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  • Evaluation of Presulfided NiMo/-Al2O3 for Hydrodeoxygenation ofMicroalgae Oil To Produce Green DieselLin Zhou* and Adeniyi Lawal

    New Jersey Center for Microchemical Systems, Department of Chemical Engineering and Materials Science, Stevens Institute ofTechnology, 1 Castle Point on Hudson, Hoboken, New Jersey 07030, United States

    ABSTRACT: In the present work, reduced presulfided NiMo/-Al2O3, the conventional hydrotreating catalyst, was evaluatedfor green diesel production via hydrodeoxygenation of unrefined microalgae oil in a microreactor, mimicking the single channelof a monolithic reactor. The effect of reactor inner diameter on space-time yield of hydrocarbon and microalgae oil conversionwas studied first to confirm the superiority of the microreactor for three-phase reactions. Based on the external and internal masstransfer limitation analyses, a range of process conditions without mass transfer limitation was determined for catalyst evaluation.The results showed that NiMo/-Al2O3 is deactivated due to the accumulation of produced oxygenated intermediates inhydrodeoxygenation reaction, and its selectivity to even-numbered carbon hydrocarbon produced from hydrodehydrationcorrelates with the catalyst activity. The catalyst activity and life can be preserved by increasing hydrogen to oil ratio, residencetime, reaction temperature, and pressure, which will decrease the adsorption of oxygenates on the catalyst surface. For thereaction condition: 500 psig H2, 360 C, H2/oil ratio of 1000 SmL/mL, and residence time of 1 s, the initial catalyst activity wasmaintained without any signs of deactivation for at least 7 h and the obtained C13 to C20 hydrocarbon yield was 56.2%, with acarbon yield of 62.7%, nearly complete conversion (98.7%) of microalgae oil, and HC(2n)/HC(2n 1) ratio of 6.

    1. INTRODUCTIONMicroalgae are ubiquitous, and although they are primarilyfound in all the oceans and seas, an area that covers 71% of theEarths surface, they also grow in freshwater bodies as well as onand in soil, rocks, ice, snow, plants, and animals.1 They growextremely rapidly and can double their biomass within 24 h,which is 10200 times faster than terrestrial oil crops.2,3Oleaginous phototropic microalgae are sunlight-driven cellfactories that convert carbon dioxide to lipids, with an averagelipid content varying between 1% and 70%, and even reachingup to 90% of dry weight under certain conditions.2,4,5 Theaccumulated oil in almost all microalgae is mainly triglycerides(>80%) with a fatty acid profile rich in C16 and C18,2 which isa substantial energy resource for liquid fuel production.Therefore, microalgae oil has been considered as a goodcandidate for low-net carbon liquid transportation fuelproduction, which has no direct competition with edible foodor oil production.However, some of the physical properties of microalgae oil

    prohibit its direct use in existing engines, such as lowflowability, high viscosity, and low volatility. In order toimprove the oils physical properties while maintaining itsheating value, an ideal upgrading process should only rearrangethe oil molecular structure while avoiding or minimizingcracking.6 On the basis of the experience of vegetable oilupgrading, the two currently practiced oil upgrading methodsare transesterification and hydrotreating, which producebiodiesel and green diesel, respectively. Biodiesel has a lowerviscosity than its parent oil, but still much higher than that ofpetrodiesel. With the cold-flow related issues unresolved,7

    biodiesel cannot be used in its pure form; rather, it is blendedwith petrodiesel, the most common product, commerciallyreferred to as B20, blends 20 vol % biodiesel with 80 vol %petrodiesel. Green diesel, on the other hand, is essentially a

    mixture of hydrocarbons, has the same chemical properties aspetrodiesel, and is compatible with all existing engines,pipelines, and infrastructure for application and distribution.Compared with biodiesel, green diesel is a more ideal substitutefor petrodiesel.In the production of green diesel from microalgae oil, oxygen

    needs to be removed from lipids, mainly triglycerides, toproduce hydrocarbons. Because the sulfur, nitrogen, andphosphorus contents in microalgae oil are really low, themain heteroatom to be removed is oxygen. For this reason, thehydrotreating of microalgae oil mainly refers to the oxygenremoval. Due to the nature of oxygen bonding in triglycerides,removal of oxygen could be achieved in two ways: (1)hydrodehydration (DHYD), in which oxygen is removed in theform of H2O and (2) hydrodecarboxylation (DCO2) orhydrodecarbonylation (DCO), in which oxygen is removed inthe form of CO2 and CO, respectively. One triglyceridemolecule produces three hydrocarbon chains.As the oxygen is present in crude oil at rather low levels, of

    the order of 0.5%, deoxygenation in petroleum refining is not ofmuch concern, and no catalysts are specifically formulated foroxygenates hydrotreating. Hence, one of the critical technicalchallenges to make the hydrodeoxygenation of microalgae oilprocess economically feasible is related to the research anddevelopment of effective catalysts. Many studies have beenperformed with commercial hydrotreating catalysts, such asCoMo, NiMo, and NiW supported on alumina, for hydro-treating of natural lipids from different sources and haverevealed that complete triglycerides conversion could be

