(713e) Rigorous Modeling of Novel Absorption-Stripping Process Configurations With Aqueous Piperazine for CO2 Capture From Natural Gas Combustion

Sachde, D. J., The University of Texas at Austin
Rochelle, G. T., The University of Texas at Austin
Madan, T., The University of Texas at Austin

This study evaluates amine
scrubbing using 8 m piperazine (PZ) for CO2 removal from the flue gas of three
natural gas-based applications. The applications and corresponding flue gas CO2
concentrations are as follows: Case 1, combined cycle (3% CO2), Case 2, combined
cycle with exhaust gas recycle (6% CO2), and Case 3, natural gas-fired boiler
(9% CO2). For each application, two bounding design scenarios were evaluated:
high capital and low operating cost (high cap-ex, low op-ex)  or low capital
and high operating cost (low cap-ex, high op-ex). Capital and operating costs
were not evaluated explicitly; rather, within the two major process areas
(absorption and stripping), equipment level design modifications and process operating
conditions were used to approximate the economic scenarios. Process and
equipment models were developed in Aspen Plus® utilizing a rigorous
thermodynamic and kinetic framework developed previously for the piperazine
solvent. The focus of this work is the development and modeling of new process
and equipment configurations for the natural gas applications.

Key developments in absorber
design included solvent recycle configurations for intercooling in the
absorber. The recycle design was used for two purposes in the natural gas
applications. First, the water content in the entering flue gas of the combined
cycle applications (Cases 1 and 2) was sufficiently low (< 7% water) that
cooling of the gas to 40°C (absorber operating temperature) would not lead to
condensation of water. This allowed the elimination of a separate direct
contact cooler (DCC) upstream of the absorber in favor of a packed section at
the bottom of the absorber with recycle of a high rate (~ 3 times the solvent
feed rate) of cool, rich solvent to couple the function of cooling the inlet
flue gas with absorption of CO2.

In addition, the liquid-to-gas
flow ratio (L/G, mole to mole basis) in the column is much lower than in the
typical coal applications (~12-14% CO2 in the flue gas) due to the
high volume of flue gas treated. The resulting energy balance in the absorber
leads to a maximum in column temperature (and corresponding equilibrium limited
mass transfer) towards the top of the column. In other words, the heat
generated by absorption of CO2 in the solvent is carried by the gas
to the top of the column. Figure 1 depicts a temperature profile for Case 1 as
an illustration of the expected temperature behavior in the gas applications.

1: Molar Flux and
temperature profiles, Case 1 (3%CO2) with DCC.  Lean loading = 0.25
mols CO2/mols alkalinity, lean amine T = 40 °C, rich gas T = 40 °C,
3:1 recycle.

The solvent-recycle design depicted
in Figure 2 is shown to cool the gas effectively by contacting it with a high
rate of cooled solvent in a well-mixed section of the column, and, in turn,
reducing the temperatures throughout the column. The high solvent rate per
wetted perimeter also provides enhancement of mass transfer (via generation of
turbulence in the recycle section). Sensitivity analyses identified solvent
recycle rates to minimize total packing requirements (capital costs) and
maximize rich loadings (minimize energy requirement in the stripping section).

Figure 2:
Case 1 and 2 Absorber PFD with Solvent Recycle, No DCC, Combined Cycle Gas

In the stripping section, an
inter-heated stripper was implemented with 5°C LMTD for the main cross
exchanger (increased exchanger area) to represent a high cap-ex, low op-ex
designs (Figure 3).

Figure 3: High cap-ex
configuration of inter-heated stripper and 5 °C LMTD cross-exchanger

The inter-heated stripper
configuration recovers additional heat from the reboiler via cross exchange of a
solvent-draw from the stripper with the rich stream leaving the reboiler. The
inter-heating also results in a cooler rich amine stream reaching the stripper
since less heat is transferred in the main cross-exchanger (i.e., the heat was
recovered in the inter-heater). The cooler rich amine stream entering the
stripper reduces the amount of water vapor lost per mole of CO2
recovered and in turn reduces irreversible heat losses in the condenser.

In addition, a simple stripper
with cold rich bypass and 10°C LMTD for the main cross exchanger (reduced exchanger
area) was implemented for the low cap-ex, high op-ex designs (Figure 4).

Figure 4: Low cap-ex configuration of simple stripper
with cold rich bypass and 10 °C LMTD cross-exchanger

In this stripper design, the column
consists of two packed sections. The cold rich amine stream leaving the
absorber is split upstream of the main-cross exchanger. A fraction of the flow
bypasses the main exchanger and is sent to the top of the stripper, reducing
the water lost per mole of CO2 in the gas leaving the stripper (as
in the inter-heated stripper design). For both stripper configurations,
analysis of the equivalent work requirement was conducted, including
sensitivity analysis identifying a range of operable lean and rich loadings for
the natural gas applications.