Prilling Technology

Seminar presented bySeminar presented byRAJESH KUMARRAJESH KUMAR

Management Trainee (Chemical)Management Trainee (Chemical)Employee ID – 0247/7164Employee ID – 0247/7164

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Prilling is widely used and simple method of commodity urea production.

The first prilling towers were concrete structures upto 40 mtr high.

They were equipped with melt disperser to spray urea urea melt , a scraper to discharged the finished product ,and fans to blow the cooling air in and out of prilling tower.

During operation the disadvantages of such conventional technology were revealed.

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These prilling towers were characterised by low height ,a static or rotating urea melt disperser and a conveyor or scraper for product discharge.

The product produced in such tower is characterised by very low quality .

The main fraction of prills is 1 to 2 mm,its content is 60-90%,prills strength is .3 to 0 kgf/prill.

Product temperature is 80 to 100 °C. Prills shouldn’t break or cake during handling and should have

uniform stable granulometric composition with sizes ranging from 2 to 3 mm.

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Height of Prilling Tower is 62.5 mtrs. With Prilling height 46 mtrs. Maximum capacity was 1000 MT/D Prills tempetrature is 55-60 °C . Still sprayers are being employed.

Product size normally varies from : +2.8 mm = nil -2.8 + 1mm= 80% -1mm = 20%

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Vibro – dispersion of Urea melt.

Prills cooling in a fluidised bed.

Exhaust air purification from Urea in a injection type scraper .

Conditioning agents added to the melt agents for Prills surface treatment.Technology for Urea dust control.

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In the majority of Urea prillers the melt jets flowing out from bucket holes are spontaneously divided into droplets of various diameters .

According to Toyo Engineering, when the urea solution droplets leave the holes of a sprayer and takes the spherical shape, a tail of urea solution is left behind which causes generation of microprills and very fine dust particles..

Partially choked holes of the sprayers due to inadequate cleaning can also form urea dust particles .

At high temperature of urea melt (> 136°C) and low partial pressure of ammonia, the following reaction occurs forming isocyanic acid.

NH2CONH2 NH3 + CHNO .As soon as it leaves the ⇄sprayer, as a result of cooling, urea is again formed. But the particle size of these urea prills are very small which causes urea dust formation. .

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Installation of Vibro-Prillers can significantly improve granulometric composition of Urea Prills.

Regular vibration of Vibro-Priller divide the melt jet coming out from bucket holes into equal size droplets.

The Vibro-Priller bucket is especially designed to reduce the product temperature by providing uniform distribution of equal sized prill across the tower section .

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Which results in maximum utilisation of the tower volume and cooling air going through it.

The Vibro-Priller is equipped with a Prilling bucket which design depends on the tower diameter and particle fall height.

The bucket holes can be directed under different angles for better uniform mixing of the melt jets providing more efficient prills cooling

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Prills of equal size are 80-95%, in some cases -99%.

Reliability and easy operation.

Possibility to operate with capacity of upto 100 TPH and more.

Environmental friendiliness since dust formation and caking are minimised.

Cost efficiency-electrical power consumption is upto 2 kwt/h.

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Prills of conventional design without fluidised bed coolers does not unable production of mono-disperse final product with size of 2-3mm(80%) and prills temperature not exceeding 50°C .

Product shipment or its storage in such condition results not only in product caking but also in degradation of granulometric composition and prills strength.

The above disadvantages can be minimised by installation of fluidised bed cooler.

After which Prills temperature does not exceed 50°C at the ambient temperature of 35°C max.

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The fluidised bed unit is based on heat and mass exchange processes between gaseous medium-air and solid material particle -Urea.

Intensive mixing of particle in the bed and tubulant air result in highly efficient heat exchange.

Reduce the temperature at the outlet of unit.

Fluidised bed can be located either in tower or next to Prilling tower on spare area.

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The expanded contact surface between solid material and cooling agent ,which increases intensity of heat exchange process.

High heat exchange characteristics ensure low temperature of product at the unit outlet-50°C max.

Continuous input and output of solid phase enables non stop operation of unit.

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Rational design.

Possibility to increase the particle size.

Product Quality improvement.

Easy and safe operation ,minimum requirement to maintenance.

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While construction or Revamp of Prilling towers special attention is paid environmental safety.

Prevention of ammonia and Urea losses as well as the sanitary impact very important for economics.

Because of high water solubility of Urea only wet type gas treatment is effective enough to be used for Urea dust trapping from the air after prilling tower and/or fluidised bed cooler before it is emmited to the atmosphere.

Air treatment unit should be of small size ,provide high purification rate at minimum hydraulic resistance and low energy consumption and highly efficient gas washers and liquid trap.

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High purification rate of exhaust air :ammonia content in treated air -40 mg/nm3 max ,Urea content -25mg/nm3.Reliable and easy operation ,minimum maintenance required.Convenient design –accessibility of all element of the dust traps.Small size of the unit.Low metal consumption.Efficient use of resources –return of trapped components to the process.

