The importance of cooling equipment

FERTILIZER COOLING TECHNOLOGY

The importance of cooling equipment

Plant operators require effective and efficient equipment for cooling the solid materials generated during fertilizer production. The main options include fluid bed coolers, rotary coolers and bulk flow coolers.

During their manufacture, fertilizer products require additional cooling as a final process step – after granulation, drying and screening but prior to bagging. It is generally necessary to cool fertilizer prills and granules close to ambient temperature to prevent the subsequent degradation of products during storage, with the caking of products in particular needing to be avoided.

For many years, fluid bed coolers and rotary coolers have been installed at fertilizer production plants worldwide as standard equipment for product cooling. Bulk flow coolers – also known as bulk solids coolers – are another attractive equipment option. These use efficient plate heat exchangers for the indirect cooling of fertilizer prills and granules.

Fluid bed coolers

This type of cooler suspends solid material in a flow of air causing it to behave as a fluid. Such fluid beds offer high cooling efficiency as the entire surface area of the material, while suspended in a fluidised state, is exposed to the flow of air. The gentle handling of materials in fluids beds is another distinct advantage.

Fluid bed coolers typically consist of a perforated horizontal plate (Figure 1). This is made to a proprietary design to ensure an even distribution of cooling air. Granular product moves through the fluid bed until it reaches a discharge weir. This can be lifted or turned to allow the bed to be emptied for cleaning. A bag collector is usually used to collect and remove dust.

Fig. 1: Fluid bed cooler: typical process configuration

The fertilizer product discharged from the granulator usually enters the cooler at between 90-100°C. The product needs to be cooled to the preferred storage temperature – generally about 40°C – to avoid caking. Two cooling stages with primary and secondary fluid bed coolers may be required, or just one single cooling step may be necessary, depending on the process.

France’s Comessa and Spanish equipment manufacturer Comspain are both major suppliers of fluid bed coolers to the fertilizer industry (Fertilizer International 437, p30). Fluid bed coolers are commonly used in urea granulation. They are a notable feature of the UFT® fluid bed granulation process offered by thyssenkrupp Fertilizer Technologies (tkFT), for example, although bulk flow cooling (BFC) can also be a good alternative (see the companion article on p40).

Fluid bed coolers may be a better option – compared to rotary coolers – in situations where physical degradation needs to be specifically avoided.

Rotary coolers

Rotary coolers work by tumbling material in a rotating drum in the presence of chilled or ambient air. The drum is set at a slight angle to so that material moves down its length under gravity. Many rotary coolers, such as the Cascade Cooler offered by Barr-Rosin (GEA Group), use a counter-flow of air to cool the cascading solid material as it moves in the opposite direction within the drum. Lifting flights maximize heat transfer by firstly raising and then dropping the material, so improving its contact with the chilled/ambient air as the drum rotates.

Rotary coolers are in widespread use in the fertilizer industry – including production processes for DAP, MAP, NPK and TSP etc. Comessa, Comspain, GEA and FEECO are notable equipment suppliers. Cooling fertilizers within a rotating drum has proved popular within the industry due to the following strengths:

• High throughput
• Ease of handling
• High availability
• Low maintenance requirements
• Flexibility – particularly the ability to cope with a wide range of particle sizes, fluctuations in material properties and the amount of material handled.

Rotary coolers are liked by fertilizer producers due to their high capacity, simplicity, robustness and reliability, according to Wisconsin-based equipment manufacturer FEECO International (see box).

Although their capital and maintenance costs and thermal efficiency are broadly comparable, rotary coolers have a number advantages and disadvantages versus fluid bed coolers, according to FEECO, including:

• Lower energy requirement
• Larger footprint
• More tolerant to feedstock variation
• Some attrition and degradation
• More easily automated.

Bulk flow coolers

Indirect cooling using heat exchange technology was first introduced several decades ago as an alternative approach to cooling fertilizer granules and prills before storage. Indirect cooling combines together the engineering science of heat transfer with the mass flow of bulk solids.

The heat exchangers used in indirect fertilizer cooling consist of a series of vertical, hollow stainless steel plates. The fertilizer granules fall slowly under gravity between these plates. Cooling water, meanwhile, flows internally through the plates in a counter-current for improved efficiency. A discharge feeder below the base of the cooler controls the outflow of product material.

Such bulk flow coolers – also known as bulk solids coolers – offer the following benefits, compared to other cooler types:

• Mass flow ensures an even temperature
• Little or no product degradation
• Efficient design – large heat transfer area in a compact unit
• Indirect cooling – no contact between cooling water and product
• No cooling air is required, thereby avoiding emissions
• Energy and equipment cost savings, as no fans, scrubbers or baghouses are required.

Canada’s Solex Thermal Science is an established major supplier of cooling equipment to the fertilizer industry. The company’s proprietary heat exchanger technology offers efficient, indirect cooling of fertilizer granules and prills. The company reports that indirect cooling technology offers dual benefits, by providing fertilizer manufacturers with a better product while also being more environmentally-friendly (see accompanying article on p36).

In certain circumstances, bulk flow coolers also provide an attractive alternative to fluid bed coolers in urea granulation plants, according to tkIS (see separate article on p40).