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Chemical Engineering and Processing 62 (2012) 1– 17

Contents lists available at SciVerse ScienceDirect

Chemical Engineering and Processing:Process Intensification

jo u rn al hom epage: www.elsev ier .com/ locate /cep

ynthesis of intensified simple column configurations for multicomponentistillations

en-Guang Rong ∗, Massimiliano Erriconstitute of Chemical Engineering, Biotechnology and Environmental Technology, University of Southern Denmark, Niels Bohrs Alle 1, DK-5230, Odense M, Denmark

r t i c l e i n f o

rticle history:eceived 28 April 2012eceived in revised form 9 October 2012ccepted 13 October 2012vailable online 23 October 2012

eywords:ulticomponent distillation

rocess intensification

a b s t r a c t

The synthesis of intensified simple column configurations (ISC) for multicomponent distillations is pre-sented. The ISC configurations use less columns and less heat exchangers than the traditional distillationconfigurations (TDC), while they keep the similar structural simplicity as the TDC configurations thateach column produces an overhead product with a condenser and a bottoms product with a reboiler.For an N-component zeotropic mixture, an easy-to-use procedure is first formulated to generate the ISCconfigurations from the simple column configurations (SC) with only sharp splits. Then, the procedure isgeneralized to produce the ISC configurations from any traditional distillation configurations (TDC) withboth sharp and sloppy splits. It is demonstrated that the procedure can explicitly modify the TDC configu-

ntensified simple column configurationsystematic procedurehermal coupling

rations step-by-step to systematically generate all the possible ISC configurations. The ISC configurationshave the potential to reduce both energy consumption and capital costs than the TDC configurations, atthe same time, they have the similar structural simplicity in terms of systems design, control and opera-tion as the traditional distillation configurations. Therefore, they constitute an advantageous alternativessubspace when looking for an optimal system for a specific application in both new design and retrofitof distillation plants.

© 2012 Elsevier B.V. All rights reserved.

. Introduction

Distillation has been the widely used separation method and istill the main workhorse in petroleum, chemical, petrochemical andany other process industries. In the transition from fossil-based

eedstocks to biomass-based feedstocks manufacturing, distillations still among the most important separation methods in many newrocess industries, including bioethanol, biodiesel and many otheriochemicals and natural products manufacturing. The reason ishat whenever applicable distillation is advantageous for any fluid

aterials separation especially in large capacity. Therefore, newistillation processes and equipment are very significant technol-

gy for future manufacturing processes, as well as for retrofittinghe existing production processes. On the other hand, to pursueustainable development of our economy and society, we need

Abbreviations: DSS, distinct separation sequence; DWC, dividing wall col-mn; HITC, heat-integrated thermally coupled configuration; ISC, intensified simpleolumn configuration; OTC, original thermally coupled configuration; SC, simple col-mn configuration with sharp splits; TDC, traditional distillation configuration withoth sharp and nonsharp splits; TES, thermodynamically equivalent structure; TSR1,ne way transport-side-rectifier; TSR2, two way transport-side-rectifier; TSS1, oneay transport-side-stripper; TSS2, two way transport-side-stripper.∗ Corresponding author. Tel.: +45 6550 7481; fax: +45 6550 7354.

E-mail address: bgr@kbm.sdu.dk (B.-G. Rong).

255-2701/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.cep.2012.10.005

maximum improve the applied technology to save resources andprotect environment.

Due to the intrinsic characteristics of distillation separations,efforts to improve distillation technology are still in twofold: oneis to reduce the energy consumption and the other is to reduce thecapital investment. This calls for process intensification principlesto achieve intensified distillation systems to save both energy andcapital costs. While in industrial situations, the mixtures to be sepa-rated very often are multicomponent, which usually call for severalcolumns in the distillation processes. The intensified distillationsystems should remain less complex to favor their design, controland operations. As indicated by Caballero and Grossmann [1], inmixtures involving more than three components, the first concernis identifying the space of all the alternative configurations in orderto extract a good (ideally the best) configuration with savings inenergy and/or capital costs without increasing the operational andcontrol complexities.

To separate a multicomponent mixture with N componentsinto N pure products, there need several columns in a distillationconfiguration. The well-known multicomponent distillation con-figurations are called simple column configurations (SC). For an

N-component mixture, there is a certain number of such simplecolumns configurations, which can be predicted from the equa-tion of Thompson and King [2]. Fig. 1 presents the two simplecolumn configurations for a ternary mixture. Such simple column

2 B.-G. Rong, M. Errico / Chemical Engineering and Processing 62 (2012) 1– 17

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ig. 1. The two simple column configurations for ternary separations: (a) directequence and (b) indirect sequence.

onfigurations were widely studied in the early stage of processynthesis [2–7], they are also the popular configurations in indus-rial distillation plants due to their simplicity in design, control andperations.

For an N-component mixture, there need N-1 columns in suchimple column configurations. In each of such simple columnonfigurations, there need N-1 condensers for the N-1 rectifyingections and N-1 reboilers for the N-1 stripping sections. Theseimple column configurations are suffered from high energy con-umption and large capital investment. The question is can wechieve intensified distillation configurations from such simple col-mn configurations which can keep similar structural simplicity inerms of design, control and operations, while they have the poten-ial to save both energy and capital costs? If so, can we develop aystematic procedure to synthesize such intensified simple columnonfigurations for any N-component mixture?

To answer these two questions, let us first see how we canhange the simple column configurations shown in Fig. 1 whichavor both energy savings and capital cost reduction. It is knownhat for the SC configurations, the condensers and reboilers asso-iated with submixtures can be eliminated and replaced withhermal coupling streams, which can produce the original ther-

ally coupled configurations (OTC) [8]. For example, Fig. 2 presentshe two thermally coupled configurations for the two SC configu-ations shown in Fig. 1. For certain cases, such thermally coupledonfigurations can reduce both energy consumption and capitalost [9].

It is also known that after introduced thermal couplings, some ofhe column sections in the OTCs are movable and can be rearrangedo produce thermodynamically equivalent structures (TES) [10]. Forxample, Fig. 3 presents the two TESs for the OTCs shown in Fig. 2.

