Method for Simultaneous Measure of Sorption and Swelling of the

Extended poromechanics for adsorption-induced swelling prediction in double porosity media: Modeling and experimental validation on activated carbon...
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Method for Simultaneous Measure of Sorption and Swelling of the Block Coal under High Gas Pressure Guoqing Chen,†,‡ Jianli Yang,*,† and Zhenyu Liu§ †

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Post Office Box 165, Taiyuan, Shanxi 030001, People’s Republic of China ‡ Graduate University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China § State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China ABSTRACT: Carbon dioxide sequestration in unmineable coalbeds combined with enhanced coalbed methane recovery is attracting attention as one of the possible methods for carbon capture and storage. Sorption and swelling characterizations are important issues for forecasting the performance of aimed coalbeds. A method for simultaneously measuring the adsorption amount of gas and the corresponding swelling ratio of coal under high gas pressure (up to 18 MPa) is proposed. A high-pressure linear variable differential transformer displacement transducer is equipped with a manometric sorption apparatus. This intentional assembly provides an important opportunity to acquire the sorption and corresponding swelling data simultaneously, which is necessary to understand clearly the relationship between sorption and swelling of the block coal sample. Results unambiguously confirm that this method can be used under supercritical CO2 and moisture conditions, with relatively high reproducibility and accuracy.

1. INTRODUCTION Carbon dioxide (CO2) sequestration in unmineable coalbeds combined with enhanced coalbed methane recovery (CO2ECBM) is considered as a possible method for carbon capture and storage (CCS).1 The injection of CO2 into coalbeds can enhance coalbed methane (CBM) recovery mainly by its competitive adsorption with methane.2 The enhanced CBM recovery can offset the high cost of CCS. Several CO2-ECBM pilot projects worldwide, including U.S.A., Canada, Poland, Japan, and China, have demonstrated the potential of the process. However, most of these projects experienced permeability reduction of the coalbed with time, leading to operational difficulties because of the loss of injectability. The permeability reduction is generally considered to be caused by the CO2 sorption induced coal swelling, which can narrow or close the coalbed fractures.3−10 Although it is well-accepted that the swelling of the coal matrix is induced by gas sorption or imbibition,11−14 including the adsorption of the gas on the coal surface and the absorption of the gas by the coal macromolecular system, the mechanism is still not quite clear. Changes in surface energy, solvation pressure, and macromolecular structure are proposed to be responsible for the geometric deformation. Nevertheless, quantitatively correlating the amount of gas adsorbed or imbibed in coal and the corresponding deformation induced is practically useful. The directly proportional relationship between the swelling ratio and the gas adsorption amount has been observed under low and moderate pressure and is commonly used in historical matching of the ECBM process.15−20 However, it works well under low- or moderatepressure conditions, and fails under high-pressure conditions.5,8,10 © 2012 American Chemical Society

Accurately measuring the adsorption amount of gas and the corresponding swelling ratio induced is crucial. Manometric (volumetric) and gravimetric methods are commonly used to measure the amount of gas adsorbed on solid materials under different pressure conditions. An accurate sample volume or a buoyancy correction is necessary to calculate the absolute adsorption amount.21−24 Unfortunately, the apparatus used in both sorption measurements usually requires powdered or crushed samples and is not equipped with sensors for measuring the sample deformation.25−27 The measure of coal swelling is typically related to a measure for block samples. To diminish the sample size effect,28 the simultaneous measure of sorption and swelling is necessary. Strain gauge 6,29 and optical methods5,7,9,10 are two commonly used methods for measuring coal deformation under high pressure. When the strain gauge method is used, a strain gauge is affixed onto the surface of the coal block by glue. Deformation of the coal sample induces a strain on the gauge, resulting in a change of its electrical resistance. Coal swelling is monitored by recording the changes of the electrical signal of the strain. Although the strain gauge method has a higher resolution, the presence of moisture in the coal sample as well as the possible interaction between supercritical CO2 and glue can make the measurement impossible. Moisture in the sample may disturb the electrical signal. The interaction of supercritical CO2 and glue may cause the strain gauge slipping from the coal surface. Furthermore, the presence of the strain gauge and glue may adsorb test gas and affect the accuracy of the sorption measurement. When the optical method is used, a camera or Received: January 19, 2012 Revised: April 8, 2012 Published: April 12, 2012 4583

