PRODUCT
`
`~
`
`CONTENTS
`
`Edito rial
`
`.
`
`IEPRA6 13(2) 93 -152 (1974
`
`ISSN 00 19- 7890
`
`June 1974
`
`PRODUCT REVIEW
`
`Thermochemical Water Decomposition Processe s
`Raul E. Chao .
`
`CATALYST SECTION
`
`Oxidation of Hydrogen Sulfide over Cobalt Molybdate and
`Related Cata lysts
`
`Robert A. Ross * and Michael R. Jeanes .
`
`Supe ra ctive Nickel-Aluminosili c ate Cat alysts for Hyd ro isomeriz ation
`and Hyd rocracking of Light Hyd rocarbo ns
`Harold E. Swift* and Edgar R. Black .
`
`Catalyst Eval ua tion for the Sim ult aneou s Red uctio n of Sulfur Dio xide
`and Nit ric Oxide by Car bon Mono xide
`Vernon N. Goetz. Ajay Sood,* and J. R. Kittrell
`POLYMER SECTION
`
`.
`
`Frictional Pro per ti es of Tetrafluoro ethylen e- Perfl uoro(propyl vinyl ether)
`Copo lymers
`
`Robert C. Bowers* and William A. Zisman .
`
`Tetrafluoroethylene-Perfluoro (propyl vinyl ether) Copolymers.
`As Water-Resistant , Thin-Film Adhesives
`
`Joseph P. Reardon* and William A. Zisman .
`
`Starch-Filled Polyvinyl Chloride Plastics-Prepar ation and Evaluation
`Richard P. Westhoff , Felix H. Otey, Charles L. Mehltretter, and
`Charles R. Russell*
`.
`
`GENERAL ARTICLES
`
`Dihydrochalcone Sweeteners: Preparation of Neohesperidin
`Dihydrochalcone
`George H. Robertson.* J. Peter Clark, and Robert Lun din .
`
`Isolation of Antitumor Alkalo ids from Cephalotaxus harringtonia
`Richard G. Po well,* S. Peter Rogovin, an d Cecil R. Smith, Jr .
`
`Deposit Formation from Deoxygenated Hydrocarbons. I.
`General Features
`
`William F. Taylor
`
`.
`
`Nylon Flammability-Effects of Thiourea, Ammonium Sulfamate,
`and Halogen Compounds
`
`Kenneth B. Gill eo .
`
`Perfluoroalkyl Ether Di-s-triazinyl Substituted Alkanes
`Thomas S. Croft, * J. L. Zollinger, and Carl E. Snyder, Jr.
`
`3,3-Bis(trifluoromethyl)oxiranes. Synthesis and Amine Reactivity
`
`James R. Griffith* and Jacques G. O'Rear
`
`.
`
`The Air Drying of Latex Coa tings
`
`Cha rles M. Hansen .
`
`93
`
`94
`
`102
`
`106
`
`110
`
`115
`
`119
`
`123
`
`125
`
`129
`
`133
`
`139
`
`144
`
`148
`
`150
`
`VO L. 13. NO.2
`June 1974
`Editorial Headquarters
`115516th St., N.W.
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`
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`
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`
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`Published by th e
`AMERICAN CHEMICAL SOC IETY
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`.
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`Seldon W. Terrant Head, Research and
`Development Departm ent
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`© Copyright 1974 by the American Chemical Society. Repro (cid:173)
`duction forbidden without permission .
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`There is no supplementary material for this issue.
