Influence of Viscosity on Liquid Flow Inside Structured Packings Christian Bradtmö ller, † Anna Janzen, ‡ Michel Crine, § Dominique Toye, § Eugeny Kenig, ‡ and Stephan Scholl* ,† † Technische Universitä t Braunschweig, Institute for Chemical and Thermal Process Engineering, Langer Kamp 7, D-38106 Braunschweig, Germany ‡ University of Paderborn, Chair of Fluid Process Engineering, D-33098 Paderborn, Germany § Universite ́ de Lie ̀ ge, Laboratoire de Ge ́ nie Chimique, Allé e de la chimie, B6C, Sart Tilman, 4000 Lie ̀ ge, Belgium ABSTRACT: In this study, X-ray computer tomography and light-induced fluorescence were applied to investigate the morphology of liquid flow inside structured packings. Fluid dynamic parameters such as liquid holdup and wetted surface were determined to study the effect of the variation of viscosity and liquid load. Flow patterns inside the packing were identified and categorized. Liquid film thickness and its distribution were analyzed on single sheets. For both methods, the measured holdup values are in good agreement, despite differences in the techniques of measurement. For the flow patterns and their relative contribution, as well as mean liquid film thickness, a strong dependency on the varied parameters was found. Furthermore, the density function of film thickness distribution changed characteristically with liquid load and viscosity. The complementary use of tomography and optical assessment allowed an improved insight into flow phenomena and the observed interdependency of physical, geometric, and operational parameters. 1. INTRODUCTION Corrugated-sheet structured packings are frequently used as column internals in distillation and absorption processes. They promote a better flow distribution and provide a high gas- liquid interfacial area, resulting in both a low pressure drop and high separation efficiency. Physical properties of the liquid phase strongly influence fluid dynamics and performance characteristics of packed columns. Rising liquid viscosity is known to increase liquid holdup and decrease the capacity, as well as separation efficiency, of structured packings. 1,2 Hence, it is necessary to understand the influence of liquid viscosity on flow phenomena inside structured packings to accurately describe such dependencies in the modeling of separation processes. X-ray computer tomography (XCT) has been shown to act as an efficient, noninvasive method to adequately display liquid distribution in packed columns. 3-10 However, it can deliver only time-averaged cross-sectional images of the irrigated packing; hence, the dynamics of two-phase fluid flow cannot be captured with this method. Moreover, water and air are usually used as working fluids in XCT studies. Hence, the applied liquid has a rather low viscosity and deviates significantly from the surface tension of organic solvents. Sidi-Boumedine and Raynal 11 investigated the influence of liquid viscosity of aqueous polymer solutions in the range of 1-20 mPa s using XCT, but in a co-current gas-liquid flow in a trickle bed reactor. Alternatively, flow patterns inside structured packings may be investigated through the liquid flow on inclined planes and single sheets. Stoter 12 studied the distribution of liquid between two sheets, thus having no optical access, while other authors 13-15 studied rivulet and film flow on inclined plates. They showed that rising viscosity can enhance spreading and, hence, influences the mass-transfer area. In most of these studies, plane and smooth surfaces were investigated. This represents a strong simplification as the metal sheets of structured packings are corrugated and often have a structured surface (texture). The latter effect was considered by Nicolaiewsky, 14 who also studied embossed surfaces and showed that higher viscosity can cause an increase of film thickness and a decrease in the wetted surface. In contrast, Ataki 16 conducted experiments with the corrugated surface of the structured packing Rombopak and observed an improve- ment of wetting. Subramanian and Wozny 17 examined single sheets with the characteristic corrugation of structured packings and smooth and embossed surfaces. However, since different working fluids were used, the effect of viscosity cannot be assessed independently. Furthermore, the liquid was distributed from a single point source, resulting in a nonideal and uneven distribution perpendicular to the flow direction. Some recent studies focused on the improvement of mass transfer through modifications of structured packing. For the absorption of CO 2 in industrial-scale processes, the reduction of the water content in the used solutions yields a large potential for the reduction of energy consumption. However, reducing the water content will increase solution viscosity. Therefore, Hu 18 and Sun 19 studied the effect of macroscopic modifications of the geometry of structured packings in absorption experiments with single sheets. Hu et al. 18 performed absorption experiments for aqueous diethanolamine solutions and found that packing modification can increase mass transfer. Kohrt et al. 20 evaluated the impact of different textured surfaces on the liquid-side mass transfer coefficient. They found a Received: May 17, 2014 Revised: August 31, 2014 Accepted: December 22, 2014 Published: December 22, 2014 Article pubs.acs.org/IECR © 2014 American Chemical Society 2803 DOI: 10.1021/ie502015y Ind. Eng. Chem. Res. 2015, 54, 2803-2815