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Introduction of conditioning agent into the Urea melt before spraying inside the tower as well as Prills treatment by an anticaking agent at the Prilling tower .

Anticaking agent is used for surface treatment of the finished product .

They are used to prevent Urea from Caking.

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Environmental friendliness –Anticaking agents do not contain the

formaldehyde. Cost –effectiveness-Low consumption

indices. High efficiency-The agents ensure

production of the Urea which is not only moistre resistant but also gains strength during storage and transportation.

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Wide capacity range at high capacities. High Process efficiency : Minimum no of dynamic

equipment (Vibro- priller ,fan ,two pumps) and process stages (melt dispersion, Prills cooling ,air treatment).

Efficient use of resources :continuous steam consumption ,low temperature cooling agent,formaldehyde containing substances are not required .

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Cost –efficiency :High Process efficiency and efficient use of resources result in minimum operation ,repair and maintenance costs.

Environmental friendliness: Minimum hazardous emission to the atmosphere without use for acid reagents.

Product Quality : Resistance to mechanical effect during shipment and storage.

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Raplacement of old buckets and prillers for Urea melt spraying with advanced vibrating Prillers.

Installation of fluidised bed cooler. Installation of wet type dust traps for air treatment

after the Prilling Tower and FB cooler. Equipment of the conditioning agent injection

section with devices for introduction of anti-caking agents for prills surface treatment.

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Dust emission from urea plants, specially from prilling towers has gained importance as this is now governed by emission regulations.

In India, first survey was carried out by FAI. The range of emission was found to be 40-1100 mg/Nm3 depending upon the type of prilling and process followed in the plants.

In most of the plants of 70’s and 80’s, dust emission are far beyond the designed limit which was 50-250 ppm.

In European and American countries, urea dust emission limit varies from 50 to 80 mg/Nm3.

In America, they have used a formula to calculate dust emission limit. For 1750 TPD urea plant, with a prilling tower handling 6,00,000 Nm3/hr of air, the limit is 80 mg/Nm3.

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1. Origin of urea dust generation is uneven particle size distribution.2. At high temperature of urea melt (> 136°C) and low partial pressure of ammonia, the following reaction occurs forming isocyanic acid. NH2CONH2 NH3 + CHNO ⇄As soon as it leaves the sprayer, as a result of cooling, urea is again formed. But the particle size of these urea prills are very small which causes urea dust formation.

3. Higher air velocity in the prilling tower can also cause urea dust generation.4. Excess of free ammonia present in urea prills makes the prills brittle and increases formation of fine urea dust particles while spurting of melt solution through the sprayers.

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5. According to Toyo Engineering, when the urea solution droplets leave the holes of a sprayer and takes the spherical shape, a tail of urea solution is left behind which causes generation of microprills and very fine dust particles.6. Partially choked holes of the sprayers due to inadequate cleaning can also form urea dust particles.7. Very high bucket speed also causes very fine urea dust generation.8. Urea dust at the bottom of the prilling tower is mainly due to impact and attrition.9. In silos, fall of prills from a height, when heaps are formed, also causes dust formation.10. Breaking of lumps generated in silos causes dust formation.

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Good operational and maintenance practices can eliminate the causes at the source of occurring.

Adding a vey small amount of mineral acid prior to prilling which neutralizes free ammonia and thereby reducing dust formation.

Installation of suitable dust abatement system so that the emission are reduced to acceptable levels. For example-

1. Wet dust scrubbers 2. Electrostatic separator 3. Dry bag filter system 4. Brink mist eliminator – 95 % efficiency 5. Beco scrubber – 60 % efficiency 6. Strip ventury scrubber – 80 % efficiency

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No.

Parameter Before Retrofit

After retrofit

1 Temperature of Commercial Urea, °C

90-100 45-50

2 Granule static strength ,kgf/gran

.4 .6

3 Granulometric composition ,Mass fraction %

-Smaller than 1 mm 3-6 1-2

-1 – 2 mm 60-80 7-20

-2 – 3 mm 20-30 80-95

-Bigger than 3 mm 2.0-2.5 1.0

4 Emission of hazardous substance atmosphere

-Ammonia ,g/s 4.084 4.084

-Urea, g/s 5.796 4.576

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Due to increasing demand for Urea with improved quality and Urea based compound fertilizer ,low cost technology for their production are very prospective nowdays.

Advanced technology of Urea for Prilling have significant advantages against conventional Prilling technology.

They meet upto date requirement for process and economical efficiency.

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Pollution free environment is a very important factor without affecting the plant load and production.

Proper operational and maintenance practices in the plant. Air emissions should monitored from time to time. Effluents should be monitored regularly. Monitoring data should be analyzed and reviewed at regular

intervals and compared with the operating standards so that any necessary corrective actions can be taken.

If required new advanced technologies should be applied for better pollution control and to minimize product loss.

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R & D Institute of Urea seminar on advanced technology in Urea Plant.

Handbook on Fertiliser technology by FAI,New Delhi.

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Thank U!

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