Document oDjd7KVeZJdVYYmMK8g05MvE

i his u one of a series of advertisements dealing u>ith basic facts about process control. Reprints of all the advertisements in this series are available. URL 17979 Basic Cascade-Control Systems disturbances to a manipulated variable such os steam flow or steam pressure make it very difficult to maintain close control of long time-constant processes. In such situations, o simple single-loop control system proves inadequate-- because the controller cannot act until after an error has developed in the primary process variable, (i.e. until a dis turbance So the manipulated variable has upset the entire process.) To insure dose control of such processes, coscode control cir cuits are generally employed to compensate immediately for disturbances to the manipulated variable. Eliminating Process Upsets In a cascade system, two controllers are connected in series --with the first (or primary) controller sensing and being ulti mately responsible for the control of the primary process variable. The output of the primary controller is the setpoint "command" signal to the secondary controller. The secondary controller, in turn, senses and regulates the manipulated variable by modulating the control valve. Thus, if there are any disturbances which would tend to upset the manipulated variable, such disturbances will be sensed immediately--and appropriate corrections will be made by the secondary controller before a disturbance can affect the primary process variable. The basic advantage of coscode control can be illustrated with a heat exchanger. In this case, the primary controller measures the outlet temperature (primary variable) and pro vides o setpoint signal to the secondary controller. The sec ondary controller measures and controls steam pressure (the manipulated variable). Thus, if the steam pressure changes, as it commonly will with variation in demand on the steamsupply line, the effect on steam pressure -to the heat exchanger will be sensed instantly, and corrected for by the secondary controller. Consequently, there will be no effect on outlet temperature. Other Cascade Applications Frequent applications for cascaded multiloop systems in clude: Froctionating-column temperature control, with a tem perature controller setting either a steam-pressure or a steam-flow controller; batch-reactor temperature control, with a batch-temperature controller setting a jacket-temper ature controller; fired-heoter temperature control, with an outlet-temperature controller setting a fuel-rate controller. Moore engineers will be pleosed to help you with any prob lem related to process control. Feel free to call for their serv ices; their time is yours--without obligation. If you are planning a cascade control loop, you should con tact the nearest office of Moore Products Co. for information about single-station cascade control. Or mark reader inquiry card for Bulletins 5019 and 5022--each contains information about "cascade control in a single station." Chemical lagmaaviqg--April 10, 1967 [MOORE] MOORE PRODUCTS CO.. Spring Housa, Pa. In Canada: Moor* Instrument Co., ltd., Rexdale, Ortt. Orel* 515 an Reader Service Card 225 URL 17980 CE REFRESHER . . . to produce a pound of vinyl chloride by a typical Process I or Process III. Values for Process III were found in the literature,8-and those for Process I were estimated from available information. Cost of reactants were assumed to be 84/lb. for acetylene, 44/lb. for ethylene, and 2.54/lb. for chlorine. Using these values, the total cost of reactants for Processes I and III were calculated, and are shown in Table I. Total cost of reactants for Process I is decidedly higher because of the relatively high price for acetylene. Operating costs, other than those for reactants, are