Energy & Fuels 2006, 20, 245-249
245
Refining Heavy Neutral Oil Paraffin by Catalytic Hydrotreatment over Ni-W/Al2O3 Catalysts Jesu´s Sa´nchez,† Marı´a Fernanda Tallafigo,‡ Miguel A. Gilarranz,*,§ and Francisco Rodrı´guez| E.T.S.I. Minas, C/Rı´os Rosas, 21, 28003 Madrid, Spain; E.T.S.I. Caminos Canales y Puertos, Ciudad UniVersitaria, 28040 Madrid, Spain; AÄ rea de Ingenierı´a Quı´mica, Facultad de Ciencias, UniVersidad Auto´ noma de Madrid, 28049 Madrid, Spain; and Departamento de Ingenierı´a Quı´mica, Facultad de Quı´mica, UniVersidad Complutense de Madrid, 28040 Madrid, Spain ReceiVed May 12, 2005. ReVised Manuscript ReceiVed October 11, 2005
Four sulfided Ni-W/Al2O3 catalysts were tested in the hydrotreatment of microcrystalline raw paraffin from the dewaxing of heavy neutral oil to obtain high grade paraffin wax. The influence of both reaction conditions (temperature, hydrogen pressure, and space velocity) and catalyst characteristics (tungsten content and porosity) on the properties of the treated paraffins was studied. A similar influence of the operating variables was found for the four catalysts studied. In general, temperatures well above 300 °C are needed to obtain transparent and colorless paraffins (Saybolt color >+30); however, rising temperature results in a higher oil content due to cracking reactions. The reaction temperature required to achieve acceptable color values can be reduced by increasing hydrogen pressure, which also reduces paraffin cracking. The rise in the tungsten load of the catalysts from 16.2 to 24.2% (w, WO3 basis) results in higher Saybolt color values, although a simultaneous increase in the extent of cracking reactions takes place and a pressure above 140 kg/cm2 is required to reduce oil generation. The enhancement in the performance of the catalyst can also be successfully accomplished by increasing simultaneously the porosity and tungsten load of the catalyst, which enables operation at milder conditions of pressure and temperature (40 kg/cm2, 310 °C) while maintaining acceptable Saybolt color and a low content of oil and carbonizable substances.
Introduction Hydrotreatment of oil wax feedstocks is the most important process for the production of high grade waxes, displacing the formerly used sulfuric acid process due to advantages such as continuous operation, high products yield, flexibility with regard to feedstocks, hydrogenation of aromatics, degradation of sulfur and nitrogen compounds, low corrosion, and reduced environmental impact.1 The raw wax feedstocks are obtained in the deparaffining of crude-oil oils in lube plants, usually by crystallization of the wax components by dilution with solvent and chilling, and subsequent refining by adsorption.2 The main purpose of wax hydrotreatment is the removal of unsaturated compounds and heteroatoms associated with hydrocarbons that confer dark color.3 Thus, the colorless and totally transparent materials of good color stability and low oil content that are required for food grade can be obtained. Likewise, the removal of heteroatoms and the conversion of aromatics to naphtenes and paraffins stabilize the product and prevent its aging.4,5 * Corresponding author. Fax: +34 91 497 35 16. E-mail:
[email protected]. † E.T.S.I. Minas. ‡ E.T.S.I. Caminos Canales y Puertos. § Universidad Auto ´ noma de Madrid. | Universidad Complutense de Madrid. (1) Koog, W. Trends in Lube Oil and Wax Production; Uhde Edeleanu GmbH Report: Alzeanu, Germany, 1999. (2) Heinrich, G. Petroleum Refining: Introduction to Refining; Technip: Paris, 1995; Vol. 1, pp 364-413. (3) Bergeron, I.; Charland, J. P.; Ternan, M. Energy Fuels 1999, 13, 686-693. (4) Callejas, M. A.; Martı´nez, M. T. Energy Fuels 1999, 13, 629-636. (5) Singh, H. Hydrocarbon Asia, Jan/Feb, 2003, pp 40-46.
