DE19825295A1 - Recovery of high purity alpha olefins from a Fischer Tropsch synthesis - Google Patents

Abstract

Etherification is carried out in stages. Following each stage, etherification products, if appropriate with other higher-boiling components, are separated as residual streams. An Independent claim is also included for a single distillation column used in the process, which includes a thermally-coupled partition.

Description

The invention relates to a method for obtaining an alpha olefin from a mainly hydrocarbon compounds containing mixture, as in the Fischer-Tropsch synthesis after at least a rough separation of heavy and / or lighter than the alpha-olefin boiling component than Feed stream, with tertiary boiling close to the alpha olefin to be recovered Olefins of the mixture after the stoichiometric addition of a light alcohol undergo catalytic etherification and one by etherification generated electricity with the alpha olefin and generated ethers and others heavy reaction products of a distillative separation heavier than the olefin boiling components is supplied.

A known process (CL Render, Z. Denga: "Sasol alpha olefins" from the series "New Processes in Chemical Technology", lecture at the ACHEMA conference in 1994) for the production of a certain alpha-olefin from a boiling cut by a fisherman -Tropsch synthesis generated liquid fraction consists of the following process steps:

• (1) Coarse separation of both light and heavy hydrocarbons.
• (2) Removal of boiling close to the desired alpha olefin tertiary olefins by catalytic etherification after Add methanol.
• (3) Distillative fine separation of low-boiling components simultaneous removal of the necessary for etherification Excess methanol.
• (4) Recovery of the methanol from the top product of ( 3 ) by extraction with water and subsequent methanol / water separation.
• (5) Distillative fine separation of heavy boiling components from the bottom product of ( 3 ), the ethers and other heavy reaction products formed in ( 2 ) also being separated at the same time.
• (6) Fine cleaning of the top product from ( 5 ) by extractive distillation.

An improvement on this known method for the production of alpha-olefins became a patent in the German Patent Office under file number 197 23 049.0 Registered. A key feature of this improved process was the Arrangement of the etherification reactor after the separation by distillation of all lower-boiling components (lower-boiling than the desired alpha-olefin).

Due to a reaction equilibrium in the etherification reactor cannot complete conversion of the tertiary olefins to the corresponding ethers can be achieved. Depending on the original content of tertiary olefins is therefore the maximum achievable purity of the alpha olefin product - even with practical complete removal of all other contaminants - hardly over 99%. (Here and the purity is given in percent by mass.) This applies to both C.L. Render and Z. Denga as well for the improved process (197 23 049.0).

An important market for linear alpha olefins opens up through their use as Copolymer in the production of plastics. For this application you need the alpha olefins should be as pure as possible, because depending on their type, impurities result in the polymerization process either "only" to product losses due to discharge contamination, loss of quality in the plastics produced or even to reduce the activity of the polymerization catalysts used. The users of alpha olefins therefore prefer the same or appropriate higher prices the better qualities. When 1-hexen z. B. was the beginning of nineties a purity of 98.5% as a very good value. Are now but already larger batches with purities of 99.3% on the market.

The method described by C. L. Render and Z. Denga is well suited from the coal liquefaction accruing fractions such. B. 1-hexenes win with a purity of 98.5%. Higher purities can only be found under progressively increasing losses in yield. The reason for this is in the Use with contained tertiary olefins whose boiling points are close to that of 1-hexene and therefore practically not with conventional separation techniques can be separated. These tertiary olefins must be in the reactor too detachable connections are implemented. In a one-stage reactor, the  Degree of implementation, however, limited by the chemical equilibrium, so that the maximum achievable purity of the 1-hexene product ultimately through the Reaction equilibrium is limited.

The object of the invention is to limit the achievable purity to approximately 99% to overcome and a simple device for the distillative separation of Propose reaction products and light alcohol after etherification.

According to the invention, this object is achieved by a method having the features of claim 1 and of a device according to claim 11 Invention are the subject of dependent claims.

It is characteristic of the process according to the invention that etherification is carried out in several stages, but at least in two stages and that after each Etherification step etherification products, optionally together with others heavier boiling components than residual currents are separated.

