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and Hydrogen Peroxide in a Semibatch Reactor. Ind. Eng. Chem. Res. 1989a, 28 (111, 1573-1580. Glaze, W. H.; Kang, J. W. Test of a Kinetic Model for the Oxidation of Organic Compounds With Ozone and Hydrogen Peroxide in a Semibatch Reactor. Ind. Eng. Chem. Res. 1989b, 28 ( l l ) , 1580-1587. Leavitt, D. D.; Horbath, J. S.; Abraham, M. A. Homogeneously Catalyzed Oxidation for the Destruction of Aqueous Organic Wastes. Enuiron. Prog. 1990, 9 (4), 222-228. Lin, P. K. H.; Sullivan, L. P.; Taguchi, G. Using Taguchi Methods in Quality Engineering. Qual. Prog. 1990, 23 (9), 55-59. Roy, R. K. A Primer On The Taguchi Method; Van-Nostrand Reinhold: New York, 1990, pp 40-71. Staehelin, J.; Hoign6, Decomposition of Ozone in Water in the Presence of Organic Solutes Acting as Promoters and Inhibitors of Radical Chain Reactions. Enuiron.Sci. Technol. 1985,19 (121, 1206-1213.
Tock, R. W.; Rege, M. A.; Bhojani, S. H. Simultaneoue Air Stripping and Advanced Oxidation Processes (AOP) for Process Water Treatment. Presented at the Summer Meeting of the American Institute of Chemical Engineers, Pittsburgh, PA, 1991; paper 28H.
* Author to whom correspondence should be addressed. Mahesh A. Rege,* Sanjay H.Bhojani Richard W.Tock, Raghu S. Narayan Department of Chemical Engineering Texas Tech University Lubbock, Texas 79409 Received for review April 8, 1991 Revised manuscript received September 11, 1991 Accepted September 27, 1991
Effects of Catalytic Hydrotreating on Light Cycle Oil Fuel Quality A pilot plant study was conducted to evaluate three commercial catalysts for hydrotreating of light cycle oil to reduce its aromatic content and improve the cetane index. The operating parameters were varied between 325 and 400 "C,1and 3 h-l, and 4 and 10 MPa at 535 L/L. The data showed that, in general, the product density and aromatic content decreased as the temperature or pressure increased or space velocity decreased. The cetane index improvement ranged from 7.3 to 10.0 for the Ni-W/A1203 catalyst and from 6.1 to 10.1 for the Ni-Mo/A1,03 catalysts. The catalyst performance was evaluated in terms of hydrodesulfurization, hydrodenitrogenation, hydrogenation, aromatic saturation, and hydrogen consumption. This study confirms that light cycle oil can be hydrotreated to improve its cetane quality, thus increasing the extent of its blending ratio into the diesel pool. Introduction The demand for high-quality middle distillates, transportation diesel, and jet fuel has grown significantly over the past decade. Currently, the diesel fuel consumption worldwide is growing at the rate of 5 % . The increased demand for diesel fuel occurred at the expense of heavy fuel oil consumption in most parts of the world. The decline in fuel oil demand, which is expected to continue in the 199(k,was a result of higher crude prices in the 19708 (Denny et al., 1990). In order to meet this increased demand for diesel fuel and to dispose off fuel oil surplus, refiners are increasing the severity of FCC operation. This would increase production of light cycle oil (LCO), which can be used as a blending component into the diesel pool. The LCO, however, has a low cetane index and higher density, sulfur, unsaturates, and aromatics. These properties adversely affect the quality of the resulting diesel fuel thus limiting its blending ratio. The refining industry faces tighter specifications in the 1990s. Current regulation in the US calls for an 80% reduction in sulfur content (from 0.25 to 0.05 w t %) of highway diesel fuel by 1993. The US has also set a minimum cetane index of 40 to maintain the aromatic content of diesel fuel at its current level of 31-34%. A t the same time, the demand for high-quality middle distillates, transportation diesel, and jet fuel is growing significantly (Denny et al., 1990, Miller, 1991). Refiners are looking for a hydrotreating strategy that will meet near-term sulfur requirement and also accommodate future aromatic specification. This would require active research in the development of selective catalysts for deep desulfurization, aromatic saturation, and cetane improvement (Wilson and Kriz, 1984a,b; Wilson et al., 1985, 1986; Nooy et al., 1986; Lee, 1991). This paper aims to determine the improvement of LCO quality by hydrotreating using commercially available catalysts under a wide range of operating conditions. The 0888-5885/91/2630-2586$02.50/0
Table I. Analysis of Light Cycle Oil DroDertv analvtical method density, g/cm8 ASTM D-1298 ASTM D-287 AP! gravity ASTM D-1218 refractive index elem analysis carbon, w t % elem analysis hydrogen, w t % 13C NMR aromatic carbon, w t % aniline point, OC ASTM D-611 Dhormann nitrogen, ppm Dhormann sulfur, wt % ASTM D-976-80 cetane index ASTM D-3710 simuld distilln, OC
IBP" 10% 30% 50% 70% 90% FBP
value 0.933 21.2 1.531 86.8 10.4 53.1 21.6 42 2.6 28.9 208 244 263 273 286 311 338
IBP, initial boiling point; FBP, final boiling point.
