Energy & Fuels 1994,8,155-162
755
Hydroisomerization and Hydrocracking of n-Hexadecane over a Platinum-Promoted Sulfated Zirconia Catalyst Robert A. Keogh,t Dennis Sparks,? Jianli Hu,$ Irving Wender,$ John W. Tierney,$ Wei Wang,t and Burtron H. Davis*yt Center for Applied Energy Research, University of Kentucky, 3572 Iron Works Pike, Lexington, Kentucky 40511, and Chemical and Petroleum Engineering Department, University of Pittsburgh, Pittsburgh, Pennsylvania 15261 Received September 13, 1993. Revised Manuscript Received January 20,1994”
The hydroisomerization and hydrocracking of n-hexadecane were studied using a sulfated zirconia catalyst containing 0.5 wt 3’% Pt. The conversions were obtained using both batch microautoclaves and continuous trickle bed reactors. Various process conditions were studied in both reactors using a reaction temperature of 150 “C. Hexadecane conversions varied from 5 to 96 wt % depending on the process conditions. At lower conversions (80%),cracking products accounted for most of the total conversion. The asymmetric distribution of the cracked products obtained with this catalyst is probably due to secondary cracking. The addition of adamantane suppressed the cracking yields at higher conversions.
Introduction
low reaction temperatures, high selectivity for isomerization, and pure primary cracking products. For the ideal Hydrocracking has been practiced in modern petroleum hydrocracking of long chain alkanes, the strengths of both refining for the production of light fuels (gasoline, diesel, the acidic and hydrogenation functions have to be carefully and jet fuel) from heavy distillate and residual-‘ since balanced. 1959when Chevron announced their isocrackingp r o ~ e s s . ~ The recent interest in solid superacid catalysts1G20 An excellent review of the development of hydrocracking suggests they have the requirements for an ideal hydrois presented by Sullivan and S ~ o t t . Hydrocracking ~ cracking catalyst provided that the strong acidity function catalysts are dual functional and contain a dispersed metal is balanced with a strong hydrogenation function. The on an acidic support.6 The relative strength of the metal addition of Pt to a sulfated ZrO2 catalyst has been shown function (hydrogenation-dehydrogenation) and the acidic to enhance the stability of the catalyst for the isomerization function may determine the nature and distribution of of butane and pentane at low temperatures.’g Because of the product^.^ Typical acidic supports in hydrocracking the relative thermodynamic stabilities, the low temperacatalysts include amorphous and crystalline silicatures favor the production of isoalkanes which have higher alumina,879alumina,lOand a number of zeolites.11 Typical octane numbers suitable for the production of high-octane hydrogenation-dehydrogenation components are noble reformulated gasoline. Wen and co-workers21studied the metals, e.g., Pt and Pd,9J2 and sulfided nonnoble metals hydrocracking of a long- chain alkane, n-hexadecane, in such as Ni, Co, and Mo.13J4The term “ideal”hydrocracking a batch reactor using a Pt/Zr02/S042-catalyst. However, was introduced to characterize the reactions of n-alkanes Wen et al. concluded that this catalyst was more effective and hydrogen over these ~ata1ysts.l~ The authors stated in hydrocracking than in hydroisomerization of the that the ideal hydrocracking of long-chain alkanes include n-hexadecane feed. They also reported that the isomerization selectivity for the short chain paraffins produced t University of Kentucky. during the cracking was at least 90%. Therefore, the f University of Pittsburgh. catalyst does not appear to be an ideal cracking catalyst a Abstract published in Advance ACS Abstracts, March 15, 1994. based on the data generated using the batch reactor and (1) Choudary, N.; Saraf, D. N. Ind. Eng. Chem. Prod. Res. Diu. 1975, 14, 74. the conditions studied by Wen. (2) A. P.. ACS .