Reactors agitated polymerization

Homogenization characteristics of a mechanically agitated polymerization reactor in terms of NO = /(Reeff = Ndfp/pe( ) have been published for various stirrer types and pseudoplastic liquids with power-law behavior (Opara, 1975, Tebel et al, 1986). The homogenization time is compared to that for Newtonian fluids (Opara, 1975), and the homogenization properties of a... 

Table 1 Jacket Surface Areas of Agitated Polymerization Reactors...

Topics that acquire special importance on the industrial scale are the quality of mixing in tanks and the residence time distribution in vessels where plug flow may be the goal. The information about agitation in tanks described for gas/liquid and slurry reactions is largely apphcable here. The relation between heat transfer and agitation also is discussed elsewhere in this Handbook. Residence time distribution is covered at length under Reactor Efficiency. A special case is that of laminar and related flow distributions characteristic of non-Newtonian fluids, which often occiu s in polymerization reactors. 

Figure 2. Conventional polymerization reactor with top-driven agitator, baffle, and wall-cooling (volumes up to 40 ms)...

It would be useful to have liquid present in the polymerization reactor that provided the advantages of a solvent but without any of the disadvantages. Sound unlikely How about water One technique, called suspension polymerization, involves adding monomer to water in a reactor, agitating the mixture rapidly so that the monomer breaks apart into very small droplets, adding an initiator that is soluble in the monomer, and heating. Each droplet acts as a microbulk polymerization, the water very effectively removes the heat of polymerization, and the resulting polymer spheres are easily separated and filtered. This process, also known as bead polymeriza-... 

Mechanically agitated slurry reactors have a wide range of applications in the chemical, biochemical, and pharmaceutical industries and, in particular, catalytic processes (see Table IV). They are commonly used for catalytic hydrogenation, oxidation, halogenation, or polymerization reactions such as... 

Selection of a polymerization reactor depends on production costs, simplicity, and energy efficiency, all of which depend on the viscosity and the non-Newtonian rheological properties of the reaction mixture (Oldshue et al, 1982 Middleman, 1977). Flow properties significantly influence the nature of the reactor, agitator size, power, and design (Gerrens, 1982). 

Since mixing and good heat transfer are of vital importance in viscous polymerization reactions, a mechanically agitated continuous stirred-tank reactor is widely used in polymerization processes. Solution polymerization, emulsion polymerization, and solid-catalyzed olefin polymerization are all carried out in a mechanically agitated slurry reactor. 

The major characteristic of a polymeric reactor that is different from most other types of reactors discussed earlier is the viscous and often non-Newtonian behavior of the fluid. Shear-dependent rheological properties cause difficulties in the estimation of the design parameters, particularly when the viscosity is also time-dependent. While significant literature on the design parameters for a mechanically agitated vessel containing power-law fluid is available, similar information for viscoelastic fluid is lacking. 

Figure 8.1. Schematic illustration of dispersion polymerization reactor, (a) two layers of immiscible liquids - monomer and dispersion fluid (e.g. water), and (b) monomer dispersion achieved by agitation (37).

A 250-ml reaction vessel was used as the polymerization reactor. Each polymerization reaction was carried out either under static conditions in a freezer, where the container was placed in a bath of glycol, or dynamically, by subjecting the container to agitation in a tank of glycol. Isoprene monomer having a purity of 99.2% was used. All polymerizations were conducted in 10-g containers and cyclohexane at 15°C with a solvent/monomer mass ratio of 9. In a typical polymerization the neodymium catalyst/diethyl aluminium chloride base varied from 150 to 500 pmol per 100 g of isoprene. 

Also, polymerization reactions are carried out in a variety of reactors including agitated batch reactors, continuous stirred tank reactors (CSTR), multizone autoclaves, loop reactors, tubular reactors, fluidized bed reactors, and a combination of these reactors. 

Some early batch polymerization reactors were built on rotating shafts to copy the action of bottle polymerizers. These reactors were expensive and difficult to maintain. They were replaced by standard stirred vessels which are commonly used today. Typical batch reactors contain an agitator that is mounted in the center of the reactor top. The reactors are often glass lined and contain one or more baffles to enhance mixing. Heat removal is accomplished by circulating a coolant through the reactor jacket. 

The heat transfer rate to the jacket of an agitated polymerization kettle is 7.4 kW/m when the polymerization temperature is 50°C and the water in the jacket is at 20°C. The kettle is made of stainless steel with a wall 12 mm thick, and there is a thin layer of polymer k = 0.16 W/m-°C) left on the wall from previous runs, (a) What is the temperature drop across the metal wall (i>) How thick would the polymer deposit have to be to account for the rest of the temperature difference (c) By what factor could the heat flux be increased by using a stainless-clad reactor with a 3-mm stainless-steel layer bonded to a 9-mm mild-steel shell ... 

Hydrocarbon oil is added to the dispersion polymerization reactor to stabilize the polytetrafluo-roethylene emulsion. Temperature and agitation control are easier in this mode than suspension polymerization. Polytetrafluoroethylene fine powder and dispersion are produced by this technique. 

---