Diffusivities for benzene

Figure 2. Arrhenius plot showing comparison of NMR and sorption diffusivities for benzene and o-xylene in NaX zeolite crystals. NMR data from (1) Germanus et al. (19) and (2) Karger and Ruthven (10). Uptake (corrected diffusivity) and tracer exchange data of Goddard (11-13) (50 pm and 100 pm NaX, 250 pm faujasite). ZLC data of Eic (15,16).

A mixture of benzene and toluene is fed as a vapor to the bottom of a distillation column. At a given point in the unit the vapor contains 80 mole % benzene while the corresponding liquid is 70 mole % benzene. Vapor pressure and diffusivity for benzene are 1.3 atm and 5.92 x 10 m /sec. The molal latent heats of vaporization are essentially the same. Find the rate of interchange of benzene and toluene, assuming a stagnant vapor layer of 0.254 cm. 

F = Function of the molecular volume of the solute. Correlations for this parameter are given in Figure 7 as a function of the parameter (j), which is an empirical constant that depends on the solvent characteristics. As points of reference for water, (j) = 1.0 for methanol, (j) = 0.82 and for benzene, (j) = 0.70. The two-film theory is convenient for describing gas-liquid mass transfer where the pollutant solute is considered to be continuously diffusing through the gas and liquid films. 

The main complication with this technique is that the mass transfer analysis is nontrivial. For example, the change of velocity as a function of the distance down the jet was not taken into account in modeling the system. The LJRR has been used to study the diffusivities of benzene and toluene in water [41] and cupric ion extraction [42,49]. 

The permeability of human skin to n-hexane has been determined in vitro in flow-through diffusion cells (Loden 1986). Pieces of full-thickness human skin were exposed to 3H -hcxane in human serum, and the appearance of label in the trans compartment measured for 0.5 or 12 hours. The skin was then sectioned with a microtome into 0.25 mm slices and the quantity of label in the skin measured. The rate of resorption (uptake of substance by the receptor fluid beneath the skin [i.e., the amount that passes through the skin]) was calculated. The rate of resorption for n-hexane through human skin was calculated to be 0.83 ( g cm2/hr). The permeability of n-hexane through human skin was much lower (approximately 100-fold) than for other chemicals tested in this study. For example, rates of resorption (in g cm2/hr) were 99 for benzene and 118 for ethylene glycol. 

FIGURE 6.11 Characteristic parametric variations of dimensionless temperature T and mass fraction m of fuel, oxygen, and products along a radius of a droplet diffusion flame in a quiescent atmosphere. j is the adiabatic, stoichiometric flame temperature, pA is the partial density of species A, and p is the total mass density. The estimated values derived for benzene are given in Section 2b. 

Diffusivity values are reported in a modified exponential form. For example, the experimentally determined diffusivity of benzene in water is 1.09x 10 cm /sec, but this value is reported as 1.09 (x 10 cm /sec). 

Fig. 3. Diffusion of benzene into H-ZSM-6 at various temperatures and for different pressure jumps.

On leaving their military bases in Czechoslovakia, the military of the old Soviet Union dumped all chemicals out onto the ground in an unbelievable show of disrespect. At one location, 50,000 kg of solvent was dumped out of a storage tank onto saturated soil. The solvent formed a pool on the surface that was visible for more than 2 weeks. As part of a hazard assessment, the Czech engineers need to know the total mass of benzene (one compound composing the solvent) that was evaporated into the air versus time for the 2-week period. Estimate and plot this mass. The mean depth of the pool just after the spill was 10 cm. Incorporate the diffusion of benzene into the saturated soil. Assume that there is no turbulence in the spilled pool. 

Further examination of equations (1) and (2) indicates that both the Knox equation and the Van Deemter equation predict a linear relationship between the value of the (B) term (the longitudinal diffusion term) and solute dlffuslvlty. A plot of the (B) term against diffusivity for benzyl acetate and hexarnethyl benzene is shown in figure 4. 

Table 6.16 shows the room temperature rate constants for the reactions of OH with some simple aromatics as well as the branching ratio for abstraction, i.e., the ratio kM/(kbi + kb2). Abstraction accounts for less than about 10% of the reaction at room temperature for those alkylbenzenes studied to date. It is noteworthy that the reactions are all quite fast, even that for benzene being within approximately two orders of magnitude of diffusion controlled. 

Similar energy minimization calculations were reported for benzene and p-xylene in silicalite (92). Diffusion coefficients were estimated from minimum energy paths through the pore. The value for benzene, 27.6 kJ/mol, is in good agreement with that of Pickett et al. (91). For the bulkier p-xylene molecule, the activation barrier was predicted to be slightly lower (23 kJ/... 

The diffusion coefficient was estimated to be 4 x 10 9 m2/s. Experimental values for benzene in faujasites range from 10 10 to 10"13 m2/s, depending on the measurement technique (24, 97). PFG-NMR measurements are the closest to the MD value, which was admitted by the authors to be a crude estimate (mainly on grounds of a short simulation and inflexible molecules). The simulation time was too short to permit a calculation of the residence times of the benzene at either the cation or the window site or inside a particular cage. The cage residence times were estimated to be at least an order of magnitude longer than those for methane in NaY zeolite (43). 

On the basis of their 2H-NMR measurements for siliceous Y and NaY zeolites, the authors estimated the simulation time needed to observe cage-to-cage migration for an MD calculation for NaY zeolite. Their estimate of 200 ns is still beyond even the longest MD simulations on the most powerful machines. It is unsurprising, therefore, that more recent theoretical investigations into diffusion of benzene have used the more appropriate TST approach. 

The calculations led to predictions of adsorption sites for the nonpolar compounds that are in good agreement with those determined experimentally. The cation site is preferred over the window site. The activation barrier for movement between two cation sites was calculated to be 30 kJ/ mol and that for movement between a cation and a window site 43 kJ/mol. Experimental measurements of activation barriers to diffusion of benzene in faujasites are between 17 and 27 kJ/mol (24). The calculations provide strong support for the mechanism of surface-mediated diffusion for all guest molecules in the limit of infinite dilution and 0 K. The MEPs show that molecules slide along the wall of the supercage, with the plane of the aromatic ring almost parallel to the pore wall. 

Coupling of diffusion and rotation applies to benzene (29) and water (80, 31) on NaX as well as for pure water (ice) (1, 32). For solid benzene however there is easy rotation about the hexad axis (28), as well as for benzene adsorbed on charcoal (33) and silica gel (34)- This author suspects that in the 13-X structure there is a three- (nearly six-) fold potential into which the benzene molecules nest nicely, most likely above the soda-lite unit. The uncoupled condition applies to SF6 and cyclohexane in NaX, as well as in the solid. Rotation temperatures are given in Table II. 

As Figure 2 indicates, ion diffusivities were in the range of 10"7 to 10"6 cm2/sec at 18 °C. So were diffusivities of benzene and urea, but water diffusivity was somewhat higher, about 0.8-3.0 X 10 6 cm2/sec. The shape of the diffusivity curves for these molecules was about the same as that... 

A similar retardation effect of cosolvent was reported previously for benzene oxidation [90, 91]. The solvent may compete with reactant for diffusion in the channels and adsorption at the active sites of TS-1 catalyst. The activity Ti-beta for 1-hexene and cydohexanol oxidations is highest in acetonitrile, which is a polar, nonprotic solvent [92]. This is in contrast with the observed enhancement of the activity of TS-1 by methanol and protic solvents [68]. These differences have been... 

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