Rajan Wilcox
Sulphur in diesel fuel creates sulphuric acid when burned, which causes corrosive wear on engine parts. This corrosion is harmful to engines and also contributes to air pollution. While sulphur can be present in diesel fuel even after refining, there are several methods to remove it, including hydrodesulfurization, adsorption desulfurization, and ultrasound-assisted oxidative desulfurization. One recent development in sulphur removal is the potassium salt method, which uses Earth-abundant materials to reduce sulphur content in diesel fuel.
Techniques to Remove Sulphur from Diesel Fuel
| Characteristics | Values |
|---|---|
| Adsorption desulphurization | Uses low-cost adsorbents to remove sulphur |
| Hydrodesulphurization (HDS) | Commonly used in refineries but is energy-intensive |
| Ultrasound-assisted oxidative desulphurization (UAOD) | Achieved 99% sulphur removal in petroleum products and 75% in diesel oil samples |
| Potassium salt method | Removes sulphur from carbon compounds in diesel fuel |
| KOSi method | Uses potassium, oxygen, and silicon to reduce sulphur content |
| Alternative techniques | Oxidation, extraction, biodesulphurization, etc. |
| Impact of sulphur in diesel fuel | Contributes to air pollution, causes corrosion in engines, and reduces vehicle lifetime |
| Global efforts | Regulations to reduce vehicle emissions have lowered allowable sulphur content |
| Sulphur removal benefits | Reduced emissions, improved air quality, extended vehicle lifetime |
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What You'll Learn
Using potassium salt
Sulfur is naturally present in crude oil and, despite the refining process reducing its levels, some of it still ends up in the resulting fuel. When that fuel is burned, sulfur dioxide (SO2) is released into the atmosphere, contributing to air pollution and acting as an indirect greenhouse gas.
A new method for removing sulfur from diesel fuel has been developed by an international team of researchers led by BP’s John W. Shabaker, UCLA’s Kendall N. Houk, and Caltech’s Robert H. Grubbs. This method employs a potassium salt, specifically potassium tert-butoxide, to induce the chemical reactions required to remove sulfur from fuel. The process also uses alkylsilanes under mild conditions to create silyl radicals that cleave the C–S bond of the heterocyclic compounds, with the silicon whisking away the sulfur.
The KOSi method, named for its elemental ingredients of potassium (K), oxygen (O), and silicon (Si), has been shown to effectively reduce sulfur levels in diesel fuel to an all-time low of 2 parts per million, down from 8 parts per million in the highest quality of diesel typically available at gas pumps. This process can be used as an additional step in the oil refinement process to eliminate the final traces of sulfur in fuels.
Potassium is an abundant and inexpensive element on Earth, making it a more environmentally friendly and cost-effective option than the rare metal catalysts traditionally used for similar reactions. The discovery of potassium salt's utility in this process was made by researchers in the Grubbs laboratory who were testing ways to break carbon-oxygen bonds without the use of a metal catalyst. They were surprised to find that the required chemical reactions were still occurring, leading to the realization that potassium salt was responsible for promoting these reactions.
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Adsorption desulphurization
Adsorptive desulfurization (ADS) is a process that uses solid adsorbents to selectively adsorb sulfur compounds from diesel fuel. The adsorbents have a high affinity for sulfur-containing molecules, allowing them to capture sulfur as the fuel passes through a bed of adsorbent material. This method is effective for removing even trace amounts of sulfur from diesel fuel.
The adsorbents used in ADS can be made from various materials, including activated carbon, activated charcoal, molecular sieves, and modified activated carbon prepared from waste products like brewer's spent grains and rubber tyres. These adsorbents are characterized using techniques such as SEM, EDX, and FTIR to ensure their capacity to desulfurize diesel effectively.
The ADS process involves optimizing the operating conditions, such as temperature and feed concentration, to maximize the adsorption of sulfur compounds. The temperature range of 30 to 80°C has been found to significantly impact the adsorption of sulfur compounds from diesel fuel using specific adsorbents. Additionally, the functional groups of the adsorbents play a crucial role in their adsorption capacity. For example, the rGO substrate, which contains abundant oxygen functional groups, promotes the dispersion of metal oxide and increases the adsorption efficiency of sulfur compounds.
After the adsorbents have reached their saturation point with sulfur, they can be regenerated for reuse. This regeneration process involves desorption, where the adsorbent is treated to release the captured sulfur compounds and restore its adsorption capacity.
ADS offers several advantages over conventional hydrodesulfurization (HDS). HDS requires high pressure, high temperature, and large amounts of hydrogen, making it an expensive and energy-intensive process. In contrast, ADS operates under milder conditions and consumes less energy. Additionally, HDS has limitations in removing certain sulfur compounds, such as thiophenes and their derivatives, which ADS can effectively address.
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Ultrasound-assisted oxidative desulfurization
The process involves the use of ultrasound to accelerate the oxidation of sulphur compounds in diesel fuel. This is achieved through a catalytic oxidative desulfurization process under phase transfer conditions and ultrasonication. The ultrasound waves create strong cavitation phenomena, which involve physical and chemical effects that improve the process efficiency and oxidation kinetics.
