Article pubs.acs.org/est
A Feasible Way to Remove the Heat during Adsorptive Methane Storage Stefan Gütlein,† Christoph Burkard,† Johannes Zeilinger,† Matthias Niedermaier,† Michael Klumpp,† Veronika Kolb,† Andreas Jess,‡ and Bastian J. M. Etzold*,† †
Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Lehrstuhl Chemische Reaktionstechnik, Egerlandstrasse 3, 91058 Erlangen, Germany ‡ Universität Bayreuth, Lehrstuhl für Chemische Verfahrenstechnik, Zentrum für Energietechnik (ZET), 95440 Bayreuth, Germany S Supporting Information *
ABSTRACT: Methane originating from biogas or natural gas is an attractive and environmentally friendly alternative to gasoline. Adsorption is seen as promising storage technology, but the heat released limits fast filling of these systems. Here a lab scale adsorptive methane storage tank, capable to study the temperature increase during fast filling, was realized. A variation of the filling time from 1 h to 31 s, showed a decrease of the storage capacity of 14% and temperature increase of 39.6 °C. The experimental data could be described in good accordance with a finite element simulation solving the transient mass, energy, and impulse balance. The simulation was further used to extrapolate temperature development in real sized car tanks and for different heat pipe scenarios, resulting in temperature rises of approximately 110 °C. It could be clearly shown, that with heat conductivity as solei mechanism the heat cannot be removed in acceptable time. By adding an outlet to the tank a feed flow cooling with methane as heat carrier was realized. This setup was proofed in simulation and lab scale experiments to be a promising technique for fast adsorbent cooling and can be crucial to leverage the full potential of adsorptive methane gas storage.
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INTRODUCTION Methane, originating from biogas or natural gas, is an attractive fuel for vehicles. Using methane as an automotive fuel exhibits considerable advantages like lower vehicle emissions and fuel costs when compared to gasoline. However, the low volumetric energy density and therefore the resulting low driving range can be problematic.1−8 The most common storage technology for increasing the energy density is compression (called also compressed natural gas, CNG) at about 20 MPa.7,8 The high pressure results in high compression costs and requires heavy, cylindrical tanks, which are difficult to integrate on-board.7 One possibility to lower the storage pressure while still maintaining high storage capacities is adsorption on microporous materials (also called adsorbed natural gas, ANG).1,5,7−10 Highly microporous carbon adsorbent as used in this study or other porous materials like metal organic frameworks show high gravimetric storage capacity.4,6,8,11 The high gravimetric density correlates with high volumetric one if the bulk density of the adsorbent is high.8 Hence, monoliths are preferred to powder beds, as the porosity of the bed lowers the density unnecessarily. Regarding carbon, monolithic adsorbents are available, for example, from pitch-12 or polymer-based13 activated carbons or carbidederived carbons (CDC),14−17 which are used in this study. In short CDCs are obtained by selective extraction of the © 2014 American Chemical Society
noncarbon species from carbides. The process is conformal and porous carbon with a tunable and narrow pore size distribution can be obtained.18−23 A good performance of CDC for adsorptive methane gas storage and high volumetric storage capacity achievable with monolithic one were reported.16 Thus, in principle ANG systems using monolithic carbonaceous adsorbents showing high gravimetric and volumetric storage capacity are available. Why only in principle? The referred high storage capacities are determined under ideal conditions. Long equilibration times, resulting in experiments up to several hours, are applied to determine isothermal, close to equilibrium conditions. However, for technical applications high filling rates are required. Filling of a vehicle at the gasoline station should be finished in several minutes. As the heat of adsorption is released and needs to be dissipated in this short period of time, a nonisothermal adsorption results and remarkable hot spots can be formed.5,9,10,24−29 Adsorption capacities at higher temperatures are lower, thus the technically achievable capacity is reduced. Despite its importance for Received: Revised: Accepted: Published: 672
August 23, 2014 November 24, 2014 December 8, 2014 December 8, 2014 dx.doi.org/10.1021/es504141t | Environ. Sci. Technol. 2015, 49, 672−678
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signal of a high pressure mass flow controller. Additionally the pressure and temperature inside of the tank is recorded with a resolution below 1 s. The tank is immersed in a temperature controlled oil bath. The dead volume of the total setup contributing to compressive storage is minimized (approximately 10 mL). To determine the adsorptive storage, two experiments are carried out. One with adsorbents, where adsorptive and compressive storage takes place in parallel and one without adsorbents, to determine the amount of solely compressive storage. Highly nonisothermal experiments are performed with high methane mass flow rates and pseudoisothermal ones with very low flow rates. Prior each measurement the adsorbent is outgassed directly in the storage tank at 120 °C for 2 h at a vacuum of 10 mbar. Detailed information about the setup and data evaluation is given in the Supporting Information (SI). Lab Scale Feed Flow Cooling Experiments. The setup for the nonisothermal experiments was modified for the feed flow cooling experiments. At the bottom of the adsorption tank a 1/16 in. stainless steel capillary was added as outflow and connected to a needle valve. A Trinamic TMCM-113 V.3.40 stepping motor is coupled with a self-built connector to the needle valve. The pressure is controlled for approx. three turns of the needle valve, which corresponds to approx. 25 000 positions of the stepping motor. Simulation. The commercial software Comsol Multyphysics (version 4.1.0.185) was used to setup the finite element simulation. Classical engineering equations like the Navier− Stokes and Brinkmann equation for the fluid flow, a first order reversible kinetic for the sorption using a Dubinin-Astakhov isotherm for temperature dependent equilibrium loading and a homogeneous single phase model for the solid and gas phase heat balance with a flow dependent effective heat conductivity were used. Further details and parameters are given in the SI.
technical realization, studies on fast and nonisothermal methane adsorption are scarce. In this work the resulting temperature rise during technical, hence fast adsorption was studied experimentally and by a chemical engineering simulation. Further, the simulation was used to deduce technical possibilities for an effective fast heat removal. The most promising approach, a feed flow cooling was further proofed in lab scale.
