Countercurrent sublimation

In this process, ammonia solution flows countercurrent to the ascending stream of CO2. Crystals of ammonium bicarbonate precipitate out when the solution becomes sufficiently saturated. The crystals are filtered or centrifuged out of the mother liquor, washed, and air-dried. Pure product may be obtained by using high purity CO2. Alternatively, high purity ammonium bicarbonate may be obtained by subliming the product formed at relatively low temperatures. 

The product must be sublimed. Thus, the boat is transferred, in a countercurrent stream of Ng, to the other end of the reactor tube and the temperature is raised to 115 C. The beginning of the sublimation is noticeable by a brown deposit at a later stage, beautiful, flexible, deep brown-black, glittering crystals, up to 15 mm. long, are formed. 

Rosenkrantz (1957) has written one of the earlier articles on the utilization of fractionation procedures with infrared analysis and has listed several types of fractionation techniques chromatography, countercurrent distribution, preferential solvent extraction, sublimation, fractional crystallization, molecular distillation, dialysis, centrifugation, electrophoresis, diffusion, and freeze-drying. He has also given references to work in which these methods have been used to fractionate a large variety of biological compounds. Elvidge and Sammes (1966) have discussed many of the techniques mentioned above. 

If the product is a solid, recrystallization is in most cases a highly selective process, and is applied directly upon removal of the ether or pentane. If the desired substance will not crystallize well at this stage, distillation, sublimation, liquid chromatography (with <30 g), preparative gas chromatography (with <20 g), or countercurrent distribution (with <100 g) may effect sufficient purification themselves, or they may be followed by recrystallization to give sufficiently pure material. For extreme purity (>99.99% has been claimed in some cases), zone refining can be used as a final step. 

The separation of a mixture containing sublimable components is carried out by fractional sublimation in countercurrent columns. The finely distributed solid phase trickles down the column countercurrently to the upflowing sublimate vapor (Fig. 7-43). For the treatment of binary mixtures, the lighter sublimable component is withdrawn from the top and the heavier sublimable component is withdrawn from the bottom of the column. Some of the lighter sublimable component is used as solid reflux at the top of the column [7.55]. 

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