts with the ever-increasing activitiesrequired to meet the stringent low sulfur, nitrogen, and aromaticsspecifications of environmentally friendly fuels. The keyparameter for further improving unit performance with highlyactive catalyst is the efficient distribution of reactants at themicrolevel, i.e., wetting of catalyst. There are several factorsthat affect the macro- and microlevel liquid distribution andflow textures. Macrolevel flow distribution is mainly affectedby inlet liquid distribution, particle shape and size of the particle,fluid velocity, and packing method. At the microlevel, liquiddistribution and flow textures are affected by start-up procedures,fluid velocity, wettability, flow modulations, and coordinationnumber of particle as reviewed by Maiti et al.4 However, theextent of uniform distribution of liquid through the catalyst bedat the microlevel is grossly affected by proper design andfunctioning of reactor internals.

The internal elements include (i) liquid entry devices, (ii) topdistribution plate, (iii) quench box, and (iv) redistributor, i.e.,redistributes the liquid and gaseous reactants evenly across eachsubsequent catalyst bed (Figure 1). The purpose of the liquidentry device and distribution tray is to establish an even liquiddistribution radially across the catalyst bed. Poor liquid distribu-tion introduces gross nonwetting in the bed, as shown in Figure2parts a-c.6 The catalyst particles on the upper left side ofpacking (Figure 2a) are not wetted and are not utilized. This isin agreement with the observations of Christensen et al.,7 Szadyand Sundaresan,8 and Marchot et al.9,10The latter authors studiedthe distribution of liquid in a laboratory trickling filter andobserved that about half of the bed cross section did not receiveany liquid and the distribution was not uniform in the remainingcross section.

* Corresponding author. Tel.: 011-2659 1020. E-mail: [email protected].

6164 Ind. Eng. Chem. Res.2007,46, 6164-6182

10.1021/ie070255m CCC: $37.00 © 2007 American Chemical SocietyPublished on Web 08/22/2007

A catalyst bed is hydraulically unstable in the sense that, ifa restriction develops somewhere in the bed, then it will alwaysbecome worse until the time of catalyst replacement. This mayhappen as a result of uneven distribution of gas and liquid inthe catalyst bed where pockets containing mainly liquid andinsufficient hydrogen can cause coking. Temperature maldis-tribution in exothermic processes generally indicates greater fluidflow in one part of the bed versus another. Rapid pressure dropbuildup sometimes reveals coking in the bed caused by regionsof stagnant flow or insufficient reactants. The restriction maybe developed because of mechanical degradation of catalystparticle or corrosion materials, pipe scales/foreign material thatentered with the feed. Fresh (not discolored) catalyst issometimes found when fixed-bed units are opened for servicingafter 2-3 years in operation, indicating flow bypassing. Thesefindings indicate that at least some aspects of fluid flow in gas-liquid distributors have not been well-understood. Yet in thepetroleum refining and other industries, public demand andgovernment regulations have dictated the removal of certaincompounds from chemical products, necessitating more severeoperation and greater need for optimal and reliable reactorperformance. Effective distribution in reactors is critical tomeeting these demands.

Most of the designs of internals in TBRs packed withmillimeter-sized particles are influenced by hardware used in apacked and trayed fractionation column. These designs are notnecessarily well-suited for trickle-bed reactors because of large

center-to-center spacing between distributors and poor liquiddischarge pattern. Moreover, with increasing demand of removalof certain specific compounds from petroleum refining products(e.g., ultralow sulfur diesel as specified in EURO III, EUROIV, and EURO V (10 ppm) norms by 2009), a greater needexists for optimum and reliable reactor performance. Forexample, in a DHDS (diesel hydro desulfurization) reactor, only1% of the untreated feed (∼1.0 wt % sulfur) mixed with theproduct because of wall flow or flow channeling keeps theproduct sulfur specification (100 wt ppm) off by∼100%, evenafter using a highly active catalyst. Effective uniform distributionof liquid in the macroscopic level is critical to meeting the abovedemands,5 and it demonstrates to have a good distributionespecially when such a low sulfur specification in the productis targeted.

Until recently, very little work has been undertaken to studyand significantly improve the performance of existing distribu-tion tray designs. Typically, catalyst manufacturers are well-equipped to test and develop new catalysts but have neither thetesting facility nor the expertise to study flow distributiondevices. Only a limited group of companies with the combinedexpertise from both catalysts manufacturing and licensing oftechnology possess these capabilities, vz. Haldor Topsoe, IFP,UOP, etc. Engineering companies do not have the facilities northe interest to undertake reactor internals development studies,which fall outside the scope of their activities. In view of therapid advances that are being realized in the area of improvementof reactor internals, it is deemed appropriate to supplement theinformation and description of varieties of liquid distributorsused in TBRs. The present review aims to discuss the differenttypes of internals used broadly in the industrial scale and inwhich improvements have been made over the years in termsof (1) distributors in large-scale reactors, (2) quench box/mixingdevice, (3) inert layer, (4) application of fluid flow modeling(CFD) in distributors studies, and (5) distributors used in thelaboratory scale. Internals for cocurrent upflow reactors are alsodiscussed in brief.

It is hoped that the paper will stimulate additional researchand development activities on design and selection of reactorinternals (mainly used in the industrial scale) with a view toobtain uniform distribution of gas and liquid on the macroscopiclevel in the case of trickle-bed reactors.

2. Distributors in Large-Scale Reactors

Most of the known designs of vapor-liquid distributors fallinto one of the four categories. The first kind of distributor isa perforated plate or sieve tray (Figure 3a). This may or maynot have notched weirs around the perforations. The tray mayalso have chimneys for vapor flow. This type of distributor isused for rough liquid distribution in conjunction with a moresophisticated final liquid distribution tray. The second commontype of liquid distribution device is a chimney tray. This deviceuses a number of standpipes, typically on a regular square ortriangular pitch pattern on a horizontal tray. The standpipes haveholes in the sides of the pipes for the passage of liquid (Figure3b). The tops of the standpipes are open to allow vapor flowdown through the center of the chimneys. The third type ofliquid distribution device is a bubble cap tray. This device usesa number of bubble caps laid out on a regular pitched patternon a horizontal tray. A cap is centered concentrically on astandpipe (Figure 3c), and sides of the cap are slotted for vaporflow. Liquid flows under the cap and, together with the vapor,flows upward in the annular area and then down through thecenter of the standpipe. The fourth type of distributor is the

Figure 1. Schematic drawing of a trickle-bed reactor used in hydrocracking.

Figure 2. (a-c) Liquid maldistribution in trickle-bed reactors.

Ind. Eng. Chem. Res., Vol. 46, No. 19, 20076165

vapor assist lift tube (Figure 3d). One leg (downflow tube) ofthe inverted “U” fits through a perforation in the support tray.The other leg (upflow tube) is shorter so that it is elevated abovethe tray. The ends of both legs are open. At the top of theinverted “U”, there is an internal opening between the legs. Thedevice thereby provides a flow path across the tray, from theinlet through the end of the short leg, with vertical flow throughthe short leg, a direction change at the top of the inverted “U”,downflow through the long leg, and discharge through the openend of the long leg below the tray. A vertical slot is cut intothe side of the short leg opposite the longer leg. The top of theslot is at or below the bottom of the internal opening betweenthe legs.

In many processes, e.g., hydroprocessing reactors, there canbe wide variations in the flow rates of vapor and liquid phasesand physical properties over time and during turndown opera-tions. Because of fabricating tolerances and the care of instal-lation, there will be unavoidable variations in the distributiontray levelness. Liquids dropping onto the distribution tray froman inlet distributor or quench zone mixer may be unevenlydistributed and could result in liquid height gradients acrossthe tray due to splashing, waves, or hydraulic head. Therefore,to have the optimized liquid distribution, the following importantelements must be considered during the design of the gas-liquid distributor tray:

(a) Drip point spacing. The dense spacing of drip points is akey parameter in optimum radial dispersion of liquid comingout of distributor. The liquid dripping on to the catalyst bedmay be visualized as a point source below each tube in the tray,and it disperses radially as it passes through the bed. So part ofthe bed may be used to compensate the larger drip point spacingtoward uniform distribution of the liquid. Therefore, for uniformdistribution of liquid, closer spacing and a greater number ofdrip points should be provided.

(b) Vaporization over the run cycle. Vaporization over therun cycle increases the vapor/liquid ratio, which can reduce theliquid level on the tray below a point where liquid can flowthrough some of the distributors. The tray design should be ableto handle various vapor/liquid ratios.

(c) Tray levelness. Tray levelness must be carefully consid-ered so that liquid does not preferentially flow through only

some of the distribution points, as shown in Figure 4a. Thedesign of the distributor should be able to overcome out-of-levelness of the tray. Fabrication tolerance, poor inclination,deflection under load, mishandling, etc. cause tray out-of-levelness. The impact of tray levelness is reduced by the choiceof a proper distributor, as shown in parts b and c of Figure 4.

(d) Vulnerability to plugging. Vulnerability to plugging bycoke or corrosion products must be considered to ensure equalliquid flow from all distribution points.

(e) Vapor-liquid mixing. Vapor-liquid mixing is also animportant feature for ensuring that the reactants reaching thecatalyst surface are at an equilibrium temperature to have auniform reaction throughout the entire catalyst bed. So thedistributor providing a higher degree of vapor/liquid mixing willbe advantageous, especially for trays located downstream ofquench zones.

(f) Pressure across the distributor. The pressure across thedistributor should be low.

In the following sections, the key features of differentdistributor designs that have been published and patented overthe years and how well these devices address the above designconsiderations are discussed.

2.1. Perforated Tray. This distributor tray is provided witha large number of liquid downflow apertures. Generally, a pool

Figure 3. Different types of distributors: (a) perforated tray, (b) multiport chimney, (c) bubble cap, and (d) vapor-lift tube.

