Energy Fuels 2010, 24, 2737–2746 Published on Web 03/31/2010
: DOI:10.1021/ef901584t
A High Temperature, High Pressure Facility for Controlled Studies of Catalytic Activity under Hydrothermal Conditions J. Becker,† L. L. Toft,† D. F. Aarup,† S. R. Villadsen,† M. Glasius,† S. B. Iversen,‡ and B. B. Iversen*,† †
Center for Energy Materials, Dept. of Chemistry and iNANO, Aarhus University, Langelandsgade 140, DK-8000 Aarhus C, Denmark, and ‡SCF Technologies a/s, Smedeholm 13B, DK-2730 Herlev, Denmark Received December 23, 2009. Revised Manuscript Received March 21, 2010
In this paper, we report on the construction of a novel test facility for evaluating catalytic processes at high temperature and high pressure. The design features make the facility well-suited for highly controlled studies of hydrothermal conversion of real biomasses with complex composition. The proof of concept is provided by bio-oil production from (i) Dried Distiller’s Grains with Solubles (DDGS), the results serving to illustrate catalyst performance, and (ii) spent coffee grounds, which exemplify constituent analysis of the as-produced bio-oil. Both studies were carried out under near-critical conditions using both homogeneous and heterogeneous catalysts.
conditions with a high throughput. The specific studies for which the reactor is currently employed concern the singlestep, catalytic, hydrothermal conversion of real biomass into biocrudeoil, a process which is being marketed under the registered trademark CatLiq. This process, which has been developed over the past 10 years, is of significant industrial interest, as it has proven itself able to convert a wide variety of biomasses into bio-oil, ranging from sewage sludge to various plant materials or protein-containing waste, including as a particular example the waste from the production of bioethanol (Dried Distiller’s Grains with Solubles or DDGS).9,10 The CatLiq process is very flexible as to the feeds that can be processed and is particularly attractive for wet biomass or waste feeds such as feeds with a moisture content of more than 50%. For such wet biomass, the heat of evaporation of water does not need to be supplied. Wet or moist wastes or biomasses are presently either treated biologically, sometimes with the aim of producing biogas,3 or dried at a high energy cost and incinerated.2 The removal of waste products with the added benefit of recovering bio-oil makes the Catliq process attractive both ecologically and economically. However, until recently, the process parameter space was not yet well examined, and the impact of temperature, pressure, feed type, and catalysts on the product yield and composition remained largely unexplored. It was the need of such studies, and the technological relevancy of exploring hydrothermal conversion processes for so-called “real biomasses” in general, which formed the specific motivation for the construction of the chemical test reactor described here.
1. Introduction The demand for clean, sustainable energy resources has been increasing rapidly throughout the world for a number of years, primarily motivated by environmental concerns and an awareness of the fact that fossil fuels are a limited resource. Much attention has been given to waste-product biomasses as an energy resource, since it is renewable, excels at low CO2 emission, and exchanges the burden of disposal with the benefit of recycling. One way of harvesting the energy is by combustion, e.g., in power plants. However, as approximately 50% of the world’s total energy demand lies in the transportation sector, which still requires liquid fuels,1 much effort has been put into the liquefaction of biomass, e.g., by thermochemical conversion2,3 such as pyrolysis methods4,5 or hydrothermal processing,6,7 often with the aid of some conversion catalyst to improve bio-oil yield.4,8 However, studying catalytic effects in such processes is complicated (i) experimentally, due to the elevated temperatures (and pressures) at which the conversions often takes place, and (ii) analytically, due to the fact that a conversion process of a “real” biomass is invariably composed of a large web of entangled reaction pathways. In this paper, we present the design and construction of a chemical reactor built for testing hydrothermal biomass conversion catalysts under well-controlled, reproducible *To whom correspondence should be addressed. E-mail: bo@ chem.au.dk. (1) Energy Information Administration. . http://www.eia.doe.gov/oiaf/ ieo/pdf/liquid_fuels.pdf (accessed Mar 2010). (2) Goyal, H. B.; Seal, D.; Saxena, R. C. Renewable Sustainable Energy Rev. 2008, 12, 504–517. (3) Huber, G. W.; Iborra, S.; Corma, A. Chem. Rev. 2006, 106, 4044– 4098. (4) Suzuki, K.; Suzuki, T.; Takahashi, Y.; Okimoto, M.; Yamada, T.; Okazaki, N.; Shimizu, Y.; Fujiwara, M. 18th Symposium of the Materials Research Society of Japan; Springer: Tokyo, 2007; pp 60-68. (5) Mohan, D.; Pittman, C. U.; Steele, P. H. Energy Fuels 2006, 20, 848–889. (6) Kruse, A.; Gawlik, A. Ind. Eng. Chem. Res. 2003, 42, 267–279. (7) Goudriaan, F.; Peferoen, D. G. R. Chem. Eng. Sci. 1990, 45, 2729– 2734. (8) Fang, Z.; Minowa, T.; Smith, R. L.; Ogi, T.; Kozinski, J. A. Ind. Eng. Chem. Res. 2004, 43, 2454–2463. r 2010 American Chemical Society
2. Equipment Design 2.1. Overview. One of the most fundamental design questions regards the basic reactor concept, i.e., the choice of its (9) Kim, Y.; Mosier, N. S.; Hendrickson, R.; Ezeji, T.; Blaschek, H.; Dien, B.; Cotta, M.; Dale B. Ladisch, M. R. Bioresour. Technol. 2008, 99, 5165–5176. (10) Belyea, R. L.; Rausch, K. D.; Tumbleson, M. E. Bioresour. Technol. 2004, 94, 293–298.
