Published: June 15, 2011

r 2011 American Chemical Society 8632 dx.doi.org/10.1021/ie102490v | Ind. Eng. Chem. Res. 2011, 50, 8632–8644

ARTICLE

pubs.acs.org/IECR

Microwave Swing Regeneration vs Temperature SwingRegeneration—Comparison of Desorption KineticsRobert Cherba�nski,*,†Magdalena Komorowska-Durka,‡Georgios D. Stefanidis,‡ and Andrzej I. Stankiewicz‡

†Chemical and Process Engineering Department, Warsaw University of Technology, ul. Wary�nskiego 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. Theexperiments were performed for two forms of the adsorbent: adsorbent bed consisting of spherical beads and adsorbent pressed inthe shape of pastilles to allow for precise temperature measurement of the solid adsorbent. In TSR the adsorbent is heated by meansof a hot inert gas stream whereas in MSR the adsorbent dissipates microwave energy into heat. It was found that MSR runs fastereven when the adsorbent temperature is much lower than the gas temperature in TSR. This implies more efficient desorption due toless energy waste in the form of heat losses and less sensible enthalpy of purge gas stream since the total gas consumption isconsiderably 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 ofthe molecular sieves after several consecutive adsorption�desorption cycles.

1. INTRODUCTION

Desorption plays a significant role in numerous integrated orcomplex processes, such as reactions carried out in adsorptivereactors,1 removal of volatile organic compounds (VOCs) andhazardous air pollutants (HAPs), or soil remediation.2,3 Selectiveand efficient desorption can significantly increase the overallefficiency of the aforementioned processes.

Three desorption methods are typically applied in laboratoryand industrial practices. These include pressure swing regenera-tion (PSR), temperature swing regeneration (TSR), and reactiveregeneration (RR).4 The PSR andTSR acronyms are used through-out, instead of the most common notation of PSA (pressureswing adsorption) and TSA (temperature swing adsorption), asthe objective of this work is to compare desorption (regeneration)steps only.

Particularly, PSR is very common, but its application be-comes inconvenient and less efficient for processes carried outat low operating pressures because additional compressorsand vacuum pumps are required. TSR cycles are usuallyconducted by providing heat by hot purge gas or superheatedsteam.3 The time needed to swing adsorbent beds over atemperature range can be relatively long as the vessel and theadsorbent bed need to be heated. Additionally, using hot purgegas leads to high dilution of the desorbed phase and thereforerequires low condensation temperatures if the adsorbateneeds to be reused. Besides, when steam is used, the adsorbentrequires drying after each regeneration step. This implies anincreased energy penalty when conventional TSR is used (as alarge amount of gas needs to be heated to high temperature)and an inherently slow process. These limitations also hold fordesorptive soil remediation and for VOCs and HAPs removalprocesses.

In view of these limitations, alternative energy transfertechniques have attracted attention.5 These include Joule’sheat generated inside the adsorbent particles by passing anelectric current through them,6 and indirect heating,5 as well asmicrowave heating.7 In this context, application of microwavesfor fast and efficient regeneration of the adsorbent bed isconsidered as an alternative heating method for intensificationof TSR. Convective heating requires use of a heating medium(for example stripping gas), whereas under microwave condi-tions heat is generated directly inside the adsorbent bed. Inprinciple, the direct interaction of microwaves with the adsor-bent (in case when the adsorbent material is heated), andselective interaction of microwaves with the adsorbate, canenable a faster process with a lower purge gas flow rate andlower process temperature that can in turn be translated intoenergy savings.

As the electromagnetic energy is converted to thermal energyinside the heated adsorbent (activated carbon or zeolite), there isheat flow from inside of the adsorbent bed (hot area) to theoutside (cool area). In contrast, when heating by hot purge gas orsuperheated steam, the temperature gradient is the opposite.Consequently, the desorbedmolecules released in the core of theadsorbent bed diffuse toward the lower temperature region morepromptly. As the diffusion toward the surface is the rate-determining step in the process, the desorption process is favoredduring microwave irradiation.8,9 The benefit of using microwaveregeneration is that heating depends on the dielectric properties

Received: December 13, 2010Accepted: June 15, 2011Revised: June 14, 2011

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of the workload (i.e., adsorbate and/or adsorbent), rather thanon the purge gas flow rate which can be minimized.10

