VOC Abatement & Odour Control Systems

Regenerative Thermal Oxidisers (RTO’s)

The oxidation of VOC’s contained in exhaust air streams involves a chemical reaction between the VOC’s and the oxygen in the air, usually at an elevated temperature, and forming carbon dioxide and water vapour.

The RTO is an extremely efficient thermal oxidiser that uses a number of ceramic or ‘hot brick’ beds to absorb heat from the exhaust gas. It then uses this captured heat to preheat the incoming process air stream and destroy air pollutants emitted in the exhaust air stream at temperatures ranging up to 1000deg C.

The ceramic beds provide primary heat recovery of up to 95% thermal efficiency enabling auto-thermal, or self-sustaining, operation of the oxidiser with very low solvent concentrations of approximately 2 g/m3, ie. Less than 5% LEL.

The primary advantage of this technique is a very high VOC destruction efficiency, often in excess of 99%, and a high thermal efficiency providing low, or near zero, gas consumption. Used throughout industry to destroy air toxins, odours, VOC’s and hazardous air pollutants discharged from industrial processes.

Tubular Electrostatic Precipitators (ESP)

An electrostatic precipitator is a filtration device that removes fine particles, such as dust, smoke and liquid droplets, from an exhaust air stream using the force of an induced electrostatic charge minimally impeding the flow of gases through the unit.

In contrast to other techniques which apply energy directly to the flowing fluid medium, an ESP applies energy only to the particulate matter being collected and therefore is very efficient in its consumption of energy in the form of electricity.

A wet electrostatic precipitator operates with saturated air streams. They are commonly used to remove liquid droplets such as sulphuric acid mist from industrial process air streams as well as for gases high in moisture content, contain combustible particulate, or having particles that are sticky in nature. The preferred and most modern type of wet ESP is a down-flow tubular design. This design allows the collected moisture and particulate to form a moving slurry that helps to keep the collection surfaces clean.

This ESP technique is ideal for the removal of high boiling point VOC’s such as machine or knitting oils, plastisol’s and web-offset printing inks in the printing, coating and textile industries. The visible emissions from these processes are often the cause of local resident complaints and are due to the high boiling point hydrocarbons and oils which, on discharge to atmosphere, will condense to form sub-micron droplets creating a highly visible emission. The abatement system favoured is a combination of Exhaust Gas Cooling (to condense the condensable components using a film condensing technique); the ‘Eurokleen’ Coalescer (to coalesce the submicron droplets into larger droplets), a Tubular ‘wet’ Electrostatic Precipitator technique (designed to remove the liquid droplets from the air stream) and finally a system to remove any remaining carry-over of droplets. Each of these techniques complements the other for the successful and cost effective removal of the visible emissions.

Any low boiling point VOC’s unlikely to fully condense at room temperature will not be removed with this abatement technique. In the event they are present and cause an odour problem to local residents then an adsorption technique is required to be included for the removal of these components – see description of this technique below.

Condensation & Coalescing Techniques

Condensation occurs when a saturated vapour comes into contact with a surface whose temperature is below the saturation temperature.

Condensation techniques are best suited for hydrocarbons and organic compounds having relatively high boiling points and high vapour phase concentrations These are commonly used in the web-offset printing, coating and textile industries where machine or knitting oils, plastisol’s and high boiling point printing inks are used. The visible emissions from these processes are often the cause of local resident complaints and are due to the high boiling point hydrocarbons and oils, which on discharge to atmosphere will condense to form sub-micron droplets creating a highly visible emission. The abatement system favoured is a combination of Exhaust Gas Cooling (to condense the condensable components using a film condensing technique); the ‘Eurokleen’ moving bed filter/coalescer (to coalesce the submicron droplets into larger droplets, collect and remove from the air stream) and a system to remove any remaining carry-over of droplets. Each of these techniques complements the other for the successful and cost effective removal of the visible emissions.

Any low boiling point VOC’s unlikely to fully condense at room temperature will not be removed with this abatement technique. In the event they are present and cause an odour problem to local residents then an adsorption technique is required to be included for the removal of these components – see description of this technique below.

Wet Scrubbers

Wet scrubbers are used for cleaning air and other gases of various pollutants and dust particles or substances considered harmful to the environment, and which may be removed or neutralized. Wet scrubbing works via the contact of target compounds or particulate matter with the scrubbing solution. Solutions may simply be water (for dust) or solutions of reagents that specifically target certain compounds. Process exhaust gas can also contain water-soluble toxic and/or corrosive gases like hydrochloric acid (HCl) or ammonia (NH3). These can be removed very well by using a wet scrubber.

The removal efficiency of pollutants is improved by increasing residence time in the scrubber or by the increase of surface area of the scrubber solution by the use of a spray nozzles, packed towers or aspirators. Wet scrubbers may increase the proportion of water in the air, resulting in a visible stack plume, if the air is discharged to atmosphere. Industrial applications for the control of odours and VOC emissions include exhaust air streams from textile heat-set stenters, printing, anaerobic digesters, waste transfer stations and food processing applications.