`* In papers with more than one author the name of the author to whom inquirie
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`
`GE-1020.001
`
`

`
`
`
`GE-1020.002
`
`GE-1020.002
`
`

`
`Deposit Formation from Deoxygenated Hydrocarbons. I. General Features
`
`William F. Taylor
`
`Esso Research and Engineer ing 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 oxy~en. De(cid:173)
`temperatures
`(150-649 C) and
`formation rate measurements have been extended to higher
`posit
`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 hy(cid:173)
`drocarbons has been the subject of considerable interest
`(Nixon, 1962). Initially, studies were carried out with air(cid:173)
`saturated hydrocarbons in a narrow, near ambient tem(cid:173)
`perature 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 majo rity of such studies were carried out with fuels
`saturated with molecular oxygen via exposure to air al(cid:173)
`though 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
`at temperatures up to 250°C at reduced pressures (Taylor,
`1968a, 1968b, 1969a, 1969b; Taylor and Wallace, 1967,
`1968). Complementary studies were carried out using tet(cid:173)
`ralin 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 oxy(cid:173)
`gen 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 con(cid:173)
`tent 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 .
`
`Experi me ntal Section
`Apparatus. The Advanced Kinetic Unit used to mea (cid:173)
`sure 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 he(cid:173)
`lium 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 con(cid:173)
`tains 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 high(cid:173)
`pressure nitrogen. The treated fuel
`is separated from the
`nitrogen drive gas by use of two individual pistons, sepa(cid:173)
`rated 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 tubu(cid:173)
`lar reactor section consisting of lf4-in. o.d. x O.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 thermo(cid:173)
`couples 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 mid(cid:173)
`point 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 re(cid:173)
`leasing 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 sec(cid:173)
`tions are analyzed for carbonaceous deposits using a mod(cid:173)
`ified LECO low carbon analyzer system (Model 734-4(0)
`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 CO2 is auto(cid:173)
`matically trapped and finally delivered to a thermal con(cid:173)
`ductivity cell. The output is integrated and recorded on a
`digital output meter as micrograms of carbon. The analyt(cid:173)
`ical system was calibrated against known standards. The
`deposit formation rate is obtained by dividing the net car(cid:173)
`bon production per section by the corresponding inner
`surface area and expressed as micrograms of carbon per
`square centimeter per 4-hr reaction time.
`Rea gen ts. 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. Chern ., Prod. Res. Develop. , Vol. 13, No.2, 1974
`
`133
`
`GE-1020.003
`
`

`
`Table I. Inspections of Hydrocarbon Jet Fuels
`
`A
`
`B
`
`C
`
`ASTM distillation, °C
`I.b.p.
`10%
`20%
`50%
`70%
`90%
`F.b.p.
`Total sulfur, ppm
`Mercaptan sulfur, ppm
`Aromatics, vol %
`Existent gum, mg /lOO ml
`Potential gum, mg /100 ml
`Peroxide number, mequiv/ 1.
`Other identification (s)
`
`o Wt % basis.
`
`169
`197
`202
`214
`229
`238
`254
`234
`<1
`19 .7 0
`0.4
`0.9
`Trace
`JP-5
`
`180
`192
`199
`212
`
`237
`257
`660
`6
`13 .7
`0.8
`7.9
`2 .2
`AFFB-9-67
`
`"
`
`PRESSURE
`CO~TROlL£R
`
`/
`
`VUT
`
`SPARGE
`'AS
`
`"
`
`WATER
`\
`
`V[ ~T
`
`S PARCI~
`VESSEL
`
`MOVEABLE
`PIS TONS
`-'"'
`
`R[ACTOR HEAT[RS
`
`ZONE 41
`
`RECEIV[R
`
`COOUR •
`
`F igu re I. Schematic of the kinetic unit used for the measurement
`of the rate of deposit formation .
`
`tubing employed in the react or section of the Advanced
`Kinetic Unit was %-in. o.d. X 0.083-in. wall stainless steel
`type 304. Prior to use the tubing is cleaned inside and out
`wit h acetone and chloroform and is blown dry with nitro(cid:173)
`gen.
