CA1142554A

CA1142554A - Process for obtaining isobutene from c.sub.4-hydrocarbon mixtures containing isobutene

Info

Publication number: CA1142554A
Application number: CA000355096A
Authority: CA
Inventors: Erwin Brunner; Eckart Schubert; Alfred Lindner; Franz Merger; Klaus Volkamer; Gerhard Sandrock; Max Strohmeyer
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 1979-07-14
Filing date: 1980-06-30
Publication date: 1983-03-08
Also published as: DE2928510A1; JPS5616425A; EP0022510A1

Abstract

ABSTRACT OF THE DISCLOSURE:

The present invention is concerned with a process for obtaining isobutene from a C4-hydrocarbon mixture contain-ing isobutene, which comprises the steps for reacting the mixture continuously with a primary C3-or C4-alcohol in the presence of an ion exchanger in its acid form as a condensing agent to form the corresponding C3- or C4-alkyl tert-butyl ether by feeding the primary C3- or C4-alcohol and the C4-hydrocarbon mixture with or without prior mixing, to an etherification reaction zone which contains the ion exchanger, while maintaining the exit temperature of the reaction mixture from the etherification reaction zone at from 25 to 65°C and the quotient of the volume of the etherification reaction zone and the throughput of the C4-hydrocarbon mixture and the primary C3- or C4-alcohol at from 0.01 to 5 hours; distil-ling the reaction mixture obtained from the etherification reaction zone in a first distillation zone, taking off as the top product without water washing a C4-hydrocarbon mixture comprising the unconverted hydrocarbons and not more than 1,000 ppm by weight of the primary C3- or C4-alcohol and taking off as the bottom product the resulting C3- or C4-alkyl tert-butyl ether, which may contain therein primary C3- or C4-alcohol which may have been added in excess;
feeding the bottom product to a second reaction zone, containing an acid catalyst, in which the C3- or C4-alkyl tert-butyl ether is decomposed at an elevated temperature to give isobutene and primary C3- and C4-alcohol; feeding the mixture of isobutene and primary C3- and C4-alcohol produced in the decomposition step to a second distillation zone, taking off as the top product without a water wash isobutene contain-ing not more than 500 ppm by weight of primary C3-or C4-alcohol and taking off as the bottom product the remaining primary C3- or C4-alcohol produced in step (c); and recycling the primary C3- or C4-alcohol which is the bottom product of step (d) to the etherification reaction zone.

Description

11'~2554 Process for obtaining isobutene from C4-hydrocarbon mixtures containing isobutene The present invention relates to a process for obtaining isobutene from a C4-hydrocarbon mixture containing isobutene, by reacting the mixture with a primary C3- or C4-alcohol, isolating the tertiary ether formed and decomposing it at an elevated temperature.
It is already known to obtain isobutene from a C4-hydrocarbon mixture by means of a sulfuric acid extraction process. In this process, highly concentrated sulfuric acid must be used and consequently expensive materials must be employed for the equipment. Since, furthermore,-side-reactions of iso-bu'tene, for example dimerization, polymerization, hydration and the like, occur during the extraction, the sulfuric acid extrac-tion process is not satisfactory in respect of yield, and of quality of the products.
A process for obtaining isobutene is also known, for example from German Patent 1,216,865 published on May 18,1966 or German Application DAS 1,934,422 published on January 15,1970 or DAS 2,011,826 published on October 1, 1970 in which a C4-hydrocarbon mixture containing isobutene is reacted with methanol in a first stage and the resulting methyl tert.-butyl ether is decomposed into methanol and isobutene in a second stage. However, the known processes have the disadvantage that methanol forms azeotropic mixtures with the C4-hydrocarbons. For example it is known from Germain-Laid-Open Application DOS 2,629,769 published January 5, 1978 and German Published Application DAS
1,934,422 that in the preparation of methyl tert.-butyl ether, when the unconverted 1~ 5~

- 2 - o.z. 0050/033~51 hydrocarbons are removed from the reaction mixture by distillation they contain about 2 % of methanol, due to the hydrocarbon/methanol azeotropes, and this methanol can only be reco~ered by expensive methods, for example by interpolating a water wash. It is a particular disadvantage that on separating by distillation the reaction mixture obtained from the decomposition s.tage and containing isobutene and methanol, the methanol and isobutene form an azeotropic mixture so that an expensive waterwash mustalso beint~lat~d into the decomposition stage (cf., for example, German Published Application n~-~
1,934,422) in order to m1ni~;7-e the loss of methanol and obtain a met~nol-fre~ isobutene, as is re~uired for.most applications.
It is true that in addition to the use of meth-anol and possibly ethanol as alc.ohols for the etherifica-tion reaction, primary alcohols in general ha~e previously been referred to as possible reactants for the conversion to the tertiary ether (cf., for exa le, German Patent 1,216,865 and German Published Applications DAS 1,934,422 and 2,011,826, already referred abo~e). However, there was a subst~ntial prejudice against the use of higher primary alcohols, for example C3- or C4-alcohols, si~ce it was known that such higher prim~ry alco~ols can easily be dehydrated to olefins under the reaction conditions of the decomposition stage, in the presence of an acid ca ~lyst. For example, German Published Application DAS 1,934,422, already referred to, expressly points out, in column 3, 1st paragraph, that methanol, which cannot ilf~255~

