(211a) Multiscale Optimization and Intensification of Natural Gas Separation and Storage

Natural gas is currently the fastest growing energy commodity and the cleanest fossil fuel with abundant reserves. Methane is the predominant component of natural gas. Significant efforts have been undertaken in the past for the separation, storage and transportation of methane. However, the following challenges remain: (i) materials with high selectivity for methane separation from natural gas often do not have high-deliverable capacity for storage, and (ii) materials with high-deliverable capacity for methane storage often do not have high selectivity for separation. In this work, we develop a combined separation and storage (CSS) technology to separate and selectively store methane at the same pressure in the same vessel, thereby essentially eliminating the need for a separate purification process. The proposed method is based on microporous materials (e.g., zeolites). While the state-of-the-art research considers materials for methane separation or storage separately [1], we propose an intensified technology enabled by advanced materials which simultaneously separate and store methane from natural gas.

Using a multi-scale systems engineering approach, we have designed a CSS process that takes a mixture of CH₄/CO₂/N₂ as feed, and stores CH₄ inside an adsorbent-packed column while venting N₂ from the system. A non-linear algebraic partial differential equation (NAPDE) model describing the conservation equations and other equations pertaining to adsorption isotherm, mass transfer driving force and pressure drop across the bed is used to study the process. In order to obtain optimal operating conditions, the detailed process model is completely discretized in both space and time into a system of nonlinear equations, whose size increases with the level of discretization. The discrete decisions related to stepwise cyclic operation together with the nonlinear equations result in a large-scale mixed integer nonlinear programming (MINLP) problem. To aid the convergence of the model, simulation-based warm starts are provided to the optimization framework. Optimization is performed to select microporous adsorbents from the database of existing materials[1] and a rank ordered list of candidate materials for different process conditions is generated. Preliminary studies reveal that for achieving simultaneous separation and storage, both purity and storage capacity of CH₄ have to be balanced by identifying the best candidate material and the corresponding operating conditions.

Our multiscale framework has the capability to simultaneously optimize the material and the process. Additionally, we will demonstrate that the framework can be used to obtain optimal properties of hypothetical materials, which can serve as a guide for future experimental efforts to design novel materials for combined gas separation and storage.

References:

[1] First, E. L.; Hasan, M. M. F.; Floudas, C. A. Discovery of Novel Zeolites for Natural Gas Purification through Combined Material Selection and Process Optimization Approach. AIChE Journal, 60(5), 1767â??1785, 2014.

[2] Siderius, D.W., Shen, V.K., Johnson III, R.D. and van Zee, R.D., Eds. â??NIST/ARPA-E Database of Novel and Emerging Adsorbent Materialsâ?, NIST Standard Reference Database Number 205, National Institute of Standards and Technology, Gaithersburg MD, 20899, http://adsorbents.nist.gov.