Method for producing tetrahydrothiophene

Abstract

The present invention relates to a process for the production of tetrahydrothiophene, which comprises the following successive stages: a) reacting 1,4-butanediol in the gas phase, in the presence of at least one catalyst and in the presence of hydrogen sulfide (H2S) to form a stream (A) comprising tetrahydrothiophene, water and possibly unreacted hydrogen sulfide; b) condensing stream (A) to obtain a tetrahydrothiophene-rich stream (B) and a gaseous effluent (C) containing the condensables possibly formed at the end of stage a); c) at least one stage of purifying stream (B), preferably by sedimentation, in which an aqueous phase, an organic phase constituting stream (D) and a gaseous effluent (E) are separated; d) optionally distilling stream (D) at least once to isolate tetrahydrothiophene from a gaseous effluent (F); e) recovering the tetrahydrothiophene separated in stages c) and optionally d); stages b) and c) may be successive or simultaneous; f) gas-liquid extraction carried out in a column to which is supplied a gas stream (G) comprising one or more of gaseous effluent (C) and / or gaseous effluent (E) and optionally gaseous effluent (F) and a liquid stream of 1,4-butanediol, to form at the column outlet a gas stream (H) and an enriched liquid stream (I) containing 1,4-butanediol; g) recycling all or part of stream (I) to reaction a).

Description

[0001] The present invention relates to a process for the production of tetrahydrothiophene incorporating the treatment of gaseous emissions. The present invention also relates to a process for treating gaseous emissions from a tetrahydrothiophene production unit.

[0002] Tetrahydrothiophene is a compound that exhibits great industrial interest. It is known as an additive in consumption gases for detecting possible leaks due to its very characteristic odor. As a result, it is present in the gases injected into the domestic supply network, in gas transportation networks, in storage units, and in the interconnections with other gas transportation networks.

[0003] The synthesis of tetrahydrothiophene (hereinafter denoted as THT) from the following in the presence of a heterogeneous catalyst: 1,4 - butanediol (hereinafter denoted as BDO) or tetrahydrofuran (denoted as THF) and hydrogen sulfide (hereinafter denoted as H2S) is known.

[0004] It is accepted in the literature that the first in - situ stage is the conversion of BDO into THF according to the following reaction:

[0005] 1,4 - BDO → THF + H2O

[0006] The second stage is the conversion of THF into THT in the presence of H2S according to the following reaction:

[0007] THF + H2S → THT + H2O

[0008] In fact, the following side reactions can occur depending on the operating conditions:

[0009] THT → dihydrothiophene + H2

[0010] THT → thiophene + 2H2

[0011] THT + H2 → propylene + CH3SH

[0012] THT + H2 → butene + H2S

[0013] In addition, this synthesis can generate degradation products such as CO2, ethylene, and C1 - C4 thiols.

[0014] Furthermore, gaseous emissions (also referred to as off-gases) are emitted throughout the production process. These emissions can contain excessive amounts of hydrogen sulfide, cracking products, intermediate compounds (such as THF), and significant amounts of THT that may potentially be introduced into the cycle. Typically, these emissions are incinerated, which results in high emissions of sulfur oxides (SO2) into the atmosphere, which can potentially cause acid rain. In fact, currently these emissions are no longer tolerable. It should be noted that these gaseous emissions themselves cannot be recycled in the main reaction, because the presence of ethylene, propylene, and butene causes very premature aging of the catalyst through the formation of gums and coke. Therefore, for recycling, these emissions must undergo a very large amount of bleeding to avoid recycling large amounts of C2 to C4 hydrocarbons. In fact, this exhaust gas also has the result of losing almost all of the THT and sulfur or oxygen compounds present in these off-gases.

[0015] In fact, from the perspective of current ecological considerations, there is indeed a need today for a process for the synthesis of THT that is more environmentally friendly while retaining high yields.

