Pore size distribution for activated carbons

Figure 6. Pore size distribution for activated carbon (left scale GAC, right scale standard material)...

Fig. 1 Pore size distributions for activated carbon, silica gel, activated alumina, two molecular sieve carbons (MSCs), and zeolite 5A. (From Ref ll)...

Setoyama, N., Suzuki, T., and Kaneko, K. (1998). Simulation study on the relationship between a high resolution a -plot and the pore size distribution for activated carbon. Carbon, 36, 1459—67. 

Figure 3. Pore size distribution for activated carbons...

Table 4.1 Effect of solvent used for dehydrochlorination of the polymer precursor on pore size distribution in activated carbon. Experimental conditions chlorinated PVC was treated with KOH at 20°C for 5 h thermal treatment conditions carbonization at 400°C for 30 min followed by activation with CO at 900°C for 5 min...

Fig. 2 Pore size distribution of activated carbon Chemviron BPL 4x10 with diffusion regimes for c-hexane...

Activated carbons for use in Hquid-phase appHcations differ from gas-phase carbons primarily in pore size distribution. Liquid-phase carbons have significantly more pore volume in the macropore range, which permits Hquids 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 Hquid-phase appHcations. Specific-grade choice is based on the isotherm (70,71) and, in some cases, bench or pilot scale evaluations of candidate carbons. 

The SAXS/TGA approach has been demonstrated to be a useful technique for time-resolution of porosity development in carbons during activation processes. Qualitative interpretation of the data obtained thus far suggests that a population balance approach focusing on the rates of production and consumption of pores as a function of size may be a fruitful approach to the development of quantitative models of activation proces.ses. These then could become useful tools for the optimization of pore size distributions for particular applications by providing descriptions and predictions of how various activating agents and time-temperature histories affect resultant pore size distributions. 

Figure 5. Typical pore size distributions for powdered activated carbons.

Chemical activation is generally carried out with uncarbonized feedstocks, gas activation generally with carbonized feedstocks. The aim of both processes is to convert the particular feedstock into a material with a high specific surface area (BET values between 400 and 2500 m /g) and the optimum pore size distribution for the required application. There are three types of pores ... 

Commercial activated carbons are generally produced in granular, bead, pellet, or extrudate forms. The particles contain a complex network of meso-macro pores (pore diameters ranging between 30 A to several microns) and micropores (pore diameter <30 A) of different shapes and sizes. The larger pores act as arteries for the gas molecules to be transported from the external gas phase to the mouth of the micropores. Most of the adsorption capacity of a gas on the carbon is created by adsorption within the micropores. Figure 22.2 shows the cumulative pore size distribution of the carbons of Table 22.2 [18]. They were also obtained from the manufacturers data sheet. 

Beside classical methods of pore size analysis, there are many advanced methods. Seaton et al. [161] proposed a method based on the mean field theory. Initially this method was less accurate in the range of small pore sizes, but even so it g ve a more realistic -way for evaluation of the pore size distribution than the classical methods based on the Kelvin equation [162]. More rigorous methods based on molecular approaches such as grand canonical Monte Carlo (GCMC) simulations [147, 163-165] and nonlocal density functional theory (NLDFT) [86, 146, 147, 161, 163-169] have been developed and their use for pore size analysis of active carbons is continuously growing. 

Fig. 13. Pore size distributions for the WV-A900, BAX 1500 and NP5 active carbons evaluated by using the NLDFT approach [171]...

Benzene adsorption it is a test mainly used for granular activated carbons. It provides very relevant information about the adsorptive capacity in gaseous phase. In many cases the adsorption-desorption isotherm is determined in order to obtain the pore-size distribution of the carbon. There is not a reference standard test, and some industries only use it for information to users. 

In this work we show that the pore size distributions of porous carbons, obtained from lignin pyrolysis and KOH activation, determined from molecular simulation and DFT are equivcdent. We suggest that the spurious minimum observed in the PSD of carbons obtained form inversion of the integral adsorption isotherms is due to artifacts of the model, such as the fact that, uniform size, infinite non-connected pores are considered. In such cases, for the pore width where transition Ifom two to three layers occurs, the difference in shape of the calculated adsorption isotherms results in an exclusion of some pore widths at the time of the inversion of the integral. Taking into account smooth changes between pore sizes, or pore connectivity may solve this problem. 

Figure 8. Pore size distributions for Ajax activated carbon from a) Fitting methane adsorption experiment data in Figure 8 and b) Using nitrogen at 77K.

A major difficulty in testing the validity of predictions from the DR equation is that independent estimates of the relevant parameters—the total micropore volume and the pore size distribution—are so often lacking. However, Marsh and Rand compared the extrapolated value for from DR plots of CO2 on a series of activated carbons, with the micropore volume estimated by the pre-adsorption of nonane. They found that except in one case, the value from the DR plot was below, often much below, the nonane figure (Table 4.9). 

---