A CO2 capture technology using carbon nanotubes with polyaspartamide surfactant
Date
2016-07-13
Authors
Ngoy, Jacob Masiala
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Abstract
Technologies for the separation of CO2 from flue gas require a feat of engineering for
efficient achievement. Various CO2 capture technologies, including absorption, adsorption,
cryogenics and membranes, have been investigated globally. The absorption technology uses
mainly alkanolamine aqueous solutions, the most common being monoethanolamine (MEA);
however, further investigation is required to circumvent its weakness due to degradation
through oxidation, material corrosion and high energy costs required for regeneration.
Attractive advantages in adsorption technology, including the ability to separate the more
diluted component in the mixture with a low energy penalty, have been a motivation for
many researchers to contribute to the advancement of adsorption technology in CO2 capture.
The challenge in CO2 adsorption technology is to design a hydrophobic and biodegradable
adsorbent with large CO2 uptake, high selectivity for CO2, adequate adsorption kinetics,
water tolerance, and to require low levels of energy for regeneration processes. The existing
adsorbent such as activated carbon, silica gel, zeolites, metal organic frameworks and others,
have been ineffective where moisture occurs in flue gas. This work provides an advanced
adsorption technology through a novel adsorbent, MWNT-PAA, designed from the noncovalent
functionalization of multi-walled carbon nanotubes (MWNTs) by polyaspartamide
(PAA) as product of amine grafted to polysuccinimide (PSI). Three types of PAA were
prepared using ethylenediamine (EDA), 1, 3 propanediamine (PDA) and monoethanolamine
(MEA) drafted to PSI to give PSI-EDA, PSI-PDA and PSI-MEA respectively. The CO2
adsorption capacity was 13.5 mg-CO2/g for PSI-PDA and 9.0 mg-CO2/g for PSI-MEA, which
decreased significantly from PSI where the CO2 adsorption capacity was 25 mg-CO2/g. PSIEDA
was selected as PAA, because the CO2 adsorption capacity was 52 mg-CO2/g which
doubled from PSI. The polymer polyethylenimine (PEI), the most commonly polymer used in
CO2 capture, was found to be non-biodegradable, while the polymer PAA showed the
presence of CONH as a biodegradable bond functionality, occurring in the MWNT-PAA, as
confirmed through Fourier Transform Infrared (FTIR) analysis. The adsorbent MWNT-PAA
was demonstrated to have a water tolerance in the temperature range 25-55 ℃, where CO2
adsorption capacity increased with the increase of water in the adsorbent. The highest CO2
adsorption capacity recorded was 71 mg-CO2/g for the moist MWNT-PAA using 100% CO2
and 65 mg-CO2/g for the mixture of 14% CO2 with air. Under the same conditions, the dry
MWNT-PAA adsorbed 70 and 46 mg-CO2/g respectively (100%, 14% CO2). The
2
regenerability efficiency of the MWNT-PAA absorbent was demonstrated at 100 ᵒC; after 10
cycles of adsorption-desorption 99% of adsorbed gas was recovered in the desorption
process. The heat flow for the thermal swing adsorption system resulted in the net release of
heat over the complete cycle; a cycle includes adsorption (heat release) and desorption (heat
absorbance). Thus, this MWNT-PAA adsorbent demonstrates an advantage in terms of
overall energy efficiency, and could be a competitive adsorbent in CO2 capture technology.
Description
A thesis submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy
Johannesburg, 2016