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EXPERIMENTAL SECTION Material. As precursor material for monolith cylindrical SiSiC (10 mm in thickness and 49 mm in length, SiSiC containing 14 wt % of free silicon, Schunk Ingenieurkeramik GmbH) was converted to monolithic CDC like described in detail previously elsewhere.17 Briefly, the chlorination was conducted in a horizontal tubular alumina reactor (length: 1300 mm, diameter: 32 mm) protected by a graphite foil heated with a controlled kiln. All gases (Linde Gas: He 5.0, Cl2 2.8, H2 5.0) were dosed via mass flow controllers. The samples were placed in the isothermal zone of the reactor. The exhaust gas is neutralized by a 30%-KOH solution. Under helium the reactor was heated up to reaction temperature (1000 °C). The reactive extraction was carried out with 1−2 mol m−3 of chlorine in helium at a superficial velocity was 0.05 m s−1. After the reactive extraction the samples were treated for 30 min in hydrogen gas flow at 1000 °C to desorb/react residual chlorine. For the feed flow experiments commercial polymer-based spherical activated carbon (PBSAC, No. 100772) provided by the company Blücher GmbH were used. The particle size was approximately 480 μm. A pore volume of 0.677 cm3 g−1 and specific surface area of 1365 m2 g−1 was determined. Characterization. For the CDC synthesis the degree of conversion was determined gravimetrically to be above 99%.30 The density was determined assuming an ideal cylinder, measuring the diameter and height and the weight of the monolith. The specific surface area was determined with N2sorption with monolithic samples of smaller size using a Quantachrome QuadrasorbSI measuring at 77 K. Subsequent data evaluation was performed with the software QuadraWin version 5.02 using Quenched Solid Density Functional Theory (QSDFT) for carbon slit pores. Magnetic Suspension Balance Experiments. Experimental investigations to determine the isothermal methane adsorption uptake were carried out in a magnetic suspension balance (Rubotherm GmbH, Bochum, Germany). In this type of balance, the sample holder couples only electromagnetically with the microbalance, which is located outside the chamber under standard ambient temperature and pressure. This special magnetic coupling setup allows measurements of mass changes of samples under elevated temperatures and pressures (T < 250 °C, p < 300 bar). For the experiment a sample of the adsorption material was placed in the measuring chamber and conditioned (dried/ cleaned under vacuum (10−3 mbar) at 150 °C). After reaching a constant mass signal, the measuring cell was cooled down to the desired temperature (22 °C, 44 °C, 80 °C) and pressurized with methane. The mass increase due to methane adsorption was detected for each measurement point after reaching steady state, hence equilibrium. This procedure was repeated for different isobaric conditions. Non-Isothermal and Pseudo-Isothermal Adsorption Experiments. The mass of methane delivered to a lab sized adsorptive storage tank (3.7 mL) is obtained by integrating the
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RESULTS AND DISCUSSION Testing Reproducibility and Accuracy of the Measurement Equipment. To measure the temperature rise and adsorption capacity at fast filling rates an experimental setup was designed and constructed in house. With the setup the final storage pressure of 3.5 MPa can be reached within hours or in less than 1 min. Details of the setup and data evaluation are given in the Experimental section and the SI. As adsorbent monolithic CDC obtained from commercially available silicon infiltrated silicon carbide (SiSiC) was used. We recently demonstrated that using this raw material very fast CDC production rates can be obtained, even for monoliths.17 The density of the monolith used, was determined to be 765 kg m−3. From low temperature N2-sorption measurement and the QSDFT model fit a specific surface area of 1410 m2 g−1 and a pore volume of 0.46 cm3 g−1 results for the adsorbent. As a novel built setup was constructed, the reproducibility and accuracy was studied first. At slow filling rate (filling time approximately 1 h) and 25 °C the storage capacity was measured nine times and a relative standard deviation of the final capacity below 0.4% resulted (see Figure S3b in the SI). During the slow filling a maximum temperature difference of 1.3 °C was recorded, hence the measurement nearly meets isothermal conditions. Such slow measurements are called pseudoisotherm, subsequently. To check the accuracy of the self-built setup pseudoisotherms obtained at 25, 40, and 80 °C were compared to equilibrium storage capacities determined 673
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with a magnetic suspension balance at selected pressures between 0.2 and 3 MPa and similar temperatures. Figure 1 shows the pseudoisotherms obtained with the selfbuilt setup. As expected at low pressures an initially strong
Figure 2. Comparison of the isothermal adsorption capacity at three different temperatures determined with a magnetic suspension balance (represented by symbols) and the self-built rig (represented by dashed lines).
Figure 1. Pseudoisotherms determined experimentally with low filling rates at three different temperatures and corresponding fit with the Dubinin-Astakhov isotherm (see SI equation S16 and S17).
increase of stored methane could be observed whereas the slope for further storage at higher pressures decreased. As adsorption is an exothermic process the final storage capacity increases at lower temperatures. In Figure 1 additional a fit of the data with the Dubinin-Astakhov isotherm (see SI equations S16 and S17, parameters: b1 = 0.158, b2 = 0 K, n = 1.84, Esorp = 10 kJ mol−1) is given and shows good accordance to the experimental data. The results of the magnetic suspension balance are compared to the ones obtained by the self-built setup (represented by the Dubinin-Astakhov fit to account for the slight temperature difference between the different measurement principles of up to 4 °C). It is worth to stress that for the self-built setup the mass of adsorbed methane is determined by integrating the flow rate over the time, hence also the error of the flow rate measurement accumulates during the integration. In case of the magnetic suspension balance the true increase in mass due to methane adsorption is measured as a final value. In this view the very good agreement between the different measuring principles observed in Figure 2, indicates a very low error occurring in the self-built setup. Temperature Increase with Faster Filling. Besides proofing the good reproducibility and accuracy of the selfbuilt setup, experiments with different filling rates were carried out, to mimic the technical adsorptive methane storage, for example, at a gasoline station. In eight experiments the filling time to reach 3.5 MPa was reduced from approximately 1 h to 31 s. The measured storage capacities are given in Figure 3a. The storage capacity achieved at maximum pressure decreases by 14% from 11.3 to 9.7 kgCH4 kg−1carbon when filling faster. For very fast filling a delay for the adsorption is observed. It takes three measurements points or 2.4 s until an increase in the methane uptake is recorded. This could either be a real kinetic effect of the adsorption or a measurement artifact for very fast filling from the mass integration or a combination of both. In Figure 3a the temperature change measured in the center of the monolith is given and shows the reason for the drop in storage capacity. Speeding up the filling, the temperature increases strongly. For the filling in 31 s the temperature rises by 39.6 °C.
