Energy Fuels 2010, 24, 5483–5492 Published on Web 09/17/2010
: DOI:10.1021/ef100589p
Observations from Heavy Residue Pyrolysis: A Novel Method To Characterize Fouling Potential and Assess Antifoulant Additive Performance Christopher A. Russell,* Simon Crozier, and Ron Sharpe Refinery and Fuels Management Research, Nalco Limited, Energy Services Division, Block 102, Cadland Road, Hardley, Hythe, Southampton SO45 3NP, United Kingdom Received May 10, 2010. Revised Manuscript Received August 31, 2010
Recently, refiners that aggressively process heavy oils and residues have been reporting disappointing profit margins as a consequence of the global economic downturn. The market is expected to recover, but this does not reduce the processing challenges that refiners will face, particularly with the probable implementation of more stringent emission targets. The following paper describes the development of technology to assist refiners to meet these bottom of the barrel upgrading challenges. Feed and residue stream monitoring tools are revealed to successfully and accurately track real unit operating conditions, providing rapid and reproducible data, currently unavailable with traditional methods. Over a stepwise severity increase, parameters of residue stability and insoluble particle content were shown to change systematically: a decrease in stability coupled with an increase in insoluble particulates. A novel feed characterization method based on a unique laboratory pyrolysis system is also described. Stability and insoluble particle data generated from laboratory stream samples exhibit very similar systematic changes to those discovered on the real unit, providing compelling evidence that the laboratory apparatus is providing a representative simulation of field thermal conversion processes, such as visbreakers and cokers. The apparatus also provides an accurate measure of surface deposition over a range of severities. Bulk insoluble particle generation coupled with surface deposition provides a unique set of parameters that describe the fouling potential of individual feeds, with data summarized in a heavy residue fouling matrix. Furthermore, the technique may be used to recommend feed-specific antifoulant additive programs. Such information has the potential to play a key role in reducing refinery energy demands and, therefore, emissions while maintaining or enhancing throughput.
formation because of uneven flow distributions.2 It is therefore to the advantage of the refinery operator to have some prior information about feed cracking behavior and tendency of feed to produce foulant during processing. Such information may permit the refinery operator to prepare for potential severity excursions by implementing a mitigation strategy that may include considered alterations to process conditions and application of antifoulant additives.3 Studies to date show no clear correlation of feed fouling tendency with primary bulk compositional parameters, such as compound class distribution [saturates, aromatics, resins, and asphaltenes (SARA)] and bulk metals. The search for such a correlation or relatively simple feed test that indicates fouling tendency has led to several research initiatives that have examined cracking behavior under controlled laboratory pyrolysis conditions. The onset of coke formation has been investigated for many years. Indeed, Wiehe discusses many of these research efforts, the broad consensus of which suggests that coke is produced as a direct byproduct of sequential polymerization and condensation reactions from lightest to heaviest fractions (maltenes, asphaltenes, and coke).4 Furthermore, a period of time prior to coke formation, termed the coke induction period, was identified. It is this phenomenon that permits thermal cracking
Introduction As conventional crude oil reserves are depleted, there has never been a greater emphasis on heavy unconventional oil and heavy residue processing efficiency.1 Because such feeds require thermal conversion, the formation of foulant material within process units, such as heat exchangers, furnace tubes, and fractionators, is relatively common. This may lead to loss in unit conversion, and in extreme cases, unscheduled shut down. Both scenarios are potentially very costly to the refiner. Furthermore, efficient process operation may promote significant energy cost savings that may contribute to refinery sustainability targets. The fouling phenomenon under discussion here concerns thermal coke formation. The formation of such foulant material accompanies the generation of cracked distillate and often occurs in furnace tubes of visbreakers and delayed cokers and, indeed, in any location where feed is maintained at around 350 °C or above for sufficient time.1 Foulant accumulation may manifest itself as notable changes to process conditions, such as increased pressure drop and hot spot *To whom correspondence should be addressed. Telephone: þ442380883919. E-mail:
[email protected]. (1) Wiehe, I. A. Process Chemistry of Petroleum Macromolecules; CRC Press (Taylor Francis Group): Boca Raton, FL, 2008. (2) Joshi, J. B.; Pandit, A. B.; Kataria, K. L.; Kulkarni, R. P.; Sawarkar, A. N.; Tandon, D.; Ram, Y.; Kumar, M. M. Petroleum residue upgradation via visbreaking: A review. Ind. Eng. Chem. Res. 2008, 47, 8960–8988. r 2010 American Chemical Society
(3) Rijkaart, M.; Vanacore, M.; Russell, C. A. Visbreaker optimization, a step change. Pet. Technol. Q. 2009, Q2. (4) Wiehe, I. A. The pendant-core building block model of petroleum residua. Energy Fuels 1994, 8, 536–544.
