Sediment Formation in Residue Hydroconversion Processes and Its

Oct 1, 2013 - ... during hydroprocessing affects mainly the equipment downstream the reactor such as atmospheric, vacuum towers, and heat exchangers...
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Sediment Formation in Residue Hydroconversion Processes and Its Correlation to Asphaltene Behavior Estrella Rogel,* Cesar Ovalles, and Ajit Pradhan Chevron Energy Technology Co, 100 Chevron Way, Richmond, California 94802, United States

Pak Leung and Nan Chen Chevron Global Downstream, 100 Chevron Way, Richmond, California 94802, United States ABSTRACT: In this work, the link between asphaltene stability and sediment formation in hydroprocessing of residua is evaluated using two recently developed techniques for asphaltene characterization (Asphaltene Solubility Profile and Solubility Fractions). Several feeds as well as products coming from commercial units and pilot plants are examined in terms of asphaltene content and asphaltene stability. The results obtained confirm the relationship between stability, structural asphaltene characteristics, and sediment formation in products. It was found that stability, asphaltene content, and nature of the feeds are key factors in sediment formation. Two sets of correlations were found: those that correlate sediment with product characteristics and those that correlate sediment with feed characteristics and operational conditions. Both sets of correlations include asphaltene solubility characteristics and content as key parameters. These findings indicate that proper asphaltene characterization of feeds can help to reduce fouling in operating units by facilitating feed selection and adjustment of operational conditions. Additionally, it has been shown that the two proposed methods can be used to monitor sediment formation during hydroconversion as well as to evaluate solvents for blending in a fast and simple way compatible with a refinery lab environment.

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INTRODUCTION Crude oils available to refiners are becoming heavier, producing in general lower quality fractions and more residue during distillation. At the same time, there is an increasing demand for cleaner transportation fuels. These two factors together drive an increasing pressure to convert residues to distillates.1,2 In particular, hydroconversion technologies represent feasible alternatives to obtain fuels from hydrocarbons resids in order to satisfy future demands. However, in residue hydroconversion processes, sediment formation can reduce on-stream factor and conversion targets significantly.3 Therefore, understanding sediment formation during hydroconversion is a key factor to improve and control the performance of this technology. In particular, enhancing predictability of sediment deposition is highly desirable because it would allow the prescreening of feeds and optimization of processing and blending strategies. In general, the tendency to form solids during hydroprocessing affects mainly the equipment downstream the reactor such as atmospheric, vacuum towers, and heat exchangers.3 Nowadays, it is clear that the formation of solids is related to changes in solubility during the processing of the residue.4−7 These changes are related to the hydroconversion process that modifies the solvent power of the material: it has been demonstrated that resins and oils are cracked and hydrogenated at a faster rate than asphaltenes. In consequence, the solvent power of oils and resins decreases, while unconverted asphaltene cores after cracking become more aromatic and condensed. All these changes induce the precipitation of the most aromatic and condensed molecules out of solution.8 The precipitation of sediment, also called “soft © 2013 American Chemical Society

coke” during hydroconversion has also been attributed to the possible liquid−liquid phase separation of the material due to the high temperature conditions in the hydroprocessing equipment.4 However, in the particular case of LC-Fining, it has been reported that the deposits downstream the reactor can be rendered back into solution at reactor operating conditions indicating that the precipitation is the result of decreasing temperature.3 Even though sediment formation is far from being completely understood, analytical studies have shed some light on the possible mechanisms of generation. It has been reported that asphaltenes in the hydroconverted products have a lower H/C and a higher aromaticity than the original asphaltenes in the feed.6−8 The analysis of sediments indicated that they are composed mainly of molecules that can be operationally classified as heptane insolubles8 and therefore related to asphaltenes in the product. In fact, a weak correlation8 has been found between the asphaltene content of converted residues and the sediment content of the products as determined by ASTM D4870.9 Asphaltene stability measurements have been reported to correlate with sediment formation. For instance, the flocculation point of H-Oil reactor effluents decreases as the amount of sediment in the sample increases.6 Practical experience has shown that sediment formation is also a function of feed characteristics and operating conditions. Received: August 12, 2013 Revised: September 30, 2013 Published: October 1, 2013 6587

