Stirred liquid phase reactors

FIGURE 7A.10 Variation of power number with impeller speed for two-phase (gas-liquid) and three-phase (gas-liquid-solid) stirred reactors. Two phase (solid-liquid) system. A, fillet formation B, disappearance of fillets C, off-bottom suspension of solids D, recirculation of mixture. Three phase (gas-liquid-sohd). A, no dispersion of gas solid settled on bottom B, gas dispersed beginning of solid suspension C, gas dispersed off-bottom suspension of solids D, recirculation of mixture. (Reproduced from Rewatkar et al. 1991 with permission from American Chemical Society. 1991, American Chemical Society.)... 

The use of an unnecessarily hot utility or heating medium should be avoided. This may have been a major factor that led to the runaway reaction at Seveso in Italy in 1976, which released toxic material over a wide area. The reactor was liquid phase and operated in a stirred tank (Fig. 9.3). It was left containing an uncompleted batch at around 160 C, well below the temperature at which a runaway reaction could start. The temperature required for a runaway reaction was around 230 C. ... 

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. 

Two complementai y reviews of this subject are by Shah et al. AIChE Journal, 28, 353-379 [1982]) and Deckwer (in de Lasa, ed.. Chemical Reactor Design andTechnology, Martinus Nijhoff, 1985, pp. 411-461). Useful comments are made by Doraiswamy and Sharma (Heterogeneous Reactions, Wiley, 1984). Charpentier (in Gianetto and Silveston, eds.. Multiphase Chemical Reactors, Hemisphere, 1986, pp. 104—151) emphasizes parameters of trickle bed and stirred tank reactors. Recommendations based on the literature are made for several design parameters namely, bubble diameter and velocity of rise, gas holdup, interfacial area, mass-transfer coefficients k a and /cl but not /cg, axial liquid-phase dispersion coefficient, and heat-transfer coefficient to the wall. The effect of vessel diameter on these parameters is insignificant when D > 0.15 m (0.49 ft), except for the dispersion coefficient. Application of these correlations is to (1) chlorination of toluene in the presence of FeCl,3 catalyst, (2) absorption of SO9 in aqueous potassium carbonate with arsenite catalyst, and (3) reaction of butene with sulfuric acid to butanol. 

In many important cases of reactions involving gas, hquid, and solid phases, the solid phase is a porous catalyst. It may be in a fixed bed or it may be suspended in the fluid mixture. In general, the reaction occurs either in the liquid phase or at the liquid/solid interface. In fixed-bed reactors the particles have diameters of about 3 mm (0.12 in) and occupy about 50 percent of the vessel volume. Diameters of suspended particles are hmited to O.I to 0.2 mm (0.004 to 0.008 in) minimum by requirements of filterability and occupy I to 10 percent of the volume in stirred vessels. 

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. 

Homogeneous reactions are those in which the reactants, products, and any catalysts used form one continuous phase (gaseous or liquid). Homogeneous gas phase reactors are almost always operated continuously, whereas liquid phase reactors may be batch or continuous. Tubular (pipeline) reactors arc normally used for homogeneous gas phase reactions (e.g., in the thermal cracking of petroleum of dichloroethane lo vinyl chloride). Both tubular and stirred tank reactors are used for homogeneous liquid phase reactions. 

In the second class, the particles are suspended in the liquid phase. Momentum may be transferred to the particles in different ways, and it is possible to distinguish between bubble-column slurry reactors (in which particles are suspended by bubble movement), stirred-slurry reactors (in which particles are suspended by bubble movement and mechanical stirring), and gas-liquid fluidized reactors (in which particles are suspended by bubble movement and cocurrent liquid flow). 

In stirred-slurry reactors, momentum is transferred to the liquid phase by mechanical stirring as well as by the movement of gas bubbles. Small particles are used in most cases, and the operation is usually carried out in tank reactors with low height-to-diameter ratios. The operation is in widespread use for processes involving liquid reactants, either batchwise or continuous— for example, for the batchwise hydrogenation of fats as referred to in Section II. 

Johnson et al. (J4) investigated the hydrogenation of a-methylstyrene catalyzed by a palladium-alumina catalyst suspended in a stirred reactor. The experimental data have recently been reinterpreted in a paper by Polejes and Hougen (P4), in which the original treatment is extended to take account of variations in catalyst loading, variations in impeller type, and variations of gas-phase composition. Empirical correlations for liquid-side resistance to gas-liquid and liquid-solid mass transfer are presented. 

