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the amount of the imbibed cornpitions and increased the intrinsic reactivity. However, the reactivity decreased abruptly in a high concentration of NaCl salts due to the limitation of the rate of ion exchange and the decrease of the degree of hydration of the active catalyst site. Acknowledgment We acknowledge the financial support of the National Science Council, Taiwan, Republic of China (Grant No. NSC 78-0402-E006-23). Literature Cited Helfferich, F. Equilibria. Zon Exchange; McGraw-Hill Book Co.: New York, 1962; Chapter 5, pp 152-156. Ohtani, N.; Regen, S. L. Influence of Aqueous Salt on Triphase Catalytic Activity. Macromolecules 1981,14, 1594-1595. Ohtani, N.; Wilkie, C. A.; Regen, S. L. Triphaw Catalysis. Influence of Percent Ring Substitution on Active Site Mobility, Macroenvironment, Microenvironment, and Efficiency. Macromolecules 1981,14,516-520. Regen, S. L. Triphase Catalysis. J. Am. Chem. SOC.1975, 97, 5956-5958. Regen, S. L. Triphase Catalysis, Applications to Organic Synthesis. J. Org. Chem. 1977,42,815-879. Regen, S. L. Triphase Catalysis. Angew. Chem., Znt. Ed. Engl. 1979, 18,421-429.
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Regen, S. L.; Besse, J. J. Liquid-Solid-Liquid Triphase Catalysis. Consideration of the Rate-Limiting Step, Role of Stirring, and Catalysts Efficiency for Simple Nucleophilic Displacement. J. Am. Chem. SOC. 1979,101,4059-4063. Regen, S. L.; Heh, J. C. K.; McLick, J. Triphase catalysis. Consideration of Catalyst and Experimental Conditions for Simple Nucleophilic Displacement Reactions. J. Org. Chem. 1979, 44, 1961-1964. Sherrington, D. C. Catalysis by Ion-Exchange Resins and Related Materiale. Polymer-Supported Reactions in Organic Synthesis; Wiley: New York, 1980, Chapter 3, pp 180-188. Wang, M. L.; Wu, H. S. Kinetic Study of the Substitution Reaction of Hexachlorocyclotriphosphazene with 2,2,2-Trifluoroethanolby Phase-Transfer Catalysis and Separation of the Distributed Products. Znd. Eng. Chem. Res. 1990a, 29,2131-2142. Wang, M. L.; Wu, H. S. Effects of Mass Transfer and Extraction of Quaternary Salts on a Substitution Reaction by Phase-Transfer Catalysis. J. Org. Chem. 1990b, 55, 2344-2350. Wang, M. L.; Yu, C. C. Effects of the Microporous Polymer-Supported Catalyst on the Allylation of 2,4-Dibromophenol. Dev. Chem. Eng. 1992, SST, 15 (l), 3-13. Wu, H. S. Study on the Displacement Reaction of Phosphazene with Trifluoroethanol by Phase Transfer Catalysis. Ph.D. Thesis, Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan, 1990. Received for review January 13, 1992 Revised manuscript received May 27, 1992 Accepted June 14, 1992
Optical Diagnostic Methods for the Study of Fuel Fouling Terence E.Parker,* Richard R. Foutter, and Wilson T.Rawlins Physical Sciences Znc., 20 New England Business Center, Andover, Massachusetts 01810
An experimental study of fuel fouling using optical measurement methods was performed. These measurements included absorption from 350 to 750 nm, scattering a t 514.5 nm, and fluorescence using a probe wavelength of 514.5 nm. Measurements were performed using a constant-temperature heating system which exited into an optical cell. Each of the measurements proved useful in monitoring changes in the test fuel, JP-4, a t test temperatures up to 775 K and pressures of 400 psig. Absorption measurements demonstrated both molecular changes in the fuel composition and a marked increase in the particulate present in the flow as a result of thermal stress. Scattering measurements indicated room temperature fuel to contain particulate with average diameters greater than 0.1 pm while thermally stressed fuel contained much larger concentrations of particulate with sizes below 0.06 pm. This work clearly illustrates the possibilities of using optical methods for monitoring the fuel fouling process. Introduction Hydrocarbon fuels, such as those used in aircraft propulsion systems, degrade when exposed to high-temperature environments and create surface deposits which can dramatically change both the heat-transfer characteristics and mechanical tolerances of critical assemblies. The surface deposition procew, known as fouling, is a poorly understood phenomenon dependent upon many parameters such as fuel composition, oxygen content, impurities such as metals and sulfur compounds, temperature, exp u r e time, surface composition,and surface morphology. The development of fuels and fuel systems for extreme temperature service in high-performance engines requires a better understanding of the fouling process. A logical step to develop this understanding is the application of optical diagnostics, developed for the study of combustion p r . 7 , to the fuel fouling problem. This paper reports lnitial results for absorption, scattering, and fluorescence measurements that have been used to monitor the changes in a thermally stressed fuel. The degradation of fuels in storage tanks and hightemperature systems has been an engineering problem for 0888-5885/92/2631-2243$03.00/0
