DE1194402B - Process for removing alkynes from a hydrocarbon mixture containing 1,3-butadiene or isoprene - Google Patents

Description

Process for removing alkynes from a 1,3-butadiene or isoprene containing hydrocarbon mixture 1,3-butadiene and isoprene are predominantly obtained by dehydrating corresponding more saturated hydrocarbons. Butadiene can e.g. B. obtained from butenes from cleavage processes or from originate from the dehydrogenation of butane using the so-called Houdry catalyst.

It is customary to use more suitable alkadienes thus obtained Separation methods, e.g. B. extractive distillation with acetonitrile to isolate.

In general, however, the products isolated in this way are available for further processing not pure enough because they still contain small amounts of alkynes, which z. B. adversely affect the polymerization.

It is known that the alkyne content with the help of selective hydrogenation to reduce, d. H. by reaction with hydrogen in the presence of a catalyst, which favors the preferred hydrogenation of the triple bond. However, if you have that want to subject the entire product obtained to this cleaning treatment are extensive Plants as well as large quantities of the relatively expensive hydrogenation catalyst necessary. Losses of the alkadienes are also not entirely avoidable.

It has now been found that the loss of alkadiene depends on the relative Concentration ratio in the hydrocarbon mixture to be cleaned depends and becomes lower when the alkyne concentration is constant at a constant alkadiene concentration elevated.

By prior enrichment of the alkyne in a fraction that is small compared to the original volume, and subsequent selective hydrogenation the above-mentioned difficulties are largely eliminated.

Accordingly, the invention relates to a method of removal of alkynes from a hydrocarbon mixture containing 1,3-butadiene or isoprene by fractional decomposition, which is characterized in that from the Hydrocarbon mixture is a fraction with an increased proportion of alkyne in itself known way by rectifying absorption extractive and / or fractionated Distillation separates off the alkynes contained therein, as usual, at least partially catalytically selectively hydrogenated and this portion is fed back into the main stream.

This mode of operation, which is very easy to carry out in practice, is made possible it to largely reduce the loss of alkadiene. In the case of the technically important Removal of vinyl acetylene from butadiene-containing mixtures can be even a minor one Profit can be made if the loss of butadiene is less than the amount which during the selective hydration tion by adding two hydrogen atoms the vinyl acetylene molecule is formed.

The method according to the invention can be carried out in various ways are and is not based on a special method of fractional decomposition or limited to a specific hydrogenation method.

In a particularly advantageous embodiment of the method takes place the separation of the fraction with increased alkyne content by rectifying absorption. Dimethyl sulfoxide is preferably used as an absorbent. If so desired can be an additional extractive distillation either before or after the rectifying Absorption can be carried out.

According to another preferred embodiment, the separation takes place by fractional distillation, the fraction with an increased proportion of alkyne is withdrawn as a side stream from the distillation column. There can be a Distillation column of any type can be used, e.g. B. one with Random packings or with exchangeable bottoms. In the latter case, a column with 80 to 160 floors preferred.

The number of floors as well as the size and other conditions of the too using the column however, to a large extent on the alkyne concentration in the hydrocarbon mixture to be purified and of the desired degree of purity of the product.

Generally a reflux ratio of 5 to 10 results in a head temperature from 40 to 60 "C and a pressure of 4.8 to 8 at abs. results are satisfactory. Either liquid or gaseous product streams can be introduced into the column will. The point where the side stream is withdrawn is to a large extent from the depending on special working conditions. In general, however, it is useful to do this Place in the lower half, preferably in the lower third of the distillation column to select.

The side stream can be either liquid or gaseous, which is im generally depends on the method used for selective hydrogenation.

The hydrogenation can be carried out, for example, in the gas phase; in many cases, however, the hydrogenation is advantageously carried out in the liquid phase.

The catalyst to be used can be varied; it contains in generally one or more metals such as nickel, cobalt, chromium, magnesium and vanadium, or metal oxides such as iron oxide as the active material.

A particularly suitable catalyst contains palladium as the active one Component, usually on an inert support such as alumina and silica. The other conditions to be selected for the hydrogenation can also be within further limits fluctuate. The hydrogenation of the liquid phase can thus be carried out using the trickle technique be performed. However, a solution of hydrogen is preferably separated off in the Fraction through a fixed bed of a palladium-containing catalyst in upwards directed current passed through it.

In the method according to the invention, the selective Product originating from hydrogenation is fed back into the main stream. When using the selective If a very high degree of purity is achieved by hydrogenation, the hydrogenation product can be used be added directly to the purified alkadiene stream that is used in the separation the fraction with an increased alkyne content is obtained. The alkyne concentration in this purified product stream is usually less than 500 parts by volume per 1,000,000 Volume parts, but through careful selection and control of working conditions both in the fractional decomposition and in the selective hydrogenation stage butadiene or isoprene-containing mixtures can be achieved that are less than Contains 100 parts of alkyne per 1,000,000 parts.

