Numerical simulation of CO2 adsorption behaviour of polyaspartamide adsorbent for post-combustion CO2 capture
Date
2017
Authors
Yoro, Kelvin Odafe
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Abstract
Climate change due to the ever-increasing emission of anthropogenic greenhouse gases arising
from the use of fossil fuels for power generation and most industrial processes is now a global
challenge. It is therefore imperative to develop strategies or modern technologies that could
mitigate the effect of global warming due to the emission of CO2. Carbon capture and storage
(CCS) is a viable option that could ensure the sustainable use of cheap fossil fuels for energy
generation with less CO2 emission. Amongst existing CCS technologies, absorption technology
using monoethanolamine (MEA) is very mature and widely embraced globally. However, the
absorption technology has a lot of challenges such as, low CO2 loading, high energy requirement
for solvent regeneration, corrosive nature etc. On this note, the adsorption technology using solid
sorbents is being considered for CO2 capture due to its competitive advantages such as
flexibility, low energy requirement for sorbent regeneration, non-corrosive nature etc. On the
other hand, adsorbents have a very vital role to play in adsorption technology and there is need to
understand the behaviour of adsorbents for CO2 capture under different operating conditions in
order to adapt them for wider applications. On this note, the study contained in this dissertation
investigated the adsorption behaviour of a novel polymer-based adsorbent (polyaspartamide)
during post-combustion CO2 capture using experimental study and mathematical modelling
approach.
Polyaspartamide is an amine-rich polymer widely used in drug delivery. In addition, its rich
amine content increases its affinity for CO2. Its porosity, thermal stability and large surface area
make it a promising material for CO2 capture. In view of this, polyaspartamide was used as the
adsorbent for post-combustion CO2 capture in this study. This dissertation investigated the
kinetic behaviour, the diffusion mechanism and rate limiting steps (mass transfer limitation)
controlling the CO2 adsorption behaviour of this adsorbent. Furthermore, effect of impurities
such as moisture and other operating variables such as temperature, pressure, inlet gas flow rate
etc. on the CO2 adsorption behaviour of polyaspartamide was also investigated. Existing
mathematical models were used to understand the kinetics and diffusion limitation of this
adsorbent during CO2 capture. Popularly used gas-solid adsorption models namely; Bohart-
Adams and Thomas model were applied in describing the breakthrough curves in order to
ascertain the equilibrium concentration and breakthrough time for CO2 to be adsorbed onto
polyaspartamide. Lagergren’s pseudo 1st and 2nd order models as well as the Avrami kinetic
models were used to describe the kinetic behaviour of polyaspartamide during post-combustion
CO2 capture. Parameter estimations needed for the design and optimization of a CO2 adsorption
system using polyaspartamide were obtained and presented in this study. The Boyd’s film
diffusion model comprising of the interparticle and intra-particle diffusion models were used to
investigate the effect of mass transfer limitations during the adsorption of CO2 onto
polyaspartamide.
Data obtained from continuous CO2 adsorption experiments were used to validate the models in
this study. The experiments were conducted using a laboratory-sized packed-bed adsorption
column at isothermal conditions. The packed bed was attached to an ABB CO2 analyser (model:
ABB-AO2020) where concentrations of CO2 at various operating conditions were obtained.
The results obtained in this study show that temperature, pressure and gas flow rate had an effect
on the adsorption behaviour of polyaspartamide (PAA) during CO2 capture. Polyaspartamide
exhibited a CO2 capture efficiency of 97.62 % at the lowest temperature of 303 K and pressure of
2 bar. The amount of CO2 adsorbed on polyaspartamide increased as the operating pressure
increased and a decrease in the adsorption temperature resulted in increased amount of CO2
adsorbed by polyaspartamide. The amounts of CO2 adsorbed on polyaspartamide were 5.9, 4.8
and 4.1 mol CO2/kg adsorbent for adsorption temperatures of 303, 318 and 333 K, respectively.
The maximum amount of CO2 adsorbed by polyaspartamide at different flow rates of 1.0, 1.5
and 2.5 ml/s of the feed gas were 7.84, 6.5 and 5.9 mmol CO2/g of adsorbent. This shows that
higher flow rates resulted in decreased amount of CO2 adsorbed by polyaspartamide because of
low residence time which eventually resulted in poor mass transfer between the adsorbent and
adsorbate. Under dry conditions, the adsorption capacity of polyaspartamide was 365.4 mg
CO2/g adsorbent and 354.1 mgCO2/g adsorbent under wet conditions. Therefore, the presence of
moisture had a negligible effect on the adsorption behaviour of polyaspartamide. This is very
common with most amine-rich polymer-based adsorbents. This could be attributed to the fact
that CO2 reacts with moisture to form carbonic acid, thereby enhancing the CO2 adsorption
capacity of the material.
In conclusion, this study confirmed that the adsorption of CO2 onto polyaspartamide is favoured
at low temperatures and high operating pressures. The adsorption of CO2 onto polyaspartamide
was governed by film diffusion according to the outcome of the Boyd’s film diffusion model. It
was also confirmed that intra-particle diffusion was the rate-limiting step controlling the
adsorption of CO2 onto polyaspartamide. According to the results from the kinetic study, it can
be inferred that lower temperatures had an incremental effect on the kinetic behaviour of
polyaspartamide, external mass transfer governed the CO2 adsorption process and the adsorption
of CO2 onto polyaspartamide was confirmed to be a physicochemical process (both
physisorption and chemisorption).
Description
A dissertation submitted to the Faculty of Engineering and the Built Environment,
University of the Witwatersrand, Johannesburg, in fulfilment of the requirements
for the degree of Master of Science in Engineering.
10 February, 2017.
Keywords
Citation
Yoro, Kelvin Odafe (2017) Numerical simulation of CO2 adsorption behaviour of polyaspartamide adsorbent for post-combustion CO2 capture, University of the Witwatersrand, Johannesburg, <http://hdl.handle.net/10539/22999>