METHOD FOR PREPARING METHYL MERCAPTAN WITH TREATMENT OF GASEOUS WASTE

Abstract

The present invention relates to a process for producing methyl mercaptan, comprising the following steps: A) methanol is reacted with hydrogen sulfide to form a stream (M), preferably in gaseous form, comprising methyl mercaptan, unreacted H.sub.2S and possibly sulfur byproducts; B) optionally, said stream (M) is condensed; C) at least one step of purification of said stream (M) is performed to obtain a stream enriched in methyl mercaptan; D) the gaseous vents resulting from said at least one purification step are recovered, said gaseous vents comprising at least one sulfur compound, preferably H.sub.2S; E) a gas-liquid extraction of said at least one sulfur compound, preferably H.sub.2S, is performed with liquid methanol so as to obtain a liquid methanol enriched in sulfur compound(s), preferably in H.sub.2S; and F) optionally, said enriched methanol is used as reagent for the reaction of step A).

Claims

1. A process for producing methyl mercaptan, comprising the following steps: A) methanol is reacted with hydrogen sulfide to form a stream (M), preferably in gaseous form, comprising methyl mercaptan, unreacted H.sub.2S and possibly sulfur byproducts; B) optionally, said stream (M) is condensed; C) at least one step of purification of said stream (M) is performed to obtain a stream enriched in methyl mercaptan; D) the gaseous vents resulting from said at least one purification step are recovered, said gaseous vents comprising at least one sulfur compound, preferably H.sub.2S; E) a gas-liquid extraction of said at least one sulfur compound, preferably H.sub.2S, is performed with liquid methanol so as to obtain a liquid methanol enriched in sulfur compound(s), preferably in H.sub.2S; and F) optionally, said enriched methanol is used as reagent for the reaction of step A).

2. The process for producing methyl mercaptan according to claim 1, in which, during step C), at least one phase separation step is performed, preferably by decantation, and/or at least one purge is performed.

3. The process for producing methyl mercaptan according to claim 1, in which the mass ratio of the gaseous vents relative to methanol in step E) is between 0.001 and 0.5, preferably between 0.005 and 0.1.

4. The process for producing methyl mercaptan according to claim 1, in which the gas-liquid extraction is performed at a temperature of between 0 C. and 80 C., for example between 5 C. and 80 C., preferably between 10 C. and 80 C., more preferentially between 20 C. and 70 C.

5. The process for producing methyl mercaptan according to claim 1, in which the gas-liquid extraction is performed at a pressure of between 4 and 60 bar absolute, preferably between 10 and 50 bar absolute.

6. The process for producing methyl mercaptan according to claim 1, in which said enriched methanol is used as reagent for the reaction of step A) as a mixture with fresh methanol.

7. The process for producing methyl mercaptan according to claim 1, in which said enriched methanol comprises between 0.1% and 20% by weight of H.sub.2S, preferably between 1% and 10% by weight of H.sub.2S, more preferentially between 1% and 5% by weight of H.sub.2S, relative to the total weight of the enriched methanol.

8. The process for producing methyl mercaptan according to claim 1, in which said gaseous vents comprise H.sub.2S, methyl mercaptan, inert compounds, possibly water and sulfur byproducts such as dimethyl sulfide and dimethyl disulfide.

9. The process for producing methyl mercaptan according to claim 1, in which the gas-liquid extraction is performed in at least one absorption column or in at least one tank, preferably with mechanical stirring.

Description

DESCRIPTION OF THE FIGURES

[0180] FIG. 1 shows an embodiment of a process for producing methyl mercaptan via the methanol route in which the vent gases E1 and E2 are recovered.

[0181] The reaction step A) is performed in a reactor (1) using methanol and H.sub.2S.

[0182] Stream (M) leaving the reactor (1) comprises MeSH, water, H.sub.2S and sulfur byproducts.

[0183] Stream (M) is condensed in a condenser (II). It is then separated in a decanter (Ill) into three streams:

[0184] a stream (N) comprising H.sub.2S;

[0185] a stream (O) comprising water; and

[0186] a stream (P) comprising MeSH, water, H.sub.2S and sulfur byproducts.

[0187] Stream (N) is purged and said purge represents the vent gas E1.

