(257d) Hybrid Duplex and Molecular Gate PSA

Keller and Kuo (1982) proposed ‘molecular gate process PSA’ (sometimes referred to as piston PSA) to fractionate binary mixtures to yield higher purity of both products and higher productivity compared to the conventional PSA based on the variants of Skartsrom cycle. It employs single bed with a piston-cylinder placed on either end of the bed for recycling gas. Hirose (1991) proposed a two bed ‘dual-reflux PSA’ to get two products of high purity.  Leavitt (1992) independently came up with the same PSA, which he termed as ‘duplex PSA’ and claimed that air can be fractionated into oxygen and nitrogen with purities in excess of 99.9 mol%. Sivakumar and Rao (2011a, 2011b,) proposed modified duplex PSA to yield high productivity. It is shown that modified duplex PSA would enhance productivity several times if the target purities are lowered to 98 mol% from 99.8 mol% for upgrading landfill gas and CO₂ capture from flue gas (Spoorthi et al. 2011 and Thakur et al. 2011). Molecular-gate and duplex PSA share the same mechanism of separation though differ entirely in their physical configuration (Sivakumar 2007). The pressure variation with time in a cycle is sinusoidal in the former whereas it is rectangular wave in the latter.  

In spite of merits of molecular gate PSA, the use of piston poses a serious constraint in its scale-up for commercial operation (Ruthven et al. 1994).On the other hand, duplex PSA has two beds and its scale-up is straight forward as its physical configuration is similar to the well established conventional PSA. However it requires a number of on-off valves and its operation is rather complex. In this work we propose a hybrid molecular gate and duplex PSA with single/multiple columns with a set of three tanks and a set of five mass flow controllers which facilitates setting any desired pressure variation with time in a cycle. 

Computer simulations were  carried out with a single adsorbent bed with several forms of  pressure variations with time for the capture of CO₂ from a synthetic flue gas mixture of 20/80 mole ratio of CO₂/N₂ over 13X zeolite. The pressure variation similar to that in the modified duplex process yielded higher productivity than those of the sinusoidal or square wave variation.

Keywords: Hybrid molecular gate, duplex PSA, CO₂ capture, flue gas

 

References

Hirose T, A simple design method of a new PSA process consisting of both rectifying and stripping sections. Proceedings of the 2^nd China-Japan-USA Symposium on Adsorption, 123. 1991.

Keller II G.E. and Kuo C.H.A. Enhanced Gas Separation by Selective Adsorption, US Patent 4,354,859,1982.

Leavitt FW. Duplex adsorption process. US Patent 5,085674. 1992.

Ruthven  DM, Farooq S, Knaeble, KS. Pressure swing adsorption. VCH publishers (UK). 1994.

Sivakumar SV, Rao DP. Modified duplex PSA.1. Sharp separation and process intensification for CO₂-N₂-13X zeolite system. Ind. Eng. Chem. Res. 2011a; 50: 3426–3436.  

Sivakumar SV, Rao DP. Modified duplex PSA.2. Sharp separation and process intensification for N₂-O₂-5A zeolite system. Ind. Eng. Chem. Res. 2011b; 50: 3437–3445.

Sivakumar SV. Sharp separation and process intensification on adsorptive separation processes. Ph.D. Thesis, Indian Institute of Technology Kanpur, India. 2007.

Spoorthi G, Thakur RS, Kaistha Nitin, Rao DP. Process intensification in PSA processes for upgrading synthetic landfill and lean natural gases. Adsorption. 2011; 17:121-133.

Thakur RS, Kaistha Nitin, Rao DP. Process intensification in duplex pressure swing adsorption. Computers and Chemical Engineering. 2011; 35: 973-983