HDS FOR FUEL CELL APPLICATIONS
Proefschrift
ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus prof. ir. K.C.A.M. Luyben, voorzitter van het College voor Promoties,
in het openbaar te verdedigen op dinsdag 8 mei 2012
om 10:00 uur door
Bandar Hussain ALSOLAMI
Master of Science in Chemical Engineering Texas A&M University
Dit proefschrift is goedgekeurd door de promotoren: Prof.dr. J.A. Moulijn
Prof.dr.ir. M. Makkee
Samenstelling promotiecommissie:
Rector Magnificus, voorzitter
Prof.dr. J.A. Moulijn, Technische Universiteit Delft, promotor Prof.dr.ir. M. Makkee, Politecnico di Torino, promotor
Prof.dr. K. Seshan, Universiteit Twente Prof.dr. J.L. Figueiredo, Universidade do Porto Prof.dr. F. Kapteijn, Technische Universiteit Delft Dr. S. Eijsbouts, Albemarle Catalysts Company Dr.ir. J.W. Gosselink, Shell Global Solutions
Prof.dr.ir. H. van Bekkum, Technische Universiteit Delft, reservelid
ISBN 978-94-6186-028-6
To my wife and children Atheer, Athaar, Ethaar, Aseel, and Hussain
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Introduction and Review ... 1
DBT Hydrodesulfurization over NiMo, CoMo, and noble metal catalysts for ultra-deep HDS at low pressure ... 35
Measurement of intrinsic kinetics for NiMo catalyst in Milli- and Micro-flow reactors ... 47
Ultra-deep HDS of 4,6-DMDBT at low pressure ... 69
Low Pressure HDS of 4,6-DMDBT; Inhibition by carbazole, naphtalene, and FAME Bio-Fuel Additives ... 85
On-site low-pressure diesel HDS for fuel cell applications: Deepening the sulfur content to ≤ 1 ppm ... 103
Summary and Evaluation ... 117
Samenvatting en evaluatie ... 129
Publications and presentations ... 141
Dankwoord ... 143
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Abstract _____________________________________________________________________The objective of this chapter is to investigate the feasibility of developing a catalytic hydrodesulfurization (HDS) process operating under low pressure and high temperature conditions to produce a near-zero sulfur content diesel suitable for fuel cell applications. As expected, it was found that decreasing the operating pressure will reduce the adsorbed hydrogen content on the catalyst surface and hence the percentage of sulfur removal (i.e. HDS activity). This is compensated by increasing the temperature which increases the catalytic activity and the hydrogen solubility, but at the same local bulk phase conditions the hydrogen coverage is decreased. Furthermore, the choice of the feedstock is an important factor. This chapter includes a comparison with other commercialized and under-development desulfurization processes, besides the conventional catalytic HDS process. Light is shed on types of HDS catalysts used to achieve low sulfur specifications.
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21.1 Background
Concerns on the environment are forcing power generators and users of electricity to consider more efficient and less environmentally damaging methods of power generation and use. Analysts within the power generation industries are suggesting that a clear market-pull is emerging towards dispersed and embedded power generation1. Fuel cells can generate required electrical power for small and medium sized residential and industrial complexes2-4. The fuel cell market is still new and definitely growing. A large hurdle for wide application is the current high investment cost. Moreover, there is a challenging opportunity to provide on-site hydrogen generators for fuel cell applications5,6 because of the lack of infrastructure for hydrogen storage, transportation, and distribution facilities5.
Liquid hydrocarbons such as gasoline or diesel are attractive as feed for fuel cell system since the infrastructure of such fuels is well established. Diesel fuel for instance is an interesting option for hydrogen production due to its high energy density, low price, and safety in handling4,7,8. In addition, a well-developed infrastructure for diesel exists. When diesel can be applied, the adoption of fuel cells in the power generation sector and the market demands will stimulate mass production and the penetration of fuel cells will be accelerated in for example the remote areas and as an auxiliary power generation in diesel truck for cooling purposes at parking places instead of idling the diesel engine .
Usually, hydrogen, which is the primary feed to the fuel cells, is produced via steam reforming followed by a water-gas shift (WGS) process. The use of gasoline or diesel fuels demands the use of a desulfurization process to reduce the sulfur content in these fuels to an acceptable level for fuel cells applications. Steam reforming has a sulfur tolerance of 1-5 ppm, while fuel cells demand much more stringent sulfur levels of 0.5 ppm to 20 ppb, depending on the fuel cell type9-13. To achieve these low sulfur levels, an ultra-deep HDS process is required. HDS process for diesel fuels usually is operated under a high pressure, i.e. ≥ 40 bar, while the other processes, viz., steam reforming, WGS, and the Fuel Cell, operate under a pressure of 5 to 10 bar6,7,14. The possibility of operating the HDS process under low pressure, i.e. 10 bar, would eliminate the need for high pressure hydrogen compressors and advanced reactor materials and some safety issues would be less important. A feasible low-pressure HDS process will increase the market share of fuel cells in the power generation sector.
1.2 Overview of desulfurization technologies
1.2.1 Catalytic Hydrodesulfurization
In recent years, most of the research, either academic or industrial focused on meeting new sulfur regulations of gasoline and diesel fuels, while maintaining fuel quality demanded by automotive industries15-43. Usually diesel fuels HDS processes operate at temperatures of 300-400 °C and pressures of 40-50 bar13. Figures 1 and 2 illustrate the HDS operating windows, in terms of reaction pressure and temperature, for recent studies carried out on model sulfur compounds and real industrial feedstocks, respectively.
As illustrated in Figure 1, only thiophene is reported to undergo desulfurization at 4-10 bar and an operating temperature of 300 to 400 °C. Clearly, more severe conditions are necessary to desulfurize dibenzothiophene (DBT) and dimethyldibenzothiophene (DMDBT) compounds. For both compounds desulfurization is only reported under a pressure range of 40 to 100 bar. The
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3 temperatures applied for DBT and DMDBT strongly overlap but there is a tendency that for DMDBT the temperature is higher than that for DBT44-62.Figure 1 - Operating pressure and temperature of research studies on HDS of thiophene,
dibenzothiophene (DBT), and dimethyldibenzothiophene (DMDBT) 44-62.
Table 1 - Required LHSV for different feedstocks15-62.
Usually model compounds studies are carried out to evaluate and understand the reaction kinetics, catalyst behavior at the micro-level, and inhibition effects of other compounds, such as organic nitrogen compounds and H2S, on the HDS reaction.
