nanomaterials

Article Application of Nanosize Molecular Sieves for Medical

Mingfei Pan, Hecham M. Omar and Sohrab Rohani *

Department of Chemical and Biochemical Engineering, Faculty of Engineering, University of Western Ontario, London, ON N6A 5B9, Canada; [email protected] (M.P.); [email protected] (H.M.O.) * Correspondence: [email protected]; Tel.: +1-519-661-4116

Received: 19 June 2017; Accepted: 21 July 2017; Published: 25 July 2017

Abstract: The development of a portable oxygen concentrator is of prime significance for patients with respiratory problems. This paper presents a portable concentrator prototype design using the /vacuum swing (PVSA) cycle with a deep evacuation step (−0.82 barg) instead of desorption with purge flow to simplify the oxygen production process. The output of the oxygen concentrator is a ~90 vol % enriched oxygen stream in a continuous adsorption and desorption cycle (cycle time ~90 s). The size of the adsorption column is 3 cm in diameter and 20 cm in length. A Li+ exchanged 13X nanosize zeolite is used as the adsorbent to selectively adsorb nitrogen from air. A dynamic model of the pressure and vacuum swing adsorption units was developed to study the pressurization and depressurization process inside the microporous area of nanosized . The describing equations were solved using COMSOL Multiphysics Chemical Engineering module. The output flow rate and oxygen concentration results from the simulation model were compared with the experimental data. Velocity and concentration profiles were obtained to study the adsorption process and optimize the operational parameters.

Keywords: nanosize zeolite; microporous adsorption; bidisperison dynamic model; portable oxygen concentrator; COMSOL Multiphysics

1. Background Air pollution is becoming a worldwide problem particularly in heavily populated cities like Beijing, Los Angeles, and Mexico City. Air pollution leads to acute and chronic respiratory problems that are becoming a growing concern from both global and individual levels. Wearing disposable face masks has become a popular method to get some protection from exposure to pollutants. However, for people with poor autonomous respiration ability, like chronic obstructive pulmonary disease (COPD) patients, it is hard to breathe through disposable face masks. As a result, there is a growing need to address the health and quality of life through a light- and portable oxygen concentrator with a medical grade oxygen supply (O2 concentration: ~88–92 vol %). Currently, oxygen concentration techniques are based on air separation processes such as cryogenic technique, membrane separation and pressure swing adsorption (PSA) that can effectively produce enriched oxygen using ambient air as the source. Although cryogenic acts as the leading process for air separation in an industrial oxygen plant, the pressure swing adsorption (PSA) technique has the advantage of reducing energy consumption for small production scales. Compared to the membrane separation technique, which requires higher pressure and expensive selective permeable materials, pressure swing adsorption using nanosize zeolite adsorbents is a more feasible for portable oxygen concentrator development. Due to the large microporous surface area inside the zeolite adsorbents, the portable oxygen concentrator using the PSA technique can adsorb nitrogen from air and output an enriched oxygen stream under high-pressure conditions. The concentrator can be regenerated by

Nanomaterials 2017, 7, 195; doi:10.3390/nano7080195 www.mdpi.com/journal/nanomaterials Nanomaterials 2017, 7, 195 2 of 19 decreasing the pressure to release the adsorbed nitrogen. In this way, a continuous oxygen stream supply can be generated to assist an individual and increase the fraction of inspired oxygen. The oxygen concentration system is usually coupled with a filtration system to remove PM 2.5 and improve the quality of product gas stream. The pressure swing adsorption process was patented by Finlayson and Sharp in 1932 (GB Patent 365092) describing an air separation process with pressurization adsorption and depressurization desorption steps [1]. In 1958, two different pressure swing adsorption units were patented: one using fractional product purge flow for desorption step (Skarstrom, 1958) and the other using pressure vacuum adsorption (PVSA) step (Montgareuil and Domine, 1958) [2,3]. The selectivity for zeolite adsorbents to adsorb nitrogen compared to oxygen is due to the interaction between electrostatic field of the cationic zeolite and the quadrupole moment of the nitrogen and oxygen. The quadrupole of nitrogen is three times higher than that of oxygen, which leads to a selective adsorption onto the zeolite surface [4]. The most common type of commercial zeolite for oxygen concentration process is zeolite 13X due to its outstanding nitrogen to oxygen adsorption selectivity. However, 13X zeolite modified with Li+ exchange method exhibits a higher nitrogen adsorption capacity at the active cation sites of the zeolite framework [5–7]. JLOX-101 LiX zeolite provided by LUOYANG JIANLONG Chemical is designed specifically for a PVSA process with high oxygen selectivity [8]. The PSA system was first installed in industrial plants as a replacement for cryogenic separation plants due to lower capital and operating costs. Portable pressure swing adsorption oxygen concentrators have been designed and developed in the past two decades due to the synthesis of more effective zeolites with higher gas adsorption capacity and selectivity. The PSA oxygen concentrator is able to provide a continuous oxygen flow using the ambient air as the gas source. Two-bed or multi-bed PSA systems are utilized alternating adsorption and desorption operations of the zeolite columns to achieve continuous oxygen separation. For on-site medical grade oxygen production, a cylinder gas source or several powerful air compressors are usually installed for the rapid pressure swing adsorption process to maintain the productivity of high purity oxygen stream [9–11]. Considering the mobility of small-scale concentrators, a variety of simplified designs would be added to the pressure swing adsorption process for system miniaturization. In this paper, we compare the performance of a portable miniature PVSA oxygen concentrator unit with two adsorption columns, an air compressor, four solenoid valves and a back-pressure regulator (total weight less than 2 kg) built in our laboratory with the simulation results under modified Montgareuil and Domine cycle using COMSOL Multiphysics Chemical Engineering module. Compared to the conventional PSA cycle using purge flow with intermediate evacuation pressure (−0.6 barg) to regenerate the adsorption column, the deep vacuum (−0.82 barg) desorption step of the PVSA cycle reduces the column and compressor size from the perspective of industrial prototype design. A dual-head air compressor is selected to generate pressure and vacuum condition simultaneously for the dual-column adsorption unit, which simplifies the gas source generation. The oxygen concentrator unit can provide a ~90 vol % enriched oxygen stream to meet the concentration requirement of medical-grade oxygen.