effect of catalyst type and operating conditions on desulfurization, denitrogenation, aromatic saturation, and cetane index improvement will be investigated. The results are discussed in light of the thermodynamic equilibrium of aromatic saturation.
Experimental Section The LCO feedstock used in this study is a representative middle distillate produced from FCC of vacuum gas oil obtained from a refinery in Saudi Arabia. The analytical results of LCO, given in Table I, shows that it contains high aromatic carbon (53.1%) and sulfur (2.6 w t % ) content, while it has low hydrogen content and cetane index. It should be noted that the aromatic content and sulfur are much higher than the diesel specifications of 31-34% and 0.25%, respectively. Three commercial hydrotreating catalysts were selected for the screening tests on the basis of recommendations 0 1991 American Chemical Society
Ind. Eng. Chem. Res., Vol. 30, No. 12, 1991 2587
I
AV ' FIC : FRN: FT : FV ' LT : LV : P : PCV: PT :
AIR OPERATED VALVE ROTAMETER FURNACE FLOW TRANSMTTER FLOW VALVE LEVEL TRAWIITTER LI(IUID VALVE P W PRESSURE CONTROLLER PRESSUK TRANSMITTER ER=:: VALVE TE ' TMR1(1 C W L E
1
WT WEIGHT SCALE WTM WET TfST M T E R
Figure 1. Schematic diagram of hydrotreating pilot plant. Table 11. Properties of the Catalysts Used catalyst
physical properties size, m m surface area, m2/g pore volume, cm3/g bulk density, kg/m3 chemical composition
A
B
Katalco NT-550
Procatalyse HR-348
C American Cyanamid HDNBO
1.6 230 0.5
1.6 145 0.43 770
3.2 155 0.4 800
16.5 2.7
22.0 3.0
bal
bal
800
Moo3 NiO
wo3
6.0 20.0 bal
of the catalyst manufacturers and published literature (Asim and Martin, 1988; Johnson, 1983; Wilson et al., 1985, 1986). These catalysts are reported to be highly active for denitrogenation, aromatic saturation, and cetane index improvement. Table I1 highlights the physical properties and chemical composition of these catalysts. Catalyst A is nickel-tungsten while B and C are nickel-molybdenum catalysts supported on y-alumina. The catalyst screening study was conducted in a continuous flow, trickle-bed hydrotreating pilot plant unit. This computer-controlled unit is capable of on-line monitoring of all process variables and unattended operation. A schematic diagram of the unit is shown in Figure 1. The 316 stainless steel reactor is 1.2 m long with an internal diameter of 2.5 cm. It was operated in isothermal mode by independent temperature control of a five-zone electric furnace. The axial temperature profile of the catalyst bed was measured by three thermocouples located in a thermowell mounted at the center of the reactor. The space above and below the catalyst bed was packed with inert a-alumina of the same dimensions to ensure homogeneous distribution of feed streams and to avoid end effects. To obtain the catalyst bed length of 36 cm, 120 mL of catalyst was diluted with equal volume of inert a-alumina. Such dilution improves the flow pattern by increasing the liquid holdup and residence time distribution and helps in achieving isothermal condition. The a-alumina was considered to be of negligible activity and
did not contribute catalytically to the LCO hydrotreating reactions. Prior to activity testing, each catalyst was dried in situ with flowing nitrogen to 150 "C for 16 h. In situ presulfiding was then carried out with kerosene spiked with 1 wt 7'0 CS2. To begin presulfiding, the catalyst bed was heated at about 25 OC/h to 200 "C with a kerosene at liquid hourly space velocity (LHSV) of 3 h-' and 4.1 MPa. The CS2-spikedfeed was then introduced and maintained for 2 h before introduction of hydrogen at a rate of 2 L/min. Temperature was then increased at about 25 "C/min to 350 "C. These conditions were maintained for 16 h to ensure complete catalyst presulfiding. The hydrotreating of LCO was carried out a t the following operating conditions: temperature 325, 350, 375, and 400 "C; LHSV 1.0,2.0, and 3.0 h-l; pressure 4.1,6.9, and 9.7 MPa. The treat gas rate for all runs was 535 L of hydrogen/L of LCO (L/L) using pure hydrogen in a once-through mode. This treat gas rate was somewhat