Zeolite Adsorption

Gas Washing

 

Cryogenic Distillation

Cryogenic separation unit are operated at extremely low temperature and high pressure to separate components according to their different boiling temperatures

Cryogenic separation is widely used commercially for purification of CO2 from streams that already have high CO2 concentration. Need for relatively high concentration of CO2 for cryogenic unit is important because in order to minimize CO2 loss from the top of the column it would be necessary to operate as close to the triple-point temperature as possible, but the minimum partial pressure of CO2 achievable in the vent gas would be 5.18 bar abs. this meant that as the concentration of other component  in the CO2/gas mixture increases the pressure of the stripping column would have to be increased in order to achieve a certain CO2 recovery from process. At 75% feed purity and 90% recovery, the column pressure would be about 26 bar pressure, for 95% recovery it would be 46 bar.

Cryogenic systems are a low temperature physical approach to separation, in which the CO2 is separated directly by phase change. This method is advantageous with respect to direct production of liquid CO2 or pure CO2 gas stream in high pressure which would be liquefied more easily. There are some difficulties for applying this method as well. For dilute CO2 stream, the refrigeration energy is high. Water has to be removed before the cryogenic cooling step to avoid blockage from freezing.

Figure: Flow sheet of cryogenic unit used for the recover of CO2

Figure above illustrates a flow sheet of cryogenic unit used for the recovery of CO2. The first CO2 rich gas stream conveyed to the cryogenic unit. The gas steam is compressed therein by a compressor to raise the pressure of the gas. The compressed gas is then cooled in two heat exchangers where it is partially condensed. The condensed liquid phase is separated from the gas phase in a flash vessel.

The results of rough design of cryogenic unit is presented in the table below:

  Table : Design summary 

Flow rate

W(kW)

Cooling water (kg/hour)

Refrigerant (kg/hr)

Vapor flow rate (m3/s)

Area
(m2)

Diameter (m)

Length (m)

200(m3/hour)

29

354

117

40

13.34

4.12

12

800(m3/hour)

62

1410

471

160

53

8.24

24