Ind. Eng. Chem. Res. 2001, 40, 553-557
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Degradation of Methanol and Methylamines to Carbon over Heated Alloy Surfaces John J. Birtill,*,† Paul Ridley,†,‡ Stephen Liddle,†,‡ Tim S. Nunney,§ and Rasmita Raval§ ICI C&P Ltd., PO Box 90, Wilton Centre, Middlesbrough, Cleveland TS90 8JE, U.K., and Leverhulme Centre for Innovative Catalysis, University of Liverpool, Liverpool L69 7ZD, U.K.
Methanol and methylamines, especially trimethylamine, can be degraded over alloy surfaces to generate byproducts and carbon deposits. The activity of various metal alloys for the degradation of methanol and methylamines to carbon has been investigated. Inconel 600, a high-Ni alloy, was found to be the most active for these degradation reactions, the cold-worked turnings yielding carbon from methanol/ammonia at 380 °C and from trimethylamine/ammonia at 430 °C. All of the other alloys tested were resistant to bulk carbon deposition from methanol/ammonia, but some of them caused bulk carbon deposition from TMA/ammonia at temperatures in the range 450-490 °C. Introduction Mono-, di-, and tri-methylamine (MMA, DMA, and TMA) are manufactured by the reaction of methanol and ammonia over solid acid catalysts such as amorphous silica/alumina or zeolites. Excess TMA can also be recycled and equilibrated with ammonia to MMA/DMA/ TMA over amorphous silica/alumina, either separately from or simultaneously with the main reaction. The process feed and recycle streams are usually heated by a combination of steam and recovered process heat (Figure 1). Electric or fired heaters can be used to boost the feed temperature in some circumstances. The skin temperature of such heaters can be significantly higher than that of the process feedstocks. The possibility of feedstock and product decomposition over hot metal surfaces (e.g., preheaters and distillation reboilers) can sometimes be overlooked in chemical process design. Thermal degradation can cause the formation of process byproducts, the buildup of carbon deposits, and even reactions with the metal surface itself such as the formation of metal carbides and nitrides. The buildup of bulk carbon deposits in electric heaters reduces their efficiency, leading ultimately to electrical failure and possibly even to the failure of equipment integrity, especially if accompanied by “metal dusting”. In one instance in a methylamines process, an electric heater with elements constructed from Inconel 600 failed after just 60 days operational exposure to a stream of recycled TMA/NH3. On subsequent inspection, the internal volume was found to be packed with carbon. The degradation of organic molecules on metal surfaces typically generates adsorbed fragments that can react with the metal surface. The subsequent reactions of these fragments with the bulk alloys has caused integrity failure in steam reforming plants.1 At high * Author for correspondence. ICI Amines Business has changed ownership. Current address: Air Products (Chemicals) Teesside Ltd., PO Box 90, Wilton Centre, Middlesbrough, Cleveland TS90 8JE, U.K. Tel: (+44) 1642-430881. E-mail:
[email protected]. † ICI C&P Ltd. ‡ On secondment from the University of Newcastle, U.K. § University of Liverpool.
Figure 1. Schematic diagram of methylamines process.
temperatures, typically >800 °C, the diffusion of carbon atoms into the bulk metal leads to carburization, the formation of carbide domains within the metal matrix. Alloys containing iron, nickel, and cobalt are liable to disintegrate into a dust of carbon, carbides, metal, and metal oxides, a catastrophic phenomenon known as metal dusting, which can occur at temperatures as low as 450 °C. Oxide films are usually protective barriers to carbon deposition, but these may well be unstable under reducing environments such as TMA/NH3. The phenomenon of carbon deposition on metal surfaces, especially metal catalyst surfaces, has been studied extensively in the past 30 years.2 Carbon deposition is catalyzed especially by iron, nickel, and cobalt. Several types of morphologies have been observed, e.g., well-ordered graphite, carbon whiskers, and metal carbides. The transportation of small metal particles at the tips of growing whiskers has been observed.2,3 The mechanism of whisker formation is believed to involve the diffusion of carbon through the metal particle under a concentration gradient, followed by precipitation at a dislocation.2 Although there is some general background knowledge of carbon deposition reactions, there is no published information about the decomposition of methylamines process feedstocks over various metal surfaces. In this work, the tendency of different alloy surfaces to catalyze the decomposition of methanol and methylamines to carbon was investigated by exposure of gaseous mixtures to metal turnings at high temperatures, such as those typical of electric heater surfaces.
