PROCESS CONTROL
Continuous Infrared Analy or Process Control A. SAVITZKY A N D E. H. WOODHULL The Perkin-Elmer Corp., Norwalk, Conn.
A.
P. WEBER,
Consulting Engineer
75 Park Row, New York, N . Y.
C
HEMICAL engineering design strives to produce plant products of desired purity with maximum yield and a t minimum cost. To satisfy purity demands, product and waste stream separations must be sharp. For maximum yield, reactions must proceed a t optimum chemical concentrations and without interference from undesirable side reactions. To produce a t minimum cost, the rate of conversion a t strategic process stages must be monitored and controlled. While control instruments for materials flow rate, temperature, pressure, and other physical properties relate their measurements t o these requirements, they cannot always account for basic process changes or fluctuations. These instruments indicate or control only the process stream environment, not the composition of the process itself. They cannot always account for variations in raw materials composition, catalyst poisoning, or unforeseen impurity accumulations. Until recently such deficiencies in instrumentation could be overcome only by sampling, running laboratory analyses, and relaying the laboratory findings back t o the plant. During this time delay the plant may have been running wastefully with heavy losses incurred, or it may have gotten out of hand while the trouble was being sought and corrected. Analytical instruments like the infrared analyzers are made to function directly in the process stream and provide a continuous and automatic composition analysis t o mitigate against sampling and laboratory time delays. They do not replace existing environmental controllers, but provide the means for resetting the controls of temperature, flow rate, or pressure. The day of complete process plant automation may not be far away, where computers will first receive the data from both the analytical and
environmental sensing elements and transmit automatically calculated corrective signals t o maintain the process controllers a t optimum settings ( 3 ) . Infrared process stream analyzers, by “fingerprinting” process stream compositions, are highly important t o the monitoring of continuous flow chemical reactor systems. I n most instances only through such on-the-spot analytical instrumentation is the conversion from batch to continuous reactor operation practicable. The well-known kinetic rate equations do not adequately define the material balance in a continuous reactor system ( 8 ) . I n these systems the primary reactant is being continuously removed by the reaction process as well as by the effluent stream from the reaction. Simultaneously the reactant is being continuously replenished by the feed stream t o the reactor. Further complication arises because in a continuous flow reactor system there does not exist any means whereby a uniform retention time in the reactor for all portions of the feed is ensured, There is short-circuiting of flow components from inlet t o outlet. The effluent from the reactor will be made up of portions which had stayed for various periods of time in the syetem. Yet, the degree of completion of reaction does vary as a function of the time spent in the system. T o control such R continuous flow system a quantitative picture of the feed and efluent compositions is mandatory, and infrared stream analyzers serve this function. Infrared absorption analysis is possible with inorganic and organic liquids and gases, except the monatomic gases. These fluids exhibit characteristic infrared absorption spectra which depend on the atomic weight, the kind of chemical bond, and the geometrical configuration of the molecules. When infrared
Figure 1. Schematic flow diagram of light hydrocarbon plant, showing possible locations of continuous analyzers
Figure 2. Schematic flow diagram of ethylene purification plant, showing possible locations of continuous analyzers
June 1956
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ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT radiation passes through a mixture of such fluids the radiation is absorbed in the wave lengths characteristic to the components and in proportion to their concentration. Knowledge of the wave lengths and the amounts of infrared energy thus absorbed provides an automatic stream analysis in a mixture of different suhstances. Such an instrument can be sensitized to analyze for only a single component in a mixture b y adjusting the optical syst'em to detect only the infrared wave length specific t o the component of interest.
laboratory spectrometer. The nondispersive analyzers do not have a dispersing means and are somewhat analogous to the laboratory filter photometer. Figure 4 shows two schematics of nondispersive instruments. The l o r e r schematic is that of the classic negative filter type (8, 6). The 'detectors are bolometers or thermopiles. The instrument is sensitized by placing the component of interest in the sensitizer cell. This gas absorbs energy at, its characteristic wave length. The instrument can then be balanced until there is no output from the detectors. If the gas of interest is placed in t,he sample cell, it will absorb a percentage of the energy in both beams. Since some of the energy a t the characteristic wave length has already been absorbed in the upper path, the change in energy reaching the detectors will be erealter for the lower detector than for the upper. This difference in detector signal is recorded as a measure of concentration. The upper schematic illustrates the Luft t'ype analyzer ( 4 ) . I n this instrument, the detector is a two-sided condenser microphone into which has been placed some of the gas of int,erest. This gas will absorb energy only a t its characterist,ic v a v e lengths. The instrument, is therefore sensitive to changes in energy only at these wave lengths with the addition or subtraction of heat resulting in a pressure change converted into an electrical outpnt by the condenser.