Figure 3. (a) Experimental results of the adsorptive storage for different filling times until 3.5 MPa is reached (66.3, 18.3, 5.43, 2.64., 1.73, 1.03, 0.73, and 0.52 min). upper: amount of adsorbed methane, lower: temperature rise in the center of the adsorbent; (b) amount of methane adsorbed at 3.5 MPa versus the average temperature during the adsorption for slow and fast filling experiments (dashed line gives the trend).
Independent from the filling rate, the temperature increase always runs through a maximum, which can be explained as most of the heat of adsorption is released in the beginning, where methane uptakes are higher. Hence, the interplay of heat generation due to adsorption and heat removal through conduction in the monolith and the tank walls needs to show a maximum in the temperature increase. It can further be observed that the maximum is shifted toward higher pressures for faster filling. Figure 3b compares the achievable storage capacity with the average temperature for pseudoisothermal and nonisothermal (fast filling) measurements. A clear correlation can be seen and 674
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there is no doubt that the decrease of methane adsorbed in case of fast filling can be attributed to the temperature increase. It needs to be pointed out, that for the lab scale system the ratio of heat generation to heat removal through the walls is lower than for a technical system, which would be used, for example, in a car. Accordingly, for technical systems the temperature effect is even more pronounced. Simulation of the Transient Adsorption Process and Feed Flow Cooling. For further insight into the nonisothermal adsorption a finite element model solving the transient impulse-, heat- and mass-transfer-balance was set up. For further details see the Experimental section and SI. First the filling process of the self-built rig was modeled and later this simulation was modified and extended to study real-size tanks. While most data was deduced from known empiric correlations, the heat of adsorption ΔHads and effective heat conductivity λeff in the adsorbent bed are unknown and therefore remain fitting parameters. Nevertheless, these parameters can be constrained by literature values. ΔHads is in the range of 10−20 kJ mol−13,5,8,24,31 and typical effective heat conductivities of porous carbons are around 0.1−1.5 W m−1 K−1.3,5,32−34 From the parameter fit ΔHads = −16.5 kJ mol−1 and λeff = 0.335 W m−1 K−1 resulted, which both is in good accordance to what is expected from literature. Figure 4 compares the experimental (continuous lines) with simulation (dashed orange lines) results for the four experi-
Figure 5. Scheme of tanks simulated (blue = gas phase, green = adsorbent, red = alumina heat pipe, gray = steal tank wall); (a) simple car sized tank; (b) car sized tank with an aluminum heat pipe lance; (c) car sized tank with aluminum heat pipe discs; (d) scheme for feed flow cooling.
of adsorbent was implemented in the finite element simulation (for details on the simulation see the SI). To study only the influence of the inner heat transfer the outer heat transfer was diminished by choosing a very high outer heat transfer coefficient. In technical realization this could be achieved by increasing the outer surface area or additional forced convection. In simulation the tank was filled to 3.5 MPa within 5 min and given another minute to cool in ambient air. Beside the simple tank, which represents the boundary case of maximum temperature rise, tanks with two different internals for improved heat removal were simulated. These internals are (i) an alumina heat pipe lance in the center of the adsorbent and (ii) heat pipe discs segmenting the adsorbent into 6 sections, while gas can pass through a hole in the middle (Figure 5b and c). Technically it could also be tried to remove the heat not with adding internals but by increasing the heat conductivity within the adsorbent, for example, by graphitic additives. To account for this option a simulation without internals but applying a 10 times higher effective heat conductivity in the adsorbent bed was performed. Further details to the real sized car tank models can be found in the SI. In Figure 6 the simulated maximum temperature increase inside the adsorbent resulting for the different cases is given vs the time. All simulations show a very fast temperature rise of approximately 60 °C in the first seven seconds. For the simple tank a continuous temperature increase of up to 135 °C is observed until the end of the 5 min of filling. In the subsequent cooling period of 1 min nearly no heat is removed and hot spots of up to 198.9 °C remain in the tank. Although in the case study of the heat pipe lance the internal surface area, available for heat removal inside the bed, is enhanced, only a minor effect in the temperature profile can be seen. For the heat pipe disc a different profile results. In the first 3 min a higher temperature rise is observed. This can be attributed to the different fluid dynamics. Here in the first segment of the adsorbent, separated by the discs, the temperature increases faster. The temperature rise after the cooling time is 106.1 and 105.4 °C for the heat pipe lance and discs, respectively. For the simple tank using an
Figure 4. Comparison of experimental results (continuous lines) and finite element simulation (orange dashed line) for four different filling times (time to reach 3.5 MPa) showing a pronounced temperature increase. upper: Time dependent pressure inside of the tank; lower: Time dependent temperature at the center of the adsorbent.
ments with highest filling rate, hence, the most interesting ones regarding pronounced temperature increase. As it can be seen the simulation describes the pressure increase over the time very accurately. The time curve of the temperature is represented correctly and all features as the fast initial rise, the maximum and the subsequent decrease are shown, although the initial slope seems not to be catched perfectly in simulation. Thus, despite the simplifications made during the simulation, it represents a good description of the nonisothermal adsorptive methane storage. After proofing the good fit of the simulation with the experimental data of the lab scale setup, the simulation was used to estimate the influence of fast filling on the temperature rise for real sized car tanks and storage of approximately 5 kg of methane. Accounting for the storage capacity this results in approximately 50 kg of adsorbent. A simple single cylindrical tank built of stainless steel (see Figure. 5a) as tank for the 50 kg 675
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which lead to an aging of the adsorbent, could be reduced. Further the enormous amount of heat released could be used externally, for for heating. One might ask where this energy made available comes from. More or less it is the energy withdrawn from the environment during desorption. When releasing the methane the adsorbents cools down slightly and heats up to ambient conditions by withdrawing energy from the environment. The temperature decrease of the adsorbent during desorption is by dimensions lower, as the time scale is by dimensions higher, compared to the filling. That is why simple “heating” with the surrounding air is working, while for the cooling during filling this does not work. Of course driving force to withdraw the energy from the environment is the pressure difference, hence, it could also be stated that some of the compression energy is recovered. Lab Scale Proof of Principle of the Feed Flow Cooling. As a proof of principle of the feed flow cooling the lab-scale tank was modified with an additional outlet. As backpressure controller a needle valve actuated by a stepping motor was used. Varying the closing speed of the needle valves at constant filling rate, different times to reach 3.5 MPa were realized. As adsorbent commercial polymer-based spherical activated carbon (PBSAC) provided by the company Blücher GmbH was used. Figure 7 gives the pressure and temperature increase over time for a completely closed valve, resembling the classical dead
Figure 6. Simulated maximum temperature rise inside the different car sized adsorptive storage tanks and for a feed flow cooling tank (see Figure 5a-d). The insert is a magnification of the upper right region. The dashed black line shows the time when 3.5 MPa are reached, filling ends and subsequent additional cooling begins.