2. Materials and Methods Figure1 shows the schematic of the oxygen concentrator using a two-column PVSA cycle. The concentrator consists of two adsorption zeolite columns, a dual-head air compressor (Parker Hannifin Corp., TTC IIS Miniature Diaphragm Pump, Hollis, NH, USA), five three-way solenoid valves (Parker Hannifin Corp., X-Valve, Hollis, NH, USA), a back-pressure regulator, an oxygen sensor (Teledyne GB300 Oxygen analyzer, Thousand Oaks, CA, USA), a surge tank, a pressure sensor (Freescale Semiconductor, MPXV7002DPT1CTND, Austin, TX, USA) and a cooling fan (avoiding the rise of the air compressor) [12–15]. The oxygen sensor is an electrochemical transducer named Micro-fuel Cell calibrated by the Faraday type calibration method based on the product gas Nanomaterials 2017, 7, 195 3 of 19 from electrolysis of water [16]. The valve sequence is shown in Table1. The concentrator operation consists of four different steps. In step 1, column 1 is pressurized by the compressor while column 2 is connected to ambient air to decrease the pressure. In step 2, the pressure inside column 1 reaches the working pressure (1.79 barg) with enriched oxygen flowing out through the backpressure regulator (set at 1.79 barg) to the surge tank. Meanwhile the pressure in column 2 decreases to vacuum (set Nanomaterials 2017, 7, 195 3 of 19 at −0.82 barg) via the air compressor to regenerate the zeolite. In steps 3 and 4, the adsorption and desorptionthe working positions pressure of (1.79 column barg) 1with and enriched 2 are reversed oxygen toflowing regenerate out through the saturated the backpressure column 1 and pressurizeregulator the regenerated (set at 1.79 barg) column to the 2. Bysurge switching tank. Meanwhile the valve the sequence pressure (stepin column 1 → 42 →decreases1), a continuous to enrichedvacuum oxygen (set stream at −0.82 can barg) be obtainedvia the air fromcompressor the surge to regenerate tank. The the flow zeolite. directions In steps of 3 theand two4, the different adsorption and desorption positions of column 1 and 2 are reversed to regenerate the saturated types ofcolumn valves 1 (normallyand pressurize open the and regenerated distributor column three-way 2. By switching solenoid the valve valves) sequence are shown (step 1 in → Figure4 → 1), 2 with two differenta continuous roles enriched in the pressure oxygen stream swing can operations. be obtained from Valve the 1 surge and 2tank. are The normally flow directions open 3-way of the valves directingtwo the different pressurizing types of flow valves and (normally countercurrent open and blowdowndistributor three-way flow while solenoid valve valves) 3 and are 4 are shown distributor 3-way valvesin Figure for 2 with pressurizing two different and roles depressurizing in the pressure swing port operations. control at Valve the column 1 and 2 are end. normally According open to the adsorption3-way isotherm valves directing of zeolite the LiX pressurizing shown in flow Figure and3 countercurrent (Santos et al., blowdown 2008), zeolite flow capacitywhile valve and 3 and selectivity 4 are distributor 3-way valves for pressurizing and depressurizing port control at the column end. of nitrogen to oxygen is higher under pressure vacuum swing range compared to conventional pressure According to the adsorption isotherm of zeolite LiX shown in Figure 3 (Santos et al., 2008), zeolite swing [17capacity]. In order and toselectivity reduce theof nitrogen compressor to oxygen size, ais dual-head higher under air compressorpressure vacuum is considered swing range to perform pressurizationcompared and to conventional vacuum operation pressure simultaneously.swing [17]. In order By to sharingreduce the the compressor central motor, size, a twodual-head diaphragms can separatelyair compressor provide is pressurizedconsidered to gasperform and evacuationpressurization pressure. and vacuum Compared operation to simultaneously. the size and total By power consumptionsharing ofthe two central diaphragm motor, two compressors, diaphragms can a separately dual-compressor provide pressurized is less bulky gas and (25% evacuation size reduction) and morepressure. efficient Compared (12% inputto the size power and total saving), power whichconsumption benefits of two the diaphr miniaturizationagm compressors, of the a dual- prototype. compressor is less bulky (25% size reduction) and more efficient (12% input power saving), which Summarizing the system design, a PVSA process that uses a vacuum for desorption and pressurization benefits the miniaturization of the prototype. Summarizing the system design, a PVSA process that for adsorptionuses a vacuum can achieve for desorption relatively and pressurization high capacity, for allowing adsorption more can achieve nitrogen relatively adsorption high capacity, while using a smallerallowing compressor more fornitrogen gas supply. adsorption while using a smaller compressor for gas supply.

FigureFigure 1. Schematics1. Schematicsof of thethe PSA oxygen oxygen concentrator. concentrator.

Nanomaterials 2017, 7, 195 4 of 19

Table 1. The valve operation sequence for the PVSA cycle. NanomaterialsNanomaterials 2017 2017, ,7 7, ,195 195 44 of of 19 19 Column 1 Column 2 Step StageTableTable 1. 1. The The valve valve operation operation sequen sequencece for for Stagethe the PVSA PVSA cycle. cycle. Valve 1 Valve 3 Valve 2 Valve 4 Column 1 Column 2 Step1 PressurizationStage P1Column 1 P1 BlowdownStage P2Column 2 P2 Step Stage Valve 1 Valve 3Stage Valve 2 Valve 4 2 Production ValveP1 1 Valve P1 3 Vacuum P2 Valve 2 Valve P1 4 1 Pressurization P1 P1 Blowdown P2 P2 1 3Pressurization Blowdown P2P1 P1 P2 Pressurization Blowdown P1 P2 P1P2 2 Production P1 P1 Vacuum P2 P1 2 4Production Vacuum P2P1 P1 P1 Production Vacuum P1 P2 P1 33 BlowdownBlowdown P2 P2 P2 P2 Pressurization Pressurization P1P1 P1 P1 P1, P2 represents valve position 1 and 2. 44 VacuumVacuum P2 P2 P1 P1 Production Production P1 P1 P1P1 P1,P1, P2 P2 represents represents valve valve position position 1 1 and and 2. 2.

FigureFigureFigure 2. Flow 2.2. FlowFlow directions directionsdirections Schematics SchematicsSchematics of ofof thethe normallynormally openopen open andand and distributordistributor distributor three-waythree-way three-way valvevalve valve underunder under pressurepressurepressure and and and vacuum vacuum vacuum conditions. conditions. conditions.

Figure 3. Adsorption equilibrium isotherms for nitrogen and oxygen onto zeolite LiX at 20 °C FigureFigure 3. Adsorption 3. Adsorption equilibrium equilibrium isothermsisotherms for nitrog nitrogenen and and oxygen oxygen onto onto zeolite zeolite LiX LiXat 20 at °C 20 ◦C. Reproduced with permission from [17]. Industrial & engineering chemistry research, 2008. ReproducedReproduced with with permission permission from from [17 [17].]. Industrial Industrial & engineer engineeringing chemistry chemistry research, research, 2008. 2008.

