Method for producing 3-methylthiopropionaldehyde

Abstract

The present invention relates to a method for producing 3-methylthiopropionaldehyde, comprising the steps of: a) providing a liquid stream (1) comprising methyl mercaptan and / or a hemithioacetal formed from 3-methylthiopropionaldehyde and methyl mercaptan; b) providing an acrolein-containing vapor stream (2), wherein the majority of said stream is acrolein and the pressure of said stream is below atmospheric pressure; c) introducing the liquid stream comprising methyl mercaptan and / or a hemithioacetal formed from 3-methylthiopropionaldehyde and methyl mercaptan of step a) and the acrolein-containing vapor stream of step b) into a reaction unit (4) by means of a gas-liquid mixer (3); and d) reacting acrolein with the hemithioacetal formed from 3-methylthiopropionaldehyde and methyl mercaptan in the reaction unit (4) of step c) to produce a 3-methylthiopropionaldehyde-containing product mixture.

Description

[Technical Field]

[0001] The present invention relates to a method for producing 3-methylthiopropionaldehyde.

[0002] 3-Methylthiopropionaldehyde, also known as methyl mercaptopropionaldehyde (MMP) or 4-thiapentanal (UN number 2785), is a key intermediate in the production of D,L-methionine and its hydroxy analog, 2-hydroxy-4-methylthiobutyric acid, also known as methionine hydroxy analog (MHA). Typically, 3-methylthiopropionaldehyde is produced by reacting acrolein with methyl mercaptan in a Michael addition reaction.

[0003] U.S. Pat. No. 4,225,516 discloses a two-stage process for producing 3-methylthiopropionaldehyde (MMP): in the first stage, acrolein is absorbed from a gas mixture of MMP, and in the second stage, the acrolein dissolved in MMP reacts with methyl mercaptan, which is continuously added to the reaction, in the presence of a catalyst at a temperature of 10 to 50°C. In the absorption step, a liquid MMP feed stream is introduced at the top of an absorption tower at a temperature ranging from about 0 to about -15°C, and the MMP feed stream is countercurrently contacted with an acrolein-containing vapor stream having a temperature ranging from about 0 to -5°C. However, the process of U.S. Pat. No. 4,225,516 has several drawbacks. First, MMP must be recycled at a very low temperature. However, this results in several drawbacks. First, the optimal reaction temperature for forming MMP from acrolein and methyl mercaptan is 60 to 90°C (see, for example, U.S. Patent Application Publication No. 2014 / 005437), which is therefore higher than the reaction temperature used in the method of U.S. Pat. No. 4,225,516. However, producing MMP at 60 to 90°C in the method of U.S. Pat. No. 4,225,516 and then recycling it at 0 to -15°C would require significantly more energy. Therefore, to produce MMP in sufficient yield at the relatively low reaction temperature of this method, the residence time in the method of U.S. Pat. No. 4,225,516 must be significantly increased. An additional disadvantage of recycling MMP at temperatures of about 0 to -15°C is that the water still contained in the acrolein-containing vapor introduced into the reaction will freeze. The ice thus formed can lead to line blockages in the process or absorb significant amounts of heat of vaporization in the subsequent MMP reaction, further exacerbating the already poor energy balance of the '516 process. As described in '516, a typical vapor stream resulting from the production of acrolein contains 48.2 mole % water.Therefore, typical process conditions of U.S. Patent No. 4,225,516 result in large amounts of wastewater and therefore high treatment / disposal costs, making the process of U.S. Patent No. 4,225,516 rather unattractive from an economic point of view.

[0004] Published U.S. Patent Application Publication No. 2012 / 0165573 discloses a method for producing MMP, in which acrolein obtained by partial gas-phase oxidation of propene is first subjected to a quenching / by-product removal step and then absorbed into MMP and reacted with free methyl mercaptan or with methyl mercaptan released from 3-methylthiopropionaldehyde and methyl mercaptan, a hemithioacetal of MMP. However, the off-gas from this process contains large amounts of organic sulfur compounds, such as MMP and methyl mercaptan, which are toxic to the catalyst used to produce acrolein. Therefore, the off-gas cannot be recycled back to the acrolein formation process. Due to environmental regulations, the sulfur-containing off-gas must be fed to a desulfurization step before being combusted in a thermal oxidizer, or the combustion gas must be fed to the desulfurization step after the thermal oxidizer. However, this incurs huge investment and operational costs, making the method of US Patent Application Publication No. 2012 / 0165573 rather unattractive from an economic point of view.

