Production of high octane gasoline components by hydroprocessing, of coalderived aromatic hydrocarbons

® 1997 Elsevier Science B. V. All rights reserved. Hydrotreatment and hydrocracking of oil fractions G.F. Froment, B. Delmon and P. Grange, editors

469

Production of high octane gasoUne components by hydroprocessing of coalderived aromatic hydrocarbons B. Demirel* and W. H. Wiser Department of Chemical and Fuels Engineering, University of Utah, Salt Lake City, UT 84112

The main objective of this work"^ was to convert aromatic compounds, representative of coal-derived liquids, into high octane compounds low in aromatics. For this study, 1-methylnaphthalene was chosen as a model compound. Experiments were performed in single and twostage systems. The model compound was treated with various catalysts at different reaction conditions in the single-stage operation. The highest conversion to isoparaffins and substituted cyclohexanes and cyclopentanes, 40.1% was achieved with a NiW/Si02-Al203 catalyst at 325 °C and lOOOpsig in 10 h with a feed to catalyst ratio of 10 to 1. In the two-stage operation, 1methylnaphthalene was hydrogenated to methyldecalins at 325 °C with almost 100% conversion in the first stage, using a NiMo/Ti02-Al203 catalyst. In the second stage, the methyldecalins underwent hydrocracking reactions at different reaction conditions using various catalysts. Pd/REX exhibited the best result for the production of high octane gasoline components without aromatics at 300-325°C.

1. INTRODUCTION Coal is the most abundant fossil fuel resource available in the world. It was the dominant energy source until replaced by petroleum and natural gas a few decades ago. However, coal is a main source of electric power in the US and still supplies 25% of the world's energy. It is widely agreed that the availability of petroleum in international trade is likely to diminish in the future. Nuclear power, natural gas, petroleum, solar energy and new technologies will not be sufficient to meet the growing energy needs of the world, and coal is expected to return to its position of dominance. Coal is not only used for a direct source of heat or producing steam for generation of electricity and a raw material in the chemical industries but also for the production of a range of gaseous and liquid fuels. During World War 11, over 100,000 barrels per day of high octane fuels were produced from coal in Germany [1]. Concerns continue to mount relative to the use of large

* present address: University of Kentucky, Center for Applied Energy Research, 3572 Iron Works Pike, Lexington, KY 40511 USA "^ This work was supported by the US Department of Energy.

470 quantities (up to 40%) of aromatic compounds in gasoline. These concerns are based on demonstrated toxicity to humans during handling of these fuels prior to combustion, as well as environmental impacts resulting from combustion of these highly aromatic fuels. Present evidence suggests that toxic components of fuels are the aromatic hydrocarbons [2]. Today's gasoline is subject to more demands and expectations than ever before. Regulatory agencies around the world are increasingly exercising their powers to regulate motor gasoline characteristics and mandate the use of deposit-control additives. The main goals are to increase octane number without addition of aromatics and to reduce emissions. The quality of gasolines can vary quite widely depending on crude source, sulfur content and octane number. It is desired to improve octane number, which is a measure of resistance to preignition, by using organic additives. Octane number or antiknock quality of a gasoline primarily depends on the type of hydrocarbons present. Aromatics have the best antiknock quality and isoparaffins, naphthenes, olefins and n-paraffins follow in succession. Refinery processes for the upgrading of gasolines aim at the conversion of other hydrocarbons into aromatics and isoparaffins. The objective of this research is to produce transportation fuel low in aromatics but exhibiting a satisfactory octane rating, from coal-derived liquids. Since coal-derived liquids are high in aromatics, the work has first focussed on conversion of aromatics to other compounds, which are observed to exhibit high octane numbers. The studies were initiated using 1methylnaphthalene as a model compound representative of aromatic compounds found in coalderived liquids. Different hydrogenation, hydroprocessing and hydrotreatment catalysts, NiMo/Ti02-Al203, NiW/Si02-Al203, NiW/Ti02-Al203, NiW/Al203, Pt/REX and Pd/REX, were examined with this feed material. Different temperatures, ranging from 200°C to 450°C and different pressures, ranging from 600 psig to 1500 psig, were investigated. Experiments were conducted in two forms in a stirred batch reactor: single-stage operation and two-stage operation, the model compound was treated with different catalysts at different temperatures. In two-stage operation, 1-methylnaphthalene was first hydrogenated to methyldecalins and subsequently treated with different hydrocracking catalysts at different reaction conditions to obtain compounds having satisfactory octane numbers.

