Agitated vessels gases

Miller, D. N., "Scale-Up of Agitated Vessels Gas-Liquid Mass Transfer," AIChE. ]. 20 (1974) 442-453. 

GAS DISPERSION IN AGITATED VESSELS. Gas is normally fed to a processing vessel through the open end of a submerged pipe, through a sparger, or through a porous ceramic or metal plate. Sometimes the gas by itself provides sufficient... 

Miller, D.N. (1974), Scale-up of agitated vessels gas-liquid mass transfer, A/Cfi Journal, 20(3) 445-453. 

Foam Production This is important in froth-flotation separations in the manufac ture of cellular elastomers, plastics, and glass and in certain special apphcations (e.g., food products, fire extinguishers). Unwanted foam can occur in process columns, in agitated vessels, and in reactors in which a gaseous product is formed it must be avoided, destroyed, or controlled. Berkman and Egloff (Emulsions and Foams, Reinhold, New York, 1941, pp. 112-152) have mentioned that foam is produced only in systems possessing the proper combination of interfacial tension, viscosity, volatihty, and concentration of solute or suspended solids. From the standpoint of gas comminution, foam production requires the creation of small biibbles in a hquid capable of sustaining foam. 

The choice of a bubble column or an agitated vessel depends primarily on the solubihty of the gas in the liquid, the corrosiveness of the liquid (often a gas compressor can be made of inexpensive material, whereas a mechanical agitator may have to be made of exotic, expensive materials), and the rate of chemical reac tion as compared with the mass-transfer rate. Bubble columns and agitated vessels are seldom used for gas absorption except in chemical reac tors. As a general rule. 

Mass Transfer Mass transfer in plate and packed gas-liquid contactors has been covered earHer in this subsection. Attention nere will be limited to deep-bed contactors (bubble columns and agitated vessels). Theory underlying mass transfer between phases is discussed in Sec. 5 of this handbook. 

Gas Holdttp in Agitated Vessels Sensei et al. (op. cit.) have also developed the following correlation for six-bladed disk-type impellers ... 

Hikita, H. and Ishikawa, H., 1969. Physical absorption in agitated vessels with a flat gas-liquid interface. Bulletin of the UniversityOsaka Prefect, A18, 427-437. 

Gas hold-up is defined as Ha = Bubble volume/Reactor volume, which is the volume of gas per unit volume of reactor. Assume the system is an agitated vessel. Let us use Richard s data to define gas hold-up 3... 

The mass transfer coefficient is expected to relate gas power per unit volume and gas terminal velocity. Measurement of gas bubble velocity is troublesome in the experimental stage of aeration. Extensive research has been conducted for an explanation of the above correlation. Gas-liquid mass transfer in low viscosity fluids in agitated vessels has been reviewed and summarised as stated in (3.5.1.7)—(3.6.2) 3... 

FIGURE 11.2 Mechanically agitated vessel with gas sparging. 

A reaction is required to be carried out between a gas and a liquid. Two different types of reactor are to be considered an agitated vessel (AV) and a packed column (PC). Devise a superstructure that will allow one of the two options to be chosen. Then describe this as integer constraints for the gas and liquid feeds and products. 

From Table 7.5, the mechanically agitated vessel gives the best performance, not only in terms of selectivity and yield but also in terms of the reactor volume. The reactor volumes in Table 7.5 are only indicative as they are based on an assumed hold-up of the gas in the reactor. 

A batch reactor is an agitated vessel in which the reactants are precharged and which is then emptied after the reaction is completed. More frequently for exothermic reactions, only part of the reactants are charged initially, and the remaining reactants and catalysts are fed on a controlled basis this is called a semi-batch operation. For highly exothermic reactions and for two-phase (gas-liquid) reactions, loop reactors with resultant smaller volumes can be used. 

The crystallizer was an agitated vessel with an Inside diameter of 9.0 cm and a volume of about 1 1. It was equlppet with four vertical baffles, a water jacket to keep the solution temperature constant, and a nozzle through which nitrogen gas was Introduced In several experiments to suspend a speed crystal more effectively In the solution. Agitation was accomplished with a 5.0 cm stainless steel marine propeller having three blades driven by a variable speed motor. 

Agitated vessels (liquid-solid systems) Below the off-bottom particle suspension state, the total solid-liquid interfacial area is not completely or efficiently utilized. Thus, the mass transfer coefficient strongly depends on the rotational speed below the critical rotational speed needed for complete suspension, and weakly depends on rotational speed above the critical value. With respect to solid-liquid reactions, the rate of the reaction increases only slowly for rotational speed above the critical value for two-phase systems where the sohd-liquid mass transfer controls the whole rate. When the reaction is the ratecontrolling step, the overall rate does not increase at all beyond this critical speed, i.e. when all the surface area is available to reaction. The same holds for gas-liquid-solid systems and the corresponding critical rotational speed. 

The existence of a stagnant laminar fluid film adjacent to the interface is not difficult to visualize, in the case where the interface is stationary, as when the fluid flows along a solid surface. However, this situation seems rather unrealistic with the fluid-fluid interface, as when the surface of the liquid in an agitated vessel is in contact with a gas phase above, or if gas bubbles move upward through a liquid, or when one liquid phase is in contact with another liquid phase in an extractor. 

For gas absorption, the equipment possibilities are generally packed columns plate distillation towers, possibly with mechanical agitation on every plate deep-bed contactors (bubble columns or sparged lagoons) and mechanically agitated vessels or lagoons. Packed towers and plate distillation columns are discussed elsewhere. Generally these... 

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