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Energy & Fuels 2000, 14, 52-55
Changes in Asphaltene Stability during Hydrotreating Jesper Bartholdy† and Simon Ivar Andersen*,‡ Haldor Topsøe A/S Research Laboratories, DK-2800 Lyngby, Denmark and Technical University of Denmark (DTU), DK-2800 Lyngby, Denmark Received June 10, 1999. Revised Manuscript Received October 19, 1999
The conversion of asphaltenes in heavy oil hydroprocessing is important as the asphaltenic fraction often contains the most refractive components. In residual oil feeds, the content of asphaltenic sulfur amounts to less than 20% of the total sulfur content; in hydrotreated oil products, however, more than 60% of the sulfur content may be asphaltenic. When hydrotreating heavy oils, both the non-asphaltenic phase and the asphaltenes undergo a chemical conversion, which may lead to unstable products as well as to increased coke lay-down on the catalyst. To be able to explain the behavior of different catalysts, the asphaltene conversion and particularly the stability of asphaltenes in hydrotreated heavy oil products were investigated. The stability of the asphaltenes was characterized by a flocculation onset titration procedure. With this method, several different concentrations of oil in toluene were titrated with heptane, and the stability parameters were derived. This study shows how the product stability is affected by changes in reaction temperature. The critical solubility parameter for the solubility of asphaltenes follows the assumed chemistry of the hydrotreating processes in question. At low severity hydroprocessing, the stability of the product increases, but when the severity is increased, the asphaltene stability is reduced.
Introduction The purpose of hydrogenation of petroleum and petroleum residua is either to convert low-grade material into valuable products by removal of undesirable elements such as sulfur or to convert residua into highvalue liquid fuels by hydrocracking. A vast number of reactions involving both cracking and hydrogenation take place during the upgrading process, and it is difficult to give an overall picture of the conversion chemistry. During catalytic conversion, the temperature of the reactor is increased to offset the deactivation of the catalyst, and thermal cracking reactions thus become more pronounced. The high temperature toward end of run can lead to the formation of insoluble material (sludge), which may cause catalyst deactivation. The stability of the asphaltenes is a fine balance between intermolecular associations and solvent-solute interactions and constitutes an important aspect of process and catalyst development. In the present work, the use of flocculation onset titration is demonstrated to be a powerful tool to follow changes in the stability of products during hydrotreating. Experimental Section Hydrotreatment. Arabian Heavy atmospheric resid was hydrotreated to various degrees in a benchscale test unit. The test equipment and the procedures used are described in a previous publication by Nielsen.1 The tests were carried out * Author to whom correspondence should be addressed at Technical University of Denmark (DTU). Phone: + 45 45 25 28 67. Fax +45 45 88 33 58. E-mail
[email protected]. † Haldor Topsøe A/S Research Laboratories. ‡ Technical University of Denmark (DTU).
Table 1. Feedstock Specification Feedstock: origin sulfur, wt % nitrogen, ppm CCR, wt % asphaltenes, C7 wt % metals, ppm
Arabian Heavy 4.31 2800 11.4 5.9 114
on a catalyst filling consisting of three catalyst types: HDM (TK-711, NiMo low metal, davg ) 170 Å), transition (TK-751, medium metal, davg ) 135 Å), and HDS (TK-771, davg ) 105 Å). The ratio of the three catalysts was fixed in all experiments (5/30/65 TK-711/TK-751/TK-771). The feedstock tested was an Arabian Heavy atmospheric resid with a composition as given in Table 1. Details are discussed below. Product Analysis. The metal content of the products was determined by ICP/OES Optima 3000 (ASTM D5708-95) and the liquid product sulfur content by an Oxford Lab-X 3000 sulfur analyzer (ASTM 4294). Nitrogen was determined by combustion analysis. The asphaltene content of the products was determined by the IP 143 method. After asphaltene analysis, the dried asphaltenes were collected and their chemical composition determined by combustion analysis. Flocculation Onset Titration. The stability of asphaltenes in oil samples can be analyzed by titration of a toluene solution of the oil sample with a nonsolvent or precipitant (heptane). The titration is continued until a solid phase appears. The onset of precipitation or incipient flocculation threshold was obtained through automatic titration of dilute toluene solutions of either asphaltenes or crude oil. The titration system consists of a Radiometer ABU93 autoburet, a Pharmacia P1 peristaltic pump, and a Beckmann spectrometer. The wavelength was 740 nm, and flow cuvettes were either 1 or 10 mm in path length. Data acquisition and system (1) Nielsen, A.; Cooper, B. H.; Jacobsen, A. C. Prepr. Pap.sAm. Chem. Soc., Div. Petrol. Chem. 1981, 26, 440.
