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High pressure excess isotherms for adsorption of oxygen and argon in a carbon molecular sieve

High pressure excess isotherms for adsorption of oxygen and argon in a carbon molecular

sieve

Lucas A.Mitchell,Trenton M.Tovar,M.Douglas LeVan

*

Department of Chemical and Biomolecular Engineering,Vanderbilt University,Nashville,TN 37235,USA A R T I C L E I N F O

Article history:

Received 30December 2013Accepted 6March 2014Available online 14March 2014

A B S T R A C T

Adsorption equilibrium data for oxygen and argon are needed for design of adsorptive separation processes to produce pure oxygen from air and also for adsorptive gas storage applications.Carbon molecular sieves may be used to accomplish a rate-based separation of oxygen and argon and,as we show,may also be useful for gas storage.Given the limited data available,particularly at high pressures,volumetric methods are applied in this paper to measure surface excess isotherms of oxygen and argon on a carbon molecular sieve,Shirasagi MSC-3R T ype 172.Temperatures are considered from 25to 100°C with pressures as high as 100bar.Isotherms are compared at 25°C,including new data measured for nitro-gen.Adsorbed-phase excess loadings are high,approaching 10mol/kg at 100bar.The oxy-gen capacity of the carbon molecular sieve at high pressure is comparable to that of a superactivated carbon on a mass basis,and it is higher on a volumetric basis.The excess adsorption isotherms are modeled using a multi-temperature Toth equation,which pro-vides an excellent description.A carbon molecular sieve is shown to be a promising adsor-bent for oxygen storage.

ó2014Elsevier Ltd.All rights reserved.

1.Introduction

The demand for pure oxygen is widespread.It is used to sus-tain life in the medical profession as well as in specialized applications such as space environments,scuba diving,and mountaineering.It is essential in the steel industry and con-tributes to the high temperatures of oxy-hydrogen and oxy-acetylene blow torches.In semiconductor fabrication,it is a component in the chemical vapor deposition of silicon diox-ide,in diffusional operations for ?lm growth,and in plasma etching and the plasma stripping of photoresistors.It is also used in a wide variety of other scienti?c,laboratory,commer-cial,and industrial applications.

Gas storage via adsorption is a targeted technology for fu-ture applications including methane and hydrogen storage in transportation vehicles.For these,the goal is to increase the

volumetric capacity of a storage vessel and to increase the margin of safety in using pressurized gases by lowering pres-sures.NASA has an interest in generating pure oxygen from spacecraft cabin air for use in backpacks at high pressure for extravehicular activity [1].The possibility exists to store oxygen in adsorptive media for this and in other applications such as for ?rst responders.

Air separation to produce oxygen or nearly pure oxygen is generally performed by two methods.Cryogenic distilla-tion is typically the source of pure oxygen,but it has high capital equipment requirements.While this is acceptable for large industrial applications,the demand for smaller sources is increasing.Adsorption processes,namely pres-sure-swing adsorption (PSA),vacuum-swing adsorption (VSA),and pressure-vacuum-swing adsorption (PVSA)?nd extensive application on more moderate scales including

https://www.sodocs.net/doc/735036341.html,/10.1016/j.carbon.2014.03.012

0008-6223/ó2014Elsevier Ltd.All rights reserved.

*Corresponding author:Address:Vanderbilt University,PMB 351604,Nashville,TN 37235,USA.Fax:+1(615)3437951.E-mail address:m.douglas.levan@https://www.sodocs.net/doc/735036341.html, (M.D.LeVan).

for medical oxygen concentrators for home and portable use.

The generation of pure oxygen from air through adsorp-tion is a dif?cult process.First,the nitrogen,carbon dioxide, and water vapor must be removed.This is commonly accom-plished using zeolites in an equilibrium-based separation. There have been many such studies for PSA[2–9],VSA[10], and PVSA[11].Nitrogen is adsorbed preferentially over oxy-gen on the zeolites.Argon is weakly adsorbed and remains with the oxygen,resulting in a product stream consisting of approximately95%oxygen and5%argon.

