Info

Figure 16.21 Impurity of A in fresh feed composition Az0B.

disturbance quite well. The purities of both products are maintained near the desired 95% specification. However, as the impurity amount is increased to 10 mol% B, the purity of the bottoms moves to the limit of 1% below the setpoint. Therefore, xB,D will deviate by more than 1% for large feed composition disturbances.

Figure 16.21 gives the responses of CS5 to 3% and 5% impurities of A in fresh feed-stream F0B. The control structure keeps the product purities within 1% of their specified values for the 3% feedstream impurity, but not for the 5% one.

16.4 DESIGN OF COLUMN/SIDE REACTOR PROCESS FOR ETHYL ACETATE SYSTEM

In this section we will investigate the design of the side reactor configuration for reactive distillation in which the reactive zone is placed at the lower section of the reactive column with no product removal from the column base. This is the type II configuration discussed in Chapter 7. Based on the optimal reactive distillation design, an improved side reactor configuration evolves gradually after going through a transitional step. The production of ethyl acetate5 is used to illustrate the design procedure.

16.4.1 Process Description

The reaction for acetic acid (HAc) esterification with ethanol (EtOH) to produce ethyl acetate (EtAc) and water (H2O) can be expressed as

The reversible reaction is described using a pseudohomogeneous model based on the mole fraction of component i (x) and catalyst weight (mcat). Model parameters are taken from Hangx et al.6 for an esterification reaction catalyzed by a Purolite CT179 ion-exchange resin. The rate expression can be written as

R = mcat x (kfxhacxetoh — knXetacXh2 o ) kF = 4.24 x 103 exp(— 48300/RT)

kB = 4.55 x 105 exp(-66200/RT) KEQ = 3.50 (T = 350 K)

This corresponds to an exothermic reaction with an equilibrium constant slighter greater than 1. In the process simulation, a catalyst density of 770kg/m3 is assumed to calculate the total volume occupied by the catalyst in a reactive tray.

5I. K. Lai, S.-B. Hung, W.-J. Hung, C.-C. Yu, M.-J. Lee, and H.-P. Huang, Design and control of reactive distillation for ethyl and isopropyl acetates production with azeotropic feeds, Chem. Eng. Sci. 62, 878 (2007). G. Hangx, G. Kwant, H. Maessen, P. Markusse, and I. Urseanu, Reaction kinetics of the esterification of ethanol and acetic acid towards ethyl acetate, Deliverable 22, Technical Report to the European Commission, Intelligent Column Internals for Reactive Separations (INTINT), 2001.

TABLE 16.5 Normal Boiling Point Ranking for Pure Components and Azeotropes

Component/Azeotrope Temp. (°C)

EtOH-EtAc-H2O 70.09

EtOH-EtAc 71.81

EtOH-H2O 78.18

EtAc 77.2

EtAc-H2O 70.37

EtOH 78.31

H2O 100

HAc 118.01

To account for nonideal vapor-liquid equilibrium and possible VLLE for this quaternary system, the NRTL model is used to calculate the activity coefficients. Model parameters are taken from Chapter 7. Vapor-phase nonideality caused by the dimerization of acetic acid is also taken into consideration using the Hayden-O'Connell second virial coefficient model. Aspen Plus built-in parameters values are used.

With the thermodynamic models available, the phase behavior can be predicted. Table 16.5 provides the normal boiling point temperatures for pure components and azeo-tropic temperatures. The phase behavior exhibits a ternary minimum boiling azeotrope: ethanol, methyl acetate, and water at 70.09 °C (Fig. 16.22). From the normal boiling point ranking, this complex system can be viewed as a ternary system conceptually, where the reactants acetic acid and ethanol can be treated as HK and IK, respectively. The ternary minimum boiling azeotrope is considered as the product, which is the LK. Thus, the reactive zone should be placed in the lower section of the reactive distillation column while the pseudoproduct (liquid with an almost azeotropic composition) is removed from the top of the column. This pseudoproduct is decanted and the water product is withdrawn from the aqueous phase, and the organic phase is further purified to obtain the acetate product. This is the exact type II configuration discussed in Chapter 7.

16.4.2 Conceptual Design

Figure 16.23 shows the reactive distillation flowsheet for EtAc production. The reactive zone is placed in the lower section of the reactive distillation column. The condensed overhead vapor from the reactive distillation column is decanted. The aqueous product is withdrawn from the decanter and the organic phase is split, with a portion returned to the reactive distillation column and a portion fed to a stripping column that produces high purity EtAc. In the reactive distillation column, because the column base has much larger residence time than a tray, the catalyst holdup in the column base is assumed to be 10 times that on a reactive tray. The two fresh feeds into the reactive distillation are the alcohol feed below the azeotropic composition (87% EtOH and 13% H2O) and industrial grade acid feed (95 mol% HAc and 5% H2O). Following the work of Lai et al.,5 the

Figure 16.22 The RCM diagram for EtOH - EtAc - H2O system.

EtOH H20

Figure 16.22 The RCM diagram for EtOH - EtAc - H2O system.

EtOH Feed 57.472 kmol/h EtOH 87.0% Water 13.0%

Or^

Duty 5113.4 kW

EtAc 1.50% EtOH 2.18% H20 96.32% ^D HAc 1.05e-03% 60.45 kmol/h ^Aqueous Product

Stripper

Duty 1979.01 kW

EtAc 99.00% EtOH 0.91% H20 0.08% HAc 1.00e-02% 47.83 kmol/h

Figure 16.23 Reactive distillation flowsheet of EtAc system.

Figure 16.24 Composition profile in reactive distillation column; (shaded area) fraction of total conversion in each tray and close to 90% of total conversion occurring in column base.

Figure 16.24 Composition profile in reactive distillation column; (shaded area) fraction of total conversion in each tray and close to 90% of total conversion occurring in column base.

optimized flowsheet consists of a reactive distillation column and a stripper as shown in Figure 16.23. The flowsheet is obtained by minimizing the TAC:

capital cost

payback period

Here, a payback time of 3 periods is used. The operating cost includes the cost of steam, cooling water, and catalyst. The capital cost is the costs of the column, trays, and heat exchangers. A catalyst life of 3 months is assumed.

The reactive distillation column has 10 reactive trays plus a reactive column base, 9 rectifying trays, and a decanter. The stripper column has 9 trays. Figure 16.24 gives the composition profile of the reactive distillation. From now on, emphasis will be placed on the reactive distillation column, and design alternatives will be sought while keeping the same design and product specifications for the stripper.

Single Reactive Tray. The composition profile in the reactive distillation column (Fig. 16.24) reveals that the concentration of the heavy reactant (HAc) remains fairly constant 90%) throughout the reactive zone, but the light reactant (EtOH) has a very low concentration in the column base 1%) and the composition decreases toward the end of the reactive zone. Of more importance, close to 90% of the total conversion occurs in the column base, which is shown in the shaded area in Figure 16.24. This naturally suggests a coupled reactor/separator configuration.7 The configuration in which all of the catalyst is placed in the column base is called the single reactive tray configuration. This flowsheet is

7C. K. Yi and W. L. Luyben, Design and control of coupled reactor/column systems. 1. A binary coupled reactor/ rectifier, Comput. Chem. Eng. 21, 25 (1997).

Figure 16.25 Catalyst loading in single reactive tray configuration versus achievable acetic acid conversion under different reflux ratios.

Reflux Ratio

Figure 16.25 Catalyst loading in single reactive tray configuration versus achievable acetic acid conversion under different reflux ratios.

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