ARTICLE pubs.acs.org/IECR
Microwave Swing Regeneration vs Temperature Swing Regeneration—Comparison of Desorption Kinetics Robert Cherbanski,*,† Magdalena Komorowska-Durka,‡ Georgios D. Stefanidis,‡ and Andrzej I. Stankiewicz‡ † ‡
Chemical and Process Engineering Department, Warsaw University of Technology, ul. Warynskiego 1, 00-645 Warszawa, Poland Process and Energy Department, Mechanical, Maritime and Materials Engineering Faculty, Delft University of Technology, Leeghwaterstraat 44, 2628 CA Delft, The Netherlands
bS Supporting Information ABSTRACT: This paper presents a comparison of microwave swing regeneration (MSR) and temperature swing regeneration (TSR) of acetone and toluene from 13X molecular sieves in terms of desorption kinetics and desorption efficiencies. The experiments were performed for two forms of the adsorbent: adsorbent bed consisting of spherical beads and adsorbent pressed in the shape of pastilles to allow for precise temperature measurement of the solid adsorbent. In TSR the adsorbent is heated by means of a hot inert gas stream whereas in MSR the adsorbent dissipates microwave energy into heat. It was found that MSR runs faster even when the adsorbent temperature is much lower than the gas temperature in TSR. This implies more efficient desorption due to less energy waste in the form of heat losses and less sensible enthalpy of purge gas stream since the total gas consumption is considerably decreased. The observed enhancement of microwave-driven desorption is more pronounced for the polar adsorbate (acetone) or high heat transfer resistances (pastilles). Finally, it was verified that microwaves do not affect the adsorption capacity of the molecular sieves after several consecutive adsorptiondesorption cycles.
1. INTRODUCTION Desorption plays a significant role in numerous integrated or complex processes, such as reactions carried out in adsorptive reactors,1 removal of volatile organic compounds (VOCs) and hazardous air pollutants (HAPs), or soil remediation.2,3 Selective and efficient desorption can significantly increase the overall efficiency of the aforementioned processes. Three desorption methods are typically applied in laboratory and industrial practices. These include pressure swing regeneration (PSR), temperature swing regeneration (TSR), and reactive regeneration (RR).4 The PSR and TSR acronyms are used throughout, instead of the most common notation of PSA (pressure swing adsorption) and TSA (temperature swing adsorption), as the objective of this work is to compare desorption (regeneration) steps only. Particularly, PSR is very common, but its application becomes inconvenient and less efficient for processes carried out at low operating pressures because additional compressors and vacuum pumps are required. TSR cycles are usually conducted by providing heat by hot purge gas or superheated steam.3 The time needed to swing adsorbent beds over a temperature range can be relatively long as the vessel and the adsorbent bed need to be heated. Additionally, using hot purge gas leads to high dilution of the desorbed phase and therefore requires low condensation temperatures if the adsorbate needs to be reused. Besides, when steam is used, the adsorbent requires drying after each regeneration step. This implies an increased energy penalty when conventional TSR is used (as a large amount of gas needs to be heated to high temperature) and an inherently slow process. These limitations also hold for desorptive soil remediation and for VOCs and HAPs removal processes. r 2011 American Chemical Society
In view of these limitations, alternative energy transfer techniques have attracted attention.5 These include Joule’s heat generated inside the adsorbent particles by passing an electric current through them,6 and indirect heating,5 as well as microwave heating.7 In this context, application of microwaves for fast and efficient regeneration of the adsorbent bed is considered as an alternative heating method for intensification of TSR. Convective heating requires use of a heating medium (for example stripping gas), whereas under microwave conditions heat is generated directly inside the adsorbent bed. In principle, the direct interaction of microwaves with the adsorbent (in case when the adsorbent material is heated), and selective interaction of microwaves with the adsorbate, can enable a faster process with a lower purge gas flow rate and lower process temperature that can in turn be translated into energy savings. As the electromagnetic energy is converted to thermal energy inside the heated