Prediction of Azeotropes in the Systems NH3-H20 and S02-H20 A. 8. van Aken, J. J. Drexhage, and J. de Swaan Arons* Koninklijke/Shell-Laboratorium. Shell Research B. V . , Amsterdam, Netherlands
The article presents some equilibrium calculations related to the removal of acidic and basic components from aqueous effluent streams by steam stripping. These have led to the discovery of azeotropes in the systems NH3-HpO and S02-H20 at very low concentrations of the first component, caused by the sharply increased ionization at high dilution. The occurrence of these azeotropes sets lower limits to the purification by stripping techniques.
Introduction The study contained in this article was initiated by a simple request for data on vapor-liquid equilibria in the system NHa-HzO. The description of this system was needed for the reduction of the ammonia content in waste water streams from 5 ppm to 35 ppb. Until then we had been used to stripping calculations where knowledge of Henry's constant of the gaseous solute was sufficient, but this question showed us how increasing ionization of the solute at strong dilution can significantly reduce volatility, sometimes even resulting in negative azeotropy, Le., formation of mixtures of lower volatility than either of the pure components. Since then, similar requests have reached us for the prevention of public water pollution, which induced us to apply the insight obtained from the former problem. We will now first give a general analysis of vapor-liquid equilibria in systems of water and ionizing volatile solutes, and then present the results for the steam stripping of very dilute aqueous ammonia and sulfur dioxide solutions.
y* is the ionic activity coefficient which has been assumed to be the same for all ionic species. Definition of a Separation Factor. For illustrative purposes and to assess the effect of ionization on the volatility behavior we define (again for the example of "3) a separation factor a as follows Y N H 3 / x N H3
f f z
yH
t
(6)
~ 0 / ~ H 2 0
in which xNHQt
=
xNHQ
+
(7 1
xNH4
and =
xH20t
xH20
+
xH30
+
(8)
xOH
and X H ~ O being a measure of the NH3 and H20 molecules, respectively, converted in the ionization reaction. Ignoring X H ~ O and XOH in (8) and introducing (1) and (2) into (6), the expression for a becomes
x",
Analysis of the Problem The factors which play a role in the volatility behavior of volatile ionizing species, not necessarily inorganic by nature, can best be recognized in dealing with an example. We will take the case of aqueous solutions of "3. For the strictly physically dissolved ammonia molecules we can write (ignoring vapor-phase nonidealities) =
YNH3P
(1)
Y*NH~XNH~H
Similarly, we write for water YH20P
=
In these equations P is the total pressure, y and x are the mole fractions of the components in the vapor and the liquid, respectively, H is Henry's constant for NH3 in Hz0, P'H~O the saturated vapor pressure of HzO, and y is the activity coefficient, the asterisk indicating that the reference state is not the pure liquid, but the infinitely dilute solution. Though the vapor only contains HzO and "3, we know that the liquid will also contain the ionic species NH4+, H 3 0 + , and OH-, their concentration being related via the charge balance (3) X N H ~ + XH,O = xOH and the equilibrium constants 2
XNH~XOH XN H 3 X H 2 0
y*
C Y =
Y*NH~H
1
(10)
+ K O X ~ Z O Y * ~ ~ 3 Y ~' ~2 O~ H/ Y ~
YH2O p0H20
At this point it is interesting to write this expression for dilute solutions. With all y's and X H ~ Oclose to one the relative volatility will be
( 2)
Y H ~ H , O ~ OH20 H ~ O ~
KO=-----.
which, with (4), results in
,
This equation clearly shows the interaction between the volatility of the physically dissolved species, H, and the ionization constant K" thereof in constituting the effective volatility. For example, in ammonia solutions at higher concentrations of "3, XOH will be much larger than K" and the solubility behavior of ammonia will be strictly according to Henry's law CY
>
= H/P',~~ 1
(12)
whence NH3 is more volatile then H20. On the other hand, if we go to infinitely dilute solutions of NH3 in HzO, the limiting value of XOH will-according to (5)-be and it can be shown that
(4)
Y*NH3YH20
and
(5) 154
Ind. Eng. Chem., Fundam., Vol.