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working principle. The most well-known design within the field of (catalytic) thermochemical biomass conversion is probably the batch autoclave concept, where a sealed steel container is used as a reaction vessel and heated to a given temperature in a furnace or a sand bath or by integrated heaters. It is employed by many groups studying real biomasses, e.g., empty palm fruit bunches, palm fruit fiber, or rice straw.11-13 While simple in construction, pure batch reactors unfortunately tend to be rather inflexible regarding parameter control, with temperature, residence time, and content composition being the only readily accessible variables. While the design may include pressure gauges, good pressure control can be hard to accomplish unless some external means of pressurization is applied to the vessel. Also, batch autoclaves usually heat slowly, which reduces the throughput of a catalyst screening and the ability to study fast processes. One notable exception in this respect is the capillary batch reactor concept, which has been used successfully for studying biomass gasification reactions by Potic et al.14 Due to the small reactor volumes involved, extremely swift heating to a set-point temperature may be obtained. However, the small volumes also necessitate very homogeneous feedstocks. As such, this reactor concept is best suited for studying model compounds, while the use of real biomasses, which may vary significantly in composition on the micro- or millimeter scale, would result in a poor reproducibility. Hence, since the conversion of real biomasses was a specific intention with this reactor facility, the capillary concept could not be used. The common alternative to a batch-type reactor is a continuous-flow reactor, which may be constructed in some specialized form such as the auger reactor,15,16 mill reactor,17 or flash-centrifuge reactor.18 While all of these concepts are efficient for a high throughput and very swift heating, the latter types (auger, mill, centrifuge, etc.) do not allow the use of heterogeneous catalyst pellets. The main use of such reactors is within the field of biomass pyrolysis. Pure-flow reactors are often used for biomass gasification,19 but with these one main problem is keeping the feed in good contact with the catalyst even at long residence times (several minutes or hours). This either requires extremely slow flow rates or an unrealistically long reactor zone and catalyst bed. For most flow reactors, residence time is limited to a maximum on the order of a few minutes. Also, if real biomasses are used as feedstock, it may be a problem to find laboratory-scale high-pressure pumps suitable for particulate matter. A further discussion of reactor
concepts has been provided by Matsumura et al. in a previous review on biomass gasification.20 In conclusion to the above considerations, a stop-flow reactor concept was chosen for the present work. Such a reactor can be opened and resealed and permits indefinite residence times and large volumes, as do batch reactors, while also including the ability to introduce biomass feeds to the reaction chamber in a temporary flow mode, thus causing each small unit volume to heat extremely rapidly.The flow capability also provides the external pressurization needed for separate pressure and temperature control. In the present case, a reactor volume of 170 mL was chosen, since this was considered sufficient for the testing of real biomass conversions while still being manageable from an operational point of view. The entire system consists of three main parts: (1) A high-pressure “hot” system, including the central reaction chamber. This is referred to as “the reactor”. It is connected to the rest of the high-pressure system through two pipes, the “reactor pipes”, extending vertically from the chamber in the upward and downward directions. (2) A high-pressure “cold” system separated from the hot reactor by two water-chilled cooling zones located on the reactor pipes and consisting of (a) the high-pressure pumps necessary for pressurizing the system independently of temperature (i.e., of the autogenous pressure) (b) the intermediate valve system which controls flows to and from the reactor (3) A low-pressure “cold” system, intended for in-line product separation in gaseous and liquid fractions. As this part of the system has not been used in the biomassconversion experiments so far, it will not be described further in this paper. Figure 1 shows each of these three sections. It should be noted that the valve system enables the flow to be established through the reactor in both the upward and downward directions, which means that feeds may be injected into or collected from the reactor via either the top or bottom reactor pipe. Depending of the density of the particular fraction being injected or collected (gas, aqueous, oil/hydrophobic, etc.), there may be specific advantages connected with using the top or bottom route. 2.2. High-Pressure “Hot” System;The Reactor. As the Catliq process operates at high temperatures and high pressures, the reactor has to be able to withstand such conditions. Moreover, a certain excess of engineering is necessary to provide “room” in the parameter space for the potential exploration of temperature- or pressure-related effects. Hence, the reactor is built as an ordinary autoclave (Figure 2), designed to meet maximum operational conditions of 500 °C at 500 bar, with an overhead to 600 °C and 600 bar. As common steels lose much of their strength at such a high temperature, the alloy X22 CrMoV was chosen for the reactor construction. This alloy is commonly used for the boiler systems at power plants and is rated for temperatures up to 600 °C. As for limiting the maximum pressure to 600 bar, two burst diaphragms were built into the design of the high-pressure system, one of which is placed immediately adjacent to the reactor (Figure 1, i/y), on the other side of the upper cooling zone. The pressure seal of the reactor itself is a stainless-steel metal gasket.