adsorbent (activated carbon or zeolite), there is heat flow from inside of the adsorbent bed (hot area) to the outside (cool area). In contrast, when heating by hot purge gas or superheated steam, the temperature gradient is the opposite. Consequently, the desorbed molecules released in the core of the adsorbent bed diffuse toward the lower temperature region more promptly. As the diffusion toward the surface is the ratedetermining step in the process, the desorption process is favored during microwave irradiation.8,9 The benefit of using microwave regeneration is that heating depends on the dielectric properties Received: December 13, 2010 Accepted: June 15, 2011 Revised: June 14, 2011 Published: June 15, 2011 8632
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Industrial & Engineering Chemistry Research of the workload (i.e., adsorbate and/or adsorbent), rather than on the purge gas flow rate which can be minimized.10 Regeneration of various types of adsorbents has been studied by means of microwave heating, including silica gel,11 zeolites with different silica to alumina ratio,12,13 activated carbon,8,9,14,15 and polymeric adsorbents.12,13 The most commonly used adsorbent for VOC removal is, indeed, activated carbon. However, carbon may pose a fire hazard in such a system. A distinctive advantage of using zeolites as adsorbent, instead of activated carbon, for VOCs removal is safer operation. In addition, zeolites have good adsorption capacities under humid conditions,16 whereas in the case of active carbon, 10% and 20% humidity may cause a decrease in adsorption activity of about 20% and 40%, respectively.17 Microwave-induced swing regeneration (MSR) is still under investigation. Overviews of state-of-the-art developments have been presented previously.7,18 Recently, Polaert et al.19 presented a broad experimental study on desorption with several types of adsorbents (activated alumina, silica, NaX, and NaY zeolites), which possess different dielectric properties and porosity. In addition, the effect of different adsorbates (water, toluene, methylcyclohexane, and n-heptane) containing seven carbon atoms with varied molecular structure and varied polarity was studied. This systematic approach has helped to identify the key controlling parameters of desorption process under microwaves. The most important conclusion from this study is that desorption is governed by the absorbed power over the course of the process. In addition, the dielectric properties of the system, consisting of the adsorbent and adsorbate as well as the temperature variations, are more important than the porous structure of the solid adsorbent and the molecular structure of adsorbates. The polarity of adsorbates is also a factor of prime importance as it determines the initial level of electromagnetic energy conversion. On the other hand, Hashisho et al.10 showed that for activated carbon fiber cloth (adsorbent), the polarity of the adsorbate does not have a significant impact on the regeneration process. Microwave regeneration can be effective for both polar and nonpolar adsorbates (methyl ethyl ketone and tetrachloroethylene). It was concluded, based on comparison of the temperature profiles for adsorbent with both loadings, that in case of low adsorbate loading, microwave heating is governed by the loss factor of the activated carbon fiber cloth. Chen et al.20 performed microwave regeneration of activated carbon loaded with toluene. The study was focused on the effect of operating conditions on the regeneration ratio. A number of process parameters such as applied power, mass of saturated activated carbon, purge gas flow rate, and irradiation time were examined. The optimal conditions for the maximum regeneration ratio (77.2%), were 500 W microwave power, 60 mL/min carrier gas flow rate, and 180 s irradiation time for saturated activated carbon mass of 5.01 g. It was concluded that the amount of activated carbon and the packing density affect the heat transfer and temperature distribution in the vessel. Moreover, high purge gas flow rates decrease the adsorbent bed temperature, whereas no purge gas leads to self-burning of the carbon. Therefore, low flow rate of purge gas is required to ensure safe operation and high regeneration ratio. Besides, microwave power has a significant effect on the regeneration ratio. More specifically, it has been shown that for 350 and 500 W of applied power, the regeneration ratio was 58% and 63%, respectively. When a higher power was applied (700 W), the regeneration ratio considerably decreased (40%).
