Latour, P. R., PhD Thesis, Purdue University, West Lafayette, Ind., 1966. Latour, P. R., Koppel, L. B., Ind. Eng. Chem. Fundam., 4, 463
GREEKLETTERS Y = E - ((1 10-’/Ka)
+
{ = concentration of sodium in CSTR
E
concentration of acetate in CSTR
=
SUBSCRIFTS
f
345-53 (1968).
final initial steady-state
= o = s =
\-.,””,.
114fi.5)
Latour, P. R., Koppel, L. B., Coughanowr, 11. R., Ind. Eng. Chem. Process Des. Develop., 6, 452-60 (1967). Latour, P. R., Koppel, L. B., Coughanowr, D. R., ibid., 7,
literature Cited
Brosilow, C. B., Naphthali, L. M., Chem. Eng. Sci., 20, 1125-38 (1965).
McAvoy, T. J., Simulation, submitted for publication, 1970. McAvoy, T. J., Hsu, E., Lowenthal, S., Ind. Eng. Chem. Process Des. Develop., 11 ( I ) , 68 (1972). Mellichamp, D. A., Coughanowr, D. R., Koppel, L. B., AIChE J., 12, 75-82 (January 1966a). Mellichamp, D. A,, Coughanowr, D. R., Koppel, L. B., ibid., 12, 83-9 (Januarv 1966b3. Nutting, D. C.: Znstrubnent Practice, 8, pp 50, 123, 221, 327, 416 (19.54). \ - - - - ,
Campbell, C. G., Scaife, R. L., Landwehr, J . C., I S A J., 5, 52-5 (July 1958). Chang,. J. W., “Proceedings of Symposium o n Automatic Control In Chemical Process and Allied Industries,” Society of Chemistry, England, Liverpool (1964). Chaplin, A. L., “Applications of Industrial pH Control,” Instruments Publishing, Pittsburgh, Pa., 1950. Chaplin, A. L., Instruments, 22, 579 (1949). Cotter, J. E., Takahashi, Y . , Chem. Eng. Symp. Ser., 59 (46), 119 (1963).
Douglas, J. M., Chem. Eng. Sci., 21, 519-32 (1966). Field, W. B., I S A J., 6 , 4 2 - 9 (January 1956). Hutchinson, J. F., hIcAvoy, T. J., Znd. Eng. Chem. Process Des. Develop., PhD Dissertation, University of Massachusetts, Amherst, Mass., 1970). Javinsky, M. A., Kadlec, R. H., Symposium on St,ability and Control of Reaction Systems: Part 11, 63rd Meeting of A.I. Ch.E., St. Loiiis, hlo., February 1968. Koppel, L. B., “Introduction to Control Theory,” pp 77-86, 315-19, 366-82, Prentice-Hall, Englewood Cliffs, N.J., 1968.
O’Connor, I. E., Denn, AI. 11..Chem. Eng. Sci., in press. Pontryagin, 1,. S., tBoltyanski, V. G., Gamkrelidge, R. V., MishProchenko, E. F., The b4athematical Theory of Optimal cesses.” Wilev‘and Sons, New York. X.Y.. 1962. Rowton,’E . E.,instrum. f‘echnol., pp 62-4 (June 1968). Shinsky, F. G., “Process Control Systems,” p 275, McGrsLwHill, New York, K.Y., 1967. Shinsky, F. G., Inslrum. Technol., pp 65-9 (June 1968). Siebenthal, C. I)., Aris, R., Chem. Eng. Sci., 19, 729 (1964). Wheeler, J . AT,, Ark, R., ibid., 25, 445 (1970). Wilson, U. S., Wylupek, W. J., ZSAJ., 15, 41-6 (July 1965). Ziegler, J., Nichols, X., Trans. A S M E , 64, 759 (1942). Ziegler, J., Nichols, N., ibid., 65, 433 (1943). I t ~ c m ~for i : review ~ December 21, 1970 ACCEPTEDMay 17, 1971 This work was supported by the Kational Science Foundation under grant GK 2982. Calculations were carried out at the Hybrid Simulation Center of the University of Massachusetts established through the support of the rl’ational Science Foundation.
Improved Computer-Compatible Process Chromatograph Graham F. Freeguard and Carl 1. Pulfordl Department of Chemical Engineering, University of Nottingham, Nottingham, NG7 ZRD, England
Modifications to a conventional process chromatograph enable continual acquisition and on-line processing of the resulting high-quality analytical data to b e undertaken during process runs. The chromatographcomputer interfacing system is used in an investigation of ethyl acrylate-styrene copolymer produced in a continuous stirred-tank reactor (CSTR). Some advantages over more conventional on-line gas chromatograph systems are discussed.
w h e n optimization of a process is required, gas chromatographic data are usually obtained by sample analysis offstream in a laboratory. On-stream analysis is desirable, but the data obtained are often of inferior quality because of the relatively unsophisticated process machine available. However, where high-quality analytical data are required from onstream analyses, one solution is modification of a process chromatograph to improve its analytical capability while
1
To whom correspondence should be addressed.
