Energy & Fuels 2001, 15, 1241-1246
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Direct Measurements of Volumetric Gas Storage Capacity and Some New Insight into Adsorbed Natural Gas Storage J. Sun,* T. D. Jarvi, L. F. Conopask, and S. Satyapal United Technologies Research Center (UTRC), 411 Silver Lane, East Hartford, Connecticut 06108
M. J. Rood Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign (UIUC), 205 North Mathews Avenue, Urbana, Illinois 61801
M. Rostam-Abadi Illinois State Geological Survey (ISGS), 615 East Peabody Drive, Champaign, Illinois 61820 Received March 22, 2001. Revised Manuscript Received July 11, 2001
We present a unique bench scale apparatus for directly measuring volumetric gas storage capacities designed at UTRC. The apparatus construction avoids gas leakage, and the analysis of experimental data prevents leakage from inflating capacity measurements. Deliverable methane storage capacities (Vd/Vs) of adsorbents are evaluated directly from experiments with this apparatus rather than calculated from gravimetric adsorption capacities. We suggest that an adsorbent for methane storage should have an optimal pore volume consisting of pores ranging from 8 to 15 Å, rather than a monodispersed 8 Å pore size distribution as calculated by recent computer simulations. This interpretation is based on our modeling results and on the fact that physical activation usually produces an adsorbent with a polydisperse distribution of pore size. In general, ultra-micropores (500 Å) is considered to have identical density as the bulk gas. However, it is difficult to estimate the methane density within mesopores (20-500 Å). Because methane is a supercritical gas, the adsorbed phase is not a liquid and may not be completely adsorbed on the adsorbent’s surface even in its micropores. Capillary condensation in the traditional sense does not take place within the pores. Therefore, uncertainties may exist in Vm/Vs estimations based on gravimetric measurements. In addition, one requires an empirical constant for methane compressibility to calculate the density of bulk gas at the storage pressure. In this communication, we propose a simple experimental approach to directly measure natural gas (6) Wojtowicz, M. A.; Smith, W. W.; Serio, M. A.; Simons, G. A.; Fuller, W. D. In Proceedings of the 23rd Biennial Conference on Carbon, State College, PA, July 13-18, 1997. (7) Parkyns, N. D.; Quinn, D. F. In Porosity in Carbons; John Wiley & Sons: New York, 1995; pp 291.
storage capacity without using empirical constants. The Vd/Vs values for two commercially available carbon adsorbents, BPL and AX-21, are measured. Modeling of pore structure and methane adsorption of these two samples is discussed in view of our storage capacity measurements to gain new insight into natural gas storage. Description of the Apparatus A schematic of the experimental apparatus for direct measurements of volumetric gas storage capacities that was designed and constructed at UTRC is presented in Figure 1. Major components include (1) a high-pressure gas supply, (2) a pressure-relief valve, (3) a 1 L gas reservoir, (4) a sample cell, (5) a pressure transducer, (6) a pressure regulator, and (7) six needle valves connected by 1/4 in. o.d. stainless steel tubing. Metal gasket face-seal fittings (VCR) are used with Ni- or Ag-plated Ni gaskets (Cajon, Inc.). Valves (A, B, C, D, and E) are severe service needle valves (SS-3NKRVCR4, Whitey). A pressure transducer (PX203-3KG5V) is used from Omega, Inc. with a digital readout (DP-25E, pressure range: 0-9999 psig). A safety valve (SS-4R3A5-E, Nupro, Inc.) is installed for emergency pressure relief. In addition, the entire test stand is mounted inside a hood. Methane gas with 99.97% purity is used for this study (Matheson Gas Products). As shown in Figure 1, gas within the system is slowly released (discharged) through the regulator R, and water is forced from the water reservoir into the beaker through a flexible tube T. The water collected in the beaker is weighed on the balance, and the equivalent volume of discharged gas is then obtained. Before discharging the stored gas from the system, low-pressure nitrogen or helium flows through valve F to remove any residual gas trapped inside tube T. There will be a change in pressure within the water reservoir due to the water level descent after gas discharge, which may be corrected using the measured water heights and the compressibility of methane as a function of pressure.1 The most straightforward way to correct for the changing pressure is to keep the water level in the reservoir at the same height before and after gas discharge using the lab-jack (Figure 1). Water displacement simplifies the measurement of discharged gas volume by measuring the mass of displaced water. The sample cell is simply a piece of 1/4 in. tubing 2.5 in. long (to minimize the system volume) capped at its end. A filter or snubber is recommended between valve C and the sample cell to prevent the sample from dispersing into the system. The system’s internal volume VT (with valves A, B, D, and E closed
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and valve C open) can be measured by filling the adsorbentfree system with helium to pressures one and then two times atmospheric pressure. The discharged gas volume, obtained by the mass of water collected, is equal to the system volume (measured as 25.7 mL) and twice the system volume, respectively. The effect of nonideal helium compressibility is negligible at pressures near atmospheric pressure. The bulk volume occupied by the adsorbent in the sample cell is denoted as v, representing the internal volume of the “storage container.” This volume is deducted from the system volume when calculating the volume of deliverable stored gas at standard temperature and pressure. The sample volume v is calculated using the sample bulk density and the sample mass m rather than using direct measurements because of the small adsorbent sample size. The typical sample size used in this study is approximately 0.5 g. Like the method presented by Wegrzyn,1 this equipment is designed to accept small samples of adsorbents and give an accurate measure of their respective methane storage capacities. Bulk density of a sample is calculated by its mass (of a batch) divided by the volume (of the same batch) measured with a graduated cylinder by gently tapping the cylinder until the volume remains constant.
Experimental Procedures and Equation for Vd/Vs Typically, a loaded sample is heated to about 100-200 °C using a heating tape wrapped around sample cell C under vacuum (
