Ind. Eng. Chem. Res. 1989,28, 1-5
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KINETICS AND CATALYSIS Catalyst Deactivation Due to Transient Behavior in Nitric Acid Production Hyo C. Lee and R. J. Farrauto* Engelhard Corporation, Research & Development, Menlo Park, Edison, New Jersey 08818
During start-up of a fixed bed nitric acid reactor, unusually high surface temperatures can be generated due to ignition of transient species. Depending on the propagation conditions of the ignition heat waves, actual catalyst surface temperature can exceed the maximum adiabatic temperature leading to catalyst melting. These surface hot spots often cannot be detected by measuring gas-phase temperatures. An example of such phenomena has been observed in the production of nitric acid over a platinum alloy oxidation gauze catalyst. More specifically, transient reactions leading to excessive surface temperatures have been observed in the platinum recovery gauze located immediately below the oxidation catalyst pad. Laboratory experiments simulating transient behavior in commercial reactors are discussed. Nitric acid is produced commercially by passing ammonia and air over an oxidation catalyst which is usually a gauze woven from platinum-rhodium alloy wire. Typically, the temperature of gas leaving the gauze ranges from about 810 to about 960 "C, most often above 850 "C. As ammonia is oxidized, platinum is slowly lost from the gauze, possibly in the form of the more volatile oxides. Rhodium is also lost, but this is not as critical a problem. "he rate of loss depends upon the type of plant. Qpically, for each ton of ammonia converted, a high-pressure plant will lose more than 1g of platinum, while lower pressure plants will lose less. Even though the rate of catalyst loss is slow when expressed in terms of weight, the cost is usually quite substantial. In many operations, the cost of platinum lost during production has been said to be the second largest expense of the operation, exceeded only by the cost of ammonia feedstock. MMYapproaches have been tried to recover some of the platinum and rhodium. Filters of various materials (Chilton, 1960) have been placed downstream from the catalyst gauze to mechanically catch and retain solid particles of platinum and rhodium. Later, it was discovered that various palladium alloys (Holtzman, 1969) had the ability to withdraw platinum-containing vapor from the gas stream. The mechanism of this withdrawal has been a subject of some controversy, but it has been theorized that, in the course of the reaction, platinum oxide is converted to the corresponding metal and subsequently alloys with the palladium-rich recovery gauze. The recovery alloys are usually used in the form of multiple sheets of woven gauze and are placed as close as possible downstream from the catalyst gauze where high temperatures facilitate solid-state diffusion of Pt into the getter gauze. Figure 1shows a typical ammonia oxidation reactor with a Pt alloy oxidation gauze followed by a Pd-rich recovery gauze. The recovery reaction can be written as PtO2 Pd Pt-Pd 02 Commercially,Pt recovery technology has been practiced successfully for almost 20 years with continuous im-
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* To whom correspondence should be addressed. 0888-5885/89/2628-0001$01.50/0
provement in the recovery efficiency. Early technology (Holtzman, 1969) provided 35-40% recoveries, while modern technology (Hatfield et al., 1983),which incorporates higher Pd content alloys and improved geometries of the individual sheets, has essentially doubled the recovery efficiency. Increasing Pd content in the recovery gauze does have a limit since, as will be shown, 100% Pd gauzes are susceptible to deformation and melting. Surprisingly, the pure Pd gauze can be melted in the reactor in spite of adiabatic steady-state temperatures 600 "C lower than the melting point of Pd. Various phenomena leading to hot spots on catalytic wire surfaces have been discussed with the emphasis on steady-state multiplicity, hysteresis, oscillation, excursion, and flickering (Tamman, 1920; Hiam et al., 1968; Ervin and Luss, 1972; Edwards et al., 1973; Hegedus, 1975). This paper presents a laboratory study in which the unique conditions leading to catalyst surface temperatures, far in excess of the gas phase and steady-state adiabatic temperature, causing considerable damage to a pure Pd recovery gauze, are described. Hypothesis The steady-state gas inlet temperature in a high-pressure (800-1000 kPa) nitric acid plant is typically 150-250 "C. However, ignition is initiated by directing a hydrogen torch (Figure 1)to the center of 1-1.5-m-diameter gauze (larger diameter gauzes are used at lower pressures). During operation of the hydrogen torch, a small amount of ammonia and air is bled into the reactor; the reaction is initiated at the hot spot. The torch is extinguished and the reaction spreads across the face of the gauze as heat is conducted radially. During this process, some of the ammonia is being converted to NO according to reaction 1 and some to Nz by reaction 2, both of which are highly exothermic: NH3 + 1.2502 = NO + 1.5H20 (A€€= -54 kcal/mol) (1) NH3 + 0.7502 = 0.5N2 + 1.5H20
(AH= -76 kcal/mol) (2)
0 1989 American Chemical Society
2 Ind. Eng. Chem. Res., Vol. 28, No. 1, 1989 Adlabatlc Temperature (oC) I -
1 I
F h S z N O x
__
*
NHS*OP = NP
I
Oxidat ion NH3+ 03 Getter PtO,+ Pd
;i;;;
Ammonia Concentration (%) In Alr Meltlng Polnt : Pt-1772 oc, Pd-1662 OC a Kanthal-1610
Figure 1. Start-up of ammonia oxidation reactor.
