STUDIES OF PARAFFIN WAX DEPOSITION ON COATED AND NON-COATED STEEL SURFACES

A novel cold baffle wax deposition (CoBWaD) stirred reactor is developed for studying deposition from wax rich hydrocarbon liquids. The reactor consists of a heated jacket to maintain the bulk temperature of the test fluid, and four PVC baffles positioned perpendicular to the cylindrical vessel wall. The baffles have a section with an exchangeable cooled surface. Fouling may thus be studied at different solid surfaces, under variable heat transfer regimes. In addition, due to the wide range of impeller rotational velocities, flow regimes ranging from laminar to highly turbulent are accessible. The apparatus is designed to enable easy and low-cost assessment of the effect of surface coatings, on paraffin wax fouling of cold surfaces. Fouling results and observations are reported for a binary test-fluid consisting of n-decane (nC10) and ntetracosane (nC24), on non-coated and coated steel surfaces, for different flow conditions and exposure times. Parameters affecting wax deposition in the CoBWaD apparatus were coating thickness (wall temperature/heat transfer), flow velocity, and surface properties. Effects of temperature, pressure or test-fluid composition were not investigated. Evidence is presented that the deposition rate-flow velocity relationship is non-monotonous, and indications are given that the wall heat flux is an important factor in wax deposition. It was also seen that one coating gave an efficient fouling protection while another promoted wax deposition. A third coating, tested at different thicknesses, demonstrated that the insulation effect of the coating layer may be more important than the surface properties. INTRODUCTION Petroleum wax crystals and precipitates in crude oil predominantly consist of n-paraffinic components. Wax crystallizes when changes in pressure and/or temperature result in super saturation. Decreased pressure will cause loss of light-end components, which act as natural solvents for the waxes, and decreased temperature gives a decreased solubility of wax molecules in the oil. Paraffin wax fouling of oil production pipelines is a costly and common problem in the petroleum industry. The wax crystallization poses three main problems in the oil industry, higher viscosity, higher yield stress for restarts after shut-in periods, and fouling of solid surfaces such as tubing, pipelines, tanks, process equipment and sucker rods. In this paper, the focus is on wax fouling of cold surfaces due to decreased liquid temperatures close to the wall. Subsea gas and oil production pipelines may typically be exposed to water temperatures below the wax appearance temperature (WAT), giving crystal and agglomerate formation of wax particles in the bulk and potentially a wax deposition problem. The WAT is defined as the temperature at which wax precipitation takes place and the oil becomes cloudy. Several methods for establishing the WAT exist, (Lira-Galeana and Hammami, 2000), e.g. the standardized ASTM D2500 for petroleum oils and ASTM D3117 for distillate fuels, the cold finger method, viscometry, calorimetry, microscopy, and others. The pour point (PP) is defined as the temperature at which the oil ceases to flow. In Figure 1 a schematic of the temperature profile of a waxy oil-cold wall system is shown; the temperature steadily increases from the cold side of the wall to the bulk liquid; a solid, but porous, wax layer forms at the cold wall, and the solid wax layer surface is at the pour-point temperature; between the PP and the wax appearance temperature there is a mushy zone of gel-like properties; Figure 1. The temperature profile, from the coolant, through the steel wall, through a wax deposit layer, and through the thermal boundary layer of the test fluid. Close to the solid wax layer there is a mushy zone where the temperature is higher than the pour point, but lower than the wax appearance temperature. Proceedings of International Conference on Heat Exchanger Fouling and Cleaning 2011 (Peer-reviewed) June 05 10, 2011, Crete Island, Greece Editors: M.R. Malayeri, A.P. Watkinson and H. Müller-Steinhagen