Zeolites separation applications

Industrial applications of zeolitic separation processes fall into the following general appHcation categories ... 

As documented in Chapter 5, zeolites are very powerful adsorbents used to separate many products from industrial process steams. In many cases, adsorption is the only separation tool when other conventional separation techniques such as distillation, extraction, membranes, crystallization and absorption are not applicable. For example, adsorption is the only process that can separate a mixture of C10-C14 olefins from a mixture of C10-C14 hydrocarbons. It has also been found that in certain processes, adsorption has many technological and economical advantages over conventional processes. This was seen, for example, when the separation of m-xylene from other Cg-aromatics by the HF-BF3 extraction process was replaced by adsorption using the UOP MX Sorbex process. Although zeolite separations have many advantages, there are some disadvantages such as complexity in the separation chemistry and the need to recover and recycle desorbents. 

The second part of the book covers zeolite adsorptive separation, adsorption mechanisms, zeolite membranes and mixed matrix membranes in Chapters 5-11. Chapter 5 summarizes the literature and reports adsorptive separation work on specific separation applications organized around the types of molecular species being separated. A series of tables provide groupings for (i) aromatics and derivatives, (ii) non-aromatic hydrocarbons, (iii) carbohydrates and organic acids, (iv) fine chemical and pharmaceuticals, (v) trace impurities removed from bulk materials. Zeolite adsorptive separation mechanisms are theorized in Chapter 6. 

However, at least for separative applications, most hopes to find consistent application of inorganic-membrane reactors lie in the development of inorganic membranes having pores of molecular dimensions (<10 A, e.g., zeolitic membranes). Such membranes should moreover be thin enough to allow reasonable permeability, defect-free, resilient, and stable from the thermal, mechanical, and chemical standpoints. Such results should not be achieved only at a lab scale (a lot of promising literature has recently appeared in this context), but should also be reproducible at a large, industrial scale. Last, but not least, such membranes should not be unacceptably expensive, in both their initial and their replacement costs. 

This cost differential can be tolerated only in applications in which polymeric membranes completely fail in the separation [78]. Demanding separation applications, where zeolite membranes could be justified, due to the high temperatures involved or the added value of the components, and have been tested at laboratory scale, are the following separation of isomers (i.e., butane isomers, xylene isomers), organic vapor separations, carbon dioxide from methane, LNG (liquefied natural gas) removal, olefines/paraffins and H2 from mixtures. In most cases, the separation is based on selective diffusion, selective adsorption, pore-blocking effects, molecular sieving, or combinations thereof. The performance or efficiency of a membrane in a mixture is determined by two parameters the separation selectivity and the permeation flux through the membrane. 

The inorganic silica membranes, also commercial, have solved the problem of thermal and chemical stability however, these membranes are only used for dehydration purposes, leaving the problem of separation of organic mixtures unsolved. As we have seen previously, due to the versatility and special feamres of zeolites, new applications in pervaporation that are not possible with other membranes could be developed with zeolite membranes. GaUego-Lizon et al. [110] compared different types of commercial available membranes zeolite NaA from SMART Chemical Company Ltd., sUica (PERVAP SMS) and polymeric (PERVAP 2202 and PERVAP 2510) both from Sulzer Chemtech GmbH, for the pervaporation of water/f-butanol mixtures. The highest water flux was obtained with the silica membrane (3.5 kg/m h) while the zeolite membrane exhibited the highest selectivity (16,000). 

In general, most of the high-separation factors reported for zeolite membranes are associated with pervaporation processes (see Section 10.5) or with vapor-separation applications where the permeated component is preferentially adsorbed. This has given rise to a variety of works in which the membranes have been used for equilibrium displacement by selective product permeation. The largest group probably corresponds to esterification processes, where hydrophilic zeolite membranes are employed to remove the product, water, replacing the extensively studied polymer membranes [187-192]. 

Hugon O, Sauvan M, Benech P, Pijolat C, and Lefebvre F. Gas separation with a zeolite filter, application to the selectivity enhancement ot chemical sensors. Sens Actuators B 2000 67(3) 235-243. 

