I n d . E n g . C h e m . R e s . 1989,28, 729-738
729
Combined Effects of Hydrogen Sulfide, Water, and Ammonia on Liquid-Phase Hydrodenitrogenation of Quinoline Selahattin Gultekin,* Mohammad Khaleeq, and Muhammad A. Al-Saleh Chemical Engineering D e p a r t m e n t , King Fahd University of Petroleum and Minerals, Dhahran 31261. S a u d i Arabia
T h e combined effects of H2S, H20, and NH3 on liquid-phase hydrodenitrogenation (HDN) of quinoline in a batch slurry reactor were studied using presulfided commerical Ni-Mo/y-A1203 catalyst, a t 7.0 M P a and 330, 350, and 375 "C. The overall and individual rate constants were determined, and the product distributions, obtained through kinetic modeling, were compared with the experimental data. The kinetics of the overall HDN of quinoline were found to be 0.8 order with respect to total nitrogen concentration, whereas individual rates were first order. Both qualitative and quantitative effects of H2S,H20,and NH3 on the HDN of quinoline were presented. The presence of H2S alone enhanced the overall HDN rate significantly. H20,as such, enhanced the overall HDN to some extent; however, the effect of H2S was dominant when H2S and H 2 0 were together. NH3 alone did not show any effect. No effect on the HDN of quinoline was found in the presence of H2S and NH3 together. The combined effects of H2S, H20,and NH3 have resulted in a decrease in hydrogenation steps, but an enhancement in hydrogenolysis steps. The net effect was an increase in the overall HDN rate. Heavy oils such as petroleum crudes and synthetic crudes derived from coal, shales, and tar sands are characterized by having substantial amounts of sulfur, nitrogen, oxygen, and organometallic compounds in addition to the polynuclear aromatics and asphaltenes. Sulfur and nitrogen compounds are undesirable, because fuels derived from them, upon burning, produce SOz and NO,, which are severe air pollutants. Moreover, they are catalyst poisons in further processing steps. Especially N compounds, due to their basic character, preferentially adsorb on active sites of the acidic catalysts used for cracking, reforming, and isomerization and hence reduce the activity of the catalyst. The removal of oxygen compounds is required to improve the stability of the commercial fuels obtained from synthetic crudes. Industrial feedstocks are hydrotreated at high temperatures (330-425 "C) under high H2 pressures (5-10 MPa) in the presence of a suitable catalyst. Hydrodesulfurization (HDS), hydrodenitrogenation (HDN), and hydrodeoxygenation (HDO) take place, to some extent, simultaneously during hydrotreatment. Some hydrodemetallation (HDM) and hydrocracking may also occur. This is an active field of research, as can be seen from the amount of scientific literature and frequent reviews (Katzer and Sivasubramanian, 1979; Laine, 1983; Ledoux, 1985; Ho, 1988) dealing with the subject. Since HDS is easier than HDN or HDO, when the reaction mixture reaches HDN conditions (more severe conditions than for HDS), there will be a substantial amount of hydrogen sulfide (the final product of HDS) and water (the final product of HDO) in addition to some ammonia (the product of easily removed nitrogen compounds) in the real hydrotreater. Therefore, mutual interaction studies among these reactions has a paramount importance. Many earlier studies have indicated that the presence of nitrogen compounds retards the HDS reactions (Desikan and Amberg, 1964; Satterfield et al., 1975; Rollman, 1977). Recent studies by Shih et al. (1977) and Satterfield and his co-workers (Satterfield and Gultekin, 1981; Satterfield and Carter, 1981; Satterfield and Yang, 1984) have reported that the presence of hydrogen sulfide enhanced the overall HDN of quinoline. The effects of
* To w h o m
correspondence should b e addressed.
water and hydrogen sulfide on the HDN of quinoline are studied by Satterfield and Smith (1986a), Gultekin et al. (1985a,b), and Satterfield et al. (1985). However, the combined effects of hydrogen sulfide, water, and ammonia on the HDN of quinoline have not yet been reported in the literature. In the present paper, the combined effects of hydrogen sulfide, water, and ammonia on the HDN of quinoline in a batch slurry reactor are reported. The results are discussed both qualitatively as well as quantitatively.
