2327
Ind. Eng. C h e m . Res. 1990,29, 2327-2332 reduces the solubility in maltene of the hydrocracked product, because the aromacity of maltene is very much decreased by the produced paraffins. Sufficient hydrogenation of the aromatic ring and extensive depolymerization are necessary for asphaltene to be soluble in the paraffin-rich maltene. Asphaltene in the starting residue exhibited rather high solubility in spite of its largest molecular weight. Its highly alkylated structure should enhance its solubility.
Conclusions Two-stage hydrocracking performs extensive hydrogenation of asphaltene in the first stage and depolymerization to increase the distillate yield (Inoue et al., 1985; Sakanishi et al., 1988; Mochida et al., 1990). In contrast, single-stage reaction at 420 "C can produce the distillate principally from the lighter portions of the residue, failing to sufficiently hydrogenate the heavier asphaltene fraction for depolymerization. And dealkylation is certainly accelerated to reduce its solubility, causing sludge formation. Literature Cited Haensel, V.; Addison, G. E. Advances in Catalytic Reforming. World Pet. Congr., 7th 1967, 4, 113.
Inoue, Y.; Kawamoto, K.; Mitarai, Y.; Takami, Y. Deactivation of Solvent Rehydrogenation Catalyst in Coal Liquefaction and its Countermeasures. Proc. Int. Conf. Coal Science, Sydney, 1985; p 193. McKenna, W. L.; Owen, G. H.; Mettick, G. R. Processing Problems in Refineries. Oil Gas J. 1964, 62, (20),106. Mochida, I.; Furuno, T.; Korai, Y.; Fujitsu, H. Studies Reveal Shot-coke Microstructure, Suggest Ways to Minimize its Formation. Oil Gas J . 1986, Feb 3, 51. Mochida, I.; Zhao, X. Z.; Sakanishi, K.; Yamamoto, S.;Takashima, H.; Uemura, S. Structure and Properties of Sludges Produced in the Catalytic Hydrocracking of Vacuum Residue. Ind. Eng. Chem. Rei. 1989,28, 418. Mochida. I.: Zhao. X. Z.: Sakanishi. K. Catalvtic Two-staee Hvdrocracking of Arabian Vacuum Residue a t a High Converiion Level without Sludge Formation. Ind. Eng. Chem. Res. 1990,29,334. Saito, K.; Shimizu, S.Hydrodesulfurization Process of Crude Oils. PETROTECH (Tokyo) 1985,8 (l), 54. Sakanishi, K.; Zhao, X. Z.; Fujitsu, H.; Mochida, I. Optimization of the Two-stage Hydrotreatment for a Coal Liquid Heavy Distillate Using Some Commercial Catalysts at Each Stage. Fuel Process. Technol. 1988, 18, 71. Symoniak, M. F.; Frost, A. C. Plugging Problems in Refineries. Oil Gas J . 1971, March 15, 76.
Received for review January 30, 1990 Revised manuscript received J u n e 18, 1990 Accepted July 9, 1990
Kinetics of the Reaction between Hydrogen and Sulfur under High-Temperature Claus Furnace Conditions Norman I. Dowling,' James B. Hyne,*,t and Dennis M. Brown1 Alberta Sulphur Research Ltd., Chemistry Department, The University of Calgary, 2500 University Drive N . W., Calgary, Alberta, Canada T2N 1N4, and Air Products and Chemicals, Inc., 7201 Hamilton Blud, Allentown, Pennsylvania 18195-1501
+
The reaction H2 (1/2)S2 H2Shas been studied as a function of temperature and residence time over the ranges 602-1290 O C and 0.03-1.5 s in the absence of a catalyst. This study shows that the combination of H2 and elemental sulfur vapor under the high-temperature conditions typical of a Claw sulfur recovery unit proceeds via a reversible homogeneous gas-phase reaction that is first order in both H2 and sulfur concentration and follows the rate law -d[H2]/dt = Itl[H2][S2]- K2[H2S] with a second-order recombination rate constant kl = 1-1 X lo3 atm-l s-l ( A , = (4.3 f 0.2) X lo6 atm-' s-,; AHl* = 26 f 1 kcal/mol) and first-order decomposition rate constant it2 = 4 X 10-4-70 s-l ( A 2= (3.6 f 1) X lo8 s-l; AH2* = 48 f 1 kcal/mol) over the temperature range studied. These findings can be used t o exploit opportunities in acid gas processing, such as effecting improved efficiencies for O2 usage in oxygen-blown Claus units and maximizing H2 content in the tail gas.
