In situ studies of carbon formation leading to metal dusting in syngas processes

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1 In situ studies of carbon formation leading to metal dusting in syngas processes Olle Söderström Department of Chemical Engineering, Lund University February 2010 Abstract Metal dusting corrosion begins with carbon deposition on the metal surface followed by diffusion of the surface carbon into the metal which eventually causes supersaturation of carbon in the metal and the metal dusting is initiated. Experiments have been carried out in a thermogravimetric analysis equipment. A screening process was conducted of 4 different materials with various alloy composition to choose for further investigation. It was found that the poorer the alloy composition, the higher the carbon formation rate. The presence of ethylene increased the carbon formation rate significantly but the ethylene content did not however seem to be decisive for the carbon formation rate. Experiments were also carried out by the means of determining limits for carbon free operation. It was found that for an atmosphere consisting of 50 % hydrogen and the rest inert nitrogen, carbon starts to form over about 18 % ethylene. It was also found that, in an atmosphere consisting of 10 % ethylene, when hydrogen was ramped from % and the rest was inert nitrogen, the carbon formation starts at about 34 % hydrogen. For a syngas mixture of 21 % carbon monoxide, 63 % hydrogen, 16-6 % steam and the rest inert nitrogen the amount of steam needed to suppress carbon formation was found to be 6 %. Introduction Damaging carbon formation is a common and well known problem in process units exposed to carburising gases, especially in syngas processes. Carbon formation in catalysts will disintegrate the catalyst pellets, influence the catalytic activity, and affect the pressure drop across reactor tubes. Carbon formation on metals can cause fouling on heat transfer surfaces and may be a precursor to metal dusting corrosion. Ultimately, the carbon formation may result in equipment failures. Thus, there is a strong motivation to understand and determine the influence of various parameters on carbon formation such as temperature, pressure and gas composition. This work has focused on carbon deposition on materials to gain some general understanding as well as to be able to set limits for safe operation with respect to metal dusting corrosion for some gas mixtures relevant to syngas processes. Theory Syngas is primarily produced by steam reforming of natural gas which mostly consists of methane but also some impurities. The general steps are that the feed gas first goes through a purification step where the gas is purified from sulphur and chlorine compounds at about 380 C before it enters the pre-reformer in which higher hydrocarbons are reformed into methane at C. After that the gas enters the steam reformer in which methane reacts with steam to form carbon monoxide and hydrogen at C. CH 4 (g) + H 2 O(g) CO(g) + 3H 2 (g)

2 Depending of the end-use of the syngas higher hydrogen content might be wanted. This is done in the water-gas shift reactor where excess steam reforms the carbon monoxide to carbon dioxide and hydrogen at about C. CO(g) + H 2 O(g) CO 2 (g) + H 2 g Carbon activity The carbon activity is a way of quantifying the thermodynamic driving force for the carbon forming reactions and it is derived from the equilibrium constants for the carbon forming reactions. In syngas processes the reactions relevant to carbon transfer from gas to metal are as follows. [1], [2] CO(g) + H 2 (g) H 2 O(g) + C(graphite) 2CO g CO 2 g + C(graphite) CH 4 g 2H 2 g + C(graphite) C n H m g m 2 H 2 g + nc(graphite) The last reaction generally describes carbon deposition from higher hydrocarbons present in the natural gas feed. An example of how to calculate the carbon activity for the reduction reaction (the topmost) is found below. a c = K pco ph 2 ph 2 O log K = 7100 T Mechanism The reaction mechanism for carbon attack that could lead to metal dusting corrosion on iron, iron-based low alloys and nickel is illustrated in figure 1 and is as follows: a) carbon transfer from the gas phase and oversaturation of the metal phase, b) Nucleation and growth of cementite, Fe 3 C, at the surface of Fe and low alloy steels, c) Nucleation and growth of graphite into the Fe 3 C, by d) C atoms attaching to the lattice planes of graphite, growing more or less perpendicular into Fe 3 C, resp. Ni and high-ni alloys (from TEM studies), e) carbon filaments Figure 1 Schematic illustration of the processes in metal dusting of iron, low alloy steels and nickel. [2] grown behind particles detached from the metal phase by the graphite growth (SEM), f) Steady state of metal dusting on Fe and low alloy steels at temperatures < 600 C, inward growth of Fe 3 C which disintegrates outward under coke formation, g) Steady state of metal dusting on Fe and low alloy steels at temperatures > 700 C, formation of an iron layer between Fe 3 C and coke, carbon diffusion through this layer and final disappearance of Fe 3 C. [2] Prevention of metal dusting Uses of coatings or changing the environment are ways of preventing metal dusting. It is mainly done through blocking the surface by either a protective oxide scale or by adsorbing sulphur. On surfaces protected by sulphur or oxides, such as Cr 2 O 3, Al 2 O 3 and spinels, adsorption and dissociation of CO, CH 4 and other hydrocarbons generally does not take place. Thus, carbon is not transferred into the metal and metal dusting is therefore not initiated. [2]

