I n d . E n g . C h e m . R e s . 1989,28, 13-20
13
Hydrodenitrogenation of Quinoline with Y-Type Zeolite Chang-yu Yu,tif William J. Hatcher, Jr.,*s$and Wolfgang Bertschs Department of Chemical Engineering and Department of Chemistry, T h e University of Alabama, Tuscaloosa, Alabama 35487
Hydrodenitrogenation (HDN) of hetero-nitrogen compounds in petroleum crudes and synthetic liquids derived from oil shale and coal is studied using quinoline as a model nitrogen compound and benzene as a diluent. Both the external and internal mass-transfer rates in the Ni-W Y-type zeolite have been calculated in order t o determine the possibility of diffusion limitations. Kinetic data were taken from a continuous flow Berty-type reactor a t 34-72 bar, 350-460 "C, and hydrogen mole fraction from 25% t o 90%. Hydrogenation of benzene solvent is negligible unless the nitrogen reactant is highly denitrogenated. Catalyst deactivation and reactivation occurred during the experimental operations. A Langmuir-Hinshelwood model was used to account for kinetic behavior, and the Redlich-Kwong equation of state (EOS) was used t o consider nonideal gas behavior. A prediction algorithm based on the simplification of the HDN network reproduced the experimental data well. The heavy oil fraction produced by coal liquefaction processes contains approximately 1-3 % nitrogen. The nitrogen content must be reduced not only to assure an environmentally acceptable NOx emission upon combustion but also to prevent nitrogen-containing compounds from poisoning the acidic function of catalysts used in upgrading these heavy oils to motor gasoline quality. Nitrogen is present in the heavy oil fraction in the form of heterocyclic compounds such as pyridine, carbazole, and quinoline (Q). Such compounds can react with hydrogen to form a hydrocarbon plus ammonia. Quinoline has received considerable study since it contains both a benzene ring and a pyridine ring. Thus, it is also chosen as the model nitrogen compound in this study. Because nitrogen removal is generally more difficult than sulfur removal, the elucidation of mechanisms of hydrodenitrogenation reactions can contribute to the development of processes for upgrading coal-derived liquids. The reaction requires a bifunctional catalyst, having the proper balance of both hydrogenation and hydrogenolysis functions. Different catalysts and kinetic models had been used with quinoline HDN studies. Aboul-Gheit and Abdou (1973) used CoMo/alumina catalyst with a power-order rate equation and concluded that the reaction path is from Q to PyTHQ (chemical names for acronyms are in Table I), then to OPA, and finally to PB. The rate-determining step is from PyTHQ to OPA. Shih et al. (1977,1978) used both oxidic and sulfided NiMo/alumina, CoMo/alumina, and NiW/alumina catalysts with both a power-order rate equation and a Langmuir-Hinshelwood model. They concluded that the major path of HDN is from Q to DHQ and then to denitrogenated hydrocarbons and ammonia. Satterfield and Cocchetto (1981), using sulfided NiMo/ alumina catalyst with a Langmuir-Hinshelwood model, also concluded that DHQ was the primary intermediate and PCH was always the major hydrocarbon product. Sanghvi and Akgerman (1983) used NiMo/Y52 (based on LZ-Y52 zeolite support) with a power-order rate equation and concluded that the reaction path for oxidic catalyst is from Q to DHQ and then to hydrocarbon products, while the reaction path for sulfided catalyst is from Q to PyTHQ and then to OPA and finally to hydrocarbon products.
* To whom
correspondence should be addressed. 'Present address: Laboratorium voor Petrochemische Techniek, Rijksuniversiteit, Gent, Belgium. Department of Chemical Engineering. f Department of Chemistry.
*
Table I. Chemical Names acronvm B MCH TO ECH EB MPCP XYL PCH PB MOHP AN MEB OHIDE IDAN MPB DHQ BzTHQ OPA
Q PYTHQ PCHAMO NH3 C2H6 H2
of Acronyms chemical name benzene methylcyclohexane toluene ethylcyclohexane ethylbenzene methylpropylcyclopentane xylene propylcyclohexane propylbenzene methyloctahydropentalene aniline methylethylbenzene octahydroindene indan methylpropylbenzene decahydroquinoline 5,6,7&tetrahydroquinoline o-propylaniline quinoline 1,2,3,4-tetrahydroquinoline propylcyclohexylamine ammonia ethane hydrogen
Not found in liquid samples.
The zeolite-based catalyst was reported more active than a commercial NiMo/alumina catalyst. Generally, conclusions from different studies agree well with each other. The most obvious differences are the HDN network and the major reaction path. These are probably due to the natural differences among catalysts and to the differences in catalyst preparation. Certainly, the ratio of the hydrogenation activity to the hydrogenolysis activity of the catalyst can affect the network and rate-determining step. The purpose of this study is to investigate the quinoline HDN reaction network and the HDN thermodynamic and kinetic behaviors on a sulfided NiW/Y82 (based on LZY82 zeolite support) catalyst under nonideal gas states. Experimental Section
The zeolite catalyst used here is a LZ-Y82 (Union Carbide) based zeolite which was ion-exchanged with nickel nitrate and impregnated with ammonium tungsten oxide. Compositions of nickel and tungsten were determined as 2 and 10 w t %, respectively. This represents about 69% ion-exchanged from the HY form. The catalyst was crushed and sieved to between 20 and 60 mesh before use.
