Mass transfer driving force for

The Driving Force for Mass Transfer. The rate of mass transfer increases as the driving force, (7 — (7, is increased. can be enhanced as follows. From Dalton s law of partial pressures... 

The mass transfer number B represents the ratio of the energy available for vaporization to the energy required for vaporization, and may be thought of as a driving force for mass transfer. It can be expressed as... 

Henry s law constant. The overall driving force for mass transfer is Ug—K ay and the rate of mass transfer across the interface is... 

The overall driving force for mass transfer is AT = Pg—Pi, where Pi is the concentration of oxygen in the liquid phase expressed as an equivalent partial pressure. For the experimental conditions, T/ 0 due to the fast, liquid-phase reaction. The oxygen pressure on the gas side varies due to the liquid head. Assume that the pressure at the top of the tank was 1 atm. Then Tg = 0.975 atm since the vapor pressure of water at 20°C should be subtracted. At the bottom of the tank, 1.0635 atm. The logarithmic mean is appropriate AT =1.018 atm. Thus, the transfer rate was... 

The interpretation is straightforward. At reaction conditions the concentration in the film is lowered by reaction, and, as a consequence, the driving force for mass transfer increases. In a homogeneous system this results in high values of Ha. In a slurry reactor this enhancement can occur if the catalyst particles are so small that they accumulate in the film layer. Table 5.4-4 summarizes expressions for the reaction rate or enhancement factor for various regimes. 

If the reaction is slow, there is a small effect on the overall mass transfer coefficient. The driving force for mass transfer will be greater than that for physical absorption alone, as a result of the dissolving gas reacting and not building up in the bulk liquid to the same extent as with pure physical absorption. 

Here Jv is the volumetric flow rate of fluid per unit surface area (the volume flux), and Js is the mass flux for a dissolved solute of interest. The driving forces for mass transfer are expressed in terms of the pressure gradient (AP) and the osmotic pressure gradient (All). The osmotic pressure (n) is related to the concentration of dissolved solutes (c) for dilute ideal solutions, this relationship is given by... 

Observe the dynamic approach to equilibrium, and show how the driving force for mass transfer changes with time. 

The driving force for mass transfer from the pellet surface is reduced as the EG and water concentrations in the gas are increased. Therefore, gas having practically no EG and moisture is used for SSP in order to maximize the reaction rate. 

Mass transfer. It is not yet possible to predict the mass transfer coefficient with a high degree of accuracy because the mechanisms of solute transfer are but imperfectly understood as discussed Light and Conway(14), Coulson and Skinner(15) and Garner and Hale 16 1. In addition, the flow in spray towers is not strictly countercurrent due to recirculation of the continuous phase, and consequently the effective overall driving force for mass transfer is not the same as that for true countercurrent flow. 

Equation (35) predicts that the mass transfer coefficient increases with increases in the screw speed and the number of parallel channels on the screw. The explanation for this is rather simple and is related to the fact that each time the film on the barrel wall is regenerated and the surface of the nip is renewed, a uniform concentration profile is reestablished, which means that the driving force for mass transfer is maximized. Since the instantaneous mass transfer rate decreases with time, mass transfer rates can be maximized by keeping the exposure time as short as possible, and... 

The decrease in the exit concentration with decreases in the extraction pressure seen in Figs. 17 and 18 is a consequence of the fact that the driving force for mass transfer is directly related to the partial pressure of the volatile component in the vapor phase, which, in this case, is constant and equal to the extraction pressure. In fact, reasonably good agreement between the data in Fig. 17 and the predictions of Eq. (38) can be obtained provided it is assumed that the dimensionless group (ki ATlk y p/L) is independent of pressure. This point is illustrated in Fig. 19, which is a plot of Eq. (38) for Pe =. The value used for (ki Aj/k v(kp/L) was chosen so as to obtain the asymptotic value of wi in Fig. 17. 

The net effect of this reduction, of course, is to increase the driving force for mass transfer in the liquid phase. 

The transfer number B in Equations 3 and 4 is Spalding s contribution. It is the driving force for mass transfer in dimensionless form. With diffusion controlling (Equation 3) ... 

We can now proceed to the second part of the calculation and find the height of packing required. Plug flow for the gas phase will be assumed because the composition of the liquid is assumed not to change, the flow pattern in the liquid does not enter into the problem. Note that using Kca requires that the driving force for mass transfer be expressed in terms of gas-phase partial pressures. 

Mean concentration driving force for mass transfer... 

Equilibrium calculations are useful in the design or operation of a flue gas desulfurization (FGD) facility and provide the necessary foundation for complex process simulation (e.g., absorber modeling) (3). Since S02 absorption into FGD slurries is a mass transfer process which is primarily limited by liquid phase resistance for most commercial applications, the solution composition, in terms of alkaline species, is very critical to the performance of the system. Accurate prediction of solution composition via equilibrium models is essential to establishing driving forces for mass transfer, and ultimately in predicting system performance. 

The most important parameter for the equipment is the gas-liquid contact area per unit volume, and the driving force for mass transfer is sufficiently large. 

It is clear that the cyclohexane conversion increases with an increasing sweep ratio y- that is, with increasing driving forces for mass transfer through the membrane. In addition, the introduction of a more diluted feed leads to an enhanced conversion. Proof of an effect of the realized product removal via the Vycor glass membrane was the fact that the achieved conversions exceeded the equilibrium conversions (shown as dotted fines in Fig. 12.10). 

The actual value of k a was measured by absorption of carbondiox-ide from air into a buffer solution of potassium-carbonate and bicarbonate. Care was taken that the mass transfer coefficient itself was not enhanced by the chemical reaction, although the composition of the buffers used guaranteed a substantial driving force for mass transfer over the whole length of the column. Literature about the subject is abundant and here referred to (11, 12, 13). 

The reactions are essentially irreversible and take place only in the acid phase. Thus transport of mass between the phases and phase equilibrium which defines the driving force for mass transfer are also important. Heterogeneous nitration is illustrated in Figure 3. 

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