e
`
`80
`
`~
`
`I
`
`IECPRA 19(1) 1-130 (1980)
`ISSN 0196-4321
`
`VOLUME 19
`NUMBER 1
`MARCH 1980
`
`.
`aJOO~D[fD8JD ~
`~[]JrnD[]Jffiffi[fD[]Jrn rnwffiDDiJD~D[f~
`ProductResearch
`and Development
`
`'lf1r
`A·
`~
`
`

`
`INDUSTRIAL &
`ENGINEERING CHEMISTRY
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`

`
`Ind. Eng. Chern. Prod. Res. Dev. 1980, 19, 65-70
`
`65
`
`10..---
`
`- - - - - - - - - - - - ,
`Temp.,) 5 0 'C
`SV .18000h-l
`: Ne-l , 2 -4mm
`Cdt.
`
`0.8
`
`0.6
`
`.~
`
`~ 04
`
`0.2
`
`NO 195 ppm
`~ 195
`•
`~ 130
`•
`~ 134 "10
`N1 balance
`
`2.0
`
`1.5
`1.0
`05
`K1SO. d dded ( '1.)
`Figure 5. Effect of K2SO. on activity of and iron oxide catalyst:
`catalyst size, 2-4 mm; SV, 18000 h-l.
`ratio klkodeclined with varying amount of K2S04 added.
`The klko ratio decreased below 0.3 with an addition of ca.
`2% K2S04; it was realized that K2S04 played a vital role
`on the catalyst activity.
`
`As was discussed so far, the main cause of the catalyst
`deactivation in a dust-free gas such as flue gas I was SOx
`in the gas. In the flue gas containing reactive dust com(cid:173)
`ponents, however, the dust component, in particular K,
`had a great influence on catalytic activity. Therefore, these
`iron oxide catalysts could be used practically for the re(cid:173)
`moval of NOx in flue gases without such a dust component
`as K.
`
`Literature Cited
`Naruse, Y., Ogasawara, T., Hata, T., Klshitaka, H., Ind. Eng. Chern., Prod.
`Res. Dev., preceding article In this Issue, 1980.
`Nelsen, F. M., Eggertsen, F. T., Anal . crem., 30(8), 1387 (1958).
`Japanese Patent, Application No. 50-127081, Research Association lor
`Industry In Japan, 1975.
`Abatement and Removal 01 NO. In the Steel
`Nishijlma, A., Kurita, M., Sato, T., Klyozuml, Y., Haglwara, H., Ueno, A., Tooo,
`N., Nippon Kagaku Kaishi, 276-282 (1979).
`Wakao, N., Smith, J. M., Chern. Eng. Sci., 17, 825-834 (1962).
`
`Received for review April 16, 1979
`Accepted September 23, 1979
`
`Deposit Formation from Deoxygenated Hydrocarbons. 4. Studies in
`Pure Compound Systems
`
`John W. Frankenfeld' and William F. Taylor
`
`Exxon Research and Engineering Company, Linden, New Jersey 07036
`
`The effects of hydrocarbon type on deposit formation in deoxygenated fuels was studied using purified hydrocarbon
`blends. The rate of deposit formation was determined at 150-650 °c in fuel blends with molecular oxygen levels
`reduced to below 1 ppm. Deposit formation rates with deoxygenated pure compound blends that did not contain
`oletins were low at low temperatures but accelerated rapidly above 500°C. Most oletins added to the fuels promoted
`deposit formation even at low temperatures but the effect varied widely with compound type. The morphology
`of the deposits obtained from deoxygenated blends was different from that observed in air-saturated fuels. The
`results are consistent with a dual mechanism for deposit formation: autoxidative oligomerization at low temperatures
`and pyrolytic breakdown at high temperatures.
`
`Introduction
`The deposit formation tendencies of jet fuel range hy(cid:173)
`drocarbons have been the subject of considerable research
`(Nixon, 1962). Initial work was carried out with air-sat(cid:173)
`urated hydrocarbons in a narrow, near ambient tempera(cid:173)
`ture range in order to investigate storage stability char(cid:173)
`acteristics. Subsequent studies were extended to higher
`temperatures in order to investigate the stability of such
`fuels when used in high-speed supersonic aircraft (Nixon,
`1962; Churchill, et al., 1966). Such studies were carried
`out mainly with fuels saturated with molecular oxygen via
`exposure to air although some limited work has been re(cid:173)
`ported with reduced oxygen containing fuels (Taylor and
`Wallace, 1967).