(11) Akhtar, J.; Kuang, S. K.; Amin, N. S. Renew. Energy 2010, 35 (6), 1220–1227. (12) Mazaheri, H.; Lee, K. T.; Bhatia, S.; Mohamed, A. R. Biores. Tech. 2010, 101 (2), 745–751. (13) Yuan, X. Z.; Li, H.; Zeng, G. M.; Tong, J. Y.; Xie, W. Energy 2007, 32, 2081–2088. (14) Potic, B.; Kersten, S. R. A.; Prins, W.; van Swaaij, W. P. M. Ind. Eng. Chem. Res. 2004, 43, 4580–4584. (15) Ingram, L.; Mohan, D.; Bricka, M.; Steele, P.; Strobel, D.; Crocker, D.; Mitchell, B.; Mohammad, J.; Cantrell, K.; Pittman, C. U. Energy Fuels 2008, 22, 614–625. (16) Hassan, E. M.; Yu, F.; Ingram, L.; Steele, P. Energy Sources, Part A 2009, 31, 1829–1839. (17) Reed, T. B. Proceedings of thermochemical conversion programme annual meeting; Golden, CO, 1988; pp 248-258. (18) Bech, N.; Larsen, M. B.; Jensen, P. A.; Dam-Johansen, K. Biomass Bioenergy 2009, 33, 999–1011. (19) Chakinala, A. G.; Brilman, D. W. F.; van Swaaij, W. P. M.; Kersten, S. R. A. Ind. Eng. Chem. Res. 2010, 49, 1113–1122. (20) Matsumura, Y.; Minowa, T.; Potic, B.; Kersten, S. R. A.; Prins, W.; van Swaaij, W. P. M.; van de Beld, B.; Elliott, D. C.; Neuenschwander, G. G.; Kruse, A.; Antal, M. J. Biomass Bioenergy 2005, 29, 269–292.
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Figure 1. Reactor high-pressure system comprised of (a) reactor, (b) primary coolers, (c) flow pump, (d) tuning pump, (e) gas booster, (f) feed/ solvent reservoirs, (g) check valves, (h) loop selector valve, (i) burst diaphragm, (j) slurry injector, (k) lower bypass valve for injector loop, (m) upper bypass valve for injector loop, (n) depressurization valve for injector system, (o) upper primary exit valve, (p) lower primary exit valve, (q) main exit valve, (r) proportional relief valve (PRV), (s) thermostatic cooler, (t) thermostatic cooling water-supply loop, (u) magnetic coupling with electric motor.
Figure 2. (a) Reactor profile viewed from the side: (1) body, (2) head piece, (3) cover, (4) counter plate, (5) ring-joint gasket, (6) gold-plated ring, (7) hexagonal socket head cap screw, (8) dowel pin. (b) Reactor profile viewed from above showing borings for cartridge heaters (A) and drains (B).
The reactor heating is provided by a total of nine cartridge heaters, each with a power of 1250 W, giving a total of 11 250 W. This power-rating was chosen to enable the system to heat rapidly and thus give a relatively welldefined initial condition even for processes which for some reason might not be operable in the flow mode and need to be run as pure batch reactions. The heaters are placed in groups of three near the outer edge of the reactor block, with drain
borings in between (Figure 2) to take care of spills in the general area, and thus reduce the risk that liquid may find its way down along the cartridge heaters and potentially shortcircuit the system. Temperature control is gained through a set of thermocouples (TCs) and a controller unit. Six TCs were built into the reactor block itself, two at the bottom, two in the middle, and two at the top. Each reading is shown individually on 2739
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displays. The controller unit reads the average of each pair, and a selector switch enables the operator to choose which of the three to use for the heating control. As the bottom TCs are those which are placed nearest the reactor center, they give the most accurate representation of the temperature inside the reaction chamber. However, as the middle TCs measure closer to the cartridge heaters, they feel the enabled heat sooner than any of the other TCs, which means that a slower, better controlled heating rate is achievable by employing the middle ones during heat-up. The top TCs are mostly used when the system operates in the pure-flow mode (with upward flow), and this is hence of lesser relevance to the normal stop-flow mode of the system. Apart from the six TCs on the reactor, an additional TC is placed on the “cold” side of each of the two primary cooling zones (see Figure 1, T7-T8) to monitor the efficiency of the chillers to stop passive heat flow from the hot reactor during operation. They also serve as a handy guide to temperature when operating the reactor in the flow mode (whether it is pure-flow or stop-flow), as the cooling capacity of the primary chillers is always exceeded to some extent under these circumstances. To enhance the cooling capacity and enable a larger flow rate through the reactor in the downward direction, which is the one commonly used for feed injection and product collection in the stop-flow mode of this system, a secondary cooling zone was built into the system downstream of the first one (Figure 1s). This secondary cooler connects to a separate water circulation system, in which the water temperature is adjustable in the range from room temperature (RT) to 75 °C. Thus, the second cooler does not necessarily cool to room temperature. Instead, it enables the collection of process products at temperatures where the bio-oil produced does not clog or thicken as much or as easily as would have been the case if the collected fractions were cooled to RT immediately. The reactor is fitted with a mechanical stirrer, which transfers rotational motion through a magnetic