Simulation of Hydrotreating of Vegetable Oil in a Slurry Bubble Column Reactor for Green Diesel Production

The purpose of this research is to obtain a mathematical model of the hydrotreating of vegetable oil in a slurry bubble column reactor to produce green diesel using an NiMo-P/Al₂O₃ catalyst. A steady state two-dimensional axisymmetric model of a slurry bubble column reactor of 2.68 m in diameter and 7.14 m in height was developed. The numerical simulation was performed using the model for the reactor operating at 34.5 bar and 598 K. Triglyceride of 5 wt% in dodecane enters the reactor with a very low velocity so that it resembles a batch operation. Hydrogen of 188 mol hydrogen/triglyceride enters the reactor with a velocity of 0.02 m/s. The simulation results show that the vegetable oil (triglyceride) conversion is 97.73%, the product yield is 83.34 wt% and the product purity is 77.23 wt%.

Green diesel; Modeling; Simulation; Slurry bubble column; Vegetable oil

The depletion of global fossil fuels accompanied by the increase in energy demand and the attention to environmental damage due to the combustion of said fuels have motivated researchers to focus on renewable energy production and utilization. Hydrogen, liquefied petroleum gases, liquefied natural gas, biomass, biodiesel, and green diesel are some alternative energy sources that are being researched and considered as possible fossil fuel replacements (Elvan et al., 2013). Green diesel is one of the most promising alternative fuels and is expected to replace petroleum diesel fuel in the future due to its superior properties.

Green diesel is a second generation biofuel and is mainly produced from vegetable oils. It consists of straight-chain alkanes with 15-18 carbon atoms, the chemical structures of which are identical to those of petroleum diesel. The significant difference between green diesel and biodiesel is in their molecular structures. While biodiesel comprises alkyl ester molecules, green diesel consists of paraffin molecules, i.e., green diesel does not contain oxygen-based molecules. This characteristic of green diesel results in high heating value and high energy density (U.S. Department of Energy, 2007). Other properties of green diesel superior to biodiesel are a higher quality cetane component (CN > 80), lower NO_x emissions, better oxidative stability, higher fulfilment of car manufacturer requirements, and higher applicability in all diesel vehicles (Kalnes et al., 2007).

Hydroprocessing or hydrotreating reaction is one of the alternative methods to produce green diesel. It is considered the most favorable method and has been applied in industries. Its mechanism includes hydrogenation, decarbonylation, decarboxylation, and hydrodeoxygenation. Hydrodeoxygenation eliminates oxygen by reacting triglycerides and free fatty acids with hydrogen to form water and n-paraffins. Decarboxylation or decarbonylation eliminates oxygen to form carbon dioxide or carbon monoxide and n-paraffins. The process involves three phases: vegetable oil (triglycerides) as the liquid phase, hydrogen as the gas phase, and a catalyst as the solid phase.

The most frequently used three-phase reactors are trickle-bed reactors and slurry reactors. A trickle-bed reactor is a type of fixed-bed reactor. A slurry reactor is a type of bubble column or stirred reactor. In slurry bubble column reactors, agitation by gas bubbles causes catalyst particles to be suspended in liquid. Slurry bubble column reactors have many advantages compared to trickle-bed reactors, such as better temperature control and heat removal, lower pressure drop (four times less than in fixed-bed reactors), the ability to use finer catalyst particles (<100 ?m) allowing for a large surface area and better mass transfer, higher yield per reactor volume, and ease of continuous addition and removal of catalyst particles (Sehabiague et al., 2008).

Experimental and kinetics studies of hydrotreating have been conducted for various feedstocks by many researchers: Sotelo-Boyas et al. (2011) used rapeseed oil with supported nickel catalysts, Kumar et al. (2014) employed stearic acid with supported nickel catalysts; Zhang et al. (2014) utilized waste cooking oil with a dispersed nanocatalyst, Manco (2014) used palm oil with supported nickel catalysts, and Landberg (2017) employed tall oil with supported nickel catalysts.

Modeling and simulation help experimental work on a particular process as they save time and cost. With modeling and simulation, one learns process behavior from which an optimal process can be designed. Although the advantages of modeling are remarkable, research on modeling of hydrotreating reactors is scarce. Research on the modeling of hydrotreating reactors has been performed by Attanatho (2012) using a microchannel reactor and Muharam et al. (2017a, 2017b) using a trickle-bed reactor.

