US2457564A - Prefractionation of hydrocarbon alkylate to eliminate propane - Google Patents

Description

DCC. 28, 1948. I* KNlEL PREFRACTIONTION OF HYDROCARBON LKYLATE T0 ELIMINATE PROPANE Filed April 29, 1942 Patented Dec. 28, 1948 PREFRACTIONATION OFHYDROCARBQN. ALKYLA'IIE T0 ELIMINATE PRLQPANE Ludwig` KnieL, Jackson Heights, N. Y., assignor to. The Lummus Company, New York, N. Yl, a;

corporation of Delaware Application April 29, 1942,. SeriaLNd.. 440,931

isoparafnic reactant from the alkyiation,system,y

particularly in the sepa-ration of; the hydrocarbon mixture discharged from the alkylation zone for the recovery oil the desired, alkylate. This problem primarily involves, fractionation difliculties in the handling oi relatively large `amounts of reactants and unreactive materials utilized in the ormationo a, relatively small amount of desired alkylate with the consequent large equipment, required to accomplish the necessary Ltra'ctionation.

Since it is necessary to have the isoparafnic reactant, e. g.,. isobutanavpresent. in an 'amount in substantial excess off that required to react with the olenic reactant, e. g., a butene, in the alkylation zone in order toy obtain a` substantial degree of alkylation, it becomes desirablefirom anec'onomic viewpoint tcrecycle the excess isoparainic reactant separated from the alkylate to the alkylation Zone to maintain the desired ratio ot isoparain. to oleiin therein. At. the same time, however, the recycling of hydrocarbons lower boiling than the isoparaffinic reactant, e.` g., propane, which is invariably introduced into the alkylation system along with the C4 fraction containing thev isobutane rand/or the `butenavis to be avoided as much as possible since, for optimum operation, itis necessary to maintain a low concentration of propane in, the, alkylation' zone.

Accordingly, itis an object oi myr invention to provide animproved method of' fractionating, the hydrocarbon mixture discharged from the alkyl'af tion- Zone in, an isoparain-oleiin alkylation systemwhereby a material saving in equiignfnent and operating costs can be efected.

It isa further object ot. my invention to provide an improved method of, separating the isobutane and the propane from, the hydrocarbon eiiluent from the' alkylation zone in an isobutanebutene alkylation system for recyclingof the isobutane to the alkylation zone and elimination cil the propane from. the alkylati'on. system.

It. is another objectof my invention to provide 260'-683.4`)` 5" 2 a convenient method of eliminating undesirable propane from an isobutane-buteneY alkylation system, Without, the use of, excessive, fractionating equipment and'with only a.` negligible loss of'valuable isobutane. Y;

Further objects` and, advantages of' my. invention Willv be apparent from the following description of' apreferred form of? embodmentthereof taken in connection. with. the accompanying drawing, which represents a typical flow sheet or. a process embodying my irwention.`

For convenience only, my invention will be. described, in connection with. the alkyl'ation of isobutane with a butene in the presence of concentrated sulfuric acid. to. form high antil'mocl` gasoline containing a major proportion of' isooct anes. It. .will be. appreciated, hoxever,4 that my invention is not. so` restricted but is applicable to, the alkylati'on of anyy `isoparailin with any olefin, Whether such materials. are, normally gas.- ecus or not, inthe presence of` any suitable alkylation catalyst to form predominantly isoparaffni'c hydrocarbons suitable for use as a high antiknock motor fuel..

According to my invention. isobntane. and bntene, in the desired proportions, containing propanev unavoidably admixed" therewith,are alkylated in the, presence of.` concentrated' sulfric acid to. form high antiknock hydrocarbons boiliing. within the range of gasoline. During the alkylationrea-ctionthe isobutane is maintained' in excessV of, that required to react with the` butene. The` resulting alkylate product is separated from the acid and is neutralized,A and.` the excess iso.'- vbntane and lower boiling hydrocarbons including propane are separated as overhead from the allwlate in. aprefractionating step. The isobutane overheadlcontains the'propane in a higher concentration than at any' other point in" the alkylation, system., and the propane is concentrated ina stream of relatively small. volume, at a. poi-nt in the alkylation system` Where it can bemost. readily eliminated therefrom by customary fractionating, operations, This overhead stream subjected to a further fractionation step to, separate the propaneirom the isobutane. The propane is eliminatedifrom the; system, and theisobutane is recycled to the alkylation zona, In. accordance with my inventiom a C4 frac'.- tion ci normally gaseous hydrocarbons, such as a. butanecut obtained from natural gas or from the. stabilization oi natural. gasoline or a C4 out of.. cracking still gases or, the like is introduced through line i0 into desobutanizer towerml'z. This feed material contains isobutane, normal ing amounts in both the overhead andthe bottoms streams according to the particular fractionating con-ditions. The isobutane and the lower boiling hydrocarbons are taken Volif-'overhead through line i6 for condensation in condenser I8. The resulting condensate is collected in accumulator '20, from' which a portion of the condensate is returned through line 2| to tower l2 as reflux.

