Process and plant for producing olefins from oxygenates

Abstract

A process for producing olefins from oxygenates includes the following steps: (i) heterogeneously catalyzed conversion of at least one oxygenate to a stream containing propylene, aromatics and cyclic olefins, (ii) at least partial hydrogenation of the aromatics and cyclic olefins to naphthenes, and (iii) at least partial recirculation of the naphthenes into the heterogeneously catalyzed conversion.

Claims

1. A process for producing olefins from oxygenates, the process comprising the following steps: (i) converting, by heterogeneous catalysis, at least one oxygenate to a stream containing propylene, aromatics, and cyclic olefins; (ii) separating the stream containing propylene, aromatics, and cyclic olefins into a C.sub.5 fraction containing propylene, a C.sub.5+ fraction containing aromatics and cyclic olefins, and an aqueous fraction containing oxygenates, wherein at least parts of the C.sub.5+ fraction are supplied to a hydrogenation in step (iii); (iii) at least partially hydrogenating the at least parts of the C.sub.5+ fraction containing aromatics and cyclic olefins to obtain naphthenes; and (iv) at least partially recirculating the naphthenes into the heterogeneously catalyzed conversion in step (i).

2. The process according to claim 1, wherein hydrogen is used as hydrogenating agent in step (iii).

3. The process according to claim 2, wherein the hydrogenation is carried out such that parts of a product stream of the hydrogenation are recirculated into the hydrogenation, and that a ratio between the recirculated product stream of the hydrogenation and the at least parts of the C.sub.5+ fraction containing aromatics and cyclic olefins lies between 1:10 and 10:1 and/or a molar excess of hydrogen lies between 200 and 5000% of the quantity theoretically necessary for the complete saturation of all contained double and aromatic bonds.

4. The process according to claim 2, wherein after step (iv) hydrogen is separated from a product stream of the hydrogenation.

5. The process according to claim 1, wherein from the aqueous fraction containing oxygenates, the oxygenates and water are separated.

6. The process according to claim 5, wherein the separated oxygenates and/or the separated water are/is at least partly recirculated into step (i).

7. The process according to claim 6, wherein the heterogeneously catalyzed conversion is effected in two stages, wherein in the first stage methanol is converted into dimethyl ether and in the second stage dimethyl ether is converted to a stream containing propylene, aromatics and cyclic olefins, and wherein methanol is recirculated to before the first stage and/or water is recirculated in the form of steam to before the second stage.

8. The process according to claim 1, wherein step (ii) further comprising separating the C.sub.5+ fraction into a C.sub.7 fraction and a C.sub.7+ fraction, and at least parts of the C.sub.7+ fraction are supplied to step iii and at least parts of the C.sub.7 fraction are recirculated to step (i).

9. A process for producing olefins from oxygenates, the process comprising the following steps: (i) converting, by heterogeneous catalysis, at least one oxygenate to a stream containing propylene, aromatics, and cyclic olefins; (ii) separating the stream containing propylene, aromatics, and cyclic olefins into a C.sub.7 fraction containing propylene and a C.sub.7+ fraction containing aromatics and cyclic olefins, wherein at least parts of the C.sub.7+ fraction are supplied to a hydrogenation in step (iii); (iii) at least partially hydrogenating the at least parts of the C.sub.7+ fraction containing aromatics and cyclic olefins to obtain naphthenes; and (iv) at least partially recirculating the naphthenes into the heterogeneously catalyzed conversion in step (i).

10. The process according to claim 9, wherein at least parts of the C.sub.7 fraction are recirculated into step (i).

11. The process according to claim 9, wherein the heterogeneously catalyzed conversion is effected in two stages, wherein in the first stage methanol is converted into dimethyl ether and in the second stage dimethyl ether is converted to a stream containing propylene, aromatics and cyclic olefins, and wherein at least parts of the C.sub.7 fraction are recirculated after the first stage and before the second stage.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, claims, and accompanying drawings. It is to be noted, however, that the drawings illustrate only several embodiments of the invention and are therefore not to be considered limiting of the invention's scope as it can admit to other equally effective embodiments.