adsorbent with heat conductivity increased by a factor of 10, the final temperature rise lowers to 94.0 °C. However, all simulations show clearly, that with heat conduction as the only mechanism for heat removal, the huge amount of heat released during adsorption cannot be dissipated effectively, independent if internals are used or not. It needs to be stated that other adsorbents might give different results, but the resulting trend is thus obvious, that the observation can be generalized. Beside heat conductivity an additional mechanism for heat removal needs to be applied. Heat transport by radiation can be neglected, as the temperature difference is too low, thus convection remains as possible mechanism. For a convective cooling a fluid needs to heat up and be transported out of the system. One might think of water pipes going through the adsorbent as a technical solution. Nevertheless, this complicates the tank design strongly, increases the dead volume of the tank and beside methane also cooling water needs to be pumped through the fuel hose at the gasoline station. An option might be to use the methane itself as heat transport fluid/cooling agent. We call this principle feed flow cooling. Figure 5d shows a flow sheet, where instead of the dead end tank with a single opening a tank with an inlet and outlet is used. Thus, methane can flow through the adsorbent, heat up, adsorb, and nonadsorbed methane leave the tank at the outlet. To increase the pressure during filling a backpressure valve is needed. This feed flow cooling mechanism with cold feed entering at the top of the cylindrical tank and leaving at the bottom was also implemented in the finite element simulation. The maximum temperature rise inside the tank resulting in the simulation is given in Figure 6. The feed flow cooling shows a temperature increase up to 90 °C in the first 50 s. Later on the maximum temperature inside of the tank stays nearly constant until the end of the filling. Once the increase in pressure stops after 5 min the still active flow of methane transports the heat very fast out of the tank and the maximum temperature difference drops during the 1 min of cooling to a final value of 40 °C. The simulation shows, that feed flow cooling is an interesting option to cool down fast filled adsorptive gas storage tanks in a reasonable time. Two further advantages can be pointed out. The temperature increases up to 388 °C during the filling, which could be used as an intrinsic baking and cleaning of the adsorbent. Hereby, the accumulation of higher boiling residues,
Figure 7. Experimental temperature rises at the center of the adsorbent and pressure increase over time for different feed flow cooling scenarios, represent by a different average pressure rise. The fasted experiment (blue line) represents a dead end tank, hence no feed flow cooling.
end tank, and several feed flow cooling scenarios differing for the average pressure increase rate. For the closed valve a temperature rise of 32.7 °C was observed. Even for the fastest feed flow cooling scenario the temperature rise drops to 12.8 °C, lowering further for the slower scenarios. Of course for this feed flow cooling experiments, the time for filling is longer and the heat production rate is thus lower than for the dead end scenario. To account for this effect the filling rate was also varied for the dead end scenario. Figure 8 shows the experimentally determined mean temperature rise for different times needed to reach 3.5 MPa for the dead end scenario as well as for the feed flow cooling: at the same filling time and thus for similar heat production rates a pronounced difference in the mean temperature rise is observable. Thus, the promising results from the simulation for a feed flow cooling could be confirmed in lab scale experiments. 676
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In further studies the possible in situ regeneration of the adsorbent during filling should be addressed as well as focusing closer on the fluid dynamics of different adsorbents and tank designs. Especially the later one was of less importance for “dead end” adsorptive gas storage and has more impact if a feed flow cooling system is used.
ASSOCIATED CONTENT
* Supporting Information S
Details on experimental setup and data evaluation for nonisothermal adsorption; Details on the simulation employed. This material is available free of charge via the Internet at http://pubs.acs.org.
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Figure 8. Experimentally determined average temperature rise during feed flow cooling and for filling a dead end tank with different filling times to reach 3.5 MPa.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +49 9131 8527430; e-mail: [email protected]. Author Contributions
S.G., C.B., J.Z., M.N., M.K., and V.K. performed experiments or simulations. A.J. carried out the magnetic suspension balance study. B.J.M.E. planned the study. All authors contributed to writing the manuscript and approved the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This report is based on a project which was funded by E.ON AG as part of the E.ON International Research Initiative. Responsibility for the content of this publication lies with the author. The authors gratefully acknowledge the funding of the German Research Council (DFG), which, within the framework of its È xcellence Initiativé supports the Cluster of Excellence E ̀ ngineering of Advanced Materialś (www.eam.unierlangen.de) at the University of Erlangen-Nürnberg. We acknowledge Schunk Ingenieurkeramik GmbH for providing SiSiC monoliths and Blücher GmbH for proving samples of activated carbon.
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ABBREVIATIONS ANG adsoptive natural gas storage CDC carbide-derived carbon CNG compressive natural gas storage PBSAC polymer bases spherical activated carbon QSDFT quenched solid density functional theory 677
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(21) Schmirler, M.; Glenk, F.; Etzold, B. J. M. In-situ thermal activation of carbide-derived carbon. Carbon 2011, 49 (11), 3679− 3686. (22) Knorr, T.; Kaiser, M.; Glenk, F.; Etzold, B. J. M. Shrinking core like fluid solid reactionsA dispersion model accounting for fluid phase volume change and solid phase particle size distributions. Chem. Eng. Sci. 2012, 69 (1), 492−502. (23) Silvestre-Albero, A.; Rico-Francés, S.; Rodríguez-Reinoso, F.; Kern, A. M.; Klumpp, M.; Etzold, B. J. M.; Silvestre-Albero, J. High selectivity of TiC-CDC for CO2/N2 separation. Carbon 2013, 59, 221−228. (24) Biloe, S.; Goetz, V.; Mauran, S. Dynamic discharge and performance of a new adsorbent for natural gas storage. AlChE J. 2001, 47 (12), 2819−2830. (25) Goetz, V.; Biloé, S. Efficient dynamic charge and discharge of an adsorbed natural gas storage system. Chem. Eng. Commun. 2005, 192, 876−896. (26) Menard, D.; Py, X.; Mazet, N. Activated carbon monolith of high thermal conductivity for adsorption processes improvementPart A: Adsorption step. Chem. Eng. Process. 2005, 44 (9), 1029−1038. (27) Menard, D. Development of thermally conductive packing for gas separation. Carbon 2003, 41 (9), 1715−1727. (28) Py, X.; Daguerre, E.; Menard, D. Composites of expanded natural graphite and in situ prepared activated carbons. Carbon 2002, 40 (8), 1255−1265. (29) Py, X.; Goetz, V.; Plantard, G. Activated carbons textural optimization for gas storage processes. Chem. Eng. Process. 2008, 47 (3), 308−315. (30) Becker, P.; Glenk, F.; Kormann, M.; Popovska, N.; Etzold, B. J. M. Chlorination of titanium carbide for the processing of nanoporous carbon: A kinetic study. Chem. Eng. J. 2010, 159 (1−3), 236−241. (31) Himeno, S.; Komatsu, T.; Fujita, S. High-pressure adsorption equilibria of methane and carbon dioxide on several activated carbons. J. Chem. Eng. Data 2005, 50 (2), 369−376. (32) Chang, K. J.; Talu, O. Behavior and performance of adsorptive natural gas storage cylinders during discharge. Appl. Therm. Eng. 1996, 16 (5 SPEC. ISS.), 359−374. (33) Tamainot-Telto, Z.; Critoph, R. E. Monolithic carbon for sorption refrigeration and heat pump applications. Appl. Therm. Eng. 2001, 21 (1), 37−52. (34) Kuwagaki, H.; Meguro, T.; Tatami, J.; Komeya, K.; Tamura, K. An improvement of thermal conduction of activated carbon by adding graphite. J. Mater. Sci. 2003, 38 (15), 3279−3284.
