integration in design of reactive distillation columns

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Chemical Engineering Science 64 (2009) 3498 -- 3509

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Chemical Engineering Science

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Towards further internal heat integration in design of reactive distillation columns—

Part IV: Application to a high-purity ethylene glycol reactive distillation column

Fanghong Zhu, Kejin Huang ∗, Shaofeng Wang, Lan Shan, Qunxiong Zhu

College of Information Science and Technology, Beijing University of Chemical Technology, Beijing 100029, People's Republic of China

A R T I C L E I N F O A B S T R A C T

Article history:

Received 19 June 2008

Received in revised form 16 March 2009Accepted 23 April 2009

Available online 3 May 2009

Keywords:

Reactive distillation

Ethylene glycol

Internal heat integration

Process design

Process dynamics

Process operation

In the first three papers of this series, it has been shown that strengthening internal heat integration

within a reactive distillation column involving reactions with high thermal effect is really effective for the

reduction of utility consumption and capital investment besides the improvement in process dynamicsand operation. One important issue that remains unstudied so far is the influences of reaction selectivity

upon the reinforcement of internal heat integration, since the reaction operation is often accompanied by

side-reactions and the maintenance of a high selectivity is extremely necessary in process synthesis and

design. A reactive distillation column synthesizing high-purity ethylene glycol through the hydration of

ethylene oxide is chosen and studied in this work. Because of the unfavorable physicochemical properties

of the reacting mixture separated (e.g., the fairly large volatility between the reactants and the existence

of a consecutive side-reaction), the process represents a challenging problem for the reinforcement of in-

ternal heat integration. Intensive comparison is conducted between the process designs with and without

the consideration of further internal heat integration between the reaction operation and the separation

operation involved, and it has been found that seeking further internal heat integration still leads to a

substantial reduction of energy requirement, in addition to a further abatement in capital investment.

Moreover, a favorable effect is again observed upon the process dynamics and operability. These striking

outcomes manifest evidently that seeking further internal heat integration should be considered in pro-

cess synthesis and design irrespective of what a reaction selectivity has been assigned.

© 2009 Elsevier Ltd. All rights reserved.

1. Introduction

In the early papers of this series, the static and dynamic effect of

seeking further internal heat integration between the reaction oper-

ation and the separation operation involved was addressed in terms

of two hypothetical ideal reactive distillation columns involving, re-

spectively, a highly exothermic and a highly endothermic reactions

(A+B↔ C+D) and a reactive distillation column synthesizing methyl

tertiary butyl ether (MTBE) from isobutylene and methanol (Huanget al., 2005, 2006, 2008). In addition to a substantial enhancement

in thermodynamic efficiency and reduction of capital investment,

process dynamics and operability was found to be improved as well

when compared with the process designs without the consideration

of further internal heat integration. One important issue that re-

mains unstudied so far is the influences of reaction selectivity upon

the reinforcement of internal heat integration, since the reaction

operation is often accompanied by several side-reactions and the

∗ Corresponding author. Tel.: +86 10 64434801; fax: +8610 64437805.

E-mail address: [email protected] (K. Huang).

0009-2509/$- see front matter © 2009 Elsevier Ltd. All rights reserved.

doi:10.1016/j.ces.2009.04.031

maintenance of a high selectivity is extremely necessary in process

synthesis and operation. Reaction selectivity is a key performance

index that evaluates the competition between the main- and side-

reactions in a process design. The stringent requirement on the re-

action selectivity might strongly affect process synthesis and design,

including certainly the configuration for internal heat integration. To

address this issue systematically, in this work we choose to study a

reactive distillation column synthesizing high-purity ethylene glycol

through the hydration of ethylene oxide. The process features irre-versible reactions with a large amount of thermal heat released (i.e.,

HR / HV ≈ 2.0 at the atmosphere pressure) and unfavorable physic-

ochemical properties (e.g., the fairly large relative volatility between

the reactants and the existence of a consecutive side-reaction). It

appears therefore to be a challenging system for examining the fea-

sibility and effectiveness of seeking further internal heat integration

upon process synthesis, design and operation.

There have already been many papers published on the synthe-

sis, design and operation of the ethylene glycol reactive distillation

columns. Okasinski and Doherty (1998) addressed the synthesis

and design of an ethylene glycol reactive distillation column and

presented useful insight into the process development. Several

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3500 F. Zhu et al. / Chemical Engineering Science 64 (2009) 3498 -- 3509

can facilitate internal heat integration between the reaction opera-

tion and the separation operation involved. It is worth mentioning

here that the distribution of catalyst is, in principle, valid for both

kinetically-controlled and equilibrium-limited reactions. However,

in the case of an equilibrium-limited reaction, if its reaction velocity

is so fast that chemical equilibrium can be reached instantaneously

on all reactive stages, then the distribution of catalyst presents no ef-

fect upon system performance, because the amount of catalyst influ-

ences both forward and backward reactions equally and gives no neteffect upon reaction equilibrium. In most cases, a sequential utiliza-

tion of these three methods (i.e., in a way similar to a single-variable

based searching method) could provide higher flexibility and more

advantages than each of them in consideration of further internal

heat integration between the reaction operation and the separation

operation involved and is therefore highly recommended during the

synthesis and design of reactive distillation columns involving reac-

tions with high thermal effect.

