Carbon dioxide removal with ammonia solutions

This carbon dioxide-free solution is usually treated in an external, weU-agitated liming tank called a "prelimer." Then the ammonium chloride reacts with milk of lime and the resultant ammonia gas is vented back to the distiller. Hot calcium chloride solution, containing residual ammonia in the form of ammonium hydroxide, flows back to a lower section of the distiller. Low pressure steam sweeps practically all of the ammonia out of the limed solution. The final solution, known as "distiller waste," contains calcium chloride, unreacted sodium chloride, and excess lime. It is diluted by the condensed steam and the water in which the lime was conveyed to the reaction. Distiller waste also contains inert soHds brought in with the lime. In some plants, calcium chloride [10045-52-4], CaCl, is recovered from part of this solution. Close control of the distillation process is requited in order to thoroughly strip carbon dioxide, avoid waste of lime, and achieve nearly complete ammonia recovery. The hot (56°C) mixture of wet ammonia and carbon dioxide leaving the top of the distiller is cooled to remove water vapor before being sent back to the ammonia absorber. 

The product stream leaving the stripper goes through an expansion valve to the low-pressure section, the operating pressure there being 5 bar. In a steam-heated rectifier, further ammonia and carbon dioxide are removed and, with some water vapour, are condensed to give a weak carbamate solution. This is pumped back to the high-pressure condenser. 

This process for production of synthetic ammonia by catalytic steam reforming of natural gas is a relatively clean process and presents no unique environmental problems. To assess the environmental impacts of a modem ammonia plant on air, water, and soil, each step in the ammonia synthesis namely, desulfurization, reforming, shift conversion, carbon dioxide removal, final purification, ammonia synthesis, and refrigeration should be examined. The sources of pollutants need to be identified and matched with cost-effective solutions for minimization/elimination by using the best available pollution control measure. 

Aminobutyric acid 516 Ammonium carbonate (450 g, 8 moles of NH3) is warmed to 55° with water (140 ml), cooled to 40°, and treated with aqueous ammonia (410 ml, 6 moles of NH3). Still at this temperature 2-bromobutyric acid (167 g) is added gradually during 30 min. The whole is set aside for 24 h at 40-50°, then ammonia and carbon dioxide are removed on the water-bath, after which the solution is concentrated in an evaporating dish until the amino add separates. After cooling, this product is filtered off and washed twice with a little methanol. Evaporation of the filtrate to 125 ml and addition of methanol (250 ml) give a second fraction. The total yield of pure amino acid is 59-62 g (57-60%). 

The dehydration of ammonium carbamate is appreciable only at temperatures above the melting point (about 150°C) and this reaction can only proceed if the combined partial pressure of ammonia and carbon dioxide exceeds the dissociation pressure of the ammonium carbamate (about 100 atmospheres at 160°C and about 300 atmospheres at 200°C). Thus commercial processes are operated in the liquid phase at 160—220°C and 180—350 atmospheres. Generally, a stoichiometric excess of ammonia is employed, molar ratios of up to 6 1 being used. The dehydration of ammonium carbamate to urea proceeds to about 50—65% in most processes. The reactor effluent therefore consists of urea, water, ammonium carbamate and the excess of ammonia. Various techniques are used for separating the components. In one process the effluent is let down in pressure and heated at about 155°C to decompose the carbamate into ammonia and carbon dioxide. The gases are removed and cooled. All the carbon dioxide present reacts with the stoichiometric amount of ammonia to re-form carbamate, which is then dissolved in a small quantity of water and returned to the reactor. The remaining ammonia is liquefled and recycled to the reactor. Fresh make-up ammonia and carbon dioxide are also introduced into the reactor. Removal of ammonium carbamate and ammonia from the reactor effluent leaves an aqueous solution of urea. The solution is partially evaporated and then urea is isolated by recrystallization. Ammonium carbamate is very corrosive and at one time it was necessary to use silver-lined equipment but now satisfactory alloy steel plant is available. Urea is a white crystalline solid, m.p. 133°C. 

The Stamicarbon process is based upon the use of carbon dioxide to strip ammonia from the reactor effluent countercurrently. This process has become the most popular with new plants. As the carbon dioxide removes the ammonia from the solution, the carbamate decomposes, leaving a minimum amount in the effluent. The solution leaves the stripper at about 150 to 180°C. The stripped gases flow to the reactor along with ammonia in an amount equivalent to the amount of carbon dioxide added for stripping. Most of the off-gas from the reactor is condensed, and inert gases are bled from the system before the condensate is returned to the base of the reactor. This process is shown in Fig. 28.19. 