However, for practical applications it is recommended to investigate catalyst performance evaluation and process optimization including actual refinery feedstocks. Figure 2 shows the operating pressure and temperature distribution utilized in recent studies on gasoline, diesel, straight run gas oil (SRGO), light cycle oil (LCO), and vacuum gas oil (VGO) feedstocks. Desulfurization of gasoline can be done at a low pressure (20 bar or less) and a temperature of 200 to 300 °C. For diesel and SRGO fuels, a higher pressure is needed (25 to 50 bar) and also a higher operating temperature (260 to 360 °C). LCO, which is a heavier feed compared to diesel, demands even more severe conditions to remove sulfur (50 to 70 bar and 300 to 380 °C). Finally, VGO is one of the heaviest streams in refineries and to desulfurize such feed it is required to operate the HDS reactor under very high pressure (70 to 150 bar) and high temperature (350 to 400 °C). The variation of operating pressure and temperature of real feedstocks shown in Figures 2 is due to variations in catalyst and in reactivity of the sulfur-containing compounds and process parameters. The variations in catalyst are contributed by several factors, such as: catalyst type, active metals type, metal/support ratio, support type, promoters, acidity, and activation procedures. Operating pressure and temperature are not the only process conditions
0 10 20 30 40 50 60 70 80 150 200 250 300 350 400 450 P (ba r) T (°C) 4,6-DMDBT DBT Thiophene
Feed type Sulfur compounds LHSV ( h-1) Gasoline & naphtha Thiophene and DBT 2-6
Diesel, SRGO, LCO C2-DBT, C3-DBT 1-4
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4 that control the HDS performance, other parameters include: liquid hourly space velocity (LHSV), hydrogen to hydrocarbon ratio, recycle gas rate, recycle gas purity, reactor configuration, and feed distribution. However, these are not very different from study to study. Table 1 illustrates this for the most important parameter, LHSV. As shown, the heavier the feed, implying a higher amount of the hindered sulfur compounds, the lower the LHSV needed for desulfurization.Figure 2 - Operating pressure and temperature of research studies on HDS of gasoline, diesel, light
cycle oil (LCO), and vacuum gas oil (VGO) feedstocks15-43.
Each refinery stream contains a range of sulfur compounds that determine its desulfurization conditions and severity. The common types of sulfur compounds found in gasoline fuel are mercaptanes (RSH), sulfides (R2S), disulfides (RSSR),
thiophene (T) and its alkylated derivatives, and benzothiophene (BT) (as shown in Figure 3). In addition to the sulfur compounds found in gasoline, diesel fuel, and light cycle oil contain alkylated derivatives of benzothiophene and dibenzothiophene. Vacuum gas oil contains the above mentioned sulfur compounds in addition to polycyclic sulfur compounds of two or more aromatic rings, such as dibenzothiophene (DBT), benzonaphthothiophene (BNT) and its derivatives, phenanthrothiophene (PT) and its derivatives, and naphthothiophenes (NT)63,64.
In the last decade, the reactivity of sulfur compounds was investigated intensively in order to understand their reaction kinetics and methods of removal. It was found that the ease of sulfur removal depends highly on the compound structure. The reactivity of sulfur compounds, under desulfurization process conditions, decreases in the following order: thiols and disulfides > thiophene > thiophene alkylated derivatives > BT > BT alkylated derivatives > DBT > 4- or 6-methyl DBT > 4,6-DMDBT > C3-DBT and C4-DBT65-73. As the ring structure increases beyond four
aromatic rings, the reactivity increases with the number of rings in the ring structure. This phenomenon is explained by the increase of possible reaction routes with the number of rings69. In order to achieve ultra-low or near-zero sulfur levels, the key point is the removal of the low-reactivity sulfur compounds, such as 4- or 6-MDBT and 4,6-DMDBT. The operating conditions, required to handle these compounds, determine the severity of the HDS process.
0 20 40 60 80 100 120 140 160 150 200 250 300 350 400 450 P (ba r) T (°C)
Light Cycle Oil (LCO) Vacuum Gas Oil (VGO) Gasoline & Naphtha
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5 Figure 3 - Reactivity of various organic sulfur compounds in HDS dependency on their ring sizes andpositions of alkyl substitution on the ring 63.
Based on recent studies of the HDS of diesel fuels, it can be concluded that if the sulfur specifications become more stringent, the HDS process has to be able to desulfurize the most steric hindered sulfur compounds. Refiners have to meet current and future sulfur levels in diesel fuel all around the world. For European Union, the sulfur content of diesel was lowered to 10 ppm since 200874-78. In USA, already in 2006 a sulfur level below 15 ppm was required, excluding small refineries. From 2010, all US refineries had implemented the 15 ppm sulfur content in diesel fuel79,80. In Japan, sulfur levels were reduced to 10 ppm in diesel fuel from 200781. The reason for desulfurization has largely been related to the legislation over the past decades. 1 to 2 % sulfur in both gasoline and diesel were causing acid rain worldwide and limits to 350 and 500 ppm S were set. Most of fuel is used in automotive applications and the emission of those combustion engines contained among others significant amounts of NOx. An aftertreatment device, a catalytic converter, downstream of the engine was decided to be obligatory to reduce NOx levels. However, for the catalytic convertors SOx are strong inhibitors and poisons and a further reduction of sulfur has been set. Depending on the country it is nowadays either 10 or 15 ppm. For fuel cell operation an even lower sulphur level is required, viz., <1 ppm. This low level cannot, however, be guaranteed by the oil companies/refineries. Due to storage at the refinery, transport by trucks to the refill station, and storage at this station, mixing with leftovers/residues in the supply chain is unavoidable. Thus, a target of sulfur <1 is a mission impossible. Dramatic development and new research ideas in the conventional diesel HDS process are essential to meet these low sulfur regulations.
1.2.2 Other desulfurization technologies
Beside the conventional HDS process several desulfurization processes are known that can achieve ultra-low or near-zero sulfur levels in gasoline and diesel fuels, presented in Figure 4. These processes can be used as standalone desulfurization processes or in conjunction with a conventional HDS process. These desulfurization technologies include, but are not limited to, catalytic distillation, alkylation, extraction, selective oxidation, adsorption, and biodesulfurization.
S S S S S S S R-SH & R-S-S-R Gaso line Ra ng e Di ese l Ra ng e Jet Ra ng e
Increase in Size & Difficulty for HDS
Re la tive Re action Ra te
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6 Figure 4 - Classification of non-conventional desulfurization technologies based on the organosulfurtransformation65.
1.2.2.1 Catalytic distillation
This technology is mainly used to desulfurize Fluid Catalytic Cracking (FCC) gasoline without major loss of octane number. The reactor is a distillation column filled with hydrotreating catalyst at different levels of the column. The light portion of gasoline, which usually contains most of the olefins and light sulfur compounds, will be distilled and desulfurized at lower operating temperature at the top section of the column82. This low severity will be enough to remove the light sulfur compounds without extensive hydrocracking or saturation of the olefinic compounds83. The bottom section of the distillation column will operate at higher temperatures allowing removal of heavier sulfur compounds of the heavier gasoline fraction. Another process configuration of catalytic distillation is based on a combination of two distillation columns. Each column is packed with a desulfurization catalyst. In the first column, the gasoline is distilled at the bottom section and the top section contains an HDS catalyst, operating at low temperature to remove sulfur from the light fraction. The untreated bottom fraction of the first column is sent to the second column30,65. This untreated portion is distilled to medium cut, which is desulfurized in the top section at medium temperatures, and a heavier cut that is desulfurized in the bottom section at higher temperatures30,83.
