Series Reactions in a CSTR

In a batch or plug-flow reactor, every fluid element spends exactly the same time in the reactor. If the series of reactions A— R— S is occurring, it is straightforward, at least conceptuaUy, to design a batch or plug-flow reactor that wiU operate at fopt (or opt), and produce a concentration of R equal to Ca . 

What happens when these reactions arc carried out in a CSTR, where mixing is intense, and not every molecule spends the same time in the reactor Ui a CSTR, the feed mixes instantaneously into the contents of the reactor and the effluent is a random sample of the contents of the reactor. Some fluid elements remain in the reactor for a very short time, and some are in the reactor for a very long time. Very few are in the reactor for exactly Topf 

4 Series Reactions in a CSTR Example I2 6 Multiple Reactions- in a C.STJi The elementary liquid-phase reactions 

Steady-State Nonisothermal Reactor Design—Flow Reactors with Heal Exchange Chapter 12 

Incrementing lempeiature in this manner is an easy way to generate R(T) and CiT) plots 

Analysis Wow We see that five steady states (SS) exist The exit concentrations and temperatures listed in Table El 2-6.2 were determined from the tabular output of the Polymath program. Steady states 1.3 and 5 are stable steady states, while 2 and 4 are 

For series reactions in steady-state operation of a CSTR, it is a matter of determining the stationary-state concentrations of species (product distribution) in or leaving each stage. 

Consider again the first-order network in reaction 5.5-la. From a material balance on A around a single-stage CSTR, 

Similarly, from a material balance on B, with cBo = 0, and input rate = output rate, 

Repeat Example 18-3 for a single-stage CSTR, and compare the results. 

The same result may be obtained from the other form of 18.3-6, in terms of kx, k2, and r. From equation 5.2-5, 

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We will now carry out this same series reaction in a CSTR,... 

Figure 2.12 Time series showing the iodide concentration in the arsenite-iodate reaction in a CSTR. The system is initially in an oxidized steady state (the potential shows a slight downward drift because of an experimental artifact). At the time s indicated by the arrows, a measured amount of acid is injected. With a small injection, the system returns to the steady state, demonstrating the stability of that state. With a larger injection, there is a transition to a second, reduced, steady state. (Adapted from De Kepper et al., 1981a.)...

Cyclic Runs. Having established the inlet ratio of H2 Buta-diene for which the steady-state yield is maximised for a given residence time, the cyclic runs were carried out such that the mean value of the feed compositions were as near as possible to their optimum steady-state values. The mole fractions of H2 and Butadiene were cycled out of phase in a symmetrical square wave fashion. Such symmetrical wave forms need not be the optimum periodic operation. Indeed, Farhad Pour et al. (7 ) demonstrated theoretically that it was possible to obtain further improvement in the selectivity of series-parallel reactions in a CSTR when asymmetrical rather than symmetrical square waves are considered. Theoretically the search for the optimum wave modulation, or the number of switches over one cycle time, can be computed by search methods or optimisation routines. However, in this work we arbitrarily limited ourselves to symmetrical square waves which are 180 out of phase accepting that such a configuration may, indeed, be quite far from the optimum periodic mode. 

It is readily apparent that equation 8.3.21 reduces to the basic design equation (equation 8.3.7) when steady-state conditions prevail. Under the presumptions that CA in undergoes a step change at time zero and that the system is isothermal, equation 8.3.21 has been solved for various reaction rate expressions. In the case of first-order reactions, solutions are available for both multiple identical CSTR s in series and individual CSTR s (12). In the case of a first-order irreversible reaction in a single CSTR, equation 8.3.21 becomes... 

The reaction, 2A C+D, is conducted in a CSTR at a feed rate of 100 cfh with an inlet concentration Ca0 = 1.5 lbmols/cuft. The specific rate in the forward direction is k = 10 amd the equilibrium constant is Ke = 16. 80% of equilibrium conversion is required, (a) Find the vessel size (b) If vessels are limited to one-tenth the size found in part (a), how many such vessels are needed in series ... 

