New Hydrodesulfurization Catalyst for Petroleum-Fed Fuel Cell

From the results of the long run, 13% Ni/ZnO catalyst was ... 1-year operation of the petroleum-fed FC cogenerations and a 1110 thousand-km running of...
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Ind. Eng. Chem. Res. 2001, 40, 2367-2370

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RESEARCH NOTES New Hydrodesulfurization Catalyst for Petroleum-Fed Fuel Cell Vehicles and Cogenerations Kinya Tawara,*,† Takeshi Nishimura,† Hikoichi Iwanami,† Tokuyoshi Nishimoto,‡ and Takashi Hasuike§ Research and Development Center, Cosmo Research Institute, 1134-2 Gongendo, Satte-shi, Saitama-ken, 340-0193 Japan, Petroleum Energy Center, 1-4-10 Ohnodai, Midori-ku, Chiba-shi, Chiba-ken, 267-0056 Japan, and Sankisangyo Company, Ltd., 775-1 Anegasaki, Ichihara-shi, Chiba-ken, 299-0111 Japan

Based on the newly found autoregenerative Ni/ZnO catalyst, a practical adsorptive hydrodesulfurization (HDS) catalyst of kerosene for the fuel cell (FC) was developed and evaluated. The content of surface Ni on the Ni/ZnO catalyst is fixed from the balance between the poisoning rate and the regenerating rate. From the results of the long run, 13% Ni/ZnO catalyst was certified as an adsorptive HDS catalyst for the kerosene-fed FC, which will keep less than 0.03 wt ppm of average sulfur for 1 year, consuming 36% of ZnO. When this catalyst is applied, a 1-year operation of the petroleum-fed FC cogenerations and a 1110 thousand-km running of the model FC vehicles are considered possible. Introduction To reduce CO2 emission from vehicles and cogeneration systems, a fuel cell (FC) is one of the most important technologies. FC vehicles using liquid H2 and cogeneration systems using methane have been demonstrated, but practical petroleum-fed FCs have not been demonstrated. To operate the steam-reforming process for the petroleum-fed FC, sulfur in the feed has to be hydrodesulfurized to less than 0.1 wt ppm.1 Methane contains simple sulfur compounds, which can be easily desulfurized using the conventional hydrodesulfurization (HDS) catalysts. Petroleum fractions are hydrodesulfurized using conventional Co(Ni)-Mo/Al2O3 catalysts in the refinery; however, gasoline or kerosene still contains 30-70 wt ppm of residual sulfur. The residual sulfur compounds in the petroleum fractions consist of heavy sulfur compounds2 that are difficult to remove by conventional catalysts.3 The limit of HDS using the conventional catalysts is 1-2 wt ppm. Only by adsorptive HDS using Ni catalyst,4 the residual sulfur has been known to be removed under the mild conditions of the FC. However, the catalyst lost practical activity after surface Ni was converted to NiS.5 We tried an idea to regenerate the sulfur-poisoned Ni catalyst in situ and found a new Ni/ZnO catalyst which can hydrodesulfurize kerosene to below 0.1 wt ppm under below 1 MPaG.6 Surface Ni on the Ni/ZnO catalyst is considered to be poisoned by HDS but to be regenerated many times by the transfer of sulfur to ZnO in a H2 atmosphere. Then, the new Ni/ZnO was named * To whom correspondence should be addressed. Fax: 8148-884-3736. E-mail: [email protected]. † Cosmo Research Institute. ‡ Petroleum Energy Center. § Sankisangyo Company, Ltd.