ZeoliteZeolite JLOX-101JLOX-101 (Ø0.4–0.8(Ø0.4–0.8 mm)mm) fromfrom JIANLOJIANLONGNG CHEMICALCHEMICAL (Yanshi,(Yanshi, Henan,Henan, China)China) waswas usedZeoliteused as as the the JLOX-101 adsorbent adsorbent (Ø0.4–0.8 inside inside the the mm) adsorption adsorption from JIANLONG column column based based CHEMICAL on on the the zeolite zeolite (Yanshi, characterization characterization Henan, China)[14]. [14]. Zeolite Zeolite was used as the adsorbent inside the adsorption column based on the zeolite characterization [14]. Zeolite Nanomaterials 2017, 7, 195 5 of 19 Nanomaterials 2017, 7, 195 5 of 19

+ JLOX-101JLOX-101 is is a a Li Li+ exchanged exchanged zeolite zeolite 13X 13X designed designed for for industrial industrial oxygen oxygen concentration. concentration. FigureFigure4 4 exhibitsexhibits the the powder powder XRD XRD (PXRD) (PXRD) patterns patterns of of zeolite zeolite JLOX-101 JLOX-101 with with two two types types of of sample sample zeolite zeolite 13X 13X fromfrom UOP UOP (Des (Des Plaines, Plaines, IL, IL, USA) USA) and and Beijing-DF Beijing-DF (Beijing, (Beijing, China) China) [ 18[18,19].,19]. The The PXRD PXRD spectra spectra of of the the collectedcollected solid solid forms forms were were obtained obtained on on a a 2500 2500 diffractometer diffractometer (D/MAX, (D/MAX, Rigaku, Rigaku, Japan) Japan) with with a a Cu Cu K Kαα radiationradiation source source ( λ(λ= = 1.5406 1.5406 Å) Å) at at 100 100 mA mA and and 40 40 kV. kV. The The samples samples were were recorded recorded at at a a scanning scanning rate rate ofof 5 5°◦ per per minute minute over over a a 2 2θθrange range of of 5–40 5–40°.The◦.The XRD XRD peak peak position position and and shape shape of of zeolite zeolite JLOX101-LiX JLOX101-LiX areare essentially essentially the the same same as as those those of of zeolite zeolite 13X, 13X, which which means means zeolite zeolite JLOX101-LiX JLOX101-LiX maintains maintains the the + intrinsicintrinsic crystalline crystalline structure structure of of zeolite zeolite 13X 13X after after the the Li Li+ ion-exchange ion-exchange method. method. Table Table2 2shows shows the the BET BET adsorptionadsorption results results of of zeolite zeolite JLOX-101 JLOX-101 with with the the sample sample 13X 13X zeolite zeolite from from Beijing-DF Beijing-DF through through ASAP ASAP 2010 (Micromeritics2010 (Micromeritics Instrument Instrument Corporation, Corporation, Norcross, No GA,rcross, USA). GA, Both USA). the BETBoth surface the BET area surface and micropore area and volumemicropore of JLOX-101 volume zeoliteof JLOX-101 are larger zeolite than are those larger of 13X than zeolite those (Table of 213X) and zeolite the adsorption (Table 2) average and the poreadsorption diameters average of zeolite pore JLOX-101 diameters and of 13X-BJ-DF zeolite JLOX-101 are 21.26 and Å and 13X-BJ-DF 22.36 Å, respectively,are 21.26 Å whichand 22.36 are inÅ, agreementrespectively, with which the microporous are in agreement nitrogen with adsorption the microporous theory of nitrogen X type zeolite. adsorption Figure theory5 illustrates of X thetype Barrett–Joiner–Halendazeolite. Figure 5 illustrates adsorption the Barrett–Joiner–Halenda cumulative pore area using adsorption nitrogen cumulative as the analysis pore adsorptive.area using Thenitrogen nitrogen as the occupies analysis most adsorptive. of the microporous The nitrogen surface occupies area most (pore of diameter the microporous <2 nm) andsurface small area part (pore of thediameter mesoporous <2 nm) surface and sma areall (porepart of diameter the mesoporous 2~50 nm). surface Figure6 areadepicts (pore the diameter particle size 2~50 distribution nm). Figure of 6 thedepicts zeolite the powder particle samples size distribution through MASTERSIZER of the zeolite 2000powder (Malvern samples Instruments through MASTERSIZER Ltd, Worcestershire, 2000 UK).(Malvern The diameter Instruments of the Ltd, vol Worceste % peakrshire, (5.75 µ m)UK). is The used diameter as the average of the vol microparticle % peak (5.75 diameter µm) is used in the as bidispersionthe average modelmicroparticle for mathematical diameter in simulation. the bidispersion model for mathematical simulation.

FigureFigure 4. 4.XRD XRD pattern pattern of of three three different different types types of of zeolites: zeolites: UOP-13X, UOP-13X, Beijing-DF-13X Beijing-DF-13X andand JLOX101-LiX.JLOX101-LiX.

TableTable 2. 2.BET BET adsorption adsorption results results of of JLOX-101 JLOX-101 and and 13X-BJ-DF. 13X-BJ-DF.

Zeolite JLOX-101 13X-BJ-DF Zeolite JLOX-101 13X-BJ-DF BET surface area (m2/g) 574.77 522.93 2 BET surfaceMicropore area (m /g) area (m2/g) 574.77515.39 477.04 522.93 2 SingleMicropore Point Adsorption area (m /g) Total Pore Volume (cm3/g) 515.390.31 0.29477.04 3 Single Point AdsorptionMicropore Total Pore Volume Volume (cm (cm3/g)/g) 0.310.24 0.22 0.29 Micropore Volume (cm3/g) 0.24 0.22 Adsorption Average Pore Diameter (Å) 21.26 22.36 Adsorption Average Pore Diameter (Å) 21.26 22.36

NanomaterialsNanomaterialsNanomaterials2017 2017 2017, 7, ,,7 1957, ,195 195 66 of of 619 19 of 19

Figure 5. BJH adsorption cumulative pore area distribution of zeolite JLOX101-LiX. FigureFigure 5. 5. BJH BJH adsorption adsorption cumulative cumulative pore pore area area distribution distribution of of zeolite zeolite JLOX101-LiX. JLOX101-LiX.

1212

1010

88

66 Volume % Volume Volume % Volume 44

22

00 0.010.01 0.1 0.1 1 1 10 10 100 100 1000 1000 ParticleParticle Size/µm Size/µm

FigureFigureFigure 6. 6. 6.Particle Particle Particle size size distributiondistribution of of zeolite zeolite JLOX-101-LiX JLOX-101-LiX JLOX-101-LiX powder powder powder sample. sample. sample.

DueDue to to the the negatively negatively charged charged zeolite zeolite surface, surface, the the polar polar compounds compounds H H2O2O and and CO CO2 2in in the the air air are are Due to the negatively charged zeolite surface, the polar compounds H O and CO in the air preferentiallypreferentially adsorbedadsorbed andand occupyoccupy thethe effectiveffectivee zeolitezeolite microporesmicropores duringduring2 thethe PSAPSA 2process.process. are preferentially adsorbed and occupy the effective zeolite micropores during the PSA process. ConventionalConventional vacuum vacuum desorptio desorptionn cannot cannot fully fully desorb desorb the the H H2O2O and and CO CO2 2molecules, molecules, which which gradually gradually Conventionaldecreasedecrease the the vacuumzeolitezeolite capacity capacity desorption under under cannot the the cyclic cyclic fully operat operat desorbionion theof of adsorption Hadsorption2O andCO and and2 desorptionmolecules, desorption steps. whichsteps. In In gradually order order decreasetoto improve improve the zeolitethe the lifespan lifespan capacity of of the the under adsorption adsorption the cyclic column, column, operation a a thin thin of layer layer adsorption of of activated activated and alumina desorptionalumina was was steps. added added In before before order to improvethethe zeolitezeolite the layer lifespanlayer as as a a of desiccantdesiccant the adsorption toto preventprevent column, zeolitezeolite a deactivation.deactivation. thin layer of TheThe activated volumevolume alumina ratioratio ofof thewasthe desiccantdesiccant added before toto thewholewhole zeolite column column layer was was as a0.15–0.20 0.15–0.20 (Chai (Chai to preventet et al., al., 2011) 2011) zeolite using using deactivation. ambient ambient air air source source The volume (0.04 (0.04 vol vol ratio % % CO CO of2 2and theand 0.4–0.8 desiccant0.4–0.8 tovolvol whole % % moisture) moisture) column [20]. [20]. was 0.15–0.20 (Chai et al., 2011) using ambient air source (0.04 vol % CO2 and 0.4–0.8 vol % moisture) [20]. Nanomaterials 2017, 7, 195 7 of 19

