EP0110924A1

EP0110924A1 - Method for the preparation of olefins with lower molecular weight

Info

Publication number: EP0110924A1
Application number: EP83901709A
Authority: EP
Inventors: Walter Schramm; Ulrich Hildebrandt; Wolfgang Baldus; Hans-Jürgen WERNICKE; Karl-Heinz Hofmann; Hans-Peter Riquarts; Ulrich Lahne; Peter HÄUSSINGER
Original assignee: Linde GmbH
Current assignee: Linde GmbH
Priority date: 1982-06-03
Filing date: 1983-06-01
Publication date: 1984-06-20
Also published as: WO1983004249A1; AU1558683A

Abstract

On les obtient par fractionnement catalytique du méthanol. Le gaz méthanol fractionné est refroidi, purifié, le cas échéant comprimé et après séparation d'une fraction C3+, amené à une étape de décomposition à basse température, au cours de laquelle on effectue une séparation C1/2 et une décomposition de la fraction C2 dans un courant d'éthane et éthylène. On décrit en particulier les conditions spéciales pour le traitement des gaz fractionnés.They are obtained by catalytic fractionation of methanol. The fractionated methanol gas is cooled, purified, if necessary compressed and after separation of a C3 + fraction, brought to a low temperature decomposition step, during which a C1 / 2 separation and a decomposition of the C2 fraction are carried out. in a stream of ethane and ethylene. The special conditions for the treatment of fractionated gases are described in particular.

Description

1 -

*

10

Procedure for. Manufacture of low molecular weight olefins

15 The invention relates to a. Process for the production of low molecular weight olefins by catalytic cleavage of methanol.

Low molecular weight olefins, in particular ethylene and propylene, but also butylene are important chemical intermediates which are required in large quantities; conventional processes for the production of these olefins are based on the splitting of hydrocarbons, for example ethane, propane, light gasoline, naphtha or kerosene. It is particularly cheap

25 Cleavage of hydrocarbon mixtures with a boiling range below approximately 200 ° C., because such uses result in relatively high olefin yields and few undesirable by-products.

* 30 Since there is a very great need for low molecular weight olefins which can lead to a shortage or an increase in the price of these inexpensive inserts, attempts have been made for some time to develop processes which are based on other inserts. Among other things, the use of heavy hydrocarbon fractions, in particular of gas oil or vacuum oil. The occurrence of large quantities of undesired fission products can, however, only be avoided in such applications by additional process steps and is therefore associated with great effort. Methods of this type are described, for example, in German Offenlegungsschrift 28 05 720, 28 15 859, 28 43 792 and 28 43 793.

In the production of low molecular weight olefins, a cracking gas is generated from the feed material used, which contains, in addition to the desired olefins, further reaction products. To separate these by-products and to isolate the olefins, the cracked gas must be subjected to extensive gas decomposition.

In the search for alternative raw materials, the catalytically accelerated exothermic splitting of methanol has already been considered. The reaction is carried out at temperatures around 400 ° C., for example between 300 and 500 ° C., and at atmospheric or moderately elevated pressure, for example between 1 and 30 bar. The lower pressure range, for example between 1 and 12 bar, in particular between 7 and 12 bar, is preferred in the production of light olefins, since the choice of higher pressures shifts the product spectrum to less desirable components.

The methanol cleavage essentially takes place in two stages. First, methanol is converted to dimethyl ether according to the reaction equation

2CH ₃ OH - H ₃ C-0-CH ₃ + H ₂ 0.

Subsequently, the dimethyl ether formed as an intermediate is converted into the desired olefin-rich cracked gas, a main reaction being the conversion into ethylene

H ₃ C-0-CH ₃ - C ₂ H ₄ + H ₂ 0

- 3 -

represents. If the reaction conditions are chosen favorably, cracked gases with an ethylene content of more than 40% by weight and comparable propylene contents can be generated. In addition, the cracked gas also contains butylene in an amount of about 1% by weight to more than 10% by weight. Furthermore, methanol fission gases are characterized by a low content of methane and hydrogen and, in particular when fissioning under low pressure, by a small proportion of higher-boiling hydrocarbons.

The investigation of the methanol cleavage has hitherto essentially been limited to the search for suitable catalysts, for which reference is made, for example, to the German laid-open specifications 29 09 927 and 29 12 068. An important step towards olefin production by methanol splitting was taken, but the peculiarities of the. Processing of the methanol cracked gas was completely neglected.

Because of the extremely high olefin content, the methanol cracked gas is particularly suitable for the production of low molecular weight olefins, since in gas separation there is already a high concentration of the products to be separated off and a small proportion of by-products. However, the cracked gas composition which differs from the usual splitting of hydrocarbons usually requires a special procedure for gas separation which is matched to the gas composition, so that the usual gas separation processes cannot be readily transferred to the process according to the invention. It was therefore an object of the invention to develop a process for the favorable processing of methanol cracked gas.

This object is achieved in that the methanol-cracked gas is cooled, cleaned, optionally compressed and, after a C ₃₊ fraction has been separated off, fed to a low-temperature gas separation in which a C./C separation and a separation the C fraction into a thanstrom and an ethylene product stream.

In the process according to the invention, the cracked gas, if it is not already generated under sufficiently high pressure, is compressed, then after precooling to temperatures between -30 and -50 ° C., for example to -35 ° C., a separation of C ₃ - Subjected hydrocarbons and higher-boiling components and then cooled to a temperature which is favorable for the isolation of a fraction of C-hydrocarbons, for example to temperatures around -100 ° C., as can be achieved by a C 1-4 cooling circuit. Ethylene is separated off as the desired product from the C ₁ -C fraction separated off, which also contains ethane in addition to ethylene. If small amounts of acetylene are present in the C ₂ fraction, these are removed for safety reasons before the ethylene-ethane separation, for example by washing or by selective hydrogenation.