Regeneration of various types of adsorbents has been studiedby means of microwave heating, including silica gel,11 zeoliteswith different silica to alumina ratio,12,13 activated carbon,8,9,14,15

and polymeric adsorbents.12,13 The most commonly used ad-sorbent for VOC removal is, indeed, activated carbon. However,carbon may pose a fire hazard in such a system. A distinctiveadvantage of using zeolites as adsorbent, instead of activatedcarbon, for VOCs removal is safer operation. In addition, zeoliteshave good adsorption capacities under humid conditions,16

whereas in the case of active carbon, 10% and 20% humiditymay cause a decrease in adsorption activity of about 20% and40%, respectively.17

Microwave-induced swing regeneration (MSR) is still underinvestigation. Overviews of state-of-the-art developments havebeen presented previously.7,18 Recently, Polaert et al.19 presenteda broad experimental study on desorption with several types ofadsorbents (activated alumina, silica, NaX, and NaY zeolites),which possess different dielectric properties and porosity. In addi-tion, the effect of different adsorbates (water, toluene, methylcy-clohexane, and n-heptane) containing seven carbon atoms withvaried molecular structure and varied polarity was studied. Thissystematic approach has helped to identify the key controllingparameters of desorption process under microwaves. The mostimportant conclusion from this study is that desorption isgoverned by the absorbed power over the course of the process.In addition, the dielectric properties of the system, consistingof the adsorbent and adsorbate as well as the temperaturevariations, are more important than the porous structure of thesolid adsorbent and the molecular structure of adsorbates. Thepolarity of adsorbates is also a factor of prime importance as itdetermines the initial level of electromagnetic energy conversion.

On the other hand, Hashisho et al.10 showed that for activatedcarbon fiber cloth (adsorbent), the polarity of the adsorbate doesnot have a significant impact on the regeneration process.Microwave regeneration can be effective for both polar and non-polar adsorbates (methyl ethyl ketone and tetrachloroethylene).It was concluded, based on comparison of the temperatureprofiles for adsorbent with both loadings, that in case of lowadsorbate loading, microwave heating is governed by the lossfactor of the activated carbon fiber cloth.

Chen et al.20 performed microwave regeneration of activatedcarbon loaded with toluene. The study was focused on the effectof operating conditions on the regeneration ratio. A number ofprocess parameters such as applied power, mass of saturatedactivated carbon, purge gas flow rate, and irradiation time wereexamined. The optimal conditions for the maximum regenera-tion ratio (77.2%), were 500 W microwave power, 60 mL/mincarrier gas flow rate, and 180 s irradiation time for saturatedactivated carbonmass of 5.01 g. It was concluded that the amountof activated carbon and the packing density affect the heattransfer and temperature distribution in the vessel. Moreover,high purge gas flow rates decrease the adsorbent bed tempera-ture, whereas no purge gas leads to self-burning of the carbon.Therefore, low flow rate of purge gas is required to ensure safeoperation and high regeneration ratio. Besides, microwave powerhas a significant effect on the regeneration ratio. More specifi-cally, it has been shown that for 350 and 500W of applied power,the regeneration ratio was 58% and 63%, respectively. When ahigher power was applied (700 W), the regeneration ratioconsiderably decreased (40%).

Han et al.21 studied the effect of microwave irradiation ondesorption of malachite green from natural zeolite. Their resultsshow that the microwave power applied in a single mode cavity(range 160�800 W), affecting the thermal conditions in theloaded bed, influences the regeneration yield of zeolite. Morespecifically, after 10 min of microwave irradiation the yield was85.8%, whereas with conventional electric thermal treatment, theyield reached 78.6, 78.2, and 81.7% after 30 min of heating attemperatures of 300, 500, and 700 �C, respectively. Additionally,the regeneration yield under microwaves changed with particlesize of zeolite. After three successive regeneration cycles, theyield decreased insignificantly from 85.8 to ∼80% due to thereduced zeolite microporosity.

Wang et al.22 reported on desorption of dye reactive red 3BSfrom carbon nanotubes under microwave irradiation over 15 minof irradiation with 500 W in a domestic oven. Decrease in theadsorption capacity to 92.8% of the original capacity wasobserved after 4 cycles, which may be due to coke formationfrom the decomposition of organic residues and changes in thepore structure; the specific surface area and pore volume werereduced by 74% and 72%, respectively.

