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Energy & Fuels 2007, 21, 1212-1216
Hydrotreater Feed Filter Fouling and Its Remedy† X. A. Wu* and K. H. Chung Syncrude Canada Ltd., Edmonton Research Center, Edmonton, Alberta, Canada T6N 1H4 ReceiVed August 11, 2006. ReVised Manuscript ReceiVed October 5, 2006
The root cause of hydrotreater feed filter fouling in a bitumen upgrading plant was revealed through a step-by-step scientific investigation. It was first confirmed that the fouling problem was related to a process flow sheet change that introduced a heavy vacuum gas oil (HVGO) stream into the coker combined gas oil (KCGO) stream prior to filtration. Characterization of the foulant and the feed indicated that the fouling reactions are likely oxidative polymerization. Iron naphthenate or naphthenic acid in the HVGO stream could act as a catalyst for such a reaction. A bench-scale oxidation test was carried out to compare the oxygen uptake rates and the C7-insoluble contents after oxidation in KCGO, KCGO plus HVGO, KCGO plus iron naphthenate, and KCGO plus naphthenic acid streams. While the oxygen uptake kinetics for these samples were similar, the C7-insoluble contents for KCGO plus HVGO and KCGO plus iron naphthenate increased significantly after oxidation compared to the base case of KCGO. No significant increase of the C7-insoluble content was observed for KCGO plus naphthenic acid, indicating that it was the iron naphthenate that catalyzed the fouling reactions. Iron naphthenate was a corrosion product in the HVGO stream, which could be eliminated by preventing corrosion in the vacuum distillation unit. The filter fouling problem indeed disappeared after the installation of corrosion-resistant equipment.
Introduction As the supply of light crude oils dwindles worldwide, heavy crude oils including bitumen extracted from the Athabasca oil sands are becoming important refinery feedstock. Athabasca bitumen produced by mining and water-based extraction processes usually contains trace amounts of fine solids, posing a challenge for refiners and upgraders. Feed filters are often installed in hydrotreater units to protect the downstream catalyst beds from plugging. In one of the bitumen upgrading plants, severe filter fouling occurred in the hydrotreater feed filter after a process flow sheet change. Preliminary foulant characterization showed that the filter fouling was likely caused by oxidative polymerization of certain polar species in gas oils in the presence of molecular oxygen leaking into the system.1 Oxidation of the petroleum process and product streams has been identified as one of the common causes for refinery fouling2-5 and fuel instability.6-12 The † Presented at the 7th International Conference on Petroleum Phase Behavior and Fouling. * To whom correspondence should be addressed: 9421-17 Avenue, Edmonton, Alberta, Canada T6N 1H4. Fax: (780) 970-6805. E-mail:
[email protected]. (1) Xu, Z.; Wang, Z.; Kung, J.; Woods, J. R.; Wu, X. A.; Kotlyar, L. S.; Sparks, B. D.; Chung, K. H. Fuel 2005, 84, 661-668. (2) Vadekar, M. Petrol. Technol. Q. 2002/2003, Winter, 87-93. (3) Watkinson, A. P.; Wilson, D. I. Exp. Therm. Fluid Sci. 1997, 14, 361-374. (4) Watkinson, A. P. Chem. Eng. Technol. 1992, 15, 82-90. (5) Crawford, J. D.; Miller, R. M. 28th Midyear Meeting of the American Petroleum Institute’s Division of Refining, Philadelphia, PA, 1963; Vol. 43, pp 106-114. (6) Wallace, T. J. AdV. Petrol. Chem. Refin. 1964, 9, 353-407. (7) Fathoni, A. Z.; Batts, B. D. Energy Fuels 1992, 6, 681-693. (8) Schrepfer, M. W.; Arnold, R. J.; Stansky, C. A. Oil Gas J. 1984, Jan 16, 79-84. (9) Sauer, R. W.; Weed, A. F.; Headington, C. E. Meeting of the Division of Petroleum Chemistry, American Chemical Society, Chicago, IL, 1958; pp 95-113. (10) Jones, E. G.; Balster, W. J. Energy Fuels 1993, 7, 968-977.
