US2384736A - Alkylation with gaseous olefinic material - Google Patents

Description

Sept. l1, 1945. F, E, FREY 2,384,736

ALKYLATION WITH GAsEoUs OLEFINIC MATERIAL Filed March 5, 194? Patented Sept. 1l, 1945 .Frederick E. Frey, Bartlesville, Okla., assignor to Phillips Petroleum Company, a corporation of Delaware Application March 3, 1942, Serial No. 433,204

This invention relates to theconversion, by an alkylation reaction in the presence of a liquid catalyst, of relatively low-boiling hydrocarbons to higher-boiling hydrocarbons. More particularly, it relates to the eificient utilization of a comparatively inert gaseous hydrocarbon stream containing a relatively small proportion of olefins as` a source of olens for use as the alkylating agent.

Prior to this invention, the feeds to practical hydrouoric acid alkylation processes have comprised relatively large proportions of reactive in'- gredients; for example, the feed used for the a1- kylation of isobutene with butylenes ordinarily contains a total of fifty per cent or more oi isobutane plus butylenes. Heretofore, `a feed having a relatively high proportion of inert diluent material has required equipment of excessive size.-

Furthermore, prior to this invention, the feeds to practical hydroiiuoric acid alkylation processes have been liquid-phase. Gaseous feeds have been used for non-catalytic or thermal alkylation, which has the disadvantage that relatively high vpressures are required.

vlarge volumes of unreactive material from an alkylation process in an early step, thereby increasing the capacity of the equipment.

Further objects and advantages of my invention will be apparent from the following description and the accompanying drawing. f

Olefin hydrocarbons are very soluble in concentrated or anhydrous hydrofluoric acid, and polymerization of olenns in concentrated hydrol fluoric acid occurs rapidly at atmospheric temperatures. Consequently, in the prior art of catalytic alkylation of hydrocarbons, there is a theory that the oleiins must not come incontact with y hydrogen iluoride except in low concentrations of ole'iins and in the presence of a large excess of alkylatable material; 'that is, the oleiins or other alkylating agents have usually been added slowly with rapid agitation to a mixture comprising hydrogen uoride and an alkylatable hydrocarbon under alkylating conditions. I have found that, in` a process for alkylating saturated hydrocarbons with oleiln hydrocarbons in the presence of lconcentrated or anhydrous hydroiluoric 55 of absorber Il, through conduit l2| and valve 22,

(Cl. 26o-683.4).

acid, the oleiins can be advantageously absorbed, under certain speciiic conditions which will be described hereinafter, by hydroiiuoric acid prior to contacting, under alkylating conditions, with an alkylatable hydrocarbon; the resulting acid solution, when used as an alkylatng agent, produces in high yield a product of high octane number suitable for 'use in aviation motor fuel. I have found that objectionable polymerization of the oleiins absorbed in hydroiiuoric acid is averted eiectively by controlling the acid-tooleiin ratio, the temperature, the -acid concentration, and the period of residence of the olefin in the acid prior to mixing with an alkylatable hydocarbon under alkylating conditions. I have found also that polymerization is controllable to some extent by adding small proportions of water, for instance, to 35 per cent, to the absorbing acid. Further, I have found that, by applying the above-mentioned discoveries to hydroiluoric acid alkylation processes, gaseous hydrocarbon streams comprising olenns in proportions of from 5 per cent or less to 50 per cent or more may be utilized advantageously as a source of alk-ylating agent in a process for the conversion of relatively low-boiling hydrocarbons to high-boiling hydrocarbons.

Understanding of some aspects of my invention may be aided by the accompanying drawing. which is a schematic now-diagram of one arrangement for practicing the invention.

A gaseous olefin-containing hydrocarbon ma- /terial is admitted through inlet Il, valve I2, and distributing means I3 near the bottom of gasphase absorber I4. In the lower section of absorber Il, which preferably is packed, the gas is passed upwardly and is contacted intimately and countercurrently with downwardly flowing concentrated hydrotiuoric acid, which absorbs olens from the gas,'and which may be admitted through inlet l5, valve IG, and distributing means I1 in the central part of absorber Il. In the upper section of absorber i4, which also preferably is packed, the gas is also passed upwardly and is contacted intimately and countercurrently with a downwardly flowing relatively dilute aqueous solution (preferably a constant-boiling mixture) of hydrofluoric acid, which absorbs vaporous hydrogen fluoride from the gas, and which may be admitted through inlet I8, valve i9, and

i distributing means 2li near the top of absorber Il. Additional details relating to the absorptlons occurring in. absorber Il are given hereinafter.

