Liquid Mixing in Stirred Tanks

The existence of solid particles suspended in the liquid caused an increase in the liquid mixing time. In contrast, aeration caused a decrease in the liquid mixing time for water, but an increase for non-Newtonian liquids [14]. 

A stirred-tank reactor equipped with a standard Rushton turbine of the following dimensions contains a liquid with density p = 1.000 gcm 3 and viscosity p = 0.013 gcm 1s 1. The tank diameter D = 2.4m, liquid depth HL = 2.4m, the impeller diameter d = 0.8m, and liquid volume = 10.85 m3. Estimate the stirrer power required and the mixing time, when the rotational stirrer speed N is 90r.p.m., that is, 1.5 s 1. 

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Liquid mixing in stirred tanks is covered in Volume 1, Chapter 7, and in several textbooks Uhl and Gray (1967), Hamby et al. (1997) and Tatterson (1991), (1993). 

The objectives of liquid mixing in stirred tanks are to (i) make the liquid concentration as uniform as possible (ii) suspend the particles or cells in the liquid (iii) disperse the liquid droplets in another immiscible liquid, as in the case of a liquid-liquid extractor (iv) disperse gas as bubbles in a liquid in the case of aerated (gassed) stirred tanks and (v) transfer heat from or to a liquid in the tank, through the tank wall, or to the wall of coiled tube installed in the tank. 

Holland, F. A. and Chapman, F. S. (1966a) Liquid Processing and Mixing in Stirred Tanks (Reihhold). Holland, F. A. and Chapman, F. S. (1966b) Pumping of Liquids (Reinhold). 

Lane, G.L., Schwarz, M.P. and Evans, G.M. (1999), CFD simulation of gas-liquid flow in stirred tank, 3rd Int. Symp. on Mixing in Industrial Processes, Japan. 

Source Holland, F. A., and Chapman, F. S. Liquid Mixing and Processing in Stirred Tanks,... 

Holland, F.A. and Chapman, F.S., 1966. Liquid mixing and processing in stirred tanks. New York Reinhold. 

Edwards ME (1985) Mixing of low viscosity liquids in stirred tanks. In Harnby N,... 

Ozonation experiments to determine kLa from such an instantaneous reaction should preferably be conducted in a so-called agitated cell in which both phases are perfectly mixed and the transfer area is determined by the geometry of the constructed interface between the gas and the liquid in the system (see Figure B 2-5 Levenspiel and Godfrey, 1974). The method has also been used in stirred tank reactors (Sotelo et al., 1990 Sotelo et al. 1991 Beltran and Gonzales 1991), but these reactors have two drawbacks ... 

When two phases are mixed together (gas-liquid, immiscible liquid-liquid), a fine dispersion of bubbles or drops and a high specific interfacial area are produced because of the intensive turbulence and shear. For this reason, resistance to interphase mass transfer is considerably smaller than in conventional equipment. In addition, a wide range of gas-liquid flow ratios can be handled, whereas in stirred tanks the gas-flow rate is often limited by the onset of flooding. Mass transfer coefficients (kLa) can be 10-100 times higher than in a stirred tank. 

Holland and Chapman—Liquid Mixing and Processing in Stirred Tanks, Reinhold. 

Bubble columns are most widely spread, followed by sparged stirred tanks. Often both are used for a certain application. Stirred tanks and bubble columns have similar characteristics with respect to mass transfer (see Table 8.1). In both reactors the liquid is well mixed. The gas phase in the bubble column shows plug-flow behavior, while in stirred tanks it is well mixed. 

In many refineries thermal cracking processes are used to convert residues into lighter products. Low value petroleum coke is a product from the more severe cracking processes. The H-Oil process made it possible to convert the asphaltenic carbonizable portion of the residue to higher value liquid products rather than coke. In the H-Oil process an ebullated bed of catalyst is used to convert lower value heavy oil into upgraded higher value products in the presence of hydrogen. The ebullated bed reactor is an expanded bed of catalyst maintained in constant motion by the upward flow of liquid. The reactor behaves as a well mixed continuously stirred tank reactor. 

The focus of this entry is limited to the design of aerobic fermenters, i.e., stirred tank and concentric tube airlift fermenters, which are commonly utilized in the bioprocessing industries. Design principles and the basic calculations are described with a couple of industrial examples. Readers with a limited background in mixing technology are referred to the gas-liquid contactor entry of this encyclopedia for a more comprehensive understanding of fluid flow and mass transfer characteristics in stirred tank reactors and bubble columns. 

In stirred tanks for gas-liquid mixing, baffles are essential to create axial flow and maintain overall... 

The previous sections describe how mixing is accomplished in a liquid phase. However, many industrial processes carried out in stirred tank reactors involve mixing of solids, gases and other liquids in a continuous liquid phase. The presence of a second phase will affect both the power consumption and the flow pattern in the tank. In the sequel, the mixing phenomena caused by the presence of gas bubbles, liquid droplets and solid particles are discussed. 

The solids are kept in suspension if the pumping capacity of the impeller causes strong enough circulation of the liquid. In most processes, complete suspension of the particles is not required. Often, so-called off-bottom suspension is sufficient, which means that all particles are moving above the bottom of the tank with some vertical velocity. Radial flow impellers are usually not very effective in suspending solid particles. Actually, about three times more power is required for a radial turbine to provide the same degree of uniformity compared to an axial turbine. This is because the radial turbines pick up particles from the bottom of the tank by the suction side of the impeller, which is only half of the total flow from the impeller. Due to the appearance of an upper and a lower circulation zone, the contents of the two zones are not sufficiently mixed. Axial impellers are therefore most frequently used for the suspension of solids in stirred tanks [65]. 

Some further special technical aspects should be mentioned. The intensive mixture of the two liquid phases is an important condition for obtaining high reaction rates. This mixing can be achieved in bubble columns, tray columns or in stirred-tank reactors. In the few publications on industrially realized two-phase reactions the stirred tank reactor is always cited, but without detailed information on the stirring device. One further possible way to increase the mass transfer between the two liquid phases is by the influence of sonification. Cornils et al. applied this technique in the hydroformylation of hexene or diisobutene and found a considerable increase in the turnover numbers [93]. Another possibility for increasing the mass transfer may be by the use of microemulsions and micellar systems [94], which can be reached by addition of certain surfactants. This aspect is discussed in Sections 3.2.4 and 4.5. The separation of catalyst compounds in two-phase systems in combination with membranes has been studied recently by Muller and Bahrmann [95],... 

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