(204f) Nanoclay-Based Solid-Amine Sorbents for Carbon Dioxide Capture

Nanoclay-Based Solid-Amine Sorbents for Carbon Dioxide Capture

            The
objective of the research project is to develop an efficient, low cost, regenerable solid sorbent for carbon dioxide adsorption
from large point sources, which can be regenerated with more realistic
recycling schemes. The main regeneration scheme that has been studied in the
literature for solid adsorbents has been temperature swing desorption while
using nitrogen as a sweep gas. This technique, while very useful in the lab to
show regeneration is possible, is not realistic as one again ends up with a
mixture of CO₂ and N₂ as an outlet stream_. In
this work we show that a low cost nanoclay-based solid sorbent can be
regenerated with more industrially relevant regeneration methods. We show that
it is possible to use pressure-swing vacuum desorption to regenerate the
sorbent. We also show that CO₂ containing moisture itself can be
used as a sweep gas with temperature swing to regenerate the sorbent.

            The
sorbent developed here is composed of a montmorillonite nanoclay, commonly used
in the production of polymer nanocomposites, grafted with commercially
available amines. Aminopropyltrimethoxysilane (APTMS)
was chemically grafted on to the edge hydroxyl groups of the
clay, and polyethylenimine (PEI) was then attached to the surface
of the clay by electrostatic interactions. FTIR analysis was used to confirm
the amine grafting. The amount of amine loaded onto the support was determined
by TGA techniques. The treated clay was initially analyzed for CO₂
adsorption in a pure CO₂ stream in a TGA. At atmospheric pressure,
the optimized adsorption temperature was determined to be between 75°C
and 85°C.
The maximum CO₂ adsorption capacity for clay treated with APTMS at
85°C
in pure CO₂ at atmospheric pressure was 6.7 wt% CO₂, but
with the additional treatment of attaching PEI to the surface of the clay the
maximum CO₂ adsorption capacity increased to 9.7 wt% CO₂.
In a more realistic simulated flue gas of 10% CO₂ and 90% N₂,
the adsorbents had essentially the same overall maximum CO­₂
adsorption capacity of 9.6% for clay treated with APTMS and PEI.

            Adsorption
studies conducted by us in pure CO₂ at room temperature and under
pressures from 275-2070 kPa showed that the average
adsorption capacity of the adsorbents did not change significantly under the
pressures examined, indicating that the uptake of CO₂ was due mainly
to chemical reaction and not due to the physical absorption of CO₂.
The average CO­₂ adsorption capacity at 2070 kPa
for clay treated with APTMS was 7.57 wt% CO₂. The combination of
APTMS and PEI treatment on the clay increased the average adsorption capacity
to 11.39 wt% CO₂. While these results are comparable to those
available in the literature for other solid sorbents, the cost of the sorbents
developed here is expected to be, as much as an order of magnitude, lower than
that of competing sorbents.

            Initial
desorption studies were conducted using pure N­₂ at 100°C as
a sweep gas, and the process was successful in completely regenerating the adsorbent.  Multiple cycles of adsorption and
desorption can be carried out with very little loss in CO₂ capture
capacity. Other regeneration schemes that we have studied that are more
commercially realistic are vacuum regeneration and regeneration using CO₂
with a small amount of water at 155°C. Vacuum regeneration was studied by first adsorbing CO₂
on the sample using a TGA and then placing the sample in a vacuum oven at 65°C-85°C
at 93 kPa vacuum for 1 hour. The sample was then
returned to the TGA were CO₂ was adsorbed onto the sample. The
amount of CO₂ adsorbed was compared and showed that vacuum
desorption is successful at regenerating the adsorbent. Regeneration using CO₂
and water was also successful in regenerating the adsorbent at the temperature
of 155°C.
The CO₂ and water studies were conducted in the TGA and water was
incorporated into the reaction gas by bubbling the CO₂
through water. Although the desorption temperature using CO₂ and
water was higher than using nitrogen, this scheme is a much more realistic way
to regenerate the adsorbent on a large scale since water can easily be
condensed out of the exit stream forming an almost pure CO₂ stream
for sequestration. This work shows that a nanoclay based solid adsorbent can be
regenerated using more industrially relevant regenerations schemes that have
not been studied much in the literature.