be dehydrated, should be used as the alcohol in order to avoid the undesirable formation of olefins in the decom-position stage.
A further substantial prejudice against the use of higher primary alcohols resulted from the fact that it was known, for example from U.S. Patent 3,170,000, especially Table I and dolumn 3, lines 20 to 31, that methanol and ethanol give substantially higher yields in the ether-ification reaction than do the higher primary alcohols, eg.
the C3- or C4-alcohols.
Because of the disadvantages and prejudices des-cribed above, the conventional process for obtaining iso-butene by decomposing the tertiary ether obtained in a first etherification stage have not found industrial use but have only remained prior art on paper, and hence the industrial production of isobutene had to depend on the use of the sulfuric acid extraction process, with all the shortcomings and disadvantages inherent in the said process.
It is an object of the present invention to provide a process for obtaining isobutene from a C4-hydrocarbon mixture containing isobutene, which does not suffer from the disadvantages of the conventional processes.
We have found that this object is achieved by a simple process for obtaining isobutene from a C4-hydrocarbon mixture containing isobutene, which comprises (a) reacting the mixture continuously with a primary C3- or C~-alcohol in the presence of an ion exchanger in its acid form as a condensing agent to form the corresponding C3- or C4-alkyl tert-butyl ether by feeding the primary C3-or C4-alcohol and the C4-hydrocarbon mixture with or without prior mixing, to an etherification rcaction zone which contains the ion exchanger, while maintaining the exit temperature

- 3 -l~ tZ551~

of the reaction mixture from the etherification reaction zone at from 25 to 65C and the quotient of the volume of the etherification reaction zone and the throughput of the C4-hydrocarbon mixture and the primary C3- or C4-alcohol at from 0.01 to 5 hours;
(b) distilling the reaction mixture obtained from the etherification reaction zone in a first distillation zone, taking off as the top product without water washing a C4-hydrocarbon mixture comprising the unconverted hydrocarbons and not more than 1,000 ppm by weight of the primary C3- or C4-alcohol and taking off as the bottom product the resulting C3- or C4-alkyl tert-butyl ether, which may contain therein primary C3- or C4-alcohol which may have been added in excess;
(c) feeding the bottom product to a second reaction zone containing an acid catalyst, in which the C3- or C4-alkyl tert-butyl ether is decomposed at an elevated temperature - to give isobutene and primary C3- and C4-alcohol;
(d) feeding the mixture of isobutene and primary C3- and C4-alcohol produced in step (c) to a second distil-lation zone, taking off as the top product without a waterwash isobutene containing not more than 500 ppm by weight of primary C3- or C4-alcohol ancl taklng orf as ~hc ~ottom procluct the remaining primary C3- or C4-alcohol produced in step (c);
and (e) recycling the primary C3- or C4- alcohol which is the bottom product of step (d) to the etherification reaction zone.
Using the novel process, a C4-hydrocarbon .
A

