Proceedings of the 6th International Conference on Process Systems Engineering (PSE ASIA)

25 - 27 June 2013, Kuala Lumpur.

Feasibility Analysis of Ethyl Acetate Reactive

Distillation with Different Catalysts in Low Tray

Efficiencies

K. C. Weng, H. Y. Lee*

Department of Chemical Engineering, National Taiwan University of Science and

Technology, Taipei 106, Taiwan.

Abstract

In this paper, the effects of different tray efficiencies () and catalysts on the design of trays has been studied for a ethyl acetate reactive distillation (RD) process. In

order to illustrate the effects of different catalysts for the design of RD process. Three

catalysts, Purolite CT179 (Hangx et al., 2001), Amberlyst35 (Tsai, 2007) and

Amberlite120 (Savkovic-Stevanovic et al., 1992) which were used in ethyl acetate

esterification reaction. The optimal tray distribution can be found by comparing with

different catalysts of each case. Moreover, there are different number of reactive trays in distillation column with different separation efficiency. It means that different

amount of catalysts is needed. Therefore, combination of this two effects to design a RD

process is an important study for industrial application. The results show that trays of

rectifying section increase are more than two times, however the trays of reactive

section are less than two times when the tray efficiencies decrease from = 1 to 0.5. Furthermore, a trend of tray increasing with different catalysts was studied under lower

tray efficiencies.

Keywords: Tray efficiency; Esterification; Reactive distillation; Reaction kinetics

1. Introduction

Ethyl acetate is an important solvent and widely uses in the chemical industries.

Most of ethyl acetate production is mainly from the esterification process of ethanol and

acetic acid. However, the ternary azeotrope with lowest boiling temperature is the

problem for product purification. According to the characteristics of residue curve and

liquid-liquid envelop, Tang et al. (2003 and 2005) proposed a novel configuration

which includes a RD with a decanter and a stripper shown in Figure 1. can achieve

industrial specifications. In conventional distillation column, number of total tray

number can be easy to estimate adversely proportional to tray efficiency (). However there is quite few studies on the effects of tray efficiency for RD processes. Even Tang

et al. (2003) had ever compared the optimal design of ideal tray condition and = 0.7 for ethyl acetate process. Their results indicated that when the tray efficiency was not

ideal, the trays of reactive and rectifying section of the RD column increased to attain

the product specifications. Furthermore, Lee et al. (2006) optimized this process by

using sulfuric acid as catalyst with tray efficiencies ranging from 1 to 0.5. Their results

showed that when the tray efficiency was reduced by 50%, the variation ratios of the

reactive and rectifying sections of column were 1.25 and 2.67 times to ideal tray

condition, respectively. It shows that the conventional design of trays adversely

854 Weng and Lee

proportional to tray efficiency is not suitable for the RD process. Because of corrosion,

the conventional catalyst, sulfuric acid is usually replaced by acidic ionic exchange

resin. And it is possible to replace new catalyst in the RD column if a higher

performance of catalyst is developed. The effect of catalysts and tray efficiency need to

be considered when design a new RD column or replacing new catalysts.

Condenser

Stripper

Decanter

RD

Column

Organic

Reflux

Aqueous Product

Feed to Stripper

Condenser

Reboiler Reboiler

HAc EtOH Acetate

NR

Nrxn NS

Figure 1. RD process configuration of ethyl acetate esterification

2. Thermodynamic and Kinetic Model

Because reactive distillation is conducted under a vapor-liquid equilibrium,

models suitable for liquid and vapor phases must be provided to allow simulations that

closely reflect the actual situations. The Hayden-OConnell model is used to describe vapor behavior because of the vapor association of acetic acid. The association

parameter can be obtained through Aspen Plus. The non-random two-liquid (NRTL)

model is used to calculate the liquid-phase activity coefficient of each component in the

liquid phase. The binary parameters of NRTL for this esterification system were from

the regression results of Tang et al. (2003). Table 1 shows the boiling points and the

azeotropic compositions from the literature data and computed result.

Table 1. Experimental data and computed result of pure components and azeotropes

Experimental data Computed result

Component Mole fraction Temp. C Mole fraction Temp. C

EtOH/EtAc/H2O (0.1126,0.5789,0.3085) 70.23 (0.1069, 0.6073, 0.2858) 70.09

*EtAc/H2O (0.6885,0.3115) 70.38 (0.6869, 0.3131) 70.37

EtOH/EtAc (0.462,0.538) 71.81 (0.4572,0.5428) 71.81

EtAc 1 77 1 77.20

EtOH/H2O (0.9037,0.0963) 78.174 (0.9016,0.0984) 78.18

EtOH 1 78.4 1 78.31

H2O 1 100 1 100.02

HAc 1 117.9 1 118.01

(*heterogeneous azeotrope)

Feasibility Analysis of Ethyl Acetate Reactive Distillation with Different Catalysts in

Low Tray Efficiencies 855

This esterification reaction is a reversible liquid-phase exothermic reaction.