Carbon Adsorbers

Solvents, volatile organic compounds (VOCs) and other contaminants in the liquid or gas phase can be adsorbed onto activated carbon.

Almost all materials containing a high fixed carbon content can potentially be activated. The most commonly used raw materials are coal (anthracite, bituminous and lignite), coconut shells, wood (both soft and hard), peat and petroleum based residues.

Most carbonaceous materials do have a certain degree of porosity and an internal surface area in the range of 10-15 m2/g. After activation, the carbon will have acquired a much increased internal surface area. The principle of adsorption is based on the highly porous structure of the granular carbon with each gram of activated carbon typically having a total surface area, including all its internal pore structure, of up to 1200 m2/g.

Contaminants are removed from the gas or liquid stream by the process of physical adsorption. Physical adsorption is dependent on the characteristics of the contaminant to be adsorbed, the temperature of the air stream to be processed, and the concentration of the contaminant in the air stream. The adsorption capacity for a particular contaminant represents the amount of the contaminant that can be adsorbed on a unit weight of activated carbon consumed at the conditions present in the application. Typical adsorption capacities for moderately adsorbed compounds range from 5 to 30 percentage of the weight of the carbon

When a carbon bed is ‘saturated’ with contaminants its efficiency of removal will reduce and would normally be removed and exchanged for new material with the exhausted carbon either disposed of or reactivated, in-situ by the carbon supplier.

Carbon adsorption is highly efficient and achieves high levels of removal, especially with VOCs and odours. However with large flows it may prove uneconomical.  A useful procedure is to combine the economy of wet scrubbing or stripping with a subsequent ‘polish’ with activated carbon.  The packed tower will remove the bulk of the contaminant while the final fraction – plus stray components not scrubbed or stripped – can then be adsorbed without prematurely exhausting the carbon bed.

Industrial applications for the control of odours and VOC emissions include exhaust air streams from textile heat-set stenters, printing, anaerobic digesters, waste transfer stations and food processing applications.

 

Bio-filtration and Bio-trickling Filters

 Bio-filtration is a pollution control technique using a bioreactor containing living material to capture and biologically degrade pollutants. Common uses include the micro biotic oxidation of contaminants in air. When specifically applied to air filtration and purification, bio-filters use microorganisms to remove air pollution. The air flows through a packed bed and the pollutant transfers into a thin biofilm on the surface of the packing material. Microorganisms including bacteria and fungi are immobilized in the biofilm and degrade the pollutant. Trickling filters and bio-scrubbers rely on a biofilm and the bacterial action in their recirculating waters.

The technology finds greatest application in treating malodorous compounds and water-soluble volatile organic compounds (VOCs). Industries employing the technology include food and animal products, off-gas from wastewater treatment facilities, pharmaceuticals, wood products manufacturing, paint and coatings application and manufacturing and resin manufacturing and application, etc. Compounds treated are typically mixed VOCs and various sulphur compounds, including hydrogen sulphide. Very large airflows may be treated although a large footprint area will be required.

One of the main challenges to optimum bio-filter operation is maintaining proper moisture throughout the system. The air is normally humidified before it enters the bed with a watering (spray) system, humidification chamber, bio-scrubber, or bio-trickling filter. Properly maintained, a natural, organic packing media like peat, vegetable mulch, bark or wood chips may last for several years but engineered, combined natural organic and synthetic component packing materials, such as leca, will generally last much longer.

A bio-filter system is a fairly simple device to construct and operate and offers a cost-effective solution provided the pollutant is biodegradable within a moderate time frame, at reasonable concentrations, and that the airstream is at an organism-viable temperature. For large volumes of air, a bio-filter may be the only cost-effective solution. There is no secondary pollution and degradation products form additional biomass, carbon dioxide and water.

 

Simdean Plasma Systems for Liquid Wastes

Total destruction of waste chemicals and gases at lower cost – a system of disposal that breaks new ground – Technologically, Environmentally & Financially.

Simdean, part of the Operational Group, have pioneered a new plasma-based system for treating hazardous waste. The system completely destroys hazardous waste whilst reducing emissions of greenhouse gases and cutting costs.

The patented plasma technology utilises high temperatures (2000°C – 4500°C) to instantaneously destroy a wide range of wastes, leaving only the constituent atoms. These are then transformed into a non-hazardous ionised gas for emission to the atmosphere and in some cases usable by-products. The system is equally effective in treating wastes in the form of liquids, slurry or contaminated gases including halogenated waste and halogen acids.

The plasma installation is capable of integrating into the client’s production line, eliminating the need to transport or store waste products, which is expensive, complicated and damaging to the environment. The process, which can easily be modified and calibrated to suit most production lines, is safe, user-friendly and simple to operate. Emissions from the system at the end of the process are well within all international standards and regulations.

The process has been developed principally for the pharmaceutical, chemical, oil, gas and chemical plant industries. Combining exceptional process efficiency with environmental benefits, lower capital costs, lower operating costs and lower maintenance costs, it delivers significant advantages over all other known treatment system.

www.simdean.co.uk

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