- Email: [email protected]
Experimental investigation and simulation of gas–liquid–liquid reactive extraction process for the production of hydrogen peroxide
Chemical Engineering Science 60 (2005) 6298 – 6306 www.elsevier.com/locate/ces
Experimental investigation and simulation of gas–liquid–liquid reactive extraction process for the production of hydrogen peroxide Shuxiang Lüa, b , Zhentao Mia,∗ , Li Wanga , Yaquan Wanga , Zehua Zhuc , Songbao Fuc a Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University,
Tianjin 300072, PR China b School of Chemical Engineering, Hebei University of Technology, Tianjin 300130, PR China cYingshan Petrochemical Plant, SINOPEC, Yueyang 414003, PR China
Received 28 October 2004; received in revised form 18 April 2005; accepted 18 April 2005 Available online 8 June 2005
Abstract The gas–liquid–liquid reactive extraction system for the production of hydrogen peroxide via anthraquinone route was investigated. The oxidation of the hydrogenated anthraquinone working solution by oxygen and the extraction of hydrogen peroxide from the working solution with deionized water were carried out simultaneously in a sieve plate column of 50 mm in diameter. The effects of the superficial velocity of oxygen on the conversion of 2-ethylanthrahydroquinone and the extraction efficiency of hydrogen peroxide were investigated, separately. The results showed that the oxidation and the extraction do not hamper each other, on the contrary, the presence of gas in the column can promote the transfer of hydrogen peroxide from the organic phase to the aqueous phase, therefore, the conversion of 2-ethylanthrahydroquinone and the extraction efficiency of hydrogen peroxide increased with the increase of gas superficial velocity. In addition, a mathematical model for the simulation of the gas–liquid–liquid reactive extraction process was developed. The predicted values were compared with the experimental data at different conditions and the agreement was found to be quite satisfactory for the production of hydrogen peroxide in a sieve plate column. 䉷 2005 Elsevier Ltd. All rights reserved. Keywords: Extraction; Simulation; Multiphase flow; Separations; Hydrogen peroxide; Sieve plate column
1. Introduction Hydrogen peroxide is widely used in almost all industrial areas, especially in the chemical industry and environmental protection (Goor and Kunkel, 1988). Since the only degradation product is water after use, by the criteria of green chemistry, hydrogen peroxide has already made
a remarkable contribution to a cleaner chemical industry (Sanderson, 2000). OH
O +
H2 OH
O
EAQH2
EAQ OH
O +
∗ Corresponding author. Key Laboratory for Green Chemical Tech-
nology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China. Tel.: +86 22 27408891; fax: +86 22 27402604. E-mail addresses: [email protected] (S. Lü), [email protected] (Z. Mi). 0009-2509/$ - see front matter 䉷 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2005.04.044
OH
(1)
+
O2 O
H2O2
(2)
By far, hydrogen peroxide has mainly been produced by a method named anthraquinone (AQ) route in which anthraquinone derivatives, usually 2-ethylanthraquinone
S. Lü et al. / Chemical Engineering Science 60 (2005) 6298 – 6306
(EAQ), dissolved in organic solvents (all together being called anthraquinone working solution) is first hydrogenated catalytically to the corresponding 2-ethylanthrahydroquinone (EAQH2 , Eq. (1)) in a slurry or fixed-bed reactor; then EAQH2 is oxidized with oxygen or air to form hydrogen peroxide and the original EAQ (Eq. (2)) in a gas–liquid reactor; and finally the hydrogen peroxide formed is extracted with water in a sieve plate extraction column. In recent years, the development of reactive separation processes has received considerable attention (Samant and Ng, 1998a,b, 1999; Sheikh et al., 1998, Krishna, 2002). The combination of reaction and separation in a single unit can improve the yield of a target product, leads to simultaneous separation of byproducts, bypass the chemical reaction equilibrium limitation and reduction of investment. In a general reactive extraction process, there are only two distinct phases, i.e., aqueous and organic phases and reactive species are present in different phases (Bart, 2000). Minotti et al. (1998) have developed a general geometric method for the design of simultaneous reaction and liquid–liquid twophase extraction on the assumption that the liquid phases leaving the extractor are in equilibrium. In this work, the oxidation reaction of EAQH2 with oxygen and the extraction of hydrogen peroxide from the anthraquinone working solution were carried out simultaneously in a sieve plate column, and the effects of the oxygen superficial velocity on the conversion of EAQH2 and the extraction efficiency of hydrogen peroxide were studied. This reactive extraction system contains three phases: gaseous (oxygen), organic (anthraquinone working solution) and aqueous phases. The gaseous phase acts as one of the reactants as well as agitating agent. Since the method of Minotti et al. (1998) cannot be used for the design and simulation of the present gas–liquid–liquid three-phase reactive extraction system, a mathematical model for this system was developed, with the effects of the reactive gas phase on both the reaction rate and the extraction efficiency being taken into account. Finally, this model was tested by the experimental data obtained in the sieve plate column.
2. Experimental section 2.1. Material and apparatus The anthraquinone working solution was a mixture of 2-ethylanthraquinone, trioctyl phosphate and aromatics of C9 –C10 , and the volume ratio of C9 –C10 aromatics to trioctyl phosphate is 3. Hydrogenation of anthraquinone to anthrahydroquinone in the working solution was carried out in a trickle bed reactor at 60 ◦ C and 0.3 MPa in the presence of an industrial Pd/Al2 O3 catalyst. Pure oxygen (99.99%) was employed. The experimental set-up is schematically shown in Fig. 1. The cylindrical glass column with diameter of 50 mm was fitted with seven sieve plates at a plate spacing of 210 mm.
6299
14
7
15
8 5
13 16
9 6
4
2 1
3
10
11 12
Fig. 1. Schematic diagram of the experimental set-up (1) water reservoir, (2) double-piston meter pump, (3) hydrogenated working solution reservoir, (4) membrane meter pump, (5) reactive extraction column, (6) dispersed phase distributor, (7) continuous phase distributor, (8) adjustable overflow tube, (9) hydrogen peroxide solution reservoir, (10) rotameter, (11) oxygen cylinder, (12) oxidized working solution reservoir, (13) gas–liquid separator, (14–16) valves.
Sieve plates were made of stainless steel with holes of 2.5 mm in diameter, corresponding to free areas of 24%. The enlarged parts at the both top and bottom of the column were 400 mm long and its diameter were 100 mm, which served as liquid–liquid separators. Oxygen and the hydrogenated anthraquinone working solution were continuously introduced into the column through a gas–liquid distributor (6) packed with 3.5 × 3.5 mm rings. Deionized water was supplied to the top of the column through a perforated plate-type distributor (7) with holes of 2 mm in diameter. The unreacted oxygen comes out through valves (14,15) at the top of the column and gas–liquid separator (13). Three sampling ports and thermocouples were situated just above the 2nd, 4th and 6th plates. Oxygen, the hydrogenated anthraquinone working solution and deionized water were used as gaseous, dispersed and continuous phases, respectively, and the flow rates of them were measured with a rotameter (10), a membrane meter pump (4) and a piston meter pump (1), respectively. Since the decomposition of hydrogen peroxide occurs easily in the presence of metal ions, the whole apparatus was passivated by nitric acid. The column temperature was kept at 50 ± 1 ◦ C through circulating water in the jacket of the column. The flow rate ranges in terms of superficial velocities were: 1.3 × 10−3 .3.0 × 10−3 m/s for the gaseous phase (Ug ), 1.2×10−3 .2.2×10−3 m/s for the dispersed phase (Ud ) and 2.4 × 10−5 .3.4 × 10−5 m/s for the continuous phase (Uc ). The physical properties of the employed system are given in Table 1.
