The invention relates to a process for producing 4,4′-dichlorodiphenyl sulfone, comprising: (a) reacting 4,4′-dichlorodiphenyl sulfoxide and an oxidizing agent in at least one carboxylic acid as solvent to obtain a reaction mixture comprising 4,4′-dichlorodiphenyl sulfone and carboxylic acid; (b) separating the reaction mixture into a first stream comprising 4,4′-dichlorodiphenyl sulfone and a second stream comprising carboxylic acid; (c) purifying the second stream comprising carboxylic acid by distilling a part of the second stream comprising carboxylic acid stripping low boilers from at least a part of the second stream comprising carboxylic acid (d) recycling the purified carboxylic acid into the reaction (a).

The invention relates to a process for obtaining 4,4′-dichlorodiphenyl sulfone from an organic mixture comprising 4,4′-dichlorodiphenyl sulfone and a linear C.sub.6 to C.sub.10 carboxylic acid as organic solvent, comprising: (a) cooling the organic mixture by (a1a) mixing the organic mixture with water in a crystallization vessel to obtain a liquid mixture; (a1b) cooling the liquid mixture obtained in (a1a) to a temperature below the saturation point of 4,4′-dichlorodiphenyl sulfone by (i) reducing the pressure in the crystallization vessel to a pressure at which the water starts to evaporate, (ii) condensing the evaporated water by cooling, (iii) mixing the condensed water into the liquid mixture in the crystallization vessel, to obtain a suspension comprising crystallized 4,4′-dichlorodiphenyl sulfone; or by (a2) bringing the organic mixture into contact with at least one coolable surface and thereby reducing the temperature in the organic mixture with a cooling rate in the range from 5 to 50 K/h until a temperature in the range from 10 to 30° C. is reached, wherein the organic mixture and the at least one coolable surface have a temperature difference which is kept during the whole cooling process in the range from 1 to 30 K to obtain a suspension comprising crystallized 4,4′-dichlorodiphenyl sulfone. (b) carrying out a solid-liquid-separation of the suspension obtained in (a1b) or in (a2) to obtain a residual moisture containing solid 4,4′-dichlorodiphenyl sulfone as product and mother liquor comprising the organic solvent and water.

The invention relates to a process for obtaining 4,4′-dichlorodiphenyl sulfone from an organic mixture comprising 4,4′-dichlorodiphenyl sulfone and a linear C.sub.6 to C.sub.10 carboxylic acid as organic solvent, comprising: (a) cooling the organic mixture by (a1a) mixing the organic mixture with water in a crystallization vessel to obtain a liquid mixture; (a1b) cooling the liquid mixture obtained in (a1a) to a temperature below the saturation point of 4,4′-dichlorodiphenyl sulfone by (i) reducing the pressure in the crystallization vessel to a pressure at which the water starts to evaporate, (ii) condensing the evaporated water by cooling, (iii) mixing the condensed water into the liquid mixture in the crystallization vessel, to obtain a suspension comprising crystallized 4,4′-dichlorodiphenyl sulfone; or by (a2) bringing the organic mixture into contact with at least one coolable surface and thereby reducing the temperature in the organic mixture with a cooling rate in the range from 5 to 50 K/h until a temperature in the range from 10 to 30° C. is reached, wherein the organic mixture and the at least one coolable surface have a temperature difference which is kept during the whole cooling process in the range from 1 to 30 K to obtain a suspension comprising crystallized 4,4′-dichlorodiphenyl sulfone. (b) carrying out a solid-liquid-separation of the suspension obtained in (a1b) or in (a2) to obtain a residual moisture containing solid 4,4′-dichlorodiphenyl sulfone as product and mother liquor comprising the organic solvent and water.

The invention relates to a process for obtaining 4,4′-dichlorodiphenyl sulfone from an organic mixture comprising 4,4′-dichlorodiphenyl sulfone and a linear C.sub.6 to C.sub.10 carboxylic acid as organic solvent, comprising: (a) cooling the organic mixture by (a1a) mixing the organic mixture with water in a crystallization vessel to obtain a liquid mixture; (a1b) cooling the liquid mixture obtained in (a1a) to a temperature below the saturation point of 4,4′-dichlorodiphenyl sulfone by (i) reducing the pressure in the crystallization vessel to a pressure at which the water starts to evaporate, (ii) condensing the evaporated water by cooling, (iii) mixing the condensed water into the liquid mixture in the crystallization vessel, to obtain a suspension comprising crystallized 4,4′-dichlorodiphenyl sulfone; or by (a2) bringing the organic mixture into contact with at least one coolable surface and thereby reducing the temperature in the organic mixture with a cooling rate in the range from 5 to 50 K/h until a temperature in the range from 10 to 30° C. is reached, wherein the organic mixture and the at least one coolable surface have a temperature difference which is kept during the whole cooling process in the range from 1 to 30 K to obtain a suspension comprising crystallized 4,4′-dichlorodiphenyl sulfone. (b) carrying out a solid-liquid-separation of the suspension obtained in (a1b) or in (a2) to obtain a residual moisture containing solid 4,4′-dichlorodiphenyl sulfone as product and mother liquor comprising the organic solvent and water.

