Ind. Eng. Chem. Res. 2003, 42, 4741-4747
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MATERIALS AND INTERFACES Gaseous Pretreatment of High-Alloy Steels Used in Ethylene Furnaces: Pretreatment of Incoloy 800 Ta-Chi Luan Degussa Taiwan, Ltd., Taipei, Taiwan
Roger E. Eckert and Lyle F. Albright* School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907
Gaseous pretreatments at high temperatures of high-alloy steels used in the coils of ethylene coils sometimes result in much improved pyrolysis operation. Recently, Nova Chemical Co. has reported pretreatments with hydrogen/steam mixtures that reduce the need to decoke from about 30-40 days to up to 515 days in both the coil and the accompanying transferline exchanger. Many pretreatments cause significant diffusion of Cr and Mn to the surface and of Fe and Ni to layers below the surface. The phenomena occurring during pretreatments and differences in resulting surface changes are still not well understood. The results obtained when Incoloy 800 coupons were pretreated were compared over a wide range of pretreatments. Temperature, time of pretreatment, and gas used all have important effects on surface compositions and suggest even more improved coils can be developed. Introduction The surface compositions of high-alloy stainless steels used in the coils of ethylene furnaces change to a high degree with use. The inner surface layers become richer in chromium (Cr) and manganese (Mn) present as oxides but lower in nickel (Ni) and iron (Fe).1 These changes of the surface composition act to reduce coke formation and collection on the inner surfaces during pyrolysis of hydrocarbons. In particular, the formation of filamentous coke is greatly reduced. Ni and Fe (and/ or their oxides) catalyze this type of coke, in which Ni and Fe particles are often embedded. Such coke is designated here as that formed by mechanism 1.2 These coke filaments are excellent collection sites for cokes formed by the other two mechanisms. In mechanism 2, tar droplets initially form in the gas phase and then collect to a considerable degree on the coil surfaces, especially if filaments are present. These droplets decompose on the surface, forming coke. Mechanism 3 is the reaction of gaseous free radicals, acetylenic compounds, dienes, etc., with free radicals on the surface of coke deposits; hence, additional coke is produced on such surfaces. Horseley and Cairns3 pretreated a high-alloy steel with gaseous mixtures of either H2/H2O, CO/CO2, or H2/H2O/CO/CO2 at 650-1000 °C for 2-4 h. The untreated steel used contained by weight 19.9% Cr, 24.6% Ni, 0.7% Nb, 0.6% Mn, 0.56% Si, 0.04% C, and the remainder Fe. Several of their pretreatments caused a significant enrichment at the surface of Cr and Mn as * To whom correspondence should be addressed. Tel.: 765494-4087. Fax: 765-494-0805. E-mail:
[email protected].
oxides. After one pretreatment, a surface layer almost 0.3 µm in depth contained about 65-70% Cr and 30-35% Mn; Fe and Ni, presumably present mainly as elements, were not detected until greater depths. These findings suggest that specific pretreatments might be developed that would result in reduced coke formation on metal surfaces. Horseley and Cairns3 explained their results as being caused by the selective oxidative capabilities of their pretreating gases. Cr and Mn oxides are more stable oxides as compared to Fe and Ni oxides. Once Cr and Mn oxides form, they essentially stop diffusing because these oxides are larger entities as compared to the metal elements. The oxides collect at and near the surfaces of the steels. Luan4 and Szechy et al.5 in the early 1990s employed pretreating mixtures of H2/H2O and CO/H2O. CO, H2, air, CO2, and other gases were also investigated. Coupons of several high-alloy steels were pretreated. Each coupon was analyzed after high-temperature pretreatments with an energy-dispersive X-ray analyzer (EDAX). Steels that have been or are used in ethylene furnaces were tested, including Incoloy 800, HK40, and HP40. Cr and Mn were transferred toward the surface in the largest amounts, but substantial amounts of Ti, Si, and Al were also often transferred, even though these latter elements are present in the initial steels in small concentrations. Considerable Ni and Fe were in the meantime transferred from the outer layer to an enriched layer, often 1 µm or more below the surface. Some pretreated surfaces were found by Luan to have reduced levels of coke formation during pyrolysis. Essentially simultaneously, Shell Oil Co. performed similar treatments with some of the same alloys.6,7
10.1021/ie030422s CCC: $25.00 © 2003 American Chemical Society Published on Web 09/06/2003
4742 Ind. Eng. Chem. Res., Vol. 42, No. 20, 2003 Table 1. EDAX Analyses of Untreated Incoloy 800 Coupons (at 15 kV in Atom %) metal
unpolished and untreated surfacea
polished and untreated surfacea
cross-cectional areab
Al Si Ti Cr Mn Fe Ni
2.0 1.3 0.3 14.9 0.3 48.0 33.3
0.7 2.0 0.4 22.8 0.8 42.8 30.3
2.3 2.5 0.8 20.7 0.7 43.3 30.0
a Average of three or four readings. b Average composition of cross-sectional areas at depths of 6-1000 µm below the polished surface of the coupon.
