Method and apparatus for the preparation of a polyolefin

Abstract

A method (100) is proposed for the preparation of a polyolefin from olefin monomers, wherein the olefin monomers are subjected to one or more polymerisation steps (13), in which a proportion of the olefin monomers are catalytically reacted to form the polyolefin, while the olefin monomers that are not reacted in the polymerisation step or steps (13) are at least partly transferred into one or more gaseous, monomer-containing purge streams (g, h), which additionally contain(s) one or more aluminium organic compounds, which comprise one or more co-catalysts used in the polymerisation step or steps (13) and/or one or more compounds formed from the co-catalyst(s). It is provided that, downstream of one or more olefin synthesis steps (21), the gaseous, monomer-containing purge stream or streams (g, h) are brought into contact with a crude gas mixture (p, r) formed using a product mixture from the olefin synthesis step or steps (21) and are subjected to a caustic wash (26) together with the crude gas mixture (p, r). The present invention also relates to a corresponding apparatus.

Claims

1. Method (100) for producing a polyolefin from olefin monomers, wherein the olefin monomers are subjected to one or more polymerisation steps (13), in which a proportion of the olefin monomers are catalytically reacted to form the polyolefin, while the olefin monomers that are not reacted in the polymerisation step or steps (13) are at least partly transferred into one or more gaseous, monomer-containing purge streams (g, h), which additionally contain(s) one or more aluminium organic compounds, which comprise one or more co-catalysts used in the polymerisation step or steps (13) and/or one or more compounds formed from the co-catalyst(s), characterised in that, downstream of one or more olefin synthesis steps (21), the gaseous, monomer-containing purge stream or streams (g, h) are brought into contact with a crude gas mixture (p, r) formed using a product mixture from the olefin synthesis step or steps (21) and are subjected to a caustic wash (26) together with the crude gas mixture (p, r).

2. Method (100) according to claim 1, wherein the gaseous, monomer-containing purge stream or streams (g, h) is or are combined with the crude gas mixture (r), downstream of one or more working-up steps (22, 23) to which the product mixture of the olefin synthesis step or steps (21) is or are subjected during the formation of the crude gas mixture (p, r).

3. Method (100) according to claim 2, wherein the working-up step or steps comprise a cooling (22) and/or a water wash (23).

4. Method (100) according to one of the preceding claims, wherein one or more washing columns (261) are used in the caustic wash (26), and wherein the gaseous, monomer-containing purge stream or streams (g, h) are brought into contact with the crude gas mixture (p, r) in or upstream of the one or more washing columns (261).

5. Method (100) according to claim 4, wherein one or more washing columns (261) are used which comprise sections separated from one another by liquid barrier trays, the number of sections being two to five, more particularly two to three.

6. Method (100) according to claim 4 or 5, wherein a plurality of washing columns (261) are used which are connected in parallel and/or in series.

7. Method (100) according to one of the preceding claims, wherein the gaseous, monomer-containing purge stream or streams (g, h) and the crude gas mixture (p, r) are brought into contact with an alkaline washing medium in the caustic wash (26), the alkaline washing medium containing sodium hydroxide in an amount of 0.5 to 20% by weight, particularly 1 to 10% by weight, particularly 1 to 6% by weight.

8. Method (100) according to one of the preceding claims, wherein the one or more aluminium organic compounds are present in the gaseous, monomer-containing purge stream or streams (g, h) in an amount of up to 5% by weight, particularly up to 2.5% by weight, particularly up to 1.25% by weight, particularly up to 0.5% by weight, particularly up to 1000 ppm by weight, based on the aluminium present.

9. Method (100) according to one of the preceding claims, wherein the gaseous, monomer-containing purge stream or streams (g, h) contain the one or more aluminium organic compounds in the form of at least one aluminium alkyl and/or in the form of at least one methylaluminoxane and/or in the form of at least one halogenated aluminium compound with the empirical formulae AIR1R2X1 and/or AIR1X1X2, wherein R1 and R2 denote branched or unbranched C1- to C12-alkyl chains and X1 and X2 denote a halogen atom, particularly triethylaluminium, and/or in the form of at least one compound formed from the above-mentioned compounds.

