METHOD AND PROCESS PLANT FOR TREATMENT OF A STREAM OF MIXED COMPOUNDS

Abstract

The invention concerns a method for treatment of a stream of mixed compounds (4a) obtained from a process (3) comprising decomposition and/or conversion of a wood or pulp material, the method comprising: feeding the stream (4a-4e) through a processing arrangement (2) comprising one or more treatment units (10, 20, 30, 40, 50) arranged to separate at least a first compound from other compounds in the stream and form a first product flow (4f) containing the first compound, wherein the one or more treatment units comprises at least a first primary separation unit (20) arranged to separate one or more compounds other than the first compound from the stream; and feeding the compounds separated from the first compound in the first primary separation unit (20) to a first auxiliary separation unit (21) so as to separate a second compound from at least one of the other compounds separated from the first compound in the first primary separation unit (20) and thereby increase the purity of said second compound and form a second product flow (22, 23) in the form of a purified second compound and/or a purified derivative of the second compound, wherein the second compound is dimethyl sulfide (DMS, CH3—S—CH3), methyl mercaptan (CH3—S—H) or acetone (CH3—CO—CH3).

Claims

1. The method for treatment of a stream of mixed compounds obtained from a process comprising decomposition and/or conversion of a wood or pulp material, the method comprising: feeding the stream through a processing arrangement comprising one or more treatment units arranged to separate methanol from other compounds in the stream and form a first product flow containing methanol, wherein the one or more treatment units comprises at least a first primary separation unit arranged to separate one or more compounds other than methanol from the stream; and feeding the compounds separated from methanol in the first primary separation unit to a first auxiliary separation unit so as to separate a second compound from at least one of the other compounds separated from methanol in the first primary separation unit and thereby increase the purity of said second compound and form a second product flow in the form of a purified second compound and/or a purified derivative of the second compound, wherein the second compound is dimethyl sulfide (DMS, CH3—S—CH3), methyl mercaptan (CH3—S—H) or acetone (CH3—CO—CH3).

2. A method according to claim 1, wherein the one or more treatment units further comprises a second primary separation unit arranged to separate methanol from other compounds in the stream, wherein the method comprises: feeding the compounds separated from methanol in the second primary separation unit to a second auxiliary separation unit so as to separate a third compound from at least one of the other compounds separated from methanol in the second primary separation unit and thereby increase the purity of said third compound and form a third product flow in the form of a purified third compound or a purified derivative of the third compound, wherein the third compound is dimethyl sulfide (DMS, CH3—S—CH3), methyl mercaptan (CH3—S—H) or acetone (CH3—CO—CH3), and wherein the second and third compounds are different compounds.

3. A method according to claim 1 wherein the processing arrangement comprising at least two treatment units arranged in series in relation to the flow of the stream containing methanol.

4. A method according to claim 1, wherein the processing arrangement is arranged to be capable of purifying methanol until it forms a first product flow in the form of a purified first compound.

5. A method according to claim 1, wherein the first primary separation unit is arranged to separate compounds from the stream that have a boiling point that is lower than that of methanol.

6. A method according to claim 1, wherein separation in the first auxiliary separation unit is based on deviating boiling points.

7. A method according to claim 2, wherein the second primary separation unit is arranged to separate compounds from the stream that have a boiling point that is lower than that of methanol.

8. A method according to claim 2, wherein separation in the second auxiliary separation unit is based on deviating boiling points.

9. A method according to claim 1, wherein methanol is methanol.

10. A method according to claim 1, wherein the second compound is dimethyl sulfide (DMS, CH3—S—CH3) or methyl mercaptan (CH3—S—H).

11. A method according to claim 2, wherein the third compound is acetone (CH3—CO—CH3).

12. A method according to claim 2, wherein the first primary separation unit is arranged upstream of the second primary separation unit.

13. A method according to claim 1, wherein the method comprises: operating the processing arrangement so as to increase a mass flow rate of the second compound in the second product flow while decreasing a mass flow rate of methanol in the first product flow.