Figure 4. Impact of tray levelness for (a) perforated-plate distributor, (b)chimney distributor, and (c) vapor-lift distributor.

6166 Ind. Eng. Chem. Res., Vol. 46, No. 19, 2007

of liquid will accumulate on the tray and cover these aperturesso that the flow of vapor through them is not possible. Normally,a large size chimney is provided to pass vapor to the tray/bedbelow this. The top of each chimney is provided with a numberof slots to act as a weir for liquid flow if the liquid on the traybuilds up and a flooding situation occurs (Figure 5a). This trayis rather simple to construct and is capable of providing thegreatest number of drip points over the cross section of thecatalyst bed. It is used in isolation for a rough distribution ofliquid or in combination with other distributors (i.e., chimneytray, bubble cap, and gas-lift tubes) for a finer distribution ofliquid. In the case of multiple beds, there is a collection traybelow the catalyst bed and a rough distributor tray below thecollection tray, which is basically a perforated tray type. Afterthe perforated tray, a second, final distributor tray is providedwith downcomers for flow of liquid and vapor onto the lowercatalyst bed. Smith et al.11 developed a perforated distributiontray with a small perforation for liquid flow along with a centralopening with cylindrical weirs for gas flow. Perforations were5-15 cm in diameter, and the total open area was sufficientfor passage of liquid and accumulation of liquid up to a certainlevel with pressure drop not exceeding 5 cm. As an example,Figure 5b shows the perforated distributor system developedby Aly et al.12 for an initial, rough distribution of liquid to thesecond distributor tray along with the chimney for vapor flow.Grott et al.13 also used a perforated tray (with a cylindrical wallat the outer periphery of the tray) for distributing liquid effluentfrom the mixing chamber. The tray has a uniformly distributedperforation of size 16 mm. Vapor passes through the annularpassageway.

The performance of this type of distribution device will notproperly satisfy all the required design considerations. Liquidon the unleveled tray will gravitate to the low points, andconsequently, the sensitivity of tray levelness will be very high.The perforation can easily become plugged by coke, corrosionproducts, or other debris carried into the reactor by the feed.

Finally, the flexibility to liquid load is very poor. Typically,this type of distribution tray can be designed to give goodperformance at either the design conditions or at turndownconditions, but not at both situations. Consequently, the trayhas a tendency to run dry as vaporization increases toward theend of the cycle. This type of design may, therefore, not beseriously considered to provide good uniform distribution.However, this type of distributor may be used as roughdistribution in a multilevel distributor system.

2.2. Chimney Tray. These types of distributors consist of ahorizontal tray fitted with vertical downpipes called risers (bothsides open-ended) having holes (liquid openings) drilled in thesides. These lateral liquid opening(s) may be at one or moreelevations with varying sizes and shapes. The total flow areaof the liquid openings is selected to hold a certain liquid levelon the tray, and the total cross-sectional area of the vaporchimneys is normally selected to obtain a low pressure dropacross the tray to ensure that the driving force for liquid flowthrough the liquid openings is mainly the static head of the liquidcolumn above the liquid opening and not the pressure dropcaused by vapor flow through the chimneys. The bulk of theliquid flow would pass through the holes as a jet, which issheared by the gas passing vertically downward. The shearingactions break up the liquid and thereby improve gas-liquidcontact before reaching the catalyst bed. This type of liquiddistributor is generally designed to control liquid level on thetray as well as proper mixing of two phases depending uponthe types of holes. Over the years, chimney trays have beenpatented along with the constant updation by several investiga-tors. Details of some of the typical chimney distributors withthe development as reported in the patent are compiled in Table1. As en example, Riopelle and Scarsdale14 disclosed in U.S.Patent No. 3,353,924 a gas-liquid distributor, consisting ofpipes with long vertical slots/notches on the sides so that liquidflow through the distributor increases as liquid level on the trayincreases (Figure 6). A simple fluid mechanical analysis of sucha device shows that the flow through the slot is proportional tothe height of the head of the liquid above the slot base raisedto the power of more than one (∼1.5). This behavior isundesirable because the 1.5 power dependence on liquid heightmakes the distributor very sensitive to variations in levelness.In addition, this device uses separate, larger chimneys for gasflow, which restricts the number of liquid irrigation points onthe tray.

Effron et al.15 disclosed in U.S. Patent No. 3,524,731 adistributor that comprises a plate having short tubes and longtubes inserted through the plate (Figure 7). The upper ends ofthe longer tubes are provided with notches and gas caps at theuppermost extremity. At low flow rate (i.e., at minimum feedthroughputs), the flow of liquid is entirely through the shorttubes with the gas flowing through the notched tubes. Uniformityof distribution is readily achieved by sizing the short tubes sothat the head of liquid existing above it is at least 38 mm. Athigher flow rates, the liquid builds up to the notches providedin the longer tubes and some of the liquid then begins to flowthrough the longer tubes. This in effect serves to “spread out”the increased flow over a greater number of points and servesto maintain the desired uniformity of distribution. The flowthrough the short tubes still remains uniform, and the gas phasestill continues to flow through the notched tubes. Thus, thenotched tubes serve two important functions: first, they act asgas chimneys to provide good uniformity in the distribution ofthe gas phase, and second, they prevent the building up of theliquid level over the plate beyond a desired height. With this

Figure 5. Distribution systems for use in multiple beds (U.S. Patent No.4,836,98912): (a) perforated tray and (b) perforated tray for roughdistribution.


feature, reactors of shorter overall lengths may be employedsince large buildups in liquid levels in the case of large turndownratios (i.e., maximum flow rate/minimum flow rate) do notoccur. The slots in the long tubes are designed so that, atmaximum flow rates, they take up to 50% of the total flowrate. Furthermore, by maintaining a head of 38 mm above theuppermost end of the shorter tubes, distribution becomesrelatively insensitive to out-of-level variations, which may occurin the transverse direction of the reactor. Overflow boxes areprovided at the end of the shorter tube to reduce the effectivedistance between drip points with slots to distribute liquid. Thepreferred configurations for the slots are triangular cutouts. Theadvantage of using such a slot, as compared to a slot ofrectangular cross section or one of triangular cross section withthe apex downwardly oriented, lies in the fact that it insuresgreater uniformity of distribution when the liquid level abovethe plate is not parallel to it because flow through the slot isproportional to the height of the head of liquid above the slotbase raised to the power<1 (∼0.5).

Grosboll et al.16 disclosed in U.S. Patent No. 4,126,540 atray deck with a hollow chimney member with circular aperturesdrilled on the riser surface (Figure 8) at different elevations.The number of chimneys ranges from about 10 to 30/m2 of tray.Each chimney has 3-8 apertures at evenly spaced intervalsaround the perimeter of the chimney. The cross-sectional areaof each aperture may range in size from 0.6 to 6.0 cm2. Thetop opening of the chimney is in the range 7.5-30 cm abovethe tray. The cross-sectional area of the top opening is 45-380cm2. Among the factors, which can be used to determine thenumbers, sizes, and locations of the apertures, are the flow rateand the composition of the liquid-vapor mixture. The centerlineof the aperture is 5-15 cm above the tray, providing settlingspace for particulate matter, if any, present in the liquid-vapormixture. The liquid level should be 3 times the diameter of thesmallest aperture. The pitch of the aperture in circumference isD or 2D of the aperture. The plate distance over the opening isat least 0.1-30 cm. The cross-sectional area of the plate is atleast two times as large as the cross-sectional area of the top

Table 1. Comparison of Different Chimney-Type Distributors

author type of distributor size details of the distributors advantages/disadvantages

Riopelle and Scarsdale14 chimney type long vertical down pipe for gas;small liquid down pipe with longvertical notch wide open at top

sensitive to liquid height, i.e., traylevelness; lesser no. of drip point asgas and liquid down pipes areseparate; can take care highturndown ratio

Effron and Hochman15 riser type long down pipes with notch at the upper end;short down pipe with overflow boxes at outlet;pitch ) 7.5-30 cm;shorter tube: dia (mm)) 10-20, height) 50;longer tube: dia (mm)) 25-50, height) 165;triangular cutouts 25 mm× 50 (height)× 6 mm

at turndown flow, occurs throughshorter tubes; at high flow, flowpasses through both long andshort tubes; can accommodatewide variation in liquid flow rate;insensitive to tray levelness anddirt deposition

Grosboll et al.16 chimney type chimney with circular hole at wall;dia of tube) 70-218 mm;10-30 no. of chimney/m2;3-8 aperture around the perimeter wall,5-15 cm above the plate;aperture size) 0.6-6 cm2

both gas and liquid passes throughsame downcomer, so larger numberof drip points possible; free ofplugging and fouling as aperture athigher level; very sensitive to liquidheight

Derr et al.17 weir type gas liquid downcomer pipe with circularholes at wall and rectangular notches atthe upper end

sensitivity to tray levelness is lessat high liquid flow rate but veryhighly sensitive at low liquid flow;uniform distribution possiblebecause of larger number of drippoints

Campagnolo et al.18 riser type;a pair ofdistribution trays

distributors at 1st tray are riser types withweir slots for liquid flow; distributors at secondtray are both riser types for both gas andliquid and liquid down pipes

much larger number of drip pointsand uniform distribution due toflow through both types ofdistributors; effect of tray levelnessis reduced because of flow throughweir slots as well

Koros et al.19 chimney type vertical slots of different lengths around thecircumference of the chimney; spray-generating device at outlet of distributors

uniform distribution due to radialcoverage of liquid spray; traylevelness less; effective at flowvariation because of existence ofslots of different lengths

Muldowney et al.20 open-ended downpipes with holesat two different levels

two types of down pipes, first type with holesat two levels and second type hasholes at higher level corresponding to firsttype; lowermost holes are at least 6 mmabove the tray

effective at different flow ratesbecause at low flow rates liquidpasses through down pipes withlower hole only, free of plugging;sensitivity to tray levelness is low

Wrisberg21 riser type open-ended tubes with aperture at variouselevations (3-4 levels);tube pitch) 50-120 mm;aperture at various levels with lowermostat 50 mm from tray

high flexibility at turn down

Muller22 down pipes withliquid conduit

downcomers with holes at wall at elevations,a reduced flow area section, a liquidconduit, and a device for improvedliquid spread

it is a combination of chimney andbubble cap type; as liquid is liftedthrough conduit, it is less susceptibleto plugging and fouling; liquidbypassing through vapor is lesscompared to bubble column; improveddistribution performance


opening. The distributors are applicable for any bed of solidparticles but particularly in a bed of solid catalyst particles withtypical catalyst size range of 0.2-12 mm. This tray hasimproved resistance to fouling and plugging since the liquidopenings are at a higher elevation, and particulate impuritiescan, therefore, settle out on the tray without plugging the liquidopenings. The drawback of chimney tray designs with liquidopenings in one elevation only is poor liquid flow range. Atlow liquid flow rates, the level will be at the liquid openings,and the liquid flow through each chimney becomes verysensitive to the variations in liquid depth, which will alwaysexist on the tray. At high liquid flow rates, liquid will overflowthe lowest elevated chimneys and cause liquid maldistribution.

