The Role of Liquid Holdup, Effective Catalyst Wetting, and Backmixing

The Role of Liquid Holdup, Effective Catalyst Wetting, and Backmixing on the Performance of a Trickle Bed Reactor for Residue Hydrodesulfurization Angelo A. Montagna Gulf Research and Development Company, Pittsburgh, Pennsylvania 15230

Yatish T. Shah* Depaltment of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 7526 1

The effect of catalyst bed length at constant liquid hourly space velocity or of liquid flowrate on the performance of a trickle bed reactor in hydrodesulfurization of 53% reduced Kuwait crude has been evaluated experimentally. Two different catalyst sizes, 7-8 mesh and 20-30 mesh (US. sieves), have been employed. The experimental results were obtained at a temperature of 750°F, a liquid hourly space velocity of 1 hr-', and pressures of 1000 and 2000 psig. The experimental data have been evaluated on the basis of the axial dispersion model of Mears (1971), the holdup model of Henry and Gilbert (1973), and the effective catalyst wetting model of Mears (1974).

Introduction model gives the values of the minimum bed length requirement in terms of Peclet number and the conversion, whereThe kinetic data obtained in a small hydroprocessing as no similar estimates are presently available based on the trickle bed reactor may be affected by possible backmixing, holdup or catalyst wetting models. The effect of bed length holdup, and incomplete catalyst wetting (Mears, 1971; on conversion can be easily illustrated graphically with the Henry and Gilbert, 1973; Mears, 1974; Paraskos et al., 1975; holdup or catalyst wetting model but the correlations of Satterfield, 1975). The roles of these effects on the reactor power n with the nature of the reaction, feed and the reacperformance should be quantitatively evaluated for proper tion temperature would be difficult. Furthermore, as pointscale-up of pilot plant data. One of the important variables ed out recently by Satterfield (1975), the exponent n will which affects backmixing, liquid holdup, and the catalyst also vary substantially depending upon the flowrate region wetting characteristics of a trickle bed reactor is the liquid being considered. At substantially high superficial liquid flowrate or length of the catalyst bed a t constant liquid velocities where liquid contacting becomes essentially comhourly space velocity. plete and the maldistribution of liquid is no longer a probMears (1971) derived the criteria for the minimum bed lem, the exponent n should approach zero. The estimations length required for a trickle bed operation to be free of of the Peclet number at low liquid flowrates and under acaxial dispersion. He showed that for shallow catalyst beds tual reaction conditions of high temperatures and pressures at constant temperature, pressure, liquid hourly space velocity, and catalyst size, the removal rate of nitrogen from are not always very accurate, thus casting some doubt on West coast gas oil increases with the catalyst bed length. the accuracy of the predictions from the axial dispersion He explained this phenomenon based on the axial dispermodel. The purpose of this paper is to evaluate the applicabilision effect. Henry and Gilbert (1973) recently explained ties of the above models to explain the catalyst bed length Mears' data for the undiluted catalyst bed on the basis of effects on the removal rates of nitrogen, sulfur, metals, and their holdup model. They suggested that, for first-order asphaltenes from 36% Kuwait atmospheric residue oil and reactions, the effect of the catalyst bed length on conversion can be accounted for by the relationship In ( C A ~ / C A ~ ) 53% reduced Kuwait crude oil. The experimental data were a L1I3; where C A and ~ C A are ~ the reactor inlet and outlet obtained under the conditions where axial dispersion, holdconcentrations of the reactant L is the length of the cataup, and the catalyst wetting effects all should be important. lyst bed. Mears (1974) proposes an almost identical relaExperimental Section ~ ) L based on incomplete tionship between In ( C A ~ I C Aand catalyst wetting effects. Paraskos et al. (1975) showed that Two atmospheric residues from the same crude, a 53% ) holds for denialthough the relationship In ( C A ~ / C A0:~ L" reduced Kuwait crude and a 36% reduced Kuwait crude, trogenation and desulfurization of various gas oils, the were analyzed in the present study. Their relevant propervalue of the power n depends significantly on the nature of ties appear in Table I. As expected, the concentrations of the feed, the reaction, and the reaction temperature. sulfur and asphaltenes are proportionately higher in the The cause for the effect of liquid velocity or the catalyst 36% than in the 53% reduced Kuwait crude. A 36% reduced bed length a t constant liquid hourly space velocity (LHSV) crude mean that the 36 volume % of the original crude is on the performance of trickle bed reactor is not yet clearly obtained as the distillate bottoms. Thus 36% reduced crude understood. The axial dispersion model of Mears, the holdis much heavier and more viscous than the 53% reduced up model of Henry and Gilbert, and the catalyst wetting crude. model of Mears all sugest that a t constant LHSV, bed Two particle sizes, 8-14 mesh and 20-30 mesh ( U S . length will not affect the reactor performance provided sieves) of the same commercially available hydrotreating that it exceeds a minimum value. The axial dispersion catalyst consisting of a group 6-group 7 metal combination Ind. Eng. Chem., Process Des. Dev., Vol. 14, No. 4, 1975

