EP0763588B1 - Process for eliminating i-alkenes from hydrocarbon mixtures - Google Patents

Description

The invention relates to a method for a special residual removal of C ₄ -C ₈ -i-alkenes from mixtures of hydrocarbons in which n- and i-alkenes are contained. Such mixtures are treated according to the invention with a cation exchanger in the H ⁺ form; the material to be treated is then separated by distillation. The process is important in the production of pure n-alkenes.

The need for a specific removal of i-alkenes exists in particular when working up distillation sections which originate from steam crackers or catalytic crackers and generally a mixture of n-alkanes, i-alkanes, n-alkenes with a terminal or internal double bond, i -Alkenes, alkydienes and acetylene compounds, if the n-alkenes are to be obtained from these sections. These cuts obtained by distillation generally represent areas of approximately the same number of carbon atoms, which only contain minor portions of the adjacent areas of carbon atom numbers. A C _{4 cut} contains only minor amounts of C ₃ hydrocarbons and C ₅ hydrocarbons. In general, in addition to the C _{4 cut} mentioned, C ₅ cuts (benzene feed) or from higher-boiling fractions after the aromatics removal, C ₅ / C ₆ cuts, C ₄ -C ₆ cuts or distillation cuts over an even larger range of carbon atoms produced including C ₇ and C ₈ hydrocarbon mixtures. After the separation of alkadienes and acetylene compounds (for example by extraction) and the subsequent separation of the i-alkenes (for example by chemical reaction), these hydrocarbon cuts primarily contain the 1-alkene corresponding to the respective number of carbon atoms, the as valuable monomer or comonomer and for the preparation of various alkylates is sought. In particular when used as a monomer or comonomer, the lowest possible content of the associated i-butene is required in order to ensure trouble-free (co) polymerization. In the case of butene-1, this maximum permissible content of i-butene is set at less than 200 ppm.

A distillative separation, for example of i-butene and butene-1, fails in many cases due to the boiling points being too close together; this is particularly pronounced in the example of the C ₄ -alkenes mentioned. Therefore, attempts have been made to convert i-butene into a higher-boiling substance by chemical conversion, which is then easier to separate by distillation. The reaction with methanol or other lower alcohols to form methyl t-butyl ether (MTBE) or its higher homologues in the presence of acidic catalysts lends itself to this. However, the raffinate with the remaining butene-1, which is formed by distillation from such a reaction mixture, is contaminated by oxygen compounds in the same boiling range, for example by dimethyl ether. Such oxygen compounds then have to be removed again, for example by an ORU system. In an ORU (Oxygen Removal Unit), oxygen compounds are removed from carbon water streams by adsorption on an active material, such as molecular sieves.

Another chemical conversion of i-butene and other i-alkenes is in their dimerization, which is also carried out in the presence of acidic catalysts becomes. However, this dimerization has the disadvantage that it can be used in addition to Diisoalken also to so-called codimers from a molecule i-alkene and one Molecule alkene-1 leads. On the one hand, the resulting diisoalkene is due to the unwanted codimer contaminated and on the other hand, alkene-1, its pure representation is desired, to a not inconsiderable extent, and the withdrawn from further use accordingly. It is therefore established technology that i-alkene, for example i-butene, cannot be completely dimerized in order to Push back formation of said codimer, and the largely remaining To achieve removal of the i-alkene by the ether formation mentioned. With that occurs however, the already mentioned difficulty of contaminating the remaining Hydrocarbon mixture with oxygen compounds.

It has now been found that it is possible to largely free hydrocarbon mixtures with a residual i-alkenes from these i-alkenes if such a mixture is treated with a cation exchanger in the H ⁺ form under the conditions mentioned below.

The invention relates to a process for removing C ₄ -C ₈ -i-alkenes from mixtures thereof containing hydrocarbons with essentially the same number of C atoms, which is characterized in that such mixtures at 0-30 ° C and 1- 25 bar, preferably 10-20 ° C. and 10-20 bar, are treated with a cation exchanger in the H ⁺ form and the material to be treated is then distilled into a top product largely free of i-alkenes and into a bottom product containing the conversion products of the i-alkenes is separated.

