Hydrogen addition to the Andrussow process for HCN synthesis

Transcription

1 Applied Catalysis A: General 201 (2000) Hydrogen addition to the Andrussow process for HCN synthesis A.S. Bodke, D.A. Olschki, L.D. Schmidt Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, USA Received 22 July 1999; received in revised form 22 October 1999; accepted 23 October 1999 Abstract The effect of hydrogen addition on HCN production by the ammoxidation of methane over Pt Rh gauze catalyst has been examined. Upon adding small amounts of H 2, the HCN selectivity increased from 74 to 82% on NH 3 basis at 35% N 2 dilution and (CH 4 +NH 3 )/O 2 =2, while selectivities on CH 4 basis were nearly unchanged. Using air instead of oxygen, the HCN selectivity increased from 76 to 86%. However, the increase in HCN selectivity with H 2 addition was accompanied by a decrease in the NH 3 conversion. H 2 addition with different (CH 4 +NH 3 )/O 2 and CH 4 /NH 3 ratios and with preheating of feed gases has also been investigated. The H 2 produced was observed to be greater than that fed for all conditions, so that with recycle the process would require no additional source of H 2. Experimental results obtained here can be qualitatively explained on the basis of a surface reaction model for Andrussow reactor described previously Elsevier Science B.V. All rights reserved. Keywords: Andrussow process; HCN synthesis; Ammoxidation 1. Introduction We have recently shown that partial oxidation of ethane C 2 H O 2 C 2 H 4 + H 2 O over supported Pt Sn bimetallic catalyst with hydrogen addition results in 85% ethylene selectivity at >70% ethane conversion [1,2]. Without H 2 addition, this process gives only 70% selectivity to olefins while most of the remaining ethane reacts with oxygen to produce CO and CO 2. Addition of appropriate amounts of H 2 increases the C 2 H 4 selectivity by over 15% because O 2 reacts with H 2 to form H 2 O instead of reacting with C 2 H 6 to form CO and CO 2. Corresponding author. address: schmi001@maroon.tc.umn.edu (L.D. Schmidt) In this paper, we investigate the addition of H 2 to the ammoxidation of methane to produce hydrogen cyanide, CH 4 + NH 3 + O 2 HCN + 2H 2 O + H 2 This process seems analogous to the partial oxidation of ethane in that it results in 70% selectivity to HCN while 30% of the NH 3 reacts with O 2 to produce N 2 and 30% of the CH 4 reacts with O 2 to produce CO and CO 2 [3,4]. Accordingly, adding H 2 may increase the HCN selectivity by reducing oxidation of NH 3 and CH 4 and allowing the reactions which produce HCN to proceed selectively. We show here that with H 2 addition HCN selectivity improved from 74 to 82% on an NH 3 basis as expected but did not increase significantly on a CH 4 basis. Therefore, H 2 addition selectively suppresses the oxidation of NH 3 to N 2 without significantly affecting CH 4 oxidation to X/00/$ see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S X(00)

2 14 A.S. Bodke et al. / Applied Catalysis A: General 201 (2000) CO and CO 2. Since NH 3 is substantially more expensive than CH 4, this could be a useful result. The H 2 obtained in the product stream was experimentally observed to be greater than that fed into the reactor, so that recycling product H 2 would be sufficient and no external source would be necessary. It should, however, be noted that the drop in NH 3 conversion was comparable to the enhancement in HCN selectivity. Hence, feeding H 2 appears to be useful only if NH 3 in the product stream is separated and recycled back to the reactor so that the overall conversion of NH 3 is high and unwanted reactions of NH 3 are minimized. subtracted from the total N 2 peak area from earlier runs with N 2 diluent to calculate the N 2 peak area corresponding to diluent. Argon could not be used as a diluent instead of nitrogen since it overlapped the CO peak in the GC spectrum. 3. Results 3.1. Ammoxidation using 35% N 2 dilution Fig. 1 shows experimental results for gases flowing at 5 slpm or a superficial velocity of 0.4 m/s, exit 2. Experimental The catalyst consisted of a set of five layers of 18 mm diameter Pt-10% Rh 80 mesh gauzes corresponding to a wire diameter of 90 m and a spacing of 320 m between adjacent wires. These gauzes were placed together randomly and packed between two alumina foam monoliths, which minimized heat losses in the axial direction. This assembly was wrapped in alumina paper and placed in an insulated quartz tube reactor to minimize radial heat losses. The experimental apparatus used here was essentially identical to that described previously for examining alternate conditions for HCN synthesis [5,6]. Gases at room temperature and 1.2 atm flowed through calibrated mass flow controllers and were pre-mixed before entering the reactor which operated autothermally at C. External heat was required for ignition, and this was provided by a Bunsen burner placed directly on the quartz tube. At a temperature of 250 C the catalyst ignited, and the exothermic heat of reaction was sufficient to maintain the catalyst temperature. The Bunsen burner was then removed and the reaction zone was insulated to achieve nearly adiabatic conditions. Product gases were analyzed by gas chromatography using a HP 5890 GC with a Haysep D column to separate the gaseous species and a TCD detector. Nitrogen was used as a diluent typically at 35% of the total flow rate. However, N 2 was also a by-product in the ammoxidation reaction. Identical experiments were, therefore, performed using argon as diluent to measure the peak size of product N 2. This was then Fig. 1. Experimental results for ammoxidation of CH 4 over five layers of 80 mesh Pt-10% Rh gauze catalyst, using 35% N 2 dilution at 5 slpm and 1.2 atm. Selectivity to HCN on ammonia basis increases from 50 to 90% while NH 3 conversion decreases from 96 to 50% as the fuel/oxygen ratio is increased from 1.8 to 2.4. On a methane basis, the HCN selectivity increases from 45 to 70% and conversion decreases from 100 to 75%.