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Han et al.21 studied the effect of microwave irradiation on desorption of malachite green from natural zeolite. Their results show that the microwave power applied in a single mode cavity (range 160800 W), affecting the thermal conditions in the loaded bed, influences the regeneration yield of zeolite. More specifically, after 10 min of microwave irradiation the yield was 85.8%, whereas with conventional electric thermal treatment, the yield reached 78.6, 78.2, and 81.7% after 30 min of heating at temperatures of 300, 500, and 700 °C, respectively. Additionally, the regeneration yield under microwaves changed with particle size of zeolite. After three successive regeneration cycles, the yield decreased insignificantly from 85.8 to ∼80% due to the reduced zeolite microporosity. Wang et al.22 reported on desorption of dye reactive red 3BS from carbon nanotubes under microwave irradiation over 15 min of irradiation with 500 W in a domestic oven. Decrease in the adsorption capacity to 92.8% of the original capacity was observed after 4 cycles, which may be due to coke formation from the decomposition of organic residues and changes in the pore structure; the specific surface area and pore volume were reduced by 74% and 72%, respectively. In multicomponent adsorbate systems, selective separation based on the polarity of adsorbates involved is possible when a transparent adsorbent is used. Microwave energy is selectively dissipated where the polar molecule is adsorbed, whereas nonpolar molecules are strongly bonded and do not desorb. From experiments with a DAY adsorbent, it was observed that as long as the polar adsorbent is present the temperature of the bed increases. At longer times, when the concentration of polar molecules in the purge gas decreases, the temperature of the fixed bed decreases as well.11 A similar conclusion was also reached for the case of a microwave-induced desorption process with a single adsorbate; that is, when a transparent adsorbent is used, microwave energy selectively couples with the polar adsorbate (i.e., water).19 Microwave transparent adsorbents (i.e., polymeric and with low silica content) are attractive because they result in longer penetration depth and more uniform heating due to the relatively low dielectric loss constant.15 Most importantly, when employing a transparent adsorbent, there is no need to heat the entire bed, which entails reduced energy consumption for heating. According to Vallee et al., the “effective” temperature of the adsorbent surface (oxides), where desorption occurs, is expected to be higher than the measured temperature of the solid bed or the purge gas temperature.23 Successive heating and cooling cycles may partially damage the carbon adsorbent resulting in reduction in the adsorption capacity. Differences in the porous structure of nonsaturated activated carbon while exposed to high temperature are caused by two effects: (1) thermal effects causing collapse of the porous structure (structural annealing),9 and (2) formation of coke inside the pores decreasing the active surface and volume of the porous structure.17 This effect is more pronounced when the adsorbent is conventionally heated.9 In contrast, microwave-induced regeneration of activated carbon restored the original adsorption capacity.15,24 Liu et al. have shown that the adsorption capacity was higher for GAC (granulated activated carbon) when regenerated under microwaves (7 cycles). This effect is related to an increase in the surface area and total pore volume, which were found to be 10% and 5% higher, respectively, than for the original GAC.25 A similar conclusion was also reached by Cha et al., where the adsorption capacity after 9 cycles increased from 10 to 30 g NOx per 100 g of char.24 8633
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Industrial & Engineering Chemistry Research Disintegration of zeolite framework is possible under hydrothermal conditions (high temperature and high water vapor pressure). Protons add to a zeolite framework causing displacement of aluminum atoms in the framework. Dealumination results in blocking of channels and cages, which decreases the adsorption capacity. In ref 26, it was shown that after each treatment under microwaves the adsorption capacity decreased by 1.5%. Under microwave heating, zeolite (type 4A and 3A) transformations to other crystalline material occur. Based on DTA analysis, these transformations require temperatures higher than 924 and 997 °C for zeolites 4A and 3A, respectively. However, the measured temperatures were 860 and 600 °C, respectively. It was concluded that the existence of cations on the 4-ring site of zeolite determines the susceptibility to microwaves.27 In the present work, kinetics of MSR and TSR are compared for two adsorptives, acetone and toluene. The comparison is made on the basis of measurements of the “apparent” temperature of the zeolite bed and the pressed zeolite material. Additionally, the zeolite adsorption capacity is studied for successive adsorptiondesorption cycles. To the best of our knowledge, this work is one of the first attempts to systematically compare kinetics of TSR and MSR for molecular sieves.
2. EXPERIMENTAL SECTION 2.1. Adsorbent and Adsorbates. 13X molecular sieves (1.0:1.0:2.5:x Na2O/Al2O3/SiO2/H2O), supplied by Soda Ma) twy
Figure 1. (Left) Pressed 13X molecular sieves (pastilles) and 13X molecular sieve beads. (Right) Teflon desorber and temperature probe axially inserted into the pastilles.
