Question 5.20: Removal of carbon dioxide from an air emission stream by wet...
Removal of carbon dioxide from an air emission stream by wet scrubbing is presented in this example. Please note the following:
1. The environmental engineering significance of carbon dioxide removal.
2. A feasible wet scrubbing system for removing carbon dioxide from an air emission stream.
3. The chemical reactions involved in scrubbing carbon dioxide using a scrubbing liquor of 25\% sodium carbonate and 4\% caustic (concentrations by weight percentage).
1. The environmental engineering significance of reducing carbon dioxide emissions has been noted previously by Wang and Lee (49). Greenhouse gases such as carbon dioxide, methane, and so forth caused global warming over the last 50 yr. Average temperatures across the world could climb between 1.4°C and 5.8°C over the coming century. Carbon dioxide emissions from industry and automobiles are the major causes of global warming. According to the United Nations Environment Programme Report released in February 2001, the long-term effects could cost the world about 304 billion U.S. dollars a year down the road. This is the result of the following projected losses: (1) human life loss and property damages as a result of more frequent tropical cyclones; (2) land loss as a result of rising sea levels; (3) damages to fishing stocks, agriculture, and water supplies; and (4) disappearance of many endangered species. Technologically, carbon dioxide is a gas that can easily be removed from the industrial stacks by a scrubbing process using any alkaline substances.
2. The following discussion presents a feasible wet scrubbing system for removing carbon dioxide from an air emission stream. Several wet scrubbing (absorption) processes are possible for carbon dioxide removal from an air emission stream. One plausible scrubbing solution is presented in Fig. 16.
This single-stage scrubbing system will remove 99.9\% of the carbon dioxide present in the airstream at these given conditions. A scrubber characteristics diagram is shown in Fig. 16. The packed tower wet scrubber, per Fig.1b, has the following characteristics:
Gas loading = 500 lb/h-ft²
Temperature = 25°C
Pressure = 1 atm
Scrubbing liquor of 25\% sodium carbonate and 4\% caustic
Packing media used Q-PAC packing in polypropylene from Lantec Products
Packing height = 24 in.
Gas loading = 500 lb/h-ft²
Liquid temperature = 75°F
3. The chemical reactions involved in scrubbing carbon dioxide are dependent on the actual scrubbing solution chosen. In this example using a scrubbing liquor of 25 \% sodium carbonate and 4 \% caustic, the chemical reactions are
CO_{2} + NaOH → NaHCO_{3}
CO_{2} + H_{2}O + Na_{2}CO_{3} → 2 NaHCO_{3}
