Inductive Heating Assisted-Catalytic Dehydrogenation of Tetralin as a Hydrogen Source for Downhole Catalytic Upgrading of Heavy Oil
The Toe-to-Heel Air Injection (THAI) combined with a catalytic add-on (CAPRI, CATalytic upgrading PRocess In-situ) have been a subject of investigation since 2002. The major challenges have been catalyst deactivation due to coke deposition and low temperatures (~ 300 °C) of the mobilised hot oil flowing over the catalyst packing around the horizontal well. Tetralin has been used to suppress coke formation and also improve upgraded oil quality due to its hydrogen-donor capability. Herein, inductive heating (IH) incorporated to the horizontal production well is investigated as one means to resolve the temperature shortfall. The effect of reaction temperature on tetralin dehydrogenation and hydrogen evolution over NiMo/Al2O3 catalyst at 250–350 °C, catalyst-to-steel ball ratio (70% v/v), 18 bar and 0.75 h−1 was investigated. As temperature increased from 250 to 350 °C, tetralin conversion increased from 40 to 88% while liberated hydrogen increased from 0.36 to 0.88 mol based on 0.61 mol of tetralin used. The evolved hydrogen in situ hydrogenated unreacted tetralin to trans and cis-decalins with the selectivity of cis-decalin slightly more at 250 °C while at 300–350 °C trans-decalin showed superior selectivity. With IH the catalyst bed temperature was closer to the desired temperature (300 °C) with a mean of 299.2 °C while conventional heating is 294.3 °C. This thermal advantage and the nonthermal effect from electromagnetic field under IH improved catalytic activity and reaction rate, though coke formation increased.
KeywordsInductive heating Catalytic upgrading Tetralin Hydrogen
Worldwide transportation energy consumption was forecast as 151 quadrillion Btu in 2040, which represents an increase of 44 quadrillion Btu from 2015 . As the world’s demand for transportation fuels is rising, securing adequate supply is the real challenge. The major source of supply, light crude oil, has peaked and is declining in reserves [2, 3]. Heavy oil and bitumen hold about 70% of the world’s oil reserves, estimated at 13 trillion barrels , which has shifted attention towards developing extraction and upgrading technologies for them. This is because under reservoir conditions, heavy oils cannot flow like light oil and, so resist flow toward the production well as they are highly-viscous and possess low American Petroleum Institute (API) gravity. Hence, upon production, additional costs are incurred for pipeline transport and surface upgrading.
The commonly used extraction methods are steam injections such as cyclic steam stimulation and Steam Assisted Gravity Drainage (SAGD). However, alternatives include in situ combustion and Toe-to-Heel Air Injection (THAI) process. For simultaneous extraction and downhole upgrading of heavy oils, a catalytic add-on (CAPRI, CAtalytic upgrading PRocess In-situ) was combined with THAI [2, 4]. In this process, about 10–15% of the original oil in place is combusted and the developed combustion front advances from toe to heel of the horizontal production well through continuous air injection via a vertical injection well. The high thermal energy released from the combustion reaction between the oil and injected air increases the temperatures of the combustion front in the range of 400–700 °C, which mobilizes the rest of the oil toward the horizontal production well aided by gravity . As the mobilized hot oil ahead of the combustion front flows over the catalyst packing around the horizontal well additional downhole upgrading occurs [2, 3, 4]. Previous studies found that the actual temperature of the mobilized hot oil passing through the catalyst packing around the horizontal well in the THAI process may not exceed 300 °C [5, 6]. However, in other studies it was found that a temperature of about 425 °C is needed to maximise catalytic upgrading [2, 3, 7, 8]. To achieve the needed 425 °C or more, inductive heating (IH) has been proposed in this study as a means to provide the additional heating required for optimum catalytic upgrading. IH has previously been integrated with a batch reactor in which paramagnetic nanoparticles such as iron coated with silica were used to convert electromagnetic field to heat for organic synthesis process . Also, applied to fluidized bed system by co-fluidization of conductive particles which are chemically inert in order to heat the process . Furthermore, IH has been incorporated with steam-assisted gravity drainage (SAGD) to expand the steam chamber and improve oil recovery . With the aid of composite catalyst pellets consisting of a conductive component and a catalytic material [12, 13], or a mixed bed of catalyst and conducting susceptors , an inductive system can be applied to generate thermal energy inside the reactor. Herein, a laboratory-scale inductive heated reactor was developed, and a mixed bed of catalyst and chemically inert steel balls susceptors used to convert electromagnetic field into heat, which increases the temperature of the catalytic bed to the reaction temperatures. This is done with an emphasis on applications in THAI–CAPRI process for heavy oil recovery and in situ upgrading. The heat is generated rapidly in the area of interest; thus minimising heat lost, lowering energy costs, maintaining uniform temperature across the catalytic bed and ensuring controllability [10, 13].