The distinct feature of the TES structures from the OTCs is that

here exist different side columns connected to the main column.he TES shown in Fig. 3a is the well-known side-rectifier column,n which there is a single-section side column to produce the

1

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Fig. 2. The original thermally coupled configurations for the SCs in Fig. 1.

(b)(a)

Fig. 3. The thermodynamically equivalent structures for the OTCs in Fig. 2.

middle product B, which we call it a product-side-rectifier-column.Also, the TES shown in Fig. 3b is the well-known side-strippercolumn, in which there is a single-section side column to producethe middle product B, which we call it a product-side-stripper-column. We will show in the following that for four-componentmixtures, we can produce TES structures in which there containsnonproduct-side-column which only serves as the transport of theintermediate submixture between the columns, from which wecan produce the intensified simple column configurations.

Similar as for ternary mixtures, the thermal coupling techniquecan be used to change any traditional distillation configurationsfor four or more component mixtures, from which we can producethe original thermally coupled configurations and thermodynami-cally equivalent structures. For example, Fig. 4 presents two SCconfigurations for a four-component distillation, and the follow-ing Figs. 5 and 6 are the generated OTC configurations and TESstructures.

From the TESs in Fig. 6, it is observed that all the side columnsdo not produce any products, they mainly function as transportof the intermediate submixture between the columns. In Fig. 6a,the side-rectifier transports the submixture BC in one-way afterthe condenser, we define it as a one-way transport-side-rectifier(TSR1). While in Fig. 6b, the side-rectifier transports the submix-ture BC in two-way through thermal coupling streams, we define itas a two-way transport-side-rectifier (TSR2). Similarly, in Fig. 6c, theside-stripper transports the submixture BC in one-way after thereboiler, we define it as a one-way transport-side-stripper (TSS1).While in Fig. 6d, the side-stripper transports the submixture BCin two-way through thermal coupling streams, we define it as atwo-way transport-side-stripper (TSS2).

Functionally, the side rectifiers in TSR1 and TSR2 in Fig. 6a andb are for the further rectifying the heaviest component D so that amixture with only intermediate components B and C is transported

to the next column. However, depending the relative volatilities ofthe feed components and product purity requirements, when therelative volatility between C and D is large and separation betweenC and D is easy, a mixture with only B and C can be withdrawn

(b)

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Fig. 4. Two SC configurations for a four-component mixture.

B.-G. Rong, M. Errico / Chemical Engineering and Processing 62 (2012) 1– 17 3

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Fig. 5. The OTC configurations generated from the SCs in Fig. 4.

nd the side column can be eliminated. Similarly, the side strip-ers in TSS1 and TSS2 in Fig. 6c and d are for the further strippinghe lightest component A so that a mixture with only intermedi-te components B and C is transported to the next column. Also,epending the relative volatilities of the feed components androduct purity requirements, when the relative volatility between

and B is large and separation between A and B is easy, a mixtureith only B and C can be withdrawn and the side column can be

liminated. Therefore, in all cases in Fig. 6, the side columns can beliminated and leave the submixture transport directly betweenhe columns. The transport of the submixture is in one-way whenn TSR1 or TSS1 is eliminated as shown in parts a and c of Fig. 7.

orrespondingly, when an TSR2 or TSS2 is eliminated, the transportf the submixture is in two-way as shown in parts b and d of Fig. 7.

For the derived configurations shown in Fig. 7, it is seen thatach column produces two products, one distillate from the top

(b)

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ig. 6. The TES structures containing nonproduct-side column from the OTCs inig. 5.

(d)

Fig. 7. The ISC configurations derived from the TES configurations shown in Fig. 6.

condenser and the other from the bottom reboiler. Therefore, theyhave the similar structural simplicity as the traditional simple col-umn configurations. However, they have reduced both the numberof columns and the number of heat exchangers compared to theircounterparts of SC configurations. Therefore, they fulfill processintensification principle that the systems can accomplish the sameseparation task with a reduced number of equipment units. Wedefine the configurations derived in Fig. 7 as the intensified simplecolumn configurations (ISC).

Synthesis of distillation configurations for multicomponent sep-arations has received considerable efforts in the last decade. Thetask is challenging because there are a large number of fea-sible configurations especially when a feed mixture containingfour or more components. In the literature, different methodshave been proposed to tackle this problem. By modifying asubmixture-network-superstructure for an N-component mixture,Agrawal [11] presented a method to draw multicomponent dis-tillation configurations with N-1 columns. Agrawal [12–14] alsopresented some observations to draw specific distillation con-figurations for four or more component mixtures. Caballero andGrossmann [1,15–17] presented a state-task-network superstruc-ture method to mathematically formulate the alternatives spaceof multicomponent distillation configurations. By first formulat-ing the feasible distinct separation sequences for an N-componentmixture a priori, Rong et al. [18,19] presented a method whichcan synthesize distillation configurations with less than N-1, equalto N-1 or more than N-1 columns. A considerable efforts havealso devoted to study the steady-state and dynamic performanceof some specific configurations [20–24]. The earlier efforts havemade significant progress in the synthesis of distillation configura-tions for a multicomponent separation. However, to the best of ourknowledge, a method aiming at synthesizing the intensified sim-ple column configurations shown in Fig. 7 for any N-componentmixture has not been attempted so far.

We have performed a detail case study for such ISC configura-tions in Fig. 7 [8]. A detail simulation for different cases have shownthat in some cases the side columns just serve as the transportation-purpose in which there is no any composition change in the side

column. While in other cases, a small tune of the number of trays inthe prior column would enough to remedy the elimination of theside column. The performance was also studied in terms of energyand capital cost savings. It was demonstrated that for the mixtures

4 ineering and Processing 62 (2012) 1– 17

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Step 1: Th e subspace of the S imple C olumn Configurations (SC ).

