968
Energy & Fuels 1993, 7, 968-977
Phenomenological Study of the Formation of Insolubles in a Jet-A Fuel E. Grant Jones* and Walter J. Balster Systems Research Laboratories, Inc., A Division of ArvinlCalspan, 2800 Indian Ripple Road, Dayton, Ohio 45440-3696 Received May 11, 1993. Revised Manuscript Received August 2, 1993e
An aviation fuel in the Jet-A class has been stressed during flow through stainless-steeltubes under near-isothermal conditions at 185 "C. The quantity of insolubles in the bulk and on the stainlesssteel surface has been measured as a function of stress duration. Surface deposits are shown to arise primarily from an adhering or reacting precursor formed initially in the bulk fuel. Other bulk insolubles do not contribute to surface foulingat this temperature. The formation of bulk and surface insolubles is shown to be tightly coupled to the measured depletion of dissolved oxygen. A mechanism is proposed to represent the inhibited oxidation based on the presence of sulfur-containingcomponents in the fuel.
Introduction Fuels serve as the primary coolant in aircraft. With enhanced aircraft performance, a greater thermal load must be dissipated into the aviation fuel by heat exchangers. Elevated temperatures cause oxidative degradation of the fuel, producing both bulk insolubles and surface deposits.' The former can lead to nozzle and filter blockage, and the latter can reduce heat-transfer efficiency. Such problems can be alleviated somewhat by operation at lower temperatures, selection of more stable aviation fuels, and the judicious use of additive packages comprising antioxidants, detergents, dispersants, and metal deactivators. A primary goal in this laboratory is to develop global computational-fluid-dynamics (CFD) models for predicting the time dependence of surface fouling under severe temperature and flow conditions that cannot be readily achieved or studied in the laboratory.2 However, the development of global CFD models requires, as a basis, an understanding of some of the simple reaction chemical kinetics associated with fuel fouling: namely, the rates of formation of both bulk and surface insolubles. The very detailed studies by Jenson et a l S 3 r 4 of model compounds such as n-hexadecane at elevated temperatures attest to the extreme diversity of product distribution and reaction pathways that are developed in relatively short reaction times. The complexity of detailed chemical analysis of reaction channels for an aviation fuel containing a broad mixture of hydrocarbons is formidable. Furthermore, it is difficult-if not impossible-to ascribe particular reaction channels to the production of insolubles. Because of these inherent difficulties, the approach to this problem taken in the present study and our previous studies is a phenomenological one, whereby the focus is on the quantity of bulk and surface insolubles without Abstract published in Aduance ACS Abstracts, October 1, 1993. (1) Hazlett, R. N. Thermal Oxidation Stability of Aviation Turbine Fuels;ASTM Monograph 1;American Society for Testing and Materials: Philadelphia, PA, 1991. (2) Reddy, K. V.; Roquemore, W. M. Prepr. Pap.-Am. Chem. SOC., Diu.Fuel Chem. 1990,35 (4), 1346-1357. (3) Jensen, R. K.; Korcek, S.;Mahoney, L. R.;Zinbo, M. J. Am. Chem. SOC.1979,101,7574-7584. (4) Jensen, R. K.; Korcek, S.;Mahoney, L. R.; Zinbo, M. J. Am. Chem. SOC. 1981, 103, 1742-1749. Q
0887-0624/93/2507-0968$04.00/0
