nitrogen and syngas

Number 313 September | October 2011

A single solution to four challenges

urea prilling plants

Nitrogen+Syngas 313 | September - October 2011� 3

Most of the urea plants around the world operate a prilling tower; the majority of these plants are now

several decades old and their capacities have been gradually pushed to about 120 to 130% of their original design capacity. These plants typically face several problems or challenges: high ammonia and dust emis-sions from the prilling tower, low prill quality, competition from granules in the market and demand from farmers for additional nutri-ents for their crops. The solution to all four challenges can be provided by Sandvik’s Rotoform pastillation system.

tower to reduce the urea dust emissions. In a limited number of cases, the dust scrubber is combined with an acid washer to reduce the ammonia emissions. These scrubber systems are only feasible in so-called forced draft prilling towers, where air fans are avail-able. Scrubber systems are not feasible in natural draft prilling towers as the pressure drop over the scrubbers is too high, unless extra air fans are installed.

The amount of urea dust and ammo-nia emissions are directly related to the load on the prilling tower (there is a linear relationship)1.The relative emission figures

A�single�solution��to�four�challengesTypical problems and challenges experienced by ageing urea prilling plants include high

ammonia and dust emissions, low prill quality, competition from granules in the market and

demand from farmers for additional nutrients for their crops. In this article, Mark Brouwer

of UreaKnowHow.com explains how the Rotoform pastillation system from Sandvik Process

Systems meets these challenges and outlines a number of additional benefits.

High emissionsIn prilling towers the urea melt falls down inside a concrete structure, cooling and crystallising against a large quantity of upward-moving air. In some prilling towers this air flow is created using fans; in others natural draft is used. However while the draft is created, the air becomes loaded with urea dust particles and ammonia, and is typically emitted into the atmosphere from the top of the prilling tower.

In some cases a dust scrubber is installed on top of or alongside the prilling

Farmers prefer pastilles to prills.


4� Nitrogen+Syngas 313 | September - October 2011

Fig 1: Sandvik Rotoform pastillation process for urea

can be influenced by the air flow, but one should realise it is not a solution but a dilu-tion of the problem.

Low prill qualityIncreasing the load on a prilling tower can have negative consequences for the prill quality. Higher moisture contents and higher temperatures cause more dust for-mation and an increased likelihood of cak-ing problems2.

Caking can be so severe that the prod-uct becomes damaged when it is loaded on trucks or cars. Part of the product will be lost and complaints from customers may follow. Measures need to be taken.

Competition from granulesThe quality of prills is significantly lower than that of granules, the main differ-ence being the lower strength and smaller size of prills. Research has shown that once a farmer has used granules, he will not choose prills again, unless the price is lower. Most new urea plants are large scale granulation plants and are located in low feedstock areas; these plants are export driven and compete with prills.

From bulk to specialty productsUrea prilling plants that are several dec-

ades old and located inland will typically have built up a strong relationship with its customers. When competitors with very low feedstock costs offer their products for a lower price, it can be very difficult to compete on price if your plant capacity is lower and your feedstock costs are higher.

However, a strong customer relation-ship does provide a competitive advan-tage. More and more farmers are starting to realise that their crop needs more nutri-ents than just nitrogen (N). Other primary plant macronutrients include phosphates (P) and potassium (K); others – carbon (C), oxygen (O) and hydrogen (H) – are avail-able through air (via the photosynthesis process) and water. Then there are the secondary macronutrients: calcium (Ca), magnesium (Mg) and sulphur (S) and the micronutrients boron (B), copper (Cu), iron (Fe), manganese (Mn), zinc (Zn), chloride(Cl), nickel (Ni), and molybdenum (Mo), which are also essential for plant growth but required in much smaller quan-tities. Finally, silicon (Si) and cobalt (Co) are also beneficial for some plants in even smaller quantities. A deficiency of any sin-gle nutrient is enough to limit growth.

In general, nitrogen application effi-ciencies are very low as between 30 and 50% of the nitrogen supplied to the soil is typically lost to air and water, causing more and more environmental problems. In Canada, for example, urea has become

the broadcast fertilizer of choice for many winter wheat growers on the Canadian Prai-ries, especially since ammonium nitrate fertilizer was removed from the market. However, the problem with surface applica-tions of urea is that the N is susceptible to ammonia volatilisation. The urea molecule, in the presence of moisture and the soil enzyme urease, converts to ammonium carbonate, which can lead to the produc-tion of ammonia gas.

Adding slow release substances to urea prills contributes to higher efficiencies, resulting in less environmental problems and less fertilizer product lost and wasted.

Producing specialty fertilizer products based on urea therefore fulfils farmers needs; these are considered to be higher value products attracting a higher price. Having a close relationship with your cus-tomers gives you the competitive advan-tage and opportunity to produce and sell higher margin specialty products.

