Ind. Eng. Chem. Res. 2004, 43, 5965-5968
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RESEARCH NOTES Thermodynamic Analysis on Pyrolusite-Absorbing Nitrogen Oxides Zhou Houzhen,*,†,‡ Long Bingqing,| and Liang Hua§ Chengdu Institute of Mountain Hazards & Environment (IMHE), Chinese Academy of Sciences (CAS) & Ministry of Water Conservancy, Chengdu 610041, China, Graduate School of Chinese Academy of Sciences, Beijing 100039, China, Chemical College, Sichuan Normal University, Chengdu 610066, China, and Southwest Institute of Technical Physics, Chengdu 610041, China
This paper has developed a new process on pyrolusite, whose valid composition is MnO2-absorbing NOx (NO and NO2). In the new process, Mn(NO3)2 is produced after absorption. It can be sold directly or can be decomposed into MnO2 and HNO3 by heating. As a product with higher added value, MnO2 can be sold directly and also recycled into the absorption system as an absorbent. The curves of log pNOx-pH at 298 K and 343 K are plotted, respectively. The purification curves of Ca, Mg, Al, and Fe at 298 K are respectively plotted. The theoretical analysis result is that pyrolusite-absorbing NOx in a subacidic solution at normal temperature can be satisfied with the Integrated Emission Standard of Air Pollutants (22.621 Pa; GB16297-1996, China). Meanwhile, in the experiment, if the temperature of the pyrolusite serum is 303 K, pH ) 4, and the solution contains 90 g L-1 of MnO2, the absorptivity for 3000 mg m-3 of NOx is 89.2%. The purity of the end product MnO2 is more than 99.995%. 1. Introduction As one of the air pollutants, NOx (NO and NO2) is controlled by promoting combustion and controlling the nitric acid production process, which is called pollution prevention. NOx is also controlled by end-of-pipe pollution control.1,2 The above measurements can decrease the emission of NOx to some extent, and some of them may be economically beneficial, but the sale of byproducts may vary with the market. Once a sluggish market appears, the byproducts will be overstocked and the removal of NOx may be affected. In this paper, a new process is introduced, in which pyrolusite (MnO2) is ground into powder. NO gas dissolving into water forms liquid NO. The liquid NO is oxidized by MnO2 in the solution, while NO2 dissolved into water forms HNO3 and NO. As mentioned above, NO is oxidized by MnO2. Then, product Mn(NO3)2 is obtained. With heating, Mn(NO3)2 may be decomposed into MnO2. As a product with higher added value, MnO2 can be sold directly and also recycled into the absorption system as an absorbent. Therefore, first, the removal of NOx occurs, which is environmentally viable. Second, higher added value product MnO2 is produced. This is of economic interest. Clarifying the thermodynamic conditions of the process may be significant for amplification of the process and its application in engineering. This paper theoretically * To whom correspondence should be addressed. Tel.: +8628-8522-3827. Fax: +86-28-8522-9892. E-mail: houzhenzhou@ yahoo.com.cn or
[email protected]. † Chinese Academy of Sciences (CAS) & Ministry of Water Conservancy. ‡ Graduate School of Chinese Academy of Sciences. § Southwest Institute of Technical Physics. | Sichuan Normal University.
analyzes the absorption and purification processes of the new process. The thermodynamic data in the paper come from refs 3-5. According to the economical benefits of the process, both [Mn2+] and [NO3-] in the solution are set as 1.0 mol L-1. 2. Thermodynamic Theoretical Analysis of the Absorption Process 2.1. Equilibria. (1) Equilibrium of MnO2-NOSolution. When the equilibrium of MnO2, NO, and solution is achieved
Mn2+ + 2H2O ) MnO2 + 4H+ + 2e-
(1)
E°1 ) +1.228 V. For
NO + 2H2O ) NO3- + 4H+ + 3e-
(2)
E°2 ) +0.956 V. For
3MnO2 + 2NO + 4H+ ) 3Mn2+ + 2NO3- + 2H2O (3) E°3 ) E°1 - E°2 ) +0.272 V. This is in accordance with
∆G° ) -nFE°cell ∆G° ) -RT ln Keq E°cell )
RT ln Keq nF
If R ) 8.314 J mol-1 K-1, T ) 298 K, F ) 96 500 C mol-1, and n ) 6, thus Keq,1 of eq 3 is 4.04 × 1027.
10.1021/ie034212r CCC: $27.50 © 2004 American Chemical Society Published on Web 08/10/2004
5966 Ind. Eng. Chem. Res., Vol. 43, No. 18, 2004
Figure 1. Curves of log pNOx-pH ([Mn2+] ) 1.0 mol L-1 and [NO3-] ) 1.0 mol L-1).
