(562bh) Kinetics and Equilibrium Study of New Activated Carbon Materials for Acetaldehyde Removal

Kinetics and equilibrium study of new activated
carbon materials for acetaldehyde removal

Simão
P. Cardoso¹, Inês S.S.  Ferreira¹, Inês Matos²,
Maria Bernardo², Márcia Ventura², Isabel M. Fonseca²,
Anabela Valente¹, Carlos M. Silva¹

¹ CICECO, Department of
Chemistry, University of Aveiro, 3810-193 Portugal

² Departamento de Química, CQFB, Universidade Nova de
Lisboa, Faculdade de Ciências e Tecnologia, 2825-114 Caparica, Portugal

*simaocardoso@ua.pt

Introduction

Volatile organic compounds (VOCs) are well-known hazardous pollutants,
present in air and water, produced from various industrial sources, including
the petroleum industry, internal combustion engines, and paints. Hazardous VOCs
are subjected to stringent legislation since concentrations as low as 100
ppmv or less may cause respiratory diseases or critical health
problems such as cancer, liver and kidneys failure [1]. Concerning to indoor
air quality, part of air pollution is related to VOCs, namely from paints, wood
preservatives, aerosol sprays, building materials cleansers and disinfectants. For
this reason, it is imperative to remove or manage VOC gases in indoor
environments where people spend long periods of time [2].

Among the numerous methods for the elimination of VOC, adsorption has
been a best option due to its efficiency and relatively low cost, its adsorbate
recovery capacity and the applicability to low concentrations. Various
adsorbents have been continuously investigated for VOC abatement, including
activated carbon, zeolites, polymers or silica [3].

The excellent properties, low cost and large specific surface areas of
porous carbonaceous materials, such as activated carbons, make these types of
adsorbents, usually in the form of pellet or granules, most attractive for
practical applications [3]. The progressive adsorption of pollutants on the
surface of the activated carbon leads to a gradual reduction in its adsorption
capacity until adsorption is no longer possible. The main goal of regeneration
is to remove these pollutants and recover the original adsorption capacity. At
indoor level, few researchers have explored ways to make adsorption-based
treatment setup reversible [2]. Common techniques employed for the regeneration
include conductive heating, hot gas purging, solvent extraction, and acid-based
treatment. Thermal regeneration is widely used at industrial scale and it is the
most advantageous for indoor air purifiers [4].

The present study deals with the development and testing of new carbonaceous
materials with high adsorption capacity, in a reversible adsorption-desorption process
for indoor air purification. Experimental data obtained from an adsorption unit
was modelled to gain insights into kinetic and equilibrium phenomena.

Adsorption measurements

A schematic representation of the adsorption unit is given in Figure 1.
The flow rate of the feed with the desired acetaldehyde concentration is
obtained by mixing the C₂H₄O/He stream with pure He in
the mixing chamber. The adsorption measurements were performed in a continuous
flow fix-bed quartz column placed in a temperature-controlled furnace, with the
main objective of determining the corresponding breakthrough curves. Before the
assays, the adsorbent was thermally treated at 180 ⁰C for 4 h under He
flow, and then the column was cooled to 25 ⁰C. Subsequently, the gaseous mixture VOC/He was
fed at 50 mL min^-1 and 2 bar, and the outlet gas concentration was
analyzed in a gas chromatograph equipped with Barrier Ionization Discharge detector (GC-BID).
For regeneration, the adsorbent was heated at 180 ⁰C for 6 h under He
flow.

Figure 1. Layout of the experimental apparatus for VOC
adsorption-adsorbent regeneration cycles.

New activated carbon materials

Biomass derived highly microporous carbons were prepared by chemical
activation. The adsorbent RHC (S_BET=2610 m²/g; pH
pzc=6.92) was prepared from rice husk, using KOH as chemical activating agent.
Adsorbent RCB3 (S_BET=2100 m²/g; pH pzc=3.0) derived from
a lignin-rich plant, namely conteira (Hedychium gardnerianum) - an invasive plant in azores island - and subjected to
chemical activation using phosphoric acid. RHC presented a somewhat neutral
surface, whereas RCB3 presented an acid pH pzc. 

k-carrageenan is a sulfated anionic polysaccharide extracted
from edible red algaes such as the specie Chondrus crispus, also
known as Irish moss. k-Carrageenan aerogel was prepared by
dissolution of the polymer into the ionic liquid [bmim][Cl]. The ionic liquid
was totally removed from the gel by successive washing steps with ethanol being further dried with
supercritical CO₂. The
BET specific surface area (S_BET) was 216 m²/g
and the pH pzc was ca. 4.

Results and discussion

Preliminary results were obtained using a commercial activated carbon
and a feed stream composed of 50 ppm acetaldehyde in He. From the measured
breakthrough curves it was possible to determine, for each sorption cycle, the
following information: the breakthrough time (based on 10 % of the inlet concentration),
stoichiometric time, adsorption capacity and regeneration efficiency. The
results listed in Table 1 were compared with data from the literature for acetaldehyde
adsorption over carbon adsorbents, being possible to conclude that this
material presents a reasonably good adsorption capacity for the first cycle [5].
Nonetheless, it was observed that along the various regeneration cycles the
adsorbent decreased significantly its capacity, corresponding to 16-22 %
regeneration efficiency, making this material not promising for several reuse
cycles. Currently, new materials are being prepared to achieve better performance
in terms of capacity, stability and reusability. Such results will be presented
during the conference.

Table
1. Breakthrough
time, adsorption capacity and regeneration efficiency of each cycle.

Acknowledgements

This work was developed within the scope of the
project CICECO-Aveiro Institute of Materials, FCT Ref. UID/CTM/50011/2019,
financed by national funds through the FCT/MCTES, and the Smart Green Homes
Project POCI-01-0247-FEDER-007678, a co-promotion between Bosch Termotecnologia
S.A. and the University of Aveiro. These projects are financed by Portugal 2020
under the Competitiveness and Internationalization Operational
Program and by the European Regional Development Fund (FEDER). Simão
P. Cardoso also thanks the post-doc
Grant (BPD/CICECO/5247/2017) financed
through the Smart Green Homes Project. 

Keywords:  activated carbon; Volatile organic compounds; adsorption

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