Process for removing water from a mixture containing methanol, water and a catalyst system The present invention relates to a process for removing water from a mixture containing metha- nol, water and a catalyst system and to a production unit for removing water from a mixture con- taining methanol, water and a catalyst system. Dialkoxymethanes, in particular dimethoxymethane (methylal), are of particular commercial in- terest. Since they are able to increase the octane number, lower soot and NOx formation, they are attractive candidates for the use as gasoline or diesel additives. Methylal currently finds use in a variety of applications including perfumes, resins, adhesives, coatings, sealants and putties. In addition, methylal is a valuable compound in pharmaceutical, cosmetic and polymer applica- tions. Therefore, new and simple processes for the production of methylal are of great interest. In order to minimize production costs, a highly active catalyst system is necessary in addition to being highly selective for the formation of methylal. Methylal can be produced by oxidation of an alcohol or the reaction of formaldehyde with the corresponding methanol. Formaldehyde itself is produced by the oxidation of methanol. Currently, in all known systems to produce methylal such as in WO 2020/161175 A1, the con- centration of dimethoxymethane (DMM), or methylal, is limited as it is in equilibrium with the formed water. Therefore, there is a need for a post-treatment to separate the components of the aqueous mixture obtained from methylal preparation process which permits to recycle the cata- lysts in order to improve methylal production and reduce the environmental impact of such pro- duction. Surprisingly, it was found that the process of the present invention according to which water is removed from a mixture comprising methanol, water and a catalyst system allows the recycling of methanol and the catalyst. This permits to reduce costs compared to other processes. Fur- ther, in view of the recycling, using a process according to the present invention permits to re- duce the CO ₂ footprint. Therefore, the present invention relates to a process for removing water from a mixture compris- ing methanol, water, a catalyst system and optionally methylal, the catalyst system comprising a catalyst and an acidic co-catalyst, the process comprising - a separation stage SM comprising -- passing a feed stream F2, comprising methanol, water and the catalyst system, through a separation unit SU comprising one or more pervaporation membranes, wherein at least one membrane, of the one or more pervaporation membranes, has a water/meth- anol pervaporation selectivity ß _pervap of at least 1, obtaining from SU - a permeate gas stream P comprising water, and optionally methanol; and - a retentate liquid stream R comprising water and methanol at a weight ratio x(R)=w(H ₂ O):w(CH ₃ OH) with x(R)<x(F2), and the catalyst system, x(F2) being the weight ratio of water to methanol in F2 with 0<x(F2)≤0.5; wherein said at least one membrane comprises a porous substrate and a porous material disposed on the substrate, wherein from 70 to 100 weight-% of the porous material con- sists of carbon; wherein, when the mixture comprises methylal, in addition to methanol, water and the catalyst system, the process further comprises - a separation stage SP, prior to SM, comprising -- passing a feed stream F1 comprising methanol, methylal, water and the cata- lyst system in a purification unit PU, wherein in PU, methylal is removed as a stream SM, SM comprising, in addition to methylal, methanol, and water is removed as a stream S _W , S _W comprising, in addition to water, metha- nol and the catalyst system, with a weight ratio x(SW)= w(H2O):w(CH3OH); -- passing the stream SW as a feed stream F2 through the separation stage SM, F2 having the same composition as S _W and x(S _W )=x(F2). Preferably, the feed stream F2 has a pressure pF2, with pF2 being in the range of from 1 to 80 bar, more preferably in the range of from 3 to 20 bar, more preferably in the range of from 5 to 15 bar. Preferably, the feed stream F2 is a gas stream. Preferably, the feed stream F2 has a temperature TF2, with TF2 being in the range of from 50 to 150 °C, preferably in the range of from 60 to 90 °C. Preferably, the at least one membrane, of the one or more pervaporation membranes, has a water/methanol pervaporation selectivity ßpervap of at least 2, more preferably in the range of from 2 to 100, more preferably in the range of from 2.1 to 50, more preferably in the range of from 2.25 to 30, more preferably in the range of from 2.5 to 10, more preferably in the range of from 3 to 8, more preferably in the range of from 4 to 6. Preferably, each pervaporation membrane of SU has a pervaporation selectivity ß _pervap of at least 1, more preferably at least 2, more preferably in the range of from 2 to 100, more prefera- bly in the range of from 2.1 to 50, more preferably in the range of from 2.25 to 30, more prefera- bly in the range of from 2.5 to 10, more preferably in the range of from 3 to 8, more preferably in the range of from 4 to 6. In the context of the present invention, the term “pervaporation membrane” means that said membrane is operated under pervaporation conditions. Preferably, the at least one membrane comprised in SU has a H2O permeance N in the range of from 0.2 to 200 Nm ³ /(m ² h bar), more preferably in the range of from 0.5 to 20 Nm ³ /(m ² h bar), more preferably in the range of from 2 to 10 Nm ³ /(m ² h bar), more preferably in the range of from 4 to 8 Nm ³ /(m ² h bar). Preferably, the at least one membrane comprised in SU has a permselectivity α(H ₂ O/CH ₃ OH) of at least 1, more preferably in the range of from 2 to 1000, more preferably in the range of from 3 to 200, more preferably in the range of from 4 to 100, more preferably in the range of from 5 to 50, more preferably in the range of from 6 to 25. Preferably, in SM the water fugacity difference across the at least one pervaporation membrane comprised in SU, calculated as (fugacity of water in the retentate stream R – (minus) fugacity of water in the permeate stream P) at constant temperature, is in the range of from 0.01 to 3 bar, more preferably in the range of from 0.05 to 1 bar, more preferably in the range of from 0.1 to 0.4 bar. Preferably, SU further comprises one or more pumps and one or more heat exchangers, in ad- dition to the one or more membranes. Preferably, SU comprises a membrane loop comprising at least one membrane M of the one or more membranes, at least one pump A of the one or more pumps and at least one heat ex- changer H of the one or more heat exchangers. Preferably, SU comprises one membrane loop as defined above. In the context of the present invention, it is conceivable that a further heat exchanger H0 and a further pump A0 located up- stream of the one membrane loop. Alternatively, preferably, SU comprises a plurality of membrane loops, more preferably from 2 to 10 membrane loops, more preferably from 2 to 5 membrane loops, more preferably 2 or 3 mem- brane loops, each membrane loop comprising at least one membrane M of the one or more membranes, at least one pump A of the one or more pumps and at least one heat exchanger H of the one or more heat exchangers. An example of SU comprising three membrane loops is il- lustrated in Figure 3, in this example it is conceivable to have a further heat exchanger H0 and a further pump A0 located upstream of the first membrane loop. Further, it is noted that in the configuration which comprises more than one loop, the concentration of water in the retentate of the most upstream loop will be higher than the concentration of the retentate exiting the second loop and so on to obtain the lowest water concentration in the final retentate R. In the context of the present invention, in case SU comprises 2 or more loops, the permeate stream P corresponds to the permeate stream coming from the most downstream loop, and the retentate stream R is the retentate stream coming from the most downstream loop. Preferably, each membrane of the one or more pervaporation membranes comprises a porous substrate and a porous material disposed on the substrate as defined herein above. Preferably, the porous substrate is made of one or more of alumina, titania, zirconia, and car- bon, more preferably