S.A. Mohammad, R.L. Robinson, Jr., K.A.M. Gasem, Oklahoma State
SPE International Conference on CO2 Capture, Storage, and Utilization,
10-12 November 2010,
New Orleans, Louisiana, USA
Traditional modeling of gas adsorption on wet coals does not include water as a
separate adsorbed component; instead, the adsorbed water is viewed as a
"pacifier" of the coal matrix. In this work, we modeled gas adsorption on wet
coals by considering water as an active component in a binary mixture.
Specifically, we used the simplified local-density/Peng-Robinson (SLD-PR) model
to investigate the effect of the water present in coals on gas adsorption under
the conditions encountered in coalbed methane (CBM) and CO2 sequestration
applications. To conduct this study, our previously acquired measurements for
high-pressure CO2 adsorption on wet coals were utilized.
When water is treated as one of the adsorbed components in a high-pressure gas
adsorption system, as many as three phases may coexist at equilibrium. To
investigate the phase behavior of CO2/water adsorbed gas mixtures on wet coals,
a new algorithm was developed to facilitate a Gibbs energy-driven multiphase
analysis of this system. The algorithm uses a phase-insertion technique, which
involves formally inserting a third (liquid) phase and solving a three-phase
flash problem, wherein the three phases are the adsorbed, bulk gas and liquid
phases. At equilibrium, the total Gibbs energy of the system is calculated
based on the phase distribution obtained at each step. This calculation is
repeated sequentially with incrementally increased amounts of the inserted
third phase. A Gibbs energy analysis for CO2/water mixed gas adsorption on wet
coals was conducted utilizing this new algorithm.
Results involving CO2/water mixtures on four well-characterized coals indicate
that the SLD-PR model is capable of representing the adsorption of these highly
asymmetric mixtures within the experimental uncertainties. The Gibbs analysis
of these coals indicates that there is a third phase present in systems that
contain large amounts of moisture. The third, water-rich phase appeared
typically at the higher pressures in the isotherm.