Amine stripper temperature profile a key to optimising net-zero performance in post-combustion carbon capture plants

I have already described how capturing 100% of the fossil carbon dioxide (CO2) in power plant and other flue gas streams is technically feasible using amine post-combustion CO2 capture with monoethanolamine (MEA), provided sufficiently low loadings (around 0.1 molCO2/molMEA) for the lean solvent entering the top of the absorber are achieved in the stripper.?Such low lean loadings have been considered to require excessive thermal energy input in the stripper reboiler but, as the graph (above left) from a?recent paper?with?Stavros Michailos?shows, the ‘inflection point’ where there is a rapid rise in the curve of specific reboiler duty (SRD) vs. lean loading can be shifted to lower lean loadings by increasing the stripper operating pressure, with a corresponding increase in reboiler temperature.?At any pressure, though, it is advantageous to operate as close to the inflection point as possible.?This will give the lowest possible lean loading for that pressure and hence reboiler temperature range, with the benefit of higher mass transfer rates in the absorber.?

More modelling and experimental investigation have subsequently been undertaken with colleagues Daniel Mullen, Muhammad Akram and Mathieu Lucquiaud?on the ?UKCCSRC PCC-CARER project and the FOCUSS project led by SSE Thermal and funded by DESNZ (formerly BEIS) under the Science and Innovation for Climate and Energy (SICE) programme and results have been exchanged with the National Carbon Capture Center (NCCC) in Wilsonville, Alabama.?This work has suggested a practical way of being able to hit that inflection point ‘sweet spot’ on operating plants. The studies were conducted using MEA but the method should be applicable to any solvent that uses a conventional stripper and has a water vapour/CO2 ratio in the vapour leaving the reboiler that increases as the lean loading is reduced and that decreases as the pressure is increased.?

The key is understanding what is happening in the stripper when the inflection point is reached.?Heat going into the reboiler both breaks the bonds between the amine and the CO2 and evaporates water vapour, to reduce the partial pressure of CO2 in equilibrium with the amine solution. This mixture of CO2 and water vapour, which is close to thermal and chemical equilibrium with the lean solvent leaving the stripper, then rises through the stripper packing and supplies heat to the downcoming solvent, partly through physical cooling but mostly through condensation of the water vapour.?If the rising CO2/water vapour mixture can be cooled to the same temperature as the rich solvent after it has entered the column (generally flashing as it does so, because it is heated to close to the lean solvent temperature in the cross-flow heat exchanger) then the CO2 leaves with as little water vapour as possible and the SRD will have the baseline value before the inflection point.?When the amount of water vapour leaving the reboiler with the CO2 rises too high, however, it cannot all be condensed in the stripper column and the level of water vapour in the CO2 at the top of the packing forming the solvent/vapour counterflow region rises.?Thus, some of the energy added in the reboiler is being wasted and the SRD starts to go up exponentially.?

The quick message in this article is that, not surprisingly, the switch from as much water vapour as possible being condensed in the stripper packing to more than necessary leaving with the CO2, as can be seen in the modelled stripper temperature profiles in the centre above and in the qualitatively very similar measured stripper temperature profiles from a series of tests at NCCC on the right.?The temperature profiles in the solvent/vapour counterflow section of the stripper packing can therefore be used to ensure that a stripper is operating at the inflection point and hence with the minimum lean loading achievable without excessive SRD at a given operating pressure.?At lean loadings higher than the inflection value the gas and liquid temperatures in the packing drop rapidly going up the column from the reboiler (the ‘Baseline region’ curves).?At lean loadings lower than the inflection point value (the ‘Exponential region’ curves) stripper packing internal temperatures stay high, falling only to an extent at the top.?In the ‘Inflection point’ region, however, the temperatures in the stripper packing fall approximately linearly from bottom to top, with the differences from the other profiles most striking around the mid-height.?

These significantly-different temperature profiles offer a robust way to assess whether a stripper is operating at a lean loading that is below, above or – as it ideally should be – at the inflection point.?Because in practice thermocouples inserted in a stripper packing cannot distinguish between liquid and gas/vapour temperatures, but see some aggregate of them, the divergence of gas/vapour and liquid temperatures at the top of the packing at loadings below the inflection point would be relatively difficult to see.?The difference in the temperature profiles along the column, with large absolute changes around mid-height, should, however, be unmistakeable.

The bottom line – don’t forget to include a reasonable number of thermocouples in the stripper column on any amine post-combustion plant with aspirations to achieve high levels of CO2 capture.?They are likely to prove useful when the operators are working to tune the plant to deliver the best possible performance.?And don’t worry too much about the thermocouple reading accuracy, the flip in temperature profile around the inflection point will be far too big to miss.

And a post-script for post-combustion CO2 capture geeks ………

It might to be thought that more stripper packing would allow more water vapour to be condensed but, beyond a certain amount to allow adequate heat and mass transfer, extra packing can give only minimal benefit.?This is because the limit on water vapour condensation is due to thermodynamics, not kinetics.?The key is the loading of the solvent entering the reboiler after heating in the stripper column.?The reboiler vapour-liquid equilibrium (VLE) defines the ratio of water vapour to CO2 leaving the reboiler at the required lean loading, but the difference between the loading of the solvent falling into the reboiler and the lean loading defines the absolute amount of CO2 and hence the amount of water vapour leaving the reboiler – and hence also the total reboiler heat input.?

At the stripper inflection point the loading of the solvent dropping into the reboiler is such that just enough water vapour is condensed by cooling the CO2/vapour mixture leaving the reboiler to the rich solvent temperature at the top of the packing to provide heat to the falling solvent to generate this lean loading (yes, it is a circular statement, and a circular physical process).?This intermediate loading is then associated with enough heat input in the reboiler, to get it to the required lean loading, that then also satisfies (as it must) the overall energy requirements for the stripper.?But at lower lean loadings than the inflection point value so much water vapour has to be produced with the CO2 that, if it was cooled to the minimum temperature set by the rich solvent, then the energy transferred to the falling solvent would result in a loading for the solvent dropping into the reboiler that would result in too low a heat input to the reboiler to satisfy the overall stripper energy requirements for the desired input and outputs.?The result is another balance – just enough water vapour is condensed to give the required intermediate lean loading at the bottom of the packing that will correspond to sufficient heat input in the reboiler to provide the overall energy requirements.?This situation, where excess water vapour is leaving with the CO2, is associated with the temperature of the rising gas/water vapour staying high until near the top of the packing – however much packing there is – and then the required amount of rapid cooling and water vapour condensation taking place in a small space at the top of the packing.

Obviously at higher lean loadings than the inflection value much less water vapour leaves the reboiler with the CO2 and it can always be condensed down to the minimum possible value – hence the level baseline in SRD values with assumed constant rich loading – but the mass transfer in the absorber suffers and hence the rich loading will be higher in practice, resulting in a higher SRD.