divid wall

64 www.cepmagazine.org May 2002 CEP

Reactions and Separations

istillation is the primary separation pro-cess used in the chemical processing in-dustries (CPI). While this unit operationhas many advantages, one drawback is

its significant energy requirement. The dividing-walldistillation column (DWC) offers an alternative toconventional distillation towers, with the possibilityof savings in both energy and capital costs.

For example, two applications of DWC technolo-gy designed by UOP are now part of a new UOPlinear alkyl benzene (LAB) complex that saved 9%of the total fractionation energy used in this com-plex. Based on a present world LAB production of2.6 million m.t./yr, and a fuel value of $10.8/millionkcal, a $12.8 million annual energy savings wouldbe possible if these DWCs were used in place ofconventional multicolumn trains in every complex.

In addition, the uninstalled equipment cost forthe total complex is reduced by about 10%. Thecapital cost savings result from the reduction in thequantity of equipment (i.e., one column, reboiler,condenser, etc., instead of two of each). There arealso indirect benefits: a DWC requires less plot areaand, therefore, shorter piping and electrical runs, asmaller storm runoff system and other associatedbenefits. The flare loads are reduced because of thesmaller heat input and less fire-case surface, leadingto a smaller flare system.

While a DWC is not suitable for every situation,clearly it can be an attractive alternative to conven-

tional distillation. This article will review recent de-velopments in DWC technology and provide guide-lines for the design of these columns. Finally, twoapplications will be discussed to illustrate the de-sign process.

DWC backgroundA DWC is not a new concept, having been intro-

duced in 1949 (1). However, lack of reliable designmethods, and concerns about the operation and con-trol of these columns have prevented theirwidespread application. Work being done in bothacademia and industry is helping to address theseconcerns. Several authors provide some good back-ground for understanding the theory behind Petlyukcolumns and/or DWCs (211). A recap is providedhere to show how a single DWC can replace an ex-isting two-column sequence.

ABC split and Petlyuk evolutionConsider a mixture consisting of three compo-

nents, A, B and C, where A is the lightest and C theheaviest. Figure 1a shows how this separationwould be accomplished in a direct sequence of twodistillation columns. For some mixtures, for in-stance when B is the major component and the splitbetween A and B is roughly as easy as the split be-tween B and C, this configuration has an inherentthermal inefficiency (Figure 2). In the first column,the concentration of B builds to a maximum at a

These distillation columns can significantlyreduce capital and energy expenses vs.conventional multicolumn arrangements.

Reduce Costs with Dividing-Wall Columns

D

Michael A. Schultz,Douglas G. Stewart,James M. Harris,Steven P. Rosenblum,Mohammed S. Shakurand Dennis E. OBrien,UOP

CEP May 2002 www.cepmagazine.org 65

tray near the bottom. On trays below this point, theamount of the heaviest component C continues to in-crease, diluting B so that its concentration profile nowdecreases on each additional tray toward the bottom ofthe column. Energy has been used to separate B to amaximum purity, but because B has not been removedat this point, it is remixed and diluted to the concentra-tion at which it is removed in the bottoms. This remix-ing effect leads to a thermal inefficiency.

Figure 1b shows a configuration that eliminates thisremixing problem. This prefractionator arrangement, orPetlyuk column (11) as it is commonly known, per-forms a sharp split between A and C in the first column,while allowing B to distribute between these twostreams. All of A and some of B are removed in theoverhead of the smaller prefractionation column, whileall of C and the remaining B are removed in the bot-toms of the prefractionation column. The upper portionof the second column then performs an A/B separation,while the lower portion separates B and C. During thedesign phase, the fraction of B separated in the over-head of the prefractionator can be set to prevent theremixing seen in the direct sequence of Figure 1a. Thethermal inefficiency has been eliminated, leading to asignificant energy saving of about 30% for a typical de-sign and can reach 50% or 60% for unconventionalones (3, 7, 12).

Figure 1b shows that the Petlyuk arrangement isthermally coupled. In other words, vapor and liquidstreams from the second (main) column are used to pro-vide vapor and liquid traffic in the prefractionator. Thissystem has only one condenser and one reboiler, andboth are attached to the second column. Because thePetlyuk arrangement has fewer pieces of major equip-ment than does the conventional two-column sequence,total capital costs may be reduced.

