Liquid diffusion pressure effects

Some Pressure Effects on Liquid Diffusion Coefficients and Equilibrium... 

Pressure effects The diffusion through liquids is governed by the number of defects or atomic-sized holes in the liquid. A high external pressme can reduce the concentration of holes and slow diffusion. Therefore, in a liquid, a diffusion-controlled rate constant also depends on the pressure. 

Very little is known about the motions of lipid bilayers at elevated pressures. Of particular interest would be the effect of pressure on lateral diffusion, which is related to biological functions such as electron transport and some hormone-receptor interactions. Pressure effects on lateral diffusion of pme lipid molecules and of other membrane components have yet to be carefully studied, however. Figure 9 shows the pressure effects on the lateral self diffusion coefficient of sonicated DPPC and POPC vesicles [86]. The lateral diffusion coefficient of DPPC in the liquid-crystalline (LC) phase decreases, almost exponentially, with increasing pressure from 1 to 300 bar at 50 °C. A sharp decrease in the D-value occurs at the LC to GI phase transition pressure. From 500 bar to 800 bar in the GI phase, the values of the lateral diffusion coefficient ( IT0 cm s ) are approximately constant. There is another sharp decrease in the value of the lateral diffusion coefficient at the GI-Gi phase transition pressure. In the Gi phase, the values of the lateral diffusion coefficient ( 1-10"" cm s ) are again approximately constant. 

Since the resistance to diffusion will be lower in the mixture critical region than that in the liquid phase it is expected that the (A B ) radical pair should be more readily diffuse apart in the critical region. Although applied hydrostatic pressure favors the recombination of (A B ) to form AB, it seems reasonable to assume that the rate of diffusion dominates the pressure effect as long as the system pressure is maintained below approximately 1,000 bar. Therefore, the formation of free radicals should be facilitated in the SCF phase, as compared with the liquid phase, and shorter reaction times are to be expected. 

Since many chemicals are processed wet and sold dry, one of the more common manufacturing steps is a drying operation (13) which involves removal of a liquid from a solid by vaporization of the liquid. Although the only basic requirement in drying is that the vapor pressure of the liquid to be evaporated be higher than its partial pressure in the gas stream, the design and operation of dryers represents a complex problem in heat transfer, fluid flow, and mass transfer. In addition to the effect of such external conditions as temperature, humidity, air flow, and state of subdivision on drying rate, the effect of internal conditions of liquid diffusion, capillary flow, equilibrium moisture content, and heat sensitivity must be considered. 

Membrane separations involve the selective solubility in a thin polymeric membrane of a component in a mixture and/or the selective diffusion of that component through the membrane. In reverse osmosis (3) applications, which entail recovery of a solvent from dissolved solutes such as in desalination of brackish or polluted water, pressures sufficient to overcome both osmotic pressure and pressure drop through the membrane must be applied. In permeation (4), osmotic pressure effects are negligible and the upstream side of the membrane can be a gas or liquid mixture. Sometimes a phase transition is involved as in the process for dehydration of isopropanol shown in Fig. 1.8. In addition, polymeric liquid surfactant and immobilized-solvent membranes have been used. 

The NMR rotating frame spin-lattice relaxation time (Tjp) method, which has been used successfully in our laboratory in studies of pressure effects on diffusion in highly viscous liquids, was used in this study to measure lateral diffusion of the phospholipid molecules in DPPC and POPC vesicles. An advantage of this method is that the diffusion coefficient is found directly from measured quantities without estimations of molecular parameters or the effects of the addition of spin or fluorescence probes to... 

Generally, diffusivity is faster and viscosity lower for supercritical fluids than for liquids. A standard value of the diffusivity of solutes in liquids is roughly 10 cm s [4] the diffusion coefficient of naphthalene in CO2 at 10 MPa and 40 °C is 1.4 10 cm s and the self-diffusion coefficient of CO2 itself is two orders of magnitude higher than that of liquids [9]. The effect of temperature and pressure on the self-diffusivity of CO2 is illustrated in Figure 6. Diffusivity is obviously a major consideration in reactions whether they be homogeneously, heterogeneously, or not catalyzed, and it will determine whether a reaction is controlled kinetically or by diffusion. 

Although elevated temperatures reduce oil viscosity and enhance diffusion, hexane vapor pressure limits the practical operating temperature of the extractor and its contents to about 55°-60°C. Higher temperatures and the consequent higher vapor pressures unduly increase the volume of vapor which the recovery systems must capture and recycle. Futhermore, if the cake temperature is at or near the boiling temperature of the solvent, a vapor phase may occur at the interface between cake fragment and solvent (mis-cella), effectively thwarting liquid diffusion. 

Notes cp is the packing fraction, We u= (gauge feed pressure pore diameter)/(wa-ter surface tension), Gz is the Graetz number, Pr is the Prandtl number. Re is the Reynolds number. Sc is the Schmidt number, Sh is the Sherwood number, is the hydraulic diameter or effective diameter, D is the liquid diffusion coefficient of the component, L is the channel length. 

Oxygen. The rotational motion of liquid water has been shown to be anisotropic by and Ti measurements. NMR spectroscopy has been used to study the dynamics of supercritical water. Temperature and pressure effects on Ti values of solvent molecules in electrolyte solutions have been examined. The diffusion coefficients of DMSO, Mc2S02, D N03 and T>2 0 in D2SO4 have been determined. ... 

When there is no external liquid film, internal zones below the dry surface of the catalyst can be fed with reactant B by liquid diffusion through the internal pore network and with reactant A directly from the gas phase. B can also reach the dry pores via the gas phase if it has a significant vapor pressure under the operating conditions. Many authors have discussed these phenomena and tried to evaluate their effects Dudukovic [13], Tan and Smith [90],... 

In designing a storage vessel for a cryogenic liquid, there are several considerations that should be made Is the system intended for long- or shortterm usage What type of liquid will be stored, and what rate of liquid loss can be tolerated Will the system be mobile or stationary What acceleration effects are anticipated for the system How much time and cost will be allowed for cooldown What will be the stratification and pressure effects on the system What type and thickness of material will be required for the inner and outer vessels and what will be the thermal conductivity of this material Other important aspects include the optimum type of insulation and the design of inner vessel supports, fill and drain lines, vapor vents, vapor diffusers, antislosh baffles, and safety devices. 

These effects of differential vapor pressures on isotope ratios are important for gases and liquids at near-ambient temperatures. As temperature rises, the differences for volatile materials become less and less. However, diffusion processes are also important, and these increase in importance as temperature rises, particularly in rocks and similar natural materials. Minerals can exchange oxygen with the atmosphere, or rocks can affect each other by diffusion of ions from one type into another and vice versa. Such changes can be used to interpret the temperatures to which rocks have been subjected during or after their formation. 

The reaction mechanisms are quite complicated in these syntheses, the kinetics depending on inter-diffusion rates in neighbouring particles, the formation of transient liquid phases, and in some cases, the vapour transport of a reactant. The presence of the latter can be detected by dre retarding effect of increased pressure in an inert surrounding atmosphere. 

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