Oxygen concentrations above 99.5% are required for several applications, mainly in the medical and aerospace fields. Two-stage pressure swing adsorption (PSA) processes, combining kinetic separation with equilibrium separation, have been developed for producing 99+% oxygen from air.
Argon and nitrogen are kinetically removed from the air feed using a CMS (carbon molecular sieve) adsorbent and the remaining nitrogen is removed using a N2/O2 selective zeolite. Despite that, two-stage processes are often unattractive, complex, and energy consuming, requiring two or more compressors/vacuum pumps. Moreover, most of the two-stage units described in literature are unable to reach the required oxygen purity of 99.5%.
There are three energy-efficient two-stage vaccuum PSA (VPSA) processes
- combining an equilibrium based PSA (EPSA) or a kinetic based PSA (KPSA) for the first stage.
- with a VPSA unit packed with the Ar/O2 selective zeolite AgLiLSX for the second stage.
Aiming to produce 99.5+% oxygen, the use of zeolite AgLiLSX allows removing argon besides nitrogen. The best two-stage VPSA configuration allowed obtaining a 99.8% oxygen stream at 6% of recovery and a 99.5+% oxygen stream at 14+% of recovery.
The sales of pressure swing adsorption (PSA) units for producing 95% of oxygen from air has increased noticeably in the past decades. There is, however, a great demand for oxygen 99+% for various industrial applications,
(a) medical applications, such as surgeries where the minimum oxygen concentration required is 99% in United States and 99.6% in Japan;
(b) military and aerospace applications where minimum concentration of 99.5% is required;
(c) semi conductor industry where concentrations higher than 99.8% is required;
(d) for metal welding and cutting processes.
Despite this increasing demand, only cryogenic separation has been recognized effective for producing high-purity oxygen(99+%). For small and medium scale production and for oxygen concentrations up to 95%, PSA is the state-of-the-arttechnology. Additionally, the low installation costs, simplicity, and easy start/stop operation of a PSA unit are major advantages that cannot be realized with the cryogenic processes.
Since the 1980s several PSA designs have been proposed for producing high-purity oxygen. The first process was disclosed by Armond in 1980 and consists of a PSA unit packed with a carbon molecular sieve (CMS) adsorbent followed by another PSA unit packed with a nitrogen selective zeolite. The CMS stage served to kinetically remove essentially argon from the air and the second stage for removing the remaining nitrogen. The process was complex and had low energy efficiency since it used three air compressors/vacuum pumps.
Following, in 1989 Miller and Theis proposed a similar two stage PSA process, where the equilibrium stage (beds packed with zeolite) comes first, followed by the kinetic stage (beds packed with CMS). The process, operating between 3.1 and 1.0bar, delivers a 99.1% oxygen product stream at atmospheric pressure. Similar processes were disclosed in the following years with small improvements compared with the original two-stage PSA process, such as introducing a buffer tank for storing enriched oxygen from the desorption step of the CMS-stage; this stored gas is then used to pressurize and feed the zeolite stage and to purge the CMS-stage, thus improving the process purity and productivity.
More recently, Lee reported a two-stage three-bed PSA unit for producing a stream of 99.2%oxygen with a recovery of 47%. This unit comprised a PSA loaded with 10X-type zeolite followed by a CMS column, running a 10-step cycle with two consecutive blow down/backfill steps. The high recovery results were possible because of the use of a very high selective CMS adsorbent. However, despite the higher recovery and purity, this was not enough to fulfill the demand of 99.5+% oxygen.
During the 1990s, new generations of highly selective adsorbents, such as LiX-, LSX-, LiLSX-type zeolites and, particularly, silver-exchanged zeolites, contributed to a significant increase in the productivity and economic efficiency of oxygen air separation by PSA-based processes.
In 1993, Knaebel and Kandybin disclosed a single-column PSA processusing a silver-exchanged mordenite. This mordenite has selectivity to argon, and the process is reported to produce astream with 99.5% of oxygen and 12% of recovery from a feed containing 95% of oxygen balanced with argon.
In 2002, Air Products and Chemicals, Inc. also patented an argon/oxygen selective zeolite, named AgLiLSX (lithium low silica X-typesilver-based zeolite), with a silver-exchanged content of 20−70mol %. The AgLiLSX zeolite is able to produce a stream of 99.1% oxygen from air in a single-stage VPSA operation, unlike present commercial zeolites that are limited to 95% oxygen (balanced mostly with argon) since they do not exhibit argon/oxygen adsorption selectivity above 1.25. This work studies three energy-efficient two-stage VPSA processes, combining an equilibrium based PSA (EPSA) or akinetic based PSA (KPSA) for the first stage; with a VPSA packed with AgLiLSX zeolite for the second stage; aiming to produce a 99.5+% oxygen stream from air. It covers the simulation and optimization of the three two-stage VPSA configurations and corresponding experimental validation. The proposed model was solved using ASPEN.