2. EXPERIMENTAL 2.1. Catalyst preparation NiMo/Ti02-Al203 catalyst prepared by incipient impregnation method contained 25 mmol of Ni and 77 mmol of Mo per 100 g of Ti02-Al203 (Engelhard). The support impregnated with an aqueous solution of ammonium molybdate ((NH4)6Mo7024.4H2), Alfa Product) followed by a solution of nickelous nitrate (Ni(Ni03)2.6H20, Fisher Scientific Co.). NiW/Ti02-Al203 and NiW/A^ Q were impregnated in a single step with an aqueous solution containing ammonium meta tungstate ((NH4)6H2Wi204o.3H20, Cerac Inc.) and nickelous nitrate. Resulting material contained 88.9 mmol of Ni and 165.6 mmol of W per 100 g of support. Each support was ground, sieved to -100 mesh and calcined at 540°C for 16 h. The impregnated sample was oven-dried at 120°C overnight and finally calcined at 540°C for 16 h. NiW/Si02-Al203(Harshaw Catalysts), Pt/REX and Pd/REX (proprietary) are commercial catalysts used for the experiments.

471 2.2. Presulfidation All catalysts except Pt/REX and Pd/REX were sulfided in a tubular reactor prior to use. Nitrogen with a flow rate of 60 ml/min purged the reactor to remove air. The catalyst was heated to 400°C under nitrogen flow and kept at this temperature and inert conditions for 1 h. The gas flow then changed to 10% (by wt) hydrogen sulfide in hydrogen. The reactor was maintained at 400 °C at the same flow rate for 2 h. Subsequently it was purged with nitrogen at the same conditions for 1 h to remove the residual amount of hydrogen sulfide and then cooed to ambient temperature. 2.3. Reaction studies Reaction experiments were conducted in a 300 cc batch reactor equipped with a magnetic drive stirrer, heating assembly, gas inlet, and gas collector. Weighed amount of 1methylnaphthalene (Aldrich Chemicals, 98%) and the sulfided catalyst in a feed to catalyst ratio of 10 to 1 were quickly duped into the reactor. The sealed reactor was repeatedly purged with nitrogen and subsequently with hydrogen to replace air, and pressurized with hydrogen and heated to a reaction temperature over about 20 minutes a slow stirring rate (80 rpm). Once the desired temperature was reached, the stirring speed was increased to 800 rpm and the reactor was remained at reaction temperature for 1-10 h. Sulfided fresh catalyst was added to the reaction mixture after 5 h for experiments longer than 5 h. After completion of the reaction, the reactor was cooled to ambient temperature and the products were analyzed. The autoclave was operated either as a sealed-off system, back filled hydrogen at room temperature, or under a constant external hydrogen pressure. Two principle conversion schemes were examined. 2.3.1. Single stage operation 1-Methylnaphthalene was fed into the autoclave with different hydroprocessing, hydrotreatment or hydrogenation catalysts. Reactions were studied under the following conditions: temperature, 325 °C; initial hydrogen pressure, 1000 psig; reaction time, 10 h; feed to catalyst ratio, 10 to 1; stirring rate, 800 rpm. 2.3.2. Two-stage operation This procedure comprised hydrogenation and hydrocracking stages. In the first stage, 1methylnaphthalene was hydrogenated to methyldecalins using a NiMo/Ti02-Al203 catalyst at 325 °C and 1000 psig hydrogen initial pressure. In the second stage, methyldecalins underwent cracking in hydrogen atmosphere at temperatures of 300 to 450 °C and hydrogen initial pressure of 700 psig. using Pt/REX and Pd/REX catalysts. 2.4. Product analysis Liquid products were identified by GC and GC/MS (Hewlett Packard 5890). DB-5 column (30 m x 0.25 mm i.d. x 1.0 /um, J&W Scientific) with a temperature program from 40 260°C. Gas products were analyzed by a Shimadzu GC-14A gas chromatograph, equipped with a flame ionization detector and Chromosorb 102 80/100 column (6'xl/8"x0.0085", Supelco). The column temperature ranged from 40 to 200°C. Scotty standard gases were used for calibration. Methyldecalins from the hydrogenation of 1 -methylnaphthalene were also identified by ^^C-NMR spectroscopy (VXR-400).

