KEITH P. LANNEAU
I,*-
E u o Sbndord Oil Co., k t o n Roug., lo.
T . . .
HE use of analytical mStrument5 for continuons process
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momtormg xs a relatively new development in control enpi-
n e e m . Some few types of simple analysers such 88 pavitomcters, pH meters, and thermal conductivity meters have seen plant application for a number of years. The general complexity and temperamental nature of the more specific and versatile laboratory anslytical devicen hae until recently prohibited their extensive use in plant installations. However, the great strides made in the field of electronics and instrumentation during World War I1 have brought about rapid and significant developments in indwfxid instrumentation during the past aeveral years. Recent papers (1-8) have pointed out the almost unlimited possibilities in the field of antomatio proeess control with the utilization of analyzers 88 primary elements in automatic feedback loops. In the petreleum industry numerous imtallations of analyzers on commercial plants have already demonstrated the value of rapid availablility of atream composition data (8, 7, 10). For the moet part installations of continuous d y s m have been made on pstreams where indication of offspecificstion atream composition permits .corrective adjustment to a pvsriable, either antomatidly or by &on of the plant opemtor. The speed withwhich both qualitative and quantitative prome8 data can be accumulated by continuous stream analyak hae suggested that continnous analytieal htrmnentsmight *d a wide variety of important wen in p r m ess m a r c h at the pilot p h t level. Two of the main f a C h S stimulath the active development and application bf d y m r s on both commercial and pilot p h t s have grown out of @war economic trends. These two factore are:
has magniiied the desirability of obtaining more rapid and accnrate andytioal data to facilitate pilot plant operations and sound interpretation of process data. This p?per discusses work on the development and application of continuous analyaers at the Esso Laboratories, Baton Rouge, La. As part of a long range research project in the &Id of ant& matic p m e w control, continuous analytical instruments have been extensively applied to large scale petroleum processing pilot plants. The role of these instruments in facilitating control of the pilot plant operations is described. Analyzer Development and Application DemonsIraha Op&abilily of Proceas and Gives Scale-Up Data Figure 1shows a view of the pilot plant area st the Esso Laboratories. One of the -in functions of the lsborstories is to operate
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Jnmasbgqualityand urity ificatiom on petroleum mJ'petrmXLics~ D I O d u d s in tbe faCe Of mo1'8 & Eompe&ion bave h d e d new incentivw for qudit and A d improvement in cornmercial Anta. ammaition control
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Nmmher 1983
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I N DU STRIA L A N D E N G I N E E R I N.G C H E M I S T R Y '
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ENGINEERING AND PROCESS DEVELOPMENT
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FWlD CRACK ING UNIT
CATALYTIC REFORMING UNIT NO 1
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I --
CENTRAL ANALYTICAL CONTROL ROOM
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RECY&’JJRY ETHYLENE
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CATALYTIC REFORMING UNIT N 0 . 2
\ / GAS AND LIQUID SAMPLE LINES
Figure 2. Diagram Showing Tie-In of Large Scale Pilot Plants to Continuous Analyzer Control Center
these laige units to demonstrate process operability, to confirm small scale pilot plant results, and to obtain engineering data for the design of commercial plants. These operations may be facilitated by the use of improved instrumental techniques. For this reason, a program for the devclopment and application of continuous analytical instruments has been instituted a t t h p labomtories. A long range result of this program will be the develop ment of better control systems for commercial refinery plmts. Inasmuch as the operation of a large pilot plant may cost several thousand dollars per day, it is desirable to obtain a masimum amount of data from the unit in a given period of operation. Some of the critical problems associated with planning and esecution of a pilot plant program are: 1. Handling and transporting large numheis of saniplcs to an analytical laboratory 2. Accumulation of vast amounts of analytical data on samples 3. Calculation and interpretation of process data 4. Planning runs for pilot units for a methodical investigation of the controlling proceas variables 5 . Selection of stable periods of unit operation a t carefully controlled conditions
