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Applied Thermal Engineering 59 (2013) 200e210

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Applied Thermal Engineering

journal homepage: www.elsevier .com/locate/apthermeng

Design and optimization of thermally coupled distillation schemesfor the trichlorosilane purification process

Nguyen Van Duc Long, Yongsoo Kwon, Moonyong Lee*

School of Chemical Engineering, Yeungnam University, Dae-dong 214-1, Gyeongsan 712-749, South Korea

h i g h l i g h t s

� A compact and energy efficient TCS purification process is proposed.� Optimal design is efficiently done by response surface methodology.� CGCC is used to evaluate the thermodynamic feasibility of implementing the heat pump.� Novel combination of internal and external heat integration is proposed.� The operating cost could be reduced by 83.01%.

a r t i c l e i n f o

Article history:Received 12 January 2013Accepted 22 May 2013Available online 29 May 2013

Keywords:Dividing wall columnTrichlorosilaneHeat pumpColumn grand composite curveHeat integrationResponse surface methodology

* Corresponding author. Tel.: þ82 53 810 2512.E-mail address: [email protected] (M. Lee).

1359-4311/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.applthermaleng.2013.05.03

a b s t r a c t

The trichlorosilane (TCS) purification process has significant energy requirements to achieve a TCS purityof 10 N. In this study, a dividing wall column was used to improve the performance of the TCS process. Aresponse surface methodology was applied to the design of the dividing wall column. The conventionaldividing wall column and top dividing wall column have significant benefits, e.g. decreasing the oper-ating cost and minimizing the total annual cost. Incorporating a heat pump in the top diving wall col-umns was also proposed to enhance the energy efficiency further. Furthermore, a column grandcomposite curve was used to evaluate the thermodynamic feasibility of implementing the heat pumpsystem into DWC. The operating cost could be reduced by 83% by novel combinations of internal andexternal heat integration: top dividing wall columns using a top vapor recompression heat pump. Acompact integrating TCS purification process was finally proposed.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

The emissions trading scheme is a very important issue for allcountries. Most carbon emissions originate from fossil fuels, andsome renewable resources are emerging as promising alternativesto fossil fuels. Among them, solar energy is considered a reasonable,natural, clean energy resource. Solar cells on the market today aremost often made from crystalline silicon because of its high light-to-energy efficiency. Trichlorosilane (TCS) is normally used in themanufacture of polycrystalline silicon metal for the semiconductorand solar cell industries. Recently, the demand for TCS hasincreased. Therefore, TCS purification processes are organizedbased on energy savings, which can decrease the price of semi-conductor as well as solar cell.

All rights reserved.5

An innovative solution to overcome the drawback of energyintensive distillation is the use of advanced process intensificationand integration techniques, such as thermally-coupled distillationcolumns, dividing-wall columns (DWC), or heat-integrated distil-lation columns [1e13].

Chemical heat pump systems have attracted considerableattention [14,15], and represent a new technology with great po-tential to reduce the energy consumption in very different sectors.They can capture rejected low-grade heat and reuse it at increasedtemperatures in a range of industrial processes. Heat pumps indistillation allow the heat of condensation released at thecondenser to be used for evaporation in the reboiler. This is aneconomic way of conserving energy when the temperature differ-ence between the overhead and bottom of the column is small andthe heat load is high. Heat pumps can also be used in grass-roots orretrofitting designs because they are easy to introduce and plantoperation is normally simpler than heat integration [16e20]. Heatpump systems have been shown to be economically feasible dueto the energy savings that can be achieved. This is not the only

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Fig. 1. Schematic diagram of the (a) direct sequence and (b) prefractionatorarrangement.

Fig. 2. Schematic diagram of the thermally-coupled distillation sequences: (a) systemwith side rectifier and (b) system with side stripper.

N.V.D. Long et al. / Applied Thermal Engineering 59 (2013) 200e210 201

advantage of these systems because heat pumps can also reducethe CO2 footprint [21]. This is an environmental advantage that canbecome increasingly important if CO2 quotas are enforced bygovernments.