many decades, and a wealth of literature is available on the subject. A brief review of some of this literature is included in the following paragraphs. In 1989, Fbquemore et al. proposed that a greater fundamental understanding of fuel fouling was necessary because of the increased fuel operating temperatures for future aircraft. This understanding would come in part by applying existing optical measurement techniques to the fuel fouling problem. In fact, an optical method was developed for monitoring fuel stability in diesel storage tanks as early as 1955 (Johnson et al,). An additional optical evaluation method for fuel fouling processes was performed by Bol'shakov and Litvinov (1968) which monitored particle formation at atmospheric pressure and temperatures up to 550 K. This work used the dependence of the angular scattering function to determine the particle diameter and then used attenuation through the optical path to determine the concentration of particulate. The fuel heating apparatus was a simple constant-volume heated flask. In general, as the fuel was heated both concentration and particle size increased. The fouling process has very notably been studied by Taylor and co-workers (Taylor and Wallace, 0 1992 American Chemical Society
2244 Ind. Eng. Chem. Res., Vol. 31, No. 9, 1992 318
odoy
Slainless Tube 7
Oplical Cell -\
Throttle
Temperalure Dump
Aulo Shulolt Sensor
Thermocouple Localions
Figure 1. Schematic diagram of the fuel-fouling apparatus.
1967,1968,1969;Taylor, 1974,1976). Results from these works include quantifying the role of oxygen in the deposition process and starting to define the importance of trace sulfur compounds for deposition processes. In addition, these works identified the existence of three temperature regimes for deposition with oxygen bearing fuels. Fuel composition effects were also examined by monitoring deposition with paraffins and paraffin/aromatic mixes. Other investigations have shown that deposition r a t a may be enhanced by the presence of deposits (Szetela et al., 1986). Optical absorption was used by Kendall and Mills (1986) to quantify the role of trace species, iron, and copper. These workers also confirmed that fuel stability could be enhanced by removing oxygen and sulfur and using a metal-deactivating additive to reduce catalytic effects from trace copper and iron in the fuel (Mills and Kendall, 1986). This sample of the literature indicates that fuel fouling is a complex process which can be affected by temperature, oxygen content, the fuel's molecular composition, trace compounds (such as sulfur, copper, and iron), and the presence of deposits. In the present paper, we examine the use of optical diagnostics to observe in situ the evolution of a fuel's molecular composition at high temperatures.
Experimental Description The experimental setup for this work is most simply discussed in two parts: the fuel handling and heating apparatus and the optical configurations used to monitor absorption, scattering, and fluorescence of the fuel. The fuel used for this study was a JP-4 manufactured and analyzed by Phillips Petroleum, and the fuel was used in its as-received condition. The dissolved gas (nitrogen and oxygen) in the fuel was not quantified. Results of a fuel analysis are included in Table I. A schematic diagram for the fuel handling and heating hardware is shown in Figure 1. The fuel is stored and pressurized in a 0.25-gal steel reservoir fitted with a neoprene bladder. The bladder is filled with nitrogen which is used to keep the fuel at a constant 400 psig pressure. A section of 0.25-in.-diameter stainless tubing is used to connect the fuel reservoir to a 0.375-in. tube which is imbedded in a heated copper block. This 0.375in. tube is 55 in. in length and connects to an optical cell manfuctured from 316 stainless steel. The interior dimension for the cell is 1in. square by 3 in. long, and four 0.75-in.-diameter optical porta for quartz windows are included at directions orthogonal to the flow. The windows which were used for the absorption measurements were manufactured from 1.25-in.-diameter, 0.5-in.-thick quartz; these windows were modified to include a 0.25-in.-deep step to allow the windows to be flush with the cell wall. Scattering and fluorescence measurements used windows which had no steps and therefore
Table I. Fuel Analysis of JP-4Used in This Study MIL-T-5624 teat resulta specifications API gravity 56.2 45.0-57.0 1A 1B max corrosion total sulfur 0.4 max 0.09 doctor test negative negative 36.0 smoke point 20 min freeze point -58 max
