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Ind. Eng. Chem. Res. 1993,32,809-817
Alternative Catalyst Supports for HCN Synthesis and NH3 Oxidation Daniel A. Hickman,+Marylin Huff> and Lanny D. Schmidt' Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455
We have examined several alternative catalyst supports consisting of various materials, geometric configurations, and catalyst loadings (Pt and Pt/Rh) for the Andrussow process for HCN synthesis and for the Ostwald process for NH3 oxidation. With increasing residence time, the HCN hydrolysis reaction decreases the amount of HCN formed. Experiments also show that the addition of Rh to a Pt-coated support slightly reduces the selectivity of HCN synthesis and that catalytically active supports can adversely affect the reaction selectivity. For the Ostwald process, increasing residence times cause the decomposition reactions of NO and NH3 to contribute and thus decrease the net NO, formation. In both HCN synthesis and NH3 oxidation, turbulent, well-mixed flow improves the product selectivity. Based on these tests, guidelines are presented for the design of an alternative Pt-coated catalyst support which gives competitive selectivities and conversions.
Introduction HCN Synthesis. HCN synthesis is carried out industrially in one of two ways. In the Degussa process (Koberstein, 19731, Pt is deposited on the walls of ceramic tubes and CH4 and NH3 react endothermically on the inside of the tubes to form HCN, CH,
+ NH3
-
HCN + 3H,
AH,,, = 61 kcal/mol (1)
-
while the temperature of the tubes is maintained at 1500 K by the combustion of CH4 outside the tubes. Approximately 90% of the NH3 is converted to HCN. The more common approach, the Andrussow process, employs a gauze catalyst (Satterfield, 1991; Twigg, 1989). The commercial catalyst, consisting of 20-50 layers of woven Pt-10 % Rh wires, is typically only a few millimeters thick and up to several meters in diameter. In this process, a mixture of CH4, "3, and air is passed over the gauze catalyst at about 2 atm. This process is exothermic with an adiabatic temperature of about 1400 K. Linear velocities of the gases are on the order of 1 m/s, giving a total contact time of about 1 me with a negligible pressure drop. The desired overall reaction for this process is CH,
-
+ NH3 + 3/20,
HCN + 3H,O AH,,, = -115 kcal/mol (2)
The feed composition (typically about 1:l:l) is chosen to be slightly fuel rich since HCN synthesis involves formation of a CN surface species from the two fuels (Hwang et al., 1987; Hwang and Schmidt, 1989) and so that the losses in selectivity due to oxidation of NH3 and CH4 are reduced: NH3 + 3/40, CH, CH,
-
'/,N2
- +
+ 3/2H,0
(3)
+ 1/20, CO + 2H,
+ 20,
CO,
AH = -75 kcal/mol AH = -8.5 kcal/mol (4)
AH = -191.8 kcal/mol (5)
2H,O
The oxidation of CHI over Pt (Hickman and Schmidt, 1992a) and Rh (Hickman and Schmidt, 1992b) has been addressed elsewhere. ~~~~~~~~~~
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* To whom correspondence should be addressed. +
Current address: The Dow Chemical Company, Midland,
MI 48674.
* Supported by DARPA-NDSEG Graduate Fellowship.
