Venturi loop reactor phase

The solid phase (catalyst/reactant) shows different behavior in different mnlti-phase reactors. In sparged reactors, there is an exponential decay of the solid concentration along the vertical axis. For stirred multiphase reactors operated at the critical speed for just suspension of the solid, N, there is a substantial variation in axial solid concentration. The speed required for achieving uniform solid concentration and the corresponding power input are relatively very high (Nienow 1969, 2000 Shaw 1992). Hence, most stirred multiphase reactors operate at rather than For venturi loop reactors, Bhutada and Pangarkar (1989) have shown that above a certain power input at which the three-phase jet reaches the reactor bottom, the solid concentration is uniform both axially and radially. In this respect, venturi loop reactor is a definitely better option (Chapter 8). 

The aforementioned discussion was general in nature and also included conventional contactors such as tray and packed columns. In the case of three-phase (G-L-S) reactions, such conventional contactors are not used. The stirred reactor is the workhorse of the fine chemicals industry. The gas-inducing reactor can be considered as an alternative to stirred reactors when a pure gas is used. However, this reactor has several drawbacks (Chapter 9). In view of this, the venturi loop reactor has been widely used as a safe and energy-efficient alternative to the conventional stirred reactor. Table 3.3 summarizes the preceding discussion in the form of a multiphase reactor selection guide. 

The venturi loop reactor is ideally suited for fast reactions involving a pure expensive gas-phase reactant and with simultaneous requirements of relatively high reaction pressure (>2MPa) and heat removal lOOkJ/mol) (Chapter 3 and Table 3.3). 

Three-phase catalytic reactions are the focus of discussion in the present chapter. Further, in most applications, the catalysts used are expensive. Therefore, effective utilization and snstenance of the catalyst activity are important aspects in the economics. This initial discnssion is based on the value of the Hatta number defined by Equation 2.4. The reactor ntilization is related to its ability to achieve the intrinsic enhancement of the reaction rate by the catalyst nsed. As discussed in Section 2.2, the intrinsic kinetics of the catalyzed reaction are nnaffected by the type of multiphase reactor. Therefore, the reactor utilization needs to be reinterpreted keeping in view the large changes in both and a that are possible with a venturi loop reactor. 

However, if this same reaction is carried out in a venturi loop reactor, the order of magnitude large values of k a in the venturi section is likely to result in a situation such that Equation 2.6 is valid. Consequently, the mass transfer limitation can be eliminated when the venturi loop reactor replaces the stirred tank type. Thus, the reaction can achieve the maximum intrinsic rate or operate at the maximum possible capacity. This matter has been briefly discussed in Section 3.4.2.4 for Uquid-phase oxidation of substituted benzenes. The solved reactor design problem in Section 8.13 shows that this is indeed the case for catalytic hydrogenation of aniline to cyclohexylamine. 

We now consider three-phase (gas-liquid-solid) reactions that constitute a major class of application of the venturi loop reactor. Figure 2.2 depicts the concentration profiles for this class of reactions. The overall rate of the gas-liquid-sohd-catalyzed reaction for a first-order reaction is given by Equation 2.16 ... 

In bubble columns, agitation of the liquid phase, and hence suspension of the catalyst is effected by the gas flow. The gas is often recycled to cause more turbulence and thus better mixing. Circulation of the liquid is often required to obtain a more uniform suspension. This can either be induced by the gas flow (airlift loop reactor) or by use of an external pump. In the latter instance it is possible to return the slurry to the reactor at a high flow rate through an ejector (Venturi tube). The local under-pressure causes the gas to be drawn into the passing stream, thus affording very efficient mixing. This type of reactor is called a Jet-loop or Venturi reactor. 

Most of the studies referred to in the previous discussion used two-phase (gas-liquid) systems. The considerations are substantially similar when the liquid jet ejector is to be used as a three-phase catalytic reactor, particularly because the catalyst loading commonly used in a venturi loop system is relatively low. In terms of mass transfer, besides gas-liquid mass transfer, solid-liquid mass transfer step assumes great importance. As discussed in Chapters 7A and 7B, factors relating to dispersion of the gas phase, suspension of solids, and concentration profile of the catalyst phase need to be addressed in the case of a three-phase reactor. 

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