lower Acidity Sulfuric Acid AIkylation
-
deve1opment
" E. C. HUGHES, D. G. STEVENS, AND FRANKLIN VEATCH THE STANDARD OIL CO. (OHIO), CLEVELAND, OHIO
During study of the alkylation process for manufacture of aviation gasoline, preliminary observations indicated the possibility of a substantial reduction in the amount of sulfuric acid consumed by reducing reactor acidity 0 ordinarily embelow the lower limit of about 90 weight 7 ployed. It was shown in both pilot scale and full scale equipment that the process could be operated successfully at 85 weight 70 titratable acidity with acid consumption of around 0.6 pound per gallon as compared to 1.2 pounds per gallon at 9270 acidity, a reduction of 50%. Minor or negligible changes in the yield and nature of the alkylate resulted, including aviation octane number ratings, with no change in operation of the unit other than a decreased feed of fresh acid. Because of the large amount of sulfuric acid consumed in the alkylation process and the difficulties involved in the disposal of large quantities of spent acid, lower reactor acidity can be an important factor in the operation of an alkylation plant.
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OMMERCIAL sulfuric acid alkylation plants which produce alkylate for aviation gasoline by the alkylation of isobutanes with butylenes normally use as fresh catalyst 98% sulfuric acid and reject 90 to 92% acid as spent catalyst, the acid diluting agent being predominantly an acid-soluble hydrocarbon complex ( 4 , 6). Preliminary observations indicated that there was little change in the nature of the reaction or the products even if the acid was spent down t o a titratable acidity of 85%. If true, this could represent a substantial saving in the over-all amount of sulfuric acid consumed in the process as well as minimize the problem of disposing of spent alkylation acid. An extensive program aimed at evaluating the alkylation process and products in this region of lower acidity was carried out on both a laboratory and plant scale. Some variables, such as reactor temperature and feed stock composition, are more easily evaluated in laboratory equipment, whereas other variables such as actual consumption of acid per gallon of alkylate are difficult to study with good accuracy in small scale equipment. The over-all objective of this work was to obtain a thorough examination of the alkylation process, particularly in the regions of 85 to 90 weight % titratable acidity of the sulfuric acid catalyst as compared to the uaual operating conditions of 98 to 90% acidity. APPARATUS
The data in this report, were obtained from two widely varying sizes of equipment. A great many experiments were carried out
in a laboratory pilot plant alkylation unit that operated continuously, except for the product fractionation, which was carried out batchwise. The full scale plant data were obtained during special periods of operation of a commercial sulfuric acid alkylation plant. LABORATORY PILOT PLANT
The laboratory alkylation unit was of conventional design and is schematically represented in Figure 1. The hydrocarbon feed was mixed alternately in 20-liter batches in nitrogen-pressured tanks and metered a t constant pressure through a precooling coil to the reactor. Control was manual. Acid was withdrawn from the fresh acid feed tank or the settler by a calibrated, adjustable pump and passed into the reactor. Reactor feed rates of both acid and hydrocarbon were maintained a t 4 liters per hour throughout the experiments. The stirred reactor (1-liter volume) contained a Turbo-type mixer with feed propellers immediately above and below it. Both acid and hydrocarbon injection points were near the center of the feed propellers. The mixer shaft rotated about 800 r.p.m. and was electrically driven. The reaction mixture passed out the top of the reactor into a settler. The reactor was immersed in a bath of water and methanol kept a t the desired temperature by an electrical refrigeration unit (not shown). The settler was also cooled by coils through which the cooled water-alcohol mixture from the refrigerated reactor bath was circulated. The hydrocarbon layer from the settler passed overhead into the product receiver on which a constant back pressure was maintained with nitrogen. The settler was fitted with an acid return line to the pump, and after the desired amount of acid was pumped into the system the fresh acid feed v a s shut off and the acid layer from the settler was recycled to the reactor. Spent acid was intermittently withdrawn from the settler and replaced by feeding fresh acid to maintain acidity at the desired level. The product was withdrawn from the receiver a t 4- to 5-hour intervals, and assed through a caustic-packed tower into a conventional bat$ debutanizer (not shown) A pressure hydrometer (not shown) cilibrated for the particular feed stocks used was employed to determine the amount of alkylate in the settler effluent a t intervals. Laboratory glass columns were used to fractionate the debutanker bottoms into depentanized light alkylate and heavy alkylate. COMMERCIAL ALKYLATION UNIT