10. Method (100) according to one of the preceding claims, wherein a hydrocarbon-containing purge washing stream (u) which is depleted in or free from the aluminium organic compound or compounds is obtained in the caustic wash (26), and wherein hydrocarbons contained in the purge washing stream (u) are fed into one or more separating steps (26), in which one or more olefin-rich fractions (x) are obtained.

11. Method (100) according to claim 10, wherein the olefin monomers which are subjected to the polymerisation step or steps (13) are prepared at least partially using the one or more olefin-rich fractions (x).

12. Method (100) according to one of the preceding claims, wherein the olefin synthesis step or steps (21) encompass at least one thermal and/or catalytic cleavage step and/or at least one step for the dehydrogenation of alkanes and/or at least one step for the oxidative coupling of methane.

13. Apparatus for the production of a polyolefin from olefin monomers having one or more polymerisation reactors which are set up so as to subject the olefin monomers to one or more polymerisation steps (13) and thereby react some of the olefin monomers catalytically to form the polyolefin, and having means which are designed to transfer the olefin monomers that are not reacted in the polymerisation step or steps (13) at least partly into one or more gaseous monomer-containing purge streams (g, h) which additionally contain one or more aluminium organic compounds, which consist of one or more co-catalysts used in the polymerisation step or steps (13), and/or one or more compounds formed from the co-catalyst or co-catalysts, characterised by means which are designed to bring the gaseous, monomer-containing purge stream or streams (g, h),downstream of one or more olefin synthesis steps (21), into contact with a crude gas mixture (p, r) formed using a product mixture from the olefin synthesis step or steps (21), and to subject it, together with the crude gas mixture (p, r), to a caustic wash (26).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] FIG. 1 shows a method according to one embodiment of the invention in the form of a schematic flow diagram.

[0038] FIG. 2 shows details of the method illustrated in FIG. 1 in the form of a schematic process flow diagram.

[0039] In the Figures, corresponding elements have been given identical reference numerals and, in the interests of clarity, the description thereof has not been repeated. In all the Figures, method steps and apparatus are indicated by numerals, whereas streams of matter are indicated by lower-case or upper-case letters.

EMBODIMENTS OF THE INVENTION

[0040] FIG. 1 shows a method according to one embodiment of the invention in the form of a schematic flow diagram. The method is generally designated 100. The method 100 comprises steps 11 to 14 for preparing a polyolefin and steps 21 to 28 for preparing olefins.

[0041] Method steps 21 to 28 for preparing the olefins are typical for a steam cracking process as described above. As mentioned previously, the method according to the invention is suitable for all olefin syntheses in which a corresponding product mixture or a crude gas mixture obtained from the product mixture is subjected to a caustic wash.

[0042] In the embodiment of the method 100 illustrated in FIG. 1 a stream a, which contains olefin monomers such as ethylene and/or propylene, is subjected to a working up and/or treatment step 11. In the working up or treatment step 11, the stream a, for example, can be brought to a suitable pressure, purified and/or temperature-controlled. The working up or treatment step 11 may also be omitted.

[0043] A stream thus obtained, now designated c, is fed into one or more polymerisation steps 13 in a suitable reactor. In addition, a stream d which may for example contain additives and/or excipients required for the polymerisation, for example one or more aluminium organic compounds which are used as co-catalysts in the polymerisation step or steps 13, is subjected to the polymerisation step or steps 13. Instead of an individual stream d, a plurality of corresponding streams may be used which may contain different additives and/or excipients. The stream d (or plurality of corresponding streams) may be subjected to a corresponding working up or treatment step 12, corresponding to the working up or treatment step 11, and be formed from a feed stream b (or a plurality of feed streams). The working up or treatment step 12 may also be omitted.