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: DOI:10.1021/ef100589p
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processes, such as visbreaking, to operate continuously, because operating severity is preferably before the end of the coke induction period, i.e., the time at temperature (severity) that a portion of feed experiences in the furnace is optimized to ensure that coke is not formed. Essentially, sensitive control of time and temperature within the furnace may permit thermal conversion units to run with limited fouling problems. However, fouling propagation may be affected by unit production targets, feed composition, and reactions under furnace conditions. Several researchers have reported methods to classify feeds in terms of fouling potential or propensity to form coke. Schabron describes a molecular weight/polarity map to examine the onset of coke formation for different heavy residues.5 Coking onset was found to coincide with a depletion in resins and, therefore, the ability of resin material to interact with asphaltene cores to maintain stability. The experiments seemed to confirm the colloidal dispersion, solution model of petroleum that is discussed in Wiehe and Kennedy.6 Furthermore, their data are used to obtain coking indexes, which may be used to estimate how close a system is to coke formation. Interestingly, one such index is based on the disappearance of the cyclohexanesoluble portion of n-heptane asphaltenes being coincident with coke formation. Each case has the potential to permit a refiner to optimize process conditions to maximize conversion or run length while limiting coke yield. The coking index concept was further expanded by Schabron and Schabron et al. to provide a novel tool to rank heavy residue with respect to fouling tendency.7,8 These indexes may be used for most residues and, of course, are unique for each residue type. An examination of residue fouling tendency may also be facilitated by investigation of reaction kinetics. Joshi et al. include an excellent summary of work that have performed detailed kinetic studies on a wide range of feeds, over a wide range of parameters.2 Here, one of the main aims is to find a correlation between feed properties, kinetic rate parameters, and activation energies. Further detailed discussion of feedspecific kinetic parameters are included in refs 9-12. Certain molecular parameters may also be used to ascertain proximity to coke formation. For example, Ogbuneke et al. discuss a novel method that is based on the increased relative abundance of a six-member ring polycyclic aromatic hydrocarbon (PAH) that is coincident with the end of the coke induction period.13 Feed composition and process conditions largely dictate feed fouling tendency, as discussed above. However, there are
further aspects and process conditions that can be just as important to fouling propagation, such as metallurgy and surface roughness. Additives may also play a crucial role in alleviating fouling problems by interactions with fouling precursors and modification of surfaces. Discussions of such additives are sparse within the literature; however, certain generic studies have been documented, many of which are discussed by Towfighi et al.14 The review concludes by suggesting that additives inhibit fouling by modifying surfaces, interfering with surface reaction processes, and promoting surface gasification of coke precursors. The formation of foulant may also be disrupted by the addition of a hydrogen donor to feed during pyrolysis. Such additives are discussed by Ogbuneke and Gould and Wiehe.15,16 Ogbuneke15 examines the effect of tetralin as an additive hydrogen donor. Experiments show that the coke induction period may be extended with the presence of excess tetralin. In low amounts, however, rapid conversion to naphthalene and to further PAH was thought to increase the potential for coke formation. The following report outlines the development of residue stream monitoring tools and pyrolysis apparatus and procedure that provides unique information on the bulk and surface fouling tendency of heavy residues that are typical feedstocks for visbreakers and cokers. The technique has developed into a multi-dimensional characterization tool that has ultimately formed the basis of a comprehensive database of fouling potential for well-known feed constituents. Furthermore, the pyrolysis technique has permitted precise examination of the antifoulant additive program potential. Experimental Section Samples. To date, 24 samples have been obtained from various refineries throughout the world, and their basic compositions are outlined in Table 1. Feeds are classified according to process unit: visbreaker or coker. Bulk Compositional Analyses. Sub-samples of feedstock were submitted for standard compositional characterization. These techniques generated data depicting metal, elemental, and compound class composition. The latter is generated by a common chromatographic technique called Iatroscan (thin-layer chromatography coupled with flame ionization detection). There is much debate on the relative merits of compound class composition separation, but for ease of access and consistency, Iatroscan was chosen. The assessment of metals content was obtained using an inductively coupled plasma-atomic adsorption spectroscopy technique, which is a slight modification of IP-377. Elemental composition was assessed using a Leco elemental analyzer. Residue Stability Analysis (RSA). Small aliquots of pyrolysis residue samples were made up into two toluene dilutions (75 and 25% vol) before being titrated with i-octane. The optically detected flocculation point (decrease in transmittance) was then used to calculate stability values for the sample. In essence, the method is a modification to American Society for Testing and Materials (ASTM) D 7157-05, because only two dilutions are required for determination of stability values. Laser Particle Size Analysis (LPA). A determination of particle count and size distribution was obtained using a standard
(5) Schabron, J. F.; Pauli, A. T.; Rovani, J. R. Molecular weight polarity map for residua pyrolysis. Fuel 2001, 80, 529–537. (6) Wiehe, I. A.; Kennedy, R. J. The oil compatibility model and crude oil incompatibility. Energy Fuels 2000, 14, 56–59. (7) Schabron, J. F.; Pauli, A. T.; Rovani, J. F., Jr.; Miknis, F. P. Predicting coke formation tendencies. Fuel 2001, 80, 1435–1446. (8) Schabron, J. F.; Pauli, A. T.; Rovani, J. F. Residua coke formation predictability maps. Fuel 2002, 81, 2227–2240. (9) Kataria, K. L.; Kulkarni, R. P.; Pandit, A. B.; Joshi, J. B.; Kumar, M. Kinetic studies of low severity visbreaking. Ind. Eng. Chem. Res. 2004, 43, 1373–1387. (10) Yue, C.; Watkinson, A. P.; Lucas, J. P.; Chung, K. H. Incipient coke formation during heating of heavy hydrocarbons. Fuel 2004, 83, 1651–1658. (11) Sawarkar, A. N.; Pandit, A. B.; Joshi, J. B. Studies in coking of Arabian mix vacuum residue. Chem. Eng. Res. Des. 2007, 85, 481–491. (12) Ebrahimi, S.; Moghaddas, J. S.; Aghjeh, M. K. R. Study on thermal cracking behavior of petroleum residue. Fuel 2008, 87, 1623– 1627. (13) Ogbuneke, K. U.; Snape, C. E.; Andresen, J. M.; Crozier, S.; Russell, C. A.; Sharpe, R. Identification of a polycyclic aromatic hydrocarbon indicator for the onset of coke formation during visbreaking of a vacuum residue. Energy Fuels 2009, 23, 2157–2163.
