Mixing in the reactor

In the first place, it is clear that not all molecules entering a reactor of holding time 6 Vjq will reside there for exactly S. It is because of the vigorous mixing, which causes some molecules to pass out of the reactor almost immediately and contribute so little to production, that the volume requirements of this type of reactor are so much higher. To determine the residence time distribution, a suitable experiment would be to inject a sharp pulse of some nonreacting tracer material at time t = 0 and measure its 

The Continuous Flow Stirred Tank Reactor Chap. 7 

If the tracer does not react, its concentration is governed by Eq. (7.1,3) with r = 0 and the suffix j dropped  

We can solve this for an arbitrary feed concentration, and if c(0) — 0, the solution is 

Suppose that a unit amount of tracer is injected during a very short interval (0, Zi), i.e., Cf t) = ti, and that 

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A continuous flow stirred tank reactor (CFSTR) differs from the batch reactor in that the feed mixture continuously enters and the outlet mixture is continuously withdrawn. There is intense mixing in the reactor to destroy any concentration and temperature differences. Heat transfer must be extremely efficient to keep the temperature of the reaction mixture equal to the temperature of the heat transfer medium. The CFSTR can either be used alone or as part of a series of battery CFSTRs as shown in Figure 4-5. If several vessels are used in series, the net effect is partial backmixing. 

Assume perfect mixing in the reactor, and because it is operated adiabatically (i.e., no exchange of heat between the reactor and its environment), Q = 0 and equation 6-53 becomes... 

F. Polyethylene polymer autoclav e type reactors usually contain 8 to 120 impellers of the same or different circulation designs on a single shaft to ensure rapid total homogeneous mixing in the reactor, w hich contains a gas at about 30,000 psi and, hence, the fluid is neither a gas or a liquid because the densities are about the same. 

In the present study, two types of the reactors were use. The configuration of the Reactor Type 1 is shown in Fig. 2. By making structure of the upper part of the reactor dual, TEMS and H2O were separately fed and mixed in the reactor. 

The term Lu / D is known as the Peclet number, Pe, and its inverse as the dispersion number. The magnitude of the Peclet number defines the degree of axial mixing in the reactor. 

In this model of non-ideal reactor mixing, a fraction, fi, of the volumetric feed rate, F, completely by-passes the mixing in the reactor. In addition, a fraction, f2, of the reactor volume, V, exists as dead space. F3 is the volumetric rate of exchange between the perfectly mixed volume Vi and the dead zone volume V2 of the reactor. 

To do this, not only must he know the chemistry of the reactions but he must know the rates at which the reactions occur and what affects those rates. The study of this is called chemical kinetics. By the proper choice of raw materials and operating conditions for the reaction stage the process designer can manipulate the ratio of products formed. One major variable is the temperature. An increase in temperature usually causes the reaction rates to increase, but some increase faster than others. Thus, the product mix in the reactor is dependent on the temperature. The pressure and the time the material spends in the reactor also affects the results. In the gaseous phase ahigh pressure will impede those steps in which the number of moles is increased and assist those in which the number of moles is decreased. A... 

Factors of importance in preventing such thermal runaway reactions are mainly related to the control of reaction velocity and temperature within suitable limits. These may involve such considerations as adequate heating and particularly cooling capacity in both liquid and vapour phases of a reaction system proportions of reactants and rates of addition (allowing for an induction period) use of solvents as diluents and to reduce viscosity of the reaction medium adequate agitation and mixing in the reactor control of reaction or distillation pressure use of an inert atmosphere. 

The mixture fraction as defined above describes turbulent mixing in the reactor and does not depend on the chemistry. However, by comparing Eqs. (45) and (46), we can note that they have exactly the same form. Thus, for the acid-base reaction, the mixture fraction is related to rA—B by... 

The value of X at the reactor outlet is a sensitive measure of the degree of mixing in the reactor. If X< 1, then mixing in the reactor is rapid compared to the second reaction in Eq. (60). In contrast, if Xx 1, then mixing is slow. 

Figure 4.30 shows the effect of the concentration of A in the feed stream, [A]o, on the concentrations of Si and B when a CSTR is considered. It can seen that when the value of [A]q is increased from 0.03 mM to 0.3 mM, repetitive oscillatory signals are obtained. Similar effects are obtained when a PFR is considered, and these are shown in Figure 4.31. Moreover, the repetitive oscillatory signal is obtained after 100 min in a PFR, whereas in the CSTR it appears earlier, due to mixing in the reactor.

