SIM 4810 CO2 Compression

Model Description

The model has a 1 barg feed of 91mol% CO2 with some other impurities. The required output from the compressor train is supercritical CO2 at a pressure of 211 barg. The outlet of the system is an injection well where the combined head of the CO2 has a bottom hole pressure of 413 barg and a formation pressure of 409 barg. The flow of CO2 through the system is 1.29 MMSCMD. 

This increase of pressure is achieved via a multi-stage compression system. There are 4 mechanically linked stages where 3 and 4 run at faster speeds via a gear box than 1 and 2 (speed ratio of 2.1:1). The compressor speed is controlled via a speed controller, whose set point comes from a primary suction pressure controller, or from a secondary stage 4 discharge over-pressure controller via a low selector. This control system aims to keep the suction pressure at a given set point but will prevent the compressor from making too much pressure into the well. There is an intercooler between each of the stages and before the CO2 goes into the well to remove heat added due to the compression. 

Each stage has an anti-surge controller that prevents that stage from going into surge by opening a recycle valve across a stage to reduce the differential pressure (head). 

Each stage also has an inlet knockout pot to remove any incompressible liquids that may be present, further processing of these fluids is not included within the model scope. 

An essential part of compressor operation is the Emergency Shutdown (ESD) system, which prevents dangerous conditions by intervening automatically. 

Phase diagrams have been provided to help the trainee understand conditions at the suction and discharge of each compressor stage. 

Compressor curves show each compressor’s performance, illustrating head vs. flow curves at various speeds, a surge line (red), and a control line (green). These curves also show the dynamic control line of the anti-surge controller. 

Economics of the process have also been provided, compressors are energy intensive machines and cost money to run and maintain. 

Learning Outcomes

• Understand the principles of CO2 compression using a four-stage centrifugal compressor and the effect of well injection on the system. 

• Gain insights into the function and importance of intercoolers, aftercoolers, knockout vessels, and anti-surge control systems. 

• Understand the function of the pressure and speed control system. 

• Carry out startup and shutdown of the process. 

• Recognise the importance and operation of an Emergency Shutdown (ESD) system in compressor operations. 

• Interpret phase diagrams to understand suction and discharge conditions at each compressor stage. 

• Analyse compressor curves to evaluate compressor performance and proximity to surge conditions and use them to understand how the anti-surge controllers operate. 

• Look at the cost of running this process and understand the wider impact of that when it comes to delivering net zero.