alumina. Preferably, the porous substrate comprises one or more channels, wherein the porous material is disposed on the surface of the walls of the one or more channels of the substrate. Preferably, the channels are made of one or more layers, the one or more layers comprising alumina and pores having a pore diameter of from 2 to 20 nm or of more than 50 nm. Prefera- bly, the outermost layer, of the channel(s), in contact with the porous material is mesoporous (2- 20 nm). Preferably, the channels comprise at least three layers, wherein each layer comprises pores having different pore diameters. Preferably, the porous material supported on the substrate comprises carbon and micropores having a pore diameter of less than 2 nm, more preferably less than 0.6 nm (also called ultrami- croporous). Preferably from 75 to 100 weight-% of the porous material consists of carbon. Preferably, the porous material supported on the substrate is obtainable or obtained by a pro- cess comprising coating a polymer slurry, more preferably a polyester slurry, onto the porous substrate and pyrolyzing the obtained polymer coating, more preferably the obtained polyester coating, on the porous substrate under inert atmosphere, obtaining a porous material supported on the porous substrate. Preferably, the pervaporation membrane is obtained or obtainable by a process comprising: (a) coating a porous substrate with a solution of ethylenically unsaturated polyester to form a polyester coating; (b) drying the polyester coating on the porous substrate by removing the solvent, obtaining a dried polyester coating on the porous substrate; (c) pyrolyzing the dry polyester coating on the porous substrate, obtaining the carbon mem- brane; wherein any one of (a), (b) and (c), each independently, is conducted more than once or wherein the sequence of (a), (b) and (c) is conducted more than once. Preferably, the pervaporation membrane is obtained or obtainable by a process according to US2012/079943 A1 (Example 1). The pervaporation membranes used in the present application are commercially available and are for example disclosed in US2022/032237 A1 and P. H. Tchoua Ngamou et al., High performance carbon molecular sieve membranes for hydrogen pu- rification and pervaporation dehydration of organic solvents, Journal of Materials Chemistry A, vol.7, no.12, 19 March 2019, p.7082-7091 (cf. Carbon tubular - Tables 3 and 4 of these docu- ments). Preferably, the pervaporation membrane is a carbon molecular sieve membrane. Preferably, the porous substrate of the at least one membrane comprised in SU has a geometry selected from the group consisting of spiral-wound, hollow fiber, plate-and-frame and single- or multichannel tubular including combinations of two or more thereof, more preferably a geometry selected from the group consisting of hollow fiber and single- or multichannel tubular including combinations of two or more thereof, more preferably a geometry selected from the group con- sisting of single- or multichannel tubular and combinations thereof, more preferably the at least one membrane comprised in SU has a geometry of multichannel tubular. Preferably, the permeate stream P is a permeate gas stream. Preferably, the permeate stream P is a permeate gas stream having a pressure pP of less than 1 bar, more preferably in the range of from 5 to 100 mbar, more preferably in the range of from 10 to 70 mbar. Preferably, the permeate stream P has a temperature in the range of from 50 °C to 180 °C, more preferably in the range of from 60 °C to 170 °C, more preferably in the range of from 80 to 140 °C. Preferably, the weight ratio x(P) is in the range of from 0.05:1 to 19:1, more preferably in the range of from 0.1:1 to 4:1, more preferably in the range of from 0.15:1 to 1:1. Preferably, the retentate stream R is a retentate liquid stream. Preferably, the retentate stream R is a retentate liquid stream having a pressure pR in the range of from 1 to 80 bar, more preferably in the range of from 3 to 20 bar, more preferably in the range of from 5 to 15 bar. Preferably, the retentate stream R is a retentate liquid stream having a temperature TR in the range of from 80 to 180 °C, more preferably in the range of from 100 to 160 °C, more preferably in the range of from 120 to 140 °C.. Preferably, the weight ratio x(R) is of at most 0.1:1, more preferably of at most 0.05:1, more preferably in the range of from 0:1 to 0.05:1. Preferably, the mixture further comprises methylal, in addition to methanol, water and the cata- lyst system and the process further comprises the stage SP prior to SM. Preferably, the purification unit PU comprises a distillation column from which the streams SM and SW are removed. Preferably, the purification unit PU is a distillation column from which the streams S _M and S _W are removed. Preferably the present invention relates to a process for removing water from a mixture compris- ing methanol, water, a catalyst system and optionally methylal, the catalyst system comprising a catalyst and an acidic co-catalyst, the process comprising - a separation stage SM comprising -- passing a feed stream F2, comprising methanol, water and the catalyst system, through a separation unit SU comprising one or more pervaporation membranes, wherein at least one membrane, of the one or more pervaporation membranes, has a water/meth- anol pervaporation selectivity ß _pervap of at least 1, obtaining from SU - a permeate gas stream P comprising water, and optionally methanol; and - a retentate liquid stream R comprising water and methanol at a weight ratio x(R)=w(H ₂ O):w(CH ₃ OH) with x(R)<x(F2), and the catalyst system, x(F2) being the weight ratio of water to methanol in F2 with 0<x(F2)≤0.5; wherein said at least one membrane comprises a porous substrate and a porous material disposed on the substrate, wherein from 70 to 100 weight-% of the porous material con- sists of carbon; wherein, when the mixture comprises methylal, in addition to methanol, water and the catalyst system, the process further comprises - a separation stage SP, prior to SM, comprising -- passing a feed stream F1 comprising methanol, methylal, water and the cata- lyst system in a distillation column comprised in purification unit PU, wherein in PU, methylal is removed as a stream SM, SM comprising, in addition to methylal, methanol, and water is removed as a stream SW, SW comprising, in addition to water, metha- nol and the catalyst system, with a weight ratio x(SW)=w(H2O):w(CH3OH); -- passing the stream S _W as a feed stream F2 through the separation stage SM, F2 having the same composition as S _W and x(S _W )=x(F2). Preferably the purification unit PU comprises one or more distillation columns, more preferably at least two distillation columns. In the context of the present invention, preferably the stream SW is a liquid stream. Preferably, the feed stream F1 has a liquid phase and a gas phase. It is also conceivable that such stream F1 be a liquid stream. Preferably, the feed stream F1, when entering PU, has a pressure p _F1 , with p _F1 being in the range of from 40 to 200 bar, more preferably in the range of from 80 to 150 bar, more preferably in the range of from 85 to 140 bar, more preferably in the range of from 90 to 130 bar. Preferably, the feed stream F1, when entering PU, has a temperature TF1, with TF1 being in the range of from 20 to 200 °C, preferably in the range of from 50 to 180 °C, more preferably in the range of from 60 to 170 °C, more preferably in the range of from 80 to 140 °C. Preferably, the weight ratio of methylal to methanol in F1 is in the range of from 0.05:1 to 0.5:1, more preferably in the range of from 0.1:1 to 0.4:1. Preferably the feed stream F1 is obtainable or obtained by a process for preparing methylal from CO ₂ . Preferably, said process for preparing methylal comprises (a) providing a gas stream G containing CO ₂ and H ₂ ; (b) providing a liquid stream SL containing methanol and the catalyst system, preferably as defined in the following; (c) introducing the gas stream G provided according to (i.1) and the liquid stream S _L provided according to (i.2) into a reactor unit RU; (d) contacting G with SL to carbon dioxide reduction conditions, obtaining a stream SMW con- taining methylal, methanol, water and the catalyst system; (e) removing S _MW from RU (f) passing SMW through a gas-liquid