Alkylation of Isobutane with Propylene COMMERCIAL PRODUCTION USING SULFURIC ACID CATALYST E. CLARENCE ODEN
AND
W. J. BURCH, JR.
Cities Seroice Refining Corporation, Lake Charles, La.
A
depropanizer overhead cut produced on the catalytic cracking recovery unit with rates as high as 4500 barrels per stream day and varying in composition from 45 to 6070 propylene, remainder principally propane with small quantities of butanes, ethane, ethylene, and methane, was charged along with isobutane and other streams containing butylenes to an alkylation unit. The olefin feed varied in composition from 0 to loo% propylene and 100 to 0% butylene and/or dimer. The alkylation was conducted in the horizontal jet-type Kellogg designed reactorsettlers with sulfuric acid as the catalyst. Yields ranged from 2000 to 6000 barrels per day of light alkylate. Data are presented on the variables affecting octane quality and acid consumption together with some data needed for process design. Typical distillation and combining ratio data for the various types of alkylate produced are also presented.
C
ONSIDERABLE pilot plant data regarding alkylation of
isobutane with propylene have been made available to petroleum refinery personnel through their medium of exchange. However, few, if any, data have been published concerning commercial alkylation with propylene. Because of the increased demand for alkylate for high antiknock fuels during and since World War 11, many olefin hydrocarbons have been evaluated as feed stock for alkylation. Alkylates are valuable for aviation gasoline blends because of their high octane numbers, high blending values, and excellent rich mixture response. Many petroleum refinery research departments alkylated isobutane with propylene on pilot plant scale just previous to and during World War 11. It is easy to understand why so much research was conducted with propylene as the raw material, since before the war little use other than for fuel had been developed for this very promising refinery byproduct. Pilot plant data indicated the alkylation of isobutane with propylene to be feasible from a commercial point of view 8s raw material was available at low cost Data indicated sulfuric acid or hydrofluoric acid to have been the most widely used catalyst for the reaction, but catalyst consumption for propylene alkylate was much higher than that required for butylene or amylene alkylate, and the octanes indicated the quality of propylene alkylate to be inferior to butylene or amylene alkylate for aviation gasoline blending. For these reasons, propylene was utilized but little for production of alkylate during World War I1since there were shortages of catalyst and isobutane, a large supply of butylene and amylene, and a great demand for the highest quality alkylate obtainable. Propylene has not, even to date, been utilized extensively for alkylation of isobutane because of the high catalyst consumption as compared with butylene or amylene as a feed stock; propylene can also be utilized profitably as polyform feed stock, polymerized with other material, and utilized for production of chemicals, such &s alcohol, glycerol, and chlorinated products. Those refineries having large alkylation capacity, large sources
of propylene, and cheap sources for production and regeneration of sulfuric acid are in a good position to alkylate isobutane with propylene on a commercial scale. PROCESS EQUIPMENT AND OPERATING PROCEDURE
Basically the alkylation equipment comprises four reactorsettlers, a refrigeration system, and the conventional towers for separating the major product streams; Figure 1 shows the major process equipment. The propane-propylene feed stream from the Girbotol unit, where hydrogen sulfide is removed, is pumped into the reactorsettler hydrocarbon feed line where it joins the deisobutanizer tower recycle, depropanizer bottoms recycle, isobutane from storage, butane-dimer and butane-butylene streams en route to the reactor-settler. The entire stream having an isobutane to olefin ratio of approximately 6 to 1, called external ratio, is cooled by water and by the reactor-settler effluent t o appioxiinately 80" F. prior to being divided into eight equal streams; two streams are charged into each of the four horizontal reactorsettlers (45 feet long by 10 feet in diameter) together with hydrocarbon acid emulsjon. The hydrocarbons and acid are intimately mixed by the two 100,000-barrels-per-day emulsion circulation pumps by being discharged through Pyrex jets spaced evenly along the emulsion discharge header below the emulsion level, Figure 1A. At this point the isobutane to olefin is approximately 115 to 1 (internal ratio). Alkylate is produced by the exothermic reactions betmeen the isobutane and the olefin in the presence of the sulfuric acid catalyst. The concentration of the acid in the reaction or emulsion zone is maintained a t about 90
PROPANE-PROPYLENE I S OBUTANE B U T W -BUTYLENE FROM STG BUTANE-DIMER
3 F, PROPANE COMPRESSOR
HIGH PRESSURE
ACL%DG::OF'ANE DE I13PANIIEH
ISOBUTANE TO STORAGE CAUSTIC
