Ammonium Carbamate decomposition

A once-through modification of a urea process is shown in Fig. 11.11. Recycle of unconverted ammonium carbamate and of ammonia and carbon dioxide formed in part from ammonium carbamate decomposition (Eq. 11.60) is practiced in other variations on this theme [64]. 

Ammonia synthesis process with decreased ammonium carbamate decomposition in the compressor. (Mitsubishi). JP 48008699 (1973). 

The reaction occurs in two steps ammonium carbamate is formed first, followed by a decomposition step of the carbamate to urea and water. The first reaction is exothermic, and the equilibrium is favored at lower temperatures and higher pressures. Higher operating pressures are also desirable for the separation absorption step that results in a higher carbamate solution concentration. A higher ammonia ratio than stoichiometric is used to compensate for the ammonia that dissolves in the melt. The reactor temperature ranges between 170-220°C at a pressure of about 200 atmospheres. 

Ammonium carbamate (solid precursor) leads to NH3 by decomposition. 

Urea, which is used for the production of urea-formaldehyde resins, is made by the in situ decomposition of ammonium carbamate, which is made by the condensation of ammonia and carbon dioxide at 200°C and 200 atm. 

Imidodisulphamide, NH2.S02.NH.S02.NH2, is the first product of the action of ammonium carbamate on pyrosulphuryl chloride.2 It is also obtained when sulphamide is heated nearly to its melting-point. Imidodisulphamide forms needle-shaped colourless crystals, melting at 160° C., and having a distinctly acid taste, since water causes rapid decomposition into sulphamide and amidosulphonic acid ... 

Ammonium carbamate is prepared from dry ice and liquid ammonia [14]. These conditions are very similar to the conditions under which we have observed the formation of amine salts. To some readers, ammonium carbamate may seem to be an exotic compound. In fact, it is manufactured industrially on a multiton scale, because on heating (usually at 100-185°C) ammonium carbamate is converted to urea and water [14-16]. Urea is important for both the agricultural and the plastics industries. The ammonium carbamate is not always isolated during urea preparation. Instead, the reactions are carried out under conditions where the carbamate is just an intermediate. Ammonium carbamate is only moderately stable and it gradually loses ammonia in air. Although the data are sparse, the rate of decomposition of carbamates in solution seems to decrease as the volatility of the parent amine decreases [17]. Free carbamic acids in solution do not decompose spontaneously to free amine and C02. Instead, the acid ionizes by reaction with water the proton is transferred from the hydronium ion to the amine and then decomposition occurs [17]. Acids catalyze the decomposition. 

The decomposition of solid ammonium carbamate, (NH4)(NH2C02), to gaseous ammonia and carbon dioxide is an endothermic reaction. 

This exothermic reaction is followed by an endothermic decomposition of the ammonium carbamate ... 

In the process, a 2 1 molar ratio of ammonia and carbon dioxide (excess ammonia) are heated in the reactor for 2 hours at 190°C and 1500 to 3000 psi (10.3 to 20.6 MPa)to form ammonium carbamate, with most of the heat of reaction carried away as useful process steam. The carbamate decomposition reaction is both slow and endothermic. The mix of unreacted reagents and carbamate flows to the reactor-decomposer. The reactor must be heated to force the reaction to proceed. For all the unreacted gases and undecomposed carbamate to be removed from the product, the urea must be heated at lower pressure (400 kPa). The reagents are reacted and... 

The reactor effiuent, consisting of urea, ammonium carbamate and unconverted reactants, is subjected, by altering the operating conditions, to a decomposition that converts part of the ammonium carbamate to carbon gaseous product is compressed and condensed. This leads to renewed formation of carbamate which is recycled to the reactor in aqueous s ution, while the excess ammonia is mixed with fresh ammonia. The entire operation is repeated to decompose aO the carbamate. The final solution obtained contains 72 to 76 per cent weight of urea, and the fmal purity desired can be obtained by a finishing treatment. 

This thermodynamically favorable step of the process is followed by an endothermic thermal decomposition of ammonium carbamate, in a concentrated solution, to give a 50-60% conversion to urea (Eq. 11.59). 

This was followed by the Stamicarbon II process, based on the decomposition of ammonium carbamate bv carbon dioxide stripping (Fig. 1.33). The urea synthesis loop features four main steps in this case ... 

Ammonia is also produced from burning of coal and bacterial decomposition of proteinaceous organic matter. Conversion of ammonia to ammonium salts will occur rapidly both in the atmosphere and in the soil but will result in less acidity of the soil. It is not known to what extent ammonia reduces excess greenhouse gases such as carbon dioxide in the atmosphere to form ammonium carbamate. Microbial assimilation of ammonium compounds consumes alkalinity of the soil as well as converting ammonium ions to nitrate. Nitrification is carried out by many aerobic bacteria. Nitrification is pH sensitive. Denitrification, which is an anaerobic process, is also pH sensitive. 

The decomposition of ammonium carbamate or ammonium carbonate in order to yield NH3 are both thought to proceed via a two-step reaction. For ammonium... 

As ammonium carbamate stores more NH3, does not produce H2O during decomposition and needs less heat for decomposition, it is preferred over ammonium carbonate. Despite the difficulties associated with the dynamically controlled decomposition of ammonium carbamate, the system was tested for commercial applications [90, 91]. A light-duty application on a 5.9 L Diesel engine was presented and the use of ammonium carbamate was protected by the name Solid SCR [92]. 

Ammonia and nitric acid have been selected as process examples in this book and are treated in detail in Sections 6.1 and 6.4, respectively. Urea is produced industrially by reaction of ammonia and CO2 via the intermediate product ammonium carbamate ([H2N-COO][NH4]). While the formation of the carbamate intermediate is exothermic and quantitative under the applied reaction conditions (200 °C, 250bar), urea forms from the intermediate by liberation of vater in a slightly endothermic equilibrium reaction. Existing process technologies differ in their ways of carbamate decomposition as well as ammonia and CO2 recycling. State-of-the-art urea plants produce up to 1.700 tons of urea per day and are often linked to ammonia plants as CO2 is a by-product of NH3 production from natural gas. 

Due to the incomplete second reaction, the reactor outlet mixture contains significant amounts of ammonium carbamate in addition to urea and water. The ammonium carbamate is usually removed by decomposing into its constituents ammonia and carbon dioxide (reverse reaction of Eq. (3.15)) via increasing temperature and decreasing pressure [12]. Stripping using ammonia or carbon dioxide also supports ammonia carbamate decomposition [12] (see also process description in Section 3.3.2) and, in addition, removes the formed ammonia and carbon dioxide from the hquid phase. 

Reactions of ammonia and carbon dioxide forming ammonium carbamate constituted the first step, which is exothermic in nature. This is followed by the endothermic decomposition of ammonium carbamate, through dehydration, in the second step to form urea. Recently, Xiang et al. [280] reported urea synthesis at atmosphere pressure and ambient temperature. 

Fig. 8.5. Proton-induced decomposition of W-substituted carbamic acid C into carbon dioxide and a primary ammonium hydrochloride—a reaction occuring in the course of acidic hydrolysis of organic isocyanates.

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