30
Ind. Eng. Chem. Prod. Res. Dev.
This capability eliminates the costly need for a separate hydrogenation stage found in a conventional oxo process. Total synthesis gas pressures influence the economics of the oxonation process since reactor vessel costs are reflected in pressure requirements. A BOC process more closely approaches the conventional oxo process (3500 psig) in its pressure requirements, i.e., about 2500-3500 psig. Unlike the conventional process that uses high pressures to stabilize the catalyst, high pressures in the BOC process are required mainly to improve olefin conversion. The Shell process is effective at lower synthesis gas pressures (ca. 1000 psig), which leads to savings in reactor costs. Any economically viable oxo process is expected to yield high overall olefin conversions and good selectivity to linear products to achieve commercialization status. A BOC process would meet these requirements. Conversion of higher olefins in a BOC process is as good as conversion levels achieved in the two major processes, although reaction rates are slower leading to increased residence time. Selectivity to form linear products in the BOC process is higher than in the conventional process but slightly lower than in the Shell process. The conventional oxo process uses pure dicobalt octacarbonyl, which is not readily adaptable to continuous recycle and must be removed from the oxo product in a separate decobalting stage. Catalyst efficiency in the conventional oxo process is far from ideal in spite of the lower catalyst cost. The initial catalyst costs for the Shell process are higher since an additional component, trialkylphosphine, has been introduced. However, the catalyst is capable of recycle, thereby improving catalyst efficiency of the process. Initial catalyst costs for a BOC
1986,25,30-33
system increase over the Shell catalyst, mainly due to cost differences between the major catalysts, i.e., M(C0)6in the BOC process and C O ~ ( C Oin) ~the Shell process; both use a trialkylphosphine and both catalyst systems are capable of recycle. A major consideration for developing the BOC system for hydroformylating high molecular weight olefins was to provide a unique and commercially viable catalyst based on domestically produced metals. If a stable international climate proves possible for imported strategic metals such as cobalt or rhodium, group 6 metals may remain shelved as potential oxo catalysts. If, however, any major disruptions in availability of strategic metals such as cobalt and rhodium (mined in South Africa) or rapidly escalating costs due to shortages should occur, a backup catalyst system should be available. The BOC system using mainly group 6 metals in combination with trace amounts of cobalt represents such a system. Registry No. Cr(CO)&PBu,), 18497-59-1;Cr(CO)6, 13007-92-6; HCO(CO)~(PBU,), 20161-43-7; tri-n-butylphosphine, 998-40-3; 1-hexene, 592-41-6; 1-heptanal, 111-71-7; 1-heptanol, 111-70-6; 1-methylhexanal, 110-43-0; 2-ethylpentanal, 22092-54-2; 1methylhexanol, 543-49-7; 2-ethylpentanol, 27522-11-8.
Literature Cited Tucci, E. R. Ind. Eng. Chem. Prod. Res. Dev. 1988, 7 , 32, 125. Tucci, E. R. Ind. Eng. Chem. Prod. Res. Dev. 1985, 2 4 , 38. Wender, I.; Pino, P. "Organic Syntheses Via Metal Carbonyls"; Wiley: New York, 1977; Vol. 2, p 197. Wolfgang, R.; McCoy, J. J.; Orchin, M. I n d . f n g . Chem. Prod. Res. Dev. 1971, 10, 142. Received f o r review February Revised m a n u s c r i p t received July Accepted September
28, 1985 18, 1985 17, 1985
Hydrodesulfurization of Petroleum Coke Deposited on Iron Ores Isao Mochida, * Toshinori Marutsuka,+ Yoro Korai, and Hiroshi Fujitsu Research Institute of Industrial Science and Department of Molecular Engineering, Faculty of Engineering Sciences, Kyushu University, Kasuga 8 16, Fukuoka, Japan
Hydrodesulfurization of the coke deposited on iron ores was studied at 650-950 O C to find procedures to reduce the sulfur content acceptable for the direct reduction of iron ores. The desulfurization of as-received coke took place at 950 O C and achieved desulfurization level of 83 % . The lower temperature decreased not only the rate of desulfurization but also the level achieved within a definite time (10 h). The many cracks found in the coke desulfurized at 950 OC suggest the importance of the accessibility of sulfur to hydogen molecules. The grinding and heat treatment at 1000 OC prior to the hydrodesulfurization enhanced the removal at 750 OC to achieve the same desulfurization level as that at 950 OC with the as-received coke. The structure and reactivity of the coke in terms of its sulfur contaminants are discussed from the view of their physical location in the lump coke.