DE150BUTANIZER
REFRIGERATION
COMPRESWR
1
I
t - 4
IRERUN HEAVY A M Y L A T E
ACID
FLYZH
FRESH STORAOE ACID
'
U SE SN TORAOE T ACID S
P
E
N
T
Figure 1. Simplified Flow Chart for Alkylation of Isobutane with Propylene and/or Butylene Using Sulfuric Acid Catalyst 2524
November 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
2525
to 93% and the emulsion is approximately 1 volume of TABLE I. REACTOR-SETTLER FEEDSTREAMS * * acid to 1 volume of hydroAnalysis, Volume % carbon. The acid concentraStream B.P.D." CH4 CZH4 CzHe CaHe CaHe ICaHs ICiHio N G H s NCaHio tion is controlled by varying Propane-propylene I106 0.3 1.2 4.5 46.3 3.9 . 10.4 2.1 0.8 the quantity of 98% white Butane-butylene6 1092 ... 0 . 0 0.0 2.6 30.5 1.9 0.0 66.0 7.6 21.3 sulfuric acid make-up. Deisobutaniser tower The temperature in the overhead 2310 0.0 0.0 0.0 11.6 0.0 78.5 0 0 9 9 Depropanizer botreactor-settler is maintained a t toms 1580 0.0 0.0 0.0 5.6 0.0 84.8 0.3 9.3 about 60" F. by an autorefrigIsobutane from eration system; the hydrostorage 574 0.0 0.0 2.4 3.9 0.0 76.2 0.3 16.8 carbons in the reactor-settlers Total 6662 axe allowed to vaporize at their a Barrels per day of feeds to one reactor-settler normal operation four reactor-settlers. vapor pressure of about 60" F., b Overhead produced from debutanizing a butdne-dimer stream returned from a butadiene plant, the pressure usually ranging between 20 t o 30 pounds per square inch gage. The vapor, rich in propane and isobutane, leaves the top of the reactor-settlers, passes through the compressor dry drum where entrained liquid is removed, then 90.0 to the four 600-hp. compressors where it is compressed to about 100 pounds per square inch gage prior to entering the butane f refrigeration condensers. Here, as much material is condensed s as possible (mostly Cq fractions), with maximum condensate 89.0 temperature of approximately 100' F. The condensate and z vapor flow to the separator drum where the condensate is drawn & off the bottom and returned to the reactor-settlers as refrigerant. 0 The vapor, largely propane, is contacted with caustic after leaving y 88.0 the drum and then flows through a caustic settler and into a high pressure dry drum. The essentially dry vapor off the dry drum is compressed by the propane compressor which discharges into the depropanizer tower for recovery of butanes. 5.8 R V P LIGHT ALKYLATE
...
...
...
e
CsHi%+ 0.0 0.6 0.0
0.0
0.4
-
8 7.0
The four reactor-settler vessels are each divided into three sections: reaction or emulsion section, acid-settling section, and final settling section to provide holding time for the effluent hydrocarbon stream. Partial settling of the hydrocarbons and acid takes place in the upper layer of the reaction zone and this mixture flows over a weir into the acid-settling section. Here the 90 to %yospent acid equivalent to the quantity of acid make-up is separated and withdrawn to spent acid storage prior to pumping to the acid plant for regeneration. The hydrocarbons from the settling section flow over a second weir into the final drawoff section which provides final acid settling and surge capacity for the essentially acid-free reactor-settler product. The reactor-settler product is pumped through the reactor feed-product exchanger, caustic mixer and settler, and water mixer and settler to the deisobutanizer tower. The last traces of any free acid carry-over are neutralized and removed in the caustic and water wash equipment. The hydrocarbon stream from the water settler is joined by any extraneous hydrocarbon stream that is charged to the deisobutanizer tower, and this combined stream is preheated to about 145' F., by a steam preheater (not shown in Figure 1) prior to its entering the tower. I n this tower a mixture consisting of propane, isobutane, and n-butane, containing approximately 83q10 isobutane is taken overhead; the remainder, butanes and alkylate, is removed in the product leaving the bottom of the tower. The overhead product from the 50-tray deisobutanizer tower, not recycled to the reactor-settlers, is joined by the compressed vapor stream discharged from the propafie compressor and is charged to the 28-tray depropanizer tower. I n this tower, propane, equal in amount to that in the feeds to the unit, is removed as perhead product. The overhead product is either utilized in processing or is burned in the fuel gas system. The depropanizer bottom product, consisting of approximately 85% isobutane, is either recycled back to the reactor-settlers to maintain proper isobutane to olefin ratio or sent to storage. The product from the bottom of the deisobutanizer tower is charged t o the debutanizer tower where essentially complete separation between butanes and alkylate is obtained. The debutanizer is a 24-tray tower and is normally operated to produce a specified Reid vapor pressure (R.v.~.) alkylate product. The total alkylate from the bottom of the debutanizer i s fractionated in a 16-tray rerun tower into overhead product comprising 95% of the
so 86.0
2
3
4
5
6
7
R E A O T O ~ TEMPERATURE
OF.
