(239e) Enrichment of Low Concentration CH4/N2 Mixture By Adsorption Process with Displacement Chromatography Technology

Enrichment of Low concentration CH4/N2 mixture by adsorption process with
displacement chromatography technology

Ying Yang¹, Kai Lu^1,2,
Donglei Qu^1,2, Ping Li^1,2*, Jianguo Yu^1,2*

1.       State Key laboratory of Chemical
Engineering, National Engineering Research Center for
Integrated Utilization of Salt Lake Resources, East China University of Science
and Technology, Meilong Road 130, Shanghai, China, 200273.

2.       State Key Laboratory of Chemical Engineering, College of Chemical
Engineering, East China University of Science and Technology, Shanghai, 200237,
China

yyang@ecust.edu.cn, liping_2007@ecust.edu.cn, jgyu@ecust.edu.cn

Abstract:

The recovery
and utilization of coal mine methane (CMM) provide a number of significant
energy, economic, safety and environmental benefits. Adsorption-based processes
are recognized to be a promising method for the enrichment of CMM because of
the advantages of low capital investment, reusable nature of the adsorbents and
straightforward automatic operation. However, the typical pressure swing
adsorption (PSA) processes have thermodynamic constraints (low adsorbed amount
and low CH₄/N₂ selectivity) on the purity of the heavy
product, thus resulting in the low performance when PSA is adopted in the
enrichment of CMM.

In this
work, the principles of displacement chromatography are employed, and different
PSA processes implemented with displacement concept have been investigated
experimentally and theoretically for the enrichment of CMM.

The
adsorption equilibrium isotherms of CH₄, N₂ and CO₂
on activated carbon(AC) are gravimetrically measured at 273 K-338 K up to 1000
kPa in a magnetic suspension balance. The Sips model, Toth model and Multisite
Langmuir model can fit the isotherms quit well. The diluted breakthrough curve
experiment and corresponding isothermal bidisperse pore model is used to study
the adsorption kinetics of CH₄/N₂/CO₂ on AC.
It is found that the micropore resistances of CH₄, N₂ and
CO₂ are the dominate resistance within AC particle. There are little
differences between the micropore diffusivities of CH₄ and N₂.

Figture 1. Schematic diagram of four-step
VPSA and CO2 displacement-VPSA cycle

The
four-step Skarstrom-type VPSA process and the four-step CO₂
displacement VPSA process on AC packed bed are compared for the separation of
low concentration CH₄/N₂ system. It is found that the
performance of CO₂ displacement VPSA process is better than that of
typical VPSA process. Furthermore, a five-step CO₂ displacement VPSA
process is proposed to solve the problem that some CO₂ would be
vented to the atmosphere in the four-step cycle. The simulations of five-step
CO₂ displacement VPSA process are carried out for CH₄/N₂
separation. With the five-step cycle, 79.3% CH₄ is captured with a
purity of 86.9% from 10% CH₄ feed gas, the productivity and energy
consumption are 0.21 molCH₄/(kgads.¡¤h) and 139.2 kJ/molCH₄,
respectively; even the 1% CH₄ feed gas can be increased to 45.8%
with 77.5% CH₄ recovery, 0.019 molCH₄/(kgads.¡¤h)
productivity and 1537.3 kJ/molCH₄ energy consumption. The results
demonstrate that CO₂ displacement is a promising technology for the
enrichment of CMM.

Keywords: Activated
carbon; IAST-Sips model; displacement chromatography technology; Vacuum pressure
swing adsorption