3 A.S. Bodke et al. / Applied Catalysis A: General 201 (2000) pressure of 1.2 atm and CH 4 /NH 3 ratio=1.1. The catalyst was initially activated over a period of 30 h using these conditions and (CH 4 +NH 3 )/O 2 =2. The HCN selectivity and NH 3 and CH 4 conversions gradually improved during this time period. Experiments with varying (CH 4 +NH 3 )/O 2 were carried out only when selectivities and conversions varied negligibly with time on-stream. On an NH 3 basis, the HCN selectivity increased from 50 to 90% and the conversion decreased from 97 to 47% as the (CH 4 +NH 3 )/O 2 ratio was changed from 1.8 to 2.4. On a CH 4 basis, the HCN selectivity increased from 45 to 70% and the conversion dropped from 100 to 77%. Fig. 1 also shows that the HCN yield increased, reached a maximum at a ratio of 2.0, and then decreased as the (CH 4 +NH 3 )/O 2 ratio was increased beyond 2.0. These results are typical and have been reproduced using several catalyst packs. They are also consistent with results reported previously [5 7]. There is typically a 5 7% variation of selectivities and conversions between catalysts which is probably due to different arrangement of the five gauzes relative to each other or to different activation conditions. Fig. 2 shows the effect of H 2 addition to this process at (CH 4 +NH 3 )/O 2 =2, CH 4 /NH 3 =1.1 and total flow rate of 5 slpm, which are typical operating conditions for an Andrussow reactor. The H 2 was added in appropriate amounts holding all other flow rates constant so that the total flow rate increased and N 2 dilution decreased with H 2 addition. On an NH 3 basis, the HCN selectivity increased from 74 to 82% while the conversion dropped from 88 to 64% with H 2 addition. No NO or NO 2 peaks were detected, and the only other N-containing product was N 2. Hence the N 2 selectivity decreased from 26 to 18% with H 2 addition. Since the drop in conversion was larger than the increase in selectivity, the HCN yield decreased from 65 to 53%. On a CH 4 basis, the HCN, CO and CO 2 selectivities were nearly unchanged, indicating that H 2 predominantly reduced NH 3 oxidation to N 2 without significantly altering CH 4 reactions to CO and CO 2. Fig. 3 shows the effect of H 2 addition at (CH 4 + NH 3 )/O 2 ratios=1.8 and 2.2, along with results shown in Fig. 2. The increase in HCN selectivity with H 2 addition was greater at smaller (CH 4 +NH 3 )/O 2 ratios, and decreases in NH 3 or CH 4 conversions were also Fig. 2. Effect of H 2 addition to CH 4 ammoxidation at fuel/oxygen=2. S HCN on NH 3 basis rises from 74 to 82% but NH 3 conversion drops from 88 to 65% with H 2 addition. Selectivities onach 4 basis are relatively unchanged. smaller. At (CH 4 +NH 3 )/O 2 =1.8 and H 2 /O 2 =0.75, the HCN selectivity and fuel conversions were comparable to those at (CH 4 +NH 3 )/O 2 =2.0 with no H 2 addition. This indicates that recycling product H 2 would allow operation at smaller (CH 4 +NH 3 )/O 2 and thus result in reduced NH 3 and CH 4 requirements. This also suggests that the optimum (CH 4 +NH 3 )/O 2 ratio for HCN production with H 2 recycle is lower than for operation without H 2 addition Ammoxidation using air We also investigated the Andrussow process using air oxidation. Fig. 4 shows results using air instead of