ll'~Z55 ~
-- 5 -- ~.Z. OGSO/C33951 raffinate which is virtually free from C3- or C4-alcohol is isolated from the reaction mixture obtained after the etherification stage, by simple distillation without interpolating a water wash, since un ~nverted primary C3- or C4-alcohol surprisingly does not form an azeotrope with the C4-hydrocarbons. In general, a C4-hydro-carbon raffinate containing not more than 1,000 ppm by weight of C3- or C4-alcohol, preferably at most 500 ppm by weight, in particular at most 100 ppm by weight, is lo taken off as the top product of the distillation.
Again, when the reaction mixture, obtained on decomposing the C3- or C4-alkyl tert.-butyl ether, is separated by distillation-into isobutene and the C3- or C4-alcohol, azeotropesof the alcohol are not formed. The C3- or C4-alcohol can therefore be recovered, without interpol-ation of a water wash, in a simple manner and virtually without losses, and be recycled to the etherification stage.
Surprisingly, the process according to the inven-tion gives isobutene in high yield, for example in a yield of more than 97 %, based on the isobutene contained in the C4-hydrocarbon mixture employed. This was unexpected since U.S. Patent 3,170,000, already referred to, states in column 3 that on using C3- or C4-alcohols only very poor yields of tertiary ether are obtained.
It is also known from U.S. Patent 3,634,535, especially column 6,that the reaction of isobutene with propanol gives the tertiary ether in a yield of only about 50 %, whilst the corresponding reaction of isobutene and ZS5i~
- 6 - O.Z. oOSo/033951 methanol gives yields of from about 90 to 95 ~. It was therefore surprising that yields of tertiary ether of more tha~ 95 % are obtained by the process according to the invention.
Isobutene-containing C4-hydrocarbon mixtures suit-able for the process of the invention are obtained, for example, from ~he thermal or catalytic cracking of petroleum products, from the manufacture of ethylene by pyrolysis of liquefied petroleum gas (LPG), naphtha, gas oil or the like, lo or from the catalytic dehydrogenation of n-butane and/or n-butene. These C4-hydrocarbon mix~ires as a rule contain-olefinic and paraffinic C4-hydrocarbons in addition to the isobutene and may also contain butadiene, for example in amounts of up to 70 per cent by weight, and higher acetylenes, eg. but-l-yne and butenyne. Butadiene-containing C4-hydrocarbon mixtures may be employed as such or after first removing the butadiene from the C4-hydrocarb~
m;xture~ for example by extraction with aselective solvent.
The C4-hydrocarbon mixtures may in addition contain C3-hydrocarbons, eg. propane, propene and propyne, for examplein amounts of up to 10 per cent by weight, In general, the C4-hydrocarbon mixtures contain from 5 to 95 per cent by weight, preferably from 10 to 90 per cent by weight, in particular from 20 to 70 per cent by weight, of isobutene.
Preferably, C4-hydrocarbon mixtures are used which in addi-tion to isobutene contain n-butane, isobutane, but-l-ene, trans-but-2-ene and cis-but-2-ene, with or without buta-1,3-diene.
The primary C3- or C4-alcohols (ie. alcohols of 3 il~2S5~

or 4 carbon atoms) to be used according to the invention are in general n-propanol, n-butanol or isobutanol, prefer-ably n-propanol or isobutanol, and especially isobutanol.
The alcohols are used, for example, as technical-grade products of conventional purity, for example of a purity of at least 95%, preferably at least 98%.
The acid condensing agents used for the etherifi-cation which represents the first stage are ion exchangers in the acid form. Examples of suitable ion exchangers are sulfonated coal, sulfonated phenol-formaldehyde resins, sulfonated resins derived from coumarone-indene condensation products and, in particular, sulfonated polystyrene resins, eg. nuclear-sulfonated crosslinked styrene-divinylbenzene copolymers. The amount of the ion exchanger is in general from 0.01 to 1 liter of bulk volume per liter of reactor volume. The ion exchangers may be used as such or on a carrier. Examples of suitable carriers are alumir,a, silica, active charcoal and plastics, eg. styrene polymers. The etherification may be carried out in, for example, stirred kettles or fixed bed reactors, the latter being preferred.
The exit temperature of the reaction mixture from the etherification zone is from 25 to 65C, preferably from 30 to 60C, ~specially from 30 to 50C. Preferably, exit temperatures which are lower than the mean temperature in the etherification stage are employed. In general, the ether-ification reaction results in not less than 90%, preferably n A

1~ 55~
- 8 - 0.2. ~O50/033951 less than 95 %, in particular not less than 96 %, conversion of the isobutene, contained in the C4-hydrocarbon mixture, to the C3- or C4-alkyl tert.-butyl ether.
The etherification according to the Ln~e~tion can be carried out under atmospheric pressure, However, it is advantageous to' wor~ under slightly superatmospheric pressure, for example at from 1.01 to 30 bar, especially from 2 to 20 bar. The isobutene-containing C4-hydro-lo carbon mixture can, depending on the pressure and tempera-ture, be employed for the reaction as a liquid or a gas.
Preferably, liquid isobutene-containing C4-hydrocarbon m;xtures are employed. ~he etherification can be carried out batchwise. In that case, the reaction times are in general from 1 minute to 5 hours. Prefer-ably, however, the etherification is carried out continu-ously, in which case the quotient of the volume of the rsaction zone (in volume units) and the throughput in volume units per hour is in general from 0.01 to 5 hours, preferably from 0 02 to 1 hour, especially from 0.03 to 1 hour.
~or the etherification reaction, the weight ratio of primary C3- or C4-alcohol to the isobutene contained in the C4-hydrocarbon mixture is in general from 100 : 1 to 1 : 1, preferably from 20 : 1 to 1.2 : 1, especially from