Another challenge with in situ catalytic upgrading of heavy oils using THAI–CAPRI is large coke deposition on the catalyst results in rapid deactivation, bed plugging and low quality of upgraded oil [14, 15]. Coke formation can be suppressed when the catalytic upgrading is carried out under a hydrogen rich environment . The difficulty, safety and cost associated with transporting hydrogen gas itself into the reservoir have shifted attention to hydrogen-donor solvents such as polycyclic compounds (i.e., cyclohexane, decalin and tetralin) . These organic liquids potentially store and transport hydrogen in a safer and less challenging manner when compared to external transport of hydrogen gas itself into the reservoir. Also, they can easily be injected into the reservoir using either the vertical injection or horizontal production wells of the THAI–CAPRI process. These polycyclic compounds can hydrogenate–dehydrogenate in the upgrading mixture, and the evolved hydrogen has been found to improve the quality of upgraded oil and minimise coke formation by halting polymerisation of free radicals from cracked heavy molecules. The dehydrogenation of tetralin to naphthalene can be used as a hydrogen source to promote in situ hydrocracking and hydrogenation reactions in the catalytic add-on (CAPRI) to THAI. Hart et al.  reported that the use of cyclohexane as hydrogen-donor solvent for downhole catalytic upgrading of heavy oil suppressed coke formation. Many studies have reported the use of tetralin as H-donor solvent [14, 17, 18], and the findings showed that coke formation is suppressed and upgraded oil quality improved. However, studies on the quantification of liberated hydrogen from tetralin are very limited even though the mechanism of hydrogen evolution through dehydrogenation of tetralin to naphthalene is well-known. These polycyclic compounds have shown the ability to suppress coke formation which is usually attributed to the participation of the evolved hydrogen in the upgrading process.
The dehydrogenation of tetralin to liberate hydrogen and produce naphthalene is faced with two challenges. The first is that dehydrogenation is an endothermic reaction that requires high temperature, and the temperature of the hot oil passing over the catalyst bed in the THAI–CAPRI is in the range of only 100 to 300 °C , which is non-uniform and also below 425 °C needed for effective catalytic upgrading. The other is that the produced naphthalene will compete with the heavy oil molecules for adsorption on the catalyst active sites [19, 20]. Since the catalyst bed under IH functions as a heat generator, the first challenge can be addressed unlike conventional heating (CH) system where the catalyst bed acts as a heat sink. The catalytic dehydrogenation of tetralin to naphthalene is commonly performed over Pd or Pt supported catalyst at high temperature [21, 22, 23]. However, Pd or Pt cannot withstand the sulphur content in heavy oil. Hence, transition metals such as Ni, Co and Mo supported on alumina and/or silica are commonly used in catalytic upgrading of heavy oil and were used in this study [24, 25]. In this study, a laboratory-scale inductive heated reactor to be combined with THAI–CAPRI process for heavy oil recovery and in situ upgrading was developed. Heat was generated inside the mixed bed of catalyst (NiMo/Al2O3) pellets and steel balls of size 3 mm which convert electromagnetic field into heat. It has been demonstrated that the mixture of NiMo/Al2O3 catalyst and steel balls (catalyst-to-steel balls ratio, CTSBR 70% v/v) inside the CAPRI reactor can rapidly heat the catalyst to 425 °C in about 25 min and enhance catalytic transformation compare to CH. The amount of hydrogen obtainable from tetralin as hydrogen-donor solvent for catalytic upgrading of heavy oil using hydrotreating (HDT) catalyst NiMo/Al2O3 was experimentally verified. The effect of reaction temperature on hydrogen evolution was examined. Furthermore, a comparative study was carried out on tetralin conversion and hydrogen evolution when the mixed bed of catalyst was heated inductively with steel balls and conventionally at varying reaction temperatures. First-order kinetics and a plug flow model were used to estimate the apparent rate constant and apparent activation energy of the reaction in both inductive and conventional heated systems.
