catalytic hydrocracking of fresh and used cooking oil

Catalytic Hydrocracking of Fresh and Used Cooking Oil

Stella Bezergianni,* Spyros Voutetakis, and Aggeliki Kalogianni

Chemical Process Engineering Research Institute (CPERI), Centre for Research & Technology Hellas(CERTH), 6th km Harilaou-Thermi Road, Thermi-Thessaloniki, Greece

Hydrocracking of vegetable oils is a prominent technology for the production of biofuels. This work comparesthe product yields and quality of hydrocracking fresh and used cooking oil under nominal operating conditions.Cracking, heteroatom removal and saturation reaction mechanisms are evaluated for both feedstock typesand for three typical hydrocracking temperatures. The assessment of both feedstocks indicates that they areboth suitable for high diesel yields with smaller kerosene/jet and gasoline/naphtha yields. As temperatureincreases, diesel selectivity increases for both feedstock types. However, the used oil feedstock exhibits higherkerosene/jet and naphtha selectivity at low temperatures (350 °C) and lower at the highest hydrocrackingtemperature (390 °C).

1. Introduction

During the past decade national and international legislationhas promoted the use of biofuels as an alternative source oftransportation energy, since their production enhances sustain-ability and economic growth. Biodiesel is the most commonbiofuel employed in Europe. Biodiesel production in particularis worthy of continued study and optimization of productionprocedures because of its environmentally beneficial attributesand its renewable nature.1 Its production (mostly via transes-terification) is mainly based on raw vegetable oil.2 Vegetableoil is produced from oil-based crops (rapeseed, soy-bean, palm,sunflower, etc.) which give moderate yields per hectare.

However, there are several considerations associated with theexisting biofuels production processes. The main byproductglycerin via the transesterification method is both an economicbut also an environmental consideration. Furthermore, largeinvestments for biodiesel production units are required in orderto ensure high efficiency.3 The most important considerationthough is the price and availability of vegetable oil, since itscost might reach up to 75% of the total biodiesel productioncost.4 The use of waste cooking oil for the production of biofuelscan compensate to the former consideration since used (fried)vegetable oil, collected from restaurants and/or homes, costs atleast 2-3 times cheaper than virgin vegetable oils.5

Several transesterification techniques and different types ofcatalysts have been employed in order to explore used cookingoil as a feedstock for biodiesel production. Alkali-catalyzedtransesterification of a single step4-6 or of a two-step process7

gives high yields at moderate methanol/oil ratios and mildtemperatures. Another interesting technology is based onheterogeneous solid catalyst-based transesterification8-10 whichemploys more environmentally benign catalysts and is effectivefor used cooking oil feedstocks, but requires higher temperatures.Enzymatic-catalysis-based transesterification exhibits significantyields at moderate operating conditions11-14 and shows sig-nificant potential.

An alternative technology for biofuels production technology,which employs the existing infrastructure of petroleum refineries,is the catalytic hydroprocessing of vegetable oil.15,16 Thistechnology has already several industrial applications.17-19 Thebiodiesel produced from hydrotreated vegetable oils has better

fuel properties than the biodiesel produced via transesterification.In addition, the use of biodiesel from hydroteated vegetable oilsimproves engine fuel economy, implying that this technologyhas a significant potential.15 Hydroprocessing of raw vegetableoil-heavy vacuum gas oil mixtures has been explored byemploying hydrotreating20 and hydrocracking21 catalysts atnominal operating conditions. Hydrocracking of used cookingoil has also been studied as a potential process for biofuelsproduction.22

This paper involves the investigation and comparison of rawand used cooking oil as hydrocracking feedstocks for biodieselproduction. In particular, the effect of reactor temperature onproduct yields and quality is studied for both feedstocks.