Derr et al.17 disclosed in U.S. Patent No. 4,126,539 a pair ofgas-liquid distributor trays to facilitate the uniform spreadingof liquid over the upper face of a catalyst bed (Figure 9). Thedistributor trays contain a series of spaced risers, which havedual functions. It permit vapor to pass the tray and also serveas liquid conduits because of weir slots cut into the sides of therisers. The upper tray is perforated by a relatively uniformlydispersed gas-phase downcomer. The gas and liquid downcom-ers are of the weir type, which maintains a desired level of liquidupon the upper tray surface throughout its cross-sectional area.In addition, the liquid downcomers are provided with one ormore liquid flows through holes, or orifices. The liquid flowsthrough the holes are sized to permit only a portion of the tray-accumulated liquid to flow through the holes with the remainingportion of the liquid overflowing the weir of each downcomerand flowing downward. This arrangement ensures the flow ofliquid through each liquid downcomer of the upper distributortray.

A second gas/liquid distributor tray is positioned beneath theupper distributor tray. The second distributor tray is providedwith gas/liquid downcomer with one or more circular liquidholes. The weir type distributors of the second tray maintain adesired liquid level on the tray so that saturated liquid andhydrogen rich gas pass downwardly through the open-endeddowncomers under flow conditions of limited pressure drop.

These liquid flows through holes are sized to permit flow ofonly a portion of the highest liquid flow permitted by the trayand ensure that some liquid always flows through eachdowncomer. At high flow rate, the liquid passes through therectangular notches and flow is proportional to the liquid headover the tray, raised to the power 0.5. This provides aminimization of sensitivity of liquid flow to variations in level.

A disadvantage of such use of this distributor would arise atlow liquid flow rates, which cause the liquid level on the trayto fall between the top and bottom of the holes. Under theseconditions, a 1.5 power dependence on liquid height instead of0.5 makes the distributor strongly sensitive to variations inlevelness. A low liquid level could be minimized by sizing thecircular holes smaller, but hole diameters< ∼6 mm would beimpractical because of the possibility of plugging. Thus, for agiven reactor, there is a minimum liquid rate for whichdownpipes with holes are effective, below which good distribu-tion cannot be guaranteed. Another problem with this distributoris that the liquid is carried past the tray by the risers and, thus,the number of points at which the liquid is introduced to theupper face of the bed is limited by the number of risers thatcan be uniformly positioned on the tray. This limitation isaggravated by the fact that the risers are of relatively largediameter. Accordingly, as the number of liquid introductionpoints is decreased, the depth to which the liquid must penetratethe catalyst bed to reach equilibrium distribution increases, andcatalyst utilization in the upper bed is thereby impaired.Additionally, because of the nature of liquid flow through weirs,the uniformity of liquid distribution affected by this type ofdesign is very sensitive to tray unevenness, introduced duringfabrication or installation.

Campagnolo et al.18 disclosed in U.S. Patent No. 4,788,040an inlet distributor system including a pair of distributor traysfor a fixed-bed catalyst reactor (Figure 10). An upper tray hasa series of risers. The risers are hollow and open above andbelow the upper distributor tray to permit vapor to pass throughthe tray, and each riser has weir slots cut into its outer surfacethrough which liquid can pass through the tray. The lower

Figure 6. Liquid downcomer pipe (from U.S. Patent No. 3,353,92414).

Figure 7. Chimney distributor with different sizes of down flow pipes(U.S. Patent No. 3,524,73115).

Figure 8. Hollow chimney member for distributing a mixed fluid stream(U.S. Patent No. 4,126,54016).

Figure 9. Chimney type distributors (from U.S. Patent No. 4,126,53917).


distributor tray has a series of risers and downpipes thereonarranged in a predetermined pattern. The pattern of risers anddownpipes provides an advantageous arrangement of pas-sageways for liquid to be distributed to the catalytic bed. Theadvantages of using this type of distributor are as follows: (1)The number of discrete liquid streams entering the upper faceof the catalyst bed is maximum. (2) The distributor is designedto ensure as nearly as possible equal liquid flow rate of eachstream, hence resulting in uniform liquid irrigation over theentire face of the bed. In this distributor, liquid flow is throughorifices in the riser and through the liquid downpipes. In bothcases, flow is proportional to the square root of the liquid heighton the distributor tray. (3) Since the flow rate through theapertures of the distributor is proportional to the square root ofliquid height, the effect of tray levelness is minimized. Bycontrast, in a distributor employing weir flow, the effect of trayirregularities is magnified.

Koros et al.19 disclosed in U.S. Patent No. 5,403,561 a mixed-phase fixed-bed reactor distributor (Figure 11), which is ahorizontal tray with vertically disposed chimneys. Thesechimneys have a first end to receive liquid and gas above thetray and a second end for distributing the liquid and gasdownwardly below the tray (Figure 11). The spray-generatingdevices for producing the conical spray are located at positionsso that the spray of the mixed fluid stream from one spray-generating device as it impinges on the top surface of the fixedbed will overlap the spray from an adjacent spray-generatingdevice. The maximum flow through the device at acceptablepressure drop and the angle of the spread of the conical spraypattern are controlled by the choice of ribbon pitch, diameterwidth, and length. The angle of the spray and the overlapdetermine the appropriate distance between the tray carryingthe spray device and the catalyst bed. Another important featureof the distributor is the self-adjusting control of uniform vaporflow through each distribution element. This control is providedby the uniform back-pressure due to the pressure drop exerted

by the flow of vapor and liquid through the conical spray-producing zone in combination with the uniform flow of liquidprovided by the chimney slots. A surprising discovery is thatthis apparatus operates over wide variations in liquid and vaporflow rates while providing excellent flow-distribution perfor-mance. In addition, when in a preferred operating mode, finedroplets ranging between 10 microns and 1000 microns areproduced. These extremely small drops are dispersed andsuspended in the vapor flow, providing the fixed bed belowthe tray with a uniform vapor/liquid flow mixture that, vianormal bed flow dynamics characteristics, will be distributeduniformly within the top entrance region of the catalyst bed.

Muldowney et al.20 disclosed in U.S. Patent No. 5,484,578 adistributor system for uniformly directing vapor and liquid acrossthe surface of a fixed bed of solids in a downflow reactorcomprising a distributor tray and open-ended downpipes (Figure12). There are two different types of downpipes with differentnumbers of holes for gas/liquid flow. A first array of thedownpipes has vertically spaced elevations of holes above thelevel of the tray. A second array of the downpipes has at leastone elevation of holes at substantially the same height abovethe level of the tray as one of the upper elevations of holes inthe first array of the downpipes. However, the second array hasno elevation of holes corresponding to the lowermost elevationof holes, and possibly other lower elevations of holes, in thefirst array of downpipes. The absence of the lowermost holesin the second array of downpipes causes the liquid flow ratethrough the distributor tray at a given liquid height to be reducedwhen that liquid height falls below the elevation of the holessecond from the bottom in the first array. This maximizes theliquid height above the lowermost holes, preserving gooddistribution even when the distributor is subject to variationsin level from one point to another. The downpipes are verticallydisposed tubes with open ends, which extend above and belowthe tray by one or more tube diameters. The lowest holes onany downpipe are suitably 0.6 cm to several cm (at the centerof the hole) above the top surface of the tray to prevent scale,sludge, or other solid matter conveyed in the liquid phase frompassing through the tray onto the solids bed below. Thus, thepresence of the downpipes ensures that a pool of liquid ismaintained on the tray. It is generally preferred that at least thebottom hole, or, more preferably, several of the holes, in thedownpipes be entirely submerged in the standing liquid.

Figure 10. Inlet distributor system (from U.S. Patent No. 4,788,04018).

Figure 11. Chimney distributor with first end for liquid receive and secondend for liquid distribution (from U.S. Patent No. 5,403,56119).

Figure 12. Distributor down pipes with holes at different levels (U.S. PatentNo. 5,484,57820).


An aspect of the present distributor is that the outlet streamsfrom the downpipes diverge into conical sprays because thestreams lose momentum to the comparatively stagnant gasbetween the distributor tray and the inerts layer located abovethe catalyst bed. The extent of divergence depends on the liquidand gas flow rates, the fluid properties, and the dimensions ofthe downpipes. On typical pitches, the conical outlet spraysapproach one another or partially overlap. For this reason, theliquid coverage at the top of the solids bed is minimallycompromised even when the second array of pipes, which haveonly one hole, are passing no liquid at all. The coverage istypically at least∼80% to∼95% of the coverage obtained whenall downpipes are passing liquid, and it can approach 100%coverage. It is preferable that the downpipes of both the firstand second arrays feature one or more notches in the top rim toconduct liquid during periods of abnormally high flow. Highflow may occur because of an interval of higher-than-designfeed rate, an unplanned surge of incoming liquid, or, much morerarely, a general rise in the liquid level on the tray due toplugging of most of the downpipe holes. The notches result inless sensitivity of liquid flow to liquid height when the tray isimperfectly leveled than would occur if the rims were unnotched.The notches may be rectangular, triangular, semicircular, or ofvarious other shapes and are distinct and unconnected to anyof the holes in the downpipes.

