Stripper Configurations for CO2 Capture by Aqueous Amines

A monumental task for improving the feasibility of carbon capture from the flue gas of coal-fired power plants is reducing the total energy cost.  A generic absorption/stripping process using amines would reduce the net power production of a coal-fired power plant by roughly 30%.  The reboiler duty, solvent pumping, and CO₂ compression account for most of this power hit.  Though alternative solvents are being studied which can improve the capture performance, the Aspen PlusŪ thermodynamic model for 9 m MEA (35wt%) developed by Hilliard is a sturdy model for a more concentrated form of the industry standard for carbon capture, 7 m MEA.  Using this model, a fair analysis of various stripper configurations can be performed.  It is proposed that increasing complexity improves performance, but with diminishing return.  Since increasing the complexity raises capital and operation costs, an optimum configuration complexity is expected.

In prior work by other authors, more complex configurations have been modeled to assess potential improvement in performance of the stripper.  Some of these configurations included Simple Stripping with Vapor Recompression, Multi-Pressure, Double Matrix, Split Product, Internal Exchange, and Flashing Feed.  In addition to these configurations that use packed columns, additional flowsheets using arrangements of equilibrium flashes are considered in this work.  This choice arose from previous results which demonstrated that the lean CO₂ loading which minimizes the energy requirement in the stripper is higher than typically considered.  For example, for 9 m MEA a typical lean loading used in operation is 0.2 with a rich loading of 0.5.  However, the optimum lean loading in the stripper has generally been in the range of 0.35-0.40 for this solvent.  This optimized lean loading reduces the solvent capacity as well as the CO₂ flux per unit solvent.  Consequently, the required height of packing drops to practically a negligible amount, making equilibrium flashes a more financially conscious option.

The double matrix configuration revealed the benefit of stripping CO₂ at different pressure levels.  High pressure stripping reduces energy consumption by reducing compression work and increasing selectivity for CO₂ over water in the vapor.  Also using a lower pressure level for stripping is beneficial to achieve the desired lean loading without using unreasonably high temperatures.  For example, MEA has a ceiling temperature of 120°C due to elevated thermal degradation rates above that temperature.  In this work, analyses have been performed for configurations with 1 pressure stage, 2 pressure stages, and 3 pressure stages.  In all cases the reboilers were run isothermally to have the closest approach to a reversible process.  This isothermal operation would also have the added benefit of only requiring one steam temperature, thereby reducing the impact on the steam turbines of the coal plant.

Multi-stage flash configurations from 1- to 3-stage flash were modeled.  In addition to the isothermal cases, the 2- and 3-stage flash configurations were modeled non-isothermally, where all stages were not necessarily heated.  The 1-stage flash exhibited poorer performance than the base case simple stripper, which can be attributed to all of the separation occurring in one irreversible step.  The performance improved with the 2- and 3-stage flash configurations as the separation became more reversible, as more CO₂ was collected at elevated pressure, and as less heat was wasted from water vapor generation in the flashes.  The multi-stage flash configurations performed well at their optimum lean loadings, but alternative configurations with packing performed better with low lean loadings.  These low lean loadings reflect cases in which the absorber does not achieve adequate performance with the lean loading optimized for the stripper.

As a consequence of increased complexity for some configurations, variables arose which could either be specified arbitrarily or optimized.  An example of one such variable was the rich solvent split for the double matrix configuration.  Another example was the distribution of vapor production for configurations with at least two stages.  The distribution of vapor production is also directly related to the pressure ratio between stages.  In previous work these values were specified using good judgment or rule of thumb.  However, in order to better understand the effect of increasing the configuration complexity, these values were varied or optimized.

Some of the final results for selected configurations are shown in Table 1 below.  The optimized lean loading and the accompanying equivalent work.  Additionally, the number of net process units for the stripper in each configuration is tabulated.  This includes separation vessels, packing sections, and heaters/reboilers.  The process unit count was reduced if a configuration eliminates standard units.  For example, the stripper with adiabatic lean flash re-compresses vapor without an intercooler, reducing the unit count by 1.

Table 1: Performance Results for Selected Stripper Configurations

Configuration	Process units	Equivalent Work	Optimum Lean Loading
		kJ/mol CO2
1-Stage flash	2	35.6	0.410
Simple Stripper	3	35.2	0.383
Stripper with adiabatic lean flash	3	34.4	0.384
2-Stage flash	4	35.1	0.390
Double matrix Flash	5	33.9	0.390
3-stage flash	6	33.6	0.375

The results of this work have elucidated several factors which improve the performance of the stripper, pumps, and CO₂ compressor.  First, operating with multiple pressure levels reduces the energy requirement.  This operation provides the benefit of stripping at high pressure, but also improves the reversibility when returning to atmospheric conditions for the absorber.  Another factor that this work confirmed was the benefit of using low-pressure vapor with high water content for additional stripping.  This was done in two different ways: contacting the vapor with cool, rich solvent, or recompressing the vapor for use in a stage with elevated pressure.  These methods condensed some of the water in the low-pressure vapor stream to reduce the total waste heat.  In the case of vapor recompression, the intercooling for the first compression stage was removed entirely, and the increase of the vapor temperature from compression replace a portion of the reboiler duty.  Lastly, the configurations which investigated adiabatic flashes or packing indicated that heating first always yielded preferable operating conditions.  As an example, only heating the second stage of the 2-stage flash produced cases where the second stage pressure was 20 times that of the first stage pressure.  Even at the optimum lean loading, the first stage pressure was only 78% of the second stage pressure.  Configurations with heat added first always resulted in the highest pressure stage first, followed by lower pressures in subsequent stages.