Decarbonisation Technology – August 2021

vapour stream. The volume fraction of CO 2 in the gas leaving the reflux accumulator is approximately 21%. The CO 2 used as refrigerant in the overhead condenser is compressed, cooled and returned to the fractionation column, where it is recovered in liquid form. A distillation process for removing CO 2 is designed

NG compressor

CO product

Natural gas

CO liquifaction system

Membrane System

Liquid CO

Feed compressor

CO compressor

Figure 5 Block flow diagram showing CO 2 liquefaction and membrane

and partially condensed in the refrigerant chillers using an external refrigerant. Next, the inlet stream is condensed and partially sub-cooled through further heat integration with streams leaving the cold section of the plant. Finally, it is flash expanded to the fractionation column at an optimum pressure designed for effective CO 2 /methane separation while avoiding CO 2 freezing issues. Vapour leaving the column overhead is further chilled and partially condensed in the overhead condenser with CO 2 refrigerant. The resultant CO 2 -rich liquid is pumped back to the fractionation column following further heat integration within the process. The refrigerant for the overhead condenser is a portion of the CO 2 product from the bottom of the fractionation column. This liquid CO 2 is then flashed to a relatively low pressure where it chills and partially condenses the overhead

with a feed gas stream under pressure is cooled by heat exchange with other streams of the process and/or external sources of the refrigeration system. The gas is condensed as it is cooled. The high-pressure liquid is expanded to an intermediate pressure, resulting in further cooling of the stream due to vaporisation occurring during expansion of the liquids. The expanded stream, comprising a mixture of liquid and vapour, is fractionated in a distillation column to separate residual methane, nitrogen and other volatile gases as overhead vapour from the CO 2 and the heavier hydrocarbon components as a bottom liquid product. A portion of the liquid CO 2 can be flash expanded to lower pressure, and then used to provide low-level refrigeration to the process streams if desired. There will be two streams leaving the CO 2 liquefaction system: the compressed liquid CO 2 stream as the product and the residue or natural

gas stream, which will be recycled back to the membrane system, as shown on the process block diagram (see Figure 5 ). The recycle natural gas stream was sent to the membrane system until the loop was converged closely. Figure 5 presents the combination of CO 2 liquefaction and a two- stage membrane system. The CO 2 liquefaction unit is designed and optimised to use energy efficiently and minimise utility consumption as any energy from the cooling and heating stream is used to cool or heat other streams. Brazed aluminum heat exchangers (BAHX), also known as plate-fin heat exchangers and cold boxes,

Figure 6 Compact design of CO 2 liquefaction heat exchanger