10.1021/ef990121o CCC: $19.00 © 2000 American Chemical Society Published on Web 12/17/1999
Changes in Asphaltene Stability during Hydrotreating
Energy & Fuels, Vol. 14, No. 1, 2000 53
Table 2. Processing Conditions during Hydroprocessing temperature
360-390 °C
H2 partial pressure LHSV H2/oil
130 bar 0.22-0.85 h-1 1100 Nl/l
control were performed using a PC. The titration rate was found to have a significant effect, and in order to minimize local precipitation during titration, the rate of heptane addition was kept low. An addition rate of 60 µL/min was found to be sufficiently low to avoid local precipitation. This rate is significantly lower than titration rates described in the literature, which are about 1 mL/min. As the titration commences, the light transmission increases due to dilution of the sample. At a certain point, the asphaltene particles flocculate to a size that scatters the light, and the turbidity of the solution increases, signifying the end point of the titration. For each sample, 3-4 titrations are made, each with different concentrations of oil dissolved in toluene to take the effect of dilution into account. A plot of precipitant/mass oil versus the solvent/ mass oil will show a linear relationship (see also Figure 4). The intercept on the y-axis gives information on the stability of the asphaltenes in the undiluted oil. The slope of the line shows the critical solubility parameter at which the asphaltenes will start to precipitate. A detailed review of the method has recently been published.2
Results and Discussion A severe hydrotreatment of Arabian Heavy AR was carried out in a laboratory bench scale test unit. The hydrotreatment was carried out with commercially available hydroprocessing catalysts. The test equipment and the catalysts used are described in the Experimental Section. Table 1 gives the analytical specification of the feedstock used during the experiment. The product sulfur level was between 0.1 and 0.8% S. Content of metals (Ni + V) was reduced to 1-45 ppm, and product asphaltenes were in the range from 0.3 to 3.6 wt %. The determination of the conversion obtained was based both on the process conditions applied and on the stage of aging of the catalyst system. The range of process conditions used during the experiments is given in Table 2. The samples generated in this study were obtained from several different runs with the same type of catalyst but of varying durationsfrom a few hours to several thousand hours. Prior to the exposure to Arabian Heavy, the catalysts were sulfided by a straight run Arabian Heavy VGO with 3% sulfur. The size of the catalyst pellets used for the test was 1/22”TL. The liquid products were analyzed for content of S, Ni + V, and asphaltenes. The analyses were carried out on the total liquid product; i.e., light products were not removed prior to analysis. From a literature study3 it was established that more than 90% of the Ni + V in the Arabian Heavy is present in the asphaltene fraction of the resid. The remaining metals are mainly found in the resins. Figure 1 gives the sample asphaltene content vs the Ni + V content of the product. The figure clearly shows that as the asphaltene content is reduced, the product Ni + V decreases. The fact that the curve has an upward bend and that the asphaltenes contain most of the metals in the feed suggests that the fraction of (2) Andersen, S. I. Energy Fuels 1999, 13, 315. (3) Reynolds, J. G. Petroleum Chemistry and Refining; Taylor and Francis: Bristol, PA, 1998.
