CA2646311A1 - Process for purifying waste sulfuric acid - Google Patents
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- CA2646311A1 CA2646311A1 CA 2646311 CA2646311A CA2646311A1 CA 2646311 A1 CA2646311 A1 CA 2646311A1 CA 2646311 CA2646311 CA 2646311 CA 2646311 A CA2646311 A CA 2646311A CA 2646311 A1 CA2646311 A1 CA 2646311A1
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- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
- C01B17/00—Sulfur; Compounds thereof
- C01B17/69—Sulfur trioxide; Sulfuric acid
- C01B17/90—Separation; Purification
- C01B17/905—Removal of organic impurities
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Abstract
A process for purifying used sulfuric acid feed acid which contains, as minor constituents, methylsulfuric acid, dimethyl ether, methanol, aliphatic and olefinic hydrocarbons, chlorinated hydrocarbons and organosilicon compounds, which comprises a two-stage organic volatilization process comprising (a) diluting the used sulfuric acid with a stream selected from water, steam and an aqueous distillate mixture to a minimum temperature of about 100°C to effect hydrolysis of methyl sulfuric acid to methanol and sulfuric acid and vaporization of methyl chloride and dimethyl ether to provide a first diluted acid and a first volatile organic vapour comprising methyl chloride and dimethyl ether; (b) removing said methyl chloride and dimethyl ether vapour; (c) passing said diluted acid to an acid stripping column in counter-current flow with steam and, optionally, water to provide a second volatile organics vapour and a second diluted sulfuric acid; (d) removing said second volatile organics vapour; and collecting said second diluted sulfuric acid. The two-stage process eliminates charring of the organic materials in the stripping column and the formation of coating of the packing and release of particulate contaminants through subsequent concentration process steps.
Description
PROCESS FOR PURIFYING WASTE SULFURIC ACID
FIELD OF THE INVENTION
The invention relates to a process for the regeneration of waste sulfuric acid contaminated with recyclable products, such as methylsulfuric acid, dimethylether, methanol, aliphatic and olefinic hydrocarbons, chlorinated hydrocarbon, and organosilicon compounds.
BACKGROUND TO THE INVENTION
Chloromethane is used in the direct synthesis of methylchlorosilanes. The chloromethane required is generally prepared from methanol and hydrogen chloride, water and, in small amounts, dimethyl ether being formed as by-products.
Chloromethane for preparing methylchlorosilanes must be carefully dried and freed from dimethyl ether.
The hydrogen chloride used for the preparation of chloromethane originates from hydrolysis reactions of methylchlorosilanes. Therefore, in the prepared chloromethane, minor constituents of methylchlorosilanes, such as aliphatic and olefinic hydrocarbons and chlorinated hydrocarbons, may also be found. If the hydrogen chloride from hydrolysis reactions of methylchlorosilanes is used directly without intermediate purification in the production of chloromethane, organosilicon compounds in accordance with the vapor pressure can additionally pass into the product.
Chloromethane can be freed from the above mentioned impurities by scrubbing with concentrated sulfuric acid. In addition to water and dimethyl ether, the remaining minor constituents, particularly olefins and organosilicon compounds are absorbed in the sulfuric acid owing to their Lewis base properties. The used sulfuric acid is approximately 75% strength by weight and must be ejected, when its absorption capacity for dimethyl ether is exhausted.
DE-A-25 03 610 describes a process for purifying sulfuric acid contaminated with methylsulfuric acid. For this purpose, used sulfuric acid originating from the purification of chloromethane is diluted with 10% to 25% by weight of water, based on the sulfuric acid content. Steam is introduced, at a temperature of 170 to 180 C. The methylsulfuric acid content is hydrolyzed to methanol and sulfuric acid and methanol is distilled off.
The process of DE-A-25 03 610 is not applicable to sulfuric acid which, in addition to methylsulfuric acid, dimethyl ether and methanol further contains additional impurities described above, such as olefins and organosilicones, since cracking processes occur owing to the high temperature. The carbon formed is solid like coke and leads to rapid blocking of apparatus components. The high acid concentration causes a slight oxidation of organic contents. This in turn releases sulfur dioxide, which produces additional off-gas pollution.
In the booklet "Schott Engineering, 1987" from Schott Engineering GmbH, Mainz, pages 12 to 17, a process for the concentration and purification of waste sulfuric acid is described, in which oxidizing agents are added in a problem-specific manner to the waste acid.
The oxidation of organic impurities described in Schott is expensive at high pollution rates, since large amounts of oxidizing agent have to be added. The oxidation of organosilicon compounds leads to colloidally distributed silica. Filtration, owing to the aggressive medium and owing to the small particle size, represents a relatively great problem. Furthermore, contacting relatively large amounts of an oxidizing agent with the sulfuric acid containing dimethyl ether comprises a considerable safety hazard.
The addition of oxidizing agents following the process described in DE-A-25 03 610 is not expedient, since the coarse coke particles are oxidized slowly and only with difficulty, owing to the small surface area in comparison to the volume.
USP 5,683,671 discloses a method for purification of sulfuric acid contaminated particularly with methylsulfuric acid. This reference seeks to address the shortcomings of DE-A-25 03 610 by in a first step diluting the sulfuric acid to at least 45%, but no more than 55% concentration with liquid water, to a maximum of 135 C. Steam is then injected at a temperature of 120 to 140 C, which hydrolyses the methylsulfuric acid component into methanol and sulfuric acid, and wherein the methanol is distilled off. In a second step an oxidizing agent is added at a temperature of between 20 and 130 C.
