Ind. Eng. Chem. Res. 2010, 49, 1991–1994
1991
Pretreatments of Coils to Minimize Coke Formation in Ethylene Furnaces Zhaobin Zhang Beijing Research Institute of Chemical Industry, SINOPEC, Beijing 100013, China
Lyle F. Albright* Forney Hall of Chemical Engineering, Purdue UniVersity, West Lafayette, Indiana 47907
Numerous techniques have been considered on how to reduce coke formation and/or coke collection on the inner surfaces of the coils in furnaces producing ethylene, propylene, etc. Such reduction would lead to several beneficial events including much reduced production costs. The techniques tested by Nova Chemical in several of their furnaces have resulted in a major reduction of coke. They have tested several pretreatments of the coils in their furnaces using several oxidizing gases at 800-1200 °C. For example, hydrogen/steam mixtures have been employed. Various pretreatments have been employed in the current study that suggest improved pretreatments are now possible. Introduction All ethylene furnaces until about 10 years ago had to be decoked after 30-60 days to remove the coke that collected in the coil. Such coke has several adverse effects. First yields of ethylene and other desired hydrocarbons are reduced. Second resistances to heat transfer from the hot combustion gases to the hydrocarbon gases being cracked increase. Hence, energy demands for operating the furnaces increase. Third, resistances to the gas flow in the coils increases as the thickness of the coke increases. Fourth when the furnaces are decoked, production of the desired products is stopped for significant time periods. Three different coking mechanisms have been identified in the coils and transfer line exchanges of the ethylene furnaces.1,2 Iron and nickel catalyze the formation of carbon (or coke) filaments, via mechanism 1. These filaments promote coke growth via mechanisms 2 and 3. These filaments act as collection sets for tiny tar droplets suspended in the gas phase (mechanism 2). These droplets then decompose to form coke and release hydrogen. In mechanism 3, the free radicals on the surfaces of the filaments react with hydrocarbon free radicals in the gas phase. As a result, the filaments grow in diameter and to some extent in height. Albright3 reported an example of the compositional changes with time (and use) on the inner surfaces of coil of an industrial furnace. The chromium content had increased sometimes by a factor as high as two. Meanwhile the iron and nickel contents at the surface had decreased substantially. At that time, no attempt was made to measure the manganese concentration; only later was it discovered that this concentration had increased several fold. Obviously, the compositions of the inner surfaces of the coils are of significant importance since the rates of coke formation and/or collection are considerably higher in new coils. The following coils have been found to have reduced coke formation in industrial furnaces: an aluminized surface produced by the Alon Processing Company4 and a siliconized surface developed by the British Petroleum Company.5 Unfortunately, the longevities of both coils were relatively short so neither was judged suitable for industrial applications. * To whom correspondence should be addressed. E-mail:
[email protected].
In 1984, Horseley and Cairns6 publicized a technique for pretreating stainless steels in the presence of a gaseous mixture of hydrogen and steam at high temperatures. As a result, the surfaces of the steel were enriched in Cr and Mn, but depleted in Fe and Ni. They indicated that oxides of Cr and Mn formed at and near the metal surface. Such oxides formed Cr-Mn spinels. Simultaneously Fe and Ni atoms migrated inwardly forming sublayers enriched with these elements. The findings suggested such pretreatments might be beneficial to coils to be employed in ethylene furnaces. Szechy, Luan, and Albright7 and Luan8 confirmed that such pretreatments were promising for ethylene furnaces. Shell Chemical Company9 also confirmed the potential benefits. Nova Chemical Company has tested pretreated coils in 41 commercial ethylene furnaces.10,11 With a newly treated coil, the time that the furnace can be operated before decoking often varied from 420 to 505 days. Shorter times occurred being about 150-200 days after the second through fourth decokings. As a comparison with nonpretreated coils, the times before decoking were about 40 days. Nova has obtained a significant number of patents on pretreated coils. The exact pretreating procedure that they employ is unknown, but it seems to be similar to ones investigated at Purdue University. Nova indicates in their publications that spinels having the composition of MnxCr3-xO4 are desired on the surface layer. Simultaneously Fe and Ni atoms migrate inward forming a sublayer enriched in these two elements. Analysis of Surface Layers of Pretreated Stainless Steels The calculation procedure developed by Luan et al.12 was used to determine the amounts of Cr, Mn, Fe, and Ni transferred to or from the surface of a pretreated steel. Energy dispersive X-ray analyzer (EDAX) readings at 10-35 kV levels were employed to determine first the average weight percent of each metal element for several layers between 0 and about 3 µm below the surface. Then a curve was drawn for that element to predict the composition as a function of depth. The EDAX results sometimes indicate that minor amounts of Si, Al, and Ti are sometimes present. To help in drawing the curve for a specific element, the concentration of these four to seven elements should add to 100% at any specific depth below the surface. The amount (A) of each element transferred to or from the surface layer can be calculated with the following equation:
10.1021/ie900271q 2010 American Chemical Society Published on Web 01/15/2010
1992
Ind. Eng. Chem. Res., Vol. 49, No. 4, 2010
A)
∫ (F)(% metal)(d micrometers) x
0
Where, x is the thickness in micrometers of the enriched or depleted surface layer, F is the initial density of the original stainless steel expressed in grams per cubic centimeter, and % metal is the change of the weight percent at a given depth. Graphical integration of the curves constructed is used to calculate values of A for each metal element for the pretreated surface. The actual grams transferred per unit surface area (cm2) can be determined as follows: g/cm2 ) A/(F × 106) In making these calculations, two assumptions are made. First the distance that the X-rays penetrate below the surface for a given kilovolt value does not change. Yet it is known that Cr and Mn oxides (and spinels) form near the surface. Second, the densities of the surface layers do not change. Although both assumptions are questionable, comparison of A values as calculated are thought to be relatively quantitative. Luan et al.12 earlier reported the results for numerous pretreatments of Incoloy 800. In the present investigation, considerably more results are reportedsfor additional stainless steels, for pretreatment times varying from 0.25 to 24 h, and for several different pretreating gases. For several pretreated surfaces, coke failed to adhere to the surface during subsequent coking experiments. This finding suggests that such a pretreatment might result in a much improved coil in an ethylene furnace.
Figure 1. Amounts of Cr, Fe, and Ni transferred at temperatures from 760 to 1000 °C for Incoloy 800: used H2/H2O ) 50 for 4 h pretreatments.
Pretreatments of Different High-Alloy Steels Figures 1 and 2 show comparisons of the calculated value of A for pretreated Incoloy 800 and HK 40. The pretreatments were made using a 50:1 molar mixture of H2/H2O at four temperatures varying from 750 to 1000 °C with a pretreatment time of 4 h. For Incoloy 800, considerably more Cr, Fe, and Ni was transferred at a given temperaturesoften by factors of 3 or more. For this stainless steel, the relative amounts of Mn transferred were lower, in the 1.1-1.8 range. The A values for Incoloy 800 are considered more accurate since five kilovolt values were available for the calculations as compared to four with HK 40. Even though the A values for Incoloy 800 are considerably higher, the compositions of the two stainless steels were fairly similar prior to pretreatments, as shown in Table 1. The results with Incoloy 800 at 760, 850, 950, and 1000 °C indicate that plots of ln A verses 1/T(K) are essentially straight lines for each of the four metal elements, i.e. Arrhenius-type plots occur. The results with HK 40 although less reliable also can be represented by similar straight line correlations having essentially identical slopes as compared to Incoloy 800. The thicknesses of the enrichment layers for Cr and Mn were also calculated using the EDAX results. These layers for Cr and Mn are defined as those having higher concentrations of Cr or Mn than were present in the nontreated steel. These thicknesses increased in each case as the amounts of Cr and Mn transferred increased. For Incoloy 800, the thickness for Cr was about 3.5 µm for the pretreatment at 1000 °C. The thickness of the enrichment layer was found to be essentially directly proportional to the amount of the metal transferred. Calculating the thicknesses of the depletion layer near the surface for Fe and Ni is considered to be less accurate as compared to the thicknesses of the enrichment layer. These thicknesses of the depletion layer are however also probably directly proportional to the amounts transferred. The thickness
Figure 2. Amounts of Cr, Mn, Fe, and Ni transferred at temperatures from 760 to 1000 °C for HK 40: used H2/H2O ) 50 for 4 h pretreatments. Table 1. Composition of Stainless Steels Prior to Pretreatments elements stainless steel
Al
Si
Ti
Cr
Mn
Fe
Ni
Incoloy 800 HK 40 HP 40
0.7 0.2 0.2
2.0 4.7 4.1
0.4
22.8 25.2 26.7
0.2 0.9 1.5
42.7 48.6 33.9
30.3 18.6 32.8
of the enrichment and depletion zones for a given pretreatment differ somewhat. For Mn enrichment at and near the surface, obviously most of the Mn diffuses from relatively large depths. For Incoloy 800, the Mn concentration near the surface sometimes was as high as 30% whereas the concentration in the untreated steel was about 0.7%. In such a case, the Mn concentration increased by a factor of over 40. It is estimated that part of the Mn had diffused from depths of 75-100 µm. A preliminary estimate can be made of the amounts of metals transferred to and from the surfaces of the stainless steel, HP 40; a H2/H2O ) 50 mixture was used at 950 °C. With this stainless steel, only three kilovolt values were employed, permitting analyses at depths up to only about 2 µm. For such pretreatments, the amounts of metals transferred with HP 40 are approximately equal to those for Incoloy 800. Times of Pretreatment Figure 3 and 4 indicate the amounts of Cr, Mn, Fe, and Ni transferred in Incoloy 800 and HK 40 respectively as the time
Ind. Eng. Chem. Res., Vol. 49, No. 4, 2010
1993
Table 2. Transfer Amounts Using Several Gases for Pretreatments of Incoloy 800 at 950 °C for 4 h pretreating gas
Cr
Mn
Fe
Ni
air CO2 CO CO2/H2O ) 50 H2/H2O ) 50
120 117 76 105 58
12 11 11 13 13
72 73 52 75 35
57 57 39 41 29
Table 3. Transfer Amounts Using Several Gases for Pretreatments of HK 40 at 950°C for 12 h
Figure 3. Amounts of Cr, Mn, Fe, and Ni transferred in Incoloy 800 at pretreatment times up to 24 h; pretreating gas used H2/H2O ) 50, at 950 °C.