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2.3.1. Determine the Volumes of Reference and Sample Cells. The volume of the reference cell (Vr) is defined as the volume between valves V1 and V2, as shown in Figure 1. Similarly, the volume of the sample cell (Vs) is designated as the volume between valves V2 and V3. These quantities are determined by a so-called helium (He) expansion method. The details are listed as follows: (a) vacuum the whole system; (b) fill the reference cell with He to a specified pressure and record the pressure as Pr1. (c) open the valve V2, release He into the sample cell, and record the pressure as Ps1. (d) the ratio of Vr/(Vr + Vs) can be determined according to the mass balance of

microscope with an image analyzer is equipped to observe the coal deformation directly through a transparent glass window. Therefore, its sensitivity and applicability are limited depending upon the experimental conditions. To improve data quality, a method for the simultaneous measure of sorption and swelling is developed. The apparatus is equipped with a high-sensitive linear variable differential transformer (LVDT) displacement transducer acting as the swelling sensor and two high-precision pressure transducers serving as the sorption sensors. This apparatus can be operated under extreme conditions, including the subcritical and supercritical CO2 conditions, as well as the moisture condition.

Vr /(Vr + Vs) = ρs1 /ρr1

(1)

where ρr1 and ρs1 are the He densities corresponding to pressures of Pr1 and Ps1 at the test temperature; (e) place a known volume stainlesssteel bar (Vssb) in the sample cell; (f) repeat the He expansion process described above and record the corresponding pressures of Pr2 and Ps2; (g) the ratio of Vr/(Vr + Vs − Vssb) is determined as

2. METHODOLOGY 2.1. Apparatus. An apparatus for the sorption study is modified to measure the sorption and swelling simultaneously. Figure 1 shows the

Vr /(Vr + Vs − Vssb) = ρs2 /ρr2

(2)

where ρr2 and ρs2 are the He densities corresponding to pressures of Pr2 and Ps2 at the test temperature; and (h) obtain the volumes of reference and sample cells by solving eqs 1 and 2 simultaneously. 2.3.2. Set the sample. Place a block coal sample in the sample holder, adjust the position of LVDT based on its scaling range, and seal them in the sample cell. 2.3.3. Determine the Void Volume. The void volume (Vvoid) is the portion of sample cell, which is free for the gas to occupy in the presence of the test sample. It can be derived according to the following steps: (a) place the sample in the sample cell; (b) repeat the He expansion process described above and recorded the pressure of Pr3 and Ps3; (c) the ratio of can be determined as: Vr /(Vvoid + Vr) = ρs3 /ρr3

(3)

where ρr3 and ρs3 are He density corresponding to pressure of Pr3 and Ps3 at the test temperature; and (d) calculate the void volume according to the following equation:

Vvoid = Vrρr3 /ρs3 − Vr

Figure 1. Schematic diagram of the apparatus for simultaneous measurement of the sorption and swelling of coal. (1-Sample cell; 2Reference cell; 3-LVDT; 4-Pressure transducer (P1); 5-Pressure transducer (P2); 6-Valve (V1); 7-Valve (V2); 8-Valve (V3); 9-Coal sample; 10-Sample holder; 11-Lower part of shell; 12-lock blade; 13screw cap; 14-Upper part of shell; 15- Fixture; 16-Water bath).