A unified notation, N r /N rea(n2) /N s(n1)∗, is employed to represent

different process designs with and without the consideration of fur-

ther internal heat integration throughout this work. N r , N rea, and N ssignify the number of stages in the rectifying, reactive and strip-

ping sections, respectively. The numbers in the parentheses, n1 and

n2, stand for the additional arrangement of reactive stages onto thestripping section and the movement of feed location of the domi-

nant feed in the reactive section. The asterisk, ∗, indicates a process

design finished after the redistribution of catalyst in the reactive sec-

tion. It is further stipulated that the dashed lines represent the static

and dynamic responses of the reactive distillation column developed

through the reinforcement of internal heat integration between the

reaction operation and the separation operation involved, and the

solid lines the static and dynamic responses of the process design

with the reactive section strictly between the rectifying section and

the stripping section (i.e., without the reinforcement of internal heat

integration). The black curves show the positive responses, and the

gray curves the negative responses.

3. Process description

Ethylene glycol (C2H6O2) is an industrially relevant product used

mainly as an anti-freezer in coolers, e.g., in motor vehicles. It is

usually produced from the reaction of ethylene oxide (C 2H4O) and

water (Eq. (1)). Ethylene oxide can react further with ethylene glycol

to produce the unwanted by-product diethylene glycol (C 4H10O3)

(Eq. (2)). Both reactions are highly exothermic and occur at moderate

temperatures, allowing production via a reactive distillation column.

In the case of a stoichiometric mole ratio between the two reactants,

a reactive distillation column can offer a higher selectivity than the

other types of reactors, e.g., a plug flow reactor (Corrigan and Miller,

1968; Stein et al., 1999). Owing to the sharp difference in relative

volatilities between the reactants: ethylene oxide and water, and

their near stoichiometric ratio in feed flow rates, the ethylene glycolreactive distillation column is often designated to operate in a total

refluxmode, andrectifying section is not necessary. Water is fed onto

the top of the reactive distillation column in order to facilitate the

suppression of the side-reaction, and the system can operate at the

atmosphere or even higher pressures. Although a higher operating

pressure favors the acceleration of the reaction velocity (Okasinski

and Doherty, 1998), additional expenditures are needed on fixed

investment and operating cost. The physiochemical properties of

the ethylene glycol reaction system can be found from Ciric and Gu

(1994) and Cardoso et al. (2000), and some of them are tabulated in

Table 1.

C2H4O(EO)+H2O(W ) → C2H6O2(EG)

HR =−80.0× 10

3

kJ / kmol

FH2O=26.3 kmol/h

2

10

14

FEO = 27.53 kmol/hVn = 668.99 kmol/h

B = 26.3 kmol/h

xEO = 0.0000

xW= 0.0026

xEG = 0.9480

xDEG = 0.0494

L1 = 721.88 kmol/h

Fig. 1. Design of the high-purity ethylene glycol reactive distillation column without

the consideration of further internal heat integration.

r (kmol m−3

s−1

)= 3.15× 1012

exp[−9547 /T ] xEO xH2O (1)

C2H4O(EO)+ C2H6O2(EG) → C4H10O3(DEG)

HR =−13.1× 103 kJ / kmol

r (kmol m−3 s−1)= 6.3× 1012 exp[−9547 /T ] xEO xEG (2)

For the simplification of simulation analysis, the following as-

sumptions have been made in this study:

(i) Vapor and liquid phases are in equilibrium on all stages.

(ii) Liquid phase is always homogeneous and no reaction occurs in

the vapor phase.

(iii) Sensible heat is negligible and the enthalpy of vaporization is

independent of composition and temperature.

(iv) No pressure drop is assumed across each stage.

In terms of the principle of mass and energy conservation in con-

junction with the given vapor–liquid equilibrium relationship, the

static and dynamic models of the high-purity ethylene glycol reac-

tive distillation column have been developed. Within the former, a

modified Newton–Raphson method is employed as the nonlinear

equation solver, and the satisfaction of component mass balance

equations on each stage as well as the attainment of the bottom

product specification is taken as the convergence criterion. Once

the composition of ethylene glycol in the bottom product has been

given, the heat duties of reboiler and condenser can be readily esti-

mated. Note that thespecification of thebottom composition of ethy-

lene glycol determines actually the reaction selectivity because the

bottom product is mainly composed of two components: ethyleneglycol and diethylene glycol (c.f., Fig. 1). By virtueof the latter, the dy-

namic behavior and controllability of the high-purity ethylene glycol

reactive distillation column can be evaluated with reference to the

given steady state operating condition in addition to the above static

calculation.

4. Process synthesis and design without the consideration of

further internal heat integration

Fig. 1 shows a basic process design of the high-purity ethylene

glycol reactive distillation column, 0/9/4. It has totally 13 stages be-

sides a total condenser (designated as stage 1 hereinafter) at the

top and a partial reboiler (designated as the last stage n hereinafter)

at the bottom. The reactive section is located above the stripping



F. Zhu et al. / Chemical Engineering Science 64 (2009) 3498 -- 3509 3501

Table 1

Physical properties and nominal operating conditions of the high-purity ethylene

glycol reactive distillation column.

Parameters Values

System pressure (MPa) 0.101

Feeding stage of water 2

Feed flow rate of water (kmolh−1) 26.3

Feed flow rate of EO (kmol h−1) 27.53

The bottom product composition of EG

(mole fraction)

0.948

Latent heat (kJkmol−1) 40×103

The maximum limit of liquid holdup (m3) 0.1

Condenser holdup (kmol) 30

Reboiler holdup (kmol) 30

Reaction rate (kmol m−3 s−1)

Main-reaction 3.15×1012 exp[–9547/T ] xEO xW

Side-reaction 6.3×1012 exp[–9547/T ] xEO xEG

Heat of reaction (kJ kmol−1)

Main-reaction −80.0×103

Side-reaction −13.1×103

Vapor–liquid equilibrium constants at

the atmosphere pressure

EO 71.9 Exp[5.720(T −469)/(T −35.9)]

Water 221.2 Exp[6.310(T −467)/(T −52.9)]

EG 77.0 Exp[9.940(T −645)/(T −71.4)]DEG 47.0Exp[10.42(T −681)/(T –80.6)]

Table 2

Process designs with and without consideration of further internal heat integration

between the reaction operation and the separation operation involved.

