Final Exam
ChE 360
5/14/11
1.
An exothermic reaction occurs in a 1500-gal reactor in which the temperature is
(15)
controlled by cooling water in the jacket. The process transfer function relating
reactor temperature T to cooling water flow Q is given by:
𝑇(𝑠) −1.4𝑒 −0.3𝑠
𝐺𝑝 (𝑠) =
=
°𝐹/ 𝑔𝑎𝑙/𝑚𝑖𝑛
𝑄(𝑠)
8𝑠 + 1
The coolant control valve is an equal percentage valve that delivers 60 GPM at a
controller output of 3 psig and is fully closed at 15 psig controller output. Time
constant and time delay have units of minutes.
(a)
Determine Gv (assume overall linear valve behavior); is the valve
(10)
air-to-open or air-to-close? What is the sign of Gv? Explain.
(b)
Several days later the reactor throughput is reduced, at which time the
(5)
flowrate is reduced by 50%. The controller was not re-tuned. What do
you expect will happen to the quality of control? Explain.
2.
Consider the liquid blending control system shown in Figure 1 where stream 1
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consists of a mixture of two components and stream 2 has x2 = 1. An overflow
line maintains a constant holdup in the tank. The control objective is to control
the blend composition, x, which can be measured on-line but there is a significant
transport time delay associated with the location of the composition measurement,
which is downstream of the blending tank. Inlet flow rate w2 can be manipulated
but its upstream pressure can vary significantly. The exit flow rate w can also be
measured. The chief disturbance variable is inlet flow rate w1, which can exhibit
very large and frequent changes.
Propose and justify two possible advanced control strategies for this blending
control problem using control techniques considered in Chapters 15 and 16 of the
text. They should also be based on the assumption that no additional control
valves are available; you do not necessarily need to use all of the sensors shown,
but you can add one inexpensive sensor if necessary. Be as specific as possible;
however, a detailed quantitative control system design is not required, although be
sure to clearly identify the MV, CV, and DV for each control loop in your
proposed control strategy.
Figure 1. Blending control system with overflow line.
3.
In the slides for Chapter 10, a chlorine vaporizer makes chlorine vapor for a
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chemical reactor. The figure below shows the P&ID.
(a)
When the reactor is shut down, the vaporizer undergoes a pressure surge
(5)
that can trip a relief valve/rupture disk on the exit line (undesirable
behavior). Why can a pressure surge occur?
(b)
Analyze the P&ID and the valve failure conditions for shutdown. What
(15)
function does each of the four valves perform? Determine whether each
valve should fail open or fail close (and state why).
(c)
Note that there is no level control. Without adding more sensors, how
(5)
could you improve control of the liquid level?
4
Liquid Chlorine
4.
A process control engineer has decided to install an automated shower control
(25)
system in a bathroom of her mansion. The design calls for a system that can
deliver 5 gallons/min (GPM) of water at 110°F (the set points) by mixing hot
water at 140°F with cold water at 70°F. Flow and temperature transmitters are
available along with control valves for adjusting the hot and cold water flow rates.
(a)
Calculate the required flow rates of hot and cold water, assuming that the density
(10)
and heat capacity of water are constant.
(b)
Calculate the relative gain array for the system and recommend a pairing of
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controlled and manipulated variables.
5.
In the operation of a manufacturing plant, operating the supply chain is a critical
(5)
component in maintaining profitability. Explain what supply chain management
entails, with a few manufacturing examples.
6.
In the BP plant accident in Texas City, TX, the malfunctioning level sensor
(5)
played a critical role. What were possible reasons for the incorrect level
measurement?