(4)

2.3.4. Obtain the Swelling and Sorption Kinetic Profiles. To obtain the swelling and sorption kinetic profiles and isotherms for test gas, the detailed steps are as follows: (a) vacuum the whole system; (b) fill the reference cell with the test gas (such as CO2 and CH4) to a specific pressure; (c) open the valve V2 for a few seconds and release the test gas into the sample cell; (d) record the axial deformation of block coal (ΔL) and the pressure change in the sample cell every 5 s until equilibrium is achieved. It can take 1−50 h to reach the equilibrium, depending upon the coal type, sample size, pressure, and water content. The swelling and sorption kinetic profiles are obtained. The swelling kinetics are recorded as the axial deformation of block coal with time, and the sorption kinetics are recorded as the pressure change in the sample cell with time. The deformation and pressure change are transferred to the swelling ratio (ε) and the excess adsorption amount (nexe), according to the following equations:

schematic diagram of the apparatus. Similar to the conventional manometric gas sorption apparatus, it consists of two stainless-steel cells (as reference and sample cells), two high-precision pressure transducers, and a set of valves. A high-sensitive LVDT displacement transducer is assembled on top of the sample cell to monitor the deformation of the block sample. The maximum measuring scale of the pressure transducer is 30 MPa, with a precision of 0.1% in terms of the full-scale value. A sample holder is placed in a sample cell to help fix the LVDT in position. A block coal is placed in the sample hold, which is adjusted to the desired position and locked during the measurement. There are small holes on the side of the sample holder to allow the test gas migrating easily. The sample and its holder are sealed in the sample cell during the experiment. The LVDT was purchased commercially. The maximum measuring scale of the LVDT used is 2 mm, with a precision of 0.1% in terms of the full-scale value. 2.2. Sample Preparation. A block coal sample of 14−17 mm in diameter and 50−70 mm in length is required by the apparatus. 2.3. Measurement Procedures. The procedures used for measuring the sorption and swelling simultaneously are similar to the procedures used for measuring the sorption alone by the manometric method.27 Instead of powder or crashed sample, a block sample is used. The swelling and sorption data are collected simultaneously.

ε = ΔL /L0 n

exe

(5)

= ntotal − ρr Vr − ρs Vvoid = Δρr Vr + Δρs Vvoid

(6)

where ΔL is the axial deformation of block coal, L0 is the original length of block coal, ntotal is the total amount of test gas in the system, ρr and ρs are the gas densities in reference and sample cells, respectively, Δρr and Δρs are the difference of gas densities in the reference and sample cells before and after gas is charged into the sample cell; and (e) repeat the steps c and d until a desired pressure is reached in the sample cell. Then a series of kinetic profiles with respect to specified pressures are obtained. 2.3.5. Obtain the Swelling and Sorption Isotherms. To obtain the swelling isotherm, plot the equilibrium swelling ratio, ε, versus the corresponding pressure, P. To obtain the sorption isotherm, plot the equilibrium excess adsorption amount, nexe, versus the corresponding 4584

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pressure, P. To correlate the swelling with the sorption, transfer the excess adsorption amount, nexe, to the absolute adsorption amount, nabs, and plot ε versus nabs.

3. EXAMPLES Care must be taken on the sample selection. It was found that the heterogeneity of the coal structure can cause a significant

Figure 3. Kinetic profiles of excess sorption and swelling of dry Coal-A after a certain amount of CO2 (a) or CH4 (b) charged into the sample cell.

Figure 2. Time-related axial deformation of the block sample and the corresponding pressure change in the sample cell after a certain amount of CO2 (a) or CH4 (b) charged into the sample cell.

