Adsorption selective carbon membranes

Fuertes, A.B., Adsorption-selective carbon membrane for gas separation, ]. Membr. Sci., 177, 9, 2000. 

They also formed the condensed polynuclear aromatic (COPNA) resin film on a porous a-alumina support tube. Next, a pinhole-free CMSM was produced by carbonization at 400-1,000°C [29], The mesopores of the COPNA-based caibon membranes did not penetrate through the total thickness of each membrane and served as channels which increased permeances by linking the micropores. CMSMs produced using COPNA and BPDA-pp ODA polyimdes showed similar permeation properties even though they had different pore stractures. This suggests that the micropores are responsible for the permselectivities of the carbonized membrane. Besides that, Fuertes [30] used phenohc resin in conjunction with the dip coating technique to prepare adsorption-selective carbon membrane supported on ceramic tubular membranes. 

The PR was also used to prepare adsorption selective carbon membranes (ASCM) [47], The main feature of this work is that the carbonized membrane is subjected to air oxidation at temperatures above 100°C for 0.5-6 h to increase the pore size... 

F uertes AB (2001) Effect of air oxidation on gas separation properties of adsorption-selective carbon membranes. Carbon 39 (5) 697-706... 

In addition, the effect of pre-treatments and post-treatments dnring the membrane fabrication process shoitld also be examined. As an example, post-oxidation treatments can be very useful to tailor the properties of a molecular sieving carbon membrane in order to make an adsorption-selective carbon membrane, which is suitable for the separation of hydrocarbon mixtures based on adsorption differences. In fact. 

In addition to the particulate adsorbents listed in Table 16-5, some adsorbents are available in structured form for specific applications. Monoliths, papers, and paint formulations have been developed for zeolites, with these driven by the development of wheels (Fig. 16-60), adsorptive refrigeration, etc. Carbon monoliths are also available as are activated carbon fibers, created from polymeric materials, and sold in the forms of fabrics, mats, felts, and papers for use in various applications including in pleated form in filters. Zeolitic and carbon membranes are also available, with the latter developed for separation by selective surface flow [Rao and Sircar, J. Membrane Sci., 85, 253 (1993)]. 

Figure 22.9 Examples of molecular engineering of carbon surface (a) isotherms for adsorption of HjO vapor on various chemically modified carbons, (b) gas drying characteristics of the modified selective surface membrane (SSF) membrane, (c) high-temperature chemisorption of CO2 on MgO-doped activated carhon.

In a simulation system, we investigate the equilibrium selective adsorption and nonequilibrium transport and separation of gas mixture in the nanoporous carbon membrane are modeled as slits from the layer structure of graphite. A schematic representation of the system used in our simulations is shown in Fig. 11.21(a) and (b), in which the origin of the coordinates is at the center of simulation box and transport takes place along the x-direction in the nonequilibrium simulations. In the equilibrium simulations, the box as shown in Fig. 11.21(a) is employed, whose size is set as 85.20 nm x 4.92 nm x (1.675 + JV) nm in x-, y-, and z-directions, respectively, where JV is the pore width, i.e. the separation distance between the centers of carbon atoms on the two layers forming a slit pore (Fig. 11.21). is the separation distance between two centers of adjacent carbon atom L is the pore length JV is the pore width, A... 

Mass transfer of gas through a porous membrane can involve several processes depending on the pore stmcture and the solid [1]. There are four different mechanisms for the transport Poiseuille flow Knndsendiflusion partial condensation/capillaiy diffusion/selective adsorption and molecular sieving [2, 3]. The transport mechanism exhibited by most of carbon membranes is the molecular sieving mechanism as shown in Fig. 2.1. The carbon membranes contain constrictions in the carbon matrix, which approach the molecular dimensions of the absorbing species [4],... 

Nguyen C, Do DD, Haraya K, Wang K (2003) The structural characterization of carbon molecular sieve membrane (CMSM) via gas adsorption. J Membr Sci 220 (1-2) 177-182 Nguyen C, Do DD (1999) Adsorption of supercritical gases in porous media Determination of micropore size distribution. J Phys Chem B 103 (33) 6900-6908 Katsaros FK, Steriotis TA, Ramanos GE, Konstantakou M, Stubos AK, Kanellopoulos NK (2007) Preparation and characterization of gas selective microporous carbon membranes. Microporous Mesoporous Mater 99 (1-2) 181-189... 

The separation factor of these organic polymer membranes is typically located in a moderate range, of around 5 and 10, but rarely higher than 20. As a rule of thumb and proven by recent publications, the membrane selectivity can be approximated as the product of the adsorption selectivity and diffusion selectivity [2]. This chapter provides a wealth of information on diffusion inside micro- and mesoporous structures using concepts and ideas that originate from Maxwell and Stefan. A molecular-level understanding of diffusion in a variety of materials such as zeolites, MOFs, covalent organic frameworks (COFs), carbon nanotubes, and cylindrical silica pores is provided with the aid of extensive data sets of molecular... 

Surface-selective flow membranes made of nanoporous carbon, which is a variation of molecular sieving membranes, were developed by Rao et al. (1992) and Rao and Sircar (1993). The membrane can be produced by coating poly(vinylidene chloride) on the inside of a macroporous alumina tube followed by carbonization to form a thin membrane layer. The mechanism of separation is by adsorption-surface-diffusion-desorption. Certain gas components in the feed are selectively adsorbed, permeated through the membrane by surface diffusion, and desorbed at the low-pressure side of the membrane. This type of membrane was used to separate H2 from a mixture of H2 and CO2 (Sircar and Rao, 2000), and their main advantage is that the product hydrogen is at the high-pressure side eliminating the need for recompression. The membrane, however, is not industrially viable because of its low overall separation selectivity. In addition, since the separation mechanism involves physical adsorption, operation at low temperatures is required. 

Adsorption systems employing molecular sieves are available for feed gases having low acid gas concentrations. Another option is based on the use of polymeric, semipermeable membranes which rely on the higher solubiHties and diffusion rates of carbon dioxide and hydrogen sulfide in the polymeric material relative to methane for membrane selectivity and separation of the various constituents. Membrane units have been designed that are effective at small and medium flow rates for the bulk removal of carbon dioxide. 

A wide range and a number of purification steps are required to make available hydrogen/synthesis gas having the desired purity that depends on use. Technology is available in many forms and combinations for specific hydrogen purification requirements. Methods include physical and chemical treatments (solvent scmbbing) low temperature (cryogenic) systems adsorption on soHds, such as active carbon, metal oxides, and molecular sieves, and various membrane systems. Composition of the raw gas and the amount of impurities that can be tolerated in the product determine the selection of the most suitable process. 

Several techniques for VOC removal have been investigated such as thermal incineration, catalytic oxidation, condensation, absorption, bio-filtration, adsorption, and membrane separation. VOCs are present in many types of waste gases and are often removed by adsorption [1]. Activated carbon (AC) is commonly used as an adsorbent of gases and vapors because of its developed surface area and large pore volumes [2]. Modification techniques for AC have been used to increase surface adsorption and hence removal capacity, as well as to improve selectivity to organic compounds [3]. 

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