However, such a high degree is in the method of the present invention the purity of the selective hydrogenation product is by no means necessary. On the contrary, in some cases it can be advantageous to carry out the hydrogenation in such a way that that only some of the alkynes are converted, so that the hydrogenation product is still considerable Contains amounts of alkynes. In such cases the hydrogenation product is preferred added to the main stream before the separation of that fraction which increased a Has alkyne content. The selection of the selective hydrogenation method is often the determining factor for the type of phase which the recycled Has electricity. Thus, the hydrogenation product can e.g. B. in the case of decomposition by fractional Distillation fed to the distillation column in a gaseous or liquid state will, where the point at which this product is introduced into the column, is preferably below the point for the removal of the side stream.

In general, the method of the invention becomes useful applied to hydrocarbon mixtures which have the same number of carbon atoms like the alkadienes have 1,3-butadiene and isoprene. Of course you have to focus of catalyst poisons, such as thiols and other sulfur compounds, are sufficiently low be. This also applies to the hydrogen to be used, which is of any origin can be. If desired, the hydrogen can be mixed with other gases, which do not adversely affect the hydrogenation, e.g. B. nitrogen. The concentration of the alkadiene in the hydrocarbon mixture to be purified can be used within wide limits vary. So z. B. mixed C4 fractions with a butadiene concentration or Cs fractions with an isoprene concentration between 20 and 50 percent by weight which come from any dehydration plants. If that The method according to the invention is applied to such crude fractions in many cases the further advantage that an impurity, e.g. B. by polymerization, the plant for the removal of alkanes and alkenes is avoided. If the present Method is applied to a starting material from which alkanes and / or Alkenes have already been separated off, it offers the advantage that only a partial amount of the original volume of material must be treated. Also suitable for this Butadiene and isoprene mixtures with a concentration of more than 90 percent by weight of alkadiene.

The invention is illustrated by the following examples.

Example 1 1001 of a gaseous C4 fraction and 720 cc of dimethyl sulfoxide as absorbent were in countercurrent at 26 "C through a column (30 trays, Length 120 cm, inner diameter 21/2 cm). The C4 fraction contained 3,200 parts of vinyl acetylene to 1,000,000 parts; the butadiene content was 7.1 percent by volume. The exhaust gas (about 901) contained 21 parts of vinyl acetylene per 1,000,000 parts and had a butadiene content of 33.5 percent by volume. The loaded absorbent was stripped with nitrogen. On a nitrogen free basis, the stripping gas had a Volume of 101 and contained 3.5 volume percent vinyl acetylene and 61.0 volume percent Butadiene.

The vinyl acetylene concentrate was then in the presence of a palladium catalyst hydrogenated at 40 ° C. and 6 at abs. The hydrogenation product contained 3200 parts of vinyl acetylene to 1,000,000 parts and was returned to the absorption column. When applied Under the above operating conditions, the butadiene loss was 5 percent by volume, calculated on the feed to the hydrogenator, which is approximately 0.5 volume percent of the feed The starting material.

Example 2 A hydrocarbon mixture (A) was fed into a distillation column with an inner diameter of 3 m and equipped with 152 replacement shelves initiated.

When the distillation was carried out, the pressure was about 6 at abs., the top temperature about 45 ° C. and the bottom temperature about 87 "C. A side stream (B) was withdrawn at a point about 23 m above the lower floor and into a Reaction vessel passed (volume 0.4 m3, length 5.7 m, internal diameter 0.3 m) for the purpose of hydrogenation in the liquid phase in an upward flow. (Print 6 kg / cm2 absolute, hydrogen supply 0.59 kg / h). From the top of the distillation column, into which the effluent stream (C) from the hydrogenation device is recycled a product was withdrawn which contained 95.5 percent by weight butadiene and 60 parts Vinylacetylene contained per 1,000,000 parts.

The other working conditions are shown in the following table. Hydrocarbon stream ABC Speed, m³. m / h ................. 40 4 4 Temperature, ° C ......................... 27 75 40 Composition in percent by volume trans-2-butene .................... .... 10.0 28.6 28.7 cis-2-butene ........................... 12.1 22.3 22.3 Butadiene ............................. 78.0 48.12 48.25 Vinyl acetylene ......................... 0.045 1.0 0.60 1-butene .............................. - - 0.2 Claims: 1. A process for removing alkynes from a hydrocarbon mixture containing 1,3-butadiene or isoprene by fractional decomposition, characterized in that a fraction with an increased alkyne content is obtained from the hydrocarbon mixture in a manner known per se by rectifying absorption, extractive and / or fractional distillation, the alkynes contained therein are at least partially selectively catalytically hydrogenated as usual, and this portion is fed back into the main stream.

Claims (1)

2. The method according to claim 1, characterized in that the separated fraction after the selective hydrogenation the main stream before implementation the fractional decomposition. 3. The method according to claim 1, characterized in that the separated fraction after selective hydrogenation at one point of the fractionated Feeds decomposition, which is below the point of consumption for the side stream.