Transfer coefficient liquid film

K Overall mass-transfer coefficient per unit area Kf Drag coefficient kL Mass-transfer coefficient, liquid film... 

Two-Film Theory, Gas-Film Mass Transfer Coefficient, Liquid-Film Mass Transfer Coefficient, Overall Mass Transfer Coefficients,... 

Bankoff [343] suggested that heat transfer coefficients in film boiling could be substantially improved by continuously removing vapor through a porous heated surface. Subsequent experimental work [344, 345] demonstrated that coefficients could be increased by as much as 150 percent, provided that a porous block was placed on the surface to stabilize the flow of liquid toward the surface. Wayner and Kestin [346] extended this concept to nucleate boiling and found that wall superheats could be maintained at about 3 K (5.4°F) for heat fluxes over 300,000 W/m2 or 95 x 103 Btu/(h-ft2). This work was extended by Raiff and Wayner [347], The need for a porous heated surface and a flow control element appears to limit the application of suction boiling. 

This equation, due to Higbie, was originally derived to describe mass transfer between rising gas bubbles and a surrounding liquid Tran. AIChE, 31,368 [1935]). It applies quite generally to situations where the contact time between the phases is short and the penetration (or depletion) depth is so small that transfer may be viewed as taking place from a plant to a semiinfinite domain. In Section 4.1.2.3 we will provide a quantitative criterion for this approach, which is also referred to as the Penetration Theory. It also describes both the short- and long-term behavior in diffusion between a plane and a semi-infinite space, and we used this property in Chapter 1, Table 1.4, to help us set upper and lower bounds to mass transfer coefficients and "film" thickness Zp j. 

Equation 28 and its liquid-phase equivalent are very general and valid in all situations. Similarly, the overall mass transfer coefficients may be made independent of the effect of bulk fiux through the films and thus nearly concentration independent for straight equilibrium lines ... 

Cg = the concentration of the saturated solution in contact with the particles, D = a diffusion coefficient (approximated by the Hquid-phase diffusivity), M = the mass of solute transferred in time t, and S = the effective thickness of the liquid film surrounding the particles. For a batch process where the total volume H of solution is assumed to remain constant, dM = V dc and... 

Vertical Tubes For the following cases Reynolds number < 2100 and is calculated by using F = Wp/ KD. The Nusselt equation for the heat-transfer coefficient for condensate films may be written in the following ways (using liquid physical properties and where L is the cooled lengm and At is — t,) ... 

With a reactive solvent, the mass-transfer coefficient may be enhanced by a factor E so that, for instance. Kg is replaced by EKg. Like specific rates of ordinary chemical reactions, such enhancements must be found experimentally. There are no generalized correlations. Some calculations have been made for idealized situations, such as complete reaction in the liquid film. Tables 23-6 and 23-7 show a few spot data. On that basis, a tower for absorption of SO9 with NaOH is smaller than that with pure water by a factor of roughly 0.317/7.0 = 0.045. Table 23-8 lists the main factors that are needed for mathematical representation of KgO in a typical case of the absorption of CO9 by aqueous mouethauolamiue. Figure 23-27 shows some of the complex behaviors of equilibria and mass-transfer coefficients for the absorption of CO9 in solutions of potassium carbonate. Other than Henry s law, p = HC, which holds for some fairly dilute solutions, there is no general form of equilibrium relation. A typically complex equation is that for CO9 in contact with sodium carbonate solutions (Harte, Baker, and Purcell, Ind. Eng. Chem., 25, 528 [1933]), which is... 

Pressure can also be controlled by variable heat transfer coefficient in the condenser. In this type of control, the condenser must have excess surface. This excess surface becomes part of the control system. One example of this is a total condenser with the accumulator running full and the level up in the condenser. If the pressure is too high, the level is lowered to provide additional cooling, and vice versa. This works on the principle of a slow moving liquid film having poorer heat transfer than a condensing vapor film. Sometimes it is necessary to put a partially flooded condenser at a steep angle rather than horizontal for proper control response. 

