The present invention relates to a process for vacuum distillation of at least one organic compound by means of a vacuum unit comprising an absorption liquid which is not identical to the compound to be distilled and forms an adduct therewith and/or is identical to the compound to be distilled.

Pumped removal of gases or distillation of compounds is typically accomplished on the industrial scale using liquid ring pumps. A liquid ring pump is composed of a cylindrical housing and a star-shaped impeller arranged in an eccentric manner therein. A non-central connection stub in the end face of the housing connects the liquid ring pump to a space from which a suction medium is to be exhausted. Within the housing of the liquid ring pump is a liquid, water in the simplest case, which, on rotation of the impeller or the housing, forms a liquid ring which is concentric with respect to the housing and seals the impeller chambers formed by the blades of the impeller. Because of the eccentric arrangement of the impeller, the blades of the impeller are immersed into the liquid ring to different depths during the rotation. The gas phase present in an impeller chamber is compressed when the blades which form the impeller chamber are immersed deeper into the liquid ring. Because of the eccentricity of the impeller, an impeller chamber with a compressed gas phase is always opposite an impeller chamber where the blades are immersed less deeply into the ring liquid, which leads to an increase in the gas volume in this impeller chamber. Through the connection stub, the suction medium is sucked into the impeller chamber adjacent to the connection stub. On rotation, therefore, the unit formed by ring liquid and impeller acts like a piston whose segments alternately compress and aspirate. A disadvantage of liquid ring pumps is that the suction medium always comes into contact with the ring liquid used as operating medium. It is therefore unavoidable that both the operating medium and the suction medium are contaminated by contact with one another. If the suction medium is a compound having potential value, the contamination with the ring liquid is a considerable problem for the reintegration or further use of this compound. This is because, in general, release of the compound removed by suction in pure form from the ring liquid is possible, if at all, only with corresponding cost and inconvenience, for example by an additional distillation as described in US Patent 4,282,013.

US Patent 4,282,013 describes a distillation of crude maleic anhydride by means of a liquid ring pump wherein the ring liquid is formed by an absorbent for maleic anhydride, followed by a further distillation in which the lower-boiling maleic anhydride is separated from the higher-boiling absorbent. However, due to the low melting point of maleic anhydride, the latter can precipitate in the absorption liquid, which then leads to fouling. As a consequence only small amounts of maleic anhydride can be absorbed in the absorption liquid. The Swiss patent 365365 discloses a process for the recovery of organic products by vacuum distillation. In this process the vacuum is generated by means of a water-jet pump. The US patent 3,223,533 also discloses a process for the vacuum distillation of an organic compound by means of a vacuum unit with a vacuum pump and an absorption unit containing water as absorption liquid, which is downstream to the vacuum unit. In the processes of these patent either the working liquid of the pump or the absorption liquid is always contacted with the organic compound to be distilled or sucked off.

However, the contamination of the operating liquid of a pump, especially the ring liquid of a liquid ring pump, with the compound removed by suction is a problem especially when the suction medium is a sulphur compound. This is because the operating medium in that case has to be disposed of or incinerated in a costly manner. An additional disadvantage in the incineration of sulphur-containing media is that the offgas has to be desulphurized to avoid sulphur dioxide emissions, which is associated with high capital and operating costs.

The problem addressed by the present invention is therefore that of providing a process for vacuum distillation of an organic compound by means of a vacuum unit, which enables complete reintegration of the organic compound to be distilled with avoidance of the formation of wastewater and with at least distinct reduction of gas emissions and without a costly desulphurization.

This problem is solved in accordance with the invention by virtue of the vacuum unit comprising an absorption liquid which is either not identical to the organic compound to be distilled and forms an addition compound therewith, which constitutes a chemical absorption, and/or the absorption liquid is identical to the organic compound to be distilled, which leads to a physical absorption. Physical absorption of the organic compound to be distilled is achieved in accordance with the invention by virtue of the vacuum unit comprising an absorption liquid identical to an organic compound to be distilled, such that the organic compound to be distilled is essentially completely absorbed in the phase formed by the absorption liquid. When the organic compound to be distilled is not identical to the absorption liquid, chemical absorption of the organic compound to be distilled is achieved by virtue of the absorption liquid forming an addition compound with the organic compound to be distilled. If different compounds are present in the gas phase aspirated, the absorption liquid is chosen such that it is different from one of the organic compounds to be distilled and forms an addition compound therewith, while it is identical to the other organic compound to be distilled and forms a common phase therewith.

