Hydrogen
Naphtha
x
.
I
St ripping Column
Nitrogen
High-pressure flow unit
I
W. A. WILSON, W. E. VORECKI, and R. V. MALO
Research Department, Standard Oil Co. (Indiana), Whiting, Ind.
Hydrodesulfurization Catalyst Studies This method for evaluating catalyst activity for sulfur and nitrogen removal from naphtha relates directly to process conditions. It can be an invaluable guide in selecting catalysts, process design, and adjustment for economical operation
SINCE
HYDROGEN has become abundant from catalytic reforming, more petroleum stocks are being hydrogen-treated (4, 6, 8). For example, virgin naphthas are pretreated to remove sulfur and nitrogen compounds that interfere with performance of reforming catalysts. Much has been published concerning removal of sulfur compounds, but little such information is known about nitrogen compounds. As more cracked naphthas are fed to reforming units, hydro-
Present address, University of California, Livermore, Calif.
treating will become even more useful; these naphthas usually contain larger amounts of sulfur and nitrogen compounds that are more difficult to remove (7, 3 ) . As hydrodesulfurization has expanded, the number of commercially available catalysts has increased. These catalysts are prepared differently, have different compositions and physical properties, and hence, different catalytic activity. Methods suggested for rating these are only qualitative and do not relate activity directly to processing conditions. A bench-scale study of process vari-
ables was undertaken to determine space velocities, temperatures, and pressures needed to remove sulfur and nitrogen from naphthas and to establish a method of catalyst evaluation that could be related directly to process conditions. Theoretical Rate of removal of contaminants such as sulfur or nitrogen compounds from petroleum streams by hydrogen treating is a function of many factors which can be broken down into three broad categories-type and concentration of contaminant, processing conditions, and VOL. 49, NO. 4
APRIL 1957
657
,
catalyst properties. case,
-E dt
As a generalized
= F(X,Y,Z)
-0.sc
where X represents a specific contaminant, Y the processing variables, and 2 the catalyst variables. The important processing variables are temperature and hydrogen partial pressure,
\
I \
c3
-I
0
If the functions of the contaminant and hydrogen are the concentrations raised to a power, a usable rate equation can be set up:
I
-2
IO
0
RECIPROCAL
where k is the specific rate constant; X, concentration of the contaminant; AH, energy of activation; R, gas content; T , absolute temperature;. P,, hydrogen partial pressure; A and B, constants, and 2, preparation, composition, and physical properties of the catalyst. For any given catalyst, 2 is a constant and can be included in the specific rate constant. Separating variables and integrating gives
where X, represents original concentration of the contaminant and t the reaction time. As a first approximation, the reaction time can be represented as t = V,/(Y,/0),where Yz is the void volume of the catalyst bed and V, the total volume of gas passed through the catalyst bed in time 8. If the perfect-gas law holds and the concentration of contaminant is small,
30
S P A C E VELOCITY,,
MOLES H C Figure 1.
Effect of space velocity
e
( N H CiN H )( R T ) ep
0 program to find the effects of process variables on contaminant removal. By holding the contaminant, hydrogenaddition rate, and catalyst constant and by varying temperature, hydrogen pressure, and space velocity one at a time, the process functions can be evaluated. When the process functions have been evaluated, the catalyst factor, 2, can be studied. An arbitrary value of 100 can be assigned to the activity of a reference catalyst, and other catalysts can be compared directly in ability to catalyze contaminant removal. Catalyst activity is an inverse function of the amount of catalyst needed to remove a given amount of contaminant. Related approaches to catalyst activity have been suggested (2, 5, 7).
A commercial catalyst containing a mixture of cobalt and molybdenum
where N H C and N H are moles of hydrocarbon and hydrogen passed over the catalyst in time 0, and P is the total pressure of the system. If it is assumed that
and the hydrogen-addition rate defined as C = NH/hTHC is introduced, Equation 4 can be written
(5)
With Equation 5 as a guide, it is now possible to design an experimental
558
on sulfur and nitrogen removal
0 Nitrogen
Experimental -V = R
50 G. CAT.*HR
INDUSTRIAL AND ENGINEERING CHEMISTRY
Sulfur
oxides supported on alumina was used. The 3/'16-inch pellets were crushed and screened to give a 4- to 6-mesh fraction. Two naphthas were used, a MidContinent virgin naphtha for the study of process variables and a thermally cracked naphtha to demonstrate the process equations developed (Table I). T o the virgin naphtha, were added 800 p.p.m. of dibenzothiophene and 150 p.p.m. of 3-methylisoquinoline. The virgin naphtha and added compounds were chosen as a feed which would give minimum coking and would contain difficult-to-remove sulfur and nitrogen compounds that could be easily analyzed, The naphtha also contained 230 p.p.m. of natural sulfur, which was more than 99y0 removed in all studies, and was disregarded. A conventional, high-pressure flow system was used. The reactor was 0.75 inch in diameter and 30 inches long. -4 20-gram catalyst charge occupied about
H Y D R O G E N IN THE PETROLEUM I N D U S T R Y 0 30
50
50
60
a 70 $
1.62
-0.8
Io3 -
10 HYDROGEN
0
"K
Figure 2. Effect of temperature on sulfur and nitrogen removal
Figure 3.
