Carbon-based solid sorbents

For more efficient utilization of MOFs sorbents, several hybrid systems based on MOFs with other solid sorbents have been investigated in the literature. The objective of having hybrid materials is to utilize the synergism between the two sorbents and therefore ultimately improve the overall performance in C02 separation. Moreover, sorbents such as activated carbons, graphenes, and CNTs provide the added feature of high surface area and easily functionalized sites which contribute to the tuning of the final properties of the composite... 

Although the preceding processes described for C02 separation are based on liquid absorbent solutions operating at low (cold methanol) to moderate temperatures (hot carbonate), C02 may also be separated by solid sorbents, typically operating at high temperatures, for example, base metal oxide such as CaO (Fan et al., 2005 Harrison and Peng, 2003) can absorb C02 at 600°C temperatures by reversible carbonation/decarbonation reaction ... 

Three general types of solid sorbents are mainly used for trapping VOCs in air inorganic sorbents like silica gels or molecular sieves, carbon-based porous materials and porous organic polymers. 

Therefore, the solid sorbent should ideally have a low affinity to water, a characteristic easily met by the porous organic polymers. In contrast, the carbon-based sorbents and molecular sieves as well as some inorganic sorbents show a comparatively high uptake of water. If such sorbents are used in high-humidity environments special measures have to be employed to remove water during or after sampling. Possible methods are ... 

Nowadays solid sorbents can be bought pre-cleaned and less effort is required before the first use. Nevertheless, all sorbents have to be cleaned thermally or by extraction to remove contaminants that formed during production, shipping or storage. For inorganic and carbon-based sorbents this is most easily accomplished by heating... 

The choice of the sorbent is dictated by the characteristics of both the analytes and their potential interferences. The sorbents most frequently employed here are silica, alkylsilane-modified silica (bonded phases), alumina, porous polymers (with and without ion-exchange groups) and carbon-based materials. One typical application is a method for the determination of hexavalent chromium in soils [10] using the on-line system depicted in Fig. 4.9. After USAL, the analytes in the leachate were directly determined or preconcentrated depending on their concentration. Concentration was performed by on-line solid-phase extraction using a laboratory-made minicolumn packed with a strong anion-exchange resin. The absolute limits of detection were 4.52 and 1.23 ng without and with preconcentration, respectively. 

There are many methods for the desulfurization of nature gas, which can be classified into dry desulfurization, wet desulfurization, and catalytic adsorption. In the dry desulfurization, some solid sorbents, such as iron oxide, zinc oxide, activated carbon (AC), zeolites, and molecular sieves, are used. In wet desulfurization method, liquid-phase chemical/physical solvent absorption systems are usually used for scrubbing H2S amine-based processes are subject to equipment corrosion, foaming, amine-solution degradation, and evaporation, and require extensive wastewater treatment. As a result, this sulfur removal technology is complex and capital intensive,44 although the processes are still employed widely in the industry. The desulfurization of coal gasification gas will be reviewed in detail in Section 5.5. In the catalytic-adsorption method, the sulfur compounds are transformed into H2S by catalytic HDS or into elemental sulfur or SOx by selective catalytic oxidation (SCO), and then, the reformed H2S and SOx are removed by the subsequent adsorption. 

On the other hand, hydrogen membranes and the reaction-based processes using solid sorbents can separate CO2 while operating at the elevated temperatures of the gasifier. Such methods utilize the carbonation/ calcination cycle of a solid metal oxide (MO). ... 

A PSA-based hydrogen purification unit can separate CO2 from hydrogen. In a PSA unit, carbon-based species, CO2, CO, and CH4, are accumulated. The tail gas from a PSA can be burned to recover heat. If CO2 capture is desired to reduce greenhouse gas emission, a CO2 absorption-based system would be attractive since it produces pure hydrogen as well as pure carbon dioxide. There are basically two types of CO2 removal processes CO2 separation based on a low temperature liquid absorbent solution, and CO2 separation based on solid sorbents at a temperature of about 600°C [43]. 

Unlike a Henry s law constant, whose value depends mainly on temperature, the value of a solid-water partition coefficient depends on the type of solid, and may also be affected by factors such as pH and ionic strength. There is considerable uncertainty in predicting Kd values for inorganic chemicals often must be measured in the laboratory. For many organic chemicals, however, a method for approximating values exists, based on the properties of the solid sorbent, particularly its concentration of organic carbon, and the properties of the sorbate. The sorbate properties necessary for predicting values are described below further estimation techniques are presented in Section 3.5. 

Perrone et al. (2001) modelled Ni(II) adsorp-tion to synthetic carbonate fluoroapatite (CaI0 ((P04)5(C03))(0H,F). The solid phase had a pHIEP of 6.3 and a ZPC of 6.4 with an SSA of 8.8m2/g, an estimated sorption site density of 3.1 sites/nm2. They conducted 8-day isotherms in closed vessels at Ni concentrations of 5 x 10-10 to 1 x 10 8 M, constant I (0.05, 0.1 or 0.5 M), constant solid phase concentrations of 10 g/dm3 at pH values of 4 to 12. As Ni sorption occurred, no significant release of Ca was seen. Sorption was reversible. Rather than precisely characterize surface functional groups, they elected to describe their sorbent surfaces using acid-base reactions for the average behaviour of all sites involved in protonation and deprotonation. Potentiametric titration data were used to estimate the constants with the FTTEQL computer code ... 

Studies on solid supports have shown that tlie pore size of the silica-based sorbents is of little importance in MSPD, but it should be considered as could vary with the sample. Particle size is of greater importance since particles as small as 3-20 m can lead to extended solvent elution times and plugged MSPD columns. However, 40 m particles with 60 Angstrom pores have been used extensively and successfully. Sorbents that have a blended range of particle size such as 40-100 m work equally well and can be used in most applications (101, 103). These materials also tend to be less expensive. Depending on the application, analysts can also use non-end-capped materials and materials with a range of carbon loading. Different applications may benefit, suffer, or be unaffected by these parameters, but workers should consider them to obtain the best extraction efficiency and the cleanest sample. 

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