6300
S. Lü et al. / Chemical Engineering Science 60 (2005) 6298 – 6306
Table 1 The physical properties of the experimental system (50 ◦ C) Parameter
d c g d c g
Value
Unit
Description
905.3 988.1 1.21 0.89 × 10−3 0.549 × 10−3 2.13 × 10−5 6.77 × 10−2 3.4 × 10−2
(kg/m3 )
Dispersed phase density (anthraquinone working solution) Continuous phase density (deionized water) Gaseous phase density (oxygen) Dispersed phase viscosity Continuous phase viscosity Gaseous phase viscosity Surface tension of continuous phase Interfacial tension between dispersed and continuous phases
(kg/m3 ) (kg/m3 ) (Pa s) (Pa s) (Pa s) (N/m) (N/m)
2.2. Procedure and analysis method In each run, the flow rates of oxygen, deionized water and the hydrogenated anthraquinone working solution were adjusted to the desired values and fed to the column. In theory, the material balance at steady-state conditions for the entire column is i o Vd (CEAQH − CEAQH ) = Vd Coo + Vc Cwo , 2 2
(3)
where Vd and Vc are the volumetric flow rates of the disi persed and continuous phases, respectively, and CEAQH and 2 o CEAQH2 are the concentration of the EAQH2 in the feed stream and the outlet stream of the dispersed phase, respectively. Coo and Cwo are the concentration of hydrogen peroxide in the outlet streams of dispersed and continuous phases, respectively. When the deviation between the value of left and right of Eq. (3) did not exceed ±5%, it is considered as the steady-state conditions were established, samples of dispersed phase and continuous phase were taken from sampling ports. Quantitative analysis of the hydrogen peroxide in aqueous phase samples was performed by titration. Each organic phase sample was extracted with deionized water for four times, and the four aqueous solutions obtained were combined together and also titrated as the aqueous samples.
3. Results and discussion 3.1. Equilibrium data The equilibrium data of hydrogen peroxide dissolving between water and anthraquinone working solution at a constant temperature (50 ◦ C) was determined with the method described in literature (Lo et al., 1983), and is shown in Fig. 2. According to these results, at the conditions the concentration of hydrogen peroxide within the range of 0.0–162 mol/m3 in anthraquinone working solution (Co ) and 0–1.2 × 104 mol/m3 in the water (Cw ), the equilibrium curve of hydrogen peroxide distributing between water and working solution is lineal and the equilibrium constant, K, is 77.4.
Fig. 2. The equilibrium data of H2 O2 dissolving between water and anthraquinone working solution.
3.2. Effects of the gaseous phase superficial velocity on the conversion of EAQH2 and the extraction efficiency of hydrogen peroxide The extraction efficiency of hydrogen peroxide may be expressed as follows: e=
nw , nr
(4)
where nr is the mole of hydrogen peroxide produced by the oxidation of EAQH2 and nw is the mole of hydrogen peroxide extracted into water. The effects of the gaseous phase superficial velocity on the conversion of EAQH2 (XE ) and the extraction efficiency of hydrogen peroxide (e) are shown in Figs. 3 and 4. It is seen that both the conversion of EAQH2 , and the extraction efficiency of hydrogen peroxide increased with the increase of the superficial velocity of gaseous phase. Such behavior is the consequences of an increase in the turbulence then momentum and mass transfer. This may also be caused by an increase in the contact time of oxygen (reactive gas) and dispersed phase (liquid reactive phase) due to their oscillatory motion and/or an increase in the holdups of dispersed and gaseous phases. (Lü et al., 2004a; Sovilj and Knezˇevic,
S. Lü et al. / Chemical Engineering Science 60 (2005) 6298 – 6306
Fig. 3. Effect of the gaseous phase superficial velocity on the conversion of EAQH2 . Ud /Uc = 50.
Fig. 4. Effect of the gaseous phase superficial velocity on the extraction efficiency of H2 O2 . Ud /Uc = 50.
1994). On the other hand, the volumetric mass-transfer coefficient of gas–liquid, kL ai , increases with the increase of the reactive gas flow rate (Danckwerts, 1970). Therefore, the oxidation reaction rate of the EAQH2 and the conversion of EAQH2 are increased. When the superficial velocity of gaseous phase is relatively high, the mean size of dispersed phase droplets within the chambers between plates become smaller, and the population density of dispersed phase droplets become higher than those related to lower superficial velocity of gaseous phase (Lü et al., 2004b). Also both the interdroplet collisions and droplets of dispersed phase break are promoted when the oxygen was introduced to the column and consequently increases surface renewal (Dehkordi, 2001, 2002). These phenomena result in the increase of the interfacial area between dispersed and continuous phases, which in turn en-
Fig. 5. The conversion profile of EAQH2 Ud = 1.7 × 10−3 m/s, Uc = 3.4 × 10−5 m/s.
6301
along the column.
Fig. 6. The extraction efficiency profile of H2 O2 along the column. Ud = 1.7 × 10−3 m/s, Uc = 3.4 × 10−5 m/s.
hances the mass transfer rate. So the extraction rate and the extraction efficiency are enhanced. A similar behavior was also observed in different types of gas-agitated extraction columns (Galkin et al., 1961; Priestley and Ellis, 1978). 3.3. Profiles of the conversion of EAQH2 and the extraction efficiency of hydrogen peroxide To investigate the profiles of the conversion of EAQH2 and the extraction efficiency of hydrogen peroxide along the column, the data of XE and e at different sampling ports were measured. The results are shown in Figs. 5 and 6. An increase in the XE and e were observed with increase of sieve plates. Such behavior is a consequence of an increase in the mean residence time of dispersed phase droplets in the column.
6302
S. Lü et al. / Chemical Engineering Science 60 (2005) 6298 – 6306
E1 y1
E2 y2
xF
Gas
Ei yi
R2 x2
Ri-1 xi-1
2
1
F
E3 y3
R1 x1
Ei+1 EN-1 yi+1 yN-1 i
EN yN N-1
Ri xi
RN-2 xN-2
s y0 N
RN-1 xN-1
RN Gas xN
outlet gaseous phase
outlet raffinate phase
liquid extraction phase (water)
Fig. 7. Gas–liquid–liquid countercurrent reactive extraction cascade.
3.4. Model development According to the stagewise model, the gas–liquid–liquid reactive extractor consists of a set of isothermal stirred tanks. (see Fig. 7). The reactive gas and liquid reactive phase (dispersed phase) flow countercurrently with the extractive phase (continuous phase). The dispersed and continuous phases are immiscible. The reactive extractor may be treated with the following assumptions: (1) Each compartment between two sieve plates is considered as a stage, which represents a perfectly mixed stirred tank with the gas phase as external agitation. (2) Reaction does not occur in the continuous phase, but only occur in the dispersed phase. (3) The two outlet liquid streams from each stage are not in phase equilibrium. The stage extraction efficiency in all stages is the same. (4) The flow rates of gaseous, dispersed and continuous phases are independent of mass-transfer, reaction and solute concentration. (5) Each stage is at known constant temperature T, and pressure P. One stage of the gas–liquid–liquid reactive extractor is shown schematically in Fig. 8. Three phases exist in each stage: the liquid reactive phase, gas reactive phase and liquid extractive phase. The oxygen must dissolve in liquid reactive phase, and oxidation reaction takes place in the liquid reactive phase, the product hydrogen peroxide is then extracted by water from the reactive phase. The liquid reactive phase and the extractive phase are immiscible, while hydrogen peroxide being distributed between the phases. The overall material balance on the solute (hydrogen peroxide) is F x F + Sy 0 + C = E1 y1 + RN xN ,
(5)
where xF and y0 are the concentration (or mole fractions) of the solute in the inlet feed stream and solvent, respectively, and F and S are the corresponding flow rate (volume or mole). y1 and xN represent concentration (or mole fractions) of the solute in the outlet extract and raffinate streams, respectively, and E1 and RN are the corresponding flow rate. C represents overall production rate in the reactive extractor due to the chemical reaction.
liquid reaction phase (hydrogenated anthraquinone working solution)
outlet extractive phase
gaseous reaction phase (oxygen) reactive phase
gaseous phase
extractive phase Fig. 8. Schematic diagram of one stage of gas–liquid–liquid reactive extraction sieve plate column for the production of hydrogen peroxide.
The compositions and flow rates of the liquid phases for the ith stage are related by the material balances: Ri−1 xi−1 + Ei+1 yi+1 + Ci = Ri xi + Ei yi (i = 1, 2, . . . , N ),
(6)
where xi and yi are the concentration (or mole fraction) of solute in the raffinate and extract streams, respectively, and Ri and Ei are the corresponding flow rates leaving ith stage. Each element of Ci , which represents the production rate in the ith stage, is related to the reaction rate, ri , and the holdup of the liquid reactive phases in ith stage, hi , by
Ci = hi ri
(i = 1, 2, . . . , N ).