[0016] An object of the present invention is to provide a process for preparing THT that enables improved management of gaseous emissions and is particularly more environmentally friendly.

[0017] Another object of the present invention is to reduce the amount of gaseous emissions, and particularly sulfur dioxide, emitted after incineration of the emissions during the production of tetrahydrothiophene.

[0018] Another object of the present invention is to provide a more economical process for the preparation of tetrahydrothiophene.

[0019] Finally, another object of the present invention is to provide a process and / or device for the treatment of gaseous emissions that can be easily incorporated into a unit for the production of tetrahydrothiophene.

[0020] The process according to the present invention meets all or some of the objectives described above.

[0021] The inventors have surprisingly found that gaseous emissions can be collected and treated by gas-liquid extraction using 1,4-butanediol as an extraction solvent. The gas (off-gas)-liquid (1,4-butanediol) extraction according to the present invention enables, in particular, the sulfur compounds and tetrahydrofuran contained in the off-gas to be introduced into the liquid 1,4-butanediol.

[0022] The term "sulfur compound" is understood to mean a compound containing at least one sulfur atom, preferably one sulfur atom. In particular, the term "sulfur compound" is understood to mean tetrahydrothiophene and sulfur-containing by-products such as thiophene, dihydrothiophene, mercaptan and possibly hydrogen sulfide, which compounds have been entrained in the off-gas during the purification phase, preferably in the condensation and / or decantation and / or distillation phase, after stage a).

[0023] Thus, the gas-liquid extraction according to the present invention makes it possible to recover sulfur compounds, and in particular tetrahydrothiophene, and tetrahydrofuran as an intermediate of the reaction.

[0024] The process according to the present invention uses one of the synthesis reactants, that is to say 1,4-butanediol, as the solvent in the gas-liquid extraction stage. Thus, the recovery of sulfur compounds and tetrahydrofuran uses the synthesis reactant, thereby optimizing this recovery route.

[0025] Very advantageously, 1,4-butanediol enriched in sulfur compounds and optionally tetrahydrofuran is used, preferably directly (for example without a purification phase), as the reactant for forming tetrahydrothiophene. Thus, the process according to the present invention makes it possible to reintroduce into the tetrahydrothiophene production process favorable compounds which have hitherto been incinerated.

[0026] The extraction stage according to the present invention exhibits the advantage of being able to be easily incorporated into a tetrahydrothiophene production unit, since it only treats the off-gas. Thus it consumes little energy and uses simple equipment.

[0027] The extraction also makes it possible to reduce the amount of off-gas treated by incineration and to greatly reduce the release of sulfur dioxide into the atmosphere.

[0028] The process according to the present invention for producing tetrahydrothiophene is thus more economical and exhibits better productivity, and at the same time is more environmentally friendly. Summary of the Invention

[0029] Thus, one aspect of the present invention is a process for producing tetrahydrothiophene, which comprises the following successive stages:

[0030] a) Reacting 1,4-butanediol in the gas phase in the presence of at least one catalyst and in the presence of hydrogen sulfide (H2S) to form a stream (A) comprising tetrahydrothiophene, water and possibly unreacted hydrogen sulfide;

[0031] b) Condensing stream (A) to obtain a stream (B) rich in tetrahydrothiophene and an off-gas (C) containing the non-condensable substances possibly formed at the end of stage a);

[0032] c) A stage of at least one purified stream (B), preferably by decantation, in which an aqueous phase, an organic phase constituting stream (D), and an off-gas (E) are separated;

[0033] d) Optionally distilling stream (D) at least once to isolate tetrahydrothiophene from off-gas (F);

[0034] e) Collecting the tetrahydrothiophene isolated in stage c) and optionally stage d);

[0035] Stages b) and c) can be successive or simultaneous;

[0036] f) Gas-liquid extraction carried out in a column to which is supplied a gas stream (G) comprising one or more gases containing off-gas (C) and / or off-gas (E) and possibly off-gas (F), and a liquid stream of 1,4-butanediol, to form at the column outlet a gas stream (H) and a liquid stream (I) containing enriched 1,4-butanediol;

[0037] g) Recycling all or part of stream (I) to reaction a). Description of the Drawings

[0038] Figure 1 is a drawing of the apparatus for implementing the process. Detailed Description

[0039] The other characteristics, aspects, objects, and advantages of the present invention will become even clearer upon reading the following description.