separation unit GLU for removing H2 from SMW, obtaining a stream S*MW depleted in H2 compared to SMW and comprising methylal, methanol, water and the catalyst system, the stream S* _MW being passed through PU as F1. Alternatively, the feed stream F1 can also preferably be obtainable or obtained by processes known in the art such as the one disclosed in WO 2020/161175. Preferably, the catalyst system comprises a catalyst for preparing methylal and an acidic co-cat- alyst for preparing methylal. Preferably, the catalyst of the catalyst system is a catalyst complex comprising a transition metal complex and at least one polydentate ligand comprising at least one P atom. Preferably the catalyst complex is a homogeneous catalyst complex, which means the catalyst complex is dissolved in the liquid reaction medium, namely methanol, under the reaction condi- tions. In other words, the catalyst complex is in the same phase as the reactants. Preferably, the transition metal of the transition metal complex is selected from the group con- sisting of ruthenium, manganese, cobalt, iron, osmium, rhodium, rhenium, iridium, nickel, plati- num and palladium, more preferably selected from the group consisting of ruthenium, manga- nese and cobalt, more preferably selected from the group consisting of ruthenium and cobalt, more preferably is ruthenium. Preferably, the transition metal complex is one or more of [Ru(acetylacetonate) ₃ ], [Ru(COD)(methylallyl)2], [Co(acetylacetonate)3], RuCl3*H2O, [Ru(p-cymene)Cl2]2, [Ru(ben- zene)Cl2]n, [Ru(CO)2Cl2]n, [Ru(CO)3Cl2]2, [RuCl3H2O], [Ru(DMSO)4Cl2], [Ru(PPh3)3(CO)(H)CI], [Ru(PPh ₃ ) ₃ (CO)Cl ₂ ], [Ru(PPh ₃ ) ₃ (CO)(H) ₂ ], [Ru(PPh ₃ ) ₃ Cl ₂ ], [Ru(Cp)(PPh ₃ ) ₂ CI], [Ru(Cp)(CO) ₂ CI], [Ru(Cp)(CO)2H], [Ru(Cp)(CO)2]2, [Ru(Cp*)(CO)2CI], [Ru(Cp*)(CO)2H], [Ru(Cp*)(CO)2]2, [Ru(in- denyl)(CO) ₂ CI], [Ru(indenyl)(CO) ₂ H], [Ru(indenyl)(CO) ₂ ] ₂ , ruthenocen, [Ru(binap)(Cl) ₂ ], [Ru(2,2'- bipyridin)2(Cl)2·H2O], [Ru(COD)(Cl)2H]2, [Ru(Cp*)(COD)CI], [Ru3(CO)12], [Ru(tetraphenylhy- droxycyclopentadienyl)(CO)2H], [Ru(PMe3)4(H)2], [Ru(PEt3)4(H)2], [Ru(Pn-Pr3)4(H)2], [Ru(Pn- Bu ₃ ) ₄ (H) ₂ ], and [Ru(Pn-octyl ₃ ) ₄ (H) ₂ ], more preferably one or more of [Ru(acetylacetonate) ₃ ] and [Ru(COD)(methylallyl) ₂ ], more preferably is [Ru(acetylacetonate) ₃ ]. Preferably, the at least one polydentate ligand comprising at least one P atom is selected from the group consisting of tris(diphenylphosphinomethyl)ethane, tris[di(p-tolyl)phosphinome- thyl]ethane, tris[di(3,5-dimethylphenyl)phosphinomethyl]ethane, tris(diphenylphosphinome- thyl)methane, tris(diphenylphosphinomethyl)amine, bis(2-diphenylphosphinoethyl)phe- nylphosphine and tris[2-(diphenylphosphino)ethyl]phosphine, more preferably selected from the group consisting of tris(diphenylphosphinomethyl)ethane, tris[di(p-tolyl)phosphinomethyl]ethane, tris[di(3,5-dimethylphenyl)phosphinomethyl]ethane and tris(diphenylphosphinomethyl)amine, more preferably is tris(diphenylphosphinomethyl)ethane, more preferably 1,1,1-tris(diphe- nylphosphinomethyl)ethane (Triphos). Preferably, the molar ratio of the transition metal complex relative to the at least one polyden- tate ligand is in the range of from :3.0 to 1:1, more preferably in the range of from 1:2.0 to 1:1, more preferably in the range of from 1:1.5 to 1:1, more preferably in the range of from 1:1.2 to 1:1. Preferably, the molar ratio of the acidic co-catalyst relative to the catalyst is in the range of from 1:50 to 1:1, more preferably in the range of from 1:20 to 1:1, more preferably in the range of from 1:10 to 1:1. Preferably, the acidic co-catalyst is a Bronsted acid or a Lewis acid, wherein more preferably the acidic co-catalyst is selected from the group consisting of methane sulfonic acid (MeSO3H), Al(OTf) ₃ , Bi(OTf) ₃ , AlCl ₃ , ZnCl ₂ , SnCl ₄ , TiCl ₄ , Fe(OTf) ₃ , HCl, H ₂ SO ₄ , tricholoroacetic acid, p- TsOH, trifluoromethanesulfonic acid and a mixture of two or more thereof, more preferably se- lected from the group consisting of methane sulfonic acid (MeSO3H), Al(OTf)3, Bi(OTf)3, H2SO4, p-TsOH, trifluoromethanesulfonic acid and a mixture of two or more thereof, more preferably se- lected from the group consisting of more preferably selected from the group consisting of MeSO3H, Al(OTf)3, H2SO4, p-TsOH, trifluoromethanesulfonic acid and mixture of two or more thereof, more preferably selected from the group consisting of MeSO3H, Al(OTf)3, p-TsOH and a mixture thereof, more preferably selected from the group consisting of MeSO ₃ H, Al(OTf) ₃ and a mixture thereof, more preferably selected from the group consisting of MeSO ₃ H and Al(OTf) ₃ . Preferably, the acidic co-catalyst is a Bronsted acid, being an acid selected from the group con- sisting of methane sulfonic acid, HCl, H ₂ SO ₄ , tricholoroacetic acid, p-TsOH, trifluoromethanesul- fonic acid and a mixture of two or more thereof, more preferably selected from the group con- sisting of methane sulfonic acid, HCl, H2SO4, tricholoroacetic acid, p-TsOH and trifluoro- methanesulfonic acid, more preferably being methane sulfonic acid. Alternatively, preferably, the acidic co-catalyst is a Lewis acid, being selected from the group consisting of Al(OTf)3, Bi(OTf)3, AlCl3, ZnCl2, SnCl4, TiCl4, Fe(OTf)3, and a mixture of two or more thereof, more preferably selected from the group consisting of Al(OTf)3, Bi(OTf)3, AlCl3, ZnCl ₂ , SnCl ₄ , TiCl ₄ and Fe(OTf) ₃ , more preferably selected from the group consisting of Al(OTf) ₃ and Bi(OTf) ₃ , more preferably being Al(OTf) ₃ . Preferably, the catalyst system comprises [Ru(acetylacetonate) ₃ ], 1,1,1-tris(diphe- nylphosphinomethyl)ethane and MeSO ₃ H. Preferably, the weight ratio x(SW)= w(H2O):w(CH3OH) is in the range of from 0.05:1 to 0.50:1, more preferably in the range of from 0.06:1 to 0.40:1, more preferably in the range of from 0.07:1 to 0.25:1. Preferably, S _W comprises water in an amount in the range of from 6 to 20 weight-%, more pref- erably in the range of from 8 to 15 weight-%, based on the weight of S _W . Preferably from 98 to 100 weight-%, more preferably from 99 to 100 weight-%, more preferably from 99.5 to 100 weight-%, more preferably from 99.9 to 100 weight-%, of S _W consist of metha- nol, water and the catalyst system. In other words, preferably, S _W substantially consists of, more preferably consists of, methanol, water and the catalyst system. Preferably, SW is essentially free of, more preferably free of, methylal. Preferably, the weight ratio of water to methanol in SW is in the range of from 0.05:1 to 0.50:1, more preferably in the range of from 0.06:1 to 0.40:1, more preferably in the range of from 0.07:1 to 0.25:1. Preferably, the stream SM, in addition to methylal and methanol, further comprises methyl for- mate. Preferably, SM comprises in the range of from 1 to 20 weight-%, more preferably in the range of from 2 to 15 weight-%, more preferably in the range of from 5 to 12 weight-%, of methanol based on the weight of S _M . Preferably, the process is a continuous process, a semi-continuous process or a batch process, more preferably a continuous process. It is conceivable that the process be computer-implemented. The present invention further relates to a production unit for removing water from a mixture comprising methanol, water and a catalyst system, the production unit comprising - a separation unit SU configured for being supplied with the feed stream F2 which com- prises methanol, water and the catalyst system, the separation unit SU comprising one or more pervaporation membranes, wherein at least one membrane, of the one or more per- vaporation membranes, has a water/methanol pervaporation selectivity ßpervap of at least 1, wherein said at least one membrane comprises a porous substrate and a porous material disposed on the substrate, wherein from 70 to 100 weight-% of the porous material con- sists of carbon, so as to obtain a permeate stream P comprising water, and optionally methanol; and a retentate stream R comprising water and methanol at a weight ratio x(R)=w(H ₂ O):w(CH ₃ OH) with x(R)<x(F2), and the catalyst system, x(F2) being the weight ratio of water to methanol in F2 with 0<x(F2)≤0.5; and - optionally a purification unit PU configured for being supplied with a feed stream F1 com- prising methanol, methylal, water and a catalyst system, for removing methylal as a stream SM, SM comprising, in