Energy & Fuels 1987,1,424-430
424
Effects of PH~s, P H 2 9 and P H ~ s / P on Hthe ~ Hydrodenitrogenation of Pyridine Robert T. Hanlon Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received February 20, 1987. Revised Manuscript Received July 15,1987
Kinetic studies conducted a t 310 "C over commercial NiMo/A1203catalyst showed that the initial hydrogenation of pyridine to piperidine is first order in PH, and is unaffected by P H 8 The subsequent hydrogenolysis conversion of piperidine to n-pentylamine is dependent on P H f i I P H 3 and increases with increasing values of this ratio. This suggests that the hydrogenolysis reaction is promoted by a suNdic surface species that exists in equilibrium with H2S,Hz, and the catalyst surface. This species is proposed to be Ni-Mo-S.
Pyridine has long been used as a model compound for studying the hydrodenitrogenation (HDN) of organonitrogen compounds commonly found in petroleum feedstocks. As shown in Scheme I, the HDN of pyridine over alumina-supported molybdenum-based catalysts proceeds via the saturation of the heterocyclic ring to form piperidine, followed by C-N bond scission to form npentylamine and subsequent removal of nitrogen to form pentane plus a m m ~ n i a . l - ~The formation of N-pentylpiperidine (NPP) has also been observed and has been proposed to arise from the equilibrium alkyl-transfer reaction of piperidine and n-~entylamine.~ The benefit of using pyridine as a model compound is that it provides a way to study comparatively the effects of operating conditions on the hydrogenation (kif) and hydrogenolysis (ki)reactions typical of HDN reaction networks. This is because the first two reaction steps described by k l f and k{ control the overall HDN rate of pyridine due to both the high relative reactivity of npentylamine (high k,l) and thermodynamic limitations of NPP formation (low kL/k{). (The k/'s presented here are pseudo-first-order rate constants and are discussed later.) The partial pressures of H, and H,S are two important variables to consider when HDN reaction networks are studied. Hydrogen, of course, is a main reactant in such networks and has been shown to play an important role in the hydrogenation reactions.2 Hydrogen's role in the hydrogenolysis reactions is less clear. The studies of Sonnemans et al.2,3were inconclusive regarding this point while quinoline HDN studies of Shih et al.4 reported on but did not explain the finding that an increase in hydrogen partial pressure seemed to slightly inhibit the hydrogenolysis reactions. Hydrogen sulfide is typically found in industrial HDN reactions as a product of the hydrodesulfurization (HDS) reactions of organosulfur compounds also found in petroleum feedstocks. The presence of HzS has been shown to markedly increase the hydrogenolysis reactions in the HDN of pyridine5 and quino(1) McIlvried, H. G. Znd. Eng. Chem.Process Des. Deu. 1971,10, 126. (2) Sonnemans, J.; VanDen Berg, G. H.; Mars, P. J.Catal. 1973, 31, 220. (3) Sonnemans, J.; Neyens, W. J.; Mars, P. J. CataE. 1974, 34, 230. (4) Shih, S. S.; Katzer, J. R.; Kwart, H.; Stiles, A. B. Prepr.-Am. Chem. SOC.,Diu. Pet. Chem. 1977,22,919. (5) Goudriaan, F.; Gierman, H.; Vlugter, J. C. J . Znst. Pet. 1973,59, 40.
0887-0624 I87 l2501-0424%01.50 ,IO I
,
~
k'l_
0
Scheme I
k C 5 H 1 1 N H 2 -C5H12 k'3
+
NH3
H Pyridine
Piperidine
Q
+
n-Pentylamine
n-Pentane
C5H11NH2
+
k'5
Ammonia
NH3
Y
C5H11 N- Pent y l p i p e r i d i n e
(
NPP)
line4v6while not affecting significantly the hydrogenation reactions. In each of the aforementioned studies, the effects of either P H o or PHzson HDN reaction kinetics were determined whde the other variable was kept constant. It would seem, however, that the effects of the two variables could be related. This would be true, for instance, if certain characteristics of the active catalyst were influenced by an equilibrium reaction between H2S,H2, and the catalyst surface. Thus, it was the goal of this study to determine the effects of not only PHzs. and P H 2 but also of the relationship between the two (i.e., the value of PH2s/PH2)on the HDN reaction kinetics of pyridine.