coupling into the high-pressure environment. As the coupling cannot function at elevated temperatures, it is placed outside the primary cooling zone, between the bottom primary and secondary cooler (Figure 1u), and a shaft which extends all the way through the bottom primary cooling zone transfers the rotational motion to the (hot) reaction chamber. To further protect the magnetic drive from the slightly increased temperatures which occur in the flow mode (with downward flow), it is also fitted with a small, water-cooling unit of it own. The stirrer unit as a whole is constructed in a way which allows exchange between different stirrer modules, the simplest of which is an ordinary paddle-wheel. Another stirrer option is a rotating rack for catalyst pellets, which allows the entire catalyst bed of the reaction chamber to spin, thus simultaneously maximizing the mixing of species in the chamber and the contact between the fluid and the catalyst. Finally, to protect the reactor from corrosion and to provide a supposedly inert chemical environment, the reaction chamber and the pipes leading to and from it have all been coated internally with a thin foil of pure gold. A similar foil covers the upper part of the stirrer shaft and the catalyst bed itself. This is particularly relevant for treating feedstocks which are rich in protein, as shown by Kruse et al.21
2.3. The High-Pressure “Cold” System. For pressurization, a station with two single-acting, pneumatic high-pressure liquid pumps was constructed, one of which has a large stroke volume and flow capacity (1.5 mL/stroke), Figure 1c, for when feed is injected or collected from the reaction chamber (termed the “flow pump”) and one with a very small stroke volume (0.6 mL/stroke) for tuning the pressure or maintaining it at specific conditions (termed the “tuning pump”), Figure 1d. Both pumps are connected to two graded-cylinder reservoirs, each able to hold 250 mL, Figure 1f. A selector allows the operator to switch between reservoirs instantaneously, changing from, e.g., solvent to a dissolved process feed even between two subsequent strokes of the pump. The pressurization station also includes a pneumatic gas booster (Figure 1e), which allows the system to be pressurized or flushed with inert gases, e.g., argon. This is an advantage during product collection, as the reaction chamber may be continuously filled with argon gas while the liquid content is withdrawn, thus maintaining pressure at the desired process value without diluting the liquid fraction and with boiling of the liquid fraction continuously suppressed. As mentioned above, the operator is free to choose whether solvent (or feed or gas) is directed to the reactor from the top or from the bottom. This is true for the pressurization process of the reactor as well. Which route is taken is decided with the valve h in Figure 1, which switches between the upper or lower pipeline leading to the reactor. Each pipeline also connects to a primary exit valve, Figure 1, o and p. These valves are used when products are collected from the reactor;which one in particular depends on whether the product should be collected through the top or the bottom reactor outlet. They also enable the flow emanating from the pressurization station to “bypass” the reactor and leave the system without ever seeing the hot zone of the facility, see Figure 3. Apart from the primary exit valves, any flow leaving the high-pressure system passes through a main exit valve (Figure 1q), which is also a selector valve for determining whether it should go to the sample-collection drain or into the low-pressure part of facility, where separation of gases and liquids might take place. If the flow is to be collected, it finally passes through a Proportional Relief Valve (PRV), which is the unit permitting the pressure of the system to be adjusted to some specific value and maintaining it within approximately 5 bar even if additional pressure is applied. This is also extremely useful during heat-up, where the density of the solvent contained inside the reactor may drop significantly, giving rise to a severe increase in the overall system pressure. One last, significant part of the “cold” high-pressure system is the slurry injector. The central part is a tube possessing a volume of 240 mL, into which slurry feeds may be loaded. The injector tube is then connected to the high-pressure system by means of Swagelok unions. Thanks to valves k and m in Figure 1, the entire section of the highpressure system in which the injector tube sits may be shut off from the rest of the system, allowing the tube to be mounted even when the reactor itself is hot and pressurized. When the tube is pressurized, e.g., after an injection run (see below), the valve n in Figure 1 furthermore enables pressure to be relieved from the injector tube exclusively (provided that valves k and m are kept shut), enabling the operator to safely dismount the injector tube without having to cool down and depressurize the reactor. The injector tube may then be cleaned, refilled, and remounted for a subsequent injection.
(21) Kruse, A.; Krupka, A.; Schwarzkopf, V.; Gamard, C.; Henningsen, T. Ind. Eng. Chem. Res. 2005, 44, 3013–3020.