addition to methylal, methanol, and for removing water as a stream SW, SW comprising, in addition to water, methanol and the catalyst system, with w(H ₂ O):w(CH ₃ OH)=x(S _W ) and a means for passing S _W as F2 in SU. Preferably, the production unit is configured for removing water from a mixture comprising meth- anol, water and a catalyst system according to the present invention. Preferably, the purification unit PU comprises a distillation column from which the streams SM and SW are removable. Preferably, the purification unit PU is a distillation column D from which the streams S _M and S _W are removed. Preferably the purification unit PU comprises one or more distillation columns, more preferably at least two distillation columns. In the context of the present invention, preferably SU further comprises one or more pumps and one or more heat exchangers, in addition to the one or more membranes. Preferably, SU comprises a membrane loop comprising at least one membrane M of the one or more membranes, at least one pump A of the one or more pumps and at least one heat ex- changer H of the one or more heat exchangers. Preferably, the at least one membrane has a geometry selected from the group consisting of spiral-wound, hollow fiber, plate-and-frame and single- or multichannel tubular including combi- nations of two or more thereof, preferably a geometry selected from the group consisting of hol- low fiber and single- or multichannel tubular including combinations of two or more thereof, more preferably a geometry selected from the group consisting of single- or multichannel tubu- lar and combinations thereof, more preferably the at least one membrane comprised in SU has a geometry of multichannel tubular. Preferably, the at least one membrane comprised in SU has a H ₂ O permeance N in the range of from 0.2 to 200 Nm ³ /(m ² h bar), more preferably in the range of from 0.5 to 20 Nm ³ /(m ² h bar), more preferably in the range of from 2 to 10 Nm ³ /(m ² h bar), more preferably in the range of from 4 to 8 Nm ³ /(m ² h bar). Preferably, the at least one membrane comprised in SU has a permselectivity α(H2O/CH3OH) of at least 1, more preferably in the range of from 2 to 1000, more preferably in the range of from 3 to 200, more preferably in the range of from 4 to 100, more preferably in the range of from 5 to 50, more preferably in the range of from 6 to 25. Preferably, the water fugacity difference across the at least one pervaporation membrane com- prised in SU, calculated as (fugacity of water in the retentate stream R – (minus) fugacity of wa- ter in the permeate stream P) at constant temperature, is in the range of from 0.01 to 3 bar, more preferably in the range of from 0.05 to 1 bar, more preferably in the range of from 0.1 to 0.4 bar. The present invention further relates to a computer program comprising instructions which, when the program is executed by the production unit according to the present invention, cause the production unit to perform the process according to the present invention. The present invention further relates to a computer-readable storage medium comprising in- structions which, when the instructions are executed by the production unit according to the pre- sent invention, cause the production unit to perform the process according to the present inven- tion. The present invention further relates to a non-transient computer-readable medium including in- structions that, when executed by one or more processors, cause the one or more processors to perform the process according to the present invention. The present invention is further illustrated by the following set of embodiments and combina- tions of embodiments resulting from the dependencies and back-references as indicated. In par- ticular, it is noted that in each instance where a range of embodiments is mentioned, for exam- ple in the context of a term such as "The process of any one of embodiments 1 to 3", every em- bodiment in this range is meant to be explicitly disclosed for the skilled person, i.e. the wording of this term is to be understood by the skilled person as being synonymous to "The process of any one of embodiments 1, 2 and 3". Further, it is explicitly noted that the following set of em- bodiments represents a suitably structured part of the general description directed to preferred aspects of the present invention, and, thus, suitably supports, but does not represent the claims of the present invention. A process for removing water from a mixture comprising methanol, water, a catalyst sys- tem and optionally methylal, the catalyst system comprising a catalyst and an acidic co- catalyst, the process comprising - a separation stage SM comprising -- passing a feed stream F2, comprising methanol, water and the catalyst sys- tem, through a separation unit SU comprising one or more pervaporation membranes, wherein at least one membrane, of the one or more pervapora- tion membranes, has a water/methanol pervaporation selectivity ß _pervap of at least 1, obtaining from SU - a permeate gas stream P comprising water, and optionally methanol; and - a retentate liquid stream R comprising water and methanol at a weight ratio x(R)=w(H2O):w(CH3OH) with x(R)<x(F2), and the catalyst system, x(F2) being the weight ratio of water to methanol in F2 with 0<x(F2)≤0.5; wherein said at least one membrane comprises a porous substrate and a po- rous material disposed on the substrate, wherein from 70 to 100 weight-% of the porous material consists of carbon; wherein, when the mixture comprises methylal, in addition to methanol, water and the cat- alyst system, the process further comprises - a separation stage SP, prior to SM, comprising -- passing a feed stream F1 comprising methanol, methylal, water and the cata- lyst system in a purification unit PU, wherein in PU, methylal is removed as a stream S _M , S _M comprising, in addition to methylal, methanol, and water is removed as a stream SW, SW comprising, in addition to water, metha- nol and the catalyst system, with a weight ratio x(S _W )=w(H ₂ O):w(CH ₃ OH); -- passing the stream S _W as a feed stream F2 through the separation stage SM, F2 having the same composition as SW and x(SW)=x(F2). The process of embodiment 1, wherein the feed stream F2 has a pressure p _F2 , with p _F2 being in the range of from 1 to 80 bar, preferably in the range of from 3 to 20 bar, more preferably in the range of from 5 to 15 bar. The process of embodiment 1 or 2, wherein the feed stream F2 has a temperature T _F2 , with TF2 being in the range of from 50 to 150 °C, preferably in the range of from 60 to 90 °C. The process of any one of embodiments 1 to 3, wherein the at least one membrane, of the one or more pervaporation membranes, has a water/methanol pervaporation selectivity ß _pervap of at least 2, preferably in the range of from 2 to 100, more preferably in the range of from 2.1 to 50, more preferably in the range of from 2.25 to 30, more preferably in the range of from 2.5 to 10, more preferably in the range of from 3 to 8, more preferably in the range of from 4 to 6. 5. The process of any one of embodiments 1 to 4, wherein the at least one membrane com- prised in SU has a H ₂ O permeance N in the range of from 0.2 to 200 Nm ³ /(m ² h bar), pref- erably in the range of from 0.5 to 20 Nm ³ /(m ² h bar), more preferably in the range of from 2 to 10 Nm ³ /(m ² h bar), more preferably in the range of from 4 to 8 Nm ³ /(m ² h bar). 6. The process of any one of embodiments 1 to 5, wherein the at least one membrane com- prised in SU has a permselectivity α(H2O/CH3OH) of at least 1, preferably in the range of from 2 to 1000, more preferably in the range of from 3 to 200, more preferably in the range of from 4 to 100, more preferably in the range of from 5 to 50, more preferably in the range of from 6 to 25. 7. The process of any one of embodiments 1 to 6, wherein in SM the water fugacity differ- ence across the at least one pervaporation membrane comprised in SU, calculated as (fugacity of water in the retentate stream R – (minus) fugacity of water in the permeate stream P) at constant temperature, is in the range of from 0.01 to 3 bar, more preferably in the range of from 0.05 to 1 bar, more preferably in the range of from 0.1 to 0.4 bar. 8. The process of any one of embodiments 1 to 7, wherein SU further comprises one or more pumps and one or more heat exchangers, in addition to the one or more mem- branes. 