PROCESS AND PLANT FOR PRODUCING OLEFINS FROM OXYGENATES

Abstract

The invention relates to a process and a plant for producing an olefins-containing hydrocarbon product by reaction of an oxygenates-containing reactant mixture, which is divided into a plurality of reactant mixture substreams, in a multi-stage oxygenate-to-olefin (OTO) synthesis reactor comprising a plurality of serially connected reaction sections comprising catalyst zones, wherein a feeding apparatus for a reactant mixture substream is arranged upstream of each catalyst zone. In each of these reaction sections a reactant mixture substream is introduced and therein under oxygenates conversion conditions converted into olefins and further hydrocarbons, wherein all reaction sections save for the first are additionally supplied with the product stream from the respective upstream reaction section. In addition at least one steam stream is introduced into at least one reaction section and at least one hydrocarbons-containing recycle stream is introduced into at least one reaction section. The OTO synthesis reactor product is fractionated in a multi-stage workup apparatus to obtain a plurality of hydrocarbon product fractions of which at least one is recycled to the OTO synthesis reactor as a recycle stream. According to the invention all reactant mixture substreams, steam streams and recycle streams are introduced into the OTO synthesis reactor in gaseous/vaporous form.

Claims

1. A process for producing an olefins-containing hydrocarbon product comprising ethylene and propylene by conversion of an oxygenates-containing reactant mixture, which is divided into a plurality of reactant mixture substreams, in a multi-stage oxygenate-to-olefin (OTO) synthesis reactor, the process comprising the following steps: (a) providing the multistage OTO synthesis reactor having a plurality of serially connected reaction sections in fluid connection with one another comprising a first reaction section and at least one subsequent reaction section which each contain catalyst zones comprising solid catalysts that are active and selective for OTO synthesis, wherein upstream of each catalyst zone a feeding apparatus for a reactant mixture substream is arranged and wherein the last reaction section in the direction of flow is in fluid connection with a conduit for discharging an OTO synthesis reactor product; (b) introducing a reactant mixture substream into each reaction section via the respective feeding apparatus, wherein the at least one subsequent reaction section is additionally supplied with the product stream from the respective upstream reaction section, introducing at least one steam stream into at least one reaction section, introducing at least one recycle stream into at least one reaction section; (c) at least partially converting the supplied oxygenates in the catalyst zones under oxygenate conversion conditions into olefins and further hydrocarbons, discharging the OTO synthesis reactor product, (d) separating the OTO synthesis reactor product in a multistage workup apparatus operating by means of thermal separation processes to obtain a plurality of hydrocarbons-containing hydrocarbon product fractions, (e) discharging an olefins-containing, in particular ethylene- and/or propylene-containing, hydrocarbon product from the workup apparatus, (f) recycling at least a portion of one or more hydrocarbon product fractions to the OTO synthesis reactor as a recycle stream or recycle streams and introducing the recycle stream(s) into at least one reaction section, wherein all reactant mixture substreams, steam streams, and recycle streams are introduced into the OTO synthesis reactor exclusively in gaseous/vaporous form.

2. The process according to claim 1, wherein all reaction sections are supplied with reactant mixture substreams on the one hand and with steam streams and/or recycle streams on the other hand.

3. The process according to claim 1, wherein at least two hydrocarbon product fractions are recycled to the OTO synthesis reactor as recycle streams and introduced thereto.

4. The process according to claim 1, wherein a hydrocarbon product fraction containing predominantly C.sub.2 to C.sub.8 hydrocarbons is introduced into the first reaction section as a recycle stream.

5. The process according to claim 1, wherein exclusively a hydrocarbon product fraction containing predominantly C.sub.2 to C.sub.4 hydrocarbons is introduced into the at least one subsequent section as a recycle stream.

6. The process according to claim 1, wherein the pressure drop over a feeding apparatus for a reactant mixture substream is less than 5 bar(a), preferably less than 3 bar(a).

7. The process according to claim 1, wherein the mass flow of the recycle stream and/or the mass flow of the steam is separately controlled or regulated for at least two reaction sections.

8. The process according to claim 1, wherein the oxygenate partial pressure inside the catalyst stage is between 0.1 and 0.5 bar(a).

9. The process according to claim 1, wherein the oxygenates-containing reactant mixture contains dimethyl ether (DME) and is produced in an etherification reactor by catalytic dehydration of methanol in the gas phase to obtain a gaseous etherification reactor product mixture comprising DME, steam and methanol vapour, wherein the gaseous etherification reactor product mixture is sent to the OTO synthesis reactor as reactant mixture without an additional separation step.

