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W O R K B O O K by
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B. F. Dudenbostel, Jr., and Wm. Priestley, Jr., Esso Research & Engineering Co.
Gas Chromatography for Process Control The gap between lab gas chromatographic analyzers and continuous stream plant analyzers i s closed
ABOUT a month ago, Esso Research & Engineering Co. released the details of a continuous plant analyzer employing gas adsorption chromatography. This analyzer is being used for the continuous analysis of propane in propylene being upgraded a t the Bayway Refinery polymerization plant. A second instrument is nearing completion and by now is probably installed in a gas absorption plant. I t was stated that these analyzers cost about $12,000 each, but the savings resulting from their installation are expected to be many times their cost per year. Previous to the use of these analyzers, poly plant samples were analyzed in the laboratory a t the rate of about 1 per day. Now a complete analysis is available every half hour. Quoting from the release, “Operators in control houses can now get this information merely by craning their necks around to read the pen ‘squiggles’ on graph paper.” Chromatography, in general, has replaced many of the infrared and mass spectrometric methods of analysis in the laboratory. Its extension into the continuous analysis field may be even greater because of the reduced maintenance factor (less electronics) and the high selectivity maintenance without loss of detection sensitivity commonly encountered in mass spectrometer and infrared analyzers. I t is also believed that this tool will be more commonly accepted by plant personnel because of their familiarity with equipment of this nature. When the project on the application of this elegant analytical tool to continuous analysis was initiated certain basic assumptions were made. 1. I t was thought that the use of bottled gas such as helium, hydrogen, or nitrogen would meet resistance in the field, since it would be necessary
to change cylinders every few weeks. Thus, the only readily available carrier gas was instrument air, continuously dried. 2. With air as a carrier gas, the use of a partitioning agent would be unsatisfactory. Since it was thought that the partitioning agent would be oxidized by the air carrier, it would lose its efficiency as a medium of separation. Therefore, gas-solid adsorption chromatography was employed. 3. T h e first two instruments were constructed from commercially available rugged components. 4. I t was decided that the instrument output should be presented in absolute percentage of components present and that normalization would not be required to give the correct answer. This restriction necessitated control of sample size to within the desired analytical accuracy. T h e simplest solution to a n accurate sample size appeared to be the use of a large sample, since this sample could be trapped between commercially available solenoid valves. 5. Because of its location in hazardous areas in a refinery, the apparatus had to be explosion proof. I t is thus understandable that this equipment is large and costly compared to most laboratory chromatographic units.
An upright cylinder vessel contains the adsorption columns, while the explosion-proof housing contains the sampling and detecting system. T h e carrier air dryer is normally located adjacent to the analyzer. T h e dryer is a commercially available unit using silica gel and is automatically regenerated periodically. T h e recorder was specially constructed because of the type of detection employed Since the use of gas-adsorption chromatography had been decided upon, a survey of the literature was carried out. I t was found that, in addition to activated carbon, silica gel and activated alumina had been used for the separation and analysis of lowboiling hydrocarbons. Complete descriptions of a gas analysis system are given by Janak in a series of papers published in Czechoslovakia. I n the Janak system, a separation of the components of a mixture in a chromatographic column is carried out using carbon dioxide as the carrier gas. T h e carbon dioxide is subsequently absorbed in caustic solution and the nonabsorbed gases are measured volumetrically. Preliminary bvork indicates that this method is simple, rapid, cheap, and potentially of high accuracy. h’umerous analyses have shown the applicability of the method to the anal>-sis of mixtures of hydro-
C3H6
n E l u t i o n Time, Minutes
Figure 1 VOL. 48, NO. 9
SEPTEMBER 1956
55A
r"xq INSTRUMENTATION
A Workbook Feature
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TO Recorder
Each Cell : 20 Pairs I-C Couples
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Carbon Heat
Flow Controls
Sample
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Air 5 Sec. On, 5 Sec. Off
Figure 2
carbons in the C Z to Cb range. I t can also be used for methane and permanent gases by changing the absorbent in the chromatographic column. The analysis can be performed to well within =tl% for each component. This accuracy corresponds closely to that obtained by conventional methods of analysis. T h e apparatus can be used without modification to analyze a number of the gas streams present in a refinery. Janak demonstrated the analysis of butane mixtures, stabilizer gases, and an ethylene feed where the concentration of methane and ethane is critical. The type of chart obtained in the case of a C1 to Cq analysis on a n alumina column is shown AMPLITUDE
RECORDER: PER CENT FULL SCALE 40
20
60
80
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in Figure 1. T h e separation of components is sufficient for good selectivity of components; and sufficient sensitivity is also available. Complete analyses seldom take longer than 30 minutes and in some cases as little as 15 minutes. Thus, in the continuous instrument described by ESSO,a 20-foot activated alumina column is employed under closely controlled thermostatic conditions. At the start of this project the design of thermal conductivity detectors had not reached a point where their durability, freedom from drift, and constancy of calibration would meet the requirements of unattended refinery operation. Also the intensit). of the signal available using air as a carrier gas was not impressive. An investigation was carried out on the feasibility of using a detector employing the principle of a water detector used for trace water analysis in hydrocarbon streams. The commercially available water analyzer employing this method of detection was described at a recent Instrumentation Society meeting, and details of this instrument have been published by the Mines Safety Appliance Co. Briefly, the method of detection is as follows : The gaseous hydrocarbon stream being analyzed is dried to an almost zero water content. This dry stream and the stream containing the water are alternately passed over beds of silica gel. The wet stream, in passing through the
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Figure 3.
56 A
Amplitude recorder: per cent full scale
INDUSTRIAL AND ENGINEERING CHEMISTRY
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gel, gives up all its water and is heated by the resulting heat of absorption. Approximately every 15 seconds the two streams are switched. The dry stream passes over the %vet gel bed, removes the water, and its temperature is lowered. At the same time the wet stream is being depleted of its water content. Thus, a temperature differential exists between the two exit streams. This differential is magnified by- using almost 30 thermocouples. One bed contains all the reference junctions and the other bed all the measuring junctions. With activated carbon in place of the silica gel used as a water detector, this gadget \vas found to be extremely sensitive to low concentration of hydrocarbons in air. A schematic diagram of the detector is shown in Figure 2 . I n Figure 3 is shown a typical recorder chart illustrating the cyclic-type signal obtained. As contrasted to the 15-second cycle employed in the ivater analyzer, it was found that 5 seconds was the optimum cycle period for the activated carbon detector. .4s can be seen from Figure 1 an average peak takes several minutes from start to finish. With this detecting system, reading every 5 seconds a large number of measurements are made. The summation of these values provides a n accurate measurement of the area of the individual components. Since the sample size is accurately controlled, this area is a direct measure of the component concentration. The final record presented at the control house is in a bar graph form with the bar height being equal to the per cent concentration of the component. Many manufacturers are planning to construct continuous chromatographic analyzers. Probably both adsorption and partition columns will be employed. Recent literature data indicate that thermal conductivity cells now available have good stability and life and that partition columns also have a long life. One manufacturer is currently testing a 6-inch long partition column for light hydrocarbon analyses. There seems to be little or no concerted effort abroad on the part of manufacturers to enter this field of continuous chromatographic analyzers.
