Can Fischer−Tropsch Syncrude Be Refined to On-Specification Diesel Fuel?

The interchangeable colloquial use of the terms “distillate” (boiling range) and “diesel fuel” (distillate meeting legislated fuel specifications), led to misleading perceptions about the suitability of Fischer−Tropsch syncrude for diesel fuel production. Two questions are addressed: Can Fischer−Tropsch syncrude be refined to diesel fuel, and which Fischer−Tropsch technology is best for maximizing distillate and ultimately diesel fuel production? The distillate yield that can be obtained from Fischer−Tropsch syncrude in an uncomplicated refinery employing only hydrocracking and/or oligomerization follows the order: Fe-LTFT > Co-LTFT > Fe-HTFT. Conversely, producing diesel fuel (not distillate) from Fe-HTFT syncrude is easier. On a molecular level, Fischer−Tropsch syncrude was found to be unsuitable for the production of EN 590:2004 diesel fuel in high yield. There is a trade-off between distillate density, cetane number, and yield, which is called the Fischer−Tropsch density−cetane−yield triangle. It is possible to meet any two of these three requirements without too much refining effort, but meeting all three with Fischer−Tropsch syncrude as feed material, is difficult. Strategies have been suggested to overcome the Fischer−Tropsch density−cetane−yield triangle and to improve the yield of on-specification diesel fuel. Some industrial implications for the standalone Fischer−Tropsch-based production of diesel fuel are discussed. High-temperature Fischer−Tropsch (HTFT) and low-temperature Fischer−Tropsch (LTFT) fuels refinery designs based on current refining technology to maximize EN 590:2004 diesel fuel are provided

Thermal Conversion Regimes for Oilsands Bitumen

Thermal conversion of oilsands bitumen at 400 °C was investigated to gain a better understanding of temporal changes in liquid properties. The work approximated a mild thermal cracking (visbreaking) process with run-lengths extending into the coking region. Reaction progress could be divided into three main regimes: (I) stable visbreaking, (II) coking visbreaking, and (III) coking. In addition to observations anticipated from the literature, the work revealed aspects of the reaction progression that was not fully appreciated before. Stable visbreaking with minimal formation of coke had a “productive” period during which viscosity decreased, while asphaltenes content and gas yield were unchanged, followed by an “unproductive” period during which the viscosity, asphaltenes content, and gas yield all increased. After the onset of precipitation of solids, the solids (“coke”) yield increased and the asphaltenes content in the liquid decreased, but the viscosity increased. The origin of increased viscosity was not due to increased asphaltenes content but due mainly to free radical addition products that remained soluble in the bulk liquid. The insolubility of molecules in the liquid and meso-phase that ultimately led to precipitation as solids and coke formation at higher temperatures, appeared to be caused by addition reactions more than as result of asphaltenes and hydrogen-depletion. The investigation does not prove this conclusively, but if this interpretation is correct, the coking limit of stable visbreaking operation at 400 °C is not as result of a solubility constraint caused by hydrogen-depletion; instead, it is the result of free radical addition that manifests as a solubility constraint leading to hydrogen-depletion. In the reaction sequence, the importance of hydrogen disproportionation of alkyl cycloalkane functionality in the bitumen was highlighted to explain the major changes, including the onset of increased gas yield and increased viscosity that was attributed to free radical addition

Ozonation of Oilsands Bitumen

The conversion of bitumen with ozone should in principle lead to oxidative degradation, as opposed to the increase in viscosity found during the reaction with oxygen because ozonation can lead to CC scission by ozonolysis. However, when bitumen was reacted with ozonated air at 140–160 °C, the viscosity increased compared to conversion with air under the same conditions. To explain these observations, the ozonation of model compounds of various hydrocarbon and heterocyclic compound classes was studied. The selectivity and nature of the oxidation products from ozonation were different from that of autoxidation. Ozonolysis that led to the ring opening of aromatics was observed. At low conversion (<10%), the addition product selectivity of the five-membered cyclic hydrocarbons, indane and indene, was less after reaction with ozonated air than with air. It appears that the observed increase in bitumen viscosity was due to the increased free radical addition product selectivity of heterocyclic compounds, which was higher after reaction with ozonated air than with air. It was also found that at higher conversion (>10%), the increase in oxidized material caused a dramatic increase in addition product selectivity during ozonation. A plausible reaction pathway for addition product formation was proposed on the basis of the Criegee mechanism. The nature of unique addition products from ozonation that were not observed after autoxidation could be explained in terms of the decomposition of the ozonide and recombination of the free radical fragments

Desulfurization of Heavy Oil–Oxidative Desulfurization (ODS) As Potential Upgrading Pathway for Oil Sands Derived Bitumen

