Article pubs.acs.org/EF
Integration of Fischer−Tropsch and Oilsands Bitumen Production Processes Arno de Klerk* Department of Chemical and Materials Engineering, University of Alberta, 9211 116th Street, Edmonton, Alberta T6G 1H9, Canada ABSTRACT: Oilsands bitumen production facilities need heat, water, and hydrogen to recover and upgrade bitumen. Hydrogen is usually derived from synthesis gas, which also provides an opportunity for Fischer−Tropsch synthesis. Heat, process, and product integration benefits were pointed out in the literature, and the literature was reviewed. New integration opportunities were identified, as well as technical aspects that should be considered in such integration. Heat integration of air separation and the impact of Fischer−Tropsch technology selection on the quality of heat integration were discussed. Integration of water management and the potential use of the Fischer−Tropsch aqueous products for bitumen recovery, demetalation, viscosity reduction, and pH management were described. Limited opportunity for integration of gas cleaning was found. Process integration during primary product separation, as well as various strategies to derive more benefit from gaseous products, was outlined. Gaseous product processing strategies that were described include the use of tail gas olefin oligomerization, improved hydrogen recovery, and opportunities related to froth treatment, such as deasphalting, that were of a more speculative nature. The lack of current understanding related to coprocessing Fischer−Tropsch wax with bitumen was highlighted. Lastly, an improvement was shown in the distillation profile of diluted bitumen produced with Fischer−Tropsch products when compared to typical industrial dilution with naphtha or natural gas condensate.
1. INTRODUCTION Oilsands bitumen recovery and upgrading,1−4 and Fischer− Tropsch processes,5,6 are normally not discussed in the same context, but there are opportunities for synergistic integration. It is useful to explore the challenges in oilsands bitumen recovery and upgrading, because these are the challenges that give rise to the integration synergies with a Fischer−Tropschbased process. (a) The foremost challenge in the recovery of bitumen from oilsands deposits is the poor fluidity of the bitumen at reservoir conditions. Although bitumen viscosity varies between deposits, oilsands bitumen has a viscosity of >100 Pa·s (>105 cP), measured at 15 °C.7 Bitumen from mined oilsands is recovered by hot water extraction, and bitumen from subsurface deposits is recovered by steam injection. The production of large quantities of steam for bitumen recovery is a characteristic of this industry. (b) Once the bitumen is recovered, poor fluidity remains an issue for bitumen transport. Two approaches to addressing this shortcoming are generally found. One approach is to produce diluted bitumen (Dilbit) by mixing the bitumen with a naphtha-range solvent to lower its viscosity sufficiently for pipeline transport. Another approach is to convert the bitumen in a bitumen upgrader facility to produce an upgraded synthetic crude oil with lower viscosity, which is suitable for transport. (c) Bitumen has a lower commercial value than benchmark crude oils, such as West Texas Intermediate. It is an extra heavy oil with density >1000 kg·m−3, with 0−5 vol % straight run naphtha yield by distillation, around 5 wt % sulfur, and a hydrogen-to-carbon molar ratio of around 1.5.7 Ways to improve the commercial value of bitumen are therefore of interest. © XXXX American Chemical Society
(d) When the bitumen is upgraded (i.e., partially refined), the high sulfur content and low hydrogen-to-carbon ratio necessitate the use of extensive material rejection by processes such as coking and deasphalting, or the use of large quantities of hydrogen during hydroprocessing.4 There is consequently a need to exploit the rejected material and/or a need to generate hydrogen. (e) Water treatment and gas cleaning are not only important utility requirements, but the volumes of water and gas to be processed per unit volume of oil produced are much more than found in typical crude oil production and processing facilities. Some of the opportunities for the synergistic integration of oilsands bitumen production and processes based on Fischer− Tropsch synthesis have been pointed out in the literature. These will be briefly reviewed. Product integration benefits have been reported, which do not require the processes for bitumen production and Fischer− Tropsch synthesis to be integrated. The purpose of product integration is to increase the commercial value of the integrated product. For example, it was found that there is a benefit to combine between 0.5 and 10 wt % wax from low temperature Fischer−Tropsch synthesis with the asphalt derived from bitumen when the combined product is used for road paving.8 An improvement was similarly noted when the wax was partially oxidized.8 In a different application, it was reported that blending 2−6 wt % Fischer−Tropsch wax with bitumencontaining formulations that are used for coatings resulted in improved chemical resistance.9 The value of product blending Received: August 2, 2016 Revised: October 17, 2016
A
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Figure 1. Concept by Davis and Matturro10,16 for the integration of Fischer−Tropsch synthesis with bitumen production and upgrading.