vThe remainder 4of the isobutane condensate.

from accumulator 20 is passed through line 23 to the alkylation reactor or chamber 25,'wherein alkylation of the isobutane with the butenes is effected in the presence ofconcentrated sulfuric acid. Butenes maybe desirablyintroduced into reactor 25 through line 26 from a suitable outside source of supply. The molal ratio of the isobutane to the butenes introduced into `reactor 25 should be at least 1:1 and preferably within the range of 2:1 to 10:1 in order to accomplish a. substantial degree of alkylation. 'I'he excess isobutane may be obtained by recycling excess isobutane within the system orv by introducing additional' isobutane through line 23 from some extraneous source. The sulfuric acid, which acts as a catalyst for the alkylation reaction, should have a concentration of between 90 and 100% and preferably of about 95 to 98%. The acid may be supplied to reactor 25 as through lines and 30. L

In reactor 25 the isobutane and the butenes are brought into v intimate contact with each other and with thesulfuric acid to effect the desired alkylation. The pressure within the reactor should be suflicient to maintain the isobutane and the butenes in liquid condition at the particular reaction temperature, which should be within the range of 0 to 100 F. and preferably within the range of 40 to 60 F. The ratio of acid to hydrocarbon is desirably maintained within'the range of l/to 11/2 volumes of acid per volume of hydrocarbon feed to the reactor. Y

Reactor 25 is desirably provided with an agitating mechanism 32 to provide the necessary intimacy of mixing. Any suitable mixing device which providesa sufficient degree of agitation may be employed. The lisobutane-butene mixture is maintained in reactor 25 for sufcient time to effect substantial alkylation of the isobutane with the butenes; Substantially complete consumption of the butenes can be effected under the proper alkylating conditions, which are Well known to the art.

The acid-hydrocarbon mixture from reactor 25 is continuously withdrawn throughline 34 into settlingY chamber 36, wherein the mixture separates into an upper hydrocarbon layer and, a lower acid layer. This acid layer is withdrawn through 4line 3l, and the greater portion thereof is preferably recycled through lines 38 and 30 to reactor 25 to effect alkylation of further isobu-l tane and butenes. A'portion of the used sulfuric acid from settling chamber 36 may be continuously discharged from the system through line 39, and fresh make-up sulfuric acid may be continuously introduced into the system through line40.

The upperhydrocarbon layer in settling chamber 36 `is withdrawn through line 42 and is passed into neutralizer 44. In this chamber the hydrocarbon mixture is treated with a neutralizing agent, e. g., an alkaline solution such as a dilute aqueous caustic soda solution, which is introduced into this chamber through line 45. The neutralizing agent servesl to neutralize the acidity of the hydrocarbons lresulting from contact thereof with the acid in reactor 25. The alkaline solution separates out in the bottom of neutralizer 44 and is continuously wthdrawn therefrom through line 46.

According to my invention, the neutralized hydrocarbon mixture is withdrawn from the top of neutralizer 4-4 through line 48 and is passed into a prefractionating tower 50. This hydrocarbon mixture comprises the alkylate formed in reactorv 25, the excess isobutane, the normal butane introduced into the system along with the isobutane and the butenes, and any propane unavoidably admixed therewith. This hydrocarbon mixture is preferably preheated in a suitable heater 5| before introduction into prefractionator 50. Tower 5 0 is operated under suchv conditions that substantially all the propane and 'a substantial proportion of the 'isobutane are separated from the remainder of the hydrocarbon mixture. Preferably, theA fractionating conditions are such that only a minimum amount or substantially'none of the normal butane is separated from the hydrocarbon mixture'V along with the isobutane and the propane. The purpose of this prefractionation treatment is to concentrate the propane in avstream of relatively small volume at a'point inthe alkylation. system where it can be most readilyeliminated therefrom by fractionation.4

