DE3220998A1 - Process for the preparation of low molecular weight olefins - Google Patents

Abstract

Low molecular weight olefins are produced by catalytic cleavage of methanol. After separation of a C3 fraction, the cleavage gas is subjected to low temperature gas decomposition using a C1/C2 separating column and an ethylene-ethane separating column. The head of the C1/C2 separating column is operated at temperatures below -95 DEG C and a part of the methane removed from the head of this column is fed back into the cleavage gas.

Description

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- LINDE AKTIENGESELLSCHAFT

(H 1316) H 82/42

BüTre 06/03/1982

Process for the production of low molecular weight olefins

The invention relates to a method for producing low molecular weight olefins.

Low molecular weight olefins, in particular ethylene and propylene, but also butylene are important chemical intermediates that are required in large quantities; common processes for producing these olefins are based on the splitting of hydrocarbons, for example ethane, propane, light petrol, naphtha or kerosene. The cleavage of hydrocarbon mixtures is particularly advantageous with a boiling range below about 200 ⁰ C, because such operations inausbeuten relatively high olefin and give little undesirable by-products.

Since there is a very great need for low molecular weight olefins, which leads to a shortage or increase in the price of these For some time now, attempts have been made to develop procedures that can be used on others Stakes are based. Among other things, the use of heavy hydrocarbon fractions, especially gas oil or vacuum gas oil. The occurrence of large amounts of undesired fission products can be

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However, only avoid sentences through additional procedural steps and is therefore associated with a lot of effort. Procedures of this type are, for example, in the German Offenlegungsschriften 28 05 720, 28 15 859, 28 43 792 and 28 43 793.

In the production of low molecular weight olefins, a cracked gas is generated from the feedstock used, which contains further reaction products in addition to the desired olefins. To separate these by-products and the isolated recovery of the olefins requires the cracking gas of an expensive gas decomposition to be carried out at low temperatures be subjected.

The invention is based on the object of developing a process for the production of olefins which can be carried out with as little effort as possible and results in a high olefin yield using feeds other than light hydrocarbons.

This object is achieved in that methanol is catalytically cleaved, the cracked gas obtained is optionally compressed, cooled and, after a C ₃₊ fraction has been separated off, fed to a low-temperature gas separation column, which has a C./(^ separation column and a C./(^ separation column as well as a C. / C ₂ _ separation column downstream ethylene-ethane separation column, from which ethylene is withdrawn as product, with a largely C _o -free methane fraction withdrawn at the top of _{the C / C 2} separation column and the C ₁ ZC _{7 separation column on} head is cooled at temperatures below -95 ⁰ C and further wherein a portion of the head of the C ₁ / C - withdrawn methane separation column is recycled.

The inventive method is based on the catalytically accelerated exothermic cleavage of methanol. - The reaction takes place at temperature

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doors to about 400 ⁰ C, for example between 300 and 500 ⁰ C, and at atmospheric or moderately elevated pressure, for example between 1 and 30 bar is performed. In the production of light olefins, preference is given to the lower pressure range, for example between 1 and 12 bar, in particular between 7 and 12 bar, since the choice of higher pressures shifts the product spectrum to less desirable components.

The methanol cleavage essentially takes place in two stages. / ~ At first there is a conversion of methanol into diraethyl ether according to the reaction equation

.._ Then the dimethyl ether formed as an intermediate implemented in 'the desired olefin-rich cracked gas, wherein one main reaction is the conversion to ethylene according to the reaction equation

H ₃ CO-CH ₃ - «- C ₂ H ₄ + H ₂ O

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represents. With a favorable choice of the reaction conditions, fission gases with an ethylene content of over 40% by weight and produce comparable propylene contents. In addition, the cracked gas also contains butylene in an amount of about 1% by weight up to more than 10% by weight. Continue to stand out

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Methanol fission gases due to a low content of methane and hydrogen as well, especially in the case of the fission under low. Pressure, due to a small proportion of higher-boiling hydrocarbons the end.

