Examples of Improving Performances of Porous Carbon Materials on Different Applications

3.1. POLLUTION CONTROL

In the last years, great attention has been paid to the removal of volatile organic compounds (VOC) at low concentration, since the presence of these pollutants is detrimental both for human health and environment, even at very low concentrations.16-19 As an example, the adsorption of benzene, toluene or their mixture at low concentration (200 ppmv for single VOC or 200 ppmv each for the mixture) have been studied at room temperature in a fixed bed reactor coupled to a mass spectrometer.20-22 Both AC and ACF have been studied and the effect of a thermal treatment to reduce most of the surface oxygen groups has been analysed.

Regarding porosity, not a clear conclusion about its importance on the VOC adsorption capacity at low concentration was found previously in the literature. This is due to the fact that porous texture characterisation of the activated carbons was not complete from the point of view of the narrow microporosity. Figure 1A shows the correlation found by our research group between the volume of narrow micropores (assessed by CO2 adsorption at 273 K) and the amount of benzene adsorbed, for both the AC20-22 and ACF. This correlation also exists for toluene. It is also important to underline that the adsorption capacities for benzene (as high as 34 g benzene/100 g AC) and toluene (as high as 64 g toluene/100 g AC) at 200 ppmv achieved by the AC prepared by hydroxide activation are higher than those of commercial samples and higher than those previously reported in the literature.21

Surface chemistry has also proved to be crucial for the adsorption capacity. Thus, Figure 1B compares the adsorption capacity of samples thermally treated, with a lower content of surface oxygen groups (including T in their nomenclature), and pristine carbons, showing that the reduction of the oxygen content in the carbons favours the adsorption capacity.21

Figure 1. (A) Relationship between the benzene adsorption capacity and the narrow micropore volume for AC and ACF; (B) Comparison between benzene adsorption for pristine and thermally treated samples.

Figure 1. (A) Relationship between the benzene adsorption capacity and the narrow micropore volume for AC and ACF; (B) Comparison between benzene adsorption for pristine and thermally treated samples.

3.2. METHANE AND HYDROGEN STORAGE

At a given pressure, a strong adsorption potential inside the micropores acting on gas molecules significantly increases the density of the adsorbed molecules in relation to the gas-phase density. This phenomenon can be exploited for enhancement of gas storage capacity through adsorption. This has been the main reason for the strong interest in using AC as a medium to reduce the pressure required to store gases such as methane and hydrogen. The search for ACs able to store large amounts of natural gas at a reasonable pressure (3.5-4 MPa), as substitute for natural gas compressed at much higher pressure (e.g. 21 MPa) has been very intense in the last years.23-26

In all previous studies23-26 it has been concluded that, in general, the higher the surface area (or micropore volume), the higher the methane adsorption capacity. In our research group, a systematic study of the performance of KOH-activated carbons in methane storage has been carried out. We concluded that, in addition to surface area and packing density, the micropore size distribution (MPSD) is also important.26 Thus, Figure 2 contains the methane uptake versus the apparent BET surface area corresponding to a series of KOH-activated carbons. The objective of this correlation was to extend the BET surface area range beyond 2,000 m2/g. The linear relationship is seen to reach a maximum for AC with a very high surface area. If only the apparent BET surface area or the micropore volume were responsible for the methane uptake, sample 5/1 (see Figure 2) should have a higher methane capacity than sample 3/1; however, the opposite behaviour is observed. These results can be explained analysing the porous texture in more detail. Thus, Table 1 contains the porous texture characterization results and the gravimetric methane adsorption capacities (up to 4 MPa) for the same samples. It can be concluded that the reason for a better performance of sample 3/1 compared to 5/1, lies in their very different MPSD, as evidenced by the values of Vdr n2-Vdr co2 (Table 1). Sample 3/1 has a narrower MPSD, which results in a higher methane uptake, consistent with the enhanced adsorption potential argument.16

0 500 1000 1500 2000 2500 3000 3500 BET (m2/g)

Figure 2. CH4 uptake at 298 K and 4 MPa in chemically activated carbons versus the BET surface area.

0 500 1000 1500 2000 2500 3000 3500 BET (m2/g)

Figure 2. CH4 uptake at 298 K and 4 MPa in chemically activated carbons versus the BET surface area.