with bitumen production from mined oilsands, hot water extraction, and froth treatment to produce a diluted bitumen.18 Concepts for the integration of Fischer−Tropsch synthesis with a bitumen upgrader were also proposed.19,20 These process proposals were based on combining a hydrogen lean synthesis gas generated by the gasification of vacuum residue or asphaltenes with supplementary hydrogen supplied or produced in different ways to create a hydrogen-rich synthesis gas feed for Fischer−Tropsch synthesis. In one process concept, the products from Fischer−Tropsch synthesis were hydroprocessed with the straight run distillates and vacuum gas oil from the bitumen.19 Co-processing of bitumen and Fischer− Tropsch material was rarely suggested. In a variation on the same concept, only the Fischer−Tropsch products were hydroprocessed and a partially upgraded oil was produced by blending the hydroprocessed Fischer−Tropsch material with straight run bitumen distillate and vacuum gas oil.20 Each one of the concepts described addressed specific needs in bitumen production and upgrading processes, or exploited specific advantages offered by Fischer−Tropsch processes. The objective of this work was to evaluate the opportunities for integration, to point out potential new opportunities for integration, to look at technology decisions, and to highlight deficiencies in current understanding that may prove to be challenges for realizing some of these opportunities. It was not a specific aim to address the process economics, although some aspects that have an impact on process economics will be discussed.
for fuels was also pointed out. Diesel fuel with an adequate cetane number can be produced by blending high cetane number, low density distillate derived from a Fischer−Tropsch process and low cetane number, high density distillate from a bitumen upgrader.10 In addition to product integration benefits, some process concepts were described for synergetic integration of a Fischer−Tropsch process with bitumen production and upgrading. The need for hydrogen during bitumen upgrading, and the observation that some H2 and CO remain after H2 recovery from synthesis gas, elicited proposals to integrate methanol and/or Fischer−Tropsch synthesis with bitumen upgrading.11,12 The potential petrochemical opportunities of this type of integration were realized, and several concepts for petrochemical production from bitumen were described, some of which included integration with a Fischer−Tropsch process.13,14 The integration of Fischer−Tropsch synthesis in a combined Fischer−Tropsch product and heavy oil/bitumen refinery was also described.15 One of the more general concepts that were proposed is shown in Figure 1, and three integration benefits were claimed.10,16 First, steam produced by synthesis gas cooling and Fischer−Tropsch synthesis was employed for the recovery of bitumen. Steam is employed in many bitumen recovery processes, but the in situ recovery of bitumen from subsurface oilsands deposits by methods such as steam assisted gravity drainage is particularly energy intensive. Bitumen recovered from oilsands deposits is very viscous. The naphtha fraction of the Fischer−Tropsch product could be used as solvent to dilute the recovered bitumen and thereby reduce its viscosity sufficiently for pipeline transport. The third integration benefit was hydrogen production. Hydrogen was separated from the synthesis gas, and it was used for hydroprocessing in the bitumen upgrader to produce bitumen with improved oil properties. Variations on the concept in Figure 1 were later developed that showed additional benefits. In one process proposal, the integration of water treatment, as well as fuel gas use for supplementary steam generation by once-through steam generators was shown, but it excluded integration with downstream bitumen upgrading.17 In another process proposal, it was shown how a Fischer−Tropsch process can be integrated
2. HEAT INTEGRATION The advantage of heat integration between a Fischer−Tropsch process and oilsands bitumen production was pointed out in the Introduction, and a number of different heat integration strategies have already been proposed.10,16−18 Closer inspection of these heat integration concepts reveal that, beyond the high level benefits that were identified, the concepts incorporated technology decisions that implicitly affected the extent of heat integration that could be achieved. The purpose of the subsequent discussion is to explore the impact of technology decisions on heat integration between a Fischer−Tropsch process and oilsands bitumen production. B