Wrisberg21 disclosed in U.S. Patent No. 5,688,445 a distribu-tor arranged above the surface of the trickle bed (Figure 13).The tray is equipped with open-ended tubular distributordowncomers with horizontal apertures at various elevations inthe tube wall of the downcomers to provide passage of liquidand gas flow through the open ends of the downcomers. Thenumber and dimensions of downcomers depend on the actualrate of gas and liquid flow introduced on the tray. In general,the height of the downcomers above the tray is at least 200mm to allow for varying liquid load without overflow of liquidthrough the open ends of the downcomers. The downcomersare typically disposed in the tray with a pitch of about 50-120mm. Horizontal apertures in the downcomers are typicallydisposed at 3-4 elevations at a minimum elevation of∼50 mmabove the bottom of the tray and at intervals of 30-40 mmbetween each aperture, which ensures high flexibility atturndown of the trickle-bed reactor. The diameter of theapertures is selected to maintain a liquid level on the tray ofabout 150-190 mm at 125% of liquid load. Preferably, thediameter of the apertures is at least 4 mm. As an importantfeature, the inner diameter of the downcomers is adjusted toprovide a Froude number (NF) < 0.35. At a Froude number<

0.35, it is ensured that the downcomers are not partially floated,which otherwise will result in an uneven flow distributionbetween the downcomers and fluctuations in velocity head lossat a varying degree of flooding. Sometimes, the tray isadditionally equipped with a number of open-ended tubular gaschimneys extending vertically for a certain height above andbelow the tray. The gas chimneys have an inner diameter, whichis larger than that of the downcomers, at least 2 times thediameter of the downcomers. The gas chimney may, further-more, be provided with apertures of the same diameter and atthe same elevations as the apertures in the downcomers.

Muller22 disclosed a distribution tray in U.S. Patent No.20060163758 for distribution of vapor and liquid across a vesselthat has downcomers with a reduced flow area section and adevice for improved liquid spread at the outlet of the downcomer(Figure 14). The distribution tray with downcomers has openupper ends for vapor inlet and open lower ends for passing thevapor and liquid through the tray. A liquid conduit is alsoprovided for each downcomer with a liquid inlet submerged inthe liquid pool on the tray, with a section for upward flow ofthe liquid and with liquid openings at more elevations. Theliquid conduit is used to transfer liquid from the liquid poolinto the downcomer. Unlike with a conventional chimney, thedriving force of the liquid flow is both liquid height and pressuredrop for the vapor entrance. The lower end of the downcomeris provided with means for improving the local liquid spreadfrom each downcomer such as vanes, baffles, ribbons, andcorrugated, flat, or curved plates with or without perforations.During operation, vapor enters the downcomer upper end. Liquidcollected on the tray flows through the inlet, upward throughthe conduit, and through the openings into the downcomer,where the liquid is mixed with downwardly flowing vapor. Thetwo-phase stream passes the reduced flow area section withincreased velocity for improved dispersion of the liquid beforethe stream flows through the device for improved liquid spread,and it then exits through the lower end of the downcomer.

From Table 1, it is observed that performance of the chimneytray has been improved with developed slot size and type alongwith use of a number of slots at several elevations. But unlikea conventional chimney, the new improved chimney type withliquid conduit, developed by Muller,22 is more promising toovercome the disadvantages of conventional chimney distribu-tors.

2.2.1. Methodology for Sizing Distributor Downpipes.Inthe following paragraphs, the sizing methodology was discussedfor downpipes having two arrays of pipes with holes at differentlevels, as disclosed by Muldowney et al.20 Sizing of downpipesinvolves more than one operating mode specified by a total gas

Figure 13. Open-ended tubular distributor (from U.S. Patent No. 5,688,-44521).

Figure 14. Down pipes with liquid conduit (from U.S. Patent No.2006016375822).


rate and a total liquid rate. The fluid rates and other fluidproperties like gas density, gas viscosity, liquid density, liquidviscosity, and liquid surface tension are determined at processconditions using applicable thermodynamics at the prevailingtemperature and pressure.

2.2.1.1. Approximate Count of Downpipes.The vesseldiameter is fixed by considerations other than fluid distribution(e.g., available space) and is assumed to be known at the outsetof the tray design process. On the basis of this diameter, anapproximate count of downpipes is determined by adopting pitchspacing. For maximum coverage, the pitch is typically chosenas small as practically possible, so that downpipes are locatedas close to each other as fabrication will permit. Commonpitches vary from 30 to 60 cm to several centimeters, dependingon the importance of maximizing coverage. The allowable pitchis typically restricted by the position of tray support beams andother internal members. Once the numbers of downpipes areknown, the gas and liquid rates per downpipe are calculated,considering at this point that all the downpipes are alike.

2.2.1.2. Diameter of the Downpipes.The next dimensionto be determined is the diameter of the downpipes. Too large adiameter limits the number of pipes on the distributor tray. Toosmall a diameter results in excessive pressure drop across thedistributor tray. Between these extremes is typically a range ofdiameters ranging from a few centimeters to<1 cm. Aconvenient pipe size is chosen for first-pass calculations withthe possibility of subsequently fine-tuning the diameter. Multiplediameters may also be used on the same tray.

2.2.1.2.a. First-Pass Calculations.The downpipes in the firstarray, that is, those having all the holes, are designed first, usingthe design case with the highest liquid flow rate. The followingequations are presented for a downpipe having holes at twoelevations, but the formulas are readily extended to pipes havingthree or more elevations of holes. As noted above, the totalhole area at a given elevation is calculated, and this total areamay be realized by any number of holes through the downpipewall at that elevation.

At any single elevation, an equation relating liquid height toliquid flow rate is

where f and g are functions readily obtained by a pressurebalance at the downpipe holes.

The physical constraint that defines the functionsf andg isequality of pressure between the liquid and the gas at twolocations: the top surface of the standing liquid and the pointin the interior of the pipe where the phases return to pressureequilibrium.

2.2.1.2.a.1. Sizing of Holes Cut in the Walls of Downpipes.In designing a two-elevation pipe such as the two-hole down-pipe, eq 1 is written once for the top hole and once for thebottom hole, that is, with different values ofH and possiblydifferent values ofA, creating two equations in the fourunknowns ashTOP, QL,TOP, hBOTTOM, andQL,BOTTOM. The othertwo equations needed to close the system are

Equation 2 requires that the liquid heights governing the topand bottom holes be the same, and eq 3 requires that the sumof the liquid flows through the top and bottom holes equals thetotal liquid flow per downpipe. The design of the first array of

pipes consists of choosing values for the areas (A) and locations(H) of the top and bottom holes for the highest expected liquidflow rate through the tray, solving eqs 1-3 by trial-and-errorto determine the liquid height (h) on the tray, and adjusting thehole areas (A) and locations (H) until the liquid height (h) issatisfactory or at a predetermined level above the top hole.

The holes in the downpipes of the second array are sized bysolving eq 1 for the area (A) of the hole in each pipe needed topass the same amount of liquid (QL) at the same liquid height(h) as for the two-hole downpipe. The calculation is againspecific to the case with the highest liquid flow rate when alldownpipes would be expected to pass liquid. This is also a trial-and-error calculation because the area (A) appears in a complexmanner in two terms of eq 1.

2.2.1.2.a.2. Number of Down Pipes.Following the holesizing for the case of the highest contemplated liquid flow tothe distributor tray, the system is evaluated for the case of thelowest expected liquid flow rate to determine what fraction ofthe downpipes should be in the second array and, thus, lackbottom holes. This evaluation is accomplished by applying eq1 to a pipe of the first array to determine what liquid flow rateper downpipe would result in the liquid height being comfortablyabove the bottom hole but below the top hole. The result willbe some valueQL* greater than the actual flow rate perdownpipeQL. The ratio of the actualQL to the targetQL* isthe fraction of pipes that must be first-array members. Theremaining pipes are designated as the second array. This stepusually requires several repetitions since the fraction of pipesin the second array preferably must correspond to a uniformgrid spacing. It is often necessary to make small adjustmentsto the holes sizes during this step. Also, it is sometimes preferredto instead try fixed fractions of second-array pipes correspondingto convenient grid spacing and to check for acceptable liquidlevel.

2.2.1.2.b. Second-Pass Calculations.Once the number ofdownpipes in each array is fixed and the hole sizes are known,the first-pass design is completed. Adjustments must be madeto the first-pass design because the foregoing calculations arebased on the assumption that gas rates through the downpipesof the first array and the second array are equal, which generallyis not accurate. The partitioning of the gas flow is determinedby the pressure drop across the first array of downpipes versusthe pressure drop across the second array of downpipes. Whenboth sets of downpipes are passing liquid, the pressure dropsacross member pipes of each set are similar, though not identicalbecause the different numbers of liquid jets in the two types ofpipes result in somewhat different degrees of shear. When onlythe first-array downpipes are passing liquid, the gas flow willslightly favor the second array of pipes because the effectiveflow area in the second array of pipes is larger because of theabsence of liquid therein. The following pressure drop equation(eq 4) provides an analysis of gas flow.

where ∆p ) pressure drop across the full length of thedownpipe,Ap ) cross-sectional area for flow in the downpipe,æ ) pressure losses in the length of the downpipe between theupper rim and the top hole, andψ ) pressure losses in the two-phase section of the downpipe between the top hole and thelower rim of the downpipe.