However, USP 5,683,671 cannot be used for sulfuric acid that also contains, in addition to methylsulfuric acid, dimethylether, and methanol, the above-mentioned
FIELD OF THE INVENTION
The invention relates to a process for the regeneration of waste sulfuric acid contaminated with recyclable products, such as methylsulfuric acid, dimethylether, methanol, aliphatic and olefinic hydrocarbons, chlorinated hydrocarbon, and organosilicon compounds.
BACKGROUND TO THE INVENTION
Chloromethane is used in the direct synthesis of methylchlorosilanes. The chloromethane required is generally prepared from methanol and hydrogen chloride, water and, in small amounts, dimethyl ether being formed as by-products.
Chloromethane for preparing methylchlorosilanes must be carefully dried and freed from dimethyl ether.
The hydrogen chloride used for the preparation of chloromethane originates from hydrolysis reactions of methylchlorosilanes. Therefore, in the prepared chloromethane, minor constituents of methylchlorosilanes, such as aliphatic and olefinic hydrocarbons and chlorinated hydrocarbons, may also be found. If the hydrogen chloride from hydrolysis reactions of methylchlorosilanes is used directly without intermediate purification in the production of chloromethane, organosilicon compounds in accordance with the vapor pressure can additionally pass into the product.
Chloromethane can be freed from the above mentioned impurities by scrubbing with concentrated sulfuric acid. In addition to water and dimethyl ether, the remaining minor constituents, particularly olefins and organosilicon compounds are absorbed in the sulfuric acid owing to their Lewis base properties. The used sulfuric acid is approximately 75% strength by weight and must be ejected, when its absorption capacity for dimethyl ether is exhausted.
DE-A-25 03 610 describes a process for purifying sulfuric acid contaminated with methylsulfuric acid. For this purpose, used sulfuric acid originating from the purification of chloromethane is diluted with 10% to 25% by weight of water, based on the sulfuric acid content. Steam is introduced, at a temperature of 170 to 180 C. The methylsulfuric acid content is hydrolyzed to methanol and sulfuric acid and methanol is distilled off.
The process of DE-A-25 03 610 is not applicable to sulfuric acid which, in addition to methylsulfuric acid, dimethyl ether and methanol further contains additional impurities described above, such as olefins and organosilicones, since cracking processes occur owing to the high temperature. The carbon formed is solid like coke and leads to rapid blocking of apparatus components. The high acid concentration causes a slight oxidation of organic contents. This in turn releases sulfur dioxide, which produces additional off-gas pollution.
In the booklet "Schott Engineering, 1987" from Schott Engineering GmbH, Mainz, pages 12 to 17, a process for the concentration and purification of waste sulfuric acid is described, in which oxidizing agents are added in a problem-specific manner to the waste acid.
The oxidation of organic impurities described in Schott is expensive at high pollution rates, since large amounts of oxidizing agent have to be added. The oxidation of organosilicon compounds leads to colloidally distributed silica. Filtration, owing to the aggressive medium and owing to the small particle size, represents a relatively great problem. Furthermore, contacting relatively large amounts of an oxidizing agent with the sulfuric acid containing dimethyl ether comprises a considerable safety hazard.
The addition of oxidizing agents following the process described in DE-A-25 03 610 is not expedient, since the coarse coke particles are oxidized slowly and only with difficulty, owing to the small surface area in comparison to the volume.
USP 5,683,671 discloses a method for purification of sulfuric acid contaminated particularly with methylsulfuric acid. This reference seeks to address the shortcomings of DE-A-25 03 610 by in a first step diluting the sulfuric acid to at least 45%, but no more than 55% concentration with liquid water, to a maximum of 135 C. Steam is then injected at a temperature of 120 to 140 C, which hydrolyses the methylsulfuric acid component into methanol and sulfuric acid, and wherein the methanol is distilled off. In a second step an oxidizing agent is added at a temperature of between 20 and 130 C.
However, USP 5,683,671 cannot be used for sulfuric acid that also contains, in addition to methylsulfuric acid, dimethylether, and methanol, the above-mentioned
2 contaminants because they are subjected to cracking at high temperatures. The resulting hydrocarbon is hard as coke and forms deposits on the equipment components.
Adding oxidation agents according to the method disclosed in DE 25 03 610 Al is also unreasonable because coarse coke particles, having a small surface area compared to the amount used, are oxidized only poorly and slowly.
United States Patent Application No. 2008/0293979A1 - Wacker Chemie AG, published 27 November 2008, defines a method for the regeneration of waste sulfuric acid contaminated with admixtures from the group consisting of methylsulfuric acid, dimethylether, methanol, aliphatic and olefinic hydrocarbons, chlorinated hydrocarbons, and organosilicon compounds, wherein the waste sulfuric acid is diluted by adding water and injecting steam to a concentration of not more than 55 wt. % based on sulfuric acid and heated at most to 135 C to boiling, concentrated at reduced pressure and at a temperature of at least 170 C to at least 80 wt. % and is subsequently reacted with an oxidizing agent.
