Mass exchange coefficient

The heat and mass exchange coefficients aH and aH were taken constant in ( 3.50). However, they often depend on local flow circumstances. Their simplified description is given in what follows. 

The heat and mass exchange coefficients are determined as the proportionality coefficients in the Newton laws (1.18) between the heat delivery into the flow Ih, Wt (admixture mass delivery IE, kg/s) and the temperature difference (concentration difference) multiplied by the value of the surface area S o of the whole body. It is known... 

APq, APi pressure drop in gas and liquid phases, atm APlg pressure drop of liquid-gas phase, atm Q gas flow rate, volume/time overall mass exchange coefficient q cross flow mass transfer coefficient in fluid bed, volume/time hquid phase flow rate, volume/time... 

Wherein C is coal particle gas absorbing concentration where diffusion radius is r, kg/m is coal particle gas absorbing concentration with equilibrium state, kg/m is the dissociate gas concentration within coal particle cranny, kg/m D is the diffusion coefficient of adsorbing gas, rrf/s a is mass exchange coefficient at coal particle surface between adsorb gas to dissociate gas, m/s r is the radius of coal particle, m. 

In this section, the new analytical technique is applied to study the mass exchange coefficient from bubble-to-emulsion phase in bubbHng fluidized beds for a case of a single bubble injection from the bottom distributor into a... 

Figure 4.62 Mass exchange coefficient K e as a function of the equivaient bubbie diameter. Reprinted from Dang et al. (2013) with permission from Elsevier.

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. 

Furthermore, for calculating the effective coefficient of quasi-diffusion in a composite (D) with the corresponding limitation of the entire process of heterogeneous mass-exchange, equations reported in Section 5.1 may be used. The high kinetic permeability of cellosorbents for large organic ions are listed in Table 16. 

One of the most rational means for displacing a broad zone is electrolyte desorption under the conditions of decreasing degree of ionization, i.e., when counterions are converted into dipolar ions, uncharged molecules and coions. This conversion corresponds to a sharp decrease in distribution coefficients of the desorbed substance. Hence, the displacement of equilibrium parame ters at a high rate of mass-exchange is one of the methods of selective stepwise chromatography. 

The cooling duty can be provided by either making the draught tube an internal heat exchanger or with a heat exchanger in an external circulation loop. The mass transfer coefficient for external loop airlift Fermenter is estimated as 8... 

Where D.. can be calculated from mass transfer coefficients or an uptake Aalf-time. For example, for air-water exchange is given by... 

Diffusion is characterized by a mass transfer coefficient U8 of 104 m/h, which can be regarded as a molecular diffusivity of 2 x 10 6 m2/h divided by a path length of 0.02 m. In practice, bioturbation may contribute substantially to this exchange process, and in shallow water current-induced turbulence may also increase the rate of transport. Diffusion in association with organic colloids is not included. The D value is thus given as Us AwZ2. 

Considerable interest has been generated in turbulence promoters for both RO and UF. Equations 4 and 5 show considerable improvements in the mass-transfer coefficient when operating UF in turbulent flow. Of course the penalty in pressure drop incurred in a turbulent flow system is much higher than in laminar flow. Another way to increase the mass-transfer is by introducing turbulence promoters in laminar flow. This procedure is practiced extensively in enhanced heat-exchanger design and is now exploited in membrane hardware design. 

Kirchner, W., F. Welter, A. Bongartz, J. Karnes, S. Schweighoefer, and U. Schurath, Trace Gas Exchange at the Air/Water Interface Measurements of Mass Accommodation Coefficients," J. Atmos. Chem., 10, 427-449 (1990). 

VOCs), and to a decrease in production yields. Quantitation of these phenomena and determination of material balances and conversion yields remain the bases for process analysis and optimisation. Two kinds of parameters are required. The first is of thermodynamic nature, i.e. phase equilibrium, which requires the vapour pressure of each pure compound involved in the system, and its activity. The second is mass-transfer coefficients related to exchanges between all phases (gas and liquids) existing in the reaction process. 

For small ion-exchange particles in water, the mass transfer coefficient decreases with increasing particle size, but is almost independent of size for particles larger than about 200 pm. 

In Fig. 42, the full-width at half maximum of the (narrower) exchange propagator provides an estimate of the effective diffusion coefficient of water molecules moving between the pore space of the catalyst and the inter-particle space of the bed. In this example, the value is 2 x lO- m s which gives a lower limit to the value for the mass transfer coefficient of 4x 10 ms This value was obtained by defining a mass transfer coefficient as Djd where d is a typical distance traveled to the surface of the catalyst that we estimate as half a typical bead dimension (approximately 500 pm). This value of the mass transfer coefficient is consistent with the reaction occurring under conditions of kinetic as opposed to mass transfer control. 

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