In the latter part of the 1990s, at least two coils that had inner surfaces with little or no Fe and Ni were developed and installed in commercial ethylene furnaces.8,9 The rates of coke formation (or coke collection) were (or are) reduced by at least 50-70%, as indicated by longer times between decokings. Even longer times between decokings were obtained by using coils pretreated using different combinations of hydrogen and hydrogen/steam mixtures.10-13 In one industrial furnace of Nova Chemicals, the time between decokings was extended from 33 to 516 days; hence, the rate of coke buildup on the surfaces was reduced, by a factor of about 15-16. In the present paper, the experimental data of Luan4 were employed to determine, for what is thought to be first time, the amounts of Cr and Mn transferred to the outer layers of Incoloy 800 due to various pretreatments; the amounts of Fe and Ni transferred from the outer surfaces to underlying layers were determined simultaneously. Comparisons indicate the effect of temperature, time, and pretreating gases. These pretreatments of Incoloy 800 reduced the levels of coke formation during pyrolysis.4 Preparation of Incoloy 800 before Pretreatment The Incoloy 800 coupons used had dimensions that averaged 20 × 6 × 1.0 mm. Most of the coupons used were first ground (and polished) using grit no. 320 SiC paper and then ground using grit no. 400 SiC paper. Such grinding first reduced the surface roughness and removed the thin surface layer enriched with Fe and Ni. Four readings at different locations were made of the unpolished surface using EDAX at 15 kV, and three readings were made for the polished surfaces. These readings are the average compositions in the top layer of about 0-0.62 µm depth for Cr and other metals. The averages of these readings are shown in columns 1 and 2 of Table 1. The polished surface had a higher Cr concentration and apparently also Mn and Si concentrations. Several cross-sectional areas were investigated at depths of 6-1000 µm below the polished surface. These cross-sectional areas were produced by sawing the coupon into two pieces. Small but apparently significant differences of the Cr and Al concentrations were noted between the polished surfaces and the crosssectional surfaces, as shown in Table 1. Analyses of other coupons indicated similar compositions. Several coupons were cut to expose the cross sections, which were then examined. The compositions at a given cross-sectional area of an Incoloy 800 coupon often differed rather significantly, at depths from 6 to 1000 µm. Black or gray areas were sometimes noted as observed by the scanning electron microscope. EDAX
analyses of such an area often indicated a quite different composition as compared to those of other areas. The cross section of one coupon indicated the following ranges of compositions at depths from 6 to 1000 µm: Cr from 20.0 to 21.2 atomic %; Fe from 40.9% (a dark spot) to 44.9%; and Ni from 28.5 to 32.1%. Significant variations were also noted for Al, Si, and Mn compositions. The Al concentrations varied by essentially a straightline relationship from 1.75% at 6 µm depth to 2.75% at 1000 µm. For the top and bottom surfaces of the coupons, similar composition variations were noted. On these latter two surfaces, the effectiveness of grinding (and polishing) could have been a factor but was probably not a major one. Effect of the Temperature of Pretreatment Luan4 pretreated Incoloy 800 coupons at temperatures varying from 660 to 1000 °C. In one series of runs, the pretreatments were made using a 50:1 molar ratio of hydrogen/steam for 4 h. EDAX results were obtained at settings from 10 to 35 kV. The EDAX reading with a given kilovolt for a specific metal indicates the average concentration of that metal extending from the surface of the coupon to a given depth below the surface. These depths have been reported4 as 0.25, 0.62, 1.10, 1.67, 2.31, 3.03, and 3.66 µm for Cr at 10, 15, 20, 25, 30, 35, and 35 kV, respectively. Similar depths have also been reported for other metals. The EDAX results obtained by Luan were used, as next explained, to predict the actual concentrations of Cr, Mn, Fe, and