Method for producing high purity isobutylene from a butane...

Chemistry of hydrocarbon compounds – Unsaturated compound synthesis – By double-bond-shift isomerization

Reexamination Certificate

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C585S668000, C585S670000

Reexamination Certificate

active

06215036

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to a process for processing a hydrocarbon cut containing essentially olefinic hydrocarbons having 4 carbon atoms per molecule and containing isobutene as well as but-1-ene and but-2-enes in a ratio corresponding substantially to thermodynamic equilibrium, said process comprising passing said cut into a distillation zone associated with a hydroisomerization reaction zone, said process being characterized in that the hydroisomerization zone is at least partly external to the distillation zone. A process of this kind permits the production of high-purity isobutene.
BACKGROUND OF THE INVENTION
Isobutene intended for polymerization must have a purity level above 99% and it must only contain traces of but-1-ene and but-2-enes (a few hundred parts per million by weight, ppm). In fact, if the impurities of isobutene are too high, the resulting polymers are less good in quality and the polymerization yield is lower. As a result it is necessary to remove from a hydrocarbon cut containing isobutene other olefinic hydrocarbons containing 4 carbon atoms per molecule. Since but-1-ene and isobutene have very close boiling points, they cannot be separated by distillation, unless considerable means are used. The other olefinic hydrocarbons having 4 carbon atoms per molecule can be separated from isobutene by distillation.
The main problem that arises when producing high-purity isobutene is thus the separation of but-1-ene from isobutene. Several ways can be contemplated to carry out this separation.
The first way consists of sulfuric acid extraction: the isobutene is selectively hydrated and thereafter regenerated by treatment of the aqueous phase. If the temperature and the concentration are properly controlled, this process results in isobutene of good purity. However, the yield does not usually exceed 90% as extraction is not complete, and dimers and oligomers are formed, leading to the formation of toxic acid sludge.
The second way consists of cracking methyl tert-butyl ether (MTBE): the isobutene is extracted from the C
4
cut by reaction with methanol in order to form MTBE. The MTBE is then cracked into methanol and isobutene on an acid catalyst. The recovery yield is generally 96%. The isobutene produced exhibits good purity but the dimethyl ether that may form during cracking has to be removed.
The third way that may be contemplated is the dehydration of tert-butyl alcohol (TBA). Methanol in the above process is replaced by water, and this leads to the formation of TBA. The isobutene is thereafter recovered by dehydration of the TBA. This way is not used commercially, mainly because TBA is very closely connected with the propylene oxide market. TBA can, depending on the processes, be a by-product of propylene oxide.
U.S. Pat. No. 5,177,283 describes a hydrocarbon conversion process comprising passing the feedstock into a fractionation zone, the top effluent being rich in one of the reactants and the bottom effluent being rich in reaction product, said process being such that a liquid flow is subjected to a side-stream draw-off, passed with a hydrogen-rich gaseous flow into a catalytic reaction zone, which gives rise to a reaction zone effluent comprising one of the reactants and the reaction product, a fraction of the gaseous part of said effluent being recycled to the reaction zone, the liquid part of said effluent being fed back into the fractionation zone, generally close to the point where it was drawn off. The hydroisomerization reaction of but-1-ene into but-2-enes is not mentioned.
SUMMARY OF THE INVENTION
The process according to the invention allows a high-purity isobutene to be produced at a low cost and with an excellent yield from an olefinic C
4
cut containing at least isobutene as well as but-1-ene and but-2-enes in a ratio corresponding substantially to thermodynamic equilibrium, generally obtained from a steam cracking process, such as the raw C
4
cut or 1 -raffinate (obtained after butadiene extraction from the raw cut), or from a catalytic cracking process. The process according to the invention is characterized by the integration of distillation and hydroisomerization operations which are arranged and carried out so as to minimize the investment cost of the process, to maximize the conversion of but-1-ene into but-2-enes and to minimize the hydrogenation of isobutene into isobutane, in order to maximize the isobutene yield. Thus, the process according to the invention can also provide at least partially the selective hydrogenation of some polyunsaturated compounds which are most frequently diene-containing or acetyl-containing, such as butadiene, vinylacetylene, methylacetylene and ethylacetylene, when such compounds are present in the feedstock, and the hydroisomerization of but-1-ene into but-2-enes (cis and trans). The but-2-ene products of this hydrogenation operation and of this hydroisomerization operation can then be separated from the isobutene by distillation, unlike but-1-ene. Compared to the other processes mentioned above, the process according to the invention has the advantage of a high isobutene yield, generally above 90%, preferably above 95% and more preferably above 98%, and of producing no oxygen-containing by-products.
The process according to the invention relates to the processing of a feedstock comprising mainly olefinic hydrocarbons having 4 carbon atoms per molecule, including isobutene as well as but-1-ene and but-2-enes in a ratio corresponding substantially to thermodynamic equilibrium, wherein said feedstock is processed in a distillation zone, which generally comprises an exhausting section and a stripping section associated with a hydroisomerization reaction zone, said process being characterized in that the hydroisomerization zone is at least partly external to the distillation zone.
The feedstock of the outer part of the reaction zone is generally drawn off at the height of a draw-off level of the distillation zone and represents at least part, preferably the major part, of the reflux liquid flowing in the distillation zone, preferably flowing in the stripping section, and more preferably flowing at an intermediate level of the stripping section, the effluent of reaction zone being at least partly, preferably mostly, reintroduced into the distillation zone at one or more reintroduction level(s), generally located in proximity to the draw-off level, i.e. substantially at the height of or substantially above or substantially below, most often substantially at the height of or substantially above one draw-off level, preferably said draw-off level, that is to say located usually at a distance from said level corresponding to a height in the range of 0 to 4 theoretical plates above or below a draw-off level, more preferably located substantially at the height of or slightly above the draw-off level, so as to ensure the continuation of the distillation. The process according to the invention makes it possible to obtain at the top of the distillation zone an effluent rich in isobutene usually of high purity, and, at the bottom of the distillation zone, effluent depleted in isobutene.
DETAILED DESCRIPTION OF THE INVENTION
The process according to the invention permits high purity isobutenes to be obtained. Any hydroisomerization reaction zone is usually such that any hydroisomerisation reaction is carried out in the presence of a hydroisomerization catalyst and in a gaseous flow comprising, preferably for the most part, hydrogen.
The feedstock which supplies the distillation zone is introduced into said zone usually at least at one level of said zone, preferably mainly at one single level of said zone. Its ratio corresponds substantially to the but-1-ene: but-2-enes thermodynamic equilibrium during introduction. One of the preferred embodiments of the process according to the invention comprises producing said feedstock from a cut comprising, for the major part thereof, olefinic hydrocarbons comprising 4 carbon atoms per molecule by processing said cut in a first hydroisomerisation zone, usually

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