1.2.2.2 Alkylation
Alkylation is an elegant desulfurization technology, used for upgrading of FCC gasoline. In the so-called OATS process (olefinic alkylation of thiophenic sulfur) olefins present in the FCC gasoline fraction are alkylated with thiophenic sulfur compounds. The FCC gasoline first passes through a pretreatment section under a low severity to remove non-thiophenic sulfur compounds84. The product stream is passed to the alkylation reactor, where thiophenic sulfur reacts with olefins and produces a higher boiling range sulfur compounds. There are also other reactions taking place, mainly aromatics alkylation and olefin polymerization. The reaction rate of thiophenic sulfur alkylation is very high compared to other reactions under the operating conditions of the alkylation process, which makes the effect of other reactions negligible. A fractionation column is used to separate the light and heavy fractions of gasoline. The light portion is essentially sulfur-free and is sent to the gasoline pool. The heavy fraction is sent to an HDS process to remove remaining sulfur
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7 compounds65,84. To desulfurize the heavy fraction, it was noted that a higher severity is required compared to conventional HDS conditions.1.2.2.3 Extraction
Extraction can be utilized in sulfur removal of gasoline and diesel fuels by applying solvents exhibiting a higher solubility for organosulfur compounds than those for sulfur-free hydrocarbons. Many studies were carried out to determine the solubility of sulfur compounds in different solvents. The solvent has to be selective and should have a boiling temperature different from that of the organosulfur compounds, allowing easy separation. The process can be operated at low temperature and pressure. The low severity operation helps to preserve the composition of the feedstock30,63,65. The process starts with a mixing tank, where the feedstock and the solvent are mixed and organosulfur is transferred from the fuel to the solvent. The mixture is then sent to a separator to separate the solvent from the desulfurized fuel. A distillation column is used for removing the organosulfur compounds from the solvent, since both have different boiling temperatures. Several extraction process technologies have been commercialized and are available in the market.
1.2.2.4 Selective oxidation
Selective oxidation is used in the desulfurization of both diesel and gasoline fuels. This process follows the same process scheme as that of the extraction process. The main difference is the use of an oxidant to oxidize the organosulfur compounds into other forms which make it easier to separate later. There are several oxidation methods reported to transform the sulfur compounds. One strategy suggests the use of peroxyacetic acid as an oxidant. The operation is conducted at stoichiometric ratio and at low temperatures (below 100 °C) and ambient pressure. Another example is based on an oxidation catalyst dissolved in an aqueous phase. The organosulfur compounds are oxidized to sulfones under atmospheric pressure and low temperatures (below 120 °C)82,85,86. The sulfones are then extracted in the aqueous solvent and can be converted further to surfactants and used for other industries, such as soap manufacturing. Another option is to use hydrogen peroxide (H2O2)86,87. A better and
cost-effective process was discovered where air was used as an oxidant88 , and a relatively cheap solvent was introduced. The study showed complete conversion of DBT, MDBT, and DMDBT to sulfones with no catalyst and only air as oxidant. Due to its high sulfur conversions and selectivity, this process could be used as a post-treatment to remove the more refractory sulfur compounds to achieve near-zero sulfur contents in the fuels.
In a recent study ultrasound energy is used to oxidize the sulfur compounds in the emulsion under low temperatures and atmospheric pressure82. Another oxidation procedure is to apply ultraviolet or visible light to the mixture in a photoreactor. Later, the mixture is separated with an extractive process82,89,90. The choice of a specific oxidation procedure depends on several factors, mainly, its selectivity and the cost of oxidant and solvent.82,85-90.
1.2.2.5 Adsorption
In the adsorption process the organosulfur compounds are selectively adsorbed on a solid adsorbent. The adsorption can take place by chemical and physical forces without changing the molecular structure, often referred to as adsorptive desulfurization91. The fuel will be sulfur-free and the adsorbent is then reactivated by
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8 washing it with a solvent to remove the sulfur compounds of the surface. Several adsorbents types have been reported, such as activated carbon, zeolites, CoMo catalysts, and silica-alumina sorbents. The type of organosulfur compound present in the fuel, i.e. DBT or 4,6-DMDBT, will have different adsorption intensity. DBT showed a higher adsorption capacity and rate compared to that of 4,6-DMDBT when using carbon aerogels or Nickel-based adsorbents10,92. When during adsorption a chemical reaction takes place, usually known as reactive adsorption, the organosulfur compounds are chemically bonded to the surface of the sorbent. The sulfur is attached to the sorbent in the form of sulfides and the fuel will be released as a sulfur-free stream. The sorbent is then regenerated and the sulfur compounds are converted to H2S, elemental sulfur, or SOx. The process is often based on a fluidized bed reactorand operates at a higher severity compared to the previous technologies. The operating temperature ranges from 340 to 410 °C, while the pressure ranges from 2 to 20 bar93. The process requires lower hydrogen consumption, compared to conventional HDS process because it does not produce large amounts of H2S, which
minimizes the recombination reactions. Since the sulfur compounds are chemically bonded to the sorbent, there is limited amount of the sulfur desulfurized via hydrogenation, and so, the hydrogen consumption is lower. During the regeneration, hydrogen could be used to remove the sulfur from the sorbent and thus H2S is
produced.
1.2.2.6 Biodesulfurization
In biodesulfurization a bioreactor is used where the fuel is mixed with an aqueous solution that contains the bacteria and other elements required for the bacteria growth. It can be implemented in gasoline and diesel fuels desulfurization. Several bacteria types have been identified that are able to biotransform the organosulfur compounds in gasoline and diesel fuels. Some of the bacteria types were found to be more selective to remove the most sterically hindered heterocyclic sulfur compounds, such as 4,6-DMDBT94-98. The bacteria are able to transform the sulfur to sulfonates, which can be used as a feedstock for surfactants production. The biodesulfurization process is a more recent technology and it needs more time to mature and be commercially acceptable to refining industry. Improvement is required in several aspects, such as: enhancing bacteria stability, achieving faster reaction kinetics, improving mass-transfer limitations, improving temperature and solvent tolerance, and increasing the bacteria selectivity towards heterocyclic sulfur compounds94,99. The most attractive characteristics of biodesulfurization are the low investment and operating costs. A major drawback is the long residence time required and the consumption of the feedstock for their own metabolism.
Most of the described desulfurization processes above are not yet mature technologies compared to the conventional HDS process, which had been in the market for more than four decades. Some of these technologies are even in the first stage of development and the ones that are developed are not yet widely commercialized in the market. Because of these reasons, in the near future new developments in the conventional HDS process will have most impact on the refining industry.
1.3 The low pressure HDS approach
The proposed approach for this thesis is to implement a low pressure high temperature (LPHT) HDS process prior to a steam reforming process for fuel cell
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9 applications as shown in Figure 5. In order to understand the applicability of the proposed scheme, it is necessary to determine the effect of low pressure operation on hydrogen solubility. Furthermore, the effect of the changes in operating temperature and pressure on HDS activity needs to be explored. The diesel feedstock could be chosen as straight-run, untreated stream directly from the refinery or as a treated stream from an existing HDS process, where the sulfur content is already highly reduced. Each stream contains different organosulfur compounds in different ratios and the reactivity of these compounds will affect the operation and behavior of the HDS process in terms of operating conditions and catalyst type. The choice of treated or untreated feedstock will be discussed also. Later in this chapter, the catalyst types available to achieve ultra-low sulfur content in diesel fuel will be discussed. The most investigated catalysts in the recent years were CoMo/Al2O3 and NiMo/Al2O3catalysts, although different metals have been investigated, including noble metals and other support materials100-123.
Figure 5 - Simplified process scheme of proposed low pressure HDS process for fuel cell applications.
