(732g) Optimal Temperature Profiles within a CCS Absorber

It is well known that absorber intercooling may be used to reduce column height and reboiler heat duty (Freguia et al. 2003, Rezazadeh et al. 2017, Plaza et al. 2010, Le Moullec et al. 2014, Walters et al. 2016). However, to date most research has focused on relatively simple intercooling geometries, and little work has been done to establish upper limits on the improvements more advanced intercooling techniques could provide.

In this work, we develop and validate a numerically efficient and flexible model of an absorption column containing monoethanolamine (MEA), similar to that developed by Saimpert et al. (2013). By coupling this model with a differential evolution optimization algorithm, we calculate the optimal temperature profile along the column’s length which minimizes the column height required for 90% capture of CO₂. The optimal temperature profile represents a tradeoff at each point in space between increasing the temperature to increase reaction kinetics and decreasing the temperature to increase the driving force for CO₂ consumption. In general, the optimal profile is not isothermal (see Figure 1), however for some simple cases analytical expressions for the optimal profile may be derived.

By comparing these optimized columns to the adiabatic case, it is possible to put an upper limit on the reduction in column height intercooling can provide. In general, this is a strong function of the liquid flowrate: at flowrates close to the minimum, the temperature bulge tends to be large and near the top of the column (Kvamsdal and Rochelle 2008), and under these circumstances the adiabatic column height is substantially larger than the minimum possible height (see Figure 1). For example, for 90% capture from a CO₂ stream containing 10mol% CO₂, when the temperature bulge is near the top the adiabatic case is typically 10-20% larger than the minimum column height. On the other hand, at larger liquid flowrates (typically around 1.5L_min - 2L_min) the temperature bulge tends to shrink in size and move towards the bottom of the column (Kvamsdal and Rochelle 2008) and under these circumstances the difference between the adiabatic and minimum possible column height tends to be marginal – typically less than 5%. The maximum possible improvement is also dependent on the inlet CO₂ concentration: for gaseous streams containing 20mol% CO₂, the adiabatic column may be more than 30% taller than column with optimized temperature profile, while for capture from a 5mol% CO₂ stream the maximum improvement intercooling could provide is correspondingly smaller.

The model was also used to investigate a range of heat integration strategies within the column. It was shown that under a wide range of circumstances the column height could be reduced to close to the minimal value simply by optimally rearranging heat within the column – moving heat away from regions where increasing the driving force is more important, and towards regions where higher temperatures are beneficial to improve reaction kinetics. The transfer of heat was consistent with both the first and second laws of thermodynamics. Such sophisticated heat integration may be possible through the use of active packings, which incorporate a heat exchange fluid within the column packing itself. For a number of cases of interest, it was also shown that traditional in-and-out intercooling (Rezazadeh et al. 2017) was able to reduce the column height to close to the minimal value, though approaches which incorporate heat redistribution within the column itself would have the benefit of reduced cooling loads.

Prepared by LLNL under Contract DE-AC52-07NA27344. This work was supported via the U.S. Department of Energy, Office of Fossil Energy, grant number FWP-FEW0225

References

Freguia, S. and Rochelle, G.T., 2003. Modeling of CO2 capture by aqueous monoethanolamine. AIChE Journal, 49(7), pp.1676-1686.

Kvamsdal, H.M. and Rochelle, G.T., 2008. Effects of the temperature bulge in CO2 absorption from flue gas by aqueous monoethanolamine. Industrial & Engineering Chemistry Research, 47(3), pp.867-875.

Le Moullec, Y., Neveux, T., Al Azki, A., Chikukwa, A. and Hoff, K.A., 2014. Process modifications for solvent-based post-combustion CO2 capture. International Journal of Greenhouse Gas Control, 31, pp.96-112.

Plaza, J.M., Chen, E. and Rochelle, G.T., 2010. Absorber intercooling in CO2 absorption by piperazine‐promoted potassium carbonate. AIChE journal, 56(4), pp.905-914.

Rezazadeh, F., Gale, W.F., Rochelle, G.T. and Sachde, D., 2017. Effectiveness of absorber intercooling for CO2 absorption from natural gas fired flue gases using monoethanolamine solvent. International Journal of Greenhouse Gas Control, 58, pp.246-255.

Saimpert, M., Puxty, G., Qureshi, S., Wardhaugh, L. and Cousins, A., 2013. A new rate based absorber and desorber modelling tool. Chemical engineering science, 96, pp.10-25.

Walters, M.S., Edgar, T.F., Rochelle, G.T., 2016. Dynamic modeling and control of an intercooled absorber for post-combustion CO2 capture. Chemical Engineering and Processing: Process Intensification 107, 1–10.