Process Control and Instrumentation
POLYSTYRENE PLANT USING THE SUSPENSION PROCESS Figure 1 shows how the plant will be instrumented and controlled An expla-
nation of why these specific schemes were used follows.
Analog vs. Digital Control
Since this is a batch system, it might be advisable to use direct digital control. Undoubtedly the throughput could be increased over that with the more traditional analog control system. However, the initial costs and maintenance expenses would also increase. To fully instrument the system would also greatly complicate the equipment required, especially for feeding the reactors (this is discussed later). An economic balance should be run to determine whether this is feasible. I feel it would not be warranted, and have chosen to instrument the plant in the traditional way. Reactor
The quality of the product is dependent upon the amount and composition of all inputs to the reactor and the temperature within the reactor. The controls con- nected with these items must keep the variables as close as possible to the desired value. This could be done entirely with instrumentation. However, this would be rather complicated, and, as noted before, would greatly increase the capital and maintenance costs. The catalyst, rubber stabilizer, and suspending agent will be weighed manually, and then charged directly to the mixing tanks. The styrene will be automatically metered into the additive mixing tanks, the rubber dissolving tanks, and the reactors. The water will be automatically metered into the reactors. This will be done with a positive displacement meter.” When the desired amount has been charged to a vessel, the meter will close a valve in the inlet line and shut off the pump supplying the material. The pump will be activated by an operator in the control room when it is time to prepare another batch. The material in the additive mix tank and the rubber-dissolving tank will be discharged into the reactor by a solenoid valve, which will be operated from the control room.
The reactants entering the system need not be purified before they are used. However, they will need to be periodically checked in the laboratory to determine that they meet the specifications set by the scope. Any material not meeting specifications will be returned to the supplier.
The temperature of the reactor could theoretically be controlled by changing the flow rate or the temperature of the water in the jacket. It will now be shown that the former is impractical. The over-all heat transfer coefficient is given in the major equipment section as around 50 BTU/hr or greater. This means that the major resistance to heat transfer is the film on the inside of the reaction vessel.
If the thermal resistance of the stainless-steel wall is ignored, then
Case Study: Instrumentation and Control 175
where inside heat transfer coefficient-reactant fluid to stainless steel wall
= outside heat transfer coefficient-heat transfer medium to stainless steel wall
= over-all heat transfer coefficient for the jacketed vessel
The over-all heat transfer coefficient will next be determined for values of the outside heat transfer coefficient that differ by a factor of 2.
When = 200 and = 60, then U = 46. When = 400 and = 60, = 52. The result of this change is a 13% increase in the amount of energy transferred. Further note that to double the outside heat transfer coefficient requires more than a doubling of the flow rate through the jacket. This indicates that the temperature in the reactor cannot be adequately controlled by changing the flow rate of water to the jacket. A change in the flow rate barely changes the rate of heat transfer. The only practical means of control is to regulate the incoming temperature. A tempered water system will be used. This requires two sources of water at different tempera- tures. One should be at a temperature below the lowest ever desired. The other should be at a temperature greater than will be needed. These two streams are then mixed together to give the desired temperature.
Cascade control, along with ratio control, is used to control the temperature. The cold-water line is to have an air-to-close control valve. In case of failure in the air supply, the valve would open fully and a runaway reaction would be prevented. The hot-water line will have an air-to-open valve for similar reasons. After the two streams are mixed, the temperature will be measured. If it is above the desired temperature, the amount of air supplied to the valves will be reduced. This will increase the cold-water flow rate, and decrease the hot-water throughput. The result will be a reduction in the inlet water temperature. The desired temperature will be determined from a measurement of the reactor temperature. A deviation from the desired temperature will cause the set point of the second controller to be changed. This will result in a change of the inlet water temperature.
The cold-water supply for the tempered water system will be ordinary cooling water. No attempt will be made to keep its temperature constant. The hot-water temperature will be maintained constant by opening and closing the steam input to the hot-water storage tank. Close control is not necessary.
