Ind. Eng. Chem. Fundam. 1984, 23, 260-264
260
Snyder, L. R.; Kirkland, J. J. "Introduction to Modern Liquid Chromatography", 2nd ed.: Wiley: New York, 1979; Chapters 15 and 16. Wankat, P. C. Ind. Eng. Chem. Fundam. 1911, 16, 468. Wankat, P. C. Proceedings Corn Refiner's Assoc. 1982 Scientific Conference, Lincolnshire, IL, June 16-18, 1982 pp 119-167. Wankat, P. C.; Ortiz, P. M. Ind. Eng. Chem. Process Des. Dev. lQ82, 2 1 , 416. Waters Assoc. "New Waters Kiloprep Process Scale Separations Systems", Milford, MA, 1983. Yau, W. W.; Kirkland. J. J.; Bly, D. D. "Modern Size Exclusion
Chromatography"; Wiley: New York, 1979; Chapter 11.
School of Chemical Engineering Purdue University West Lufuyette, Indiana 47907
Phillip C. Wankat
Received f o r review February 22, 1983 Accepted August 10, 1983
Role of HNOp in the Absorption of Nitrogen Oxides in Alkaline Solutions Nitrous acid formed in the gas phase has an important effect on the rates of absorption of mixtures of nitrogen oxides in alkaline solutions. I t is only if its contribution is taken into account that the behavior of industrial absorption towers can be simulated. The gas-phase mass transfer of HNOl largely determines the height of alkaline scrubbing units and the efficiency of plants for the production of nitrites.
Introduction There are two major classes of industrial operations where nitrous acid formed in the gas phase contributes significantly to the absorption process: (i) alkaline scrubbing of NO, containing gases such as flue gas, tail gas from plants for the manufacture of nitric acid, off-gas from steel pickling plants, and organic nitration plants; the main goal of the operation here is the removal of nitrogen oxides with very little or no concern for the quality of the product; (ii) production of nitrites by absorption of NO, in alkali. The process has been widely used for the production of sodium nitrite and is further gaining considerable importance for the manufacture of calcium nitrite (Makoto et al., 1979, 1982). The main objective in this case is the achievement of high yields in nitrites that would minimize the expenses for subsequent separation processes and the loss of fixed nitrogen in nitrates. The following gas-phase reactions occur when NO and NO2 are mixed in presence of oxygen and water vapor NO
+ '/202
-+
NO2
2N02 + N204 NO
+ NO2
N203
+ NO2 + H2O + 2HN02 3N02 + H 2 0 + 2HN03 + NO NO
Table I. Gas Phase Reactions equil (b)
Kb =' P = 5.94 P2
(c)
K, = P
(C?
(d? (e)
Reaction a is essentially irreversible at low temperature and consists of a slow termolecular process. Its overall activation energy is negative and its rate is known to decrease at increased temperature. The rate constant for reaction a is given by Bodenstein (1922). The equilibrium constants for reactions b to e are given in Table I. The numerical values were taken from selected sources as indicated. Reactions b and c are known to be fast and always at equilibrium for any practical consideration. The rate of reaction d has been measured experimentally in several different occasions, but considerable disagreement exists among the various reported data (Wayne and Yost, 1951; England and Corcoran, 1975; Chan et al., 1976). England and Corcoran give a value of Izd = 2.06 X g-mol/(cm3)(s)(atm3)at 25 OC for the thirdorder rate constant claiming an accuracy of *50%. Limited information is available on the kinetics of reaction e, but its equilibrium is known and nitric acid vapor is present in very low concentrations in the gas mixtures. 0196-4313/84/1023-0260$01.50/0
=
(d)
ref
exp(- 689 1.61 ) T
X
4.18 x
lo-'
exp(- 4869.0
T
PIP2
)
4723.0 K d = -= 1.85 x 10." e x p ( 7 ) P 1P 2Pw
Bodenstein and Boes (1922). Hoftijzer and Kwanten (197 2). Giaque (1942). a
a
b c
Hisatsune (1961) Forsythe and
NO and NO2 have very low reactivity in water and alkaline solutions. N204 reacts in alkaline solutions according to
(a) (b)
equilibrium constant, atrn-l
N204+ 20H-
+
NO2- + NO3- + HzO
(f)
When NO2,in equilibrium with N204,is brought in contact with water or dilute nitric acid, equal amounts of HNOl and H N 0 3 are formed according to the reaction N z 0 4+ H 2 0 H N 0 2 + HNO, (g)
-
Down to about 200 ppm of NO2 the major transporting species is the Nz04formed in the gas phase. The hydration of N204is a fast pseudo-first-order reaction with a rate constant of about 290 s-l (Sherwood et al., 1975). When NO2 is absorbed in alkaline solutions, reaction g is still the rate-controlling step. Gray and Joffe (1955), Chambers and Sherwood (1937), Kameoka and Pigford (19771, and Takeuchi and Yamanaka (1978) found the absorption in alkaline solutions only slightly different from that in water. The reverse reactions are, however, suppressed owing to the formation of nitrite ion. Nz03reacts in alkaline solutions according to
N203 + 20H-
-.+
2N02- + HzO
(h)
The process is likely to be controlled again by the hydrolytic reaction of N,03. Corriveau (1971) determined a rate constant of about 1.2 X lo4 s-l at 25 "C. In contrast to nitric acid, nitrous acid is relatively weak but at pH greater than about 6 it can be considered to be completely dissociated. In alkaline solutions, therefore, 0 1984
American Chemical Society
Ind. Eng. Chem. Fundam., Vol. 23, No. 2, 1984
261
the HN02 formed in the gas phase and transferred to the gas-liquid interface undergoes an instantaneous proton transfer reaction "02
+ OH-
-
NO
