Figure 7.2 - Open Loop Response to a Step Change in the Feed Composition
The transient response to a 5% increase in reflux rate is shown in Figure 7.3. The same column was used and pressure and reboiler duty were again held constant. The initial transient was fast but only visible before the permanent changes were asserted. Without an increase in reboiler duty, the column was effectively quenched by the extra reflux and ethanol was shifted to the reboiler. The temporary increase in separation efficiency in the rectification section (due to increased vapour and liquid flows) reduced the amount of ethanol lost in the distillate and had a very short positive effect on the overall conversion. However, this effect was quickly overshadowed by the loss of ethanol from the reaction zone. Both conversion and ether purity moved quickly away from acceptable values indicating the importance of maintaining the reboiler duty close to the optimum. Changes in the distillate product became evident more quickly than in the bottoms product, indicating the damping effect of column hold-up.
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