[0005] Published patent application U.S. Patent Application Publication No. 2014 / 0005437 also discloses a method for producing MMP. In this method, (A) a mixture of propene and an inert diluent gas is first oxidized with air to produce an oxidation reaction gas mixture containing acrolein and by-products, followed by (B) quenching to produce a gas stream containing acrolein and a quench liquid containing residual acrolein and by-products. Next, (C) acrolein is recovered by stripping from the quench liquid at the bottom of the quench, and the stripped acrolein is returned to the quench. (D) A first portion of the acrolein-containing gas stream from the quench is absorbed with water to produce an aqueous acrolein solution and a non-condensable gas stream containing inert gas. (D1) At least a portion of the non-condensable gas stream is recycled to propene oxidation to provide gas, thereby reducing the oxygen concentration in the feed gas for the acrolein reaction. Next, (E) acrolein is distilled from the aqueous acrolein solution of the absorption step to obtain an acrolein-free aqueous bottom product, and (E1) the distilled acrolein thus obtained is condensed. (F) The condensed acrolein thus obtained and a second portion of the acrolein-containing gas stream from the quench are reacted with methyl mercaptan in a mixture containing 3-methylthiopropionaldehyde and at least one hemithioacetal of methyl mercaptan and 3-methylthiopropionaldehyde to obtain the desired 3-methylthiopropionaldehyde. However, the method of U.S. Patent Application Publication No. 2014 / 0005437 has several drawbacks. First, the step (D) of absorbing the quenched acrolein-containing gas stream into water and the step (E) of distilling acrolein from the aqueous acrolein solution thus obtained have the serious drawback of producing an acrolein-containing liquid stream. Acrolein, especially liquid acrolein, is one of the most dangerous chemicals. However, the acrolein-containing liquid stream of US Patent Application Publication No. 2014 / 0005437 is not immediately further processed into other, less hazardous compounds.Therefore, the process of U.S. Patent Application Publication No. 2014 / 0005437 sets high standards for safety measures regarding the handling of the acrolein-containing liquid stream. In addition, acrolein in liquid form is highly susceptible to polymerization, and therefore a stabilizer must be added to the acrolein-containing liquid stream. Furthermore, the off-gas from the MMP-forming reaction still contains sulfur-containing compounds and therefore must be fed to desulfurization before thermal oxidation, or the combustion gas of the off-gas must be fed to desulfurization after thermal oxidation. However, the desulfurization and thermal oxidation of the off-gas from the MMP-forming reaction of U.S. Patent Application Publication No. 2014 / 0005437 is quite expensive due to the large volume of the off-gas stream.

[0006] Therefore, there is a need for a process for producing 3-methylthiopropionaldehyde that does not involve the condensation of acrolein and that produces off-gases with a significantly reduced content of sulfur-containing compounds compared to prior art processes.

[0007] It has been found that this need can be met by using an acrolein-containing vapor stream that is mostly acrolein and has a pressure lower than atmospheric pressure, and introducing the acrolein-containing vapor stream and a liquid stream containing methyl mercaptan (MC) and / or 3-methylthiopropionaldehyde (MMP) and a hemithioacetal formed from methyl mercaptan (MC) (MMP-MC) into a reaction unit using a gas-liquid mixer, where acrolein is reacted with methyl mercaptan and / or 3-methylthiopropionaldehyde and the hemithioacetal formed from methyl mercaptan to produce 3-methylthiopropionaldehyde.

[0008] The subject of the present invention is therefore a process for the preparation of 3-methylthiopropionaldehyde, comprising the following steps: a) providing a liquid stream (1) comprising methyl mercaptan and / or a hemithioacetal formed from 3-methylthiopropionaldehyde and methyl mercaptan; b) providing an acrolein-containing vapor stream (2), wherein the majority of said stream is acrolein and the pressure of said stream is below atmospheric pressure; c) introducing the liquid stream containing methyl mercaptan and / or hemithioacetal formed from 3-methylthiopropionaldehyde and methyl mercaptan from step a) and the acrolein-containing vapor stream from step b) into a reaction unit (4) via a gas-liquid mixer (3); d) reacting acrolein with methyl mercaptan and / or hemithioacetal formed from 3-methylthiopropionaldehyde and methyl mercaptan in reaction unit (4) of step c) to produce a 3-methylthiopropionaldehyde-containing product mixture. The method includes:

[0009] The MMP formation method according to the technical teachings of U.S. Patent Application Publication No. 2012 / 0165573 produces about 7,400 kg / h of off-gas per ton of acrolein, which contains about 50 kg / h of sulfur-containing components per ton of acrolein. Therefore, this off-gas must be treated in a thermal oxidizer with SO2 removal (STO), which is very cumbersome. In comparison, the MMP formation method according to the present invention produces only about 0.8 kg / h of off-gas per ton of acrolein, which contains only about 0.050 kg / h of sulfur-containing components per ton of acrolein. Therefore, the off-gas from the method according to the present invention must also be treated in a thermal oxidizer with SO2 removal (STO). However, the off-gas from the method according to the present invention is only 0.1%, or 1 / 1000, of the off-gas of U.S. Patent Application Publication No. 2012 / 0165573. More importantly, compared to the off-gas of U.S. Patent Application Publication No. 2012 / 0165573, the off-gas from the process of the present invention also contains only 0.1%, or 1,000 times less, of the off-gas of U.S. Patent Application Publication No. 2012 / 0165573. Thus, the amount of sulfur-containing components in this off-gas is significantly reduced compared to U.S. Patent Application Publication No. 2012 / 0165573. This is also illustrated in Figure 3, where the upper description represents the process of U.S. Patent Application Publication No. 2012 / 0165573 and the lower description represents the process of the present invention. While the process of U.S. Patent Application Publication No. 2012 / 0165573 produces a significantly larger sulfur-containing off-gas stream, the process of the present invention produces a significantly smaller sulfur-containing off-gas stream. As a result, the treatment of this off-gas is much less cumbersome than the treatment of the off-gas of the method of US Patent Application Publication No. 2012 / 0165573.

[0010] A further advantage of the method of the present invention is that it allows for absorption / desorption in the acrolein-forming process prior to the MMP-forming reaction. The released off-gas contains non-condensable gases, which can be recycled to the acrolein-forming reaction, thereby reducing the oxygen partial pressure below a safety threshold. Therefore, only the remaining non-condensable gases from the off-gas of the method of the present invention must be fed to a simple thermal oxidizer. In contrast, the method of U.S. Patent Application Publication No. 2012 / 0165573 does not allow for such an absorption / desorption step. Therefore, it contains large amounts of organic sulfur compounds, such as MMP and methyl mercaptan, which are toxic to the catalyst used in the acrolein-forming process. Therefore, the off-gas cannot be recycled back to the acrolein-forming process. Rather, all off-gas from the method of U.S. Patent Application Publication No. 2012 / 0165573 must be fed to a thermal oxidizer with SO2 removal (STO). However, this makes the approach of US Patent Application Publication No. 2012 / 0165573 unattractive in terms of investment and operational costs.