Integrating the prefractionation column into thesame shell as the main column can further reduce theamount of equipment. This is the DWC (Figure 1c). As-suming that heat transfer across the dividing wall is

negligible, a DWC is thermodynamically equivalent toa Petlyuk column. When compared to a conventionaltwo-column system, a capital cost savings of up to 30%is typical (3, 7, 12).

As discussed above, a DWC can be used to separatethree products in a single column. In evaluatingwhether a DWC is a viable option, consider the thermo-dynamic properties, as well as the composition of thestream to be separated, in addition to the product re-quirements. Based on this information, some guidelinesare useful to determine whether a DWC is a good can-didate for accomplishing a particular separation:

Product purity: the purity of the middle product isgreater than can be achieved in a simple sidedraw column.Therefore, when a high-purity middle product is desired, aDWC should be considered. If strict purity specificationsare not required for the middle product, a simple sidedrawcolumn may be sufficient for the task. However, even forthis case, a DWC may be advantageous as it may accom-

A B

C

A

C

B

A

C

B

a. Direct Sequence b. Petlyuk Column c. Dividing-WallColumn

Figure 1. Separating three components by distillationcan be done in a variety of arrangementswith one or two towers.

Component B Mol Fraction

Component BProfile inColumn 2

Component BProfile inColumn 1

RemixingOccurs inColumn 1

Co

lum

n T

ray

Top

Bottom0 1.0

Figure 2. Remixing of Component B occurs in a conventional, two-column direct-sequence arrangement.

plish the separation in a smaller column using less energycompared to the simple sidedraw setup.

Feed composition: Component B should be in ex-cess, and components A and C should be present in fair-ly equal quantities. A typical rule of thumb is that aDWC is most advantageous when the feed consists ofabout 6070 mol% B, with A and C then making up therest of the feed in roughly equal amounts. It is impor-tant that this rule not be applied indiscriminately,though, because the relative volatility of the compo-nents is important as well. The relative volatility ij is afactor indicating the difficulty of separation of twocomponents. The larger the value of this parameter, theeasier is a separation for a given system.

Relative volatility: When B is a significant portion ofthe feed, a DWC can be advantageous as long as the splitbetween A and B is at least as difficult as that between Band C (11). When the A/B split is fairly easy relative to theB/C split, the DWCs advantages may not be great enoughto justify its selection over a simple direct sequence.

Revamp possibilities: To increase the throughputthrough an existing simple sidestream column, a divid-ing wall can be inserted through a portion of the col-umn. This is an extension of the first rule concerningproduct purity. In this case, however, if increasedthroughput is desired through an existing simple-sidestream column, it may be effected by inserting a di-viding wall through a portion of the column.

As with any guidelines, there are exceptions, butthese can be useful during the screening process.

The case against DWCsAlthough a DWC may offer the potential for a sav-

ings in both capital and energy costs, there are somesituations in which two-column separations are prefer-able (3, 6). For instance, a DWC contains a single con-denser and reboiler to provide the entire reflux liquidand boilup vapor to the column. The condenser operatesat the coldest temperature required for the separation,while the reboiler operates at the hottest temperature.Compare this to the two-column sequence, in which thereboiler on the first column and the condenser on thesecond are at intermediate temperatures, so some of theduty can be supplied at intermediate levels. This maybe advantageous for heat integration purposes, or ifless-expensive intermediate duties are available.

Further, the two columns may require significantlydifferent operating pressures for reasons that may in-clude restrictions on overhead or bottoms tempera-tures, due to the available duties or restrictions on thebottoms temperature because of fear of degradation orpolymerization. The flexibility of operating at distinct-ly different design pressures might outweigh the sav-ings possible with a DWC. Additionally, a DWC willlikely be taller and have a larger diameter than either

of the two conventional columns, and may surpass con-struction restrictions for a single tower. One solution tothis problem is the use of high-performance trays.Such trays have a high capacity and efficiency, and canbe spaced close together (as little as 300 mm), reduc-ing the diameter and height of the DWC. Therefore, asin any design problem, it is necessary to evaluate theconstraints and tradeoffs before proceeding with amore-detailed design.