Experimental Section The experimentswere carried out in a fiied-bed tubular reactor as described previously.' Liquid reactant was pumped at a fixed rate into a specially designed heated section of tubing, where it evenly vaporized (without boiling) into a premixed H,S/H, gas stream coming from a gas cylinder. Such an even vaporization provided a steady reactant flow rate to the reactor. The reactor employed w a ~a stainleas-steel tube (6.85mm inside tube diameter) consisting of a 70 mm long preheat section and a 80 mm long exit section, each filled with quartz crystals (0.2-0.25 mm diameter) and held in place by glass wool, and a 125 mm long reactor section filled with an evenly distributed 1:l mixture by volume of catalyst and quartz crystals, both of 0.2-0.25 mm diameter. A total of 1.44 g (2.3 mL) of a BASF NiMo/A1203 catalyst (2.4 wt YO Ni; 10.0 wt % Mo) was used. A thermoelement was placed directly in the gas stream after the reactor section for good temperature (6)Yang,S. H.; Satterfield, C. N. Ind. Eng. Chem. Process Des. Deu. 1984, 23, 20.
(7) Reitemeyer, H. 0. Dissertation, University of Karlsruhe, 1970.
0 1987 American Chemical Society
Energy & Fuels, Vol. I, No. 5, 1987 425
Hydrodenitrogenation of Pyridine Table I. Numbers Designating (P, ,P, ) Conditions under Which Experiments %ere kun condition
"I
8 0
B a ~
1.25 2.50 5.00 10.00
PHfi =
% a; 1
4
8.0 kPa 2 3 5 8
PHfi =
Pnfi = 32.0 kPa
16.0 kPa
0
e
i
0
Pyridine
A Piperidine 0 n-Pentylamine
BO
0 N-Pentylpiperidine
iNPPl
50
3
7 9
6
control. Calculations showed that heat- or mass-transfer limitations in this system were insignificant. A regulating valve and a needle valve placed in series downstream from the reactor were used together to control the gas flow rate. A sampleport was located after the needle valve from which gas samples could be taken by using a heated (- 100 OC) 2.5-mL glass syringe. The heating of the syringe as well as the heating of the lines (- 200 "C) from the vaporizer down to and including the sample port prevented condensationof the organiccompounds. A glass receptacle cooled with liquid N2and then a cadmium acetate aqueous bath were used after the sample port to condense out the organic compounds and to wash out the H2S,respectively. The volumetric flow rate of the remaining H2 gas stream was measured by a soap film bubble flowmeter. The gas samples were injected directly into a Siemans gas chromatograph (L350), which contained a 4-m packed glass column (Tritonx100 10 wt % + KOH 1wt % on Volaspher A2) and a FID. The GC was operated isothermally at 160 "C. The relative response factors for the compounds were determined by using standardized liquid mixtures. Since the FID could not detect NH3,only the carbon-containing compounds were analyzed for. Thus, the gas compositions were calculated in percent carbon, representing the fraction of carbon atoms in the reactant feed system that were reacted to a specified compound. The concentration of ammonia was determined from the reaction stoichiometry shown earlier. The assumption inherent in these calculations was that all of the carbon-containingcompounds in the product stream were accounted for, which was reasonable. In general, at each condition of PHfiand PHz a series of at least five experiments at varying reactant space times (catalyst weight/molar flow rate of reactant) were conducted. The liquid and gas flow rates were first set at the highest desired values (lowest reactant space time). Gas samples were taken and analyzed every 15min until a steady state was observed. This usually required three to four samples (3/4 to 1h). Then the liquid and gas flow rates were reduced proportionately to the next desired reactant space time so as to maintain constant inlet partial pressure of the reactant. Gas samples were taken again until a steady state was observed. This procedure was repeated for each space time.