Process design 0/9/4 0/9(2)/4(4)* 0/9(2)/6(4)

Number of stages (including

condenser and reboiler)

15 15 17

EO feed location 10 8 8

Distribution of catalyst (m3)

1 Condenser Condenser Condenser

2 0.1 0.043 0.043

3 0.1 0.043 0.043

4 0.1 0.043 0.043

5 0.1 0.043 0.043

6 0.1 0.043 0.043

7 0.1 0.043 0.043

8 0.1 0.043 0.043

9 0.1 0.1 0.1

10 0.1 0.1 0.1

11 0 0.1 0.1

12 0 0.1 0.1

13 0 0.1 0.1

14 0 0.1 0.1

15 Reboiler Reboiler 0

16 – – 0

17 – – Reboiler

section with the fresh ethylene oxide fed onto the bottom stage of

the reactive section. It is readily understood that no further inter-nal heat integration has been considered in this process design (i.e.,

no overlap between the reactive section and the stripping section

in view of the highly exothermic reactions involved). Tables 1 and 2

show the nominal operating conditions of the high-purity ethylene

glycol reactive distillation column. The liquid holdup is restrained to

be not greater than 0.1 m3 here so that excessive pressure drop can

be avoided on the reactive stages. Note that a fairly high reaction se-

lectivity (i.e., X EG /X DEG=19.19, where X EG is the molar concentration

of ethylene glycol and X DEG the molar concentration of diethylene

glycol in the bottom product) is assigned between the main- and

side-reactions in the nominal steady state.

The steady state profiles of temperature, vapor and liquid flow

rates, and liquid composition are shown in Fig. 2. Similar simulation

results were also presented by Ciric and Gu (1994), Baur et al. (1999),

Stage number

T [ K ]

0/9/4 0/9(2)/4(4)*

Stage number

L / V [ k m o l / h ]

L V

360

390

420

450

480

0

200

400

600

800

0

0.2

0.4

0.6

0.8

1

Stage number

X [ m o l e f r a c t i o n ]

EO water

EG DEG

1 3 5 7 9 11 13 15

1 3 5 7 9 11 13 15

1 3 5 7 9 11 13 15

Fig. 2. Steady state profiles: (a) temperature, (b) vapor and liquid flow rates, (c)

liquid composition (process designs: 0/9/4 and 0/9(2)/4(4)*).

Chen et al. (2000), Cardoso et al. (2000), Jackson and Grossmann

(2001) and Al-Arfaj and Luyben (2002).

5. Process synthesis and design with the consideration of

further internal heat integration

For the high-purity ethylene glycol reactive distillation column,

there exist two factors that make the reinforcement of internal heatintegration fairly difficult between the reaction operation and the

separation operation involved. One is the fairly large relative volatil-

ity between the reactants: ethylene oxide and water, which spans a

considerable range from 14.16 to 93.73 along the height of the re-

active distillation column. Ethylene oxide exists mainly in the vapor

phase and its liquid composition is extremely small throughout the

column, thereby limiting the effect of superimposing the reactive

section onto the stripping section. The other is the existence of a

consecutive side-reaction (i.e., Eq. (2)). As the reaction rate increases

more rapidly with temperature than that of the main-reaction (i.e.,

Eq. (1)), one can readily imagine that the allowance of these reac-

tions to occur simultaneously in the stripping section is likely to be

confined severely as long as the stringent reaction selectivity has to

be maintained.




In order to show clearly the effect of seeking further internal heat

integration between the reaction operation and the separation oper-

ation involved, one has to guarantee that the total amount of catalyst

is maintained constant in all situations examined, although the ex-

act value should be subject to a detailed optimization search. Fig. 3

demonstrates the effect of the three methods outlined in Section 2

Reactive stages superimposed on the stripping section

H e a t d u t y [ M W ]

Ascending EO feed location


7

7.3

7.6

7.9

8.2

7.2

7.5

7.8

8.1

6.4

6.8

7.2

7.6

8

Catalyst on the reactive stages superimposed

on the stripping section [m3]


0 1 2 3 4

0 1 2 3 4 5

0.06 0.08 0.1 0.12 0.14

Fig. 3. Effect of seeking further internal heat integration on the high-purity ethylene

glycol reactive distillation column: (a) superimposition of reactive stages onto thestripping section, (b) relocation of EO feeding location, (c) redistribution of catalyst

among reactive stages.

Table 3

Effect of seeking further internal heat integration upon the high-purity ethylene glycol reactive distillation column.