A first special feature of the process according to the invention takes into account the small proportion of methane and hydrogen in the methanol cracked gas and the resulting problems when performing the C./C ₂ separation. In 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. For this it is necessary.

to sufficiently cool the top of the separation column, for example by feeding in a cold reflux liquid or by indirect cooling in a top condenser. The head cooling can be carried out, for example, by indirect cooling with ethylene boiling under low pressure. Another type of head cooling preferred when carrying out the process according to the invention is that methane is separated from the cracked gas at temperatures below -95 ° C is used as the cooling medium. In addition to indirect cooling in a head cooler, direct cooling is particularly preferred, in which cryogenic liquid methane is applied to the top of the separation column as reflux. The liquid methane is separated from the cracked gas at temperatures below about -95 ° C., that is to say at a lower temperature level than can be achieved by a C ₂ cooling circuit. The temperature of the cooling liquid can be between -120 and -150 ° C, for example around -130 ° C. Due to the low methanol content of the cracked gas of a methanol crack, it is not easily possible to use a sufficient amount for the C./C ₂ separation column to provide methane as a cooling liquid. It is therefore an essential feature of this variant of the invention that at least part of the methane fraction which is drawn off at the top of the C./C separation column is recycled. Just the amount. Methane, which is not required for the procedure in the low temperature section, is drawn off and used, for example, as heating gas. In order to recycle the methane, it is led, for example after its cold contents have been released to process streams to be cooled, into a suitable pressure stage of the cracked gas compression. Since the C./C ₂ separation usually takes place at elevated pressure, approximately between 8 and 15 bar, for example at 10 bar, the full compression ratio is not required when the methane is recompressed.

- 6 -

The methane can be recycled in various ways; for example, it can be returned to the top of the C./C ₂ separation column in a separate circuit. This type of recycling has the advantage that there is no mixing of the methane with other fractions, but requires a special compressor and additional heat exchangers or separate heat exchanger sections. It is therefore in many cases easier to guide the medium to be recycled into the cracked gas at a suitable pressure stage and to further process it together with it.

In a favorable further development of this variant of the invention, another part of the low-boiling components of the cracked gas is used for the condensation of the low-boiling components serving as return for the C./C ₂ separation column. A suitable refrigerant is a condensate which is obtained when the cracked gas is cooled in the last pressure stage of a multi-stage C refrigeration cycle, that is to say at temperatures between approximately -80 ° C. and approximately -95 ° C. The condensate formed is rich in methane and also contains small amounts of hydrocarbons. This condensate is expanded under • ^• cooling and warmed again in the indirect heat exchange with the remaining, only methane and optionally hydrogen-containing ga§- ^{shaped portion of the cracked gas and ver¬ evaporated. After the condensate has thus delivered the required for the generation of methane retrace cold, it will, after further heating by indirect heat exchange with process streams to be cooled again, preferably the crude gas supplied to and reused ^{'Zerlegungs¬} the subject methods. In this way, the C ₂ hydrocarbons still present in this fraction, in particular ethylene, are not lost.

- ύl.s- - 7 -

A second special feature of the method according to the invention is the utilization of the ethane obtained in the ethylene-ethane separation. Since this gas, which is relatively worthless in itself, is a favorable use for thermal cleavage for the production of low molecular weight olefins, it is subjected to such a cleavage in order to increase the olefin yield of the process according to the invention. This cleavage, which is carried out, for example, at temperatures between 750 and 900 ° C in a heated tubular reactor with very short residence times, for example between 0.1 and 1 sec, and in the presence of water vapor, provides an ethylene-rich cracking gas which is used to prevent secondary reactions is cooled very quickly and is then broken down together with the cracked gas from the methanol cracking.

In an advantageous further development of this Verfahrensvarian¬ te is a C ₃ fraction obtained in a corresponding manner from ^'the separated after the pre-cooling C-, fraction and a propylene-propane separation subjected. While the separated propylene is directly available as a product stream, the propane is used as an insert for a thermal cracking, so that, like the ethane fraction, propane, which can essentially only be used as exhaust gas, is converted into an ethylene-rich gas by thermal cracking put. The product gas obtained in the propane splitting, which is carried out under similar conditions as the ethane splitting, is also decomposed together with the methanol splitting gas after it has cooled. The thermal cleavage of the separated propane can under certain circumstances be carried out together with the thermal cleavage of the ethane fraction, as a result of which a plant for carrying out the process can be reduced by one thermal cleavage stage. In many cases, the methanol splitting can also be coupled with a conventional process for splitting gaseous or liquid hydrocarbons under normal conditions. Even in such a case, it is expedient to process the cracking gas obtained by such an additional thermal cracking together with the cracking gas from the cracking of methanol, since the considerable effort for an additional gas separation can thereby be avoided. If such additional thermal cleavage is provided, a propane fraction which may have been separated and thermally cleaved can also be processed together with the feed stream to be cleaved additionally. Which fractions are to be subjected to the thermal cleavage individually or separately from one another is determined in the individual case from the nature of the starting materials and the volume flows obtained on the basis of a customary economic analysis.

The cracked gases occurring in the process according to the invention usually contain. Impurities which would interfere with the gas separation to be carried out at low temperatures. While the cracking gases from the thermal cracking contain higher-boiling hydrocarbons and sulfur compounds as undesirable constituents, this is especially the case with dimethyl ether methanol, which has not been reacted. The proportion of this component, which can lead to difficulties in the gas decomposition to be carried out at low temperatures, depends on the reaction conditions present in each case and can vary considerably, for example in the range between 1 and and 25% by weight, in some cases even outside this range. It is therefore expedient to separate these components from the respective fission gases before they are processed together.