Compared to homogeneous acidic catalysts, such as the sulfuric acid used in previous

studies, heterogeneous acidic solid catalyst ion-exchange resins possess has certain

advantages, such as low corrosiveness and ease of catalyst replacement in the column.

Therefore, ion-exchange resins have been widely applied in recent years. The ethyl

acetate esterification reaction in this study involved three kinds of ion-exchange resins.

The reaction kinetic models are shown in Table 2. The kinetic model of catalysts

CT179, Amberlyst 35, and Amberlite 120 were based on Hangs et al. (2001), the master

thesis of Tsai (2007), and Savkovic-Stevanovic et al. (1992), respectively.

Table 2. Kinetic models for three catalysts

System

(Catalyst) Kinetic model

k1 (T=363K)

Keq (T=363K)

(i)

Heterogeneous

(Purolite CT179)

Pseudo-homogeneous model

r = mcat(k1xHAc1.5xEtOH k2xEtAcxH2O)

k1= 4.24103 e (48300/RT )

k2= 4.55105 e (66200/RT )

4.7510-4

[kmol/(kgcats)] 3.50

(ii)

Heterogeneous

(Amberlite120)

Pseudo-homogeneous model

r = k1CHAcCEtOH k2CEtAcCH2O k1=9.7810

-4e(17.46/T)

k2=3.6810-4e(16.53/T)

9.32110-4

[m3/kmols] 2.65

(iii)

Heterogeneous

(Amberlyst 35)

r = mcat(k1CHAcCEtOH k2CEtAcCH2O) k1=6.147e

-(5673.35/T)

k2=6.8310-1e-(5268.02/T)

1.00210-6

[m6/kmolkgcats

]

2.95

*R = 8.314[kJ/kmol/K], T[K], r [kmol/s], mcat [kgcat], Ci [kmol/m3], xi[mole

fraction],a[activity].

3. Feasibility of Tray Adjustment Strategy For the Ethyl Acetate RD Process

In the steady-state process simulation, the feed rate of the 95 mol% acetic acid

as 50.8 kmol/h, and the feed rate of the ethanol was 57.472 kmol/h. To reduce the feed

cost, the concentration was set to 87 mol%, near the azeotrope, with a fixed feed ratio.

The specifications of product are ethyl acetate with a concentration of 99 mol% and the

concentration of impurity (acetic acid) lower than 0.01 mol%.

To observe the effects of tray efficiency, a simple assumption is setting tray

efficiency the same in the whole column. Then the stripper is assumed no reaction

occurrence. So that the stage number of stripper is like conventional column inversely

proportional to the tray efficiency. The stage numbers of rectifying and reactive section

are obtained by the optimal design of minimum total annual cost (TAC).

3.1. Fundamental Design: Ideal Tray Condition ( = 1) To observe the tray number differences between ideal trays and those with lower

tray efficiencies, we must first optimize the processes when = 1 with various catalysts. Because the optimized configuration of ideal trays with CT179 was proposed by Lai et

al. (2007). The optimization steps of the ideal trays with the other two catalysts are

shown below:

1. Set the number of trays in the reactive section (Nrxn). 2. guess a number of trays in the rectifying section (Nr). 3. guess a number of trays in the stripper (Ns).

856 Weng and Lee

4. Adjust the organic reflux rate and the reboiler duty until the product achieves the specifications.

5. Return to Step 3 and adjust the tray number of the stripper until the TAC reaches the minimum.

6. Return to Step 2 and adjust the tray number of the rectifying section until the TAC reaches the minimum.

7. Return to Step 1 and adjust the tray number of the reactive section until the TAC reaches the minimum.

Table 3 shows the optimization results of RD processes with various catalyst

when = 1. The results indicated that the reflux rate and diameter of the reactive distillation column are different for each catalytic system. The tray number of the

rectifying section is similar for processes with three kinds of catalysts. However, the

tray number of the reactive section is not like rectifying section. There are largest

amount of catalysts for the column using Amberlite 120 as catalyst.

Table 3. Optimal result for different catalysts ideal system.