Figure 3. Schematic flow diagram of aromatics separation plant, showing possible locations of continuous analyzers
For example, in the natural gasoline plant illustrated schematically in Figure I, analyzer 1 measuies the concentration of hydrocarbons heavier than methane in the absorber overhead to check absorber efficiency. Analyzers 2a and 2b indicate ethane purity and ethane loss in the bottoms. The remaining analyzers perfoim similar functions on the other towers. The data provided permit operation a t the most profitable level a t all times. Infrared analytical control at five points in the ethylene plant outlined in Figure 2 provides maximum purity and recovery of product. First, by ethylene analysis of the feed stock for accounting purposes and proces- control; secondly, ethylene analysis beyond the crackers fol optimum efficiency; thirdly, ethylene analysis of absorber off-gas for absorber efficiency; fourthly, ethylene analysis in ethvlene tovier bottoms for fractionation efficiency; fifthly, end-point analysis for product purity. I n the last case, we do not analyze dilectly for the ethylene but rather for the impurities. Figure 3 demonstrates the possible placement of infrared analyzers in an aromatics separation unit. Analyzer la monitors benzene purity in the overhead n hile analyzer 1 b measures benzene loss in the bottoms of the benzene tower. The process operator uses these data to adjust t h e temperature and flow rate in the tover so that benzene loss in the bottoms is held t o a minimum consistent with the overhead product purity. I n a plant n-ith a 300-barrel/day benzene capacity, a 2 7 , increase in recovery pays for the analyzers in less than 6 months. Of particular importance is the continuous analysis of p-xylene purity with analyzer 2, where p-xylene is being separated from other xylenes. I n many cases, where the analysis is for the same component in different places, a single analyzer can handle several streams on a 1-minute/point basis.
Principles of Infrared Analyses Infrared continuous analyzers are generally grouped into two main classes-nondispersive and dispersive. The dispersive analyzers contain a prism or grating t o spread the radiation from a source into a spectrum. They are analogous t o the familiar
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A
t3 Figure 4. Schematic of nondispersive analyzers A. 5.
Luft type analyzer Negative filter type
Samples are introduced only in one side in contrast t o the negative analyzer. Any absorption a t the characteristic wave length by gas of interest will cause a different amount of energy to reach one of the two chambers. The difference causes a signal t o appear a t the output of the condenser microphone. There are pros and cons t o both these schemes. Primarily, the negative filter analyzer, the lower diagram, allows for better compensation of interfering components. T h a t is, if there is a material in the sample which has absorption bands that overlap the gas of interest, presence of this material in the sample cell will give the same kind of signal as does the gas of interest. This type of interference can often be eliminated by placing some of the interfering gas in the filter cell. This is true also in the Luft type instrument. However, filtering is effective only up t o a certain point. From then on, the introduction of interfering gas into the filtering cell has very little effect. I n the negative filter analyzer, some of this gas is also placed in the compensator cell to further reduce the interference.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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The prime disadvantage of the negative analyzer is that the black body detector is seeing energy over a far wider wave length range than is required by the problem. Because of this superfluous energy, the negative analyzer is always detecting a very small signal in the presence of a very large background, thus imposing severe stability requirements on the optical, electronic, and niechanical components of the system. Further problems, such as sensitivity to ambient and sample temperatures, are introduced by d.c. operation and these instruments usually are operated in this fashion.
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Figure 5.
