Gas-phase heterogeneous Catalytic Cracking (GCC)

Gas-phase heterogeneous Catalytic Cracking (GCC): catalyst re-activation improves monetary and CO2-balances of waste fat-derived biofuels Volker Heil, Axel Kraft, Andreas Menne, Christoph A. Unger, Anna Fastabend, Karl Meller, Marko Juricev-Spiric, Juan Pablo Zúñiga Ruiz Fraunhofer UMSICHT, Oberhausen, Germany [email protected] Manuscript of a lecture held at the 10th International Colloquium Fuels Conventional and Future Energy for Automobiles January 20-22, 2015, Esslingen / Germany Manuscript volume: ISBN 978-3-943563-16-0 Summary Gas-phase heterogeneous Catalytic Cracking (GCC) of fatty oils and fatty acids over activated carbon (AC) produces high-quality biofuels and bio-based blending components for automotive (diesel and gasoline), aviation (jet-fuel) and shipping (bunker fuel) uses. Tolerance of GCC concerning feedstock composition and impurities is intrinsically very high, and product purification efforts are intrinsically low, so already medium-sized plants become commercially feasible. No hydrogen is needed for the core process, and the catalyst is affordable and of high, world-wide availability. The main drawback so far has been the fact that the catalyst lifetime is restricted: after enabling the decarboxylation of up to 7 times its own weight, the activated carbons pores are blocked. Simply burning the coke (like in FCC processes) would destroy the AC. Even at prices of a few Euro/kg, this renders the AC a crucial part of financial balancing. A multiple re-activation-cycle, involving gas-phase reactions at 900 °C, proves the feasibility of repeated AC recovery for the process. The results show that the drawback of pore blocking can clearly be overcome. As re-activation of AC requires much less energy-consumption as its production resulting in a lower GHG-impact, it renders GCC-derived fuels and blending components even more favorable not only from a financial, but as well from an environmental point of view and minimizes the carbon footprint of these products.

1. A short history of GCC In 1927 and 1932, Portail i and Oppenheimii filed processes to convert vegetable oils into lighter components over activated carbon (AC) blocks or fibers, used as catalyst. The feedstock was lead to the catalyst under pressure in the liquid state, in order to decompose at temperatures of 300 °C and higher. The China Vegetable Oil Corporation, Shanghai, was ready to report ‘Catalyzed liquid-phase catalytic cracking’ of cottonseed, soybean, rapeseed and tung oil over AlCl3, lime, magnesin and caustic soda at 300350 °C in 1947 iii. In 1969, Argauer and Landolt filed a ‘Crystalline zeolite ZSM-5 and method of preparing the same’, which was published in 1972 iv. Shortly afterwards, these catalysts were used experimentally to catalytically crack plant oils v. Using this zeolite became quite common during the following decades; other catalysts used were H-mordenite, H-Y zeoliote, silicalite, alurninum-pillared clay (AL-PILC) and silicaalumina vi.

GCC - 10th Int. Coll. Fuels, Esslingen, 2015

In the early 2000s, an alternative process set-up was developed at Fraunhofer UMSICHT, Germany. Activated carbon catalysts were used to catalytically decompose vegetable fats and oils in the gas phase at temperatures especially preferred of 450-550 °C vii. 2. GCC over activated carbon This Gas-phase heterogeneous Catalytic Cracking (GCC) over activated carbon is a way to convert triglycerides and Free Fatty Acids – FFA – into hydrocarbons. No external hydrogen is added. The fatty acids’ carboxylic groups are separated from the longchained part of the fatty acids as well as (in case of triglycerides) from the glycerol backbone, forming CO2 (decarboxylation) or CO (decarbonylation). Simultaneously, a part of the feedstock ‘cokes’: it turns to substances with an increased C/H-ratio, while the majority of the (decaboxylated or decarbonylated) fatty acids forms paraffins, thus reaching a reduced C/Hratio. Coping with this coking is the main focus of the experimental work described in this article. To a certain extent, even double bonds of unsaturated fatty acids are saturated. As C16- and C18-fatty acids are the most common ones, the primary liquid products 1

are C15- and C17-alkanes next to alkylated benzenes. Secondary liquid products, formed by secondary cracking reactions, cover the kerosene and gasoline boiling range. Fig. 1 outlines the basic process, consisting of evaporation, catalytic reaction and cooling / product separation.