Is the conversion corresponding to the thermodynamic equilibrium z. B. at 70%, then 30% of the tertiary remain after the first etherification step Olefins of the feed stream in the etherification product stream. After the removal the ethers formed in the first etherification step can in the second Etherification step under the same reaction conditions again up to 70% of the remaining olefins are implemented. Overall, this results in sales of up to to 91%. The remaining impurities in the alpha-olefin product can thereby be significantly reduced.

After the rough separation of the heavier and / or lighter boilers from the The feed stream and prior to the multi-stage etherification can be a distillative Fine separation can be made from low boilers.

Alternatively, however, after the rough separation, the heavier and / or lighter Boiling from the feed stream and before the multi-stage etherification distillative fine separation of light and heavy boilers become. Which alternative is cheaper depends on the  Composition of the feed stream and the type of rough separation of the heavier and / or lighter than the alpha-olefin boiling components.

Methanol can be used as the light alcohol.

After the last etherification step, namely after the separation of the Etherification products and possibly other heavy components can be light alcohol in a further distillation step as an azeotropic mixture separated and thereby a current can be obtained, which is either immediately as Product used or further cleaning steps, for example one Extractive distillation and / or is subjected to adsorption. Is an advantage here that the distilled stream the alpha-olefin before more Contains cleaning steps with high purity with high yield.

The azeotropic mixture is advantageous when the light alcohol is added at least partially used before the multi-stage etherification. This lowers the Costs for the equipment to be used in the method according to the invention Alcohol.

Part of the azeotropic mixture can be the second and / or one of the following Etherification steps are supplied. This is also how the operating costs can be for lowering alcohol.

Those in the separation of the etherification products and any other heavier components after the second and / or after one or more Residual streams occurring downstream of the etherification steps can be converted into one lower section of a column for separating the etherification products of the first etherification step. The advantage is that only the etherification products in this column exactly with the corresponding effort must be separated from the alpha olefin and still achieve a high yield becomes.

The residual flows occurring in the flow direction before the last etherification step can use a lower section of a column to separate the Etherification products of the last etherification step are supplied. Here  The advantage is that an exact separation of the etherification products only after the last etherification step.

The light alcohol separated off as an azeotropic mixture can be used as the liquid phase obtained and a gas phase are separated therefrom, the gas phase contains mainly light ether, which results in a side reaction from the alcohol can form.

The separation of the gas phase with the light ethers prevents the enrichment this light ether in the recycle of the azeotropic mixture and it works favorably on the purity of the alpha olefin to be obtained, if the at In some cases, etherification also used side reactions with formation supported by light ether.

Characteristic of the inventive device for performing the last distillation step according to claim 1 and the distillation to separate the light alcohol according to claim 5 is that the device only a single column contains and that the column is designed as a thermally coupled dividing wall column is. In this way, investment costs and energy costs are saved.

The invention is based on embodiments with schematic representations explained in more detail in four figures. Every etherification is called a reactor and each Distillation shown as a column. Equivalent parts in the figures are the same Reference numerals shown.

Fig. 1 shows an inventive two-stage etherification of tertiary olefins and separation of the reaction products.

FIG. 2 shows the process from FIG. 1 with upstream and downstream process steps and a modification of the separation of the reaction products.

FIG. 3 shows the process according to FIG. 2 with a further modification of the separation of the reaction products.

Fig. 4 shows a dividing wall column according to the invention.

Fig. 1 shows schematically an etherification according to the invention, in the example in two stages carried out and separation of reaction products. A stream 1 with an alpha-olefin and tertiary olefins is added over a stoichiometric amount of a light alcohol 2 and the total stream 3 thus formed is fed into a 1st reactor 4 for etherification of the tertiary olefins. An exit stream 5 with ethers formed in the 1st reactor 4 is fed to a 1st column 6 , from which a bottom product 7 is drawn off with the ethers and a top product 8 is obtained with the alpha-olefin and with tertiary olefins not reacted in the 1st reactor 4 . This top product 8 of the 1st column 6 is fed into a 2nd reactor 9 . An exit stream 10 with ethers formed in the second reactor 9 is fed to a second column 11 , from which a bottom product 12 is drawn off with further ethers and a top product 13 is obtained with the alpha-olefin and only a few tertiary olefins which have not been converted in the second reactor 9 becomes.

