Stirred vessels

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. 

Ma.nufa.cture. In a typical process, a solution of sodium carbonate is allowed to percolate downward through a series of absorption towers through which sulfur dioxide is passed countercurrently. The solution leaving the towers is chiefly sodium bisulfite of typically 27 wt % combined sulfur dioxide content. The solution is then mn into a stirred vessel where aqueous sodium carbonate or sodium hydroxide is added to the point where the bisulfite is fully converted to sulfite. The solution may be filtered if necessary to attain the required product grade. A pure grade of anhydrous sodium sulfite can then be crystallized above 40°C because the solubiUty decreases with increasing temperature. 

In a typical batch operation, carbon disulfide is added to four molar equivalents of 25—30 wt % aqueous ammonia in a stirred vessel, which is kept closed for the first one to two hours. The reaction is moderately exothermic and requires cooling. After two to three hours, when substantially all of the disulfide has reacted, the reaction mixture is heated to decompose dithiocarbamate and trithiocarbonate and vented to an absorption system to collect ammonia, hydrogen sulfide, and any unreacted carbon disulfide. 

Batch-stirred vessels are most often used in treating material with powdered activated carbon (72). The type of carbon, contact time, and amount of carbon vary with the desired degree of purification. The efficiency of activated carbon may be improved by applying continuous, countercurrent carbon—Hquid flow with multiple stages (Fig. 3). Carbon is separated from the Hquid at each stage by settling or filtration. Filter aids such as diatomaceous earth are sometimes used to improve filtration. 

In this section, we consider the transient adsorption of a solute from a dilute solution in a constant-volume, well-mixed batch system or, equivalently, adsorption of a pure gas. The solutions provided can approximate the response of a stirred vessel containing suspended adsorbent particles, or that of a very short adsorption bed. Uniform, spherical particles of radius are assumed. These particles, initially of uniform adsorbate concentration, are assumed to be exposed to a step change in concentration of the external fluid. 

Impeller Reynolds Number The presence or absence of turbulence in an impeller-stirred vessel can be correlated with an impeller Reynolds number defined... 

Turbulent Flow in Stirred Vessels Turbulence parameters such as intensity and scale of turbulence, correlation coefficients, and... 

Suspensions of fine sohds may have pseudoplastic or plastic-flow properties. When they are in laminar flow in a stirred vessel, motion in remote parts of the vessel where shear rates are low may become negligible or cease completely. To compensate for this behavior of slurries, large-diameter impellers or paddles are used, with (D /Df) > 0.6, where Df is the tank diameter. In some cases, for example, with some anchors, > 0.95 Df. Two or more paddles may be used in deep tanks to avoid stagnant regions in slurries. 

The vertical vibratoiy mill has good wear values and a low-noise output. It has an unfavorable residence-time distribution, since in continuous operation it behaves like a well-stirred vessel. Tube mills are better for continuous operation. The mill volume of the vertical mill cannot be arbitrarily scaled up because the static load of the upper media, especially with steel beads, prevents thorough energy introduction into the lower layers. Larger throughputs can therefore only be obtained by using more mill troughs, as in tube mills. 

Stirred Vessels Gases may be dispersed in hquids by spargers or nozzles and redispersed by packing or trays. More intensive dispersion and redispersion is obtained by mechanical agitation. At the same time, the agitation will improve heat transfer and will keep catalyst particles in suspension if necessaiy. Power inputs of 0.6 to 2.0 kW/m (3.05 to 10.15 np/1,000 gal) are suitable. 

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 most common heterogeneous catalytic reaction is hydrogenation. Most laboratory hydrogenations are done on liquid or solid substrates and usually in solution with a slurried catalyst. Therefore the most common batch reactor is a stirred vessel, usually a stirred autoclave (see Figure 2.1.1 for a typical example). In this system a gaseous compound, like hydrogen, must react at elevated pressure to accelerate the process. 

Figure 4-4 shows a semi-batch reactor with outside circulation and the addition of one reactant through the pump. Semi-batch reactors have some reactants that are charged into the reactor at time zero, while other reactants are added during the reaction. The reactor has no outlet stream. Some reactions are unsuited to either batch or continuous operation in a stirred vessel because the heat liberated during the reaction may cause dangerous conditions. Under these... 