-,-Bolten. -. . . ,. .. - Monom. 1976. No. 171.’ 714. The objective of this work is to study the hydrocracking (3) Chen, N. Y. Oil 61 Gas J: 1968,66, 151. (4) ORear, D. J. Ind. Eng. Chem. Res. 1987,26, 2377. and hydroisomerization of n-hexadecane in both batch (5) Sullivan, R. F.; Scott, J. W. ACS Symp. Ser. 1987, No. 222, 293. microautoclaves and continuous trickle bed reactors. The (6) Sullivan. R. F.: Mever, J. A. ACS Symp. Ser. 1975, No. 20, 28. results obtained using the batch reactors serve to define (7) WeitkAp, J. ACS-Symp. Ser. 1975, No. 20, 1. (8)Nace, D. M. Ind. Eng. Chem. Prod. Res. Diu. 1969,8, 24. the activity and selectivity needed to screen the catalyst (9) Van Hook,W. A.; Emmett, P. H. J. Am. Chem. SOC.1962,14,4410. prepared in-house. In addition, various process param(10) Speight, J. G. The Chemistry and Technology of Petroleum,
Chemical Industries; Marcel Dekker, Inc.: New York, 1980; Vol. 3. (11) Bard, W. J.; Huffman, H. C. 8th World Petr. Congr., Moscow Prepr. PD, 1971, 12 (I), 119. (12) Conradt, H. L.; Garwood, W. E. Ind. Eng. Chem. 1960,52, 113. (13) Beuther, H.; Larsen, 0. A. Ind. Eng. Chem. Process Des. Deu. 1965,4, 177. (14) Voorhies,
A.;Smith W. M. Advances in Petroleum Chemistry and Refining, Interscience Publishers, New York, 1964; Vol. 8, p 169. Reitmeyer, H. 0.;Weitkamp, J. J.Erdol (15) Pichler, H.; Schulz, H.; Kohle-Erdgas-Petrochem. 1972,25,494. 0887-0624/94/2508-0755$04.5QIQ
(16) Yori, J. C.; Luy, J. C.; Parera, J. M. Catal. Today 1989,5, 493. (17) Garin, F.;Andreamasino, D.; Abdulaamad, A.; Sommer, J. J.Catal. 1991,131, 199. (18) Nukano, Y.; Izuka, T.; Hattori, H.; Tanabe, K. J . Catal. 1979,57, 1. (19) Hosi,T.;Shimidzu,T.;Itoh,S.;Baba,S.;Takaoka,H.Prepr.-Am. Chem. Soc., Diu. Pet. Chem. 1988, 33 (I), 562. (20) Yamaguchi, T.; Jin,T.;Tanabe, K. J.Phys. Chem. 1986,90,3148. (21)Wen, M. Y.; Wender, I.;Tierney, J. W. Energy Fuels 1990,4,372.
0 1994 American Chemical Society
Keogh et al.
756 Energy &Fuels, Vol. 8, No. 3, 1994 Needle Valve
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eters (e.g., pressure, residence times, etc.) may be quickly evaluated in the batch reactor and then serve to define the conditions for the experiments that follow in the trickle bed reactors. The activity and selectivity data obtained using the trickle bed reactors in a continuous mode are more representative of an industrial process and allow an evaluation of the catalyst for longer run times (50-100 h on-stream).
Experimental Section Catalyst Preparation. Hydrous zirconia was precipitated from a 1.14-L, 0.5 M ZrCL (Alfa Products) solution at a pH of 10.5 by rapidly adding 1.4 L of 15 N NHdOH while vigorously stirring. The resulting precipitate was washed with water by repeated filtration/reslurrying cycles until a negative test for chloride ions was obtained. Analysis of the last wash indicated a chloride content of 3 ppm. The filter cake was dried at 120 "C overnight. The dried filter cake was ground and redried for 1h prior to the addition of sulfate. The dried sample was sulfated using 10 mL of 1 N HzS04 per gram of ZrOz. The slurry was stirred for 1 h, after which the sulfated ZrOz was filtered without further washing. The sample was dried at 120 "C for 2 h prior to impregnation with HzPtCh. The Pt/ZrOz/SOd" catalyst was dried overnight at 120 "C and stored in a vacuum desiccator. The chemical analysis of this catalyst showed the catalyst contained 0.57 wt % Pt and 2.56 wt %
s.