3 Material and Method Experiments have been conducted in a thermogravimetric analysis, TGA, equipment provided by Haldor Topsøe A/S. In the TGA, weight changes of a material can be measured as a function of time under controlled gas composition and temperature. The temperature is controlled with a thermocouple connected to a regulator and the gas composition by well calibrated 5850 TR mass flow controllers from Brooks. The weight changes are recorded with a Mark 2CT5 Microforce Balance from C.I. Electronics Ltd. able to measure changes with a precision of 0.01 mg in the sample weight. The experiments are fully automated using a number of softwares. All experimental steps are set up in an Excel sheet. LabVIEW is used to read the instructions from the Excel file and pass the commands onto Automation Tool-kit that continuously instructs the hardware how to run the TGA equipment. With the various gas lines connected to the rig it was possible to run with a gas composed of CH 4, CO, C 2 H 4 /C 4 H 10, H 2, N 2 and steam. Sample preparation Four materials were tested; two stainless steels and two carbon steels. Their alloy compositions are found in table 1. Table 1 Alloy composition of the materials tested. SS- 308H SS- 316 Fe-2.25Cr- 1.0Mo Fe-1.25Cr- 0.5Mo Fe 66, Cr Ni Mo C < Mn <2 < Si <0.75 < P <0.045 < S <0.03 <0.03 <0.035 <0.035 N - < The Fe-1.25Cr-0.5Mo carbon steel samples were cut from a larger plate of approximately 500x200x4 mm. Since this plate was rusty it had to be ground and polished which was done with an electrical grinder using first P50 grit sand paper to make a coarse grinding getting the rust off and then P120 grit sand paper to polish the scratches. Then samples of dimensions 10x4x2 mm weighing about 1 g were cut out with an angle grinder. The Fe-2.25Cr-1.0Mo carbon steel was delivered in a long rod with a rectangular cross-section of 2x1.5 mm with a coarse grey surface. The stainless steels 308 H and 316 were delivered in long cylindrical rods of 2 mm in diameter with smooth metallic-shiny surface. Samples were cut to a weight of about 1 g and bent into a U-shape so that they would not fall through the bottom of the sample basket. Before inserting the samples in the reactor they were washed with soap in distilled water, wiped with paper and then also cleaned with n-hexane on a piece of paper. The sample was placed in a glass basket using a clean pincer and the glass basket was hung on a quartz hook in the reactor connected to the micro scale. Results and Discussion Screening of materials A material susceptible to carbon formation within 24 hour experiments is sought. Therefore a screening process was conducted of two stainless steels and two carbon steels with various alloy compositions to choose for further investigation. It was found that the poorer the alloy composition, the higher the carbon formation rate, as seen in table 2. The carbon steel Fe-1.25Cr-0.5Mo was chosen for further experiments.