`This laboratory has conducted an extensive study of the
`variableswhich control the kinetics of deposit formation
`from hydrocarbons exposed to such high-temperature
`stress and the factors which may help overcome this in(cid:173)
`stability.
`Initially these studies were carried out with
`air-saturated jet fuels (Taylor, 1969, Taylor and Wallace,
`1967). More recently they have been extended to deoxy(cid:173)
`genated systems. The improvements in fuel stability which
`accrue on deoxygenation were pointed out by Taylor
`(1974). However,in certain poor quality fuels the expected
`
`enhancement of stability by deoxygenation did not occur.
`This observation led to a study of the effects of trace
`impurities, likely to be present in the poor quality fuels,
`on deposit formation to determine whether such impurities
`were negating the beneficial effects of molecular oxygen
`removal. Taylor (1976) found that certain sulfur con(cid:173)
`taining compounds could be highly deleterious to high(cid:173)
`temperature stability in deoxygenated fuels. Taylor and
`Frankenfeld (1978) studied nitrogen and oxygen containing
`impurities and found that the nitrogen compounds studied
`were nondeleterious at high temperature but certain of
`them led to sludge formation during storage under ambient
`conditions. Many of the oxygen compounds, on the other
`hand, were found to be moderately to severely deleterious
`to high-temperature stability in deoxygenated JP-5.
`In this paper the effects of hydrocarbon type on deposit
`formation in deoxygenated fuels are discussed. The fuels
`used in these studies were blends of pure hydrocarbons,
`representative of those found in actual jet fuels.
`In ad(cid:173)
`dition to the influences of individual hydrocarbons, the
`effects of interactions between hydrocarbon types were
`investigated. Finally, the effects of dissolved oxygen (02)
`level on the morphology of high-temperature deposits in
`pure hydrocarbon blends are discussed in light of the
`
`0196-4321/80/1219-0065$01.00/0
`
`© 1980 American Chemical Society
`
`

`
`1
`2
`
`3
`
`4
`
`5
`
`6
`
`7
`
`8
`
`9
`
`66
`
`Ind. Eng. Chern. Prod. Res. Dev., Vol. 19, No.1, 1980
`
`possible mechanisms for deposit formation under condi(cid:173)
`tions of high-temperature stress.
`Experimental Section
`Apparatus. A schematic of the Advanced Kinetic Unit
`used to measure the rate of deposit formation was shown
`previously (Taylor, 1974). The molecular oxygen content
`of the fuel to be tested was adjusted by sparging at at(cid:173)
`mospheric pressure using helium. The treated fuel was
`passed through an oxygen sensor cell and delivered to a
`double piston fuel delivery cylinder. The oxygen sensor
`cell contained a polarographic sensor and the oxygen
`content of the total fuel was monitored. Oxygen analyses
`were also made on selected samples using a thermal con(cid:173)
`ductivity gas chromatographic analyzer. The fuel was
`delivered to the unit by means of high-pressure nitrogen.
`The treated fuel was separated from the nitrogen drive gas
`by use of two individual pistons, separated by a small water
`layer. The fuel then passed through a heated tubular
`reactor section consisting of a 0.25 in. o.d., 0.083 in. wall
`S.S. 304 tube which was contained inside four individually
`controlled heaters. Each heater zone was approximately
`12 in. long and was controlled by a proportional temper(cid:173)
`ature controller. Unit pressure was controlled by means
`of a Mity-Mite controller.
`The rate of deposit formation was measured after a 4-h
`run. The reactor tube was cut into 16 sections, each 3 in.
`long, (4 sections per reaction zone) and the tube sections
`were analyzed for carbonaceous deposits using a modified
`LECO low carbon analyzer system (Taylor, 1974). The
`analytical system was calibrated against known standards.