0888-5885/89/2628-0013$01.50/0 0 1989 American Chemical Society
Ind. Eng. Chem. Res., Vol. 28, No. 1, 1989
14
of the liquid products listed in Table I except MPCP, MOHP, and PCHAM (PCHAM was not detected in the GC/MS analyses) were purchased and used to spike liquid products in order to make positive identification with the result of GC/MS.
L L'
~-
.
1-2.; -p--
:rr
rhhp
>_-
CJ-
Figure 1. Schematic diagram of HDN experimental apparatus.
This HDN study was carried out in a continuous flow, fixed catalyst bed, Berty-type gradientless reactor (Autoclave Engineers). A 4-g, 20/60-mesh single charge of presulfided NiW/Y82 zeolite catalyst was used for all HDN experiments. The catalyst activity for each experiment can be determined from periodic standard runs at 406 "C, 35 bar, 80 mol % hydrogen, and 445 (g.h)/mol of contact time ( W I F ) . In order to simulate making fuels from heavy oil fractions, a liquid feed composition of 94.2 mol % solvent (benzene) and 5.8 mol % nitrogen-containing heterocyclic compound (either Q, PyTHQ, or BzTHQ) was used. With this ratio, the nitrogen element in the liquid feed is about 1 w t 5%. Gas feed was pure hydrogen gas. Molar ratios in the feed were 10-160 for hydrogen/heterocyclic nitrogen compound and 0.6-10 for hydrogen/liquid feed. The liquid and gas feeds were mixed and preheated before entering the reactor. Preheater temperature was controlled at a temperature higher than the mixture dew point. Estimated dew points were predicted by Chao-Seader correlations. The schematic diagram of this HDN experiment apparatus is shown in Figure 1. Reactor conditions were set at temperatures from 350 to 460 "C and pressures from 34 to 72 bar. A t least three liquid product samples were collected from valve V2 during each experiment. A steady state was claimed when three consecutive liquid product samples gave approximately the same product distributions. Typical line-out times were 1-3 h, and typical balance periods were 30-60 min. After each experiment, a liquid sample was also collected from tank T 2 and analyzed to compare with samples from V2. This should further confirm an approach to steady state during the experiment. An overall mass balance was evaluated by comparing the amount of liquid feed and the amount collected from V2 and liquid sample tank "2. Qualitative chemical analysis of gas products was carried out with an on-line GC (VA-1400) equipped with a 15-ft X 1/8-in. Porapak Q column and a thermal conductivity detector. Liquid product samples were quantitatively analyzed with an off-line GC (HP-5880) equipped with a flame ionization detector. A 30-m X 0.532-mm column (J&W Scientific Inc.) coated with 1.5-pm thickness of DB-5 was implemented for liquid product analyses. A 15-m X 0.53-mm column (ALLTECH Associate Inc.) coated with 1.2-pm thickness of SUPEROX was used to analyze the solvent quantitatively. Liquid products were identified by GC/MS with a computer data base. Only those products with higher concentrations were picked as model products. The chemical names and corresponding acronyms of picked model products are shown in Table 1. All
External a n d I n t e r n a l Mass T r a n s f e r In order to prove the Berty-type reactor is well-mixed and gradientless, Berty (1974) had evaluated the mass flow in the reactor. He determined that external mass-transfer limitations will not exist if the impeller speed is above the critical value. It was determined that impeller speeds used in this study were greater than 50 times this critical value. External mass-transfer limitations also can be investigated by comparing the product conversions at different impeller speeds. Experimental results showed there is no significant difference, 94% compared to 93.2%, in the quinoline conversion when the revolution rate was increased from 1250 to 1500 rpm. The slight decrease is probably due to analytical error. An analysis of possible internal mass-transfer limitations requires a knowledge of the intraparticle diffusivity at reaction conditions. Since there is no comprehensive theory available for diffusion in zeolites, it is impossible to predict diffusivity values at reaction conditions with confidence. An estimate was made by measuring counterdiffusivities of liquid PB-Q at low temperatures in HY82 zeolite by the methods of Satterfield and Cheng (1972) and Moore and Katzer (1972). The Weisz-Prater criterion states that there are no pore diffusion limitations when @ is much smaller than one: = (r,)ot,&2/DeCsS
(1)
When the effective diffusivity was extrapolated to reaction temperature, and the highest measured reaction rate was used, the calculated @ was much less than one. However, these results are inconclusive due to the extent of extrapolation required to estimate De at reaction conditions. Thermodynamic Analysis of Reactions A chemical reaction equilibrium analysis may help draw a possible map for the quinoline HDN network analysis. It may also help explain the kinetic study later. Free energies of quinoline HDN compounds have been discussed by Cocchetto and Satterfield (1976). Because there were more products found in this quinoline HDN study, due to a different catalyst and different reaction conditions, a free-energy study with some different approaches was taken here. For lack of experimental data, the van Krevelen group contribution model was used here to predict free energy of the model compounds. Group parameters -C and N from Cocchetto and Satterfield were chosen for use because they were correlated with known experimental data of naphthalene, pyridine, pyrrole, piperidine, and pyrrolidine. With this modification, the accuracy of some free-energy predictions is expected to be improved. In this study, mole fractions of hydrogen in the reactor may range from 25% to 90% while mole fractions of benzene solvent may range from 9% to 70%, depending on the feed rate and reaction conditions. Nonideal gas behavior was expected due to high pressure and high mole fractions of high boiling point compounds in the HDN reactions. Fugacity coefficients were calculated from the Redlich-Kwong equation of state. Since there are some thermodynamic similarities among groups of compounds, only Q, PyTHQ, BzTHQ, DHQ, PB, PCH, C2H6, NH3, H2, and B need be considered to represent the thermodynamic activities in the reaction. Other compounds were
Ind. Eng. Chem. Res., Vol. 28, No. 1, 1989 15 n
w
NHE
(4) desorption of product R such as PyTHQ, BzTHQ, DHQ, OPA, or PCHAM from u-type active sites, then A+u=Au (3.1) H2 v = H2v (3.2) Au + nH2v = Ru + nv (3.3) Ru=R+u (3.4) Rate expressions for these four steps are (4.1) -ri = kAPAPu4A& - kArPAu4Au (4.2) -rz = kHPH2Pv&dv - kHrPHzV4HzV
+
PCH slTUO w Ec!i& Figure 2. Suggested quinoline HDN network.
