Liquid purification or separation – Processes – Separating
Patent
1990-08-22
1992-04-28
Dawson, Robert A.
Liquid purification or separation
Processes
Separating
23302T, 210737, 423212, 423242, 423551, 423553, B01D 100
Patent
active
051086255
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a process for purifying aqueous buffer solutions used for absorption of sulphur dioxide (SO.sub.2) from exhaust gases. "Purifying" includes in this case also the recovery of buffers from a drawn off liquid flow. Such a drawing off is in many cases necessary in order to prevent impurities which accumulate in the process liquor, from building up to too high concentrations. The most important impurity will be sodium sulphate (Na.sub.2 SO.sub.4). but other components may also be present.
Impure solutions of the above type are for instance obtained when phosphate, citrate or adipate buffers are used for SO.sub.2 recovery from exhaust gases in coal heated boiler plants. In such uses SO.sub.2 is absorbed in the buffer solution to form bisulphite ions (HSO.sub.3.sup.-). In such process acidic hydronium ions (H.sub.3 O.sup.+) are liberated which are then taken up by buffer ions to form more of the acidic buffer components. The following reactions are utilized in the absorption plant when for instance phosphate ions represent the buffer system: +HSO.sub.3.sup.-.sub.(l) ( 2) PO.sub.4.sup.-.sub.(l) +H.sub.2 O.sub.(l) ( 3) =HSO.sub.3.sup.-.sub.(l) +H.sub.2 PO.sub.4.sup.-.sub.(l)
From the absorption tower the solution is conveyed to the regeneration plant, in which the liquid in this case is evaporated according to Norwegian Patent Application 85.3622. In the process SO.sub.2 is driven off together with vaporized water by reversing the reactions (1)-(3), and a concentration and precipitation of Na.sub.2 HPO.sub.4 takes place and possibly also of the oxidation product Na2SO.sub.4, as the buffers are usually employed in the form of Na salts: +(n+1)H.sub.2 O.sub.(g) +Na.sub.2 HPO.sub.4(s)
If citrate or adipate buffers are used instead of phosphate buffers, the regeneration does not take place by evaporation but by regular stripping with open steam in counter current towers. Then the regeneration takes place without any essential change in the liquid volume, by reversing the reactions (1)-(3), in which citrate ions and citric acid (Ci.sup.3-, HCi.sub.2-, H.sub.2 Ci.sup.-, H.sub.3 Ci) or adipate ions and adipic acid (Ad.sup.2-, HAd.sup.-, H.sub.2 Ad) are included in the reactions (3) instead of the phosphate ions (HPO.sub.4.sup.2-, H.sub.2 PO.sub.4.sup.-).
For all said three processes the oxidation loss of SO.sub.2 is very small. Nevertheless, some increase of Na.sub.2 SO.sub.4 in the liquid must be expected, also because the gas from which SO.sub.2 is removed, often contains small concentrations of SO.sub.3 which are taken up by the buffer solution and which must be neutralized with a Na base (NaOH or Na.sub.2 CO.sub.3) An obvious possibility to maintain the Na.sub.2 SO.sub.4 concentration at an acceptable low level, would be to draw off a small flow of process liquid and replace this by fresh buffer solution. For a 500 MW power plant with 0.25 vol % SO.sub.2 in the exhaust gas and a sulphate formation corresponding to 0.25% of the amount of SO.sub.2, the amount of sulphate will amount to approx. 0.5 kmol/h. If it is desired to keep the sulphate concentration as low as 0.5 kmol/m.sup.3 only by removing process liquid, 1 m.sup.3 liquid per hour must be drawn off. With typically 3 moles per liter of Na.sub.2 HPO.sub.4 in the buffer at a price of approx. US $ 1.2 per kilogram, the loss of Na.sub.2 HPO.sub.4 alone will represent US $ 511 per hour. Even with this low estimate for the sulphate formation, such a drawing off of process liquid to remove Na.sub.2 SO.sub.4 from the system will lead to unacceptably high losses of Na.sub.2 HPO.sub.4. In addition it may represent considerable environmental problems. This illustrates that there exists a need for being able to remove Na.sub.2 SO.sub.4 selectively from the process liquid, i.e. without losing essential amounts of valuable buffer.
The buffer systems discussed herein are all salts of polyvalent acids. The SO.sub.2 uptake in the absorption tower displaces the buffer systems towards the more acidic buffer components. When reference is made to a selectiv
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Derwent Abstract No. 83-819244/46.
Abstract For WO 87/01612, Mar. 26, 1987.
Dawson Robert A.
Kim Sun Uk
Sintef
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