14. A method according to claim 10, wherein the method comprises: recirculating a portion of the stream in a recirculation flow line from a point in the processing arrangement downstream of a separation unit where DMS and/or methyl mercaptan is separated from the stream, to a point in the processing arrangement upstream of the separation unit (20) where DMS and/or methyl mercaptan is separated from the stream, wherein the recirculated portion of the stream during recirculation is fed through a DMS/methyl mercaptan production reactor to which is fed, via a supply line (71), one or more sulphur compounds capable of reacting with methanol or another compound in stream so as to form DMS and/or methyl mercaptan.

15. Method according to claim 1, wherein the second compound is DMS, and wherein the method comprises: feeding purified DMS from the first auxiliary separation unit to an oxidizing unit so as to oxidize DMS and form a purified derivative of DMS in the form of dimethyl sulfoxide (DMSO).

16. A method according to claim 10, wherein the method comprises: feeding a flow containing DMS and/or methyl mercaptan from a lignin treatment arrangement to a point in the processing arrangement upstream of a separation unit (20) where DMS and/or methyl mercaptan is separated from the stream, wherein the lignin treatment arrangement is arranged to react at least a portion of a flow of incoming lignin obtained from a wood or pulp decomposition and/or conversion process with sulphur from an incoming flow of sulphur compounds so as to form DMS and/or methyl mercaptan.

17. A method according to claim 1, wherein the stream of mixed compounds is obtained from black liquor used for digestion of wood material in pulp production in a pulp mill.

18. Use of a process plant for carrying out the method of claim 1, the process plant comprising: a processing arrangement comprising one or more treatment units configured to, when the stream is fed through the processing arrangement, separate methanol from other compounds in the stream and form a first product flow containing methanol, wherein the one or more treatment units comprises at least a first primary separation unit arranged to separate one or more compounds other than methanol from the stream; the processing arrangement further comprising a first auxiliary separation unit arranged in association with the first primary separation unit so as to receive the compounds separated from methanol in the first primary separation unit, the first auxiliary separation unit being configured to separate a second compound from at least one of the other compounds separated from methanol in the first primary separation unit and thereby increase the purity of said second compound and form a second product flow in the form of a purified second compound and/or a purified derivative of the second compound.

19. Use of a process plant according to claim 18, wherein the one or more treatment units further comprises a second primary separation unit arranged to separate methanol from other compounds in the stream, and wherein the processing arrangement further comprising a second auxiliary separation unit arranged in association with the second primary separation unit so as to receive the compounds separated from methanol in the second primary separation unit, the second auxiliary separation unit being configured to separate a third compound from at least one of the other compounds separated from methanol in the second primary separation unit and thereby increase the purity of said third compound and form a third product flow in the form of a purified third compound and/or a purified derivative of the third compound.

20. Use of a process plant according to claim 18, wherein the processing arrangement comprises at least two treatment units arranged in series in relation to the flow of the stream containing methanol.

21. Use of a process plant according to claim 18, wherein the processing arrangement is arranged to be capable of purifying methanol until it forms a first product flow in the form of a purified first compound.

22. Use of a process plant according to claim 18, wherein the first primary separation unit is arranged to separate compounds from the stream that have a boiling point that is lower than that of methanol.

23. Use of a process plant according to claim 18, wherein the first auxiliary separation unit is arranged to separate compounds based on deviating boiling points.

24. Use of a process plant according to claim 19, wherein the second primary separation unit is arranged to separate compounds from the stream that have a boiling point that is lower than that of methanol.

25. Use of a process plant according to claim 19, wherein the second auxiliary separation unit is arranged to separate compounds based on deviating boiling points.

26. Use of a process plant according to claim 19, wherein the first primary separation unit is arranged upstream of the second primary separation unit.

27. Use of a process plant according to claim 18, wherein the processing arrangement is configured to be operated so as to increase a mass flow rate of the second product flow while decreasing a mass flow rate of the first product flow.