472 3. RESULTS AND DISCUSSION Reaction conditions were determined after screening studies with various hydroprocessing, hydrotreatment and hydrogenation catalysts. All conditions remained the same for all experiments changing only the catalyst. The total conversion was defined as Feed - Feed in product %Total Conversion = 100 x — Feed Liquids were weighed just after completion of the reaction, and gas amounts were calculated from gas chromatograms. Liquid product distribution was calculated based on converted material. The products subdivided into four groups. (a) Cycloalkanes (methyldecalins, cyclohexanes and cyclopentanes which are mostly alkyl substituted and 'other cycloalkanes' which include decalins, octahydroindenes, bicycloheptanes) (b) Alkanes {normal and branched) (c) Alkenes and cycloalkenes (d) Aromatics {Methyltetralins, and 'other aromatics' which include bigger than one ring aromatics). For the single stage operation, the reaction temperature was chosen as 325 °C, remaining the pressure and reaction time the same, since screening test reactions at high temperatures resulted in cracking to aromatics. Results are shown in Figure 1. Almost 100% conversion was achieved with each catalyst. The only difference observed was in the depth of ring hydrogenation with the catalysts used. NiMo/Ti02-Al203 catalyst exhibited the hydrogenation activity followed by NiW/Al203 and NiW/Ti02-Al203 catalysts. The aromatics observed with these catalysts were methyltetralins ranging from 0.8 to 28.6%. Pt/REX and Pd/REX exhibited very high cracking activity. NiW/Si02-Al203 catalyst presented quite different results from the other catalysts and showed both hydrogenation and cracking activities. The apparent difference lies in the support material, where Si02-Al203 gives the highest yield of monocyclics. In terms of molecular size or boiling range and octane number, substituted cyclopentanes and cyclohexanes are very desirable components for gasoline. In general, cycloalkanes exhibit higher octane numbers than most of the isoparaffins. The highest octane numbers are observed when the substituent is a methyl group, with the octane number dropping rapidly as the length of the substituent group increases beyond a single carbon atom. The octane number is slightly higher when a cyclic compound contains only one methyl group rather than two methyl groups, and when both methyl groups are attached to the same carbon atom than when the methyl groups are bound to separate carbon atoms [3]. The most effective catalyst for the production of the desirable compounds in a single stage is NiW/Ti02-Al203 catalyst (Harshaw Catalysts). With the conversions to liquids being essentially 100%, this catalyst yielded 38.5% of the 1-methylnaphthalene by weight of cyclopentanes and cyclohexanes in 10 h at 325°C besides forming methyldecalins (15.3%) and 'other' cycloalkanes (17.2%), which are mostly octahydroindenes, decalins and bicyclo[2.2.1]heptanes. Only 1,6% of branched alkanes were observed. Methyltetralins, one ring aromatics and 'other' aromatics are 1.2%, 12.2% and 1.6%, respectively.

473 1-MethyInaphthalene, 3 2 5 ^ ; , 1000 psig, 10 h D Methyldecalins H Cyclohexanes + Cyclopentanes 11 Other Cycloalkanes n Normal Alkanes D Branched Alkanes HI Alkenes + Cycloalkenes M Methyltetralins • Other Aromatics

NiMo/Ti02-Al203

NiW/Ti02-Al203

NiW/Al203

NiW/Si02-Al203

Pt/REX

Pd/REX _L 20

J40

-1. 60

80

100

wt% Of liquid

Figure 1. Yield and distribution of liquid products from hydrogenation/hydrocracking of 1methylnaphthalene using different hydroprocessing catalysts. Difference to 100% is the feed.

