Hydrocarbon adsorption and separation

Zeolites are widely used to adsorb hydrocarbons, including unwanted volatile organic compounds (VOCs) such as solvent vapour and unburnt fuel in automobile engines. They are also used in refineries and petrochemical plants to separate and purify products on a large scale. The zeolites are chosen on the basis of cost, capacity, resistance to process conditions and, particularly in petrochemical applications, for their ability to act as true molecular sieves. 

Zeolites also have a key role as hydrocarbon adsorbents in the context of petrochemical production, in the separation of specific compounds or classes of compounds from complex mixtures that derive from the refining of crude oil 

The separation of linear and branched alkanes is also of importance in the process known as dewaxing, in which the removal of normal alkanes makes the product hydrocarbon less viscous and reduces the so-called pour point temperature. Such processes can be combined with catalytic isomerisations to optimise the value of oil fractions (Chapter 8). Linear paraffins are also separated using a zeolite-based process from kerosene fractions to give reactants for the synthesis of linear alkylbenzene sulfonate anionic surfactants, which are both cost effective and biodegradable. 

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CO2, and in hydrocarbon adsorption and separation. Adsorption-based separation is an important process for hydrocarbon mixtures in the petrochemical industry. While zeolite-based molecular sieves have been successfully applied for such commercial applications since the middle of the past century, " MMOFs offer distinct advantages, thanks to their adjustable pore dimensions, unique pore geometries, and functionalized pore surfaces. In this chapter, we give a concise summary of the various aspects of hydrocarbon and alcohol adsorption and separation using MMOFs as the adsorbent. 

C2H4), alkynes (general formula C H2 , 2, example acetylene, C2H2) and aromatic hydrocarbons, such as toluene and xylenes. Among the many types of hydrocarbon adsorption and separations, separations involving small hydrocarbons (Cj C4), isomers of hexane, and Cg-aromatics (orr/ro-xylene or oX, wcra-xylene or wX, /7ara-xylene or / X, and ethyl benzene or EB) have received considerable attention because of their role in various... 

Chromatographic Analysis.—Chromatographic adsorption-analysis, the most delicate method of separation of closely related compounds, depends on the simultaneous adsorption and separation of mixtures of organic compounds, such as natural dyes, biochemical products, isomerides, hydrocarbons, etc., in suitable solvents such as petroleum, ether, chloroform, carbon disulphide and water. 

AC materials as porous materials with very high surface areas and large pore volume have been widely used in deodorization, decolorization, purification of drinking water, treatment of wastewater, and adsorption and separation of various organic and inorganic chemicals. Recently, some carbon materials have been reported for adsorptive desulfurization of liquid hydrocarbon fuels. 

A sharp increase of the retention volumes Vn, and heats of adsorption of saturated hydrocarbons on tetramethylammonium montmorillonite in the Henry region as compared with natural mineral (Table 5) shows that the adsorption and separation of these molecules take place in chink-like micropores having the width of 0.45 nm on the (internal) surface of (CH3)4N - montmorillonite [39]. 

Adsorption and separation processes involve also the active sites existing on the external surface of (CH3)4N - montmorillonite, their role being more important in the case of adsorption of isoparaffins and cyclohexane molecules. This is indicated by a significantly smaller differences between the specific retention volumes of iso- and cycloparaffins on natural and tetramethylammonium samples than the difference in Vm values characteristic for n-paraffines on the same adsorbents. Thus, the tetramethylammonium montmorillonite adsorbs n-alkanes selectively from the mixtures containing iso- and cycloparaffines, which is confirmed by the values of relative retention volumes for such hydrocarbon pairs as, for instance, n-heptane / 2,4- dimethylpentane. These can be easily calculated from the data presented in Table 5. 

The anomalous changes in the adsorption and thermodynamic characteristics of hydrocarbons on montmorillonite indicates explicitly that, at low surface coverage, the adsorption and separation of hydrocarbons take place in the surface micropores on the lateral faces of the mineral. With the increase in the amount of adsorbed modifier, the processing degree of the mineral packets by the organic substance also increases, resulting in the increase of the retention volume of hydrocarbons, and corresponding increase of Henry s... 

The ability of microporous solids to act as high-capacity molecular sieves has long been exploited in a wide range of applications in adsorption and separation. The electrostatic interactions of the traditional cationic forms of aluminosilicates are well suited for the uptake of polar molecules (such as H2O) and are also able to separate oxygen from air. The development of microporous solids with varied chemistry has enabled adsorption and diffusion properties to be finely tuned for particular technologies. Pure silica zeolite polymorphs such as silicalite have particular importance, because they enable separation on the basis of a different range of polarity and on molecular size the absence of aluminium in the framework also prevents the presence of unwanted acidity, so adsorbed hydrocarbons do not undergo any catalytic transformation. 

In this final section of this report, we would like to draw a structure property relationship in the FMOF series. Now, looking through the applications of the FMOFs, many of them has been exploited for gas adsorption, storage, gas/hydrocarbon separation, and so on. In this regard, depending on the end applications of these MOFs, they are divided into two parts (i) FMOFs for gas adsorption and separation and (ii) FMOFs for hydrocarbon uptake. 

Physisorption-based adsorption and separation processes are of primary interest for various applications, including H2 storage, carbon capture and sequestration, and hydrocarbon separations. Physisorption based separation of adsorbate mixtures can occur through a variety of mechanisms, including equilibrium, steric, kinetic, or some combination of these in more complex systems. 

The adsorption and separation properties of a particular adsorbent largely depend on the adsorbate, including its morphology, composition, polarity, polarizability, diffusivity, and other properties. In the following sections we will describe adsorption and separation in MMOFs, focusing mainly on industrially important hydrocarbons and alcohols. We also discuss the concepts of commensurate and incommensurate adsorption in MMOFs and their implications toward adsorption-related properties. 

Because carbon has a natural affinity for adsorption of heavy hydrocarbon species and polar molecules, CMS membranes need to be used at a sufficiently high temperature to eliminate contribution/interference of the adsorption. In contrast, strong adsorption of heavier molecules may be used to separate those species by adsorption as discussed earlier by the SSF mechanism (Rao and Sircar, 1993b). The SSF carbon membranes typically have pore dimensions much greater than those needed for CMS membranes since the separation is based on the adsorbed species effectively blocking permeation of other components (Fuertes, 2000). Carbon membranes are resistant to contaminants such as H2S and are thermally stable and can be used at higher temperatures compared to the polymeric membranes. For the synthesis gas environment, the hydrothermal stability of carbon in the presence of steam will be a concern limiting its operation temperature. 

Adsorption chromatography on silica is well suited to the separation of less polar compounds such as polyaromatic hydrocarbons, fats and oils, and for the separation of isomers or compounds with differing functional groups. 

Separation can also be accomplished by solvent extraction, adsorption, and crystallization. Solvent extraction is accomplished by selectively dissolving certain hydrocarbon components. Adsorption is similar to solvent extraction but uses a solid to separate out various components selectively based on their tendency to adhere to the surface of the solid adsorbent. Crystallization uses the differing melting points of the components during cooling, which causes some of its compounds to solidify or crystallize, and separate out of the liquid. 

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