fouling reactions can be catalyzed by iron or other transition metals5-7,13-15 and carboxylic acids to various degrees.16-18 The feed to the fouled filter after the process flow sheet change contained relatively high concentrations of both naphthenic acid, originating from bitumen-derived heavy vacuum gas oil, and iron naphthenate, a corrosion product from the reaction between naphthenic acid and steel pipes in the vacuum distillation unit at high temperatures. The literature data were inadequate for revealing the filter fouling mechanism. Hence, it was imperative to develop an experimental program to identify the fouling mechanism including the catalytic functions of various contaminants in the feed. Background and Investigation Plan Figure 1a illustrates a backwash hydrotreater feed filter system. In filtration mode, oil flows into the tubular filter elements, laying down solids on the filter mesh surfaces to form filter cakes. In backwash mode, filtered oil flows inside out, pushing the filter cakes into a collection vessel.19 The filter elements should be clean as shown in Figure 1a after each backwash cycle. When the filter is fouled, however, sticky materials glue filter cakes to the mesh surfaces and the foulant (11) Oswald, A. A.; Noel, F. J. Chem. Eng. Data 1961, 6, 294-301. (12) Frankenfeld, J. W.; Taylor, W. F.; Brinkman, D. W. Ind. Eng. Chem. Prod. Res. DeV. 1983, 22, 608-614. (13) Denisov, E. T.; Emanuel, N. M. Russ. Chem. ReV. 1960, 29, 645662. (14) Uri, N. AdV. Chem. Ser. 1962, 36, 102-112. (15) Pickard, J. M.; Jones, E. G. Energy Fuels 1997, 11, 1232-1236. (16) Cooney, J. V.; Beal, E. J.; Beaver, B. D. Fuel Sci. Technol. 1986, 4, 1-18. (17) Morris, R. E.; Flohr, K. W. 2nd International Conference on LongTerm Storage Stabilities of Liquid Fuels, San Antonio, TX, 1986; pp 744758. (18) Frankenfeld, J. W.; Taylor, W. F.; Brinkman, D. W. Ind. Eng. Chem. Prod. Res. DeV. 1983, 22, 615-621. (19) Chung, K. H.; Chan, E. Oil Gas J. 2000, Jan 31, 70-72.
10.1021/ef060372e CCC: $37.00 © 2007 American Chemical Society Published on Web 11/11/2006
Hydrotreater Feed Filter Fouling and Its Remedy
Energy & Fuels, Vol. 21, No. 3, 2007 1213
Figure 3. Diagram of a filtration simulation device.
common fouling mechanism, KCGO and HVGO are fully compatible. In addition, HVGO is virtually solids-free and does not contribute to the solids loading on the filter. It was therefore decided to carry out a thorough investigation by first confirming that mixing KCGO and HVGO indeed causes fouling, then proposing a fouling mechanism based on foulant and stream characterization, and finally verifying the mechanism through a simulated oxidation test. Experimental Section
Figure 1. Clean (a) and fouled (b) mesh filters on a hydrotreater feed stream. The arrows in (a) indicate the direction of flow in filtration mode. Solids laying down on the mesh surface can be backwashed by reversing the direction of oil flow. The thick and sticky foulant in (b), however, cannot be removed by backwashing.
Figure 2. Simplified flow diagram of a bitumen upgrading plant. Dashed lines represent the configuration change in late 1999.
