Carbon dioxide, purification compounds

After bulk removal of carbon monoxide in the shift conversion section and of carbon dioxide in the carbon dioxide removal section, the synthesis gas still contains typically 0.2-0.5 vol% carbon monoxide and 0.01-0.2 vol% carbon dioxide. These compounds must, together with any water present, be removed quantitatively, i.e. to low ppm levels, before the gas can be admitted to the synthesis converter, because all oxygen containing compounds are poisons to the ammonia synthesis catalysts [238]. The most important technologies for this final purification are discussed in the following. 

Final Purification. Oxygen containing compounds (CO, CO2, H2O) poison the ammonia synthesis catalyst and must be effectively removed or converted to inert species before entering the synthesis loop. Additionally, the presence of carbon dioxide in the synthesis gas can lead to the formation of ammonium carbamate, which can cause fouHng and stress-corrosion cracking in the compressor. Most plants use methanation to convert carbon oxides to methane. Cryogenic processes that are suitable for purification of synthesis gas have also been developed. 

All the above-mentioned initiators are very sensitive towards substances with active hydrogen. Care must therefore be taken to exclude acids, water, thiols, amines, and acetylene derivatives. Oxygen, carbon dioxide, carbon monoxide, carbonyl compounds, and alkyl halides which can react with the initiator, also interfere with the reaction. Careful purification and drying of the starting materials and apparatus is, therefore, absolutely essential, especially when dealing with living polymers (see Example 3-19). 

Calcium carbonate is obtained from natural limestone deposits. The purified compound, known as precipitated calcium carbonate, is synthesized from limestone. Limestone is calcined to calcium oxide and carbon dioxide in a kiln. The products are recombined after purification. Calcium oxide is hydrated... 

The preparation and purification of this compound is identical with that of cycZoselenobutane, the aS-tetramethylene dibromide in the latter case now being replaced by 27 grams of ae-pentamethylene dibromide. The product need only be distilled under ordinary pressure in a stream of carbon dioxide to effect purification. It is a colourless or very faintly yellow liquid, B.pt. 158° C. at 759 mm., nv 1 5475 at 18° C., density 1 409 at 12 5° C., 1 899 at 20° C., 1 892 at 26° C., and 1 884 at 32 2° C., whence d ° - 1 424 0-001236f. The compound has similar properties to those described for o/cZoselenobutane. The mercurichloride, C6H10Se. HgCl2, forms white, feathery needles, M.pt. 175° to 176° C. cycloSelenipentane I 1 -dichloride,... 

In further purification, carbon dioxide, residual carbon monoxide, and sulfur compounds (only present in the synthesis gas from partial oxidation) have to be removed as they are not only a useless ballast but above all poisons for the ammonia synthesis catalyst. 

Microcystins are an increasingly important group of bioactive compounds, produced mainly by planktonic cyanobacteria. They are a family of cyclic heptapeptides that cause both acute and chronic toxicity. Purified microcystins are utilized in a range of research applications. This review summarizes the isolation of microcystins from the cyanobacteria by supercritical fluid extraction (SFE). The microcystins can be successfully extracted when a modifier is used in supercritical carbon dioxide fluid. The advantage of the method is that the sample handling steps are minimized, thus reducing possible losses of microcystin and saving extraction and purification time. 

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