8
9
EXTERNAL RATIO,
Figure 2.
IO (VOL.
II
I2
1 3 1 4 1 6
%I
External I s o b u t a n e Olefin Ratio against I-C Octane Rating
total feed, called light alkylate, and a bottom product comprising approximately 5% of the total alkylate charged, referred to as heavy alkylate. The heavy alkylate is used for pump gland oil make-up and blending into motor gasoline. The approximate compositions of the feeds to the reactorsettlers are shown in Table I. PROCESS VARIABLES
I n alkylation reactions using sulfuric acid as a catalyst there are two major factors affecting the economics of the process: 1. Alkylate qualit which is evaluated by its octane rating, distillation range, anfR.v.p. as a gasoline blending stock, particularl for aviation gasoline. 2. &id consumption, which represents a major operating cost in an alkylation unit of this type. There are several secondary variables that affect the two major factors in the economics of alkylation reactor-settlers; those most often considered in the literature are: 1. External ratio, which is the ratio of isobutane in the fresh feed to the reactor-settlers divided by the amount of olefin in the fresh feed 2. Internal ratio which is equivalent t o the ratio of isobutane to olefin a t the point of olefin injection; this includes the isobutane in the charge plus the emulsion recycle 3. Percentage of propylene in the olefin feed 4. Catalyst concentration 5 . Reactor-settler temperature 6. Contact time (production rate) 7. Diluents in the feed 8. Ratio of acid to hydrocarbon
Many attempts were made to correlate the commercial plant data just as collected, but in all cases it was impossible to obtain correlations that would exactly duplicate or substantiate pilot plant data. However, by utilizing pilot plant data as a guide for-. estimating correction factors to be applied to the many variable$ '
INDUSTRIAL AND ENGINEERING CHEMISTRY
2526
thnt affmt the alkylation reaxtion, it was possible to correct for the effect of some variables and thereby obtain the effect of others. Thc effect of each variable was changed as commercial data indicated until curves were obtained that seemed to correlate all commercial unit data most accurately. All data presented arc based on commercial unit, control samples taken daily with no effort made to selpct spccial samples. 3.0
0.0
1
Figtire 3.
I IO
I
1
20
30
I
%I
SPENT ACID 92 40 50 60 70 80 90 100 PERCENT PROPYLENE ON OLEFIN FEED { VOL % i
Acid Corrswnption against I'erceritapr of Propylene in Olefin Feed
EXTERNAL RATIO. Bwmae of thc feed rates available and the special design of the cquipment, an isobutane to olefin external ratio of approxiniately 6.0 whq maintained most of the time. For this reason all corrections of data were based on a n external ratio of 6.0. As is indicated by Figure 2, changes abovc or below this point (6.0) are quite effective on the octane rating of the alkylate produced. As the external ratio exceeds 9, the incremental improvement in oc Lane rating rapidly decreases. Conversely, the oppositc is indicated for an external ratio below 5 . The poinh shown on the graph are scattered but the slope and general position of the curve nere in accord with a similar correlation of A.S.T.M. octane values against external ratio. Pilot plant and commercial data indicate INTERNAL RATIO. that the slope of the external ratio against octane rating curve and the internal ratio against octane rating curve are essentially the same. Only data for the external ratio are shown here. I n the light of comparison, an external ratio of 6.0 is equivalent t o an internal ratio of approximately 115, for average operation and maximum emulsion recycle. PERCENTAGE O F PROPYLENE
I N TOTAL OL hI 'IY b I Y h U 1 )
Considerable data were rorrelated on the effect of the pelt-nlage of propylene in the olefin feed on the quality of alkylate produced. In this study, the percentage of propylenc in the feed varied froni 0 to loo%, but the majority of these data substantiated pilot plant data in that the quantity of acid consumed increased as the percentage of propylene increased. Figure 3 shows the relation existing between the percentage of propylene in the feed and the acid consumption, in pounds of sulfuric acid consumed per gallon of total true alkylate produced (total true aikplate = the total debutanized alkylate less Cg in the feed and gland oil used for lubrication). The acid consumption was corrected the basis of 98.0% fresh acid and 92.0% qwnt acid which represented the average of plant operation for the period studied. The octane rating of the light alkylate decreased as the percentage propylene in the feed increased (Figure 4 , REACTOR-SETTLER TEMPERATURE. Substantially all data represented covered a reactor-settler temperature range of 80" t o 70" F. The higher the reactor-settler temperature the lower the octane rating of the alkylate produced. It was impossible &omaintain the most desirable temperature because the capacity of the refrigeration unit mas limited, but the reactor-settlers were maintained at the lowest temperature possible with the refrigeration available. A plot of acid consumption against