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In fact, a weak correlation8 has been reported between asphaltene content of feeds and the sediment content in the product. However, this correlation was built using data of materials converted using the same H-Oil unit, and, therefore, it is affected by particular operational conditions and very difficult to generalize. Thermodynamic modeling3 of sediment deposition relates the H/C ratio of the feed to the sediment content determined by ASTM D4870. In an early detailed study, the amount of sediment produced in laboratory hydrotreating experiments could be correlated to asphaltene structural characteristics of the feeds and the ratio of heptane insolubles to pentane insoluble-heptane soluble material.10 Unfortunately, this method to predict sediment content requires significant and tedious lab work. In general, it has been shown that asphaltene characteristics and, in particular, solubility are linked to sediment formation. Rather recently, new methods have been developed11−13 to evaluate asphaltene precipitation tendencies as well as asphaltene contents. Those methods are known as the Asphaltene Solubility Profile and Solubility Fractions. It has been shown that these methods correlate well with the aromaticity of the samples, a finding that is compatible with the relationship between structural characteristics and solubility.13,14 In this work, we apply these techniques to evaluate the link between asphaltene characteristics and sediment content. First of all, a series of preliminary tests were performed to check the relationship between stability, structural asphaltene characteristics, and sediment formation in hydroprocessed products. In the second place, the influence of feed characteristics on sediment formation was studied under diverse hydroconversion operational conditions. Two sets of correlations were found: those that correlate sediment with product characteristics and those that correlate sediment with feed characteristics and operational conditions. The end goals of this work are to gain predictive capabilities as well as to facilitate feed selection and adjustment of operational parameters in residue hydroconversion processes.

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peak area is the analytical signal. Results are reported as weight percent for all three elements. High-Resolution Solid-State NMR. Measurements were performed on a Bruker Advance 500 spectrometer operating at static magnetic field of 11.7 T (resonance frequency for 13C at 125.77 MHz). Samples were packed into 4 mm zirconia rotors and spun typically at 8 kHz. The samples were characterized using 13C CP-MAS NMR experiments. All spectra were obtained after obtaining Hartmann− Hahn match, using 1H−13C contact time, typically 2.5 ms, and recycle delay of 2.0 s. A total of 2048 free induction decay (FID) signals were accumulated, and the FID were processed with exponential apodization using a line broadening of 200 Hz. The measured chemical shift was recorded relative to tetramethylsilane (TMS) as a secondary reference. The description of structural parameters and 13C NMR chemical shift ranges are reported elsewhere.16 Argonne Premium Coal sample obtained from Premium Coal Sample Program at Argonne National Laboratory was used as a reference sample while deriving the structural parameters. Solubility Profile of Asphaltenes by On-Column Method. A solution of the sample in methylene chloride is injected in a column packed with an inert material using n-heptane as the mobile phase. This solvent induces the precipitation of asphaltenes and, as a consequence, their retention in the column. The first eluted fraction from the column is the maltenes which is soluble in n-heptane. After all this fraction has eluted, the mobile phase is changed gradually from pure n-heptane to 90/10 methylene chloride/methanol and then to 100% methanol.11,12 This procedure gradually redissolves the asphaltenes from the easy to dissolve (low solubility parameter) to the hard to dissolve (high solubility parameter). Asphaltenes were quantified using an Evaporative Light Scattering Detector (ELSD). The flow rates used were kept constant during all the experiments. The volumetric flow of the mobile phase was 4.0 mL/min, and 3.5 L/ min of nitrogen was used for the nebulizing gas of the ELSD. The analysis took 35 min for completion. Duplicates are run for each sample to ensure good quality of results. The injected volume is 80 μL in all the experiments. The result of this analysis is a curve that represents the solubility distribution of the asphaltenes. It has been shown before that the solubility distribution of the asphaltenes can be quantified and correlated to asphaltene stability11 and can be used to evaluate asphaltene content.12 The broadness of the solubility profile is related to the stability of the sample, and it is quantified in terms of the stability parameter ΔPS. This parameter is measured as the difference in time between the time corresponding to the maximum of the first peak and the time at which 75% of the sample (in terms of area) has eluted.17 The asphaltene concentration can be determined based on the properties of the eluted asphaltene solubility profile. The signal should be deconvoluted in two peaks by a peak fitting procedure. It has been found previously that the area of the second peak is related to the mass of gravimetric asphaltenes (obtained by ASTM D6560)18 according to