[0040] It is stipulated that the expressions "from... to..." and "between... and..." used in this specification should be understood to include each of the recited end values.

[0041] The term "enriched 1,4-butanediol" is understood to mean 1,4-butanediol obtained after the gas-liquid extraction according to the present invention, that is, after stage (f).

[0042] In particular, the enriched 1,4-butanediol is a composition comprising 1,4-butanediol and at least one sulfur compound, preferably comprising 1,4-butanediol and tetrahydrothiophene, and possibly sulfur-containing by-products such as thiophene and dihydrothiophene, and possibly tetrahydrofuran. In particular, relative to the total weight of the enriched 1,4-butanediol, the enriched 1,4-butanediol contains between 0.1% by weight and 20% by weight of tetrahydrothiophene, preferably between 1% by weight and 10% by weight, more preferably between 1% by weight and 5% by weight of tetrahydrothiophene.

[0043] More particularly, the term "enriched 1,4-butanediol" is understood to mean a composition comprising: ​

[0044] - 1,4-butanediol, preferably at least 50% by weight of 1,4-butanediol, more preferably at least 80% by weight, even more preferably at least 90% by weight of 1,4-butanediol, relative to the total weight of the composition;

[0045] - tetrahydrothiophene, preferably between 0.1% by weight and 20% by weight of tetrahydrothiophene, more preferably between 1% by weight and 10% by weight, even more preferably between 1% by weight and 5% by weight of tetrahydrothiophene, relative to the total weight of the composition;

[0046] - possible sulfur-containing by-products, preferably dihydrothiophene and / or thiophene;

[0047] - possible tetrahydrofuran;

[0048] - possible hydrogen sulfide;

[0049] - possible water; and

[0050] - possible non-condensable compounds.

[0051] Thus, the composition may comprise:

[0052] - between 80% by weight and 99% by weight of 1,4-butanediol, relative to the total weight of the composition, and

[0053] - between 0.1% by weight and 10% by weight, preferably between 1% by weight and 5% by weight of tetrahydrothiophene, relative to the total weight of the composition.

[0054] The term "off-gas" or "gaseous emissions" is understood to mean a gas phase which contains at least one sulfur compound as defined above and is collected in particular after at least one separation or purification stage of the stream (A) after reaction a).

[0055] The off-gas, and in particular streams (C), (E) and (F), contains at least one sulfur compound and possible tetrahydrofuran.

[0056] These off-gases may contain, and in fact even consist of: tetrahydrothiophene, tetrahydrofuran, non-condensable compounds, water and sulfur-containing by-products, possible hydrogen sulfide.

[0057] These off-gases can be obtained from the overhead streams and / or percolates of condensers, decanters and / or distillation columns.

[0058] In particular, the off-gas or gaseous emissions according to the invention are generally considered as waste and are generally sent to an incinerator.

[0059] The term "non-condensable compound" is understood to mean a compound that remains in the gaseous state under the conditions of the process stages. As non-condensable compounds, CH4, CO, CO2, H2 and N2 may be mentioned.

[0060] The process according to the invention comprises the successive stages mentioned above: stages a) to g). The process may comprise intermediate purification stages.

[0061] Stage a) - Reaction

[0062] During stage a), 1,4-butanediol, preferably in the gaseous state, reacts with hydrogen sulfide to form stream (A), which contains tetrahydrothiophene, water, possibly unreacted hydrogen sulfide, possibly tetrahydrofuran and possibly sulfur-containing by-products. Stream (A) may also contain unreacted 1,4-butanediol. Olefins and / or non-condensable compounds may also be present in stream (A).

