2088
I N D U S T R I A L ANI) E N G I N E E R I N G C H E M I S T R Y AIR CONDITIONING AND OTHER SERVICES
The entire office and laboratory sections are air conditioned with a centrally controlled circulating air system. The temperature is controlled within close limits by automatic steam coils or cooling coils in the air duct. The humidity is controlled between 40 and 60% relative humidity by a water spray in the air duct to add moisture and a relative-humidity control to start the refrigerating compressor at the upper limit. When the refrigerating unit is operating to reduce humidity the cooling coils and heating coils may be operating simultaneously. The air ducts and all service piping are carried in the space between the suspended ceiling and the roof. Large hoods are provided in the laboratories, Roof ventilating fans service the hoods and ceiling vents in the toilets. To compensate for the air exhausted from the hoods and rest rooms the air-conditioning system provides for the admiasion of 33% fresh air. Heat in the building is supplied by an automatic, low pressure gas-fired boiler. Electricity is s u p plied to the building a t 2300 volts and is converted to either 110, 220, or 440 volts for laboratory use. Thus, the services provided in the laboratory and pilot plant areas where needed are gas, cold water, hot water, steam, air, and IlO-, 220-, or 440-volt electricity. I n the laboratory area all services are carried over the ceiling and in the wood stud walls. There is no exposed piping. In the end laboratmy the services are carried to the center desk and wall in a covered pipe trench under the floor. Safety showers are provided in each laboratory and the whole building is protected from fire by a complete sprinkler system. The controlled-conditions testing room is kept a t 73' F. * 8.5' and 50 * 2% relative humidity by means of a separate air-conditioning system also located in the upstairs equipment room. Laboratory furniture, benches, and tables are made of standard units all constructed of hardwoods.
Vol. 42, No. 10
The building as finally constructed accomplishes all the desired objectives. Choice of proper unit module, floor plan, architectural treatment, and materials of construction has made it p o s sible to build a small industrial research laboratory that compares favorably with the larger and more elaborate laboratories. LITERATURE CITED
Adams, C. S., IND.ENQ.CHEM., 39,457-61 (1947). Bailar, J. C., Jr., J . C h m . Education, 24, 327-8 (1947). Beach, D. M., TND. ENQ.CHEM., 39,448-53 (1947). Bown, Ralph, and Rose, R. S., Jr., C h a . Inds., 65, 352-3 (September 1949). (5) Cairns. R. W., IND.ENQ.CHEM., 39,440-3 (1947). (6) Case, L. O., J . Chem. Education, 24, 338-40 (1947). (7) Cavelti, J. E., Ibid., 24,324-7 (1947). ( 8 ) Darby, G. M., Roberts, E. J., and Grothe, J. D., IND.ENQ. CHEM.,39,453-6 (1947). (9) Dowswell, H. R., J . Chem. Education, 24,350-3 (1947). (10) Garden, N. B., IND.ENQ.CHEM., 41, 237-8 (1949). (11) Hard, C. D., J.Chem. Education, 24,333-7 (1947). . , 2021-7 (1948). (12) Japs, A. B., IND.ENQ.C H ~ M40, (1) (2) (3) (4)
(13) Levy, H. A., Ibid., 41,248-50 (1949). (14) Lewis, H. F., J . Chem. Education, 24,320-3 (1947). (15) Lynch, A. A., IND.ENG.CHEM.,40, 2011-13 (1948). (16) Marvin, G. G., J . Chem. Education, 24,329-32 (1947). (17) Norris, W. P., IND.ENG.CHEM.,41, 231-2 (1949). (18) Rassweiler, C. F., J.Chem. Education, 24, 346-50 (1947). 41, 244-8 (1949). (19) Rice, C. N., IND.ENG.CHEM., (20) Rochow, E. G . , Chemistry &Industry, 61,986-7 (1947). 41, 1657-64 (21) Sjorgren, C. N., and Deal, J. M., IND.ENG.CHEM., (1949). (22) Smith, P. C., Ibid., 39, 440-7 (1947). (23) Swarthout, J. A., Ibid., 41, 227-8 (1949). (24) Ibid., pp. 233-6. (25) Thompkins, P. C., and Levi, H. A., Ibid., 41, 228-31 (1949). (26) Ibid., pp. 239-44. (27) Van Arsdel, W. B., and Eskew, R. K., Ibid., 40,2014-20 (1948). (28) Weber, H. C., J . Chem. Education, 24,341-6 (1947). RBCEIVED March 8. 1950.