as the “autoregenerative catalyst” in a previous paper.6 Therefore, the molecular ratio of sulfur to surface Ni on the Ni/ZnO catalyst exceeds 1 by the end of the life. The finding of the autoregenerative Ni/ZnO catalyst was interesting as a target of fundamental chemistry; however, the original Ni/ZnO catalyst could not realize practical life for the petroleum-fed FC for 1 year, because the surface area (Brunauer-Emmett-Teller, BET) of the original Ni/ZnO catalyst was small (15-18 m2/g). Whether a practical Ni/ZnO catalyst having a 1-year life was possible or not, therefore, became the most important subject as a target of the industry and engineering. The object of this paper is to develop industrial models of the Ni/ZnO catalyst having a wide BET and to test the practical life. The conventional ZnO adsorbent is used usually until it adsorbs 45 mol % (18 wt %/ZnO wt) of sulfur/1 mol of ZnO. The same adsorbing capacity of sulfur should be required in the Ni/ZnO catalyst. As a catalyst and an adsorbent, Ni/ZnO has the double functions, so it was defined as an “adsorptive HDS catalyst”.6 Ni is the active component of the HDS catalyst, and ZnO is the acceptor of H2S. In a high content of Ni, a high activity will be realized, but a long life will not be expected because of the low content of ZnO. On the other hand, in a high content of ZnO, a long life will be expected, but a high activity will not be realized because of the low content of Ni. When the tradeoff was controlled, the realization of a highly active and longlife adsorptive HDS catalyst was studied in this paper and the realization of a petroleum-fed FC was certified. Another object of this paper is to simulate the necessary volume of the HDS catalyst for petroleum-fed FC vehicles and cogenerations. The case study using the phosphoric acid FC (PAFC) was carried out, which was tested successfully in many fields using methane. The

10.1021/ie000453c CCC: $20.00 © 2001 American Chemical Society Published on Web 04/21/2001

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simulation was carried out corresponding to the heaviest duty for the HDS catalyst, which is the constant full power operation for 1 year. The HDS catalyst can be replaced at the 1-year maintenance period. Some prototypes of the FC vehicle are being demonstrated by automakers. They are, however, powered by liquid H2 or H2 from the reforming of methanol. Technologies of a petroleum-fed FC must be developed for the practical commercialization of the new vehicles. The key technology is the complete desulfurization of the petroleum fractions. We propose the new HDS catalyst as the key technology. The necessary volume of the HDS catalyst for the model of the petroleum-fed FC vehicle using a 10-kW polymer electrolyte FC (PEFC) stack was calculated corresponding to a constant running at 100 km/h. The life of the HDS catalyst was simulated from the running conditions. Experimental Section Theory. The total amount of surface Ni in a usual Ni/Al2O3 catalyst does not change under the usual reaction conditions. The total amount of surface Ni in the Ni/ZnO catalyst, however, changes in the progress of the reaction. Estimation of the change of the active Ni content in the course of run is important for the effective application of HDS. Equation 1 shows the material balance of

dCsurf Ni/dt ) k1Csurf NiSaCZnOb/L - k2CS(LHSV)

(1)

poisoning surface Ni and regenerating surface Ni using apparent rate equations, where Csurf Ni is the content of the active surface Ni, Csurf NiS is the content of the surface NiS, CZnO is the content of ZnO, L is the average diffusion resistance of H2S, CS is the content of sulfur in the feed, LHSV is the liquid hourly space velocity of fed kerosene, k1 and k2 are the rate constants, and a and b are the orders of the reactions. As the acceptor of H2S, the surface ZnO is assumed to act at first. Because an unexpectedly large quantity of ZnO is consumed until the end of the life, the content of ZnO is used instead of the content of the surface ZnO. The rate of regeneration is linked to the contents of Csurf NiS and CZnO. Therefore, the apparent rate of the regenerating reaction is expressed as k1Csurf NiSaCZnOb/L, which decreases according to the consumption of the acceptor of H2S. k2CS(LHSV) shows the poisoning rate of the active surface Ni, which is considered as constant in the fixed reaction conditions using the fixed feed rate of kerosene under the fixed LHSV in all of the ages of the reaction. Equation 1 does not, however, mean the kinetic model of regeneration. The kinetic model should be broken down into some steps, which may consist of adsorption of H2 on the catalyst surface, surface reaction on the catalyst, desorption of H2S from Ni, transport of H2S to the surface of ZnO, reaction of H2S with ZnO, and desorption of H2. Elution curves and isotherms studied by Oliphant et al.7 may be helpful for the analysis of one of these steps. In the balanced state, dCsurf Ni/dt ) 0; then the content of the surface NiS is expressed as eq 2. Therefore, the