[0063] Thus, gas streams of hydrogen sulfide and 1,4-butanediol, each in gaseous form, are introduced into the reactor. These reactants may reach the reactor via the same stream or separately. Preferably, the reactants reach via the same stream, and the hydrogen sulfide stream is preheated and enables the vaporization of the liquid stream of 1,4-butanediol.

[0064] The reaction takes place in the gas phase and preferably continuously. One or more catalysts may be located in a fixed bed in a reactor with trays, which is multi-tubular or non-multi-tubular, isothermal or adiabatic. A multi-zone reactor with the same or different catalysts, with or without multiple injections of BDO, or multiple reactors in series may also be used.

[0065] The reaction temperature may be between 200 °C and 450 °C, preferably between 200 °C and 380 °C. Preferably, the reaction temperature is between 200 °C and 360 °C. Above this temperature, the catalyst may be physically damaged (especially by sintering and coking). This temperature may vary depending on the duration of operation of the catalyst, which may be partially deactivated over time.

[0066] The pressure range in the reactor may be from atmospheric pressure to 50 bar, preferably from 1 to 30 bar, and more preferably from 1 to 10 bar.

[0067] The molar ratio of hydrogen sulfide / 1,4-butanediol may be between 1 and 100, preferably between 1 and 50, and more particularly between 1 and 10. Hydrogen sulfide is preferably in excess relative to 1,4-butanediol.

[0068] The flow rate of BDO, relative to the amount of catalyst, can be between 0.005 and 10 kg / h / kg of catalyst, preferably between 0.1 and 5 kg / h / kg of catalyst, more preferably between 0.1 and 1 kg of BDO / h / kg of catalyst.

[0069] The reactor contains a catalyst for the reaction for forming tetrahydrothiophene, preferably in the gas phase. Among the catalysts that can be used, silica, alumina or aluminosilicate can be used. Preferably, the catalyst is alumina, which is more or less doped with alkali metal or alkaline earth metal oxides, preferably undoped.

[0070] Preferably, the catalyst used is such an alumina that contains less than 0.3 wt% of Na2O content relative to the total weight of the catalyst, such as Spheralite 505 sold by Axens.

[0071] A stream (A) is thus obtained, which contains tetrahydrothiophene, water, possibly unreacted hydrogen sulfide, possibly tetrahydrofuran and possibly sulfur-containing by-products.

[0072] Stage b) - Condensation

[0073] By any conventional technique, preferably by one or more condensers or economizers, the stream (A) from stage a) is subjected to a condensation stage. During condensation, the stream (A) is cooled particularly as low as possible to maximize the removal of water. Preferably, the stream (A) is condensed at a temperature between 20 °C and 70 °C, for example between 30 °C and 60 °C. The condensation stage makes it possible to obtain a stream (B) rich in tetrahydrothiophene and an off-gas (C), which contains the non-condensable substances possibly formed at the end of stage a) and contains at least one sulfur compound and possibly tetrahydrofuran. Preferably, the off-gas (C) contains tetrahydrothiophene, the non-condensable substances possibly formed at the end of stage a), possibly unreacted hydrogen sulfide, possibly tetrahydrofuran and possibly sulfur-containing by-products.

[0074] Stage c) - Purification, preferably by decantation

[0075] The stream (B) rich in tetrahydrothiophene is subjected to a purification stage. Preferably, it is a decantation stage to separate the organic phase from the aqueous phase containing the water generated during reaction a). In addition, the off-gas is also removed during this stage. Thus, the organic phase (D) containing tetrahydrothiophene is separated from the aqueous phase. The off-gas is collected in a gas stream designated as (E), and the gas stream designated as (E) contains at least one sulfur compound and possibly tetrahydrofuran. Preferably, the gas stream (E) contains tetrahydrothiophene, possibly unreacted hydrogen sulfide, possibly tetrahydrofuran and possibly sulfur-containing by-products.