cannot be removed by backwashing (see Figure 1b). Costly ex situ manual cleaning must be carried out on each individual filter element. The start of filter fouling in the bitumen upgrading plant coincided with a process configuration change. Figure 2 shows a simplified flow diagram of bitumen processing. Before filter fouling occurred, the hydrotreater feed was coker combined gas oil (KCGO), which is a mixture of coker heavy and light gas oils. The filter performed as per design specification for 4 years.19 With the installation of a vacuum distillation unit, heavy vacuum gas oil (HVGO) was blended to KCGO in a volume ratio of 1:4 and filter fouling occurred. Initially, the incompatibility of feedstock causing phase separation of certain oil components from the mixture was considered. A compatibility test showed that, contrary to this
Materials. KCGO, HVGO, and hydrotreater feed were obtained from the commercial Syncrude bitumen upgrading plant. The hydrotreater feed comprised approximately 80% KCGO and 20% HVGO. Naphthenic acid (Fluka) and ferric naphthenate (ICN Biomedicals) were used as additives without further purification. Solvents used in this study (chloroform, n-heptane, etc.) were HPLC-grade reagents, supplied by Fisher Scientific. Methods. 1. Filtration Simulation. A filtration simulation skid was constructed at the University of British Columbia to confirm the hypothesis that fouling is caused by mixing KCGO and HVGO (see Figure 3). The feed, about 60 L in volume, was heated to 100 °C and homogenized with an impeller. It was then pumped through a disk filter containing a filter element identical to that used at the commercial plant. The flux was kept constant at 0.1 m/min (or 2.7 gpm/ft2), similar to the plant condition. The pressure drop (∆P) across the filter over the time was monitored. The filtration terminated when ∆P reached 138 kPa (or 20 psi). 2. Characterization of Foulant and Feed Streams. Filter foulant was rinsed with chloroform at room temperature to extract the sticky material acting as an adhesive. Chloroform was subsequently evaporated in an argon atmosphere. The chloroform-soluble (CS) fraction of the foulant was submitted to the Institute for Chemical Process and Environmental Technology, National Research Council of Canada (ICPET, NRC) for chemical analyses. The analytical techniques included CHNS/O elemental analysis, X-ray photoelectron spectroscopy (XPS), and gel-permeation chromatography (GPC). Detailed instrumental parameters and procedures were described elsewhere.1 Hydrotreater feed samples were also characterized at ICPET, NRC using the aforementioned techniques. Both the bulk sample and an extracted polar fraction were analyzed. The extraction was carried out in a preparative high-performance liquid chromatographic (HPLC) column using hexane as the mobile phase. The polar fraction was eluted with tert-butyl methyl ether. 3. Oxidation Test. Figure 4 shows the schematic of the oxidation test apparatus. The test oil, about 100 g, was heated to a constant temperature, 95, 110.4, 130.5, or 150.5 °C, in a silicone oil bath while bubbled with argon to remove all residual air in the oil and the overhead gas. After the test oil temperature reached the set value, two three-way valves were turned to the circulation loop and the overhead gas started to circulate through the test oil at 12 mL/s. Because the circulation loop had not been purged, the initial air content in the system was about 1/10 of the atmosphere. This low oxygen environment mimicked the plant condition in case of a
1214 Energy & Fuels, Vol. 21, No. 3, 2007
Wu and Chung Table 1. Foulant and Feed Stream Characterization Results (wt %) elemental analysis
Figure 5. Pressure-drop buildup curves for simulated filtration of KCGO (9), HVGO (b), and their mixture ([).
minor air leak. A polarographic oxygen analyzer (Orbisphere 3650EX analyzer with 2956A membrane), calibrated in air with a barometer measurement, monitored the oxygen partial pressure in the overhead gas. The test terminated when 95% of the initial oxygen had reacted with the test oil. The oils tested were KCGO, KCGO plus HVGO, KCGO plus ferric naphthenate, and KCGO plus naphthenic acid. C7 insolubles were extracted from the oxidized oils by diluting the oil with n-heptane in a ratio of 1:40 by weight and filtering the mixture through a 0.45 µm filter disk (Millipore HAWP04700), after letting the mixture stand for 4 h. A test to increase the waiting time to 16 h did not yield more C7 insoluble. The C7 insolubles were washed on the filter disk with n-heptane 3 times, dried in a vacuum oven at 60 °C for 5 h, and weighed with a 0.1 mg resolution balance. Dried C7 insolubles were analyzed with a photoacoustic Fourier transform infrared (PA-FTIR) spectrometer (Thermo-Nicolet Magna 560 spectrometer with a MTEC model 300 photoacoutic cell). Prior to the analysis, the sample underwent two-stage purging with helium to remove CO2 and moisture in the instrument.
