Colloids and Surfaces A: Physicochem. Eng. Aspects 241 (2004) 59–65
Sorption equilibrium properties of nitrous oxide
on low-silicon x-type zeolites
Martin Bülow
a,∗
, Dongmin Shen
b
, Sudhakar R. Jale
a
a
BOC PGS Technology, 100 Mountain Ave., Murray Hill, NJ 07974, USA
b
DataXplore, Inc., 38 Snyder Ave., Berkeley Heights, NJ 07922, USA
Abstract
Sorption thermodynamic functions of nitrous oxide, N
2
O, are described for zeolites NaLSX and CaLSX in shapes of clay-bound beads.
They were determined by the sorption-isosteric method (SIM) over complete ranges of sorption-phase concentration and compared with those
for carbon dioxide, CO
2
, as reported earlier for the same NaLSX sorbent. Complementary information was obtained for N
2
O and CO
2
on
zeolites NaLSX, CaLSX and LiLSX by Monte Carlo simulation experiments using a MSI Cerius
2
software package.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Sorption thermodynamics; Isosteric measurements; Monte Carlo simulation; LSX zeolites; Nitrous oxide; Carbon dioxide
1. Introduction
Nitrous oxide, N
2
O, is a “greenhouse gas”. Its earth-atmo-
spheric concentration (currently ≈2 ppm) increases steadily
by ca. (0.2–0.3%) p.a. This is caused mainly by antropogenic
activities and emissions from chemical processes, e.g., syn-
thesis of adipic acid for Nylon-66, automotive power gener-
ation and wastewater treatment. N
2
O is very stable in air; its
lifetime amounts to ca. 150 years. Removal of N
2
O from air
streams in front of cryogenic air-separation units (ASU) [1]
is critical as that of water, H
2
O, and carbon dioxide, CO
2
.
Excess of N
2
O in ASU may lead to plugging of tubes and
heat exchangers and to contamination of products, specifi-
cally of noble gases. As the concentration of N
2
O in air in-
creases further, the current regime of air-prepurification units
(PPU) in front of ASU may become inadequate, since N
2
O
cannot be removed easily by existing technologies. Break-
through curves for N
2
O in a CaA-zeolite bed at 8000 ppm
N
2
O and various CO
2
concentrations in air [2] indicate that
CO
2
displaces N
2
O as the mass-transfer front progresses
along the adsorber bed. This finding questions the usage of
CaA zeolite as single sorbent for N
2
O removal. Although
N
2
O and CO
2
have identical molecular weights, sorption
∗
Corresponding author. Tel.: +1-908-771-6210;
fax: +1-908-771-6113.
E-mail address: martin.bulow@boc.com (M. Bülow).
interaction of N
2
O with zeolite NaX modifications, espe-
cially with those of the low-silicon X zeolite, e.g., NaLSX
[3], deviates from that of CO
2
due to differences in specific
properties of these gases such as their dipole and quadrupole
moments. It is important to understand sorption thermody-
namics of N
2
O on PPU sorbents and to assess differences in
sorption properties between N
2
O and CO
2
. Such knowledge
will allow for development of novel materials and separa-
tion processes to purify ASU-feed gases.
2. Experimental
2.1. Sorption-isosteric method and sorbents
The sorption-isosteric method (SIM) was used to de-
termine sorption-thermodynamic data for N
2
O on zeolites
NaLSX and CaLSX as dependencies on sorption-phase
concentration, n. This technique as well as its utilization
for an investigation of sorption-thermodynamic properties
of CO
2
on identical NaLSX beads were described in detail
in refs. [4–9]. To evaluate differential values of sorption
enthalpy, H, also called isosteric sorption heat, standard
sorption entropy, S
◦
, and standard Gibbs free sorption
energy, G
◦
, the following equations [6,10] are used:
ln p = constant -
q
isosteric
(n)
RT
(1)
0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.colsurfa.2004.04.039