HCSTR

HCSTR = homogeneous continuous stirred tank reactor. 

Figure lOJ.b-2 Weight distribution in HCSTR parameter is conversion, x Pfi at zero conversion = 1000 (from Gerrens [20]). 

Balances for an HCSTR have been discussed above (Equations 3.96-3.99). In a SCSTR, the average value at the exit of the reactor of any additive quantity, X (conversion of monomer(s), initiator, moments of living and dead chains) is given by the following equation ... 

The MWD is the same as the instantaneous MWD in a batch reactor at the same temperature, initiation rate and conversion (for linear chains). In a batch polymerization, since monomer/radical concentrations and rate constants for termination may vary significantly with conversion, the cumulative MWD would be broader than in a comparable HCSTR. 

Real fiow/mixing regimes in polymer reactors will always lie somewhere in between the extremes of HCSTR and SCSTR. The best one can do is calculate upper and lower bounds. The usefulness of these calculations will depend on two factors, the difference between the upper and lower bound and how close the real fluid behaves to one of... 

In practice, if transfer reactions (to small molecules) govern molecular weight development, molecular-weight averages for HCSTR, SCSTR and batch reactors are almost identical. 

From Gerrens [1976]. BR = batch reactor PER = plug flow reactor HCSTR = homogeneous continuous stirred tank reactor = perfectly mixed flow reactor of this chapter SCSTR = segregated continuous stirred tank reactor. 

Weight distribution of polymer in HCSTR parameter conversion, )c, number average degree of polymerization at zero conversion = 1000. From Gerrens [1976]. 

The MWD resulting from semi-batch operations of a stirred tank reactor with monomer feed under various conditions is treated in Refs. 77-82. In a homogeneous continuous stirred tank reactor (HCSTR), the steady-state concentrations of monomer and initiator can be derived from the monomer and initiator mass bal-... 

In a CSTR, the narrow Poisson distribution resulting from the chemistry is superimposed by the broad residence time distribution of the HCSTR - chains can leave the reactor after seconds of growth as very short chains, but there are also chains which reside in the reactor for very long time, growing to very long molecules. This results in a Schulz-Flory distribution [83] [Eqs. (37)-(40)j, where t is the mean residence time and [M] is the steady state-monomer concentration. 

Automatic Continuous Online Monitoring of Polymerization Reactions was adapted to monitoring a homogeneous continuous stirred tank reactor (HCSTR) to verify the quantitative predictions concerning f, M, and r, as a function of the flow and kinetic parameters, to determine the kinetic parameters themselves, to ascertain the ideality of mixing in the reactor, to assess the effects of feed and reactor fluctuations, and to approximate a fully continuous tube-type reactor [38],... 

In an HCSTR in which monomer and initiator are fed into the reactor at the same flow rate r, at which material is removed, a steady-state condition is reached in which the reactor contents will remain at constant valnes of conversion, polydispersity, etc. 

The HCSTR used in Reference [38] allowed for variation of flow rates, initiator, and other reagent concentrations. The rate constant a could be determined in batch polymerization reactions monitored by ACOMR The system was used to investigate the effects on/, M, and t] of changing the different flow and concentration variables. 

The approach to the steady state is important in industry because it controls the rate at which product grade crossovers occur in continuous reactors. For the HCSTR, Equations indicate how c,(0, cjt), and c(f) vary as functions of time for different combinations of flow, polymerization, and concentration conditions. 

Whereas the previous experiments showed how steady-state HCSTR operation is approached and can be maintained, there is eonsiderable practical interest in being able to monitor the effects of drift in reactor conditions. 

HCSTR Flory Flory Wider than Flory... 

Kumar, A., MWD of Reversible ARB Polymerization in HCSTRs with Monomers Exhibiting Unequal Reactivity, Macromolecule, 20, 220-226. 

Chapter 3 derived the MWD of the polymer in batch reactors Section 4.2 has shown that the flashing of condensation product does not affect the distribution. We have already observed that an HCSTR is a continuous reactor that is employed when large throughputs are required. 

Here, is the rate of flashing of the condensation product from the HCSTR. The parameter 9 is known as the reactor residence time and is equal to V/F, where F is the volumetric flow rate of the feed. We have already shown that, for batch reactors, the first moment, 2, of the MWD is time invariant It can similarly be proved using Eqs. (4.3.2a) and (4.3.2b) that the same is true for HCSTRs. This fact means that... 

Consider the following computational scheme to find the MWD of the polymer formed in HCSTRs. First, we find out whether the condensation product is evaporating. If it is, [W] and Aq in the product stream are determined and is calculated using Eq. (4.3.14). However, if is zero or negative, there is no flashing of the condensation product, and we evaluate Ag and [W] using Eqs. (4.3.8) and (4.3.7). Once these are known, the MWD is determined through Eq. (4.3.6) by sequential computations. 

This chapter has discussed the analysis of reactors for step-growth polymerization assuming the equal reactivity hypothesis to be valid. Polymerization involves an infinite set of elementary reactions under the assumption of this hypothesis, the polymerization can be equivalently represented by the reaction of functional groups. The analysis of a batch (or tubular) reactor shows that the polymer formed in the reactor cannot have a polydispersity index (PDI) greater than 2. However, the PDI can be increased beyond this value if the polymer is recycled or if an HCSTR is used for polymerization. A comparison of the kinetic model with experimental data shows that the deviation between the two exists because of (1) several side reactions that must be accounted for, (2) chain-length-dependent reactivity, (3) unequal reactivity of various functional groups, or (4) comphca-tions caused by mass transfer effects. 

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