The gauze catalyst is supported from below by a ceramic structure which is immediately followed by a heat exchanger which cools the products before the highly unstable HCN can react according to the reaction HCN + H,O
-
NH3 + CO
AH298 = -12 kcal/mol (6)
The HCN synthesis process operates adiabatically at 1400 K, with HCN yields around 60-70% based on NH3 fed (Satterfield, 1980). Ammonia is usually recovered from the product gas and recycled. An early study of a pilot plant reactor (Pan and Roth, 1968) examined the effect of feed composition on reaction selectivities. These studies demonstrated that the optimal feed composition is a compromise between selectivity of HCN formation based on NH3 reacted and overall HCN production. For example, for a fixed fuel-to-air ratio, the maximum selectivity is obtained at a higher CHJNH3 ratio than the maximum overall HCN production. These maxima in selectivity and HCN production occurred near minima in the surface and product gas temperatures. At higher fractions of fuel 3"( and CHI) for a given NH3/ CHI ratio, the amount of unreacted NH3 (ammonia leakage) increased. However, the amount of NH3 converted to Na by reaction 3 decreased with increasing fuel levels. In the case of NH3 oxidation, in the absence of CHI, virtually all NH3 was oxidized. The addition of CH4 inhibited this reaction, but in fuel-lean conditions this inhibition is less pronounced than in fuel-rich conditions. The kinetics of HCN synthesis over polycrystalline Pt and Rh foils have been examined at low pressures, with the rates being fit to a form of Langmuir-Hinshelwood kinetics (Hasenberg and Schmidt, 1985,1986,1987). These rate expressionshave been used in an atmospheric-preseure model that simulates quite well the performance of an Andrussow reactor using 13 simultaneous reactions (Waletzko and Schmidt, 1988). Several studies have addressed the microstructural properties of gauze catalysts used for HCN synthesis (Pan, 1971; Schmidt and Luss, 1971; Cowans et al., 1990). The gauze catalyst takes about 60 h to reach its maximum conversion of NH3 to HCN and then slowly deactivates with time (Pan, 1971). The initial activation is accompanied by a roughening of the catalyst surface which increases the catalytic activity. Presumably, as carbonaceous deposits accumulate, the number of active sites decrease and the gauze deactivates. Although this process operates at significantly higher temperatures than the Ostwald process for NH3 oxidation (1100 K), the loss of
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810 Ind. Eng. Chem. Res., Vol. 32, No. 5, 1993
Pt from the gauze is significantly smaller in the Andrussow process because the formation of volatile PtO2 requires a strongly oxidizing environment (Satterfield, 1980). The literature is generally limited to studies of the gauze catalysts used in industry, with few studies of alternative supported catalysts (Hutchings, 1986). The purpose of this study was to test several alternative catalyst configurations as substitutes for the gauze catalyst in the Andrussow process. This search for a viable alternative was motivated by several factors. 1. Because the gauze is composed of pure noble metals, the capital cost associated with a gauze pack (Pt-10% Rh) for a reactor about 1m in diameter is on the order of $1 million, and a large portion of this cost is due to the Rh content of the gauze. Platinum currently costa about $380 per troy ounce, while the cost of Rh in recent years has beenabout 10times that of Pt (Espinosa, 1991). However, Rh is added to the gauze because it increases the mechanical strength of the gauze, not necessarily because of its catalytic properties. Thus, by coating an inexpensive substrate with a thin layer of pure Pt, one should expect toreduce the catalyst capital cost by an order of magnitude. 2. The conversion and selectivity obtained in the current Andrussow process leave much room for improvement. In fact, a large portion of the capital and operating costs for this process are associated with separation of NH3 from the product stream, which typically contains 1-3% "3. Thus, an alternative catalyst would find favor if it were able to give equivalent or superior HCN conversion levels while reducing the amount of NH3 in the product stream. 3. The examination of catalyst supports having different geometries, material properties, and catalyst loadings should yield valuable information pertaining to the influence of these properties on the performance of an HCN synthesis reactor. The various geometries and materials of the catalyst supports result in a wide array of characteristic flow patterns and thermal properties. In addition, samples can be prepared with a variety of noble metal loadings and compositions. Examining the performance of these various catalysts and supports should yield some valuable insights into the relative importance of the physical and chemical phenomena that occur in the Andrussow process. 4. Finally, the geometry of a catalyst support can have a significant impact on the selectivity of a heterogeneous process by affecting the mass-transfer rate from the gas phase to the catalyst surface (Hickman and Schmidt, 1992~).Comparing the HCN synthesis selectivities of several catalyst geometries should yield additional insights into this effect. NH3 Oxidation. The Ostwald process, the superoxidation of NH3 to NO, is the key step in the production of nitric acid. Nitric oxide and NO2 are then absorbed in HzO to form HN03 (Satterfield, 1980). The current industrial process uses Pt-10% Rh gauze as the catalyst. The gauze pack is typically 4 m in diameter and may contain as many as 40 layers of gauze. The reaction takes place at 1100 K and atmospheric pressure. With a feed composition of 10% NH3 in air, the process is as high as 97 % selective to NO, with nearly complete This reaction is extremely fast with conversion of "3. a typical surface contact time of 1ms (Satterfield, 1991; Honti, 1976). The desired overall reaction for this process is the superoxidation of NH3 to NO. The excess 0 2 is consumed by homogeneous reaction with NO to form NO2.