a
categorized by 1of these 10 model compounds. The results from final data analyses showed that the fugacity coefficients at different reaction conditions varied from 1.02 to 1.25 for hydrogen and 0.5 to 0.99 for hydrocarbons. This indicates that the nonideality in this reaction system cannot be ignored. A general equation form, A + nH2 = R + S, can represent any of the quinoline HDN reactions where A and R represent the hydrocarbon reactants and products, including Q, PyTHQ, BzTHQ, DHQ, PCH, PB, etc. H2 represents hydrogen. S represents ammonia and light hydrocarbons (LHC) including methane, ethane, and propane. Equation 2 gives the thermodynamic equilibrium ratio of reactant A and product R.
A reaction was determined as irreversible if the equilibrium ratio YR/YAis smaller than 1 X lo9 or greater than 1 X lo3 for all reaction conditions; otherwise, it was determined as reversible. Estimated equilibrium ratios at the extremes of temperatures, pressures, and hydrogen mole fractions of this study showed that hydrogenation reactions are reversible and hydrogenolysis reactions are irreversible. During the equilibrium ratio calculations, the fugacity coefficients for hydrogen, hydrocarbons, and light hydrocarbons were assumed as 1.1, 0.9, and 1.0, respectively. The equilibrium constant (Keq)is from estimated standard free energy. Figure 2 is the suggested quinoline HDN network according to the chemical and thermodynamic analyses and the results of the former quinoline HDN studies. Kinetic Model Mechanism It is often assumed that hydrogen is adsorbed on the metallic ion-exchanged sites, while basic nitrogen compounds are adsorbed on the acidic support. Sonnemans et al. (1973) confirmed this assumption from pyridine HDN. They found that the strongly adsorbed nitrogen bases do not poison the hydrogen chemisorption. Satterfield and Cocchetto (1981) also made the same conclusion from the result of catalyst deactivation. The hydrogenation was 20% deactivated while hydrogenolysis was 60% deactivated after 400 h of operation. Another assumption made here is that hydrogen was in chemisorption equilibrium on the catalyst surface due to its high partial pressure (11-62 bar) in this HDN study. Consider a hydrogenation reaction or a hydrogenolysis reaction without cracking (ring opening) on the NiW/Y82 zeolite surface: A nH2 = R If the surface reaction consists of four elementary steps, (1) adsorption of nitrogen hydrogens such as Q, PyTHQ, BzTHQ, or DHQ on u-type active sites; (2) adsorption of hydrogen on v-type active sites; (3) reaction between adsorbed A and possible neighboring adsorbed hydrogen; and
+
-r3
= ksPAuPH2vn4Au4H2vn - ksrlPRuPvn4~u4vn(4.3)
(4.4) kRPRPu&du The active sites u and v and the adsorbed compound Au, Ru, and H2v are all in solid or solidlike state. Their activities are equal to one at all temperatures and for-wide ranges of pressure. Therefore, fugacity coefficients &, @,, &Au, 4 R u , and &Hfi are equal to one. Because hydrogen was assumed to be in chemisorption equilibrium, P, can be expressed by the Langmuir isotherm: -r4
=
kRrPRu&=tu -
p v
= P:/(l
+ KHZfH2)
(5)
Substituting (5) into (4.2) and using matrix algebra to solve (4.1), (4.3), and (4.4) simultaneously yields the rate equation
16 Ind. Eng. Chem. Res., Vol. 28, No. 1, 1989 0
-1
\
.