The catalyst was activated prior to testing in either the batch microautoclave or the continuous trickle bed reactor using a temperature of 725 "C for 2 h in air. After activation, the catalyst contained 0.72 w t % S and had a BET surface area of 55 m2/g. The hot, activated catalyst was placed in the dried batch reactors immediately after activation. The reactors were cooled in a desiccator prior to the addition of n-hexadecane for the activity studies. The activated catalyst was cooled in a desiccator prior to loading the trickle bed reactor and activity testing. Batch Experiments. In a typical batch reactor experiment, 2 g of activated catalyst and 4 g of anhydrous n-hexadecane (Aldrich) were loaded in the reactor. The reactor was purged with hydrogen and then pressurized to the desired final Hz pressure. The reactor was heated to 150 "C in a fluidized sand bath and shaken vertically at 400 cpm for the desired residence time. After the experiment, the reactor was rapidly cooled (typically,less than 1 min) to near room temperature and vented to the gas collectionassemblyfor analysis. The reaction products were filtered and stored in a freezer prior to analysis. Continuous Trickle Bed Experiments. A schematic of the trickle bed reactor system used in these experiments is shown in Figure 1. The catalyst was activated and cooled, and typically 8.00g of catalyst was loadedinto the reactor as quicklyas possible. The reactor was installed into the furnace and purged with
Figure 2. Effect of feed to catalyst ratio on the conversion of hexadecane (150 "C,30 min, 250 psig of Hz) in the batch reactors. hydrogen by pressurizing the system (ca. 300 psig) and slowly venting to atmosphere. This cycle was repeated four times, leaving the reactor pressurized on the last cycle and adjusting the back pressure regulator to obtain the desired pressure. A hydrogenflow rate was established using the mass flow controller to obtain a hydrogen to n-hexadecane molar feed ratio of 1.5 to 1.0. After establishing pressure and hydrogen flow set points were established, the furnace was turned on and a steady-state temperature of 150 "C was obtained in approximately 1.5 h. A t this point the syringe pump was turned on to provide the desired feedrate. Liquid samples were taken from the product receiver to establish steady-state conversions for the process variables studied. Analyses. The liquid products were analyzed using a 60 m X 0.32 m DB-5capillary column (0.25 fim thickness) installed in an HP 5890 Series I1 gas chromatograph. Identification of the compounds was accomplishedby using standards and/or by GC/ MS. Gas analyses were performed on a Carle 311H gas chromatograph configured to analyze Hz,CO,, 02,Nz, CHI, and Cz through Cg paraffins and olefins. Quantification of the componentswas done using a primary gas standard (Matheson).
Results A. BatchReactor Studies. The objective of the batch reactor studies were to verify the activity of the catalyst and to establish an initial set of parameters for further study in the trickle bed reactor experiments. The parameters studied, feed to catalyst ratio, pressure, residence time and the effect of adamantane addition, were varied to obtain a large range of conversions which would determine the selectivity changes with conversion. Feed-to-Catalyst Ratio. The initialexperimenta in the batch reactors studied the effect of the feed to catalyst ratio on the conversion, isomerization, and cracking of n-hexadecane (n-Cl6). Data were obtained using a temperature of 150 "C, a residence time of 30 min and a H2 pressure of 250 psig (ambient). The feed to catalyst ratio was varied from 1to 6 by maintaining 2 g of the catalyst and varying the amount of n-Cl6 added to the reactor. The conversion, cracking and isomerization yields are shown in Figure 2 as a function of the feedxatalyst ratio. For the purpose of this discussion, conversion is defined as the disappearance of n-C16 in the liquid phase, isomerization as the yield of iso-CU compounds, and cracking as the yield of compounds with carbon numbers less than 16.
Hydrocracking of n-Hexadecane
Energy & Fuels, Vol. 8, No. 3, 1994 757 L
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As expected, the n-C16 conversionincreases with smaller feedxatalyst ratios. The n-Cu is almost completely converted (97.5 5% conversion) using a 1:l feed-to-catalyst ratio. The yields of cracked products parallel the trend observed for conversion. At the highest conversion, the cracked product yield accounts for 95% of the conversion of the hexadecane and very little isomerization is observed. The yields of iso-Cl6 products remain fairly constant for the ratios of 2-6; however, at a ratio of 1, the yield of iso-C16 compounds decreased significantly. The yields of isomerization (iso-cl~ compounds) products are equal to or greater than the yield of cracked products only at intermediate to low conversions of the n-hexadecane. The distributions of cracked products by carbon number obtained using different feed-to-catalyst ratios are given in Figure 3. No C1, Cz, C14, CIS, or compounds greater than (216 were detected in the products. At catalyst to feed ratios of 1-3, the maximum yield of the cracked products (conversionsof 46-97 % ) occursat carbon number 7. The maximum yield occurred at carbon number 8 using the highest feed to catalyst ratio (lowest conversion,30 % 1. All of the cracked product distributions are asymmetrical. These results indicate that pure cracking is not occurring using these conditions even at low and intermediate conversions. The selectivity to is0 paraffins for the cracked products is over 90% for all of the feedxatalyst ratios studied. The iso/normal ratios of the cracked products are shown in Figure 4. The ratios are similar at the lower carbon numbers (4-8) for the different feed to catalyst ratios; however, at the higher carbon numbers (9-12), there is a lowering of the is0 to normal ratio for the 1:l feedxatalyst experiment. The is0 to normal ratio remains fairly constant for this carbon number range (8-12) using this ratio. The is0 to normal ratio increases with carbon number in the cracked products obtained with the feed to catalyst ratios of 3-6. Effect of Pressure. The effect of hydrogen pressure was studied using a temperature of 150 OC,a residence time of 15 min, and a feed to catalyst ratio of 2:l. The conversion,isomerization, and crackingyields as a function of pressure are shown in Figure 5. The conversion increased from 2 to 70 w t % as the pressure increased from 14.7 to 250 psig. Increasing the pressure further to
Carbon Number
Figure 4. Ratio of is0 to normal paraffinsin the cracked products using differentfeed to catalystratios (sameconditionsas in Figure 2).