30
20
PRESSURE, ATM.
Effect of hydrogen pressure on nitrogen removal
0 Nitrogen 0 Sulfur
2.5 inches of this length. A coaxial thermowell with a sliding thermocouple was used to measure catalyst bed temperatures. The liquid product was separated from the gas under pressure and was stripped countercurrently with nitrogen to free it of hydrogen sulfide and ammonia. The start-up procedure consisted of charging catalyst, heating under hydrogen, pressuring with hydrogen, and starting oil flow. Because removal of sulfur increased and removal of nitrogen decreased at first, about 50 to 70 hours were allowed for catalyst activity to reach an equilibrium value. Sulfur was determined by combustion in a lamp (ASTM method D 1266-55T), absorption of the oxides in hydrogen peroxide, oxidation to sulfuric acid, and turbidimetric determination of the sulfate. Total nitrogen was determined by a micro-Kjeldahl method. Basic nitrogen was determined by titration with perchloric acid in benzene-glacial acetic acid. In general the process conditions used were: space velocity, 0.05 mole of hydrocarbon per gram of catalyst
per hour; temperature, 371' C.; hydrogen pressure, 11.4 atm.; and hydrogen addition rate, 1.2 moles per mole of hydrocarbon. Each was varied in turn while the others were held constant. The variables were assumed to be independent of each other and the study was not extended far enough to determine interaction factors.
Effects of Process Variables Effect of space velocity on removal of sulfur and nitrogen is plotted on Figure 1. A straight line was found for both sulfur and nitrogen removal : the reactions are thus fi& order and follow the form, log-X = - K x
x,
sv
where X represents either the sulfur or nitrogen compound and SV the space velocity. Effect of temperature is plotted in Figure 2 over the range from 345' to 395' C. An Arrhenius-type temperature function was assumed. For sulfur removal, the calculated energy of activation was 3.8 kcal. per mole
- 0.5
,
..?.
where AHa is 3.8 kcal. per mole and C is 1.2 moies of hydrogen per mole of hydrocarbon. The rate constant, K,, is also a function of pressure and is 1.81 at 11.4 atm. of hydrogen. For nitrogen removal,
/
- E L L \
70
- 80
J
a
XI2 0 0
~
and for nitrogen removal was 20 kcal. per mole. This low value for sulfur removal indicates clearly a diffusionrate controlled reaction. Effect of hydrogen pressure on nitrogen removal is plotted in Figure 3. The semilogarithmic plot can be represented by a straight line passing through N / N , = 1. The data for sulfur removal scattered badly and are not included in this plot. Effects of space velocity, temperature, and hydrogen pressure can be combined into equations similar to Equation 5. For sulfur removal,