For the two-stage operation, the data clearly indicate that methyldecalins are the intermediate compounds in the formation of either isoparaffins or cycloalkanes from methylnaphthalenes. Therefore methyldecalins are preferred feed for the second stage processing. In the first stage of the two stage operation, 1-methylnaphthalene was hydrogenated to methyldecalins with the catalyst, NiMo/Ti02-Al203, shown the highest hydrogenation activity at single stage experiments given in Figure 1. Figure 2 summarizes the results under different reaction conditions, keeping only the temperature constant at 325 °C. A total conversion of 100% was achieved and the yields of methyldecalins were over 92% for each experiment. The yield of methyldecalins only changed from 97.2% to 98.6% as the pressure was increased from 1000 psig to 1500 psig. When only the feed to the ratio was changed from 10/1 to 5/1, 99.5% conversion to methyldecalins was accomplished. In each case, the amount of cyclohexanes and cyclopentanes decreased with an increase in pressure and a decrease in the feed to catalyst ratio due to cracking to small hydrocarbons. Very small proportions of methyltetralins were observed. In the second stage of the two stage operation, methyldecalins from hydrogenation of 1methylnaphthalene were used as feeds with Pt/REX and Pd/REX catalysts after several catalyst screening experiments. Figure 3 shows the yield and distribution of liquid from hydrocracking of methyldecalins as a function of temperature, using Pt/REX. Figure 4 also shows the results with Pd/REX catalyst under the same conditions. As apparent from Figures 3 and 4, these catalysts show almost identical performance and the following discussion is valid for both metals. For both catalysts, total conversion increased as the temperature increased from 300°C to 450°C. The amounts of gas products also increased as a result of cracking reactions at higher temperatures. The yields of cyclohexanes and cylopentanes reached a maximum value (55.2% with Pt/REX and 68.8% with Pd/REX) at 325 °C and started to decrease above this temperature. The decrease of 'other' cycloalkanes and cycloalkenes indicates that they underwent cracking at

474 1-Methylnaphthalene, NiMo/Ti02-Al203, 325°C D Methyldecalins U Cyclohexanes + Cyclopentanes M Other Cycloalkanes M Aikanes M Alkenes + Cycloalkenes SI Methyltetralins • Other Aromatics

1000 psig.1 Oh, 10/1

1500psig, 10h, 10/1

lOOOpsig, 10h.5/1

]

1500psig, 8h.5/1

1500psig, 8h,4/1

0

J

I

20

40 60 wt % of liquid

L--J

\

^

I

80

^

1 L

100

Figure 2. Effects of different variable combinations on the yield and distribution of liquid products from the hydrogenation of 1-methylnaphthalene using NiMo/Ti02-Al203. Difference to 100% is the feed.

Methyldecalins, Pt/REX, 700 psig, 1 h

300

325

350

400

450 40 60 wt % of liquid

Figure 3. Yield and distribution of liquid products from hydrocracking of methyldecalins as a function of temperature using Pt/REX catalyst. Difference to 100% is the feed.

475 Methyldecalins, Pd/REX, 700 psig, 1 h

o o

2 3

s. E

|2

40

60 wt % of liquid

80

Figure 4. Yield and distribution of liquid products from hydrocracking of methyldecalins as a function of temperature using Pd/REX catalyst. Difference to 100% is the feed.

higher temperatures faster than cyclohexanes and cyclopentanes, and their yields were less than 1% at 400-450°C. The amount total alkanes, particularly branched, increased with temperature for both catalysts, but the increase in aromatics is a prominent result of higher reaction temperature. This can be understood from the increase in cyclohexane dehydrogenation and intermediate methyltetralin hydrocracking rates with an increase in temperature. The above results could form the basis for the effective production of desirable components for the reformulated gasoline from a feed rich in two-ring and possibly multiring aromatics. It was observed that the gaseous fractions from the hydrocracking of 1 -methylnaphthalene and methyldecalins are predominantly composed of butanes and pentanes, depending on the type of catalyst and the reaction temperature. The C4-C5 hydrocarbons may be alkylated or polymerized to form isoparaffins in the gasoline boiling or used directly as fuel gas [4]. In one stage operation, the highest yields of methane were obtained with NiMo/Ti02AI2O3, NiW/Ti02-A^0^ and NiW/AJ Q , because these catalysts have higher hydrogenation activity compared to NiW/Si02-Al203, Pt/REX and Pd/REX cracking catalysts as discussed in connection with Figure 1. The highest amount of propane was achieved with Pt/REX and Pd/REX, and isobutane was the highest with NiW/Si02-Al203. In the first step of the two-stage operation, the high yield of methane was reached because of the strong hydrogenation activity of the NiMo/Ti02-Al203 catalyst. In the second step, Pt/REX and Pd/REX showed the highest amounts of isobutanes at 300°C and 325°C. These amounts started to decrease when the temperature was increased. At the optimum condition for the production of cyclohexanes, cyclopentanes and isoparaffins, 325 °C and 700 psig, the amount of gases and the yields of isobutane were about 4% and 64%, respectively with these catalysts.