Results and Discussion Hypotheses for Filter Fouling. KCGO, HVGO, and 80% KCGO plus 20% HVGO samples were tested in the filtration simulation device shown in Figure 3. The ∆P data across the filter were plotted in Figure 5. Pressure-drop buildup was observed for the KCGO but not for the HVGO. This is expected because HVGO is virtually solids-free. After blending, however, one would expect an intermediate rate of pressure-drop buildup because of intermediate amounts of solids in the blend oil, assuming that no chemical or physical changes occurred. The observed rate of ∆P increase for the blend was higher than either of its components, indicating that certain changes did occur and mixing KCGO and HVGO likely contributed to filter fouling. The analytical results for the aforementioned CS fraction of the foulant, the hydrotreater feed, and its HPLC-extracted polar
GPC
sample
H/C
N
S
O
foulant (CS fraction) hydrotreater feed (whole) hydrotreater feed (polar fraction)
1.17
2.2
8.0
7.6
74
0 40a 0
a
Figure 4. Bench-scale setup for the mild oxidation test on gas oils.
XPS pyrrolic N
1.58
0.4
4.1
1
60a
1.20
2.4
5.0
2.8
80
pyridinic N
Mw 790
260
Estimated from basic nitrogen analysis data.
fraction were listed in Table 1. The composition of the foulant fraction resembles that of the polar fraction of the feed, except for its oxygen content and GPC molecular weight, but is significantly different from that of the whole feed. In particular, both the foulant fraction and the polar fraction of the feed contain approximately 2.3 wt % nitrogen, which is substantially higher than that of a typical bitumen-derived sample. Nitrogen speciation with XPS indicated that these concentrated nitrogen species are predominantly pyrrolic compounds, which are susceptible to oxidative polymerization according to the literature.11,12,18,20 This oxidative fouling mechanism is in agreement with the analytical data of the foulant, showing high oxygen content and high molecular weight. Hence, the pyrrolic nitrogen species in the feed are probably the main fouling precursors. Copolymerization of pyrroles and sulfur species, such as sulfoxides,11 is also likely due to the high sulfur content in the foulant. Because KCGO is a cracked product, olefins might be involved in the fouling reactions as well. Because of the complexity of all possible reactions, only a general autoxidation scheme19 is proposed here where R‚ represents free radicals of
initiation RH + O2 f R‚ + ‚O2H RO2H f RO‚ + ‚OH propagation R‚ + O2 f RO2‚ R‚ (Cn) + R (unsaturated Cm) f R‚ (Cn+m) abstraction RO2‚ + RH f RO2H + R‚ RO‚ + RH f ROH + R‚ termination R‚ (or RO2‚) + R‚ (or RO2‚) f oxygenated oligomers f foulant organic molecules, including pyrroles, sulfur species, oxygenates, olefins, and saturates. Oxygenated oligomers likely become oil-insoluble foulant through rearrangement or further addition. Pyrrolic nitrogen species and other polar species are primarily present in KCGO. HVGO may contribute to the fouling via three possible routes. HVGO could supply molecular oxygen to the system because vacuum distillation units are susceptible to air leaks.21 This hypothesis, however, was refuted after a low dissolved oxygen concentration was found in a sealed HVGO sample taken from the commercial plant.22 The other two mechanisms are related to catalytic functions of iron naph(20) Speight, J. G., Ed. Petroleum Chemistry and Refining; Taylor and Francis: Washington, D.C., 1998; p 217. (21) Reich, L.; Stivala, S. S. Autoxidation of Hydrocarbons and PolyolefinssKinetics and Mechanism; Marcel Dekker, Inc.: New York, 1969; p 32. (22) Wu, X. A.; Chung, K. H. Ind. Eng. Chem. Res. 2006, 45, 37073710.
Hydrotreater Feed Filter Fouling and Its Remedy
Energy & Fuels, Vol. 21, No. 3, 2007 1215
Table 2. Apparent Rate Constants of Oxygen Uptake, kapp, in KCGO with Various Additives under Different Conditions case
sample
description
temperature (°C)
kapp × 104 (s-1)
1 2 3 4 5 6 7 8
KCGO KCGO plus HVGO KCGO plus ferric naphthenate KCGO plus naphthenic acid hydrotreater feed hydrotreater feed hydrotreater feed hydrotreater feed
100 g of KCGO 16 g of HVGO plus 85 g of KCGO 3.9 mg of ferric naphthenatea plus 100 g of KCGO 3 g of naphthenic acida plus 97 g of KCGO ∼80 g of KCGO and ∼20 g of HVGO blended at the plant same as above same as above same as above
110.4 110.4 110.4 110.4 95 110.4 130.5 150.5
2.02 2.05 2.00 1.94 1.38 1.89 2.66 3.49
a
Additive dosages are approximately 4 times the concentrations in the hydrotreater feed.