(2) FIG. 1 shows the schematic representation of a usual MTP process, and

(3) FIG. 2 shows the schematic representation of the process according to an embodiment of the invention.

DETAILED DESCRIPTION

(4) FIG. 1 shows the MTP production according to the prior art. Via conduits 1 and 2, methanol is introduced into a reactor 3 in which the methanol is at least partly converted to dimethyl ether. Via conduits 4, 5 and 6, the dimethyl ether is withdrawn and supplied to a second reactor 7 in which the dimethyl ether together with steam is converted to olefins. The olefin stream thus obtained contains propylene and other olefins, but also cyclic olefins and aromatics.

(5) Via conduit 8, the product stream obtained is introduced into the cooling device 9. There, a gaseous phase separates from a liquid phase. The gaseous phase contains the C.sub.5 fraction and is supplied to a compressor 11 via conduit 10. The gaseous fraction obtained in the compressor 11 is supplied to a distillation column 14 via the conduits 12 and 13. In this distillation column 14, the C.sub.3 fraction is separated from the C.sub.4+ fraction.

(6) Via conduit 20, the C.sub.3 fraction is supplied to a column 21 in which the C.sub.2 fraction is withdrawn over the head. Via conduit 22 and conduit 52, the C.sub.2 fraction gets back into conduit 5 and from there can be guided via conduit 6 into the reactor 7, so that here the desired product propylene is at least partly produced by olefin interconversion. To avoid an enrichment of inert light gaseous components such as methane or CO.sub.x in the circuit, a small partial quantity of the stream from conduit 22 can be removed from the system as purge via a non-illustrated conduit. Furthermore, the C.sub.3 fraction is withdrawn from the column 21 via conduit 23 and supplied to a column 24. In this column 24, the desired target product propylene is distilled off over the head and withdrawn via conduit 25, while in the bottom further compounds with three carbon atoms are left and are withdrawn via conduit 26.

(7) Via conduit 15, the bottom product of the column 14 is withdrawn from the column 14 as C.sub.4 fraction, and via the conduits 51 and 52 it is likewise recirculated to before the conversion of the ether to olefins in conduit 5, in order to further increase the yield of propylene by olefin interconversion. To avoid an enrichment of butane (a component inert for the conversion in the reactor) in the circuit, a small partial quantity of the stream from conduit 15 can be removed from the system as purge via a non-illustrated conduit.

(8) The liquid fraction obtained in the cooler 9 is supplied to a separator 31 via conduit 30. The aqueous phase separated in the separator 31 also contains oxygenates (when using methanol as educt, above all methanol) and is supplied to a column 33 via conduit 32.

(9) From the bottom of the column 33 water is discharged via conduit 34. Furthermore, steam is withdrawn from the column 33 via conduit 36 and fed into conduit 4, from where the steam gets into the reactor 7 via conduit 5 and conduit 6, in which reactor it is used as educt for the conversion of the dimethyl ether to olefins.

(10) The top product of the column 33, at least one oxygenate, preferably methanol, is fed into the conduit 1 via conduit 35 and thus gets into the reactor 3 via conduit 2. When methanol is used as educt, recovered methanol together with the methanol fed in as educt thus is converted to dimethyl ether. Alternatively, the oxygenate also can directly be returned into the reactor 7 together with the steam via conduit 36.

(11) The organic phase withdrawn from the separator contains the C.sub.5+ fraction, which is discharged via conduit 41 and passed on via a pump (not shown). To this C.sub.5+ fraction, the liquid fraction obtained from the compressor 11 at 15-25 bar then is also admixed via conduit 40. The combined streams then are introduced into a column 43 via conduit 42. From the head of the column 43, the C.sub.4 fraction is introduced via conduit 44 into the conduit 12, from where it is fed into the column 14 together with the gaseous part from the compressor 11 via conduit 13.