[0011] The use of an acrolein-containing vapor stream has the advantage that the presence of liquefied acrolein is avoided or at least significantly reduced in the process according to the present invention. This has the advantage that in step d), acrolein immediately reacts with methyl mercaptan and / or hemithioacetal formed from 3-methylthiopropionaldehyde and methyl mercaptan to produce 3-methylthiopropionaldehyde. Thus, in contrast to the prior art processes, there is no liquefied highly concentrated acrolein in the process according to the present invention. Rather, if acrolein is not present at all in the liquid phase, its concentration in the liquid phase is quite low, particularly less than 10% by mass.

[0012] Furthermore, the use of an acrolein-containing vapor stream has the general advantage that in the event of a leak in a process plant for producing MMPs, liquid acrolein, which is significantly more hazardous than gaseous acrolein, is not exposed to the environment. The use of an acrolein-containing vapor stream having a pressure lower than atmospheric pressure has the further advantage that in the event of a leak in said plant, release of acrolein from a process plant for producing MMPs is avoided or at least significantly reduced.

[0013] According to step c) of the process according to the invention, the liquid stream (1) provided in step a) and the vapor stream (2) provided in step b) are introduced into the reaction unit by means of a gas-liquid mixer, which means that the two streams are brought into contact and mixed in the gas-liquid mixer (3) before being introduced into the reaction unit (4), i.e. the two streams are mixed in step c).

[0014] In principle, the reaction unit of step c) is not subject to any restrictions and can therefore be any type of reaction unit suitable for producing 3-methylthiopropionaldehyde from acrolein and methyl mercaptan, such as a continuous stirred tank reactor (CSTR), a series of CSTRs, or a plug flow reactor. Nevertheless, it is preferred that the reaction unit is a continuous stirred tank reactor (CSTR), since this allows a sufficiently long residence time for a high conversion of the reactants and a high yield of 3-methylthiopropionaldehyde. The 3-methylthiopropionaldehyde-containing product mixture obtained in step d) is then withdrawn from the reaction unit (4) as a 3-methylthiopropionaldehyde-containing stream (5).

[0015] In the context of the present invention, the term majority of the stream is used as known to the person skilled in the art and indicates that the acrolein-containing vapor stream provided in step b) contains more than 50% by weight of acrolein.

[0016] The higher the acrolein content in the acrolein-containing vapor stream provided in step b), the better. Specifically, the higher the acrolein content in the acrolein-containing vapor stream provided in step b), the faster the MMP-forming reaction in step d) proceeds. Preferably, the acrolein-containing vapor stream in step b) contains at least 55% by mass, at least 60% by mass, at least 65% by mass, at least 70% by mass, at least 75% by mass, at least 80% by mass, at least 85% by mass, at least 90% by mass, at least 95% by mass, or in extreme cases, 100% by mass of acrolein.

[0017] In an embodiment of the process according to the invention, the acrolein-containing vapor stream of step b) contains at least 70% by weight of acrolein.

[0018] In the context of the present invention, the term subatmospheric pressure is used as known to those skilled in the art and refers to any absolute pressure below 1,013.25 hPa (or 1,013 mbar). In relation to the pressure in the process, the term subatmospheric pressure is used as known to those skilled in the art and refers to any absolute pressure below 1,013 mbar. In the context of the present invention, the terms bar or mbar are used as known to those skilled in the art and refer to absolute pressure, which is zero-referenced to a perfect vacuum using an absolute scale so as to be equal to the gauge pressure plus atmospheric pressure. In the following, the term subatmospheric pressure is also used equivalently to reduced pressure.

[0019] In another embodiment of the process according to the invention, the pressure of the acrolein-containing vapor stream in step b) is in the range from 300 to 950 mbar.

[0020] In principle, the process according to the invention is not limited with respect to the supply of an acrolein-containing vapor stream, the majority of which is acrolein, as long as the acrolein-containing vapor stream thus supplied provides the advantages of the invention. For example, the acrolein-containing vapor stream can be obtained by the following steps: i) feeding an aqueous acrolein solution, or optionally an aqueous acrolein solution free from gases that are difficult to condense, into a distillation column equipped with at least one reboiler at the bottom and at least one condenser at the top, ii) withdrawing a liquid mixture substantially comprising water at the bottom of the distillation column; iii) withdrawing a gas mixture substantially comprising acrolein and water at the top of the distillation column; iv) cooling the gas mixture of step iii) in a condenser to a temperature that makes it possible to obtain, on the one hand, an aqueous condensate and, on the other hand, an acrolein-rich gas mixture; and v) extracting acrolein from the acrolein-rich gas mixture of step iv). The method may include:

[0021] However, the acrolein-containing gas stream withdrawn from step v) may still contain varying amounts of water, which may condense during the process of the invention.

[0022] It is therefore preferred to provide an acrolein-containing vapor stream by a method for purifying acrolein, the method comprising: b1) dividing an acrolein-containing liquid feed stream into at least a first liquid stream and a second liquid stream; b2) introducing a first liquid stream having a temperature T1 into the distillation column at a location between the top and bottom of the distillation column; b3) introducing a second liquid stream having a temperature T2 into the distillation column at an upper portion of the distillation column; b4) withdrawing an acrolein-rich overhead vapor stream from the distillation column and an acrolein-depleted bottoms stream from the distillation column. Including, In this case, the temperature T2 of the second liquid stream of step c) is lower than the temperature T1 of the first liquid stream (2) of step b), and the process for purifying acrolein is carried out under reduced pressure, preferably at a pressure of 300 to 950 mbar.

