Abstract:
A method of making sodium carbonate and/or sodium bicarbonate is disclosed in which carbon dioxide gas is reacted with an aqueous solution sodium hydroxide solution in the presence of a compound of the formula (I): Na + [X—O] −  where X is Cl, Br, or I.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. patent application Ser. No. 14/015,606, filed Aug. 30, 2013, which claims priority to U.S. Patent Application No. 61/803,587, filed Mar. 20, 2013, both of which are incorporated herein by reference in their entireties. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Sodium carbonate is a widely-used product, with many applications in foods, as well as many other applications such as in cleaning, textile, and other industrial and commercial applications. Much of worldwide sodium carbonate production uses a process known as the “Solvay process”, which involves the reaction of carbon dioxide produced by thermal decomposition of calcium carbonate, with a solution of sodium chloride and ammonia to produce sodium bicarbonate, followed by thermal decomposition of the sodium bicarbonate to form sodium carbonate. However, high capital costs involved with building and operating the Solvay process have invited alternative approaches for making sodium carbonate. 
         [0003]    One such alternative type of process is described in U.S. Pat. No. 7,708,972, U.S. Pat. No. 8,202,659, and US Patent Application Publication 2010/0147698 A1, the disclosure of each of which is incorporated by reference in its entirety herein. This process involves a carbonation reaction between carbon dioxide and an aqueous sodium hydroxide brine solution produced by electrolysis of a sodium chloride solution. Other features of this process include generation of carbon dioxide for the carbonation reaction by reacting calcium carbonate (e.g., from limestone) with hydrochloric acid, and generating the hydrochloric acid by reacting chlorine gas and hydrogen gas produced by the electrolysis. Additional features include separation and purification of the sodium carbonate produced by the carbonation reaction, and recovery and recycle of sodium chloride from the electrolysis byproducts. Although this process can be effective, it also has certain limitations, such as requiring high levels of sodium hydroxide (e.g., at least 25 wt. % concentration) and/or high temperature levels (e.g., above 100° C. or above 110° C.) in order to achieve target efficiency levels. 
         [0004]    Many other processes have been used or proposed for the production of sodium carbonate, and each of them has its own advantages and disadvantages. However, new and different processes and systems for producing sodium carbonate, which may afford opportunities for improved performance, cost, reliability, process variation tolerance, etc., are always welcome in the art. 
       SUMMARY OF THE INVENTION 
       [0005]    According to one aspect, a method of making sodium carbonate and/or sodium bicarbonate comprises reacting carbon dioxide gas with an aqueous solution comprising sodium hydroxide in the presence of a compound of the formula (I): Na + [X—O] −  where X is Cl, Br, or I. 
         [0006]    According to further aspects, the aqueous solution comprising sodium hydroxide is generated by electrolysis of a solution comprising sodium chloride. In further aspects, the compound according to formula (I) is sodium hypochlorite, which can be generated by reaction of water and chlorine gas formed by electrolysis of a sodium chloride solution. 
         [0007]    According to another aspect, a system for producing sodium carbonate comprises: (a) an electrical cell reactor for electrolysis of a solution comprising sodium chloride, comprising a cathode chamber and an anode chamber separated by a membrane, an outlet connected to the anode chamber configured for collecting chlorine gas from the anode chamber, and an outlet connected to the cathode chamber configured for collecting an aqueous solution comprising sodium hydroxide from the cathode chamber; (b) a reactor for reacting a liquid comprising the aqueous solution collected from the cathode chamber in (a) with a gas comprising chlorine collected from the anode chamber in (a) to produce sodium hypochlorite; and (c) a carbonation reactor for reacting an aqueous liquid comprising the aqueous solution collected from the cathode chamber in (a) and sodium hypochlorite produced in (b) with a gas comprising carbon dioxide to produce sodium carbonate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The foregoing and other features, and advantages of the invention are described in the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0009]      FIG. 1  schematically depicts a representative embodiment of a process and system for producing sodium carbonate; 
           [0010]      FIG. 2  schematically depicts another representative embodiment of a process and system for producing sodium carbonate; and 
           [0011]      FIG. 3  schematically depicts another representative embodiment of a process and system for producing sodium carbonate. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    Turning now to the figures, an exemplary system and process for making sodium carbonate is schematically depicted. As shown in  FIG. 1 , system/process  10  is shown with carbonation reactor  12 , into which is fed an aqueous solution  16  comprising sodium hydroxide and a compound according to formula (I). The nucleophilic reaction between the carbon dioxide and the aqueous sodium hydroxide produces sodium carbonate product  18 . As used herein, “sodium carbonate” is defined broadly to include not only anhydrous sodium carbonate, but also hydrated sodium carbonates. The reaction between carbon dioxide and aqueous sodium hydroxide produces hydrated sodium carbonates and/or sodium bicarbonate. In some embodiments, hydrated sodium carbonates can be further processed to purify and subject the hydrated sodium carbonates to heat-induced desiccation to produce purified anhydrous sodium carbonate, and the sodium bicarbonate can be subjected to a heat-induced decomposition reaction to form sodium carbonate plus water and carbon dioxide. 
