Study on the Adsorptive Removal of CO2 in Indoor Spaces
Keywords: CO2, Adsorption, Indoor Space, Subway, Cabin
1 Introduction
Nowadays, CO2 is considered one of the main
reasons of the global warming effect, and there
have been many studies for capturing CO2.
However, capturing CO2 from ambient air has
not gained much attention though there are some
cases it is needed. For example, in subway,
ventilation is not easy because the outdoor
tunnel air is mostly severely polluted by various
particulate matters. Or, on very hot or summer
day, the ventilation may severely drop thermal
comfort of residents. In general, low
concentration of CO2 is not a serious
environmental concern. However, exposure to
high concentration of CO2 may cause various
adverse effects, e.g., acidosis, dizziness,
drowsiness, nausea, or headaches. In United
States, average exposure for healthy adults
during work day should not exceed 5,000 ppm.
In Korea, CO2 level should not exceed 1,000
ppm for public spaces, e.g., underground and
train station, underground shopping area, bus
terminal, airport, library, museum, art gallery,
hospital, nursery, and children day care center.
However, CO2 concentration of subway cabin
often exceeds 4,000 ppm during rush hour in
Korea. Therefore, development of ambient CO2
adsorption technology is required. In this study,
an ambient CO2 adsorbent, zeolite embedded
with LiOH (LEZ), was prepared, and CO2
adsorption performance was tested.
2 Materials/Methods
LEZ-13X was prepared by mixing zeolite 13X
with LiOH, while LEZ-5A was prepared by
using zeolite 5A instead of zeolite 13X. Zeolite
and LiOH was ground into powder smaller than
250 μm, and mixed as various portions. For
conventional experiment, LEZ-13X and LEZ5A, prepared with a weight portion of 0.25
(zeolite) : 0.75 (LiOH) were used. The mixed
powders were pasted and pelletized after adding
water. The median size of the pellet was around
4 mm. Prepared pellets were dried under various
temperature condition to investigate the effect of
drying temperature. Typically, prepared LEZ13X and LEZ-5A was dried at 50 ℃ for 24 h.
CO2 adsorption of prepared sample was tested
by using an adsorption reactor shown in Figure
1. The adsorption reactor was made of stainless
steel tube. The length of the reactor was 250 mm,
the inner radius was 13 mm, and the inner
volume was 33 mL. 10 g of prepared sample
was loaded in this reactor. Desired flow rate
(typically 3 L/min) of N2 gas with desired
concentration (typically 5,000 ppm) of CO2 was
flown into a mixer for the complete mixing, and
this mixed gas was flown into the adsorption
reactor. CO2 concentration of outflow gas was
monitored to calculate how much CO2 was
adsorbed on the adsorbent. CO2 concentration
was monitored by non-dispersive infrared sensor
(SenseAir, Sweden). The calibration curve of
the used sensor was made by using a certified
CO2 measuring instrument (Thermo Scientific,
USA).
Figure 1: Experimental set-up for measuring
CO2 adsorption performances of prepared
adsorbents.
3 Results and Discussion
Figure 2 shows the effect of LiOH content in
prepared LEZ-13X and LEZ-5A on the CO2
adsorption capability. The amount of adsorbed
CO2 increased according to the increase of
LiOH content. CO2 adsorption was linearly
proportional to the content of LiOH. Therefore,
CO2 adsorption of prepared LEZ-13X and LEZ5A sample was governed by LiOH in the
prepared sample. Because zeolite is one of the
most common CO2 adsorbent, zeolite was
expected to adsorb CO2. However, CO2
adsorption capacity decreased when more
zeolite was used for the preparation of sample,
in other words, CO2 adsorption capacity
decreased when less LiOH was applied. Raw
material, zeolite 13X and zeolite 5A, can adsorb
CO2 adsorption capacity between LEZ-13X and
LEX-5A was not different from each other. It
means CO2 adsorption capacity is directly
related with LiOH, not with zeolite. As reported
in previous studies, base can play an important
role in adsorbing CO2. In the same way, CO2
adsorption was strongly dependent on the
chemical reaction between LiOH and CO2. The
reaction between LiOH and CO2 is as following:
2LiOH + CO2 → Li2CO3 + H2O
(1)
By this reaction, Li2CO3 and water with heat is
produced. Because this reaction is an
irreversible acid-base neutralization reaction, the
regeneration of Li2CO3 to LiOH is not easy.
Therefore, the regeneration of used LEZ-13X
and LEZ-5A is another problem to be solved in
our further study.
The pelletized LEZ-13X and LEZ-5A was dried
under various temperature condition from 25 ºC
to 300 ºC and the result was presented in Fig. 5.
CO2 adsorption capacity was maximal when the
drying temperature was around 50 ºC. However,
when the drying temperature was higher than
100 ºC, it decreased drastically. The reason for
this phenomenon is not clear as for now, but it
seems that it is closely related with the water in
the prepared LEZ-13X and LEZ-5A. When the
drying temperature is higher than 100 ºC, the
water molecule boils and vaporized may cause
unknown physical or chemical reaction. We
suggest a possibility that the hot water in LEZ13X and LEZ-5A dissolute LiOH on the surface
of the pellet, and it is vaporized with LiOH in
the air, removing active OH- sites from the
surface of LEZ-13X and LEZ-5A.
7
Adsorbed CO2 (μmol/g)
CO2, but, in this study, during the preparation of
LEZ-13X and LEZ-5A, the pore structure of
zeolite was destroyed in the process of grinding
zeolites.
LEZ-5A
LEZ-13X
6
(c)
5
4
3
2
1
0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
LiOH Content
Figure 2: CO2 adsorption performances of LZE13X and LZE-5A with various contents of LiOH.
4 Conclusions
Zeolite modified with LiOH was prepared and
their CO2 adsorbent performance was tested.
Prepared CO2 adsorbent showed good CO2
adsorption performance. CO2 adsorption was
ascribed to the LiOH in the adsorbent because
the amount of adsorbed CO2 increased
according to the increase of LiOH content.
Prepared CO2 adsorbent is expected to be used
for the fast and efficient removal of indoor CO2.
However, the regeneration of used adsorbent is a
remaining problem to be solved.
5 References
Korean Ministry of Environment. Indoor air
quality guideline for public transportations
in Korea.
The U.S. Occupational Safety and Health
Administration.
Zhao C., Deng H., Li Y., and Liu Z. 2010.
Photodegradation of oxytetracycline in
aqueous by 5A and 13X loaded with TiO2
under UV irradiation, Journal of Hazardous
Materials 176, 884–892.
Zhao Z., Cui X., Ma J., and Li R. 2007.
Adsorption of carbon dioxide on alkalimodified
zeolite
13X
adsorbents.
International Journal of Greenhouse Gas
Control 1, 355-359.