[0076] Preferably, the stream (B) is separated at a temperature between 20 °C and 70 °C, preferably between 30 °C and 60 °C. The pressure can be an absolute pressure between 1 and 40 bar.

[0077] Stage d) - Possible distillation

[0078] The stream (D), i.e. the organic phase obtained from the purification stage c), can be subjected to one or more distillations. One or more successive distillations make it possible to remove compounds heavier than THT and / or compounds lighter than THT.

[0079] According to one embodiment of the invention, two distillation columns can be used in series, the first for separating the heavy compounds and the second for separating the light compounds.

[0080] According to another embodiment of the distillation stage, the distillation is carried out in a dividing wall column of the DWC type, said dividing wall column comprising 3 outlets: the top fraction contains light impurities, the bottom fraction contains heavy impurities and the middle fraction contains purified THT.

[0081] The distillation of stage d) can be carried out at an absolute pressure between 0.05 and 75 bar, preferably between 1 and 30 bar, more preferably between 1 and 5 bar.

[0082] The distillation of stage d) can be carried out at a temperature between 20 °C and 200 °C, preferably between 40 °C and 160 °C, more preferably between 60 °C and 140 °C.

[0083] The distillation can be carried out in any known type of distillation column. It can be a column with trays (e.g. trays with caps, trays with valves or perforated trays) or with packing (e.g. with random or structured packing). The distillation can be carried out in a plate column which preferably comprises between 5 and 50 trays, more preferably between 10 and 40 trays, for example between 25 and 30 trays. The distillation can also be carried out in a dividing wall column (DWC, dividing wall column). The dividing wall can be fixed or movable, for example with structured or random packing.

[0084] Stage e) - Recovery of the tetrahydrothiophene separated in stage c) and / or d)

[0085] The tetrahydrothiophene is recovered at the end of stage c) and / or d) in order to be able to be stored or to be used directly in further synthesis stages.

[0086] Stage f) - Liquid-gas extraction

[0087] The purpose of the liquid-gas extraction stage is to recover the off-gases produced after the reaction stage a). A single gas stream or the entire gas stream of the production unit can be recovered.

[0088] Preferably, the following off-gases can be withdrawn and incorporated into stream (G), which off-gases contain the non-condensable (C) obtained from the condensation stage b), the off-gas (E) obtained from the purification stage c), and the possible gaseous fraction (F) obtained from the distillation stage d). It would not deviate from the scope of the invention if streams (C), (E), and (F) reached the liquid-gas extraction unit individually.

[0089] The liquid-gas extraction stage is carried out in a unit comprising at least one column. The column is fed with one or more gas streams and a liquid stream of 1,4-butanediol. At the column outlet, a gas stream (H) comprising non-condensable compounds and possibly hydrogen sulfide is generated, as well as a liquid stream (I) comprising enriched 1,4-butanediol.

[0090] The gas-liquid extraction can be carried out in at least one absorption column or at least one tank, preferably a tank with mechanical stirring. The absorption column is in particular selected from packed columns (e.g., random or structured packings), plate columns, bubble columns, spray columns, and falling film columns, and indeed even a tank with mechanical stirring. Preferably, one or more packed columns are used, e.g., between 1 and 10 columns. Multiple absorption columns can be used in parallel or in series. Preferably, the gas-liquid extraction is carried out in at least one absorption column or at least one tank, preferably a tank with mechanical stirring.

[0091] The flow rates of the gas (off-gas) phase and the liquid (1,4-butanediol) phase depend on the type and number of columns. The off-gas and liquid 1,4-butanediol enter the absorption column in cocurrent or countercurrent, preferably countercurrent. For example, regardless of whether the liquid phase is recycled to the column, the off-gas arrives via the bottom of the column and the liquid 1,4-butanediol arrives via the top of the column.

