Gas Phase Applications

Solvent Recovery. Most of the activated carbon used in gas-phase applications is employed to prevent the release of volatile organic compounds into the atmosphere. Much of this use has been in response to environmental regulations, but recovery and recycling of solvents from a range of industrial processes such as printing, coating, and extmsion of fibers also provides substantial economic benefits. 

Material balances, often an energy balance, and occasionally a momentum balance are needed to describe an adsorption process. These are written in various forms depending on the specific application and desire for simplicity or rigor. Reasonably general material balances for various processes are given below. An energy balance is developed for a fixea bed for gas-phase application and simphfied for liquid-phase application. Momentum balances for pressure drop in packed beds are given in Sec. 6. 

Traditionally, active carbons are made in particulate form, either as powders (particle size < 100 pm, with an average diameter of -20 pm) or granules (particle size in the range 100 pm to several mm). The main precursor materials for particulate active carbons, PAC, are wood, coal, lignite, nutshells especially from coconuts, and peat. In 1985, 360 kt of such precursors (including 36 % wood and 28 % coal) were used to make active carbons [10], of which nearly 80 % were used in liquid-phase applications, with the rest being used in gas-phase applications. Important factors in the selection of a precursor material for an active carbon include availability and cost, carbon yield and inorganic (mainly mineral) matter content, and ease of activation. 

Gas phase applications of activated carbon fall into the main categories of separation, gas storage, and catalysis. These applications account for about 20% of the total use of activated carbon, with the majority using either granular or pellet type. Table 3 shows the major gas phase applications, again along with 1987 consumption levels. 

Granular Activated Carbon (GAC) - irregular shaped particles with sizes ranging from 0.2 to 5 mm. This type is used in both liquid and gas phase applications. 

Pelleted Activated Carbon - extruded and cylindrical shaped with diameters from 0.8 to 5 mm. These are mainly used for gas phase applications because of their low pressure drop, high mechanical strength and low dust content. 

With powder activated earbon, in most cases, the carbon is dosed into the liquid, mixed and then removed by a filtration process. In some cases, two or more mixing steps are used to optimise the use of powder carbon. Powder activated carbon is used in a wide range of liquid phase applications and some specific gas phase applications such as Incinerator flue gas treatment and where it is bonded into filters sueh as fabrics for personnel protection. 

While conventional monoliths contain parallel channels, in practice, systems are often made from alternate layers that allow lighter structures with better mass transfer characteristics in gas-phase applications, see Figure 9.6 showing interconnected flow paths. They are usually made from metal, mostly Fecralloy , Kanthal , or stainless steel, and widely used in autocatalysts and in environmental... 

Ti02. However, in the case of ZnO, photocorrosion frequently occurs with the illumination of UV lights. This phenomenon is considered as one of the main reasons for the decreased photocatalytic activity of ZnO in aqueous solutions [209,210], But for gas-phase applications, this disadvantage should not exist [211], Additionally, some studies have confirmed that ZnO exhibits a better efficiency than Ti02 in the photocatalytic degradation of some dyes, even in aqueous solutions [212,213],... 

A variety of photocatalyst supports has been examined experimentally. Dip-coated glass slides or plates have been used in many experimental systems as a simple lab-scale supported photocatalyst system. Coated glass offers many of tlte important features of a supported photocatalyst while still offering relatively simple preparation. Honeycomb monoliths, widely used as commercial catalyst supports for a variety of gas-phase applications, have also been examined as photocatalyst supports (Fig. 3). Although these monoliths offer good stability and excellent throughput, providing illumination for the photocatalyst inside the monolith channels can be problematic [41,42]. Randomly structured support materials, like fiber-based filters, reticulated foams, and similar materials, have been used... 

Production capacity was almost equally split between powdered and nonpowdered activated carbon products. Powdered activated carbon, a less expensive form used in liquid-phase applications, is generally used once and then disposed of. In some cases, however, granular and shaped products are regenerated and reused (35). In 1990 production capacity for granular and shaped products was split with about two-thirds for liquid-phase and one-third for gas-phase applications (37). 

Most commercial adsorbents for gas-phase applications arc employed in tlie form of pellets, beads, or other granular shapes, typically about 1.5 to 3.2 mm in diameter. Most commonly, these adsorbents are packed into fixed beds through which (lie gaseous feed mixtures are passed. Normally, Ihe process is conducted in a cyclic manner. When Ihe capacity of Ihe bed is exhausted, the feed flow is stopped to terminate the loading step of the process, the bed is treated to remove the adsorbed molecules in a separate regeneration step, and Ihe cycle is then repealed. 

It is, of course, the last effect - selective adsorption (or absorption) of the species of interest - which is the key for chemical sensing the first two effects can be regarded as nonspecific interferences. There are, however, situations in which these two effects cannot be ignored, even in gas phase applications. Thus, for an AT-cut crystal at 50 °C, (4.14) is... 

Figure 4 Shaped channels in metal monoliths in order to increase mass transfer in gas-phase applications.

Mesoporosity (near or larger than 30 A) is desirable for liquid phase applications, whereas smaller pore sizes (10 to 25 A) are required for gas-phase applications (Yang, 1997). 

For gas-phase applications, carbon adsorbents are regularly applied in the shape of hard granules, hard pellets, fiber, cloths, and monoliths since these prevent an extreme pressure drop [77,78],... 

As pointed out in Section A9.3.2.1, most of the CMRs for gas-phase applications require selective permeation through the membrane. The aim of this section is to briefly describe the different gas transport processes through a porous membrane. 

Electron spin resonance (e.s.r.) spectroscopy, applied to free radicals in condensed phases, is a long established technique with several commercially available spectrometers. The gas phase applications we will describe have little in common with condensed phase studies, and are much more a part of rotational spectroscopy. However, the experimental methods used for condensed phase studies can be applied to the study of gases with rather little change, so it is appropriate first to describe a typical microwave magnetic resonance spectrometer, as illustrated schematically in figure 9.1. 

Although the activity of Ti-Beta is much lower than that of Al-Beta, it was found to have a high tolerance for water. The latter property, which is related to a higher hydrophobic character combined with the absence of Brpnsted acid sites, allows the use of Ti-Beta in gas-phase application at increased reaction temperatures where Br0nsted acid-catalysed site reactions would otherwise have become significant.77... 

Pore size is also related to surface area and thus to adsorbent capacity, particularly for gas-phase adsorption. Because the total surface area of a given mass of adsorbent increases with decreasing pore size, only materials containing micropores and small mesopores (nanometer diameters) have sufficient capacity to be useful as practical adsorbents for gas-phase applications. Micropore diameters are less than 2 mn mesopore diameters are between 2 and 50 nm and macropores diameters are greater than 50 mn, by IUPAC classification (40). 

The practical adsorbents used in most gas phase applications are limited to the following types, classified by their amorphous or crystalline nature. 

Drying. The single most common gas phase application for TSA is drying. The natural gas, chemical, and cryogenics industries all use zeolites, silica gel, and activated alumina to dry streams. Adsorbents are even found in mufflers. 

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