Experimental analysis in different batch operating units for process intensification: methyl acetate production case study

As reported in the previous section, five experiments are conducted for the present evaluation study. The first experiment is conducted with equimolar quantities of reactants in a batch reactor at 40 °C (without condensation of the product) to serve as a base case for comparison. The equilibrium conversion obtained is 69 %. Even though the reaction is exothermic in nature and equilibrium constant is found to be almost constant with temperature, and therefore the equilibrium conversion is almost constant at different temperatures. The purity obtained corresponds to the equilibrium composition (wt fractions—methyl acetate: 0.555, methanol: 0.108, water: 0.135, acetic acid: 0.202). The next two experiments are conducted in simple reactive batch distillation assembly using molar ratios of acetic acid to methanol as 1:1 and 1:1.7, whereas the last two experiments are conducted in the multi-stage reactive batch distillation column for the same set of molar ratios. The equipment mentioned in the previous section is employed and the procedure reported earlier is followed. The results obtained in all the cases are summarized in Table 3. The compositions of different condensate cuts in the simple reactive batch distillation operation for two molar ratios along with the reboiler temperatures show that for a feed molar ratio of 1:1, 80 % conversion is achieved, whereas when the molar ratio is increased to 1:1.7, the conversion increased to 90 % with respect to acetic acid.

Full size table

Table 3 also illustrates that although conversion has improved with use of excess methanol, purity with respect to methyl acetate in the latter case has decreased almost by 18 %. It can be observed that two azeotropic mixtures exist at boiling points lower than the pure products as evident from Table 2. The product compositions in Table 3 show that they do not correspond to one single azeotrope, but are a combination of both the azeotropes. Moreover, the minimum boiling azeotrope consists of methyl acetate and methanol as its components. Therefore, when methanol is available, methyl acetate combines with methanol to generate the minimum boiling azeotrope, which is of lower purity with respect to methyl acetate. Table 3 clearly shows that the first azeotrope is dominant when excess methanol is used, whereas with equimolar reactants, reasonable quantity of second azeotrope is present in the product. This can be justified by the fact that methanol available for the formation of first azeotrope is limited in case of equimolar reactant quantities.

Operation in a simple reactive batch distillation is equivalent to a single stage for separation and does not have provision for refluxing part of the condensate product to enhance separation. Therefore, the next two experiments are conducted in a multi-stage reactive batch distillation column with 15 trays and partial reflux (25 %). The results obtained during these two experiments with 1:1 and 1:1.7 molar ratios are also reported in Table 3. The conversions obtained in a multi-stage reactive batch distillation for 1:1 and 1:1.7 molar ratios are found to be 89 and 100 %, respectively. The purity of product with respect to methyl acetate is found to be 0.89 in the equimolar case, whereas it dropped to 0.70 when excess methanol was used. The reason for this decrease has already been discussed earlier. Compared to the performance in a single stage, the conversions obtained in multi-stage operation are found to be considerably increased, whereas there is only marginal improvement with respect to product purity. Higher conversions can be explained by the increased separation that can be achieved in multi-stage operation, thereby leading to a greater shift in equilibrium to the right compared to single-stage operation. The product purity obtained is comparable due to different operating times; however, the quantity of distillate collected in multi-stage operation is higher than in single-stage operation.

The results obtained in all the five experiments, as illustrated in Table 3, show comparison of all the cases using three different batch units and two feed molar ratios. As discussed earlier, the increase in conversions and decrease of methyl acetate purity with increase in methanol in the initial reaction mixture for cases 2 and 3 is evident, as we move from left to right in Table 3, whereas there is negligible change in product purity in case 1. This can be explained by the fact that no boiling of the mixture is involved in this case, and hence the presence of different azeotropes does not arise. Comparing the three cases for each molar ratio, it is clear that the conversions as well as product purities progressively increase from case 1 to case 3. This improvement is due to the incorporation of process intensification options of single-stage integrated reaction–separation in case 2 and multi-stage integrated reaction–separation with partial reflux in case 3. Therefore, this study illustrates that it is possible to achieve a trade-off between conversion and purity of methyl acetate, leading to high conversion with reasonably high product purity, in a multi-stage reactive batch distillation using optimized quantity of reactant methanol.

The analysis of results obtained above for methyl acetate case study can easily be extended for the class of reactions considered in the present study. While carrying out reactive-separation (single-stage/multi-stage), there are two factors playing opposing roles in achieving high conversion and high product purity. Based on condition (c) reported in “Process intensification schemes in batch mode”, part of the reactant also leaves along with the desired product as the first product cut, and therefore stoichiometric quantities of reactants cannot result in 100 % conversion with respect to the reactants. The product obtained is a combination of the first and second azeotropes [from conditions (c) and (d) reported in “Process intensification schemes in batch mode”], and the product purity is expected to be in between the compositions of the two azeotropes. On the other hand, if the reactant constituting the first azeotrope is taken in excess, 100 % conversion can be achieved with respect to the limiting reactant. However, the product obtained is dominated by the first azeotrope, and therefore the product purity is lower compared to the case where stoichiometric quantities of reactants are taken.