The mixture of hydrocarbons to be treated is preferably a C ₄ -, a C ₅ -C ₆ - or a C ₄ -C ₆ distillation fraction.

In a particularly preferred manner, the mixture of hydrocarbons to be treated is a C ₄ fraction.

The process according to the invention is described below using the example of the treatment of a C ₄ fraction; However, it is easily applicable by the person skilled in the art to other distillation cuts up to C ₈ hydrocarbons, taking into account the corresponding boiling point positions.

In petrochemical plants with a steam cracker or a catalytic cracker, for example, the outflowing mixture of cracked hydrocarbons is subjected to a distillative separation, in which a C ₄ hydrocarbon stream is generally collected separately. This C _{4 cut} is generally subjected to the extraction of the valuable material butadiene. The C ₄ -hydrocarbon mixture flowing out here is known as raffinate I.

This raffinate I is used for the chemical conversion of i-butene either Dimerization, including further oligomerization, or t-alkyl ether formation subjected. A resulting distillate, freed from i-butene, is in the generally referred to as raffinate II. Raffinate II is used for further extraction of the valuable materials butene-1 and butene-2.

The following table shows a typical composition of raffinate I or II of a steam cracker or catalytic cracker: Steam cracker catalytic. Crackers C ₄ cut Raff.I Raff. II i-butane 3rd 6 37 n-butane 11 22 13 i-butene 43 0.8 15 Butene-1 24th 11 11.5 trans-butene-2 10th 37 12th cis-butene-2 8th 22 10.5 Butadiene-1,3 <0.2 0 0.5 Others (C ₃ , C ₅ ) <0.8 <1.2 0.5

According to the invention, it is now possible to use largely already i-butene depleted raffinate II continues without passing through an etherification plant To deplete i-butene to a level of less than 200 ppm without the Switching on etherification with the risk of undesirable formation Oxygen compounds are required.

Cation exchangers for the process according to the invention are polymers made from acrylic compounds or styrene which are reacted with crosslinking agents, such as divinylbenzene, to give insoluble resins, or crosslinked phenol-formaldehyde condensates which contain sulfonic acid groups, carboxyl groups or phosphonic acid groups. In addition, ion exchangers with inorganic carrier material such as functionalized siloxanes can also be used. Styrene-divinyl resins with sulfonic acid groups are preferably used. Such resins have long been known to the person skilled in the art. For the use according to the invention, such resins are present in the H ⁺ form at least 50 equivalent%, preferably at least 80 equivalent%, particularly preferably at least 90 equivalent%. Such resins are common commercial products, for example for water desalination.

The process according to the invention can in principle be carried out batchwise or continuously. In the batchwise operation, the hydrocarbon mixture containing one or more i-alkenes is treated with a cation exchanger of the type described, if appropriate under such a pressure at which the hydrocarbon mixture is liquid. The desired removal of the i-alkenes can be checked by taking samples after specified reaction times. However, the process according to the invention is preferably carried out continuously, using shaft reactors or tubular reactors which are filled with the cation exchanger in the form of a fixed bed. However, in the case of continuous operation, it is likewise possible to keep the cation exchanger in the form of a moving bed in which the hydrocarbon mixture to be treated flows through the reactor from bottom to top. In many cases, it is sufficient to pass the hydrocarbon mixture to be treated through only one reactor. Due to the simple construction of the reactors, particularly low investment costs are to be used here. However, it is also possible to carry out the process according to the invention using 2 or more reactors with a cation exchanger in the H ⁺ form. For example, it may be advantageous to separate the major part of i-butene in a first reactor under more stringent reaction conditions, the formation of codimer with loss of butene-1 not yet being too significant, and in a second step under milder conditions to push the remaining i-butene content down to the required specification, and under milder reaction conditions the loss of butene-1 by formation of codimer or by acid-catalytic isomerization to other alkenes can also be adequately avoided.

The process according to the invention is preferred at a temperature of 0-30 ° C from 10-20 ° C. The pressure under which the method according to the invention is carried out to the extent that the mixture to be treated of Hydrocarbons taking into account the desired reaction temperature must be kept in the liquid state; otherwise the pressure is not critical. The pressure range to be set according to the invention is therefore 1-30 bar, preferably 10-20 bar; in a particularly preferred manner, how it sets at the desired reaction temperature worked. The Reaction mixture can move the reactor both from the bottom up and from the top flow downwards.

