The utilization of natural zeolite materials in the elimination of polluting gases is investigated. Carbon monoxide pollution is emphasized because its concentration may reach dangerous levels in places such as vehicle tunnels, underground parking lots, etc. The elimination of carbon monoxide is also of interest in some industrial processes relating to the production of pure gases

Alfani, F.

Greco, G., Jr.

Iroio, G.

English

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PHYSICAL ADSORP210M ON NATURAL AND SYNTHETIC
ZEOLITSS (National Aeronautics and Space
Adainistration) 16 p HC 102/AF A01 CSCL 13B	 Unclas
65/45 30475
REMOVAL OF CARBON MONOXIDE - PHYSICAL
ADSORPTION ON NATURAL AND SYNTHETIC
ZEOLITES
Francesco Alfani, Guido Grecq Jr. and
Gabriele Iorio
Translation of "Abbatimento di ossido di carbonio -
Assorbimento fisico su zeolitt sintetiche a natur-
als" Inquinamento, Vol. 20, May 1978, pp 51-55.
4
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
WASHBGTON, D.C. 20546
	 JULY 1982
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PHYSICAL ADSORPTION ON NATURAL AND.SYNTHE-
TIC ZEOLITES.
Francesco Alfani, Guido Greco,Jr.
and Gabriele Iorio	 N. +acum *46
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Translation of "Abbatimento di ossido di carbonio -
Assorbimento fisico su zeoliti sintetiche a naturals"
Inquinamento, V ol. 20, May 1978, PP. 51-55. (A78-46489)	 f
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	 14
REMOVAL OF CARBON MONOXIDE.
PHYSICAL ADSORPTION ON
NATURAL AND SYNTHETIC ZEOLITES.
Francesco Alfani, Guido Greco Jr.
and Gabriele Iorio
3
(Institute of Principles of Chemical Engineer-
ing, University of Naples)
The main purpose of the investigations the results of which
are described in this article is that of verifying the possibil- /5*
ity of using natural 2eolite materials in the elimination of pol-
luting gases. In particular, attention is focused on pollution
with carbon monoxide, whose concentration may reach dangerous levels
in places such as vehicle tunnels, underground parking lots, etc.
The elimination of carbon monoxide is also of interest in some in-
dustrial processes relating to the production of pure gases.
As regards more directly the problems of atmospheric pollution,
the technique most widely used up to now is based on the effect of
ventilation plants.
In the more specific area of the environmental conditioning
of vehicle tunnels, where the use of axial accelerators is con-
stant, there are limits of application deriving from the fact that
it is impossible to exceed maximum values in the speed of the air,
due to problems of safety in case of fire and at the costs of plant
and management [1].
Consequently, for the purpose of improving the control of at-
mospheric pollution it is necessary to use techniques of "in sit4"
*Numbers in margin indicate foreign pagination.
1
ORIGI14AL PAGE IS
OF poolt QUALITY
removal, such as, for instance, selective adsorption on suitable
adsorption beds.
The choice of these materials is obviously prompted by econo-
mic considerations related t• o their costs and their adsorption
properties.
In this article, attention is drawn particularly to the physi-
cal adsorption of carbon monoxide on natural tufas, abundantly
available in the countryside and therefore characterized by low
cost.
The adsorption characteristics of these materials were compared
with those of synthetic molecular sieves, characterized obviously
by much higher costs. The experimental tests were carried out with
recourse to a dynamic technique described elsewhere.
Adsorption Materials
The specimens of natural tufas used in the investigation come
from caves located in the province of Avellino. The content of
zeolite fraction (cabasite) is about 65% weight and the weighted
chemical composition, determined in conditions of saturation (en-
vironmental humidity, 54.5%), is given in Table 1 [3].
Table 1: Weighted chemical composition of cabasitic tufa.
SA'^	 50,62
A603	 16,05
Fe103	 3,75
FeO	 0,35
TA	 0,46
MnO	 0,11
PA	 0,15
Ca0	 4,29
MgO	 1,33
Na:O	 2,55
K20	 7,02
H2O	 13,32
:
-2-
Before they are used, the tufa specimens are ground and
