Polymerization in tubular reactors

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. 

Solution Polymerization. Solution polymerization of vinyl acetate is carried out mainly as an intermediate step to the manufacture of poly(vinyl alcohol). A small amount of solution-polymerized vinyl acetate is prepared for the merchant market. When solution polymerization is carried out, the solvent acts as a chain-transfer agent, and depending on its transfer constant, has an effect on the molecular weight of the product. The rate of polymerization is also affected by the solvent but not in the same way as the degree of polymerization. The reactivity of the solvent-derived radical plays an important part. Chain-transfer constants for solvents in vinyl acetate polymerizations have been tabulated (13). Continuous solution polymers of poly(vinyl acetate) in tubular reactors have been prepared at high yield and throughput (73,74). 

Experimental work with styrene in tubular reactors has been reported (39) where viscosities were relatively low due to conversions below 32%. However, Lynn ( ) has concluded that a laminar flow tubular reactor for styrene polymerization is probably technically infeasible due to the distortion in velocity... 

The first section of this chapter describes the most important high pressure process run under homogeneous conditions to manufacture Low Density PolyEthylene (LDPE). The radical polymerization of ethylene to LDPE is carried out in tubular reactors or in stirred autoclaves. Tubular reactors exhibit higher capacities than stirred autoclaves. The latter are preferred to produce ethylene copolymers having a higher comonomer content. 

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. 

The objective was to develop a model for continuous emulsion polymerization of styrene in tubular reactors which predicts the radial and axial profiles of temperature and concentration, and to verify the model using a 240 ft. long, 1/2 in. OD Stainless Steel Tubular reactor. The mathematical model (solved by numerical techniques on a digital computer and based on Smith-Ewart kinetics) accurately predicts the experimental conversion, except at low conversions. Hiqh soap level (1.0%) and low temperature (less than 70°C) permitted the reactor to perform without plugging, giving a uniform latex of 30% solids and up to 90% conversion, with a particle size of about 1000 K and a molecular weight of about 2 X 10 . 

DEPOSITION OF FAST-POLYMERIZING MONOMER IN TUBULAR REACTOR... 

Kolhapure and Fox, R. (1999), CFD analysis of micromixing effects on polymerization in tubular low-density polyethylene reactors, Chem. Eng. Sci., 54, 3233-3242. 

Example 10.8 The production of high-pressure low-density polyethylene is carried out in tubular reactors of typical dimensions 2.5 cm diameter and 1 km long at 250°C and 2500 atm. The conversion per pass is 30% and the flow rate is 40,000 kg/h. Assuming that the polymerization reaction is first order in ethylene concentration, estimate the value of the polymerization rate constant. 

Olefin polymerization in batch reactors is not common. Laboratory-scale high-throughput reactors are perhaps one of the few examples of such reactors applied to olefin polymerization. Some olefin polymerization tubular reactors can also be treated as batch reactors, where a polymerization-time to reactor-length transformation can be made and directly applied to the equations derived above if the tubular reactor has plug-flow residence time. 

Computational fluid dynamics (CFD) approaches are emerging as alternative detailed tools for examining polymerization systems with complex mixing and reactor components. Recent examples on LDPE cases include Kolhapure and Fox [118], micromixing effects in tubular reactors Zhou etal. [119], tubular (and autoclave) reactors Wells and Ray [120], analysis of imperfect mixing effects applicable to many reactive flow systems, including LDPE autoclaves and Buchelli etal. [121], fouling effects. 

In the third stage, polymerization and water removal continue in a pressurized vessel, with conditions that change with time (for batch nylon 6,6 production) or with position (for continuous nylon 6,6 production in tubular reactors [14] or in a series of back-mixed reactors) as shown in Figure 7.8. High pressure is required to maintain a sufficiently high... 

Multi-objective optimization procedures were used for the simultaneous maximization of monomer conversion and minimization of side products during low-density polyethylene polymerizations performed in tubular reactors under steady-state conditions [170]. Genetic algorithms were used to compute the Pareto sets. Multi-objective optimization procedures were also used for the simultaneous maximization of molecular weight averages and minimization of batch times in epoxy semibatch polymerizations [171]. In this case, monomer feed rates were used as the manipulated variable. 

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