(185b) Mechanistic Model Development of Amine Hydrochloride Salts Production in a Confined Impinging Jet Reactor

The synthesis of polyurethane involves a series of undesired side reactions that result in precipitation of highly insoluble amine hydrochloride (AHC) salts. The production of these salts means loss of starting material and costly reprocessing. Using joint experimental and theoretical studies it is necessary to get a better understanding of these reactions, and identify conditions when the formation of precipitates is either mitigated or the size of the particles being produced is negligibly small.

The formation of AHC salts during the precipitation reaction between 4,4'-methylene dianiline (MDA) and hydrogen chloride (HCl) has been studied experimentally in a confined impinging jet reactor (CIJR) [N. F. Ershad 2013. Concentration and mixing effects on the production of amine hydrochloride salts in a confined impinging jet reactor. MSc thesis, University of Alberta, Edmonton]. A mechanistic numerical model that describes the experimental processes in CIJR is now developed. The model involves three simultaneously occurring phenomena: chemical reactions, crystallization, and mixing of reacting species and precipitated particles. Chemical reactions lead to the supersaturation buildup. This supersaturation is relieved by particle nucleation and growth.  Agglomeration of the particles due to collisions resulting from Brownian motion and shear-induced collisions is also taken into account. As soon as the particles appear, the transient behaviour of the particle size distribution (PSD) is tracked by solution of the population balance equations. The equations governing reaction crystallization and PSD evolution are coupled with a mixing model that incorporates meso and micro mixing effects in the CIJR. Within the mixing model, the reactor is split into several interacting zones characterized by different values of the energy dissipation rate, entrainment from the surroundings, and residence times.

The empirical parameters of the model are obtained by fitting the numerical results and experimental observations in terms of the size of the particles. The unknown kinetics of the chemical reactions are determined such that the simulations mimic the following experimental result: the particles can be either methylene dianiline monohydrochloride MDA.HCl or methylene dianiline dihydrochloride MDA.2HCl. The particle species strongly depend on the local concentration of amine (the MDA blend strength). The model allows us to investigate the effect of local HCl concentration (percent excess of HCl) and mixing intensity (injection flow rate) on the size and composition of the particles and develop a guideline on how to achieve and control a desired PSD of a certain salt type throughout the process.