Industry reviewWhile theoretical studies have shown the economic

advantages of DWCs in certain circumstances, industryhas been hesitant to build these columns. One reasonmay be a lack of understanding of their design and con-trol. In recent years, several academic groups have re-searched this area (14, 15, 16). One group set up apilot-scale column to study controllability and operabil-ity (17, 18). This work has contributed to a better un-derstanding of the design and control, and therefore agrowing acceptance of DWCs within industry.

In 1985, BASF constructed and started up what is be-lieved to be the first commercial DWC. BASF is also be-lieved to be the leader in the total number of such columnsin existence, with roughly 25 DWCs operating today (3).

Various consulting and engineering and constructionfirms offer design and construction services for DWCs.Kellogg Brown & Root has designed at least two DWCsfor BP, at least one of which is used to split a lightkerosene stream (13). Sumitomo Heavy Industries hasbeen involved in designing at least six columns forundisclosed customers (the firm refers to its technologyas a Column in Column) using an onsite pilot-plantfacility (19). Linde AG has recently constructed theworlds largest DWC for Sasol, an estimated 107-m talland 5-m in dia. Finally, Krupp Uhde has designed a col-umn to remove benzene from pyrolysis gasoline forVeba Oel. A similar column by Krupp Uhde will startup for Chevron in the near future (19).

DWCs for detergent manufactureDWC technology is suitable for use in the separation

of streams in plants that produce detergents and aromat-ics, as well as in refining, hydroprocessing and reform-ing operations, among others. Two applications for deter-gent manufacture are discussed here to illustrate thesteps involved in designing and constructing a DWC.

Once an application has been identified, the next stepis to model the column. A general outline of the ap-proach is given below. The explanation here will referonly to the simple A, B, C split described previously.

Steady-state modelingThe first step in studying a DWC is developing a stat-

ic or steady-state simulation. This can be done using pro-



prietary software or with standard packages, such asAspen Technologys Aspen Plus or HyprotechsHYSYS.Process. The latter do not include a basic divid-ing-wall column in their library of functions, althoughthe user can build custom fractionation columns. Thisfeature was used to model the DWC as a combination ofindividual tray sections, connected by internal vapor andliquid streams. For a conventional DWC, four columnsections are needed: one for each of the sections aboveand below the wall, and one for each of the sections oneither side of it. This DWC has five degrees of freedom(condenser, reboiler and three product streams). Typicalspecifications for design are three compositions plus thesplit-fraction of the vapor below the wall, and the split-fraction of the liquid above the wall.

Once the initial static simulation is developed andconverges to the desired product specifications, the nextstep is to optimize the design. Column sections can bedesigned, similar to a conventional column, to balancethe capital vs. energy cost trade-offs. This is more complicatedthan with a conventional columnbecause liquid from the upperrectification section splits to ei-ther side of the wall. The refluxrate must be sufficient to satisfythe separation on either side.

As already noted, the key tothe DWC advantage is that themiddle component B is split inthe prefractionation section, sothat some B travels above thewall, while the remaining amountmoves below the wall and outwith the sidedraw stream. The split can be set to mini-mize the total reboiler-duty requirement by eitheradding or removing trays above or below the feed point,or changing the amount of liquid reflux that is sent tothe feed-side of the column.

Consider a typical example, where B is roughly6070 mol% of the feed, A and C are present in equalproportions, and the difficulty of the split between Aand B is roughly the same as that for the split betweenB and C. In the prefractionation section, the optimal de-sign would result in B distributing so that half is recov-ered above the wall and the other half below the wall. Ifthe feed deviates from any of these conditions, it maybe necessary to take more than half of B either above orbelow the wall to optimize the design. Finally, both thevapor and liquid splits must be optimized, which willhave a significant impact on the required reboiler duty.

Dynamic modelingNext, a dynamic study is performed to ensure the

successful commercialization of a DWC. One reason is

to develop suitable control schemes for this unconven-tional system. Another reason is that a dynamic studyhelps in understanding the column operation.

