The composition, x, of a distillation product stream is measured by an online gas chromatograph (GC) which has a fixed analysis time. The reading provided by the GC is used 1+105 by an ideal PI controller, Gpl 205 which manipulates reflux rate, Re, to control composition. The control performance was very poor. A system identification exercise was then conducted to analyse the problem. This revealed that the analyser required 10 minutes to return a reading, while the transfer function between composition and reflux flow rate has a gain of 2 and a time constant of 5 minutes. a) Draw a schematic of the control loop [5 marks] b) What are the proportional gain and integral time of the controller? [3 marks] c) Your boss looks at the data in the description above and states that the poor performance is caused by the measurement delay. Perform a simple analysis to prove to yourself that what your boss said is true. [you may use the approximation 2-os exp(-os) ] 2 + os [10 marks) d) It is suggested that the Smith predictor could be used to improve the control of product composition i Draw a schematic of the Smith-predictor control strategy. (5 marks] ii Explain the rationale behind the Smith-predictor control strategy [5 marks] Show, through analysis, that the Smith-predictor can indeed improve control of process with large time delays. [10 marks] 111. The composition, x, of a distillation product stream is measured by an online gas chromatograph (GC) which has a fixed analysis time. The reading provided by the GC is used 1+105 by an ideal PI controller, Gpl 205 which manipulates reflux rate, Re, to control composition. The control performance was very poor. A system identification exercise was then conducted to analyse the problem. This revealed that the analyser required 10 minutes to return a reading, while the transfer function between composition and reflux flow rate has a gain of 2 and a time constant of 5 minutes. a) Draw a schematic of the control loop [5 marks] b) What are the proportional gain and integral time of the controller? [3 marks] c) Your boss looks at the data in the description above and states that the poor performance is caused by the measurement delay. Perform a simple analysis to prove to yourself that what your boss said is true. [you may use the approximation 2-os exp(-os) ] 2 + os [10 marks) d) It is suggested that the Smith predictor could be used to improve the control of product composition i Draw a schematic of the Smith-predictor control strategy. (5 marks] ii Explain the rationale behind the Smith-predictor control strategy [5 marks] Show, through analysis, that the Smith-predictor can indeed improve control of process with large time delays. [10 marks] 111
