DESCRIPTION OF THE PRIOR ART The oligomerization of olefins such as isobutylene using acidic catalysts is a known reaction.

As described in U. S. 3,760,026, a number of catalysts are known for this reaction including cold sulfuric acid, phosphoric acid on Kieselguhr, silica/alumina sometimes promoted with Ni, Co, Fe, Pt or Pd; activated natural clays plus activating substances such as ZnO metallic phosphates such as those of iron (III) and cerium optionally supported on carriers such as activated carbon, bauxite, activated carbon alone and with metal halides such as TiC12 heteropolyacids such as silicotungstic acid on silica gel and phosphomolybdic acid; BF3H3PO3 ; dihydroxyfluroboric acid HF and fluorides or oxyfluorides of S, Se, N, P, Mo, Te, W, V and Si boiling below 300°C ; BFs dimethyl ether complexes ; BFs hydrocarbon complexes ; BFs SO2 ; and AfCis with catalysts such as dimethyl ether, HC1, and nitromethane. These catalysts and dimerization processes, including operating conditions, are known in the art.

An especially preferred catalyst is a sulfonic acid-type ion exchange resin such as Amberlyst A-35. U. S. Patent 4,447,668 describes isobutylene dimerization using A-35 with methyl t-butyl ether as solvent.

Considerations associated with the isobutylene dimerization involve removal of the substantial heat of reaction and the requirement that high selectivity to the dimer product be maintained. U. S. Patent 5,877,372 is relevant to these considerations.

SUMMARY OF THE INVENTION In accordance with the present invention, a multistage process is provided for the dimerization of an isoalkene such as isobutylene. A particular feature of the process of this invention is the separation of isoalkene dimer product between dimerization stages in order to avoid the formation of unwanted higher polymers through reaction of isoalkene with dimer. In this way, high overall conversion of isoalkene can be achieved without formation of excessive amounts of higher isoalkene polymer.

BRIEF DESCRIPTION OF THE DRAWING The accompanying drawing is a schematic representation of an especially preferred practice of the invention.

DETAILED DESCRIPTION With reference to the drawing and the process represented therein, the following description relates to the conversion of isobutylene to diisobutylene although it will be understood that the invention is applicable to the dimerization of other isoalkenes such as isoamylene. Isobutylene feed which illustratively is derived from tertiary butanol by dehydration or more preferably from a refinery fluid catalytic cracking C4 stream or olefin stream cracking plant derived C4 stream or C4 stream from dehydrogenation is fed via line 1 to dimerization zone 2 wherein isobutylene is dimerized. Also fed to zone 2 via line 3 is a tertiary butanol recycle stream, the tertiary butanol serving as a selectivity enhancing modifier during dimerization in especially preferred practice.

In zone 2, the isobutylene containing feed is contacted with a solid dimerization catalyst, preferably a sulfonic acid resin catalyst such as Amberlyst A-15 of Rohm & Haas at dimerization conditions whereby exceedingly high selectivity to dimer is achieved. As a critical feature of the present invention the isobutylene conversion in reaction zone 2 is maintained at considerably less than 100% in order that the advantages of the invention be achieved. Reaction conditions including temperature, pressure, flow rate

(space velocity) and the like are regulated so as to provide a maximum isobutylene conversion in zone 2 of 85%, preferably a maximum conversion of 60%. Usually conversion in the range 40 to 85% is suitable. In the various dimerization steps conventional dimerization equipment and procedures can be used. Packed column reactors with external cooling of a pump around stream can be used; also, packed tubes with an external cooling agent are also suitable.

The reaction mixture is removed via line 4 from reactor 2 with a portion recycled via line 5 through heat exchanger 6 back to zone 2 via line 7. By removing heat from the recycle stream in this way the dimerization exotherm can conveniently be controlled.

The net reaction product mixture passes via line 8 to distillation column 9 wherein the reaction mixture is distille to separate a heavy fraction comprised of diisobutylene product from lighter components of the reaction mixture including unreacted isobutylene. The lighter fraction passes via line 10 to reaction zone 11 wherein the mixture is contacted at dimerization conditions with solid dimerization catalyst, preferably the same catalyst employed in reaction zone 2. In zone 11, conversion of isobutylene in the feed through line 10 preferably is 80% or more, and the reaction product mixture is removed via line 12 and passed to distillation zone 13. Reaction mixture recycle and cooling to control the exotherm in reactor 11 are also provided, these are not shown.

Product diisobutylene is removed from zone 13 via line 14 and is suitably combined with the similar product stream which is removed from zone 9 via line 15 and the combined stream is sent to such further treatment as might be appropriate such as hydrogenation to isooctane (not shown).

Although distillation zone 13 is illustrated as a single zone, it will be understood that a plurality of columns can be used which collectibily are represented by zone 13.

Overhead from zone 13 representing unreacted C4 components of the feed is removed via line 16 for use in reactions such as alkylation or for fuel.