         [0013]    Turning now to  FIG. 2 , further detail is shown of an exemplary embodiment in which an aqueous solution of sodium chloride is subjected to hydrolysis to generate the sodium hydroxide for use in the carbonation reaction. As shown in  FIG. 2 , a system/process  20  is shown with an electrolysis cell reactor  22  having an anode chamber  24  and a cathode chamber  26  separated by a membrane  25  that is impermeable to water, but selectively permeable to ions. Anode chamber  24  is initially charged with a solution or brine of sodium chloride and cathode chamber  26  is initially charged with water. The cathode and anode are subjected to a difference in electronegative potential through an external circuit to drive the electrolysis reaction, as is well-known in the art. The resulting electrochemical reactions produce hydroxide ions in the cathode chamber  26 , which combine with sodium ions that migrate from the anode chamber  25  through the membrane  25  to form sodium hydroxide. The resulting aqueous sodium hydroxide solution  16  exits the cathode chamber  26 , is combined with a compound according to formula (I) from stream  44 , and is delivered to the carbonation reactor  12 . A hydrogen stream  28  is also produced in the cathode chamber  26 . The anode chamber  24  produces a chlorine gas stream  30  and a dilute brine solution stream  17 . The hydrogen stream  28  and the chlorine gas stream  30  are delivered to HCl reactor  32 , where they are reacted together in a highly exothermic reaction to form hydrogen chloride stream, which is delivered to CO 2 -generating reactor  38 . Heat generated by the HCl reactor  32  can be used to provide heat for other parts of the process or for generating electricity, as is known in the art. The hydrogen chloride stream  34  is delivered to the CO 2 -generating reactor  38 , where it reacts with calcium carbonate, which can be provided by limestone as a raw material. The reaction of hydrogen chloride with calcium carbonate produces a carbon dioxide stream  14 , which is delivered to the carbonation reactor  12 . The CO 2 -generating reactor  38  also produces calcium chloride  39 , which can be disposed of as a waste stream and/or used in other process steps such as purification (e.g., desulfation) of brine compositions for use in the electrolysis reaction. 
         [0014]    In some embodiments, the products of the electrolysis reaction can also be used to generate sodium hypochlorite as the compound according to formula (I) for the carbonation reaction. As shown in  FIG. 2 , a portion of the chlorine gas stream  30  is directed to sodium hypochlorite reactor  42 , where it is reacted with aqueous sodium hydroxide solution an electrolysis reaction to form sodium hypochlorite. This reaction is well-known in the art and does not require further detailed explanation. The aqueous sodium hydroxide solution for the sodium hypochlorite reaction can be provided by diverting a portion of the aqueous sodium hydroxide solution  16  produced by the electrolysis cell reactor  22  to the sodium hypochlorite reactor  42 . Water  40  can be added to provide the appropriate concentration level of sodium hydroxide in the sodium hypochlorite reactor  42 . Sodium hypochlorite in stream  44  produced by the sodium hypochlorite reactor  42  is then introduced to the aqueous sodium hydroxide solution  16  upstream of the carbonation reactor  12  for use in the carbonation reaction. 
         [0015]    Of course, the crude hydrated sodium carbonate  18  produced by the carbonation reactor is typically subjected to further purification processing in order to meet product specification targets. Such processing is shown in more detail in system  50  of  FIG. 3 , along with details about the provision of sodium chloride and water to the electrolysis cell reactor  22 . As shown in  FIG. 3 , the crude sodium carbonate stream  18 , which includes the reacted aqueous solution  16  and particles of hydrated sodium carbonate is delivered to separator  52 , which separates the hydrated sodium carbonate particles  18 ′ from the mother liquor  62 . The hydrated sodium carbonate particles  18 ′ are delivered to drying vessel  54 , where heated air stream  56  subjects the hydrated sodium carbonate particles  18 ′ to desiccation processing. Heat applied in the drying vessel  54  can also thermally decompose any sodium bicarbonate formed in the carbonation reactor  12  to form sodium carbonate, water, and CO 2 . Air/water vapor stream  58  is exhausted from the drying vessel  54 , while anhydrous sodium carbonate  18 ″ is produced as a final product. 