higher than commercial units so as to reduce catalyst deactivation and to maintain good mass flow velocities which in turn help good flow distribution through trickle-bed pilot plant reactors. The material balance closure and experimental error for this study were within &5%. The chemical composition and physical properties of LCO feedstock and products were monitored by 13C NMFt spectroscopy, elemental hydrogen and carbon analysis, density, refractive index, and GC simulated distillation as shown in Table I. Total sulfur and nitrogen were analyzed by using the Dhormann coulometric technique. API gravity (G)was determined using the ASTM D-287 method and midboiling temperature (M) was calculated by the ASTM D-3710 method. The hydrogen consumption was calculated from that consumed by desulfurization, denitrogenation, and the.increase in the hydrogen content of the product. Cetane index (CI) was calculated using the ASTM D-976 method by the following equation: CI = 65.01(10g M)2 + 0.192G(log M) + 0.016C2 1.809 X 10-4w - 420.34 (1)
Results and Discussion Each of the catalysts was subjected to eight experimental runs in order to investigate the effect of operating con-
2588 Ind. Eng. Chem. Res., Vol. 30, No. 12, 1991 Table 111. Results of LCO Hydrotreating over Catalyst A operating conditions 1 2 run no. 325 350 temp, "C 6.9 press., MPa 6.9 2.0 2.0 LHSV, h-' product properties 873.5 density, kg/m3 870.3 1.489 refractive index 1.486 31.0 aniline point, OC 33.0 1.645 H/C atomic ratio 1.663 32.0 29.0 aromatic carbon, wt % 0.25 sulfur, wt % 0.18 1.2 C B; H/C ratio, A C > B; HDS,A C B; aromatic saturation, A > C B; cetane index improvement, A C > B; H2 consumption, A > C B. These results show that although Ni-W catalyst A is more active for aromatic saturation, catalyst C (nickelmolybdenum) can give similar cetane improvement at lower hydrogen consumption. These conclusions were also reported by Wilson and Kriz (1984) using similar catalysts for hydrotreating middle distillate derived from Athabasca bitumen. A t higher temperatures the phase change would
--
-
- -
-
-
result in cracking reaction thus leading to a decrease in the boiling point and increasing gas yield. Since the products at 400 "C did not significantly increase by hydrocracking, it is expected that the thermodynamic equilibrium contributed to the decrease in aromatic saturation. Application of these catalysts in a conventional refinery is faced by a lower available operating prewure (3-5 MPa), temperature (360 "C), and H2/HC ratio (below 80).Cetane improvement by hydrotreating is favored at high severity which involves a high hydrogen consumption. The hydrogenation severity must be adjusted to an economic scenario taking into account operating cost versus the advantages of incorporating more cracked product in the diesel pool.
Conclusions The effect of hydrotreating on LCO fuel quality has been investigated over three commercial catalysts under typical refinery conditions. The data showed that in general product quality (density, sulfur, nitrogen, H/C atomic ratio, aromatic saturation, and boiling point range) improved as the temperature and pressure increased or space velocity decreased. The optimum conditions for the three catalysts were found to be 350 "C, LHSV of 1.0 h-l, 6.9 MPa, and 535 L/L which are beyond the capacity of conventional hydrotreaters. The applicability of these results industrially could be limited by the high severity and hydrogen treat gas rate. The nickel-tungsten catalyst is more active for hydrogenation and aromatic reduction, but has higher hydrogen consumption. However, in terms of cetane improvement, a nickel-molybdenum type catalyst was of comparable performance. Acknowledgment This work is part of KFUPM/RI Project No. 11071 supported by The Research Institute of King Fahd University of Petroleum and Minerals. We thank Messrs. A. M. Aitani, M. A. B. Siddiqui, and M. A. Ali for their participation in experimental work and product analysis. We are also thankful to Mr. Khalid Nahas, Jeddah Oil Refinery, for supplying the LCO used in this study. Registry
No.