(14) Towfighi, J.; Sadrameli, M.; Niaei, A. Coke formation mechanisms and coke inhibiting methods in pyrolysis furnaces. J. Chem. Eng. Jpn. 2002, 35, 923–937. (15) Ogbuneke, K. U. Fundamental understanding of coke formation and the influence of additives in visbreaking. Ph.D. Thesis, School of Chemical and Environmental Engineering, The University of Nottingham, Nottingham, U.K., 2007. (16) Gould, K. A.; Wiehe, I. A. Natural hydrogen donors in petroleum resids. Energy Fuels 2007, 21, 1199–1204.
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visbreaker 01 visbreaker 02 visbreaker 03 visbreaker 04 visbreaker 05 visbreaker 06 visbreaker 07 visbreaker 08 visbreaker 09 coker 01 coker 02 coker 03 coker 04 coker 05 coker 06 coker 07 coker 08 coker 09 coker 10 visbreaker 10 coker 11 coker 12 coker 13 visbreaker 11
12.8 18.2 6.0 2.4 5.7 4.3 2.8 1.4 17.6 79.4 41.9 49.9 4.5 5.7 4.2 3.5 7.0 3.3 3.1 3.3 1.8 3.2 1.2 0.8
13.2 5.7 5.5 4.5 3.7 17.4 4.5 7.4 8 23 32 19 19 20.1 23.2 19.3 22.9 19.4 20.3 5 11.1 14.2 2.3 11.6
toluene insolubles saturates (ppm) (%) 49.5 46 52 52.8 45 47.9 45.6 46.9 49.4 43 48 47 42.9 45.9 47.9 44.3 36.8 51 47.3 36.6 45.6 43.6 51.1 58.6
aromatics (%)
Conversion rate in % min-1 (relative laboratory severity).
0.74 0.80 0.71 0.69 0.69 0.83 1.11 0.78 0.82 0.68 0.70 0.58 0.95 0.92 0.86 0.78 0.88 0.94 0.99 1.03 0.67 0.70 0.78 0.68
feed
a
conversion ratea 32.3 34.8 27.6 25.2 34.4 30.7 32.6 30.8 31.8 26 17 27 28.7 25.7 24.4 28.8 30.1 17.6 20.2 41.6 34.8 32.5 27 25.9
resins (%) 5 13.5 14.9 17.5 16.9 4 17.3 14.9 10.8 8 4 7 9.4 8.3 4.5 7.6 10.1 12 12.2 16.8 8.5 9.7 19.6 3.9
asphaltenes (%) 0.4 2.8 4.4 4.7 2.8 0.6 3.0 0.8 2.7 0.4 0.4 0.4 0.7 0.7 0.7 0.4 0.4 3.6 3.5 1.7 1.1 1.0 4.5 0.9
1.15 0.33 0.28 0.31 0.29 1.43 0.42 2.18 0.27 1.30 1.43 3.75 1.50 1.17 0.34 0.42 2.80 0.38 0.31 0.26 0.11 0.21 0.31 0.33
23 7 26 33 62 20 84 37 54 13 10 15 30 28 10 11 14 47 38 89 4 5 28 15
S Ni (%) Ni/V (ppm)
13 16
22 3 9 12 53 23 67 13 65 16 7 13 8 7 8 5 8 63 10 26
Fe (ppm) 28 3 14 9 31 5 48 53 26 9 5 14 8 9 11 14 10 7 7 10 9 14 16
Na (ppm)
1
1
3
2
3 2
2 1 4
3
Zn (ppm)
3 4
5 1 3 7 3 2
1 2 4 6 104 2 5 10 4 9 24
47
Al (ppm)
Table 1. Bulk Compositional Data for the Heavy Residue Sample Set
3 2 9
9 17 15 3 12 11 4 11 4 9 7 9 10 3 3 8
60 2
Ca (ppm)
2
15
2 53 2
3 4 3 13 11
2 5 12
11
K (ppm)
1
2 2
2
2
Mg (ppm)
20 21 93 108 214 14 200 17 200 10 7 4 20 24 29 26 5 125 123 348 35 24 91 45
11 16 108 4 11 20 9 27 20 3 7 4 5 6 6 7 4 2 8 15
53 2 2
4
1
1 2 1
3
V Si P (ppm) (ppm) (ppm)
272 38 145 165 390 110 642 129 382 97 53 109 125 80 79 123 60 258 190 506 52 48 163 105
total metal
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: DOI:10.1021/ef100589p
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particle counter. Samples of feed and pyrolysis residue were first diluted in toluene to 0.1% vol prior to flow through the counter. The machine provides a numbered distribution over a set particle size range. With further manipulation, an approximate particle volume was calculated by assuming a spherical particle shape. Pyrolysis Apparatus and Procedure. The pyrolysis apparatus used in this study is a modified high-temperature-high-pressure batch autoclave reactor. The main features include constant pressure operation, distillation of cracked products, additive injection into the reactor during pyrolysis, and quantification of the surface deposit via stainless-steel microreactor inserts. Reactor insert material is composed of stainless-steel (SS-304) woven wire, which has a mesh size of 70 and wire diameter of 0.15 mm. The inserts are first prepared by cutting into 30 100 mm sections, dimensions that permit the precise fit onto the reactor inside wall. Further preparation involves a multi-stage cleaning procedure that begins with acid etching in concentrated HCl (38%), followed by washing with deionized water, methanol, and dichloromethane, before being dried under a gentle stream of nitrogen. The reactor is a 100 mL Parr bench-mounted stirred vessel manufactured in Hastelloy C276 and fitted with a high-torque sealed magnetic stirrer drive on the removable head. Feed is added to the reactor once the preweighed micro-insert has been carefully placed inside toward the bottom of the chamber. Additives may be dosed into the feed at this stage if necessary. The reactor is assembled and leak-tested and, if successful, purged with nitrogen. The reactor is then pressurized, and the heater system is turned on. As the temperature increases, the stirring speed is increased, from 440 to 980 rpm. The reactor is further heated to a temperature of 350 °C, where it is maintained for 10 min. This temperature represents the feed temperature prior to entering a typical furnace unit. Furthermore, time at temperature permits feed constituents to equilibrate prior to the cracking onset. The temperature