In multicomponent adsorbate systems, selective separationbased on the polarity of adsorbates involved is possible when atransparent adsorbent is used. Microwave energy is selectivelydissipated where the polar molecule is adsorbed, whereas non-polar molecules are strongly bonded and do not desorb. Fromexperiments with a DAY adsorbent, it was observed that as longas the polar adsorbent is present the temperature of the bedincreases. At longer times, when the concentration of polarmolecules in the purge gas decreases, the temperature of thefixed bed decreases as well.11 A similar conclusion was alsoreached for the case of a microwave-induced desorption processwith a single adsorbate; that is, when a transparent adsorbent isused, microwave energy selectively couples with the polaradsorbate (i.e., water).19 Microwave transparent adsorbents(i.e., polymeric and with low silica content) are attractive becausethey result in longer penetration depth andmore uniform heatingdue to the relatively low dielectric loss constant.15 Most im-portantly, when employing a transparent adsorbent, there is noneed to heat the entire bed, which entails reduced energyconsumption for heating. According to Vallee et al., the “effec-tive” temperature of the adsorbent surface (oxides), wheredesorption occurs, is expected to be higher than the measuredtemperature of the solid bed or the purge gas temperature.23

Successive heating and cooling cycles may partially damage thecarbon adsorbent resulting in reduction in the adsorption capacity.Differences in the porous structure of nonsaturated activated carbonwhile exposed to high temperature are caused by two effects: (1)thermal effects causing collapse of the porous structure (structuralannealing),9 and (2) formation of coke inside the pores decreasingthe active surface and volume of the porous structure.17 This effect ismore pronounced when the adsorbent is conventionally heated.9 Incontrast, microwave-induced regeneration of activated carbon re-stored the original adsorption capacity.15,24

Liu et al. have shown that the adsorption capacity was higherfor GAC (granulated activated carbon) when regenerated undermicrowaves (7 cycles). This effect is related to an increase in thesurface area and total pore volume, which were found to be 10%and 5% higher, respectively, than for the original GAC.25 Asimilar conclusion was also reached by Cha et al., where theadsorption capacity after 9 cycles increased from 10 to 30 g NOxper 100 g of char.24

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Disintegration of zeolite framework is possible under hydro-thermal conditions (high temperature and high water vaporpressure). Protons add to a zeolite framework causing displace-ment of aluminum atoms in the framework. Dealuminationresults in blocking of channels and cages, which decreases theadsorption capacity. In ref 26, it was shown that after each treatmentunder microwaves the adsorption capacity decreased by 1.5%.

Under microwave heating, zeolite (type 4A and 3A) transfor-mations to other crystalline material occur. Based on DTAanalysis, these transformations require temperatures higher than924 and 997 �C for zeolites 4A and 3A, respectively. However,the measured temperatures were 860 and 600 �C, respectively. Itwas concluded that the existence of cations on the 4-ring site ofzeolite determines the susceptibility to microwaves.27

In the present work, kinetics of MSR and TSR are comparedfor two adsorptives, acetone and toluene. The comparison ismade on the basis of measurements of the “apparent” tempera-ture of the zeolite bed and the pressed zeolite material. Addi-tionally, the zeolite adsorption capacity is studied for successiveadsorption�desorption cycles. To the best of our knowledge,this work is one of the first attempts to systematically comparekinetics 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

(Ciech S.A.), were used in the adsorption�desorption experi-ments. The adsorbent has a spherical shapewith diameter 3�6mm.These spherical beads were used in one set of experiments.Another set of experiments was performed with 13X pastillesprepared by pressing the molecular sieve beads at elevatedpressures in a hydraulic press (Figure 1). The motivation formaking pastilles is concerned with appropriate temperaturemonitoring in MSRs. More specifically, in the zeolite bed, thecontact of the temperature sensor with the zeolite beads is looseand, thus, the recorded temperature is most likely gas-phasetemperature, which is lower than the temperature of the solidadsorbent beads. On the other hand, the pastilles have beenengineered such that the temperature sensor is in contact withthe adsorbent itself so that adsorbent temperature is recorded.Concerning the adsorbates, toluene (CAS 108-88-3. ACS re-agent,g99.7% (GC)) and acetone (CAS 67-64-1. ACS reagent,g99.5% (GC)), purchased from Fluka, were utilized. Heliumwas used as purge gas (purityg99.999%, delivered by Multax, PL).2.2. Setups.Two experimental setups were utilized to perform

(1) adsorption from the gas phase (Figure 2), and (2) microwaveswing regeneration (MSR) and temperature swing regeneration(TSR) (Figure 3). Themultimodemicrowave oven was operatedand the heating cord was switched off for MSRs, whereas themultimode microwave oven was switched off and the heatingcord was operated for TSRs.The setups for desorption experiments consist of a gas

chromatograph GC-2014 (Shimadzu Corp., Japan), a laboratorymultimode microwave oven (Plazmatronika, type RM800pcPoland), two mass flow controllers (BetaErg, Poland), an ultra-thermostat (Polyscience 9506, USA), a fiber optic (FO) sensorwith 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 afurnace with PID control (Wulkan, Poland) and a balanceAG204 (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 sampleinjection. An insulated heated valve enclosure allows the valve to

Figure 1. (Left) Pressed 13X molecular sieves (pastilles) and 13Xmolecular sieve beads. (Right) Teflon desorber and temperature probeaxially inserted into the pastilles.