The commercial unit was a conventional sulfuric acid alkylation unit with an initial designed capacity of 225 barrels per day of alkylate output. Besides details of construction, the major difference between the plant unit and the pilot plant unit consisted of the autorefrigeration nature of the plant reactor. The low reactor temperature was maintained by light hydrocarbons evaporated from the reactor mixture instead of by mechanical refrigeration used in the pilot model unit. The reactor itself was divided into four compartments, with the baffles so ar1447
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 43, No. 6 hoduat Eeoeiver
b
l
f-
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7
Control Valve Cooled Reaotor ,
_Cooled
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LAdjustable Prmrp
LQdrocaarbon Feed Tanks
Aaid
Aold F e d Tank
Figure 1. Alkylation Unit
ranged that the hydrocarbon acid emulsion flowed from one end of the reactor to the other. The f i s t three compartments were equipped with propeller-type mixers acting as both mixers and internal emulsion recirculating pumps. The fourth compartment was the primary acid hydrocarbon settler. Although the unit had a designed capacity of 225 barrels per day, modifications were incorporated which enabled production up to 1600 barrels per day during the wartime period. Additional discussion of the unit and modifications is given by Ryan and Stevens (9). EXPERIMENTAL
In general, the followPILOT PLANT OPBRATINQ CONDITIONS. ing operating conditions were maintained during the pilot plant work: Ratio of ieobutflne (%methylpropane) to olefin weight in reactor feed, 4.8 to 5.0. Volume ratio of acid to hydrocarbon in reactor, 1 Reactor temperature, 40' to 45' F. (except where temperature was being investigated) Settler temperature, 45" to 50" F. (except where temperature was being investigated) Reaction time, 7 to 8 minutes Acid settling time, 15 to 20 minutes Reactor pressure, high enough to ensure liquid phase operation. Varied from 65 pounds per square inch a t 45" F. to 100 pounds per square inch at 105' F.
FEEDSTOCK ANALYSES. The feed stock used in the pilot plant unit varied somewhat with experiments a t different times because refinery streams were used as source material. Table I gives feed atock analyses which may be considered as typical
except, in cases where special olefins were being investigated. Analyses are based on Podbielniak distillation (5') and infmred spectroscopy methods (3) common in the industry.
ID ANALYSES. A t intervals acid samples were withdrawn from the settler and immediately centrifuged to separate entrained hydrocarbons. The density of the centrifuged acid was obtained with a Westphal balance and a sample was immediately diluted for titratable acidity determination. The remainder of the centrifuged acid was frozen with solid carbon dioxide and s u p plied to the analytical group for water (by Karl Fischer reagent, I ) and hydrocarbon content analyses. The method for determining hydrocarbon content of the acid was developed by the analytical group and involved potassium iodate oxidation of the organic matter under suitable conditions with determination of the amount of potassium iodate consumed (IO). The equivalents of oxygen consumed were converted to a weight per cent hydrocarbon by assuming oxidation of methylene group to carbon
TABLE I. FEED STOCK ANALYSES Weight % Saturates Propane Isobutane n-Butane Isopentene n-Pentane Olefins Isobutylene n-Butylenes Isobutane-olefin weight ratio
88.4
5.6
55.6 20.7 3.9
2.7 11.6
2.1 9.5 4.8
INDUSTRIAL AND ENGINEERING CHEMISTRY
June 1951
dioxide and water. This assumption is not entirely accurate, but served to determine the relative order of magnitude of hydrocarbon contents of different samples. HYDROCARBON PRODUCT WORK-UP. The debutanized bottoms were transferred to laborator lass columns and fractionated to obtain the light depentanizeza kylate of 104" to 370" F. boiling range for engine rating.