`The deposit formation rate was 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 four hour reaction time.
`Reagents. The individual hydrocarbons employed were
`obtained from commercial sources and were the highest
`quality available. They were purified further by passing
`them over both activated alumina and finely divided silica
`gel and stored under nitrogen in the absence of light before
`blending and use.
`All fuel blends were made by adding the stated per(cid:173)
`centages (by weight) of the various pure components ac(cid:173)
`companied by thorough mixing. The resulting blends were
`deoxygenated as described above and tested immediately.
`Results
`The effects of various pure hydrocarbons on deposit
`formation in deoxygenated fuels was studied by preparing
`blends of highly purified compounds added consecutively
`to make an increasingly complex mixture. Blends con(cid:173)
`taining up to six components were studied. The pure
`compounds chosen were representative of known classes
`of hydrocarbons found in jet fuels and were employed at
`levels approximating their normal occurrence. The most
`prevalent hydrocarbon types in jet fuels of the JP-5 type
`are normal paraffins, branched paraffins, substituted cy(cid:173)
`cloparaffins (naphthenes), and substituted aromatics.
`Their relative concentrations vary considerably from one
`fuel to another depending on crude oil source and pro(cid:173)
`cessing. However, a representative average is 25% normal
`paraffins, 25% branched paraffins, 30% naphthenes, and
`20% substituted aromatics (Taylor, 1973). The blends
`employed in this study are shown in Table I. The four(cid:173)
`component blends 4 and 5 were used as base stocks for
`studies of other hydrocarbon types which would bepresent
`in small quantities. Aromatic compounds were added to
`these blends by backing out an equivalent amount of
`sec-butylbenzene while naphthenes were added in place
`of isopropylcyclohexane. Olefins were added in place of
`
`100
`50
`50
`35
`35
`30
`25
`25
`20
`30
`25
`25
`20
`30
`25
`25
`15
`30
`5
`25
`25
`20
`25
`5
`25
`25
`15
`25
`5
`5
`25
`25
`15
`30
`5
`25
`25
`15
`30
`5
`25
`25
`10
`30
`5
`5
`
`Table I. Composition of Pure Hydrocarbon Blends
`wt%
`bl end no .
`compound(s )
`n -do de ca ne
`n-dodeca ne
`2,2,5-tri met hylhexane
`n-dodecane
`2,2,5-trimethylhexane
`sec- butylbenzene
`n-do deca ne
`2,2,5-trimethylhexane
`sec-butylbenzene
`iso propycyclohexane
`n-do decane
`2,2,4-tri methylpe ntane
`sec-but ylbenzene
`iso pro pycyclo hexane
`n-dodec an e
`2, 2,4-t r imethylhexane
`sec -bu t y lben zen e
`iso pro pylcyclo he xa ne
`naphthalen e
`n-dodecan e
`2 ,2,4-t rimethylhe xane
`sec-butyl ben zene
`isopropyl cycl ohex ane
`d ecalin
`n -dodec an e
`2,2 ,4-t r im et h ylhexane
`sec-but yl benzen e
`isoprop ylc ycl ohexa ne
`naphthalene
`decalin
`n-dodeca ne
`2,2, 4-tri methylpe nt ane
`sec-but ylben zen e
`isopropylc ycl ohexane
`tetra lin
`n-do decane
`2 ,2,4-trimethylpen t an e
`se c-b ut ylbe n ze ne
`isopropyi cyclohex ane
`indan
`n-dodecane
`2,2,4-trimethylpentane
`sec-butylb enzene
`isopropylcyclohexane
`t etralin
`ind an
`
`10
`
`11
`
`equivalent amounts of normal and branched paraffins.
`In general, differences in deposit formation rates among
`the pure compound blends were very small under deoxy(cid:173)
`genated conditions. This is illustrated by the Arrhenius
`plots shown in Figure 1. These plots relate the deposit
`formation rates to the temperature. A binary blend, a
`four-component blend, and a six-component blend are
`shown and all exhibit similar curves. In fact , all deoxy(cid:173)
`genated blends of compounds shown in Table I showed the
`same general behavior, i.e., relatively low deposit rates and
`flat Arrhenius curves until the temperature reached 538
`DC (1000 OF), at which point there was a sharp increase
`in carbonaceous deposits (Figure 1). The small differences
`that were observed all occurred at temperatures above 538
`DC.