28. Use of a process plant according to claim 18, wherein the processing arrangement comprises: a recirculation flow line arranged to recirculate a portion of the stream from a point in the processing arrangement downstream of a primary separation unit where DMS and/or methyl mercaptan is separated from the stream, to a point in the processing arrangement upstream of the primary separation unit where DMS and/or methyl mercaptan is separated from the stream; a DMS/methyl mercaptan production reactor (60) through which the portion of the stream is fed during recirculation, and a supply line for feeding one or more sulphur compounds to the DMS/methyl mercaptan production reactor, wherein the sulphur compounds are capable of reacting with methanol or another compound in stream so as to form DMS and/or methyl mercaptan.

29. Use of a process plant according to claim 18, wherein the processing arrangement comprises an oxidizing unit arranged downstream the first auxiliary separation unit so as to allow oxidation of the second compound and form a purified derivative of the second compound.

30. Use of a process plant according to claim 18, wherein the process plant comprises a lignin treatment arrangement arranged to react at least a portion of a flow of incoming lignin obtained from a wood or pulp decomposition and/or conversion process with sulphur from an incoming flow of sulphur compounds so as to form DMS and/or methyl mercaptan, wherein the process plant further comprises a flow line for feeding DMS and/or methyl mercaptan from the lignin treatment arrangement to a point in the processing arrangement upstream of a separation unit (20) where DMS and/or methyl mercaptan is separated from the stream.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0066] In the description of the invention given below reference is made to the following figure, in which:

[0067] FIG. 1 schematically shows a process for producing “Product 1”, “Product 2”, “Product 3” and/or “Product 4” from a raw material stream.

[0068] FIG. 2 schematically shows a process for producing DMS and/or DMSO, and acetone from a raw material stream in the form of crude methanol.

[0069] FIG. 3 schematically shows a process for producing additional amounts of DMS and/or DMSO from a raw material stream in the form of crude methanol, wherein the process also comprises DMS produced from lignin.

[0070] FIG. 4 schematically shows a process for producing additional amounts of DMS and/or DMSO from a raw material stream in the form of crude methanol, wherein also some methanol is converted to DMS.

[0071] FIG. 5 schematically shows a process for producing DMS and/or DMSO from a raw material stream in the form of contaminated methanol.

[0072] FIG. 6 schematically shows an example of a process plant according to this disclosure.

DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION

Example 1

[0073] With reference to FIG. 1, a process within the biorefinery concept (see e.g. “Biorefinery” in FIG. 1), in accordance with the present invention, is described. Thus, in FIG. 1, a process, within the biorefinery concept, for producing “Product 1” (e.g. methanol, for example, pure methanol), “Product 2” (e.g. acetone, for example, acetone), “Product 3” (for example, DMS) and/or “Product 4” (for example, DMSO and/or DMSO.sub.2), i.e. a process for producing one, or more, compounds according to formula (I). In some embodiments, “Product 1”, “Product 2”, “Product 3”, and/or “Product 4” (see FIG. 1) are formed and being comprised in said material composition of said process. In embodiments, “Product 1”, “Product 2”, and/or “Product 3” (see FIG. 1) may be one, or more, compounds of formula (II) and may be comprised in said retrieving composition of said process. Further, said retrieving composition, comprising one, or more, of “Product 1”, “Product 2”, and “Product 3” (see FIG. 1) may be retrieved from “Raw material stream” (see FIG. 1), i.e. from said raw material composition. Furthermore, “By-product gases” (see FIG. 1), e.g. reduced sulfur gases, may be separated in the process. Further, with reference to FIG. 1, a process is described wherein said process comprises a selection of said treatment step, wherein the treatment step comprises one, or more, of a reaction step, a synthezising step, an oxidation step, a separation step, a stripping step, and a distillation step. Said selected treatment step may, e.g., be performed at, for example, “Biorefinery” in FIG. 1 and/or “Treatment” in FIG. 1.