Prior to steady-state conversion, sigiiificant amounts of the feed NH, and air pass unreacted through the Pt oxidation gauze and may react on the Pd-rich recovery gauze. Additionally, some NO also passes through the reactor due to reaction 1at the hot spot. Thus, the recovery gauze may be exposed to both NH, and NO. It is hypothesized that the reaction of NH, and NO at the Pd surface, which is reported to be much more effective for NH3 and NO interactions than Pt (Pignet and Schmidt, 1975),contributes to the observed melting of the recovery gauze. The maximum temperature for an exothermic reaction is derived from the equilibrium adiabatic temperature rise of a stoichiometric gas mixture for a given reaction. The following reactions, at ammonia oxidation conditions, were considered for the estimation of a maximum temperature rise as a function of ammonia feed concentrations: AHRxN,
major reaction NH3 + 1.2502 = NO + 1.5H2O
kcal/mol -54 AHRXN,
side reactions NH3 + 0.7502 = 0.5 N2 + 1.5H2O NH3 + 0 2 = O.5N2O + 1.5H2O NH3 = 0.5N2 + 1.5H2 NH3 + 1.7502 = NO2 + 1.5H2O NH3 + 1.5NO = 1.25N2 + 1.5H2O
kcal/mol -76 -66 +11 -68 -104
reaction no. (1) reaction no. (2) (3)
(4) (5) (6)
Accordingly, the equilibrium adiabatic temperature profiles, at a given air preheat temperature, were derived as a function of the ammonia/air ratio, in order to examine the possibility of catalyst deformation due to overfueling (NH3in excess of stoichiometry). Figure 2 illustrates that ammonia oxidation reaction to NO, at 10.5%, which is typical of a high-pressure plant (reactions 1and 5 above), can yield a maximum temperature of 940 "C, while the maximum temperature for ammonia decomposition in the presence of air (reaction 2) is estimated to be only 1200 "C, far below the 1552 "C melting point for Pd. An alternative hypothesis was therefore necessary to explain the melting of Pd gauzes. As was stated above, steady-state reactions will not lead to temperatures sufficient to melt pure Pd; thus the high temperatures experienced must be caused by non-steady-state transient behavior. Commercial experience suggested that recovery gauze deformation and melting occurred primarily during start-up. The simulation of plant experience in the laboratory was accomplished by cutting a hole in the oxidation
oc
Figure 2. Adiabatic flame temperature for ammonia/oxygen reactions (at To= 218 "C and P = 1070 kPa). SIDE VIEW
I FLOW 9OWlORh
WITH 1/2" CENTER
6 HOLE AT THE
Figure 3. Simulation of NH3 bypass in laboratory test.
gauze to generate ammonia bypass analogous to the unreacted NH3which passes through the cold portions of the oxidation gauze. The remaining portion of the oxidation gauze pack converts the feed NH3 to NO, simulating the spot heated by the hydrogen torch. In this manner, both NH3 and NO were fed to the recovery gauze located immediately below the oxidation gauze pack. The arrangement is shown in Figure 3.