is then increased to 410 °C at a rate of 10 °C min-1. This cracking temperature was predetermined as a consequence of research performed in collaboration with the University of Nottingham.13,15,17-20 Although pressures may vary from unit to unit, 10 bar was chosen to allow for a direct comparison of cracking distributions. Also for this reason, a temperature of 410 °C was used throughout all characterization experiments. The temperature is maintained at 410 °C for a predetermined length of time. Complete characterization of feed is obtained by performing several pyrolysis experiments of increasing time increments, typically 10, 20, 30, and up to 90 min in duration. Once the predetermined cracking time is completed, the reactor is rapidly cooled back down to 350 °C at 15 °C min-1, thereby quenching any further cracking reactions. The reactor is then drained of pressure through a cooled-coiled trap that permits the collection of cracked distillate. Once reactor pressure has been diminished, complete distillation is facilitated by nitrogen bleed through for 10 min at 20 mL min-1. The reactor is then cooled and disassembled. Reactor micro-inserts are removed and sequentially washed in solvents of increasing polarity, cyclohexane, toluene, and
Figure 1. Relationship between asphaltene and sulfur contents for the heavy residue sample set.
dichloromethane, to reveal three deposit layers, termed loose, medium, and hard. The latter layer contains the material under discussion in the present paper. Tar products were examined in terms of particle volume and size distribution and in terms of intrinsic stability, described above.
Results and Discussion Bulk Composition. The identification of an indicative fouling potential parameter has led to the development of many novel analytical techniques. Some of these methods have become relatively routine for characterizing crude oils and, indeed, heavy residues. Part of the reason for routine analysis of these parameters is that, for some feeds, fouling tendency may be related to one or more of these features, e.g., sulfur and asphaltene content. Bulk analyses of the heavy residue samples under consideration in the present study are shown in Table 1. A variety of visbreaker and coker feeds are displayed, and importantly, the sample set covers a range of sulfur and asphaltene contents. These parameters are often associated with fouling tendency. Figure 1 displays the relationship between asphaltene and sulfur contents. For certain feeds, high asphaltene and sulfur contents correlate well with fouling tendency; for example, field experience indicates 40% Arab heavy blend (designated visbreaker 04 in Table 1) to be relatively high fouling. Furthermore, a measure of total solids (toluene insolubles) present in the feed may be linked to fouling tendency because of their implication in catalytic coke growth. On a similar basis, high metals content has also been associated with high fouling tendency. An indication of the variation of feed conversion under laboratory pyrolysis conditions is listed in Table 1. A conversion at each designated pyrolysis time is calculated as the difference in percent residual pyrolysis tar product, i.e., the tar remaining in the reactor after pyrolysis, and 100. For every feed, conversion increases linearly with pyrolysis time, providing an excellent data fit and, therefore, some indication of the conversion rate, which is listed in Table 1. Interestingly, there appears to be a broad correlation of the conversion rate and asphaltene content: the conversion rate generally increases with an increasing asphaltene content. Perhaps, the conversion rate increases faster with a high asphaltene content because there are relatively more heteroatom bonds, especially sulfur, that are easier to break than carbon-carbon bonds. Indeed, a broad relationship of sulfur and asphaltene content is displayed in Figure 1; feeds
(17) Russell, C. A.; Snape, C. E.; Crozier, S.; Kikabhai, T.; Sharpe, R. Biological marker compound transformations as an extremely sensitive measure of cracking in visbreaking. Prepr. Symp.;Am. Chem. Soc., Div. Fuel Chem. 2004, 49, 492–495. (18) Ogbuneke, K. U.; Snape, C. E.; Andresen, J. M.; Crozier, S.; Kikabhai, T.; Sharpe, R. Coke formation mechanisms during visbreaking of a vacuum resid. Prepr. Symp.;Am. Chem. Soc., Div. Fuel Chem. 2005, 50, 272–274. (19) Ogbuneke, K. U.; Snape, C. E.; Andresen, J. M.; Crozier, S.; Sharpe, R. The potential of PAHs to predict coke yields in visbreaking. Prepr. Symp.;Am. Chem. Soc., Div. Fuel Chem. 2006, 51, 229–230. (20) Ogbuneke, K. U.; Snape, C. E.; Andresen, J. M.; Russell, C.; Crozier, S.; Sharpe, R. Effect of different additives on coke yields from the thermal cracking of vacuum residue. Prepr. Symp.;Am. Chem. Soc., Div. Fuel Chem. 2007, 52, 118–120.