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Figure 5. Effect of pressure on the conversion and product distribution of hexadecane (150 "C, 15 min, 2:l n-Cla:catalyst) in the batch reactors. 750 psig increases the conversion from 70 to 83%. The cracking yields parallel the trend observed for conversion. The production of iso-Cle compounds (isomerization) increases with a pressure increase from 14.7 to 100 psig; however, further increases in pressure does not significantly change the yield of isomerization products. As the pressure of the reactor is increased, there is a general shift to lower carbon number where the maximum yield is obtained (Figure 6). The carbon numbers for the maximum yield for the 250 and 500 psi data are the same. All of the carbon number distributions appear to be asymmetrical for the data obtained by varying the feed to catalyst ratios; however, the CSC~products have a larger experimental error. They are present primarily in the gas phase and this causes a greater experimental error in their determination due to the estimation of the system volume and pressure measurements. The selectivity for isoparaffins was 90 % for all the products obtained at different pressures. The is0 to normal paraffii ratios for each carbon number is given in Figure 7; the spike a t C13 is most likely due to experimental error in determining the amount of the normal isomer due to ita very low concentration. This ratio increases with increasing carbon number for all of the pressures studied.
Keogh et al.
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758 Energy & Fuels, Vol. 8, No. 3, 1994 201
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Figure 6. Distribution of cracked products by carbon number using different reactor pressures (same conditions as in Figure 5). 45
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For a mechanism that involves bifunctional catalysis with an alkene intermediate it is expected that the alkene concentration, and thereby the isomerization/cracking conversion, would decrease with increasing hydrogen pressure. This decrease in conversion was not observed in this study: rather the conversion increased with increasing hydrogen partial pressure. Furthermore, the selectivity shifted toward more cracking relative to isomerization as the pressure increased. However, there was only a slight, if any, change in the selectivity for carbon number products within the cracked products. In addition, there was essentially no change in the isoalkaneln-alkane ratio for the cracked products as the hydrogen partial pressure was increased. This implies that there are at least two reaction mechanisms, and that at least one of these has a strong dependence on hydrogen pressure. Effectof Residence Time. The effect of residence time was studied at two pressures (250 and 500 psig) using a temperature of 150 "C and a feed to catalyst ratio of 2. At a pressure of 500 psig, the conversion of n-hexadecane significantly increases with an increase in residence time from 5 to 30 min (Figure 8). At the 30-min residence time, 96% of the n-hexadecane has been converted. The cracking yields also parallel the trend observed for the conversion so that cracking accounts for 95% of the
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Hydrocracking of n-Hexadecane
Energy & Fuels, Vol. 8, No. 3, 1994 769 t
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8 to 6 with increasing residence time. The yield obtained at the maximum carbon number also increases with increasing residence time. Similar to the data previously presented, the carbon number distributions do not appear to be symmetrical. The is0 paraffin selectivity of the cracked products is over 90% for all the samples obtained (Figure 10). The ratios are similar for the carbon numbers from 4 to 6; however, for the carbon numbers from 8 to 12, the ratios decrease with increasing residence time. The is0 to normal paraffin ratios increase with increasing carbon numbers in the cracked products using the 5- and 15-min residence times. The ratio obtained at the 30-min residence time remains fairly constant in the 8-13 carbon range. The effect of residence time was also studied at a hydrogen pressure of 250 psig (Figure 8). The conversion of n-hexadecane increases as the residence time increases from 5 to 15 min. The conversion did not significantly increase by increasing the residence time to 30 min. Similar to the data obtained at the 500 psig of hydrogen pressure, the cracking yields parallel the conversion data. At the lowest residence time, the isomerization yields are similar to the cracking yield. Further increases in the residence time did not substantially increase the yield of iso-Cls compounds. The carbon number distributions of the cracked products, similar to the previous data presented, are asymmetrical (Figure 9b). The carbon number where the maximum yield occurs decreases with increasing conversion. The maximum occurs a t a carbon number of 8 at the 5-min residence time and 7 for the 15-and 30- min residence times. The maximum yields for the three residence times were similar. The is0 paraffin selectivity was greater than 90% for all carbon numbers in the cracked products obtained using the different residence times. The is0 to normal paraffin ratio of the individual carbon numbers obtained at 250 psig exhibit essentially the same trend as obtained at 500 psig (Figure 10). Effect of Adamantune on Cracking Yields. The data obtained using the Pt/ZrOz/SOrcatalyst from the previous experiments suggest that the yields of isomerization and cracking are approximately the same only at the low to intermediate conversion levels. The isomerization and cracking yields based upon n-Cl6 conversion, independent of reaction conditions, are shown in Figure 11. The lines drawn in Figure 11are an interpolation function and do