The solutionThe solution to all four of these challenges can be met by Sandvik’s Rotoform pastil-lation system.

Figure 1 shows a simplified flow scheme of the Rotoform system.

The feed to the Rotoform unit is urea melt with a concentration of 99.6 wt-% and in existing urea plants can be branched off


Fig 2: Rotating shell delivers droplets of the required size

Urea pastilles on the steel cooling belt


from the urea evaporation section down-stream of the urea melt pumps.

Urea is introduced under pressure (2-3 barg) in molten form to the drop former. The Rotoform HS (High Speed) drop former, patented by Sandvik, consists of a heated stator and a perforated rotating shell that turns concentrically around the stator to deposit drops of urea across the full width of the belt. The circumferential speed of the Rotoform unit is synchronised with the speed of the steel belt cooler ensuring that the drops are deposited on the belt without deformation and, after solidification, results in regular pastilles

with an optimum shape. The rotating shell (Fig.2) contains

rows of small holes which are sized to deliver the required product size. The heat released during crystallisation and cooling is transferred by the stainless steel belt to the cooling water. The cooling water is sprayed against the belt underside, absorbs the heat and is collected in pans, cooled in a cooling system (cooling tower) and returned to the Rotoform units.

Under no circumstances can the cool-ing water come into contact with the urea product.

The use of formaldehyde is not neces-

sary in this technology to realise pastilles with a high crushing strength. The pastilles are very uniform.

After solidification the pastilles are smoothly released from the steel belt via an oscillating scraper. The product then falls directly onto a conveyor belt for trans-fer to storage. The section above the mov-ing steel belt is enclosed with a hood and vented to an existing vent system.

There are no large air flows involved in this technology and there is no visible urea dust emission. The ammonia vapours that are produced can easily be captured in a simple atmospheric absorber; this results

Below left: Rotoform skid mounting equipment

showing the hood which captures and ammonia and

dust emissions. Right: Front view of installation of

urea plant with four Rotoform units.


0

200

400

600

800

1,000

1,200

1,400

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39

prills, t/d pastilles, t/d

Fig 3: Production capacity of prills and pastilles in t/d vs age of the plant in years

0

20

40

60

80

100

120

140

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39

dust emission, mg/Nm3 NH3 emission, mg/Nm3

Fig 4: Dust and NH3 emission from prilling in mg/Nm3 vs age of the plant in years

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39

Excellent

Good

Problematic

Fig 5: Prill quality vs. age of the plant in years

6� Nitrogen+Syngas 313 | September - October 2011

Prills Granules Pastilles

Average diameter, mm 1.5-1.9 2-4 2-5

Moisture, wt-% 0.15-0.30 0.10 0.10

Formaldehyde content, wt-% 0.1-0.3 0.3-0.55 Only when (in case of export) required

Shape Spherical Spherical Split-pea

Crushing strength, N 12 (1.7 mm) 40 (3 mm) 40-75 (3 mm)

Product temperature, °C 60-80 50-60 40-45

Table 1: Typical properties prills, granules and pastillesin negligible emissions of ammonia and urea, a unique feature of this technology.

By installing one or more Rotoform lines in parallel with a prilling tower, the load on the prilling tower will be reduced, leading to lower ammonia and urea dust emissions from the prilling tower and a higher prill quality. For example, reducing the load on the prilling tower from 1,300 to 1,000 t/d by installing two Rotoform lines in parallel will reduce the ammonia and dust emission figures by approximately 23% (300/1300).

Let’s show an imaginative but realistic example how the switch from prills can take place.

Figure 3 shows the production capac-ity of a typical urea plant over time. The plant has a design capacity of 1,000 t/d prills and during its first years will be able to increase its actual production to some 1,300 t/d prills, typically by making use of the design margins and by creep debot-tlenecking technologies. Dust and ammo-nia emissions from the prilling tower will increase in line with the increase in pro-duction as these are linear related.

So, as plant capacity increases from 1,000 t/d to 1,300 t/d, dust emissions will increase from 100 mg/Nm3 to 130 mg/Nm3 and ammonia emissions from 80 mg/Nm3 to 104 mg/Nm3, as can be seen in Fig.4.

When pastilles are produced the load on the prilling tower will be reduced, as will the dust and ammonia emission figures. In this way one can reach emission figures which are acceptable to local governmen-tal regulations.

Furthermore, while the load on the prilling tower decreases, the prill quality increases. For example, Figure 5 shows that after start up of the plant the prill qual-ity was considered good. After increasing the load on the plant and prilling tower the prill quality became problematic. Too much dust, high moisture content, high prill temperatures, low strength and cak-ing troubled the production and sales of the prills. As pastille production reduced the load on the prill tower, the prill quality became good and even excellent again.

