Hydrodesulfurisation is extremely important to meet strict environmental sulfur dioxide emission regulations and protect catalyst used in refineries. Hydrodesulfurisation is a hydrogenation reaction where sulfur is removed from a sulfur compound. Cobalt molybdenum is a catalyst which can be used in the process which is usually carried out in a fixed bed reactor. Hydrodesulfurisation is the subject of vast amount of research to improve catalyst performance and operating conditions to meet environmental regulations as there get stricter but also keep the costs of hydrodesulfurisation at a minimum.
Background and process importance
Hydrodesulfurisation is the removal of sulfur from petroleum fuel products and is of very high importance for a variety of reasons such as preventing air pollution and catalyst poisoning.
Hydrodesulfurisation is required to prevent air pollution by sulfur dioxide caused by the fuels upon combustion. Petroleum fuel combustion causes sulphur dioxide production which pollutes the air and causes acid rain. Acid rain can have serious effects on the environment such as forests, building, lakes and rivers. Sulfur is present as thiols, sulfides, disulfides and thiophenes in oil feedstock's and can be removed using hydrodesulfurisation (Stirling,2000). Fuel oil composed of residual petroleum fractions in the middle east origin which contain 3-5% sulfur yield sulfur dioxide into the atmosphere upon combustion (Shell international petroleum company limited ,1983).
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Sulfur in the feed to reformer units can poison the catalyst being used in the process. Hydrodesulfurisation can be used to prevent poisoning of the catalyst being used by removing sulphur from the feed stock (Konings, 1980). Hydrodesulfurisation has been extensively used in industry to treat naphtha as feedstock to meet the sulphur specifications of less than 1ppm to protect the platinum catalyst.
Sulfur emissions poison the noble metal catalyst used in catalytic converters in automobile. Poisoning of the catalyst would decrease the effectiveness of the catalytic converter therefore reducing control of emissions such as carbon monoxide and nitrogen oxide (Ocelli et al.,2001).Hydrodesulfurisation of fuels helps reduce the amount of sulfur that can poison the catalytic converter of an automobile.
Sulphur dioxide (SO2) can have a direct effect on human health. It can cause irritation, coughing and a feeling of chest tightness, which may cause the airways to narrow. Fuel combustion accounted for more than 90% of UK sulfur dioxide emissions in 2008 with the two main sources being the combustion of solid fuel and petroleum products"( Murells et al 2010).
As the graph above shows combustion in energy and transformation industry causes the majority of SO2 emissions however there has been a significant decrease in the amount of so2 produced as limits are placed on SO2 emissions allowed. All petroleum refineries now use hydrodesulfurisation units.
Time Series of SO2 Emissions (Mtonnes) and the ceiling to be achieved in 2010.
SourceÂ :Murells,T.P. et al.,(2010).
Process details and economic evaluation
Hydrodesulfurisation is one of the fundamental hydrotreating processes of refining and petrochemical industries it removes organically bound sulfur from sulfur containing chain molecules in crude or distillate by conversion to hydrogen sulfide which is typically achieved by reaction with hydrogen over non-noble metal sulfided supported catalysts. (Edward,S.E. et al.,2007 & Shell international petroleum company limited ,1983).
An example of a hydrodesulfurisation reaction is given below:
RS + Î-2 catalyst RÎ- + Î-2S (Crabtree,R.H. et al.,2007)
Catalyst: Co-Mo (Cobalt molybdenum) Alumina support
R is a hydrocarbon group
C16H33SH +H2 C16+H34 + H2S (Shell international petroleum company limited ,1983)
Hydrodesulfurisation typically takes place in a fixed bed reactor. The type of reactor used depends on the crude oil fraction to be treated. Light fractions are vaporized and passed
through a fixed bed reactor. Trickle bed reactors are used for heavier feedstock which can not be vaporised (Chiusoli et al, 2006). Hydrodesulfurisation of petroleum oils was the first large-scale application of trickle bed reactors commercialized in 1955. A large commercial reactor may have 20 to 25 m of total depth of catalyst, and may be up to 3-m diameter or above in several beds of 3- to 6-m depth. Bed depth is often limited by pressure drop, the catalyst crush strength, and the maximum adiabatic temperature increase for stable operation (Green and Perry.,2008).