Deasphalting Crude Residuum for Catalytic Cracker Commercial Production Using Horizontal Settlers and Propane Solvent E. CLARENCE ODEN AND EDWARD L. FORET Cities Service Refining Corporation, Lake Charles, La. D a t a obtained on a commercial propane deasphalting unit for approximately 4 years of operation are presented. An average of 20,000 barrels per day of crude residuum was charged to the propane deasphalting unit to produce approximately 75 volume % catalytic cracker feed stock and 25% asphalt. Mixed base Gulf Coastal crudes varying from sweet to sour were processed on topping units at coil outlets varying from 650" to 740' F. to produce feed stock for the deasphalting unit. The data include curves correlating the effect of operating variables on the quantity and quality of products produced. Methods of predicting yields and quality of products based on the typical analyses of feed stock are presented in the form of graphs. The degree of purity of the propane solvent, propane to oil ratios and their effect on yields and quality of product, methods of propane recovery, and losses normally incurred are discussed. Included in the discussion are a description of the unit, the operating limitations, flexibility of operations, capacity limitations, maintenance requirements,
service factor, and operating labor requirements. A process flow sheet is included with typical operating conditions that will be helpful to process engineers,
T
HERE are two systems, vacuum distillation and propane deasphalting, in commercial use for removing valuable heavy gas oils from petroleum residues. These systems are very competitive, based on the number of commercial installations of each. This report is being presented principally to present data and correlations useful for predicting yields of asphalt and deasphalted gas oil (the valuable heavy gm oil extracted from asphaltic petroleum residues) and quantities and qualities when produced from petroleum residues by means of propane extraction. The propane deasphalting system is believed to have some advantages over the more common vacuum distillation system of removing heavy gas oil from petroleum residues. Pilot plant data obtained prior to and during World War I1 indicated that a propane deasphalting unit could compete with
2089
INDUSTRIAL AND ENGINEERING CHEMISTRY
October 1950
CONOLNSLR
-----. I
i
I.
,---o
I
PURQL QAS COOLER
PROPANE STORAQI
4
r
b
I
I
I
I
I
I I I I I I
c I
I I
I 1 I
I
I I I I I I
I
I
I
I
I
I
I
I
Figure 1.
I
I
I I I I I
I I I I I
I---
I I
I
I
I
Simplified Propane Deasphalting Unit Flow Diagram
the more familiar vacuum still for removing valuable heavy gas oils from petroleum residues. Interest was created in propane deasphalting when i t was noted that crude rmiduum could be propane deasphalted in horirontal settlers at considerably lesa than the cost of vacuum distilling the residues, and the soft asphalt* could be blended to heavy residual fuel oils. Small investment in equipment and higher"service factor with lower maintenance cost were other good features of propane deasphalting using horizontal settlers. In the production of distillate from asphaltic crudes by vacuum distillation the oil and asphalts are unavoidably subjected to rather drastic heat treatment and some thermal decomposition of the high molecular weight hydrocarbons takes place. This is avoided in propane deasphalting since separation of products occurs a t relatively low temperatures. Another advantage of propane deasphalting is the fact that liquid propane has a greater affinity for paraffinic hydrocarbons than for naphthenic hydrocarbons in the reduced crude. For a given reduced crude the same yield of aaphalt might be obtained by distillation and by propane deasphalting, but the oil produced by vacuum distillac tion will include more naphthenic constituents. Propanedeasphalted gas oil has lese naphthenic material and will include instead very high boiling paraflinic material which is an excellent catalytic cracking feed stock normally lost in asphaltic residue produced by vacuum distillation (6). The development of fluid catalytic cracking in recent years has increased the demand for good quality virgin feeds since they produce maximum gasoline and light fuel oils. The most desir-
able feed stock for a catalytic cracking unit seems to be a high Diesel index, virgin heavy gas oil that contains no materials that would contaminate the catalyst; but since there is approximately 16% of the average Gulf Coastal crude heavier than virgin heavy gas oil, all except the very heaviest part of this crude residuum must be c&talyticallycracked, if catalytic crackers are available, in order to show maximum economics on refinery operation. This situation exists in the refinery where these data were collected. The crude residuum is propane deasphalted, then the deasphalted gas oil is catalytically cracked over a relatively cheap catalyst. Fluid Catalytic cracking units are generally limited in cracking capacity by coke burning capacity; therefore, a clean feed stock that produces minimum coke is desirable. Contamination of catalyst is a problem recognized by all refiners who crack deasphalted gas oil; but this problem has been partially solved in this case by cracking the feed stock over low-value natural catalyst and reject synthetic catalyst from other units, using a high addition rate. Commercial propane deasphalting units employing horizontal settlers have been in operation several years, but very little commercial data on these units have been published. This report presents correlations on quality and quantity of products obtained and the effect of operating variables on commercial operation of a propane deasphalting unit employing horizontal settlers. Commercial data from March 1946 through September 1949 are presented in the form of points which represent monthly
2090
INDUSTRIAL AND ENGINEERING CHEMISTRY
(SUS. AT 210 * E l
Figure 2.
Coil O u t l e t T e n i p e r a t u r r 19s.
