dwc column simulation

chemical engineering research and design 8 7 ( 2 0 0 9 ) 4760

Contents lists available at ScienceDirect

Chemical Engineering Research and Design

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2008 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Keywords: Dividing-Wall Column; Design; Retrotting; HYSYS; Fully Thermally Coupled Distillation System

1. In

Distillationtions andestimated1999). Motlation, resements thacost. Any reconomicaof reduced

Columngurationssequence,2002), andworldwidecling the vcoupled arvapor/liquple columnThermallyPetyluk Sycolumns. I

CorrespoE-mail aReceived

0263-8762/$doi:10.1016troduction

columns are used for about 95% of liquid separa-the energy use from this process accounts for an3% of worlds energy consumption (Hewitt et al.,ivated by this large energy requirement for distil-archers have developed various column arrange-t can bring in savings in both energy and capitaleduction of energy consumption will not only bringl benets but also environmental benets in termsusage of fossil fuels and its associated emissions.congurations vary from simple to complex con-

. Simple column congurations refer to directindirect sequence and distributed sequence (Shah,are the conventional column systems operated

today. Complex columnarrangements refer to recy-apor and/or liquid, heat integration, etc. Thermallyrangements are realized by setting up two-wayid ows between different columns of the sim-congurations. Reported studies reveal that FullyCoupled Distillation System (FTCDS, also called

stem) provides the maximum energy reduction inn most cases, this is implemented in the form of

nding author. Fax: +65 6779 1936.ddress: [email protected] (G.P. Rangaiah).29 February 2008; Received in revised form25 June 2008;Accepted27 June2008

a Dividing-Wall Column (DWC) in which both columns arehoused in a single shell. This reduces not only the energyconsumption but also space and investment requirementscompared to the conventional column system.

The DWC has been known for several decades since itspatent by Wright (1949). Petyluk et al. (1965) introducedthe thermal coupling for separating ternary mixtures andpresented a fully thermally coupled congurationPetylukcolumn. Amminudin et al. (2001) noted the industrial accep-tance and commercialization of DWC by organizations suchas BASF AG, M.W. Kellogg (together with BP, later known asBP Amoco), and Sumitomo Heavy Industries Co. together withKyowa Yuka. Linde AG constructed the worlds largest DWCfor Sasol, an estimated 107m tall and 5m in diameter (Schultzet al., 2002). Recently, Adrian et al. (2004) reported that BASFoperates about 30 DWCs worldwide in their plants.

Fidkowski and Krolikowski (1987) have established that thePetyluk system requires the least energy among all the optionsfor a three-product system. This key advantage is also applica-ble for a DWC, which has the minimum vapor ow rate for theparticular separation compared to the conventional system.Triantafyllou and Smith (1992), and Agrawal and Fidkowski(1998) have shown that, for a mixture of three components,

see front matter 2008 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved./j.cherd.2008.06.013journa l homepage: www.e lsev ie

tting conventional column syng-Wall Columns

kumar, G.P. Rangaiah

of Chemical & Biomolecular Engineering, National University of Sin7576, Republic of Singapore

b s t r a c t

istillation, the most common separation process in chemical proc

ividing-Wall Column (DWC), which works on the basis of Fully Th

hosen for this study due to its lower energy consumption compare

bjective of this study is to investigate the potential of retrotting co

r separating ternary mixtures into three products, to DWCs. Fo

re selected and conventional 2-column systems are designed, wh

e plants. Then, retrotting these systems to DWC is studied. Res

ystems to DWCs is very attractive both economically and for its reems to

re, Engineering Drive 4,

dustries, requires signicant energy inputs.

ally Coupled Distillation System (FTCDS), is

the conventional column system. The main

tional 2-column (C2C) systems in operation

, six applications of industrial importance

re assumed to be currently in operation in

how that retrotting the existing 2-column

d energy requirements.

48 chemical engineering research and design 8 7 ( 2 0 0 9 ) 4760

the DWC reduces the total vapor ow by 1050% comparedto conventiReduction iand condenFurthermocompared tcolumn (C2as well as o

Despiteof DWCs, ltheir opera(Schultz etthat it is imalthough itdesign of tusually leftuid and vapresistancesstudies ofthat the DW

Kolbe ansavings gaiumn systemand 40% inments are dand associetc. This wgapore whechemical acompetitioenergy usaexisting eqpossibly wiinvestmentably considconventioning energy

In thisto DWCs istions. Firstthese applare designeconventiondetailed teapplicationemployed fthe six appusing HYSYventional 2by more ththe six app

2. De

Design of Dbasic equatcommerciaerally usepart of shoous simulaPlus. Chaptto processtions are al

HYting

s (e.g2; Dseden astep

s simt spe

om thon. Tal prnt. Its intepsn fors simzatiothese

Sh

ed bin Fientaernalumnrecy101-n moaryequivns T-101A and T-101B together are equivalent to thecolumn of the FTCDS (see Figs. 2 and 3). Importantly,eam Side 1 and Side 2 shall have the same purity soeir ows can be added; the combined Side stream rep-s the side stream (side draw in the main column of, see Fig. 2). In the shortcut column T-100 in Fig. 1, thenser is a partial condenser, and Distillate 1 is a vapor.t column T-100 is simulated assuming a percentage oflights in Bottom 1 and 1% heavies in Distillate 1. Then,

n T-101A is simulated by specifying the middle compo-eavies in Distillate) and top component (lights in Sideer the purity requirements provided in the example orcess data from the plant. Similarly, T-101B is simulatedide 2 (heavies in top) and Bottom (lights, which is thecomponent, in Bottom) specications. The specica-f lights in Bottom 1 and/or heavies in Distillate 1 ofonal systems using direct and indirect sequences.n vapor ow contributes to lower duties of reboilerser, and consequently capital and operating costs.

re, DWC uses only 1 reboiler and 1 condenser wheno 2 reboilers and 2 condensers for a conventional 2-C) system. This would add to the savings in capitalperating cost.the economic edge and lower energy requirementsack of reliable design methods and concerns ontion have prevented their commercial applicational., 2002). Abdul Mutalib and Smith (1998a) notedpractical to manipulate the vapor split in a DWCis easier to manipulate the liquid split via specialhe liquid distributor. However, liquid split is alsouncontrolled, and the operating values of both liq-or splits result from the natural balancing of owinside the column. Simulation and experimental

Abdul Mutalib and Smith (1998a,b), demonstrateC can be operated successfully.d Wenzel (2004) have stated the following averagened by using a DWC against a conventional col-: 25% in investment cost, 35% in operating cost

space requirement. The savings in space require-ue to reduction in number of reboilers, condensersated equipments such as pumps, their supports,ill be of particular interest in locations such as Sin-re land is limited. Thus, DWC is attractive to manynd related industries in the current scenario ofn and environmental concerns, in order to reducege for distillation. One way is to make use of theuipments and operate the plant more efciently,th minor modications and small investment. Anwith payback period of less than 3 years is favor-ered by the management. Retrotting an existingal columnsystem to aDWChaspotential for reduc-and an acceptable payback period.investigation, retrotting conventional columnsanalyzed by considering six industrial applica-

, conventional 2-column systems are designed forications. Then, both new and retrotted DWCsd; the latter uses the existing equipment in theal 2-column system as much as possible. Finally,chno-economic analysis of retrotting for the sixs is carried out. The process simulator, HYSYS isor simulation and optimization of all columns forlications. A step-by-step procedure for DWC designS is described. Results show that retrotting con--column systems to DWCs reduce operating costan 30% with an acceptable payback period, for alllications studied.

sign procedure for DWC using HYSYS

WCs has been studied in the open literature usingions (e.g., Triantafyllou and Smith, 1992) and usingl simulators (e.g., Kim, 2006). Investigators gen-FenskeUnderwoodGilliland (FUG) equations asrtcut method for initialization followed by rigor-tion with simulators such as HYSYS and Aspener 4 in Seider et al. (2003) provides a good overviewsimulation using these simulators. The FUG equa-so used in the shortcutmethod in simulators. Some

Fig. 1simula

studieal., 2002001) uand th

TheDWC iproducable frsituatiphysicthe plaponenmain stillatiorigorouoptimiDWC;

2.1.