While higher-boiling components obtained from the cracking gas obtained in the thermal cracking are separated by condensation in the course of cooling and acid gases, in particular hydrogen sulfide, by washing, which is preferably carried out as part of the cracking gas compression between two compressor stages at an intermediate pressure, the removal of the dimethyl ether from the methanol cracked gas is preferably also carried out at an elevated pressure. Provided that the methanol decomposition is carried out at an elevated pressure, for example at pressures 5-12 bar, carried out, it is possible urchzuführen the gap gas purification at this pressure ^'. When the thermally obtained cracked gas is cleaned at a corresponding or lower pressure level, the cleaned cracked gases can then be further processed together in the course of the cracked gas compression.

In order to eliminate the disruptive influence of the dimethyl ether on the cracking gas decomposition, according to a further special aspect of the process according to the invention, the methanol cracking gas is washed, in which, in addition to the dimethyl ether, annoying carbon dioxide is washed out. The cleaned cracked gas, which then essentially only contains C-- to C ^ -hydrocarbons, can then be broken down in a low-temperature gas separation plant.

In a preferred embodiment of this variant of the invention, the washing is carried out at superatmospheric pressure, since the gas volume to be cleaned and the amount of washing liquid required are then considerably reduced, which in turn leads to significantly smaller components and thus cost savings. .

In a further advantageous embodiment of this variant, the detergent loaded with diethyl ether and carbon dioxide, after it has been removed from the washing stage, is subjected to partial regeneration by relaxation or heating. The low-boiling components dimethyl ether and carbon dioxide then at least partially outgas from the washing liquid. After these components, which can be used for any suitable purpose, have been separated off, the detergent can then be regenerated in a relatively simple manner. In particular, it is expedient to carry out the relaxation of the detergent up to the pressure of the methanol cleavage, since the gas fraction obtained in the • expansion and rich in dimethyl ether can then be returned directly to the cleavage.

The separation of the dimethyl ether and the carbon dioxide from - 1 1 -

the cracked gas can be carried out with any suitable detergent, for example with methanol, ethanol, water or mixtures thereof.

In a particularly preferred embodiment, the washing is carried out with methanol. A major advantage of this procedure is that methanol is used anyway as a gap insert and is therefore introduced into the process as an insert anyway. It is therefore not necessary to provide an additional substance as a detergent. Washing with methanol is also particularly advantageous because the loaded detergent does not have to be regenerated in a separate process step, but can be fed directly to the methanol cleavage. Since the unreacted dimethyl ether is returned to the cleavage in this way, the yield of desired cleavage products is also increased.

As already mentioned, the methanol cleavage essentially comprises two partial reactions, namely first the conversion of methanol to dimethyl ether and then the generation of the cracked gas from the dimethyl ether. These two partial reactions can take place in a single reactor if a suitable catalyst is used. However, it is also possible to largely separate the individual reactions from one another and to carry them out in different reactors. If the methanol cleavage contains a special reactor for the cleavage of the intermediate dimethyl ether, it is particularly expedient to introduce the gas resulting from the degassing of the detergent directly into this second reaction stage. After degassing the detergent as liquid, partially regenerated washing methanol can then be fed to the first reaction stage together with fresh methanol. The sub-fractions of the detergent that occur during the relaxation are thus each - 1 2 -

returned to the cheapest places in the splitting process.

If the methanol cleavage is carried out in a one-step process, it is in many cases advantageous to convert the dimethyl ether-rich gas obtained in the degassing of the detergent in a separate cleavage step connected in parallel with the methanol cleavage. As a result, not only can optimal conditions for the conversion of dimethyl ether to the cracked gas be achieved, but moreover keeping the dimethyl ether away from the first reaction stage of the methanol cleavage also has a favorable influence on the reaction equilibrium present there. The fission gases formed in each case in the parallel cleavages can then be combined and further processed together.

Most of the carbon dioxide that is washed out of the cracked gas is contained in the components that degas from the detergent. When this fraction is returned to the cleavage, the presence of carbon dioxide within certain limits is very favorable, since it is possible in this way to prevent the cleavage reactor from coking prematurely. However, if such a large proportion of carbon dioxide is washed out that the reactor is exposed to an undesirably high level of carbon dioxide, it may be more advantageous to separate the carbon dioxide from the dimethyl ether and to reduce the risk of coking in the reactor by either only a part of the existing carbon dioxide or water is led to the cleavage.

The separation of the carbon dioxide from the outgassed fraction, which essentially contains carbon dioxide and dimethyl ether, can be achieved by washing with water. It a wash water laden with dimethyl ether is obtained which, after relaxation, can be passed directly to the cleavage, while the carbon dioxide which has not been washed out is drawn off as residual gas.

Another feature of the invention finally relates to a special type of cooling of the cracked gas in cases in which a hydrocarbon cracking is also carried out in parallel with the methanol cracking. In this case, the special process control is characterized in that the cooling of the methanol cracked gases takes place at least partially with the generation of steam under increased pressure, the cracked gas of the thermal cracking is cooled at least partially with the generation of steam under increased pressure , the cooled cracked gases are compressed if necessary and jointly fed to the cracked gas decomposition, the steam obtained during the cooling of the cracked gases being expanded to perform the work and the energy thus obtained is at least partially used for compressing the cracked gases.

This process variant is based on two different cracking inserts, namely on the one hand the usual thermal cracking of hydrocarbons and on the other hand the catalytic cracking of methanol. The use of the two feed streams permits a particularly high flexibility of the process, since the change in the relative feed amount enables rapid adaptation to different conditions of the raw material supply.

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The thermal cleavage is carried out under customary reaction conditions, that is to say in an externally heated tubular reactor at temperatures between 750 and 900 ° C., for example at 830 ° C., with short residence times of 0.1 to 1 sec. In Cracking furnace and at atmospheric or slightly elevated pressure. The very reactive gas that occurs at high temperature is immediately quenched to temperatures at which undesired side reactions no longer occur.

