Three phase continuous reactions

While most laboratory scale reactions are run in a batch mode there are a large number of commercially important processes that are run in a continuous manner. Since some of the continuous reactors described in Chapter 6 are potentially useful for laboratory scale processes and many lab scale reactions will have to be adapted to large scale continuous flow systems, a knowledge of the effect that reaction variables can have on heterogeneously catalyzed reactions run in a continuous manner is important. 

---

The three-phase continuous countercurrent fluidized-bed reactor and the spouted-bed reactor have been used on the laboratory scale. Pruden and Weber88 have shown that the countercurrent mode of operation for hydrogenation of a-methyl styrene performs better than the cocurrent fixed-bed operation under similar reaction conditions. 

The key issue in effective catalytic oxidation of organics is finding a suitable catalyst. Oxidation of aqueous phenol solutions by using different transition metal oxides as heterogeneous catalysts is already known [4-6]. On the other hand, the potential of molecular sieves to catalyze oxidative phenol destruction has not been examined yet. The objective of this contribution is to provide kinetic and mechanistic data on the catalytic liquid-phase oxidation of aqueous phenol solutions obtained in the presence of various transition metal oxides and molecular sieves. The reaction was studied in a semibatch slurry as well as two-and three-phase continuous-flow reactors. Another matter of concern was the chemical stability of catalysts under the reaction conditions. 

To summarize, transient kinetic experiments are an established and valuable tool in the investigation of heterogeneously catalysed gas phase reactions. For liquid-phase systems, transient studies are much more rare than for gas-phase systems. It is probably related to slower dynamics and the fact that the intrinsic kinetic phenomena can be obscured by mass transfer effects and catalyst deactivation. As an illustration (Figure 8.11) we will consider three-phase continuous hydrogenation of an organic compound leading to two products over a metal catalyst on a structured support (knitted silica). 

For the strongly contracting phase volumes associated with chemical reactions, the three-dimensional continuous-... 

Ma.nufa.cture. Isophorone is produced by aldol condensation of acetone under alkaline conditions. Severe reaction conditions are requited to effect the condensation and partial dehydration of three molecules of acetone, and consequendy raw material iaefftciency to by-products is limited by employing low conversions. Both Hquid- and vapor-phase continuous technologies are practiced (186,193,194). 

The effect of physical processes on reactor performance is more complex than for two-phase systems because both gas-liquid and liquid-solid interphase transport effects may be coupled with the intrinsic rate. The most common types of three-phase reactors are the slurry and trickle-bed reactors. These have found wide applications in the petroleum industry. A slurry reactor is a multi-phase flow reactor in which the reactant gas is bubbled through a solution containing solid catalyst particles. The reactor may operate continuously as a steady flow system with respect to both gas and liquid phases. Alternatively, a fixed charge of liquid is initially added to the stirred vessel, and the gas is continuously added such that the reactor is batch with respect to the liquid phase. This method is used in some hydrogenation reactions such as hydrogenation of oils in a slurry of nickel catalyst particles. Figure 4-15 shows a slurry-type reactor used for polymerization of ethylene in a sluiTy of solid catalyst particles in a solvent of cyclohexane. 

Table 11.4 lists reactors used for systems with two fluid phases. The gas-liquid case is typical, but most of these reactors can be used for liquid-liquid systems as well. Stirred tanks and packed columns are also used for three-phase systems where the third phase is a catal5hic solid. The equipment listed in Table 11.4 is also used for separation processes, but our interest is on reactions and on steady-state, continuous flow. 

Column reactors can contain a draft tube - possibly filled with a packing characterized by low pressure drop - or be coupled with a loop tube, to make the gas recirculating within the reaction zone (see Fig. 5.4-9). In recent years, the Buss loop reactor has found many applications in two- and three-phase processes About 200 Buss loop systems are now in operation worldwide, also in fine chemicals plants. This is due to the high mass-transfer rate between the gas and the liquid phase. The Buss loop reactor can be operated semibatch-wise or continuously. As a semibach reactor it is mostly used for catalytic hydrogenations. 

The kinetics of a liquid-phase chemical reaction are investigated in a laboratory-scale continuous stirred-tank reactor. The stoichiometric equation for the reaction is A 2P and it is irreversible. The reactor is a single vessel which contains 3.25 x 10 3 m3 of liquid when it is filled just to the level of the outflow. In operation, the contents of the reactor are well stirred and uniform in composition. The concentration of the reactant A in the feed stream is 0.5 kmol/m3. Results of three steady-state runs are ... 

This term is restricted here to equipment in which finely divided solids in suspension interact with gases. Solids fluidized by liquids are called slurries. Three phase fluidized mixtures occur in some coal liquefaction and petroleum treating processes. In dense phase gas-solid fluidization, a fairly definite bed level is maintained in dilute phase systems the solid is entrained continuously through the reaction zone and is separated out in a subsequent zone. 

The typical PCR run (Fig. 7.1) consists of three cycles denaturation (1-2 min at 3 94°C), primer annealing (1-2 min at 50-55°C), and extension (1-2 min at 72°C). This design also requires optimization for each particular PCR. The desired blunt-ended duplex product does not appear until after the third cycle, whereupon it accumulates exponentially in subsequent cycles. The number of cycles required will depend on the efficiency of the reaction per cycle. Once the desired product has reached about 1012 copies, PCR efficiency drops significantly, and product stops amassing exponentially. This is the plateau phase continuing PCR beyond this point often results in contaminating by-products rather than more product (Cha and Thilly, 1993). 

As schematically shown in Figure 7a, initial PEVD reaction and product nucleation occurs at the three-phase boundary of solid electrolyte (E), working electrode (W) and the sink vapor phase (S) which contains vapor phase reactant (B). Only here are all reactants available for the half-cell electrochemical reaction at the sink side of a PEVD system. Although the ionic and electronic species can sometimes surface diffuse at elevated temperature to other sites to react with (B) in the vapor phase, the supply of the reactants continuously along the diffusion route is less feasible and the nuclei are too small to be stabilized under normal PEVD conditions. Only along the three phase boundary line are all the reactants available for further growth to stabilize the nuclei. Consequently, initial deposition in a PEVD process is restricted to certain areas on a substrate where all reactants for the sink electrochemical reaction are available. 

---