Inert sweep gas on the

Our objective was to clarify the reaction mechanisms that determine the observed alkene/alkane ratios under various conditions, and the results are reported here. When oil shale is pyrolyzed either isothermally or nonisothermally, the hydrocarbon and hydrogen concentrations are all time dependent. To determine if the alkene-alkane-hydrogen system is at equilibrium, we heated oil shale at a constant rate and measured the C to C3 hydrocarbons and hydrogen over time. We also measured the effect of an inert sweep gas on the time-dependent ethene/ethane and propene/propane ratios and the integral 1-alkene/n-alkane ratios in the oil. We determined that the C2H4-C2H6-H2 system is not at thermal equilibrium and interpret our results in terms of a nonequilibrium free-radical mechanism proposed by Raley (8). 

Figure 5. Effect of inert sweep gas on the time-dependent ethene/ethane ratio for oil shale heated at 1.5°C/min under autogenous conditions. The slow-sweep sample size and flow rate were 28 g and 50 cm3/min, respectively. The fast-sweep sample size and flow rate were 14 g and 100 cm3/min, respectively. Most of the ethene and ethane was evolved between 400° and 500°C.

Figure 9. Effect of temperature and inert sweep gas on the ethene/ethane ratio from retorting oil shale. Results are shown for work at LLNL and LETC. The temperature dependence of the ethene/ethane ratio can be characterized by an activation energy of about 11 kcal/mol.

A distinction might be made, in fact, when the MR is used to carry out a catalytic reaction, considering whether the membrane itself has a catalytic function or not. In the case of MRs for H2 production, most of the membranes used are permselective, which allows the selective removal of H2 from the reaction volume under the effect of a driving force. This is a function of the species partial pressures on both the membrane sides and can be created by means of an inert sweep gas in the permeate compartment (nitrogen, helium, water, etc.), or with the application of a pressure difference between the retentate and permeate sides. 

In the discussion of concentration polarization to this point, the assumption is made that the volume flux through the membrane is large, so the concentration on the permeate side of the membrane is determined by the ratio of the component fluxes. This assumption is almost always true for liquid separation processes, such as ultrafiltration or reverse osmosis, but must be modified in a few gas separation and pervaporation processes. In these processes, a lateral flow of gas is sometimes used to change the composition of the gas on the permeate side of the membrane. Figure 4.14 illustrates a laboratory gas permeation experiment using this effect. As the pressurized feed gas mixture is passed over the membrane surface, certain components permeate the membrane. On the permeate side of the membrane, a lateral flow of helium or other inert gas sweeps the permeate from the membrane surface. In the absence of the sweep gas, the composition of the gas mixture on the permeate side of the membrane is determined by the flow of components from the feed. If a large flow of sweep gas is used, the partial... 

The drawback of using an external permeate-side sweep gas to lower the partial pressure on the permeate side of the membrane for an industrial process is that the sweep gas and permeating component must subsequently be separated. In some cases this may not be difficult some processes that have been suggested but rarely used are shown in Figure 4.15. In these examples, the separation of the sweep gas and the permeating component is achieved by condensation. If the permeating gas is itself easily condensed, an inert gas such as nitrogen can be used as the sweep [18], An alternative is a condensable vapor such as steam [19-21],... 

Oil coking can be minimized by decreasing the liquid-phase residence time, which, in turn, can be accomplished by using high pyrolysis temperatures and/or an inert sweep gas (9). Because 1-alkene/n-alkane ratios depend on pyrolysis temperature and heating rate, they are good indicators of oil coking (16, 25,... 

Figure 6. Effect of inert sweep gas during retorting on the 1 -alkene/n-alkane ratios in shale oil. The ratios at the peaks were determined on samples from capillary column GC by total-ion MS.

The effect of reactant loss on membrane reactor performance was explained nicely in a study by Harold et al [5.25], who compared conversion during the cyclohexane dehydrogenation reaction in a PBMR equipped with different types of membranes. The results are shown in Fig. 5.4, which shows the cyclohexane conversion in the reactor as a function of the ratio of permeation to reaction rates (proportional to the ratio of a characteristic time for reaction in the packed bed to a characteristic time for transport through the membrane). Curves 1 and 2 correspond to mesoporous membranes with a Knudsen (H2/cyclohexane) separation factor. Curves 3 and 4 are for microporous membranes with a separation factor of 100, and curves 5 and 6 correspond to dense metal membranes with an infinite separation factor. The odd numbered curves correspond to using an inert sweep gas flow rate equal to the cyclohexane flow, whereas for the even numbered curves the sweep to cyclohexane flow ratio is 10. 

