Which parameter - temperature or pressure - has more impact in a diesel hydrotreating unit in producing higher quality in terms of cetane no and product sulfur?
Answers
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19/01/2009
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SARVESH KUMAR, INDIAN OIL CORPORATION R&D CENTRE, kumarsarvesh@iocl.co.in
At low pressure (30-40 bar) diesel hydrodesulfurization is governed by hydrogenolysis route, where temperature has more impact for reducing bulk of the sulfur. Residual sulfur content below 500 ppm are refractory in nature and governed by hydrogenation route, which requires high pressure (70-100 bar. At high pressure hydrotreating, aromatics are also saturated which enhances cetane number.
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17/01/2009
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keith bowers, B and B Consulting, kebowers47@gmail.com
Neither reactor temperature nor pressure can be considered independently. Achieving the required low levels of sulfur, together with high cetane is not a simple issue of raising reactor temperature or increasing hydrogen partial pressure or system total pressure. While increasing reactor temperature is a common method of increasing sulfur removal in conventional hydrotreaters, that might actually INCREASE product sulfur when attempting to make <15 PPM S in diesel.
Often different catalysts are required in the top and bottom beds to remove disulfides made when aromatic rings are opened in the middle of the reactor, and they react with H2S made in the top beds from simple mercaptans. Keeping H2S levels low in the recycle gas is important to keep the re-make mercaptans reaction rate low.
It is very important to identify the sulfur 'species' --(what kind and size of hydrocarbon has the sulfur attached) before planning how to remove it. Different catalysts are needed to remove sulfur from compounds that have the sulfur hidden from the catalyst active sites than those that easily remove simple mercaptan sulfur. The SHAPE of the sulfur containing molecule is an important factor in achieving the reaction conditions to remove the sulfur.
Catalyst aromatics saturation activity is critical. Hydrogen partial pressure must be sufficient to keep those reactions supported. Temperature changes rate of reaction, not equilibrium so much, but increases light ends make with higher temp. Another often unappreciated factor in achieving ULSD specs of <15 PPM is avoiding contamination in tankage and transfer pumps and piping. One major refiner found they could meet the new lower targets if only they could eliminate contamination from leaking heat exchangers!! Remember 20 PPM is equivalent to just 2 grains of red rice in a liter of white rice grains!
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17/01/2009
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Muhammad Muzaffar Malik Sarfaraz Ahmed Malik, BPPL, muzaffar_malik1@yahoo.com
The pressure drop increment of hydrotreater is an indication of flow restriction (like clogging or coke lumps formation in catalyst bed) that may lead to channeling and improper hydrotreating. While reactor bed temperature (within optimum ranges) expedites mercaptans conversion to H2S. Thus increasing temperature has more effect on cetane index and product sulfur.
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16/01/2009
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Samuel Sinanan, IPSL, Samuel.Sinanan@ipsl.co.tt
For desulphurisation reactions, temperature has the greatest impact as this affects the rate constant of the HDS reactions. For improving cetane where saturation of aromatics is required the kinetics are not as straightforward as HDS reactions. Saturation is favoured by high hydrogen partial pressures and fairly low temperatures. For monoaromatics, saturation is limited by equilibrium, with the equilibrium temperature dependent on the pressure.
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