[0023] In another embodiment of the process according to the present invention, step c) comprises mixing the liquid stream containing methyl mercaptan and / or hemithioacetal formed from 3-methylthiopropionaldehyde and methyl mercaptan from step a) with the acrolein-containing vapor stream from step b) in a gas-liquid mixer for step c). Preferably, the methyl mercaptan- and / or hemithioacetal-containing liquid stream from step a) and the acrolein-containing vapor stream from step b) are introduced simultaneously into the gas-liquid mixer for step c), thereby achieving the best possible mixing of the two streams.

[0024] In principle, the process according to the present invention is not subject to any limitations regarding the gas-liquid mixing device in step b). Therefore, any conceivable gas-liquid mixing device can be used in step b) as long as it is compatible with the specific conditions of the process according to the present invention and is suitable for achieving the effects of the process according to the present invention. For example, the gas-liquid mixing device in step b) can be a static or dynamic mixing device, particularly a mixing device with or without mechanical stirring. Examples of mixing devices with mechanical stirring are a stirred line mixer, a stirred vessel, and a rotary impeller. Examples of mixing devices without mechanical mixing are a jet mixer, an injector, an orifice and a mixing nozzle, and a filling tube. A suitable gas-liquid mixing device can also reduce the required pressure of the acrolein-containing vapor stream in step b) to below atmospheric pressure. Preferably, the reduced pressure in step b), i.e., a reduced pressure below atmospheric pressure, can be generated in the gas-liquid mixing device in step c). For example, a jet mixer can be used into which a methyl mercaptan and / or hemithioacetal-containing liquid stream is introduced. The flow of the methyl mercaptan and / or hemithioacetal-containing liquid stream generates a reduced pressure for the acrolein-containing vapor stream of step b). Additionally or alternatively, the reduced pressure in step b) can be generated downstream of the reaction unit of step c) by a suitable device, such as a vacuum pump.

[0025] In a further embodiment of the process according to the invention, the pressure of the acrolein-containing vapor stream of step b) is generated in the gas-liquid mixing device of step c) and / or by means of a vacuum generator downstream of the reaction unit of step c).

[0026] The reaction of acrolein with methyl mercaptan and / or the hemithioacetal formed from 3-methylthiopropionaldehyde and methyl mercaptan to produce 3-methylthiopropionaldehyde proceeds in the liquid phase. Therefore, acrolein must be absorbed from the vapor phase of the vapor stream of step b) into the liquid phase of the liquid stream of step a). Absorption of acrolein from the vapor phase into the liquid phase is improved if acrolein has a low vapor pressure in the liquid phase. This has the effect of reducing the amount of acrolein evaporating from the liquid phase. Because the vapor pressure of a substance is a function of its temperature and its concentration, it is preferable that the acrolein-containing vapor stream of step b) have a low temperature. On the other hand, the reaction of step d) in which acrolein reacts with methyl mercaptan to produce 3-methylthiopropionaldehyde shows acceptable yields only at higher temperatures. Both conditions are met if the temperature of step d) is high enough for the reaction to MMP and the concentration of acrolein in the liquid phase is low enough to limit the vapor pressure to the pressure value in the MMP reactor. At a pressure of 600 mbar, the condensation temperature of acrolein in the vapor stream is 38°C. At a temperature of 50°C and a pressure of 600 mbar, the acrolein concentration should be less than 50%, and at a temperature of 90°C, it should be less than 11%. At a pressure of 800 mbar, the condensation temperature of acrolein in the vapor stream is 46°C. At a temperature of 50°C and a pressure of 800 mbar, the acrolein concentration should be less than 78%, and at a temperature of 90°C, it should be less than 16%.

[0027] In one embodiment of the process according to the invention, the temperature in step d) is higher than the condensation temperature of acrolein in the vapor stream of step b).

[0028] Preferably, the temperature in step d) is in the range of 50 to 90°C, which allows for the production of 3-methylthiopropionaldehyde in a very good yield. In particular, the temperature in step d) is in the range of 60 to 80°C, which is considered to be the optimum reaction temperature for the MMP-forming reaction in step d).

[0029] As explained above, this is advantageous when the temperature in step d) is higher than the condensation temperature of acrolein in the vapor stream of step b) Thus, taking into account the condensation temperature of acrolein in the preferred pressure range of 300 to 950 mbar, it is preferred that the temperature of the acrolein-containing vapor stream of step b) is in the range of from 20 to less than 50°C, in particular from 20 to 40°C.

[0030] In a further embodiment of the process according to the invention, the temperature of the acrolein-containing vapor stream in step b) is in the range of from 20 to less than 50°C.

[0031] Preferably, the temperature of the liquid stream containing methyl mercaptan and / or the hemithioacetal formed from 3-methylthiopropionaldehyde and methyl mercaptan in step d) is in the range of 50 to 90°C, and the temperature of the acrolein-containing vapor stream in step b) is in the range of 20 to less than 50°C.

