Reactor selection, multiphase reactions

In the foregoing, only points that are relevant to multiphase reactions/reactors are dealt. Points 1,2,6,9, and 12 are synonymous, implying (directly or indirectly) high selectivity of conversion of a given reactant to the desired product. The limit of selective processes is the acid-base (electron transfer) reaction. This reaction goes not only to completion but is also instantaneous (Section 1.7). The same cannot be... 

Microlevel. The starting point in multiphase reactor selection is the determination of the best particle size (catalyst particles, bubbles, and droplets). The size of catalyst particles should be such that utilization of the catalyst is as high as possible. A measure of catalyst utilization is the effectiveness factor q (see Sections 3.4.1 and 5.4.3) that is inversely related to the Thiele modulus (Eqn. 5.4-78). Generally, the effectiveness factor for Thiele moduli less than 0.5 are sufficiently high, exceeding 0.9. For the reaction under consideration, the particles size should be so small that these limits are met. 

There are of course a number of well established alternatives to the stirred tank used in the large scale chemical industries, and strategies have been proposed for selection of the best reactor for a given multiphase reaction The nature of flow and contacting in these reactors differs significantly. Industrial applications are largely dominated however by a small number of these ... 

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. 

Various parameters must be considered when selecting a reactor for multiphase reactions, such as the number of phases involved, the differences in the physical properties of the participating phases, the post-reaction separation, the inherent reaction nature (stoichiometry of reactants, intrinsic reaction rate, isothermal/ adiabatic conditions, etc.), the residence time required and the mass and heat transfer characteristics of the reactor For a given reaction system, the first four aspects are usually controlled to only a limited extent, if at aH, while the remainder serve as design variables to optimize reactor performance. High rates of heat and mass transfer improve effective rates and selectivities and the elimination of transport resistances, in particular for the rapid catalytic reactions, enables the reaction to achieve its chemical potential in the optimal temperature and concentration window. Transport processes can be ameliorated by greater heat exchange or interfadal surface areas and short diffusion paths. These are easily attained in microstructured reactors. 

Figure 1.2 Exploitable features of membrane reactors, (a) Enhancing the conversion of a reversible reaction in a packed-bed inert membrane reactor, (b) Enhancing the conversion of a reversible reaction in a catalytic membrane reactor, (c) Preventing slip in a reaction requiring stoichiometric feeds, (d) Enhancing the rate of a multiphase reaction, (e) Energetic, thermodynamic, or kinetic coupling of two reactions run on opposite sides of a membrane, (f) Hybrid of fixed-bed reactor (PER) and selective inert membrane reactor (IMR-P) in series. 79...

Figure 13.2 Types of membrane reactors, (a) IMR-P, (b) IMMR-P, (c) hollow membrane tube reactor with catalyst in shell (another version of IMR-P), (d) fluidized-bed Inert selective membrane reactor (IMR-F), (e) CMR-E, (f) CMR-P, (g) catalytic nonreactive membrane reactor (CNMR-E), (h) catalytic nonselective hollow membrane reactor (CNHMR-E) for multiphase reactions G = gas, L = liquid, and (i) immobilized-enzyme membrane reactor (lEMR). (Adapted from Shao, X., Xu, S., and Govind, R AlChE Symp. Ser., 268, 1, 1989.)...

Future Trends in Reactor Technology The technical reactors introduced here so far are those used today in common industrial processes. Of course, research and development activities in past decades have led to new reactor concepts that may have advantages with respect to process intensification, higher selectivities, and safety and environmental aspects. Such novel developments in catalytic reactor technology are, for example, monolithic reactors for multiphase reactions, microreactors to improve mass and heat transfer, membrane reactors to overcome thermodynamic and kinetic constraints, or multifunctional reactors combining a chemical reaction with heat transfer or with the separation in one instead of two units. It is beyond the scope of this textbook to cover all the details of these new fascinating reactor concepts, but for those who are interested in a brief outline we summarize important aspects in Section 4.10.8. 

We regard the essential aspects of chemical reaction engineering to include multiple reactions, energy management, and catalytic processes so we regard the first seven chapters as the core material in a course. Then the final five chapters consider topics such as environmental, polymer, sohds, biological, and combustion reactions and reactors, subjects that may be considered optional in an introductory course. We recommend that an instmctor attempt to complete the first seven chapters within perhaps 3/4 of a term to allow time to select from these topics and chapters. The final chapter on multiphase reactors is of course very important, but our intent is only to introduce some of the ideas that are important in its design. 

The mass balances [Eqs. (Al) and (A2)] assume plug-flow behavior for both the gas/vapor and liquid phases. However, real flow behavior is much more complex and constitutes a fundamental issue in multiphase reactor design. It has a strong influence on the reactor performance, for example, due to back-mixing of both phases, which is responsible for significant effects on the reaction rates and product selectivity. Possible development of stagnant zones results in secondary undesired reactions. To ensure an optimum model development for CD processes, experimental studies on the nonideal flow behavior in the catalytic packing MULTIPAK are performed (168). 

The selection of a reaction system for multiphase reactors is a complicated matter that cannot be covered in the limited space of this book. Here we mention the comprehensive paper of Krishna and Sie (1994) addressing this topic. 

---