Step 2: Select an SC from the above subs pace

Step 3: Generation of the Original Th ermal ly Coupled Configurations (OTC) for the selected

Step 4: Generation of the Thermodynam ically Equivalent Struc tures (TES) for the OTCs in

Step 5: Id entify the T ES structures whic h fulfi l the two cri teria

Step 6: Generation of the ISC confi guratio ns by el iminating the transport si de col umns in the ide ntified TES s tructures

Step 7: Any remaining SC configurations to be exam ine d? If yes go t o s tep 2, if no go to st ep

Step 8: Summarize all the generated ISC confi gurations

in the SC configurations in Fig. 9, 1,3,5,7 represent the recti-fying sections and 2,4,6,8 represent the stripping sections.

Step 2: Select an SC configuration. Let us select the SC configurationof Fig. 9k.

Table 1The 14 simple column sequences for a five-component mixture.

Sequence no. Simple column sequences

1-(a) ABCD/E(1) → ABC/D(2) → AB/C(3) → A/B(4)2-(b) ABCD/E(1) → ABC/D(2) → A/BC(3) → B/C(4)3-(c) ABCD/E(1) → AB/CD(2) → A/B(3) → C/D(4)4-(d) ABCD/E(1) → A/BCD(2) → B/CD(3) → C/D(4)5-(e) ABCD/E(1) → A/BCD(2) → BC/D(3) → B/C(4)6-(f) ABC/DE(1) → D/E(2) → A/BC(3) → B/C(4)7-(g) ABC/DE(1) → D/E(2) → AB/C(3) → A/B(4)8-(h) AB/CDE(1) → A/B(2) → CD/E(3) → C/D(4)9-(i) AB/CDE(1) → A/B(2) → C/DE(3) → D/E(4)10-(j) A/BCDE(1) → BCD/E(2) → BC/D(3) → B/C(4)

B.-G. Rong, M. Errico / Chemical Eng

tudied, the ISC configurations shown in Fig. 7 have similar or betternergy performance than the traditional simple column configura-ions, most importantly, significant capital cost reductions werechieved in all studied cases [8]. Together with their simplicity inavor of system’s design, control and operations, such ISC systemsre among the most important alternatives for multicomponentistillation in both grassroot design and plant retrofit. Therefore,

t is significant to develop a systematic procedure for synthesis ofuch ISC systems for any number of component mixtures. In theollowing, we will present such a systematic procedure.

. The systematic procedure for synthesis of intensifiedimple column configurations

For four-component mixtures, after examining all the five sim-le column configurations, we found that the configurations shown

n Fig. 7 are the only possible ISC configurations. However, for fiver more component mixtures, the number of simple column con-gurations increases rapidly with the number of components, thus,ore such ISC configurations can be expected. There certainly need

systematic procedure to synthesize such ISC configurations forny N-component mixture. In the following, we will present such

systematic procedure with a five-component mixture as an illus-ration example.

From the derivation of the ISC configurations for quaternaryixtures shown in Fig. 7, it is clear that the ISC configurations are

btained from the corresponding TES structures shown in Fig. 6y directly eliminating the single-section transport side columns.here are two criteria of such TES structures that would finally leado the ISC configurations.

riterion 1: The TES must contain the single-section transport sidecolumns.

riterion 2: Except the transport side columns, each of the othercolumns in the TES must keep the simple column fea-tures that each column produces an overhead productwith a condenser and a bottom product with a reboiler.

Also, from Fig. 6 it is observed that for TES structures fulfill thesewo criteria, there are four cases for the single-section transportide columns which can produce the ISC configurations:

ase 1: A TES structure with one-way transport-side-rectifier(TSR1)

ase 2: A TES structure with two-way transport-side-rectifier(TSR2)

ase 3: A TES structure with one-way transport-side-stripper(TSS1)

ase 4: A TES structure with two-way transport-side-stripper(TSS2)

Therefore, to achieve the possible ISC configurations for an N-omponent mixture, we need systematically examining the TEStructures and identifying those TES structures fulfilling the tworiteria. To do so, a systematic procedure is presented in Fig. 8,rom which the possible ISC alternatives can be synthesized step-y-step. Without further clarification, the systematic procedure isiming at identifying those TES structures that fulfill the above tworiteria, at the same time they contain at least one of the four cases

f the transport side columns.

In the following, the procedure is illustrated for the synthe-is of ISC configurations for a five-component mixture, which ispplicable to a mixture with any number of components.

Fig. 8. The systematic procedure for synthesis of ISCs from simple column configu-rations.

Step 1: The SC configurations for a five-component mixture. Intotal, there are 14 simple column sequences for a five-component mixture which are illustrated in Table 1. Eachsequence consists of four individual splits which are num-bered in the brackets. The 14 corresponding simple columnconfigurations are presented in Fig. 9. The numbering ofthe column sections in a SC configuration is based on theindividual splits of its separation sequence. The first split isalways for the fresh feed. Each individual split designatesone rectifying section and one stripping section. Therefore,

11-(k) A/BCDE(1) → BCD/E(2) → B/CD(3) → C/D(4)12-(l) A/BCDE(1) → BC/DE(2) → B/C(3) → D/E(4)13-(m) A/BCDE(1) → B/CDE(2) → CD/E(3) → C/D(4)14-(n) A/BCDE(1) → B/CDE(2) → C/DE(3) → D/E(4)

B.-G. Rong, M. Errico / Chemical Engineering and Processing 62 (2012) 1– 17 5

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Fig. 9. The 14 simple column configurations for a five-component mixture.

6 B.-G. Rong, M. Errico / Chemical Engineering and Processing 62 (2012) 1– 17

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B.-G. Rong, M. Errico / Chemical Engineering and Processing 62 (2012) 1– 17 7

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Fig. 12. The ISC configurations with one-way t

tep 3: Generate the OTCs for the selected SC configuration.

For any traditional distillation configurations, all of possibleriginal thermally coupled configurations can be generated by

eplacing the heat exchangers associated with submixtures indi-idually or combinatorially [10]. For SC configuration in Fig. 9k,here are three heat exchangers associated with submixtures: theeboiler associated with BCDE, the condenser associated with BCD,

rt from the SCs for a five-component mixture.

the reboiler associated with CD. Totally, 7 OTCs can be generatedwhich are presented in Fig. 10.