Reduced Crude Viscosity
averitgeb. These data were used with pilot plant data as the h a w of all correlations. PROCESS EQUIP'MENT AND OPERATING PROCEDURE
Flow through the deasphalting unit a t the Tutwiler Refinery of the Cities Service Refining Corporation in Lake Charles, La., is shown in the process flow diagram, Figure 1. This propane deasphalting unit has been described previously in an article by Kraemer (8). The crude residuum from the Topping units is picked up from intermediate storage at an average rate of approximately 20,000 barrels per stream day by two turbine-driven, four-stage centrifugal pumps. Suction is taken on the propane storage drum by two turbine-driven, threeletage, high-speed centrifugal pumps. Each propane stream oins one of the residuum streams in a propane to oil ratio ky volume of between 2.5 to 1 and 5 to 1. These streams are mixed prior to entering a tubular-type feed preheater. Since the residuum enters the unit at approximately 190" F. and the propane cools the mixture only a few degrees below the desired temperature, the duty of the preheaters is small. During the summer months, when higher cooling water temperatures are experienced and the residuum enters at a higher temperature, it has been found necessary to cool the mixture by using water in the preheaters to obtain the desired settler temperature. After leaving the preheaters, the propane-oil mixture enters two settlers connected in parallel where separation into an oil-rich and an asphalt-rich phase occurs. These settlers are 8 feet inside di, ameter by 45 feet drums mounted at an angle of approximately 6 from the horizontal. The interiors are arranged with perforated bafftes set a t an angle and spaced a t regular intervals for the purpose of obtaining better separation between the gas oil-propane phase and the asphalt-propane phase. Pressure of approximately 500 pounds per square inch gage is maintained on the settlers by means of pressure-recorder controllers on the deasphalted ga8 oil effluent streams. Li uid level recorders indicate the level between the asphalt an3 gas oil phases. This level is maintained within the desired limits by changing the setting on flow recording controllers which vary the amount of the asphalt phase withdrawn from the bottom of the settler. After the removal of the two phases from the settlers, the process becomes one of stripping the propane from the products and recovering propane for recycling. The asphalt product, carrying with it an a proximately equal volume of propane, is withdrawn from the fowest point of the settlers and is fed directly to the gas-fired asphalt furnace in two parallel streams. The mixture of asphalt and propane is heated by the furnace to approximate1 385" F. The two streams join on leaving the furnace and discxarge into the asphalt-high pressure flash drum. Here a major part of the propane is flashed directly to the propane condensers where it is condensed and returned to the propane storage drum for recycling. The asphalt from the flash drum is fed to the asphalt stripper where the remaining propane is stripped out by steam a t a reduced pressure. The steam and propane vapors are taken overhead to the jet condenser where the steam is condensed and removed. The propane vapor flows from the jet condenser to one of the two 600-hp. t,wo-
Vol. 42, No. 10
stage compressors where it is compressed to the 285 pounds per square inch ga e that is required to force it through the ropane condensers and back into the propane storage drum. &he aphalt is removed from the bottom of the stripper by a steam driven reciprocating pump. C d e oil from the catalytic cracker6 is blended directly into the disccarge of the asphalt product pump in the proper proportion to produce No. 6 fuel oil. The deasphalted gas oil-propane phase is withdrawn overheitd from the settlers and flows in two parallel stream to the deasphalted oil-low pressure steam kettles and then to the deasphalted oil-high pressure steam kettles. In these kettles, a major portion of the propane is flashed directly to the propane condensers. The deasphalted gas oil streams from the high pressure steam kettlw join and discharge into the deasphalted oil stripper where the remaining propane is stripped out with steam a t a reduced pressure. The steam and propane vapors are taken overhead to the jet condenser, which also services the asphaIt stripper. From the jet condenser the propane flows to compressors and then to the condensers and back to the propane storage drum. The propane-free deasphalted gas oil is removed from the bottom of the stripper by a steam-driven reciprocating pump anti may either be segregated in intermediate storage tanks or be blended in various proportions with virgin gas oil from the topping units to prepare feed for the catalytic cracking units. DESIGN AND OPERATING CONSIDERATIONS
MATERIALS OF CONSTRUCTION. The construction of the equipment necessary for the production of catalytic cracker feed stock from reduced crude consists almost entirely of carbon steel drums, settlers, strippers, etc. The pumps are cast steel with all moving parts of alloy. All piping is of flanged and welded steel construction. The furnace tubes are constructed of carbon steel in thr convection and radiant sections. Tubes in the condensers are of admiralty metal. OPERATING CONSIDERATION. Rased on limited vacuum distillstion data available to the authors, these operating data indicate that a propane deasphalting unit employing horizontal settlers could be operated at considerably less cost per barrel of residuum rocessed than a vacuum unit. Detailed operating costs and f a t a on vacuum units are not available to the authors; therefore, a thorough. comparison is not possible. Maintenance on the two horizontal settlers is negligible. The internal parts of the settlers should be designed to allow ease of ins ection and cleaning. eh! ' low and high pressure steam kettles which have the tubee inserted through the bottom with the vapor space above have given very little trouble. An occasional leak has developed in the t u b e but not more than would normally be expected with this type equipment
.
30
96
eo
IS
IO
I00
Figure 3.