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to cooling of hot air from the air compressor is considerable.6 When a cryogenic air separation unit is present in association with production of bitumen from mined oilsands, it provides an opportunity to produce hot water by utilizing the low grade heat from hot air cooling, instead of using steam to produce hot water. This heat integration opportunity was not pointed out for the integration of mined oilsands bitumen production with a Fischer−Tropsch process.18 2.2. Water Gas Shift Conversion. The water gas shift reaction can be employed to produce additional H2 for hydrogen recovery, or to increase the H2:CO ratio of the synthesis gas for indirect liquefaction. When the objective is to produce additional H2 for hydrogen recovery, the water gas shift conversion will be operated in a way to maximize the H2:CO ratio. When the objective is to produce synthesis gas for Fischer−Tropsch synthesis, the aim would be to produce a H2:CO ratio close to the usage ratio around 2:1, but this depends on the Fischer−Tropsch technology (Section 2.3). The forward water gas shift reaction to increase the H2 content of the gas is exothermic, and the heat of reaction of the water gas shift reaction is ΔHr,25 °C = −41 kJ·mol−1.21 The steam pressure that can be generated from removal of reaction heat depends on the operating temperature of the water gas shift conversion. With clean synthesis gas, a low or high temperature “sweet” water gas shift process might be employed. From a heat integration point of view, a high temperature water gas shift is preferred, even though the equilibrium for H2 production is less favorable. When water gas shift conversion is employed in association with gasification of heavy materials, such as petroleum coke or asphaltenes, the synthesis gas must first be desulfurized. Even so, the clean synthesis might still contain parts per million levels of sulfur-containing compounds, in which case a high temperature “sour” water gas shift process must be employed.23 Although the water gas shift process consumes steam as a process feed, it also produces steam as utility due to the exothermic nature of the reaction and heat recovery during gas cooling. For example, heat integration of a high temperature water gas shift reactor within the synthesis generation unit of an oilsands bitumen upgrader is shown in the book by Gray.4 2.3. Fischer−Tropsch Synthesis. The operating temperature of Fischer−Tropsch synthesis determines the steam pressure that can be generated. About 20% of the calorific value of the synthesis gas feed is released as reaction heat during Fischer−Tropsch synthesis.5 Hence, from a heat integration perspective, Fischer−Tropsch synthesis at a higher temperature is much preferred. This is related to the saturated steam pressure that can be produced at a given temperature (Table 1).
2.1. Synthesis Gas Generation. In the oilsands industry, synthesis gas is generated mainly for the production of hydrogen. Synthesis gas is generated both by steam methane reforming and by gasification of heavy products from bitumen upgrading.4 Reforming and gasification are endothermic processes, and energy in the form of heat must be supplied to these processes. The technology that is selected for synthesis gas generation, as well as the method in which energy is supplied, affects the heat integration opportunities. Steam methane reforming using a tubular steam reformer is supplied with heat energy from fuel combustion, external to the process stream being reformed. Air is typically used as oxidant. Steam is generated by heat recovery from the hot synthesis gas, and from the hot flue gas exiting the tubular reformer furnace. Steam methane reforming by a partial oxidative reformer, such as an autothermal reformer, is supplied with heat by partial combustion of the process feed. Steam is generated by cooling the hot synthesis gas that exits the autothermal reformer. The “steam export problem” in steam methane reforming is wellknown,21 but in association with bitumen production the excess steam is not a problem, but a benefit. Synthesis gas can also be generated by gasification of heavy products, such as the products from carbon rejection processes employed in bitumen upgrading. For example, entrained flow gasifiers are industrially employed for the gasification of asphaltenes from a solvent deasphalting process in the Long Lake Upgrader to produce synthesis gas.4 The synthesis gas produced in this way has a H2:CO molar ratio close to 1:1, which is considered low. It is, in principle, also possible to produce synthesis gas from other materials, such as biomass, or organic waste products. It will be later shown that integration opportunities are not eroded by producing a synthesis gas with low H2:CO ratio. Another source of heat that is associated with partial oxidative reformers and gasifiers is the heat that originates from the supply of the oxidant (Figure 2). In these
Table 1. Saturated Steam Pressures at Temperatures Relevant for Heat Recovery from Fischer-Tropsch Synthesis
Figure 2. Partial oxidative reforming heat integration opportunities with bitumen production. These are (1) hot water from air compressor cooling, and (2) steam from synthesis gas cooling.