(7)
Santacesaria et al. (1987, 1999) have investigated the kinetic aspects of the oxidation of hydrogenated anthraquinone working solution in a gas–liquid reactor. They reported that the reaction rate of the oxidation of hydrogenated anthraquinone working solution by atmospheric pressure oxygen was governed by the transport of oxygen through the gas–liquid interface (Hâncu and Beckman, 2000). Kinetics studies of the oxidation of EAQH2 in oxygen–anthraquinone working solution–water three-phase system have shown that the reaction rate can be described by (Lü, 2004) r = kL ai
PO2 , HO2
(8)
where r is the reaction rate, kL ai is gas–liquid mass-transfer coefficient, PO2 is the partial pressure of oxygen, and HO2 is the Henry’s law constant of oxygen. At a constant temperature of T = 50 ◦ C and atmospheric pressure, the value HO2 can be found in the literature (Santacesaria et al., 1987).
S. Lü et al. / Chemical Engineering Science 60 (2005) 6298 – 6306
In a sieve plate column, the gas–liquid mass-transfer coefficient is determined using the equation of Sharma et al. (1967, 1969). kL ai = 0.042f −2.2 (d Ud )0.6 Ug1.2 ,
(9)
where f is the fractional free area of the sieve plate, d is the density of dispersed phase (liquid reactive phase). The holdup of the liquid reactive phase for air–water–anthraquinone working solution three-phase system is influenced by the flow rates of all phases (Lü et al., 2004a). The liquid reactive phase holdup in each stage, hi , is assumed to be the same and is calculated using following equation (Lü et al., 2004a). 0.38 0.52 1/4 1/4 hi = 9.41 Ug c Ud c g g 1/4 −0.58 0.18 d × exp −10.65Uc c g c c 1/2 −0.39 g × Hc c , (10) where c is the density of continuous phase (extractive phase), d and c are the viscosities of dispersed and continuous phases, respectively. is density difference between the continuous and the dispersed phases, g is the gravity acceleration, Hc is plate spacing, and is interfacial tension between dispersed and continuous phases. Through Eqs. (9) and (10), effect of reactive gas phase on the reaction was taken into account in the model. The overall production rate in the reactive extractor is the sum of the production rates covering all of the stages:
C=
N
Ci
(i = 1, 2, . . . , N ).
(11)
i=1
Since composition of the liquid leaving each stage is not in liquid–liquid equilibrium, the relations of them are determined by the stage extraction efficiency as follows: ei =
yi − yi+1 yi∗ − yi+1
yi∗ = Kx i
(i = 1, 2, . . . , N),
(i = 1, 2, . . . , N ),
(12) (13)
where yi∗ is equilibrium concentration of the extract phase leaving the ith stage, ei is the stage extraction efficiency in the ith stage and K is equilibrium constant. Studies of the stage extraction efficiency in gas–liquid– liquid three-phase system have shown that the stage extraction efficiency is influenced by the flow rates of all phases (Lü et al., 2004b). It can be related to the flow rates of dispersed, continuous and gaseous phases by Eq. (14): 0.5 U 0.58 c −3 Hc e = 7.94 × 10 (14) exp 0.14Ug , 0.35 U d0 d
6303
where d0 is sieve hole diameter. Through Eq. (14), effect of reactive gas phase on the extraction efficiency was taken into account in the model. Since the two liquid phases are immiscible, the flow rates of liquid phases are constant in each stage, i.e., F = R1 = R2 = · · · = RN = R, S = E1 = E2 = · · · = EN = E. The number of independent variables can be determined by degrees-of-freedom analysis method. The variables are E, R, Ug , N, e, xi , yi , hi , ri and Ci for i = 1, 2, . . . , N. There are 2(N + 1) concentrations; N holdups of reactive phase; N production rates; N reaction rates. The total number of variables is 2(N + 1) + 3N + 5. The independent equations are material balance equations (Eq. (6)) in N stages, the relations of the production rate and the holdup of reactive phase (Eq. (7)) in N stages, and the relations of xi and yi (Eqs. (12)–(14) are not independent) in N stages. This gives a total of 3N independent equations. Therefore, there are 2(N + 1) + 5 degrees of freedom. Normally, the solvent and the feed flow rate, S and F, the gas phase flow rate, Ug , and the concentrations of the feed stream and solvent, xF , y0 are known or specified. The values of hi and e can be determined by Eqs. (10) and (14), respectively. Therefore, an additional N + 1 design variables must be specified to satisfy the conditions to solve this model. The remaining 1 design variable is chosen from the variables N, xN and y1 . 3.5. Simulation of the gas–liquid–liquid reactive extraction process 3.5.1. Comparison of the experimental and calculated data Fig. 9 shows the concentration of hydrogen peroxide in anthraquinone working solution at different sampling ports, Co , at the same superficial velocities of dispersed and continuous phases, but different superficial velocities of gas phase. The zero sieve plate is the bottom of the column, from which the hydrogenated anthraquinone working solution was fed into the column. At this position, EAQH2 is still not contacted with the oxygen, and the reaction does not take place, thus, there is no hydrogen peroxide being produced and the value of Co is zero. Co increases as the number of the sieve plates is increased up to about 4, and then Co is decreased with the further increase of the sieve plates. This behavior is the consequence of the co-action between the reaction and the extraction. The calculated results with the model developed above are shown in Fig. 9 as solid line. The experimental data obtained at different conditions agree well with the calculated data, and the average relative deviation between the experimental and the calculated data is less than 15%. 3.5.2. Simulation of gas–liquid–liquid reactive extraction process for the production of hydrogen peroxide in a sieve plate column For the purpose of design, the simulation of the gas–liquid–liquid reactive extraction process for
6304
S. Lü et al. / Chemical Engineering Science 60 (2005) 6298 – 6306
Fig. 9. Comparison of calculated and experimental concentration of hydrogen peroxide in anthraquinone working solution for different superficial velocity of reactive gas: (a) Ud = 2.12 × 10−3 m/s, Uc = 4.2 × 10−5 m/s, Ug = 2.02 × 10−3 m/s and CA0 = 126 mol/m3 , (b) Ud = 2.12 × 10−3 m/s, Uc = 4.2 × 10−5 m/s, Ug = 2.72 × 10−3 m/s and CA0 = 134 mol/m3 , (c) Ud = 2.12 × 10−3 m/s, Uc = 4.2 × 10−5 m/s, Ug = 2.99 × 10−3 m/s and CA0 =137 mol/m3 , (d) Ud =2.12×10−3 m/s, Uc =4.2×10−5 m/s, Ug =3.63×10−3 m/s and CA0 =118 mol/m3 . • Experimental data; — Calculated data.
production of hydrogen peroxide in a sieve plate column was performed. For given Ud , Uc and the original of concentration of EAQH2 , CA0 = 255.8 mol/m3 , the conversion of EAQH2 , XE = 0.99, and the concentration of hydrogen peroxide in outlet of raffinate phase, CoN = 2.9 mol/m3 , are specified to simulate the number of the sieve plates needed for producing hydrogen peroxide. The superficial velocity of the reactive gas (oxygen) is assumed to be the same in each stage of the column. In fact, the superficial velocity of the reactive gas will decrease with the increase of the sieve plates, because it will be consumed with the proceeding of reaction. To keep the superficial velocity of the reactive gas has no much change, the reactive gas may be introduced in column at several positions along the height of the column. The superficial velocity of the reactive gas introduced from the bottom of the column could not be too high for the reason of flooding. The simulated results are given in Figs. 10 and 11. Figs. 10 and 11 show the concentration profile of hydrogen peroxide in the liquid reactive phase (anthraquinone working solution) and the aqueous phase at various superficial velocity of the reactive gas, respectively. The number of the sieve plates needed decreased as the superficial velocity of the reactive gaseous phase was increased. An increase in the superficial velocity of gaseous reactive phase results in the increase in the overall plate extraction efficiency (Eq. (14)) and the holdup of the liquid reactive phase (Eq. (10)), which in turn enhances both the extraction and the reaction rate.
Fig. 10. Simulated concentration profile of hydrogen peroxide in anthraquinone working solution along the column. Ud = 1.7 × 10−3 m/s, Uc = 3.4 × 10−5 m/s, CA0 = 255.8 mol/m3 .
