(115a) Modeling and Optimization of Simultaneous Natural Gas Purification and Storage

Natural gas is the fastest growing energy source with applications in power generation, vehicular transportation and chemicals production, among others. It is projected to account for more than a quarter of the total primary energy consumption by 2035.¹ To meet the increased demand, we need to tap into both conventional and unconventional sources of methane, which is the key component of natural gas. Since many of the unconventional sources are stranded, transportation becomes a key issue.² Moreover, many methane sources including shale gas, coal bed methane and landfill gas have impurities (e.g. CO₂, N₂, H₂S) as high as 70%.To transport methane through pipelines, the impurity content has to be less than 3-4%.^3,4 Traditionally, methane is first separated from the impurities, then compressed and transported. Each of the steps in the methane supply chain is energy and capital intensive. For instance, the operating cost of absorption-based CO₂ removal from methane is about $ 0.18-0.3/MMscf.⁵ The cost of cryogenic distillation to remove N₂ from methane is even higher ($ 0.3-1.0/MMscf).⁶ Pressure swing adsorption (PSA)⁷ using microporous materials (e.g., zeolites) is a cost-effective alternative with operating  cost ranging from $0.04-0.66/MMBtu.⁸ The operating cost of natural gas transportation through pipelines ranges from $0.3-0.5 /MMBtu per 1000 miles depending on the transportation pressure and flow rate.⁹

Due to the standalone development of separation and transportation technologies, excess energy is consumed. For instance, PSA processes often involve high pressure adsorption for purification, vacuum depressurization for column regeneration and then re-pressurization of the product gas for pipeline transport. Energy is consumed at each pressure change. There is a potential for energy savings if the purification and storage steps are combined and carried out in the same vessel. Thus, our objective is to streamline the current separate purification, storage and distribution entities in the supply chain into fewer steps resulting in reduced energy costs. We propose a combined separation and storage (CSS) technology to purify and store methane in the same vessel which can eliminate the need for separate purification and storage. Microporous materials such as zeolites and metal organic frameworks (MOFs) can be used as adsorbents due to their good preferential adsorption and storage capabilities. 

In the CSS process, feed gas is sent to the column packed with adsorbent and the impurities are vented out from the other end while the product gas (CH₄) gets stored in the adsorbent column till saturation is reached at that pressure. We model the adsorption of CH₄ in the column using a non-linear algebraic partial differential equation model^8,11,12 which is discretized over space and time to a system of algebraic equations. The completely discretized model is then optimized to identify optimal operating conditions and unit specifications (e.g. adsorption pressure, adsorption time, length of column) for obtaining maximum storage capacity and purity of CH₄and minimum energy cost for a particular material.  To further improve  the CSS technology, we identify candidate materials which have high storage capacity while meeting the requisite purity specifications at reduced cost.

References:

1)  Energy Outlook 2035, British Petroleum, February 2015.

2)  Rufford, T., Smart, S., Watson, G.C.Y., Graham, B.F., Boxall, J., Diniz da Costa, J.C. and May, E.F. “The removal of CO₂ and N₂ from natural gas: A review of conventional and emerging process technologies”, Journal of Petroleum Science and Engineering 94-95 (2012): 123-154.

3)  Tagliabue M., Farrusseng D., Valencia S., Aguado S., Ravon U., Rizzo C., Corma A. and Mirodatos. C. “Natural Gas Treating by Selective Adsorption: Material Science and Chemical Engineering Interplay” Chemical Engineering Journal155(3) (2009): 553–566.

4)  Baker, R.W. and Lokhandwala, K.A. “Natural gas processing with membranes: an overview”. Industrial and Engineering Chemistry Research47(7) (2008):2109-2121.

5)  Peters, L., Hussain, A., Follmann, M., Melin, T. and Hägg, M.B. “CO₂ removal from natural gas by employing amine absorption and membrane technology - A technical and economical analysis” Chemical Engineering Science 172(2-3) (2011): 952-960.

6)  “Nitrogen Removal from Natural Gas”. Membrane Technology and Research, Inc., Menlo Park, CA, United States (1997). http://www.osti.gov/scitech/servlets/purl/493341.

7)  Ruthven D.M., Principles of Adsorption and Adsorption Processes, John Wiley & Sons, New York, NY, USA (1984).

8)  First, E.L., Hasan, M.M.F. and Floudas, C.A., “Discovery of Novel Zeolites for Natural Gas Purification through Combined Material Screening and Process optimization.” Journal of American Institute of Chemical Engineers. 60 (2014): 1767–1785.

9)  Comot-Gandolphe, S., Appert O., Dickel, R., Chabrelie, M-F. and Rojey, A., “The challenges of further cost reductions for new supply options (Pipeline,LNG,GTL)”, 22^nd World Gas Conference, Tokyo, Japan, (2003).

10) Ko, D., Siriwardene, R. and Biegler L., “Optimization of pressure swing adsorption and fractionated vacuum pressure swing adsorption processes for CO₂ capture”, Industrial and Engineering Chemistry Research44(2005): 8084-8094.

11) Hasan, M.M.F., Baliban, R.C., Elia, J.A. and Floudas, C.A. “Modeling, Simulation and Optimization of Post-combustion CO₂ Capture for Variable Feed CO₂ Concentration and Feed Flow. Part 2: Chemical Absorption and Membrane Processes” Industrial & Engineering Chemistry Research51(48) (2012): 15665–15682.

12) Hasan, M.M.F., First, E.L. and Floudas, C.A. “Cost-Effective CO₂ capture based on in silico screening of zeolites and process optimization”, Physical Chemistry Chemical Physics 15(2013): 17601-17618.