In the hydrotreatment processes the waxes are contacted with hydrogen at high temperatures and relatively elevated pressures in the presence of a catalyst. Under these conditions, several reactions such as isomerization, cracking, dehydrocyclization, cyclization, aromatization, and hydrogenolysis take place, being the extension of such reactions influenced by the characteristics of raw paraffin feedstock, the reaction conditions, and the catalytic system employed.6,7 As a result of such reactions, the heterocyclic compounds are hydrogenated and the heteroatoms contained in the feedstock are either removed in the gas phase (H2S, NH3, H2O) or deposited on the catalyst surface (NiSx, VSx).8 On the other hand, large hydrocarbon molecules are hydrogenated and cracked. In the case of n-paraffins, the skeletal isomerization to give branched isoparaffins can also be of importance,9 because it lowers the melting point and reduces the stiffness of the treated paraffin.7 The higher tendency of multibranched isoalkanes to crack determines a relatively low occurrence of multibranched compounds,7 although it has been found that the extent of isomerization reactions increases with temperature, resulting in a higher iso/n paraffin ratio and promoting the yield of polybranched products.10 (6) Ali, L. I.; Ali, A. A.; Aboul-Fotouh, S. M.; Aboul-Gheit, A. K. Appl. Catal., A 2001, 205, 129-146. (7) Calemma, V.; Peratello, S.; Stroppa, F.; Giardino, R.; Perego, C. Ind. Eng. Chem. Res. 2004, 43, 934-940. (8) Eijbouts, S. Deactivation, regeneration and recycling of hydroprocessing catalysts. In EnVironmental Catalysis; Janssen, F. J. J. G., van Santen, R. A., Eds.; Imperial College Press: London, 1999; pp 345-362. (9) Liu, Y.; Liu, C.; Que, G. Energy Fuels 2004, 16, 531-353. (10) Calemma, V.; Peratello, S.; Perego, C. Appl. Catal., A 2000, 190, 207-218.
10.1021/ef050143c CCC: $33.50 © 2006 American Chemical Society Published on Web 11/11/2005
246 Energy & Fuels, Vol. 20, No. 1, 2006
Cracking is a major concern in the hydrotreatment of paraffins since it reduces long-chain paraffins yield and increases the low molecular weight compounds content, usually grouped by the term oil. When linear paraffins are subjected to hydrotreatment, the cracking reactions always take place at a certain extent.10,11 It has been reported that rising temperature and reaction time result in higher cracking12 together with a higher conversion to isoparaffins due to a higher contribution of isomerization reactions. Likewise, high temperatures shift the thermodynamic equilibrium toward aromatics formation.13 On the other hand, the cracking of long-chain linear paraffins has been reported to take place in a lower extent at high pressure, in contrast to the behavior observed for light paraffins.10,14 The use of bimetallic catalysts is a common practice in hydrotreatment processes. The catalysts based on noble metals are known to show a higher selectivity toward the hydrogenation of aromatics and heteroatoms,10 although those based on Ni, Co, W, and Mo are usually employed to treat feedstocks with a high concentration of nitrogen and sulfur.15 Ni-Mo and Ni-W catalysts combine a high selectivity in the hydrogenation of aromatics and nitrogen compounds and a good performance for hydrodesulfurization.16,17 Ni-Co and Mo-W catalysts are activated by presulfiding in situ or ex situ to obtain