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. 

Collisional break-up of erystals suspended in stirred vessels may oeeur as a result of eollision between erystal-crystal, erystal-impeller or erystal-vessel, and has been deseribed by many authors e.g. Ottens and de Jong (1973), Kuboi etal. (1984), Mazzarotta (1992). 

Conti, R. and Nienow, A.W., 1980. Particle Abrasion at High Solids Concentration in Stirred Vessels - II. Chemical Engineering Science, 35, 543-547. 

Kuboi, R., Nienow, A.W. and Conti, R., 1984. Mechanical Attrition of Crystals in Stirred Vessels. In Industrial Crystallization 84. Eds. S.J. Jancic and E.J. de Jong, Amsterdam Elsevier Science Publishers B.V. 

Mazzarotta, B., Di Cave, S. and Bonifazi, G., 1996. Influence of time on crystal attrition in a stirred vessel. American Institution of Chemical Engineers Journal, 42, 3554-3558. 

Mersmann and Geisler, R., 1991. DeteiTnination of the local turbulent energy dissipation rates in stirred vessels and its significance for different mixing tasks. In 4th World Congress of Chemical Engineering. Karlsruhe, Germany. 

Ottens, E.P.K and de Jong, E.J., 1973. A Model for Secondary Nucleatioii in a Stirred Vessel Cooling Crystallizer. Industrial and Engineering Chemistry Eundamentals, 12, 179-184. 

Ranade, V.V., 1997. An efficient computational model for simulating flow in stirred vessels a case of Rushton turbine. Chemical Engineering Science, 52, 4473-4484. 

Fermentation usually occurs in a conventional stirred vessel at 30°C (with cooling) and vigorous aeration. The process from start to finish can take as littie as 24 hours thus absolute sterilisation is not crudal. However, several processes reuse the mycelium many times and in these circumstances dean conditions are a minimum requirement... 

Non-stirred, aerated vessels are used in the process for traditional products such as wine, beer and cheese production. Most of the newly found bioprocesses require microbial growth in an aerated and agitated system. The percentage distribution of aerated and stirred vessels for bioreactor applications is shown in Table 6.1. The performances of various bioreactor systems are compared in Table 6.2. Since these processes are kinetically controlled, transport phenomena are of minor importance. 

TABLE 6.1. Percentage of distribution aerated and stirred vessel in bioreactor application... 

In an airlift fermenter, mixing is accomplished without any mechanical agitation. An airlift fermenter is used for tissue culture, because the tissues are shear sensitive and normal mixing is not possible. With the airlift, because the shear levels are significantly lower than in stirred vessels, it is suitable for tissue culture. The gas is sparged only up to the part of the vessel cross section called the riser. Gas is held up, fluid density decreases causing liquid in the riser to move upwards and the bubble-free liquid to circulate through the down-comer. The liquid circulates in airlift reactors as a result of the density difference between riser and down-comer. 

Equations (299) and (300) depict the input-output relationships for the concentrations and the temperature in each phase for a given continuous steady-flow dispersed system. Therefore, (299) and (300) can be used in predicting the input-output relationships for a multistage multicomponent gas-liquid system with several continuous stirred vessels in series. 

In a falling film evaporator (4) a water-paraffin mixture is distilled off and completely pumped back to the reactor. The resulting product is separated into a 60% sulfuric acid fraction and paraffin-containing alkanesulfonic acid (5), which is bleached by hydrogen peroxide (6). In a stirred vessel (7) the alkanesulfonic acid is neutralized by 50% sodium hydroxide solution until the pH is exactly 7. The composition of the neutralized product is also given in Table 2. 

The degassed raw acid is thoroughly mixed with an extraction solvent such as higher alcohols and pumped to a separator (5) where an aqueous solution of approximately 20% sulfuric acid separates as the lower phase. The upper solvent phase containing the sulfonic acid is neutralized with 50% caustic in a stirred vessel (6). The resulting mixture has the composition listed in Table 5. 

In other cases, modifications were conducted to modify the standard manual shaking procedure. Hoffman et al. [225] modified the method substituting manual shaking by a special stirring vessel. 

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