The present invention therefore provides a process for vacuum distillation of at least one organic compound by means of a vacuum unit comprising a vacuum pump and an absorption apparatus connected upstream of the vacuum pump and comprising an absorption liquid according to the general formula (I) R -CHO where R is an alkyl, alkenyl or alkynyl group which is unsubstituted or substituted by an H3C-O- or H3C-S- group and has 1 to 12 carbon atoms, wherein the absorption liquid is different from one of the organic compounds to be distilled and forms an addition compound therewith, wherein said organic compound to be distilled corresponds to the general formula (II) R ² -L -R ³ , where L is an alkyl, alkenyl or alkynyl group having 1 to 12 carbon atoms and R ² and R ³ are each independently a hydrogen atom or an HO- or HS-group, provided that at least one of R ² and R ³ is an HO- or HS-group, provided that at least one of R ² and R ³ is an HO- or HS-group, and/or the absorption liquid is identical to one of the organic compounds to be distilled.

In the alternative that the absorption liquid is different from one of the organic compounds to be distilled, preferably, only one of R ² and R ³ is an HO- or HS-group. With regard to the construction of the absorption apparatus, the process according to the invention is not subject to any restrictions. Preferably, the absorption apparatus is a scrubber as typically used in chemical processing plants. This has the advantage that at least partial continuous exchange of the absorption liquid is possible even during the vacuum distillation, by virtue of the absorption liquid enriched with the organic compound to be distilled and/or the addition compound being removed from the scrubber, preferably with employment of a vacuum, and being replaced by an amount of new absorption liquid, i.e. not contaminated with a chemically and/or physically absorbent organic compound, corresponding to the amount of the absorption liquid removed.

In one embodiment of the process according to the invention, the absorption apparatus is therefore a scrubber.

In the context of the present invention, the term "addition compound" or "adduct" is used to refer to all composite molecules which are formed by addition of two compounds to form a covalent bond, with or without simultaneous elimination of water. Examples of addition compounds in the context of the present invention are the products from a Michael addition, such as preferably the 3- methylthiopropionaldehyde Michael adduct formed from acrolein and methyl mercaptan, the hemiacetals formed from alcohols and carbonyl compounds, and the corresponding hemithioacetals and hemimercaptals respectively formed from thiols and mercaptans with carbonyl compounds, such as preferably 1 ,3-bis(methylthio)propan-1-ol which is formed from 3-methylthiopropionaldehyde and mercaptan.

When the absorption liquid is identical to the organic compound to be distilled, this leads to an increase in concentration of the compound in question. If the organic compound to be distilled is different from the absorption liquid, it forms an adduct with the absorption liquid. This adduct may itself be a product of value which can either be recycled into the upstream process or sent to another process. However, the process according to the invention is not restricted to distilling just a single compound by means of the vacuum generated by the vacuum unit. Instead, the process according to the invention is also applicable to cases where more than one compound is distilled off, with one organic compound to be distilled being identical to the absorption liquid, and another organic compound to be distilled being different from the absorption liquid and forming an adduct therewith. For all these cases, aldehydes are suitable as absorption liquids. This is because saturated and/or unsaturated aldehydes react with alcohols and mercaptans to give Michael adducts, hemiacetals and the corresponding hemimercaptals or hemithioacetals. In addition, aldehydes with alkyl, alkenyl or alkynyl groups having 2 to 12 carbon atoms have low melting points and a simultaneously high boiling point. Under the conditions under which absorption liquids are used, compounds of this kind are liquid in most cases and therefore usable as absorption liquid or as a constituent of an absorption liquid. Only in exceptional cases may additional cooling be required.

Therefore, in the process according to the invention, the absorption liquid corresponds to the general formula (I) R -CHO, and R is an alkyl, alkenyl or alkynyl group which is unsubstituted or substituted by a CH3O- or CH3S- group and has 1 to 12 carbon atoms.

The presence of CH3O- or CH3S- groups in the absorption liquid used increases its boiling point. This is advantageous in order to be able to effectively absorb the distilled organic compound as it flows into the absorption liquid at high temperatures from the top of the distillation column, without the absorption liquid simultaneously getting into the region of its boiling point or even beginning to boil.

In an embodiment of the process according to the invention, the absorption liquid corresponds to the general formula (I) R -CHO, and R is an alkyl, alkenyl or alkynyl group which is substituted by a CH3O- or CH3S- group and has 1 to 12 carbon atoms.

Preferably, the R radical of the aldehyde of the general formula (I) has 1 to 6 carbon atoms; further preferably, the R radical of the aldehyde contains 2 to 4 carbon atoms. Moreover, linear alkyl, alkenyl or alkynyl groups are preferred over branched groups. More preferably, the R radical is therefore a linear alkyl group having 1 to 6 carbon atoms, especially having 2 to 4 carbon atoms, substituted by a CH3O- or CH3S- group.

Preferably, the vacuum unit in the process according to the invention has a downstream phase separator, in order to separate the compounds that have not been absorbed in the absorption liquid from the absorption liquid and/or from the addition compound formed from the distilled organic compound and the absorption liquid. In the context of the present invention, the use of 3- methylthiopropionaldehyde as absorption liquid is particularly advantageous. This is because the boiling temperature of 3-methylthiopropionaldehyde at standard pressure is 165°C and is therefore well above the boiling temperature of water, which is typically used as ring liquid in liquid ring pumps and as suction medium in jet pumps. The use of 3-methylthiopropionaldehyde or higher homologues as absorption liquid therefore avoids transfer of the absorption liquid to the gas phase.