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Because of the extraordinarily high olefin content, the methanol is cracked gas particularly suitable for the production of low molecular weight olefins, as it is used in gas separation from the outset there is a high concentration of the products to be separated and a small proportion of by-products. The opposite different from the usual splitting of hydrocarbons

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However, the corresponding cracked gas composition usually requires a Special gas separation procedure, tailored to the gas composition, so that the usual gas separation processes are not readily transferable to the method according to the invention. In particular, the low proportion requires of methane and hydrogen in the cracked gas, a change in the procedure in the low-temperature part of the gas separation.

In the present process, the cracked gas, if it is not already generated under sufficiently high pressure, compressed, then after a pre-cooling to temperatures from -30 to -50 ⁰ C, for example -35 ⁰ C, a separation of C -, - cabbage water toffen and higher boiling components is subjected, and thereafter, cooled to a for the isolation of a fraction of C ₂ hydrocarbons convenient temperature, for example at temperatures of about -95 ⁰ C as they are reachable by a C ₂ -Kältekreislauf. Ethylene is separated off as the desired product from the separated C fraction, which contains ethylene as well as ethane. If the C ₂ fraction contains small amounts of acetylene, these are removed for safety reasons before the ethylene-ethane separation, for example by washing or by selective hydrogenation.

A special feature of the process according to the invention consists in carrying out the C ₁ / C ^ separation. _{During this separation, a C 2} -free fraction should be taken off at the top of the separation column, since ethylene contained in this fraction would lead directly to a reduction in the product yield.

To do this, it is necessary to cool the head of the separation column sufficiently, for example by applying a cold reflux liquid or by indirect cooling in a top condenser. The head can be cooled, for example, by indirect cooling with ethylene boiling under low pressure. Another, when carrying out the method according to the invention preferred type of head cooling is that methane, the

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is separated from the cracked gas at temperatures below -9 5 ⁰ C, is used as a cooling medium. In addition to indirect cooling in a head cooler, direct cooling is particularly preferred, in which the head of the separation column is subjected to deeply cold liquid methane as a return. The liquid methane is accessible from the cracked gas at temperatures below about -95 ⁰ C, that is at a lower temperature level than by a C "-Kältekreislauf separated. The temperature of the cooling liquid may be -120 to -150 ⁰ C, for example at about -130 ⁰ C. Due to the low methane content of the reformed gas to a methanol decomposition, it is not readily possible for the C / C_-separation column a sufficient amount of Provide methane as a coolant. It is therefore an essential feature of the process according to the invention that at least part of the methane fraction which is sucked off at the top of the C./C ^ separation column is returned. Only the amount of methane that is not required for the procedure in the low-temperature part is subtracted and used, for example, as heating gas. To the

If the methane is recirculated, for example after its refrigeration has been released to process streams to be cooled, it is passed into a suitable pressure stage of the cracked gas compression. Since the C ₁ / C separation usually takes place at increased pressure, approximately between S and 15 bar, for example at 10 bar, the full compression ratio is not required for the recompression of the methane.

The methane can be recycled in various ways; for example, it can be returned to the top of the C./C ^ separation column in a separate circuit. These The type of recirculation has the advantage that the methane is not mixed with other fractions, but it is required a special compressor and additional heat exchangers or separate heat exchanger cross-sections. That's why it is in many Julien it is easier to use the recycled methane

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at a suitable pressure level into the cracked gas and further process it together with it.

In a favorable development of the method according to the invention, another part of the low-boiling components of the cracked gas is used to condense the low-boiling components serving as return for the Cj / Cj separation column. A suitable refrigerant is a condensate of a multi-stage during the cooling of the reformed gas in the last pressure stage C ₃ -KaItekreislaufs, that at temperatures between about -80 ⁰ C and about -95 ⁰ C is obtained. The condensate formed is rich in methane and also contains small amounts of C ^ hydrocarbons. This condensate is expanded while cooling and in indirect heat exchange with the remaining gaseous part of the cracked gas, which only contains methane and possibly hydrogen, is reheated and evaporated after further heating in indirect heat exchange with process streams to be cooled, preferably fed back to the raw gas compressor and subjected again to the decomposition process. In this way the (^ hydrocarbons, especially ethylene, still contained in this fraction are not lost.

