Heat exchanger fouling by petroleum asphaltenes
Creators: name: Asomaning, Samuel
Publisher: name: University of British Columbia
Year: 2009

description: Fouling during petroleum processing is a well documented problem. The increased use of heavy crude oils, which are rich in asphaltenes, to meet increasing energy demands has brought the severity of the problem to the fore. Petroleum asphaltenes are generally held as the main precursors of fouling by heavy crude oils. Asphaltenes are higher molecular weight entities defined as the pentane insoluble-benzene soluble fraction of heavy crude oil. Cold Lake heavy oil with a 16 wt.%. asphaltene content was used to study the mechanism of fouling by petroleum asphaltenes in various carrier solvents - a fuel oil, a hydrotreated aliphatic solvent (paraflex), and mixtures of fuel oil and xylene or pentane - on a model heat transfer surface. A solution of 10 wt.%. heavy oil in a carrier solvent was circulated around the fouling loop, with an HTRI annular probe as the heat transfer surface, over 48 hour periods. The probe was electrically heated. Surface and bulk temperatures as well as electrical measurements were made. The thermal fouling resistances and initial fouling rates were determined from these temperature and power measurements. Heat fluxes of 91-332 kW/m² were used and surface temperatures at time zero ranged from 160 to 245°C. Except for runs designed to study bulk temperature effects, a constant bulk temperature of 85°C was used. Surface and bulk temperature, solvent type, heavy oil concentration, flow and heteroatom compound effects have been established. Deposit formation increased with increasing surface temperature. For the runs under nitrogen atmosphere with 10 % heavy oil in fuel oil, the initial fouling rate almost doubled for every 15°C increase in surface temperature and an apparent activation energy of 89 kJ/mol. was determined. At a constant surface temperature, the thermal fouling rate and final fouling resistance decreased with increasing bulk velocity. Fouling was found to have an unusual dependence on the concentration of heavy oil via the fluid viscosity and surface temperature. Fouling was dependent on the aromatic composition of the carrier solvent. Fouling rates determined for the 10 wt%. heavy oil in fuel oil were slightly lower than corresponding experiments with paraflex as carrier fluid. Addition of xylene to the fuel oil carrier solvent resulted in a decrease in the test fluid viscosity and a decrease in fouling rates at a fixed bulk velocity. At 50 wt%. xylene in carrier solvent, little or no measurable fouling was observed. Addition of small amounts of pentane to the heavy oil-fuel oil mixtures increased fouling markedly. In heavy oil-fuel mixtures at a constant initial surface temperature of 220°C, fouling was appreciable up to bulk temperatures of 85°C. Above 85°C and up to a bulk temperature of 140°C, the fouling propensity of the heavy oil dropped to almost zero. Within the narrow Reynolds number range of 1800 to 2600, fouling rate decreased with increasing bulk velocity. The heteroatom compounds tested did not result in increases in fouling at initial surface temperatures of 185 to 220°C. For thiophenol at 220°C, some inhibitory effects were observed. By contrast, saturation of the heavy oil-fuel oil mixture with air rather than nitrogen caused a dramatic increase in fouling. Isothermal batch heating experiments were employed to study changes in fluid composition with time. It was shown that solids of composition similar to asphaltenes were present in suspension in the heavy oil-fuel oil mixtures. The concentration decreased with increasing bulk temperature and with xylene addition, and increased with pentane addition. Chemical analysis confirmed that deposits from the fouling runs comprised asphaltene-like species. While surface temperature appears to play an important role in thermal fouling of asphaltenes, the results of the present study point to the solubility or incompatibility of asphaltenes as the likely mechanism for the deposit formation. Under conditions where asphaltenes are readily soluble, little or no deposit forms on the heat transfer surface irrespective of asphaltene concentration. Where precipitation of asphaltenes is favoured, fouling is severe. A colloidal instability index taken from the literature was found to correlate the fouling behaviour of heavy oil-fuel oil mixtures.