TABLE 1. Porous texture characterisation and gravimetric methane adsorption capacity (up to 4 MPa) for two KOH AC.

Sample

BET

VDR N2

VDR CO2

VDR N2-VDR CO2

CH4 uptake

(m2/g)

(cm3/g)

(cm3/g)

(cm3/g)

(mmol/g)

3/1

2,758

1.35

0.72

0.63

12

5/1

3,350

1.48

0.67

0.81

11.6

From a practical point of view, to increase the methane adsorption capacity we need to develop not only micropore volume, but also we have to control carefully the micropore size distribution. Thus, samples with high surface areas and very narrow micropore size distribution are required for this application. For this reason, in our studies of methane storage, KOH instead of NaOH was used as the activating agent for the preparation of ACs due to the narrower MPSD that can be obtained by KOH.

In relation to high-pressure storage of H2 on activated carbons at room temperature, it must be remarked that this requirement is much more difficult than that of methane storage, due to the much lower H2 uptake (either per unit mass of AC or per unit volume). However, to get the maximum H2 uptake, the properties of the adsorbent that need to be optimised are very similar to those analysed for methane storage. More details of our results about hydrogen storage in carbon materials are not included here because a specific paper of our research group on this topic is also included in this book.

3.3. SPACE CRIOCOOLERS

The European Space Agency's (ESA) Darwin Mission is a future space interferometer dedicated to search for terrestrial planets in orbit around other stars. The interferometric imaging of astrophysical objects will be accomplished via four free-flying telescopes and a central hub. To guarantee proper mechanical stability, any vibration of the optical system (with its integrated 4.5 K cryocoolers) cannot be tolerated. To reach such low temperature, a two-stage vibration-free sorption He/H2 cooler has to be designed with a suitable AC.27 In this section, an example corresponding to the development of such an adsorbent is presented.

A sorption cooler has two parts: (i) a cold stage; and (ii) a sorption compressor. The cold stage consists of a counterflow heat exchanger and a Joule-Thomson expansion valve. This cold stage works as a typical refrigerator. The high-pressure gas needed comes from the sorption compressor. A sorption compressor can be described as a thermodynamic engine that transfers thermal energy to the compressed gas in a system without moving parts. Its operation is based on the principle that large amounts of gas can be adsorbed on certain solids such as highly porous ACs. The amount of gas adsorbed is a function of temperature and pressure. If a pressure contain-ner is filled with an adsorbent and gas is adsorbed at low temperature and pressure, then high pressure can be produced inside the closed vessel by an increase in the adsorbent temperature. Subsequently, a controlled gas flow out of the vessel can be maintained at high pressure by a further increase in temperature until most of the gas is desorbed.

ACs are obviously very interesting candidates for this application. They have to satisfy three essential requirements: (i) a large adsorption capacity per mass of adsorbent; (ii) a minimum void volume; and (iii) very good mechanical properties.

In order to optimise AC properties for this application, studies with samples prepared from different raw materials and using different activation processes have been carried out. To predict their adsorption performance, helium adsorption isotherms at different temperatures and pressures have been obtained. Figure 3 shows these isotherms corresponding to an AC prepared by anthracite activation. According to the conditions needed in the compressor stage, we are interested in obtaining a material with a maximum helium adsorption capacity at 2 bar and 50 K (adsorption stage) and a minimum adsorption capacity at 13 bar and 120 K (desorption stage). The best adsorption capacity has been obtained with an activated carbon monolith with relatively high development of porosity and, most importantly, with high narrow micropore volume assessed by CO2 adsorption at 273 K. This material also presents quite high density (0.7 g/cm3). A very high activation degree of the material is not desired because the density of the material becomes low. In addition this activated carbon monolith presents a low pressure drop and a low thermal expansion, it has very good mechanical properties and it is easy to machine, which makes this material promising for adsorption compressor applications.

Pressure (bar)

Figure 3. Helium adsorption isotherms at different temperatures corresponding to an activated carbon prepared by KOH activation of an anthracite. The numbers 1-4 correspond to each step of a complete cycle of a cell.

Pressure (bar)

Figure 3. Helium adsorption isotherms at different temperatures corresponding to an activated carbon prepared by KOH activation of an anthracite. The numbers 1-4 correspond to each step of a complete cycle of a cell.

0 0

Post a comment