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not represent a kinetic model. The data show that at low to intermediate conversions(ca. 2-50% ) the isomerization yields are slightly higher than the cracking yields. For conversions greater than 50%,the cracking yields show a rapid increafie with n-hexadecane conversion, and the isomerization yields show a correspondingdecrease. Based on a mechanism that involves an olefin intermediate, these data could indicate that the hydrogenation and acidic functions are not well balanced using any of the process conditions studied for the long-chainparaffin, hexadecane. For this mechanism the acidic function of this catalyst would be much stronger than the hydrogenation function which produces the high cracking yields at the higher conversion levels. In a recent patent,22it was reported that the addition of adamantane to a normal paraffin feed suppressed the cracking yields. The data in the patent indicate that the addition of 0.1-0.8 wt % adamantane to the feed significantlyreduced the cracking yields when using a Pt/ZrOz/ SO4 catalyst and n-heptane or n-octane as the feedstock. The effect of adding adamantane to a n-C16 feed with respect to cracking and isomerization yields was studied using a reaction temperature of 150 "C, a residence time of 15 min, 500 psig of Hz, and a feed to catalyst ratio of 2 to 1. The amount of adamantane added to the n-Cl6 was varied from approximately 0.5 to 5.0 wt % The addition of adamantane did not substantially change the n-hexadecane conversion but did impact product selectivity (Figure 12). At the lower concentrations of adamantane in the n-hexadecane (0.5 and 0.8 wt %), no substantial change was observed in the cracking or isomerizationyields as was reported in the patent for n-heptane and n-octane conversion. However, at a concentration of adamantane of 4.93 wt % , the cracking yield decreased from 59.68 to 36.89% and the isomerization yield showed a corresponding increase. The added adamantane (all concentrations) did not isomerize or crack during the conversion of n-hexadecane. The amounts of adamantane recovered in the products were within experimental error of their concentration in the feed. The lack of conversion adamantane was also reported by Iglesia and c o - w ~ r k e r s . ~ ~
.
(22) US.Patent 5,157,199,1992. (23) Iglesia, E.;Soled, S.L.;Kramer, G. M. J . Catal. 1993, 144, 238.
Keogh et al.
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Figure 13. A comparison of the distribution of the cracked products of hexadecane with and without adamantane added (same conditions as in Figure 15).
As expected, the addition of 4.93 wt % adamantane caused a change in the distribution of the cracked products (Figure 13). The carbon number at which the maximum yield was obtained shifted from 6 (without adamantane) to 8 (with 5 wt 5% adamantane). In addition, higher yields of cracked products at the higher carbon numbers are obtained with the adamantane added. The addition of adamantane also increased the is0 to normal paraffin ratios in the higher carbon number compounds when compared to the distribution (Figure 14) obtained without added adamantane. B. ContinuousTrickle Bed Studies. The objectives of the trickle bed experiments were to establish the effect of a number of parameters (pressure residence time and HZto feed ratio) in the conversion and selectivity of the catalyst in a continuous feed process. The initial variation in each parameter was based on the data obtained in the batch reactors. Further variations in the parameters were done based on the data obtained during the trickle bed runs to obtain a wide range of conversions determine the relationship between selectivity and conversion. Effect of Pressure. The effect of reactor pressure was studied by varying the pressure from 100 to 1000 psig
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using a temperature of 150 "C, a molar Hz to n-Cle ratio of 3 and a WHSV = 1.0. The increase in conversion(Figure 15) with increasing reactor pressure is similar to that observed in the batch reactors. However, the conversions in the trickle bed reactor indicated a continuous increase with increasing pressure and were slightly lower than those obtained in the batch reactors. Parallel to the increase in the conversion, the cracking yields increased, and the fraction which accounted for the majority of the increase in cracking yield was the Cs-Cg fraction (gasoline). The isomerization (iso-cls)yields did not change significantly with increasing reactor pressure. The carbonnumber distribution of the cracked products (Figure 16)obtained in the trickle bed reactor experiments were similar to those with the batch reactor. The
Hydrocracking of n-Hexadecane
Energy & Fuels, Vol. 8, No. 3, 1994 761
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WHSV, g h r per gram catalyst
Figure 18. Conversion of hexadecane as a function of WHSV (150 OC, 370 psig, 3:l Hz/n-C16).