Then,the argon must be removed to produce a stream of puri?ed oxygen.This is a more dif?cult separation than the one for oxygen and nitrogen.There have been studies based on zeolites[6,10,12],but argon does not show an appreciable difference in isotherm loadings from oxygen.A carbon molec-ular sieve(CMS)separates gases based on differences in mass transfer rates through constricted pores.This adsorbent is well suited for the separation of oxygen and argon,as the mass transfer rate of argon is approximately60times slower than that of oxygen[13].

There is a need for adsorption equilibrium data and descriptive equations to address design needs for separation and storage processes involving oxygen and argon at high pressures.While there have been prior equilibrium studies of oxygen and argon adsorption on CMS materials[4,14–19], the pressures do not exceed20bar near room temperature (293to313K)or5bar for a broader temperature range.

In this paper,adsorption equilibria of oxygen and argon are reported for a CMS adsorbent,Shirasagi MSC-3R T ype 172.The data were measured using a volumetric system de-signed for oxygen service and cover the temperature range of25–100°C and pressures as high as100bar.For oxygen, because of safety concerns,only the25°C isotherm was mea-sured to100bar,with higher temperature isotherms mea-sured to12bar.A high pressure nitrogen isotherm at25°C was also measured for comparison.The data are represented as excess adsorption isotherms and are analyzed using a tra-ditional temperature-dependent isotherm model,allowing for accurate prediction of adsorption loadings over wide ranges of temperatures and pressures.Finally,the data for oxygen, argon,and nitrogen are compared with loadings measured on other adsorbents,and the capability for adsorptive storage of oxygen is evaluated.

This paper reports the highest pressure measurements to date of oxygen and argon isotherms on a carbon molecular sieve and is the?rst to examine the potential of the material for oxygen storage.

2.Materials and methods

2.1.Materials

Shirasagi MSC-3R T ype172carbon molecular sieve(lot M398) was supplied by Japan EnviroChemicals,Ltd.It is a coconut shell-based material and was in1.8mm pellet form.This mate-rial was chosen originally because of its ability to separate oxy-gen and argon on a rate-selective basis.All gases were ultrahigh purity(99.99%)and obtained from Airgas and Air Liquide.2.2.Apparatus and procedures

The volumetric apparatus and procedures used in this work have been described previously[1].The adsorbent sample was degassed?rst using a Micromeritics ASAP2020porosi-meter to determine the adsorbent mass.Approximately4g of sample was heated to100°C for1h under vacuum and then held at300°C for an additional10h under vacuum.After the dry sample mass was measured,the sample was loaded into the adsorbent bed of the volumetric apparatus,where it was regenerated again by heating at300°C under vacuum overnight.To determine the accessible volume on the sample side of the apparatus,helium expansions were performed at the highest measured isotherm temperature(100°C)to re-duce any potential helium adsorption effects.The sample was then regenerated a?nal time at300°C under vacuum overnight.

All of the data presented in this paper were obtained using a single charge of CMS.It was regenerated in situ between iso-therm measurements by heating to200°C under vacuum. Data were measured in the following order:(1)oxygen iso-therms in order of increasing temperature to12bar,(2)argon isotherms in order of increasing temperature to100bar,(3) the25°C oxygen isotherm from10to100bar,and(4)the 25°C nitrogen isotherm to100bar.

3.Results and discussion

3.1.Measured isotherms

Adsorption isotherms for oxygen and argon are shown in Figs.1and2,respectively,with data for oxygen,argon,and nitrogen tabulated in Tables1–3.All adsorbed quantities are excess adsorption,calculated as in our previous study[1]. Compressibility factors for all gases were calculated using the commercial NIST REFPROP program.