Introduction Recovery of elemental sulfur from H2S during sour natural gas processing is conventionally carried out via the modified Claus process (eqs 1and 2 coupled to show the 3H2S
+ (3R)Oz
2H2S
+
-
SO2
2H2S
+
(3/X)S,
SO2
+
+
2H20
H20
(1)
(2)
sequence of combustion (1)and catalytic (2) steps). This process consists of partial combustion of the H2S in the acid gas feed to form SO2 within a high-temperature front-end thermal stage followed by reaction of H2S and 'Alberta Sulphur Research Ltd. *Air Products a n d Chemicals, Inc.
0888-5885/90f 2629-2327$02.50f 0
SO2 over an alumina catalyst at lower temperatures to effect elemental sulfur production. Although the modified Claus process has remained relatively unaltered since its introduction (Baehr, 1938), further modifications to the basic process have been introduced in order to increase the plant capacity. For example, oxygen enrichment of air for combustion of the acid gas feed leads, at 100% 02,to the Claus oxygen-based process expansion (Goar et al., 1985) or COPE technology. A schematic diagram of the thermal stage of a Claus sulfur recovery unit (SRU),including COPE modifications, is shown in Figure 1. This use of oxygen in Claus operations also provides other benefits resulting from the higher temperatures within the furnace. Although a limit is imposed on the operating temperature by the refractory (1500 "C), some of these benefits are exploitable but require careful consideration of the complexities of the thermal stage reaction chemistry. The appearance of molecular H2 in the waste heat boiler (WHB) outlet stream of a Claus plant is believed to result 0 1990 American Chemical Society
2328 Ind. Eng. Chem. Res., Vol. 29, No. 12, 1990 Table I. Analyzed Hi Concentrations in the Exit Stream and Calculated %H, Consumption Valuesn reactor tube diameter 0.4 cm 0.2 cm 0.6 cm mol 70 70H? mol % % H, mol % % Hz mol % exit consumption consumption exit temp, "C exit exit consumption 602 0.78 0.94 0.0 (0.06) 0.93 1.3 (0.23) 0.90 4.6 (0.52) 6.2 (0.21) 602 0.93 1.2 (0.05) 0.88 0.83 0.58 12.2 (0.47) 815 0.91 3.7 (0.05) 0.77 17.6 (0.19) 0.61 0.18 35.6 (0.42) 930 52.8 (0.17) 0.78 16.8 (0.04) 0.44 0.19 0.08 79.9 (0.38) 1038 56.9 (0.16) 0.22 0.47 50.0 (0.04) 0.11 0.19 79.9 (0.35) 1146 69.5 (0.14) 0.29 0.23 0.35 62.4 (0.04) 0.13 76.1 (0.32) 1290 66.5 (0.13) 0.14 0.41 56.6 (0.03) 0.31 0.24 75.0 (0.29) ~~~~
1.0 cm % H,
consumption 17.2 (1.45) 38.7 (1.31) 80.8 (1.17) 91.1 (1.06) 88.4 (0.97) 86.1 (0.90) 85.0 (0.81)
"Initial inlet H2concentration = 0.94 mol %. %HZconsumption = [(mol 70H, inlet-mol % Hzexit)/mol % H2 inlet] 100. Values in parentheses are the corresponding temperature adjusted hot-zone residence times (in s). COMBUSTION AlR102
STEAM
1 REACTION FURNACE
GAS FEED
~
wns
(sed
t,
4
1.13
i
1
1
1
I
I
A TEMPERATURE MOOERANT
REFRACTORY LINING
T BOILER FEED WATER
Figure 1. Schematic diagram of the thermal stage of a Claus sulfur recovery unit.