In eq 4,æ is a function describing pressure losses in the lengthof the downpipe between the upper rim and the top hole, which

h ) H + f(A, FL)QL2 + g(A,FL, FG, QG)QL (1)

hTOP ) hBOTTOM (2)

QL,TOP + QL,BOTTOM ) QL (3)

∆p ) æ(Ap,QG, FG, µG) +ψ(Ap, QG, QL, FG, FL, µG, µL, σL) (4)


is specific to the numbers and types of notches in the upperrim. The functionψ is a function describing pressure losses inthe two-phase section between the top hole and the bottom endof the downpipe, which is specific to the numbers, elevations,and relative positions around the pipe circumference of the holes.Gas flow to each type of downpipe is determined for a givendesign case by writing eq 4 once for a first-array downpipe andonce for a second-array downpipe. For the highest-flow case,the values ofQL will be substantially equal for each array pipe,but for other design cases, and in particular for the low-flowcase, the values ofQL will differ significantly. The values ofQG to be used are those used in eq 1 for liquid height, whichare equal for the first and second arrays during the initialcalculation. Using eq 4, the pressure drop across the first arrayand second array of downpipes are computed. When the pressuredrops are equal, the design is consistent and complete.

However, after the first-pass calculation, the pressure dropscannot be equal because the gas rates were assumed to be equaleven for the low-flow case when the second array of downpipesis inactive. Thus, an outer loop of iteration must be undertakenin which the gas flow rate to the first array of downpipes isguessed, the gas flow rate to the second array of downpipes isobtained as the difference between the guessed first-array gasflow and the total gas flow, and all of the foregoing calculationsare repeated until the degree of gas partitioning is arrived at,which reconciles the liquid height and pressure drop equationsfor all design cases. This procedure is not practical for handcalculation and is preferably executed by a digital computer. Asuitable numerical technique for solving these equations is aNewton-Raphson method.

Successful completion of the design procedure occurs whendownpipe diameters and locations, hole sizes and locations, andother details as noted above give a liquid height above thebottom elevation of holes for all design cases and above higherelevations of holes in as many design cases as possible. Thefundamental requirements for realizing the maximum benefitsof the present invention are that the bottom holes of the firstarray be submerged in all cases and that the downpipes of thefirst array be arranged on the tray to provide maximal coveragewhen the pipes of the second array are inactive.

2.3. Bubble Cap Tray.These distributors have a completelydifferent mode of operation than the chimney type of distributiontrays. While the static liquid head is the driving force for liquiddistribution on the chimney distribution trays, the driving forcefor liquid distribution on the bubble cap tray is the vapor flow.The bubble cap distributor consists of a horizontal tray. Bubblecaps are provided for vapor and liquid flows across the tray.Each bubble cap is an inverted U-shaped flow conduit consistingof upflow channel(s) and downflow channel(s). The lower partof each upflow channel is provided with one or more lateralvapor openings, typically vertical slots or inverted V-notches.Each downflow channel extends through the tray plate. Thevapor passes through the lateral vapor openings in the lowerpart of each upflow channel and thereby generates a pressuredrop from the vapor space above the tray to the inside of theupflow channel. Because of this pressure drop, liquid is liftedup into the upflow channel and mixed with the vapor and thetwo-phase mixture flows up through the upflow channel, overan internal weir, and down through the downflow channel andexits the distribution unit below the tray.

An example of a traditional bubble cap distribution tray isgiven in U.S. Patent No. 3,218,249 by Ballard et al.23 (Figure15). A cap is mounted on the riser. The bottom of the cap isslotted equidistantly. The depth of the slot should preferably

be from∼3 mm to∼12 mm of the depth of the cap itself, andthe width of the slots should be from about1/32 to about1/8 ofthe cap cross-sectional diameter. Other type of slots like V-notchare also used. The top of the slots is maintained below thebottom of the upper rim of the downcomer. A good practice isto maintain a clearance of at least 6 mm between the top of theslot and the top of the riser. The top of the downcomer may beslotted or left unslotted. The bottom of the downcomer slotsshould be maintained above the top of the cap slot. Usually,the downcomer will be from∼7.5 to∼15 cm in cross-sectionaldiameter, although a downcomer as small as 3.8 cm or less isless typically used. Sometimes a bubble cap with a riser flaredat the bottom is advantageous for vapor-liquid distribution. Theliquid tends to flow down the riser wall, particularly at lowervapor rates, and the flared section causes the liquid to bedisengaged from the riser in a conical pattern, thereby achievinga greater distribution of the liquid over the cross section of thevessel.

In operating a reactor with a bubble cap tray, a mixed phaseis introduced in the reactor through the inlet conduit and sparger,which distributes the liquid onto the tray with a minimum ofsplashing and erosion. The liquid phase, disengaged from thevapor phase by gravity, fills up on a tray to a level below theslot depth in the downcomer caps, with such level beingdetermined primarily by the gas flow rate per cap. It is, ofcourse, necessary that some of the slot openings be exposedabove the liquid surface to permit the passage of vapor. Wherethe caps have no slots, the liquid level on the tray will be belowthe bottom rims of the caps for the same reason. Where unslottedcaps are used, the clearance between the bottom rim and thetray must be maintained to accommodate the passage of gasand liquid. The pressure drop through the distribution tray inthe reactor, which is normally quite small, forces the feed vaporthrough the slots to flow upwardly through the annulus, reversedirection, and then flow downwardly through the downcomer.The vapor, because of the forces acting on it as a result of beingfed to the reactor under pressure and then forced into thecontacting zone through the tortuous cap and downcomer paths,is in constant turbulence as it contacts the liquid in the reservoiron the distribution tray in the vicinity of the downcomers. Forthis reason, the vapor entrains liquid with it as it passes throughthe cap slots or under the downcomer cap and transports itthrough the downcomers from whence it is discharged into thecontacting zone. The liquid on the distribution tray seeks itsown equilibrium level, as dictated by the design of the apparatus,and it is, thus, not necessary for operability to achieve optimumdesign efficiency for the distribution tray. The reactor will beoperative so long as the liquid level on the tray does not sealoff all openings in the downcomer caps.

Shih and Christolini24 disclosed in U.S. Patent No. 5,158,-714 bubble caps with apertures comprising two rectangularopenings superimposed upon and at substantially right angles

Figure 15. Modified bubble cap (from U.S. Patent No. 3,218,24923).


to one another, forming an opening in the shape of a cross, thatis, the central points of the two rectangular openings arecoincident and preferably are on the axis of the cap. Typically,one of the rectangular openings is larger than the other, i.e.,one has a longer length than the other opening.

Jacobs et al.25 disclosed in U.S. Patent No. 6,098,965 adistribution apparatus that includes a plate with a number ofbubble caps. The bubble caps include a riser and a spaced-apartcap. The riser has a top and a bottom, and the riser is securednear the bottom to the redistribution plate. A passageway isdefined between the top and bottom and provides a means offluid communication across the redistribution plate. Preferably,the cap has a plurality of spaced-apart slots to allow the flowof fluids through the cap and into the annulus formed by thecap and the riser.

Muller26 disclosed in U.S. Patent No. 6,769,672 a bubble capassembly having differently configured fluid-flow paths fordifferent resistances to the fluid passageway. This createsdifferent vapor flow rates and liquid flow rates in differentupflow channels. The shape, dimensions, and location of thepassageway, the cross-sectional size and shape of the upflowchannel, the length of the channel, the relative height of thetop of the upflow channel, the roughness of the surfaces incontact with the fluid, and the presence of restrictions in thefluid-flow path, including inside the downcomer, all put theresistance in the overall flow path of fluids. As an example,the difference between the liquid flow rates through at leasttwo flow paths of the apparatus may vary from∼30% to∼8200% for different process conditions. The invention im-proves the uniformity of liquid distribution over the cross sectionof the vessel despite differences in elevation of liquid levels onthe distribution tray or changes in the vapor and or liquid flowrates through the reactor.

Nelson et al.27 disclosed in U.S. Patent No. 6,984,365 a bubblecap distributor where a spacer is located between the riser andthe cap (to maintain a gap between the top end of the riser andthe cap) and a deflector baffle is placed below the outlet of thepassageway. The deflector baffle redirects the majority of thefluid flowing downwardly from the riser passageway, so thatthe fluid forms a relatively wide spray pattern over thedownstream catalyst bed.

In general, the bubble cap design is less sensitive to the traylevelness and shows stable sensitivity over a very broad rangeof liquid loadings compared to the case of the chimneydistributor. Also it provides intimate mixing between gas andliquid because of its vapor-assist liquid sweeping quality. Butbecause of its larger diameter compared to the chimney type,fewer drip points could be accommodated in the tray.

2.4. Vapor-Lift Tube. Gamborg and Jensen28 disclosed inU.S. Patent No. 5,942,162 a liquid-vapor distribution devicefor use in two-phase cocurrent downflow vessels with horizontaltray being fitted with a vapor-lift tube (Figure 16). One leg(downflow tube) of the inverted “U” fits through a perforationin the support tray. The other leg (upflow tube) is shorter sothat it is elevated above the tray. The ends of both legs areopen. At the top of the inverted “U”, there is an internal openingbetween the legs. The device thereby provides a flow path acrossthe tray, with inlet through the end of the short leg, verticalflow through the short leg, direction change at the top of theinverted “U”, downflow through the long leg, and dischargethrough the open end of the long leg below the tray. A verticalslot is cut into the side of the short leg opposite the longer leg.The top of the slot is at or below the bottom of the internal

opening between the legs. Alternatively, two or more slots couldbe cut into the short leg sides adjacent to or opposite the longerleg.

In operation, a liquid level will be established on the tray.The liquid level on the vapor-lift tube will be above the bottomof the short leg but below the top of the slot in the short leg.Vapor will pass through the slot in the short leg, creating apressure drop between the inside and outside of the vapor-lifttube. Because of the lower pressure inside the vapor-lift tube,the liquid level will be higher inside than outside the vapor-lifttube. The vapor and liquid will mix in the shorter leg with thevapor lifting the liquid to flow up and over the connecting wallbetween the shorter and longer legs. Liquid will partiallydisengage while flowing over the connecting wall and downthe longer leg. At the opening under the tray, the liquid andvapor will further disengage with the liquid draining off thedrip edge.

