Why is air injection so important in indirect fertilizer cooling?
Unveiling the not-so-secret weapon to avoid caking
Author: Jill Caskey
Air injection plays a critical role when indirectly cooling fertilizer.
Small amounts of purge air at precise points during the heat transfer process are crucial in mitigating caking prior to packaging, storage and transport.
But why does caking occur in the first place?
Because most fertilizers are hygroscopic. This means at a precise humidity (critical relative humidity, or CRH), they will start to absorb moisture. This can result in a fertilizer granule or prill that becomes soft and sticky, or starts to crack and creates dust.
The main fertilizers where caking is of concern are NPKs, Urea and MAP/DAP products. But let’s not forget about those fertilizer salts as well, such as ammonium chloride and potash. We tend to label these as “high-maintenance fertilizers”, or HMFs.
So where does this moisture come from? For those who want to geek out with me, keep reading. Otherwise, feel free to skip the next section.
As the fertilizer enters the vertical plate cooler, air is entrained within the product, filling the pore space. Say the air is entering at the fertilizer temperature of 75°C and a relative humidity of 35%. It will contain about 96 g of water per kg of dry air. As it’s cooled, to say 40°C, the air will only be able to hold 49 g of water per kg of dry air at 100% relative humidity. This means 47 g of water per kilogram of dry air will condense out of the air as it is cooled. This is where the moisture comes from.
Indirect fertilizer cooling
Take a situation where you’re indirectly cooling fertilizer with vertical plate technology. The material flows downward by gravity between the heat exchanger plates. A cooling fluid flows through the plates to cool the fertilizer by conduction (counter-current to product flow).
In addition to the water vapour that can condense out of the air, the moisture in a hygroscopic fertilizer will also transfer from the product into the air in the pore space as it is cooled, contributing to the total water load. If the amount of water vapour contained in the pore space air is above the dew point temperature, condensation will occur at the cold plate surface. Add to that the potential of dust being present, and you get ugly caking!
That’s where air injection comes in. By injecting a small amount of purge air at a target dew point –which is below the temperature of the fluid-cooled plates – we can prevent condensation from forming to avoid caking in the heat exchanger.
Air is not used as the cooling medium in indirect plate cooling. This means less than 2,000 Nm3/h or 1,200 SCFM of purge air is needed at a particular location in the cooler. The design ensures the water temperature profile and plate temperatures are always above the dew point of the air in the void space.
Not only does this process eliminate condensation and caking, but it requires less air compared to direct-contact fertilizer coolers, such as fluid beds. And less air means less energy required.
For example, industry estimates peg power consumption of up to 1.7 MW for cooling 100 tph of fertilizer using a fluid bed. This can be translated to annual electrical energy costs of approximately $744,000 US.
By indirectly cooling fertilizer with vertical plate technology, operators can reduce power consumption to 170kW for the same fertilizer capacity. This results in annual energy savings of more than $600,000 US.
Want to learn more? Contact a Solex representative today.
About the expert
Jill Caskey, Global Sales Director, Solex Thermal Science
Jill joined Solex in 2011, with the past eight years focused as a technical inside sales representative responsible for lead qualification and technical equipment design (thermally and mechanically). Jill offers broad experience with many markets and application guidelines, including in-depth experience with fertilizer applications and equipment design. Contact Jill.
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