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'The Co-Mo/Al2O3 hydrodesulfurisation catalyst is prepared by impregnation of a high surface area alumina support with aqueous solutions of Co(NO3)2-6H2O and (NH4)6Mo7O24.The catalyst typically contains 1-4 wt% Co and 8-16 wt% Mo. The catalyst precursor is then dried and calcined to give the supported mixed Co/Mo oxide which is then activated by sulfiding in hydrogen and a sulfur-containing feed (stirling,2000)'. MoS2 is dispersed in the form of nanoparticles on an inexpensive support such as alumina. Nano particles are used as there give the catalyst a greater surface area for reaction. Using a alumina support gives high catalytic activity from the high surface/volume ratio and also good mechanical properties for industrial applications. Without a support the catalyst nano particles would not work in fixed bed or trickle bed reactors as the particles would be suspended in the gas or liquid phase and would not be retained by normal filters. The catalyst also includes another metal such as nickel or cobalt acting as a promoter (Chiusoli et al, 2006).
Space velocity (LHSV) (hr -1 )
Shell international petroleum company, 1983
300 c572 -380C 716f
Gary and Handwerk , 1994
Gas and oil
Co-Mo or Mo-Ni
Green and Perry,2008
1.5-8 Operating conditions for hydrodesulfurisation in a refinery taken from various sources are shown in figure 2 below
Increasing temperature and hydrogen partial pressure increases sulfur removal however excessive temperatures are to be avoided because although there increase sulfur removal there also increase coke formation which decreases catalyst activity. Over the life of the catalyst temperature is increased to achieve a constant conversion. Once the catalyst is no longer active as it has been poisoned it is regenerated or discarded. An increase in space velocity reduces conversion, hydrogen consumption and coke formation ( Green and Perry 2008; Gary and Handwerk ,1994).
The reactor pressure is optimized to increase the solubility of the hydrogen and minimize catalyst deactivation due to coking ( Green and Perry, 2008). The increase in partial pressure of hydrogen increases the HDS rate and decreases the coke deposits on the catalyst. Reduced coke deposits would increase the catalyst life as they would be less fouling. Higher operating pressure also increases the hydrodesulfurisation rate and therefore less catalyst is used for the same desulfurization rate. Operating at higher pressures can increase the feed throughput of the unit while maintaining the given desulfurization rate (Parkash, 2003). Operating at high pressures can result in decrease in money spent on replacing catalyst as the catalyst would last longer if there are not used or poisoned as much. Operating at high pressures would however result in high operating cost and more expensive reactors being needed to withstand higher pressures.
Catalyst typically used for hydrodesulfurisation are Co/Mo and Ni/Mo (Edward et al,2007) . Cobalt molybdenum catalyst oxides on alumina catalyst have proven to be highly selective easy to regenerate and resistant to poisons. Cobalt molybdenum catalyst will reduce a given amount of sulphur at less severe operating conditions with a lower hydrogen, this is because nickel molybdenum catalyst have a higher hydrogenation activity than cobalt molybdenum (Gary and Handwerk 1994).It would be more economic to use cobalt molybdenum catalyst than nickel molybdenum catalyst as money could be saved on process energy and catalyst replacement cost.
Catalysts are gradually deactivated by metal sulfides formed from the metal compounds in the feed. The deposition of metal eventually causes complete blocking of the catalyst pores this leads to reactants no longer being able to reach the catalyst active site and so desulfurization can no longer occur. Narrower pores are blocked quicker by metals in the feed however there do supply a greater surface area for desulfurization. A tailor made catalyst can be made for optimum desulfurization and metal tolerance. Catalyst at the start of the reactor where metal content is high in the oil could have larger pores for greater metal tolerance whereas catalyst at the end of the reactor where there is less metal could be tailored to have smaller pores as there is not much metal tolerance needed at this stage and greater desulfurization could occur as a result of smaller pores. Effective applications of tailor made catalyst can lead to a substantial reduction in the amount of catalyst required for a given duty making the process more economic. (Shell international petroleum company limited ,1983). 'Catalyst consumption varies from 0.003 kg/m3 to 0.02 kg/m3 depending upon the severity of operation and the gravity and metal contents in the feed. The catalyst replacement cost is $0.25/m3Â feed' (Gary and Handwerk,1994).