    Received: October 8, 2014Revised: December 5, 2014Published: December 5, 2014

    Article

    pubs.acs.org/EF

    2014 American Chemical Society 262 dx.doi.org/10.1021/ef502258q | Energy Fuels 2015, 29, 262272

    pubs.acs.org/EF

  • achieved over these catalysts.813 These conventional hydro-treating catalysts are more active in sulfided form than in oxideform.14 The sulfidation creates active sites that can play a role inthe rupture of the carbonheteroatom bond.15 Toba et al.studied the hydrotreating of waste cooking oil in both batchreactor (7 MPa, 250350 C) and fixed-bed flow reactor (5MPa, 350 C) using CoMo, NiMo, and NiW catalysts. In theirstudy, the NiMo and NiW catalysts showed high and stablehydrogenation activity, whereas deactivation was observedwhen using the CoMo catalyst. The NiW catalyst favorsmore hydrodecarboxylation or hydrodecarbonylation thanhydrodehydration.16 Senol also reported that the NiMo catalystshowed a higher activity than CoMo in hydrodehydration andhydrogenation reactions.15 Kubicka and Kaluza studied the Ni,Mo, and NiMo sulfided catalysts in deoxygenation of rapeseedoil at 260280 C, 3.5 MPa, and 0.254 h1 in a fixed-bedreactor. They found bimetallic NiMo catalysts showed higheractivity and yields of hydrocarbons than monometallic catalyst;NiMo yielded a mixture of decarboxylation and hydro-dehydration products, whereas Ni yielded only decarboxylationproducts, and Mo yielded almost exclusively hydrodehydrationhydrocarbon products.17 Da Rocha Filho et al. studied thehydrotreating of different vegetable oils in a batch reactor usingNiMo catalyst and reported a 6676 wt % n-alkanes yield after2 h at 360 C and 14 MPa.18 Huber et al. conducted thehydrotreating of sunflower oil over NiMo catalyst in a fixed-bedreactor under reaction conditions as follows: temperature 350C, pressure 50 bar H2, LHSV 5.2 h

    1, and H2 to oil ratio of1600 SmL/mL. The maximum carbon yield in C15C18alkanes they obtained was 71%, which is 75% of the maximumtheoretical yield of 95%.19 Peng et al. studied the hydrotreatingof microalgae oil in batch mode with 10 wt % Ni/HBeta at 260C and 40 bar and obtained 78 wt % yield of liquid alkanes after8 h of reaction time. They also obtained almost identical resultsfrom their trickle-bed reactor system under identical exper-imental conditions.3 As discussed later, in the present study,presulfided NiMo/Al2O3 catalyst was studied for hydrotreatingof microalgae oil as the baseline for further catalyst screeningwork.Although very high oil conversions and hydrocarbon yields

    were obtained in the studies mentioned above, which wereconducted in batch reactors or trickle-bed reactors, it should benoted that the space-time yield (STY) was extremely low dueto the severe heat and mass transfer limitations in theconventional macroreactor system. The low STY implies thesize of the reactor and all associated ancillary equipment will beunrealistically large, driving up both the capital and operatingcost. Therefore, a cost-effective upgrading process demands adifferent and innovative approach to reactor design forhydrotreating. Monolith reactors are being studied as asubstitute for conventional multiphase reactors, such astrickle-bed reactors, slurry reactors, and slurry bubble columnreactors for gasliquidsolid reactions due to their superiorhydrodynamics.20 Monolith substrate is usually made fromceramic, but metallic monolith has also been developed forhighly exothermic reactions, which enable the coupling of thereactor with a heat exchanger to effectively remove heat intransverse direction and control reaction temperature. Themonolith reactor features a honeycomb structure comprisingthousands of finely divided flow passages that extend throughthe whole reactor. The cross-sectional dimensions of eachchannel are determined by cell density (cells per square inch,cpsi), typically ranging between 100 to 1200 cpsi. Considering

    the fact that the typical values for wall thickness range between0.006 and 0.05 cm, the monolith reactor has correspondingsubmillimeter and, thus, microchannel dimensions, and suchmicrochannels mimic independent microreactors.Many previous studies2123 of microreactors applied to

    different reactions showed that the mass and heat transfer rateswere greatly enhanced and were often several orders ofmagnitude greater than those achievable in conventionalreactors. This is attributed to a combination of short diffusiondistance and the existence of a flow pattern of alternati