Table3 shows the physical properties of the zeolite, desiccant and adsorption column. The bed voidage and particle voidage are estimated from the voidage variation study and single point BET adsorption total pore volume, respectively [21]. The adsorption constants for zeolite column are fitted with the experimental data based on the single particle adsorption isotherms of LiX zeolite. The PVSA system operates between 1.79 barg for adsorption and −0.82 barg for desorption under a compressor flow. The time for different steps of the column test is summarized to maintain a stable oxygen flow and achieve a complete regeneration of column in each cycle. The experimental performance of the column would be compared to the model prediction in the following paragraphs.

Table 3. Characteristics of adsorbent, desiccant and adsorption column [17,22–25].

Adsorbent Zeolite LiX Unit Average zeolite particle size 600 µm Average microparticle size 5.75 µm Zeolite bulk density 790 kg/m3 Particle voidage, ε p 0.35 1 Maximum surface excess of N , qe 3.42 mol/kg 2 N2 Maximum surface excess of O , qe 6.06 mol/kg 2 O2

Adsorption constant, bN2 0.09 1/bar

Adsorption constant, bO2 0.02 1/bar Pore tortuosity, τP 3 - BET Surface Area 574.77 m2/g Micropore Volume 0.240007 cm3/g Adsorption Average Pore Diameter 21.2609 Å Desiccant Type Activated Alumina Particle density 765 kg/m3 Average desiccant particle size 600 µm Adsorption Column - - Length 10 cm Inside radius 1.5 cm Column voidage, εc 0.36 1 Material aluminum -

3. Mathematical Model

3.1. Transport and Adsorption Equations In order to study the performance of the adsorption column, a three-dimensional model was developed using COMSOL Multiphysics. The equations were solved using the finite element method. Due to the cylindrical geometry of the adsorption column, a two-dimensional axi-symmetrical approximation was used to decrease the computational cost. The following assumptions were made in the model equations:

(a) The air source is assumed to be a binary ideal gas with 79% N2 and 21% O2. (b) Uniform bed voidage and particle diameter. (c) The stabilized gas temperature at inlet and outlet is measured to be 22.5 and 23.8 ◦C by thermocouple, checking the temperature dependent adsorption model (Santos et al., 2008), the |q ◦ −q ◦ | nitrogen and oxygen capacity difference (error = i,22.5 C i,23.8 C × 100%) at 1.79 barg is calculated qi,22.5◦C to be 1.47% and 1.03%, respectively, which is negligible compared to the adsorption equilibrium equations, so the system is assumed to be isothermal [17]. (d) Gravity effects on fluid flow are negligible.

The commercial zeolite particles are produced by aggregation of nanosize particles (zeolite powder) using a binder. The structure of the zeolite particles is shown in Figure7. In the Nanomaterials 2017, 7, 195 8 of 19

NanomaterialsThe commercial2017, 7, 195 zeolite particles are produced by aggregation of nanosize particles (zeolite8 of 19 powder) using a binder. The structure of the zeolite particles is shown in Figure 7. In the pressure swing process, the high-pressure flow will first disperse from the column void to the macropore area pressureinside the swing nanosize process, zeolite the particles high-pressure represented flow willby the first macropore disperse fromdispersion the column equations. void The to thegas macroporeadsorption area usually inside happens the nanosize at the zeolitemicropore particles surface represented of the zeolite by the microparticles, macropore dispersion which is equations.simulated Theby the gas adsorptionadsorption usuallymass transfer happens equations at the micropore in the surfacemodel. ofBased the zeolite on the microparticles, characteristics which of the is simulatedcommercial by zeolites, the adsorption the mass mass transfer transfer of adsorpti equationson inmodel the model. is controlled Based onby thea set characteristics of bidispersion of theequations commercial [26]. zeolites, the mass transfer of adsorption model is controlled by a set of bidispersion equations [26].

FigureFigure 7.7. Schematics ofof thethe zeolitezeolite particleparticle structure.structure.

TheThe velocityvelocity profile profile of of the the gas gas stream stream was developedwas developed in the in Free the and Free Porous and MediaPorous Flow Media Interface Flow ofInterface COMSOL of COMSOL Multiphysics Multiphysics using the Navier–Stokes using the Navier Equation–Stokes to Equation describe theto describe flow in open the flow regions, in open and theregions, Brinkman and the equation Brinkman to describe equation the to flow describe in porous the flow media. in porous media. TheThe flowflow inin zeolitezeolite microporemicropore areaarea isis governedgoverned byby aa setset ofof thethe continuitycontinuity equationequation andand thethe momentummomentum equation,equation, whichwhich togethertogether formform thethe BrinkmanBrinkman equationsequations (COMSOL,(COMSOL, 2016)2016) [[27]:27]: ∂ ∂ ε ρ + ∇ ρ = ((εcρ)) + ∇(ρ(u)u =) Q Q (1)(1) ∂t∂t c br br        ρ ρ∂u∂ u   1 T 2   −Q1  Qbr +u (u · ∇) u = −∇p + ∇ · 1  µ(∇u + (∇Tu) 2− µ(∇ · u)I) −−1 κ µbr + u (2)  + (u ⋅ ∇ )  = −∇ p + ∇ ⋅  μ (∇u + (∇u) − μ (∇ ⋅ u )I)  −  κ μ + u 2 (2) εc ε ∂t ∂t εcε  ε εc 3 3   ε 2  εc c  c   c    c  3 where ρρis the density of the fluid (kg/m ),3εc isε the column voidage (–), t is the time (s), Qbr is the where is the density of the fluid (kg/m ), is the column voidage (–), is the time (s), Qbr volumetric mass source of the fluid phase (kg/m3/s),c κ denotes the permeability of the porous medium 3 κ (mis the2), µvolumetricis the dynamic mass viscositysource of ofthe the fluid fluid phase (kg/(m (kg/m·s)),/s), p is the denotes gauge pressurethe permeability of the fluid of the flow porous (Pa), 2 μ andmediumu is the (m Darcy), velocity is the dynamic (m/s). viscosity of the fluid (kg/(m·s)), p is the gauge pressure of the fluid flowThe (Pa), permeability and u is the of theDarcy packed velocity bed (m/s). of spherical particles can be described by the Blake–Kozeny equationThe (Raopermeability et al., 2009) of the [22 packed]: bed of spherical particles can be described by the Blake–Kozeny equation (Rao et al., 2009) [22]: d2 ε κ = ( c )2 (3) 1502 1 −εεc d 2 where d is the average diameter of zeoliteκ particles= ( (m).c ) −ε (3) The mass source term Qbr is defined as the150 mass1 depositc rate of nitrogen and oxygen inside the adsorbent to simulate the adsorption process (Rao, 2013) [28]. where d is the average diameter of zeolite particles (m). 2 The mass source term Qbr is defined as the mass deposit rate of nitrogen and oxygen inside Qbr = −∑ ki Mi(ci − cpi) (4) the adsorbent to simulate the adsorption processi=1 (Rao, 2013) [28].

