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Journal of Natural Gas Chemistry 15(2006)63-69
SCIENCE PRESS
Article
Study on Catalytic Wet Oxidation of HaS into Sulfur on Fe/Cu Catalyst Junfeng Zhang, Zhiquan Tong* Environmental Engineering Department of Xiangtan University, Xiangtan 411105, China
[ Manuscript received November 21, 2005; revised December 21, 2005 J
Abstract: A wet catalytic oxidation at room temperature was investigated with solution containing ferric, ferrous and cupric ions for H2S removal. The experiments were carried out in a two step process, and the results obtained show that the removal efficiency of H2S can always reach 100% in a 300 mm scrubbing column with four sieve plates, and the regeneration of ferric ions in 200 mm bubble column can match the consumed ferric species in absorption. Removal of HzS, production of elemental sulfur and regeneration of ferric, cupric ions can all be accomplished at the same time. No raw material is consumed except 0 2 in flue gas or air, the process has no secondary pollution and no problem of catalyst degradation and congestion. Key words: wet oxidation; HzS; catalytic; sulfur; ferric ion; ferrous ion; cupric ion
1. Introduction The removal of H2S from biogas, natural or industrial gas streams is an important environmental concern, since H2S is extremely hazardous to human health and corrosive, besides its strong unpleasant smell. It is produced during the anaerobic treatment of waste water and during effluent treatment in some industries such as paper and sour products or in petrochemical plants and natural gas refineries [l]. Various approaches such as alkaline/amine scrubbing, gas incineration, chemical oxidation by NaClO, Cl02, H202, KMn04 and gas phase oxidation by Cl02, adsorption, bio-filtration have been advocated over the years to remove or at least absorb hydrogen sulfide. However, all the methods have similar disadvantages, such as the cost implication being a huge investment or a costly operation. Another method employable to destroy hydrogen sulfide is through H2S oxidation in aqueous ferric sulfate and ferric sulfate solutions, but its removal efficiency level is not high enough[2].
In recent years gas desulfurization processes based on either a V5+/V4+ couple or a Fe3+/Fe2+ couple [3] with chemicals for stabilizing the vanadium, or iron [4,5]have received increasing attention in the natural gas and oil refining industry. But there are many unknowns and questions regarding the performance, cost, operational ability, and reliability or stability of the chelate employed. The use of microorganisms to oxidize HzS, producing sulfate or elemental sulfur as a consequence of complete or incomplete metabolism, respectively, has been considered a potential alternative for application on a large scale. The most used bacteria in these investigations belong to the genus Thiobacillus, mainly T . denitri f icans. These chemolithotrophic species use sulfur and its reduced forms (including H2S) as the energy source for growth. Thiobacillus ferrooxidans also oxidizes ferrous iron to ferric iron to obtain energy for its cellular processes [l]. In general, the possible advantages of a biotechnological process would be low energy demand for the process operation and reduced consumption of chemicals, but the initial investment
* Corresponding author. Tel: 0732-8292504, E-mail: tzq@ xtu,edu.cn.
64
Junfeng Zhang et aJ./ Journal of Natural Gas Chemistry Vol. 15 No. 1 2006
of capital will be considerable due to the low rate of removal of microorganisms [6]. In this study, a novel process for H2S gas treatment based on two steps is investigated, corresponding to absorption with chemical reaction in solution with ferric and cupric ions (whereas the cupric ion is converted to cupric sulfide, then the ferric ion oxidizes cupric sulfide to elemental sulfur), and oxidation of ferrous ions in the solution to produce ferric ions. The oxidation of ferrous ion, produced during the absorption step, to ferric ion involves the catalytic activity of the cupric ion [7], and hence the regeneration of the absorbing solution is fast. The reactions are: H2S(,) H2Sp) HSH+ (1)
+
+
HS-
+ S2- + H+
(2)
+ s2- cus 1
(3)
cu2+ 2Fe3+
+ S2-
+
--j
4
So 1 +2Fe2+
+ CuS + So + 2Fe2+ + Cu2+ 4Fe2+ + 0 2 + 4H+ -+ 4Fe3+ + 2H20 2Fe3+
sieve plates (each of them has 260 holes with the diameter of 5 mm) and one rotating-stream-tray evaporator for removing water carried by gas stream. The regeneration of residual ferric ion was carried out in a bubble column with 200 mm diameter. The process of the experimental setup is shown in Figure 1.