9. The process of embodiment 8, wherein SU comprises a membrane loop comprising at least one membrane M of the one or more membranes, at least one pump A of the one or more pumps and at least one heat exchanger H of the one or more heat exchangers. 10. The process of any one of embodiments 1 to 9, wherein the porous substrate of the at least one membrane comprised in SU has a geometry selected from the group consisting of spiral-wound, hollow fiber, plate-and-frame and single- or multichannel tubular including combinations of two or more thereof, preferably a geometry selected from the group con- sisting of hollow fiber and single- or multichannel tubular including combinations of two or more thereof, more preferably a geometry selected from the group consisting of single- or multichannel tubular and combinations thereof, more preferably the at least one mem- brane comprised in SU has a geometry of multichannel tubular. 11. The process of any one of embodiments 1 to 10, wherein the permeate stream P is a per- meate gas stream having a pressure p _P of less than 1 bar, preferably in the range of from 5 to 100 mbar, more preferably in the range of from 10 to 70 mbar. 12. The process of any one of embodiments 1 to 11, wherein the weight ratio x(P) is in the range of from 0.05:1 to 19:1, preferably in the range of from 0.1:1 to 4:1, more preferably in the range of from 0.15:1 to 1:1. 13. The process of any one of embodiments 1 to 12, wherein the retentate stream R is a re- tentate liquid stream having a pressure p _R in the range of from 1 to 80 bar, preferably in the range of from 3 to 20 bar, more preferably in the range of from 5 to 15 bar. 14. The process of any one of embodiments 1 to 13, wherein the retentate stream R is a re- tentate liquid stream having a temperature TR in the range of from 80 to 180 °C, preferably in the range of from 100 to 160 °C, more preferably in the range of from 120 to 140 °C. 15. The process of any one of embodiments 1 to 14, wherein the weight ratio x(R) is of at most 0.1:1, preferably of at most 0.05:1, more preferably in the range of from 0:1 to 0.05:1. 16. The process of any one of embodiments 1 to 15, wherein the purification unit PU com- prises a distillation column from which the streams SM and SW are removed. 17. The process of any one of embodiments 1 to 16, wherein the feed stream F1 has a liquid phase and a gas phase. 18. The process of any one of embodiments 1 to 17, wherein the feed stream F1, when enter- ing PU, has a pressure pF1, with pF1 being in the range of from 40 to 200 bar, preferably in the range of from 80 to 150 bar, more preferably in the range of from 85 to 140 bar, more preferably in the range of from 90 to 130 bar. 19. The process of any one of embodiments 1 to 18, wherein the feed stream F1, when enter- ing PU, has a temperature T _F1 , with T _F1 being in the range of from 20 to 200 °C, preferably in the range of from 50 to 180 °C, more preferably in the range of from 60 to 170 °C, more preferably in the range of from 80 to 140 °C. 20. The process of any one of embodiments 1 to 19, wherein the catalyst of the catalyst sys- tem is a catalyst complex comprising a transition metal complex and at least one polyden- tate ligand comprising at least one P atom. 21. The process of embodiment 20, wherein the transition metal of the transition metal com- plex is selected from the group consisting of ruthenium, manganese, cobalt, iron, osmium, rhodium, rhenium, iridium, nickel, platinum and palladium, preferably selected from the group consisting of ruthenium, manganese and cobalt, more preferably selected from the group consisting of ruthenium and cobalt, more preferably is ruthenium. The process of embodiment 20 or 21, wherein the transition metal complex is one or more of [Ru(acetylacetonate) ₃ ], [Ru(COD)(methylallyl) ₂ ], [Co(acetylacetonate) [Ru(p-cymene)Cl2]2, [Ru(benzene)Cl2]n, [Ru(CO)2Cl2]n, [Ru(CO)3Cl2]2, [Ru(DMSO)4Cl2], [Ru(PPh3)3(CO)(H)CI], [Ru(PPh3)3(CO)Cl2], [Ru(PPh3)3 [Ru(PPh ₃ ) ₃ Cl ₂ ], [Ru(Cp)(PPh ₃ ) ₂ CI], [Ru(Cp)(CO) ₂ CI], [Ru(Cp)(CO) ₂ H], [Ru(Cp*)(CO) ₂ CI], [Ru(Cp*)(CO) ₂ H], [Ru(Cp*)(CO) ₂ ] ₂ , [Ru(indenyl)(CO) denyl)(CO)2H], [Ru(indenyl)(CO)2]2, ruthenocen, [Ru(binap)(Cl)2], [Ru(2,2'-bipyri- din) ₂ (Cl) ₂ ·H ₂ O], [Ru(COD)(Cl) ₂ H] ₂ , [Ru(Cp*)(COD)CI], [Ru ₃ (CO) ₁₂ ], [Ru(tetraphenylhy- droxycyclopentadienyl)(CO) ₂ H], [Ru(PMe ₃ ) ₄ (H) ₂ ], [Ru(PEt ₃ ) ₄ (H) ₂ ], [Ru(Pn-Pr ₃ ) ₄ (H) ₂ ], [Ru(Pn-Bu3)4(H)2], and [Ru(Pn-octyl3)4(H)2], preferably one or more of [Ru(acety- lacetonate)3] and [Ru(COD)(methylallyl)2], more preferably [Ru(acetylacetonate)3]. The process of any one of embodiments 20 to 22, wherein the at least one polydentate ligand comprising at least one P atom is selected from the group consisting of tris(diphe- nylphosphinomethyl)ethane, tris[di(p-tolyl)phosphinomethyl]ethane, tris[di(3,5-dime- thylphenyl)phosphinomethyl]ethane, tris(diphenylphosphinomethyl)methane, tris(diphe- nylphosphinomethyl)amine, bis(2-diphenylphosphinoethyl)phenylphosphine and tris[2-(di- phenylphosphino)ethyl]phosphine, preferably selected from the group consisting of tris(di- phenylphosphinomethyl)ethane, tris[di(p-tolyl)phosphinomethyl]ethane, tris[di(3,5-dime- thylphenyl)phosphinomethyl]ethane and tris(diphenylphosphinomethyl)amine, more pref- erably is tris(diphenylphosphinomethyl)ethane, more preferably 1,1,1-tris(diphe- nylphosphinomethyl)ethane (Triphos). The process of any one of embodiments 20 to 23, wherein the molar ratio of the transition metal complex relative to the at least one polydentate ligand is in the range of from :3.0 to 1:1, preferably in the range of from 1:2.0 to 1:1, more preferably in the range of from 1:1.5 to 1:1, more preferably in the range of from 1:1.2 to 1:1. The process of any one of embodiments 1 to 24, wherein the molar ratio of the acidic co- catalyst relative to the catalyst is in the range of from 1:50 to 1:1, preferably in the range of from 1:20 to 1:1, more preferably in the range of from 1:10 to 1:1. The process of any one of embodiments 1 to 25, wherein the acidic co-catalyst is a Bronsted acid or a Lewis acid, wherein preferably the acidic co-catalyst is selected from the group consisting of methane sulfonic acid (MeSO3H), Al(OTf)3, Bi(OTf)3, AlCl3, ZnCl2, SnCl ₄ , TiCl ₄ , Fe(OTf) ₃ , HCl, H ₂ SO ₄ , tricholoroacetic acid, p-TsOH, trifluoromethanesulfonic acid and a mixture of two or more thereof, preferably selected from the group consisting of methane sulfonic acid (MeSO3H), Al(OTf)3, Bi(OTf)3, H2SO4, p-TsOH, trifluoromethanesul- fonic acid and a mixture of two or more thereof, more preferably selected from the group consisting of MeSO ₃ H, Al(OTf) ₃ , H ₂ SO ₄ , p-TsOH, trifluoromethanesulfonic acid and mix- ture of two or more thereof, more preferably selected from the group consisting of MeSO3H, Al(OTf)3, p-TsOH and a mixture thereof, more preferably selected from the group consisting of MeSO3H, Al(OTf)3 and a mixture thereof, more preferably selected from the group consisting of MeSO ₃ H and Al(OTf) ₃ . 27. The process of any one of embodiments 1 to 26, wherein the catalyst system comprises [Ru(acetylacetonate) ₃ ], 1,1,1-tris(diphenylphosphinomethyl)ethane and MeSO ₃ H. 28. The process of any one of embodiments 1 to 27, wherein the weight ratio x(SW)= w(H ₂ O):w(CH ₃ OH) is in the range of from 0.05:1 to 0.50:1, preferably in the range of from 0.06:1 to 0.40:1, more preferably in the range of from 0.07:1 to 0.25:1. 29. The process of any one of embodiments 1 to 28, wherein SW comprises water in an amount in the range of from 6 to 20 weight-%, preferably in the range of from 8 to 15 weight-%, based on the weight of S _W . 30. The process of any one of embodiments 1 to 29, wherein from 98 to 100 weight-%, prefer- ably from 99 to 100 weight-%, more preferably from 99.5 to 100 weight-%, more prefera- bly from 99.9 to 100 weight-%, of SW consist of methanol, water and the catalyst system. 31. The process of any one of embodiments 1 to 30, wherein the weight ratio of water to methanol in S _W is in the range of from 0.05:1 to 0.50:1, preferably in the range of from 0.06:1 to 0.40:1, more preferably in the range of from 0.07:1 to 0.25:1. 32. The process of any one of embodiments 1 to 31, wherein the stream S _M , in addition to methylal and methanol, further comprises methyl formate. 