As notshownrepresare intcut coto the(Liquidcolumnecess100 iscolummainthe strthat thresentFTCDScondestream

Firssay 1%columnent (h1) as pthe prowith Smiddletions oSYS ow sheet with three shortcut columns fora FTCDS.

., Triantafyllou and Smith, 1992; Muralikrishna etunnebier and Pantelides, 1999; Amminudin et al.,mathematical models to design the FTCDS/DWCsimulator to conrm the design.s involved in the design and implementation of ailar to a conventional column. The feed conditions,cications, column operating conditions are avail-e plant or the given examples depending upon thehe thermodynamic model to be used for predictingoperties may be available from the process team inf not, this shall be selected depending on the com-and operating conditions for the application. Thein the design of a FTCDS/DWC are: (a) shortcut dis-nding initial estimates of variables required forulation, (b) rigorous simulation of the FTCDS, (c)

n of the system, and (d) design and sizing of theare outlined below.

ortcut distillation

y Amminudin et al. (2001), three shortcut columnsg. 1 arenearly equivalent to a FTCDS in Fig. 2. In thistion, recycle streams in a FTCDS are not explicit butl; rectifying liquid and stripping vapor in the short-

for the prefractionator respectively correspondcled liquid and vapor streams to column T-100Out and Vapor101-Out in Fig. 2). We use the three-del in Fig. 1 for shortcut calculations to obtain theestimates for rigorous simulation. The column T-alent to the prefractionator of the FTCDS, and the

chemical engineering research and design 8 7 ( 2 0 0 9 ) 4760 49

Fig. 2 Final (converged) FTCDS by rigoro

column T-100 are varied by trial and error to arrive at nearlyequal composition of Side 1 and Side 2.

2.2. Rigorous simulation

Once shortulated rigoget accuratinvolve lesthan FUG erate-basedmation of meach stageequations irate-basedmodels arequate for nRigorous siequilibrium

This seclation. FirstasHYSYS, athe thermoproperties iulation envhence requin Fig. 2. Thability of in

Fig. 3 Simthe prefrac

(shown inFfor Liquid1T-100 cannofeeds to it.

Theprefand can be

d th. Theamsquid

forcycleheet:iquidted

100-OtletvideLiqu

cessan fopor1s Va

me sgive

101Bs simfrom

tagetionrtcuin F

umnicatcut estimates are completed, FTCDS has to be sim-rously using HYSYS (Fig. 2) or some other way, toe results. The approaches for rigorous simulations number of assumptions and are more realisticquations; they are: equilibrium-stage models andmodels. The former uses mass, equilibrium, sum-ole fractions and enthalpy (MESH) equations for

. The resulting set of large number of nonlinears solved using a suitable numerical method. Themodels use mass transfer rates. Equilibrium-stagemore common in simulators andare generally ade-early ideal distillation systems (Seider et al., 2003).mulation used in the present study is based on the-stage model.tion presents the steps involved in rigorous simu-, open a new case/le in the process simulator suchdd the components involved in theprocess, choosedynamic models (for predicting required physicalncluding phase behaviour) and proceed to the sim-ironment. The FTCDS has two recycle loops andires two recycle blocks for its simulation as showne placement of recycle blocks is guided by the avail-itial estimates for Vapor101-In and Liquid101-In

and ad(Fig. 2)rial strand LiBottomthe reow s2 for LconnecLiquidand ou

ProIn andthe nelocatioand Vasame athe salationsand T-rigorou1 (bothfeed ssimulaon shoshown

Col5 specilarity of vapor ows in FTCDS and DWC; PF istionator.

simulationin the coluA typical cand bottomeach side pucts (Vaporwhich canrates.

ProvidecomponentSide, heavyThe ow radded as thcalculationand Vapor1us simulation.

ig. 2) fromshortcut calculation.Note that estimates01-Out and Vapor101-Out are not available; hence,t be solved since these 2 streamsare the additional

ractionator doesnothave a reboiler and condenser,simulated by an absorber column. Hence, selecte absorber column into the ow sheet as T-100en, add distillation unit for T-101, and the mate-Feed, Vapor100-Out, Liquid100-Out, Vapor101-Out

101-Out for column T-100, and Distillate, Side andcolumn T-101 as shown in Fig. 3. To initializestream, a recycle block has to be added to theone RCY-1 for Vapor100-Out and another RCY-100-Out. Vapor100-Out and Vapor101-In shall be

as inlet and outlet to RCY-1 respectively. Similarlyut and Liquid101-In shall be connected as inlet

to RCY-2 respectively.the data from shortcut estimates to the Vapor101-id101-In. Dene the columns by the connectingry streams, providing the number of stages, stager Vapor101-In, Liquid101-In, Side, Liquid101-Out01-Out. The stage number for Liquid101-Out is thepor101-In entry stage. Similarly, Vapor101-Out hastage number as Liquid101-In. The shortcut calcu-an estimate for the number of stages for T-101A

, which are equivalent to the main column T-101 inulation. The feed stage of Distillate 1 and Bottomshortcut estimation) is taken respectively as the

for Vapor101-In and Liquid101-In in the rigorous. Similarly, Side draw location was selected basedt calculations. The complete the ow sheet is asig. 2.T-101 has 5 degrees of freedom (DOF) and henceions shall be provided. This is in the context of

given conditions of all feeds, stage/tray pressuresmn, number of stages, feed stage and condenser.olumn with a total condenser, reboiler, distillates products has 2 DOF (e.g., see Seider et al., 2003);roduct adds 1 to DOF. Since T-101 has 3 side prod-101-Out, Side and Liquid101-Out), its DOF 5 (=2+3),be used for specifying product purities and/or ow

the product purity in each stream: namely, lightpurity in Distillate, middle component purity incomponent purity in Bottom, as 3 specications.

ate of Liquid101-Out and Vapor101-Out shall bee remaining 2 specications. Activate simulations and converge T-101. Now, streams Liquid101-Out01-Out are calculated and available for further cal-


culations. Column T-100 has no degree of freedom and so nospecication to be given. This column is analogous to T-100simulated via shortcut calculations. Hence, provide the num-ber of stages, feed location and pressure prole, etc. from theinitial estimates obtained from shortcut estimation. The num-ber of stages in T-100 (i.e., prefractionator) is assumed to be thesame as the number of stages along the dividing wall, and isestimated based on the sumof the number of stages below thefeed stagestage in T-converge af

2.3. Op

The designsimulationto be optimet al. (2006is the majmizing rebominimizesof columnsmizing theduty are: (1rate, (3) Feetion of Liqulocation ofNo. of stagis carried ovarying easequentiall100 and T-as their effof Rangaiahfour stepsoptimum.

