ASN/UAS via sulphuric acid process

 

Highlights of the technology

In the production of Sulphur based fertilizers, usually the Ammonium Sulphate crystals are used to provide the Sulphur content. The second way is to directly synthesize Ammonium Sulphate from the reaction of Ammonia gas with Sulphuric acid, which gives a significant economic advantage when the sulphuric acid and the ammonia are available, especially for the production of high S content fertilizers like ASN 26:13 or UAS 33:12.

The presented Technology offers the opportunity to manufacture both products quality.

AS Synthesis

The pipe reactor technology is proposed to synthesize Ammonium Sulphate (AS) from sulphuric acid and ammonia.

The pipe reactor consists of:

  • a tubular reactor under pressure with a mixing chamber in which the NH3 gas and the sulphuric acid are mixed,
  • a flash tank where the steam produced by the reaction is separated from the hot AS slurry.

and its main advantages are:

  • Obtaining a AS slurry with very fine crystals within a very short time,
  • Flexibility of the adjustment of the final water content,
  • Easiness of operating (start-up of the plant, shut down of the plant),
  • Reduced maintenance cost due to the reactor simplicity.

Granulation process

The heart of the granulation process is a Drum Granulator composed of:

  • a rotary drum consisting in a cylindrical horizontal drum turning around its axis and fitted with lifters,
  • a fluidized table installed inside the drum and fed with hot air.

In the granulator, two operations take place simultaneously:

  • size increase,
  • crystallization and drying.

Those operations are obtained as follows: The lifters bring the seed material to the upper part of the drum from where it falls on the surface of the fluidized bed table. On the table the product is dried. The slope of the table allows the product to fall down along the table into the lower part of the drum. While falling, it is coated with ASN or UAS slurry that is sprayed through nozzles disposed in the granulator. The lifters return the freshly coated material towards the upper part of the drum to be further crystallized and dried on the fluidization table. The same cycle is then repeated as many times as necessary to reach the desired grain size.

Raw material control

The tiny AS particles formed in pipe reactor are immediately dispersed into Ammonium nitrate or Urea solution at the lower part of flash tank. A uniform ASN or UAS slurry formed, and its water content is precisely controlled to adjust the slurry physical properties.

Recycle Ratio Control

Due to the high efficiency of the granulation principle, it is easy to achieve a stable growth pattern of the granule size. To this effect, it is necessary to maintain a selected stable amount of recycling which provides the seed required for granulation and the basis for the equilibrium of the energy balance. Accordingly, the solid recycle is composed of fines from screens, crushed over-size granules from the crusher and part of the on-size granules production. Reliable crusher operation and efficient screens are thus important for the optimization of the recycling ratio and a high production rate.

Product cooling

The cooling of the product is achieved in a bulk flow heat exchanger where the cooling medium is water. The amount of gaseous emission in the plant is much lower than using a fluid bed cooler.

Scrubbing system

A scrubber system is installed for elimination of dusts and ammonia, in respect with EFMA-BAT recommendations or other applicable local standards.

The scrubbing solution is fully recycled to the Slurry Tank. The plant thus does not generate any liquid waste in normal operation.

Single train typical productions

 

Typical figures
  ANS 26:13 or UAS 33:12
1. Raw materials efficiency >99.9%
2. Water content ≤ 0.5%
3. Granulometry (adjustable) 90% min between 2 to 4 mm
4. Hardness (for granules 3mm) 4.0 kg

5. Utilities (per ton product)

Electrical power (kWh/t)

Medium Pressure Steam (kg/t)

 

~ 28 kWh/t

~ 310 kg/t

6. Nominal capacity per single train 300 000 mtpa

7. Pollution control

Ammonia in atm.

Dusts in atm.

Liquid effluent

 

<30 mg/ Nm3

<30 mg/ Nm3

No liquid effluent

 

Process Description

AS Synthesis

The sulphuric acid and ammonia are proportionally introduced into the pipe reactor (R3100) together with some process water or condensate. The synthesized solution is then flashed in the flash tank (S3101), from where the vapor is separated from the AS slurry.

The vapor flows out from the upper part of the flash tank and is condensed in the vapor condenser (E3102).

The AS slurry falls down to the lower part of flash tank and is immediately mixed with Ammonium nitrate solution for ASN production or urea solution for UAS production.

