Column PSA process

Figure k. Light gas enrichment and recovery in product as a function of product cut in single-column PSA process. 

Fig. 8 Schematic drawings of (A) three-column TSA and (B) two-column PSA processes.

In the PSA process, the reformer product gas is passed through an adsorption column, where impurities are preferentially adsorbed. After a set time, the feed gas to the reactor column is diverted to a parallel column. At this stage, the first column is depressurized, allowing the bed regeneration. It is a cyclic process, the two columns being alternately pressurized and depressurized [15]. 

The equations for a local equilibrium cell model of pressure swing adsorption processes with linear isotherms have been derived. These equations may be used to describe any PSA cycle composed of pressurization and blowdown steps and steps with flow at constant pressure. The use of the equations was illustrated by obtaining solutions for a single-column recovery process and a two-column recovery and purification process. The single-column process was superior in enrichment and recovery of the light component at large product cuts. The two-column process was superior at small cuts ... 

The Sumitomo-BF PSA process uses carbon molecular sieves (CMS) as the selective adsorbent, CMS has a higher capacity of adsorption than zeolites for methane and oxygen, and it is considered to be advantageous for hydrogen purification. If dirty raw gases are fed to this process, minor amounts of heavy hydrocarbon components such as aromatics are likely to cause deterioration of the adsorbents. To remove the heavy hydrocarbons, prefilter columns that contain activated carbon are placed upstream of the main CMS adsorbent beds4. 

The basic concept of a H2 PSA process is relatively simple. The impurities from the H2-containing feed gas mixture are selectively adsorbed on a micro- and meso-porous solid adsorbent (zeolites, activated carbons, silica and alumina gels) at a relatively high pressure by contacting the feed gas with the solid in a packed column of... 

The adsorbent particles are normally used as beads, extrudates, or granules (-0.1 -0.3 cm equivalent diameters) in conventional H2 PSA processes. The particle diameters can be further reduced to increase the feed gas impurity mass transfer rates into the adsorbent at the cost of increased column pressure drop, which adversely affects the separation performance. The particle diameters, however, cannot be reduced indefinitely and adsorption kinetics can become limiting for very fast cycles48 New adsorbent configurations that offer (i) substantially less resistance to gas flow inside an adsorber and, thus, less pressure drop (ii) exhibit very fast impurity mass transfer coefficients and (iii) minimize channeling are the preferred materials for RPSA systems. At the same time, the working capacity of the material must be high and the void volume must be small in order to minimize the adsorber size and maximize the product recovery. Various materials satisfy many of the requirements fisted above, but not all of them simultaneously. 

Figure 13 is a schematic flow diagram for a two column Skarstrom type [36] PSA drier. The process can be used to obtain very dry product gas (< —60 C dew point). The product purity depends on the type of alumina used, feed gas pressure and dry purge gas quantity. Typically a practical PSA drier uses 15-30% of product gas as purge. The typical total cycle time for a PSA process is 2-4 minutes. 

Activated carbon columns are frequently used to remove trace or dilute organic impurities from an inert nonadsorbing gas. These impurities may consist of toxic compounds, odor-forming compounds, solvent vapors, volatile organic compounds, etc. A TSA process is generally used for these applications. However, both TSA and PSA processes are used for the particular application of solvent vapor removal and recovery which are described in Section 22.4.1.1... 

Figure 22.3(b) is a schematic drawing of a two-column embodiment of a PSA process for solvent vapor recovery, (i) The solvent-laden feed gas is passed through the carbon column at a superambient pressure level (P ) in order to produce a solvent-free effluent gas at pressure P. (ii) The column is then countercurrently depressurized to a near ambient pressure level (Pp) to desorb a part of the adsorbed solvents. It is then (iii) countercurrently purged at pressure Pd with a part of the solvent-free gas produced by step (i) in order to further desorb the solvent, (iv) Finally, the column is countercurrently pressurized from Pp to P with another part of the solvent-free gas produced during step (i). The... 

Figure 22.6(a) is a schematic representation of a nine-column Poly Bed PSA process developed by the Union Carbide Corporation, USA, for the Hj purification application [16]. The parallel columns are packed with a layer of an activated carbon in the feed end followed by a layer of 5A zeolite at the product end. The PSA cycle has 11 sequential steps consisting of (i) adsorption at feed pressure to produce the H2 product at the same pressure, (ii) four cocurrent... 

Figure 22.10 Adsorption column profiles for C2H4-CH4 separation by a pressure swing adsorption (PSA) process using activated carbon (a) gas phase C2H4 mole fi-actions, (b) C2H4 loadings on the adsorbent.

Since basic equilibria, kinetic and fixed-bed data of sorption of nCs and nCe in pellets of 5A zeolite were obtained, we are able to simulate a cyclic PSA process for the separation of n/iso-paraffins. The case selected is the patent data shown by Minkkinen et al. [3]. In such process, isomerisation of Cs/Ce normal paraffins with recycling of normal paraffins is described. The recycling is performed in a selective adsorption containing 38Kg of 5A zeolite pellets. In the selective adsorption (lenght=4m i.d=12.7cm) unit a PSA cycle takes place at 300°C. Adsorption phase occurs at a total pressure of 15 bars with a duration of 6 minutes. Desorption phase is performed in 6 minutes at 2 bars countercurrent to adsorption with a fraction of the iCs rich product. To obtain continuous operation two columns are used. The effluent of the isomerisation reactor contains approximately 13.9 mole % nCs and 4.6 mole % nCo. The performance of the unit... 

We estimated the N2 desorption characteristics from these zeolites under two idealized but common concepts of operation of PSA processes. They are (a) isothermal evacuation of an adsorbent column which is initially equilibrated with a binary gas mixture of N2 (yi=0,79) and O2 (y2=0.21), and (b) isothermal and isobaric desorption of pure N2 from an adsorbent column by flowing a stream of pure O2 (called purging) through the column. The adsorbers are initially at a pressure of one atmosphere and at a temperature of 30 C in both cases. Analytical model solutions are available for the above described desorption processes when they are carried out under local equilibrium conditions and when the adsorbates follow Langmuir isotherms [1,6]. 

Consequently, water must be removed from the ambient air before the zeolite can carry out the N2-O2 separation. This is typically done by using a water selective desiccant like activated alumina or NaX zeolite for selectively removing the water from air. The desiccant generally forms an integral part of the zeolite PSA process for air separation (a layer of the desiccant followed by the zeolite layer in the same column or in two separate columns). The ratio of the... 

---