Nanomaterials 2017, 7, 195 9 of 19

where ki is the macropore mass transfer rate of nitrogen and oxygen into zeolite particles (1/s), Mi is the molecular weight of N2 and O2 (kg/mol), ci and cpi are the column gas concentration and particle concentration of nitrogen and oxygen, respectively (mol/m3)[29].

e 60Di,j k = (5) i K · d2

e 2 where Di,j are the molecular diffusivity of N2 and O2 into the zeolite pore area (m /s), K is the dimensionless Henry’s law constant for N2 and O2 (–) [30]. The molecular gas diffusivity was defined by the Chapman–Enskog Equation [31,32]. The parameters used in the Chapman–Enskog equation are shown in Table4. s 5.953 × 10−24 T3 T3 e = + Di,j 2 (6) pσijΩD Mi Mj where σij is the average collision diameter of components i and j (m), ΩD is the collision integral (–), Mi is the molecular mass of component i (kg/mol) and Mj is the molecular mass of component j (kg/mol).

σi + σj σ = (7) ij 2 where σi is the collision diameter of component i (m) and σj is the collision diameter of components j (m). 1.060 0.193 1.036 1.765 ΩD = + + + (8) 0.156 0.476TN 1.530TN 3.894TN TN e e e where TN is the standardized temperature (–).

T TN = (9) q εi q εi kb,i kb,i where εi is the characteristic energy of component i (J), εj is the characteristic energy of component j (J), kb,i is the Boltzman’s constant of component i (J/K) and kb,j is the Boltzman’s constant of component j (J/K). The pressure drop is given by the Ergun equation [33]

∂p 150µ (1 − ε )2 1.75ρ (1 − ε ) = c + c | | 2 2 u 3 u u (10) ∂z dp εc dp ε where z represents the coordinate axis of the gas flow (m). Input flow rate F is a function of pressure fitted with experimental data:

F(p) = 2.478 − 0.7251p (11) where F is the flow rate of inlet gas (L/min). The gas distribution is solved in the Transport of Diluted Species in Porous Media Interface of COMSOL Multiphysics with convection and dispersion equations (COMSOL, 2016) [27].

∂ci (1 − εc) ∂cpi ∂qi − ∇(DD · ∇ci) + ∇(uci) = (ε p + ρp ) (12) ∂t εc ∂t ∂t Nanomaterials 2017, 7, 195 10 of 19

The total gas mass deposit rate from column void to zeolite particles are calculated from the combination of particle concentration dispersion rate and microparticle adsorption rate.

∂cpi ∂q ε + ρ i = k (c − c ) (13) p ∂t p ∂t i i pi ∂q i = k (q∗ − q ) (14) ∂t pi i i (b Pω )1/ni q∗ = qe i i (15) i i 2 1/nj 1 + ∑ (bjPωj) j=1 e DD = 0.7Di,j + 0.5dpu (16) where kpi is the micropore mass transfer rate of nitrogen and oxygen (1/s), ρp is the density of 3 ∗ zeolite particle (kg/m ), qi and qi are the equilibrium and actual adsorbed species to the zeolite, e respectively (mol/kg), ωi is the volume fraction of N2 and O2 (–), qi is the maximum surface excess of gas component onto zeolite pore area (mol/kg), bi is the adsorption constant of N2 and O2 (1/bar), 2 DD is the axial dispersion coefficient (m /s).

60De = i kpi 2 (17) K · dp   ε p 1 e = Di  1 1  (18) τp e + e Dk,i Di,j e 2 e where Di is the effective intra particle diffusivity of N2 and O2 into zeolite pore area (m /s), Dk,i is 2 the effective Knudsen diffusivity of N2 and O2 into the zeolite pore area (m /s) [34–36], τp is the pore tortuosity (–) [25]. The effective Knudsen diffusivity is given by [37]: s e dp 8RT Dk,i = (19) 3 πMi where Mi is the molecular mass of nitrogen and oxygen (kg/mol), R is the ideal gas constant (J·K−1·mol−1), T is the column temperature (K). Initial condition of adsorption step:

At t = 0, the initial pressure (gauge) = −0.82 (barg), the inflow is set at p = 1.79 (barg), ωN2 = 0.79; The initial adsorbed nitrogen concentration = 0.3172 (mol/kg), initial adsorbed oxygen concentration = 0.02564 (mol/kg); Initial condition of desorption step:

At t = 0, the initial pressure = 1.79 (barg), the inflow is set at p (gauge) = −0.82 (barg), ωN2 = 0.79; The initial adsorbed nitrogen concentration = 1.333 (mol/kg), the initial adsorbed oxygen concentration = 0.1329 (mol/kg). Nanomaterials 2017, 7, 195 11 of 19

Initial condition of adsorption step:

At t = 0, the initial pressure (gauge) = −0.82 (barg), the inflow is set at p = 1.79 (barg), ω = 0.79; N2 The initial adsorbed nitrogen concentration = 0.3172 (mol/kg), initial adsorbed oxygen concentration = 0.02564 (mol/kg); Initial condition of desorption step:

At t = 0, the initial pressure = 1.79 {barg}, the inflow is set at p (gauge) = −0.82 (barg), ω = 0.79; N2 The initial adsorbed nitrogen concentration = 1.333 (mol/kg), the initial adsorbed oxygen concentration = 0.1329 (mol/kg).

Table 4. Characteristics parameters of nitrogen and oxygen [38–40].

Adsorbent Zeolite LiXUnit Reference Nanomaterials 2017Dynamic, 7, 195 viscosity of the fluid, μ 1.84 × 10−5 Pa·s Smits et al., 200611 of 19 1.5 × 10−2 - Dimensionless Henry’s law constant of N2, KN2 Sander, 2015 Table 4. Characteristics parameters of nitrogen3.2 × 10− and2 oxygen- [ 38–40]. Dimensionless Henry’s law constant of O2, KO2 3.67 10−10 m Average collisionAdsorbent diameter of N2, σ Zeolite LiX Unit Reference N2 Dynamic viscosity of the fluid, µ 1.84 × 10−5 Pa·s Smits et al., 2006 −26.06 10−10 m DimensionlessAverage collision Henry’s lawdiameter constant of of O N2, ,σK 1.5 × 10 - 2 NO2 2 −2 Sander, 2015 Dimensionless Henry’s law constant of O2, KO2 3.2 × 10 - ε 99.8 −10 K CharacteristicAverage collision energy diameter of N of2, N2, σN2 k 3.67 10 m N2 b,N2 −10 Average collision diameter of O2, σO2 6.06 10 m Bird et al., 2007 Characteristic energy of N , ε /k 99.8113 K K Characteristic energy of O2 2, Nε2 b,N2 k N2 b,N2 Characteristic energy of O2, εN2 /kb,N2 113 K Bird et al., 2007 −3 MolecularMolecular Weight, Weight, MN2 MN2 28.01 28.01 10 10kg/mol−3 kg/mol −3 Molecular Weight, M 31.99 10 kg/mol−3 Molecular Weight,O2 MO2 31.99 10 kg/mol Gas constant, R 8.31 J/(K·mol) Gas constant, R 8.31 J/(K·mol)

3.2. Geometry and Meshing of the Finite Element Algorithm The geometrygeometry ofof thethe column column used used for for the the model model simulation simulation was was a cylindera cylinder with with a height a height of 20 of cm20 andcm and a diameter a diameter of 3of cm. 3 cm. The The gas gas source source entered entered at at the the bottom bottom of of the the column column andand enrichedenriched oxygenoxygen stream exitedexited atat thethe toptop ofof thethe columncolumn shownshown inin FigureFigure8 8a.a.