prob ably not significantly different for the two processes. As a rule, total production costs per pound of vinyl chloride are less for Process III. Unless the cost of acetylene relative to that of ethylene is significantly reduced, Process I will likely be of lesser importance in the future. Companies that have cheap supplies of both ethylene and hydrogen chloride may And it advantageous to use a simplified Process III. They would use a com bination of oxychlorination of ethylene plus pyrolysis of dichloroethane. Process II is claimed to produce cheaper vinyl chlo ride than can be produced by Process !.* *,8> 32 Costs of acetylene and ethylene are cheaper than in Proc ess I. Capital costs for a Process II plant are signif icantly greater because a naphtha pyrolysis unit must be installed and because dilute hydrocarbon streams complicate to some extent the separation and reaction phases of the process. A Kureha plant to produce 66 million Ib./yr. of vinyl chloride had a total investment cost of ?4,150,000.18 It is estimated that a plant if scaled up to 200 million lb./yr. would cost about $9 million. Although detailed and comparable cost data for Processes II and III are not available, both processes seem to offer significant advantages in terms of pro duction costs for most American companies as com pared to previous vinyl chloride processes. Many fac tors, including cost and availability of reactants, would have to be thoroughly considered before a final deci sion could be made as to the preferred process. Process IV will be seriously considered by many companies in the future because of its apparently in creased simplicity and high yields of products. Con siderable industrial research is known to be occurring in this area. Up to now, the Kellogg process is the only one in which a significant amount of design data have been reported. A plant based on this process would likely be relatively simple and cheaper than one based on Process III. Process IV also has the advantage of increased flexibility of operation. Variations in avail abilities of chlorine and hydrogen chloride, or changes in relative demands for vinyl chloride and 1,2-dichloroethane, can be more easily handled than in Process III. Because of the complexity of the plant, a balanced process requires excellent instrumentation for control purposes. An upset in any portion of the plant tends to upset the remainder of it. Methods of starting up the unit, and maintaining the desired optimum con ditions as the catalyst ages and as flows and yields vary, require careful planning. Kureha has described in considerable detail its methods of handling this problem.13 References 1. Albright, L. F., Vinyl Chloride Processes. Chem. Ena., Mar. 27, 1967, p. 123. 2. Barton, D. H. R. and Howlett, K. E,, Kinetics of the Dehy- drochlorlnation of Substituted Hydrocarbons, J. Chem. 8oe., 148-164 (1949). 3. Braeonier. F. F.. Manufacture of Vinyl Chloride Starting With Naphtha, Oxygen and Chlorine, Chem. Age India. June 1963, p. 463. ' Braeonier, F. F., How S.B.A. Makes Vinyl Chloride, Hydro carbon Proceae., Nov. 1964, p, 140. 5. Buckley, J. A., Vinyl Chloride via Direct Chlorination and Oxychlorination, Chem. Eng., Nov. 21, 1966, pp. 102-104. 6. Burke, D. P. and Miller, R., Oxychlorination, Chem. Week, Aug. 22. 1964, pp. 93-118. 7. Carroll, R. T. and Dewitt, E. J. (to B. F. Goodrich Co.), Oiychlorination of Lower Alkanes," U.S. Patent 3,173,962 (Mar. 16, 1665). 101 108^ StroX* at Calvert City, Chem. Week, Aug, 29, 1964, pp. 9. _E. I. du Pont de Nemours A Co.. Inc., "Chemical Process and Catalyst," British Patent 941,363 (Nov. 13, 196$); Belgium Patent 614,630 (Sept. I, 1963). 10. Edwards, E. F, and Weaver, T., New Route to Vinyl Chloride. Chem. Eng. Progr., Jan. 1965, p. 21. 