initial regard for the detailed chemistry. In such a treatment the global formation of insolubles is monitored as a function of reaction time or fuel-stress duration at a fixed temperature. Insolubles are quantified by standard surface-carbon burnoff of filters and stainless-steel surfaces. This approach has been pursued in this laboratory in both static616and dynamic' studies at elevated temperatures ( N 185 "C) where degradation is dominated by autoxidation reaction chemistry and the effects of pyrolysis can be neglected. Resulta reported herein relate to dynamic tests in which fuel is passed slowly through stainless-steeltubes that are maintained at a fixed temperature by surface contact within a heated copper block. The important features of the current heat-exchanger tests are the very slow flow rates and long path lengths which ensure that the bulk fuel and wall temperatures are equivalent and that reaction takes place under near-isothermal conditions. The residence time of the fuel within the tube is the chemical reaction time or the stress duration. Numerous studiesS1l have been reported which employed high wall temperatures, fast fuel flow, and concomitant large variations in temperature both along and across the tubes for approximation of certain engineering aspects of the fouling processes. The current experiments were initiated to minimize complications arising from both fluid dynamics and temperature nonuniformity and to study the chemical degradation of the fuel as a function of time at a fixed (5) Jones, E. G.; Balster, W. J. ASME Paper 92-GT-122; American Society of Mechanical Engineers: New York, 1992. (6) Jones, E. G.; Balster, W. J.; Anderson, S. D. Prep.-Am. Chem. SOC., Diu.Pet. Chem. 1992,37 (2), 393-402. (7) Jones, E. G.;Balster, W. J.; Post, M. E. Degradation of a Jet-AFuel in a Single-Pass Heat Exchanger. ASMEpaper 93-GT-334, presented at the 38th International Gas Turbine and Aeroengine Congress and Exposition, Cincinnati, OH, May 24-27, 1993, acceDted for Dublication in Tram. ASME. (8)Heneghan, S.P.; Martel, C. R.; Williams, T. F.;Ballal, D. R. ASME Paper 92-GT-106;American Societyof MechanicalEngineers: New York, 1992. To be published in Tram. ASME. (9) Marteney, P. J.: SDadaccini, L. L. J. Ena. - Gas Turbines Power 1986,108,648I653. (10) Giovanetti, A. J.; Szetela, E. J. AIAA Paper 86-0525; American Institute of Aeronautics and Astronautics: Washington, DC, 1986. (11) Chin, J. S.;Lefebvre, A. H.; Sun,F. T.-Y. ASME Paper 91-GT114; American Society of Mechanical Engineers: New York, 1992.
0 1993 American Chemical Society
Formation of Insolubles in a Jet-A Fuel
Energy & Fuels, Vol. 7, No. 6,1993 969
SYRINGE PUMP ISCO, 500 cc
CONTROLLER
Constant Flow
TUBING CHANNELS 0.125 (2) 0.25 (2)
HIGH TEMPERATURE BLOCK 185OC Cu (rect. 3 2 x 3 ~ 3 )
ROOM TEMPERATURE BLOCK (optional) 3 O o C , (cyl. 3 2 x 2 - in diam.)
0.45 pm Ag Filter
(optional)
I REGULATOR 350 ps1
Figure 1. Schematic diagram of the near-isothermal flowing test rig (NIFTR) in single-pass configuration.
temperature. Fuel entering the heat exchanger contains a fixed amount of oxygen determined by the level of oxygen in air-saturated fuel at room temperature. Since this is the only source of oxygen for subsequent reactions, the conversion of fuel to oxygenated products is limited, and product buildup is minimal. An aviation fuel in the Jet-A class, designated POSF-2827, was chosen for study. This has served as a laboratory baseline fuel that meets USAF specifications and has been reported* to form deposits in single-pass heat-exchanger tests at 300 "C. The paper is structured as follows. After a description of the experimental apparatus and techniques, the Results and Discussionsection treats, in turn, data format, surface deposition, filtered insolubles (qualitative),total insolubles (quantitative), oxygen consumption, and reaction mechanism. Data will be presented showing the existence of a unique, stress-time-dependentdeposition profile which correlates closely with the consumption of oxygen. Results will be shown that demonstrate the bulk origin of surface depositsvia the existence of a deposition precursor initially formed in the bulk fuel. This precursor will be shown to differ from other classes of bulk insolubles that do not contribute to surface fouling. Finally, the role of oxygen and its relation to the observed deposition is addressed in terms of the presence of autocatalysis and the global Arrhenius parameters describing oxygen consumption. The implication of these data to the problems of fuel fouling are summarized in the Conclusions.