Commercial HDT catalyst NiMo/Al2O3 quadra-lobe shaped (Akzo-Nobel) was used. The tetralin (99.5% purity) used in this study was purchased from Sigma Aldrich, UK and n-hexadecane (99%, Alfa Aesar, UK) as solvent. 80 g of tetralin was added to 1 L of n-hexadecane stirred for 2 h by magnetic stirrer to form a solution used for the reaction.
2.2 CAPRI Reactor
The liquid product was analysed using a FID equipped gas chromatograph (GC) 6890N and a capillary column HP-5 (30 m × 0.320 mm × 0.25 μm). The catalyst surface area, pore volume and size were determined by nitrogen adsorption technique using Micrometritics Analytical Instrument ASAP® 2010, using the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) equations according to ASTM C1274. Bruker AXS GmbH (D8 Advanced XRD, Karlsruhe, Germany) was used to collect the X-ray diffraction (XRD) data of the catalyst using Cu Kα radiation from 10 to 80° 2θ and a step width size of 0.025°. Consequently, Hitachi TM3030Plus tabletop scanning electron microscope equipped energy dispersive X-ray spectroscopy (EDS) was used to examine the morphology of catalyst and quantify the elemental composition. Micromeritics AutoChem II 2920 analyser was used to determine the number and the strength of the catalyst acid sites through temperature programmed desorption using tert-butylamine [(CH3)3CNH2]. The coke deposited on the catalyst after reaction was determined using a thermogravimetric analyser, TGA (TG 209 F1 Iris® instrument, Netzsch-Geratebau GmbH) under air atmosphere with flowrate 50 mL min−1, ramp temperature 25 to 900 °C step increase 10 °C min−1 and held at 900 °C for 8 min. The gaseous product was analysed online using Agilent 7890A GC configured as a refinery gas analyzer.
3 Results and Discussion
The conversion of tetralin and the respective selectivity of the dehydrogenation products are defined by the equations.
3.1 Catalyst Characterization
Physicochemical properties of NiMo/Al2O3 catalyst
Specific surface area (m2 g−1)
Pore volume (cm3 g−1)
Pore diameter (nm)
Weak acid site at 280 °C [mmol adsorbed (CH3)3CNH2 g−1]
Strong acid site at 480 °C [mmol adsorbed (CH3)3CNH2 g−1]
3.2 Catalyst Bed Temperature Profiles
The measured temperature by the thermocouple is approximately that of the heated catalytic bed, the difference between 60% and 70% (v/v) catalyst/steel ratio affirmed this observation. Hence, the measured temperature is the heating of the steel balls majorly. When the steel balls are packed with the catalyst, they create a thermal conducting network. Therefore, the significant difference in heating rate between 60% and 70% (v/v) can be attributed to the percolation threshold for random sphere packing in 3D which is about 0.3. This means that the steel balls will form a percolating cluster and connectivity across the packed bed above 30% fractional occupancy and thus allow thermal conduction across the steel balls majorly.
3.3 Conversion, Selectivity and Liberated Hydrogen
The major product of the dehydrogenation reaction is naphthalene with selectivity of approximately 98% for all reaction temperatures investigated. The selectivity of cis-decalin is slightly more over the early minutes of experiment at 250 °C. However, with time-on-stream, the ratio of cis to trans-decalin changes from 2.4:1 to 1:1 while at 300–350 °C trans-decalin showed superior selectivity than cis-decalin (ratio of 1:2). This trend is due to the instability of cis-decalin and its isomerisation to a more thermally stable trans-decalin at high temperatures.
The formation of cis and trans decalins in the dehydrogenation of tetralin is an indication of the hydrogenation reaction promoted by the evolved hydrogen mainly due to naphthalene formation. The amount of decalin produced is very small, when compared to naphthalene produced, because of the small contact time remaining after naphthalene and hydrogen have been formed. The evolved hydrogen hydrogenates some residual tetralin, under the prevailing pressure, as its trickles down the catalyst bed. The reaction is also dependent on the prevailing partial pressure of hydrogen within the reactor. Additionally, the hydrogenation of tetralin to decalin is an exothermic one which is thermodynamically unfavourable at high reaction temperatures.