Csurf NiS ) {(k2/k1)CS(LHSV)L/CZnOb}1/a

(2)

content of the surface NiS increases in a high LHSV or in a high sulfur content in the feed. Equation 3 expresses the content of the active surface Ni in the catalyst, where Csurf Ni0 is the initial con-

Table 1. Properties of Ni/ZnO Catalysts ABD Ni ZnO Al2O3 BET surface Ni ratio catalyst (g/mL) (wt %) (wt %) (wt %) (m2/g) (mol %/total Ni) CDSC-1 CDSC-2

0.80 0.84

5.30 13.4

86.0 78.4

8.70 8.20

93 95

21 12

tent of the active surface Ni. The regenerating rate

Csurf Ni ) Csurf Ni0 - {(k2/k1)CS(LHSV)L/CZnOb}1/a (3) k1Csurf NiSaCZnOb/L becomes slower and the content of the active surface Ni on the catalyst becomes smaller gradually according to the consumption of ZnO in the progress of the run. By a sudden change of reaction conditions, the balance of Ni and NiS on the catalyst surface changes slowly according to eq 3 and attains a new equilibrium after several hundreds of hours, because the rate of the regenerating reaction and the rate of accumulation of NiS is not fast. In a low Csurf Ni0, remaining Csurf Ni becomes low, and the HDS ability becomes low after attaining equilibrium. Therefore, eq 3 is as slippery as an eel, and the analysis of all of the parameters in eq 3 is not easy. Only the outline of the equations is shown here. Materials and Methods. Kerosene containing 51 wt ppm of sulfur was used. H2 (99.99%), N2, CO2, and a 1:1 mixture of CO and CH4 gases were obtained from Nippon Sanso Co. Nickel nitrate hexahydrate (JIS grade 1), zinc nitrate hexahydrate (JIS grade 1), ammonium carbonate (JIS grade 1), and ammonium water (JIS grade 1) were supplied by Kanto Chemical Co., Inc. The coprecipitated Ni/ZnO catalysts (CDSC-1 and CDSC-2) were prepared with the same procedure as that used in the previous paper6 except alumina gel was mixed at the coprecipitation and extruded in 1.5 mm diameter size. The properties of the catalysts are shown in Table 1. The metal content of the catalysts was analyzed by an inductively coupled plasma (ICP) emission spectrochemical analyzer ICPS-200 provided by Shimadzu Corp. The total sulfur determination was carried out by 856/825R-d/1003, which is supported by ASTM D40405 and provided by Houston Atlas, and the limit of the original detection of 30 ppb was improved to 0.3 ppb in this laboratory. The surface Ni on the catalysts was measured by NO chemisorption using temperatureprogrammed desorption mass spectroscopy (TPD-MS) provided by Bel Japan, Inc. BET was measured by BELSORP 28 provided by Bel Japan, Inc. The long run evaluations packed with 100 mL of catalyst were carried out using flow reactors, which were designed and constructed as the bench test plants and could be operated automatically, without operators, at night and holidays. The catalysts were reduced before the reaction in a flow of 48 L/h of H2 at 360 °C for 20 h at 0.6 MPaA. The electric conversion efficiency of the FC to the combustion heat of kerosene, heat of combustion, and density of kerosene are taken as 36%, 41.8 kJ/g, and 0.79 g/mL, respectively. Therefore, the consumed kerosene for generating 1 kW/h ()3600 kJ/h) of electricity is 303 mL/h [)(3600 kJ/h)/(41.8 kJ/g)/(0.79 g/mL)/0.36 ratio]. Taking the accepting capacity of sulfur on ZnO and the sulfur content in kerosene as 18 wt % and 70 wt ppm, respectively, the necessary quantity of ZnO for accepting sulfur in the fed kerosene for 1 year is calculated as 815 g ()303 mL/h × 24 h × 365 days × 0.79 g/mL × 70 × 10-6 wt ppm/0.18 ratio). Then, the

Ind. Eng. Chem. Res., Vol. 40, No. 10, 2001 2369

Csurf Ni ) Rk3

(6)

of Csurf Ni and Csurf Nii as shown in eqs 7 and 8, where Csurf Nii is the content of the initial surface Ni, k3i is the initial rate constant, CSi is the treated sulfur content at the initial step, and R is a constant. The increment

