Replacement of Air Inter-coolers in the Ammonia Plant

Background

The Ammonia is synthesised by reacting Hydrogen generated by reforming of hydrocarbons and Nitrogen in the atmospheric air in the presence of a catalyst at higher pressure to synthesise Ammonia. The atmospheric air is supplied to the reactor by a battery of air compressors. These compressors are very important for the operation of the plant and hence are rightly referred to as the heart of a fertiliser plant. The efficiency of these compressors therefore play a very important role in the efficiency of the whole plant. This case study describes about a project implemented in the compressor section of a fertiliser plant.

Previous status

In a 1,00,000 ton per annum capacity Ammonia plant, the air requirements of the Ammonia converter were being met by two numbers of oil lubricated 4 stage reciprocating compressors. The compressors were provided with inter-coolers with finned tubes and were laid in a horizontal fashion. The oil in the air from cylinders used to plug the gap between the fins and reduce the heat transfer. The exit air from the inter-cooler used to be at 55 – 58°C as against the design of 42°C. The capacity of the subsequent stages was getting reduced leading to loss of Ammonia production.

Energy saving project

The inter-coolers for the compressor was replaced with finless tubes and laid in a vertical fashion.

Implementation methodology & time frame

The implementation of this project was taken up parallely while the plant was operating. The replacement was done for one compressor first and the second compressor was taken up subsequently. The implementation and the subsequent operation did not pose any problem.

Benefits of the Project

The replacement of the horizontal fin type cooler with vertical finless cooler resulted in reduction of exit air temperature to around 45°C. There was a reduction of power to the extent of 45 kW.

Financial Analysis

This amounted to an annual monetary saving of Rs 0.85 million. The power saving alone has been considered. The investment made was around Rs 2.0 million. The simple payback period for this project was 28 months.

Benefits of vertical finless cooler

• Exit air temperature reduces
• Reduction in power consumption

Cost benefit analysis

• Annual Savings - Rs. 0.85 million
• Investment - Rs. 2.0 million
• Simple payback - 28 months