The material to be treated is then separated by distillation. The top product obtained is a hydrocarbon mixture largely freed of i-alkene, which can be used in particular for further working up to alkene-1. The bottom product obtained is the alkenes with an internal double bond and a higher-boiling mixture which contains the oligomerization products of the removed i-alkenes. However, the distillation can also be carried out in such a way that the alkene-1 and the alkenes with an internal double bond are obtained as the top product and only the higher-boiling oligomerization product remains in the bottom. In the case of a C ₄ distillation cut, the oligomerization products are essentially C ₈ hydrocarbons (dimers) which are 90% by weight and C ₁₂ -, C ₁₆ - or even higher oligomerization products which are at most 10 % By weight are present. The composition of the oligomer is strongly dependent on the composition of the feed mixture and the reaction conditions; in general, all possible reaction products arise from the i-alkene with the other alkenes in accordance with the associated reaction rate constants.

The bottom product obtained can be worked up to pure recyclables. For the Case that a raffinate which has already largely been freed from i-alkene is used as the starting point which is used to comply with the desired specifications of other i-alkene must be freed, the bottom product represents only a small amount, the Refurbishment is not worthwhile, but this is advantageous for the feed for the steam cracker or can be fed to a catalytic cracker.

The method according to the invention is used in an LHSV (Liquid Hourly Space Velocity) of 0.1-5 liters of hydrocarbon mixture containing i-alkenes per Liters of cation exchanger per hour, preferably 1-2 l / l · h, carried out. In continue preferably the LHSV and the reaction temperature are so dependent on the content of i-alkenes in the hydrocarbon mixture or from the desired residual content The i-alkene in the treatment mixture is adjusted to be as complete as possible Sales of i-alkene is achieved with as little loss of 1-alkene as possible.

Examples Examples 1 and 2

A continuous laboratory apparatus consisting of a preheater, a temperature-controlled double-jacket reactor and a separator was filled with 150 ml of cation exchanger in the H ⁺ form (Lewatit SPC 118 from Bayer AG). A C ₄ raffinate II from a steam cracker was passed through the apparatus at a temperature and an LHSV, as indicated in the following Table 1, and at an equal pressure of 15 bar in both experiments, the removal of the i- Butene up to a value of below 200 ppm (below the detectability limit) the slight loss of butene-1 also given in Table 1 occurred. After a running time of 7 hours, the reaction product from the last hour was collected and examined by gas chromatography. This information can also be found in Table 1 below. example 1 Example 2 LHSV 2nd 1 Temp. (° C) 50 40 commitment product commitment product i-butene (% by weight) 0.62 nn 1.83 nn Butene-1 (% by weight) 9.77 8.23 8.60 6.54 Butene-2 (% by weight) 61.56 61.71 58.51 58.17 nn = undetectable (detection limit <100 ppm)

Claims (6)

1. Process for removing C₄-C₈-isoalkenes from mixtures comprising these with hydrocarbons having essentially the same number of carbon atoms, characterized in that such mixtures are treated at 0-30°C and 1-25 bar with a cation exchanger in the H⁺ form and the treated material is then fractionated by distillation into an overhead product largely freed from isoalkenes and a bottom product comprising the isoalkene-conversion product.
2. Process according to Claim 1, characterized in that the hydrocarbon mixture to be treated is a C₄, a C₅-C₆ or a C₄-C₆ distillation fraction.
3. Process according to Claim 2, characterized in that the hydrocarbon mixture to be treated is a C₄ fraction.
4. Process according to Claim 1, characterized in that, with a continuous process procedure, a liquid hourly space velocity (LHSV) of 0.1-5, preferably 1-2, litres of mixture to be treated per litre of cation exchanger per hour is established.
5. Process according to Claim 4, characterized in that an LHSV is established as a function of isoalkene (isobutene) content and as a function of 1-alkene (1-butene) content.
6. Process according to Claim 3, characterized in that a reaction temperature of 10-20°C is set.