Control Structure CS1

Figure 12.4 shows a control structure that uses two direct composition measurements. Bottoms product purity (mol% C) is measured and controlled by vapor boilup. The composition of A on tray 5 is measured and controlled by the flowrate of fresh feed F0A. Throughput is set by flow controlling fresh feed F0B. Reflux-drum level is controlled by manipulating the reflux flowrate, and base level is controlled by manipulating the bottoms flowrate.

Tray 5 is selected because this is the location where the composition of A is changing rapidly from tray to tray. The steady-state composition is 38 mol% A. Two 0.5-min firstorder composition measurement lags are inserted in both composition control loops. Composition transmitter spans are 20mol%. All valves are 50% open at steady state.

The controllers are tuned by running relay-feedback tests. Values of ultimate gains and periods are provided in Table 12.2. Tyreus-Luyben tuning is used in most cases, but some loops are detuned to give larger closed-loop damping coefficients.

Results for the CS1 control structure are given in Figures 12.5-12.8. In Figure 12.5 the disturbance is a positive 20% step change in the throughput handle, VS, at 10min. The

12.1 TERNARY SYSTEM WITHOUT INERTS 297 TABLE 12.2 Controller Tuning Parameters for Ternary Without Inerts

CS1 CS2 CS3

Was this article helpful?

0 0

Post a comment