-
NH,
-
+ 5/40, NO + 3/2H,0
-
NO + '/,O,
NO,
AH = -54 kcal/mol (7)
AH = -14 kcal/mol
(8)
There are several reactions that occur on the catalyst which compete with these to reduce the overall NO, selectivity. NH,
-
+ 3/40,'/,N, + ,/,H,O NO
AH = -75 kcal/mol
'/2N, + '/,02AH = -22 kcal/mol
(9) (10)
NH3 + 3/2N0 5/4N,+ 3/2H20 AH = -104 kcal/mol (11)
In order to maximize the production of NO,, the contributions of these reactions must be reduced. We compare the effectiveness of a foam monolith supported Pt catalyst to the industry standard Pt-10% Rh gauze catalyst. We evaluate the catalyst effectiveness as a function of feed composition, reaction temperature, and flow rate.
Experimental Section Catalysts. Catalyst configurations of four basic types were examined: Pt-10% Rh gauzes, Pt-coated A1203 foam monoliths, Pt-coated cordierite (a magnesium aluminosilicate) extruded monoliths, and Pt-coated metal monoliths. Systematic studies of these four groups permitted the evaluation of the effects of several parameters on the production of HCN. For the NH3 oxidation studies, the gauze and foam monoliths were compared. As shown in Table I, important parameters include the thermal conductivity of the substrate, the flow patterns in the catalyst support, and the dimensions of the channels within the catalyst. In addition, samples containing a variety of total loadings and noble metal compositions were prepared and tested. The gauze catalysts were 40- and 80-mesh (40and 80 wires per inch) woven wire samples which were cut into 18-mm-diameter circles and stacked together to form a single gauze pack 1-10 layers thick. They were typically sandwiched between two extruded ceramic supports. The foam monoliths were a-Al203samples with an open cellular, spongelike structure. We used samples with 3050 pores per inch (ppi) which were cut with a core drill into 17-mm-diameter cylinders 2-20 mm long. A coating of Pt or Pt/Rh was then applied directly to the alumina by a technique involving organometallic deposition. Relatively high loadings of noble metals were used. The foam monolith samples used in this work had loadings which varied from 2 to 20 wt % noble metal. Scanning electron microscopy (SEM)micrographs of these catalysts before and after use revealed that the catalyst formed large crystallites ( 1pm) on the support with the metal covering a significant fraction of the support surface. The cordierite extruded monoliths, having 400 square cells/in2,were similar to those used in automotive catalytic converters. However, instead of using an alumina washcoat as in the catalytic converter, these catalyst supports were loaded directly with 12-14 w t % Pt in the same manner as the foam monoliths. Because these extruded monoliths consist of several straight, parallel channels, the flow in these monoliths was laminar (with entrance effects) at the flow rates studied. The metal monoliths were prepared by electroplating a strip (- 1cm wide) of the metal with Pt. The Pt loadings were typically 15 wt % , and the metal support compositions included various iron-based alloys (such as N
-
Ind. Eng. Chem. Res., Vol. 32, No. 5, 1993 811 Table 1. Characteristics of Catalysts support
material
thermal conductivity
gauze extruded monolith
Pt-10% Rh wires cordierite
high low
foam monolith metal monolith
alumina various iron- and cobalt-based alloys, nickel
low high
geometry woven mesh uniform; straight channels cellular corrugated, concentric cylinders
characteristic channel diameter (mm)
flow characteristics