Figure 2. Schematic diagram of the experimental setup for adsorption.

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be operated at a constant temperature (120 �C). A temperaturecontroller 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 vaporsample flows through the external loop while the carrier gas flowsdirectly through the chromatographic column. When the valve isswitched to the other position, the sample contained in the sampleloop 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. 700W) operating at 2.45 GHz. The cavity is equipped witha microwave choked outlet, which is needed to introduce thepurge gas and to evacuate a vapor-laden gas stream to/from themicrowave applicator through Teflon tubing. The tubing wasconnected to a Teflon cylinder (the desorber, i.d. 14 mm), whichwas fixed to a Teflon plate lying at the bottom of the microwaveapplicator. Additionally, stainless steel tubing, outside of themicrowave cavity, that drives the vapor-laden gas stream to thetwo position valve was thermostatted at 100 �C to avoid vaporcondensation. Due to the limited penetration depth of micro-waves into the solid body of the adsorbent, the Teflon desorberhas its inner diameter safely lower than the calculated penetrationdepth of microwaves into the 13X molecular sieve beads(Dp = 17.3 cm, calculated from eq 1,28,29).

Dp ¼ λ02π

ffiffiffiffiffiffiffiffiffiffið2ε0rÞp 1ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

1 +ε00rε0r

!2vuut � 1

0B@

1CA

vuuuutð1Þ

where ε0r and ε00r are set to 2.55 and 0.18, respectively.30

A heated stainless steel coil was used for gas preheating in theTSR experiments, and the gas flow rate in TSR and MSR wascontrolled at a constant value by mass flow controllers. Theelectric furnace (max. 2400 W) equipped with a PID controllerwas used for degassing the adsorbent at a constant temperature.Weighing of the outgassed, loaded, and regenerated adsorbentswas 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 adsor-bate concentrations in the purge gas. The calibration curves were

obtained according to the standard external method by compar-ing the chromatographic responses of the reference samples withtheir concentrations in the gas (see Supporting Information).These calibration curves were determined for the temperature atwhich the external sample loop was operated (393.15 K). Theadsorbate concentration in the gas at 393.15 K (c393.15) iscalculated, according to eq 2, from the known concentration(cr) of the compound in the reference solution, the volume (ν) ofthe solution microsample (5 μL) injected into the GC by aHamilton syringe, and the volumetric capacity of the thermo-statted external sample loop (1 cm3):

c393:15 ¼ crν1 cm3

ð2Þ2.4. Adsorption. Prior to adsorption, the 13X molecular

sieves were outgassed in the furnace by heating at 300 �C for4 h. Then the outgassed adsorbent was weighed and immediatelyplaced into the U-shaped glass adsorber vessel (see Figure 2).The adsorber and the bubbling washer were thermostattedduring adsorption at 20 �C. A fixed adsorbate concentration inthe gas was achieved by dilution of the vapor-laden gas streamleaving the bubbling washer with the inert gas. The diluted vaporstream flowed into the adsorber, where the adsorbent bed wasplaced. The gas stream leaving the adsorber was analyzed by theGC and the chromatographic signal was recalculated to obtainthe adsorbate concentration in the gas using the appropriatecalibration curve. Adsorption was terminated after an overnightprocess when subsequent signals from the FID detector remainedconstant with time. Then, the loaded adsorbent was weighed. Therelative vapor pressure of the adsorbate and the adsorption capacitywere obtained from the following calculations:• Relative vapor pressure of an adsorbate at 293.15 K:

pp0

¼ 293:15RM

c293:15

p0ð3Þ

where c293.15 = c393.15(393.15)/(293.15)• Adsorptive capacity (kg/kg):

q ¼ ma

madsð4Þ

Figure 3. Schematic diagram of the experimental setup formicrowave/conventional desorption (themicrowave ovenwas operated duringMSR and theheating cord was operated during TSR).