11. Fontana. C. M., Gorin, E.. Kidder, O. A. and Kinney. R. E., Oxygen Equilibrium Pressures and Oxide Solubility in the Melt, Ind. Eng. Chem., 44, 369 (1952). 12. Fontana, C. M., Gorin, E.. Kidder, Q. A. and Meredith, C. 8., Ternary System Cuprous Chloride-Cupric Chloride-Potas sium Chloride and Its Equilibrium Chlorine Pressures Ind. Eng. Chem., 44. 363 (1952). 13. Fontana. C. M., Gorin. E. and Meredith, C. S-, Kinetics of Oxygen Absorption by the Melt, Ind. Eng. Chem., 44. 373 (1952). _ 14. Friend, L, Wender, 1* and Yarse, J. C,, Liquid-Phase Oxychlorination Provides High Selectivity Route to Vinyl Chlor ide, paper given at Division of Petroleum Chemistry. American Chemical Society Meeting, New York, Sept. 11-16, 1966. 15. Gomi, S., Japan's New Vinyl Chloride Process, Hydro carbon Proceae., Nov. 1964, p. 165. 16. B. F. Goodrich Co., "Preparation of Vinyl Chloride," British Patent 938,824 (Oct. 9. 1963). 17. Gorin, E., Fontana, C. M. and Kidder. G. A.. Chlorination of Methane With Copper Chloride Melts, Ind. Eng. Chem., 44, 2128-2138 (1948). 18. Heinemann. H., Miller. K. D. and Spector, M. L, Olefin Chlorination in Homogeneous Aqueous Copper Chloride Solu tions, paper given at Division of Petroleum Chemistry. American Chemical Society Meeting, New York, Sept 11-16, 1966. 19. Flowsheets. Hydrocarbon Proceae., Nov. 1968, pp. 239-240 : Nov. 1966. pp. 198, 288-290. _ 20. Imperial Chemical Industries. Ltd., "Chlorohydrocarbons." Belgium Patent 632,044 (Nov. 18. 1963). 21. Kapralova, G. A. and Semenov, N. N., Study of the Mechan ism of 1.2-dichloroethane Decomposition by the Calorimetric Method. Zh. Fie. Khim., 17, 7$ (1963). 22. M. W. Kellogg Co., private communication, 1966. 23. Krehler, H. (to Farbwerke Hoeohat). "Process of Prepar ing Vinyl Chloride," U.S. Patent 2.724,006 (Nov. 15. 1955). 24. Nalr, K. S.. Notes on the Manufacture of Commercially Important Products, Chem. Age India, May 1966. p. 382. 25. Robert. C. P., Electrolytic Recovery of Chlorine From Hydrogen Chloride, Chem. Eng. Progr.. 44, 456 (1950), 26. Shell International Research, "Halogenatlon of Hydro carbons." British Patent 907,435 (Oct. 3. 1962). 27. Societe Beige de L'Axote, "Vinyl Chloride," French Patent 1,290,953 (Mar. 12, 1962). 28. Societe Beige de L'Asote, "Process for the Preparation of Vinyl Chloride," British Patent 954.791 (Apr. 8. 1964). 29. Vulcan Materials Co.. "Oxychlorination Process," British Patent 980.983 (Jan. 20. 1965). 30. Vulcan Materials Co., 1, 2-Dichloroethane by Oxychlori nation Reaction. Report (1966). 31. Wacker-Chemie gmbH., "Prevention of Vinyl Chloride De composition During the Catalytic Dissociation of Dichloroe thane." Belgium Patent 610,498 (May 21, 1962); German Patent 1,135,451 (Nov. 25, 1960). 32. Washlml, K. and Asaktira, M., Computer Control of a Vinyl Chloride Plant Provides Process Optimisation, Chem. Eng., Oct, 24. 1966. pp, 133-138; Nov. 21, 1966. pp, 121-126. 33. Wolf, F., Runge, F. and Korn, R.. The Catalytic Oxidation of Hydrogen Chloride to Chlorine With Oxygen, Z. Anorg. u. Allgem. Chem., 48, 304 (i960). Key Concepts for This Article Active <8) Passive <) Inpat/Peedetsck (DOstpst/Product (2) Reviewing Processes* Acetylene* Analyzing Chemistry* Ethylene* Vinyl Chlorine* chloride* Hydrogen Hyndartoiocnh*lorl- achnlhoyrdidreo,us* Vinyl chloride* Ethylene dichloride* Special Agent ,(44), Chlorination* Air* Pyrolysis* Oxygen* Reactors* Copper chloride* Carbon* (Words In bold are role indicators; numbers correspond to EJC- AlChE system except for Role 8 modification. Asterisks mark key concepts suggested for indexing. Others are added to improve reading as an abstract. Indexing is described In Chem. Eng., Oct. II, 1965, p. 187 ; or you may order Key Concept Reprint. 594, using Reader Service Postcard.) 226 April 10.1967--Chemical Engiiwaring