10. The process according to claim 9, wherein the oxygenates-containing reactant mixture has a DME content between 50% and 70% by weight, preferably between 55% and 60% by weight.

11. The process according to claim 9, wherein the absolute pressure of the oxygenates-containing reaction mixture before introduction into the OTO synthesis reactor is less than 7 bar(a), preferably less than 6 bar(a), and the temperature of the oxygenates-containing reaction mixture is set such that the temperature is at least 5 C., preferably at least 10 C., above the dew point at this pressure.

12. The process according to claim 9, wherein the absolute pressure of the oxygenates-containing reaction mixture before introduction into the OTO synthesis reactor is less than 7 bar(a), preferably less than 6 bar(a), and the temperature of the oxygenates-containing reaction mixture is at least 140 C., preferably at least 150 C.

13. A plant for producing an olefins-containing hydrocarbon product comprising ethylene and propylene by conversion of an oxygenates-containing reactant mixture, which is divided into a plurality of reactant mixture substreams, in a multi-stage oxygenate-to-olefin (OTO) synthesis reactor comprising the following constituents: (a) a multistage OTO synthesis reactor having a plurality of serially connected reaction sections in fluid connection with one another comprising a first reaction section and at least one subsequent reaction section which each contain catalyst zones comprising solid catalysts that are active and selective for OTO synthesis, wherein upstream of each catalyst zone a feeding apparatus for a reactant mixture substream is arranged and wherein the last reaction section in the direction of flow is in fluid connection with a conduit for discharging an OTO synthesis reactor product; (b) means for introducing a reactant mixture substream into each reaction section via the respective feeding apparatus, means for introducing at least one steam stream into at least one reaction section, means for introducing at least one recycle stream into at least one reaction section, (c) means for adjusting oxygenate conversion conditions, means for discharging the OTO synthesis reactor product, (d) a multi-stage workup apparatus operating by means of thermal separation processes and suitable for separating the OTO synthesis reactor product into a plurality of hydrocarbons-containing hydrocarbon product fractions, means for introducing the OTO synthesis reactor product into the workup apparatus, (e) means for discharging an olefins-containing, in particular ethylene- and/or propylene-containing, hydrocarbon product from the workup apparatus, (f) means for recycling at least a portion of one or more hydrocarbon product fractions obtained in the workup apparatus to the OTO synthesis reactor as a recycle stream or recycle streams and means for introducing the recycle stream(s) into at least one reaction section, wherein all means recited under (b) are configured such that all reactant mixture substreams, steam streams and recycle streams are introduceable into the OTO synthesis reactor in gaseous/vaporous form.

14. The plant according to claim 13, wherein all reaction sections are provided with means for introducing reactant mixture substreams on the one hand and with means for introducing steam streams and/or recycle streams on the other hand.

15. The plant according to claim 13, wherein the workup apparatus comprises a plurality of separation stages in which different hydrocarbon fractions are obtained and further comprises at least two recycle conduits which recycle from different separation stages to the OTO synthesis reactor and which are connected to different means for introducing recycle streams into the reaction sections.

16. The plant according to claim 15, wherein a first separation stage is connected to the first reaction section via a first recycle conduit and in that a second separation stage is connected to at least one subsequent reaction section via a second recycle conduit.

17. The plant according to claim 13, wherein it further comprises an etherification reactor arranged upstream of the OTO synthesis reactor which is configured such that by catalytic dehydration of methanol in the gas phase a gaseous, oxygenate-containing reactant mixture that can be sent to the OTO synthesis reactor without an additional separation step is obtainable.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0057] The invention is more particularly elucidated hereinbelow by way of an example without limiting the subject matter of the invention. Further features, advantages and possible applications of the invention will be apparent from the following description of the working example in conjunction with the drawings.

[0058] FIG. 1 shows a schematic diagram of an exemplary embodiment of the process according to the invention/the plant according to the invention,

[0059] FIG. 2 shows a schematic detailed diagram of the OTO synthesis reactor with the accompanying feed distribution system in an exemplary configuration.

DETAILED DESCRIPTION OF THE INVENTION

[0060] A preferred embodiment of the process according to an embodiment of the invention is characterized in that all reaction sections are supplied with reactant mixture substreams on the one hand and with steam streams and/or recycle streams on the other hand. These measures ensure a particularly uniform distribution of the reactant components over the reaction sections, thus resulting in very good temperature control in the catalyst zones. In addition, the partial pressure of the reactants is uniformly kept at a low level which results in a reduced partial pressure of the reactant components in the individual catalyst zones. The reduced partial pressure of the reactants increases the selectivity of the OTO reaction toward desired target components such as in particular ethylene and propylene. The reactants in the abovementioned context include not only the oxygenates supplied to the reaction sections but also the hydrocarbons, in particular olefins, recycled to the reaction sections via the recycle streams which may likewise be converted into the abovementioned target components.