Fig. 2 shows schematically the process of Fig. 1 with upstream and downstream process steps and a modification of the separation of the reaction products. The process is described using a case study for the production of 1-hexene.

A crude hexene fraction 15 , which contains 50% of 1-hexene, for example, and is obtained by preceding process steps, is fed into the column 14 . Except for low residual contents, all components with a lower boiling point than 1-hexene are removed overhead as stream 16 . Methanol 2 is added to the bottom product 1 of the column 14 , which is freed of all lower-boiling components, in front of the reactor 4 , in a more than stoichiometric ratio to the tertiary olefins to be reacted. In steady state equilibrium, only as much methanol 2 is supplied to the process from the outside as is actually converted. The stoichiometric excess is created by recycling methanol from a downstream process section described later.

In reactor 4 , the majority of the tertiary olefins - insofar as the approximation to the chemical equilibrium permits - are already converted to heavy ethers with methanol. Since the ethers thus formed boil significantly higher than 1-hexene, they can be separated off in column 6 via the bottom in stream 7 . Depending on the sharpness of the cuts already carried out further upstream, other components which boil more heavily than 1-hexene are also separated off together with the ethers.

Since the reaction products of the first reactor 4 are separated from the crude hexene 5 in the column 6 , the etherification reaction can continue in the reactor 9 until a chemical equilibrium is reached again. Around 80% of the tertiary olefins can already be converted in a single-stage reactor. Thus, with the two-stage arrangement under the same process conditions, a total turnover of approximately 96% can be achieved. With a third etherification stage (again consisting of reactor and subsequent separation of the reaction products), a total conversion of over 99% could be achieved, but usually the additional effort cannot be economically justified.

With much less effort, the overall conversion can be improved somewhat by setting an even higher methanol excess in the second reactor 9 by branching off a partial stream 18 of the above-mentioned methanol recycle and adding it to the crude hexene stream 8 before the reactor 9 . This optional return is shown in dashed lines in Fig. 2.

The heavy ethers generated in the reactor 9 are separated from the outlet stream 10 of the reactor 9 in the column 11 . Since heavy components are generated both in the reactor 4 and in the reactor 9 , it is advantageous to combine the separation of the heavy reaction products of one of the reactors with the fine separation of the other high boilers still contained in the feed stream 15 to the column 14 .

In the example shown in FIG. 2, this fine separation is already carried out after the first reactor 4 . In this case, it may make economic sense to separate the 1-hexene from the bottom product 12 of the column 11 only relatively roughly, for example to a residual content of between 10% and 50%, but the bottom product of the column 11 in the lower part of the column 6 attributable (dash-dotted line in Fig. 2) and then there to accomplish the fine separation of the 1-hexene from the heavy components.

Fig. 3 shows schematically the process as in Fig. 2 but with a further modification of the separation of the reaction products.

Combining the fine separation of the other high boilers with the separation of the reaction products of the first reactor has the advantage that the current through the other parts of the plant is thereby minimized. However, under certain boundary conditions it may be more favorable to provide the fine separation of the high boilers only after the second or last reactor stage 9 ( FIG. 3). This is especially the case when side reactions such as. B. run isomerizations that lead to heavy but close to the 1-hexene boiling components. In an analogous manner as already described above for the example of FIG. 2, it may be economically sensible in the example of FIG. 3 that the 1-hexene from the bottom product of column 6 is only relatively coarse, for example to a residual content between 10% and Separate 50%, but to introduce the bottom product of column 6 into the lower part of column 11 (dash-dotted line in FIG. 3) and then to achieve fine separation of 1-hexene from the heavy components.