Step 1ORecall thathat for dLiquid101Out. Feednd the ochosen Vaand Liquireboiler dStep 2OStarting fof Liquid1draw rateachieve ththe lowesend of thiStep 3Oplocation otion, the d

been varied, and the solution corresponding to the lowestreboiler duty is taken as the optimum and carried forward tothe next step.Step 4Optimize feed location of column T-100: The feed locationof column T-100 is varied, and, for each chosen feed location,the draw rates of Vapor101-Out and Liquid101-Out in T-101are varied to nd the design with the lowest reboiler duty.

effein Tfromoileal efmini

Siz

ext sr anoptiprore idare s

herefracenerspectn, anthaiamen thaelocy (Siuse

veloc

K1

K1 iseloc

0.8

lum

4va

G isy inof 0aresformis forutor

Table 1 C

Example

BTX separaDepropaniAlkanes sein T-101A and the number of stages above the feed101B (see Fig. 1). The complete ow sheet shouldter several iterations due to the two recycle blocks.

timization

obtained by shortcut calculations and rigorousis generally not the optimum, and hence it needsized. The studies of Lek et al. (2004) and Rangaiah) on distillation columns show that operating costor part of the total annual cost and that mini-iler/condenser duties (or equivalently reux ratio)operating cost leading to nearly optimal design. Hence, the FTCDS design is optimized by mini-reboiler duty. The design variables affecting this) Liquid101-Out draw rate, (2) Vapor101-Out drawd location of Vapor101-In to T-101, (4) Feed loca-id101-In to T-101, (5) Side draw location, (6) FeedT-100, (7) No. of stages in column T-100 and (8)es in column T-101. Optimization of the FTCDSut via a four-step procedure outlined below, by

ch of the rst six variables systematically andy. The last two variables (number of stages in T-101 given by shortcut calculations) are not variedect is expected to be marginal based on the studyet al. (2006). Hence, the resulting design after the

is close to the optimum but may not be the global

ptimize feed location of Vapor101-In to column T-101:t the feed stage for Vapor101-In is the same as

rawing out Liquid101-Out; similarly, the feed stage-In is the same as that for drawing out Vapor101-stage for Vapor101-In is varied in suitable steps toptimum feed location to column T-101. For eachpor101-In feed stage, ow rates of Vapor101-Outd101-Out are varied and optimized for the lowestuty.ptimize feed location of Liquid101-In to column T-101:rom the optimal solution in step 1, feed location01-In is varied; for each selected feed location, thes of Vapor101-Out and Liquid101-Out are varied toe lowest reboiler duty. The data corresponding tot reboiler duty will be taken as the optimum at thes step.timize side draw location in column T-101: Next, drawf Side stream is varied. For each selected draw loca-raw rate of Vapor101-Out and Liquid101-Out has

Theshownbe seenthe rebminimto the

2.4.

The nreboilefor thetrationthem aa DWCever, tthe pre

In gthe recolumensureumn dcolumating vvelocitity wasvapor

Vmax =

whereating v

Vact =

The co

D =

wheredensitmentsthe nehenceEq. (1)distrib

Reboiler duty at different steps in the optimization of the DW

Prior to optimization After step 1

tion 1,531 1367zer/debutanizer 33,660 9436paration 1,053 952ct of the optimization steps on reboiler duty isable 1 for three applications discussed later. It can

this table that there is a considerable reduction inr duty during the rst step. Subsequent steps havefect which indicates that the reboiler duty is closemum after the 4 steps.

ing of column, condenser and reboiler for DWC

tep after the optimization is to size the column,d condenser for the DWC. The simulation resultsmized FTCDS shall be used for DWC as the concen-le, heat duties and performance data for both ofentical. The sizing calculations for the columns inimilar to the conventional 2-column system. How-is slight difference in sizing the DWC as it housestionator as well, and is described below.al, column diameter should be sufcient to handleive maximum vapor and liquid ow rates in thed depends on mainly vapor ow rate. This will alsot the pressure drop is in the acceptable range. Col-ter is determined by the ooding condition of thet xes the upper limit on vapor velocity. The oper-ity is normally between 70 and 90% of the oodingnnott, 2005). In this study, 80%of the ooding veloc-d as the operating vapor velocity, Vact. The oodingity, Vmax can be estimated from the correlation:

L vv

(1)

a constant which is taken to be 0.07m/s. The oper-ity is

Vmax (2)

n diameter, D shall be calculated by:

G

pVact(3)

the vapor ow rate in kg/s, and vap is the vaporkg/m3. The column is usually fabricated in incre-.5 ft; so, the calculated diameter is rounded up tot 0.5 ft. This results in a lower vapor velocity ands a more conservative estimate. The factor K1 insieve trays. Other internals are downcomers, feed

s and liquid/vapor draw-off pipes. Cost of these is

After step 2 After step 3 After step 4

1357 1355 13559435 9409 9409934 934 934


minor compared to that of column shell and trays, and henceit is neglect

Fig. 3 shFTCDS andthe DWC istom sectionvapor loadtom sectionvapor owow from tow from tWhen theform the D

It is obvDiameter o(D2) of the Fumn sizingDWC is thethe main cter of the mshould be smiddle andtion. Diamebased on vmiddle andbigger thandiameter, ostep diamemer case, tbe used.

The cosdenser areof estimatithe distillaumn(s), rebtrays is takassumingupdated wiical Enginetotal moduthe columnall the costetc. (Turtonof steam (foThe electrismall and nform the sizfollowed by

3. Re

A search ofcations of Dfew other acolumn; thtion. All ththese appliselected touationof reDesign conlisted in Ta

In thisretrotted

application. C2C has the nearly optimal design for a newtion

, it wrotor inns.g coowf they is irottrly, euse wseleidd

site iWC.e avectioen tddler oft then essumning tquipumnt at sals astalays hntaltheor sitionl secationt tomen

n toremtheboileed tootti

Ben

provBTX

ter 0.yste, thehe dexisin th

n secsec

telysert fortheepened.ows the similarity of the vapor ow rates betweenDWC. Both the main column of the FTCDS anddivided into three sections: top, middle and bot-s. In the FTCDS, vapor ow rate Vt represents thein the top section, Vb the vapor load in the bot-, V1 the vapor ow to the prefractionator, V2 theto the main column middle section, V3 the vaporhe prefractionator to the main column, V4 vaporhe main column middle section to its top section.prefractionator is housed within a single shell toWC, the corresponding vapor ow rates apply.ious from Fig. 3 that V1 +V2 =Vb and V3 +V4 =Vt.f the prefractionator (D1) and of the main columnTCDS shall be arrived as per the conventional col-. Cross-sectional area of the middle section of then the sum of the area of the prefractionator andolumn, which can be used to estimate the diame-iddle section of the DWC. Nevertheless, the DWC

ized for themaximumvapor load in each of the top,bottoms sections, available from rigorous simula-ter of each of the three sections shall be calculatedapor rate similar to the conventional column. The

bottom sections will be usually the same as orthe top section. Depending upon the change in

ne can choose either to use a single diameter orter (top section with a smaller diameter). In the for-he larger diameter of all the three sections should

t correlations for column, trays, reboiler and con-taken from Turton et al. (2003). The costing consistsng capital and operating costs. The capital cost fortion system consists of total module cost of col-oiler and condenser. The costing for number ofen with respect to the higher number of stages70% efciency. The cost correlations should beth the current cost index; for this study, the Chem-ering Plant Cost Index (CEPCI) of 560 was used. Thele cost is 18% more than the bare module cost of, trays, reboiler and condenser, which accounts forfor the associated instruments, piping, structures,et al., 2003). Operating cost is the sum of the costr reboilers) and cooling medium (for condensers).city, which is required for circulation pumps, iseglected. An Excel program was prepared to per-ing of the columns, trays, reboilers and condenserscosting for the equipments and utilities.

sults and discussion

the open literature had revealed that several appli-WCs were successfully studied as new design. Applications involve a side draw stream in a singleese can be tried in a DWC for efcient separa-ese examples are summarized in Table 2. Out ofcations and from our own industry source, 6 wereinvestigate and discuss the techno-economic eval-trotting conventional 2-columnsystems toDWCs.ditions and specications for these applications areble 3, which form the basis for the study.study, three columnsC2C, new DWC and

DWC (R-DWC) are designed for each selected

convencationfor retdirecticatioexistinvaporratio ovelocitfor retSimilafor re-

Theof themcut atof R-Dshall bThis sand ththe minumbeto meeDWC. Ithe col

Durother ethe colried ouinternafter inpast, trhorizodue totime fproducbe welinstallried ourequirecolumwill bewhereThe reinstallin retr

3.1.