10.1021/ie0005856 CCC: $20.00 © 2001 American Chemical Society Published on Web 12/15/2000
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Ind. Eng. Chem. Res., Vol. 40, No. 2, 2001
Figure 4. Free energy of decomposition of TMA. Data from ref 4, adjusted to 19 bar assuming ideal gas behavior. Figure 2. Free energy change of methanol decomposition. Data from ref 4, adjusted to 19 bar assuming ideal gas behavior.
trile (CH3CN) can be formed from the decomposition of TMA but not DMA. Dehydrogenation of the amines to imines is another possibility, although no thermodynamic data were available for such reactions. In addition, ammonia is unstable at 19 bar with respect to N2/H2. Alloys with high nickel contents are sometimes selected to avoid nitriding of hot metal surfaces in the presence of ammonia. Thus, it is no surprise that the feedstocks and recycled products in methylamines plants can sometimes degrade to generate carbon and other byproducts. There are many reactions that are thermodynamically favorable. However, the actual rates of these decomposition reactions depend also on the catalytic properties of the hot metal surfaces, and therefore, suitable metal surfaces for heater elements should have low catalytic activities for these decomposition reactions. Experimental Section
Figure 3. Free energy of formation of methylamines. Data from ref 4, adjusted to 19 bar assuming ideal gas behavior.
Various alloys were screened in a search for catalytic inactivity, a reversal of the usual aims in catalyst screening. Thermodynamic and Kinetic Aspects of Methylamines Formation Under “thermodynamic control”, the methylamines formation reactions proceed to near-complete methanol conversion and equilibrium between ammonia and the three methylamine products, MMA, DMA, and TMA. In fact, the methylamines feedstocks and products are all thermodynamically unstable with respect both to their elements and to other simple compounds. Methanol becomes progressively more unstable with increasing temperature with respect to CO/H2, CO2/C/ H2 (Boudouard reaction), and C/H2/H2O, as summarized in Figure 2. Even the equilibrium decomposition of methanol into its elements C/H2/O2 is capable of yielding significant carbon deposits from flowing feed streams above 500 °C (See Supporting Information, Table 6). Figure 3 shows that all three methylamines become increasingly less stable with respect to C/N2/H2 as temperature is increased, with TMA being the least stable. MMA can also decompose to HCN or methane without forming carbon. DMA and, even more so, TMA can decompose to yield carbon as well as other smaller molecules such as amines and nitriles. Figure 4 shows the possible decomposition products of TMA. Acetoni-
The alloy metal was cut into turnings on a titaniumtipped lathe. The average thickness of the turnings was 0.25 mm. Some samples were heated overnight at 1000 °C in air in order to determine the effect of annealing the cold-worked surfaces. These samples generally had a matte appearance after treatment and were tested without polishing. The surface composition of the metal turnings was measured by XPS. The reactivity of the alloys was investigated by passing gaseous feedstock mixtures over the alloy turnings mounted in a microreactor tube mounted in a close-fitting, electrically heated, metal block. The microreactor tube was made from glass-lined stainless steel tubing with dimensions 10 mm i.d. × 270 mm length. The glass lining was necessary because, in other tests, carbon was found to form on the walls of metal tubes. An internal thermowell made from 316 stainless steel was used to calibrate the internal temperature of the reactor tube along its length against the external block temperature. The temperature increased along the pre-bed zone, which acted as a preheat zone. The central zone (length, ∼15 cm; volume, ∼12 mL) was nearly isothermal. The metal turnings were mounted to occupy approximately half of the central zone. The internal thermowell was present in some tests but was omitted from tests with TMA above 450 °C because of the formation of carbon on the thermowell above this temperature. The space above and below the metal turnings was packed with small glass beads (ballotini). The possibility of homogeneous gas-phase reactions was investigated in additional comparative studies using only glass beads as the reactor packing.
