HUMG
ADVENCED PROGRAM IN CHEMICAL ENGINEERING
ECH157 – MIDTERM EXAM (7/2017)
Problem 1 :
For each of the following questions there is only one best answer. Clearly circle the correct answer and justify your choice
Unlike feedback control , the implementation of feed- forward control does not require the use of a sensor.
True
False
The implementation of feedback control requires the availability of a mathematical model to predict the input/ output relationship
True
False
The three control loops shown in the figure below are all feedback control loops
/
True
False
The steady state gain of a linear process is a dimensionless constant that captures how much the input influences the output.
True
False
If the input to a stable first-order process is a sinusoidal function of time, then the output response of this system is also a sinusoidal function of time with the same frequency
True
False
Problem 2 :
Consider the following three heat exchanger control instrumentation diagrams. For each diagram ( a. b and c ) the objective is to maintain a desired cold stream temperature. Since the cold stream exiting the exchanger is fed to a reactor, it is important that the stream temperature never be substantially higher than the set-point
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For each diagram , determine if the process gain relating the manipulated input to the measured output positive or negative , and explain why.
Which design ( a, b and c ) will have the fastest dynamic behavior ? why
Problem 3 :
Consider the following step responses of two different processes. Solving for y(t), determine whether each output response is stable or unstable and which it is oscillatory or not :
Y(s) = 2/(s(s2+4s+3))
Y(s) = 2/(s(s2+4s+8))
Explain your reasoning in each case.
Problem 4 :
A series of three isothermal constant – volume mixing tanks are shown below. A composition analyzer collects measurements of the concentration at the exit , C.A3 which are used to adjust the feed concentration , C,A0
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Determine which of the following transfer function models could possibly describe the input/ output dynamics . Justify your answer
G(s)=1/((2s+1)(s2+s+1))
G(s)=e^(-s)/(s+1)2
G(s)=as/(s+1)2
G(s)=-1/((s+1)(2s+1)(3s+1))
G(s)=1/((s+1)(s2+s-1))
Based on your answer to part (a) , determine the final value of the output response when the input is subject to a unit step change.
Problem 5 :
Consider the cascade of two identical liquid storage tanks in series, where qi, q1 and q2 denote the volumetric flow rates of the streams , and h1 and h2 represent the liquid levels in the two tanks , respectively . Two level sensor ( LT-1 and LT-2) are used to measure the levels in the tanks. The process is initially at steady state , and a step decrease of magnitude 1.0L/s is introduced in the feed flowrate at time t=0
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Which plot corresponds to the response of which level sensor ? why ?
Determine the valve resistances, R1 and R2 ( assume they are both linear ) , and the cross-sectional areas of the tanks. Shown your calculations.
What is the overall transfer function between LT-2 and qi ?
What would be the long-time response of LT-1 to a ramp disturbance in the feed flow rate with a ramp rate of 2.0 L/s2?