FOULING IN HIGH SOLID CONTENT SUSPENSION – EFFECT OF NUCLEATING AIR AND BACKGROUND ELECTROLYTE

Particulate depositions formed on process surfaces of e.g. heat exchangers is a common problem in industry. Deposited material causes economical losses due to increased energy consumption, maintenance and cleaning. The particle deposition is often considered to include transport to the surface and adhesion on the surface dominated by surface forces such as DLVO interactions in aqueous systems. The fouling consists of complex phenomena that are not easy to distinguish experimentally. In this work, the effect of nucleation of dissolved air and the effect of background electrolyte on fouling are studied experimentally with slurries containing 6–50 wt-% of calcite (CaCO3) particles, the primary size being about 0.8 μm. The experiments are performed in a laboratory set-up having a rectangular flow channel with heated walls. The bulk fluid temperature, heat flux, flow rate and solid content are varied in the experiments. Fouling is found to be induced by nucleating air at temperatures below boiling temperatures. The nucleation of air is eliminated by increasing the pressure in further experiments. The background electrolyte (MgCl2) is found to have a strong effect on the dispersion properties and the fouling rate. INTRODUCTION Particle deposition mechanisms consisting of particle transport to the wall and particle adhesion on the wall have been widely studied (Epstein, 1997; Turner, 1993). It has been observed that the particle deposition rate has a maximum when particle charge is in a minimum or near zero (Hermansson, 1977) and a maximum may occur even when the particle and surface are oppositely charged (Williamsson et al., 1988). Adomeit and Renz (1996) introduced a method for particle transport and adhesion combined to a numerical model and further developed model accounting for hydrodynamically induced forces. For particle deposition, only a few studies have been done under boiling conditions (Turner and Klimas, 2000; Wen and Melendres, 1998; Bramson, 1995). Even fewer have been conducted in conditions where the nucleation of dissolved air has an effect on fouling (Yang et al., 2000). In some cases, the stability of the dispersed fluid is disturbed and undesirable fouling occurs. In this study, a suspension containing ground particles of calcium carbonate is used in order to study particle deposition on a heated wall and how nucleating air and added background electrolyte have a marked effect on the mass deposition rate. The DLVO theory describes the force between charged surfaces interacting through a liquid medium. It combines the effects of the van der Waals attraction and the electrostatic repulsion due to the so called double layer of counterions. The theory is named after Derjaguin and Landau, Verwey and Overbeek. The XDLVO theory is an extension with the Lewis Acid-Base interactions that have marked effect on surfaces interacting in aqueous medium (van Oss, 1994). XDLVO theory was used in determining the forces affecting the particle, from which mass transfer coefficient for the particle was calculated according to methods of Spielman and Friedlander (1974) and Elimelech (1995). The CFD model (Ojaniemi et al. 2008) utilizing the mass transfer coefficients formed, is being validated using the experimental data of this study where background electrolyte was used and nucleation of air was eliminated. EXPERIMENTAL SET-UP AND PROCEDURE Experimental set-up As crystals or particles deposit on the heat transfer surface, the overall heat transfer coefficient decreases. When the heat flux and wall temperature are constant, deposition of a resistive layer on the wall can be measured from the elevation of temperature by the thermocouple situated in the joint between heating block and a test material sheet forming a wall to the flow channel. A schematic representation of the experimental set-up is given in Fig. 1. The construction material of the test setup is AISI 316L. The set-up consists of a vessel of 150 dm Proceedings of International Conference on Heat Exchanger Fouling and Cleaning VIII 2009 (Peer-reviewed) June 14-19, 2009, Schladming, Austria Editors: H. Müller-Steinhagen, M.R. Malayeri and A.P. Watkinson