(60d) Polymerization in a Spray Dryer – Designing a Pre-Reaction before Atomization to Boost the Reaction in the Spray

Abstract

Powder polymers with well defined size and a spherical shape are commonly generated by the use of spraying devices, e. g. spray dryer. Here polymers in solution or suspensions of polymers are atomized to small droplets and dried to particles. A huge advantage of the high specific surface area after atomization is the fast drying rate. In addition the low process time ensures a gentle drying at a high process temperature but low surface temperature, so glass transition temperature of the polymer will not be reached. Usually the particle size differs between 10 to 500 µm otherwise the solid handling is difficult or the size of the dryer device would exceed the economical benefit. The process of spray drying couples three different operations - atomization, drying and solid formation, all of which are in competition. Because of the fast drying rate a short retention time < 10 s is applicable. A new process is created if a reactive system is sprayed in a common spray dryer. So a fourth, a reactive step, polymerization in droplets, is added and completes a one step process from a monomer solution to a ready powder polymer.

But the short retention time combined with the kinetics of the reaction is the bottle neck of the process. To handle the kinetics problem the reaction is split in a reaction starting within the nozzle similar to a plug flow reactor and a progressing reaction in the droplets like small batch reactors. From a safety point of view a runaway of the reaction in the droplet is of no consequence because these small volumes are uncoupled in an aerosol and have a high specific surface area to dissipate the exothermic heat.

The spraying of a polymerizing system is the motivation of this project. But first the pre-reaction within the nozzle done by a special nozzle is focus of this research. The nozzle is a 3-fluid nozzle and handles the two solutions of the monomer and initiator which get mixed internal in a co-flow. The mixing in nozzle is sensitive because the flow regime is laminar and a radial mass transfer is not existent.  Therefore a static mixer is installed and its optimal design and flow parameter of the feeding liquids are tested with an optical measuring device. By the use of a transparent pipe, LED and CMOS camera, the transmitted light is characterized by the use of the Beer–Lambert law. The results present the radial concentration of tracer, a water soluble dye, at certain lengths of the pipe and different flow regime. An orifice with a small diameter (1 mm) shows homogeneous concentrations over almost the whole diameter.

With the use of a third fluid, gaseous nitrogen, the liquid gets atomized at the tip of the nozzle. Nitrogen substitutes pressured air because oxygen will react with the radicals. A double jacket over nearly the whole nozzle heats up the mixture until the reaction temperature is reached. During the heating the thermal initiator decomposes and a radical activation of the monomers sets in. The monomers form chains and consequently polymers. With the propagation of the monomer chains the viscosity of the liquid increases. If the viscosity is low enough the liquid gets atomized outside the nozzle by the high pressured nitrogen.

An important property is consequently the viscosity of the mixture and therefore the grade of polymerization. With the use of a modified rheometer the function of viscosity over reaction time of the monomer / initiator solutions is obtained. The method called rheokinetics was published by Kulichikhin and Malkin 1996 [1]. The material of this research is acrylic acid and its acrylates solved in water. An azo-initator is added to start the reaction. Polymers of acrylic acid are wide applied because of their electrolytic property e.g. as super absorbing polymers. The reaction is classified as a radical polymerization and has fast kinetics which is very positive concerning the low retention time left in a spray dryer. But a huge disadvantage is the high safety request, if it will not react only in the droplets of the spray, especially during the pre-reaction within the nozzle. Thus a step wise development of the process is necessary.

Based on the polymerization in the rheometer correlations are done to describe the viscosity over time dependent on concentration of initiator and monomer. With potential approximations the function of viscosity at the beginning of the reaction is described empirically. Viscosity function is combined with further theoretically and semi-empirical values, so that the development of temperature, conversion and viscosity over the length of the nozzle is presented. With this reliable tool to handle and optimize the pre-reaction the next step is to operate in a common spray dryer. A first test of the special nozzle in a spray dryer is therefore presented.

References

[1]     S. G. Kulichikhin and A. Y. Malkin, Rheokinetics: Rheological Transformations in Synthesis and Reactions of Oligomers and Polymers. Basel: Hüthig & Wepf Verlag (1996)