470

Table I. Feedstock Inspections

Gravity “API Sulfur (wt 5%) Nitrogen (wt %) Nickel (ppm) Vanadium (ppm) Carbon residue (wt %) Pentane insolubles

3 6% Reduced Kuwait Crude

53% Reduced Kuwait Crude

11.4 4.67 0.31 21

14.8 4.07 0.23 15

65

56

12 9.2

9 7.9

on alumina from the same lot, were used in the present study. The catalyst bed volumes ranged from 50 to 400 cm3, with corresponding bed lengths of 10.5 to 84.2 cm. The catalyst was packed in a stainless steel reactor 2.54 cm i.d. X 96.5 cm in internal dimensions, equipped with a 0.635 cm 0.d. axial thermowell mounted in the center of the reactor. In all runs provisions were made for ensuring homogeneous distribution of the feed streams over the active catalyst bed by packing the space above and below the catalyst bed with inert packing of the same dimensions. Also, a specially designed liquid distributor was used to avoid channeling of the liquid flow. A glass model simulation of the reactor system indicated no channeling effect (i.e., selective flow of liquid on one side of the bed) within the reactor. The reactor system was a steam jacketed, once-through bench scale unit. The residual oil feed and hydrogen were premixed before entering the reactor. Cocurrent oil and hydrogen flows were employed in these downflow experiments. The bulk of the light gases were separated from the liquid product in a high-pressure separator, with the balance of the gases being removed in a low-pressure separator. The product liquid was analyzed for the reactants, i.e., sulfur, nickel, vanadium, and asphaltenes. The gas was analyzed for HzS, ammonia, hydrogen, and light hydrocarbons by mass spectrometry. The liquid weights, the product gas analysis, and the gas rates were used to calculate the C4+ liquid recovery. The C4+ liquid recoveries for the various runs ranged from about 102 to 104% by weight of liquid feed. These values indicate that an acceptable material balance was obtained during the experiments. The runs with the 36% reduced Kuwait crude employed the 8 X 14 mesh catalyst and the runs with the 53% reduced Kuwait crude were made with the 20 X 30 mesh catalyst. All the runs with 36% reduced Kuwait crude were performed at 75OoF, 2000 psig hydrogen partial pressure, 1 LHSV and 5000 SCF of Hz per bbl of feed for a duration of 24 hr. The first 8 hr of each run was discarded since that is the time required for the equipment to stabilize the achieve steady state. The runs with 53% reduced Kuwait crude were made at 750°F, 1 LHSV, 5000 SCF of H2 per bbl of feed and hydrogen partial pressures of 1000 and 2000 psig. All runs except those using 200 cm3 of catalyst with the 36% reduced Kuwait crude were duplicated and the average of the results are presented here. The sulfur content of the liquids was measured by a high-temperature combustion, LECO, technique to an accuracy of 4~0.03%sulfur. The nickel and vanadium contents were measured by X-ray fluorescence to an accuracy of &lo%. The asphaltene content (normal pentane insolubles) was determined by ASTM D-893 to a reporducibility of f15%. The carbon residue was also measured to a reproducibility of 4~15%. 480