4 Table 2 Shows the rate of carbon formation on tested materials at pch 4 =1.0 and T=750 C. Material Rate(mg C/(h*g mat )) SS SS-308H Fe-2.25Cr-1.0Mo Fe-1.25Cr-0.5Mo Ethylene influencing the carbon formation rate from methane The influence of ethylene on the carbon formation rate from methane was studied. This is because the natural gas feed contains higher hydrocarbons which affect the carbon growth and ethylene was chosen to represent the higher hydrocarbons since it is relatively reactive. It was found that the presence of ethylene increased the carbon formation rate from methane but it was independent of the concentration, see figure 2. carbon starts to grow. Since hydrogen suppresses the thermal cracking, the hydrogen content has to be taken into account as well. For a gas consisting of 50 % H 2, C 2 H 4 ramping from 0-20 % in 10 hours and the rest inert N 2, carbon starts to grow at 18 % C 2 H 4, see figure 3. Figure 3 Shows that the carbon growth starts at 18 % C 2 H 4 in an atmosphere consisting of 50 % H 2 and the rest inert N 2. Another approach to determining the limit for safe operation is to keep the ethylene composition fixed and ramp the hydrogen from a level where the hydrogen completely suppresses carbon formation downwards until the carbon starts growing on the surface. For a gas consisting of 10 % C 2 H 4, H 2 ramping from % in 8 hours and the rest inert N 2, carbon starts to grow at 34 % H 2, see figure 4. Figure 2 Shows how the carbon formation from methane is influenced by the ethylene composition of the gas. The gas flow through the reactor consists of 60 % CH 4, 30 % C 2 H 4 and 10 % H 2 during initiation and then steps through 15, 10, 5, 1 % C 2 H 4 where the rest is CH 4. Limits for safe operation In experiments conducted to set limits for safe operation the materials were pre-treated by heating in an atmosphere consisting of 50 % H 2 and 50 % N 2 from C at 4 C/min. The heating in hydrogen partly reduces the protective oxide layer providing a more susceptible surface for carbon formation. It is also providing more homogenous surface conditions which give more consistent results. An interesting point when studying higher hydrocarbons is at what concentration the Figure 4 Shows that carbon grows below 34 % H 2 in an atmosphere consisting of 10 % C 2 H 4 and the rest inert N 2. The product from the primary reformer is syngas at about 900 C that needs to be cooled. Since steam suppresses the reduction reaction of carbon monoxide a safe operation limit for steam is sought. For a gas consisting of 21 % CO, 63 % H 2, steam ramping from 16-6

5 % in 10 hours and the rest inert N 2, carbon starts to grow at 6 % steam, see figure 5. Figure 5 Shows that carbon grows below 6 % steam in an atmosphere consisting of 21 % CO, 63 % H 2 and the rest inert N 2. Conclusions A screening of various alloys showed that the composition of the alloy has large influence on the carbon formation rate. It was shown that the poorer the alloy, the higher the carbon formation rate. It was also shown that the presence of a small amount of ethylene in methane has a large influence on the carbon formation rate. An addition of x % ethylene in pure methane increases the rate to the double. However, the addition of more ethylene does not have much effect on the carbon formation rate. Limits for carbon free operation have been investigated. For an atmosphere of 50 % hydrogen one cannot go above 18 % ethylene, where the rest is inert nitrogen, before carbon starts to form (a c =8.4*10 11 ). And for an atmosphere of 10 % ethylene one cannot go below 34 % hydrogen, where the rest is inert nitrogen, before carbon starts to form (a c =10.2*10 11 ). For a syngas with an atmosphere of 63 % hydrogen, 21 % carbon monoxide one cannot go below 6 % steam, where the rest is inert nitrogen, before the carbon starts to form (a c =28.5). This shows that it is irrelevant to compare carbon activities from different carbon forming reactions with one another. It also shows that even though there is an enormous thermodynamic potential for a carbon forming reaction, the kinetics for the reaction is extremely decisive to determine if carbon will grow. Sources of error One of the largest sources of error that is also the hardest to estimate is the incubation time since it is no reading from the TGA indicating how fast the carbon transfer from the gas to the metal is before carbon protrusions starts growing. Even though there is thermodynamic potential for the carbon to form, it could take years before the material is sufficiently saturated with carbon for the cementite to form, which is often the case in real process equipment. Also, in the experiments determining the carbon free operation limits, the process gas compositions are constantly varied. This means that the incubation time is constantly varied. This may have an impact on when carbon starts to grow, hence the carbon free operation limits presented in this work. The kinetics in the carbon forming reactions is very slow which results in that the reactions are far from equilibrium when carbon starts to form. For example, in the experiments determining the lower limits of steam for carbon free operation, the slow kinetics of the carbon formation reactions may give an underestimate of the lower limit of steam at which carbon starts to grow. Future work This project has focused on gaining general knowledge of the influence of various gas components not earlier treated in literature and should therefore be considered a prestudy. Determining limits for carbon free operation proved to be very time consuming and since this project is set for a limited time, further work is needed in order to develop a severity function for carbon formation on materials. Preferably, this could be done in a

6 pilot able to conduct experiments under industrial conditions. References [1] M. L. Holland and H. J. de Bruyn, Metal dusting failures in methane reforming plant, Int. J. Pres. Vessels & Piping, 1996, 66, pp [2] H. J. Grabke, Metal dusting, Material and corrosion, 2003, 54, pp