Analysis of a high temperature fouling unit for heavy hydrocarbon fractions
Creators: name: Simard, Martial
Publisher: name: University of British Columbia
Year: 2009

description: With the depletion and increase in the price of light crude oil, the conversion of heavy oils and bitumens into distillable fractions in upgrading units has become an important source in meeting the demand for fuels and petrochemical products. Fouling, or the deposition of any undesirable material on heat exchangers surfaces, is a costly operational problem in conventional refineries because it increases the resistance to heat transmission and flow. The precipitation problem is worse with heavy petroleum streams because of the higher concentration of fouling precursors such as asphaltenes and polar heteroatomic species and the higher temperatures of processing required to convert the high molecular weight components. At temperatures above 300°C, heavy oil streams undergo thermal decomposition or free-radical reactions which lead to the formation of coke defined as toluene insoluble carbonaceous solid which fouls processing units. In order to design and operate upgrading units with minimum coke deposition, the chemical, thermal, and fluid mechanical factors causing the problem must be known. A research project which involved kinetic and thermal fouling studies on pitch, gas oils, and their blends was initiated to generate such information. As part of this research, a recirculation flow loop had been constructed to study coke deposition during flow through a vertical tube placed in an electrically heated fluidized bed. The present work was conducted to develop the apparatus, to analyze its behaviour, to evaluate the tendency of pitch-gas oils blends to form coke and to assess the capability of the unit to detect it under the appropriate conditions of bulk and surface temperatures. A series of fouling experiments was carried out by recirculating a 50:50% vol. pitch-coker heavy gas oil blend over 11-56 hour periods with average bulk fluid temperatures of 200-375°C, tube side velocities of 0.3-2.2 m/s (laminar flow), and average fluid bed temperatures in the range of 500-615° C. A number of improvements to the high-temperature unit were made to reach the desired temperature conditions, to provide the necessary measurements for adequate interpretation of results, and to increase the quality of the data. In order to determine the flow regime and to account for possible viscosity changes during recirculation, the density of the above mixture and the viscosity of pitch, gas oils, and their blends were measured over a wide range of temperatures. Additional tests were performed to monitor the viscosity change during recirculation of the test fluid and also the change in the amount of toluene insolubles in the 50:50% vol. pitch-heavy gas oil blend. Coke deposition was determined by mass deposition and by thermal measurements. Interpretation of the latter was made by use of different data analysis techniques and of empirical correlations found between process variables. Moreover, in order to see whether the absence of fouling observed was due to the fouling unit and/or to the nature of the test fluid, a blend of de-asphalted oil known to give measurable fouling rates in turbulent flow and in similar periods of time as for the coking experiments was studied. Finally, measurements of heat transfer coefficients were made in order to estimate the sensitivity of the unit to detect fouling by thermal measurements. Correlations were developed over a wide temperature range for the viscosity (80- 310°C) and density (60-145°C) prediction of any blend of pitch and gas oil. As a result, the flow regime could be determined and the viscosity drop observed in some of the runs revealed a possible bias effect on the mass flowrate measurement. Moreover, the use of different data analysis techniques and empirical equations found between process variables confirmed the liquid flowrate variations as the reason for the observed changes in the thermal resistance for the runs with the 50:50% vol. pitch-gas oil blend. The liquid flowrate variations were essentially eliminated by modifying the configuration of the bypass circuit. Also, the thermal resistance of the fluid bed was found to be as high as 38% of the total resistance, which substantially reduced the sensitivity of the unit to detect coke deposition. Finally, no significant fouling was observed both from the thermal and mass deposition measurements with the 50:50% vol. pitch-gas oil blend over the range of conditions mentioned above. As for the experiments involving a blend of de-asphalted oil, the evidence obtained was judged insufficient to draw a definite conclusion as whether sufficient fouling actually took place.