`
`Results
`Since previous measurements of the rate of deposit for(cid:173)
`mation were limited to relatively low temperatures and
`pressures, a series of runs were made with air-saturated
`fuels at higher
`temperatures and pressures in the Ad (cid:173)
`vanced Kinetic Unit prior
`to conducting studies with
`deoxygenated fuels . Runs were made in the Ad vanced Ki (cid:173)
`netic Unit with fuels A. B. and E on an air-saturated
`basis at 69 atm over the temperature range of l50 -540°C.
`the first with tem(cid:173)
`Two runs were made with each fuel,
`perature zones at l50 -3l5°C and the second with temper(cid:173)
`ature zones at 371-540°C. Results of these runs with fuels
`A and B are shown in Arrhenius plots in Figures 2 and 3.
`Similar results were obtained with fuel E. A complex Ar(cid:173)
`rhenius plot was obtained in all cases. In general, the de(cid:173)
`posit
`formation rate wit h these air-saturated fuels
`rose
`with increasing temperature, dropped sharply in a transi(cid:173)
`tion zone covering the approximate 350-425°C range, and
`t hen again increased wit h rising temperature. Apparent
`activation energies,
`exclusive of
`the
`transition zone,
`ranged from 5 to 20 kcal Zmol.
`T he effect of deoxygenation on a spectrum of jet fuel
`investigated.
`Inspec(cid:173)
`range hydrocarbon fuels was next
`tions on these fuels are shown in Table I. All of these
`fuels, wit h t he exception of fuel A, have been evaluated
`previously in a variety of test devices on an air-saturated
`basis (Goodman and Bradley. 1970).
`In general,
`these
`indicated t hat
`t he stability of these fuels
`prior
`resu lts
`
`134
`
`Ind. Eng. Chern.. Prod. Res. Develop.. Vol. 13. No .2. 1974
`
`Figure 2. Air-saturated fuel A (64 ppm of O2 ) at 69 atm: • . tem(cid:173)
`perature zones at 150-315°C ; _ . temperature zones at 3il-538°C.
`
`ranged from a very good quality (fue l C) to a very poor
`quality (fuel F). These fuels were all first evaluated in th e
`Advanced Kinetic Unit on an air-saturated basis at 69
`atm over the temperature range of l50-3I5°C. Results of
`t he
`these tests are summarized in Table II. In gene ral,
`ranking of these fuels on an air-saturated basis corre(cid:173)
`sponds to previous evaluations. T he rate of deposit form a(cid:173)
`tion of these fuels was next measured in t he Advanced
`Kinetic Unit on a deoxygenated basis at the identical con(cid:173)
`ditions employed previously on an air-saturated basis.
`Total deposits were markedly reduced by removal of mo(cid:173)
`lecular oxygen with all fuels with the exception of poor
`quality fuel F. A comparison of t he de posit
`formation
`rates obtained on an air-saturated us. a deoxygenated
`basis is shown in Figure 4 for fuel E, whic h is typical of
`the results obtained with fuels A t hrough E. A sim ilar
`
`Fuel
`
`D
`
`206
`212
`
`219
`223
`232
`239
`<0.2
`
`200
`208
`210
`220
`
`239
`257
`3
`
`2 .5
`0
`0
`Nil
`AFFB-1l-68
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`1.80
`0 .2
`0 .2
`Nil
`P&W523
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`166
`174
`178
`187
`
`229
`258
`190
`10
`8 .9
`0 .2
`0.6
`0.4
`AFFB-8-67
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`174
`188
`199
`215
`225
`238
`263
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`
`GE-1020.004
`
`

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`Figure 3. Air-saturated fuel B (58 ppm of O2 ) at 69 atm: .
`, tem(cid:173)
`peraturezonesat 150-315°C; _, temperature zonesat 37l-538°C.
`
`lOOOj" K
`Figure 4. Fuel E at 69 atm : .
`, air-saturated (69 ppm of O2 ) ; A,
`deoxygenated (0.3 ppm of O2 ) .
