Thermal decomposition of ammonium salts of nicotinic acid and its analogs in the liquid phase

Abstract

The ability of ammonium salts to undergo thermal decom- position with the formation of gaseous ammonia and free acid is well known (I - 3). The thermal decomposition can proceed both in the solid phase by the heating of the dry salt in a stream of inert gas (2 - 4), and in the liquid phase by heating the aque- ous solution of the salt (5, 6). The liquid-phase process has great industrial significance -- it is employed in the regeneration of ammonia in the cyclic phosphate process (5 - 7), as well as in the chemico-pharma- ceutical industry for the distillation of ammonia-containing waste water (8, 9). In the technology of vitamin PP, this proc- ess is utilized to obtain nicotinic acid from the solution of am- monium nicotinate (10, 11). In our opinion, the given method for the decomposition of ammonium salts can serve as the basis for the practically waste-flee method for the isolation of some carboxylic acids widely applied in the pharmaceutical industry by the ammo- niacal hydrolysis of the corresponding nit'riles. When the temperature of the solution is raised, the equi- librium of the reaction (2) is shiRed to the right. The free am- monia thereby formed can be distilled off with the water vapor, and the non-volatile acid will remain in the liquid phase. According to the existing classification, the given proc- ess pertains to reaction-distillation (chemodesorption) proc- esses ( 12). At the present time, processes of the chemodesorp- tion of ammonia have been studied fragmentarily, and only from positions of the equilibrium of the hydrolysis reaction (2) (5, 6, 10). The object of the present work was to study the main ki- netic features of the process of the chemodesorption of ammo- nia from aqueous solutions of ammonium salts. The process was conducted in a special glass distillation assembly in conformity with recommendations ( 13). The con- struction of the assembly allowed air to be bubbled through the solution being distilled and/or the stirring of the solution with a stirrer, as well as the dispensing of water into it through a dropping funnel. Analysis of the distillate and the residue from the distillation was performed by the method ofpotentiometric titration and direct ionometry using a"Radelkis" selective am- monia electrode. The detailed technology of the analysis ap- plied was described in ( 14). Experiments to study the rate of thermal decomposition were performed using the irreversible regime with the constant distillation of ammonia from the reaction flask with the water vapor. The volume of the solution in the flask was thereby kept constant with the dispensing of water through the dropping funnel in the amount equal to that removed for the analysis of the distillate. The rate of the process was determined from the initial conversion when the degree of conversion of the ammo- nium salt was less than 2 rel. %. For all the investigated salts, the rate of the process was constant in the range of degrees of conversion from 0 to 3%. In order to study the influence of diffusion factors on the process of liquid-phase thermal decomposition, we conducted a special series of experiments with ammonium nicotinate. It was shown that, in the range of concentrations of the solution 0.1 - 0.4 M at 100~ neither mechanical stirring with the stir- rer at 100 - 500 rpm nor the bubbling of air through the system