Table 4for thediameDWC sR-DWCsince tin thestagescolumthe topseparacondenthe costion ofdays dal column; however, in the actual industrial appli-ill be the existing columns in the plant, identiedting evaluation. The conguration can either bedirect sequence depending upon the product spec-For retrotting the C2C to a DWC (R-DWC), thelumn is checked for the design parameters andrate of DWC; this includes checking to see if theactual vapor velocity to maximum/ooding vaporn the acceptable range. This is the deciding factoring one of the two columns in the C2C as a DWC.xisting reboilers and condensers should be checkedith minimal modications.cted, existing column ismodiedwith the additionle section,whichhouses the dividingwall. It will bento 2 portions to form the top and bottom sectionsMiddle section of R-DWC is shop fabricated andailable before the actual retrotting at site starts.n is rst welded to the top of the bottom sectionhe removed top portion will be welded on top ofsection to complete the column. In this exercise,stages in the existing column shall be sufcientrequired stages in top and bottom sections of theence, the existing column is used fully leavingwithmodication of adding the middle section only.he shutdown of the existing plant, the piping andments around the columns are disconnected from. Subsequently, the trays installation would be car-ite. As per the industrial practice, trays and columnre installed in the column in vertical position (i.e.,lation of the column at site). However, in the recentave been installed in shop or at site (i.e., column in

position) before installation of the column. This istime constraint, which industries are facing; extrate installation means late startup and associatedloss. Before the column installation, trays shouldured to avoid dislocation during transport and/orof the column. The necessary testing will be car-satisfy the safety and local statutory regulationts in force. In case there is no place around theeffect the modications, then the main columnoved from the existing location to another locationnecessary modications and testing can be done.rs and condensers are re-used and/or new onescomplete the retrotting. Practical considerations

ng are outlined in Appendix A.

zene, toluene, xylene (BTX) application

ides the comparison of C2C, new DWC and R-DWCapplication. The C2C system has 2 columns of

9 and 0.8m, with 31 and 40 stages respectively. Them has only one column with diameter of 0.9m. Inexisting column 1 (C-100) from the plant is used

iameter requirement is met. The number of stagesting column is not sufcient to satisfy the totale top and bottom section of the DWC. Hence, ation with 2 trays is shop fabricated and added totion during the retrotting. The middle section isshop fabricated and assembled at site. The existingand reboiler can be re-used. Hence, costing showsadditional middle section and trays. Such an addi-section can take place in a short duration of 510ding upon the site condition. From Table 4, it can

52ch

emica

len

gin

eering

research

and

desig

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7(2

00

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Table 2 Reported and potential applications of DWC in the open literature

Reference Column conguration (s) Description/details Remarks

Kaibel (1987) DWC and a conventionalcolumn conguration

Application studied is separating a mixture of n-hexane, n-heptane andn-octane. DWC is compared with the conventional column conguration ofdirect sequence.

Triantafyllou and Smith (1992) FTCDS and DWC Application used is separation of close-boiling mixtures of C4sseparate1-butene from the feed mixture containing i-butane,1-butene, n-butane,trans-2-butene and cis-2-butene.

Concluded that DWC will have capital cost savingsin addition to all the savings realized by FTCDS.

Fidkowski et al. (1993) Conventional columncongurationdirectsequence

Applications cited as examples are separation of (1)acetaldehydemethanolwater, (2) acetonechloroformbenzene and (3)ethanolwaterethylene glycol system

These applications are potential for using andevaluating DWC.

Abdul Mutalib and Smith (1998a,b) DWC Application in the rst paper is the separation of methanol, iso-propanol andbutanol. The second paper deals with simulation and pilot plant studies for thesame application.

Agrawal and Fidkowski (1998) FTCDS Application considered is the separation of air to produce argon for FTCDS. Concluded that partial thermal coupling isadvantageous compared to FTCDS for cryogenicapplications.

Dunnebier and Pantelides (1999) In addition to the application used by Triantafyllou and Smith (1992), a casestudy on separation of alkane mixtures containing 2-methylbutane, pentane,hexane and heptane with DWC is discussed.

Hairston et al. (1999) DWC Application referred to is the separation of pure ethyl acetate from the mixtureof lower and higher boiling impurities developed by Sumitomo for Kyowa YukaLtd., Japan.

Application commercialized with a DWC.

Amminudin et al. (2001) FTCDS and DWC Case study is the replacement of a conventional depropanizer and debutanizerby a DWC.

Rev et al. (2001) FTCDS and conventionalcolumn system

Case study is the separation of a mixture of ethanol, n-propanol and n-butanolfor comparing energy consumption and cost advantages of FTCDS system withthe conventional column system with or without heat integration.

Shah (2002) FTCDS and DWC Separation of LPG from mixed pentanes and a heavy end in a light-end renerydistillation train.

Muralikrishna et al. (2002) DWC Separation of equimolar mixture of benzene, toluene and o-xylene.Schultz et al. (2002) DWC One application suggested is the linear alkyl benzene (LAB) complex and

another is pre-fractionation of kerosene within a LAB complex.Blancarte-Palacios et al. (2003),

Harlvorsen and Skogestad(2003), and Jimenez et al. (2003)

FTCDS Blancarte-Palacios et al. (2003) considered a mixture of n-pentane, n-hexane,n-heptane and n-octane using a FTCDS. Harlvorsen and Skogestad (2003), andJiminez et al. (2003) considered a mixture n-pentane, n-hexane and n-heptaneonly.

Adrian et al. (2004) DWC Separation of a mixture with butanol (15%), pentanol (70%) and hexanol (15%).Bruggemann and Marquardt (2004) FTCDS Separation of benzene, toluene and ethyl benzene (BTE) mixture.Kolbe and Wenzel (2004) DWC Reduction of benzene in gasoline and separation of benzene, toluene and xylene

(BTX) mixture.Discussed retrotting an existing column andbenets achieved.

Kim (2006) FTCDS and DWC Two applications referred here. One is the fractionation of BTX and the other isthe gas concentration process to produce gas products from a gas mixturecontaining methane, ethane, propene (in lights), propane (as middlecomponent), i-butane, i-butene, n-butane, i-pentane and n-pentane in heavies.

BTX production is one of the largest energyconsumers in a cracker plant.

Rangaiah et al. (2006) Conventional columnwith a side product

Three applications: (1) mixture of benzene, toluene and biphenyl, (2) mixture ofalkanes, and (3) mixture of benzene, toulene and p-xylene.