Figure 1. Ni +V content in feed and product vs amount of asphaltenes. Table 3. Chemical Composition of Asphaltenes reaction temperature, °C
N, wt %
C, wt %
H, wt %
S, wt %
O, wt %
359.4 359.8 359.8 359.8 369.7 378.8 379.2 379.3 380.0 389.0 394.8 399.8 401.5
1.26 1.25 1.21 1.24 1.26 1.26 1.07 1.09 1.25 1.07 1.21 1.23 1.24
83.23 83.90 83.37 83.21 83.12 82.87 83.38 83.57 84.23 84.01 84.93 85.19 88.73
7.90 7.90 8.15 8.03 7.81 7.62 7.96 7.75 7.62 7.67 7.15 7.22 6.75
7.47 6.51 6.97 6.95 7.09 7.59 6.20 7.24 6.16 6.6 5.33 4.98 2.86
1.33 1.39 1.34 1.40 1.33 1.15 1.34 1.52 1.48 1.32 2.40 1.68 1.67
feed
1.20
83.17
7.61
8.16
1.53
asphaltenic metals increases as the metals are converted, i.e., asphaltenic metals are the least reactive of the metals present in the feed. The composition of the asphaltenes was determined by elemental analysis. The major constituents of the asphaltenes were C, H, S, O, N, V, and Ni (in order of decreasing abundance); however, in this study only C, H, S, O, and N of the asphaltenes were determined. The chemical composition product of the asphaltenes is given in Table 3. Despite the different temperature used in the tests, the sulfur, nitrogen, and oxygen contents of the samples did not change significantly except for the samples converted at the highest operating temperatures (at or above 394.8 °C). In these samples, the sulfur level was reduced significantly. At low-temperature treatment, the composition of asphaltenes remained the same as that of the feed. However, at a higher temperature of operation, the composition changed as the sulfur content in the asphaltenic fraction decreased. This might indicate a significant structural change of the asphaltenes. The H/C molar ratio, however, decreased slightly as the temperature of operation increased. This is shown in Figure 2. Similar findings were reported by Stanislaus.4 The decreasing H/C ratio is attributed to a lowering of the molecular weight by cracking. The conversion of the asphaltenes is suggested to be a scission of side chains and cracking of naphthenes, leaving the aromatic structure unaffected. As discussed below, these changes will significantly lower (4) Stanislaus, A.; Absi-Halabi, M.; Khan, Z. Chem. Ind. 1994, 58, 159.
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Energy & Fuels, Vol. 14, No. 1, 2000
Figure 2. Product asphaltene molar H/C ratio for products desulfurized at different temperatures.
Bartholdy and Andersen
Figure 4. Flocculation titration of feed and product. Intercept with axis indicates degree of product stability. (O/2/0) feed and (b) hydrotreated product.
Figure 3. Fraction of asphaltenic sulfur in the product vs sulfur in total liquid product.
the product stability. The sulfur content of the asphaltenes did not change in proportion to the change in reaction temperature, but as can be seen in Figure 3, the fraction of sulfur associated with the asphaltenes steadily increased as the content of product sulfur was reduced (higher operating temperature). The asphaltenes contained less than 20% of the sulfur in the feed, which means that the fraction of asphaltenic sulfur increases as the sulfur is removed from the oil. This indicates that the sulfur is difficult to process at the conditions given. Hydrotreatment can change the asphaltenes to become less soluble either by removing their aliphatic side chains or by hydrogenating the oil so that it becomes a poor solvent for the asphaltenes. During severe hydrotreatment it is observed that asphaltenes spontaneously precipitate from the product. The asphaltene precipitation (sludge formation) is undesirable when upgrading residual oil in the industry. The formation of sludge is related to hydrocracking, i.e., mostly a problem toward end of run when the operating temperature is high. The product stability can be determined in several ways: e.g., by microscopic inspection of the product,5 by P-number determination,6 or by “Hot Filtration Test” (SMS 2696). Other methods are reviewed in ref 2. In this study we used flocculation onset titration using heptane as titrant.2 This method is superior to other methods available in that it gives information on both product stability and critical solubility parameters for the asphaltenes. (5) Mochida I.; Zhao, X. Z.; Sakanishi, K. Ind. Eng. Chem. Res. 1990, 29, 2324. (6) Street, R. D.; George, S. E.; Boardman, S.; Bahn, O. K.; den Ouden, J. Hydrocarbon Eng. Sept. 1997, 53.
