Mass/heat transfer resistance

Correlations of heat and mass-transfer rates are fairly well developed and can be incorporated in models of a reaction process, but the chemical rate data must be determined individually. The most useful rate data are at constant temperature, under conditions where external mass transfer resistance has been avoided, and with small particles... 

Part 1. Presentation of the model. Int J Heat Mass Transfer 47 3375-3385 Tiselj I, Hetsroni G, Mavko B, Mosyak A, Pogrebnyak E, Segal Z (2004) Effect of axial conduction on the heat transfer in micro-channels Int J Heat Mass Transfer 47 2551-2565 Triplett KA, Ghiaasiaan SM, Abdel-Khalik SI, Sadowski DL (1999) Gas-liquid two-phase flow in microchannels. Part I. Two-phase flow patterns. Int J Multiphase Flow 25 377-394 Tsai J-H, Lin L (2002) Transient thermal bubble formation on polysihcon micro-resisters. J Heat Transfer 124 375-382... 

When the mass transfer resistances are eliminated, the various gas-phase concentrations become equal a/(/, r, z) = j(r, z) = a(r, z). The very small particle size means that heat transfer resistances are minimized so that the catalyst particles are isothermal. The recycle reactor of Figure 4.2 is an excellent means for measuring the intrinsic kinetics of a finely ground catalyst. At high recycle rates, the system behaves as a CSTR. It is sometimes called a gradientless reactor since there are no composition and temperature gradients in the catalyst bed or in a catalyst particle. 

Mass and heat transfer to the walls in turbulent flows is a complex mixture of molecular transport and transport by turbulent eddies. The generally assumed analogy between mass and heat transfer by assuming Sh = Nu, is not valid for turbulent flows [26]. Simulations and measurements have shown that there is a laminar film close to the surface where most of the mass transfer resistance for high Sc liquids is located. This fUm is located below y+ = 1 and for low Sc fluids, and for heat transfer the whole boundary layer is important [27]. 

Empirical methods approximate methods, in which the resistance to heat transfer is considered to control the rate of condensation, and the mass transfer resistance is neglected. Design methods have been published by Silver (1947), Bell and Ghaly... 

Boyko, L. D. and Kruzhilin, G. N. (1967) Ini. J. Heat Mass Transfer 10, 361. Heat transfer and hydraulic resistance during condensation of steam in a horizontal tube and in a bundle of tubes. 

Dimensionless numbers in mixing, 16 685 used in convection heat-transfer analysis, 73 246-247 Dimensionless parameter, external mass transfer resistance and, 25 290-292 Dimensionless reactor design formulation, 21 350... 

The hot-wire anemometer can, with suitable cahbration, accurately measure velocities from about 0.15 m/s (0.5 fl/s) to supersonic velocities and detect velocity fluctuations with frequencies up to 200,000 Hz. Eairly rugged, inexpensive units can be built for the measurement of mean velocities in the range of 0.15 to 30 m/s (about 0.5 to 100 ft/s). More elaborate, compensated units are commercially available for use in unsteady flow and turbulence measurements. In cahbrating a hotwire anemometer, it is preferable to use the same gas, temperature, and pressure as will be encountered in the intended apphcation. In this case the quantity I RJAt can be plotted against /v, where I = hot-wire current, = hot-wire resistance. At = difference between the wire temperature and the gas bulk temperature, and V = mean local velocity. A procedure is given by Wasan and Raid [Am. Inst. Chem. Eng. J., 17, 729-731 (1971)] for use when it is impractical to calibrate with the same gas composition or conditions of temperature and pressure. Andrews, Rradley, and Hundy [Int. J. Heat Mass Transfer, 15, 1765-1786 (1972)] give a cahbration correlation for measurement... 