The evolved hydrogen produced from tetralin dehydrogenation was estimated from the amount of naphthalene produced excluding dialin (1,2-dihydronaphthalene) product (based on reaction Scheme 1, tetralin = naphthalene + 2H2 and excluding teralin = dialin + H2). Figure 5d shows the hydrogen production against time-on-stream over NiMo/Al2O3 catalyst at 250, 300 and 350 °C. The liberated hydrogen over the catalyst increases with time-on-stream and reaction temperature. The mean amounts of liberated hydrogen gas at the respective reaction temperatures are as follows 298.4 mmol (250 °C), 645.3 mmol (300 °C) and 752.6 mmol (350 °C). An increase in the optimum amount of liberated hydrogen from 360 to 876 mmol can be observed at 350 °C. On the other hand, the amount of tetralin converted at each reaction temperature ranges from 141 to 305 mmol (250 °C), 330 to 485 mmol (300 °C) and 345 to 543 mmol (350 °C) relative to 605 mmol of tetralin feedstock. The corresponding amounts of decalins produced are thus; at 250 °C (trans: 0.54–2.6 mmol, cis: 2.3–2.5 mmol), 300 °C (trans: 3.4–7.3 mmol, cis: 2–3.5 mmol) and 350 °C (trans: 1–10 mmol, cis: 3.65–5 mmol), respectively. This suggests that in situ hydrogenation of tetralin by the liberated hydrogen gas is negligible compared with the extent of dehydrogenation to naphthalene. This is because of temperature increase and limited hydrogen gas favours the dehydrogenation of tetralin to naphthalene as shown in Fig. 5a–c. Additionally, once tetralin dehydrogenate to hydrogen and naphthalene, the contact time between the hydrogen and the residual tetralin is insufficient to ensure reasonable conversion to trans and cis decalins in a fixed-bed continuous process.
The analysis of the gaseous product from the reaction showed the presence of 13.8 vol% (250 °C), 20.2 vol% (300 °C) and 37.6 vol% (350 °C) hydrogen. Most of this hydrogen is due to the formation of naphthalene from tetralin (selectivity of 98%) as only a negligible amount of 1,2-dihydronaphthalene was observed from the liquid product analysis. This evolved hydrogen under catalytic upgrading of heavy oil with tetralin as H-donor can promote hydrocracking and hydrogenation reactions to stabilise free hydrocarbon radicals. The formation of trans and cis-decalins in the liquid product affirmed the occurrence of the hydrogenation reaction due to the liberated hydrogen (Fig. 5a–c). The increased selectivity of trans and cis-decalins, as reaction temperature increased, can be ascribed to the increased moles of hydrogen evolved as temperature increased from 250 to 350 °C (Fig. 5d). In the light of this, hydrogen partial pressure would have increased also, thereby increasing the hydrogenation of tetralin to trans and cis decalins by the available hydrogen.
3.4 Fixed-Bed Reaction Kinetics
As expected an increase in reaction temperature, increases the rate of tetralin dehydrogenation, which is consistent with the results shown in Fig. 5a–c. The apparent activation energy was found as 30 kJ mol−1. It is believed that a catalyst with higher activity shows lower activation energy, this means that the reaction is mass transfer limited. This is because of the large dimensions of the catalyst particles (1.4 mm × 1.21 mm) and length (5 ± 2.1 mm) limiting the diffusion of reactants through the pores to the catalyst active sites. Additionally, the reactor internal diameter (20 mm) is significantly larger than the size of the catalyst particles. An activation energy in a similar range was reported by Chen et al.  for Mo supported on coal char as catalyst for tetralin dehydrogenation, though the rate constant was almost same, the activation energy obtained with NiMo/Al2O3 used in this study was slightly lower.
