Ind. Eng. Chem. Prod. Res. Dev. 1980, 79, 65-70
08 -
SV
18000h-'
Cat
N-1, 2-4mm
As was discussed so far, the main cause of the catalyst deactivation in a dust-free gas such as flue gas I was SO, in the gas. In the flue gas containing reactive dust components, 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 removal of NO, in flue gases without such a dust component as K.
I
Temp 3 5 0 'C
65
Literature Cited 0
05
10
15
Naruse, Y . , Ogasawara, T., Hata, T., Kishitaka, H., Ind. Eng. Cbem., Prod. Res. Dev., preceding article in this issue, 1980. Nelsen, F. M.. Eggertsen. F. T., Anal. Chem., 30(8), 1387 (1958). Japanese Patent, Application No. 50-127081, Research Association for Abatement and Removal of NO, in the Steel Industry in Japan, 1975. Nishijima, A., Kurita, M., Sato, T., Kiyozumi, Y., Hagiwara, H., Ueno, A,, Todo, N., Nippon Kagaku Kaisbi, 276-282 (1979). Wakao, N., Smith, J. M.. Cbem. Eng. Sci., 17, 825-834 (1962).
20
K,SOL a d d e d ( % )
F i g u r e 5. E f f e c t o f K2S04o n a c t i v i t y catalyst size, 2-4 mm; S V , 18 000 h-l.
of a n d iron oxide catalyst:
ratio k / ko declined with varying amount of K2S@,added. The k/ko ratio decreased below 0.3 with an addition of ca. 2% K2S04;it was realized that K2S04played a vital role on the catalyst activity.
Received f o r review April 16, 1979 Accepted S e p t e m b e r 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 OC in fuel blends with molecular oxygen levels reduced to below 1 ppm. Deposit formation rates with deoxygenated pure compound blends that did not contain olefins were low at low temperatures but accelerated rapidiy above 500 O C . Most olefins 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 oligomerizationat low temperatures and pyrolytic breakdown at high temperatures.
Introduction The deposit formation tendencies of jet fuel range hydrocarbons have been the subject of considerable research (Nixon, 1962). Initial work was carried out with air-saturated hydrocarbons in a narrow, near ambient temperature range in order to investigate storage stability characteristics. 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 reported with reduced oxygen containing fuels (Taylor and Wallace, 1967). This laboratory has conducted an extensive study of the variables which control the kinetics of deposit formation from hydrocarbons exposed to such high-temperature stress and the factors which may help overcome this instability. Initially these studies were carried out with air-saturated jet fuels, (Taylor, 1969, Taylor and Wallace, 1967). More recently they have been extended to deoxygenated systems. The improvements in fuel stability which accrue on deoxygenation were pointed out by Taylor (1974). However, in certain poor quality fuels the expected 0196-4321/80/1219-0065$01.00/0
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 containing compounds could be highly deleterious to hightemperature 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 addition to the influences of individual hydrocarbons, the effects of interactions between hydrocarbon types were investigated. Finally, the effects of dissolved oxygen (0,) level on the morphology of high-temperature deposits in pure hydrocarbon blends are discussed in light of the C
1980 American Chemical Society
66
Ind. Eng. Chem. Prod.
Res. Dev.,
Vol. 19, No. 1, 1980
possible mechanisms for deposit formation under conditions 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 atmospheric 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 conductivity 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 1 2 in. long and was controlled by a proportional temperature controller. Unit pressure was controlled by means of a Mity-Mite controller. The rate of deposit formaticn 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 percentages (by weight) of the various pure components accompanied 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 containing 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 cycloparaffins (naphthenes), and substituted aromatics. Their relative concentrations vary considerably from one fuel to another depending on crude oil source and processing. 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 fourcomponent blends 4 and 5 were used as base stocks for studies of other hydrocarbon types which would be present 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
Table I.
Composition of Pure Hydrocarbon Blends
blend no.
compound(s)
wt %
n-dodecane n-do de ca n e 2,2,54rimethylhexane
3
n-dodecane 2,2,5-trimethylhexane see-but ylbenzene
4
n-dodecane 2,2,5 -trimethylhexane see-but ylbenzene isopropyc yclo hexane
5
n-dodecane 2,2,4-trimethylpentane see-but ylbenzene isopropycyclohexane
6
n -dodecan e 2,2,4-trimethylhexane sec-but ylbenzene isopropylcyclohexane naphthalene
7
n-dodecane 2,2,4-trimethylhexane see-butylbenzene isopropylcyclohexane d eca li n
8
n-dodecane 2,2,4-trimethylhexane sec-butylbenzene isopropylcyclohexane naphthalene de ca li n
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
9
n-dodecane 2,2,4-trimethylpentane see-butylbenzene isopropylcyclohexane tetralin
25 25 15 30 5
10
n-dodecane 2,2,4-trimethylpentane sec-but ylbenzene isopropylcyclohexane indan
25 25 15 30
11
n-dodecane 2,2,4-trimethylpentane see-but ylbenzene isopropylcyclohexane tetralin indan
25 2 %5 10 30
1
2
3
5 3
equivalent amounts of normal and branched paraffins. In general, differences in deposit formation rates among the pure compound blends were very small under deoxygenated 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 deoxygenated 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 "C (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 "C. The effects of condensed ring aromatic-napthenic compounds 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 naphthalene and decalin are shown in Figure 1. Similar plots were obtained in the case of indan and tetralin. In general,
67
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 1, 1980
+ c t
1
i
CL
1-
C00"C
5:O"C
400'C
l:LL1TLI_.GL__LLJJL+7" ..20 1.30 1.40 ..50 1.60
5 1 10
1000
,.
I
I
"(
Figure 1. Deoxygenated pure compound blends at 69 atm: 0 , two component blend (50% ,n-dodecane,5 0 7 ~trimethylhexane); 0 , sixcomponent blend (5% naphthalene, 5% decalin, 25% n-dodecane, 25% trimethylhexane, 25% isopropylcyclohexane, 15% sec-butylbenzene).
0
120
130
,
I I 15C
1.40
,
I
I
-
160
70
lOC0 "K
Figure 2. Deoxygenated fuels at 69 atm: 0 , JP-5 fuel; 0 , blend no. 4 (Table I).
Table 11. Effect o f Deoxygenation o n a Pure Compound I3lend Compared t o Actual JP-5 (Temperature Range 371-538 "C)"
fuel
JP-5 blend A C
O2 c o n t e n t , p p m
total carbonaceousb d e p , pig
dep at low temp,b (