In steady-state modeling of a distillation column,thermodynamics are a key concern. The column is de-signed using the appropriate property package and set-ting the feed flowrate and composition, as well as thecolumn operating conditions, to meet the product speci-fications. Different considerations are important for adynamic model. These involve the physical processesoccurring in the column and include the weir heightsfor the trays, devices such as trapout trays and the wallstatic-head considerations, plus the actual equipmentrequired to make the column work.

The dynamic simulation should be thought of as atrue operating plant. Some preliminary equipment de-signs are developed using the steady-state modeling. Ifthe steady-state model has a condenser that producesproduct streams and reflux, the dynamic model must in-

clude the overhead exchanger,the overhead accumulator andthe reflux and/or product pumps.Control instrumentation is re-quired and must be tuned. Fur-ther, the dynamic simulationshould model the hydraulic net-work. Control valves need to beincluded in the lines as appropri-ate, and additional valves may bemodeled to account for pressuredrops. The dry-tray pressure dropacross the column is determinedby accounting for the static head.Equivalent weir heights and tray

spacings must be included to accurately model liquidand vapor holdup when using theoretical stages.

The model must next be validated before using it topredict the columns dynamic behavior. One way ofdoing this is operating the dynamic model until it re-produces the results of the steady-state simulation. Thisdynamic steady-state can then serve as a baselinestarting point for all subsequent testing. In reaching thedynamic steady-state, the controllers can be tuned andstrip charts configured to record key process variables.

Once a dynamic model has been created, other disci-plines should critique and try to improve it. Variousgroups such as operations, reactor design, instrumenta-tion, etc., bring a unique perspective to the problem.After proving that the system will work, further testingshould be done to determine the optimum location forthe control points. Once a final design is set, the designteam holds a final review meeting before anyone sizesand selects items such as rotating equipment, instru-mentation and piping. The work process involves thefollowing steps:


DWCs typically separatea feed into three

products. The middleone is purer then that

from a sidedraw column.

1. The design engineer responsible for process engi-neering creates the static model of the process.

2. Taking the model from Step 1, the dynamic simu-lator is used to match the steady-state model.

3. The dynamic model includes control schemes rec-ommended by technical services personnel and the pro-cess control specialists. Optional features are offered atthis stage to determine the best approach. These optionsmay include actions such as moving the temperature-control points to better anticipate compositionalchanges within the tower. By having a working modelof the system, each new idea is tested and the resultsare saved for review.

4. The dynamic simulator is used to determine theeffectiveness of the proposed control scheme. Once thismodel has been debugged, alternative control featuresand process upsets are modeled.

5. At minimum, the product quality should be dis-turbed to verify that the control instrumentation will ad-equately measure off-specification conditions. Difficul-ties in selecting the proper measurement/control pointshould be resolved.

6. The results of this exercise should be discussedwith the project team during a meeting to review theprocess flow diagram. Prior to starting project work,the flow scheme, including all control requirements,should be fixed and explained to the team by the pro-cess control specialist. It may be necessary to modifythe control scheme during this step.

7. The technology is incorporated into the commer-cial design as engineered specifications are developedfor the plant.

Control system and instrumentationBecause a DWC is not a traditional piece of process

equipment, the tools used to evaluate the controlscheme should be reviewed in detail to ensure that theyare appropriate for the tower. In addition, the sophisti-cation necessary for the process model, the best simula-tor package, and the required amount of control schemetesting must be reviewed, as well. The dynamic simula-tor can be adapted to study control systems in a con-cise, structured and organized fashion.

Once the best control scheme is determined, deci-sions about the instrumentation details are important.Many questions surround the issue of liquid/vapor traf-fic on each side of the wall.

Direct control of the liquid to each side of the wallis preferred. All liquid from above the wall is removedfrom the column, and then controlled so that a knownflow returns to each side of the wall. Direct control ofthe vapor split was not required for the applicationsthat we modeled. If vapor-split control were needed, aspecial tray would have needed be installed to handlethis requirement.

Kerosene prefractionation columnOne application of a DWC within UOP is for a pre-

fractionation of kerosene within a LAB complex. Thisis a full-range kerosene, containing C7C16 hydrocar-bons. The separation removes a heart cut of mid-range material, typically in the range of C11C13. Look-ing at the generic layout in Figure 1, then A = C10 andlighter, B = C11C13 and C = C14 and heavier.