4 : 1 to 1.3 : 1.
- The reaction mixture which is obtained from the etherification reaction zone and which as a rule still contains excess primary C3- or C4-alcohol which had been t ~. 'S' ' r~ ~
_ 9 ~ 33 3 ~: 5 1 added for the etherificat~on reaction ls separated by distiilation, without ~nterpolating a water wash, and the top product taken off is a C4-hy8rDcarDon raf-firAte substantially free from isobutene, the isobutere content in general being not more than 5 percent by weight, preferably not more than 2.5 percent by weight, especially not more than 1.5 percert by weight. Preferably, the top product taken off is a C4-hydrocarbon raf~inate con-taining not more than 200 ppm by weight of ~3- or C4-alkyl tert.-butyl ether and/or di-C3-aLkyl or di-C4-alkyl ether.
The bottom product from the distillation of the reaction mixture obtained after the etherification con-sists of the C~- or C4-alkyl tert.-butyl ether which may or may not still contain excess primary C3- or C4-alcohol.
Advantageously, a bottom product cor~taining ~ot more thar.
1,000 ppm by weight, preferably not more than 500 ppm ~y weight, especially not more than 100 ppm by weight, of C4-hydrocarbons is taken off.
mereafter, in the second stage of the process, the tertiary ether obtained is decomposed into isobutenG
and primary C3- or C4-alcohol in the p-esence o an acid catalyst at ele~ated temperatures. The starting material for the decomposition can be a tertia~ ether ~hich is virtually free from C3- or C4-alcohol and which has been obtained, for example, by using, for the etherlfication, an amount of C3- or C4-alcohol corresponding to at most the stoichiometrically required amount of alcohol, or ~y remov-ing (for example by distillation) excess added pri~ary C -10 ~ 33 ~`' or C4-alcohol from ~he bottom product obtained after distil-lation of the etherification reaction mixtu~e. Prefer-ably, howe~er, ~e tertiary ether obtained as the bottom product after remc~ing the ~4-hydrocarbon raffinate by distillation is employed for the decomposition without further removal of any excess C3- or C4-alcohol which may be present. Alternatively, it is ~lso possible to rem-ove only a part of the excess C~- or C4-alcohol. In general, the C3- or C4-alkyl tert.-butyl ether formed is o used in the decomposition stage without addition of water.
To carry-Qut the decomposition, the tertiary ether is ~aporized and brought into contact with the acid catalyst in the vapor phase. Examples of suitable acid catalysts are ion exchangers in the acid form, eg. sulfonated coal, sulfonated phenol-formaldehyde resins, sulfonated resins derived from coumarone-indene condensation products and, in particular,sulfonated polystyrene resins, eg. nuclear-sulfonated, crosslir~ed styrene-divinylbenzene copolymers.
Other catalysts which may be used advantageously are solid phosphoric acid catalysts which comprise monophospho-ric acid or preferably polyphosphoric acid on a solid carrier. Ex~mples of suitable carriers for the ~hos-phoric acid catalysts are alumina, silica, active charcoal, kieselguhr or pumice. Silica gel is the prefe~red carrier .
Other suit~ble acid catalysts are acid metal sul-fates, eg, sodium bisulfate, calcium bisulfate, alum~num sulfates, nickel sulfate, copper sulfate, cobalt sulfate, - 11 - 3.~ C50~3~951 c~dmium sulfate and stron~ium sulfate. These acid sulfates ~ay ~e used unsupported but are prefer-ably used on a carrier. Examples of cuitable carriersare silica gel, active charcoal, alumina and pumice.
Further suitable catalysts for the decompositior~
are silica gel and alumina used by themsel~es.
In a further embodiment of th~ process according to the invention, a metal phosphate, especially a metal hydro-gen phosphate, is used as the acid decomposition catalyst.