When carrying out the process according to the invention, it is expedient to subject the methanol _{cracked gas to a purification before cooling and C_ +} separation, with the dimethyl ether and carbon dioxide contained in the cracked gas being separated off in particular. Otherwise, these components would lead to difficulties in the low-temperature gas separation process. In an advantageous further development of this process step, it is proposed that a wash at

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a higher pressure than the methanol cleavage is carried out. This is particularly of advantage if the methanol decomposition at low pressures, for example between 1 and 7 bar is carried out, since the laundry can then be carried out at a higher pressure under substantially more favorable conditions. If, on the other hand, the methanol cleavage is carried out at pressures between 7 and 12 bar, the washing can also be carried out at the pressure of the cleavage.

In a further embodiment of the process according to the invention, the C_ ₊ fraction separated from the cracked gas after the precooling can be broken down into a C ₃ ~ fraction and into higher-boiling components; considerable amounts of propylene can be obtained from the CU fraction as a further process product, while butylene can also be separated off as a further olefinic product from the heavier fraction. If more propylene is obtained in the C ₃ fraction than is desired, this fraction can be _{subjected to a C 3} disproportionation either before or after the propylene fractionation, with ethylene and butylene being essentially formed from the propylene.

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Further details of the method according to the invention are provided explained on the basis of the exemplary embodiments shown schematically in the figures. Show it:

Fig. 1 is a block diagram showing the main method steps for olefins by cleavage methanol / and

Fig. 2 shows a specially designed for the C./C separation Conduct of proceedings.

In the process according to FIG. 1, a mixture of methanol and water is fed to the reactor 2 via line 1 and is converted therein catalytically at temperatures of 400 ^° C. and at a pressure of 3 bar to form a cracked gas. The addition of water to the methanol is useful in order to prevent or at least delay coking of the catalyst particles in the reactor. The methanol cracked gas is withdrawn via line 3, compressed to an intermediate pressure in the compressor 4 and then fed via line 5 to a purification stage 6 in which the methyl ether contained in the cracked gas and any carbon dioxide present are separated off. The purified cracked gas then reaches the cracked gas compressor 8 via line 7 and is there compressed to the pressure of the low-temperature gas decomposition, for example a pressure between 22 and 30 bar. The cracked gas then passes via line 9 into a pre-cooling 10, in which it is ^{cooled to a temperature between about -3 0 and -50 0} C, for example by a multi-stage propane circuit to temperatures of -35 ⁰ C. The condensing during the pre-cooling ( ^ Hydrocarbons and higher-boiling components are then separated off in the C / C separation 11 and drawn off via line 12. The remaining C ₂ fraction is transferred to the low-temperature part 13, which is

temperatures below the pre-cooling up to about -120 to

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-150 ⁰ C works, led. In the low-temperature part 13, a fraction of C and C ~ hydrocarbons is obtained in liquid form and is drawn off via line 14. Components remaining in gaseous form, which essentially contain methane and hydrogen as well as small amounts of (^ -hydrocarbons, are withdrawn via line 15 and, if necessary separately from one another at different temperature levels, can be fed to various uses after they have been heated ^ -containing fraction are returned to the cracked gas compressor in order to obtain olefin contained therein.

The fraction withdrawn via line 14 is fed to the decomposition section 16, in which demethanization takes place.

The methane which has been separated off is drawn off via line 17 and, after reheating, is used, for example, as heating gas. The remaining C ₂ fraction reaches a purification stage 19 via line 18, in which acetylene, if any, contained in the cracked gas is separated off. The cracked gas then passes via line 20 into the ethylene-ethane separation 21, from which ethylene is discharged as a product stream via line 22 and ethane is discharged via line 23. The ethane is heated and thermally cracked in a bar Äthanspaltofen 24 in the presence of steam at temperatures of 760-900 ⁰ C, for example at 830 ⁰ C and under a pressure of about. 7 After a dwell time of, for example, 0.5 seconds, the hot cracked gas emerges from the heated tubular reactor 24 via line 25 and is immediately cooled in a quenching device 26 to such an extent that secondary reactions no longer occur. After further cooling and separation of high-boiling fission products in 27, the remaining fission gas is fed to a first compressor stage 28 and compressed to an intermediate pressure. Since the ethane cleavage gas contains small amounts of carbon dioxide, which interferes with the low-temperature part, it is sent to a purification stage 29.

leads. The purified cracked gas is then combined with the methanol cracked gas from line 7 and in the already disassembled manner described.