Although DWCs can potentially to reduce energy use and in-vestment costs, their design and optimization is challenging,involving solving multivariable problems with variables thatinteract with each other, which need to be optimized simulta-neously. Statistical approaches have revealed the potential for this,with the structures of dividing wall columns being optimized usinga response surface methodology (RSM) [4]. This method can beimplemented easily and efficiently using HYSYS and MINITAB.Another statistical approach, factorial design, is used widely inexperimental studies [7,22e25]. These approaches can obtain in-formation about each factor studied as well as the interactionsbetween them fromminimal experimental data. In addition, its useis economical through the reduced number of simulations and timesavings.

The aim of this study was to enhance the performance of thecrude TCS purification steps in TCS processing using DWC andheat integration. The response surface methodology was used todetermine the optimum structure of DWC. The integration ofelectrically driven heat pumps in DWC was considered. A columngrand composite curve was used to indicate the thermodynamicfeasibility of implementing a heat pump system to DWC.A compact integrating TCS purification process was finallyproposed.

1.1. Dividing wall column

Distillation is the primary separation process used in chemicalprocessing industries. Although this unit operation has manyadvantages, one of the major drawbacks is its large energy re-quirements [26], which can influence the overall plant profit-ability significantly. Therefore, the increasing cost of energy hasforced industry to reduce its energy consumption. In addition, thetendencies to rationalize and reduce the use of fossil energy re-sources, as well as the tighter environmental regulations havegenerated the need to employ new and efficient separationmethods [27,28].

Fig. 1a shows the direct conventional distillation sequence forternary distillation. The system contains a reboiler and condenserin every distillation column. The number of product flows in thecolumns is always the same, namely, two. On the other hand, in amore novel distillation scheme, i.e. a prefractionator scheme(Fig. 1b) [28], there is one extra product flow, known as the sidestream, in the second column of the column sequence. Thermally-coupled distillation systems can be constructed through theimplementation of two interconnecting streams (one in the vaporphase and the other in the liquid phase) between the two columns.Such systems have attracted significant attention for the separation

N.V.D. Long et al. / Applied Thermal Engineering 59 (2013) 200e210202

of ternary mixtures, and have been shown to provide significantenergy savings with respect to both conventional direct and indi-rect distillation sequences. The three most widely analyzedthermally-coupled systems are the sequence with a side rectifier,sequence with a side stripper and a fully thermally coupled system(or Petlyuk column), which are illustrated in Figs. 2a,b and 3a,respectively [29e32]. These sequences have been shown to provideenergy savings of up to 30% compared to conventional direct andindirect distillation sequences [33e35].

On the other hand, the Petlyuk column undergoes strong in-teractions between the two columns because of their thermalintegration, which creates difficulties in design and operation. Inparticular, the Petlyuk system includes a vapor stream that passesfrom the first column (or prefractionator) to the main column, andanother that moves in the opposite direction. This situation re-quires that the pressure at the bottom of the prefractionator belower than that of the main column, whereas the pressure at thetop should be higher than that of the main column [35], which isperceived to be difficult to control. To solve this operational prob-lem and reduce the capital cost, a vertical wall is installed in thecentral section of the column to divide the column into a pre-fractionator and a main column, as shown schematically in Fig. 3b.This arrangement is called the dividing wall column (DWC), which

Fig. 3. Schematic diagram of the (a) Petlyuk

is conceptually the same as the Petlyuk column due to their ther-modynamically equivalent arrangements [36]. The top dividingwall column (TDWC) or bottom dividing wall column (BDWC) canbe produced bymoving the dividingwall to the top or bottomof thecolumn, respectively.

1.2. Thermal heat pump integration

In spite of the potential of a heat pump to improve the perfor-mance of a distillation column, only a few guidelines are currentlyavailable to help a process design engineer. Distillation columns arethermodynamic systems comprising a heat source (condenser) andheat sink (reboiler). The proper integration of a heat pump into abackground process is clearly understood in terms of the heatsource and heat sink graphical representation of a process [37].Such representation, known as the grand composite curve, showsthe point, termed the pinch, where the division between these twozones occurs. A heat pump is a system that takes low level heatfrom a source and delivers it to a sink where a higher temperaturelevel is required. The proper integration of a heat pump is said to beacross the pinch [38,39].