[0061] In a further preferred embodiment of the process according to the invention at least two hydrocarbon product fractions are recycled to the OTO synthesis reactor as recycle streams and introduced thereto. This allows the different properties of different hydrocarbon product fractions to be better utilized. Thus, hydrocarbon product fractions containing low molecular weight, low-boiling hydrocarbons are more suitable as gaseous coolant streams than higher molecular weight, higher-boiling hydrocarbons owing to their low dew point. On the other hand the latter have a higher potential as reactive components since especially higher molecular weight olefins having carbon numbers greater than four are particularly easily converted by catalytic cracking over the OTO synthesis catalyst into low molecular weight olefins such as ethylene and propylene in particular. Nevertheless the hydrocarbons in the product fractions containing low molecular weight, low-boiling hydrocarbons are also partially converted into low molecular weight olefins, albeit to a lesser extent than olefin-containing fractions having higher molecular weight, higher-boiling hydrocarbons.

[0062] It is particularly preferable when a hydrocarbon product fraction containing predominantly C.sub.2 to C.sub.8 hydrocarbons is introduced into the first reaction section as a recycle stream. This fraction has both good coolant properties and a high proportion of higher molecular weight components such as olefins as reactive components. Introducing this fraction into the first reaction section is particularly advantageous since this maximizes the residence time of this fraction in the OTO reactor, thus allowing a particularly extensive conversion of the reactive components into low molecular weight olefins such as ethylene and propylene in particular.

[0063] In a development of the two abovementioned particular embodiments of the process according to the invention exclusively a hydrocarbon product fraction containing predominantly C.sub.2 to C.sub.4 hydrocarbons is introduced into the at least one subsequent reaction section as recycle stream.

[0064] This utilizes the good coolant properties of the low molecular weight, low-boiling hydrocarbons particularly effectively while simultaneously maximizing the residence time of the hydrocarbon product fraction containing predominantly C.sub.2 to C.sub.8 hydrocarbons in the OTO reactor, thus allowing a particularly extensive conversion of the reactive components into low molecular weight olefins such as ethylene and propylene in particular.

[0065] A further, preferred embodiment of the process according to the invention provides that the pressure drop over a feeding apparatus for a reactant mixture substream is less than 5 bar(a), preferably less than 3 bar(a). Operating experience and further investigations have shown that these pressure drops are markedly below those occurring when adding liquid/gaseous oxygenate mixtures using two-fluid nozzles in prior art processes. This makes it possible to use lower pressures in the DME reactor and the equipment parts arranged upstream thereof, thus allowing a more cost-effective design.

[0066] In a further aspect the process according to the invention is characterized in that the mass flow of the recycle stream and/or the mass flow of the steam is separately controlled or regulated for at least two reaction sections. Particularly flexible operation of the OTO synthesis reactor is thereby made possible and thermal fluctuations may be readily compensated.

[0067] A further preferred embodiment of the process according to the invention is characterized in that the oxygenate partial pressure inside the catalyst stage is between 0.1 and 0.5 bar(a). Investigations have shown that establishing these oxygenate partial pressures results in a particularly advantageous reactor productivity since this achieves a compromise between a high selectivity for short-chain olefins such as ethylene and propylene on the one hand (favoured by lowest possible oxygenate partial pressure) and a high oxygenate throughput on the other hand (favoured by highest possible oxygenate partial pressure).

[0068] In a further aspect the process according to the invention is characterized in that the oxygenates-containing reactant mixture contains dimethyl ether (DME) and is produced in an etherification reactor by catalytic dehydration of methanol in the gas phase to obtain a gaseous etherification reactor product mixture comprising DME, steam and methanol vapour, wherein the gaseous etherification reactor product mixture is sent to the OTO synthesis reactor as reactant mixture without an additional separation step. It is advantageous when there is no separation of the etherification reactor product mixture into a gas phase and a liquid phase that must be sent to the OTO synthesis reactor and applied thereto separately. This saves cooling energy, a corresponding separation apparatus is omitted and the conduit system is simplified.