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(Ciech S.A.), were used in the adsorptiondesorption experiments. The adsorbent has a spherical shape with diameter 36 mm. These spherical beads were used in one set of experiments. Another set of experiments was performed with 13X pastilles prepared by pressing the molecular sieve beads at elevated pressures in a hydraulic press (Figure 1). The motivation for making pastilles is concerned with appropriate temperature monitoring in MSRs. More specifically, in the zeolite bed, the contact of the temperature sensor with the zeolite beads is loose and, thus, the recorded temperature is most likely gas-phase temperature, which is lower than the temperature of the solid adsorbent beads. On the other hand, the pastilles have been engineered such that the temperature sensor is in contact with the adsorbent itself so that adsorbent temperature is recorded. Concerning the adsorbates, toluene (CAS 108-88-3. ACS reagent, g99.7% (GC)) and acetone (CAS 67-64-1. ACS reagent, g99.5% (GC)), purchased from Fluka, were utilized. Helium was used as purge gas (purity g99.999%, delivered by Multax, PL). 2.2. Setups. Two experimental setups were utilized to perform (1) adsorption from the gas phase (Figure 2), and (2) microwave swing regeneration (MSR) and temperature swing regeneration (TSR) (Figure 3). The multimode microwave oven was operated and the heating cord was switched off for MSRs, whereas the multimode microwave oven was switched off and the heating cord was operated for TSRs. The setups for desorption experiments consist of a gas chromatograph GC-2014 (Shimadzu Corp., Japan), a laboratory multimode microwave oven (Plazmatronika, type RM800pc Poland), two mass flow controllers (BetaErg, Poland), an ultrathermostat (Polyscience 9506, USA), a fiber optic (FO) sensor with a multichannel signal conditioner (FISO Technologies, Canada), a heated stainless steel coil, and a personal computer. Additional equipment, not shown in Figures 2 and 3, comprise a furnace with PID control (Wulkan, Poland) and a balance AG204 (Mettler Toledo, Switzerland). The off-gas concentration was measured online by the gas chromatograph equipped with a flame ionization detector (FID) and a two-position valve (6 port valve, Valco Instruments Co. Inc.) with an external sample loop (1 cm3) for automated sample injection. An insulated heated valve enclosure allows the valve to
Figure 2. Schematic diagram of the experimental setup for adsorption. 8634
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Figure 3. Schematic diagram of the experimental setup for microwave/conventional desorption (the microwave oven was operated during MSR and the heating cord was operated during TSR).
be operated at a constant temperature (120 °C). A temperature controller is used to stabilize the temperature to the desired level. The valve is actuated with a two-position standard electric actuator (Valco Instruments Co. Inc.). Using the equipment, the vapor sample flows through the external loop while the carrier gas flows directly through the chromatographic column. When the valve is switched to the other position, the sample contained in the sample loop and the valve flow passage is injected into the column. The desorber, made of Teflon, is placed in a multimode cavity of the microwave oven with a continuous variable power source (max. 700 W) operating at 2.45 GHz. The cavity is equipped with a microwave choked outlet, which is needed to introduce the purge gas and to evacuate a vapor-laden gas stream to/from the microwave applicator through Teflon tubing. The tubing was connected to a Teflon cylinder (the desorber, i.d. 14 mm), which was fixed to a Teflon plate lying at the bottom of the microwave applicator. Additionally, stainless steel tubing, outside of the microwave cavity, that drives the vapor-laden gas stream to the two position valve was thermostatted at 100 °C to avoid vapor condensation. Due to the limited penetration depth of microwaves into the solid body of the adsorbent, the Teflon desorber has its inner diameter safely lower than the calculated penetration depth of microwaves into the 13X molecular sieve beads (Dp = 17.3 cm, calculated from eq 1,28,29). Dp ¼