Due to the pore constrictions introduced during manufac-turing,rates of uptake on CMS materials are generally slow compared to adsorbents developed for equilibrium-based separations.Time constants for the rate of adsorption on Shirasagi MSC-3R T ype172,based on results of separate experiments performed using a frequency response method [13],are about2min for oxygen,1h for nitrogen,and2h for argon;these correspond to times near the middle of an up-take curve.To approach adsorption equilibrium fairly closely, oxygen took hours and argon and nitrogen took about a day. We allowed at least48h for equilibration for all gases before recording any?nal measurements.For oxygen at low temper-atures and pressures,we allowed up to200h for equilibration, because after a relatively rapid initial uptake and pressure reduction,a very slow exponential decline to a slightly lower pressure was observed(i.e.,$1%drop in pressure between48 and200h).This is possibly due to the ultimate transport of oxygen through tight pore constrictions,which were too nar-row for argon or nitrogen to pass through.It could also be due to a chemisorption process involving a small fraction of the carbon surface.We note that the aging of CMS adsorbents in oxygen containing environments has not been conclusively established,although it is recognized for cellulose-based CMS

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membranes [19];studies have been directed toward stabiliz-ing CMS adsorbents by hydrogen treatment,which may re-duce signi?cant oxygen chemisorption,should it occur [20].We also note that our data were reproducible after regenera-tion (see Fig.1).

As shown in Figs.1and 2,the oxygen and argon isotherms are linear at pressures up to about 100kPa.A line of slope unity is shown in the ?gures to emphasize this linearity.The decrease in slopes of the isotherms is easily apparent

by a pressure of 103kPa,with this decrease being smooth and gradual.Adsorbed-phase loadings for both oxygen and argon are near 10mol/kg at 25°C and 104kPa,with argon having a slightly higher loading.The oxygen isotherms ap-pear to be more temperature sensitive than those for argon,as the loadings for oxygen decrease more with increasing temperature.

Isotherms for oxygen,argon,and nitrogen at 25°C are compared in Fig.3.The three gases have similar loadings across the entire pressure range,with argon having slightly higher loadings than oxygen or nitrogen.Also,all three gases have nearly linear isotherms up to 100kPa.This linearity suggests that there is little interaction of molecules in the adsorbed phase,so adsorbed-phase concentrations in a mixture of the gases should be described reasonably well by partial pressures and pure gas isotherms.

3.2.

Isotherm model

Adsorption equilibrium models can provide accurate descrip-tions of the temperature and pressure dependence of data over wide ranges.Many such models are available,and the temperature dependent Toth equation [21]is adopted here.The Toth isotherm is n ?

n s bP ?1tebP Tt

e1T

where n s is the saturation loading,b describes the adsorption af?nity,and t is a measure of adsorbent homogeneity.Tem-perature dependences are given by

n s ?n 0exp v 1àT

T 0

!

e2T

b ?b 0exp Q 0T 0

à1 !e3T

t ?t 0ta 1àT 0

e4T

where v and a are empirical parameters,and Q is the isos-teric heat of adsorption in the Henry’s law limit.The nitro-gen isotherm was modeled using the basic Toth isotherm given by Eq.(1).Using T 0?273:15K as the reference tem-perature,the Toth parameters for all three gases were ob-tained via a least squares analysis and are given in Table 4.Solid curves using the parameters for oxygen and argon are plotted in Figs.1and 2,and they describe the data well.

3.3.Isosteric heat of adsorption

The isosteric heat of adsorption for a pure component can be calculated using the Clausius-Clapeyron-type equation

D H ads ?zRT 2@eln P T@T n e5TFor the Toth isotherm,the isosteric heat of adsorption with z ?1is [21]

D H ads ?Q àa RT 0t ln ebP Tà1tebP Tt ??ln bP 1tebP T

t àá1=t 24358<:9=;e6

T

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Isosteric heats for oxygen and argon at 25and 100°C on MSC-3R T ype 172are shown as a function of loading in Fig.4.They are only weakly temperature dependent over our range of interest.The isosteric heats are constant over the linear range of the isotherms and decrease slightly as the slopes of the isotherms decrease.We obtain isosteric heats at zero loading of approximately 12.5kJ/mol for oxygen and 8.9kJ/mol for argon.These agree reasonably well with respective predicted values of 10.7kJ/mol and 9.0kJ/mol [15],although measured values for other carbon molecular sieves are higher,16kJ/mol for oxygen [22]and 18kJ/mol for argon [15].

https://www.sodocs.net/doc/735036341.html,parison with other adsorbents

Adsorption isotherms for oxygen on various adsorbents at 25°C are shown in Fig.5.Shirasagi MSC-3R T ype 172gives high loadings,similar to those of a superactivated carbon [23],and the highest at 104kPa.The Takeda 3A CMS [18]and BPL activated carbon give similar loadings that are somewhat lower at high pressures than MSC-3R T ype 172and the super-activated carbon.The 13X zeolite [1]and the titanosilicates [24,25]give comparatively low loadings.It should be noted that other superactivated carbons or high surface area tem-plated carbons have not been tested for oxygen adsorption and may have quite high capacities.