Experimental Apparatus and Procedure The apparatus (Figure 1)used in this work is identical with that described by Gultekin et al. (1985a,b). The reactor is 1-L autoclave (Autoclave Engineers) with the necessary accessories like a Magnedrive I1 agitator, catalyst injection tubing (loader), a sampling line, and a hydrogen line. The catalyst, commercial Ni-Mo/y-Alz03 (HDS-3 TRILOBE, American Cynamid) ground and sieved to 140-170 mesh (96-106 hm), was presulfided a t 348 "C under a flow of 10% HzS in Hz gas for 4.5 h. Presulfided catalyst was charged into the loader together with quinoline (and CSz, H20, and diaminoethane, when required) and n-hexadecane (about 15 mL). Heating was started after the reactor was loaded with 485 mL of hexadecane, which was the carrier oil in the reactor. When the required temperature was reached, the catalyst-reactant feed from the loader was pushed into the reactor with the help of H2 pressure. The reactor was then brought to 7.0-MPa pressure. This was taken as the zero time of the reaction. The stirrer speed was set at 1500 rpm in order to minimize external mass-transfer resistance. Liquid samples, averaging about 1.5 mL each, were then periodically withdrawn from the reactor and collected. The sampling line was flushed before each sampling. A porous stainless steel filter was set at the opening of the sampling line inside the reactor to filter off the catalyst. The liquid samples were analyzed on a Varian 3700 gas chromatograph using a thermionic nitrogen specific detector. A glass column (Chromosorb 103,2 m) was capable of separating the peaks of different nitrogen compounds. The experiments are performed at three different temperatures (330,350,and 375 "C) under a hydrogen pressure of 7.0 MPa. In order to elucidate the individual and
0888-5885/89/2628-0~29$01.50/0 0 1989 American Chemical Society
730 Ind. Eng. Chem. Res., Vol. 28, No. 6, 1989 Q
PyTHQ
H
I
i NH3
+
HC
H
BzTHQ
DHQ
Figure 2. Quinoline HDN network Q, quinoline; PyTHQ, 1,2,3,4DHQ, tetrahydroquinoline; BzTHQ, 5,6,7,8-tetrahydroquinoline; decahydroquinoline; and HC, hydrocarbons.
-:1 F i g u r e 1. Experimental setup. T a b l e I. ExDerimental Design f o r B a t c h Reactor Systema ~
wt %
run
R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 R15 R16 R17 R18 R19
temp, "C 330 350 375 330 350 375 330 350 375 330 350 375 375 375 375 375 375 375 330
wt % Q 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.o 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
wt 7'0 CS2 wt % H 2 0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.04' 0.0 0.04b 0.04b 0.0 0.04 0.02c 0.04 0.02c 0.02' 0.04 0.04 0.02 0.02 0.04 0.02 0.04 0.02 0.08 0.04 0.04 0.04 0.02 0.04 0.00 0.00 0.02 0.00 0.00 0.00 0.00
DAE 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.03d 0.03d 0.03d 0.03 0.03 0.06 0.03 0.00 0.03 0.03
Total H2 pressure = 7.0 MPa; presulfided Ni-Mo/r-A120s catalyst (1.0 wt % ) = 3.9 g; volume of n-hexadecane per batch = 500 mL. '0.04 w t % CS2 corresponds to 34 kPa of H2S partial pressure. '0.02 w t % H 2 0 corresponds to 34 kPa of H 2 0 partial pressure. d0.03 w t % diaminoethane (DAE) corresponds to 34 kPa of NH3 partial pressure in the reactor. (I
combined effects of hydrogen sulfide, water, and ammonia on the kinetics of the catalytic hydrodenitrogenation of quinoline, various sets of feeds were used a t different compositions and temperatures as shown in Table I, which reveals the detailed experimental design for the batch reactor system.
Results and Discussion The major reaction intermediates detected by the gas chromatographic technique were quinoline (Q), 1,2,3,4tetrahydroquinoline (PyTHQ), 5,6,7&tetrahydroquinoline (BzTHQ), and decahydroquinoline (DHQ). (See the reaction network, Figure 2). Similar intermediate products were reported by Gultekin et al. (1985a,b). However, Satterfield and Yang (1984) have reported trace amounts of o-propylaniline as an additional intermediate. Their
0
1
1
3
5
4
6
1
1 I Y E (HR)
F i g u r e 3. HDN of quinoline at 7.0 MPa: effects of H2S, H 2 0 , and NH3 reproducibility.
initial feed contains 5 wt % quinoline, whereas ours contains 1 wt %. No peaks of o-propylaniline or ammonia were observed on the chromatogram; no attempts were made to detect the other hydrocarbons such as propylcyclohexane and propylbenzene. Conversion is expressed in percent HDN defined as %HDN =
cO
- CN
~
CO
100
where Co is the initial quinoline concentration and CN is the total concentration of all nitrogen compounds except ammonia. In order to test the consistency of the experimental data, three different runs (R6, R9, and R11;see Table I) were reproduced (Figure 3). The results are reproducible within an accuracy of 5%. In Figure 4, percent nitrogen removal (%HDN) is plotted versus time, where the feed is 1.0 wt. % quinoline in a-hexadecane (carrier) at 330,350, and 375 "C. It is also evident from the plot that %HDN increases with increasing the temperature of the reaction as per the Arrhenius law. Effect of Hydrogen Sulfide. The addition of 0.04 wt % carbon disulfide in the feed produces, in situ, 34 kPa
Ind. Eng. Chem. Res., Vol. 28, No. 6, 1989 731 100
no
-
z
== 6 1 r
= Y
"
-
10
20
0
0
1
2
J
1 1I Y
5
E
6
7
8
9
(HR)
Figure 4. HDN of quinoline: effects of temperature and H2S.
