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Simulation of Process Control with Computer ,Why “fly” blind when an analog up blues?” Here, a computer equipment to an existing system
T H E process industry is unique in that the control design of its various loops, whether simple or complicated, has grown a little like Topsy. First, there existed only manual control; then, with the advent of pneumatic control devices, simple off-on and proportional control was applied. Finally, with few realizing what the mathematical meanings were, integral and derivative modes of control, or a combination of both, were added until the present day pneumatic controllers offer almost any control mode known and used in the most highly technical field of electronics. Early process equipment was designed, or perhaps overdesigned, with large
storage capacities to minimize upsets of all types. In the 1930’s when more advanced instruments became available, it was unfortunate that their capabilities were not fully understood. Even today the application of control theory lags in the process industry, thus preventing savings in investment in process equipment by better designed control which would, in many cases, elimhe need for large storage capacity. analog computer arose out of a need to determine ,entire system as well as control system design before the fact instead of after the fact, as was past practice. The aircraft and missile industry gives the greatest testimony to
Analog can help you avoid “startd the best way of adding
this, as it is doubtful that any aircraft designer would build a plane or missile today without having “flown” it first on a computer of some type. Why then should the process industry be different? If processes were “flown” before being built, instead of afterward, the reward would be immediate profit instead of days or weeks of “start-up blues.” Adding Equipment
The illustration described here involves just such a situation. A new piece of equipment was to be added to an existing system. Figure 1 shows that there were four independent stripperscrubber systems. The liauid from each scrubber was fed to its stri‘pper and then recirculated to its scrubber. Because stripper capacity was more than ade-
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CONTROL SCHEM
Figure 1. Schematic diagram of the original system of strippers and scrubbers shows existing controls proposed new equipment addition
Figur of the new scrubb VOL. 50, NO. 11
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4 Figure 3. Control scheme B for the system after addition of the new scrubber
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Figure 4.
quate, controls were to be applied to take advantage of this available capacity and only a scrubber would be added. The problem was then to split the stream from the new scrubber into four equal parts to divide its load equally among the four strippers, separate out the proper portion from each stripper base, and then reblend it back to the new scrubber. Two possible schemes of control instrumentation are immediately evident (Figures 2 and 3). Scheme A (Figure 2) involved the level controls on three strippers to apportion approximately one fifth of the flow from each of these strippers to the new scrubber, thus constituting three fourths of the total feed. A flow controller was installed to control the total stream by making up the other one fourth from the stripper. The excess liquid introduced into the cycle at the scrubbers was removed from the system by a level control on the fourth stripper. The liquid to each existing scrubber was flow-controlled so that their cycles remained unchanged and were subject only to the presence of liquid in the vessels. Scheme B (Figure 3) involved flow controls on all four streams from the strippers to the new scrubber so that, in essence, the feed to the new scrubber was still flow-controlled, as in scheme A. Liquid level controls were used on each stripper to remove excess liquid in the system, and flow controllers were used to control the major streams back to each independent scrubber, as in scheme
The computer schematic diagram simulates the entire system
Controls for scheme A are shown, but because the schematic diagram for scheme B i s similar to that of A it has been omitted
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A. Which scheme is best? The instrumentation decided upon was essentially the same in each case so that, in this instance, cost was not an important factor; hence, controllability was the criterion by which a decision could be made. Simulating Schemes
I t was a simple matter to simulate the two schemes on an analog computer. Each scrubber and stripper presented two elements for simplified simulation : A holdup, or dead time, from the time liquid was introduced at the top until it reached the base, and a liquid level capacity in the base. The entire schematic for plan A is shown in Figure 4.
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4 Figure 5. This simulated pattern of flow from the base of a scrubber resulted from a slugged condition
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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b Figure 6. Various levels and flows
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through the system resulting from a simulated slug in the No. 1 scrubber, control scheme A, are recorded here
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I n scheme A, each stripper is simulated as a hold-up (dead time) and a capacity. A liquid level controller controls approximately one fifth of the withdrawal from each stripper, while a constant four-fifths withdrawal sufficiently characterizes the flow control back to each independent scrubber so that these individual scrubbers and their controls need not be simulated. The flow controller to the new scrubber accepts three fourths of its input from the first three level-controlled effluents while it controls the other one fourth from stripper No. 4. The level controller on the No. 4 stripper, which accumulates all of the excess liquid in the system, withdraws it separately. The new scrubber is simulated, as described, with its own hold-up, level control, and flow distribution of one fourth of its effluent to each stripper. To test the controllability of scheme A it was determined from previous operating experience that the worst upset that cduld occur in the system would be a slugged scrubber. I t was necessary to see which control scheme would introduce the least upset into the whole system. This, then, was the design criterion by which a decision could be made. A pattern of flow gradients from any given scrubber, resulting from a slugged condition, was worked out from experimental information. This may be introduced into any of the four strippers. Figure 6 shows the response of the whole system to such an upset, illustrating the situation if the upset occurs in any of the first three. If the upset should occur in the No. 4 stripper, the recovery would be the same as any of the strippers with the control setup in scheme B. Going to scheme B, the instrumentation was rearranged (Figure 3), The prescribed upset was introduced into the same stripper as above, as the response of the system as a whole is identical no matter where the upset is infroduced. Figure 7 shows the results graphically. Obviously, scheme B is the superior system of control. The worst upset to the system, a slugged scrubber, is confined to the scrubber-stripper pair where it occurs. Scheme A, on the
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b Figure 7. Control scheme B was used to record levels and flows VOL. 50, bJ0. 11
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Figure3 8 shows a reproduction of the flow variation caused b y a simulated slug in the operating system in the field
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These level and flow changes actually took place during the upset reproduced in Figure 8
The instrumentation is control scheme A a n d the similarity b e t w e e n these records a n d those in Figure 6 shows the correctness of the simulation
other hand, shows the upset being passed from the No. 1 stripper where it is introduced to the No. 4 stripper and back to the new scrubber. This is undesirable as the effect might be to upset the new scrubber, or the other strippers, which could compound the difficulty. O n the other hand, scheme B confines the upset to one loop. The other three stripperscrubber pairs, and the new scrubber, do not even realize that an upset has occurred. In addition to being able to determine which control scheme is best, it was also possible to see how the entire installation would react under any set of conditions. From the original simulated 100%. throttling ranges for the level controls on the strippers, it was possible
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to determine how much worse the upset would be with 50% or narrower throttling ranges, thus graphically illustrating the proper use of proportional control where its application is designed to make full use of the capacity of the vessel to which it is applied. Figures 8 and 9 are records reproduced from the actual (operating) system after the new scrubber was installed. The instrumentation is connected according to cohtrol scheme A. The similarity of the level and flow patterns with those in Figure 6 shows that the simulated model represented the true system accurately . This simulation technique may be applied to almost any problem in the process industry just as it is now being
INDUSTRIAL AND ENGINEERING CHEMISTRY
applied in the aircraft and missile industries. It is generally true that simplifying assumptions must be made as the amount of analog equipment may be limited or the complete mathematical representation of the process may not be known. It is axiomatic that results subject to either of these limitations are far better than no results a t all. Why fly blind or perhaps crack up on first attempt when there is such a powerful tool to get rid of some of the “bugs” before the system is even built? RECEIVED for review .4pril 25, 1958 ACCEPTED July 29, 1958 Division of Industrial and Engineering Chemistry, Symposium on Computers in the Chemical World, 133rd Meeting, ACS, San Francisco, Calif., April 1958.
