In with the old

Activated carbon is defined as a microporous adsorbent made from a carbonaceous material. Simply put, it appears solid, but is actually a network of inter-linked fissures and pores with an extremely high internal surface area - around 1,000m2/g. Camfil's Dr Chris Ecob puts carbon - an old material with a new future - under the microscope.

Filtration using activated carbon is a recognised technique in the air cleaning industry. In general, however, the underlying mechanisms and particularly the potential of the technique are not well understood. The element carbon has been in existence since the dawn of time. It has a diverse presence in our world and coal will be the most familiar example. Diamonds, which have a very ordered structure, are the purest form of carbon. It is less well known that the molecular building blocks in all animal and plant tissue are formed from chains of carbon atoms. It is from these sources, particularly the botanical materials, that another form of carbon, activated carbon, has been derived.

Pore structure
It is a consequence of the complex internal pore structure that activated carbon is used in the purification and filtration of gases. Contaminant gas molecules in an air stream are able to enter the large pores at the surface of the carbon and, through a process called diffusion, move towards the internal surface of the smallest pores. When a gas molecule collides with the carbon surface at a suitable site, an attraction is formed and it will be retained. Gas molecules are extremely small and they are most strongly adsorbed in the smallest diameter pores. Commercial carbons that traditionally satisfy these criteria are manufactured from coconut shell and coal. The first major use of activated carbon in air filtration was during the First World War when it was used in respirators to combat war agents such as chlorine, mustard gas and phosgene. Eighty years later, activated carbon is extensively used in applications where the removal of odours and toxic or corrosive gases from air is required.

As we move into the next century, changes in technology and markets are imminent. Today, many of the problems to which carbon filters are applied as a solution are poorly defined. The reason is simple: it is costly to analyse an airstream contaminated with a complex mixture of gases or vapours. Only in relatively few applications where there might be a requirement for compliance with statutory regulations will such work be undertaken. This situation will change. In the "Comfort" sector, as public concern towards such issues as "Sick Building Syndrome" increases, so will the need to clearly identify the harmful airborne agents. In the "Clean Process" sector there are industries that have now realised that it is not just particulate but chemical or molecular contamination that can seriously affect their processes.

The prime example is the semiconductor manufacturing industry, for which Camfil has developed the "Gigasorb" filter. Camfil also have available the "Gigacheck" monitoring system to more simply determine molecular contamination in airstreams. As a mirror of recent developments in the particulate filter market, the requirement is increasingly for more efficient and more compact carbon filters that have a lower impact on the environment throughout their total life (manufacture, use and disposal). Of particular concern is pressure loss, which has a significant effect on energy costs throughout the filter life.

New forms
These requirements translate into the need for new forms of active carbon and novel filter designs. Active carbons derived from synthetic materials such as polymer fibres and resins rather than botanical material will become increasingly important. In such materials, the potential exists to exert more influence on the structure of the carbon during the manufacturing process. This allows adsorbents to be made with more specific properties than is possible today. More importantly, producing activated carbon in forms that have high external surface area-to-volume ratios, such as fine fibres or small diameter moulded beads, allows very rapid adsorption dynamics. Less of these materials are required than conventional granular materials to achieve similar adsorption rates. Filter sizes become smaller, and because novel honeycomb and pleated configurations are possible, lower pressure loss values are achieved.

Filters based on activated carbon fibres are now used in medical applications and extensively in photocopiers for the removal of ozone. In the copier application, filter sizes and operational pressure loss values have typically been reduced by 25% and 75% respectively when compared to traditional devices using granular adsorbents.

To support their carbon filter activities, Camfil has constructed a new test facility at the company's headquarters in Sweden. This facility has the capability to test life-scale ventilation filter systems against a wide range of inorganic gases and organic vapours. To simulate the varied environmental conditions experienced in the global market, the test rig will deliver up to 4,000m3/h of air in the range of 10-50ÂșC and 10-100% relative humidity. Clearly this facility is unique in the filtration industry and demonstrates a commitment to providing world class products for the control of gas-phase pollutants in any environment.

Waiting to inhale: 20,000 to 1
Hickson & Welch, supplier of fine organic chemical intermediates, has reduced releases to atmosphere from 400 tons per annum to 12 tons per annum in a period of three years. Due, in part, to Waterlink Sutcliffe Carbons of Ashton-in-Makerfield. At the eight final release points at which the carbon is used, the mean release has been reduced to an undetectable amount of less than 1mg/m3.