`
`Table II. The Effect of Deoxygenation on Deposit
`Formation with Different Fuels
`
`O2 content,
`ppm
`
`Fuel
`A
`
`B
`
`C
`
`D
`
`E
`
`F
`
`Total carbonaceous deposits·
`As ppm
`based on
`ug of carbon
`total fuel
`64
`2,404
`1.24
`0.1
`0 .16
`315
`58
`2.05
`3,992
`0.1
`655
`0.34
`75
`373
`0.20
`0.7
`257
`0 .13
`74
`2.43
`4,613
`0.1
`882
`0.46
`2,872
`69
`1.51
`0.3
`0.31
`589
`57
`4 .21
`8,157
`1.4
`37,265
`19 .2
`• Cumulative deposits produced in 4 hr in the Advanced
`Kinetic Unit. Conditions: 69 atm; zone 1, 149°C; zone 2,
`204°C; zone 3, 260°C; zone 4, 316°C.
`
`IOOO,--
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`
`comparison for fuel F, which did not exhibit reduced de(cid:173)
`posit formation rates with deoxygenation, is shown in Fig(cid:173)
`ure 5. A summary of results obtained is shown in Table II.
`In the temperature range of 150-315°C, apparent activa(cid:173)
`tion energies for the air-saturated fuels ranged from 5 to
`15 kcal /rnol, whereas on a deoxygenated basis the appar(cid:173)
`ent activation energies were less than 5 kcal /rnol.
`The effect of deoxygenation on deposit formation rates
`at temperatures above 315°C was investigated with fuels
`A and B. Additional runs were made with fuel A both at
`371-538°C and 482-649°C, and with fuel B at 371 to
`538°C. Results of these runs with fuel A is shown in Fig(cid:173)
`ure 6 and with fuel B in Figure 7. It can be seen that with
`deoxygenation fuel A continued to exhibit relatively low
`deposit formation rates (e.g., less than 100 (J1.g/cm2)/4 hr)
`until approximately 593°C. A comparison with the data
`
`IOOO j" K
`Figure 5. Fuel F at 69 atm: .
`, air-saturated (57 ppm of O2 ) ; A ,
`deoxygenated (0.3ppm of O2 ) .
`
`obtained on an air-saturated basis indicates that this de(cid:173)
`posit rate was first reached with fuel A at 288°C. The ap(cid:173)
`parent activation energy for deposit formation with fuel A
`rose continuously over the temperature range, from less
`than 5 kcal/rnol to 40 kcal/mol. In contrast to fuel A, fuel
`B exhibited a complex Arrhenius plot with a distinct rate
`maxima at approximately 438°C. Fuel B first exhibited a
`100 (J1.g/cm2 ) / 4 hr deposit formation rate at 427°C on a
`deoxygenated basis and at 299°C on an air -saturated
`
`Ind . Eng. Chem ., Prod. Res. Develop., Vol. 13, No.2, 1974
`
`135
`
`GE-1020.005
`
`

`
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`I .()())
`
`"'0
`x
`'"
`=: 100
`~
`N e
`u
`is
`
`10
`
`::J
`....
`~
`z
`
`s
`-c
`"
`p
`....
`
`g~
`
`2.0 L------'' --
`1. 20
`1. 00
`
`----'' --
`1.40
`
`----'l.--
`1. 60
`
`- L--
`1. 80
`
`-
`
`' -- - ' - - '
`2.20 2. 30
`2.00
`
`looo / 'K
`Figure 6. Deoxygenated fuel A at 69 atm : &, temperature zones
`at 1S0- 31SoC and 0.1 ppm of O2 ; _, temperature zones at 371(cid:173)
`S38°C and 0.4 ppm of O2 ; e , temperature zones at 482-649°C and
`0.3 ppm of O2 •
`
`basis. The maximum apparent activation energy for de(cid:173)
`posit formation exhibited by fuel B was 45 kcal /rnol in the
`370-427°C temperature range.