1.3.1 Hydrogen solubility
Hydrogen is the essential component for the removal of sulfur compounds in HDS process. In order to apprehend the role of hydrogen in HDS reactions, the reaction pathways should be understood for the various organosulfur compounds during the HDS process.
Figure 6 - Reaction pathways for the desulfurization of thiophene, DBT, and 4,6-DMDBT
compounds124.
Figure 6 shows the reaction schemes for HDS of thiophene, DBT, and 4,6-DMDBT. The desulfurization reactions of sulfur compounds generally occur via two pathways: 1) the ‘direct desulfurization’ (DDS) and 2) the ‘hydrogenation’ (HYD). The direct desulfurization route removes the sulfur without hydrogenation/saturation
S S Thiophene DDS HY D Fa st S S DDS HY D S lo w DBT S S DDS HY D S lo w 4,6-DMDBT
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10 of the aromatic structure attached to it and the hydrogenation route starts with first hydrogenation/saturation of the organic structure prior to the sulfur removal.When sulfur is removed from thiophene through the direct desulfurization route, butadiene is produced. When the HYD route is operative, the reaction product is tetrahydro-thiophene (THTH). Subsequently, the sulfur is removed, via C-S bond cleavage to produce either butene or butane. It is difficult to distinguish between the direct desulfurization and hydrogenation pathways in the HDS of thiophene, because any butadiene formed is rapidly hydrogenated into butane/ butane124.
For DBT, the product of the direct route is biphenyl (BPh), whereas the hydrogenation path produces tetrahydro-DBT (THDBT), via hydrogenation of one of the aromatic rings, followed by conversion to cyclohexylbenzene (CHB). Because the reaction of BPh to CHB is very slow, it is easy to discriminate between the direct desulfurization and the hydrogenation routes of DBT124. In the case of 4,6-DMDBT, the direct desulfurization path produces a 3,3´-dimethylbiphenyl (DMBP) compound. The hydrogenation route product is 4,6-DiMethyl-TetraHydro DBT (DM-TH-DBT), which is converted to 3,3΄-dimethyl-cyclohexylbenzene (DMCHB) after sulfur removal through C-S bond scission91,93. As in the desulfurization of DBT, the hydrogenation of 3,3´-DMBP to 3,3´-DMCHB is a slow reaction, which allows studying the behavior of each reaction route individually, i.e. the selectivity is directly measured. The hydrogenation and direct desulfurization reaction pathways are widely discussed in the literature. However, there are several not so widely discussed additional intermediate reaction pathways and byproducts, such as isomerization, demethylation, and C-C bond scission, as suggested in detailed kinetic studies, especially for the HDS reactions of DBT and 4,6-DMDBT52,53,91,125.
As shown in Figure 6, the hydrogen amount required for the direct desulfurization and the hydrogenation reaction routes are significantly different. In the direct desulfurization reaction, the sulfur is first removed via C-S bond cleavage, where hydrogen is not required to complete the reaction. Then the removed sulfur reacts with hydrogen to form H2S gas, which implies that the hydrogen amount
required to complete this path is relatively low. However, in the hydrogenation reactions, hydrogen is consumed in order to hydrogenate the organic structure first. Then, the sulfur is removed from the saturated/hydrogenated structure, via C-S bond scission also. Finally, as in the direct route, the sulfur is converted to H2S gas. In this
case, the needed hydrogen amount is relatively high. Thus, under the low hydrogen availability conditions of this study, the hydrogenation reactions might be severely affected in comparison to the direct desulfurization route and it is to be expected that achievement of close to full conversion of the hindered alkylated DBTs is more difficult than of DBT.
In order for the hydrogen to participate in the HDS reactions, it has firstly to diffuse through the liquid phase, which could be any hydrocarbon stream such as diesel or gasoline fuels, to reach the catalyst surface. The hydrogen is subsequently activated, dissociated and adsorbed on the catalyst surface. The low hydrogen solubility and/or slow mass transfer to reach the catalyst surface may lead to hydrogen limited conditions126. The solubility of hydrogen in organic solvents is rather well known. Figure 7 demonstrates the effect of the hydrogen pressure on its solubility at a temperature of 350 °C for hexadecane, bicyclohexyl, tetralin, and 1-methylnaphthalene solvents126. As shown in Figure 7, the general trend is that as the total hydrogen pressure increases, the hydrogen solubility increases close to linearly, regardless of the hydrocarbon solvent used. At low pressure of 10 bar, the hydrogen
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11 solubility in all solvents is only 0.02 to 0.05 mole fraction126. For 1-methylnaphthalene, the hydrogen solubility is 0.07 mole fraction at a pressure of 100 bar. This hydrogen solubility triples as the pressure increased to 260 bar. In the case of tetralin, it behaves similar to 1-methylnaphthalene solvent. At 100 bar hydrogen pressure, the hydrogen solubility is around 0.08 mole fraction and it triples as the pressure is increased to 260 bar. The hydrogen solubility in bicyclohexyl is higher compared to previous solvents; it is about 0.14 mole fraction at 100 bar total pressure and 0.32 mole fraction at a hydrogen pressure of 260 bar. At a fixed pressure and temperature, from the four solvents used, the hydrogen solubility is the highest in hexadecane solvent. The solubility is 0.24 mole fraction at 100 bar and increases to 0.45 hydrogen mole fraction at a total pressure of 260 bar. So, at any fixed temperature and pressure, the hydrogen solubility decreases in the following order, according to the structure of the solvent: hexadecane > bicyclohexyl > tetralin > 1-methylnaphthalene126.The data of hydrogen solubility in different hydrocarbon solvents is relevant information. Interestingly, also data is available for real feedstock. Figure 8 shows the hydrogen solubility as a function of total pressure for light cycle oil (LCO) at 316 °C. Even though the pressure range in Figure 8a is much smaller compared to Figure 7, the same solubility trend is observed, where the hydrogen solubility increases as the hydrogen pressure increases. Increasing the operating temperature increases the hydrogen solubility127, as shown in Figure 8b for vacuum gas oil.
Figure 7 - Hydrogen solubility versus hydrogen partial pressure for various hydrocarbon solvents at
operating temperature of 350 °C126.
For instance, at 50 bar and 130 °C, molar fractionhydrogen solubility in HVGO and LVGO was about 0.038 and 0.047. When the temperature increased to 250 °C, the solubility of both fuels increased to about 0.073.
At a pressure of 50 bar, the hydrogen solubility in LCO is about 0.05 (316o C), while the solubility in hexadecane, bicyclohexyl, tetralin, and 1-methylnaphthalene is about 0.15, 0.06, 0.025, and 0.025 (350o C), respectively. So, LCO shows higher solubility than both tetralin, and 1-methylnaphthalene; because the solubility increases with temperature the difference at the same temperature would be even higher. When compared with hexadecane (350o C), LCO (316o C) shows a lower solubility at only one-third of the value. By referring to figure 8b, a temperature increase from 130 to 250 °C leads to a less than double hydrogen solubility. So, at the same temperature the solubility in LCO is lower than that in hexadecane. In LCO and bicyclohexyl the hydrogen solubility will not differ much.
