Carbon dioxide removal with alkali carbonate solutions

Petit A process for removing hydrogen sulfide and hydrogen cyanide from gas streams by scrubbing with an alkali carbonate solution and regenerating the liquor with carbon dioxide. Invented by T. P. L. Petit. 

Manufacture. Aqueous sodium hydroxide, sodium bicarbonate, sodium carbonate, or sodium sulfite solution are treated with sulfur dioxide to produce sodium metabisulfite solution. In one operation, the mother liquor from the previous batch is reinforced with additional sodium carbonate, which need not be totally in solution, and then is treated with sulfur dioxide (341,342). In some plants, the reaction is conducted in a series of two or more stainless steel vessels or columns in which the sulfur dioxide is passed countercurrent to the alkali. The solution is cooled and the sodium metabisulfite is removed by centrifuging or filtration. Rapid drying, eg, in a stream-heated shelf dryer or a flash dryer, avoids excessive decomposition or oxidation to which moist sodium metabisulfite is susceptible. 

Hydroxymercuri phenolphthalein.— To a solution of 3 grams of phenolphthalein in 25 c.c. Normal sodium hydroxide, about 125 c.c. of water are added and the solution boiled with 3 grams of yellow mercuric oxide for three hours. The mixture is then allowed to settle, the liquor poured off and centrifuged to remove traces of sediment. Carbon dioxide is passed in, and the mercurated product separates as a purj>lish, milky precipitate, which is separated by centrifuging. It is washed with alcohol and dried at 110 C. It is insoluble in all the usual solvents, except glacial acetic acid, in which it gives turbid solutions. It cannot be crystallised, and gives the same colour with alkali as phenolphthalein. 

Sodium 4-methylphenoxide solution was dehydrated azeotropically with chlorobenzene, and the filtered solid was dried in an oven, where it soon ignited and glowed locally. This continued for 30 min after it was removed from the oven. A substituted potassium phenoxide, prepared differently, also ignited on heating. Finely divided and moist alkali phenoxides may be prone to vigorous oxidation (or perhaps reaction with carbon dioxide) when heated in air. 

B. 2,5-Dimethylmandelic acid. A mixture prepared from 140 g. (0.5 mole) of 2,5-dimethylphenylhydroxymalonic ester and a cold solution of 140 g. of potassium hydroxide in 560 ml. of water in a 1-1. round-bottomed flask is warmed on a steam bath for 5 hours. The alkaline solution is cooled and extracted with one 100-ml. portion of ether to remove any material not soluble in alkali. The alkaline solution is acidified with 300 ml. of concentrated hydrochloric acid and then warmed on a steam bath and stirred for 2 hours (or until there is no further evidence that carbon dioxide is escaping). The mixture is cooled, the oily layer is extracted with ether, the ethereal solution is dried with anhydrous sodium sulfate, and the ether is distilled under partial vacuum (Note 3). The oily residue is crystallized from benzene. The yield of 2,5-dimethyImandelic acid melting at 116.5-117° is 55-63 g. (63-70%) (Note 4). 

Reaction XXXVI. Condensation of Carbon Tetrachloride with Phenols and simultaneous Hydrolysis (Tiemann-Reimer). (B., 10, 2185.)—This reaction is closely analogous to that of the formation of hydroxy-aldehydes by means of chloroform and caustic alkali (see p. 104). A mixture of a phenol, carbon tetrachloride and caustic soda or caustic potash solution is boiled. Condensation occurs, chiefly in the para-position, but small amounts of the ortho-acids are also formed. The product, after the excess of carbon tetrachloride has been removed, is saturated with carbon dioxide and the unchanged phenol extracted with ether. The hydroxy acids are then precipitated by acidification with hydrochloric acid. 

Methyl-2-ethylpropane-l,3-diol.42 To a mixture of 46 g. of methyl-ethylacetaldehyde and 82 g. of formalin (40%) is added with cooling a 12% alcoholic solution of pqtassium hydroxide containing 23 g. of potassium hydroxide. After twelve hours the excess alkali is neutralized by passing in carbon dioxide, and most of the alcohol is removed by distillation. The aqueous residue is extracted thoroughly with ether, and after removal of the ether the producl is distilled. The yield of the glycol, b.p. 218-220°, is nearly the theoretical amount.. It melts at 42°. 

While the distillation is continuing, two or three crystals of barium chloride are added to the distillate in the first Erlenmeyer flask. This distillate is then boiled gently for several seconds to remove carbon dioxide as well as to ascertain if any turbidity appears (Note 6). If the distillate remains clear, it is titrated with standard 0.01 N sodium hydroxide solution after addition of four to five drops of the phenolphthalein solution (Note 7). All further distillates are treated as described above. If less than 4ml of the standard alkali solution are required for the titration of the first distillate, the next 10ml of distillate is similarly titrated. If the acetic acid in the initial distillate consumes more than 4ml of standard alkali then three additional 5-ml distillates are collected and individually titrated as described above. When the indicator changes color upon the addition of the first drop of the alkaline solution, the analysis is complete. 

The silver oxide used may be the commercially available material or that freshly prepared by adding, with stirring, 8.8 g. of sodium hydroxide in 80 ml. of water to a solution of 34 g. of shver nitrate in 200 ml. of water. The precipitate is collected by filtration and washed with water to remove the bulk of the alkali. The wet cake is used directly for preparation of the quaternary hydroxide. The strongly basic quaternary hydroxide solution should be protected from excessive exposure to air because of carbon dioxide absorption. 

It is difficult to remove the hydrazobenzene from the iron sludge at this point, and a number of modifications of this process have consequently been advocated. One method involves the removal of azobenzene and reducing this compound by zinc in alcoholic alkaline solution at 60 C. The reduction mass is filtered, and the zinc residues are boiled up with fresh alcohol. The filtrate separates into two layers, of which the lower contains aqueous sodium zincate, while the upper is an alcoholic solution of hydrazobenzene. The alcoholic layer is separated and saturated with carbon dioxide to precipitate the alkali. After filtering, the alcoholic solution is evaporated to obtain the hydrazobenzene, for which practically quantitative yields have been claimed. 

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