Continuous operation stirred tank-CSTR

Due to the behavior of the membrane reactor as a continuously operated stirred tank reactor (CSTR) [2], it can be used effectively to suppress side-reactions, for example the noncatalyzed reduction yielding the racemate in the oxazaborolidine reaction [11]. 

CSTR continuously operated stirred tank reactor... 

A continuously operated stirred tank reactor (CSTR) for use up to 3 kbar and 300"C is shown in Fig. 4.10. The reactor is equipped with a fairly large sapphire window (visual observation and spectroscopic analysis. Spectroscopic studies may be conducted using a reflectance technique developed by Franck and Roth probing light enters the cell, passes through a sample layer with a precisely known thickness, and is reflected from a mirror which is positioned inside the fluid under investiga-... 

Ideal Continuously Operated Stirred Tank Reactor (CSTR)... 

In Figures 3.4 and 3.5, the RTDs of ideal reactors are presented together with the RTD of a real reactor. The ideal, continuously operated stirred tank reactor (CSTR) has the broadest RTD between all reactor types. The most probable residence time for an entering volume element is t = 0. After a mean residence time t = t), 37% of the tracer injected at time t = 0 is still present in the reactor. After five mean residence times, a residue of about 1% still remains in the reactor. This means that at least five mean residence times must pass after a change in the inlet conditions before the CSTR effectively reaches its new stationary state. 

The cascade consists of a series of ideal continuously operated stirred tank reactors, CSTR, connected one after the other. The outlet function of one CSTR is... 

In order to quantify the influence of the gas flow rate on the residence time of the particles a simple model can be used that represents the horizontal apparatus by a series of continuously operated stirred tank reactors (CSTRs). The principle of this model is illustrated in Fig. 7.40. The size (length) and number of the tanks express the intensity of back-mixing (mixing in the direction of solids transport). They are flctitious for an open process chamber (as in Fig. 7.38), but may correspond to... 

Various laboratory reactors have been described in the literature [3, 11-13]. The most simple one is the packed bed tubular reactor where an amount of catalyst is held between plugs of quartz wool or wire mesh screens which the reactants pass through, preferably in plug flow . For low conversions this reactor is operated in the differential mode, for high conversions over the catalyst bed in the integral mode. By recirculation of the reactor exit flow one can approach a well mixed reactor system, the continuous flow stirred tank reactor (CSTR). This can be done either externally or internally [11, 12]. Without inlet and outlet feed, this reactor becomes a batch reactor, where the composition changes as a function of time (transient operation), in contrast with the steady state operation of the continuous flow reactors. 

Impurities, added or unintentional, can have a major effect on rates of nucleation and crystal growth. Table 4-1 shows the effect of an impurity, structurally similar to the crystallizing solute, added to an all-growth crystallization (separation of stereoisomers. Examples 7-6 and 11-6). The data for a continuous stirred tank (CSTR) operation show a sevenfold decrease in the first order growth rate constant as a result of addition of this impurity to prevent nucleation of the undesired isomer. 

Impinging jet crystallization was discussed in Section 9.11 above. One configuration of this type of operation is shown in Fig. 9-22. The impinging jet contacting device delivers its product to an age vessel (either batch or continued stirred tank CSTR) to provide an age time, required for most compounds, to allow diffusion of mother Uquors from the droplets (actually nucleated solids with trapped... 

Bench-scale kinetic experiments can be conducted in batch, continuous stirred-tank (CSTR), tubular plug-flow, or differential reactors. The last of these can be operated with once-through flow or recycle. Advantages and disadvantages of the various types are discussed. In particular Batch reactors are inexpensive, but require attention to rapid attainment of reaction conditions at start CSTRs are excellent for gas-liquid, but less so for gas-phase reactions tubular reactors are especially suited for reactions of heterogeneous catalysis and differential reactors operated "once through" are best for measurement of initial rates. 

Compare these results to those of Equation 2.22 for the same reactions in a batch reactor. The CSTR solutions do not require special forms when some of the rate constants are equal. Intermediate components B and C will exhibit maximum concentration at particular values of t, and a plot of outlet concentrations versus t is qualitatively similar to the behavior shown in Figure 2.2. However, the value for t that gives a maximum in a CSTR will be different than the value of t that gives a maximum in a PFR. For the normal case of bi = 0, the value of t that maximizes bojn is a root mean, fniax = 1/V a s > rather than the log mean of Equation 2.23. The best possible yield of B is lower in a CSTR than in a PFR or batch reactor. Continuous flow stirred tank reactors are almost always worse in terms of selectivity because the entire reactor operates under conditions that favor production of undesired byproducts. 

