Membranes Partial condensers

When a mixture in a reactor effluent contains components with a wide range of volatilities, then a partial condensation from the vapor phase or a partial vaporization from the liquid phase followed by a simple phase split often can produce a good separation. If the vapor from such a phase split is difficult to condense, then further separation needs to be carried out in a vapor separation unit such as a membrane. 

The net work of 0.52 MW for the separation section assumes that we perform all separations mechanically, i.e., with compressors and semipermeable membranes. In reality we use evaporation, partial condensation, and distillation to separate the components. We can use Eq. (5.8) to estimate how much heat, from 75 psia steam, we must supply to an ideal separation device to provide the necessary separation work. Again, this is only a rough estimate of the required heat. 

Pervaporation have been considered an interesting alternative process for the current industrial options for aroma recovery, distillation, partial condensation, solvent extraction, adsorption, or a combination thereof. It is considered a basic unit operation with significant potential for the solution of various environmental and energetic processes (moderate temperatures). This separation process is based on a selective transport through a dense membrane (polymeric or ceramic) associated with a recovery of the permeate from the vapour phase. A feed liquid mixture contacts one side of a membrane the permeate is removed as a vapour from the other side. Transport through... 

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. 

A large problem in polymer electrolyte membrane fuel cell operation is a possible partial condensation of water vapor when temperature gradients are present in the fuel cell and a dual-phase water system develops. 

Heuristic 11 Separate vapor mixtures using partial condensation, cryogenic distillation, absorption, adsorption, membrane separation and/or desublimation. The selection among these alternatives is considered in Chapter 7. 

As usually identical membranes are employed for both liquid and vaporous feed mixtures the partial condensation of vapor on the membrane in vapor permeation will have no detrimental effect on the performance of either the membrane or the process. It can even be proven that the highest performance can be obtained with the same membrane, when a mixture of hquid and vapor is direcdy used as a feed [33]. Condensation of the vaporous portion supphes the heat necessary for the evaporation of the permeate and thus temperature polarization is avoided. As the volume of the vapor phase will exceed that of the hquid a strong mixing effect will occur at the membrane surface, reducing concentration polarization too. 

During fuel cell operation, water is formed by the electrochemical reactions at the cathode. It will partly be transported by diffusion through the electrolyte membrane to the anode side, other parts of the water are removed through the gas diffusion layer. As it can be expected, water will partially condense into the pore structure of the GDL, particularly under the ribs of the flow field, from which it is removed either by evaporation or by liquid transport. Synchrotron imaging and tomography are powerful tools to study the productirm, accumulation and removal of water [61-66]. 

Alternatively, the capillary network model constitutes a significant improvement over the aforementioned mentioned tortuosity model, since it can provide realistic modeling, especially for systems involving membranes partially blocked by condensed vapors. In this model the degree of connectivity of the pores, z, is replacing the less tangible tortuosity factor t. The estimation of the z can be based on gas and condensed vapor relative permeability measurements, presented in this section. 

Mass transfer of gas through a porous membrane can involve several processes depending on the pore stmcture and the solid [1]. There are four different mechanisms for the transport Poiseuille flow Knndsendiflusion partial condensation/capillaiy diffusion/selective adsorption and molecular sieving [2, 3]. The transport mechanism exhibited by most of carbon membranes is the molecular sieving mechanism as shown in Fig. 2.1. The carbon membranes contain constrictions in the carbon matrix, which approach the molecular dimensions of the absorbing species [4],... 

Gas transport can occur in porous membranes by four different idealized mechanisms Knudsen diffusion, partial condensation/diffiision, selective adsorption/diffusion, and molecular sieving (Rao and Sirkar, 1993a,b). Knudsen diffusion occurs when the mean free path of the molecule is greater than the size of the pore therefore, the di sing gas molecule collides more often with the pore wall than with other molecules. Knudsen diffusion can be described using Eq. (23.1) (Hines and Maddox, 1985) ... 

Plasticization Gas solubility in the membrane is one of the factors governing its permeation, but the other factor, diffusivity, is not always independent of solubility. If the solubility of a gas in a polymer is too high, plasticization and swelhng result, and the critical structure that controls diffusion selectivity is disrupted. These effects are particularly troublesome with condensable gases, and are most often noticed when the partial pressure of CO9 or H9S is high. H9 and He do not show this effect This problem is well known, but its manifestation is not always immediate. 

Process Description Pervaporation is a separation process in which a liquid mixture contacts a nonporous permselective membrane. One component is transported through the membrane preferentially. It evaporates on the downstream side of the membrane leaving as a vapor. The name is a contraction of permeation and evaporation. Permeation is induced by lowering partial pressure of the permeating component, usually by vacuum or occasionally with a sweep gas. The permeate is then condensed or recovered. Thus, three steps are necessary Sorption of the permeating components into the membrane, diffusive transport across the nonporous membrane, then desorption into the permeate space, with a heat effect. Pervaporation membranes are chosen for high selectivity, and the permeate is often highly purified. 

Transition-metal nanopartides are of fundamental interest and technological importance because of their applications to catalysis [22,104-107]. Synthetic routes to metal nanopartides include evaporation and condensation, and chemical or electrochemical reduction of metal salts in the presence of stabilizers [104,105,108-110]. The purpose of the stabilizers, which include polymers, ligands, and surfactants, is to control particle size and prevent agglomeration. However, stabilizers also passivate cluster surfaces. For some applications, such as catalysis, it is desirable to prepare small, stable, but not-fully-passivated, particles so that substrates can access the encapsulated clusters. Another promising method for preparing clusters and colloids involves the use of templates, such as reverse micelles [111,112] and porous membranes [106,113,114]. However, even this approach results in at least partial passivation and mass transfer limitations unless the template is removed. Unfortunately, removal of the template may re-... 

These porous structures may hinder the transport of solutes away from the membrane downstream surface, causing a local increase of the solute partial pressure and hence a decrease of the driving force (19.1). Eventually, solute condensation may occur if the solutes local partial pressure surmounts its saturation vapour pressure. This problem becomes particularly relevant when dealing with high-boiling aroma compounds [14] and when pressure drop in the downstream circuit increases owing to poor module design. 

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