[0069] In a further aspect the process according to the invention is characterized in that the oxygenates-containing reactant mixture contains dimethyl ether (DME) and is produced in an etherification reactor by catalytic dehydration of methanol in the gas phase to obtain a gaseous etherification reactor product mixture comprising DME, steam and methanol vapour, wherein the gaseous etherification reactor product mixture is sent to the OTO synthesis reactor as a reactant mixture without an additional separation step and wherein the oxygenates-containing reactant mixture has a DME content between 50% and 70% by weight, preferably between 55% and 60% by weight. Investigations have shown that these oxygenate contents in the reactant mixture may be particularly readily processed in the downstream OTO synthesis reactor.

[0070] In a further aspect the process according to the invention is characterized in that the oxygenates-containing reactant mixture contains dimethyl ether (DME) and is produced in an etherification reactor by catalytic dehydration of methanol in the gas phase to obtain a gaseous etherification reactor product mixture comprising DME, steam and methanol vapour, wherein the gaseous etherification reactor product mixture is sent to the OTO synthesis reactor as a reactant mixture without an additional separation step and wherein the absolute pressure of the oxygenates-containing reactant mixture before introduction into the OTO synthesis reactor is less than 7 bar(a), preferably less than 6 bar(a), and the temperature of the oxygenates-containing reactant mixture is set such that it is at least 5 C., preferably at least 10 C., above the dew point at this pressure. Investigations have shown that these process conditions allow long-lasting, stable operation of the process without premature catalyst deactivation and while simultaneously achieving a good yield of low molecular weight olefins such as ethylene and propylene in particular.

[0071] In a further aspect the process according to the invention is characterized in that the oxygenates-containing reactant mixture contains dimethyl ether (DME) and is produced in an etherification reactor by catalytic dehydration of methanol in the gas phase to obtain a gaseous etherification reactor product mixture comprising DME, steam and methanol vapour, wherein the gaseous etherification reactor product mixture is sent to the OTO synthesis reactor as a reactant mixture without an additional separation step and wherein the absolute pressure of the oxygenates-containing reactant mixture before introduction into the OTO synthesis reactor is less than 7 bar(a), preferably less than 6 bar(a), and the temperature of the oxygenates-containing reactant mixture is at least 140 C., preferably at least 150 C. Investigations have shown that these process conditions allow particularly long-lasting, stable operation of the process without premature catalyst deactivation and while simultaneously achieving a very good yield of low molecular weight olefins such as ethylene and propylene in particular.

[0072] In a further aspect the process according to the invention is characterized in that a first reaction section and five subsequent reaction sections are present.

[0073] In a further aspect the process according to the invention is characterized in that the conversion in the OTO synthesis reactor is carried out at temperatures of 300 C. to 600 C., preferably at temperatures of 360 C. to 550 C., most preferably at temperatures of 400 C. to 500 C.

[0074] In a further aspect the process according to the invention is characterized in that the conversion in the OTO synthesis reactor is carried out at pressures of 0.1 to 20 bar, absolute, preferably at pressures of 0.5 to 5 bar, absolute, most preferably at pressures of 1 to 3 bar, absolute.

[0075] In a further aspect the process according to the invention is characterized in that the catalyst zones in the reaction sections contain a granular, shape-selective zeolite catalyst of the pentasil type, preferably ZSM-5, in the form of a fixed bed.

[0076] In a particular aspect of the plant according to the invention all reaction sections are provided with means for introducing reactant mixture substreams on the one hand and with means for introducing steam streams and/or recycle streams on the other hand. These constructional features ensure a particularly uniform distribution of the reactant components over the reaction sections, thus resulting in very good temperature control in the catalyst zones. In addition, the partial pressure of the reactants is uniformly kept at a low level which results in a reduced partial pressure of the reactant components in the individual catalyst zones. The reduced partial pressure of the reactants increases the selectivity of the OTO reaction toward desired target components such as in particular ethylene and propylene. The reactants in the abovementioned context include not only the oxygenates supplied to the reaction sections but also the hydrocarbons, in particular olefins, recycled to the reaction sections via the recycle streams which may likewise be converted into the abovementioned target components.

[0077] It is preferable when the plant according to the invention comprises a workup apparatus having a plurality of separation stages in which different hydrocarbon fractions are obtained and further comprises at least two recycle conduits which recycle from different separation stages to the OTO synthesis reactor and which are connected to different means for introducing recycle streams into the reaction sections. This makes it possible for at least two hydrocarbon product fractions to be recycled to the OTO synthesis reactor as recycle streams and introduced thereto. This allows the different properties of different hydrocarbon product fractions to be better utilized. Thus, hydrocarbon product fractions containing low molecular weight, low-boiling hydrocarbons are more suitable as gaseous coolant streams than higher molecular weight, higher-boiling hydrocarbons owing to their low dew point. On the other hand the latter have a higher potential as reactive components since especially higher molecular weight olefins having carbon numbers greater than four are particularly easily converted by catalytic cracking over the OTO synthesis catalyst into low molecular weight olefins such as ethylene and propylene in particular. Nevertheless the hydrocarbons in the product fractions containing low molecular weight, low-boiling hydrocarbons are also partially converted into low molecular weight olefins, albeit to a lesser extent than olefin-containing fractions having higher molecular weight, higher-boiling hydrocarbons.

