CHNG 3003 - PROCESS DYNAMICS AND CONTROL II

TUTORIAL #6 - To be held Monday, April 2/07 in the Chem. Engng. Design Office.

Consider the problem of controlling temperature in the third tank of the following system by manipulating the heat input to the first:

A liquid mixture of ethanol and n-pentane (15 mole % EtOH) at 40 C and 1000 kPa is charged to the process with volumetric flowrate 30 m3/hr. A small flow of air (100 kg/hr) is added to each vessel at the same temperature and pressure. The tanks are 10 m3

vertical cylinders.

The pressure and level control valves are linear-trim and designed for a pressure drop of 300 kPa at the nominal flowrates. At the initial steady-state, the liquid level in the tanks is 50% of scale and the coil is supplying 1.97 106 kJ/hr of heat to the first vessel. When fully opened, the heat medium control valve can pass enough fluid to transfer

kJ/hr.

HeatMedium

TI

PC1

LC1

Air

Offgas

Air

Feed

PC2

Offgas

LC2

Air

PC3

Offgas

LC3

LiquidEffluent

TC1

The bottoms product pumps deliver a pressure rise of 300 kPa with an efficiency of 75%. The range of the temperature transmitters is 0 – 100 C and the pressure transmitters are calibrated for 500 - 1100 kPa. The temperature control loop is subject to a measurement delay of 10 minutes.

(a) Build a dynamic model of this process using HYSYS.Plant. Choose a value of the proportional gain which will ensure that the tank levels remain in the range 45 – 55%. Tune the remaining controllers until satisfactory servo response has been achieved.

(b) Simulate 5 hours of PID control of the Tank 3 temperature, introducing a –10 C step change in the feed temperature at time t = 10 min. The setpoint of the temperature controller is to remain fixed at its initial value of 75 C. Export your results to Excel and estimate the integral of the squared error (ISE) for TC1.

(c) Switch the temperature controller to manual and implement a 5% step increase in controller output. Fit a first-order-plus-deadtime model to this reaction curve.

(d) With TC1 on manual, make a 10 C step decrease in the process fluid inlet temperature and fit a first-order-plus-deadtime model to this response.

(e) Apply the “IMC Design” feature of the HYSYS PID controller block to determine and for the temperature controller. Repeat part (b) using these parameters and

compare your results to those obtained in part (h) of Tutorial #4.

(f) Apply the “Autotuner” feature of the HYSYS PID controller block to tune TC1 and repeat part (b).

(g) As indicated in the process schematic, a second thermocouple has been installed to provide a continuous measurement of the inlet temperature. Design a feedforward controller which compensates for the effect of feed temperature on the measured outlet temperature. Simulate the performance of the combined feedforward/feedback control scheme for the step decrease in feed temperature and compare your results to those obtained in part (i) of Tutorial #4.

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