Liquid transport diffusion

Transport numbers for several non-haloaluminate ionic liquids generated from ionic liquid self-diffusion coefficients are listed in Table 3.6-7. The interesting, and still open, question is whether the NMR-generated transport numbers provide the same measure of the fraction of current carried by an ion as the electrochemically... 

If a liquid system containing at least two components is not in thermodynamic equilibrium due to concentration inhomogenities, transport of matter occurs. This process is called mutual diffusion. Other synonyms are chemical diffusion, interdiffusion, transport diffusion, and, in the case of systems with two components, binary diffusion. 

GL 21] [no reactor] [P 22] A linear increase in conversion with increasing diffusion coefficient was observed [73], This shows that liquid transport of hydrogen to the catalyst has a dominant role. 

Transport Properties Although the densities of SCFs can approach those of conventional liquids, transport properties are more favorable because viscosities remain lower and diffusion coefficients remain higher. Furthermore, CO2 diffuses through condensed-liquid phases (e.g., adsorbents and polymers) faster than do typical solvents which have larger molecular sizes. For example, at 35°C the estimated pyrene diffusion coefficient in polymethylmethacrylate increases by 4 orders of magnitude when the CO2 content is increased from 8 to 17 wt % with pressure [Cao, Johnston, and Webber, Macromolecules, 38(4), 1335-1340 (2005)]. 

In conclusion, at an intermediate optimum thickness, a diffusion layer allows for (1) gas diffusion toward the CL, (2) liquid transport from the CL toward the flow field channels, (3) good contact with both the bipolar plate... 

In this section, we will briefly discuss different testing techniques that are widely used to measure most of the important mass transport properties of fhe diffusion layers. It is important to note that these techniques can also be used with MPLs. The first subsection will explain methods that deal with properties that affect both gas and liquid mass transport, and the other two subsections will discuss only techniques that measure gas and liquid transport properties, respectively. 

As stated earlier, CEP and CC are the most common materials used in the PEM and direct liquid fuel cell due fo fheir nature, it is critical to understand how their porosity, pore size distribution, and capillary flow (and pressures) affecf fhe cell s overall performance. In addition to these properties, pressure drop measurements between the inlet and outlet streams of fuel cells are widely used as an indication of the liquid and gas transport within different diffusion layers. In fhis section, we will discuss the main methods used to measure and determine these properties that play such an important role in the improvement of bofh gas and liquid transport mechanisms. 

Besides gas and liquid transport in the diffusion media, there is also electronic conduction. Most models neglect this due to the high conductivity of the carbon in the diffusion media, although it can become a limiting factor due to geometry or diffusion-media composition. For those that take it into account. Ohm s law is used... 

Facilitated transport of penicilHn-G in a SLM system using tetrabutyl ammonium hydrogen sulfate and various amines as carriers and dichloromethane, butyl acetate, etc., as the solvents has been reported [57,58]. Tertiary and secondary amines were found to be more efficient carriers in view of their easy accessibility for back extraction, the extraction being faciUtated by co-transport of a proton. The effects of flow rates, carrier concentrations, initial penicilHn-G concentration, and pH of feed and stripping phases on transport rate of penicillin-G was investigated. Under optimized pH conditions, i. e., extraction at pH 6.0-6.5 and re-extraction at pH 7.0, no decomposition of peniciUin-G occurred. The same SLM system has been applied for selective separation of penicilHn-G from a mixture containing phenyl acetic acid with a maximum separation factor of 1.8 under a liquid membrane diffusion controlled mechanism [59]. Tsikas et al. [60] studied the combined extraction of peniciUin-G and enzymatic hydrolysis of 6-aminopenicillanic acid (6-APA) in a hollow fiber carrier (Amberlite LA-2) mediated SLM system. 

The main mass transport resistance in liquid fluidized beds of relatively small particles lies in the liquid film. Thus, for ion exchange and adsorption on small particles, the mass transfer limitation provides a simple liquid-film diffusion-controlled mass transfer process (Hausmann el al., 2000 Menoud et al., 1998). The same holds for catalysis. 

Concentration modulation experiments have been reported for applications to heterogeneous catalysis (48). The experimental implementation was accomplished by periodically flowing solutions with different (reactant) concentrations over the catalyst immobilized on the IRE. Fast concentration modulation in the liquid phase is limited by mass transport (diffusion and convection), and an appropriately designed cell is essential. The cell depicted in Fig. 12 has two tubes ending at the same inlet (65). This has the advantage that backmixing in the tubing upstream of the cell can be avoided. With this cell, concentration modulation periods of about 10 s were achieved (45,65). 

Porous Membrane DS Devices. The applicability of a simple tubular DS based on a porous hydrophobic PTFE membrane tube was demonstrated for the collection of S02 (dilute H202 was used as the scrubber liquid, and conductometric detection was used) (46). The parameters of available tubular membranes that are important in determining the overall behavior of such a device include the following First, the fractional surface porosity, which is typically between 0.4 and 0.7 and represents the probability of an analyte gas molecule entering a pore in the event of a collision with the wall. Second, wall thickness, which is typically between 25 and 1000 xm and determines, together with the pore tortuosity (a measure of how convoluted the path is from one side of the membrane to the other), the overall diffusion distance from one side of the wall to the other. If uptake probability at the air-liquid interface in the pore is not the controlling factor, then items 1 and 2 together determine the collection efficiency. The transport of the analyte gas molecule takes place within the pores, in the gas phase. This process is far faster than the situation with a hydrophilic membrane the relaxation time is well below 100 ms, and the overall response time may in fact be determined by liquid-phase diffusion in the boundary layer within the lumen of the membrane tube, by liquid-phase dispersion within the... 