sifted; the adsorption tests were accomplished on the fraction
between 10 and 20 mesh.
The comparison tests were conducted on the following syn-
thetic zeolites (BDH):
3 A (1/8" cylindrical pellets)
4 A (4-8 mesh spherical pellets)
5 A (1/8" cylindrical pellets)
13 X (1/16" cylindrical pellets).
Wherever the possible presence of external resistance to
the transport of matter was determined, some tests were carried
out initially using both tufas and molecular sieves in the form
of powder. The results of these preliminary tests show clearly
that the external resistances are negligible. Since all the ma-
terials used show a high absorption capacity as compared with the
water vapor, they were pre-treated in a current of He for 12 hours
at 250°C.
In the adsorption tests, gaseous mixtures were used pre-con-
stituted of He/CO (SIO) with the following volumetric compositions:
0.97%; 10.31%; 19.70%.
The choice of helium as transport gas was imposed by the adop-
tion of a detector with thermal conductivity [2].
Experimental Technique
The technique used to determine the isotherms of adsorption
was described in detail in [2]. In its general form, it consists
in the analysis of the transient response of a column filled with
3
the adsorbing material and stressed with a step of known ampli-
tude in the concentration of carbon monoxide. The adsorbing col-
umn is maintained initially in a flow of He. At the time t = 09
the step of amplitude c° of CO is sent obtaining a response in /52
terms of `-he concentration of the effluent CO as shown in Fig.
1.
The signal remains at the value corresponding to pure He for
a time equal to r2 which indicates the complete adsorption of CO.
"Subsequently the latter reaches quickly the final value correspon-
ding to a concentration of CO equal to c o
 (feed value) indicating
thus that there is saturation of the adsorbing bed. The interval
0-r1 (see Fig. 1) corresponds to the delay induced by the connec-
tions between the column and the measurement element. Therefore
only the area(r2--rl) c o is connected to the amount of CO adsorbed.
The form of the response curves, which is independent of the
partial pressures of CO in gaseous phase and the velocity of flow,
indicates that the transport of matter inside the single adsorbing
grain occurs with negligible resistance and the adsorption kinetics
are fast.
As regards the measurement element used, it is proper to em-
phasize the considerable sensitivity which made it possible in
the experimental conditions adopted to measure a minimum amount
of CO adsorbed equal to 10 -6 gmoles/g adsorber. This made it pos-
sible to determine the adsorbing capacities up to a tempprature
of 350 0C with a partial CO pressure of 0.5 atm.
Fig. 2 shows a flow diagram of the experimental equipment used.
Discussion of the Results
Figs. 3, 4 and 5 give the adsorption isotherm of carbon monoxide
4
on tufa and 5A and 13 X zeolites in terms of gmoles of CO ad-
sorbed per gram of adsorber against the
OF POOR QUALITY
Fig. 1 Step response of the adsorb-
ing column: concentration of CO in
gaseous phase over time.
Tv TWW"Uto
0 O"Od"
SN *tvo4 a edwo de
Ftf Miuntem 6 pwUU
OPT TrawAton di
peniom dt"wwwute
PT Trambnom 6
Pn OWW
RC Cotaw di
►iferimente
MC Coto w di mkwa
Fig. 2 Flow diagram of the equip-
ment TU thermostat; D detector; SV
solenoid valve; FV range gauge; DPT
transducers of differential pressure;
PT pressure transducer; RC reference
column; MC measurement column.
5
_1-
• To	 it
T= 20%
1cr • T= 53"C
o . T=102'C
1^,ata►
ORIGINAL PASS
OF POOR QUALrry
to°
a^
ell"
2 t5t
0t
3 t
ie-,!,T- : 103 t
10^	 li 1Q1	 10°
PO
 , Atm
Fig. 3 Isotherms of adsorption of
CO on tufa.
Fig. 4: Isotherms of adsorption
of CO on 5 A zeolite.
partial pressure of CO in gaseous phase. An analysis of the results
shows that the adsorption capacity of the tufa is approximately in
the ratio of 1:5 and 1*2 with regard to that of the 5A and 13X zeo-
lites, respectively, in the entire range of temperatures and par-
-	 z7 7 -
tial pressures of CO explored.
These results are justified on the basis of the indications
given in [161, according to which the concentrations of active