These systems can also be represented graphically on a tem-peratureeenthalpy diagram known as the column grand composite

column and (b) dividing wall column.

Fig. 4. A three-column distillation system for the initial design of the DWC structure.

Table 1Feed conditions of the crude TCS.

Feed conditions

Component Mass flow (kg/h)

Hydrochloric acid (HCl) 80Dichlorosilane (DCS) 250Trichlorosilane (TCS) 17,400Silicon tetrachloride (STC) 1000Heavies 360Temperature (�C) 60Pressure (bar) 6.47


curve (CGCC) [37,40e45]. For a fixed product composition, numberof stages and reflux ratio, this diagram shows the steady statethermodynamic operating profile of the column as well as thecorresponding condenser and reboiler heat duties. Similar to

Fig. 5. Simplified flow sheet illustrating the sep

overall background processes, the thermodynamic division be-tween the two zones is a point called the column pinch point [37].

To make a heat pump installation profitable in distillationprocesses, the system must operate between small temperaturedifferences [37]. A heat pump operates by extracting a givenamount of heat from a source at a relatively low temperature anddelivering a larger amount of heat to a sink with a higher tem-perature by consuming high quality energy (electrical or me-chanical) to achieve the task. The opportunities for theimplementation of a heat pump on the top of the column or anintermediate heat pumping system are determined by the shape ofthe CGCC. A flat profile on both sides above and below the pinchpoint indicates that the process is suitable for recovering the wasteheat from the rectification section and reusing it in the strippingsection of the column.

If an inspection of the CGCC shows that the columnenergy requirements can be met by a heat pump operating be-tween the stages above and below the CGCC pinch pointacross relatively small temperature differences, there is a goodpossibility of improving the energetic efficiency of the system.A more detailed description about CGCC can be found elsewhere[37].

aration train of five conventional columns.

Table 2Column hydraulics, energy performance, and product specifications of the existingcolumns sequence.

C-201 C-202 C-203 C-204 C-205

Number of trays 45 40 40 45 40Condenser duty (kW) 1824 1998 453 1160 2224Reboiler duty (kW) 2469 1492 1068 1184 2208Tray type Bubble cap Bubble cap Sieve Packed PackedColumn diameter (m) 1.40 1.30 0.66 1.52 1.96Number of flow paths 1 1 1Tray spacing (mm) 609.6 609.6 609.6HETP (mm) 566.9 611.9Max flooding (%) 80.99 79.76 79.33 80.94 80.45

Purity (mass fractions %)

TCS 99.99999999 (10 N)Chemical grade TCS 99.50STC 97.00

Fig. 6. Optimization plot.


2. Design and optimization methodology

2.1. Design

The initial DWC design is achieved using a shortcut designprocedure [46] based on the conventional column configurationshown in Fig. 4. In this conventional configuration, the first columncorresponds to the prefractionator of the DWC. The rectifying sec-tion of the second column and the stripping section of the thirdcolumn represent the top and bottom sections of the DWC,respectively. The stripping section of the second column and therectifying section of the third column are equivalent to the dividingwall section of the DWC. The bottom stream from the second col-umn and the top stream from the third column correspond to theside stream of the DWC. Consequently, the DWC can be divided intofour sections: a prefractionator for the feed mixture; top and bot-tom sections above and below the dividing wall; and a dividingwallsection.

2.2. Optimization

The response surface methodology (RSM) is a general techniquefor an empirical study of the relationships between the measuredresponses and independent input variables. A response surface isnormally a polynomial, whose coefficients are extracted by a simpleleast-square fit to the experimental data. The RSM is quite powerfulbecause, in addition to modeling, it can also optimize the condi-tions of the process under investigation [4,47]. Normally, a low-order polynomial is used in some regions of the independent var-iables [22]. A simple first-order model can be used as an approxi-mating function if the response is modeled well by a linear functionof the independent variables. On the other hand, a polynomial ofhigher degree, such as a second-order model, must be used if thereis curvature in the system. According to the sparsity-of effects

Table 3Coded levels of factors (CDWC).

