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, wherein the absorption liquid is not identical to the compound to be distilled and forms an addition compound therewith and/or the absorption liquid is identical to the compound to be distilled. The present invention further also relates to the use of a specific absorption liquid as lubricant in a liquid ring pump and to a liquid ring pump comprising a specific absorption liquid. Pumped removal of gases or distillation of compounds is accomplished on the industrial scale using a number of different pumps. According to the principle of their physical action, pumps for generating vacuum are divided into gas-binding vacuum pumps and gas transfer pumps. Gas transfer pumps achieve vacuum pressures down to less than 10 ³ bar, which is entirely sufficient for most industrial scale applications in the chemical industry, especially vacuum distillation. Therefore, for industrial scale applications in the chemical industry, generally only gas transfer pumps are used. With regard to their design, gas transfer pumps are divided into positive displacement pumps and ejector pumps or jet pumps.

A feature common to all positive displacement pumps is an encapsulated (closed) expansion chamber, the size of which changes cyclically during the operation cycle. The operation cycle of a positive displacement pump can be divided into four operation cycle phases: priming, conveying (compression), exhaust and pressure change. The sucking and exhaust phase is also referred to as the change in load on the low-pressure side and on the high-pressure side. In the case of positive displacement pumps, the gas present in the receiver enters an expansion chamber formed by pistons, rotors or vanes, the expansion chamber is closed, the gas is possibly compressed and then is expelled. The mechanical elements within wet-running pumps are sealed with respect to one another by a liquid.

A liquid ring pump consists 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 aspirated. Within the housing of the liquid ring pump is a liquid working medium, 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 two 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. As a result, the blades of the impeller form chambers of different size, which are also referred to as impeller chambers. The gas phase present in an impeller chamber is compressed when the blades which form an 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. The terms "jet pump" and "ejector pumps" refer to pumps where the pumping action is generated by a fluid jet, typically a liquid jet. The liquid used here is called the motive medium or working medium. By exchange of momentum with the fluid jet, another medium, called the suction medium, is aspirated, accelerated and compressed or conveyed if it is under sufficient pressure. The conveying of the suction medium proceeds in the following steps: The motive medium passes at maximum velocity from a narrow motive nozzle into a much larger mixing chamber. As it does so, a dynamic pressure drop arises according to Bernoulli's law. Therefore, the pressure in the flow is lower than standard pressure. In the mixing chamber, the motive jet meets the suction medium present in the mixing chamber, which is usually under standard pressure. After exiting from the nozzle, the motive jet at first acts like a free jet: Internal friction and turbulences give rise to shear stress at the interface between the fast motive medium and the much slower suction medium. This stress brings about a transfer of momentum, the effect of which is that the suction medium is accelerated and entrained. The widening of the motive jet and the aspiration of the suction medium slow down the jet, meaning that dynamic pressure is converted to static pressure. Since the suction medium is accelerated in the mixing chamber, according to Bernoulli's law, a pressure drop also arises for the suction medium, i.e. a suction effect which conveys further suction medium from a space which is adjacent via a suction stub, provided that a sufficiently high minimum pressure exists therein.

A general disadvantage of positive displacement pumps and especially of jet pumps is that the working medium or the motive medium always comes into contact with the suction medium. In the aspiration of a medium by means of these pump types, it is therefore unavoidable that the working medium or motive medium is contaminated by the suction medium, or the suction medium contaminated by the working medium or motive medium leaves the pump. If the suction medium is a compound of potential value, contamination with the working medium or motive medium is a considerable problem. This is because, for reintegration or supply to other uses, the compound removed by suction has to be released again in pure form from the working medium or motive medium. In practice, this is possible only at corresponding cost and inconvenience, if at all, 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. US patent 3,954,567 discloses a process, in which solutions of phosgene, an organic solvent and nitrogen or oxygen compounds which have a higher boiling point than the solvent are evaporated in a vacuum which is produced by a liquid ring pump operated on a phosgene-containing solvent as working fluid, the used working fluid being subsequently returned to the process. However, due to the high reactivity of phosgene this process is strictly limited to the use of solvent molecules without any functional groups, in particular molecules without carbonyl or carboxylic groups.

