Dynamic adsorption capacity

Commercially available ACs were conformed as honeycomb monoliths with a magnesium silicate clay as binder. The textural and mechemical properties of the raw materials and the monolith composites were determined. These results were analysed together with the dynamic adsorption capacities towards o-dichlorobenzene (o-DCB) a chlorinated probe molecule used to simulate a dioxin. With this data, criteria by which the dynamic adsorption capacity could be related to the textural properties of the adsorption imits were established. 

All of the monoliths produced were dried at 150°C and fractions subsequently heat-treated at either 500°C or 850°C in a nitrogen atmosphere. The static and dynamic adsorption capacities of these monolithic composites at 30°C were determined using o-DCB as a probe molecule. This molecule was chosen since it may be considered as approximately corresponding to half a molecule of tetrachloro-dibenzene dioxin (TCDD), the most toxic isomer of the dioxin family. 

From the results shown in Table 3 and the textural characteristics of the adsorbents presented in Table 2 several conclusions may be drawn. Under these dynamic adsorption conditions with a contact time of <1 second the most favourable conditions lead to the amount adsorbed before breakthrough being equivalent to the micropore volume of the adsorbent. Thus, it would appear that the organic is stored in the micropores. However, this condition may only be met if other textural properties of the monolithic adsorbent are also present. For samples ChFc-m/a and ChFc-m/b, with external areas of c.l20 m g the dynamic adsorption capacity was equal to the micropore volume. But for sample ChFc-m/c with an external area of 90 m g the dynamic adsorption capacity was only 75% of the micropore volume. Thus, a high external area was necessary. This was confirmed by the results obtained with NoGp-m/b and NoGp-m/c where the dynamic capacity was equal to the micropore volume even when the external area was reduced to 110 m g. ... 

For sample EAAw-m/b although the external area was 120 m g the dynamic capacity was only c. 50% of the micropore volume. However, this sample had a narrow threshold diameter (0.9 pm) compared to the previous two series. Thus, it would appear that the presence of wide pores that aid internal diffusion into the monolith walls was also important. This was confirmed by the results with samples EAN-m/b and EAN-m/c that only gave dynamic adsorption capacities of 50% and 1% of their micropore volumes, respectively. These samples had low external areas 90 m g and 50 mV and narrow threshold diameters (0.3 pm). 

The dynamic adsorption capacity of activated carbon containing monoliths has been shown to be equivalent to the micropore volume. However, this condition can only be met when the external area is above c. 100 m g" and the threshold diameter wide. In systems with no micropore volume or poor internal diffusion due to a low external surface area and narrow threshold diameter the breakthrough point is reached when c. 9% of the external area is covered. Future work will concentrate on using higher linear velocities and adsorption temperatures md different monolith geometries (wall thickness and channel width) in order to study the internal diffusion limitations of these types of adsorption units. 

The dynamic adsorption capacity of the annular bed was calculated by integrating the concentration vs. time curves up to a point close to the dynamic equilibrium time. Typical results of the dynamic adsorption capacity as a function of the SO2 inlet partial pressure, the feed flow rate and the feed concentration are shown in Figures 3 to 5. The effect of temperature on the dynamic adsorption capacity is shown in Figure 6 where the higher temperature data were taken from reference (5). 

The efficiency of the dynamic adsorption process was defined as the ratio of the dynamic adsorption capacity to the static... 

Figure 3. The effect of SO2 concentration on the dynamic adsorption capacity...

The dynamic adsorption capacity of the bed ranged from 23.2 mg SO2 per gram of adsorbent, at 890 mm Hg, 180°C and 7 liters/minute, to 101.6 mg SO2 per gram of adsorbent, at 1610 mm Hg, 100°C and 5 liters per minute. It increased with the SO2 feed concentration, or partial pressure, and decreasing flow rate, as can be seen in Figures 3 to 5. The dynamic adsorption capacity decreased markedly with increasing temperature as can be seen in Figure 6. 

The effect of temperature on the efficiency of dynamic adsorption at constant total pressure is illustrated in Table I. As can be seen, the efficiency reaches a maximum value at 140°C at each level of the total pressure. This result may be attributed to two opposing effects. Increasing the temperature increases the adsorption rate but decreases the dynamic adsorption capacity. At high temperatures the second effect becomes more dominant resulting in lower efficiencies. 

The static and dynamic adsorption capacity of the ACF increased on impregnation with PABA. The breakthrough time for formaldehyde and the amount adsorbed also increased on impregnation. The removal of formaldehyde involved both physisorption and chemisorption. The chemisorption was suggested to be due to the introduction of the amino groups in the impregnant. 

In practical operations, maximum capacity of adsorbent cannot be fully utilized because of mass transfer effiects involved in actual fluid-solid contacting processes. In order to estimate practical or dynamic adsorption capacity, however, it is essential, first of all. to have information on adsorption equilibrium. Then kinetic analyses are conducted based on rate processes depending on types of contacting processes. The most typical of the rate steps in solid adsorbents is the intraparticle diffusion which is treated in the next chapter. 

When solar dryers are integrated into the complex energy system of a farm, adsorbent beds can also be utilized as auxiliary units. Adsorbent materials have to be regenerated for exploiting their dynamic adsorption capacities. The regeneration temperature is... 

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