pretreating gas
Cr
Mn
Fe
Ni
CO H2/H2O ) 3 H2/H2O ) 50
27 39 72
6 10 14
20 33 49
10 15 23
a given time of pretreatment varied in the following decreasing order: Cr, Fe, Ni, and Mn. These pretreatments provide new information on the phenomena occurring at and near the surfaces of the steels. Initially, the transfers of Cr and Mn toward the surface are through relatively pure metal atoms. Both of these transfers are apparently relatively rapid. Meanwhile, the oxide layer formsschromium oxides are initially predominant building up to concentrations of 50-70%. These oxides are both heavier and physically larger in size. Transfers of elemental metals in this layer are apparently slower. Eventually the Cr concentration builds up to about 70% and then decreases as the manganese concentration increases to 30%. Elemental Mn is apparently diffusing through the oxide layer. At the surface, the oxides of chromium and manganese react to form spinels.10,11 Diffusion of oxygen from the pretreating gases to sublayers below the surfaces of the steels is apparently of little or no importance. Different Pretreating Gases
Figure 4. Amounts of Cr, Mn, Fe, and Ni transferred in HK 40 at pretreatment times up to 24 h; pretreating gas used H2/H2O ) 50, at 950 °C.
Comparative runs were made to determine the amounts of metal transferred in Incoloy 800 using five different pretreating gases at 950 °C for 4 h. The amounts transferred sometimes differed substantially as indicated in Table 2. For Cr, Fe, and Ni, the amounts transferred varied by a factor of over 2 for air (having the most transfer) and H2/H2O ) 50 (having the least). Yet the A of Mn transferred varied only between 11 and 13 for the five gases (air, CO2, CO, CO/H2O ) 50, and H2/H2O ) 50). Air obviously contained the largest concentration of oxygen or oxygen free radicals. The similar transfer of Mn can be explained in part at least by the much greater distances that it is transferred in Incoloy 800. Such transfer is apparently a ratecontrolling step. Three gases were used to pretreat the stainless steel HK 40 at 950 °C for 12 h. As indicated in Table 3, the largest transfers of all four metals occurred with H2/H2O ) 50 whereas the smallest transfers occurred with CO. H2/H2O ) 3 resulted in intermediate transfer amounts. Yet, larger transfer amounts occurred with CO pretreatments of Incoloy 800, as shown in Table 2. Coking Experiments
of pretreatment increased to 24 h. These pretreatments were with a 50:1 molar ratio of hydrogen to steam at 950 °C. Very large changes of the metal composition occur at and near the surface in the first hour. For the remainder of the run, the rates of metals transferred were much slower and relatively constant. For these experiments, much larger amounts of metals transferred with Incoloy 800. The amounts of metal transferred for
Several pretreated coupons were first weighed and then positioned in the laboratory tubular furnace for coking experiments. There is a 3:1 molar ratio of ethane and steam was passed over each coupon for 1.5 h at 850 °C causing some coke to collect on the coupon. Various coupons were tested as follows: (a) Each coupon was weighed after coking. The weight gain after coking was considered to be the weight of the coke
1994
Ind. Eng. Chem. Res., Vol. 49, No. 4, 2010
collected on the coupon. Knowing the dimensions of the coupon, the amount of coke collected per squared centimeter of the coupon was calculated. The coupon was handled carefully to ensure that no coke was lost during handling. (b) The coupon was then brushed with a soft nylon brush. Sometimes 80-100% of the coke was removed due to brushing. Visual observations suggest that more coke deposited on the top side of the coupon as compared to the bottom. Scanning electron microscopy (SEM) photographs indicated that globular coke particles were prevalent on the top side. Apparently tar droplets suspended in the gas phase collected primarily on the top side. These droplets after being deposited on the top surface then decomposed forming globular coke deposits. Apparently more coke on a relative basis could be brushed from the top side as compared to the bottom. During brushing, some metal particles can spall from the surface, but for the coupons examined, spalling was likely of minor importance. (c) Several coupons were further