476 3.1. Potential for high octane gasoline without aromatics It was shown that 38.5% substituted cyclohexanes and cyclopentanes were produced by single stage operation at 325 °C over NiW/Si02-Al203 along with 1.6% of isoparaffins and 12.2% alkenes and cycloalkenes. For the two-stage operation, the highest yield of substituted cyclohexanes and cyclopentanes was also achieved at 325 °C with Pd/REX as catalyst and methyldecalins as feed. These results for both operations are summarized in Table 1. Alkenes and cycloalkenes in the product from the single stage operation can be hydrogenated and converted to isoparaffins, cyclohexanes and cyclopentanes. With recycling of methyldecalins from the first stage to the second stage, these intermediates can be converted in the presence of Pd/REX as catalyst to high octane substituted cyclohexanes and cyclopentanes. In this case, 6.4% cycloalkanes, 0.9% isoparaffins and 0.5% hydrogenated alkenes and cycloalkenes will be formed. For the potential production of high octane gasoline, the yield will reach 60.1% of methylnaphthalene converted to high octane compounds. The results are summarized in Table 2.

Table 1. Summary of the results from single and two-stage conversion of 1-methylnaphthalene over NiW/Si02-Al203 and Pd/REX catalysts.

Liquid Compounds

Total Conversion, wt % Cycloalkanes Methyldecalins Cycloalkanes and Cyclopentanes Others Alkanes Normal Branched Alkenes and Cycloalkenes Aromatics Methyltetralins Others

Product,

wt%

One stage Operation

Tw(HStage Operation

1 -Methylnaphthelene NiW/Si02-Al203 325°C, lOOOpsig, lOh

Methyldecalins Pd/REX 325°C, 700 psig, 1 h

99.9

75.9

15.3 38.5 17.2

68.4 10.7

0.2 1.6 12.2

1.3 9.6 7.6

1.2 13.8

0.4 2.0

477 Table 2. Potential for the production of high octane gasoline components from 1-methylnaphthalene.

From First Stage;

wt%

(T = 325°C, NiW/Si02-Al203) Substituted Cycloalkanes and Cyclopentanes

38.5

IsoparafFms

1.6

Hydrogenated alkenes and cycloalkenes

12.2

From Second Stage; (T = 325°C, Pd/REX) Substituted Cycloalkanes and Cyclopentanes

6.4

IsoparafFms

0.9

Hydrogenated alkenes and cycloalkenes

0.5

Total

60.1

4. CONCLUSION The highest conversion to isoparaffins and substituted cyclohexanes and cyclopentanes, 40.1%, was achieved at 325°C with NiW/Si02-Al203 catalyst for the single stage operation. Almost 100% conversion to methyldecalins was accomplished at 325 °C by hydrogenation of 1-methylnaphthalene, using NiMo/Ti02-Al203 in the first stage of the two-stage operation. Pt/REX and Pd/REX catalysts are very promising catalysts for hydrocracking of methyldecalins without forming aromatics, and Pd/REX appeared the most effective catalyst for the production of reformulated gasoline in a two-stage operation. The total yield of isoparaffins and substituted cyclohexanes and cyclopentanes is 78% at the same temperature with a negligible amount of aromatics. Gas products from single stage operation range from mostly methane to propane, depending on reaction temperatures and type of catalyst. NiW/Si02-Al203 catalyst produced the highest yield of isobutane for the single stage operation whereas Pd/REX catalyst presented the highest yield of isobutane in the second stage of the two-stage operation. In the first step of the latter, gas products are very rich in methane as a result of the strong hydrogenation activity of the catalyst. Considering the yields of isobutanes and isopentanes for possible alkylation to gasolinesize isoparaffin molecules, and the yields of substituted cycloalkanes, the experimental results showed promise to reduce the aromatic content of coal-derived liquids to very low values, while yielding a gasoline product of octane number of 85 or higher. Such an octane number rating is

478 based on the substituted cycloalkanes and isoparaffins, both produced directly from aromatics, and with alkylation of the isobutanes and isopentanes to follow. A potential conversion of 60.1% appears possible for the production of high octane gasoline from 1-methylnaphthalene in a twostage operation. REFERENCES 1. Kleinpeter, J.H. in 'Coal Liquefaction Products Volume T, (Ed H.D. Schultz), John Wiley, New York, 1983, p.5 2. Barbir, F. and Veziroglu, T. N. in 'Clean Utilization of Coal', (Ed Y. Yuriim), Kluwer Academic, Dordrecht, 1992, p. 131 3. 'Knocking Characteristics of Pure Hydrocarbons', American Petroleum Institute Research Project 45, ASTM, Philadelphia, 1958 4. 'NESTE - From Oil to Plastics', (Ed K. Hastbacka), Neste OY, Porvoo, 1993