(4) Do
(5) F i g u r e 1. E x p e r i m e n t a l s e t u p
(6)
Definitely, the following overall reaction is performed: 2H2S 0 2 -+ 2s' J, +2H20 (7)
+
Chemical absorption of hydrogen sulfide involves reactions (1)-(3), where reaction (4) and (5), and mainly ( 5 ) , produce elemental sulfur. The regeneration of cupric ion and ferric ion may be represented by reactions (5) and (6). In this way, the desulfurization process can be regarded as a catalytic reaction of hydrogen sulfide with oxygen in flue gas or air. Reactions (3) and (5) take place quickly, while reaction (6) is comparatively slow to keep up with the others, and hence an additional reactor must be adopted to complete reaction (6), irrespective of the oxygen presence in flue gas. The absorption solution is simple and reliable, with no consumption of raw material except 0 2 and the process has no secondary pollution nor any problem of degradation of catalytic activity. 2. Experimental The absorbing solution was prepared to contain 60 g/L ferric ion and 80 g/L cupric ion, and 30 g/L ferrous ion was also added for effective utilization of oxygen and accelerating the regeneration of ferric ion. A series of absorption experiments were conducted in a column of 300 mm diameter with four
1-HZS bottle, 2-Column, 3-Circulating tank, 4-Pump, 5-Flow meter, 6-Pressure taps, 7-Compressor, 8-Ferric regenerating reactor, +Vent blower
The samples of H2S were absorbed by zinc acetate solution, and titrated with 0.1 mol/L iodine solution. Cupric ion was analyzed with iodine evaluating method using ammonium fluoride as efficient masking agent and the ferrous and ferric ions in adsorbing liquid were analyzed by employing electrochemical methods 181.
3. Results and discussion 3.1 Wet catalytic oxidation of H2S H2S (0.7 m3/h) and air (700 m3/h) were introduced into the scrubbing column at room temperature under 3.0 L/m3 liquid-gas ratio (L/G). Simultaneously, air (15 m3/h) was introduced into the bubble column to regenerate ferric ion consumed in the absorption process. Figure 2 shows the H2S removal efficiency and the concentrations of cupric ion and ferrous ion with the reaction time. No sulfur oxides were detected in the exit gas streams, indicating that the oxidation of H2S to SO2 is prevented in the wet catalytic oxidation. The Fe/Cu catalyst keeps the H2S removal efficiency at almost 100% till the termination of experiment while the concentration of cupric
65
Journal of Natural Gas (2hemistr.y Vol. 15 No. 1 2006
ion and ferric ion still retained nearly the initial Values. No CuS was found accumulated, and the process is expected to continue smoothly. The released sulfur accumulated on the surface of circulating tank, at the
same time the absorbent in it becomes homogeneous quickly, and block of floating sulfur can be taken out for further purification.
100
-
1.00
-
-
-
-
(a) I1
x
2 .z
8E
f?
f
0.8
-
0.6
-
.
80 7 ,
-
60n
-
cl
DO
E
2
~
3 -
0
"
"
"
~
"
~
'
"
'
~
'
"
~
'
"
'
"
'
'
centration under similar conditions as in the former was shown in Figure 3. Figure 3 shows that the desulfurization efficiency can always reach 100% even to 0.3% (v/v) HzS,when the H2S concentration increases further, cupric ion must be increased to ensure the perfect removal efficiency.
-
ID
20
0
'
H2S removal efficiency of different H2S inlet con-
1.o
-
-
-
-
--
(b)
-
TI
'a 40:
0.2
-
h
v
0.4
-
1 -
-
-
0
-Fe2' -&-Fe3' '
t C U 2 ' ~ ~ '
~
'
'
'
'
~
'
'
'
~
1
~
'
1 1
0 0.001
0.002 0.003 H2S concentration
0.004
Figure 3. HzS removal efficiency of different H2S inlet concentration
3.2 Selectivity for H2S to sulfur on Fe/Cu catalyst
IR spectrometer was employed for the analysis of elemental sulfur during the production. The solid samples were dried at room temperature in a vacuum before the measurement. There is a peak at
'
1622 cm-' in the IR spectrum (Figure 4(1)) showing the presence of s-0 bond in the production before purification, while that no S-S bond can be detected shows the sulfur produced is elemental sulfur, not polymeric sulfur, and that the 1622 peak disappeared in IR spectrum (Figure 4(2)) of the stripped sample means that stream stripping can purify the production effectively. The concentrations of sulfate ions and sulfite ions in the liquid samples were analyzed by an ion chromatograph (Dionex 2000i). The total sulfur in the liquid sample and the solid sample were analyzed by a barium titration method, the results are shown in Table 1. L
0.2
'
o ~ ~ " ' ~ " " " " ' ~ " " ' " 0
1000
2000
3000
Wavenumber (cm-l )
Figure 4. IR of production elemental sulfur (1) Before purification, (2) A f t e r purification
4000
~
~
~
'
66
Junfeng Zhang et al./ Journal of Natural Gas Chemistry Vol. 15 No. 1 2006
Table 1. Results of analysis ~
Solid production (%)
Absorption solution (g/L)
s/so;1.18
s/so:0.73
c( Fe3+)
C(CU2+)
CFe
cus
CUCl
S
60
80
90
0
0
100
The amount of sulfate ions and sulfite ions in the liquid sample after 4 h reaction was 94 g (sulfur) and 58 g (sulfur) respectively. The total sulfur in the liquid and the solid sample after 4 h was 4 kg. It indicates that the wet oxidation of H2S to sulfur is much selective and the selectivity is over 96.2%, which was calculated using the following equation: Sulfur selectivity (%) = Moles of sulfur produced/moles of H2S reacted x 100%
3.3. Discussion Room temperature catalytic oxidation of H2S has been attempted with solid catalysis for several years, but carbons have been known to be most effective for room temperature oxidation of H2S. Recently, it was reported that the hydrodarco-activated carbon was capable of adsorbing 0.66 gsulfur/gcarbon in air [9]. It is shown that the Fe/Cu catalyst developed can completely remove H2S continuously, this is the first report to show that ordinary homogeneous solution with Cu2+, Fe2+ and Fe3+ can be much more effective than any other room temperature catalytic oxidation of H2S. Different from the wet oxidation of H2S by Fe/Cu catalyst, it has been reported that the oxidation from Fe2+ to Fe3+ by 0 2 is more difficult than the reduction from Fe3+ to Fe2+ by H2S in a liquid redox process using Fe-chelating agent as the catalyst. Liquid redox process uses the reduction/oxidation cycle depicted by reactions (4) and (6). Due t o the increasing concentration of H+ with the increasing of ferric ion (more ferric ion will hydrolyze and produce H+), moreover, excessive H+ trends to prevent reactions (1)and (2), the Fe-chelating agent must be used to enhance the removal efficiency of H2S and higher partial pressure of 0 2 should be used to increase the oxidation rate of Fe2+ in Fe-chelating system. This is because of the difficulty of the oxidation of ferrous ion, while the oxidation from Fe2+ to Fe3+ by 0 2 in Fe/Cu catalytic system occurs much more quickly due to the catalysis of Cu2+ [7]. Moreover, few Fe/Cu system with Cu2+ can remove H2S more completely than the complicated chelatings.