technologies, partial combustion is performed using purified O2 to limit the coprocessing of inert gases. The purified O2 is usually obtained from an air separation unit. In the air separation unit the air is compressed to around 0.6 MPa. The hot compressed air must be cooled down from 100 °C, first with cooling water, and then with chilled water, before it can enter the cryogenic section.22 In a normal cooling water system that employs evaporative cooling, the water consumption due C
Temperature (°C)
Saturated steam pressure (MPa)
190 200 210 220 230 240 260 280 300
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the olefins. Strictly speaking, the same is true of the Fischer− Tropsch distillate (Table 2), which would also require some form of hydroprocessing to decrease its olefin content before it can be blended with the bitumen. The olefin content of the lighter fractions is higher, and irrespective of the Fischer−Tropsch technology, the olefin content of the liquid petroleum gas (C3−C4) and light naphtha (C5−C6) fractions is upward of 50% (Table 2). Additional liquid products can be produced from the olefins in the liquid petroleum gas. The gaseous olefins are readily converted to naphtha and distillate by olefin oligomerization technology. Solid phosphoric acid- and ZSM-5 catalyst-based technologies are industrially used in combination with Fischer−Tropsch synthesis for this purpose.26 Little is known about the compatibility of bitumen with oxygenates (mostly alcohols, ketones, and aliphatic carboxylic acids) that are present in straight run Fischer−Tropsch products. Hydrotreating of straight run Fischer−Tropsch products before being mixed with bitumen is therefore prudent, since with appropriate hydrotreating conditions, oxygenates will also be converted. Olefin oligomerization, olefin hydrogenation, and oxygenate hydrogenation are all very exothermic reactions. Heat recovery from these processes can contribute to the overall heat export during the integration of a Fischer−Tropsch process with oilsands bitumen production. Thus, far, these heat integration opportunities have been overlooked.
The heat exchange area that is available in the Fischer−Tropsch reactor in relation to the heat duty determines the minimum temperature difference required for heat exchange. The actual temperature that can be achieved for steam generation is lower than the Fischer−Tropsch synthesis temperature in order to provide the temperature difference to drive the heat flow. When a cobalt-based Fischer−Tropsch technology is employed, the saturated steam pressure that can be generated is only around 1−2 MPa, because synthesis is conducted at below 230 °C. Of all the Fischer−Tropsch technologies, cobaltbased Fischer−Tropsch synthesis provides the least benefit in terms of heat integration. When iron-based Fischer−Tropsch technology is employed, synthesis can be performed at higher temperatures. Steam pressures up to 8 MPa can be generated when using iron-based high temperature Fischer−Tropsch synthesis. The pressure at which the steam can be generated appears not to have been considered for the integration with bitumen production. A number of the integration concepts10,16,19,20 were developed specifically with low temperature Fischer−Tropsch synthesis in mind, which produces a product that contains a meaningful wax fraction and less gaseous products. In at least some of the concepts, there was a need for synthesis gas conditioning to adjust the H2:CO ratio,19,20 which indicated that the concept was developed for cobalt-based Fischer−Tropsch synthesis. Unlike iron-based Fischer− Tropsch catalysts, cobalt-based Fischer−Tropsch catalysts do not catalyze the water gas shift reaction and the H2:CO ratio of the synthesis gas feed must be close to the usage ratio of around 2:1.5,6 Steam and hot water can be generated from Fischer− Tropsch product condensation and