The bottom composition of a pilot-scale, methanol–water distillation column is measured using an on-line gas chromatograph (GC). The composition measurement is sent to a PI controller that adjusts the steam flow rate to the reboiler. Recently, the control-loop performance has deteriorated: the

Question

The bottom composition of a pilot-scale, methanol–water distillation column is measured using an on-line gas chromatograph (GC). The composition measurement is sent to a PI controller that adjusts the steam flow rate to the reboiler. Recently, the control-loop performance has deteriorated: the closed-loop composition response is more oscillatory and the period of oscillation is much larger than usual. The troubleshooting strategy used in Example 12.9 has indicated that neither a cyclic disturbance nor a sticking control valve is the source of the problem. According to the maintenance records, the filter in the sample line has been replaced recently. Suggest additional diagnostic tests that should be considered before controller re-tuning is performed as a last resort.

Accepted Answer

The combination of a more oscillatory response and a larger period of oscillation could occur if the time delay associated with the composition measurement has increased. For example, the transport delay associated with the sampling line to the GC would increase if the flow rate in the sampling line decreased. A decrease could occur due to a partial blockage in the line, or perhaps due to the new filter. Thus, the filter and the sample line should be inspected.
If the old filter had inadvertently been replaced with a new filter that had a smaller sieve size, a larger pressure drop would occur, and the downstream pressure would decrease. Consequently, the liquid velocity in the sample line would decrease, and the transport delay would increase. As a result, the composition control loop would become more oscillatory and exhibit a longer period of oscillation.

Question 12.11: The bottom composition of a pilot-scale, methanol–water dist...

This Question has been Answered.

Already have an account?

Login

Related Answered Questions