Scientific Journal of Frontier Chemical Development June 2013, Volume 3, Issue 2, PP.30-35
Mass Transfer and Reaction Kinetics of PH3 Absorption with Pd(Ⅱ)-Cu(Ⅱ) in Double-Stirred Cell Guangfei Qu, Yixing Ma, Junyan Li, Yilu Lin, and Ping Ning* Faculty of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming, 650500, China #Email: email@example.com
Abstract The solution of Pd(Ⅱ)-Cu(Ⅱ) was used as a self-screened catalyst of liquid phase catalytic oxidation to remove phosphine. Mass transfer and reaction kinetics were studied in a double-stirred cell. The results indicated that the system was an interfacial instantaneous reaction. Gas-side and liquid-side mass transfer coefficient were 3.88×10-7 mol•(m2•Pa•s)-1 and 6.74×10-7 mol•(m2•Pa•s)-1 (303.15 K) respectively. The rate constants of the first-order reaction were over the order of 105 under the experiment temperatures. Ha number was 552.56, and practical enhancement factor was 593.63. Keywords: Posphine; Catalytic Oxidation; Kinetics
1 INTRODUCTION Phosphine (PH3) is a colourless, highly toxic, cancerogenic gas, with characteristic of stench and highly reactive activity, whose emission will cause air pollution and be harmful to environment and human health. At present, there are some removal methods of low concentration PH3, including adsorption and chemi-adsorption mostly. Impregnated activated carbon catalytic oxidation method [1-4] and the temperature and pressure swing adsorption method  are typical adsorption methods. The former has several disadvantages that activated carbon consumption is large and the reuse of the catalyzer is difficult. While the disadvantages of the temperature and pressure swing adsorption are that it needs complex process, huge investment and energy cost. Chemical absorption is mainly based on the methods of chemical oxidation absorption, which utilizes sodium hypophosphite, potassium permanganate sulfuric acid and hydrogen peroxide [6, 7] reacting with PH3 for removal. However, chemical oxidate adsorption method has some disadvantages, for instance, consumption of huge oxidants, difficulty in operation, and the fluctuating efficiency evident with the change of oxidant concentration. In our perliminary study [8, 9], one Pd (Ⅱ)-Cu(Ⅱ) catalyst, qualified with a good effect for removing PH3, has been selected from transition metal ions. The experiment results showed that the catalyst can remove PH3 from yellow phosphorus tail gas at atmospheric pressure, in low temperature and in microaerobic conditions. It was also demonstrated that the catalyst only needs simple equipments and its consumption is low. What's more, the catalyst holds good quality characters, such as strong selectivity, high purification efficiency, longer stability time, lower reaction temperature, without additional heating equipment, lower energy consumption and separateness of reaction products. While it still lacks the kinetics of mass transfer and reaction for this method to remove PH3, resulting in the limited role in enlarging pilot-plant test. Our thesis attempts to make a study on mass transfer and reaction kinetics of PH3 absorption in double-stirred cell, including regional of kinetics, gas-side and liquid-side mass transfer, reaction rate constant, characteristic number of macrokinetics, etc, in order to provide basic kinetics datas for application of industrial. * Corresponding author: Ping Ning, Professor, Doctoral Tutor, E-mail: firstname.lastname@example.org This research was funded by the National Natural Science Foundation of China (51008147) and the Yunnan Natural Science Foundation (2009ZC026M) - 30 www.sjfcd.org
2 EXPERIMENT EQUIPMENTS AND PROCESS A double-stirred cell was used in the experiments (Fig.1). Gas phase and liquid phase were stirred independently by tow stirrers. PH3, N2 and O2 which came from steel cylinders were depressurized by pressure-relief valves, and then measured by passing through a glass rotameter. After that, gases were mixed in a mixer then entered into the double-stirred cell and reacted. The samples were analysed after mixed. Gases after reaction were also sampled and analysed then discharged after the absorption of KMnO4 solution finally. Continuous vapor process and intermittent liquid process were involved in the investigation. Beside, U-tube manometer and super thermostat water bath were used to measure the system pressure and to control the reactor temperature, respectively.