In a further embodiment of the process according to the invention, the absorption liquid is therefore 3-methylthiopropionaldehyde or a higher homologue thereof, wherein said higher homologue differs from the 3-methylthiopropionaldeyhde by an additional CH2 unit in the alkyl group. In the context of the present invention, a higher homologue of 3-methylthiopropionaldehyde is understood to mean a compound which is structurally similar to this aldehyde and differs from the 3- methylthiopropionaldehyde only by an additional CH2 unit in the alkyl group. The higher homologue of 3-methylthiopropionaldehyde has an alkyl group having 3 to 12 carbon atoms, preferably having 3 to 6 carbon atoms, especially having 3 or 4 carbon atoms.

In another embodiment of the process according to the invention, the absorption liquid is H3C-S-R-CHO, with R being an alkyl group having 3 to 12 carbon atoms. 3-Methylthiopropionaldehyde is additionally an important intermediate in the preparation of D,L- methionine and of the Methionine Hydroxy Analogue 2-hydroxy-4-methylisobutyric acid (also known by the abbreviation MHA). Methionine is an essential amino acid which is used primarily as a supplement in animal feeds, and MHA is a liquid substitute for methionine, but has much lower bioavailability than methionine.

3-Methylthiopropionaldehyde (3-methylmercaptojDropionaldehyde, MMP) is prepared by the (catalysed) reaction of methyl mercaptan and acrolein at a pressure of typically 1 bar and a temperature of typically 20°C to 100°C, it being possible to react methyl mercaptan and acrolein with one another either in gaseous or liquid form. 3-Methylthiopropionaldehyde which is obtained by this reaction has a much higher boiling point (165°C to 166°C) than the methyl mercaptan (boiling point: about 6°C) and acrolein (boiling point: 52°C) starting materials. Therefore, the crude MMP formed can be readily separated from the other volatile substances in the product mixture and purified further by distillation. In the distillation, the 3-methylthiopropionaldehyde is separated in gaseous form from the high boilers and liquefied again in a condenser. This distillation is conducted under reduced pressure, the consequence of which is that lower temperatures are required for the distillation than when the distillation has been conducted under standard pressure. This has the advantage that the formation of high boilers is avoided. Typically, for the distillation of 3-methylthiopropionaldehyde, pressures of less than 100 mbar and temperatures of 50°C to 100°C are employed. The purified MMP can subsequently be sent to the preparation of methionine.

Typically, for generation of the reduced pressure or vacuum in the distillation of 3- methylthiopropionaldehyde, conventional gas ejectors or vacuum pumps are used. However, a disadvantage of gas ejectors, also referred to as vacuum ejectors, as already mentioned at the outset, is that the ejector medium always comes into contact with the process medium in question, which unavoidably results in contamination of the ejector medium. In the case of distillation of 3- methylthiopropionaldehyde with water as ejector medium in what is called a waterjet pump, the water has to be disposed of in a costly manner because of the contamination by sulphur-containing MMP, for example by incineration. An additional disadvantage in the incineration of sulphur-containing media is that the offgas has to be desulphurized to avoid sulphur dioxide emissions. However, these additional steps are associated with high capital and operating costs. By comparison, the process according to the invention, especially in the case of use of 3- methylthiopropionaldehyde as absorption liquid, has the advantage that gaseous 3- methylthiopropionaldehyde is physically absorbed in an identical absorption liquid from the gas mixture that has been aspirated or distilled off from the distillation. The absorption liquid need therefore not be disposed of in a complex manner as in the case of water, but can either be recycled directly into the process for preparing 3-methylthiopropionaldehyde or sent to other uses. In this case, no further purification of the absorption liquid is required either, since no troublesome components are present in the absorption liquid. Uncondensable or unabsorbable gases from the aspirated gas mixture are removed in a phase separator downstream of the vacuum unit and sent to thermal disposal.

The process according to the invention serves for vacuum distillation of at least one organic compound. Therefore, either a single compound or two or more compounds can be subjected to the vacuum distillation. The simplest case involves just a single organic compound to be distilled, which is additionally identical to the absorption liquid. This has the great advantage that a liquid consisting ideally entirely of the absorption liquid or at least a liquid comprising essentially the absorption liquid leaves the vacuum unit. As a result, a subsequent separation of any low-boiling impurities present from the absorption liquid becomes much simpler than in the case of a complex muiticomponent mixture. Ideally, however, no further purification of the absorption liquid is required. In the case of two or more organic compounds which are distilled, it is likewise advantageous when one of the organic compounds to be distilled is identical to the absorption liquid. This is because, in these cases too, the number of compounds to be separated from one another is reduced by one component compared to the processes known from the prior art. In one embodiment of the process according to the invention, therefore, the organic compound to be distilled or one of the organic compounds to be distilled is identical to the absorption liquid.

