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 authors 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.9,10 The 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: nigamkdp@gmail.com.

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.

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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.

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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 outthe 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.

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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

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).

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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).

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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