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with high asphaltenes invariably have high sulfur content and, therefore, a higher conversion rate. Initial toluene insolubles and metals content are also displayed in Table 1. Although certain aspects of bulk composition have been implicated or associated with fouling tendency, there are others where the potential reasons are not clear at all. For example, Albacora blend is labeled on Figure 1 and designated coker 06 in Table 1. According to bulk compositional data, the feed blend consists of relatively low sulfur and asphaltenes. However, processing experiences in the field have shown the feed to exhibit very high fouling tendency. Intrinsic Stability of Pyrolysis Residues. There is a vast distribution of molecular species in a typical heavy residue, and scientists in the field continue to endeavor to ascertain definitive structures for even a relatively small proportion, not withstanding the evolution and transformation of such molecules with thermal action. However, there have been several studies that have shed some light on the type of transformation mechanisms that occur. For instance, Russell et al. describe how the relative changes to common biomarker parameters in the maltene fractions of undistilled residues may be used to track cracking reactions in visbreakers and, indeed, may assist in the calculation of residence time distributions.17 Ogbuneke et al. explain that the detection of certain PAHs correlate well with the onset of coke formation, providing a potential method to aid the defense against unwanted fouling episodes.13 However, both techniques require extensive sample work up time and require expensive analytical equipment. Although such techniques are essential for the understanding of fundamental molecular transformations, they are not very convenient for rapid sample turn around necessary for sensitive unit process monitoring. Therefore, it is more suitable to regard molecular interactions and transformations during thermal conversion of heavy residue by considering a relatively more simpler approach. Wiehe describes a model where petroleum macromolecules in fractions of unconverted and converted heavy oil may be represented by a distribution of pendant and core building blocks.4 For example, the saturate fraction of a heavy residue may be almost entirely composed of longchain-like pendants, whereas asphaltenes have a greater proportion of aromatic cores. Manipulation of the model constituents can help further interpret fraction interactions and, therefore, the relative susceptibility of asphaltene-type material (foulant precursors) to precipitate out of solution, deposit on process surfaces, and eventually form foulant. The stability of asphaltene-like material within the process stream decreases with increasing process severity. Furthermore, the stability parameter is a key specification for cracked product from visbreakers. Therefore, unit severity is often monitored with respect to this value. Thus, it is vitally important to obtain rapid feedback to facilitate high resolution and sensitivity to maintain unit optimization. The experimental method to determine such information involves precipitation of asphaltenes using n-alkane solvent. Specifically, the method under investigation here involves slight modification to ASTM D 7157-05. The Nalco residue stability analyzer instrument is a two-channel system, which allows for a very rapid test. Although intrinsic stability as strictly defined in ASTM D 7157-05 requires three dilutions rather than two, the accuracy and reliability of the RSA instrument of Nalco are such that it produces equivalent to those obtained from the ASTM D 7157-05 method. Typically,
results fall within a 95% confidence interval for the reproducibility of the method. In accordance with ASTM D 7157-05, three parameters are reported: (1) S is the intrinsic stability of the oil. This is an indication of the stability or available solvency power of an oil with respect to precipitation of asphaltenes. (2) Sa is the peptizability or ability of the asphaltenes to remain in a colloidal dispersion. Sa is linked to the solubility of the asphaltenes, the length and number of the aromatic chains, or degree of ring condensation. (3) So is the “aromatic” equivalent of the oil. It is a measure of the solvency power of the oil with respect to asphaltene solubility. The apparatus was recently used at a European refinery to monitor and analyze residue stream (visbroken tar) stability during visbreaker optimization. At each scenario, a measure of relative unit severity (minutes) was obtained by estimating a relative furnace residence time (combining feed rate, soaker pressure, and temperature), furnace outlet temperature, and soaker pressure data with residence time, activation energy, and universal gas constant in the following equation:1 relative unit severity ðminÞ ¼ ðrelative residence timeÞ EA 1 1 e R T 700 where EA is the activation energy taken as 40 kcal/mol, R is the gas constant (0.001 987 kcal mol-1 K-1), and T is the temperature (K). Stability measurements could therefore be compared over a series of severity increments or scenarios. As severity is increased, the intrinsic stability of resulting visbroken tar systematically decreases (Figure 2a). At a particular stability, insoluble coke is detected by LPA of the residue, which will be examined later. The stability of laboratory generated vistars are shown in Figure 2b, where two examples are displayed. Once more, the x axis is represented by a severity term, relative laboratory severity. Here, a reaction rate constant is calculated from the Arrhenius equation for each second that the feed is at the designated pyrolysis temperature relative laboratory severity ðminÞ ¼ kt where t is the time in minutes and k=Ae((-EA)/(RT)), where EA is the activation energy taken as 34 kcal/mol, A is the preexponential factor taken as 2 1010 s-1, R is the gas constant (0.001 987 kcal mol-1 K-1), and T is the temperature (K). Therefore, for a pyrolysis experiment consisting of 20 min at 410 °C, the relative laboratory severity would be equal to k multiplied by 20 to give relative laboratory severity (minutes). Similarities to field data are encouraging, with intrinsic stability systematically decreasing with increasing pyrolysis severity for both feeds. The range of stability values is much broader for the laboratory vistars because very low severities are investigated. It would be extremely rare to obtain equivalent samples from a real unit, because of the method by which units are brought on line and the cost incurred by running at low severity/low conversion. Therefore, although the unit samples here are obtained from a visbreaker optimization exercise, the severity range is relatively narrow compared to that covered in the laboratory. Consequently, the stability range is also relatively narrow. The initial stability of the visbreaker feeds are markedly different, with feed B much more stable than feed A. Above a relative laboratory 5487
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Figure 3. Tracking asphaltene solubility (Sa) over (a) visbreaker severity optimization and (b) residue laboratory pyrolysis series.