Figure 8. (a) Schematics of adsorption column; (b) refined mesh with axis of symmetry; (c) full Figure 8. (a) Schematics of adsorption column; (b) refined mesh with axis of symmetry; (c) full model geometry.

The computational mesh was a swept rectangular mesh. The shape of the rectangular mesh fitsfits both the shape of the axial gas dispersion pattern and the boundary layer condition. In order to findfind an appropriate meshmesh size,size, aa meshmesh refinementrefinement studystudy waswas undertakenundertaken withwith thethe differencedifference function:function:

ω (mesh ) − ω (mesh ) O2 x O2 f inest %Di f f erence = × 100%, ( ) ωO2 mesh f inest

( ) ( ) where ωO2 meshx is the oxygen vol % of outflow at the mesh size x (mm) and ωO2 mesh f inest is the oxygen vol % of outflow at the finest mesh size (mm). Figure9 shows the results of the mesh refinement study. The relative tolerance was set to 10 −6 and largest %Difference of all the time points for each mesh size was evaluated. All the mesh sizes below 1 mm were fine enough using the criterion of less than 1% difference. The mesh size = 1 mm was selected and used for all the simulations. The refined mesh and geometry are shown in Figure8b,c. Nanomaterials 2017, 7, 195 12 of 19

|ω (mesh ) −ω (mesh ) | O2 x O2 finest %Difference = ×100% , ω (mesh ) O2 finest where ω (mesh ) is the oxygen vol % of outflow at the mesh size x (mm) and ω (mesh ) is O2 x O2 finest the oxygen vol % of outflow at the finest mesh size (mm). Figure 9 shows the results of the mesh refinement study. The relative tolerance was set to 10−6 and largest %Difference of all the time points for each mesh size was evaluated. All the mesh sizes below 1 mm were fine enough using the criterion of less than 1% difference. The mesh size = 1 mm was selected and used for all the simulations. The refined mesh and geometry are shown in Figure Nanomaterials 2017, 7, 195 12 of 19 8b,c.

7 6.31 6

5

4

3 %Difference 2 1.39 0.89 1 0.18 0 0 0.5 1 1.5 2 Mesh Size/mm

Figure 9. Mesh refinement refinement study results.

4. Results and Discussion

4.1. Concentrator Output The photograph photograph of of the the oxygen oxygen concentrator concentrator is shown is shown in Figure in Figure 10 with 10 withthe in-house the in-house made mademicrocontroller microcontroller circuit circuit for the for thecontinuous continuous production. production. The The concentrator concentrator is istested tested with with a vacuum/pressure swing swing cycle cycle between between −−0.820.82 barg barg and and 1.79 1.79 barg. barg. The The flowrate flowrate of ofoutput output stream stream is is1.128 1.128 L/min L/min and and the the average average oxygen oxygen concentration concentration peak peak is is 91.64 91.64 vol vol %. The The output oxygen concentration is almost stable at 1.79 barg, which means the desorption under a vacuum is effective and the desorption time is longlong enoughenough toto regenerateregenerate thethe zeolite.zeolite. The enriched continuous oxygen stream can be conserved in a storage tank to produceproduce a steady enriched air stream. The cyclic oxygen production test shows that the oxygen concentratio concentrationn decreased gradually to 80 vol % % after after 300 300 cycles. cycles. The adsorption column could be replaced every month for commercial portable oxygen concentrator development and thethe deactivateddeactivated zeolitezeolite adsorbentadsorbent couldcould be be regenerated regenerated in in the the oven oven at at 450 450◦C °C with with a continuousaNanomaterials continuous 2017 nitrogen nitrogen, 7, 195 flow flow to to remove remove the the desorbed desorbed contaminants. contaminants. 13 of 19

Figure 10. 10. PhotographPhotograph of oxygen of oxygen concentrator concentrator prototype prototype (a) front side, (a) (b front) back side, side, ( (cb)) Oxygen back side, (analyzer.c) Oxygen analyzer.

4.2. Model Validation Figure 11 compares the experimental flow rate with the simulation results. The experimental and model output flowrates are 1.226 L/min and 1.24 L/min, respectively. The difference of the experimental data and simulation results is 1.1%, which means the modified Free and Porous Media Flow Interface is able to represent the outlet flow condition. Before 21 s, the adsorption column is in the pressurization stage without any gas flowing out through the outlet. The flow rate of the outlet is maintained at 1.226 L/min at the production stage, which means the same amounts of mass flow are entering and exiting the adsorption column to maintain the adsorption pressure after t = 21 s.

Figure 11. Comparison of the experimental and simulation results of the column outlet flow rate.

Figure 12 shows the outlet pressure profile at different times during the adsorption step in the Free and Porous Media Flow Interface. The difference between the simulation results and the experimental data is negligible, which is significant for the adsorption function in the Transport of Diluted Species in Porous Media Interface. According to the outlet pressure and flowrate profile in

Nanomaterials 2017, 7, 195 13 of 19

Nanomaterials 2017, 7, 195 13 of 19 Figure 10. Photograph of oxygen concentrator prototype (a) front side, (b) back side, (c) Oxygen analyzer.

4.2. Model4.2. Validation Model Validation Figure 11Figure compares 11 compares the experimentalthe experimental flow flow rate rate withwith the the simulation simulation results. results. The experimental The experimental and modeland outputmodel output flowrates flowrates are 1.226are 1.226 L/min L/min and and 1.241.24 L/min, respectively. respectively. The Thedifference difference of the of the experimentalexperimental data and data simulation and simulation results results is is 1.1%, 1.1%, whichwhich means means the the modified modified Free Free and Porous and Porous Media Media Flow InterfaceFlow Interface is able is to able represent to represent the the outlet outlet flow flow condition. condition. Before Before 21 21s, the s, theadsorption adsorption column column is in is in the pressurization stage without any gas flowing out through the outlet. The flow rate of the outlet is the pressurizationmaintained at stage 1.226 without L/min at anythe production gas flowing stage, out which through means the the outlet. same amounts The flow of mass rate offlow the are outlet is maintainedentering at 1.226 and exiting L/min the at adsorption the production column stage,to maintain which the means adsorption the pressure same amounts after t = 21 of s. mass flow are entering and exiting the adsorption column to maintain the adsorption pressure after t = 21 s.

