Polycondensation reactor

By the 1960s, it was already well known that the polycondensation rate can be enhanced considerably if the low-molecular-weight by-products are removed at a sufficient rate. Many patent applications for polycondensation reactors describe inventions especially suited for the efficient removal of volatile components. Nevertheless, a good understanding of the interaction of chemical reactions and mass transport was still lacking at that time. 

Due to different residence times needed for the esterification and the polycondensation steps, the industrial-batch polycondensation process is designed with two main reactors, i.e. one esterification reactor and one or two parallel polycondensation reactors (Figure 2.34). 

Figure 2.35 Discontinuous polycondensation reactors (a) conventional design for capacities up to 35t/d (b) novel discontinuous disc-ring reactor for capacities up to 100t/d [2]. From manufacturer s literature published by Zimmer AG and reproduced with permission...

The special requirements for esterification and polycondensation reactor design result from the understanding of kinetics and mass transport as discussed above... 

Cheong, S. I. and Choi, K. Y., Modeling of a continuous rotating disk polycondensation reactor for the synthesis of thermoplastic polyesters,. /. Appl. Polym. Sci., 61, 763-773 (1996). 

Many polymer reactions, for example, are highly exothermic, so the temperature control concepts outlined in this book must be applied. At the same time, controlling just the temperature in a polymer reactor may not adequately satisfy the economic objectives of the plant, since many of the desired polymer product properties (molecular weight, composition, etc.) are created within the polymerization reactor. These key properties must be controlled using other process parameters (i.e. vessel pressure in a polycondensation reactor or chain transfer agent composition in a free-radical polymerization reactor). 

Besides these laboratory experiments, the analysis of industrial reactors may also reveal segregation effects, as for instance in reactors for free radical polymerization of ethylene where the initiator feedstream is likely to be mixed by an erosive process (175). Polymerization and polycondensation reactors offers an especially interesting field for future applications of micromixing. 

Many polycondensation reactor designs provide large surface areas for mass transfer of condensation byproducts out of the polymer phase. In some processes, high vacuum is applied to encourage byproduct removal. 

Kinetic models are applied to the design and simulation of polycondensation reactors, so that overall reactor performance can be predicted and reactor operation can be optimized. 

Example 7 (Two-phase model for a continuous finishing-stage reactor) In designing a finishing-stage polycondensation reactor, the following important factors need to be... 

Figure 7.6 Schematic diagram of a continuous two-phase finishing-stage melt polycondensation reactor for PET production.

Kim et al. [19] fabricated PET/clay composites using a two-step in-situ polymerization method. In the first step, a slurry mixture of monomer (purified tereph-thalic acid and ethylene glycol), polycondensation catalyst, clays, and some additives was kept at 250 °C for 5-6h in the esterification step. Then, it was transferred to a polycondensation reactor until the intrinsic viscosity (IV) value reached 0.6dlg" . Then, the materials were pelletized. Furthermore, a solid-state polymerization (SSP) process is carried out to conduct the polymerization process further. SSP was carried out at between 220 and 145 °C for around 8h until the IV reached 0.8dlg . ... 

The design and operation of most polycondensation reactors for devolatilization of volatile by-products (namely vented extruders) is clearly a very difficult problem because of the complexity of the flows (with or without foaming). Further progress is likely to require a heavy use of computational fluid dynamics, as simplified models seem to have arrived at their limits. 

Figure I Basic layout of a pilot plant for production of oligo(ethylene terephthalate)s in a continuous process. 1 Polycondensation reactor 2 punched metal walk 3 condenser ...

Pure terephthalic acid (PTA) is formed by the oxidation of para-xylene. Life cycle inventory data for production of PTA are from Boustead S0), Data on the esterification process were estimated from a process model developed by SRI (6). The process includes two esterification reactors followed by a three column polycondensation reactor train. 

Steady-state model of a stirred-tank polycondensation reactor... 

C.3. In the polycondensation reactor example above, we reduced the number of equations by deriving moment equations. This required a closure approximation to estimate the value of A.3 given A,o, A,i, A.2. Test this approximation by solving the complete set of population balance equations... 

Ester interchange is favored for terephthalates because the free acid is very insoluble and difficult to incorporate into a reaction system. Two stages are used In the first, methanol is displaced from the terephthalic acid ester by the diol, and in the second, the excess diol is driven off at high temperatures and low pressures. A continuous process (Figure 17.1) starts with a molar ratio of dimethyl terephthal-ate to glycol of 1 1.7. Methanol is removed from the horizontally sectioned vessel over a period of 4 h with a rise in temperature to 245°C at a pressure of 1 atm. In two polycondensation reactors, the pressures are about 0.020 and 0.001 atm, and the exit temperatures are 270°C and 280°C, respectively. The resultant polymer melt is low enough in viscosity that pumps can be used to extrude fibers directly or to make chips as an intermediate form for storage. 

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