Hydrogen sulfide is a toxic and odorous compound present in biogas produced by the anaerobic digestion of biosolids and other organic materials. Reactive absorption of hydrogen sulfide into aqueous ferric sulfate solution is a direct process for H2S removal and Sulfur recovery. Apart from sulfur, only H2O is generated in the process, and consequently, no waste treatment facilities are required. A distinct advantage of the process is that the reaction of H2S with is so rapid and complete that there remains no danger of discharging toxic waste gas. Effective operation parameters on this process considered. Results show that high temperature and low pressure are suitable for absorption reaction. Variation of hydrogen sulfide concentration and Fe3+ concentration with time in absorption reaction shown that the reaction of ferric sulfate and hydrogen sulfide is first order with respect to the both reactant. At low Fe2(SO4) concentration the absorption rate of H2S increase with increasing the Fe2(SO4)3 concentration. At higher concentration, a decrease in the absorption rate was found. At higher concentration of Fe2(SO4)3, the ionic strength and viscosity of solution increase remarkably resulting in a decrease of solubility, diffusivity and hence absorption rate.
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Key words: Absorption, Biological process, Fe2(SO4)3, Hydrogen sulfide, Reactive absorption
Hydrogen sulfide is being produced by many industrial activities such as petroleum refining, natural gas and petrochemical plants, viscose rayon manufacturing craft, pulp manufacturing, food processing, aerobic and anaerobic wastewater treatments and many other industries (Wang and Pei, 2012). The presence of H2S in gaseous streams proscribes the direct use of these gases because of its toxic properties, the formation of SO2 upon combustion, and the problems it usually gives in downstream processing (Ter Maat et al., 2005). Upon inhalation, hydrogen sulfide reacts with enzymes in the blood stream and inhibits cellular respiration resulting in pulmonary paralysis, sudden collapse, and death. Continuous exposure to low (15-50 ppm) concentrations will generally cause irritation to mucous membranes and may also cause headaches, dizziness, and nausea. Higher concentrations (200-300 ppm) may result in respiratory arrest leading to coma and unconsciousness (Syed et al., 2006). This means that it is often necessary to remove H2S from gas stream prior to use.
Nowadays, researchers and scientists have been focused on developing effective, stable, and practicable methods for hydrogen sulfide removal from gaseous streams. Various solid materials have been developed to capture H2S from a number of industrial gas effluent streams (Wang et al., 2008; Katoh et al., 1995; Jianwen et al., 2011; Liang et al., 2011; Li et al., 2007; Feng et al., 2009; Nguyen-Thanh et al., 2005a). Several commercial techniques are available for the removal of H2S, including aqueous NaCl (Duan et al., 2007), iron-based sorbents (Xie et al., 2010; Wang et al., 2012), activated carbon (Nguyen-Thanh and Bandosz, 2005b), Fe2O3, metal oxides (Ko et al., 2005; Van Dijk et al., 2011), and removal of H2S using aqueous red mud (Pandey et al., 2003) which is a caustic waste product of alumina industry (Sahu et al., 2011). Most of the process use gas-liquid contactors in which the H2S is absorbed into a complexing reagent to give either another dissolved sulfide containing component and problems are the degradation of the solvent (Ebrahimi et al., 2003). Iron (Fe) is an excellent oxidizing agent to convert H2S to elemental sulfur (S). During the1960s, intensive researches commenced in England focused on increasing the solubility of elemental Fe3+ in aqueous solutions and they realized that Iron is an excellent oxidizing agent to convert H2S to elemental sulfur (S0) (Heguy and Nagl, 2003). However, there is always research for the development of effective and low-cost method.
In this study, the reactive absorption of hydrogen sulfide into aqueous ferric sulfate solution has been studied and design calculations for equipment have been done. An aqueous Fe2(SO4)3 solution used an absorbent in this process. H2S absorbed and oxidized to elemental sulfur, and at the same time, Fe3+ is reduced to Fe2+ according to Eq.1.
H2S + 2Fe3+â†' S0 + 2Fe2+ + 2H+ (1)
Elemental sulfur is removed from the solution by separator and the reactant Fe3+ is regenerated from Fe2+ by oxidation in reactor to the following reaction (Eq. 2):
Fe2+ + H+ + 0.25 O2 â†' Fe3+ + 0.5 H2O (2)
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Depending on the gas flow rate and the efficiency required several types of absorbers are suitable, such as jet scrubbers, bubble-cap towers or packed towers. The sulfur separator can include filter press, settler and sulfur melters, depending on quality of elemental sulfur required. Apart from sulfur, only H2O is generated in the process. Thus, this process does not need waste treatment facilities (Ebrahimi et al., 2003). A distinct advantage of this process is that the reaction of H2S with Fe2(SO4)3, is so rapid and complete that there remains no danger of discharging toxic waste gas. The process is schematically depicted in Fig. 1.
