Practical Aspects

During actual use of a gas mask canister the flow through the activated carbon bed is pulsating, i.e. during inhalation air is drawn through the canister, while during exhalation there is a stand-still of air in the canister. In approval tests for filters mostly a constant flow pattern is applied. The time-averaged gas velocity has to be equal when applying a constant continuous flow or a pulsating flow for a fair comparison of the breakthrough behavior for both cases. Figure 1 shows the respiration pattern with a breathing cycle time of 4 s, that is, during 2 s air is flowing through the canister and the next 2 s the air is still.

*To whom correspondence should be addressed. E-mail: [email protected]

Breakthrough measurements applying continuous and pulsating flows were performed using toluene on shallow activated carbon beds (Norit R1). Toluene is a good representative for the type of vapors for which activated carbon forms a suitable adsorbent. Pulsating flow was studied by using the positive halves of a sinusoidal flow pattern, which closely resembles the actual breathing pattern. Figure 2 shows breakthrough profiles of toluene for constant and pulsating flow, where a bed length was applied of 1.5 cm, a toluene concentration of 7.5 g/m3, and the time averaged superficial gas velocity was 0.127 m/s. The observed difference between the two breakthrough profiles is very typical: pulsating (breather) flows are found to be less favorable for the breakthrough behavior compared to a constant flow pattern.1 Furthermore, the difference between the two becomes larger for low concentrations. Since in gas masks the low concentration range is of extreme importance in case of high toxic compounds, this difference becomes even more important. The influence of flow rate on the mass transfer from the bulk gas phase to the surface of the adsorbent particles is ultimately responsible for the difference in breakthrough times.1 Clearly, pulsating flow is a factor to take into account if one requires a reliable assessment of the performance of an activated carbon canister in practice.

Figure 1. The respiration pattern is represented by a sine wave pattern (solid line), which closely resembles the actual breathing pattern. The dashed line is the corresponding constant flow.

A second practical aspect that is discussed is the risk of desorption.2 There exists a tendency in industry to equip respiratory protective devices (RPD) with a blower device, which turns them into power assisted devices. The function of the blower is to continuously draw air through the canister

Figure 1. The respiration pattern is represented by a sine wave pattern (solid line), which closely resembles the actual breathing pattern. The dashed line is the corresponding constant flow.

and to blow the air into the protective part around the head of the user. The reason for this approach is obvious: in RPD it is important to have as low a pressure drop as possible. This ensures both a better protection and a higher comfort for the user. This development of equipping RPD with a blower device entails the hazard of desorption. Because air is continuously drawn through the canister, also during exhalation, the possibility increases that adsorbed contaminants are desorbed from the activated carbon. The occurence of desorption was examined under various conditions in order to explore whether or not significant risks exist.

Figure 2. Breakthrough profiles of toluene on Norit R1; comparison between pulsating (breather) and constant flow. The symbols represent the experimental data, the solid lines represent the simulation results.

Figure 2. Breakthrough profiles of toluene on Norit R1; comparison between pulsating (breather) and constant flow. The symbols represent the experimental data, the solid lines represent the simulation results.

The existence of a desorption risk was examined by means of a series of sorption experiments in which the same procedure was followed. During a certain period of time a bed of activated carbon was exposed to an air flow containing a contaminant. After this period the supply of the contaminant was stopped while a flow of clean air was continued. The effluent contaminant concentration was monitored during the entire experiment. From the viewpoint of a possible occurrence of a desorption risk it is especially interesting what happens after the supply of contaminant has been stopped. The sorption experiments were carried out with cyclohexane and Norit R1 carbon. The bed height was 2 cm and the total flow rate was 7.5 L/min, which corresponds to a superficial gas velocity of 6.4 cm/s. The entire carbon bed was divided into two separate beds, in series, of 1 cm each. The cyclohexane concentration was monitored gas chromato-graphically after each bed, providing extra information about the progress of the concentration front.

Figure 3 shows the results of experiments where challenge times for cyclohexane were applied of 10, 20, and 30 min. In all three cases cyclo-hexane was positively detected after the first part of the bed. The point at which the feed of cyclohexane was stopped, is visible as well: the concentration rises less fast or decreases even at that moment. The system is flushed with clean air that causes the concentration to rise less fast or even to decrease initially. Subsequently, previously adsorbed cyclohexane starts to release: desorption occurs and the concentration shows a further increase. A longer challenge time results in a higher concentration. Although at 20 and 30 min challenge time the concentration rises above the breakthrough criterion of 5 mg/m3, this is measured halfway through the carbon bed (1 cm). The concentration at the end of the bed is of more importance, as filter canisters are equipped with carbon beds that typically have a bed length of 2 cm. Only in case of a challenge time of 30 min cyclohexane was detected at the end of the bed. This occurred, though, only after more than 200 min; nevertheless, the concentration did rise above the breakthrough criterion of 5 mg/m3. For 10 and 20 min challenge time no cyclohexane was detected at the end of the bed even after 10-12 h of clean air flow, i.e. the concentration is below the detection limit of 0.01 mg/m3.

10000

Feed concentration

1000

0.001

1 10 100 1000 Time (minutes)

Figure 3. Breakthrough profiles of cyclohexane on Norit R1. The feed concentration was 5.63 g/m3, supplied during 10, 20, and 30 min; the carbon was dried and the feed humidity was <10%. The concentration jumps are a consequence of a period of air standstill.

For a more detailed discussion of the results of this study (including experimental results at different conditions) is referred to the recent publication by Linders et al.2 It is concluded that physisorbed contaminants

Feed concentration

Feed concentration

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10 min, 1.0 cm 20 min, 1.0 cm 20 min, 1.0 cm 20 min, 2.0 cm

15 h

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15 h

1 10 100 1000 Time (minutes)

Figure 3. Breakthrough profiles of cyclohexane on Norit R1. The feed concentration was 5.63 g/m3, supplied during 10, 20, and 30 min; the carbon was dried and the feed humidity was <10%. The concentration jumps are a consequence of a period of air standstill.

may be released from activated carbon filters in significant concentrations once the influent concentration of the contaminant has been reduced to zero. A redistribution of physisorbed contaminants over the activated carbon bed occurs in a period of rest (e.g. one night or more), which may lead to augmented release of contaminant when the filter is re-used. Under humid conditions the desorption of physisorbed contaminants occurs more rapidly than under dry conditions. Because desorption phenomena pose a risk for the users of RPD, it is advisable to address this somehow in all approval tests for vapor filters.

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