Process intensification in PSA processes for upgrading synthetic landfill and lean natural gases

Abstract

Process intensification aims at reducing the size of equipment by orders of magnitude and is actively perused in separation processes. Its feasibility in Pressure Swing Adsorption (PSA) processes has been explored. A 4-bed PSA and a 3-bed PSA, which emulate the moving bed processes, and duplex PSA and a modified duplex PSA have been selected for the exploratory studies. Simulation studies on the separation of a mixture of CH 4 -CO 2 over 5A zeolite were carried out to compare the performance of these processes. An index has been proposed to quantify the process intensification. The 3-bed PSA and the modified duplex PSA exhibited superior performance compared to the other two for a purity of 99.9 mol% of both the products. However, the performances of the processes other than duplex were comparable when purities were set at 95 mol%. In 3-bed PSA a modest process intensification of four times reduction in size and two times reduction in energy requirement appears to be feasible if benchmarked against the PSA based on the variant of the Skarstrom cycle. Notations bLangmuir parameter (m 3 /mol) c Concentration in gas phase (mol/m 3 ) C A Particle diameter (mm) D Diameter of bed (mm) D L0 Dispersion coefficient (m 2 /s) D M Molecular diffusivity (m 2 /s) E Energy (kJ/mol·feed) f Friction factor g c Gravitational constant (m/s 2 ) I P I , I P I Index for process intensification (thousand US $/y·(mol/s)) k i Linear driving force constant (s −1 ) L Bed length (m) L e Equivalent length of the valve (m) n Number of moles N Number of components P Pressure (bar) P Productivity of CH 4 (LSTP/h·kg of adsorbent) P H Adsorption pressure (bar) P I Intermediate desorption pressure (bar) P L Desorption pressure (bar) P 1 , P 2 Pressures used in (9) (bar) q Amount adsorbed in solid phase (mol/m 3 ) q e Amount adsorbed in solid phase at equilibrium with gas phase (mol/m 3 ) q s Saturation constant (mol/m 3 ) Q T Total amount adsorbed in solid phase (mol/m 3 ) R Universal Gas constant (bar·m 3 /mol·K) 122 Adsorption (2011) 17: 121-133

Biogas upgrading – technology overview, comparison and perspectives for the future

“…PSA unit characteristics include feed pressure, purging pressure, adsorbent, cycle time, and column interconnectedness, among other things. When using several columns there are many ways of modifying the process cycle to increase the yield of methane from raw biogas to upgraded gas, reduce the methane loss and reduce energy demand …”

Capture of low grade methane from nitrogen gas using dual-reflux pressure swing adsorption

“…Recently, Bhatt et al have performed an experimental investigation of DR-PSA with a split-column design, where each adsorbent bed is separated into two columns in series [24]. DR-PSA processes have features similar to a continuous distillation process; for example when both gas species are competing for adsorption sites the phase equilibria (vapor-solid) is akin to vapor-liquid equilibria [23], and the use of heavy (extract) and light (raffinate) refluxes through the adsorbent beds is similar to the use of liquid and vapor refluxes in a distillation process. In this paper we demonstrate experimentally a DR-PSA process to separate methane and nitrogen mixtures into a high-purity nitrogen stream, which can be safely vented to the atmosphere, and simultaneously an enriched methane product.…”

“…The DR-PSA processes (also known as duplex PSA) studied by Diagne et al [10,17,18], Ritter and co-workers [19,20], Webley and Kearns [21], Thakur et al [22] and Spoorthi et al [23] combined the stripping and enriching PSA cycles into a single two-bed system, where the feed gas is fed to an intermediate position along the adsorbent bed and some of each product stream is refluxed into the respective ends of the two adsorption columns. Recently, Bhatt et al have performed an experimental investigation of DR-PSA with a split-column design, where each adsorbent bed is separated into two columns in series [24].…”

Adsorption equilibria and kinetics of CH4 and N2 on commercial zeolites and carbons

“…Adsorption based processes are well-established technologies for the separation of gas mixtures in, for example, the air separation industry [1], the hydrogen production industry [2][3][4], and the capture of carbon dioxide from flue gases [5][6][7][8][9]. In the past several decades, adsorption based processes for separating nitrogen and methane have attracted significant attention in areas such as natural gas production [8,10], landfill gas upgrading [11,12], coalbed methane enrichment [13], and coal mine methane/ventilation air methane purification [14].…”