7
NATURE O F PRODUCT AT LOW ACIDITY ALKYLATION
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Points of particular interest under low acidity conditions would be the aviation octane number ratings of the light alkylate produced, as well as changes in product distribution and yield. Such data are primarily available from the pilot plant runs with oonfirming data from the full-scale plant operation in some cases. TITRATABLE ACIDITYVERSUS OCTANERATINGS. Figure 2 shows the change in the rich mixture octane rating of the light alkylate as titratable acidity of the alkylation acid was dropped over a wide range with no change in operating temperature or feed stock. The octane rating decreased as acidity decreased, but not so much as might be expected. Over the range of interest from 92 t o 85% acidity the drop in rich mixture octane rating corresponded roughly to 0.3 ml. of tetraethyllead per gallon (8 2.3 a t 93% titratable acidity t o S 2.0 a t 85% by pilot unit data). Even at 76% titratable acidity, a light alkylate of relatively high quality was produced. Other changes which occurred below 80% acidity suggest 85% acidity a8 a good operating minimum, however. The octane rating of the full scale plant alkylate was higher than the pilot unit alkylate. The magnitude of the change over the region examined, however, seemed to be similar between the pilot unit and the plant unit. Considerable additional detail is available which indicates that over the operating region of 93 to 85% titratable acidity the following approximate changes in A.S.T.M. octane ratings (t?)may be expected:
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became heavier as titratable acidity was lowered. This distribution will vary according to the out points used in separating the light alkylate. In this work the material boiling between 104" and 370" F. was taken as light alkylate. From these data a product a t %yo titratable acidity would contain 10% heavy alkylate, while alkylate produced a t 85y0titratable acidity would contain 15% heavy alkylate. This decrease in relative quantity of light alkylate might be an important factor when maximum production of aviation gasoline is desired, but would be less important in motor gasoline manufacture, since total and light alkylate have nearly the same F-2 octane number. Again 85y0 titratable acidity seems a good working minimum because the curve falk off somewhat more sharply below that acidity. ACID CHANGES AT LOW ACIDITY ALKYLATION
One of the most attractive features of ACID CONSUMPTION. low acidity alkylation is reduced acid consumption. Decreased acid consumption is beneficial, not only from the standpoint of over-all process cost, but also because of the diminished amount of spent alkylation acid which must be disposed of. Disposal of spent alkylation acid presents a problem in certain areas and occasionally appears in danger of controlling production rates Any minor changes which will diminish the acid disposal problem are of interest t o the plant operator. The variation of acid consumption with catalyst acidity level as measured in the pilot unit is shown in Figure 5 along with some full scale plant data under similar operating conditions and with roughly similar feed stocks. Acid consumption fell off rapidly as the acidity was lowered t o a minimum somewhere around 80 to 82% and then rose again sharply. Again operation of the unit a t around 85y0 would seem to be close to the minimum with an appreciable safety factor from the region wheix? consumption increases sharply. Appreciably better results were obtained again in the full scale unit as compared to the pilot unit. Oper-
F-4 rating. Loss of less than 0.3 mi. of tetraethyllead P 2 rating. Loss of less than 1.0 octane number F-1 rating. Loss of less than 1.0 octane number
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TITRATABLE ACIDITYVERSUS TOTAL ALKYLATB YIELD. Figure 3 shows the decrease in total alkylate yield as the acidity of the catalyst phase was lowered in the pilot unit. Yield of total alkylate decreased slowly with titratable acidity down to about 85y0and then seemed ta fall off more rapidly. Two points available from plant data, however, indicated no drop in total yield between 93 and 86.9% acidity. This discrepancy, if real, probably is again due to the superior performance characteristics of the full scale unit as compared to the pilot unit. While no full scale plant data are available in the acidity regions below 85%, it is predicted that the total alkylate yield will fall off substantially, although the break point may be somewhat lower with the full scale unit than the piiot unit. TITRATABLEACIDITYVERSUS PRODUCT DISTRIBUTION. As shown in Figure 4, pilot unit data indicated that the alkylate
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180-
OPILOT UNIT DATA XPLANT W T A
1 78,
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82
84
Figure 3.
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88 W T % TITRATABLE ACIDITY
56
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9b
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92
I
94
Total Alkylate Yield us. Acidity
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X P L A N T DATA O P I L O T P L A N T DATA
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Figure 2.
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90 90 ACIDITY
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Titratable Acidity us. F-4Rating
'
100
'
78
80
Figure 4.
82 84 86 88 W T % TITRATABLE ACIDITY
90
92
Acidity us. Product Distribution
94
INDUSTRIAL AND ENGINEERING CHEMISTRY
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ating the full scale unit during a 16-day test period at 86.9% titratable acidity gave an acid consumption of 0.56 pound per gallon of total alkylate compared to a 1-year average of 1.24 pounds per gallon a t a titratable acidity of 93.2%, or a reduction in acid consumption greater than 50%. This waa accomplished without any major changes in the alkylation unit operation. *4 decreased rate of fresh input and spent acid withdrawal &as all that was required. Less leeway is available, however, in operating the unit, as catalyst acidity must be watched very closely to keep it from getting into the region where acidity will fall off too rapidly to be controlled easily.