In the C ₂ / C ₃ separation 11, a C ₃ ^ fraction is obtained in line 12, which is broken down at 30 into a C ₃ fraction and into heavier hydrocarbons, which are drawn off via line 31. The C ₃ fraction is withdrawn via line 3 2 and, optionally after a selective hydrogenation of more unsaturated compounds, fed to a propylene-propane separation 33. While propylene is withdrawn as a process product via line 34, the propane passes via line 35 into a thermal cleavage stage 36 and is thermally converted there under similar conditions as the ethane in the cleavage stage 24. The cracked gas arrives via line

37 into the quench cooler 26, where it is cooled together with the ethane cracking gas from line 25 and processed further.

In FIG. 2, the process is carried out in the low-temperature part the cracked gas separation shown in more detail. Via line

38 purified cracked gas, which can also contain thermal cracked gas in addition to methanol cracked gas, is fed to the compressor 8 and compressed to the pressure of the gas separation, for example to 25 bar. The cracked gas then reaches the pre-cooling 10 via line 9, where it is cooled against process streams to be heated and an evaporating refrigerant which is not shown in the figure and which is circulated. In the process, C ₃ and higher boiling hydrocarbons condense and are separated from the cracked gas in the separator 39 and drawn off via line 40. The gaseous remaining components are fed via line 41 to a heat exchanger 4 2 and cooled against cold process streams as well as against evaporating ethylene at elevated pressure in a refrigeration cycle to a temperature of about -60 ⁰ C.

Components that condense in the process are removed in the separator 43

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separated and fed via line AA and valve 45 into the lower region of the C. / C separation column 46.

The remaining gaseous components in separator 43 pass via line 47 into a further heat exchanger 48 and evaporating ethylene are cooled in a refrigeration cycle further to about -80 ⁰ C against cold process streams and at medium pressure. The components condensing in the process are separated off in the separator 49 and also into the C./C ₂ separation column 46 via line 50 and valve 51. fed in. Because of the greater content of lower-boiling components, the fraction is introduced into the C ₁ / (! Separation column at a point above the aforementioned feed point.

The gas phase in the separator 49 consists essentially of _{components boiling lower than C 2} hydrocarbons, in particular methane and hydrogen, and still has a low content of (^ hydrocarbons

SO gas is withdrawn via line 52 and cooled in heat exchanger 53 against depressurized evaporating ethylene in a refrigeration cycle as well as against cold process streams to a temperature of about -95 ⁰ C. This results in a methane-rich condensate which also contains the C_ hydrocarbons still contained in line 52. This condensate is separated off in the separator 54, drawn off via line 55, subcooled in the heat exchanger 56 and then depressurized in the valve 57 while cooling. The cold thus obtained is transferred to the fraction remaining in gaseous form in separator 54, which was withdrawn via line 58 and initially cooled in heat exchanger 56 against cold process streams before it is further cooled in heat exchanger 59 against the relaxed evaporating condensate and fed to a separator 60. The condensate evaporating in the heat exchanger 59 is after heating in the heat exchangers 56, 53, 48, 42 and 10 to

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withdrawn and after heating in the heat exchangers 53, 48, 4 2 and 10 released as heating gas. Because the low methane content of a methanol cracked gas in the separators 54 and 60 only leads to small quantities of condensate which, on the one hand, are in the δ heat exchanger 59 inadequate cooling effect and, on the other hand, inadequate return flow for the C./C,> separation column result in the separator 60, part of the top product the separation column 46 is branched off after heating and returned to the raw gas via line 70. The repatriation this fraction is expediently carried out between two compression stages of the compressor 8 at the pressure level of the separation column 46, for example at pressures around 10 bar. The methane returned via line 70 is largely converted into the heat exchangers 10, 42 and 48 during the cooling process remain in the gas phase and thus increase the methane fraction ii of the deep freezing, so that this measure in the separators 54 and 60 there are sufficient amounts of condensate.

