Alicja ZAWADZKA1, Grzegorz WIELGOSINSKI1
INVESTIGATION OF CATIONS INFLUENCE ON SPEED OF ABSORPTION WITH CHEMICAL REACTION. 2. ABSORPTION OF NO2 WPŁYW KATIONU NA SZYBKOŚĆ ABSORPCJI Z REAKCJĄ CHEMICZNĄ. 2. ABSORPCJA NO2 Summary: The effect of various absorbents, i.e. aqueous solutions of sodium hydroxide, potassium hydroxide, ammonium hydroxide, lithium hydroxide, sodium bicarbonate, potassium bicarbonate, sodium hydrocarbonate and ammonium hydrocarbonate on the rate of absorption of NO2 was examined. Investigations were carried out in the same apparatus as earlier. The effect of solution concentration and impeller speed on the rate of absorption was examined and next, according to equations corresponding to the pseudo-first order reaction, the chemical reaction rate constants for each absorbent were determined and finally the chemical reaction rate constants obtained for each absorbent were compared. Key words: nitrogen dioxide, absorption with chemical reaction, reaction kinetics
Introduction Many methods of neutralisation of flue gases containing nitrogen dioxide have been described in the literature. Most of these methods refer, however, to the denitrifcation of exhaust (hot) gases. There are also other sources of NOx emission to the atmosphere, first of all in chemical industry where the emission of nitrogen oxides with a stream of gases at temperature close to ambient is small. The deNOx methods used in the power industry are not suitable for these gases. Thus, in order to reduce the emission, most often the absorption in carbonate or bicarbonate solutions, is used. Our aim was to compare the rate of NO2 absorption in water solutions of sodium hydroxide, potassium hydroxide, lithium hydroxide, ammonium hydroxide sodium bicarbonate, potassium bicarbonate sodium hydrocarbonate and ammonium hydrocarbonate from the point of view of the effect of cations on process kinetics. In the available literature on denitrifcation processes no such comparison was made.
Course of reaction Nitric oxides formed in the processes of nitration and denitrification are mainly nitric oxide NO and nitrogen dioxide NO2. At ambient temperature, in excess air, the reaction of nitric oxide oxidation to nitrogen dioxide takes place very fast, according to the reaction equation: 2 NO + O2 → 2 NO2 (1) Nitrogen dioxide reveals dimerisation ability at temperatures below 100ºC, according to the reaction equation: 2 NO2 → N 2O4 (2) so, at the temperature of around 20ºC, about 85% of nitrogen dioxide occurs in the form of dimer N2O4. The absorption of nitric oxides – in this case dimer N2O4 in water alkaline solutions, can be treated as gas absorption A that is subjected to chemical reaction in the liquid phase. Hence, the reaction taking place in the system is, e.g. for water NaOH solutions: k
N 2O4 + 2 NaOH → NaNO2 + NaNO3 + H 2 O
1
(3)
Faculty of Process and Environmental Engineering Technical University of Lodz, Wólczańska 175, 90-924 Lodz, Poland, Phone: +48 42 6313795, fax: +48 42 6368133 e-mail:
[email protected]
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The absorption with a simple second-order chemical reaction carried out according to Whitman’s film theory was used in the present study.
Experimental The kinetics of NO2/N2O4 absorption in aqueous carbonate and bicarbonate solutions was investigated in a mixer with a flat contact surface, which was a copy of Danckwerts’ apparatus [1]. Schematic of the apparatus is shown in Figure 1. The absorption rate was studied in aqueous solutions of sodium hydroxide (NaOH), potassium hydroxide (KOH), lithium hydroxide (LiOH), ammonium hydroxide (NH4OH), sodium bicarbonate (Na2CO3), potassium bicarbonate (K2CO3), sodium hydrocarbonate (NaHCO3) and ammonium hydrocarbonate (NH4HCO3). The aim of these investigations was to compare the process rates and to determine the effect of cation type on the process kinetics. First, the effect of such factors as the impeller rotations frequency and concentration of the aqueous hydroxide solution on the rate of absorption with a chemical reaction was defined. The objective of these investigations was to find if in the process conditions the reaction of NO2/ N2O4 with a hydroxides or carbonates took place in the pseudo-first-order region. As a result, it was found that the rotations frequency in the liquid phase had no effect on the process rate, while the impact of hydroxide concentration was significant. The above conclusions allowed us to hypothesise that the reaction of NO2/N2O4 with respective hydroxides took place most probably in the pseudo-first-order reaction regime which enabled a relatively simple determination of the reaction rate constant. Making the basic mass balance for a reactor it can be written that a change in the hydroxide mass in the reactor is equal to the stream of absorbed nitrogen dioxide: dc V ⋅ B = RA ⋅ F (3) dτ Boundary conditions for this equation are as follows:
τ=0
cB = cB 0
(4)