[0032] According to the present invention, a liquid stream containing methyl mercaptan and / or 3-methylthiopropionaldehyde and a hemithioacetal formed from methyl mercaptan is supplied in step a) and introduced into a reaction unit in step c), where the methyl mercaptan and / or 3-methylthiopropionaldehyde and a hemithioacetal formed from methyl mercaptan contained in the liquid stream react with acrolein to produce 3-methylthiopropionaldehyde. In principle, the process according to the present invention is not subject to any restrictions regarding the concentration of methyl mercaptan and / or hemithioacetal in the liquid stream of step a). Thus, the methyl mercaptan and / or hemithioacetal-containing liquid stream of step a) can have any conceivable concentration of methyl mercaptan and / or hemithioacetal. Nevertheless, it is preferred that the majority of the methyl mercaptan and / or hemithioacetal-containing liquid stream is methyl mercaptan and / or hemithioacetal. Therefore, the methyl mercaptan and / or hemithioacetal-containing liquid stream of step a) preferably contains more than 50% by weight of methyl mercaptan and / or hemithioacetal. Preferably, the methyl mercaptan and / or hemithioacetal-containing liquid stream of step a) contains at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, or in extreme cases, 100% by weight of methyl mercaptan and / or hemithioacetal. It can be expected that increasing the methyl mercaptan and / or hemithioacetal concentration as much as possible will be beneficial for a high yield of 3-methylthiopropionaldehyde in the reaction of step d). However, the heat of condensation released by the absorption of acrolein from the gas phase into the liquid phase, which leads to an increase in the reaction mixture of step d), and the heat of reaction released in the MMP-forming reaction must be taken into account. However, this will favor the evaporation of acrolein and methyl mercaptan from the liquid phase.Because the reaction of acrolein with methyl mercaptan and / or hemithioacetal to 3-methylthiopropionaldehyde proceeds in the liquid phase, vaporization of the reactants from the liquid to the vapor phase would result in a loss of 3-methylthiopropionaldehyde yield. Alternatively, it might be beneficial to cool the methyl mercaptan and / or hemithioacetal-containing liquid stream of step a) to a temperature that compensates for the temperature increase caused by the condensation and heat of reaction in steps c) and / or d), especially step d). However, this would require significant cooling energy, and in the worst case scenario, it would also be necessary to heat the reaction mixture of step d) to the temperature range optimal for the MMP formation reaction. Therefore, it is beneficial if the methyl mercaptan and / or hemithioacetal-containing liquid stream of step a) also contains a solvent having a boiling point at least 20°C higher than that of acrolein, i.e., a solvent having a boiling point of 53°C at a pressure of 1,013 mbar. Solvents with this characteristic are able to absorb the heat of condensation and reaction very well, thus avoiding or at least significantly reducing the evaporation of volatile reactants from the liquid phase to the vapor phase. Preferably, the solvent has a boiling point in the range of 73 to 200°C, 73 to 190°C, 73 to 180°C, or 73 to 170°C.

[0033] In one embodiment of the process according to the invention, the methyl mercaptan and / or hemithioacetal-containing liquid stream formed from 3-methylthiopropionaldehyde and methyl mercaptan in step a) also contains a solvent having a boiling temperature at least 20° C. higher than the boiling temperature of acrolein.

[0034] In another embodiment, step a) of the process according to the invention further comprises a step a') of mixing the methyl mercaptan and / or hemithioacetal-containing fluid stream formed from 3-methylthiopropionaldehyde and methyl mercaptan with a solvent having a boiling temperature at least 20° C. higher than the boiling temperature of acrolein to provide a methyl mercaptan-containing liquid stream of step a).

[0035] Despite the specific temperature and the presence of additional solvent in the streams of step a) and / or step b), a part of the acrolein and / or methyl mercaptan may evaporate from the liquid phase into the vapor phase, which will result in a loss of yield of 3-methylthiopropionaldehyde. Therefore, in order to keep said losses as low as possible, it is advantageous to wash the acrolein and / or methyl mercaptan, if contained in the off-gas from the reaction of step d), with a solvent having a boiling temperature at least 20° C. higher than that of acrolein, and to feed the liquid stream containing acrolein and / or methyl mercaptan thus obtained to step c) and / or step d).

[0036] In a further embodiment, the method according to the present invention comprises: e1) washing the acrolein and / or methyl mercaptan, if contained in the off-gas from the reaction of step d), with a solvent having a boiling temperature at least 20° C. higher than the boiling temperature of acrolein to obtain an acrolein and / or methyl mercaptan-containing liquid stream; and e2) feeding the acrolein and / or methyl mercaptan-containing liquid stream obtained in step e1) to step c) and / or step d). Further includes:

[0037] Additionally or alternatively, it is also conceivable to wash the vapor phase in reaction unit (4) with a solvent having a boiling temperature at least 20° C. higher than the boiling temperature of acrolein. In that case, any methyl mercaptan and / or acrolein evaporated from the liquid phase into the vapor phase can be washed / fed directly back into the reaction in step d).