lo These phosphates may also contain phosphoric acid i~ excess over the amount corresponding to the stoich~ometric compo-sition of the acid metal phosphate, for example in anexcess of up to 6~%, preferably from l +o 50 ~, in particular fromlO to 20 %. Examples of such metal phosphates are mag-nesium phosphates, calcium phosphates, strontium phosphates, barium phosphates, manganese phosphates, nickel phosphates, copper phosphates, cobalt phosphates, cadmium phosphates, iron(II) phosphates, chromium phosphates ~nd in ~articular aluminum phosphates, ~he metal phcspnate cat21ystca~beused as such or on a carrier. Examples of suitable carriers are alumina, silica, acti~e charcoal and zinc oxide.
The amount of the acid catalyst is ln Oeneral ~rom about O.Olto lkg,preferably from aboutO.03 toO.3 k~, ~e~
kg of tertiary ether passed thh~ough the reactor per hour.
- Preferably, fixed bed reactors are used cr ~he decomposi-- tion of the tertiary ether.
me decomposition temperature of the ~ertiary ether varies wi+~h the nature of the acid catalyst and with the contact time, but is Ln general from 50 to 350QC, preferably from 80 to 30QC, in particular from 100 to 250C. Tf a metal phosphate or phosphoric acid catalyst ls used as the decomposition catalyst, the decomposition is in general carried out at from 80 to 350C, preferably from 90 to 260C, especially from 170 to 210C.
me contact time of the vaporized tertiary ether is advantageously from 0 1 to 20 seconds, preferably ~rom 1 to lo 10 seconds.
~he decomposition of the tertiary e~her c~n be carried out under atmospheric pressure, but is i~ general carried out u~der superatmospheric pressure, for example at up to ~0 bar, preferably up to 20 bar. Advantage-ously, the decomposition of the tertiary ether is carried out under pressures of from 2 to 15 bar, preferably from 3 to 12 bar, especiaily from 4 to 12 bar. H~wever, the decomposition can also be carried out under reduced pressure.
The decomposition of the terliary ether ~ay be carried out batchwise but is preferabiy carried out con-tinuously.
me reaction mixture obtained from the decompo-sition, which contains isobutene and primary C3- or C4-alcohol as the reaction products, is fed to a second distillation zone, in which isobutene containing not more than 500, prefer2bly no~ more than 100, especially not more than 50, ppm by weight of primary C3- or C~-alcohol is taken off as the top product~ wit~out inter?olati~.g 1 1 ~ >'~
- 13 - o.~. o~o~o33? 1 a water wash. AdvantageousiyJ a top product which is ` not less than 99.3 ~ by weight pure) preferably not less than ~9.5 ~ by weight pure, especially not less ~han 99.7 % by weight pure, and which contains the di-C3-alkyl ether or di-C4-alkyl ether, which may be formed in very small amounts as a by-product, and/or the C3- or C4-alkyl tert.-butyl ether, in an amour.t of at most 100 ppm by weight, preferably at most 50 ppm by weight~ especially at most 20 ppm by weight, is taken off. ~he pr~ ary lo C3- or C4-alcohol obtai~ed as the bottom product from the second distillation zone is recycled to the etherification reaction zone. Preferably, the content of di-C3-aL~yl ether or di-C4-alkyl ether, which may be formed in very small amounts a~ a by-product but accumulates in the recycled prima-y C3- or C4-alcohol, is restricted to from 2 to 20 % by weight in the latter.
In the novel process, it can be advantageous, if isobutanol is used as the C4-alcohol, to bleed off a part of the stream of isobutanol in order to remove any impurities which may have accumulated, in whi~h case the ble~stream is advantageouslytakenfrom the side of '~he second distillation zone, or from the bo~tom product +æken off the second distillation zone. Advantageously, the isobutanol ~leed-stream is in general from 0.1 to 10 ~ by weight, preferably from 0.5 to 5 % by weight, of the total isobutanol stream. ~he isobutanol bleed-stream taken off to remove any diisobutyl ether formed advantageously contains from 3 ! 0 40 % by weight, prefer-ably ~rom 5 to ~ ~ by weight, especi~lly ~rom 10 to 30 ~

