Shifting from discontinuous to continuous (BtoC) processes is one of the most studied topics in the current chemical engineering research mainly because it implies a huge impact on safety due to the strong reduction of the reactor size (which leads to intrinsically safe processes).
The aim of this work is to carry out a high viscosity, high monomer content (70% w/w) emulsion tetrapolymerization reaction in a series of Continuously Stirred Tank Reactors (CSTRs).
Emulsion polymerizations are usually carried out in semi-batch (SB) reactors, by dosing a mix of micelles swollen with unreacted monomer (which can be called “premix”) on a water-based buffered solution. The reaction is highly exothermic thus a good temperature control can be maintained in an industrial reactor using long dosing times.
The proposed BtoC procedure is quite simple, and it is based on the total accomplishment of three main constraints: 1) process productivity must be increased with respect to the SB recipe; 2) reactors workup must be reduced; 3) no loss of product during the start-up phase of the continuous system (this constraint also includes avoiding runaway reactions triggering). The approach suggests to: 1) divide the SB recipe in two main phases; 2) start-up the series of CSTRs following the first part of the SB recipe; 3) go to continuous by activating a pump to transfer the content of the first reactor in the second one; 4) fill the second reactor of the series following the second part of the SB recipe; 5) go to continuous by unloading the content of the second reactor in the final workup reactor. Such an approach was tested with a recipe of industrial interest, based on butyl acrylate. The experimental setup was carried out with a battery of 2 CSTRs, 5 pumps, and a tank for the final product. Multiple samples were collected overtime and analyzed, tracking free monomer content and particle size. From this study, we found that the process started up using the proposed BtoC procedure reached a steady state which was not acceptable from a product quality point of view. Anyway, the continuous process could be carried out for a limited amount of time (this means that it was possible to run a semi-continuous process) by optimizing dosing times and reactors volume. Under such conditions, it was possible to run a semi-continuous synthesis without loss of product due to the start-up phase, in smaller and easier to control reactors than the semi-batch ones.
Keywords: high viscosity; emulsion polymerizations; industrial scale optimization; runaway reactions; batch to continuous procedure.