High Intensity Sound Enhances Calcination and CO2 Capture of

Jul 19, 2016 - In this work, we show that the application of high intensity sound to limestone and dolomite beds in a CaL reactor enhances significant...
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High Intensity Sound Enhances Calcination and CO2 Capture of Limestone and Dolomite at Ca-Looping Conditions J. M. P. Ebri, J. M. Valverde,* and M. A. S. Quintanilla Faculty of Physics, University of Seville, Avenida Reina Mercedes s/n, 41012 Seville, Spain ABSTRACT: The calcium looping (CaL) process, based on the calcination/ carbonation of CaCO3, has emerged in the last years as a potentially low cost technology for CO2 capture. In this work, we show that the application of high intensity sound to limestone and dolomite beds in a CaL reactor enhances significantly their multicycle CO2 capture capacity. Experimental tests have been carried out in which pulsed sound waves are applied either during the calcination stage of each CaL cycle or in the carbonation stage. The effect of sound is to intensify the transfer of heat, mass, and momentum and becomes more marked when sound is applied during calcination by promoting CaO regeneration. The application of sound slows the decay of capture capacity with the number of cycles and, if other factors are kept equal, reduces the energy requirement to capture a unit mass of CO2.

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INTRODUCTION The calcium looping (CaL) process is being widely studied in the last years as a potential technique to capture CO2 from exhaust gases using cyclic carbonation and calcination of a dry particulate sorbent based on CaO.1−5 In this process, the CO2 loaded gas is passed through a bed of CaO particles at atmospheric pressure in a carbonator reactor operated at around 650 °C. At this temperature, the kinetics of carbonation is fast enough as required by industrial applications, whereas the equilibrium CO2 concentration is acceptably low (around 1% vol.), which allows for a high CO2 capture efficiency.6 The carbonated solids are then circulated into a second gas−solid reactor (calciner) operated at temperatures above 900 °C where regeneration of the sorbent takes place at a sufficiently fast rate. To be economically viable, the CaO based sorbent must be obtained from cheap and abundantly available materials such as natural limestone (almost pure CaCO3) or dolomite (CaMg(CO3)2).7 In order to maximize the efficiency of the process, fast reaction rates are desirable, which translates into the necessity of a large surface area of the sorbent particles as well as efficient transfer of heat and mass in the reactors. Gas fluidized beds are operated in the fast fluidization regime (gas velocities above m/s) in the case of postcombustion CO2 capture, which favors the gas−solids contacting efficiency.3 On the other hand, in precombustion capture conditions, either fixed or bubbling beds operated at gas velocities of a few cm/s are more appropriate although in this regimes the transfer of heat and mass would be hampered.8 The usually poor thermal conductivity in a fixed bed leads to local heating in exothermic reactions (such as carbonation) and local cooling in endothermic reactions (such as calcination), which slows down gas−solid heterogeneous reactions at high temperature. Even in the case of efficient heat/mass transfer, the capture capacity of limestone or dolomite derived CaO is seen in © 2016 American Chemical Society

thermogravimetric analysis (TGA) experiments to decrease irreversibly with the number of carbonation-calcination cycles mainly because of the reduction of the available surface area for fast carbonation.9 The main factor responsible for the loss of surface area is the sintering experienced by the particles due to the high mobility of the atoms at the elevated temperature needed to achieve full decarbonation and regenerate the sorbent during calcination.10,11 This mechanism becomes significant at temperatures above 900 °C. Thus, a desirable measure to slow the decay of the capture efficiency of the sorbent caused by sintering and to decrease the energy penalty would be to operate the CaL process at lower calcination temperatures.6 In regards to the use of dolomite, although it yields a smaller percentage of CaO per unit weight than limestone, during first calcination it decomposes into a mixture of CaO and MgO,12 the latter grains being inert at CaL conditions.13 As the CaO active grains are dispersed in an inert MgO skeleton, this would act as a backbone that mitigates the drastic loss of CaO surface area by sintering.14 In previous experiments on the capture of CO2 at CaL conditions using particles of natural limestone (CaCO3), we have found that the application of a high intensity acoustic field (typically about 140 dB) during carbonation enhances the CaO carbonation rate,15,16 probably because the agitation of the particles produced by the acoustic field helps to homogenize the passage of the gas through the fixed bed and also because the acoustic flow field enhances the heat and mass transfer at the gas−solid boundary of those particles large enough to be unmovable by the sound wave due to acoustic streaming.17 In this work, we investigate the effect of high intensity pulsed Received: Revised: Accepted: Published: 8671