7 12 4 5
FIG. 1 EXPERIMENTAL APPARATUS 1,2,3.PH3，N2，O2 cylinder；4.gas control valve；5.flow meter；6.buffer；7.three-way cock；8.buncher；9.magnetic force gearing； 10.stirrer；11.water jacket inlet；12.water jacket outlet；13.KMnO4
3 REGIONAL OF MASS TRANSFER AND REACTION KINETICS OF PH3 ABSORPTION Regional kinetics should be confirmed preferentially when a transfer and reaction system was studied. Based on simple classification, the PH3 absorption reaction belongs to mass transfer control type. More stringent classification could be measured according to regional of kinetics. On the basis of membrane model, factors which effect regional of kinetics are given in Table 1. In the research, reaction of regional kinetics can be measured by analyzing various variables in the table. Experimental investigation (Table 2) of influencing factors was designed. The basis conditions of experiment were as follows: T 303.15 K , Pin 68.09 Pa , C pd ( ) 5.0 10
1 mol L , C pd ( ) : CCu ( ) 1 : 10, VL 100ml , nG 200r / min, n L 150r / min
TABLE 1 THE FACTORS INFLUENCING THE ABSORPTION REACTION
concentration of reactant B in bulk of liquid intensity of pressure of A in bulk of gas interfacial areas liquid volume
A + + + －
liquid-side mass-transfer coefficient
Zone of mass transfer and reaction kinetics B C D E F G + + + － ？ ？ + + + + ？ ？ + + + + + + + + + － － － －
H + + + －
second-order rate constant + + + + ? ? + － G Note: regional of dynamic A—instantaneous reaction; B—interface instantaneous reaction; C—rapid reaction; D—pseudo-m order rapid reaction; E—medium speed reaction; F—pseudo-n order medium speed reaction; G—slow reaction; H—extremely slow reaction of liquid phase; "+" means being effected by the factor; "-" means not being effected by the factor; "?" means being effected by the factor probably with the same rate equation. k
It was demonstrated in Table 2 that the volume variation of absorption solution had no effects on absorption rates of gases. Therefore, E~H can be excluded based on Table 1. By changing the stirring speed of liquid phase, it can be discovered that liquid-side mass transfer coefficients didn't affect the reaction rate k1. So A and C region could be ruled out. Depending on the earlier work, in the process of Pd(Ⅱ)-Cu(Ⅱ) catalytic oxidizing PH3, Pd(Ⅱ) reacted with PH3 to remove PH3, and the reaction between Cu(Ⅱ) and O2 achieved the regeneration of Pd(Ⅱ). It can be seen - 31 www.sjfcd.org
from Fig. 2, when the catalysts proportion of CPd(Ⅱ) and CCu(Ⅱ) were firmly maintained as 1:10, and the concentration of Pd(Ⅱ) was improved from 1.0×10-3 mol/L to 2.0×10-2 mol/L, the absorption of PH3 ratio had no remarkable change with the different concentration catalysts of PH3. That is, the absorption of PH3 with catalysts is zero-order reaction. It can be found from the results that the concentration of liquid phase has no effect on reaction ratio. Thus the D region could be excluded as well. From all above, the reaction that PH3 was absorbed by Pd(Ⅱ)-Cu(Ⅱ) solution belongs to interface instantaneous reaction. TABLE 2 THE FCATORS INFUENCES THE ABSORPTION OF PH3 BY PD(Π)-CU(Π)
Date of parameters Absorbing rate/mol．(m2．s)-1
nL r min1
FIG. 2 RELATIONSHIP OF CONCENTRATION OF CATALYZER
FIG. 3 EFFECT OF CONCENTRATION OF O2 ON ABSORPTION RATE
4 EXPERIMENTAL FOR KINETIC STUDY 4.1 Effect of Concentration of O2 on Absorption Rate From previous experiment , the oxygen content of mixture gases will affect the stabilization of absorption seriously. However, it was inferred that the oxygen content of mix gases would affect the regeneration of catalysts rather than the process of PH3 removed by catalysts. To confirm the inference, the influence on absorption ratio was investigated because of oxygen content. The experiment results were demonstrated in the Fig. 3. As it can be seen from the graph that the absorption of PH3 ratio didn’t change sharply when the φO2 of mixture ranged from 0% to 20%. It was illustrated that oxygen content has little effect on absorption ratio, which is analogous to the research of Sue-min Chang . While, in order to ensure the stability of catalysts, experiment employed the φO2 of mixture as 20%.
