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Enhancing CO2 uptake in a falling film absorption tower using printed hydrophobic patterns

Pratanu Roy1 Du T Nguyen1 Katherine Hornbostel2 Joshuah K Stolaroff1

1Lawrence Livermore National Laboratory, Livermore, California, USA

2University of Pittsburgh, Pittsburgh, Pennsylvania, USA

Abstract Falling film absorbers are frequently proposed for capturing CO2 from flue gas using liquid solvents, such as MEA. In this type of flow configuration, the liquid solvent trickles down the packed column while the flue gas is flowed upward through the column. The purpose of the column is to enhance the contact between the gas and liquid phases to promote mass transfer between the two phases. One of the primary limitations of the conventional falling film technology is liquid maldistribution over the absorber packing surface. When the liquid is not properly distributed, undesirable flow patterns are created with patches of non-wetted areas, which do not participate in the absorption process. The non-wetted area can be reduced by increasing the flow rate of the liquid. However, once the absorber surface is fully wetted, the gas absorption rate does not increase with subsequent increase in flow rate. At this point, the interfacial mass transfer is primarily dominated by the film dynamics. Minimizing the liquid maldistribution and maximizing the gas-liquid interfacial area would enhance the CO2 mass transfer rate, thereby improving the energy efficiency and reducing the operational cost of the absorber. We propose a novel approach that varies the wettability of the absorber surface, which continuously perturbs the liquid flow and therefore enhances the gas-liquid interfacial area. We demonstrate that printing a pattern of hydrophobic patches onto a hydrophilic surface reduces liquid maldistribution and improves the mass transfer coefficient by 10-20%. The flow dynamics and interfacial mass transfer are studied here with a combination of computational and experimental tests for sodium carbonate solution flowing down an inclined plate. The computational modeling was performed using the TransFORT code, which is a parallel finite volume code to solve the mass, momentum and species transport equations. A level set method was used to capture the liquid-gas interface. Different combinations of patterns and contact angles for the hydrophilic and hydrophobic zones were tested and compared. Figure 1 shows the instantaneous liquid distribution for one such combination. Patterns that produced high gas-liquid interfacial based on this model were selected for experimental study. For the experiments, functionalized advanced packings were fabricated by using direct ink writing (DIW) to pattern hydrophobic silicone patches onto glass plates. Based on the model optimization results, 1mm x10mm patches were printed onto the glass plate with a vertical pitch spacing of 10mm, as shown in Figure 2. Inclined glass plates with and without this surface pattern were then tested experimentally with multiple trials. Dilute (1 weight %) K2CO3 solution was flowed over these inclined plates using a peristaltic pump to recirculate the water, and pure CO2 was flowed over the wetted surface for over an hour. The CO2 absorption rate was determined as a function of time for each trial by tracking the pH in the water collected at the bottom of the plate. Based on these

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experimental results, the plates with hydrophobic patches absorbed ~20% more CO2 than the standard plates. This result combined with the computational results indicate that perturbing the flow of solvent in a packed column using hydrophobic patches can increase the rate of CO2 absorption by up to 20%, resulting in a smaller absorption tower for a given power plant.

Figure 1: Instantaneous liquid distribution in alternating hydrophobic patterns (red zones) on a hydrophilic plate

Figure 2: Picture of K2CO3 solution flowing over an inclined glass plate with a hydrophobic pattern printed onto the surface. Acknowledgement This work was performed under the auspices of the U. S. Department of Energy by Lawrence Livermore National Laboratory under the contract DE-AC52-07NA27344.