Carbon capture and storage (CCS) and direct air capture (DAC) are necessary technologies in the achievement of global climate targets. Innovative ways to enhance CCS and DAC (by making them cheaper, reducing the land area required, smaller energy requirements, etc.) are being studied to facilitate accelerated large-scale, deployment of CCS and DAC globally.
One promising innovation for implementing CCS and/or DAC is using crossflow absorbers. Crossflow absorbers can operate at a wider range of fluid flowrates than counterflow absorbers, therefore, larger volumes of gas can be processed faster. Crossflow absorbers also have pressure drops that are much lower than those of counterflow absorbers, meaning their operating costs are lower. Additionally, crossflow absorbers can be built in a modular fashion (i.e., they are flexible and do not need to be tall, vertical columns) that allows them to be adapted to the needs of each specific location with minimal visual impacts.
However, liquid entrainment and liquid carryover are significant drawbacks of crossflow absorbers. Both these phenomena can reduce mass transfer efficiency, as well as result in the need for more rigorous downstream processes to condense and collect the solvent from the gas phase to prevent solvent loss to the atmosphere. Previous studies have shown that liquid entrainment and liquid carryover may be countered by changes to the packing geometry (Lavalle et al., 2018). The packing that is available for use in crossflow gas-liquid contactors is designed for cooling tower applications and is optimised for the heat transfer processes that take place in cooling towers, rather than the mass transfer processes that are needed for CCS (Holmes and Keith, 2012).
A pilot-scale test rig was developed and used to experimentally investigate the hydrodynamic and mass transfer performance of crossflow absorbers. The experimental test campaign used an air and sodium hydroxide system and the obtained data was analysed using response surface methodology (RSM). The crossflow absorber was found to have pressure drops 16% to 50% smaller than those in counterflow absorbers, and liquid holdups -50% to 200% different from those in counterflow, with the difference in liquid holdups increasing with decreasing liquid loads irrespective of packing set. Lower pressure drops can result in a lower energy requirement. The mass transfer performance of the crossflow absorber was found to be at most 15% smaller than that of counterflow absorbers.
Sulzer’s Mellapak 250Y.PP structured packing was investigated to determine its performance in crossflow absorbers, along with five other packing sets produced by tilting the Mellapak 250Y.PP at different angles to investigate the effect of tilting the packing on its performance and determine whether the packing can be optimised for use in crossflow absorbers. Out of the structured packing sets that were tested, the set tilted at 82° was found to be the one with the best performance.
The crossflow absorber’s operating conditions were optimised to maximise effective interfacial mass transfer area and minimise pressure drop. The optimum operating conditions were found to be 5.06 m/sec gas flowrate and 1.17 L/m2·sec liquid flowrate, which is equivalent to an L/G ratio of 0.3, for Sulzer Mellapak 250Y.PP packing that has been tilted 90±8°.
Finally, empirical equations for pressure drop, liquid holdup, and effective interfacial mass transfer area were developed from the experimental data collected from the crossflow absorber test rig. These empirical equations were incorporated in a preliminary rate-based model that had been developed by previous members of the CCS research group. These empirical equations reduced the model’s average absolute relative deviation (AARD) from 216% to 3.3% for pressure drop, from 512,854% to 9,305% for liquid holdup, and from 43% to 30% for effective interfacial mass transfer area
