Jun 29, 2017 - NaX (UOP-APG 13X), were investigated by means of the isosteric sorption technique [1-3]. Full sets of sorption thermodynamic functions, viz., ...
Sorption Thermodynamics of Carbon Dioxide on Faujasite-type Zeolites NaX and NaLSX. A Comparative Isosteric Study by
Martin Bülow and Dongmin Shen
The BOC Group Technical Center Murray Hill, NJ, August 20, 1997
CONTENTS
Summary Introduction Experimental Results and Discussion Sorption Thermodynamic Data for Carbon Dioxide - NaLSX
3 3 3 7
Zeolite and Carbon Dioxide - NaX Zeolite Comparison of Carbon Dioxide Sorption Thermodynamics for NaLSX and NaX Systems Sorption Enthalpy Sorption Entropy Gibbs Free Sorption Energy Calculation of Sorption Isotherms Conclusions References Acknowledgments Attachnent Basic sorption-thermodynamic relationships
7
2
17 17 19 20 21 25 26 27 28
SUMMARY Sorption equilibria for carbon dioxide (CO2) on two sodium faujasite type zeolites for PPU TSA purposes, NaLSX (UOP-Na,KLSX; Na+ vs. K+ cation exchange performed at BOC) and NaX (UOP-APG 13X), were investigated by means of the isosteric sorption technique [1-3]. Full sets of sorption thermodynamic functions, viz., of enthalpy, entropy and Gibbs free energy of sorption as concentration dependences were derived from experimental sorption isosteres. The data show a remarkable advantage of the BOC proprietary PPU TSA sorbent, NaLSX [4], over the “traditional” material, NaX, e.g., APG 13X of UOP, utilized in many old and new PPU TSA plants up-front air-separation units. The sorption system CO2 / NaLSX exhibits a remarkably structured concentration dependence of thermodynamic functions, in opposite to that for the CO 2 / NaX system for which these functions change smoothly with concentration without specific features, in accordance with information from literature [5-7]. In the concentration region of saturation of both solid pore systems, the changes of thermodynamic functions approach values that are typical of the CO2 sublimation phase transition. However, for none of the CO2-zeolite systems investigated, a phase transition takes place within the micropore structure. Therefore, data collected allows one to calculate any sorption isotherms for given regions of temperature and sorbate pressure that were physically meaningful. Sorption data compiled in this report, represent a data set recommended to be used by BOC for the design of TSA PPU plants. INTRODUCTION During recent years, the development of a novel BOC proprietary PPU TSA molecular sieve [4] has been one of the challenges to the Adsorption Team at GTC. Such a sorbent is meant to replace the NaX-type zeolite sorbent (APG 13X of UOP) currently used in BOC’s air prepurification units (PPUs) that work over a wide range of conditions. Sorption isotherm data for the CO2 / NaX (UOP-APG 13X) system were extensively collected and compiled in [8]. Although some information on sorption heats is also given therein, a comprehensive description of sorption thermodynamics of this system is missing but still needed. For the sorption system CO2 / NaLSX no information on sorption thermodynamics is available so far either in literature or from other sources. The thermodynamic functions considered in this report, are the changes of enthalpy, entropy and Gibbs free energy of CO2 as dependences on its concentration in the sorption phase. Their determination and comparison with those for the two sorbents, NaLSX and NaX, represent the main goals of this investigation. EXPERIMENTAL In accordance with fundamentals of physical sorption, sorption isosteres are straight lines at constant sorbate concentration, n, in plots of ln p vs. 1/T, if no phase transition occurs in the experimental region of physical parameters. Therein, p and T 3
denote, respectively, sorbate equilibrium pressure and absolute temperature. If isosteres, or data sets (T, p, n), are available over the entire sorbate concentration region, e.g., by means of the experimental set-up as outlined in Figure 1, sorption equilibrium is comprehensively and fully described. To evaluate sorption thermodynamic functions, i.e., the sorption enthalpy or isosteric heat of sorption, q = - H , standard sorption entropy, S°, and standard Gibbs free sorption energy, G°, textbook equations are utilized, cf., Refs. [1-3] and Attachment: -H = qisosteric = R (ln p / T -1)n,
(1)
S° = H/T + R ln (p / p),
(2)
G° = H - TS°,
(3)
G° = - RT ln(p / p),
with p = 760 torr.
(4)
As already described in detail. cf., [9,10], the principle of an isosteric experiment is to maintain a (nearly) constant value of sorption phase concentration while equilibrium pressure is measured as temperature function. Repeating the procedure for various sorbate concentrations, n, after subsequent dosing, sorption thermodynamic functions are obtained as concentration functions [1-3]. For the actual systems,
1 7
5 X5
X7
X6
X1
X12
X10
X3
X11 X14
3 X2
4
6
11
X15 X13
X4
12
2
8
X8
X9
14
9
13 10
15
T=const.
Figure 1. Scheme of the isosteric apparatus as described in [9]. (1. Gas supply, 2. Circulating pump, 3&4. Gas cylinders, 5&6. Pressure sensors, 7. Mass spectrometer, 8. Sample holder, 9&10. Cryostat, 11-15, Vacuum systems.)
the ratio of volumes for sample holder and sorbent is < 5. During experiments over the entire temperature and pressure ranges, > 99 % of sorbate mass dosed into the system did remain in the sorption phase. The isosteric condition, n const, in the sorption phase is, therefore, obeyed. The maximum filling of the pore system, i.e., of micro-, meso- and macropores of the solid material, i.e., saturation concentration, ns,, is reached if, usually, an isostere approaches/coincides with the function ps vs. 1/Ts 4
where ps denotes the saturation pressure of the sorbate at a given temperature, Ts. In the given case of CO2 sorption, the criterion is different, viz., isosteres at highest sorbate concentration have to approach the sublimation curve for CO2. This gives an additional opportunity for a consistency check for the experimental method utilized, i.e., proof of accuracy of the isosteric technique by measuring the sublimation curve of CO2 in absence of sorbent, as shown in Figure 2. The changes of enthalpy, 25.26 KJ/mol, and entropy, - 129.57 J/mol K, typical of sublimation were determined isosterically. They agree well with corresponding literature data [11] that amount to 25.23 KJ/mol and - 129.63 J/mol K, respectively. In terms of sorption enthalpy, the accuracy of the current technique is c. ± 25 J/mol. Adsorption isosteres of CO without adsorbents 2
100
p, torr
10
Y=A+B*X
1
0.1 5.50
Parameter Value Error -------------------------------------------------A 9.6599 0.04597 B -1320.8939 6.94164 -------------------------------------------------R SD N P --------------------------------------------------0.99971 0.01427 23