In prior art processes, organic compounds are released in a typical packed stripping column as a combined vapour/liquid stream as the waste sulfuric acid stream enters the hot column. Since the column is generally under an inert nitrogen blanket to avoid a flammable atmosphere, the organic materials tend to char/form carbon/coke deposits when the organic materials contact the hot packing. This charring requires the packing to be periodically cleaned but also results in the charred materials passing out of the column with the diluted acid for subsequent treatments.
However, although the aforesaid US2008/0293979A1 process may provide an improvement in the art, there still remains the problem of charring and particulate carbon formation when spent sulfuric acid contacts the surfaces of the stripping column and its packing when the feed acid is diluted by water/steam therein, to release the volatile organic materials. There is, therefore, a need to provide an improved process for purifying waste sulfuric acid containing minor constituents of the aforesaid organic contaminants.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an improved process for the purification of waste sulfuric acid contaminated with recyclable products, such as
Adding oxidation agents according to the method disclosed in DE 25 03 610 Al is also unreasonable because coarse coke particles, having a small surface area compared to the amount used, are oxidized only poorly and slowly.
United States Patent Application No. 2008/0293979A1 - Wacker Chemie AG, published 27 November 2008, defines a method for the regeneration of waste sulfuric acid contaminated with admixtures from the group consisting of methylsulfuric acid, dimethylether, methanol, aliphatic and olefinic hydrocarbons, chlorinated hydrocarbons, and organosilicon compounds, wherein the waste sulfuric acid is diluted by adding water and injecting steam to a concentration of not more than 55 wt. % based on sulfuric acid and heated at most to 135 C to boiling, concentrated at reduced pressure and at a temperature of at least 170 C to at least 80 wt. % and is subsequently reacted with an oxidizing agent.
In prior art processes, organic compounds are released in a typical packed stripping column as a combined vapour/liquid stream as the waste sulfuric acid stream enters the hot column. Since the column is generally under an inert nitrogen blanket to avoid a flammable atmosphere, the organic materials tend to char/form carbon/coke deposits when the organic materials contact the hot packing. This charring requires the packing to be periodically cleaned but also results in the charred materials passing out of the column with the diluted acid for subsequent treatments.
However, although the aforesaid US2008/0293979A1 process may provide an improvement in the art, there still remains the problem of charring and particulate carbon formation when spent sulfuric acid contacts the surfaces of the stripping column and its packing when the feed acid is diluted by water/steam therein, to release the volatile organic materials. There is, therefore, a need to provide an improved process for purifying waste sulfuric acid containing minor constituents of the aforesaid organic contaminants.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an improved process for the purification of waste sulfuric acid contaminated with recyclable products, such as
3 methylsulfuric acid, dimethylether, methanol, aliphatic and olefinic hydrocarbons, chlorinated hydrocarbons, and organosilicon compounds, which process reduces or eliminates charring of the organic materials which contaminates the surface of equipment, particularly, stripping columns, as well as being passed on to subsequent process steps.
Accordingly, in one aspect the invention provides a process for purifying used sulfuric acid feed acid which contains, as minor constituents, methylsulfuric acid, dimethyl ether, methanol, aliphatic and olefinic hydrocarbons, chlorinated hydrocarbons and organosilicon compounds which comprises a two-stage organic compound volatilization process comprising (a) diluting the used sulfuric acid with a stream selected from water, steam and an aqueous distillate mixture to a minimum temperature of about 100 C to effect hydrolysis of methylsulfuric acid to methanol and sulfuric acid and vaporization of methyl chloride, dimethyl ether and a portion of the released methanol to provide a first diluted acid and a first volatile organic vapour comprising methyl chloride and dimethyl ether and a portion of the released methanol;
(b) removing said methyl chloride and dimethyl ether and methanol vapour;
(c) passing said diluted acid to an acid stripping column in counter-current flow with steam and, optionally, water to provide a second volatile organics vapour and a second diluted sulfuric acid;
(d) removing said second volatile organics vapour; and (e) collecting said second diluted sulfuric acid.
Preferably, the used sulfuric acid is diluted in step (a) to not less than 50 wt.% and more preferably, not less than 57 wt.%.
Thus, in the practice of the present two-stage invention, contaminated waste sulfuric acid is diluted external of the stripping column in a first stage with water, steam and/or, most preferably, distillate liquors to a temperature of about 115 C
generally resulting from the heat of dilution. This external pre-dilution stage is, preferably, effected in a static mixer and subsequently in a small vessel and results in release of organic material vapours and venting thereof for subsequent combination with the vapours leaving the stripping column of use in the second dilution stage. Thus, these organic materials
Accordingly, in one aspect the invention provides a process for purifying used sulfuric acid feed acid which contains, as minor constituents, methylsulfuric acid, dimethyl ether, methanol, aliphatic and olefinic hydrocarbons, chlorinated hydrocarbons and organosilicon compounds which comprises a two-stage organic compound volatilization process comprising (a) diluting the used sulfuric acid with a stream selected from water, steam and an aqueous distillate mixture to a minimum temperature of about 100 C to effect hydrolysis of methylsulfuric acid to methanol and sulfuric acid and vaporization of methyl chloride, dimethyl ether and a portion of the released methanol to provide a first diluted acid and a first volatile organic vapour comprising methyl chloride and dimethyl ether and a portion of the released methanol;
(b) removing said methyl chloride and dimethyl ether and methanol vapour;
(c) passing said diluted acid to an acid stripping column in counter-current flow with steam and, optionally, water to provide a second volatile organics vapour and a second diluted sulfuric acid;
(d) removing said second volatile organics vapour; and (e) collecting said second diluted sulfuric acid.