Steps 4 and 5: Generate the TES structures and identify the TES

structures fulfilling the two criteria.

For any original thermally coupled configuration, there are acertain number of movable column sections, which are designated

8 B.-G. Rong, M. Errico / Chemical Engineering and Processing 62 (2012) 1– 17

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2

6

3

4

7

8

AB

A

BC

DE

1

23

4

57

8

AB C

D

E

CD

BCDE

ABCDE

(a) (b) (c)

(d) (e)

(g) (h)

B

C

D

A

E

1

2

35

67

8CD

BCD

ABCDE

ABCDE

E

AB

C

D

1

2

4

5

6

7

8BC

BCD

(f)

(i)

C

D

7

8CD

B5

6BCD

E

ABCDE1

2

4

A

ABCDE1

2

A

B

E

3

4

6

7

8

C

D

CD

BCDE

1

2

4

5

6

7

8

A

B

CD

DE

BC

ABCDE

ranspo

bsb7twp

(j) (k)

Fig. 13. The ISC configurations with two-way t

y the introduced thermal couplings. By recombining the columnections through moving the movable section individually or com-inatorially, all of the TES structures can be generated [25]. For the

OTCs in Fig. 10, totally 19 thermodynamically equivalent struc-ures are generated, among which there are four TES structureshich fulfill the two criteria for generation of the ISCs, which areresented in Fig. 11.

(l)E

rt from the SCs for a five-component mixture.

Step 6: Generation of the ISC configurations from the identified TESstructures in step 5. It is obvious that each of the identi-fied TES structures shown in Fig. 11 contains a transport

side column. Fig. 11a contains a one-way transport-side-rectifier (TSR1) (Case 1). Fig. 11b contains a one-waytransport-side-stripper (TSS1) (Case 3). Fig. 11c containsa two-way transport-side-rectifier (TSR2) (Case 2). And

B.-G. Rong, M. Errico / Chemical Engineering and Processing 62 (2012) 1– 17 9

ABC

A

B

C

BC

DEF

D

E

F

ABCDEF

3

41

2

5

6

7

89

10DE DE

ABC

A

B

C

BC

DEF

D

E

F

ABCDEF

3

41

2

5

6

7

89

10

D

EDEF

BC

7

8

ABC 4

A

B

C

F

5

ABCDEF

3

1

2

6

9

10DE

D

7

8

A B

C

BC

F

1

3

2

6

ABCDEF

DE

E

9

10

(a) (b)

(c) (d)

Fig. 14. An ISC configuration for a six-component mixture. (a) SC, (b) OTC, (c) TES, (d) ISC.

S

S

m

Fig. 11d contains a two-way transport-side-stripper (TSS2)(Case 4). By eliminating the transport side columns, the cor-responding ISC configurations are readily generated whichare presented in Fig. 12i and j for the one-way transportISCs, and in Fig. 13i and j for the two-way transport ISCs.

tep 7: Select another SC and repeat from step 2 to step 6 until allof the SC configurations are selected.

tep 8: Summarize all the generated ISC configurations. Fig. 12presents all the ISC configurations with one-way transportof the intermediate submixture, while all the ISC configu-

rations with two-way transport are presented in Fig. 13.

In total, 12 ISC configurations with one-way transport of a sub-ixture between two columns are produced as shown in Fig. 12.

ABCD

2

1 4

3

ABC

A

BCD

5

6 C

8

7BC

B

D

AABCD

2

1 4

3

ABC

A

BCD

5

6 C

8

7BC

B

D(a) (b)

Fig. 15. The early obtained configurations for the separation sequence of e

Exactly, 12 ISC configurations with two-way transport of a sub-mixture between two columns are produced as shown in Fig. 13.Except the difference of one-way or two-way transport of thesame submixture, the corresponding ISC configurations betweenFigs. 12 and 13 are exactly the same structures with the sameindividual columns. However, each one-way transport ISC and itscorresponding two-way transport ISC are produced from their cor-responding TES structures which are produced from the distinctOTC configurations. Revisiting Figs. 4–7, this is easy to understandthat a one-way transport side-rectifier transports a submixture

with a condenser, and this condenser can always be replaced witha two-way thermal coupling that gives the OTC configuration toproduce the two-way transport side-rectifier. Similarly, a one-waytransport side-stripper transports a submixture with a reboiler,

ABCD

2

1 4

3ABC

A

BCD

5

6 C

8

7BC

B

D

BCD

2

1 4

3ABC

A

BCD

5

6 C

8

7BC

B

D(c) (d)

xample 1: (a) TDC (one-way), (b) TDC (two-way), (c) OTC, (d) HITC.

10 B.-G. Rong, M. Errico / Chemical Engineering and Processing 62 (2012) 1– 17

ABCD

2

1

6

5

4

3A

ABC

BCD

D

BC

9

7B

8BC

C

ABCD

2

1

6

5

4

3A

ABC

BCD

D

BC

9

7B

8BC

C

ABCD

2

1

6

5

4

3A

ABC

BCD

D

BC

9

7B

8BC

C

ABCD

2

1

6

5

4

3

D

ABC

BCDC

BC

9

7

A

8BC

BABCD

2

1

6

5

4

3A

ABC

BCD

D

BC

9

7B

8BC

C

(a) (b) (c)

(d) (e)

TC. (a

act

sacfi

wrcnep

Fig. 16. Traditional distillation configuration and its O

nd this reboiler can always be replaced with a two-way thermaloupling that gives the OTC configuration to produce the two-wayransport side-stripper.

For any N-component mixture, there are a certain number ofimple column configurations [1]. Therefore, it can be claimed thatpplying the procedure presented in Fig. 8, all the possible ISConfigurations can be systematically generated from the SC con-gurations for a mixture with any number of components.