The compression of the cracked gas to the pressure of the gas separation, for example to pressures between 20 and 35 bar, is associated with a high expenditure of energy. According to the invention, this energy is applied at least in part by the fact that high-pressure steam, which is generated during the cooling of the two fission gases obtained separately from one another, is expanded on work strips. Steam generation from the cracking gas of the thermal cracking can take place in the usual way, that is to say by indirect heat exchange of evaporating water with the hot cracking gases emerging from the tubular reactor. The heat of reaction obtained in the catalytic methanol cleavage can be used in various ways to generate steam. For example, the reactor can be designed in one or more stages, the hot escaping fission gases entering into indirect heat exchange with a cooling medium, or cooling can already take place in the reactor itself, for example by embedding cooling tubes in a catalyst bed or by rinsing tubes filled with catalyst with a coolant.

The steam generation in connection with the methanol cleavage can take place directly, that is to say by direct cooling of the reactor or the hot cracking gases emerging from the reactor with boiling water. Under certain circumstances, however, this process can lead to considerable temperature differences between the heat-exchanging media and thus high thermal stresses in the heat accumulation used in each case.

QMPI _ - 1 5 -

1 shear occur. In many cases, it is therefore beneficial to generate the steam indirectly with the interposition of a heat transfer medium. In such a case, a heat transfer medium is absorbed by the heat generated during the reaction.

5 heated and then cooled again in a steam generator against water boiling under pressure. The cooled heat transfer medium can be heated again against heat of reaction and in the

• Be guided in a cycle.

10 Suitable heat transfer media for such a configuration of the _. Invention are, for example, circulating salt melts. Another advantageous heat transfer medium is a quench oil, which is used in the usual cooling of the cracked gases by thermal cracking of hydrocarbons. Such quench oil. is a high-boiling hydrocarbon fraction which is injected into the pre-cooled cracked gas from the thermal cracking, which results in a further reduction in the temperature of the cracked gas under the condensation of high-boiling cracked products. The quench oil is then withdrawn from the cracked gas and, after cooling in an open circuit, returned to the cracked gas.

A favorable embodiment of the method according to the invention

25 provides that a portion of the quench oil used in the cooling of the thermal cracking gas is drawn off and conducted as a heat carrier for removing the heat of reaction of the methanol cracking in the circuit. A portion of fresh quench oil is continuously added to this cycle. 30 conducts as well as a corresponding amount of excess

Quench oil removed. This prevents decomposition products which form from the quench oil circulated at high terroera acids from settling and lead to inadequate heat dissipation. The quench oil removed from the circuit can be returned to the quench oil circuit be returned to the thermal cleavage. The amount of the freshly added or withdrawn quench oil can vary within a wide range, taking into account the respective special conditions. A proportion of 3 to 30% by weight of fresh quench oil in the heat transfer circuit is favorable; the fresh proportion should preferably be between 5 and 25% by weight, in particular between 10 and 20% by weight.

For the process according to the invention to be successful, it is not necessary for all of the above-mentioned embodiments of the invention to be carried out. Rather, each embodiment alone or in any combination with other embodiments offers significant advantages in the processing of methanol cracked gases.

Further details of the invention are explained below on the basis of the exemplary embodiments shown schematically in the figures.

Show it:

FIG. 1 shows a block diagram which shows the essential process steps for olefin recovery by methanol cleavage,

FIG. 2 shows a procedure specifically designed for C 1 / C ₂ separation,

Figures process procedures specially designed for the purification of the methanol cracked gas, wherein ^• in

FIG. 3 the methanol splitting takes place in one stage, in - 17 -

FIG. 4 the methanol splitting takes place in two splitting stages connected in series, and in

FIG. 5 shows a modification of the method according to FIG. 4, and

FIG. 6 shows a procedure specifically designed for cracked gas cooling.

In the process according to FIG. 1, a mixture of methanol and water is fed to reactor 2 via line 1 and is converted catalytically therein to a cracking gas at temperatures of 400 ° C. and at a pressure of 3 bar. The addition of water to the methanol is expedient in order to prevent or at least delay the catalyst particles from killing 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 cleaning stage 6, in which the dimethyl ether contained in the cracked gas and any carbon dioxide present are separated off. The cleaned cracked gas then passes via line 7 to the cracking gas pressor 8 and is compressed there to the pressure of the low-temperature gas separation, for example a pressure between 22 and 30 bar. The cracked gas then passes through line 9 into a pre-cooling 10, in which it is cooled to a temperature between about -30 and -50 ° C, for example by a multi-stage propane circuit to temperatures of -35 ° C. The during the pre-cooling The condensing C hydrocarbons and higher-boiling constituents are then separated off in the C / C separation 11 and taken off via line 12. The remaining C ₂ _ fraction is in the low-temperature part 13, which at temperatures below the pre-cooling up to about -120 to

^ ^j ? .EA OMPI - 18 -

1 -150 ° C works, led. A fraction of C. and C ₂ hydrocarbons is obtained in liquid form in the low-temperature section 13 and is drawn off via line 14. Gaseous components which are essentially methane

5 and contain hydrogen as well as still small amounts of ^ -Kohlenwasser¬, withdrawn via line 15 and can, if necessary separately from one another at different temperature levels, be used for different purposes after heating. For example

10 a C ₂ -containing fraction is returned to the cracking gas compressor in order to recover the olefin contained therein.

^ -

The fraction drawn off via line 14 is fed to the decomposition part 16, in which demethanization takes place.