US patent 3,248,233 discloses a process for recovering essence from orange or other fruit juice concentration for return to the juice after concentration so as to impart thereto the natural flavour and in particular to such recovery from concentration of the juice in vacuum concentrating equipment. However, the ring sealing liquid in the vacuum pump of this process is water, which must be distilled of from the concentrated essence.

The contamination of the working medium or motive medium with the suction medium aspirated is a great problem especially when the suction medium is a sulphur compound. This is because the contaminated working or suction medium then has to be disposed of at high cost, for example by incineration. However, an additional disadvantage in the incineration of sulphur compounds is that the offgas has to be desulphurized to prevent sulphur dioxide emissions. This is generally associated with high capital and operating costs.

It was therefore an object of the present invention to provide a process for vacuum distillation of organic compounds which enables complete reintegration of the compound distilled off with avoidance of the formation of wastewater and with at least distinct reduction of gas emissions and without a costly desulphurization.

This object is achieved in accordance with the invention by conducting the vacuum distillation by means of a vacuum unit comprising an absorption liquid which allows physical or chemical absorption of an organic compound to be distilled or simultaneously physical and chemical absorption of organic liquids to be distilled.

Physical absorption of the 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 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 liquid 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 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 h C-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, 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 the distilled, preferably, only one of R ² and R ³ is an HO- or HS-group. 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 methyl mercaptan. The term "vacuum unit" is used in the context of the present invention to refer to all pumps by which a reduced pressure can be generated for a vacuum distillation, including any additional apparatuses required for chemical and/or physical absorption of distilled organic compounds.

In technical terms, strictly speaking, vacuum pumps are not pumps but compressors. They therefore constitute a condensation stage for the compound distilled off. When the absorption liquid in the vacuum unit is identical to the compound distilled off, this leads to an increase in concentration of the compound in question. If the compound distilled off 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 merely to aspirating and absorbing individual compounds 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 not just distilled off but also absorbed, one compound being identical to the absorption liquid and another compound being different from the absorption liquid and forming an adduct therewith. For all these cases, aldehydes are suitable as absorption liquids. Aldehydes, and especially those wherein the alkyl, alkenyl or alkynyl groups containing 1 to 12 carbon atoms are either unsubstituted or substituted by an H3C-O- or H3C-S- group, have comparatively low melting points and comparatively high boiling points. Aldehydes are therefore usable over a wide temperature range as absorption liquids in vacuum distillation. Additional cooling of the absorption liquid is therefore not generally required, but can additionally be conducted to reduce the release of any organic compound distilled off that has not already been physically absorbed in the absorption liquid, and to reduce the thermal stress for the absorbent. In the case that the aldehyde used as absorption liquid is identical to an organic compound to be distilled, therefore, the organic compound distilled off is absorbed in the phase formed by the aldehyde in question. When the organic compound to be distilled is an alcohol and/or a mercaptan, the aldehyde used as absorption liquid forms an addition compound with the organic compound to be distilled.

