Odours & particulate emissions need a good scrub

To meet legislation for odour and particulate emissions, a well known poultry feed producer from the north of England needed to improve on the performance of a cyclone system that was found to be inadequately handling emissions.To solve the problem, Nairb Air Ltd was commissioned to produce an Impinjet wet scrubbing system based on a similar installation system which is already operating successfully for another feed mill in the south of England.

Scrubber design

The commissioned scrubber now installed has been designed to draw the odour and remaining carry over from the capped cyclone exhaust and feed it into a spray zone chamber before passing it through two fluidised plates at high velocity.

The liquor that is used in the system is a blend of alkali and water. The water mist that it is generated by the process is then drawn through two moisture eliminators before being exhausted to atmosphere.

The input temperature of the airstream from the cyclone is approximately 60°C, the operating volume is 12,000cfm. The exhaust fan is driven by a 45KW motor.

To keep operating noise to a minimum, an acoustic enclosure is fitted around the fan and a diffuser is used to contain air noise from fan discharge prior to entering the stack.

Commenting on the Impinjet system the mill’s plant engineer stated: “As well as being very successful, the Impinjet has also proved to be particularly good choice due to its self cleaning characteristics, especially considering the nature of pelleted feed of which we produce up to 1,000 tonnes a week.”

Nairb Air also reports that results of in-depth studies by its independent testing authorities of its systems, installed in many leading animal feed mills, have shown that the chemical scrubbing of odours can be achieved effectively to well within acceptable limits at a much lower cost than by incineration or the adsorption process.

Nairb Air has been requested to assist many large animal feed mills to come up with solutions to remove odours produced by the processing of animal feed which drift in prevailing winds and can cause local community concerns.

The odour was created when the drying and cooling process extracts moisture from pelleted feed before storage. This process generates large amounts of odorous vapour which used to be vented directly to the atmosphere.

Portable pilot plant

The studies were undertaken where a portable wet scrubbing pilot plant for assessment purposes was provided by Nairb Air.

This was fitted into the normal processing system to allow regular samples of stack emissions to be taken to ascertain the effectiveness of the wet scrubbing principle for this type of odour control. The pilot plant ran for 14 days to allow engineers sufficient data to assess the scrubber’s effectiveness.

Nairb Air says that results proved so successful that it was commissioned to produce a purpose made Impinjet wet scrubbing system. It takes air and vapour into the scrubbing unit by backward inclined centrifugal fan driven by a 55KW motor which provides a duty of 36,500m3 /hr.

Perforated plates

The odour-laden vapour is routed by the fan into the inlet of the scrubbing vessel. The first stage of the scrubbing cycle preconditions the vapour by drenching it with a fine spray of liquor before it travels up inside of the scrubber shell where it accelerates through horizontal impingement plates. The plates are perforated with hundreds of small holes which support a continuous moving layer of scrubbing liquor.

The vapour is pushed through the plate and broken into minute streams which impinge upon targets submerged in the liquid to maximise liquid contact absorption. Liquid is retained and recirculated continuously.

The scrubbing liquid is simply disposed of by discharging into the drains and fully meets with local water authority’s requirements.

A sound attenuator is fitted at the base of the discharge stack to keep noise to minimum levels that are also laid down by the local authorities.

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