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One-step hydrotreatment process for conversion of vegetable oils to renewable diesel and LPG

Researchers from Japan and Mexico have developed a one-step hydrotreatment process over catalysts containing Ni-Mo and solid acids for the conversion of vegetable oils (Jatropha, palm and canola) to renewable diesel and LPG fuels. A paper on their work appears in the ACS journal Energy & Fuels.

Jatropha oil produced the highest yield (wt %) of renewable diesel (83.5%), with 4.9% LPG. Palm oil came second with a renewable diesel yield of 82.1% (LPG 5.4%); and canola was third with an 81.4% renewable diesel yield and 5.7% LPG. The resulting renewable diesel properties varied slightly—e.g., pour points from -10 to -15 °C; density (g/mL) at 25 °C from 0.78 to 0.79; and viscosity at 30 °C (mPA/s) from 3.82 to 4.22.

The new paper is a follow-on to their earlier work (Liu 2009) reporting the conversion of Jatropha oil containing 15 wt % free fatty acids (FFAs) to renewable diesel by hydrotreatment in a fixed-bed reactor over sulfided Ni–Mo/SiO2–Al2O3 catalyst with functions of hydrogenation, deoxygenation, and isomerization/cracking.

In recent years, the hydrotreatment of vegetable oils to produce hydrocarbons has been studied worldwide and an extensive research has been performed to search suitable reactors and catalysts. When vegetable oils are treated at high temperatures and high pressures without a catalyst, vegetable oils can be converted to the mixtures of paraffins, cycloparaffins, and aromatic hydrocarbons but a relatively large amount of fatty acids remains in the products. Solid acids (such as H-ZSM-5, SO4/ZrO2, and so on) can convert vegetable oils to the mixtures of gasoline, kerosene, light gas oil, gas oil, and long residue in the hydrocracking of vegetable oils. Industrial fluid catalytic cracking (FCC) catalysts can convert vegetable oils to gasoline distillated hydrocarbons in the FCC unit under the FCC conditions.

As for the BHD [biohydrogenated diesel, i.e., renewable, drop-in diesel] production, two types of catalysts have been reported as effective catalysts in converting vegetable oils to diesel distillated hydrocarbons: noble metal catalysts (such as supported Pd, Pt, and so on) and sulfided bimetal catalysts (such as Ni-Mo, Co-Mo, Ni-W, and so on).

...We proposed the concept that BHD fuel should [have] a chemical composition and physical properties similar to those of the normal diesel for the first time in the previous work. However, it needs further research to investigate how to achieve the concept satisfactorily. Many aspects for the reaction and catalysts should be clarified, such as the influences of various acidic supports, the influences of various vegetable oils, the influences of various reaction conditions, and so on. We had tried to combine Pt with various solid acids to achieve the catalyst’s multiple functions (hydrogenation-dehydrogenation, hydroisomerization, and hydrocracking) in the hydrotreatment of long-chain hydrocarbons.

This paper focuses on the combination of sulfided Ni-Mo and various solid acids to achieve hydrogenation, deoxygenation, hydroisomerization, and hydrocracking for the hydrotreatment of vegetable oils. The influences of solid acidity, oil composition, and reaction conditions have been thoroughly investigated, and the chemistry in the hydrotreatment of vegetable oil has also been discussed in this work.

— Liu 2011

The team used a stainless steel tubular reactor (i.d., 1 cm; length, 50 cm) for loading catalyst and a furnace for heating the tubular reactor. The vegetable oil was pushed into the reactor in a constant rate by a high-pressured microfeeder, while a mixed gas containing 90% H2 and 10% Ar was introduced into the reactor from a high-pressure H2 cylinder and the flow rate was controlled by a mass flow controller.

The pressure in the reaction system was controlled by a back-pressure regulator. A cold trap (soaked in a tank of ice water) was set between the reactor exit and the back-pressure regulator to collect liquid products. The standard reaction conditions were: catalyst amount, 1 g; reaction temperature, 350 °C; H2 pressure, 4 MPa; liquid hourly space velocity (LHSV), 7.6 h; ratio of H2 to oil in feed, 800 mL/mL, in which the H2 volume was described in the conditions of standard temperature and pressure (STP).

Triglycerides and free fatty acids underwent the hydrogenation and deoxidization at the same time during the reaction. The various vegetable oils (Jatropha oil, palm oil, and canola oil) were converted to mixed paraffins by the one-step hydrotreatment process although they contained quite different amounts of free fatty acids, the team noted.

The Ni-Mo/SiO2 formed n-C18H38, n-C17H36, n-C16H34, and n-C15H32 as predominant products in the hydrotreatment of Jatropha oil. These long normal hydrocarbons had high melting points and thus gave the liquid hydrocarbon product over Ni-Mo/SiO2 a high pour point of 20 °C.

Neither Ni-Mo/H-Y nor Ni-Mo/H-ZSM-5 was suitable for producing diesel-range hydrocarbons from Jatropha oil because a large amount of gasoline-range hydrocarbons was formed on the strong acid sites of zeolites. When SiO2-Al2O3 was used as a support for the Ni-Mo catalyst, the pour point of the liquid hydrocarbon product decreased to −10 °C by converting some C15–C18 n-paraffins to iso-paraffins and light paraffins on SiO2-Al2O3.

Because SiO2-Al2O3 had a proper solid acidic strength, the researchers concluded, both the chemical composition and the pour point of liquid hydrocarbon product over Ni-Mo/SiO2-Al2O3 were similar to those of a normal diesel bought from a fueling station.

The glycerin groups in the vegetable oils were converted to propane over Ni-Mo/SiO2-Al2O3 by the hydrogenation and deoxidization.

Vegetable oils (Jatropha oil, palm oil, and canola oil) were convert to green BHD and LPG fuel by a one-step hydrotreatment process over the catalysts containing Ni-Mo and solid acids. SiO2-Al2O3 was a suitable support for Ni-Mo to produce BHD from vegetable oils. Ni-Mo/SiO2-Al2O3 was a trifunctional catalyst with abilities of hydrogenation, deoxygenation, and isomerization/cracking. After all C=C bonds were hydrogenated in the first step and both free fatty acids and triglycerides were deoxygenated in the second step, vegetable oils were converted to C15–C18 n-paraffins and propane. SiO2 -Al2O3 had a proper acidic strength for the isomerization/cracking of C15–C18 n-paraffins in the third step. Vegetable oils can be convert to BHD and LPG over Ni-Mo/SiO2 -Al2O3 no matter how many C=C unsaturated bonds and how many free fatty acids they contained. The liquid hydrocarbon product can be directly used as a BHD fuel in current diesel engines, and the gas hydrocarbon product can be used as a LPG fuel.

—Liu 2011

Resources

  • Yanyong Liu, Rogelio Sotelo-Boyás, Kazuhisa Murata, Tomoaki Minowa, Kinya Sakanishi (2011) Hydrotreatment of Vegetable Oils to Produce Bio-Hydrogenated Diesel and Liquefied Petroleum Gas Fuel over Catalysts Containing Sulfided Ni–Mo and Solid Acids. Energy & Fuels Article ASAP doi: 10.1021/ef200889e

  • Yanyong Liu, Rogelio Sotelo-Boyás, Kazuhisa Murata, Tomoaki Minowa and Kinya Sakanishi (2009) Hydrotreatment of Jatropha Oil to Produce Green Diesel over Trifunctional Ni–Mo/SiO2–Al2O3 Catalyst. Chem. Lett. doi: 10.1246/cl.2009.552

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