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The influence of microwave irradiation on the adsorptioncapacity q (kg adsorbate/kg adsorbent) of the 13X molecularsieves (spherical beads) was examined by performing severaladsorption and desorption cycles using the same adsorbent bedfor each adsorbate. Table 1 presents the conditions of theperformed desorption experiments and the results for twoadsorbates: acetone and toluene. Six adsorption�desorptioncycles with each adsorbate were completed. After saturation ineach cycle, the adsorbent bed was partly regenerated. The timespans for microwave desorption of acetone and toluene were 56and 112 min, respectively. The initial relative vapor pressure ofthe adsorbate (calculated from eq 3) in each cycle was in therange 0.67�0.77 (Pa/Pa) for acetone and 0.69�0.74 for toluene(listed in the fourth column of Table 1). Constant power level of

203 W was used in each experiment. The volumetric flow rate ofthe purge gas (helium) was constant for each experiment andequal to 10 Ncm3/min. The adsorption capacity of the 13Xmolecular sieves for acetone and toluene (fifth column, Table 1)after each regeneration cycle remained approximately constantat ∼0.12 (kg/kg) and ∼0.14 (kg/kg), respectively. It can beconcluded that microwaves do not change the capacity of themolecular sieves at the conditions studied (relatively lowtemperature). However, a more detailed investigation of thestructure at higher temperature has to be done to exclude changes inmorphology. These results are in line with results for activatedcarbon for consecutive regeneration cycles undermicrowave heatingwhere the adsorption capacity was higher in comparison toconventional heating conditions.8,9,15 In the rightmost column ofTable 1, desorption efficiency coefficients (md/ma, fraction ofadsorbed mass that is desorbed) are listed. For the microwaveexperiments with acetone and toluene as adsorbates the value is inthe range of 39.0�56.9% and 47.7�81.9%.2.5. Desorption. For comparison of desorption kinetics

between TSR and MSR two sets of experiments were performedwith acetone (A) and toluene (T) as adsorbate using the twodifferent forms of adsorbent described in Section 2.1, that is,spherical beads and specially prepared pastilles (p). Any experi-ment for each combination of these parameters (heating mode,adsorbate, and adsorbent) was repeated at least twice. For eachexperiment, a fresh portion of degassed adsorbent was used. Themass of unloaded adsorbent was∼2.68 g in the case of beads andin the range 2.38�2.78 g in the case of pastilles (column 4,Table 2), while the adsorption capacity q (kg/kg, column six) isin the range 0.144�0.206. The initial relative vapor pressure ofthe adsorbate (calculated from eq 3) in each cycle was in therange 0.85�1.0 (Pa/Pa) for acetone and 0.13�0.27 for toluene(fifth column, Table 2). The MSR experiments were performedwith a constant power level of 140 W. In TSR, the power was

Table 1. Process Conditions for the Performed ConsecutiveAdsorption�Desorption Cycle Experiments underMicrowaves

name adsorbate

adsorption�desorption cycle p/p0 (-) q (kg/kg)

md/ma

(%)

A1 acetone I 0.77 0.120 48.7

A2 acetone II 0.69 0.125 48.4

A3 acetone III 0.67 0.124 39.0

A4 acetone IV 0.67 0.127 42.3

A5 acetone V 0.71 0.125 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

Table 2. Operating Conditions of the Desorption Experiments: MSR, Microwave Swing Regeneration; TSR, Temperature SwingRegeneration

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 beads acetone 2.6810 0.97 0.183 140 18

A4/TSR beads acetone 2.6869 0.96 0.177 - 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 pastilles acetone 2.7815 0.85 0.171 - 2

T1/MSR beads toluene 2.6836 0.13 0.155 140 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 pastilles toluene 2.4277 0.24 0.147 - 2

T9p/MSR pastilles toluene 2.3813 0.27 0.144 140 2

T10p/TSR pastilles toluene 2.4515 0.27 0.154 - 2

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controlled manually in order to achieve temperature profilescomparable to MSR. MSRs and TSRs were conducted at the samevolumetric flow rate of the purge gas (2000 Ncm3/min), chosen sothat it provides turbulent conditions to mitigate external masstransfer limitations and allow for investigation of desorption kinetics.Finally, TSR is performed by means of a hot purge gas heatedconventionally in a coil pipe, whereas MSR is performed by heatingthe 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 two-position valve and were examined in the GC. Desorption wasterminated either after 2 min or after 18 min. Then the partlyregenerated adsorbent was weighed immediately. A comparison ofdesorption kinetics was carried out by employing desorption rateprofiles, 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 c393:15393:15

T + 273:15ð5Þ

2. The volumetric flow rate of the purge gas at the desorptiontemperature is calculated as

_QT ¼ _Q 273:15T + 273:15273:15

ð6Þ

whereQ·273.15 (displayed on the mass flow controller) is the

volumetric flow rate of the purge gas at the normaltemperature (273.15 K).

