Recover
ulfur Dioxide from Waste Gases' Effect of Solvent Concentration on Capacity and Steam Requirements of Ammonium Sulfite-Bisulfite Solutions H. F. JOHNSTONE University of Illinois, Urbana, Ill.
I
N PREVIOUS papers (I, 2) in this series2the equilibrium partial vapor pressures of
solutions in the ammonia-sulfur dioxide-water system were reported. The application of these solutions as solvents for sulfur dioxide in a cyclic method involving regeneration by heating was pointed out, and a method for estimating the minimum steam requirements was developed. When the concentration of sulfur dioxide in the original gases is low and the humidity and temperature are high, there is an optimum concentration of the solvent which produces the maximum capacity and requires the smallest quantity of steam. This optimum is due to the increase in the vapor pressure of ammonia as the concentration of the solution increases, an effect which limits the extent of regeneration and thus offsets the increase in capacity and decrease in steam requirements encountered normally in solutions in which the base is not volatile. The increase in steam requirement with increase in ammonia vapor pressure is due to the greater tendency t o hold the sulfur dioxide in solution in the regenerator, even though the actual vapor pressure of the latter a t any given temperature also increases w i t h' concentration. These relations may be understood by exBecause of the vapor pressure of ama m i n a t i o n of t h e monia over sulfite-bisulfitesolutions durequilibrium diagrams ing regeneration, there is an optimum and equations in the concentration of the ammonia in the previous paper. The existence of the solution which produces the maximum optimum concentracapacity and requires the minimum tions when the abquantity of steam for regeneration. This sorption temperature
0
C -CONCENTRATION OF NHs MOLES/100 flOLES Hfl
concentration is a function of the composition of the raw gas and of other operating conditions. For dilute gases with high humidity, the optimum concentration may be less than half the concentration of the saturated salt solution.
FIGURE1. EFFECTOF AMMONIA CONCENTRATION ON CAPACITY OF SULFITE-BISULFITE SOLU1 Presented before the Division of Industrial and Engineering Chemistry TIONS FOR ABSORBING at the 94th Meeting of the American Chemical Society, Rochester, N. Y., SULFURDIOXIDEFROM September 6 t o 10, 1937. Published b y permission of the Director of the GASES CONTAININQ 0.3 Engineering Experiment Station, University of Illinois. This paper oonPER CENT SULFURDItaina part of the results obtained on the conperative researoh projeot, Cane OXIDE, ASSUMINGEQUI34, with the Utilities Research Commission of Chioago. LIBRIUM Is REACHED IN 2 The third paper in the series appeared in March, 1937, pages 286 to 297. THE SCRUBBER 1396
DECEMBER, 1937
INDUSTRIAL AND ENGINEERING CHEMISTRY
FIGURE2. EFFECT OF AMMONIA CONCENTRATION ON STEAM REQUIREMENTS FOR RE-
pa02
1397
M
- C)Z-
(25
c-s
where M is a function of the temperature only, GENERATION O F SULFITE-BISULFITE SOLUlog M = 6.866 - 2369 TIONS FOR ABSORBING SULFUR DIOXIDE FROM T G A s E s CONTAINING 0.3 PERCENTSULFUR The maximum concentration of sulfur dioxide in the soluD I o x I D E, ASSUMING. tion leaving the scrubber, then, is given by: EQUILIBRIUM Is R E A C H E DI N T H E SCRUBBER (3)
-
Solving for S,: 0
is at 35OC.(95'F.) or above, is shown in F i g u r e s 1 a n d 2. Many industrial g a s e s , particularly those resulting from c o m b u s t i o n, have w e t - b u 1b temperatures in this range. It will be o b s e r v e d that, for the conditions represented, the concentrations of ammonia producing the highest capacity for absorbing sulfur dioxide coincide with those requiring the smallest amounts of steam for regeneration. The curves in Figures 1and 2 are based on an arbitrary set of operating conditions chosen only for comparison. The solution is assumed to leave the scrubber saturated with respect to gases containing 0.3 per cent sulfur dioxide. Regeneration is carried to the point where the vapor pressure of ammonia over the lean solution a t the stated temperature equals that of sulfur dioxide. The vapors leaving the regenerator are assumed to be in equilibrium with the influent solution. The graphical derivation of these curves is tedious and inexact, involving as it does reading off data from several graphs. The derivation given in this paper of the optimum concentration is applicable to the general case and may be used in predicting the capacity of any solution of ammonium sulfitebisulfite for absorbing sulfur dioxide from dilute gases. The operating conditions affecting the optimum concentrations of ammonia in the solvent are: (1) concentration of sulfur dioxide in the gases being treated, (2) efficiency of the scrubber, (3) temperature of solution leaving the scrubber, (4)temperature of regeneration, and (5) efficiency of regeneration. The solution cannot leave the scrubber completely saturated with the sulfur dioxide in the entering gas. The degree of saturation depends on the size and on the efficiency of the scrubber. Items 1 and 2, however, may be combined by designating the equilibrium sulfur dioxide pressure over the solution leaving the scrubber a t the scrubber temperature. As previously shown, the relation between this equilibrium pressure and the composition of the solution over the ranges in question is as appears in Equation 1.