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A Feasible Way to Remove the Heat during Adsorptive Methane Storage Stefan Gütlein,† Christoph Burkard,† Johannes Zeilinger,† Matthias Niedermaier,† Michael Klumpp,† Veronika Kolb,† Andreas Jess,‡ and Bastian J. M. Etzold*,† †
Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Lehrstuhl Chemische Reaktionstechnik, Egerlandstrasse 3, 91058 Erlangen, Germany ‡ Universität Bayreuth, Lehrstuhl für Chemische Verfahrenstechnik, Zentrum für Energietechnik (ZET), 95440 Bayreuth, Germany S Supporting Information *
ABSTRACT: Methane originating from biogas or natural gas is an attractive and environmentally friendly alternative to gasoline. Adsorption is seen as promising storage technology, but the heat released limits fast filling of these systems. Here a lab scale adsorptive methane storage tank, capable to study the temperature increase during fast filling, was realized. A variation of the filling time from 1 h to 31 s, showed a decrease of the storage capacity of 14% and temperature increase of 39.6 °C. The experimental data could be described in good accordance with a finite element simulation solving the transient mass, energy, and impulse balance. The simulation was further used to extrapolate temperature development in real sized car tanks and for different heat pipe scenarios, resulting in temperature rises of approximately 110 °C. It could be clearly shown, that with heat conductivity as solei mechanism the heat cannot be removed in acceptable time. By adding an outlet to the tank a feed flow cooling with methane as heat carrier was realized. This setup was proofed in simulation and lab scale experiments to be a promising technique for fast adsorbent cooling and can be crucial to leverage the full potential of adsorptive methane gas storage.
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INTRODUCTION Methane, originating from biogas or natural gas, is an attractive fuel for vehicles. Using methane as an automotive fuel exhibits considerable advantages like lower vehicle emissions and fuel costs when compared to gasoline. However, the low volumetric energy density and therefore the resulting low driving range can be problematic.1−8 The most common storage technology for increasing the energy density is compression (called also compressed natural gas, CNG) at about 20 MPa.7,8 The high pressure results in high compression costs and requires heavy, cylindrical tanks, which are difficult to integrate on-board.7 One possibility to lower the storage pressure while still maintaining high storage capacities is adsorption on microporous materials (also called adsorbed natural gas, ANG).1,5,7−10 Highly microporous carbon adsorbent as used in this study or other porous materials like metal organic frameworks show high gravimetric storage capacity.4,6,8,11 The high gravimetric density correlates with high volumetric one if the bulk density of the adsorbent is high.8 Hence, monoliths are preferred to powder beds, as the porosity of the bed lowers the density unnecessarily. Regarding carbon, monolithic adsorbents are available, for example, from pitch-12 or polymer-based13 activated carbons or carbidederived carbons (CDC),14−17 which are used in this study. In short CDCs are obtained by selective extraction of the © 2014 American Chemical Society
noncarbon species from carbides. The process is conformal and porous carbon with a tunable and narrow pore size distribution can be obtained.18−23 A good performance of CDC for adsorptive methane gas storage and high volumetric storage capacity achievable with monolithic one were reported.16 Thus, in principle ANG systems using monolithic carbonaceous adsorbents showing high gravimetric and volumetric storage capacity are available. Why only in principle? The referred high storage capacities are determined under ideal conditions. Long equilibration times, resulting in experiments up to several hours, are applied to determine isothermal, close to equilibrium conditions. However, for technical applications high filling rates are required. Filling of a vehicle at the gasoline station should be finished in several minutes. As the heat of adsorption is released and needs to be dissipated in this short period of time, a nonisothermal adsorption results and remarkable hot spots can be formed.5,9,10,24−29 Adsorption capacities at higher temperatures are lower, thus the technically achievable capacity is reduced. Despite its importance for Received: Revised: Accepted: Published: 672
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signal of a high pressure mass flow controller. Additionally the pressure and temperature inside of the tank is recorded with a resolution below 1 s. The tank is immersed in a temperature controlled oil bath. The dead volume of the total setup contributing to compressive storage is minimized (approximately 10 mL). To determine the adsorptive storage, two experiments are carried out. One with adsorbents, where adsorptive and compressive storage takes place in parallel and one without adsorbents, to determine the amount of solely compressive storage. Highly nonisothermal experiments are performed with high methane mass flow rates and pseudoisothermal ones with very low flow rates. Prior each measurement the adsorbent is outgassed directly in the storage tank at 120 °C for 2 h at a vacuum of 10 mbar. Detailed information about the setup and data evaluation is given in the Supporting Information (SI). Lab Scale Feed Flow Cooling Experiments. The setup for the nonisothermal experiments was modified for the feed flow cooling experiments. At the bottom of the adsorption tank a 1/16 in. stainless steel capillary was added as outflow and connected to a needle valve. A Trinamic TMCM-113 V.3.40 stepping motor is coupled with a self-built connector to the needle valve. The pressure is controlled for approx. three turns of the needle valve, which corresponds to approx. 25 000 positions of the stepping motor. Simulation. The commercial software Comsol Multyphysics (version 4.1.0.185) was used to setup the finite element simulation. Classical engineering equations like the Navier− Stokes and Brinkmann equation for the fluid flow, a first order reversible kinetic for the sorption using a Dubinin-Astakhov isotherm for temperature dependent equilibrium loading and a homogeneous single phase model for the solid and gas phase heat balance with a flow dependent effective heat conductivity were used. Further details and parameters are given in the SI.
technical realization, studies on fast and nonisothermal methane adsorption are scarce. In this work the resulting temperature rise during technical, hence fast adsorption was studied experimentally and by a chemical engineering simulation. Further, the simulation was used to deduce technical possibilities for an effective fast heat removal. The most promising approach, a feed flow cooling was further proofed in lab scale.