3. Based on eqs 5 and 6, the total mass of desorbed adsorbateis calculated as

md ¼ _Q 273:15393:15273:15

Z t

0c393:15dt ð7Þ

Finally, the desorption rate is:

r ¼ Δmd

Δtð8Þ

A verification of the calculation method was performed bycomparing the total mass of desorbed adsorbate calculated byeq 7 with gravimetric measurements. The results of the compar-isons 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 theuncontrolled desorption over the time period taken to placethe FO sensor into the pastilles or beads of the loadedadsorbent, after the latter was weighed and before thedesorption process got started. Furthermore, the differencesobserved in desorbed mass (GC and balance) among re-peated experiments performed with the same adsorbate, thesame form of adsorbent (beads or pastilles), and for the sametime (2 or 18 min) are due to (A) differences in the adsorbateloadings (e.g., in Table 2, q (kg/kg) = 0.157 and 0.183 forexperiments A1 and A3, respectively) and (B) differences inthe temporal temperature profiles as will be presented in thefollowing graphs.Desorption Rates with 13X Beads. In desorption experiments

with 13X beads the temperature profiles obtained from MSR arefollowed in TSR with appropriate preheating of the purge gas.Practically, the temperature measurements in MSR and TSRwere realized by inserting an FO sensor into the adsorbent bed.One must bear in mind that MSR and TSR employ different heattransfer mechanisms. In TSR heat is introduced into the adsor-bent with a preheated gas stream, whereas in MSR heat isproduced within the adsorbent volumetrically. As it was men-tioned before (Section 2.1), the utilized technique of tempera-ture measurement provides information concerning gas-phase

Figure 4. Comparison of desorbed mass between TSR and MSR of 13X beads and pastilles from toluene (nonpolar). See also Table 2 for the specificconditions of each experiment.

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temperature which is the reference temperature in this set ofexperiments. It must be stressed that the same value of referencetemperature implies a higher adsorbent temperature in MSRthan in TSR and, in turn, the higher adsorbent temperatureenhances the diffusivity of adsorbates in the pores of theadsorbent. The results presented in Figures 6 and 8 showdesorption rate profiles of acetone and toluene for MSR andTSR. Figures 7 and 9 show the corresponding temperatureprofiles. 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 sameadsorbent form (e.g., A1/MSR, A3/MSR, A5/MSR) differ from

each other due to somewhat varying solid-phase concentrationsof adsorbate and desorption temperatures.The comparison of the two heating methods based on

Figures 6�9 reveals the following:(1) Themaximumacetonedesorption rates forMSRsare apparently

higher than the maximum desorption rates for the corre-sponding TSRs. This holds even when two TSR runs (A6/TSA, A7/TSA) were carried out at higher temperatures thanthe temperaturesmeasured in theMSRs (see Figures 8 and 9).

(2) The toluene desorption rates with MSR and TSR at 140W are approximately equal although the temperaturesrecorded for all TSRs are clearly higher than the tem-peratures for MSRs (see Figures 8 and 9).

Figure 5. Comparison of desorbed mass between TSR and MSR of 13X beads and pastilles from acetone (polar). See also Table 2 for the specificconditions 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 presentedin Figure 7.

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(3) There are evident differences in the adsorbent bedtemperatures after 2min ofmicrowave regeneration whenthe adsorbent is loaded with acetone and toluene (seeFigures 7 and 9). This implies selective interaction ofmicrowaves with the good microwave absorber (acetone),which is favorable from the energy efficiency point of view.

Desorption Rates with 13X Pastilles. In desorption experi-ments with 13X pastilles, the adsorbent temperature profilesobtained from MSRs are again followed in TSRs by appropriatepreheating of the purge gas. In the case of 13X pastilles, though, adifferent temperature measurement approach was considered,compared to the case of zeolite beads. In particular, the adsorbenttemperature was measured by the FO sensor which was placedaxially 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 theadsorbent pastilles and recording the temperature profile startingfrom the same conditions as in the main desorption experiment.The current approach concerning temperature measurementswas 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 11and 13 present the corresponding temperature profiles. MSRswere 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 theheat transfer limitations pertaining to convective heating (seeFigures 11 and 13). Besides, it is observed that the adsorbenttemperatures are lower in acetone desorption than in toluenedesorption despite the gas-phase temperatures being higher in

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 forMSRs at 140W andTSRs of 13X beads carried out under the temperature conditions presented inFigure 9.