Fig. 2: Energy yield (see Eq. 1) of bench-scale GCCexperiments with different bio-based oils and waste fats over AC at 450 °C reaction temperature. Energy content ◊ product i Energy content ◊ feed total

Energy yield i = ◊

1

Energy content = mass ⋅ LHV

3. Catalyst de-activation by pore filling Fig. 1: The process scheme of GCC

Therefore, catalytic cracking over activated carbon is an ideal pathway to transform wastes and fatty acidcontaining by-products from plant oil processing into high-quality fuel components.

As the remaining volatiles formed in the catalyst are stripped out with nitrogen after the feeding stops, the Catalyst mass increase can mainly be regarded as coking products, remaining inside the pores of the activated carbon. They are formed during the whole process time, gradually reducing the pore capacity and finally deactivating the catalyst. To be precise, it could be stated that the process is not a real catalysis because altering the ‘catalyst’ by coking is part of the process. Obviously, this does not go along with the definition that a catalyst does not change in the process. In case of activated carbon derived by gas activation, catalyst consumption is about 15 wt.-% of the feedstock.

‘CRACKING’ and ‘SNG and LPG’, two projects funded by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU) in the framework of the funding program ‘Biomass for energy’, investigated the catalytic cracking of bio-based waste fats, plant oil processing by-products and inedible vegetable oils in bench scale (60 ml/h feed). Selected experiments in pilot scale (3 kg/h feed) confirm the potential of a scale-up towards industrial applications viii,ix.

In order to investigate the change of the pore system by coking during GCC, an activated carbon, prepared by chemical activation, was used for catalytic cracking of mixed waste fats. 6.7 g AC per g of fat was used. For pore structure analysis, N2-adsorption at 77 K was performed, using a ‘Sorptomatic 1,900’ of Porotec, Hofheim/ Taunus. Cumulative specific surface and cumulative specific pore volume were derived according to Horvarth and Kawazoe x. Fig. 3 presents the results for the fresh and used catalystviii.

Fig. 2 shows the energy yield according to Eq. 1 of the product compartments OLP (Organic Liquid Product), OGP (Organic part of the Gas Product), CO2+CO+H2 (the inorganic part of the gas product), Water and Catalyst mass increase. The results were derived in bench-scale with different bio-based oils and waste fats over AC at 450 °C reaction temperature.

Both cumulative specific surface and cumulative specific pore volume of the micropores decrease by an order of magnitude during GCC experiments. Afterwards, the catalytic activity is nearly gone, and breakthrough of fatty acids can be observed.

One of the major advantages of the process is its robustness against feedstock compositions and impurities. It works with triglycerides as well as 100 % free fatty acids, water content is only restricted by economic reasons, and non-volatile components remain in the evaporation unit and do not reach the reactor at all.


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are led in down flow through a fixed-bed reactor of 670 mm length and 40 mm inner diameter. The products are cooled down in a multistage cooler system. Analytics are done via GC-MS 1 for the liquid and GC-FID/HCD 2 for the gaseous products. b. Re-activation plant Re-activation was done in a bench-scale, electrically heated rotary kiln with nitrogen as inert gas. The reaction tube of the rotary kiln consists of austenitic steel 1.4828, heat-resistant up to 1.050 °C. Its total length is 1.150 mm with a central activation chamber, 530 mm in length and 72 mm inner diameter, and two peripheral tube sections. Heating is done by an electric clamshell furnace.

Activated carbon fresh used Specific surface Specific pore volume

c. Materials and conditions Mixed waste fats, commercially collected from restaurants and food industry, homogenised and filtered, were used as input material. Like shown in Fig. 2, this feedstock can lead to energetic yields of nearly 80 % for the OLP. Tab. 1 summarises the properties of the feedstock material that consists mainly of used cooking oils and similar substances.