Figure 2. Tracking intrinsic stability (S value) evolution over (a) visbreaker severity optimization and (b) residue laboratory pyrolysis series.
severity of around 20, both feeds appear to converge on a maximum instability value of around 1.2, which is consistent with real unit operation. Changes to asphaltene solubility are displayed in panels a and b of Figure 3 for real plant data and laboratory vistars, respectively. As expected, there is a systematic decrease in Sa with severity as the asphaltenes become more insoluble, i.e., larger and greater in number, with many more cores relative to pendants, as they move to completely insoluble coke. Once again, the field samples exhibit a smaller range of values; however, both field and laboratory data exhibit similar trends. Changes to the maltene fraction of pyrolysis residues are not as straightforward as those discussed above. They can vary quite dramatically from feed to feed, as shown in panels a and b of Figure 4. Examining the laboratory generated vistars first (Figure 4b), the solvency power of the oil increases with severity in the case of feed A, suggesting that the residue contains a greater amount of aromatic species relative to non-solvent aliphatic hydrocarbons. Confirmation of this observation is revealed because the distillate associated with feed A contains relatively more aliphatic to aromatic species, as determined by whole-fraction GC-MS. Conversely, the So of feed B decreases with increasing severity, suggesting that the residue is less aromatic. Indeed, the distillate fractions contain a relatively greater proportion of aromatic species to aliphatic as determined by whole-fraction GC-MS. The profile observed from the plant vistars follows a unique trend, first decreasing and then increasing. Interestingly, the samples appear to cover a similar range to the laboratory vistars. Assessment of Bulk Insoluble Particulates. As alluded to above, the unsustainable formation of increasingly insoluble asphaltenes may be accompanied by the significant formation
Figure 4. Tracking oil solvency power (So) over (a) visbreaker optimization and (b) residue laboratory pyrolysis series.
of insoluble coke. Rapid feedback and resolution is once again essential for real process unit (i.e., visbreaker) monitoring. Traditionally, a significant insoluble sediment is determined by a process called hot filtration (ASTM D 4870), a process that is often laborious and cumbersome and offers relatively poor reproducibility and sensitivity. Here, a measure 5488
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Figure 5. Particulate evolution with increasing severity over visbreaker optimization survey and under laboratory conditions.
Figure 6. Typical surface depositional profile revealing distinct stages of formation.