Introduction Upgrading of heavily contaminated petroleum residue is a present target for the petroleum industry. KKI (Kobe Steel, Ltd., Koa Oil Co., Ltd., and Idemitsu Kosan Co., Ltd.) proposed its pyrolysis catalyzed on the fluidized iron ore, where its heavy end was deposited as the coke (Mori et al., 1982). The coke is further utilized as the reductant in the successive step for the direct reduction of the iron ore. A problem to be solved is to reduce the sulfur content in the coke since the sulfur reacts with the iron ore during 'Department of Molecular Engineering. 0196-4321/86/1225-0030$01.50/0
the reduction to form iron sulfide, which deteriorates the quality of the sponge iron for steel production. Although some desulfurization processes of petroleum cokes have been reported (Vrbanovic, 1983; Brandt and Kapner, 1984), hydrodesulfurization appears most promising (Takanari and Tanaka, 1973; Ohashi and Sugawara, 1984). In the present study, hydrodesulfurization of the coke deposited on the iron ore was investigated to find procedures to reduce the sulfur content acceptable for the steel industry at lower temperatures than that where the reduction of iron ore may take place. The reactivity of sulfur can be principally correlated to its location in the coke, since the pyrolysis may liberate reactive organic sulfur 0 1986 American
Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25. No. 1. 1986 31
Table I. Chemical Properties (wt W )of Iron Ore (Rio Doee) total Fe FeO SiO, AW, CaO 64.48
0.11
5.49
0.06
1.23
Me
,0.06
P
S
0.036
0.026
Fe,O* 92.i8"
__
total oxwen' 27.75
"Oxygen ratio in iron oxide 1001
Table 11. Soecific Surface Area of KWC-IO
I
IPVPI ... .. of ..
KWC-IO original HDS at 150 OC 950 "C HDS at 750 OC after heat treatment (1000 "C, 10 m i d HDS at 750 "C (ground KWC-IO)b
SSA, m2/g
HDS,"wt 70
1.42 1.90 24.7 17.9
41 83 78
7.7
82
'HDS = hydrodesulfurization. bPPartielesize distribution, 100
5
10
100 pm; mean particle size, 45 p m .
28
T i m (hr)
Figure 1. Desulfurization profiles of KWC-IO at variable temperatures: 950 ' c (0); 850 "c(0); 750 'c (0); 650 'c (A).
leaving only heteropolycyclic ones in the coke. Thus, the accessibility of sulfur in lump coke to hydrogen is the major factor in determining the level of sulfur removal. Experimental Section Materials. Kuwait coke deposited on the iron ore (KWC-IO) produced by the KKI process was supplied by Kobe Steel, Ltd. (Elemental analyses: C, 24.46 wt %; H, 0.73 w t %; N, 0.44 wt %; S, 2.36 wt %.) Chemical properties of the iron ore (Rio Doce) are shown in Table I. Hydrodesulfurization. The coke sample in a packed bed was heated in an IR image furnace (ULVAC RHLE45N) under nitrogen flow to the prescribed temperature, where the gas stream was switched to hydrogen for desulfurization (normal flow rate, 48 mL/min). Sulfur removed as HzS was trapped as CdS in aqueous Cd(OAc), to be quantified. Sulfur in the coke was also analyzed by automatic sulfur determination apparatus (TAMEC QSA2). Two procedures provided the consistent results. KWC-IO (particle size distribution, 7W230 r m ; mean particle size, 140 rm) was ground by an automatic miller and by hand in a mortar with a pestle. Particle sizes of cokes after grinding were estimated by S E M ground KWC-IO (distribution, 10-1M) rm; mean, 45 wm); ground KWC-IO after hydrodesulfurization (distribution, 10-80 rm; mean, 40 rm). Observation by S c a n n i n g Electron Microscopy (SEM) a n d X-ray Micro Analyzer (XMA). Hydro-
desulfurization was observed by SEM (JEOL JSM-25s). Sulfur distribution on the coke surface was analyzed by XMA (Kevex UX 7100). Surface Areas of Cokes. Surface areas of cokes were measured by BET using nitrogen. Results Hydrodesulfurization of As-Received Cokes. Figure 1 shows the hydrodesulfurization profiles of as-received KWC-IO a t several temperatures. The desulfurization took place rather rapidly in an initial few hours and then slowed and finally leveled off a t the respective levels a t these temperatures, although both initial rates and final levels of desulfurization were very dependent on the temperature. At 950 "C, the bulk of the desulfurization took place rapidly, within 2 h 77% of the sulfur was removed, whereas removal reached only 60% a t 850 "C after 10 h and only 37% at 650 OC after 28 h. Surface areas of cokes are summarized in Table 11, which indicates a significant increase after desulfurization especially a t 950 "C. Figure 2 shows the SEM microphotographs of coke surface before and after the hydrodesulfurization a t 650 and 950 "C. The surface of the original coke was not smooth; however, neither pores nor cracks were observable (Figure 2a). In contrast, the desulfurization induced on the surface many cracks, of which number and size were critically dependent upon the desulfurization temperature: a higher temperature induced more cracks of larger width and length. XMA observation of the coke before and after the desulfurization at 950 "C indicated no increase of sulfur in
ET
C)
,
l0OP
,
Figure 2. Microphotographs of KWC-IO surface: (a) as-received coke; (b) after desulfurization at 650 ' C (c) after desulfurization at 950 OC.
