Distillation phase transfer processes

Ordinary diffusion involves molecular mixing caused by the random motion of molecules. It is much more pronounced in gases and Hquids than in soHds. The effects of diffusion in fluids are also greatly affected by convection or turbulence. These phenomena are involved in mass-transfer processes, and therefore in separation processes (see Mass transfer Separation systems synthesis). In chemical engineering, the term diffusional unit operations normally refers to the separation processes in which mass is transferred from one phase to another, often across a fluid interface, and in which diffusion is considered to be the rate-controlling mechanism. Thus, the standard unit operations such as distillation (qv), drying (qv), and the sorption processes, as well as the less conventional separation processes, are usually classified under this heading (see Absorption Adsorption Adsorption, gas separation Adsorption, liquid separation). 

In processing, it is frequently necessary to separate a mixture into its components and, in a physical process, differences in a particular property are exploited as the basis for the separation process. Thus, fractional distillation depends on differences in volatility. gas absorption on differences in solubility of the gases in a selective absorbent and, similarly, liquid-liquid extraction is based on on the selectivity of an immiscible liquid solvent for one of the constituents. The rate at which the process takes place is dependent both on the driving force (concentration difference) and on the mass transfer resistance. In most of these applications, mass transfer takes place across a phase boundary where the concentrations on either side of the interface are related by the phase equilibrium relationship. Where a chemical reaction takes place during the course of the mass transfer process, the overall transfer rate depends on both the chemical kinetics of the reaction and on the mass transfer resistance, and it is important to understand the relative significance of these two factors in any practical application. 

The concept of a transfer unit for a countercurrent mass transfer process, introduced in Volume 1, is developed further for distillation in packed columns in Section 11.11. The number of transfer units is defined as the integrated value of the ratio of the change in composition to the driving force. Thus, considering the vapour phase, the number of overall gas transfer, units Nog is given by ... 

In contrast to continuous packed bed columns, each stage, whether cocurrent or countercurrent, can be considered to be at equilibrium for many multi-phase mass-transfer processes such as distillation, absorption, extraction etc. Such stages are usually called ideal stages . 

Besides fluid mechanics, thermal processes also include mass transfer processes (e.g. absorption or desorption of a gas in a liquid, extraction between two liquid phases, dissolution of solids in liquids) and/or heat transfer processes (energy uptake, cooling, heating, drying). In the case of thermal separation processes, such as distillation, rectification, extraction, and so on, mass transfer between the respective phases is subject to thermodynamic laws (phase equilibria) which are obviously not scale dependent. Therefore, one should not be surprised if there are no scale-up rules for the pure rectification process, unless the hydrodynamics of the mass transfer in plate and packed columns are under consideration. If a separation operation (e.g. drying of hygroscopic materials, electrophoresis, etc.) involves simultaneous mass and heat transfer, both of which are scale-dependent, the scale-up is particularly difficult because these two processes obey different laws. 

Gas-liquid processes that involve several components in each phase include many chemical reactions, distillation, and transfer of one or more species from a gas to a liquid (absorption or scrubbing) or vice versa (stripping). 

Membrane processes termed as osmotic distillation or membrane distillation could be shown to be applications of membrane contactor technology also. Both of these processes are based on gas membranes. Osmotic distillation, sometimes called osmotic evaporation, involves transfer of water vapor across a gas-fiUed membrane, the process is driven by a difference in water vapor pressure maintained across the membrane [58-59] by separate aqueous hquids. Membrane distillation is a process where water vapor transfer is driven solely by a temperature difference across the gas-fiUed membrane [60-61]. Water evaporates from a hot aqueous phase and condenses on a cooler surface. This process may be useful in desalinating water or producing pure water if a good natural source of warm water is available, such as in a geothermal process. 

The transfer of mass within a fluid mixture or across a phase boundary is a process that plays a major role in various engineering and physiological applications. Typical operations where mass transfer is the dominant step are falling film evaporation and reaction, total and partial condensation, distillation and absorption in packed columns, liquid-liquid extraction, multiphase reactors, membrane separation, etc. The various mass transfer processes are classified according to equilibrium separation processes and rate-governed separation processes. Fig. 1 lists some of the prominent mass transfer operations showing the physical or chemical principle upon which the processes are based. 

It is interesting to note that although the transfer process is predominantly gas-phase mass transfer controlled, there is a finite contribution from the liquid-phase transfer resistance. In multicomponent distillation, it is our experience that it is not safe to ignore the liquid-phase resistance even when for similar operating conditions for a binary system, the liquid-phase resistance is negligible. 

In the Shell Middle Distillates Synthesis (SMDS) process starting from natural gas, the reactor configuration chosen for the first commercial unit in Malaysia, successfully commercialized in 1993, is the multi-tubular downflow trickle bed with catalyst inside the tubes (Sie et al., 1991) see Fig. 30e. Because of the enormous exothermicity of the synthesis reaction and the relatively poor heat transfer an extremely large heat transfer area is required. The reactor volume is largely governed by the installable heat transfer area in a vessel of given volume. Use of the multi-tubular three-phase fluidized bed or slurry reactor (see Fig. 30k and 301) provides much better heat transfer characteristics (an improvement of a factor of five over fixed bed units) and could lead to considerably lower reactor volumes. However, the anticipated scale-up problems with three-phase... 

In processes where a condensing vapor or vapor from a liquid phase moves through an inert gas, eg, condensation in the presence of air, drying, humidification, crystallization (qv), and boiling of a multicomponent liquid, mass-transfer as well as heat-transfer effects are important (see Air conditioning Distillation Evaporation). Such processes are discussed elsewhere in the Encyclopedia, but the primary emphasis is on either the heat transfer or the mass transfer taking place. Herein the interactions between heat and mass transfer in such processes are discussed, and applications to humidification, dehumidification, and water cooling are developed. These same principles are applicable to other operations. 

Separation operations are interphase mass transfer processes because they involve the creation, by the addition of heat as in distillation or of a mass separation agent as in absorption or extraction, of a second phase, and the subsequent selective separation of chemical components in what was originally a one-phase mixture by mass transfer to the newly created phase. The thermodynamic basis for the design of equilibrium staged equipment such as distillation and extraction columns are introduced in this chapter. Various flow arrangements for multiphase, staged contactors are considered. 

The third fundamental transfer process, mass transfer, occurs in distillation, absorption, drying, liquid-liquid extraction, adsorption, and membrane processes. When mass is being transferred from one distinct phase to aiiother or through a single phase, the basic mechanisms are the same whether the phase is a gas, liquid, or solid. This was also shown in heat transfer, where the transfer of heat by conduction followed Fourier s law in a gas, solid, or liquid. 

I. Introduction to absorption. As discussed briefly in Section 10.IB, absorption is a ma s-transfer process in which a vapor solute. 4 in a gas mixture is absorbed by means of a liquid in which the solute is more or less soluble. The gas mixture consists mainly of an inert gas and the solute. The liquid also is primarily immiscible in the gas phase i.e., its vaporization into the gas phase is relatively slight. A typical example is absorption of the solute ammonia from an air-ammonia mixture by water. Subsequently, the solute is recovered from the solution by distillation. In the reverse process of desorption or stripping, the same principles and equations hold. 

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