April 26, 2016 July 11, 2016 July 19, 2016 July 19, 2016 DOI: 10.1021/acs.iecr.6b01616 Ind. Eng. Chem. Res. 2016, 55, 8671−8678

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Industrial & Engineering Chemistry Research

Figure 1. Sketch of the experimental setup. 1: Gas cylinder of 15% CO2/85% N2 vol/vol). 2: Gas cylinder of compressed dry air. 3: Mass flow controllers. 4: Safety maximum pressure regulator. 5: Mass flow controller readout. 6: Oven temperature controller. 7: Oven. 8: Quartz fritted filter. 9: Quartz reactor. 10: Teflon adapter. 11: Acoustic waveguide. 12: Silicone elastic membrane. 13: Microphone. 14: Low frequency loudspeaker. 15: Differential pressure transducer. 16: Particle filter. 17: Mass flow meter. 18: CO2 gas analyzer. 19: Electric amplifier. 20: Electric signal generator. 21: Oscilloscope. 22: Data acquisition and control.

means of the mass flow controllers. The gas exiting the reactor passes through a gas analyzer and a flow meter to monitor its composition. A pressure gauge connected to the inlet port of the reactor is used to measure the total pressure drop across the reactor. An electric signal generator and amplifier are used to excite the loudspeaker to produce the low frequency sound wave that reaches the reactor trough a closed guide. A silicone elastic membrane inside the guide with good sound transmission prevents gas exchange between the reactor and the sound guide. The frequency of the sound was fixed to 130 Hz, and the sound level intensity was 157 dB, which is above the minimum intensity generally observed (∼140−150 dB) for the intensification of fluidized bed processes by sound waves.15 In the experiments, pulsed sound was employed to clearly appreciate the effect of sound application and to allow for refrigeration of the loudspeaker. Thus, the sound generator is turned on for 5 min and turned off for another 5 min cyclically. We have tested the effects of pulsed sound when it is applied during the calcination stage only (PULCAL tests) and during the carbonation stage only (PULCAR tests). The reactor was heated by an electric furnace monitored by a PID temperature controller. Temperature is kept at 610 °C for 45 min during the carbonation stage and at 900 °C for 30 min during the calcination stage, with a 15 min intermediate period between the calcination and carbonation stages in which the reactor is vented using dry air while the temperature drops from the calcination to the carbonation temperature. The whole measuring process is automated, and the protocols are controlled by a computer to ensure reproducibility of measuring conditions.

sound on the multicycle capture performance of limestone and dolomite beds operated at CaL conditions. As will be seen, a quick response to the application of sound is observed, and both carbonation and calcination are notably enhanced by high intensity sound waves. Our results indicate that the decrease in the calcination kinetics at relatively low temperatures could be counteracted by the application of high intensity sound, which serves to enhance substantially CaO regeneration.

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EXPERIMENTAL SETUP AND PROCEDURE The experimental setup used in this work is schematized in Figure 1. The reactor consists of a cylindrical quartz vessel (45 mm inner diameter) with a gas distributor made of a quartz plate fitted at its base (2 mm thick, 16 to 40 μm pore size). Dry compressed air or a mixture of 15% CO2 and 85% N2 is supplied to the inlet port of the reactor by means of two mass flow controllers and a set of valves. For carbonation, a mixture of 15% CO2+85% N2 is passed through the reactor while dry compressed air is used for calcination. Whereas in typical industrial operation calcination is performed in 90% CO2, the use of dry air during calcination allowed us to calculate the amount of CaO in the sorbent from the amount of CO2 in the effluent gas from the reactor. Since an increase in the concentration of CO2 inside the reactor during calcination would decrease the rate of calcination, the capture capacities obtained in our experiments are expected to be larger than the ones that would be obtained had calcination being performed in typical industrial operation conditions. During both carbonation and calcination, a total gas flow of 2000 cm3/ min in standard conditions (25 °C, 101.3 kPa) is kept fixed by 8672

DOI: 10.1021/acs.iecr.6b01616 Ind. Eng. Chem. Res. 2016, 55, 8671−8678

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Industrial & Engineering Chemistry Research

accordance with the reaction in eq 3 and around 53 g of CaO from the stoichiometry of dolomite decomposition (eq 4).

