Energy & Fuels 1991,5,835-839
835
Upgrading of Coprocessed Liquid: Kinetics and Internal Mass-Transfer Effects H. A. Rangwala,* Z. M. George, and A. H. Hardin Alberta Research Council, Devon, Alberta, TOC 1EO Canada Received February 8, 1991. Revised Manuscript Received July 30,1991
Results are reported for catalytic upgrading of a 343 "C) av mol w t
0.922 22.0 2.46 0.580 1.52 11.0 33.0 54.0 397.0
Simulated distillation analyses (ASTM-D2887).
Table 11. Properties of Catalyst (Ni-Mo-r-AlzOl) metal loading, wt % NiO
Moo3 characteristics surface area, m2/g pore volume, cm3/g av pore radius, nm pellet density, g/cm3 particle size, mm pellet (as received) powder (crushed and sieved) 20/30 US mesh 30/50 US mesh 50/80 US mesh
2.8 14.0 200.0 0.43 4.3 1.14 1.59 0.72 0.44 0.24
substantial diffusional limitation is detrimental to the catalyst life.spg There are no direct measurements of catalyst effectiveness factors for catalytic upgrading of coal-derived liquids. To obtain a quantitative idea about intraparticle diffusion, effectiveness factors are usually calculated assuming the validity of simplified pore diffusional models.1° In this study, the upgrading of a
d
c,
u al
'c 'c
W
-Equallon2
c'
0 A
For HDN (exp.rlm*nlal valueas) For HDS ( ~ X p n m O ~ lValUO8) al
O.i+
i
L.0
0.1
0
Figure 3. Comparison of measured catalyst effectiveness factors with those predicted using the single pore model. Table V. Calculated Values of
and 7 from Experimental
Data @
runno. 5 6 7 8 8A
ll
d,mm
HDS
HDN
HDS
HDN
0.24 0.44 0.72 1.59 1.59
0.06 0.17 0.35 1.19 1.13
0.02 0.06 0.15 0.57 0.61
1.0 0.87 0.66 0.46 0.43
1.0 0.91 0.86 0.65 0.69
almost comparable to those obtained by White et a1.18 Thus, for Upgrading of coprocessed liquids, internal mass transfer is important, in the next section, catalyst effectiveness factors from experimental data will be compared with those predicted from the single pore model. The model will be applied to extract an intrinsic rate constant and, hence, intrinsic activation energy for these reactions. Internal Mass-TransferEffects. The results in Table IV show that rate constants are dependent on particle size. The catalyst effectiveness factor for the size 0.24 mm is shown later to be unity. This implies that the rate constants for the smallest particle size represent the intrinsic reaction rates. The experimental values of the catalyst effectiveness factors for all the other particle sizes are obtained by dividing the measured rates by those for the smallest size. The results are summarized in Table V. The corresponding values of Thiele parameters are calculated by using the intrinsic rate constants and effective m2/s diffusivities of 10 X 10-lom2/s for HDS and 5 X for HDN reactions. The latter values were obtained by trial and error so as to provide the best fit between the measurned q and predicted values of q using eq 2. Figure 3 shows a comparison o f t measured in this study and those predicted by using eq 2. The agreement between the measured values and theoretical curve in Figure 3 is reasonably good. The experimentally measured q for catalyst pellets are 0.46 and 0.65 for HDS and HDN reactions and confirm that there is substantial internal mass-transfer effect during upgrading of this particular coprocessed feedstock. This also explains the low E, values obtained in this study. The calculated values of @ using our measured rates and the above diffusivities values are given in Table V. The values of CP for the catalyst pellet are substantially higher than 0.1 and show a significant internal mass-transfer
limitation for upgrading of the feedstock using pellets. However, for the smallest particle investigated, viz., 0.24mm size, the value of @ is considerably less than 0.1 and the measured rates for this particle size can be assumed as the intrinsic reaction rates. This supports the assumption made in calculating experimental values of rl in the above paragraph. De values of 0.3 X 10-loto 0.8 X 10-lo mz/s have been reported by Philippopoulos et for HDS of atmospheric residue. Trytten et a1.20have reported De values in the to 6 X 10-lo mz/s for HDN and HDS range 0.43 X of different fractions of Syncrude Coker Gas Oil. Incidently, the last value corresponds to a fraction similar to our reaction product. If one considers quinoline and thiophene as typical reactant species, the De values for these species are 9.5 X 10-lo and 13.2 X m2/s, respectively (see Appendix). Hence, the Devalues used to confirm eq 2, viz., 5 X 10-lomz/s for HDN and 10 X 10-lo mz/s for HDS, agree reasonably well with published values for similar feedstocks and with calculated values for typical model compounds. Since the intrinsic values of rate constant can be written as k'= k / q (4) we can iterate q from eq 2 using measured values of k and above De values after correcting the latter for temperature. The intrinsic rate constants obtained by such iteration techniques are also shown in Figure 2. The intrinsic activation energy value E for HDS and for HDN reactions are 57 and 44 kJ/mol, respectively. This result is surprising. It shows that the true activation energies for upgrading this particular feedstock are indeed low. Weisz and Prater2' give the following approximate relationship between E , E,, and q E/Ea=2-q (5) With the experimental values of q and E, for HDS and HDN, the values of E predicted from eq 5 are 55.4 and 44.6 (19) Philippopoulos,C.; Papaya",
-97. , d1.5 -
N. Ind. Eng. Chem. Res. 1988,
-I.