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Figure 3. P&I diagram of the high-pressure system displaying a reactor bypass through the upper loop (blue) and lower loop (red).
2.4. Process Run Overview (Feed Introduction). The introduction of a dissolved feed (e.g., an aqueous glucose solution) to the reactor is done directly from the flow pump, using one of the two reservoirs. Prior to injection, the reactor is filled with pure solvent (usually water), preheated, and pressurized. Upon commencing feed injection, the selector valve Figure 1h is momentarily closed, the selector switch on the pump reservoirs is set so that the pump draws liquid from the feed reservoir, and the pressure is increased slightly, usually 10-15 bar above the reactor set point. Note that the pressure increase is confined to the pressurization station at this stage. What remains is now for the operator to choose whether injection should take place through the upper or lower reactor inlet. Assuming, by way of example, that the bottom inlet is chosen, the upper primary exit valve (Figure 1o) is opened, the lower one is sealed (Figure 1p), and the selector valve h is opened in the direction of the bottom reactor inlet. Once there is connection between the pressurization station and the reactor, the excess pressure in the former will be relieved, as the PRV is still adjusted to the set-point pressure, which is lower than the pressure now provided from the pressurization station. The resulting pressure drop is immediately felt by the flow pump, which leaps into action in an effort to compensate. This in turn establishes a flow along the pathway shown in Figure 4. The amount of feed injected may be read off the grading on the feed reservoir. When the desired volume has been consumed, the selector valve h in Figure 1 is closed, thus disrupting the flow pathway, and the reactor pressure instantly relaxes to the set-point value once again. The overall injection procedure may be completed within 1-2 min, depending on the volume of feed involved. For the injection of feeds which are nonsoluble (slurries), the slurry injector is used. The injector tube is filled with the feed, and a small, special-made piston is added to one end before the tube is resealed. The tube is mounted between valves k and m, in Figure 1, so that the piston end connects to the pressurization station. Once mounted, pressurized (pure)
solvent from the pressurization station is then applied to the tube by opening the valves h and k (the relief valve n is kept closed). From this point on, the injection procedure is very similar to the one described above. While keeping the valve m closed, the pressure provided by the flow pump is increased slightly, and when valve m is then subsequently opened, the pressure drop (down to the system pressure set by the PRV) will activate the pump, establishing a flow pathway from the pressurization station through the injector, through the reactor, to the collection drain (see Figure 5). The piston inside the injector begins to move under the excess pressure from the solvent behind it, forcing the slurry in front of it out of the tube and down into the hot reactor. The volume of transferred slurry is equal to the volume of pure solvent consumed by the pump and may be read as the grading of the solvent reservoir. When the desired volume has been transferred, the valve m, in Figure 1, is closed, separating the injector from the reactor and causing the pressure to normalize to the value set on the PRV. Whether or not the injector is used is of course determined from the feed in question. In either case, each small unit volume of feed is heated from RT to a temperature very near the set point of the reactor upon passing through the primary cooling zone, a transition which takes 3-4 s at usual flow rates. This is vital when dealing with biomass-decomposition reactions, in order to leap across the temperature interval where the formation of tar and coke is favored (200-300 °C).6,22 2.5. Design Summary. The design of the present catalytic test facility provides the operator with good control of the temperature and pressure within a wide parameter space (up to 500 °C, 500 bar) and with a routine accuracy within 5 °C and 5 bar. Pressure is controlled independently of temperature, and the system may be pressurized with liquids or gases alike, which is particularly useful during product collection, as the loss of system pressure may be compensated (22) Zhang, B.; Keitz, M. v.; Kenneth, V. Appl. Biochem. Biotechnol. 2008, 174, 143–150.
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Figure 4. Flow pathway at injection of a dissolved feed through the lower loop of the reactor system.
Figure 5. Flow pathway at injection of feed using slurry injector. The outline shows the solvent path (red) and feed path (blue).
