1. Absorption Processes to Remove Hazardous H

₂

S from Gas Streams

This technology uses both chemical and physical solvents and is one of the most preferred to remove H₂S from these gas streams. The use of chemical solvents mitigates the presence of H₂S in the corresponding phase. Among these chemicals, alkanolamine solutions know a wide usefulness because, together with toxic H₂S, the solvent removes CO₂ ^[1][6]. One step ahead the use of conventional solvents, ionic liquids are considered as their reliable alternatives. The well-known singularities or properties that these chemicals make are gaining positions on H₂S removal ^[2][7]. The use of mixtures (AAILs) of tertiary amines and ionic liquids (amino acid) to remove H₂S was investigated in ^[3][8]. Among these compounds, tetramethylammonium arginine and tetramethylammonium glycine, [N111][Arg] and [N1111][Gly], respectively, presented H₂S removal rates of 100%. The rate of gas removal is increased when transferring protons between the ionic liquid and the tertiary amine. Larger-scale applications of the method seemed to be unpractical due to the costs and complex synthesis of the ionic liquids. In term of costs, deep eutectic solvents (DESs) are an option to ionic liquids. Interaction between H₂S and DES (basic) does not imply chemical reaction. Functionalizing these deep eutectic solvents with some chemicals, including amines, and oxidizing reagents produced an increase in the absorptive and regenerative properties of the compounds ^[4][9]. The addition of polyethyleneimine to deep eutectic solvents was investigated ^[5][10], the mixture having 90% H₂S removal efficiencies after four consecutive absorption–regeneration cycles. Nanofluids also emerged as potential absorbents for H₂S. These absorbents are formed by dispersion of several inorganic compounds (SiO₂, Al₂O₃) in the form of nanoparticles or graphene oxide and carbon nanotubes into organic solvents (monoethanolamine, diethanolamine, etc.) ^[6][11]. Removal of H₂S via the use of Fe-Monoethanolamine-BmimCl solution with inert nanoparticles (SiO₂) presented good perspectives, though H₂S removal efficiency decreased after continuous use (cycles) ^[7][12]. Some of the difficulties presented by conventional solvents appeared to be resolved with the use of these ionic liquids and deep eutectic solvents; however, some of the properties of these compounds, i.e., high viscosity, can be considered a drawback since it reduces mass transfer coefficients and thus causes an increase in energy demand ^[8][18]. The use of mixtures of these compounds can improve the removal of the toxic gas; however, the greatest negative impact of these mixtures is the lack of maintaining the removal properties after continuous use ^[9][19]. A new perspective has been raised in terms of improving the regeneration step to avoid this being lost in the absorption properties. Various triethylenetetramine functionalized ionic liquids (TETAH-ILs) are mixed with ethylene glycol (EG) to investigate their performance on H₂S absorption ^[10][20]. The results showed that the mixture 10% [TETAH][BF4]-EG presented the best absorption capacities (1.128 mol H₂S/mol IL) at 30 °C and 100 mL/min. Removal of H₂S is attributed to the formation of H₂S-IL compounds via hydrogen bonds. A bubble absorption column is used to investigate hydrodynamics of CO₂ and H₂S removal from pure water and water containing nanofluids dispersed with neat and OH- and NH₂-functionalized multiwalled carbon nanotubes ^[11][21]. Sodium dodecyl sulfate is used as surfactant and stabilizer. Maximum CO₂ removal and H₂S removal are found to be 0.0038 mmol/m²·s and 0.056 mmol/m²·s using NH₂-MWCNTs/nanofluid, respectively. The improvement in the absorption properties of diethanolamine with respect to the removal of H₂S from sour industrial off gas was investigated in ^[12][22]. The influence of lean amine H₂S impurity (LAHI), lean amine temperature (LAT), and column pressure (CP) on H₂S removal was studied. The increase of LAHI and LAT is detrimental for H₂S removal from the gas stream and LAT, LAHI (83%) being the key factor on H₂S removal, whereas LAT (15%) and CP (2%) have a minor impact on this efficiency. The next investigation used machine-learning operations to study the solubility of H₂S in fifteen ionic liquids ^[13][23]. No less than six machine-learning operations were used, and the respective results were compared. The conclusions showed that the least-squares support vector machine predicted H₂S solubility into the ionic liquids well with R² (0.99798), RMSE (0.01079), MSE (0.00012), RRSE (6.35%), RAE (4.35%), MAE (0.0060), and AARD (4.03). It was found that H₂S solubility decreased with temperature and has a direct dependence with the pressure. Ionic liquids such as [OMIM][Tf2N] are the best choice for H₂S capture. The next investigation ^[14][24] explored various aspects of the processing and technologies used in acid gas removal (AGR). The work summarized processing by chemical absorption and mechanisms involved in the removal process; it also showed the main amine-based solvents currently used in such tasks. Absorption by physical methods is also discussed, summarizing pros and cons of the most used absorbents. Industrial applications of AGR processes were considered. The removal of H₂S from industrial gas streams using an iron/copper bimetallic catalytic oxidation desulfurization system was investigated in ^[15][25]. The absorbent is formed by adding N-methyl pyrrolidone (NMP) and CuCl₂ aqueous solution to an iron-based ionic liquid (Fe-IL). The acidity and viscosity of the system are greatly reduced by addition of NMP and water, improving gas–liquid mass transfer efficiency. The presence of the copper(II) salt increased the oxidative properties of the solution, allowing for the improvement of the Fe³⁺ catalytic influence on the oxidation of H₂S to the monomer form of sulfur.