0 50 100 150 200 250 300 0.0 0.1 0.2 0.3 0.4 0.5 Hexadecane Bicyclohexyl Tetralin 1-methylnaphthalene H2
solubility (mol frac
tion)
P
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12 Figure 8 - a) Hydrogen solubility versus total hydrogen pressure for light cycle oil (LCO) at operatingtemperature of 316 °C. b) Hydrogen solubility in two types of petroleum fractions, LVGO and HVGO,
versus pressure127.
1.3.2 Effect of pressure on HDS activity
Table 2 shows the effect of hydrogen pressure on the HDS activity of DBT and 4,6-DMDBT model compounds. It differentiates between the direct desulfurization and hydrogenation routes activities, as shown in Figure 6. For the total HDS activity, it is shown that as the hydrogen pressure increases, the total activity also increases for both compounds128. As expected, the individual activities for 4,6-DMDBT are much lower compared to DBT at each pressure value due to its steric hindered structure. The same behavior is seen for the hydrogenation reaction activities for both compounds. For the direct desulfurization pathway, DBT exhibited the same general trend, where the activity increased as the pressure increases. Furthermore, the increase of the hydrogen pressure had no significant effect on the direct desulfurization reaction of 4,6-DMDBT compound.
Table 2 - Effect of hydrogen pressure on the transformation of DBT and 4,6-DMDBT128.
Note: Sulfided commercial NiMo/alumina catalyst at 340 °C. Reactant pressure = 0.1 bar; H2S pressure = 0.5 bar. rT = total activity; rDDS = activity of the direct desulfurization pathway; rHYD = activity of the hydrogenation pathway (mol h-1 kg-1). Conversions = 10-30 mol-%.
The increased activity behavior of the HDS process with increasing the operating pressure is also shown in a real feedstock. Table 3 shows the effect of the operating pressure on the total sulfur content and other properties of naphtha feedstock. The sulfur content of the feed was reduced from 1236 ppm to 152 ppm at 400 °C and 5 bar. Further increase in the pressure to 10 and 20 bar could achieve sulfur contents of 53 and 21 ppm, respectively. As expected, as the hydrogen pressure increases, the sulfur content in the product will be reduced further129. It can be noted
0 10 20 30 40 50 60 0.00 0.02 0.04 0.06 H2 solubility (mol fraction) P H2 (bar) a
PH2 (bar) Ptotal (bar) rT (mol h-1 kg-1) rDDS rHYD rDDS/rHYD DBT 20 20.6 6.7 5.2 1.3 4.0 30 30.6 9.7 7.7 2.0 3.8 40 40.6 13.0 9.6 2.7 0.4 4,6-DMDBT 20 20.6 1.2 0.3 0.9 0.3 30 30.6 1.7 0.3 1.4 0.2 40 40.6 3.1 0.4 2.6 0.2 3.6
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13 also that the octane number decreases, as the pressure increases. This is mainly due to the hydrogenation/saturation of both aromatics and olefins compounds.Table 3 - Total pressure effect on the properties of hydrotreated naphtha129.
Note: Feed had 92.1 research octane number (RON) and 0.12 wt.% sulfur content. Catalyst was
NiMo/HZSM-5 with 0.43 wt.% Ni and 2.8 wt.% Mo. Reaction temperature is 400 °C. LHSV: 3 h-1. H2/feed ratio: 150 Nm3 m-3.
Figure 9 - Sulfur removal (XS) versus time-on-stream for LCO at different operating pressures127. T: 343 °C, LHSV: 1 h-1.
Figure 9 shows the sulfur removal dependency with the time-on-stream (TOS) of LCO feedstock at 343 °C and using a CoNiMo/Al2O3-SiO2 catalyst at different
operating pressures. The LCO feedstock had an API gravity of 10.6 °API and contained a 1.46 wt-% of total sulfur and 554 ppm of organic nitrogen compounds. LHSV and hydrogen to hydrocarbon ration were kept constant at 1 h-1 and 180 Nm3 m-3 of liquid feed, respectively. At an operating pressure of 24 bar, 94% of HDS conversion was achieved. When the pressure dropped to 15 bar, the HDS activity only lost one percent. As the pressure was reduced further to 8 bar, about 90% of HDS conversion could be still accomplished127. So, even though the operating pressure was reduced to one-third of its original value, the sulfur removal only lost around five percent relatively. This indicates that reducing the operating pressure of the HDS
Pressure (bar) Feed 3 5 10 20 RON change - 0.2 -1.1 -2.8 -6.4 Sufur (ppm) 1235.6 164.3 152.0 53.1 21.0 Component (wt-%) aromatics 14.3 24.0 23.4 20.9 15.7 iso-paraffin 27.1 28.9 33.1 37.7 39.1 naphthalene 10.9 10.0 10.9 11.3 12.0 olefin 41.1 26.3 20.5 16.5 15.0 paraffin 4.1 8.1 8.9 11.1 14.9 unidentified 2.5 2.7 3.2 2.5 3.3 80 85 90 95 100 100 200 300 400 500 P = 24 bars P = 8 bars P = 15 bars TOS (h) XS (%)
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14 process does not necessarily have a catastrophic impact on the HDS activity and that other parameters could have more effect on the activity than the operating pressure.Figure 10 illustrates the hydrogen partial pressure effect on the sulfur content in the product stream. The feedstock used is 25% LCO and 75% SRGO blend, which had a sulfur content of 1.52 wt.% and a total aromatic content of 29.9 wt.%. The study was carried at two reaction temperatures with a difference of 20 °C. A CoMo and a NiMo catalyst were applied in this comparison. The effect of the hydrogen pressure on the sulfur content is minimal for the CoMo catalyst. On the other hand, the NiMo catalyst exhibited a much higher response to pressure increase and could achieve the same sulfur levels as the CoMo catalyst when the pressure increased by almost 60% of its original value. At 20 °C higher temperature, both catalysts achieved much lower sulfur contents. When the hydrogen pressure is increased at this temperature, the NiMo catalyst showed a slightly higher HDS activity compared to that of the CoMo catalyst130. This confirms that the hydrogenation pathway has high hydrogen pressure dependency and the direct desulfurization route has a lower dependency on the hydrogen partial pressure. In Table 1 data is presented for the desulfurization of both DBT and 4,6-DMDBT compounds also catalyzed by a NiMo catalyst. The data showed that the direct desulfurization and the hydrogenation reactions activity for DBT increased as the hydrogen pressure increased. The activity of the direct desulfurization pathway of 4,6-DMDBT was, however, almost constant as the hydrogen pressure increases. Thus, it is to be expected that the sulfur-containing compounds, left after HDS, consists preferentially of higher concentrations of steric hindered heterocyclic organosulfur compounds, such as 4,6-DMDBT.