Potential applications for DWC.

chem

ical

engin

eering

research

and

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476053

Table 3 Characteristics of the applications studied

No. Components Feed composition(mole fraction)

Feed conditions Specications of top, side andbottom products (mole%)

Other conditions Reference

1 BTX separationBenzene 0.33 100kgmol/h Benzene: 99.5% Column pressure: 10 atm Bek-Pedersen and Gani

(2004), Rangaiah et al. (2006)Toluene 0.33 10atm Toluene: 91% Total condenserp-Xylene 0.34 Saturated liquid p-Xylene: 92% PengRobinson Model

2 BTE separationBenzene 0.33 100kgmol/h Benzene: 99.5% Column pressure: 1.75 bar From our industry sourceToluene 0.33 1.75bar Toluene: 96% Total condenserEthyl Benzene 0.34 Saturated liquid Ethyl benzene: 96% PengRobinson Model

3 Depropanizer/debutanizerEthylene 0.0128 1600kgmol/h Recovery of Column pressure: 14.0 bar Amminudin et al. (2001)Propene 0.076 14.9 bar n-Propane: 94% Total condenserPropane 0.2312 90% liquid Butane products: 95% MP steam at 5bari-Butane 0.1443 Pentane products: 97% Cooling water at 25 Ci-Butene 0.2683 SRK Modeln-Butane 0.0409i-Pentane 0.094n-Pentane 0.1008n-Hexane 0.0317

4 EWE separationEthanol 0.33 100kgmol/h Ethanol: 81.8% Column pressure: 1 atm From our industry sourceWater 0.33 1.5 bar Water: 99.6% Total condenserEthyl glycol 0.34 Saturated liquid EG: 84.6% NRTL Model

5 EPB separationEthanol 0.1 300kgmol/h Ethanol: 99% Column pressure: 1 atm Emtir et al. (2001)1-Propanol 0.8 1 atm 1-Propanol: 99% Total condenser1-Butanol 0.1 Saturated liquid 1-Butanol: 99% NRTL Model

6 Alkanes separationn-Pentane 0.34 100kgmol/h n-Pentane: 99.5% Column pressure: 500kPa Bek-Pedersen and Gani

(2004), Rangaiah et al. (2006)n-Hexane 0.33 510kPa n-Hexane: 87.5% Total condensern-Heptane 0.33 Saturated liquid n-Heptane: 88% PengRobinson Model


Table 4 Comparison of conventional 2-columns system, new DWC and retrotted DWC for BTX application

Sizing/cost details Conventional 2-column system DWC (new) DWC (retro)

(with units in brackets) C-100 C-101 Total T-101 R-101

No. of stages 31 40 68 68No. of stages (top, middle and bottom) 17, 35, 16 17, 35, 16Diameter of column (m) 0.9 0.8 0.9 0.9Height of column (m) 24 30 51 51Condenser duty (kW) 1,476 876 2,352 1,306 1,306Reboiler duty (kW) 1,508 890 2,398 1,355 1,355Ratio of Vact/Vmax 0.8 0.8 0.8 0.8

Bare module costColumn (US$) 223,746 203,501 427,247 395,453 300,260Sieve trays (US$) 74,092 90,127 164,219 162,525 111,224Condenser (US$) 26,344 22,545 48,889 26,403 0Reboiler (US$) 64,010 48,511 112,521 60,378 0

Capital costTotal module cost (US$) 480,739 450,903 931,642 795,154 504,562

Operating costCooling water (US$/year) 95,009 56,394 151,403 84,066 84,066Steam (US$/year) 333,873 197,047 530,920 299,999 299,999Total (US$/year) 428,882 253,441 682,323 384,065 384,065

Total annual cost (US$/year) 868,652 543,096 484,977

be seen theimplementin the capiclear directplants. Fora DWC; froUS$ 298,25This attracthe equipm

3.2. Ben

Table 5 proseparation.1.5m, with

ne clumnr diathe m.8 usof 0.7highy. Homiddand

, usinlow

7 trar oftal s

Table 5

Sizing/cos

(with unit

No. of stagNo. of stagDiameter oHeight of cCondenserReboiler duRatio of Va

Bare moduColumnSieve traCost of cCost of re

Capital cosTotal mo

OperatingCooling mSteam coTotal (US

Total annure is savings of about 43% in the operating cost bying a new DWC in addition to savings of about 14%tal cost, compared to the C2C system. This gives aion to go for a DWC against the C2C system for newexisting plants, the C2C system can be retrotted tom Table 4, the resulting operating cost savings are8/year and the payback period is only 20 months.tive payback period is due to the re-use of most ofents.

zene, toluene, ethyl benzene (BTE) application

vides the comparison of the three cases for the BTEThe C2C systemhas 2 columns of diameter 1.1 and51 and 35 stages respectively. The DWC system has

only oing cosmalleity tothan 0rangewith avelocittional/1.2m),Hencegives awith 3numbethe toComparison of conventional 2-columns system, new DWC and r

t details Conventional 2-column system

s in brackets) C-100 C-101 To

es 51 35es (top, middle and bottom)f column (m) 1.1 1.5olumn (m) 41 31duty (kW) 1,262 753 2ty (kW) 1,282 776 2

ct/Vmax 0.8 0.8

le cost(US$) 331,968 446,086 778ys (US$) 140,837 134,749 275ondenser (US$) 33,748 29,657 63boiler (US$) 58,605 45,348 103

tdule cost (US$) 697,840 808,125 1,505

costedium cost (US$) 81,234 48,489 129

st (US$) 283,837 171,896 455$/year) 365,071 220,385 585

al cost (US$/year) 886olumn with 1.2m diameter. In R-DWC, the exist-C-100 from the plant is used despite the slightly

meter. In this case, the ratio of actual vapor veloc-aximum velocity is 0.9, which is slightly higher

ed for other columns but still within acceptable0.9 (Sinnott, 2005). Another option is to use C-101er diameter which will give a much lower vaporwever the cost of investment is high for the addi-le section due to the larger diameter (1.5m versusthe payback period works out to be 41 months.g C-100 satises technical requirements and alsoer payback period. The additional column sectionys has to be added as the middle section. Thestages in C-100 is more than sufcient to satisfytages in the top and bottom sections of R-DWC.etrotted DWC for BTE application

DWC (new) DWC (retro)

tal T-101 T-101

65 6515, 37, 13 15, 37, 131.2 1.152 52

,015 1,293 1,293,058 1,335 1,335

0.8 0.9

,054 479,111 249,658,586 194,421 102,452,405 38,461 0,953 59,895 0

,965 951,648 431,757

,723 83,229 83,229,733 295,571 295,571,456 378,800 378,800

,649 569,130 465,152


Table 6 nd rapplicatio

Sizing/cos tem

(with unit

No. of stagNo. of stagDiameter oHeight of cCondenserReboiler duRatio of Vac

Bare moduColumn 6,1Sieve tra 4Condens 4Reboiler 3

Capital cosTotal mo 9,1

Operating cCooling w 2,0Steam (U 3,4Total (US 5,4

Total annu 7,2

The existinthe costingtrays. From35% in thetion to saviC2C systemagainst theC2C systemresulting oppayback peapplication

3.3. De

The C2C focolumns ofDWC systemexisting coto a R-DWCmaximumsufcient fosection witbe re-usedfor a new cthat thereimplementretrottingwhich is gobig, duties othe energy