of hydrogen sulfide partial pressure. The presence of hydrogen sulfide in the reaction atmosphere enhances the overall HDN rate a t all temperatures (Figure 4). Satterfield and Gultekin (1981) reported that hydrogen sulfide, if present during hydrodenitrogenation, enhances the hydrogenolysis reactions and inhibits hydrogenation reactions to some degree. The net effect is the enhancement of the overall HDN rate. A quantitative discussion on this effect is given in the latter part of this paper. Individual Effects of Water and Ammonia. Water (0.02 wt 70)is added to the feed in order to generate 34 kPa of partial pressure. The presence of water alone at 375 " C (Figure 5) enhanced the overall HDN rate to some extent. However, the enhancement was lower than that in the presence of hydrogen sulfide alone. It can be postulated that water is less acidic than hydrogen sulfide when they are adsorbed on the catalyst surface. Moreover, hydrogenolysis reactions proceed faster with increasing the surface acidity; as a result hydrogen sulfide has a more pronounced enhancement effect than water. The presence of ammonia alone (34-kPa partial pressure), generated in situ by 0.03 wt % diaminoethane, did not affect the overall HDN of quinoline (Figure 6). Ammonia due to its more volatility and low solubility in the solvent hexadecane may not adsorb on the catalyst surface. As a result, no net effect of ammonia was found. Combined Effects of Hydrogen Sulfide and Water. The enhancement in the presence of hydrogen sulfide and water together, at all temperatures, was to the same extent as in the presence of hydrogen sulfide alone (Figures 7-9). In other words, water has no effect in the presence of hydrogen sulfide on the overall HDN of quinoline. We might reason that most of the surface might have been covered by hydrogen sulfide preferentially rather than water. The analogous behavior of water and hydrogen sulfide was reported by Satterfield et al. (1985) and Satterfield and Smith (1986a). Combined Effects of Hydrogen Sulfide and Ammonia. Ammonia, as such, did not affect the overall HDN; however, at 375 "C, no net effect on the HDN of quinoline was observed in the presence of hydrogen sulfide and am-
0
1
3
2
1
5
E (HR) Figure 5. Individual effects of H2S, HzO, and NH3 a t 375 1I Y
* *
p o % t r A T 330 c P tNH3 A I 330 C P ONLY AT 375 C
0
P tHZS A I 375 C
0
0 t H H J A T 375 C 0 t H Z S t H H 3 AI 3 7 5 C
A
+
0
1
1
J
k
5
6
7
d
OC.
9
E (HR) Figure 6. Effects of NH3 and H2S on HDN of quinoline. 1I Y
monia (Figure 6). It is hypothesized that hydrogen sulfide and ammonia may react with each other in the bulk phase, forming ammonium hydrosulfide, NHIHS (AGzse = -12.1 kcal/mol), which is stable under the reaction conditions. Combined Effects of Hydrogen Sulfide, Water, and Ammonia. At 350 and 375 "C, the presence of hydrogen sulfide, water, and ammonia (simultaneously) enhanced the overall HDN to the same extent as in the presence of H2S alone (Figure 8 and 9), whereas at 330 "C the combined effect has shown slightly lower enhancement (Figure 7) than that in the presence of hydrogen sulfide alone. Therefore, the interaction among H2S, HzO, and NH, may also play an important role in the overall HDN of quino-
732 Ind. Eng. Chem. Res., Vol. 28, No. 6, 1989
loo
i
0
1
1
5
4
5
6
7
8
9
Y E (HR) Figure 7. Effects of H2S, H20, and NH3 at 330 "C.
0
I
T I
2
3 1I Y
k
5
k
5
E (HR)
Figure 9. Effects of H2S, H 2 0 , and NH, at 375 "C. 100
IO
z r
60
t
z Y
"
E
40
0 . .
20
0 o
t
2
3
4 T I Y E
5
6
7
n
9
(HR)
0
I
2
J T I Y E (HR)
Figure 8. Effects of H2S, HzO, and NH, at 350 "C.
Figure 10. Effects of increasing the H2S concentration at 375 "C.
line. Hence, the combined effects were further studied by changing the partial pressures of H2S, H20, and NH3 individually. The effects of increasing the concentrations of hydrogen sulfide, water, and ammonia are shown in Figures 10-12, respectively (see Table I for feed conditions). Doubling the partial pressure of hydrogen sulfide to 68 kPa in the system did not show further enhancement (Figure 10) on the overall HDN. Since hydrogen sulfide is available in excess, a part of it may react with the ammonia in the bulk phase, and the remaining hydrogen sulfide is more than sufficient to cover the catalyst surface completely. In other words, enhancement has reached its maximum. In Figure
11, when the concentration of water was doubled, the enhancement effect was almost the same as that in the presence of water alone (see Figure 5 ) . When the concentration of water increases, it may be competing with hydrogen sulfide for adsorption on the catalyst surface, since water causes less acidity than hydrogen sulfide on the surface, and hence the overall HDN enhancement is slightly lowered. In other words, enhancement in this case may be due to water alone. Similarly, when the concentration of the ammonia was doubled, the enhancement effect was to the same extent as that in the presence of water alone. At this stage, it can be concluded that whole hydrogen sulfide may have been consumed in the bulk-
Ind. Eng. Chem. Res., Vol. 28, No. 6, 1989 733
DHO BZ-THP
A c U E LL
w __I
0
=
0.6
O
0.5
0
Q
PY-THO
0.k 0.3 0.2 0.1
0.0
0
1
2
3
4
5
6
7
a
9
1 I Y E (HR)
Figure 13. Product distribution a t 330 "C; Q feed only.
t
0 ONLY
A
0 tHZStH2OtNHS
0
4 tHZSt 2H20tNH3
1.0
t DHO
BZ-THO
A
I
0
3
1
k
5
1 I Y E (HR) Figure 11. Effects of increasing the H20concentration a t 375 "C.
0
3
1
1
k
0
0
0
PI-THO
5
6
7
1 I Y E (HA)
Figure 14. Product distribution a t 350 "C; Q feed only.
