Experimentation void volume determination

Experimental assessment of the column void volume proved to be critical since the solute retention volume approaches the void volume as pressure is increased. Following the recommendations of Kobayashi (24), we used an unretained solute, methane, for this measurement. Values for the void volume determined over an extended pressure range were 1.8 and 0.5 ml. for the crosslinked resin and alumina columns, respectively. These figures were in excellent agreement with void volume approximations of 1.4 and 0.45 ml. based upon the geometric volume of the column assuming a porosity of 0.6 for the packed beds. 

Zeolite A. The structure of zeolite A contains two types of voids (1) the a cage, 11.4 A in diameter, and (2) the P cage (or sodalite unit), 6.6 A in diameter (7). Table I compares experimentally determined pore volumes of zeolite A with the void volume as calculated from the structure (no influence of cations considered). Since the sodium zeolite A does not adsorb normal paraffins, data are included for the calcium-exchanged form. Also shown in column 5 is the void fraction, Vi, as calculated by... 

The two parameters are determined experimentally by measuring the bed void volume at different linear velocities and a double-log plot of evsU.n and Ut are found as slope and intercept of a linear regression of experimental data in this plot. Additionally the parameters may be evaluated from literature correlations, which give a good estimate of the range of flow rates applicable for the fluidization of a certain matrix. Martin et al. [19] used two dimensionless... 

The void volume or void time is a very important parameter, and its correct determination could be critical for the interpretation of the experimental results. 

Other Experimental Methods. It is probably suitable to discuss here column porous structure. Porous space of a conventional packed column consists of the interparticle volume (Vip—space around particles of packing) and pore volume (Vp— space inside porous particles). The sum of those two constitutes the column void volume. The void volume marker ( unretained ) should be able to evenly distribute itself in these volumes while moving through the column. Only in this case the statistical center mass of its peak will represent the true volume of the Uquid phase in the column. In other words, its chromatographic behavior should be similar to that of the eluent molecules in a monocomponent eluent. If a chosen void volume marker compound has some preferential interaction with the stationary phase compared to that of the eluent molecules, it will show positive retention and could not be used as void marker. If on the other hand it has weaker interaction, it will be excluded from the adsorbent surface and will elute faster than the real void time, meaning that it also could not be used. For any analytical applications (when no thermodynamic dependences are not extracted from experimental data), 10% or 15% error in the determination of the void volume are acceptable. It is generally recommended to avoid elution of the component of interest with a retention factor lower than 1.5. Accurate methods for the determination of the column void volume are discussed in Chapter 2. 

According to Rosenbaum (14), the experimentally determined dry wool density could actually be the density of a polymeric material containing "small static voids" originally occupied by the water molecules in the native state of the hydrated protein for wool, Rosenbaum s "specific volume without voids" is 7% smaller than the experimental specific volume. 

Because the extraction efficiency was determined by the direct comparison of dye concentration in the spiked dyebadi before and after the extraction, the higher SFE recoveries (e.g. efficiency >99%) should have relative standard deviations <1%. For the purpose of this study, >99% of recovery is sufficient to illustrate the effectiveness of the SFE technique. According to our experiments, no decomposition or breakdown of these disperse dyes was observed during SFE at the specified experimental conditions described atove. The restrictor flow rates of SC-CO2 often dominate the success of SFE, and can be varied to provide information on the dynamics of the extraction process. It is known that if the flow of supercritical fluid is sufficient to sweep the ceil void volume, the effectiveness of the extraction is enhanced. In fact, changing the flow rate is a simple way to determine the extraction efficiency (7). In this study, no obvious difference in extraction efficiency was observed at the SC-C02 flow rate of 2.0, and S.O mL/min. It is also noted that SFE of samples with high concentrations of water tends to plug fused silica restrictors 19). Therefore, a restrictor temperature controller was used in our experiments to avoid restrictor plugging. 

The remainder of this section is devoted to a discussion of the experimental techniques used to determine the other physical properties of catalysts that are of primary interest for reactor design purposes the void volume and the pore size distribution. 

WSRC has developed a special code, FLOWTRAN-TF, based on the conservation of mass, energy, and momentum to account for two-phase flow, heat transfer effects, and cross-rib gap flows in assembly subchannels. The heat conduction models developed for FLOWTRAN-FI have been incorporated in FLOWTRAN-TF. Each subchannel coolant node has radially adjacent fuel surface temperature nodes to accommodate the heat transfer in the cell.. Rib fin effects are also handled in the same manner as they are in FLOWTRAN-FI. In order to initiate the computation, an air void fraction must be assigned to the computational cell above the fuel. This is done by assuming an air void volume in the first (top) axial cell as adjusted by an experimentally determined partitioning factor. Results from FLOWTRAN-TF have been shown to be relatively insensitive to the value assigned. Two-phase flow across the ribs is modeled by the application of an assumed partition factor based on values given in the literature. 

From the pure component data it is possible to calculate the expected a behavior as a function of temperature for a blend of the two polymers using the equation ah = sas + < rar, where the subscripts b, s, and r refer to the blend, polystyrene, and rubber, respectively, and the < s represent the volume fractions of the two components in the blend. The calculated curves (Figure 7) are reasonably smooth and exhibit only the polystyrene Tg. The calculated curve for TR-41-2445 is in good agreement with that found experimentally for the solution-blended material. The only significant difference is that below the polystyrene Tg the calculated values of a are about 0.5 X 10 4 deg1 lower than the experimentally determined data points. This may be attributable to the density differences in the samples, particularly for the blended material where density variations and void formation can occur at the interfaces between the polymer phases. 

The examples above demonstrate satisfactory agreement between the calculated results and the experimental data. This shows that the initial approximate assumptions are reasonable. In most cases, the one-term Equation 5 is applicable for the description of adsorption equilibria on zeolites, particularly for zeolites with small voids (zeolite L, chaba-site, erionite, mordenite) for which, in adsorption of hydrocarbons, n = 3 as a rule. The concept of the volume filling of micropores makes it possible to describe adsorption equilibria over sufficiently wide ranges of temperatures and pressures (using fs instead of Ps) with the use of only 3 experimentally determined (usually from 1 adsorption isotherm for the average temperature) constants. Wo, A, and n. The constant n requires only a tentative estimation, since it is expressed by an integer. 

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