Comparing the ISC configurations shown in Figs. 12 and 13ith the SC configurations in Fig. 9, each ISC configuration has

educed one column and reduced two heat exchangers than the

orresponding SC. However, for feed mixture with more compo-ents, the ISC configurations can reduce more columns and heatxchangers from the corresponding SC configurations. For exam-le, for the separation sequence for a six-component mixture:

ABCD

2

1

6

5

4

3A

ABC

BCD

D

BC

9

7B

8BC

C

ABCD

2

1

6

5

4

3A

ABC

BCD

D

BC

9

7B

8BC

C

AB

Fig. 17. The TES structures obtain

) Traditional distillation configuration, (b)–(e) OTCs.

ABC/DEF(1) → A/BC(2) → DE/F(3) → B/C(4) → D/E(5) (the numberin bracket is the order of the individual splits, which designate thenumbering of columns sections in its distillation configuration.),Fig. 14 showed that the ISC configuration can reduce two columnsand four heat exchangers compared to the SC configuration.

2.1. Structural features of the ISC configurations from SCs withonly sharp splits

As discussed above, the similarity between the ISC configura-tions and their corresponding SC configurations exist in that each

column produces an overhead product with a condenser and abottoms product with a reboiler. However, the ISC configurationsare generated by modifying the corresponding SC configurations,they have some different features from the SCs. From the ISC

CD

2

1

6

5

4

3A

ABC

BCD

D

BC

9

7B

8BC

C

B

ABCD

2

1

6

5

4

3A

ABC

BCD

D

BC

9

7

8BC

C

ed from the OTCs in Fig. 16.

B.-G. Rong, M. Errico / Chemical Engineering and Processing 62 (2012) 1– 17 11

ABCD

2

1

6

3

A

D

BC

9

7 B

8

BC

C

ABCD

2

1

6

3

A

D

BC

9

7 B

8

BC

C

ABCD

2

1

6

3

A

D

BC

9

7 B

8

BC

C

ABCD

2

1

6

3

A

D

BC

9

7 B

8

BC

C

gurat

cs

a

b

de

stt

3t

swtFsmsr

Fig. 18. The derived ISC confi

onfigurations derived in Figs. 7, 12, 13 and 14d, the followingtructural features can be observed:

) An ISC configuration uses less than N-1 columns for an N-component distillation.

) Each column has an overhead condenser and a bottom reboiler.c) There exist submixtures transferring between intermediate

locations of columns. An intermediate submixture can transportbetween two columns in one-way or in two-way.

) A column in an ISC can have more than two sections.) Each product is obtained once in an ISC.

Now, taking these structural features into consideration, we willhow that the procedure in Fig. 8 can be generalized to synthesizehe ISC configurations from the traditional distillation configura-ions containing both sharp and sloppy splits.

. A generalized procedure for synthesis of ISCs from anyraditional distillation configurations

It is known that the SC configurations are obtained from theeparation sequences with only sharp splits. A separation sequenceith solely sharp splits constitutes a unique distillation configura-

ion in which the individual columns are arranged sequencially (seeig. 9). It is also known that we can formulate distinct separation

equences containing nonsharp (sloppy) split(s) to accomplish aulticomponent separation. For example, 22 functionally distinct

eparation sequences were formulated for a four-component sepa-ation, among which there are 17 separation sequences containing

1

2

ABCDE

3

4

5

6

7

8

9

10

A

D

E

B

C

ABCD

ABC

BCD

BC

A

((a)

Fig. 19. Initial traditional distillation configurations: (a) TDC with 4 co

ions from the TESs in Fig. 17.

at least one sloppy split [18,26]. There we have demonstratedthat from each of the functionally distinct separation sequences,a traditional distillation configuration (TDC) and its possible OTCconfigurations can be generated. It must be indicated that, for aformulated separation sequence containing sloppy split(s), thereexist flexibility to combine the individual columns for the individ-ual splits. For an N-component mixture, depending on the differentways to combine the individual columns, a traditional distillationconfiguration (TDC) from a formulated separation sequence con-taining sloppy split(s) can have its final structure with less than N-1,or exactly N-1, or more than N-1 columns [18,19]. As long as the wayto combine the individual columns is determined, the traditionaldistillation configuration is readily obtained, and subsequently, allof its possible OTCs and TESs can be produced [18,19,25]. So far,no attempt has been made to generate the ISC configurations fromthe different TDC configurations with sloppy splits. Now, the ques-tion is can the systematic procedure be generalized to produce theISC configurations from the traditional distillation configurationscontaining sloppy split(s)? In the following, we will answer thisquestion with two examples, which showed that the systematicprocedure can be generalized to produce the ISC configurationsfrom any TDC configurations.

3.1. Example 1: generation of ISC configurations fromfour-component TDC containing sloppy splits

For a four-component mixture ABCD, let us examine one of the22 functionally distinct separation sequences [18]. The distinct sep-aration sequence with the first sloppy split for the feed mixture

A

E

ABCD1

2

BCDE 4

3

7

6

5

ABC

BCD

D

11

9B

10BC

C8

BC

b)

lumns (equal to N-1), (b) TDC with 5 columns (more than N-1).

12 B.-G. Rong, M. Errico / Chemical Engineering and Processing 62 (2012) 1– 17

5

6

7

8

9

104

ABCDE

3

1

2

A

E

B

D

C

BC

ABC

ABCD

BCD

ABC5

6

7

8

9

10

A

B

D

C

BC

BCD

E

ABCDE

3

1

2

ABC5

6

7

8

9

10

A

ABCDE

3

1

2

E

B

D

C

BC

BCD

3

1

2

A

D

6

5

7

8ABCDE

9

10

B

C

E

BC

ABC

ABCD

BCD

4

A

D

E

B

C

ABCD

ABC

BCD

BCABCDE

3

4

5

6

7

8

9

101

2

E

ABCDE

D

B

C

ABC

BCD

BC

5

6

7

8

9

10

3

41

2

ABCD

A

(b)(a) (c)

(d) (e) (f)