A tertiary butanol containing stream is removed from zone 13 via line 3 and is

recycled to zone 2 with a purge being removed via line 17. A particular feature of the process is that an azeotrope of tertiary butyl alcohol and diisobutylene having a composition by weight of about 60% tertiary butyl alcohol and 40% diisobutylene is removed from zone 13 via line 3 and recycled.

Improved overall selectivities are achieved at high overall isobutylene conversion by practice of the invention as compared to systems which call for high isobutylene conversion in a single step. It is essential not only that partial isobutylene conversion be achieved in the first reaction but that product diisobutylene formed in the first reaction be separated prior to subsequent isobutylene reaction in order that the advantages of the invention be realized.

Of course, more than a series of two reaction zones can be employed with dimer separation between zones.

The dimerization reaction in the various reaction zones is carried out in accordance with known procedures such as those described in U. S.

5,877,372.

The production of tertiary butyl alcohol by means of the Oxirane process is well known and widely practiced on an industrial scale. See, for example, U. S. 3,351,635.

Likewise, the dehydration of tertiary butanol to form isobutylene is well known as is the dehydrogentaiton of various C4 fractions. See, for example, U. S. Patents 5,625,109,3,510,538,4,165,343, and 4,155,945.

In the dimerization of isobutylene in accordance with the present invention, tertiary butanol can be employed as a selectivity enhancing modifier and this results in a substantial improvement in reaction selectivity to the dimer as compared to operation without this modifier. Methyl tertiary butylene can similarly be used.

Also an isoalkane such as isooctane or the like hydrocarbon can be employed as a diluent to further enhance reaction selectivity by reducing isobutylene feed concentration, and to aid in removal of the reaction exotherm although this is neither necessary nor preferred.

In general, known oligomerization catalysts and conditions can be

employed in the several oligomerization steps. Suitable conditions include temperatures broadly in the range 0 to 200°C, preferably 10 to 120°C, and the use of pressures sufficient to maintain the liquid phase, illustratively above 250 psig, e. g. 50-500 psig.

Known dimerization catalysts can be used including those described in prior art such as U. S. 3,760,026. The use of sulfonic acid type ion exchange resins such as Amberlyst A-15, Dowex 50 and the like is especially preferred.

The following example illustrates the invention with particular reference to the process shown in the drawing.

Example Referring to the accompanying Figure, a C4 feed stream passes at the rate of 1000 Ibs/hr via line 1 to reactor 2. The feed stream composition by volume is 40% isobutylene, 50% n-butenes, and 10% others.

Reactor 2 is a packed column reactor packed with Amberlyst A-35 catalyst.

Also passed to reactor 2 at the rate of 85 Ibs/hr via line 3 is a 60/40 weight ratio tertiary butyl alcohol and diisobutylene azeotropic mixture from column 13. In reactor 2 dimerization reaction conditions of 100°C and 350 psig are maintained, the liquid hourly space velocity of the total feed mixture is 2 hr~1 and in reactor 2 about 70% of the isobutylene is reacted.

The net reaction product mixture comprised by weight of 11 % unreacted isobutylene, 46% n-butenes, 5% tertiary butyl alcool, 29% diisobutylene, and 10% others passes via line 8 to distillation column 9. An overhead stream is removed at 45°C and 70 psig and passes at the rate of 805 Ibs/hr via line 10 to reactor 11. The composition by weight of this overhead stream is about 15% isobutylene, 63% n-butenes, 6% tertiary butyl alcool, 4% isooctene, and 12% others.

A bottoms stream is removed via line 15 from distillation zone 9 at the rate of 280 Ibs/hr and comprises by weight 96% diisobutylene and 4% others, i. e trimer, crosspolymers and the like.

Reactor 11 is a packed column reactor containing A-35 catalyst also.

In reactor 11 the isobutylene is dimerized at 100°C and 350 psig, feed liquid hourly space velocity is 2 hr'and isobutylene conversion therein is 80%.

The reaction mixture stream comprised by weight of 62% n-butenes, 3% isobutylene, 6% tertiary butyl alcool, 16% diisobutylene and 13% others passes at the rate of 805 Ibs/hr to distillation column 13 wherein the reaction mixture is distille. An overhead stream is removed at 45°C and 70 psig via line 16 at the rate of 624 Ibs/hr. This stream has the following composition by weight: 4% isobutylene, 80% n-C4 hydrocarbons, and 16% others and preferably passes to an alkylation zone (not shown).

In this example, tertiary butyl alcohol is removed as sidedraw.

A sidestream azeotropic mixture comprised of tertiary butyl alcohol and diisobutylene is removed via line 3 and recycled to reactor 2 as above described.

A bottoms diisobutylene stream is removed at 185°C and 75 psig via line 14 at the rate of 181 Ibs/hr. This stream comprises by weight 96% diisobutylene and 4% others, mainly higher polymer.

As can be seen, through practice of the process of the invention about 94% of isobutylene fed is converted to higher molecular product at a selectivity to diisobutylene of 96%.

When it is considered that at a comparable isobutylene conversion level but in a one-step reaction selectivity to diisobutylene is significantly less, the advantages which result from practice of the invention become apparent.