'I'he olefin-depleted gas is passed from the tcp to washer 23, wherein it is contacted with substantially pure water, which is admitted through conduit 24, having valve 25, and which absorbs any remaining small proportions of hydrogen uoride. The olefin-depleted, hydrogen uoridefree gas is withdrawn through outlet 26 having valve 2li,

The oleiin-in-acidphase is passed from the bottom of absorber lli, through conduit 28 and valve 28, to alkylator 30, wherein, under alkylating conditions, such as 30 to 150 F., a pressure sufficient to maintain all components in the liquid phase, and for a time of about one to thirty or more minutes, it is agitated with an alkylatable hydrocarbon, such as isobutane or isopentane, which may be admitted through inlet 3| and valve 32. To increase the amount or the strength of the catalyst, additional concentrated or anhydrous hydrofluoric acid may be advantageously admitted to the alkylator. as through conduit 33 and valve 34 and-/or valve 35.

The resulting mixture passes as uthe eiuent from alkylator 30, through conduit 36 having valve 3l, to separator 38. wherein it is separated,

by gravitational or centrifugal means, into two liquid phases.

Of these phases, the lighter or hydrocarbon phase is passed through conduit 39 having valve di) to fractionator 4 l, by which it is separated into the following six fractions: (1) a very minor fraction, comprising non-condensable gases, which is withdrawn through outlet [i2 having valve 33; (2) a minor fraction comprising an azeotropic mixture of hydrogen fiuoride and relatively low-boiling hydrocarbons. such as propane and butane, which passes via valve it and conduit 55 to separator 55; (3) a comparatively large fraction comprising unreacted readily alkylatable hydrocarbons, such as isobutane or isopentane. which is recycled via conduit il, pump 38, and valve 49 to alkylatoi` 30; (4) a fraction comprising relatively diiiicultly. alkylatable hydrocarbons, such as normal butane, which is removed trough outlet 55 having valve 5I; (5) a substantial fraction of the desired highly-branched paraflnic reaction products boiling within the gasoline range, which is withdrawn through outlet.52 havingvalve 53; and (6) a high-boiling bottom fraction or oil, comprising polymers and other byproducts. which iswithdrawn through outlet 55 having valve 55.

The heavier or hydrofluoric acid phase from separator 3B is recycled to absorber it and/or alkylator 35 by way of conduit 56 and valve 57 and/or valve 95, respectively. Preferably. however, a part of it is passed through conduit 53 and valve 59 to acid-fractionation unit 55, wherein it is separated into the following three frac- Y tions (1) a major fraction, comprising substantially anhydrous hydrogen fluoride, which is recycled via conduit 6I, pump .52, and valve 63 and/or valve 35 to oleiin absorber l5 and/or alkylator 3i), respectively; (2) a fraction. compris-- ing a constant-boiling or maximum-boiling mixture of water and hydrogen uoride which is passed partly via conduit 55 and valve 65 to the In separator, by cooling and gravitational or centrifugal means, the aforementioned azeotropic mixture from fractionator 6| is separated into two liquid phases. The lighter or hydrocarbon phase is withdrawn through outlet 'H having valve 12, and the heavier or hydrouoric acid phase is passed through conduit T3 and valve lll to fractionator 66.

Fractionator 68, operated at a pressure lower than Vin fractionator 60, separates the constantboiling mixture from fractionator 60 into two fractions: (1) a minor fraction of substantially pure water, most of which is recycled via conduit l5, pump 16', and valve 25 to washer 23, but a part of which may be withdrawn through outlet 18 having valve 19, to compensate for excess water entering the system with the various feeds or otherwise, and a part of which, if desired, may be recycled to absorber I4 through valve 11; and (2) a major fraction, comprising a constantboiling or maximum-boiling mixture of hydrogen fluoride and water, which, because of the effect of a decrease in pressure upon the composition of a constant-boiling mixture, is relatively richer in hydrogen fluoride than the constantboiling mixture from fractionator 60, and which is recycled via conduit 80, pump 8|, and valve 82 to fractionator 60. 'I'he pressure in fractionator 68 is preferably of the order of one atmosphere or less, whereas that in fractionator 60 is preferably several atmospheres or more.