Results The effects of the partial pressures of hydrogen sulfide and hydrogen on the HDN of pyridine were studied at 310 "C,a reactant partial pressure of 60.0 kPa and space times ( W/Fp,,o) based on a pyridine feed rate only of 40.0-483.0 h g of catalyst (fresh catalyst before sulfiding)/(g mol of pyridine). Seta of experiments were conducted at each of nine different conditions of PHaand pH,.The designations of these ( P H pPHJconditions are shown in Table I. One set of experiments was also conducted under condition 6 with a pyridine partial pressure of 30.0 kPa. Experiments using each of the intermediates, piperidine or n-pentylamine, were conducted at 310 "Cand over a range of space times, (PHa, P& conditions, and reactant partial pressures. Catalyst Activity. The fresh catalyst was presulfided for about 8 h at 400 "C and 0.2 MPa absolute pressure by using a 10%/90% H2S/H2gas mixture at a flow rate of 6 L of gas (STP)/h. The catalyst was then contacted with pyridine or piperidine feed at 310 "C and varying contact times, reactant partial pressures, and (PHa,PH conditions for about 130 h. The experiments reporte here were conducted after this time period. The steady-state activity
d
m
40
I-
? I-
O 0 3
P -0
50
100
150
200
250
300
350
400
SPACE TIME, gm c a t - h r l g m o l p y r i d i n e
Figure 1. Product distributions for pyridine HDN under condition 6. Pyridine inlet partial pressure: 60.0 kPa.
SPACE TIME, g m c a t - h r l g m o l p y r i d i n e
Figure 2. Increase in conversion with increase in pyridine inlet partial pressures at each space time under condition 6.
PH ,MPa
- Condition
----
0'
50
100
3 Condition 8
~ , ~ ~ , k P a
~~
2 50 10 0 0
150 200 250 300 350 SPACE TIME gm c a t - h r l g m o l p y r i d i n e
8.0 8 0
400
~
450
500
Figure 3. Effects of an increase in hydrogen partial pressure at constant hydrogen sulfide partial pressure on pyridine HDN product distribution. Pyridine inlet partial pressure: 60.0 kPa. of the catalyst over the course of the experiments was verified by operating the first and last experiments under the same conditions (PHa= 32.0 kPa, PHz= 5.0 MPa, Ppy,o = 60.0 kPa, W/FPy,, = 40.0 h g of cat./(g mol of pyridine) and noting the reproducibility of the pyridine conversion (73% in the f i i t experiment and 74% in the last). A single charge of catalyst was used for all of the experiments. Hydrodenitrogenation of Pyridine. In Figure 1 is shown a representative product distribution on a percent carbon basis (i.e., ammonia free) for the HDN of pyridine under condition 6. (The curves drawn in Figure 1as well as those drawn in Figures 2-4 are simulated product distributions based on the kinetic modeling presented in the Discussion. The curve in Figure 5 is not simulated and was drawn merely to guide the eye.) As can be seen the data are consistent with the reaction mechanism described earlier; pyridine is first hydrogenated to piperidine, which undergoes hydrogenolysis to form n-pentylamine and then ammonia plus hydrocarbons. The product N-pentyi-
Hanlon
426 Energy & Fuels, Vol. 1, No. 5, 1987 io
1I
Table 11. Effect of Pressure on Piperidine Conversion" piperidine conversion pHz? MPa PH,,s= 8.0 kPa P H 0 s = 32.0 kPa 2.50 0.35 0.45 10.00 0.26 0.34
PH2 MPa PHZs,kPa
-
Pyridine
70.
! i
2
__
---
f
sot
\
Condition 4 Condition 7
~
5 00
4 0
5 OD
32 0
/
a The conversion seems to be independent of PH2s or P H 2 at constant PH~SIPH~ and increases with increasing values of P H z s / P H 2 . W/Fpip,o= 80.0 g of cat. h/(g mol of piperidine). Ppip,o = 60.0 kPa. ""I
I 0 0
n-pentylamina Inlet Partial Pressure, k P a
0 70'
SPACE TIME, Q m c a t - h r l g m o l p y r i d i n e
Figure 4. Effect of an increase in hydrogen sulfide partial pressure at constant hydrogen partial pressure. This does not affect pyridine conversion but increases piperidine conversion. Pyridine inlet partial pressure: 60.0 kPa.
e
60 501
15 0
3
b
30 0 45 7 75 8 149 3
f
312 5
0
a
0
0
0
0 0
__
I :\ 6
e
801
0
60
3
50
m E
Piperidine
PH2MPa PH2SkPa ~~
A n m CCoonnddlit i o n 3 9
10 2 05 0
3 82 00
'
ob
50
id0
105 0 0
' 2 000
6
2
SPACE TIME, Q m o a t - h r l g m o l p e n t y l a m i n e
Figure 6. n-Pentylamine conversions at different space times and feed partial pressures under condition 6.