Bulk polymerization. This is the simplest method. Monomers and initiator are mixed in the reactor shown in Figure 22-5 and heated to the right temperature. The bulk process is suitable for condensation polymers because the heat of reaction is low (it gives off less heat).. ... 

We win develop mass balances in terms of mixing in the reactor. In one limit the reactor is stirred sufficiently to mix the fluid completely, and in the other limit the fluid is completely unmtxed. In any other situation the fluid is partially mixed, and one cannot specify the composition without a detailed description of the fluid mechanics. We wiU consider these nonideal reactors in Chapter 8, but until then all reactors wiU be assumed to be either completely mixed or completely unmixed. 

A fluid bed process for oxyacylation of olefins has been described (4). The fluid bed process overcomes some of the disadvantages in the fixed bed operation to produce vinyl acetate. In the fluid bed process the catalyst is continuously homogeneously mixed in the reactor resulting in a significant improvement in the homogeneous addition of the promoter even if it is introduced through a single outlet. 

Experimental determination of the mass transfer coefficient is based on the appropriate mass balance on the specific reactor used (Figure B 1-2). The simpler the reactor system is, the simpler the mass balance model for evaluating the experimental results can be. For example, if mixing in the reactor deviates too far from ideality, kL is no longer uniform throughout the reactor. Neither method as described below can then be used. Instead a more complicated model of the mixing zones in the reactor would be necessary (Linek, 1987 Stockinger, 1995). 

Aluminum Chloride-Based Alkylation. An improved aluminum chloride-based process was developed by Monsanto in the 1970s. Using a presynthesized aluminum chloride complex and operating the reactor at higher temperature and pressure, the catalyst inventory is reduced to below its solubility in the reaction mixture. The reactants and the catalyst complex are mixed in the reactor to form a homogeneous liquid. The transalkylation... 

Temperature control is important in cooling. Basically, temperature control of large reactors operates the same way as in small reactors. However, if mixing in the reactor is poor—i.e., if the overturn is too... 

Operating with the expanded bed allows the processing of heavy feedstocks, such as atmospheric residua, vacuum residua, and oil sand bitumen. The catalyst in the reactor behaves like fluid that enables the catalyst to be added to and withdrawn from the reactor during operation. The reactor conditions are near isothermal because the heat of reaction is absorbed by the cold fresh feed immediately owing to thorough mixing in the reactors. 

The coil takes up volume in the vessel, so the size of the reactor will have to be larger to provide the same holdup for reaction. If the coil volume is too high a fraction of the total volume, the mixing in the reactor may be adversely affected. 

There is perfect mixing in the reactor. This implies no spatial variations of rate in the reactor, and the composition of the exit stream is the same as the composition anywhere in the reactor. 

Air and wastewater are mixed in the reactor chamber where oxidation of organics takes place. This reactor is an original design and has granted the patent P9500388. 

Polyterpene resins are related to the oldest reported polymerization, as they were first observed in 1789 by Bishop Watson by treatment of turpentine with sulfuric acid [92]. Commercial polyterpene resins are synthesized by cationic polymerization of /3- and a-pinenes extracted from turpentine, of rf,/-limonene (dipentene) derived from kraft-paper manufacture, and of d-limonene extracted from citrus peels as a by-product of juice industry [1,80,82,93]. The batch or continuous processes are similar for the three monomers. The solution polymerization is generally performed in mixed xylenes or high boiling aromatic solvent, at 30-55° C, with AlCl3-adventi-tious water initiation. The purified feedstream (72-95% purity, depending on monomer) is mixed in the reactor with solvent and powdered A1C13 (2—4 wt% with respect to monomer), and then stirred for 30-60 min. After completion of the reaction, the catalyst is deactivated by hydrolysis, and evolved HC1 is eliminated by alkaline aqueous washes. The organic solution is then dried, and the solvent is separated from the resin by distillation. 

The intensive rapid solid mixing in the reactor leads to a wide range of residence times of individual particles in the reactor for continuous operations this gives poorer performance since the removal of fully reacted particles will inevitably be associated with removal of unreacted carbon for catalytic reactions, the movement of porous catalyst contributes to the backmixing of gaseous reactant. 

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