In an alternative embodiment of the process according to the invention, one of the organic compounds to be distilled is identical to the absorption liquid and another one of the organic compounds to be distilled is different from the absorption liquid.

However, the process according to the invention is not restricted to the distillation of a single organic compound. Instead, the process according to the invention also allows the distillation of two or more organic compounds, of which either only one compound or even all the compounds is/are different from the absorption liquid. According to the invention, an organic compound which is to be distilled and is different from the absorption liquid forms an addition compound with the absorption liquid. Appropriately, an organic compound which is to be distilled and is different from the absorption liquid contains an HO- or SH- group. This is because compounds having an HO- or SH- group can add onto a double bond of the alkenyl radical of an absorption liquid according to the invention in a Michael addition reaction, especially when the compound is an α,β-unsaturated carbonyl compound, or add onto the carbonyl function of an absorption liquid according to the invention to form a hemiacetal or hemimercaptal.

Therefore, in the process according to the invention, therefore one of the organic compounds to be distilled is different from the absorption liquid and corresponds to the general formula (II) R ² -L -R ³ where L is an alkyl, alkenyl or alkynyl group having 1 to 12 carbon atoms and R ² and R ³ are each independently a hydrogen atom or an HO- or HS- group, provided that at least one of R ² and R ³ is an HO- or HS-group. In a further embodiment of the process according to the invention, one of the organic compounds to be distilled is different from the absorption liquid and corresponds to the general formula (II) R ² -L ¹ - R ³ where L is an alkyl group having 1 to 12 carbon atoms and R ² and R ³ are each independently a hydrogen atom or an HO- or HS- group, provided that at least one of R ² and R ³ is an HO- or HS- group.

Preferably, L is a linear alkyl group having 1 to 6 carbon atoms, preferably having 1 to 4 carbon atoms. Further preferably, one of the R ² and R ³ radicals is a hydrogen atom and the other radical is an HO- or HS- group. In the context of the present invention, moreover, it has been found that the use of 3- methylthiopropionaldehyde as absorption liquid in the vacuum distillation can drastically reduce the content of methyl mercaptan in the gas phase. This is attributed to the fact that 3- methylthiopropionaldehyde forms what is called a hemimercaptal with methyl mercaptan. These are semi-stable chemical compounds of aldehydes with thiols. The reaction to give this hemimercaptal takes place very rapidly in the liquid phase, and so virtually all the methyl mercaptan from the gas phase is absorbed in the liquid phase in chemically bound form as an addition compound. Therefore, as shown by the table which follows, the vapour pressure of methyl mercaptan over a mixture of 3- methylthiopropionaldehyde and methyl mercaptan is much lower than over other liquid phases, for example over mixtures of water and methyl mercaptan or of methanol and methyl mercaptan. Over a solution of 5.7 per cent by weight of methyl mercaptan in 3-methylthiopropionaldehyde, a vapour pressure of 14 mbar relative to ambient pressure (1.014 bar) is measured at a temperature of 20°C. By comparison, at the same temperature, the vapour pressure over a solution of only 2.5 per cent by weight of methyl mercaptan in methanol is already 500 mbar relative to ambient pressure (1.500 bara), and over a solution of only 1.2 per cent by weight of methyl mercaptan in water is actually 700 mbar relative to ambient pressure (1.700 bara).

In a further preferred embodiment of the process according to the invention, the compound to be distilled is therefore methyl mercaptan.

Table 1 : Vapour pressures over mixtures of methyl mercaptan (MC) in various solvents (MMP

= 3-methylthiopropionaldehyde, H2O = water and MeOH = methanol) A continuous physical and/or chemical absorption of the organic compound to be distilled in the absorption liquid ultimately leads to enrichment of the organic compound in the absorption liquid. Increasing enrichment of absorbed compounds has an adverse effect on the further absorption capacity of the organic compound in question. This is because the amount of the chemically absorbable organic compound is limited by the amount of the absorption liquid present. It should also be taken into account that the addition compound formed as a consequence of the chemical absorption is formed in an equilibrium reaction. Which side the equilibrium for the formation of the addition compound is on depends on the equilibrium constant for the reaction in question and the concentration of the components present in equilibrium. In the case of a high concentration of the addition compound compared to the absorption liquid, therefore, reverse reactions of the addition compound in the direction of the organic compound and the absorption liquid cannot be ruled out.