As CaO precursors we have used sieved natural limestone and dolomite particles supplied to us by a local quarry (Taljedi S.L., Gilena, Spain). Only the fraction of particles with mesh sizes dp in the range 710 μm ≥ dp ≥ 425 μm was loaded in the reactor. The minimum fluidization velocity Umf can be estimated by equating the weight of the sample per unit cross sectional area of the reactor to the pressure drop through the packed bed of particles as described by the Carman-Kozeny equation18

CaCO3 → CaO + CO2

(3)

CaMg(CO3)2 → CaO + MgO + 2CO2

(4)

After the precalcination stage the samples were subjected to a total number of 20 carbonation-calcination cycles in each one of the three operation modes (no sound, PULCAR, and PULCAL). Every experimental run was started by using a fresh sample of material.

mg = Δp A

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EXPERIMENTAL RESULTS AND DISCUSSION Figures 2 and 3 show the time evolution of the CO2 %vol concentration measured in the effluent gas of the reactor (CO2 breakthrough curve) during the calcination and carbonation

2 μg Umf ρg Umf ϕ2 ϕ = 150H + 1.75H (1 − ϕ)3 (esd p)2 (1 − ϕ)3 esd p

(1)

where ϕ is the solid fraction of the particle bed in the reactor, H is its height, es is the sphericity of the particles, μg is the gas viscosity, and ρg is the gas density. Since the solid fraction of the particles is given by ϕ = m/(ρpAH), where ρp is the particle density, substituting in eq 1 yields the following algebraic equation from which the mimimum fluidization velocity can be evaluated: 1 − 150

2 μg Umf ρp gρg Umf ϕ ϕ 1.75 − =0 (1 − ϕ)3 ρp g (esd p)2 (1 − ϕ)3 esd p

(2)

The minimum fluidization velocity depends on the temperature inside the reactor for the gas density ρg and viscosity μg both depend on temperature and, in our experiment, on the chemical composition of the gas passing across the reactor. For simplicity, we substitute in eq 2 the gas density ρg and viscosity μg of air, at the temperature of carbonation (T = 610 o C), ρg = 0.40 kg/m3 and μg = 3.85 × 10−5 Pa.s, while at the temperature of calcination (T = 900 °C), ρg = 0.30 kg/m3 and μg = 4.56 × 10−5 Pa.s. Assuming ϕ ≃ 0.6 for the solid fraction and es = 1 for the sphericity of the particles, ρp = 2710 kg/m3 during calcination (the density of solid CaCO3) and ρp = 3350 kg/m3 during carbonation (the density of solid CaO), eq 2 yields 7.4 cm/s < Umf < 20.5 cm/s for calcination and 10.7 cm/ s < Umf < 29.3 cm/s for carbonation, where the bounds on Umf are given by substitution of the limits in the range of particle sizes used in our experiments. A gas flow of 2000 cm3/min at standard conditions gives a superficial gas velocity U = 2.10 cm/s inside the reactor when the reactor is at 25 °C. However, due to the changes in gas density with temperature, the corresponding gas velocities are U = 6.20 cm/s at carbonation temperature (610 °C) and U = 8.26 cm/s at calcination temperature (900 °C). Therefore, we may conclude that in our experiment the bed is operated at fixed regime, being close to minimum fluidization during calcination. For the range of particle size used in the experiments, the particles have large enough inertia not to be moved by the low frequency sound wave applied.19 Thus, the main effect of sound would be due to the acoustic streaming developed on the surface of the particles, which would arguably enhance the gas−solid transfer of heat, mass and momentum.19 In all the runs, the mass of fresh particles was 178 g. The sample introduced in the reactor is heated at 900 °C during 30 min for precalcination under dry air. A complete calcination of the sample would yield 100 g of CaO in the case of limestone in