3.5 Coke Formation
The respective coke content of the catalyst after 6 h of reaction was determined as 10.2% (250 °C), 15.6% (300 °C) and 20.3% (350 °C). This represents about 5% coke increase for every 50 °C increase in reaction temperature. This increased coke formation is the result of the increased formation of the more strongly adsorbing and more condensable naphthalene compound as reaction temperature increases (Figs. 5a–c, 6a).
In spite of the observed coke formation, when incorporated with the THAI process, tetralin can keep the asphaltenes in the heavy oil in solution, suppressing their precipitation, polymerisation and condensation to coke while they undergo cracking . The use of methane gas and cyclohexane solvent as hydrogen-donors for catalytic upgrading of heavy oil in comparison to the use of a pure hydrogen gas had been reported in the literature by the authors [16, 30]. It was found that the addition of hydrogen gas improved the upgraded oil quality more than methane gas , while the use of cyclohexane solvent improved the upgraded oil quality more than hydrogen gas at cyclohexane/oil ratio of 0.08 . In summary, coke formation was suppressed as follows cyclohexane > hydrogen gas > methane gas. This is because of the ability of the produced aromatic compounds due to liberation of hydrogen gas from cyclohexane to keep the asphaltene molecules solubilized inhibiting precipitation and aggregation. Potentially, the quantity of hydrogen gas liberated from tetralin dehydrogenation (Fig. 5d) can support in situ catalytic hydroprocessing of heavy oil. Since it has been proven that the liberated hydrogen gas was beneficial in capping free radicals from cracked heavy molecules such as poly-aromatic and resins in situ and consequently halt their polymerisation and condensation into coke and increase catalyst longevity [15, 31].
3.6 Inductive Against Conventional Heating
3.6.1 Catalytic Bed Temperature
Under IH, heat flows from the steel balls to the catalyst pellets and the surrounding fluid, while, with CH it flows from the reactor wall to the surrounding fluid and to the catalyst. The heat generated within the catalyst bed by the steel balls is rapidly transferred to the catalyst and the surrounding fluid because of their intimacy. In the light of this, the actual reaction temperature at the surface of catalyst should be about the same as the surrounding fluid. In other words, the reaction occurred nearer to, or at, the desired temperature at the surface of catalyst, whereas with CH, heat is transferred from the surrounding fluid to the catalyst which increases the temperature gradient between the fluid and the catalyst. Hence, the temperature at the surface of the catalyst was about 5 °C lower than the desired experimental temperature (300 °C).
3.6.2 Conversion, Selectivity and Liberated Hydrogen
Converse to the above trends, no significant difference was observed for naphthalene selectivity under IH and CH methods, selectivity greater than 98% was achieved for all the investigated temperatures. Under IH, a thermally stable trans-decalin was produced more than unstable cis-decalin while under CH more cis-decalin was produced. This can be attributed to the difference in temperature at the surface of the catalyst and the activity induced by the heating technique. At 300 °C, the selectivity of trans and cis-decalins under CH and IH were mirror opposite. Whilst at 350 and 380 °C the selectivity of cis-decalin was approximately the same for CH and IH. However, more hydrogen was liberated from tetralin when the reaction was carried out under IH than CH (Fig. 9c).
Thermodynamically, a system that generates heat from within favours tetralin conversion and hydrogen evolution due to dehydrogenation, compared with a system in which the catalyst bed function as a heat sink (Fig. 8). The activity of the catalyst under IH would have been enhanced in addition to the thin temperature gradient between the catalyst and the surrounding fluid. Polarization of reactant molecules, improved catalyst activity, and enhanced molecular diffusion or transport of species in the reaction medium have been proposed as some of the non-thermal effects of electromagnetic field on heterogeneous catalysis carried out under inductive or microwave heating . These non-thermal effects could have improved the activity of the catalyst under IH. The closeness of the steel balls to the catalyst pellets induced rapid heat transfer in the inductive heated catalytic bed which would have promoted faster products (naphthalene, trans and cis decalins) desorption from the catalyst active sites and lower the strong adsorptive energy of naphthalene [22, 32]. This condition could have increased conversion of tetralin and dehydrogenation under IH. Hence, the low tetralin conversion observed under CH can be attributed to the effect of slow desorption of produced naphthalene from the catalyst surface due to the large temperature gradient between the catalyst and the surrounding fluid (Fig. 8). This could have created heat and mass transfer effect which decrease the catalyst activity under CH . In the case of IH with steel balls, the steel balls rapidly transfer heat to the surface of the catalyst and the bulk solvent whereas in CH heat transfer from the bulk solvent to the catalyst is slow (Fig. 8). Hence, under IH naphthalene desorption from the catalyst active sites is enhanced and the strong adsorptive energy of aromatic hydrocarbons lowered .