Ind. Eng. Cham., Process Des. Dev., Vol. 14, No. 4, 1975

Results The extent of desulfurization (%), and the product concentrations of nitrogen (% wt), carbon residue (% wt), pentane insolubles (% wt), nickel (pprn), and vanadium (ppm) obtained with the 36% reduced Kuwait crude a t 750°F, 2000 psig and 1 LHSV with the 8-14 mesh catalyst as a function of the catalyst bed length are presented in Figures la, to If, respectively. These results show the pronounced effect of catalyst bed length on the conversions of all reactants. We have evaluated these results by the axial dispersion, the holdup, and the catalyst wetting models. Axial Dispersion Model In order to evaluate the experimental data shown in Figures l a to If by the axial dispersion model, we need the kinetic constants and the effectiveness factors for the various reactions. At small deviations from plug flow and for firstorder reactions, one can express the relation between outlet and inlet concentrations as (Wehner and Wilhelm, 1959)

Here k is the intrinsic kinetic constant, 7 is the effectiveness factor, LHSV is the liquid hourly space velocity, d, is the catalyst diameter, L is the length of the catalyst bed, and Ped is the Peclet number based on the particle diameter. Equation 1 can be rewritten as

where the subscript p on the ratio CA~/CA,,represents the condition under plug flow. Equation 2 can be rearranged as (3) The reported literature information indicates that Ped

a

La

where a has been reported to be greater than zero. Hochman and Effron (1969) give a = 0.50 whereas Sater and Levenspiel (1966) report a = 0.747 f 0.147. It is clear that, ,)L according to eq (31, the log-log plot of In ( C A ~ / C A ~vs. should be a straight line with a slope greater than unity. The values of C A ~can be obtained from the results of experiments with a reactor where the catalyst bed length is ~) 1971). greater than 2Od,/Ped In ( C A ~ / C A(Mears, The applicability of the axial dispersion model to the present set of data was evaluated in two different ways. First, the values of minimum L required for elimination of backmixing as predicted by Mear’s criterion (1971) are summarized in Table 11. The Peclet numbers for these calculations were obtained from the correlation of Hochman and Effron (1969) which indicates that Ped = 0.042Re0.50. Although this correlation was derived from the experimental data in the range of 4 < Re < 80, we have extrapolated its use to a considerably low Reynolds numbers. It should be noted that this extrapolation could be in considerable error. It appears from the experimental data shown in Figures l a to If that the conversions may be assumed to be leveled off somewhere between bed lengths of 40 and 80 cm. Since Mears’ criterion predicts conservative values for minimum L and considering the above-mentioned inaccuracy involved in the prediction of Ped the agreement between predicted and experimental values of Lmin appears to be reasonable. Secondly, the validity of eq 3 for the present set of data was examined. Log-log plots of In ( C A ~ I C A ~vs., ) L for all the reactions are shown in Figure 2. These plots indicated

50

LlOUlD FLOWRATE ( c c / h r l 200 300

100

I wc

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1

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50 8

400

100 I

200

300

400

I

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0

TEMP 750'F PRESSURE = 2 0 O O p s l g LHSV = I H, = 5000 SCFIBbl

7 1

0 PRODUCT PENTANE INSOLUBLES WT 70

%

DESULFURIZATION

KEY -

TEMP = 7 5 0 ° F PRESSURE = 2000 psig Lnsv = I Hz = 5 0 0 0 S C F l E b l

.t

IO

20

60

40

80

(la1 % D E S U L F U R I Z A T I O N VERSUS CATALYST BED LENGTH AND L l O U l D FLOWRATE

50

0.20

PRODUCT NITROGEN WT %

00

200

300

I

I

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I I I 40 60 80 CATALYST B E 0 LENGTH, (cm) (Id) PRODUCT PENTANE INSOLUBLES VERSUS CATALYST BE0 LENGTH AND L l O U l D FLOWRATE

21 IO

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L I O U I D FLOWRATE ( c c l h r l 700 300

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1

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L I OU I D FLOWR ATE (cc/hr) 200 300

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5 41

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TEMP = 7 5 0 ° F PRESSURE = 2000 p r i g LHSV = I HZ = 5 0 0 0 SCF/Bbl

6

60 80 (le) PRODUCT NICKEL VERSUS CATALYST BED LENGTH AND L I Q U I D FLOWRATE

400

-

CARBON RESIDUES WT %

40

30

KEY

7 k

20

80

BED L E N G T H ( c m l (Ib) PRODUCT N I T R O G E N VERSUS CATALYST BED LENGTH AND L l O U l D FLDWRATE

e,

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I 20

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CATALYST BED LENGTH (cml (If) PRODUCT VANADIUM VERSUS CATALYST BE0 LENGTH AND LlOUlD FLOWRATE

(IC) PRODUCT CARBON RESIDUE VERSUS CATALYST BED LENGTH Am) LlOUlO FLDWRATE

Figure 1. Experimental data for 36% KATB; 8-14 mesh. 10.