Hickson & Welch specialises in difficult chemistry, producing multipurpose intermediates in a variety of multi-stage chemical reactions. These include nitrations, chlorinations, hydrogenation, phosgenation and similar types of complex reactions, in 16 chemical production facilities at its Castleford site. The reactions are essentially based on toluene and are carried out on organic solvents.

Fire risk exists due to the nature of many organic solvents, and to prevent this the company uses nitrogen gas to exclude oxygen from the reacting mass. As a consequence inert gases carry significant concentrations of solvents in the discharges that require eliminating. The solvents within the equipment require to be changed regularly and consequently a multi-purpose abatement system is used.

Gas throughput In operation, the gas flow follows a condensation process, using low temperature calcium brine, which removes 90% of solvents. The gas throughput, almost 900m3/hr and at 20,000mg/m3 concentrations, first enters one of two identical in-series cylindrically shaped static beds, which each hold 750kg of carbon on supporting internal grills. During its progress through the first bed the gas has its solvent concentration reduced to a maximum of 300mg/m3 and subsequently to less than one, on passage through the second cylinder.

The solvent concentration levels are monitored daily and when the maximum 300mg/m3 level is reached at the first carbon bed discharge point, this indicates the bed is becoming exhausted. The exhausted bed is replaced, flow is reversed, and the secondary bed becomes the first. It is then returned to Waterlink Sutcliffe Carbon where the spent carbon is regenerated. The rate of carbon adsorber exchange varies from daily with the toxic, volatile and low release dichloromethane to every three months with toluene.

Carbon cost considerations
Specialist contract chemical manufacturer, Witton Chemical Co, knew that it needed to replace the worn out local exhaust ventilation (LEV) system in its multi-purpose batch manufacturing plant, but the choice of abatement for the new system was large and complicated. The new system had to both capture fumes efficiently and remove a wide range of materials from the exhaust air stream to prevent the release of materials to the environment. The problem of removing contaminants from the air stream was a challenge because of the large number of materials involved, and particularly because one, trimethylamine (TMA), is amongst the most odorous materials known (the odour threshold for TMA is 0.00021ppm, 0.0005mg/m3).

Since the system was to be installed in a manufacturing plant, the extraction arms needed to be robust and able to resist the occasional brush with machinery and heavy objects. Several types of abatement plant were considered. Given that the concentration of compounds in the air stream was on average little more than 5mg/m3, incineration was discounted on the grounds of both the high capital cost and operating cost. With such low concentrations of materials in the air stream it is very unlikely that an incinerator would have reached its autothermal level, and so fuel costs would have been prohibitively expensive. Wet scrubbing was considered but this is inefficient for removing VOCs. A system that combined wet scrubbing with adsorption, oxidation or incineration would again have a high capital cost. Cryocondensation was ruled out because the high airflows involved would have meant that large amounts of liquid nitrogen would have been required to cool the system. Because only small amounts of a complicated mixture of materials were involved, there was little point in installing plant to recover the VOCs from the air stream.

Contaminant concentration Camfil was able to offer either a Biovox system (a biotrickling filter) or a deep bed carbon system. For many applications where the average concentration of compounds is less than 2,000mg/m3, Biovox is cheaper to install and operate than an incinerator. For this application, however, the average concentration of contaminants was too low even for a Biovox. Camfil therefore had only the option of offering a deep carbon bed system which would be both cost-effective and would give Witton the performance required in capturing the wide range of materials that might enter the extraction system. A good incentive to promote clean working practices and good housekeeping in the factory is that these reduce the carbon usage in the filter. Thus, designing systems to handle and use chemicals without spills and fumes is not only safer but also cheaper.

The system chosen was a vertical deep bed carbon filter, seen as most appropriate because it had a two-stage bed deploying a base carbon in the first stage, which would capture the heavy molecules, and a specially impregnated carbon as the second stage to adsorb and chemically bind the lighter molecules like TMA. The complete LEV system comprises articulated collection arms mounted at 12 locations around the production area, extract ducting, prefilter system, carbon filter, fan and stack. The materials chosen were 316 stainless steel, polypropylene and polyester elastomer.



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