Experimental Section A schematic diagram of the catalytic ammonia oxidation reactor system is illustrated in Figure 4. The pilot reactor is designed to simulate commercial operations over a wide range of throughput conditions. The reaction system consists of two catalytic beds whose inside walls are covered with a 3.25-cm-diameter quartz liner in order to prevent ammonia decomposition at high temperature on the walls of the reactor. A proprietary mixing device for reactants is placed 25 cm above the catalyst bed. The catalyst pack is comprised of 10 layers of a Pt alloy gauze (80 mesh X 0.076-mm diameter) followed by anywhere from six to nine layers of a pure Pd recovery gauze with interstitial separator screens made of a high-temperature base metal alloy such as Kanthal (tradename of the Kanthal Corp.). The catalyst bed, with an effective diameter of 25 mm, had a 12.5-mm-diameter hole punched at the center as described above and shown in Figure 3. An adjustable thermocouple was embedded at the top section of the recovery gauze bed. In addition, interstitial separator screens, placed between adjacent recovery gauzes, were coated with a temperature-indicating paint called Tempilak (tradename of Tempi1 Div., Big Three Ind., Inc.; available up to a melting point of 1371 "C), in order to detect local hot spots. Sheathed thermocouples protected by quartz tubing were also used to
Ind. Eng. Chem. Res., Vol. 28, No. 1, 1989 3 INTERMITTENT NH3 SURGE
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INLEL TC
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TEMP.
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WATER COOLED PROBE INLET SAMPLE
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CONCENTRATION NH3
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GAUZE HOLDER OUTLET PROBE c
NO ,
SAMPLE
co
c
NO SAMPLE
--
Test Conditions : Tin 150 OC
--
P 6 9 0 KPa M a s s Flux 12 Kg/mP-sec NH3 F e e d 11%
11Y 1
HIGH TEMPERATURE GASKETS AT LOCATIONS 1,2,3 8 4 EXHAUST
Figure 4. Schematic diagram of the reactor. QUASI-STEADY STATE
I
0
TIME
Figure 5. Temperature variation at ammonia oxidation conditions.
monitor the inlet and outlet bulk gas temperatures approximately 25 mm from the catalyst bed. Detailed flow schemes, with the sampling system, are described by Heck et al. (1982).
Results and Discussion 1. High-TemperatureTest at Steady State. Laboratory tests were carried out to examine temperature rises in the recovery gauze bed when ammonia feed concentration was gradually increased after ignition. No unusual temperature rises were noted during the test. The gauze temperature followed the bulk gas temperature downstream of the reactor as illustrated in Figure 5. The test was continued by increasing the ammonia feed concentration gradually up to 13.5%, far in excess of the typical 10.5% used commercially. After exposure of the catalyst and recovery gauze to an ammonia concentration of 13.5%, no sign of physical deformation was noted. Recovery gauze temperatures measured underneath the edge of the oxidation gauze hole were about 50-140 "C higher than the bulk gas temperature but with no unusual temperature overshooting. Assuming 70% of the ammonia feed was converted to NO, coupled with 30% for ammonia
Figure 6. Laboratory simulation of transient high-temperature conditions.
decomposition over Pd, the maximum temperature is estimated to be 1250 "C, still far below the melting point of the Pd recovery gauze, eliminating steady-state behavior as a cause for melting. 2. Transient High-TemperatureTests. Three dynamic modes were simulated in the laboratory to examine whether excessively high temperatures would occur during the following test conditions: A. Intermittent NH, Surge. A steady flow of 11% NH3 and air was established through the reactor arranged as in Figure 3. Once temperatures leveled out, the ammonia flow was pulsed to 12.5% for approximately 5 s as shown in Figure 6. Such intermittent ammonia surges were repeated with higher ammonia feed concentrations. The bulk gas exit temperature increased to 1200-1230 " C with a 13.5% ammonia feed. Visual inspection of the individual Pd gauze revealed that the bottom layers melted, while no melting was noted in the oxidation gauzes (see Figure 7 , gauzes on left, separator screens on right). In fact, the size of the burned holes in the recovery gauze bed grew progressively larger down through the pack, an observation consistent with field experiences. Furthermore, the gauze melting seemed to be initiated at the boundary region of the gauze below the 12.5-mm-diameter hole where high reactant (ammonia) and product (nitric oxides) interactions are postulated to occur. No melting of the recovery gauze was observed below the hole outside the catalyst gauze pack, as shown in Figure 7. The interstitial separator screen at the bottom fused to the Pd. It appeared, from temperature-indicating paints on the separator screen, that hot spots (dark color) were apparent at the outside edge and center zone for the top and bottom sections of the recovery gauze pack, respectively. A relatively low temperature zone (white color) was visible at the center of the top section where it is postulated that ammonia and NO, react. B. High Ammonia Start-up. A series of ignition tests were conducted with a square wave pulse of 5-10% ammonia to examine excessively high temperature rises during ignition conditions (Figure 8A). The ignition test was also repeated after shutdown to examine the effect of severe thermal cycling on gauze deformation. After the air flow and preheat temperature were established, the ammonia supply line was quickly opened,
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Ind. Eng. Chem. Res., Vol. 28, No. 1, 1989 "*i
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NH3
CONCENTRATION ndi tiona Tin 300 OC Press 690 KPa Mass Flux 12 KgImP-sec
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0
TOP GAUZE
I TEMP.
