(397c) Simulation Study of Capturing CO2 from Syngas after Water Gas Shift Reaction By Pressure Swing Adsorption

Abstract:

Since global warming is getting
worse, the United Nations signed the Paris Agreement in April 2016 to control
the rate of temperature rise. The agreement including restricting the increase
of temperature within 1.5~2 C, setting a fund for climate change and balancing
CO­₂ emission/nature consumption before 2050. Due to the legal
validity of this agreement, countries show their determination to solve global
warming.

  There are four major technologies to
capture carbon dioxide: chemical absorption, cryogenic separation, membrane
separation and adsorption. Among these technologies, PSA has advantages of low
cost and easy to operate. PSA can separate different gas mixture by various
adsorbents, which makes it very competitive.

  This simulation research studies the
separation and concentration of carbon dioxide from syngas after water gas
shift reaction (41.4% carbon dioxide, 1.3% carbon monoxide and 57.3% hydrogen
after water removal) by pressure swing adsorption (PSA) process. The UOP 13X
zeolite is used as adsorbent. Langmuir-Freundlich isotherm equation is used to
regress the isotherm data to obtain the parameters of CO₂, N₂,
CO and H₂. The mass transfer coefficient of linear driving force
(LDF) model was calculated by theory. In order to test the reliability of the
simulation program, we compare the results of simulation program with
breakthrough curve experimental data (Figure 1), desorption curve experimental
data (Figure 2) and the results of single-bed four-step PSA process
experiments. The verification shows that the simulation
program can be trusted.

  Finally, a dual-bed six-step PSA process
(Figure 3) with beds of inside diameter 2.32 cm is employed for syngas feed to
find the optimal operating conditions and for temperature from 373 K to 413 K
because syngas is usually at high temperature. The operating variables such as
feed pressure, adsorption time/vacuum time, adsorption time/cocurrent
depressurization time, pressure equalization time, bed length, vacuum pressure
and feed/surrounding temperature are discussed.

The results show that when feed
pressure increases, total amount of feed gas increases and so does the amount
of gas adsorbed, which makes CO₂ purity increase, but makes recovery
decrease because of more gas exhaust from top stream. As adsorption time/vacuum
time increase, adsorber adsorbs more CO₂ and is regenerated better
which make CO₂ purity increase and slight influence on CO₂
recovery. While adsorption time/cocurrent depressurization time increase, more
weak adsorptive gas exits from top stream, which makes CO₂ purity
increase but recovery decrease. There is only slight influence on CO₂
purity and recovery while pressure equalization time increases. 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 CO exits at cocurrent depressurization step, but recovery
decreases because CO₂ also exits at cocurrent depressurization step.
The last variable is feed and surrounding temperature. While temperature
increases, less CO₂ adsorb, which makes CO₂ purity
decrease but CO₂ recovery increase slightly.

After discussing the operation
variables, the optimal operating conditions are feed pressure 3.95 atm, bed
length 110 cm, vacuum pressure 0.05 atm, adsorption time 160 s, cocurrent
depressurization time 10 s, vacuum time 130 s, pressure equalization time 5 s
and feed temperature 393 K. The results of final conditions are 95.3% purity
and 97.6% recovery of CO₂ in bottom product (Figure 4) and, at the same time, hydrogen with 97.6% purity and 98.3%
recovery is obtained.

Figure
1. Breakthrough curve of CO₂ of gas mixture.

Figure
2. Desorption curve of CO₂.

Figure
3. Schematic diagram of dual-bed six-step PSA process.

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