Absorption - Introduction to Carbon Capture and Sequestration - page 219

processes that have been investigated, amine scrubbing is the most
developed technology for carbon capture. Amine scrubbing was invented
in 1930 by R. R. Bottoms [5.1], who fi led a patent on the process, which
was immediately put into commercial use to separate CO 2 from hydro-
gen and natural gas [5.14]. With increasing concern over climate change
in the 1980s, amine-scrubbing was applied for carbon capture in small-
scale gas and coal-fi red power plants [5.15]. Amine-scrubbing is now
used in small 20 MW combustion plants, but has not yet been demon-
strated in a commercial-scale plant.
Currently, aqueous monoethanolamine (MEA) is the solvent of choice
for CO 2 scrubbing. Carbon dioxide reacts with MEA to primarily form a
carbamate [5.16]. There are also a series of reactions involving water and
CO 2 alone which lead to a small amount of CO 2 capture. For this
response, we compared the solvent effectiveness in a set of absorption
and stripping columns which use water alone to one that uses a 30%
MEA solution in water. The effectiveness of each solvent is determined
by comparing the size of the column and solvent fl ow rates which would
be needed to complete a desired separation of CO 2 from fl ue gas and the
associated capital cost.
Before we get started with our calculation, we defi ned in Figure 5.8.1
a few key variables for our absorption and stripper column designs. We
assume our analysis will be applied to a typical commercial-scale 500
MW coal-fi red power plant, which produces a fl ue gas stream consisting
of 12% CO 2 ( y N + 1 = 0.12), and a total fl ue gas fl ow rate of 9.5 x 10 7 mol/

680 m 3 /sec . The carrier gas fl ow rate entering the absorption column
V i A is defi ned as (1- y CO 2 , fl ue ) multiplied by the total fl ue gas fl ow rate,
which gives us 8.3 x 10 7 moles per hour. We already know the absorption
column is designed to remove 90% of the CO 2 in the fl ue gas stream.
Based on that criterion and the 12% initial mixture composition, we cal-
culate that the gas composition exiting the absorption column ( y 1 A ) will be
approximately 0.012% CO 2 . The solvent from the absorption column is
sent to a stripping column, where the CO 2 is removed from the solvent
by increasing the temperature, producing a relatively pure stream of CO 2 .
The absorption column operates at a cooler temperature (40°C) than the
stripping column (120°C) to enhance the capture of CO 2 in the absorber
and release of CO 2 in the stripper.
We can obtain an intuitive sense of why a 30% MEA solution would
be more effective at CO 2 capture than pure water by fi rst analyzing the

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