I50 zoo 250 REDUCED CRUDE VISCOSITY [ S U S AT 2IO'F.l
is E a
5 300 350
Yields a t S t a n d a r d C o n d i t i o n s
The propane condensers consist of 10 bundles, 5 banks connected in parallel with 2 bundles in each bank connected in seriep with water flowing countercurrent to the hydrocarbons. These condensers require more maintenance than any other process equipment employed. The condensers use river water in the tubes while condensing propane vapors on the shell side. Heavy oil entrained in the propane vapor is deposited on the condenser tube8 and reduces the propane circulation by decreasing the heat transfer coefficient. This problem is partially solved by washing the shell side with heated heavy cycle oil, a stream from the catalytic cracking units that may have a boiling range of 550' to
October 1950
INDUSTRIAL AND ENGINEERING CHEMISTRY
$50" F. Heavy cycle oil is an excellent solvent for cleaning the
ropane condensers, probably because of ita aromatic content. $he solvent oil is heated, then pumped through the exchanger being cleaned. The tube sides of the condensers often become fouled with marine growth. Backwaahin with water a t high pressure for 15 minu t e ~removes most of the Sebris from the tubes. Total time for cleaning a bank of condensers is approximately 4 hours if the heavy c cle oil is heated while the condenser tubes are W i g backwasied. Sight glasses proved to be impractical due to fouling, and try cocks have been installed for determining liquid levels. The heavy oils in the system would con a1 in the small instrument lines a t normal temperatures; theregre, a flushing oil system using kerosene or light gas oil is used in instrument lines where necessary. The reciprocating product pumps have become vapor locked at times as a result of poor stripping of propane in the as oil strip pen; however, increased steam rates or cleaning &e strippers normally overcomes this. It would be desirable to re lace reciprocating pum s with centrifugal pum s if at all possibg since metering of pro&cta is difficult due to ffwtuations which are inherent with reciprocating pumps. The jet condenser operating with river water serves a dual purpose in condensing the strippin$ steam and cooling the ropane vapors prior to compression. Difficulty is encountered onpy when the salt content of the river water becomes excessive and causes salt carry-over into the mist extractors. Salt deposita decrease the efficiency of the mist extractors and cause salt carry-over to the propane compressors. Salt de sits result in burned out compressor valves. Well water or cgmically treated water, if at all possible, would be desirable in the jet condenser since it would eliminate normal difficulties encountered. Asphalt furnace difficulties were practically eliminated when the salt content of asphalt was reduced as a result of crude ded t i n g units being installed. The propane storage drum is provided with a purge as cooler which prevents excess losses of propane and removes ehane and noncondensables from the system. The amount of material vented is normally about 50,000 cubic feet per da and is equal to the make-u solvent taken into the system. dydrogen sulfide present in tge system is concentrated near the purge gas system where corrosion is greatest. PROCESS VARIABLES
Primary Variables. FEEDSTOCK. Feed stock for the propane deaaphalting unit consists of crude residuum which is the bottom product on a crude topping unit. Crude residuum will be referred to as reduced crude in this report in keeping with the authors' standard deaspbalting unit terminology. In propane deasphalting reduced crude for catalytic cracker feed stock, the quality is the major variable encountered. Average crude in this study consists of approximately '20% West Texas sour crude and 80% mixed naphthenic and paraffiic base sweet crudes. At times the refinery procesaea segregated sweet crudes which contain very little asphalt. These crudes are processed primarily for high octane light naphthas and possefls minimum heavy ends. These variables affect viscosity and carbon residue of the reduced crude aa well as the salt and sulfur content. The viscosity of reduced crude varies with the extent of topping as determined by the furnace coil outlet temperature at the topping units. Heat available and the mechanical condition of the topping units determine the allowable furnace coil outlet temperature which, in turn, determines the percentage of reduced crude produced based on crude. The fact that a minimum of crude segregation is done in the refinery makes classification of reduced crudes difficult. The Viscosity of the average reduced crude increases with furnace coil outlet temperature as shown in Figure 2; therefore, viscosity is a measure of the extent of topping on the average reduced crude. The effecta of various types of crudes are shown by the deviation of the pointa from the curve shown as average. The many changes that occur in topping operations and the various types of reduced crudes encountered make estimating deaaphalted gas oil and asphalt yields from a propane deaaphalting unit difficult. A reasonably accurate method is necassary
2091
since production eetimates on refinery operation must be made months in advance of Bctual production. After an extensive study, the viscosity of reduced crude, Saybolt seconds, Universal (S.S.U.) at 210' F., was found to be the only routine laboratory test that was indicative of catalytic cracker feed stock yields when proceaeing average reduced crude. Figure 3 indicates that yields of deasphalted gas oil and BB phalt, on a commercial propane deasphalting unit employing horizontal settlers, are dependent primarily on the reduced crude viscosity and the settler temperatures for average reduced crude in the range of the operating conditions considered in this report. This relationship was found to hold with pilot plant data also, but pilot plant asphalt yields were slightly higher for a given viscosity of feed stock. Commercial data were corrected to standard Conditions with the aid of pilot plant data obtained at the refinery. Standard conditions for purposes of this report are: (4) propane to oil ratio of 4 to 1; (a) average molecular weight of solvent of 44.1; and (c) settler temperatures below the critical ae determined by viscosity of the feed stock. S m a TEMPERATURE. From Figure 3 it can be seen that at constant viscosity, propane to oil ratio, pressure, and average molecular weight of solvent, deasphalted gas oil yield is inversely proportional to the settler temperature and is approximately a straight-line function in the range of the commercial data considered. Asphalt, under the same conditions, increases with incressingsettler temperature. Fig(lre 4 shows that a critical settler temperature, dependent upon the viscosity of the reduced crude feed stock, appears to exist. Data on this relationship from the pilot plant and data published by Bray and Bahlke (3) agree. The data indicab that at settler temperatures above the critical for a feed stock, the deasphalted gas oil yield decreases rapidly and the effect of the propane to oil ratio on yield revemea itself-Le., increasing propane to oil ratio increases deaaphaltad gas oil yield. The data plotted by Bray and Bahlke ($1 are predominantly above the critical settler temperature for the feed stock considered,
100
Figure 4.