The separated isobutane and propane are taken off overhead through line 52 forcondensation in condenser 54. The resulting condensate is collected in accumulator 55, fromv which' a sufficient portion of the condensate is returned through line 5B to tower- 50 as reflux. The amount of reflux necessary to leffect this prefractionation of propane 'and isobutane is surprisingly small, and a reflux ratio of about 1:1 to 1.5:1 (ratio of reflux to net overhead product) is usually sumcient. .f

Further in accordance with my invention, the remainder ofthe condensate Yfrom accumulator 55 is passed through line l58 to depropanizer 60, wherein the propane is separated from the isobutane admixed therewith. The separated isobutane is removed fromtower '60 as a bottoms stream through line 6| and is desirably admixed with the C4 hydrocarbon charge in line 23 passing to reactor 25. In this way a portion of the excess isobutane lis recycled to reactor 25 to maintain the necessary excess of isobutane vtherein. The propane is removed as the overhead stream from tower through line 62 for condensation in condenser G3, the condensate from which is collected in accumulator 64. This cona densation is carried outto provide only suflicient reflux for tower 60, which reflux is supplied to rtower 60 through` line 65. The uncondensed proacer,

portion of the propane-isobutane stream in line 58 may be passed directly to reactor 25' through line Blaccording to the amount of propane that must be eliminated.

The bottoms stream from prefractionator 50 is passed through line 68 into debutanizzery tower 10. This -bottoms stream comprisesv the alkylate admixed with some isobutane and the normal butane. In debutanizer tower T the alkylate. is separated from the isobutane andA the normal butane, which are removed overhead through line l2 for condensation in condenser 13. The resulting condensate is' collected in accumulator 1'4, from which asuicient portion of the condensate is returned through line 15 to reflux tower 110. rPhe remainder of the condensate is recycled through line 16 to deisobutanizer towerV |12vv for separation of the isobutane from the normal butane. The isobutanethus recovered supplies some of the excess isobutane required in reactor 2-5. If desired, line 11 may beprovided for recycling of a portion or all of the isobutane-in line ltv directly` to the reactor when the percentage of normal butane in this iscbutane stream is relatively small.

The alkylateis. removed from; the` bottom of debutanizer tower through line 1,3, intok alkylate ractionator Bil., If desired, the ractionating conditions in tower 10 may `be maintained in such fashion that a portion of the butane is included in the alkylate bottoms stream to4 give the 'resulting motor fuel fraction any desired volatility. I'n tower 8l! thealkylate is fractionated under conditions suitable ior production of the desired high antiknock gasoline, which is4 removed overhead through line- 8l for condensation in condenser 82. A portion of the condensate is returned through line 83 as reflux for tower 80. The remainder of the condensate is Withdrawn from the system through line 84 and comprises the desired high antiknock aviation grade motor iuel. The higher boiling alkylate product is removed from the bottom oi tower 8u through line 85 for disposal as desired.

In operation, it is generally desirable to maintain the proportion of propane in the reactor 25 below 3% by weight since the propane is inert as far as the alkylation reaction is concerned and since it also adversely affects the yield of alkylate if it is present in too great a proportion during alkylation. Accordingly, unless the propane is continuously vented or eliminated from the alkylation system in some fashion, it will be continuously recycled along with the isobutane and will gradually accumulate in the system and thereby adversely alect the alkylation reaction. It should be noted that the amount of propane vented through line 6B controls all of the fractionating operations prior to this point.

My invention provides a method of concentrating the propane in a stream of relatively small volume at a point in the alkylation system where it can be most readily eliminated. If the hydrocarbon eiiluent stream in line 48 is 'fractionated in one step solely for the purpose of removing the propane overhead, a relatively large tower will be necessary. Furthermore, a relatively high reflux ratio will be required in order to prevent a substantial amount of isobutane from being carried overhead with the propane and to prevent loss of isobutane. In addition, a substantial supply of heat will be necessary in order to provide the requisite stripping eilect to strip the relatively small amount oi propane from the relatively large amount of bottoms.

Wilt-h my invention, these disadvantages, can be avoided, and a superior separation canfbe ob.-` tai-nedi at the same time. lThe fractionation in pretractionator 5B1 is carried out such that a substantial proportion of the isobutane is carried ofi overhead with the propane and only sufficient refluxA is required to prevent the removal oi any normal butane in the overhead stream. In this manner, the propane is concentrated at a point inthe alkylation system where it can be most readily eliminated therefromr in a stream having a total volumel considerably-less than the volume of the hydrocarbon eilluent in line 48. Accordingly. the easeoi` separation of the propane from the isobutane indepropanizer tower 5B materiallyy reduces the heating loadand the other operating requirements necessary for the elmination of the propane from the system. The stripping of, the propane from the isobutane in tower Sr is considerably less diicult because of the much smaller amount of material being handled in the elimination or the propane. Furthermore, the amountof isobutanelost with the eliminated propane is reduced to asubstantially negligibleminimum.