Finally, if you operate a forced draft prilling tower, the power consumption per tonne of urea will reduce by about 9 kWh/t as the Rotoform unit consumes less power than a forced draft prilling tower.

The quality of the pastilles is bet-ter than the quality of prills. It is similar and in some aspects even better than the quality of granules. Table 1 provides an overview of the product properties of


Fig 6: Flow scheme to produce urea + AS

Blending unit to produce urea +AS


AN + AS 15% NPK Sulphur bentonite 10%

Urea + KCl 20% Urea + KCl 43% Urea + sulphur

Urea Urea + AS 20% Urea + AS 40%

prills, granules and pastilles. For example, the strength is similar to the strength of granules even without any formaldehyde. The size of the Rotoform product is the same as for granules, moreover the size can be easily varied between 1 and 5 mm. The pastilles are extremely uniform, even more than granules. It goes without saying that the Rotoform product quality is signifi-cantly better than prills. Farmers who have applied the pastilles have been very satis-fied and were prepared to pay a premium price above prills.

From Table 1 one can conclude that granules and pastilles have some sig-nificant advantages over prills: Granules and pastilles can be produced with larger average diameter (which can be specified over a relatively wide range) and are signifi-cantly stronger than prills.

A Rotoform system with a typical capac-ity of 130-140 t/d can be used to produce all kinds of specialty urea products in addi-tion to fertilizer grade urea. The following products have been already produced with

a Rotoform pastillation system:l fertilizer grade urea l technical urea for urea-formaldehyde,

melamine, ad blue productionl urea blended with macronutrientsl urea blended with micronutrientsl urea with ammonium sulphatel ammonium nitratel ammonium nitrate derivatives (KCl,

Zeolite, AS < 10% and NPK) l NPK complex fertilizer from nitrate and

urea routel calcium nitratel magnesium nitratel sulphur bentonite

The production processes for these specialty urea products is fairly simple and straightforward. In some cases a liquid additive can be simply injected into the urea melt, in other cases a blending unit is applied to grind and mix the solid addi-tives in the urea melt (Fig.6).

For example, for urea plus ammonium sulphate (AS), urea melt and solid AS are mixed and grinded in two steps in a blend-

ing unit, which is a standard packaged unit. Up to 40 wt-% AS has been mixed with urea with excellent results.

The introduction of specialty urea prod-ucts can be realised smoothly by start-ing off with a single Rotoform line, which can be brought into and out of production whenever required, and then by adding additional lines in parallel when market demand increases. The pictures above show various specialty urea products. nх

References1. 2010 02 Brouwer UreaKnowHow.com Dust

Emission of Prilling Towers.

2. 2010 01 Brouwer UreaKnowHow.com Forms of Urea & Background of Caking.

3. 2010 03 Baeder SPS UreaKnowHow.com Rotoformer Pastilles the Sustainable Pre-mium product.

4. 2010 04 Baeder SPS UreaKnowHow.com Rotoformer applications.

5. IFA website.

6. Nitrogen & Syngas 2011 Presentation, Kumar Swamy, Sandvik Process Systems.

Are you interested in granulation of urea, urea & mixtures, ammonium nitrate and mixtures and sulphur bentonite?

Take advantage of Sandvik Process Systems’ proven Rotoform granulation system and start producing high-quality granules in a more efficient, cost effective and environmentally-friendly manner.

The process, already the world’s most widely used system forsulphur solidification, is ideal for debottlenecking or revamping urea and other fertilizer processing facilities, enabling fast, controlled production with no need for recycling, scrubbing or crushing.

Highly efficient process• From liquid to solid in a single step• High quality, virtually dust-free pastilles • Suitable for urea and nitrate fertilizers, technical urea,

AdBlue, animal feed, speciality urea products and more.

Environmentally-friendly production• Indirect heat transfer – no contact between product and

cooling media • No separate air cleaning necessary; low dust/ammonia

emissions• Low sound emissions

Significant commercial advantages • Initial investment costs significantly lower than

competitive systems • Very low energy consumption: 4kW/t for urea granulation • No use of chemicals such as formaldehyde.

Sandvik Process Systems P. O. Box 42 62 • D-70719 Fellbach/Germany Tel: +49 (0) 711-51 05-0 • Fax: +49 (0) 711-51 05-196 • www.processsystems.sandvik.com

E-mail: [email protected]

Urea + sulphur Sulphur bentonite

Urea + AS Urea

Upgrade granulation of urea, sulphur bentonite

and other fertilizers

Upgrade granulation of urea, sulphur bentonite

and other fertilizers

PS230_ENG_Fert_210x297 17.10.11 10:47 Seite 1

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