System CT179 Amberlyst35 Amberlite120

Column Configuration RD Stripper RD Stripper RD Stripper

Total no. of trays including

the reboiler 20 10 29 12 60 11

Trays of stripping section 10 12 11

Trays of reactive section 11 17 50

Trays of rectifying section 9 12 10

Range of reactive section 10~20 13~29 11~60

Reflux rate (kmol/hr) 395.1 303.23 334.63

Column diameter (m) 1.84 1.22 1.61 1.21 1.68 1.21

Total volume of catalyst (m3) 2.44 2.41 6.01

3.2. Conventional Tray Adjustment Strategy

Because the industrial tray efficiency of distillation column is low, a = 0.5 was regarded as close to the actual situation. Few studies have explored whether the

conventional strategy of increasing the number of trays at low tray efficiencies is

suitable for the reactive distillation column. Therefore, we gave the tray number

adjustment using the conventional strategy first. Notice conventional strategy means

each section of tray number is to estimate adversely proportional to tray efficiency. The

optimization of the ideal trays of three catalysts was used as the base cases.

Table 4. Simulation result for non-ideal (= 0.5) system using conventional strategy

System CT179 Amberlyst35 Amberlite120

Nr 18 24 20

Nrxn 22 34 100

Ns 20 24 22

Reflux flow (kmol/hr) 1440 318.43 654.4

RD reboiler duty (kW) 16429.6 4330.7 7951.9

Xproduct (mole fraction)

HAc 9.810-4 1.010-4 1.5310-4 EtOH 8.710-3 9.710-3 9.5510-3 EtAc 0.99 0.99 0.99

H2O 3.210-4 2.210-4 2.9310-4

Feasibility Analysis of Ethyl Acetate Reactive Distillation with Different Catalysts in

Low Tray Efficiencies 857

When = 0.5, the number of trays in the reactive, rectification, and stripping sections was adjusted to twice that of the ideal trays. The process simulation result is

shown in Table 4. The acetic acid composition of product cannot reach its specification

for CT179 and Amberlite 120 processes. It means that reaction rate decrease. Based on

these results, when the tray efficiency is low, using the conventional tray number

adjustment strategy of distillation column on the reactive distillation column produces

problems regarding the use of some kinds of catalysts.

3.3. Observation at Non-ideal Tray ( = 0.9 ~ 0.5) Conditions Because the conventional adjustment strategy of distillation for column tray

numbers is not applicable by using some catalysts with low tray efficiency. The tray

efficiency effect for the tray number of the reactive distillation column must be

reconsidered. Therefore, we optimized systems of low tray efficiencies with different

catalysts to observe the optimum trays of each section. The test method for tray

efficiencies was based on the ideal trays for each catalyst and adjusted to lower values

with an interval of 0.1. Subsequently, we conducted the optimization of each tray

efficiency system to observe the relationship between the tray number and efficiency

under non-ideal situations. The tray efficiency was adjusted to no less than 0.5. The

optimization steps for non-ideal situations are shown as follows:

1. The number of trays in the stripper (Ns) is based on the ideal tray efficiency and increases adversely proportional to the tray efficiency using round up.

2. Provide an initial estimated value for the tray number of the reactive section (Nrxn). 3. Provide an initial estimated value for the tray number of the rectifying section (Nr). 4. Adjust the organic reflux flow and reboiler duty until the specifications are met. 5. Return to Step 3 and adjust the tray number of the rectifying section until achieving

the minimum TAC.

6. Return to Step 2 and adjust the tray number of the reactive section until achieving the minimum TAC.

To facilitate the observation of tray number variation in each section of the

system using different catalyst, we mapped the tray number variation proportions to

each tray efficiency of the reactive and stripping sections. Figure 2. shows the variation

proportions of tray numbers of the reactive and rectifying section using different

catalysts at low tray efficiencies. The observation results show that when the tray

efficiency was halved, the tray number variation of the rectifying section using various

catalysts was greater than twice that of the basis. The results indicated that the variation

of reactive trays were smaller than the proportion of two. This phenomenon is the same

as the result of Lee et al., (2006).

(a) (b)

Figure 2. The ratio of (a) rectifying (b) reactive trays for different catalysts

858 Weng and Lee

4. Conclusion

In this study, the effects of lower tray efficiencies and three kinds of catalysts

on the design of trays has been studied for a ethyl acetate reactive distillation (RD)

process. It is found that the conventional adjustment strategy of distillation column is

not suitable for the ethyl acetate reactive distillation column. There are the phenomena

that more trays in rectifying section and less trays in reactive section for a reactive

distillation column with inverse ratio of lower tray efficiency. The results show that

trays of rectifying section increase are more than two times however, the trays of

reactive section are less than two times when the tray efficiencies decrease from = 1 to

0.5. And for observing the tray change of rectifying section with tray efficiencies, the

trend of tray number increasing can be found.

Acknowledgment

This financial supports from the National Science Council of R.O.C under grant

No: NSC101-2218-E-011-011 which is gratefully acknowledged.

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