near-RT (20-70 °C, depending on operator choice). Hence, the overall margin on the duration of a process run is approximately 1-2 min, which is satisfactory for most runs, as each of these normally takes 30-60 min. The system is adaptable to suit very short runs as well, but as this feature has not been used in the present studies, the matter will not be discussed further here. Reservoirs connected to the pumps can provide either solvent or dissolved feed. Nonsoluble feeds may be injected into the reactor by means of the slurry injector unit, which may be exchanged and refilled even while the reactor is hot
for with (argon) gas, thus preventing boiling in the reaction chamber. Thanks to the stop-flow capability of the reactor, feed may be injected into a preheated, pressurized reactor, giving rise to a very rapid temperature increase on each small unit volume of feed. As the injection may usually be completed within 1 min, the procedure constitutes a well-defined temporal starting point for a process run. Likewise, at the end of a run, the product may be collected within a similar time window, and each small unit volume of the product stream is cooled within seconds from the process temperature to 2742
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e.g., for the decomposition of fatty acids. DDGS is a waste product from the fermentation process in ethanol production and consists mainly of cellulose, protein, fatty acids, and starch, with specific contents depending on the raw-material used for the particular ethanol-production process. This particular batch of DDGS was delivered through SCF Technologies a/s from Agroetanol (Sweden). A fatty-acid content of 4.02 ( 0.28 w% was determined by extraction with pentane. Beyond this, no detailed analysis of the DDGS composition has been made, since the present studies are comparative. The use of coffee grounds for biomass feedstock is explorative in nature, aimed at a basic comparison with the DDGS feed. In this paper, it mainly serves to illustrate constituent analysis of the bio-oil. The study was motivated by the interesting notion that great amounts of bio-oil (predicted 1.3 billion L26) can be created from spent coffee. This feedstock should be well suited to the CatLiq process, considering the high surface area and high water content (65-80%), which makes it expensive to incinerate. Extraction of the oil content has been conducted before,26 but never previously (to the best of our knowledge) by the use of a hydrothermal liquefaction process, as is used in this work. Prior to all experiments, a slurry was made of 20 wt % DDGS in water. A total of 10 wt % K2CO3 (Sigma-Aldrich) was added to the feed as homogeneous catalyst. ZrO2catalyst pellets were supplied by SCF Technologies a/s. Three experimental runs of three consecutive experiments each were conducted, two with DDGS and one with coffee grounds. All used the slurry injector, according to the experimental procedure outlined above. The effect of the heterogeneous ZrO2-catalyst was tested with the DDGS feed, of which the first run used a catalyst loading equal to 900 m2 total surface area present in the reactor, while the second run was done without heterogeneous catalyst. The run using coffee grounds also used 900 m2 of ZrO2 catalyst. In each of the nine experiments, 200 mL of slurry was used (total injection time approximately 120 s), and the subsequent run time was 45 min. The process temperature and pressure were 350 °C and 250 bar, respectively. At the end of each run, the reactor was flushed with 200 mL of water (total injection time approximately 90 s.) and then emptied by displacing the liquid from the reaction chamber with pressurized argon gas. The chamber was then filled with fresh, demineralized water and flushed for 4-5 min before the next injection commenced. The heterogeneous catalyst was deliberately not replaced between the three experiments of the first run. The rationale was to monitor the conversion efficiency, which often varies initially when a “fresh” heterogeneous catalyst is used, and hence to obtain a more realistic picture of the catalytic effect. 3.2. Illustrative Examples. All the collected products had an oil layer floating on top (Figure 7A) of the aqueous phase, and another one resting at the bottom. To obtain comparative results of yield, which should also include oil components emulsified in the aqueous phase, an extraction procedure was developed inspired by Karag€ oz et al.27 First, the upper oil layer was isolated manually, and the mass determined. The aqueous phase was carefully separated
Figure 6. Photo of the reactor.
and running. As it appears, the facility overall allows multiple test runs to be carried out in sequence without cooling down or depressurizing, as any product recovery may be followed by a thorough flushing and refilling of the reactor with pure solvent. As the reaction chamber and all its devices (stirrer, catalyst bed, etc.) are plated with gold foil, the catalytic processes takes place under presumably inert, noncatalytic conditions. This also significantly reduces the risk that corrosion products coming from the steel walls of the reaction chamber may end up in the chamber during a process run. Particularly, nickel, which might leach from the reactor walls, is a concern in this respect, as it has proven to be an active catalyst in biomass degradation reactions.8,23,24 A photo of the reactor is shown in Figure 6. In summary, this reactor has been developed and built with the aim of studying biomass conversions into bio-oil from the “applied science” perspective, hence the focus on the conversion of real biomasses. Its main purpose is to distinguish effective catalysts from ineffective ones on the basis of simple output parameters such as oil yield, product compositions, etc., and to do such investigations in an efficient manner and with the rigid parameter control necessary for the potential transfer of results to other systems, e.g., pilot-scale plants. Due to the complex chemistry involved in the conversion of real biomasses, it is not a primary target to study specific reactions in any detail beyond basic constituent analysis of the as-produced oils. 3. Results and Discussion 3.1. Experimental Procedure. As part of an ongoing study, the catalytic effect of monoclinic zirconia (m-ZrO2) on the conversion of (i) Dried Distiller’s Grains with Solubles (DDGS)9,10 and (ii) spent coffee grounds into bio-oil has been explored. Catalytic effects of ZrO2 under hydrothermal conditions have been reported previously in the literature, (23) Sina g, A.; Kruse, A.; Rathert, J. Ind. Eng. Chem. Res. 2004, 43, 502–508. (24) Minowa, T.; Zhen, F.; Ogi, T. 4th International Symposium on Supercritical Fluids, Sendai, Japan, May 11-14, 1997; Elsevier Science: Sendai 1997; pp 253-259. (25) Watanabe, M.; Iida, T.; Inomata, H. Energy Convers. Manage. 2006, 47, 3344–3350.
(26) Kondamudi, N. M.; Susanta, K.; Misra, M. J. Agric. Food Chem. 2008, 56, 11757–11760. (27) Karag€ oz, S.; Bhaskar, T.; Muto, A.; Sakata, Y.; Uddin, M. A. Energy Fuels 2004, 18, 234–241.
