Fouling of Some Canadian Crude Oils

A thermal fouling study was undertaken using three sour Canadian crude oils. Experiments were carried out in a recirculation fouling loop, equipped with an annular (HTRI) electrically heated probe. Fluids at pressures of about 10001340 kPa under a nitrogen atmosphere were re-circulated at a velocity of 0.75 m/s for periods of 48 hours, and the decline in heat transfer coefficient followed under conditions of constant heat flux. Bulk temperatures were varied over the range 200-285°C, and initial surface temperatures from 300 to 380°C. Heat fluxes were in the range of 265-485 kW/m. Surface temperature effects on fouling of the three oils were compared, and fouling activation energies estimated. For the lightest oil, a more detailed study of velocity, and bulk and surface temperature effects was carried out. Fouling rate decreased slightly with increasing velocity. Fouling rate increased with both surface and bulk temperatures; a rough correlation was developed using a modified film temperature weighted more heavily on the surface temperature. Deposits showed high concentrations of sulphur and of minerals, indicating the importance of iron sulphide deposition. INTRODUCTION Fouling in crude oil pre-heat trains is costly in terms of energy, maintenance and lost production. Although crude oil fouling has been a subject of much study [1-3], there are few investigations of fouling under controlled conditions which give guidance on predicting fouling rates for different oils under non-coking conditions. Reported causes of fouling from crude oils include i) impurities such as water, rust and other particulates ii) gum or polymeric species formed through oxidation of reactive species in the oils iii) insoluble asphaltenes from self-incompatible oils or from blending iv) iron sulphide formation v) coke formation due to reactions of polar fractions These factors (i) to (v) become progressively more important as the oil temperature is raisedi.e., factor (i) can predominate at lower temperatures in the preheat train, whereas factors (iv) and (v) become more important near the furnace inlet temperature. This research focused on three Canadian crude oils, all of which have significant sulphur content. For two of the oils, only a few results are reported, whereas the other oil was subject to more extensive study. MATERIALS Properties of the three oils are listed in Table 1. Cold Lake crude arises from heavy oil production, and is high in C7 asphaltene content (8.6 %), and is substantially higher in viscosity, and in organic sulphur content (3.7 %) than the other two oils. Midale, has properties intermediate between those of Cold Lake and Light Sour Blend, with asphaltenes of 5%, and sulphur of 2.5%. Light Sour Blend has a viscosity a factor of 13 below that of Cold Lake, asphaltenes of 2 %, and a sulphur content of 1.3%. Oils were provided by Shell Canada Ltd. The oils were characterized [4] using the Automated Flocculation Titrimeter (AFT), to determine the insolubility number (IN) and the solubility blending number SBN [5]. The results are listed below in Table 2. Viscosity at the film temperature was estimated using several methods [6-8], and the value from [8] which was close to the average prediction was used. APPARATUS A fouling loop was constructed for this research which could operate at pressures to 2 MPa, and at bulk temperatures of up to 285 °C. The feed tank (Fig. 1) has a capacity of 7.5-L of oil, and is equipped with external band heaters. 1 Srinivasan and Watkinson: Fouling of Canadian Crude Oils Published by ECI Digital Archives, 2003 Table 1 Bench mark crude analytical data. (as supplied by Shell Canada Limited) S.No Analysis CLK MDL LSB