Robustness of the Proposed Batch Operation

Vinyl acetate is the best entrainer for acetic acid dehydration using heteroazeotropic batch distillation system. There are only several operating variables needed to be set for this simple batch operating sequence to work. They are the setpoint of the middle tray temperature control loop at 91.73°C, the entrainer preloading amounts of 1.0 kmol, and the reboiler duty fixed at 0.16 GJ/h. In this section, the settings of these three operating variables will be altered to see if the proposed batch operation is robust enough. 

We first examine the effect of changing the setpoint of the middle tray temperature control loop. Because the desirable top vapor of this column is the vinyl acetate-water two component azeotrope (at 65.45°C) and the bottom is the pure acetic acid product (n.b.p. at 118.01°C), the setpoint value of this middle temperature control loop was set to be the average of the two temperatures at 91.73°C. Table 13.7 shows five simulation runs with this temperature setpoint altered firom 71.73 to 111.73°C in increments of 10°C. This is a very wide range of values for setting this setpoint. However, even with this wide range of 

The total batch time and the recoveries of the three products are pretty much the same with the wide 10°C mismatches of temperature upward or downward (101.73 or 81.73°C). Only when this setpoint is set to be too low at 71.73°C does the batch operation fail with the organic phase totally refluxed but still not able to reach the temperature setpoint. At this low temperature setpoint value causing total refluxing of the organic phase, Strategy A discussed in Section 13.3.2 should be implemented with two sequential steps in the batch sequence (aqueous product recovery step and followed by the entrainer-acetic acid recovery step). However, because our proposed procedure only allows for simultaneous recovery of the three products, the batch operation failed. 

The penalty for setting the setpoint value too high is not very severe with only a little increase in the total batch time and an increase in the acetic acid loss toward the top of the column. The reason for the wide tolerance of the setting of the middle tray temperature setpoint is because thermodynamically the bottom product will approach the heavy boiler (pure acetic acid) and the top vapor will approach the lightest boiler (vinyl acetate-water two-component azeotrope). Thus, as long as the middle tray temperature setpoint is set between these two temperatures, and also allowing for enough trays above and below the middle tray, the desirable separation will be achieved. 

Finally we examine the effect of changing the reboiler duty. Table 13.9 summarizes the six simulation results with the reboiler duty changing from 0.14 to 0.19 GJ/h with 0.16 GJ/h as the base case. The compositions of the decanter organic phase, aqueous phase, and the bottom are all the same with the varying of the reboiler duty. The recovery percentages of the acetic acid, water, and the entrainer (vinyl acetate) are also the same, no matter of what reboiler duty is used. As expected, the total batch time decreases as the reboiler duty increases. 

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