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Cracked gas returned in line 38. By returning this stream to the decomposition process, the C ₂ hydrocarbons still contained in the condensate of the separator 54 are also used, which increases the yield of valuable

S len components increased.

In the separator 60 a condensate falls at temperatures between about -120 and -150 ⁰ C, for example at -130 ⁰ C, at which consists essentially of methane. The uncondensed portion of the cracked gas only contains hydrogen and methane. It is withdrawn via line 61 and, after releasing its cold content in the heat exchangers 56, 53, 48, 42 and 10, can be given off as heating gas, for example. The condensed methane is drawn off via line 62 and applied via valve 63 to the top of the C./C2 separation column as reflux liquid. The use of a pure and cryogenic methane fraction as return for the C./C ₂ separation column ensures that a methane fraction is drawn off at the top of the separation column 46 via line 64 which contains almost no (^ - hydrocarbons. In the bottom of the separation column 46 a pure C ₂ fraction is obtained, which is fed via line 65 and valve 66 into an ethylene-ethane separating column 67. If the C ₂ fraction from the bottom of the separating column 46 still contains traces of acetylene, between the two separating columns A device for separating off the acetylene is connected. In the separation column 67, ethylene is obtained as the top product and drawn off via line 68. This fraction is released as product after being heated against process streams to be cooled is withdrawn and thermally cleaved after heating, for example according to the procedure shown in FIG.

The top product of the C ₁ / (^ - separation column 46 is via line

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Claims (8)

Claims IS 1 .j A process for the production of low molecular weight olefins, \ _y characterized in that methanol is catalytically cleaved, the cracked gas obtained is optionally compressed, cooled and, after a C ₃₊ fraction has been separated off, is fed to a low-temperature gas decomposition which has a Cj / C ^ Separation column as well as an ethylene-ethane separation column downstream of the bottom of the C./C separation column, from which ethylene is withdrawn as a product, with a largely C ₂ free at the top of the C./C separation column methane fraction is withdrawn and the C./C is cooled -Trennsäule ₂ at the head at temperatures below -95 ⁰ C and further wherein a portion of the ₂ ~ separation column drawn from the head of C./C methane is recycled. 2. The method according to claim!, Characterized in that the top of the C. / C - separation column with liquid methane, which was separated from the cracked gas ^{at temperatures below -95 0 C, is cooled.} 3. The method according to claim 1 or 2, characterized in that the portion to be returned of the _{methane withdrawn from the top of the C./C 2} separation column is returned to the cracked gas. Shape. 5723 7.73 copy 4. The method according to any one of claims 1 to 3 / characterized in that the _{cracked gas after the C 3+} separation is subjected to a multi-stage partial condensation that the ^{condensates occurring at temperatures up to about -80 0} C in the C./C ₂ Separation column that the condensate obtained after further cooling to temperatures below about -9 5 ⁰ C is separated from the remaining methane-rich gas and, after expansion, is used for cooling and partial condensation of the remaining gas, the condensate thus formed for cooling the top of the C./C ₂ separation column is used. 5. The method according to any one of claims 1 to 4, characterized in that that the cracked gas before the C, separation of a wash to remove dimethyl ether and carbon dioxide is subjected. 6. The method according to claim 5, characterized in that the washing is carried out at a higher pressure than the methanol cleavage . 7. The method according to any one of claims 1 to 6, characterized in that the separated from the cracked gas C ₃₊ fraction of a C ₃ / C. Separation and the resulting C ₃ fraction is subjected to a disproportionation. 8. The method according to any one of claims 1 to 7, characterized in that the methanol cleavage is carried out at a pressure between 1 and 30 bar, in particular between 7 and 12 bar.