Sorbents, commercial zeolites

Effects of cation sites can be best illustrated with the important system of N2/O2 on type X zeolites. Na/X (or 13X) has been used commercially for air separation since the 1970 s. Li/LSX is the best sorbent commercially available today (Chao, 1989). Mixed-cation AgLi/LSX (with 1-3% Ag cations) has been shown to be even better than Li/LSX (Yang and Hutson, 1998 Hutson, Rege, and Yang, 1999). 

In the sorption experiments, dye solutions were added to different quantities of sorbents into glass-stoppered bottles and subsequently placed on a shaker for 24 h at 28 2°C. From the initial concentrations of sorbents (g/L) and dyes (mg/L), the amounts adsorbed in the sorbent were measured. Percent removal of dyes over synthesised zeolite as a function of contact time, shown in Figure A.4. The amounts sorbed were determined by the difference between initial and final concentrations and expressed as mg of dye/g of sorbent. Under the conditions of the experiments, all systems approached equilibrium within 15 h of contact time. The adsorption capacity of synthesised zeolite was higher due to larger pore size and surface area compared to commercial zeolite. The molecular size of methyl orange facilitates adsorption, resulting in higher adsorption capacity than methylene blue and safranine T. Reduced adsorption in synthesised zeolite (SZ) is due to the inability of the molecule to penetrate all the internal pore structures and less available surface. 

There are only four types of sorbents that have dominated the commercial use of adsorption activated carbon, molecular-sieve zeolites, sihca gel, and activated alumina. Estimates of worldwide annual sales of these sorbents are as follows (Humphry and Keller, 1997) ... 

During the last decade, there have been several published accounts on using adsorption for liquid fuel desulfurization. Commercially available sorbents (i.e., zeolites, activated carbon and activated alumina) were used in all of these studies. Weitkamp et al. 

First results on n-complexation sorbents for desulfurization with Ag-Y and Cu(I)-Y zeolites have been reported recently [3,4]. In this work, we included the known commercial sorbents such as Na-Y, Na-ZSMS, H-USY, activated carbon and activated alumina (Alcoa Selexsorb) and made a direct comparison with Cu(l)-Y and Ag-Y which were the sorbents with n-complexation capability. Thiophene and benzene vapors were used as the model system for desulfurization. Although most of these studies can be applied directly to liquid phase problems, Cu-Y (auto-reduced) and Ag-Y zeolites were also used to separate liquid mixtures of thiophene/benzene, thiophene/n-octane, and thiophene/benzene/n-octane at room temperature and atmospheric pressure using fixed-bed adsorption/breakthrough techniques. These mixtures were chosen to understand the adsorption behavior of sulfur compounds present in hydrocarbon liquid mixtures and to study the performance of the adsorbents in the desulfurization of transportation fuels. Moreover, a technique for regeneration of the adsorbents was developed in this study [4]. 

Molecular sieve zeolites have become established as an area of scientific research and as commercial materials for use as sorbents and catalysts. Continuing studies on their synthesis, structure, and sorption properties will, undoubtedly, lead to broader application. In addition, crystalline zeolites offer one of the best vehicles for studying the fundamentals of heterogeneous catalysis. Several discoveries reported at this conference point toward new fields of investigation and potential commercial utility. These include phosphorus substitution into the silicon-aluminum framework, the structural modifications leading to ultrastable faujasite, and the catalytic properties of sodium mordenite. 

An excellent review and detailed coverage on commercial adsorbents and new adsorbent materials has been presented by Yang in his newly published monograph on adsorbents.A very brief overview of existing commercial adsorbents is given here. Commercial sorbents that have been used in large-scale adsorptive separation and purification processes include activated carbon, zeolites, activated alumina, silica gel, and polymeric adsorbents. Although the worldwide sales of sorbent materials are relatively small as compared with other chemical commodities, sorbents and adsorption processes play a very important role in many process industries. The estimated worldwide sales of these sorbents are as follows ... 