tem improve the selectivity and the stability of the instrument. Thus, the most generally useful nondispersive analyzer is one which combines the so-called negative filter type of arrangement with a selective detector, and this combination is provided by several manufacturers. Figure 5 is a schematic of one such combination instrument (9). Examining only paths P%and PS, one can see the source, the sample cell which lies across the two beams, the compensator cell, the sensitizer cell, the filter cell, and the selective detector of this particular instrument. A third pathway for the radiation to follow, P1, has been added. It is chopped in phase with the sensitizer beam so that, as far as the detector is concerned, the energy coming down this path is in the same phase as the energy coming down the sensitizer path. This third path which is termed the ‘hulling” path allows the use of an optical feedback arrangement, employing a tapered trimmer to produce a radiation null a t the detector. If the attenuator were placed in either of the other beams, it would not be possible to achieve the present high order of discrimination. The loop response to be discussed later is that of the detector 4- elecattenuator optics. tronics Historically, almost all of the published literature and a very large proportion of current field experience deals with the older type of instruments. Thus, many analytical problems which have been shelved should be re-examined. Many of them can be solved today and with greater stability, sensitivity, and accuracy than was possible on even the relatively easy problems of a dozen years ago. The separation of isobutane from n-butane is a typical example. Here, it is desirable t o know the concentration in the range of about 0 t o 15% isobutane in, mainly, n-butane with some isopentane impurity present. If there were no isopentane present, this would be a very simple problem and actually has been solved where isopentane concentration was specified as never rising above 1% (6). However, it was never solved in the cases where isopentane concentration was much greater, since isopentane has strong spectral overlaps with isobutane. For the range of 0 t o 70% isopentane and 0 t o 90% n-butane, we found the problem truly difficult. However, it was solved on a nondispersive instrument using a selective detector and considerable compensation. Interestingly enough, the problem was also attempted by a dispersive type of continuous analyzer ( 7 ) . This instrument is capable of compensating for spectral overlap by the fact that it makes use of the laboratory technique called the base-line den-
Schematic of Tri-Non analyzer, showing complete closed loop
The standard Luft type analyzer is limited in regard to the compensation feature. As long as only the single-sided sample cell is used, compensation is not possible. However, the instrument’s use of the selective detector means that it measures only a t the wave lengths of interest. Therefore, most of the signal it receives is useful and results in a direct and very large gain in signal to noise ratio, of the order of 15 times for even a Rimple carbon dioxide analysis. A further stability factor is t h a t these systems are always chopped (that is, ax. amplification is used), resulting in lower ambient temperature sensitivity and little or no sensitivity to sample temperature change. The solution, then, is t o link these two instruments-the negative filter type has the advantage of allowing compensation, but the black detectors and d.c. operation make for a relatively unstable instrument. The selective detector and chopped sys-
DISPERSION UNIT (SHOWN ROTATED AT 90.)
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MAIN ASSEMBLY
LITTROW IMAGE 2
LITTROW IMAGE 1
Figure 6. June 1956
Schematic of Bichromator dispersive analyzer
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ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT sity method, employing a sample wave length and a reference wave length. Figure 6 shows the optical schematic of such an instrument. The portion in the upper left'-hand corner resembles the scheniatic of the conventional laboratory infmred spectrometer. Going backward through this system, there is a thermocouple, detector, the exit slit, a flat mirror, a collimating mirror, prism, and Littrow mirror. The essential difference between this inEtrument and a laboratory instrument is that here there are two Littrow mirrors, one determining the sample n.ave length and the other determining the reference wave length. Sample cell is placed a t the intermediate slit image t o minimize any drifts due t o dirt or fogging of the cell v-indows.
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Dynamic Analysis Initially, the infrared analyzer 15 as used only for measurement; hence the dynamics of the instrument and its associated sampling system were of fairly minor importance. However, with its increased use as a measuring elemefit for closed loop control the dynamic response has become of practical interest. Sampsle System Because sampling system requirements vary from one application t o another, one cannot describe a "standard" system. However, in general, a system mill consist of the necessary sample lines, filters to be sure an optically clear sample reaches the analyzer pressure regulator for gases, and solenoid valves if a single analyzer is to operate on more than one stream. A pump or aspirator may be required in special cases. The response of the sample line itself to a step change in concent'ration of the gas of interest is quite easy t'o visualhe. Since the pressure drop across the line remains constant the flow volume will remain constant if the viscosity remains unchanged (for laminar flow) or if the density remains constant (in t'he turbulent flow region). For the particular experiments t o be described, about 1% isobutane was introduced into a nitrogen stream as a step change. The resulting change in viscoeity or density will affect the volume flow rate b y less t,han 1/2v/o. Hence, the flow rate may be considered essentially constant and the average velocity with which a new concentration of sample proceeds down the tube is merely
sNE. PLE I
many applications which can be solved only on one or the other instrument.