difference even if the samples are from the same piece of coal lump (see section 4.1). As examples, therefore, the block coal samples used, if no specification, are handmade from the same anthracite lump and pretreated with supercritical CO2 (45 °C, 10 MPa) for more than 24 h to eliminate the effect of the irreversible change induced by high-pressure CO2.30 To release adsorbed gas and moisture, the block coal samples were dried at 110 °C under a vacuum condition for more than 24 h prior to use. 3.1. Analysis of Dry Coal Sample. A block coal sample with 16 mm in diameter and 68.7 mm in length, namely, CoalA, was pretreated and used as an example. Figure 2 represents the time-related axial deformation of the block sample and the corresponding pressure change in the sample cell after a certain amount of CO2 or CH4 charged into the sample cell (refer to 2.3.4). According to eqs 5 and 6, the correlated kinetics of the swelling ratio and excess adsorbtion amount are presented in Figure 3. The corresponding density values are obtained from the U.S. National Institute of Standards and Technology (NIST) Chemistry WebBook.31 The swelling and excess sorption isotherms were shown in Figure 4 (refer to 2.3.5). The swelling isotherm of dry Coal-A under the pressure of CH4 is increased monotonously with the increase of the pressure under the experimental pressure range

Figure 4. Swelling and sorption isotherms of dry Coal-A under CO2 (a) and CH4 (b) pressure at 45 °C (the lines are the Langmuir fitting results).

while the swelling isotherm of dry Coal-A under the pressure of CO2 reaches a maximum value at a pressure of around 12 MPa 4585

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Figure 7. Normalized swelling kinetics (with the equilibrium pressure about 1 MPa) profiles measured by the strain gauge and LVDT methods.

Figure 5. Relationship between the swelling ratio and absolute adsorption amount of dry Coal-A under CO2(a) and CH4(b) pressure at 45 °C.

Figure 6. Comparison of the swelling isotherms of dry Coal-A measured by the strain gauge with that by the LVDT (under the pressure of CO2 at 45 °C; ∗a new strain gauge was used for the second test).

Figure 8. Swelling and sorption isotherms of dry and moist (4.84 wt %) Coal-B under CO2(a) and CH4(b) pressure at 45 °C (the swelling ratio was measured by LVDT).

are other theories or mathematical equations that can be used in the modeling.13,14 Figure 5 correlates the gas adsorption amount with the swelling ratio for dry Coal-A according to the procedure 2.3.5, along with the pressure profile. The experiments were carried out under CO2 and CH4 pressures at 45 °C. The excess adsorption amount is transferred to the absolute adsorption amount. The calculation details are described elsewhere.23 It can be seen that the swelling ratio is approximately proportional to the absolute adsorption amount in a region where the absolute adsorption amount is less than 2 mmol/g for CO2 and 1.5 mmol/g for CH4 (the gas pressure is lower than 4 MPa for CO2 and 5 MPa for CH4). Beyond this region, the slope of the

and then decreased slightly with the increase of the pressure. The swelling induced by CO2 is higher than that induced by CH4. The swelling phenomena presented are consistent with that reported in refs 5 and 7. The excess sorption isotherm of CH4 monotonously increased with the increase of the pressure, while the excess sorption isotherm of CO2 reaches a maximum value at the pressure of around 7 MPa and then decreased sharply with the further increase of the pressure. The sorption phenomena presented are consistent with that reported in refs 24−26. The lines tracing the data points in Figure 4 are modeling lines according to the Langmuir equation.32 There 4586

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Figure 9. Swelling kinetics of moist (4.84 wt %) Coal-B under CO2 pressure measured by strain gauge (a),under CH4 pressure measured by strain gauge (b), under CO2 pressure measured by LVDT(c) , and under CH4 pressure measured by LVDT (d).

length, namely, Coal-B, is used as an example. It was pretreated under supercritical CO2 and dried or moisturized33 to the designated water content (4.84 wt %) prior to use. The swelling and sorption isotherms of dry and moist Coal-B under the pressure of CO2 and CH4 are compared in Figure 8. The isotherms are obtained at 45 °C. It shows that the swelling ratio and the excess adsorption amount for moist sample are lower than that for the dry sample. Figure 9 compares a set of swelling kinetics for the moist sample by strain gauge and LVDT measurements. A significant influence of moisture is observed for the strain gauge measurement.