EXPERIMENTAL SECTION

Materials. Thirty-one hydroprocessed products were obtained from pilot plants and commercial operation units. Three different pilot plant configurations were used: one fixed bed reactor, two fixed bed reactors, and two ebullated bed reactors. Six feeds were also analyzed. Details of feeds and product characteristics will be provided in the next sections. Methylene chloride, methanol, n-heptane, and THF HPLC grade solvents were purchased from Fisher Scientific and used without further purification. Sediment Content and Micro-Carbon Residue. Sediment content was determined for all the products using the standard test ASTM D4870.9 Micro-Carbon Residue was determined by ASTM D 4530.15 Elemental Analysis. Carbon, hydrogen, and nitrogen (CHN) analysis was carried out with a Carlo Erba model 1108 analyzer. Approximately 1 mg to 5 mg of sample is weighed into a tin cup for nonvolatile samples or a piece of indium tubing for volatile samples. The sample is combusted with oxygen in helium carrier gas to produce nitrogen oxides, carbon dioxide, and water as the combustion products of nitrogen, carbon, and hydrogen, respectively. The nitrogen oxides are reduced to nitrogen gas, and excess oxygen is consumed in a copper reduction tube, leaving only N2, CO2, and H2O in the carrier gas stream. These combustion products are separated by gas chromatography and quantitatively determined with a thermal conductivity detector. Detector output is integrated, and the integrated

Log M = a log A − b

(1)

where M and A are the mass and the area, respectively, while a and b are constants obtained by a calibration procedure.12 Separation of Asphaltenes in Solubility Fractions. As in the solubility profile analysis, the sample is dissolved in methylene chloride and injected in a column packed with an inert material using n-heptane as the mobile phase. Maltenes (heptane solubles) elute from the column as the first peak. The asphaltenes remained precipitated in the column and were fractionated according to their respective solubilities by switching the mobile phase in successive steps to solvents of increasing solubility parameters: (1) 10 min after the injection of the sample, the mobile phase was switched to a blend of 15% methylene chloride/85% n-heptane (Solubility Parameter of 16.05 MPa0.5), (2) 20 min after the injection of the sample, the mobile phase was switched to a blend of 30% methylene chloride/70% n-heptane (Solubility Parameter of 18.8 MPa0.5), (3) 30 min after the injection of the sample, the mobile phase was switched to 100% methylene 6588

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chloride (Solubility Parameter of 20.3 MPa0.5), and (4) 40 min after the injection of the sample, the mobile phase was switched to a blend of 10% methanol/90% methylene chloride (Solubility Parameter of 21.23 MPa0.5). After 10 additional minutes, the solvent was switched again to n-heptane. The eluted fractions are quantified using an Evaporative Light Scanning Detector (ELSD) as described elsewhere.13 This method provides a solubility distribution of the asphaltenes in terms of four fractions and correlates well with the asphaltene amount determined by the gravimetric test ASTM D 6560.18 In order to determine the asphaltene content using the Solubility Fraction Method, the area of each asphaltene signal is calibrated using the same procedure as previously described.19 Then, all the percentages of asphaltenes of the different signals are added together. The asphaltene content obtained by this methodology correlates with the value obtained by gravimetric analysis (ASTM D 6560).13 The stability (STB) is evaluated as the ratio between the areas of the different peaks