Figure 6. Kinetic data of oxygen uptake in KCGO-based samples at 110.4 °C: (a) 100% KCGO, (b) 84% KCGO plus 16% HVGO, (c) KCGO plus 39 mg/kg of ferric naphthenate, and (d) 97% KCGO plus 3% naphthenic acid. All data are presented in two segments because of the limitation of the data-logging system of the oxygen analyzer. kapp in Table 2 is based on the average slope of the two segments.
Figure 7. Arrhenius plot for the oxygen uptake reaction in a hydrotreater feed oil in the temperature range of 95-150 °C. The diamonds are experimental data. They fit a line with the formula: y ) -2.61x - 1.78, r2 ) 0.9981. The apparent activation energy from the slope is 21.7 kJ/mol.
thenate13-15 and naphthenate acid16-18 in hydroperoxide (RO2H) decomposition or other fouling reactions. Both components were relatively abundant in HVGO and are virtually absent in KCGO. The following section discusses the investigation in determining which of these two components was the catalyst for fouling reactions. Fouling Mechanism and Its Remedy. Four KCGO-based samples with various additives including HVGO, iron naphthenate, and naphthenic acid were oxidized in an apparatus designed to simulate the mild oxidation reactions at the plant (see Figure 4). The data of oxygen uptake by the test oils were plotted in Figure 6 and summarized in Table 2. The reactions are first-order with respect to the oxygen partial pressure in the overhead gas, PO2, which can be expressed as
and CL is the molar concentration of O2 dissolved in oil. Because dnO2 ) b dPO2 (with b being a constant), assuming that the overhead gas volume is much larger than the oil volume, eq 2 can be rewritten as
-
dPO2 dt
) kappPO2
(1)
where kapp is the apparent rate constant, determined to be 2 × 10-4 s-1 regardless of the test oil composition. An Arrhenius plot was generated for a hydrotreater feed; i.e., KCGO blended with HVGO at the plant (see Figure 7). The apparent activation energy, Ea,app, was determined to be 21.7 kJ/mol. This low value seems to indicate that the kinetics is diffusion-controlled rather than reaction-controlled. However, it was found that kapp is proportional to the test oil volume, VL. Because the test oil was well-agitated, the diffusion limit, if any, likely occurred at the oil-gas interface. With a near-constant bubbling rate and bubble surface area, kapp should be independent of the oil volume in the diffusion-controlled case. In contrast, if the kinetics is reaction-controlled
-
dnO2 dt
) kintCLVL
(2)
where nO2 is the total amount of O2 (in moles) in the system, kint is the intrinsic rate constant for the oxygen uptake reaction,
-
dPO2 dt
)
kintVL P k Hb O2
(3)
where kH is Henry’s law constant. Therefore, kapp is proportional to VL, supporting the reaction-controlled case. The low value of Ea,app could be explained by the fact that the activation energy for the reaction, Ea,int, equals Ea,app plus the heat of solution of O2 in the gas oil. The latter has not been determined, but a crude estimation based on the enthalpy of vaporization for O2 at its boiling point, 6.8 kJ/mol,23 yields Ea,int ) 28.5 kJ/mol. This value is indeed comparable to the literature data of Ea,int of the Athabasca bitumen and the Athabasca maltene, 36.5 and 26.8 kJ/mol, respectively, determined in the temperature range of 137-207 °C.24 The kinetics of oxygen uptake in gas oils were not further studied because the data in Table 2 clearly indicated that the difference in fouling propensities of KCGO samples mixed with different additives cannot be revealed using this method. A study in the literature showed that oxygen uptake or firststage oxidation is not catalyzed by iron or other contaminants but that the subsequent formation of fouling deposits is.25 A quick method to estimate the tendency of an oil to form fouling deposits induced by oxidation is to determine its C7-insoluble concentrations before and after the oxidation test. Molecules participating in the oxidative polymerization likely become less soluble in n-heptane and contribute to the increase of the C7(23) Lide, D. R., Ed. CRC Handbook of Chemistry and Physics, 86th ed.; Taylor and Francis: Boca Raton, FL, 2005; pp 6-97. (24) Jha, K. N.; Rao, P. M.; Strausz, O. P. Prepr. Pap.-Am. Chem. Soc., DiV. Fuel Chem. 1978, 23, 91-97. (25) Mochida, I.; Sakanishi, K.; Fujitsu, H. Oil Gas J. 1986, Nov 17, 58-63.