- 14 - ~ J
by weight, of diisobutyl ether.
In an advantageous embodiment of the process, the isobutano~ ble ~ stream is dehydrated in the con~en-tional manner in ~he presence of a denydrating catalyst, resulting in the dehydration of not only the isobutanol but also the diisobutyl ether, &nd thereby additionally increasing the yield of isobutene.
Advantageously, the dehydration is carried out in the gas phase over a catalyst. Examples of suitable catalysts are silica gel, thorium oxide, tit ium(lV~
oxide ànd especially all~mina. In general, the dehydra-tion is carried out at from 250 to 450C, preferabl~ from 300 to 400C. It can be advantageous to carry out the dehydration in the presence of water, which may or may not be added for the purpose.
The Figure diagrammatically shows a~ illustrative embod~ment of the process according to the invention.
The isobutene-containing C4-hydrocarbon mixture (fed through line 1) and the primary C3- or C4-alcohol (fed through line 2) are mixed, and the resulting mixture is passed through line 3-to the etherification reaction 4, which contains the ion exchanger. Advantageously, tAe reactor is a fixed bed reactor, eg. a flow t~be or a loop reactor or a combination of both types.
However, other types of reactor, for example a stirred kettle or astirred kettle cascade, can also be used.
The reaction mixture obtained is taken from the reactor - through line 5 and fed to a first distillat on colu~Ln 6.
At the top of the distillation column, substæ~tially - 15 - ~ . O~SCfO339~1 isobutene-free C4-hydrocarbon raffinate is taken off through !ine 7. The tertiary ether which is obtained as the bottom product of the distillation column 6 and which may contain excess primary C3- or C4-alcohol is next fed to the vaporizer 9 through line 8, and after vaporization is passed through line 10 into the reactor 11 which contains the acid catalyst. This reactor is in general a fixed bed reactor. The mixture of isobutene and primary C~- or C4-alcohol taken from reactor 11 is passed through line 12 lo into the distillation column 13 where very pure isobutene is obtained as the top product, which is taken off through line 14. The C3- or C4-alcohol obtained as the bottom product is returned to the etherification reactor 4 through lines 15 and 2, where necessary after replenishing the C~-or C4-alcohol through line 16. Advantageously, a small pleed-stre~ontainirg C3- or C4-alcohol is taken off through line 17 to remove any impurities formed, eg. diiso-butyl ether, diisobutene or triisobutene. If isobutanol is used as the C~- or C4-alcohol, thisbleed-stream can be fed to a dehydration reactor, where additional isobutene is obtained, Using the process according to the invention, very pure isobutene is obtained, which in particular is suitable for the manufacture of high molecular weight polymers of isobutene.
The Examples which follow illustrate the lnvention, - me etherification ~ carried out using a C4-hydro-carbon mixture which consis~d of the residue (raffinate) of - - 16 - o.~. O~S0/v339~1 a C4-fraction~ obtained from an ethylene production instal-lation, from which the butadiene had been ex~racted .
After the extraction of the butadiene, the C4-hydrocarbon mixture had the following composition:
isobutane 1 9 % by volume n-butane 8.1 % by ~olume isobutene 46.o % by volume but-l-ene 26.7 % by volume trans-but-2-ene 10.1 % by volume cis-~ut-2-ene 7.0 % by volume buta-1,3-diene 0.2 % by volume Per hour, a mixture of 258 g of this C4-hydrocarbon mixture and 320 ml of isobutanol was introduced into a stainless steel tubular reaction vessel which contained 254 ml of a sulfonated styrene-divinylbenzene copolymer resin in the hydrogen form (Lewatit SPC 118, particle size 0.8 - 1 mm). A
temperature of 40C and a pr~ssure of 12 bar were maintained in the reaction vessel. ~he reaction mixture obtained 2c was fed to a distillation column, and at the top of the column a butene/butane raffinate containing less than 2 per cent by weight of isobutene was obtained. The raffinate was -rirtually free from isobutanol ard could therefore be used directly, without additional purification operations (for example without interpolation of a water was~, as the starting material for further reactions.
At the bottom of the column, 500 ml per hour of isobutyl tert~-butyl ether, which still contained 24 3 per cent by weight, based on the mixture, of excess isobutanol, were * (trademark) - 17 - ~ 050JC3~9~1 taken off and fed to a vaporizer. The vaporized iso-butyl tert.-butyl ether, heated to lgOC, was cracked by pàssing it into a tubular crack~ng reactor which contaired ph~sphoricacid on silica (20 %excess ofbhosph~ic acid)as the cracking catalys~; cracking took place at 190C, giving iso-butene and isobutanol. ~he crac~ed reaction product W2S
passed into a second distillation column, at the top of which 115 g per hour of ~ery pure isobutene of the following composition were obtained;
isobutene 99.85 % by weight isobutane 730 ppm by weight butane 3 ppm by weight but-l-ene 420 ppm by weight trans-but-2-ene 190 ppm by weight cis-but-2-ene 160 ppm by weight buta-1,3-diene 19 ppm by weight me yield of isobutene, based on isobutene origin-ally contained in the C4-hydrocarbon mixture em~loyed, was 97.7 %. At the bottom of the second distillat on column, 320 ml per hour of isobutanol were obtained, and this material was recycled to the etherification reaction.
It proYed possible to effect a 4-fold increase in the throughput of C4-hydrocarbon mixture and isobut&nol through the etherification reactor, with virtually no change in the purity of the butene/butane raffinate obtained on distillation, and of the product, containing isobutyl - tert.-butyl ether, obtained at the bottom of the distil-lation column.