Thermodynamic nonidealities are considered both in the transport equations (A10) and in the equilibrium relationships at the phase interface. Because electrolytes are present in the system, the liquid-phase diffusion coefficients should be corrected to account for the specific transport properties of electrolyte solutions. 

The liquid-phase diffusion coefficients are found with the Nemst-Hartley equation (193), which describes the transport properties in weak electrolyte systems. The gas-phase diffusion coefficients are estimated according to the... 

P 67] Simulations were made following experiments made previously [156], Therein 0.11 mM Rhodamine B solutions in 20 mM carbonate buffer were mixed with the same carbonate buffer. For the buffer solution, the physical properties of water were approximated. For Rhodamine B, a diffusion coefficient of 2.8 10-6 cm2 s-1 was taken. Electroosmotic flow was applied for liquid transport. For all of the walls in the domain the electroosmotic (EO) mobility was set to 3.4 10-4 cm2 V-1 s 1, which corresponds to a zeta potential (Q of-44.1 mV. The electric field in the outlet channel was 1160 V cm-1. The Reynolds number was 0.22. The electric field strength was set low in order to decrease diffusive (pre-)mixing prior to the groove structure. 

Multiphase reactors include, for instance, gas-liquid-solid and gas-liq-uid-liquid reactions. In many important cases, reactions between gases and liquids occur in the presence of a porous solid catalyst. The reaction typically occurs at a catalytic site on the solid surface. The kinetics and transport steps include dissolution of gas into the liquid, transport of dissolved gas to the catalyst particle surface, and diffusion and reaction in the catalyst particle. Say the concentration of dissolved gas A in equilibrium with the gas-phase concentration of A is CaLt. Neglecting the gas-phase resistance, the series of rates involved are from the liquid side of the gas-liquid interface to the bulk liquid where the concentration is CaL, and from the bulk liquid to the surface of catalyst where the concentration is C0 and where the reaction rate is r wkC",. At steady state,... 

The rate of solute diffusion in liquids in porous materials becomes observably lower than would be expected from Equation 3.27 when the solute molecular size becomes large with respect to the pore size. This phenomena can be important for catalytic processing of heavy liquid petroleum fractions [25]. In the case of liquid transport through finely porous materials, the effective diffusivity can be expressed as [26]... 

IONIC LIQUIDS FOR DIFFUSION STUDIES AND IONIC TRANSPORT BEHAVIOR... 

This result has been quoted in the latest paper by Swalin (30) as a support for a cooperative diffusion theory. The physical image is also in agreement with the liquid transport model recently derived by Rahman (27) from computer experiments. 

At a relatively low temperature (e.g., near OX), some gases undergo capillary condensation and become liquid occupying the pores. The associated transport is then governed by pseudo-liquid phase diffusion. When other gases do not dissolve in the condensed component(sX separation of gases occurs. Two examples are SO2/H2 [Barrer, 1965] and H2S/H2 [Kameyama, et al. 1979] systems in which SO2 and H2S, respectively, condense in the pores and diffuse across the membrane while H2 in both cases is blocked from the pores as it does not dissolve in either liquid SO2 or H2S. 

Kimura and Sourirajan have offered a theory of preferential adsorption of materials at interfaces to describe liquid phase, selective transport processes in portms membranes. Lonsdale et al. have ofiered a simpler explanation of the transport behavior of asymmetric membranes which lack significant porosity in the dense surface layer. Their solution-diffusion model seems to adequately describe the cases for liquid transport considered to date. Similarly gas transport should be de-scribable in terms of a solution-diffusion model in cases where the thin dense membrane skin acts as the transport moderating element. 

In the bubble column the velocity profile of recirculating liquid is shown in Fig. 27, where the momentum of the mixed gas and liquid phases diffuses radially, controlled by the turbulent kinematic viscosity Pf When I/l = 0 (essentially no liquid feed), there is still an intense recirculation flow inside the column. If a tracer solution is introduced at a given cross section of the column, the solution diffuses radially with the radial diffusion coefficient Er and axially with the axial diffusion coefficient E. At the same time the tracer solution is transported axially Iby the recirculating liquid flow. Thus, the tracer material disperses axially by virtue of both the axial diffusivity and the combined effect of radial diffusion and the radial velocity profile. 

Estimation of liquid-side mass transfer coefficients for gas-liquid transport through the upper surface of the film can be most readily made by assumption of a purely diffusive transfer. Under these conditions, a mass transfer coefficient can be approximated using the following equation for mass transfer ... 

SILP systems have proven to be interesting not only for catalysis but also in separation technologies [128]. In particular, the use of supported ionic liquids can facilitate selective transport of substrates across membranes. Supported liquid membranes (SLMs) have the advantage of liquid phase diffusivities, which are higher than those observed in polymers and grant proportionally higher permeabilities. The use of a supported ionic liquid, due to their stability and negligible vapor pressure, allow us to overcome the lack of stability caused by volatilization of the transport liquid. SLMs have been applied, for example, in the selective separation of aromatic hydrocarbons [129] and CO2 separation [130, 131]. 

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