DHP A
20
: If
BZ-THO
o
a
0
PI-THO
0 ONLY
-
4 tHZStH20tNHS o 4 t H Z S t H 2 0 t 2NHJ A
'
J
0 0
I
1
3
+
0
5
1 I Y E (HR)
Figure 12. Effects of increasing the NH3 concentration a t 375 O C .
phase reaction with enough ammonia available to form ammonium hydrosulfide. Therefore, the resulting enhancement is attributed to water. Product Distribution. The distributions of nitrogen intermediate concentrations with respect to reaction time are shown in Figures 13-15 for three temperatures (330, 350, and 375 "C)when quinoline only was used as the feed a t 7.0 MPa. Theoretical lines, simulated by the kinetic model (see details later), are drawn through the experimental data points. In the presence of sulfided catalyst and under the reaction conditions, quinoline is hydrogenated to form PyTHQ, which is a fast, reversible reaction (Satterfield et al., 1978) that reaches equilibrium almost
1
3
2
k
5
1 I Y E (HR)
Figure 15. Product distribution a t 375 "C; Q feed only.
instantaneously. As the reaction proceeds with time, eventually both the quinoline and PyTHQ concentrations decrease in proportion. It is also evident from Figures 13-15 that as the temperature increases the concentration of quinoline becomes high and the concentration of PyTHQ becomes low, which is the result of the thermodynamic equilibrium effect between Q and PyTHQ. The intermediate decahydroquinoline (DHQ) concentration, at first, increases substantially and decreases gradually depending on the temperature, which is very typical for intermediates. Decahydroquinoline concentration is higher (Figure 13) at 330 "C and decreases rapidly with increasing temperature (Figures 14 and 15). It can be postulated here that a t low temperature (330 "C)
734 Ind. Eng. Chem. Res., Vol. 28, No. 6, 1989
1 I
1.0
0.9
DHC!
0.1
BZ-THQ
A
o
0.7
Q PI-THQ
0
0.6 I
zos -. --' 0 . k
A
I = 350
0.3
t
I= 375
A
x L
0
1
7
3
4
5
6
1
8
I
9
1 I M E (HR)
Figure 16. Product distribution at 330 "C; Q
c c
+ HzS feed. 0.2
0.1
0 0
PY-THQ
0
, o
i
z
3
4
5
,
--
-T---~r--T-
6
7
a
9
7
B
9
1I Y E ( H R )
Figure 19. 0.8-order fit plots for Q feed only. 0.9
o.a 0
1
1
J
5
I
7
6
0 1
1 I M E (HR)
Figure 17. Product distribution at 350 "C; Q
+ HZS feed. 0.6
/
I
:05
OH0 A
o
B
BZ-THO
O
+/
-' 0 . 4
Q
v
I
pr-rtto
0.3
0.2
0.1
m
,
O
I
1
3
v-
1
5
0.0
1 I U E (HR)
Figure 18. Product distribution at 375 " C ;Q
+ HzS feed.
the hydrogenolysis reaction may be rate limiting. The product BzTHQ concentration, a t first, reaches a lower maximum, decreases with time, and will appear until the end of the reaction. The effect of H2S on the product distribution can be observed in Figures 16-18. At all temperatures, H2S lowered the concentration of DHQ and increased the concentration of BzTHQ. This is because hydrogen sulfide enhances the hydrogenolysis reaction rate and inhibits hydrogenations (Satterfield and Gultekin, 1981). Overall Kinetics. Most studies find that nitrogen removal is pseudo first order with respect to the total nitrogen concentration (McIlvried, 1971; Satterfield and Cocchetto, 1981; Gultekin et al., 1985a,b;Gates et al., 1978).
O
l
l
J
4
5
6
T I Y E ( H R )
Figure 20. 0.8-order fit plots for Q
+ H2S feed.
Unlike previous kinetic studies of quinoline hydrodenitrogenation, in this study the rate of nitrogen removal was not in general pseudo first order, but was less than first order. In addition, when the data were fit to the power law kinetic model, the apparent order of the reaction changed with temperature. A reasonable fit with 0.8 as the order with respect to nitrogen concentration at three different temperatures (330,350, and 375 "C) for various feeds (Q only, Figure 19; Q + H2S,Figure 20; Q + H,S + H20, Figure 21; and Q + H,S + H,O + NH3, Figure 22) are given and the corresponding Arrhenius plots are shown in Figure 23.