3.4. Eth

For the EWdiameter 0(Table 7). Tonly 1.1m.ing column

r veable,tingg cos ofDWCives astemtedis noent

vingive (3Comparison of conventional 2-columns system, new DWC an

t details Conventional 2-column sys

s in brackets) C-100 C-101

es 32 32es (top, middle and bottom)f column (m) 2.3 2.3olumn (m) 33 33duty (kW) 8,058 7,756ty (kW) 7,376 8,064

t/Vmax 0.8 0.8

le cost(US$) 3,084,705 3,019,246ys (US$) 241,941 241,941er (US$) 352,127 118,133(US$) 175,706 186,877

tdule cost (US$) 4,754,046 4,388,871

ostater (US$/year) 1,037,374 998,495S$/year) 1,633,056 1,785,381$/year) 2,670,430 2,783,876

al cost (US$/year)

g condenser and reboiler can be re-used. Hence,shows the cost for additional middle section andTable 5, it can be seen there is savings of aboutoperating cost by implementing a DWC in addi-ngs of about 36% in capital cost, compared to the. This gives a clear direction to go for a DWCC2C for new plants. For the existing plants, thecan be retrotted to a R-DWC; from Table 5, theerating cost savings are US$ 206,656/year and theriod is 25 months, which is attractive for the BTE.

propaniser/debutaniser application

if vapoacceptretrotexistinsavinga newThis gC2C syretrotwhichinvestmThe saattractr this depropanizer/debutanizer application has 2diameter 2.3m with 32 stages each (Table 6). Thehas only one column with diameter of 2.4m. The

lumn 2 (T-101) from the plant can be retrottedassuming that a vapor velocity of 0.9 times the

is acceptable. The 32 stages in C-101 is more thanr both the bottomand top sections, and themiddle

h 40 trays has to be added. The existing reboiler canbut not the existing condenser, and hence costingondenser is included. From Table 6, it can be seenis savings of about 39% in the operating cost bying a new DWC or R-DWC. The payback period forthe existing C2C system to a R-DWC is 32 monthsod and acceptable. The column capacity being veryf the reboiler and condenser are very large, and sosaving potential is high.

anol, water and ethyl glycol (EWE) application

E application, the C2C system has 2 columns of.9 and 0.5m, with 30 and 7 stages respectivelyhe DWC system has one column with diameter ofFor retrotting to a R-DWC, diameter of the exist-(C-100) is not sufcient for use as a DWC even

rial of coninstead of strays stillmand henceexisting C2period of 33

3.5. Eth

For EPB appter 2.6 andThe DWC hexisting cofor retrottis only 2.3is more thaand bottomreboiler canadditionalseen that tby implemecapital costtion to go fThe paybacto a R-DWCetrotted DWC for depropanizer/debutanizer


Total T-101 R-101

68 6810, 40, 18 10, 40, 182.4 2.367 67

15,814 9,685 9,68515,440 9,409 9,409

0.8 0.9

03,951 6,981,134 3,459,10683,882 557,481 306,96370,260 764,654 764,65462,583 209,211 0

42,917 10,489,082 5,595,675

35,869 1,246,831 1,246,83118,437 2,083,165 2,083,16554,306 3,329,996 3,329,996

82,889 5,427,813 1,119,135

locity of 0.9 times the maximum vapor velocity iswhich leaves the only option of a new column for

. The existing reboiler can be re-used but not thendenser. From Table 7, it can be seen that there isabout 31% in the operating cost by implementingcompared to the conventional 2-column system.clear direction to go for a DWC against using thefor new plants. The existing C2C system can be

to a DWC but the payback period is 49 months,t that attractive. Themain reasons being thehighercost and fewer saving (in terms of the amount).

s in terms of percentage for operating cost is still1%). One alternate solution is using cheaper mate-struction instead of stainless steel. Carbon steeltainless steel as the material for the column (withade of stainless steel)would reduce the capital costthe payback period. In such a case, retrotting theC system to a R-DWC is attractive with a paybackmonths.

anol, propanol and butanol (EPB) application

lication, the C2C system has 2 columns of diame-1.5m, with 52 and 32 stages respectively (Table 8).ave only one column with diameter of 2.3m. Thelumn 1 (C-100) with diameter of 2.6m can be useding since the diameter requirement for the DWCm. The number of stages in the existing columnn sufcient to satisfy the total stages in the topsections of the DWC. The existing condenser andbe re-used. Hence, the costing shows the cost for

middle section and trays. From Table 8, it can behere is savings of about 43% in the operating costnting a DWC in addition to savings of about 30% incompared to aC2C system. This gives a clear direc-or a DWC against the C2C system for new plants.k period for retrotting the conventional columnis only 18 months.


Table 7 Comparison of conventional 2-columns system, new DWC and retrotted DWC for EWE application



No. of stages 30 7 35 35No. of stages (top, middle and bottom) 17, 9, 9 17, 9, 9Diameter of column (m) 0.9 0.5 1.1 1.1Height of column (m) 24 7 29 29Condenser duty (kW) 1,259 202 1,461 1,033 1,033Reboiler duty (kW) 1,289 141 1,430 973 973Ratio of Vact/Vmax 0.8 0.8 0.8 0.9

Bare module costColumn (US$) 162,944 38,098 201,042 252,769 252,769Sieve trays (US$) 71,702 22,612 94,314 96,653 96,653Condenser (US$) 45,708 22,108 67,816 73,828 73,828Reboiler (US$) 58,776 22,699 81,475 50,731 0




3.6. Alkanes (pentane, hexane, heptane) application

Table 9 proand R-DWCcolumns oftively. AnewFor R-DWCratio of actity is morecolumn. ThFrom Tablein the opersavings of a

tem. This gives a clear direction to go for a DWC against thestemcanonth

An

rotts sho

maasespar

be ob

Table 8

Sizing/cos

(with unit

No. of stagNo. of stagDiameter oHeight of cCondenserReboiler duRatio of Va

Bare moduColumnSieve traCondensReboiler

Capital cosTotal mo

OperatingCooling wSteam (UTotal (US

Total annuvides the comparison of C2C system, new DWCfor the alkanes separation. The C2C system has 2diameter 0.8m each, with 38 and 21 stages respec-DWChas only one columnwithdiameter of 0.9m.

, the existing column is tried for retrotting but theual vapor velocity to the maximum vapor veloc-than 0.9, which leaves the only option of a newe existing condenser and reboiler can be re-used.9, it can be seen that there is savings of about 37%ating cost by implementing a DWC in addition tobout 20% in capital cost, compared to the C2C sys-

C2C sysystemis 32 m

3.7.

For retcationthe sumthree care comIt canComparison of conventional 2-columns system, new DWC and r

t details Conventional 2-column system

s in brackets) C-100 C-101 T

es 52 32es (top, middle and bottom)f column (m) 2.6 1.5olumn (m) 52 29duty (kW) 6,881 2,889ty (kW) 8,012 2,984

ct/Vmax 0.8 0.8

le cost(US$) 2,039,313 418,948 2,4ys (US$) 499,787 123,199 6er (US$) 102,878 73,679 1(US$) 183,749 95,349 2

tdule cost (US$) 3,479,941 880,907 4,3

costater (US$/year) 442,924 185,963 6S$/year) 1,773,868 660,662 2,4$/year) 2,216,792 846,625 3,0

al cost (US$/year) 3,9for new plants. For the existing plants, the C2Cbe retrotted to a R-DWC and the payback periods, which is attractive.