Ind. Eng. Chem. Res., Vol. 40, No. 2, 2001 555 Table 1. Typical Composition and Melting Ranges of Alloys Tested in This Work
alloys 316 stainless Inconel 600 Incoloy 800 Incoloy 825 Haynes 120 Haynes 230 9:1 Chrom-Moly
relative surface compositiona (atom %)
alloy composition (nominal wt %)
melting range (°C)
Ni
Cr
Fe
1371-1399 1354-1413 1357-1385 1370-1400 13001300-1370
10-14 70-75 32 38-42 37 ∼62
16-18 14-17 20 19-23 25 21 8-10
∼68 6-10 48 38 ∼36 2 90
Mo
W
2-3 1.5-3 0.5 1.3 1
0 14
Mn
other
Ni
Cr
Fe
490 380 >490
air-treated TCarb (°C) 390 470 >490 470 470 430 >490a
a Air-treated versions of Incoloys 800 and 825, Haynes were tested with MeOH/NH3.
was a sharp transition temperature within a range of 5 °C above which carbon deposition occurred. At increased feed-rates, i.e., lower residence times, the transition temperature increased slightly. At lower feedrates, there was still no particulate carbon deposition at 425 °C, although the beads were discolored. The byproducts collected in aqueous solution included MMA, DMA, and acetonitrile. The GC analytical results were not very systematic, however. The determination of byproducts would have excluded any methane or HCN, if present. Only trace amounts of formaldehyde derivatives were detected up to 440 °C. Surprisingly, the air-treated turnings showed carbon deposition at a lower temperature than the untreated turnings, i.e., 390 °C. However, this finding was not investigated further. (c) Reaction of Monomethylamine and Ammonia over Inconel 600. This reaction was carried out in a glass-lined reactor without a thermowell. The MMA/ NH3 feed was premixed in the molar ratio 3:1 (N/C ) 1.3). In this case, no comparative test was carried out with glass beads alone. Analytical investigations for other byproducts were not carried out. The results are summarized in Table 4. It was concluded that monomethylamine degrades to carbon over Inconel 600 around 460 °C, which is about 30 °C hotter than trimethylamine. This can be attributed to the lower thermodynamic stability of TMA compared to that of MMA or to an easier mechanistic pathway for TMA decomposition because of its greater number of methyl groups. (d) Relative Performance of Other Alloys. The relative tendency of all of the alloys investigated to lay down carbon is summarized in Table 5. The approximate lowest temperature for bulk carbon formation for each alloy is given in brackets. Note that, in general, the post-alloy glass beads were blackened at slightly lower temperatures than were required for bulk carbon formation, thus implying some tar formation. TMA/NH3 decomposed to carbon over all of the alloys tested except for Incoloys 800 and 825. Methanol/NH3 did not decompose to yield bulk carbon over any alloy except for Inconel 600. For four out of the five alloys tested, carbon deposition from TMA/NH3 was observed at slightly lower temperatures for the air-treated alloys compared to those for the cold-worked alloys. Detailed results are summarized in the Supporting Information (Tables 9 and 10).
Figure 5. Electron microscopy of carbon deposits formed over Inconel 600. Source: plant electrical heater. Magnification: 340 000×. Note Ni particle at end of carbon fiber.
(e) Morphology of Carbon Deposits. Carbon deposits from laboratory experiments on Inconel 600 and from plant Inconel 600 electric heater elements were inspected by transmission electron microscopy, and the morphologies were found to be similar, with mainly filamentous carbon observed in each case. Small particles of nickel metal were observed at the extreme ends of some filaments. EDX analysis confirmed that the particles were nickel. The plant sample is shown in Figure 5. Discussion The results of this work show that methylamines are stable to homogeneous decomposition in the gas phase to around 490 °C, but in common with other organic compounds, they decompose to generate carbon on metal surfaces at lower temperatures. The fundamental characteristics of the adsorption and decomposition of methylamines on single-crystal metal surfaces have been reported elsewhere.5-8 The generally accepted mechanism for decomposition of organic compounds and carbon growth involves stepwise adsorption onto the metal surface, fragmentation to simpler intermediate species and eventually adsorbed carbon or “carbide”, and/or dissolution of carbon into the metal and growth of carbon filaments.2 The filamentous carbon might, itself, promote further carbon deposition.9 The initial decomposition of methylamines on the alloy surface takes place at low temperatures, and surface carbon residues occur as low as 40-110 °C for MMA5-8 and even lower for TMA.8 Diffusion of carbon into the bulk becomes appreciable above 430 °C for Ni(111),5-8 the same temperature at which TMA decomposes to bulk carbon over Inconel 600 turnings. The generation of bulk carbon on alloys is associated with their Ni content, but the overall composition of the alloy, including its Cr content, is also important. The worst, i.e., most active, alloy out of those tested was Inconel 600 (>70% Ni,