Only the positive root is of interest. The operation of the regenerator is somewhat arbitrary, but it will be assumed here that regeneration will be carried to the point a t which the vapor pressure of ammonia equals the vapor pressure of sulfur dioxide over the solution. As shown in the previous paper @), this represents the approximate limit of regeneration. The equilibrium vapor pressure of ammonia is given by an equation analogous to Equation 1, PNH, =
- S) -c
N.C(C 2s
(5)
where N is also a function of temperature only, log N = 1 3 . 6 8 0
-4987 T
Equating (1) and (5) for the value of 8,:
(7) The ratio X,/C, therefore, is a function of temperature only and is independent of the value of C. Although Equation 7 indicates that the effect of temperature is a complex function, it happens that in the range of practical interest-via., from 70" to 120" C., a linear relation exists (Figure 3).
FIGURE3. EFFECTOF TEMPERATURE OF REGENERATION ON COMPOSITION OF REGENERATED SOLUTIONS
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INDUSTRIAL AND ENGINEERING CHEMISTRY
The equation of the line is: S,/C = 0.00219t,
+ 0.587
(8)
The capacity of the solution is defined as the pounds of sulfur dioxide recovered per 100 pounds of solution entering the scrubber, or 6400 (S, - X,) W = 1800 17C 648, (9)
+
+
Substituting the values of S, and S, from Equations 4 and 8, W
= 800
(
d
m - C(0.696 + 0.0175t,) - a , )
1800
+ 0.14trC+ 54.6C
Equation 10 may be used for determining the capacity of any solution of known ammonia concentration under fixed conditions of operation, giving the values of the parameters n and t,. p,EQUlLlBI?IUM
TABLEI. OPTIMUMCONCENTRATIONS OF SOLUTIONS FOR ABSORBING SULFUR DIOXIDE FROM GASESCONTAINING 0.3 PERCENTSULFUR DIOXIDE iMoles NHa/100 Moles He0 at Absorption Temp. of: 40" C. 45OC. 50OC. 550 (24.3) 20.4 17.1 14.0 17.7 14.7 12.0 (25.7) 21.2 18.5 15.2 12.5 10.2 90 22.4 15.9 13.0 10.4 8.6 100 19.4 1 3 . 6 1 0 . 9 9 . 0 7.1 110 16.8 120 14.2 11.3 9.0 7.3 5.8 a Values in parentheses are above the saturation concentration of ammonium aulfite.
Regeneration Temp., C . 70 80
c.
35°C. (28.8)*
Differentiating Equation 10 in respect to C and equating to zero, we have, after simplifying, 7 2 0 0 ~- 0.14a2t, - 54.6~' ~ ~ . 218.4) - - u C ~(0.56t, 1260 31.5tr - 0.14 at, - 5 4 . 6 ~
+
+
du-
(11)
Although Equation 11 appears to be complicated, numerical examples can be solved easily for C,,,., so that the optimum concentration can be found {or any conditions of operation. A table of values covering practically all such conditions when the vapor pressure of sulfur dioxide over the solution leaving the scrubber is 2.3 mm. (corresponding to equilibrium with a 0.3 per cent gas) is s h o w n in Table I. Since the saturated concentration of ammonium sulfite exists at approximately 22.5 moles of ammonia per 100 moles of water at 25' C. values of c g r e a t e r t h a n this value obviously cannot be used. When the absorption temperature is relatively high, the table shows that the o p t i m u m a, 9D ll?o concentration of the REGENERATION TEMPERATURE tk. solution may be less FIGURE4. EFFECTOF TEMPERA- than half of the conTURE OF ABSORPTION AND OF REc e n t r a t i o n of the QENERATION ON OPTIMUM SOLUsaturated salt soluTION CONCENTRATION FOR GASES 0.3 PERCENTSULFUR tion. CONTAINING DIOXIDE These d a t a are plotted in Figure 4 in order to show the effect of the temperature of absorption and of regeneration on the optimum value of C. Since the constant
PARTIAL PRESSURE OVER SOLLITION LEAVING SCRUBBER
mm
WFRf-IIRY
FIGURE5. EFFECTOF GAS COMPOSITION ON OPTIMUM SOLUTIOX CONCENTRATION FOR ABSORPTION AT 45' C.
a is directly proportional to the concentration of sulfur dioxide in the gases, assuming equilibrium is reached, it is possible to use the data of Table I to show the effect of changing the gas composition. This is shown in Figure 5, for which the absorption temperature is held constant a t 45" C. It is evident from Figures 4 and 5 that the optimum concentration of ammonia (a) decreases as the absorption temperature decreases, (b) decreases as the regeneration temperature decreases, (c) increases as the sulfur dioxide concentration of the gas increases, and (d) increases as the efficiency of the scrubber increases the saturation of the solution.
Nomenclature a C
M ,N
= ratio psoZ/Mat temperature of absorption = concentration of "8, moles per 100 moles HaO
constants in vapor pressure equations for SO2and NHs, respectively psoZ = equilibrium vapor pressure of SO2, mm. Hg ~ N = H equilibrium ~ vapor pressure of ammonia, mm. Hg S = concentration of SOZ,moles per 100 moles HzO S, = concentration of SO2 in solution leaving the absorber 8. = concentration of SO2 in solution for which pso, = p", at regeneration temperature t. = temperature of solution leaving absorber, O C. t, = temperature of solution leaving regenerator, ' C. W = capacity of solution for abosrbing SOZ, lb. per 100 lb. solution entering scrubber =
Literature Cited (1) Johnstone, IND.ENQ.CHEM.,27, 587 (1935). (2) Johnstone and Keyes, Ibid.,27, 659 (1935). RECEIVED July 12, 1937.
From Banka Tin