A problem with the use of liquid ring pumps in vacuum distillation is the occurrence of cavitation, especially when the liquid ring pump is operated close to the vapour pressure of the ring liquid. If the ring liquid evaporates, bubbles form in the liquid during this operation, which are also referred to as cavitation bubbles. This is because the evaporated ring liquid requires a much larger space than the ring liquid in the liquid state. If the pressure in the ring liquid rises again, the evaporation process ends. The ring liquid vapour formed in the cavitation bubble then condenses at the outer wall of the vapour bubble, and the vapour bubbles already formed then collapse again within a very short time. The space previously required by the vapour phase abruptly becomes smaller, and the ring liquid flows in an implosive manner into the space made available. This gives rise to pressure surges in the ring liquid, which only last for a short time but can have an intensity in the order of magnitude of several hundreds of MPa. This operation therefore gives rise to pressure waves with very high peak pressures. If imploding vapour bubbles are close to or directly at a solid wall, for example the impeller blades of the liquid ring pump, the implosion gives rise to a liquid jet, called a micro jet, which hits the wall or the impeller blade at (very) high velocity. This leads both to material wear, to a considerable degree in some cases, and to intensive emission of sound. The abrupt compressive stress on the material of wall and impeller blade is so high that the occurrence of cavitation leads to crater-like removal of material. The reduced pressure generated by the liquid ring pump brings about lowering of the vapour pressure of the ring liquid within the liquid ring pump. In the operation of a liquid ring pump, the ring liquid used therefore already evaporates at a lower temperature than at the standard pressure of 1013 mbar. This promotes the occurrence of cavitation in the liquid ring pump. This problem is solved in accordance with the invention by using, as absorption liquid, an aldehyde wherein the alkyl, alkenyl or alkynyl group comprising 1 to 12 carbon atoms is substituted by an H3C- O- or H3C-S- group. The additional H3C-O- or H3C-S- group increases the boiling point of the aldehyde, which avoids or at least partly reduces the occurrence of cavitation. Moreover, the additional H3C-O- or H3C-S- group increases the dissolving properties of the aldehyde, which also contributes to avoidance or at least reduction of the occurrence of solid deposits which can promote the formation of cavitation. In one embodiment of the process according to the invention, the absorption liquid corresponds to the general formula (I) R -CHO where R is an alkyl, alkenyl or alkynyl group which is substituted by an H3C-O- or H3C-S- 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, alkenyl or alkynyl group having 1 to 6 carbon atoms, especially having 2 to 4 carbon atoms, substituted by an H3C-O- or H3C-S- 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-methylthiopropionaldehyde 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. 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 multicomponent 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.

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, only 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 process according to the invention, it has been found to be particularly advantageous when the organic compound which is to be distilled and is different from the absorption liquid includes an HS- group. This is because organic compounds having HS- groups form a hemimercaptal as addition compound with aldehydes. The use of the absorbents according to the invention as operating media in vacuum units, for example jet pumps, liquid ring pumps and positive displacement pumps, in place of water therefore avoids not only contamination of the operating medium with a sulphur compound but also the costly and inconvenient disposal of the contaminated operating medium.

In a further preferred embodiment of the process according to the invention, therefore, the organic compound which is to be distilled and is different from the absorption liquid has an 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).

More preferably, the organic compound which is to be distilled that is different from the absorption liquid is methyl mercaptan.

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

= 3-methylthiopropionaldehyde, H2O = water and MeOH = methanol).

According to the definition of the invention, the term "vacuum unit" encompasses both vacuum pumps as such and vacuum pumps having an upstream apparatus containing an absorption liquid according to the invention for chemical and/or physical absorption of the organic compounds to be distilled. With regard to the choice of pump, the process according to the invention is not restricted to individual pump types. Instead, all pump types which can generate a reduced pressure sufficient for the vacuum distillation are usable as vacuum unit. In particular, suitable pump types for the process according to the invention are all of those which work with a liquid operating medium or motive medium. These are especially jet pumps, positive displacement pumps, positive displacement blowers or Roots blowers, liquid ring pumps, and rotary vane and external vane pumps. Also usable in the process according to the invention, in addition, are dry-running positive displacement pumps, i.e. those which work without a liquid operating medium or are not sealed by a liquid, called dry runners, or molecular pumps, turbomolecular pumps, cryopumps and sorption pumps, provided that these pumps have an upstream apparatus comprising an absorption liquid according to the invention for chemical and/or physical absorption of distilled organic compounds.

In one embodiment of the process according to the invention, therefore, the vacuum unit is a pump selected from the group consisting of jet pumps, positive displacement pumps, positive displacement blowers, liquid ring pumps, rotary vane and external vane pumps, molecular pumps, turbomolecular pumps, cryopumps and sorption pumps.

In a preferred embodiment of the process according to the invention, the positive displacement pump, if it is a dry-running pump, or the molecular or turbomolecular pump, cryopump or sorption pump has an upstream apparatus comprising an absorption liquid according to the invention. Preferably, the upstream apparatus comprising an absorption liquid according to the invention is a scrubber.

