Using Isobutyl Acetate as the Entrainer

Isobutyl acetate was found in Chapter 9 to be the best entrainer for acetic acid dehydration in the continuous process. Several patents have also shown isobutyl acetate to be a favorable entrainer for the separation of acetic acid and water (Costantini et al. and Partcn and Ure ). However, the closeness of the normal boiling-point temperature of acetic acid (118.01°C) and isobutyl acetate (116.40°C) is definitely troublesome for the second step of operating Strategy A (separation of acetic acid and entrainer) in the batch system. In fact, in the paper by Christensen and Olson and also by Biuguet et al., double azeotropes were found for the mixtore of acetic acid and isobutyl acetate in the experimental data because the two normal boiling points are so close together. 

The main reason for the failure of using isobutyl acetate as the entrainer is because of the poor separation capability between isobutyl acetate and acetic acid. This factor did not have to be considered at all in the continuous process as discussed in Chapter 9. However, it is important in the heteroazeotropic batch distillation. 

In the following section, two alternative entrainers considered in the literature will be compared for the purpose of acetic acid dehydration. They are vinyl acetate, which was 

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Figure 9.4 RCM, LLE envelope, and material balance lines of acetic acid-water system using isobutyl acetate as the entrainer.

The heterogeneous azeotropic column system using isobutyl acetate as the entrainer will be studied in detail in this section. The overall control strategy of this system will be developed in order to maintain bottom and top product specifications in spite of feed flowrate and feed composition changes. In the control strategy development, we will assume no online composition measurement is available. The composition control loops will be inferred by some tray temperature control strategy. This type of control strategy can easily be implemented in industry for wider applications. 

The comparison of the minimum attainable TAC for these three systems, as well as the acetic acid dehydration system without any entrainer, is shown in Table 9.13. From the table, one can observe that the no-entrainer system has the highest TAC and the isobutyl acetate system is most favorable for this feed composition and product specification requirements. The TAC for the isobutyl acetate system is only about 55% of the no-entrainer system, which shows that a large saving can be made by using the isobutyl acetate system. Notice that this finding is in general agreement with the industrial applications. (See patents by Costantini et al. and Parten and Ure ). The above two patents also found isobutyl acetate as a favorable entrainer for the separation of acetic acid and water. Another earher patent by Othmer found w-propyl acetate to be useful as an entrainer for this system. The patent by Mitsui Petrochemical Industries found n-butyl acetate to be... 

The aqueous phase should contain as little entrainer as feasible. The reason is because the aqueous phase stream will be drawn out of the system, and any entrainer loss should be compensated by the makeup stream in Figure 9.2. This will correspond to a stream cost of the system as seen in Table 9.13. The makeup flowrate can acmally be estimated using the outer molar balance envelope in Figure 9.2, assuming ideal conditions, as explained previously. In this regard, an isobutyl acetate system results in the least makeup flowrate while an ethyl acetate system requires the most makeup flowrate. This is confirmed by Tables 9.10 to 9.12. 

In Chapter 9, three candidate acetates were studied in detailed process simulations to determine the best entrainer for this system. The three candidate acetates considered were ethyl acetate, isobutyl acetate, and n-butyl acetate. Rigorous process simulations were performed to find the optimum design and operating conditions of these three entrainer systems. In each entrainer system, the total annual cost (TAC) was used as the objective function to be minimized. From a comparison of the minimized TAC, the isobutyl acetate was selected as the best entrainer with the lowest TAC and operating utility cost. 

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