The number of the sieve plates needed is 50 at a rather small gaseous phase superficial velocity, Ug = 1.31 × 10−3 m/s. In conventional AQ process, the oxidation reaction and the extraction take place in two vessels, the oxidation reactor and the extraction column, separately. The two vessels are quite sizable, typical diameters and heights of a sieve plate column are 2–3 m and 10–20 m respectively, with 50–80 plates.
S. Lü et al. / Chemical Engineering Science 60 (2005) 6298 – 6306 i CEAQH 2 o CEAQH 2
Coo Cwo Co CoN Cw
Fig. 11. Simulated concentration profile of hydrogen peroxide in aqueous phase along the column: Ud = 1.7 × 10−3 m/s, Uc = 3.4 × 10−5 m/s, CA0 = 255.8 mol/m3 .
4. Conclusions The simultaneous oxidation of EAQH2 in the anthraquinone working solution by oxygen and the extraction of hydrogen peroxide from the anthraquinone working solution have been studied in a sieve plate column. The effects of the superficial velocities of oxygen on the conversion of EAQH2 and the extraction efficiency of hydrogen peroxide were investigated. It was found that the superficial velocity of the gaseous phase had positive effects on both the conversion of EAQH2 and the extraction efficiency of hydrogen peroxide; the oxidation reaction of EAQH2 and the extraction of hydrogen peroxide do not hamper, but, promote each other. A mathematical model has been developed for gas–liquid– liquid reactive extraction based on the stagewise theory. The simulation of the gas–liquid–liquid reactive extraction process for the production of hydrogen peroxide in a sieve plate column has been conducted using this model. The results showed that at same conditions, the number of the stages needed decreased as the superficial velocity of the reactive gaseous phase increased. The increase in the superficial velocity of reactive gaseous phase resulted in the increase in both reaction rate and extraction rate. The simulation results using this model showed that the calculated and the experimental data agreed well in a sieve plate column. It is feasible to produce hydrogen peroxide by reactive extraction process in one sieve plate column used in conventional AQ process. The model may also be used for the design and simulation of other kind of gas–liquid–liquid reactive extraction processes. Notation
C CA0
do e E f g h Hc HO2 kL ai K nr nw N P PO2 r R S T U V x XE y y∗
concentration of the EAQH2 in the feed stream of the dispersed phase, mol/m3 concentration of the EAQH2 in the outlet stream of the dispersed phase, mol/m3 concentration of hydrogen peroxide in the outlet of dispersed phase, mol/m3 concentration of hydrogen peroxide in the outlet of continuous phase, mol/m3 concentration of hydrogen peroxide in anthraquinone working solution, mol/m3 concentration of hydrogen peroxide in the outlet of raffinate phase, mol/m3 concentration of hydrogen peroxide in aqueous phase, mol/m3 sieve hole diameter, m stage extraction efficiency extract stream flow rate, mol/s or m3 /s fractional free area of the sieve plate acceleration due to gravity, m/s2 holdup of extractive phase plate spacing, m Henry’s law constant of oxygen, atm · m3 /mol liquid phase volumetric mass transfer coefficient, 1/s equilibrium constant the mole of the hydrogen peroxide formed by reaction, mol the mole of the hydrogen peroxide extracted into water, mol number of stages or sieve plates pressure, atm partial pressure of oxygen, atm reaction rate raffinate stream flow rate, mol/s or m3 /s solvent stream flow rate, mol/s or m3 /s temperature, ◦ C superficial velocity, m/s volumetric flow rates, m3 /s concentration of solute in raffinate phase, mole fraction or mol/m3 conversion of EAQH2 concentration of solute in extract phase, mole fraction or mol/m3 equilibrium concentration of solute in extract phase, mole fraction or mol/m3
Greek letters
interfacial tension, N/m density difference between the continuous and the dispersed phases, kg/m3 viscosity, Pa s density, kg/m3
Subscripts production rate the original concentration of EAQH2 , mol/m3
6305
c d
continuous phase dispersed phase
6306
F g i
S. Lü et al. / Chemical Engineering Science 60 (2005) 6298 – 6306
feed gaseous phase stage
Acknowledgements The support of the National Key Fundamental Research Development Program of China (No. G2000048005) for this research is gratefully acknowledged. References Bart, H.J., 2000. Reactive Extraction. Springer, Berlin. pp. 51–53. Danckwerts, P.V., 1970. Gas–Liquid Reactions. McGraw-Hill, New York. Dehkordi, A.M., 2001. Novel type of impinging stream contactor for liquid–liquid extraction. Industrial and Engineering Chemistry Research 40, 681–684. Dehkordi, A.M., 2002. Experimental investigation of an air-operated twoimpinging-streams reactor for copper extraction processes. Industrial and Engineering Chemistry Research 10, 2512–2520. Galkin, N.P., Goriainov, N.E., Tikhomirov, V.B., Fedorov, V.D., 1961. The mechanism of distribution of liquids in a plate extractor. Journal of Nuclear Energy (A and B) 14, 132–133. Goor, G., Kunkel, W., 1988. Ullmann’s encyclopedia of industrial chemistry, fifth ed. vol. A13. VCH, Weinheim, pp. 443–446. Hâncu, D., Beckman, E.J., 2000. Production of hydrogen peroxide in liquid CO2 . 3. oxidation of CO2 -Philic anthrahydroquinones. Industrial and Engineering Chemistry Research 39, 2843–2848. Krishna, R., 2002. Reactive separations: more ways to skin a cat. Chemical Engineering Science 57, 1491–1504. Lo, T.C., Baird, M.H.I., Hanson, C., 1983. Handbook of Solvent Extraction. Wiley, New York. p. 15. Lü, S., 2004. Study on gas–liquid–liquid reactive extraction process for the production of hydrogen peroxide. Ph.D. Thesis, Tianjin University, Tianjin, P.R. China.
Lü, S., Wang, L., Liu, J., Yang, T., Wang, Y., Mi, Z., 2004a. Holdups of dispersed and gas phase in gas-agitated sieve plate extraction column. The Chinese Journal of Process Engineering 4, 193–197. Lü, S., Mi, Z., Wang, Y., Wang, L., 2004b. Experimental investigated and simulation of a gas-agitated sieve plate column. Chemical Engineering and Technology 27, 903–908. Minotti, M., Doherty, M.F., Malone, M.F., 1998. Design for simultaneous reaction and liquid–liquid extraction. Industrial and Engineering Chemistry Research 37, 4748–4756. Priestley, R., Ellis, S.R.M., 1978. Gas agitated liquid extraction columns. Chemistry and Industry 7, 757–760. Samant, K.D., Ng, K.M., 1998a. Synthesis of extractive reaction process. A.I.Ch.E. Journal 44, 1363–1381. Samant, K.D., Ng, K.M., 1998b. Effect of kinetics and mass transfer on design of extractive reaction process. A.I.Ch.E. Journal 44, 2212–2228. Samant, K.D., Ng, K.M., 1999. Systematic development of extractive reaction process. Chemical Engineering and Technology 22, 877–880. Sanderson, W.R., 2000. Clear industrial processes using hydrogen peroxide. Pure and Applied Chemistry 72, 1289–1304. Santacesaria, E., Ferro, R., Ricci, S., Carrà, S., 1987. Kinetic aspects in the oxidation of hydrogenated 2-ethyltetrahydroanthraquinone. Industrial and Engineering Chemistry Research 26, 155–159. Santacesaria, E., Serio, M.D., Russo, A., Leone, U., Velotti, R., 1999. Kinetic and catalytic aspects in the hydrogen peroxide production via anthraquinone. Chemical Engineering Science 54, 2799–2806. Sharma, M.M., Gupta, R.K., 1967. Mass transfer characteristics of plate columns without downcomer. Transactions of the Institution of Chemical Engineers 45, T169–T175. Sharma, M.M., Mashelker, R.A., Mehta, V.D., 1969. Mass transfer in plate columns. British Chemical Engineering 14, 70–76. Sheikh, J., Kershenbaum, L.S., Alpay, E., 1998. Analytical basis for separation enhanced reaction in continuous flow processes. Chemical Engineering Science 53, 2933–2939. Sovilj, M., Knezˇevic, G., 1994. Gas-agitated liquid–liquid extraction in a spray column. Collection Czechoslovak Chemical Communications 59, 2235–2243.
