(590b) Numerical and Experimental Investigation of Liquid Film Flows On Packings in Absorbers for Post-Combustion CO2 Capture

Global warming and greenhouse gas emission reduction have become a major issue in the world. It is necessary to develop Carbon Capture and Storage (CCS) technology for coal fired power plants in order to reduce greenhouse gas emission. According to these backgrounds, Post-Combustion CO₂ Capture (PCC) technology has the great potential as the economical and efficient technology for reducing CO₂ emissions, which can be applied into both retro-fit and new construction of coal fired power plants and can control the CO₂ capture rate.

Gas-liquid interfacial flows, such as the flue gas and liquid solvent, are applied in CO₂ absorbers for PCC in order to capture CO₂ from the flue gas into liquid solvent. Efficient control of these liquid solvent flows by using packing elements in packed columns is important to increase the gas-liquid interfacial area and the mass transfer rate between the gas and the liquid. In typical packed columns, liquid is fed from the top of the packing element, and then the liquid film flow is formed on packing element surfaces. Simultaneously, gas is fed from the bottom of the packing element and makes contact counter-currently flows with the liquid film. There are two types of packing elements: structured packing and random packing. Structured packing elements can increase the gas-liquid interfacial area efficiency and the throughput of gas flow capacity more than random packing elements.

Control of liquid film flows by using packing elements is one of the key design factors in packed columns. In particular, the channeling flow of liquid significantly reduces the gas-liquid interfacial area. To prevent this phenomenon, it is very important to predict the detailed behavior of liquid film flows for the design and development of packing elements. Therefore, the present study focuses on gas-liquid interfacial flows on an inclined wall, which is the model simplified from typical structured packing elements in absorbers for PCC.

There have previously been numerous theoretical, numerical and experimental studies on liquid film flows. These previous efforts have produced useful findings such as the liquid film shape and thickness, liquid hold-up (the amount of liquid held by a packing element), gas pressure drops, validation of Computational Fluid Dynamics (CFD) etc. However, most of these studies concerned liquid film flows on smooth wall surfaces, but the effects of wall surface texture treatments on liquid film flows have not yet been clarified. Furthermore, detailed descriptions of the transition phenomena between the film flow and the rivulet flow, as well as how such phenomena are affected by wall surface texture treatments, are still lacking.

In what follows, this study develops a three-dimensional numerical simulation technique using CFD with the Volume of Fluid (VOF) model as well as a lab-scale experimental testing technique. We investigate that the effects of wall surface texture treatments on the interfacial flow through the comparison of two geometry cases (smooth wall and wavy wall). Furthermore, our fundamental technics are applied to develop our advanced design of surface texture treatments and packing geometry. The absorption column tests using the CO₂-NaOH system is carried out to measure the performance of the absorption rate and the gas pressure drop. Our new findings in this paper include the followings.

(1) With a typical texture geometry used in a commercial industrial packing element, the present numerical and experimental results comparing the two geometry cases (smooth wall and wavy wall) show that the surface texture treatments can help to prevent liquid channeling and can increase the wetted area quantitatively. The main reason for the increase of the wetted area on the wavy wall is that the liquid film break-up is inhibited due to the spreading of the liquid flow in spanwise direction by the wavy wall geometry.

(2) The numerical simulation results, on both of smooth and wavy walls, agree well with the experimental results at points such as the gas-liquid interfacial surface shape and the wetted area. These validations demonstrate that the present simulation, which uses the VOF model, is capable of predicting gas-liquid interfacial flows on an inclined plate with a high accuracy in a reasonable calculation time period.

(3) Our advanced design of surface texture treatments and packing geometry is developed by using present numerical and experimental techniques. The absorption column tests using the CO₂-NaOH system show that our advanced packings trail-manufactured with two types of specified surface areas (300 m2/m3 and 200 m2/m3) have higher absorption performance and lower gas pressure drop by comparing to several conventional structured packings.

In an ongoing project, IHI has constructed a post-combustion capture pilot plant at its Aioi Works (capacity: 20 tons of CO₂/day). This pilot plant can be operated in combination with the co-located coal combustion test facility, which makes it possible to carry out operational evaluations using actual flue gas from coal-fired boiler. Sequential test operations were started at this pilot plant from June, 2012. On the next step, our advanced packing systems will be evaluated by using this pilot plant in 2013.