the socalled sulfidic catalysts, which show a higher activity.8,17 The support of the catalyst also plays an important role in the hydrotreatment reactions due to the significance of the acidic sites in the isomerizations and cracking reactions.10,17 Alumina is the most used support due to the good removal of heteroatoms and color achieved with alumina-based catalysts, although other supports have been tested to improve characteristics such as dispersion, deactivation behavior, and reducibility of the precursor.18 Catalysts supported on alumina are more active in hydrodesulfurization, probably due to a more efficient pathway for the reaction, whereas catalysts based on molecular sieves exhibit characteristics such as a low generation of isoparaffins in hydroisomerization reactions due to spatial constraints.11,19 In a former work,20 alumina-supported catalysts based on a Ni-Mo and Ni-W active phase were evaluated for the hydrotreatment of microcrystalline raw paraffin from the dewaxing of heavy neutral oil. It was found that Ni-W catalysts require lower temperature to achieve acceptable Saybolt color and oil content values. In the present work, we tested four different Ni-W catalysts to learn about the influence of operating variables and catalyst characteristics on the properties of hydrotrated paraffin. The color, oil, and carbonizable substances contents were measured by standard tests and were considered to evaluate the treated paraffins. These properties are commonly used in the industrial practice to assess the suitability of paraffins for high grade applications. (11) Walendziewski, J.; Pniak, B. Appl. Catal., A 2003, 250, 39-47. (12) Barton, D. G.; Soled, S. L.; Meitzner, G. D.; Fuentes, G. A.; Iglesia, J. J. Catal. 1999, 181, 57-72. (13) Yang, H.; Wilson, M.; Fairbridge, C.; Ring, Z. Energy Fuels 2002, 16, 855-863. (14) Froment, G. F. Catal. Today 1987, 1, 455-473. (15) Bej, S. K.; Dalai, A. K.; Adjaye, J. Energy Fuels 2001, 15, 11031109. (16) Grange, P.; Vanhaeren, X. Catal. Today 1997, 36, 375-391. (17) Prins, R. Hydrotreating Reactions. In Handbook of Heterogeneous Catalysis; VCH: Weinheim, Germany, 1997; pp 1908-1928. (18) Luck, F. Bull. Soc. Chim. Belg. 1991, 100, 781-800. (19) Campelo, J. M.; Lafont, F.; Marinas, J. M. Appl. Catal., A 1998, 170, 139-144. (20) Sanchez, J.; Tallafigo, M. F.; Gilarranz, M. A.; Rodrı´guez, F. Energy Fuels 2004, 15, 1492-1499.
Sa´ nchez et al. Table 1. Properties of the Raw Paraffin Feedstocks oil content (%, w; ASTM-D-721) viscosity at 210 °F (cSt; ASTM-D-445) Saybolt color (ASTM-D-156) ASTM color (ASTM-D-1500) needle penetration at 25 °C (mm; ASTM-D-1321) carbonizable substances test (ASTM-D-612)
0.36-1.12 5.84-6.70 +30 0.36 0.10 pass
350 100 0.77 +22 0.46 0.11 pass
350 100 1.0 +18 0.46 0.04 pass
350 140 0.7 +28 0.38 0.04 pass
380 100 0.78 +29 0.39 0.09 not pass
1
1-2
3-4
6-7
4-5
1-2
1-2
3-4
1-2
1-2
1-2
5-6
0.10
0.16
0.39
0.73
0.61
0.18
0.21
0.72
0.20
0.24
0.23
0.72
porosity, although the strength of this catalyst can be considered as acceptable for commercial use. Prior to the paraffin hydrotreatment reactions the catalysts were sulfided in situ during 5 h with a 4.2% terc-butyl disulfide solution in heavy gas-oil and under a hydrogen flow corresponding to a hydrogen-to-liquid volumetric ratio of 100. The treatment was carried out at 320 °C, 11 kg/cm2, and a space velocity of 1 h-1.