[0038] A suitable solvent in the context of the process of the present invention is 3-methylthiopropionaldehyde (MMP), and therefore the reaction product itself has a boiling point of 165-166°C under standard conditions, i.e., at a standard pressure of 1,013 mbar. The relatively high boiling point and therefore high heat absorption capacity of the 3-methylthiopropionaldehyde-containing stream effectively absorbs the heat of condensation released upon mixing of the liquid methyl mercaptan and / or hemithioacetal-containing stream with gaseous acrolein, as well as the heat of reaction released during the reaction of methyl mercaptan and / or hemithioacetal with acrolein to produce 3-methylthiopropionaldehyde. This effectively avoids or significantly reduces evaporation of the reactants from the liquid phase to the vapor phase. In addition, methyl mercaptan is believed to react with 3-methylthiopropionaldehyde in an equilibrium reaction to form the corresponding hemithioacetal of 3-methylthiopropionaldehyde, i.e., 1,3-bis(methylthio)-1-propanol. The equilibrium reaction involves the continuous introduction of methyl mercaptan into 3-methylthiopropionaldehyde and the continuous release of methyl mercaptan and 3-methylthiopropionaldehyde from the hemithioacetal, with the formation of the corresponding hemithioacetal of 3-methylthiopropionaldehyde. Therefore, a portion of methyl mercaptan is always present as free, i.e., unbound or chemically unbound, methyl mercaptan in this equilibrium state, and is believed to react with acrolein to form 3-methylthiopropionaldehyde. As free methyl mercaptan is reduced at equilibrium, additional methyl mercaptan is released from the hemithioacetal and becomes available again to further react with acrolein to produce 3-methylthiopropionaldehyde. Thus, the use of 3-methylthiopropionaldehyde as a liquid solvent for the methyl mercaptan-containing stream and as a solvent in the MMP-forming reaction of step d) improves not only the physical solubility but also the chemical solubility of methyl mercaptan in the liquid stream of step a).Therefore, the high heat capacity and solubility properties of the 3-methylthiopropionaldehyde-containing stream are believed to avoid or at least significantly reduce the evaporation of methyl mercaptan and acrolein upon condensation and / or release of heat of reaction.

[0039] In one embodiment of the process according to the invention, the methyl mercaptan and / or hemithioacetal-containing liquid stream formed from 3-methylthiopropionaldehyde and methyl mercaptan in step a) also contains a solvent which is 3-methylthiopropionaldehyde.

[0040] In the simplest case, the solvent 3-methylthiopropionaldehyde, i.e., the 3-methylthiopropionaldehyde-containing product mixture obtained in step d), is removed from the process according to the invention and then mixed with a stream containing or consisting of methyl mercaptan to provide a methyl mercaptan-containing liquid stream (1). To maximize the solubility of methyl mercaptan in 3-methylthiopropionaldehyde, the 3-methylthiopropionaldehyde is conditioned before mixing with methyl mercaptan. Additionally or alternatively, part of the conditioned 3-methylthiopropionaldehyde-containing stream can be recycled around the bottom pump back to the reaction unit.

[0041] In another embodiment, the method according to the present invention comprises the steps of: f1) withdrawing the 3-methylthiopropionaldehyde-containing product mixture obtained in step d) from the reaction unit as a 3-methylthiopropionaldehyde-containing stream; f2) feeding all or at least a portion of the 3-methylthiopropionaldehyde-containing stream from step f1) to a heat exchanger to provide a conditioned 3-methylthiopropionaldehyde-containing stream; and f3) supplying a portion of the 3-methylthiopropionaldehyde-containing stream prepared in step f2) to the reaction unit (4) or mixing a portion of the 3-methylthiopropionaldehyde-containing stream prepared in step f2) with methyl mercaptan and / or a hemithioacetal formed from 3-methylthiopropionaldehyde and methyl mercaptan to supply a liquid stream (1) containing methyl mercaptan and / or a hemithioacetal formed from 3-methylthiopropionaldehyde and methyl mercaptan. Further includes:

[0042] In a preferred embodiment, the 3-methylthiopropionaldehyde-containing stream is adjusted in step f2) to a temperature in the range of 20 to 40°C.

[0043] In that case, the 3-methylthiopropionaldehyde-containing stream is particularly suitable for being mixed with methyl mercaptan and / or hemithioacetals formed from 3-methylthiopropionaldehyde and methyl mercaptan.

[0044] The 3-methylthiopropionaldehyde-containing product mixture obtained in step d) is withdrawn from the reaction unit as a 3-methylthiopropionaldehyde-containing stream, recycled back to the reaction unit, and / or used as a solvent for methyl mercaptan and / or hemithioacetal formed from 3-methylthiopropionaldehyde and methyl mercaptan, and already contains a relatively high concentration of MMP. Therefore, this stream does not need to be purified before being recycled back to the reaction and / or used as a solvent for methyl mercaptan. Rather, it is even more preferable that this stream not be purified before being recycled and / or used as a solvent, since the reaction can then be completed with respect to the unreacted acrolein still contained in this stream.

[0045] If the methyl mercaptan and / or hemithioacetal-containing liquid stream formed from 3-methylthiopropionaldehyde and methyl mercaptan in step a) contains a solvent different from 3-methylthiopropionaldehyde, the process according to the invention further comprises a step of separating said solvent from the 3-methylthiopropionaldehyde-containing product mixture obtained from step d).

[0046] In most prior art processes, to avoid polymerization of unreacted acrolein, either an at least equimolar ratio of methyl mercaptan to acrolein or a very slight excess of methyl acrolein relative to acrolein is used in the production of 3-methylthiopropionaldehyde. These prior art processes involve the use of a liquefied acrolein stream and a liquid methyl mercaptan stream. While accurate metering of the liquid stream is easily achieved, accurate metering of the vapor stream is not always possible. Therefore, using an at least equimolar ratio of methyl mercaptan to acrolein or a very slight excess of methyl acrolein relative to acrolein is highly likely to lead to (high) methyl mercaptan over-dosing in the process according to the present invention. However, this would result in a waste of raw materials. It may be expected that the methyl mercaptan / acrolein ratio could be subsequently corrected. However, this does not make sense from an economic point of view.

[0047] Thus, in contrast to the prior art processes, in the process according to the invention there is preferably an excess of acrolein relative to methyl mercaptan and / or the hemithioacetal of methyl mercaptan and 3-methylthiopropionaldehyde, said ratio being able to be adjusted by adding methyl mercaptan in a second reactor downstream of the first reactor.