Ni, 7440-02-0; Mo, 7439-98-7; W, 7440-33-7.
Literature Cited Asim, M. Y.; Martin, J. F. Hydrotreatment of Light Cycle Oils for Improved Color and Color Stability; Proceedings of the Akzo Catalyst Symposium '88, Scheveningen, The Netherlands; Akzo Chemicals Division: Amersfoort, The Netherlands, 1988; paper H-10. Danaher, W. J.; Palmer, L. D. Chemical Changes and Ignition Quality Improvement Resulting from Hydrotreating Light Cycle Oil. Fuel 1988,67 (IO), 1441-1445. Denny, R. F.; Gilsdorf, N. L.; Hibbs, F. M.; Houde, E. J.; Reno, M. E.; Silverman, R. P. The Refining Challenge for the 1990'sProduct Slate, Product Quality and Processing Efficiency. Proceedings of the UOP Technology Conference, Istanbul, Turkey; UOP: Des Plaines, IL, 1990; pp 1-16. Edgar, M. D. Hydrotreating Compression Focus of Pilot Plant Study Program. Oil Gas. J. 1978, 76 (33), 102-106. Fairbridge, C.; Kriz, J. F. Hydroprocessing of Coal-Derived Middle Distillate. Fuel Sci. Technol. Znt. 1986, 4 (2), 171-189. Johnson, A. D. Study Shows Marginal Cetane Gains from Hydrotreating. Oil Gas. J. 1983, 81 (21), 78-82, Lee, S. L. How to Produce Clean Diesel Fuel. Proceedings of the Akro Catalysts Symposium '91,Scheveningen, The Netherlands; Akzo Chemicals Division: Amersfoort, The Netherlands, 1991; paper H-3. Martin, J. M. Cetane Improvement by Hydrotreating. Presented a t Ketjen Catalysts Technical Seminar, Pasadena, TX, 1983. Miller, J. W. New Specifications for Transportation Fuels. Proceedings of the Akzo Catalysts Symposium '91, Scheveningen,
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The Netherlands; Akzo Chemicals Division: Amersfoort, The Netherlands, 1991; paper H-1. Moore, P. K.; Akgerman, A. Comments on Upgrading of Middle Distillate Fractions of Syncrude from Athabasca Oil Sands. Fuel 1985, 64, 721-722. Nooy, F. M.; Lee, S. L.; Yoes, J. R. Applications of Ketjenfine-840. Proceedings of the Ketjen Catalyst Symposium, Scheveningen, The Netherlands; Akzo Chemicals Division: Amersfoort, The Netherlands, 1986; paper H-3. Wilson, M. F.; Kriz, J. F. Upgrading of Middle Distillate Fractions of Syncrudes from Athabasca Oil Sands. Prepr.-Am. Chem. SOC.,Diu. Pet. Chem. 1983, 28, 640-649. Wilson, M. F.; Kriz, J. F. Selected Aspects of Catalytic Refining of Middle Distillates from Athabasca Syncrudes. Prepr.-Am. Chem. Soc., Diu. Pet. Chem. 1984a, 29, 284-291. Wilson, M. F.; Kriz, J. F. Upgrading of Middle Distillate Fraction of Syncrude from Athabasca Oil Sands. Fuel 198413, 63 (21, 190-196.