Factor Levels

�1 0 1Top section (N1) 8 11 14Middle section (N2) 29 31 33Bottom section (N3) 3 6 9Feed section (N4) 9 12 15Side product section (N5) 6 9 12

principle in the response surface methodology, main (single factor)and quadratic effects and two-factor interactions are likely to be themost significant effects, and the higher order interactions arenegligible [7]. In other words, higher order interactions, such asthree-factor interactions, are quite rare and are considered to beresidual, which are dispersed randomly. Almost all RSM problemsuse one or both of these models. A polynomial model is unlikely tobe a reasonable approximation of the true functional relationshipover the entire space of independent variables, but usually workswell for a relatively small region [22].

After an initial setting of the DWC structure, the main designvariables including the internal vapor (FV) and liquid (FL) flows tothe prefractionator, the number of trays in the top (N1), middle(N2), bottom (N3), feed (N4) and side product (N5) sections wereoptimized. The number of trays in each side of the dividing wallwere fixed at N2. After determining the preliminary ranges of thevariables through single-factor testing, a BoxeBehnken design [22]was employed under the RSM to analyze how the variables inter-acted as well as to optimize the column structure by maximizingthe total annual cost (TAC) saving. This design is formed bycombining 2k factorials with incomplete block designs [22]: Theresulting designs are usually very efficient in terms of the numberof required runs, and they are either rotatable or nearly rotatable.Rotability is a very important property in the selection of RSM.

Fig. 7. Simplified flow sheet illustrating the CDWC (C-202 & C-203).

Table 4Utilities cost data.

Utility Price ($/GJ)

Cooling water 0.35Steam 6.08Electricity 16.80


Since the purpose of RSM is optimization and the location of theoptimum is unknown prior to running the experiment, it makessense to use a design that provides equal precision of estimation inall direction. Notice that the BoxeBehnken design is a sphericaldesign, with all points lying on a sphere of radius

ffiffiffi2

p. Also, the Boxe

Behnken design does not contain any points at the vertices of thecubic region created by the upper and lower limits for each variable.This could be advantageous when the points on the corners of thecube represent factor-level combinations that are prohibitivelyexpensive or impossible to test because of physical processconstraints.

Simulation run data were fitted to a second-order polynomialmodel and regression coefficients were obtained. The generalizedsecond-order polynomial model used in the response surfaceanalysis is as follows:

Y ¼ b0 þXki¼1

biXi þXki¼1

biiX2i þ

XXi

Table 5Coded levels of factors (TDWC).

Factor Levels

�1 0 1Top section (N1) 50 55 60Bottom section (N2) 9 12 15Feed section (N3) 6 8 10


minimize the TAC, i.e. maximize the TAC saving over the conven-tional sequence.

The largest total annual cost saving (23.60%) was found at the�0.2929, �0.4141, �0.2727, �0.3131 and �0.3535 coded levels ofthe number of trays in the top, middle, bottom, feed and sideproduct sections, respectively (Fig. 6). The variables’ naturalvalues can be derived from their coded levels. FV and FL werethen optimized to maximize the total annualized cost saving,while satisfying product purity required. The simulationresults show that the optimal liquid split ratio (RL) and vaporsplit ratio (RV) were 0.68 and 0.72, respectively. Note that thesplit is defined as the ratio between the flow into the pre-fractionator to the total flow into the dividing wall section. Fig. 7shows a simplified flow sheet illustrating the resulting DWCsystem. The result from rigorous simulations also showed thatthe DWC can save up to 25.25% and 23.31% in terms of the

Fig. 9. Contour plots of interactions between

operating costs and total annualized costs, respectively. Theutility cost data used in this study were taken from Ref. [49] andare listed in Table 4.

Several simulation runs were carried out to validate the three-dimensional response surface plots predicted by the RSM. Fig. 8compares the three-dimensional response surface plots of inter-action between N1 and N2 evaluated by the RSM and by rigoroussimulation. It is apparent from the figure that the three-dimensional response surface plots by the RSM are quite similarto the actual ones.