alysis of the results

ing the C2C system to a R-DWC, most of the appli-w the techno-economic viability. Table 10 providesry of the results including the costingdetails for theof all applications. The condenser duties, which

able to reboiler duties, are not shown in this table.served from the payback periods in Table 10 thatetrotted DWC for EPB application


otal T-101 R-101

60 6010, 40, 18 10, 40, 182.3 2.659 59

9,770 5,187 5,18710,996 6,316 6,316

0.8 0.9

58,261 1,736,263 1,270,24722,986 453,640 336,39576,557 110,320 079,098 155,557 0

60,848 3,025,223 1,970,067

28,887 333,883 333,88334,530 1,398,371 1,398,37163,417 1,732,254 1,732,254

35,586 2,337,299 2,126,267


Table 9 Comparison of conventional 2-columns system, new DWC and retrotted DWC for alkanes application



No. of stages 38 21 48 48No. of stages (top, middle and bottom) 14, 23, 11 14, 23, 11Diameter of column (m) 0.8 0.8 0.9 0.9Height of column (m) 28 17 37 37Condenser duty (kW) 753 666 1,419 858 858Reboiler duty (kW) 817 672 1,489 934 934Ratio of Vact/Vmax 0.8 0.8 0.8 0.9

Bare module costColumn (US$) 155,489 109,679 265,168 232,565 232,565Sieve trays (US$) 85,621 47,317 132,938 114,723 114,723Condenser (US$) 30,856 23,410 54,266 32,749 0Reboiler (US$) 46,488 42,312 88,800 49,708 0




retrotting the C2C system to a DWC is attractive for BTX, BTE,depropanizer, EPB and alkanes separations. For EWE separa-tion, the payback period is slightly higher (4 years) and notattractive due to higher investment though the percentagesavings in the operating cost is comparable with other appli-cations. Factors which affect the investment and hence thepayback period have been analyzed, and alternate solutionshave beenThese are d

3.7.1. Capacity of the plantTable 10 presents retrotting results corresponding to thecapacities of the existing plants. Here, the longer paybackperiod in some cases is because of limited use of existingequipment. Table 11 presents retrotting results at capacitiesthat maximize re-use of existing equipment. Recall that theexisting columns could not be used for retrotting to DWCs (at

eed ction

Table 10 appl

Applicatio

S$/ye

BTXC2CDWCR-DWC

BTEC2CDWCR-DWC

DepropanizC2CDWCR-DWC

EWEC2CDWCR-DWC

EPBC2CDWCR-DWC

AlkanesC2CDWCR-DWCproposed in order to reduce the payback period.iscussed below.

100% fcalcula

Summary of the techno-economics of various cases for all

ns and cases Reboiler duty(kW)

Investmentcost US$

U2,398 931,642 682,321,355 795,154 384,061,355 504,562 384,06

2,058 1,505,965 585,451,335 951,648 378,801,335 431,757 378,80

er/debutanizer15,440 9,142,917 5,454,309,409 10,489,082 3,329,999,409 5,595,675 3,329,99

1,430 553,056 410,60973 596,064 281,80973 531,198 281,80

10,996 4,360,848 3,063,416,316 3,025,223 1,732,256,316 1,970,067 1,732,25

1,489 671,089 420,95934 531,279 262,05934 425,846 262,05apacity) for EWE and alkanes applications. A quickshows that, at reduced capacities, the existing col-

ications

Operating cost Payback period(months)

ar Savings

US$/year %35 298,258 43.7%5 298,258 43.7% 20

60 206,656 35.3%0 206,656 35.3% 25

66 2,124,309 38.9%6 2,124,309 38.9% 32

66 128,800 31%6 128,800 31% 49

74 1,331,162 43.5%4 1,331,162 43.5% 18

35 158,897 37.7%5 158,897 37.7% 32


Table 11 Cost per unit feed rate at capacity which maximizes re-use of existing equipment

Description Units BTX BTE Depropanizer/debutanizer

EWE EPB Alkanes

Feed ow rate at 100% kg/h 9228 9228 90,170 4405 18,030 8604Capacity factor % 100% 100% 100% 75% 100% 80%Reboiler duty kW/(kg/h of feed) 0.15 0.14 0.10 0.22 0.35 0.11Investment US$/(kg/h of feed) 55 47 62 45 109 28Operating cost per year US$/(kg/h of feed)/year 42 41 37 64 96 30Savings per year US$/(kg/h of feed)/year 32 22 24 29 74 18Payback period Months 20 25 30 19 18 18

umn can be utilized and hence results in a better paybackperiod. The reduced capacity factor is shown in Table 11; theinvestment cost shown in this table for each application corre-sponds to the reduced capacity (as per the capacity factor). Inaddition, Tunit feed ranies may bproduct deuations, retdesirable pperiod.

It is alsocan be retroity but the cmore thanbe studiedability of upof the increthe operatiput. In theabove, thewith 32 staDWC is 2.4of the existing at reduthe maximvelocity ofexisting coto DWC atcases, the padditionaloperating aoriginal desmore feed.nes separat80% of the o60% more f

3.7.2. Material of constructionMaterial of construction of equipments invariably affects thecapital cost. Material of construction for a column is onegrade less than that for the internals such as trays, pack-

d dids indues reqhe soof caas rerodut. Suver

12 gistrucS refpropk pe

3 appation

redttracbe

esulte. S

attrastrue in mn br.

Releminanplachas

Table 12 em talkanes s

Descriptio

Depropanidebutani

EWE

Alkanesable 11 gives required investment and savings perte. Note that distillation columns in some compa-e operating at lower capacities depending on themand and market conditions. Hence, for such sit-rotting the existing C2C system to a DWC may berovided the investment has an acceptable payback

worthwhile to analyze if both the existing columnstted to twoDWCs. Eachmayhave different capac-apacity of both columns put together is likely to bethe existing capacity eventually. This option has toin detail taking into account product demand andstream and downstream equipments to take careased capacity. The benets are not just savings inng cost but also signicantly increased product out-depropanizer/debutanizer application discussed

existing plant has 2 columns of diameter of 2.3mges. The diameter requirement for retrotting tom, which is very close to the existing diameter. Oneing 2 columns can be retrotted to a DWC operat-ced capacity based on vapor velocity of 0.8 timesum velocity. Alternately, as stated above, if vapor0.9 times the maximum velocity is acceptable, onelumn with a diameter of 2.3m can be retrotted100% of the original design capacity. In both theseayback period works out to less than 3 years. As anoption, if both the columns are retrotted to DWCst 90 or 100% (depending on vapor velocity) of theign capacity, then the plant can process 80 or 100%Similarly, the 2 columns in the C2C system for alka-ion can be retrotted to 2 DWCs, each operating atriginal design capacity; then the plant can processeed.

ing anof uicasesproceswith tmadegradenow pmarkequalityTableof conand Cfor depaybacotherappliccan bevery ashouldmay rexercismoreof conchangthis careboile

3.7.3.The chmainteand reabove

Effect of material of construction for retrotting a C2C systeparationn Material for column(with SS internals)

Investment (US$) Operat(US$

zer/zer

SS 5,595,675 3,329CS lined with SS 4,124,038 3,329CS 2,652,402 3,329

SS 531,198 281CS lined with SS 440,021 281CS 348,845 281

SS 425,846 262CS lined with SS 343,614 262CS 261,381 262stributors, depending upon the corrosive naturethe process. This is seen widely in industrial

to economic reasons. In some cases where theuires higher grade materials, internals are madelid material of that grade. However, columns arerbon steel and then inner-lined with the higherquired for the process conditions. Certain millsce cladded materials and supply directly to thech cladded materials from the mill exhibit highy similar to the solid material of the same grade.ves a summary of the payback period if materialtion is varied; here, SS refers to stainless steelers to carbon steel. Table 12 presents data onlyanizer/debutanizer, EWE and alkanes where theriod for retrotting to a R-DWC was higher thanlications. For example, depropanizer/debutanizerwith SS, the payback period is 32 months which

uced to 15 months by using CS, which is in thetive range. Hence, the material of constructionre-evaluated considering all possibilities, whichin more economic benets for the retrotting

imilarly, EWE and alkanes applications also turnctive for retrotting with the change in materialction. The results in Table 12 are based on theaterial of construction of column only. If required,

e extended to column internals, condenser and

ated issuescal/petrochemical plants usually have an annualce period of 1014 days, during which major repairement are carried out. The retrotting discussedbeen assumed to take place within this mainte-

o a DWC: depropanizer/debutanizer, EWE anding cost/year)