Preferably, the used sulfuric acid is diluted in step (a) to not less than 50 wt.% and more preferably, not less than 57 wt.%.
Thus, in the practice of the present two-stage invention, contaminated waste sulfuric acid is diluted external of the stripping column in a first stage with water, steam and/or, most preferably, distillate liquors to a temperature of about 115 C
generally resulting from the heat of dilution. This external pre-dilution stage is, preferably, effected in a static mixer and subsequently in a small vessel and results in release of organic material vapours and venting thereof for subsequent combination with the vapours leaving the stripping column of use in the second dilution stage. Thus, these organic materials
4 have no, or only a reduced, opportunity to contact the hot packing surfaces in the inert atmosphere of the stripping column.
The sulfuric acid regeneration and purification process and plant according to the invention, as hereinabove defined, includes a steam stripping column, preferably, followed by a two-stage, steam heated sulfuric acid concentrator (SAC) designed to produce 96%
product acid or whichever strength is suitable for recycle to the precursor process. The contaminated organic vapours from the dilution and stripping steps are, subsequently, concentrated in an overheads fractionation column to recover an organic rich overhead stream and a bottom distillate stream containing only trace amounts of organic components.
The organic rich overhead stream can be further separated by partial condensation in a methanol rich liquid stream for potential recycle and a dimethyl ether rich vapour stream for recycle or incineration. The distillate stream recovered from the bottom of the fractionation column can be used as dilution in the preceding dilution step.
Accordingly, in a preferred aspect, the invention further provides a process as hereinabove defined comprising (f) collecting said vapour selected from the group consisting of step (b) and step (d);
(g) fractionating said vapour of step (f) to produce both a liquid methanol stream and a dimethyl ether rich vapour stream overhead and a liquid distillate bottoms stream.
Preferably, the process as hereinabove defined comprises the use of distillates mixture, defined as a portion of the condensed aqueous stream evaporated during the subsequent sulfuric acid concentration (SAC) stages.
Preferably, the process as hereinabove defined has said first diluted acid of about 115 C and wherein said first diluted acid and said second diluted sulfuric acid is at least 57wt. %.
In more detail, feed acid contaminated with volatile organic compounds is diluted by a stream of water, steam or, preferably, distillate or mixtures thereof in a static mixer and subsequently in a small, vented vessel to about 60 wt.%. It is believed that the dilution is sufficient to result in the hydrolysis of the methylsulfuric acid with the formation of methanol and sulfuric acid, wherein the heat of dilution raises the temperature of the acid to
The sulfuric acid regeneration and purification process and plant according to the invention, as hereinabove defined, includes a steam stripping column, preferably, followed by a two-stage, steam heated sulfuric acid concentrator (SAC) designed to produce 96%
product acid or whichever strength is suitable for recycle to the precursor process. The contaminated organic vapours from the dilution and stripping steps are, subsequently, concentrated in an overheads fractionation column to recover an organic rich overhead stream and a bottom distillate stream containing only trace amounts of organic components.
The organic rich overhead stream can be further separated by partial condensation in a methanol rich liquid stream for potential recycle and a dimethyl ether rich vapour stream for recycle or incineration. The distillate stream recovered from the bottom of the fractionation column can be used as dilution in the preceding dilution step.
Accordingly, in a preferred aspect, the invention further provides a process as hereinabove defined comprising (f) collecting said vapour selected from the group consisting of step (b) and step (d);
(g) fractionating said vapour of step (f) to produce both a liquid methanol stream and a dimethyl ether rich vapour stream overhead and a liquid distillate bottoms stream.
Preferably, the process as hereinabove defined comprises the use of distillates mixture, defined as a portion of the condensed aqueous stream evaporated during the subsequent sulfuric acid concentration (SAC) stages.
Preferably, the process as hereinabove defined has said first diluted acid of about 115 C and wherein said first diluted acid and said second diluted sulfuric acid is at least 57wt. %.
In more detail, feed acid contaminated with volatile organic compounds is diluted by a stream of water, steam or, preferably, distillate or mixtures thereof in a static mixer and subsequently in a small, vented vessel to about 60 wt.%. It is believed that the dilution is sufficient to result in the hydrolysis of the methylsulfuric acid with the formation of methanol and sulfuric acid, wherein the heat of dilution raises the temperature of the acid to
5 approximately 115 C. Most of the methyl chloride, and dimethyl ether and a portion of the released methanol are vaporized and removed from the system. If acid weaker than design is to be processed, steam can be injected into the acid in the static mixer to ensure that the acid fed to the stripping column is above 100 C.
The diluted acid is passed down the packed stripping column, counter current to the stripping steam. The temperature rise caused by dilution and steam addition results in the volatilization of any methanol and other organic compounds carried over from the first stage, in the packing, which are then carried to the top of the column by the stripping steam. Preferably, part of the stripping steam is generated internally in the column, by concentrating the column bottoms to approximately 65 wt.% in a natural thermosyphon stripping column reboiler. The reboiler circulation loop includes a stripper retention tank to give additional residence time for the 65% acid. Low pressure steam is injected below the packing of the column to supplement the steam generated in the reboiler.
Preferably, operation of the stripping column is carried out under partial vacuum to reduce processing temperatures and further reduction in charring of organic compounds not removed under stage 1.