right in the peaks indicate a more difficult to dissolve material. In Figure 1, these shifts indicate that the hydroprocessed product contains a significant amount of difficult-to-dissolve asphaltenes that seem not to be present in the feed. On the other hand, in Figure 1 the recovered sediment is enriched in the less soluble asphaltenes as indicated by the shift to the right in the second portion of the solubility profile. Therefore, the sediment contains proportionately more low solubility asphaltenes than the product. The comparison of the first peaks in the figure indicates a decreasing amount of easyto-dissolve asphaltenes in the following order: feed, hydroprocessed product, sediment. In fact, the sediment contains only a small portion of the easy-to-dissolve asphaltenes which are represented by the first peak. It has been shown before12 that this fraction is key in keeping the rest of the asphaltenes in solution. Additionally, it is clear from the comparison of product and sediment that only a fraction of the asphaltenes present in the product goes into the recovered sediment obtained by filtration. This indicates that sediment particles larger than 1.7 μm retained during filtration are mainly composed of difficult to dissolve asphaltenes. Based on this result, it seems plausible to expect that the less soluble asphaltenes tend to agglomerate to form larger particles than the more soluble counterparts. Stability of Products and Molecular Characteristics of Converted Asphaltenes. A vacuum residue was hydroprocessed in a continuous flow pilot plant unit composed of a series of two ebullated bed reactors. An alumina supported catalyst was used for these tests. General characteristics of the feed and products can be seen in Table 1 as a function of time

STB = (Fraction(1) + Fraction(2)) /(Fraction(3) + Fraction(4))

(2)

where Fraction(1) and Fraction(2) represent the areas of the first two peaks, while Fraction(3) and Fraction(4) represent the areas of the last two peaks. This stability measurement comes from the idea mentioned several times before that the more soluble asphaltenes can act as peptizing agents for the least soluble asphaltenes.20−22 In fact, the higher the STB values, the more stable the material.

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RESULTS AND DISCUSSION Comparison of Sediment and Asphaltene Solubility Characteristics. Figure 1 shows a comparison of the

Table 1. Characteristics of Feed and Products Obtained at Different Run Times run time (h)

reactor temp (°C)

sediment content (ppm) ASTM D4870

asphaltene content (wt %) ASTM D6560

Micro-Carbon Residue (wt %) ASTM D4530

0 368 536 824 968 1064

-404 408 410 410 410

0 223 813 1639 1998 7567

5.49 3.12 3.42 3.95 4.40 4.02

13.14 9.29 8.93 9.12 9.49 8.52

on stream. The conversion level was kept in the range 38 ± 4% (1000 °F + fraction) by increasing the reactor temperature. The other conditions were kept constant throughout the experiment (LHSV = 0.2 h−1, Total Pressure = 2400 Psi). The stability of the products was followed at different time intervals during the run using the solubility profile method. Figure 2 shows the solubility profile of feed and products taken at different times of the experiment. The changes observed in the curves are characterized by the gradual shift of the shoulder in the profile to the right as the time on stream increases up to a point when a differentiable second peak appears (968 h). This indicates that heptane asphaltenes, which are represented by the second portion of the profile,12 are becoming less and less soluble in the medium. Increasing temperatures and less active catalyst are the two main causes for this behavior. In later operation stages, temperature was kept constant so the decrease in solubility can be attributed exclusively to the decreased activity of the aged catalyst. During hydroprocessing, reactions involving scission of side chains, cracking of naphthenic moieties, and hydrogenation

Figure 1. Comparison of asphaltene solubility profiles of Feed AA, hydroprocessed material (Product AA), and its sediment recovered by filtration.

asphaltene solubility profiles of a feed (Feed AA), hydroprocessed product (Product AA), and its recovered sediment. The hydroprocessed product was obtained from a commercial hydroprocessing unit. The sediment was separated from the hydroprocessed product using the filtration procedure described by ASTM4870. Figure 1 shows that the three profiles are remarkably different. There is a significant shift of the response to the right following the order: feed-productsediment. In the solubility profile analysis, the sample elutes as a function of its solubility in solvent blends having gradually increasing solvent power. In other words, the solvent power increases as the time increases. This means that shifts to the 6589

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Figure 2. Solubility profile of feed and products obtained at different run times.

occur at different rates for asphaltenes and the rest of the sample. Maltenes and resins react first becoming more saturated, so their solvent power decreases. Asphaltenes lose side chains becoming more aromatic and more difficult to solubilize. The result is the precipitation of asphaltenes as sediment.6,8 In the case of increasing temperature, cracking reactions are enhanced. In the case of the aged catalyst, as its ability to hydrogenate decreases, cracking reactions continue to occur but less and less hydrogenation is happening making asphaltenes more and more aromatic. As a consequence, the product becomes less and less stable and produces more sediment as it can be seen in Figure 3 where stability parameter ΔPS together with sediment content are plotted as function of operation time.