900 300 VISCOSITY, WB 4T 0 I O V .
400
Reduced Crude Critical Temperature vn. Viscosity
PBOPANE M OIL RATIO. Increasing propane to oil ratio at constant settler temperature decreases deasphalted gas oil yields and increases asphalt yields when the settler temperatwe is below the critical. Normally, settler temperatures are below the critical in this refinery. Figure 5 shows the yield correction to standard conditions. The propane to oil ratio has very little effect at 5" below the critical temperature and reverses itself at 5 O above the critical temperature on the average reduced crude; therefore, no correction was applied to data within ' 5 of the critical settler temperature. AVERAQ~ MOLECULAB WIOIQHT OF SOLVENT.The solvent used in the commercial propane deasphalting unit consistg of a mixture of propane with varying amounta of light hydrocarbons. The average mo!ecular weight of the solvent varies between 43
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
2092
3
4
I
Figure 5.
5
PROPANE TO OIL RATIO BELOW CRITICAL TEMPERATURE)
Correction to Actual Propane to Oil Ratio
and 46. The solvent is usually lighter during the winter months because of the cooling water affecting over-all refinery operation which results in higher contamination with material lighter than propane. The authors' pilot plant and commercial data both show that a lighter solvent precipitates more asphalt for average reduced crude (Figure 6). Data by Bray and Bahlke (8) showed ethane to precipitate 3.6 times more asphalt than propane, and butane to precipitate only 0.45 as much as propane. PRESSVRE OF SETTLER.Since all data in this report were at a pressure of approximately 500 pounds per square inch gage, pressure will not be considered a variable. Pilot plant data indicate that increasing pressure above the bubble point of propane increases the deasphalted gas oil yield. It appears that superimposed pressure has the equivalent effect of decreased settler temperature. Secondary Variables. QUALITY OF MAKE-UPPROPANE. The source of propane is the alkylation unit depropanizer tower overhead stream. Variation in cooling water temperature from winter to summer affects the tower operation; therefore, the wmposition of the overhead stream varies. During the winter the stream often contains w much as 10 mole % material that is lighter than propane, but in the summer the total butanes may be a~ high as 15 mole % with very little material lighter than propane. The molecular weight of the solvent can be Controlled t~ some extent by venting the light material from the propane storage drum and removing the heavy material from the bottom of the propane strippers. Since no definite effect of molecular weight on the quality of deasphalted gas oil (at constant yield) has been established, the solvent in the system is purged of material lighter or heavier than propane. Material lighter than propane decreases the condensation temperature of the solvent, thereby decreasing the temperature difference between the solvent and the cooling water, This limits the capacity of the condensers and the propane circulation rates on the unit. Light material causes excess pressures on the propane storage drum which vents to a safety flare at approximately 275 pounds per square inch gage. PROPANE CIRCULATION.Propane circulation rates might be affected by several factors: 1. Condition and size of the propane condensers 2. Temperature of cooling water
3. Composition of the solvent 4. Optimum propane to oil ratio for feed stock being processed 5. Use of propane circulation as a means of controlling settler temperature 6.. Minimum propane to oil ratio necessary of - - for _urecipitation _ asphalt 7. Feed rates limiting amount of total material to aettlers 8. Preaeure limitation on equlpment
Vol. 42, No. 10
The limitation of processing equipment in this refinery prohibits the use of factors 4, 5, and 6. The effect of propane circulation on yields is calculated from the propane to oil ratio correction previously explained for settler temperatures below the critical on average reduced crude being deasphalted. COOLING WATERTEMPERATURE. River wa6er used in the refinery for cooling and condensing varies in temperature from 95' F. in summer operation to 45' F. in winter operation. This is a major factor that affects propane circulation since the propane condensers use river water. This change in circulation affects t,he quality of products as previously explained. The cooling water during summer operation limits the settler temperature, especially when highly viscous reduced crudes are being processed. Reduced crude coming directly from the t o p ping units is a t approximately 190' F. and enters the feed preheaters or coolers (where river water is the cooling medium) prior to entering the settlers.
I
I 41
Figure 6.