Experimental Section The Jet-A fuel in this study was used as received, without filtration. It contains 0.079% (wt) of sulfur and has a jet fuel thermal oxidation test (JFTOT) breakpoint of 539 K. The fuel was pumped through stainless-steel (304)tubing clamped tightly in a heated copper block. Twosizes of tubing were used: namely,
0.125-in. o.d., 0.085-in. i.d. and 0.25411. o.d., 0.18-in. i.d. Tubing was ultrasonically cleaned using 10% Blue Gold solution12 and was rinsed with distilled water and methanol. The criterion for clean tubing was minimal surface carbon as measured using a LECO RC-412 surface-carbon analyzer. The background level of carbon on 2-in. sections (0.125-in.0.d.)was typically about 10 pg, exhibited variation alongthe tubes and also with tubing batch, and was not subtracted from the observed signals. Figure 1is a schematic diagram of the near-isothermal flowing test rig (NIFTR). The system consists of a syringe pump which forces fuel through a 32-in.-long heated tube section, through a filter housing containing a 47-mm (0.45-pm) silver membrane filter, and finally through a backpressure regulator which maintains a minimumpressureof 350psi. In selectedexperiments a 32-in.Jongcooled sectionwas inserted, and filtration was shifted downstream as shownin Figure 1. The majoreffect of this section is to provide additional cooling time prior to filtration. When additional fuel stress time was required, a third configuration was used in which the tubing was looped back through a second channel in the same heated block, and the membrane filter was accordingly moved downstream. For a flow rate of 1.8 cm3/ min-which, in the current study, represents a worst-case situation for achieving isothermal conditions-at least 75 % of the heated section has equivalent wall and bulk-fuel temperatures.' The NIFTR was operated in the horizontal configuration; under the current test conditions, deposition differences in horizontal and vertical configurations could not be detected? Some discrimination against the collection of bulk particles is expected due to gravitational effects, particularly at the slowest flows. Followinga 6-h test the tube was allowed to drain overnight, and the heated region was cut into 16 2-in. sections. These sections were subsequently rinsed with heptane, placed in a vacuum oven (115 "C)for a minimum of 12 h, and subjected to surface-carbon analysis. The in-line filters were sectioned into two equal pieces;one was rinsed with heptane and the other with heptane followedby acetone. On occasion,one of the halves was (12)Blue Gold Company, Ashland, OH.
Jones and Balster
970 Energy & Fuels, Vol. 7, No. 6,1993 Table I. Nomenclature
IS IG SG
insoluble solids (insolublein both heptane and acetone) insoluble gums (insoluble in heptane, soluble in acetone) soluble gums (soluble in heptane, the implication being that filterables are insoluble with respect to stressed fuel but may be soluble in unstressed fuel, Le., heptane)
left unrinsed for comparison purposes. The filters were then subjected to the same vacuum-ovenand surface-carbon-analysis treatment as the tube sections. Basedupon the residual following rinsing, the filtered insolubles were subdivided into three categories-IS, IG, and SG-as defined in Table I. The quantity of bulk insolublespresent on the fiiters as IS was obtained directly from the heptane/acetone-rinsedfilter. The quantity present as IG was determinedfrom the heptane-rinsedfilter and the quantity of IS. The quantity present as SG was estimated from unrinsed filters and the quantity of IS and IG. It should be noted that filterables in these experimentsare defined with respect to 0.45pm filters. It is expected that both quantity and distribution of insolubles are functions of pore size. Surface deposits were not specified as IS or IG but rather as the total IS + IG. Previous static flask experiments employing stainless-steel discs suspendedin the fuel at 185"C yielded surface deposits composed primarily of IS (>80%).6 Two types of experimentswere performed. In the first,surface deposits and bulk insolubles were quantified over a series of fuel flow rates using a heated block at 185 "C. A 6-h experimental test time was sufficient to produce measurable insolubles. In separate experiments the residual amount of dissolved oxygen was measured for a series of flow rates which provided different stressintervals in the heated section. Residence-timecalculations were based on tube dimensions and plug flow, assuming a 15% volume expansion at 185 "C. An in-line Hewlett-Packard gas chromatographmodified by Rubey et al.13was used for dissolvedoxygen measurements. Data are presented as percent residual oxygen as a function of residence time or stress duration, with 100%correspondingto fuel saturated with respect to air at room temperature. Based on recent measurements by Striebich," the saturated value for POSF-2827fuel is 64.9 ppm (w/w). All signals were corrected for naturally occurring argon which is not chromatographically distinguishable from oxygen.