[0078] In the finally elucidated embodiment it is particularly preferable when a first separation stage is connected to the first reaction section via a first recycle conduit and when a second separation stage is connected to at least one subsequent reaction section via a second recycle conduit. This makes it possible in particular for a hydrocarbon product fraction containing predominantly C.sub.2 to C.sub.8 hydrocarbons to be introduced into the first reaction section via the first recycle conduit as recycle stream and for exclusively a hydrocarbon product fraction containing predominantly C.sub.2 to C.sub.4 hydrocarbons to be introduced into the at least one subsequent reaction section via the second recycle conduit as recycle stream. This utilizes the good coolant properties of the low molecular weight, low-boiling hydrocarbons in the carbon number range C.sub.2 to C.sub.4 particularly effectively while simultaneously maximizing the residence time of the hydrocarbon product fraction containing predominantly C.sub.2 to C.sub.8 hydrocarbons in the OTO reactor, thus allowing a particularly extensive conversion of the reactive components into low molecular weight olefins such as ethylene and propylene in particular.

[0079] A further aspect of the plant according to the invention is characterized in that it further comprises an etherification reactor arranged upstream of the OTO synthesis reactor which is configured such that by catalytic dehydration of methanol in the gas phase a gaseous, oxygenates-containing reactant mixture that can be sent to the OTO synthesis reactor without an additional separation step is obtainable. It is advantageous when there is no separation of the etherification reactor product mixture into a gas phase and a liquid phase that must be sent to the OTO synthesis reactor and applied thereto separately. This saves cooling energy, a corresponding separation apparatus is omitted and the conduit system is simplified.

[0080] FIG. 1 shows a schematic diagram of an exemplary embodiment of the process according to the invention/the plant according to the invention for producing an olefins-containing hydrocarbon product comprising in particular the short-chain olefins ethylene and propylene as value products by conversion of an oxygenates-containing reactant mixture. To produce the oxygenates-containing reactant mixture initially methanol vapour, optionally in conjunction with steam as diluent, is applied via conduit 1 to the dehydration reactor (DME reactor) 2 which has been filled with a dumped fixed bed of a commercially available dehydration catalyst. Effected over this catalyst is a heterogeneously catalysed partial conversion of the methanol to dimethyl ether (DME) under dehydration conditions known to those skilled in the art.

[0081] In certain embodiments of the present invention, the obtained gaseous product mixture from the dehydration reactor, which comprises not only DME but also unconverted methanol and steam, can be applied without cooling and phase separation but rather still in gaseous form by means of conduit 3 directly to the OTO synthesis reactor 6, which in the present case comprises six reaction sections. Division into six reactant mixture substreams and distribution thereof to the six reaction sections is carried out using the conduit system 3a to 3f. In addition, via the conduit system 4a to 4f steam may be supplied and likewise distributed over the six reaction sections. Finally, via the conduit system 5a to 5f a gas stream containing predominantly C.sub.2 hydrocarbons is recycled to the OTO synthesis reactor and distributed over the six reaction sections. The gas distributor system shown in FIG. 1 is to be understood as being purely schematic. In particular embodiments the individual gas typesreactant mixture, steam, hydrocarbon recycle streammay be applied to the reaction sections either separately or premixed. Premixing of the gas streams is preferable since this reduces the partial pressure of the reactive components, thus resulting in improved temperature management of the OTO synthesis reactor and improved selectivity for short-chain olefins. Possible operating modes of the reactor include those in which said reactor is supplied either with oxygenate-containing reactant mixture and steam as diluent or with oxygenate-containing reactant mixture and a hydrocarbon recycle stream as diluent or with oxygenate-containing reactant mixture and both steam and a hydrocarbon recycle stream as diluent. The latter operating mode is preferred especially when the steam content in conduit 3 and the amount of the hydrocarbon recycle stream are not yet sufficient to allow adequate temperature control and partial pressure adjustment in the reaction sections. It provides the greatest flexibility among the elucidated operating modes.