Experimental investigation and simulation of gas–liquid–liquid reactive extraction process for the production of hydrogen peroxide Shuxiang Lüa, b , Zhentao Mia,∗ , Li Wanga , Yaquan Wanga , Zehua Zhuc , Songbao Fuc a Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University,
Tianjin 300072, PR China b School of Chemical Engineering, Hebei University of Technology, Tianjin 300130, PR China cYingshan Petrochemical Plant, SINOPEC, Yueyang 414003, PR China
Received 28 October 2004; received in revised form 18 April 2005; accepted 18 April 2005 Available online 8 June 2005
Abstract The gas–liquid–liquid reactive extraction system for the production of hydrogen peroxide via anthraquinone route was investigated. The oxidation of the hydrogenated anthraquinone working solution by oxygen and the extraction of hydrogen peroxide from the working solution with deionized water were carried out simultaneously in a sieve plate column of 50 mm in diameter. The effects of the superficial velocity of oxygen on the conversion of 2-ethylanthrahydroquinone and the extraction efficiency of hydrogen peroxide were investigated, separately. The results showed that the oxidation and the extraction do not hamper each other, on the contrary, the presence of gas in the column can promote the transfer of hydrogen peroxide from the organic phase to the aqueous phase, therefore, the conversion of 2-ethylanthrahydroquinone and the extraction efficiency of hydrogen peroxide increased with the increase of gas superficial velocity. In addition, a mathematical model for the simulation of the gas–liquid–liquid reactive extraction process was developed. The predicted values were compared with the experimental data at different conditions and the agreement was found to be quite satisfactory for the production of hydrogen peroxide in a sieve plate column. 䉷 2005 Elsevier Ltd. All rights reserved. Keywords: Extraction; Simulation; Multiphase flow; Separations; Hydrogen peroxide; Sieve plate column
1. Introduction Hydrogen peroxide is widely used in almost all industrial areas, especially in the chemical industry and environmental protection (Goor and Kunkel, 1988). Since the only degradation product is water after use, by the criteria of green chemistry, hydrogen peroxide has already made
a remarkable contribution to a cleaner chemical industry (Sanderson, 2000). OH
O +
H2 OH
O
EAQH2
EAQ OH
O +
∗ Corresponding author. Key Laboratory for Green Chemical Tech-
nology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China. Tel.: +86 22 27408891; fax: +86 22 27402604. E-mail addresses: [email protected] (S. Lü), [email protected] (Z. Mi). 0009-2509/$ - see front matter 䉷 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2005.04.044
OH
(1)
+
O2 O
H2O2
(2)
By far, hydrogen peroxide has mainly been produced by a method named anthraquinone (AQ) route in which anthraquinone derivatives, usually 2-ethylanthraquinone
S. Lü et al. / Chemical Engineering Science 60 (2005) 6298 – 6306
(EAQ), dissolved in organic solvents (all together being called anthraquinone working solution) is first hydrogenated catalytically to the corresponding 2-ethylanthrahydroquinone (EAQH2 , Eq. (1)) in a slurry or fixed-bed reactor; then EAQH2 is oxidized with oxygen or air to form hydrogen peroxide and the original EAQ (Eq. (2)) in a gas–liquid reactor; and finally the hydrogen peroxide formed is extracted with water in a sieve plate extraction column. In recent years, the development of reactive separation processes has received considerable attention (Samant and Ng, 1998a,b, 1999; Sheikh et al., 1998, Krishna, 2002). The combination of reaction and separation in a single unit can improve the yield of a target product, leads to simultaneous separation of byproducts, bypass the chemical reaction equilibrium limitation and reduction of investment. In a general reactive extraction process, there are only two distinct phases, i.e., aqueous and organic phases and reactive species are present in different phases (Bart, 2000). Minotti et al. (1998) have developed a general geometric method for the design of simultaneous reaction and liquid–liquid twophase extraction on the assumption that the liquid phases leaving the extractor are in equilibrium. In this work, the oxidation reaction of EAQH2 with oxygen and the extraction of hydrogen peroxide from the anthraquinone working solution were carried out simultaneously in a sieve plate column, and the effects of the oxygen superficial velocity on the conversion of EAQH2 and the extraction efficiency of hydrogen peroxide were studied. This reactive extraction system contains three phases: gaseous (oxygen), organic (anthraquinone working solution) and aqueous phases. The gaseous phase acts as one of the reactants as well as agitating agent. Since the method of Minotti et al. (1998) cannot be used for the design and simulation of the present gas–liquid–liquid three-phase reactive extraction system, a mathematical model for this system was developed, with the effects of the reactive gas phase on both the reaction rate and the extraction efficiency being taken into account. Finally, this model was tested by the experimental data obtained in the sieve plate column.
2. Experimental section 2.1. Material and apparatus The anthraquinone working solution was a mixture of 2-ethylanthraquinone, trioctyl phosphate and aromatics of C9 –C10 , and the volume ratio of C9 –C10 aromatics to trioctyl phosphate is 3. Hydrogenation of anthraquinone to anthrahydroquinone in the working solution was carried out in a trickle bed reactor at 60 ◦ C and 0.3 MPa in the presence of an industrial Pd/Al2 O3 catalyst. Pure oxygen (99.99%) was employed. The experimental set-up is schematically shown in Fig. 1. The cylindrical glass column with diameter of 50 mm was fitted with seven sieve plates at a plate spacing of 210 mm.
6299
14
7
15
8 5
13 16
9 6
4
2 1
3
10
11 12
Fig. 1. Schematic diagram of the experimental set-up (1) water reservoir, (2) double-piston meter pump, (3) hydrogenated working solution reservoir, (4) membrane meter pump, (5) reactive extraction column, (6) dispersed phase distributor, (7) continuous phase distributor, (8) adjustable overflow tube, (9) hydrogen peroxide solution reservoir, (10) rotameter, (11) oxygen cylinder, (12) oxidized working solution reservoir, (13) gas–liquid separator, (14–16) valves.
Sieve plates were made of stainless steel with holes of 2.5 mm in diameter, corresponding to free areas of 24%. The enlarged parts at the both top and bottom of the column were 400 mm long and its diameter were 100 mm, which served as liquid–liquid separators. Oxygen and the hydrogenated anthraquinone working solution were continuously introduced into the column through a gas–liquid distributor (6) packed with 3.5 × 3.5 mm rings. Deionized water was supplied to the top of the column through a perforated plate-type distributor (7) with holes of 2 mm in diameter. The unreacted oxygen comes out through valves (14,15) at the top of the column and gas–liquid separator (13). Three sampling ports and thermocouples were situated just above the 2nd, 4th and 6th plates. Oxygen, the hydrogenated anthraquinone working solution and deionized water were used as gaseous, dispersed and continuous phases, respectively, and the flow rates of them were measured with a rotameter (10), a membrane meter pump (4) and a piston meter pump (1), respectively. Since the decomposition of hydrogen peroxide occurs easily in the presence of metal ions, the whole apparatus was passivated by nitric acid. The column temperature was kept at 50 ± 1 ◦ C through circulating water in the jacket of the column. The flow rate ranges in terms of superficial velocities were: 1.3 × 10−3 .3.0 × 10−3 m/s for the gaseous phase (Ug ), 1.2×10−3 .2.2×10−3 m/s for the dispersed phase (Ud ) and 2.4 × 10−5 .3.4 × 10−5 m/s for the continuous phase (Uc ). The physical properties of the employed system are given in Table 1.
