Reactors for Polymerization

Figure 13.9 Evolution of the temperature and cooling water flow rate in a 75-m3 polymerization reactor. For about lOOmin, the maximum cooling capacity is reached.

Along with micromixing, the control of macromixing or RTD is also important for efficient operation of the polymerization reactor. For continuous polymerization, the reactor should accomplish the following requirements ... 

For implementation of the state estimation algorithm, it is necessary to define the estimator gain. Several state estimator algorithms have been proposed in the literature to calculate K. Table 8.4 highlights the most common state estimator algorithms with the respective strengths and weaknesses. Table 8.4 also presents some examples of implementations in polymerization reactors. For illustrative purposes the extended Kalman filter (EKF) will be briefly shown below. 

Twin Screw Extruders as Polymerization Reactors for a Free-Radical Homopolymerization... 

Until about the mid-1990s, polyethylene research laboratories usually possessed several polymerization systems depending on the size of the research program. It was common procedure that each investigator had a dedicated polymerization reactor for individual research. These singlebatch reactors were about 1-5 liter in capacity and ran in either a slurry or solution mode. Often these individual reactors would be used to carry out a designed experiment in which two or three process variables would be investigated at two levels each. This required 4-8 separate experiments which would take about one week to carry out in the case of three variables at two levels, or 8 individual experiments. 

The material balances in a semibatch polymerization reactor for a copolymer system are as follows ... 

A better way of avoiding the problems of heat transfer associated with bulk polymerization on an industrial scale is to use suspension polymerization. This is essentially a bulk polymerization in which the reaction mixture is suspended as droplets in an inert medium. The initiator, monomer and polymer must be insoluble in the suspension medium which usually is water. A solution of initiator in monomer is prepared and then added to the pre-heated aqueous suspension medium. Droplets of the organic phase are formed and maintained in suspension by the use of (i) vigorous agitation throughout the reaction and (ii) dispersion stabilizers dissolved in the aqueous phase (e.g. surfactants and/or low molar mass polymers such as poly (vinyl alcohol) or hydroxymethylcellulose). The low viscosity of the aqueous continuous phase and the high surface area of the dispersed droplets provide for good heat transfer. Each droplet acts as a small bulk polymerization reactor for which the normal kinetics apply and polymer is... 

II. CONTINUOUS POLYMERIZATION REACTORS FOR FREE-RADICAL POLYMERIZATION OF VINYL MONOMERS... 

Figure 47 Instrumental setup of the FT-Raman probe, F-Raman spectrometer, and polymerization reactor for the on-line control of the SBS triblock copolymerization (see text).

Before we can explore how reactor conditions can be chosen, we require some measure of reactor performance. For polymerization reactors, the most important measure of performance is the distribution of molecular weights in the polymer product. The distribution of molecular weights dictates the mechanical properties of the polymer. For other types of reactors, three important parameters are used to describe their performance ... 

Some slurry processes use continuous stirred tank reactors and relatively heavy solvents (57) these ate employed by such companies as Hoechst, Montedison, Mitsubishi, Dow, and Nissan. In the Hoechst process (Eig. 4), hexane is used as the diluent. Reactors usually operate at 80—90°C and a total pressure of 1—3 MPa (10—30 psi). The solvent, ethylene, catalyst components, and hydrogen are all continuously fed into the reactor. The residence time of catalyst particles in the reactor is two to three hours. The polymer slurry may be transferred into a smaller reactor for post-polymerization. In most cases, molecular weight of polymer is controlled by the addition of hydrogen to both reactors. After the slurry exits the second reactor, the total charge is separated by a centrifuge into a Hquid stream and soHd polymer. The solvent is then steam-stripped from wet polymer, purified, and returned to the main reactor the wet polymer is dried and pelletized. Variations of this process are widely used throughout the world. 

Processes for HDPE with Broad MWD. Synthesis of HDPE with a relatively high molecular weight and a very broad MWD (broader than that of HDPE prepared with chromium oxide catalysts) can be achieved by two separate approaches. The first is to use mixed catalysts containing two types of active centers with widely different properties (50—55) the second is to employ two or more polymerization reactors in a series. In the second approach, polymerization conditions in each reactor are set drastically differendy in order to produce, within each polymer particle, an essential mixture of macromolecules with vasdy different molecular weights. Special plants, both slurry and gas-phase, can produce such resins (74,91—94). 

Hydroxyhydroquinone and pyrogaHol can be used for lining reactors for vinyl chloride suspension polymerization to prevent formation of polymer deposits on the reactor walls (98). Hydroxyhydroquinone and certain of its derivatives are useful as auxiUary developers for silver haUde emulsions in photographic material their action is based on the dye diffusion-transfer process. The transferred picture has good contrast and stain-free highlights (99). 5-Acylhydroxyhydroquinones are useful as stabilizer components for poly(alkylene oxide)s (100). 

Flexible batch. Both the formula and the processing instructions can change from batch to batch. Emulsion polymerization reactors are a good example of a flexible batch facility. The recipe for each produc t must detail Both the raw materials required and how conditions within the reac tor must be sequenced in order to make the desired product. 

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. 

A VCM (vinyl chloride monomer) production unit uses three vertically mounted agitated reactors for the polymerization of vinyl chloride. Crude material balances infer about 8 to 10% monomer losses. Describe how you would go about assessing whether these losses are due to leaks such as fugitive air emissions. Be specific in recommending procedures and instruments. 

Figure 12-10. The Inventa-Fisher process for producing nylon 6 from caprolactam (1) Melting station, (2,3) polymerization reactors, (4) extruder, (5) Intermediate vessel, (6) extraction column, (7,8) extraction columns, (9) cooling silo.

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