tested. After brushing, the coupon was returned to the furnace and contacted with a 3:1 molar mixture of H2/H2O at 900 °C for 2 h. Such a pretreatment acted to further decoke the coupon as indicated by a decrease in weight of the coupon. (d) For several coupons, coking followed decoking was repeated. For one coupon, this sequence was repeated four times. In this case, the fraction of coke that could be removed by brushing increased to about 100%. These coking tests indicated several promising results. First, the pretreatments of the coupon decreased the amount of coke deposited on the surface. Second the fraction of the coke that was removed by brushing increased toward 100% as the coupon was repeatedly coked then decoked. Third pretreating the coupon with CO at 850 °C for 24 h resulted in essentially no adherence of coke to the coupon. The lack of adherence suggests that filamentons coke formation is small at most. Such a finding is expected when Fe and Ni concentrations on the surfaces to the steel are small. The above results suggest that in industrial furnaces coke collection on the surfaces of the coil may be negligible with the proper pretreatments. The high velocity gases in industrial coils may remove all the coke from the surfaces. The resulting entrained coke or coke precursor would be transferred to and through the transfer line exchangers to the following scrubbing tower. Conclusions Although the current results are promising, additional research is still needed to determine the most promising pretreatment
technique. There is still the need to determine why the rates of metal transfer during a pretreatment sometimes vary so much for different high alloy steels. The size and character of the metal grains in the steel may be a factor. Determining the preferred composition of the Cr-Mn spinels is also still needed. More data are yet needed in the importance of so-called trace metals (Ti, Si, Al, etc.) in the stainless steel. Finally, the current results suggest that improved decoking procedures can be developed that will minimize coke formation and/or collection after decoking. Literature Cited (1) Baker, R. T. K.; Harris, F. S. Chemistry and Physics of Carbon; Walker, P.L., Thrower, P.A., Eds.; Marcel Dekker, Inc.: New York, 1978; Vol. 14, Chapter 1. (2) Albright, L. F.; Marek, J. C. Mechanistic Model for Formation of Coke in Pyrolysis Units Producing Ethylene. Ind. Eng. Chem. Res. 1988, 27, 755. (3) Albright, L. F. Metal Diffusion from Furnace Tubes Depends on Location. Oil Gas J. 1988, (Aug 15), 69–75. (4) Albright, L. F.; McGill, W. A. Aluminized Ethylene Furnaces Tubes Extend Operating Life. Oil Gas J. 1987, (Aug 31), 46. (5) Brown, D. E.; Clark, J. T. K.; Foster, A. J.; McCarroll, J. J.; Sims, M. L. Inhibition of Coke Formation in Ethylene Steam Cracking. In Coke Formation on Metal Surfaces; Albright, L.F., Baker, R.T.K., Eds.; ACS Symposium Series, no. 202; American Chemical Society: Washington, DC, 1982; Chapter 2, p 23. (6) Horsely, G. W.; Cairns, J. The Inhibition of Carbon Depositor on Stainless Steels by Prior Selective Oxidation. Appl. Surf. Sci. 1984, 18, 273–286. (7) Szechy, G.; Luan, T. C.; Albright, L. F. Pretreatment of High Alloy Steels to Minimize Coking on Ethylene Furnace. In NoVel Production Methods for Ethylene, Light Hydrocarbons, and Aromatics; Albright, L.F., Crynes, B.L., Nowak, S., Eds.; Marcel Dekker: New York, 1992; Chapter 18, pp 341-359. (8) Luan, T. C. Reduction of Coke Deposition in Ethylene Furnaces. Ph.D. Thesis, Purdue University, West Lafayette, IN, August 1993. (9) John, R. G.; Ngan, D. Y. Heat Treatment of High-Temperature Steels. U.K. Patent Application 2,234,530A, June 28, 1990. (10) Gyorffy, M.; Benum, L.; Sakamoto, N. Increased Run Length and Furnace Performance with Kobota and Nova Chemicals ANK 40 Anticoking Technology. AIChE 18th Ethylene Producers’ Conference, AIChE Spring Meeting, Orlando, FL, April 23-27, 2006. (11) Saunders, R.; Gyorffy, M. ANK 400 Anticoking Technlogy. Seminar, Houston, TX, Nov 30, 2006. (12) Luan, T. C.; Eckert, R. E.; Albright, L. F. Gaseous Pretreatment of High Alloy Steels Used in Ethylene Furnaces: Pretreatment of Incoloy 800. Ind. Eng. Chem. Res. 2003, 42, 4741–4747.
ReceiVed for reView February 18, 2009 ReVised manuscript receiVed September 25, 2009 Accepted November 24, 2009 IE900271Q