Fig.6shows the 25°C adsorption isotherm for argon on MSC-3R T ype 172compared to 25°C argon isotherms on other adsorbents.MSC-3R T ype 172gives the highest loadings at high pressures.The 5A zeolite [26]and BPL activated carbon give similar loadings,with both having higher loadings than Takeda 3A CMS.MSC-3R T ype 172has higher isotherm slopes than Takeda 3A CMS [18].The titanosilicates [24,25]give com-paratively low loadings,which are similar to those for oxygen on these adsorbents.

Fig.7shows 25°C adsorption isotherms for nitrogen on various adsorbents.MSC-3R T ype 172followed closely by BPL activated carbon give the highest loadings for isotherms

Table 1–Oxygen excess adsorption data on MSC-3R T ype 172.25°C 50°C 75°C 100°C P (kPa)n (mol/kg)P (kPa)n (mol/kg)P (kPa)n (mol/kg)P (kPa)n (mol/kg)7.01·10à1 1.96·10à38.09·10à1 1.12·10à3 4.46·10à1 4.28·10à4 5.51·10à1 3.76·10à49.82·10à1 2.74·10à3 1.48 2.55·10à3 1.14 1.26·10à3 1.299.15·10à41.58 4.39·10à3 3.20 5.76·10à3 3.20 3.44·10à3 3.60 2.36·10à33.81 1.10·10à210.6 1.98·10à28.309.15·10à39.61 6.59·10à311.5 3.35·10à228.5 4.67·10à229.3 3.37·10à230.1 2.44·10à230.18.96·10à2147 2.45·10à1134 1.62·10à1190 1.46·10à1131 3.78·10à1391 6.19·10à1350 4.07·10à1456 3.41·10à13609.68·10à11210

1.79

1190

1.09

1260

9.05·10à1

1049 2.49981 2.422210 4.223480 5.4358707.4911,100

9.08

Table 2–Argon excess adsorption data on MSC-3R T ype 172.25°C 50°C 75°C 100°C P (kPa)n (mol/kg)P (kPa)n (mol/kg)P (kPa)n (mol/kg)P (kPa)n (mol/kg)34.5 1.11·10à148.3 1.05·10à166.9 1.07·10à158.67.43·10à296.5 2.95·10à1116 2.44·10à1143 2.28·10à1141 1.73·10à12767.88·10à13557.17·10à1330 5.02·10à1379 4.80·10à1889 2.16965 1.81965 1.381100 1.262210 4.442320 3.532310 2.762690 2.5555207.925650 6.345380 5.125860 4.4610,700

10.3

9100

7.99

10,200

7.23

8620

5.67

Table 3–Nitrogen excess adsorption data on MSC-3R T ype 172.25°C P (kPa)n (mol/kg)44.8 1.24·10à1117 3.13·10à13177.47·10à1965 1.892320 3.735520 6.6411,000

8.54

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measured to high pressure.The isotherm for13X zeolite[1], also measured in our laboratory,and Takeda3A CMS[18]give similar loadings for nitrogen.The capacity of MSC-3R T ype 172is three times that of13X zeolite at104kPa.The capacities of the titanosilicates[24,25]at high pressure are not apparent.

Although both are carbons,there are notable differences between MSC-3R T ype172and BPL.BPL is a coal-based carbon with a surface area of approximately1200m2/g and a median pore width of approximately12A?,which depends on the method used in porosity analysis[27].The MSC-3R is a coco-nut shell-based carbon with a surface area of approximately 750m2/g and two average pore widths of3.5and6.0A?,based on a similar material[19].Although BPL has a higher surface area,the reduced average pore size in MSC-3R created during production results in higher excess adsorbate densities at high pressures.