of toluene-insoluble foulant material is obtained using a laser particle counter that operates on a light-scattering basis. With some manipulation of the raw count data (assuming spherical particles with a density of 1 g mL-1), particle volume and mass may be calculated. Figure 5 reveals the particle evolution with increasing severity for both unit- and laboratorygenerated vistars. It is clear that all profiles are different, and indeed, this is the case for all feeds examined in the laboratory; there is a unique relationship between particle generation and apparent asphaltene stability for every feed. It also appears that the unit-generated vistars are slightly offset from the laboratory samples, because at high S values, there is an appreciably higher volume of particles. This is probably a consequence of real feed and process apparatus differences. Of course, the generation of insoluble material in the bulk liquid phase is not only dependent upon asphaltene destabilization but may also be associated with inorganic constituents and other coking seeds. Therefore, the profiles indicate the importance of monitoring both stability and particle content, because the generation of problematic insoluble material may be independent of apparent asphaltene stability. Upon closer inspection, toluene-insoluble particle formation profiles generally reveal three distinct stages, with others revealing the first two at the very least. The first stage is termed the coke induction period, which is followed by an initial stage of particle formation and then a significant stage of relatively more rapid particle formation at very high severities. Each stage may be assigned a numerical value, for example, induction period duration in relative laboratory severity units and initial stage rate. The coke induction feature has been widely observed by many researchers over the years. The examination of this feature makes visbreaking processes possible, because operational severity is maintained as close to the end of this period as practicable to maintain maximum conversion while avoiding significant fouling and off-spec products. Assessment of the asphaltene stability and toluene-insoluble concentration provide essential information for the operation of thermal conversion units, such as visbreakers. However, these features are observed from the bulk phase and tell very little about surface fouling. There are many instances in the field where product specifications are within tolerated thresholds, but furnace tube skin point temperatures are accelerating toward maxima, indicating significant fouling. In such situations, severity has to be reduced, which in turn reduces conversion. Fouling may also dramatically
limit the length of run. Therefore, examination of bulk liquid parameters may not assist the prevention of costly fouling excursions. Heavy Residue Characterization by Bulk and Surface Foulant Systematics. The assessment of bulk parameters, such as asphaltene stability and insoluble sediment content, may not provide enough information to ascertain overall fouling characteristics. It would be useful, therefore, to obtain some idea of heavy residue feed surface fouling tendency. For example, if visbreaker operators were planning to bring a relatively unknown feed blend online, having an estimate of relative fouling tendency with respect to product stability, particle content, and surface fouling tendency may be beneficial for unit operational planning, a plan that may additionally include an antifoulant additive treatment program. Many heavy residue feeds have been characterized using the laboratory pyrolysis apparatus, as discussed previously with reference to Table 1. A typical depositional profile is displayed in Figure 6, which is very similar to that revealed by bulk particle formation. Again, there are three distinct stages present that have been designated as follows: (1) The induction period is defined by analytical detection limits and is thought to represent a period of foulant precursor transformations (e.g., decrease in asphaltene solubility via sidechain cleavage, condensation and dehydrogenation of resins, and changes in maltene solvency), initially resulting in negligible generation and deposit. The duration of induction period is unique for each feed. (2) The initial stage is represented by a gradual rate of deposition. The rate is also unique for each feed. Over this stage, catalytic growth is thought to be the prevailing coke formation mechanism. (3) The significant stage is also characterized by rate and onset severity. At these severities, however, catalytic growth plays an ever-diminishing role, as the foulant film becomes thicker. Instead, radical formation is thought to play a key role in increasing surface foulant thickness and hardness and bulk liquid sediment. In general, the heavy residue feeds fall into one of three relative fouling categories, examples of which are displayed in panels a-c of Figure 7. Feeds that may be considered low-medium fouling (Figure 7a) exhibit a relatively long induction period, followed by significant particle growth in the bulk phase as opposed to considerable deposition on the surface, i.e., a higher rate of formation in the bulk fluid relative to the surface. 5489
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Figure 8. Fouling database covering a wide variety of heavy residues (axes are defined in the text).
low asphaltene and sulfur contents, e.g., Albacora blend in Figure 1. There are many potential reasons why these particular heavy residue streams behave in such an aggressive way with respect to surface fouling, and thus far, there are no obvious features revealed by various routine analytical techniques. However, the fact that each feed exhibits unique foulant systematics suggests that a relative ranking system could be implemented. As described previously, certain numerical values may be assigned to the stages of particle formation in the bulk liquid and surface deposition. A relatively low fouling feed exhibits greater preference for foulant formation in the bulk fluid (Figure 7a), whereas a high fouling feed has a short induction period coupled with a high rate of foulant formation (Figure 7b). In the closed system reactor that is the basis for the work here, the affinity for foulant to form on surfaces as opposed to retention within the bulk liquid is intimately connected. Also, because visbreakers and other thermal conversion units operate toward the end of the induction period and the beginning of the initial stage, it was decided to calculate the ratio of the initial stage rate/ induction period duration in relative laboratory severity units. Calculating this ratio for both bulk and surface foulant systematics provides bulk and surface fouling factors. When plotted together, the data for all heavy residues may be compared (Figure 8). The samples fall into one of two distinct groups, labeled 1 and 2. Samples in group 1 typically exhibit surface coke deposition induction periods of greater than 1 relative laboratory severity unit. Samples closest to the origin of x and y are typified by long induction periods and relatively low initial stage rates, giving low surface and bulk fouling factors. The bulk liquid fouling factor changes markedly with a systematic increase through group 1 samples. Group 1 samples are considered to reside on a normal fouling tendency trend, with high fouling toward the top of the arrow and low fouling toward the bottom. Samples that reside in group 2 are typified by the fouling systematics displayed in Figure 7c, where surface fouling seems to propagate almost immediately. These feeds are highly problematic, because no clue to their surface fouling nature is provided by the bulk liquid particle volume. The database is relatively large and continuously growing as more potentially problematic visbreaker and coker feeds come online. The classification scheme provides a useful tool for the refiner to attain some indication of relative fouling tendency.
Figure 7. (a) Typical low-medium, (b) typical high, and (c) typical very high foulant systematics profile.
