Towards environmentally friendly carbon filtration

Bigneat has considerable experience in the design, manufacture, validation and maintenance of enclosures for containing hazards such as particulates and chemical fumes.

Their cabinets are mostly of a ductless design incorporating carbon filters for fume removal.

These work by filtering air drawn through the filter, removing chemicals, whether acid, base, solvents or otherwise, and returning it clean to the laboratory.

Other fume cupboards that extract air away via ducting are ideal for bulk fume applications.

These are built into all new laboratories as a matter of course.

However, in the situation where more safety cabinet capacity is required, a recirculatory filtration cabinet can prove to be the ideal low cost solution.

More and more laboratories are using this type of cabinet.

With the ever-increasing demands for improved air quality in the workplace, removal of fumes is essential.

In view of this growing market several years ago, Bigneat made a commercial decision to develop an in-house range of filters for the adsorption of a wide range of chemical challenges.

However, customer concerns regarding the environment and employee safety raised the level of the challenge considerably.

Development work for these filters was carried out in the laboratories of the Department of Applied Chemistry at the University of Portsmouth, which has 40 years experience in the field of porous carbon solids.

Many commercial filters using impregnated carbons contain chemicals that are undesirable from personnel safety and environmental viewpoints. Hexavalent chromium (the 'baddie' in the Erin Brockovitch film), in the form of potassium chromate, is a good impregnant for acid-retaining carbons, but is also toxic, corrosive to skin and eyes, and carcinogenic.

Copper (II) chloride, used to treat carbons for ammonia, amine and other alkaline species retention, presents additional hazards to health and to the environment.

Therefore, alternative, low-hazard impregnants for the new Bigneat range of filters to be named Chemcap were investigated and subsequently used.

Carbon, element 6 in the Periodic Table, is remarkable for the diversity and utility of its forms.

As graphite it slows down neutrons in nuclear reactors and lubricates at high temperature; as diamond it sparkles and captivates, as well as providing industrial cutting power; as Buckminsterfullerene, or buckyballs, it is providing the electronics and pharmaceutical industries with a wealth of fascinating possibilities.

But these allotropic forms of carbon are only the beginning; the element also finds widespread use as activated carbon, a highly porous, high surface area material with an almost limitless range of uses as a purifier of liquids and gases.

With starting materials as diverse as coal and coconut shells, the use of crude forms of activated carbons stretches back into antiquity.

Large scale use of the more sophisticated product only began in the 20th century, with the need to protect troops from the poisonous gases used in WW1. Military applications have progressed considerably since then, and modern nuclear, biological and chemical (NBC) protection is usually based around forms of activated carbon.

Activated carbon is used in the purification of water, for removing organic species and excess chlorine, in solvent recovery to prevent loss of hydrocarbons to the atmosphere, and in air conditioning plant to remove trace odours.

Modern cars have carbon filters installed in fuel systems to prevent vapour loss to the environment, and some varieties of intoxicating drink sold in Scandinavia are made by the addition of flavours to alcohol previously rendered tasteless by treatment with activated carbon.

From an occupational safety point of view, activated carbon is indispensable, and is capable of retaining a wide range of organic solvents, acids, alkalis and species such as formaldehyde.

However it is necessary to impregnate the carbon with suitable chemicals to maximise filtration performance.

The development of the Bigneat Chemcap filter range was carried out with reference to a number of very important criteria, namely: filters had to give high retention capacity; and chemical treatments for enhanced acid or alkali retention had to be non-hazardous to operatives undertaking the carbon impregnation work and to the environment when exhausted carbon is sent for landfill.

When used in conjunction with the Bigneat range of ductless fume hoods, filter performance had to conform to the requirements of internationally recognised test procedures, such as AFNOR and BS 7989:2001; and rigorous quality control procedures had to be developed for all stages of filter production, from activated carbon manufacture through to the performance of the final product.

In the course of the Chemcap filter development work at Portsmouth University, the performance of carbons, filters and fume hoods has been assessed with around 150 different challenge chemicals.

The Chemcap filters currently in use provide outstanding retention capacities for solvents (type OS), acids (type H+), alkalis (type AM).

In addition, type MP filters give excellent all-round performance in circumstances where solvents, acids and alkalis are used in the enclosure, and type F is a special filter for formaldehyde retention.

The new Bigneat cabinets in combination with Chemcap cabinets offer effective containment and high levels of operator safety.

But how is this proven? As far as conformity to international specifications is concerned, Bigneat fume cabinets are assessed against the French Afnor test and the British Standard for ductless fume hoods.

The Afnor NF X 15-211 standard for enclosures of Class 2 requires that their filtration performance be assessed in two phases of operation.

In Phase 1, normal operation, the amount of toxic material retained by the filtration system is determined at the point at which the concentration of the material in the exhaust gases reaches 1 % of the VME for that material.

In Phase 2, the detection phase, the additional amount of toxic material retained by the filtration system is determined at the point at which the concentration of the material in the exhaust gases reaches 50% of the VME.

A challenge concentration of 200ppm in the air stream is required throughout the test.

With propan-2-ol as the challenge, the Bigneat system gave a retention capacity of 350 grams in Phase 1 and an additional 1130 grams retention in Phase 2.

The British Standard, BS 7989: 2001, requires a capacity type test to be carried out, using propan-2-ol challenge at a much higher concentration than in the Afnor test, ie, 800ppm compared to 200ppm.

The Bigneat enclosure fitted with a type OS filter had no trouble in complying with the requirement that no more than 40ppm of solvent should be emitted with the exhaust gas during the evaporation of 1 litre of solvent, and that no more than 400ppm should be emitted during the evaporation of two litres of solvent.

The excellent performance of current Chemcap filters is backed up by a policy of continuing product development, and the company says it is very willing to discuss special filtration requirements with potential customers.