Maximum Recovery of Gas Compression Waste Energy
Authors: Ansari N. A. , Salu S.  A. 
Published: May 2016

Abstract
This paper presents a revised energy efficient compression scheme to maximize recovery of wasted energy from a conventional gas compression scheme, which is highly prone to wastage of substantial amounts of energy at different stages in the process.
A typical compression process requires a discharge gas cooler and an anti-surge control system. The common means of cooling the compressor discharge gas is by using either fin-fan air coolers or a cooling water system that eventually dissipates the heat energy of the gas to the environment. Similarly, in a fixed speed driven compression system, significant energy is wasted due to recycling of compressed gas to the suction through the anti-surge control system to satisfy fluctuations in production rates.
In the revised scheme, the compressor discharge gas fin-fan cooler/water cooler will be replaced by an evaporator of an advanced Special Rankine Cycle (SRC) to capture thermal energy from hot discharge gas and convert it to power output in a turbine. A refrigerant mixture that will be thermodynamically efficient at the operating temperature ranges and environmentally friendly will be required. Combining the SRC discharge gas cooling with the use of a compressor driver that has full variable speed capability will greatly optimise compression energy recovery. Use of gas turbine drives will provide additional opportunity to recover waste heat from the driver's flue gas in the SRC. The proposed energy efficient gas compression scheme will also enable lowering the compressors suction temperature to the minimum temperature possible to reduce compression energy requirement.
A case study of applying the proposed scheme in a Saudi Aramco gas compression plant is presented with an energy saving analysis. Overall energy recovery of about 45% is achievable.
The new scheme will provide significant economic and environmental benefits as a result of the substantial energy recovery.

Introduction
A gas compressor is mechanical equipment that is utilized to increase pressure of a gas stream from a low pressure to a higher pressure within certain compression ratio. Compressors action to increase gas pressure is achieved by reducing its volume in order to either transport the fluid through a pipeline or boost the pressure to what is required for downstream processing. Due to the limitation in gas compression ratio that is achievable in a single compression stage, a multistage compression system is usually required in most cases in order to achieve the required final downstream pressure in a plant. Every stage of a gas compression required other process equipment to function and perform effectively. Figure-1 shows a process flow scheme representing a typical gas compression stage with the associated equipment. The process flow scheme begins with a suction liquid knock-out drum which is provided to remove very fine liquid droplets from the gas stream feeding the gas compressor. The suction liquid knock-out drum is usually equipped with internal demisting device to remove droplets and particles of very few microns size which are detrimental to the compressor impeller. The gas is then fed to the compressor to increase its pressure to the required discharge pressure.