Physical sorbents for carbon dioxide separation and removal were extensively studied by industrial gas companies. Zeolite 13X, activated alumina, and their improved versions are typically used for removing carbon dioxide and moisture from air in either a TSA or a PSA process. The sorption temperatures for these applications are usually close to ambient temperature. There are a few studies on adsorption of carbon dioxide at high temperatures. The carbon dioxide adsorption isotherms on two commercial sorbents hydrotalcite-like compounds, EXM911 and activated alumina made by LaRoche Industries, are displayed in Fig. 8.F23,i24] shown in Fig. 8, LaRoche activated alumina has a higher carbon dioxide capacity than the EXM911 at 300° C. However, the adsorption capacities on both sorbents are too low for any practical applications in carbon dioxide sorption at high temperature. Conventional physical sorbents are basically not effective for carbon dioxide capture at flue gas temperature (> 400°C). There is a need to develop effective sorbents that can adsorb carbon dioxide at flue gas temperature to significantly reduce the gas volume to be treated for carbon sequestration. 

Existing commercial sorbents including activated carbon, zeolites, activated alumina, and silica gels will 2. 

Table 7.1 lists the typical sorbents used their uses as well as strengths and weaknesses. The four major commercial adsorbents are the following zeolite molecular sieves (zms), activated alumina, silica gel, and activated carbon. The surfaces of activated alumina and most molecular-sieve zeolites are hydrophilic, and will preferentially adsorb water over organic molecules. Silicalite, which is a hydrophobic zeolite, is the main exception. Activated carbon, on the other hand, preferentially adsorbs organic and non-polar or weakly polar compounds over water. The surface of silica gel is somewhere in between these limits and has affinity for both water and organics. Detailed information about each of these classes of adsorbents can be found in Refs. [1,4, 6, 7]. 

Future applications of adsorption are limited by the availability of new and better sorbents. Ideally, the sorbent shonld be tailored with specific attributes to meet the needs of each specific application. Development of better sorbents can also improve the performance of the cnrrent commercial processes. A good example is the invention of the LiX (Si/Al = 1) zeolite (Chao, 1989). Air separation has been performed by pressnre swing adsorption, and the generic sorbents 13X (i.e., NaX) and 5A (i.e., CaA) zeolites were nsed prior to this invention. By switching from NaX to LiX (Si/Al = 1), the prodnctivity of oxygen increased instantly by 1.4-2.7 times and the power consnmption reduced by 21-27% depending on the operating conditions nsed (Leavitt, 1995). 

Table 1.1 shows examples of commercial applications of these sorbents. Both bulk separation and purification processes are given. Here bulk separation is defined (by Keller, 1983) as having the concentration of the adsorbed component above 10 wt % in the feed. For purification, the concentration of the adsorbed component is generally <2 wt % in the feed. The liquid-phase bulk separations that use the zeolites listed in Table 1.1 are accomplished with the simulated moving bed process. Not included in Table 1.1 are many liquid-phase bioseparations... 

Purification of aliphatics by the removal of aromatics is important in the petrochemical industry as well as for pollution control. In a typical benzene removal process, a combination of extraction and distillation is used (Meyers, 1986). Improvements by other processes have been considered, such as pervapora-tion (Hao et al., 1997), liquid membranes (Li, 1968 Li, 1971a and 1971b), and adsorption by temperature swing adsorption (TSA) in the liquid phase (Matz and Knaebel, 1990). In the work of Matz and Knaebel, commercially available sorbents were used silica gel, activated alumina, activated carbon, zeolite 13X,... 

Commercial sorbents have been considered for isotope separations. These include D2-H2 on alumina at 77.4 K (Katorski and White, 1964 King and Benson, 1966), D2-DT-T2 on 13X zeolite at various low temperatures (Maienschein... 

Prior to ca. 1980, PSA systems were used with both adsorption and desorption pressures well above atmospheric. These systems were low in capital (due to simplicity) but high in power consumption (since both N2 and O2 in the feed are compressed compared with vacuum swing adsorption (VSA) where only the waste gas is evacuated). The availability of improved sorbents and lower cost vacuum equipment led to the development of VS A, which is typically operated with adsorption pressure slightly above atmospheric and desorption pressure of typically 0.2 atm. A further breakthrough occurred in 1989, with the invention of LiLSX zeolite (low silica X, with Si/Al = 1) (Chao, 1989). The LiLSX is currently the best commercial sorbent for air separation and will be discussed... 

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