[0082] It is also possible as a particular embodiment of the invention to supply steam and hydrocarbon to one or more reaction sections, the oxygenate content being reduced to zero in extreme cases save for a small value. This optimizes the conversion of specific recycle streams or else hydrocarbon-containing streams from other processes may be incorporated. It is especially preferred when these reactant mixture substreams are added to the downstream reaction sections of the OTO reactor, particularly preferably to the last reaction section, having an oxygenate content that has been reduced or reduced to zero.

[0083] Supply of the first reaction section with C.sub.2 hydrocarbons via the conduit path 5a may optionally also be omitted since the first reaction section is already being supplied with a hydrocarbon recycle stream via conduit 22.

[0084] The conversion of the oxygenates and hydrocarbon reactive components in the reaction sections of the OTO synthesis reactor is effected under oxygenate conversion conditions known to those skilled in the art and disclosed in the relevant literature. To this end the reaction sections are provided with catalyst zones provided with fixed dumped beds of a commercially available olefin synthesis catalyst.

[0085] The product mixture of the OTO synthesis reactor is discharged therefrom via conduit 7 and supplied to the multistage product workup which is shown in FIG. 1 merely in highly schematic form and is subsequently elucidated only to the extent required for understanding the present invention. Initially carried out in quench stage 8 is a cooling of the product mixture below the dew point and subsequently a phase separation into an aqueous phase discharged via conduit 9 as well as into a gaseous phase and into a liquid phase which each contain predominantly hydrocarbons, are discharged via conduits 10 and 11 from the quench stage and are both applied to a distillation column 12 known as a debutanizer.

[0086] The debutanizer distillation column 12 separates the hydrocarbon stream supplied via conduits 10 and 11 by fractionating distillation. Discharged from the column 12 as the bottoms product is a hydrocarbon fraction containing hydrocarbons having four or more carbon atoms (C.sub.4+ fraction). Said fraction is supplied via conduit 13 to a workup apparatus for heavy hydrocarbon fractions 14. The further separation of the hydrocarbon mixture is carried out therein by means of a plurality of separating operations, for example multistage distillation, extraction, extractive distillation.

[0087] The tops product from the column 12 forms a hydrocarbon fraction containing hydrocarbons having four or less carbon atoms (C.sub.4 fraction). This fraction also contains hitherto unconverted oxygenates. It is discharged from column 12 via conduit 15 and applied to a distillation column 16 known as a depropanizer.

[0088] The depropanizer distillation column 16 separates the hydrocarbon stream supplied via conduit 15 by fractionating distillation. Discharged from the column 16 as the bottoms product is a hydrocarbon fraction containing hydrocarbons having four carbon atoms and unconverted oxygenates (C.sub.4O fraction). Said fraction is supplied via conduit 17 to the workup apparatus for heavy hydrocarbon fractions 14. The further separation of the hydrocarbon mixture is carried out therein by means of a plurality of separating operations, for example multistage distillation, extraction, extractive distillation.

[0089] The tops product from the column 16 forms a hydrocarbon fraction containing hydrocarbons having three or less carbon atoms (C.sub.3 fraction). It is discharged from column 16 via conduit 18 and applied to a distillation column 19 known as a deethanizer.

[0090] The deethanizer distillation column 19 separates the hydrocarbon stream supplied via conduit 18 by fractionating distillation. Discharged from the column 19 as a bottoms product is a hydrocarbon fraction which comprises hydrocarbons having three carbon atoms and thus comprises not only propane but also the target product propylene. It is supplied via conduit 20 to a workup apparatus (not shown) in which propane and propylene are separated by distillation and which contains optionally further workup stages so that the target product propylene is obtainable in pure form.

[0091] The tops product from the column 19 forms a hydrocarbon fraction containing hydrocarbons having two or less carbon atoms (C.sub.2 fraction). It is discharged from column 19 via conduit 5 and after further optional workup or conditioning steps (not shown) is separated into a substream which is discharged from the process as a purge stream via a conduit (not shown). If desired, ethylene may also be obtained from the purge stream as a pure product by workup steps that are known per se. From the remaining proportion a smaller substream is removed as purge and the remaining stream of the C.sub.2 fraction is recycled to the OTO synthesis reactor via conduit 5.

[0092] The OTO synthesis reactor 200 shown schematically in FIG. 2 for conversion of DME into olefins is in the form of a fixed bed reactor having a plurality of reaction sections 200a-200f which each contain zones of a catalyst reactive and selective for OTO synthesis. It is advantageous to provide at least three, preferably at least four, most preferably, as shown in FIG. 2, six, catalyst stages. This embodiment of the OTO synthesis reactor is an advantageous compromise. Yet more reaction sections would further reduce the reaction enthalpy liberated per section and would therefore be advantageous for temperature control of the reactor; however the increasing capital costs and increasing control complexity would be disadvantageous.