-2
k
-3
Y
M 2
-4
,L--
-5 10U
50 L
ne
i)"
I >G Feed S L I C ~
'OC I
Figure 3. Activity of NiW/Y82 catalyst: (A)quinoline conversion; (A)denitrogenation.
rium constant, and the activation energies of parameters. Statistically, the rate-determining step can be determined by model discrimination from the regression of experimental data with (8)-(10). However, in the case of no one rate-controlling step, general (6) has to be analyzed and more parameters will be involved. Reactor Blank Test and Solvent Cracking Test A quinoline HDN experiment was done without catalyst in order to understand the net effect of catalyst. From the product analysis, PyTHQ was found very close to the equilibrium value, while BzTHQ was found much lower than the equilibrium value. This may imply that the reaction from Q to PyTHQ is a very fast reaction even without catalyst. Traces of PCH and MEB were also detected a t a very low liquid feed rate. To check the cracking of solvent, catalytic reactions of pure benzene and benzene with quinoline were compared at 400 OC and 35 bar. Experimental results showed that very little hexane was found in the product when a trace of quinoline was mixed with benzene and about 20-fold increase of hexane was found when pure benzene is hydrogenated. This indicates that nitrogen compounds prohibit the adsorption of benzene on the catalyst surface and thus prevent the benzene reaction. Benzene also had been reported by Satterfield et al. (1978) not to affect the HDN of quinoline. Catalyst Activity The activity of the presulfided NiW/Y82 catalyst was checked periodically by reacting quinoline at 406 "C, 35 bar, 80 mol 70hydrogen, and catalyst contact time (W/F) of 445 (g.h)/mol. The quinoline conversion and denitrogenation ratio were used as indicators of hydrogenation activity and hydrogenolysis activity, respectively. Figure 3 illustrates catalyst activities at different times on feed. Quinoline conversion was 66.60 f 3.37, while quinoline denitrogenation was 23.94 f 4.68 during the whole experimental period. From the statistical t-test analysis, there is no significant relation between catalyst activities and catalyst life time. Both of the activities were thus assumed constant for all the HDN experiments. Since the experimental procedure involved periodic interruptions in the liquid feed and maintaining the catalyst under pure hydrogen during the interruptions, no conclusions can be
U
2du
4bu
WIF
do 0
soii
1000
g-hlgmol )
Figure 4. log Kf of Q + 2H2 = BzTHQ versus W / F . Feed, temperature ("C), pressure (bar): (A)Q, 408,35; (A)Q, 408,52; ( 0 )Q, BzTHQ, 357,71; (v) Q, 357,36 and 71; (0) Q, 456,36; 408,70; (0) (0)Q and PyTHQ, 456, 70 and 72. Dashed lines, 357 "C; heavy lines, 408 "C; thin lines, 456 "C.
drawn about the long-term activity maintenance of this catalyst. Quinoline HDN Network Analysis Experimental data of the ratio f R / u d H 2 f l ) , named Kf afterward, were plotted against WIF to search for the possible equilibrium constant Keqand to understand the effects of temperature, pressure, and contact time. The reaction between Q and PyTHQ was very fast and attained the same point either from feed of Q, PyTHQ, or BzTHQ. Equilibrium was reached a t almost all the reaction conditions. The reaction between PyTHQ and DHQ was determined as irreversible because the Kf values appeared to be determined by reaction rate and not by equilibrium. Since the hydrogenolysis rate of PyTHQ is much higher than that of DHQ (discussed later), the observed WIFinvariant Kf values of Q to BzTHQ and RzTHQ to DHQ in the quinoline hydrogenation loop can be nearly equal to the K,, values. Figure 4 shows that the equilibrium between Q and BzTHQ was slowly reached from both feeds at low temperature. The Kf value reached equilibrium fast at medium temperature with high pressure and at high temperature with both low and high pressures. It then declined as W/F goes higher. The net reaction direction of BzTHQ + 3H2 = DHQ varied in different conditions. At 408 "C and 36 bar, DHQ generated from PyTHQ is more than the equilibrium concentration with BzTHQ; hence, the net reaction direction is from DHQ to BzTHQ. At high pressure and high temperature, DHQ turns lower than the equilibrium concentration with BzTHQ due to faster reaction from Q to BzTHQ; therefore, the net reaction direction is from BzTHQ to DHQ. The trends of getting W/F-invariant Kf values among reactions P B + 3H2 = PCH, EB + 3H2 = ECH, and IDAN + 3H2 = OHIDE were also observed at medium and high temperatures. An example is shown in Figure 5 . Kinetically, it is inconclusive how close these W/F-invariant K f values are to their Keqvalues due to the lack of reaction rate constants related to these compounds. However, they appear very comparable to the estimated Kq values. Analysis of these Kf distributions suggests that PCH and OHIDE were
Ind. Eng. Chem. Res., Vol. 28, No. 1, 1989 17 -4.5
-2
P -3
A -5.0
-4
xw - 5
3
Y
?p
c
-
-1
-5.5
-6
-7
-6.U
0
200
400
WIF
600
800
1000
1.35
1.40
1.50
1.45
( g-hlgmal )
Figure 5. log Kfof PB + 3H2 = PCH versus W/F. Q, feed; 408 OC: (A)36 bar, (A) 52 bar, (0)70 bar.
Figure 7. Comparisons of equilibrium constants. Solid lines, this study; dashed lines, estimated. (A)PB + 3H2 = PCH; (0)EB 3H2 = ECH; (0) IDAN + 3H2 = OHIDE.
