Improving energy performance of instrument air system
Real-time case study wherein extended adsorption cycle times has led to reduced fuel firing in a captive power plant and effective utilisation of energy
Subhosree Chakraborty and A Natarajan Engineers India Limited
T he underlying takeaway of the COP28 meeting, the United Nations Climate Change Conference held in Dubai, UAE, in December 2023, stressed the need for greater energy efficiency and better energy utilisation processes. Such declarations are a positive development for the energy industry, especially considering that energy- efficient solutions were historically not prioritised due to the possibility of low return rates. The introduction of carbon costs (carbon incentives) can play a positive role in improv- ing overall economics. This study will discuss an innovative approach to deter- mining the cyclic switchover of the instrument air dryers in a refinery/petrochemical complex based on actual neces - sity. For this purpose, a dew point analyser is specifically installed to monitor and regulate the sequence for dryer changeover. As per case studies performed in-house, the implemen- tation of a dew point demand controller can lead to power savings of 11-27% for an increase in tower changeover time from one hour to three hours. This power saving would reduce the overall power requirement of the com- plex and subsequently result in less fuel gas being fired in the complex’s captive power plant (CPP). A decrease in fuel gas consumption would subsequently reduce the CO₂ emissions to the environment, in line with the decarbonisa- tion policy of the Government of India.
Compressed air is one of the costliest utilities in any indus- try. According to the Sankey diagram (see Figure 1 ), for 100% electrical input energy, only around 15% is obtained as the useful output. This low return on electrically derived energy raises significant concerns, highlighting the need for energy optimisation in compressed air plant systems. One such area of energy conservation is utilising the heat of compression in air drying technology. The hot air from the oil-free air compressor, at temperatures of 120°C or higher, can be used directly to regenerate the desiccant bed in the compressed air dryer. The main advantage of a heat of compression (HOC) type compressed air dryer is the energy conservation and heat recovery that can be achieved. The energy that would otherwise be wasted in a conventional air dryer’s after- cooler is now used to reactivate the desiccant. HOC-type air drying is an established practice that has been followed in many fields over a considerable period. The focus now is to identify opportunities for further improving the energy efficiency of these existing facilities. The following approach will be discussed, wherein the cyclic switchover of the air dryers will be determined on actual necessity rather than a predefined set point. For this purpose, a dew point analyser is specifically installed to monitor and regulate the sequence for dryer changeover. Compressed air system Compressed air is generated at a centralised location in the plant and distributed to the various users through head- ers. Two qualities of compressed air are produced and distributed: • Plant air is compressed air cooled to ambient tempera- ture. Though it does not contain any entrained water drop- lets, it is saturated with water vapour at supply conditions. • Instrument air is used to operate various instruments in the facility and to purge some control panels. Instrument air is compressed air cooled to ambient temperature and dried to remove water vapour to meet stringent atmospheric dew point requirements (dew point at atmospheric pressure: -40°C). Moisture in instrument air can cause problems in the operation of pneumatic systems, solenoid valves, and other instruments and can adversely affect the process or prod- uct being manufactured. Thus, it is essential to dehumidify
5%
Mechanical losses
80%
Heat of compression
Electrical energy (100 kW)
100%
15%
Useful output
Air compressor
Figure 1 Sankey diagram depicts relatively low level of use- ful output from electrical energy
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PTQ Q1 2025
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