(239f) Simulation Study of Concentrating High Purity CO2 from Syngas after Oxy-Fuel Combustion By Pressure Swing Adsorption Process

Abstract:

Exhaust
emissions from industrial activities contain large amounts of carbon dioxide,
which causes serious greenhouse effect, resulting in climate change which will
endanger our living environment. According to the
investigation of Intergovernmental Panel on Climate Change (IPCC), CO₂
is the most important greenhouse gas from human activities.
Therefore, to reduce the emission of CO₂ is a major task nowadays.

Pressure swing adsorption (PSA) plays an important
role in the separation of gas mixtures and concentration. Comparing to other
separation method such as absorption, membrane separation, and cryogenic
separation, PSA has the advantages of low cost and being easy to operate. As
more and more novel adsorbents are synthesized, PSA is a potential method to
slow down the global warming.

The system of capturing CO₂
can be divided into three categories by combustion types: pre-combustion
capture, post-combustion capture and oxy-fuel combustion.

This
simulation research studies concentrating carbon dioxide from the syngas of a
gasifier after oxy-fuel combustion and dehydration with composition of 95% CO₂
and 5% N₂
by pressure swing adsorption process, so that the concentrated carbon dioxide
can be captured and utilized/stored to reduce greenhouse gas emission. High
purity carbon dioxide is mainly applied in the areas of laser, semiconductor
wafer cleaning, and critical extraction of medicine, etc. The zeolite 13X is
used as adsorbent. First, the experimental adsorption isotherm data were
regressed to obtain the parameters of Langmuir-Freundlich isotherm equation. The
mass transfer coefficient of linear driving force (LDF) model was calculated by
theory and verified by breakthrough curve and desorption curve experiments,
shown in Figure 1 and Figure 2. Then we verified the simulation program by
comparison with the data of a single-bed four-step process experiment. The verification shows the
accuracy of the simulation program.

Finally, the four-bed
twenty-four-step and four-bed twenty-step PSA processes for syngas after oxy-fuel
combustion and dehydration are studied to find the optimal operating
conditions. The PSA processes and operating variables such as feed pressure, feed
temperature, step time, bed length, and vacuum pressure were discussed.

The results show that the four-bed twenty-step PSA process without tank and with
recycle shown in Figure 3 is the best process for getting high purity of carbon
dioxide. When feed pressure
increases, total amount of feed gas increases and so does the amount of gas be
adsorbed, which makes CO₂ product purity increase, but makes
recovery decrease because of more gas exhaust from top stream. While
temperature increases, the selectivity of CO₂ to N₂
increasing with temperature,
which makes CO₂ purity increase. As bed length increases, more gas
is adsorbed and less gas exits at cocurrent depressurization step, which makes
CO₂ purity decrease but recovery increase. When vacuum pressure increases, CO₂
purity increases because more N₂
exits at cocurrent depressurization step, but recovery decreases because CO₂
also exits at cocurrent depressurization step.

After discussing the
operation variables, the optimal operating conditions are feed pressure 5.00
atm, vacuum
pressure 0.238 atm, adsorption
time 231 s, cocurrent depressurizaton time 129 s, vacuum time 486 s, pressurization equilibrium time 204 s, and
temperature 338.14 K. The simulation results of optimal operating
conditions are 99.99994% purity and 9.81% recovery of carbon dioxide at bottom
product of four-bed twenty-step PSA process without tank and with recycle shown
in Figure 4.

Figure 1. Breakthrough curve of CO₂ for 50% CO₂ and 50% N₂ gas mixture.

Figure 2. Desorption curve of CO₂ by pure helium.

Figure 3. Schematic diagram of
four-bed twenty-step PSA process.

Figure 4. Schematic diagram of
optimal results for CO₂
capture from syngas feed.