∆Csurf NiS ) Csurf Nii - Csurf Ni ) R(k3i - k3)

(7)

∆Csurf NiS ) [R(LHSV)/(n-1)](1/CSin-1 - 1/CSn-1) (8) Figure 1. Sulfur in treated kerosene and index of surface NiS as a function of reaction time in the accelerated long runs of Ni/ZnO catalysts: (2, b) CDSC-1, ([, ×) CDSC-2. Temperature: CDSC1, 300 °C; CDSC-2, 270-280 (T2)-290 (T3) °C. Total pressure: 0.60 MPaA. Initial sulfur: 51 wt ppm.

of the surface NiS content, also, can be expressed as a simple index equation 9.

necessary volume of the catalyst is calculated as 1200 mL (815 g/0.784 ZnO ratio/0.84 ABD for CDSC-2) for catalysts using the apparent bulk density (ABD) and the content of ZnO listed in Table 1. Therefore, LHSV for these catalysts in the practical system is 0.25 h-1 [)(303 mL/h)/(1200 mL)]. In this long run, 2 times the accelerated LHSV ) 0.50 h-1 was used.

The changes of the surface NiS content index in the accelerated long runs of CDSC-1 and CDSC-2 are shown in Figure 1, where CSi is set at the second data (100 h), when the methanation reaction was suppressed. In both cases, equilibrium of eq 3 is attained at about 900 h. Csurf NiS in CDSC-1 is somewhat higher than that in CDSC-2. k2CS(LHSV) is constant in both catalysts, but k1 may be higher at 300 °C (CDSC-1) than at 270 °C (CDSC-2). However, in an excess content of ZnO before half of the catalyst life, the difference of the effects of ZnO content between both catalysts may not be detected. Therefore, the content of the surface NiS in CDSC-2 becomes somewhat higher at that temperature. However, the deactivation rate of the HDS corresponds to the decreasing rate of the remaining surface Ni content. CDSC-2 showed a long life. At T2, the temperature was raised to 280 °C from 270 °C, and at T3, the temperature was raised to 290 °C from 280 °C. After these temperature changes, it needed 300-500 h to attain a new equilibrium. CDSC-2 adsorbed, in the accelerated half-year test period, nearly the planned quantity of sulfur for the 1-year maximal power operation of the FC. Until this age, CDSC-2 adsorbed 18.9 g of sulfur/100 mL of catalyst, which corresponds to 36.4 mol % consumption of ZnO (14.3 wt % of sulfur/ZnO wt); this value is regarded as enough ZnO capacity for practical uses. The average sulfur level was 0.060 wt ppm in the run. Assuming the content of the active surface Ni Csurf Ni is fixed constant even under the half LHSV, the average sulfur level under the half LHSV is calculated as 0.030 wt ppm (second order). If CDSC-2 is used under the half LHSV of this test for 9400 h (4700 × 2), the same quantity of sulfur as that on this test will be accumulated on CDSC-2. However, the remaining active surface Ni content Csurf Ni in eq 3 under the half LHSV is regarded as larger than that under this test, and then the average sulfur level is considered as below 0.030 ppm. Therefore, CDSC-2 is considered as operable for 1 year sufficiently under the practical LHSV (0.25 h-1). As a conclusion, CDSC-2 was selected as a practical catalyst of adsorptive HDS for FC. To the best of our knowledge, this is the first time that the 1-year life of the practical adsorptive HDS catalyst of petroleum fractions heavier than gasoline for the FC was demonstrated. The HDS activity of CDSC-1 shrunk under these conditions. The remaining active surface Ni content at the middle age is considered to be very small in CDSC-1 because of the small quantity of the original surface Ni