4
1
TIME
I GAUZETEMP.
I
TIME
Figure 8. (A, top) Ammonia concentration profile during high ammonia start-up. (B, bottom) Temperature profile during high ammonia start-up. Adiabatlc TemDerature (oC)
INTER MEDIATE
GAUZE
BOlTOM GAUZE
0
1 2 3 4
6 8
7 0
9 101112181416
Ammonia Feed Conoentratlon (lb)
Figure 9. Adiabatic flame temperature for ammonia/oxygen reactions (at To = 218 "C and P = 1070 kPa).
Figure 7. Laboratory getter gauzes melted during NH3 surge test.
providing a 5% ammonia pulse to the catalyst bed. After 1min, the ammonia feed was stopped for 2-3 min, followed by another pulse. This was repeated and then followed by higher ammonia pulses of 10% until the recovery gauze deformed. The minimum ammonia feed required for the initiation of gauze deformation was found to be around 7.5%. The gauze temperature was measured during high ammonia ignition tests by embedding a thermocouple in the Pd recovery gauze pack, near the boundary area below the hole in the oxidation gauze. A sharp temperature rise was noted immediately after ammonia was injected, as shown in Figure 8B. The maximum surface temperature was a t least 300 "C higher than the exit bulk gas temperature and much higher than the adiabatic temperature calculated above.
Conclusions Literature data (Pignet and Schmidt, 1975) for ammonia oxidations over Pt, Pd, and Rh indicate that Pd is the most active catalyst (but not necessarily selective to NO,) for ammonia conversions, especially at low temperature. Thus, the ammonia which has bypassed the less active Pt gauze during start-up in commercial plants can ignite over the Pd recovery gauze. Bypass ammonia can also react with the NO, produced a t the hot spot of the Pt oxidation gauze, leading to the highly exothermic reaction (6). This reaction (Figure 9), which is 2-fold as exothermic as the desirable reaction (l), -104 versus -54 kcal/mol, occurs a t the Pd surface. Heat removed from the surface to the bulk gas is slow, resulting in melting of the Pd recovery gauze, especially under transient dynamic conditions typical of start-up in commercial plants. A hydrogen torch is used to light-off the gauze during the start-up of commercial ammonia oxidation reactors. Ammonia bypass through the oxidation gauze pack is inevitable until the entire gauze surface becomes hot. In large-size industrial reactors, it is observed that 1-2 min is required for complete propagation of the ignition wave
Ind. Eng. Chem. Res. 1989,28, 5-9 NH, t AIR
TEMP. NU.
B U R N E D HOLE D U E TO EXCESSIVE GAUZE TEMPERATURE
]
TEMP.
+ AIR
dNz
OXIDATION G A U Z E
N.0
1 NO
Figure 10. Ignition delay in the oxidation pack.
during start-up. In addition, Barelko et al. (1978) analyzed the speed for the propagation of ignition heat waves on Pt wires during the oxidation of ammonia. Accordingly, considerable ammonia bypass through the catalyst gauze can occur during the ignition period. I t is likely that bypass ammonia can react over Pd with the NO, produced by oxidation of ammonia at the gauze site heated by the hydrogen torch. The transient reaction of ammonia and nitrogen oxides is strongly catalyzed by palladium, resulting in highly exothermic reactions exceeding adiabatic equilibrium temperatures, leading to melting of the Pd recovery gauze. This has been observed at ammonia concentrations as low as 7.5% where the adiabatic temperature is far below that required to melt the gauze. This model is exhibited in Figure 10. Process and compositional modifications to improve the resistance of the getter gauze to deformation have been proposed by Lee (1986) and Farrauto et al. (1986). It should be noted, however, that the gauze surface temperature approaches the adiabatic equilibrium temperature, under heat-transfer-limiting conditions, which is substantially higher than the bulk gas temperature. Thus, significant temperature gradients between the catalyst surface and bulk gas stream exist, and any local variations in NH,/air can cause catalyst temperature flickering. Furthermore, in the case of overfueling beyond the stoichiometric ratio (14.38%),a temperature excursion at the catalyst surface is quite possible, causing catalyst
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deformation by exceeding the adiabatic equilibrium temperature due to the limitations of the heat release rate from the catalyst surface to the bulk gas stream. The worst case for excessive a”onia/air ratio is often caused by liquid ammonia entrainment to a catalyst bed. But under those circumstances, both the oxidation and the recovery gauze are observed to melt. Hence, overfueling cannot explain the melting phenomena discussed in this paper. Registry No. NH3, 7664-41-7; Pt, 7440-06-4; Pd, 7440-05-3; NO, 10102-43-9.