cooling. Steam and hot water from product condensation are also potential sources of heat integration with bitumen production. The nature of the integration is analogous to the integration described in Section 2.1. This type of utility-based heat integration may not be the most efficient use of the heat in this part of the process, because the hot products can be heat integrated directly with an atmospheric distillation unit.24 2.4. Fischer−Tropsch Product Workup. No previous mention was made of heat integration opportunities from Fischer−Tropsch product workup. This might be a consequence of the superficial treatment of this topic. Many of the integration concepts that were referred to in the Introduction point out that the naphtha fraction from Fischer−Tropsch synthesis is a useful diluent for bitumen to decrease the bitumen viscosity. However, straight run Fischer−Tropsch naphtha is olefinic (Table 2),25,26 and there is a limitation on the olefin content of bitumen for pipeline transport.27 The straight run Fischer−Tropsch naphtha cannot be used as diluent for bitumen without first being hydrotreated to saturate
3. WATER MANAGEMENT Water management in Fischer−Tropsch facilities is important and can be of comparable size in relation to oil production as in oilsands bitumen facilities. The fresh water intake in a typical oilsands bitumen production facility is typically of the order 0.5−3 m3 water per m3 bitumen, depending on the process and its operation.1 The fresh water intake can be of a brackish nature. The fresh water intake for a Fischer−Tropsch coal-toliquids facility with closed-loop cooling is of the order 2−3 m3 water per m3 oil product, but it is generally lower in gas-toliquids facilities.6 In both processes, much of the water is cleaned, recycled, and reused, so that the volume of water handled in the process is about an order of magnitude more than the fresh water intake. Much of the water required in a Fischer−Tropsch process is required for cooling, either to produce a hot cooling water return or to produce steam. Much of the water required during oilsands bitumen production is required for heating, either hot water to facilitate bitumen extraction or steam for extraction and in situ subsurface bitumen recovery. There is an obvious benefit to integrate hot water and steam production in a Fischer−Tropsch process with the hot water and steam needs in oilsands bitumen production. Synergies in heat management were described separately (see Section 2). Other synergies related to water management are shown in Figure 3. 3.1. Integration of Water Treatment. In general, little work has been done to explore potential synergies that could be derived from the integration of the water streams from a Fischer−Tropsch process and that are found in oilsands bitumen production. Apart from the gain in economy-of-scale when integrating water treatment, there might also be advantages related to the differences in the composition of the wastewater streams. The three main sources of water that require treatment in a Fischer−Tropsch facility are the water needed during synthesis
Table 2. Olefin Content of Typical Straight Run Syncrudes from Fischer-Tropsch Synthesis Olefin content (wt %) Description
Fe-HTFTa
Fe-LTFTb
Co-LTFTc
liquefied petroleum gas (C3−C4) naphtha (C5−C10) distillate (C11−C22)
88 77 69
77 63 29
65 39 5
a
Iron-based high temperature Fischer−Tropsch synthesis. bIron-based low temperature Fischer−Tropsch synthesis. cCobalt-based low temperature Fischer−Tropsch synthesis. D
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Figure 3. Integration of Fischer−Tropsch water treatment with bitumen production has several potential synergies. These are (1) reduced emulsion layer, (2) improved desalting, demetalation, and decreased bitumen viscosity, (3) partial asphaltenes to maltenes conversion, and (4) lower chemical use and economy-of-scale.