2. Methodology

For this study a small-scale pilot plant hydroprocessing unitof CPERI/CERTH was employed. This hydroprocessing unithas been employed for hydrotreating (HDS, HDN) and hydro-cracking of various feedstocks, both of fossil and biobasedorigin. It mainly consists of a feed system, a fixed-bed reactorsystem and a product separation system, as schematicallydepicted in Figure 1; however, the unit is described in moredetail in the author’s previous work.21 The main feedstock ismixed with high pressure hydrogen and enters the fixed bedreactor where the feed molecules undergo hydrotreating and/orhydrocracking reactions. The product exits the reactor in a mixedgas-liquid phase and is cooled before it enters a highpressure-low temperature separator, where the gas and liquidphases separate.

For the evaluation of the hydrocracking effectiveness, bothfeed and product analysis is performed. The total liquid productand feedstock characterization involves several measurementsincluding simulated distillation (Agilent 6890N-GC), density

* To whom correspondence should be addressed. Tel.: +30-2310-498315. Fax: +30-2310-498380. E-mail: [email protected].

Figure 1. Simplified schematic diagram of CPERI/CERTH hydroprocessingpilot plant.

Ind. Eng. Chem. Res. 2009, 48, 8402–84068402

10.1021/ie900445m CCC: $40.75 2009 American Chemical SocietyPublished on Web 08/17/2009

(Anton-Paar DMA 4500), sulfur and nitrogen (Antek 5000),carbon (CHN LECO 800), and hydrogen (Oxford InstrumentsNMR MQA 7020). Once total carbon, hydrogen, sulfur, andnitrogen weight percent are determined, the oxygen concentra-tion is indirectly determined, assuming it is the only significantelement contained in the product. The gaseous product isanalyzed offline via a Hewlett-Packard 5890 Series II-GC,equipped with two detectors: a thermal conductivity detector(TCD) and a flame ionization detector (FID). The TCD is usedfor the analysis of H2, CO, CO2, O2, N2, and H2S while theFID is used for CH4 and C2-C6, hydrocarbons.

For all experiments in this study, the same commercialhydrocracking catalyst was employed. The catalyst was pre-sulphided according to the catalyst provider’s recommendedprocedure. Furthermore, in order to maintain constant catalystactivity, DMDS (di-methyl-di-sulfide) and TBA (tetra-butyl-amine) were added to achieve a specific sulfur and nitrogenconcentration in each feedstock (∼2 wt % S and ∼700 wppmN). Each experiment (condition) was considered complete whenthe reactions reached steady state, usually after 5-6 days onstream. This was verified by monitoring the product densitydaily. Once the product density was stabilized, the individualeffects of each experiment were considered stable and the studycomplete. The product collected during the last day of each studywas analyzed in detail, as it represented that particular condition.

In order to analyze the effectiveness of hydrocrackingreactions, hydrocracking conversion is utilized, which is definedas the percentage of the heavy fraction of feed which has beenconverted to lighter products during hydrocracking:

where feed360+ and product360+ are the weight percent of thefeed and product respectively which have a boiling point higherthan 360 °C.

Furthermore, in order to measure the hydrocracking effective-ness toward the production of a particular product instead ofother products, the measure of selectivity is employed. Selectiv-ity can be defined for different products (for example, diesel,gasoline, etc.) based on the boiling point range which definesthese products. The selectivities of diesel, kerosene/jet, andnaphtha production are defined in the following equations:

where feed360+ and product360+ are the weight percent of thefeed and product, respectively, which have a boiling point higherthan 360 °C, feed180-360 and product180-360 are the weight percentof the feed and product, respectively, which have a boiling pointbetween 180 and 360 °C (diesel molecules), feed170-270 andproduct170-270 are the weight percent of the feed and product,respectively, which have a boiling point between 170 and 270°C (kerosene/jet molecules), and feed40-200 and product82-200

are the weight percent of the feed and product, respectively,which have a boiling point between 40 and 200 °C (naphthamolecules).

