TECHNOLOGY
CONSOLE SETTINGS.
Set point (knobs) and proportional and reset (switches) adjustments are on a console
Monsanto Tries Direct Digital Control Uses RW-300 computer in experimental operation, bypasses conventional instruments to control distillation columns Another threshold has been reached in the application of computers to process control. Monsanto Chemical and TRW Computers, in an experimental operation at the former's Texas City, Tex., ethylene plant, have proved the feasibility of using computers to per-
form direct digital control of a process, replacing conventional instruments in the control loops. Involved in the experiment are 10 control loops, making up the complete control of two consecutive distillation columns—the debutanizer and
Computer Uses This Executive Routine
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depropanizer—in the process. The computer system, a Thompson Ramo Wooldridge RW-300, was installed in parallel with the existing control system in order to make a comparison between the two. The computer system dramatically outperforms the conventional control. Flows, for example, are smoother, and response to control is faster. However, Monsanto's project manager Dr. T. J. Williams points out, currently available instruments have a number of improvements over older models such as those in the ethylene plant. Thus, the computer's performance is not directly comparable to today's instruments. Still, computer operation is estimated to be at least as good. And that could be very good, economically. Based on currently expected costs, the economics of direct digital control will probably be most attractive in a larger plant—one with, say, about 200 control loops. Capital savings of between $500 and $1000 per control loop would readily be possible, according to Dr. Williams. Lack of computer hardware designed for direct control places actual application of the technique at least
several years away. But it could well have a major impact on future control system design. If the low equipment costs currently talked about in the computer industry should ever be achieved, the economic break-even point for direct digital control could drop to perhaps 25 to 50 loops. Has Two Advantages. The computer system has two main advantages that make for improved control over that of conventional systems. Both advantages involve the control modes, in this case proportional and reset. Derivative, a third mode often used in control, is not needed with the computer system. Proportional action is often included in a control signal, since a control valve operating from fully open to fully closed in direct proportion to the control signal (generally 3 to 15 p.s.i.g. for pneumatics) can lead to sluggish process response. Proportional action, in effect, specifies the range of control signal that will give a fully open to fully closed valve. It must, however, be an optimum value, since too narrow a range will cause the process to respond so quickly it will cycle. Reset action adds a further refinemen. A process variable cannot be brought to the set point, following a deviation, with proportional action alone. It will assume a new level, offset in one direction or the other from the set point. Reset counters this by, in effect, adding corrective action as long as a deviation from the desired set point exists. One of the computer's advantages is that proportional and reset modes are noninteracting. In the control equation for conventional controllers the proportional and reset terms might be added, multiplied, divided, or a combination of these. The result is that a change in one causes some change in the other, making precise control difficult. In the digital computer, the control equation is simply: Output = K p (e + 2K r e) + Kj In the equation, Kp is the proportional term, Kr the reset term, and e the error. The term Kx is a basis to keep the output at a base point equivalent to 9 p.s.i.g., since the signal is later converted to pneumatic. In the computer system, K„ and Kr are separately adjustable. And—the second advantage of the system—they are both almost infinitely variable. This combination of features, Dr. Wil-
liams explains, leads to much more "fine tuning" of each control loop. Cascade Loops Included. The control loops involved in the Monsanto experiment include two temperatures, one pressure, two levels, and three flows. In addition, two cascade loops —one a level-flow, the other a levelpressure—were involved. The computer is connected in parallel to the controller in each loop. Pneumatic signals are transduced to electric currents, which are converted to voltages and then to digital signals for the computer. Outputs from the computer follow this procedure in reverse. A hand valve in the output pneumatic line can be used to valve in either the controller or computer. Also in the output pneumatic line is a solenoid air valve. Should the computer fail to compute in an alloted time, a watchdog timer in the computer actuates the solenoid valve, which closes and holds air pressure in the line to the air-operated process control valve. This allows about 15 min. to take action before air bleeds off the control valve. The timer in the computer, Dr. Williams says, is set for 2 sec. to give the computer time for auxiliary functions such as printing or scanning for changes in set points or in values for Kp and Kr. The time must be long enough to get computations done, but not so long that it would affect the process. The executive routine for the computer operation is such that the computer squeezes in the auxiliary functions in whatever time it has left after making the control computations. While the 10 loops involved only take up 1500 to 2000 words of the RW-300's 8000-word memory, it is the maximum number of loops that can be handled by the machine in the time alloted. A faster machine could handle more loops. Other Machines Needed. Before direct digital control can be put into actual use, new computers must be developed. This type of control requires a computer with reliability upwards of 99.95% (two to four hours a year down-time). The current average with optimizing computers, such as that used at Monsanto's Luling, La., ammonia plant, is about 99.5% (about 40 hr. a year down-time), Dr. Williams says. However, it shouldn't be too difficult to develop a direct control computer with greater reliability, since it needn't be as complex.