-- 18 ~ 050fO33951 COMP.~RA~IVE E~PLE1 The etherification was carried out as described in Example 1, at 40C, but employing the corresponding stoichi-ometric amountof methanol instead of isobutanol With a throughput, of starting mixture, of 2 liters/h per liter of reactor volume, the residual content of isobutene in the bute~e/butane raffinate obtained after distillation was more than 30 per cent by weight. In addition, this raf~inate contained more than 1.5 mole ~ of methanol, which was washed out of the raffinate by treatment with water.
The methanol was recovered from the methanol-water mixture obtained a~ter the water wash, and was recycled to the etherification reaction. ~e butene/butaneraffinatewas thendried inorder to remove the entrained water.
In contrast, when using isobutanol (as described in Example 1) Lnstead o~ methanol, a butene/butane raffinate containing less than 1 ppm of isobutanol is obtained by simple distillation.

The ether cleavage was carried out as described in Example 1, but instead of isobutyl-tert.-butyl ether, methyl tert,butyl ether was used.
Impure isobutene having the following composition was obtained at the top of the distillation column:
isobutene 97.0 ~ by weight othe~ hydrocarbons 0.15 ~ by weight methanol 2.45 % by weight dimethyl ether 0.4 X by weight me isobutene obtained contained 2.45 ~ by weight of methanol and, in addition, 0.4 ~ by we'ght of d~methyl - 19 ~ 050/033951 ether, whilst in the isobutene obtained according to Example 1 the content of isobutanol and of diisobutyl ether wasbelow the limit of detectability (4 ppm).
For many applications, for example for use as a starting material for cationic polymerization with boron fluoride, the isobutene obtained in the Comparative Example must be subjected to additional purification operations, whilst the very pure isobutene obtained accordLng to Example 1 can be employed directly. In lo order to obtain, from the isobutene produced in Comparati~e Example 2, an isobutene of similar purity to that obtained in Example 1, it would be necessary, for example, to carry out the following additional process steps.
1. Distillation of the isobutene to remove the dimethyl ether.
2. Subsequent extraction of the isobutene with water to remove methanol.
3. Drying the resulting moist isobutene.
4. Distillation of the methanol-water mixture, obtained from the extraction, in order to recover the methanol.

~he etherification was carried out using the C4-cut from an ethylene production installation. The C4-hydro-carbon mixture had the following composition:
butane 3.65 % by weight isobutane 1.41 % by weight but-l-ene 20.44 ~ by weight isobutene 23.52 ~ by weight - 20 - 0. . OOSo/033951 trans-but~2-ene 4.95 % by weight cis-but-2-ene 3.15 ~ by wei~ht buta-1,3-diene 42.31 % by weight buta-1,2-diene 0.10 % by weight but-l-yne 0.11 % by weight butenyne 0.36 % by weight Per hour, a mixture of 457 g of this hydrocarbon cut and 320 ml of isobutanol was reacted as described in Example 1. me isobutene content of the butene/butane raffir~te obtained after distillation was 1.0 per cent by weight.
The bottom product from the first distillation was vaporized and then passed Lnto a tubular cracking reactor, where the isobutyl tert.-butyl ether was cracked, at 190C, to give isobutene and ~sobutanol. me isobutene (107 g per hour) taken of~ at the top of the downstream distil-lation stage had the following composition:
butane 0.012 % by weight isobutane ~.041 % by weight but-l-ene 0.042 % by weight isobutene 99.332 % by weight trans-but-2-ene0.09 % by weight cis-but-2-ene 0.11 % by weight buta-1,3-diene0.36 % by weight buta-1,2-diene0.007 % by weight but-l-yne 0.0032% by weight butPnyne 0.0028 % by weight In spite of the C4-hydrocarbon mixtur_ used as the starting material having a buta-1,3-diene content of - 21 - ~ O~/033951 42.31 per cent by weight, the content of buta-1,3-diene in the isobutene product was only 0.~6 per cent by weight.
Equally, the concentrations of buta-1,2-diene, but-l-yne and butenyne were greatly reduced.
Th~ yield of isobutene, based on isobutene contained in the C4-hydrocarbon mixture employed, was 96.5 %. The isobutanol, which was recovered virtually completely from the bottom of the second distillation column, was recycled to the etherification reaction.
EXAMPT`~ 3 The etherification was carried out using a C4-hydro-carbon mixture which c ~ isted of the residue (raffinate) of a C4-fraction, obtained from an ethylene production instal-lation, from which the butadiene had been extracted.
After the extraction of ~he butadiene, the C4-hydrocarbon mixture had the following composition:
isobutane 1 9 % by volume n-butane 8.1 % by volume isobutene 46.0 % by volume but-l-ene 26.7 ~ by volume trans-but-2-ene 10.1 % by volume cis-but-2-ene 7.0 % by volume butadiene 0.2 % by volume Per hour, a mixture of 250 g of this C4-hydrocarbon mixture and 266 ml of n-propa~ol was introduced into a stainless steel tubular reaction vessel which contained 138 ml o~ a sulfonated styrene-divinylbenz2~e copolymer resin in the hydroge~
form (Lewatit SPC 118, particle size 0 8 - 1 mm). A