It is possible to counteract these impairments by, before, during or after a vacuum distillation, removing the absorption liquid from the absorption apparatus at regular time intervals and supplying the absorption apparatus with fresh absorption liquid comprising none of the organic compounds to be distilled in physically and/or chemically absorbed form. This exchange of absorption liquid, i.e. the removal of absorption liquid contaminated with at least one absorbed organic compound from the absorption apparatus and the supply of fresh absorption liquid not yet contaminated with a distilled organic compound to the absorption apparatus, can be effected discontinuously, continuously or continuously in batches. A continuous exchange in batches is understood in the context of the present invention to mean an exchange of absorption liquid which is continuous for a particular period of time and is interrupted at irregular time intervals.

At least once the absorption capacity of the absorption liquid is exhausted, the old or contaminated absorption liquid must be exchanged for a new absorption liquid. New absorption liquid in this connection is understood to mean an absorption liquid according to the invention which at least does not comprise any adduct or any addition compound according to the present invention. Preferably, the new absorption liquid does not contain any organic compounds to be distilled in physically and/or chemically absorbed form. Exhaustion of the absorption capacity has been attained at least once a compound which should actually be chemically absorbed in the absorption liquid is present in the liquid phase leaving the absorption apparatus. The presence of such a compound in the liquid phase is detectable or determinable by suitable analysis methods such as IR, UV-vis, NMR or other suitable methods. Continuous exchange of the old or contaminated absorption liquid for new uncontaminated absorption liquid has the advantage that the vacuum distillation does not have to be stopped for exchange of the absorption liquid - as would be the case for batchwise exchange of the absorption liquid.

In one embodiment of the process according to the invention, therefore, the absorption liquid enriched with an absorbed organic compound is removed discontinuously, continuously or continuously in batches from the absorption apparatus, and the absorption apparatus is supplied discontinuously, continuously or continuously in batches with absorption liquid comprising none of the organic compounds to be distilled in physically and/or chemically absorbed form.

Continuous exchange of the absorption liquid contaminated with a distilled organic compound for absorption liquid not containing any of the organic compounds to be distilled in physically and/or chemically absorbed form has the advantage that the vacuum distillation does not have to be stopped for exchange of the absorption liquid - as would be the case for discontinuous exchange of the absorption liquid.

Preferably, the exchange is therefore continuous, in order to fundamentally rule out any impairments either with regard to the absorption or with regard to the pump performance.

Additionally or alternatively, the absorption liquid contaminated with a distilled organic compound can be exchanged for an absorption liquid other than the first absorption liquid when a first organic compound has been removed completely from a mixture by vacuum distillation, and a further organic compound other than the first compound is to be distilled thereafter.

In an alternative or additional embodiment of the process according to the invention, the absorption liquid enriched with an absorbed organic compound is removed from the absorption apparatus and the absorption apparatus is supplied with an absorption liquid other than the first absorption liquid when, on completion of physical and/or chemical absorption of one or more first organic compounds, a further organic compound other than these compounds is to be distilled.

By lowering the operating temperature of the absorption unit and especially of the absorption liquid thereof, the efficiency of the chemical absorption of the organic compounds, which are to be distilled and are different from the absorption liquid, is increased. This is attributed to the fact that lowering of the temperature of the absorption liquid improves the condensation of the organic compounds distilled off. Since the formation of addition compounds from the absorption liquid according to the invention and the organic compounds which are to be distilled and are different from the absorption liquid preferably takes place in the liquid phase, the chemical absorption is thus automatically also optimized. Preferably, therefore, the absorption liquid according to the invention is cooled in the process according to the invention. The cooling of the absorption liquid can be effected either indirectly, i.e. by convection, i.e. by cooling of the absorption apparatus in question which is supplied with the absorption liquid as operating medium, or directly, i.e. by cooling of the absorption liquid prior to entry into the absorption apparatus.

Preferably, the absorption liquid is cooled in circulation. In the context of the process according to the invention, circulation cooling is possible especially when supply and removal of absorption liquid to and from the absorption apparatus is effected in a continuous manner. Circulation of an absorption liquid according to the invention preferably comprises the removal of an absorption liquid with an organic compound physically and/or chemically absorbed therein from the absorption apparatus, the separation of the unabsorbed and/or uncondensed compounds from the absorption liquid in a phase separator and the recycling of the absorption liquid to the absorption apparatus, optionally accompanied by the supplementation of additional absorption liquid when the amount of absorption liquid remaining after the removal of the addition compound is less than the minimum amount required for proper operation of the absorption apparatus. In the phase separator, the unabsorbed components are separated from the absorption liquid. The components which form the addition compound, i.e. especially methyl mercaptan and 3-methylthiopropionaldehyde, remain in the absorption liquid; other unabsorbed compounds do not remain in the absorption liquid. The hemithioacetal is too stable to be able to be broken back down in a phase separator to its individual methyl mercaptan and 3-methylthiopropionaldehyde constituents. The simplest and best way of breaking down the hemithioacetal is to react it with acrolein to obtain 3-methylthiopropionaldehyde. The reaction of 1 mol of hemithioacetal with 1 mol of acrolein gives two moles of 3- methylthiopropionaldehyde. Preferably, the efficiency of the chemical absorption is enhanced by cooling the temperature of the absorption liquid down to 40 +/- 5°C to 20 +/- 5°C.