Parameters Condenser/Reboiler duty [MW] Comparison (%)

Process design without further internal heat integration: 0/9/4 8.021/7.433 100/100

Seeking further internal heat integration

Superimposing four reactive stages onto the stripping section: 0/9/4(4) 7.616/7.029 94.951/94.565

Ascending EO feed location by two stages: 0/9(2)/4 7.955/7.367 99.177/99.112

Combinatorial use of the above methods: 0/9(2)/4(4) 7.611/7.024 94.888/94.498

Redistribution of catalyst: 0/9(2)/4(4)* 7.246/6.658 90.338/89.574

Final process design: 0/9(2)/6(4) 7.246/6.658 90.338/89.574

for the reinforcement of internal heat integration in the process de-

sign, 0/9/4. It is readily seen that all the three methods can be effec-

tive in improving the thermodynamic efficiency of the high-purity

ethylene glycol reactive distillation column. With the superimposi-

tion of the reactive section onto the stripping section, the heat duties

of condenser and reboiler decrease monotonically, and it appears

reasonable to allow the reactions to occur on all the stages in the

stripping section (c.f., Fig. 3a). Similar arrangements of the reactive

section were also reported by a number of researchers in terms of a MINLP optimization formulation (Papalexandri and Pistikopoulos,

1996; Cardoso et al., 2000; Jackson and Grossmann, 2001). These

realities imply that the fairly high reaction selectivity poses hardly

limitation on the reinforcement of internal heat integration between

the reaction operation and the separation operation involved. As far

as the feeding location of fresh ethylene oxide is concerned, the min-

imal energy requirement is reached when it has been fixed at stage

8 (c.f., Fig. 3b). Away from this stage will lead to the degradation in

the thermodynamic efficiency. The distribution of catalyst displays

a strong effect upon the thermodynamic efficiency, and more cata-

lyst should be employed in the common section where the reactive

section has been superimposed onto the stripping section (c.f.,

Fig. 3c). Here, for the simplification of computational analysis, the

catalyst distribution is assumed uniform above and below the EOfeed location, respectively, i.e., a two dimensional search is con-

ducted in this work. Although the optimal distribution of catalyst is

found to be 0.12 m3 per stage blow the EO feed location, the feasible

solution becomes 0.1 m3 per stage owing to the constraint that the

maximally permissible amount of catalyst should not exceed 0.1 m3

per stage. In Table 3, the detailed effect of seeking further internal


operation involved is summarized in terms of a close comparison

with the basic process design, 0/9/4. The replacement of all sepa-

rating stages with reactive ones in the stripping section (i.e., in the

process design, 0/9/4(4)) leads to a reduction of heat duty by 5.049%

in the condenser and 5.435% in the reboiler. Feeding ethylene oxide

onto stage 8 instead of 10 (i.e., in theprocess design, 0/9(2)/4) secures

a reduction of heat duty by 0.823% in the condenser and 0.888% inthe reboiler. Combinatorial application of these two methods results

in a better process design: 0/9(2)/4(4), than those by either of the

methods, with a reduction of heat duty by 5.112% in the condenser

and 5.502% in the reboiler. Redistribution of catalyst in the process

design, 0/9(2)/4(4)*, abates further energy requirement, giving rise

to a reduction of heat duty by 9.662% in the condenser and 10.426%

in the reboiler. The substantial improvement in the thermodynamic

efficiency signifies definitely the paramount importance of consider-

ing further internal heat integration between the reaction operation

and the separation operation involved in the high-purity ethylene

glycol reactive distillation column. In Fig. 2, the steady state profiles

of temperature, vapor and liquid flow rates, and liquid composition

are also illustrated for the resultant process design, 0/9(2)/4(4)*.

In comparison with the basic process design, 0/9/4, there appear

almost no substantial changes in the temperature and liquid com-

position profiles, and only have the liquid and vapor flow rates been




FH2O= 26.3 kmol/h

2

8

16

FEO= 27.53 kmol/h

L1 = 652.15 kmol/h

14 Vn = 599.62 kmol/h

B = 26.3 kmol/hxEO = 0.0000

xW= 0.0026

xEG = 0.9480

xDEG = 0.0494

Fig. 4. Design of the high-purity ethylene glycol reactive distillation column with

the consideration of further internal heat integration.

reduced, implying a simultaneous abatement in capital investmentand operating cost.

Remember the fact that seeking further internal heat integration

in a reactive and non-reactive distillation column reduces actually

the driving forces of mass transfer in the rectifying/stripping section

where internal/external heat integration has been arranged (Melles

et al., 2000; Nakaiwa et al., 2003; Huang et al., 2006), it is there-

fore necessary to make an appropriate compensation to the mass

transfer driving forces during the synthesis and design of the high-

purity ethylene glycol reactive distillation column. A straightforward

method is to add a number of separating stages to the stripping sec-

tion, and this should be determined in terms of a detailed simulation

analysis. In this situation, two separating stages have been found to

be enough for the stripping section, leading to a final process de-

sign: 0/9(2)/6(4). The final process configuration of the high-purityethylene glycol reactive distillation column is depicted in Fig. 4, and

its nominal steady state operating conditions are also tabulated in

Table 2. In Fig. 5, the profiles of temperature, liquid and vapor flow

rates and liquid composition are illustrated for the process design,

0/9(2)/6(4). In comparison with the basic process design, 0/9/4, the

reboiler heat duty has been reduced by 10.426%, in addition to a

9.662% abatement in the condenser heat duty (c.f., Table 3). The ex-

tremely small improvement in the thermodynamic efficiency from

the process designs, 0/9(2)/4(4)* to 0/9(2)/6(4), implies, however, the

dominant effect of the combination between the reaction operation

and the separation operation involved upon the synthesis and design

of the high-purity ethylene glycol reactive distillation column.