+
-2
-3
1 &-4
=a-
’/
u
+
MEBi TO *Pa, XYL
+
”CH
-1
Figure 8. Modified quinoline HDN network after kinetic and chemical analyses.
-5
-b
1.35
1.55
1.45
1000/T
(
1/K
1.65
)
Figure 6. Comparisons of equilibrium constants. Thick lines, this study; thin lines, Cocchetto and Satterfield (1976); dashed lines, Q + 2H2 = BzTHQ; (0) Bz estimated. (A)Q + 2H2 = PyTHQ; (0) + 3H2 = DHQ.
formed quicker or earlier than P B and IDAN because Kf values are from high to low while approaching the equilibrium values. Very possibly, the majority of PB is from dehydrogenation of PCH, and much less from direct hydrogenolysis of OPA. Thermodynamically, the reaction between PCH and P B is reversible, and PB is favored a t high temperature. I t is confirmed in the experimental results. Similarly, IDAN was mostly formed from OHIDE and was favored at high temperature. Other final products, including ECH, EB, MEB, MPB, XYL, TO, MOHP, MPCP, and MCH, are probably derived from cracking, dehydrogenation, isomerization, and disproportion reactions of PCH, PB, and OHIDE. EB may also be a dehydrogenated product of ECH. Total mole percents of PCH and P B in the liquid ranged from 1% to 28%, while quinoline denitrogenation increased from 1.6% to 94% in this study. Total amounts of ECH and EB ranged from less than 0.01% to 15% in the same period. Figure 5 shows that the K f curves a t high-pressure conditions approach
equilibrium values faster than that at low-pressure conditions. This may suggest that more PB and IDAN are formed from OPA a t high-pressure reaction conditions, though the major reaction path, from OPA to PCHAM, is not changed. Figures 6 and 7 show comparisons among experimental equilibrium constants, estimated theoretical equilibrium constants, and some equilibrium constaiits reported by Satterfield and Cocchetto (1981). Estimated heats of reaction were computed from the van’t Hoff equation. They are 27.6, 9.5, and 44.6 kcal/mol for Q to PyTHQ, Q to BzTHQ, and BzTHQ to DHQ, respectively. Estimated heats of reaction for the final products, PB to PCH, EB to ECH, IDAN to OHIDE, etc., are omitted since their accuracy is considerably less than those reported above, either due to smaller mole fractions in the liquid products or due to incomplete data for kinetic study. In review of the suggested reaction network in Figure 2, some corrections can be made from the kinetic and products analyses. First, the reaction of PyTHQ to DHQ had been determined to be irreversible. Second, because PCHAM was not found in the liquid samples, the reaction from OPA to PCHAM can be kinetically simplified as irreversible. Absence of PCHAM agreed with the thermodynamic analysis that the reactions after PCHAM are irreversible. This absence also suggests that the reaction rates from PCHAM to PCH and to OHIDE are much higher than from OPA and DHQ to PCHAM. The quinoline HDN network after these corrections is illustrated in Figure 8.
18 Ind. Eng. Chem. Res., Vol. 28, No. 1, 1989 Table 11. Comparison of Model Parameters with Other Studies _..________ source this work a' bk parameters 37 42 3d E," kcal/mol 40 E,b kcal/mol 38 33 45 31' KHz 0.025/atm g g h 16.8/atm g g 6.4 X lo6 g oil/mol Kf E 5.03 kcal/mol g g g 6.0 2.0 1.0 KPCHQIKQ 3.0 1.0 1.0 1.0 KB~THQ/KQ1.0 6.0 2.0 1.0 KDHQIKQ 9.6 1.0 1.0 1.0 KOPAIKQ 1.0 0.25 0.7 1.0 KNHSIKQ 0.7 R2' 0.90 g g g
*
* PyTHQ + H2 = OPA. DHQ + H2 = PCHAM. At 400 "C. Averaged heat of adsorption of nitrogen heterocyclics and ammonia. eReaction in presence of H2S. fFrom first-order rate equation. 8Not reported or discussed. h0.8/psi for (1)and 0.08/ psi for (2). ' R 2 = Multiple coefficient of determination. 'Source a = Satterfield and Cocchetto (1981). Source b = Satterfield and Yang (1984). 'Source c = Shih et al. 11977).