`The effects of condensed ring aromatic-napthenic com(cid:173)
`pounds were evaluated using naphthalene, tetralin, decalin,
`and indan as model compounds. Compounds of these
`types are known to be present in JP-5 type fuels (Taylor,
`1973). Arrhenius plots for the blends containing naph(cid:173)
`thalene and decalin are shown in Figure 1. Similar plots
`were obtained in the case of indan and tetralin. In general,
`
`

`
`1000r--
`
`-
`
`-
`
`-
`
`-
`
`-
`
`-
`
`- - -- -- --
`
`- - - - ,
`
`1000 r - - - - - - - - - - - - - - -- - - --,
`
`Ind. Eng. Chern. Prod. Res. Dev., Vol. 19, No.1, 1980
`
`67
`
`100
`
`10
`
`'"0:
`::>g
`cr
`<,
`
`5<,
`
`u ~
`
`UJ...
`'"0:
`..
`'":;
`0:a
`u,
`!:::
`'"~
`UJa
`
`ZQ.
`
`N
`
`500 ·C
`400 ·C
`bOO ·C
`5 '------'---'----'-----'----_J....L--'---_----'----'--_---'---'-------'-_--'---__--.J
`I. 70
`·1 .b O
`1.50
`1.10
`1.20
`1.30
`1.40
`1000j"K
`Figure 2. Deoxygenated fuels at 69 atm:
`4 (Table I).
`
`e, JP-5 fuel; 0, blend no.
`
`.---
`
`,,,
`
`\
`
`\
`
`\
`
`\
`
`\
`
`\
`
`\.,
`
`\
`
`\
`
`\
`
`I
`I
`
`II
`
`I
`I
`
`1 0 0 0 r - - - - - - -
`
`"
`
`\
`
`\
`
`\ •
`\
`
`-,
`
`'"~
`~
`a
`
`100
`
`f
`I
`"I
`I
`I
`I
`I
`
`0°
`
`-.
`
`.." .
`
`"
`• • ~ ....
`.... ~.'
`
`I
`
`I
`
`.'.
`
`10
`
`600·C
`
`500·C
`
`1. 6 0
`
`. 70
`
`Figure 3. Air-saturated fuels at 69 atm: e, JP-5 fuel (64 ppm of
`O2) ; 0, blend no. 4 (Table I, 77 ppm of O2) ,
`
`ppm to 77 ppm of O2 for blend no. 4 compared to more
`than an eightfold increase over the same range of oxygen
`levels for JP-5 (Table 11). Arrhenius plots for both fuels
`on an air-saturated basis are given in Figure 3. Signifi(cid:173)
`cantly higher deposit formation rates, over the entire
`temperature range, were encountered with the actual JP-5
`fuel under these conditions.
`The effects of various olefms, representative of materials
`known to be present in many JP-5 type fuels, were eval(cid:173)
`uated using blend 4. The olefins were added at 2 wt %,
`typical levels for JP-5 fuels (Taylor, 1973). One percent
`each of the n-paraffin and branched paraffin were backed
`out. The results with monoolefins are given in Table III
`and with diolefins in Table IV. The total deposits over
`
`100
`
`10
`
`o
`
`bOO·C
`
`500·C
`
`400 ·C
`
`'";:
`
`i
`'"5
`
`u
`L.-
`
`ae
`
`-
`'"....
`""cr
`2....
`""::;
`cr::
`....
`;;;
`.
`'"a
`
`z5
`
`aa
`
`1 .30
`
`1 .50
`
`r.so
`
`l. 70
`
`1 .40
`1000l" K
`Figure 1. Deoxygenated pure compound blends at 69 atm: e, two
`component blend (50% n-dodecane, 50% trimethylhexane); 0, six(cid:173)
`component blend (5% naphthalene, 5% decalin, 25% n-dodecane,
`25% trimethylhexane, 25% isopropylcyclohexane, 15% sec-butyl(cid:173)
`benzene).