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6 X

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that, within the accuracy of the data, In (CAJCA~,)for all reactants are proportional to L-". Although the powder n varies slightly with the nature of the reaction, it is always greater than 1 as it should be. In this test, CAO(,for each reaction is assumed to be equal to the outlet concentration for the largest catalyst bed.

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UICKEL VAUADIUY SULFUR CARBDU RESIDUE PENTANE IUSOLUBLES UITIKIGEU

\CR

1

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10.

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Figure 2. Correlation of experimental data (36% KATB feed) by the axial dispersion model.

Holdup and Catalyst Wetting Models According to the holdup model of Henry and Gilbert (1973) and the catalyst wetting model of Mears (1974), our data can be correlated with a log-log plot of In (CA~/CAJ vs. L . The plots for the present data are shown in Figure 3. As shown by this figure, the present data are correlated well by both the holdup and catalyst wetting models. Although the slopes of the plots for the various reactions are about the same, they are about 0.2, i.e., slightly lower than % as suggested by Henry and Gilbert and 0.32 as suggested by Mears (1974). The experimental results with the 20-30 mesh catalyst Ind. Eng. Chem., Process Des. Dev., Vol. 14. No. 4. 1975

481

10

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Figure 4. Bed length effect on feed 20-30 mesh size catalyst.

%

desulfurization for 53% KATB

particles (53% reduced Kuwait crude feed) were obtained with catalyst bed lengths of approximately 24, 48, 72, and 97 cm. Some typical results showing the catalyst bed length (or liquid flowrate) effect on extent of desulfurization at two different pressures but at the same temperature, LHSV, and hydrogen rate are described in Figures 4a and 4b. Very similar results were also obtained for other reactions. These results clearly indicate that the axial dispersion, holdup, and catalyst wetting effects were not important in these experiments. The minimum catalyst bed lengths required for the elimination of backmixing were calculated using Mears’ criterion. For the sulfur removal reaction and at pressures of 1000 and 2000 psig these results are described in Table 111. The results indicate that Mears’ criterion predict significantly larger values of Lmin 482

Ind. Eng. Chem., Process Des. Dev., Vol. 14, No. 4, 1975

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than the ones obtained experimentally. The conservative values of minimum length calculated from the Mears criterion could probably be a result of the extrapolation of Hochman and Effron (1969) correlation to considerably small Reynolds numbers. Recently, Paraskos et al. (1975) showed that for LHSV = 1and temperature = 750°F, the percentage desulfurization should be dependent on the catalyst bed length. The discrepancy between their predictions and the present data may be due to the difference in the catalyst particle size used in these two works. The catalyst particle size used in the present study was approximately two to three times smaller than the one used by Paraskos et al. (1975). The liquid holdup, effective catalyst wetting and backmixing effects all should be less important at smaller particle size.

Discussion and Conclusions For hydrodesulfurization of 53% reduced Kuwait crude, Paraskos et al. (1975) analyzed the effect of variation in LHSV onto the conversions of sulfur, metals, and nitrogen removal reactions. Here we analyze similar effects of catalyst bed length or superficial liquid velocity. The results for 36% reduced Kuwait crude are well explained by the holdup model of Henry and Gilbert and the catalyst wetting model of Mears. The superficial mass rate used in the present experiments was approximately 0.02 kg/(m2)(sec) up to about 0.16 kg/(m2)(sec). Figure 4 of Satterfield (1975) show that at these rates, the effective catalyst wetting is rather small. Thus, the Mears wetting model perhaps gives the most logical explanation of the observed phenomenon for 36% reduced Kuwait crude. However, the results for 53% reduced Kuwait crude are somewhat surprising and cannot be very well explained by the holdup or the catalyst wetting model. Although not intended experimentally, it appears that the liquid distribution within the catalyst bed was quite uniform and all the catalyst particles were effectively wetted during the experiments with this stock. Paraskos et al. (1975) pointed out that, according to the axial dispersion model, the log-log plot of In (CA~/CAJVS. L would not be a straight line. However, in this paper we investigated the application of the axial dispersion model in a slightly different way and found that the model reasonably well correlates the present set of data for 36% re-

