| OCR Text |
Show Table 2 Values of equilibrium constants at 25°C.H* is for pH = 8. S02 data from ref. (14). C02 data from ref. (15) Constant H (M atm-I ) KI (M) K2 (M) H * (M atm-I ) 1.242 0.0132 6.24 x 1Q-8 1.19 X 107 0.031 4.3 X 10-7 5.61 X 10-11 1.371 Carbon dioxide dissolved in sea-water will be present predominantly in the form of the bicarbonate ion. Dissolved S02 is rapidly oxidised to sulphate in sea-water. The reaction involves oxidation by 02 which is significantly enhanced by the high level of chloride present in sea-water (4,5). The mechanism is still not fully clear but it appears to involve free radical reactions (4). SEA-WATER COMPOSITION Sea-water is a concentrated electrolyte solution containing high levels of sulphate, bicarbonate, chloride and other ions. Average sea-water composition is given in Table 3 and the scrubbing of flue gases would have little impact on the concentration of major ions. Trace metal concentrations in sea-water are low and a potential cause of concern is pollution of the seas bX toxic metals present in dust partIcles in the flue gas. The extent of this pollution will depend on the nature and concentration of toxic metals in the flue gas and it can be minimised by installing dust collection equipment upstream of the absorption tower. An important property of sea-water is its buffer ca1?acity, which is responsible for the ability of sea-water to withstand significant changes ill pH. This buffenng action is due mainly to the carbonic acid-bicarbonate equilibrium and to a lesser extent the boric acid-borate equilibrium. Over the geological time scale the pH of sea-water is also regulated by equilibria involving suspended clay minerals (6). Absorption and oxidation of S02 in sea-water will lower its pH and limit the further uptake of S02, the solubility of which decreases with increasing acidity. The pH of the liquid effluent from a sea-water scrubbing plant depends on the liquid/gas ratIo and the concentration of S02, C02 and HCI in the flue gas. High L/G ratios would result in high pH and the effluent could be discharged to the sea without further treatment. Low L/G ratios will result in an acidic effluent which would require dilution with cooling water from the condensers or addition of lime prior to discharge. The liquid effluent could also be aerated in order to expel the absorbed C02 thus raising the effluent pH. This would also aid the oxidation of S02 to sulphate, minimising the release of S(iv) species to the sea. 4 |