correlation profile decreases significantly and the swelling ratios approach to a plateau or equilibrium. In addition, the swelling isotherms and kinetics measured by LVDT are compared to that measured by the strain gauge. The experimental details of the strain gauge method can be found elsewhere.29 Figure 6 compares the swelling isotherms from the data measured by strain gauge and LVDT for dry Coal-A under the pressure of CO2. The difference between the isotherms from the two methods is apparent. The use of glue is a fatal drawback. It is common understanding that glue and coal have different expansion factors upon exposure to high-pressure CO2. At the sub- or supercritical state, CO2 acts as a solvent and interacts with glue in some extent. These probably are reasons which cause the difference. The sharp decrease of the swelling ratio near supercritical pressure caused by the softening of glue. Normalized swelling kinetics (with the equilibrium pressure about 1 MPa) measured by both methods for the same block coal sample (dry Coal-A) as well as the different block coal samples (dry Coal-A, dry Coal-B, dry Coal-C, dry Coal-D, and dry Coal-E) from the same coal lump are compared in Figure 7. The normalized swelling data (εt/εe) are used for easy comparison, where εt and εe are the swelling ratio at time t and the equilibrium, respectively. The swelling ratio approaches to equilibrium in 5 h when the LVDT is used and in 12 h when the strain gauge is used. The delayed responds by the strain gauge is apparent. 3.2. Analysis of Moist Coal Sample. In practice, coalbed is usually exposed to water. Hence, the sorption and swelling measurements under the moisture condition are sometimes required. A coal block of 16.3 mm in diameter and 69.8 mm in

4. REPRODUCIBILITY AND ACCURACY 4.1. Reproducibility. Figure 10a is the swelling and sorption isotherms for dry Coal-A under the pressure of CO2 at 45 °C. Coal-A was pretreated under supercritical CO2 and dried at 110 °C under vacuum before each measurement to release adsorbed gas and moisture. The closed symbol represents the first measurement, and the open symbol represents the second measurement. The relatively high reproducibility can be observed. Figure 10b is the swelling and sorption isotherms for different block coal samples (dry Coal-A, dry Coal-B, and dry Coal-C), from a same coal lump under the pressure of CO2 at 45 °C. The samples are also pretreated under supercritical CO2 and dried under vacuum at 110 °C to release adsorbed gas and moisture. The results imply that the effect of heterogeneity of the sample is notable. 4.2. Accuracy. LVDT is a commercially mature product. It is based on the principle of electromagnetic induction and sensitive to the geometry deformation. The output voltage is 4587

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Figure 10. Comparison of swelling and excess sorption isotherms for dry Coal-A (a) and for different block coal samples from a same coal lump (b). (under the pressure of CO2).

Figure 11. (a) Comparison of the excess sorption isotherms of dry Coal-A under the pressure of CO2 by this study with that from ref 34; (b) comparison of the excess sorption isotherms of FILTRASORB400 activated carbon under the pressure of CO2 by this study with that reported in ref 35(b).

correlated with the deformation of the object. The accuracy of LVDT is ensured by a frequent standard calibration. The CO2 sorption isotherms of a coal were compared for the difference between block coal and crushed coal in Figure 11a. The results from the block coal sample, Coal-A, in this study are compared to the results from the crushed sample (4−10 mm) by Han et al.34 The block and crushed samples are from the same coal lump. Furthermore, the powder of an activated carbon (FILTRASORB 400) is used as a standard sorbent to evaluate the reliability of the sorption measurement in this study. The CO2 sorption isotherms of FILTRASORB 400 activated carbon are measured and are compared to those measured by Gensterblum et al.35 (Figure 11b). No significant differences in the results are observed.

swelling ratio is approximately proportional to the adsorption amount in the region when the adsorption amount is lower than 2 mmol/g for CO2 and 1.5 mmol/g for CH4 of coal, which is corresponding to a pressure lower than 4 MPa for CO2 and 5 MPa for CH4. The adsorption amount beyond this region, the relationship is no longer linear. In agreement with the finding reported in the literature, both the swelling ratio and the excess adsorption amount of moist coal measured in this study are lower than those for dry coal.