6. Open-loop process dynamics

Fig. 6 shows the open-loop transient responses of the high-purity

ethylene glycol reactive distillation columns with and without fur-

ther internal heat integration between thereactionoperation andthe

separation operation involved when they are subject to a ± 33.45

kmol/h step change in the bottom vapor flow rate, respectively. As

can be seen, the high-purity ethylene glycol reactive distillation col-

umn is characterized by severe non-minimum phase behavior (i.e.,

initial inverse responses) and an under-damped response. In terms

of the close comparison between the process designs: 0/9/4 and

0/9(2)/6(4), one can readily understand that seeking further internal

heat integration helps to suppress the severity of the non-minimum

phase behavior although its effect appears to be quite marginal. The

process design, 0/9(2)/6(4), exhibits a smaller time constant than

Stage number

T [ K ]

Stage number

L / V [ k m o l / h ]

L V

360

390

420

450

480

0

200

400

600

800

0

0.2

0.4

0.6

0.8

1

Stage number

X [ m o l e f r a c t i o n ]

EO WaterEG DEG

1 5 9 13 17

1 5 9 13 17

1 5 9 13 17

Fig. 5. Steady state profiles: (a) temperature, (b) vapor and liquid flow rates, (c)

liquid composition (process design: 0/9(2)/6(4)).

0.75

0.85

0.95

Time [h]

X

E G

[ m o l e f r a c t i o n ]

–33.45

+33.45

0 5 10 15 20

Fig. 6. Open-loop transient responses of the high-purity ethylene glycol reactive

distillation columns with and without further internal heat integration when they are

subject to a ± 33.45 kmol/h step change in the bottom vapor flow rate, respectively.

Grey curves: negative responses; black curves: positive responses.

the process design, 0/9/4, despite the fact that a relatively larger de-

gree of under-dampness is observed in the former than in the latter.

These facts imply that the reinforcement of internal heat integration

gives no adverse effect to process dynamics.




0.92

0.93

0.94

0.95

0.96

Time [h]

X

E G

[ m o l e f r a c t i o n ]

+2%

–2%

0 10 20 30 40


distillation columns with and without further internal heat integration when they are

in the face of a ± 2% step change in the feed flow rate of fresh water, respectively.

Grey curves: negative responses; black curves: positive responses.

The occurrence of the non-minimum phase behavior is closely

related to the variation of water composition in the reactive section.

The increase/decrease of the heat duty of reboiler reduces/enhances

immediately the water composition in the reactive section and thus

theconversion rate of ethylene glycol. This is whythe bottom compo-

sition of ethylene glycol always exhibits an initial inverse response.

With the accumulation/depletion of water in the top condenser, the

level controller gradually increases/decreases the reflux flow rate

and thus the water composition in the reactive section, leading to

relatively slow variations in the conversion rate of ethylene glycol.

The variation caused by the reflux flow rate lasts for a relatively

long time and becomes the dominant one soon after the changes in

the bottom vapor flow rate. The interaction of these two conflicting

factors is, in fact, the principal reason for the occurrence of the non-

minimum phase behavior, and Eq. (3) shows the detailed mechanism

by assuming a first-order lag and an integrating process for the initial

inverse response and the one by the reflux flow rate, respectively.

K LT LS

− K V T V S+ 1

= (K LT V − K V T L)S+ K LT LS(T V S+ 1)

(3.1)

K LT V − K V T L<0 (3.2)

where T L T V and K L ≈ K V (because of the extremely small steady

state gain between the bottom composition of ethylene glycol and

the reboiler heat duty).

With the reinforcement of internal heat integration between

the reaction operation and the separation operation involved, the

dynamics associated with reflux flow rate is enhanced in rapidity

because of the reduction in the internal overflows, thus reducing

somehow the severity of the non-minimum phase behavior.

In Fig. 7, the open-loop transient responses of the high-purity

ethylene glycol reactive distillation columns with and without fur-

ther internal heat integration between the reaction operation andthe separation operation involved are depicted when they are in

the face of a ± 2% step change in the feed flow rate of fresh water,

respectively. In the circumstance of positive responses, fairly small

difference could be found between the process designs: 0/9/4 and

0/9(2)/6(4). In the case of negative responses, however, great dif-

ference could be observed between them. The process design with

further internal heat integration, 0/9(2)/6(4), could operate near the

desired steady state for a much longer time than the process design

without further internal heat integration, 0/9/4. This phenomenon

indicates that the former is less sensitive than the latter in the face

of disturbances from the feed flow rate of fresh water. The difference

in open-loop transient behaviors has been aroused by the different

characteristics of the interaction between the reaction operation and

the separation operation involved. After a certain period of time has

0.925

0.935

0.945

0.955

Time [h]

X

E G [ m o l e f r a c t i o n ]

–2%

+2%

0 10 20 30 40


distillation columns with and without further internal heat integration when they

are subject to a ± 2% step change in the feed flow rate of fresh ethylene oxide,

respectively. Grey curves: negative responses; black curves: positive responses.

elapsed, the two process designs will finally reach the same steady

state, though it is not indicated in the figure.

Fig. 8 illustrates the open-loop transient responses of the high-

purity ethylene glycol reactive distillation columns with and withoutfurther internal heat integration between the reaction operation and

the separation operation involved when they are subject to a ± 2%

step change in the feed flow rate of fresh ethylene oxide, respec-

tively. For the dynamic responses to the negative change in the feed

flow rate of fresh ethylene oxide, difference can hardly be noticed

between the process designs: 0/9/4 and 0/9(2)/6(4). For the dynamic

responses to the positive change in the feed flow rate of fresh ethy-

lene oxide, however, sharp difference has been found between them.

