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by B. F. Dudenbostel, Jr., and Wm. Priestley, Jr., Esso Research & Engineering Co.
Developments in Process Control Analysis in 1956 Uniform, high quality production can be best achieved by application of exact measurements on a continuous basis
I HIS is intended to be a survey of the major developments during the past year in the field of process control analysis. T h e future applications of process control analysis is certain to be accentuated by the following trends: 1. In the petroleum field, the replacement of empirical product specification tests by product composition specifications. 2. In the petrochemical field, the greatly accelerated production of feed stocks, intermediates, and chemical products of high purity. 3. The transition from batch to continuous manufacturing in the chemical industries. 4. The complete automation of plants which cannot be carried out without continuous analyzers. In the past few years, the use of electronic equipment in the plant has become widespread. T h e size of equipment now available has simplified e x p l o s i o n p r o o f i n g , although this has not as yet been done in the case of many types of instruments. Among the process control analytical techniques which have either been initiated or extended in the past year are those of mass spectrometry, infrared, ultraviolet, gas chromatography, flow colorimetry, refractive index measurements, and moisture analysis. O n e of the fields which is in use in many locations and in the process of further development is that of mass spectrometric process control. This technique provides an added area of information over continuous ultraviolet, nondispersion infrared,
and other analytical methods, giving data on molecular weight distribution. This has made possible the continuous analysis of complex multicomponent streams with a single instrument. O n e of the earliest applications in process control was that of nondispersion infrared analysis. T h e use of nondispersion infrared analyzers in plant streams for composition monitoring or control often pays for their cost in several months, as a result of improved process control. Better quality of product, reduced losses of valuable stream components, better unit efficiency, fewer plant upsets, and smoother operation are typical benefits. T h e usual profits from successful application far exceed those of the instrument manufacturer. About 1000 process control, nondispersion, infrared analyzers have been sold. T h e current annual sales of about 200 instruments are an indication of interest and the use of this equipment, in spite of the difficulties that have been encountered by both the instrument manufacturer and the user. As the uses of these instruments become more widely known in process industries, more nondispersion infrared analyzers will be applied to plant streams. T h e major problems encountered in the utilization of this technique have been sample handling and instrument maintenance. Of the total instruments produced, about 2 5 % are used in the medical field, and 5 5 % in the petroleum, chemical, and petrochemical field ; the re-
maining 2 0 % arc spread out through many other industries. Perhaps the oldest application of continuous analysis by spectrometry is in ultraviolet analysis. Quantitative determinations are made on a wide variety of both organic and inorganic compounds, including unsaturated hydrocarbons, ketones, benzene derivatives, naphthalenes, quinoncs, nitrates, and halogens. Heterocyclics, aromatics, and dicnes have strong ultraviolet absorptivitics, and are readily determined by ultraviolet analyzers. T h e biggest impetus to the application of this technique has been the development of commercial equipment, described in a previous Instrumentation article (March 1956, p. 81 A). M a n y products, of course, arc quality-controlled by their water content. T h e water level, even in trace quantities of feed stocks to many processes, particularly catalytic, is very critical. M a n y approaches to the problem of water analysis to process control have been discussed in previous Instrumentation articles in this journal. These analyzers have utilized various water properties, such as dew point, electrical resistance, infrared absorption, heat of adsorption, dielectric constant, and weight loss on evaporation. T w o new approaches to the problem of water determination for process control were discussed this year in the Instrumentation section of this journal. O n e of these (May 1956, p. 81 A) was nuclear magnetic resonance spectrometry, which is useful for determinations above the 1 % VOL. 49, NO. 1
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JANUARY 1957
99 A
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Circle No. 100 A on Readers' Service Card, page 139 A
100 A
INSTRUMENTATION
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A Workbook Feature
level in liquid a n d solid products. This, although so far being applied to process control applications in t h e laboratory, is also applicable for t h e continuous analysis of liquid samples directly in the field. T h e second analytical method utilizes the principle of electrolysis for detection of water content. This analyzer is capable of trace moisture detection (p.p.m. level) in gaseous streams. Continuous Plant Analyzers
T h e advent of continuous plant analyzers employing gas adsorption chromatography has excited the imagination of those in the field of process control analysis. These analyzers are currently being used for the continuous determination of propane and propylene in polymerization plants. Another instrument is being used in gas absorption plants. These analyzers are relatively expensive today (SI0,000 to $12,000 each), but the yearly savings resulting from their installation are apparently many times their cost. Previous to the use of these analyzers, samples were analyzed in the laboratory at t h e rate of about one per day. Now, complete analyses are available every half hour. Chromatography has replaced m a n y of the infrared a n d mass spectrometric methods of analysis in the laboratory. Its extension into the continuous analysis field m a y be even greater because of the reduced maintenance factor (less electronics) and the high selectivity maintenance, without loss of detection sensitivity which is commonly encountered in mass spectrometer a n d infrared analyzers. It is also believed that this tool will be more commonly accepted by plant personnel because of their familiarity with equipment of this nature. M a n y manufacturers arc constructing continuous chromatographic analyzers. Both adsorption and partition columns arc being employed. Recent literature data indicate that thermal conductivity cells now available have good stability, a n d a long life, which is an essential item in the utilization of this equipment. W e are just in the infancy of the application of this technique in the field of process control analysis.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
A color specification has been a guide to product quality for m a n y years in many industries. Color is a good criterion of product quality in m a n y cases. I n the past, this specification was tested in the laboratory. Today, with the advent of many continuous processes a n d the corresponding continuous analyzers, color specifications can be met by continuously monitoring plant streams. Among the many applications currently being used in thefield of continuous flow of colorimeters are t h e control of processes through the measurement of turbidity. T h e color rating of many products, such as refined vegetable oils, also c a n be carried out by continuous flow colorimetry. Once again, the availability of good commercial equipment has extended the application of this technique to process control.
Refractometers
Today, a n u m b e r of explosionproof, rugged process monitoring refractometers are on the market. T h e various applications to which this technique may be applied were discussed in a previous Instrumentation article (October 1956, p . 51 A ) . While refractometry has been a p plied to many process streams, there has been a limitation to the scope of these applications. M a n y processes arc amenable to monitoring and control by refractive index; however, because these streams a r e only slightly transparent or even opaque, the current instruments cannot be used. For example, in the manufacture of high boiling petroleum products, a relation between refractive index a n d quality exists. These streams arc too dark to be analyzed by transmission instruments, but will certainly be amenable to analysis when satisfactory equipment becomes available. We have indicated during the past year that uniform, high quality production can be best achieved through the application of exact measurements, which have replaced empirical tests a n d process control. This, of course, can be done to the greatest advantage if these tests a r e carried out on a continuous basis. M a r k e d savings in manufacturing costs a r e thus achieved.
