(142f) Numerical and Experimental Study On Liquid Film Flows On Packing Elements in Absorbers

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 the 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.

As such descriptions may contain clues for clarifying the mechanism of the channeling phenomenon, this study develops a three-dimensional numerical prediction technique using CFD with the Volume of Fluid (VOF) model as well as a lab-scale experimental testing technique. In this study, the effects of the change of the liquid flow rate and the wall surface texture treatments on the transition between the film flow and the rivulet flow are investigated. Our new findings in this paper include the followings.

(1) An interesting hysteresis phenomenon in the flow transition between the film flow and the rivulet flow on the inclined plate is discovered during the liquid flow rate (or Weber number) increases and then decreases. We think that the main reason for the hysteresis in the transition of the flow patterns is that the transition processes between the film flow and the rivulet flow are different during the liquid flow rate increases and then decreases, and such transitions depend strongly on the history of the liquid flow (the history of changes in interfacial surface shape). This finding about the transition phenomenon of flow patterns can be applied into other studies to grasp the details of transition processes as well as to facilitate the control of liquid flow rates and the development of packing elements for assuring the efficient gas-liquid interfacial area.

(2) The simulation results agree well with the experimental results at points such as the critical Weber number, which causes the transition between the film flow and the rivulet flow as well as the gas-liquid interfacial surface shape. 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) The present simulations show quantitatively that surface texture treatments can help to prevent the liquid channeling and can increase the wetted area through the comparison of two geometry cases (with and without the wavy surface texture).

On the next step, using this prediction technique, we will systematically investigate the effects of surface texture treatments of packing elements on the gas-liquid interfacial flow. We will also apply these numerical and experimental predictions into the gas-liquid interfacial flow on packing elements with more complicated geometries under similar conditions of the amine-based CO₂ capture process in order to develop the advance design of packing elements, which have the high absorption performance and the low gas pressure loss.