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EXPERIMENTAL SECTION Material. As precursor material for monolith cylindrical SiSiC (10 mm in thickness and 49 mm in length, SiSiC containing 14 wt % of free silicon, Schunk Ingenieurkeramik GmbH) was converted to monolithic CDC like described in detail previously elsewhere.17 Briefly, the chlorination was conducted in a horizontal tubular alumina reactor (length: 1300 mm, diameter: 32 mm) protected by a graphite foil heated with a controlled kiln. All gases (Linde Gas: He 5.0, Cl2 2.8, H2 5.0) were dosed via mass flow controllers. The samples were placed in the isothermal zone of the reactor. The exhaust gas is neutralized by a 30%-KOH solution. Under helium the reactor was heated up to reaction temperature (1000 °C). The reactive extraction was carried out with 1−2 mol m−3 of chlorine in helium at a superficial velocity was 0.05 m s−1. After the reactive extraction the samples were treated for 30 min in hydrogen gas flow at 1000 °C to desorb/react residual chlorine. For the feed flow experiments commercial polymer-based spherical activated carbon (PBSAC, No. 100772) provided by the company Blücher GmbH were used. The particle size was approximately 480 μm. A pore volume of 0.677 cm3 g−1 and specific surface area of 1365 m2 g−1 was determined. Characterization. For the CDC synthesis the degree of conversion was determined gravimetrically to be above 99%.30 The density was determined assuming an ideal cylinder, measuring the diameter and height and the weight of the monolith. The specific surface area was determined with N2sorption with monolithic samples of smaller size using a Quantachrome QuadrasorbSI measuring at 77 K. Subsequent data evaluation was performed with the software QuadraWin version 5.02 using Quenched Solid Density Functional Theory (QSDFT) for carbon slit pores. Magnetic Suspension Balance Experiments. Experimental investigations to determine the isothermal methane adsorption uptake were carried out in a magnetic suspension balance (Rubotherm GmbH, Bochum, Germany). In this type of balance, the sample holder couples only electromagnetically with the microbalance, which is located outside the chamber under standard ambient temperature and pressure. This special magnetic coupling setup allows measurements of mass changes of samples under elevated temperatures and pressures (T < 250 °C, p < 300 bar). For the experiment a sample of the adsorption material was placed in the measuring chamber and conditioned (dried/ cleaned under vacuum (10−3 mbar) at 150 °C). After reaching a constant mass signal, the measuring cell was cooled down to the desired temperature (22 °C, 44 °C, 80 °C) and pressurized with methane. The mass increase due to methane adsorption was detected for each measurement point after reaching steady state, hence equilibrium. This procedure was repeated for different isobaric conditions. Non-Isothermal and Pseudo-Isothermal Adsorption Experiments. The mass of methane delivered to a lab sized adsorptive storage tank (3.7 mL) is obtained by integrating the
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RESULTS AND DISCUSSION Testing Reproducibility and Accuracy of the Measurement Equipment. To measure the temperature rise and adsorption capacity at fast filling rates an experimental setup was designed and constructed in house. With the setup the final storage pressure of 3.5 MPa can be reached within hours or in less than 1 min. Details of the setup and data evaluation are given in the Experimental section and the SI. As adsorbent monolithic CDC obtained from commercially available silicon infiltrated silicon carbide (SiSiC) was used. We recently demonstrated that using this raw material very fast CDC production rates can be obtained, even for monoliths.17 The density of the monolith used, was determined to be 765 kg m−3. From low temperature N2-sorption measurement and the QSDFT model fit a specific surface area of 1410 m2 g−1 and a pore volume of 0.46 cm3 g−1 results for the adsorbent. As a novel built setup was constructed, the reproducibility and accuracy was studied first. At slow filling rate (filling time approximately 1 h) and 25 °C the storage capacity was measured nine times and a relative standard deviation of the final capacity below 0.4% resulted (see Figure S3b in the SI). During the slow filling a maximum temperature difference of 1.3 °C was recorded, hence the measurement nearly meets isothermal conditions. Such slow measurements are called pseudoisotherm, subsequently. To check the accuracy of the self-built setup pseudoisotherms obtained at 25, 40, and 80 °C were compared to equilibrium storage capacities determined 673
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with a magnetic suspension balance at selected pressures between 0.2 and 3 MPa and similar temperatures. Figure 1 shows the pseudoisotherms obtained with the selfbuilt setup. As expected at low pressures an initially strong
Figure 2. Comparison of the isothermal adsorption capacity at three different temperatures determined with a magnetic suspension balance (represented by symbols) and the self-built rig (represented by dashed lines).
Figure 1. Pseudoisotherms determined experimentally with low filling rates at three different temperatures and corresponding fit with the Dubinin-Astakhov isotherm (see SI equation S16 and S17).