Savings(US$/year)

Payback period(months)

,997 2,124,310 32,997 2,124,310 23,997 2,124,310 15

,806 128,800 49,806 128,800 41,806 128,800 33

,055 158,898 32,055 158,898 26,055 158,898 20


nance period, and so production loss during the retrottingtime has noutside theproductionevaluation.duced inceenergy confor details,(accesseding energymore favortechno-ecoretrottingings and avor capacity

4. Co

Retrottingapplicationenergy conlation. TherequiremenDWCs for ntial for retroperating cnew as weThe returnvery good iback periothe paybacing this by2 columnsreduces toconstructioto carbon stem can bein this andretrottingsumption aprocesses.

Appendiximplicatio

While retromore difcthe retrot

techno-e existing location experien

retrottin project m

plan.

Of the abstudy descrout with athe retrottheManageare briey d

A.1. Existing plant condition

uipms co

ondiitablenicasider

ocati

stillaor pipdicad. Thcolume pck toe secportaant tspaceat aove

er reanstps.

xper

encetingin

ith thsafes shnecspecmoamm

checdes ato prs pnmecpecick ifion.n 1)experote to

tions

rojec

matingt mat. Th, rese inal timpleot been considered. If the retrotting is carriedregular maintenance period, then the associatedloss needs to be considered for techno-economicOn the other hand, several countries have intro-ntives and tax rebates to industries investing inservation projects. Singapore is one such country;see http://www.nccc.gov.sg/incentive/home.shtmin June 2008). These incentives and increas-cost, which make retrotting to a DWC even

able, are not taken into account in the abovenomic evaluation. Other potential benets froman existing C2C system to a DWC are space sav-ailability of a spare column for other applicationsincrease.

nclusions

the C2C systems to DWCs for several industrials has been studied in this work, to reduce thesumption and hence the operating cost of distil-results of this study conrm the lower energyts and consequently cost savings due to use ofew plants as well as show the signicant poten-otting existing C2C systems. The savings in theost are more than 30% in all the applications forll as retrotted DWC compared to a C2C system.s from retrotting a C2C system to a DWC aren 5 of the 6 applications considered, with a pay-d of less than 3 years. In the EWE application,k period is about 4 years. Two ways of reduc-changing material of construction or retrottingto 2 DWCs, are discussed. The payback period33 months for the EWE application if material ofn for the column (and not internals) is changedteel. Alternately, capacity of the separation sys-increased by retrotting both columns to DWCsother applications. In general, energy savings byto a DWC translate to reduction of fossil fuels con-nd thus contribute to sustainability of chemical

A. Retrot procedure andns at site

t is an attractive option, its implementation isult than the new plants. The factors, which affectting exercise, are:

conomic feasibility of retrotting operation;plant condition;of the existing columns and exchangers;ced contractors and other resources to performg at site;anagement with a clear retrotting execution

ove, the rst one has already been discussed in theibed in the main text. It is very important to comegood and reliable report on techno-economics ofting and project execution plan to present them toment/Board and obtain approval. The other factorsiscussed below.

The eqsystemtheir cnot sumechabe con

A.2. L

The ditures for moaffecteto thehold ththis baor somare imimportIf theparkedity andAnothfor dowfor pum

A.3. E

Experiretrotas planable whelp atractorout thewith reting orX-ray/ging tothe cojectedsuch aas thetries, sto cheoperatDivisiowhichthe reters havregula

A.4. P

Projectretrotprojecprojecbudgetessencminimthe coment to be retrotted as well as the equipments/nnected with it should be checked/monitored fortion to perform the new operation. If any of them is

from the point of process/thermal design and/orl stability/integrity, then such equipment have toed for modications and added to the cost.

on of the existing equipment for retrotting

tion column, for example, may have some struc-e supports. If this equipment is subject to removaltion, then the piping attached to it may also bee piping has to be separated from the attachmentsn, and temporary supports have to be provided to

iping until completion of the modication to putits original position. If the column has to be liftedtions need to be replaced, the location and accessnt to plan the crane and other type of facility. It iso know where to park the crane to lift the column.is not available near the column, then it will bedistance, the crane should have sufcient capac-

rhang to handle the column from such a distance.sonwhich location affects is the process condition

ream equipments such as net pump suction head

ienced contractors and resources

d contractors and resources ensure smooth; they will take adequate safety measures as welldetail on the execution plan. The resources avail-em in terms of people, machines and equipmentand successful retrotting. The experienced con-all be technically sound to understand and carryessary testing as per the local statutory regulationst to that industry. For example, hot welding, cut-

dication require non-destructive testing such asa ray, ultrasonic testing and dye penetration test-

k if the quality of the welding is acceptable as pernd regulations. Subsequently, the column is sub-essure testing such as hydrostatic testing, leak testeumatic testing to check for possible leaks as wellhanical integrity of the equipment. In some indus-al testing such as helium testing has to be donethe equipment is sound and good for startup andThere are codes and standards (ASME Section VIII,available for a repair or rework of the equipments,rienced contractors, will be aware of to completeting in full compliance. The vessels and exchang-be re-registered for operation as per the statutory.

t management and clear retrotting plan

nagement plays a key role in preparing a clearplan. The project plan is usually prepared by thenagement team to achieve the objectives of thee project management monitors and controls theource planning and time schedule. Time is thethe retrot operation as this has to be done in thee. A shutdown should be planned to ensure that

te operation can be carried out successfully meet-


ing the budget, available resources and within the plannedtime. Otherwise, this will result in additional cost in terms ofproduction loss.

Planning of resources, appointing the right contractors,evaluating the technical and commercial risks, etc. are keyactivities of planning. All the affected systems need to be putback for proper operation. In addition, the statutory approvalsfrom the local authorities are required if certain equipmentssuch as columns, heat exchangers, piping, etc. are to be mod-ied at site. Coordination and communication plans have tobe drafted and executed properly. Budget depends upon thecomplete planning of the operation, contingency, etc. Orga-nizing all the lead activities within the estimated cost leadsthe project to the successful completion.

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Retrofitting conventional column systems to dividing-Wall ColumnsIntroductionDesign procedure for DWC using HYSYSShortcut distillationRigorous simulationOptimizationSizing of column, condenser and reboiler for DWC

Results and discussionBenzene, toluene, xylene (BTX) applicationBenzene, toluene, ethyl benzene (BTE) applicationDepropaniser/debutaniser applicationEthanol, water and ethyl glycol (EWE) applicationEthanol, propanol and butanol (EPB) applicationAlkanes (pentane, hexane, heptane) applicationAnalysis of the resultsCapacity of the plantMaterial of constructionRelated issues

ConclusionsRetrofit procedure and implications at siteExisting plant conditionLocation of the existing equipment for retrofittingExperienced contractors and resourcesProject management and clear retrofitting plan

References

dwc column simulation

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