A dilute aqueous solution of hydrogen uoride is withdrawn from washer 23 and is passed through valve 83 and conduit 8B to fractionator 58. If desired, this solution may be passed to an intermediate fractionator, not shown in the drawing, to be separated into two fractions, (1) Water and (2) a constant-boiling mixture, only the latter of which would be passed to fractionator 5U.

The olefin-containing feed to my process may be any normally gaseous hydrocarbon material which boils appreciably below hydrogen uoride. It preferably contains olens heavier than ethylene, such as propylene, butylenes, or amylenes. Preferably the proportion of total olens is in the range from about 5 to about 50 per cent by volume of the feed. Feed material containing proportions of olens smaller than 5 per centrequires handling excessive volumes of diluent material; feed material containing proportions of oleiins larger than 50 per cent may usually be admitted directly to the alkylation unit. Fractions containing propane and lighter, including 5 per cent or more propylene are especially suitable for use as feed to my process.

Many modifications of the absorbing system are possible, such as, for example, a series of Iwashers in which the olefin-containing feed material is contacted in succession with concentrated hydroiiuoric acid, one or more aqueous acid solutions inthe order of decreasing concentration, and nally pure water. The one or more concentrated acid extracts, which contain dissolved olefins, may be passed to the alkylator; the one or more dilute acid extracts, which are substantially pure aqueous solutions of hydrogen uoride, may be passed to an acid-recovery unit, or

.anacid eiuent from one washer may be used as the absorbing liquid in an absorber using a` more'concentrated solution of acid. In another 4,modification a centrifugal-type contactor maybe used to contact the gaseous olefin-containing material with concentrated hydrouoric acid, whereby the olens are absorbed `by the acid; the olefin-depleted gas from the centrifugal-type contactor may then be washed with dilute aqueous hydrofluoric acid and/or water to recover vaporous hydrogen fluoride. .V

In general, the absorption system has the following three functions: (1) to transfer oleflns from a gas phase to a liquid hydroiluoric acid phase in a short time without objectionable polymerlzation; (2) to wash out and recover organically combined iluorine from the olefin-depleted gas; and (3) to wash out and recover hydrogen fluoride vapors from the olefin-depleted gas. The manner in which the absorption system embodied in the accompanying drawing accomplishes these functions may -be visualized as follows: In absorber il, in a short space immediately above gas-distributing means I3, the oleflns are absorbed by liquid concentrated hydrogen fluoride, of which an appreciable amount is vaporized. Some alkyl iluorides formed by theaddition of hydrogen fluoride to oleflns are also present in the vapor phase. In the zone next above thisl and below distributing means l1, fresh countercurrently-flowing liquid concentrated hydroiluorlo acid absorbs the vaporous alkyl fluorides. In the upper section of absorber i4, dilute aqueous countercurrently lflowing hydrofluoric acid extracts most of the hydrogen uoride vapor, and

in washer 2'), water extracts substantially all the remaining hydrogen fluoride.

The quantity of water used in washer 23 is preferably about 1 to 10 per cent by weight of the olen-depleted gas. The optimum proportion, of course, depends upon the concentration of hydrogen fluoride in the ellluent gas from abi sorber it. The aqueous hydrogen fluoride introd-need through distributing rmeans 20 near the top of absorber ll is preferably a maximum-boiling solution; the amount is preferably about to 20 per cent by weight of the hydrocarbon gas xbeing treated; Using smaller proportions does not remove the vaporous lhydrogen fluoride sufllciently completely, and consequently overloads -washer E3, whereas using larger proportions introduces so much water as to aect adversely the subsequent alkylation step. Thequantity of concentrated hydrofiuoric acid introduced through distribution means ll at the middle of the absorber preferably is from about one-fourth to about four times the Weight of hydrocarbon treated and should be at least five times the weight of aqueous solution introduced at the 4top of the absorber. Smaller proportions are likely to cause excessive polymerization of olens or else to be excessively diluted by the aqueous solution introduced at the top of the absorber, whereas larger proportions may produce unnecessarily high acid-to-hydrocarbon ratios subsequently in the' alkylator. In general, the optimum proportions and concentrations of the various solutions will depend somewhat upon the particular hydrocarbons used, the operating conditions, and the designof the equipment, but suitable proportions may he readily determined by trial.