(12) Via conduit 45, the bottom product of the column 43, which contains the C.sub.5+ fraction, is guided into the column 46. From the bottom of the column 46, the C.sub.7+ fraction is withdrawn into the conduits 47 and 48.

(13) Over the head of the column 46, the C.sub.5/C.sub.6 fraction obtained is recycled via the conduits 49, 50, 51 and 52, in that it is recirculated into the conduit 5. Parts of the C.sub.5 and C.sub.6 fraction are supplied to the conduit 47 via conduit 53 and discharged from the process via conduit 48 (purge). The stream leaving the process via conduit 48 represents the MTP gasoline.

(14) FIG. 2 schematically shows the procedure of the process according to the invention. Up to the plant component 45, the process according to the invention corresponds to the process already known from the prior art.

(15) Via conduit 45, the stream however is introduced into a column 60 in which the C.sub.7 fraction instead of the C.sub.5/C.sub.6 fraction is withdrawn over the head. In an advantageous configuration of the column 60, this column 60 is operated as extractive distillation and supplied with an additional stream which has advantageous chemical and physical properties, so that an even better separation between olefins in the head and aromatics and cyclic olefins in the bottom is possible. The stream used as extracting agent for example can be an olefin or a stream rich in aromatics, which preferably is produced and recirculated within the plant. The operating principle is based on the fact that either the olefins in the head or the aromatics in the bottom are enriched.

(16) Via the conduits 80, 81 and 83, the C.sub.7 fraction is recirculated into the conduit 5 and thus before the reactor 7 for the conversion of the dimethyl ether to olefins, so that this stream can be subjected to an olefin interconversion. A small partial quantity of the stream 80 is removed from the system as purge via conduit 82, in order to limit the enrichment of inert components such as hexanes and heptanes in the circuit.

(17) Due to the recirculation of the C.sub.7 stream, the propylene-yield olefin interconversion on the one hand is increased analogous to the following model reaction equation:
C.sub.7H.sub.14.fwdarw.C.sub.3H.sub.6+C.sub.4H.sub.8

(18) The propylene yield also is increased indirectly, since the conversion of the C.sub.7 olefins proceeds endothermally, which in an advantageously adiabatically operated reactor reduces the increase in temperature, so that the selectivity of the total conversion to propylene is increased.

(19) Via conduit 61, the C.sub.7+ fraction is separated from the column 60, which then is supplied to the hydrogenation reactor 63 via conduit 62. The liquid hydrocarbon stream is heated to 20 to 250 C. and via a pressure of 2 to 45 bar mixed with hydrogen. The stream resulting therefrom then is introduced into a reactor 63 filled with a suitable hydrogenation catalyst.

(20) The reactor 63 for example can be a constructively simple fixed-bed reactor, but there can also be used reactors with internal cooling, of the single- or multistage type. Noble metals just like nickel, palladium, platinum or mixtures thereof on carrier materials such as activated carbon, silica or alumina can be used as catalysts.

(21) To ensure an advantageous configuration of the process in energetic terms, the hydrogenation reactor 63 is operated at approximately the same temperature as the bottom of the column 60, whereby an otherwise necessary heat exchanger between these two plant sections can be omitted, whereby the investment and operating costs of the process are reduced and the economy is improved.

(22) After passage through the reactor 63, the cyclic olefins and the aromatics have been hydrogenated to naphthenes. Existing open-chain paraffins largely show an inert behavior in the hydrogenation. Possibly left small residual amounts of open-chain olefins are hydrogenated to the corresponding open-chain paraffins.

(23) Via conduit 64, the product obtained is supplied to a cooling 65. In this cooling 65, the liquid phase at the same time is separated from the gas phase. The liquid phase is withdrawn from the cooling 65 via conduit 70. Since the hydrogenation is very exothermal, a part of the liquid phase is guided back into the hydrogenation reactor 63 via conduit 71. A dilution of the reaction mixture and thus an uncontrolled heating thereby can be avoided.