The solvents most used in carbon dioxide removal from ammonia synthesis gas can be characterized according to the nature of the absorption process. Chemical absorption, i.e. processes where the carbon dioxide reacts with the solvent by a chemical reaction which is reversed in the solvent regeneration stage, is most often based on the use of alkanolamines, mainly MEA (mono-ethanolamine) [273], or hot solutions of potassium carbonate [274] as solvents. 

Several revamp options are available for modification of the carbon dioxide removal section depending on the type of carbon dioxide removal process. The processes mostly used in ammonia plants are chemical absorption processes based on either hot potassium carbonate (HPC) such as Benfield, or Vetrocoke, or amine solutions such as MEA. The chemical carbon dioxide removal processes may be improved or replaced with a physical process in which the absorbent is regenerated by simply flashing off carbon dioxide. In this way the need for regeneration heat may be reduced or eliminated. A physical carbon dioxide removal system may result in energy savings of 0.01-0.35 Gcal/MT ammonia. 

A.lkanolamine Process. Carbon dioxide is an acidic gas that reacts reversibly with aqueous alkaline solution to form a carbonate adduct. This adduct decomposes upon the addition of low level heat faciUtating CO2 removal. An aqueous solution of 15—20 wt % monoethanolamine (MEA) was the standard method for removing CO2 in early ammonia plants. 

This ammonia is recycled to the reactor via a compressor and a heater. Liquid ammonia is used as reflux on the top of the absorber. The net amount of carbon dioxide formed in the reactor is removed as bottom product from the absorber in the form of a weak ammonium carbamate solution, which is concentrated in a desorber-washing column system. The bottom product of this washing column is a concentrated ammonium carbamate solution which is reprocessed in a urea plant. The top product, pure ammonia, is Hquefted and used as reflux together with Hquid makeup ammonia. The desorber bottom product, practically pure water, is used in the quench system in addition to the recycled mother Hquor. 

About 14 g of choline chloride are stirred with a solution of about 20 g of phosgene in 100 g of chloroform for about two hours at room temperature. The mixture becomes a two-phase liquid mixture. Hydrochloric acid and excess phosgene are removed by distillation in vacuo. Chloroform is added to the syrup, and the mixture is then added to a solution of excess ammonia in chloroform which was cooled with solid carbon dioxide-acetone. The mixture is... 

Kasuganobiosamine (4) and Kasugamycinic Acid (9a) by cold Alkaline Hydrolysis. Kasugamycin hydrochloride (622 mg., 1.43 mmoles) was dissolved in 5 ml. of water free from carbon dioxide and 50 ml. of water saturated with barium hydroxide was added. The solution was allowed to stand at room temperature for 36 hours. Ammonia (0.30 mmole) was produced and barium oxalate (199 mg., 0.80 mmole) was obtained. After removal of barium oxalate by filtering, the filtrate was neutralized with dry ice. After removal of barium carbonate by filtering, the filtrate was adjusted to pH 7.0 and placed on a column of Amberlite CG-50 (ammonium form, 1.5 x 22 cm.), allowed to pass with a rate of... 

The submitters used 55 cc. of hydrochloric acid at this point the checkers stated that this amount was insufficient to neutralize the mixture. The purpose of acidification at this point is not to liberate the cyclobutanedicarboxylic acid, but merely to remove carbonates and excess potassium hydroxide. After the carbon dioxide has been expelled, the solution is made alkaline with ammonia hence a great excess of hydrochloric acid should be avoided. The submitters used only enough hydrochloric acid to make the solution acid to litmus. After the solution has been made basic with ammonia, barium chloride solution is added until there is no further precipitation of barium malonate. 

The solution obtained, after removal of insoluble barium sulphate, is yellow, strongly alkaline, and decomposes on heating with loss of ammonia. It readily absorbs carbon dioxide, forming a soluble carbonate. 

The substance is best prepared by dissolving cobaltous carbonate in the smallest possible quantity of hydrochloric acid, treating the cold solution with a mixture of concentrated aqueous ammonia and ammonium carbonate, and oxidising by means of a stream of air drawn through the liquid. When oxidation is complete ammonium chloride is added and the whole evaporated to a syrup dilute hydrochloric acid is added to remove carbon dioxide, and the liquid is saturated with ammonia gas to decompose any tetrammino-salt formed. On the addition of concentrated hydrochloric acid the salt crystallises out on cooling.6... 

Rh (NHo) 5C1] (OH) 2, is produced by mixing the chloride with moist silver oxide. Silver chloride is precipitated, and a strongly alkaline liquid produced containing the hydroxide. The base absorbs carbon dioxide from the air, removes ammonia from ammonium salts, and precipitates metallic hydroxides from solutions of the metallic salts. It is only known in solution, and on evaporation of the liquid it is slowly transformed in the cold, more rapidly on heating, into a mixture of aquo-pentammino-rhodium chloride and aquo-pentammino-rhodium hydroxide, thus ... 

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