The process design, 0/9(2)/6(4), can again operate around the desired

steady state for a longer time than the process design, 0/9/4, imply-

ing a certain degree of improvement in process dynamics aroused by

the reinforcement of internal heat integration between the reaction

operation and the separation operation involved. After a certain pe-

riod of time has elapsed, the two process designs will finally reachthe same steady state, though it is not indicated here.

7. Operability evaluation

The decentralized control schemes are depicted in Fig. 9. As can

be seen, the levels of condenser and reboiler are controlled, respec-

tively, with the reflux flow rate and bottom product flow rate, and

two proportional-only (P) controllers are adopted, here. The bottom

composition of ethylene glycol is controlled with the reboiler heat

duty, and a PI controller is employed. The water composition on the

sensitive stage is controlled with the feed flow rate of fresh water,

keeping the stoichiometric balance between the reactants, and a P

controller is used. While the sensitive stage locates on stage 15 for

the process design with further internal heat integration, 0/9(2)/6(4),it is on stage 13 for the process design without further internal heat

integration, 0/9/4 (c.f., Figs. 2 and 5). The flow rate of ethylene ox-

ide is designated as the production rate handle, and a first-order

lag with a time constant of 5.0 min is assumed in all concentration

measurements.

The decentralized control systems have been tuned with the fol-

lowing procedure. They are considered to be optimal for the respec-

tive process designs. For the water composition loops, the first-order

plus a time-delay transfer function models are developed based on

the step responses obtained through the perturbation of fresh wa-

ter flow rate. According to the Ziegler–Nichols rule, the proportional

gain can then be estimated as

K c = T W / (K W × dW ) (4)




FC

LC

H2O

LCB, xEGCC

PC

EO

2

14

10

CC 13

FC

LC

H2O

LCB, xEGCC

PC

EO

CC

2

16

8

14

Fig. 9. Control schemes of the high-purity ethylene glycol reactive distillation

columns: (a) 0/9/4, (b) 0/9(2)/6(4).

where T W , K W , and dW are the time constant, process gain, and time-

delay of the developed transfer function models, respectively.

The bottom composition controllers are tuned in terms of an

experimental design approach. The ultimate gain and the ultimate

frequency are found by increasing the gain of a P controller until sus-

tained oscillations occur. Then, the Ziegler–Nichols settings are cal-

culated for the control loops. Finally, a detuning factor f is searched

for that makes all composition control loops have appropriate damp-

ing coefficients

K c = K ZN /f (5.1)

T I = T ZN × f (5.2)

where K c and T I represent the proportional gain and integral time,

respectively, and K ZN and T ZN denote the Ziegler–Nichols settings.

All the controller parameters are listed in Table 4. Notice that

while the P controller for the reboiler level is set to be 0.28, the

one for the condenser level is chosen to be 4.5. The tight inventory

control of the top condenser can facilitate the suppression of the

non-minimum behavior of the high-purity ethylene glycol reactive

distillation column (Huang et al., 2009).

The regulatory responses of the high-purity ethylene glycol re-

active distillation columns with and without further internal heat

integration between the reaction operation and the separation op-

eration involved are presented in Fig. 10 when they have been

upset by a ± 2% step change in the feed flow rate of fresh ethy-

Table 4

Controller parameters for the high-purity ethylene glycol reactive distillation column.

Distilation

column

Control loops

Condenser level

control loop

Reboiler level

control loop

Bottom EG

control loop

Stoichiometric balance

control loop

0/9/4

K C 4.5 0.28 8.0 12.35

T I 0 0 0.03 0

0/9(2)/6(4)K C 4.5 0.28 8.0 15.91

T I 0 0 0.03 0

lene oxide, respectively. As can be seen, the process design with

further internal heat integration, 0/9(2)/6(4), compares favorably

with the process design without further internal process integra-

tion, 0/9/4, displaying not only smaller maximum deviations, but

also shorter settling times. At the new steady states reached, the

former maintains its higher thermodynamic efficiency than the

latter.

Fig. 11 displays the regulatory responses of the high-purity ethy-

lene glycol reactive distillation columns with and without further

internal heat integration between the reaction operation and theseparation operation involved when the reactant of ethylene oxide is

changed from a pure component flow of ethylene oxide (i.e., Z EO=1.0

and Z W = 0.0) into a mixture flow of ethylene oxide and water (i.e.,

Z EO=0.9 and Z W = 0.1). In this case, oscillatory responses are exhib-

ited in both the process designs. With the consideration of further

internal heat integration in the process design, 0/9(2)/6(4), the de-

gree of oscillation has been suppressed considerably, resulting in a

substantial reduction in settling time.

In Fig. 12, the servo responses of the high-purity ethylene gly-

col reactive distillation columns with and without further internal


operation involved are illustrated when they have been upset by a

± 0.01 step change in the set-point of bottom control loop, respec-

tively. Again, it can be seen that the process design, 0/9(2)/6(4), out-performs, 0/9/4, with a reduced degree of oscillations. Much long

settling times are needed to reach the desired steady state in both

the process designs, and it is due to the improper selection of oper-

ating region (i.e., too high a conversion rate for ethylene glycol leads

to an extremely small process gains between the bottom composi-

tion of ethylene glycol and the reboiler heat duty). In such an oper-

ating region with a bottom composition of ethylene glycol as high as

94.8 mol%, severe non-linearity and non-minimum phase behavior

occurs, degrading further the process operability. These simulation

outcomes are in excellent agreement with our insights gained from

the studies of open-loop process dynamics.