Figure 10 - Effect of hydrogen partial pressure on the sulfur content of a feedstock containing 25%
LCO and 75% SRGO130. P: 30 bar. T and LHSV are not reported. 1.3.3 Effect of temperature on HDS
Figure 11 expresses the effect of operating temperature on the product sulfur content during the HDS of feed blend of 25% LCO and 75% SRGO, which is the same feed used to generate Figure 10. The experiments were carried out at a constant pressure of 30 bar. The hydrogen to oil ratio and LHSV were held constant during the testing. Two types of CoMo catalysts are used, where the one noted with number two (II) is a new version of CoMo catalyst, CoMoII. A NiMo catalyst was utilized for comparison purposes. The general trend, shown in Figure 11, is that as the temperature increases, the HDS activity increases too. Between both CoMo catalysts, CoMoII always had a higher HDS activity, which means that a 6 to 8°C lower
0 50 100 150 200 250 300 100 110 120 130 140 150 160 Tbase+ 20 C Tbase PH2(% base) Su lf u r (pp m ) CoMo II NiMo
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15 temperature is sufficient to achieve the same sulfur level compared to CoMoI catalyst130. For NiMo catalyst, the activity was lower than both CoMo catalysts, at low temperatures. As the temperature increased, the HDS activity of the NiMo catalyst increased too.Figure 11 - Operating temperature versus product sulfur content for 25% LCO and 75% SRGO feed
blend using different catalyst types130. P: 30 bar.
Figure 12 - Sulfur content in product versus operating temperatures at different LHSV for the HDS of
SRGO and CGO feedstocks131. P: 30 bar.
Figure 12 demonstrates the effect of operating temperature on the total sulfur content in the product after desulfurization of SRGO and coker gas oil (CGO) feedstocks at two different LHSV values. The organic sulfur and nitrogen contents of the SRGO feedstock were 1.4 wt-% and 60 ppm, respectively. The CGO feed contained 0.76 wt-% organic sulfur and 0.1 wt-% nitrogen contents. A commercial CoMo/Al2O3 catalyst was used for the desulfurization of both feedstocks. Total
reactor pressure and hydrogen to oil ratio were held constant at 29.5 bar and 200, respectively.
For SRGO feed, at LHSV of 4 h-1 and a reactor temperature of 320 °C, the sulfur could be reduced to 5400 ppm in the product. Increasing the temperature to 340 °C decreased the sulfur content to around 3100 ppm. The sulfur level is further reduced to 1100 ppm when the temperature is elevated to 360 °C. In order to achieve
0 40 80 120 160 200 240 0 2 4 6 8 10 12 14 CoMo I CoMo II NiMo T ( C) + base Sulfu r (ppm ) 0.E+00 2.E+03 4.E+03 6.E+03 300 320 340 360 380 400 SRGO; LHSV = 4 T ( C) Sul fur in P roduct (ppm) SRGO; LHSV = 2 CGO; LHSV = 4 CGO; LHSV = 2
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16 the required sulfur specification in the hydrotreated SRGO of 350 ppm in the product, not only the temperature had to be increased to 360 °C but also the LHSV had to be reduced by 50% in order to accomplish this sulfur level of 350 ppm131. In the case of CGO, a higher temperature of 375°C is required to achieve the sulfur specification of 350 ppm at the same LHSV value. This shows that the CGO has a higher resistance to desulfurization compared to SRGO, even though the CGO feed had half the sulfur content compared to SRGO. This could be related to the inhibition effect of organonitrogen compounds on the HDS reaction, since CGO has an organic nitrogen content of 1,020 ppm compared to only 60 ppm nitrogen content in the SRGO feedstock132-138. As mentioned before, organonitrogen compounds content and structure play a major role in the HDS reactions. Unfortunately, regarding the chemical structure of the nitrogen compounds no information was reported.Table 4 - Increase in catalyst activity (or reactor temperature) required to achieve different reduction
amount in the sulfur content of the diesel product130
Note: 500 ppm is chosen as the base case and LHSV is kept constant. The data are for a typical CoMo
catalyst.
Table 4 shows the required increase in catalyst activity or operating temperature in order to achieve different sulfur specifications in diesel fuel. The data were generated under constant LHSV and using a typical CoMo catalyst. The reference is chosen to be 500 ppm sulfur content in the diesel. It illustrates that dramatic improvements in catalyst activity are needed to achieve the required reductions in the sulfur specifications in real diesel fuels. In order to reach 200 ppm and 100 ppm sulfur content in diesel, the CoMo catalyst activity has been doubled, and tripled, respectively. The catalyst activity is needed to be increased by four times to reduce the sulfur content to 50 ppm130. Increasing reactor temperature can compensate for low catalyst activity. It should be noted that increasing the reactor temperature is not feasible in most refining HDS plants, because usually the operating window of the HDS reactor is only about 330 to 350 °C between the start-of-run and end-of-run temperatures.
In summary, the operating pressure has an important effect on the rates of desulfurization reactions. Higher operating pressures increase hydrogen solubility and hence the availability of adsorbed hydrogen on the catalyst surface for the reaction, increasing the activity of the HDS reactions. However, decreasing the operating pressure does not have a dramatic effect on the HDS reactions. Compared to higher pressures higher reactor temperatures have a more pronounced effect on the HDS process activity. There are other operating parameters that can also increase the activity of HDS process, such as reduction of the LHSV. The choice of the catalyst type is also an important factor that can be used to increase the activity and selectivity of the HDS process. It should be taken into account that organonitrogen compounds play an inhibition role on the HDS reactions and reduce the activity of the process
139-Product sulfur (ppm) Required catalytic activity (%) Required T (ºC)
500 100 0
350 130 +7
200 190 +17
100 300 +29
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17144
. These compounds compete with the sulfur compounds for active sites on the catalyst surface145. Nitrogen compounds are usually divided into two categories: basic and non-basic, examples of both categories are shown in scheme 1. The basic nitrogen compounds have been reported to be stronger inhibitors for the HDS reactions than the non-basic compounds138. However, strong inhibition by non-basic nitrogen compounds has also been reported144, either due to hydrogenation reactions occurring during this process, that might lead to the formation of basic species, or to the strong adsorption of the non-basic compounds over the support surface. After standard HDS, it is reported that the organic nitrogen compounds that survived the first hydrotreatment are partially or fully hydrogenated heterocyclic compounds that have a relatively high inhibition effect on the HDS reactions22,126.
Scheme 1 – Typical basic and non-basic nitrogen compounds identified in diesel fuels146. 1.3.4 Effect of feedstock on HDS activity
The feedstock properties and characteristics have a great impact on the HDS process. Lighter feedstocks, in terms of API gravity, tend to contain less organic sulfur and nitrogen compounds content and a very small amount or none of the steric hindered heterocyclic compounds. These feedstocks pose a lower severity on the HDS process. However, heavier feedstocks require higher severity HDS conditions due to their higher content of sulfur and nitrogen. Moreover, they contain a higher fraction of refractory organic sulfur and nitrogen compounds and multi-ring aromatics. Even for the same category fuel, such as diesel, there are different grades and cut points, for example, heavy diesel, where the end-point is higher than that of normal, light diesel, which has lower initial boiling point, and full range diesel fuels. Moreover, the diesel could be used as straight-run, untreated feed or pre-hydrotreated stream. The origin of the fuel affects its sulfur compounds content and types and hence the HDS process behavior, including the H2S partial pressure. Several studies have been carried out in
order to elucidate the relation between the feedstock characteristics and the reactivity of HDS. Many property-reactivity correlations for HDS of different feedstocks have been established. The purpose here is not to validate and compare these correlations, but to show that feedstock properties have an effect on the HDS reactions and to show that the proper choice of the feedstock is an important factor.