4. Mathematical model 4.1. Absorption reaction
A one-dimensional, isothermal two-phase model based on the two-film theory was developed for the description of hydrogen sulfide absorption into aqueous cupric solutions. Based on an assumption of the isothermal condition, the essential behavior of the gasliquid reactor may be depicted. The model supposes that the transport in liquid and gas films is represented by diffusion only. According to this model, the absorption quality of H2S (simplified as A) can be expressed as:
NA= KGA(PA + YCBL)
(8)
where y = EADLB, CBLrepresents the concentration DLA of cupric ion. Because reaction (3) is very fast, the concentration of A at reaction surface p ~ i = O ,the CB, can be expressed as follows: ~
PA~GADLA = CBL(critica1) ~LADLB Equation (8) shows that when CBLincreases from 0 to C ~ ~ ( ~ ~ the i t ireaction ~ ~ q , surface moves from the outside of liquid to the interface of gas and liquid film, and over much of the CBLrange it has no significant effect on the reaction velocity. It should be noted that since the concentration of cupric ion is high enough in these experiments, much more than CBL(critical), increasing it does not increase N A appreciably. At the same time, the masstransfer in gas film of A is the control step of the whole process, then equation (8) can be expressed by:
CBL=
NA= ~
G *AP A
(9)
Equation (9) shows that the absorption of hydrogen sulfide in the aqueous cupric solutions is controlled by the capability of reactor employed and the operating conditions. The methods can enhance the transmitting characteristic and they are helpful to the experiments. And kGA is determined by the space velocity of flue gas G and absorbent L , therefore, ~ G A can be defined as: kG = a ~ p ~ r (10)
67
Journal of Natural Gas Chemistry Vol. 15 No. 1 2006
The absorption experiments for determining ~ G A were done at 25 "C when p~ was 100 Pa in the column with one plate as shown in Figure 1, and the experimental results are listed in Table 2.
Assuming the granules are even spheres, W = 4/3.rrr3p,, A = 471-r~. The conversion of cupric sulfide 2 was calculated using the following equations:
Table 2. Experimental results for determining k G A
0.275
G/(m3/h) 500 500
0.290
500
0.320
600 700
NA/(kmol/h) 0.260
0.365
LI(LIh) 1000 1500 2000 1500 1500
A series of copper sulfide leaching experiments were carried out using ferric chloride solution by Mu Yufeng [lo], and the surface reaction was found to be the control step. Hence equation (11) can be expressed by:
Based on Table 2, the value of a , ,8, y can be calculated to be 4 . 4 lop6, ~ 0.87, 0.14 respectively. 4.2. Leaching of cupric sulfide
,
I
At the same time,
A shrinking-core model of cupric sulfide leaching with ferric ion was built for studying the reaction kinetics of equation (5). In the experiments carried out in stirred tank reactor shown in Figure 4, cupric sulfide was prepared by the reaction between diluted aqueous solutions of CuSO4 (reagent grade) and NazS (reagent grade), other reagents were also reagent grade, the reactor employed was operated intermittently as batch reactor (BR). The leaching velocity of cupric sulfide TB can be expressed by: 1 dW rB=--'(11) M dt In this formula, W is the weight of cupric sulfide (kg), A4 is molecular weight of cupric sulfide (g/mol). 0.6 I
I 30'C 40 'c v 50°C A
20
40
t / min
E RT
(13)
The CuS leaching results were shown in Figure 5(a), the k' under different temperatures can be obtained from the line slopes shown in Figure 5(a), and the relationship between Ink' and 1/T was shown in Figure 5(b), by linear regression analysis the apparent activation energy E is determined to be 41.57 kJ/mol. In the same way, the order of ferric concentration is found to be 0.5219 based on Figure 6(a) and Figure 6(b). The rate equation for the leaching process fitted to experimental results was as follows:
-3r------
-4
0.4
0
5 = koexp(--)
60
31
32 10000 / T
33
Figure 5. Relationship between l - ( l - ~ ) ' and / ~ t under different reaction temperatures (a), the relationship between Ink' and 10000/T (b) c(,,3+)=1.5 mol/L, r0=-125 pm +75 pm
68
Junfeng Zhang et al./ Journal of Natural Gas Chemistry Vol. 15 No. 1 2006
0.20 -
0.15
(a)
0
A
-
v
1.5 molL 1.0 molL 0.5 m o m
I
L
r
0.05 -
0
10
20
30
40
50
-0.8
-0.6
-0.4
t I min
Figure 6. The relationship between l - ( l - z ) l l 3 between Ink' and lnc(Fe3+) (b) T=30 "c,r0=-125 pm +75 pm
-0.2
0.0
0.2
0.4
0.6
In c ( F ~ "
and t of different concentration of cFes+ (a), relationship
In this formula, R=8.314 J/(mol.K), t is reaction time (min). Formula (14) is applicable under the conditions as follows: 30 "C
Gas inlet
Hot water inlet z
Liquid sample point
Cold water outlet
P
Figure 7. Stirred tank reactor with heater
The reaction velocity can be expressed by:
4.3. Regeneration of ferric ion
Reaction (6) is very important because it determines the continuation of the desulfurization process. So the kinetic study of reaction (6) must be done for purposes of further engineering application. The experiments were carried out in a stirred cell reactor as shown in Figure 7. The concentration of oxygen in the input and output was measured by KMSOOCS Multifunctional Analyzer (Kane Co. England Company), the temperature was determined by the temperature of hot water flowing in interlayer, and the composition of reaction solution was the same as in desulfurzation scrubbing. All reagents used were of reagent grade and the pH value of system was adjusted by appended hydrochloric acid and sodium hydroxide. Since a stirring speed of more than 80 rpm would affect the reaction adversely, experiments of oxidizing ferrous were run at a stirring speed of 80 rpm.
Therefore, systematic experiments must be done to identify the reaction order of every reactant. By assuming that the kinetic constant k varies with temperature according to an Arrhenius equation, the activation energy can be calculated as shown in section 4.2. The results show that E is 12.35 kJ/mol, less than 82.46 kJ/mol obtained by Bouboukas [ll],this is because the accelerating function of the cupric ion for the oxidation of ferrous by oxygen is remarkable. Based on multiple linear regressions analysis to the experimental results gained at room temperature (shown in Table 3), formula (15) can be expressed by: d~~,z+ 12.35 x lo3 0.6 = -1260exp()%2+ CH+ coz 8.314T dt (16) In formula (16), t is reacting time (min), T is thermodynamic temperature (K), c is concentration (mol/L).
69
Journal of Natural Gas Chemistry Vol. 15 No. 1 2006
Table 3. Partial results of ferric regeneration experiments
1
0.0019
2
0.0022
3
0.0045
0.510
C(H+ 1/ ( m o w ) 0.32 0.208 0.41
......
......
......
......
......
30
0.0045
0.605
0.42
3 . 0 9 l~o r 5
w/(mol/ (L.min))
c(Fe2+)/(mol/L) 0.411 0.606
5. Conclusions The Fe/Cu catalytic system was developed for the HZS oxidation to sulfur by a wet process at a low room temperature. The removal efficiency of HzS can always reach 100% and the sulfur selectivity of Fe/Cu catalyst is perfect. No raw material will be consumed except 0 2 (in flue gas or air) and the process has no secondary pollution and no problems of degradation and congestion. The capacity of the equipments employed is potentially large and their structure is simple.
