Open Loop Transients

It is becoming increasingly important to consider dynamic responses in the design phase of major engineering projects to realise the benefits of integrating control system design with process design. Process control considerations can influence equipment requirements (especially instrumentation) and can, in exceptional cases, negate the effectiveness of a design that is satisfactory at steady state but difficult to control. This is particularly true for processes such as MTBE and ETBE reactive distillation, where instability and hysteresis effects may be present (e.g. Jacobs and Krishna, 1993; Nijhuis et al., 1993). Step Change in Feed Rate

The open loop transient response to a 5% increase in feed rate with pressure, reboiler duty and reflux rate held constant is shown in Figure 7.1. Without an increase in heat input, there was a significant increase in the bottoms flow rate as the extra feed material left the column almost entirely with the ether product. The composition of the bottom product shifted to include the additional light material and the temperature decreased accordingly as the reboiler remained in phase equilibrium. Less ethanol was returned to the reaction zone that subsequently reduced the isobutene conversion.


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