In one embodiment of the process according to the invention, the vacuum unit is a liquid ring pump wherein the ring liquid comprises the absorption liquid according to the invention or consists of the absorption liquid according to the invention.

Since pumps, especially liquid ring pumps, are condensation stages, the at least one organic compound to be distilled is condensed in the pump after distillation, which firstly improves the physical and/or chemical absorption of the organic compound in the absorption liquid because of the greater volume available for the absorption. 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 in the case of encapsulated pumps and especially in the case of those pumps sealed with a liquid comprising or consisting of the absorption liquid leads to a reduction in the gas volume available for the vacuum generation. In the medium or long term, this leads to impairment of the performance of the pump. Moreover, the enrichment of a chemically absorbed organic compound also affects 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 vacuum unit at regular time intervals and supplying the vacuum unit with 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 vacuum unit and the supply of fresh absorption liquid not yet contaminated with a distilled organic compound to the vacuum unit, 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 should 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 vacuum unit, and the vacuum unit 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 also 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. Lowering the operating temperature of the vacuum units and especially of the absorption liquid used therein as operating medium or ring liquid or as part of the operating medium or ring liquid increases the efficiency of the chemical absorption of the organic compounds which are to be distilled and are different from the absorption liquid. 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 an organic compound which is to be distilled and is different from the absorption liquid in accordance with the invention 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 pump type in question which as supplied with the absorption liquid as operating medium, or directly, i.e. by cooling of the absorption liquid prior to entry into the pump. 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 pump 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 pump, 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 pump, 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 pump. In the phase separator, the unabsorbed and/or uncondensed 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 vacuum unit. 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 alkyi alcohol or alkyi 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 alkyi alcohol or alkyi 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 vacuum unit to a chemical production process, it is possible to increase the yield of compounds formed in the production process in question.

In one embodiment of the process according to the invention, therefore, the absorption liquid enriched with an absorbed organic compound is removed from the vacuum unit and is sent to a chemical production process.

If 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, the absorption liquid removed from the vacuum unit is preferably sent to a process for preparing 3-methylthiopropionaldehyde from acrolein and methyl mercaptan.

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.

In the context of the present invention, it has also been found that the aldehydes used in accordance with the invention as a constituent of a ring liquid in liquid ring pumps also have lubricating properties in the operation of the liquid ring pumps. Irrespective of their property of acting as absorption liquid for aldehydes, alcohols and thiols, they can therefore additionally also be used as lubricant additives to other ring liquids in liquid ring pumps. The present invention therefore further provides for the use of compounds of the general formula (I) R -CHO as lubricants in liquid ring pumps, 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.

The present invention therefore further also provides a process for operating a liquid ring pump, comprising the step of adding a compound of the general formula (I) R -CHO as lubricant to the ring liquid of the liquid ring pump or the step of charging a liquid ring pump with a compound of 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. The present invention additionally also further provides a liquid ring pump comprising a ring liquid, characterized in that the ring liquid comprises or consists of an absorption liquid according to the invention.

In the use according to the invention, the process according to the invention for operating a liquid ring pump and the liquid ring pump according to the invention, the R radical of the compound of the general formula (I) is preferably an alkyl, alkenyl or alkynyl group which is substituted by an H3C-O- or H3C-S- group and has 1 to 12 carbon atoms, preferably 1 to 6 or 2 to 4 carbon atoms. Moreover, linear groups are preferred over branched alkyl, alkenyl or alkynyl groups. Preferably, the R radical of the compound of the general formula (I) is a linear alkyl radical which is substituted by an H3C-S- group and has 1 to 12, preferably 1 to 6 or 2 to 4 carbon atoms. More preferably, the aldehyde of the general formula (I) is 3-methylthiopropionaldehyde or a higher homologue thereof.