Wilson, M. F.; Fisher, I. P.; Kriz, J. F. Hydrogenation of Aromatic Compounds in Synthetic Crude Distillates Catalyzed by Sulfided Ni-W/A1203. J . Catal. 1985,95, 155-166. Wilson, M. F.; Fisher, I. P.; Kriz, J. F. Cetane Improvement of Middle Distillate from Athabasca Syncrudes by Catalytic Hydroprocessing. Ind. Eng. Chem. Prod. Res. Deu. 1986, 25 (4), 505-511. J a m a l A. Anabtawi,* Syed A. Ali Petroleum and Gas Technology Division T h e Research Institute King Fahd University of Petroleum and Minerals Dhahran 31261, Saudi Arabia
Received for review December 3, 1990 Revised manuscript received June 11, 1991 Accepted September 13, 1991
Reaction of Trace Mercury in Natural Gas with Dilute Polysulfide Solutions in a Packed Column The natural gas produced around the world can contain traces of mercury which have to be removed. I t is difficult t o purify gas to desired mercury levels using conventional techniques. By scrubbing with dilute polysulfide solution, the residual mercury in the gas can be removed from about 0.1 to below 0.01 ppb, a reduction of 90%. In the system, the gas is passed through a packed tower wetted with a solution containing 3 ppm of polysulfide salt. Stainless steel packings are effective for this application. In addition to promoting gas-liquid contact, the stainless steel packings adsorb and concentrate polysulfides which react with Hg in the gas to form insoluble HgS, and thus remove Hg from the gas.
Introduction Natural gases found around the world contain trace amounts of mercury (Phannenstiel et al., 1975). The mercury contents were reported to be 200-300,180,50-80, 1-9 and 0.005-0.04 ppb (parts per billion) for gases from Sumatra, Groningen, Algeria, the Middle East, and North America, respectively (Bodle et al., 1980). For gas with a molecular weight of 22.4, the Hg concentration of 1 pg/m3(NTP) equals 1ppb. Thus, for natural gases having a molecular weight of about 22, Hg concentrations in terms of pg/m3(NTP) are close to that in ppb. Mercury can be a major source of concern in liquefied natural gas (LNG) plants (Leeper, 1980), and many potential methods for Hg removal from natural gas have been suggested (Bodle et al., 1980). For example, sulfur-impregnated activated carbon at about 175 OF was found to be better than other non-carbon-based materials (Situmorang and Muchlis, 1986; Biscan et al., 1986). The concentration of mercury in the reactor effluent was found to be temperature dependent and was determined by the thermodynamic equilibrium at the prevailing reaction temperature. At 79.4 "C,the equilibrium concentration of mercury in the effluent was found to be 0.06 ppb. In an effort to reduce the residual mercury concentration to below 0.01 ppb, the present study was undertaken to investigate the reaction of mercury with aqueous solutions of polysulfides at room temperature. Reaction of Hg and S. Sulfur reacts with mercury reversibly in the gas to form solid HgS HgS(s) * 2Hg(g) + S&) Keq F? (PHg)2(PS,) where K is the thermodynamic equilibrium constant (atm3)an! P h and Ps2 are partial pressures of Hg and S2 (atm), respectively. 0888-5885/91/2630-2592$02.50/0
The equilibrium constants, KB9,over the temperature range of 610-650 K have been reported by Kelley (1937). Based on Kelley's data, the plot of In Kegvs 1000/T is a straight line (see Figure 1)and linear extrapolation to a lower temperature could be justified. In addition, this relationship has been verified for a commercial operation at 340 K. Thus, Kw can be represented as follows: In Keq= -44544/T
+ 51.068
where T is temperature, K. In order to reduce the concentration of Hg in the treated gas to below 0.01 ppb, the reaction temperature must be decreased to lower the equilibrium constant. However, at low temperatures, the reaction rate becomes low. One approach to increase the rate of reaction is to create a large surface area of sulfur for reaction by dispersing it on a high surface area carrier such as activated carbon (Biscan et al., 1986). However, when it is operated at low temperatures, the mass-transfer zone in the adsorption bed increases, which not only necessitates a large bed but also diminishes the adsorption capacity of the adsorbent. Another approach is to dissolve sulfur in water and use the sulfurcontaining water to remove the mercury from the gas by scrubbing. The solubility of sulfur in pure water at 20 OC is low, at 1.9 X mol of SB/kg,or 4.86 ppb (Boulegue, 1978), but it can be increased by increasing the pH of the water to >7. In alkaline water, the solubility of sulfur increases by the formation of polysulfides, Sz2-,and much of the sulfur in the solution exists as polysulfides. To increase the pH of the solution, NaOH, KOH, and amines such as monoethanol amine, diethanol amine or ammonia can be used. Polysulfide ions react with mercury to give HgS, according to
0 1991 American Chemical Society
