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17.5e-2.755 QUESTION 6 Consider the typical control system for the double effect eviporator shown in Figure 5. Manipulating the steam in the first effect controls the composition of the product out of the last effect (control loop not shown). Step testing carried out on the double effect evaporator gives the relationship between the concentrated solution's composition and the steam flow rate as: 9p 5.05 + 1 while the relationship between the concentrated solution's composition and the feed density is given as: 12.8e-1.65 2.55 + 1 The sensor/transmitter has a dynamics expressed by: 9. Valve dynamics can be assumed negligible 0.12 0.008s + 1 Vapour Vapour Stem Food Consentrated Solution Condensate Condensate Figure 5 Double Effect Evaporator (a) Show the composition control scheme (8 marks) (b) Identify the output, manipulated and disturbance variables (3 marks) (c) Draw a complete block diagram of the closed loop system with the transfer function of each block indicated (8 marks) (d) What should be the failsafe position of the control valve used for composition control? (1 mark)