3.2.2. Integrating C-204 and C-2053.2.2.1. Top dividing wall column (TDWC). Some preliminarysimulation runs were performed to determine the levels of thevariables. Corresponding to the changes of factor value, the simu-lated response values (TAC saving) were recorded. The presence ofcurvature indicates that the simulation region is close to the opti-mum. Table 5 lists the factors and levels used in the RSM basedoptimization. The TDWC parameters were optimized over 15simulation runs. For each run, the internal vapor and liquid flows tothe prefractionator were varied to optimize TAC saving and to meetthe required product purity and recovery. Fig. 9 shows contourplots of the interactions between main variables. As a result, 64trays were used in the design of the TDWC (Fig. 10). The use of aTDWC can save up 20.61%, and 19.30% in terms of operating costand TAC, respectively.

main design variables’, N1, N2, and N3.

Enthalpy deficit (KW)

-300 0 300 600 900 1200 1500 1800 2100 2400 2700 3000

Tem

pera

ture

(oC

)

49

50

51

52

53

54

55

56

main columnprefractionator

Condenser 1

Reboiler

Condenser 2

(b)

Enthalpy deficit (KW)

-300 0 300 600 900 1200 1500 1800 2100 2400 2700 3000

Stag

e

0

10

20

30

40

50

60

70

main columnprefractionator

(a)

Fig. 11. CGCC for TDWC.

Fig. 12. Simplified flow sheet illustrating the TDWC system with two top vaporrecompression heat pump systems.

Fig. 10. Simplified flow sheet illustrating the TDWC (C-204 & C-205).


3.2.2.2. TDWC with top vapor recompression heat pump.Fig. 11 shows the CGCC for the TDWC. The distance between thepoints on the curve and the vertical axis indicates the heat loadrequired for each stage. The distance between the points on thecurve and vertical axis indicates the heat load required for eachstage (Fig. 11b). The horizontal distances between the far endpoints of the curve above and below the pinch represent thereboiler and condenser heat duties, respectively [37]. In this case,the column pinch point is located at stage number 51, which cor-responds to the bottom of the dividing wall. An inspection of theCGCC shows that the temperature difference between the top ofthe column and the reboiler stage is small, and the heat load ishigh (Fig. 11a), which indicates that there is a possibility ofimproving the energetic efficiency of the system through theimplementation of heat pump.

Vapor from the top of both the prefractionator and maincolumn can be compressed to a pressure of 2.84 bar (increasingtheir vapor temperature), and then condensed in the column’sreboiler through indirect contact with the liquid in the column’sbottom (Fig. 12). This allows the heat of the condensing vapor toassist in vaporization at the bottom of the column via a heatexchanger. Note that the pressure of the outlet compressor wasadjusted to obtain a minimum approach in the heat exchanger of10 �C. Compared to the conventional column sequence, thermalintegration of the two heat pump systems in TDWC can reducethe operating cost and TAC by up to 83.01 and 55.81%, respec-tively. The outlet stream of heat exchanger still remains 78 kW.However, this stream is at relatively low temperatures, whichis not good enough to use for evaporation in the reboiler. Inthe case this heat is utilized, the duty of the compressorand pressure ratio increase from 114 kW to 149 kW and from 1.56to 1.77, respectively to satisfy minimum approach in the heatexchanger of 10 �C. As a result, the TAC saving reduces from 55.81to 54.82%.

3.2.3. Final proposed sequenceIntegrating the TCS purification process (Fig. 13) can make a

plant more efficient and reduce costs. Using the modified sequencecan save up 58.20 and 42.25% of the operating cost and TAC,respectively. In addition, the space required for the plant can also bereduced dramatically.

Fig. 13. Simplified flow sheet illustrating the proposed sequence.


4. Conclusions

A proposed sequence, including the DWC, was designed andoptimized for a more energy-efficient TCS purification process. Ashortcut method determined a suitable initial structure for theDWC, which was then optimized response surface methodology.The proposed method could be implemented simply and efficientlyin HYSYS and Minitab. The simulated results were in good agree-ment with the predicted values. The use of DWC can achieve manybenefits as well as decrease the TAC significantly. Incorporating aheat pump into the TDWC, which includes internal and externalheat integration, is a very attractive option for improving energysavings. The column grand composite curve was used effectively toindicate the thermodynamic feasibility of the implementation ofheat pump system into the DWC. The TDWC with top vaporrecompression showed 83.01% reductions in energy consumptioncompared to a conventional column sequence. The processcompactness is an additional benefit from DWC application.

