Ethane sorption

Kinetics of Ethane Sorption on 4A Molecular Sieve Crystal Powder and Pellets... 

The Dc values found in this work may be compared with the value of 4.8 X 10 cm /sec reported previously by Brandt and Rudloff (3), who studied ethane sorption by 4A crystals at 22.9 °C but in the absence of helium carrier gas. The present data at 25.2°C indicate (for an average radius of 1.39 microns) that Dc is 4.6 X 10 cm /sec, in excellent agreement with the earlier work. However, the activation energy reported by Brandt and Rudloff was 7.4 Kcal/mole, as compared with the value of 5.66 Kcal/mole found here. 

The results in this study have demonstrated clearly that the rate of adsorption and desorption of ethane at low concentrations on 4A molecular sieves in the absence of binder is controlled by intracrystalline diffusion of the ethane. Furthermore, the diffusion process may be characterized by Pick s law and an effective diffusivity dependent only on temperature, and applicable to both adsorption and desorption. It may be expected, therefore, that such micropore diffusion also determines the rates of ethane sorption with commercial 4A pellets containing clay... 

Many simple systems that could be expected to form ideal Hquid mixtures are reasonably predicted by extending pure-species adsorption equiUbrium data to a multicomponent equation. The potential theory has been extended to binary mixtures of several hydrocarbons on activated carbon by assuming an ideal mixture (99) and to hydrocarbons on activated carbon and carbon molecular sieves, and to O2 and N2 on 5A and lOX zeoHtes (100). Mixture isotherms predicted by lAST agree with experimental data for methane + ethane and for ethylene + CO2 on activated carbon, and for CO + O2 and for propane + propylene on siUca gel (36). A statistical thermodynamic model has been successfully appHed to equiUbrium isotherms of several nonpolar species on 5A zeoHte, to predict multicomponent sorption equiUbria from the Henry constants for the pure components (26). A set of equations that incorporate surface heterogeneity into the lAST model provides a means for predicting multicomponent equiUbria, but the agreement is only good up to 50% surface saturation (9). 

Prediction of the breakthrough performance of molecular sieve adsorption columns requires solution of the appropriate mass-transfer rate equation with boundary conditions imposed by the differential fluid phase mass balance. For systems which obey a Langmuir isotherm and for which the controlling resistance to mass transfer is macropore or zeolitic diffusion, the set of nonlinear equations must be solved numerically. Solutions have been obtained for saturation and regeneration of molecular sieve adsorption columns. Predicted breakthrough curves are compared with experimental data for sorption of ethane and ethylene on type A zeolite, and the model satisfactorily describes column performance. Under comparable conditions, column regeneration is slower than saturation. This is a consequence of non-linearities of the system and does not imply any difference in intrinsic rate constants. 

In order to understand further the behavior for the four harbinger compounds as to their adsorption and desorption pathways, and to test the validity of reversible sorption, a series of experiments were performed whereby the adsorption step was followed by successive desorption using distilled water. The same four compounds (1,4-dichlorobenzene, 1,2-dichloroethane, 1,1,2-trichlor-ethane, and 1,1,2,2-tetrachloroethane) and a Pleistocene clay were used for the... 

In the presence of D2 gas there is an accelerated addition reaction to form ethanes and a simultaneous exchange to form substituted deutero-ethylenes. The addition usually predominates at low temperatures, while the exchange begins to predominate above 90" C. One of the interesting observations on the addition at low temperatures is that the initial product of the reaction of C2H4 + D2 is C2H6. This would seem to indicate a pre-equilibrium between the pool of I) atoms formed from dissociative sorption of D2 and the H atoms dissociated from sorbed C2H4. It is further... 

The sorption of ethane from dilute mixtures with helium by 4A sieve crystal powder and pellets made without binder has been studied with a microbalance in a flow system at temperatures between 25° and 117°C. Results show clearly that intracrystalline diffusion is the rate-controlling process and that it is represented well by a Pick s law diffusion model. Transient adsorption and desorption are characterized by the same effective diffusivity with an activation energy of 5660 cal/gram mole. 