Because
Keq,1 )
[Mn2+]3[NO3-]2 pNO2[H+]4
if [Mn2+] ) 1.0 mol L-1, [NO3-] ) 1.0 mol L-1, and pH ) 5, thus partial pressure pNO ) 1.57 × 10-4 Pa. (2) Equilibrium of NO-NO2-Solution. When the equilibrium of NO, NO2, and solution is achieved
NO2 + H2O ) HNO3 + H+ + e-
(4)
E°4 ) +0.775 V. The reaction of NO2 converting to NO is
3NO2 + H2O ) 2H+ + 2NO3- + NO
(5)
This is in accordance with (2) and (4). If R ) 8.314 J mol-1 K-1, T ) 298 K, F ) 96 500 C mol-1, and n ) 3, thus Keq,2 of eq 5 is 1.53 × 109. If [NO3-] ) 1.0 mol L-1, pH ) 5, and partial pressure pNO ) 1.57 × 10-4 Pa, thus partial pressure pNO2 ) 1.01 × 10-9 Pa and partial pressure pNOx ) pNO + pNO2 ) 1.57 × 10-4 Pa. (3) If T ) 343 K, the other conditions (R, F, and n) are the same as the conditions in (1); thus, Keq,1 of eq 3 is 9.65 × 1023. If [Mn2+] ) 1.0 mol L-1, [NO3-] ) 1.0 mol L-1, and pH ) 5, thus partial pressure pNO ) 1.02 × 10-2 Pa. (4) If T ) 343 K, the other conditions (R, F, and n) are the same as the conditions in (2); so Keq,2 of eq 5 is 9.55 × 107. If [NO3-] ) 1.0 mol L-1, pH ) 5, and partial pressure pNO ) 1.02 × 10-2 Pa, thus partial pressure pNO2 ) 3.26 × 10-8 Pa and partial pressure pNOx ) 1.02 × 10-2 Pa. 2.2. Thermodynamic Theoretical Analysis on the Absorption Process. According to the equilibria in section 2.1, the curves of log pNOx-pH at normal temperature (298 K) and at higher temperature (343 K) have been plotted (Figure 1). The absorption temperature is under 343 K for engineering control and economical benefits. In Figure 1, we see, at normal temperature under subacidic conditions (pH e 6.5), that NOx can be absorbed by MnO2, and emission can be satisfied with the Integrated Emission Standard of Air Pollutants (22.621 Pa; GB16297-1996, China). At higher
Figure 2. Neutralizing purification curves of Al and Fe by MnCO3 (298 K).
temperature, it needs stronger acidity. This means that the purification cost of the absorption solution must be increased. Of course, the dynamics herein has not been taken into account. Experiments show that, at normal temperature or even lower temperature, the speed of MnO2 absorbing NOx is able to meet the requirements for the process.6 From the above analysis, a conclusion can be reached. It is possible in thermodynamics for MnO2 to absorb NOx. The absorption process is more favorable at normal temperature than at higher temperature. Meanwhile, the absorption is more efficient in a more acidic solution. However, the reactions are conducted in a subacidic solution for the experimental equipment. (Data of Figures 1-3 are given in the Supporting Information.) 3. Purification of the Absorption Solution Besides MnO2, pyrolusite contains such impurities as Fe2O3, Al2O3, CaO, MgO, etc. When MnO2 reacts with NOx, some of the impurities may dissolve and form nitrates. Because these nitrates have an impact on the purification of the product, the impurities must be removed. In the first step of purification, Fe and Al will be purified by neutralizing hydrolysis, in which MnCO3 is the hydrolytic reagent. In the second step of purification, Ca and Mg will be removed, in which H3PO4 is the hydrolytic reagent. 3.1. Dealuminum by Hydrolysis.
Al3+ + H2O ) Al(OH)2+ + H+
(6)
According to ∆G° ) -RT ln Keq, hydrolysis constant Kh(1) ) 1.06 × 10-5, so pH ) 4.97 + log [Al(OH)2+] - log [Al3+].
Al(OH)2+ + 2H2O ) Al(OH)3 + 2H+
(7)
Hydrolysis constant Kh(2) ) 1.13 × 1024, so pH ) -12.03 - (1/2) log [Al(OH)2+].
[Al3+]t ) [Al3+] + [Al(OH)2+] ) 10-19.08-3pH + 10-24.05-2pH (8) 3.2. Deferrum by Hydrolysis.
Fe3+ + H2O ) Fe(OH)2+ + H+
(9)
Ind. Eng. Chem. Res., Vol. 43, No. 18, 2004 5967
Hydrolysis constant Kh(1) ) 6.81 × 10-3, so pH ) 2.17 + log [Fe(OH)2+] - log [Fe3+].
Fe(OH)2+ + H2O ) Fe(OH)2+ + H+
(10)
Hydrolysis constant Kh(2) ) 1.01 × 10-5, so pH ) 5.00 + log [Fe(OH)2+] - log [Fe(OH)2+].