Inclusion of a partial concentration step to 65 wt.% in the stripping column enables a high ratio of stripping steam/acid feed to be economically achieved which still further enhances organics material removal.
Fractionation of the combined overheads vapour from the dilution vessel and the stripping column provides a useful methanol liquid stream, a dimethyl ether rich vapour stream for recycle or incineration and an almost organic free bottoms distillate stream for recycle.
The acid leaving the bottom of the stripping column, which contains only small amounts of residual organics, flows by gravity to an ODU (Organics Destruction Unit) First Reactor. In ODU First Reactor, provision is, preferably, made to, optionally, add a solution of oxidizing agent, generally hydrogen peroxide in acid, to the stripper bottoms to improve the colour and further reduce the organics level in the stripper bottoms. Acid overflows the ODU First Reactor and enters a first evaporation stage. There are, preferably, 2 evaporation stages in series in the preferred SAC system but the number of stages may vary from one to multiple stages depending on circumstances. In the case of 2 stage
The diluted acid is passed down the packed stripping column, counter current to the stripping steam. The temperature rise caused by dilution and steam addition results in the volatilization of any methanol and other organic compounds carried over from the first stage, in the packing, which are then carried to the top of the column by the stripping steam. Preferably, part of the stripping steam is generated internally in the column, by concentrating the column bottoms to approximately 65 wt.% in a natural thermosyphon stripping column reboiler. The reboiler circulation loop includes a stripper retention tank to give additional residence time for the 65% acid. Low pressure steam is injected below the packing of the column to supplement the steam generated in the reboiler.
Preferably, operation of the stripping column is carried out under partial vacuum to reduce processing temperatures and further reduction in charring of organic compounds not removed under stage 1.
Inclusion of a partial concentration step to 65 wt.% in the stripping column enables a high ratio of stripping steam/acid feed to be economically achieved which still further enhances organics material removal.
Fractionation of the combined overheads vapour from the dilution vessel and the stripping column provides a useful methanol liquid stream, a dimethyl ether rich vapour stream for recycle or incineration and an almost organic free bottoms distillate stream for recycle.
The acid leaving the bottom of the stripping column, which contains only small amounts of residual organics, flows by gravity to an ODU (Organics Destruction Unit) First Reactor. In ODU First Reactor, provision is, preferably, made to, optionally, add a solution of oxidizing agent, generally hydrogen peroxide in acid, to the stripper bottoms to improve the colour and further reduce the organics level in the stripper bottoms. Acid overflows the ODU First Reactor and enters a first evaporation stage. There are, preferably, 2 evaporation stages in series in the preferred SAC system but the number of stages may vary from one to multiple stages depending on circumstances. In the case of 2 stage
6 operation acid flows between the two stages by gravity and is concentrated to about 88 wt.% H2SO4 in Stage 1 and overflows to Stage 2 at about 175 C via an ODU
Second Reactor which provides additional residence time. Additional hydrogen peroxide is, optionally, added prior to ODU Second Reactor and the oxidation of any remaining organics is completed therein. The final product acid at 96 wt % H2S04 or other desired strength flows by gravity from the second stage via a feed/product interchanges and into a product drum. After cooling the product acid is, optionally, passed through a filter to remove any particles of silica which may have been formed by the oxidation of the organosilicon compounds, before being pumped to final storage Vapours generated in the evaporator(s) are condensed in one or more overhead condensers. These vapours are substantially water vapour containing small amounts of sulphuric acid. Design techniques familiar to those skilled in the art are applied to minimize the amount of sulphuric acid which reports to the final condensed aqueous condensate stream The SAC system described above operates under vacuum most typically, although not always, drawn by steam ejectors. The design of the SAC system and selection of operating conditions are optimized by means familiar to those skilled in the art to achieve the desired product strength and quality with minimum acid loss to effluent.
The combined condensed aqueous stream from the acid concentration stages, together with that from the steam ejector condensers and the fractionation column bottoms stream, comprises the distillate stream, a portion of which is preferentially used for the initial feed acid dilution step in order to reduce water consumption and effluent load.
BRIEF DESCRIPTION OF THE DRAWINGS
In order that the invention may be better understood, preferred embodiments will now be described by way of example only with reference to the accompanying drawing wherein Fig. 1 is a diagrammatic process and plant layout according to the invention.
Second Reactor which provides additional residence time. Additional hydrogen peroxide is, optionally, added prior to ODU Second Reactor and the oxidation of any remaining organics is completed therein. The final product acid at 96 wt % H2S04 or other desired strength flows by gravity from the second stage via a feed/product interchanges and into a product drum. After cooling the product acid is, optionally, passed through a filter to remove any particles of silica which may have been formed by the oxidation of the organosilicon compounds, before being pumped to final storage Vapours generated in the evaporator(s) are condensed in one or more overhead condensers. These vapours are substantially water vapour containing small amounts of sulphuric acid. Design techniques familiar to those skilled in the art are applied to minimize the amount of sulphuric acid which reports to the final condensed aqueous condensate stream The SAC system described above operates under vacuum most typically, although not always, drawn by steam ejectors. The design of the SAC system and selection of operating conditions are optimized by means familiar to those skilled in the art to achieve the desired product strength and quality with minimum acid loss to effluent.
The combined condensed aqueous stream from the acid concentration stages, together with that from the steam ejector condensers and the fractionation column bottoms stream, comprises the distillate stream, a portion of which is preferentially used for the initial feed acid dilution step in order to reduce water consumption and effluent load.