The analytical data needed in pilot plant process and engineering studies are generally of three distinct types: data required for calculation of material balance for a selected operating period; control inspections of process streams to permit setting up and maintaining desircd operating conditions; and feed inspections and analyses and tests showing quality and yield of producti obtained during the selected period. It is important that all three of these types of data be made available as quickly as possible. Sormally it is not feasible to shut down a large pilot unit between peiiods selected for data TT ork-up. Therefore costly “blank” runs can be avoided R hen successive work-up operating periods can follow without dclay. Use of continuous analyzeis on the pilot units has definitely proved valuable in thir iespect. Mass Spectrometer Is Indispensable for Gas Analysis
Table I lists the various types of‘ instruments which have been used in t’his woyk. On inost pilot units continuous measurement of gas and liquid stream gravities with conventional methods of flow and volume measurement have facilitated operations hy providing readily available data for preliminary material balance calculations. The gravitomet,ers and other simple types of analyzers, such as thermal conductivity meters, refractometers, and dielectric constant, analyzers, measure simple physical propertics of hydrocarbon mixtures. These propert.ies often correlate satisfactorily xith concent,rat,ionof a key constituent or class of compounds in a given plant stream. Thus such types of analyzers have also found extensive use in supplying, on a current basis, analyt,ical data for product, quality indication and for composition control of intermediate or by-product, process streams. The Four-Point (COP,CO, 02,H2) gas analyzer, which is a combination thermal conductivity and chemical analyzer, has been valuable for the, analysis of carbon dioxide, carbon monoxide, and oxygen in streams that contain products of carbon combustion.
Because of the natuie of these problems, a partial decentralization of analytical facilities for the laboratory was indicated--i e , direct automatic sampling and analysis by instruments on the plants appeared to offer certain advantages. A program of applying continuous analyzers to the pilot plants was then undertaken with the following general objectives: 1. T o minimize problems associated with sampling 2. T o provide accurate analytical data with less delay to permit immediate work-up of unit data and planning of successiw unit runs 3. T o provide an effective and instantaneous means of ohserving effects of process variables 4. T o provide new control techniques for setting up and maintaining desired unit operating conditions for periods selected for data accumulation 5 . To provide instrumental methods of increasing safety for personnel and expansive process equipment
Table 1.
Types of Continuous Analyzers Installed in Pilot Plants at Esso Laboratories
Gas and liquid gravitometers Thermal conductivity analyzers Refractometers Dielectric constant recorders Four-Point (Cog, CO, 0 2 , Hs) gas analyzers Paramagnetic 0 2 recorders Infrared and ultraviolet analyzers Mass spectrometers Figure 3.
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l a r g e Catalytic Reforming Pilot Plant
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 45, No. 11
PILOT PLANTS
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This analyzer has been used for control data t o operate regenerators in fluid catalytic pilot units and to supply carbon burning data for final material balance calculations on such units. The oxygen recorders used measure the paramagnetic properties of the gas sample. These analyzers have been applied particularly for safety purposes, where excessive oxygen concentrations in process streams create a hazard. I n many instances, where an end point, or intermediate composition control analysis is called for in a process, the qualitative and quantitative identification of one or more specific compounds has been necessary. For this purpose the infrared nondispersive gas analyzer has been useful. I t s application in industrial plants is described extensively in the literature (6,5, 7 ) . The infrared analyzer has been sensitized to a number of individual compounds such as carbon monoxide, carbon dioxide, and most of the light hydrocarbon gases. It has served primarily as a carbon monoxide analyzer in this program. In pilot plant work, where a variety of different analytical problems are encountered and where complete hydrocarbon gas analyses are usually necessary for a final data work-up, the mass spectrometer is by far the most versatile and important instrument for gas analysis. A t Esso Laboratories mass spectrometers have been applied to continuous plant analysis to provide data a t pilot units for material balance calculations for operational control. The instruments have been used for complete hydrocarbon gas analysis and for key constituent monitoring. This work has previously been reported by Starr and coworkers (9). Central location of Analyzers Has Proved Practicable for Group of Pilot Plants