Figure 11. Comparison of the experimental and simulation results of the column outlet flow rate. Figure 11. Comparison of the experimental and simulation results of the column outlet flow rate.

Figure 12Figure shows 12 shows the outlet the outlet pressure pressure profile profile at different at different times times during during thethe adsorptionadsorption step step in inthe the Free and PorousFree Mediaand Porous Flow Media Interface. Flow The Interface. difference The diff betweenerence thebetween simulation the simulation results andresults the and experimental the experimental data is negligible, which is significant for the adsorption function in the Transport of data isNanomaterials negligible, 2017 which, 7, 195 is significant for the adsorption function in the Transport of Diluted14 of 19 Species Diluted Species in Porous Media Interface. According to the outlet pressure and flowrate profile in in PorousFigures Media 11 and Interface. 12, the model According is able to represent the outlet the pressure pressurization and flowratestep flow condition profile in of Figures a portable 11 and 12, the model is able to represent the pressurization step flow condition of a portable oxygen concentrator oxygen concentrator with an air compressor instead of a constant flowrate simulation. with an air compressor instead of a constant flowrate simulation.

Figure 12. Comparison of the experimental and simulation results of the column outlet pressure. Figure 12. Comparison of the experimental and simulation results of the column outlet pressure.

FigureFigure 13 shows 13 shows the the internal internal gas gas velocity velocity profil profilee at at different different times times during during the adsorption the adsorption step. step. The ‘arrow’The ‘arrow’ surface surface represents represents the the direction direction of of velocity velocity vector.vector. When When the the bed bed is isbeing being pressurized, pressurized, the the velocity decreases from the inlet to the outlet due to the closed outlet boundary condition (Figure 13a,b). The velocity distribution at the production stage (Figure 13c) is uniform, which means the column is saturated and concentrated gas is transported from the outlet of the column at a constant pressure.

Figure 13. Internal velocity profile of adsorption column with inlet distributor at time = (a) 1 s; (b) 10 s; (c) 30 s.

Nanomaterials 2017, 7, 195 14 of 19

Figures 11 and 12, the model is able to represent the pressurization step flow condition of a portable oxygen concentrator with an air compressor instead of a constant flowrate simulation.

Figure 12. Comparison of the experimental and simulation results of the column outlet pressure. Nanomaterials 2017, 7, 195 14 of 19 Figure 13 shows the internal gas velocity profile at different times during the adsorption step. The ‘arrow’ surface represents the direction of velocity vector. When the bed is being pressurized, velocitythe decreases velocity fromdecreases the inletfrom the to theinlet outlet to the dueoutlet to due the to closed the closed outlet outlet boundary boundary condition condition (Figure(Figure 13a,b). The velocity13a,b). distribution The velocity atdistribution the production at the production stage (Figure stage 13(Figurec) is uniform,13c) is uniform, which which means means the columnthe is column is saturated and concentrated gas is transported from the outlet of the column at a constant saturatedpressure. and concentrated gas is transported from the outlet of the column at a constant pressure.

Figure 13.FigureInternal 13. Internal velocity velocity profile profile of adsorption of adsorption column column withwith inlet distributor distributor at time at time = (a) = 1 (s;a )(b 1) s;10 ( b) 10 s; (c) 30 s. s; (c) 30 s. Nanomaterials 2017, 7, 195 15 of 19

Figure 14 shows the experimental and the simulation results of the outlet oxygen concentration at Figure 14 shows the experimental and the simulation results of the outlet oxygen concentration adsorption pressure = 1.79 barg and desorption pressure = −0.82 barg. The enriched oxygen product at adsorption pressure = 1.79 barg and desorption pressure = −0.82 barg. The enriched oxygen product starts tostarts flow to out flow of out the of columnthe column at att = t = 21 21 s s and and thethe outlet outlet oxygen oxygen concentration concentration in the inproduct the product flow flow reachesreaches the peak the atpeakt = at 35 t = s. 35 The s. The experimental experimental datadata shows shows the the same same enriched enriched tendency tendency of oxygen of oxygenin in the outflow.the outflow. Compared Compared to the to the simulation simulation result, result, a sl slightight difference difference of the of theoxygen oxygen breakthrough breakthrough curve curve is observedis observed from thefrom experimental the experimental oxygen oxygen sensor sensor data. This This could could be explained be explained by the by inaccuracy the inaccuracy of of the multicomponentthe multicomponent Sips Sips adsorption adsorption equations equations due to to the the complexity complexity of the of heterogeneous the heterogeneous zeolite zeolite porous surface. porous surface.

Figure 14. FigureExperimental 14. Experimental outflow outflow oxygenoxygen concentr concentrationation with with simulation simulation results. results.

Two typesTwo oftypes inlet of inlet flow flow distribution distribution were were investigatedinvestigated for for the the column column adsorption adsorption test. Figure test. 15a Figure 15a illustrates the common central flow from tubing to the column inlet without inlet distributor and illustrates the common central flow from tubing to the column inlet without inlet distributor and Figure 15b illustrates the distributed inflow with an inlet gas distributor. Figure 16 shows the N2 Figure 15concentrationb illustrates profiles the distributedof two different inflow inflowwith types anat different inlet gas times distributor. during the pressurization Figure 16 shows step. the N2 The distribution of the central flow is not uniform with a radial dispersion from column axis to the boundary while the distributed flow is closed to an ideal plug flow with negligible radial dispersion. Figure 17 compares the outflow concentration for distributed inflow and central flow. The results are in accordance with the experimental results in that the pressurization time of the column with central inflow is 1.6 s longer than that of the column with the distributed flow. The inlet gas distributor is significant for the inflow air and decreases the pressurization time for the adsorption cycle.

Figure 15. Schematics of inflow type at the inlet of the adsorption column: (a) central flow without inlet distributor; (b) distributed flow with inlet distributor.

Nanomaterials 2017, 7, 195 15 of 19

Figure 14 shows the experimental and the simulation results of the outlet oxygen concentration at adsorption pressure = 1.79 barg and desorption pressure = −0.82 barg. The enriched oxygen product starts to flow out of the column at t = 21 s and the outlet oxygen concentration in the product flow reaches the peak at t = 35 s. The experimental data shows the same enriched tendency of oxygen in the outflow. Compared to the simulation result, a slight difference of the oxygen breakthrough curve is observed from the experimental oxygen sensor data. This could be explained by the inaccuracy of the multicomponent Sips adsorption equations due to the complexity of the heterogeneous zeolite porous surface.

