Urea finishing technologies: selecting the optimal solution

FERTILIZER FINISHING

Urea finishing technologies: selecting the optimal solution

Granulation technology generates a premium urea end-product in large volumes capable of withstanding lengthy storage and extreme shipping conditions. Prilling technology, meanwhile, given its typically lower investment cost, can be an attractive option for smaller-scale urea producers supplying local markets. Stamicarbon’s Dr Wilfried Dirkx, Ahmed Shams and Branislav Manic explore the technology options for urea finishing.

The Pardis III granulation plant.

PHOTO: STAMICARBON

Urea production continues to grow globally in response to rising fertilizer demand. At the same time, urea finishing plants are becoming more technologically advanced and their capacities are expanding to keep up with this ever-increasing demand.

Selecting the most suitable finishing technology, from the various options available, should depend on the specific configuration of each plant, its scale, and market requirements.

Fluid bed granulation, for example, is often the preferred choice for most largescale plants, especially when urea is produced for export and/or transported over long distances. This preference is due to the superior strength, handling, and shipping behaviour of the granules generated by fluid bed granulation.

Large-scale urea plants located in countries with abundant, low-cost feedstocks benefit from competitive production economics that enable them to manufacture urea for the export market at a low cost and high margin. The use of large capacity granulation plants, which can reach up to 5.000 t/d, also enables these urea producers to enjoy the economies of scale.

Prilling technology, in contrast, offers specific advantages – including capex savings, energy efficiency and operational simplicity – for smaller-scale urea plants targeting local markets.

Stamicarbon, the nitrogen technology licensor of the Maire engineering group, has extensive experience in licensing urea granulation and prilling plants for various markets. The company, by offering continuous innovation and a wide choice of technologies, can provide its customers with an optimal fertilizer finishing solution, one that is tailored to meet their specific needs, production objectives and market demands.

Scaling up granulation technology

In Stamicarbon’s fluid bed granulation process, concentrated urea melt (98.5 wt%) is introduced to the granulator unit via specially designed spray nozzles (see photo). These proprietary nozzles coat each seed granule with a thin film of urea melt, layer upon layer, until granules grow to reach the required product diameter.

Stamicarbon’s fluidised bed design delivers considerable opex savings, compared to other granulation processes, as it reduces the formaldehyde content of the final product and generates less dust. This allows the granulation plant to continuously operate for 2-3 months without a shutdown for cleaning. Stamicarbon’s MicroMist™ Venturi (MMV) Scrubber can also be installed to meet the latest environmental regulations, being capable of bringing down urea dust emissions to below 10 mg/Nm3 .

Stamicarbon began developing fluid bed technology for urea granulation plants in the 1970s. This eventually resulted in the commissioning of the company’s first pilot granulation plant for Grodno Azot in Belarus in 1998. This project involved the conversion of an existing small granulation unit (280 t/d) to Stamicarbon’s new Granulation Design.

Stamicarbon’s fluidised bed granulator incorporates proprietary film spraying nozzles.

PHOTO: STAMICARBON

Stamicarbon later scaled up its fluid bed technology during a revamp of a Canadian urea plant in 2003. Two existing granulation lines (625 t/d each), originally designed by another licensor and supplying the local market, were converted to Stamicarbon’s Granulation Design as part of this revamp project.

Subsequently, Stamicarbon’s first grassroots urea granulation plant entered production in Egypt in 2006. This plant, initially designed with a capacity of 2,000 t/d, was later revamped to run at a capacity above 2,500 t/d.

In recent times, more than 20 granulation plants based on Stamicarbon’s Granulation Design have been licensed and put into operation as part of the company’s LAUNCH FINISH™ series. These plants have all met strict performance guarantees, despite having different configurations, capacities, ambient conditions, emissions and product requirements.

Currently, the Pardis III plant in Iran (see first page photo) is the largest operational granulation plant based on Stamicarbon’s standard Granulation Design. This started up in 2018 and can operate at 110 percent of its nameplate capacity of 3,250 t/d with a turndown ratio of 60 percent.