Example 2

[0074] DMS can be produced starting from a raw material composition in accordance with the present process. The process here in Example 2 may further comprise treatment of lignin in black liquor with sulfur, or with other sources of sulfur such as hydrogen sulfide, and methyl mercaptane, e.g. at 200-250° C., i.e. treatment steps of the process.

Example 3

[0075] From a stripper top (for example, “Distillation” with “Volatiles″-stream in FIG. 1 of SE 539 579 C2) a DMS rich stream gas phase, was characterized to contain see table 1, where (MM) is methyl mercaptan, where H2S is hydrogen sulfide, where (DMDS) is dimethyl disulfide and (%) is weight %. Here in Example 3, a process comprising a DMS rich stream gas phase from such a stripper top, is described. Thus, Example 3 refers to the stripper top in FIG. 5, i.e. the stripper top where “contaminated methanol stream”, i.e. the raw material composition, is going in from the left and “methanol composition” is going out at the bottom, and “DMS”, i.e. a retrieving composition, or a material composition, is going out in the top.

TABLE-US-00001 Sample MM (%) H2S (%) DMS (%) DMDS (%) 1 5.82 2.45 2.33 0.01 2 6.22 3.58 3.59 0.01

[0076] This stream, i.e. DMS rich stream gas phase, (see “DMS” in FIG. 5) can be purified by further separation in a condenser (see “Cooling” in FIG. 5), i.e. treatment step of the process. Further, methyl mercaptan, with a boiling point of 6° C., is going out from the top of said condenser, see “Cooling” and “Methyl mercaptan” in FIG. 5. (The condensed stream may then also be further purified by another stripper column and another condenser.) After being purified by the further separation in the condenser (see “Cooling” in FIG. 5), the purified DMS may be oxidized with nitrogen dioxide in nitrogen gas (see “NO.sub.2/N.sub.2” in FIG. 5) to DMSO (see “DMSO” in FIG. 5), and the DMSO may be further purified (see “purification” in FIG. 5). During the oxidation nitrogen oxide is formed which reacted back to NO.sub.2 with addition of ammonia. Thus, when the compound of formula (I) is DMSO, said treatment step comprises an oxidation step wherein DMS has been oxidized.

Example 4

[0077] From an acetone column total condenser (for example, “Distillation” with “Acetone+impurities″-stream in FIG. 1 of SE 539 579 C2), an acetone rich stream was characterized to contain see table 2, where (%) is weight %. Here in Example 4, a process is described wherein the raw material composition may be such an acetone rich stream.

TABLE-US-00002 Sample MeOH (%) EtOH (%) Acetone (%) 2 70 0 30

[0078] Thus, in Example 4 refer to “Crude methanol”, i.e. a raw material composition which is going in to the “Biorefinery” from the left, see “Biorefinery” in FIG. 2. The “Biorefinery” in FIG. 2 is comprised in the process. Moreover, reduced sulfur gases may be separated, see “Reduced sulfur gases” in FIG. 2, from the acetone rich stream. The acetone rich stream can be purified by further separation in a condenser (see “Pure acetone” in FIG. 2). (The condensed stream may be further purified by another stripper column and another condenser.) The biorefinery of Example 4 (“Biorefinery” in FIG. 2) may further comprise production of DMS, see “DMS” in FIG. 2, DMSO/DMSO.sub.2, see “DMSO/DMSO.sub.2” in FIG. 2, involving a treatment step, e.g. comprising an oxidation step see “Treatment” in FIG. 2. Furthermore, the biorefinery of Example 4 (“Biorefinery” in FIG. 2), may also further comprise production of acetone, see “Pure acetone” in FIG. 2.

Example 5

[0079] Example 5 describes a process involving production of DMS (see “DMS” in FIG. 3) and a reactive lignin product (see “Reactive lignin stream” in FIG. 3), comprising an added stream of DMS (see “Stream rich in DMS” in FIG. 3). The Example 5 (see FIG. 3) comprises all parts of Example 4 (except notation that reduced sulfur gases may be separated see “Reduced sulfur gases” in FIG. 2), and Example 5 further also comprises lignin treatment (see “Lignin treatment” in FIG. 3) rendering said reactive lignin product and producing said added stream of DMS. The lignin treatment comprises stream of lignin (see “Lignin stream” in FIG. 3) addition of internal, or external, sulfur source (see “Sulfur source (internal or external)” in FIG. 3).