`
`Table II. Effect of Deoxygenation on a Pure Compound
`Blend Compared to Actual JP-5
`(Temperature Range 371-538 °C)O
`
`fuel
`JP-5
`
`blend 4 C
`
`0 , content, ppm
`64 (air sat urated)
`1.6
`0.4
`77 (air saturated)
`6.4
`0 .3
`
`total
`carbona-
`ceo us?
`dep,/lg
`11085
`3843
`1485
`1963
`1599
`1522
`
`dep at
`low
`temp,"
`«483
`°C), /lg
`8766
`3113
`774
`993
`534
`533
`
`69 atm ; zone 1, 371 °C; zone 2 ,
`a Other conditions :
`b Cumulative
`427 °C; zone 3, 483 °C; zone 4, 538 °C.
`deposits for 4-h run ; "low temperature" deposits from
`C See Table I.
`first two temperature zones only.
`
`the blends containing these condensed ring compounds
`showed the same behavior as the standard blends alone,
`i.e., relatively low deposit rates until the temperature
`reached 538°C (1000 OF) at which point there was a sharp
`increase in deposits (Figure 1). There appeared to be no
`unusual effects due to the addition of these compounds.
`Deposit formation rates over the temperature range of
`371-600 °C (700-1100 OF) for the four-component blend
`4 (Table I) are contrasted with those obtained from an
`actual JP-5 fuel in Table II and in the Arrhenius plots in
`Figures 2 and 3. When the fuels were deoxygenated (02
`content 1 ppm) the deposit formation rates were similar
`at temperatures of 500 °C or lower (Figure 2). At higher
`temperatures, the JP-5 appeared slightly more stable. The
`fuelsvaried considerably, however, in their response to the
`dissolved oxygen concentration. In both cases the highest
`deposit formation rates were observed on an air-saturated
`basis. However, the sensitivity to oxygen content was
`much greater in the case of the actual JP-5. Thus, only
`a 27% increase in deposits was observed in going from 0.3
`
`

`
`68
`
`Ind. Eng. Chern. Prod. Res. Dev., Vol. 19, No.1, 1980
`
`Table III. Effects of Monoolefins and Aromatic
`Olefins on Deposit Formation in a Deoxygenated
`Pure Hydrocarbon Blend (Temperature
`Range 150-650 0C)a
`
`-o;:
`
`0 ,
`content,
`ppm
`0.1
`
`total
`carbona-
`ceous
`dep ,/lgb
`
`7 000
`
`dep at
`low
`temp
`«483
`°C), /lg
`21;8
`
`0 .1
`
`9600
`
`299
`
`0 .1
`
`2964
`
`8 00
`
`1
`
`11 350
`
`5 35
`
`olefin
`1-dodecene
`CH,(CH,) 7CH=CH,
`cyclohexene
`
`0o
`
`-methylstyrene
`
`o -i= CH2
`CH,
`
`~
`
`indene
`
`00~ I
`
`allylbenzene
`o-CH2CH= CH2
`
`0.3
`
`2272
`
`300
`
`Figure 4. Deoxygenated pure compound no. 4 (Table I) at 69 atm :
`e, with 2% e -methylstyrene: 0 , with 2% allylbenzene.
`
`none
`
`0 .1
`
`68 43
`
`296
`
`a Standard blend No .6 (Table II) ; olefins added to 2 .0
`wt % by backing out 1.0% ea ch of n-paraffin and
`branched paraffin ; conditions as in Table II.
`b Total
`deposits over 4-h run.
