Gas-liquid systems heat transfer

Two-Phase (Gas-Liquid) System Heat Transfer and Hydraulics, An Annotated Bibliography, by R. R. Kepple and T. V. Tung, ANL-6734, USAEC Report (1963). 

It contains six chapters related to the overall characteristics of the cooling systems single-phase and gas-liquid flow, heat transfer and boiling in channels of different geometries. 

The choice of scale-up technique depends on the particular system. As a general guide, constant tip speed is used where suspended solids are involved, where heat is transferred to a coil or jacket, and for miscible liquids. Constant power per unit volume is used with immiscible liquids, emulsions, pastes and gas-liquid systems. Constant tip speed seems more appropriate in this case, and hence the rotor speed should be 0.66 Hz. The... 

Collier, J. G., and D. J. Pulling, 1962, Heat Transfer to Two-Phase Gas-Liquid System, Part II, Further Data on Steam-Water Mixtures in the Liquid Dispersed Region in an Annulus, UK Rep. AERE-R-3809, Harwell, England. (4)... 

If the no-phase-change restriction does not rigorously apply, a simple design procedure can be formulated based on the results discussed in Section III, where it is shown that thermal equilibrium is quickly achieved in gas-liquid systems because of the large heat effects associated with evaporation or condensation. Although the total mass transfer between the phases may be small, it is not unrealistic to assume that the gas and liquid phases have the same temperature at each axial position. 

In many of these operations the engineer is concerned primarily with prediction of pressure losses. However, the heat transfer rate through the tube wall into the gas or the liquid phase is also of major concern in heat-exchange equipment. In the design of chemical reactors for heterogeneous gas-liquid systems, it is necessary to be able to predict not only pressure drops and rates of heat transfer into or out of the channel, but also the rates of mass transfer from the gas into the liquid phase. 

It is important to study the bubble rise velocity and its radial profile in a gas-liquid system as these are closely related to the hydrodynamics, and mass and heat transfer [25]. Bubble rise velocity and its radial profile have also significant influences on gas and liquid residence time distributions. A suitable bubble rise velocity and radial profile can improve production efficiency. Bubble rise velocities in a... 

From the area of thermal process engineering, the mass and heat transfer in stirred vessels and in bubble columns is treated. In the case of mass transfer in the gas/liquid system, coalescence phenomena are also dealt with in detail. The problem of simultaneous mass and heat transfer is discussed in association with film drying and in continuous electrophoresis. 

System with random fluxes is defined as the nonequilibrium system where the fluxes of substance, heat, etc. change randomly. One can cite numerous examples of such systems turbulent gas-liquid systems with intensive heat/mass transfer, turbulent fluids containing dispersed solids, etc. In the case of pore formation, such situation is realized when the heat fluxes change randomly because of air fluidization or mechanical mixing. All macroscopic measured parameters of stationary turbulent flows, like their pressure, temperature, excess (free) energy, entropy, etc. do not change with time, while their values and directions in different spots of the flows can vary significantly. 

Heat transfer in slurry reactors follows the same behavior as that described for, gas-liquid systems as long as liquid properties are appropriately substituted by the slurry properties. 

Droplet vaporization is a phenomenon occurring in a gas-liquid system, although only recently have serious efforts been made towards understanding the various hquid-phase processes and their influence on the overall behavior. The problem is a complex, yet interesting and important one. Fundamental research in the interdisciplinary areas of fluid mechanics, chemical kinetics, phase equilibrium analysis, and heat and mass transfer are required to achieve a good understanding of the problem. The following discussions may substantiate this point and stimulate future research efforts. 

In adiabatic cooling, a warm gas is brought into contact with a cold liquid, causing the gas to cool and some of the liquid to evaporate. Heat is transferred from the gas to the liquid but no heat is transferred between the gas-liquid system and its surroundings (hence adiabatic cooling). Some common processes of this type are described below. 

In this context it is emphasized that in chemical reaction engineering a detailed description of the movement of the interfaces have not been considered important even for gas-liquid systems, in the sense that the complexity of such an approach will lead to impracticable computational costs and little gain in understanding and physical modeling of the important chemical processes. Henceforth, if otherwise not explicitly stated, for the examination of the engineering heat and mass transfer theories both the h3rpothetical films and the embedded interface are assumed to be stagnant, = 0. However,... 

Heat transfer coefficients in gas-liquid systems are generally lower than those in the liquid alone (Karcz, 1999), a result of the reduction in power consumption in the presence of gas. The heat transfer coefficient is approximately proportional to (powerf. ... 

In any catalytic system not only chemical reactions per se but mass and heat transfer effects should be considered. For example, mass and heat transfer effects are present inside the porous catalyst particles as well as at the surrounding fluid films. In addition, heat transfer from and to the catalytic reactor gives an essential contribution to the energy balance. The core of modelling a two-phase catalytic reactor is the catalyst particle, namely simultaneous reaction and diffusion in the pores of the particle should be accounted for. These effects are completely analogous to reaction-diffusion effects in liquid films appearing in gas-liquid systems. Thus, the formulae presented in the next section are valid for both catalytic reactions and gas-liquid processes. 

The transfer of heat in a fluid may be brought about by conduction, convection, diffusion, and radiation. In this section we shall consider the transfer of heat in fluids by conduction alone. The transfer of heat by convection does not give rise to any new transport property. It is discussed in Section 3.2 in connection with the equations of change and, in particular, in connection with the energy transport in a system resulting from work and heat added to the fluid system. Heat transfer can also take place because of the interdiffusion of various species. As with convection this phenomenon does not introduce any new transport property. It is present only in mixtures of fluids and is therefore properly discussed in connection with mass diffusion in multicomponent mixtures. The transport of heat by radiation may be ascribed to a photon gas, and a close analogy exists between such radiative transfer processes and molecular transport of heat, particularly in optically dense media. However, our primary concern is with liquid flows, so we do not consider radiative transfer because of its limited role in such systems. 

Solubilities and diffusivities of gas are practically always required for design of gas-liquid process and obtaining solubility and diffusivity data for the gas-liquid system under consideration may be a chalenging problem so wide is the range of solutes and solvent the chemical engineer or researcher may encounter. Moreover the choice of a suitable gas-liquid contactor is also a question of matching these data, those concerning the reaction kinetics and the physical kinetics characteristics of the proposed reactor, i.e., specific gas-liquid interfacial area, heat and mass transfer coefficients and gas or liquid holdup. Some considerations on solubility and diffusivity will be proposed in part 1 of this review and on gas-liquid mass transfer in part 2. 

It is therefore advantageous to switch from gas-liquid to liquid-liquid reactions if the overall rate is more pronouncedly increased by the interfacial area rather than the mass transfer coefficient. Also, unlike gas-liquid systems, the heat removal in exothermic liquid-liquid reactions may be carried out under refluxing conditions so that heat transfer is very efficiently carried out in an overhead condenser. 

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