Results and Discussion Evaluations. The catalyst should be evaluated by the long-life test under the practical reaction conditions, and the practical target should be to pass a 1-year life test under the practical LHSV, keeping below 0.1 wt ppm of treated sulfur. However, we carried out the test under 2 times the accelerated LHSV. The adsorbed sulfur on the catalyst corresponding to 1 year can be attained in the half-year test. However, the content of the active surface Ni is considered to be reduced severely as assumed from eq 3 in the course of the accelerated long run. The runs were carried out under the reaction conditions: reaction pressure ) 0.60 MPaA; feed rate of kerosene ) 50 mL/h; feed rate of H2, CO2, and CO + CH4 ) 15, 4.9, and 0.40 NL/h, respectively; reaction temperature of CDSC-1 and CDSC-2 ) 300 and 270 (start) °C. The mixture of CO and CH4 gases was used to simulate H2 from the FC system. The sulfur contents in the treated kerosene versus the catalyst age are shown in Figure 1. The HDS activity of CDSC-2 within the initial 20 h is reduced because of the consumption of H2 by a strong methanation reaction, which falls after that period. However, CDSC-1, containing a small initial surface Ni, does not show the strong methanation reaction. The sulfur balance after the long runs is listed in Table 2. A change of the surface NiS content corresponds to the change of HDS activity. The rate of HDS of a petroleum fraction, containing mixed sulfur compounds, is expressed as eq 4, where k3 is the rate constant and n ()2.2 for this HDS) is the order of the reaction. Equation 4 is reformed to eq 5, where CSf is the sulfur

-dCS/dt ) k3CSn

(4)

content of the feed. The surface Ni Csurf Ni reflects the

k3 ) [LHSV/(n - 1)](1/CSn-1 - 1/CSfn-1)

(5)

rate of reaction. Then, Csurf Ni can be related to the reaction constant k3 and expressed as eq 6. The increment of surface NiS ∆Csurf NiS is equal to the difference

index ∆Csurf NiS ) 1/CSin-1 - 1/CSn-1

(9)

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Table 2. Sulfur Balance on Accelerated Long Runs Using Ni/ZnO Catalysts catalyst

Ni

cumulative run (h)

cumulative S (mol/100 mL of catalyst)

S/Ni (mol %)

S/ZnO (mol %)

S/surface Ni (mol %)

CDSC-1 CDSC-2

0.0753 0.186

800 4700

0.050 0.30

67 160

5.7 36

310 1400

content. Then, the treated sulfur could not keep 0.1 wt ppm after only 600 h. The treated sulfur level of CDSC-1 reached 0.25 wt ppm of equilibrium at 900 h. Within that time, only 6.4% of ZnO was consumed. The reaction temperature of CDSC-1 was higher (300 °C) than that (270-290 °C) of CDSC-2. High temperature is considered to show a high HDS activity and high regeneration rate. However, the temperature advantage could not recover the disadvantage of the low content of the initial Ni content. The large quantity of ZnO still remained as the active acceptor of H2S in CDSC-1. To check the remaining activity of CDSC-1, the run was continued, stopping the kerosene feed from 900 to 950 h and restarting it at 950 h. The sulfur in the product was improved to 0.057 wt ppm at 965 h. From these results, the enough remaining activity of CDSC-1 after the 900 h of run was made clear. Therefore, the result is not considered as the end of the life. CDSC-1 is expected to have a longer life in the lower LHSV or in the lower sulfur-containing kerosene. Simulations for the Petroleum-Fed Cogenerations and Vehicles. This catalyst was made clear to be applicable for the adsorptive HDS of naphtha and kerosene-fed FC, which usually could not be operable using these feedstocks. Cogenerations and FC vehicles will be applicable using this catalyst. The HDS section of the kerosene-fed cogeneration for A kW is considered operable for 1 year using 1.2 A L of CDSC-2 catalyst as calculated before. The HDS section of the kerosene-fed FC cogeneration of 50 and 200 kW, for example, will be realized using 60 and 240 L of the Ni/ZnO catalyst, respectively. Some original models of the FC vehicle (FCV) were designed with a 10 kW PEFC stack. The efficiency of the steam-reforming unit for hydrocarbon is lower than that for methanol, because a high-temperature reaction is necessary to shift the methane-rich equilibrium to a H2-rich equilibrium. The efficiency of the reformer is defined as the ratio of the produced H2 divided by the stoichiometrical value from the input hydrocarbon. An efficiency of 70% is usual for the small reformer. In this efficiency, 8.64 mL of gasoline is necessary for 22.4 L of H2 production. A model of H2-fed FCV with 100 kg of alloy, which holds 20 000 L of H2 for 250 km of driving, was demonstrated. Using the above gasoline-fed reformer, 20 000 L of H2 is supplied by 7.7 L of gasoline. If the gasoline-fed FCV is designed in the same weight as the H2-fed FCV, the fuel consumption of the gasolinefed FCV is 32 km/L. The practical FCVs are estimated to be equipped with 10-80 kW of FC stacks. If a B kW (3.60 × 103 B kJ/h) FC stack that has 30% electric efficiency is powered by gasoline that has 3.26 × 104 kJ/L of combustion heat, the maximum gasoline feed is 0.368 B L/h [)(3.60 × 103 B kJ/h)/0.3 ratio/(3.26 × 104 kJ/L)]. Because the HDS catalyst has to be operated at LHSV