It will be appreciated that my invention is not necessarily limited to the use of concentrated Suluric, acid as the alkylation catalyst. Other suitable substances for electing this reaction comprise aluminum chloride, phosphoric acid, hydrouoric acid,l and the like. y

Although I have described` a preferred' i'orm o i embodiment of my invention, I am aware that modifications.. may. be madey thereto; therefore, only,I such limitationsasappear in the claims, ap- Qeldedr hereinafter should be made.

I claim:

1. The process oi alkylating an isoparan with a normally gaseous olefin to form high antiknock motor fuel, which comprises fractionating a feed stock containing the isoparafl'in and unreactive hydrocarbons both higher boiling and lower boiling than the isoparafn to separate the isoparaiiin and the lower boiling hydrocarbons as overhead therefrom, contacting the isoparain overhead with the normally gaseous olefin in the presence of an alkylation catalyst to effect alkylation of the isoparaifin with the oleiin, maintaining an excess of iso-v parain over that required to react with the olen during alkylation, separating the resulting hydrocarbon mixture from the alkylation catalyst, subjecting the hydrocarbon mixture to a prefractionation to separate the lower boiling unreactive hydrocarbons and a portion only of the excess isoparaflin as overhead therefrom to concentrate the unreactive hydrocarbons in such overhead, passing such overhead to a zone independent of the Zone of said prefractionation and there fractionating such overhead alone under fractionating conditions independent of iractionating conditions within the prefractionation zone to separate the unreactive lower boiling hydrocarbons from the excess isoparan therein, eliminating the separated unreactive hydrocarbons from the system, fractionating the remainder of said hydrocarbon mixture to separate the remaining isoparaiiin from the alkylate, recycling the isoparafn recovered in the system to the alkylation zone to maintain the excess of isoparaiiin therein, and iractionating the alkylate to recover a high antiknock motor fuel.

2. The process of alkylating isobutane with a butene to form high antiknock motor fuel, which comprises fractionatinga C4 fraction containing propane unavoidably admixed therewith to separate the isobutane and propane as overhead from the normal butane, contacting the isobutane overhead with a butene in the presence of concentrated sulfuric acid to eiect alkylation of the isobutane with the butene, maintaining an excess of isobutane over that required to react with the butene during alkylation, separating the resulting hydrocarbon mixture from the sulfuric acid, heating the hydrocarbon mixture and subjecting it to a prefractionation to separate the propane and a portion only of the isobutane as overhead therefrom and to concentrate the propane in such overhead, supplying only sulicient reflux to the prefractionating zone to Yprevent separation of hydrocarbons higher boiling than the isobutane, passing such overhead to a zone independent of said prefractionation zone and there fractionating such overhead alone under i fractionating conditions independent of fractionating conditions within the prefractionation zone to separate the propane from the isobutane, eliminating the propane from the system, fractionating the remainder of said hydrocarbon mixture to separate the remaining isobutane from the alkylate, recycling 4the separated isobutane to the alkylation zone to maintain the desired excess of isobutane therein, and fractionating the alkylate to recover a high antiknock motor fuel.

3. In the process of alkylating isobutane with a normally gaseous olefin in the presence of concentrated sulfuric acid to form high antiknock motor fuel wherein the isobutane is admixed with propane and wherein the isobutane is present in excess of that required to react With the yzone to separate the propane from the isobutane admixed therewith, eliminating the propane from the system, recycling the separated isobutane to the alkylation zone, and vfractionating the remaining hydrocarbon` eiiluent to recover a high antiknock motor fueLthe reilux ratio in the prefractionating zone being maintained between about 1:1 and 1.5:1.

`- Y LUDWIG KNIEL.

REFERENCES CITED The Vfollowing references are of. record in the le ofl this patent:

UNITED STATES PATENTS Number Name Date 2,172,560 Kemp et al Sept. 12, 1939 `2,281,248 Putney Apr. 28, 1942 2,283,603 Goldsby et al May 19, 1942 2,286,504 Parker June 16, 1942 2,338,272 Westenberg Jan. 4, 1944 Gardner Apr. 4, 1944

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