The vapor-lift tube device is similar to the bubble cap devicein concept but has several advantages. Since the vapor-lift tubedevice is smaller, more can be placed on a distribution tray toachieve better distribution of liquid. Furthermore, because ofthe smaller size of the distributor, more wall coverage isachievable. Overall wetting efficiency below the tray is betterwith a smaller pitch than with a larger pitch. The bubble capdesign tray is limited to relatively large spacing; increasing thenumber of bubble caps with reduced spacing would increasethe number of distribution points but would negatively impacton the liquid/vapor flow relationships through each cap. Usingmore bubble caps would require making the bubble caps smallerwith either smaller slots or fewer slots. Using smaller slots isnot practical since there is a minimum slot size for foulingconsiderations. Using fewer slots is not desirable since that maylead to channeling of the vapor in the annulus and less efficientcontacting with the liquid phase. A further advantage for thevapor-lift tube device is that its simplicity makes it easier andless costly to fabricate in the optimal size prescribed by theprocess conditions.

Another advantage of a vapor-lift tube device according tothe invention over a chimney type design is the significantlywider turndown range possible with the vapor-lift tube. As theliquid flow decreases, a properly designed chimney must eitherbecome taller or have smaller holes drilled in the side. Thisproblem has been solved by the vapor-lift tube design as thissystem is self-controlling in the regard that the effective slotareas are changed as a result of the actual liquid flow. The theorybehind this self-controlling system is that the pressure dropequation below needs to be fulfilled for all liquid loads.

Figure 16. Vapor-lift tube (from U.S. Patent No. 5,942,16228).

∆Pslot(h) ) ∆Pstaticvapor(h) + ∆Pstatic

upflow(h) + ∆Pfrictionvapor (h) +

∆Psfrictionupflow (h)


By studying this equation in detail, it can also easily beconcluded that overflow at high liquid rates would not bepossible because the actual liquid level cannot be higher thanthe top of the slot. If the liquid level was assumed to be coveringthe slot, it would be forced down by the vapor to fulfill theabove pressure balance. A further advantage of the vapor-lifttube over the chimney-type design is the increased contactingof the liquid and vapor phases. The intimate contacting thatoccurs in the upflow portion of the vapor-lift tube providescloser approaches to thermal and compositional equilibrium thanwould be achieved in the chimney tray. Another problemcommonly seen in the industry is plugging by coke or corrosionproducts. The vapor-lift tube is less vulnerable compared toperforated plates and chimney trays to fouling because the highgas velocities in the slot would tend to clean and remove cokeand corrosion products.

3. Quench Box

In a cocurrent downflow trickle-bed reactor, the liquid phaseis typically mixed with a gas or vapor phase and the mixture ispassed over a particulate catalyst maintained in a packed bedin a downflow reactor. Because of chemical reaction, heat isproduced and reactant is depleted from the vapor phase andbecomes rich with additional components along the reactorlength. So it is required to quench the effluent and to add reactantat different locations. The quench box is provided in betweentwo beds for liquid cross mixing, distribution correction, andproduct side-draws. The key component of the quench box are(a) a quench box conduit with a sparger, (b) a quench tray orcollection tray, (c) a mixing chamber, and (d) distributor trays.A typical quench box system is shown in Figure 17.

Although some disclosures of quench box design are availablein the patent literature, a compact quench box design still needsto be developed to promote the required intimate mixingbetween the quench stream and the hot reaction two-phase fluidstream while reducing capital investment or unit downtimeduring catalyst change out. The savings on reactor height canbe used to load an incremental catalyst volume to improve theperformance of the reactor or to reduce the total weight or thecapital investment of the reactor.

Scott29 disclosed in U.S. Patent No. 4,133,645 (Figure 18) amethod and a distributor device for effecting the uniformdistribution of a mixed-phase vapor/liquid reactant stream acrossthe fixed bed of catalyst particles. Mixed-phase reactants orcomponents are first separated into a principally vapor phaseand a principally liquid phase. These separated phases are thenremixed in a manner that creates vapor/liquid froth, with thelatter being redistributed to the upper surface of the bed of

catalyst particles. Briefly, the distributor comprises three an-nular-form catalyst-free volumes zones. In the vapor-liquidseparation zone, the mixed phase is separated into an upwardlyflowing substantially liquid-free vapor phase and a downwardlyflowing liquid phase. The flow of the liquid phase is reversedupwardly and a quasi-stagnant pool is formed by overflowinga cylindrical wall or weir onto a horizontal perforated plate.Vapor-phase flow is also reversed, to assume a downwarddirection into the catalyst-free area below the quasi-stagnantliquid pool. The vapor passes upwardly through the perforationsand through the liquid pool to form a vapor-liquid froth andthe area of the device is called the remixing zone. Vapor quenchstream is injected through a perforated toroidal ring into theseparated vapor-phase proximate towards to the locus of vapor-flow reversal. The froth is directed through the downcomer ofthe tray to the catalyst bed below.

As disclosed in U.S. Patent No. 4,836,989, Aly et al.12

developed a distributor system with an improved vapor/liquidcontact and distribution. As shown in Figure 19, a collectiontray beneath the catalyst support grid is there to collect the liquidleaving the upper catalyst bed. The vapor is injected though aspider-type distributor to provide a uniform initial distributionof the injected vapor. Spillways are provided in a collector trayto permit a pool of liquid to accumulate on the tray beforepassing through the spillways into the mixing chamber. Thespillways comprise upstanding downcomers, which provide apassage for the downflowing liquid as well as for the vapor.The spillways have outlets beneath the collector tray, whichface sideways and tangentially into an annular mixing chamber.

Figure 17. Typical quench box.

Figure 18. Distributor/redistributor or quench box system (from U.S. PatentNo. 4,133,64529).

Figure 19. Distributor/redistributor or quench box system (from U.S. PatentNo. 4,836,98912).


The mixing chamber comprises a cylindrical, vertical-wallportion, which is fixed to the collection tray and a lower, annulartray with an upstanding rim for providing a pool of liquid inthe mixing chamber. The side-facing outlets of spillways imparta rotary or swirling motion to the liquid in the mixing chamber,which promotes good intermixing and temperature equilibriumof the liquid at this point. The liquid spills over the edge or rimand falls downward onto the deflector fixed to the roughdistributor tray. The disclosed design does not provide for acompact quench box design in view of the requirement for thebend of the quench pipe at the centerline of the reactor and thetwo-tray assembly for the distributor system.

Pedersen et al.30 disclosed in U.S. Patent No. 5,462,719 amethod and apparatus for mixing and distributing fluids in areactor (Figure 20) where the ratio of liquid to vapor is relativelylow. The apparatus forms a mixing zone in the column intowhich a first reactant (e.g., a process stream having both gasand liquid phases) flows vertically downward. The secondreactant (e.g., quench gas) is introduced into the mixing zoneand flows radially to intercept the downflowing process stream.Because of the construction of the mixing zone, a tortuous pathis provided that thoroughly mixes the different gas phases andthe liquid phase by several different mechanisms, i.e., (a) radialjet mixing; (b) perpendicular mixing of the liquid and gasphases; (c) jet/turbulent mixing; (d) high-velocity swirl jetmixing and atomizing of the liquid phase; (e) turbulent swirlmixing after the liquid is atomized; (f) additional turbulent jetmixing; and (g) energy dissipation of the flow as the mixedstream is distributed radially outward across the column throughfirst a sieve tray and then a riser tray.

Jacobs et al.25 disclosed in U.S. Patent No. 6,098,965 a reactordistribution apparatus and quench zone mixing apparatus. Aquench zone mixing apparatus that occupies a low vertical height

and has an improved mixing efficiency and fluid distributionacross the catalyst surface includes a swirl chamber, a roughdistribution network, and a distribution apparatus. In the swirlchamber, reactant fluid from a catalyst bed above is thoroughlymixed with a quench fluid by a swirling action. The mixed fluidsexit the swirl chamber through an aperture to the roughdistribution system where the fluids are radially distributedoutward across the vessel to the distribution apparatus. Thedistribution apparatus includes a plate with a number of bubblecaps and associated drip trays that multiply the liquid drip streamfrom the bubble caps to further symmetrically distribute thefluids across the catalyst surface. Alternatively, deflector bafflesmay be associated with the bubble caps to provide a wider andmore uniform liquid distribution below the plate. The distribu-tion apparatus can be used in the reaction vessel without theswirl chamber and rough distribution system, e.g., at the top ofa vessel.

Chou31 disclosed in U.S. Patent No. 20020172632 a compactquench box design for a cocurrent downflow fixed-bed reactor.It comprises quench pipe assembly for gas injection, a collectiontray fitted with ramps, a mixing chamber, and a distributiontray. Ramps help in radial mixing of the two phases and bringthe liquid from the collection tray to the mixing chamber. Vaporcoming out of the mixing chamber redistributes radially ontothe final distributor tray from the center of the reactor. In thisdesign, the location of the quench gas assembly is away fromthe centerline of the reactor, which eliminates the need todisassemble the quench distributor during the unloading of thecatalyst.

Nelson et al.27 disclosed in U.S. Patent No. 6,984,365 aquench zone mixing apparatus with improved mixing efficiency.They have put baffles inside the swirl chamber to stabilize thevapor- and liquid-phase vortices. This reduces the requiredoverall height of the swirl chamber, provides a wide operating

Figure 20. Distributor/redistributor or quench box system (from U.S. Patent No. 5,462,71930).

Figure 21. Sensitivity of different distributors with changes in liquid flowrate.35

Figure 22. Industrial vapor-liquid distributors improvements.36


range for vapor and liquid throughput, and promotes turbulence/mixing within each of the fluid phases.