process employed. One of the remaining challenges in Fischer− Tropsch refinery technology development is finding an efficient method to use or recover the carboxylic acids dissolved in the acid water. Bitumen upgrading can potentially benefit from the Fischer− Tropsch acid water, which has a high chemical oxygen demand due to the organic acids but virtually no dissolved inorganic solids. It was found that treatment of oilsands materials with acids could facilitate demetalation.28 Demetalation has downstream benefits related to reduced fouling. Some direct product quality benefits were also noted when treatment with acidified water was applied to asphaltenes processing, which resulted in 8% n-pentane precipitated asphaltenes to maltenes conversion.28 It was further found that acid treatment could decrease bitumen viscosity, with a viscosity decrease of the order 5 times, from 10 to 2 Pa·s, being found for bitumen viscosity measured at 60 °C.29 The acid water provides an opportunity of treating bitumen with a low inorganic solids content water that would otherwise only be available after extensive water treatment. Bitumen recovery from mined oilsands deposits benefits from the hot water being slightly alkaline, and industrially, the pH is adjusted by addition of NaOH.1,31 Conversely, bitumen and bitumen derived fractions appear to benefit from treatment with acidic water.28,29 Hence, there might be benefits of using Fischer−Tropsch acid water during bitumen−water separation and desalting, partly due to its low inorganic solids content and partly due to its acidity, which would reduce the extent of emulsion formation compared to alkaline water. When retrofitting existing processes, specific attention should, however, be paid to the metallurgy of the process equipment to confirm material compatibility. 3.3. Organic Products from Water Treatment. Potential benefits that are of a more speculative nature are those related to the organic products found in the Fischer−Tropsch aqueous product. Once the carboxylic acids are neutralized, the amphiphilic properties of the propionates and butyrates make these compounds surface active. One of the reasons cited for operating at higher pH during bitumen recovery from oilsands deposits is the conversion of naphthenic acids present in the bitumen to naphthenates, which also have surface active
gas generation, the water produced during Fischer−Tropsch synthesis, and the water required for cooling.6 Water, in the form of steam, is a feed material for synthesis gas generation. The water that is not consumed during the reforming or gasification process is condensed and phase separated from the synthesis gas. The same is true for water gas shift conversion if it is included as part of the process design. Excess steam is co-fed with the synthesis gas to the water gas shift reactor, and the unconverted steam is condensed and recovered. In both cases the water has a low inorganic total dissolved solids content but may have some dissolved organic materials. The water produced during Fischer−Tropsch synthesis is condensed with the organic products and phase separated. The Fischer−Tropsch process-affected water (also called the “Fischer−Tropsch aqueous product” or “reaction water”) has a low inorganic solids content but invariably contains dissolved organic materials and is acidic in nature. Oilsands process-affected water is alkaline and has a high total dissolved solids content.1,30,31 The compositions of the process-affected water from oilsands mining and in situ subsurface recovery of bitumen are different. The composition of the water is also affected by the geology of the oilsands deposit. The alkaline nature of the oilsands process-affected water, and the acidic nature of the Fischer−Tropsch process-affected water, could be used for mutual neutralization. This is of particular benefit when dealing with water containing corrosive short chain carboxylic acids from Fischer−Tropsch synthesis. The acidity in the Fischer−Tropsch process-affected water could also be used to replace chemical addition in some water cleaning technologies, for example, for the regeneration of ion exchange resins. 3.2. Fischer−Tropsch Acid Water. A major product from Fischer−Tropsch synthesis is water produced with the oil products during synthesis. The first step in the treatment of the Fischer−Tropsch aqueous product is recovery of the lighter boiling oxygenates by distillation.26 The bottom product from distillation is called acid water, because it contains water with dissolved carboxylic acids, with the concentration being in the range 0.1−2%, depending on the type of Fischer−Tropsch E
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Figure 4. Integration of process gas cleaning and synthesis gas cleaning before Fischer−Tropsch synthesis typical of adding Fischer−Tropsch to an existing bitumen upgrading facility with acid gas treatment.
