Carbon dioxide transport coefficients

Carbon Dioxide Transport. Measuring the permeation of carbon dioxide occurs far less often than measuring the permeation of oxygen or water. A variety of methods ate used however, the simplest method uses the Permatran-C instmment (Modem Controls, Inc.). In this method, air is circulated past a test film in a loop that includes an infrared detector. Carbon dioxide is appHed to the other side of the film. AH the carbon dioxide that permeates through the film is captured in the loop. As the experiment progresses, the carbon dioxide concentration increases. First, there is a transient period before the steady-state rate is achieved. The steady-state rate is achieved when the concentration of carbon dioxide increases at a constant rate. This rate is used to calculate the permeabiUty. Figure 18 shows how the diffusion coefficient can be deterrnined in this type of experiment. The time lag is substituted into equation 21. The solubiUty coefficient can be calculated with equation 2. 

The first hypothesis seems unlikely to be true in view of the rather wide variation in the ratio of carbon dioxide s kinetic diameter to the diameter of the intracrystalline pores (about 0.87, 0.77 and 0.39 for 4A, 5A and 13X, respectively (1J2)). The alternative hypothesis, however, (additional dif-fusional modes through the macropore spaces) could be interpreted in terms of transport along the crystal surfaces comprising the "walls" of the macropore spaces. This surface diffusion would act in an additive manner to the effective Maxwell-Knudsen diffusion coefficient, thus reducing the overall resistance to mass transfer within the macropores. 

In Section I we introduce the gas-polymer-matrix model for gas sorption and transport in polymers (10, LI), which is based on the experimental evidence that even permanent gases interact with the polymeric chains, resulting in changes in the solubility and diffusion coefficients. Just as the dynamic properties of the matrix depend on gas-polymer-matrix composition, the matrix model predicts that the solubility and diffusion coefficients depend on gas concentration in the polymer. We present a mathematical description of the sorption and transport of gases in polymers (10, 11) that is based on the thermodynamic analysis of solubility (12), on the statistical mechanical model of diffusion (13), and on the theory of corresponding states (14). In Section II we use the matrix model to analyze the sorption, permeability and time-lag data for carbon dioxide in polycarbonate, and compare this analysis with the dual-mode model analysis (15). In Section III we comment on the physical implication of the gas-polymer-matrix model. 

Gas transport coefficients of PVC and PVC modified with pyridine groups were studied. It was found that there is a strong time dependence of the permeability and diffusivity of oxygen, nitrogen, carbon dioxide and methane in membranes prepared by solvent casting of PVC and pyridine modified PVC. There is a two-fold reduction for PVC of the diffusion coefficients during the first two days,... 

The properties of fluids under supercritical conditions are considered ideal for extracting substances from exhausted activated carbons. Two supercritical fluids are of particular interest, carbon dioxide and water. Carbon dioxide has a low critical temperature of 304 K and a moderate critical pressure of 73 bar, while water has a critical temperature of 647 K and a critical pressure of 220 bar. The character of water at supercritical conditions changes from one that supports only ionic species at ambient conditions to one that dissolves paraffins, aromatics, gases and salts [65]. These supercritical fluids exhibit densities similar to those of liquids (high solvent strengths) and diffusion coefficients similar to those of gases (excellent transport characteristics), enabling them to effectively dissolve and/or desorb contaminants from the carbon surface and to easily enter/exit even the smallest pores and carry away any... 

Furthermore, a new force field for carbon dioxide will be developed that reproduces bulk densities, vapor-liquid equilibrium data, and overcritical transport properties (e.g., diffusion coefficients and viscosities) simultaneously. New force... 

A cell for characterising the diffusion of small molecules through thin polymer films using attenuated total reflectance (ATR) FTIR spectroscopy was described. The cell was designed to be used with precast (commercially extruded) polymer films, thus enabling the as-processed transport properties of the film to be studied. The cell was used to measure the diffusion of carbon dioxide, amyl acetate and limonene, and simultaneous diffusion of the individual components from a 50/50 mixture of amyl acetate and limonene through the thin polymer films (HDPE, LDPE and PS). Diffusion coefficients measured with the ATR technique compared favourably with values obtained from gravimetric measurements with the same penetrants and polymer samples. 20 refs. 

The maximum value of extract concentration in the supercritical solvent is given by the equilibrium solubility of extractable components in the supercritical solvent. Due to different transport resistances and equilibrium distribution coefficients for different compounds, these may be successively extracted at different rates. The maximum concentration of extract in the supercritical solvent may then be a function of extraction time. A typical example is the extraction of water subsequently to non polar compounds by supercritical carbon dioxide. 

The effect of copolymer composition on free volume and gas permeability of PECT copolymers as well as PET and PCT homopolymers was studied by Hill et al. (97). The free volume was studied by positron annihilation lifetime spectroscopy (PALS) in order to determine the relative size and concentration of free volume cavities in the copolymers. The logarithm of the permeability to oxygen and carbon dioxide increased linearly with the %mol content of 1,4-CHDM units in the copolymer, which was in agreement with the free volume cavity size and relative concentration observed by PALS measurements. Light et al. (98) studied the effect of sub-T relaxations on the gas transport properties of PET, PCT and PECT polyesters. They observed that modification of PET with 1,4-CHDM increased the magnitude of the p-relaxation, as well as the diffusion and solubility coefficients for oxygen and CO. ... 

Figure 14.3 Transport parameters for water, oxygen and carbon dioxide with EVA series and EVA/PVC blends, (a) Permeability coefficients (b) Diffusivity coefficients (c) Solubility coefficients.

Coefficients of the equadon of state and of the equation for transport properties are stored for each substance. Parameters of the critical point and coefficients of equations for calculadon of the ideal-gas functions, the saturated vapor pressure and the melting pressure are kept also. The thermal properties in the single-phase region and on the phase-equilibrium lines can be calculated on the basis of well-known relations with use of these coefficients. The system contains data for 30 reference substances monatomic and diatomic gases, air, water and steam, carbon dioxide, ammonia, paraffin hydrocarbons (up to octane), ethylene (ethene), propylene (propene), benzene and toluene. The system can calculate the thermophysical properties of poorly investigated gases and liquids and of multicomponent mixtures also on the basis of data for reference substances. 

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