8. Discussions

Owing to the combination of reaction operation and separation

operation within one framework, the synthesis and design of re-

active distillation columns become much more complicated than

those of conventional distillation columns. In spite of the fact that

seeking further internal heat integration is actually a single-variable

based searching method, it can effectively deal with the multi-

variable nature of the synthesis and design of reactive distillation

columns involving reactions with high thermal effect. In compar-

ison with the currently available methods for process synthesis

and design, e.g., the MINLP formulation, it appears extremely sim-

ple in principle, fairly easy to adopt in applications, and requires

no complicated mathematical vehicles. Even for the high-purity

ethylene glycol reactive distillation column that contains reactions

with extremely unfavorable physicochemical properties (e.g., the




Time [h]

X E G [ m o l e f r a c t i o n ]

Time [h]

V n

[ k m o l / h ]

0.93

0.94

0.95

0.96

0.3

0.35

0.4

0.45

0.5

Time [h]

X W [ m o l e f r a c t i o n ]

560

590

620

650

680

710

25.6

26

26.4

26.8

27.2

Time [h]

F W [

k m o l / h ]

620

660

700

740

Time [h]

R R [ k m o l / h ]

+2%

–2%

+2%

–2%

+2%

–2%

+2%

+2%

–2%

–2%

–2%

+2%

–2%

–2%

0 4 8 12 16 20

0 4 8 12 16 20 0 4 8 12 16 20

0 4 8 12 16 20

0 4 8 12 16 20

Fig. 10. Regulatory responses of the high-purity ethylene glycol reactive distillation columns with and without further internal heat integration when they have been upset

by a ± 2% step change in the feed flow rate of fresh ethylene oxide, respectively: (a) composition of ethylene glycol in the bottom product, (b) bottom vapor flow rate, (c)

composition of water on the sensitive stage, (d) feed flow rate of fresh water, (e) reflux flow rate. Grey curves: negative responses; black curves: positive responses.

fairly large relative volatility between ethylene oxide and water)

and the existence of a consecutive side-reaction, it can give rise to

a satisfactory process design with not only higher thermodynamic

efficiency and lower capital investment but also further improved

dynamics and operability than the conventional design practice,

reflecting its inherent reliability and robustness. These striking

results imply that the presence of high reaction selectivity does

not confine very much the reinforcement of internal heat integra-

tion between the reaction operation and the separation operation

involved.

It is worth investigating the intricate mechanism that seeking fur-

ther internal heat integration could improve process dynamics andoperability for the high-purity ethylene glycol reactive distillation

column. Generally speaking, the way to integrate the reaction opera-

tion and the separation operation involved is one of the primary fac-

tors that determine the dynamics and operability of a reactive distil-

lation column. Unfortunately, very few systematic methods are now

available for process synthesis and design based on dynamic perfor-

mance indexes. Seeking further internal heat integration searches

for the optimum process design in terms of an economic objec-

tive (i.e., thermodynamic efficiency). Too strong/weak internal heat

integration can certainly not yield a process design with high ther-

modynamic efficiency because of the interactions between the re-

action operation and the separation operation involved. The same is

also true for the resultant process dynamics and operability. There-

fore, seeking further internal heat integration works essentially to

enhance the thermodynamic efficiency through compromising the

reaction operation and the separation operation involved (Huang

et al., 2007). This is the major reason why process dynamics and op-

erability can be improved simultaneously along with the enhance-

ment of the thermodynamic efficiency for the high-purity ethylene

glycol reactive distillation column.

It is imperative to gain further insight into the fact that the

presence of a high reaction selectivity does not confine very much

the reinforcement of internal heat integration between the reaction

operation and the separation operation involved. Although the con-

clusion has been derived from the specific case study of the high-

purity ethylene glycol reactive distillation column, it is consideredto be of general significance, irrespective of what kinds of reaction

kinetics (e.g., exothermic or endothermic, series, parallel, or even

combined, etc.) involved in the reactive distillation columns. It is

certainly impossible to prove this corollary theoretically due to the

existence of innumerable reaction systems and their corresponding

process designs, the inherent characteristics of internal heat inte-

gration may, however, allow us giving the above deduction. As has

already been demonstrated in the previous and the current papers,

the reinforcement of internal heat integration helps to balance the

interaction between the reaction operation and the separation oper-

ation involved. It is the resultant coordinated relationship that pro-

vides additional flexibility for keeping the process innovation from

the violation of the reaction selectivity given in the basic process

design.




Time [h] Time [h]

0.84

0.88

0.92

0.96

1

0.3

0.5

0.7

0.9

Time [h]


X W [ m

o l e f r a c t i o n ]

550

600

650

700

750

800

20

22

24

26

28

Time [h]

V n

[ k m o l / h ]

F W [ k

m o l / h ]

600

650

700

750

800

Time [h]

R R [ k m o l / h ]

0 10 20 30 40

0 10 20 30 40 0 10 20 30 40

0 10 20 30 40

0 10 20 30 40

Fig. 11. Regulatory responses of the high-purity ethylene glycol reactive distillation columns with and without further internal heat integration when the fresh reactant of

ethylene oxide is changed from a pure component flow ( Z EO = 1.0 and Z W = 0.0) into a mixture flow of reactants ethylene oxide and water ( Z EO = 0.9 and Z W = 0.1): (a)

composition of ethylene glycol in the bottom product, (b) bottom vapor flow rate, (c) composition of water on the sensitive stage, (d) feed flow rate of fresh water, (e)

reflux flow rate.

9. Conclusions

Synthesis, design and operation of the high-purity ethylene gly-

col reactive distillation column has been studied in this work, and

the following remarks have been reached on the reinforcement of

internal heat integration between the reaction operation and the

separation operation involved.

