(740e) Purification Process Technology of Coal Bed Methane (CBM) | AIChE

(740e) Purification Process Technology of Coal Bed Methane (CBM)

Authors 

Row, S. W., GS e&C
Ahn, E. S., GS E&C
Won, W., Sogang Univ.



Natural gas adsorbed on coal beds underground is called Coal Bed Methane (CBM). The CBM compositions are mainly methane and other impurities such as carbon dioxide, ethane, and so on. The compositions may vary according to the CBM site.

To use the CBM as an energy source, gas treating processes to remove sour gas may be required depending on the purposes of the natural gas utilization. This study focuses on the development of the energy saving and flexible separation / purification processes of CBM.

At first, we compared three main methods for separation processes, i.e., absorptions, adsorptions, and membranes through simulations and experiments.

Our simulation of absorption process using Pro-II software shows that the sour gas mole fractions of CO2 and H2S in the purified gas stream are less than 10 and 5 ppm respectively when the feed gas (CH4 rich gas) stream consists of CO2 (~10%) and H2S (~5%). The absorption processes using amine as a solvent have an excellent purification performance and is generally adopted in the large scale gas treatment plants requiring a highly purified gas product.

The simulation (gPROMS software) and experiments of a pressure swing adsorption (PSA) process were performed. When the adsorbent is carbon molecular sieve (CMS) and the feed gas consists of 10% CO2 and 90% CH4, the purified product purity is more than 98% and the recovery is about 80%. The purity can be enhanced up to more than 99% CH4. The adsorption process using CMS has a very good purification performance and is generally used in the small to medium large scale gas treatment processes.

We checked membrane performances through the simulation (gPROMS software) and experiments. When the feed gas consists of 10% CO2 and 90% CH4, the purified product gas (CH4) has more than 97% purity. The membrane process has a relatively low purification performance but a simple and compact configuration compared with the other processes such as the absorption and adsorption processes. The membrane process is mostly used in a small scale process but recently a somewhat large scale membrane process is under the development and commercialized.

This study focuses on the two processes – adsorption and membrane processes – as candidates for the conceptual design work, considering the utilization purposes and amount of the purified CBM gas as well as transportation problems of the pilot plant facilities to the corresponding site in our R&D program. Here, the scale of CBM feed gas is expected as a 10~2500 Nm3/hr.

With the two processes of adsorption and membrane processes, we performed a conceptual design study for the gas treatment by comparing eight types of candidate process configurations. The candidate processes involve the single processes (PSA only or membrane only) and hybrid processes consisting of PSA and membrane systems. The power consumptions per the corresponding product (purified gas) flow rate are compared, and a suitable process configuration is adopted in view of power consumption, purity, and recovery of the product gas. A hybrid process with a recycle stream is better than single process in view of the power consumption, the purity, and the recovery of product gas (CH4).

As a conclusion, the conceptual design is performed for the CBM purification processes adopting PSA and membrane processes, resulting in the development of the energy efficient hybrid system of PSA and membrane, which can achieve more than 97% purity and 95.5% recovery of CH4. Also we checked the pressure vacuum swing adsorption process that can acquire more than 97% purity and 90% recovery of CH4. As a result, a basic design package for the CH4 gas purification process was developed to construct a pilot plant to purify CBM feed gas.

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