Vol. 43, No. 6
revealed an extremely strong absorption in the 295 to 310 mp region. The over-all spectrum was generally similar to that of known conjugated triene or tetraene materials such as occur in natural drying oils--e.g., tung oil-when dissolved in concentrated sulfuric acid. The extremely intense ultraviolet absorption probably was due to the presence of conjugated polyene material produced by "conjunct polymerization" ( 5 )or hydrogcbn evchange activity of the acid. The reason why sulfuric acid finally ceases to alkylate as acidity is lowered under normal operating conditions is the build-up of this acid-soluble hydrocarbon complex diluent rather than a build-up of water in the acid (6). Some confusion exists in the literature because of failure to identify the diluent. The belicf that acid below 90% acidity will not alkylate effectively is probably true for acid which has water aa the diluent, but is not true when the acid is diluted with the normal products produced in the alkylation reaction (8). EFFECT OF TEMPERATURE A T LOW ACIDITY ALKYLATIOh
Figure 7 shows what occurred to the yield and product diytribution in the low acidity alkylation region when the temperature was increased from 45' to 70" F. This temperature incre74
76
78
85
82
a4
W T % T -RATAB-E
Figure 5.
Aciditj
z's.
a0
aa
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92
%
ACIUI-Y
Acid Consumption
WATER- AND ACID-SOLUBLEHYDROCARBON CONTEXTOF C ID. The nature of the spent alkylation acid is of considerable interest. Figure 6 shows the changes which occurred in acidsoluble hydrocarbon and water content of the catalyst phase as acidity was lowered. Both pilot plant and full scale unit data are given. Water content of the catalyst phase remained essentially constant, even down to acidities as low as 80%. Commercial 98% sulfuric acid was used in both the plant and pilot unit work. KOwater was produced in the process by oxidation reactions.
Figure 7 .
Effect of Temperature on Product Yield and Distribution 0
0 ?I_3T L N T PA-A DA-A X P L A \hTT CATDATA
WT % -l-?A-ASLC
Figure 6.
AC
C -Y
Acid-Soluble Hydrocarbon and Water Contents us. .4cidity
ment seemed to have no effect on thc product distribution irk the acidity regions around 85 %, the product gradually becoming heavier in both cases. The yield of total alkylate, however, n as affected and the data would suggest that one could operate a plant down into the region of 82 to 83% acidity if the temperature were raised to 70" F. Total yield of product would then be about as good as is normally obtained at the higher acidity regions, although the product would be somewhat heaviw . Presumably acid consumption a t the 83% acidity region would he loryer than that obtained a t S5%, unless the increase in temperature also increased acid consumption. Examination of acid conmmption in the lower acidity regions at slightly incieased temperatures would be of interest, but accurate data are unavailnhle. EFFECT OF FEED STOCK CORIPOSITIOK
Figure 6 shows that a t 80% titratable acidity the catalyst phase contained about 10% of acid-soluble hydrocarbons and 2% of water. This left about 8% of material unaccounted for, which probably was acid tied up in the form of sulfonate and/or esters d i i c h were not titrated under thr conditions of determination of titratable acidity. The nature of the acid-soluble hydrocarbon complex is uncertain, but it undoubtedly is a highly unsaturated material. Utraviolet absorption examination of the spent alkylation acid
The promotional effect of isobutylene in the ordinary alkylating conditions has long been suspected. At 80% titratable acidity the effect was particularly striking (Table 11). t-nder identical operating conditions a feed stock in which 18% of the butylenes was present as isobutylene gave a much higher yield of total alkylate and a much lower acid consumption than a feed stock containing no isobutylene. The very high acid consumption in the second case is thought to be due to formation of butyl sulfates, although no experimental support is available.
