(562ar) Energy-Efficient Thermal Regeneration of Biphasic Solvents in a Laboratory CO2 Desorption System

A
novel biphasic solvent-enabled CO₂ absorption process (BiCAP) is under
development for post-combustion carbon capture (PCC) as a promising alternative
to the conventional monoethanolamine (MEA) process. The BiCAP features the
regulated formation of dual liquid phases during CO₂ absorption and
the regeneration of the CO₂ rich phase only with reduced mass and
elevated CO₂ loading for CO₂ desorption. The process has
great potential in significantly reducing the energy use compared with the MEA
process.

The
biphasic solvents employed in the BiCAP are a new class of amine-based water-lean
blends. Two biphasic solvents, BiS1 and BiS2 were selected for this study. The CO₂
absorption capacity of either BiS1 or BiS2 is more than 30% higher than that of
the benchmark 30 wt% MEA. Because of the absorbed CO₂ concentrated in
the rich phase, their CO₂ desorption capacity is more than twice that
of MEA.

A
10 kWe laboratory experimental system (Figure 1) consisting mainly of a flash
vessel and a stripping column was fabricated to investigate the CO₂
desorption performance of biphasic solvents. The
system is rated at 300 psig at 200 °C. The flash vessel is a 5-inch
inner diameter (ID) and 2-ft height cylinder with a heating coil inside that
uses steam as a heat source. The stripper is a 2-inch ID and 9-ft height column
with stainless steel Pro-Pak random packing, attached to a sump section with a heating
coil inside that uses steam to reboil the solvent. The CO₂
desorption system can be operated under two modes: the single stripper mode,
under which the flash is bypassed; and the sequential flash and stripper mode.

In
the CO₂ desorption experiments, for either BiS1 or BiS2, the CO₂
rich phase was separated from the biphasic solvent mixture loaded with the
required amount of CO₂, and then used as the feed solution. The 30
wt% aqueous MEA solution was tested as a reference. The MEA solution is
monophasic and was used directly for desorption experiments after being loaded
with the required amount of CO₂.

In
the single stripper experiments, the stripping of either BiS1 or BiS2 solvent
was conducted at 120, 135, and 150 ℃. At these temperatures, the
stripping pressures were controlled at 160-280 kPa, 260-470 kPa, and 450-860
kPa, respectively, resulting in the rate of CO₂ recovery varying from
approximately 90% to 30%. The stripping temperature as high as 150 ℃ was
adopted for the biphasic solvents because they have demonstrated extremely high
thermal stability in our previous studies. For the comparison, the MEA solution
was tested at either 110 or 120 ℃ to reflect its typical stripping temperature
range with the pressure controlled at 120-160 kPa or 140-260 kPa.

The
heat duty in the single stripper generally decreased with increasing stripper
pressure at the constant reboiler temperature, mainly because the stripping
heat associated with water vapor loss in the product CO₂ stream was reduced
at a higher pressure. The estimated heat duty ranged between 2,500 and 3,900
kJ/kg CO₂ for BiS1 and between 2,000 and 2,800 kJ/kg CO₂ for
BiS2. In comparison, the MEA required the heat duty of 4,200 to 4,800 kJ/kg CO₂
under comparable conditions. The results of heat duty obtained at 150 °C are displayed in Figure 2(a). The
low heat duty for the biphasic solvents is a result of low sensible heat use due
to reduced solvent mass and low stripping heat due to reduced water vapor loss
in the product CO₂ stream.

Under
the sequential flash and stripper test mode, the stripper reboiler temperature remained
at 120, 135, or 150 ℃ for BiS1 and BiS2, and 110 or 120 ℃ for MEA. The
flash temperature was controlled at approximately 10 ℃ lower than that of
the reboiler. Because of the relatively high level of CO₂ loading in
the flash vessel, the flash pressures for BiS1 or BiS2 could be elevated to at
least 0.6 bar higher than their respective stripping pressures, which were
controlled to be comparable to those in the single stripper mode. Throughout
the flash vessel and the stripper, the total CO₂ recovery rate
achieved ranged between 40% and 80% in the different experiments. Among the
total CO₂ recovery, 21% to 57% could be attributed to the flash
vessel depending on the test conditions.

The
heat duty in either the flash or stripper decreased with increasing operating
pressure when the operating temperature remained unchanged. The heat duty in
the flash was lower than that in the stripper, primarily because it was operated
at higher pressure and lower temperature and thus lowered the stripping heat
associated with water vapor loss. The overall heat duty weighted over both
devices was estimated at 2,500-3,100 kJ/kg CO₂ for BiS1 and
2,200-2,700 kJ/kg CO₂ for BiS2, compared with the heat duty of
3,500-5,400 kJ/kg CO₂ for MEA. Figure 2(b) shows the results of heat
duty obtained under 140/150 °C
(for BiS1 and BiS2) or 110/120 °C
(for MEA) flash/stripping. As mentioned above, the low heat duty for the
biphasic solvents was attributable to their low sensible and stripping heat
uses.

Compared
with the single stripper, the overall heat duty incurred in the sequential
flash and stripper system was comparable. However, because of the elevated
flash pressure, the CO₂ stream from the flash required less
compression work, which thus reduced the overall work requirement.

The
experimental results revealed that the selected biphasic solvents could lower
the heat duty of CO₂ desorption by 30%-45% compared with the benchmark
MEA solution. In particular, the biphasic solvent BiS2 could reduce the
regeneration heat duty to as low as 2,000-2,200 kJ/kg CO₂ desorbed. Additional
work is ongoing to investigate different desorption process configurations to
further reduce the energy use of CO₂ capture for the biphasic
solvents.    

Figure 1.
Schematic diagram of the laboratory flash and stripping system for CO₂
desorption

Figure
2. Heat duty of CO₂ desorption operating under (a) single stripper
mode at the reboiler temperature of 150 ℃ for the biphasic solvents and
120 ℃ for MEA and (b) sequential flash and stripper mode at the
flash/reboiler temperatures of 140/150 ℃ for the biphasic solvents and
110/120 ℃ for MEA.