increase of stored methane could be observed whereas the slope for further storage at higher pressures decreased. As adsorption is an exothermic process the final storage capacity increases at lower temperatures. In Figure 1 additional a fit of the data with the Dubinin-Astakhov isotherm (see SI equations S16 and S17, parameters: b1 = 0.158, b2 = 0 K, n = 1.84, Esorp = 10 kJ mol−1) is given and shows good accordance to the experimental data. The results of the magnetic suspension balance are compared to the ones obtained by the self-built setup (represented by the Dubinin-Astakhov fit to account for the slight temperature difference between the different measurement principles of up to 4 °C). It is worth to stress that for the self-built setup the mass of adsorbed methane is determined by integrating the flow rate over the time, hence also the error of the flow rate measurement accumulates during the integration. In case of the magnetic suspension balance the true increase in mass due to methane adsorption is measured as a final value. In this view the very good agreement between the different measuring principles observed in Figure 2, indicates a very low error occurring in the self-built setup. Temperature Increase with Faster Filling. Besides proofing the good reproducibility and accuracy of the selfbuilt setup, experiments with different filling rates were carried out, to mimic the technical adsorptive methane storage, for example, at a gasoline station. In eight experiments the filling time to reach 3.5 MPa was reduced from approximately 1 h to 31 s. The measured storage capacities are given in Figure 3a. The storage capacity achieved at maximum pressure decreases by 14% from 11.3 to 9.7 kgCH4 kg−1carbon when filling faster. For very fast filling a delay for the adsorption is observed. It takes three measurements points or 2.4 s until an increase in the methane uptake is recorded. This could either be a real kinetic effect of the adsorption or a measurement artifact for very fast filling from the mass integration or a combination of both. In Figure 3a the temperature change measured in the center of the monolith is given and shows the reason for the drop in storage capacity. Speeding up the filling, the temperature increases strongly. For the filling in 31 s the temperature rises by 39.6 °C.
Figure 3. (a) Experimental results of the adsorptive storage for different filling times until 3.5 MPa is reached (66.3, 18.3, 5.43, 2.64., 1.73, 1.03, 0.73, and 0.52 min). upper: amount of adsorbed methane, lower: temperature rise in the center of the adsorbent; (b) amount of methane adsorbed at 3.5 MPa versus the average temperature during the adsorption for slow and fast filling experiments (dashed line gives the trend).
Independent from the filling rate, the temperature increase always runs through a maximum, which can be explained as most of the heat of adsorption is released in the beginning, where methane uptakes are higher. Hence, the interplay of heat generation due to adsorption and heat removal through conduction in the monolith and the tank walls needs to show a maximum in the temperature increase. It can further be observed that the maximum is shifted toward higher pressures for faster filling. Figure 3b compares the achievable storage capacity with the average temperature for pseudoisothermal and nonisothermal (fast filling) measurements. A clear correlation can be seen and 674
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there is no doubt that the decrease of methane adsorbed in case of fast filling can be attributed to the temperature increase. It needs to be pointed out, that for the lab scale system the ratio of heat generation to heat removal through the walls is lower than for a technical system, which would be used, for example, in a car. Accordingly, for technical systems the temperature effect is even more pronounced. Simulation of the Transient Adsorption Process and Feed Flow Cooling. For further insight into the nonisothermal adsorption a finite element model solving the transient impulse-, heat- and mass-transfer-balance was set up. For further details see the Experimental section and SI. First the filling process of the self-built rig was modeled and later this simulation was modified and extended to study real-size tanks. While most data was deduced from known empiric correlations, the heat of adsorption ΔHads and effective heat conductivity λeff in the adsorbent bed are unknown and therefore remain fitting parameters. Nevertheless, these parameters can be constrained by literature values. ΔHads is in the range of 10−20 kJ mol−13,5,8,24,31 and typical effective heat conductivities of porous carbons are around 0.1−1.5 W m−1 K−1.3,5,32−34 From the parameter fit ΔHads = −16.5 kJ mol−1 and λeff = 0.335 W m−1 K−1 resulted, which both is in good accordance to what is expected from literature. Figure 4 compares the experimental (continuous lines) with simulation (dashed orange lines) results for the four experi-
Figure 5. Scheme of tanks simulated (blue = gas phase, green = adsorbent, red = alumina heat pipe, gray = steal tank wall); (a) simple car sized tank; (b) car sized tank with an aluminum heat pipe lance; (c) car sized tank with aluminum heat pipe discs; (d) scheme for feed flow cooling.
of adsorbent was implemented in the finite element simulation (for details on the simulation see the SI). To study only the influence of the inner heat transfer the outer heat transfer was diminished by choosing a very high outer heat transfer coefficient. In technical realization this could be achieved by increasing the outer surface area or additional forced convection. In simulation the tank was filled to 3.5 MPa within 5 min and given another minute to cool in ambient air. Beside the simple tank, which represents the boundary case of maximum temperature rise, tanks with two different internals for improved heat removal were simulated. These internals are (i) an alumina heat pipe lance in the center of the adsorbent and (ii) heat pipe discs segmenting the adsorbent into 6 sections, while gas can pass through a hole in the middle (Figure 5b and c). Technically it could also be tried to remove the heat not with adding internals but by increasing the heat conductivity within the adsorbent, for example, by graphitic additives. To account for this option a simulation without internals but applying a 10 times higher effective heat conductivity in the adsorbent bed was performed. Further details to the real sized car tank models can be found in the SI. In Figure 6 the simulated maximum temperature increase inside the adsorbent resulting for the different cases is given vs the time. All simulations show a very fast temperature rise of approximately 60 °C in the first seven seconds. For the simple tank a continuous temperature increase of up to 135 °C is observed until the end of the 5 min of filling. In the subsequent cooling period of 1 min nearly no heat is removed and hot spots of up to 198.9 °C remain in the tank. Although in the case study of the heat pipe lance the internal surface area, available for heat removal inside the bed, is enhanced, only a minor effect in the temperature profile can be seen. For the heat pipe disc a different profile results. In the first 3 min a higher temperature rise is observed. This can be attributed to the different fluid dynamics. Here in the first segment of the adsorbent, separated by the discs, the temperature increases faster. The temperature rise after the cooling time is 106.1 and 105.4 °C for the heat pipe lance and discs, respectively. For the simple tank using an
Figure 4. Comparison of experimental results (continuous lines) and finite element simulation (orange dashed line) for four different filling times (time to reach 3.5 MPa) showing a pronounced temperature increase. upper: Time dependent pressure inside of the tank; lower: Time dependent temperature at the center of the adsorbent.