Marigliano et al. performed further modelling work for methane steam reforming and water-gas shift in membrane reactors [415]. They defined the sweep factor I as the ratio of flow rates of inert gas on the permeate side to the flow rate of methane on the reaction side of the membrane ... 

The presence of the equilibrium [Eq. (8)] K 10 ) was detected by the effect of Pb +, Ba +, and Ca on the rate of Eq. (9), as they reduce the equilibrium concentration of CrOi - by precipitation and thereby increase the decomposition. The formulation of the product as NOa" " in NOa solvent, and not as molecular NaOs, seems reasonable for an ionic melt and the relatively large stability evident the initial decomposition may take place via N2O6, however, since the rate is dependent on the flow rate of the inert sweep gas used (Duke and Iverson, 1958). 

Other workers do the opposite and add catalyst to the solvent (which again may be cooled) after first sweeping the flask with inert gas to remove air. It appears that if catalyst and solvent are mixed without removal of air (which is certainly not advised) fires are more likely to occur when catalyst is added to the solvent. Catalyst particles falling through organic vapor cannot be eflectively cooled and may enter the liquid glowing. On the other hand, when solvent is added rapidly to the catalyst, any tendency of the catalyst to heat is limited by quenching with a massive amount of liquid. 

The Damkdhler-Peclet product also had an impact on performance the optimal value ranged from is 1.0 x 10 at 773K, to 1.0 x 10 at 873K. Little or no improvement was observed when the pressure in the tube was larger than the pressure in the shell, and no improvement was seen when the shell pressure exceeded the tube pressure. When the inert gas sweep rate was increased, the membrane reactor improved until the amount of sweep gas to reactant gas was approximately one hundred as seen in Figure 3. Once again there was an asymptotic limit to the amount of enhancement seen. There was no improvement when the permeabilities of any other component were increased over the permeability of methane. 

This example illustrates the distillation of a binary mixture in an open-batch distillery with flowing sweep gas and pervaporation by having a porous plate floating on top of the liquid hold up, as shown in Fig. 4.20. The porous plate was made from inert sintered metal with various pore sizes between 100 and 1 mfi, and had a thickness of 1 mm. The porosity was 40 % and the tortuosity factor was about 2. This results in an effective liquid phase mass transfer coefficient of about hiq = 2 X 10-7 m s-i, which results in Kiiq = 1.9 X 10 22. Therefore, one would expect the distillation process to be nonselective - that is, Si = xi - xi = 0. 

The original motivation for studying the thermal properties of cellulosic chars came from our study on bulk cellulose pyrolysis under conditions simulating those existing in a fire. In such a situation, the flame over the surface of the solid supplies heat to the pyrolyzing solid. In our work, the radiative and conductive feedback of heat from the flame to the surface was simulated using radiant heaters. The experiments were carried out in an inert gas environment, to maintain as well-defined a heat transfer environment as possible, free from complications due to actual combustion heat sources. A convective How of the inert gas was used to sweep away volatiles from the vicinity of the surface, and the heat transfer effects of the sweep gas were also taken into account. 

A small amount of dry [(C6H5)3PH]3[LnCl6] (where Ln is any of the lanthanides) is placed in the apparatus, and the preparation2 is carried out in the manner described for bromo complexes in Sec. B. Preparation on a 500-mg. or smaller scale improves the ease with which the last of the excess HI is removed from the product. After thoroughly sweeping out HI, the moisture- and oxygen-sensitive product is handled in dry inert gas in the dry-bag. The yield is essentially 100%, and Cl- analysis (by x-ray fluorescence) indicates <1% residual Cl. The pyridinium salts can also be prepared in this manner. 

A membrane reactor using a H-ZSM5 membrane was used by Bernal et al. [3.42] to carry out the esterification reaction of acetic acid with ethanol. An equimolar etha-nol/acetic acid liquid mixture was fed in the membrane interior, while He gas was used as an inert sweep on the shell-side. In this particular application the membrane, itself, provides the catalysis for the reaction. NaA and T-type zeolite membranes have been utilized for esterification reactions in a PVMR and in a vapor permeation membrane reactor (VPMR) by Tanaka et al [3.43, 3.44]. Both membranes are hydrophilic and show good separation characteristics towards a number of alcohols. The NaA membrane was used to study the oleic/acid esterification in a vapor permeation membrane reactor (VPMR) at 383... 

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