Figure 4. a. Molecular parameters of extracted heptane asphaltenes as a function of run time. b. Solubility parameter of heptane asphaltenes calculated using the solubility profile of the products11,12 as a function of aromaticity of extracted heptane asphaltenes.

alkyl chains are observed. This result is consistent with previous studies. This increase in aromaticity is the main cause in the decreasing solubility of the asphaltene. In fact, a correlation of the aromaticity with the solubility parameter calculated for heptane asphaltenes using solubility profile analysis11 was found as it can be seen in Figure 4b. An excellent correlation factor of 0.99 was found for these hydroprocessed samples. Based on these findings, it can be concluded that the solubility profile analysis and other similar solubility techniques can be useful to monitor petroleum residue hydrotreating and, in particular, the changes on asphaltene characteristics during hydroprocessing. Effect of Blending on Sediment Content. In order to evaluate if a simple correlation was possible between stability and sediment content, Product A from a commercial residue hydroconversion unit was blended in different proportions with several solvents, and the sediment contents were measured using ASTM D4870. Table 2 shows the characteristics of Product A and prepared blends. Asphaltene solubility profiles of the blends and original product were obtained to determine asphaltene stability and content. It was found that the stability parameter (ΔPS) correlates linearly with sediment content as can be seen in Figure 5

Figure 3. Sediment content and asphaltene stability (ΔPS) as a function of run time.

Converted asphaltenes from three different products and asphaltenes from the feed were isolated and analyzed by CP/ MAS NMR. This technique seems to be appropriate to compare aromaticity of different asphaltenes as a function of run time.23 Data was analyzed following a procedure developed for coal samples.16 In Figure 4a, aromaticities of the converted asphaltenes are plotted as a function of the run time. As expected, a significant increase in aromaticity and a decrease in the average size of the

SED = − 13880 + 5791.1ΔPS 6590

(3)

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similar asphaltene stability but completely different asphaltene content have different sediment content. Therefore, it is important to take into consideration asphaltene content as well as asphaltene stability when evaluating sediment. Linear regression analysis was used to evaluate the correlation between sediment content and asphaltene content and stability. Using the results of the solubility profile test on the samples, the following correlation was found

Table 2. Blends of Product A with Different Solvents blend A B C D E F G

solvent none buffer oil buffer oil heavy cycle oil heavy cycle oil light cycle oil light cycle oil

A (% wt)

solvent (% wt)

sediment content (ppm) ASTM D4870

100 95 90 95

0 5 10 5

4716 4590 4334 3838

90

10

3112

95

5

3604

90

10

3167

SED = − 7982 + 2570ΔPS + 642ASPC1

(4)

where SED represents the sediment content, and ΔPS and ASPC1 are the asphaltene stability and content measured by the solubility profile test, respectively. The correlation coefficient is 0.9319, and the comparison between calculated and experimental sediment contents can be seen in Figure 6.

Figure 5. Sediment content as a function of asphaltene stability ΔPS for blends of Product A and different solvents. Figure 6. Predicted sediment content calculated using eq 4 (data obtained by using the solubility profile method11,12) as a function of the experimental sediment content.

where SED represents sediment content. The correlation coefficient is 0.7886. According to this correlation, the sediment content would be zero when ΔPS is around 2.40. This value is reasonable in comparison with experimental evidence that indicates that samples with values larger than 2.0−2.2 are becoming unstable and might exhibit precipitation issues.11,12 In a previous section, it was shown that the solubility tests correlate with structural changes in the converted asphaltenes. The results described in this section indicate that the solubility tests can also be used to reflect changes in the solvent power of the maltenes and that those changes are linked to the sediment content of the sample. Sediment Formation As a Function of Product Characteristics. Several products were collected at diverse run times from different pilot plant experiments. In this way, the set of samples includes products coming from different feeds as well as obtained under different operational conditions. As explained earlier, products came from fixed bed reactors as well as ebullated bed reactors, all units working in a continuous fashion (i.e., no batch experiments). Nineteen samples were analyzed using solubility profile11,12 and solubility fractioning13 tests. Stability and asphaltene content were determined using both tests. In the previous section, it has been shown that the stability of the material is correlated to its sediment amount. However, asphaltene stability is not the only factor that determines the amount of sediment. Previous works have shown that there is also a correlation between asphaltene and sediment content.7,10 In fact, practical experience indicates that two materials with