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Because of the varied analytical data requirements of the pilot plants, the task of effectively instrumenting a group of such plants with continuous analytical devices would involve many installations of a number of different types of instruments. In addition t o the considerable installation costs of such an undertaking, the problem of maintaining the large number of analyzers would be troublesome. As a result it was decided that it would be more feasible to have a group of continuous analyzers located in a central analytical room in the pilot plant area such that sample streams from each of several plants could be piped to the instruments. This has proved to be a sound approach t o the problem. Figure 2 is tt block diagram indicating how four large scale pilot plants have sample streams tied in to a central analyzer room. Each of the four plants is individually equipped with certain onsite analyzers for which full-time service is required or for which installation costs are low. Table I1 lists the instruments installed in the analyzer room. The manufacturer of each instrumentis indicated. The analyzer room is actually located a t a large catalytic reforming pilot plant, which is the No. 1 unit in Figure 2. Figure 3 shows the reforming plant. Photographs of the instruments are shown in Figures 4, 5, and 6. The Process Monitor mass spectrometer (6, Q), which is the work-
Table II. Continuous Analytical Instruments Located in Central Analyzer Room for a Pilot Plant Area Instrument Process Monitor mass spectrometer for multicomponent gas analysis Mass spectrometer for key constituent analysis (modified leak detector) Four-Point gas analyzer (COZ,CO, 02, Hd Recording gas gravitometer Recordine liquid gravitometer Thermal conductivity recordmg analyzer Recording refractometer
November 1953
Hg
Manufacturer Consolidated Eneineering Corp. Consolidated Engineering Corp. Cambridge Instrument Co., Inc. Aroco-Anubis American Recording Chbrt Co. Arcco-Anubis American Recording Chkrt Co. Cambridge Instrument Co., Inc. Built at Esso Laboratories
Figure 4. Process Monitor Mass Spectrometer Providing Complete Multicomponent Analysis of Plant Gas Streams
horse instrument in the group, is adapted for providing complete analyses of hydrocarbon gas streams from both reforming pilot units and the fluid cracking unit. The instrument, shown in Figure 4, has also served as a key constituent monitor for light gas streams from the ethylene recovery plant. Although some of the sample lines to the analyzer room from the pilot units are approximately 200 yards long, sample holdup has not been a problem. Continuous gas sample streams from each plant can flow a t high velocity through l/rinch pipe t o by-pass a manifold a t the analyzer room and then return to the plant. At the manifold (Figure 7 ) solenoid valves permit either automatic or manual selection of the stream to be sampled by the Process Monitor mass spectrometer. Provision is also made for introducing to the instrument snap gas samples caught a t any of the plants. Some of the applications of this instrument have been described in detail by Starr et al. (9). Other instruments in the central room have also been applied t o continuous analysis of sample streams from a t least two plants. The gravitometers, Four-Point gas analyzer, the hydrogen analyzer, and the refractometer have monitored streams from both of the catalytic reforming units. The continuous refractometer is a reflection type instrument which was designed and built a t the laboratories for this application. The liquid stream sampling systems are similar to the gas sample plan in that a fast flow by-pass to a manifold a t the analyzer room reduces sample holdup time, Some important advantages for consolidating the location of the group of analyzers might be noted:
1. At a reasonable cost per analyzer, a superior instrument environment has been provided. The analyzer room of weatherproof brick, equipped with air conditioning, results in improved instrument stability and service factor. At the same time the relatively nonhazardous location provided for the electronic equipment minimizes the objections to the nonexplosion-proof character of the instruments. 2. The objectionable piping of hydrocarbon samples into the plant control rooms has been avoided; yet, where desirable, the analyzer readings are readily indicated in the control rooms on remote recorders.