λ0 1 pffiffiffiffiffiffiffiffiffiffi vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 0vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 u 0 2π ð2εr Þ u u !2 00 uB u C u@ t1 + ε r 1A t ε0r
ð1Þ
where ε0 r and ε00 r are set to 2.55 and 0.18, respectively.30 A heated stainless steel coil was used for gas preheating in the TSR experiments, and the gas flow rate in TSR and MSR was controlled at a constant value by mass flow controllers. The electric furnace (max. 2400 W) equipped with a PID controller was used for degassing the adsorbent at a constant temperature. Weighing of the outgassed, loaded, and regenerated adsorbents was carried out using a scale having readability of 0.1 mg. 2.3. Calibration Curves. Two calibration curves, one for each adsorbate (acetone and toluene), were made to calculate adsorbate concentrations in the purge gas. The calibration curves were
obtained according to the standard external method by comparing the chromatographic responses of the reference samples with their concentrations in the gas (see Supporting Information). These calibration curves were determined for the temperature at which the external sample loop was operated (393.15 K). The adsorbate concentration in the gas at 393.15 K (c393.15) is calculated, according to eq 2, from the known concentration (cr) of the compound in the reference solution, the volume (ν) of the solution microsample (5 μL) injected into the GC by a Hamilton syringe, and the volumetric capacity of the thermostatted external sample loop (1 cm3): cr ν c393:15 ¼ ð2Þ 1 cm3 2.4. Adsorption. Prior to adsorption, the 13X molecular sieves were outgassed in the furnace by heating at 300 °C for 4 h. Then the outgassed adsorbent was weighed and immediately placed into the U-shaped glass adsorber vessel (see Figure 2). The adsorber and the bubbling washer were thermostatted during adsorption at 20 °C. A fixed adsorbate concentration in the gas was achieved by dilution of the vapor-laden gas stream leaving the bubbling washer with the inert gas. The diluted vapor stream flowed into the adsorber, where the adsorbent bed was placed. The gas stream leaving the adsorber was analyzed by the GC and the chromatographic signal was recalculated to obtain the adsorbate concentration in the gas using the appropriate calibration curve. Adsorption was terminated after an overnight process when subsequent signals from the FID detector remained constant with time. Then, the loaded adsorbent was weighed. The relative vapor pressure of the adsorbate and the adsorption capacity were obtained from the following calculations: • Relative vapor pressure of an adsorbate at 293.15 K: p R c293:15 ¼ 293:15 p0 M p0
ð3Þ
where c293.15 = c393.15(393.15)/(293.15) • Adsorptive capacity (kg/kg):
q¼ 8635
ma mads
ð4Þ
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The influence of microwave irradiation on the adsorption capacity q (kg adsorbate/kg adsorbent) of the 13X molecular sieves (spherical beads) was examined by performing several adsorption and desorption cycles using the same adsorbent bed for each adsorbate. Table 1 presents the conditions of the performed desorption experiments and the results for two adsorbates: acetone and toluene. Six adsorptiondesorption cycles with each adsorbate were completed. After saturation in each cycle, the adsorbent bed was partly regenerated. The time spans for microwave desorption of acetone and toluene were 56 and 112 min, respectively. The initial relative vapor pressure of the adsorbate (calculated from eq 3) in each cycle was in the range 0.670.77 (Pa/Pa) for acetone and 0.690.74 for toluene (listed in the fourth column of Table 1). Constant power level of Table 1. Process Conditions for the Performed Consecutive AdsorptionDesorption Cycle Experiments under Microwaves md/ma