References [l] Oprime M E A G, Garcia 0, Cardoso A A. Process Biochem, 2001, 37(2): 111 [2] Eng S J, Motekaitis R J, Martell A E. Inorg Chim
c(Od/(moW) 2.05~10-5 3.0~10-~ 4 . 4 10-5 ~
Acta, 2000, 299(1): 9 [3] Iliuta I, Larachi F. Chem Eng Sci, 2003, 58(23-24): 5305 [4] Jun K-D, Joo 0-S, Cho S-H, Han S-H. Appl Catal A , 2003, 240(1-2): 235 [5] Zhang H, Tong Zh Q. Natural Science Journal of X i angtan University, 2003, 25(1): 53 [6] Pagella C, De Faveri D M. Chem Eng Sci, 2000, 55(12): 2185 [7] Zhang J F, Tong Zh Q. ACTA Sci Circum, 2005, 25 (4): 497 [8] Bond A M, Pfund B V, Newman 0 M G. Anal Chim Acta, 1993, 277(1): 145 [9] Bagreev A, Adib F, Bandosz T J. Carbon, 2001, 39(12): 1897 [lo] Mu Y F. Journal of Kunming Institue of Technology, 1995, 20(5): 11 [ll] Bouboukas G, Gaunand A, Renon H. Hydrometallurgy, 1987, 19(1): 25
Journal of Natural Gas Chemistry 15(2006)63-69
SCIENCE PRESS
Article
Study on Catalytic Wet Oxidation of HaS into Sulfur on Fe/Cu Catalyst Junfeng Zhang, Zhiquan Tong* Environmental Engineering Department of Xiangtan University, Xiangtan 411105, China
[ Manuscript received November 21, 2005; revised December 21, 2005 J
Abstract: A wet catalytic oxidation at room temperature was investigated with solution containing ferric, ferrous and cupric ions for H2S removal. The experiments were carried out in a two step process, and the results obtained show that the removal efficiency of H2S can always reach 100% in a 300 mm scrubbing column with four sieve plates, and the regeneration of ferric ions in 200 mm bubble column can match the consumed ferric species in absorption. Removal of HzS, production of elemental sulfur and regeneration of ferric, cupric ions can all be accomplished at the same time. No raw material is consumed except 0 2 in flue gas or air, the process has no secondary pollution and no problem of catalyst degradation and congestion. Key words: wet oxidation; HzS; catalytic; sulfur; ferric ion; ferrous ion; cupric ion
1. Introduction The removal of H2S from biogas, natural or industrial gas streams is an important environmental concern, since H2S is extremely hazardous to human health and corrosive, besides its strong unpleasant smell. It is produced during the anaerobic treatment of waste water and during effluent treatment in some industries such as paper and sour products or in petrochemical plants and natural gas refineries [l]. Various approaches such as alkaline/amine scrubbing, gas incineration, chemical oxidation by NaClO, Cl02, H202, KMn04 and gas phase oxidation by Cl02, adsorption, bio-filtration have been advocated over the years to remove or at least absorb hydrogen sulfide. However, all the methods have similar disadvantages, such as the cost implication being a huge investment or a costly operation. Another method employable to destroy hydrogen sulfide is through H2S oxidation in aqueous ferric sulfate and ferric sulfate solutions, but its removal efficiency level is not high enough[2].
In recent years gas desulfurization processes based on either a V5+/V4+ couple or a Fe3+/Fe2+ couple [3] with chemicals for stabilizing the vanadium, or iron [4,5]have received increasing attention in the natural gas and oil refining industry. But there are many unknowns and questions regarding the performance, cost, operational ability, and reliability or stability of the chelate employed. The use of microorganisms to oxidize HzS, producing sulfate or elemental sulfur as a consequence of complete or incomplete metabolism, respectively, has been considered a potential alternative for application on a large scale. The most used bacteria in these investigations belong to the genus Thiobacillus, mainly T . denitri f icans. These chemolithotrophic species use sulfur and its reduced forms (including H2S) as the energy source for growth. Thiobacillus ferrooxidans also oxidizes ferrous iron to ferric iron to obtain energy for its cellular processes [l]. In general, the possible advantages of a biotechnological process would be low energy demand for the process operation and reduced consumption of chemicals, but the initial investment
* Corresponding author. Tel: 0732-8292504, E-mail: tzq@ xtu,edu.cn.
64
Junfeng Zhang et aJ./ Journal of Natural Gas Chemistry Vol. 15 No. 1 2006
of capital will be considerable due to the low rate of removal of microorganisms [6]. In this study, a novel process for H2S gas treatment based on two steps is investigated, corresponding to absorption with chemical reaction in solution with ferric and cupric ions (whereas the cupric ion is converted to cupric sulfide, then the ferric ion oxidizes cupric sulfide to elemental sulfur), and oxidation of ferrous ions in the solution to produce ferric ions. The oxidation of ferrous ion, produced during the absorption step, to ferric ion involves the catalytic activity of the cupric ion [7], and hence the regeneration of the absorbing solution is fast. The reactions are: H2S(,) H2Sp) HSH+ (1)
+
+
HS-
+ S2- + H+
(2)
+ s2- cus 1
(3)
cu2+ 2Fe3+
+ S2-
+
--j
4
So 1 +2Fe2+
+ CuS + So + 2Fe2+ + Cu2+ 4Fe2+ + 0 2 + 4H+ -+ 4Fe3+ + 2H20 2Fe3+
sieve plates (each of them has 260 holes with the diameter of 5 mm) and one rotating-stream-tray evaporator for removing water carried by gas stream. The regeneration of residual ferric ion was carried out in a bubble column with 200 mm diameter. The process of the experimental setup is shown in Figure 1.