The heart-cut makes up roughly 60% of the kerosene,making this separation a good candidate for a DWC. Pu-rity specifications are set both on the amounts of n-C10and n-C14 in the middle product. A third specification isset on the recovery of n-C11n-C13 in the middle prod-uct. A study found that it is not possible to successfullyoperate and control a DWC with purity specifications onboth the light and heavy components in the sidedrawproduct (14), which appears to preclude the use of aDWC for this application. However, this study consid-ered distillation of pure chemical species. In each case,the light, medium and heavy key components were adja-cent, meaning that there are no components that boil attemperatures in between those of the keys. Also, A, Band C represent distinct chemical species, instead of arange of species, as with kerosene prefractionation.

For the kerosene prefractionation column consideredhere, the feed is fractionated into three product streams.The key components are the normal paraffins in theC10C14 range. Normal paraffins are desired in the rangeof C11C13, while n-C10 is the light impurity and n-C14 isthe heavy impurity. Hundreds of species are present thatboil at temperatures between the light and middle prod-ucts, and between the middle and heavy products. Thesedistributing species are not important to the productquality, as they will be removed in a downstream separa-tion. The column can therefore be designed so that then-C10 and n-C14 will meet purity requirements.

Table 1 shows the capital and energy savings that re-sult by replacing the conventional direct sequence witha DWC in a typical system. The DWC yields an energysaving of 30% and a capital saving of 28%, both ofwhich roughly meet the rule-of-thumb for typical DWCdesigns. Because of this attractive cost saving, theDWC is now a standard offering at UOP for thekerosene prefractionation column in an LAB complex.

PEP fractionationHere, the DWC is part of the fractionation section

within our Pacol Enhancement Process (PEP) process



Capital Cost Energy Cost

Two-Column Sequence 1.0 1.0

DWC 0.72 0.7

Table 1. Cost savings for kerosene dividing-wall column.

unit, a sieve separation process that removes C7+ aro-matics from a desired C7+ olefin/paraffin mixture (theC7+ olefins) (20). This is a batch process that usestwo regenerate streams to purge and desorb the sievebeds. The regenerated effluent streams are fractionatedin a DWC to purify and remove the desorbed C7+ aro-matics and to purify and recycle pentane, one of the re-generate streams. A third product of the fractionation isa combination of the desorbant component, benzene,and washed C7+ olefins. The DWC employs a noveltrap tray and external refluxing scheme to prevent theC7+ aromatics from mixing with the co-boiling compo-nent, the C7+ olefins. Figure 3 compares the conven-tional two-column fractionation system to the singleDWC. In this figure, Component A is pentane, Compo-nent B is benzene, Component C comprises the C7+olefins and Component D, the C7+ aromatics.

Comparing this design to a standard configuration asshown in Figure 1, a typical DWC candidate wouldstart with a single feed stream that requires fractiona-tion into three product streams, each with it own dis-tinct, non-overlapping boiling range. The DWC in thePEP process has three product streams, two feeds, twoexternal reflux streams and two co-boiling groups thatmust be withdrawn separately in two of the productstreams. This DWC therefore did not start as a typicalcandidate that could follow conventional rules for such

a design. By building on previous work by others in thisarea, conventional design methods for DWCs wereadapted to this system. Table 2 shows the economic ad-vantage of the PEP DWC. This column uses high-per-formance trays, spaced at 310 mm.

Modeling the columnThe steady-state model consists of prefractionation,

stripping and product sections (Figure 4). When con-verting this model to a dynamic simulation, a fewchanges were necessary. First, the dividing wall actual-ly begins just below Stage 18 of the main section andends just above Stage 4 of the stripping section. Toproperly investigate the effect of the liquid split abovethe wall and the vapor split below the wall, it makessense to split the main column and the bottom fraction-ation section each into two sections, one below the walland one above it.