Figure 13 shows the observed and predicted HDS reactivity of thirteen straight run, untreated middle distillate feedstocks21. The predicted data were generated using the correlation: HDS reactivity α (API)2.18
(DBTs)-0.31 (N)-0.2, where DBTs is the total content of DBT compounds and its alkylated derivatives, including 4,6-DMDBT; and N is the nitrogen content. Each feedstock was characterized by 24 physical and chemical properties. The HDS reactivity was measured under 18.3 bar of pressure, 343 °C reaction temperature, and 0.74 h-1 LHSV. A CoMo/Al2O3 catalyst was used in
the reactivity measurements. The suggested correlation could predict the experimental
Carbazole Indole
Acridine Aniline Quinoline Pyridine
Non-basic nitrogen
Basic nitrogen
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18 reactivity values closely. So, this indicates that the reactivity of HDS process of untreated middle distillates is mainly affected by the API gravity, steric hindered organic sulfur compounds content, and to a lesser extent the organic nitrogen content. The API gravity is, in this study, far the most important factor in determining the HDS reactivity21. The API gravity represents to a certain extent the heaviness of the feedstock, as shown in table 5, the higher the API the lighter the fraction. The conclusion, that the heavier the feedstock the lower the reactivity, is not surprising. It should be noted that not only the structure changes with API, but also the hydrogen solubility: in heavier feedstocks the hydrogen solubility is larger compared to lighter feedstocks. Apparently, the negative influence of the chemical composition at lower API is not fully compensated by the higher hydrogen solubility.Table 5 – Typical API gravity values for petroleum fractions.
Figure 13 - Observed versus predicted HDS reactivity of 13 different straight run, untreated middle
distillate feedstocks based on the model: HDS reactivity α (API)2.18 (DBTs)-0.31 (N)-0.2. P: 17 bar, LHSV: 0.74 h-1, T: 343 °C21.
Kagami et al. also concluded that feed density, nitrogen content, and the 90% distillation temperature are valid parameters to model the HDS activity of light gas oil147. This effect of feed density or API could be explained due to components of the hydrocarbons in the feed. Yuan et al. used molecular dynamics simulation to predict hydrogen solubility in heavy hydrocarbons148,149. It was found that the hydrogen solubility increased in the order: paraffins (squalane) < mono-aromatics (benzene) < di-aromatics (naphthalene). However, the hydrogen solubility decreased in 4-ring aromatics (chrysene). This decrease of hydrogen solubility also was noticed when heavier feedstocks than diesel were tested. Cai et al. found that the hydrogen solubility decreased in following order: LVGO > HVGO > vacuum residue of Athabasca oil150. Riazi et al. showed also higher hydrogen solubility of LVGO
Fraction API gravity
Typical gas oil 40
Light cycle gas oil (LCO) 33
Diesel 45
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19 compared to HVGO151. In summary, hydrogen solubility will increase with the feed density to a certain extent, but when the feed becomes very heavy, the hydrogen solubility will decrease. Thus, in middle distillates the H2 solubility is relatively highwhen compared to other fractions
Usually pre-hydrotreated feedstocks have lower organic sulfur and nitrogen contents and different species distribution compared to untreated feeds. Figure 14 illustrates the reactivity of the HDS process for 13 pre-hydrotreated middle distillates22. The feedstocks were characterized for several properties as in the previous study. The HDS reactivity was measured using a NiMo/Al2O3 catalyst, and
an operating pressure of 46 bar and temperature of 330 °C. The experiments were carried out with constant hydrogen to oil ration of 85 Nm3 m-3and LHSV of 0.65-1.2 h-1. It was found that the HDS reactivity of pre-hydrotreated distillates can be correlated linearly with the organic nitrogen content of the feedstock. The correlation used: HDS reactivity = a - bNf, where a and b are constants and Nf is the
pre-hydrotreated feed organic nitrogen content22. Since most of the easy-to-desulfurize sulfur compounds are already removed, the sulfur compounds remained must be the sterically hindered heterocyclic compounds.
Figure 14 - HDS first order rate constant versus nitrogen feed content of 13 different pre-hydrotreated
distillates; based on the correlation: HDS reactivity α a - bNf, where a and b are constants and Nf is the nitrogen content in the feed22. T: 330 °C. P: 45 bar. LHSV: 0.65-1.2 h-1.
For comparison of figures 13 and 14, a distinct difference should be recognized, where different feedstocks are used. Figure 13 represent untreated straight-run feedstocks, however, figure 14 represent hydrotreated feedstocks. Three properties of the feedstocks are summarized in Table 6. As shown in figure 13, API played a major role in the model, because the range of API was very large, but for the treated feeds, the API range was much smaller and the effect of nitrogen content became more apparent, as shown in figure 14.
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20 Another option for feedstock is to use a fractional evaporation column before the HDS process152, as shown in Figure 15. The purpose of the fractionation column is to remove the part of the fuel that contains refractory sulfur, such as BT and its alkylated derivatives and DBT and its alkylated derivatives, prior to the HDS reactor. The degree of evaporation depends on the boiling range of the fuel, sulfur content, and the type of sulfur compounds in the fuel. The vaporous light fraction is fed to the HDS reactor for desulfurization with a typical HDS catalyst, where the reaction is carried out in the gas phase. Because there is no refractory sulfur in this feed, it is relatively easy to remove essentially all remaining sulfur compounds and achieve sulfur levels of less than 1 ppm. The HDS reactor is operated under low severity conditions. Finally, the product is passed through a zinc oxide bed to remove H2S gasprior to enter the steam reformer and shift reactor, where hydrogen is produced for the fuel cell application. All the reactors are operated under the same pressure and temperature, except for the fractional evaporation temperature where it starts at a lower temperature. The heavy part of the feedstock, which was rejected by the evaporation column, can be used for heating purposes. This process configuration may be applicable for new small-scale operation. However, it is not economically feasible for a medium or large scale HDS process, since a significant fraction of the fuel will be rejected by the evaporation process and redirected to be used for heating purposes.
Figure 15 - Schematic process diagram of fractional evaporation column prior to the HDS reactor152.
1.4 Catalyst selection
The new sulfur regulations in hydrocarbon fuels have initiated massive developments in the HDS catalysis in order to develop new or improve existing catalysts to achieve these regulations. HDS catalysts contain active metal(s) with/without a promoter(s) on the surface of a support or mixed-supports structure. These catalysts can be classified according to their active metals type, support type, promoter function, predominant HDS reactions route (e.g. hydrogenation or direct desulfurization), or the feedstock they are treating (e.g. gasoline, diesel, LCO, or VGO). There are several types of supports used for HDS catalysts, such as Al2O3,
SiO2, TiO2, ZrO2, and zeolites. Figure 16 shows a classification of HDS catalyst
according to the type of support. The supports were categorized as: alumina supports, mixed-oxide alumina supports, and other support types153-187.
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21 Alumina supports have been used extensively in catalyst manufacturing, especially in HDS, because of its outstanding textural and mechanical properties, and low costs66. One of the most appealing and important characteristics of alumina supports is their ability to restore catalytic activity via regeneration even after intensive use under the hydrotreating conditions. Alumina support is not an inert material and it can interact with metals and promoters deposited on its surface. Moreover, alumina supports possess acid catalytic sites that can participate in the HDS reactions.Alumina supported catalysts are usually prepared by impregnation of the active metals, such as Mo or W, with promoters, such as Co or Ni, onto the alumina surface using either pore filling or incipient wetness procedures188. The prepared catalyst is subsequently dried and calcined, among others to disperse the active metals and promoters onto the alumina support surface. The metal dispersion is an important characteristic of the finished catalyst and it depends on several factors, such as impregnation procedure, solution concentration, pH of the solution, and calcination temperature. Usually,, in-situ pre-sulfiding is required to convert the active metals from their oxide form to the sulfide form, which is the active phase of the HDS catalysts. Two of the most used alumina-supported catalysts in HDS are CoMo/Al2O3
and NiMo/Al2O3.