The crude hexene stream 13 resulting from the last removal of heavier-boiling components still contains the stoichiometric excess of methanol. Since methanol has an azeotropic point with hexene, the excess methanol in column 21 cannot be separated off in pure form but only as an azeotropic methanol / hexene mixture. It is at least partially returned to the reactor 4 .

The bottom product 22 of the column 21 is a pre-cleaned witch fraction with a witch content of about 90%, which can be brought to the desired final purity in further purification steps (not shown in FIG. 3).

Fig. 4 schematically shows a dividing wall column according to the invention.

Columns 11 and 21 of FIG. 2 and FIG. 3 can be combined in a single dividing wall column. As a result, larger apparatuses are required in the apparatus for the method according to the invention, but fewer in total.

example Obtaining 1-hexene

Based on pure 1-hexene, the reactor insert typically contains approx. 5 mol% tertiary olefins boiling closely to 1-hexene. Assuming that in one single stage reactor due to chemical equilibrium limitation a maximum of 80% can be converted to ethers with methanol and neglects the in minor reactions also taking place to a smaller extent (e.g. isomerizations), then the 1-hexene product contains at least 1 mol% of these impurities, because it is practically no longer separated off due to its location can be.

Now separate the reaction products, the ethers, after the first reactor can in a next while maintaining the other reaction conditions Reaction stage again 80% of the tertiary remaining after the first reactor Olefins are converted to ethers, so that then only in the 1-hexene product 0.2% tertiary olefins are included.

Claims (11)

1. A process for the production of an alpha-olefin from a mixture containing mainly hydrocarbon compounds, as occurs in Fischer-Tropsch synthesis after an at least rough separation of heavier and / or lighter components than the alpha-olefin, as a feed stream, whereby close tertiary olefins of the mixture boiling on the alpha-olefin to be recovered are subjected to catalytic etherification after the stoichiometric addition of a light alcohol and a stream obtained by the etherification with the alpha-olefin and ethers and other heavy reaction products produced by distillation is more difficult to separate than the olefin which is boiling by distillation Components are supplied, characterized in that the etherification is carried out in several stages, but at least in two stages, and that etherification products, if appropriate together with other higher-boiling components, are separated off as residual streams after each etherification step. 2. The method according to claim 1, characterized in that after the rough Separation of the heavy and / or light boilers from the feed stream and Before the multi-stage etherification, a fine separation by distillation of light Boiling is made. 3. The method according to claim 1, characterized in that after the rough Separation of the heavy and / or light boilers from the feed stream and Before the multi-stage etherification, a fine separation by distillation of light and heavier boilers is made. 4. The method according to claim 1, characterized in that as a light alcohol Methanol is used. 5. The method according to claim 1, characterized in that after the last Etherification step, namely after the separation of the etherification products and possibly other heavy components in another Distillation step, the light alcohol by distillation as an azeotropic mixture separated and thereby a current is obtained, which is either immediately as Product used or further cleaning steps, for example one Extractive distillation and / or is subjected to adsorption.   6. The method according to claim 5, characterized in that the azeotropic Mixture when adding the light alcohol before the multi-stage etherification is used at least in part. 7. The method and claim 5, characterized in that part of the azeotropic Mix it at least the second and / or one of the following Etherification steps is supplied. 8. The method according to claim 1, characterized in that the separation the etherification products and possibly other heavy components ten after the second and / or after one or more downstream Residual flows occurring in a lower section of an etherification step Column for the separation of the etherification products of the first Etherification step. 9. The method according to claim 1, characterized in that the in Flow direction before the last etherification step a lower section of a column for separating the Etherification products of the last etherification step are supplied. 10. The method according to claim 5, characterized in that the as an azeotrope Mixture of separated light alcohol is obtained as the liquid phase and that a gas phase is separated therefrom, the gas phase mainly contains light ethers, which form in a side reaction from the alcohol can. 11. Device for carrying out the last distillation step after Claim 1 and the distillation to remove the light alcohol after Claim 5, characterized in that the device only one Column contains and that the column as a thermally coupled Partition column is executed.