“Multi-temperature” method for high-pressure sorption measurements on moist shales

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  • Multi-temperature method for high-pressure sorption measurements onmoist shalesMatus Gasparik, Amin Ghanizadeh, Yves Gensterblum, and Bernhard M. Krooss

    Citation: Rev. Sci. Instrum. 84, 085116 (2013); doi: 10.1063/1.4817643 View online: http://dx.doi.org/10.1063/1.4817643 View Table of Contents: http://rsi.aip.org/resource/1/RSINAK/v84/i8 Published by the AIP Publishing LLC.

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  • REVIEW OF SCIENTIFIC INSTRUMENTS 84, 085116 (2013)

    Multi-temperature method for high-pressure sorption measurementson moist shales

    Matus Gasparik, Amin Ghanizadeh, Yves Gensterblum, and Bernhard M. Kroossa)Energy and Mineral Resources Group (EMR), Institute of Geology and Geochemistry of Petroleum and Coal,Lochnerstr. 4-20, RWTH Aachen University, 52056 Aachen, Germany

    (Received 21 April 2013; accepted 22 July 2013; published online 22 August 2013)A simple and effective experimental approach has been developed and tested to study the temperaturedependence of high-pressure methane sorption in moist organic-rich shales. This method, denotedas multi-temperature (short multi-T) method, enables measuring multiple isotherms at varyingtemperatures in a single run. The measurement of individual sorption isotherms at different tem-peratures takes place in a closed system ensuring that the moisture content remains constant. Themulti-T method was successfully tested for methane sorption on an organic-rich shale sample. Ex-cess sorption isotherms for methane were measured at pressures of up to 25 MPa and at temperaturesof 318.1 K, 338.1 K, and 348.1 K on dry and moisture-equilibrated samples. The measured isothermswere parameterized with a 3-parameter Langmuir-based excess sorption function, from which ther-modynamic sorption parameters (enthalpy and entropy of adsorption) were obtained. Using these, weshow that by taking explicitly into account water vapor as molecular species in the gas phase withtemperature-dependent water vapor pressure during the experiment, more meaningful results are ob-tained with respect to thermodynamical considerations. The proposed method can be applied to anyadsorbent system (coals, shales, industrial adsorbents) and any supercritical gas (e.g., CH4, CO2) andis particularly suitable for sorption measurements using the manometric (volumetric) method. 2013AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4817643]

    I. INTRODUCTION

    There is a considerable research interest in sorption prop-erties of coals, and more recently shales, stimulated by theireconomic potential (coal bed methane, shale gas) and devel-opment of concepts to mitigate the climate change throughcapture and storage of CO2 in geologic formations. The cru-cial role of moisture in sorption process is widely recog-nized in the literature. The sorption capacities to methaneand carbon dioxide were shown to decrease significantly inthe presence of moisture in coals (Joubert et al., 1973, 1974;Krooss et al., 2002; Hildenbrand et al., 2006; Busch et al.,2006; Siemons and Busch, 2007; Crosdale et al., 2008; Dayet al., 2008; Ozdemir and Schroeder, 2009; Battistutta et al.,2012; vbov et al., 2012), shales (Busch et al., 2008;Ross and Bustin, 2009), and activated carbons (e.g.,Gensterblum et al., 2009; Billemont et al., 2011). The detri-mental effect of moisture on the sorption capacity of gas(CH4, CO2) is attributed to water and gas molecules compet-ing for sorption sites or by simple volumetric displacement.On the other hand, some studies on methane sorption in ac-tivated carbons suggested that the moisture can enhance thesorption capacity through the formation of methane hydrateunder specific experimental conditions (Zhou et al., 2002;Miyawaki et al., 1998).

    Published high-pressure sorption data for moist coals arecommonly reported as sorption isotherms at given moisturecontents on coal samples that were moisture-equilibrated atsome specified relative humidity conditions. There is, how-