Results and Discussion Data Format. As discussed above, deposition data are acquired as microgramsof carbon on each of 16 2-in. heated tube sections. The basic assumption made in such studies is that the deposita have a uniform elemental composition along the tube, i.e., the amount of deposit is proportional to the measured surface carbon. Based upon previous analyses, carbon constitutes -80% of the total mass. Throughout this paper reference is made to deposited carbon and filtered carbon; the term carbon refers to the 80% carbon content of the carbonaceous insolubles. Resolution of the surface data is limited by the length of the tube sections, which should be reflected by presentation of the deposition data in histogram format; however, for ease of comparison these data are presented as points centered at the middle of each tube. One presentation of typical deposition data (flow rate 0.50 cm3/min) where the weight of carbon (pg) is plotted as a function of the tube number is given in Figure 2a. Under isothermal conditions and a fixed flow rate, distance along the tube can be related to residence time or stress duration, assuming plug flow and a 15% volume expansion. Thus, (13) Rubey, W.A.; Tissandier, M. D.; Striebich, R. C.; Tirey, D. A,; Anderson, S. D. Prep.-Am. Chem. Soc., Diu. Pet. Chem. 1992,37 (2),
371-376. (14) Striebich,
R.C.Private communication.
7 0 ' " '
IC
70
'
a
.
.
.
'
'
I
b
- 3.0
4 2.0
TUBE NUMBER (2.117 Sections)
STRESS DURATION (mln)
Figure 2. Data format: 185 "C, flow rate of 0.50 cm3/min.(a) Carbon vs tube number and (b) time-averaged deposition rates vs fuel stress duration. Figure 2a could be plotted equally well as micrograms of carbon as a function of fuel-stress duration. It is important to distinguish two time frames. First is the experiment test time (6 h), representing an integration period to collect measurable deposits; this time along with flow determines the total quantity of fuel passed through the tube. Second is the fuel-stress duration, reaction time, or residence time in the heated block (1-30 min for the current data); this time controls the reaction conversion. Deposition data can also be expressed as an experimenttime-averagedcarbon-depositionrate. First, a rate in units of micrograms per unit stress time per cubic centimeter of fuel passed is calculated by dividing the micrograms of carbon on a tube section by the fuel residence time in that section and then by the total amount of fuel passed through the system. This is equivalent to micrograms per unit experiment time per unit volume of tube section. Data plotted in this manner are shown in Figure 2b with respect to the ordinate scale on the left. When viewed as a histogram, the area under the curve represents the total micrograms of surface carbon per cubic centimeter of fuel passed. Deposition results can then be readily compared with filter data which are experimentally integrated to yield micrograms of filtered carbon per cubic centimeter of fuel passed. A second way of expressingthe experimenttime-averaged deposition rate is to divide the micrograms of carbon on a tube section by the internal surface area of the tube section and then by the total experiment test time. This format has been employed conventionally in the literature; however, its use with data covering broad temperature ranges is questionable. The units in this case are micrograms per hour per square centimeter. Data plotted in this manner are shown in Figure 2b with respect to the ordinate scale on the right. The two rate expressions differ only by the surface-to-volume ratio ( 4 / d ) . With isothermal data the convention employed here is to plot as in Figure 2b with respect to the ordinate scale on the left, except when comparing tubes of different diameter and then as in Figure 2b with respect to the ordinate scale on the right. Surface Deposition. Figure 3 shows the residual dissolved oxygen at 185 O C and the 6-h-averaged surface deposition for seven different fuel flow rates--O.lO,O.125, 0.25,0.50,0.75, 1.0, and 1.88cm3/min. In each case, fuel saturated with respect to air at room temperature was passed through 0.125-in. tubes. In contrast, the blank shown in Figure 4 was obtained at a flow of 0.25 cm3/min using He-spargedfuel under conditions where the dimolved
Formation of Insolubles in a Jet-A Fuel
Energy &Fuels, Vol. 7, No. 6, 1993 971
O
A
185OC .
+
1.88
0
0.75
I
0.125-in (Flow
185 OC
tubing 0.5)
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X
m0
40
% N
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0' 0
.