[0093] Supplying with dimethyl ether as the oxygenate is carried out by dividing the reactant stream in conduit 201 into the individual reactant substreams in conduits 201a to 201f Simultaneously via conduits 211a to 211f all reaction sections are supplied with a C.sub.2 hydrocarbons-containing recycle gas; as elucidated with reference to FIG. 1 this may be a substream of the tops product from the deethanizer. Furthermore, via conduit 212 the first reaction sections are supplied with a C.sub.4 to C.sub.6 hydrocarbons-containing recycle gas obtained by working up the bottoms products from the debutanizer and the depropanizer. The latter may also contain proportions of unconverted DME which are likewise recycled to the OTO synthesis reactor. All of the streams applied to the reactor 200 may be combined also with steam; alternatively or in addition steam may be added to one or more reaction sections via feed conduits (not shown). This is advantageous especially when the steam stream is to be controlled separately from the reactant substreams or recycle streams for improved temperature control. It is essential and characterizing to the invention that all of these material streams are applied to the OTO synthesis reactor in gaseous form. This may be achieved for example by choosing the temperature for the C2 hydrocarbons-containing recycle gas of between 0 C. and 50 C. and for the steam of between 100 C. and 220 C. Due to the proportion of higher-boiling hydrocarbons the temperature of the C4- to C6-hydrocarbons-containing recycle gas must be higher than that of the first recycle gas; it is essential that the temperature is safely above the dew point which depends on the precise composition of the fraction.

[0094] The individual reaction sections are arranged in series. By mixing the cold input gas with the hot product gas exiting the preceding catalyst stage the latter is cooled and may therefore react in the desired temperature range with the admixed dimethyl ether and the reactive components in the recycle gas in the subsequent reaction stage.

[0095] Mixing of a dimethyl ether-containing reactant substream and recycle gas is shown exemplarily in the last stage 200f. A flow controller 203b and the control valve 203a assigned thereto are used to adjust the reactant substream such that the desired oxygenate amount is introduced into the reaction section 200f. The cold reactant substream supplied via conduit 201f does already achieve a certain cooling when this stream mixes with the product stream from the upstream reaction section 200e. In addition, C2-containing recycle gas and/or steam may be added via valve 204a so that via the temperature controller 204b the desired target temperature of the exit stream from the reaction section is also achieved.

[0096] This temperature and reaction management concept is advantageously implemented in the same way for all other reaction sections but at least for the reaction sections 2 to 6. The entry and exit temperatures for the respective stage are flexible and easily adjustable via the quantity ratio of the respective DME and recycling streams. It is thus possible to establish over the entire reactor a temperature profile optimal for a maximum ethylene and/or propylene yield.

Numerical Examples

[0097] Specifically a reactor as shown in FIG. 2 may be advantageously operated with the following settings:

[0098] A preselected temperature level may be established over the reaction sections 200a and 200e of the reactor and in the next reaction section additional cooling with oxygenate, a recycle gas consisting predominantly of C.sub.2 hydrocarbons having a preferred temperature between 120 C. and 160 C. and/or process steam may be minimized. The temperatures in the reaction sections 200a and 200e are preferably between 470 C. and 500 C. All of the material streams added to the reaction sections are gaseous and were measured such that per reaction section virtually the same temperature increase is obtained as in a process according to the prior art with the same six-stage reactor but biphasic supply of the reactant mixture in gas/liquid form via two-fluid nozzles.

[0099] In the last reaction section 200f a reduced conversion of DME/a largely flat temperature interval is established over the reaction section. According to the invention the temperature profile in the reaction section 200f then varies for example between 480 C. and 500 C. Thus at maximum temperature and low reformation from DME a very largely complete reaction of the C.sub.2 to C.sub.4 olefins present in the reaction gas to afford propylene is achieved. Comparable settings are possible in a prior art configuration of the reactor only to a limited extent since the exothermicity of the corresponding reaction in the presence of oxygenates requires low entry temperatures.

[0100] Cooling the oxygenate and recycle gas streams to 120 C. to 160 C. results in efficient cooling of the product gas upon introduction of the oxygenate recycle gas mixture into the reaction sections. In example 1 reported hereinbelow in table 1 the use of about 84% by weight of ethylene in the recycle gas and without steam introduction results in the process data summarized therein with regard to cooling in the individual reaction sections.

[0101] In place of the above-described recycle gas stream cooling may also be achieved by admixing process steam with the oxygenate stream via a separate side feed before application to the respective reaction section as reported hereinbelow in table 2 as example 2.