Sorption Kinetics. The adsorption and desorption data were analyzed in terms of a model based on the following main assumptions. Micropore diffusion within the sieve crystals is the rate-controlling process. Diffusion may be described by Fick s law for spherical particle geometry with a constant micropore diffusivity. The helium present in the system is inert and plays no direct role in the sorption or diffusion process. Sorption occurs under isothermal conditions. Sorption equilibrium is maintained at the crystal surface, which is subjected to a step change in gas composition. These assumptions lead to the following relation for the amount of ethane adsorbed or desorbed by a single particle as a function of time (Crank, 4). 

Experiments were first performed using sieve pellets containing an imbedded thermocouple and with gas concentrations up to 10% ethane. These showed temperature variations of less than 0.5 °C during sorption, thus confirming the isothermal assumption. 

In this work, a gas chromatographic system has been used to obtain binary sorption equilibria for nitrogen-oxygen, methane-ethane and propane-cyclopropane mixtures on 5A molecular sieve. The experimental measurements have also been compared with the predictions from the statistical thermodynamic model of Ruthven (11). The model is capable of predicting binary sorption equilibria from the parameters derived from the pure-component isotherm measurements. 

Figure 2. (a) Pure-component isotherms for ethane and methane at 298K. o- Gravimetric data of Loughlin (14) (Data for methane are at 273K). (b) Binary sorption isotherms for ethane-methane... 

Moreover, the macro - and micro -FTIR techniques enable us to obtain spectra in situ from a working catalyst, since the cells used (see Sect. 2.1.1) may be operated as flow-through reactors. Thus, coking of zeolite catalysts upon reaction of ethane or ethylbenzene was investigated in situ, and the decrease of diffusivities (e.g., of benzene) in the coking samples was measured as a function of the amount of coke deposited [15]. Similarly, the sorption of para-, meta-, and ortho-diethylbenzene from the gas phase into H-ZSM-5... 

ABSTRACT. An isosteric sorption system has been used to study the sorption of methane, ethane, ethene, propane, N2 and CO2 and some of their binary mixtures in silicalite-1. Isotherms of some of these sorbates have been determined at equilibrium pressures up to 20 atmospheres. Isosteric heats of sorption have been obtained from the slopes of the isosteres. Separation factors calculated from the Henry s Law constants determined from the initial slopes of the single conq>onent isotherms are found to be in good agreement with experim tal separation factors. The Langmuir-Freundlich equation has be used to lit the single component data and the Ideal Adsorbed Solution theory has be used to predict a binary sorption isotherm from the respective single component data. Comparison of the sorption behaviour of the hydrocarbons in silicalite-1 and NaY zeolites has been made. 

The sorption of ethane, efoene and propane in silicalite-1 has been studied over foe temperature range 0-50 C. This study is part of a larger programme designed to establish foe preferred zeolite for foe s aration of specific pairs of lower saturated and imsaturated hydrocarixrns and their branched isomos by Pressure Swing Adsorption (PSA) methods. These studies are, of course, also of importance in foe development of a b er understanding of foe fundamentals of hydrocarbon/zeolite sorption interactions. 

The isosteres obtained for foe sorption of these hydrocarbons were linear and all of a similar quality to foose presmted in Figure 2 for ethane sorbed in silicalite-I. 

The isosteric heats of sorption, Qst, obtained from the slopes of the isosteres are plotted in Figures 4 as a function of coverage for ethane, ethene and propane sorbed in silicalite-1. It will be shown later that the slope of the isosteres obtained from the isosteric apparatus used in these studies increases when die corrections referred to above are allowed for. However, for these hydrocarbon/silicalite-1 systems the corrections are too small to affect the slopes significantly so the isosteric heats shown in Figure 4 can be accepted as accurate. 

Table 1 shows that the initial heat of sorption of ethene in NaY of ethene is 13 kJ mof greater than that of ethane and some 7 kJ mof greater than that of propane. Thus the specific interaction eigy of the double bond in die hi electric fields which exist in the NaY supercage... 

The heats of sorption of ethane and propane in silicalite are some 5r6 kJ mof greater than the corresponding heats in NaY. The Na cations present in and Sm sites of the supercages of NaY introduce a polarization energy contribution to these heats of soiption of hydrocarbons in NaY but this contribution is smaller than the increase in the net dispersion-repulsion energy in silicalite due to the closer fit of these molecules in die smaller channels and intersections in this latter framework. In the case of the smaller CH molecule the difference is... 

---