         [0016]      FIG. 3  also shows further details about processing of the liquid product of the anode chamber  24  of the electrolysis cell reactor  22 . This liquid is depleted of sodium ions that migrated to the cathode chamber  26  and of chloride ions that formed chlorine gas stream  30 , and can be referred to as a dilute brine. The dilute brine stream  17  is delivered to purification section  19 , where one or more purification stages can involve desulfation, dechlorination, and/or dechloratation. The purified dilute brine  21  is delivered to sodium chloride source  66  such as a salt storage pile. The mother liquor  62  from the separator  52  is also delivered to the sodium chloride source  66 , along with water  64 , to generate saturated sodium chloride brine  68 . The saturated sodium chloride brine  68  is delivered to purification section  70 , where it can be subjected to purification stages including desulfation, calcium removal, and/or magnesium removal (depending on the purity of the sodium chloride source  66 ), after which it may be subjected to mechanical vapor compression to re-saturate the solution with sodium chloride. Purified saturated sodium chloride solution  72  is delivered to the cathode chamber  26 , and water  23  is delivered to anode chamber  24  of the electrolysis cell reactor  22 . 
         [0017]    Although the present invention is not bound to or limited by any particular theory of operation, the carbonation reaction of carbon dioxide with aqueous sodium hydroxide is believed to proceed by nucleophilic addition to the electrophilic carbonyl groups on the carbon dioxide molecule. Again, not being bound by any particular theory of operation, the compound according to formula (I) such as sodium hypochlorite is believed to assist in the nucleophilic attack on the carbonyl groups to create the reactive carbonyl anion. Almost any amount of sodium hypochlorite can be used, with exemplary amounts ranging from 0.1 to 10 wt. % based on the total weight of the aqueous sodium hydroxide solution, more specifically from 0.5 to 5 wt. %, and even more specifically from 1 to 2 wt. %. 
         [0018]    The amount of sodium hydroxide used for the carbonation reaction can also vary widely. Although the compound according to formula (I) can be effectively used in conjunction with sodium hydroxide levels in excess of 25 wt. % such as the sodium hydroxide levels described in the above-referenced U.S. Pat. No. 7,708,972, it was quite surprising that the formula (I) compound could, in some embodiments, help to provide sufficient reactivity so that lower levels of sodium hydroxide can be used. In some embodiments, the sodium hydroxide level is less than 35 wt. %, more specifically less than 25 wt. %, even more specifically less than 23 wt. %, and even more specifically less than 20 wt. %, based on the total weight of the aqueous solution. In some embodiments, the molar ratio of [formula (I) compound]:[NaOH] can range from 2.69×10 −3 :1.0 to 2.69×10 −1  :1.0, more specifically from 1.34×10 −2 :1.0 to 1.34×10 −1  :1.0, and even more specifically from 2.69×10 −2 :1.0 to 5.36×10 −2 :1.0. 
         [0019]    The temperature of the carbonation reaction can also vary widely. Although formula (I) compounds such as sodium hypochlorite can be effectively used in conjunction with reaction temperatures in excess of 100° C. as described in the above-referenced US 2009/0260993 A1, it was quite surprising that the formula (I) compound could, in some embodiments, help to provide sufficient reactivity so that lower temperatures can be used. In some embodiments, the reaction temperature is less than 100° C., more specifically less than 105° C., even more specifically less than 110° C., and even more specifically less than 115° C. In some embodiments, exemplary reaction temperatures can range from 25° C. to 150° C. , more specifically from 50° C. to 100° C., and even more specifically from 70° C. to 85° C. 
         [0020]    The invention is further described below in the following non-limiting example(s). 
       EXAMPLES 
       [0021]    A reaction mixture was prepared by diluting 1000 mL of a (5% m/m) sodium hypochlorite solution with 3000 mL of water followed by the addition of 1000 grams of sodium hydroxide. The resulting solution was subsequently reacted with carbon dioxide gas which was introduced to the reaction mixture using an aerator. The reaction was conducted at atmospheric pressure at an autogenously generated temperature ranging from 60-85° C. The reaction was allowed to proceed for 90 minutes or until the production of sodium carbonate caused the formation of a mixture so dense that carbon dioxide could no longer be percolated through the sodium carbonate slurry. The sodium carbonate was recovered from the reaction vessel and dried in an oven at approximately 150° C. or in some instances the samples were dried using a microwave oven. The dried samples were heated further at 250° C. to dehydrate the sodium carbonate to its anhydrous form prior to analysis by acid titration. The isolated samples were determined to have greater than 99.5% purity. 
         [0022]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description.