6300
S. Lü et al. / Chemical Engineering Science 60 (2005) 6298 – 6306
Table 1 The physical properties of the experimental system (50 ◦ C) Parameter
d c g d c g
Value
Unit
Description
905.3 988.1 1.21 0.89 × 10−3 0.549 × 10−3 2.13 × 10−5 6.77 × 10−2 3.4 × 10−2
(kg/m3 )
Dispersed phase density (anthraquinone working solution) Continuous phase density (deionized water) Gaseous phase density (oxygen) Dispersed phase viscosity Continuous phase viscosity Gaseous phase viscosity Surface tension of continuous phase Interfacial tension between dispersed and continuous phases
(kg/m3 ) (kg/m3 ) (Pa s) (Pa s) (Pa s) (N/m) (N/m)
2.2. Procedure and analysis method In each run, the flow rates of oxygen, deionized water and the hydrogenated anthraquinone working solution were adjusted to the desired values and fed to the column. In theory, the material balance at steady-state conditions for the entire column is i o Vd (CEAQH − CEAQH ) = Vd Coo + Vc Cwo , 2 2
(3)
where Vd and Vc are the volumetric flow rates of the disi persed and continuous phases, respectively, and CEAQH and 2 o CEAQH2 are the concentration of the EAQH2 in the feed stream and the outlet stream of the dispersed phase, respectively. Coo and Cwo are the concentration of hydrogen peroxide in the outlet streams of dispersed and continuous phases, respectively. When the deviation between the value of left and right of Eq. (3) did not exceed ±5%, it is considered as the steady-state conditions were established, samples of dispersed phase and continuous phase were taken from sampling ports. Quantitative analysis of the hydrogen peroxide in aqueous phase samples was performed by titration. Each organic phase sample was extracted with deionized water for four times, and the four aqueous solutions obtained were combined together and also titrated as the aqueous samples.
3. Results and discussion 3.1. Equilibrium data The equilibrium data of hydrogen peroxide dissolving between water and anthraquinone working solution at a constant temperature (50 ◦ C) was determined with the method described in literature (Lo et al., 1983), and is shown in Fig. 2. According to these results, at the conditions the concentration of hydrogen peroxide within the range of 0.0–162 mol/m3 in anthraquinone working solution (Co ) and 0–1.2 × 104 mol/m3 in the water (Cw ), the equilibrium curve of hydrogen peroxide distributing between water and working solution is lineal and the equilibrium constant, K, is 77.4.
Fig. 2. The equilibrium data of H2 O2 dissolving between water and anthraquinone working solution.
3.2. Effects of the gaseous phase superficial velocity on the conversion of EAQH2 and the extraction efficiency of hydrogen peroxide The extraction efficiency of hydrogen peroxide may be expressed as follows: e=
nw , nr
(4)
where nr is the mole of hydrogen peroxide produced by the oxidation of EAQH2 and nw is the mole of hydrogen peroxide extracted into water. The effects of the gaseous phase superficial velocity on the conversion of EAQH2 (XE ) and the extraction efficiency of hydrogen peroxide (e) are shown in Figs. 3 and 4. It is seen that both the conversion of EAQH2 , and the extraction efficiency of hydrogen peroxide increased with the increase of the superficial velocity of gaseous phase. Such behavior is the consequences of an increase in the turbulence then momentum and mass transfer. This may also be caused by an increase in the contact time of oxygen (reactive gas) and dispersed phase (liquid reactive phase) due to their oscillatory motion and/or an increase in the holdups of dispersed and gaseous phases. (Lü et al., 2004a; Sovilj and Knezˇevic,
S. Lü et al. / Chemical Engineering Science 60 (2005) 6298 – 6306
Fig. 3. Effect of the gaseous phase superficial velocity on the conversion of EAQH2 . Ud /Uc = 50.
Fig. 4. Effect of the gaseous phase superficial velocity on the extraction efficiency of H2 O2 . Ud /Uc = 50.
1994). On the other hand, the volumetric mass-transfer coefficient of gas–liquid, kL ai , increases with the increase of the reactive gas flow rate (Danckwerts, 1970). Therefore, the oxidation reaction rate of the EAQH2 and the conversion of EAQH2 are increased. When the superficial velocity of gaseous phase is relatively high, the mean size of dispersed phase droplets within the chambers between plates become smaller, and the population density of dispersed phase droplets become higher than those related to lower superficial velocity of gaseous phase (Lü et al., 2004b). Also both the interdroplet collisions and droplets of dispersed phase break are promoted when the oxygen was introduced to the column and consequently increases surface renewal (Dehkordi, 2001, 2002). These phenomena result in the increase of the interfacial area between dispersed and continuous phases, which in turn en-
Fig. 5. The conversion profile of EAQH2 Ud = 1.7 × 10−3 m/s, Uc = 3.4 × 10−5 m/s.
6301
along the column.
Fig. 6. The extraction efficiency profile of H2 O2 along the column. Ud = 1.7 × 10−3 m/s, Uc = 3.4 × 10−5 m/s.
hances the mass transfer rate. So the extraction rate and the extraction efficiency are enhanced. A similar behavior was also observed in different types of gas-agitated extraction columns (Galkin et al., 1961; Priestley and Ellis, 1978). 3.3. Profiles of the conversion of EAQH2 and the extraction efficiency of hydrogen peroxide To investigate the profiles of the conversion of EAQH2 and the extraction efficiency of hydrogen peroxide along the column, the data of XE and e at different sampling ports were measured. The results are shown in Figs. 5 and 6. An increase in the XE and e were observed with increase of sieve plates. Such behavior is a consequence of an increase in the mean residence time of dispersed phase droplets in the column.
6302
S. Lü et al. / Chemical Engineering Science 60 (2005) 6298 – 6306
E1 y1
E2 y2
xF
Gas
Ei yi
R2 x2
Ri-1 xi-1
2
1
F
E3 y3
R1 x1
Ei+1 EN-1 yi+1 yN-1 i
EN yN N-1
Ri xi
RN-2 xN-2
s y0 N
RN-1 xN-1
RN Gas xN
outlet gaseous phase
outlet raffinate phase
liquid extraction phase (water)
Fig. 7. Gas–liquid–liquid countercurrent reactive extraction cascade.
3.4. Model development According to the stagewise model, the gas–liquid–liquid reactive extractor consists of a set of isothermal stirred tanks. (see Fig. 7). The reactive gas and liquid reactive phase (dispersed phase) flow countercurrently with the extractive phase (continuous phase). The dispersed and continuous phases are immiscible. The reactive extractor may be treated with the following assumptions: (1) Each compartment between two sieve plates is considered as a stage, which represents a perfectly mixed stirred tank with the gas phase as external agitation. (2) Reaction does not occur in the continuous phase, but only occur in the dispersed phase. (3) The two outlet liquid streams from each stage are not in phase equilibrium. The stage extraction efficiency in all stages is the same. (4) The flow rates of gaseous, dispersed and continuous phases are independent of mass-transfer, reaction and solute concentration. (5) Each stage is at known constant temperature T, and pressure P. One stage of the gas–liquid–liquid reactive extractor is shown schematically in Fig. 8. Three phases exist in each stage: the liquid reactive phase, gas reactive phase and liquid extractive phase. The oxygen must dissolve in liquid reactive phase, and oxidation reaction takes place in the liquid reactive phase, the product hydrogen peroxide is then extracted by water from the reactive phase. The liquid reactive phase and the extractive phase are immiscible, while hydrogen peroxide being distributed between the phases. The overall material balance on the solute (hydrogen peroxide) is F x F + Sy 0 + C = E1 y1 + RN xN ,
(5)
where xF and y0 are the concentration (or mole fractions) of the solute in the inlet feed stream and solvent, respectively, and F and S are the corresponding flow rate (volume or mole). y1 and xN represent concentration (or mole fractions) of the solute in the outlet extract and raffinate streams, respectively, and E1 and RN are the corresponding flow rate. C represents overall production rate in the reactive extractor due to the chemical reaction.
liquid reaction phase (hydrogenated anthraquinone working solution)
outlet extractive phase
gaseous reaction phase (oxygen) reactive phase
gaseous phase
extractive phase Fig. 8. Schematic diagram of one stage of gas–liquid–liquid reactive extraction sieve plate column for the production of hydrogen peroxide.
The compositions and flow rates of the liquid phases for the ith stage are related by the material balances: Ri−1 xi−1 + Ei+1 yi+1 + Ci = Ri xi + Ei yi (i = 1, 2, . . . , N ),
(6)
where xi and yi are the concentration (or mole fraction) of solute in the raffinate and extract streams, respectively, and Ri and Ei are the corresponding flow rates leaving ith stage. Each element of Ci , which represents the production rate in the ith stage, is related to the reaction rate, ri , and the holdup of the liquid reactive phases in ith stage, hi , by
Ci = hi ri
(i = 1, 2, . . . , N ).
