Transport parameter determination

In this work preliminary vapour permeation measurements were carried out with two different species, the rather bulky dichloromethane (DCM) molecules and the much smaller methanol molecules. Two typical permeation curves are displayed in Figure 4.8. The transport parameters, determined on the basis of the tangent method and Equations (4.9)-(4.11), are listed in Table 4.3. It contains the parameters dehned above as well as solubility C in the membrane in equilibrium with the feed pressure of penetrants. 

Vazquez, MI Benavente, J. A study of temperature effect on chemical, structural and transport parameters determined for two different regenerated cellulose membranes. Journal of Membrane Science, 2003,219, 59-67. 

The problem addressed here is that of expressing mass transfer coefficients that apply to turbulent flow conditions in a tube in terms of appropriate dimensionless groups. To implement Step 1, both fluid mechanical and transport properties must be taken into account. The former determine the degree of turbulence or ability to form eddies, and hence the rate at which mass is transported to or from the tubular wall, whereas the transport parameter determines the rate at which mass is conveyed through the film adjacent to the interface. It is proposed to use velocity v, density p, and viscosity p as the fluid mechanical properties, as each of these parameters either promotes or resists the formation of eddies. Transport through the film is determined by only one parameter, the diffusivity of the conveyed species, hi addition to these factors, we expect pipe diameter to play a role because it determines the distance over which the mass is to be transported and plays a role as well in the degree of turbulence generated in the system. 

Analytical solutions for the closure problem in particular unit cells made of two concentric circles have been developed by Chang [68,69] and extended by Hadden et al. [145], In order to use the solution of the potential equation in the determination of the effective transport parameters for the species continuity equation, the deviations of the potential in the unit cell, defined by... 

If initial solute uptake rate is determined from intestinal tissue incubated in drug solution, uptake must be normalized for intestinal tissue weight. Alternative capacity normalizations are required for vesicular or cellular uptake of solute (see Section VII). Cellular transport parameters can be defined either in terms of kinetic rate-time constants or in terms of concentration normalized flux [Eq. (5)]. Relationships between kinetic and transport descriptions can be made on the basis of information on solute transport distances. Note that division of Eq. (11) or (12) by transport distance defines a transport resistance of reciprocal permeability (conductance). 

Imanidis G, C Waldner, C Mettler, H Leuenberger. (1996). An improved diffusion cell design for determining drug transport parameters across cultured cell monolayers. J Pharm Sci 85 1196-1203. 

Acrylonitrile is both readily volatile in air (0.13 atm at 23° C) (Mabey et al. 1982) and highly soluble in water (79,000 mg/L) (Klein et al. 1957). These characteristics dominate the behavior of acrylonitrile in the environment. While present in air, acrylonitrile has little tendency to adsorb to particulate matter (Cupitt 1980), so air transport of volatilized material is determined mainly by wind speed and direction. Similarly, acrylonitrile dissolved in water has only a low tendency to adsorb to suspended soils or sediments (Roy and Griffin 1985), so surface transport is determined by water flow parameters. Based on its relatively high water solubility, acrylonitrile is expected to be higly mobile in moist soils. In addition, acrylonitrile may penetrate into groundwater from surface spills or from contaminated surface water. The high vapor pressure indicates that evaporation from dry soil samples is expected to occur rapidly (EPA 1987). 

The main goal of this chapter is to review the most widely used modeling techniques to analyze sorption/desorption data generated for environmental systems. Since the definition of sorption/desorption (i.e., a mass-transfer mechanism) process requires the determination of the rate at which equilibrium is approached, some important aspects of chemical kinetics and modeling of sorption/desorption mechanisms for solid phase systems are discussed. In addition, the background theory and experimental techniques for the different sorption/ desorption processes are considered. Estimations of transport parameters for organic pollutants from laboratory studies are also presented and evaluated. 

Determine the transport parameters that control pollutant migration through the subsurface environment (i.e., diffusion/dispersion and diffusion coefficients)... 

Simulation and predictive modeling of contaminant transport in the environment are only as good as the data input used in these models. Field methods differ from laboratory methods in that an increase in the scale of measurement relative to most laboratory methods is involved. Determination of transport parameters (i. e., transmission coefficients) must also use actual contaminant chemical species and field solid phase samples if realistic values are to be specified for the transport models. The choice of type of test, e.g., leaching cells and diffusion tests, depends on personal preference and availability of material. No test is significantly better than another. Most of the tests for diffusion evaluation are flawed to a certain extent. 