(20) Trytten, L. C.; Gray, M. R.; Sanford, E.C . Ind. Eng. Chem. Res. 1990, 29, 725. (21) Weisz, P. B.; Prater, C . D. Ado. Catal. 1954, 6, 144.
Upgrading of Coprocessed Liquid
Energy & Fuels, Vol. 5, No. 6, 1991 839
kJ/mol for HDS and HDN, respectively. These values agree very well with those obtained from estimated intrinsic rate constants. The E, values are low, but not equal to half of the corresponding intrinsic values, if the reactions were taking place in the diffusion regime only. This indicates that the upgrading, using catalyst pellets, occurs in an intermediate regime where both internal mass transfer and kinetics are important. Conclusions 1. The catalyst effectiveness factors determined in this study show that HDS and HDN reactions during catalytic upgrading of this particular coprocessed liquids are partially limited by internal mass-transfer effects. 2. The upgrading reactions for this feedstock and catalyst occur in the intermediate regime where diffusion and kinetics are both equally important. 3. The single pore model is shown to be applicable for this feedstock in determining catalyst effectiveness factors provided intrinsic rate data and effective diffusivity values are available. Alternatively, at least two experimental data for different catalyst sizes should be available. Acknowledgment. We thank the Alberta Research Council for their financial support for this work, and Drs. Rahimi and Fouda of CANMET for supplying the feedstock. Technical assistance for experimental work was provided by Mr. P. Ho-Si. Appendix The effective diffusivity of reactant species in the porous catalyst can be calculated from the following relationz2
D, = Do(c/7) exp(-4.6X)
(AI)
where Do is the molecular diffusivity of the species, c / 7 is the ratio of catalyst porosity to tortuosity, and X is the ratio of solute to pore diameter. For the purpose of estimating De, we assume that the nitrogen and the sulfur species in the feedstock may be represented by quinoline and thiophene, respectively. The average molecular weight of the product liquid, as calculated from simulated distillation analyses, is about 240 kg/kmol and the viscosity of the (22) Satterfield, C. N.; Colten, C. K.; Pitcher, W. H., Jr. AIChE J. 1973, 19, 628.
product liquid a t the reaction condition of 375 OC, estimated from literature,= is about 0.25 cP. Using this data and Wilke-Chang's c~rrelation'~ for molecular diffusivity, Do values for quinoline and thiophene in the reaction product a t 375 OC are 1.5 X 10" and 2.1 X 10" mz/s, respectively. The t/7, based on data given by Satterfieldls for typical hydrotreating catalysts, is about 0.1. The critical molecular diameters for quinoline and thiophene based on estimated critical volumes are in the range 0.8-1.0 nm. These values correspond to X 0.1. Based on the above values of Do, e/?, and A, the effective diffusivities De of quinoline and thiophene a t the reaction condition of 375 "C are 9.5 X and 13.2 X 10-lo mz/s, respectively.
-
Nomenclature specific surface area of catalyst, m2/g catalyst particle size, mm molecular diffusivity, m2/s effective diffusivity, m2/s activation energy for intrinsic rate constant, kJ/mol activation energy for apparent rate constant, kJ/ E. mol k apparent reaction rate constant, cm3/(s.kg of catalyst) k' intrinsic reaction rate constant, cm3/(s.kgof catalyst) liquid feed rate at 23 O C , cm3/s QF liquid product rate at 23 "C,cm3/s QP average pore radius, nm R S external surface area of catalyst pellet, cm2 catalyst pellet volume, cm3 fractional sulfur or nitrogen conversion Greek Letters catalyst porosity 9 catalyst effectiveness factor 9 space time, (*kg of catalyst)/cm3 x ratio of solute diameter to catalyst pore diameter P catalyst pellet density, g/cm3 7 catalyst tortuosity 4 Thiele parameter al dimensionless number defined by eq 3 Registry No. Ni, 7440-02-0; Mo, 7439-98-7. a d Do
3
$
(23) Teonopouloe, C.; Heidman, J. L.; Hwang, S. C. Thermodynamic and Transport Properties of Coal Liquids, John Wiley: New York,1986; Chapter 9.