BRIEF DESCRIPTION OF THE DRAWINGS
In order that the invention may be better understood, preferred embodiments will now be described by way of example only with reference to the accompanying drawing wherein Fig. 1 is a diagrammatic process and plant layout according to the invention.
7 DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Fig. 1 shows generally as 10, a two-stage process for eliminating or reducing the amount of charring and carbon particulate formation formed in sulfuric acid purification processes of the prior art.
Fig. 1 shows a spent 80 wt. % sulfuric acid feed stream 12 fed to an acid mixing and dilution vessel 14 into which is passed an aqueous distillate mixture 16 to generate a 60 wt. % sulfuric acid at about 115 C and effect hydrolysis of the methylsulfuric acid.
Volatile organic materials, such as methanol, methyl chloride and dimethyl ether, are removed through conduit 18 and the diluted acid passed through conduit 20 to packed stripping column 22 in counter current flow to steam stream 24.
The volatile organic vapour stream 18 and the overheads stream 54 from column are combined and enter the overheads fractionation system.
Part of the stripping steam is generated internally in column 22, by concentrating the column bottoms to approximately 65 wt.% in a natural thermosyphon stripping column reboiler 26. The reboiler circulation loop includes a stripper retention tank or drum 28 to give additional residence time for the 65% acid. Low pressure steam 24 is injected below the packing of column 22 to supplement steam 30 generated in reboiler 26.
The acid leaving the bottom of stripping column 22, which contains only small amounts of residual organics, flows by gravity 32 to an ODU (Organics Destruction Unit) First Reactor 34. In ODU First Reactor 34, provision is, preferably, made to, optionally, add a solution of oxidizing agent, generally, hydrogen peroxide 36 in acid to the stripper bottoms 38 to improve the colour and further reduce the organics level in stripper bottoms 38. Acid overflows 40 and enters a first evaporation stage 42. There are, preferably, 2 evaporation stages 42, 44 in series in the preferred Sulphuric Acid Concentration (SAC) system. Acid flows between the two stages by gravity and is concentrated to about 88 wt.% H2SO4 in Stage 1 and overflows to Stage 2 at about 175 C via an ODU
Second Reactor 46 which provides additional residence time. Additional hydrogen peroxide 48 is, optionally, added prior to ODU Second Reactor 46 and the oxidation of any remaining organics is completed therein. The final product acid at 96 wt % H2SO4 flows by gravity 50
Fig. 1 shows generally as 10, a two-stage process for eliminating or reducing the amount of charring and carbon particulate formation formed in sulfuric acid purification processes of the prior art.
Fig. 1 shows a spent 80 wt. % sulfuric acid feed stream 12 fed to an acid mixing and dilution vessel 14 into which is passed an aqueous distillate mixture 16 to generate a 60 wt. % sulfuric acid at about 115 C and effect hydrolysis of the methylsulfuric acid.
Volatile organic materials, such as methanol, methyl chloride and dimethyl ether, are removed through conduit 18 and the diluted acid passed through conduit 20 to packed stripping column 22 in counter current flow to steam stream 24.
The volatile organic vapour stream 18 and the overheads stream 54 from column are combined and enter the overheads fractionation system.
Part of the stripping steam is generated internally in column 22, by concentrating the column bottoms to approximately 65 wt.% in a natural thermosyphon stripping column reboiler 26. The reboiler circulation loop includes a stripper retention tank or drum 28 to give additional residence time for the 65% acid. Low pressure steam 24 is injected below the packing of column 22 to supplement steam 30 generated in reboiler 26.
The acid leaving the bottom of stripping column 22, which contains only small amounts of residual organics, flows by gravity 32 to an ODU (Organics Destruction Unit) First Reactor 34. In ODU First Reactor 34, provision is, preferably, made to, optionally, add a solution of oxidizing agent, generally, hydrogen peroxide 36 in acid to the stripper bottoms 38 to improve the colour and further reduce the organics level in stripper bottoms 38. Acid overflows 40 and enters a first evaporation stage 42. There are, preferably, 2 evaporation stages 42, 44 in series in the preferred Sulphuric Acid Concentration (SAC) system. Acid flows between the two stages by gravity and is concentrated to about 88 wt.% H2SO4 in Stage 1 and overflows to Stage 2 at about 175 C via an ODU
Second Reactor 46 which provides additional residence time. Additional hydrogen peroxide 48 is, optionally, added prior to ODU Second Reactor 46 and the oxidation of any remaining organics is completed therein. The final product acid at 96 wt % H2SO4 flows by gravity 50
8 from the second stage into a product drum 52. After cooling, this cooled 96 wt.% acid may then be returned to a silicones plant on level control. The concentrated, cooled acid may optionally be passed through a filter to remove any residual particles of inorganic matter produced by the acid processing.
Each SAC evaporator 42, 44, preferably, consists of a tantalum bayonet heat exchanger in a natural circulation thermosyphon loop with a separator vessel mounted above.
EXAMPLES
In the following examples a spent sulfuric acid, from the production of silicones was used having the following composition:-Sulfuric acid 80.0% by weight Water 3.5% by weight Dimethyl ether 12.0% by weight Methyl sulfuric acid 3.5% by weight Methanol 1.0% by weight Methyl chloride Trace Organosilicon compounds Trace Total organic carbon (TOC) > 7.0% by weight (70,000 ppm) The process steps described were carried out continuously.