Johnson DA and Amidon GL (1988) Determination of Intrinsic Membrane Transport Parameters from Perfused Intestine Experiments a Boundary Layer Approach to Estimating the Aqueous and Unbiased Membrane Permeabilities. J Theor Biol 131 pp 93-106. 

Quantitative analysis relies on a highly probable mechanistic hypothesis and determines as many as possible kinetic, thermodynamic, and/or transport parameters for the various steps. This is often a complex problem, since the values of the parameters are usually correlated, their relation to experimental data is nonlinear, and the data contain artifacts and statistical errors [40, 41]. 

In Fig. 1, various elements involved with the development of detailed chemical kinetic mechanisms are illustrated. Generally, the objective of this effort is to predict macroscopic phenomena, e.g., species concentration profiles and heat release in a chemical reactor, from the knowledge of fundamental chemical and physical parameters, together with a mathematical model of the process. Some of the fundamental chemical parameters of interest are the thermochemistry of species, i.e., standard state heats of formation (A//f(To)), and absolute entropies (S(Tq)), and temperature-dependent specific heats (Cp(7)), and the rate parameter constants A, n, and E, for the associated elementary reactions (see Eq. (1)). As noted above, evaluated compilations exist for the determination of these parameters. Fundamental physical parameters of interest may be the Lennard-Jones parameters (e/ic, c), dipole moments (fi), polarizabilities (a), and rotational relaxation numbers (z ,) that are necessary for the calculation of transport parameters such as the viscosity (fx) and the thermal conductivity (k) of the mixture and species diffusion coefficients (Dij). These data, together with their associated uncertainties, are then used in modeling the macroscopic behavior of the chemically reacting system. The model is then subjected to sensitivity analysis to identify its elements that are most important in influencing predictions. 

Generally, the transport parameter used is the logarithm of the partition coefficient of the bas (bioactive substance) or some quantity derived from it. The partition coefficient is almost always determined between water and 1-octanol. Parameters obtained by chromatographic methods are being used with increasing frequency however. The term in t2 is introduced to account for the frequently observed parabolic dependence of a data set of bas on the transport parameter. Models other than... 

Reverse-Osmosis Experiments. All reverse-osmosis experiments were performed with continuous-flow cells. Each membrane was subjected to an initial pure water pressure of 2068 kPag (300 psig) for 2 h pure water was used as feed to minimize the compaction effect. The specifications of all the membranes in terms of the solute transport parameter [(Dam/ 6)Naci]> the pure water permeability constant (A), the separation, and the product rate (PR) are given in Table I. These were determined by Kimura-Sourirajan analysis (7) of experimental reverse-osmosis data with sodium chloride solution at a feed concentration of 0.06 m unless otherwise stated. All other reverse-osmosis experiments were carried out at laboratory temperature (23-25 °C), an operating pressure of 1724 kPag (250 psig), a feed concentration of 100 ppm, and a feed flow rate >400 cmVmin. The fraction solute separation (/) is defined as follows ... 

Gillham, R. W., Starr, R.C. Miller, D.J. (1990). A device for in situ determination of geochemical transport parameters. 2. Biochemical reactions. GroundWater, 28(6), 858-62. 

According to this kinetic model the collision efficiency factor p can be evaluated from experimentally determined coagulation rate constants (Equation 2) when the transport parameters, KBT, rj are known (Equation 3). It has been shown recently that more complex rate laws, similarly corresponding to second order reactions, can be derived for the coagulation rate of polydisperse suspensions. When used to describe only the effects in the total number of particles of a heterodisperse suspension, Equations 2 and 3 are valid approximations (4). 

Of the four coefficients L, Lhh, aA, and ah, only three are independent. In order to determine them, we need a third experimental transport parameter which is chosen to be the tracer diffusion coefficient of the cations. The transport coefficient LAA, in terms of >A is (see Section 5.2)... 

The theoretical characterization of the response of this kind of membrane systems in electrochemical techniques is very interesting for determining thermodynamics and transport parameters of ions. 

---