Fig. 1 shows one embodiment of the process according to the invention using a liquid ring pump. 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 liquid ring pump (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 conduits (6) and (15). Even though the 3- methylthiopropionaldehyde has been 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 ring liquid of the liquid ring pump. The methyl mercaptan forms the corresponding hemimercaptal with 3-methylthiopropionaldehyde. Via the conduit (9), the ring liquid and uncondensable or unabsorbed gases are guided from the liquid ring pump (8) to a separator (10). In addition, via the conduits (6) and (18), a portion of the purified 3-methylthiopropionaldehyde from the heat exchanger (5), where it has been condensed out, is introduced into the separator (10). The low-boiling impurities in the vapour phase of the ring liquid which has been sent to the separator (10) remain in the vapour phase and are removed via the conduit (17). The liquid phase comprising 3-methylthiopropionaldehyde and hemimercaptal is subsequently recycled into the process for preparing 3-methylthiopropionaldehyde via the conduits (1 1 ) and (13). The liquid phase comprising 3-methylthiopropionaldehyde is partly recycled to the liquid ring pump. Prior to entry into the liquid ring pump, the liquid phase used as ring liquid is cooled down to the desired temperature with the aid of the ring liquid cooler (14).

The present invention is further described by the following items:

1. Process for vacuum distillation of at least one organic compound by means of a vacuum unit 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, and/or the absorption liquid is identical to one of the organic compounds to be distilled.

2. Process according to item 1 , wherein the absorption liquid corresponds to the general formula (I) R -CHO where R is an alkyl, alkenyl or alkynyl group which is substituted by an H3C-O- or H3C-S- group and has 1 to 12 carbon atoms. Process according to item 1 or 2, 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 3, 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 4, 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 4, 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 6, 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 7, wherein the organic compound which is different from the absorption liquid and is to be distilled has an HS- group. Process according to any one of items 1 to 8, wherein the vacuum unit is a pump selected from the group consisting of jet pumps, positive displacement pumps, positive displacement blowers, liquid ring pumps, rotary vane and external vane pumps, molecular and turbomolecular pumps, cryopumps and sorption pumps. Process according to item 9, wherein the positive displacement pump, if it runs dry, or the molecular or turbomolecular pump, cryopump or sorption pump is connected to an upstream apparatus having an absorption liquid according to any one of items 1 to 4. Process according to item 9, wherein the vacuum unit is a liquid ring pump wherein the ring liquid comprises an absorption liquid according to any one of items 1 to 4 or consists of an absorption liquid according to any one of items 1 to 4. Process according to any one of items 1 to 1 1 , wherein the absorption liquid enriched with an absorbent organic compound is removed discontinuously, continuously or continuously in batches from the vacuum unit, and the vacuum unit 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. 13. Process according to any one of items 1 to 12, wherein the absorption liquid enriched with an absorbed organic compound is removed from the vacuum unit and is sent to a chemical production process.

14. Use of compounds of the general formula (I) R -CHO as lubricants in liquid ring pumps, 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. 15. Liquid ring pump comprising a ring liquid, characterized in that the ring liquid comprises or consists of an absorption liquid according to any one of items 1 to 4.

The present invention is briefly presented in the example which follows. 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 as shown, for example, in Fig. 1 , with a liquid ring pump wherein the ring liquid comprises 3-methylthiopropionaldehyde. Before being fed into the liquid ring pump, the composition of the ring liquid was determined by gas chromatography. For a duration of one hour, the liquid ring pump comprising this ring liquid was used to suck in a gas phase comprising 3-methylthiopropionaldehyde and methyl mercaptan. After an operating time of one hour, the ring liquid was removed from the liquid ring pump 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 ring liquid. The hemimercaptal formed from 3-methylthiopropionaldehyde and methyl mercaptan which was mentioned further above is thermally unstable and was detected as methyl mercaptan in the gas chromatography.

Table 2: Results of the gas chromatography analysis of a ring liquid A liquid stream of approximately 104 relative mass units entered the liquid ring pump, and then a liquid stream of approximately 142 relative mass units left the liquid ring pump. 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 liquid ring pump. 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 ring 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.