(4) Do
(5) F i g u r e 1. E x p e r i m e n t a l s e t u p
(6)
Definitely, the following overall reaction is performed: 2H2S 0 2 -+ 2s' J, +2H20 (7)
+
Chemical absorption of hydrogen sulfide involves reactions (1)-(3), where reaction (4) and (5), and mainly ( 5 ) , produce elemental sulfur. The regeneration of cupric ion and ferric ion may be represented by reactions (5) and (6). In this way, the desulfurization process can be regarded as a catalytic reaction of hydrogen sulfide with oxygen in flue gas or air. Reactions (3) and (5) take place quickly, while reaction (6) is comparatively slow to keep up with the others, and hence an additional reactor must be adopted to complete reaction (6), irrespective of the oxygen presence in flue gas. The absorption solution is simple and reliable, with no consumption of raw material except 0 2 and the process has no secondary pollution nor any problem of degradation of catalytic activity. 2. Experimental The absorbing solution was prepared to contain 60 g/L ferric ion and 80 g/L cupric ion, and 30 g/L ferrous ion was also added for effective utilization of oxygen and accelerating the regeneration of ferric ion. A series of absorption experiments were conducted in a column of 300 mm diameter with four
1-HZS bottle, 2-Column, 3-Circulating tank, 4-Pump, 5-Flow meter, 6-Pressure taps, 7-Compressor, 8-Ferric regenerating reactor, +Vent blower
The samples of H2S were absorbed by zinc acetate solution, and titrated with 0.1 mol/L iodine solution. Cupric ion was analyzed with iodine evaluating method using ammonium fluoride as efficient masking agent and the ferrous and ferric ions in adsorbing liquid were analyzed by employing electrochemical methods 181.
3. Results and discussion 3.1 Wet catalytic oxidation of H2S H2S (0.7 m3/h) and air (700 m3/h) were introduced into the scrubbing column at room temperature under 3.0 L/m3 liquid-gas ratio (L/G). Simultaneously, air (15 m3/h) was introduced into the bubble column to regenerate ferric ion consumed in the absorption process. Figure 2 shows the H2S removal efficiency and the concentrations of cupric ion and ferrous ion with the reaction time. No sulfur oxides were detected in the exit gas streams, indicating that the oxidation of H2S to SO2 is prevented in the wet catalytic oxidation. The Fe/Cu catalyst keeps the H2S removal efficiency at almost 100% till the termination of experiment while the concentration of cupric
65
Journal of Natural Gas (2hemistr.y Vol. 15 No. 1 2006
ion and ferric ion still retained nearly the initial Values. No CuS was found accumulated, and the process is expected to continue smoothly. The released sulfur accumulated on the surface of circulating tank, at the
same time the absorbent in it becomes homogeneous quickly, and block of floating sulfur can be taken out for further purification.
100
-
1.00
-
-
-
-
(a) I1
x
2 .z
8E
f?
f
0.8
-
0.6
-
.
80 7 ,
-
60n
-
cl
DO
E
2
~
3 -
0
"
"
"
~
"
~
'
"
'
~
'
"
~
'
"
'
"
'
'
centration under similar conditions as in the former was shown in Figure 3. Figure 3 shows that the desulfurization efficiency can always reach 100% even to 0.3% (v/v) HzS,when the H2S concentration increases further, cupric ion must be increased to ensure the perfect removal efficiency.
-
ID
20
0
'
H2S removal efficiency of different H2S inlet con-
1.o
-
-
-
-
--
(b)
-
TI
'a 40:
0.2
-
h
v
0.4
-
1 -
-
-
0
-Fe2' -&-Fe3' '
t C U 2 ' ~ ~ '
~
'
'
'
'
~
'
'
'
~
1
~
'
1 1
0 0.001
0.002 0.003 H2S concentration
0.004
Figure 3. HzS removal efficiency of different H2S inlet concentration
3.2 Selectivity for H2S to sulfur on Fe/Cu catalyst
IR spectrometer was employed for the analysis of elemental sulfur during the production. The solid samples were dried at room temperature in a vacuum before the measurement. There is a peak at
'
1622 cm-' in the IR spectrum (Figure 4(1)) showing the presence of s-0 bond in the production before purification, while that no S-S bond can be detected shows the sulfur produced is elemental sulfur, not polymeric sulfur, and that the 1622 peak disappeared in IR spectrum (Figure 4(2)) of the stripped sample means that stream stripping can purify the production effectively. The concentrations of sulfate ions and sulfite ions in the liquid samples were analyzed by an ion chromatograph (Dionex 2000i). The total sulfur in the liquid sample and the solid sample were analyzed by a barium titration method, the results are shown in Table 1. L
0.2
'
o ~ ~ " ' ~ " " " " ' ~ " " ' " 0
1000
2000
3000
Wavenumber (cm-l )
Figure 4. IR of production elemental sulfur (1) Before purification, (2) A f t e r purification
4000
~
~
~
'
66
Junfeng Zhang et al./ Journal of Natural Gas Chemistry Vol. 15 No. 1 2006
Table 1. Results of analysis ~
Solid production (%)
Absorption solution (g/L)
s/so;1.18
s/so:0.73
c( Fe3+)
C(CU2+)
CFe
cus
CUCl
S
60
80
90
0
0
100
The amount of sulfate ions and sulfite ions in the liquid sample after 4 h reaction was 94 g (sulfur) and 58 g (sulfur) respectively. The total sulfur in the liquid and the solid sample after 4 h was 4 kg. It indicates that the wet oxidation of H2S to sulfur is much selective and the selectivity is over 96.2%, which was calculated using the following equation: Sulfur selectivity (%) = Moles of sulfur produced/moles of H2S reacted x 100%
3.3. Discussion Room temperature catalytic oxidation of H2S has been attempted with solid catalysis for several years, but carbons have been known to be most effective for room temperature oxidation of H2S. Recently, it was reported that the hydrodarco-activated carbon was capable of adsorbing 0.66 gsulfur/gcarbon in air [9]. It is shown that the Fe/Cu catalyst developed can completely remove H2S continuously, this is the first report to show that ordinary homogeneous solution with Cu2+, Fe2+ and Fe3+ can be much more effective than any other room temperature catalytic oxidation of H2S. Different from the wet oxidation of H2S by Fe/Cu catalyst, it has been reported that the oxidation from Fe2+ to Fe3+ by 0 2 is more difficult than the reduction from Fe3+ to Fe2+ by H2S in a liquid redox process using Fe-chelating agent as the catalyst. Liquid redox process uses the reduction/oxidation cycle depicted by reactions (4) and (6). Due t o the increasing concentration of H+ with the increasing of ferric ion (more ferric ion will hydrolyze and produce H+), moreover, excessive H+ trends to prevent reactions (1)and (2), the Fe-chelating agent must be used to enhance the removal efficiency of H2S and higher partial pressure of 0 2 should be used to increase the oxidation rate of Fe2+ in Fe-chelating system. This is because of the difficulty of the oxidation of ferrous ion, while the oxidation from Fe2+ to Fe3+ by 0 2 in Fe/Cu catalytic system occurs much more quickly due to the catalysis of Cu2+ [7]. Moreover, few Fe/Cu system with Cu2+ can remove H2S more completely than the complicated chelatings.