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The main cause of catalyst deactivation in hydrodesulfurisation is coke deposition. Catalyst regeneration can be used to restore catalyst activity and remove coke. carbonaceous species can be eliminated using an oxidizing atmosphere at a temperature of 450-550Â Â°C. An average activity loss of 11% is observed after one regeneration for Hydrodesulfurisation catalyst CoMo catalysts used in European ultra low sulfur diesel units. This shows the excellent quality of regenerated catalyst achievable. Regenerating catalyst is extremely useful as it can slow down refineries procurement expenses (Dufresne,2007).
BASF Hydrodesulfurisation Catalyst is based on the proven combination of cobalt and molybdenum catalysts on an alumina carrier. BASF also use titanium in the aluminium and they claim it leads to better performance and longer life of the catalyst. Titanium promotion leads to more dispersed Co-Mo creating more surface and a higher activity ( BASF, 2007).Using a catalyst with a higher surface and higher activity could ultimately save money on the cost of hydrodesulfurisation as the catalyst would be more effective and last longer. Figure 3 shows the composition of Cobalt and Molybdenum catalyst used by BASF for hydrodesulfurisation.
Figure 3 Source: ( BASF, 2007)
A substantial amount of research is carried out in hydrodesulfurisation catalyst to make them more efficient to make hydrodesulfurisation more economic and meet the sulfur reduction requirements. Ultra low sulfur diesel requires very low sulfur composition such as 50ppm therefore highly active catalyst are needed. The diagram below shows the different Catalyst components and their key roles which could be the subject of research to increase catalyst activity in hydrodesulfurisation.
Figure 4 Source: (Anthony,S.et al.,2010)
Research has been done into improving preparation techniques to increase catalyst activity. Vakros et al (2007) found that using equilibrium deposition filtration instead of the conventional non dry impreganation step can lead to 30% increase activity of the hydrodesulfurisation CoMo/alumina parent catalyst.
In 1991 it was found that TiO2 supported MoS2 and ZrO2 supported mos2 supported catalysts have respectively three to five times higher hydrodesulfurisation and hydrogenation activities than alumina-supported ones with an equivalent Mo loading per nm2. However, before 1991, the specific surface area of such oxides remained below 100m2/g (after calcination at 773 K) this restricted the interests of such supports. However within the last decade there has been a large improvement in the preparation of theses oxides resulting in supports with larger pore diameters and higher specific surface area.Chiyoda cooperation developed a novel method of producing a tio2 support equivalent to the conventional alumina support. (Breysse et al, 2003).
The production of Nebula catalyst has been a breakthrough in catalyst activity improvement since its introduction 2001.Nebula was jointly discovered by Exxon mobile research company and Akzo nobel and reaches four times the catalyst activity of the traditional co-mo catalysts.It is of excellent use as it can help existing units meet future product sulfur specifications as it can allow a reactor designed to reduce sulfur to 500ppm to reduce sulfur to 50ppm.Nebula can also maintain product specifications whilst running at lower temperatures. This could save money on capital expenses and running costs. ( Eijsbouts.S.et al.,2003).However the Nebula catalyst cost, hydrogen consumption and other operating costs are very high this could offset the savings in capital expenses (Anthony,S.et al.,2010).
Although significant progress has already been made in improving hydrodesulfurisation there is no doubt that the hydrodesulfurisation process and catalyst used will continue to be the subject of intense research to try and optimise processes to meet stricter environmental regulations of so2 emission at a low cost.
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