7 8 Ind.
Eng. Chem. Process Des. Develop., Vol. 1 1 , No. 1 , 1972
leaving unaltered its function as a process machine. Extending the dynamic range of the machine and improving the output facilities are more efficient and easier to achieve than making a laboratory instrument suitable for plant use. Generally, such improvements alone will be sufficient for normal purposes. However, if computer facilities exist and on-line data processing is desirable, the software problems and computer costs can be less formidable with the suggested arrangements than with a modified laboratory instrument if the usually adopted procedures (Briggs, 1967) for laboratory instruments are followed.
Apparatus
The chromatograph available for this project was a convent~ional machine fully purged for use in hazardous areas and comparatively recent'ly available a t the time. The frequency of the analysis (typically 10 min) is governed by a synchronous motor driving a bank of adjustable cams which control the sequeiitial operation of the analysis. These cams control mirroswjtches which activate the circuits for cycle start aiid stop, recorder auto zero, automatic sample injection, stream change selection, and column switching, and the six peak gate circuits which control the activation of t.he recorder for the individual recording of required component peaks when wing a bar graph facilit'y. Each gate is adjusted so that it only alloivs the detector response to a single peak t,hrough to the recorder, and this response may be intlividually attenuated with the precision potentiometer associated with each gate. All automatic sampling and switching is done with ~)neumatically operated sampling valves (1.C.I Ltd., 1956) which operat,e by the movement of a grooved PTFE block over open p o r h in a stainless st,eel plate. The detector signal i. fed to a N a r k 3 Kent, Recorder (fsd 2.5 mV) by means of a 0-5 V retransmission slidewire in the recorder to the signal bias unit in a Kent process control unit. This modular system consists of a 0-1 V signal bias unit, two-stage peak store, chromatographic process controller, and a single-channel servo-foIlowerjalarm. The bias unit is used t o back off the chromatograph detector standing voltage before the signal is fed to the logic modules. 1his proce5s control unit controls the reactor output composition by monitoring the ratio between two selected peaks in each chromatogram and by using the difference bebn-een this ratio and a preset desired value to drive a conventional elect'roi)iieuniat,ic converter which alters the set point of a diaphragm valve controlling t8heflow of one of the feed streams. I n the present work a n immediat'e requirement was a n effective increase in the detect,or sensitivity. This was achieved a factor of 103-the attenuator ranges by extending-by available to the detector signal prior to amplification b y nieans of additional precision resistor and capacitor networks. l h e selection of detector sensitivity for a given peak was made automatic by hardware additions to the chromatograph. Two ext'ra cams with appropriat,ely cut profiles were fitted to the synchronous motor drive to select range and sensitivity within a range, respectively. Xicroswitches activated by these cams route =I= 10 V (depending on t,he switch condition) t,o t'wo illput change devices in the computer interface. Soft'ware detection of the states of the two devices enables the appropriate correct,ioii factor to be applied to the detector output'. When a n instrument is linked on-line to a computer, interfacing problems arise because of t,he basic differences in the philosophies governing the design of the two machines (Brown, 1968). I n the present case the chromatograph and computer (a Digit>alEquipment Corp. PDP8 with 4K store and a patchboard interface constructed in the department) were not compatible primarily because of the different earth potentials of the two machiiies, the chromatograph "eart,h" being, in fact, the 50-cyrle mains neutral line. Consequently, no direct elect.rica1 connection was possible. However, to minimize the computer time required for this esercise, a system for the transfer of information about the state of the analysis was 1iecesr:iry. The cliromatograph I'riniary triggering voltages are 12 Ir and 24 V floating, aiid these are rout,ed through the niicror 7
, 7
switches controlled b y the cam operation. The voltngea are necessary for the automatic operation of the chromntogrnph and the process coiitrol logic and are present a t the start and , at' sample injection, and during peak emergence. By suitable extension of the chromat,ograph circuits, these voltages can be used to activate relays whose contact positions will t.hen reflect, but not interfere with, the occurrence of t'he events mentioned. The relay contacts are connected to Schmitt trigger interrupts. With a simple interrupt-servicing software routine, i t is possible to define uniquely all significant event,s in each chromatogram wit'hout a direct electrical link occurring between chromatograph and computer. By employment of this system of interrupts instead of continual software monit,oring of the continuous detector output, access to the computer can be restricted to a small percentage of the total analysis time without loss of relevant information. Also, by identification of the start of each analysis, the effects of a single-faulty analysis are minimized. Each analysis is completely independent of all other analyses made during t'he run, and, b y use of t,he internal marker technique, depends only on t,hree prerun calibration analyses to calculate absolute concentrat,ions. With this technique the absolute