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the former case. The difference is due to the lower heat ofevaporation of toluene compared to acetone (401 vs 515 kJ/kg,respectively, at 50 �C) and the higher desorption rates of acetonecompared 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 tem-perature is the key factor ruling desorption kinetics, as it governsthe diffusivity of the adsorbates in the pores of the adsorbent. Inthe performed experiments, the temperature of the adsorbent issignificantly lower in TSR compared to MSR, in which heat isdirectly and instantaneously dissipated inside the solid structureof the adsorbent. Therefore, the higher adsorbent temperature inMSR, compared to TSR, is responsible for the observed en-hancement of the microwave-driven regeneration (Figure 10).

However, in the case of toluene desorption, when comparing thecorresponding temperature profiles of 13X pastilles in TSRs andMSRs, the cause of the observed enhancement in MSR is not soevident. Clearly, the toluene desorption rates in MSRs are higherthan the corresponding desorption rates in TSRs for times over0.5 min (Figure 12); this is in keeping with the higher adsorbenttemperature in MSRs after 0.5 min (Figure 13). On the otherhand, it is unexpected that despite the lower adsorbent tempera-ture inMSRs for desorption times below 0.5 min (Figure 13), thedesorption rates in MSRs are higher than those in TSRs in thecorresponding time interval (<0.5 min, Figure 12).More detailedinvestigation must be undertaken to fully explain this.There are also visible differences in the temperatures after 2min of

microwave regeneration when the adsorbent is loaded with acetone

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 conditionspresented in Figure 11.

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and toluene, confirming the selective interaction of microwaves withthe strong absorber (acetone) as discussed in the previous section.Desorption Efficiencies. The obtained results regarding de-

sorption kinetics were also confirmed by independent gravi-metric measurements for short 2-min experiments and for longer18-min experiments (see Table 2). Based on thesemeasurementsa desorption efficiency coefficient is defined as

ϕ ¼ md

ma100% ð9Þ

In eq 9, md is the desorbed mass of the adsorbate after 2 or 18min and ma is the initially adsorbed mass of the adsorbate. Thecoefficient describes the total desorption efficiency and is pre-sented in Table 3.The results show that in the case of acetone desorption from

the zeolite beads the desorption efficiency is ∼2.5 times higherunder microwave irradiation for 18 min. This is consistent withthe higher microwave-driven desorption rates shown in theprevious graphs. Besides, the comparison of acetone and toluenedesorption efficiencies under microwaves for 2 min shows that

Figure 11. Temperature profiles obtained for MSRs at 140 W and TSRs of 13X pastilles from acetone. Solid lines connecting filled symbols: pastilletemperatures in MSRs. Dashed lines connecting open symbols: gas temperatures in TSRs. Dashed lines connecting half-filled symbols: pastilletemperatures in TSRs.

Figure 12. Comparison of toluene desorption rates for MSRs at 140 W and TSRs of 13X pastilles carried out under the temperature conditionspresented in Figure 13.

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the desorption efficiencies for toluene are more than four timeslower than those for acetone. Furthermore, toluene desorptionefficiencies under microwaves are slightly higher (1.2�1.5 times)compared to those under conventional heating (by means of ahot gas stream). Similar trends have been found also in case ofacetone and toluene desorption from the 13X pastilles. It isstressed that microwave-driven desorption of a polar adsorbatecan render the process multifold faster as compared to conven-tionally heated desorption. Moreover, in the case of nonpolaradsorbates, the microwave desorption can be advantageous, espe-cially when high heat transfer resistances are expected (e.g., pellets ofadsorbent). Faster desorption is equivalent to more efficientdesorption due to less energy waste in the form of (1) heat lossesto the surroundings and (2) sensible enthalpy of purge gas streamsince the total gas consumption is considerably decreased.Energy Consumption. It is emphasized here that the micro-

wave unit was not optimized to minimize the total energy

consumption, and energy consumption values for MSR andTSR have not been registered. Rather, the objective of this workwas to study the influence of microwave irradiation on desorp-tion efficiency. Albeit higher desorption efficiencies imply fasterdesorption process and lower energy input to the desorber, thetotal energy consumption in the process is also determined bythe magnetron efficiency and the efficiency of conversion ofelectromagnetic energy to heat. More specifically, due to thelow ratio of desorber volume to multimode cavity volume, theefficiency of conversion of electromagnetic power into heat in thedesorber loading is expected to be low. In Coss et al. it was shownthat only 27% of the electromagnetic power generated wasabsorbed due to the small sample in the waveguide.15 Largerdesorber volume and minimization of (1) heat losses from thedesorber with appropriate insulation and (2) the reflectedmicrowave power from the cavity to the magnetron isolator bymeans of impedance matching circuit hardware (e.g., a stubtuner) are necessary to improve the total energy efficiency of thesystem. These aspects, however, are outside the scope of this work.