Fig. 3: Specific surface and specific pore volumex of micropores in an activated carbon catalyst before and after GCCviii. Re-activation experiments based on desorption and extraction completely failed, indicating that catalyst deactivation is due to pore filling with carbon and/or carbon-rich, non-volatile cracking products.

Tab. 1: Mixed waste input fat properties

4. Activated carbon recovery Burning the coke deposits, the usual way to re-activate non-carbon porous catalysts, would burn the activated carbon matrix along with the deposits. Instead, reactivation with steam or CO2 at up to 1000 °C can be performed, involving the endothermic activation reactions given in Eq. 2, which is only valid at sufficiently high temperatures.

2

Density (70°C) [kg/l] 0.9 Water content [%] 0.1

Kin. Viscosity (70°C) [mm2/s] 18.8 Acid value [g/mg] 14.0

Iodine value [g/100g] 78.3

As catalyst, a formed charcoal-based activated carbon of 4 mm diameter, prepared by steam activation, was used. Tab. 2 summarizes some characteristic data; more data are available in literature 3.

In industrial scale, water and carbon dioxide can be recovered along with the consumed heat by oxidation of the gaseous products with in-blown air xi. 5. Experimental set-up a. GCC plant Bench-scale GCC preparation starts with preheating the feedstock to 70 °C and the mixing with a magnetic stirrer. A peristaltic pump leads 60 g/h oil or fat into the evaporator, a steel tube of 780 mm length and 40 mm inner diameter, mounted with a decline of 25°. Together with nitrogen as carrier substance, the vapors GCC - 10th Int. Coll. Fuels, Esslingen, 2015

1

Gas chromatograph with mass spectrometer Gas chromatograph with flame ionization detector and heat conductivity detector 3 viii , table A – 2, catalyst ‚AK4‘ 2

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Tab. 2: Fresh AC catalyst properties Specific pore Specific surface Iodine volumex area xii,xiii number xiv (micropores) according to Brunauer, according to Emmett and Horvath Teller and Kawazoe 3 [cm /g] [m2/g] [mg/g] 0.54 1 254 >1100 In Tab. 3, the GCC-parameters of the test series reported here are given. Tab. 3: GCC-parameters of re-activation test series Experimen- Feedstock Feedstock Reaction tal time flow rate preheating temp. temp. [h] [g/min] [° C] [° C] 5 1 70 450 Liquidsampling cooling temp. [° C] 5

N2 flow rate

N2 temp.

[l/min] 100

[° C] 600

Catalystbed volume

ml 374

Re-activation was done over 3 h at 900 °C under nitrogen-diluted steam atmosphere. d. Test-cycle routine

Fig. 4: Mass balance of GCC-experiments with mixed waste fats over repeatedly re-activated AC (90 vol.-%), blended with fresh AC (10 vol.-%) Not only the yield of the OLP rises with the reactivation cycles; its density rises slightly, too, as shown in Fig. 5. This is partly due to the fact that

concentration of long-chain hydrocarbons (e.g. heptadecane: density ρ=0.777 kg/l) increased at expense of the short chain hydrocarbons (e.g. heptane: density ρ=0.68 kg/l) over the test series.

In order to investigate the effect of repeated re-activation of GCC-catalyst-performance, a series of GCCexperiments with subsequent re-activation was performed. The set-up was based on the conservative assumption that 90 % of the activated carbon catalyst can be re-activated in long-term operation. First, fresh activated carbon was used and afterwards re-activated. In all following experiments, 10 vol.-% of the catalyst-bed volume were filled with fresh AC, while the remaining 90 % were filled up with the reactivated carbon of the previous cycle. The catalyst was mixed, used for GCC and re-activated subsequently. Nine of these cycles were performed. 6. Results and discussion Fig. 5: Density of the OLP shown in Fig. 4 As Fig. 4 shows, the only real observable effect of reactivation cycles on the mass balances was that the OLP-yield increased slightly with time.


The results show that multiple re-activation of the AC catalyst is possible. Even though activated carbon is by far not as costly as the catalysts of comparable biofuels technologies and is world-wide available without restrictions to few producers, this fact clearly helps to improve the economics of the overall process.

