The Continuous Reactor

Specific reactor characteristics depend on the particular use of the reactor as a laboratory, pilot plant, or industrial unit. AH reactors have in common selected characteristics of four basic reactor types the weH-stirred batch reactor, the semibatch reactor, the continuous-flow stirred-tank reactor, and the tubular reactor (Fig. 1). A reactor may be represented by or modeled after one or a combination of these. SuitabHity of a model depends on the extent to which the impacts of the reactions, and thermal and transport processes, are predicted for conditions outside of the database used in developing the model (1-4). 

We have introduced four main reactor types in this chapter the batch reactor, the continuous-stirred-tank reactor (C TR), the semi-batch reactor, and the plug-flow reactor (PFR). Table 4.3 summarizes the mole... 

In models for continuous ideal reactors, the continuously stirred tank reactor, CSTR, and the plug flow tube reactor, PFTR, are distinguished. Both are shown schematically in Figure 4-2. Both are characterized by a simultaneous input feed of educts and solvents on one side and a removal of the reaction mixture on the other side with a constant reaction rate. As a consequence, the reaction volume remains constant in both reactors throughout the reaction period. 

Adding a tracer spike to an ideal mixed flow reactor produces a step increase in the tracer concentration just as in the batch reactor case, but in this case the tracer s concentration declines over time as the reactor s effluent carries it away. If the amount of solution in the reactor remains constant and no more tracer is added by the feed solution or by generation in the reactor, the continuity equation simplifies to make the rate of accumulation equal to the rate of loss by the effluent flow. 

In this example we consider a cell population in a batch stirred reactor where the cells are distributed according to their age, denoted t, ranging between 0 and 00. The main purpose of this example is to demonstrate the boundary condition that arises. Each cell beyond a certain age has a constant rate of division, say, k. The division of the cell of age t results in the loss of that cell, but also in the gain of two new cells of age zero each. The only particle state variable is the cell age t. No external coordinates are needed because the population is in a well-stirred reactor. The continuous phase is assumed to have no explicit influence on the cells, presumably because the necessary nutrients are present in saturating proportions. We again have the particle state domain = [0, 00). 

Feed purification generally involves absorption, adsorption, extraction, and/or distillation. Reaction involves agitated batch, agitated semibatch, continuous stirred tank, or continuous flow reactors. The continuous flow reactors may be empty or contain a mass of solid catalyst. Product separation and purification involves distillation in the petrochemical industry or extraction and crystallization in the extractive metallurgy and pharmaceutical industries absorption is used to a lesser extent. 

The following example deals with the effect of fouling by coke deposition in an isothermal reactor. The continuity equation for the reactant A may be written in terms of mole fractions, assuming that both the density and the number of moles remain constant [see Froment and Bischoff, 1961] ... 

The mass transfer resistances in a multiphase reactor at the gas-liquid and liquid-solid interfaces are shown in Figure 7.1 (Smith 1981). In the case of a slurry reactor, the continuous gas-phase shown in the figure should be replaced by a gas bubble. If equilibrium exists at the gas-liquid boundary,... 

In the second model (Fig. 2.16) the continuous well-stirred model, feed and product takeoff are continuous, and the reactor contents are assumed to he perfectly mixed. This leads to uniform composition and temperature throughout. Because of the perfect mixing, a fluid element can leave at the instant it enters the reactor or stay for an extended period. The residence time of individual fluid elements in the reactor varies. 

By contrast, if the reactor is continuous well-mixed, then the reactor is isothermal. This behavior is typical of stirred tanks used for liquid-phase reactions or fluidized-bed reactors used for gas-phase reactions. The mixing causes the temperature in the reactor to be effectively uniform. 

Single-reaction-step processes have been studied. However, higher selectivity is possible by optimizing catalyst composition and reaction conditions for each of these two steps (40,41). This more efficient utilization of raw material has led to two separate oxidation stages in all commercial faciUties. A two-step continuous process without isolation of the intermediate acrolein was first described by the Toyo Soda Company (42). A mixture of propylene, air, and steam is converted to acrolein in the first reactor. The effluent from the first reactor is then passed directiy to the second reactor where the acrolein is oxidized to acryUc acid. The products are absorbed in water to give about 30—60% aqueous acryUc acid in about 80—85% yield based on propylene. 

A continuous bleed is taken from the reactor to remove high boilers. Values contained in this bleed are recovered in the bleed stripper and the distillate from this operation is recycled to the esterification reactor. The bleed stripper residue is a mixture of high boiling organic material and sulfuric acid, which is recovered for recycle in a waste sulfuric acid plant. 