Ind. Eng. Chem. Res., Vol. 28, No. 6, 1989 735 0
0.9
0.8 0.7
-1
0.6
0.5 r 0
z
.Y.
'
c Y
- 0.5 I
A
350
c
+
I = 375
c
-1
0
I = 330 C
0.4
2
0 0
0 ONLY P tHlS
0.2 t
P tH2StH20
0
0 tHZStHlOtNH3
0.1
A C I I V A I I O ~ E N E R G Y = 31 .O K C A L / U O L 0.0
0
1
1
3
4
5
1I YE
(HR)
Figure 21. 0.8-order fit plots for Q
6
7
8
9
+ HzS + HzO feed.
0.6
0.5
x 22
I = 330 C
- 0.k
A
1. 350
c
0.3
t
I = 375
c
1
0.2
0.1 0.0
1
1
1
4
5
6
7
8
9
1IYE ( H R )
Figure 22. 0.8-order fit plots for Q
0 OOll
70. Therefore, a t high concentrations of feed, near zero order prevails and at low concentrations near first order. The activation energies obtained from Arrhenius plots varied slightly with the feed; however, they are near 31 kcal/mol. The value is slightly larger than previously reported (25 kcal/mol for HDS-9 catalyst) by Shih et al. (1977). Kinetic Modeling. The reaction network for quinoline HDN has been well established in the literature (Shih et al., 1977; Satterfield and Cocchetto, 1981; Gioia and Lee, 1986). In view of the nitrogen intermediates that appeared, the network proposed (Figure 2) will be similar to that given by Shih et al. (1977), except that in this study opropylaniline was absent. A Langmuir-Hinshelwood type of kinetic expression with the surface reaction as the rate-determining step was used to analyze the experimental data. A few other simplifying assumptions include (i) there is no competition between H2and all the other N compounds for adsorption sites on the catalyst surface, (ii) adsorption equilibrium is always established between the surface and bulk phases, and (iii) there are equal adsorptivities for all nitrogen compounds including ammonia. The generalized rate expression for each reaction step in the network will be
0.7
0
0 0017
0 0016
Figure 23. Arrhenius plot and activation energy.
0.1
I
0 0015
1 / l (l/K)
0.Q
-
0 OOlk
+ HzS + HzO + NH3 feed.
In the literature, non-first-order denitrogenation kinetics were reported by Tait and Hensley (1981) for hydrotreatment of shale oil with strong acidic zeolite catalysts. Miller and Hineman (1984) have determined under what conditions non-first-order kinetics prevails for denitrogenation of quinoline. It was reported that when the initial concentration of N compounds in the feed is high and when the nitrogen removal is near completion, lower than first order prevails depending on the temperature. Satterfield and Smith (1986b) also reported the overall order to be nearly zero. However, their initial feed concentration of quinoline was 5.0 wt 70,whereas in our case it is 1.0 wt
:,c~pH2n
r11
=
(1 + E K C J ( 1
+ KH~PH,)"'
(1)
where k',] is the intrinsic rate constant for the ith compound in the j t h reaction. Since H, pressure is always maintained constant in the system, due to the first assumption, the terms containing H2both in the numerator and denominator could be lumped together with the rate constant. Also, because of the third assumption, the rate expression could be further simplified to r11 = k1,G (2) where k'$H,"/(1 + KH~PH~)"' k, = 1 + KNCN~
736 Ind. Eng. Chem. Res., Vol. 28, No. 6, 1989 Table 11. Rate Constants (k;X 128 mol of Q/h/a of Catalyst) feed Q
+ H2S
Q (1) 330
Q + H2S (2) 330
+ H2O (3) 330
0.0288 0.0365 0.4104 0.2393 0.2885 0.0608
0.0321 0.0484 0.3332 0.1673 0.6536 0.0855
0.0441 0.0515 0.3287 0.1696 0.5815 0.0859
Q + H2S + H2O + NH3 (4) 330
Q + NH3 (5)
Q (6) 350
Q + H2S (7)
330
350
350
0.0436 0.0431 0.2824 0.1899 0.5890 0.0750
0.0008 0.0375 0.3650 0.2243 0.3836 0.0641
0.1906 0.1755 0.8319 0.6761 1.0240 0,1889
0.2822 0.1775 0.6891 0.4441 2.1108 0.2194
0.1741 0.1712 0.7215 0.5494 1.8490 0.2561
Q + H2S
+ H2O (8)
Q + H2S + H,O + NHq (9) 350 0.3908 0.1402 0.5620 0.6185 1.5551 0.2308
feed
Q (10) Q + H2S (11) temp,
375
375
0.4702 0.7885 1.0836 1.6603 3.9166 0.4195
0.6044 0.6828 0.9958 1.3011 4.7971 0.4713
Q + H2S + H20 (12) 375
Q + H2S + H,O+ NH3 (13) 375
0.4364 0.7776 1.2132 1.4835 5.3893 0.4713
0.6677 0.7076 1.0813 1.3252 5.4214 0.4774
Q
+ NH3
(14) 375
Q
+ H2O (15) 375
+ 2H2S + H2O +
+ 2H20 +
NH3 (17) 375
NH3 (18) 375
Q + H2S +H,O + 2NH3 (19) 375
0.0423 0.6968 1.1314 1.2077 5.4277 0.4066
1.0948 0.6274 0.8670 1.1100 5.2173 0.4837
0.5309 0.7781 1.1472 1.4694 4.4268 0.4418
0.1539 0.8139 1.1522 1.6217 4.4864 0.4303
Q + H2S + NH, (7) 375
Q + 2H2S + H20 NH, (run 13)b(8) 375
+-2H26+ NH3 (run 14)b(9) 375
Q + H2S + H 2 0 + 2NH3 (run 15)b(10) 375
2.33 0.80 0.80 0.67 1.33 1.15
1.13 0.99 1.06 0.89 1.13 1.05
0.33 1.03 1.06 0.98 1.14 1.03
Q + H2S + NH3 (16) 375
Q
Q+H#
QC
kl k2 k3 k4 k5 k~~
0.0645 0.8101 1.1744 1.6192 4.5794 0.4338
0.3664 0.8082 1.1024 1.5358 3.6570 0.4136
"Overall rate constant [kT X 128 mol Q ' . l / h / g of cat].