Figure 2. a): CO2 breakthrough curve during calcination in the 19th cycle. The temperature profile during calcination is similar for all calcination conditions, and it is shown in the inset. b): CO2 breakthrough curve during carbonation in the 20th cycle. Both plots are for limestone derived CaO under various operation modes: “No sound”: no sound applied; PULCAR: pulsed sound applied during carbonation; PULCAL: pulsed sound applied during calcination. Gray bands indicate the time over which high intensity sound is applied. 8673

DOI: 10.1021/acs.iecr.6b01616 Ind. Eng. Chem. Res. 2016, 55, 8671−8678

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Industrial & Engineering Chemistry Research

modes in the carbonation cycle during the first minutes (despite sound is off in PULCAL operation). Likewise the CO2 breakthrough measured during calcination for the PULCAR mode is well above that measured in the absence of sound (despite the sound is off in PULCAR mode). The observation that PULCAL operation causes an increase in the capture rate of CO2 during the carbonation cycle can be explained by the enhancement of decarbonation in the previous calcination stage, which promotes CaO regeneration. Thus, PULCAL operation is the mode for which the capture capacity of the sorbent is mostly enhanced. Figure 4 and Figure 5 show the CO2 breakthrough curves for several carbonation cycles of experimental runs operated in the

Figure 3. a): CO2 breakthrough curve during calcination in the 19th cycle. The temperature profile during calcination is similar for all calcination conditions, and it is shown in the inset. b): CO2 breakthrough curve during carbonation in the 20th cycle. Both plots are for dolomitic CaO under various operation modes: “No sound”: no sound applied; PUCAR: pulsed sound applied during carbonation; PULCAL: pulsed sound applied during calcination. Gray bands indicate the time over which high intensity sound is applied.

Figure 4. CO2 breakthrough curves for several carbonation cycles under a) “no sound” and b) PULCAL operation modes for limestone derived CaO.

PULCAL and “no sound” modes. As may be seen, the effect of PULCAL operation mode on the capture rate during the carbonation stage is much more marked for limestone derived CaO (Figure 2) than for the dolomitic CaO as might be expected since the MgO inert skeleton would serve already to promote mass/heat transfer in the case of dolomite in the absence of sound as compared to lime. The total amount of CO2 captured at each cycle mCO2 has been obtained from the CO2 breakthrough curves measured in the carbonation stage according to the equation

stages of the 19th and 20th cycles, respectively, for limestone (Figure 2) and dolomite (Figure 3). The gray bands in the figures indicate the time span of sound application for the PULCAL data (pulsed sound applied only in calcination stages) and the PULCAR data (pulsed sound applied only in carbonation stages). As can be seen, the application of high intensity sound has a significant effect on enhancing both carbonation and calcination even when applied near the end of the stages. This effect is more marked for the PULCAL operation mode, in which local maxima of the CO 2 concentration measured in the effluent gas are seen to nearly coincide with the application of sound. Moreover, the effect of sound application is further prolonged to the part of the stage when it is off. As can be seen in Figures 2 and 3, the CO2 breakthrough curve obtained for the PULCAL mode remains neatly below the curve obtained for the other two operation

mCO2 =

MCO2 V mo ,CO2

∫t

tout

o (pin Fino − pout Fout )dt

in

where MCO2 is the molar volume of CO2 at standard 8674

(5)

mass of CO2, Vom,CO2 is the molar conditions, Foin and Foout are the mass DOI: 10.1021/acs.iecr.6b01616 Ind. Eng. Chem. Res. 2016, 55, 8671−8678

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Industrial & Engineering Chemistry Research

Figure 5. CO2 breakthrough curves for several carbonation cycles under a) “no sound” and b) PULCAL operation modes for dolomite derived CaO.

flow rate of gas entering and exiting the reactor, respectively (measured by the flow meters at standard conditions), pin and pout are the volume fraction of CO2 in the gas stream into and out of the reactor, and tin and tout are the times of initiation and end of the carbonation cycle. The capture capacity is then calculated as mCO2 C= msorb (6)

