Separation process adsorptive drying

Major industrial adsorption processes using zeolite adsorbents may be classified as follows (I) hydrocarbon separation processes, (II) drying gases and liquids, (III) separation and purification of industrial streams, (IV) pollution control applications, and (V) nonregenerative applications. Some important commercial processes in each of these areas are discussed briefly. 

For foam separation processes, adsorption takes place in solution, the essential basis exists for solute separation by foaming. Foam consists of gas bubbles separated by thin liquid films. The liquid films are often formed by the mutual approach of two already existing liquid surfaces (e.g., two bubbles below the surface). Foam structures may vary between two extreme situations. The first is wet foam, which consists of nearly spherical bubbles separated by rather thick liquid films. The second is dry foam, which may develop from the first type as a result of drainage (i.e., foam drainage). 

This part, on applications, covers the following unit operations 8. Evaporation 9. Drying of Process Materials 10. Stage and Continuous Gas-Liquid Separation Processes (humidification, absorption) 11. Vapor-Liquid Separation Processes (distillation) 12. Liquid—Liquid and Fluid-Solid Separation Processes (adsorption, ion exchange, extraction, leaching, crystallization) 13. Membrane Separation Processes (dialysis, gas separation, reverse osmosis, ultrafiltration) 14. Mechanical-Physical Separation Processes (filtration, settling, centrifugal separation, mechanical size reduction). 

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). 

The traditional application of adsorption in the process industries has been as a means of removing trace impurities from gas or liquid streams. Examples include the removal of H2S from hydrocarbon streams before processing, the drying and removal of C02 from natural gas, and the removal of organic compounds from waste water. In these examples the adsorbed component has little value and is generally not recovered. Such processes are generally referred to as purification processes, as distinct from bulk separations, in which a mixture is separated into two (or more) streams, each enriched in a valuable component, which is recovered. The application of adsorption to bulk separations is a more recent development that was stimulated to a significant extent by the rapid... 

Numerous applications of this type of contactors were proposed for a nmnber of separation processes (e.g., gas purification, adsorption, and drying), as well as for multifunctional reactors. Although the feasibility of some of these ideas was confirmed on the bench or pilot-plant scale, the only process that has been realized on industrial scale up to date was for heat recovery. This application of gas-flowing sohds-fixed bed contactors has been efficiently exploited in France since 1965. 

Pick s first law describes equimolar diffusion, in which all components of the system may diffuse independent from each other. During thermal separation processes, matter is transported through phase boundaries. If a phase boundary is selectively permeable to one component, only one-directional diffusion is possible (an especially important case for absorption, adsorption, and drying). For one-directional diffusion, Stefan s law gives... 

This book provides a clear fundamental development of the technology of important separation processes. As indicated by the title the book deals with separation processes in which heat is an input to the complete process of separating the constituents of a mixture. The flow of heat in the process is clear in distillation, crystallization and drying but is not so obvious in absorption, extraction and adsorption, where the heat flow is required to regenerate the solvent or adsorbent. 

The PSA cycle makes use of the simple fact that the partial pressure of adsorbate in the gas phase can be reduced by lowering the total pressure. Pressure reduction can thus be used to regenerate adsorbent that has been loaded with adsorbate at an elevated pressure. Since it is not necessary to heat or cool the bed between or during the adsorption and desorption steps, very rapid cycling is possible. The process is now widely used for hydrogen purification, air separation, hydrocarbon separation, and air drying, and new applications are under development. 

Despite the wide choice of mass transfer models which are available, the simplest and most popular adsorption rate expression is the linear driving force model because it represents actual processes reasonably well and reduces the computational effort required. An example of how this and various other simplifications and empirical correlations can be incorporated into the design and analysis of pressure swing adsorption processes is provided by White and Barkley (1989). The example used is the drying of air. Examples of how simplifying assumptions can aid the modelling of PSA air separation processes is provided by Knaebel and Hill (1985) and by Kayser and Knaebel (1989). Further information on cycle models can be found in Ruthven (1984), Yang (1987), Ruthven (1990) and Ruthven et al. (1994). 

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