- 90
-1.0-
J
- 95
>
0
3
a
0 0 J
- 1.5 -2.0
I
0
20
40
RECIPROCAL SPACE VELOCITY,
60
99
0 MOLES HG
IC HYDROGEN
20 PRESSURE, A T M .
30
Figure 4. Effect of space velocity in hydrotreating thermal naphtha
Figure 5. Effect of hydrogen pressure in hydrotreating thermal naphtha
0 Nitrogen
0 Nitrogen 0 Sulfur
0
Sulfur
VOL. 49, NO. 4
0
APRIL 1957
659
L
2
A P
150-
> 0 5
w
E
z
100
AA
A
-
‘
A A 6,
A
A
D
W
A
0
0
a
!= z
-
0 Figure 6.
25 NITROGEN
75
50 REMOVAL,
01 0,
IO0
Hydrodesulfurization of Thermal Naphtha
Experiments using thermal naphtha were carried out to demonstrate forms of equations developed for removing the added sulfur and nitrogen compounds. Conditions were essentially the same. The main variation was in hydrogen addition rate which was lowered to 1 mole per mole of hydrocarbon. Figure 4 shows the effect of space velocity on contaminant removal at 7.3 atm. hydrogen pressure. A straightline fit was obtained with the assumption of first-order rate equations. Figure 5 shows the effect of hydrogen pressure a t 0.017 space velocity. The data for nitrogen fit a straight line passing through the origin. For sulfur, the data do not seem to be directed toward the origin. This complexity is probably caused by a combination of thermal and catalytic removal that cannot be separated by this study. As shown in Figure 6, sulfur removal is easy compared with removal of nitrogen. When only 40% of the nitrogen has been removed, 90% of the sulfur is gone. When 75y0of the nitrogen has been taken out, less than 0.5% of the sulfur remains. I n most cases, if processes are designed to remove most of the nitrogen, removal of the sulfur will be ensured. Evaluation of Catalyst Activity
In deriving Equation 5, the catalyst factor, 2, was a constant for any given
I
Figure 7.
catalyst and was included in the specific rate constant. Also, removals of sulfur and nitrogen were found to be first-order reactions with respect to contaminant concentration. For a specific contaminant
x log -
-K‘y
x,= sv ~
when the reaction is carried out at constant temperature, hydrogen pressure, and hydrogen-addition rate, and SV is the space velocity of the hydrocarbon. A new variable, catalyst activity, CA, can now be introduced so that K x = Ki(C‘4)
If a given catalyst, designated as a reference, is given a n arbitrary value of 100, a catalyst activity scale is defined. This activity is directly proportional to the specific rate constant. It is inversely proportional to the quantity of catalyst required to remove a given amount of sulfur or nitrogen. For example, if the catalyst used during the present study is designated as the reference, Figure 1 becomes an activity scale merely by multiplying the abscissa by 100. The catalyst of unknown activity is used for treating the reference naphtha at the reference conditions given above. The abscissa for the amount of sulfur or nitrogen removal w-ith the unknown is multiplied by the space velocity and this product is the catalyst activity. Results of using this method for several commercial catalysts are shown in Figure 7. For this comparison, the reference catalyst was in the form of 3/16inch cylindrical pellets instead of 4 to 6 mesh. The two types of activity scatter widely with no apparent relationship
INDUSTRIAL AND ENGINEERING CHEMISTRY
I
I
50 SULFUR
‘/e
Hydrotreating thermal naphtha
where AH,, is 20.1 kcal. per mole, KNis 8.2 X IO3, and C is 1.2 moles of hydrogen per mole of hydrocarbon.
1
A
t
0
660
50
I 100
I
I
I50
I
200
REMOVAL
Activities of commercial catalysts
between them. From the plot it is not difficult to pick the catalyst to best do a desired job of removing sulfur or nitrogen or both. Conclusion
Agreement between the process-variable study and the thermal-naphtha stud)- suggests that observed catalyst activities probably apply to naphthas in general. These activities, therefore, aid greatly in catalyst research, process design, and catalyst buying. Process studies also assist in design of units for hydrotreating naphthas. With relationships between the variables established, several process designs can be made; operating conditions can then be determined on the best economic basis. General forms of the equations will also allow process designs to be made with minimum experimental work for stocks other than naphthas. literature Cited (1) Am. Petroleum Inst. Research Project 48A, Rept. PR-13, March 1954. (2) Conn, M. E., Connelly, G. C., IND. ENO.CHEM. 39,1138 (1 947). ( 3 ) Deal, V. D., Anal. Chem. 25, 426 (1953). (4) Eckhouse, J. G., Gerald, A. J., de Rosset, A. J., Oil Gas J . 53, No. 17, 81 (1954). ( 5 ) Hougan, 0. A., “Reaction Kinetics in Chemical Engineering,” Chap. IV, Am. Inst. of Chem. Engrs., New York, 1951. ( 6 ) Patterson, A. C., Jones, M. C. K., Oil Gas J.53, No. 35, 94 (1955). (7) Rescorla, A. R., Ottenweller, J. H., Freeman, R. S., Anal. Chem. 20, 196 11948). (8) Zimmirschied, W. J., Hunt, R. A,, Wilson, W. A,, Petroleum Rejner 34, N o . 5, 153 (1955).
RECEIVED for review September 17, 1956 ACCEPTED January 24, 1957
