Liquid-phase applications

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. 

Liquid phase applications account for nearly 80% of the total use of activated carbon. Activated carbon used in liquid phase applications typically have a high fraction of pores in the macropore (>50nm) range. This is to permit the liquid phase molecules to diffuse more rapidly into the rest of the pore structure [15]. 

The principal liquid phase applications, the type of carbon used, and 1987 consumption levels are presented in Table 2. 

In liquid-phase applications, transfer of the adsorbate from the bulk solution to the carbon particle must proceed through two stages first, via transfer of the adsorbate from the bulk liquid to the surface of the carbon particle, and second, by migration... 

Powder Activated Carbon (PAC) - pulverized carbon with a size predominantly less than 0.18mm (US Mesh 80). These are mainly used in liquid phase applications and for flue gas treatment. 

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. 

Applications of carbon adsorption go far beyond conventional water treatment applications which we will discuss in a general sense shortly. Table 8 provides a summary of the key applications of carbon adsorption systems for liquid phase applications. 

Table 8. Liquid Phase Applications of Carbon Adsorption.

Accordingly, chip micro reaction systems are frequently described in the literature. Most of them are made of silicon (see, e.g., [19, 56-62]) Glass can be manufactured by similar routes as for silicon and could hence constitute gas-phase micro reactors however, the glass chip micro reactors described so far were made for liquid-phase applications (see, e.g., [63-70]). 

Microfiltration with uniform transmembrane pressure Commercial and developing liquid phase applications Gas separations using inorganic membranes... 

Granular activated carbon (GAC) GAC is designated by mesh sizes such as 8 X 20, 20 X 40, or 8 X 30 for liquid-phase applications and 4 X 6, 4 X 8, or 4 X 10 for vapor-phase applications. GAC is used for the removal of toxic organic compounds from groundwater and industrial wastewater. 

Coconut-shell-based GACs These have a high portion of micropores and present surface areas generally over 1000 m2/g and apparent densities of about 0.50 g/cm3. Being manufactured mainly from vegetative material, they do not exhibit the fully developed pore structure of coal-based carbons. They are used in both vapor- and liquid-phase applications. Coconut shell-based carbon is slightly more expensive to produce than coal-based GAC, since about only 2% of the raw material is recoverable as GAC, versus 8-9% for coal-based carbons. In Table 4.1, the basic properties of common materials used in the manufacture of activated carbon ate presented. 

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). 

Activated carbons for use in liquid-phase applications differ from gas-phase carbons primarily in pore size distribution. Liquid-phase carbons have significantly more pore volume in the macropore range, which permits liquids to diffuse more rapidly into the mesopores and micropores (69). The larger pores also promote greater adsorption of large molecules, either impurities or products, in many liquid-phase applications. Specific-grade choice is based on the isotherm (70,71) and, in some cases, bench or pilot scale evaluations of candidate carbons. 

The total activated carbon consumption for liquid-phase applications in the United States in 1987 was estimated to be about 76,700 t, which accounted for nearly 80% of the total activated carbon use. The consumption by application is summarized in Table 5 (74). 

Potable Water Treatment. Treatment of drinking water accounts for about 24% of the total activated carbon used in liquid-phase applications (74). Rivers, lakes, and groundwater from wells, the most common drinking water sources, are often contaminated with bacteria, viruses, natural vegetation decay products, halogenated materials, and volatile organic compounds. Normal water disinfection and filtration treatment steps remove or destroy the bulk of these materials (75). However, treatment by activated carbon is an important additional step in many plants to remove toxic and other organic materials (76—78) for safety and palatability. 

In the liquid phase applications, the solid acts either as an acid catalyst or as a dessicant. 

These considerations can be extended to reversible processes. They also apply to single phase, liquid systems. For the case, rather common in heterogeneous catalysts, in which one reactant is in a gas phase and the others and the products are in a liquid phase, application of the principles given above is straightforward provided that there is mass transfer equilibrium between gas phase and liquid phase, i.e., the fugacity of the reactant in the gas phase is identical with its fugacity in the liquid phase. In such case, a power rate law for an irreversible reaction of the form... 

Adhesion has been achieved on these oxides through a variety of silane precoat treatment processes. Solutions of y-aminopropyltriethoxysilane, y-methacryloxytrimethoxysilane, and y-glycidoxytrimethoxysilane applied to thermal oxide substrates all improved resist image adhesion for conventioanl positive photoresists Mallinckrodt Multisurf also works well. Conventional liquid-phase application of HMDS, however, was not adequate for the latter three tougher substrates listed above it did provide adequate photoresist adhesion for thermal oxides, however. For the last three substrates, a double... 

Device designed primarily with liquid phase applications in mind, but can be used for gas phase sensing. dWith slight modification by the grating. 

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). 

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