Figure 14. Experimental outflow oxygen concentration with simulation results. Nanomaterials 2017, 7, 195 15 of 19 Two types of inlet flow distribution were investigated for the column adsorption test. Figure 15a illustrates the common central flow from tubing to the column inlet without inlet distributor and concentrationFigure 15b profiles illustrates of twothe distributed different inflow inflow with types an atinlet different gas distributor. times during Figure 16 the shows pressurization the N2 step. The distributionconcentration of profiles the central of two flowdifferent isnot inflow uniform types at with different a radial times dispersionduring the pressurization from column step. axis to the boundaryThe distribution while the distributedof the central flowflow is not closed uniform to an with ideal a radial plug dispersion flow with from negligible column axis radial to the dispersion. boundary while the distributed flow is closed to an ideal plug flow with negligible radial dispersion. Figure 17 compares the outflow concentration for distributed inflow and central flow. The results are Figure 17 compares the outflow concentration for distributed inflow and central flow. The results are in accordancein accordance with with the the experimental experimental results results in in that that the the pressurization pressurization time timeof the ofcolumn the column with central with central inflowinflow is 1.6 is s 1.6 longer s longer than than that that of of the the columncolumn wi withth the the distributed distributed flow. flow. The inlet The gas inlet distributor gas distributor is is significantsignificant for the for inflowthe inflow air air and and decreases decreases the pressurization pressurization time time for the for adsorption the adsorption cycle. cycle.

FigureFigure 15. Schematics 15. Schematics of of inflow inflow typetype at at the the inlet inlet of the of theadsorption adsorption column: column: (a) central (a )flow central without flow without inlet distributor; (b) distributed flow with inlet distributor. Nanomaterialsinlet distributor; 2017, 7 (, b195) distributed flow with inlet distributor. 16 of 19

Figure 16. (a) N2 concentration profile without inlet distributor at t = 1 s; (b) N2 concentration profile Figure 16. (a)N2 concentration profile without inlet distributor at t = 1 s; (b)N2 concentration profile without inlet distributor at t = 3 s; (c) N2 concentration profile without inlet distributor at t = 6 s; (d)

without inletN2 concentration distributor atprofilet =3 with s; (c )Ninlet2 distributorconcentration at t =profile 1 s; (e) withoutN2 concentration inlet distributor profile with at inlett = 6 s; (d)N2 concentrationdistributor profile at t with = 3 s; inlet(f) N2 distributorconcentration atprofilet = 1with s;( inlete)N distributor2 concentration at t = 6 s. profile with inlet distributor at t = 3 s; (f)N2 concentration profile with inlet distributor at t = 6 s.

Figure 17. Simulation results of outflow oxygen concentration for different inflow type.

Nanomaterials 2017, 7, 195 16 of 19

Figure 16. (a) N2 concentration profile without inlet distributor at t = 1 s; (b) N2 concentration profile

without inlet distributor at t = 3 s; (c) N2 concentration profile without inlet distributor at t = 6 s; (d)

N2 concentration profile with inlet distributor at t = 1 s; (e) N2 concentration profile with inlet

Nanomaterialsdistributor2017 at, 7 t, 195= 3 s; (f) N2 concentration profile with inlet distributor at t = 6 s. 16 of 19

Nanomaterials 2017, 7, 195 17 of 19 Figure 17. Simulation results of outflow oxygen concentration for different inflow type. Figure 17. Simulation results of outflow oxygen concentration for different inflow type. Figure 18 shows the concentration peak of the oxygen output breakthrough curve under differentFigure vacuum 18 shows swing the range. concentration The nitrogen peak of capaci the oxygenty of the output zeolite breakthrough decreases drastically curve under when different the pressurevacuum swingof vacuum range. port The increases nitrogen from capacity 18 kPa of the (abs) zeolite to 34 decreases kPa (abs). drastically The deep whenvacuum the desorption pressure of stepvacuum is able port to increasesincrease the from productivity 18 kPa (abs) of tothe 34 ad kPasorption (abs). Thecolumn. deep The vacuum simulation desorption results step also is exhibit able to aincrease good fitting the productivity precision of ofthe the experimental adsorption column.data with The the simulation maximum resultsdifference also 0.67%. exhibit a good fitting precision of the experimental data with the maximum difference 0.67%.

Figure 18. Output oxygen concentration peak from different vacuum swing range to 1.79 bar Figure 18. Output oxygen concentration peak from different vacuum swing range to 1.79 bar gauge gauge pressure. pressure.

FigureFigure 1919 showsshows the the average average surface surface excess excess of the of zeolite the zeolite surface surface at the adsorption at the adsorption and desorption and desorptionstage. The stage. amount The ofamount N2 and of ON2 andadsorbed O2 adsorbed to the to zeolite the zeolite particle particle gradually gradually decreases decreases when when the theadsorption adsorption column column is connected is connected to the to suction the suction port of port the air of compressor the air compressor to create vacuumto create desorption vacuum desorptionconditions insideconditions the column. inside Nthe2 and column. O2 are releasedN2 and fromO2 are the released zeolite during from thethe blowdown zeolite during and purge the blowdownstages, which and generates purge stages, a clean which bed forgenerates the next a clean adsorption bed for cycle. the next adsorption cycle.

Figure 19. Simulation results of average adsorbed nitrogen and oxygen amount to the zeolite particles.

Nanomaterials 2017, 7, 195 17 of 19

Figure 18 shows the concentration peak of the oxygen output breakthrough curve under different vacuum swing range. The nitrogen capacity of the zeolite decreases drastically when the pressure of vacuum port increases from 18 kPa (abs) to 34 kPa (abs). The deep vacuum desorption step is able to increase the productivity of the adsorption column. The simulation results also exhibit a good fitting precision of the experimental data with the maximum difference 0.67%.

Figure 18. Output oxygen concentration peak from different vacuum swing range to 1.79 bar gauge pressure.

Figure 19 shows the average surface excess of the zeolite surface at the adsorption and desorption stage. The amount of N2 and O2 adsorbed to the zeolite particle gradually decreases when the adsorption column is connected to the suction port of the air compressor to create vacuum desorption conditions inside the column. N2 and O2 are released from the zeolite during the Nanomaterials 2017, 7, 195 17 of 19 blowdown and purge stages, which generates a clean bed for the next adsorption cycle.

Figure 19. SimulationSimulation results results of of average average adsorbed adsorbed nitrogen and oxygen amount to the zeolite particles.

5. Conclusions A portable two-column pressure vacuum swing adsorption (PVSA) oxygen concentrator was built in our laboratory and its performance was modeled by solving the continuity equations and the momentum equation in COMSOL Multiphysics. The model was able to simulate the microporous adsorption of nitrogen under a dry and CO2 free compressor flow. The velocity and oxygen concentration profiles were investigated at every time step (∆t = 0.1 s) during the four operation stages: pressurization, production, blowdown, and desorption. With the process optimization, it was possible to get better operational parameters and design characteristics to improve the performance of the concentrator units and meet the terminal requirements of oxygen supply (O2 concentration ~88–92 vol %). The valves and the compressor of the prototype are controlled by a microcontroller program for continuous oxygen production. The concentrator is a lightweight, miniature unit suitable for outside oxygen supplement applications.

Acknowledgments: The authors would like to acknowledge the support provided by the Natural Sciences and Engineering Research Council of Canada (NSERC) and Ontario Centres of Excellence (OCE). Author Contributions: Mingfei Pan performed the experiment, developed the model, ran the simulations and wrote the manuscript. Hecham Omar helped with data analysis and writing of the manuscript. The work was supervised by Sohrab Rohani. The draft of the manuscript was reviewed, revised and submitted by Sohrab Rohani. Conflicts of Interest: The authors declare no conflict of interest.

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