Heavy oil usually contains percentage levels of sulfur. Most of the sulfur in heavy oil is found in bulky thiophenic structures. Thiophenic sulfur is difficult to remove by catalytic hydrodesulfurization, but it can readily be oxidized. The sulfoxides and sulfones produced from sulfur oxidation can be solvent extracted from the heavy oil as a result of their increased polarity. Oxidative desulfurization of heavy oil was studied using Canadian Cold Lake bitumen (5% S, 1100 kg/m³), with air as oxidant. At the conditions investigated, namely, autoxidation at 145–175 °C followed by water washing, 46–47% of the sulfur in the bitumen could be removed. This is equivalent to >20 kg sulfur per ton oil. Part of the sulfur was removed as SO₂ and part as water extracted sulfur-containing compounds. Lower autoxidation temperatures led to better desulfurization. The main challenge was to prevent free radical addition reactions that cause a viscosity increase and bitumen hardening. Autoxidation of undiluted bitumen and bitumen–water mixtures resulted in hardening. Hardening was prevented when bitumen was diluted with naphtha (<i>n</i>-heptane). However, the oxidized sulfur compounds could not be extracted with water from the bitumen–heptane phase, and some material was precipitated as a result of solvent deasphalting. Oxidation selectivity was studied using a model dibenzothiophene and <i>n</i>-heptane mixture. Some precipitation was also observed, and the chemistry was analogous to the precipitation chemistry (gum formation) that undermines storage stability of transportation fuels

Thermal Behavior of Potassium C₁–C₁₂ <i>n</i>‑Alkanoates and Its Relevance to Fischer–Tropsch

The thermal behavior of potassium C₁–C₁₂ <i>n</i>-alkanoates (K-carboxylates) were studied over the temperature range <i>T</i>/K = (243 to 873). A number of problems in industrial Fischer–Tropsch facilities were attributed to these compounds, but this study also revealed some beneficial effect that may directly be related to the thermal behavior of potassium methanoate. The unusually low melting point of potassium methanoate, <i>T</i>/K = (442.2 ± 0.3), combined with its thermal stability to <i>T</i>/K ≈ (693), may explain the ease of distribution of the potassium promoter of iron-based Fischer–Tropsch catalysts during synthesis even when potassium promoter is added separately. The C₂–C₁₂ K-carboxylates were all thermally stable at temperatures <i>T</i>/K ≤ (713), and significant mass loss was not observed at <i>T</i>/K ≤ (748). The thermal stability and high melting point of potassium propanoate, <i>T</i>/K = (636.9 ± 0.3) and potassium butanoate, <i>T</i>/K = (623.1 ± 0.3), in particular caused these compounds to be prone to cause pressure drop problems in refining units. The C₄–C₁₂ K-carboxylates melted to a liquid crystal phase first, before clearing at higher temperature. One or more solid–solid transitions were observed in all of the K-carboxylates, with the exception of potassium hexanoate

Heterocyclic Addition Reactions during Low Temperature Autoxidation

Low temperature air oxidation affects many aspects of the fuels business: weathering, storage stability, hardening, and fouling. Addition reactions taking place during the autoxidation of different heterocyclic compound classes were studied at 130 °C. This investigation reports on the conversion, product selectivity, nature of addition products, and plausible mechanisms that would explain the observations subsequent to the oxidation of indole, 2,3-dihydroindole, quinoline, benzofuran, 2,3-dihydrobenzofuran, and thianaphthene. Of these, indole is known to be prone to oxidative addition. Among the five-membered heterocyclic compounds, the propensity to form oxidative addition products increased in the order: S ≪ O < N. Addition took place mainly through C–C bond formation. In comparison to hydrocarbons, it was surprising to find that indole and 2,3-dihydroindole were less prone to oxidative addition than the five-membered naphtheno-aromatic hydrocarbon analogues indene and indan. On the basis of the work, some implications for low temperature oxidation processes, and free radical processes in general, are discussed

In Situ Measurement of Liquid Phase Oxygen during Oxidation

In liquid phase autoxidation of hydrocarbons, oxygen availability in the liquid phase affects reaction rate and product selectivity. Instead of relying on engineering predictions, this work set out to measure in situ oxygen availability in the liquid phase under oxidation conditions. This was achieved by employing an oxygen sensitive material submersed in the liquid and measuring the change in fluorescence decay with a fluorometer. Mass transfer coefficients (<i>k</i>_L) of oxygen in benzene and in indan were 3.0 × 10^–6 and 1.3 × 10^–6 ± 0.1 × 10^–6 m/s, respectively, at 50 °C and 19.2 kPa O₂ partial pressure. This enabled the calculation of the maximum oxygen transfer rate and it matched the experimental observations well. Liquid phase oxygen content increased until the oxygen transfer rate and the oxygen consumption rate were balanced, i.e. reached dynamic equilibrium. At high oxygen consumption rate, the oxygen in the liquid phase was consumed and the oxygen consumption rate matched the maximum oxygen transfer rate. These changes could be monitored in situ over time during oxidation. It was further found that liquid phase oxidation violated key assumptions underlying engineering predictions based on the Hatta number. Notably, the kinetic constant for oxygen consumption changed with time, oxidation did not reach “steady state”, and the relationship between oxygen consumption and oxidation rate varied over time