`
`Table IV. Effects of Diolefins on Total Deposits in a
`Deoxygenated Pure Hydrocarbon Blend (Temperature
`Range 150-650 0C) a
`
`dep at
`low
`total
`temp
`carbona-
`0 ,
`con-
`ceous «483
`°Cl'
`tent
`dep,
`ppm /lg OCb
`0.2
`2640
`
`/lg
`289
`
`0 .2
`
`13442
`
`1880
`
`0.1
`
`12000
`
`300
`
`1.4
`
`3998
`
`201
`
`0 .1
`
`3105
`
`88 3
`
`0.2
`
`2965
`
`247
`
`0.2
`
`2965
`
`247
`
`olefin
`1,3-nonadiene
`CH ,( CH, ).CH=CHCH=CH,
`1,8-nonadiene
`CH, =CH(CH,) ,CH=CH,
`1,7-octadiene
`CH , = CH(CH ,).CH =CH,
`2,6-octadiene
`CH ,CH=CH( CH ,), CH =CHCH,
`divinylbenzene"
`o-CH= CH2
`CH= CH2
`1,3-cyclohexadi ene
`
`o
`
`4-vinylcyclohexene
`o-CH= CH2
`none
`
`a Standard blend no . 6 (Table III) ; olefins added at 2
`wt % by backing out each of n-paraffin and branched
`paraffin ; run conditions.
`b Cumulative deposits over 4-h
`run.
`C Mixed isomers.
`
`the 4-h run at 427-600 °C (S00-1100 OF) are shown as well
`as the deposits formed in the low-temperature regime. The
`results illustrate the very different behavior exhibited by
`
`olefins of different structure. Aliphatic and naphthenic
`monoolefins such as 1-dodecene and cyclohexene were
`non deleterious to mildly deleterious to fuel stability.
`Aromatic substituted monoolefins varied considerably in
`their influence on deposit formation. Indene was highly
`deleterious both in total deposits and in the low-temper(cid:173)
`ature regime. Allylbenzene was nondeleterious, affording
`significantly less deposits than the blend itself. The con(cid:173)
`jugated aromatic olefin, o-methylstyrene, was intermediate
`in its effect. Although the blend with this olefin present
`produced less total deposits, the deposit rate at low tem(cid:173)
`peratures was much higher . This difference in behavior
`is illustrated by the Arrhenius plots in Figure 4 as well as
`the data in Table III. The tendency toward low temper(cid:173)
`ature deposit formation exhibited by a-methylstyrene is
`potentially quite deleterious to long-term storage stability
`of jet fuels. Allylbenzene, on the other hand, does not show
`this tendency.
`Structural effects were also significant in determining
`the behavior of diolefins (Table IV). Thus, the a,w-di(cid:173)
`olefins, 1,S-nonadiene and 1,7-octadiene, were quite de(cid:173)
`leterious at low temperatures. This is illustrated by the
`Arrhenius plots in Figure 5 where 1,S-nonadiene is con(cid:173)
`trasted with 1,3-nonadiene. The former gives significantly
`greater deposit rates at temperatures from 150 to 500 "C
`(300-930 OF). The cyclic diolefins, 1,3-cyclohexanediene
`and 4-vinylcyclohexene, were both relatively stable al(cid:173)
`though the former gave lower deposit rates. Divinyl(cid:173)
`benzene behaved in similar fashion to a-methylstyrene
`with which it is structurally related. This material gave
`relatively low overall deposits but showed a high deposit
`formation rate at low temperatures.
`A single acetylene, 1-decyne, was tested, also at the 2
`wt % level. As expected, this compound was highly
`unstable, exhibiting total deposits more than six times
`those of the base blend. This is higher than has been
`observed with peroxides (Taylor and Frankenfeld, 1975)
`or disulfides (Taylor, 1976)and represented easily the most
`unstable deoxygenated fuel blend component yet studied.
`The dissolved oxygen level exerts a significant influence
`on the morphology of deposits obtained from pure com(cid:173)
`pound blends. Electron micrographs of deposits obtained
`from fuels with varying oxygen contents are shown in
`Figure 6. The samples were taken from the corresponding
`section of the reaction tube. The local deposit formation
`
`.~.
`
`0 ___
`
`0
`
`BLEND NO • •
`ALONE
`
`100
`
`10
`
`5
`
`4 3
`
`e~ v
`
`N'
`5
`
`<,
`
`u u
`
`,
`
`0 "
`
`'>-
`""'"
`>-""::;
`
`zQ
`
`tx:
`0
`u,
`
`~
`
`'"0
`"'
`
`Q.