Example 1 Spent sulfuric acid and water streams at room temperature were metered continuously into the top of a glass column at rates calculated to give a diluted acid stream of 57 wt % acid strength. The column was hot-oil jacketed and was 1" diameter and 36' length packed with 3/16' ceramic Intalox saddles. The column was operated at slight below atmospheric pressure under an inert atmosphere.
Live steam was injected at the bottom of the column at a temperature of about 105 C. The acid leaving the bottom of the column flowed by gravity into a glass flask located in a heating mantle which served as a reboiler. The reboiler was maintained at a temperature at which the acid was concentrated to about 65 wt % acid strength.
The steam generated in this concentration step was returned to the bottom of the column to act as additional stripping steam.
Each SAC evaporator 42, 44, preferably, consists of a tantalum bayonet heat exchanger in a natural circulation thermosyphon loop with a separator vessel mounted above.
EXAMPLES
In the following examples a spent sulfuric acid, from the production of silicones was used having the following composition:-Sulfuric acid 80.0% by weight Water 3.5% by weight Dimethyl ether 12.0% by weight Methyl sulfuric acid 3.5% by weight Methanol 1.0% by weight Methyl chloride Trace Organosilicon compounds Trace Total organic carbon (TOC) > 7.0% by weight (70,000 ppm) The process steps described were carried out continuously.
Example 1 Spent sulfuric acid and water streams at room temperature were metered continuously into the top of a glass column at rates calculated to give a diluted acid stream of 57 wt % acid strength. The column was hot-oil jacketed and was 1" diameter and 36' length packed with 3/16' ceramic Intalox saddles. The column was operated at slight below atmospheric pressure under an inert atmosphere.
Live steam was injected at the bottom of the column at a temperature of about 105 C. The acid leaving the bottom of the column flowed by gravity into a glass flask located in a heating mantle which served as a reboiler. The reboiler was maintained at a temperature at which the acid was concentrated to about 65 wt % acid strength.
The steam generated in this concentration step was returned to the bottom of the column to act as additional stripping steam.
9 The product acid from the stripping column/reboiler apparatus was a clear light amber colour compared to the clear dark amber colour of the acid feed.
Depending on the exact combination of operating conditions and stripping steam/acid feed ratios employed the product acid Total Organic Carbon (TOC) content was reduced to between from 650 ppm to 2700 ppm.
After completion of a number of experiments the column was dismantled and evidence of charring and carbon deposition was observed at the feed point locations at the top of the column and on the heated walls of the column.
Example 2 Spent sulfuric acid and water streams were metered continuously at room temperature into an agitated mixing vessel mounted externally to the stripping column at rates calculated to give a diluted acid stream of 57 wt. % acid strength at about 115 C. The vapours generated in the mixing vessel were vented from the vessel and combined with vapours leaving the top of the stripping column. The diluted acid formed in the mixing vessel flowed by gravity to the top of the stripping column which was operated at 560 mmHg abs. pressure and under an inert atmosphere.
Live steam was injected at the bottom of the column at a temperature of about 105 C. The acid leaving the bottom of the column flowed by gravity into a glass flask located in a heating mantle which served as a reboiler. The reboiler was maintained at a temperature at which the acid was concentrated to about 65 wt. % acid strength. This temperature varied according to the pressure at which the column was operated.
The steam generated in this concentration step was returned to the bottom of the column to act as additional stripping stream.
The product acid from the stripping column/reboiler apparatus was a clear light amber colour compared to the dark amber colour of the acid feed. Depending on the exact combination of operating conditions and stripping steam/acid feed ratios employed the product acid TOC was reduced to as low as 60 ppm.
This stripper product acid was then continuously concentrated by multi-stage vacuum concentration to a 96 wt.% strength product suitable for recycling to a silicones manufacturing process. A suitable quality product in terms of TOC content was obtained without the necessity for the addition of an oxidizing agent.
After completion of a number of experiments, the column was inspected and no charring or carbon deposition was found to have occurred.
Although this disclosure has described and illustrated certain preferred embodiments of the invention, it is to be understood that the invention is not restricted to those particular embodiments. Rather, the invention includes all embodiments which are functional or mechanical equivalents of the specific embodiments and features that have been described and illustrated.
Depending on the exact combination of operating conditions and stripping steam/acid feed ratios employed the product acid Total Organic Carbon (TOC) content was reduced to between from 650 ppm to 2700 ppm.
After completion of a number of experiments the column was dismantled and evidence of charring and carbon deposition was observed at the feed point locations at the top of the column and on the heated walls of the column.
Example 2 Spent sulfuric acid and water streams were metered continuously at room temperature into an agitated mixing vessel mounted externally to the stripping column at rates calculated to give a diluted acid stream of 57 wt. % acid strength at about 115 C. The vapours generated in the mixing vessel were vented from the vessel and combined with vapours leaving the top of the stripping column. The diluted acid formed in the mixing vessel flowed by gravity to the top of the stripping column which was operated at 560 mmHg abs. pressure and under an inert atmosphere.
Live steam was injected at the bottom of the column at a temperature of about 105 C. The acid leaving the bottom of the column flowed by gravity into a glass flask located in a heating mantle which served as a reboiler. The reboiler was maintained at a temperature at which the acid was concentrated to about 65 wt. % acid strength. This temperature varied according to the pressure at which the column was operated.