Figure 4. Screen shot of the steady-state simulation of the PEP DWC. Afew changes were needed to use this model in the dynamic simulation.KEY:Feed StreamsRaw Pentane From Purge: Pentane (purge material) and desired C7+ olefin/paraffin productBenzene, Benzene 2: Raw benzene streams used as external reflux streamsFrom Desorb Step: Benzene (desorbant) and heavy aromatic material

Product StreamsPentane: Purified pentaneBenzene/C7+Olefins: Sent to downstream processingAromatics: Undesired heavy aromatic material

EquipmentPrefracTS: Feed side of the DWCMainTS: Section above the wall, plus the section on the product side of the wall above thesidedraw streamStripTS: Section below the wall, plus the section on the product side of the wall below thesidedraw stream

a. Conventional Two-Column Design

b. Single Column Design

D

A

BC

BCB

D

ABD

A

B

B

B

C

A

BC

A

D

A

Figure 3. PEP fractionation via a dividing-wall column offers advantagesover using two columns in series.

Capital Cost Energy Cost

Two-Column Sequence 1.0 1.0

DWC 0.65 0.5

Table 2. Cost savings for PEP dividing-wall column.

Liquid from the newly created rectification sectionabove the wall is fed to the upper product section, sim-ulating the total trapout tray above the wall. Vapor fromthe newly created stripping section splits and feeds thebottom of the prefractionation and lower-product sec-tions. Liquid from the newly created upper product sec-tion is drawn out of the tower by a total trapout tray.This trapout tray can be modeled as a vessel with alevel controller.

The generic overhead condenser is removed and re-placed with an air cooler, accumulator vessel and re-flux/product pump. Other feed and product pump cir-cuits are added. One unique aspect of the PEP separa-tion is the cyclic nature of the feed composition. Thiscould not be adequately modeled with a steady-statemodel, and, therefore, was programmed into the dy-namic model. Finally, complete control instrumentationfor all lines was included.

Handling upsetsOnce the dynamic model was configured and validat-

ed, testing began, in which the column behavior wasstudied under normal and upset conditions. Offspecproduct must be either eliminated or minimizedthroughout any reasonable operation or upset. Differentcontrol configurations are tested under various upsetconditions. The response to feed changes, loss of re-flux, reaction to weather changes, etc., are among theupset conditions tested. As a result of this testing, acontrol system was designed to react quickly to upsetsand be easily implemented and operated.

As already mentioned, the column design can be fur-ther optimized during the dynamic simulation study.For instance, trays can be removed from a section tostudy the effect on product quality. Opportunities foroptimization that were not apparent from the steady-state model were identified during the dynamic study ofthe PEP DWC. A few items to consider beyond theproduct-quality goal of this column were:

Incorporating the feed system into the model tosimulate real-life upsets that are expected.

Maintaining a reasonable heat balance. Evaluating the level of vaporization common dur-

ing each feed upset.The heat evaluation was also important to the suc-

cess of this column, and required a study independentof all other reviews. Thermal fluctuations come frommany sources. Because the feed fluctuates cyclically, aconstant influence could not be anticipated. This varia-tion causes fluctuations in the product streams, requir-ing a more complex model than that used in the staticapproach. A study evaluated the option of reboiling ei-ther side of the wall independently. This showedpromise during the theoretical stage of the project, butwas less attractive as the operating principles became



Literature Cited1. Wright, R. O., U.S. Patent 2,471,134, Standard Oil Development

Co., Elizabeth, NJ (May 24, 1949).2. Ennenbach, F., et al., Divided-Wall Columns A Novel Distilla-

tion Concept, Petroleum Technology Quarterly, pp. 97103 (Au-tumn 2000).

3. Becker, H., et al., Partitioned Distillation Columns Why, Whenand How, Chem. Eng., 108 (1), pp. 6874 (Jan. 2001).

4. Dnnebier, G., and C. C. Pantelides, Optimal Design of Thermal-ly Coupled Distillation Columns, Ind. Eng. Chem. Res., 38, pp.162176 (1999).