Figure 6. CO2 capture capacity for a) limestone and b) dolomite derived sorbents in the absence of sound and when a pulsed sound is applied during the carbonation stage (PULCAR) and in the calcination stage (PULCAL).

where mCO2 is the mass of CO2 captured during the carbonation cycle (calculated using eq 5), and msorb is the total mass of sorbent available at the beginning of the carbonation cycle (only CaO in the case of the lime and CaO plus MgO for dolomitic lime). The MgO, once formed, does not react with CO2 in the range of temperatures and partial pressures of CO2 used in our experiments,13 and therefore it can be considered an inert material. Thus, the maximum theoretical value of the capture capacity for lime is Cmax lim = 0.78, whereas for the dolomitic lime is Cmax dol = 0.46, where we have used that the dolomite used in our experiments has roughly a CaO:MgO molar ratio equal to 1. Figure 6 shows the capture capacity C as a function of the cycle number measured at the end of each carbonation stage. As can be observed, the capture capacity is for both sorbents well below the maximum uptake capacity specially for the first few cycles mainly due to incomplete decarbonation under fixed bed conditions, which severely hinder the transfer of heat and mass. The capture capacity increases in the first cycles as subsequent calcinations allow for a higher degree of CaO regeneration. After a few cycles, all the available CaO is

regenerated during the calcination stage and the capture capacity reaches a maximum, after which it follows a gradual decline with the cycle number arguably caused by a loss of surface area of the sorbent due to sintering in the calcination stages. It is also seen in Figure 6 that the decline of capture capacity is mitigated for the dolomite derived sorbent as might be expected since the inert MgO present in the dolomite-based sorbent acts as a skeleton that supports the active CaO surface area. Thus, although the dolomite derived sorbent has a smaller percentage of CaO than lime, its capture capacity is larger in accordance with previous results obtained from thermogravimetric tests.20 Further insight on the effect of pulsed sound can be obtained from the measurements of the pressure drop across the bed. Figure 7 shows the time evolution of the pressure drop across the reactor for the 10th calcination stage of limestone derived CaO under PULCAL operation together with the evolution of the % CO2 in the effluent gas. Figure 8 shows a similar plot for the second and fifth carbonation of limestone derived CaO operated under PULCAR mode. Remarkably, under PULCAL operation mode, the pressure drop across the reactor follows 8675

DOI: 10.1021/acs.iecr.6b01616 Ind. Eng. Chem. Res. 2016, 55, 8671−8678

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Industrial & Engineering Chemistry Research

Figure 7. Total pressure drop across the reactor during the 10th calcination cycle of limestone derived CaO in PULCAL operation mode as a function of time together with the %vol CO2 of the gas leaving the reactor. Gray bands indicate the periods of application of high intensity sound. The inset shows the temperature profile during calcination.

Figure 8. Total gas pressure drop measured across the reactor during the second and fifth carbonation cycle of limestone derived CaO in PULCAR operation mode as a function of time together with the %vol CO2 of the gas leaving the reactor. Gray bands indicate the periods of application of sound.

8676

DOI: 10.1021/acs.iecr.6b01616 Ind. Eng. Chem. Res. 2016, 55, 8671−8678

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Industrial & Engineering Chemistry Research

operation mode. Since the loudspeaker is on only for 50% of the time during calcination cycles (30 min) in PULCAL operation and carbonation cycles (45 min) in PULCAR operation, that yields an energy consumption of 0.23 MJ during a carbonation - venting - calcination - venting period in PULCAL operation and 0.34 MJ in PULCAR operation. The total energy used in each carbonation - venting - calcination venting period must be divided by the mass of CO2 captured during the carbonation cycle in order to compare the energy efficiency of each mode of operation and each type of sorbent: results for the 20th carbonation cycle are presented in Table 1.