Stamicarbon originally secured the contract for the Pardis III project in 2011. The granulation unit is connected to a fertilizergrade urea plant. This uses Stamicarbon’s Pool Condenser Design and operates under difficult ambient conditions. The plant’s performance has met customer expectations, due to the reliability of this design, and achieves an on-stream time of more than two months during extreme heat conditions in summer.

Constant improvement

Stamicarbon first introduced its Optimized Granulation Design in 2008 (Figure 1). This new design has reduced the capex and opex of urea granulation plants, as it requires fewer equipment items while maintaining reliable performance and high on-stream times.

In particular, the elimination of the two main fluidisation fans delivers significant cost savings by cutting power consumption. Increasing the length of the granulator’s cooling zone, compared to the standard design, also allows three other major equipment changes:

• Firstly, the elimination of the fluid-bed granulator cooler

• Secondly, the replacement of the fluidbed product cooler with a solids bulk flow cooler

• Finally, the elimination of the granulator cooler scrubber and its accompanying pumps and fans.

Instead, product granules are cooled further in the fourth compartment of the granulator. Only a small crusher feed cooler is necessary, separate to the fluid bed cooler.

The Optimized Granulator Design is characterised by:

• The use of film spraying nozzles

• Low urea dust formation during granulation

• Reduced formaldehyde content in the final product

• High product quality.

The fewer equipment items necessary significantly lowers the plant’s footprint and reduces the overall investment cost. Cumulative savings from lower transportation and insurance costs, together with the reduced footprint and construction costs, all contribute to a lower total capex. Less operational equipment also delivers a maintenance cost saving and lower opex.

Fig. 1: Flowsheet for Stamicarbon’s fluidised bed granulation process

The first plant based on the Optimized Granulation Design was commissioned in 2015 for Shahjalal in Bangladesh. This 1,760 t/d capacity plant operates reliably with low maintenance costs and has met expectations on reduced power consumption. The quality of the granulated urea produced also exceeds standard commercial requirements, despite a formaldehyde content of less than 0.3 percent in humid ambient conditions.

Acidic scrubbing

Any ammonia present in the urea melt is released during the crystallisation process in the granulator. Acidic scrubbing is therefore required to efficiently capture this ammonia and prevent its release into the atmosphere.

Acid scrubbing is achieved, after the initial dust scrubbing stage, by bringing ammonia-laden air into contact with sulphuric or nitric acid injected into a circulating aqueous solution. The circulated acid reacts with ammonia, effectively reducing its concentration in the exhaust air, to generate an ammonium salt. This can be sent outside battery limits (OSBL) or, by applying sulphuric acid, incorporated into the urea end product. Valuably, this eliminates waste streams and avoids their disposal at the plant’s battery limit.

The salt generated by the scrubbing system – for example, liquid urea ammonium sulphate (UAS) – is in the form of a solution containing about 55 weight-percent (wt-%) water. This cannot be directly mixed with the main urea melt feed entering the granulator (via nozzles) as this only contains 1.5 wt-% water.

Instead, the water content of the UAS solution needs to be controlled and reduced using a a dedicated evaporation step (as shown in Figure 1). This allows the recycled UAS solution to be introduced into the granulator together with urea melt. The resulting granules can be sold as standard urea as the sulphur content of the final product is minimal (typically about 0.05-0.1 wt-% sulphur).

This scrubbing configuration can address the increased need for sulphur as a plant nutrient by concentrating and recycling liquid UAS so it leaves the granulation plant in a solid form by combining with molten urea. Additionally, Stamicarbon has developed a unique UAS process that allows urea to be granulated with much higher concentrations of ammonium sulphate. This adds solid ammonium sulphate to molten urea using a flexible modular process. Existing granulation plants can be adapted to this new design configuration via relatively minor modifications, upgrades to construction materials and the addition of some extra equipment.

Fig. 2: Flowchart for Stamicarbon’s prilling process

Granulation capacities of up to 5,000 t/d

In recent decades, there has been a rapid increase in projects which increase the maximum capacity of operational plants. Stamicabon’s practical experiences with large capacity urea granulation plants (above 3,000 t/d) have been positive to date, these having met the overall onstream requirements of customers.