Preferably, the absorption system is fitted with a cooling means to extract the heat of absorption of oleflns in the hydrogen fluoride. The vapor pressure of hydrogen fluoride increases rapidly with increase in temperature above room temperature. Thus the load on the hydrogen fluoride recovery part of the absorption .system is decreased by the use of lowl temperatures. Also, as h as been pointed out already, the extent of undesirable side reactions is reduced .by lowering the temperature of the oleiln-in-acid phase. A suitable cooling means, for example, consists of one or more cold-water coils situated within the absorber and/or the acid washers.

I have found that the rate of polymerization of olefins in hydrolluoric acid is reduced considerably if some water is present in the acid. However, although alkylation with hydroiluoric acid containing up to about ten per cent water produces in high yield a product'of good quality, both the quality and the yield may be somewhat inferior to those obtained by alkylation with acid containing less water. Therefore, it is advantageous to have a. higher concentration of water present in the acid phase of the olefin-extraction step than in the acid phase of the alkylation step. This desirable end can be attained in a modification 'of my process in which the combined hydrouoric acid phases introduced to the olefin-extraction step have a relatively lower concentration, such as to 90 per cent by weight, the balance being water, and in .which a proportion of substantially anhydrous Vhydroflucric acid large enough to raise the average concentration to the desired value, at least per cent and preferably higher, is introduced in the alkylatlon step. This modification of my invention is advantageous in instances in which unusually large tendencies to polymerization are encountered. Usually, however, in order to simplify the operation and to maintain a high acid-to-olefln ratio in' the absorption operation, it is preferable to introduce all the hydrogen fluoride, in concentration sufficient to obtain efllcient operation of the alkylation step, into the absorption step, and to depend upon control of time, temperature, and acid-to-oleiin ratio to avert undesirable polymerization reactions.

The preferred time interval for passing the olens through the absorption system to the inlet of the-alkylator is in the range 5 to 60 seconds; the preferred temperature is in the range 30 to F.; and the preferred ratio of hydrofluoric acid to olefin is in the range 10:1 to 20:1 by weight. These variables are mutually interdependent; for example, at the lower temperature and the lower acid-to-olefin ratio or longer time interval, approximately the same results are obtained as at higher temperature and a higher acid-to-oleiln ratio or a shorter time interval. Good results can be obtained outside the ranges given, -but these ranges are preferred'for practical reasons as follows: The preferred time intervals are easily obtainable with absorption or extraction equipment of conventional design; the preferred temperatures are near atmospheric and are therefore obtainable with little or no refrigeration; the preferred acid-to-olen ratios are suitable for operation of the alkylation step. The pressure in the absorption system should be sufficient to maintain a liquid hydrogen fluoride phase. and preferably in the. range 40 to 200 pounds per square inch. The exact optimum conditions for any particular case may be readily determined by trial.

In some cases, the step of recovering hydrogen fluoride from the maximum-boiling mixture may be optional. The separation is difficult even by the method embodied within the flow-diagram of the accompanying drawing, of distilling alternately at'diferent pressures. If this step is not practiced, advantage may be taken of the fact that aqueous solutions of hydrogen fluoride have a commercial value and can be disposed of for other uses than alkylation. In any case, the operating'conditions' of my process may beso controlled that only a relatively small proportion of i cent.

eiiluent is about 11.2 by weight. In the ralkylator given:

Example I A depropanizer overhead fraction comprising 13.5 per cent propylene by weight is fed to a system of the type already described. In the ab=Y sonption step, the average temperature is 53 F.; the pressure is 95 pounds per square inch; the average time of residence of the propylene in the extractor and the conduit leading to the alkylator is about 20 seconds; a proportion of maximum-boiling aqueous hydrogen fluoride equivalent to 10.4 per cent by weight of the total oleiinic feed is introduced at the top of the absorber, and. a proportion of substantially anhydrous hydrofluoric acid equivalent to 220 per cent by weight ofthe total olefin feed is introduced at the middle of the absorber. The average effective concentration of the hydroiiuoric acid is about 95 per cent, the other per cent being water. The acidto-olefin ratio in the emuent from the absorber is about 17.1 by weight. In the alkylator, the conditions are: temperature, 66 F.; average time of residence, 14 minutes; pressure, 45 pounds per square inch; rapid agitation with isobutane, which is introduced into the alkylator at a rate' of about 7 moles per mole of olen introduced. The alkylator eilluent is separated by gravity into an acid phase and a. hydrocarbon phase. About half of the acid phase is recycled to the middle of the absorber for use as absorbing liquid and the other half is passed to an acid recovery system of the type already described in connection with the accompanying drawing. The hydrocarbon phase is subjected to debutanization, from which an is'obutane fraction is recycled to the alkylator. The normally liquid debutanized alkyl ate is characterized as follows.:

Yield, wt. per cent vof olein 209 Oleiln content 0.0 Composition, vol. per cent: l

Ca 95" C.) 76.1 Ca (95-125C.) 19.9 Ca 125 C.) 4.0

An aviation-gasoline cut is obtained from this alkylate in a yield of 99 per cent by volume; it

has an ASTM octane number of' 90.7, which isv increased to 100.4 by the addition of 1 cc. of tetraethyl lead per gallon. t

Example II A refinery gas containing 9.3 per cent by weight butylenes, 6.7 per cent by weight propylene, some normal butane, some propane, and a substantial -proportlon of ethane, methane, and hydrogen is mum-boiling aqueous hydrogen fluoride, -intro.

duced at the top of the absorber, equivalent tc 8.5

assarse the conditions are: temperature 97-l04 F.; average reaction time, 4.75 minutes; pressure 115 pounds per square inch; rapid agitation with isobutane which is introduced into the aiwlator at a rate of about 11.5 moles per mole of oleiin introduced. The alkylator eilluent is separated by gravity into an acid phase and a hydrocarbon phase. About half of the acid phase is recycled to the middle of the absorber for use as absorb ing liquid, and the other half is passed to an acid= recovery system of the type already described in connection with the accompanying drawing. The hydrocarbon phase is subjected to debutanization, from which an isobutane fraction is recycled t0 the aihlator. The normally liquid debutanised alkylate is characterized as follows:

Yield, wt. percent of oleiln Olen content, wt. percent 0.08 Organic fluorine, wt. percent 0.0008 Composition, vol. percent:

Ct( 95 C.) 10.3 Ca(95125 C.) 85.3 Ca( 125 C.) 1.4

An aviation-gasoline cut is obtained from this alkylate in a yield of practically 100.00 per cent by gaseous oleiinic feed material without the necesy sity for liquefying said material.. Further, my invention makes possible the recovery, and the utilization as alkylating agent for hydrofluoric per cent by weight ofthe total oleflnic feed; and

a proportion of substantially anhydrous hydroiluoric acid, introduced at the middle of the ab sorber, equivalent to 180 per cent byweight of the total oleflnic feed. The average eiiective concentration of the hydrofluuric acid is about 95 per The acid-to-oleiin ratio in the absorber acid allq'lation, of oletlnspresent only in small proportions, such as ve percent or even lem, in hydrocarbon. streams. Still further. my invention provides for substantially completely recovering from the olen-depleted hydrocarbon 'stream hydrogen uoride vaporized from the absorbing liquid.

Obviously, there `are many variations and modificationsI oi my invention, only a few of f which are described herein. Additional Dumps. valves, fractionators, coolers, and the like, such as are well-known in the art, may be used wherever they are desirable or convenient. .It is intended that the foregoing speciilc examples and description of particular apparatus and operating conditions should illustrate the principles v of my invention and the preferred modes ci' practicing it, but that the scope of the invention ils to be limited only by the appended claims. I c aim: 1. In a process for` `lating an alkylatable hydrocarbon in the prese ce of concentratedilydroiiuoric acid, the imp ,ovement' which comprises contacting in an olefin absorption step ai gaseous hydrocarbon material containing olens of three to four carbon atoms per molecule with' liquid -aqueous hydrofluoric v"acid having an acid concentration of 65 to 90 per cent to eiiect a dissolution of said olefins in said liquid hydrofiuoric I acid, said contacting also resulting ,in formation of organic duerme-containing compounds which are retained in the gaseous phase, separating from said contacting a liquid hydrouoric acid yphase containing dissolved cleans, contacting resulting unabsorbed gaseswith a relatively dilute liquid aqueous hydronuoric acid to absorb' containing higher-boiling hydrocarbons produced hydrogen fluoride, adding a resulting aqueous medium to the aforesaid liquid concentrated hydronuoric acid, contacting unabsorbed gases with water to absorb remaining vaporous hydrogen iluoride thereby forming a weak aqueous hydroiluoric acid, discharging resulting olefinfree and hydrogen iiuoride-free unabsorbed gases from the system, passing said weak hydrofluorlc acid solution to a low pressure fractional distillation means and separating same into water and stronger aqueous hydrouoric acid, passing at least a portion of said water back to the last saidv absorption step, passing said stronger hydrofiuoric acid to a high pressure fractional distillation means, admixing with the ai'oresaid oleiln-containing hydrouoric acid phase suillcient more highly concentrated hydronuoric acid to produce an acid of alkylating strength and containing more than 90 per cent of hydrogen iluoride, and contacting'the same with an alkylatable hydrocarbon under alkylation conditions to eilect an alkylation 'of said alkylatable hydrocarbon, removing from eilluents ot said alkylation a liquid hydroiluoric acid phase and a hydrocarbon phase, recovering from said hydrocarbon phase a fraction containing hydrocarbons resulting from said alkylation as a product oi' the process, passing at least a portion of the last said hydroiuoric acid phase to the aforesaid high pressure fractional distillation means, recovering from said high pressure means a substantially pure highly concentrated hydrofluoric acid and passing same to said alkylation step,