(7)
Santacesaria et al. (1987, 1999) have investigated the kinetic aspects of the oxidation of hydrogenated anthraquinone working solution in a gas–liquid reactor. They reported that the reaction rate of the oxidation of hydrogenated anthraquinone working solution by atmospheric pressure oxygen was governed by the transport of oxygen through the gas–liquid interface (Hâncu and Beckman, 2000). Kinetics studies of the oxidation of EAQH2 in oxygen–anthraquinone working solution–water three-phase system have shown that the reaction rate can be described by (Lü, 2004) r = kL ai
PO2 , HO2
(8)
where r is the reaction rate, kL ai is gas–liquid mass-transfer coefficient, PO2 is the partial pressure of oxygen, and HO2 is the Henry’s law constant of oxygen. At a constant temperature of T = 50 ◦ C and atmospheric pressure, the value HO2 can be found in the literature (Santacesaria et al., 1987).
S. Lü et al. / Chemical Engineering Science 60 (2005) 6298 – 6306
In a sieve plate column, the gas–liquid mass-transfer coefficient is determined using the equation of Sharma et al. (1967, 1969). kL ai = 0.042f −2.2 (d Ud )0.6 Ug1.2 ,
(9)
where f is the fractional free area of the sieve plate, d is the density of dispersed phase (liquid reactive phase). The holdup of the liquid reactive phase for air–water–anthraquinone working solution three-phase system is influenced by the flow rates of all phases (Lü et al., 2004a). The liquid reactive phase holdup in each stage, hi , is assumed to be the same and is calculated using following equation (Lü et al., 2004a). 0.38 0.52 1/4 1/4 hi = 9.41 Ug c Ud c g g 1/4 −0.58 0.18 d × exp −10.65Uc c g c c 1/2 −0.39 g × Hc c , (10) where c is the density of continuous phase (extractive phase), d and c are the viscosities of dispersed and continuous phases, respectively. is density difference between the continuous and the dispersed phases, g is the gravity acceleration, Hc is plate spacing, and is interfacial tension between dispersed and continuous phases. Through Eqs. (9) and (10), effect of reactive gas phase on the reaction was taken into account in the model. The overall production rate in the reactive extractor is the sum of the production rates covering all of the stages:
C=
N
Ci
(i = 1, 2, . . . , N ).
(11)
i=1
Since composition of the liquid leaving each stage is not in liquid–liquid equilibrium, the relations of them are determined by the stage extraction efficiency as follows: ei =
yi − yi+1 yi∗ − yi+1
yi∗ = Kx i
(i = 1, 2, . . . , N),
(i = 1, 2, . . . , N ),
(12) (13)
where yi∗ is equilibrium concentration of the extract phase leaving the ith stage, ei is the stage extraction efficiency in the ith stage and K is equilibrium constant. Studies of the stage extraction efficiency in gas–liquid– liquid three-phase system have shown that the stage extraction efficiency is influenced by the flow rates of all phases (Lü et al., 2004b). It can be related to the flow rates of dispersed, continuous and gaseous phases by Eq. (14): 0.5 U 0.58 c −3 Hc e = 7.94 × 10 (14) exp 0.14Ug , 0.35 U d0 d
6303
where d0 is sieve hole diameter. Through Eq. (14), effect of reactive gas phase on the extraction efficiency was taken into account in the model. Since the two liquid phases are immiscible, the flow rates of liquid phases are constant in each stage, i.e., F = R1 = R2 = · · · = RN = R, S = E1 = E2 = · · · = EN = E. The number of independent variables can be determined by degrees-of-freedom analysis method. The variables are E, R, Ug , N, e, xi , yi , hi , ri and Ci for i = 1, 2, . . . , N. There are 2(N + 1) concentrations; N holdups of reactive phase; N production rates; N reaction rates. The total number of variables is 2(N + 1) + 3N + 5. The independent equations are material balance equations (Eq. (6)) in N stages, the relations of the production rate and the holdup of reactive phase (Eq. (7)) in N stages, and the relations of xi and yi (Eqs. (12)–(14) are not independent) in N stages. This gives a total of 3N independent equations. Therefore, there are 2(N + 1) + 5 degrees of freedom. Normally, the solvent and the feed flow rate, S and F, the gas phase flow rate, Ug , and the concentrations of the feed stream and solvent, xF , y0 are known or specified. The values of hi and e can be determined by Eqs. (10) and (14), respectively. Therefore, an additional N + 1 design variables must be specified to satisfy the conditions to solve this model. The remaining 1 design variable is chosen from the variables N, xN and y1 . 3.5. Simulation of the gas–liquid–liquid reactive extraction process 3.5.1. Comparison of the experimental and calculated data Fig. 9 shows the concentration of hydrogen peroxide in anthraquinone working solution at different sampling ports, Co , at the same superficial velocities of dispersed and continuous phases, but different superficial velocities of gas phase. The zero sieve plate is the bottom of the column, from which the hydrogenated anthraquinone working solution was fed into the column. At this position, EAQH2 is still not contacted with the oxygen, and the reaction does not take place, thus, there is no hydrogen peroxide being produced and the value of Co is zero. Co increases as the number of the sieve plates is increased up to about 4, and then Co is decreased with the further increase of the sieve plates. This behavior is the consequence of the co-action between the reaction and the extraction. The calculated results with the model developed above are shown in Fig. 9 as solid line. The experimental data obtained at different conditions agree well with the calculated data, and the average relative deviation between the experimental and the calculated data is less than 15%. 3.5.2. Simulation of gas–liquid–liquid reactive extraction process for the production of hydrogen peroxide in a sieve plate column For the purpose of design, the simulation of the gas–liquid–liquid reactive extraction process for
6304
S. Lü et al. / Chemical Engineering Science 60 (2005) 6298 – 6306
Fig. 9. Comparison of calculated and experimental concentration of hydrogen peroxide in anthraquinone working solution for different superficial velocity of reactive gas: (a) Ud = 2.12 × 10−3 m/s, Uc = 4.2 × 10−5 m/s, Ug = 2.02 × 10−3 m/s and CA0 = 126 mol/m3 , (b) Ud = 2.12 × 10−3 m/s, Uc = 4.2 × 10−5 m/s, Ug = 2.72 × 10−3 m/s and CA0 = 134 mol/m3 , (c) Ud = 2.12 × 10−3 m/s, Uc = 4.2 × 10−5 m/s, Ug = 2.99 × 10−3 m/s and CA0 =137 mol/m3 , (d) Ud =2.12×10−3 m/s, Uc =4.2×10−5 m/s, Ug =3.63×10−3 m/s and CA0 =118 mol/m3 . • Experimental data; — Calculated data.
production of hydrogen peroxide in a sieve plate column was performed. For given Ud , Uc and the original of concentration of EAQH2 , CA0 = 255.8 mol/m3 , the conversion of EAQH2 , XE = 0.99, and the concentration of hydrogen peroxide in outlet of raffinate phase, CoN = 2.9 mol/m3 , are specified to simulate the number of the sieve plates needed for producing hydrogen peroxide. The superficial velocity of the reactive gas (oxygen) is assumed to be the same in each stage of the column. In fact, the superficial velocity of the reactive gas will decrease with the increase of the sieve plates, because it will be consumed with the proceeding of reaction. To keep the superficial velocity of the reactive gas has no much change, the reactive gas may be introduced in column at several positions along the height of the column. The superficial velocity of the reactive gas introduced from the bottom of the column could not be too high for the reason of flooding. The simulated results are given in Figs. 10 and 11. Figs. 10 and 11 show the concentration profile of hydrogen peroxide in the liquid reactive phase (anthraquinone working solution) and the aqueous phase at various superficial velocity of the reactive gas, respectively. The number of the sieve plates needed decreased as the superficial velocity of the reactive gaseous phase was increased. An increase in the superficial velocity of gaseous reactive phase results in the increase in the overall plate extraction efficiency (Eq. (14)) and the holdup of the liquid reactive phase (Eq. (10)), which in turn enhances both the extraction and the reaction rate.
Fig. 10. Simulated concentration profile of hydrogen peroxide in anthraquinone working solution along the column. Ud = 1.7 × 10−3 m/s, Uc = 3.4 × 10−5 m/s, CA0 = 255.8 mol/m3 .