32
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 1, 1986 i
4
0
Figure 3. Effects of grinding on desulfurization a t 850 "C: (0) nontreatment; ( 0 )grinding treatment.
-'-I-
-I
5 10 Time(hr) Figure 5. Influence of pretreatment heating rate on desulfurization a t 750 "C. Pretreatment: 1000 "C, 10 min.
F t v
-I
1000°C
, (
KWC-IO
0
5
10
1 I 4
10 14 Time(hr) Figure 4. Effects of grinding on desulfurization a t some temperatures: 850 "c ( 0 ) ;750 "c (@; 650 "c (A).
Figure 6. Influence of pretreatment temperature and time on desulfurization at 750 "C. Pretreatment: 100 "C/min, 10 min.
the iron region by the reaction. No formation of FeS may be due to excess hydrogen under the present conditions. Influence of Grinding on the Hydrodesulfurization. The hydrodesulfurization profiles of KWC-IO at 850 "C are illustrated in Figure 3, where the influences of grinding are indicated. As described above, the desulfurization ratio of the as received coke leveled off at 60% at 850 "C after 10 h, where the cokes were ground into smaller particles (diameter 10-80 pm, average 40 pm) before the second desulfurization. The second treatment after grinding allowed further desulfurization of 20%, the total desulfurization (80%) at this temperature being comparable to that achieved at 950 O C shown in Figure 1. The ground coke before the desulfurization was very reactive at 850 "C, allowing the high level of desulfurization in the initial 1 h where 72% of the sulfur was removed. The desulfurization rate after 1 h was very slow and gave the final desulfurization level of 75%. The rate of initial desulfurization (within 30 min) of ground coke was the same as the as-received one. The levels of desulfurization of these ground KWC-IO cokes were slightly but distinctly different, corresponding to the sizes and surface areas of ground cokes (Table 11). The desulfurization profiles of ground KWC-IO cokes at some temperatures are shown in Figure 4. Although the desulfurization rate after the initial 4 h was very dependent on the temperature, as described above, the desulfurization level was much the same (75--80%) regardless of the temperature. Influence of Preheating Treatment in Nitrogen Atmosphere. The desulfurization profiles of KWC-IO preheated at 1000 "C in nitrogen atmosphere are illustrated in Figure 5, where the desulfurization was carried out at 750 "C. The desulfurization was very much enhanced by the heat treatment and achieved removal of sulfur of about 70%. The level of desulfurization was comparable to that of the as-received coke at 950 "C. The
major desulfurization took place within 2.5 h. The heating rate did not essentially influence the desulfurization level. The heat treatment increased the surface area to the level observed after the desulfurization at 950 "C, as shown in Table 11. The desulfurization profiles of cokes preheated for 10 min at some temperatures are illustrated in Figure 6, where the hydrodesulfurization was carried out at 750 "C. The temperature of the preheating treatment was very influential on the final desulfurization level, 78% removal being achieved a t 750 "C after the heat treatment at 1000 "C, while the treatment at 800 "C was not effective at all. The heat treatment at 900 "C was effective in achieving the desulfurization level of 60%. Longer heat treatment (60 min) at the same temperature further increased the level by 8% to 68%. It is noted that the initial rates within 30 min were the same regardless of the heat treatment. Discussion Coke Structure and Desulfurization Mechanism. The extent of sulfur removal by hydrogen from the coke was found to be strongly dependent on the desulfurization temperatures in the present study. It is worthwhile to discuss the reactivity of sulfur in the coke against hydrogen molecules, which may dissociate into atoms in the vicinity of a sulfur atom on the carbon surface. The sulfur in the coke produced around 500 "C are assumed to be present almost exclusively in the polycondensed aromatic rings, since those of the other types may be eliminated easily during the pyrolysis. Thus, their chemical structure may be estimated to be rather similar, so that the reactivity of sulfurs is concluded to be influenced principally by their location in the lump coke, indicating their differentiation by the physical accessibility against gaseous hydrogen. The sulfur accessible to hydrogen can be removed within a definite time, although the rate is influenced by temperature, as usual for any chemical reaction. In contrast, the
0
5
Time (hr)
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 1, 1986
/Crack
Figure 7. Model of a coke grain in terms of its sulfur contaminants.