.
'
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4
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' 8
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6
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Y'
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STRESS DURATION (min)
Figure 3. POSF-2827fuel initially saturated with respect to air at room temperature. Deposition rate on 0.125-in.-0.d.tubing at a series of flow rates (cma/min) vs stress duration at 185 O C . Superimposed is dissolved oxygen vs stress duration. 185 OC He BLANK
"0
2
4
6
8
10
12
STRESS DURATION (mln)
Figure 4. POSF-2827 fuel initially sparged with He. Deposition rate on 0.125-in.-0.d. tubing at a flow rate of 0.25 cm3/min vs stress duration at 185 O C . oxygen is estimated a t less than 5% of the air-saturated value. The blank includes all non-oxygen-related surface contributions. Clearly, the observed deposition in Figure 3 arises primarily from the presence of dissolved oxygen. This is additionally confirmed by the oxygen consumption and the existence of a deposition rate that increases, passes through a maximum, and returns to baseline. Deposition is complete after 10 min of stressing. Notein Figure 3 that a t the three fastest flow rates, only the initial portions of the deposition profile are collected. Such conditions do not provide sufficient stress time for completion of either the oxygen consumption or the
"0
5
10
15
20
25
STRESS DURATION (min)
Figure 5. POSF-2827fuel initially saturated with respect to air at room temperature. Comparison of deposition on 0.125-in.-o.d. and 0.25-in.-0.d. tubing. deposition process within the 32-in. heated tube. At the other extreme, data obtained at the slowest flow rate (0.10 cm3/min) extend past 25 min; hence, about one-half of the data points exceed the plot range. The data set for a flow rate of 0.50cm3/minwas obtained from two passes through the heated block in order to provide 10 min of stressing. The observed deposition processes are consistent with the autoxidative basis of fuel fouling. Maxima have been reported in most tube deposition studies;however, the existence of a large temperature drop along the tube has made it difficult to ascertain whether the maximum arises from oxygen consumption or from temperature-related chemical-kinetic changes. Isothermal conditions simplify the interpretation. The fuel becomes totally depleted of oxygen, and deposition processes can no longer be maintained. The fact that the deposition profile is approximately independent of fuel flow rate implies that the deposition maps a bulk-fuel reaction-time-dependent process. This indicates that surface deposita originate in the bulk fuel. The existence of a deposition-rate maximum that occurs after complete oxygen consumption indicates a delay in the deposition process. This may arise, in part, from the time required to complete chemical reactions, e.g., to convert peroxy radicals, ROO', into surface deposita. If surface deposits originate primarily from chemical reactions occurring in the bulk, then the time necessary for diffusion to the wall may be significant. That is, the walls may serve as a collector, with an inherent delay due to diffusion. The possibility of bulk origin of the surface deposita can also be checked by comparing the deposition observed in Figure 3 on 0.125-in. tubes with that from similar experiments on 0.25-in. tubes. Presumably, the mean time to diffuse to the wall will be greater in the larger-diameter tubes and will be manifested by a broadened deposition profile. Figure 5 shows this comparison for a flow rate of 0.50 cm3/min. The shift and broadening seen in Figure 5, when considered with the oxygen-lossdata in Figure 3,
972 Energy & Fuels, Vol. 7,No. 6,1993