[0102] A further option in a further embodiment of the invention is that of combining a recycle gas stream and a process steam stream with an oxygenate stream and applying them to a reaction section together as shown hereinbelow in table 3 as example 3.

TABLE-US-00001 TABLE 1 Cooling of individual reaction sections using DME and C.sub.2 recycle gas (Example 1) Cooling demand Cooling Cooling by upstream of by DME C.sub.2 recycle section (gas) gas Reaction section #1 0% Reaction section #2 100% 95.3% 4.7% Reaction section #3 100% 88.9% 11.1% Reaction section #4 100% 84.0% 16.0% Reaction section #5 100% 80.4% 19.6% Reaction section #6 100% 77.6% 22.4%

TABLE-US-00002 TABLE 2 Cooling of individual reaction section using DME and process steam (Example 2) Cooling demand Cooling Cooling by upstream of by DME process section (gas) steam Reaction section #1 0% Reaction section #2 100% 89.0% 11.0% Reaction section #3 100% 84.7% 15.3% Reaction section #4 100% 81.8% 18.2% Reaction section #5 100% 79.7% 20.3% Reaction section #6 100% 78.3% 21.7%

[0103] The lower entry temperatures also reduce the partial pressures of the individual reactants as summarized for the above three examples and compared with a prior art embodiment hereinbelow in table 4.

[0104] Under otherwise comparable conditions an OTO plant based on a gaseous DME reactant stream and gaseous diluents can achieve an up to 2% higher propylene selectivity than a comparative plant according to the prior art. The selectivity increase is achieved due to the abovementioned reduction in the partial pressure of the reactive components and also due to the fact that the reaction temperatures can be kept in the optimal range in the individual reaction sections by the temperature management according to the invention.

TABLE-US-00003 TABLE 3 Cooling of individual reaction section using DME, C.sub.2-recycle gas and process steam (Example 3) Cooling demand Cooling Cooling by Cooling by upstream of by DME C.sub.2 recycle process section (gas) gas steam Reaction 0% section #1 Reaction 100% 87.3% 5.9% 6.8% section #2 Reaction 100% 81.2% 12.4% 6.4% section #3 Reaction 100% 76.5% 17.4% 6.1% section #4 Reaction 100% 73.0% 21.1% 5.9% section #5 Reaction 100% 70.3% 23.9% 5.7% section #6

TABLE-US-00004 TABLE 4 Partial pressures of the reactants upon entry and exit for individual reaction sections (all pressures in bar(a)). Compar- P.sub.React = reactants ative ex. Example 1 Example 2 Example 3 partial pressure (prior art) (invention) (invention) (invention) P.sub.React to 0.438 bar 0.320 bar 0.486 bar 0.322 bar section #1 P.sub.React from 0.357 bar 0.250 bar 0.395 bar 0.251 bar section #1 P.sub.React to 0.410 bar 0.318 bar 0.446 bar 0.319 bar section #2 P.sub.React from 0.331 bar 0.249 bar 0.359 bar 0.248 bar section #2 P.sub.React to 0.381 bar 0.317 bar 0.404 bar 0.317 bar section #3 P.sub.React from 0.303 bar 0.246 bar 0.321 bar 0.245 bar section #3 P.sub.React to 0.351 bar 0.315 bar 0.363 bar 0.312 bar section #4 P.sub.React from 0.274 bar 0.241 bar 0.283 bar 0.239 bar section #4 P.sub.React to 0.321 bar 0.309 bar 0.322 bar 0.305 bar section #5 P.sub.React from 0.245 bar 0.234 bar 0.246 bar 0.231 bar section #5 P.sub.React to 0.292 bar 0.284 bar 0.302 bar 0.297 bar section #6 P.sub.React from 0.216 bar 0.210 bar 0.224 bar 0.220 bar section #6

LIST OF REFERENCE NUMERALS

[0105] 1 conduit [0106] 2 DME reactor [0107] 3-5 conduit [0108] 6 OTO synthesis reactor [0109] 7 conduit [0110] 8 quench [0111] 9-11 conduit [0112] 12 separating column (debutanizer) [0113] 13 conduit [0114] 14 workup apparatus [0115] 15 conduit [0116] 16 separating column (depropanizer) [0117] 17-18 conduit [0118] 19 separating column (deethanizer) [0119] 20-22 conduit [0120] 200 OTO synthesis reactor [0121] 201 conduit [0122] 203a, 204a control valve [0123] 203b flow meter [0124] 204b temperature measurement [0125] 205 conduit [0126] 211-212 conduit