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Table 1. Oil Yields and TOC Values from the DDGS Experiments for Different Catalyst Loads m-ZrO2 catalyst (total m2)
oil yield (% of DDGS)
TOC (g/L)
1/1 2/1 3/1 1/2 2/2 3/2
900 900 900 0 0 0
17.08 ( 0.42 22.68 ( 0.56 24.53 ( 0.61 11.71 ( 0.29 17.03 ( 0.43 8.97 ( 0.57
14.9 14.4 (;)a 30.9 33.0 45.9
a
m-ZrO2 catalyst (total m2)
oil yield (% of DDGS)
TOC (g/L)
1/3 2/3 3/3
900 900 900
20.4 31.8 30.7
5.85 8.08 4.82
phenomenon is naturally a subject of some concern, and this initial study is therefore now being expanded with comprehensive experimental runs, to achieve an improved experimental basis. The results of these experiments will be published elsewhere. Table 2 show the oil yields from the experiments using coffee grounds as feed. Yields are similar to those achieved with the DDGS feedstock, indicating that spent coffee is an attractive feedstock for bio-oil production through hydrothermal liquefaction. It is worth noticing the low TOC values, which can be interpreted as a really effective biomass conversion to bio-oil or as higher gas promotion. These initial results have now motivated a further investigation which includes uncatalyzed experiments to test the effect of ZrO2 with this feedstock. The bio-oils produced from the coffee grounds feedstock were analyzed by gas chromatography coupled with mass spectrometry (GC-MS, Clarus 500, PerkinElmer, USA) after dilution to approximately 7.5 g/L using ethanol. Individual compounds were separated using a ZB-5 column (60 m, 0.25 mm I.D., film thickness of 0.25 μm, Phenomenex), helium as the carrier gas (flow 1 mL/min), and a splitmode injection of 1:9. The temperature gradient was the following: start at 70 °C, 1 °C/min for 30 min, kept at 100 °C (5 min), 4 °C/min to 250 °C, and finally 15 °C/min to a final temperature of 300 °C. The total run time was 83 min. The MS was running in electron impact mode (70 eV). Internal standards were 4-bromotoluene and 2-bromoethyl benzene. Further details of the analytical method are described by Glasius et al.29 Figure 8 shows an example of a total ion chromatogram (TIC) from GC-MS analysis of bio-oil from coffee grounds produced in the present facility. Most compounds are tentatively identified from library search matches. The peak numbers correspond to the numbers in Figure 8. The peak areas denote the contribution of the compound to the total peak area in the chromatogram. The chromatogram shows about 100 compounds, but a majority of these are only found at trace levels, since 30 peaks contribute with 94% of the total peak area. Specific compounds accounting for 93% of the total area were identified by comparison with the NIST library of mass spectra (Table 3). The compounds contributing most to the total peak area were palmitic acid (C16:0) > linoleic acid (C18:2) > phenol. Authentic standards confirmed the identification of these compounds. Even though a comparison of peak areas is somewhat inaccurate without time-consuming analysis of authentic standards to give calibration curves, the results provide an indication of the distribution. It is surprising that half of the total peak area originates from only two compounds, palmitic acid and linoleic acid. This indicates that bio-oil from used coffee grounds has potential for further processing to isolate fatty acids that are of interest for biofuel production.
Figure 7. (A) Example of as-collected top-oil from DDGS floating on the aqueous fraction. (B) Extraction of emulsified oil constituents from the aqueous phase (first extraction).
experiment/run
experiment/ run
An experimental error made this TOC determination unreliable.
from the lower oil layer by decantation; 20 mL was taken aside for Total Organic Carbon (TOC) analysis (Hach and Lange DR2800 w. TOC-kit LCK 387) and the remainder acidified to pH = 2 with 4 M HCl and extracted two times with 100 mL of diethyl ether (Aldrich), Figure 7B. The combined ether phases were then dried with anhydrous sodium sulfate and evaporated on a rotation evaporator (Heidolph Laborota 4001), after which the mass of the extraction yield was determined. The bottom oil layer was shaken vigorously with 20 mL of demineralized water to remove salt contents, and the aqueous phase was then acidified and extracted with 25 mL of diethyl ether to recover any emulsified oil constituents and the yield determined as outlined above. The oil fraction of the bottom layer was dissolved in acetone, evaporated on rotation evaporator, and the mass of the yield determined. The total oil yield of any experiment is the sum of these four components. The top-layer oil, the aqueous-phase oil, and the combined oil extracted from the bottom layer were then each dissolved in ethanol, since this slows down aging phenomena such as repolymerization.28 The oil-yield and TOC results are shown in Table 1. Though the trend is not entirely unambiguous, there is a clear indication that the ZrO2 catalyst has a positive effect on the oil yield. The increase in yield from experiment 1/1 to 3/1 is a sign of the gradual activation of the catalyst. Assuming that variations in gas evolution and ash formation are small, a decreased oil yield should correspond to an increased TOC value. This trend is indeed generally observed, though experiment 2/2 is an outlier in this respect (also regarding the oil yield). A likely explanation may be that more DDGS has been caught in the reactor upon injection, causing an artificial increase of both values. However, any outlier (28) Diebold, J. P. A Review of the Chemical and Physical Mechanisms of the Storage Stability of Fast Pyrolysis Bio-Oils. http://www. p2pays.org/ref/19/18946.pdf (accessed Mar 2010).