High fouling feeds are characterized by a relatively shorter induction period followed by significant foulant generation on both the surface and in the bulk liquid phase (Figure 7b). Heavy residue feeds that exhibit these foulant systematics have to be used with great care to avoid rampant fouling during processing. In some instances, processing problems may be predicted by examining the bulk composition, i.e., if the feed has a particularly high asphaltene and sulfur content, as discussed previously. However, more often than not, such information alone cannot reliably indicate residue processing fouling tendency. There is a third scenario that is often encountered by our laboratory when problematic feeds are submitted for characterization, and this is summarized in Figure 7c. Here, surface fouling is initiated almost immediately, accompanied by a relatively high rate of deposition. Significant bulk particle generation occurs after a long induction time, very similar to that of the low fouling scenario. Therefore, the foulant has a greater affinity for surface deposition over formation in bulk liquid. Furthermore, such feeds offer no indication of their potentially serious processing issues from bulk compositional analyses. For example, these feeds may have relatively 5490
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Figure 9. Composition of laboratory deposits.
Correlation of Laboratory and Field Deposit Composition. The composition of the field deposit can vary widely. It is important to ascertain that the deposit generated in the laboratory is similar to that found in the field, with positive correlation giving credibility to the relative fouling tendency ranking system and to any positive antifoulant additive performance demonstrated in the laboratory. Figure 9 displays two examples of feed that exhibit very different field foulant compositions. The first concerns an oil sands upgrading facility in Canada, which uses a vacuum furnace. The second example concerns a coker furnace. Reports from the field confirm that the deposit from the oil sands upgrader contains significant contribution from iron sulfides. It is also important to mention that the severity is much lower than that of visbreaking and coking. The deposit from the coker furnace more resembles standard carbon-rich coke. Both feed samples were characterized using the laboratory pyrolysis rig. Experimental conditions were slightly different for each feed because the severity of oil sands upgrading is relatively lower than coking. Interestingly, the deposit on the reactor insert from the oil sands upgrader feed was slightly more than that generated from the coker feed, even though the pyrolysis temperature was relatively lower (40 min at 390 °C, cf. 40 min at 410 °C, respectively). This correlates well with unit deposit observations. However, it is the composition of the laboratory deposits that are most encouraging. The deposit from the oil sands upgrader feed is dominated by Fe and S, which correlates well with the unit deposit. Furthermore, the Cr content is relatively lower than for the coker deposit, suggesting that the deposit is relatively thicker and relatively more Fe is contained as iron sulfides. The coker furnace feed generated a laboratory deposit more representative of standard, carbon-dominated, hard coke. The compositions were determined by scanning electron microscopy-energy-dispersive spectrometry (SEM-EDS), and included in Figure 9 are some representative photomicrographs. The morphology of the deposits are significantly different, relating to their primary composition. There is noteworthy surface roughness for the oil sands feed deposit relative to the coker sample. Such surface roughness is thought to promote accelerated fouling and may relate to the higher fouling rate of this feed. The evidence presented here gives increased confidence that deposit formation mechanisms under laboratory conditions are similar to those in the field.
Figure 10. Typical example of the depositional profile (red), including modification because of the antifoulant additive treatment program (blue).
Development and Application of Antifoulant Additives. Previous aspects of this discussion describe similarities between laboratory- and field-generated vistars, associated foulant formation mechanisms, and the classification of various feeds according to foulant systematics. Such information has permitted laboratory development of effective antifoulant additives with confidence for application to field units. In essence, research has shown that, under laboratory conditions, using both fresh and precoked reactor inserts, a combination of surface modification/passivation and high-temperature coke suppressant addition is a very potent combination for furnace tube foulant reduction. Furthermore, certain additives are effective with certain feeds; therefore, products can be tailored for individual feed requirements. An example of how surface passivation and high-temperature coke suppressant can effect surface deposition is displayed in Figure 10. Here, reactor inserts were first passivated at a temperature under an inert mineral oil. Once the feed had reached the cracking temperature, a 5 min precoking time was observed prior to injection of high-temperature coke suppressant (cf. offset data points reflect 5 min precoking time). The antifoulant chemistry has effectively increased the induction period and reduced the initial stage rate of deposition, even on precoked surfaces (5 min precoking time in Figure 10). Translated to real unit operation, application of additives may facilitate increased processing efficiency and, therefore, extend the length of run. 5491
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classified into two groups by the authors. (9) The database has the potential to provide refinery operators with essential information on common residue blends, including those from opportunity crudes, prior to online processing. (10) Good correlation of laboratory and field foulant deposit composition may permit further insight into formation mechanisms and additive interactions. (11) A combination of surface modification/ passivation and high-temperature coke suppressant is demonstrated to effectively increase the induction period and reduce the depositional rate, which may translate to unit process to facilitate efficient energy savings in the form of an increased length of run and a decreased decoking frequency.
Conclusions (1) Bulk composition of residue feed is not sufficient to elucidate fouling potential. (2) Assessment of asphaltene destabilization (S value) facilitates rapid sample turn around without compromising accuracy and reproducibility. (3) Examination of S values from a visbreaker optimization program revealed good sensitivity to severity increase. (4) Unit vistar severity profile compared well to laboratory-generated S-value profiles. (5) Similarly, insoluble particulates present in bulk vistars displayed good correlation between the unit and laboratory. (6) Three distinct stages of surface deposition have been identified, and are common for all feeds: induction period, initial stage, and significant stage. (7) Differentiation of affinity for surface deposition over bulk particle generation is observed under laboratory conditions, permitting relative ranking of feed fouling tendency. (8) Residue feeds have been
Acknowledgment. The authors thank Nalco Energy Services Research and Development and, particularly, European Sales and Marketing Teams for their continued support.
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