sulfur located inside the lump coke is difficult to contact to the gaseous hydrogen and is not removed within a definite time unless the structural changes of the lump coke take place. Hydrogen molecules can penetrate into the pores and cracks of the coke if they are large enough. Such a coke structure in terms of sulfur location is schematically illustrated in Figure 7, where the sulfur atoms can be classified into three categories according to their location; on the outer surface, on the walls of pores and cracks, and inside the coke surrounded by the dense wall. According to the scheme described above, the level of desulfurization achieved within a definite time is principally defined by the accessibility to hydrogen or effective surface area of the coke since the sulfurs are distributed rather evenly through the coke, although the rate of desulfurization is strongly dependent upon the temperature. The variable level of desulfurization at variable temperatures is thus explained in terms of variable extents of crack development due to the thermal expansion and shrinkage at the reaction temperature, as clearly indicated by Figure 2. Such an understanding leads directly to the procedures for the enhancement of desulfurization level; the principle is to enlarge the accessibility. Both grinding and heat treatment in the inert atmosphere increased the extent of desulfurization, allowing a similar achievement at 750 "C to that at 950 "C with the as-received coke. Another approach of partial gasification to increase the accessibility will be described in a following study. Roles of Iron Ore in the Hydrodesulfurization. Hydrogen atoms produced by the dissociation of hydrogen
33
molecules may react with a sulfur in the heterocyclic ring as described above. Such dissociation may take place on the active site of coke (surface radicals) at elevated temperatures or on some metallic impurities. The ore, which contains a considerable amount of iron oxide as some reduced forms, may behave as a catalyst for the activation of hydrogen molecule. The hydrogen atom produced on the catalyst can spill over to the sulfur atom to be liberated in hydrogen sulfide (Sermon and Bond, 1973). The catalytic roles of iron ore for the hydrogen activation have been extensively studied in coal liquefaction (Morita et al., 1980; Okada et al., 1984) and gasification (Walker et al., 1968; McKee, 1981). Another role of iron ore is to induce pores and cracks according to its gasification activity when partially reduced (McKee, 1981) and to differences in its thermal expansion relative to that of the coke (Mrozowski, 1956),respectively. The former aspect of enhancement of the desulfurization is now in progress. Further study is required to establish its major role.
Acknowledgment We are grateful to Kobe Steel, Ltd., and Koa Oil Co., Ltd., for their financial support and to Drs. N. Mori and E. Miura for their helpful discussion.
Literature Cited Brandt, H. H.; Kapner, R. S . Prepr.-Am. Chem. SOC.,Div. Pet. Chem. 1984,2 9 , 453. McKee, D. W. Chem. Phys. Carbon 1981, 16, 1 . Mori, K.; Kaneko, D.; Taniuchi, M.; Morimitsu, T.; Ijiri, R. Pan-Pac. Synfuels Conf. 1982 1982, 2,574. Morita, M.; Hashimoto, T.; Sato, S . ; Toyoshima, H.; Itoh, T. J . Jpn. Chem. SOC. 1980, 6,931. Mrozowski, S . R o c . Conf. Carbon, Ist, 2nd 1956,31. Ohashi, H.; Sugawara, T. "Research on Coal Liquefaction and Gasification"; Ministry of Education, Science and Culture, Japan, 1984; Spey 12, 135. Okada, T.; Fukuyama, T.; Takekawa, T.; Matsubara, K. J . Fuel SOC.Jpn. 1984, IO), 853. Sermon, P. A,; Bond, G. C. Cafal.Rev 1973,8,211. Takanari, N.; Tanaka, T. J . Jpn. Pet. Insf. 1973, 16(5),59. Vrbanovic, 2. High Temp.-High Pressures 1983, 15, 107. Walker, P. L.,Jr.; Shelef, M.; Anderson, R. A. Chem. Phys. Carbon 1966,4 , 287.
Received for review November 27, 1984 Revised manuscript received July 24, 1985 Accepted October 23, 1985