Figure 5. Stability of hydrotreated products.
Three batches of feed were used during the study. A flocculation onset titration was carried out for each batch. Figure 4 gives the titration data for the three feed batches. As can be seen, the data are identical. This verifies not only the uniformity of the feedstock used in this study but also the reproducibility of the titration method used. The data points in Figure 4 show a straight-line correlation. Extrapolation of the regression line gives a positive intercept with the y-axis, indicating that the feed is stable (at zero dilution with toluene it is necessary to add heptane to the feed to make the asphaltenes precipitate). On the other hand, severe hydrotreatment gives unstable products. This is seen for the hydrotreated product in Figure 4. The extrapolation of the product data gave a negative intercept, indicating an unstable product (solids were detected in the sample). A titration similar to the one described above was carried out for the other samples of this study. The products became unstable as the reaction temperature was increased. This can be seen from Figure 5, which shows that the value of the intercept becomes more negative. In the temperature region around 380 °C there is an abrupt change in stability. This is assumed to be caused by a change in reaction chemistry in that the cracking reactions become dominant, e.g., dealkylation, leading to more condensed asphaltenic structures (H/C decrease) and a more aliphatic malthene fraction. As the malthene fraction becomes more aliphatic, the asphaltenes become more
Changes in Asphaltene Stability during Hydrotreating
and more insoluble.5 From this study we are, however, unable to deduct if the changed stability is a result of the increased hydrocracking or whether the temperature influences the stability directly. Product samples generated at the low reaction temperature were found to be unstable, but hardly any precipitate was found in the product. For the higher temperature products, a separate solid phase could easily be seen in the sample. Molecular weights were measured for the feed and for two product samples taken at 378.8 and 389.0 °C, showing a decline in molecular weight from initially 499 to 427 and 371 g/mol, respectively. The change between 379 and 389 °C was related to an increase in cracking reactions, leading to a more alkane rich “solvent” matrix. The slope of the regression lines in Figure 4 can, as reported by Andersen,2 be converted into a critical solubility parameter for the solvent-precipitant mixture at which the least soluble asphaltene will precipitate or separate. The critical solubility parameter is related mainly to the chemistry of these asphaltenes as dilute solutions are investigated here. Also on the basis of concepts of the theory of regular solution, the critical solubility parameter may be directly related to the properties of the asphaltenes, assuming that immiscibility occurs at a fixed solubility parameter difference as observed for polymer systems. The critical solubility parameter was determined and correlated with the process temperature (see Figure 6). As the temperature increased, the critical solubility parameter also increased, indicating that the asphaltene solubility parameter increases. This is in agreement with the reduction in H/C, indicating a more condensed hydrocarbon structure. It is well-known that solubility parameters of hydrocarbons for pure compounds increase with condensation.7 The findings presented here verify that the critical solubility parameter can be related to (7) Speight, J. G. In Asphaltenes and Asphalts 1; Yen, T.F., Chilingarian, G.V., Eds.; Development in Petroleum Science 40 A, Elsevier: New York, 1994; p. 57.
Energy & Fuels, Vol. 14, No. 1, 2000 55
Figure 6. Critical solubility of hydrotreated products.
the physical state and properties of the asphaltenes, assuming that interactions with other oil components can be neglected. Conclusions The product stability of feed and product oil was determined after severe hydrotreatment by use of a flocculation onset titration procedure. The method indicates that the products become unstable, and thats above approximately 380 °C, where cracking reactions become dominantsthe product stability drops rapidly. There are indications to the effect that sulfur remaining in the product is to be found predominantly in the asphaltenic fraction. As the reaction temperature is increased, the H/C ratio of the asphaltenes is reduced. This change is also reflected in the critical solubility parameter for the onset of asphaltene precipitation. The flocculation onset titration procedure was proven to be a simple and reliable tool for measurement of product changes during operation. With the present feedstock, the instability of the products was mainly related to the significant change in oil solvent properties given by the intercept of the titration plot. EF990121O