    a)Electronic mail: bernhard.krooss@emr.rwth-aachen.de

    ever, limited data available up to date on the temperature de-pendence of sorption capacity for coals and none for shales.The major experimental difficulty lies in the fact that smallchanges in the moisture content between the individual mea-surements at respective temperatures can greatly affect themethane sorption capacity. Hence, the resulting changes insorption capacity do not depend on temperature alone. Thechanges may arise during evacuation cycles if isothermsare measured on a single sample in a consecutive manner,or when split samples moisturized at the same relative hu-midity conditions are used to measure each isotherm. Inter-laboratory studies on CO2 sorption on moist coals (Goodmanet al., 2004, 2007) suggested that much of the variation insorption capacity between individual laboratories results fromdifferences in moisture contents. Modifications of the exper-imental sorption setup can enable some control on the mois-ture content of the adsorbent sample in the sample cell. Forexample, Billemont et al. (2011) modified their gravimetricRubotherm setup to allow in situ moisture-equilibration ofthe sample directly in the sample cell. The gravimetric setupsare particularly suitable for this approach as they are basedon direct measurement of weight change and, this way, thein situ moisture content can be directly measured prior to thestart of the sorption experiment. This approach is, however,not applicable for manometric (volumetric) sorption devices.

    Here, we present a simple experimental approach formanometric apparatus to measure sorption isotherms atdifferent temperatures under closed system conditions. Thus,it enables to keep the moisture constant at all temperatures.In addition, the measuring times can be significantly reducedas the sample need not be de-gassed between individual

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  • 085116-2 Gasparik et al. Rev. Sci. Instrum. 84, 085116 (2013)

    FIG. 1. Schematic representation of the high-pressure manometric sorptionapparatus.

    measurements at different temperatures. This multi-temperature technique was successfully tested on dry andmoisture-equilibrated samples. Excess sorption isothermsfor methane were measured at pressures of up to 25 MPaand temperatures of 318 K, 338.1 K, and 348.1 K on adry and moisture-equilibrated organic-rich shale sample.The measured isotherms can be accurately represented bya 3-parameter function, which is based on the Langmuirfunction for absolute sorption and a density term relatingexcess and absolute sorption. The results are discussed withrespect to the thermodynamic parameters derived from thetemperature dependency of the Langmuir pressure constant.

    II. METHODS

    A. Manometric high-pressure sorption setupA schematic representation of the manometric sorption

    setup (also referred to as Sievert-type apparatus) is shown inFigure 1. It consists of two stainless steel chamberssamplecell (SC) and reference cell (RC), shut-off valves and apressure gauge connected via 1/16 in. tubing. The appara-tus is connected to methane and helium gas supplies with agas purity of 99.995% and 99.999%, respectively. Two high-pressure VICI R valves (v1 and v2) are used as shut-off valvesin front of the RC cell and between RC and SC. Due to theirconstruction and operation mode, the dead volume of thesevalves is the same irrespective of the position. A three-portvalve (v3) is used to switch between the gas supply and avacuum pump. Valves v1 and v2 are operated by computer-controlled electric actuators. A high-precision piezoresistivepressure transmitter (Tecsis P3382; all metal, no polymerseals) with a 25 MPa range attached to the reference cell is

    used to monitor the pressure. The accuracy given by the ven-dor is 0.05% of full scale value (= 0.0125 MPa). The samplecell is sealed by metal face seal fittings (VCR R, Swagelok)as described in Checchetto et al. (2004) using nickel gas-kets with an integrated 0.5 m filter. Both cells including thevalves v1 and v2 and the pressure transmitter are kept at con-stant temperature using a gas chromatograph (GC) oven. Thetemperature stability is within 0.1 K. Temperature readingsare taken from a Pt-100 (class 1/10 B) resistance temperaturedetector (RTD) with an estimated accuracy of 0.1 K.

    The volumes of the reference and the sample cell weredetermined by helium expansion. Multiple gas expansionsinto the empty sample cell and into the sample cell containinga stainless steel cylinder of accurately known volume (refer-ence volume) were performed up to 10 MPa. For the experi-mental setup used in this study the volumes of the reference(Vrc) and the sample cell (Vsc) were 1.350 0.007 cm3 and11.35 0.05 cm3, respectively.

    B. Samples and sample preparation

    High-pressure sorption isotherms were measured on adry and moisture-equilibrated shale sample that was crushedand sieved to 0.51.0 mm particle size. The basic geo-chemical properties of the studied sample are listed inTable I. The drying procedure consisted of pre-drying thesample overnight in a vacuum oven and additional dryingwas performed in situ at 383 K under vacuum after the sam-ple was transferred into the sample cell. This in situ dryingis necessary to remove any moisture taken up by the sam-ple in contact with air humidity during its placement into thesample cell since even small amounts of moisture can have asignificant effect on the sorption capacity. The moist samplewas prepared by moisture-equilibration at room temperaturein an evacuated desiccator over a saturated salt solution ofK2SO4 under constant relative pressure of water vapor (p/p0= 0.97). The moisture content (m.c.) was calculated fromEq. (1):

    m.c. = (mme mdry)mme

    100%. (1)

    Here, mme and mdry are the weight of moisture-equilibratedand dry samples, respectively.

