Info

In Table 28, the difference of interaction parameters among three cases is apparent.

In Table 28, the difference of interaction parameters among three cases is apparent.

Moreover, the average deviation Ayt in the third case is very small. The reason may be that this case is approximate to the actual situation, and the VLE model can accurately describe the real activity coefficients. Besides, it manifests that the influence of two-molecule aggregation and reversible chemical reaction on the interaction parameters can't be ignored.

By using EQ stage model, the extractive distillation process of separating water and acetic acid is simulated. In the simulation three cases are concerned, i.e. 1. not considering two-molecule aggregation and reversible chemical reaction (EQ1 model); 2. only considering reversible chemical reaction (EQ2 model); 3. considering two-molecule aggregation and reversible chemical reaction simultaneously (EQ3 model).

The experimental data, as well as the calculated results from the EQ stage models (EQ1, EQ2 and EQ3 models), are listed in Table 29. It is shown that the values of EQ3 model are closer to experimental results than those of EQ1 and EQ2 models. That means that EQ3 model reflects the real state of the system of water / acetic acid / tributylamine more accurately.

In order to go a further step to investigate the difference among EQ1, EQ2 and EQ3 models, the composition and temperature distributions along the extractive distillation column under the same operation condition are given in Tables 30 and 31, where subscripts 1, 2, 3 and 4 represent water, acetic acid, tributylamine and salt produced by the reaction, respectively. The tray is numbered from the bottom to the top. It can be seen from Table 30 that the calculated results of EQ1 model correspond well with those of EQ2 model on a large part of trays. However, in the vicinity of the tray feeding the mixture of acetic acid and water, i.e. from No. 9 to No. 15, the difference of liquid composition distribution is enlarged, which may be due to the relatively great influence of the feed mixture on the chemical reaction between acetic acid and tributylamine. But the difference of the composition and temperature distributions between EQ3 model and EQ1 (or EQ2) model is apparent. This indicates that the influence of two-molecule aggregation is more obvious than that of reversible chemical reaction. In summary, EQ3 model is more suitable for the design and optimization of extractive distillation process than EQ1 and EQ2 models.

On the other hand, the effect of the solvent, tributylamine, on the separation of water and acetic acid can be verified by simulation. It can be seen from Table 29 that water composition at the bottom of the extractive distillation column is nearly equal to zero. That is to say, the mixture of water and acetic acid can be effectively separated by extractive distillation with tributylamine as the separating agent.

7.2. NEQ stage model

The NEQ stage model for extractive distillation should follow the philosophy of rate-based models for conventional distillation. Unfortunately, it has been rarely reported on the NEQ stage model for extractive distillation. The reason may be that is building an NEQ stage model for extractive distillation process isn't as straightforward as it is for the EQ stage model where, at most, if the chemical reaction is involved, we need to simply add an equation to take account of the effect of chemical reaction on a tray or section of packing.

:omparison of the experimental and calculated values_

No. Tiop / °C Water concentration at the top, wt% __Exp. EQ1 EQ2 EQ3 Exp. EQ1 EQ2 EQ3

Was this article helpful?

0 0

Post a comment