Regression of Kinetic Model Parameters The whole quinoline HDN network can be divided into two parts. Part I is the SATURATION part because quinoline has to be hydrogenated to either PyTHQ or DHQ before reactions of hydrogenolysis take place. Part I1 is a combination of hydrogenolysis and further hydrogenation. From the product analysis, OPA was only about 2 mol % , AN was not found in most cases, and PCHAM was too low to be detected. Therefore, part I1 can be treated as mixture of denitrogenated aromatics and cyclics. This part is thus named the HYDRODENITOGENATED part. It is very hard to find model parameters of part I since much of the data were a t or close to equilibrium. There was not enough nonequilibrium data to determine parameter values of the rate equation. On the other hand, hydrogenolysis reactions, from PyTHQ to OPA and from DHQ to PCHAM, are irreversible and can be regressed. Data for these reactions were regressed with the irreversible forms of (8)-(10). Subsequent reactions after these two hydrogenolysis reactions will not affect the regression result because the total mole number of denitrogenated products is a constant. Nonlinear regression with the Marquardt algorithm was used for all regression work. The irreversible surface reaction-controlling model, (1l),had the largest regression
...11
(11)
coefficient. Regressed model parameters are given in Table 11. Some model parameters from Shih et al. and from Satterfield and co-workers are also listed. The activation energy of the rate constant k,,, which is related to the heat of reaction, is very similar among these studies. Other parameters are not comparable due to assumptions of different kinetic models. The regression results revealed that the major quinoline HDN path is from Q to PyTHQ, then to OPA, then to PCHAM, and finally to denitrogenated hydrocarbons. The same reaction route was reported by Sanghvi and Akgerman (1983) for sulfided NiMo/Y52 catalyst. In contrast, Cocchetto and Satterfield (1976) reported that DHQ is the main intermediate with sulfided NiMo/alumina. However, they also reported that OPA reacted faster than DHQ while feeding pure individual compounds. The reaction of OPA primarily hydrogenated through PCHAM to PCH rather than by hydrogenolysis to P B directly. These two
Table 111. Multiple Coefficients of Determinant for Q HDN Prediction product multiple coeff 0.985 Q 0.947 PyTHQ BzTHQ 0.771 0.968 DHQ aromatics" 0.782 0.864 cyclicsb aromatics + cyclics 0.839 Q conversn 0.969 denitrogenation 0.847 "Aromatics = P B + EB + MB + XYL N. bCyclics = PCH + ECH + MCH MPCP.
+ MEB + MPB + IDA+ OHIDE + MOHP +
conclusions by Cocchetto and Satterfield were observed in this study. It is believed that the reaction route is determined by the strength competition between hydrogenolysis and hydrogenation. As discussed before, hydrogenolysis is related to the acidic catalyst support, while hydrogenation is related to the ion-exchanged metallic compound. Since a zeolite catalyst has a higher acidic site density and a stronger electric field than alumina-type catalysts, its hydrogenolysis ability is expected to be higher too. This may explain the differences in main reaction routes reported from different studies. Prediction of Product Distributions Several assumptions are made in developing an algorithm to predict product distributions. First, due to the fact that chemical equilibria were approached in many cases, all reactions in the SATURATION part of the quinoline HDN network were assumed in equilibrium. Second, cyclics and aromatics in the HYDRODENITROGENATED part of HDN network were assumed in the same equilibrium state as the major products PCH and PB. Third, products an AN and OPA are neglected. Fourth, the quinoline denitrogenation rate is predicted from (11). Fifth, total consumption of hydrogen in the HYDRODENITROGENATED part is averaged from PCH and ECH for cyclics and averaged from P B and EB for aromatics. In other words, about 4.5 mol of hydrogen is consumed to produce 1 mol of aromatics from quinoline, and about 7.5 mol of hydrogen is consumed to produce 1mol of cyclics from quinoline. Last, the nonideal gas state in the reactor is represented by a mixture of Q, PyTHQ, BzTHQ, DHQ, PCH, PB, B, NH3, and H2. Multiple coefficients of determinants of predicted data for each compound are listed in Table 111. Deviations between experimental data and predicted data may be caused from the assumption that Q, PyTHQ, BzTHQ, and DHQ are always in equilibrium. The assumption about hydrogen consumption in the reactor also causes errors, especially when the feed rate ratio of hydrogen to heterocyclic nitrogen compound is low. Predicted and experimental product distributions a t 456 "C and 36 bar were plotted in Figure 9 as an example. The comparison between experimental and predicted data for all quinoline HDN experiments is shown in Figure 10. Both Q and PyTHQ are predicted more accurately than other model compounds at any condition. This is because PyTHQ was the primary intermediate for the overall HDN reaction. Model parameters regressed from (11) should fit well with the kinetic behavior of PyTHQ. Quinoline was quickly in equilibrium with PyTHQ due to the very fast reaction rates. This equilibrium was included in the prediction algorithm; therefore, quinoline can be predicted
Ind. Eng. Chem. Res., Vol. 28, No. 1, 1989 19
W/F
( g-hlgmol )
Figure 9. Predicted product distribution: 456 ' C , 36 bar. Solid curves, model prediction. (A)Q, (0) PflHQ, (0) BzTHQ, (A)DHQ, (D) LHC.
/
0
20
40 E x p e r i m e n t a l Data
60
80
100
( % )
Figure 10. Comparisons of predicted and experimental data.
as accurately as PyTHQ. This prediction algorithm is also capable of showing higher PyTHQ than BzTHQ a t low temperature and higher BzTHQ than PyTHQ at high temperature. The predicted curve of each compound also has the same shape as that of experimental data. Prediction of the system at medium temperature, 408 "C, and medium pressure, 52 bar, gave the best accuracy. This probably is due to the nature of regression. For instance, prediction of light hydrocarbons (LHC) tended to be higher than the experimental data at low pressure and tended to be lower than the experimental data at high pressure, but fit very well a t a medium pressure, 52 bar.