Figure 4-S Ca, Cb, and Cc versus k x for series reactions B — C in a CSTR. Plots shown are for kij k = 0.1, 1.0, and 10. The yield to form B exhibits a maximum at a particular residence time r, ax, but this maximum is lower than in a PFTR.

The other mode of flow reaction employs one or more stirred tanks in series, which is called a continuous stirred tank (CSTR) battery. The rate of reaction in a single tank is... 

If series reactions are conducted in a CSTR, the concentrations in the reactor can be adjusted to influence selectivity and conversion. Because the production of the undesirable product C depends on the concentration of the desired product B, this concentration should be kept small. The reactor can be operated with low conversion (small concentration of B). 

Batch suspension reactors are, theoretically, the kinetic equivalent of water-cooled mass reactors. The major new problems are stabilization of the viscous polymer drops, prediction of particle size distribution, etc. Particle size distribution was found to be determined early in the polymerization by Hopff et al. (28, 29,40). Church and Shinnar (12) applied turbulence theory to explain the stabilization of suspension polymers by the combined action of protective colloids and turbulent flow forces. Suspension polymerization in a CSTR without coalescence is a prime example of the segregated CSTR treated by Tadmor and Biesenberger (51) and is discussed below. In a series of papers, Goldsmith and Amundson (23) and Luss and Amundson (39) studied the unique control and stability problems which arise from the existence of the two-phase reaction system. 

Figure 7.3 also compares the evolution of the concentrations of the intermediate Q and the product in case of two first-order reactions in series in a CSTR with that in a batch or plug flow reactor. For constant density, the mass balance for the reaction components in a CSTR are ... 

Series reactions take place in a CSTR with heat effects. [1st Ed. P9-23]... 

A series parallel reaction takes place in a CSTR (Bequette, 1998).[12] The governing equations are ... 

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. 

Nylon 6,6 is a polycondensation polymer. To develop polymeric properties, step-growth polymers require a high degree of conversion, which cannot be obtained in a CSTR. Putting a number of CSTRs in series makes high extents of reaction possible. 

A very sharp minimum in the pH/time curve has been observed for the IO3/SO3" system, and when reaction was carried out in a CSTR a series of oscillations occurred. The oxidation of HS by acidic BrOj can exhibit oscillating behavior, and the BrOj/SCN" system in HCIO4 in a CSTR exhibits sustained oscillations. 

E17.5 One desires to process a reversible reaction 2A< P + Q at constant volume in a CSTR reactor or several reactors in series. Pure reactant A is fed under flow of 3.5 m /h and initial concentration of 48 kmol/m. The forward reaction rate constant is equal to 0.75 m /(kmol h) and the equilibrium constant K" = 16. If the final conversion is desired to be 85% of the equilibrium conversion, calculate the volume of a single CSTR. If the capacity of the available reactors is only 5% of the calculated volume, how many CSTR reactors in series would be needed ... 

Aris et al. have primarily analyzed whether the steady-state multiplicity features in a CSTR arising from a cubic rate law also can arise for a series of successive bimolecular reactions [26]. Aris et al. have showed that the steady-state equations for a CSTR with bimolecular reactions scheme reduces to that with a cubic reaction scheme when two parameters e(=k,Cg/k j) and K(=kjC /k j) arising in system equations for the bimolecular reactions tend to zero. Aris et al. have shown that the general multiplicity feature of the CSTR for bimolecular reactions is similar to that of the molecular reactions only at smaller value of e and K. The behavior is considerably different at larger values of e and K. Chidambaram has evaluated the effect of these two parameters (e and K) on the periodic operation of an isothermal plug flow reactor [18]. 

The overall selectivity decreases with an increase in total conversion for both CSTR and PFR. For any particular conversion overall selectivity Ob is always higher in a PFR than in a CSTR as shown in Figure 3.18. Thus, the design of a reactor for a series reaction is a trade-off between conversion and selectivity. 

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