[0048] In yet another embodiment of the process according to the invention, the molar ratio of methyl mercaptan and / or the hemithioacetal of methyl mercaptan and 3-methylthiopropionaldehyde to acrolein in step c) and / or step d) is less than 1.

[0049] Preferably, acrolein is present in the process according to the invention in only a slight excess relative to methyl mercaptan and / or the hemithioacetal of methyl mercaptan and 3-methylthiopropionaldehyde, i.e., 1,3-bis(methylthio)-1-propanol. Preferably, the molar ratio of acrolein to methyl mercaptan and / or 1,3-bis(methylthio)-1-propanol is in the range of 1.001 to 1.9, in particular 1.001 to 1.8, 1.001 to 1.007, 1.001 to 1.6, 1.001 to 1.5, 1.001 to 1.4, 1.001 to 1.3, 1.001 to 1.2 or 1.001 to 1.1.

[0050] If acrolein is still present in the 3-methylthiopropionaldehyde-containing stream (5) withdrawn from the reaction unit (4), the MMP-forming reaction can be completed by (i) recycling all or part of the 3-methylthiopropionaldehyde-containing stream (5) to the reaction and / or using it as a solvent for methyl mercaptan and / or the hemithioacetal formed from 3-methylthiopropionaldehyde and methyl mercaptan, as described above, or (ii) feeding all or part of the 3-methylthiopropionaldehyde-containing stream (5) to a second reactor downstream of the reaction unit of step d). In option (ii), the methyl mercaptan and / or the hemithioacetal formed from 3-methylthiopropionaldehyde and methyl mercaptan-containing stream can be accurately metered and then added to the 3-methylthiopropionaldehyde-containing stream. To complete the MMP-forming reaction, it is preferred herein to use an excess of methyl mercaptan and / or a hemithioacetal formed from 3-methylthiopropionaldehyde and methyl mercaptan relative to acrolein.

[0051] The invention is further illustrated by the following figures and examples. [Brief explanation of the drawings]

[0052] [Figure 1] 1 is a diagram illustrating a process according to the present invention, where the numbers indicate the following: (1) a liquid stream containing methyl mercaptan and / or a hemithioacetal formed from 3-methylthiopropionaldehyde and methyl mercaptan, (2) an acrolein-containing vapor stream, (3) a gas-liquid mixing device, (4) a reaction unit, (5) a 3-methylthiopropionaldehyde-containing product stream, (6) a scrubber, and (7) an off-gas stream. [Figure 2]FIG. 1 shows one embodiment of a process according to the present invention, in which the numbers (1) through (7) have the same meaning as in FIG. 1, with additional numbers indicating: (8) partial MMP recycle stream, (9) process MMP stream, (10) MMP recycle stream around the bottoms pump, (11) partial MMP recycle stream, (12) heat exchanger, (13) cooled partial MMP recycle stream, and (14) a liquid stream comprising methyl mercaptan and / or hemithioacetal formed from 3-methylthiopropionaldehyde and methyl mercaptan. [Figure 3] FIG. 1 shows a comparison of the prior art method of US Patent Application Publication No. 2012 / 0165573 (above) with the method of the present invention (below).

[0053] Working Example: The examples herein are carried out using computational models of processes based on the individual processes of the examples. Process modeling is an established and reliable methodology used by engineers to simulate complex chemical processes before building an actual plant. In the context of the examples herein, commercially available modeling software Aspen Plus® (Aspen Technology, Inc. 20 Crosby Roads, Bedford, Massachusetts 01730, USA) was used in combination with physical property data available from public databases.

[0054] 1. Comparative Example: The production of MMP is simulated using modeling software Aspen Plus® for a manufacturing method according to the technical teachings of U.S. Patent Application Publication No. 2012 / 0165573: a stream of a gaseous mixture containing acrolein is absorbed into a reactive absorbent in a mixture of MMP, methyl mercaptan, and a hemithioacetal formed from 3-methylthiopropionaldehyde and methyl mercaptan, which is then reacted with methyl mercaptan to produce 3-methylthiopropionaldehyde.

[0055] MMP reacts with free methyl mercaptan or with methyl mercaptan released from the hemithioacetal formed from 3-methylthiopropionaldehyde and methyl mercaptan MMP. After being absorbed from the gas phase into the liquid phase, the absorbed acrolein reacts with methyl mercaptan and / or the hemithioacetal formed from 3-methylthiopropionaldehyde and methyl mercaptan to produce MMP. The resulting MMP-containing liquid stream is withdrawn from the reaction unit and subjected to purification to remove gaseous impurities and by-products. The process produces approximately 7,400 kg / h of off-gas per ton of acrolein, along with 50 kg / h of sulfur-containing components per ton of acrolein, based on the data in Tables 1 and 4 of U.S. Patent Application Publication No. 2012 / 0165573. The specific composition of this off-gas and the flow rates of these components are shown in Table 1 below. [Table 1]