14, No. 3, 1975
In other words, the increasing influence of ionization on dilution of the system will lead to azeotropy. This can be
demonstrated to be also the case for SO2 by the expression
where H is Henry's constant for SO2 in HzO and K1" and Kz" are the ionization constants of SOz. In fact, with proper knowledge of Henry's constants and ionization constants, eq 13 and 14 will help to predict the decrease in relative volatility on dilution. For a number of compounds this is shown in Table I. The Case of One Basic a n d One Acidic Volatile Solute. In the case where the pH of the solution is not exclusively determined by the volatile solute, as we assumed up to now, eq 13 for a has to be corrected by using for X H ~ Oor XOH the proper value instead of G. This consideration is of interest in the problem of simultaneous dissolution in water of HzS and "3. From Table I and eq 13 we see that HzS, owing to its high Henry's constant and small dissociation constants under normal conditions, is much more volatile than "3. When the ammonia concentration increases, however, the relative volatility will reverse owing to the high p H value imposed by "3, both volatilities a t that point being still greater than that of water. When the ammonia concentration increases further, HzS may even become less volatile than water, as can be understood from eq 15
Strictly speaking, one should account for the fact that despite X H ~ S = 0, the high concentration of NH3 will make X H ~ O < 1 and activity coefficients different from unity. However, since the foregoing is restricted to very dilute solutions, these effects can be ignored. The Effect of Added Salts. Until now we have ignored the activity coefficients y in expression 10 for the relative volatility. This was allowed because we restricted ourselves to dilute solutions where the influence of y is insignificant. Things become different, however, as soon as we start adding salts. Let us first establish that the activity coefficients Y * N H ~ , Y H ~ O ,and y + should be mutually consistent, i.e., they should have been obtained (by partial differentiation) from a proper expression for the excess Gibbs' free energy that accounts for all sorts of interactions in the solution, such as Coulomb forces, dispersion forces, etc. This has not always been achieved, however, and generally speaking, in practice one deals with these activity coefficients separately. Thus, the Debye-Huckel theory for electrolyte solutions gives an expression for y + in very dilute solutions (Lewis and Randall, 1961) In yt = A47 in which the ionic strength
(16)
(where zi = charge of ionic species i) and A is dependent on the temperature and the dielectric constant t of the medium. For more concentrated solutions (16) can be empirically extended in terms of I. If we add salt, yi will first start to decrease according to the Debye-Huckel theory, from the value of 1 in the infinitely dilute solution, in general go through a minimum value which may be in the range from for instance 0.2 to 0.7 at I = 0.1-1.0, and then rise again, possibly exceeding 1.
Looking again a t (10) and knowing that ~ * N Hand ~ YH,O do not change drastically, we can see how the addi-
Table I. Relative Volatilities of Ionic Species a t Infinite Dilution in Water
NH3
100 15" 15 4.8" 6.7 x lo-' 25 3 X l o m 3 " 0.1 3.2" 1.6 x 10-5 100 1540" 1540 6.50 10.0" 1.1 x 103 SOz 100 316b 3 16 2.6" 7.8' 9.1 x lo-' COz 25 1690d 5.4 x io4 10.3" 1.0 x io4 HCN 100 53f 53 8" 5.2 x lo1 a These values have been obtained by Dr. P. L. Chueh of Shell Dev. Co., by computer analysis of basic data published by Brosheer et al. (1947), Clark (1966), Lange (1961), Pohl (1962), Seidell and Linke (1965), and Selleck (1952). Correlated from Rabe and Harris (1963). Extrapolated from data published by Perrin (1969). Correlated from data published by National Research Council (1926-1933). e Robinson and Stokes (1968). Extrapolated from Timmermans (1960). HF H,S
f
