Overall Mass Transfer Coefficients

Equation 28 and its liquid-phase equivalent are very general and valid in all situations. Similarly, the overall mass transfer coefficients may be made independent of the effect of bulk fiux through the films and thus nearly concentration independent for straight equilibrium lines ... 

K a overall mass-transfer coefficient per volume of contacting column ... 

Comparison of Eqs. (5-255) and (5-257) shows that for systems in which the eqmlibrium line is straight, the overall mass transfer coefficients are related to each other by the equation... 

Overall Mass-Transfer Coejficient Tatterson (op. cit.) and Zlokarnik (op. cit.) have summarized the hterature covering overall mass-transfer coefficients. There is much scatter in the experimental data because the presence of surface-ac tive agents and electrolytes have a significant effect on the mass transfer. The correlation of Van t Riet [Ind. Eng Chem. Process Des. Dev., 18(3), 357 (1979)] is recommended ... 

Kqa = Gas film overall mass transfer coefficient, lb mols/hr (fP) (ATM)... 

Overall Mass Transfer Coefficient C02/Na0H System Metal Tower Packings... 

Overall Mass Transfer Coefficient COa/NaOH System ... 

Kp, = Overall mass transfer coefficient, g/m s a = Surface area per unit volume of adsorbent particle, mVm ... 

The relationships between the local and overall mass transfer coefficients are ... 

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... 

Let us define two overall mass transfer coefficients one for the rich phase, Ky, and one for the lean phase, Kj,. Hence, the rate of interphase mass transfer for... 

Correlations for estimating overall mass-transfer coefficients can be found in McCabe et al. (1993), Perry and Green (1984), Geankoplis (1983), Henley and Seader (1981), King (1980) and Treybal (1980). 

Overall mass transfer coefficient based on liquid phase, lb mol/ (hr) (ft ) (lb mol/ft ) Overall mass transfer coefficient based on liquid film controlling, lb mol (hr) (ft ) (lb mol/ft3)... 

Given this evaporation rate, the overall mass transfer coefficient may then be calculated from Equation 4-391,... 

K Overall mass-transfer coefficient per unit area Kf Drag coefficient kL Mass-transfer coefficient, liquid film... 

When Kh is a function of composition, the concept of overall mass transfer coefficient stops being useful. Instead, the overall resistance to mass transfer is divided between two him resistances, one for each phase. This is done by assuming that equilibrium is achieved at the interface. The equilibrium values are related by a function having the form of Henry s law ... 

We have elected to use the overall mass transfer coefficient Ki, which is based on the equivalent liquid-phase driving force OglKu — ai, but this choice was... 

Solution The experimental conditions are consistent with Equation (11.26) so that kiAi was measured. The experimental result was reported as KgAi because the overall mass transfer coefficient was based on the equivalent gas-phase driving force expressed in partial pressure units rather than concentration units. Because a pure gas was sparged, kg oo and Kj = k . Equation (11.3) relates Kg and Ki through Henry s law constant. 

In model equations, Uf denotes the linear velocity in the positive direction of z, z is the distance in flow direction with total length zr, C is concentration of fuel, s represents the void volume per unit volume of canister, and t is time. In addition to that, A, is the overall mass transfer coefficient, a, denotes the interfacial area for mass transfer ifom the fluid to the solid phase, ah denotes the interfacial area for heat transfer, p is density of each phase, Cp is heat capacity for a unit mass, hs is heat transfer coefficient, T is temperature, P is pressure, and AHi represents heat of adsorption. The subscript d refers bulk phase, s is solid phase of adsorbent, i is the component index. The superscript represents the equilibrium concentration. 

The cyclohexene hydrogenation is a well-studied process especially in conventional trickle-bed reactors (see original citations in [11,12]) and thus serves well as a model reaction. In particular, flow-pattern maps were derived and kinetics were determined. In addition, mass transfer can be analysed quantitatively for new reactor concepts and processing conditions, as overall mass transfer coefficients were determined and energy dissipations are known. In lieu of benchmarking micro-reactor performance to that of conventional equipment such as trickle-bed reactors, such a knowledge base facilitates proper, reliable and detailed comparison. 

GL 16] ]R 12] ]P 15] Using a simple thin-film model for mass transfer, values for the overall mass transfer coefficient were determined for both micro-channel processing and laboratory trickle-bed reactors [11]. The value for micro-reactor processing (fCL = 5-15 s ) exceeds the performance of the laboratory tool Ki a = 0.01-0.08 s ) [11, 12], However, more energy has to be spent for that purpose (see the next section). 

A lower-bound approximation for the overall mass transfer coefficient, Kj a = 2-6 s , is also given in [11]. 

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