S u m m a r y and Conclusions The major path of quinoline HDN reactions with the use of sulfided NiW/Y82 catalyst was determined statistically as from Q to PyTHQ, then to OPA, then to PCHAM, and finally to denitrogenated hydrocarbons. In all the experimental conditions, PyTHQ is found in equilibrium with Q instantaneously, P B is found less than or equal to the equilibrium value with PCH, OPA is found
very low, and PCHAM is too low to be detected. This suggests that the rate-limiting step of denitrogenation is the irreversible reaction from PyTHQ to OPA, which agreed well with Sanghvi and Akgerman (1983). At high temperature, aromatic compounds are thermodynamically more favored than their corresponding cyclics; for example, PB is more favored than PCH, and IDAN is more favored than OHIDE. At high pressure, more P B and IDAN are formed from OPA, though it is not the major HDN path. Other aromatics and cyclics such as EB, ECH, MEB, MPB, XYL, IDAN, MOHP, etc., are derived from PB, PCH, and OHIDE. With these considerations, the optimal condition of fast HDN rate coupled with minimum hydrogen consumption in this study is at high reaction temperature and pressure. Hydrogenolysis data can be well regressed with a Langmuir-Hinshelwood model. The adsorption strength of quinoline nitrogen products is in the order DHQ > PyTHQ > Q, BzTHQ, OPA > NH3. Ignoring the adsorption factors may give a poor kinetic analysis. The use of a solvent together with a wide range of hydrogen mole percents may give nonideal gas states in the reaction. Corrections with a suitable EOS model are necessary in the reaction thermodynamic and kinetic analyses. Nomenclature A = reaction reactant B = lumped kinetic parameter in @)-(lo) C," = concentration on catalyst external surface De = effective diffusivity F = molar flow rate of heterocyclic nitrogen compound in the feed Keq= thermodynamic reaction equilibrium constant Kf = fugacity ratio of reaction Ki = adsorption equilibrium constant of compound i K,, = surface reaction equilibrium constant L = distance between pores in particle P = system pressure Pi,= adsorbed compound i on type-u active site expressed in pressure form Pi,= adsorbed compound i on type-v active site expressed in pressure form P," = total type-u active sites expressed in pressure form P,t = total type-v active sites expressed in pressure form R = reaction product S = ammonia or light hydrocarbons in (2) or ammonia in (8)-(11) T = adsorbing nitrogen species in multicomponent system W = catalyst mass Yi= mole fraction of compound i fi = fugacity of compound i ki = adsorption constant of compound i k[ = desorption constant of compound i k,, = forward surface reaction rate constant &' = reverse surface reaction rate constant n = stoichiometric number of hydrogen or reaction order of hydrogen r = reaction rate rv = reaction rate based on catalyst unit volume u = catalyst type-u active site v = summation of reaction stoichiometric numbers in (2) or catalyst type-v active site Greek Symbols = Weisz-Prater parameter c$~ = fugacity coefficient of compound i € & g i s t 4 NO,MCH, 108-87-2;EB, 100-41-4;MPCP, 3728-57-2; PCH, 1678-92-8;MOHP, 3868-64-2; AN, 62-53-3;MEB, 2555014-5;IDAN, 4 ~ 1 - 7MPB, ; 28729-54-6; DHQ, 2051-28-7; BZTHQ, 10500-57-9; OPA, 1821-39-2; Q, 91-22-5; PyTHQ, 635-46-1; PCHAM, 6850-40-4;Ni, 7440-02-0; W, 7440-33-7.
I n d . E n g . Chem. R e s . 1989,28, 20-27
20
Literature Cited Aboul-Gheit, A. K.; Abdou, I. K. The HDN of Petroleum Model Nitrogen Compounds. J . Inst. Pet. 1973, 59, 188-194. Berty, J. M. Reactor for Vapor-Phase Catalytic Studies. Chem. Eng. Prog. 1974, 7O(May), 78-84. Cocchetto, J. F.; Satterfield, C. N. Thermodynamic Equilibria of Selected Heterocyclic Nitrogen Compounds with Their Hydrogenation Derivatives. Ind. Eng. Chem. Process. Dev. Des. 1976, 15, 272-277. Moore, R. M.; Katzer, J. R. Counterdiffusion of Liquid Hydrocarbons in Type Y Zeolite: Effect of Molecular Size, Molecular Type, and Direction of Diffusion. AICHE J . 1972, 18, 816-824. Sanghvi, B. S.; Akgerman, A. Quinoline Denitrogenation on Zeolite Based Catalysts. Spring National Meeting of AICHE, Houston, March 1983, paper 56d. Satterfield, C. N.; Cheng, C. S. Liquid Counterdiffusion of Selected Aromatics and Naphthenic Hydrocarbons in Type Y Zeolite. AICHE J . 1972, 18, 724-728. Satterfield, C. N.; Cocchetto, J. F. Reaction Network and Kinetics of the Vapor Phase Catalytic HDN of Quinoline. Ind. Eng. Chem.