Table 111. Effects of H,S, HaO,and NHI on Rate Constants (Ri= k,/ki,) feed Q
temp.
Q (1) Q t H2S (2) 375 375
375
Q + H2S + H20 + NH3 (4) 375
Q + NH3 (5) 375
0.93 0.99 1.12 0.89 1.38 1.12
1.42 0.90 1.00 0.80 1.38 1.14
0.78 1.03 1.02 0.93 0.94 0.98
+ H2S
+ H20 (3)
Q
+ H 2 0 (6) 375
+
o c
Rl R2 R3 R4
1.00 1.00 1.00
R5
1.oo 1.oo
1.29 0.87 0.92 0.78 1.23
RTa
1.00
1.12
0.14 1.03 1.08 0.98 1.17 1.03
0.10
0.88 1.04 0.73 1.38 0.97
Effect on overall rate constant. *See Table I.
where KN is the adsorption equilibrium constant for N compounds and CN is the initial concentration of nitrogen compounds (quinoline). Since Q and PyTHQ are in equilibrium during the reaction, for modeling purposes the reaction network can be altered to Xi
A-
NH3
+
HC
where [AI = [Q + PyTHQl [B] = [BzTHQ] [D] = [DHQ] Each of these surface reactions is assumed to follow first-order kinetics with respect to nitrogen compounds (other orders gave negative rate constants). Therefore, the governing differential equations for this model will be
where Yi is mole fraction of nitrogen compounds in the system. In order to solve the above differential equations for the rate constants, eq 3 could be integrated analytically, and
the constant k, + k2 + k3 is obtained by using a nonlinear regression algorithm on a computer. The differential eq 4 and 5 are solved (simultaneously) for the rate constants, using the H-J-B method proposed by Himmelblau et al. (1967), which is a well-known powerful technique for kinetic analysis of complex reactions. This model fit the experimental data very well, and the rate constants are presented in Table 11. The rate constants found in this way were used to produce the product distributions of each compound as shown Figures 13-18. The Arrhenius plots are straight lines for all the rate constants (Figures 24 and 25) except for k,, where the points are scattered. The rate constant k, is evaluated by subtracting the values of h2 and k , from k , + k2 + k3, which is obtained separately. As a result, the precision of evaluating k, is less. Effects of H2S, HzO, and NH3 on Rate Constants. The individual as well as combined effects of H2S, H 2 0 , and NH3 on each rate constant a t 375 "C are shown in Table 111. For example, in the second column, R1 is defined as R1 = k l Q + H * S / k l Q o n l y It can be observed (Table 111) that the presence of hydrogen sulfide enhanced the hydrogenolysis rate constants (k, and k5) by 20-30% and slightly reduced (8-20%) the hydrogenation rate constants (kz, ks, and k4). This is in agreement with the literature (Satterfield and Gultekin, 1981; Satterfield and Yang, 1984; Gultekin et al., 1985a,b).
