Deposit Formation from Deoxygenated Hydrocarbons. 1. General Features William F. Taylor Esso Research and Engineering Company, Government Research Laboratory. Linden. New Jersey 07036
The study of the variables which control the rate of deposit formation from hydrocarbon jet fuels exposed to high-temperature stress is extended to the effect of the removal of dissolved molecular oxygen. Deposit formation rate measurements have been extended to higher temperatures (150-649°C) and pressures (18-69 atm) than previously employed in fuel stability studies. With most fuels, removal of molecular oxygen markedly lowered the rate of deposit formation. However, the poorest quality fuel tested did not exhibit lower deposit formation rates with deoxygenation. With the majority of fuels the greatest reduction in deposit formation rate with deoxygenation occured in the liquid phase. Pressure exerted a complex effect on deposit formation.
Introduction The deposit formation tendencies of jet fuel range hydrocarbons has been the subject of considerable interest (Nixon, 1962). Initially, studies were carried out with airsaturated hydrocarbons in a narrow, near ambient temperature range in order to investigate storage stability characteristics. Subsequently, studies were extended to higher temperatures in order to investigate the stability of such fuels when used in high-speed supersonic aircraft. The majority of such studies were carried out with fuels saturated with molecular oxygen uia exposure to air although some limited work has been reported with reduced oxygen containing fuels (Nixon and Henderson, 1966; Taylor and Wallace, 1967). This laboratory completed an extended study of the variables which control the kinetics of the deposit formation process in air-saturated jet fuels a t temperatures up to 250°C at reduced pressures (Taylor, 1968a, 1968b, 1969a, 1969b; Taylor and Wallace, 1967, 1968). Complementary studies were carried out using tetralin as a model jet fuel range hydrocarbon (Taylor, 1970a, 1970b, 1972). We are now extending our study of the kinetics of deposit formation to deoxygenated jet fuel range hydrocarbons ( i e . , fuel in which the molecular oxygen content has been drastically reduced). The range of conditions investigated has also been extended to include temperatures up to 649°C and pressures up to 69 atm. This paper discusses the effect of variables such as fuel type, temperature, pressure, and molecular oxygen content on the rate of deposit formation from jet fuel range hydrocarbons over this range of conditions. Subsequent work will discuss the effect of factors such as the presence of trace impurity sulfur, nitrogen, and oxygen compounds on deposit formation in deoxygenated fuels. Experimental Section Apparatus. The Advanced Kinetic Unit used to measure the rate of deposit formation is shown in a schematic drawing in Figure 1. The molecular oxygen content of the fuel to be tested is adjusted in a fuel treatment vessel by sparging the fuel at atmospheric pressure using either helium or air. Following this, the treated fuel is passed through an oxygen sensor cell and delivered to a double piston fuel delivery cylinder. The oxygen sensor cell contains a polarographic sensor and the oxygen content of the total fuel is monitored by a Beckman Model 778 oxygen analyzer as it passes through the cell. Oxygen analyses were also made on selected samples using a Model 154 Perkin-Elmer thermal conductivity gas chromatographic analyzer using a molecular sieve column preceded by a guard chamber to remove the hydrocarbon portion. The
fuel delivery cyclinder is a chrome plated hydraulic piston accumulator purchased from Liquidonics Inc., Westbury, N. Y. The fuel is delivered to the unit by means of highpressure nitrogen. The treated fuel is separated from the nitrogen drive gas by use of two individual pistons, separated by a small water layer which is employed to detect possible leaks. Each sliding piston contains two Teflon O-ring seals. The fuel then passes through a heated tubular reactor section consisting of yd-in. 0.d. x 0.083-in. wall stainless steel type 304 tube which is contained inside of four individually controlled heaters. The tube itself is contained inside of a thick-walled pipe, which has slots machined out so as to hold a series of sheathed thermocouples firmly against the outside wall of the tube. Each heater zone is approximately 12 in. in length and contains both a control thermocouple positioned at the zone midpoint and a movable read-out thermocouple. Bulk fuel temperature is measured at the exit. Each zone heater is controlled by a proportional temperature controller (West Instrument Corp., digital set point unit model JYSCR). After the fuel leaves the heated reactor sector it is cooled and then passes into a high-pressure receiver where it is kept under nitrogen pressure. Unit pressure is controlled by means of a MITY-MITE type of pressure controller releasing to vent, The rate of deposit formation is measured after a 4-hr run. First the reactor tube is cut into 16 sections, each 3 in. long (four sections per reaction zone). The tube sections are analyzed for carbonaceous deposits using a modified LECO low carbon analyzer system (Model 734-400) obtained from Laboratory Equipment Corp., St. Joseph Mich. The tube specimens are placed inside a quartz tube contained in a laboratory tube furnace where the deposits are allowed to react with oxygen. The effluent is passed through a catalytic converter which reacts any carbon monoxide to carbon dioxide, and this stream is then passed into the LECO analyzer where the COz is automatically trapped and finally delivered to a thermal conductivity cell. The output is integrated and recorded on a digital output meter as micrograms of carbon. The analytical system was calibrated against known standards. The deposit formation rate is obtained by dividing the net carbon production per section by the corresponding inner surface area and expressed as micrograms of carbon per square centimeter per 4-hr reaction time. Reagents. Because of the wide possible variation in jet fuel composition, six fuels were chosen to represent a spectrum of stability levels. Inspections on these fuels are shown in Table I. The existent gum, potential gum, and peroxide number inspections are all current results. The Ind. Eng. Chem., Prod. Res. Develop., Vol. 13,No. 2, 1974
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Table I. Inspections of Hydrocarbon Jet Fuels Fuel A
B
C
E
D
F
ASTM distillation, "C 1.b.p. 10%
20 % 50 % 70 % 90 % F.b.p. Total sulfur, ppm Mercaptan sulfur, ppm Aromatics, vol % Existent gum, mg/100 ml Potential gum, mg/100 ml Peroxide number, mequiv/l. Other identification(s)
169 197 202 214 229 238 254 234