properties.31 The use of carboxylates from the Fischer− Tropsch process could be applied in a similar role. There are also potential benefits from coprocessing during bitumen upgrading. It was demonstrated that naphthenic acids retarded the onset of asphaltenes precipitation and that it was related to the modification of the solubility parameter of the mixture and not a specific chemical interaction.32 The aliphatic carboxylic acids might be useful in the same role, either as acids, or as carboxylates. A specific application Fischer−Tropsch oxygenates that is relevant to oilsands bitumen upgrading is the use of oxygenates for the retardation of the onset of coking during visbreaking. Wiehe33 described the steps leading to coking, with the formation of a second phase that led to coking, which took place in this second phase. If the solubility parameter of the bitumen is increased, the formation of a second phase and, hence, the onset of coking should be suppressed. The same phenomenon was noted when using very aromatic feed materials that also increased the solubility parameter and delayed the onset of coking.34 Using acids that might survive the thermal conversion could be detrimental by increasing the total acid number of the final product. Alternatively, the corresponding carboxylates can be used. Depending on how the reaction chemistry of the carboxylates is tailored, the metal used as counterion for the carboxylates can be selected to ketonize at high temperature to provide an increase in solubility parameter as ketones. Additionally some of the ketones might be incorporated into the oil product. The lighter oxygenates in the Fischer−Tropsch aqueous product is alcohol-rich. As explained in the previous paragraph, alcohols could be used as solubility parameter modifiers during high temperature operation for the same reasons. It is not clear to what extent the alcohols would be converted and whether the alcohols would participate in the reaction chemistry at high temperature. It is likely that the alcohols would be reactive. At moderate reaction temperatures, no benefit was found when using methanol for Friedel−Crafts alkylation of oilsands derived materials with FeCl3, but this might have been due to the catalyst, rather than the potential benefit of reacting alcohols with bitumen.35 In other work, some hydrolysis was noted,28 and with a more appropriate choice of catalyst, the alcohol-rich material might assist in bitumen upgrading through addition reactions. The alcohols could also be used in conjunction with the water employed for bitumen recovery, or during bitumen liberation and separation to change the water properties, such as suggested by Cobb.36 The naphthenic acids remaining in the oilsands processaffected water are a major source of toxicity.37 The solubility of the phenolic and naphthenic acid components from the bitumen in water is increased at higher pH, i.e. more alkaline conditions.38 It is speculated that there might be an opportunity to change the oil−water partition after bitumen recovery to reduce the naphthenic acid and phenolic content of the
oilsands process-affected water by treating it with polar and/or acidic components from Fischer−Tropsch synthesis.
4. GAS CLEANING The main gas cleaning processes found in association with Fischer−Tropsch processes are desulfurization and CO2 removal. Gas desulfurization is imperative, because sulfur is a Fischer−Tropsch catalyst poison.5,6 The need for CO2 removal depends on the Fischer−Tropsch gas loop design. When an iron-based Fischer−Tropsch catalyst is employed, the water gas shift reaction is also catalyzed. The amount of CO2 in the synthesis gas will determine whether forward or reverse water gas shift takes place. This in turn will affect the decision of whether CO2 should be selectively removed from the synthesis gas or not. When a cobalt-based Fischer−Tropsch catalyst is employed, no water gas shift takes place during Fischer−Tropsch synthesis. It was found that CO2 does not participate in Fischer−Tropsch synthesis over a cobalt-based catalyst, but that it is hydrogenated to methane.39 Hence, it might be worthwhile to remove CO2 after synthesis gas generation, both to decrease gas volume and to reduce methane selectivity during cobalt-based Fischer−Tropsch synthesis. Bitumen production is energy intensive, with much of the energy being required to produce steam for bitumen recovery. Cleaning of flue gas from combustion processes to generate steam is not normally required when low sulfur fuels are employed, such as natural gas or low sulfur fuel gas. Gas cleaning is required for process gas streams that are produced during bitumen upgrading processes, mainly to remove hydrogen sulfide. Integrating desulfurization of process gas streams from bitumen upgrading with desulfurization of synthesis gas for the Fischer−Tropsch process is possible, but practical only if the process gas will become a co-feed to the Fischer−Tropsch process. Despite the apparent integration synergy, in many instances it will not be practical to integrate gas cleaning of the different process streams. Process gas streams produced during bitumen conversion processes have a high sulfur content. The high sulfur content gas streams require a different type of gas cleaning technology than can be employed for low sulfur content gas streams. High sulfur-content gas cleaning typically requires liquid absorption technologies, such as amine absorption, or cold methanol absorption (e.g., Rectisol technology).40 Low sulfur-content gas cleaning can be performed using solid adsorption, such as using a packed bed of ZnO.41 Integration of bitumen process gas cleaning with synthesis gas cleaning might be considered in cases where the process gas is co-fed to a gasification unit converting high sulfur feed materials, such as petroleum coke, asphaltenes, or coal. The desulfurization requirements for gas cleaning aimed at air pollution control are not as stringent as they are for gas F
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Energy & Fuels cleaning aimed at Fischer−Tropsch synthesis. Bitumen upgrading facilities typically employ amine adsorption,4 which would reduce the sulfur content in the gas to