15 Separated methane is withdrawn via line 17 and used after heating, for example as a heating gas. The remaining C ₂ fraction passes via line 18 into a purification stage 19, in which acetylene, if any, contained in the cracked gas is separated off. Then it arrives

20 fission gas via line 20 into the ethylene-ethane separation 21, from which ethylene is discharged as a product stream via line 22 and ethane via line 23. The ethane is heated and in an ethane cracking furnace 24 in the presence of steam at temperatures between 760 and 900 ° C, for example at

25 830 ° C, and thermally split under a pressure of about 7 bar. After a dwell time of, for example, 0.5 seconds, the hot cracked gas emerges from the heated tube reactor 24 via line 25 and is immediately cooled in a quenching device 26 to such an extent that no secondary

30 reactions more occur. After a 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 cracking gas contains small amounts of disruptive parts in the low temperature section

35 contains carbon dioxide, it is added to a cleaning stage 29 - 1 9 -

leads. The cleaned cracked gas is then combined with the methanol cracked gas from line 7 and broken down in the manner already described.

In the C ₂ / C ₃ separation 11, a C, fraction occurs in line 12, which at -30 is broken down into a C ₃ fraction and heavier hydrocarbons which are drawn off via line 31. The C ₃ fraction is drawn off via line 32 and, if appropriate after a selective hydrogenation of more unsaturated compounds, a propylene-propane

Separation 33 fed. While propylene is withdrawn via line 34 as a process product, the propane passes through line 35 into a thermal cracking stage 36 and is thermally converted there under conditions similar to the ethane in cracking stage 24. The cracked gas passes through the line

37 into the quench cooler 26 and is cooled and processed there together with the ethane split gas from line 25.

2 shows the process control in the low-temperature part of the cracked gas separation in more detail, via line

38, cleaned cracked gas, which in addition to methanol cracked gas may also contain thermal 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 passes through line 9 into the pre-cooling 10, where it is cooled against process streams to be heated and an evaporating and circulating cooling medium (not shown in the figure). 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 components are fed via line 41 to a heat exchanger 42 and cooled to a temperature of about -60 ° C. in a refrigeration cycle against cold process flows and against ethylene evaporating at elevated pressure. Condensing components are in the separator 43 separated and fed via line 44 and valve 45 into the lower region of the C./C ₂ separation column 46.

The components remaining in gaseous form in the separator 43 reach a further heat exchanger 48 via line 47 and are further cooled to about -80 ° C. in a refrigeration cycle against cold process streams and ethylene evaporating at medium pressure. The components which condense in this process are separated in the separator 49 and likewise fed into the C./C ₂ separation column 46 via line 50 and valve 51. Because of the greater content of lower-boiling constituents, the fraction is introduced into the C. / C ₂ separation column at a point above the feed point mentioned above.

The gas phase in the separator 49 is comprised essentially of lower than C ₂ hydrocarbons ^"boiling components, ie, in particular from methane and hydrogen, and still has a low content of C ₂ hydrocarbons from. This gas is withdrawn via line 52 and heat exchanger 53 against pressurelessly evaporating ethylene in a refrigeration cycle and against cold process streams cooled down to a temperature of about -95 ° C. This produces a methane-rich condensate which also contains the C ₂ -hydrocarbons still contained in line 52. This condensate separated in the separator 54, drawn off via line 55, subcooled in the heat exchanger 56 and then relaxed with cooling in the valve 57. The cold thus obtained is released to the fraction remaining in gaseous form in the separator 54, which is drawn off via line 58 and initially in the Heat exchanger 56 was cooled against cold process streams before being counteracted in heat exchanger 59 the expanded evaporating condensate is cooled further and fed to a separator 60. The condensate evaporating in the heat exchanger 59 becomes, after being heated in the heat exchangers 56, 53, 48, 42 and 10 - 21 -

Fission gas returned in line 38. By recycling this stream into the decomposition process, the ^ hydrocarbons still contained in the condensate of the separator 54 are also used, which increases the yield of valuable components.

At temperatures between approximately -120 and -150 ° C, for example at -130 ° C, a condensate is formed in the separator 60, which essentially consists of methane. The uncondensed fraction of the cracked gas only contains

Hydrogen and methane. It is drawn off via line 61 and, after its cold contents have been released, can be released in the heat exchangers 56, 53, 48, 42 and 10, for example as heating gas. The condensed methane is drawn off via line 62 and fed via valve 63 to the top of the C./^ separating column as reflux liquid. The use of a pure and cryogenic methane fraction as reflux for the C./C ₂ separation column ensures that a methane fraction which contains almost no C_-hydrocarbons is withdrawn at the top of the separation column 46 via line 64. in the

At 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 separation column 67. If the C ₂ fraction from the bottom of the separation column 46 still contains traces of acetylene, a device for separating the acetylene is connected between the two separation columns. In the separating column 67, ethylene is obtained as the top product and drawn off via line 68. This fraction is released as a product after heating against process streams to be cooled. At the bottom of the separating column 67, a pure ethane fraction is obtained, which is drawn off via line 69 and, after heating, is thermally split, for example in accordance with the procedure shown in FIG. 1.

The top product of the C./C- _j separation column 46 is via line 64

' - 22 -

deducted and given off after heating in the heat exchangers 53, 48, 42 and 10 as heating gas. Since the low methane content of a methanol cracked gas in the separators 54 and 60 only leads to small amounts of condensate which, on the one hand, do not result in a sufficient cooling effect in the heat exchanger 59 and, on the other hand, do not result in a sufficient return quantity for the C./C ₂ separation column in the separator 60 , a portion of the top product of the separation column 46 is branched off after heating and returned to the raw gas via line 70. This fraction is expediently returned between two compression stages of the compressor 8 at the pressure level of the separation column 46, that is to say, for example, at pressures of around 10 bar. The methane returned via line 70 will remain largely in the gas phase during cooling in the heat exchangers 10, 42 and 48 and thus increase the methane content in the deep-freezer, so that sufficient measures of condensate in the separators 54 and 60 are achieved by this measure attack.