3. Results

For this study, a series of experiments was conductedaiming to identify the effects of utilizing fresh versus usedcommercial cooking oil feedstocks for hydrocracking in orderto produce hybrid biofuels. The fresh cooking oil was aconventional commercial cooking oil (sunflower oil), whilethe used cooking oil was obtained mostly from localrestaurants as well as households after extensively being usedfor frying. The comparison of the fresh and used cookingoil is presented via Table 1.

From the comparison of the characteristics of the two oils inTable 1, it is apparent that the fresh cooking oil is not toodifferent from the used cooking oil. The used cooking oil densityis slightly higher than that of the fresh cooking oil, as cookingoil undergoes thermolytic, oxidative, and hydrolytic reactions.23

Interestingly, the used cooking oil has higher sulfur and nitrogencontent, which are most likely caused by the hydrolysis andoxidation of existing sulfur and nitrogen compounds containedin food items that were fried. Another small difference isobserved in the bromine index between the two oils, whichshows higher values for fresh cooking oil over the used one.This indicates that fresh cooking oil contains a slightly highernumber of unsaturated bonds over the used one. This observationis in agreement with literature,24 according to which usedcooking oil exhibits polarity which increases upon repetitivefrying.

The two types of cooking oil were used as feedstocks in thehydroprocessing pilot plant unit of CPERI/CERTH (Figure 1).Two experimental runs were conducted in order to study thetwo feedstocks. Both experimental runs employed the samecommercial hydrocracking catalyst and were conducted atidentical conditions, i.e. reactor temperatures 350, 370, and 390°C, system pressure 2000 psig (13789.5 kPa), liquid hourly spacevelocity (LHSV) 1.5 h-1, and H2-to-liquid feed ratio (H2/oil)of 6000 scfb (1068 nm3/m3).

3.1. Product Yields. The hydrocracking temperature is themost dominant operating parameter which defines catalystperformance as well as catalyst life. In this study three typicalhydrocracking temperatures (350, 370, and 390 °C) are assessedin terms of hydrocracking activity as well as product yields andqualities.

The comparison of the product yields (naphtha/gasoline anddiesel) resulting from hydrocracking of the used and freshcooking oil is given in Figures 2 and 3, respectively, for thethree reactor temperatures studied. The product yields aredetermined by the simulation distillation data of the differentproducts, considering the boiling range of naphtha/gasoline(40-200 °C) and diesel (180-360 °C). Diesel production ismostly favored as it is easily observed by comparing the twofigures at all reactor temperatures. Furthermore, the naphtha/

conversion(%) )feed360+ - product360+

feed360+× 100 (1)

diesel selectivity(%) )product180-360 - feed180-360

feed360+ - product360+× 100

(2)

kero/jet selectivity(%) )product170-270 - feed170-270


(3)

naphtha selectivity(%) )product40-200 - feed40-200


(4)

Table 1. Properties of Fresh and Used Cooking Oil

fresh cooking oil used cooking oil

density (kg/m3) 891.4 896.6S (wppm) 0.9 38N (wppm) 0.69 47.42H (wt %) 11.62 11.62C (wt %) 76.36 76.74O (wt %) 12.02 11.6355refractive index 1.45513 1.45511bromine index 49.2 46.60

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gasoline yield increases with temperature, as higher temperaturesfavor cracking and therefore the production of lighter molecules,while diesel yields are certainly not favored. Interestingly usedcooking oil exhibits higher naphtha/gasoline yields at moderatetemperatures, while fresh cooking oil exhibits higher dieselproductivity at all temperatures. This difference in naphtha/gasoline and diesel productivities is up to a point expected asused cooking oil contains molecules of higher polarity, whichenable the ion-exchange on the catalyst surface and, therefore,the overall cracking activity.