Breivik et al.32 disclosed in U.S. Patent No. 20060257300 aquench box comprising gas injection line, a collector tray,spillway collectors, a mixing chamber, and an impingement platebelow the mixing chamber followed by distributor trays toimprove the distribution, quenching, and gas-liquid mixing.A pool of liquid accumulates in the collector tray and istransferred to spillways, which are provided with outlets thatimpart a rotary movement to the exiting fluid. The vortex mixingchamber mixes the reactant fluids in a compartment where thefluids are swirled together. The fluids exit the mixing chamberby overflow in a weir and pass through a central orifice at thebottom. The fluids then drop onto the impingement plate, whichredirects the flow radially underneath the mixing chamber. The

impingement plate is located at a distance above the firstdistributor tray to provide free flow of liquid underneath.

3.1. What is Happening after the Distributor. The use ofinert layer particles as the top layer is common in commercialpacked columns,33 which attempts to compensate for poor liquiddistribution and prevents disruption of the upper surface of thecatalyst bed. The inert layer is basically chemically inert granularmaterial, and hence, the character of the material depends onthe nature of the reaction prevailing in the reactor bed.Tasochatzidis et al.34 detected a slight improvement of liquiddistribution by means of a conductance probe using two layersof large-size inert material (12 mm and 10 mm alumina ball)as the top layer in a bed of 3 cm cylindrical particles. Anyuneven distribution due to the distributor or the top layer cancause a change in the downstream flow pattern.

Figure 23. Distributor for upflow reactor (a) distributor in details and (b) quench (U.S. Patent No. 6,554,99438).

Figure 24. Liquid mass velocity contours of a radial slice at a 5 ft depth (11 ft diameter reactor) for (a) riser tray, (b) conventional bubble cap tray, and(c) modified bubble cap tray (from Jacobs and Milliken49).

Figure 25. Liquid mass velocity contours of a radial slice at a 40 ft depth (11 ft diameter reactor) for (a) riser tray, (b) conventional bubble cap tray, and(c) modified bubble cap tray (from Jacobs and Milliken49).


4. Comparison of Distributors

4.1. Sensitivity.All known distributors fall into two types,gravity and gas-lift type distributors. Because of the differentoperating principles, the two types would have differentcharacteristics, as shown in Figure 21.35 It could be seen fromFigure 21 that the vapor-lift distributor shows excellent sensitiv-ity over a wide operating range, i.e., overflow at high liquidrates can be avoided by the gas-lift principle.

4.2. Performance.Patel et al.36 compared the advantages anddisadvantages (Table 2) of various distributors with respect todistributor spacing density, level sensitivity, liquid turndownratio, flexibility to vapor/liquid ratio, and liquid/vapor mixingcapability. The vapor-lift tube was found to be much better thanothers. It is also observed that more and more gas-lift distributorsare being used, and use is expected to be increased in the future,as shown in Figure 22.36

5. Internals for Upflow Reactors

Sometimes cocurrent upflow reactors are used because ofnumerous advantages over downflow reactors, especially whenit necessary to ensure complete external wetting at the sametime as having high liquid holdup for a liquid-limited reaction(Dudukovic et al.37). However, it is not widely practicedcommercially because of the difficulty in designing and manag-ing such a system. Reynolds38 disclosed in U.S. Patent No. 6,-554,994 a fluid distributing means and quench system forcocurrent upflow of gas and liquid in a fixed-bed hydrotreaterwith layered catalyst (Figure 23). The fluid distribution meansmay take a number of forms, for example, screens, grids,perforated plates, etc. The fluid distribution means serves twoprimary functions. It is intended to distribute the fluids passingupwardly through the reactor evenly across the horizontal planeof the catalyst layer. It also serves to ensure the break-up of

Table 2. Comparison of Distributors

distributor typedistributor

spacing densitylevel

sensitivityliquid

turndown ratioflexibility

vapor/liquid ratioliquid/vapor

mixing capability

perforated plate best worst worst worst worstsimple chimney average poor poor poor poormultiport chimney average average average average poorbubble cap worst average good good bestgas-lift best best best best best

Table 3. Liquid Dispersion for Different Distributors

distributor type riser conventional bubble cap tray modified bubble cap tray

layoutpitch (in) 20.3 cm 17 cm 17 cmpattern square triangular triangularno. of distributors

2.4 m diameter reactor 90 151 1713.35 m diameter reactor 184 301 3454.26 m diameter reactor 304 499 559

liquid discharge pattern uniformly over 2.5 cmdiameter circle

50% uniformly over 3.8 cm diameter circle;50% uniformly over surrounding2.0 cm annular ring

uniformly over 10 cm diameterannular ring 0.25 cm wide

Table 4. Distributors Used in Lab Scale

authors distributor details column and packing details flow range,kg/m2 s phase distribution

Herskowitz andSmith52

capillary tubes; 86 tubes (0.1 cm i.d.)arranged uniformlyover thecross section; the tubes were1 cm long and located 0.5 cmabove the top of the bed

column diameter/height (mm):40.8,114/260-700; packingdiameter (mm): granular,2.52-11.1; spherical, 3.0-9.5;cylindrical, 3.8-8.9

L ) 1-5;G ) 0.0014-0.07

the gas through the annulussurrounding the centralliquid feed tube

Levec et al.53 capillary tubes; their distributorconsists of the capillaries thatwere placed between two Pertinaxplates; 550 capillaries at 6.0 mmpitch with 0.9 mm i.d. nd 3.0 cmlength through which water waspumped into the column

plexiglas column; diameter/height(cm) ) 17.2/130;D/dp ) 28-57

G ) 0.0-0.37;L ) 0.06-25.7

the bottom plate had circular holesaround the capillaries; the airintroduced to the chamberformed between the platesand it exited the distributorthrough these holes

Ravindra et al.54 99 stainless steel capillarytubes were used

rectangular (60 mm× 80 mm);height) 200 mm; packing

diameter (mm)) 1.6-6.3;glass beads and aluminaparticles

L ) 1-8;G ) 0.05

to introduce liquid as a pointinlet; stainless steel tube of0.4 cm was employedinstead of capillary tubes

Saroha et al.55 ladder type; the liquid distributorwas made of 6.4 mm i.d.stainless steel tubes to which3.2 mm i.d. tubes were attached;there were 37 1.5 mm holesarranged in a square pitch of2 cm

column diameter/height(mm) ) 152/550;packing diameter (mm):1.5extrudates

L ) 0.7-5;G ) 0-0.027

both gas and liquid flow throughdistributor

Li et al.56 121 tubes 2 mm diameter, whichwere supported by two plates;the upper plate was perforatedwith 121 holes, and the lowerone was perforated with180 holes

rectangular (120× 120 mm)height) variable up to 500 mm;packing diameter (mm)) 1.5polypropylene trilobe, spheres

UG ) 0-35 mm/s;UL ) 2.3 mm/s

liquid and gas flowed throughthe tubes and the remainingholes


large gas bubbles and the optimal mixing of the fluids.Preferably, it also is designed to serve the secondary functionsof supporting the catalyst layer and preventing the mixing ofthe catalyst particles at the interface between any two adjacentcatalyst layers.

The fluid distribution plate is perforated with a numbers ofholes with tubes or risers that have a deflector on their lowerends to prevent the direct passage of large gas bubbles into thebore of the riser, assuring optimal distribution of the fluids. Eachriser has at least one opening in its sidewall. Vapor passes intothe bore of the riser through the openings and the liquid aroundthe deflector and through the opening at the bottom of the risers,up through the perforations in the fluid distribution plate, intothe space, and upward through the catalyst support grid intothe immediately adjacent layer of catalyst. Since many of thereactions taking place in the bed of catalyst are exothermic,there is a quench line into the catalyst bed in an optimal location,

i.e., directly below the fluid distributing means, to ensure theeven distribution of quench fluids in each catalyst layer toprevent hot spots.

Hatem and Corinne39 disclosed in Patent No. WO2004033084a distributor system for a fixed-bed upflow reactor to producebisphenol from acetone and phenol. The distributor comprisesa manifold and distribution arms extended laterally frommanifold. The manifold and distribution arms have perforationsfor fluid distribution.

6. Applications of CFD in Distributor Studies

A liquid distributor located above the packed bed is used todistribute liquid and gas uniformly, thereby dictating how andwhere the downflowing liquid enters the top of the catalystsurface. Various distributor designs have been developed in anattempt to obtain a uniform distribution of liquid for optimumpacking performance based on drip point density distributionand geometric uniformity.40-42 The distributor tray designs aremostly influenced by hardware used in packed and trayedfractionation columns. The design of the distributor is commonlyextrapolated from experiments carried out at small scale withfluids at low pressure and low temperature, usually air and waterat ambient conditions, which do not correspond to industrialconditions and may not be representative. Numerical modelingis used to fill the gap between cold-flow experiments and realprocess conditions. Stanek et al.43 formulated a mathematicalmodel of the effect of flow distribution on the progress ofcatalytic reaction with heat effects. The numerical solution wasobtained by the finite difference method and predicted that theentrance-region flow pattern plays a significant role in affectingconversion. Stanek44 analyzed the impact of nonuniformity ofinitial velocity and pressure field on maldistribution of liquid,mass transfer, heat transfer, etc. by formulating a mathematicalmodel. It was observed that these effects should be interpretedcorrectly and appropriate measures should be taken in the designof equipment for cost savings.

In the past decade, the CFD model has been significantlyenhanced and calculation speed has been greatly increased, sothat CFD has actually been more and more used to simulatehydrodynamics in complex flow systems such as a bubblecolumn and a fluidized bed. Sapre et al.45 have also developeda novel flow-distribution technique to measure the point-to-pointflow in a hot reactor, which could be used for verification andvalidation of numerical results. Fewer works have been pub-lished for flow behavior occurring within internals like distribut-ing trays or mixing boxes. Van Baten and Krishna46 havesimulated the flows on sieve trays in the Eulerian framework.Their simulations could capture chaotic tray hydrodynamics andreveal several liquid circulation patterns. Raynal and Harter47

studied two-phase flow through a downcomer (chimney type)experimentally and numerically. They used both 2D and 3Dsimulations using the volume of fluid (VOF) approach to capturetwo-phase flow with an interface clearly identified. Theyobserved good agreement between simulated and experimentalflow pictures captured through a high-magnification charge-coupled device (CCD) camera.