[0044] In the polymerisation step or steps 13, depending on the polymerisation yield, some of the olefin monomers supplied in the form of the stream c are reacted to form a polyolefin. The method is equally suitable for the production of homo- and heteropolymers. In the polymerisation step or steps 13 a stream e is obtained which contains the corresponding polyolefin, for example in liquid form and/or in the form of a granulate.

[0045] The stream e is fed into a degassing or gas purging step 14 where it is substantially or completely freed from any monomers and other short-chained hydrocarbons that are still present. Corresponding compounds may also be obtained from polymerisation step 13, as illustrated by the stream g, by being drawn off from a reactor, for example. In the degassing step 14, or as early as the polymerisation step 13, a purge gas stream i, such as nitrogen, for example, may be used, to flush around or through the polyolefin. In this way, the olefin monomers, as well as other compounds contained in the stream e, such as short-chain paraffins which are formed in the polymerisation step or steps 13, are transferred into a gaseous stream g and/or h. Both the stream g and the stream h, which are referred to here as monomer-containing purge streams, contain amounts of a co-catalyst used in the polymerisation step or steps 13, in addition to the monomers and optionally short-chained hydrocarbons as well as the purge gas of stream i, such as nitrogen, as mentioned above. They are therefore not suitable for feeding directly into process steps in which water is used at a neutral pH, since, as previously stated, this can be expected to result in the formation of gel or solids. The monomer-containing purge stream or streams g and/or h may also be combined to form a combined stream k and are fed into a caustic wash 26 in the embodiment shown.

[0046] In process step or steps 21 to 28 for producing olefins, a stream I, typically together with at least one recycled stream y, is subjected to one or more olefin synthesis steps 21. In the olefin synthesis step or steps 21, which are carried out in one or more cracking furnaces in the embodiment shown, a vapour stream m is also used. As already mentioned, the method according to the invention is also suitable for other olefin syntheses in which an olefin synthesis step 21 is carried out catalytically and, if desired, no vapour stream m is used. In the olefin synthesis step or steps a product mixture is obtained, as illustrated by the stream n.

[0047] The product mixture n, a so-called cracking gas in the case of a steam cracking process, is fed into one or more working-up steps 22, 23. For example, the stream n is first cooled in a cooling step 22, for example by means of a linear cooler and/or using so-called quenching oil, thus producing a stream o. A stream of higher-molecular compounds may be separated off as early as the cooling step 22, although this is not illustrated separately. The stream o can then be subjected to a water wash 23, for example, by passing the stream o in countercurrent to a water stream. The stream o is further cooled by means of this stream of water and higher molecular compounds in the stream o such as pyrolysis gasoline and other compounds, for example, can be washed out. In the water wash 23, a stream of water q may be obtained which is fed into a steam generator 24. The above-mentioned vapour stream m is obtained in the steam generator 24.

[0048] A stream p obtained in the water wash can then be subjected to a compression step 25. In the terminology used here, as already mentioned, streams formed in any way from a product mixture of the stream n, such as the stream p, are referred to as crude gas mixtures. The compression step 25 can be carried out for example using a multi-stage compressor into which fluid streams can be fed and removed at different pressure stages. For details, reference may be made to the above-mentioned article Ethylene in Ullmann's Encyclopedia of Industrial Chemistry. For example, a stream r may be taken from the compression step 25 at a suitable pressure; in the terminology used in this application this is a crude gas mixture from the olefin synthesis step or steps 21.

[0049] The stream r is subjected to a caustic wash 26, which is illustrated in detail in FIG. 2. The caustic wash serves to wash so-called acid gases, particularly hydrogen sulphide and carbon dioxide, out of the fluid of the stream r. According to the embodiment of the invention shown in FIG. 1, the monomer-containing purge streams g, h or a combined stream formed therefrom are simultaneously fed into the caustic wash 26, as is also shown in detail in FIG. 2. According to the embodiment of the invention shown here, the aluminium organic compound or compounds contained in the monomer-containing purge stream or streams g, h, or in the combined stream k, is or are also washed out in the caustic wash in addition to the above-mentioned acid gases.