`
`0
`
`

`
`Ind. Eng. Chern. Prod. Res. Dev., Vol. 19, No.1, 1980
`
`69
`
`A. Air Saturated. 11 PPM 0,
`
`(Run No. 92; 29.1 uv.C!cm2/4 hr)
`
`0
`
`0
`
`0 o
`
`1000
`
`'"~
`
`0
`~
`
`100
`
`~~O~
`
`,II
`:\
`\
`~ au . . , .c.,
`
`. 0
`
`\
`
`\
`
`10
`
`0
`
`/
`
`\
`00 '
`•
`.... o .~ ~,
`•
`
`......•
`
`'"5
`
`u
`u,
`
`0 w
`
`I-"'"z
`Q
`I-
`<::;;
`tx
`-
`!:
`'"0
`
`0"
`
`0-
`
`W0
`
`1.30
`
`371"C
`1 . 50
`
`260·C
`1.90
`
`2 . 10
`
`2.30
`
`1 . 70
`100 0/"K
`Figure 5. Deoxygenated pure compound blend no. 4 (Table I) at
`69atm: e, with 2% 1,3-nonadiene; 0 , with 2% 1,8-nonadiene.
`
`rate was nearly the same for this tube section in the three
`runs. The air-saturated fuel (Figure 6A) contains nu(cid:173)
`merous spherical particles 1500-3000 A in diameter on a
`solid varnish-like background. As the dissolved O2 content
`decreases (Figures 6B and 6C), the number of spherical
`particles is greatly diminished and the deposits become
`more varnish-like in character.
`Discussion
`The mechanism of dep osit formation in air-saturated
`fuels has been discussed by a number of workers (Mayo,
`1968; Boss and Hazlett, 1969; Taylor, 1969, 1972, 1974).
`It isclear that deposits at low temperature «500 °C) form
`as the result of complex free radical liquid phase aut(cid:173)
`oxidative reactions while high-temperature deposits are
`mainly caused by pyrolysis. Where such information is
`available it appears that tendency to form deposits at lower
`temperatures for various hydrocarbons follows their sus(cid:173)
`ceptibility to autoxidative attack (Mayo, 1968; Taylor,
`1969). The present results as well as those presented by
`Taylor (1974) indicate that different reactions occur and
`different orders of reactivity obtain under oxygen-poor
`conditions. Taylor (1969) reported the effects of fuel
`composition on deposit formation in air-saturated fuels.
`He found that branched paraffins increased and most
`aromatics significantly decreased total deposit formation
`when blended with normal paraffins.
`In contrast, the
`results discussed here (Figure 1) show little, if any , dif(cid:173)
`ference among such blends when they are deoxygenated.
`The effects of condensed ring compounds also show con(cid:173)
`siderable variation between air-saturated and deoxygen(cid:173)
`ated fuels. For example, tetralin reduced deposits by more
`than 80% in air-saturated blends (Taylor, 1969). In de(cid:173)
`oxygenated fuels, tetralin was only slightly inhibitory. In
`fact, tetralin was one of the most effective inhibitors
`studiedby Taylor (1969) , presumably because it possesses
`four benzylic hydrogen atoms which inhibit cooxidation
`reactions involving hydrogen abstraction by peroxy radicals
`(Ingold, 1967). Apparently thi s effect is important only
`when an excess of oxygen (and, therefore, peroxy radicals)
`ispresent. Indan, which also has four benzylic hydrogens,
`
`B.
`
`6.1.. P
`
`9.l. (Run No. 93; 25.0 ugC/cm 2/ 4 hr)
`
`c.
`0.3 PPM 02 (Run No. 91; 25.0 ';1gClc.m2/4 hr)
`Figure 6. Electron micrographs of dep osits from pure compound
`blend no. 4 (Table I) at variou s dissolved oxygen levels: A, air sat(cid:173)
`urated (77 ppm of O2) ; B, 6.4 ppm of O2; C, <1 ppm of O2; all at
`5000X magnification.
`
`might be expected to be as strong an inhibitor as tetralin.
`However,in these studies, where oxygen was largely absent,
`it, as well as tetralin, exhibited virtually no influence on
`deposit formation.