Process Des. Deu. 1981, 20, 53-62. Satterfield, C. N.; Yang, S. H. Catalytic HDN of Quinoline in a Trickle-Bed Reactor. Comparison with Vapor Phase Reaction. Ind. Eng. Chem. Process Des. Deu. 1984,23, 11-19. Satterfield, C. N.; Modell, M.; Hites, R. A.; Declerck, C. J. Intermediate Reactions in the Catalytic HDN of Quinoline. Ind. Eng. Chem. Process Des. Deu. 1978, 17, 141-148. Shih, S. S.; Katzer, J. R.; Kwart, H.; Stiles, A. B. Quinoline Hydrodenitrogenation: Reaction Network and Kinetics. Prepr. Pap.Am. Chem. SOC.,Diu. Pet. Chem. 1977,22,919-940. Shih, S . S.; Reiff, E.; Zawadzki, R.; Katzer, J. R. Effect of Catalyst Composition on Quinoline and Acridine HDN. Prepr. Pup.-Am. Chem. Soc., Diu. Fuel. Chem. 1978,23,99-106. Sonnemans, G. H.; Berg, V. D.; Mars, P. J. The Mechanism of Pyridine Hydrogenolysis on Molybdenum-Containing Catalysts: 11. Hydrogenation of Pyridine to Piperidine. J . Catal. 1973,31, 220-230. Received for review March 4, 1988 Revised manuscript received September 13, 1988 Accepted October 11, 1988
An Investigation of the Mechanisms of Flue Gas Desulfurization by In-Duct Dry Sorbent Injection Mark
R. Stouffer,* Heeyoung Yoon, and F r a n c i s P. B u r k e
Consolidation Coal C o m p a n y , Research & Development, 4000 Brownsuille Road, Library, Pennsylvania 15129
T h e mechanisms of SOz capture by hydrated lime particles entrained in humid flue gas were investigated in pilot-scale tests. T h e mechanisms are applicable t o emerging in-duct dry sorbent injection processes, which involve dry sorbent injection and flue gas humidification by water spraying downstream of the air preheater in a coal-fired boiler system. SO2 removals were measured over 2-s contact time a t approaches t o the adiabatic saturation temperature ranging from 5 to 35 K, in the presence and absence of evaporating water droplets. Hydrated lime removed significant SOz from humid flue gas in the absence of water droplets. The amount of SO2capture by this gas-solid reaction increased with increasing humidity and was proportional to the layers of absorbed moisture on lime t h a t would be in equilibrium with the humid flue gas. The presence of evaporating water droplets significantly enhanced the SOz removal and, thus, indicated that strong particle-droplet interactions occur. This enhancement was greater with larger droplets. Emerging in-duct dry sorbent injection technology may provide low-cost retrofit SO2control options for existing coal-fired power plants (Yoon et al., 1986; Statnick et al., 1987). In-duct desulfurization processes currently under active development include the Coolside process (Consolidation Coal Company) and the HALT process (Dravo Lime Company). The technology involves injection of a dry sorbent, typically hydrated lime, in conjunction with flue gas humidification by water spraying in the ductwork downstream of the air preheater, but ahead of the particulate collection system in a coal-fired boiler system. With a humid flue gas, SO2 is removed by the entrained sorbent particles in the duct and by the sorbent bed in the particulate collector (ESP or baghouse). Since sorbent residence time in the duct is very short (typically 1-3 s), a highly active sorbent is needed for significant SO2 removal during entrainment. If the existing ductwork and particulate collector are used for SO2capture and the dry waste is disposed with the fly ash, these processes have inherently low capital costs. The Coolside process has been developed through laboratory studies, 1-MW-scale field tests (Yoon et al., 1985a,b), and 0.1-MW pilot studies (Yoon et al., 1988). A full-scale (100-MW) utility demonstration is scheduled for 1989 at the Ohio Edison Edgewater power station. The Dravo HALT process has been developed through 0.1-MW pilot tests and a 5-MW “proof-of-concept’’project (Forsythe and Kaiser, 1985; Babu et a]., 1986). 0888-5885/89/2628-0020$01.50/0
Figure 1 is a schematic diagram of the Coolside desulfurization process. Hydrated lime is injected dry in the duct downstream of the air preheater, and the flue gas is subsequently humidified to a close approach to adiabatic saturation by finely atomized water sprays. With the injection of sorbent upstream of the water sprays, sorbent particles may interact with evaporating droplets. An alternate process configuration, which is used in the HALT process, involves injection of sorbent downstream of the water sprays after droplet evaporation. In the Coolside process, a water-soluble additive (for example, a sodiumbased compound) is injected with the humidification water to enhance sorbent activity and utilization. Additionally, since the waste is dry, spent sorbent can be recycled to allow more complete sorbent utilization. In the 1-MW Coolside field tests in 1984 (Yoon et al., 1985a,b),SO2removals of up to 80% were observed across the humidifier and ESP, using commercial hydrated lime as the sorbent (with no recycle) and NaOH as an additive. The highest removals were obtained at 11-14 K approach to adiabatic saturation, 2 / 1 Ca(OH)2/S02mol ratio, and 0.2/1 NaOH/Ca(OH)2mol ratio. Sorbent utilizations were as great as 40%. Initial pilot-scale tests confirmed the results of the 1MW Coolside field tests. The 0.1-MW pilot test unit consists of a 21-cm-i.d. pilot humidifier and a pilot baghouse. Simulated flue gas is humidified by using a commercial atomizing nozzle. The objective of our pilot unit 0 1989 American Chemical Society
