(352f) Henry Constants of Krypton on 4A, 5A and 13X and Saturation Loadings on 5A and 13x Zeolites

Literature data from sixteen studies on 4A, 5A, and 13X zeolites are analyzed to evaluate Henry constants and saturation loadings. The Henry constant data are linear on a van’t Hoff plot and similar with each other. The saturation data below the critical adsorbate reduced temperature, T_CAR, is modeled using the Rackett equation and the zeolite structure. The subT_CAR model predictions for krypton are excellent. The superT_CAR data for krypton are linear with excellent regression coefficients. Intersection of the linear superT_CAR plots with the appropriate model prediction curves gave a T_CAR of 0.68 for krypton on 5A and 0.85 on 13X zeolites.

THEORETICAL MODEL

The equilibrium relationship in physical adsorption is highly affected by the characteristics of the adsorbents. At low pressure and constant temperature, the loading q is linearly related to the system pressure p by Equation 1 where q is expressed in terms of molecule, mass or molar units per unit volume or mass. The coefficient K is Henry's constant. The temperature dependence of the Henry constant obeys the van’t Hoff equation given in Equation 2 where ΔH₀ represents the differences in enthalpy between adsorbed and free liquid or gaseous states. Neglecting differences in heat capacity between the phases, equation 2 may be integrated to yield equation 3 where K₀ is a constant pre-exponential factor.

The theoretical saturation loading in zeolites may be calculated from first principles for zeolite crystals assuming 100% accessibility for the adsorbate and the Rackett model for the sorbate density¹ as given in equation 4 which is applicable in the range 0 to T_CAR, the critical adsorbate reduced temperature. Here ε_Z is the zeolite void fraction for the α cage only, and ρ_Z is the zeolite framework crystallographic density. Z_RA is a particular constant for the modified Rackett equation; values are given in the paper by Spencer & Danner². An alternative expression for equation 4 is given in equation 5 where q_max,c is the theoretical loading at critical liquid-vapor conditions, as given in equation 6.

The sorbate often appears to change phase at a temperature at or below the critical adsorbate temperature, or at a reduced temperature of 1 or less. We have named this temperature T_CAR, the critical adsorbate temperature. In addition, we define subT_CAR and superT_CAR regions as below or above T_CAR. The literature indicates that T_CAR is less than the three-dimensional critical temperature. For the present, T_CAR will have to be obtained experimentally. There are two aspects of supercritical q_max adsorption that warrant consideration; first, at what T_r does the transition from subcritical to supercritical q_max adsorption loading occur, and secondly a model for supercritical gas adsorption is needed.

A model is reported in our other paper at this meeting³. The model equations reduce to equations 7 and 8. The new terms in these equations are V_b, the specific volume of liquid krypton at the normal boiling point, V_ads, the specific volume of adsorbed krypton, W, the expansivity of the adsorbed krypton, and T_bR, the reduced normal boiling point temperature. For krypton, T_bR = 0.57. To calculate q_max, knowledge of the framework voidage for the α or large cage, density, and saturation temperature and saturation pressure between the boiling point and the critical point temperature and expansivity W are required.

RESULTS AND DISCUSSION

Adsorption isotherms are extracted from the literature. References are available. Data from 16 different studies are included ranging from a T_r of 0.43 to 3.45. Adsorption isotherms for krypton are plotted on a log-log plot for all three zeolites. The isotherms on all three zeolites are generally consistent in shape and position except for the isotherms of Vermesse et al.⁴ on 5A and 13X zeolites at a T_r of 1.42. These isotherms are measured by an unusual technique, and are exceptional in that they keep on rising to very high loadings at extremely high reduced pressure of approximately 40, never levelling off. As this is not reasonable, this isotherm is not included in further consideration.

The isotherms are considered for saturation loadings and Henry Law region. In these plots, Henry constant data exists for four isotherms on 4A zeolite, 26 isotherms on 5A and 32 isotherms on 13X. The Henry constant data extends between 12 and 16 decades of loading, which is a very wide range not usually observed. Saturation loadings exist for nine isotherms on 5A and 13 on 13X.

The Henry constants are plotted on a van’t Hoff plot in Fig. 1 for the three zeolites. The data are remarkably similar across the zeolites. The regression results for -∆H and K_o are presented in Table 1, along with a few representative values reported in the literature. The results reported are a compilation of all the other studies. The t ratio for 4A is 6.66 which is statistically less significant than the t ratio’s for 5A and 13X at 73.69 and 76.29 respectively.

The maximum saturation loadings for krypton on the three zeolites are plotted in Fig. 2 as q_max versus T_r. These plots are separated into two regions, a subT_CAR region to the left and a superT_CAR region to the right. Note that T_CAR is in general unknown and varies for different adsorbent/adsorbate systems. It is evaluated through observation and by calculating the intersection of the superT_CAR linear fit with the predictive subT_CAR model

in Fig. 2, the data for 5A zeolite are clearly separated into a subT_CAR region at and below a T_r of 0.65, and a superT_CAR region above this point. The subT_CAR data are in excellent agreement with the predictive model. Also in Fig. 2, the data for 13X zeolite appear to separate into a subT_CAR region at and below a T_r of 0.87 and a superT_CAR region at and above this point. The subT_CAR data are in excellent agreement with the predictive model for the alpha cage only. The superT_CAR data for both 5A and 13X follow the same linear pattern of Equations 7 and 8. These equations are plotted for all the superT_CAR data in Fig. 3. The slope is fixed at -1 and the intercept regressed as 0.4651. This is below the predicted value of 0.57.

CONCLUSIONS

A van’t Hoff plot for the 4A, 5A and 13X data indicate similarity between the zeolites, with heats of adsorption between 15.5 and 24.0 kJ/mole. Saturation data as a function of reduced temperature was consistent with the proposed subT_CAR model and superT_CAR models.

ACKNOWLEDGEMENTS

The authors wish to acknowledge the support of the American University of Sharjah, and Georgia Institute of Technology in the development of this paper.

REFERENCES

1. Loughlin, K. F. and Abouelnasr, D. M. 2009, Adsorption, Vol. 15, pp. 521-533.
2. Spencer, C.T. and Danner, R.P. 2, 1972, J. Chem. Eng. Data, Vol. 17, pp. 236-240.
3. Loughlin, K. F. and Abouelnasr, D. M., A New Theory for Adsorbate Specific Volumes and Saturation Loadings on 4A, 5A and 13X Zeolites AIChE 2021 presentation, Boston, Ma.
4. Vermesse, J., Vidal, D. and Malbrunot, P. 17, 1996, Langmuir, Vol. 12, pp. 4190-4196.