The hydrocracking effectiveness, as it is differentiated forthe two feedstocks, can also be identified by comparing theconversion and product selectivities in Table 2. The conversionand product selectivities are calculated from the distillation data

of each feed and product by employing eqs 1-4. With respectto conversion, hydrocracking is more severe for the freshcooking oil at all reaction temperatures, which is expectedbecause the fresh cooking oil contains a higher degree ofsaturated molecules and is also slightly lighter in terms of densitythan the used cooking oil feedstock. This shows that the amountof large molecules (with boiling point > 360 °C) which isconverted to smaller and more useful molecules (with boilingpoint < 360 °C) is higher for fresh cooking oil. Furthermoreconversion increases with temperature for both feedstocksconsidered, which is also excepted as temperature favorscracking reactions that mainly define conversion.

Regarding product selectivities, the comparison differs foreach product. Diesel selectivity is decreasing as temperatureincreases, which is expected as increasing temperature causesa higher degree of cracking reactions which leads to crackingnot only feedstock molecules but also diesel molecules intolighter ones. On the other hand kerosene/jet and naphthaselectivies increase for higher temperatures due to theaforementioned effect. The two different trends are observedfor both feedstocks and were also reported in the literature.21

Diesel selectivies for the two feedstocks do not show anyappreciable difference at any hydrocracking temperature.However, in the case of the kerosene/jet and naphthaselectivity, the used oil feedstock shows higher values at lowtemperatures (350 °C) and lower values at the highesthydrocracking temperature (390 °C).

3.2. Heteroatom Removal. Even though sulfur and nitrogenare contained in insignificant amounts in the two feedstocks(see Table 1), sulfur (DMDS) and nitrogen (TBA) additives areartificially added in the feedstock to regulate catalyst activity,a typical procedure in hydroprocessing pilot plants. Besidescracking of heavy molecules to lighter ones, heteroatom removal(mainly sulfur, nitrogen, and oxygen) is also a significantmeasure of the overall hydrocracking effectiveness, as heteroa-toms are not desired in the final products. The extent ofheteroatom removal is expressed in Figure 4 as the percentageof the sulfur, nitrogen, and oxygen contained in the feed whichhas been removed during hydrocracking of each feedstock.Oxygen on the other hand is naturally contained as thefeedstocks consist of esters, ketones, etc. Among the threeelements, nitrogen is the most easily removed for both feed-stocks, with its removal percent being over 99.5% for all cases.Sulfur is also effectively removed for both feedstocks, but usedcooking oil exhibits a slightly higher extent. This difference isclearly seen in Table 3 with the product sulfur always beingbelow 300 wppm in the case of used cooking oil, while itexceeds 400 wppm for fresh cooking oil. Sulfur removalimproves significantly for increasing temperatures in the caseof used cooking oil, exhibiting a similar trend with conversion.Finally, in terms of oxygen removal both feedstocks areperforming similarly, even though the fresh cooking oil containsless oxygen initially.

3.3. Saturation. Another important reaction mechanism ofall hydroprocessing processes is the saturation of double bonds,which enables the cracking reactions to take effect. Both usedand fresh cooking oil contain a large amount of double bonds,indirectly indicated by the bromine index of the two feedstocks(Table 1). Hydrocracking of the two oils enables a high degreeof saturation, as it is clearly depicted in Table 4, with freshcooking oil showing the smaller bromine index values. Interest-ingly, however, the bromine index increases with increasinghydrocracking temperature for both cases of used and freshcooking oil. This implies that saturation is not favored by

Figure 2. Gasoline yield of hydrocracking fresh and used cooking oil at threedifferent temperatures. All experiments were performed at P ) 2000 psig(13789.5 kPa), LHSV ) 1.5 h-1, and H2/oil ) 6000 scfb (1068 nm3/m3).

Figure 3. Diesel yield of hydrocracking fresh and used cooking oil at threedifferent temperatures. All experiments were performed at P ) 2000 psig(13789.5 kPa), LHSV ) 1.5 h-1, and H2/oil ) 6000 scfb (1068 nm3/m3).