Some authors also studied the impact of two-phase flowdistributors on catalyst utilization depending on drip pointspacing, liquid discharge pattern, and wall coverage. The effectof feed distribution on the macroscopic flow structure wasstudied using the numerical simulations by several authors.48-51

Jiang et al.50-51 have simulated numerically the impact of initialliquid entry from a single-point source, a two-point source, anduniform distribution, using the discrete cell model approach

Figure 26. Different type of lab distributors (a) capillary tubes used byLevec et al.53 and Ravindra et al.;54 and (b) ladder type used by Saroha etal.55


(DCM) in trickle-bed reactors. They predicted that the numberof liquid channels formed in the nonprewetted bed correspondsto the number of liquid point sources. They evaluated behaviorof the packed bed with initial uniform liquid distribution. It wasalso observed that, although initial distribution was uniform,after a few particle diameters of axial distance, the channelformation starts.

Jacobs and Milliken49 quantified the impact of center-to-centerspacing, wall coverage, and liquid-discharge pattern on catalystutilization using computational fluid dynamics (CFD) for someof the recent advanced gas-liquid distributors. These distributorsare riser tray, conventional bubble cap tray, and modified bubblecap type, and these trays differed for their liquid dischargepattern, pitch, and numbers of distributor points. They evaluatedthree tray designs, i.e., riser tray, conventional bubble cap tray,and modified bubble cap tray with assumed liquid dischargepattern. The trays were evaluated for reactor diameters of 2.4,3.35, and 4.26 m to cover the majority of installations. Theyassumed different discharge patterns (shown in Table 3) fordifferent distributors. Using CFD modeling, contour plots wereobtained for different liquid dispersions. Figures 24 and 25represent extent of liquid dispersion at inerts/catalyst interfaceat different bed depths. Homogeneity represents plug flow; darkrepresents no flow, and spotted is for local flow. Because ofthe ring-shaped discharge pattern and increased wall coverage,the modified bubble cap tray design quickly approaches plugflow, in comparison with the other two designs. The design ofa distributor optimized within laboratory standard experimentalranges may not match the design requirements for industrialflow conditions. CFD has become mature to capture flowbehavior and guide for design strategy.

7. Laboratory Scale

Distributors used in laboratory-scale reactors are mostly forcold-flow studies. The only purpose of these is to ensure uniformliquid distribution because they are free from other criteria thatoccur for large-scale reactors. Different types of small but simpledistributors have been reported by several authors for generationof experimental data (Herskowitz,52 Levec et al.,53 Ravindra etal.,54 Saroha et al.,55 and Li and co-workers56,57). These aremainly (i) capillary type, where provisions are made for liquidand gas flow separately, (ii) ladder type, with a perforation inthe branch tube for mixed-phase flow or perforated plate (Figure26). Table 4 shows some details about the various types ofdistributors used in different studies.

8. Conclusions

From the foregoing, the following conclusions regardingliquid distribution and flow texture can be drawn:

• A good liquid distributor at the top of the catalyst bed isvital for uniform liquid distribution, which is influenced bydistributor spacing, liquid discharge pattern, and off-pitchdistributor placement near the wall.

• Mainly four types of distributors are used, namely,perforated plate, multiport chimney, bubble cap, and vapor-lifttube.

• A perforated plate or sieve tray is simple to construct andis capable of providing the maximum number of drip points,but it is very sensitive to tray levelness, is vulnerable to dirtdeposits, is less flexible to liquid load, and has a tendency torun dry toward the end of the cycle.

• The most commonly used distributor in the past for lesscritical service is the multiport chimney type. Vapor is allowed

to pass through the top opening and liquid passes through weepholes cut into the side of the riser. This tray has less sensitivityto tray levelness, increased tolerance to dirt deposits, greaterflexibility to changing vapor/liquid ratio, and fewer chances torun dry toward the end of the cycle. Sometimes attachment ofa liquid conduit to the chimney improves performance over thatof a conventional chimney.

• Sizing of the multiport downcomers can be done followingthe design methodology as outlined by Muldowney et al.20

• Some of the key design components of a multiport chimneydistributor are number of chimneys, apertures around theperimeter of the chimney, the cross-sectional area of eachaperture, the top opening of the chimney, the centerline of theaperture above the tray, the liquid level on the tray, the pitchof the aperture, etc.

• Sometimes, a spray-generating device such as a helicalribbon is attached at the end of downcomer to produce a conicalspray. The mixed fluid stream from one spray-generating devicewill overlap with the spray from an adjacent spray-generatingdevice. The angle of the spread of the conical spray pattern iscontrolled by the choice of ribbon pitch, diameter width, andlength. The angle of the spray and overlap determines theappropriate distance between the tray carrying the spray deviceand the catalyst bed.

• In a bubble cap distributor, vapor passing through slots cutin the bubble cap aspirates liquid held up on the tray, carryingit over a central downcomer. It shows stable sensitivity over abroad range of liquid loadings compared to the case of thechimney tray. However, the only disadvantage is the size ofthe bubble cap, which introduces wider spacing between drippoints and a lower number of drip points, causing less catalystutilization.

• The performance of a gas-lift distributor is found to be morepromising compared to other distributors because it providesintimate mixing of vapor and liquid, is less vulnerable to fouling,is insensitive to tray levelness, can distribute liquid uniformlyat large turndown ratio, and can accommodate a larger numberof drip points compared to the case of the bubble cap distributor.With proper design, the vapor-lift tube device will reduce theliquid flow difference between vapor-lift tubes at differentelevations better than what can be achieved with chimney typeand bubble cap designs. The most commonly installed distributorin the industry today is the gas-lift distributor, especially forapplications where optimal catalyst performance is mandatory.

• In a cocurrent downflow trickle-bed reactor, because ofchemical reaction, heat is produced and reactant is depleted fromthe vapor phase and becomes rich with additional componentsalong the reactor length. So it is required to quench the effluentand to add reactant at different locations. A quench box isprovided in between two beds for liquid cross mixing, distribu-tion correction, and product side-draws. The key component ofthe quench box are a quench box conduit with a sparger, aquench tray or collection tray, a mixing chamber, roughdistributor trays, and trays for final distribution.

• In the case of an upflow packed-bed reactor, a perforatedplate fitted with a number of riser tubes is used to produce auniform distribution of fluids. Also, the quench fluid is usedthrough the perforated quench tube between the beds to preventhot spots.

• Very few studies have been reported for CFD analysis ofdistributors. Uniform liquid distribution is influenced by dis-tributor spacing, liquid discharge pattern, and off-pitch distribu-tor placement near the wall. A modified bubble cap typedistributor, simulated using CFD, results in the highest catalyst


utilization because of improved ring-shaped discharge patternand increased wall coverage. Using 2/3 layers of large-size inertmaterial as the top layer in the bed and a redistributor in thebed improves the liquid distribution.

• Various types of distributors, simple in nature and withprovision of separate gas and liquid entry, are used in smalllaboratory-scale reactors to ensure uniform distribution of liquid.

• Various types of distributors were compared with theiradvantages and disadvantages. It is found that gas-lift tubes arebest with respect to distributor spacing density, level sensitivity,liquid turndown ratio, flexibility to vapor/liquid ratio, liquid/vapor mixing capability, and level sensitivity. The vapor-lifttube was found to be much better than the others.

9. Recommendations for Future Work

• A high-quality distributor should be developed based oncold-flow modeling and supported by CFD calculations forcommercial operating range with special emphasis on thedischarge pattern.

• The distributor with a spray-generating device at thedowncomer end should be developed using a cold-flow studyand backup support with CFD.

• The advantages of a vapor-lift distributor over a chimneytype design are significant. More work with this type of designshould be done with cold-flow testing as well as with CFDapplication.

• A design methodology should be developed for the vapor-lift tube distributor.

• CFD studies are recommended to be performed for thequench box.

We hope that the above conclusions will further help indesigning/choosing internals, especially distributors, and recom-mendations made for future work will further demystify the flowdistribution in trickle-bed reactors. Both these aspects (conclu-sions and recommendations for future work) taken together willprovide a holistic approach in the design of good distributorsfor uniform distribution of two phases in a cocurrent trickle-bed reactor. We strongly feel that the quality of transportationfuel has to be further improved beyond EURO III and IV (5-10 ppmw) to have a much cleaner and greener environment.We hope that the present manuscript focuses our attention tofurther understanding of microlevel phenomena in TBRs, whichwill facilitate uniform liquid distribution to meet the stringentsulfur specification in the future (5-10 ppmw) in transportationfuel.

Acknowledgment

We wish to acknowledge the support of Center for HighTechnology, Ministry of Petroleum and Natural Gas, Govern-ment of India, for providing research facilities in the area oftrickle-bed reactors (TBRs).

Nomenclature

a ) effective interfacial area per volume of bedas ) packing specific area per volume of bedA ) total area of the hole(s) at an elevationD ) diameter of tubeg ) acceleration due to gravityh ) liquid height above top surface of trayH ) height of hole center above top surface of trayL ) local liquid velocityQL ) liquid volume flow rates per downpipeQG ) gas volume flow rates per downpipe

NF ) Froude no. (V/(g D)0.5)V ) superficial liquid velocity in the downcomers

Greek Letters

FL ) liquid densitiesFG ) gas densitiesσL ) liquid surface tension

Subscripts

L ) liquidG ) gas

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ReceiVed for reView February 18, 2007ReVised manuscript receiVed May 31, 2007

AcceptedJune 21, 2007

IE070255M


distributor- trickle bed reactor

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