4. Mathematical model 4.1. Absorption reaction
A one-dimensional, isothermal two-phase model based on the two-film theory was developed for the description of hydrogen sulfide absorption into aqueous cupric solutions. Based on an assumption of the isothermal condition, the essential behavior of the gasliquid reactor may be depicted. The model supposes that the transport in liquid and gas films is represented by diffusion only. According to this model, the absorption quality of H2S (simplified as A) can be expressed as:
NA= KGA(PA + YCBL)
(8)
where y = EADLB, CBLrepresents the concentration DLA of cupric ion. Because reaction (3) is very fast, the concentration of A at reaction surface p ~ i = O ,the CB, can be expressed as follows: ~
PA~GADLA = CBL(critica1) ~LADLB Equation (8) shows that when CBLincreases from 0 to C ~ ~ ( ~ ~ the i t ireaction ~ ~ q , surface moves from the outside of liquid to the interface of gas and liquid film, and over much of the CBLrange it has no significant effect on the reaction velocity. It should be noted that since the concentration of cupric ion is high enough in these experiments, much more than CBL(critical), increasing it does not increase N A appreciably. At the same time, the masstransfer in gas film of A is the control step of the whole process, then equation (8) can be expressed by:
CBL=
NA= ~
G *AP A
(9)
Equation (9) shows that the absorption of hydrogen sulfide in the aqueous cupric solutions is controlled by the capability of reactor employed and the operating conditions. The methods can enhance the transmitting characteristic and they are helpful to the experiments. And kGA is determined by the space velocity of flue gas G and absorbent L , therefore, ~ G A can be defined as: kG = a ~ p ~ r (10)
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Journal of Natural Gas Chemistry Vol. 15 No. 1 2006
The absorption experiments for determining ~ G A were done at 25 "C when p~ was 100 Pa in the column with one plate as shown in Figure 1, and the experimental results are listed in Table 2.
Assuming the granules are even spheres, W = 4/3.rrr3p,, A = 471-r~. The conversion of cupric sulfide 2 was calculated using the following equations:
Table 2. Experimental results for determining k G A
0.275
G/(m3/h) 500 500
0.290
500
0.320
600 700
NA/(kmol/h) 0.260
0.365
LI(LIh) 1000 1500 2000 1500 1500
A series of copper sulfide leaching experiments were carried out using ferric chloride solution by Mu Yufeng [lo], and the surface reaction was found to be the control step. Hence equation (11) can be expressed by:
Based on Table 2, the value of a , ,8, y can be calculated to be 4 . 4 lop6, ~ 0.87, 0.14 respectively. 4.2. Leaching of cupric sulfide
,
I
At the same time,
A shrinking-core model of cupric sulfide leaching with ferric ion was built for studying the reaction kinetics of equation (5). In the experiments carried out in stirred tank reactor shown in Figure 4, cupric sulfide was prepared by the reaction between diluted aqueous solutions of CuSO4 (reagent grade) and NazS (reagent grade), other reagents were also reagent grade, the reactor employed was operated intermittently as batch reactor (BR). The leaching velocity of cupric sulfide TB can be expressed by: 1 dW rB=--'(11) M dt In this formula, W is the weight of cupric sulfide (kg), A4 is molecular weight of cupric sulfide (g/mol). 0.6 I
I 30'C 40 'c v 50°C A
20
40
t / min
E RT
(13)
The CuS leaching results were shown in Figure 5(a), the k' under different temperatures can be obtained from the line slopes shown in Figure 5(a), and the relationship between Ink' and 1/T was shown in Figure 5(b), by linear regression analysis the apparent activation energy E is determined to be 41.57 kJ/mol. In the same way, the order of ferric concentration is found to be 0.5219 based on Figure 6(a) and Figure 6(b). The rate equation for the leaching process fitted to experimental results was as follows:
-3r------
-4
0.4
0
5 = koexp(--)
60
31
32 10000 / T
33
Figure 5. Relationship between l - ( l - ~ ) ' and / ~ t under different reaction temperatures (a), the relationship between Ink' and 10000/T (b) c(,,3+)=1.5 mol/L, r0=-125 pm +75 pm