[0080] The lignin treatment (see “Lignin treatment” in FIG. 3), comprising e.g. an alkaline treated black liquor, rendering said reactive lignin product, here a lignin with more reactive phenolic groups.

Example 6

[0081] Production of DMS comprising an internal, or external, source of sulfur. Example 6 describes a process for production of DMS (see “DMS” in FIG. 4) with a recycling of a stream of DMS (see “Stream rich in DMS” in FIG. 4) to the biorefinery (see “Biorefinery” in FIG. 4).

[0082] The Example 6 (see FIG. 4) comprises all parts of Example 4 (except notation that reduced sulfur gases may be separated see “Reduced sulfur gases” in FIG. 2), and Example 6 may further also comprise addition of internal, or external, sulfur source (see “Sulfur source (internal or external)” in FIG. 4) for reaction (see “Reactor” in FIG. 4) with pure methanol (see “Pure methanol” in FIG. 4), as well as, said recycling of a stream of DMS (see “Stream rich in DMS” in FIG. 4) to the raw material composition (see “Crude methanol” in FIG. 4) and to said biorefinery (see “Biorefinery” in FIG. 4).

[0083] FIG. 6 shows, in a schematic view, an example of a method and process plant 1 according to this disclosure. As can be seen in FIG. 6 the process plant 1 is configured for treatment of a stream of mixed compounds 4a obtained from a process 3 comprising decomposition and/or conversion of a wood or pulp material. Although not shown in FIG. 6, the process 3 may be configured for black liquor digestion of wood material and the stream of mixed compounds 4a may be a condensate from evaporation of such black liquor. The process 3 may form part of the process plant 1 and the process plant 1 may be an integrated plant comprising both a “biorefinery” (see below) and a pulp mill.

[0084] The process plant 1 comprises a processing arrangement 2, i.e. an arrangement that may be denoted “biorefinery”. FIG. 6 indicates that the processing arrangement 2 may comprises five treatment units 10, 20, 30, 40, 50 configured to, when the stream 4a, 4b, 4c, 4d, 4e is fed through the processing arrangement 2, separate at least a first compound from other compounds in the stream and form a first product flow 4f containing the first compound. The treatment units 10-50 may be of different type and number.

[0085] In this particular example the treatment units include a first and a second primary separation unit 20, 40. The other treatment units 10, 30 and 50 are in this example optional but are indicated as it in many cases would be suitable to include further treatment units, such as further separation units that may form further product flows and/or units for acidic treatment of the flow or similar. For the principle of this disclosure only one of the first and second primary separation units 20, 40 is required.

[0086] The stream of mixed compounds is denoted 4a when fed to the processing arrangement 2. As the stream passes the treatment units 10-50 the composition and mass flow rate of the stream typically changes since some compounds is removed, such as more volatile compounds, and some compounds may be added, such as acid. This change of the stream is indicated by the notations 4b-4f used after the treatment units 10-50. As further described below, one or more compounds including “the first compound” will generally remain in the stream 4a-4f and form an outgoing first product flow 4f. This first product flow 4f may be a flow of a (to some degree) purified first product or may be a mix of compounds that might be used for combustion and thus for heating purposes (possibly after having removed water in an additional treatment unit, if not already removed in e.g. treatment unit 50). In the example of FIG. 6 the first compound is methanol, and product flows other than the first product flow relate to more volatile compounds that are separated from (or produced from) methanol present in the “main” flow 4a-4f.

[0087] The first primary separation unit 20 is arranged to separate one or more compounds other than the first compound from the stream. In this example the first primary separation unit 20 is a distillation unit that separates compounds from the stream that have a lower boiling point than the first compound.