`The effect of intermediate oxygen level was investigated
`with fuels A and B at 69 atm over the temperature range
`371-538°C. Results are summarized in Table III. The larg(cid:173)
`est effect of oxygen concentration on the deposit forma(cid:173)
`tion rate occurs at lower temperatures in the liquid phase
`just before the " t ransit ion" zone exhibited by air-saturat(cid:173)
`ed fuels . The change in deposit formation rate with oxy(cid:173)
`gen content with the two fuels is also different. Fuel A ex(cid:173)
`hibits markedly different
`total deposits at 0.8 and 1.6
`ppm of O2 whereas total deposits with fuel B were similar
`at 0.3 and 14.6 ppm of O2 •
`The effect of total pressure was investigated with fuels
`A and B over the temperature range 371-538°C. The com(cid:173)
`parison was made between 18 and 69 atm. Results of this
`comparison are summarized in Table IV. It can be seen
`that
`the effect of pressure is complex, with pressures
`greater than 18 atm resulting in lower total deposits with
`fuel B and with essentially no change in total deposits
`with fuel A.
`Several analyses were made of the oxygen content of de(cid:173)
`posits formed from a deoxygenated fuel. In order to elimi(cid:173)
`nate possible contamination of the deposit as a result of
`the normal procedure of sawing the thick-walled tube into
`sections, a length of Nicrome wire was placed inside the
`heated length of the reactor tube. Fuel B, rigorously deox(cid:173)
`ygenated, was employed at 69 atm with temperatures at
`371-482°C in a normal 4-hr run. Both visual observations
`and carbon analysis indicated the presence of deposits on
`the wire . Samples of such deposits from different runs
`were analyzed for oxygen content using the method devel(cid:173)
`oped by Meade, et at. (1967) . In addition, nitrogen analy(cid:173)
`sis was determined by the method of Nightingale and
`
`136
`
`Ind . Eng. Chern .. Prod . Res. Develop ., Vol. 13, No.2, 1974
`
`•
`
`•
`2.0 '-:-::-__L-__L-_ _ L-_ _ L-__L-_---l
`1.2 0
`1. 80
`1.60
`1.40
`2. 40
`2.20
`2.00
`
`r.ooo/-c
`Figure 7. Deoxygenated fuel B at 69 atm : e , temperature zones
`at 1S0- 31S"C and 0.1 ppm of O2 ; _ , temperature zones at 371(cid:173)
`S38°C and 0.3 ppm of O2 •
`
`(1962), and sulfur content was determined by
`Walker
`combustion followed by a barium perchlorate titration.
`No significant quantities of S or N were found in the de(cid:173)
`posit samples. The average oxygen content of the deposit
`samples was 3 wt %, which is considerably lower than the
`oxygen content of typical autoxidative hydrocarbon fuel
`deposits, which range from 15 to 30 wt % (Nixon, 1962).
`
`Discussion
`The formation of deposits from jet fuel range hydrocar(cid:173)
`bons involves complex chemical and physical processes. In
`air-saturated hydrocarbons a complex sequence of reac(cid:173)
`tions takes place at even moderate temperatures (Boss
`and Hazlett, 1969). It is clear that deposits form as the
`end result of free radical, chain reactions involving molec(cid:173)
`ular oxygen, although the detailed reactions leading to the
`formation of actual deposits have not been elucidated. A
`number of studies have shown that sediments and depos(cid:173)
`its formed as a result of hydrocarbon autoxidative reac(cid:173)
`tions contain high concentrations of oxygen and lesser but
`significant amounts of sulfur and nitrogen in a relatively
`low molecular weight species, i.e., below 600 (Nixon, 1962;
`Schwartz, et al., 1964; Zrelov, 1966) . Thus,
`in liquid
`is the change in
`phase autoxidative deposit formation it
`solvent character resulting from the incorporation of oxy(cid:173)
`gen, nitrogen, and sulfur into the deposit species which is
`important rather than an increase in molecular weight.