adsorption p/p0 (-)
q (kg/kg)
(%)
I
0.77
0.120
48.7
II
0.69
0.125
48.4
acetone
III
0.67
0.124
39.0
acetone acetone
IV V
0.67 0.71
0.127 0.125
42.3 56.9
A6
acetone
VI
0.72
0.124
49.5
T1
toluene
I
0.74
0.145
47.7
T2
toluene
II
0.69
0.144
62.4
T3
toluene
III
0.69
0.146
81.9
T4
toluene
IV
0.71
0.140
49.0
T5
toluene
V
0.69
0.144
78.1
T6
toluene
VI
0.70
0.142
73.7
name
adsorbate
A1
acetone
A2
acetone
A3 A4 A5
desorption cycle
203 W was used in each experiment. The volumetric flow rate of the purge gas (helium) was constant for each experiment and equal to 10 Ncm3/min. The adsorption capacity of the 13X molecular sieves for acetone and toluene (fifth column, Table 1) after each regeneration cycle remained approximately constant at ∼0.12 (kg/kg) and ∼0.14 (kg/kg), respectively. It can be concluded that microwaves do not change the capacity of the molecular sieves at the conditions studied (relatively low temperature). However, a more detailed investigation of the structure at higher temperature has to be done to exclude changes in morphology. These results are in line with results for activated carbon for consecutive regeneration cycles under microwave heating where the adsorption capacity was higher in comparison to conventional heating conditions.8,9,15 In the rightmost column of Table 1, desorption efficiency coefficients (md/ma, fraction of adsorbed mass that is desorbed) are listed. For the microwave experiments with acetone and toluene as adsorbates the value is in the range of 39.056.9% and 47.781.9%. 2.5. Desorption. For comparison of desorption kinetics between TSR and MSR two sets of experiments were performed with acetone (A) and toluene (T) as adsorbate using the two different forms of adsorbent described in Section 2.1, that is, spherical beads and specially prepared pastilles (p). Any experiment for each combination of these parameters (heating mode, adsorbate, and adsorbent) was repeated at least twice. For each experiment, a fresh portion of degassed adsorbent was used. The mass of unloaded adsorbent was ∼2.68 g in the case of beads and in the range 2.382.78 g in the case of pastilles (column 4, Table 2), while the adsorption capacity q (kg/kg, column six) is in the range 0.1440.206. The initial relative vapor pressure of the adsorbate (calculated from eq 3) in each cycle was in the range 0.851.0 (Pa/Pa) for acetone and 0.130.27 for toluene (fifth column, Table 2). The MSR experiments were performed with a constant power level of 140 W. In TSR, the power was
Table 2. Operating Conditions of the Desorption Experiments: MSR, Microwave Swing Regeneration; TSR, Temperature Swing Regeneration name
adsorbent form
adsorbate
mads (g)
p/p0 (-)
q (kg/kg)
PMW (W)
desorption time (min)
A1/MSR
beads
acetone
2.6865
1.00
0.157
140
18
A2/TSR
beads
acetone
2.6808
0.96
0.191
-
18
A3/MSR A4/TSR
beads beads
acetone acetone
2.6810 2.6869
0.97 0.96
0.183 0.177
140 -
18 18
A5/MSR
beads
acetone
2.6890
0.98
0.203
140
2
A6/TSR
beads
acetone
2.6873
0.97
0.205
-
2
A7/TSR
beads
acetone
2.6810
0.92
0.206
-
2
A8p/MSR
pastilles
acetone
2.6141
0.93
0.173
140
2
A9p/TSR
pastilles
acetone
2.7050
0.91
0.138
-
2
A10p/MSR
pastilles
acetone
2.7733
0.91
0.164
140
2
A11p/TSR T1/MSR
pastilles beads
acetone toluene
2.7815 2.6836
0.85 0.13
0.171 0.155
140
2 2
T2/TSR
beads
toluene
2.6880
0.22
0.159
-
2
T3/MSR
beads
toluene
2.6838
0.20
0.156
140
2
T4/TSR
beads
toluene
2.6877
0.23
0.165
-
2
T5/MSR
beads
toluene
2.6893
0.23
0.161
140
2
T6/TSR
beads
toluene
2.6860
0.22
0.160
-
2
T7p/MSR
pastilles
toluene
2.5425
0.25
0.144
140
2
T8p/TSR T9p/MSR
pastilles pastilles
toluene toluene
2.4277 2.3813
0.24 0.27
0.147 0.144
140
2 2
T10p/TSR
pastilles
toluene
2.4515
0.27
0.154
-
2
8636
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Figure 4. Comparison of desorbed mass between TSR and MSR of 13X beads and pastilles from toluene (nonpolar). See also Table 2 for the specific conditions of each experiment.