[0188] Stream (P) is distilled in a distillation column (IV) to remove the H.sub.2S (stream (R) at the top of the column) and to obtain a stream (S) at the bottom of the column comprising MeSH, water and sulfur byproducts. Stream (S) is then distilled in a distillation column (V) to obtain a stream (U) at the bottom of the column comprising the sulfur byproducts and a stream (T) at the top of the column comprising the MeSH and the water. Stream (T) is then separated in a decanter (VI) into a stream (V) comprising MeSH and water and a stream (W) comprising water. The vent gas from this decanter is recovered and represents the vent gas E2.

[0189] FIG. 2 shows one embodiment of the drying process in which the vent gas E3 is recovered.

[0190] Stream (A) enters distillation column (1). Stream (A) is distilled in column (1). The distillate (B) is recovered at the top of the column in gaseous form. The distillate (B) is then condensed in a condenser (2) where it is recovered in two-phase liquid form (condensate (C)). The condensate (C) then settles in the decanter (3) so as to obtain:

[0191] an aqueous phase (D); and

[0192] an organic phase (E).

[0193] The organic phase (E) then serves as reflux for the distillation column (1).

[0194] After decantation, the vent gas E3 is recovered by stripping with an inert gas from the aqueous phase (D).

[0195] The dried methyl mercaptan is recovered at the bottom of column (1) (stream (F)).

[0196] The expression between X and X includes the limits mentioned, unless mentioned otherwise.

[0197] The examples that follow illustrate the present invention but are not in any way limiting.

EXAMPLES

Example 1: Comparative Example, without Gas-Liquid Extraction

[0198] The conditions are as follows:

[0199] In a methyl mercaptan production unit, the vent gases E1, E2 and E3 were recovered as mentioned in FIGS. 1 and 2.

[0200] After recovery, the vent gases E1, E2 and E3 are combined and their composition is as follows:

TABLE-US-00001 TABLE 1 Amount (weight % relative to the total Component weight of the vent gases) H.sub.2S 74.5 MeSH 16.9 Inert matter 8.2 Water 0.3 Sulfur byproducts (DMS and DMDS) 0.1 TOTAL 100

[0201] For 100 tonnes/day of methyl mercaptan produced, 3 tonnes/day of these vent gases are produced and must be incinerated.

[0202] Thus, for a unit producing 100 000 tonnes/year of methyl mercaptan, these vent gases can represent up to 3000 tonnes/year to be incinerated. Their incineration leads to 5000 tonnes/year of SO.sub.2 emissions.

Example 2: Example in Accordance with the Invention, with Gas-Liquid Extraction

[0203] The vent gases E1, E2 and E3 are recovered in the same way as for Example 1 and the composition of the three combined vent gases is the same:

TABLE-US-00002 TABLE 2 Amount (weight % relative to the total Component weight of the vent gases) H.sub.2S 74.5 MeSH 16.9 Inert matter 8.2 Water 0.3 Sulfur byproducts (DMS and DMDS) 0.1 TOTAL 100

[0204] The combined vent gases are sent to a methanol absorber in order to perform the gas-liquid extraction according to the invention.

[0205] Said extraction is performed in a packed absorption column, the gas stream of the vent gases arriving at the bottom of the column and the liquid methanol arriving at the top of the column counter-currentwise.

[0206] The temperature is 46 C. and the pressure is 27 bar absolute.

[0207] The mass ratio of the gaseous vents to the methanol is 0.05.

[0208] The enriched methanol is recovered at the bottom of the column with the following composition:

TABLE-US-00003 TABLE 3 Component Amount (weight %) Methanol 95.3 H.sub.2S 3.4 MeSH 1 Inert matter 0.14 Water 0.14 Sulfur byproducts (DMS and DMDS) 100

[0209] This absorption makes it possible to recover more than 99% of the H.sub.2S, MeSH and sulfur byproducts that were destined for incineration without this absorber. The inert matter is almost not absorbed by methanol.

[0210] This enriched methanol is then mixed with fresh methanol (methanol that has not undergone the extraction step) to be sent to the reactor where the methyl mercaptan is produced from methanol and H.sub.2S.

[0211] For a unit producing 100 000 tonnes/year of methyl mercaptan, this treatment of the vent gases makes it possible to recycle 2250 tonnes/year of H.sub.2S and to recover an additional 500 tonnes/year of methyl mercaptan.

[0212] Furthermore, the sulfur products remaining in the vent gases are in negligible amount: there is no longer any release of SO.sub.2.