3.5.Adsorptive storage of oxygen

The possibility of using an adsorbent for gas storage can be explored using an adsorption isotherm and the physical prop-erties of the adsorbent.The total volumetric capacity for a mass of adsorbent is given by[28]

q tot?q b nt 0ce7Twith

0? tve1à Te8T

where q b is the bulk density of the packing, 0is total voidage (with contributions from packing interstices and v from par-ticle porosity),and c?P=ezRTTis the gas molar density.

Fig.8shows two curves for25°C:the oxygen density in an unpacked vessel and the oxygen density in a vessel packed with MSC-3R T ype172CMS.We used q b?705kg/m3, =0.35,and v?0:46;the value of q b corresponds to the center of the manufacturer’s range(680–730kg/m3)[29],and the va-lue of v corresponds to a similar material(Shirasagi MSC-3K T ype162)[19].Oxygen gas is nearly ideal over the entire pres-sure range,with the compressibility factor reaching0.95at 104kPa.Over the linear range of the isotherm,the packed ves-sel contains5.3times the amount of oxygen as an unpacked vessel.At103kPa,the packed vessel contains about4.3times, and at104kPa it contains about43%more.Expressed another way,for vessels of the same volume,vessels packed with adsorbent at230kPa and4650kPa would contain the same amount of oxygen as unpacked vessels at103kPa and 104kPa,respectively.

The performance of MSC-3R T ype172CMS for oxygen stor-age on a volumetric basis will exceed that of a superactivated carbon.Although the isotherms on a mass of adsorbent basis are similar on the two adsorbents,the CMS is denser than the superactivated carbon,and thus gives a much larger contribu-tion from the?rst term on the right side of Eq.(7).For exam-ple,the particle density of AX-21superactivated carbon[30]is 700g/cm3,which corresponds to a bulk density of455kg/m3 using ?0:35.

Table4–Model parameters for multi-temperature Toth equation.

n0(mol/kg)v b0(kPaà1)Q=eRT0Tt0a

Oxygen14.4 1.23 3.69·10à2 5.52 1.00 1.89·10à2 Argon18.50.589 2.43·10à2 3.890.858 1.38·10à2 Nitrogen a15.8 1.74·10à20.778

a Nitrogen parameters are n

s ;b,and t as shown in Eq.(1)

.

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4.Conclusions

Surface excess isotherms for oxygen and argon adsorbed on Shirasagi MSC-3R T ype 172carbon molecular sieve over the temperature range 25–100°C and pressures up to 104kPa have been measured.These are the highest pressure oxygen and argon isotherms reported for a carbon molecular sieve.Excess loadings for oxygen and argon approach 10mol/kg at 104kPa and 25°C.Oxygen,argon,and nitrogen isotherms have simi-lar loadings and linear slopes up to 100kPa at 25°C.

The measured isotherm data have been analyzed using traditional methods.The oxygen and argon data were mod-eled with a multi-temperature Toth equation,while the nitro-gen data were modeled with the classic Toth isotherm.The isosteric heats of adsorption were determined to be 12.5kJ/mol for oxygen and 8.9kJ/mol for argon over the linear ranges of the 25°C isotherms.

The isotherms for oxygen,argon,and nitrogen adsorption on Shirasagi MSC-3R T ype 172have been compared to iso-therms in the literature for these gases on other adsorbents.The CMS has the highest loadings and isotherm slopes at 104kPa,including somewhat higher loadings than a superac-tivated carbon.

The high capacities of the adsorbent suggest potential as an adsorbent in the production and storage of pure oxygen.Calculations performed for oxygen storage indicate that volu-metric density is increased over bulk gas by a factor of more than 5at low pressure,dropping to 4.3at 103kPa and 1.43at 104kPa.While less important in a separation process,as a precautionary measure for storage,the potential chemisorp-tion of oxygen on a carbon surface and the possible creation of gaseous impurities need further study,particularly if the oxygen is to be used for breathing.

Acknowledgment

Financial support for this research was provided by NASA Cooperative Agreement NNX09AW24A and NSBRI funded pro-ject SMST02002under NASA Cooperative Agreement NCC

9-58.

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