tion of salt will initially decrease the volatility of NH3 relative to that of water, but that a further increase in salt concentration will help to make a even larger than one, thus breaking the azeotrope. (Strictly speaking, in the case of a = 1 in the system NHs-HzO in the presence of salt we have no azeotrope. Vaporization of NH3 and HzO will change the composition of the liquid and even the NHs-HzO ratio due to the change of I.) In this respect the azeotrope of HC1 and HzO is an interesting case. Owing to the high K" of HCl, th'is compound itself takes care of the increase in I with HC1 concentration. Adding salt to the liquid azeotrope immediately increases the volatility of HCl relative to HzO, without going through a minimum first, because a t the azeotropic composition the value of I due to HCl alone is already so high that y + values are much larger than l . Conversely, in an aqueous salt solution of an ionic strength equal to that of the liquid HCl-HZO azeotrope the limiting relative volatility of HC1 will be very close to 1. The Addition of Nonelectrolytes. We have now established that, if we disregard the temperature, the relative volatility is governed by three factors: Henry's constant H,the ionization constant(s) K", and the ionic strength I. On addition of a nonelectrolyte, for instance, methanol to an aqueous solution of a volatile ionizing compound, the dielectric constant of the medium will change. This will affect H, because the physical solubility of, for instance, NH3 in H20 will be different from that in methanol because of the difference in intermolecular interactions. I t will also affect K" because there will be a change in fugacity if we transfer an ion from an infinitely dilute solution in H20 to one in methanol. Finally, according to (16), y* is affected, because A is inversely proportional to the dielectric constant. Together these effects will in general increase the volatility of the ionizing solute. Steam Stripping of Very Dilute Aqueous Ammonia a n d Sulfur Dioxide Solutions Equilibrium Curve and Operating Line. The vapor-liquid equilibrium curve for dilute aqueous ammonia solutions has been calculated from the following equations
Henry's law pNH3 = H.[NH,]*
(17)
ionization constant K,,' = [NH '][OH-]
"1 autoprotolysis constant Kwo = [H,O'][OH'] c h a r g e balance [NH4+] + [H,O+] = [OH'] Ind. Eng. Chern., Fundam., Vol. 14, No. 3, 1975
(19) (20) 155
Table 11. Degree of Ionization as a Function of Total Ammonia Concentration ( t = 100 "C)
105 103
0.001
0.016 0.150 0.456 0.950
10 1 0
Table 111. Degree of Ionization as a Function of Temperature and Concentration of Total SO2 i x,
PPmw 103
a. wb
Figure 1. Equilibrium curve for dilute aqueous ammonia solu-
lo2 10 5 2 1 0
40°C
100°C
0.565 0.882 0.985 0.992 0.997 0.998 > 0,999
0.385 0.746 0.956 0.977 0.990 0.995 >0.999
tions ( t = l 0 O O C ) .
1 I I
I I I I
2
I I I I I
t J - t h TRAY I
I
I I I I
I
on
a
1
10 TOTAL SOz.
100 p~mr
Figure 2. Volatility of SO2 relative to water at 100°C.
the values of H,Kbo and Kw"being known from the literature. For a particular value of P N H ~ , [NHs] is calculated from (17); [OH-] and [H30+] can be expressed in unknown [NH4+] by (18) and (19), respectively, and, on the correct assumption that, in very dilute solutions, activities are practically equal to concentrations, inserted in (20), which is then solved for [NH4+]. Now the total ammonia concentration in the liquid [NH3]a = [NH3] [NH4+], is known. Instead of .working with a P N H ~ [NH3]a diagram we used a y-x diagram, in which y and x are expressed in parts per billion (ppb), according to
+
Y =
PNHs/P;
x = [",IJ55.6
( P = P"*o= 1 bar, [NH3]a is in mol/l). The shape of the equilibrium curve will be seen from Figure 1. At high concentrations we may write Y N H ~= HXNH~ = HX(1 - i) 156
Ind. Eng. Chem., Fundam., Vol. 14. No. 3, 1975
HX
6@-
L - v , Y,
Figure 3. Material-balanceenvelopes for the stripping column. ( x = X N H ~+ X N H ~ , H = 15 bars at lWC), because the degree of ionization i is