The two promising candidates are adsorbent monoliths and adsorbent sheets. The fabrication of activated carbon and zeolite monoliths are reported in the literature. Zeolite monoliths have also been tested for air separation application by PSA.50 51 However, the use of monoliths for use in H2 PSA is not known to the authors. Monoliths having very high cell density (several hundred to thousand cells per square inch) will be necessary in order to have fast adsorption kinetics as well as reasonable bulk density for a PSA application. Manufacture of such monoliths is complex, and they are not yet commercially available. Gas channeling through the monoliths can also be a problem.52 Adsorbent sheets have been used for air separation by RPSA.53 54 The thickness of the adsorbent sheets and the space between the... 

Zeolites are traditionally used in catalysis/purification and separation applications in the petrochemical industry but are rapidly finding new uses. This section discusses membranes for low-dielectric-constant, corrosion-resistant, hydrophilic and antimicrobial, and pervaporation applications. 

Hydrophilic and antimicrobial zeolite coatings have also been shown to be effective for gravity-independent water-separation applications on manned spacecraft. Condensing... 

In GSC, separation occurs based on differences in the adsorption of the various components in the sample onto the solid adsorbent. While GSC may not offer as much flexibility in stationary phase functionality as GLC, it has its own advantages. For separation applications, advantages include higher available operating temperatures, higher column efficiencies, and no stationary phase leakage. Typical solid phases for GSC include zeolites, silica gel, activated alumina, carbon, carbon molecular sieves, diatomites, and porous polymers. 

The selection of a suitable zeolite adsorbent for CO2 removal from flue gas (mixture of CO2 and N2) has been carried out. The limiting heats of adsorption, Henry s Law constants for CO2 and N2, CO2 pure component adsorption isotherms and expected working capacity curves for Pressure Swing Adsorption (PSA) separation application were determined. The results show that the most promising adsorbent characteristics are a near linear CO2 isotherm and a low Si02/Al203 ratio with a cation in the zeolite structure that has strong electrostatic interaction. 

There are relatively few studies found in litaature concerning gas separation applications with type-A zeolite mon-branes (0.41 x 0.41 mn). With tubular supports, the best results found for gas separation are 8.33 for a H2/n-bntane equimolar mixture with a Hj permeance value of 7 x lO mol/Cm s Pa)... 

Zeolites are crystalline aluminosiUcates characterized by a structure comprising a three-dimensional pore system and regular framework formed by linked TO4 tetrahedral (T = Si, Al) with different morphological and physico-chemical properties. Due to their impressive selectivity and uniform pore structure, they have very efficient molecular sieving properties, and are able to separate molecules based on size and shape. Zeolite powders, films and membranes are widely used in catalysis, adsorption and separation applications (McLeary et al, 2006 Pina et al., 2011). Zeolites are cheap and widely available due to their abundance in both natural and synthetic forms. The application of zeolites in the membrane field is growing very fast, and has been the subject of increased research focus during the last few decades (McLeary et al., 2006). 

Since 2004, many articles on preparation of zeolite MMs have been published, on such areas as MFI or Sil-1 zeolite etched on the Si substrate for gas separation applications, and MMRs for KCR and fine chemical synthesis (Coronas and Santamaria, 2004 Kwan et al, 2010 Wan et al., 2001 Yeung et al, 2005). Coronas and Santamaria (2004) have reported on the use of zeolite films and interfaces in micro-scale and portable applications, including the removal of volatile organic compounds from indoor air, recovery of catalysts in homogeneous reactions, zeolitic microreactors and microseparators, for example. Moreover, zeolite coated microreactors and microseparators exhibit high surface-to-volume ratio, and are capable of high productivity as a result of the good contact between reactants and catalyst wall. 

Pure gas isotherms for adsorption of N2 and O2 on five commercial zeolites (NaX, 5A, Na- Mordenite, CaX and CaLSX) at two different temperatures are reported. The isotherms can be described by the Langmuir model in the range of the data. Mixed gas Langmuir model is used to evaluate the relative N2 adsorption and desorption characteristics for these zeolites in connection with air separation application by the pressure swing adsorption (PSA) concepts. 

The key adsorptive properties of the zeolites for air separation applications are (a) specific nitrogen adsorption capacity and selectivity of adsorption of Nz over Oz and Ar as functions of gas phase pressure, temperature and composition, (h) isosteric heats of adsorption of Nz and Oz as functions of adsorbate loadings, (c) desorption characteristics of Nz and Oz from the zeolite under various conditions of operation, and (d) kinetics of ad(de)sorption of Nz and Oz on the zeolite. 

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