Literature Cited Barelko, V.; Kurochka, I. I.; Merzhanov, A. G.; Shkadinsii, K. G. “Investigation of Travelling Waves on Catalytic Wires”. Chem. Eng. Sci. 1978, 33, 805-811. Chilton, T. H. “The Manufacture of Nitric Acid by the Oxidation of Ammonia”. Chem. Eng. Prog. Monogr. Ser. 1960,3, 56. Edwards, W. M.; Worley, F. L., Jr.; Luss, D. “Temperature Fluctuations of Catalytic Wires and Gauzes-11. Experimental Study of Butane Oxidation on Platinum Wires”. Chem. Eng. Sci. 1973, 28, 1479-1491. Ervin, M. A,; Luss, D. “TemperatureFluctuations of Catalytic Wires and Gauzes-I. Theoretical Investigation”. Chem. Eng. Sci. 1972, 27, 339-346. Farrauto, R. J.; Lee, H. C.; Hatfield, W. R. “Low Temperature Light-off Ammonia Oxidation”. Filed for US Patent Application No. 06887578, 1986. Hatfield, W. R.; Heck, R. M.; Hsiung, T. “Method for Recovering Platinum in a Nitric Acid Plant”. US Patent 4412859, 1983. Heck, R. M.; Bonacci, J.; Hsiung, T. “A New Research Pilot Plant Unit for Ammonia Oxidation Processes and Some Gauze Data Comparisons for Nitric Acid Process”. Znd. Eng. Chem. Process Des. Deu. 1982, 21(1), 73-79. Hegedus, L. L. “Temperature Excursions in Catalytic Monoliths”. AZChE J . 1975,21(5), 849-853. Hiam, L.; Wise, H.; Chaikin, S. “Catalytic Oxidation of Hydrocarbons on Platinum”. J. Catal. 1968,10, 272-276. Holtzman, H. “Platinum Recovery in Ammonia Oxidation Plant”. Platinum Met. Rev. 1969, 13, 2-8. Lee, H. C. “Platinum Recovery using Perforation Resistant Gauzes”. Filed for U.K. Patent Application No. 86 306 362.4, 1986. Pignet, T.; Schmidt, L. D. “Kinetics of NH3 Oxidation on Pt, Rh and Pd”. J. Catal. 1975, 40, 212-225. Tamman, G. Z. 2.Anorg. Allg. Chem. 1920, 111, 90.
Received for review May 6, 1988 Accepted September 6, 1988
Kinetics of Absorption of Carbon Monoxide in Aqueous Solutions of Sodium Hydroxide and Aqueous Calcium Hydroxide Slurries Anand V. Patwardhan and Man Mohan Sharma* Department of Chemical Technology, University of Bombay, Matunga, Bombay 400 019, India
The kinetics of absorption of carbon monoxide in aqueous sodium hydroxide solutions and aqueous calcium hydroxide slurries was studied in a stirred autoclave with a plane gas-liquid interface, in the temperature range 100-160 “C. The partial pressure of carbon monoxide was varied from 10 to 100 atm. In the case of aqueous sodium hydroxide solutions, the reaction was found to occur in the diffusion film and was first order in carbon monoxide; the reaction was also found to be first order in hydroxyl ion concentration. The values of the second-order rate constant were found to be in the range 3-141 m3/(kmol.s). In the case of calcium hydroxide slurries, the values of the second-order rate constant were found to be in the range 2-60 m3/(kmol-s). The absorption of carbon monoxide in aqueous solutions of sodium hydroxide and aqueous slurries of calcium hydroxide is relevant for the manufacture of sodium or calcium formate/formic acid.
Previous Studies Khalifa (1965) has suggested a mechanism for the for0888-5885/89/2628-0005$01.50/0
mation of formate ion from carbon monoxide and hydroxyl ion. The mechanism involves attack on the carbon atom of the carbon monoxide molecule by a lone pair of electrons of the hydroxyl group. The complex which is formed is of a resonating type and rearranges to give formate ion. Carbon monoxide in gases available from a variety of sources, which do not contain carbon dioxide, has been 0 1989 American Chemical Society