1216 Energy & Fuels, Vol. 21, No. 3, 2007
Wu and Chung
Table 3. C7-Insoluble Contents in Gas Oils after Oxidation with Various Additives
a
case
sample
C7 insoluble in total oil sample (mg/kg of oil)
0 1 2 3 4
original KCGO KCGO KCGO plus HVGO KCGO plus ferric naphthenate KCGO plus naphthenic acid
1062 ( 147a 1210 ( 34a 1446 ( 168a 1598 ( 198a 1241
normalized C7 insoluble (mg/kg of KCGO)
increase of normalized C7 insoluble (%)
increase of 1260 cm-1 band strength (%)
1062 1210 1718 1598 1279
0 14 62 50 20
0 16 63 42 25
The error was calculated on the basis of two measurements at the 95% confidence level.
Figure 8. PA-FTIR spectra of C7 insolubles from the original KCGO (0), the oxidized KCGO (1), and the oxidized KCGO plus ferric naphthenate (3). The spectra were adjusted to keep the peak height at 1377 cm-1 identical for quantitative comparison of the 1260 cm-1 band.
insoluble yield in the test oil. The C7-insoluble concentrations for the KCGO-based samples after the oxidation test are shown in Table 3. Because none of the additives generate C7 insolubles by themselves, the C7-insoluble concentrations were normalized on the basis of their KCGO contents. Using the amount of C7 insoluble in the KCGO prior to oxidation as the base case (0), the increased amount of C7 insoluble in the KCGO after oxidation (case 1) is relatively small, 14%. Both KCGO plus HVGO (case 2) and KCGO plus ferric naphthenate (case 3) have substantially higher increases of their C7 insolubles, 62 and 50%, respectively, as compared to case 1. KCGO plus naphthenic acid (case 4), however, is not significantly different from case 1 (20 versus 14%). To confirm that the increase in C7-insoluble yields is indeed related to oxidation, the C7 insolubles extracted from the samples in the aforementioned test were analyzed using the PA-FTIR technique (see Figure 8). The overall variations in the spectra of all cases are small, probably because of the bulk of C7 insoluble derived from the heavy fractions of bitumen and unaltered under the mild condition of 110 °C. However, a closer look at the 1200-1300 cm-1 region revealed a noticeable difference in the strength of the band at 1260 cm-1, which is assigned to aryl-O stretching in aromatic ethers/phenols or C-O stretching in esters.26 The increase in the 1260 cm-1 band (26) Lin-Vien, D.; Colthup, N. B.; Fateley, W. G.; Grasselli, J. G. The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic Press: San Diego, CA, 1991; p 65.
heights for various cases follows the general trend of the increase in the C7-insoluble yields (see Table 3). The data clearly indicate that it is the iron naphthenate rather than the naphthenic acid in HVGO that catalyzed the fouling reactions in KCGO. Understanding the root cause of fouling enabled us to determine the most cost-effective solution. In this case, it was to avoid corrosion and eliminate the corrosion product, iron naphthenate, from the HVGO stream. The hydrotreater feed filter fouling indeed disappeared shortly after the installation of corrosion-resistant pipes in the vacuum distillation unit. Conclusions It has been found that the hydrotreater feed filter fouling is related to blending HVGO to KCGO after a process flow sheet change. The fouling mechanism is oxidative polymerization, with molecular oxygen leaking into the system. The KCGO provided pyrrolic and other polar species as fouling precursors, and the HVGO provided iron naphthenate as a fouling catalyst. Because the latter was a corrosion product, the filter fouling problem has been solved after eliminating the upstream corrosion problem. Acknowledgment. The authors acknowledge Dr. P. Watkinson’s group at the University of British Columbia, Dr. L. Kotlyar’s group at ICPET, NRC, and Edwin Chan at Syncrude for their contributions to this investigation. EF060372E
