Recovery of Sulfur Dioxide J
from Waste Gases The partial vapor pressures have been measured a t 35", 50", 70", and 90" C. over a long range of concentration of ammonia and relative concentrations of sulfur dioxide in solutions of the ammonia-sulfur dioxidewater system. For the concentrations encountered in the cyclic process of absorption of sulfur dioxide from waste gases and regeneration by distillation, the partial pressures may be expressed by the formulas :
Equilibrium Partial Vapor Pressures -
(2s - C)* PSOl =
l%.l
c-s
over Solutions of the Ammonia-
Sulfur Dioxide-Water System H. F. JOHNSTONE University of Illinois, Urbana, Ill.
only of historical interest, being one of the first suggested for the cyclic recovery method ( 8 ) , but also, because of the high molar solubility of ammonium salts, it seems to offer the greatest possibility of development for very dilute gases. I n this paper, the vapor pressure data are given for the entire range of concentration of ammonia and relative concentration of sulfur dioxide that may be encountered in t h e cycle, and also for the entire temperature range of the process. The effect of various other electrolytes that might influence the temperature coefficient of the vapor pressure has also been studied. I n a later paper in this series a discussion of other hydrolytic equilibria will be given in connection with vapor pressure data of other sulfite-bisulfite systems. The series will also include data on the design of the regenerator and on the actual operation of a pilot plant on some of these systems.
C(C - S) andpNHl= !W -2s - c I
where C is the concentratiem of ammonia in moles per 100 moles of water, and S is the concentration of sulfur dioxide. The values of the constants are given by log -%.l = 3.865 - 2369/T and log AT= 13.680 - 4987/T. The partial pressure of water follows Raoult's law. In the range of concentration considered, the effect of temperature on the vapor pressure of sulfur dioxide is practically independent of the nature of any completely ionized electrolytes present. The capacity of these ammonia-sulfiir dioxide solutions for absorbing sulfur dioxide from 0.3 per cent gases may be as high as 8 pounds per 100 pounds of solution. Subsequent papers will deal with the principal item of cost-namely, the steam required for regeneration and also a complete cost summary derived from data obtained during the operation of a commercial-scale plant.
Apparatus and Method The equilibrium vapor pressures mere measured a t 35 O , SOo, 70", and 90" C. by the dynamic method. The apparatus used is shown in Figure 1: -4steady stream of urified nitrogen was passed from the aspirator bottle, A , througg the saturator, B, which consisted of six Bichowsky tubes (I). The gas, saturated with the vapor of ammonia, sulfur dioxide, and a-ater, then passed through the heated side arm of the saturator into a weighed Vanier bulb. The latter contained a standard solution of iodine (0.05 to 0.2 N , according to the vapor pressure of sulfur dioxide) in 3 N sulfuric acid and 10 per cent potassium iodide in the outer part, and anhydrone in the inner tube. Four of these saturators and absorbers were connected in series so that the vapor pressures could be measured at the four temperatures with a single measurement of the quantity of inert gas and a single analysis of the solution being studied. The nitrogen, after leaving the absorber for the 90" C. thermostat, passed first through a protective anhydrone tube and then through a humidifier into the aspirator bottle, C. The total quantity of dry inert gas was measured by the weight of water flowing from this bottle, with suitable corrections for temperature and pressure. The rates of flow of water from bottles A and C were adjusted so that they were approximately equal, and so that a slight pressure was maintained on the saturators in the first two thermostats and a slight vacuum was maintained on those in the third and fourth thermostats. These pressures were read at frequent intervals on mercury manometers and mere used for correcting the barometric pressures read at the same time.
ANY of the methods which have been proposed for the recovery of sulfur dioxide from dilute waste gases are based on the absorption of the gas by aqueous solutions of either sulfite or sulfite and free alkali, and subsequent regeneration of the solution with release of the dissolved gas by heating, either with or without a vacuum. Because of the absence of quantitative data on the equilibrium vapor pressures over these sulfite-bisulfite systems and the importance of such data for the design of both the absorber and the regenerator, a series of vapor pressure measurements has been made on the ammonia-sulfur dioxide-water system. This system is not
The vapor pressure of each constituent was found from equations, such as 587
INDUSTRIAL AND ENGINEERIKG CHEMISTRY
588
VOL. 27, NO. 5 la
Apparatuj for
nsol
V a p o r P r e 5 g u r e Mea ~ u r e r n e n r 5
+ + + fwt P where n = moles of constituent designated by subscript Psoz =
nsoz
~ N H B
~
H
~
O ’
(1)
P = corrected barometric pressure