Stamicarbon has also investigated how to design even larger capacity granulation plants – without compromising its proven design philosophy and product quality. Results suggest that scaling up the Optimized Granulation Design to 5,000 t/d now look feasible, as the main operational and manufacturing challenges have been overcome and suitable supporting measures have also been identified.

Based on the outcome of these investigations, Stamicarbon has determined that a single-train 5,000 t/d configuration is preferable, versus a dual-train configuration of 2500 t/d each, as this should deliver a capex saving on the total investment of around 30 percent. Currently, Stamicarbon is looking forward to developing this largescale granulation concept further.

In 2019, Stamicarbon licensed its first single-line 4,000 t/d urea granulation plant. This project has entered construction, having completed its design phase, and is equipped with the MMV Scrubber to comply with environmental regulations. In 2022 and 2023, Stamicarbon also secured licensing contracts for two more 4,000 t/d urea granulation plants for a customer in Africa.

Cost-effective prilling for local markets

Prilling stands out as the most cost-effective finishing method and is a particularly attractive option for producers wishing to supply high-quality fertilizers for their local market. Several considerations do, however, need to be taken in to account before selecting conventional prilling technology, including product specifications and process challenges.

While their nutrient value is identical, prills are weaker than granules, with a crushing strength below 1.5 kg, and are therefore more vulnerable to breakage during handling and storage. The lower strength of prills, by contributing to moisture absorption and caking during storage, and potentially segregation during handling, can lead to product losses prior to application. Furthermore, the prilling process itself can generate considerable amounts of dust, sometimes exceeding one percent of production output. All of these challenges therefore need to be addressed for prilling technology to be successfully adopted.

Addressing prilling challenges

Stamicarbon, as a leading urea finishing technology provider, has developed a prilling technology that overcomes most of these challenges. This generates prills of the highest quality for urea producers while ensuring that the capex and opex of the finishing process remain highly competitive.

The rotating prilling buckets used in Stamicarbon’s prilling process (Figure 2) are designed to evenly distribute liquid urea droplets in the prilling tower. These droplets crystallise into prills as they fall through a prevailing airflow moving in counter-current. Stamicarbon’s process design offers several advantages, including:

• A sharp size distribution

• Uniform prill distribution across the entire prilling tower area

• Improved heat transfer between falling prills and the airflow

• Lower prill temperatures

• Suitability for larger plant capacities.

Prilling machines built to Stamicarbon’s design are effective over a wide operating range and help to reduce the particulate emissions from prilling towers. Indeed, their dust generation, being significantly lower than granulation units, contributes to plant efficiency.

Overall, Stamicarbon’s ‘next-generation’ prilling tower design, when used in combination with Jet Venturi™ scrubbing technology, can produce high-quality, large-diameter urea prills while substantially reducing particulate matter emissions, plume opacity and ammonia emissions.

Operational experience with urea plants in China has shown that Stamicarbon’s prilling process works excellently in combination with its Ultra Low Energy melt technology, thereby helping to achieve the lowest overall plant energy consumption. The latest contract for a Stamicarbon Ultra-Low Energy urea plant in China, announced in October 2023, will incorporate the company’s prilling design. The plant, located in Shouguang city in Shandong province, is the company’s eighth urea licensing and equipment supply contract in China.

Conclusion

The choice between granulation and prilling for urea finishing typically depends on the plant’s specific requirements, budget and market needs. Granulation technology provides a premium end-product in large volumes that can withstand lengthy storage and extreme shipping conditions. This makes granulation technology more attractive for urea producers located in lowcost, feedstock-rich regions who are aiming to export high-value commodity products.

In contrast, installing a prilling plant, given its typically lower investment cost, can be an attractive option for smaller-scale producers in other locations supplying specific markets. Despite its technological challenges, prilling can be a viable option for many urea producers and, with the right technology in place, offers benefits in terms of cost, energy efficiency and operational simplicity.

For each and every individual situation, Stamicarbon can identify a tailored finishing process and find the right solution – by offering hands-on expertise in granulation and prilling technologies that caters to all the diverse segments of the urea market.