`Scanning electron microscope studies of a number of au(cid:173)
`toxidative jet fuel deposit specimens have shown that
`such deposits generally are microspherical particles ap(cid:173)
`proximately 1000 A in diameter (Schirmer, 1970). Thus, it
`is clear that complex chemical and physical processes
`both participate in the overall deposit formation process.
`One of the salient features of the deposit formation pro(cid:173)
`cess with air-saturated fuels over the 150-540°C range at
`
`GE-1020.006
`
`

`
`Table III. The Effect of Intermediate Oxygen Content on Deposit Formation
`Deposit formation rate,
`[~g of C/cm')/4 hrl X 106•
`At 354 °C,
`At 502°C,
`liquid phase
`vapor phase
`
`Oxygen content, ppm of 0 ,
`
`Fuel
`A
`
`Total carbonaceous deposits"
`As ppm based
`Jlg of carbon
`on total fuel
`0.77
`1,485
`14.7
`21.4
`0.4
`1,586
`0 .82
`31.8
`18.8
`0.8
`3,843
`19.6
`283
`1.6
`1.98
`11,085
`630
`64 (a ir-sat urated)
`5 .71
`50.3
`4,739
`2.43
`61.5
`15 .1
`0 .3
`4 ,431
`63.1
`36 .5
`14 .6
`2 .28
`58 (air-sat ur ate d)
`257
`212
`9,105
`4 .68
`n Other conditions: 69 atm; zone I, 371 °C; zone 2, 427 °C; zone 3, 482°C; zone 4, 538°C. b Cummulative deposits produced
`in 4 hr in the Advanced Kinetic Unit.
`
`B
`
`Fuel
`A
`
`Table IV. Effect of Pressure on Deposit Formation
`Total carbonaceous deposits'
`As ppm based
`Pressure,
`atm
`on total fuel
`ug of carbon
`1320
`18
`0.68
`1485
`69
`0.77
`3 .64
`7008
`18
`45
`3578
`1.84
`69
`4739
`2.43
`<Cumulative deposits produced in 4 hr in the Advanced
`Kinetic Unit. Conditions: zone I, 371 °C; zone 2, 427 °C;
`zone 3, 482°C; zone 4, 538°C; oxygen content 0.3 to 0.8
`ppm.
`
`B
`
`69 atm is the complex Arrhenius plot which results from
`the sharp drop in rates in the 350-430°C range. Previous
`studies at sub-atmospheric pressures have shown that as a
`hydrocarbon jet fuel is heated, the rate of autoxidative de(cid:173)
`posit formation increases until the liquid phase is lost, at
`which point the rate of deposit formation drops sharply
`(Taylor and Wallace, 1967). It would appear that much of
`this drop can be attributed to a reduction in the autoxida(cid:173)
`tive reaction rate caused by a lowering of the concentra(cid:173)
`tion of reactive species as the system passes from the liq(cid:173)
`uid phase to the vapor phase (Mayo, 1968). With increas(cid:173)
`ing temperature the autoxidative rate constants continue
`to increase, so that ultimately this concentration effect is
`overcome and the overall rate of reaction again increases.
`Calculations indicate that a hydrocarbon jet fuel mix (cid:173)
`ture similar to fuels A and B has a critical point of 410°C
`and 22.4 atm (Nixon, et al., 1970). Thus, the sharp drop
`in autoxidative deposit formation rates observed in the
`present study would appear to reflect an effect of the
`transition between the liquid phase and the supercritical
`vapor phase at the 69 atm employed.