Fe(OH)2+ + H2O ) Fe(OH)3 + H+
(11)
Hydrolysis constant Kh(3) ) 5.58 × 103, so pH ) -3.75 - log [Fe(OH)2+].
[Fe3+]t ) [Fe3+] + [Fe(OH)2+] + [Fe(OH)2+] ) 103.42-3pH + 101.25-2pH + 10-3.75-pH (12) According to the above equilibria, the curves of log [Me]t-pH on ferrum and aluminum hydrolysis can be plotted (Figure 2). Figure 2 shows that if 2.5 e pH e 5.5, ferrum and aluminum in solution will automatically hydrolyze to [Al]t ) 10-27 mol L-1 and [Fe]t ) 0.000 26 mol L-1. Thus, ferrum and aluminum can be simultaneously removed, and the [Mn]t in solution is not affected. Experiments, however, show that the removal of ferrum and aluminum by MnCO3 should be conducted under intense stirring conditions.6 3.3. Removal of Calcium by H3PO4. When the equilibrium of Ca3(PO4)2 and solution is achieved, the equilibria of the following reactions are simultaneously achieved:
H3PO4 ) H+ + H2PO4-
H2PO4- ) H+ + HPO42-
(14)
Ka(2) ) 6.23 × 10-8 pH ) 7.21 - log [H2PO4-] + log [HPO42-] HPO42- ) H+ + PO43-
(15)
Ka(3) ) 2.2 × 10-13
[PO43-]t ) [H3PO4] + [H2PO4-] + [HPO42-] + [PO43-] ) [PO43-](1 + 1012.66-pH + 1019.87-2pH + 1021.99-3pH) (16) Ca3(PO4)2 ) 3Ca2+ + 2PO43-
x{
[Mg2+] )
x{ 3
}
1.97 × 10-64(1 + 1012.66-pH + 1019.87-2pH + 1021.99-3pH)2 [PO43-]t2
(18)
According to the above equilibria, the curves of log [Me]t-pH on purifying calcium and magnesium by H3PO4 are plotted (Figure 3). Figure 3 indicates that calcium and magnesium in solution can be removed by H3PO4 to [Ca] ) 0.000 15 mol L-1 and [Mg] ) 10-14 mol L-1 if pH g 5.5, while [Mn]t in solution is not affected.
It is feasible in thermodynamics for pyrolusite to absorb NOx. There is no need for heating during absorption. It is theoretically available to purify ferrum and aluminum by MnCO3 with a neutralization hydrolysis method, and to remove calcium and magnesium by H3PO4, when [PO43-]t ) 0.01 mol L-1 only. The process is conducted at a lower cost, and the quality of the product is ensured. Acknowledgment The authors are grateful to Professor O ¨ rebro, Sweden, for his/her supportive comments that have helped us a great deal. The authors acknowledge financial support provided by Education Bureau of Sichuan Province, Sichuan, China. The authors also thank Dr. Chen Yong for his language assistance.
pH ) 12.66 - log [HPO42-] + log [PO43-]
}
6.22 × 10-34(1 + 1012.66-pH + 1019.87-2pH + 1021.99-3pH)2 [PO43-]t2
Mg3(PO4)2 ) 3Mg2+ + 2PO43-
4. Conclusion
pH ) 2.12 - log [H3PO4] + log [H2PO4-]
3
3.4. Removal of Magnesium by H3PO4. Like removal of calcium by H3PO4:
(13)
Ka(1) ) 7.52 × 10-3
[Ca2+] )
Figure 3. Purification curves of Ca and Mg by H3PO4 ([PO43-]t ) 0.01 mol L-1 and 298 K).
(17)
Supporting Information Available: Data of Figures 1-3. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Zhu, T. X. Explore the Controlling Methods of Nitric Acid Tail Gas. J. Appl. Chem. Ind. (China) 1997, 26 (3), 8-10.
5968 Ind. Eng. Chem. Res., Vol. 43, No. 18, 2004 (2) Jiang, Z. H. Raise Nitric Acid Productivity and Control NOx Effluents with Direct O2 Injection Method. J. Shanghai Chem. Ind. (China) 1999, 24 (6), 23-25. (3) Latimer, W. M. The Oxidation States of the Elements and Their Potentials in Aqueous Solution; Prentice Hall: Englewood Cliffs, New Jersey, 1952. (4) Liang, Y. J.; Che, Y. C. Manual of abio-thermodynamics (China); Northeastern University Press: Shenyang, China, 1993. (5) Teaching & Research Section of Physical Chemistry in Central South Institute of Technology. Physical Chemistry (China); Central South Institute of Technology: Changsha, China, 1979.
(6) Zhou, H. Z.; Liang, H. A New Process Study on Comprehensive Utilization of NOx from Nitric Acid Plant. Wuhan Univ. J. Nat. Sci. 2003, 3B (8), 1001-1006.
Received for review October 27, 2003 Revised manuscript received May 8, 2004 Accepted July 19, 2004 IE034212R
