Elsevier

Biomass and Bioenergy

Volume 119, December 2018, Pages 31-36
Biomass and Bioenergy

Research paper
Hydroprocessing of lipids extracted from marine microalgae Nannochloropsis sp. over sulfided CoMoP/Al2O3 catalyst

https://doi.org/10.1016/j.biombioe.2018.08.011Get rights and content

Highlights

  • Neutral Algal lipids hydrogenation favors at lower pressure (~ 50 bar), however higher pressure (>50 bar) favors cracking.

  • 50% Kerosene yield was obtained at 375°C Temperature and 120 bar H2 Pressure over hydrotreatingsulfided CoMoP catalyst.

  • Petrochemical feedstocks like aromatics (35%) and cycloalkanes (10%) form by hydroconversion of Nannochloropsis sp. algae.

  • The activation energy for the conversion of algae oil was 14.96 KJmol over CoMoP/Al2O3 catalyst.

Abstract

Hydroprocessing of neutral lipids extracted from Nannochloropsis sp. algal biomass was studied over sulfided cobalt molybdenum phosphorus/aluminium oxide (CoMoP/Al2O3) catalyst in a batch reactor at 300–375 °C with H2 at 50–120 bar. A clear light yellowish green product was obtained, containing 6–26% gasoline, 35 50% kerosene, 8–40% diesel, with 10–80% alkanes, 1–10% cycloalkanes, and 5–35% aromatics, with a maximum of 10% char formation in the process. Power law kinetic model was validated with experimental results. A kinetic study shows a pseudo 1st order reaction with respect to the neutral algae lipids oil concentration, with the activation energy for algal oil conversion over CoMoP/Al2O3 catalyst was 14.96 kJ/mol. Activation energies for the formation of diesel (125 kJ/mol) and kerosene (146 kJ/mol) were higher than for the heavy hydrocarbons such as PAH (7 kJ/mol) and alkanes (64 kJ/mol) products.

Introduction

There is much continued interest in the sustainable production of renewable biofuels to displace fossil fuels. These renewable biofuels are being developed broadly considering two key factors: 1) minimizing competition with food production and 2) compatibility with current infrastructure [1,2]. First generation biofuels, i.e., ethanol and biodiesel from sugar or starch and vegetable oils respectively, requires the cultivation of crops on land that could be otherwise used for food production [2]. In contrast, second generation biofuels can be produced either from crops or food wastes. These include biodiesel from waste oil or tallow and lignocellulosic ethanol from forestry or crop wastes. More recently, algae have been considered as a promising feedstock for third generation biofuels, as they can be produced at high productivity on otherwise non-arable land. Moreover, many algal biomasses can be cultivated to have high oil (triacylglyceride) content, and can produce oil at much higher productivity than other land-based oil-crops [3,4]. These are a hydrophyte containing chlorophyll without stems and roots and are categorized into two verities, based on size, i.e., macroalgae (seaweed) and microalgae [5]. Algae can be cultivated in the non agricultural land by using any kind of water (brackish, salty, west water, open ponds or in closed photobioreactors) with ample nutrients. They grow rapidly compared to other crops and plants, and its volume and size get double in just 24 h by consuming CO2 as a feed [[4], [5], [6]]. Unfortunately, the conventional biofuels produced from these crops, ethanol, and biodiesel, require engine modification for their use in present vehicular engines. There is a need to develop second and third generation biofuels that are compatible with current infrastructure.

Currently, mainly three approaches are being used to produce algae based biofuels. The first process involves extracting lipids from algal biomass cell, followed by the transesterification of triglycerides (TAGs) and alcohol into fatty acids alkyl esters (biodiesel). The second technique employs hydrothermal liquefaction (HTL) at high pressure (50–200 bar) and temperature (250–450 °C) to produce water-insoluble bio-crude oil [7]. The third technique is pyrolysis, which thermally degrades algae biomass at 300–700 °C in the absence of oxygen, resulting in the production of bio-oils (aqueous and organic phase), solid residue (char/coke), and gases (bio-gas). These processes require the final bio-oil/crude to be upgraded using hydrogen, before being used in conventional transportation engines. Hydroprocessing of the bio-oil/crude may produce drop-in biofuels which are compatible with engines and can be used directly without any upgrading.

Many researchers have studied the hydroprocessing of bio-crude produced via hydrothermal liquefaction of algae over various catalysts including sulfided CoMo/γ-alumina, HZSM-5 at temperatures between 400 and 500 °C. Transportation oil produced between 55 and 85%, however, yield decreases on increasing temperature. The highest reported yield was achieved using a combination of Ru/C and Raney/Ni catalysts [[8], [9], [10], [11]].

Today, the direct conversion of algae lipids to drop-in fuels has yet to be investigated. In comparison to hydrothermal liquefaction, the extraction of algae lipids allow the other valuable biomass components, in particular, the proteins, pigments, and long-chain polyunsaturated fatty acids, to be recovered [12]. The direct conversion of these lipids to a drop-in fuel would be a major improvement over the conventional approach of conversion to biodiesel via transesterification.

The lipids extracted from microalgae using a wet extraction process are predominantly triacylglycerides, but also contain waxes, sterols, free fatty acids, monoglycerides, diglycerides, phospholipids, glycolipids and chlorophyll as impurities. It is therefore of interest to determine whether a catalytic process can be developed, that can successfully convert this complex feedstock to suitable drop-in fuels. This study investigates for the first time the hydroprocessing of crude lipids extracted from the commercially relevant marine microalga Nannochloropsis sp. over a sulphided form of CoMoP/Al2O3 catalyst [13].

Section snippets

Catalyst preparation

The catalyst CoMoP/Al2O3 was prepared by conventional wet impregnation method on commercial mesoporous extrudates of γ-Al2O3 (BET surface area = 298 m2 g-1, BJH pore size = 6.1 nm, pore volume = 1.1 ml g−1). 1.89 g of (NH4)6Mo7O24.4H2O, as molybdenum precursor, 1.55 g Co (NO3)2.6H2O as cobalt precursor and 0.16 g of 85% aqueous H3PO4 as Phosporous precursor were used during catalyst synthesis. An ammonical solution of molybdenum precursor, an aqueous solution of cobalt precursor, were added

Catalyst characterization

The BET surface area and pore volume of the sulphided catalyst were found to be 298 m2/g and 0.5 ml g−1 respectively. BJH analysis (mean pore size) showed a narrow pore size distribution with a mean pore size of 7.0 nm (Supporting Fig. 1).

Many researchers have identified the monolayer morphology of MoS2 slabs with 5–8 nm length visible in micrographs of sulphided CoMo/Al2O3 catalyst [16,17]. This indicates that monolayer of CoMoS clusters exists in the active phase, with a slab-like morphology

Conclusions

Algal lipid was successfully hydroprocessed over sulfided CoMoP/Al2O3 catalyst. The results show that almost complete conversion of neutral lipids was achieved (375 °C and 50 bar H2), by forming 6% of gasoline, 35% of kerosene, and 87% of alkanes. Pressures of 50 bar and temperatures of 375 °C were the best for minimizing undesired coking, and to prolong the catalyst life. Increased formation of aromatics (35%) and cycloalkanes (10%) during the hydroconversion of marine microalgae

Acknowledgments

We are thankful to the Hydroprocessing Laboratory of CSIR-IIP to support and help in experimental and analytical work. We are also thankful to CSIT-IIP for funding the research work under OLP 134919 (OLP- 0893).

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