the same trend than the CO2 breakthrough curve as the CO2 released by the sorbent contributes to the net gas flow through the material in the reactor. On top of this general trend, the pressure drop is seen to increase temporarily during the sound pulses. In PULCAR operation mode (Figure 8) the first application of the sound generally coincides with a sharp change in the pressure drop, but the increase in the pressure drop during sound pulses is not as marked as in the case of PULCAL mode. The application of high intensity sound would cause an increase in the pressure drop across a particle bed even if the particles are not movable by the sound wave (as is the case of our experiments) due to acoustic streaming.17,19 In our experiment, acoustic streaming is expected to be caused by attenuation of the sound wave in the packed bed due to enhanced viscous dissipation on the surface of the particles.19,21 Numerical simulations of acoustic streaming show the generation of a net flow of gas in the pores of the material away from the incoming wave.21 This flow is driven by the linear momentum carried by the sound wave, and its average velocity is much smaller than the velocity amplitude of the sound wave.22 If the results of those numerical simulations were applicable to our experiment, the flow due to acoustic streaming would be superimposed to the net flow of gas through the reactor. Since the net flow of gas in the reactor travels in the upward direction while the incoming sound wave travels in the downward direction (see Figure 1), the result of this superposition would lead to a tortuous pattern of streamlines within the pores, resulting in greater viscous friction between the gas and particles and thus in a larger pressure drop across the reactor. This phenomenon would be at the same time responsible for the enhancement of heat transport and gas diffusion that promote carbonation and calcination at high temperatures. Therefore, under these assumptions the changes in the pressure drop when the high intensity sound is applied would give a measure of the intensity of the resulting acoustic streaming. According to our measurements, this indicates that acoustic streaming would be more intense when the sound is applied during calcination than during carbonation. A possible explanation for the greater intensity of acoustic streaming during calcination is that the larger temperatures used during calcination lead to a higher gas viscosity compared to carbonation, resulting in larger absorption of the sound wave in the solid surfaces and thus enhanced acoustic streaming. The application of high intensity sound imposes an additional energy cost that needs to be compensated for by the increase in the capture capacity if the overall efficiency of the CO2 capture process is to be improved. In our experiments, most of the energy consumed is used to power the oven. Our oven is rated at 4500 W for a maximum temperature of 1100 °C. Assuming for simplicity that the power consumption of the oven is proportional to the difference between the set temperature of the oven and the temperature of the lab (25 °C aprox.), at carbonation (610 °C), the oven needs a power of 2450 W, while at calcination (900 °C) the oven needs 3663 W. Those power needs result in an energy consumption of 17.62 MJ during a carbonation - venting - calcination - venting period, taking into account the duration of each stage and the fact that the temperature of the oven is set to 610 °C during venting. The maximum power that the loudspeaker can demand is 250 W according to its manufacturer specifications, and although we do not use it in the experiment at its full power, we will use that figure in order to make estimations of the efficiency of each

Table 1. Estimated Energy Consumption of Our Experiment during the 20th Carbonation-Calcination for the Different Modes of Operation of Our Experimental Setupa material

operation

mCO2 (g)

Uoven (MJ)

Usound (MJ)

Utotal/mCO2 (MJ/g)

limestone limestone limestone dolomite dolomite dolomite

no sound PULCAR PULCAL no sound PULCAR PULCAL

14.21 19.06 20.20 20.70 22.16 23.78

17.62 17.62 17.62 17.62 17.62 17.62

0 0.34 0.23 0 0.34 0.23

1.24 0.94 0.88 0.85 0.81 0.75

a

The columns Uoven and Usound give the energy consumed by the oven and the loudspeaker, respectively, during a carbonation - venting calcination - venting period of operation. The column mCO2 gives the mass of CO2 captured in the 20th carbonation.

In terms of energy consumed per gram of CO2 captured, the dolomite based sorbent is more energy efficient than the limestone one and either PULCAR and PULCAL operation are more efficient than operation without sound, with PULCAL being the most energy efficient method of operation for both dolomite and limestone based sorbents. Finally, it must be mentioned that the energy consumed for sound generation is, on its largest part, wasted as heat in the loudspeaker rather than being transferred to the sound wave emitted by the loudspeaker. (This is a fact common to all loudspeakers and not a particular fact of the model used in the experiment.) At room temperature, a sound wave of intensity 140 dB (equal to 100 W/m2) has an energy density of only 0.29 J/m3, and these values become smaller at larger temperatures due to the increase in the sound speed with gas temperature. While the loudspeaker is on, its energy output as sound must balance the energy lost in the cavity formed by the reactor and the sound wave. To make an estimation, as solid surfaces tend to be good reflectors of sound, we can assume sound energy is lost only by absorption in the particles (in fact, as we have discussed, this is what drives acoustic streaming in the bed). For the cross sectional area of our bed (15.90 cm2), this means only 0.16 W are absorbed by the bed, which is a small amount of the power consumption of the loudspeaker. Therefore, it is expected that if the experiment is scaled up, there would not be much need to increase the energy spent on sound generation provided measures are taken to ensure the sound waves are enclosed in the reactor and hence all the sound energy is absorbed by the particles rather than allowing it to escape out of the reactor.

