Forced gas permeation

Three general test procedures used to measure the permeability of plastic films are the absolute pressure method, the isostatic method, and the quasi-isostatic method. The absolute pressure method (ASTM D 1434, Gas Transmission Rate of Plastic Film and Sheeting) is used when no gas other than the permeant in question is present. Between the two chambers a pressure differential provides the driving force for permeation. Here the change in pressure on the volume of the low-pressure chamber measures the permeation rate. 

Gas separations by distillation are energy-consuming processes. The driving force for the gas permeation is only the pressure difference between two compartments separated by the membrane. The permeation is governed by two parameters—diffusion and solubility ... 

As sweep gas flow rate is increased, the performance of the reactor improves until the flow rate is about one thousand times the reactant flow rate. The concentration of all species, but most importantly formaldehyde decreases in the shell side of the reactor as this happens. This increases the driving force for permeation of all species. After increasing this flow rate to a certain point further increases in inert gas flow rate do not change the concentration gradient of any species along the reactor because the shell concentrations of all species is... 

In this section the solution-diffusion model is used to describe transport in dialysis, reverse osmosis, gas permeation and pervaporation membranes. The resulting equations, linking the driving forces of pressure and concentration with flow, are then shown to be consistent with experimental observations. 

The second step in the process is permeation of components i and j through the membrane this step is equivalent to conventional gas separation. The driving force for permeation is the difference in the vapor pressures of the components in the feed and permeate vapors. The separation achieved in this step, /fmem, can be defined as the ratio of the components in the permeate vapor to the ratio of the components in the feed vapor... 

In the crossflow module illustrated in Figure 8.5(a), the pooled permeate stream has a water concentration of 1.88%. The counterflow module illustrated in Figure 8.5(b) performs substantially better, providing a pooled permeate stream with a concentration of 3.49%. Not only does the counterflow module perform the separation twice as well, it also requires only about half the membrane area. This improvement is achieved because the gas permeating the membrane at the residue end of the module contains much less water than the gas permeating the membrane at the feed end of the module. Permeate counterflow dilutes the permeate gas at the feed end of the module with low-concentration permeate gas from the residue end of the module. This increases the water concentration driving force across the membrane and so increases the water flux. 

Depending on reaction and permeation conditions, two modes of FC can be defined natural and forced filtration. In the first case, infiltration flow is a consequence of the natural pressure gradient between the atmosphere (typically constant) and the reaction zone. In the second case, forced gas flow is induced by... 

In liquid-expulsion permporometry [8], the porous solid is saturated with a liquid and by application of a pressure difference across the sample the liquid is forced out of the largest pores. The rate of gas permeating through these pores is then measured. Then, the pressure difference is increased which frees another pores, etc. As a result, pore-size distribution is obtained. 

Pervaporation and vapor permeation (and gas permeation) are closely related processes and are characterized by generating a permeate in the vapor state. In this situation, the driving force for permeation of a particular component approximates very closely to the difference in partial vapor pressure of that component across the membrane. Because the pressure on the back-side of the membrane is low, almost all of the faster permeating component can be removed from the feed. The process purifies the feed by removing the faster permeating component. The product from the process is the retentate and the concentrated impurity is the permeate. 

More recently, Teramoto et al. [24-25] referred to the use of a novel facihtated transport membrane for gas separation in which a carrier was supphed to the feed side (high-pressure side) and it was forced to permeate through a membrane to the permeate side (low-pressure side), and then the permeated carrier solution was recirculated to the feed side. Since the membrane was always wet with the carrier solution, the membrane became very stable with no open or unfilled pores present which usually caused membrane unselectively in traditional SLM. This new type of membrane has been named a bulk flow liquid membrane (BFLM). The membrane resulted to be stable over a discontinuous one-month testing period. 

In gas permeation the driving force across the membrane may be expressed in terms of concentrations, but is usually defined in terms of partial pressures. The partial pressures of component i are and/ 2i in the bulk gas phase, andand... 

The driving force in gas permeation may be expressed in terms of the difference between a component partial pressure on the residue side and the permeate side of the membrane. The feed is introduced to the separator at a high pressure, while the permeate side is controlled at a low pressure. Examples 18.3 and 18.4 use the perfect mixing model for the performance evaluation and the design of two gas permeation processes. 

Because of the phase change associated with the process and the non-ideal liquid-phase solutions (i.e., organic/water), the modeling of pervaporation cannot be accomplished using a solution-diffusion approach. Wijmans and Baker [14] express the driving force for permeation in terms of a vapor partial pressure difference. Because pressures on the both sides of the membrane are low, the gas phase follows the ideal gas law. The liquid on the feed side of the membrane is generally non-ideal. 

When membrane reactors are applied to separate a gaseous product from the reaction zone, the driving force for permeation can be obtained by reducing the partial pressure of the product at the permeate side using vacuum (Fig. 9.5), sending a sweep gas into the permeate (Fig. 9.6), or by increasing the partial pressure in the feed stream (Fig. 9.7). 

Gas permeation is used to separate gas mixtures, for example, hydrogen fixjm methane. High pressures on the order of 500 psia are used to force the molecules through a dense polymer membrane, which is packaged in pressure-vessel modules, each containing up to 4,000 ft of membrane surface area. Membrane modules cost approximately 35/ft of membrane surface area. Multiple modules are arranged in parallel to achieve the desired total membrane area. 

Pervaporation, vapor permeation and gas permeation are very closely related processes. In aU three cases the driving force for the transport of matter through the membrane is a gradient in the chemical potential that can best be described by a gradient in partial vapor pressure of the components. The separation is governed by the physical-chemical affinity between the membrane material and the species to be passed through and thus by sorption and solubility phenomena. The transport through the membrane is affected by diffusion and the differences in the diffiisivities of the different components in the membrane can play an important role for the separation efficiency, too. All three processes are best described by the solution-diffusion mechanism , their main differences are determined by the phase state and the thermodynamic conditions of the feed mixture and the condensability of the permeate. 

Mass flux and mass transfer coefficient per unit dimensionless driving force surface loading reciprocal gas permeation unit... 

K. C. Khulbe, G. Chowdhury, B. Kruczek, R. Vujosevic, T. Matsuura, G. Lamarche, Characterization of the PPO dense membrane prepared at different temperatures by ESR, atomic force microscope and gas permeation, J. Membr. ScL, 126, 115 (1997). 

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