Tubular polymerizations

Suspension Polymerization. At very low levels of stabilizer, eg, 0.1 wt %, the polymer does not form a creamy dispersion that stays indefinitely suspended in the aqueous phase but forms small beads that setde and may be easily separated by filtration (qv) (69). This suspension or pearl polymerization process has been used to prepare polymers for adhesive and coating appHcations and for conversion to poly(vinyl alcohol). Products in bead form are available from several commercial suppHers of PVAc resins. Suspension polymerizations are carried out with monomer-soluble initiators predominantly, with low levels of stabilizers. Suspension copolymerization processes for the production of vinyl acetate—ethylene bead products have been described and the properties of the copolymers determined (70). Continuous tubular polymerization of vinyl acetate in suspension (71,72) yields stable dispersions of beads with narrow particle size distributions at high yields. 

Equation (l) shows the rate of polymerization is controlled by the radical concentration and as described by Equation (2) the rate of generation of free radicals is controlled by the initiation rate. In addition. Equation (3) shows this rate of generation is controlled by the initiator and initiator concentration. Further, the rate of initiation controls the rate of propagation which controls the rate of generation of heat. This combined with the heat transfer controls the reaction temperature and the value of the various reaction rate constants of the kinetic mechanism. Through these events it becomes obvious that the initiator is a prime control variable in the tubular polymerization reaction system. 

The initiator types, however, are characterized by these parameters, and since the effect of pressure is small (, 9.) and the tubular polymerization of ethylene is undertaken within a narrow range of pressure, the descriptive constant becomes... 

Thermal runaway. Temperature control in a tubular polymerizer depends on convective diffusion of heat. This becomes difficult in a large-diameter tube, and temperatures may rise to a point where a thermal runaway becomes inevitable. 

Consider a laminar flow tubular polymerizer with cooling at the tube wall. At what radial position will a hotspot develop at the tube wall, at the centerline, or at an intermediate radius Justify your answer. Will the situation change with heating at the wall ... 

Open-tubular Polymeric surfactants Neutral hydrophilic layer, e.g.,... 

Suspension Polymerization. The suspension or pearl polymerization process has been used to prepare polymers for adhesive and coaling applications and for conversion to poly(vinyl alcohol). Suspension polymerization are carried out with monomer-soiubie initiators predominantly, with low levels of stabilizers Continuous tubular polymerization of vinyl acetate in suspension yields stable dispersions of beads with narrow particle size distributions at high yields. 

Many processes involving the flow of a reactive liquid along a tube are known, which makes theoretical analysis of the flow of polymerizing liquids, i.e., fluids with time-dependent rheological properties, relevant to the tubular polymerization reactors and similar plant equipment used in reactive processing. 

There are some fundamental investigations devoted to analysis of the flow in tubular polymerization reactors where the viscosity of the final product has a limit (viscosity < >) i.e., the reactive mass is fluid up to the end of the process. As a zero approximation, flow can be considered to be one-dimensional, for which it is assumed that the velocity is constant across the tube cross-section. This is a model of an ideal plug reactor, and it is very far from reality. A model with a Poiseuille velocity profile (parabolic for a Newtonian liquid) at each cross-section is a first approximation, but again this is a very rough model, which does not reflect the inherent interactions between the kinetics of the chemical reaction, the changes in viscosity of the reactive liquid, and the changes in temperature and velocity profiles along the reactor. 

APPENDIX 13.1 LUMPED PARAMETER MODEL OF A TUBULAR POLYMERIZER... 

Initial results on the two zone wall temperature optimization of bulk polymerizers in tubular reactors shows that the molecular weight distribution and product quality can be controlled for conversion levels aroung 20%. Further investigations into the use of optimal wall temperatures in tubular polymerizers are underway. 

Polystyrene can be easily prepared by emulsion or suspension techniques. Harkins (1 ), Smith and Ewart(2) and Garden ( ) have described the mechanisms of emulsTon polymerization in batch reactors, and the results have been extended to a series of continuous stirred tank reactors (CSTR)( o Much information on continuous emulsion reactors Ts documented in the patent literature, with such innovations as use of a seed latex (5), use of pulsatile flow to reduce plugging of the tube ( ), and turbulent flow to reduce plugging (7 ). Feldon (8) discusses the tubular polymerization of SBR rubber wTth laminar flow (at Reynolds numbers of 660). There have been recent studies on continuous stirred tank reactors utilizing Smith-Ewart kinetics in a single CSTR ( ) as well as predictions of particle size distribution (10). Continuous tubular reactors have been examined for non-polymeric reactions (1 1 ) and polymeric reactions (12.1 31 The objective of this study was to develop a model for the continuous emulsion polymerization of styrene in a tubular reactor, and to verify the model with experimental data. 

Afonso MD and DePinho MN. Nanofiltration of bleaching pulp and paper effluents in tubular polymeric membranes. Sep. Sci. Techn. 1997 32(16) 2641-2658. 

C. E. Wyman and L. F. Carter, A numerical model for tubular polymerization reactors, AIChE Symp. 

In a detailed model for tubular polymerization reactors, Hamer and Ray [24] showed that the viscosity and monomer conversion at the wall... 

In this review, the rte<4dnetic ai )toach is presented, complete mathematical and physical statements of the preplan are given, and the operation of a tubular polymerization reactor is analyzed as an example. The fundamental necessity of using the rheokinetic approach, whenever there is a sharp growth in the viscosity, is demonstrated. The trends of further investigation are presented. 

Radial Distributions in Tubular Polymerization Reactors. Low-Temperature Process... 

A sharp distortion of profiles and overshoot of reactants along the axis are negative features of the operation of a tubular polymerization reactor. Its utilization is more or less effective only if the viscosity does not grow too strongly. If this growth amounts... 

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