ASN or UAS slurry preparation

ASN or UAS slurry coming from flash tank (S3101) falls by gravity into the homogenizer tank (T3201), which receives also the dusts from the granulation loop and the wash solution from air scrubbing.

Some solid additives required for the production are dosed and fed to the homogenizer tank.

The homogenizer tank is fitted with a steam coil to keep the slurry temperature, and an agitator (A3201) ensures the mixing to give a homogenous slurry.

A small amount of ammonia is injected for controlling the pH in the homogenizer tank if necessary.

The superheated ASN or UAS slurry is sent to Drum Granulator (B3201) by means of Slurry Pump (P3201).

Granulation and drying

The granulation and partial drying are achieved in the Drum Granulator (B3201).

The slurry is injected by specific nozzles mounted on a spraying header. The number and position of nozzles are defined in order to spray the slurry uniformly on the curtains falling from the fluidised table. The temperature and pressure conditions of spraying are designed for insuring the optimum layering of liquid on the solid, in order that onion skin granulation is optimized.

At the inlet of the Drum Granulator, the Recycling Loop Conveyor (W3306) feeds the Drum Granulator with the recycled solid from the screens (S3303) and the Crusher (B3302). The granulometry and quantity of recycled product are controlled by the operation of the Crusher and recycling of on-size product, in order to achieve the control of the size growth inside the Drum Granulator.

The air flow rate to the Drum Granulator is well controlled by means of the air Blower (K3202).

The final drying of granules is done in a rotary drum dryer to ensure a very low moisture content and thus improve the handling and storage properties of product.

The off-gases leaving the Dryer Drum (B3301) pass through the dryer Cyclones (S3307) for the removal of dusts and then directed to the Drum Granulator.

Recycling loop

The Dryer Outlet Conveyor (W3301) discharges to the Elevator (W3302) which transports the granules to the Product Screens (S3303).

On the upper mesh of the screens, the oversize granules are separated and fall directly into the Crusher (B3302). The broken oversize granules drop to the Recycling Loop Conveyor (W3306).The on-size product grade granule stream is split in two parts: the required production output goes to the product cooler (E3401) through conveyors (W3305) and (W3401), whereas the excess flow is returned to the recycling loop conveyor (W3306).

Product cooling and coating

After cooling, the cold product is transferred to the Coating Drum (B3401) via the Cooler Outlet Conveyor (W3402).

In the Coating Drum (B3401), the product is coated with a liquid anticaking agent, before being sent to the storage outside battery limits.

Gas scrubbing

The off-gases leaving the Drum Granulator (B3201) are directed to the Scrubber (S3501), which treats also exhaust vents coming of the condenser (E3102) and Slurry Tank (T3201).

In the scrubber, the gases are washed by an acidic solution, in order to eliminate the remaining dusts and to neutralise the ammonia. The Scrubber Pumps (P3501) ensures the circulation of the acidic solution.

The solution in the Scrubber (S3501) is maintained under pH control by injection of nitric acid. To maintain the concentration of wash solution, a part of the washing solution is sent to the homogenizer tank (T3201) under flow rate control and the level in the scrubber (S3501) is controlled by process condensate coming from the condensates Tank (T3101) and addition of demineralised water from Battery Limits.

De-dusting System

A comprehensive plant de-dusting system is provided to ensure a clean working environment.

A centrally located de-dusting system is provided to recover the dust from the equipment de-dusting points. The major part of dust emitting locations in the process such as conveyor transfer points, elevator discharges, screens and crusher are connected to the de-dusting system by a series of ducts. Each de-dusting point is maintained at a slightly negative pressure by the action of the de-dusting Fan (K3503) which draws the contaminated air through the de-dusting Filter (S3503). The recovered dust is returned to the homogenizer Tank (T3201). In order to prevent clogging of dust recovery at the de-dusting points, each of these vents is fed with hot air coming from the Hot Air Fan (K3502) and de-dusting Air Heater (E3501).

Collect of solid waste

The granulation plant must be kept out of humid atmosphere: for this purpose, the presence of liquid inside the building shall be avoided as far as possible and building is maintained closed and heated. Consequently, the major part of solid waste coming from leakages or cleaning of handling equipment are recovered as solid and fully recycled inside the process.

A small remaining part which could remain on the ground floor, be collected in a pit and recycled as liquid solution to the Scrubber (S3501).