τ=t cB = cB Under the assumption that the reaction takes place in the pseudo-first-order region [2], when the rate of absorption can be described by the following equation: R A = c A* ⋅ k ⋅ c B ⋅ D A (5) after separation of variables, integration within the limits according to boundary conditions (4) and upon transformation, a relation used to determine the reaction rate constant is obtained:] 2
2 2 ⋅ V c B 0 − c B ⋅ ⋅ (6) c ∗ ⋅ F ∆τ A For such assumptions a series of studies on NO2/N2O4 absorption in water solutions of NaOH, KOH, LiOH, NH4OH, Na2CO3, K2CO3, NaHCO3 and NH4HCO3 were made at process temperature range from 290 to 310 K. For each experiment performed batch-wise, a change of sulphite concentration in the reaction mass in time was measured. The content of nitrites in the reaction mass was analysed by the iodometric method. In this method nitrites react stoichiometrically with potassium permanganate. Determinations are made by adding excess standard potassium permanganate solution to the alkaline solution containing nitrites, then, it is acidified with sulphuric acid and after ten minutes potassium iodide is added. The separated iodine is titrated with sodium thiosulphate in the presence of starch as an indicator. On the basis of the investigations the value of reaction rate constant was calculated and the dependence of the reaction rate constant on temperature was determined using the Arrhenius equation in the form: E k = k o ⋅ exp − (18) R ⋅T
1 k= DA
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Then, the values of reaction rate constant and energy of activation were determined by the linear regression method. Table 1. Results of identification of reaction kinetics of nitric oxides with water solutions of hydroxides, carbonates and bicarbonates. Substance
ko
E/R
NaOH KOH LiOH NH4OH Na2CO3 K2CO3 NaHCO3 NH4HCO3
16 45 8,5 52 38 31 44 67
5820 14500 3800 17700 12100 10000 13800 20100
Correlation coefficient r 0.98 0.95 0.91 0.93 0.96 0.94 0.92 0.96
A comparison of rate constants of the tested reactions of NO2/N2O4 with water hydroxide solutions (NaOH, KOH, LiOH and NH4OH) is shown in Fig.1, and with water solutions of carbonates and bicarbonates (Na2CO3, K2CO3, NaHCO3 and NH4HCO3) in Fig. 2. 3
k [m /kmol 0,050 s]
NaO H
0,045 0,040
NH4O H
0,035
LiO H
0,030
KO H
0,025 0,020 0,015 0,010 0,005 0,000 286 287
288 289 290
291 292 293 294
295 296 297
298 299
T [K]
Fig.1. Comparison of reaction rate constants NO2/N2O4 with water solutions of NaOH, KOH, LiOH and NH4OH. 3
k [m /kmol 0,40 s] 0,35
K2CO 3 Na2C O 3
0,30
NH4HC O 3
0,25
NaHCO 3
0,20 0,15 0,10 0,05 0,00 287
288
289
290
291
292
293
294
295
296
T [K]
Fig. 2. Comparison of reaction rate constants NO2/N2O4 with water solutions of Na2CO3, K2CO3, NaHCO3 and NH4HCO3.
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Comparison of absorbent efficiency It was found that in the group of hydroxide solutions the rate constant of the chemical reaction between NO2/N2O4 and KOH is the highest, while in the group of carbonates and bicarbonates the rate constants of chemical reaction between N2O4 a NH4HCO3. Analysis of the results allows us to make an additional observation – if the reaction rate constants for hydroxide solutions are put in a decreasing sequence, we will obtain: k(KOH) > k(NaOH) > k(LiOH) > k(NH4OH) A similar series can be formed for carbonate solutions: k(NH4HCO3) > k(NaHCO3) > k(Na2CO3) > k(Na2CO3) This means that the effect of a cation on the reaction rate is significant and analogous for both hydroxide and carbonate solutions.
Conclusions The process of nitrogen dioxide (NO2/N2O4) absorption in the aqueous solution of selected hydroxides was studied. The investigations were carried out in a tank with flat contact surface which was a copy of Danckwerts apparatus. It was found that all chemical reactions took place in the pseudofirst-order region and that the reaction rate constants and their dependence on temperature were determined for all reactions. Then, the values of determined reaction rate constants were compared.
References 1. 2.
Danckwerts P. V., Gilham A. J.: Trans. Instn Chem. Eng. 1966, 44, 42. Charpentier J. R.: Trans. Instn Chem. Eng. 1982, 60, 131.
WPŁYW KATIONU NA SZYBKOŚĆ ABSORPCJI Z REAKCJĄ CHEMICZNĄ. 2. ABSORPCJA NO2 Streszczenie Badano wpływ rodzaju absorbenta na szybkość absorpcji z reakcją chemiczną dwutlenku siarki w wodnych roztworach wodorotlenku sodowego, wodorotlenku potasowego, wodorotlenku amonowego, wodorotlenku litowego, węglanu sodowego, węglanu potasowego, wodorowęglanu sodowego i wodorowęglanu amonowego. Badania prowadzono w szklanym absorberze z płaską powierzchnią kontaktu stosowanym w poprzednich badaniach. Określono wpływ częstości obrotowej mieszadła i stężenia absorbenta w fazie ciekłej na szybkość absorpcji z reakcja chemiczną a następnie korzystając z rozwiązań równań dyfuzji z reakcją chemiczną dla przypadku reakcji pseudo pierwszego rzędu wyznaczono wartość stałej szybkości reakcji oraz jej zależność od temperatury. na koniec porównano wartości stałej szybkości reakcji dla wszystkich badanych absorbentów. Słowa kluczowe: dwutlenek azotu, absorpcja z reakcją chemiczną, kinetyka reakcji
Author’s version of the manuscript published in: Ecological Chemistry and Engineering, 2004, 11, (1), 55-60