4 16 A.S. Bodke et al. / Applied Catalysis A: General 201 (2000) Fig. 3. H 2 addition at various fuel/oxygen ratios with 35% N 2 dilution. Effect of H 2 is more pronounced at lower (CH 4 +NH 3 )/O 2. 35% N 2 dilution while other conditions were similar to those in Fig. 1. The HCN selectivity increased and fuel conversions decreased as the (CH 4 +NH 3 )/O 2 ratio was increased from 1.6 to 2.2. These trends were similar to those using 35% N 2, although comparable selectivity and conversions were obtained at lower (CH 4 +NH 3 )/O 2 ratios. The maximum in HCN selectivity occurred at (CH 4 +NH 3 )/O instead of 2.0. Fig. 5 shows the effect of H 2 addition to the Andrussow process using air and at (CH 4 +NH 3 )/O 2 =1.8. The HCN selectivity increased from 76 to 86% on an NH 3 basis while conversion decreased from 90 to 70% with H 2 addition. The HCN yield increased from 69 to 73% up to H 2 /O and then decreased with further H 2 addition. On a CH 4 basis, the HCN selectivity and yield did not vary significantly although the CH 4 conversion decreased from 95 to 87% Effect of preheat and CH 4 /NH 3 ratio Fig. 6 compares CH 4 ammoxidation with and without preheat. For preheating the feed gases, a heating tape was wound around the section of the reactor tube preceding the catalyst and electrically heated up to 250 C. It can be seen that HCN selectivities were lower but both NH 3 and CH 4 conversions were higher when feed gases were preheated. Fig. 7 shows the effect of preheat with H 2 addition. The (CH 4 +NH 3 )/O 2 was maintained at 2 and the H 2 /O 2 ratio was varied with and without preheat. HCN selectivities and conversions were both lower

5 A.S. Bodke et al. / Applied Catalysis A: General 201 (2000) Fig. 4. Ammoxidation of CH 4 using air. Trends are similar to results using 35% N 2 dilution, but the maxima in HCN selectivity occurs at 1.8 instead of 2. Fig. 5. Effect of H 2 addition to ammoxidation using air and at fuel/oxygen=1.8. HCN selectivity increases from 76 to 87% on NH 3 basis while NH 3 conversion falls from 90 to 70% with H 2 addition. Selectivities on CH 4 basis are relatively unchanged. upon application of preheat compared to results without preheat. This is in contrast to the higher conversions obtained upon preheating without H 2 addition as observed in Fig. 6. Hence preheating the feed gases would not be beneficial with hydrogen recycle in the Andrussow process. Fig. 8 compares H 2 addition with varying CH 4 /NH 3 ratios at (CH 4 +NH 3 )/O 2 =2. The improvement in HCN selectivity by H 2 addition was greater at lower CH 4 /NH 3 ratio. However, the initial HCN selectivity without H 2 addition was greater at higher CH 4 /NH 3 ratios. For example, the HCN selectivity at CH 4 /NH 3 =1 and H 2 /O 2 =0.75 was lower than that at 1.1 without H 2 addition. Hence, the optimum CH 4 /NH 3 ratio should be maintained at 1.1 with H 2 addition. 4. Discussion 4.1. H 2 recycle Fig. 9 compares the amount of H 2 formed to that fed at (CH 4 +NH 3 )/O 2 =2 and CH 4 /NH 3 =1.1. For no H 2 addition, 1.3 mol of H 2 are formed per mole of O 2 in the feed. The amount of H 2 produced is greater than that fed for all H 2 /O 2 ratios up to 1.5. Hence for H 2 addition within this ratio, feedback from the