imply that almost all of the observed oxygen-related deposition originates from the bulk fuel. Shown in Figure 3 are additional data obtained at a 0.25-cm3tmin flow rate which confirm that the deposition process for the largerdiameter tubes is also independent of flow rate. In summary, data a t 185 "C support the premise that surface deposition arises mainly from oxygen-dependent formation of a product within the bulk fuel which diffuses to the tube walls and either adheres or reacts to leave a residue. This product is defined here as a deposition precursor, P. The total surface carbon per cubic centimeter of fuel passed through the heat exchanger is approximately constant and independent of flow. The area under the profile in Figure 3 for 0.125-in. tubes is about 3 pg/cm3. Significantly, the total deposition on the 0.25-in. tube is the same within experimental uncertainty. This indicates that P is not appreciably consumed by competitive processes during the additional diffusion time to the wall and implies that it can be quantitatively evaluated. Contributions to deposition related to nonoxidative processes are difficult to evaluate because of the large background. As mentioned earlier, no background corrections have been made. Such effects would be included along with some contribution from the residual oxygen in the blank run in Figure 4. Approximately one-half of the total carbon in Figure 4 arises from a clean-tube background. The remainder comprises two contributions, one being the presence of some residual oxygen not eliminated during sparging and the other being related to fuel exposure. The magnitude of the second contribution is proportional to the internal surface area and does not appear to be related to exposure or stress time. Filtered Insolubles (Qualitative). For each of the above flow rates, the corresponding bulk filtered insolubles have been quantified. Two distinct situations become evident. First, in all cases where surface deposition has been completed within the heated tube (i.e., slow flow rates or long stress duration), the total filtered carbon is 2 pg/cm3, with equal amounts of IS and IG being present on the filter. Figure 6 shows the surface-carbon-burnoff dependence which is the detected carbon-dioxide signal formed during the linear programmed heating of the filtered sample in pure oxygen. The fuel flow rate was O.lOcm3/min for obtaining this filtered sample. Note that the burnoff dependences are nondescript, having equal abundance (area under curve) of IS and IG. In the second situation which arises when the surface-deposition process has not been completed within the heated tube (for example, a flow rate of 1.0 cm3/min),the IG portion of the filterables is dominant and has a unique bimodal carbonburnoff dependence as shown in Figure 7. Similar profiles are obtained for IG in cases where the preceding surface deposition fails to return to baseline. The filtered material in these cases is almost entirely the deposition precursor, P, which has passed through the hot tube before diffusion to the walls. Thus, the filter data further support the bulk origin of oxygen-related surface deposits in this fuel and, in addition, place P in the category of an IG that is capable of being collected on 0.45-pm filters. In recent experiments a second filter having a pore size of 0.20 pm has been placed in series. Under the current experimental conditions, less than 10% of P is collected on the second filter. Thus, the mean precursor diameter must be on the order of 0.5 pm or larger at the point of filtration. Since some cooling of the fuel has occurred, no information
Jones and Balster 400
300
-
0 3.
Y
s=
200
U
4
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100
0 0
200
400
600
800
TEMPERATURE ( C)
Figure 6. Output from RC-412 surface-carbon burnoff of a filtered sample. Samplecollectedusingair-saturated POSF-2827 fuel and a flow rate of 0.10 cmg/min at 185 OC. 400
300
200
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n-
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TEMPERATURE ( OC)
Figure 7. Output from RC-412 surface-carbon burnoff of a filtered sample.Samplecollectedwing air-saturated POSF-2827 fuel and a flow rate of 1.0 cm3/min at 185 OC. concerning the in-situ precursor size at 185 "C can be obtained from these data. A question arises concerning whether the precursor to deposition at 185 "C may also cause surface deposits at other temperatures. This question has been addressed for deposition on 30 "C stainless-steel surfaces by passing the fuel first through the hot tube at 185 O C under conditions where P will be present in the effluent and then immediately through a 32-in. tube maintained in a room-temperature copper block. The recorded temperature was -30 "C. The results of two such experiments are given in Figure 8. Both data sets show a sharp discontinuity between the two blocks. Evidently, no
Energy & Fuels, Vol. 7, No. 6, 1993 973
Formation of Insolubles in a Jet-A Fuel
808 30 O C
185OC
@
0 Flow0.50cc/min
60
d
40
ab"..
