JND’s indirect cocoa nib dryer & roaster in hot demand

Any process equipment that costs less to run is always of interest, so when JND developed a drying and roasting system that was environmentally friendly, this generated of a lot of interest in the cocoa industry. Roy Marriott, Business Manager of JND Technologies Ltd, Retford, explains why JND’s indirect cocoa nib dryer/roasters are in hot demand from both new and existing clients.

What is so different about JND’s new system?

JND has long been known in the cocoa industry for high capacity continuous conditioning, drying and roasting equipment, but increasingly stringent emission regulations have made the industry look for more cost-effective ways of producing quality nibs using less energy and requiring less expensive odour and particulate treatment of process gas streams.

That’s why JND developed the new generation Indirect Cocoa Nib Dryers/Roasters, which separate the energy source required to dry and roast from the venting air (sometimes known as “fluff” gas).  This separation enables the main processing air to be continuously re-circulated and topped up to by closed loop in-duct burners as this gas flow never comes into direct contact with the product flow.

It is still necessary to pass a small amount of preheated air through the Dryer/Roaster shell in order to remove any evaporated moisture and prevent any re-condensation in the machines or the extract ductwork. The more traditional direct machines use the process air to dry, roast and remove the moisture, and usually they are one-pass systems involving heat being exhausted to the atmosphere. Usually where plants are located near to residential areas, all of this air also has to be de-dusted and possibly de-odorised; both very expensive operations.

How does the system work?

The Indirect Cocoa Nib Dryer/Roaster comprises of a rotating metal tube that passes through an external heating chamber. The chamber is heated by hot gasses that pass through the annular space between the tube and the chamber wall.  This hot gas is usually produced by a duct mounted burner and re-circulated by a hot gas fan. However, it is possible to use hot gasses from other sources since the heating gas does not come into contact with the product.

Heat is transferred from the hot gas to the outer wall of the tube. It is then conducted through the tube wall to the inside surface where it heats the bed of material, principally by conduction from the hot metal surface. The bed of material is continuously turned (rolled not lifted and dropped) by the drum rotation.

The bed travels along the tube to the discharge end due to the combination of the slope of the tube and the slow rotation. The depth of the material bed and consequently its retention time is maintained by adjustable vanes in a weir ring fitted in the discharge section. Roasted nibs overflow into the bottom section of he material discharge hood and are then discharged to a product conveyor system.

To what degree does feed moisture content affect the thermal efficiency?

Whereas the thermal efficiency of the more traditional direct units varies according to the moisture content of the feed (the efficiency decreases as the moisture content of the feed increases), the indirect units are more constant and typically up to 65%.

The indirect units can be zoned along their length and although the number of zones is not fixed, three or four zones is usual depending upon the profile of the end product required.  Each zone has a gas inlet and exhaust connection arranged diametrically opposite and at each end of the zone high heat transfer fins secured to the main shell ensure maximum process gas turbulence. The overall effect is that of a cross-flow unit which helps to maintain the temperature difference (driving force) between the process gasses and the cocoa nibs as they pass through the unit.  An additional benefit of the indirect unit is that there is no issue with products of combustion coming into contact with the nibs.

What do customers need to consider when upgrading?

In directly heated units the process gas is forced through a mechanically agitated bed of nibs resulting in high drying rates even at low processing temperatures. With the indirect unit all the process energy passes through the wall of the main shell so that the heat transfer is slower per unit residence time (don’t forget the process gasses are totally re-circulated). This means that the indirect units usually take up a bigger footprint and it is often difficult replacing a more traditional direct unit with an indirect one in an existing space.

About JND Technologies…

JND Technologies (www.jnd.co.uk) specialises in rotary thermal process echnologies for drying and calcining materials as diverse as foodstuffs to minerals and also produces a range of mineral sizing equipment. JND Technologies is a division of Langley Holdings PLC, (www.langleyholdings.com), a UK based, globally operating engineering group which employs over 2500 people worldwide and has revenues in 2007 of around €500 million.