Results and Discussion Table 3 summarizes the characteristics of the paraffin samples treated over Ni6W16D25 catalyst under different conditions. Temperature shows a noticeable influence on the Saybolt color of the treated samples. For the runs carried out at a reaction pressure of 100 kg/cm2 and when a temperature of 315 °C is employed, a very poor Saybolt color value of -11 is obtained despite the relatively severe pressure conditions. As the temperature is increased to 350 °C, the Saybolt color value increases dramatically up to +26, which is indicative of a good removal of the sulfur, nitrogen, and aromatic compounds that contribute to dark color. Further increase in the reaction temperature up to 375 °C results in >+30 Saybolt color values, although the oil content also increases, showing the occurrence of cracking reactions. Likewise, at 375 °C and 100 kg/cm2, a failure in the carbonizable substances test was observed due to the high oil content of the paraffin treated and the generation of branched paraffins. To obtain the >+30 Saybolt color value required for food grade paraffin with an acceptable oil content, the pressure must be increased up to 140 kg/cm2, which enables a good removal of dark color compounds due to a higher availability of hydrogen at the catalyst surface, whereas excessive cracking is prevented. The effect of pressure in the prevention of long-chain paraffins cracking has been attributed to the inverse dependence of cracking rate constant on hydrogen pressure,10 contrary to the behavior displayed by lighter paraffins.14 A pressure of 140 kg/cm2 also makes it possible to reduce to 350 °C the reaction temperature needed for a >+30 Saybolt color, which contributes to less cracking and lower content of carbonizable substances as well. The important dependence of color on pressure can also be seen from the results of the run carried out at 60 kg/cm2, where the Saybolt color drops to a value of +20 for the reaction at 350 °C (see Figure 1). Rising reaction pressure and temperature can contribute to increase slightly the space velocity required for high Saybolt
color values, which would result in the use of a lower load of catalyst. Thus, for a pressure of 140 kg/cm2 and a temperature of 375 °C, a substantial increase in the space velocity from 0.5 to 0.78 h-1 only leads to a slight drop in Saybolt color to +29, with good values for oil and carbonizable substances contents. The sensitivity of Saybolt color to the modifications in space velocity becomes more important as pressure and temperature are reduced. It is interesting to indicate that, in a previous work,20 where a heavy neutral oil paraffin from another source was hydrotreated with a catalyst of characteristics similar to those of Ni6W16D25, it was reported that at 100 kg/cm2 a reaction temperature as low as 300 °C was enough to achieve >+30 Saybolt color values. This fact is indicative of the very important influence of paraffin source on its treatability. The results shown in Table 4 for catalyst Ni6W24D25 indicate that when the tungsten load is increased from 16.2 to 24.2% (w, WO3 basis) the destruction of dark color compounds only increases slightly when compared to the results for catalyst Ni6W16D25 (Table 3). Thus, Figure 1 shows that at 350 °C, 100 kg/cm2, and a space velocity of 0.5 h-1, the Saybolt color value only increases from +26 to +28. The same difference can be observed for the reaction runs at 350 °C and 60 kg/cm2 (i.e., the Saybolt color increases from +20 to +22). A possible explanation for the moderate improvement in color removal
Figure 1. Saybolt color and oil content increase of hydrotreated paraffin for catalysts Ni6W16D25, Ni6W24D25, and Ni6W12D30 (350 °C, 0.5 h-1).
248 Energy & Fuels, Vol. 20, No. 1, 2006
Sa´ nchez et al.