1 I I I I 42 43 44 45 46 AVERAGE MOLECULAR WEIGWT OF SOCVENT
47
Correction Factor us. Solvent Molecular Weight
Correlating Yields and QualitJrof Products. For purposes of
this report the following will be defined: Penetration of asphalt is defined as the consistency of a bituminous material expressed as the distance that a standard needle vertical1 penetrates a sample of the asphalt under known conditions of Lading, time, and tem rature (I). The Conradson carbon resiEe test is a means of determining the relative carbon-forming tendencies of oil ( 1 ) . Retention is defined in this report as the per cent of the Conradson carbon residue removed from the reduced crude and retained in the asphalt. The difference in value between deasphalted gas oil and No. 6 fuel oil must always be taken into consideration before deciding upon operating conditions for the deasphalting unit, but as the relative value of No. 6 fuel oil increases the value of asphalt increases. Maintaining optimum feed rates to the catalytic cracker, which is limited by coke burning capacity, may cause variation in yields because of the changes that occur in feed stock quality. Pilot plant data indicate that Conradson carbon residue goes directly to coke in a catalytic cracker; therefore, minimum carbon residue is desirable in a catalytic cracker feed stock. Carbon residue on average reduced crude increases-with visaosity, S.S.U. at 210' F., as shown in Figure 7. Considerable data on which this report is based were obtained before any electrical desalting equipment was installed at which time salt on reduced crude was often 400 pounds per IO00 barrels.
October 1980
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
Figure 8. Figure 7.
Sulfur Removed by Asphalt US. Asphalt Yield
Carbon Residue us. Reduced Crude Viscosity
The salt content for present operation now averages approximtely 30 pounds per lo00 barrels. The deasphalting unit mwes tw a desalting unit in that at least 96% of salt goes to the asphalt; therefore, salt is rarely ever found in the deasphalted gaa oil in quantities greater than 1 pound per lo00 barrels. Before equipment was installed in the refinery for desalting crude the deasphalting unit was actually a propane desalting and deasphalting unit, Since practically all the salt left in the crude residue waa removed with the asphalt when subjected to propane dewphalting operations. Sulfur varies with the various feed stocks and is known to be at a maximum in this refinery when proceclsing West Texas crudes. Figure 8 shows that sulfur in deasphlted gas oil decreases with increasing asphalt production for any one reduced crude. It is desirable to have minimum sulfur in deasphalted gas oil. Sulfur limits catalytic cracking in that sulfur causes increased coke deposits on the catalyst which utilizes additional air for combustion. The production of hydrogen sulfide which results when cracking
Figure 9.
2093
Retention us, Carbon Residue on Asphalt
feed stocks containing large amounts of sulfur causes severe COFrosion in product recovery equipment. Minimum asphalt is desirable when operating for m i n i u m residual fuel oil production. Minimum asphalt, usually low penetration asphalt, may cause operating difficulties at times due to difficdties in pumping and blending. Asphalt is blended with cycle stooks from catalytic crackers to produce No. 6 fuel oil whioh has a gravity specification. At constant cycle oil rates fop blending, the quality of asphalt may be changed purpoeely in or. der to facilitate blending. The effeot of propane circulation on quality of asphalt and de. asphalted gas oil produced is shown in Figures 9 and 10 on feed stock when quality, in respect to carbon residue, io a funotien of reduced crude viscosity. Figure 9 was drawn from pilot plant data on 23 volume yonduced crude. It can be seen that retention is increased by (a) increasing asphalt yield and/or increasing propane to oil ratio and (b) increasing propane to oil ratio at constant akphalt yield. At constant propane to oil ratio retention increases aa a semilogarithmic function of asphalt yield and is 100% when a11 feed goes to asphalt. The per cent carbon residue on asphalt is calculated by difference and decreases as retention of carbon residue in aaphalt increases at constant propane to oil ratio and increases as asphalt yield decreases at constant propane to oil ratio. Figure 10 waa plotted from commercial data and represents
ASPHILT YIELD, V0L.X
Figure 10. Retention us. Asphalt Yield
TABLE I. C~R~EIATED VERSUSACIWAL DEASPHALTEII GAS OIL YIELDS Deaqphalted
Deea halted caa il Yield Conelated, Aatual, vol. % vol. %
8
Date
Qaa Od Quality
Correlated wt. 5% carbon reaidue
Actual wt. 5% carbon residue
2.6 2.6 2.6 2.7 2.7 2.9
Asphalt Quality CorreActual lated penetra. penetration tion
1946"
Marchb
Aprilb May b June Julv Auk. 8ept.b Oct.
Nov.
Dee.
95 70 130 95 120 48 72 43 0 0
43 35 70 57 87 65 50 42 42 32
0
28 44 57 51 50 82 82 83 76 119 200 20 1
08.0 70.8 70.0 76.5 71.5 74.4 74.5 76.2 82.5 93.8
69.5 78.4 76.1 82.4 72.2 78.7 81.1 79.4 84.0 92.5
2.9 2.1 2.8
2.1 2.5 2.7 2.8 3.1 3.1 3.4 3.2 2.7 2.5
94.5 89.9 87.0 91.8 90.6 87.8 85.0 90.3 87.2 90.4 86.3 66.9
91.4 88.0 89.0 90.2 90.1 89.3 88.5 91.1 90.2 89.7 14.0 07.9
3.4 3.2 2.9 3.1 3.4 3.3 3.3 3.5 2.8 3.0 3.1 3.0
2.6 2.8 2.9 2.8 2.9 3.2 3.5 3.3 2.7 2.7 2.6 2.5
300
73.7 74.4 83.1 72.0 73.0 74.0 71.3 74.9 76.4 72.0 70.6 71.3
75.8 77.1 78.4 70.4 74.7 75.4 70.2 78.4 81.3 74.5 70.5 69.5
2.8 2.4 2.5 2.8 2.9
3.1 2.6 2.7 2.9 2.9 3.5 3.6 3.8 2.9 2.9 2.2 2.9
300 300 110 170 190 180 300 275 290 300 300 300
3.0
1947
Jan. Feb. March April May June July Aug. %pt. Oct.