Batch or semicontinuous processing is appropriate for many laboratory scale operations. However, continuous processes are typically used by industry to allow higher throughput with smaller equipment size. To gain scale-up information, the importance of relatively inexpensive but fully operable pilot plants (or miniplants) cannot be overestimated. Many types of continuous reactors (fixed bed, fluidized bed, continuous stirred tank CSTR) can be adapted for high pressure service. 

We consider that the chemical process studied takes place in an isothermal, continuous-flow stirred tank reactor (CSTR) operated with a constant total flow rate and with constant input concentrations. We assume that at least one of the chemicals entering the system is available in two different forms, unlabeled and labeled, respectively, and that the kinetic isotope effect can be neglected, that is, the kinetic parameters are the same for the labeled... 

Case 2 Continuous Flow-Stirred Tank. For operation of an individual CSTR operating at steady state, material balances on each product species yield the following relations ... 

Consider the reaction system and production requirements discussed in Illustration 10.1. Consider the possibility of using one or more continuous flow stirred-tank reactors operating in series. If each CSTR is to operate at 163 C and if the feed stream is to consist of pure A entering at 20 C, determine the reactor volumes and heat transfer requirements for (1) a single CSTR and (2) three identical CSTRs in series. 

The F t) curve for a system consisting of a plug flow reactor followed by a continuous-flow stirred-tank reactor is identical to that of a system in which the CSTR precedes the PFR. Show that the overall fraction conversions obtained in these two combinations are identical for the case of an irreversible first-order reaction. Assume isothermal operation. 

The continuous-flow stirred tank reactor (CSTR), which is sometimes also more accurately abbreviated as a CFSTR, is a form of a continuous reactor that is often viewed as the opposite of a PFR in terms of mixing. That is, perfect mixing is assumed to occur in a CSTR so that the contents inside the CSTR vessel are evenly distributed and that the reactor operates at a single concentration. Under this assumption, the CSTR effluent concentration is identical to the concentration inside the reactor itself. (This behavior is very different from... 

Reactors are mostly not isothermal, as heat is consumed or released, and perfect mixing or a perfect heat exchange with the surrounding is impossible. However, some reactors are almost isothermal, such as, for example, a well-mixed continuous stirred tank reactor (CSTR). In a batchwise operated stirred tank or in a plug-flow reactor (PFR), isothermal conditions with regard to reaction or residence time (axial position), respectively, are hard to realize. However, the assumption of an isothermal system is helpful for a first examination of reactor types as it simplifies the equations and we can focus on concentration and mixing effects only. Thus, here, we inspect isothermal reactors. Thermal effects are considered in Section 4.10.3. 

Despite the higher cost compared with ordinary catalysts, such as sulfuric or hydrochloric acid, the cation exchangers present several features that make their use economical. The abiHty to use these agents in a fixed-bed reactor operation makes them attractive for a continuous process (50,51). Cation-exchange catalysts can be used also in continuous stirred tank reactor (CSTR) operation. 

Over 25 years ago the coking factor of the radiant coil was empirically correlated to operating conditions (48). It has been assumed that the mass transfer of coke precursors from the bulk of the gas to the walls was controlling the rate of deposition (39). Kinetic models (24,49,50) were developed based on the chemical reaction at the wall as a controlling step. Bench-scale data (51—53) appear to indicate that a chemical reaction controls. However, flow regimes of bench-scale reactors are so different from the commercial furnaces that scale-up of bench-scale results caimot be confidently appHed to commercial furnaces. For example. Figure 3 shows the coke deposited on a controlled cylindrical specimen in a continuous stirred tank reactor (CSTR) and the rate of coke deposition. The deposition rate decreases with time and attains a pseudo steady value. Though this is achieved in a matter of rninutes in bench-scale reactors, it takes a few days in a commercial furnace. 

In previous studies, the main tool for process improvement was the tubular reactor. This small version of an industrial reactor tube had to be operated at less severe conditions than the industrial-size reactor. Even then, isothermal conditions could never be achieved and kinetic interpretation was ambiguous. Obviously, better tools and techniques were needed for every part of the project. In particular, a better experimental reactor had to be developed that could produce more precise results at well defined conditions. By that time many home-built recycle reactors (RRs), spinning basket reactors and other laboratory continuous stirred tank reactors (CSTRs) were in use and the subject of publications. Most of these served the original author and his reaction well but few could generate the mass velocities used in actual production units. 

There are a variety of ways of accomplishing a particular unit operation. Alternative types of process equipment have different inherently safer characteristics such as inventory, operating conditions, operating techniques, mechanical complexity, and forgiveness (i.e., the process/unit operation is inclined to move itself toward a safe region, rather than unsafe). For example, to complete a reaction step, the designer could select a continuous stirred tank reactor (CSTR), a small tubular reactor, or a distillation tower to process the reaction. 

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