Table 11. Predicted Values of Minimum Catalyst Bed Length Required for Elimination of Backmixing (Feed conditions: temperature = 750"F, pressure = 2000 psig, LHSV = 1hr-l; charge stock: 36% Kuwait atmospheric residue; catalyst size: 8-14 mesh; 24 hr operation; fluid properties: p = 0.93 g/cm3; p = 0.15 cP) Lmioufrom Mears' criteria, cm

Catalyst Catalyst bed vol, cm3 length, cm

Re

Ped

Sulfur

Nickle

Vanadium Nitrogen

Carbon residue

Pentane insolubles

~

150 81 79 0.023 189 127 50 10.16 0.31 0.033 172 92 125 60 74 100 20.32 0.62 0.047 147 77 96 200 40.64 1.24 55 65 0.066 93 71 79 400 81.28 2.48 33 43 a L,in varies with the bed length because at constant LHSV, as bed length is varied, the liquid flowrate is also varied.

72 76 53

.*.

Table 111. Predicted Values of Minimum Catalyst Bed Length Required for Elimination of Backmixing (Feed conditions: temperature = 750"F, pressure = 1000 and 2000 psig, LHSV = 1hr-l; charge stock: 53% Kuwait atmospheric residue; catalyst size: 20-30 mesh; 24 hr operation; fluid properties: p = 0.9 g/cm3, p = 0.12 cP) Lminfrom Mears' criteria for sulfur removal reaction, cm

Catajyst vol, cm3

Catalyst bed length, cm

P=

P=

Re

Ped

1000 psig

2000 psig

100 200 300 400

24.13 48.26 72.39 96.52

0.15 0.30 0.45 0.60

0.016 0.023 0.028 0.033

155

...

110

125 102 86

duced crude. The applicability of the model to the data for 53% reduced Kuwait crude is, however, only fair. It should be emphasized that the applicability of axial dispersion model to the present set of data depend on the validity of the extrapolation of the Peclet number correlation of Hochman and Effron (1969) to low Reynolds numbers. Some further experimental evidence in this regard is needed. One can thus conclude that all three models i.e., axial dispersion, holdup, and catalyst wetting equally well explain the present data for 36% reduced Kuwait crude. However, the data with the 53% reduced Kuwait crude are only moderately correlated by the axial dispersion model. Without the knowledge of holdup-and catalyst wetting characteristics in the experiments with 53% reduced Kuwait crude, it is hard to explain the conversions data with this residual oil on the basis of the holdup or the catalyst wetting model. Backmixing, liquid holdup and effective catalyst wetting all appear to be strongly dependent on the catalyst particle size or the viscosity of the feedstock. Acknowledgment The contribution of S. L. Peake and H. G. McIllried for part of the experimental work presented in this paper is gratefully acknowledged. The authors also gratefully acknowledge several constructive comments of Professor C. N. Satterfield on this paper.

90 76

Nomenclature C A= ~ reactor inlet concentration of species A, g/cm3 CA,,= reactor outlet concentration of species A, g/cm3 d, = catalyst particle diameter, cm K = intrinsic rate constant, l k r L = length of the catalyst bed, cm LHSV = liquid hourly space velocity, l k r Ped = Peclet number based on the catalyst diameter, dimensionless Re = Reynolds number based on the catalyst diameter, dimensionless Subscript p = plug flow condition

Literature Cited Henry, H. C.. Gilbert, J. B., I d . Eng. Chem., Process Des. Dev., 12, 328 (1973). Hochman, J. M., Effron, E., Ind. Eng. Chem., Fundam.. 8 , 6 3 (1969). Mears. D. E., Chem. Eng. Sci., 26, 1361 (1971). Mears, D. E., in H. M. Hulburt, "Chemical Reaction Engineering 11, ACS Monograph Series 133, p 218, 1974. Paraskos, J. A.. Frayer. J. A,, Shah, Y. T., to be published in Ind. Eng. Chem., Process Des. Dev., 1975. Sater, V. E., Levenspiel, O., Ind. Eng. Chem.. Fundam., 5 , 86 (1966). Satterfield, C. N.. AK3h.E J., 21 (2). 209 (1975). Wehner. J. E.,Wilhelm, R. W., Chem. Eng. Sci., 6, 89 (1959).

Received for review March 3,1975 Accepted June 30,1975

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