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ISRN Chemical Engineering Most works on PSA processes have shown that normally the purity and recovery present a trade-off for the design. In the case of recovering the less adsorbed gas, if more purge is used, more of the contaminants can be desorbed from the column and purity increases, but since more light gas is exiting from the “bottom end,” light-gas recovery is smaller. A similar effect is observed for the utilization of the rinse step and purity and recovery of the more adsorbed gas. However, other strategies are valid to improve process recovery without seriously affecting the purity. The case of Polybed PSA for H2 purification is a good example [65]. The units built until 1975 were having 4 columns and the recovery of H2 was around 60%. Nowadays, PSA unit with 12 columns are found [65] and up to 16 columns were patented [135] with H2 recovery close to 90%. When the number of columns is increased, more pressure equalization steps can be performed and thus less hydrogen is lost with the contaminants, increasing its recovery. The developments in the PSA process presented above were mainly motivated to improve the purity and the recovery of the target product(s). Nowadays, several new applications of PSA as an alternative technology are still in the stage of finding proper cycle configuration (step scheduling and times, number of columns, etc.). Other appli- cations in more established markets are intending to improve either the unit size and/or the energetic consumption of the separation. 5. The Role of the Adsorbent in PSA The development of materials science in the last 60 years was quite intense. The result was the discovery of many porous materials, from all kind of zeolites and mesoporous materials [136–141] to the most diverse surfaces in activated carbons [142–145] and lately the high-surface area coordination polymers [146–151]. However, as strange as it may seem, only few materials are used in PSA units nowadays. A review of adsorption properties of the different mate- rials is out of the scope of this work, but good databases with adsorption properties of different gases on several adsorbents can be found [16, 152, 153]. What is important to mention is that a material to be used in PSA should be easily regenerated. It is frequent to find in literature adsorbents with a very high capacity, particularly at low pressures. Normally the isotherms of gases on such adsorbents are “rectangular”: very steep at low pressures and quite flat after a certain pressure. Defining the “cyclic capacity” as the difference of loading between the high and low pressures of the PSA cycle, the only way to have an acceptable cyclic capacity is making blowdown at very high vacuum. The direct implication of using such conditions is that the power consumption increases rapidly. So, materials showing linear or slightly nonlinear isotherms are preferred in PSA design. One frequent case is to have a multicomponent mixture of gases and that the number of compounds to be separated cannot be removed by a single adsorbent. The solution to this problem was found for the case of H2 purification from methane steam reforming. In this application, H2 is mixed with H2 O, CO2 , CO, unconverted CH4 , and possibly other 5 gases like N2 . Activated carbon can be used to remove H2 O and CO2 quite selectively but the loading for CO is rather limited for small partial pressures. It is thus common practice to use different layers of adsorbents to increase the loading of CO in the same column. This approach has also been applied in other separations [66, 70, 79, 154–160]. Consecutive layers of adsorbents can also be used to improve the productivity of kinetic adsorbents by adding a material that can be easily regenerated after the kinetic adsorbent [161, 162]. Other important aspect regarding the material properties for PSA applications is the diffusion of the different gases through its porous structure. There are different types of “resistances” to diffuse from the bulk gas phase to the adsorption site [4, 5]. They are: boundary layer around the adsorbent particle, and resistances in the macro-meso pores, mouth of the micropores, and micropores (or crystals). In some applications however, these mass transfer “prob- lems” have become part of the solution. In fact, if the diffusional resistance of one of the components of the mixture is very large, this gas will take so long to adsorb that can be separated from other gas that diffuses faster through the pores. The “kinetic processes” were recognized soon [28]. In fact, materials like zeolites are called “molecular sieves” because of this effect [136]. Another example of kinetic materials is the carbon molecular sieves (CMS) [29–31, 33, 38, 163–167]. A CMS is prepared by contracting the pores of an activated carbon to limit the adsorption of some molecules. Its first utilization was for air separation to separate O2 from N2. An extreme example of resistance to diffusion is the molecular exclusion like in the Isosiv process [5, 97–99]. In the Isosiv process, n-paraffins are selectively adsorbed in zeolite 5A, while isoparaffins are kinetically excluded from the zeolite crystals. Most recently, several inorganic materials have proved to be useful for kinetic separations [34, 36, 168–173]. A special kind of titanosilicates, ETS-4, cation exchanged with alkali- earth metals can be used for kinetic separations [35, 41, 174, 175]. In these materials, the pore size can be tuned with a very high accuracy by thermal treatment of the sample. Many studies have confirmed that CH4 can be excluded from the structure while gases like H2 S, CO2 , and specially N2 can be adsorbed [43, 176, 177]. 6. Advances in Process Engineering From all the main advances in process engineering, the most challenging one is the development of cyclic strategies that can improve the performance indicators of the PSA. Despite the performance of the material, the design of a PSA process requires several engineering decisions that should be taken sometimes with a very deep impact in terms of performance indicators. The main drawback of the engineering of a PSA process is that it is quite task consuming (and normally iterative). With modern computers, the design of the PSA cycle can be carried out by modelling different scenarios. There are different degrees of complexity to define a PSA model,PDF Image | Advances in Pressure Swing Adsorption for Gas Separation
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