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V O L . F L O W RATE SAMPLE C E L L V O L . VOL. FLOW RATE
TIME (b) Figure 7. Schematic of continuous sampling system ( a ) ; ideal response of analyzer to a step change a t the sample line input (b)
Average velocity =
The isobutane problem was solved on this instrument to approximately the same accuracy that was provided by the nondispersive analyzer. In this particular case, the choice between the two instruments could be made on the basis of price alone and actually was of concern only to the instrument manufacturer; hence, the selection of the nondispersive type. Although there is often an application overlap between the two instruments \\?here one may do the analysis as n-ell as the other, there are
On this basis the response of sample tube of a given length to a step change in concentration is that of a pure dead time, To, given by
To =
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Length - Sample line volume Average velocity Volume flow rate
When a volume is introduced in the sample line from such components as filters and traps, it is approximately true that the dead time is increased by an amount equal t o the volume of the
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Figure 8. Comparison of calculated and experimental system response to step change of isobutane concentration-laminar flow 1050
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Figure 9. Comparison of calculated and experimental system response to step change of isobutane concentration-turbulent flow
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 48, No. 6
PROCESS CONTROL
Figure 10.
Aerial view of American Cycnamid’s Fortier plant, showing number and location of Tri-Ncn analyzers
element divided by the volume flow through it. Hence, total dead time, 2’01 = Total sample volume to sample cells/volume flow rate. The sample cell is shown schematically in Figure 7 as a cylinder with the sample stream entering a t one end and emerging at the other. The radiation beam also enters a t one end through a n infrared transmitting window and proceeds by multiple reflections from the highly polished cell interior t o the infrared transmitting exit window a t the other end. For this particular condition of noncollimated and nondispersed radiation, the radiation transmission of the cell is reduced approximately linearly with concentration of gas of interest in the cell. Furthermore, as long as the gas molecule is in the cell whether near the entrance or exit ports, it reduces the cell transmission by the same amount. The response to a step change of gas concentration a t the entrance of the sample cell will then be a linear decrease in cell transmission until the new concentration has proceeded the length of the cell and has completely filled the cell with this concentration. With these simplifying assumptions the response of an ideal analyzer with a flat frequency response is shown in Figure 7b.
T,, the integrating time in seconds
=
Sample cell volume Volume flow rate in sample cell We see that the sample cells act as an integrator which, after time TI, saturates and gives a constant reading. By employing either the Poiseuille equation for laminar flow or Fanning’s equation for turbulent flow, it becomes possible to compute the dead time, TO, accurately.
To To
=
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< 2000 > 2000
d/2Pf It is clear that a step change in concentration a t the entrance June 1956
of the sample tubing does not result in a sharp concentration front a t the entrance of the sample cell. I n the case of laminar flow, the concentration front will be parabolic, whereas for turbulent flow the diffusion and mixing throughout the length of the tube will be considerable. The effect of a streamlined front is t o round off the sharp corners of the response curve. The Tri-Non analyzer is in itself a closed loop device, and its dynamic response may be obtained directly by applying servomechanism theory.
Experimental Data The lower curve in Figure 8 is the experimentally measured combined response of the sample line, sample cells, and analyzer for a flow rate of 0.02 cubic feet/minute and Re-520. The upper curve shows the computed concentration change in the sample cell. Dead time, To, equals 102 seconds, and TI yields approximately 16.6 seconds. The experimentally measured dead time is about 98 seconds, and the response is within 1% of its final value in about 114 seconds. Thus, a computed T I of 16.6 seconds is a reasonable approximation to the actual response. The difference represents the dynamics of the instrument itself. Figure 9 shows further experimental response for higher flow in the turbulent region. Here again, calculation of both To and T I gives an accurate approximation t o the concentration change, AC, which takes place in the sample cells. For the experimental condition shown (Figure 9 j where To GS 4 seconds and TI = 2 seconds the response is fast enough so t h a t the overshoot of the instrument photometer servo becomes apparent in the experimental curve. Since the dead time term is the predominant factor in the measuring system which affects closed loop stability, every attempt should be made t o reduce the dead time t o a minimum. This can be done by keeping the sample line short and the flow rate through the sample line high. If the analyzer must be located a considerable distance from the process and only a limited pressure drop across the sample line is available, the dead time can be greatly reduced by bringing the sample close to the analyzer by a larger diameter by-pass pipe. The sample
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ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
Figure 1 1 .
Outdoor location of 50-p.p.rn. analyzers for ammonia plant
CO and COS
line is tapped into this pipe close t o the instrument; hence a small flow rate through the analyzer is permissible XJ, ithout excessive dead time. I n fact, for liquid analyses, the by-pass is actually built into the sample cell.