5. Hernndez, S., and A. Jimnez, Design of Energy-Efficient Pet-lyuk Systems, Comp. Chem. Eng., 23, pp. 10051010 (1999).

6. Lestak, F., and C. Collins, Advanced Distillation Saves Energyand Capital, Chem. Eng., 104 (7), pp. 7276 (July 1997).

7. Triantafyllou, C., and R. Smith, The Design and Optimisation ofFully Thermally Coupled Distillation Columns, Trans. IChemE, 70,Part A, pp. 118132 (Mar. 1992).

8. Carlberg, N. A., and A. W. Westerberg, Temperature-Heat Dia-grams for Complex Columns: Part 3: Underwoods Method for thePetlyuk Configuration, Ind. Eng. Chem. Res., 28, pp. 13861397(1989).

9. Agrawal, R., and Z. T. Fidkowski, Thermodynamically EfficientSystems for Ternary Distillation, Ind. Eng. Chem. Res., 38, pp.20652074 (1999).

10. Nikolaides, I. P., and M. M. Malone, Approximate Design andOptimization of a Thermally Coupled Distillation Column with Pre-fractionation, Ind. Eng. Chem. Res., 27, pp. 811818 (1988).

11. Glinos, K., and M. F. Malone, Optimality Regions for ComplexColumn Alternatives in Distillation Systems, Chem. Eng. Res. Des.,66, pp. 229240 (1988).

12. Petlyuk, F. B., et al., Thermodynamically Optimal Method for Sep-arating Multicomponent Mixtures, Int. Chem. Eng., 5, pp. 555561(1965).

13. Parkinson, G., et al., The Divide in Distillation, Chem. Eng., 106(4), pp. 3235 (Apr. 1999).

14. Halvorsen, I. J., and S. Skogestad, Optimal Operation of PetlyukDistillation: Steady-State Behavior, J. Process Control, 9, pp.407424 (1999).

15. Wolff, E. A., and S. Skogestad, Operation of Integrated Three-Product (Petlyuk) Columns, Ind. Eng. Chem. Res., 34, pp.20942103 (1995).

16. Serra, M., et al., Control and Optimization of the Divided WallColumn, Chem. Eng. and Proc., 38, pp. 549562 (1999).

17. Abdul Mutalib, M. I., and R. Smith, Operation and Control of Di-viding Wall Distillation Columns: Part 1: Degrees of Freedom andDynamic Simulation, Trans. IChemE, 76, Part A, pp. 308318(1998).

18. Abdul Mutalib, M. I., et al., Operation and Control of DividingWall Distillation Columns: Part 2: Simulation and Pilot Plant StudiesUsing Temperature Control, Trans. IChemE, 76, Part A. pp.319334 (1998).

19. Parkinson, G., Drip and Drop in Column Internals, Chem. Eng.,107 (7), pp. 2731 (July 2000).

20. Schultz, M. A., et al., Design and Control of a Dividing Wall Dis-tillation Column for the Fractionation Section in the Pacol Enhance-ment Process (PEP), paper presented at 2001 Spring AIChE Nation-al Meeting, Houston, TX (Apr. 2226, 2001).

more apparent. A single reboiler with a stable heatsource was selected as the best way to meet the require-ments of this fractionation. As proposed, the design ad-justs the vapor to the wall indirectly by manipulatingthe liquid traffic on each side of it. A small upset in thedifferential pressure across the trays will make the nec-essary vapor flowrate adjustment. The major benefit ofthis is the columns ability to meet the challenges im-posed by variable feeds.

Shell fabrication and tray designAdding a vertical partition to a conventional distilla-

tion column presents some challenges to fabricating theshell and trays. If the temperature gradient across thewall is too great, it may be necessary to install an insu-lated wall to prevent heat transfer that could affect theseparation. Additional manways may be needed in aDWC, so that the column is accessible on either side ofthe wall.

The wall within the DWC effectively changes thetray design, creating two distinct noncircular sections.Also, if the wall is not exactly in the center of the col-umn, the tray design becomes nonsymmetrical. A high-performance tray is used and is suited for this applica-tion because it has a 90-deg. tray-to-tray orientationthat makes designing easy within the noncircular sec-tion and asymmetrical sections. The tray is designed foruniform flow distribution across the tray deck, which isimportant when the section is noncircular.