[0092] This type of system (which enables gas-liquid extraction) is generally referred to as an "absorber" and, in the context of the present invention, as a "1,4-butanediol absorber".

[0093] The gas-liquid extraction can be carried out at a temperature between 25 °C and 200 °C, preferably between 25 °C and 150 °C, preferably between 25 °C and 100 °C, more preferably between 40 °C and 90 °C. The gas-liquid extraction is carried out at an absolute pressure between 1 and 40 bar, preferably between 1 and 10 bar, even more preferably between 1 and 5 bar.

[0094] The weight ratio of the off-gas to 1,4-butanediol can be between 0.001 and 10, preferably between 0.1 and 3, more preferably between 0.5 and 2.

[0095] Stage f) allows sulfur compounds and possibly tetrahydrofuran to be fed into liquid 1,4 - butanediol and reduces, and in fact even avoids, the emission of SO2 into the atmosphere after incineration.

[0096] The exhaust gas thus treated (i.e., at least one of its sulfur compounds has been absorbed in 1,4 - butanediol) can then be collected, optionally incinerated, and released into the atmosphere with a reduced content of SO2 after incineration: preferably, the exhaust gas contains little or no SO2 after incineration.

[0097] According to an embodiment of the process of the present invention, the gas stream (H) can be recycled in reaction stage a). Depending on the amount of hydrogen sulfide introduced into reaction a), the gas stream (H) can be rich in hydrogen sulfide. This stream can be compressed using a compressor to be subsequently introduced into the reactor of stage a). A bleed can be present in this feed line to remove possible impurities. This additional recycling stage also makes it possible to reduce the production of sulfur dioxide generated by the incineration of these exhaust gases and to reduce the amount of fresh hydrogen sulfide introduced during stage a).

[0098] Preferably, relative to the total weight of the enriched 1,4 - butanediol, the enriched 1,4 - butanediol contains between 0.1% by weight and 20% by weight of tetrahydrothiophene, preferably between 1% by weight and 10% by weight of tetrahydrothiophene, more preferably between 1% by weight and 5% by weight of tetrahydrothiophene.

[0099] Stage g) - Recycling

[0100] The stream (I) containing the enriched 1,4 - butanediol is directed in whole or in part to stage a). It is thus used as a reactant. Optionally, it is introduced as a mixture with fresh 1,4 - butanediol. Preferably, the entire stream (I) is recycled in stage a). Preferably, stream (I) is involved in stage a) without adding fresh 1,4 - butanediol.

[0101] The term "fresh 1,4 - butanediol" is understood to mean 1,4 - butanediol that is not enriched in the context of the present invention, i.e., 1,4 - butanediol that has not undergone the gas - liquid extraction according to the present invention.

[0102] Thus, stream (I) makes it possible to recycle the tetrahydrothiophene and other sulfur compounds that have been fed into the exhaust gas. This stream also makes it possible to recycle the tetrahydrofuran that may be present in the exhaust gas.

[0103] The present invention also relates to a process for treating exhaust gas discharged from a unit for producing tetrahydrothiophene from 1,4 - butanediol and hydrogen sulfide, the process comprising the following stages:

[0104] -Collect the off-gas from the unit for the production of tetrahydrothiophene, said off-gas containing at least one sulfur compound (preferably tetrahydrothiophene) and possibly tetrahydrofuran;

[0105] -Perform gas-liquid extraction of said at least one sulfur compound, preferably tetrahydrothiophene, with liquid 1,4-butanediol to obtain a liquid 1,4-butanediol enriched in sulfur compound (preferably tetrahydrothiophene) and possibly tetrahydrofuran;

[0106] -Optionally use said enriched liquid 1,4-butanediol as a reactant for the reaction to produce tetrahydrothiophene from 1,4-butanediol and hydrogen sulfide.