[0056] Example according to the present invention: Using the modeling software Aspen Plus®, the production of MMP was simulated for the production process shown in Figure 2: A methyl mercaptan (MC)-containing liquid stream (1) containing approximately 82% by weight of methyl mercaptan and hemithioacetal of MMP and approximately 3% by weight of HO and an acrolein-containing vapor stream (2) having a pressure of approximately 0.60 bar and containing approximately 92.6% by weight of acrolein, approximately 2% by weight of acetaldehyde, and approximately 5% by weight of water are introduced into a reaction unit (4) via a gas-liquid mixer (3). In the reaction unit (4), acrolein is reacted with methyl mercaptan at a temperature of approximately 64°C to produce a 3-methylmercaptopropionaldehyde-containing product mixture. Compared to the process of U.S. Patent Application Publication No. 2012 / 0165573, the process of the present invention produces only approximately 0.8 kg / h of off-gas per ton of acrolein. In other words, the off-gas from the process of the present invention is only 0.01% of the off-gas from the process of U.S. Patent Application Publication No. 2012 / 0165573. Furthermore, the off-gas contains only 0.05 kg / h of sulfur-containing components per ton of acrolein, which is only 0.1% of the sulfur-containing components in the off-gas from the process of U.S. Patent Application Publication No. 2012 / 0165573. The composition of the off-gas from the process of the present invention and the flow rates of those components are shown in Table 2 below. [Table 2]

Claims

1. A method for producing 3-methylthiopropionaldehyde, comprising the steps of: a) Methyl mercaptan and / or Hemithioacetal formed from 3-methylthiopropionaldehyde and methyl mercaptan providing a liquid stream (1) comprising b) providing an acrolein-containing vapor stream (2), wherein the majority of said stream is acrolein and the pressure of said stream is below atmospheric pressure; c) introducing the liquid stream containing methyl mercaptan and / or hemithioacetal formed from 3-methylthiopropionaldehyde and methyl mercaptan from step a) and the acrolein-containing vapor stream from step b) into a reaction unit (4) via a gas-liquid mixer (3); and d) reacting acrolein with methyl mercaptan and / or hemithioacetals formed from 3-methylthiopropionaldehyde and methyl mercaptan in reaction unit (4) of step c) to produce a 3-methylthiopropionaldehyde-containing product mixture. The method comprising:

2. 2. The process of claim 1, wherein the acrolein-containing vapor stream of step b) contains at least 70% by weight of acrolein.

3. 3. The process according to claim 1 or 2, wherein the pressure of the acrolein-containing vapor stream of step b) is in the range of from 300 to 950 mbar.

4. 3. The method according to claim 1 or 2, wherein step c) comprises mixing the liquid stream containing methyl mercaptan and / or hemithioacetal formed from 3-methylthiopropionaldehyde and methyl mercaptan from step a) with the acrolein-containing vapor stream from step b) in the gas-liquid mixing device of step c).

5. 3. The process according to claim 1 or 2, wherein the pressure of the acrolein-containing vapor stream of step b) is generated in the gas-liquid mixing device of step c) and / or by a vacuum generating device downstream of the reaction unit of step c).

6. 3. The process of claim 1 or 2, wherein the temperature of step d) is higher than the condensation temperature of acrolein in the vapor stream of step b).

7. 3. The process according to claim 1 or 2, wherein the temperature of the liquid stream comprising methyl mercaptan and / or hemithioacetal formed from 3-methylthiopropionaldehyde and methyl mercaptan in step d) is in the range of 50 to 90°C.

8. 3. The process of claim 1 or 2, wherein the temperature of the acrolein-containing vapor stream of step b) is in the range of from 20 to less than 50°C.

9. 3. The process according to claim 1 or 2, wherein the liquid stream comprising methyl mercaptan and / or hemithioacetal formed from 3-methylthiopropionaldehyde and methyl mercaptan in step a) also comprises a solvent having a boiling temperature at least 20° C. higher than the boiling temperature of acrolein.

10. e1) if acrolein and / or methyl mercaptan are contained in the off-gas from the reaction of step d), washing them with a solvent having a boiling temperature at least 20° C. higher than the boiling temperature of acrolein to obtain an acrolein and / or methyl mercaptan-containing liquid stream; and e2) feeding the acrolein and / or methyl mercaptan-containing liquid stream obtained in step e1) to step c) and / or step d).

3. The method of claim 1 or 2, further comprising:

11. 3. The process according to claim 1 or 2, wherein the liquid stream comprising methyl mercaptan and / or hemithioacetal formed from 3-methylthiopropionaldehyde and methyl mercaptan in step a) also contains a solvent which is 3-methylthiopropionaldehyde.

12. f1) withdrawing the 3-methylthiopropionaldehyde-containing product mixture obtained in step d) from the reaction unit as a 3-methylthiopropionaldehyde-containing stream; f2) feeding all or at least a portion of the 3-methylthiopropionaldehyde-containing stream from step f1) to a heat exchanger to provide a conditioned 3-methylthiopropionaldehyde-containing stream; and f3) feeding a portion of the prepared 3-methylthiopropionaldehyde-containing stream of step f2) to a reaction unit (4) or mixing a portion of the prepared 3-methylthiopropionaldehyde-containing stream of step f2) with methyl mercaptan and / or a hemithioacetal formed from 3-methylthiopropionaldehyde and methyl mercaptan to provide a methyl mercaptan-containing liquid stream (1).

3. The method of claim 1 or 2, further comprising:

13. 13. The process according to claim 12, wherein the 3-methylthiopropionaldehyde-containing stream is adjusted in step f2) to a temperature in the range of from 20 to 40°C.

14. 3. The process according to claim 1 or 2, further comprising, when the liquid stream comprising methyl mercaptan and / or hemithioacetal formed from 3-methylthiopropionaldehyde and methyl mercaptan in step a) contains a solvent different from 3-methylthiopropionaldehyde, separating said solvent from the 3-methylthiopropionaldehyde-containing product mixture obtained from step d).

15. 3. The process according to claim 1, wherein the molar ratio of methyl mercaptan and / or the hemithioacetal formed from methyl mercaptan and 3-methylthiopropionaldehyde to acrolein in step c) and / or d) is less than 1.

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