6 18 A.S. Bodke et al. / Applied Catalysis A: General 201 (2000) Fig. 6. Effect of preheating the feed gases of HCN production. Preheat decreases the HCN selectivity and increases fuel conversion on both NH 3 and CH 4 basis. product stream would be sufficient and no external source would be necessary. Upon increasing H 2 /O 2 in the feed from 0 to 1.75, H 2 produced increased only from 1.3 to 1.5. Hence, the amount of H 2 produced appears to be nearly unchanged and independent of the H 2 added in the feed. This suggests that most of the H 2 added must be reacting with O 2 to produce H 2 O and that the H 2 in the product stream must be formed by the ammoxidation reaction between CH 4 and NH 3, CH 4 +NH 3 HCN + 3H Mechanism of HCN production and H 2 addition The kinetics of HCN synthesis over polycrystalline Pt and Rh foils at pressures between 0.01 and 10 Torr have been previously examined in detail [8,9]. Rates of reactions were measured between 600 and 1500 K in a steady flow reactor and were fit to the Langmuir Hinshelwood form. Rate expressions obtained were used in an atmospheric pressure model consisting of 13 simultaneous equations [10]. Predicted HCN selectivities agreed quite well with those observed in commercial reactors, and a distinct optimum with the feed composition was obtained near that observed experimentally. HCN production was postulated to be a true surface reaction between CH 4 and NH 3 on Pt and Rh catalysts and HCN yields could be explained nearly quantitatively without invoking any homogeneous steps or any intermediates involving oxygenated species.

7 A.S. Bodke et al. / Applied Catalysis A: General 201 (2000) Fig. 7. Effect of preheat with H 2 addition. Preheating reduces both HCN selectivity and fuel conversion for all H 2 /O 2 ratios. With addition of O 2, the HCN production fell while NO rose to become the dominant product [11]. The rate of reaction of O 2 with NH 3 to produce NO was observed to be much faster than the rate of reaction of O 2 with CH 4 to form CO. Although NO could oxidize further to form N 2, it was also capable of reacting with CH 4 to produce HCN. These experiments suggested that in an industrial Andrussow reactor, HCN is produced mostly by CH 4 +NH 3 HCN+3H 2 and CH 4 +NO HCN+(1/2)H 2 +H 2 O reactions. Pt and Rh are also well known as excellent catalysts for H 2 oxidation [12] and H 2 added to a CH 4 NH 3 O 2 mixture over these catalysts should quickly react with O 2 to form H 2 O. Since the rate of NH 3 oxidation is significantly faster than that of CH 4 oxidation, a large part of O 2 in the feed must be reacting with NH 3. Hence H 2 addition should predominantly reduce the amount of NH 3 reacting via the oxidation pathway to form NO, which would eventually lead to the formation of either N 2 through complete oxidation or HCN via reaction with CH 4. This explains the decrease in NH 3 conversion with H 2 addition. It also explains the reduction in the selectivity of N 2 and a consequent increase in the HCN selectivity upon adding H 2. Since only a small fraction of O 2 reacts with CH 4, the larger effect of H 2 addition would be on the NH 3 +O 2 reaction instead of the CH 4 +O 2 reaction. This is consistent with the experimental observation that H 2 addition does not significantly affect selectivities on a C-atom basis. The trend of increasing HCN selectivity and decreasing NH 3 conversion with H 2 addition is similar

8 20 A.S. Bodke et al. / Applied Catalysis A: General 201 (2000) Fig. 8. H 2 addition to HCN production at (CH 4 +NH 3 )/O 2 =2, 35% N 2 dilution and varying CH 4 /NH 3 ratios. The increase in HCN selectivity is higher and drop in conversion is lower at CH 4 /NH 3 =1.0 compared to 1.1 or 1.2. to the trend with increasing fuel/oxygen ratio. In other words, H 2 acts as an alternative to NH 3 or CH 4 for reaction with O 2. Similarly, the effect of H 2 addition on HCN selectivity on N-atom basis is also comparable to that of increasing CH 4 /NH 3 ratio as shown in Fig Equilibrium predictions Fig. 9. Comparison of the amount of H 2 formed to that fed in the Andrussow process at (CH 4 +NH 3 )/O 2 =2 and CH 4 +NH 3 =1.1. For H 2 /O 2 <1.5, the process produces more H 2 than fed, and no additional H 2 source would be required. This process operates at very high temperatures of C. The large selectivities to HCN obtained here are surprising since thermodynamic considerations predict that HCN should be very unstable under these conditions and should react immediately to produce CO, CO 2 or N 2. Fig. 10 shows equilibrium selectivities to CO, CO 2,N 2 and HCN for various feed compositions. Panels A and B show the equilibrium