-- 22 -- ~ . 0050/03 3a.~ 1 temperature of 40C and a pressure of 12 bar were maintained in the reaction vessel. me reaction mixture obtained was fed to a distillation colllmn, and at the top of the column a butene/butane raffinate contai~ing 2 per cent by weight of isobutene was obtained. ~he raffinate was virtually free from propanol a~d could therefore be used directly, without additional purification operations (for example without interpolation of a water wa~, as the starting material for further reactions.
At the bottom of thedis~llatior cn~l~,428ml perhour ofpropyl tert.-butyl ether, which still contained 27 per cent by weight, based on the mixture, of excess propanol, we~ taken off and fed to a vaporizer. The vaporized propyl tert.-butyl ether, heated to 170C, was cracked by passing it irto a tubular crac~ing reactor which contained 20 % of H3P04 on heat-treated silica gel as the cracking catalyst;
cracking took place at ~rom 180 to 200C, gi~ing isobutene and propanol. The cracked reaction product was passed i~to a second distillation column, at the top of which 95.7 g per hour of very pure isobutene of the following composition were obtained:
isobutene 99.9 % by weight isobutane 300 ppm by weight butane 100 ppm by weight but-l-ene 100 ppm by weight trans-but-2-ene 100 ppm by weight cis-but-2-ene 100 ppm by weight buta-1,3-diene 100 ppm by weight At the bottom of the second distillation column, -- 23 -- ~ . C~S0/033~51 287 ml per hour of propanol, containing 7~2 % of propyl tert.-butyl ether, were obtained. It was possible to recycle this mixture to the etherification reaction, where-by the amou~t of isobutene obtained could be Lncreased to 112 g per hour.
The yield of isobutene, based on isobutene cont~lned in the C4-hydrocarbon mixture employed, was 97.5 ~ if the entire bottom product was recycled to the second di~t l-lation column.

Drawing

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for obtaining isobutene from a C4-hydrocar-bon mixture containing isobutene, which comprises (a) reacting the mixture continuously with a primary C3- or C4-alcohol in the presence of an ion exchan-ger in its acid form as a condensing agent to form the corresponding C3- or C4-alkyl tert-butyl ether by feeding the primary C3- or C4-alcohol and the C4-hydrocarbon mixture with or without prior mixing, to an etherification reaction zone which contains the ion exchanger, while maintaining the exit temperature of the reaction mixture from the ether-rification reaction zone at from 25 to 65°C and the quotient of the volume of the etherification reaction zone and the throughput of the C4-hydrocar-bon mixture and the primary C3- or C4- alcohol at from 0.01 to 5 hours;
(b) distilling the reaction mixture obtained from the etherification reaction zone in a first distillation zone, taking off as the top product without water washing a C4-hydrocarbon mixture comprising the unconverted hydrocarbons and not more than 1,000 ppm by weight of the primary C3- or C4- alcohol and taking off as the bottom product the resulting C3-or C4-alkyl tert-butyl ether, which may contain the-rein primary C3- or C4-alcohol which may have been added in excess;
(c) feeding the bottom product to a second reaction zone, containing an acid catalyst, in which the C3- or C4-alkyl tert-butyl ether is decomposed at an elevated temperature to give isobutene and primary C3- and C4-alcohol;
(d) feeding the mixture of isobutene and primary C3- and C4-alcohol produced in step (c) to a second distil-lation zone, taking off as the top product without a water wash isobutene containing not more than 500 ppm by weight of primary C3- or C4-alcohol and taking off as the bottom product the remaining prima-ry C3- or C4-alcohol produced in step (c); and (e) recycling the primary C3- or C4- alcohol which is the bottom product of step (d) to the etherification reaction zone.

2. The process of claim 1 wherein the top product taken off the first distillation zone is a C4-hydrocarbon mixture, comprising the unconverted hydrocarbons and containing not more than 200 ppm by weight of C3- or C4-alkyl tert-butyl ether or di-C3-alkyl ether or di-C4-alkyl ether or mixtures thereof.

3. The process of claim 1 wherein a bottom product com-prising the C3- and/or C4-alkyl tert-butyl ether formed and containing not more than 1,000 ppm by weight of C4-hydrocarbon is taken off the first distillation zone.

4. The process of claim 1 wherein the bottom product obtained from the first distillation zone and containing the C3- or C4-alkyl tert-butyl ether formed is employed, without separation from the primary C3- or C4-alcohol contained in the said bottom product, as the starting material for the decomposition stage.

5. The process of claim 1 wherein the decomposition of the C3- or C4-alkyl tert-butyl ether is carried out out under a pressure of from 2 to 15 bars.

6. The process of claim 1 wherein, in the second distil-lation zone, isobutene which is not less than 99.3%
by weight pure and contains not more than 100 ppm by weight of di-C3-alkyl ether or di-C4-alkyl ether and/or C3- or C4-alkyl tert-butyl ether is taken off as the top product, without interpolating a water wash.

7. The process of claim 1 wherein the content, in the primary C3- or C4-alcohol recycled to the etherification reaction zone, of di-C3-alkyl ether or di-C4-alkyl ether which may form in very small amounts as the by-product but accumulates in the recycled primary C3- or C4-alcohol is restricted to from 2 to 20% by weight.

8. The process of claim 1 wherein, when using isobutanol as the primary C4-alcohol, a small isobutanol bleed-stream containing from 3 to 40% by weight of diisobutyl ether is taken off the side of the second distillation zone or from the bottom product of the second distillation zone and is dehydrated at an elevated temperature in the presence of a dehydration catalyst.