CoMo-based alumina catalysts are well known for their high preference for the direct desulfurization pathway and, associated herewith, their low hydrogenation activity. Generally, CoMo-based catalysts perform relatively well for cracked feedstocks under deep desulfurization conditions, since these feedstocks have very high sulfur contents, mainly of Th, BT, and DBT compounds130. Several efforts have been made to improve the CoMo/Al2O3 activity by incorporating more hydrogenation
capabilities. Higher activity could be achieved by increasing the metal loading onto the support by increasing the alumina support surface area. Additionally, using better metal loading techniques could improve the dispersion of the active metals and hence the catalyst activity. Manipulating the acidity level of the alumina support by addition of other promoters, such as phosphorus or fluorine, can increase the HDS activity because of an increase in the number of acid sites and a higher metal dispersion188. The structure of the sulfided phase depends on the details of the synthesis and the sulfiding temperature used for catalyst activation. At low temperatures the so-called CoMoI (often called Type I) is found while at higher temperatures the so-called CoMoII (often called Type II) is created. Several studies indicated that the CoMoII has a higher activity compared to that of CoMoI66.
NiMo-based alumina catalysts normally show higher hydrogenation capability as compared to that of CoMo/Al2O3 catalysts189. For naphthalene hydrogenation the
activity of NiMo-based catalysts was around 2 to 2.5 times higher than that of CoMo catalysts. This high hydrogenation activity affects the overall HDS performance of NiMo catalysts when the feed has high aromatic contents. When naphthalene is added to the feed, it was found that the HDS activity of the NiMo catalysts was strongly reduced in comparison to that of CoMo catalysts188. Addition of fluorine190-194 and phosphorus195-200 to the alumina support material increased the active metal dispersion and hence higher NiMo-HDS performance could be achieved.
Other oxides are mixed with the alumina support in order to achieve different support properties. For example, Mo on TiO2 and ZrO2 supports show relatively high
desulfurization and hydrogenation activities compared to Mo on alumina support, but TiO2 and ZrO2 supports possess a too low specific surface area to use them for HDS
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22 catalysts66. This low surface area could be overcome by coating TiO2 and ZrO2 on analumina support. Another example is applying a mixed Bi2O3-Al2O3 oxide support
which did not show great improvements on the direct desulfurization or hydrogenation reactions but it increased the cracking activity of the catalyst201. Zeolites, such as USY, HY, HZSM-5, and SiO2 are also used in combination with
alumina support in order to improve the acidity of the alumina support. A high acidity can improve the isomerization and demethylation reactions, which transform low reactivity steric hindered sulfur compounds to more reactive forms66. It has to be noted that if the acidity is highly increased, other undesirable side reactions, such as cracking, can take place.
Figure 16 - Classification of HDS catalysts based on support type122-137.
Other support materials, beside alumina support, are used in developing HDS catalysts, such as zeolites, mesoporous materials (MCM-41), SiO2, and activated
carbon. Many catalyst improvements could be accomplished by utilizing such materials. For example, zeolites can offer a well-defined and ordered pore structure on a molecular level which could help in increasing the active metal dispersion and hence the catalytic activity. MCM-41 has a very high surface area which can be used to increase the active metal loading, without access restriction for bulky molecules as in zeolitic structures66. Carbon supports have many advantages such as minimized metal-support interaction, which enhances the metal dispersion besides large specific surface area and controlled pore volume. The catalytic performance of carbon-supported metals depends highly on the preparation procedures and the nature of the carbon support157.
It is striking that in many studies the active phases are applied without support material, e.g. MoS2, Mo2C, RuS2. The advantage of using bulk metals is due to their
hydrogenation reaction selectivity in addition to the high loading of the active phase. The promoter is incorporated into the active metals via molecular complexes synthesis or direct reaction126. Not surprisingly, the catalytic behavior of these bulk metals strongly depends on their synthesis methods.
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23 In summary several combinations of promoters, active metals, and supports for HDS catalyst development have been reported. The choice of a certain combination depends on a large variety of parameters, such as the type of feedstock, the type of sulfur and nitrogen compounds, operating conditions, cost, and reducibility of production.1.5 Aims and Thesis outline
The objective of this thesis is to investigate the feasibility of developing a catalytic HDS process operating under low pressure and high temperature (LPHT) conditions to produce in a low-cost reactor configuration a near-zero sulfur content diesel suitable for fuel cell applications. The low pressure operation reduces the investment cost in the HDS reactor, eliminates the need for hydrogen compression and also introduces a higher safety factor which is particularly important for small scale applications9 and which usually contributes substantially to the equipment cost. Operating under lower hydrogen pressure reduces the severity of the HDS process which reduces again the operating costs since continuous monitoring will not be needed. With this, higher process automation levels can be integrated into the system to achieve higher operation simplicity and user friendly configuration for untrained users. Since the product of the HDS process will be fed directly to the steam reformer, the desulfurized diesel quality and specifications, such as, for example, API gravity, cetane number, and aromatic content will not be of significance at this point. As a consequence, the HDS reactor can be operated at higher reaction temperature than that of conventional industrial operation. Since steam reformers usually operate in the temperature range of 600 to 1000 °C, the operating cost associated with higher temperatures for the HDS reactor will be minimal202,203.
The catalytic HDS technology is a multi-variable process and, as shown in this chapter, several factors can affect the performance of the HDS process, such as operating conditions, catalyst type, and feedstock type and composition. Operating conditions variables include temperature, pressure, LHSV, hydrogen to oil ratio; feedstock variables include feed type, origin, aromatics content, sulfur content and types, and nitrogen content and types. The catalyst variables include support type, active metals, promoters, and synthesis methods. Some of these variables are highly related and can exhibit a positive synergy but they also can cancel. The objective of the chapter was to investigate the applicability of a low pressure, high temperature HDS process. As expected, decreasing the operating pressure will reduce the adsorbed hydrogen content on the catalyst surface and hence the HDS activity. However, increasing the temperature increases the hydrogen solubility and the hydrogen availably for reaction. Furthermore, higher temperatures can improve low-activity catalysts to achieve lower sulfur contents and the choice of the feedstock, either treated or untreated, is an important factor.
The thesis was divided into two major parts: model compounds and real feedstocks, as shown in Figure 17. The main theme of the thesis is to investigate the HDS reactions under low pressure operation, i.e. 10 bar. In Chapter 1, an overview of the recent advances in HDS is presented with emphasis on low-pressure, high temperature operation. In Chapter 2, the comparison of different catalysts used in the HDS process is performed. In this chapter, the possibility of achieving very low sulfur levels using the phosphorous promoted NiMo catalyst at 10 bar pressure, when most literature suggests noble metal catalysts, is presented and discussed. In Chapter 3, the