Future thoughts and conclusionsAdvances in the theory of design, control and opera-

tion of a DWC have contributed to a better understand-ing of these columns and have led to commercial devel-opments. As the base of commercial experience contin-ues to grow, the number of applications should increasefor both conventional and unconventional cases. Com-puter models will be an important part of this process,as advanced tools allow a more-complete analysis ofthese columns.

Chemical engineers should look for unconventionalapplications of DWC technology. Simply because afractionation system does not meet the single-feed,three-key-component, three-product criteria, does notmean that DWC technology cannot be adapted to theseparation. The UOP application for the PEP DWC isan example of this.

As academic research improves the understanding of di-viding-wall columns, and industry finds new applications,these columns should become more common in the plant.Although it took roughly 50 years for the DWC to gainlimited acceptance by some companies, perhaps the next50 years will be a time when the DWC becomes a standardpiece of equipment in the CPI. CEP


MICHAEL A. SCHULTZ is a process research specialist in the UOPEngineering Science Skill Center (25 East Algonquin Road, DesPlaines, IL 60017-5017; Phone: (847) 375-7895; Fax: (847) 375-7904;E-mail: [email protected]). He has four years of experience withUOP in a variety of process research projects. His work focuses ontechnical and economic analysis, process development, and processoptimization of both developing and existing process technologies. Healso serves as a resource for the assessment and application of UOPsdividing-wall column technology, as well as for other fractionationissues. Schultz holds a BS in chemical engineering from the Univ. ofMichigan-Ann Arbor, and a PhD in chemical engineering from the Univ.of Massachusetts-Amherst. He is a member of AIChE.

DOUGLAS G. STEWART is a senior design engineer in the UOPEngineering Dept. (Phone: (847) 391-2904; E-mail:[email protected]). He has 24 years of experience with UOP,including field service and engineering dept. assignments in a varietyof UOP process technologies. He currently is involved in UOPsdetergent process technologies. Stewart holds a BS in chemicalengineering from the Univ. of Minnesota.

JAMES W. HARRIS is a process control coordinator in the UOP TechnicalServices Dept. Skill Center (Phone: (847) 391-1366; E-mail:[email protected]). He has 12 years of experience with UOP,including field service and engineering dept. assignments in a varietyof UOP process technologies. He is a technical leader for safety-instrumented-system applications; additional activities includesupporting training for new-hire instrument engineers, interfacingwith field service personnel on technical issues, maintaining companyprocedures for specifying instrumentation, and developing newprocedures to sustain technical leadership of the company. Harrisholds a BS in chemical engineering from SUNY-Buffalo. He is amember of AIChE, as well as ISA, and is an active member of the ISASP84 committee and APIs subcommittee on instrumentation andcontrol systems.

STEVEN P. ROSENBLUM is a senior process modeling specialist in theUOP Engineering Dept. (Phone: (847) 375-7182; E-mail:[email protected]). He has 25 years of experience in the CPI withthe last five at UOP. During his tenure at UOP, he has undertakenadvanced process control and optimization, and engineering dept.assignments in a variety of UOP process technologies. He serves as anapplications-support group leader for all outside vendor simulationsoftware. Rosenblum holds a BS in chemical engineering from CornellUniv. and an MS in chemical engineering from Northeastern Univ.

MOHAMED S. M. SHAKUR is a product manager in the processequipment group of UOP (Tonawanda, NY; Phone: (716) 879-7601; E-mail: [email protected]). He has 13 years of experience with UOP,mainly in the distillation design group, focusing on the design andcommercialization of column internals, column startup,troubleshooting and optimization. He markets UOP column internalsworldwide and develops new products to complete the UOP productportfolio of high-performance column internals. Shakur holds a BEngin chemical engineering from the City College of New York.

DENNIS OBRIEN is a detergents technology specialist in the UOPEngineering Dept. (Phone: (847) 391-2802; E-mail:[email protected]). He has 31 years of experience with UOP, in thefield service, engineering and computer departments, withassignments in a variety of UOP process technologies. He currently isa technical specialist for detergents and provides engineering supportfor adsorbents. OBrien holds a BS in chemical engineering from theUniv. of Tulsa, and a MBA from Roosevelt Univ. He is a member ofAIChE.

divid wall

Documents