Reactions three phase continuous processes

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. 

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. 

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. 

In a first approximation, the new methods correspond to the conventional solvent techniques of supported catalysts (cf Section 3.1.1.3), liquid biphasic catalysis (cf Section 3.1.1.1), and thermomorphic ( smart ) catalysts. One major difference relates to the number of reaction phases and the mass transfer between them. Owing to their miscibility with reaction gases, the use of an SCF will reduce the number of phases and potential mass transfer barriers in processes such as hydrogenation, carbonylations, oxidation, etc. For example, hydroformylation in a conventional liquid biphasic system is in fact a three-phase reaction (g/1/1), whereas it is a two-phase process (sc/1) if an SCF is used. The resulting elimination of mass transfer limitations can lead to increased reaction rates and selectiv-ities and can also facilitate continuous flow processes. Most importantly, however, the techniques summarized in Table 2 can provide entirely new solutions to catalyst immobilization which are not available with the established set of liquid solvents. 

Chemisorption reactions in soils, which are two-dimensional surface processes, can rarely be separated experimentally from the three-dimensional nucleation and precipitation reactions. It is perhaps best to view the removal of adsorbate ions from solution, broadly termed sorption, as a continuous process that ranges from chemisorption (at the low end of solubility) to precipitation (at the high end of solubility). Unless a new solid phase can be detected, the onset of precipitation and termination of chemisorption during sorption is usually not recognized by experimentalists. For this reason, an understanding of sorption necessitates some knowledge of precipitation reactions, which will be outlined here. 

Calcium carbide is manufactured today in a dosed furnace12 lined internally with refractory bricks and equipped with three electrodes positioned in a triangular layout, fabricated in situ with coke and lime hues from the processes (Sodeberg electrodes). These electrodes are suspended vertically above the furnace and introduced progressively into the litne/coal mixture, in which they cause partial fusion and mutual reaction. They are continuous, but usually have hollow cores to allow the injection of raw material fines from the feed or dost removal (Knapsack plant). They axe supplied with three-phase an. power at a voltage of 100 to 250 V, with a current density less than 10 A/cm2 of electrode surface area. Due to the reaction s poor energy eflidency, electricity consumption may be as high as 3.3 kWh/kg of carbide. 

In the beginning of the cumene oxidation process, oxidation was carried out as a three-phase reaction [25—27], the so-called wet oxidation. In addition to cumene and air, an aqueous sodium carbonate solution was continuously added to the reactors to extract and neutralize organic acids that are formed during oxidation. Phenolchemie, now INEOS Phenol, was the first to operate reactors without adding any caustic soda or sodium carbonate [28], which is called dry oxidation. Such a process is easier to handle and even leads to higher yields. 

A variety of ion exchange resins with strong and weak acid, weak base, and quaternary ammonium ion functionality are available in bead form well suited for filtration from reaction mixtures and for use in continuous flow processes. They have been used for >30 years in flow systems for water deionization. Sulfonic acid resins are already used on a large scale as catalysts for the addition of methanol to isobutylene to form methyl terr-butyl ether, for the hydration of propene to isopropyl alcohol, and for a variety of smaller scale processes. Tertiary amine resins have been used as catalysts for the addition of alcohols to isocyanates to form urethanes. The quaternary ammonium ion resins could be used as reagents with any of a large number of counter ions, and as catalysts in two and three phase reaction mixtures, although the author is not aware of any commercial process of this sort at present. 

Analogously to batch distillation and the RCM, the simplest means of reactive distillation occurs in a still where reaction and phase separation simultaneously take place in the same unit. Additionally, we can choose to add a mixing stream to this still, and the overall process thus consists of three different phenomena chemical reaction, vapor liquid equilibrium, and mixing. Such a system is referred to as a simple reactive distillation setup. This setup is shown in Figure 8.1 where a stream of flowrate F and composition Xp enters a continuously stirred tank reactor (CSTR) in which one or more chemical reaction(s) take place in the liquid phase with a certain reaction rate r =f(kf, x, v) where v represents the stoichiometric coefficients of the reaction. Reactants generally have negative stoichiometric coefficients, while products have positive coefficients. For example, the reaction 2A + B 3C can... 

Catalysts have been bonded to insoluble polymeric matrices to allow, in principle, noticeable simplifications of PTC the catalyst is a third insoluble phase which, at the end of the reaction, can be isolated by simple filtration and then recycled, thus avoiding the tedious processes of distillation, chromatographic separation, and so on. This is of potential interest from the industrial point of view, due to the possibility of carrying on both discontinuous processes with a dispersed catalyst and continuous processes with a catalyst on a fixed bed. This technique was introduced by Regen, and called three-phase-catalysis ... 

Second, a study performed by Lonza (Visp, Switzerland), dealing with current benefits and drawbacks of microreaction technology, was presented [5]. The authors depicted the kind of reaction that prevails in the FCPI and classified them in three main classes. Type A, B and C (very fast, rapid and slow reactions, respectively). Thqr reasoned that up to 50% of the reactions performed at Lonza could benefit from continuous processing. For 44% of lhem a microreactor would be the preferred reaction device. However, the handling of solids reduces the number of reaction candidates to less than 20%. The authors therefore emphasized the development of multi-purpose microreactor modules that can deal with solid phases. Further, Roberge et al. [5] emphasized the potential of MRT to reduce labor costs by highly automated and efficient processes. 

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