The present study aims to obtain a mathematical model of the hydrotreating of vegetable oil to produce green diesel in a slurry bubble column reactor using an NiMo-P/Al₂O₃ catalyst and to understand the hydrotreatment behavior in the reactor. 

The simulation results using the model developed in this research show that the performance of the slurry bubble column reactor producing green diesel via hydrotreating of vegetable oil is excellent. The TG conversion, the green diesel yield, and the green diesel purity are 97.73%, 83.34%, and 77.23%, respectively.

We express our gratitude to the University of Indonesia, which funded this research through the Hibah Publikasi Internasional Terindeks untuk Tugas Akhir Mahasiswa (PITTA) scheme under the contract No 2573/UN.2.R3.1/HKP.05.00/2018.

Attanatho, L., 2012. Performances and Kinetic Studies of Hydrotreating of Bio-oils in Microreactor. Dissertation, Doctoral, Oregon State University, United States

Deckwer, W.D., Serpemen, Y., 1982. Modeling the Fischer-Tropsch Synthesis in the Slurry Phase. Industrial & Engineering Chemistry Process Design and Development, Volume 21(2), pp. 231-241

Elvan, S., Dimaggio, C., Kim, M., Salley, S.O., Simon, K.Y., 2013. Catalytic Conversion of Brown Grease to Green Diesel via Decarboxylation over Activated Carbon Supported Palladium Catalyst. Industrial & Engineering Chemistry Research, Volume 52(33), pp. 11527-11536

Kalnes, T., Marker, T., Shonnard, D.R., 2007. Green Diesel: A Second Generation Biofuel. International Journal of Chemical Reactor Engineering, Volume 5, pp. 1-11

Kumar, P., Yenumala, S.R., Maity, S.K, Shee, D., 2014. Kinetics of Hydrodeoxygenation of Stearic Acid using Supported Nickel Catalysts: Effects of Supports. Applied Catalysis A: General, Volume 471, pp. 28-38

Landberg, K., 2017. Experimental and Kinetic Modelling Study of Hydrodeoxygenation of Tall Oil to Renewable Fuel. Master’s Thesis, Graduate Program, Chalmers University of Technology, Sweden

Manco, J.F.V., 2014. Conceptual Design of a Palm Oil Hydrotreatment Reactor for Commercial Diesel Production. Master’s Thesis, Graduate Program, Universidad Nacional de Colombia, Medellín, Colombia

Muharam, Y., Nugraha, O.A., 2017a. Prediction of the Effects of the Inlet Velocity and the Reactor Length on the Performance of a Trickle-Bed Reactor for Renewable Diesel Production. Advanced Science Letters, Volume 23, pp. 5609–5614

Muharam, Y., Nugraha, O.A., Leonardi, D., 2017b. Modelling of a Hydrotreating Reactor to Produce Renewable Diesel from Non-edible Vegetable Oils. Chemical Engineering Transactions, Volume 56, pp. 1561-1566

Nader, A.A., 2008. Simulation of Gas-to-Liquid (GTL) Process in Slurry Bubble Column Reactor. Master’s Thesis, Graduate Program, University of Technology, Iraq

Schweitzer, J.M., Viguié, J.C., 2009. Reactor Modeling of a Slurry Bubble Column for Fischer-Tropsch Synthesis. Oil & Gas Science and Technology, Volume 64(1), pp. 63-77

Sehabiague, L., Lemoine, R., Behkish, A., Heintz, Y.J., Sanoja, M., Oukaci, R., Morsi, B.I., 2008. Modeling and Optimization of a Large-scale Slurry Bubble Column Reactor for Producing 10,000 bbl/day of Fischer-Tropsch Liquid Hydrocarbons. Journal of the Chinese Institute of Chemical Engineers, Volume 39(2), pp. 169-179

Sotelo-Boyás, R., Liu, Y., Minowa, T., 2011. Renewable Diesel Production from the Hydrotreating of Rapeseed Oil with Pt/Zeolite and NiMo/Al₂O₃ Catalysts. Industrial & Engineering Chemistry Research, Volume 50(5), pp. 2791-2799

U.S. Department of Energy, 2007. Basic Research Needs: Catalysis for Energy. Basic Energy Sciences Workshop, Maryland, United States

Zhang, H., Lin, H., Wang, W., Zheng, Y., Hu, P., 2014. Hydroprocessing of Waste Cooking Oil over a Dispersed Nano Catalyst: Kinetics Study and Temperature Effect. Applied Catalysis B: Environmental, Volume 150-151, pp. 238-248