cations in the physical adsorption are 7.1, 3.8 9 5.7, 2.9 moles/g
for zeolites of type A. cabasites and 13 X. respectively.
But we must take into account
	 the fact that the specimens
of natural tufa used in the present study contain only about 65%
in the zeolite fraction.
On the other hand, some initial tests conducted on zeolites of
type 4A and 3A at ambiental temperature and partial CO pressure of
0.2 atm indicated that the amount of CO adsorbed is practically nil.
This behavior which is quite different from the tufa and zeolites
5A and 13 X, on one hand, and 3A and 4A zeolites, on the other,
with regard to the physical adsorption of CO may be attributed
to the different chemical composition and the different structural
characteristics of the materials used. Actually the actual dia-
meter of the pores [51 is equal to 4.1 A for zeolites of the type
Linde A and 7.4 A for fau,jasite (13 X) and 3'.7-4.2 A for cabasite.
Therefore, the average molecular dimensions of CO being 2.8 A,
there should be no steric obstacles to the penetration of the CO
molecule in any of the adsorbing materials studied. But as regards
the chemical composition of the zeolites, it should be noted that
those of the type Linde A are prepared by inserting, by ion exchange
In the initial porous structure, Ca++ cations in the 5A. Na+ in the
4A and K+ in the 3A. The average ionic radius of such cations in-
creases in the order indicated; moreover with regard to the balance
of charges, the concentration of the Ca ++ ions in the 5A is half of
that of the Na+ and K+ in 4A. and 3A. It is therefore plausible
that the ion exchange with ions of sodium and potassium reduces con-
siderably the initial diameter of the pores, thus causing the total
disappearance of the adsorbing properties of the zeolites 4A and
7
ORIGINAL PACE IS
OF POOR QUALMY
if	
o T • 20'
• T a 50t
M	 • T a 70'0
o Tit 100'C
PCO.Atm
Fig. 5 Isotherms of adsorption of CO on
13 X zeolite.
10°
tQ'
E
a^
1Q=
4 =icr*g DOi eal eM.n4.85 krAl t moles
• . = io'	 . =4.80 .
c . a 2-Ue .	 • -5.04 .
• . 0 7.10'5
ie
2.5	 3.0	 15	 4.0
t1/T)•101 . °tC^
I
Fig. 6 Partial pressure of CO over
1/T for adsorption of CO on tufa with
constant values of concentration of
the adsorbate.
AM.,.=767 kcut/q *0l° o
it 7.48	 -	 •
-7,42	 a
=7,10
	 •
E
A
o^
•	 - 1.5 .10'" -	 »
OWN" PARE IS
OFQUMM.
3A with regard to the molecule of CO. The results obtained in
the present study, insofar as they concern the zaolites of the
Linde A type, are consistent with what is observed in [6] in
which it is emphasized how a zeolite A in the calcic form shows
adsorption properties superior by far to those of the same zeo-
lite in the sodic form.
I
2.5
	 3.0	 as	 4.0
Figure 7: Partial pressure of CO
over 1 /T for the adsorption of CO
on 5A zeolite with constant values
of the concentrations of the adsor-
bate.
The experimental properties have made it possible to eval-
uate the isoster i c heats of adsorption. Figs. 6 9 7, and 8
is
9
OW" PAGE IS
OF MW QUALM
show.the curves of partial pressure of CO in the gaseous form over
the inverse of the absolute temperature in accordance to differ-
ent amounts of CO adsorbed. The data show high consistency with
relation [7,8]:
OIX Pu	 dH.,.
0 (1/T) FA	 R
In conformance with data in the literature [13, 14 9 151 the
isoteric heat of adsorption proves to depend on the amount of ad-
sorbate and on the type of cation occurring in the zeolite struc-
ture [10]. The dependence of &H of adsorption on the concentra-
tion of CO adsorbed is plotted in Fig. 9 for tufa. Under the
experimental conditions adopted in this study, the values o
AHads
 vary between 4.80 and 5.53 kcal/gmole for tufa, and be-
tween 7.10 and 7.67 for 13 X zeolite. These values are not
unlike those indicated in [9], where a value was indicated of
AHads equal to 6.9 Kcal/gmole for the adsorption of CO on NaX.
The increase of oHads with the increase of the concentra-
tion of the CO adsorbed indicates that the forces of interaction
of adsorbate to adsorbate are dominant with regard to the forces
of adsorbate-adsorber dispersion just as is expected for the
adsorption of relatively small linear molecules such as those of