Nov.
D0c.b
0 110
30
70 140 180 125 150 85 150
1948
Jan. Feb. March April Mayb Juneb Suly Aug. Sept.b 0ct.b Nov.6
Dee. 1949
3.0 3.3 3.0 2.5 2.9 2.8 3 1
Feb. March .4pril May aune July
when larger percentages of carbon residue are calculated to be in wphalt. Curves A and B represent pilot plant and commercial data, respectively. Carbon residue on asphalt (calculated by difference) might be thought of as an indication of the efficiency o f separation of the deasphalted gas oil and asphalt phases in propane deasphalting with horizontal settlers. Figure 12 indicates that the carbon residue on deasphalted gaa oil is dependent upon the carbon residue of the feed stock (reduced crude) and the asphalt yield on reduced crude. Increasing the propane-oil ratio increases retention (see Figure 10) which decreases the carbon residue on the deasphalted gas oil produced. The above relationships hold only when the settler temperature is below the critical which is limited by viscosity, as shown in Figure 4. Table I lists all actual and correlated yields obtained from commercial data from March 1946 through Septembpr 1949. The 1946 data are the least accurate since there were practically no solvelit analyses during that year. All data shown in Table I were correlated from average viscosity of feed stock on average
H
I
r l
I
I
I
I
I
I
I
I
I
l
1
l
295 300 234 170 235 176 300 300 285 300 300
300
02.7 2.5 300 3.1 258 82.1 2.7 100 04 2.8 84.2 2.1 37 72 2.9 79.3 8.1 175 157 3.2 87.9 58 2.9 3.3 77 81.1 130 3.1 3.4 98 2.9 150 77.0 3.0 104 74.5 2.8 180 2.9 127 AUK. 82.1 3.0 150 2.9 121 Sept. a 1940 data not corrected for inolecular weight of solvent. b Settler temperature near critical; therefore, no correction for propane to
Jan.
Vol. 42, No. 10
INDUSTRIAL AND ENGINEERING CHEMISTRY
2094
61.0 77.6 83.8 78.7 87.3 80.2 77.0 73.0 81.6
A--7.0 CARBON RESIDUE, WT X ON REDUCED CRUDE 8-6.0 C-5.0
IO 15 PO 25 30 ASPHALT YIELD, V 0 L . g ON REDUCED CRUDE
Figure 12. Carbon Residue on Deasphalted Gas Oil vs. Asphalt Yield at Standard Conditions
oil on yields was applied.
average reduced crude. The curves represent the actual average of all commercial data available. The curves are in agreement with pilot plant data in that they extrapolate to 100% retention when average reduced crude produces 100% asphalt. Figure 11 is a correlation between calculated carbon residue on wphalt Rnd ssphalt penetrat,ion. Asphalt produced is harder
reduced crude for each month. Variables such as types of crudes, salt and sulfur content of reduced crude, and feed rates varying from design rates have been neglected. AI1 yields are based on calendar days rather than stream days; but since the two figures were usually almost equal, the error was considered to be negligible. The correlated deasphalted gas oil yields are well within the accuracy of the data, as is the Conradson carbon residue predicted on deasphalted gas oil. The penetration of asphalt doe8 not correlate very well on hard asphalts but is in better agreement on the soft asphalt. This is explained on the basis of correlations for different feed stocks as shown by Figure 11, curves A and B. SUMMARI
PERCENT CARBON RESIWE ON ASPHALT (WT. X CALCULATED)
Figure 11. Asphalt Penetration v8. Per Cent Carbon Residue on Asphalt
Viscosity S.S.U.at 210' F., of average Gulf Coastal reduced crude (crude residuum) can be used as a close approximation in determining the deasphalted gas oil and asphalt yields on a commercial propane deasphalting unit employing a definite set of operating conditions in the horizontal settlers. A critical settler temperature, dependent upon viscosity of the feed stock, appears to exist. At constant propane to oil ratios and for settler temperatures above the critical temperature range, the deasphalted gas oil yield is no longer a straight-line function of temperature. At constant viscosity of feed stock deasphalted below the critical settler temperature, deasphalted gas oil yields decrease with increasing settler temperatures. Increasing propane to oil ratios above the minimum required for as halt precipitation to occur decreases the deasphalted gas oil yiepd when operatin below the critical settler temperature of the feed stock, has no e%ect on yields a t the critical temperature,
Octobe~l9so
INDUSTRIAL AND ENGINEERING CHEMISTRY
and increases the deasphalted gas oil yield above the critical settler temperature. Increasin the avera e molecular weight of the solvent increases the deaspht!ted gas yield on reduced crude and, conversely, deoreasin the average molecular weight of the solvent decreases the deaa %altedgas oil yield. The dnradson carbon residue of deasphalted &asoil produced on average crude is decreased with increasing ylelds of aa halt and/or increasing propane to oil ratios when propane deasptaltin is conducted iihorizontal settlers. %he sulfur content in deasphalted gas oil is dependent upon the oulfur content in reduced crude and the per cent yield of asphalt produced in horizontal settlers. The penetration of asphalt produced from average Gulf Coastal reduced crude ap ars to be dependent u p p the calculated r cent carbon resige in asphalt and efficiency of separation o g h e asphalt and deasphalted gas dl phases.