(f) ISC

(svclsmrongmTgmTctiWtnctpt

veidfht

Fig. 20. (a)–(b) OTCs, (c)–(d) TESs, (e)–

ABCD), then the sharp splits of A/BC and BC/D, finally the sharpplit of B/C: ABCD(1) → A/BC(2) → BC/D(3) → B/C(4). The four indi-idual splits will designate four individual columns each with aondenser and a reboiler. Conventionally, in order to have a distil-ation configuration with exactly the same number of columns asimple column configurations (N-1 columns for an N-componentixture), some of the individual columns can be combined when a

eboiler of one column has the same product (a single componentr a submixture) as a condenser of another column. Such combi-ation of individual columns can through heat-integration whichives the traditional distillation configuration with one-way com-unication of the intermediate submixture as shown in Fig. 15a.

he combination can also through thermal coupling streams whichive the traditional distillation configuration with two-way com-unication of the intermediate submixture as shown in Fig. 15b.

he other condensers and reboilers associated with submixturesan then be replaced with thermal couplings which give the originalhermally coupled configuration as shown in Fig. 15c and the heat-ntegrated thermally coupled configuration as shown in Fig. 15d.

e have made the combination of the individual columns for all ofhe synthesized distillation configurations for four or more compo-ent mixtures in our earlier works [18,19]. Note that for the TDConfigurations in Fig. 15a and b, when examining the TES struc-ures for their OTCs in Fig. 15c and d, there is no opportunity toroduce the ISC configurations with less than N-1 columns (lesshan 3 columns).

As discussed above, the earlier strategy to combine the indi-idual columns through heat-integration or thermal coupling wasmployed determinedly as it can reduce the number of columnsn the final distillation configurations, which did give the final

istillation configurations with N-1 or less than N-1 columnsor an N-component separation [19]. However, we will showereby that leaving the individual columns stand alone in theraditional distillation configuration will give the flexibility in the

configurations for the TDC of Fig. 19a.

thermodynamically equivalent structures, which will create theunique opportunity to synthesize the ISC configurations.

Fig. 16a presents the traditional distillation configuration for thesame separation sequence of Fig. 15a. Each separation split uses onecolumn with a condenser and a reboiler. Column 1 for the sloppysplit ABCD, column 2 for the sharp split A/BC, column 3 for thesharp split BC/D, and finally column 4 for the split B/C. Comparedto Fig. 15a, column 2 and column 3 are not combined, rather theystand alone with their respective reboiler BC and condenser BC.Obviously, reboiler BC and condenser BC produce the same sub-mixture BC and need to enter into the same column 4. However,the two streams of BC submixture are produced from two differ-ent columns, when their composition difference is significant, it isthermodynamically more efficient that they enter into two differ-ent locations of column 4. The one rich in B enters to the uppersection 7, and the one rich in C enters to the lower section 8, whichmakes column section 9 between the two feed locations. For suchcase, this section 9 can prevent remixing of the two BC streamswhich will reduce the number of stages needed in column 4. Whenthe two streams of BC submixture have similar compositions, theycan enter into the same location of column 4, which makes columnsection 9 unnecessary.

Obviously, compared to Fig. 15a, the traditional configuration ofFig. 16a has one more column. Remember, this initial configurationjust serves as the starting point, from which we will derive the ISCconfigurations. As mentioned above, the configuration in Fig. 16awill give flexibility to produce TES structures, from which the finalISC configurations will be produced. Again, we need first generatethe OTC configurations, which are straightforward by replacing theheat exchangers associated with submixtures with thermal cou-

pling streams. Fig. 16b is the OTC by replacing condenser ABC andreboiler BCD of Fig. 16a. The reboiler BC and condenser BC of Fig. 16acan be replaced with thermal coupling streams individually orsimultaneously, which generated the OTCs in Figs. 16(c)–(e). Now,

B.-G. Rong, M. Errico / Chemical Engineering and Processing 62 (2012) 1– 17 13

E

ABCDE

ABCD

4

3

8

7

6

5A

ABC

BCD

D

BC

11

9B

10BC

C

1

2

E

ABCD4

3

8

7

6

5A

ABC

BCD

D

BC

11

9B

10BC

C

1

2

ABCDE4

3

8

7

6

5A

ABC

BCD

D

BC

11

9B

10BC

C

ABCD1

2

ABCDE

E

ABCD1

2

ABCDE 4

3

8

7

6

5

D

ABC

BCDC

BC

11

9

A

10BC

B

E

4

3

8

7

6

5A

ABC

BCD

D

BC

11

9B

10BC

CE

ABCD

ABCDE 1

2

4

3

8

7

6

5A

ABC

BCD

D

BC

11

9B

10BC

C

ABCD1

2

ABCDE

E

E

ABCDE

ABCD

4

3

8

7

6

5A

ABC

BCD

D

BC

11

9B

10BC

C

1

2

4

3

8

7

6

5A

ABC

BCD

D

BC

11

9B

10BC

CE

ABCD

ABCDE 1

2

(a) (b) (c)

(d) (e) (f)

(g) (h)

ations

fitaltItIi

dgaatscvflwes

Fig. 21. The OTC configur

rom the OTCs, we can generate the TES structures which are shownn Fig. 17 respectively. From the TES structures, we can observehat there exist the single-section transport side columns whichre the same four cases of transport side columns summarized ear-ier. Therefore, they can be eliminated and leaving the submixturesransporting directly between the columns, which generated theSC configurations as shown in Fig. 18. Similarly, the one-way orwo-way transports of the submixture between the columns in theSC configurations are determined by the type of the side columnsn the TES structures.

From this example, it is shown that, starting from a traditionalistillation configuration with sloppy split(s), the procedure can beeneralized to produce the ISC configurations and the major stepsre the same as for the SC configurations. From this example, welso learned that it is not an issue whether the initial traditional dis-illation configuration has N-1 columns or more than N-1 columnsince it just serves as a starting point to derive the new intensifiedonfigurations. For this case example, it is better to keep the indi-idual columns stand alone (more than N-1 columns), which gives

exibility with more opportunity to derive intensified systemsith less than N-1 columns. To further confirm this generalization,

xample 2 is selected for a separation sequence containing sloppyplits for a five-component mixture.

from the TDC of Fig. 19b.