The temperature of the water entering the reactor will be controlled by sparging steam directly into the feed tank. The entering styrene temperature is to be control- led by manipulating the steam pressure on the shell side of the styrene heat exchanger.
Styrene Storage Tanks
The styrene storage tanks will be equipped with level indicators and a high-level alarm and switch. If the level in the tanks becomes too high, the feed will automati- cally be switched to one of the other tanks. The temperature of the styrene will be monitored. If the temperature should exceed the operator will be alerted by an alarm bell. He can then take any action deemed appropriate.
FROM
c u - 2 0 1
P - 4 0 6
W . D . TO DRYER
AIR
TO REACTOR
TO STORAGE
Special Symbols and Unlisted Equipment appearing in Figure D-306 Tank supplying hot water to the tempered water system; FICZ-401 Voltage regulator on a variable-speed motor, which is manipulated to maintain a constant flow rate to the centrifuge; TRCZ-405 Voltage regulator on the blower, which is manipulated to maintain a constant exiting wet-bulb temperature for the dryer; SIZ-501 Synchronized motor on the cutter, which is controlled by the extruder screw speed.
Wash Tank
The water and hydrochloric acid charged to the wash tanks will be regulated in the same way as the amounts of styrene and water are to be metered into the reactors.
Centrifuge
Centrifuges are designed for a given feed rate, and the rate will be maintained, close to that value by varying the speed of the centrifugal pumps. The rate of rotation of the centrifuge must also be controlled. Some type of warning and shutdown system should be included. It should indicate when there are excessive vibrations and when there has been a failure of some needed It should alert an operator by ringing a bell or causing a siren to blow, and safely shut the system down. Often these devices are supplied by the centrifuge manufacturer. In our case, it will be specified that these controls are to be included with the centrifuge when it is purchased.
Dryer
To prevent bubble formation during extrusion, the polystyrene leaving the dryer must contain less than 0.05% water (see the scope). It would be desirable if the moisture content of the polystyrene could be continuously measured, directly or indirectly. Unfortunately, this cannot be done reliably, so instead the mass and heat-transfer driving forces will be controlled. The dry and wet bulb temperatures of the gases leaving the dryer will be measured and used to control the inlet temperature and throughput of the air. The temperature of the polymer cannot exceed or the heat distortion properties will be affected. Therefore, the exit air temperature. will be controlled at 185°F. If it gets too high, the inlet air tempera- ture will be reduced using a cascade control system. The temperature of air leaving the air heat exchanger will be controlled by regulating the pressure (and, hence, the temperature) of the steam in the jacket of the air heat exchanger. The desired pressure will be determined by the exit temperature of the airstream from the dryer. The flow rate of the air through the system will be controlled by the wet-bulb temperature of the airstream leaving the dryer. The set point will be determined experimentally during startup.
An averaging means of control will be used to regulate the feed rate to the dryer. Extruder
An extruder is a complicated device to control. Often the barrel is divided into three sections, and the temperature at the exit of each section determines the additional amount of electrical energy to be supplied. Most of the energy for heating is provided by the screw. The throughput is usually set by the rate at which the screw rotates, and is maintained constant. Work is currently being done on the effect of extruder operating conditions on product quality. Preliminary conclusions indicate that conditions should be kept as constant as possible if reproducible results are desired.
Case Study: Instrumentation and Control 179
Usually when an extruder is purchased the controls are included. Therefore, they will not be indicated on the control diagram.
Cutter and Water Bath
When the polystyrene leaves the dryer, it is in strands of in. diameter. These strands are cooled in awaterbath, and then slicedinto in. lengths. The cutter must be synchronized with the extruder output so that the correct length is obtained. The water bath must not be allowed to exceed a given temperature. Usually it is controlled somewhat below this temperature by regulating the rate at which the cooling water enters. The level of the fluid and the material balance is determined by an overflow pipe on the tank.
Product and Testing Storage
The product and testing storage silos will have high-level alarms that automati- cally switch the feed to another vessel when a given level is exceeded.