[0107] All elements of the process for treating the off-gas are those defined for the process for the production of tetrahydrothiophene according to the present invention. Description of the Drawings

[0109] Appendix Figure 1 is a diagram showing an industrial plant for the production of tetrahydrothiophene according to the present invention.

[0110] Reaction stage a) starts in reactor R1 from completely or partially enriched 1,4-butanediol and hydrogen sulfide.

[0111] The alcohol stream enters reactor R1 via pipeline 1. The hydrogen sulfide stream enters reactor R1 via pipeline 2. The stream A leaving reactor R1 via pipeline 3 contains tetrahydrothiophene, water, possibly unreacted hydrogen sulfide, possibly tetrahydrofuran, possibly sulfur-containing by-products, possibly olefins, and possibly non-condensable substances.

[0112] Condensation stage b) is carried out in device R2 (such as a condenser). Device R2 is fed with stream A via pipeline 3. The stream B containing liquid tetrahydrothiophene is discharged from device R2 via pipeline 4, and the off-gas C is discharged via pipeline 5.

[0113] Decantation stage c) is carried out in device R3. The aqueous phase is separated and removed via pipeline 6, the stream D containing tetrahydrothiophene is separated and discharged via pipeline 7, and the off-gas E is removed via pipeline 8.

[0114] Distillation stage d) is carried out in device R4. When using a column of DWC type, heavy impurities are removed via pipeline 9, tetrahydrothiophene is isolated via pipeline 10, and light impurities F are removed via pipeline 11.

[0115] The gas-liquid extraction stage f) is carried out in apparatus R5. The apparatus is fed with liquid 1,4-butanediol via line 12 and with off-gas G via line 13. Line 13 collects all the off-gases and gaseous impurities generated during the separation and purification stages of the process, that is to say, line 5 collects off-gas C from the condensation stage b), line 8 collects off-gas E from the decantation stage c), and line 11 collects the gaseous fraction F from the distillation stage d). At the outlet of apparatus R5, off-gas H is removed via line 14, and a stream I of enriched liquid 1,4-butanediol, which is particularly enriched in tetrahydrothiophene, is discharged via line 15.

[0116] Line 15 conveys the enriched 1,4-butanediol to reactor R1 for use as a reactant.

[0117] Depending on the amount of off-gas H collected and their composition (components, composition) (depending on whether hydrogen sulfide was introduced in excess in reaction a)), these off-gases can be collected and reintroduced into stage a).

[0118] The invention also relates to the use of a gas-liquid extraction apparatus in a process for the production of tetrahydrothiophene, as defined above, said gas-liquid extraction apparatus being used for the collection and recycling of off-gases produced after purification of the crude reaction product.

[0119] The following examples serve to illustrate the invention, but are in no way limiting.

[0120] Examples

[0121] The process according to the invention is carried out. The gas-liquid extraction stage is carried out in a Norton IMTP absorption column having a diameter of 400 mm and a packing height of 4 m. The pressure inside the column is 3 bar absolute, with the bottom temperature being between 75 °C and 80 °C and the top temperature being 64 °C. The weight ratio of off-gas to 1,4-butanediol is 1.13.

[0122] Analysis is carried out on the gas streams at the inlet (stream entering via line 13 in Figure 1 ) and at the outlet (stream leaving via line 14 in Figure 1 ) of the reactor for gas-liquid extraction for carrying out stage f) of the process. The absorption efficiency can thus be calculated. These measurement results are shown in Table 1 below.

[0123] Absorption efficiency <![CDATA[H2S]]> 2.4％ <![CDATA[CO2]]> 0.8％ <![CDATA[H2O]]> 90.8％ <![CDATA[C2 to C4 olefins]]> 0.9％ <![CDATA[C1 to C4 mercaptans]]> 15.4％ DHT / Thiophene 37.1％ THF 45.1％ THT 94.8％

[0124] Table 1

[0125] This test shows that 94.8% of the THT present in the off-gas is absorbed by BDO and is thus collected.

