Figure 3 Lanxess CISA CO 2 capture plant in South Africa
Courtesy of Shell
but the economic optimum is around 95% in most cases. CO₂ absorption is exothermic, which results in a temperature bulge in the absorption column. As high temperatures are detrimental to efficient absorption, at high inlet CO₂ concentrations, an intercooler is used to remove heat from the system and maintain an operating temperature profile favourable to absorption: a draw-off tray collects absorbent from the upper packing beds, which is pumped through the intercooler before returning to the lower packing bed. The upper section of the absorber is a water- wash system that ensures the emissions of solvent and degradation products to the atmosphere are minimised. The water-wash system is usually sufficient to meet the most stringent emissions specifications. However, depending on the project requirements and feed gas characteristics, an aerosol mitigation device can be added downstream of the absorber. The rich absorbent, loaded with CO₂, is regenerated in a stripping column using structured packing to promote mass transfer and reboilers to generate the stripping steam. A condenser is used to condense the stripping steam from the overhead vapours, and the pure (water-saturated) CO₂ is released for downstream treatment. A lean-rich heat exchanger is used to recover heat from the hot
lean absorbent exiting the stripper to preheat the rich absorbent before it enters the column. The lean absorbent exiting the lean-rich exchanger is further cooled in the lean absorbent cooler and is then sent back to the absorber to absorb more CO₂. Existing Cansolv CO₂ plants are shown in Figures 2 and 3 . Ethylene plant considerations for a plant operating with a post-combustion CC unit Flue gas from ethylene plant furnaces is very low in sulphur and low in particulate matter, two good points for post-combustion CC with absorbent technology like Cansolv. The fuel gas composition can change depending on the ethylene plant feedstock, so there is typically a difference in the hydrogen fraction in the fuel gas between liquids and gas crackers; this results in a difference in the ratio of H₂O to CO₂ in the flue gas. For gas crackers, the fuel to the furnaces is normally high in hydrogen, which means the flue gas contains a relatively high amount of water vapour and a lower CO₂ content (8.5 wt%). However, this is still attractive for absorbent technology. If the ethylene plant is located in desert regions, dust and sand in the combustion (ambient) air may contribute to higher particulate levels in the flue gas than for European/American crackers, so it would be prudent to consider higher
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