News 2008

Munich, March 6, 2008

High pressure air assists the production of fertilizers

Nobody would expect that for many years now, compressed air has taken on an important part in the small, but nevertheless very important sector of urea production within the fertilizer industry. We would like to show our readers, that BAUER KOMPRESSOREN, the specialists in the high pressure sector, are experts this field and that their knowledge in the application of compressed air has enabled them to become competent partners all over the world.
More than 60 years of experience developing and producing piston compressors certainly sets standards in quality and performance. It is at these industrial locations that BAUER custom-made compressed air units offer conditions that guarantee optimal supply and maximum economic viability.

History

Production plant from UREA

Due to fast-growing populations on our continents, it is no longer possible to cultivate all agricultural products without the use of fertilizers. As well as the most well-known classical nitrogenous fertilizers, like for example, the ammonium salts just to name one group, we would like to particularly look into the use of compressed air in the production of urea. Urea is a white crystalline body, made up of 46% nitrogen and is increasingly used in the agricultural industry.

Thanks to its particular advantages, urea has surpassed ammonium nitrate as a fertilizer for many years now.

Just to name a few, these are advantages:

Urea can be used on soil as a solid material, in a watery solution or as a spray on certain kinds of grain.

If used properly, with urea you can expect bigger harvests than with other nitrogenous fertilizers.

The chemical formula is: CO(NH2)2

The industrial production of UREA takes place in large manufacturing plants all over the world. In order to produce the starting materials like ammonia, carbon dioxide, nitrogen, hydrogen etc. , large petrochemical plants and air separation plants surround the actual fertilizer production plants. Naphtha, for example, is first cracked into methane at approx. 930 °C and 5 bar, then into C02 and H2 . By adding N2 (produced from air separation) you then obtain NH3. These production plants, planned and built by large, wellknown chemical engineering consulting companies, turn out up to 3000 t urea/day. Plants with an even bigger daily output are being planned. In some case these large plants are simply built anywhere; however power supplies like natural gas or crude oil and water do have to be provided. These materials have to produce electrical energy as well, to operate motors and other kinds of auxiliary machinery. In addition, large docks have to be built or expanded so that the produced goods can be transported by ship or the equivalent tracks have to be built for rail transportation.

Furthermore, strict nature conservancy stipulations have to be followed to limit excessive burden on the environment by discharging sewage into rivers, for example, or polluting the air with NH3, NOX, CO2 and sulphur etc.

Hence well-equipped testing laboratories for measuring the amount of pollution in the air and water are automatically part of a manufacturing location.

Die industrielle Herstellung

Production

Fluid ammoniac and gaseous carbon dioxide are used as starting materials when producing urea. These substances are put into a reactor. The walls of this tower, that is over 27 meters high, consist mainly of 2 metal layers with a wall thickness of approx. 6 mm; the outer wall is made of carbon steel and the inner wall of stainless steel.

Under heat (approx. 230 °C) and pressure (approx. 180 bar) synthetic ammonium carbamate is formed, which dehydration, with the addition of acid and heat, dissolves into urea and water. After removing more water (evaporation) and further processing, it is then available as a white granulate in the shops.

Why compressed air?

i 22.0 of Nagarnjuna Fertilizer

In the reactor the two combinations cause an acidic reaction.

This means that, without the influx of compressed air, the stainless steel layer would be destroyed by the ammoniac within a foreseeable time.

The continuous presence of compressed air in the reactor makes sure that the chrome contained in the stainless steel (up to 24%) oxidizes into Cr03 - a very resistant, hard and brown looking layer - so that oxidation and reduction (caused by the ammoniac) stay in equilibrium.

Since industrial processes run round the clock, the reactor has to be supplied continuously with high pressure air that has to have an exact defined residual oil content. No particular requirement is made to the dryness of the air. A pressure dew point of approx. 30/40 °C at 180 bar is normally sufficient for most units; this is achieved with the 4 stage compression of the BAUER units.

Purity of compressed air and material requisition

Process technology requires that the following minimum values are set: No more than 7 ppm residual oil content. Avoid copper seals and copper alloys, if possible.

It is possible to keep within the required limits of residual oil content with just a small effort.
After each single stage, already in the compressor, the compressed air is automatically drained and the oil removed.

The air is drained again and the oil removed (also vaporous oil particles are transformed into drops and eliminated) by simply installing a water/air heat exchanger that first cools down the warm compressed air, then the oil vapour from 40 °C to approx. 25/28 °C.
The condensed oil is easily filtered out of the compressed air by a downstream deep-bed filter (coalescence principle) with automatic condensate drainage. The remaining oil contents in the compressed air (approx. 7 ppm) end up in the reactor, however this amount does not cause any considerable oil deposit in the system. Some other large projects require that no more than 5 ppm residual oil content is left in the compressed air. In such cases, the compressed air has to be pre-dried in a regeneration dryer and then the oil removed in an activated carbon adsorbent.

The demands made to the seals are not a problem for BAUER units, since the copper seals do not come in contact with the compressed air.

Choice of compressor and control system

Urea production plants are planned and built with various annual output capacities, therefore various compressor models with different delivery quantities are used.

The following BAUER models have proven themselves in industrial use and can certainly be recommended:

I 150-7,5/11-VERTICUS
with a delivery rate of 320 - 420 l/min
= equivalent to UREA production 875 MTPD

I 22.0-15 with 650 l/min
= equivalent 1700 MTPD and

I 28.0-55 with 2500 l/min
= equivalent 2500 MTPD

The last two are horizontal units.

To supply one reactor usually 2-3 units of the same kind are used. They run around the clock on a daily basis and, to ensure an even work-load, the control centre switches them on and off automatically. Rough industrial use, like high ambient temperatures, frequent sand storms etc. demands high quality products.

By monitoring, for example, pressure and temperature of each single stage, we can make sure the predetermined parameters are not exceeded. This is done by BAUER B-CONTROL; a programmable electronic compressor control with a display. In the case of faults the unit is switched off, the fault is indicated on the display and simultaneously sent to the higher priority control centre.

Special versions

Urea-Container

Occasionally some projects require that the electrical controls are made in the explosion proof version, since the compressors are placed in locations in which, at times during normal service, an explosive gas/air mixture can form.

When, for example, the compressors are placed outside, they (including control and purification systems) have to be put in 10 or 20 feet containers. In such cases and, for example, in very warm and sandy countries, it is important to make sure there is an efficient air in- and outlet system and that the sand is completely extracted into the container’s ventilation canal.
When solving such problems, BAUER KOMPRESSOREN can rely on many years of experience and practical use.

Conclusion

It is clear to see that compressed air as a medium, has taken over an extremely important role in this very important sector of urea production and that we are sure to hear about this in the near future too. One should not oversee the economic advantage either, since the cost of repairing corroded synthesis towers exceeds by lengths the investment and running costs of the installed compressors.

Author:
L. Kühlwein, graduate engineer, project engineer in the industrial sector