The number of the sieve plates needed is 50 at a rather small gaseous phase superficial velocity, Ug = 1.31 × 10−3 m/s. In conventional AQ process, the oxidation reaction and the extraction take place in two vessels, the oxidation reactor and the extraction column, separately. The two vessels are quite sizable, typical diameters and heights of a sieve plate column are 2–3 m and 10–20 m respectively, with 50–80 plates.
S. Lü et al. / Chemical Engineering Science 60 (2005) 6298 – 6306 i CEAQH 2 o CEAQH 2
Coo Cwo Co CoN Cw
Fig. 11. Simulated concentration profile of hydrogen peroxide in aqueous phase along the column: Ud = 1.7 × 10−3 m/s, Uc = 3.4 × 10−5 m/s, CA0 = 255.8 mol/m3 .
4. Conclusions The simultaneous oxidation of EAQH2 in the anthraquinone working solution by oxygen and the extraction of hydrogen peroxide from the anthraquinone working solution have been studied in a sieve plate column. The effects of the superficial velocities of oxygen on the conversion of EAQH2 and the extraction efficiency of hydrogen peroxide were investigated. It was found that the superficial velocity of the gaseous phase had positive effects on both the conversion of EAQH2 and the extraction efficiency of hydrogen peroxide; the oxidation reaction of EAQH2 and the extraction of hydrogen peroxide do not hamper, but, promote each other. A mathematical model has been developed for gas–liquid– liquid reactive extraction based on the stagewise theory. The simulation of the gas–liquid–liquid reactive extraction process for the production of hydrogen peroxide in a sieve plate column has been conducted using this model. The results showed that at same conditions, the number of the stages needed decreased as the superficial velocity of the reactive gaseous phase increased. The increase in the superficial velocity of reactive gaseous phase resulted in the increase in both reaction rate and extraction rate. The simulation results using this model showed that the calculated and the experimental data agreed well in a sieve plate column. It is feasible to produce hydrogen peroxide by reactive extraction process in one sieve plate column used in conventional AQ process. The model may also be used for the design and simulation of other kind of gas–liquid–liquid reactive extraction processes. Notation
C CA0
do e E f g h Hc HO2 kL ai K nr nw N P PO2 r R S T U V x XE y y∗
concentration of the EAQH2 in the feed stream of the dispersed phase, mol/m3 concentration of the EAQH2 in the outlet stream of the dispersed phase, mol/m3 concentration of hydrogen peroxide in the outlet of dispersed phase, mol/m3 concentration of hydrogen peroxide in the outlet of continuous phase, mol/m3 concentration of hydrogen peroxide in anthraquinone working solution, mol/m3 concentration of hydrogen peroxide in the outlet of raffinate phase, mol/m3 concentration of hydrogen peroxide in aqueous phase, mol/m3 sieve hole diameter, m stage extraction efficiency extract stream flow rate, mol/s or m3 /s fractional free area of the sieve plate acceleration due to gravity, m/s2 holdup of extractive phase plate spacing, m Henry’s law constant of oxygen, atm · m3 /mol liquid phase volumetric mass transfer coefficient, 1/s equilibrium constant the mole of the hydrogen peroxide formed by reaction, mol the mole of the hydrogen peroxide extracted into water, mol number of stages or sieve plates pressure, atm partial pressure of oxygen, atm reaction rate raffinate stream flow rate, mol/s or m3 /s solvent stream flow rate, mol/s or m3 /s temperature, ◦ C superficial velocity, m/s volumetric flow rates, m3 /s concentration of solute in raffinate phase, mole fraction or mol/m3 conversion of EAQH2 concentration of solute in extract phase, mole fraction or mol/m3 equilibrium concentration of solute in extract phase, mole fraction or mol/m3
Greek letters
interfacial tension, N/m density difference between the continuous and the dispersed phases, kg/m3 viscosity, Pa s density, kg/m3
Subscripts production rate the original concentration of EAQH2 , mol/m3
6305
c d
continuous phase dispersed phase
6306
F g i
S. Lü et al. / Chemical Engineering Science 60 (2005) 6298 – 6306
feed gaseous phase stage
Acknowledgements The support of the National Key Fundamental Research Development Program of China (No. G2000048005) for this research is gratefully acknowledged. References Bart, H.J., 2000. Reactive Extraction. Springer, Berlin. pp. 51–53. Danckwerts, P.V., 1970. Gas–Liquid Reactions. McGraw-Hill, New York. Dehkordi, A.M., 2001. Novel type of impinging stream contactor for liquid–liquid extraction. Industrial and Engineering Chemistry Research 40, 681–684. Dehkordi, A.M., 2002. Experimental investigation of an air-operated twoimpinging-streams reactor for copper extraction processes. Industrial and Engineering Chemistry Research 10, 2512–2520. Galkin, N.P., Goriainov, N.E., Tikhomirov, V.B., Fedorov, V.D., 1961. The mechanism of distribution of liquids in a plate extractor. Journal of Nuclear Energy (A and B) 14, 132–133. Goor, G., Kunkel, W., 1988. Ullmann’s encyclopedia of industrial chemistry, fifth ed. vol. A13. VCH, Weinheim, pp. 443–446. Hâncu, D., Beckman, E.J., 2000. Production of hydrogen peroxide in liquid CO2 . 3. oxidation of CO2 -Philic anthrahydroquinones. Industrial and Engineering Chemistry Research 39, 2843–2848. Krishna, R., 2002. Reactive separations: more ways to skin a cat. Chemical Engineering Science 57, 1491–1504. Lo, T.C., Baird, M.H.I., Hanson, C., 1983. Handbook of Solvent Extraction. Wiley, New York. p. 15. Lü, S., 2004. Study on gas–liquid–liquid reactive extraction process for the production of hydrogen peroxide. Ph.D. Thesis, Tianjin University, Tianjin, P.R. China.
Lü, S., Wang, L., Liu, J., Yang, T., Wang, Y., Mi, Z., 2004a. Holdups of dispersed and gas phase in gas-agitated sieve plate extraction column. The Chinese Journal of Process Engineering 4, 193–197. Lü, S., Mi, Z., Wang, Y., Wang, L., 2004b. Experimental investigated and simulation of a gas-agitated sieve plate column. Chemical Engineering and Technology 27, 903–908. Minotti, M., Doherty, M.F., Malone, M.F., 1998. Design for simultaneous reaction and liquid–liquid extraction. Industrial and Engineering Chemistry Research 37, 4748–4756. Priestley, R., Ellis, S.R.M., 1978. Gas agitated liquid extraction columns. Chemistry and Industry 7, 757–760. Samant, K.D., Ng, K.M., 1998a. Synthesis of extractive reaction process. A.I.Ch.E. Journal 44, 1363–1381. Samant, K.D., Ng, K.M., 1998b. Effect of kinetics and mass transfer on design of extractive reaction process. A.I.Ch.E. Journal 44, 2212–2228. Samant, K.D., Ng, K.M., 1999. Systematic development of extractive reaction process. Chemical Engineering and Technology 22, 877–880. Sanderson, W.R., 2000. Clear industrial processes using hydrogen peroxide. Pure and Applied Chemistry 72, 1289–1304. Santacesaria, E., Ferro, R., Ricci, S., Carrà, S., 1987. Kinetic aspects in the oxidation of hydrogenated 2-ethyltetrahydroanthraquinone. Industrial and Engineering Chemistry Research 26, 155–159. Santacesaria, E., Serio, M.D., Russo, A., Leone, U., Velotti, R., 1999. Kinetic and catalytic aspects in the hydrogen peroxide production via anthraquinone. Chemical Engineering Science 54, 2799–2806. Sharma, M.M., Gupta, R.K., 1967. Mass transfer characteristics of plate columns without downcomer. Transactions of the Institution of Chemical Engineers 45, T169–T175. Sharma, M.M., Mashelker, R.A., Mehta, V.D., 1969. Mass transfer in plate columns. British Chemical Engineering 14, 70–76. Sheikh, J., Kershenbaum, L.S., Alpay, E., 1998. Analytical basis for separation enhanced reaction in continuous flow processes. Chemical Engineering Science 53, 2933–2939. Sovilj, M., Knezˇevic, G., 1994. Gas-agitated liquid–liquid extraction in a spray column. Collection Czechoslovak Chemical Communications 59, 2235–2243.