The weights of sulfur dioxide, ammonia, and water were found, respectively, by (1) titration of the iodine solution with thiosulfate, (2) distillation of the ammonia from this solution, after addition of excess alkali, into standard acid, and (3) difference, from the total weight increase of the Vanier bulb. The accuracy of this method for the determination of the vapor pressure of these three constituents has been demonstrated by Terres and Hahn, who measured the partial vapor pressures of solutions of ammonium sulfite and of saturated ammonium bisulfite solutions (22). The apparatus used was checked by measuring the vapor pressure of water a t the four temperatures used. The results agreed with the accepted values within 0.5 per cent. In general, two determinations, and a t times four, were made on each solution. These were accepted when they checked within 2 mm. a t the high temperatures and within 0.2 mm. a t the low temperatures. At very low vapor pressures, especially in the case of ammonia, the method of analysis of the solution in the Vanier bulb was not sufficiently accurate to give good results, but since these data were of little interest they were not refined. Greater accuracy was also obtainable nThen the results warranted it by maintaining a slower rate of gas flow through the absorbers. Normally this was held at 1200 cc. per hour. The solutions were made up by adding the predetermined quantity of sulfur dioxide to standard solutions of ammonium hydroxide. The sulfur dioxide was obtained from cylinders of refrigeration-grade material. The ammonium hydroxide was the ordinary laboratory c. P. grade. The other chemicals used were the “Analyzed” grade or were recrystallized from the technical product. The solutions were stabilized against oxidation by the addi-
tion of 0.1 per cent hydroquinone. With this inhibitor, only a trace of sulfate could be detected after several weeks, whereas, in the absence of it, noticeable oxidation took place within a few hours. The solutions were analyzed by the usual methods, the samples being taken by weight so that the results could be expressed in terms of moles per 100 moles of water. These units were chosen because of the necessity of wing water as the reference substance, and the ease of converting to the mole fraction basis, which is required in some of the calculations. Data The vapor pressure data obtained are shown in Tables I to 111. The three series of solutions, A, B, and C, represent three concentrations of ammonia, with increasing concentrations of sulfur dioxide. According to the solubility data of Terres and Hahn (11) and of Ishikawa and Hagisawa (S), the solutions represented by A are approximately the maximum concentration of ammonia that may be used without danger of crystallization in some part of the operation cycle. The D series of solutions contains the same concentration of available ammonia as the C series, with an additional amount present as ammonium sulfate. These data, therefore, show the effect of oxidation of the sulfite of a solution containing somewhat more ammonia than the B solutions. The data in Table I1 indicate the effect of temperature on the partial vapor pressures for a number of modified ammonia solutions, rrhile those of Table 111 show the partial pressure data for sulfite-bisulfite solutions of other bases. Effect of Concentration The effect of the concentration of sulfur dioxide and ammonia on the partial pressures of these two constituents over the simple ammonia solutions can best be discussed by reference to the two equilibria:
MAY, 1935
INDUSTRIAL AND ENGIR EERING CHEMISTRY
589
methyl amine, differ from those for the ammonia solutions, apparently because of a specific ion effect. If the total concentration of dissolved sulfur dioxide in moles per 100 moles of water is represented by S and that of ammonia by C, it is possible to find an expression for the partial pressures in terms of the stoichiometric concentrations of these constituents from the equations,
It may be shown that the equilibrium constants for these reactions are
+ [HSOa-I + ISOa--l + ["(+I
S = [HzSOI]
+ [NHdf] = [HSOa-] + 2[SOj--] + [OH-]
[H+]
where the quantities in the brackets now become the actual ion concentrations. Some variations in the values of M and N are to be expected as the total salt concentration of the solution is changed. While such effects are somewhat apparent from the data, it will be shown later that!, over a considerable range of concentration, a single value for each of the constants a t any one temperature will reproduce the actual vapor pressures within 10 per cent for all of the ammonia solutions studied. The constants for other sulfite-bisulfite solutions, however, such as those of sodium, potassium, and
Solution A-2
(10)
For the concentration range in which we are interested and for the p H range between 4.5 and 6,' it, is permissible to make the simplifying assumptions (1) that the concentrations of the unionized molecules of ammonia and sulfur dioxide (or the hydrated forms of these molecules) are negligible compared to those of the ionized portions and (2) that the concentrations of hydrogen and hydroxyl ions are likewise negligible compared to the high concentration of ammonium and of sulfite and bisulfite ions. Within the limitations imposed, the ratio of the neglected quantities to those retained is of the order of 10-4 or smaller. With these simplifications, solution of Equations 8 to 10 gives: [HSOs-] = 3 - C, [SOa--] = C - S , [ S " q + ] (2s - C)* whence p 8 0 2 = M -(C - 8)
=
(9)
and the equation for the electroneutrality of the solution,
where k, and IC, are the Henry's law constants for sulfur dioxide and ammonia, respectively, and K1, RP,Kat and K , are, respectively, the ionization constants for the first and second ionization stage of sulfurous acid, for the ionization of ammonium hydroxide, and for water The quantities in parentheses represent the activities of the ions. I n the salt concentrations here considered, these may differ widely from the actual ion concentrations. Since the known laws for the activity coefficients obviously cannot be applied, it is necessary to determine empirical values for the constants in order t o be able to use these important interpolation formulas. The vapor pressures may then be expressed by
s
(8)
[NHaOH]
C
= C
(11)
For solutions in which a strong acid is present which may be considered completely ionized, such as sulfuric acid,* the int.erpolation equations become: (2s - C -S
psoa = 'IfC 1
9
+ nA)2
- ?&A
The p H of ammonium bisulfite solutions ia approximately 4.1. The aecond ionization of sulfuric acid is complete above a pH of 4.5.
OVER SOLUTIONS OF SULFUR DIOXIDE,AMMONIA,LVD WATER TABLEI. PARTIALVAPOR PRESSURES soz/ c = KHa/
100 l
