Heat effects CSTRs with

The problems of monomer recovery, reaction medium viscosity, and control of reaction heat are effectively dealt with by the process design of Montedison Fibre (53). This process produces polymer of exceptionally high density, so although the polymer is stiU swollen with monomer, the medium viscosity remains low because the amount of monomer absorbed in the porous areas of the polymer particles is greatly reduced. The process is carried out in a CSTR with a residence time, such that the product k jd x. Q is greater than or equal to 1. is the initiator decomposition rate constant. This condition controls the autocatalytic nature of the reaction because the catalyst and residence time combination assures that the catalyst is almost totally expended in the reactor. 

For the perfectly mixed continuous reactor, the CSTR, the ratio VT/ Fy only represents the mean residence time, /p,av however, it is still possible to compare the performance of the CSTR with the performance of the BR by letting the mean residence time fp av = t. Interestingly, when the reaction rate shows a positive dependence on reactants concentration, the BR is more effective than the CSTR. This is because the batch reactor experiences all the system compositions between initial and final values, whereas the CSTR operates at the final composition, where the reaction rate is smaller (under the above hypotheses). Finally, one can compare the two continuous reactors under steady-state conditions. The CSTR allows a more stable operation because of back-mixing, which however reduces the chemical performance, whereas the PFR is suitable for large heat transfer but suffers from larger friction losses. 

There are several other aspects about CSTRs with exothermic reactions that should be mentioned at this point. The first involves the temperature of the feed. The colder the feed, the less heat must be transferred from the reactor. So control would be expected to be improved. However, as we will see in Chapter 3, a cold feed can produce some interesting dynamics for instance, an increase in feed flowrate initially decreases reactor temperature because of the sensible-heat effect. But as the reactant concentration in the reactor increases, the temperature eventually increases. A reactor temperature runaway can result if the cold feed quenches the reaction and reactant concentration builds to a very high level before the reaction lights off. ... 

In some situations the dynamics of the cooling system may be such that effective temperature control cannot be accomplished by manipulation of the coolant side. This could be the situation for fluidized beds using air coolers to cool the recirculating gases or for jacketed CSTRs with thick reactor walls. The solution to this problem is to balance the rate of heat generation with the net rate of removal by adjusting a reactant concentration or the catalyst flow. Such a scheme is shown in Fig. 4.24. 

One important oscillating system—namely, the methylamine decomposition on noble metal wires (24,143,227,228)—belongs to this class of ther-mokinetic blocking/reactivation models. This reaction is unique in several ways. It is the only endothermic oscillator (-1-150 kJ/mol), and it is the only unimolecular reaction that displays oscillations caused by sur ce effects. [The oscillating N2O decomposition, reported by Hugo (5) in 1968, does not oscillate because of the instability of the surface reaction, but rather due to the instability of a CSTR when certain heat and mass transfer conditions exist. Any reaction with similar rate and heat effects would oscillate under such circumstances.] This reaction is also the most vigorous oscillator yet observed and displays frequencies of up to 10 Hz and amplitudes approaching 500 K. Moreover, because the reaction oscillates at temperatures of around 1000 K, the oscillations can actually be observed visually as the metal catalyst heats and cools. 

Parallel re dons take place in a CSTR with heat effects. [1st Ed. P9-21]... 

The solution of the nonlinear optimization problem (PIO) gives us a lower bound on the objective function for the flowsheet. However, the cross-flow model may not be sufficient for the network, and we need to check for reactor extensions that improve our objective function beyond those available from the cross-flow reactor. We have already considered nonisothermal systems in the previous section. However, for simultaneous reactor energy synthesis, the dimensionality of the problem increases with each iteration of the algorithm in Fig. 8 because the heat effects in the reactor affect the heat integration of the process streams. Here, we check for CSTR extensions from the convex hull of the cross-flow reactor model, in much the same spirit as the illustration in Fig. 5, except that all the flowsheet constraints are included in each iteration. A CSTR extension to the convex hull of the cross-flow reactor constitutes the addition of the following terms to (PIO) in order to maximize (2) instead of [Pg.279]

