Gases adsorption onto solids

Each time a gas is close to a solid, it binds to the surface of this solid this is the adsorption phenomenon. More precisely, there are two types of adsorption  

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Adsorption onto solid beds is another method of acid gas removal from reformer off-gas. Typically, adsorption beds are best suited for low to medium concentrations of H2S and can be made selective for its removal. This type of operation is classified into two types of processes which are nonregenerable and regenerable. The nonregenerable packed beds usually operate at temperatures above 350°C and are made out of ZnO which reacts irreversibly with H2S. These processes are capable of reducing the sulfur content to less than 0.1 ppm. 

On the other hand, the Kelvin equation has been extensively used in research on gas adsorption onto porous solids (see Sections 8.4 and 8.5) and capillary condensation. 

Examples of gas adsorption onto porous solids which obey the Langmuir equation are CH4 - sodalite and Ar - sodalite (Barrer and Vaughan 1971). Other systems such as Kr - carbon (Sykes and Thomas 1960) and QHs - 5A zeolite (Ruthven and Loughlin 1972) show apparent conformity to the Langmuir equation, as indeed do many other gas-solid systems, but on closer examination reveal departures, especially at coverages approaching saturation of the surface and at raised temperatures. Linearization of the Langmuir equation, of which one form is... 

One way to control gaseous pollutants like SO2 and SO3 is to remove the gases from fuel exhaust systems by absorption into a liquid solution or by adsorption onto a solid material. Absorption involves dissolving the gas in a liquid while adsorption is a surface phenomenon. In each case, a subsequent chemical reaction can occur to further trap the pollutant. Lime and limestone are two solid materials that effectively attract sulfur dioxide gas to their surfaces. The ensuing chemical reaction converts the gaseous pollutant to a solid nontoxic substance that can be collected and disposed or used in another industry. 

The adsorption of inert gases onto solid materials represents the most widely used method for the determination of surface area, although other methods are available [6,7]. The BET method, developed by Brunauer, Emmett, and Teller [8], is generally used for gas adsorption surface area measurements. 

A typical N2 adsorption measurement versus relative pressure over a solid that has both micropores and mesopores first involves essentially a mono-layer coverage of the surface up to point B shown in isotherm IV (lUPAC classification) in Figure 13.1. Up to and near point B the isotherm is similar to a Langmuir isotherm for which equilibrium is established between molecules adsorbing from the gas phase onto the bare surface and molecules desorbing from the adsorbed layer. The volume of adsorbed N2 that covers a monolayer volume, hence the surface area of N2 can then be determined from the slope of the linearized Langmuir plot when P/V is plotted against P ... 

As already mentioned, the first step in any heterogeneous catalytic reaction is the adsorption of a gas molecule onto a solid surface. Adsorption heat measurements can provide information about the adsorption process not available using other surface analytical tools. For example, differential heat measurements can provide valuable insights into sites distribution on the catalyst surface as well as quantitative information on the changes in catalyst particle surface chemistry that result from changes in particle size or catalyst support material [148-150],... 

If an SOC is absorbed into a liquid organic layer on the particle, a relationship between Kp and pL that is similar to that developed for adsorption onto a solid can be derived (Pankow et al., 1994a, 1994b). In this case, the gas-particle partitioning coefficient for the / th compound is defined as... 

Powder injection applies many of the principles of pulmonary delivery of dry powders to the lungs The drug has to be in the form of very small particles, is dispensed from a reservoir, and is delivered as an aerosol i.e., particles are dispersed in a gas. Liquid or dissolved drug can be delivered by precipitation or adsorption onto carrier particles. The big difference with pulmonary delivery is the momentum at which the particles are delivered. Driven by a high-pressure helium gas stream, the particles travel fast enough to penetrate the outer layer of the skin, the stratum corneum. The design of devices to deliver needle-free injection of solids was pioneered by researchers at the University of Oxford who founded PowderJect Pharmaceuticals PLC in 1993 (now PowderMed Ltd.) to develop the only powder-based technology so far. Since that... 