The steam generated in this concentration step was returned to the bottom of the column to act as additional stripping stream.
The product acid from the stripping column/reboiler apparatus was a clear light amber colour compared to the dark amber colour of the acid feed. Depending on the exact combination of operating conditions and stripping steam/acid feed ratios employed the product acid TOC was reduced to as low as 60 ppm.
This stripper product acid was then continuously concentrated by multi-stage vacuum concentration to a 96 wt.% strength product suitable for recycling to a silicones manufacturing process. A suitable quality product in terms of TOC content was obtained without the necessity for the addition of an oxidizing agent.
After completion of a number of experiments, the column was inspected and no charring or carbon deposition was found to have occurred.
Although this disclosure has described and illustrated certain preferred embodiments of the invention, it is to be understood that the invention is not restricted to those particular embodiments. Rather, the invention includes all embodiments which are functional or mechanical equivalents of the specific embodiments and features that have been described and illustrated.
Claims (9)
1. A process for purifying used sulfuric acid feed acid which contains, as minor constituents, methylsulfuric acid, dimethyl ether, methanol, aliphatic and olefinic hydrocarbons, chlorinated hydrocarbons and organosilicon compounds, which comprises a two-stage organic volatilization process comprising (a) diluting the used sulfuric acid with a stream selected from water, steam and an aqueous distillate mixture to a minimum temperature of about 100°C to effect hydrolysis of methyl sulfuric acid to methanol and sulfuric acid and vaporization of methyl chloride, dimethyl ether and a portion of the released methanol to provide a first diluted acid and a first volatile organic vapour comprising methyl chloride and dimethyl ether and a portion of the released methanol;
(b) removing said methyl chloride, dimethyl ether and methanol vapour ;
(c) passing said diluted acid to an acid stripping column in counter-current flow with steam and, optionally, water to provide a second volatile organics vapour and a second diluted sulfuric acid;
(d) removing said second volatile organics vapour; and (e) collecting said second diluted sulfuric acid.
(b) removing said methyl chloride, dimethyl ether and methanol vapour ;
(c) passing said diluted acid to an acid stripping column in counter-current flow with steam and, optionally, water to provide a second volatile organics vapour and a second diluted sulfuric acid;
(d) removing said second volatile organics vapour; and (e) collecting said second diluted sulfuric acid.
2. A process as claimed in claim 1 comprising diluting said used sulfuric acid to a concentration of not less than 50 wt.%.
3. A process as claimed in claim 2 comprising diluting said used sulfuric acid to a concentration of not less than 57 wt.%.
4. A process as claimed in claim 3 comprising diluting said used sulfuric acid to a concentration of about 60 wt.%.
5. A process as claimed in any one of claims 1 to 4 wherein said dilution stream comprises said distillate mixture.
6. A process as claimed in any one of claims 1 to 5 wherein said first diluted acid is at about 115°C.
7. A process as claimed in any one of claims 1 to 6 wherein said second diluted sulfuric acid is at least 57 wt. %.
8. A process as claimed in any one of claims 1 to 7 comprising treating said diluted acid with said steam, and, optionally, water in step (b) under a partial vacuum.
9. A process as claimed in any one of claims 1 to 8 comprising (f) collecting said vapour selected from the group consisting of step (b) and step (d);
(g) fractionating said vapour of step (f) to produce a liquid methanol stream and a dimethyl ether rich vapour stream overhead and a water stream containing only trace amount of methanol as a bottoms stream.
(g) fractionating said vapour of step (f) to produce a liquid methanol stream and a dimethyl ether rich vapour stream overhead and a water stream containing only trace amount of methanol as a bottoms stream.
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| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2646311 CA2646311A1 (en) | 2008-12-11 | 2008-12-11 | Process for purifying waste sulfuric acid |
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| Application Number | Priority Date | Filing Date | Title |
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| CA 2646311 CA2646311A1 (en) | 2008-12-11 | 2008-12-11 | Process for purifying waste sulfuric acid |
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Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN109052336A (en) * | 2018-09-17 | 2018-12-21 | 杭州东日节能技术有限公司 | A kind of dry by-product dilute sulfuric acid purification enrichment facility of chloromethanes and technique |
| CN109694043A (en) * | 2018-12-27 | 2019-04-30 | 镇江江南化工有限公司 | A kind of method that Waste Sulfuric Acid integrated treatment recycles |
| CN111362234A (en) * | 2020-04-16 | 2020-07-03 | 浙江嘉福新材料科技有限公司 | NMST byproduct sulfuric acid concentration equipment and process thereof |
-
2008
- 2008-12-11 CA CA 2646311 patent/CA2646311A1/en not_active Abandoned
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN109052336A (en) * | 2018-09-17 | 2018-12-21 | 杭州东日节能技术有限公司 | A kind of dry by-product dilute sulfuric acid purification enrichment facility of chloromethanes and technique |
| CN109694043A (en) * | 2018-12-27 | 2019-04-30 | 镇江江南化工有限公司 | A kind of method that Waste Sulfuric Acid integrated treatment recycles |
| CN111362234A (en) * | 2020-04-16 | 2020-07-03 | 浙江嘉福新材料科技有限公司 | NMST byproduct sulfuric acid concentration equipment and process thereof |
| CN111362234B (en) * | 2020-04-16 | 2024-06-07 | 浙江嘉福新材料科技有限公司 | NMST byproduct sulfuric acid concentration equipment and process thereof |
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