The invention relates to a process for working up a composition comprising solid 4,4′-dichlorodiphenyl sulfoxide and a solvent, wherein the amount of solvent in the composition is in the range between 5 and 25 wt % based on the total mass of the composition by washing the composition using a carboxylic acid until the amount of solvent in the composition is below 1.5 wt % based on the total amount of the composition after washing.

The invention relates to a process for working up a composition comprising solid 4,4′-dichlorodiphenyl sulfoxide and a solvent, wherein the amount of solvent in the composition is in the range between 5 and 25 wt % based on the total mass of the composition by washing the composition using a carboxylic acid until the amount of solvent in the composition is below 1.5 wt % based on the total amount of the composition after washing.

The invention relates to a process for working up a composition comprising solid 4,4′-dichlorodiphenyl sulfoxide and a solvent, wherein the amount of solvent in the composition is in the range between 5 and 25 wt % based on the total mass of the composition by washing the composition using a carboxylic acid until the amount of solvent in the composition is below 1.5 wt % based on the total amount of the composition after washing.

The invention relates to a process for producing 4,4′-dichlorodiphenyl sulfoxide comprising: (I) reacting thionyl chloride, chlorobenzene and aluminum chloride in a molar ratio of thionyl chloride:chlorobenzene:aluminum chloride of 1:(6 to 9):(1 to 1.5) at a temperature in the range from 0 to below 20° C., forming an intermediate reaction product and hydrogen chloride; (II) mixing aqueous hydrochloric acid and the intermediate reaction product at a temperature in the range from 70 to 110° C. to obtain an organic phase comprising 4,4′-dichlorodiphenyl sulfoxide and an aqueous phase; (III) cooling the organic phase comprising the 4,4′-dichlorodiphenyl sulfoxide to a temperature below the saturation point of 4,4′-dichlorodiphenyl sulfoxide to obtain a suspension comprising crystallized 4,4′-dichlorodiphenyl sulfoxide; (IV) solid-liquid-separation of the suspension to obtain a residual moisture containing solid 4,4′-dichlorodiphenyl sulfoxide comprising crystallized 4,4′-dichlorodiphenyl sulfoxide and mother liquor.

The invention relates to a process for producing 4,4′-dichlorodiphenyl sulfoxide comprising: (I) reacting thionyl chloride, chlorobenzene and aluminum chloride in a molar ratio of thionyl chloride:chlorobenzene:aluminum chloride of 1:(6 to 9):(1 to 1.5) at a temperature in the range from 0 to below 20° C., forming an intermediate reaction product and hydrogen chloride; (II) mixing aqueous hydrochloric acid and the intermediate reaction product at a temperature in the range from 70 to 110° C. to obtain an organic phase comprising 4,4′-dichlorodiphenyl sulfoxide and an aqueous phase; (III) cooling the organic phase comprising the 4,4′-dichlorodiphenyl sulfoxide to a temperature below the saturation point of 4,4′-dichlorodiphenyl sulfoxide to obtain a suspension comprising crystallized 4,4′-dichlorodiphenyl sulfoxide; (IV) solid-liquid-separation of the suspension to obtain a residual moisture containing solid 4,4′-dichlorodiphenyl sulfoxide comprising crystallized 4,4′-dichlorodiphenyl sulfoxide and mother liquor.

A method of distilling a liquid containing dimethyl sulfoxide using a distillation system, including mixing sodium carbonate with a liquid containing dimethyl sulfoxide at a bottom of the distillation system such that sodium carbonate accounts for 0.005% to 25% by weight relative to 100% by weight of the liquid containing dimethyl sulfoxide and sodium carbonate in total at the bottom of the distillation system and heating the bottom; and obtaining a distillate containing dimethyl sulfoxide at a position lower than a position at which the liquid containing dimethyl sulfoxide is introduced and higher than a position of the heating portion.