In a similar fashion, a correlation was found using the results of the asphaltene fraction method13 SED = − 506 − 3706STB + 430ASPC2

(5)

where SED represents the sediment content, and STB (eq 2) and ASPC2 are the asphaltene stability and content measured by the solubility fraction test, respectively. The correlation coefficient is 0.9113, and the comparison between calculated and actual sediment can be seen in Figure 7. Based on the results obtained in this section, it can be concluded that sediment content can be effectively predicted using the asphaltene stability and content of the sample, regardless of the method used, i.e. solubility profile or solubility fraction. This can be explained based on the fact that asphaltene stability determines the tendency for precipitation, while the asphaltene content determines the potential precipitate amount. In principle, any of the conventional techniques employed to measure asphaltene stability (i.e., titration techniques) or asphaltene content can be used for a successful correlation with the sediment content. However, these traditional approaches might not be practical since it would require two separate tests, and this can be time-consuming i.e. the use of ASTM D 6560 to determine asphaltene content takes several hours. In this sense, the two methods herein proposed are practical and fast ways (less than one hour) to measure asphaltene content and asphaltene stability in just one test. 6591

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Table 3. Sediment Content As a Function of Operational Variables Used for Hydroprocessing of Different Resids

Figure 7. Predicted sediment content calculated using eq 5 (data obtained by using the asphaltene fraction method13) as a function of the experimental sediment content.

feed

linear hourly space velocity (LHSV) (h−1)

temp of the reactor (°F)

conversiona (% vol)

cut point of the product (°F)

sediment content (ppm)

1 2 2 2 3 4 4 5 5 6 6

0.27 0.15 0.15 0.15 1.1 0.4 0.4 0.13 0.13 0.17 0.5

810 774 782 791 825 800 815 757 773 795 795

85 72 78 82 58 57 64 53 76 76 52

975 1075 1075 1075 975 1000 1000 1050 1050 1000 1000

190 100 1000 2000 570 1110 2750 500 3500 3600 100

a

Conversion refers to the resid conversion that is heavier than the cut point.

Table 4. Characteristics of Feeds Sediment Formation As a Function of Feed Characteristics. It is widely believed that asphaltene content and characteristics of the feed have a great impact in the formation of sediment during hydroprocessing.24,25 For instance, it has been reported that the sediment formation in the product can be reduced significantly if the asphaltenes are previously removed from the feed,10 and a weak correlation has been found between asphaltene content in the feeds and sediment content in the products.7 Also, it has been shown that the addition of highly aromatic diluents to the feed helped in reducing the amount of sediment.26 These findings can be interpreted as an indication that perhaps a more stable feed can contribute to a lower sediment formation. This idea is also supported by a phenomenological correlation10 that links feed characteristics with sediment generation during hydroprocessing of residues. In the later correlation, the variables with the largest impact were condensation degree of aromatic rings present and the ratio of pentane soluble-heptane solubles to heptane insolubles. These parameters are linked to the solubility of the asphaltenes in the medium and, therefore, linked to the stability of the material.10 Another important aspect to consider on the effect of feeds in sediment formation is the influence of certain operating and unit design factors.5 These parameters include many aspects linked to the design of the reactor, separators, catalyst selection, etc. In order to simplify the study, a series of six different feeds were processed in the same pilot plant unit using the same catalyst but under slightly different operating conditions. A total of ten products were analyzed, and their sediment content was measured by ASTM D 4860.9 Table 3 summarizes operational conditions and sediment content of the products. The feeds were analyzed using the solubility fractionation test, and their stability and asphaltene content were calculated. Characterization of feeds is reported in Table 4. Regression calculations were performed using the data presented in Table 3 together with the asphaltene characteristics of the feed. After an optimization process to select the minimum number of variables, the best fitting was