4.2 Effect of Catalyst Proportion on Reaction Rate In experiment, the mole concentration of Pd in the catalysts at 5×10-3 mol/L was kept while changing the concentration of Cu to study the influence of different catalyst proportion on reaction rate. From Fig. 4, it can be observed that the changes of catalyst proportion had no effect on the initial reaction rate, while the rate decreased uninterruptedly with the reaction. However, the time of stable reaction rate of catalysts can exceed 60 min when the proportion of CPd(Ⅱ) and CCu(Ⅱ) was 1:8. When the proportion of CPd(Ⅱ) and CCu(Ⅱ) was 1:10, catalysts were more stable although the strengthening effect is weak. Therefore, it can be considered that the catalytic system can gain preferable stability when the proportion of CPd(Ⅱ) and CCu(Ⅱ) was 1:10. In that way, the reliability of kinetics data could be guaranteed. - 32 www.sjfcd.org
FIG. 4 THE ABSORPTION RATE CURVE AT DIFFEREN PROPORTION
FIG. 5 EFFECT OF CONCENTRATION OF PH3 ON ABSORPTION RATE
4.3 Confirmation of Pseudo-first Order Reaction and Second Order Reaction The detail determination method of gas-side mass transfer coefficient and liquid-side mass transfer coefficient can be seen from the reference . From the test, the gas-side mass transfer coefficient and liquid-side mass transfer coefficient were 3.88×10-7 mol•(m2•Pa•s)-1 (303.15K) and 6.74×10-7 mol•(m2•Pa•s)-1 (303.15K) respectively. In addition, via the phase equilibrium relation of interface and mass transfer formula (1) and (2), a formula (3) can be achieved. Ci Hpi NpH 3 kG ( pG pi ) Ci H（pG NpH 3 / kG）
(1) (2) (3)
For double-stirred cell, the perfect mixing coefficient was closed to 1 when the stirring speeds of gas phase were 120~800 r•min-1 and 100~187 r•min-1 for liquid phase. In the investigation, the stirring speed of gas phase and liquid phase were 300 r•min-1 and 150 r•min-1 respectively. So the concentration of outlet PH3 can be regarded as the concentration of main reaction. The catalysts concentration was fixed as 5×10-3 mol/L, and the input concentration of PH3 changed and then the output concentration of PH3 was measured, thus relevant absorption rate can be gained by means of material balance calculation, the interface concentration of PH3 could be calculated by formula (3). Experiments above were repeated with different temperatures and results were represented in Fig. 5. From the picture, the interface concentration of PH3 varied directly as reaction rate in range of temperature. That is, for the PH3, absorption reaction was first order reaction. The slope of the line in the Fig. 5 was the rate constant of the pseudo-first order reaction. According to the Arrhenius formula (4)，by taking different temperatures and corresponding reaction constants to form equation, the activation energy Ea=14.35 kJ/mol and the pre-exponential factor k0=1.99×108 can be calculated. The activation energy of activated carbon catalytic oxidation of PH3 is 63.1 kJ/mol , therefore, the energy of gas-liquid phase absorption process was far lower than that of gas-solid absorption process. k ko e
Ea / RT
4.4 Characteristic Number Study on Macrokinetics 1) Study on Ha Number Ha number is the basis characteristic number of gas-liquid phase reaction macrokinetics, whose computational formula is as formula (5)  Ha
DA k1 kL 2
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According to data of experiment stated previously, Ha was 552.56 when T was 303.15 K. For Ha>3, the intrinsic chemical reaction rate was so fast that the mass transfer rate was promoted by up to 552.56 times. 2) Study on Chemical Enhancement Factors Practical enhancement factor E of mass transfer efficiency accompanied with chemical reaction can be expressed as formula (6). Combined with the experiment data, a certain practical enhancement factor derived from formula (6) was 593.63. Moreover, different enhancement factors could be acquired according to the application of different gas-liquid reaction mass transfer models. For membrane model, the relation between E and Ha is as (7). The enhancement factor in theory was 536.37 combining formula (6).
N PH 3 k L C A*
Ha tanh Ha
The deviation between theory enhancement factor and practical enhancement was 9.65% based on membrane model. It's 2.88% between theory enhancement and Ha number.
5 CONCLUSIONS In conclusion, the results of the experiment indicated that the gas-liquid reaction, PH3 absorbed by Pd(Ⅱ)-Cu(Ⅱ) solution, belongs to interfacial reaction. The reaction can be confirmed as zero order reaction for Pd and pseudo-first order reaction for absorption of PH3. The rate constant for the first-order reaction was over the order of 105. Characteristic numbers of macrokinetics were confirmed as well. These base data will be very useful for application of industrial. In follow-up work, mechanism study and analysis will be studied in details. It’s important to gain better result of PH3 removal.
SYMBOL DESCRIPTION T — thermodynamic temperature, K P — partial pressure of solute A in gas phase, Pa CPD(II) — concentration of palladium(Ⅱ) ion , mol•L-1 Ccu(II) — concentration of ion, mol•L-1 VL — volume of holding of liquid phase, mL nG — stirring speed of gas phase, r•min-1 nL — stirring speed of liquid phase, r•min-1 CPD(II) — concentration of palladium(Ⅱ) ion , mol•L-1 Ccu(II) — concentration of ion, mol•L-1 A — interfacial area, m2 kL — liquid phase mass transfer coefficient, m•s-1 kG — gas phase mass transfer coefficient, mol•m-2•Pa-1•s-1 φ O 2 — volume fraction of O2 in gas phase Ci — concentration of interfacial PH3, mol•L-1 H — solubility coefficient, mol•m-3•Pa-1 k0 — pre-exponential factor N PH 3 — absorption rate of PH3, mol•m-2•s-1 Ha — H a number Ea — intrinsic chemical reaction activation energy, kJ•mol-1 D — diffusion coefficient of PH3 in the liquid phase, m-2•s-1 kL — liquid phase mass transfer coefficient, m•s-1 - 34 www.sjfcd.org
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engineering from Kunming University of Science interests:
Ma, Yunnan, 1988.12, Doctoral student. E-mail:
Li, Shanxi, 1974, Doctoral student. E-mail:
pollution control of solid waste, cyclic economy, cleaner production. E-mail: email@example.com
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Guangfei Qu, Yixing Ma, Junyan Li, Yilu Lin, Ping Ning