KINETICS OF DEPOSIT FORMATION FROM HYDROCARBON FUELS A T HIGH TEMPERATURES General Features of the Process W I L L I A M
F. T A Y L O R A N D T H O M A S J . W A L L A C E
Government Research Laboratory, Esso Research and Engineering Co., Linden, .V, J . 07036
Initial results are reported from studies designed to elucidate the variables which control the rate of deposit formation from hydrocarbon jet fuels exposed to high temperature stress. The exclusion of oxygen suppresses deposit formation. Deposits are formed much more rapidly when a surface is wetted with a condensed fuel phase. Various jet fuels differ in their base reactivity at lower temperatures and their ability to maintain the deposit formation process at sequentially higher temperatures. The over-all deposit formation process exhibits an apparent activation energy of 10 kcal. per mole and a 0.2-order dependence on oxygen partial pressure at moderate oxygen partial pressure levels. Trace levels of sulfur compounds also influence the deposit formation process. The deposits formed are nonvolatile and insoluble in jet fuel hydrocarbons and are undoubtedly formed as a result of a complex free radical autoxidation process.
I
high speed supersonic aircraft, aerodynamic heating causes metal skin temperatures to rise considerably above those encountered in subsonic aircraft. I t has been estimated for a plane such as the Supersonic Transport (Mach 2.7) that the temperature of exterior surfaces can rise to the 450' to 500' F. range, and the temperature of an uninsulated fuel tank could rise to 430' F. (Chemical Week, 1967). Other studies have shown that hydrocarbon jet fuels exposed to such high temperature stress can degrade and form deposits (Churchill et al., 1966). One particular problem area is the formation of deposits in fuel wing tanks which contain puddles of residual liquid hydrocarbon and hydrocarbon vapors. Such deposits may flake off and contaminate the bulk fuel and cause malN A
PROBE REACTOR
CONTR VALVE
functions in the fuel system components. This paper report the initial results of a study designed to elucidate the variables that control the rate of deposit formation from hydrocarbon jet fuels a t the high temperatures potentially present in the wing tank of a supersonic aircraft. Experimental
Apparatus. A schematic drawing of the phase study unit, which was used to study the effect of phase type on the rate of deposit formation, is shown in Figure 1. The main section consists of a heated reactor section, A , which contains a cylindrical glass reactor 5.7 cm. in inside diameter and 74 cm. in length. Inside the reactor sections is a glass probe reactor finger, B, 3.8 cm. in outside diameter and 68 cm. in length. The temperature of the vapor space surrounding the probe
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THEATER HEAT T R A N S F E R FLUID B A T H
/ L I N E WOUND W I T H T A P E HEATERS
Figure 1. 258
Schematic flow plan of phase study unit
l&EC PRODUCT RESEARCH A N D DEVELOPMENT
reactor is controlled by a jacket heater, C. T h e temperature of the probe reactor finger is regulated independently by a heat transfer medium (Dow Corning 210H fluid), which is heated to a controlled temperature in an external heater and circulated within the probe reactor finger. T h e temperature of the probe finger is measured by means of five equally spaced iron-constantan thermocouples contained in a glass thermowell positioned against the inner wall of the probe finger and extending its entire length. T h e jet fuel was fed to the unit a t the rate of 50 cc. per hour and the gas flow (Nz or air) a t the rate of 200 cc. per minute (STP). I n the absence of oxygen, the jet fuel was de-oxygenated in situ with a constant stream of Nzfor 48 hours prior to admission to the reactor. T h e jet fuel was vaporized in a preheater, mixed with the air (or SZ), and then passed upflow through the reactor section. T h e effluent jet fuel product is collected in a trap system. T h e discharge side of the reactor is connected to a vacuum pump. I n runs carried out in the presence of a metal surface, cylindrical sleeves of 304 stainless steel (10 cm. in length) were tightly fitted over the probe finger reactor. T h e kinetic unit used to measure the rate of deposit formation is shown in a schematic drawing in Figure 2. T h e main section of the unit consists of a glass tubular reactor 3.8 cm. in diameter and 100 cm. in length. T h e reactor volume is approximately 1130 cc. T h e unit has five separate reactor heaters. each independently regulated by its own Gardsman temperature control. T h e reactor is maintained on a slight incline and liquid fuel flows down the reactor in the presence of a constant stream of air. T h e liquid fuel flow, rate is fixed at 125 cc. per hour and the gas flow is fixed a t 5 liters per minute. T h e total run time employed is 4 hours. T h e reactor heaters are controlled so that the fuel encounters a sequence of rising temperature zones as it flows down the reactor. Carefully weighed metal strips approximately 1.0 cm. wide by 10 cm. long are positioned in the center section of each heater zone. T h e average residence time of the flowing hydrocarbon (in the absence of loss by vaporization) is 14 seconds per strip, the time it takes the hydrocarbon stream to flow the length of a single strip. This was measured a t ambient temperature and pressure by measuring the time it took a n actual jet fuel to travel the length of the reactor. A thermowell extends down the length of the tubular reactor, and a n individual thermocouple is positioned in each zone. T h e reactor heaters are separated by a small distance, so that each reactor sector can be inspected visually; a t other times this zone is wrapped with insulation tape to prevent excessive heat loss. T h e unit is designed so that it can be operated under a controlled, reduced pressure. T h e liquid fuel is presaturated with air or a n air-nitrogen mixture prior to admission to the reactor section. At the conclusion of a run, hydrocarbon fuel is shut off and a full vacuum is applied to the unit (,,
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C O TRAP -78’C.
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Figure 2. Schematic flow plan of the kinetic unit for the measurement of rate of deposit formation VOL. 6
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Table 1. Physical Inspections of Hydrocarbon Jet Fuels Fuel A B c D E
Distillation. ASTM.
Loss,
' F.
5
Recovery,
yG
Residual, 5 Total sulfur, p.p.m. wt. Mercaptan sulfur, p.p.m. wt. Total nitrogen, p.p.m. Basic nitrogen, p.p.m. Peroxide No., millequiv. of 0 2 per liter Additives added to fuel Paraffin, naphthene, aromatic distribution, wt. 4°C" Paraffins Pr'aphthenes (cycloparaffins) Noncondensed 2-ring condensed 3-ring condensed Total
Aromatics .Alkyl benzenes Indans Naphthalenes Total Grand total
334 356 374 394 419 454 484 99.0
...
1. o 760
5