33. The process of any one of embodiments 1 to 32, being a continuous process, a semi-con- tinuous process or a batch process, preferably a continuous process. 34. The process of any one of embodiments 1 to 33, wherein the process is computer-imple- mented. 35. A production unit for removing water from a mixture comprising methanol, water and a catalyst system, the production unit comprising - a separation unit SU configured for being supplied with the feed stream F2 which comprises methanol, water and the catalyst system, the separation unit SU compris- ing one or more pervaporation membranes, wherein at least one membrane, of the one or more pervaporation membranes, has a water/methanol pervaporation selec- tivity ß _pervap of at least 1, wherein said at least one membrane comprises a porous substrate and a porous material disposed on the substrate, wherein from 70 to 100 weight-% of the porous material consists of carbon, so as to obtain a permeate stream P comprising water, and optionally methanol; and a retentate stream R com- prising water and methanol at a weight ratio x(R)=w(H ₂ O):w(CH ₃ OH) with x(R)<x(F2), and the catalyst system, x(F2) being the weight ratio of water to metha- nol in F2 with 0<x(F2)≤0.5; and - optionally a purification unit PU configured for being supplied with a feed stream F1 comprising methanol, methylal, water and a catalyst system, for removing methylal as a stream S _M , S _M comprising, in addition to methylal, methanol, and for removing water as a stream SW, SW comprising, in addition to water, methanol and the catalyst system, with a weight ratio x(S _W )=w(H ₂ O):w(CH ₃ OH) and a means for passing S _W as F2 in SU. 36. The production unit of embodiment 35, wherein the production unit is configured for re- moving water from a mixture comprising methanol, water and a catalyst system according to any one of embodiments 1 to 34. 37. The production unit of embodiment 35 or 36, wherein the purification unit PU comprises a distillation column from which the streams S _M and S _W are removable. 38. The production unit of any one of embodiments 35 to 37, wherein SU further comprises one or more pumps and one or more heat exchangers, in addition to the one or more membranes. 39. The production unit of embodiment 38, wherein SU comprises a membrane loop compris- ing at least one membrane M of the one or more membranes, at least one pump A of the one or more pumps and at least one heat exchanger H of the one or more heat exchang- ers. 40. The production unit of any one of embodiments 35 to 39, wherein the at least one mem- brane has a geometry selected from the group consisting of spiral-wound, hollow fiber, plate-and-frame and single- or multichannel tubular including combinations of two or more thereof, preferably a geometry selected from the group consisting of hollow fiber and sin- gle- or multichannel tubular including combinations of two or more thereof, more prefera- bly a geometry selected from the group consisting of single- or multichannel tubular and combinations thereof, more preferably the at least one membrane comprised in SU has a geometry of multichannel tubular. 41. The production unit of any one of embodiments 35 to 40, wherein the at least one mem- brane comprised in SU has a H ₂ O permeance N in the range of from 0.2 to 200 Nm ³ /(m ² h bar), preferably in the range of from 0.5 to 20 Nm ³ /(m ² h bar), more preferably in the range of from 2 to 10 Nm ³ /(m ² h bar), more preferably in the range of from 4 to 8 Nm ³ /(m ² h bar). 42. The production unit of any one of embodiments 35 to 41, wherein the at least one mem- brane comprised in SU has a permselectivity α(H2O/CH3OH) of at least 1, preferably in the range of from 2 to 1000, more preferably in the range of from 3 to 200, more prefera- bly in the range of from 4 to 100, more preferably in the range of from 5 to 50, more pref- erably in the range of from 6 to 25. 43. The production unit of any one of embodiments 35 to 42, wherein the water fugacity differ- ence across the at least one pervaporation membrane comprised in SU, calculated as (fu- gacity of water in the retentate stream R – (minus) fugacity of water in the permeate stream P) at constant temperature, is in the range of from 0.01 to 3 bar, preferably in the range of from 0.05 to 1 bar, more preferably in the range of from 0.1 to 0.4 bar. 44. A computer program comprising instructions which, when the program is executed by the production unit according to any one of embodiments 35 to 43, cause the production unit to perform the process according to any one of embodiments 1 to 34. 45. A computer-readable storage medium comprising instructions which, when the instruc- tions are executed by the production unit according to any one of embodiments 35 to 43, cause the production unit to perform the process according to any one of embodiments 1 to 34. 46. A non-transient computer-readable medium including instructions that, when executed by one or more processors, cause the one or more processors to perform the process ac- cording to any one of embodiments 1 to 34. In the context of the present invention, the terms “methylal” and “dimethoxymethane” are used interchangeably. In the context of the present invention, the abbreviation “Cp” stands for cyclopentadienyl, the abbreviation “Cp*” stands for pentamethylcycopentadienyl and the abbreviation “binap” stands for 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl. The term „bar“ as used in the context of the present invention refers to „bar(abs)“. In the context of the present invention, the term “gas stream” means that the stream has a gas phase. In the context of the present invention, a term “X is one or more of A, B and C”, wherein X is a given feature and each of A, B and C stands for specific realization of said feature, is to be un- derstood as disclosing that X is either A, or B, or C, or A and B, or A and C, or B and C, or A and B and C. In this regard, it is noted that the skilled person is capable of transfer to above ab- stract term to a concrete example, e.g. where X is a chemical element and A, B and C are con- crete elements such as Li, Na, and K, or X is a temperature and A, B and C are concrete tem- peratures such as 10 °C, 20 °C, and 30 °C. In this regard, it is further noted that the skilled per- son is capable of extending the above term to less specific realizations of said feature, e.g. “X is one or more of A and B” disclosing that X is either A, or B, or A and B, or to more specific reali- zations of said feature, e.g. “X is one or more of A, B, C and D”, disclosing that X is either A, or B, or C, or D, or A and B, or A and C, or A and D, or B and C, or B and D, or C and D, or A and B and C, or A and B and D, or B and C and D, or A and B and C and D. The present invention is further illustrated by the following examples. Examples The following abbreviations are used in the examples: MeFo: methyl formate DMM: dimethoxymethane Triphos: 1,1,1-tris(diphenylphosphinomethyl)ethane COD: 1,5-cyclooctadiene Ph: phenyl Tol: tolyl All components used in the examples are commercially available components. Pervaporation membrane The pervaporation membrane used in the inventive examples is an single channel carbon mo- lecular sieve membrane obtained from Fraunhofer IKTS prepared according to a process de- scribed in Example 1 of US 2012/079943A1. Said membrane has a length of 25 cm, an inner diameter of 7 mm and an outer diameter of 10 mm. The binary pervaporation selectivity ßpervap for water/methanol measured according to the procedure outlined in 1.3 was of 3.6. Analytics 1.1 GC Analysis of reaction mixtures Retention times of methylal and methyl formate were determined by comparison to samples of the authentic materials. Relative response factors of methylal and methyl formate were deter- mined by calibration against 1,4-dioxane as internal standard. The relative quantities of the two species in the crude reaction mixture were thus determined according to the peak areas and de- termined response factors. Methylal and methyl formate contents were analyzed by gas chromatography according to the following procedure: To a 0.75 gram aliquot of the crude reaction mixture was added 0.25 g 1,4- dioxane as an internal standard. The sample was then analyzed according to the following GC method: GC column: CP-Sil 5 CB for formaldehyde 60 m x ID 320 micrometers x 8 micrometers Carrier gas: He Inlet temperature: 250 °C – Pressure: 3.138 bar – Split ratio: 50:1 Constant flow rate: 8 mL/min – Injection volume: 1 microliter Oven temperature: 45 °C for 3 min, then increase of 10°C/ min ramp rate up to 225 °C, then at 25 °C for 8.25 min (total run time = 29.25 min) Flame Ionization Detector (FID) 290°C 1.2 Water content analysis - Karl Fischer Analysis The water content of the samples was determined by a calibrated volumetric one-component Karl-Fischer titration using a Metrohm Dosimat 805 with 803 Ti Stand, with which the endpoint of the titration is detected by electrometric indication. Methanol was used as the solvent and Hy- dranalTM Composite 5 (Honeywell) was used as the Karl-Fischer titrant. The presence of traces of water in the methanol was compensated for by dosing titrant until the water was reacted away. Then, about 100-400 mg sample was accurately weighted and added to the solvent and the titration was performed until the endpoint was reached. The mass fraction of water is calcu- lated based on the amount of titrant dosed using the Metrohm software. 