If one of the organic compounds distilled off in the process according to the invention is identical to the absorption liquid, physical absorption of this organic compound results in enrichment of the absorption liquid during the performance of the process according to the invention in the absorption apparatus. In a preferred embodiment of the process according to the invention, the absorption liquids used in accordance with the invention are aldehydes having an HS- or HO- group, preferably 3-methylthiopropionaldehyde or a higher homologue thereof. Such compounds are generally valuable synthesis units for the production of products of economic significance. The addition compounds which are formed in the process according to the invention by chemical absorption of an organic compound other than the absorption liquid with the absorption liquid are also valuable synthesis units for further conversions to products of economic significance. More particularly, the hemiacetals or hemimercaptans formed by addition of an alkyl alcohol or alkyl mercaptan onto the carbonyl function of an aldehyde are important synthesis units. This is because these addition compounds formed in an equilibrium reaction can again release the alkyl alcohol or alkyl mercaptan in solution. For example, the hemimercaptal formed from 3-methylthiopropionaldehyde and methyl mercaptan can release methyl mercaptan again in solution. The methyl mercaptan released can form further 3-methylthiopropionaldehyde in a separate reaction with acrolein, the latter being an important synthesis unit in the production of methionine, a product of economic significance. Preferably, therefore, the absorption liquid is 3-methylthiopropionaldehyde and the organic compound which is to be distilled and is different from the absorption liquid is methyl mercaptan. By sending the absorption liquid removed from the absorption apparatus to a chemical production process, it is possible to increase the yield of compounds formed in the production process in question.

In a preferred embodiment of the process according to the invention, the absorption liquid removed from the absorption apparatus is sent to a chemical production process.

In a further preferred embodiment, the absorption liquid removed from the absorption apparatus comprises an addition compound formed from 3-methylthiopropionaldehyde and methyl mercaptan. Advantageously, the absorption liquid removed from the absorption apparatus can be sent to a process for preparing 3-methylthiopropionaldehyde. Since the addition compound present in the absorption liquid, the hemimercaptal, is not thermodynamically stable but is always present in equilibrium in proportion to its 3-methylthiopropionaldehyde and methyl mercaptan starting materials, the production process is thus supplied with the required starting materials in pure form. This especially has the advantage that there is no requirement in the absorption apparatus for any additional absorption medium which would then have to be removed again from the recycled stream in a costly and inconvenient manner.

In a particularly preferred embodiment of the process according to the invention, the absorption liquid removed from the absorption apparatus is sent to a process for preparing 3- methylthiopropionaldehyde.

Depending on the individual boiling points of the organic compounds to be distilled, it is not always necessary, that the vacuum unit is preceded by a distillation column. For example, methylmercaptan, which adds in a Michael-addition to the carbon carbon double bond of acrolein under formation of 3- methylthiopropionaldehyd has a very low boiling point of 6°C and a high vapour pressure of 170 kPa at 20°C. By comparison, the reaction product 3-methylthiopropionaldehyd has a boiling point of 165 to 166°C. Thus, for a separation of methylmercaptan from a 3-methylthiopropionaldehyd containing mixture it is only necessary to apply a vacuum with which the methylmercaptan is sucked off. Notwithstanding, it is advantageous that the vacuum unit in the process according to the present invention is preceded by a fractionation, in particular a distillation column or a quencher unit. This allows a broader possibility of application of the process according to the present invention.

Preferably, the process according to the present invention is preceded by a fractionation, such as a distillation in order to achieve a better separation of the compounds to be distilled. As outlined above, the absorption liquids according to the invention are suitable for use in a vacuum unit having a vacuum pump and an absorption apparatus which comprises an absorption liquid and is connected upstream of the vacuum pump.

The present invention therefore also further provides a vacuum unit comprising a vacuum pump and an absorption apparatus connected upstream of the vacuum pump, wherein the absorption apparatus comprises an absorption liquid according to the invention. Preferably, the absorption apparatus of the vacuum unit is a scrubber.

A further object of the present invention is the use of a vacuum unit comprising a vacuum pump and an absorption apparatus connected upstream of the vacuum pump for the vacuum distillation of an organic compound, wherein the absorption apparatus comprises an absorption liquid according to the process of the present invention.