Ni at depths up to at least 3.0 µm. The EDAX reading for Cr at 10 kV was assumed to be the average Cr percent in the 0-0.25 µm layer; the validity of this assumption will be discussed later. The EDAX reading at 15 kV was assumed to be the average Cr composition in the layer from 0 to 0.62 µm layer. By taking a Cr balance, the amount of Cr in the 0.25-0.62 µm layer can be calculated. Hence, the average Cr concentration in this layer can be calculated. This calculation procedure was continued to determine the average concentrations in layers with depths up to 3.03 or even 3.66 µm. This procedure was also employed for the other metals. Curves were then constructed to predict the Cr, Mn, Fe, and Ni compositions versus depth at 660, 760, 860, 950, and 1000 °C. The following were plotted: average composition in each layer versus the midpoint (or middepth) of the layer, as shown in Figures 1-4. The following were all considered in constructing the curves: (a) Each predicted curve was drawn so that for each layer the area of the curve above the average composition equals the area below. Consequently, whenever an average composition point as plotted in Figures 1, 3, and 4 is close to a maximum or a minimum, the predicted curve needs to miss that point. For other points, the curve often passes through or close to them. (b) The sum of Cr, Mn, Fe, and Ni concentrations as predicted for a specific depth should be about 96-98%. The additional 2-4% is the sum of concentrations for Al, Si, and Ti. To accomplish this, the thickness of the total enriched layer of Cr is essentially equal to the thicknesses of the total depleted layers for Fe and Ni. The thickness of the enriched layer for Mn is less, often significantly. (c) For each metal, curves at different temperatures have similar shapes and slopes. In Figure 1 at 860, 950, and 1000 °C and possibly 760 °C, the Cr concentrations exhibited maxima of about
Ind. Eng. Chem. Res., Vol. 42, No. 20, 2003 4743
Figure 1. Predicted Cr concentration as a function of the depth for Incoloy 800 coupons pretreated with H2/H2O ) 50 after 4 h at temperatures of 660-1000 °C.
Figure 2. Predicted Mn concentration as a function of the depth for Incoloy 800 coupons pretreated wtih H2/H2O ) 50 after 4 h at temperatures of 660-1000 °C.
78-80%. The Cr concentration in the untreated coupon was about 22.8%; see Table 1. Hence, the Cr concentration had been enriched at the maximum by a factor of about 3.4-3.5. The maxima occurred at increased depths as the pretreatment temperature increased. For runs at 860, 950, and 1000 °C, Cr concentrations decreased to about 60-70% as the surface was approached. At depths greater than the maxima, the Cr concentrations decreased, eventually to minimum concentrations of about 16%. These Cr-depleted zones started at depths of about 0.45-3 µm for pretreatment temperatures of 660-1000 °C, respectively. Figure 2 shows that considerable Mn enrichment occurred near the surfaces of the coupons. With increased temperatures of pretreatment, the Mn concentrations increased from about 0.8% (see Table 1) to at least 6% at 660 °C and to 29% at 1000 °C. Even higher concentrations may have occurred in the layer extending from 0 to 0.1 µm. Mn concentrations probably increased especially with higher temperatures by a factor of at least 35. The enrichment layers for Mn tended to be much thinner as compared to the Cr enrichment layer. The Mn enrichment zones varied from 0-0.2 µm at 660 °C to 0-1.4 µm at 1000 °C. Based on Mn material balance considerations, the Mn depletion zone (with Mn concentrations 50 µm on occasion. As shown in Figures 3 and 4, considerable Fe and Ni were transferred inward from the surface layers as a result of the pretreatments. At or near the surface, the concentrations of both Fe and Ni were reduced from
Figure 3. Predicted Fe concentration as a function of the depth for Incoloy 800 coupons pretreated wtih H2/H2O ) 50 after 4 h at temperatures of 660-1000 °C.
Figure 4. Predicted Ni concentration as a function of the depth for Incoloy 800 coupons pretreated wtih H2/H2O ) 50 after 4 h at temperatures of 660-1000 °C.
about 42.8% and 30.3%, respectively (see Table 1), to