`The stability of a given fuel is often referred to in terms
`of a "breakpoint temperature" concept, which is defined
`at the temperature at which the measure of the rate of de(cid:173)
`(Ed(cid:173)
`posit formation first exceeds some arbitrary level
`wards, 1972). If all fuels exhibited the same approximate
`apparent activation energy for the rate of deposit forma(cid:173)
`tion, such a technique would rank fuels the same over the
`entire temperature range and would be essentially equiva(cid:173)
`lent to a comparison based on relative preexponential rate
`constants. It can be seen that the rate of deposit forma(cid:173)
`tion often exhibits a complex behavior with both air-satu(cid:173)
`rated and deoxygenated fuels, with apparent activation
`energies varying from negative values up to approximately
`45 kcal/rnol. Thus, the use of "breakpoint temperatures"
`to compare fuels is obviously, at best, an oversimplified
`procedure, and the relative ran kings of fuels can be in(cid:173)
`fluenced by the "breakpoint" conditions which are select(cid:173)
`ed.
`
`the drastic reduction in mo(cid:173)
`With the majority of fuels,
`lecular oxygen content (i. e., less than 1 ppm) resulted in a
`marked reduction in the rate of deposit formation. The
`analysis of deposits also indicates that the nature of de(cid:173)
`posits formed in a deoxygenated fuel
`is different
`from
`those formed in an air-saturated fuel, suggesting that such
`deposits are formed as a result of different classes of reac(cid:173)
`tions. Although removal of molecular oxygen reduced de(cid:173)
`posit formation rates somewhat in the vapor phase, by far
`the greatest reduction occured in the liquid phase where
`rates were often reduced by a factor of 10 or better. Free
`radical, autoxidative reactions must clearly dominate the
`complex series of chemical
`reactions which ultimately
`leads to the formation of deposits. Thus, at low,
`liquid
`phase temperatures, removal of molecular oxygen reduces
`deposit formation by suppressing the hydrocarbon autox(cid:173)
`idative reactions which are the dominant cause of deposit
`formation. At higher temperatures new hydrocarbon reac(cid:173)
`tions begin to assume importance. Jet fuel range hydro(cid:173)
`carbons start to exhibit measurable rates of pyrolysis at
`temperatures above approximately 350°C (Fabuss, et al.,
`1964). Although such reactions are normally termed "ho(cid:173)
`mogeneous," the catalytic effect of surfaces in pyrolysis
`studies has been shown to be important (Taylor, et al.,
`1969). The predominate products from pyrolysis reactions
`have molecular weight equivalent to or less than the par(cid:173)
`ent hydrocarbon, although higher molecular species have
`been reported (Fabuss, et al., 1962). The production of
`olefins which occurs in pyrolysis reactions would be ex(cid:173)
`pected to be deleterious in that it could readily lead to
`condensation reactions, particularly at higher pressures.
`In the present study increasing the total pressure from
`18 to 69 atm had a complex effect on deposit formation.
`With fuel A higher pressures had no effect on deposit for(cid:173)
`mation whereas with fuel B,
`it resulted in lower deposit
`formation rates. In a liquid phase environment, as pointed
`out previously,
`it
`is a change in solvent character rather
`than an increase in molecular weight which influences the
`deposit formation process. In a vapor phase environment
`it could be expected that this situation would be reversed.
`Although this should clearly be the case at lower pres(cid:173)
`sures,
`the situation at pressures above the critical pres(cid:173)
`sure is not as clear. "Supercritical" gases can exert sol(cid:173)
`vent properties much like liquids, particularly as their
`density approaches that of a liquid at high pressures (Gid(cid:173)
`dings, et al., 1968). Such solvent properties are a sensitive
`function of fluid density and thus of pressure. It would be
`expected that enhanced solvent character at higher pres(cid:173)
`sures would reduce the formation of deposits. In contrast,
`pressure generally exerts an accelerating influence on py(cid:173)
`increase
`reactions. Higher pressures, of course.
`rolysis
`contact time at otherwise fixed conditions in a flow reac(cid:173)
`tion system. Thus, the effect of pressure on the overall de(cid:173)
`posit formation 'process would appear to be complex, par-
`
`Ind . Eng. Chem ., Prod. Res. Develop., Vol. 13, No .2, 1974
`
`137
`
`GE-1020.007
`
`

`
`Ind. Eng.
`
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