Description: Description: paper master fig
Fig. 1: Process scheme of process for H2S removal, 1- Absorber, 2- Oxidizer, 3- Solid-liquid separator.
2. PROCESS DESIGN CALCULATIONS
In this study it was assumed that a packed tower used as absorber, continuous filter used as solid-liquid separator to separate sulfur from outlet liquid, and a back mixing reactor used as oxidizer to regenerate Fe2+ to Fe3+. According to these assumptions and for known gas flow rate (100000 kg/day sour gas, contain4% v/v H2S), design calculations have been done. Calculations have been done for a packed tower with 2 inches raschig rings as packing and 0.1 molar Fe2(SO4)3 concentration. Onda's method used to calculate kG and kL, gas and liquid side mass transfer coefficient (Eqs. 3-5).
The height of liquid and gas-film transfer units (HG, HL) and overall gas-phase transfer unit are obtained from Eqs. 6-8.
And after calculations, tower's height (Z) calculated (Eq. 9).
Subsequently, Diameter and height of absorption tower are 3.6 ft and 12.5 ft and pressure drop is 0.35 in H2O/ft packing (Sinnott, 2005; Kolev, 2006; Richardson et al., 2003).
In this process assumed that Oxidizer is a back mixing reactor. According to this assume, calculations have been done for oxidizer. Fe3+ is regenerated from Fe2+ according to equation 2 and it is relatively slow reaction. The reaction is first order with respect to oxygen and is 0.536 with respect to Fe2+, as follow (Eq. 10):
Consequently, Oxidizer's height and diameter are 2.45 ft and 4.9 ft (Treybal, 1980; Levenspiel, 1999; Perry and Green, 1997).
3. RESULTS AND DISCUSSION
3.1. Variations of reactant concentration
As other researchers found before (Ebrahimi et al., 2003), this experimental results also showed that the absorption reaction of hydrogen sulfide by ferric sulfate solution appears irreversibly and is first order with regard to both reactants. Variations of concentration profile of the reactants calculated in recent studies. These profiles had shown in Fig.2 and Fig.3.
Fig.2: Fe3+ concentration profile.
Recent studies have resulted at low Fe2(SO4)3 concentrations (up to 0.3 M) the absorption rate of H2S increase with total Fe2(SO4)3 concentration. At higher Fe2(SO4)3 concentrations a decrease in the absorption rate was found. At higher concentration of Fe2(SO4)3 the ionic strength and viscosity of the solution increase remarkably resulting in a decrease of the solubility, diffusivity and liquid side mass transfer coefficient of H2S and hence absorption rate (Ebrahimi et al., 2003).
Fig.3: H2S concentration profile.
3.2. Influence of temperature and pressure on the process
It is important to have an exact control on effective parameters in reaction to have high efficiency and low cost and energy consumption. Therefore, some effective parameters in this process are considered. Results shown that high temperature and low pressure is appropriate condition for absorption reaction as shown in Fig. 4 and Fig. 5.
Fig.4: Influence of temperature on H2S concentration in outlet
Fig.5: Influence pressure on H2S concentration in outlet
The preferred treatment method for sulfur containing gas streams depends on the source of the gas. H2S removal from gas streams using aqueous ferric solution as absorbent has been studied theoretically and calculations for absorber and oxidizer have been done and their dimensions are obtained for a known sour gas flow rate. The absorption reaction is first ordered with respect to both H2S and ferric iron concentration and influence of temperature and pressure on H2S absorption show that in high pressure and low temperature the process should have higher efficiency and optimum concentration of ferric sulfate should be used in the process due to increasing the ionic strength and viscosity of the solution at higher concentration of Fe2(SO4)3, consequently, resulting in a decrease of the solubility, diffusivity and liquid side mass transfer coefficient of H2S and hence absorption rate.
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a Surface area of interface per unit volume of column, m2/m3
Surface tension, J/m2
Viscosity of liquid, Ns/m2
Viscosity of liquid, Ns/m2
Density of liquid, kg/m3
Density of gas, kg/m3
Liquid phase diffusivity, m2/s
Vapour phase diffusivity, m2/s
Liquid side mass transfer coefficient, m/s
Gas side mass transfer coefficient, kmol/m2.atm.s
Height of transfer unit-liquid film, m
Height of transfer unit-gas film, m
HOG Height of transfer unit-overall (gas concentrations), m
Overall liquid-phase transfer coefficient, m/s
Overall gas-phase transfer coefficient, s/m
Gm Molar rate of gas per unit cross-section, kmol/m2.s
Lm Molar rate of liquid per unit cross-section, kmol/m2.s
Liquid flow rate (mass) per unit cross-section, kg/m2.s
Gas flow rate (mass) per unit cross-section, kg/m2.s
dp Packing size, m
CT Total molar concentration, kmol/m3
R Universal gas constant, J/kmol.K
Z Height of packed column, m
g Acceleration due to gravity, m/s2
P Total pressure, N/m2
T Absolute temperature, K