concentration of the marker compound is known and unchanging. Consequent,ly, it is possible by simple ratios of peak height's or peak areas to determine the absolute coucent'rations of other components irrespective of any variation in sample size between analyses. The masimum signal at, the recorder is 2.5 mV. Rather than amplifying this for use as the input signal, it is advantageous to use directly the 0-+4.5 V signal available a t the bias unit output representative of that part of the detector output corresponding to a n emerging peak. This signal is used directly to supply the analog to digital converter (ADC). The digitalization rate is 20 sec-', sufficient to follow every significant change in the detector response. The signal from the detector is digitalized only vhen peaks are emerging. A permanent record of the analysis is available a t the t,eletype, but, temporary records are available by use of two 10-bit digital to analog converters (D;lC's) in the interface. These devices output the current reactant concentrat,ions to digital voltmeters or pen recorders. Application
An int'erest'iiig application of the system has been in a detailed investigation of the copolymerization, in toluene solution, of styrene and ethyl acrylate in a CSTR.Precise values for reactant concentrations and the kinetic reactivity ratios are necessary to predict operat'ing concent'rations for the production of copolymer of specific composition. Usually these conceiit,rations are initially determined in the laboratory from lowconversion batch experiments. However, during subsequent scale-up and continuous operation, some adjustment to the monomer ratio from that initially predicted may be necessary because of either the scale of operation or the variability of feed stock quality. The analysis was performed with nitrogen as the carrier gas, a 10-ft 10% SE30 column at' llO"C, and a flame ionization detector. Hexane, which is accurately metered a t a constant proportion into the react,or output as it enters the chromatograph, is used as a n internal marker and thus eliminates all dependence upon the reproducibility of t,he sample size. The peak gates are set SO that only t'he monomers and hexane are recorded, in the order hexane, ethyl acrylate, styrene. The program is stored on magnetic tape and is self-initiating when called. Certain initial data are required. These are inInd. Eng. Chem. Process Des. Develop., Vol. 1 1 , No. 1 , 1972
79
.A
7 .
.-
6 -
3
5 -
b
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Figure 1. Weight percent of ethyl acrylate vs. time
70
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BY PROCESS CHROMATOGRAPH
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Figure 2. Comparison of theoretical and experimental attainments of steady state
putted by t’eletype with the computer in a conversational mode and are the monomer concentrations, the ratio of the two monomers required in the polymer produced-i.e., the required mer ratio-and a tolerance figure, the purpose of which is t,o enable a check of the function of the reactor, its ancillaries, and the chromatograph to be made. When three successive analyses of the unreacted reactor output are within t,he required tolerance, a caption for the subsequent output is printed. This indicates that the complete tern is functioning satisfactorily and that the polymerization may be initiated. The mean of the three analyses is used to determine the constants necessary for the calculation of the absolute concen80
Ind. Eng. Chem. Process Des. Develop., Vol. 1 1 , No. 1, 1972
trations in each sample analyzed. At present t8heparameters outputted are the residual monomer concentrations, polymer concentration, actual mer ratio, the difference between the actual and required mer ratios, and t,he results of two algorithms which enable the reactivity ratios to be calculated. By allowing the “process control” aspect of the chromatograph to function unhindered, the computer time available to the experiment is used primarily for calculating kinetic parameters of bhe polymerization. Figure 1 shows a plot of a typical variation of the ethyl acrylate concentration in the react,or output from start-up to equilibrium by use of data taken from t’he computer out-
put. S o t e that the ol~viouslyspurious result a t X has had no effect upon subsequent' calculat'ions. Figure 2 shows the results of part of an invei;t,igationinto the at,t:tinment of t8hesteady state of t,lie reactor. Data from the computer output are shown toget,her with some results obtained by a mai;s balance (by weighing) on the reactor discharge. Also shown is R predicted attainment of the steady state derived b y w e of t'he process equations and updated kinetic parameters. ,In important feature of tlie technique is tlie ease with which such a n investigation can he carried out, as a matter of routinr. during normal production.
Efficient use has been made of computer time b y retaining tlie existing control system of the chromatograph (which incidentally maintains the off-line capability) and by using additional hardware rather than software to monitor the state of the a n a l p i s and to determine the periods of data acquisition. Conclusions
The syst'em has funct,ioned satisfactorily for 18 months aiid has been extensively used. I n the event of a computer hardware failure, tlie chromatograph mill still perform all essential analytical and coiit'rol operations. However, in practice the Discussion The program has been written specifically for the ~ x ~ r t ~ i c i i l a r main failures have been in t'he unmodified aspects of the chromatograph, aiid this reinforces the philosophy that instrudescribed, but I)ec:iu