from the rapid thermal cracking of H2S to H2 and sulfur, which also occurs at the high temperatures (900-1200 "C) that typically exist within the reaction furnace. Thermodynamically, this equilibrium (eq 3) is expected to favor I
d
(3)
endothermic H2S decomposition even more at the higher temperatures, up to 1500 "C, during oxygen-blown operation. This should lead in principle to increased H2 and sulfur levels within the furnace as a result of application of oxygen enrichment techniques. Available equilibrium and kinetic results for the H2S decomposition reaction (Raymont, 1974) suggests that the reverse reaction between H2 and sulfur becomes the dominant rate process as lower temperatures are imposed. This suggests that the cooling of the hot gases leaving the reaction furnace and entering the WHB will cause recombination to be favored, effecting a reduction in the overall sulfur conversion within the furnace and removal of potentially useful H, from the tail gas. In oxygen-blown operation, this recombination would result in less than optimum sulfur production for a given oxygen consumption, or a lower efficiency for O2 usage. Whereas the high-temperature decomposition of H2Shas been reasonably well studied (Randall and Bichowsky, 1918; Raymont, 1974; Fukuda et al., 1978; Chivers et al., 1980),direct investigation of the reaction between H2 and sulfur is only reported in the literature at lower temperatures (Norrish and Rideal, 1923; Zel'venskii et al., 1961). In the work described here, the kinetics of this reaction are studied at high temperatures such as those encountered under Claus thermal stage conditions. The study also addresses the feasibility of using a rapid thermal quench technique, which would minimize the reassociation of H, and sulfur in the WHB zone of a Claus SRU, thus improving O2 efficiencies and increasing H, levels in the tail gas during oxygen-blown operation.
Experimental Section Apparatus and Procedures. The flow reactor design used in this study consisted of unpacked small diameter
/
,' 5
'
I
I
,'
,
/
I
400
800
800
1000
1200
1400
1000
Temperature ("C)
Figure 2. Experimental 70Hzconsumption versus temperature for various residence times for the reaction of hydrogen and sulfur.
(0.2-1.0-cm) quartz tubes housed in a high-temperature electric furnace (Mini-Brute Model MB71, Thermco Inc.) with an operating range of 400-1300 "C over a hot-zone length of 45 cm with ramp-up and cool-down end zones of 30 cm in length. This reactor design was used to study the reaction between H, and sulfur vapor at various hotzone temperatures between 602 and 1290 "C (Table I), which gave temperature-adjusted residence times within the hot zone in the range 0.03-1.5 s. Initial temperature ramp-up and cool-down from the hot zone prevented strict isothermal operation of the reactor over its entire length. This complication in the measured extent of reaction arising from reactor temperature end effects was allowed for in the analysis of the experimental data by using a modeling approach as opposed to the more classical type of kinetic analysis. To assist analysis of the experimental data, all runs were performed under dilute conditions with concentrations of H2and sulfur vapor relatively low to those existing in Clam furnace gas. A large sulfur-to-H2 ratio was also used to determine the rate dependency on H2 concentration. A commercially supplied experimental gas mixture, 0.94 mol '70 H, in argon, was used in all runs at a constant metered
Ind. Eng. Chem. Res., Vol. 29, No. 12, 1990 2329
0.94
0.7
4
c,
C,
(mole%) 0.70
(mole%) 0.94
0.60
0.50 0.22
0.25
01 0.0
I
0.1
0.2
0.3
I
0.4
-log
c,
0.5
0.6
0.7
I
Figure 3. First-order dependence with respect to hydrogen for the H, + S2reaction at a temperature of 815 "C.
flow rate of 500 mL/min a t 21 "C. Sulfur pick-up was achieved by passing this gas mixture across a vessel containing liquid sulfur a t 250 "C. The amount of sulfur picked up was a function of the temperature of this vessel for a constant flow rate. The ultimate mole fraction of S2 in the gas phase a t the furnace temperature was determined from the mass of sulfur condensed in the postreactor trap, the constant volumetric flow rate, 2nd the length of flow time. A constant sulfur concentration in the reactant gas stream of 6 mol %, expressed as Sz, corresponding to a sulfur vapor partial pressure of 6 X atm (totalsystem pressure of 1atm), was achieved by using this method. This concentration is sufficiently large in relation to the H, concentration (0.88 mol % after adjustment for sulfur pick-up) to satisfy the condition of excess sulfur. This permits the extent of reaction between the H, and sulfur to be conveniently followed as a function of the change in H2 concentration. Experiments to determine the kinetic order with respect to H2 were conducted a t a temperature of 815 "C by using a 0.4-cm-diameter reactor and varying the concentration of H, in the inlet stream. Concentrations below 0.94 mol % (see Figure 3) were prepared by blending the commercial mixture with a pure argon stream while maintaining a constant total volumetric flow rate of 500 mL/ min and confirmed by GC analysis of the blended stream. Experiments to determine the kinetic order with respect to sulfur were similarly carried out at a temperature of 815 "C by varying the concentration of S2 in the gas phase while maintaining this species in large excess over the H2. Variable concentrations of sulfur, corresponding to 9.0 and 12.0 mol % expressed as S z , were achieved for sulfur pick-up vessel temperature settings of 275 and 300 "C, respectively. While the precise concentration of H2 in the gas phase is a function of the amount of sulfur pick-up, as both inlet and exit Hz concentrations are determined in the absence of sulfur, it is permissible to use these values in the kinetic analysis.