[0088] A first auxiliary separation unit 21 is arranged in association with the first primary separation unit 20 so as to receive the compounds separated from the first compound in the first primary separation unit 20. The first auxiliary separation unit 21 is configured to, based on deviating boiling points, separate a second compound from at least one of the other compounds separated from the first compound in the first primary separation unit 20. The first auxiliary separation unit 21 may thus be a cooler/condenser. As a result of the treatment in the first auxiliary separation unit 21, the purity of the second compound increases and form a second product flow 22, 23 in the form of a purified second compound and/or a purified derivative of the second compound. See further explanation below regarding different variants for the second product flow(s).

[0089] As mentioned above, the treatment units further include the second primary separation unit 40, that is located downstream of the first separation unit 20 (in relation to the stream 4a-4f) and that also is a distillation unit arranged to separate at least the first compound from other compounds in the stream based on deviating boiling points.

[0090] The processing arrangement 2 further comprises a second auxiliary separation unit 41 arranged in association with the second primary separation unit 40 so as to receive the compounds separated from the first compound in the second primary separation unit 40. The second auxiliary separation unit 41 is configured to separate, again based on deviating boiling points, a third compound from at least one of the other compounds separated from the first compound in the second primary separation unit 40. As a result, the purity of the third compound increases and it forms a third product flow 42 in the form of a purified third compound and/or a purified derivative of the third compound.

[0091] The processing arrangement 2 is further arranged to be capable of purifying the first compound until it forms the first product flow 4f in the form of a purified first compound. As explained on various places in this disclosure, it may be beneficial to produce also a purified first compound but it is not necessary that a purified first compound is produced.

[0092] In the example of FIG. 6, product flows 22, 23, 42 and 4f are intended to indicate outflow of products that are used in some way outside of the processing arrangement 2. These products, or a portion of the product flow, may alternatively be used for various purposes within the processing arrangement 2.

[0093] As mentioned above, the first compound in the example of FIG. 6 is methanol. Further, the second compound is exemplified as dimethyl sulfide (DMS, CH3—S—CH3) or methyl mercaptan (CH3—S—H), while the third compound is exemplified as acetone (CH3—CO—CH3). Depending on the composition of the incoming stream 4a and the particular design of the processing arrangement 2 and the process plant 1, other compounds may be regarded as forming the first, second and third compounds.

[0094] Depending on the particular design of the processing arrangement 2, flows 22, 23 and 25 can represent different compounds, and at least one of these flows is optional, as is also an oxidizer 24 arranged downstream the first auxiliary separation unit 21. In one example, flow 22 represents purified methyl mercaptan, flow 23 represents purified DMS (optionally oxidized to DMSO in oxidizer 24), and flow 25 represents some other compounds (such as H.sub.2S) separated from the stream 4b in the first primary separation unit 20. In another example flow 22 represents purified DMS, flow 23 is optional or represents purified DMSO (i.e. a portion of the DMS is fed to oxidizer 24) and flow 25 represents some other compounds, including e.g. methyl mercaptan and H.sub.2S, separated from the stream 4b in the first primary separation unit 20. The flow 25 may serve as a source of sulphur and be fed to a “sulphur source” schematically indicated in FIG. 6 with reference number 70.

[0095] Besides product flows of DMS, DMSO, methyl mercaptan and sulphur compounds, including variants 22 and 23 of the second product flow, the method/process plant 1 of FIG. 6 is capable of generating product flows of purified methanol (first product flow 4f) and of purified acetone (third product flow 42).

