Mass transfer driving force

The use of molal humidity as the mass-transfer driving force is conventional and convenient because of the development of humidity data for, especially, the air—water system. The mass-transfer coefficient must be expressed in consistent units. 

Although the right-hand side of Eq. (14-60) remains valid even when chemical reactions are extremely slow, the mass-transfer driving force may become increasingly small, until finally c — Cj. For extremely slow first-order irreversible reactions, the following rate expression can be derived from Eq. (14-60) ... 

Effect on mean mass-transfer driving force... 

Whenever die rich and the lean phases are not in equilibrium, an interphase concentration gradient and a mass-transfer driving force develop leading to a net transfer of the solute from the rich phase to the lean phase. A common method of describing the rates of interphase mass transfer involves the use of overall mass-transfer coefficients which are based on the difference between the bulk concentration of the solute in one phase and its equilibrium concentration in the other phase. Suppose that the bulk concentradons of a pollutant in the rich and the lean phases are yi and Xj, respectively. For die case of linear equilibrium, the pollutant concnetration in the lean phase which is in equilibrium with y is given by... 

Fl urf 2.13 Using mass transfer driving force to trade off fixed cost versus operating cost. 

Of the contacting patterns in Figure 7.3, countercurrent packed beds offer the largest mass transfer driving force and agitated tanks the lowest. 

Targeting in time interval (5.5-6 h) is shown in Fig. 12.24a. The associated block diagram is depicted in Fig. 12.24b. Since no contaminant load is removed from the B wash, dispensing with used water as effluent would certainly amount to inefficient use of available mass transfer driving forces in the system, as this water could still be reused in the next batch cycles if not reusable in the subsequent time intervals within... 

As we shall see, towers have the largest mass transfer driving force and in this respect have an advantage over tanks. Agitated tanks have the smallest driving force. 

Remark As indicated earlier, in most heterogeneous systems the mass-transfer driving force between phase I and phase II includes the concentration in phase I minus the concentration in phase I which is in equilibrium with the concentration in phase II (or vice versa), i.e.,... 

The acceleration of mass transfer due to chemical reactions in the interfacial region is often accounted for via the so-called enhancement factors [19, 26, 27]. These parameters are defined as a relationship between the mass transfer rate with reaction and mass transfer rate without reaction, assuming the same mass transfer driving force. 

Selectivity. In many cases it is desired to remove H2S selectively from a gas stream, rejecting CO2 to the highest possible extent. It is, therefore, useful to introduce the selectivity factor S, being a yardstick for the process selectivity independent of mass transfer driving forces [8] ... 

Bcomb mass transfer driving force with combustion Rvap = mass transfer driving force without combustion c = effective normalized reactant concentration Cp = specific heat (per g of mixture)... 

Countercurrent operation is the most widely used absorption equipment arrangement. As the gas flow increases at constant liquid flow, liquid holdup must increase. The maximum gas flow is limited by the pressure drop and the liquid holdup that will build up to flooding. Contact time is controlled by the bed depth and the gas velocity. In countercurrent flow mass transfer driving force is maximum at the gas entrance and liquid exit. Cocurrent operation can be carried out at high gas velocities because there is no flooding limit. In fact, liquid holdup decreases as velocity increases. However, the mass transfer driving force is smaller than in countercurrent operation. 

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