Hydrotreating Technologies – High Quality and Cleaner Fuels
Over the years, in face of the rising pollution levels associated with the technological development and the rise in petroleum consumption, the environment legislation has become increasingly severe.
Restrictions on SOx and NOx emissions induced the necessity to higher technology development that can allow reducing the contaminants levels in the petroleum derivates, mainly sulfur and nitrogen. Normally, the concentration of contaminants increases with the density of the petroleum derivate.
A lot of technologies were applied to reduce the contaminants levels in the petroleum derivates, for example, the kerosene treating with Clay, the adsorption of sulfur compounds over black carbon and the recognized treatments Bender and Merox. The mentioned technologies show limitations, mainly when the concentration of contaminants is high.
The hydrotreating technology (treatment with hydrogen) was studied by many researchers in the refining industry and academic sector over the decades and, currently, is practically impossible to attend the petroleum derivates specifications without these streams passing through the hydrotreating unit.
The hydrotreating process involves a series of chemical reactions between hydrogen and organic compounds containing the contaminants (N, S, O, etc.). According to the target contaminant of the hydrotreating, the process can be called hydrodesulfurization (removing S), hydrodenitrogenation (removing N), hydrodeoxygenation (removing O) or hydrodearomatization when the main objective is to saturate of aromatic compounds, among others.
The most commons hydrotreating forms are hydrodesulfurization (where the objective is to remove compounds like benzothiophene, dibenzothiophene, etc.) and the hydrodenitrogenation (removing porphyrins, quinolines, etc.) These compounds, besides provoke emissions of SOx and NOx when are burned, produce in the derivates acidity, color, and chemical instability.
The main chemical reactions associated with the hydrotreating process can be represented like below:
R-CH=CH2 + H2 → R-CH2-CH3 (Olefins Saturation)
R-SH + H2 → R-H + H2S (Hydrodesulfurization)
R-NH2 + H2 → R-H + NH3 (Hydrodenitrogenation)
R-OH + H2 → R-H + H2O (Hydrodeoxygenation)
Where R represents a hydrocarbon.
The hydrotreating process is normally conducted in fixed bed reactors and the most applied catalysts are Cobalt (Co), Nickel (Ni), Molybdenum (Mo) and Tungsten (W), commonly in association with then and supported in alumina (Al2O3). The association Co/Mo is applied in reactions that need lower reactional severity like hydrodesulfurization, while the catalyst Ni/Mo is normally applied in reactions that need higher severity, like hydrodenitrogenation and aromatics saturation.
The hydrotreating is applied in the finishing of the final products like gasoline, diesel or kerosene or like intermediate step in the refining scheme in refineries to prepare feed charges to other processes like Residues Fluid Catalytic Cracking (RFCC) or Hydrocracking (HCC) where the main objective is to protect the catalyst applied in these processes.
The basic process flow is similar to the various hydrotreating processes (hydrodesulfurization, hydrodenitrogenation, etc.), however, the process severity, determined by variables like hydrogen partial pressure, temperature and catalyst vary and the contaminants removal is affected.
The hydrotreatment process units are optimized aiming a equilibrium between cited operational variables, because chemical reactions are exothermic and the decontrolled raising in the temperature can affect negatively the reactional equilibrium besides it’s possible the sintering of the catalysts, to minimize this risk normally the hydrotreating reactors have points between the catalyst beds where are injected hydrogen in lower temperature (quench lines) to permit a better control of the reactor temperature.
Figure 1 shows a typical arrangement for a hydrotreating process unit with a single separating vessel.
Figure 1 – Basic Process Flow Diagram for Low Severity Hydrotreating Process Units
The configuration with a single separating vessel is normally applied in lower severity units, like hydrodesulfurization units. This arrangement is possible in this case because under reduced pressures the difference between water and hydrocarbons properties is large and the separation process needs reducing contact areas, so a single vessel can realize the separation process.
Higher severity units, like process units dedicated to treating unstable streams (Light Cycle Oil, Coke Gas Oil, etc.) or with the objective to remove nitrogen or aromatics saturation, operates with two separating vessels like presented in Figure 2.
In this case, the difference between water and hydrocarbons properties is small and the phase separation process needs higher interface area so, two separating vessels are applied, one under high pressure where the separation among liquid and gaseous phase (H2, H2S, NH3 and light hydrocarbons) occurs and other under low pressure where the separation between aqueous and hydrocarbon phase is promoted, apart from the separation of the remaining gases.
Figure 2 – Basic Process Flow Diagram for High Severity Hydrotreating Process Units
For lower severity units the temperatures applied are about 300 to 350 oC and pressures varying between 20 to 40 bar, in addition of lower residence times. Units with high severity operate under temperatures 350 to 400 oC and pressures varying from 40 to 130 bar.
Like aforementioned, great efforts was employed in the hydrotreating technology development, however, technology licensers like Axens, UOP, Exxon Mobil, CB&I, Lummus, Haldor Topsoe, Albemarle among others, still invest in researches to improve the technology, mainly in the development of new arrangements that can minimize the hydrogen consumption (high cost raw material) and that apply lower cost catalysts and more resistant to deactivation process.
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