CO on zeolites [16]. The adsorption tests conducted on tufa show
that, with the increase of the partial pressure of CO, the ssiount
adsorbed at different temperatures (see Fig. 3) tends to an
asymptomatic value which may be expressed as a function of the
temperature, by the relation:
?' (T) _ ?" (T.) exp (— a (T — TO 
in which a is equal to 0.0174°K -1 . This behavior was attributed
10
i`
ANAL PAGE 13
OF POOR QVALMY
le	 o 6M^,S
6
0
2
kai /4utlea
• • t &91 r	 w
0 w A47 M	 M
^i
E
ap
0 4o:2•segmolea/g
•	 = 90'
2.S	 a0	 3.So .1 4,0(VT)•.d, K
Fig. E: Partial pressure of CO
against 1/T for adsorption of CO
on 13 X zeolite with constant
values of the concentration of
adsorbate.
m
a
5^0
4.6 •	 =Wlea/
D	 2	 4	 6
Fig. 9: Dependance of the isosteric
heat of adsorption of CO on tufa on the
amount adsorbed.
WN
11
[11, 12, 131 to the filling of the volumes of the mi.croporous
structure of the zeolite. Actually a thermodynamic analysis con-
ducted on this adsorption model allows the theoretical determin-
ation of the above indicated relation between the degree of fill-
ing of the microporcus volume and the temperature, which involves
the thermal coefficient a of limiting adsorption which is prac-
tically constant for a broad range of temperatures.
Conclusions
The linear variation of the adsorption isotherms, in accor-
dance with the lower values of the partial pressure of CO explored,
makes it possible to foresee the value of the amount of carbon mon-
oxide adsorbed at the level of its maximum permissible concentra-
tion. The results of the adsorption tests carried out both on
commercial zeolites and on tufa show how the adsorption capacity
is rather limited in every case.
It is found that a direct application of the technique of
elimination by physical adsorption of CO on systems of consider-
able potentials is difficult to implement. In any case, the ad-
sorption properties of synthetic zeolites can be increased con-
siderably [4] by means of ionic exchange which permits the sub-
stitution of the ions K + , Na+ , Ca++ with ions of iron, cobalt and
nickel able to form chemical bonds with the carbon monoxide.
In this respect an interesting characteristic of the country
tufas is that of containing in their structure iron oxides which,
after being reduced to metallic iron, could increase considerably
the adsorption powers. In any rase, even for the natural zeolites
occurring in the tufa, it is possible to adopt ion exchange tech-
niques identical to those used for synthetic zeolites for insert-
ing in the lattice the ion species needed.
Experimental tests along these lines are now in progress.
r	 -
12
Acknowledgments
The authors thank engineer Carlo Notari and Mr. Pietro Na-
politano for their collaboration in the accomplishment of the
experimental tests and Doctors Aiello and Colella for furnishing
specimens of tufa and for the exchange of ideas.
4
Symbols
A = weight of adsorption bed
c = concentration in gaseous phase
B = concentration in adsorbate phase
c*= limiting concentration of adsorbate
p = partial pressure
R = universal gas constant
t = time
T = temperature
To= .reference temperature
Greek symbols
g
gmoles/cm3
gmoles/g
gmoles/g
atm
cal/gmol°K
min
OK
OK
AHads = thermal coefficient	 °K-1
a	 = isosteric adsorption head 	 Kcal/gmole
T 1	 = transport delay	 min
T 2	 = duration of step response 	 min
Bibliography
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[4) Breck D.W. et al., U.S. Patent 3, 185, 540, 1965.
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Industry, London, 1958.
13
♦ i#i
[6] Breck D.W., Eversole W.Q., Milton R.M., Reed T.B., Thomas T.L.,
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171 Brunauer S., The adsorption of Gases and Vapours, Princeton Univer-
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[12] Bering B.P., Dubinin M.M., Serpinsky V.V., J. Colloid Interface
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[13] Dubinin M.M., Proceedings Ind. Int. Conf. on Adsorption-Desorp-
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14


Removal of carbon monoxide. Physical adsorption on natural and synthetic zeolites

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