    C. Measuring procedureThe manometric technique of high-pressure sorption

    measurements has been described in detail elsewhere(Mavor et al., 1990; Krooss et al., 2002; Busch et al., 2003;Gensterblum et al., 2009; Van Hemert et al., 2009). The

    TABLE I. Basic geochemical properties of the studied shale sample. The equilibrium moisture content is calculated by Eq. (1). Helium densities for dry andmoist sample were obtained from the void volume measurements with He at 318.1 K on dry and moisture-equilibrated samples, respectively.

    XRD (wt. %)

    Total organic Vitrinite Quartz and Total Eq. moisture He-density, He-density,carbon, TOC (wt. %) reflectance(VRr%) feldspars clays Carbonates (wt. %) dry (g/cm3) moist (g/cm3)

    5.7 2.4 38.7 59.0 0.6 2.69 2.556 0.002 2.476 0.001

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  • 085116-3 Gasparik et al. Rev. Sci. Instrum. 84, 085116 (2013)

    FIG. 2. Measuring program used in the multi-T method showing thechange in pressure (a) and the variation in temperature (b) during the firstfour gas injection steps. From the equilibrium data at respective temperaturesthe individual isotherms are calculated.

    multi-T technique described here differs from the conven-tional measurement in that the temperature is varied in a con-trolled manner during the sorption experiment. This is illus-trated in Figure 2 which shows the change in pressure andtemperature during the first four measurement steps in thesorption experiment. Initially, the reference cell is loaded withgas (methane) at T = 318.1 K. After reaching thermal equi-librium the gas is then expanded into the sample cell and al-lowed to equilibrate with the sample. When no more gas up-take is observed the temperature is increased from 318.1 K to338.1 K. During this period, a new equilibrium is establishedcorresponding to the change in pressure in a constant vol-ume and simultaneous change in the sorption capacity upontemperature increase. The temperature is then increased to348.1 K and decreased finally to 318.1 K for time intervalsnecessary to reach the equilibrium. This way, during a singlegas injection, data points corresponding to three isotherms at318.1 K, 338.1 K, and 348.1 K can be determined simultane-ously. This procedure is repeated for consecutive steps untilthe equilibrium pressure in the sample cell reaches 25 MPa.The restoration of the initial temperature of 318.1 K at the endof each temperature program cycle is not a necessity and theloading of the reference cell can be performed at 318.1 K and348.1 K in an alternating manner. Here, the isotherms for theinitial and the final 318.1 K are compared for consistency.

    The benefit of the multi-T experimental procedure is thatthe water content within the system remains constant at alltemperatures. This ensures that (1) the change in the moisturecontent of the sample with temperature is negligible, providedthe void volume is sufficiently small, or that, at least, (2) itcan be accounted for by simple mass-balance considerations.Moreover, the measuring time can be significantly reducedcompared to consecutive isotherm measurements. This comesmainly from the fact that in the latter approach the measuringspeed is limited by the need to degas the sample after each in-dividual sorption test. This can be a very lengthy process formoist coal and shale samplesespecially, if care is given not

    to lose moisture during the degassing process (e.g., by cool-ing the sample cell and short evacuation intervals followed bymonitoring of the degassing).

    D. Calculation of the excess sorption

    The excess sorption (Gibbs surface excess) for the ithstep is calculated as a difference between the total mass ofgas transferred into the sample cell and the mass of gas occu-pying the void volume (V 0void ) at density CH4eq correspondingto (p,T) in the sample cell:

    mCH4excess,i = mCH4transf erred,i V 0voidCH4eq,i . (2)

    The mass of methane transferred from the reference cell intothe sample cell for N successive injection steps is calculatedfrom:

    mCH4transf erred,i = Vrc

    Ni=1

    (

    CH4rc,i CH4eq,i

    ), (3)

    where Vrc is the volume of the reference cell; CH4rc and CH4eqare, respectively, the density of methane in the reference cellduring the loading step and the equilibration step. In the multi-T experiment the temperature during the loading of the refer-ence cell (T0) was always equal to 318.1 K. Normalized todry sample weight (mdry), the excess sorbed mass (mCH4excess),...

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