Ind. Eng. Chem. Res., Vol. 28, No. 6, 1989 737 2
- -
1
0
Y
"
breaking in this case is via DHQ. When H2S and HzO are present together, kl decreased slightly (R, = 0.93) and k5 increased markedly (R5= 1.38). The value of kz did not change, k3 increased by 12% , and k4 decreased by 1170, indicating that most of the HDN followed the reaction DHQ NH3 + HC. Satterfield and Smith PyTHQ (1986a,b) also reported that the presence of water in the system can affect differently the path of the quinoline HDN reaction network. Ammonia (34 kPa) in the system apparently did not affect the rate constants except for kl (R, = 0.78). The presence of NH, and H2S together inhibited the hydrogenation steps (R, = 0.88, R4 = 0.73), and among the hydrogenolysis reactions, k l decreased (Rl = 0.10) while k5 increased (R5= 1.39). Consequently, no apparent effect on the overall rate constant kT (RT = 0.97) is found. Hence, it is hypothesized that NH, and H2S may react in the bulk phase to some extent, thereby affecting individual rate constants, leaving the overall HDN unaffected. The combined effects of H2S, HzO, and NH3 are still complex; i.e., the hydrogenation rate constants decreased, while the hydrogenolysis rate constants increased considerably (R, = 1.42, R5 = 1.38), and the net (gross) effect is the enhancement of the overall HDN (RT = 1.14), as in the presence of hydrogen sulfide alone. Therefore, the combined effect phenomenon is further explored by changing the concentrations of H2S, HzO, and NH, individually while keeping the others constant. When the amount of hydrogen sulfide doubled (keeping water and ammonia constant), the hydrogenation rate constants further decreased while the hydrogenolysis rate constants increased substantially, but the result on the overall HDN enhancement did not change. Therefore, hydrogen sulfide, available in excess, partly reacts with ammonia and the remaining interacts preferably with the catalyst surface. When the concentration of water doubled (keeping the other species constant), the hydrogenation rate constants were unaffected; however, the hydrogenolysis rate constants increased marginally, and consequently, the overall HDN was enhanced (RT = 1.05) to the same extent as in the presence of water alone. This can also support our proposition that H2S and NH, react in the bulk phase, and water in excess interacts preferentially with the surface. Finally, when the ammonia concentration doubled, the mild enhancement effect (RT = 1.03) is the same as that in the presence of water alone, indicating that all the H2S may have been consumed in the bulk reaction (ammonia in excess) and the resulting slight enhancement could be attributed to the water in the system.
-1
0 2
-2
-3 0
-I
0.0014
0.0015
0.0017
0.0016 1 / T
0.0018
(lfi)
Figure 24. Arrhenius plots for K1 and K3;Q feed only. 2
1
0
0
K2
A
Kk
0
K5
Y
u -1 0 2
-2
-3
-k
0 0014
0 0015
G 0016 1/ T
0 0011
0 0018
(I/K)
Figure 25. Arrhenius plots for K2,K4, and K6: Q feed only.
Satterfield and Yang (1984) proposed two types of catalytic sites on sulfided Ni-Mo/yAlz03 catalyst: site I is a sulfur anion vacancy associated with Mo metal and is responsible for both hydrogenolysis and hydrogenation reactions; site I1 is a Brcansted acid type which is responsible for the hydrogenolysis reaction. Hydrogen sulfide interacts with sulfur anion vacancies (site I) to convert into Bransted acid type (site 11),which might be the cause of the enhancement. The presence of HzO alone in the system reduced 12, drastically ( R , = 0.14), whereas k, has significantly increased (R5= 1.17), indicating that most of the C-N bond
Conclusions The presence of H2S (34 kPa) enhances the HDN rate significantly at all temperatures. Increasing the partial pressure of H2S did not increase the HDN rate further. Water alone enhanced the HDN rate slightly, while the effect of H2S was dominant when they were together. The presence of NH3 alone did not effect the HDN rate, and no effect on the HDN of quinoline was found in the presence of H2S and NH, together. The combined effects of H2S,HzO, and NH, on the HDN of quinoline show the enhancement to be equal to that in the presence of hydrogen sulfide alone. The overall HDN kinetics (of quinoline) showed 0.8 order with respect to total nitrogen concentration. However, the individual reaction steps showed first-order kinetics. The effects of H2S,HzO, and NH3 on individual as well as overall rate constants were quantified. At equal partial pressures (34 kPa) of H2S, HzO, and NH, (combined effect), the hydrogenation rate constants decreased,
I n d . Eng. Chem. R e s . 1989, 28, 738-742
738
while the hydrogenolysis rate constants increased considerably, and the overall result was the enhancement of the HDN.
Acknowledgment The financial support provided by the King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia, for research project CH/EFFECTS/102 is greatly acknowledged. Registry No. Q, 91-22-5; PyTHQ, 635-46-1; BzTHQ, 1050057-9; DHQ, 2051-28-7; Ni, 7440-02-0; Mo, 7439-98-7; H,S, 778306-4; HZO, 7732-18-5; NHB, 7664-41-7.
Literature Cited Desikan, P.; Amberg, C. H. Catalytic Hydrodesulphurization of Thiophene: V. The Hydrothiophenes. Selective Poisoning and Acidity of the Catalyst Surface. Can. J . Chem. Eng. 1964,42,843. Gates, B. C.; Katzer, J. R.; Olson, T. H.; Kwart, H.; Stiles, A. B. Kinetics and Mechanism of Desulfurization and Denitrogenization of Coal Derived Liquids. DOE Report, 1978; University of Delaware, Newark. Gioia, F.; Lee, V. Ind. Eng. Chem. Process Des. Dev. 1986, 25, 918. Gultekin, S.; Al-Ohali, M. S.; Al-Saleh, M. A. Arab. J. Sci. Eng. 1985a, 10, 265-272. Gultekin, S.; Al-Ohali, M. S.; Al-Saleh, M. A. Arab. J . Sci. Eng. 1985b, 10, 273-280. Ho, T. C. Catal. Rev. 1988, 30, 117-160. Himmelblau, P. M.; Jones, C. R.; Bischoff, K. B. Ind. Eng. Chem. Fundam. 1967, 6 , 539. Katzer, J. R.; Sivasubramanian, R. Catal. Reu. 1979, 20, 155.
