Problem No. 39: Warehouse curing for urea granules

• Enclosed building capable of protecting the product from the weather and sunlight
• Temperature between 5 and 50°C
• Protect the fertilizer from moisture, which can cause lumps and dust, affecting spreading capabilities
• Good stock management e.g. carried out on a first in first out basis.

Muhammad Kashif Naseem from SABIC, Saudi Arabia starts this round table discussion: Can anyone explain the warehouse curing phenomenon for granulated urea product and the maximum time required for this, particularly in the summer season in the Gulf region with a product outlet temperature of around 50°C?

Mark Brouwer from UreaKnowHow.com replies: Intentional aging or curing of fertilizer in a storage pile prior to bagging or bulk shipment is referred to as curing. Chemical reactions that cause caking bonds proceed to near completion during the curing period. The heat of reaction retained in the curing pile speeds up the completion of the reactions. After curing there is reduced tendency for additional bonds to develop. What is your experience of the time required for curing?

Akbar Ali from SAFCO-SABIC, Saudi Arabia raises a good question: In urea granulation the curing time is 60 to 72 hours. Can we understand from your comments that the length of curing time is due to chemical reactions causing caking bonds or is there any other reason behind this? Have you any literature on curing time for urea granules showing how we can decide on the curing time for a specific product temperature?

Muhammad Kashif Naseem from SABIC, Saudi Arabia replies: Almost three days is considered normal, but can we reduce this or is there any method available to reduce this warehouse curing time?

Easa Norozipour from KHPC, Iran answers the last question: As we know, curing is a phenomenon for stabilisation of a humid and high temperature product like urea. In this case, I think the curing time also depends on the following:

• water content in the final product;
• amount of ammonia content in the product;
• temperature of the final product;
• the delta temperature between warehouse and environment;
• height of the urea pile in the warehouse;
• relative humidity of the environment;
• size of the final product.

All of these items can influence the curing time of the final product. In our experience three days is the normal time. Zaheer Abbas from SAFCO, Saudi Arabia shares his experiences: In my experience, our final check is the temperature differential between the urea granules and warehouse ambient conditions. Whatever the humidity outside, it will finally affect the temperature differential. If we are able to maintain the recommended temperature differential (dT=+-5°C), the curing time will be automatically adjusted. I would recommend taking some temperature measurements before deciding on the curing time for any warehouse. Other contributing factors are final cooler efficiency and the difference between wet bulb and dry bulb temperature inside of warehouse.

Mark Brouwer from UreaKnowHow.com provides more information: Another important consideration is that caking behaviour is in part determined by the amount of dust and fines in the product.

Easa Norozipour from KHPC, Iran shares more information: Sometimes near the Gulf the humidity is very high and we have experienced a lot of water droplets in and around the urea belt conveyer. As we know, there aren’t any warehouse and belt conveyer channels with 100% sealing. Therefore the relative humidity of the environment can increase the caking tendency. The mesh size (dust) of the final product is also an important factor as mentioned already by Mark.

Akbar Ali from SAFCO-SABIC, Saudi Arabia asks a question: Do you have any written information regarding this subject? I could not find anything in the Stamicarbon documents.

Janusz Mac´kowski from ZCh”Police”, Poland provides a reference: At theTenth Stamicarbon Urea Symposium 2004 (Round Table Presentation) there was an interesting article “Improvements in the physical quality of urea prills”. I think it is also related to this topic.

Nick Sutherland from SOLEX Thermal Science Inc., UK provides some useful information: As we know, urea (and all other fertilizers) are hygroscopic and as such if sent to the storage area while still at a temperature above that of the local ambient conditions any moisture in the air will be attracted to the product, this is obviously a problem in climates where the humidity is high and if the product has even a small amount of dust then “caking” is likely to occur.

To reduce this situation I recommend you consider the Solex Bulk Flow Cooler which can cool the product closer to the ambient temperature with the added benefit of not generating any further dust, in fact the unit, in conjunction with a de-dusting system, can reduce the dust load.

Muhammad Kashif Naseem from SABIC, Saudi Arabia contributes to the discussion: Product dust formation is influenced by:

• moisture or humidity;
• differential temperature of atmosphere and product;
• free ammonia in product;
• strength of product;
• pressure in final product system (prilling tower + granulator scrubber);
• frictional resistance of the conveying system. Methods to prevent dust formation include:
• UF-85;
• Urecoat/Ureasoft;
• seeding technology;
• suitable temperature and pressure in final concentrator.

Mark Brouwer from UreaKnowHow.com provides more information: Freshly produced urea granules need some kind of “curing time” to reach their final anticaking quality. Normally 1 or 2 days will do the job.

The product leaving the granulator has been screened to remove the undersize and oversize particles in order to produce an end product of typically 95% between 2 and 4 mm. That means that your end product is a mixture of granules from 2 mm up to 4 mm diameter. That seems to be very close but in terms of weight per granule the difference is huge. The weight of a 4 mm granule is about 8 times higher than the weight of a 2 mm granule. Because of this big weight difference, a 4 mm granule will be more difficult to fully dry out than a 2 mm granule. The result is that a 2 mm granule has a lower moisture content than a 4 mm granule. The moisture figure you get from the laboratory during production is the average moisture of the granule mixture. You can easily check this by determining the moisture content of the fines recycle product and of the oversize before crushing. You will find a very big difference.

When you dump your freshly produced end product on a pile in storage, this moisture difference will slowly disappear: finer product will pick up moisture from the bigger product. This moisture “migration” will cause some caking in the fresh pile. Once the moisture migration is finished and you break the slight caking (during shipping) this initial caking will not happen again.

Of course that does not mean that your product will be 100% cake-free: the normal rules remain, low moisture is better than high moisture, low temperature is better than high temperature, a low pile is better than a high pile, etc.

Adel Tabei from Pardis Petrochemiical Company, Iran raises new questions: Is it possible for the caking phenomena to occur during shipping (at high humidity condition) after sufficient curing time of urea in the warehouse? Or if we have a sufficient curing time in the warehouse, will that prevent caking during shipping? Or are these not related?

Gholamali Soroush from Shiraz Petrochemical Complex, Iran replies to the raised question: Besides the drying time, the urea curing process depends on humidity, temperature, piling height, and so on. Even if there are no changes in the temperature and humidity (good sealing) from the warehouse to the destination, there may be some reduction in urea quality that reduces the urea crushing strength. Urea powdering (degradation) is related to the granule formation step and the quality of the anti-caking agent. In some destinations, the environmental dew point (humidity and temperature) may have less impact on the urea than the warehouse. However, if urea encounters a reduction in temperature and an increase of humidity from loading port to destination, some moisture can become trapped in the urea granule. Humid air can also diffuse into the urea granule leading to caking. For these reasons, besides the control of all the usual parameters, I suggest using good quality urea formaldehyde, and spraying anti-caking agents after the granulation step to ensure customer satisfaction.