) 0.25 h-1, the necessary volume of HDS catalyst is 1.47 B L [)(0.368 B L/h)/0.25 h-1]. If the fuel consumption of the gasoline-fed FCV is C km/L, the catalyst life is 3460 BC km ()0.368 B L/h × 9400 h × C km/L). In the case of the above model FCV (B ) 10 kW, C ) 32 km/L), the necessary volume of the HDS catalyst and the catalyst life is 14.7 L and 1110 thousand km, respectively. The life of the HDS catalyst is excess for the life of FCV. However, this volume of catalyst is necessary to keep the sulfur level for the life of the vehicle. Conclusions A 13% Ni/ZnO catalyst was certified as an adsorptive HDS catalyst for the kerosene-fed FC, which will keep less than 0.03 wt ppm of the average sulfur for 1 year. Applying this catalyst, 1-year operation of petroleumfed FC cogenerations and 1110 thousand-km running of FC vehicles are considered possible. Acknowledgment This research was carried out as a project of the Petroleum Energy Center, with a subsidy from the Ministry of International Trade and Industry, Japan. Literature Cited (1) Nielsen, B. Poisoning of nickel catalysts by arsenic. Appl. Catal. 1984, 11 (1), 123-138. (2) Tajima, H.; Kabe, T.; Ishihara, A. Separation and analysis of sulfur-containing polyaromatic hydrocarbons in light oil. Bunseki Kagaku 1993, 42, 67-75. (3) Zhang, Q.; Isihara, A.; Yashima, H.; Qian, W.; Tsutsui, H.; Kabe, T. Deep Desulfurization of Light Oil (Part 5) Hydrodesulfurization of Methyl-substituted Benzothiophenes and Dibenzothiophenes in Light Gas Oil Catalyzed by Various Sulfided CoMo/Al2O3 and Ni-Mo/Al2O3 Catalyst. Sekiyu Gakkaishi 1997, 40 (1), 29-34. (4) Eberly, P. E., Jr.; Brannon, J. H. Guard Bed for the Removal of Sulfur and Nickel from Feeds Previously Contacted with Nickel Containing Sulfur Adsorption Catalysis. U.S. Patent 4,446,005, May 1, 1984. (5) Tawara, K.; Imai, J.; Iwanami, H. Ultra-deep Hydrodesulfurization of Kerosene for fuel Cell System (Part 1) Evaluations of Conventional Catalysts. Sekiyu Gakkaishi 2000, 43 (2), 105113. (6) Tawara, K.; Nishimura, T.; Iwanami, H. Ultra-deep Hydrodesulfurization of Kerosene for fuel Cell System (Part 2) Regeneration of Sulfur-poisoned Nickel Catalyst in Hydrogen and finding of autoregenerative Nickel Catalyst. Sekiyu Gakkaishi 2000, 43 (2), 114-120. (7) Oliphant, J. L.; Fowler, R. W.; Pannell, R. B. Bartholomew, C. H. Chemisorption of Hydrogen Sulfide on Nickel and Ruthenium Catalysts. I. Desorption Isotherms. J. Catal. 1978, 51, 220242.

Received for review May 4, 2000 Revised manuscript received February 22, 2001 Accepted February 23, 2001 IE000453C