Venturi loop reactors

Minimum suspension criteria for multiimpeller reactors are given by Dutta and Pangarkar [33], Data for venturi-loop reactors are limited [34-35], However, because of continuously sucking the slurry away from the bottom by the recycle pump, possibly not many problems on solids settling occur. 

In venturi loop reactors the gas flow rate is not an independent variable. Both gas flow rate and holdup are very sensitive to the venturi design parameters and gas and liquid properties. [8, 53-56]. 

Applying high energy input reactors such as venturi loop reactors may have an extra bcnificial effect. Hence, slurry reactors which fit onto a table and which still have a capacity of kilotons per year are probably within reach, provided the heat effects involved in the reaction can be handled. The heat transfer in slurry reactors using built-in tubes, has been studied [40-43, 124-127],... 

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. 

Criteria Trickle bed Stirred tank reactor Gas- inducing reactor Venturi loop reactor Sparged reactors Microstructured Structured trickle beds ... 

Duveen (1998) has suggested the use of a venturi loop reactor for oxidations with pure oxygen in a manner exactly analogous to the Praxair Uquid oxidation reactor. The operation in a dead-end mode has been claimed to produce practically no vent gas. It must be noted here that burning of acetic acid and the consequent products cannot be avoided. CO and other products formed must be purged. To this extent, the operation is likely to be similar to the Praxair liquid oxidation reactor. 

These results will be compared with those for the venturi loop reactor in Section 8.13. 

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). 

In the following sections, a brief overview of the areas of application and advantages of the venturi loop reactor is presented. 

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 ... 

TABLE 8.1 Industrially Important Reactions Carried Out in Venturi Loop Reactors with Relative Merits Over Stirred Reactors... 

Enhanced yield of cyclohexylamine Yield increased from 81% in conventional stirred reactor to 93.5% (please refer to Section 8.13, Worked Examples for Design of Venturi Loop Reactor Hydrogenation of Aniline to Cyclohexylamine) Relatively short time of 4h at a lower pressure of 2.5 MPa. Unreacted sugar as low as 0.03% Catalyst consumption reduced by 80% as compared to conventional stirred reactor... 

Batch time lowered by a factor of 2 catalyst loading reduced by factor of 2 Solventless system increases productivity as compared to conventional stirred tank reactor. Reaction time reduced by 80% yield increased from 83 to 94% Appropriate design of venturi loop reactor reduces chlorination batch time by a factor of 5-7 from 20 to 40 h required in a conventional stirred tank reactor. Highly efficient heat transfer in the external loop affords nearly isothermal operation at 45-50 °C, resulting in a high yield (more than 90%) of 2,4-dichloro phenol while suppressing formation of 2,6 dichloro phenol. Efficient removal of reaction by-product (hydrogen chloride) in the external gas circuit (Fig. 8.3) improves the yield Almost complete conversion of methanol in less than 1 h... 

ADVANTAGES OF THE VENTURI LOOP REACTOR A DETAILED COMPARISON... 

Pretreatment of exhausts containing volatile organic compounds to ensure stable operation of biofilters venturi loop reactor using aqueous surfactant solution yields stable operation of biofilters despite concentration excursions of the volatile organic compounds Park et al. (2008)... 

Increase in total pressure implies a higher capital cost of the reactor and is therefore not the optimum solution. As mentioned earlier, the gas-liquid mass transfer and solid-liquid mass transfer coefficients offered by a venturi loop reactor are at least an order of magnitude higher than their nearest competitor, the conventional stirred tank reactor or gas-inducing reactor. Therefore, if the gas-liquid mass transfer coefficient can be so increased by a factor of 10 through the use of a venturi loop reactor, then the sole reason for excessive reactor pressure to achieve the required higher value of mass transfer rate vanishes. In conclusion, the venturi loop reactor can allow operation at much lower pressures as compared to the stirred reactor or its variants. 

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