`The addition of monoolefins to the synthetic blends
`tended to increase the rate of deposit formation in most
`cases (Table III). Again structural effects are noteworthy
`but quite different from those observed under air-saturated
`conditions. Thus, allylbenzene significantly retarded
`sediment formation under deoxygenated conditions and
`afforded significantly lower levels of deposits than either
`the standard blend alone or the blend containing the
`monoolefin , I-dodecene. Taylor (1969) encountered just
`the opposite effects in air-saturated systems. In addition,
`Taylor found that vinylcyclohexene was much more re(cid:173)
`active toward deposit formation than vinylcyclohexaneand
`the conjugated diolefin, 7-methyl-3-methylene-l,6-octa(cid:173)
`diene , was significantly more reactive than either vinyl(cid:173)
`cyclohexane or 1-decene in air-saturated fuels. Taylor's
`results correlate well with the relative rates of oxidation
`for such olefins (Mayo, 1968). The present results, in an
`oxygen-poor environment, are quite different. For exam(cid:173)
`ple, the conjugated diolefins, 1,3-nonadiene, and 1,3-
`
`

`
`70
`
`Ind. Eng. Chern. Prod. Res. Dev., Vol. 19, No.1, 1980
`
`cyclohexadiene, were relatively inert while the a,w-di(cid:173)
`olefins, 1,8-nonadiene and 1,7-octadiene, were quite de(cid:173)
`leterious to fuel stability (Table IV). Intermediate between
`these extremes were the nonconjugated diolefins with at
`least one internal double bond. Clearly, the present results
`in oxygen-poor environments, show just the opposite ef(cid:173)
`fects from those observed under air saturated conditions.
`Taylor (1974) has suggested that these differences can
`be explained on the basis of different mechanisms of de(cid:173)
`posit formation; autoxidation which occurs in the low(cid:173)
`temperature regime and pyrolysis which predominates at
`high temperatures. Thus, in deoxygenated fuels, low(cid:173)
`temperature, autoxidative reactions may not be of prime
`importance; rather, pyrolysis becomes the prevalent overall
`mechanism. Thus, very little deposits obtained at tem(cid:173)
`peratures below that which pyrolysis occurs.
`This view is supported by the Arrhenius plots shown in
`Figures 1 and 3. Significant deposits in all deoxygenated
`runs appear only at high temperatures where pyrolysis
`becomes important. Fabuss et a1. (1964) report that jet
`fuel range hydrocarbons start to exhibit measurable rates
`of pyrolysis above 350°C. The air-saturated fuels (Figure
`3) show a greater tendency toward autoxidative deposit
`formation in the low-temperature regime. The much
`greater sensitivity to oxygen level exhibited by actual JP-5
`can be explained on the basis of fuel composition. The
`minor components of the JP-5, olefins and sulfur, nitrogen,
`and oxygen compounds, are more readily attacked by
`molecular oxygen (Nixon, 1962; Mayo, 1968;Taylor, 1974).
`This leads to higher rates of autoxidative decomposition
`for JP-5 relative to the four-component pure compound
`blend at all levels of dissolved oxygen.
`The unusual shapes of the Arrhenius plots for a-me(cid:173)
`thylstyrene (Figure 4) may be explained similarly. Mayo
`(1968) found that a-methylstyrene was oxidized almost 30
`times as rapidly as allylbenzene and more than 200 times
`as rapidly as a terminal monoolefm such as l-octene. Thus,
`it appears that, even at the very low levels of dissolved
`oxygen employed in this study, a-methylstyrene reacts at
`an appreciable rate in the low-temperature, autoxidative
`reaction, regime. Blends containing o-methylstyrene, on
`the other hand, undergo high-temperature pyrolysis at a
`slower rate than the standard blend alone (Figure 4). This
`is additional evidence for the dual mechanism of deposit
`formation with high-temperature pyrolysis predominating
`in deoxygenated systems.
`Autoxidative deposits, formed in air-saturated fuels,
`contain as much as 30% oxygen (Taylor, 1974; Nixon ,
`1962). Analysis of deposits from deoxygenated fuels
`showed only 3-6% oxygen (Taylor, 1974; Taylor, 1973)