Feb-2025

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

Article Summary

The 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 improving overall economics.

This study will discuss an innovative approach to determining the cyclic switchover of the instrument air dryers in a refinery/petrochemical complex based on actual necessity. 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 implementation 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 complex 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 decarbonisation policy of the Government of India.

Compressed air is one of the costliest utilities in any industry. 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 aftercooler 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 headers. Two qualities of compressed air are produced and distributed:
• Plant air is compressed air cooled to ambient temperature. Though it does not contain any entrained water droplets, 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 product being manufactured. Thus, it is essential to dehumidify instrument air and meet its dew point specification before usage. This is achieved with the help of an instrument air dryer, which is explained in the subsequent section.

Instrument air dryer
An instrument air dryer is a piece of equipment or machinery used to dehumidify compressed (process) air to significantly lessen or eliminate moisture in the air stream. The most common types of dryers being used are based on the adsorption principle, where the drying agent is a spherical or granular form of material known as a ‘desiccant’ (see Figure 2). The most common drying agents are silica gel, molecular sieve, and activated alumina.

The dryer selection is based on the mode of regeneration. Generally, the air dryer utilises two towers: one containing a desiccant that removes moisture from the air stream from the compressor, while the other regenerates the used desiccant. The main distinction between types of instrument air dryers is based on the regeneration process, specifically the ‘heat-reactivated regeneration system’ and the ‘heatless regeneration system.’

In the heat regeneration system, the adsorbent is reused after desorbing its water vapour through either of the means, such as:
• Direct heating of the adsorbent bed by passing hot air, which is heated by an electrical heater.
• Indirect heating of adsorbent bed by embedded steam coils in the adsorber through which steam is passed.
• Heating of adsorbent bed through the hot compressed air (before air compressor aftercooler), utilising HOC.

Instrument air dryer prevalent practice
Energy conservation by an HOC-type compressed air dryer is a breakthrough in compressed air drying technology. The hot air from the oil-free air compressor at 120°C or higher is used directly to regenerate the desiccant bed in the compressed air dryer (see Figure 3).

After regeneration, this air is cooled to 40°C in the water-cooled aftercooler and then dried in the second tower. Thus, the use of electrical heaters is avoided. The main advantages of HOC-type compressed air drying are energy conservation and heat recovery, which were wasted in the aftercooler. Conventional ‘no purge loss type’ air dryers are now being used to reactivate the desiccant.

Fixed-cycle dryer
The instrument air dryers are designed for a fixed cycle time, where the switching from adsorption to regeneration mode takes place after a predefined set point to reach cycle time. Dryers are mainly designed to function at maximum operating conditions, the highest flow rate, the highest temperature, and the lowest pressure.

Fixed-cycle dryers are constantly switching towers and regenerating based on worst-case scenario, whereas the actual operating conditions may be quite different. With a fixed-cycle dryer, regeneration is constant and designed for the maximum incoming water load. In reality, the average amount of moisture entering the dryer is less than the design. However, the fixed-cycle dryer cannot take advantage of a reduced water load since it continues to regenerate at a predefined cycle time.

Regulating dew point – an innovative approach
The dew point demand controller makes it possible to reduce the operating cost of any dryer by regulating the dew point rather than regulating the sequence through a fixed time-based control panel. The outlet air dew point will determine the operating cycle time of the dryer. If the dew point is higher than the desired level, the adsorption cycle can be extended beyond eight hours until the required dew point of -40°C is achieved. The controllers can be set for the specific dew point desired in the system. Changeover would take place only at the adjusted dew point.