In the exemplary embodiment shown in FIG. 3, fresh methanol is introduced via line 1 and at 2 is divided into a partial stream 3 to be fed to the cleavage and a partial stream 4 to be fed to the laundry. The amount of the partial stream drawn off via line 4 depends on the detergent requirement in the laundry and can, if appropriate, be the

- 23 -

make up the entire flow of fresh methanol. The methanol enters the upper area of the washing column 5 and washes dimethyl ether and carbon dioxide out of the rising cracking gases introduced via line 6. The detergent loaded with these components is drawn off via line 7 and in valve 8 from the pressure of the laundry, which is, for example, in the range between 7 and 10 bar, to the pressure of the methanol cleavage, for example to a pressure between 1 and 2 bar, relaxed. Components which degas during expansion and essentially consist of dimethyl ether and carbon dioxide are separated off in separator 9. The methanol partially regenerated in this way is discharged via line 10 and fed together with the partial stream 3 to the methanol cleavage 11 via line 12. Water is also fed into line 12 via line 13 in order to largely suppress coking of the catalyst during methanol cleavage. The reactor 11 contains a catalyst which is suitable both for the conversion of methanol into dimethyl ether and for the further conversion to the desired cracking gas. The exothermic reaction is kept at a temperature of approximately 400 ° C. by suitable control measures and the cracked gas is then cooled and drawn off via line 14. When the cracked gas is cooled, water condensing is separated in the separator 15 and drawn off via line 16. A partial flow of the water reaches the head of the washing column 5 via line 17 and washes methanol vapors in the upper area of the washing column from the cleaned cracked gas, so that the cracked gas withdrawn via line 18 no longer contains any disturbing impurities. Another partial flow 19 of the separated condensate can, for example, be led to line 13 and re-enter the split 11. The gas emerging from the separator 15 passes via line 20 to the compressor 21, in which it is compressed to the pressure of the methanol wash, and then via line 6 into the washing column 5.

ι.

O? I The cleaned fission gas withdrawn via line 18 is then fed to a low-temperature gas separation unit and separated into the individual product streams.

The gaseous fraction obtained in the separator 9, which essentially consists of dimethyl ether and carbon dioxide, is drawn off via line 22 and fed to a separate cracking stage 23, in which a conversion of dimethyl ether to cracked gas takes place. The cleavage takes place under conditions similar to those in the cleavage 11, but at lower reaction temperatures of about 300 ° C. The escaping cracked gas is withdrawn via line 24 and mixed with the cracked gas in line 14.

The carbon dioxide content of the gas in line 22 suppresses the tendency to coke in the reactor 23. If the amount of carbon dioxide in this gas fraction is not sufficient to provide adequate protection against the coking, a further partial flow of the in the separator can be provided via line 25 15 recovered water can be fed into the gas in line 22.

In another embodiment of the invention, the methanol-water mixture separated in the separator 9 can be divided into two partial streams, the portion withdrawn via line 10 in turn being fed to the methanol cleavage, while a partial stream withdrawn via line 26 through the pump 27 as a pre-loaded detergent is conveyed into an area of the washing column 5 below the feed point of the line 4.

The embodiment shown in FIG. 4 differs from that of FIG. 3 in that the cleavage does not take place in two reactors operated in parallel, but in two series-connected reactors. The

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O PI Mixture of methanol and water to be fed via line 12 to the methanol cleavage is subjected to dimethyl ether synthesis in a first reactor 28, whereupon the reaction product enters via line 29 into the second reaction stage 30, in which a dimethyl ether cleavage takes place. The fission gas generated in the process is further processed in the manner already described. The gas fraction obtained in the separator 9 is fed via line 22 to the input of the second reaction stage 30.

The embodiment shown in FIG. 5 differs from that of FIG. 4 in that carbon dioxide is separated from the gas fraction obtained during the partial regeneration of the detergent. The loaded detergent withdrawn via line 7 from the bottom of the washing column is heated in the heat exchanger 31, as a result of which carbon dioxide and dimethyl ether largely pass into the gas phase. The separator 9, in which the degassing components are separated from the detergent, can be designed as a decanter in order to separate higher hydrocarbons washed out from the cracked gas. Such a design of the separator 9 is moreover also possible in the preceding exemplary embodiments.

The gaseous fraction drawn off from the separator 9 via line 22, which essentially consists of dimethyl ether and carbon dioxide, is fed into the lower region of a washing column 32. 'Dimethyl ether is washed out of this gas with water introduced via line 33, so that carbon dioxide is withdrawn as residual gas via line 34 at the top of the column 32. The water used for the washing can be a partial flow of the water which is separated from the cracked gas in the separator 15. In the bottom of column 32, a mixture of water and diethyl ether is obtained in the liquid phase, which is drawn off via line 35, expanded in valve 36 to the pressure of the cleavage and after mixing is fed to the gap stage 30 with the gas obtained in line 29. , - -

The separation of the carbon dioxide can be carried out in a corresponding manner when carrying out a process according to the exemplary embodiment in FIG. 3.