The embodiments of the process according to the present invention also apply to the use according to the present invention. Fig. 1 shows one embodiment of the process according to the invention using a vacuum unit with an upstream absorption apparatus: By a distillation under reduced pressure, the high-boiling impurities are removed from a stream comprising 3-methylthiopropionaldehyde which comes from production. For this purpose, a liquid stream comprising 3-methylthiopropionaldehyde from production is introduced via the conduit (1 ) into a pressure-reduced distillation column (2). The reduced pressure in the column (2) is generated by means of a vacuum unit with an upstream absorption apparatus (8) connected to the top of the column via the conduits (4) and (7). The high-boiling impurities are removed from the bottom of the column (2) via the conduit (3). The 3-methylthiopropionaldehyde is removed via the top of the column and passed via the conduit (4) to the heat exchanger (5), where it is condensed. The liquid 3-methylthiopropionaldehyde obtained in such a way is sent to further uses via the conduit (6). Even though the 3-methylthiopropionaldehyde is fed to the heat exchanger (5), a gas phase comprising small amounts of 3-methylthiopropionaldehyde and methyl mercaptan and low-boiling impurities from the preparation of the aldehyde still nevertheless passes through the conduit (7) into the vacuum system. Both the gaseous 3-methylthiopropionaldehyde and the gaseous methyl mercaptan are condensed and absorbed in the 3-methylthiopropionaldehyde-containing absorption liquid of the absorption apparatus (9) of the vacuum pump (1 1 ). The methyl mercaptan forms the corresponding hemimercaptal with 3-methylthiopropionaldehyde. The vacuum pump (1 1 ) in one case is a standard rotary vane pump, and in the other case is a gas injector or waterjet pump. Via the conduits (12) and (13), the absorption liquid and the hemimercaptal present therein are removed from the absorption apparatus of the vacuum unit and sent to further uses.

The present invention is further described by the following items: Process for vacuum distillation of at least one organic compound by means of a vacuum unit comprising a vacuum pump and an absorption apparatus connected upstream of the vacuum pump and comprising an absorption liquid according to the general formula (I) R -CHO where R is an alkyl, alkenyl or alkynyl group which is substituted or unsubstituted by an H3C-O- or h C-S-group and has 1 to 12 carbon atoms, wherein the absorption liquid is different from one of the compounds to be distilled and forms an addition compound therewith, wherein said organic compound to be distilled corresponds to the general formula (II) R ² -L -R ³ where L is an alkyl, alkenyl or alkynyl group having 1 to 12 carbon atoms and R ² and R ³ are each independently a hydrogen atom or an HO- or HS-group, provided that at least one of R ² and R ³ is an HO- or HS-group, and/or the absorption liquid is identical to one of the compounds to be distilled. Process according to item 1 , wherein the absorption apparatus is a scrubber. Process according to item 1 or 2, wherein the absorption liquid corresponds to the general formula (I) R -CHO, and R is an alkyl, alkenyl or alkynyl group which is substituted by a CH3O- or CH3S- group and has 1 to 12 carbon atoms. Process according to any one of items 1 to 3, wherein the absorption liquid is 3- methylthiopropionaldehyde or a higher homologue thereof, wherein said higher homologue differs from the 3-methylthiopropionaldehyde by an additional CH2 unit in the alkyl group. Process according to any of items 1 to 4, wherein the absorption liquid is H3C-S-R-CHO, with R being an alkyl group having 3 to 12 carbon atoms. Process according to any one of items 1 to 5, wherein the organic compound to be distilled or one of the organic compounds to be distilled is identical to the absorption liquid. Process according to any one of items 1 to 5, wherein one of the organic compounds to be distilled is identical to the absorption liquid and another one of the organic compounds to be distilled is different from the absorption liquid. Process according to any one of items 1 to 7, wherein one of the organic compounds to be distilled is different from the absorption liquid and corresponds to the general formula (II) R ² - L -R ³ where L is an alkyl group having 1 to 12 carbon atoms and R ² and R ³ are each independently a hydrogen atom or an HO- or HS- group, provided that at least one of R ² and R ³ is an HO- or HS-group. Process according to item 8, wherein the compound to be distilled is methyl mercaptan. Process according to any one of items 1 to 9, wherein the absorption liquid enriched with an absorbed organic compound is removed discontinuously, continuously or continuously in batches from the absorption apparatus, and the absorption apparatus is supplied discontinuously, continuously or continuously in batches with absorption liquid comprising none of the organic compounds to be distilled in physically and/or chemically absorbed form. Process according to any one of items 1 to 10, wherein the absorption liquid enriched with an absorbed organic compound is removed from the absorption apparatus and the absorption apparatus is supplied with an absorption liquid other than the first absorption liquid when, on completion of physical and/or chemical absorption of one or more first organic compounds, a further organic compound other than these compounds is to be distilled. Process according to item 10 or 1 1 , wherein the absorption liquid removed from the absorption apparatus is sent to a chemical production process. Process according to any one of items 10 to 12, wherein the absorption liquid removed from the absorption apparatus comprises an addition compound formed from 3- methylthiopropionaldehyde and methyl mercaptan. Process according to item 12 or 13, wherein the absorption liquid is sent to a process for preparing 3-methylthiopropionaldehyde. Vacuum unit comprising a vacuum pump and an absorption apparatus connected upstream of the vacuum pump, wherein the absorption apparatus comprises an absorption liquid according to any one of items 1 to 5. Use of a vacuum unit comprising a vacuum pump and an absorption apparatus connected upstream of the vacuum pump for the vacuum distillation of an organic compound, wherein the absorption apparatus comprises an absorption liquid according to any of items 1 to 5. Use according to item 16, wherein at least one organic compound is subjected to the vacuum distillation, wherein the absorption liquid is different from one of the compounds to be distilled and forms an addition compound therewith, wherein said organic compound to be distilled corresponds to the general formula (II) R ² -L -R ³ where L is an alkyl, alkenyl or alkynyl group having 1 to 12 carbon atoms and R ² and R ³ are each independently a hydrogen atom or an HO- or HS-group, provided that at least one of R ² and R ³ is an HO- or HS-group, and/or the absorption liquid is identical to one of the compounds to be distilled.