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Junfeng Zhang et al./ Journal of Natural Gas Chemistry Vol. 15 No. 1 2006
0.20 -
0.15
(a)
0
A
-
v
1.5 molL 1.0 molL 0.5 m o m
I
L
r
0.05 -
0
10
20
30
40
50
-0.8
-0.6
-0.4
t I min
Figure 6. The relationship between l - ( l - z ) l l 3 between Ink' and lnc(Fe3+) (b) T=30 "c,r0=-125 pm +75 pm
-0.2
0.0
0.2
0.4
0.6
In c ( F ~ "
and t of different concentration of cFes+ (a), relationship
In this formula, R=8.314 J/(mol.K), t is reaction time (min). Formula (14) is applicable under the conditions as follows: 30 "C
Gas inlet
Hot water inlet z
Liquid sample point
Cold water outlet
P
Figure 7. Stirred tank reactor with heater
The reaction velocity can be expressed by:
4.3. Regeneration of ferric ion
Reaction (6) is very important because it determines the continuation of the desulfurization process. So the kinetic study of reaction (6) must be done for purposes of further engineering application. The experiments were carried out in a stirred cell reactor as shown in Figure 7. The concentration of oxygen in the input and output was measured by KMSOOCS Multifunctional Analyzer (Kane Co. England Company), the temperature was determined by the temperature of hot water flowing in interlayer, and the composition of reaction solution was the same as in desulfurzation scrubbing. All reagents used were of reagent grade and the pH value of system was adjusted by appended hydrochloric acid and sodium hydroxide. Since a stirring speed of more than 80 rpm would affect the reaction adversely, experiments of oxidizing ferrous were run at a stirring speed of 80 rpm.
Therefore, systematic experiments must be done to identify the reaction order of every reactant. By assuming that the kinetic constant k varies with temperature according to an Arrhenius equation, the activation energy can be calculated as shown in section 4.2. The results show that E is 12.35 kJ/mol, less than 82.46 kJ/mol obtained by Bouboukas [ll],this is because the accelerating function of the cupric ion for the oxidation of ferrous by oxygen is remarkable. Based on multiple linear regressions analysis to the experimental results gained at room temperature (shown in Table 3), formula (15) can be expressed by: d~~,z+ 12.35 x lo3 0.6 = -1260exp()%2+ CH+ coz 8.314T dt (16) In formula (16), t is reacting time (min), T is thermodynamic temperature (K), c is concentration (mol/L).
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Journal of Natural Gas Chemistry Vol. 15 No. 1 2006
Table 3. Partial results of ferric regeneration experiments
1
0.0019
2
0.0022
3
0.0045
0.510
C(H+ 1/ ( m o w ) 0.32 0.208 0.41
......
......
......
......
......
30
0.0045
0.605
0.42
3 . 0 9 l~o r 5
w/(mol/ (L.min))
c(Fe2+)/(mol/L) 0.411 0.606
5. Conclusions The Fe/Cu catalytic system was developed for the HZS oxidation to sulfur by a wet process at a low room temperature. The removal efficiency of HzS can always reach 100% and the sulfur selectivity of Fe/Cu catalyst is perfect. No raw material will be consumed except 0 2 (in flue gas or air) and the process has no secondary pollution and no problems of degradation and congestion. The capacity of the equipments employed is potentially large and their structure is simple.
References [l] Oprime M E A G, Garcia 0, Cardoso A A. Process Biochem, 2001, 37(2): 111 [2] Eng S J, Motekaitis R J, Martell A E. Inorg Chim
c(Od/(moW) 2.05~10-5 3.0~10-~ 4 . 4 10-5 ~
Acta, 2000, 299(1): 9 [3] Iliuta I, Larachi F. Chem Eng Sci, 2003, 58(23-24): 5305 [4] Jun K-D, Joo 0-S, Cho S-H, Han S-H. Appl Catal A , 2003, 240(1-2): 235 [5] Zhang H, Tong Zh Q. Natural Science Journal of X i angtan University, 2003, 25(1): 53 [6] Pagella C, De Faveri D M. Chem Eng Sci, 2000, 55(12): 2185 [7] Zhang J F, Tong Zh Q. ACTA Sci Circum, 2005, 25 (4): 497 [8] Bond A M, Pfund B V, Newman 0 M G. Anal Chim Acta, 1993, 277(1): 145 [9] Bagreev A, Adib F, Bandosz T J. Carbon, 2001, 39(12): 1897 [lo] Mu Y F. Journal of Kunming Institue of Technology, 1995, 20(5): 11 [ll] Bouboukas G, Gaunand A, Renon H. Hydrometallurgy, 1987, 19(1): 25