Condensation of the sulfur vapor following pick-up was prevented by maintaining the flow line between the sulfur pick-up vessel and high-temperature furnace at a temperature of 450 "C using a high-temperature heating tape (Thermolyne) and J-type thermocouple with digital readout device (Trendicator 400A, Doric). The flow line exiting the high-temperature furnace and ahead of the sulfur knock-out was similarly heated. Preliminary investigation at a temperature of 500 "C had shown reaction between H, and sulfur to be very slow at this temperature. Unreacted sulfur was removed from the exiting gas stream prior to sampling by passage through a cooled glass wool trap. Duplicate samples of the gas phase were collected for analysis over consecutive 5-min intervals by using a manifold and side-stream sampler arrangement. Analysis of the exit gas stream for both product H a and residual H2 was performed by gas chromatography. Hydrogen sulfide analysis was carried out by using a Varian Model 3700 gas chromatograph (He carrier, 30 mL/min) equipped with a Poropak Q column (5 f t X in., 80 "C isothermal) and thermal conductivity (TC) detector. Hydrogen analysis was performed on a Hewlett-Packard Model 5700A gas chromatograph (Ar carrier, 15 mL/min) equipped with a 5-A molecular sieve column (6 ft X 1/8 in., 30 "C isothermal) and TC detector. Experimental Design. I t is well-known that Claus furnace gas is a relatively complex mixture of species containing, in addition to H, and sulfur, other species such as HzS, SOz, CO, CO,, COS, CS,, and H,O vapor. The large number of possible interactions between the various species in such a mixture precludes its use to directly investigate the Hz + sulfur reaction. In this study, therefore, the reaction between H,and sulfur was isolated by excluding all other species in order to better examine its kinetics under conditions typical of the thermal stage of a Claus plant.
Results and Discussion H, Consumption as a Function of Temperature. Experimental mole percent H, concentrations found in the exit stream and calculated percent H2 consumption values for the reaction H2 + (1/2)S, H2S (4) are shown in Table I along with the corresponding reaction temperature and the hot-zone residence time for each reactor/ temperature combination. These data are plotted as a series of curves in Figure 2, where each curve represents the results obtained from a single reactor. The hot-zone residence times shown correspond to the average for each reactor over the temperature range studied. This is necessary in order to accommodate the variation in residence time for any particular reactor as a function of the hot-zone temperature (see Table I). Inspection of the experimental curves in Figure 2 shows that the reaction of H, with sulfur under the conditions studied must be fast, as even at very short average residence times of 0.045 s more than half of the initial H, present has already been consumed at a temperature of 1100 "C. The effect of increasing the residence time up to the range more commonly associated with the Claus thermal stage (0.5-1.5 s) is to cause this consumption to rise as high as 90% at temperatures as low as 900 "C. The maximum in Hz consumption further suggests that the kinetics of the reaction system are complex. The reversibility of the reaction shown in eq 4 above is well established. Work by Raymont (1975), among others, demonstrated the thermal decomposition of H2S, and Doumani (1944) showed that there was a facile reduction
2330 Ind. Eng. Chem. Res., Vol. 29, No. 12, 1990 Table 11. Analyzed H2S Concentrations in the Exit Stream and Calculated %H,S Production Values" reactor tube diameter 1.0 cm 0.6 cm 0.4 cm 0.2 cm mol % 5%H2S mol 70 % HPS mol % % H2S mol % % H2S exit production exit production exit production temp. "C exit production