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ENGINEERING AND PROCESS DEVELOPMENT
Figure 5. Continuous Mass Spectrometer Monitoring C j Content of Stabilizer Overhead Stream a t Catalytic Reforming Pilot Plant
3. hfaintenance of the instruments, located in a group, has become more effective. 4. Certain elements of the sampling systems for the various plants have been incorporated in a common system a t the analyzer room. This applies particularly to filters, dryers, rotameters, water settlers, safety valves, and flow regulators. The result of this consolidation is a reduction in total installation costs for the analyzers and improved maintenance of the sampling equipment. 5 . The versatility of a single mass spectrometer adapted for multicomponent analysis of continuous plant streams has been fully utilized by application of the instrument to analyses for several plants. This maximum utilization of the spectrometer fully justifies the services of an analyst who calculates data from the machine and prepares tabulated composite analyses for material balance calculations immediately following controlled runs a t the various pilot plants.
analyzers is to provide the three types of analytical data required for expeditious operation of the pilot unit. Data for Material Balance. Preliminary material balance calculations are based on the integrated values from the charts of the gas and liquid gravitometers and the associated flow meters and volume measurements of accumulated liquid. These data are supplemented for final material balance calculations by composite hydrocarbon gas analyses from the Process Monitor mass spectrometer. The monitor analyses are available shortly after a selected controlled operating period on the unit is terminated. Both “make” gas streams-i.e., the recycle gas and stabilizer overhead-are monitored by the spectrometer. The instrument, as originally constructed by the manufacturer, was designed for monitoring of three constituents in a gas stream (6). Extensive modifications made to the instrument a t these laboratories have adapted it to complete multicomponent analysis of gas streams. The mode of operation of this instrument, described by S t a n et al. (Q),will be briefly reviewed here. The two gas streams, sampled continuously, are alternately and automatically admitted to the spectrometer every 15 minutes, First the recycle stream is admitted to the instrument and allowed to flush lines for 3 minutes. Then the instrument scans the mass spectrum in 12 minutes and records the properly attenuated mass peak signals. The stabilizer overhead stream is next sampled automatically, the lines are flushed, and the spectrum recorded. I n this manner, data for complete analysis of either stream are available every 30 minutes. During an extended period of operation on the plant the data accumulated thusly may roll up on the recorder chart. When a given period of 8 to 12 hours is selected for material balance work-up, an analyst selects the appropriate spectra from the recorder and calculates a “composite” or average analysis of each of the gas streams for the period. An electronic computer is located in the analyzer room to facilitate the calculation of the tabulated and averaged spectra. Typical analyses reported for the recycle and stabilizer overhead stream are shown in Table 111. Usually these composite analyses can be obtained within 2 or 3 hours after a material balance period is terminated.
Conventional Instruments Furnish All Essential Information for large Catalytic Reforming Unit
Flow Plan. The role of continuous analyzers in facilitating pilot plant operations a t Esso Laboratories is best illustrated by a study of their extensive application a t the large catalytic reforming unit ( N o . 1 unit in Figure 2) shown in Figure 3. I n Figure 8 a flow diagram of this plant, which has been described by Shepardson, Tyson, and Voorhies (a), shows the particular plant stream or streams monitored b y each instrument listed in Table 11. The flow plan of the plant is, of course, highly simplified. Brieflv, in this process for upgrading low octane naphthas, the naphtha feed contacts a fluid catalyst bed in the reactor, and total product passes overhead to a partial condenser where the small amount of heavy hydrocarbon polymer is removed. In a second condenser light gas is flashed off and partially recycled. I n the last tower the reformed naphtha liquid product is stabilized before passing t o an accumulator tank, and C, and lighter gases pass overhead. At the left of Figure 8 the catalyst flow is shown. Spent catalyst flows from the reactor to the regenerator where combustion flue gases go overhead and regenerated catalyst is returned to the reactor. Figure 8 indicates the extent to which the important gas and liquid streams are monitored by analytical instruments at the plant. iilthough not shown in the diagram, all streams are accurately metered by various types of conventional flow and volume measuring devices. The net effect of the continuous 2384
Figure 6.