Acknowledgments

This study was supported by Basic Science Research Programthrough the National Research Foundation of Korea (NRF) fundedby the Ministry of Education, Science and Technology(2012012532).

Nomenclature

A area [ft2]B bottom product flow [kg/h]BDWC bottom dividing wall columnBC bare cost [$]BMC updated bare module cost [$]Csteam cost of the steam [$]CCW cost of cooling water [$]Celectricity cost of electricity [$]CGCC column grand composite curveD top product flow [kg/h]DWC dividing wall columnDCS dichlorosilanF feed flow [kg/h]

FL internal liquid flows to the prefractionator [kg/h]FV internal vapor flows to the prefractionator [kg/h]HCl hydrochloric acidi fractional interest rateL length [ft]MF module factorMPF material and pressure factorN number of trays [$]n number of years [year]Op operating costRSM response surface methodologyRL liquid split ratioRV vapor split ratioS side product flow [kg/h]STC silicon tetrachlorideTAC total annual cost [$]TCS trichlorosilaneTDWC top dividing wall columnUF update factor

Subscripts and superscriptsL liquidV vapor

Appendix A. Column cost correlations

a. Sizing the column: The column diameter was determined un-der a column flooding condition that fixed the upper limit ofthe vapor velocity. The operating velocity is normally 70e90%of the flooding velocity [50,51]. Here, 80% of the flooding ve-locity was used.

b. Capital cost: Guthrie’s modular method was applied [52]. Theinvestment cost for conventional distillation is the total cost ofthe column and auxiliary equipment, such as reboilers andcondensers. For the DWC, it includes the additional dividingwall cost. In this study, the Chemical Engineering Plant CostIndex of 575.4 was used for cost updating.

Tray stack ¼ ðN � 1Þ � tray spacing (2)


Total height ¼ tray stackþ extra feed space

þ Disengagementþ skirt height (3)

Updated bare module cost ðBMCÞ¼ UF� BC� ðMPFþMF� 1Þ (4)

where UF is the update factor

UF ¼ present cost indexbase cost index

(5)

BC is bare cost.For vessels:

� � � �

BC ¼ BC0 �

LL0

a

� DD0

b

(6)

For the heat exchanger:

BC ¼ BC0 ��AA0

�a(7)

Area of heat exchanger; A ¼ QUDT

(8)

For the compressor:

BC ¼ BC0 ��SS0

�a(9)

where MPF is the material and pressure factor; MF is the modulefactor (typical value), which is affected by the base cost. D, L and Aare the diameter, length and area, respectively. S is the brakehorsepower.

Updated bare module cost for tray stack ðBMCÞ¼ UF� BC�MPF (10)

The material and pressure factor : MPF ¼ Fm þ Fs þ Fm(11)

c. Operating cost (Op):

Op ¼ Csteam þ CCW þ Celectricity (12)

where Csteam is the cost of the steam; CCW is the cost of coolingwater; and Celectricity is the cost of electricity.d. Total annual cost (TAC) [53]

n

TAC ¼ capital cost� ið1þ iÞð1þ iÞn � 1þ Op (13)

where i is the fractional interest rate per year and n is the number ofyears.

Appendix B. Supplementary material

Supplementary material associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.applthermaleng.2013.05.035.

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Design and optimization of thermally coupled distillation schemes for the trichlorosilane purification process1 Introduction1.1 Dividing wall column1.2 Thermal heat pump integration

2 Design and optimization methodology2.1 Design2.2 Optimization

3 Case study3.1 Conventional distillation sequence3.2 Proposed sequence3.2.1 Integrating C-202 and C-2033.2.2 Integrating C-204 and C-2053.2.2.1 Top dividing wall column (TDWC)3.2.2.2 TDWC with top vapor recompression heat pump

3.2.3 Final proposed sequence

4 ConclusionsAcknowledgmentsNomenclatureAppendix A Column cost correlationsAppendix B Supplementary materialReferences