Hog, Hql = Height of transfer unit based on overall gas or liquid film coefficients, ft Gm, Ljn = Gas or liquid mass velocity, lb mols/(hr) (ft ) Kga, Kla = Gas or liquid mass transfer coefficients, consistent units... 

An important mixing operation involves bringing different molecular species together to obtain a chemical reaction. The components may be miscible liquids, immiscible liquids, solid particles and a liquid, a gas and a liquid, a gas and solid particles, or two gases. In some cases, temperature differences exist between an equipment surface and the bulk fluid, or between the suspended particles and the continuous phase fluid. The same mechanisms that enhance mass transfer by reducing the film thickness are used to promote heat transfer by increasing the temperature gradient in the film. These mechanisms are bulk flow, eddy diffusion, and molecular diffusion. The performance of equipment in which heat transfer occurs is expressed in terms of forced convective heat transfer coefficients. 

Liquid film mass transfer coefficient ( b moles) (cu ft)/(sec) (sq ft) (lb mole)... 

Nql = number of transfer units, based on overall liquid film coefficients Z = height of packing, ft... 

For same conditions as (1.) Some data are reported as individual gas or liquid film coefficients or transfer unit heights. However, it is often possible to use it as overall data if the conditions are understood. 

Pave = average total pressure in tower, atmospheres Hl = height of liquid film transfer unit, ft Hg = height of gas film transfer unit, ft a = effective interfacial area for contacting gas and liquid phases, ft /ft. Because this is very difficult to evaluate, it is usually retained as a part of the coefficient such as Kca, Ki a, kca, and k.La. 

Overall mass transfer coefficient based on liquid phase, lb mol/ (hr) (ft ) (lb mol/ft ) Overall mass transfer coefficient based on liquid film controlling, lb mol (hr) (ft ) (lb mol/ft3)... 

Figure 10-50C. Tube-side (inside tubes) liquid film heat transfer coefficient for Dowtherm . A fluid inside pipes/tubes, turbulent flow only. Note h= average film coefficient, Btu/hr-ft -°F d = inside tube diameter, in. G = mass velocity, Ib/sec/ft v = fluid velocity, ft/sec k = thermal conductivity, Btu/hr (ft )(°F/ft) n, = viscosity, lb/(hr)(ft) Cp = specific heat, Btu/(lb)(°F). (Used by permission Engineering Manual for Dowtherm Heat Transfer Fluids, 1991. The Dow Chemical Co.)...

Figure 10-50D. Tube-side (inside pipes or tubes) liquid film heat transfer coefficient for Dowtherm A and E at various temperatures. (Used by permission Engineering Manual for Heat Transfer Fluids, 1991. The Dow Chemical Co.)...

X = distance film has fallen g = gravitational constant Pi = liquid density = latent heat of vaporization JL = liquid viscosity k = liquid thermal conductivity AT = temperature difference = (Tb bbi,p i -NrUj = local Nusselt number, h x/k, h = local heat transfer coefficient... 

Sinek and Young present a design procedure for predicting liquid-side falling film heat transfer coefficients within 20% and overall coefficients within 10%. 

The thesis of Steward indicates that the overall liquid film and mass transfer coefficients were functions of the gas flow rate and the column pressure and are independent of the liquid flow rate and inlet air temperature. The gas film heat transfer coefficient was found to be a function only of the air flow rate. 

The carbon source affects oxygen demand. In penicillin production, oxygen demand for glucose is 4.9 mol 1 1 h-1. The lactose concentration is 6.7 mol 1 1 h 1, sucrose is 13.4 mol l-1 h. The yield of oxygen per mole of carbon source for CH4 is YQjC = 1.34, T0j/C for Paraffins = 1, and Y(> /c for hydrocarbon (CH20)n = 0.4. The mass transfer coefficient k,a is for gas-liquid reactions, and the film thickness where the mass transfer takes place is 8... 

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