Adsorption of hydrocarbons

Fig. Ill-16. Surface tension lowering of water at 15°C due to adsorption of hydrocarbons. , n-pentane A, 2,2,4-trimethylpentane O, n-hexane x, n-heptane A, n-octane. (From Ref. 133.)...

Fig. 3. Adsorption of hydrocarbons by 2eobtes is much greater for unsaturated hydrocarbons whose molecules contain double or triple bonds. From top to bottom, the curves show adsorption (at 150°C) of propjiene, ethylene, acetjiene, and isobutjiene (unsaturated) and propane, ethane, and methane...

Mangun, C. L., Daley, M. A., Braatz, R. D. and Economy, J., Effect of pore size on adsorption of hydrocarbons in phenolic-based activated carbon fibers. Carbon, 1998,36(12), 123 129. 

The adsorption of hydrocarbons by activated carbon is characterized by the development of adsorption isotherms, adsorption mass and energy balances, and dynamic adsorption zone flow through a fixed bed. 

When type X is utQized, in any of its ion exchange forms, for dehydration or possibly for sweetening (sulfur removal), there is little likelihood that the intracrystalline diffusion will be the dominant resistance to mass transfer. Large aromatic sulfurs would of course be an exception. When type X is used for adsorption of hydrocarbons or aromatics then it is possible that the micro-pore diffusion might dominate. When type A is used there is always a distinct possibility that intra-crystalline diffusion will be slow and may dominate the mass transfer, even for relatively small molecules. This is especially true when the chosen structure is a K A or type 3A. Selection of other small pore structures, for separations or purification applications can also create situations where the dominant resistance is found in the crystaUites. 

Normura,. and Shima, H. (2008) Adsorption of hydrocarbons and formations of carbocations over zeolites smdied by IR spectroscopy. 

Heats of adsorption measurements do not lead to very specific interpretation since the isosteric heat of adsorption (AH) arises from both nonspecific interactions, which occur in all cases of adsorption, and from specific interactions with the hydroxy groups nevertheless, valuable conclusions about the binding forces can be deduced. Saturated hydrocarbons, e.g., n-pentane, have a value of — AH of 8.0 kcal/mole, while saturated ethers have values of around 16 kcal/mole.14 Probably dispersion forces only are involved in the former case and additional specific interaction with the silanol-OH occurs in the second case. On graphite, where there is no specific interaction, the heats of adsorption of hydrocarbons and ethers are very similar.17 The heat of adsorption of furan (11 kcal/mole) is 5 kcal/mole less than that of tetrahydrofuran this again indicates the effect that delocalization of electrons by the double bonds has on the binding forces.14... 

In Table IV is presented a brief review of the literature relating to surface phenomena. In recent years much interest has been shown in the adsorption of hydrocarbons upon solids. No effort has been made to include references to analytical methods based upon selective adsorption. This process is often employed in the purification of hydrocarbons and in some cases is superior to fractionation. The work of Lewis and Gilliland (45-47) reviews the status of the techniques and data relating to the adsorption of petroleum upon solid surfaces. The increasing importance of such techniques is evidenced by the recent development of commercial processes (5, 80) for the separation of hydrocarbons based on adsorption. 

Characterization Data for Oxide-Supported Metal Catalysts Employed by Research Groups Studying the Adsorption of Hydrocarbons by Infrared (IR) or Raman (Ra) Spectroscopy... 

G. Engelhardt, M. Magi, and E. Lippmaa, Workshop proceedings, Adsorption of hydrocarbons in microporous adsorbants II, Eberswalde, GDR, 1982, 2, 1. 

During the first minutes of the experiment, measurement of the coke content is disturbed by several transient effects adsorption of hydrocarbons, pressure stabilization and gradual displacement of the pretreatment gas by the feed. To determine the total amount of coke deposited on the catalyst, the coked catalyst is stabilised at the end of the experiment in the pretreatment gas. The weight difference between the uncoked catalyst and the coked catalyst gives the total amount of non-desorbable products. 

The adsorption of hydrocarbon molecules on Si surfaces is an interesting topic of study under various viewpoints. For example, a thin hydrocarbon film coating Si may be applied as a low dielectric in microelectronics and may passivate the surface if covalent bonds are formed between Si atoms and the adsorbate species. Further, unsaturated hydrocarbons play an important role as precursor species for chemical vapor deposition (CVD) of diamond - like films on the Si surface, and of silicon carbide (SiC). 

These procedures make use of the differences between (a) the strong specific interactions given by certain polar molecules (e.g. n-butanol) with polar surface sites and (b) the non-specific interactions given by the adsorption of hydrocarbon molecules (e.g. n-C32H66) on the graphitic basal plane. 

The activation energy for adsorption of hydrocarbons on nickel is about 40 kJ/mole (76) and the activation energy for hydrocracking is in the range 160-200 kJ/mole (77). This means that below the temperature, Tp, at which Ta > rn, i.e. rp > 0, gum formation takes place. To ensure trouble free operation the prereformer must operate within the temperature window of T < T < Tc. 

Surface Area (SA, mf/g). The snrface area is the measure of the catalyst activity (as long as the same catalyst types are compared) and has a strong effect on the performance of an Flnidized Catalytic Cracking Unit (FCCU). High surface area also results in increased adsorption of hydrocarbons, and a higher steam rate in the stripper may be reqnired. The zeolite and matrix surface areas of a catalyst can be analysed separately. Matrix pores provide access of the hydrocarbons to the active zeolite sites and matrix surface area often correlates with the bottoms conversion activity of the catalyst or the Light Cycle Oil (LCO) yield at constant conversion. 

The examples above demonstrate satisfactory agreement between the calculated results and the experimental data. This shows that the initial approximate assumptions are reasonable. In most cases, the one-term Equation 5 is applicable for the description of adsorption equilibria on zeolites, particularly for zeolites with small voids (zeolite L, chaba-site, erionite, mordenite) for which, in adsorption of hydrocarbons, n = 3 as a rule. The concept of the volume filling of micropores makes it possible to describe adsorption equilibria over sufficiently wide ranges of temperatures and pressures (using fs instead of Ps) with the use of only 3 experimentally determined (usually from 1 adsorption isotherm for the average temperature) constants. Wo, A, and n. The constant n requires only a tentative estimation, since it is expressed by an integer. 

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