Vol. 41, No. i l
reactor-settler temperature fdiled to show it definite trend. This is believed to have been attributable to the rather narrow range of reactor-settler temperature, as a result of limited refrigeration capacity. Figure 5 showing the relation between octane rating and reactor-settler temperature indicates a decrease of 0.35 octane number per 10' F. reactor-settler temperature. Pilot plant data indicate that the slope of this curve may be too great. However, both 1-C and A.S.T.R.I. clear octanes when plotted against reactor-settler temperature indicated the magnitude shown, A C I D CONCENTRATION. The concentration of the fresh aulfuric acid charged to the reactor-settlers normally averages 98% while the concentration of the spent or reject acid varied from 89 to 94% with most of the data around 92%. Figure 6 shows the relation between octane rating and the concentration of the spent acid. These data indicate no decrease in octane per decrease in acid concentration within the range of this study. While it is fairly well established that there is essentially no change in octane rating for a change in acid concentration in the reactors from 04 to 91%, the small quantity of data below 91% would indicate, but not firmly establish, the same to be true to as low as 87y0 It is anticipated that this curve will shorn a break when the point of polymerization is reached and (it is hoped) that future data will definitely establish this point. As a large percentage of the cost of production of alkylate is in the cost of sulfuric acid, i t is important to keep i t s consumption as lorn as possible; therefore, the data indicated by Figure 6 are imporLant in commercial operation. DILwxwrs IN FEED. The pcrcentage of diluenls in the feed was small, making it difficult to obtain any definite correlations which might exist between the octane quality and diluent considered, with the exception of propane. With refrigerating limitations, propane in the feed accentuated this difficulty resulting in the operation a t increased reactor-settler temperature There was definite indication that ethylene reacted with sulfuric acid inasmuch as the quantity entering the unit normally exceeded the quantity leaving the unit. 92 91
90 89
88 a7
e6
5.8 RVP LIGHT ALKYLATE
When the alkyhtion of isobutane with propyleiie was first undertaken tho amount of hydrogen sulfide and mercaptan in the feed caused the acid consumption to be extremely high. This situation was soon remedied by the installation of a conventional Girbotol unit for the removal of hydrogen sulfide. Since increased reactor-settler temperatures show a decrease in octane quality, this study substantiates previous work by Kniel ( 4 ) in that quantities of propane in excess of 3% to the reactorsettler would be harmful. It was expected that methanc and ethane diluents would be even more harmful than propane, but the quantities of ethane and methane in the feed to the reactorsettlers were of insufficient magnitude to permit correlations.
November 1949
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CONTACT TIME. Inasmuch as contact time (a range df 20 to 35 minutes for these data) is a direct function of production
-w
I
(normal conditions), a curve was plotted showing the effect of production rate against octane quality. There is a definite trend as shown by Figure 7 that as the production rate increased while holding emulsion recycle rate constant, the quality of the alkylate decreased; this substantiates pilot plant data. Production rates of light alkylate normally vary between 2000 to 6000 barrels per stream day, with the average approximately 4500 barrels per stream day. The reactor-settlers used RATIOOF ACIDTO HYDROCARBON. in the commercial unit, on which these data were based, were designed for an acid to hydrocarbon ratio of 1volume per volume and no effort was made to study the effects of this variable. Operations a t 1 to 1 ratio gave no operating difficulties as to acid carry-over; the ratio was sufficiently high to yield a good quality alkylate, comparable to the experimental data presented by Linn and Grosse (6) for hydrogen fluoride alkylation of isobutane and propylene.
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