In the aspect of process synthesis and design, seeking further in-

ternal heat integration between the reaction operation and the sep-

aration operation involved enhances the thermodynamic efficiency

substantially in addition to a further reduction of capital investment.

Although the reaction system displays extremely unfavorable physic-ochemical properties (e.g., the fairly large relative volatility between

the reactants: EO and H2O) and a very high specification of reaction

selectivity (i.e., X EG /X DEG = 19.19), no special restrictions have been

encountered in the reinforcement of internal heat integration be-

tween the reaction operation and the separation operation involved,

demonstrating its high reliability and applicability to the synthesis

and design of reactive distillation columns involving reactions with

high thermal effect.

In the aspect of process dynamics and operation, seeking further

internal heat integration between the reaction operation and the

separation operation involved improves the process dynamics and

lessens the difficulties in process operation. The stringent specifica-

tion of the reaction selectivity poses no limitation on such improve-

ment and the intensified synergism between the reaction operation

and the separation operation involved still accounts for the simulta-

neous improvement in process design and operation.

Although these outcomes have been derived in terms of the high-

purity ethylene glycol reactive distillation column, they are consid-

ered to be of general significance to any other complicated reactive

distillation columns with a high reaction selectivity. It is the resul-

tant coordinated relationship between the reaction operation and

the separation operation involved that provides additional flexibility

for keeping the process innovation from violating the given reaction

selectivity. Current work is now underway to ascertain systemati-

cally this corollary with other kinds of reactive distillation columns

involving reactions with high thermal effect.So far, synthesis, design and operation of reactive distilla-

tion columns involving reactions with high thermal effect (i.e.,

HR / HV >1.0) have been exclusively studied in our work.

For the reactive distillation columns involving reactions with

considerably (i.e., 0.05HR / HV 1.0) or negligibly/no (i.e.,

0HR / HV <0.05) thermal effect, however, relatively few studies

have been performed. For these processes, internal mass integra-

tion should be considered either simultaneously with the internal

heat integration or exclusively between the reaction operation and

the separation operation involved in process synthesis and design.

Similar to the concept of internal heat integration, internal mass

integration signifies the impact of mass coupling between the re-

action operation and the separation operation involved upon the

thermodynamic efficiency of a reactive distillation column. Prudent




Time [h] Time [h]

0.936

0.944

0.952

0.96

0.34

0.36

0.38

0.4

0.42

0.44


X w

[ m o l e f r a c i t o n ]

500

550

600

650

700

750

26

26.4

26.8

27.2

27.6

Time [h] Time [h]

V n

[ k m o l / h ]

F w

[ k m o l / h ]

600

650

700

750

800

Time [h]

R R [ k m o l / h ]

+0.01

–0.01

+0.01

–0.01

–0.01

+0.01

+0.01

–0.01

+0.01

–0.01

–0.01

+0.01

–0.01

+0.01

0 4 8 12 16 20

0 4 8 12 16 20 0 4 8 12 16 20

0 4 8 12 16 20

0 4 8 12 16 20

Fig. 12. Servo responses of the high-purity ethylene glycol reactive distillation columns with and without further internal heat integration when they have been upset

by a ± 0.01 step change in the set-point of bottom control loop, respectively: (a) composition of ethylene glycol in the bottom product, (b) bottom vapor flow rate, (c)

composition of water on the sensitive stage, (d) feed flow rate of fresh water, (e) reflux flow rate. Grey curves: negative responses; black curves: positive responses.

consideration of internal mass integration is also likely to reduce

capital investment and utility consumption, simultaneously. Prin-

ciples for the reinforcement of internal mass integration should

be derived and it is meaningful to see whether the single-variable

based searching method is still feasible in these situations. Our re-

cent work has demonstrated the utmost importance of deepening

internal mass integration during the synthesis and design of reac-

tive distillation columns involving reactions with negligible or no

thermal effect (Sun et al., 2008).

Notation

B bottom withdrawal, kmols−1

CC composition controller

dW time-delay of responses of water composition on the sen-

sitive stage to the changes in water flow rate, s

f detuning factor

F feed flow rate of reactants, kmols−1

FC flow rate controller

HR heat of reaction, kJ kmol−1

HV heat of vaporization, kJ kmol−1

K c proportional gain

K eq vapor–liquid equilibrium constant

K L static gain of responses of bottom EG composition to the

changes in reflux flow rate

K V static gain of initial inverse responses of bottom EG com-

position to the changes in vapor flow rate

K W static gain of responses of water composition on the sen-

sitive stage to the changes in water flow rate

L liquid flow rate, kmol s−1

LC level controller

n1 number of stages

n2 movement of feed location

n number of stages

PC pressure controller

r reaction rate, kmol s−1

RR reflux rate, kmol s−1

S symbol of Laplace transformation

T temperature, K

T I integral time, sT L time constant of responses of bottom EG composition to

the changes in reflux flow rate, s

T V time constant of initial inverse responses of bottom EG

composition to the changes in vapor flow rate, s

T W time constant of responses of water composition on the

sensitive stage to the changes in water flow rate, s

V vapor flow rate, kmol s−1

X liquid composition

Z feed composition

Subscripts

DEG diethylene glycol

EO ethylene oxide




EG ethylene glycol

r rectifying section

rea reactive section

s stripping section

W water

ZN Ziegler–Nichols settings

Acknowledgments

The project is financially co-supported by The National Science

Foundation of China (Grant number: 20776011) and The Scientific

Research Foundation for the Returned Overseas Chinese Scholars,

State Education Ministry.

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