Gas-phase mass transfer coefficient

Mass transfer involves establishing a transfer between the elementary regions of the reactor and between individual phases (interfacial mass transfer coefficients gas phase mass transfer, liquid phase mass transfer, mass transfer with reaction, liquid-solid mass transfer), as well as other elementary phenomena and processes connected with mass transfer gas phase phenomena and processes (gas hold-up, bubble size, interfacial area and bubble coalescence/redispersion), volumetric mass transfer and power consumption during mass transfer (2). 

A spray cyclone is based on the same idea as tiie bubble cyclone. A gas is introduced with a high velocity (say, 40 m/s) tangentionally into the cyclone, and liquid is sprayed in the axis (see figure 4.14). The liquid drops travel with great speed towards the wall, and flow downward as a thin film. Because of the greatly enhanced gas phase mass transfer coefficients, gas phase components can be separated on the basis of their effective liquid phase mass transfer coefficients (Schrauwen, 1986). These effects are discussed further in section 5.42.2 see eq. (5.45). 

Liquid-phase mass transfer coefficient Gas-liquid interfacial area per unit volume of dispersion Gas volume fraction in dispersion Diffusivity of cyanogen in solution Henry law coefficient... 

The rate of mass transfer was given by equation T14. Mass transfer between gas phase and liquid phase (Eq. T16) was only considered in this study. Mass transfer rate (Eq. T15) between gas phase and liquid phase can be expressed in terms of the volumetric mass transfer coefficient and the concentration difference between gas-liquid interface phase and liquid phase. Where, kia (sec ) is the volumetric mass transfer coefficient and is the concentration of the yxth species at gas-liquid interface phase which can be defined by Henry s law. The fugacity of a very dilute species in a liquid phase is linearly proportional to its mole fraction at low mole fractions. 

First, the overall mass transfer coefRcient k a of the microreactor was estimated to be 3-8 s [43]. For intensified gas liquid contactors, kj a can reach 3 s while bubble columns and agitated tanks do not exceed 0.2 s Reducing the flow rate and, accordingly, the liquid film thickness is a means of further increasing kj a, which is limited, however, by liquid dry-out at very thin films. Despite such large mass transfer coefficients, gas-liquid microreactors such as the falling film device may still operate between mass transfer and kinetic control regimes, as fundamental simulation studies on the carbon dioxide absorption have demonstrated [44]. Distinct concentration profiles in the liquid, and even gas, phase are predicted. 

Intrinsic rate constant Mass transfer coefficient (gas, liquid phase)... 

The relationship of the overall gas-phase mass transfer coefficient to the individual film coefficients maybe found from equations 4 and 5, assuming a straight equiHbrium line ... 

The rate of mass transfer,/, is then assumed to be proportional to the concentration differences existing within each phase, the surface area between the phases,, and a coefficient (the gas or Hquid film mass transfer coefficient, k or respectively) which relates the three. Thus... 

Under equiUbrium or near-equiUbrium conditions, the distribution of volatile species between gas and water phases can be described in terms of Henry s law. The rate of transfer of a compound across the water-gas phase boundary can be characterized by a mass-transfer coefficient and the activity gradient at the air—water interface. In addition, these substance-specific coefficients depend on the turbulence, interfacial area, and other conditions of the aquatic systems. They may be related to the exchange constant of oxygen as a reference substance for a system-independent parameter reaeration coefficients are often known for individual rivers and lakes. 

Gas-phase mass-transfer coefficient for dilute systems kmoP[(s-m )(kPa solute partial pressure)] lbmol/[(h-fF)lbf/in solute partial pressure)]... 

The overall gas-phase and hquid-phase mass-transfer coefficients for concentrated systems are computed according to the following equations ... 

When it is known that Hqg varies appreciably within the tower, this term must be placed inside the integr in Eqs. (5-277) and (5-278) for accurate calculations of hf. For example, the packed-tower design equation in terms of the overall gas-phase mass-transfer coefficient for absorption would be expressed as follows ... 

The Shei wood-number relation for gas-phase mass-transfer coefficients as represented by the film diffusion model in Eq. (5-286) can be rearrangecTas follows ... 

The important point to note here is that the gas-phase mass-transfer coefficient fcc depends principally upon the transport properties of the fluid (Nsc) 3nd the hydrodynamics of the particular system involved (Nrc). It also is important to recognize that specific mass-transfer correlations can be derived only in conjunction with the investigator s particular assumptions concerning the numerical values of the effective interfacial area a of the packing. 

For the liquid-phase mass-transfer coefficient /cl, the effects of total system pressure can be ignored for all practical purposes. Thus, when using Kq and /cl for the design of gas absorbers or strippers, the primary pressure effects to consider will be those which affect the equilibrium curves and the values of m. If the pressure changes affect the hydrodynamics, then Icq, and a can all change significantly. 

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