One specific implementation of UAOD involves the use of a MoO3/Al2O3 catalyst and H2O2 combined with ultrasonication. This system has been shown to significantly enhance the process efficiency, with above 98% conversion of DBT in model diesel containing 1000 μg/g sulphur. The ultrasound-assisted process achieved this conversion in just 30 minutes, compared to 55% conversion in the silent process.
Another study utilized green synthesized aluminum terephthalate [MIL-53(Al)] as a catalyst for UAOD. This catalyst was synthesized hydrothermally using waste polyethylene terephthalate bottles (PET) as a source of terephthalic acid, an organic linker. The catalytic activity of the prepared Al-MOF was evaluated in the oxidative desulfurization (ODS) of both modelled and real crude oil samples.
UAOD is an effective method for obtaining ultra-low sulphur diesel fuel, and it offers a cost-effective alternative to other desulfurization processes.
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Hydrodesulfurization
The HDS process involves treating the feedstocks at high temperatures (290-455°C) and high pressures (150-3000 psi) in the presence of hydrogen gas and metal catalysts such as CoMo/Al2O3 or NiMo/Al2O3. This process converts organic sulfur compounds into hydrogen sulfide (H2S) gas. The produced H2S gas is then captured and removed, and it can be further converted into byproduct sulfur or sulfuric acid.
HDS is an important technology for refineries to meet strict environmental regulations regarding sulfur emissions. For diesel fuel, regulations have been implemented to reduce sulfur content to less than 10 mg/L in Europe since 2009 and less than 15 mg/L in the US since 2006. The latest environmental standards, such as ultra-low-sulfur diesel (ULSD), require very deep hydrodesulfurization to achieve even lower sulfur levels.
While HDS is effective at removing sulfides and disulfides, it has limitations in removing certain compounds, such as thiophenes and their derivatives, which are more challenging to refine due to their substituted rings. Additionally, the high temperatures and pressures required make the HDS process energy-intensive and costly. As a result, researchers are exploring alternative methods, such as adsorption, oxidation, extraction, and biodesulfurization, to complement or enhance the sulfur removal process.
Overall, hydrodesulfurization plays a critical role in the petroleum industry's efforts to produce cleaner fuels, reduce emissions, and comply with environmental regulations by removing sulfur compounds from diesel fuel and other petroleum products.
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Alternative techniques to hydrodesulfurization
Hydrodesulfurization (HDS) is a widely used method for removing sulfur from diesel fuel. However, it has some limitations, such as high energy requirements, elevated temperatures, and pressure, which make the process uneconomical and energy-intensive. As a result, there is a need for alternative methods that can achieve ultradeep desulfurization without these drawbacks.
Adsorption Desulfurization
Adsorption desulfurization is a process that utilizes low-cost adsorbents to remove sulfur from diesel fuel. The process is influenced by factors such as sulfur feed concentration, space velocity, functional groups of the adsorbents, and temperature. The use of specific substrates, such as rGO, can enhance the adsorption efficiency of sulfur compounds by promoting the dispersion of metal oxides.
Oxidation
Oxidation techniques involve using chemical or biological processes to remove sulfur from diesel fuel. These methods offer an alternative to HDS and can be effective in reducing sulfur compounds.
Extraction
Extraction methods, such as extractive desulfurization (EDS), physically separate sulfur compounds from diesel fuel. These techniques have gained interest due to their ability to achieve deep desulfurization without the harsh conditions required by HDS.
Biodesulfurization
Biodesulfurization employs biological processes to remove sulfur from diesel fuel. This approach has been explored as an alternative to HDS, aiming to reduce sulfur content and minimize environmental impact.
Potassium Salt Method
Researchers at Caltech have recently developed a method using potassium salt to induce chemical reactions that remove sulfur from diesel fuel. This approach is more environmentally friendly and cost-effective than traditional methods using rare metal catalysts. The process effectively eliminates sulfur from carbon compounds, reducing sulfur levels to extremely low concentrations.
These alternative techniques offer potential solutions to the limitations of hydrodesulfurization, providing more economical and environmentally friendly ways to remove sulfur from diesel fuel.
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Frequently asked questions
The current EU standard for sulphur content in diesel fuel is 10 parts per million (ppm).
Hydrodesulphurization (HDS) is the most commonly used method in refineries. However, it requires high temperatures and pressures, making it energy-intensive. Other methods include ultrasound-assisted oxidative desulphurization (UAOD) and adsorption desulphurization using low-cost adsorbents.
Removing sulphur from diesel fuel helps to reduce air pollution and extend the lifetime of vehicles' catalytic converters, which control tailpipe emissions. It also reduces corrosive wear on engine parts caused by sulphuric acid formation during fuel combustion.
Researchers from Caltech and BP have developed a new technique called KOSi, which uses potassium salt and Earth-abundant materials (potassium, oxygen, and silicon) to remove sulphur from diesel fuel. This method is more effective and environmentally friendly than traditional methods.