maximum yields occurred at about carbon number of 6. The carbon number distributions were asymmetrical and were similar to those obtained in the batch reactor experiments. The ratios of is0 to normal paraffins for each carbon number (Figure 17) show that although the majority of the products are branched alkanes in both reactors, there are differences in the trends. In the batch reactor experiments, the ratio sharply increases with the carbon number of the products when compared to those obtained in the trickle bed experiments. In the batch experiments, the highest iso-to-normal ratios were obtained at the highest total pressure. The opposite was observed in the trickle bed experiments. The differences observed may be due to the slightly different processing conditions employed in each reactor type such as residence time. Effect of Residence Time. The effect of residence time (WHSV)was studied by holding constant the temperature (150 "C), pressure (370 psig), and the molar Hz:n-Cls ratio 3. The results of these experiments (Figure 18) are similar to the conversion of n-Cl6 in the batch reactor. The carbon number distribution of the cracked products and the iso/normal ratios for each carbon number are typical of the results shown in Figures 16 and 17. The maximum yields occur at carbon number 6 or 7 depending on the degree of conversion. The selectivity to branched hydrocarbons for each carbon number in the product distribution is similar to that shown in Figure 17. Typical of the is0 to normal ratio data obtained in the trickle bed experiments, there is only a marginal increase in this ratio with increasing carbon number of the cracked products. Effect of Hz:n-C16 Molar Ratio. The effect of the Hz: n-C16 molar ratio in the conversion of mC16 was studied using a temperature of 150 OC, a WHSV = 1and 100 psig of hydrogen. The conversion, isomerization, and cracking
Figure 19. Conversion of hexadecane as a function of the molar Hz:n-C16 (150 "C, 100 psig, WHSV = 1).
yields are shown in Figure 19. The conversion of n-Cu increases with increasing Hz:n-Cle ratios and is similar to the results obtained by increasing the total reactor pressure. The amount of cracking also increases with increasing molar ratio and parallels the trend in conversion. The isomerization yields remain fairly independent of the H2:n-Cls ratio. The carbon number distribution of the cracked products exhibits a maximum yield at carbon number 6 or 7, depending on the degree of conversion. The pattern is asymmetrical as observed in all of the previous experiments. The high selectivity to branched alkanes for each carbon number of the cracked products is similar to those obtained in the previous experiments.
Discussion The classical theory of acid catalyzed cracking dates to 1949. Greensfelder, Voge, and Good2* and Thomas25 suggested that a normal paraffin undergoes conversion via a carbenium (designated carbonium ion at that time) ion intermediate by /3 scission to form a linear olefin and another carbenium ion which may be transformed to a hydrocarbon by hydride transfer from another reactant molecule. The products of these processesare linear unless a carbenium ion undergoesisomerizationprior to /3-scission and/or hydride transfer. The data obtained using the Pt/ZrOzSOd catalyst and n-hexadecane indicate a strong preference for branched alkanes in the product (e.g., Figures 4, 7, and 9). There is a unique relationship between the isomerization yields (iso-Cl~compounds) and the conversion of hexadecane (Figure 11). The data indicates that, at low conversions (95%)can be obtained at medium hydrogen pressures (ca. 500 psig) and a temperature of 150 OC. However, at conversions greater than 50 w t % , the majority of the product slate is accounted for in the cracked products. At lower conversions, isomerization yields are slightly higher or equal to the cracking yields. To obtain high yields of branched c16 compounds at the higher conversions, adamantane can be added to the normal paraffin feed. However, for this hydrocarbon, higher concentrations of adamantane are required than those reported for the n-C8 and n-C7 hydrocarbons.22