Packaging
The feed tanks for all the packaging systems will be equipped with a high-level alarm and a high-level feed shutoff.
The automatic bagging and pall&zing unit will be purchased with all the con- trols attached. The drum- and carton-filling stations will require accurate weighing devices that automatically meter a prescribed amount into each container. The other operations will be essentially manual.
The bulk loading station will be capable of automatically weighing any set amount of material into a truck or hopper car.
Conveying Systems
The conveying systems will be operated manually. Ion Exchange
The flow rate through the ion exchanger will be determined using averaging control based on the amount of deionized water in the storage tanks. The controls will be purchased with the unit, not designated separately.
Steam Generator
The complete generating system, including controls, will be purchased as a package.
Water Purification
The rate of water pumped from the river will be determined by the water level in the storage facilities following the sand filter.
Water Distribution
tank (D-305) gets below a prescribed level, an alarm will sound. The operator will then start pumping water from the deionized water storage tanks into them.
All the water from the steam condensate lines and the water baths (D-501) will be pumped into the hot-water storage tanks, unless they are full. In that case, a control valve will divert the water from the water baths into the wash-water tanks. An overflow pipe will send any excess water from these tanks to the waste treatment facilities.
References
Smith, W.M.: Manufacture of Plastics, Reinhold, New York, 1964, 308.
2. Brown, J.E.: “Onstream Process Analyzers,”Chemical Engineering, May 6, 1968, p. 164. 3. “Process Instrument Elements,”Chemical Engineering, June 2, 1969, pp. 137-164.
4. Instrument Symbols and Identification, 1, Instrument Society of America, Pittsburgh, 1973. 5. Buckley, P.S.: “A System Approach to Process Design,” presented to the Richmond, Va.,
March 21, 1967.
6. Buckley, P.S.: “Impact of Process Control Advances on Process Design,” Chemical Engineering
Progress, Aug. 1964, pp. 62-67.
7. Buckley, P.S.: Techniques of Process Control, Wiley, New York, 1964.
8. Rijnsdorp, J.E.: “Chemical Process Systems and Automatic Control,” Chemical Engineering Progress, July, 1967, pp.
9. Buckley, P.S.: “Impact of Process Control Advances on Process Design,” talk given at the Process Control Workshop in Memphis, Tenn., Feb. 1964.
IO. Gould, L.A.: Chemical Process Control: Theory and Applications, Addison Wesley, Reading,
Mass., 1969, 183.
11. Friedman, P.G., Moore, J.A.: “For Process Control Select the Key Variable,” Chemical Engineering, June 12, 1972, p. 85.
12. “How to Cash in on Process Control Computers,” Chemical Week, Apr. 20, 1968, p. 71. 13. Moore, J.F., Gardner, N.F.: “Process Control in the Chemical Engineering, June 2, 1969, 94. 14. Lawrence, J.A., Buster, A.A.: “Computer Process Interface,”Chemical Engineering, June 26, 1972, p.
102.
15. Baker, W., Weber, J.C.: “Direct Digital Control of Batch Processes Pays Off,” Chemical Engineering, Dec. 15, 1969, p. 121.
16. Huters, W.A.: “Process Control System Planning and Analysis,”Chemical Engineering Progress, Apr. 1968, p. 47.
17. Spolidoro, E.F.: “Comparing Positive Displacement Meters,” Chemical Engineering, June 3, 1968, p. 9 1 .
18. Landis, D.M.: “Process Control of Centrifuge Operations,”Chemical Engineering Progress, Jan. 1970, p. 51.
Additional References
Coughanowr, D.R., Koppel, L.B.: Process Systems Analysis and Control, McGraw-Hill, New York, 1965. Buckley, P.S.: Techniques of Process Control, Wiley, New York, 1964.
Gould, L.A.: “Chemical Process Control: Theory and Applications,” Addison Wesley, Reading, Mass., 1969.
Issue of Chemical Engineering: “Instrumentation and Process Control,” Sept. 11, 1972 (lists equipment manufacturers).