ments with highest filling rate, hence, the most interesting ones regarding pronounced temperature increase. As it can be seen the simulation describes the pressure increase over the time very accurately. The time curve of the temperature is represented correctly and all features as the fast initial rise, the maximum and the subsequent decrease are shown, although the initial slope seems not to be catched perfectly in simulation. Thus, despite the simplifications made during the simulation, it represents a good description of the nonisothermal adsorptive methane storage. After proofing the good fit of the simulation with the experimental data of the lab scale setup, the simulation was used to estimate the influence of fast filling on the temperature rise for real sized car tanks and storage of approximately 5 kg of methane. Accounting for the storage capacity this results in approximately 50 kg of adsorbent. A simple single cylindrical tank built of stainless steel (see Figure. 5a) as tank for the 50 kg 675
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which lead to an aging of the adsorbent, could be reduced. Further the enormous amount of heat released could be used externally, for for heating. One might ask where this energy made available comes from. More or less it is the energy withdrawn from the environment during desorption. When releasing the methane the adsorbents cools down slightly and heats up to ambient conditions by withdrawing energy from the environment. The temperature decrease of the adsorbent during desorption is by dimensions lower, as the time scale is by dimensions higher, compared to the filling. That is why simple “heating” with the surrounding air is working, while for the cooling during filling this does not work. Of course driving force to withdraw the energy from the environment is the pressure difference, hence, it could also be stated that some of the compression energy is recovered. Lab Scale Proof of Principle of the Feed Flow Cooling. As a proof of principle of the feed flow cooling the lab-scale tank was modified with an additional outlet. As backpressure controller a needle valve actuated by a stepping motor was used. Varying the closing speed of the needle valves at constant filling rate, different times to reach 3.5 MPa were realized. As adsorbent commercial polymer-based spherical activated carbon (PBSAC) provided by the company Blücher GmbH was used. Figure 7 gives the pressure and temperature increase over time for a completely closed valve, resembling the classical dead
Figure 6. Simulated maximum temperature rise inside the different car sized adsorptive storage tanks and for a feed flow cooling tank (see Figure 5a-d). The insert is a magnification of the upper right region. The dashed black line shows the time when 3.5 MPa are reached, filling ends and subsequent additional cooling begins.
adsorbent with heat conductivity increased by a factor of 10, the final temperature rise lowers to 94.0 °C. However, all simulations show clearly, that with heat conduction as the only mechanism for heat removal, the huge amount of heat released during adsorption cannot be dissipated effectively, independent if internals are used or not. It needs to be stated that other adsorbents might give different results, but the resulting trend is thus obvious, that the observation can be generalized. Beside heat conductivity an additional mechanism for heat removal needs to be applied. Heat transport by radiation can be neglected, as the temperature difference is too low, thus convection remains as possible mechanism. For a convective cooling a fluid needs to heat up and be transported out of the system. One might think of water pipes going through the adsorbent as a technical solution. Nevertheless, this complicates the tank design strongly, increases the dead volume of the tank and beside methane also cooling water needs to be pumped through the fuel hose at the gasoline station. An option might be to use the methane itself as heat transport fluid/cooling agent. We call this principle feed flow cooling. Figure 5d shows a flow sheet, where instead of the dead end tank with a single opening a tank with an inlet and outlet is used. Thus, methane can flow through the adsorbent, heat up, adsorb, and nonadsorbed methane leave the tank at the outlet. To increase the pressure during filling a backpressure valve is needed. This feed flow cooling mechanism with cold feed entering at the top of the cylindrical tank and leaving at the bottom was also implemented in the finite element simulation. The maximum temperature rise inside the tank resulting in the simulation is given in Figure 6. The feed flow cooling shows a temperature increase up to 90 °C in the first 50 s. Later on the maximum temperature inside of the tank stays nearly constant until the end of the filling. Once the increase in pressure stops after 5 min the still active flow of methane transports the heat very fast out of the tank and the maximum temperature difference drops during the 1 min of cooling to a final value of 40 °C. The simulation shows, that feed flow cooling is an interesting option to cool down fast filled adsorptive gas storage tanks in a reasonable time. Two further advantages can be pointed out. The temperature increases up to 388 °C during the filling, which could be used as an intrinsic baking and cleaning of the adsorbent. Hereby, the accumulation of higher boiling residues,
Figure 7. Experimental temperature rises at the center of the adsorbent and pressure increase over time for different feed flow cooling scenarios, represent by a different average pressure rise. The fasted experiment (blue line) represents a dead end tank, hence no feed flow cooling.
end tank, and several feed flow cooling scenarios differing for the average pressure increase rate. For the closed valve a temperature rise of 32.7 °C was observed. Even for the fastest feed flow cooling scenario the temperature rise drops to 12.8 °C, lowering further for the slower scenarios. Of course for this feed flow cooling experiments, the time for filling is longer and the heat production rate is thus lower than for the dead end scenario. To account for this effect the filling rate was also varied for the dead end scenario. Figure 8 shows the experimentally determined mean temperature rise for different times needed to reach 3.5 MPa for the dead end scenario as well as for the feed flow cooling: at the same filling time and thus for similar heat production rates a pronounced difference in the mean temperature rise is observable. Thus, the promising results from the simulation for a feed flow cooling could be confirmed in lab scale experiments. 676
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In further studies the possible in situ regeneration of the adsorbent during filling should be addressed as well as focusing closer on the fluid dynamics of different adsorbents and tank designs. Especially the later one was of less importance for “dead end” adsorptive gas storage and has more impact if a feed flow cooling system is used.
ASSOCIATED CONTENT
* Supporting Information S
Details on experimental setup and data evaluation for nonisothermal adsorption; Details on the simulation employed. This material is available free of charge via the Internet at http://pubs.acs.org.
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Figure 8. Experimentally determined average temperature rise during feed flow cooling and for filling a dead end tank with different filling times to reach 3.5 MPa.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +49 9131 8527430; e-mail: [email protected]. Author Contributions
S.G., C.B., J.Z., M.N., M.K., and V.K. performed experiments or simulations. A.J. carried out the magnetic suspension balance study. B.J.M.E. planned the study. All authors contributed to writing the manuscript and approved the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This report is based on a project which was funded by E.ON AG as part of the E.ON International Research Initiative. Responsibility for the content of this publication lies with the author. The authors gratefully acknowledge the funding of the German Research Council (DFG), which, within the framework of its È xcellence Initiativé supports the Cluster of Excellence E ̀ ngineering of Advanced Materialś (www.eam.unierlangen.de) at the University of Erlangen-Nürnberg. We acknowledge Schunk Ingenieurkeramik GmbH for providing SiSiC monoliths and Blücher GmbH for proving samples of activated carbon.
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ABBREVIATIONS ANG adsoptive natural gas storage CDC carbide-derived carbon CNG compressive natural gas storage PBSAC polymer bases spherical activated carbon QSDFT quenched solid density functional theory 677
dx.doi.org/10.1021/es504141t | Environ. Sci. Technol. 2015, 49, 672−678
Environmental Science & Technology
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