Shear-induced coagulation

This can be explained by the fact that the flow in the CCTVFR became closer to plug flow as the Taylor number was dropped closer to. Therefore, the steady-state particle number and the steady-state monomer conversion could be arbitrarily varied by simply varying the rotational speed of the inner cylinder. Moreover, no oscillations were observed, and the rotational speed of the inner cylinder could be kept low, so that the possibility of shear-induced coagulation could be decreased. Therefore, a CCTVFR with these characteristics is considered to be highly suitable as a pre-reactor for a continuous emulsion polymerization process. In the case of the continuous emulsion polymerization of VAc carried out with the same CCTVFR, however, the situation was quite different [365]. Oscillations in monomer conversion were observed, and almost no appreciable increase in steady-state monomer conversion occurred even when the rotational speed of the inner cylinder was decreased to a value close to. Why the kinetic behavior with VAc is so different to that with St cannot be explained at present. 

The concentration of latex emulsions was one of the early applications suggested for UF. Unfortunately, not all latexes are amenable to processing with UF. Many latexes are unstable under the high shear induced by pumps or even in the thin channels of UF modules. However, the use of diaphragm pumps and careful control of hydrodynamic shear within the module helps prevent coagulation. 

Latexes should be stable under zero-shear conditions at room temperature. However, when latex is pumped from the reactor to a tank, or from tank to tank, it is subjected to shear forces that can cause shear-induced coagulation of the latex particles. Additional surfactant, called a post-stabiliser (296), is often added to latexes to improve shear stability. Also, in cold climates, latexes may be exposed to extreme temperatures that cause the aqueous phase to begin to freeze, thereby compressing the latex particles together, and causing coagulation (358). Poststabilisers are also added to improve freeze-thaw stability and electrolyte tolerance. 

Colloidal instability (coagulation) (291) in latexes can become an issue when the latex is sheared through pumping or mixing (shear-induced coagulation) when the latex is frozen and then thawed (freeze-thaw stability) when contaminants or additives are... 

Once particles are present in a volume of gas, they collide and agglomerate by different processes. The coagulation process leads to substantial changes in particle size distribution with time. Coagulation may be induced by any mechanism that involves a relative velocity between particles. Such processes include Brownian motion, shearing flow of fluid, turbulent motion, and differential particle motion associated with external force fields. The theory of particle collisions is quite complicated even if each of these mechanisms is isolated and treated separately. 

The continuous loop reactor is likely to be the only tubular reactor used on commercial production of emulsion polymers [71], although its use is limited to production of vinyl acetate homopolymers and copolymers (with ethylene and VeovalO) [77-79]. A continuous-loop reactor consists of a tubular loop that connects the inlet and the outlet of a recycle pump. These reactors combine the heat transfer characteristics of a tubular reactor with the RTD of a CSTR. The main drawback of this reactor is that the requirements for the mechanical stability of the latex are stringent because the recycling pump may induce shear coagulation. 

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