The emulsion process can be modified for the continuous production of latex. One such process (68) uses two stirred-tank reactors in series, followed by insulated hold-tanks. During continuous operation, 60% of the monomers are continuously charged to the first reactor with the remainder going into the second reactor. Surfactant is added only to the first reactor. The residence time is 2.5 h for the first reactor where the temperature is maintained at 65°C for 92% conversion. The second reactor is held at 68°C for a residence time of 2 h and conversion of 95%. 

After the mbber latex is produced, it is subjected to further polymerization in the presence of styrene (CgHg) and acrylonitrile (C H N) monomers to produce the ABS latex. This can be done in batch, semibatch, or continuous reactors. The other ingredients required for this polymerization are similar to those required for the mbber latex reaction. 

M ass Process. In the mass (or bulk) (83) ABS process the polymerization is conducted in a monomer medium rather than in water. This process usually consists of a series of two or more continuous reactors. The mbber used in this process is most commonly a solution-polymerized linear polybutadiene (or copolymer containing sytrene), although some mass processes utilize emulsion-polymerized ABS with a high mbber content for the mbber component (84). If a linear mbber is used, a solution of the mbber in the monomers is prepared for feeding to the reactor system. If emulsion ABS is used as the source of mbber, a dispersion of the ABS in the monomers is usually prepared after the water has been removed from the ABS latex. 

If a linear mbber is used as a feedstock for the mass process (85), the mbber becomes insoluble in the mixture of monomers and SAN polymer which is formed in the reactors, and discrete mbber particles are formed. This is referred to as phase inversion since the continuous phase shifts from mbber to SAN. Grafting of some of the SAN onto the mbber particles occurs as in the emulsion process. Typically, the mass-produced mbber particles are larger (0.5 to 5 llm) than those of emulsion-based ABS (0.1 to 1 llm) and contain much larger internal occlusions of SAN polymer. The reaction recipe can include polymerization initiators, chain-transfer agents, and other additives. Diluents are sometimes used to reduce the viscosity of the monomer and polymer mixture to faciUtate processing at high conversion. The product from the reactor system is devolatilized to remove the unreacted monomers and is then pelletized. Equipment used for devolatilization includes single- and twin-screw extmders, and flash and thin film evaporators. Unreacted monomers are recovered for recycle to the reactors to improve the process yield. 

The alcoholysis reaction may be carried out either batchwise or continuously by treating the triglyceride with an excess of methanol for 30—60 min in a well-agitated reactor. The reactants are then allowed to settle and the glycerol [56-81-5] is recovered in methanol solution in the lower layer. The sodium methoxide and excess methanol are removed from the methyl ester, which then maybe fed directiy to the hydrogenolysis process. Alternatively, the ester may be distilled to remove unreacted material and other impurities, or fractionated into different cuts. Practionation of either the methyl ester or of the product following hydrogenolysis provides alcohols that have narrow carbon-chain distributions. 

The following conditions are stipulated the catalyst decomposition rate constant must be one hour or greater the residence time of the continuous reactor must be sufficient to decompose the catalyst to at least 50% of the feed level the catalyst concentration must be greater than or equal to 0.002 x Q, where the residence time, is expressed in hours. An upper limit on the rate of radical formation was also noted that is, when the rate of radical formation is greater than the addition rate of the primary radicals to the monomers, initiation efficiency is reduced by the recombination of primary radicals. 

Chlorine Trifluoride. Chlorine trifluoride is produced commercially by the continuous gas-phase reaction of fluorine and chlorine ia a nickel reactor at ca 290°C. The ratio of fluorine to chlorine is maintained slightly in excess of 3 1 to promote conversion of the chlorine monofluoride to chlorine trifluoride. Sufficient time ia the reactor must be provided to maintain high conversions to chlorine trifluoride. Temperature control is also critical because the equiHbrium shift of chlorine trifluoride to chlorine monofluoride and fluorine is significant at elevated temperatures. 

The process options reflect the broad range of compositions and gas volumes that must be processed. Both batch processes and continuous processes are used. Batch processes are used when the daily production of sulfur is small and of the order of 10 kg. When the daily sulfur production is higher, of the order of 45 kg, continuous processes are usually more economical. Using batch processes, regeneration of the absorbant or adsorbant is carried out in the primary reactor. Using continuous processes, absorption of the acid gases occurs in one vessel and acid gas recovery and solvent regeneration occur in a separate reactor. 

Reaction conditions depend on the composition of the bauxite ore, and particularly on whether it contains primarily gibbsite, Al(OH)2, or boehmite [1318-23-6] AlOOH. The dissolution process is conducted in large, stirred vessels or alternatively in a tubular reactor. The process originated as a batch process, but has been converted to a continuous one, using a series of stirred tank reactors or a tubular reactor. 

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