In the exemplary embodiment shown in FIG. 6, a hydrocarbon, for example ethane, propane, naphtha or a gas oil, is fed via line 1 together with steam to a cracking furnace 2 for the thermal cracking. The cracking furnace 2 is a tube reactor in which the cracking insert is carried out through the tubes which are heated from the outside. The energy required for the endothermic reaction is provided by the combustion of a heating medium supplied via line 3. Hot flue gases emerge from the cracking furnace via line 4 and are fed to a chimney 6 after heat recovery in the heat exchanger 5 and possibly further heat exchangers (not shown in the figure). Those emerging from the cracking furnace 2 via line 7

Fission gases are fed directly into a first heat exchanger 8 and cooled to such an extent against water boiling under high pressure that there are no longer any undesirable side reactions. Depending on the type of cracking insert, the precooled cracked gas usually has a temperature between 350 and 480 ° C. In order to generate as much energy as possible in the form of high-pressure from the cracked gas, the lowest possible outlet temperature from the heat exchanger 8 is aimed at, whereby however the dew point of the gap gases must not be undercut in order to avoid contamination or laying of the heat exchanger and the subsequent lines. A cooled quench oil is injected into the pre-cooled cracked gas drawn off via line 9 at 10 for further cooling. Due to the direct heat exchange with the quench oil, the cracked gas is cooled further to below the dew point, so that high-boiling cracked products condense. The mixture of cracked gas and quench oil is passed into an oil wash 11, in which condensed constituents are washed out of the cracked gas stream, for this purpose a high-boiling hydrocarbon fraction is fed to the top of the oil wash column 11 via line 12. The cracked gas, which now has a temperature of between about 100 and 120 ° C., is drawn off from the top of the oil washing column 11 via line 13 and, after further cooling and optionally cleaning 14, is fed via line 15 to the cracked gas compressor 16. The compressed cracked gas then passes via line 17 into the low-temperature gas separation, not shown in the drawing.

A methanol splitting is carried out in parallel with the splitting of the hydrocarbons, for which purpose a mixture of methanol and water is fed to the reactor 19 via line 18. The catalytic conversion takes place at temperatures of 400 ° C. and at a pressure of 3 bar in a reactor cooled internally by cooling tubes 20. The addition of water to the methanol is expedient in order to prevent coking of the catalyst particles in the reactor or at least to delay them. The methanol cracked gas is withdrawn via line 21 and, after cooling, optionally a cleaning treatment 22 is fed via line 23 to the cracked gas compressor 16 and compressed together with the cracked gas from the thermal cracking.

For cooling the cracked gases of the two cracks 2 and 19

OMPI a quench oil cycle is maintained. Quench oil injected into the cracked gas in line 9 is separated off in the oil wash column 11 and withdrawn from the bottom of the column 11 together with condensed constituents of the cracked gas via line 24. While quench oil in an amount that the in. Splitting gas corresponds to the freshly obtained amount, which is discharged from the part of the system under consideration here, the circulated portion is conveyed via line 25 and the pump 26 into a heat exchanger 27, in which the splitting gas is supplied to a via line 28 and the Line 29 drawn cooling medium, for example water boiling under medium pressure, is cooled. The cooled quench oil is then injected via line 30 and valve 31 at 10 into hot cracked gas in line 9. Before this, however, a partial flow is branched off via line 32 and control valve 33 and fed to a quench oil circuit which flows through the heat exchanger 20 of the methanol splitting and removes the heat of reaction from the methanol splitting. The quench oil circuit consists of a circulation pump 34, the downstream heat exchanger 20, in which the quench oil is heated, and the heat exchanger 35, in which the hot quench oil is cooled against water boiling under high pressure. From the cooled quench oil, which exits the heat exchanger 35 via line 36, a partial flow is drawn off via line 37 and valve 38, which is returned to line 30 at 39. The amount of quench oil withdrawn from the circuit via line 37 corresponds to the amount of fresh quench oil supplied via line 32.

The steam system of the process shown in FIG. 6 contains a steam drum 40, which is fed via line 41 with water under high pressure, which has been heated to close to the boiling point by heat exchange with hot process streams. Water is conducted via line 42 to the heat exchanger 8 and partially steams therein. The boiling

1 steam-water mixture then returns via line 43 to Dampftrom el 40. In the same way, water withdrawn via line 44 is evaporated in the heat exchanger 35 against hot quench oil and returned via line 45 to the steam drum. The steam generated passes through line 46 into the heat exchanger 5, in which it is overheated against hot flue gases, and is then fed via line 47 to a turbine and expanded to perform work. The expanded steam is withdrawn via line 49. It can be preheated after total condensation, for example, and pumped back into the steam drum 40 via line 41.

The energy obtained during the work-relieving expansion of the steam in the turbine 48 is fed to the fission gas compressor 16 via a shaft 50 and used to drive it.

The -Spaltgasverdich _¬ schematically illustrated in Figure 6 tung 16 and the work-performing expansion of the fes 48 Damp can each be carried out in several stages. For example, steam generated in the steam drum 40 at a pressure of approximately 110 bar can first be heated to an average pressure of approximately 40 bar, then in a further relaxation stage to a pressure of approximately 15 bar and finally to a low pressure. for example in the range from 2 to 5 bar. Correspondingly ^" , the cracked gas compression can also be divided into three or four compressor stages.

Claims

-3o - 'claims

1. Process for the production of low molecular weight olefins by catalytic cleavage of methanol, characterized in that the methanol cracked gas is cooled, cleaned, optionally compressed and, after separation of a C ₃₊ fraction, fed to a low-temperature gas separation in which a C - _J / C - separation and a decomposition of the C ₂ fraction into an ethanol stream and an ethylene product stream takes place.

!. A method according to claim 1, characterized in that the low-temperature gas separation contains a C ^ / C ^ separation column and an ethylene-ethane separation column connected downstream of the bottom of the C __, / C2 separation column, the head of the C./C ₂ - Separation column a largely C ₂ ^_ free methane fraction is drawn off and the C./C ₂ separation column is cooled at the head at temperatures below -95 ° C. and part of the methane drawn off from the top of the C ₁ C separation column is also recycled . - 3 -

A method according to claim 2, characterized in that the head of the C_./C ₂ separation column is cooled with liquid methane which was separated from the cracked gas at temperatures below -95 ° C.

Method according to claim 2 or 3, characterized in that the portion of the methane to be recycled drawn off from the top of the C./C ₂ separation column is returned to the cracked gas.