oi
4
2095
and to recognize the cooperation of Carl W. Kraemer and Cecil Hutchinson of the Operating Department. The authors gratefully acknowledge the indispensable assistance of Charles E Smith, Jr., and Elmer N. Coulter in preparing the graphs. LITERATURE CITED
(1) Am. boo. Teeting Materiale Committee, “A.S.T.M. Standards on Petroleum Products and Lubrioanta,” Philadelphia, 1948. (2) Bray, Ulrio B., and Bahlke, W. H., in “The Science of Petroleum,”
by Dunstan, A. E.,Nash, A. W., Tizard, Henry, and Brooke, B. T.,Vol. 111, pp. 1966-71, London, Oxford University Prees, 1938.
(3) (4)
Kraemer, C. W., Oil Om J., 44, No. 47,228-33 (1946). Thompson, F. E. A., in “The Science of Petroleum,” by Dunrttan, A. E., Nash, A. W., Tizrtrd, Henry, and Brooks, B. T.. Vol. 111, p. 1853, London, Oxford University Press, 1938.
ACKNOWLEDGMENT
The authors wish to expreas their appreciation to the Cities Service Refining Corporation for permission to publish this paper
RECBIVBDDeoember 22, 1949. Presented et the Fifth Southweat Redo& Meeting of the A M B R I QCHBMIOAL ~ SOOIBTY, Oklahoma Ciay, Okla Deoember 8 t o 10, 1949.
Purification of Argon M. W. MALLETT Battelle Memorial Znatituw, Columbus, Ohio
A
method of purifying argon in quantities permitting a
flow rate of 15 liters or less per hour is described. The.oxygen and nitrogen impurities of commercial grade argon are gettered by titanium at 850’ C. Tests showed the. treated argon to be sufficiently free of active gases to prevent tarnishing of highly reactive heated uranium.
I
NERT atmospheres must be used during fabrication and heat
treatment of various metals, in particular titanium and airconium and their alloys. Argon is commonly used for this purpose. However, the commercial grades are of doubtful and varying purity (99.7 to 99.9%); the principal contaminant is nitrogen, plus about 0.001% hydrogen and 0.001% oxygen. Newton (I) haa pointed out that uranium metal is an exclellent scavenger for chemically active gages. Since uranium is a v d able in rather limited quantities, a more readily obtainable material was sought.
short aa possible. From the drying tower, the gas passes into a fused silica tube about 1 inch in diameter, whose central portion is filled with titanium powder (-20 60 mesh) for a length of about 10 inches. The powder is hcld in place by wads (0.5 inch long) of steel wool which are compressed somewhat 80 the powder will not run through them. Such a charge will contain about 160 grams of titanium powder. The flow r a t e and C+ pacities given here are based on a tube of these dimensions. For smaller charges, the flow rates and capacities will be proportionJly lower. Titanium, aa purchased, almost invariably has a high hydrogen content which must be removed by heating a t 850’ C in a vacuum for several hours. If hydrogen is not objectionable, the titanium can be used, as received, to remove oxygen and nitrogen. The titanium reaction chamber is heated at 850’ C. by a resistance-wound furnace when in use. Although there is mme evidence that a much faater flow rate is permissible, best resulb will be obtained with a rate of 15 liters or less of argon per hour. The titanium charge described will be enough to purify one third
+
PURIFICATION METHOD
Nearly all the commonly accepted gettering materials such as calcium and magnesium metals, barium-aluminum, bariummagnesium, and magnesium-lithium alloys were tested. Argon waa pasaed through a bed of the getter a t various flow rates. The temperature of the getter was varied, However, the treated argon invariably caused tarnishing, in 1 hour, of a polished test strip of uranium heated at 600’ C. Finally, titanium-metal powder waa found to purify the argon to the extent that the test strip showed no tarnish in 1.5 hours a t 600’ C. 3ecause the chemical properties of titanium and zirconium are similar, airconium should be equally satisfactory. However, titanium is to be preferred because it is more readily available, and, on a weight basis, it has twice the capacity of zirconium. A purification train capable of servicing a tight system, not exceeding about 5 liters in free volume, is shown in Figure 1. The argon cylinder is connected to a safety bubbler containing concentrated sulfuric acid or mineral oil. This seal relieves train pressures over 1 atmosphere. Next, the gas is passed through a drying tower containing anhydroua magnesium perchlorate. Rubber tubing connections should be aa few and as
ANHYDROUS MAGNESIUM pcAw4RAT€ RADIATION B M L E
y,
4 c e S q OR MINERAL OIL FLOW
RATE: IS LITERS PER
W R MAXIMUM
Figure 1. Train for Purifying Argon