Table 4. Characteristics of Raw Paraffin Hydrotreated over Catalyst Ni6W24D25 temperature (°C) pressure (kg/cm2) space velocity (h-1) Saybolt color initial oil content (%, w) oil content increase (∆%, w) carbonizable substances ASTM-D-612 carbonizable substances CFR-LN-6 T, variant B
350 60 0.5 +22 0.36 0.22 not pass 1
350 100 0.5 +28 0.36 0.25 pass
350 140 0.5 >+30 0.36 0.09 pass
3-4 1
350 200 0.5 >+30 0.36 -0.12 pass 1
350 100 1.0 +16 0.36 -0.02 not pass 4-5
300 200 0.5 +19 0.36 -0.01 pass 1
could be the reduction in surface area and pore volume caused by the impregnation with a higher metallic load. Thus, in a previous work it was reported that during the preparation of the catalyst pore mouths are obstructed by the deposition of the active phase and the microporosity is substantially reduced, although it was found that the microporosity is of relatively low relevance in the hydrotreatment of raw paraffins.20 In contrast to the poor influence on Saybolt color, the higher tungsten load of catalyst Ni6W24D25 gives rise to a significantly higher cracking of paraffins, which in general leads to a higher oil content and a rise in carbonizable substances. As in the case of catalyst Ni6W16D25, for catalyst Ni6W24D25 increasing pressure prevents paraffin cracking and oil generation. However, at 140 kg/cm2 the oil and carbonizable substances content is still excessive and higher pressures are needed. It can be seen that at 200 kg/cm2 treated paraffin with very low oil content could be obtained, and even a reduction in oil content takes place, which can be interpreted as the hydroisomerization of branched paraffins and/or as the cracking of light paraffins and the subsequent removal of the products lighter than C7 in the stripping column. Another common characteristic of catalysts Ni6W24D25 and Ni6W16D25 is the important decrease in Saybolt color value with increasing space velocity, as can be seen from the runs carried out at 350 °C, 100 kg/cm2, and 0.5-1.0 h-1 (see Tables 3 and 4). On the other hand, the generation of oil decreases substantially with increasing space velocity, which is in agreement with the higher cracking degree reported for a long reaction time.12 Therefore, the comparison of catalysts Ni6W24D25 and Ni6W16D25 indicates that the use of high loads of tungsten leads to an undesirable rise in paraffin oil content that is not
accompanied by an important improvement in Saybolt color. To prevent oil generation a very high pressure would be needed. Catalyst Ni6W12D30 was prepared with a load of active phase similar to that of catalyst Ni6W16D25 and from a support of higher porosity and surface area. Thus, the runs with this catalyst were intended to study the influence of porosity by comparison with catalyst Ni6W16D25. The results obtained can be found in Table 5. In general, it can be seen that the higher porosity of catalyst Ni6W12D30 results in a higher color removal, providing Saybolt color values of >+30 at 100 kg/cm2 for a reaction temperature of 45 °C lower than that required for catalyst Ni6W16D25. The increase in color removal seems to be the result of the better access of paraffins and dark color compounds to the pores of the catalyst, even despite the higher diameter of the extrudates (3 versus 2.5 mm), such access being favored by the relevant porosity of the catalyst. The catalyst of higher surface area is also expected to have a higher number of positions for the anchorage of the metal precursor, thus leading to a higher dispersion. Another interesting characteristic of catalyst Ni6W12D30 is the selectivity toward the destruction of color contributing compounds. Thus, during the hydrogenation a very low increase in the oil content by cracking reactions takes place, and even slightly lower values than those for catalyst Ni6W16D25 can be seen (Figure 1), although the difference cannot be considered as significant. The carbonizable substances content can also be considered as comparable, even though the raw paraffin feedstocks employed in the runs with catalyst Ni6W12D30 have in general a higher oil content. The role of pressure in the prevention of cracking reactions can be clearly seen when the runs at 40 and 100 kg/cm2 are compared at 350 °C (see Figure 1). At this temperature, a reaction pressure higher than 100 kg/cm2 would not be needed to obtain paraffin of good characteristics; however, a pressure of 140 kg/cm2 would make it possible to increase the space velocity up to 1.0 h-1 while maintaining a Saybolt color of +30 and providing a reduction in oil content. Thus, in these conditions the results for carbonizable substances would be slightly better. This reduction of carbonizable substances with increasing space velocity can also be observed at 100 kg/cm2. The combined effect of porosity and content of active phase can be studied from results of the runs carried out with catalyst