401
or
10
1
50
100
I50
260
SPACE TIME, gm c a t - h r l g m o l
250
300
350
400
piperidine
Figure 5. Effect of pressure on piperidine conversion. The conversion seems to be independent of PH# or PH,under constant PHB/PHZconditions. Piperidine inlet partial pressure: 60.0 kPa. piperidine (NPP) is formed in a side reaction of piperidine and n-pentylamine and decomposes in the reverse reaction. Figure 1shows that the concentrations of n-pentylamine and NPP were quite low. Indeed, when the data from all of the pyridine HDN experiments are considered, the concentration of n-pentylamine in the product stream never exceeded 11%carbon and that of NPP never exceeded 8% carbon (4 mol %). These results were most likely due to the high relative reactivity of n-pentylamine in the first case and equilibrium limitations of NPP formation in the second. Figures 2-4 show how different operating variables affected the pyridine HDN product distribution. To make the effects easier to see, only the concentrations of the two dominant nitrogen compounds, pyridine and piperidine, are plotted. In Figure 2 is shown the effect of the inlet pressure of pyridine, Ppycon the product distribution under condition 6. At each space time an increase in Ppy,o from 30.0 to 60.0 kPa resulted in an increase in the conversion of pyridine, suggesting that the catalyst surface was not completely covered by N-containing compounds; the increased inlet partial pressure of the reactant resulted in an increased surface coverage of pyridine and thus an increased reaction rate and conversion. In Figure 3 the data from conditions 3 and 8 are plotted to show how P H z affected pyridine HDN at constant P H 2 s . Increasing PH2 by a factor of 4 caused a significant increase in the conversion of pyridine, as expected since the pyridine reaction is a hydrogenation reaction. Figure 4 shows how an increase in PHs at constant PH2 (conditions 4 and 7) affected the product distribution.
While the hydrogenation reaction does not appear to have been affected, the piperidine hydrogenolysis reaction increased noticeably. Hydrodenitrogenation of Piperidine. Experiments with piperidine feed were conducted under conditions 3, 6, and 9 (constant H2S/H2 = 0.0032) for varying contact times and a reactant partial pressure of 60.0 kPa. At each condition studied the formation of pyridine was negligible. In Figure 5 are shown the product distributions for the two extreme conditions, condition 9 with high PHB and PH2 and condition 3 with low PHps and PH2.The data seem to indicate that the hydrogenolysis of piperidine is unaffected by the absolute value of PHzSor PHzso long as the value of the ratio P H 2 s / P H 2 remains constant. To pursue this finding further single experiments were conducted a t a contact time of 80.0 h g of cat./(g mol of piperidine) and under two other P H 2 S / P H 2 conditions. The resulting piperidine conversions are compared in Table I1 to those obtained under conditions 3 and 9 for the same contact time. As can be seen, the conversion of piperidine is dependent on the ratio P H z s / P H ? and increases with increasing values of this ratio. This result is consistent with the results shown in Figure 4. Hydrodenitrogenation of n -Pentylamine. In Figure 6 are plotted the conversions of n-pentylamine against space time a t a constant inlet n-pentylamine partial pressure of 30.0 kPa. Also shown in the plot are the conversions of n-pentylamine for varying feed partial pressures a t a constant space time of 65.0 h g of cat./(g mol of n-pentylamine). As was the case for pyridine in Figure 2, an increase in the initial pressure of the reactant resulted in an increased conversion. The high relative reactivity of n-pentylamine is reflected by the lower space time and lower inlet reactant partial pressure required for conversions comparable to those of pyridine and piperidine. Discussion The kinetic modeling of the pyridine HDN reaction network is based on the reactions described earlier. The reverse reaction where piperidine dehydrogenates to form pyridine is neglected since our studies starting with a pure
Energy & Fuels, Vol. 1, No. 5, 1987 427