3.2. Example 2: generation of ISC configurations from afive-component TDC containing sloppy splits

For a five-component mixture, following the systematic proce-dure in Fig. 8, the whole set of the ISC configurations was derivedfrom the subspace of the SC configurations which were shown inFigs. 12 and 13. Again, like four-component mixtures, except the 14separation sequences with only sharp splits, there are a large num-ber of functionally distinct separation sequences containing sloppysplits for five-component separations. In our earlier works, we haveformulated 719 functionally distinct separation sequences [26],later, with additional constraints we have derived 569 function-ally distinct separation sequences for a five-component separation[27]. When combining the individual columns for the individualsplits in the separation sequences to produce the traditional dis-tillation configurations, similar as for four-component mixture,depending on the ways to combine the individual columns, thetraditional distillation configurations can have less than N-1 (lessthan 4), exactly N-1 (equal to 4) or more than N-1 (more than

4) columns [19,26,27]. The objective of example 2 is to exam-ine that for any functionally distinct separation sequence for afive-component mixture, whether the procedure can be general-ized to generate the ISC configurations. To do so, let us pick up

14 B.-G. Rong, M. Errico / Chemical Engineering and Processing 62 (2012) 1– 17

E

ABCD

4

8

7

6

A

ABC

BCD

D

BC

11

9B

10BC

C

1

2

3

5

ABCDE

E

ABCD

4

8

7

6

A

ABC

BCD

D

BC

11

9B

10BC

C

1

2

3

5

ABCDE

E

ABCD

4

8

7

6

A

ABC

BCD

D

BC

11

9B

10BC

C

1

2

3

5

ABCDE

E

ABCD

4

8

7

6

A

ABC

BCD

D

BC

11

9B

10BC

C

1

2

3

5

ABCDE

(a) (b) (c)

4

3

8

7

6

5A

ABC

BCD

D

BC

11

9B

10BC

C

ABCD

ABCDE1

2

E

4

3

8

7

6

5A

ABC

BCD

D

BC

11

9B

10BC

C

ABCD1

2

ABCDE

E

4

3

8

7

6

5A

ABC

BCD

D

BC

11

9

B

10BC

C

ABCD1

2

ABCDE

E

B

4

3

8

7

6

5A

ABC

BCD

D

BC

11

9

10BC

C

ABCD1

2

ABCDE

E

(d) (e) (f)

(g) (h)

om th

aAiaf

itstwctFa5a5cma

Fig. 22. The TES configurations fr

separation sequence which is composed of five individual splits:BCD/E(1) → ABCD(2) → A/BC(3) → BC/D(4) → B/C(5). Correspond-

ngly, there will need five individual columns each with a condensernd a reboiler: column 1 for ABCD/E, column 2 for ABCD, column 3or A/BC, column 4 for BC/D, and finally column 5 for B/C.

Again, depending on the determined ways to combine thendividual columns, there will have different initial traditional dis-illation configurations with different number of columns. We willhow hereby that if the objective of the initial distillation configura-ions is to serve as a starting point to generate the ISC configurationsith fewer columns, it is significant to determine the way to

ombine the individual columns. Fig. 19 presents two initial tradi-ional distillation configurations for the above separation sequence.ig. 19a contains exactly 4 columns (equal to N-1) due to columns 3nd 4 are combined as a single column. However, Fig. 19b contains

columns (more than N-1) due to columns 3 and 4 are standinglone. Again, the feed locations of the two BC streams to column

depend on their compositions. Next, start from each of the TDConfigurations in Fig. 19, we will show how their respective ulti-ate ISC configurations can be generated by generalization of the

bove procedure.

e corresponding OTCs of Fig. 21.

Let us start from the TDC with 4 columns (equal to N-1) ofFig. 19a, two OTCs are generated as shown in Fig. 20a and b. TheirTES structures are generated as shown in Fig. 20c and d. Obviously,these two TES structures fulfill the two criteria that there exists thesingle-section transport side column, and each of the other columnskeeps the simple column features that each column produces anoverhead product with a condenser and a bottom product witha reboiler. As expected, the transport side columns can be elimi-nated and the corresponding ISC configurations are then producedas shown in Fig. 20e and f. Note that the other possible OTCs andtheir TES structures are not shown in Fig. 20 since they cannot pro-duce the ISC configurations. As a result, only two ISC configurationsare generated from the TDC of Fig. 19a.

Now, let us examine the TDC with 5 columns (more than N-1) shown in Fig. 19b. Again, first the OTCs are generated whichare shown in parts a to h of Fig. 21. Then, their corresponding TESstructures are generated which are shown in parts a to h of Fig. 22.

These TES structures are all fulfill the two criteria to generate thecorresponding ISC configurations. Finally, the corresponding ISCconfigurations are produced which are shown in parts a to h ofFig. 23. Also, the other possible OTCs and their TES structures which

B.-G. Rong, M. Errico / Chemical Engineering and Processing 62 (2012) 1– 17 15

E

8

7

A

BCD

D

BC

11

9B

10BC

C

1

2

3

5

ABCDE BC

8

7

A

BCD

D

9B

C

1

2

3

5

ABCDE

E

BC

11

10

E

8

7

A

BCD

D

BC

11

9B

10BC

C

1

2

3

5

ABCDE

E

8

7

A

BCD

D

BC

11

9B

10BC

C

1

2

3

5

ABCDE

4

3

8

5

A

D

BC

11

9 B

10

BC

C

ABCD1

2

ABCDE

E

4

3

8

5

A

D

BC

11

9 B

10

BC

C

ABCD1

2

ABCDE

E

4

3

8

5

A

D

BC

11

9 B

10

BC

C

ABCD1

2

ABCDE

E

4

3

8

5

A

D

BC

11

9 B

10

BC

C

ABCD1

2

ABCDE

E

(a) (b) (c)

(d) (e) (f)

(g) (h)

om th

cITchlFs

wriuumwpt

ta

Fig. 23. The ISC configurations fr

annot produce ISC configurations are not shown in Figs. 21 and 22.n total, 8 ISC configurations are generated from the TDC of Fig. 19b.herefore, compared to the initial TDC shown in Fig. 19a, leavingolumns 3 and 4 standing alone in the initial TDC shown in Fig. 19bas created more opportunities to generate the intensified distil-

ation systems. Note that also, the respective ISCs generated fromig. 19a and b have the same number of 3 columns in their finaltructures (less than N-1).

