Wax deposition from kerosene onto cooled surfaces
Creators: name: Ghedamu, Michael Abraha
Publisher: name: University of British Columbia
Year: 2009

description: Wax fouling is a major problem in some oil refineries. The main objective of this project was to test different surfaces with the aim of eliminating or at least reducing wax deposits in heat exchangers. Wax is separated in oil refineries by cooling the wax-laden petroleum stream in chillers and then scraping off the deposited wax mechanically from the surfaces of the heat exchangers (chillers). The solid wax is separated from the liquid petroleum stream by means of filters. An experimental test rig was set up to study ways of eliminating or reducing wax deposits by changing some of the operating conditions as well as the surface type of the heat transfer area. A double pipe heat exchanger 0.72 m long with inner tube (ID=9.96 mm, OD=12.45 mm) and outer pipe (ID=25.4 mm) was used. The solution tested was wax dissolved in kerosene, which flowed through the annular section while the cooling water flowed countercurrently in the inner tube. The effects of flow velocity of waxkerosene, of bulk temperature, of wax-kerosene concentration and of heat transfer surface type have been studied. Two types of wax were used: refined wax and slack wax. The surfaces used were: uncoated stainless steel, sand-blasted stainless steel, chrome-plated stainless steel, n-C18 silane-coated chrome-plated stainless steel, Heresite Si 57 E coated stainless steel (shiny), Heresite P-400/L-66 coated stainless steel (dull) and n-C18 silane coated stainless steel. The cloud point for each wax-kerosene concentration investigated (5, 10, 15 and 20 wt. % wax) was measured using ASTM procedures. The rheology of wax-kerosene was also investigated to determine if the mixtures were Newtonian or non-Newtonian. All mixtures were found to be Newtonian. The mixture viscosity was determined at temperatures from the cloud point upwards at each concentration. A Kern-Seaton (1959) equation was used to determine R*[sub f] and Θ[sub c] from the resistance vs. time experimental data. The wax deposit showed a decrease in R*[sub f] with increasing Re, with increasing T[sub b] and with decreasing concentration. Similar results were found by Bott and Gudmundsson (1977b). From the plots of R*[sub f] vs. Re, the hierarchy in increasing R*[sub f] was found to be: Heresite-coated stainless steel (dull and shiny) &lt; n-C18 silane coated stainless steel &lt; n-C18 silane-coated chrome-plated stainless steel &lt; chrome-plated stainless steel &lt; uncoated stainless steel &lt; sand-blasted stainless steel. A similar hierarchy with four of the seven tubes was shown with respect to R*[sub f] vs. T[sub b]. That plastics show a lower wax deposit compared to metal surfaces has been shown by previous investigations. After some deposition had occurred, the removal of wax chunks from the surface and occasional bare patches were visually observed on all tubes except the two Heresite-coated tubes and the sand-blasted stainless steel. The phenomenon of deposit sliding was observed on the chrome-plated stainless steel, where the sliding velocity was recorded. The concentration and bulk temperature of a petroleum stream may be fixed by refinery conditions. However, a lower wax deposit on heat transfer surfaces can be obtained by using a smooth surface material which has a low affinity for wax, and high flow velocity or turbulence.