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5. CONCLUSION A methodological study on the simultaneous measurement of sorption and deformation of the block coal sample under high gas pressure is achieved by equipping a high-pressure LVDT displacement transducer in a manometric sorption measurement apparatus. The main advantages of this method are that (1) it enables a direct correlation of the two parameters (adsorption amount and swelling ratio) and (2) it allows us to make a swelling measurement under moisture or sub- and supercritical CO2 condition. Dry and moist block coal samples from an anthracite lump were tested as examples. Reproducible data with high accuracy under a simulated coalbed condition unambiguously reveal the possible application of the proposed method as a promising candidate in sorption and swelling characterization of coal under high gas pressure. The phenomena presented in this work are consistent with that reported in the literature. The

AUTHOR INFORMATION

Corresponding Author

*Telephone/Fax: +86-351-4048571. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

We gratefully acknowledge the financial support from the Natural Science Foundation of China (21176244) and the National Basic Research Program of China (2010CB227003), as well as the financial support from Shell International Exploration and Production B.V. and the MOVECBM Project under the 6th Framework Program of the European Commission. The authors are greatly indebted to Professor Bijiang Zhang for his valuable suggestions. 4588

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(24) Siemons, N.; Busch, A. Measurement and interpretation of supercritical CO2 sorption on various coals. Int. J. Coal Geol. 2007, 69, 229−242. (25) Busch, A.; Gensterblum, Y.; Krooss, B. M. High-pressure sorption of nitrogen, carbon dioxide, and their mixtures on Argonne Premium coals. Energy Fuels 2007, 21, 1640−1645. (26) Ottiger, S.; Pini, R.; Storti, G.; et al. Adsorption of pure carbon dioxide and methane on dry coal from the Sulcis Coal Province (SW Sardinia, Italy). Environ. Prog. 2006, 25, 355−364. (27) Ozdemir, E.; Morsi, B. I.; Schroeder, K. CO2 adsorption capacity of argonne premium coals. Fuel 2004, 83, 1085−1094. (28) Gruszkiewicz, M. S.; Naney, M. T.; Blencoe, J. G.; et al. Adsorption kinetics of CO2, CH4, and their equimolar mixture on coal from the Black Warrior Basin, west-central Alabama. Int. J. Coal Geol. 2009, 77, 23−33. (29) Harpalani, S.; Schraufnagel, R. A. Shrinkage of coal matrix with release of gas and its impact on permeability of coal. Fuel 1990, 69, 551−556. (30) He, J.; Shi, Y.; Ahn, S.; et al. Adsorption and desorption of CO2 on Korean coal under subcritical to supercritical conditions. J. Phys. Chem. B 2010, 114, 4854−4861. (31) National Institute of Standards and Technology (NIST). NIST Chemistry WebBook; NIST: Gaithersburg, MD, 2011; http://webbook. nist.gov/chemistry/fluid/ (accessed Jan 19, 2012) (32) Langmuir, I. The adsorption of gases on plane surfaces of glass, mica and platinum. J. Am. Chem. Soc. 1918, 40 (9), 1361−1403. (33) Krooss, B. M.; van Bergen, F.; Gensterblum, Y.; et al. Highpressure methane and carbon dioxide adsorption on dry and moistureequilibrated Pennsylvanian coals. Int. J. Coal Geol. 2002, 51 (2), 69− 92. (34) Han, F.; Busch, A.; Krooss, B. M.; et al. CH4 and CO2 sorption isotherms and kinetics for different size fractions of two coals. Fuel 2012, DOI: 10.1016/j.fuel.2011.12.014. (35) Gensterblum, Y.; van Hemert, P.; Billemont, P.; et al. European inter-laboratory comparison of high pressure CO2 sorption isotherms. I: Activated carbon. Carbon 2009, 47, 2958−2969.

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