API

S (% wt)

N (ppm)

Ni (ppm)

V (ppm)

1 2 3 4 5 6

3.3 3.5 6.1 5.0 7.4 4.0

6.6 5.8 5.6 5.4 3.0 4.1

5682 4237 3927 3691 6041 5707

104 50 33 43 66 76

270 153 1063 137 211 216

where SED is the sediment content, LHSV represents the linear hourly space velocity, T represents the temperature of the reactor (°F), STB represents the stability as determined by the asphaltene solubility fraction method (eq 2), and ASPC2 is the asphaltene content. The correlation coefficient is 0.8942. Figure 8 shows a comparison between the sediment content calculated using eq 6 and the experimental sediment content determined by ASTM D4870.9 These results indicate that sediment formation can be effectively estimated using a combination of process variables (LHSV and T) and the characteristics of the feed. Specifically, the asphaltene content and stability.

Figure 8. Predicted sediment content in products calculated using eq 6 (data obtained by using the asphaltene fraction method13 on feeds) as a function of the experiment sediment content.

SED = − 94176.7 − 4697*LHSV + 136*T − 2797*STB − 475*ASPC2

feed

(6) 6592

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Notes

It can be reasoned that the amount of sediment depends on the initial stability stage of the feed, and depending on the process conditions (i.e., temperature and LHSV) less stable stages will be reached at longer times on stream. The feed stability represents the initial solubility gap in terms of the difference between the solvent power of the maltenes and the solubility characteristics of asphaltenes. In other words, this gap can be envisioned as the difference between the solubility parameter of maltenes and the asphaltenes. As discussed before, this initial solubility gap becomes wider and wider as the hydroprocessing occurs depending on the operating conditions including catalyst, plant design, residence time, etc. However, the amount of sediment formed for two different feeds under the same operation conditions (as found in many commercial operations) should depend mainly on their initial asphaltene stabilities and contents. From a practical point of view, this indicates that asphaltene characterization of the feeds can be used to predict sediment formation after hydroprocessing, and, therefore, it could be a useful tool to better control commercial hydrotreatment units. However, for any characterization method to be useful in a refinery environment, it should be done in a fast and simple way. The methodologies proposed in this work comply with these requirements.

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CONCLUSIONS

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AUTHOR INFORMATION

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS E. Rogel, C. Ovalles, and A. Pradhan wish to thank Chevron Energy Technology Company and, in particular, the Measurement and Chemistry Focus Area for providing funding and the permission to publish this paper. Dr. M. Moir’s enlightening comments and suggestions are highly appreciated. This work was also possible thanks to generous donations of samples from our partners in Chevron Global Downstream.

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REFERENCES

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The cumulative evidence presented in this work indicates that sediment formation in residue hydroconversion is strongly linked to asphaltene content and solubility characteristics. Sediment amounts are correlated to the stability and content of asphaltenes in the materials. Changes in the chemical structure of asphaltenes and maltenes under hydroprocessing conditions are the main contributors to the low solubility of asphaltenes, and therefore, to sediment formation. In particular, it has been shown that the decrease in asphaltene solubility during hydroconversion is linked to aromaticity changes due to hydrocracking. These changes can be easily quantified using the Solubility Profile Method. Similarly, changes in the solvent power of diluents can be monitored successfully by evaluating the solubility characteristics of the produced blends. It has also been shown that the initial stability and concentration of asphaltenes in the feed are key factors in determining the amount of sediment formed in the product after hydroconversion. The feed stability represents the initial solubility gap in terms of the difference between the solvent power of the maltenes and the solubility characteristics of asphaltenes. This initial solubility gap becomes wider and wider as the hydroprocessing reactions proceed depending on the operating conditions. Therefore, under the same operation conditions sediment formation should depend mainly on the initial asphaltene stability of the feed. In this work, two methods are proposed that can help in reducing fouling during residue hydroprocessing in a simple and fast way compatible with their use in a refinery environment. These methods can be used to monitor sediment formation during hydroconversion and to evaluate solvents for blending, selecting, and optimizing feeds.

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dx.doi.org/10.1021/ef401614a | Energy Fuels 2013, 27, 6587−6593