Table 5. Characteristics of Raw Paraffin Hydrotreated over Catalyst Ni6W12D30 temperature (°C) pressure (kg/cm2) space velocity (h-1) Saybolt color initial oil content (%, w) oil content increase (∆%, w) carbonizable substances ASTM-D-612 carbonizable substances CFR-LN-6 T, variant B carbonizable substances CFR-LN-6 T, variant C
300 100 0.5 +4 0.49 0.01 pass
315 100 0.61 +20 0.84 0.02 pass
330 100 0.5 >+30 0.63 -0.04 pass
350 100 0.5 >+30 0.67 0.09 pass
330 40 0.57 +21 0.84 0.06 pass
350 40 0.5 +27 0.82 0.16 not pass
350 140 0.5 >+30 0.65 0.05 pass
350 140 1.0 >+30 1.12 -0.02 pass
350 100 1.0 +27 0.90 0.02 pass
1-2
1
2
2-3
3-4
5-6
3-4
1-2
1-2
0.20
0.11
0.29
0.34
0.37
0.70
0.42
0.23
0.21
Table 6. Characteristics of Raw Paraffin Hydrotreated over Catalyst Ni4W28D20 temperature (°C) pressure (kg/cm2) space velocity (h-1) Saybolt color initial oil content (%, w) oil content increase (∆%, w) carbonizable substances ASTM-D-612 carbonizable substances CFR-LN-6 T, variant B
290 100 0.5 +28 0.81 0.02 pass
310 100 0.5 >+30 0.85 -0.03 pass
330 100 0.5 >+30 0.84 0.05 pass
350 100 0.5 >+30 0.79 0.09 pass
350 100 0.76 >+30 0.89 0.15 pass
350 100 1.0 >+30 1.01 0.10 pass
290 40 0.5 +24 0.61 0.02 pass
310 40 0.5 >+30 0.62 0.09 pass
350 40 0.5 >+30 0.95 0.20 not pass
330 40 1.0 +20 0.60 0.09 pass
340 40 1.0 +26 0.66 0.02 not pass
310 80 0.5 >+30 0.81 0.04 pass
310 60 0.5 >+30 0.35 0.04 pass
+30 0.93 0.08 pass
310 1.0 +20 1.05 0.04 pass
330 1.0 >+30 0.70 0.01 pass
330 1.6 +18 1.00 0.01 pass
350 1.55 +30 0.84 0.03 pass
290 0.5 +30 0.89 -0.06 pass
1-2
1-2
+30 value is maintained when space velocity is increased slightly from 0.5 to 0.79 h-1, but for 1.0 h-1 the Saybolt color drops to a +20 value. Thus, the increase allowable for the space velocity depends on reaction temperature. To maintain Saybolt color values of +30 at 330 °C, the space velocity can be increased up to 1.0 h-1 and at 350 °C values as high as 1.5 could be employed. It is also interesting to indicate that the use of high pressure (e.g., 140 kg/cm2) results in an important limitation of the generation of oil. Conclusions The catalysts tested showed a similar dependence on operating variables, with a high influence of temperature on Saybolt color. The Saybolt color of the paraffins increases significantly at high temperatures at the expense of an increase in oil content. In general, temperatures well above 300 °C are needed to obtain paraffins with Saybolt color of >+30 and low carbonizable substances content. Rising pressure favors the hydrotreatment of dark color compounds and reduces oil content due to the prevention of cracking reactions. The catalysts studied are very sensitive to changes in space velocity; in most cases, important reductions in Saybolt color were observed for a space velocity higher than 0.5 h-1. For the catalysts prepared from the same support, the rise in the tungsten load from 16.2 to 24.2% (WO3 basis) only resulted in a slight improvement in Saybolt color value, probably as a consequence of the loss of surface area, whereas the generation of oil increased significantly. To prevent oil generation, very high pressures, around 200 kg/cm2, would be needed. Better results were obtained by increasing the porosity of the catalyst and maintaining the metal load. In these conditions, paraffins of >+30 Saybolt color were obtained even when the reaction temperature was lowered to 310 °C, which also enabled an interesting reduction in the oil content. Likewise, when 350 °C and 140 kg/cm2 were employed, it was possible to increase the space velocity from 0.5 to 1.0 h-1 without significant changes in color. Finally, the simultaneous increase in the porosity of the catalyst and in the concentration of tungsten enabled additional reduction in the reaction temperature required. Thus, temperatures of 310 °C were enough even for the reactions carried out at 40 kg/cm2. EF050143C
