In tackling the challenge of carbon footprint and carbon emissions reduction, the liquefaction of CO2 has emerged as one of the most effective methods in transporting captured CO2 and supplying it for use in other industries.
Compressors are essential machinery for CO2 liquefaction. In terms of the process, the traditional (and also most cost friendly) liquefaction method includes the use of external refrigerants such as ammonia (the main purpose of applying an external refrigerant to the process being that the refrigerant isn’t in contact with the CO2 during liquefaction).
In liquefaction processes using ammonia as a refrigerant, CO2 is compressed and liquified after using the ammonia as cooling medium. In a next step, it is transported to the storage location. During the process, CO2 first runs through a separator, where water and other condensed gases are removed from the CO2. After being sent to compression, ammonia is used as cooling medium with help of the heat exchanger, while also being compressed after evaporation and later cooled (thus completing the cycle).
Even though this liquefaction process is, by definition, a cost-feasible solution, capital expenditures (CAPEX) still are an weighty factor in the overall investment decision. This cost is mainly impacted by the compressors needed in the process.
In the application discussed in the paper, the two compression duties (CO2 and ammonia) can be handled in different sections of just one single-skid compressor, instead of two separate units. This makes the compressor footprint in the liquefaction plant more compact, robust and cost-efficient.
Like CAPEX, operational expenditures (OPEX) are primarily defined by the compressors (especially the power consumption cost of the liquefaction process). This is why compressor considerations are essential for plant operators: The combined compressor solution’s package is more optimized in terms of size and equipment cost compared to alternative compression solutions (where CO2 and ammonia are handled on separate compressors units).
Using the examples of current plants in Europe, the paper will discuss the design concept and performance data of handling the two duties through a single compressor. Specifically, the author will be considering the different inlet and outlet conditions for the ammonia cycle and CO2 cycle, respectively, describing how this impacts compressor design and performance (speeds, flow management etc.)
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