The typical bioreactor is a two-phase stirred tank. It is a three-phase stirred tank if the cells are counted as a separate phase, but they are usually lumped with the aqueous phase that contains the microbes, dissolved nutrients, and soluble products. The gas phase supplies oxygen and removes by-product CO2. The most common operating mode is batch with respect to biomass, batch or fed-batch with respect to nutrients, and fed-batch with respect to oxygen. Reactor aeration is discussed in Chapter 11. This present section concentrates on reaction models for the liquid phase. 

A mass balance for an arbitrary liquid-phase component in the stirred tank reactor is thus written as follows dci... 

There is an additional point to be made about this type of processing. Many gas-phase processes are carried out in a continuous-flow manner on the macro scale, as industrial or laboratory-scale processes. Hence already the conventional processes resemble the flow sheets of micro-reactor processing, i.e. there is similarity between macro and micro processing. This is a fimdamental difference from most liquid-phase reactions that are performed typically batch-wise, e.g. using stirred glass vessels in the laboratory or stirred steel tanks in industrial pilot or production plants. 

The catalysts most frequently used are based on noble metals (mainly palladium and platinum) on various supports, or on nickel catalysts (mainly Raney type). Hydrogenations are generally performed in the liquid phase, under relatively mild conditions of temperature and pressure (1—40 bar). Most processes are performed batch-wise using powder catalysts in stirred tank or loop-type reactors with sizes up to 10 m . 

Stirred suspensions of droplets have proven to be a popular approach for studying the kinetics of liquid-liquid reactions [54-57]. The basic principle is that one liquid phase takes the form of droplets in the other phase when two immiscible liquids are dispersed. The droplet size can be controlled by changing the agitator speed. For droplets with a diameter < 0.15 cm the inside of the drop is essentially stagnant [54], so that mass transfer to the inside surface of the droplet occurs only by diffusion. In many cases, this technique can lack the necessary control over both the interfacial area and the transport step for determination of fundamental interfacial processes [3], but is still of some value as it reproduces conditions in industrial reactors. 

Reactions were carried out in liquid phase in a well-stirred (1000 rpm) high-pressure reactor (Parr Instruments, 300 mL) at 30 bar and 150°C with 370 mg catalyst for two hours, unless otherwise specified. The feed consisted of the amine with slight excess of ketone at ketone/amine-group molar ratio of 1.6 while maintaining a reaction volume of about 150 mL. In a typical experiment, 576 mmol of aniline is mixed with 920 mmol of cyclohexanone and 370 mg of BS2 catalyst in the 300 mL reactor. The reactor is closed and then pressure-tested to 50 psi above operating pressure to ensure that the system is leak proof Once pressure-tested, the headspace is replaced... 

A reaction between organic compounds is carried out in the liquid phase in a stirred-tank reactor in the presence of excess formaldehyde. The organic reactants are nonvolatile in comparison with the formaldehyde. The reactor is vented to atmosphere via an absorber to scrub any organic material carried from the reactor. The absorber is fed with freshwater and the water from the absorber rejected to effluent. The major contaminant in the aqueous waste from the absorber is formaldehyde. 

Stirred tank reactors are employed when it is necessary to handle gas bubbles, solids, or a second liquid suspended in a continuous liquid phase. One often finds that the rates of such reactions are strongly dependent on the degree of dispersion of the second phase, which in turn depends on the level of agitation. 

Large stirred tank reactors are generally not suited for use at high pressures because of mechanical strength limitations. They are used mainly for liquid phase reaction systems at low or medium pressures when appreciable residence times are required. 

Other advantages of the tubular reactor relative to stirred tanks include suitability for use at higher pressures and temperatures, and the fact that severe energy transfer constraints may be readily surmounted using this configuration. The tubular reactor is usually employed for liquid phase reactions when relatively short residence times are needed to effect the desired chemical transformation. It is the reactor of choice for continuous gas phase operations. 

Continuous flow stirred tank reactors are normally just what the name implies—tanks into which reactants flow and from which a product stream is removed on a continuous basis. CFSTR, CSTR, C-star, and back-mix reactor are only a few of the names applied to the idealized stirred tank flow reactor. We will use the letters CSTR as a shorthand notation in this textbook. The virtues of a stirred tank reactor lie in its simplicity of construction and the relative ease with which it may be controlled. These reactors are used primarily for carrying out liquid phase reactions in the organic chemicals... 

Since one is almost always concerned with liquid phase reactions when dealing with stirred tank reactors, the assumption of constant fluid density is usually appropriate. In this case equation 8.3.6 can be written as... 

The following reaction sequence takes place in the liquid phase in a continuous stirred tank reactor. 

The decomposition reaction A -> B + C occurs in the liquid phase. It has been suggested that your company produce C from a stream containing equimolar concentrations of A and B by using two continuous stirred tank reactors in series. Both reactors have the same volume. The reaction is first-order with respect to A and zero-order with respect to B and C. Each reactor... 

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