[0126] Depending on the production of the unit, if the charging of 38 kg / h of THT is considered, then subsequently the emission of 28 kg / h of SO2 after incineration is avoided, i.e., 224 tons / year (for an operation of 8000 h / year).

[0127] Another very large advantage of this absorption is the non-absorption of olefins. This is because more than 99% of the olefins pass through the absorber and are almost removed by the percolation of the leaving gas phase and do not recycle in the reaction.

[0128] Another advantage is also the non-absorption of the non-condensable (CO2) obtained from the pyrolysis products. This is because the absorption of this non-condensable by the BDO absorption column would lead to the accumulation of non-condensables, which is completely unacceptable for the process.

[0129] Regarding THF as a reaction intermediate, at least 45% of this product is charged in these effluents, and it is known that the content of THF in these effluents increases with the aging of the catalyst.

[0130] Regarding the other partially absorbed compounds, except for hydrogen sulfide which will react at the level of the main reaction, their recycling will result in their reappearance in the purification stage downstream of the main reaction.

Claims

1. A process for producing tetrahydrothiophene, which comprises the following successive stages: a) reacting 1,4 - butanediol in the gas phase, in the presence of at least one catalyst and in the presence of hydrogen sulfide (H2S) to form a stream (A) comprising tetrahydrothiophene, water and possibly unreacted hydrogen sulfide; b) condensing stream (A) to obtain a stream (B) rich in tetrahydrothiophene and an off - gas (C) containing possibly formed non - condensable substances at the end of stage a); c) at least one stage of purifying stream (B), in which an aqueous phase, an organic phase constituting stream (D) and an off - gas (E) are separated; d) optionally distilling stream (D) at least once to isolate tetrahydrothiophene from an off - gas (F); e) collecting the tetrahydrothiophene isolated in stage c) and optionally in stage d); Stages b) and c) can be successive or simultaneous; f) gas - liquid extraction carried out in a column, to which is supplied a gas stream (G) comprising one or more of off - gas (C) and / or off - gas (E) and possibly off - gas (F) and a liquid stream of 1,4 - butanediol, to form at the column outlet a gas stream (H) and a liquid stream (I) comprising enriched 1,4 - butanediol; g) recycling all or part of stream (I) to reaction a).

2. The process according to claim 1, characterized in that During stage f), the weight ratio of the off - gas to 1,4 - butanediol is between 0.001 and 10.

3. The process according to claim 1 or 2, characterized in that The gas - liquid extraction is carried out at a temperature between 25°C and 200°C.

4. The process according to claim 1 or 2, characterized in that The gas - liquid extraction is carried out at an absolute pressure between 1 and 40 bar.

5. The process according to claim 1 or 2, characterized in that, During stage g), the entire stream (I) is recycled in stage a).

6. The process according to claim 1 or 2, characterized in that Relative to the total weight of the enriched 1,4 - butanediol, the enriched 1,4 - butanediol contains between 0.1% by weight and 20% by weight of tetrahydrothiophene.

7. The process according to claim 1 or 2, characterized in that The off - gas (C) obtained from stage a) contains tetrahydrothiophene, possibly formed non - condensable substances at the end of stage a), possibly unreacted hydrogen sulfide, possibly tetrahydrofuran and possibly sulfur - containing by - products.

8. The process according to claim 1 or 2, characterized in that The gas - liquid extraction is carried out in at least one absorption column or at least one tank.

9. The process according to claim 1 or 2, characterized in that The stream (H) originating from the gas - liquid extraction stage is recycled in stage a).

10. Use of a gas - liquid extraction device for collecting and recycling off - gases produced after purification of a crude reaction product in a process for producing tetrahydrothiophene as defined in any one of claims 1 to 9.

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