0
0
20
0
0
2
4
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STRESS DURATION (mln) (Total 185 C and 30 C)
Figure 8. Deposition rate in successive tubes at 185 and 30 "C vs total tube residence time. Flow rates are 0.75 and 0.50 cm3/ min. Vertical lines mark the transition from the hot to the cold block, i.e., 3.4 min (flow 0.75 cm3/min) and 5.2 min (flow 0.50 cmg/min). significant deposition occurs on the cold tube because P either does not diffuse to the wall or, upon arrival, does not adhere or react; this dilemma cannot be resolved from the currently available data. The fact that P is still present in the effluent from the cold tube is confirmed by the filter data. In both experiments IG are dominant and the surface-carbon dependences appear as in Figure 7. It should be noted that the baseline deposition in the cold tube includes the same non-oxygen-related contributions observed in the heated sections. Total Insolubles (Quantitative). Arguments made previously can be clarified by consideringthe total quantity of carbon, i.e., surface carbon plus filtered carbon produced
under different oxidative-stress conditions. The total filtered carbon is obtained directly from the filter burnoff, and the total surface carbon is obtained by taking the sum of the total carbon above baseline on each tube section or integrating the area under the profiles in Figure 3. Integration excludes the baseline or nonoxygen-related contribution. Since different quantities of fuel are passed through the tube at each flow rate, the data are normalized per cubic centimeter of fuel. Figure 9a shows the dependence of the total carbon and also that fraction of carbon contained on the fiiter as a function of fuel residence time in the tube or total stress duration. Note that for short stress times, most of the carbonaceous material is swept through to the filter with minimal loss to the walls. For stress times longer than 10 min, P is lost to the walls, and only nonadhering bulk particles are filtered. About 40% of the total filterables fall into the nonadhering category, and 60% are characterized as P. In Figure 9a the total carbon increases with stress time up to 5 min, at which time the oxygen has been totally consumed (see Figure 3); the total carbon remains approximately constant over the next 20 min of additional stressing. Earlier it was concluded that a direct correlation exists between the surface deposits and the consumption of dissolved oxygen. The data in Figure 9a show a correlation between the total carbon (bulk + surface) and the presence of dissolved oxygen. The near-linear rise in total carbon as a function of total stress duration reflects the fact that an increasingly larger fraction of the oxygen is consumed over the first 5 min of reaction. The time behavior of total carbon is not surprising if one considers that the degradation is via autoxidation at this temperature. However, it is surprising that both surfacedeposition precursors, P, and bulk particles have achieved a size that exceeds 0.45 pm in such short reaction times. Clearly, the observed insolubles are early products in the autoxidation chemistry. If the oxygen is totally consumed in fuel saturated initially a t room temperature with respect to air, then the maximum amount of insolubles formed from 1 cm3 of POSF-2827 aviation fuel is 5 pg. The partitioning shown
Fraction of Total Carbon in Bulk ('10)
8
b -
a
7
TOTALCARBON
21# "
IS( Bu Ik )
0
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TOTAL STRESS DURATION (min)
30
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TOTAL STRESS DURATION
"n)
Figure 9. (a) Total carbon and fraction in bulk vs total stress duration at 185 O C . (b) Total carbon, IS (bulk),and IG (bulk + precursor) vs total stress duration at 185 O C .
974 Energy & Fuels, Vol. 7,No. 6, 1993
Jones and Balster
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in Figure 9a indicates that 60 % , or 3 pg, is P and 40 5% , or 2 pg, is nonadhering bulk insolubles. In Figure 9b the distribution of IS and IG collected on the filter is superimposed on the total-carbon dependence. The IS which do not adhere to the tube surface are detected after an induction period of a few minutes and rapidly increase to 1 pg. After consumption of oxygen is complete, the quantity of IS does not increase over an additional 20-min period of stressing. This is in contrast to the behavior of the IG. For the first few minutes, IG constitute almost 100% of the total collected carbon, exhibiting a maximum at around 4-5 min where the oxygen is totally consumed and finally dropping to a constant level of -1 pg. The maximum arises from that fraction of IG that leads to surface fouling, i.e., P. For short residence times these precursors are swept through to the filter. At longer residence times within the heated tube, they are lost to the walls; as a result the filtered amount drops to a constant level of 1pg which constitutes the nonadhering portion of the IG. Again, the quantity of this material does not increase with an additional 20 min of stressing, i.e., after complete oxygen consumption. Note that for long residence times, the ratio of IS to IG is about unity. In summary, 1cm3of air-saturated fuel has the potential (provided the oxygen is totally consumed) to produce a total of 5 pg of insolubles: 3 pg of a surface-fouling precursor, P, in the form of an IG, 1pg of a nonsurfacefouling IG, and 1 pg of a non-surface-fouling IS. The contribution from inherent insolubles present in the initial fuel is