controlled manually in order to achieve temperature profiles comparable to MSR. MSRs and TSRs were conducted at the same volumetric flow rate of the purge gas (2000 Ncm3/min), chosen so that it provides turbulent conditions to mitigate external mass transfer limitations and allow for investigation of desorption kinetics. Finally, TSR is performed by means of a hot purge gas heated conventionally in a coil pipe, whereas MSR is performed by heating the 13X molecular sieve beads in a multimode microwave oven. The desorption experiments were started immediately after placing the loaded adsorbent into the desorber. During desorption, the vapor samples were taken automatically every 30 s by the twoposition valve and were examined in the GC. Desorption was terminated either after 2 min or after 18 min. Then the partly regenerated adsorbent was weighed immediately. A comparison of desorption kinetics was carried out by employing desorption rate profiles, which are generated through the following calculation steps: 1. The adsorbate concentration in the purge gas is calculated at the desorption temperature T: cT ¼ c393:15
393:15 T + 273:15
ð5Þ
2. The volumetric flow rate of the purge gas at the desorption temperature is calculated as T + 273:15 Q_ T ¼ Q_ 273:15 273:15
ð6Þ
· where Q273.15 (displayed on the mass flow controller) is the volumetric flow rate of the purge gas at the normal temperature (273.15 K). 3. Based on eqs 5 and 6, the total mass of desorbed adsorbate is calculated as Z 393:15 t 393:15 c dt ð7Þ md ¼ Q_ 273:15 273:15 0
Finally, the desorption rate is: r ¼
Δmd Δt
ð8Þ
A verification of the calculation method was performed by comparing the total mass of desorbed adsorbate calculated by eq 7 with gravimetric measurements. The results of the comparisons for toluene and acetone are presented in Figures 4 and 5, respectively. Figures 4 and 5 show discrepancy between gravimetric and chromatographic analysis for some experiments (T7p/MSR, A1/MSR, A3/MSR). The reason for that may be traced to the uncontrolled desorption over the time period taken to place the FO sensor into the pastilles or beads of the loaded adsorbent, after the latter was weighed and before the desorption process got started. Furthermore, the differences observed in desorbed mass (GC and balance) among repeated experiments performed with the same adsorbate, the same form of adsorbent (beads or pastilles), and for the same time (2 or 18 min) are due to (A) differences in the adsorbate loadings (e.g., in Table 2, q (kg/kg) = 0.157 and 0.183 for experiments A1 and A3, respectively) and (B) differences in the temporal temperature profiles as will be presented in the following graphs. Desorption Rates with 13X Beads. In desorption experiments with 13X beads the temperature profiles obtained from MSR are followed in TSR with appropriate preheating of the purge gas. Practically, the temperature measurements in MSR and TSR were realized by inserting an FO sensor into the adsorbent bed. One must bear in mind that MSR and TSR employ different heat transfer mechanisms. In TSR heat is introduced into the adsorbent with a preheated gas stream, whereas in MSR heat is produced within the adsorbent volumetrically. As it was mentioned before (Section 2.1), the utilized technique of temperature measurement provides information concerning gas-phase 8637
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Figure 5. Comparison of desorbed mass between TSR and MSR of 13X beads and pastilles from acetone (polar). See also Table 2 for the specific conditions of each experiment.
Figure 6. Comparison of acetone desorption rates for MSRs at 140 W and TSRs of 13X beads carried out under the temperature conditions presented in Figure 7.
temperature which is the reference temperature in this set of experiments. It must be stressed that the same value of reference temperature implies a higher adsorbent temperature in MSR than in TSR and, in turn, the higher adsorbent temperature enhances the diffusivity of adsorbates in the pores of the adsorbent. The results presented in Figures 6 and 8 show desorption rate profiles of acetone and toluene for MSR and TSR. Figures 7 and 9 show the corresponding temperature profiles. MSRs were performed at 140 W of microwave power (power generated by the magnetron). It is noted here that the desorption rate profiles for the experiments performed with the same adsorbate and the same adsorbent form (e.g., A1/MSR, A3/MSR, A5/MSR) differ from
each other due to somewhat varying solid-phase concentrations of adsorbate and desorption temperatures. The comparison of the two heating methods based on Figures 69 reveals the following: (1) The maximum acetone desorption rates for MSRs are apparently higher than the maximum desorption rates for the corresponding TSRs. This holds even when two TSR runs (A6/ TSA, A7/TSA) were carried out at higher temperatures than the temperatures measured in the MSRs (see Figures 8 and 9). (2) The toluene desorption rates with MSR and TSR at 140 W are approximately equal although the temperatures recorded for all TSRs are clearly higher than the temperatures for MSRs (see Figures 8 and 9). 8638
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Figure 7. Temperature profiles obtained for MSRs at 140 W (solid symbols) and TSRs (open symbols) of 13X beads from acetone.