Method according to one of ^' Claims 1 to 4, characterized in that the cracked gas is subjected to a multi-stage partial condensation after the C- ₊ separation, that the condensates occurring at temperatures down to about -80 ° C enter the C_. / C ₂ ~ separating column that the condensate obtained after further cooling to temperatures below about -95 ° 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 being used Cooling the head of the C./C ₂ separation column is used.

Method according to one of claims 1 to 5, characterized ge indicates that the Athan stream generated during the decomposition of the C ₂ ~ fraction is warmed up and drawn off as an insert for thermal cracking, the gas obtained during the thermal cracking is cooled, into the methanol crack ¬ gas and is disassembled together with this. - -

7 .. Method according to one of claims 1 to 6, characterized denotes ge that obtained in the C ₂ / C ₃₊ fraction C_ separation of a C _j / C, and the butchering of er¬ thereby maintained C ₃ Fraction of a propylene-propane separation subjected to separated propylene as the product and separated propane as the insert for a thermal cracking, the cracking gas obtained in the thermal cracking cooled, led into the cracking gas from the methanol cracking and together with this is disassembled.

8. The method according to any one of claims 1 to 7, characterized ge indicates that an additional thermal cleavage of gaseous or liquid hydrocarbons is provided under normal conditions and that cracking gas obtained in this cleavage is mixed with the usual cracking gas after cooling.

9. The method according to claim 7 and 8, characterized in that the separated propane is passed into the additional thermal cleavage.

10. The method according to any one of claims 6 to 9, characterized in that the methanol cracking gas is released at a higher pressure than the cracking gas of the thermal cracking (s) and that the cracking gas of the thermal cracking (s) is mixed with the methanol cracked gas during the cracking gas compression.

11. The method according to any one of claims 1 to 10, character- ized in that a wash for removing dimethyl ether and carbon dioxide is carried out to purify the methanol cracked gas.

CMPI

12. The method according to claim 11, characterized in that the washing is carried out at a pressure between 1 and 30 bar, preferably between 7 and 12 bar.

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13. The method according to claim 11 or 12, characterized gekenn¬ characterized in that the loaded detergent from the laundry is relaxed to a lower pressure and / or heated and gassing components are separated.

14. The method according to claim 13, characterized in that the detergent is expanded to the pressure of the methanol cleavage.

15. The method according to any one of claims 11 to 14, characterized in that the washing is carried out with methanol as a detergent.

16. The method according to claim 15, characterized in that at least a partial stream of the methanol to be split is used as a detergent and the loaded detergent is fed to the cleavage.

17. The method according to any one of claims 11 to 16, characterized in that the methanol cleavage is carried out in two stages and that the components outgassing from the loaded detergent are at least partially passed into the second stage of the methanol cleavage.

18. The method according to any one of claims 13 to 16, characterized in that when using methanol as a detergent, the remaining methane-rich liquid after degassing the loaded Metha¬ nols is passed into the first stage of the methanol cleavage.

OMPI -34 -

19. The method according to claim 17 or 18, characterized in that the ^' two stages of the methanol cleavage are connected in series, in the first stage a conversion of methanol into dimethyl ether and in the second stage a conversion of dimethyl ether into the cracked gas.

20 method ^" according to claim 17 or 18, characterized gekenn¬ characterized in that the two stages of methanol cleavage are connected in parallel, in the first stage a conversion of methanol into cracked gas and i in the second stage a conversion of dimethyl ether into cracked gas, whereupon the two fission gases are combined and further processed together.

21. The method according to claim 13, characterized in that the components degassed from the heated loaded detergent are subjected to a water wash and the wash water loaded with dimethyl ether is expanded and fed to the methanol cleavage.

22. The method according to claim 16 and 21, characterized gekennzeich¬ net that the wash water loaded with dimethyl ether is passed into the second stage of the cleavage.

23. The method according to any one of claims 1 to 22, characterized ge indicates that the cooling of the methanol cracked gas takes place at least partially with generation of steam under increased pressure, that in a parallel operated cracking plant under normal conditions gaseous or liquid hydrocarbons thermally be split, the cracked gas generated in this process is cooled at least partially with the generation of steam under increased pressure, the cooled cracked gases gases are optionally compressed and jointly fed to the cracking gas decomposition, the steam obtained in the cooling of the cracking gases relaxing while performing the work and the energy obtained in this way being used at least in part to compress the cracking gas.

24. The method according to any one of claims 1 to 23, characterized ge indicates that the heat of reaction of the Methanolspal¬ device is removed by indirect heat exchange with water boiling under pressure or by indirect

Heat exchange with a heat transfer medium, after which the heat transfer medium heated by the heat of reaction is cooled by indirect heat exchange with water boiling under pressure.

25. The method according to claim 24, characterized in that circulated salt melts are used as heat carriers.

26. The method according to claim 24, wherein the cooling of the cracked gas of the thermal cracking comprises a direct heat exchange with a quench oil, the quench oil being separated from the cracked gas after the heat exchange, cooled and partially circulated, characterized in that that a partial stream of quench oil is used as a heat carrier for the removal of the heat of reaction of the methanol cleavage.

27. The method according to claim 26, characterized in that the quench oil used as heat carrier for the removal of the heat of reaction of the methanol cleavage is conducted in the circuit, fresh quench oil being fed continuously to the circuit and a corresponding amount of excess quench oil being removed. ¹ 28. The method according to claim 26, characterized in that the quench oil withdrawn from the circuit is returned for cooling the cracked gas of the thermal cracking.

29. The method according to claim 27 or 28, characterized gekenn¬ characterized in that the circulated quench oil contains 3 to 30% by weight, preferably 5 to 25% by weight, in particular 10 to 20% by weight of fresh quenching oil.

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