INDUSTRIAL AND ENGINEERING CHEMISTRY
June 1951
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CONCLUSIONS
TABLE IT. EFFECT OF ISOBUTYLENE Olefin Feed 187 of butylenes as isobutylene O%’isobutylene in butylenes
Total Alkylate Yield, Wt. Olefin Fed 184 141
Acid Consumption, Lb./Gal. Alkylate 0.62 1.99
Conditions Temperature 45’ F. Iaobutane/olkfin ratio, 4 . 5 Titratable acidity, weight 70,80
t
The results in Table I1 indicate that low acidity alkylation, a t least in regions as low as 80%. should not be attempted without a considerable amount of isobutylene in the feed stock olefins. NATURE OF HEAVY ALKYLATE AT LOW ACIDITY ALKYLATION
‘t
The general conclusion from this work is that for commercial sulfuric acid alkylation plants alkylating isobutane with C, olefins, operation of the plant in the titratable acidity region around 85y0instead of the usual 90 to 92% would result in substantial savings in sulfuric acid and consequently a decrease in the amount of spent acid to dispose of. This saving would be gained a t little or no decrease in alkylate yield or quality. The olefin feed stock should be carefully watched to ensure that it contains an appreciable quantity of isobutylene a t all times. There is also good indication that raising the reactor temperature up into the 70” F. region might regain what little is lost on the alkylate yield and quality a t the lower temperatures. The spent acid was shown t o be spent, not because of n-ater built up in the acid but because of build-up of an acid-soluble hvdrocarbon complex.
ACKNOWLEDGMENT
I
49 0 HE Ar Y ALKYLATE 0 0 LIGHT LIGHT ALKYLATE ALKYLATE
z 2 W
1
92
The authors wish t o acknowledge the support of the work and the release of this publication by The Standard Oil Co. (Ohio). Among the large number of engineers, operators, and analysts who contributed to the work, special mention of the following should be made: J. J. Szabo, G. G. Thompson, Valeria Elersich, E. P. Kropp, Gladys McConkey, E. G. Glass, Jr., J. J. Wolnik, and the Sohio Research Analytical Group.
,
88 86 84 W T % TITRATABLE: ACDITY
90
82
I
LITERATURE CITED
Figure 8. Low Acidity Alkylate Bromine Numbers As it was shown that the relative amount of heavy alkylate increase# a t low acidity and the above data indicate that the heavy alkylate becomes increasingly olefinic, it is evident that the alkylation process tends toward a mixed alkylation and polymerization process as the titratable acidity is decreased. The polymer formed, however, was for the most part heavy enough to be in the heavy alkylate-i.e., above 3‘70”F.-rather than in the light alkylate. Total alkylate yield then decreases as acidity decreases because more of the olefin is polymerized t o heavy material and is not alkylating the full amount of isobutane. Therefore, at low acidity conditions not so much isobutane will be consumed per unit of olefin fed as is usual a t higher acidity opera tion.
(1) Almy, E. G., Griffin, W.C., and Wilcox, C. S., IND. ENG.CHEM., ANAL.ED.,1 2 , 3 9 2 (1940). (2) Am. SOC. Testing Materials, Philadelphia, Pa., “A.S.T.M. Manual of Engine Test Methods for Rating Fuels,” Designations D 909-47T, D 357-47, D 908-47T (1948). (3) Brattain, R. R., Calif. Oil WorEd PetroEeum Ind., 36, No. 2, 9-10, 12,14-17 (1943). (4) Egloff, G., and Hulla, G., “Alkylation of Alkanes,’’ACS AMonograph 107, New York, Reinhold Publishing Gorp., 1948. (5) Ipatieff, V. N., and Pines, H., IND.ENG.CHEM.,28, 684 (1936). (6) Nelson, W. L., “Petroleum Refinery Engineering,” p. 659, New York, McGraw-Hill Book Co., 1949. (7) Podbielniak, W. J., IND. ENG.CHEM.,ANAL.ED.,5, 172 (1933). (8) Ruthruff, R. F., U. S. Patent 2,316,108 (April 6, 1943). (9) Ryan, G. B., and Stevens, D. G., Natl. Petroleum News, 38, No. 23, R411 (1946). (10) Williams, R. J.. Rohrnian, E., and Christensen, B. E., b. A m . Chem. Soc., 59, 291-3 (1937).
RECEIVED September 15,
1950.
* * * * * In July, INDUSTRIAL AND ENGINEERING CHEMISTRY will publish the Symposium on Radioactive Wastes which was presented at the September 1950 meeting of ACS in Chicago. This symposium was organized to provide background information in the field of radioactivity and to cover experience to date in handling wastes problems in this field; some of the data may be valuable for civilian defense use. The papers t o be published include reports on the natural radioactivity of water; removal of radioactive isotopes from water; removal of plutonium from laundry wastes; and liquid waste disposal at Oak Ridge Laboratory.