Figure 8. Comparison of toluene desorption rates for MSRs at 140 W and TSRs of 13X beads carried out under the temperature conditions presented in Figure 9.
(3) There are evident differences in the adsorbent bed temperatures after 2 min of microwave regeneration when the adsorbent is loaded with acetone and toluene (see Figures 7 and 9). This implies selective interaction of microwaves with the good microwave absorber (acetone), which is favorable from the energy efficiency point of view. Desorption Rates with 13X Pastilles. In desorption experiments with 13X pastilles, the adsorbent temperature profiles obtained from MSRs are again followed in TSRs by appropriate preheating of the purge gas. In the case of 13X pastilles, though, a different temperature measurement approach was considered, compared to the case of zeolite beads. In particular, the adsorbent temperature was measured by the FO sensor which was placed axially in the specially prepared adsorbent pastilles (see Figure 1), whereas the gas temperature was measured by the same FO
sensor fixed at the gas inlet of the desorber after removing the adsorbent pastilles and recording the temperature profile starting from the same conditions as in the main desorption experiment. The current approach concerning temperature measurements was applied to obtain the surface adsorbent temperature. As in the previous section, Figures 10 and 12 show desorption rate profiles of acetone and toluene, respectively, and Figures 11 and 13 present the corresponding temperature profiles. MSRs were again performed at 140 W of magnetron power. As expected, the temperatures of 13X pastilles in TSRs are significantly lower than the gas-phase temperatures due to the heat transfer limitations pertaining to convective heating (see Figures 11 and 13). Besides, it is observed that the adsorbent temperatures are lower in acetone desorption than in toluene desorption despite the gas-phase temperatures being higher in 8639
dx.doi.org/10.1021/ie102490v |Ind. Eng. Chem. Res. 2011, 50, 8632–8644
Industrial & Engineering Chemistry Research
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Figure 9. Temperature profiles obtained for MSRs at 140 W (solid symbols) and TSRs (open symbols) of 13X beads from toluene.
Figure 10. Comparison of acetone desorption rates for MSRs at 140 W and TSRs of 13X pastilles carried out under the temperature conditions presented in Figure 11.
the former case. The difference is due to the lower heat of evaporation of toluene compared to acetone (401 vs 515 kJ/kg, respectively, at 50 °C) and the higher desorption rates of acetone compared to toluene (see Figures 10 and 12). Because a very high gas flow rate is utilized in the desorber to mitigate external mass transfer limitations, the adsorbent temperature is the key factor ruling desorption kinetics, as it governs the diffusivity of the adsorbates in the pores of the adsorbent. In the performed experiments, the temperature of the adsorbent is significantly lower in TSR compared to MSR, in which heat is directly and instantaneously dissipated inside the solid structure of the adsorbent. Therefore, the higher adsorbent temperature in MSR, compared to TSR, is responsible for the observed enhancement of the microwave-driven regeneration (Figure 10).
However, in the case of toluene desorption, when comparing the corresponding temperature profiles of 13X pastilles in TSRs and MSRs, the cause of the observed enhancement in MSR is not so evident. Clearly, the toluene desorption rates in MSRs are higher than the corresponding desorption rates in TSRs for times over 0.5 min (Figure 12); this is in keeping with the higher adsorbent temperature in MSRs after 0.5 min (Figure 13). On the other hand, it is unexpected that despite the lower adsorbent temperature in MSRs for desorption times below 0.5 min (Figure 13), the desorption rates in MSRs are higher than those in TSRs in the corresponding time interval (
