CSTR space time

Since a CSTR operates at or close to uniform conditions of temperature and composition, its kinetic and product parameters can usually be predicted more accurately and controlled with greater ease. The CSTR can often be operated at a selected conversion level to optimize space-time yield, or where a particular product parameter is especially favored. 

It is important to understand that the time constant xp of a process, say, a stirred tank is not the same as the space time x. Review this point with the stirred-tank heater example in Chapter 2. Further, derive the time constant of a continuous flow stirred-tank reactor (CSTR) with a first-order chemical reaction... 

Unlike the situation in the PFR, there is always a simple relationship between the mean residence time and the reactor space time for a CSTR. Since one normally associates a liquid feed stream with these reactors, volumetric expansion effects are usually negligible (SA = 0). 

In order to reduce the disparities in volume or space time requirements between an individual CSTR and a plug flow reactor, batteries or cascades of stirred tank reactors ard employed. These reactor networks consist of a number of stirred tank reactors confiected in series with the effluent from one reactor serving as the input to the next. Although the concentration is uniform within any one reactor, there is a progressive decrease in reactant concentration as ohe moves from the initial tank to the final tank in the cascade. In effect one has stepwise variations in composition as he moves from onfe CSTR to another. Figure 8.9 illustrates the stepwise variations typical of reactor cascades for different numbers of CSTR s in series. In the general nonisothermal case one will also en-... 

Each of these problems will be considered in turn. Consider the three ideal CSTR s shown in Figure 8.11. The characteristic space times of these reactors may differ widely. Note that the direction of flow is from right to left. The first step in the analysis requires the preparation of a plot o>f reaction rate versus reactant concentration based on experimental data (i.e., the generation of a graphical representation of equation 8.3.30). It is presented as curve I in Figure 8.11. 

In this case there are two intermediate unspecified reactant concentrations instead of just the single intermediate concentration encountered in Case II. At least one of these concentrations must be determined if one is to be able to appropriately size the reactors. In principle one may follow the procedure used in Case II where the design equations for each CSTR are written and the reactor space times then equated. This procedure gives three equations and three unknowns (VRl9 fBl, and fB2). Thus, for the first reactor,... 

For a CSTR equal in volume to the tubular reactor, one moves along a line of constant kCB0T in Figure 8.16 in order to determine the conversions accomplished in cascades composed of different numbers of reactors but with the same overall space time. The intersection of the line/cCg0T = 19.6 and the curve for N = 1 gives fB = 0.80. 

If a monomer solution at a concentration of 1 mole/liter is fed to a CSTR at 0 °C, determine the space time necessary to achieve a conversion corresponding to 90% of the equilibrium value. If the reactor volume is 100 liters, what is the corresponding volumetric flow rate ... 

Different reactor networks can give rise to the same residence time distribution function. For example, a CSTR characterized by a space time Tj followed by a PFR characterized by a space time t2 has an F(t) curve that is identical to that of these two reactors operated in the reverse order. Consequently, the F(t) curve alone is not sufficient, in general, to permit one to determine the conversion in a nonideal reactor. As a result, several mathematical models of reactor performance have been developed to provide estimates of the conversion levels in nonideal reactors. These models vary in their degree of complexity and range of applicability. In this textbook we will confine the discussion to models in which a single parameter is used to characterize the nonideal flow pattern. Multiparameter models have been developed for handling more complex situations (e.g., that which prevails in a fluidized bed reactor), but these are beyond the scope of this textbook. [See Levenspiel (2) and Himmelblau and Bischoff (4).]... 

Use the F(t) curve for two identical CSTR s in series and the segregated flow model to predict the conversion achieved for a first-order reaction with k = 0.4 ksec-1. The space time for an individual reactor is 0.9 ksec. Check your results using an analysis for two CSTR s in series. 

Calculate the mean residence time (t) and space time (t) for reaction in a CSTR for each of the following cases, and explain any difference between (f) and t ... 

Second, for a flow reactor, such as a CSTR, the mean residence time and space time are equal, since q = q0 ... 

The following example illustrates a similar comparison through time quantities space time r for a CSTR, and t for a BR. 

For a CSTR, from Section 18.3.2, the mean residence time (equal to the space time... 

Consider the conversion of methanol in a 50-L reactor (volume of catalyst) similar to that shown in Figure 1.2 (which operates like a CSTR). The reactor contains 800 g of catalyst (zeolite H-ZSM5), and the space time through the reactor is 0.1 h The methanol feed rate is 1.3 kg h-1. For each reaction temperature, determine the yield and selectivity to each olefin, and comment on your results. 

Consider the flow mixing through three identical ideal CSTRs in series. Each tank has a space time, r, or mean residence time, Mj, of 2 min. An idealised impulse of tracer is made in the inlet to the first tank what tracer response will be observed from the third tank ... 

The packed-bed reactor configuration commonly employed with immobilized enzymes yields a higher degree of conversion or higher space-time yield than a CSTR, the typical configuration for soluble enzymes. Examples are the nitrile hydratase-catalyzed process to 5-cyanovaleramide (5-CVAM) (Chapter 7, Section 7.1.1.3) or the decarboxylation of D,L-aspartate to D-aspartate, L-alanine, and C02 (Section 7.2.2.5). 

Time is still an important variable for continuous systems, but it is modified to relate to the steady-state conditions that exist in the reactor. This time variable is referred to as space time. Space time is the reactor volume divided by the inlet volumetric flow rate. In other words, it is the time required to process one reactor volume of feed material. Since concentration versus real time remains constant during the course of a CSTR reaction, rate-data acquisition requires dividing the difference in concentration from the inlet to the outlet by the space time for the particular reactor operating conditions. 

Figure 15. Effect of segregation on polymerization of styrene in cyclohexane solution. Standard CSTR with h baffles and a 6-blade turbine, V = 670 cm, T = 75 °C. Dispersion Index DI vs. space time. Influence of agitation speed. Curves S (segregated flow) and M (well-micromixed flow) calculated from batch experiments. Initiator PERKAD0X l6, A = 0.033 mol L - -, kd = 5 x 10 5 s-1, f = 0.85 Mq = 6.65 mol L l, SQ = 2.22 mol IT1.

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