Oxidative Ring-Opening of Aromatics: Decomposition of Biphenyl Carboxylic Acids and Zinc Biphenyl Carboxylates

Ring-opening conversion of multinuclear aromatics can be used to upgrade heavy aromatic oils to lighter products, and it is usually performed reductively with H₂. Oxidative ring-opening is an alternative strategy that involves three steps: (i) oxidation of multinuclear aromatics to quinonoids, (ii) further oxidation and ring-opening to produce aromatic carboxylic acids, and (iii) decarboxylation of aromatic carboxylic acids. In the last step, decomposition by ketonization is an undesirable side reaction that leads to a ring-closed product. Selectivity control during aromatic carboxylic acid decomposition was investigated using biphenyl-2-carboxylic acid, biphenyl-2,2′-dicarboxylic acid, zinc­(II) biphenyl-2-carboxylate, and zinc­(II) biphenyl-2,2′-dicarboxylate. The reaction networks of thermal decomposition of the aromatic carboxylic acids were determined. Decomposition of biphenyl-2-carboxylic acid took place mainly by decarboxylation to produce biphenyl, dehydration and ring-closure to produce fluorenone, and the formation of diphenic anhydride as intermediate product leading to fluorenone. Decomposition of biphenyl-2,2′-dicarboxylic acid proceeded through decarboxylation to biphenyl-2-carboxylic acid as intermediate, as well as two seemingly related pathways, leading to the formation of a hydroxy-fluorenone and a cyclic trione. Over the temperature range from 340 °C to 400 °C, thermal decomposition invariably resulted in a higher ketonization than decarboxylation selectivity. Decomposition of the analogous zinc carboxylates demonstrated that ketonization could be suppressed and the most abundant products were biphenyl > fluorenone > fluorene. It was possible to achieve a biphenyl (decarboxylation) to fluorenone (ketonization) selectivity ratio of 17:1 during batch reactor decomposition of zinc­(II) biphenyl-2,2′-dicarboxylate at 380 °C. Reaction stoichiometry indicated that water should affect selectivity, which is consistent with observations in the literature, but this aspect was not investigated further

Hydrocarbon Addition Reactions during Low-Temperature Autoxidation of Oilsands Bitumen

Low-temperature oxidation of bitumen with air in the temperature range of 130–160 °C was investigated. Of particular interest were the addition reactions taking place during oxidation, which contributed to the observed increase in viscosity of oxidized bitumen. During the autoxidation of bitumen, the relative aliphatic to aromatic loss-ratio of hydrogen increased from 18:1 to 30:1 when the temperature was increased from 140 °C to 150 °C and then remained almost the same at 160 °C. It coincided with a bitumen oxidation selectivity change reported in the literature. The hydrocarbon class responsible for most addition reactions during bitumen oxidation is the naphthenic-aromatic class. A model compound oxidation study at 130 °C found no addition products during paraffin oxidation, low addition product selectivity for naphthenic and alkylaromatic compounds, and no measurable oxidation of aromatics without alkyl groups. It was proposed that the dominant pathway for addition reactions of hydrocarbons is hydrogen disproportionation of free radicals to produce olefins. Free-radical addition to olefins through the formation of C–C bonds explained all of the oxidation selectivity observations from the model compound studies, as well as the addition products identified from their mass spectra. It could also be applied to explain the bitumen oxidation results in these and other studies

Viscosity Changes during Mild Oxidation of Oilsands-Derived Bitumen: Solvent Effects and Selectivity

The low-temperature oxidation of oilsands bitumen was investigated to determine how viscosity was affected by oxidation and whether oxidative hardening could be suppressed by solvent dilution. This work was performed to support the development of an oxidative desulfurization process, but it could also find application in processes for bitumen recovery by <i>in situ</i> low-temperature oxidation. The experimental investigation was conducted at 140–200 °C and near atmospheric pressure with oilsands-derived bitumen, air, and mesitylene as the solvent. Solvent dilution decreased the bitumen viscosity through diluent action, but it did not suppress oxidative hardening of the bitumen. In fact, with the presence of a solvent, the bitumen viscosity increased more than by oxidation of bitumen on its own. This could be explained in terms of easier hydrogen abstraction from bitumen relative to the solvent, which increased the probability of addition reactions. The increase in viscosity with oxidation extent was also investigated, and apparently conflicting reports in the literature were reconciled. At a constant temperature, different periods of near constant viscosity increase with the increase in the oxygen consumption were identified. It was also found that, during the first free radical chain-propagation-dominated oxidation period, the extent of viscosity increase was different at different oxidation temperatures for the same level of O₂ consumption. Oxidative hardening is not just related to oxidation extent, but it is also affected by changes in oxidation selectivity because of the conditions at which the oxidation was performed