1.3 Pervaporation selectivity ßpervap of a given membrane The pervaporation selectivity for water/methanol was calculated according to the following equation: retentate are the mole fractions (mol/mol) of the respective components. The binary pervaporation selectivity ß _pervap of a given membrane at a molar concentration of 25 mol-% is determined as follows: a feed stream with a binary composition of about 30 mol-% wa- ter and 70 mol-% methanol is filled into a batch pervaporation unit equipped with a pervapora- tion membrane. At the permeate side of the membrane, a vacuum of 70±15 mbar is drawn. Then, the feed stream is circulated across the pervaporation membrane at a crossflow velocity of 2.4 m/s and heated to 120 °C. The permeate was condensed using a permeate condenser operated at a temperature of 1 °C. Any permeate collected during the heat-up phase was dis- carded. After reaching the desired temperature, samples are taken from retentate and permeate simultaneously. Since ßpervap may vary with concentration and since it is impossible to measure ßpervap at an ex- act targeted concentration, the value for ßpervap at Xwater,retentate = 25 mol-% water is obtained through a linear least squares fit of ß _pervap vs X _{water,retentate} (mol/mol). The thus obtained pervapo- ration selectivity for the membrane in this example is 3.6. 120.4 0.074 2.4 0.359 0.779 0.56 3.53 6.31 119.7 0.075 2.4 0.349 0.758 0.54 3.13 5.84 120.4 0.075 2.4 0.337 0.750 0.51 2.99 5.89 120.5 0.077 2.4 0.318 0.731 0.47 2.72 5.84 118.8 0.087 2.4 0.318 0.694 0.47 2.27 4.87 118.2 0.086 2.4 0.297 0.659 0.42 1.94 4.59 118.1 0.084 2.4 0.278 0.642 0.38 1.80 4.66 118.3 0.084 2.4 0.266 0.579 0.36 1.38 3.80 119.1 0.062 2.5 0.216 0.478 0.27 0.91 3.33 119.5 0.062 2.5 0.213 0.440 0.27 0.79 2.91 119.3 0.062 2.5 0.211 0.413 0.27 0.70 2.63 119.3 0.060 2.5 0.189 0.332 0.23 0.50 2.13 119.3 0.059 2.5 0.186 0.268 0.23 0.37 1.60 1.4 Permeance N The permeance N of a component i is defined as the flux of component i defined by the fugacity difference Δfi of component i over the membrane, Δfi being equal to fi,retentate – fi,permeate The fugacity (also called vapor pressure) of component i at the retentate side can be calculated as: ^^ _^^ ^{^^ ^^ ^^} = ^^ _^^ ^^ _^^ ^^ _^ ^{^} ^ ^{^ ^^ ^^} with ^^ _^^ being the activity coefficient, ^^ _^^ being the mole fraction and ^^ _^ ^{^} ^ ^{^ ^^ ^^} being the saturated va- por pressure (for the pure component i) and at the permeate side as: ^^ ^{^^ ^^ ^^ ^^} = ^^ ^{^^ ^^ ^^ ^^} ^^ ^^ ^^ ^^ ^ _{^ ^^} = ^^ _^^ ^^ _{^^ ^^ ^^ ^^ ^^} with ^^ ^^ ^^ ^^ ^^ ^ _^ being the mole fraction and ^^ _{^^ ^^ ^^ ^^ ^^} being the total permeate pressure. 1.5 Permselectivity α The permselectivity α(H2O/CH3OH) is defined as the ratio of the permeances of water and Example 1: Acid co-catalysts evaluation According to Example 1, methylal was synthesized from CO ₂ , H ₂ in presence of methanol, the Ru-catalyst source Ru(COD)(methylallyl) ₂ , a ligand Triphos and an acidic co-catalyst. The reac- tion is illustrated by Equation I below. In an argon filled glovebox, the ruthenium precursor, Ru(COD)(methylallyl)2 (0.041 g, 1.0 eq., 0.127 mmol), triphos ligand (0.083 g, 1.05 eq, 0.133 mmol) and the requisite acidic co-catalyst (4 eq., 0.510 mmol) were charged into a round bottom flask and dissolved in MeOH (78.9 g). This catalyst/solvent mixture was then transferred to a nitrogen flushed steel autoclave (300 mL inner volume) with an overhead stirrer set at 800 rpm, the autoclave sealed and flushed with ni- trogen gas. The autoclave was then charged with 20 bar CO2 gas pressure and warmed to 30 °C for 30 min. The mass increase of this CO ₂ dosing was noted as the basis for determining the reaction yield (typically 20 bar corresponded to a CO ₂ mass of 7-10 g). H ₂ gas was then added to the autoclave reactor and the autoclave heated to the required reaction temperature (100 °C). The hydrogen gas dosing was controlled so as to obtain a total pressure of 120 bar at 100 °C. The autoclave was once more sealed, and stirred (800 rpm) at 100 °C for the required reaction time (8 h). No further gas dosing was done during the course of the reaction. Heating was then stopped and the autoclave allowed to cool to ambient temperature before being transferred to an ice cooling bath (ca.0 °C). The reactor pressure was then slowly released to minimize loss of the volatile reaction products. The crude reaction mixture was analyzed by gas chromatog- raphy to determine conversion and selectivity for the desired product, methylal. Results are shown below in Table 1. Table 1 *methylal selectivity [%] = methylal [GC wt.%] / (methylal [GC wt.%] + methyl formate [GC wt.%])*100 As may be taken from Table 1, different acidic co-catalysts can be used in combination with the Ru-catalyst. It has been found according to the present invention that, in addition to the known Al(OTf)3 co-catalyst, Bi(OTf)3, MeSO3H and p-TsOH demonstrate great performance as well. Brønsted acids have also been investigated (see entries 1.3, 1.4 and 1.7) but only methanesul- fonic acid (MSA) results in a success as when using trifluoroacetic acid (TFA) as a co-catalyst, no conversion was obtained. We may also note that H2SO4 and triflic acid can also be used as co-catalysts (entries 1.6 and 1.8) while no conversion was obtained using HNO3 and very few with BF ₃ (50% solution in MeOH). Example 2: Ru source evaluation According to Example 2, methylal was synthesized from CO ₂ , H ₂ in presence of methanol, a Ru- catalyst source, a ligand triphos and MeSO3H as co-catalyst according to the same proce- dure/recipe as described in Example 1 and with the same molar ratios. The reaction is illus- trated by Equation II below. Equation II: Table 2 *methylal selectivity [%] = methylal [GC wt.%] / (methylal [GC wt.%] + methyl formate [GC wt.%])*100 † Experiment performed with double the standard quantitites of both ruthenium precursor and triphos ligand As may be taken from Table 2, different sources of Ru-catalyst under different conditions can be used in order to successfully obtained methylal. It has been found according to the present in- vention that Ru(COD)(methylallyl) ₂ can easily be replaced by Ru(acac) ₃ , a Ru-source cheaper than Ru(COD)(methylallyl)2 while exhibiting also improved results with a selectivity to methylal of 85% with a reaction time of only 4 hours. Example 3: Water removal according to the present invention A stream S _W having the following composition (mass fraction) was prepared 89.6 % MeOH; 10.0 % H ₂ O; 0.40 % catalyst system: Ru(acac) ₃ / triphos / Al(OTf) ₃ (Ru(acac) ₃ : triphos: Al(OTf) ₃ molar ratio = 1: 1.05: 4). To do so, all components were mixed and heated to 50 °C for 1 hour. This ensured that the triphos ligand and Al(OTf)3 dissolved fully in methanol prior to passing through the membrane. For this example, the experiment was performed in a batch mode. The stream SW was then passed through a separation unit. In particular, about 3 kg of S _W was fed into a batch vessel and the stream was then pumped over a pervaporation membrane at a crossflow velocity of 2.5 m/s and heated to 120 °C. The retentate was flowed back to the batch vessel for progressively re- moving water over the pervaporation membrane. The experiment was run for 48 hours. The permeate gas stream P had a pressure of 70 mbar comprising water and methanol. Samples from the retentate streams (R_1-R_7) and a final retentate liquid stream R_8 being depleted in water compared to SW and comprising methanol and the catalyst system were obtained. The water content of the retentate at different time was measured (R_1-R_8). The present mem- brane separation method using this stream S _W did not result in an increase in pressure and was operated successfully until completion, i.e. less than 2 wt.