Typical Installation A typical well integrated installation of continuous analyzers is in American Cyanamid's large plant near New Orleans, La. The principal product of the plant, built by Chemical Construction Co., is acrylonit,rile. Figure 10 is an aerial view of the plant shoIying the various units and the number of nondkpersive analyzers in each. The acrylonitrile is made from hydrogen cyanide and acetylene ( 1 ) . The acetylene unit was developed by Badisclie .inilin SodaFabrilc in Germany. I n the process, natural gas and oxygen are reacted in burners of unique design. Principal reaction product's are carbon dioxide, carbon monoxide, hydrogen, and acetylene. Oxygen for the process is supplied from an air separation plant which also produces nitrogen. Part of this nitrogen, plus hydrogen from the acetylene unit are reacted to make ammonia as a by-product. This unique acetylene process is a highly critical one, demanding precise operating conditions for maximum yield and minimum danger of explosion. I n the acetylene plant, a pair of
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Tri-Xon (Perkin-Elmer Corp.) analyzers is mounted near each of the two groups of four burners. One analyzer in each pair determines methane concentration in the burner product and the other, acetylene. An automatic sampling system successively admits a small portion of gas from each of t'he burners into the analyzers and the results are recorded on a chart for each analyzer in a control room tiso floors below. Sample cycling time is 8 minutes for four burners. From the burners, the product stream passes through filters for soot removal and then into compressors. It is important' t o know exact acetylene concentration a t this point since acetylene cannot be compressed beyond a certain pressure without danger of a violent explosion. A t the next step, the acetylene absorber, another analyzer samples the off-gas. This gas goes to the ammonia plant, and acetylene concentration must be less than 0.3%. Two other analyzers monitor efficiency of carbon dioxide removal in the purification tower. One determines carbon dioxide concentration half w y up the tolyer, and the other measures carbon dioxide in the acetylene product, where it is held to less than a few tenths of 1%. Instruments are mounted Fell up on the tower structures to minimize the sample lags referred to previously. The two analyzers in Figure 11 are installed in the ammonia unit of the Fortier plant. Their task is to monitor the concentra,tion of oxygen-containing compounds in the feed streams to the ammonia reactor. Such compounds n-ould quickly poison the catalyst R-ere they allowed to reach the reactor. One analyzer checks on the efficiency of carbon dioxide removal in the caustic poda scrubbers. I-Iere carbon diovide level is held in the 5-p.p.m. range. The other analyzer measures traces of carbon monoxide a t the outlet of the caustic scruhbers. Scale range of each of these instruments is 0 to 50 p.p.m. of impurity a t 100 pounds/ square inch gage. The shelter provides the necessary protection from sun and rain. After over a year of operation, the operators of the Fortier plant have formed a number of iniportant conclusions as to the value of their infrared analyzers. 1. First, and most important the analyzers provide in the control rooni a practically instantaneous indication of stream conditions. Thus, the operating men can see immediately the effects of any changes they make in the operating variables of the process so that top utilization of material is ensured, and product purity is maintained. 2. There is ample warning of any dangerous situations developing in the process-important t o safety of personnel and equipment. 3. There is the ability to make more efficient use of control laboratory personnel, since the mstallation of these continuoue analyzers has allowed the control laboratory to extend the scope of its operations.
Litasahwe Cite (1) Chem. Eng. 62,258 (September 1955). (2) Fa$tie, W. C., Pfund, A. I€., J . O p t . SOC.A m r . 37, 762 (1947). (3) IND.ENG.CHEW46,1371-1435 (1954). (4) Luft. K. F., Zeits, F., Tech. P h y s i k 5 , 9 7 (1943). ( 5 ) Martin, E. L., Thoinas, B. U., IND.ENG.CFIEX 46, 1352 (1054). (6) Patterson, W. A,, Chern. Eng. 59, 132-6 (September 1952). (7) Savitaky, A., Rresky, D. R..ISD.EXG.C?iin.M. 46, 1352 (1984). (8) Weber, A. P.. Chem. Eng. Progr. 44, 26 (1 953). (9) Woodhull, E. E., Siegler. E. II.,Gobcov, €I., IND. ENG.CHEM. 46, 1396 (1954).
RECEIVED for review
January 26, 1956.
INDUSTRIAL AND ENGINEERING CHEMISTRY
ACCEPTEDApril 6, 1958.
voi. 48, N ~ 6.
