Aqueous Urea Decomposition Reactor: Reaction Modeling and Scale Up

Abstract

This Article will illustrate an application of the principles discussed in previous sections such as scale-up, reaction modeling, and rate coefficient estimation. The example to be discussed involves the production of ammonia gas by decomposition of an aqueous solution of urea reactant at about 40 wt% urea to be used for reduction of NOx in Power plants. A large commercial size reactor of the kettle type (Figure 1) was designed based on data gathered from smaller size demonstration reactors and a laboratory scale unit. The reactor was designed to decompose an aqueous solution of urea into ammonia (2,000 lbs/hr) and carbon dioxide gas. Ammonia gas was to be used as a reducing agent in a power plant. Steam is used as the source of heat since the decomposition is endothermic. (1) Unfortunately, the commercial unit's performance was poor and the reactor had to operate at higher temperatures relatively speaking. The smaller units (400 lbs/hr ammonia magnitude) operated at 300°F (about 100 psig pressure) while the larger commercial units operated at 320°F. Nevertheless, the commercial reactors did not achieve design conversion of urea. (2) Also the reactor was sized with a large safety factor because of uncertainty in the scale-up from the smaller units. (3) There seemed to be discrepancies in the performance data that had been collected from the lab and demonstration size reactors. (4) There were issues with the scale-up of the urea reactant into the kettle reactor. The main objectives of the analysis are: To review the basis for scale-up and then design of the reactor to improve performance to achieve higher conversion of urea to ammonia gas Model and recommend proper urea feed reactant assembly into the kettle reactor at commercial sizes. Review the chemical kinetics for the reactor design. As well as to be able to reduce the size of the reactor for cost savings. Aqueous urea decomposition chemistry Without the development of relevant chemistry for aqueous urea decomposition, the reactor modeling and design would mean very little. This this is the first step in the analysis [1]. The overall decomposition of urea in water is known to occur according to the following reaction, NH 2 CONH 2 (aq) + H 2 O (l) < = = > 2NH 3 (g) + CO 2 (g) (1) 113 H kJ mol ∆ = + / However, there is evidence that the reaction takes place in two major steps [2,3], NH 2 CONH 2 (aq) + H 2 O (l) < = = >NH 4 COONH 2 (aq) (2) 23 H kJ mol ∆ = − / which leads to the formation of ammonium carbamate, and NH 4 COONH 2 (aq) < = = > 2NH 3 (g) + CO 2 (g) (3) 136 H kJ mol ∆ = + / (By difference) Where decomposition of carbamate leads to ammonia and carbon dioxide in the gas phase. The overall reaction (1) is highly endothermic, or +113 kJ/mol. Similarly, the decomposition of carbamate to ammonia (reaction 3) is endothermic or +136 kJ/mol. The formation of Carbamate by reaction (2) is slightly exothermic. The literature value for the heat of reaction (1) represents 809 btu/ lb Urea. This value is lower by 12% than the value used during design of the commercial units. The source of this difference could not be determined.