[0096] As an example, the second primary separation unit 40 in the form of a distillation unit may be operated at around 61-62° C. (if atmospheric pressure) to evaporate and separate acetone (boiling point 56° C.) from e.g. methanol (boiling point 64.7° C.) and ethanol (boiling point 78.4° C.). Also water will then remain in the liquid phase. Remaining volatile compounds in the stream 4d, such as DMS (boiling point 37° C.), methyl mercaptan (boiling point 6.0° C.) and H.sub.2S (boiling point -60.3° C.) follow the acetone, as also some of the methanol. Flow 43 in FIG. 6 can be seen as representing volatile sulphur compounds that can be fed to the sulphur source 70. Flow 43 can alternatively be seen as representing methanol or a mix of compounds separated from acetone. Also the methanol may be separated from the acetone in the second auxiliary separation unit 41 (or in a not shown additional auxiliary separation unit) so as to purify acetone in the third product flow 42.

[0097] The processing arrangement 2 is further configured to be operated so as to increase a mass flow rate of the second and/or third product flow while decreasing a mass flow rate of the first product flow. This can, for instance, be done by adjusting temperature/pressure of the first and/or second primary separation units 20, 40 so that a larger fraction of the incoming stream 4b, 4d is separated and fed to the corresponding auxiliary separation unit 21, 41. By operating the auxiliary separation units 21, 41 properly, and possibly by adding further separation units, this provides for the possibility to generate a larger mass flow rate of purified DMS (and/or DMSO/metyl mercaptan) and purified acetone, while the mass flow rate of purified methanol decreases (since some methanol will be removed from the main stream 4). The method may focus completely on the output of purified second and/or third compound and no purified methanol needs to be produced.

[0098] As shown in FIG. 6, the exemplified process plant 1 further comprises a recirculation flow line 61 arranged to recirculate a portion of the stream 4c-4f from a point in the processing arrangement 2 downstream of the first primary separation unit 20, where DMS and/or methyl mercaptan is separated from the stream, to a point in the processing arrangement 2 upstream of the first primary separation unit 20. In this case the recirculation flow line 61 starts downstream the last treatment unit 50 where the methanol may be purified to a relatively high degree, but it could instead start further upstream, even directly downstream the first primary separation unit 20 (at stream 4c).

[0099] As seen in FIG. 6 it is further included a DMS/methyl mercaptan production reactor 60 through which the portion of the stream 4c-4f is fed during recirculation. Further, a supply line 71 is arranged for feeding one or more sulphur compounds, such as H.sub.2S, to the DMS/methyl mercaptan production reactor 60, wherein the sulphur compounds are capable of reacting with methanol present in the recirculated flow so as to form DMS and/or methyl mercaptan.

[0100] This way additional amounts of DMS or methyl mercaptan are produced from methanol and sulphur compounds and introduced in the stream 4b upstream of the first primary separation unit 20. The sulphur compounds are preferably obtained at least partly from flow 25 and/or flow 43 as mentioned above. The additional amounts of DMS or methyl mercaptan can then be separated in the first primary separation unit 20 and then further purified as described above.

[0101] The process plant 1 of FIG. 6 further comprises a lignin treatment arrangement 80 arranged to react at least a portion of a flow of incoming lignin 81, obtained from a wood or pulp decomposition and/or conversion process, with sulphur from an incoming flow 72 of sulphur compounds so as to form DMS and/or methyl mercaptan (as well as reactive lignin 82, which may be used for other purposes not directly related to the method of this disclosure). The flow 72 of sulphur compounds may include H.sub.2S and is preferably obtained at least partly from flow 25 (see above).

[0102] The process plant 1 further comprises a flow line 83 for feeding DMS and/or methyl mercaptan from the lignin treatment arrangement 80 to a point in the processing arrangement 2 upstream of the first primary a separation unit 20 where DMS and/or methyl mercaptan is separated from the stream. Additional DMS (or DMSO or methyl mercaptan) can thus be produced, separated and purified in line with what is described above.

[0103] The invention is not limited by the embodiments described above but can be modified in various ways within the scope of the claims. Besides the many modifications and variations already described above in relation to the example shown in FIG. 6, it may be mentioned that the recirculation arrangement 60, 61 and the lignin treatment arrangement 80, 83 are optional features. Further, another variant of an oxidized derivative of DMS, besides DMSO, is dimethyl sulfone (DMSO.sub.2).