9 A.S. Bodke et al. / Applied Catalysis A: General 201 (2000) Fig. 10. Equilibrium prediction of product composition for reacting CH 4 NH 3 O 2 H 2 mixtures. Panels A and B show C and N atom selectivities as a function of the (CH 4 +NH 3 )/O 2 ratio. Panels C and D show C and N atom selectivities as a function of the H 2 /O 2 ratio at (CH 4 +NH 3 )/O 2 =2. selectivities on C and N-atom basis, respectively, with varying (CH 4 +NH 3 )/O 2 ratios, for CH 4 /NH 3 =1.1 and 35% N 2 dilution. Similarly panels C and D show equilibrium selectivities as a function of varying H 2 /O 2 ratios for (CH 4 +NH 3 )/O 2 =2. For all feed compositions, we first calculated the adiabatic temperature if the inlet gases would react to form the product mixture experimentally obtained using the Pt-10% Rh catalyst. Using the inlet composition and this adiabatic temperature, the product composition was then calculated assuming that the feed gases went to equilibrium. Only gas phase products were considered and solid phase species such as solid carbon were neglected. For all compositions of the CH 4 NH 3 O 2 H 2 feed mixture, equilibrium predicts that most of the CH 4 reacts with O 2 to form CO and CO 2 while most of the NH 3 forms N 2. HCN selectivities on the basis of CH 4 or NH 3 are less than 0.001%. CH 4,NH 3 and O 2 are always completely converted and selectivities vary only such that the stoichiometric balance between C, N, O and H species is obeyed. For example, with increasing (CH 4 +NH 3 )/O 2, the ratio of C/O increases thus increasing the amount of CO produced and decreasing the amount of CO 2 produced at equilibrium. Hence, we see that HCN should be highly unstable under experimental conditions and that these experimental results are far from equilibrium Flames and explosions Experiments involving reactions of hydrogen oxygen mixtures at high temperatures may be thought

10 22 A.S. Bodke et al. / Applied Catalysis A: General 201 (2000) to be dangerous since these mixtures are flammable in the gas phase over a very wide range of compositions [13]. However, we observed no evidence of homogeneous flames or explosions for all H 2 /O 2 ratios even at very high temperatures. This suggests that H 2 and O 2 must be quickly reacting on the catalyst surface and the heat of this reaction must be effectively removed by the production of HCN by the ammoxidation reaction between CH 4 and NH 3. This process illustrates how the heat produced by a very exothermic reaction can be utilized to drive the desired energy consuming reaction without explosions or large temperature excursions. Another example of such a reaction is the oxidative dehydrogenation of C 2 H 6 to C 2 H 4, where we have previously demonstrated that C 2 H 4 selectivities up to 90% can be obtained over Pt Sn bimetallic catalysts by H 2 addition [1,2]. 5. Summary Addition of H 2 to the Andrussow process increases the selectivity to HCN by 10% on the basis of NH 3 but does not significantly affect selectivities on the basis of CH 4. This could be useful since NH 3 is more expensive than CH 4, which is abundantly available via natural gas. H 2 produced through this reaction is more than that fed, so that with recycle the process requires no additional source for H 2. H 2 addition should also allow operation at leaner (CH 4 +NH 3 )/O 2 ratios and thus decrease the overall requirement of NH 3 and CH 4 for given HCN production. However, NH 3 conversion decreases with H 2 addition and hence feeding H 2 would only be useful if NH 3 is separated from the product stream and recycled back to the reactor. Acknowledgements This research was supported by NSF under Grant No. CTS References [1] A.S. Bodke, et al., Science 285 (5427) (1999) 712. [2] A.S. Bodke, L.D. Schmidt, J. Catal., submitted for publication. [3] B.Y. Pan, R.G. Roth, I&EC Proc. Design Dev. 7 (1968) 53. [4] B. Pan, US Patent No. 3,370,919 (1968). [5] D.A. Hickman, M.C. Huff, L.D. Schmidt, Ind. Eng. Chem. Res. 32 (1993) 809. [6] S.S. Bharadwaj, L.D. Schmidt, Ind. Eng. Chem. Res. 35 (1996) [7] A.G. Dietz, L.D. Schmidt, in press. [8] D. Hasenberg, L.D. Schmidt, J. Catal. 97 (1986) 156. [9] D. Hasenberg, L.D. Schmidt, J. Catal. 91 (1985) 113. [10] N. Weletzko, L.D. Schmidt, AIChE J. 34 (1987) [11] D. Hasenberg, L.D. Schmidt, J. Catal. 104 (1987) 441. [12] W.R. Williams, C.M. Marks, L.D. Schmidt, J. Phys. Chem. 96 (1992) [13] B. Lewis, G. von Elbe, Combustion, Flames and Explosions of Gases, 3rd Edition, Academic Press, Orlando, Florida, 1987.