From this example, again it is demonstrated that the TDCsith more than N-1 columns should not be prematurely excluded,

ather they are the preferred alternatives to generate the ultimatentensified distillation systems with fewer columns. The reason isndoubtedly that they have kept the flexibility to rearrange the col-mn sections in their subsequent TES structures, which can createore opportunities to produce the intensified distillation systemsith fewer columns. It is also once again demonstrated that therocedure can be generalized to produce the ISC configurations and

he major steps are the same as for the SC configurations.

Summarize all the cases in the above sections, we have learnedhat starting from any TDC there are the same two major steps topproach the final ISCs: the step to generate the OTCs from the

e corresponding TESs of Fig. 22.

TDCs, and the step to generate the TESs from the OTCs. It is a uni-versal mechanism to produce the corresponding OTCs for any TDC[10,18]. It is also a universal mechanism to produce the correspond-ing TESs for any OTC [10,25]. Therefore, it can be claimed that theprocedure is readily generalized to produce the ISC configurationsfor any TDC of a formulated distinct separation sequence. Finally, aflowchart is formulated for the generalized procedure as illustratedin Fig. 24.

3.3. Structural features of the ISC configurations from TDCscontaining nonsharp splits

Reviewing the structural features of the ISC configurations sum-marized in Section 2.1, it is observed that the ISCs generated fromthe TDCs including nonsharp splits have the same structural fea-tures as the ISCs generated from the SCs with only sharp splits.A final issue is noteworthy that the two-way transport ISC con-

figurations have no structural degrees of freedom to rearrange thecolumn sections. This is easy to understand that, unlike the thermalcoupling two-way communications in OTCs, the two-way transportof a submixture in an ISC is created though eliminating the side

16 B.-G. Rong, M. Errico / Chemical Engineering and Processing 62 (2012) 1– 17

Step 1. Formulate a distinct separation sequence (DSS) by introducing a distinct set

of intended individual splits for the given mixture, assign an individual column for

each of the intended individual splits (both sharp and sloppy splits).

Step 2. Determine the traditional distillation configurations (TDCs) by sequencing or

combining the individual columns for the introduced individual splits in step 1.

Step 3. Select a TDC from the generated TDCs in step 2 for the formulated distinct

separation sequence.

Step 4. Generate the original thermally coupled configurations (OTCs) for the

selected TDC in step 3.

Step 0. Given an N-component mixture which does not form azeotropes during

distillation separation.

Step 5. Generate the thermodynamically equivalent structures (TESs) for the OTCs

generated in step 4.

Step 6. Identify the TES structures among the TESs generated in step 5 that fulfill

the two criteria.

Step 7. Generate the intensified simple configurations (ISCs) by eliminating the

transport side columns in the identified TES structures in step 6. Store the generated

ISCs in step 10.

Step 8. Select a remaining TDC and repeat steps from 4 to 7 until all the generated

TDCs in step 2 for the formulated distinct separation sequence have been selected.

Step 9. Do you want to examine another distinct separation sequence (DSS)? If yes

go to step 1, if no go to step 10. Repeat this cycle until all the feasible separation

sequences have been examined.

Step 10. Summarize all the generated ISC configurations and stop.

inten

ct

4

csaasccctlacc

Fig. 24. The generalized procedure for systematic synthesis of the

olumns. Therefore, the ISCs with two-way transports of submix-ures cannot produce thermodynamically equivalent structures.

. Conclusions

A systematic procedure for synthesis of the intensified simpleolumn configurations for multicomponent distillation is pre-ented. Each column in an ISC produces an overhead product with

condenser and a bottoms product with a reboiler. The key mech-nism to derive such ISC configurations is to identify those TEStructures in which there exist transport-side-columns. The ISConfigurations are generated by eliminating the transport sideolumns and leave the submixtures directly transport between theolumns. The ISC configurations use fewer columns than the tradi-ional distillation configurations. Furthermore, they keep the simi-

ar structural simplicity as the traditional distillation configurationsnd are amenable to system’s design, control and operations. Forertain cases, they can reduce both energy consumption and capitalosts compared to the traditional distillation configurations.

sified simple column configurations for an N-component mixture.

The procedure is simple and easy-to-use. It is straightforwardto explicitly modify the distillation configurations step-by-step. Itcan generate the ISC configurations from the traditional distillationconfigurations with both sharp and nonsharp splits. In this work, inorder to generate the ISC configurations, the flexibility to combinethe individual columns in the traditional distillation configurationsis also explored. It is found that, for an N-component mixture, theTDCs with more than N-1 columns can create more opportunities togenerate the ISC configurations. Therefore, when the TDCs are serv-ing as the starting point to generate the final ISCs, it is necessary thatthe TDCs with both N-1 and more than N-1 columns be included.This will guarantee that all of the possible ISC configurations aresynthesized.

It can be concluded that the ISC configurations constitute anew subspace which is unique in terms of intensified struc-

tures and simplicity compared to the existing subspaces formulticomponent distillations, for example, SCs, TDCs, OTCs, TESs,dividing-wall-column (DWCs), side-draw-product schemes, heat-integrated schemes, among others. The focus of this paper has

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B.-G. Rong, M. Errico / Chemical Eng

een on the systematic procedure and the generation of this newubspace. Future efforts will focus on the development of heuris-ics as well as screening and optimization methods in terms ofoth steady-state and dynamic performance by comparing withhe existing subspaces, which are important information for opti-

al design, control, and operation of such ISC configurations forpecific applications.

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