3.6.3 First-Order Kinetic Plot
Due to the lower temperatures experienced at the catalyst surface with CH system (i.e., the actual temperature in the catalyst bed is lower than the experimental temperature as shown in Fig. 8), the kinetics are slower compared to that observed with IH. Additionally, the activity of the catalyst could have been limited by adsorbed naphthalene product under CH. The rate constant under CH at 350 °C (~ 0.020 min−1) is closely comparable to 0.027 min−1 obtained under IH at 300 °C (Fig. 6a), which is in agreement with tetralin conversion results shown in Fig. 5b (IH at 300 °C) and Fig. 9b (CH at 350 °C).
Tetralin dehydrogenation as a hydrogen source for underground catalytic upgrading of heavy oil was studied in a fixed-bed reactor that mimics the catalyst packing around the horizontal production well (CAPRI) of the THAI process under induction heating at 300-380 °C, 18 bar, 0.75 h−1 and 70% (v/v) CTSBR using NiMo/Al2O3 catalyst. The results were compared to those achieved under CH. It was found that under the same operating conditions, higher tetralin conversion and more hydrogen liberation was observed in the inductive-heated system compared with those achieved with the conventional system. Additionally, the catalyst benefited from enhanced activity under the IH because of the non-thermal effect of the eddy current and magnetic fields. The temperature gradient between the surrounding fluid and catalyst was larger under conventional-heated system than inductive-heated counterpart. At 350 °C about 88% tetralin conversion was observed, 0.605 mol of tetralin produced about 0.876 mol of hydrogen mainly through dehydrogenation to naphthalene. This liberated hydrogen has the potential to promote hydroprocessing reactions when combined with in situ catalytic upgrading of heavy oil provided the partial pressure of hydrogen is favourable.
The authors acknowledge, with thanks, funding from Engineering and Physical Science Research Council (EPSRC), United Kingdom with Grant Number EP/N032985/1. Data in this paper is available free of charge via https://edata.bham.ac.uk/.
Compliance with Ethical Standards
Conflict of interest
The authors declare no conflict of interest.
This paper has been prepared in compliance with Ethical Standards. The corresponding author confirms this paper conforms to ethical standards and the project from which it is derived was granted ethical approval from the University of Birmingham. No humans or animals were used in the course of this research.
The corresponding author has the informed consent of all co-authors in submitting this paper.
- 1.EIA (2017) Global transportation energy consumption: examination of scenarios to 2040 using ITEDD. https://www.eia.gov/analysis/studies/transportation/scenarios/pdf/globaltransportation.pdf. Accessed 06/02/2019
- 4.Ayasse C, Greaves M, Turta A (2002) Oilfield in situ hydrocarbon upgrading process. US Patent 6412557 B1Google Scholar
- 5.Petrobank Energy and Resources Ltd. (2007) Strength in heavy oil: growing THAI™ globally. online:https://www.knotia.ca/kstore/productinfo/fric08/PDFs/Petrobank%20Energy%20and%20Resources%20Ltd.%20AR_2007.pdf. Accessed 29 Feb 2018
- 8.Xia TX, Greaves M (2001) 3-D physical model studies of downhole catalytic upgrading of Wolf Lake heavy oil using THAI. In: Paper 2001-17 presented at the Petroleum Society’s Canadian international petroleum conference 2001, Calgary, Alberta, Canada, 12–14 JuneGoogle Scholar
- 25.Hart A, Omajali JB, Murray AJ, Macaskie LE, Greaves M, Wood J (2016) Comparison of the effects of dispersed noble metal (Pd) biomass supported catalysts with typical hydrogenation (Pd/C, Pd/Al2O3) and hydrotreatment catalysts (CoMo/Al2O3) for in situ heavy oil upgrading with Toe-to-Heel Air Injection (THAI). Fuel 180:367–376CrossRefGoogle Scholar
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.