Then, assume that the reaction takes place in a fixed bed of 1.61 m diameter and 16.1 m height, under contact time of 5 min, and the inlet temperature of gas being 50 °C, for different CO inlet concentration (several runs). Estimate the conversion of CO in an isothermal and adiabatic fixed-bed reactor and under the following assumptions isobaric process, negligible external mass transfer resistance, and approximately constant heat capacity of air (cp = 1 kJ/kg K) and heat of reaction (AH = -67,636 cal/mol). The inlet temperature of the reaction mixture is 50 °C and its composition is 79% N2 and approximately 21% 02, while the inlet CO concentration varies from 180-4000 ppm (mg/kgair) (for each individual ran). 

The relationships between the overall mass transfer coefficient and the film mass transfer coefficients in both phases are not as simple as in the case of heat transfer, for the following reason. Unlike the temperature distribution curves in heat transfer between two phases, the concentration curves of the diffusing component in the two phases are discontinuous at the interface. The relationship between the interfacial concentrations in the two phases depends on the solubility of the diffusing component. Incidentally, it is known that there exists no resistance to mass transfer at the interface, except when a surface-active substance accumulates at the interface to give additional mass transfer resistance. 

For liquid-phase catalytic or enzymatic reactions, catalysts or enzymes are used as homogeneous solutes in the hquid, or as sohd particles suspended in the hquid phase. In the latter case, (i) the particles per se may be catalysts (ii) the catalysts or enzymes are uniformly distributed within inert particles or (hi) the catalysts or enzymes exist at the surface of pores, inside the particles. In such heterogeneous catalytic or enzymatic systems, a variety of factors that include the mass transfer of reactants and products, heat effects accompanying the reactions, and/or some surface phenomena, may affect the apparent reaction rates. For example, in situation (iii) above, the reactants must move to the catalytic reaction sites within catalyst particles by various mechanisms of diffusion through the pores. In general, the apparent rates of reactions with catalyst or enzymatic particles are lower than the intrinsic reaction rates this is due to the various mass transfer resistances, as is discussed below. 

Generally speaking, the design of TBR requires knowledge of hydrodynamics and flow regimes, pressure-drop, hold-ups of the phases, interfacial areas and mass-transfer resistances, heat transfer, dispersion and back-mixing, residence time distribution, and segregation of the phases. 

Possibility of operating partially or wholly in the vapour phase by varying the liquid flowrate according to catalyst wetting, heat of vaporization, and mass-transfer resistances in the liquid phase. 

Here we consider a spherical catalyst pellet with negligible intraparticle mass- and negligible heat-transfer resistances. Such a pellet is nonporous with a high thermal conductivity and with external mass and heat transfer resistances only between the surface of the pellet and the bulk fluid. Thus only the external heat- and mass-transfer resistances are considered in developing the pellet equations that calculate the effectiveness factor rj at every point along the length of the reactor. 

Gloski, D., Glicksman, L. and Decker, N. (1984). Thermal Resistance at a Surface in Contact with Fluidized Bed Particles. Int. J. Heat Mass Transfer, 27,599. 

In order to avoid the unfavorable process conditions, different flue-gas treatment processes for combustion plants based on catalytic filters were developed, which combine fly-ash removal with SCR of ISKh with NH3 [4—8], The advantages of these processes are space and treatment-cost savings, reduced internal and external mass transfer resistances compared to honeycomb SCR catalysts, heat recovery from offgases with good efficiency, and low corrosion problems due to the removal of both dust and NOx at high temperatures. 

Finally, it must be pointed out that the adsorbent when it makes contact with a binary mixture, one component is selectively adsorbed by the solid adsorbent. In the flowing fluid, a trace of an adsorbable species is adsorbed from a relatively inert carrier. In addition, the heat effects can be ignored, as a result, isothermal conditions can be taken [9,103], The flow is fed at the top of the bed at a constant flow rate, and under conditions such that mass-transfer resistance is insignificant [2,103],... 

Minkowycz, W.J. and Sparrow, E.M., Condensation Heat Transfer in the Presence of Non-condensables, Interface Resistance, Superheating, Variable Properties and Diffusion, Int. J. Heat Mass Transfer, Vol. 9, p. 1125,1966. 

---