3. CONCLUSIONS

Microwave swing regeneration (MSR) and temperature swingregeneration (TSR) of a polar and a nonpolar compound(acetone and toluene, respectively) from 13X molecular sieveshave been compared in terms of desorption rates and desorptionefficiencies. It was found that irrespective of the dielectricproperties of the adsorbates, 13X molecular sieves absorbmicrowaves producing heat inside the adsorbent. Therefore,they form a suitable adsorbent for microwave desorption ofpolar and nonpolar compounds. Furthermore, due to the alter-native heating mechanism in MSR, the process runs faster evenwhen the adsorbent temperature is much lower than the gastemperature in TSR. The direct and instantaneous heating of theadsorbent in MSR results in higher desorption rates and higher

Figure 13. Temperature profiles obtained for MSRs at 140 W and TSRs of 13X pastilles from toluene. Solid lines connecting filled symbols: pastilletemperatures in MSRs. Dashed lines connecting open symbols: gas temperatures in TSRs. Dashed lines connecting half-filled symbols: pastilletemperatures in TSRs.

Table 3. Comparison of Desorption Efficiency Coefficientsfor MSR and TSR (Values of the Coefficient: for 2-minexperiments in parentheses, for 18-min experiments in squarebrackets)

adsorbate ϕ [%] for MSR ϕ [%] for TSR

beads acetone A1 [67.1] A2 [26.8]

A3 [67.4] A4 [29.5]

A5 (20.3) A6 (12.7)

toluene T1 (4.5) T2 (3.2)

T3 (3.7) T4 (2.7)

T5 (4.2) T6 (3.5)

pastilles acetone A8p (18.1) A9p (12.3)

A10p (16.5) A11p (8.0)

toluene T7p (6.3) T8p (1.3)

T9p (5.1) T10p (2.5)

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desorption efficiencies than in TSR that become more pro-nounced when the adsorbate is polar or when high heat transferresistances are present in TSR. This shows the significantpotential of microwave technology to enable more efficientdesorption processes due to less energy waste in the form ofheat losses and sensible enthalpy of purge gas stream since thetotal gas consumption is considerably decreased. It is remarked,however, that in order for this potential to have a positive impacton the total energy consumption, sufficiently high efficiency ofconversion of electromagnetic energy in the cavity to heat insidethe desorber is necessary through appropriate microwave en-gineering and process design. Finally, it was verified that micro-waves do not affect the adsorption capacity of the molecularsieves after several consecutive adsorption�desorption cycles.

’ASSOCIATED CONTENT

bS Supporting Information. GC calibration curves for to-luene and acetone. This material is available free of charge via theInternet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*E-mail: R.Cherbanski@ichip.pw.edu.pl.

’ACKNOWLEDGMENT

This study has been performed with a financial support of theMinistry of Science and Higher Education (Poland) within aframe of the scientific grant N208 003 32/0611. M.K.-D., G.D.S.,and A.I.S. acknowledge the Dutch Ministry of Economic Affairsand SenterNovem for financial support through the EOS-LT04033 project grant.

’NOMENCLATUREcr = concentration of a compound in a reference solution (g/cm3)cT = adsorbate concentration in the purge gas at temperature

T (g/cm3)Dp = penetration depth of microwaves (cm)M = molecular weight (g/mol)mads = mass of adsorbent (g)ma = adsorbed mass of an adsorbate (g)md = desorbed mass of an adsorbate (g)PMW = microwave power generated by magnetron (W)(p)/(p0) = relative vapor pressure of an adsorbate (-)q = adsorption capacity (kg/kg)Q·T = volumetric flow rate of purge gas at temperature T

(Ncm3/min)Q·He = volumetric flow rate of purge gas (Ncm3/min)

r = desorption rate (g/min)R = gas constant 8.314472(15) (J/mol/K)T = temperature (K or �C)t = desorption time (min)j = desorption efficiency coefficient (%)λ0 = wavelength of microwave radiation (cm)ε = permittivity (F/m)ε0 = free space permittivity (F/m)ε0r = relative dielectric constant (-)ε00r = relative dielectric loss (-)ν = volume of the solution sample (cm3)

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