(24) Via conduits 73 and 74, parts of this stream are discharged together with the partial stream 82 of the C.sub.7 product, in order to control the enrichment of inert components in the circuit. Due to its chemical composition of largely paraffinic components, the stream 74 also can be referred to as naphtha. The remaining stream of the cycloparaffins is recirculated into conduit 5 via conduits 75, 81, 82 and 83.

(25) The gaseous part from the cooling stage 65 is fed back into conduit 61 via conduit 66, from where it gets into the hydrogenation reactor 63 via conduit 62. Since this hydrogen stream also contains formed light gases such as methane, a partial stream also must again be removed from the circuit at this point via conduit 67 (purge).

(26) Due to the two recirculations via conduit 71 and conduit 66, a typical quantity ratio of 1 to 10 tons of hydrogenated liquid product per ton of non-hydrogenated feed is obtained in the hydrogenation reactor 63, and a molar excess of hydrogen in the amount of 2 to 50 times the theoretical hydrogen quantity required for the complete saturation of all double and aromatic bonds.

(27) The liquid product which is withdrawn via conduit 70 substantially consists of open-chain and cyclic paraffins, which upon recirculation into the reactor 7 are converted to propylene and other short-chain olefins. The open-chain paraffins, which likewise are contained in this stream, are inert and serve the dilution of the reaction mixture, whereby the amount of the added steam can be reduced. This possible reduction of the steam addition has the additional advantage to prolong the useful life of the catalyst, since its irreversible deactivation by dealuminization at the lower steam partial pressure thus obtained is slowed down.

(28) The proportions of the streams discharged from the process usually are less than 10 vol-%, based on the respective total stream, i.e. e.g. 82/80<10 vol-%, 73/75<10 vol-%, 68/66<10 vol-%.

(29) The process shown in FIG. 2 has the advantage that the core system of a usual MTP process as shown in FIG. 1 remains the same and substantially need not newly be designed in terms of engineering. As a result, the quantity of the original MTP gasoline is distinctly reduced and there is obtained a smaller amount of naphtha instead of the usually contained MTP gasoline. The propylene yield of the entire process can distinctly be increased thereby.

Example

(30) Table 1 summarizes the mass balances over the respective plant limits for the prior art process (FIG. 1) and for the improvement according to the invention (FIG. 2):

(31) TABLE-US-00001 TABLE 1 Process Process acc. to FIG. 1 acc. to FIG. 2 [t/h] [t/h] Feed Methanol 208.3 Methanol 208.3 Products Propylene 59.3 Propylene 71.3 MTP gasoline 21.5 MTP naphtha 7.4 Others* 10.4 Others* 12.5 Water 117.1 Water 117.1 *LPG (=C.sub.3/C.sub.4 paraffins and olefins) and so-called light ends (i.e. methane, CO.sub.x, ethane and ethylene)

(32) It can clearly be seen that the improvement of the process according to the invention leads to a distinct increase of the propylene yield.

(33) While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

(34) The singular forms a, an and the include plural referents, unless the context clearly dictates otherwise.

(35) Comprising in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of comprising). Comprising as used herein may be replaced by the more limited transitional terms consisting essentially of and consisting of unless otherwise indicated herein.

(36) Providing in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary.

(37) Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

(38) Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.

(39) All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited.

LIST OF REFERENCE NUMERALS

(40) 1, 2 conduit 3 reactor 4-6 conduit 7 reactor 8 conduit 9 cooler 10 conduit 11 compressor 12, 13 conduit 14 distillation column 15 conduit 20 conduit 21 distillation column 22, 23 conduit 24 distillation column 25, 26 conduit 30 conduit 31 separator 32 conduit 33 column 34-36 conduit 40-42 conduit 46 distillation column 44, 45 conduit 46 distillation column 47-53 conduit 60 distillation column 61, 62 conduit 63 hydrogenation reactor 64 conduit 65 cooling stage 66, 67 conduit 70-75 conduit 80-83 conduit