I
MONTGOMERY PHISTER, Jr.
Thompson-Ramo-Wooldridge Products Co., Los Angeles, Calif.
Controlling a Process with a Computer In the next few years digital computer control systems will take their place with other tools in the process engineer's kit, and will become widely used in direct control of continuous and batch processes in the chemical, petroleum, and related industries. This new tool will make possible the automatic supervision of such processes, and the resulting "tight" control will provide improvements in plant efficiency and product quality sufficient to pay for the control system in a relatively short time. The following report will help acquaint the reader with some of the planning which must be done and the problems which must be solved in getting a working system into operation.
XYZ CHEMICAL CO. October 12, 1958 Report on "Should We Use a Computer for Process Control?r1 TRANSMITTAL: FI'om: R. J . Jones, Manager, Engineering Development Section M. T. Whitaker, Vice President, Chairman, Executive To : Planning Committee Dear Mike: Thought you might like to see a copy of this report which has been circulating among our engineers and which has also been through Research. The report was written in question and answer style-the questions we posed we anticipated o u r various technical managers might want answeredand the answers themselves came in part from some of our men and the computer people. As you can see from some of the side remarks, o u r preliminary study didn't anticipate all the questions which might be asked, but after going over the report, I'm sure you will agree that the answers to the questions raised indicate we ought to give serious thought to application of computer control to some of our processes. Regards, Bob
-
Should We Use a Computer f o r P r o c e s s Control? How do we harness the digital computer to the process?
A . l3akr-k
u O mY %e.la**
w
The control system designers must write three sets of equations. First, the equation expressing the 'lprofit'lgained from operating the process. This enables the control system to run the process so this profit is a maximum in the face of variations in raw materials, ambient conditions, product demand, etc. Secondly, equations must be written relating the process variables. These include material and energy balances as well as equations describing chemical reactions. The third set expresses the constraints which exist on process variables because of limitations or restrictions on the process. What about solving these equations? The solutions are found by determining what values of the controllable variables-flow rates, temperatures, pres-
1 624
INDUSTRIAL AND ENGINEERING CHEMISTRY
79$L&y&&,
sures, levels, etc.-the operator can control directly by adjusting conpoints, corresponding to it. These values are, in general, functions of variables over which the operator has no control_* --raw ailability, breakdowns, etc. ct anv vpfrinaell benefits, such as logging and bookkeeping? Definitely. Because it has available in digital form all the important and ocess variables, it is easy ita1 control system to carry out many auxiliary functions, some of which are handled by conventional logging or "data processing" systems. Here are some examples:. periodic print-out of instrument readings, periodic print-out of process analytical data, calculation and print-out o verages and total flows, and comp tion and recording of accounting data. Can a computer run checks on our processes and instruments? Yes, and what's more important, these checks can be made automatically and periodically. For instance, checks can be made on the process to look for abnormal, unstable, or hazardous conditions which can be detected by an analysis of the values and rates of change of a number of related instrument readings. The computer can even run a check on itself, to determine whether its inputoutput system is maintaining its prescribed accuracy and to check the arithmetic operations carried out by the computer. How do we make our measurements? Most measurements can be made with conventional instruments. For measurements involving chemical and physical properties, chromatographs, spectrometers, viscometers, gravitometers, etc., can be used.
How will we handle calibration of our analytical instruments? It can be arranged for the computer to introduce a sample of known composition periodically into the instrument. The computer can measure the characteristics of this known sample, and adjust the correction coefficients it uses in interpreting this information, simultaneously printing out a record of the coprection made so that the operator can see how rapidly the instrument is drifting. This calibration can be done automatically its frequently as desired.
0.4.
What problems are we apt to run into on "start UD" of the comwter control s m t em? VOL. 50, NOI. 11
NOVEMBER 1958
1625
First, to minimize problems, a set of procedures must be carefully planned for this period. The measuring instruments may be connected to the computer, and a special computer program used which checks out these connections and logs specified process instrument readings. A modified form of the control program may then be employed which performs exactly the functions the complete control system is to carry out, except that it prints out or otherwise indicates process operating guides, rather than making adjustments directly on process instruments. When sufficient time has elapsed, the operators will be confident that the computer is operating reliably, and the computer outputs can be connected so that the computer makes adjustments directly on the process.
#-&
?
-7UiL
What part does the computer play in process start-up or shutdown? The computer may take some part in either operation, either directly by carrying out part of the associated routine procedures, or indirectly by printing out pertinent information for use by operating personnel during start-up and shutdown. A plan can be devised, for example, which requires the computing control system to measure the progress of the operation and to print out notification to the operator as soon as certain critical conditions exist. Can the computer be made to take mecia1 Pction in certain situations? ~~~
It can, by programming a series of priorities. F o r instance, it may be desirable to make it possible for the operator to initiate a l o g of process data at any time when he finds that hazardous or unusual conditions exist, by constructing a master coordinating routine whose job it is to determine the sequence with which the various functions are performed, and to ensure that high-priority functions interrupt lower priority ones under the appropriate conditions. How much will digital computer control cost us? The installed cost of a typical system in an existing process is likely to be from $200,000 to $300,000,depending on the number of inputs and outputs required and the special instrumentation necessary. What benefits can we expect? The principal benefits accruing from improved control arise in three areas: increased productivity (better yields), reduced operating costs (material, fuel, steam, catalysts, etc.), improved product quality. 1626
INDUSTRIAL AND ENGINEERING CHEMISTRY
fi
f &
kQ'Clo/