Figure 7.42 Types of gas sorption isotherm - microporous solids are characterised by a type I isotherm. Type II corresponds to macroporous materials with point B being the point at which monolayer coverage is complete. Type III is similar to type II but with adsorbate-adsorbate interactions playing an important role. Type IV corresponds to mesoporous industrial materials with the hysteresis arising from capillary condensation. The limiting adsorption at high P/P0 is a characteristic feature. Type V is uncommon. It is related to type III with weak adsorbent-adsorbate interactions. Type VI represents multilayer adsorption onto a uniform, non-porous surface with each step size representing the layer capacity (reproduced by permission of IUPAC).

Gas chromatography achieves separation based on solute partitioning into a thin liquid coating or adsorption onto a solid stationary phase material, whereas LC uses these and other mechanisms to achieve separation. The type of stationary phase and, in some cases, mobile phase determines the mechanism to achieve separation. Descriptions that follow are of the mechanisms and materials used most often. 

The extent of adsorption of gases onto solid surfaces can be determined experimentally using a wide variety of apparatus and techniques, and the literature on this subject is extensive. In general, measurements fall into one of two categories either the volume of the gas adsorbed is determined manometrically, or gravimetric methods are used, where the mass adsorbed on the solid is determined directly. 

The above adsorption isotherm equation is known as the Langmuir equation. Originally it was derived by other means and for another system, namely for the adsorption from the gas phase onto a solid adsorbent. The surface of the latter contained fixed sites onto which the attachment of molecules of adsorbing substance occurred (i.e., the localized adsorption took place). 

PROBABLE FATE photolysis very little specific data, but photolysis may claim some of the dissolved compound, atmospheric and aquatic photolytic half-life 4.4-13 hrs, subject to near surface, direct photolysis with a half-life of 4.4 hrs, if released to air, it will be subject to direct photolysis, although adsorption may affect the rate, reaction with photochemically produced hydroxyl radicals gives an estimated half-life of gas phase crysene of 1.25 hrs oxidation chlonne and/or ozone in sufficient quantities may oxidize chrysene, photooxidation half-life in air 0.802-8.02 hrs hydrolysis not an important process volatilization probably too slow to compete with adsorption as a transport process, will not appreciably evaporate sorption adsorption onto suspended solids and sediment is the dominant transport process if released to soil or to water, expected to adsorb very strongly to the soil biological processes short-term bioaccumulation, metabolization and biodegradation are the principal fates... 

In addition to gases produced naturally in the environment, estuaries tend to be enriched in byproducts of industry and other human activity. A few studies have investigated volatile organic pollutants such as chlorinated hydrocarbons (chloroform, tet-rachloromethane, 1,1-dichloroethane, 1,2-dichlor-oethane, 1,1,1-trichloroethane, trichloroethylene and tetrachloroethylene) and monocyclic aromatic hydrocarbons (benzene, toluene, ethylbenzene, o-xylene and m- and p-xylene). Concentrations of VOCs are controlled primarily by the location of the sources, dilution of river water with clean marine water within the estuary, gas exchange, and in some cases, adsorption onto suspended or settling solids. In some cases (for example, chloroform) there also may be natural biotic sources of the gas. Volatilization to the atmosphere can be an important cleansing mechanism for the estuary system. Since the only estuaries studied to date are heavily impacted by human activity (the Elbe and... 

Thermal desorption from a solid phase microextraction (SPME) fiber has shown considerable potential for selectively introducing semivolatile chemicals into an IMS. ° The SPME approach is a simple design patterned after the early platinum wire introduction thermal desorption system described. With SPME, semivolatile compounds are extracted by either absorption or adsorption onto a nonvolatile polymeric coating or solid sorbent phase that has been coated onto a small fiber. Normally, the adsorption liber is housed in the needle of a syringe to permit puncture of a sample bottle septum and to protect the fiber from contamination during transfer of the fiber from the sample to the IMS instrument. After the analytes are adsorbed onto the SPME fiber, the fiber is retracted into the needle and then injected in a normal syringe technique such that the fiber is extended into the heated region of the IMS and the analytes are desorbed from the fiber into the clean carrier gas of the IMS. 

Acetone can be removed from acetone-air mixtures using simple countercurrent cascades, by adsorption onto charcoal (Foust et al. 1980). We wish to find the required number of equilibrium stages to reduce a gas stream carrying 0.222 kg acetone per kg air to a value 0.0202 kg acetone per kg air. Qean charcoal = 0) enters the system at 2.5 kg/sec, and the air rate is constant at 3.5 kg/sec. Equilibrium between the solid and gas can be taken to obey the Langmuir-type relationship... 

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