Crystallization from solution recovery period

Sodium hydrosulfite is produced through the Formate process where sodium formate solution, sodium hydroxide, and liquid sulfur dioxide reacted in the presence of a recycled stream of methanol solvent. Other products are sodium sulfite, sodium bicarbonate, and carbon monoxide. In the reactor, sodium hydrosulfite is precipitated to form a slurry of sodium hydrosulfite in the solution of methanol, methyl formate, and other coproducts. The mixture is sent to a pressurized filter system to recover sodium hydrosulfite crystals that are dried in a steam-heated rotary drier before being packaged. Heat supply in this process is highly monitored in order not to decompose sodium hydrosulfite to sulfite. Purging is periodically carried out on the recycle stream, particularly those involving methanol, to avoid excessive buildup of impurities. Also, vaporized methanol from the drying process and liquors from the filtration process are recycled to the solvent recovery system to improve the efficiency of the plant. 

B. m-Nitrobenzazide. In a 2-1. round-bottomed flask fitted with an efficient mechanical stirrer is placed a solution of 78 g. (1.2 moles) of commercial sodium azide in 500 ml. of water (Note 3). The flask is surrounded by a water bath kept at 20-25°. The stirrer is started, and over a period of about 1 hour a solution of 185.5 g. (1 mole) of m-nitrobenzoyl chloride in 300 ml. of acetone (previously dried over anhydrous potassium carbonate) is added from a dropping funnel. wz-Nitrobenzazide separates at once as a white precipitate. Stirring is continued for 30 minutes after the addition is complete then 500 ml. of water is added and the reaction mixture stirred for an additional 30 minutes. The azide is separated on a suction filter, washed with water, and dried in the air. The yield of crude product, m.p. 68°, is 189 g. (98%) (Note 4). It may be recrystallized from a mixture of equal parts of benzene and ligroin (b.p. 100-140°), when the temperature is kept below 50° (Note 5). The product thus obtained consists of almost colorless crystals, m.p. 68-69° (Note 6), the recovery being 80-90% (Note 7). 

CH8)8NBH2C1 forms white crystals, m.p. 84-85°C., easily soluble in benzene and hot carbon tetrachloride and somewhat less soluble in cold carbon tetrachloride. The compound may be precipitated from carbon tetrachloride solution with cyclohexane to obtain good recovery of product and may be sublimed in vacuo at 60°C. Heating to 150°C. in an evacuated tube causes decomposition to (CH8)8NBHC12 and other products. (CH8) 8NBH2C1 is only difficultly soluble in water and hydrolyzes in cold aqueous methanol only slowly, even if the solution is made basic or acidic. The solid can be handled in the atmosphere without difficulty for short periods of time (a day or less). 

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