(29) Glasius, M.; Villadsen, S. R.; Forsberg, R.; Becker, J.; Rudolf, A.; Iversen, S. B.; Iversen, B. B. In preparation.
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Figure 8. GC-MS total ion chromatogram of bio-oil produced from coffee grounds. Peak numbers are shown for peaks included in the data analysis in Table 3. Note the difference between scales. Table 3. Identification of Compounds in the 30 Largest GC-MS Peaks in Bio-Oil from Coffee Grounds peak (number)
RT (min)
peak area (%)
compound
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
8.030 8.566 8.796 11.141 12.884 14.623 15.740 16.576 16.742 18.690 21.844 22.345 23.397 23.848 24.640 24.815 25.877 26.158 29.058 34.002 42.733 43.895 55.813 67.826 68.287 71.883 72.159 72.264 72.705 73.612
0.3 0.3 0.8 4.2 0.5 1.2 9.2 0.4 2.1 0.4 0.8 0.6 0.7 2.1 1.3 0.8 0.5 0.5 0.8 0.6 0.3 0.4 1.0 32.4 5.2 18.8 1.1 4.3 1.9 0.3
2-ethoxytetrahydrofuran 2-methylcyclopentanone 3-methylcyclopentanone and isovaleric acid 2-methyl-2-cyclopentenone unknown, multiple peaks possibly 3-methyl-2-cyclopentenone phenol 3,4-dimethyl-2-cyclopenten-1-one 2,3-dimethyl-2-cyclopenten-1-one 4,4-dimethyl-2-cyclopenten-1-one o-cresol 3,4,5-trimethyl-2-cyclopenten-1-one 3-ethyl-2-cyclopenten-1-one p-cresol (major) and m-cresol (minor) likely trimethyl-cyclopentanone and unknown (two peaks) 2,3,4-trimethyl-2-cyclopenten-1-one 5-ethyl-2-furaldehyde MS similar to peak #17 MS similar to peak #17 p-ethylphenol possibly 4,5,6,7-tetrahydro-2-indanone 2-indanone and 1-indanone 2,6-di-tert-butyl and p-cresol palmitic acid palmitic acid, ethyl ester linoleic acid unknown; very likely C18:2 derivative oleic acid 9-octadecenamide, hexadecanamide and probably stearic acid, ethyl ester unknown
Table 4. Overview of the Contribution of Major Compound Classes to the Total Peak Area in Figure 8 peak area
phenols
indanones
pentacyclic ketones
fatty acids
amides
other
not identified
13.7%
0.7%
12.8%
61.8%
1.9%
2.1%
7.0%
Other fatty acids were also present in the bio-oil, so altogether fatty acids contributed to about two-thirds of the total peak area and were thus the major group of compounds (Table 4). Other major groups included phenols and pentacyclic ketones. Detection of ethyl esters of fatty acids can be attributed to esterification between fatty acids and the storage medium ethanol. Further analytical work involves analysis of authentic standards of identified compounds, in order to accurately quantify the contribution of each compound, as well as derivatization of fatty acids for improved GC-analysis.
4. Conclusion A test facility has been constructed for catalytic processes taking place at high temperatures and high pressures. The stop-flow design as well as other features of the facility provides good control over the three main parameters, temperature, pressure, and residence time. Furthermore, the gold lining of the reaction chamber provides a chemically and catalytically inert reaction environment. The design makes the facility well-suited, e.g., for studying hydrothermal conversion of biomasses into bio-oil. As a proof of concept, 2745
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: DOI:10.1021/ef901584t
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results on the conversion of DDGS and spent coffee grounds into a fatty-acid-rich bio-oil were given. The overall process is a promising technology, which in the future may play an important role in waste removal as well as production of CO2neutral oil for “green” electricity production or automotive purposes. In this initial exploration, a positive effect of the ZrO2 catalyst is revealed in the form of improved oil yield, but further studies are necessary to reach comprehensive conclusions. The study shows that the new test facility is well suited for optimization studies of high pressure and high
temperature catalytic reactions, and a very large parameter space potentially can be explored. Acknowledgment. The authors gratefully acknowledge the help of Palle Kjær Christensen, Erik Ejler and Eigil Hald for their extensive help with the construction and mounting of this facility, including the machining of the reactor itself. Preben Sottrup from SCF Technologies is acknowledged for his help with the electrical systems. This work was supported by the Danish Advanced Technology Foundation, the Danish National Research Foundation and the Danish Strategic Research Council.
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