MIXING FOR POLYMERISATION REACTORS
There are many measures that are used to determine the end use quality of polymers including, among others, physical, mechanical and rheological properties. On a chemical level these properties are determined by the molecular weight and the molecular weight distribution of the polymer and, in the case of co-polymers, the distribution of monomers within the polymer molecule itself. Mixing issues that can affect these properties include:
- Poor dispersion of initiators resulting in a wider molecular weight distribution. This is often encountered when scaling-up a process and is usually addressed by increasing the initiator usage, increasing raw material costs.
- In emulsion and suspension polymerisations, changes in particle size and particle size distribution can change their process ability in equipment downstream from the reactor such as filters and driers. Again, this is usually addressed by increasing surfactant usage which increases raw material costs.
In both these cases the problem is usually created by not maintaining the critical mixing rate, relative to the reaction rate, on scale-up. Philadelphia Mixing Solutions and Mixing Solutions has many years of experience analysing these issues and providing solutions that can address them.
Another common problem encountered during polymerisations is fouling where product builds up on the walls of the reactor and the blades of the impellers. This reduces the heat transfer capability of the system with the polymer acting as an insulator, reduces the available volume and, if there is significant build-up on the impeller blades, can generate large out-of-balance forces which can cause mechanical problems. If fouling does occur the reactor must be taken out of service for cleaning reducing its utility and increasing operating costs. In severe cases large lumps of the fouling material can fall off the wall onto the impellers causing severe damage requiring replacement of blades and shafts.
Philadelphia Mixing Solutions has developed a mixing system that increases the internal recirculation within the reactor ensuring that the velocity of fluid at the wall is high and can reduce buildup of fouling while ensuring that the mixing conditions required to produce the desired product qualities are maintained.