Laine, R. M. Catal. Rev. 1983, 25, 459. Ledoux, M. J. Catal. 1985, 125-148. McIlvried, H. G. Kinetics of the Hydrodenitrification of Pyridine. Ind. Eng. Chem. Process Des. Dev. 1971, 10, 125. Miller, J. T.; Hineman, M. F. J . Catal. 1984, 85, 117. Rollmann, L. D. J . Catal. 1977, 46, 243. Satterfield, C. N.; Cocchetto, J. F. Ind. Eng. Chem. Process Des. Deo. 1981, 20, 53. Satterfield, C. N.; Gultekin, S. Effect of Hydrogen Sulfide on the Catalytic Hydrodenitrogenation of Quinoline. Ind. Eng. Chem. Process Des. Dev. 1981, 20, 62. Satterfield, C. N.; Carter, D. L. Ind. Eng. Chem. Process Des. Deu. 1981, 20, 538. Satterfield, C. N.; Smith, C. M. Ind. Eng. Chem. Process Des. Deu. 1986a, 25, 942. Satterfield, C. N.; Smith, C. M. Chem. Eng. Sci. 198613, 41, 839. Satterfield, C. N.; Yang, S.H. Ind. Eng. Chem. Process Des. Dev. 1984, 23, 11. Satterfield, C. N.; Modell, M.; Mayer, J. F. AIChE J. 1975,21, 1100. Satterfield, C. N.; Modell, M.; Hites, R. A.; Declerck, C. .J. Ind. Eng. Chem. Process Des. Dev. 1978, 17, 141. Satterfield, C. N.; Smith, C. M.; Ingalls, M. Ind. Eng. Chem. Process Des. Deu. 1985, 24, 100. Shih, S. S.;Katzer, J . R.; Kwart, H.; Stiles, A. B. Quinoline Hydrodenitrogenation: Reaction Network and Kinetics. Prepr.-Am. Chem. Soc., Diu. Pet. Chem. 1977, 22, 919. Tait, A. M.; Hensley, A. L. Evaluation of Hydrocracking catalysts for Conversion of Whole Shale Oil into High Yields of Jet Fuel, DOD: AFWAL-TR-81-2098, 1981.
Received for review February 9, 1988 Revised manuscript received December 1, 1988 Accepted January 20, 1989
Chemical Kinetics and Reaction Mechanism of Thermal Decomposition of Aluminum Hydroxide and Magnesium Hydroxide at High Temperatures (973-1123 K) Ienwhei Chen,* Shuh-kwei Hwang, and Shyan Chen D e p a r t m e n t of Chemical Engineering, T a t u n g I n s t i t u t e o f Technology, Taipei, T a i w a n , Republic of China
A study on chemical kinetics of thermal decomposition for aluminum hydroxide and magnesium hydroxide to form alumina and magnesia at high temperatures ranging from 973 to 1123 K was carried out by the use of instruments such as the Lindberg box furnace, X-ray diffractometer, and thermogravimetric analyzer. An empirical reaction rate equation was obtained by the differential method of analysis. The apparent activation energies are found to be 30.6 and 50.9 kJ/mol, while the reaction orders with respect to aluminum hydroxide and magnesium hydroxide are 0.45 and 0.55, respectively. A sequential reaction mechanism has been proposed and discussed. Alumina and magnesia have been widely used as a support of many catalysts; in addition, they have the power to catalyze chemical reactions such as condensation, dehydration, and hydrolysis (Windholz, 1983; Hightower, 1975). As usual, y-alumina and magnesia are formed by decomposition of aluminum hydroxide and magnesium hydroxide at temperatures ranging from 773 to 1123 K (Kirk-Othmer, 1978; Miyahara et al., 1981; Hattori et al., 1976). The specific surface area and the densities of yalumina and magnesia are affected by the decomposition temperature. This decomposition process is really necessary in many preparations of y-alumina and magnesium oxide supported catalysts, such as precipitation, coprecipitation, impregnation, and compounding. The reaction mechanisms of thermal decomposition of aluminum hydroxides and magnesium hydroxides remain * To whom correspondence should be addressed.
a matter of uncertainty. This study deals with the kinetic investigation of the decomposition reaction of aluminum hydroxide to y-alumina and that of magnesium hydroxide to magnesia directly at high temperatures ranging from 973 to 1123 K.
Experimental Section Aluminum hydroxide (Kokusan) and magnesium hydroxide (Kokusan) were dried in an oven (Kwang Shen) at 373 K to remove physically adsorbed water before they were used in the kinetic investigation. After drying, the aluminum hydroxide and magnesium hydroxide samples were put in a Lindberg box furnace at different temperatures ranging from 973 to 1123 K at an interval of 50 K, and they were weighed at each appropriate time interval. The reactant and product were determined for identification by using X-ray diffractometer (Rigaku D/Max-3A) with Fe K a radiation. The thermogravimetric analysis
08S8-5885/89/2628-0738$01.50/0 0 1989 American Chemical Society