-% residual water content in the reten- tate as shown below (Table 3). Table 3 Water analysis results of the retentate at different time Example 4: Water removal according to the present invention A stream SW having the following composition (mass fraction) was prepared 89.8 wt.-% MeOH; 10.0 wt.-% H ₂ O; 0.20 wt.-% of the catalyst system: Ru(acac) ₃ / triphos / methanesulfonic acid (Ru: triphos: MeSO3H molar ratio = 1: 1: 3). To do so, all components were mixed and heated to 50 °C for 1 hour. This ensured that the triphos ligand and Al(OTf)3 dissolved fully in methanol prior to passing through the membrane. For this example, the experiment was performed in a batch mode. The stream S _W was then passed through a separation unit SU. In particular, 4.0kg of SW was fed into a batch vessel and the stream was then pumped over a pervaporation membrane at a crossflow velocity of 2.5 m/s and heated to 120 °C. The retentate stream was flowed back to the batch vessel for continuously removing water over the pervaporation membrane. The experiments was run for 45 hours. The permeate gas stream P had a pressure of 70 mbar comprising water and methanol. A final retentate liquid stream R_7 being depleted in water compared to S _W and comprising methanol, water and the catalyst system. The water content of the retentate at different time was measured (R_1-R_7). The present membrane separation method using this stream SW did not result in an increase in pressure and was operated successfully until completion, i.e. about 2% residual water content in the retentate R as shown below (Table 4). Table 4 Water analysis results of the retentate at different time Reference Example 1 Process for preparing methylal not according to the present inven- tion A stream S _MW having the following composition (mass fraction) was prepared 84.6 wt.-% MeOH; 7 wt.-% Methylal; 1 wt.-% MeFo; 6.9 wt.-% H2O; 0.1 % HCO2H; 0.4 wt.-% catalyst system: Ru(COD)(methylallyl) ₂ / triphos / Al(OTf) ₃ (Ru(COD)(methylallyl) ₂ : triphos: Al(OTf) ₃ molar ratio = 1: 1.05: 4). To do so, all components were mixed and heated to 50 °C for 1 hour. This ensured that the triphos ligand and Al(OTf)3 dissolved fully in methanol prior to passing through the membrane. While according to the invention, methylal is removed from SMW prior to passing through a sepa- ration SU. For Reference Example 1, it has been decided to pass the mixture S _MW comprising methylal first through a separation unit, comprising a pervaporation membrane, to remove water and to remove methylal via distillation in a purification unit PU afterwards.3.5 kg of SMW was used to run the experiment using the pervaporation membrane. Upon subjecting the reaction mixture to the conditions of the membrane separation, namely 120 ºC at 8 bar N ₂ , it was proved that applying such reaction conditions was impossible. Indeed, already upon reaching a temper- ature of 90 °C, a pressure build-up of 12 bar was registered. Because of the continuously rising pressure, the pressure was relieved from the system to avoid an undesired discharge of the unit contents over a safety valve. Over the course of a further 5 hours, the pressure needed to be released frequently to prevent a pressure build-up. The pressure increase continued after reaching a steady temperature, indicating that not only physical effects were taking place, but that a pressure increase continued due to the release of gaseous components. This was con- firmed by analysis results: for 5 samples, all taken within the first 3 hours after heating, the con- tent of methylal decreased rapidly, from 7.0 wt.-% at the start of the experiment, to 2.65 wt.-% after 1.5 h after heating started, to 0.87 wt.-% after 2 hours (see Figure 3). The rapid decrease in concentration is presumably due to a combination of physical release through venting and thermal degradation under presence of the catalyst (and absence of hydrogen and CO2). At the end of the experiments, no methylal could be detected in the retentate anymore. The content of methyl formate decreased correspondingly. Thus, this example clearly demonstrates that the order of the steps of the process of the present invention is of importance. Reference Example 2 Process removing water from a mixture comprising methanol, methylal, water and a catalyst system not according to the present invention – using NaA zeolite as a membrane The pervaporation membrane used in Reference Example 2 is a single-channel NaA zeolite membrane obtained from Fraunhofer IKTS. Said membrane has a length of 25 cm, an inner di- ameter of 7 mm and an outer diameter of 10 mm. The binary pervaporation selectivity ß _pervap for water/methanol measured according to the procedure outlined in 1.3 was of 779, demonstrating the superior selectivity of the NaA membrane in binary mixtures. An experiment was ran using the above-described NaA membrane instead of a pervaporation membrane comprising a carbon porous material as defined in the present invention under simi- lar conditions as above. Heating started only 30 minutes after filling the separation unit at room temperature with the feed mixture given in Table 10. The set point for the heating was at 120 °C. Only 14 minutes after the heating started (T = 57.3 °C), the permeate vessel discharged for the first time. In the first 32 min., the permeate vessel discharged for a further 3 times (nor- mally, no permeate discharge is registered up until this temperature, and an increase in level is barely noticeable). After 32 min., when heating was only at 70 °C, the unit tripped because the permeate vessel reached its maximum level alarm. The experiment needed to be stopped, since the flow rates registered where not compatible with the unit. Only a single sample could be obtained because of the short time of the experiments. The low water concentration in the permeate and the resulting low selectivity further make clear that the NaA membrane was detri- mentally affected by the presence of the methanesulfonic acid. This clearly illustrates that a pervaporation membrane comprising a porous material supported on a porous substrate, wherein from 70 to 100 weight-% of the porous material consists of car- bon is mandatory in order to be able to separate water from methanol and the catalyst system. Table 10: Feed mixture Brief description of the figure Figure 1 is a schematic representation of a production unit used for the process according to preferred embodiments of the invention. The production unit comprises a purification unit PU and a separation unit SU, preferably a per- vaporation membrane. The aqueous liquid feed stream F1 was passed through the purification unit PU obtaining a stream S _M comprising methylal and methanol, and a stream S _W comprising methanol, water and the catalyst system. The liquid stream S _W was passed through SU, a mem- brane, preferably a pervaporation membrane, obtaining a permeate stream P comprising water and a retentate stream R comprising methanol and the catalyst system. The mixture of metha- nol and the catalyst system comprised in R can be recycled and used for the production of methylal. The water removed as P from SU can be further treated and used in different process or used to create heat for the process of the present invention. Figure 2 represents the feed pressure and temperature as a function of experiment time for Ref. Example 1 not according to the present invention Figure 3 represents an arrangement of SU according to embodiments of the present inven- tion comprising three loops. Cited literature - US 2012/079943A1 - US2022/032237 A1 - P. H. Tchoua Ngamou et al., High performance carbon molecular sieve membranes for hydrogen purification and pervaporation dehydratation of organic solvents, Journal of Materials Chemistry A, vol.7, no.12, 19 March 2019, p.7082-7091