Reactor temperature, polymerization

Emulsion Process. The emulsion polymerization process utilizes water as a continuous phase with the reactants suspended as microscopic particles. This low viscosity system allows facile mixing and heat transfer for control purposes. An emulsifier is generally employed to stabilize the water insoluble monomers and other reactants, and to prevent reactor fouling. With SAN the system is composed of water, monomers, chain-transfer agents for molecular weight control, emulsifiers, and initiators. Both batch and semibatch processes are employed. Copolymerization is normally carried out at 60 to 100°C to conversions of - 97%. Lower temperature polymerization can be achieved with redox-initiator systems (51). 

The polymerization of monomers to form hydrocarbon resins is typically carried out by either the direct addition of catalyst to a hydrocarbon fraction or by the addition of feed to a solvent—catalyst slurry or solution. Most commercial manufacturers use a continuous polymerization process as opposed to a batch process. Reactor temperatures are typically in the range of 0—120°C. 

More recent process research aimed at anionic PS is that of BASF AG. Unlike the Dow Process, the BASF process utilizes continuous linear-flow reactors (LFR) with no back-mixing to make narrow polydispersity resins. This process consists of a series alternating reactors and heat exchangers (Fig. 22). Inside the reactors, the polymerization exotherm carries the temperature from 30°C at the inlet to 90°C at the outlet. The heat exchangers then take the temperature back down to 30°C. This process, which requires no solvent, results in the formation of narrow polydispersity PS. 

When ethylene is polymerized, the reactor temperature should be well controlled to avoid the endothermic decomposition of ethylene to carbon, methane, and hydrogen ... 

By maintaining the first-stage reactor just beyond the phase inversion point, the dispersed rubber phase is relatively rich in dissolved styrene. As polymerization subsequently proceeds in the LFR s, the dissolved styrene will react to form either a graft copolymer with the rubber or a homopolymer. The latter will remain within the rubber droplet as a separate occluded phase. Achieving the first-stage reactor conversion and temperature by recycling a portion of the hot second reactor effluent may permit simplification of the first reactor temperature control system. 

Figure 1. Typical reactor temperature profile for continuous addition polymerization a plug-flow tubular reactor. Kinetic parameters for the initiator 1 = 10 ppm Ea = 32.921 kcal/mol In = 26.492 In sec f = 0.5. Reactor parameter [(4hT r)/ (DpCp)] = 5148.2. [(Cp) = heat capacity of the reaction mixture (p) = density of the reaction mixture (h) = overall heat-transfer coefficient (Tf) = reactor jacket temperature (r) = reactor residence time (D) = reactor diameter].

The ability to manipulate reactor temperature profile in the polymerization tubular reactor is very important since it directly relates to conversion and resin product properties. This is often done by using different initiators at various concentrations and at different reactor jacket temperature. The reactor temperature response in terms of the difference between the jacket temperature and the peak temperature (0=Tp-Tj) is plotted in Figure 2 as a function of the jacket temperature for various inlet initiator concentrations. The temperature response not only depends on the jacket temperature but also, for certain combinations of the variables, it is very sensitive to the jacket temperature. 

Errors in the Temperature Measurement During Polymerizations Runs. The internal reactor temperatures measured during Runs 2,... 

Heat is applied to the reactor to further concentrate the reactants and to supply the energy to activate the polymerization reactions. At the outset, the reactor temperature and pressure rise rapidly. Sensor measurements indicate the existence of a temperature gradient having as much as a 40°C difference between material at the top and at the bottom of the reactor. Shortly after the pressure reaches its setpoint, the entire mixture boils and the temperature gradient disappears. The solution is postulated to be well mixed at this time. The cumulative amount of water removed is one indication of the extent of polymerization. 

In this section we develop a scheme implemented in a continuous polymerization reactor to regulate polydispersity by tracking periodic conversion profiles and maintaining stable temperature conditions. Oscillatory conversion is tracked by manipulating the initiator feedrate while the heat exchange rate is used to regulate reactor temperature. 

Notice that the outlet flow rate, q, may differ from the feed rate, qf, due to density variations during the polymerization. Moreover, the rate constants depend on reactor temperature and are described by the following Arrhenius expressions. 

Autoclave reactors are operated adiabatically, which means that the heat of reaction must be removed by the fresh ethylene entering the reactor. The conversion is related, therefore, to the difference in temperature between the feed and the reactor temperature. This limits the conversion to 15 - 20%. Taking into account the fact that the percentage conversion, Ax, can be approximated by using the average specific heat, cp, the enthalpy of the polymerization,... 

Figure 11. Simulated start-up of vinyl acetate polymerization at low emulsifier level (0.01 mol/L H>0) under closed-loop control with arbitrarily selected controller tuning constants and manipulation of reactor temperature at initiator concentration of 0.005 mol/L conversion in R1—STD feedback (-------------) vs. DTC (------)...

Emulsion Polymerization in a CSTR. Emulsion polymerization is usually carried out isothermally in batch or continuous stirred tank reactors. Temperature control is much easier than for bulk or solution polymerization because the small (. 5 Jim) polymer particles, which are the locus of reaction, are suspended in a continuous aqueous medium as shown in Figure 5. This complex, multiphase reactor also shows multiple steady states under isothermal conditions. Gerrens and coworkers at BASF seem to be the first to report these phenomena both computationally and experimentally. Figure 6 (taken from ref. (253)) plots the autocatalytic behavior of the reaction rate for styrene polymerization vs. monomer conversion in the reactor. The intersection... 

From an operational point of view, the choice of an appropriate polymerization reactor depends on six requirements temperature control mixing product accumulation and reactor foul-up follow-up separation processes the desired form of the product and safety. Heats of polymerization are typically high, so that maintaining the reactor at a desired temperature level is not always a simple task. Temperature can become spatially nonuniform and globally out of control (causing inconsistency of the reaction medium). Nonuniformity in temperature can lead to localized zones of poor mixing or even dead zones. In a polymerization reactor, temperature, mixing, viscosity,... 

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