and separating also from said high pressure' means an aqueous hydroiluoric acid fraction and Passing same to the aforesaid low pressure irac-l tional distillation means.

2. In a process for alkylatinga low-boiling isoparamn by reaction with a normally gaseous oleiln in the presence oi a hydroiluoric acid alkylation catalyst, the improvement which comprises passing an oleiln-containing hydrocarbon gas to a low point or an absorption zone, passing to a high point of said absorption zone as an absorption liquid aqueous hydroiluoric acid containing substantially less than 65 per cent by weight of hydrogen iluorlde, passing to an intermediate Point of said absorption zone concentrated liquid hydroiluoric acid in an amount such that the amount of hydrogen-fluoride in the absorption liquid initially contacting said hydrocarbon gas is between 65 and 90 per cent by weight, removing oienn-iree hydrocarbon gases and from the bottom oi said absorption zone a resulting rich hydrouoric acidabsorption liquid, intimately adq mixing in an alkylation zone said rich absorption liquid, sumcient additional hydrogen iluoride to from eliuents ot-said sone a hydrocarbon fraction from the top of said absorption zone unabsorbed 'l by alkylation in said zone.

3. A process for removing oleiins from an oleiin-containing hydrocarbon gas which comprises passing a gaseous oleiln-paraln mixture to a low point of an absorption zone, passing to a high point of said absorption zone as an absorption liquid aqueous hydroiluoric acid containing substantially less than per cent by weight of hydrogen fluoride, passing to an intermediate point of said absorption zone concentrated liquid hydroiluoric acid in an amount such that the amount of hydrogen uoride in the absorption liquid initially contacting said hydrocarbon gas is between 65 and 90 per cent by weight, removing from the top of said absorption zone unabsorbed olen-free hydrocarbon gases and washing' said olen-iree'hydrocarbon gasesv withy Water to remove-residual hydrogen iiuoride, andrecovering a paralnic gas free from oleiins as a product of the process.

4. In a process for alkylating a low-boiling alkylatable hydrocarbon by reaction 4with a nor'- mally gaseous oletln in the presence of a hydrofluoric acid alkylation catalyst, the improvement which comprises passing an olefin-containing hydrocarbon gas to 'a low point of an absorption zone,.passing to a high point of said absorption zone as an absorption liquid aqueous hydrofluoric acid containing substantially less than 65 per cent by weight of hydrogen fluoride, passing to an intermediate point of said absorption zone concentrated liquid hydrouoric acid in an amount such that the amount of hydrogen iiuoride in the absorption liquid initially contacting said hydrocarbon gas is betweenv65 and 90 'per cent by weight, removing from the top of said absorption zone unabsorbed .olen-free hydrocarbon gases and from the bottom of said absorption zone a resulting rich hydroiluoric acid absorption liquid, intimately admixing'in an alkylation zone said rich absorption liquid, suihcient additional hydrogen fluoride to raise the hydrogen iluoride concentration of said liquid above per cent by weight, and a low-boiling alkylatable hydrocarbon, maintaining alkylating conditions in said alkylation zone, and recovering from eilluents of said zone a hydrocarbon fraction containing higher-boiling hydrocarbons produced by alkylation in said zone.

5. In the process of claim 3 the additional improvement which comprises passing a resulting highly aqueous liquid hydroiluoric acid from the last said washing step to a distillation step and separating same into an acid-free water fraction and a more highly concentrated aqueous hydrouoric acid fraction containing substantially less than 65 per cent by weight of hydrogen uoride, returning said acid-free waterl to the lastv said washing step, and passing said more highly concentrated aqueous acid to a high point o! said absorption zone.

' FREDERICK E. ERES?.l

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