Liquid film coefficient

Liquid-Film Coefficients (Physical) and (Reactive) The gas-side resistance can be eliminated by employing a pure gas, thus leaving the liquid film as the only resistance. Alternatively, after the gas-film resistance has been found experimentally or from corre-... 

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... 

Expressions of this type can be written for both gas and liquid films in which the absorption coefficients are the gas- and liquid-film coefficients, respectively. The driving force across the gas film is given by the difference between the actual partial pressure of the soluble gas and that at the interface, v/hile the driving force across the liquid film is given by the difference between the concentration of the soluble gas at the interface and that in the main bulk of liquid. 

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. 

It is seen that the liquid film coefficient is generally considerably higher than the gas... 

Overall mass transfer coefficients are only constant when both liquid film coefficients are constant and also when the slope of the equilibrium line is constant. Thus, for a non-linear equilibrium relationship, the overall mass transfer coefficient will vary with concentration. How would you implement this effect into the program ... 

It is known that the overall liquid transfer coefficient K a for absorption of S02 in water in a column is 0.003 kmol/sm3 (kmol/m3). Obtain an expression for the overall liquid-film coefficient KLa for absorption of NH3 in water in the same equipment using the same water and gas rates. The diffusivities of S02 and NH3 in air at 273 K are 0.103 and 0.170 cm2/s. S02 dissolves in water, and Henry s constant /7 is equal to 50 (kN/m2)/(kmol/m3). All the data are expressed for the same temperature. 

Liquid fertilizers, potassium orthophosphates in, 20 637 Liquid-film coefficient, 15 695 Liquid filtration, 11 322-323 Liquid flavor forms, 11 576-577 Liquid flow control, in variable-conductance heat pipes, 13 233 Liquid fluidization, 11 791-792 Liquid food ingredients, encapsulated,... 

Figure 12.7. Variation of liquid-film coefficient with liquid flow for the absorption of oxygen in water...

Davidson, J. E, Cullen, E. J., Hanson, D., and Roberts, D. Trans. Inst. Chem. Eng. 37 (1959) 122. The hold-up and liquid film coefficient of packed towers. Part I. Behaviour of a string of spheres. 

Cryder, D. S. and Gilliland, E. R. Ind. Eng. Chem. 24 (1932) 1382-7. Heat transmission from metal surfaces to boiling liquids. I. Effect of physical properties of boiling liquid on liquid film coefficient. 

In contrast, Tamburrino and Gulliver (2002) related liquid film coefficient from Gulliver and Halverson (1989) and Lau (1975) to their measurements of Hanratty s P for open-channel flows in a flume. They could not get a A l relationship. Instead, their result was the following ... 

Table E8.7.1 Transfer enhancement ratio for a range of liquid film coefficients common on the ocean surface...

From equation (E8.7.4), we can see that an equivalent, bulk, liquid-film coefficient, Ke, would be... 

The influence of wind is predominant in determining the liquid film coefficient for lakes, reservoirs, oceans, and many estuaries. Wind creates a shear on the water surface and generates turbulence below and on the water surface. Thus, this section deals with the measurement and prediction of the wind influence on liquid film coefficient. 

The results of both and Rn measurements of liquid film coefficient versus wind velocity are plotted in Figure 9.4, along with two parameterizations that will be discussed in prediction of wind influence. 

Batch Technique. As with river reaeration measurements, tracers can also be put into lakes, estuaries, and oceans to measure the influence of wind on liquid film coefficient. If we have a volatile tracer in a lake with a well-established mixed layer, for example, we can apply the same batch reactor equation from Section 6.A, as though we had a well-mixed tank ... 

This is generally seen as an indication that wind is an important driving factor for lakes, estuaries, and oceans, ft has been shown that breaking waves and water surface slicks are important (Asher and Wanninkhof, 1998), and there are other parameters -such as mean square water surface slope - that have been proposed as better indicators (Jahne, 1991). The problem is that our ability to predict these indicators from wind velocity measurements have not been developed and tested for liquid film coefficient. 

One other measurement technique that has been used to measure Kl over a shorter time period, and is thus more responsive to changes in wind velocity, is the controlled flux technique (Haupecker et al., 1995). This technique uses radiated energy that is turned into heat within a few microns under the water surface as a proxy tracer. The rate at which this heat diffuses into the water column is related to the liquid film coefficient for heat, and, through the Prandtl-Schmidt number analogy, for mass as well. One problem is that a theory for heat/mass transfer is required, and Danckwert s surface renewal theory may not apply to the low Prandtl numbers of heat transfer (Atmane et al., 2004). The controlled flux technique is close to being viable for short-period field measurements of the liquid film coefficient. 

Of course, both of the two coefficients, C and Klo are some combination of the processes considered when equation (8.87) through (8.102) were developed, and are a function of liquid film coefficient across both the bubbles and the free surface, bubble and water surface interfacial area, hydrostatic pressure, the mole ratio of gas in the bubbles, and equilibrium with the atmosphere. These two coefficients, however, can be valuable in the design of an aeration system, as long as (1) the arrangement of diffusers in the water body or tank is similar to the application and (2) the depth of the test is the same as the application. Significant deviations from these two criteria will cause errors in the application of the tests to the field. 

The last boundary condition is the stoichiometric requirement that one mole of A consume one mole of B. The constants determined from these boundary conditions are given by Sherwood and Pigford (S9, p. 336) and some sample concentration profiles are shown the results are interpreted in terms of the liquid film coefficients. 

In cases where the major resistance is in the liquid phase, the ratio RL/ RT= 1 and the simplification can be made that the over-all coefficient is equal to the liquid film coefficient. Which resistance dominates has to be determined from the ratio kLa / (kGa Hc) (Table 3-3). For compounds with a low Hc such as semi-volatile organic compounds, both resistances can be important (Libra, 1993). In oxygen transfer the liquid-side resistance dominates and KLa = kLa. This is also true for most of the cases in ozone mass transfer, unless there is strong mass transfer enhancement by very fast or instantaneous reactions of... 

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