Corona Chemistry Holds Promise for Coal's Future GE links coal with natural gas, hammers out chemicals from coal in corona reactor Corona discharge chemistry may restore coal to its former position of grandeur, Mr. John Coffman of General Electric's Schenectady, N.Y., general engineering labs told the 47th national meeting of the American Institute of Chemical Engineers, Baltimore, Md. At the same time, however, a note of caution not to expect much impact on the coal industry from chemical uses in the near future was sounded by Dr. Donor Lion of Booz, Allen & Hamilton, Inc. Mr. Coffman and his co-workers believe that hydrogen formed in a highvoltage, high-frequency corona will react with coal by hydrogenative scission, giving a spectrum of chemicals ranging from soft, resinous materials to gaseous hydrocarbons which can be used as a substitute for natural gas. In addition, they believe that such an electrochemical process can be made commercially attractive under prevailing economic conditions. Mr. Coffman defines corona as a soft electrical discharge which is prevented from assuming the characteristics of an arc by electrode geometry or the presence of a solid dielectric barrier between electrodes. In the GE corona experiments, he says, finely powdered coal suspended in a hydrocarbon oil saturated with hydrogen at one atmosphere pressure is converted by corona to a number of chemical products. While all of the products of reaction have not been identified, Mr. Coffman predicts, from analogy to materials of similar structure, that at 200° C , a spectrum of aromatic materials, typified by phenol, will be obtained. At somewhat lower temperatures, the action will be less drastic and the principal product might be a soft, resinous material. At higher temperatures, say 350° C , the scission reactions will be more drastic, and the products mostly gaseous hydrocarbons, he adds. In one proposed reactor, bubbles created by pushing hydrogen through a sintered metal sparger encounter the corona from the high voltage electrode. The bubbles prevent the corona discharge from reaching the ground electrode. Radicals form in dynamic gas JUNE
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Hydrogen Balances This Way
eight product problems
EMULSIFYING THICKENING SUSPENDING STABILIZING
GELLING PLASTICIZING
BINDING FILM-FORMING one simple solution
pockets which develop adjacent to the high voltage electrode and diffuse into the liquid, reacting with the material there. The products thus formed can build up to substantial concentrations because they are relatively unattacked by the corona, Mr. Coffman says. He believes that the hydrogen in coal is too valuable to burn and proposes a commercial plant, utilizing coal as the only raw material to produce chemicals and electricity. In such a plant, a coal feed stream would be split into two parts. One part of the coal would be pulverized, made into an oily paste with still bottoms from the previous run, then fed to the reactor. Another part would be stripped of its hydrogen and methane by low temperature carbonization, and the gases also fed to the reactor. The coke or char from the stripping operation would be burned to supply steam for a turbine powering a high frequency generator and, in some cases, a standard 60-cycle unit. The plant can be thought of as a combined energy-chemical-fuel operation, highly flexible and capable of adjustment to the particular market to be served, Mr. Coffman adds. To make gaseous fuel as the primary product, three times as much coal would go through the stripping operation than through the pulverizer, the output of the plant being divided perhaps equally in value between electrical power and gaseous fuel. There is plenty of incentive for going forward with the corona discharge work. Coal production in the U.S. has slipped from 650 million tons per year in 1920 to an annual production of about 450 million tons last year.
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Gaseous Fuel. No one holds much hope for chemical uses providing a demand for appreciable quantities of coal in the near future. Dr. Lion cites disappointing horizons for tonnage coal requirements resulting from chemical processing in the years to come. Dr. Lion gives two reasons for making such a bleak prophecy. Even if all presently produced synthetic organic chemicals in the U.S. were made from coal, the amount of coal required would be of the order of 20 to 25 million tons—a figure representing only about 5CA of total annual coal production in this country. A second drawback to chemical uses for coal (given present technology and competitive factors) is that single plants using coal as the basic raw material for chemical production would have to be so large that the chemicals produced would, in most cases, exceed total annual demand for them. In the long run, however, Dr. Lion says, conversion of coal to gaseous and liquid materials is often regarded as a likely prospect. Substantial conversion of coal to gaseous and liquid materials could expand coal utilization enormously. For example, meeting 10% of current annual natural gas requirements from coal would alone involve some 50 million tons of coal, he estimates. GE will use a recently awarded contract (C&EN, April 9, page 23) from the Office of Coal Research for $750,000 to determine if the corona process can be commercialized to supply gaseous fuel. At present, corona coal experiments have only been in effect about six weeks and the results are inconclusive.