[0050] A stream, designated s in FIG. 1, processed in this manner using the caustic wash is fed into the compression step 25 or into a compressor used in the compression step 25, at a suitable pressure stage and is further compressed therein. In the compression step 25, in addition to a stream u, which is fed into a drying step 27, for example, a stream t (condensate) of higher molecular compounds may also be obtained. A stream obtained by means of the drying step 27, now designated v, can be sent into a separation means 28 in which the compounds contained in the stream v are converted into different fractions or corresponding streams w, x and/or y. The separation 28 may be carried out in any desired manner. At least some of the fractions or streams formed in the separation 28 may be recycled back into the olefin synthesis step or steps 21, as illustrated here by stream y. In particular, these are saturated hydrocarbons, for example, which are not suitable for polymerisation. Further fractions, such as aromatic compounds, may also be removed from the process as required, as illustrated here by stream w.

[0051] A stream, here designated x, contains ethylene and/or propylene, for example, and can be subjected, as feed stream a, to the working up and/or treatment steps 11 upstream of the polymerisation step or steps 13. It will be understood that the streams x and a may also be decoupled, so that, for example, monomer obtained in the separation means 28 can be stored intermediately, or step a need not consist exclulsively, or at all, of the compounds contained in the stream x. Also, the stream x may also be only partially converted into a feed stream a, in which case a partial stream of x is utilised in some other way.

[0052] As already mentioned, FIG. 2 shows details of the caustic wash 26. The central component of an apparatus used in the caustic wash 26 is a washing column 261 which is illustrated here with four sections. The sections are not separately designated. The sections are separated from one another by means of liquid barrier trays, which are not separately designated either. The lowest section of the washing column 261 is usually provided with a partition wall which makes it possible to separate the spent lye from the washing lye that is to be recycled. The spent lye and washing lye are comparable in their compositions, but the partition wall makes it possible to separate off a floating organic phase and preferably convey it into the spent lye.

[0053] In the washing column 261. a basic washing medium is used which is introduced, by means of pumps 262 in the form of the streams designated A, into an upper region of the three lower sections of the washing column 261 and is drawn off above the respective liquid barrier tray. In the topmost section of the washing column 261, a water stream B may be used, which is also supplied by means of a suitable pump 263. A fresh water stream is illustrated in the form of the stream E, while a purge water stream is illustrated in the form of the stream F. By suitable adjustment of the streams E and F it can be ensured that the stream B always has an adequate washing capacity. Fresh washing medium can be supplied in the form of the streams C and D and can be stored intermediately in a storage tank 264.

[0054] As shown in FIG. 2, the crude gas mixture designated r in FIG. 1 which is partially compressed in the compression step 25 is temperature-controlled by means of a heat exchanger 265. Upstream and/or downstream of the heat exchanger 265 the stream r may be combined with the monomer-containing purge stream or streams g, h or a corresponding combined stream k, thereby forming a stream which is designated G here. It should be understood that all the streams designated g, h in FIG. 2 each illustrate alternative feed points for gaseous, monomer-containing purge streams. Thus, one or more monomer-containing purge streams can also be fed directly into the washing column 261, for example below the feed point for the stream r or the stream G. Depending on the washing out required, they can also be fed into a column section located above.

[0055] As a result of the caustic wash by means of the streams A and the final water wash by means of the stream B, a stream H can be drawn off at the top of the washing column 261 which is free from, or substantially free from unwanted compounds such as the above-mentioned so-called sour gases, but also of the aluminium organic compound or compounds. A corresponding stream H can be temperature-controlled by means of a heat exchanger 266 and further treated in the form of the stream s, as explained in connection with FIG. 1.

[0056] A purge stream I, which may also contain oil-like compounds, inter alia, can be removed from the sump of the lowest section of the washing column 261. These can be discharged in the form of a stream K using an oil separator 267. The remaining stream, here designated L, can be released through a relief valve 268 and transferred into a degassing container 269. In the degassing container 269, volatile compounds can be converted into the gaseous phase and drawn off in the form of the stream M. Degassed spent lye can be discharged in the form of the stream N and is typically sent for disposal.