Instrument Installations in Central Analyzer Room
(Left t o right). Gas gravitometer, refractometer, liquid gravitometer, and Cambridge Four-Point and hydrogen analyzers
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 45, No. 11
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Additional data required for final material balCg AND LIGHTER GASES ance calculations are supplied by the Four-Point (4PT. THERM. COND.) gas analyzer. This instrument, which records the concentration of carbon dioxide, carbon monoxide, and oxygen in the metered flue gas stream permits immediate calculation of the weight of carbon burned during a selected balance period. Analytical Data for Operational Control. The Four-Point analyzer also provides important information for operational control of the regenerator. Upsets in catalyst circulation or excessive oxygen rates are reflected on the recorder chart within 1 or 2 minutes. Although the instrument is located in the analyzer room, for convenience of the plant operator the recorder is remotely located in the plant control room. The instrument shown in Figure 5, which monitors the stabilizer overhead gas stream, is a modified leak detector mass spectrometer equipped with a special spectrometer tube and Figure 8. Catalytic Reforming Pilot Plant Flow Plan Showing Continuous continuous sampling system, This instrument Analytical Instruments has also been described previously (9). Compared to the other larger mass spectrometer, this instrument and its application are relatively simple. The analyzer is focused for mass 7 2 , which causes the circular Table 111. Typical Hydrocarbon Gas Analyses Obtained chart recorder to give a continuous reading of the total C5 from Process Monitor Mass Spectrometer a t Catalytic hydrocarbon content of the gas stream. The recorder span is Reforming Plant O-lO% C5. This analyzer has been used strictly for control Stabilizer Stabilizer purposes. Normally it is desired to operate the stabilizer tower Recycle Overhead Recycle Overhead CornStream Gas ComStream, Gas, in such a manner that a small constant amount of C5 hydrocarbon pound Mole % Mole'% pound Mole % Mole % is carried overhead with the lighter gases. This control tends Do fix the vapor pressure of the liquid product drawn from the bottom of the to%-er. Experience has shown that the plant operator can manually control the top temperature of the tower to regulate
Figure 7. Process Monitor Mass Spectrometer Control Panel and Automatic Sample Manifold for Plant Streams
November 1953
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for a constant Cg reading on the spectrometer chart. This would be a particularly effective application for automatic feedback control. I n practice, however, no automatic controls have been associated with the analyzers on the reforming pilot plant. The incentive for the entirely automatic feature is considerably less than on a commercial plant where operations are usually conducted a t a single set of specified conditions. Figure 9 shows that the recorders of the hydrogen analyzer and the continuous refractometer are also located remotely in the plant control room because of their control functions. The hydrogen analyzer gives a continuous indication of hydrogea recycle rate to the reactor, and the refractometer monitors the liquid product for any change in composition. Applied to the product stream, the refractometer is a good indicator of stability of operation of the plant. Figure 9 shows a chart selected from the instrument during a start-up period. The curve very clearly indicates irregularities in plant performance during start-up. This was caused by unsatisfactory level control in the partial condenser. Polymer periodically passing over into the liquid product has caused increases in the refractive index. I n a similar manner, it has been observed on occasion that the refractometer responds t o upsets in the stabilizer tower, when light gases are ineffectively separated from the liquid product. A period of stable plant operation is also reflected by the instrument, as may be seen by the refractive index curve for the last 12 hours of the chart. I n addition to its service for accumulating gas analyses for material balance calculations, the Process Monitor mass spectrometer has been useful in providing control information. Almost any irregularity or instability in operation of the plant is reflected in the gas analyses. A simple inspection of certain
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
2385
ENGINEERING AND PROCESS DEVELOPMENT This is shown in Figure 10. This type of end point quality control analysis is particularly valuable in studying the individual effects of critical process variables. Conclusions
Advantages for consolidation of continuous analyzers in a central room for the pilot plant area have already been enumerated, and the important role of the analyzers in operation of the large catalytic reforming pilot unit has been described. It is interesting to note some of the general results of the entire program of application of analyzers.