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As the temperatures of GCC and re-activation differ considerably, process concepts with separate units for both steps seem to be favorable for commercial plants.

iv

Argauer, R. J. & Landolt, G. R. Crystalline zeolite ZSM-5 and method of preparing the same. US3702886A, 1969.

7. Summary and outlook

v

Haag, W.; Rodenald, P. G.; Weisz, P. B.: Catalytic production of aromatics and olefins from plant materials. In: Symposium on alternate feedstocks for petrochemicals presented before the division of petroleum chemistry, inc., American Chemical Society, San Francisco meeting, August 24-29, 1980, 650–656.

vi

Katikaneni, S. P. R. Adjaye, J. D. & Bakhshi, N. N. Catalytic Conversion of Canola Oil to Fuel and Chemicals over Various Cracking Catalysts, The Canadian Journal of Chemical Engineering 73, 484–497 (1995).

vii

Cinquemani, C. Heil, V. Jakob, J. & Weber, A. Process for converting of raw materials and waste materials containing oil or fat in a composition containing hydrocarbons. EP1489157B1, 2004. English version: DK/EP1489157T3

Gas-phase heterogeneous Catalytic Cracking of biobased fats and oils at ambient pressure over activated carbon offers a novel way towards bio-based fuels and fuel blending components. Up to 80 % energy yield of the Organic Liquid Product can be achieved. Catalyst re-activation is possible via gas activation. Nine re-activation cycles were demonstrated without any sign of performance loss of the catalyst. This fact clearly helps to improve the economics of the overall process. Experiments in larger scale, with AC-reuse-ratios higher than 90 %, and with more test cycles could be carried out in order to explore the edge of re-activation possibilities. Special investigations, including optimization of the reactivation procedure, could enable the GCC operators to purposefully use effects like the increase of OLP density with repeated re-activation for product design. 8. Acknowledgement Parts of the investigations were funded by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU) within the funding programme ‘Biomass for energy’ in two projects: • Screening von bio-based wastes for the conversion into gasoline- and Diesel-fuels by catalytic cracking (03KB007) • SNG and LPG from bio-based wastes – technical feasibility and application potential (03KB028 )

viii Heil, Volker et al., Screening von biogenen Abfallsubstanzen zur Umwandlung in Benzin- und Dieselkraftstoffe durch katalytisches Cracken, final report of the project BMU-03KB028 (BMU: Federal Ministry for the Environment, Nature Conservation and Nuclear Safety), May 2012. http://publica.fraunhofer.de/eprints/ urn:nbn:de:0011-n-2083342.pdf ix

Heil, Volker et al., SNG und LPG aus biogenen Reststoffen – technische Machbarkeit und Verwertungspotenzial, final report of the project BMU-03KB028, May 2012. http://publica.fraunhofer.de/ eprints/urn:nbn:de:0011-n-2052263.pdf

x

DIN 66135-4, Partikelmesstechnik Mikroporenanalyse mittels Gasadsorption - Teil 4: Bestimmung der Porenverteilung nach Horvath-Kawazoe und Saito-Foley, 2004

Sole responsibility of this lecture lies with the authors; the BMU is not responsible for any use that may be made of the information herein.

xi

Kienle, H. von; Bäder, E. (1980): Aktivkohle und ihre industrielle Anwendung. Publisher: Enke, Stuttgart/Germany:

References

xii

S. Brunauer, P. H. Emmett und E. Teller, Adsorption of Gases in Multimolecular Layers, Journal of the American Chemical Society, 2, 1938, 60, 309-319

We thank the BMU for the financial support.

i

Portail, F. C. F. Method of converting by catalysis mineral and vegetable oils. US1,842,197A, 1927.

ii

Oppenheim, R. Method of converting by catalysis mineral and vegetable oils. US 1,960,951, 1932.

iii

Chang, C.-C. & Wan, S.-W. China's Motor Fuels from Tung Oil, Industrial and Engineering Chemistry 39, 1543–1548 (1947).


xiii DIN 66131, Bestimmung der spezifischen Oberfläche von Feststoffen durch Gasadsorption nach Brunauer, Emmett und Teller (BET), 1993 xiv European Council of Chemical Manufacturers' Federations (CEFIC), Test methods for activated carbon, Brussels, 1986

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