From Figure 2-6. wc note a very important observation The total volume to achieve 80% conversion for five CSTRs of equal volume in series is roughly the same as the volume of a PFR, As wc make the volume of each CSTR smaller and increase the number of CSTRs, the total volume of the CSTRs in series and the volume of the PFR will become identical. That is, we can model a PFR with a large number of CSTRs in series. This concept of using many CSTRs in series to model a PFR will be used later in a number of situations, such as modeling catalyst decay in packed-bed reactors or transient heat effects in PFRs. 

From these three case.s. (I) adiabatic PFR and CSTR, (2) PFR and PBR with heat effects, and (3) CSTR with heat effects, one can see how one couples the energy balances and mole balances. In principle, one could simply use Table 8-1 to apply to different reactors and reaction systems w ithout further discussion, However, understanding the derivation of the.se equations w ill greatly facilitate their proper application and evaluation to various reactors and reaction systems. Ctmsequenily, the following Sections 8.2. 8.3, 8,4. 8.6, and 8,8 will derive the equations given in Table 8-1. 

Example H-5 Piwluetion of Ac etic Anhydride Example 8-9 CSTR with CooUna Coil Example H-IO Parallel Reaction in a PER with Heat Effects Example H-H Multiple Reactions in a CSTR... 

PFR and CSTR with and witliout a Heal E.schanger Multiple Steady States Unsteady-Si.ite Heat Effects (Ch. 9) Reactor Safely Catalysis (Ch, 10)... 

P8-28- This problem concerns series reaction with heat effects in a CSTR. The instruction could extend the problem statement in tlie text to ask the students to carry out a parameter sensitivity. E.g. increase/decrease/ j and 2- This problem could be alternated with P8-31, which involves parallel reactions. 

Figure 5.2 gives the response of a one-CSTR process for step changes in feed rate from 100 to 150 lb-mo 1/hr and from 100 to 50 Ib-mol/hr for the system with k = 0.5 and 95 percent conversion. When feed rate is increased, the temperature in the reactor initially decreases. This is due to the sensible heat effect of the colder feed (70°F versus 140°F). After about five minutes, the temperature starts to increase because the concentration of reactant has increased, which increases the rate of reaction. The maximum temperature deviation is only 0.06°F, but it takes over five hours to return to the setpoint because of the slow change in reactor concentration and the large reset time. 

Average temperature of coolant = T,y Figure 4.26 A CSTR in steady-state operation with heat effects. 

During polymerization with a CSTR, the monomer and the other components of the polymerization recipe are fed continuously into the reactor while the polymerization product mixture is continually withdrawn from the reactor. The application of the CSTR in suitable polymerization processes reduces, to some extent, the heat removal problems encountered in batch and tubular reactors due to the cooling effect from the addition of cold feed and the removal of the heat of reaction with the effluent. Even though the supporting equipment requirements may be relatively substantial, continuous stirred tank reactors are economically attractive for industrial production and consistent product quality. 

The heat associated with a specific polymerization reaction depends on the temperatures of both the monomers and polymer. A standard basis that is consistent for treating polymerization heat effects results when the products of polymerization and the monomers are all at the same temperature. Consider a calorimeter method of measurements of heat of polymerization of monomers. The initiator is mixed with the monomer, and the system is a continuous flow CSTR. The polymerization reactions take place in the CSTR. The polymerization products enter a devolatilizer where the monomers are vaporized and removed from the product mix and recycled back to the reactors. The CSTR is water cooled to bring the monomers/polymer to the reactor temperature. There is no shaft work performed by the process. The CSTR is built, so that changes in potential and kinetic energy are negligible. The first law of thermodynamics for open systems can be written for the system as... 

---