OCR Text |
Show As it is freely available in abundance sea-water is an attractive scrubbing solution for many industrial plants. Furthermore, sea-water is a buffered and concentrated solution containing high levels of the very same ions which are produced by the dissolution of flue gases from fossil fuel combustion. Absorption of sulphur dioxide (S02), carbon dioxide (C02) and hydrogen chloride (HCI) results in sulphate (S042-), bicarbonate (HC03-) and chloride (CI-) ions respectively. In principle, these ions may be discharged to the oceans in considerable quantities without any foreseeable damage to marine ecosystems, considering that they are major constituents of sea-water. GAS/LIQUID EQUILIBRIUM Flue gas desulphurisation has generally been achieved inside counter-current absorption towers, wherein pollutants are transferred from bulk gas phase into aqueous droplets or films. The following theoretical discussion is applicable to all mass transfer operations including spargers in which pollutant gases are transferred from bubbles into a bulk aqueous phase. Absorption of S02 and C02 involves HeJ?IY.'s Law equilibrium followed by rapid dissociation as described by reactions (i), (ii) and (iii): H X02 + H20 ~ =-- X02.H2O (i) X02.H2O --== Kl ::::-. HX03- + H+ (ii) K2 :::::::::. HX03- --== X032- + H+ (iii) where X is either S or C, H is Henry's Law constant and Kl and K2 are the first and second dissociation constants respectively. The equilibrium constants are defined in terms of activities: H = = = ~03-·~+ ~02.H20 ~032-.~+ ~X03- (iv) (v) (vi) Henry's Law constant, H, is usually defined in terms of concentration, however for a neutral species such as X02.H20 we may assume an activity coefficient of unity and equate activity with concentration. An additional equilibrium constant relating to the total of dissolved species may also be defined: H * = H {l + K1/aH+ + K1.K2/(aH+)2} (vii) H* is termed the pseudo - Henry's Law constant, and aH+ is the activity of the hydrogen ion: aH+ = lo-pH (viii) 2 |