The Use of Activated Carbon as a Desiccant for Hopcalite type Carbon Monoxide Catalysts.

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1 The Use of Activated Carbon as a Desiccant for Hopcalite type Carbon Monoxide Catalysts. Dr Mike Williams 1 and Heidi Marsh 1 Chemical Safety, Fuels and Environment Group Cranfield University m.r.williams@cranfield.ac.uk

2 Project This work started as a final year project for our Applied Science degree course and the initial work was done in Such projects have to have sufficient experimental scope to provide the student with a piece of work that enable them to gain their own results and interpret them, based on about 10 days of lab work, once a week.

3 Background There have been military fatalities from carbon monoxide poisoning, in tents and vehicles and while demilitarising ammunition. Sources of carbon monoxide are poorly functioning heaters, heaters taken into confined spaces with little ventilation and from the products of explosion of high explosives and burning propellants. Military respirators offer no protection against carbon monoxide.

4 Idea The idea behind this project was that catalysts for carbon monoxide oxidation (in general) need dry air or protecting against poisons (or both). Many respirator canisters (especially military ones) would stop many of the poisons (such as acid gases) and some have been designed to allow two or more canisters to be connected in series. The question for Heidi to answer was could a military respirator canister be used to protect an oxidation catalyst placed behind it?

5 In Practice It was too complicated to test the full scope of this in the student project, mainly because of the difficulty in making acid gas concentrations and carbon monoxide concentrations together once a week. It was possible to look at catalysts that didn t like water and to see if there was enough drying power in carbon to extend their lifetime. This presentation is not about the catalysts. This presentation is about activated carbon.

6 Materials dstl Porton Down donated some ASC carbon. Molecular Products donated some Hopcalite. When Heidi did her work these were relatively new. When I checked some of the results and filled in the gaps this summer they were 10 years old. All I did was dry them in flowing nitrogen at 120 C for 2 hours.

7 Water Adsorption from Vacuum This was a repeat of Heidi s work I took more time over it (it took three days to complete most of the adsorption branch).

8 Conditions for breakthrough tests. Work was done in Va tubes (3.14 cm 2 cross sectional area) at 1 l min -1 linear flow This is equivalent to 30 l min -1 through a filter roughly the size of the S6/S10 canisters. Dry air was drawn from a cylinder. Humid air from in-house controlled flow system checked against dew point meter. Carbon monoxide concentration of 1000 ppm produced by flow mixing calibrated pressure drop for CO with mass flow controlled for air. 1 l/min drawn from larger flow through rotameter to Va tube.

9 Detection Air leaving the Va tube was directed to a small flask containing the detectors. Water breakthrough - Kane May humidity meter calibrated against dew point meter. Carbon Monoxide City technology Citicell, 0 to 1000 ppm ma output. Response time of the latter is of the order of 10 seconds. The humidity meter was slower to change from humid to dry (over a minute).

10 Experimental Set-up Rotameter, 1 l/min Air, 5 l/min Test Bed Vessel with detectors. Carbon Monoxide, 5 ml/min Waste Stream, 4 l/min

11 Water Penetration 80% RH 1 l/min at 25 C.

12 Effectiveness of Hopcalite, Dry Air

13 Critical Bed Depth of Hopcalite Decay is not first order with bed mass (related to residence time) but pre-heating in bigger beds may explain this.

14 Using Carbon as Desiccant, 50% RH

15 Water Breakthrough Hopcalite itself acts as a drying agent. Carbon monoxide penetrates before water.

16 Using Carbon as Desiccant 75% RH

17 Effect of Carbon Layer on Breakthrough Time Run Mass Hopcalite (g) Mass Carbon (g) Time to 50ppm Breakthrough (mins 2 cm dry 6.6 none >40 minutes 2cm 50% 6.6 None cm 50% + ASC cm 75% 6.7 None cm 75% + ASC

18 Water Adsorption from Vacuum Reminder the capacity of carbon for water depends on relative humidity. It doubles between 50% RH and 75% RH.

19 Conclusions It is expected that impregnated carbons such as those used in military respirators will delay many of the materials that poison carbon monoxide catalysts from reaching the catalyst if used in front of the catalyst. They will also delay the arrival of water vapour. Because of the nature of the water adsorption isotherm on carbon the protection may be roughly the same for all humidities.