Recoverv of Sulfur Dioxide from Waste Gases J
Distillation of a Three-Component System Ammonia--Sulfur Dioxide-Water A method is described for calculating the number of theoretical plates, or the value of the diffusional potential, in the regeneration of a solution of ammonium sulfitebisulfite saturatcld with respect to a dilute sulfur diolide gas. While the sulfur dioxide is less lolatile than the water, its relative concentration in the vapor may be increased by the stripping action of the vapor countercurrent to the solution, provided no reflux is returned to the top of the column. The iize of the regenerator and quantity of steam required is estimated for
a number of feed concentrations for the two cases of distillation (1)by direct steam and (2) by the indirect application of heat to the solution. The steam required for a single-effect nonrecompression regenerator varies froin 4 to 16 pounds per pound of sulfur dioxide recovered, depending on the composition of the dilute waste gases and on other conditions of operation. The results obtained on a 12 X 81 inch (30.4 X 206 cm.) column packed with 1-inch (25-mm.) Raschig rings are discussed.
1%.F. .JOHNSTOSEAND D. B. K E l X S The const,aiits ancl S :Ire actually HE reco\.ery of sulfur functions of the solubility constants ot’ dioxide from a soluUniversity of Illiiiois, sulfur dioxide and ammonia, ancl of the tion of ainnioniuni Urbana, 111. ionization constants of t8heacid and base, sulfite-liwlfite saturatetl with respect and of n-ater. The effect of temperature on the d u e s of J1 to a dilute gas present,s a unique and interesting problem in and S is given hy the equations: distillation. h graphical method for computing the number of equilibrium plates required in a fract,ionating column for multiple-component hydrocarbon mixtures on t’hebasis of ideal solution laws has been described by Bronx et’al. (1j. The comSulfur Dioxide, the Less Volatile Constituent plexity arising froin conipoiind format,ion in the salt solution discussed here obviously requires further consideration, and Assuming the validity of Dalton’s lam for the composition greatly increaser the tlifficulty of the problem. In this paper of the vapor, the mole fraction of each c,omponent.in the vapor a method for computing the liquid and vapor compositions may be computed for any liquid composition froin Equations throughout the rcgenerator will be described, based on theusual 1 to 5 . The representation of the complete system requires assumptions made in the t,heory of distillation. This inet’hod a t’hree-dimensional diagram in which the mole fraction of’ unfortunately does not lend itself to graphical solution so that’ each component in the vapor is represented by an equilihiuiii the calculations required hecome long and t.ediou:. It is a surface. The contour lines for sulfur dioxide and ammonia method, hen-ever, which has been extremely usefiil in the defor several concentrations of ammonia are shown in Figure I . sign of a regenerat,or for a pilot plant operating on the reIt is apparent that because of the existence of the compounds covery of sulfur dioxide from waste gases, and it’s use for tlie mole fractions of both ainni mia and sulfur dioxide in the similar three-coinponeiit, teins for which equilibrium data vapor are less than the corresponding mole fractions in the exist, such as the carboante-bicarbonate system (a),should solution, when the composition of the latter lies between approre to be of import,ance. proximately that of the sulfite and that of the bisulfite. hliove and below this range these components, sulfur dioxide Equilibrium Diagram and ammonia, respectively, become inore volatile than the water, and therefore the composition of the vapor becomes For the sulfite-bisulfite equilibrium, suitable interpolation richer in one of these coiistit’uents than in the corresponding equations giving the vapor pressure of each comoonent have solution. been derived from a conaideration of the reactions involved In the regeneration of a scrubbing solution saturated in ( 3 ) . Over a limited range of relat,ive concentrations these respect to a dilute sulfur dioxide gas, the concentration of the are as follows: sulfur dioxide never exceeds the Ijisulfite st,age and actually lies somewhat’below it. I n order to obtain as high a ratio o i sulfur dioxide to water vapor as possible, to reduce the quantity of steam required for the regeneration, Tve are faced with the proposition of concentrating the less volatile constituent (sulfur dioxide) in the vapor phase. The regeneration differs 659
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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process as described is, therefore, inoperable. The calculations on i t will be given, however, with the understanding that the results represent the lower limit of steam consumption. A flow sheet for a regenerator operating on direct steam is shown in Figure 2. I n this diagram, F moles of feed are introduced a t the top, and L moles of the regenerated liquor are withdrawn a t the bottom. If the usual assumption of equal inolal latent heats is made, for an adiabatic process moles of vapor enter the bottom and V moles, of different composition, leave the top. This assumption is only approximately rigid in this case because the molal latent heat of the ammonia in the solution is greater than the molal latent heats of sulfur dioxide and water, the latter two being about equal. Since the quantity of ammonia that is removed is negligible compared to the number of moles of sulfur dioxide and water, the constancy of the vapor volume throughout the tower still holds. Since the number of moles of vapor is constant, the same must be true for the liquid, or F = L. Material balances for the individual components, sulfur dioxide, ammonia, and water, are also shown in Fiiure 2. I n the F x+,iiile to plot the steam required for regeneration of any t'eeti -c~Iutioii. Ruc'li a plot is shovn in Figwe 8, where the ordillate. express the cwncentrat,ion of sulfur dioxide in niole- per 1UO moles of water. for a fixed ainmoriia concentration of L"'.L'T mole- per 100 iiioles of water. Such a solution repre-nit- approxiinately the highest concentrat ion of aninioiiin tlist can be used and for certain absorption temperature. givr> the greate.t calmcity for absorbing sulfur dioxide 11. Tlie concentration of sulfur dioxide in ted hot11 b y the temperature ai, which the absorptii in take-: place and h y the concent,ratioii of sulfur diositle in tlie dilute ga.. 'These re1ationship.c are 5how-n in Figure ! I . It i.. i i i nv 1)osd>letil cleteiviine the compositioli of the feed to tlie regenerator, S,, for ally teniperat,ure of absorption arid of sulfur dioxide in the Rase.. From this antity of steam required for regeneration either nietliod of regeneration from Figure 8. -1. :in example. sup1)o-e that gases containing 0.4 per cent rulfiw dinxicle are being scrubbed, with the solution leaving the Fcru1)i)er a t 33" C. From Figure 9. the coin~ ) o s i t i o l iof the s o l u t i o n leaving the regenerator i20.44 niiiles of sulfur diosidc per 100 moles or^water. The q u w t i t y of steam required for regeneration at 90" C'. i- -Iionn by Figure 8 to he 4.1 poLlnc~s ll'r p ~ ~ i i ii idf Fulfur dioxide recovered vhen the regeneration ii- niade by direct allplicatiiiii ot' -teain, and 4.35 pound- per pound 11-lien the mlutioii i- iieated liy illdirect ine~ii.. These e-tiniate-. i d emir+, are iiiade (111 tlie 1i:i.i. Of a .ingleeffect regrilerator ~ritlri~iir vapor w~iii11re,-sioii. C ' o i i s i d e r a l i l r -aving C R I I lie iiiatlr if' wcli a bpteiii iverr utilized . 1111t the iiit't II od? of c:i 1c t i 1 a t i o n pre-c,ii tetl 11ere \v{ii11(1 reiliain 1111c I] n ii w