Hydrodenitrogenation of Pyridine piperidine feed showed negligible formation of pyridine at 310 OC. NPP is assumed to form from the alkyl-transfer reaction between piperidine and n-pentylamine and to decompose in the reverse reaction. This assumption follows from the work of Sonnemans et aL3 It is noted here that the influence of the reactions involving NPP is small when k,' and k i are determined due to the low levels of NPP that were formed. Adsorption studies by Sonnemans et using pyridine, piperidine, and ammonia and HDN kinetic studies of quinoline by Satterfield and Cocchettosas well as our own qualitative observations all suggest that the equations describing the various reactions in a HDN reaction network are complex functions of the initial reactant partial pressure and of the strong competitive adsorption characteristics of the basic nitrogen-containing compounds. Thus it seems appropriate to use Langmuir-Hinshelwood kinetic expressions in analyzing the data. The use of such expressions requires that the adsorption equilibrium constant (Ki)for each N-containing compound be determined. To accomplish this a procedure is used whereby the simpler reactions in the HDN reaction network are modeled first to determine the K:s of the corresponding compounds. These K{s are then used in modeling the more complex reactions. This procedure follows closely that used by Satterfield and Cocchetto.s The kinetics of the HDN of n-pentylamine (PA) are considered here first. Assuming that the reaction is first order in P A and that Langmuir-Hinshelwood kinetics apply, kinetic eq 1can be written, where rpAis the reaction
~~KPAPPA (1) 1 + KPAPPA + KAMPAM rate of n-pentylamine, Ki is the adsorption equilibrium constant of compound i, Pi is the partial pressure of compound i, k{ is the pseudo-first-order reaction rate constant, which is a function of T, PF2,and PH2s, and AM stands for ammonia. Other assumptions inherent in eq 1 are that hydrogen and the hydrocarbon products do not competitively adsorb with the N-containing compounds. The former assumption is based on sdsorption studies by Sonnemans et a1.2 using hydroge. and nitrogen bases. Since a large excess of hydrogen exists during the reaction and thus the change in the total number of moles is negligible, the equation P A M = P p A , O- P p A is valid, where PPA,O is the inlet partial pressure of n-pentylamine. Substitution of this relationship with eq 1 into the reaction equation for an ideal plug-flow reactor and integration a t constant temperature and hydrogen sulfide and hydrogen partial pressures results in eq 2, where FpA,O and X p A are -rpA
ki(
=
w, FPA,O
=
0
1
I
L
45
KAM- 5 0 kPa-'
W/FpA,o
-
SPACE TIME, gm c a t - h r / g m o l pentylamlne
Figure 7. Comparison of kinetic models for n-pentylamine reaction under condition 6. Definition of symbols is given in Figure 6.
describe pyridine HDN. (Note: the time variable used by Sonnemans et al. was defined as mP/at where m = catalyst mass, P = total pressure, and @t = total molar flow rate including hydrogen to the reactor. This is equal to WPpA,O/FpA,O, assuming constant molar flow rate.) The data for the HDN of n-pentylamine are plotted in the top half of Figure 7 according to eq 2, assuming K p A P p A + K A M P m >> 1 and K p A = K A M . The plot shows that, under the conditions of this study, (1)the catalyst surface is not completely covered by the N-containing compounds since the conversion is dependent on the partial pressure of n-pentylamine and (2) ammonia adsorbs less strongly than n-pentylamine since with increasing space time the conversion of n-pentylamine increases more than predicted for equal adsorptivities. The values for K p A and KAMare iterated on in eq 2 until the right-hand side of the equation best correlates to a linear function of ( W/Fpko) and passes through the origin. The values of 1000 kPa-' and 50 kPa-' for K p A and K A M , respectively, best fit the data as shown in the lower half of Figure 7. The pseudo rate constant, k i , can be determined from the slope of this line. Equations similar to eq 2 are then written to describe the reactions occurring when one starts with a pure piperidine feed. With the reaction rate of n-pentylamine as an example, eq 3 applies. Dividing the numerator and
= ( k i K P i P P i p - k i K P A P P A - k4/KPAPPAKPi$Pip + ~BIKNPPPNPPKAMPAM) / (1 + K P i p P P i p + KPAPPA + K N P P ~ N+PK P AM~AM (3))
rPA
(2)
the inlet molar flow rate and conversion of n-pentylamine, respectively, and W is the weight of the catalyst. It can be shown that if the catalyst surface is completely covered by the N-containing compounds (1