Use according to item 16 or 17, wherein the absorption apparatus is a scrubber. 19. Use according to any one of items 16 to 18, wherein one of the organic compounds to be distilled is identical to the absorption liquid and another one of the organic compounds to be distilled is different from the absorption liquid.

20. Process according to any one of items 16 to 19, wherein one of the organic compounds to be distilled is different from the absorption liquid and corresponds to the general formula (II) R ² -L -R ³ where L is an alkyl group having 1 to 12 carbon atoms and R ² and R ³ are each independently a hydrogen atom or an HO- or HS- group, provided that at least one of R ² and R ³ is an HO- or HS-group.

21. Process according to item 20, wherein the compound to be distilled is methyl mercaptan.

22. Process according to any one of items 16 to 21 , wherein the absorption liquid enriched with an absorbed organic compound is removed discontinuously, continuously or continuously in batches from the absorption apparatus, and the absorption apparatus is supplied discontinuously, continuously or continuously in batches with absorption liquid comprising none of the organic compounds to be distilled in physically and/or chemically absorbed form.

23. Process according to any one of items 16 to 22, wherein the absorption liquid enriched with an absorbed organic compound is removed from the absorption apparatus and the absorption apparatus is supplied with an absorption liquid other than the first absorption liquid when, on completion of physical and/or chemical absorption of one or more first organic compounds, a further organic compound other than these compounds is to be distilled.

24. Process according to item 22 or 23, wherein the absorption liquid removed from the absorption apparatus is sent to a chemical production process.

25. Process according to any one of items 22 to 24, wherein the absorption liquid removed from the absorption apparatus comprises an addition compound formed from 3- methylthiopropionaldehyde and methyl mercaptan.

26. Process according to item 24 or 25, wherein the absorption liquid is sent to a process for preparing 3-methylthiopropionaldehyde.

Example:

3-Methylthiopropionaldehyde was prepared in a reactor from acrolein and methyl mercaptan. A liquid stream comprising 3-methylthiopropionaldehyde and methyl mercaptan from the production of the aldehyde was subjected to a vacuum distillation in an arrangement, for example as shown in Fig. 1 , with a vacuum pump (rotary vane pump or gas ejected/water jet pump) with an upstream 3- methylthiopropionaldehyde-containing absorption apparatus. Before being fed into the absorption apparatus, the composition of the absorption liquid was determined by gas chromatography. For a duration of one hour, the vacuum unit comprising this absorption liquid was used to suck in a gas phase comprising 3-methylthiopropionaldehyde and methyl mercaptan. After an operating time of one hour, the absorption liquid was removed from the absorption apparatus of the vacuum unit and the composition thereof was again determined by gas chromatography. The analysis results are compiled in Table 2. The figures for the masses in this table are relative. The reference chosen, with a value of 100 mass units, was the content of 3-methylthiopropionaldehyde in the absorption liquid. The hemimercaptal formed from 3-methylthiopropionaldehyde and methyl mercaptan which was mentioned further up is thermally unstable and is detected as methyl mercaptan in the gas chromatography.

Table 2: Results of the gas chromatography analysis of an absorption liquid

A liquid stream of approximately 104 relative mass units entered the absorption apparatus, and then a liquid stream of approximately 142 relative mass units left the absorption apparatus. The difference in mass between the two liquid streams resulted almost entirely from the chemical and physical absorption of 3-methylthiopropionaldehyde and methyl mercaptan from the gas stream sucked in by the vacuum unit. Within an operating time of one hour, approximately 18.5 relative mass units of methyl mercaptan and approximately 12.5 relative mass units of 3-methylthiopropionaldehyde were recovered in physically or chemically absorbed form in the absorption liquid. At a load of 6500 relative mass units of 3-methylthiopropionaldehyde, this would correspond to a loss of approximately 0.5%, which would lead to considerable losses in an industrial scale process such as the preparation of methionine, for which 3-methylthiopropionaldehyde is an essential precursor. Moreover, about 6.5 relative mass units of water were also recovered, which could be reintegrated without any problem and hence did not need to be disposed of.