Table 2. Conversion and Selectivities of Hydrocracking Fresh andUsed Cooking Oil at Three Different Temperaturesa

350 °C 370 °C 390 °C

used fresh used fresh used fresh

conversion 72.62 80.37 73.97 78.41 81.88 85.09diesel selectivity 98.12 98.87 94.30 95.31 92.04 91.04kero/jet selectivity 6.21 5.57 8.66 8.67 20.04 22.24naphtha selectivity 3.04 2.24 7.24 6.12 12.17 12.68

a All experiments were performed at P ) 2000 psig (13789.5 kPa),LHSV ) 1.5 h-1, and H2/oil ) 6000 scfb (1068 nm3/m3).

8404 Ind. Eng. Chem. Res., Vol. 48, No. 18, 2009

temperature, which can be expected as saturation is a competingreaction mechanism to the cracking one.

The saturation extent can also be observed via the carbon-to-hydrogen ratio (C/H). In Table 5 the C/H ratio of bothfeedstocks and their corresponding products are given, for thethree different hydrocracking temperatures, where it is observedthat the C/H ratio is decreased for both used and fresh cookingoil feedstocks. However, the hydrocracking products of the usedcooking oil have a slightly higher C/H ratio, compared to theproducts of the fresh cooking oil. Moreover, the C/H ratioincreases with hydrocracking temperature, indicating that tem-perature does not favor saturation.

4. Conclusions

Hydrocracking of vegetable oils is a prominent process forthe production of hybrid biofuels. This work regards both freshand used vegetable cooking oils for hydrocracking via acommercial catalyst, considering three typical hydrocrackingtemperatures. Both feedstocks are able to give good productyields and quality, while exhibiting preference to diesel produc-tion. Nevertheless, diesel production is not favored by hydro-cracking temperature, which increases the production of lighterproducts (kerosene/jet and naphtha/gasoline). In terms ofheteroatom removal, nitrogen is the most easily removedelement in both used and fresh cooking oil. Sulfur however ismore easily removed from used cooking oil while oxygen isremoved adequately for both used and fresh cooking oil. Finallya large degree of saturation is achieved via hydrocracking ofboth oils, which decreases however with temperature, assaturation reactions compete with the cracking ones.

Literature Cited

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Figure 4. Heteroatom (sulfur, nitrogen, and oxygen) removal percent via hydrocracking of used and fresh cooking oil at three hydrocracking temperatures.All experiments were performed at P ) 2000 psig (13789.5 kPa), LHSV ) 1.5 h-1, and H2/oil ) 6000 scfb (1068 nm3/m3).

Table 3. Heteroatom (Sulfur, Nitrogen, and Oxygen) RemovalPercent via Hydrocracking of Used and Fresh Cooking Oil at ThreeHydrocracking Temperaturesa

product

feed 350 °C 370 °C 390 °C

used cooking oil S (wppm) 27070 278.6 156.8 109.2N (wppm) 551.70 1.95 1.31 0.10O (wt %) 12.58 1.86 0.57 0.47

fresh cooking oil S (wppm) 26690 1090 431.5 1140N (wppm) 642.4 0.09 0.02 0.08O (wt %) 11.57 0.01 0.66 -0.11


Table 4. Comparison of Bromine Index of Hydrocracking Feed andProducts at Three Different Hydrocracking Temperatures UsingFresh and Used Cooking Oila

product

feed 350 °C 370 °C 390 °C

used 49100 158.2 224.4 425fresh 47400 19.3 56.1 167


Table 5. Comparison of Carbon-to-Hydrogen (C/H) Ratio ofHydrocracking Feed and Products at Three DifferentHydrocracking Temperatures Using Fresh and Used Cooking Oila

ProductFeed 350 °C 370 °C 390 °C

Used 6.50 5.71 5.79 5.81Fresh 6.53 5.73 5.72 5.77


Ind. Eng. Chem. Res., Vol. 48, No. 18, 2009 8405

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ReceiVed for reView March 18, 2009ReVised manuscript receiVed July 21, 2009

Accepted August 1, 2009

IE900445M

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catalytic hydrocracking of fresh and used cooking oil

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