Tubular reactor monomer conversion

Figure 7. Monomer conversion vs, polymerization time in the helical tubular reactor laminar flow regime...

Figure 10. Monomer conversion rates as a function of emulsion Reynolds number for straight and helical tubular reactors...

FIGURE 13.7 Performance of a laminar flow, tubular reactor for the bulk polymerization of styrene Tin = 35°C and F = 1 h. (a) Stability regions, (b) Monomer-conversion within the stable region. 

The intense heat dissipated by viscous flow near the walls of a tubular reactor leads to an increase in local temperature and acceleration of the chemical reaction, which also promotes an increase in temperature the local situation then propagates to the axis of the tubular reactor. This effect, which was discovered theoretically, may occur in practice in the flow of a highly viscous liquid with relatively weak dependence of viscosity on degree of conversion. However, it is questionable whether this approach could be applied to the flow of ethylene in a tubular reactor as was proposed in the original publication.199 In turbulent flow of a monomer, the near-wall zone is not physically distinct in a hydrodynamic sense, while for a laminar flow the growth of viscosity leads to a directly opposite tendency - a slowing-down of the flow near the walls. In addition, the nature of the viscosity-versus-conversion dependence rj(P) also influences the results of theoretical calculations. For example, although this factor was included in the calculations in Ref.,200 it did not affect the flow patterns because of the rather weak q(P) dependence for the system that was analyzed. 

Tubular Reactor Studies. The first run in the tubular reactor was with the same recipe as for Seed I in Table I, but the conversion was very low, and there were two distinct phases. The residence time in the tube was equal to the batch reaction time. Apparently the more nearly constant temperature of the tubular reactor prevented rapid polymerization. In the next run, initiator and emulsifier levels were doubled, but still conversion was low, although phase separation was not so severe. With seed latex and still more emulsifier, Run I shown in Table II, monomer conversions of about 60% were obtained at 50 minutes average residence time in the reactor. No phase separation was evident, but later tests indicated that some phase separation was occurring. 

The monomer conversion in the tubular reactor was regularly measured during the run, but it did not always reach a steady state value, even after reaction times equivalent to four residence times. At low conversions, steady state was normally obtained. At exit conversions between approximately 30 and 60%, conversion increased to a maximum at two residence... 

A mathematical model was developed, able to predict monomer conversion and temperature profiles of industrial tubular reactors for the production of low-density polyethylene, in different operating conditions. 

In the literature many studies on LDPE tubular reactors are found (2-6).All these studies present models of the tubular reactor, able to predict the influence, on monomer conversion and temperature profiles, of selected variables such as initiator concentration and jacket temperature. With the exception of the models of Mullikin, that is an analog computer model of an idealized plug-flow reactor, and of Schoenemann and Thies, for which insufficient details are given, all the other models developed so far appear to have some limitations either in the basic hypotheses or in the fields of application. 

Ihara and Yasuda investigated the deposition behavior of methane in the medium-sized tubular reactor with 13.56 MHz radio frequency discharge [2]. They observed that the critical WjFM value, WjFM), for methane was 8GJ/kg, and nearly 100% of monomers were converted to the plasma polymer beyond this critical WjFM value. As shown in Figure 19.5, the critical WjFM value of perfluoropropene in the small reactor is around 6GJ/kg and the DjFM is 15%, and the corresponding value in the medium is around 4GJ/kg, and the maximal conversion is around 30%. In the large reactor, (WIFM) is about 1 GJ/kg and the maximal DjFM is 20%. The lower value of the critical WjFM for C3F6 than that for CH4 is explained in Chapter 7. 

In a detailed model for tubular polymerization reactors, Hamer and Ray [24] showed that the viscosity and monomer conversion at the wall... 

Solution In radical chain reactions, the overall rate of polymerization, Rp, and the number-average degree of polymerization, X , are functions of the initiator concentration [I], the monomer concentration [M], and also the temperature via the temperature dependence of the individual rate constants. At constant [M] and [I], the Schulz-Flory MWD is produced. However, if [M] and [I] vary with time, a number of Schulz-Flory distributions overlap and thus a broader MWD is produced. In the ideal CSTR [M] and [I] are constant and the temperature is relatively uniform. Consequently, chain polymerizations in CSTR produce the narrowest possible MWD. In the batch reactor, [M] and [I] vary with time (decrease with conversion) while in the tubular reactor [M] and [I] vary with position in the reactor and the temperature increases with tube radius. These variations cause a shift in X with conversion and consequently a broadening of MWD. 

Rexene Co. and Philips Petroleum Co. first developed the bulk polymerization process with the first-generation TiCU catalyst [8,11,70]. It was then commercialized by Dart Industries in 1964. The reactor feed contains 10-30% propylene in the liquid phase. A mixture of hexane and isopropanol was employed for the removal of catalyst residue as well as the amorphous polypropylene. The process step of removing residual catalyst was later eliminated after the high-efficiency catalyst was adopted, constituting the so-called liquid pool process. Subsequently, Philips and Sumitomo companies further developed the liquid-phase polymerization process. This process enhances the reaction rate, catalyst efficiency, monomer conversion, and therefore results in high productivity. It also eliminates the need for solvent recovery and reduces environmental pollution. However, the process is somewhat complicated by the unreacted monomer, which has to be first vaporized and then liquefied before it is reused. The reaction vessel must be designed to operate under high pressures. In most cases, this process employs autoclaves for batch operation and tubular reactors for continuous operation. 

The Dowlex process by Dow Chemicals is the dominant process in solution polymerization, but Dow does not license this technology to other companies (Figure 2.39). The Dowlex process uses two CSTRs in series with a high boiling hydrocarbon solvent. Other competing processes include the DSM process and the Sclairtech process by Nova Chemicals. In some configurations, these processes may also have tubular reactors operated in series with the CSTR to complete monomer conversion. 

Many papers have been published in the last 20 years or so on modeling and simulation analysis of tubular reactors. It is difficult to make a clear statement on the validity of these analyses usually because of the lack of experimental verifications. When the velocity profile varies along the tube, a prediction of reactor performance is not much more complex theoretically, but its application to real systems is very difficult (if not impossible) because of lack of information on how the velocity profile changes along the tube at high monomer conversions (and viscosities). 

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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