Figure 9. Chart from Continuous Refractometer Monitoring Liquid Product Stream at Catalytic Reforming Pilot Plant
peaks on the instrument chart, while not sufficient for accurate analysis, permits observation of changes in gas composition. Use of the spectrometer in determining when stream composition is remaining constant has assisted in the selection of periods of stable plant operation for complete accumulation of process data and determination of material balance. End-Point Quality Control Data. Inasmuch as the purpose of the reforming process is to upgrade 10%- octane naphthas, it is highly desirable to have preliminary indications of product quality during pilot plant runs. Usually there is a delay of at least several hours or even days aftei a run before octane ratings from laboratory knock tests on the liquid product are reported. Means-hile it is necessary that operation of the large pilot plant continue. The continuous refractometer has been of considerable value in giving an instantaneous indication of product quality. During periods when stable plant operation is obtained, the refractometer reading correlates reasonably ell with Research Clear Octane rating determined for the liquid product.
For a commercial plant installation it is a fairly straightforward matter to calculate estimated annual savings resulting from use of a continuous analyzer. I n pilot plant work, where production of a marketable product is usually not the objective, recurrent savings resulting from applications of analyzers are more difficult to estimate. However, it has been demonstrated that saving of a few days operation of a large pilot plant can justify the cost of one or even several analyzer installations. While the direct cost of analytical data has not been substantially reduced by continuous instruments on the pilot plants, the analyzers have played an important role in the increasing effort to improve speed and accuracy of measurements for control and investigation of critical variables in process research. I n addition to these factors, which have facilitated the opeiations of the pilot plant organization a t Esso Laboratories, the program of development and application of continuous analyzers has provided much valuable information relating to future uses of these instruments in automatic control systems for large scale refinery plants. Acknowledgment
The author wishes to acknowledge the work of C. E. Starr, Jr., and E. M. Charlet and their efforts in planning and coordinating the analyzer program. Much of the engineering and development work was done by B. ISodge and Ti. M. Davidson.
100
a
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1. Problems associated with transporting large numburs of samples to a distant analytical laboratory have been drastirally reduced. The continuous systems are an improved and more accurate technique for “composite sampling.” Errors from rhanges in gas composition of samples handled in tanks, barrels, and bottles have been eliminated. 2. Many costly delays in obtaining analytical data have been eliminated. Over-all cost of pilot plant operations has been reduced through more efficient use of on-stream operating time. 3. I n many instances use of t h r instruments particularly adapted to the plant streams has resulted in improved accuracy of analytical data. 4. A higher degree of control of pilot plant operating conditions has been attained. 5. The analyzers have frequently asPisted in the interpretation of the effects of process variables.
98
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literature Cited
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k! 3
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fl) i2) (3) (4)
W M
CL W
2 3 , 5 5 1 (1951).
m
4 z
92
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5z
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0
88
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50
60
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RECORDER CHART DIVISIONS
Figure 10. Correlation of Continuous Refractometer Readings with Octane Number of Product from Reforming Pilot Plant
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Awes. E.. S c i . Am.. 187. 82 (Seuteniber 1952). &ern. W e e k , 71, 28 (De;. 27, 1952). Draffen, J. F., Petroleum Refinel.,31, 121 (December 1952). Jamison, N. C., Kohler, T. R., and Koppius, 0. G., Anal. Chwn ,
( 5 ) Munch, R. H.. IND. EKG.CHEX.,4 4 , 9 3 A (October 1952) ( 6 ) Robinson, C. F., etaZ.,Instrzc.rn~?i.ts,2 4 , 2 2 1 (1951). (7) Serrill, J. L., Jr., OiZGasJ., 51,84 (September 1952). (8) Shepardson, R. X i . , Tyson, C. W., and Voorhies, A., Jr., pwsented a t 17th Mid-year Meeting of A.P.I., Division of Refining, San Francisco, Calif. (Xiay 1952). ( 9 ) Starr, C. E., Jr., Johnson, F. B., Purdy, K. hl., Charlet, E. A I . , a,nd Lanneau, K. P., present,ed a t X I I t h Int,ernational Congress of Pure and applied Chemistry, Fuel, Gas, and Petroleum Chemistry Section (September 1951); Proc. Am. Petroleum Inst., 3 2 M (111). 224 (1952). (10) Thomas, B. W., Petmleum RP,fmet,30, 81 (1951). ACCEPTED September 8, lYj3. RECEIVED for review June 2 2 , 1953.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 45, No. 11