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CONCLUSIONS In this work we have shown that application of pulsed sound either during calcination or carbonation enhances the capture 8677

DOI: 10.1021/acs.iecr.6b01616 Ind. Eng. Chem. Res. 2016, 55, 8671−8678

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(13) Readman, J.; Blom, R. The Use of in situ Powder X-ray Diffraction in the Investigation of Dolomite as a Potential Reversible High-Temperature CO2 Sorbent. Phys. Chem. Chem. Phys. 2005, 7, 1214. (14) Li, L.; King, D. L.; Nie, Z.; Howard, C. Magnesia-Stabilized Calcium Oxide Absorbents with Improved Durability for High Temperature CO2 Capture. Ind. Eng. Chem. Res. 2009, 48, 10604. (15) Valverde, J. M.; Raganati, F.; Quintanilla, M. A. S.; Ebri, J. M. P.; Ammendola, P.; Chirone, R. Enhancement of CO2 Capture at Calooping Conditions by High-Intensity Acoustic Fields. Appl. Energy 2013, 111, 538. (16) Valverde, J.; Ebri, J. M. P.; Quintanilla, M. A. S. Acoustic Streaming Enhances the Multicyclic CO2 Capture of Natural Limestone at Ca-looping Conditions. Environ. Sci. Technol. 2013, 47, 9538. (17) Valverde, J. Convection and Fluidization in Oscillatory Granular Flows: The Role of Acoustic Streaming. Eur. Phys. J. E: Soft Matter Biol. Phys. 2015, 38, 38. (18) Gidaspow, D. Multiphase flow and fluidization: continuum and kinetic theory descriptions; Academic Press: 1994. (19) Valverde, J. M. Acoustic Streaming in Gas-Fluidized Beds of Small Particles. Soft Matter 2013, 9, 8792. (20) Valverde, J.; Sanchez-Jimenez, P.; Perez-Maqueda, L. Ca-looping for Postcombustion CO2 Capture: A Comparative Analysis on the Performances of Dolomite and Limestone. Appl. Energy 2015, 138, 202. (21) Haydock, D.; Yeomans, J. Lattice Boltzmann Simulations of Attenuation-Driven Acoustic Streaming. J. Phys. A: Math. Gen. 2003, 36, 5683. (22) Haydock, D.; Yeomans, J. Acoustic Enhancement of Diffusion in a Porous Material. Ultrasonics 2003, 41, 531.

capacity of natural limestone and dolomite beds operated at calcium-looping conditions. The application of sound on unmovable particles leads to acoustic streaming resulting from the frictional dissipation of energy in a boundary layer nearby the solids, which promotes heat and mass transfer thus accelerating both carbonation and calcination. Acoustic streaming enhances also momentum transfer as seen from the increase of the pressure drop across the bed when sound is applied. This effect is more marked when the pulsed sound is applied during the calcination stage arguably due to the relatively higher temperature, which promotes further the gas viscosity and therefore acoustic streaming.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Andalusian Regional Government Junta de Andalucia (Contract FQM-5735), Spanish Government Agency Ministerio de Economia y Competitividad and FEDER Funds (Contract CTQ201452763-C2-2-R). The Microscopy, Functional Characterization and X-ray services of the Innovation, Technology and Research Center of the University of Seville (CITIUS) are gratefully acknowledged.

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DOI: 10.1021/acs.iecr.6b01616 Ind. Eng. Chem. Res. 2016, 55, 8671−8678