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INSTRUMENTATION
W O R K B O O K
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by B. F. Dudenbostel, Jr., and Wm. Priestley, Jr., Esso Research & Engineering Co.
Application of Process Control Analysis to Petroleum Refining Continuous analytical monitoring can be applied to four basic processes: fractionation, transformation, purification, blending β \ PETROLEUM refinery is the factory where the raw material—crude pe troleum—is converted into usable finished products. It is the manu facturing phase of the oil industry. This phase is of importance here be cause of the applications of process control analysis to the refining process. The function of the refinery is to convert crude oil into the finished products required by the market, in the most efficient, but most profit able manner. The methods em ployed will vary widely, of course, from one refinery to another, de pending on the crude process, the nature and location of the market, the type of equipment available, and many other factors. However,
the application of continuous analysis to refining process is most advan tageous costwise, regardless of these factors. The various refining processes will be mentioned in general terms, merely to indicate where certain process control analyses can be utilized. The first category in refining is fractionation or distillation. This method of physically separating a mixture of compounds was the earliest process used in petroleum re fining, and today is still one of the most important. However, since it is not normally possible to separate complex petroleum mixtures into individual compounds, these mix tures are segregated into fractions,
each of which is characterized by a carefully controlled boiling range. This, of course, suggests the utilization of continuous monitoring of these components, which will be discussed later. These fractions may then be further processed or utilized by various refinery operations. A second basic type of process, essentially a chemical one, consists of converting or transforming certain of the fractions into products of higher commercial value and utility. There are many ways of doing this, but all consist fundamentally of altering the molecular structure of the components. Once again, because of these changes, application of physical methods makes continuous monitoring possible. The var-
RUDE OIL
GAS OIL
DISTILLATION
CATALY" CRACKING II
Control Regeneration!
Entraînaient by Flow Colorimetry
0, CO Detn.
Temperature, Pressure, Flow
1
LIGHT NAPHTHA
'
LIGHT ENDS
Gasoline Cut Point By UV
DEHYDROGENATION Composition1 ft./., 60s Chromatography
CompositionOctane No. by ft./., UV, M.S.
Fundamental Refinery O p e r a t i o n
SEPARAI
SEPARATION
Composition by R.I. Gas'Chrom., HtO Content Analyzer
Composition' ft./, UV, Flash Point by Cont. Anal.
POLYMER 12
ALKYLA1
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IANAL YSI s|
VOL. 48, NO. 11
IOCTAIME NO.I
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NOVEMBER 1956
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INSTRUMENTATION
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A Workbook Feature
ious transformation processes include catalytic cracking, where high boil ing fractions may be cracked to form lighter, more valuable products. What might be considered the re verse process is the conversion of gaseous products by polymerization and alkylation processes to high quality liquid products which may be blended into the gasoline. Cer tain processes, such as catalytic re forming, include cracking and poly merization along with dehydrogenation, hydrogénation, and isomerization reactions. In all of these, changes which occur in the molecules result in differences in physical properties. Thus, these processes may be continuously monitored. In nearly all cases, the various products or fractions produced by the processes mentioned above contain certain objectionable constituents or impurities. This leads to the third basic category or refining, purification. This is no more than the removal of unwanted components or the conversion to innocuous or less desirable products or compounds. Removal of these impurities may be accomplished by physical methods such as extraction processes. Many such methods are amenable to continuous analysis. The fourth basic category is the blending of the finished fractions into commercially salable products such as motor gasoline, kerosine, lube oils, bunker fuels, etc. These are the fundamental operations of a refinery, and each is amenable to continuous analytical monitoring. Certain of these are already controlled in such a fashion. The accompanying illustration points out a few of these applications. The first step in refining a crude oil is the distillation process. This is a simple pipe still distillation, atmospheric and vacuum. In addition to the normally utilized temperature, pressure, and flow indicators, flow colorimetry is an important monitoring device for this operation. Continuous measurement of the color and density of the flowing sample makes it possible to follow the operation of these stills through a determination of the amount of entrainment in the overhead streams. It is essential that the material—sediment, metallic components, and the like—be re50 A
moved from the stocks. For simplicity we may say that the products of this distillation include gases, light and heavy naphtha fractions, and gas oil fractions. The gaseous fractions may be monitored by continuous refractometry and by gas chromatography. In actual refinery operation, these gaseous products are, of course, blended with those from other processes such as will be briefly mentioned later. These gaseous fractions are often rich in components, such as normal and isobutane, which are amenable to continuous monitoring by continuous refractometry or gas chromatography. Process control refractometers are available commercially. Gas chromatographic analyzers are not yet available commercially, but they will soon appear on the market. The feasibility of the application of this analytical technique has been demonstrated in the field. It is possible by either method to detect changes of the order of 0 . 1 % in the composition of gas streams. The light naphtha fraction is a very important one in this refining operation. Among the processes to which it may be subjected are those of separation and dehydrogenation. Separation processes may be used to isolate pure compounds for blending in gasolines or for use as solvent material. It is possible to utilize continuous refractometry, ultraviolet or mass spectrometry, and similar measurements for control. As mentioned in a previous Instrumentation Workbook Feature, light boiling materials cannot be tolerated in the preparation of certain solvent mixtures. Mass spectrometry or other techniques can be used to determine the light component accurately. This can then be related to the flash point, an important property of such solvents. The dehydrogenation process is an important one in the production of high quality, high octane motor gasolines. In this case the octane number of the product is the important characteristic. Since in most of these processes the aromatic hydrocarbon content is the important one, in so far as octane number is concerned, its determination on a continuous basis serves as a means to monitor the process. The aromatic
INDUSTRIAL AND ENGINEERING CHEMISTRY
content can be determined continuously in many ways. Methods include dielectric constant measurements, refractive index, ultraviolet absorption, or mass spectrometric determination. This has also been mentioned in a previous Instrumentation Workbook Feature. The gas oil fraction from the original pipe still distillation serves as a feed stock for catalytic cracking. The catalytic cracking operation itself utilizes continuous analysis. The control of the regeneration process can be followed by the determination of oxygen and carbon monoxide on a continuous basis. The "after burning" is controlled by continuously monitoring the carbon monoxide by infrared spectrometry. Among the products of catalytic cracking are light ends and gasoline fractions. The light ends fraction may be separated by further fractionation, during which the hydrocarbon composition can be monitored by continuous refractometry or gas chromatography. These light components may be recombined to form high octane gasoline products by such processes as polymerization and alkylation. The feeds to these processes must have closely controlled water content for optimum catalytic processing. This is determined on a continuous basis by one or another of the commercially available water analyzers. In many cases, water contents in the order of a few to a few hundred parts per million are critical. These feeds are also monitored by instruments such as continuously recording gas chromatographic units. The feeds to such units can be monitored in a batch continuous method by gas chromatography. A complete analysis for ethane, ethylene, propane, propylene, normal and isobutane, and butènes can be made directly on the plant stream. Once again, the products are of high octane value. Particularly in the case of alkylation, it should be a simple matter to monitor the product by means of continuous analysis. Thus, the few steps in the refining operation discussed here show that the application of process control is being utilized to great advantage. Undoubtedly, in the future many other refining steps will be continuously monitored.