2. Adsorption Processes to Remove Hazardous H

₂

S from Gas Streams

Various materials: metal oxides, zeolites, activated carbon, metal organic frameworks (MOFs), biochar, mesoporous silica, ash, and composite materials have been investigated in the removal of H₂S from gas streams. Selection of these materials is based in some of their properties, such as elevated uptake capacity, selectivity, and thermal and mechanical stability with respect to the removal of this toxic gas ^[16][26]. A mixture of biosolid (sewage) and surfactant (pluronic surfactant F127, heated at 950 °C) was used for the adsorption of H₂S in ^[17][27]. It was shown that the presence of the surfactant increased the mesopore volume and carbon content of the biosolid, whereas the treatment at 950 °C developed the micropores, leading to the dispersal of the catalytic sites (Ca and Fe oxides) responsible for the gas adsorption, and increased the nitrogen atoms of pyrrolitic nature. Besides adsorption, the catalytic sites provide an environment to promote the oxidation of H₂S to elemental sulfur. As a consequence, the breakthrough capacity is greatly improved (250% better than that presented by the biosolid alone); moreover, this capacity (221.2 mg/g) is better than those derived from other adsorbents such as STIX^® (North Tonawanda, NY, USA) (201 mg/g), S-208 (36 mg/g), or Centaur^® (Mumbai, India) (176 mg/g). Metal oxides presented have good properties for adsorbing H₂S; however, there is a continuous effort to improve their characteristics in this important field. Adsorption of H₂S by molybdenum(IV) oxide nanoparticles is described in the literature ^[18][28]. The best results are achieved at 0.081 and 0.074 g H₂S/g, at a temperature of 85 °C, pressure of 16 bar, and superficial velocity of 0.018 m/s. The above capacities are obtained when the adsorbent presents non-spherical and spherical shapes and using an initial H₂S concentration of 43 ppm in the feed gas stream. MnO₂ is the most effective adsorbent compared with other composite nano-sized metal oxides such as NiO/TiO_2, CoO/TiO₂, graphite oxide/ZnO, and CuO/TiO₂. Zeolites are other group of adsorbent materials of a wide use in several fields; some of the properties of these materials, i.e., elevated porosity and surface areas and porosities, are responsible for the wide use of these materials, including the removal of H₂S. Y and ZSM-5 zeolites’ properties (including surface and pore structure) have been modified by the use of magnetite nanoparticles ^[19][29], being the performance, with respect to the H₂S removal, of both adsorbents at the temperatures in the 100–300 °C range at a pilot scale. Activated carbons and biochars are other types of adsorbents with active adsorption sites. Jute thread waste (a cellulosic biomass) activated with KOH served to yield a N–S-rich nanoporous carbon ^[20][34]. The carbon has a pore volume of up to 1.50 cm³/g and a surface area of up to 2580 m²/g. Within this material, increasing the pressure increased the H₂S adsorption capacity by 1 bar (19.1 mmol/g), 10 bar (32.6 mmol/g), and 35 bar (45.0 mmol/g). Again, the presence of pyrrolic-N atoms in the material helps with H₂S uptake onto it, this being attributable to hydrogen bonds; at the same time, negatively charged sulfur atoms around the pyrrolic-N atoms are responsible for a physisorption process. This material presented an 84% of adsorption regeneration after five cycles. Using a peanut shell as precursor, impregnated with copper and activated by KOH, an activated carbon was formed and used to investigate the removal of H₂S ^[20][34]. The best sample of the activated carbon has 1523.2 m²/g of surface area, with 97.6 mg/g of gas uptake. The non-linear Langmuir isotherm model best fits the experimental results: where [H₂S]_e,a is the gas uptake at an elapsed time, [H₂S]_m,a is the maximum monolayer uptake capacity, K_L is Langmuir constant, which is related to the affinity, and [H₂S]_e is the gas uptake at the equilibrium. The kinetic displayed pseudo-first-order trend: In the above equation, [H₂S]_t is the uptake at an elapsed time, [H₂S]_e is the gas uptake at the equilibrium, and k₁ is the rate constant of the pseudo-first-order model. Both film diffusion and intraparticle models are responsible for H₂S uptake onto the adsorbent. After five cycles, the adsorbent maintains a gas removal rate of 90%. Biochar, biomass ashes, and sewage sludge (incinerated) are used to compare their performance on H₂S from a biogas ^[21][35]. Sewage sludge is discarded due to its surface characteristics and low porosity. Biochar exhibited the highest adsorption capacity of 171.8 mg/g, its low density translated into low volumetric adsorption capacity, limiting its scale-up. Biomass ashes have the highest volumetric adsorption uptakes, 22.2–38.3 mg/cm³ (35.6–78.2 mg/g), attributable to their porosity properties and the presence of mineral oxides responsible for the catalytic H₂S oxidation. Biochar has the highest adsorption uptake (171.8 mg/g), but its use at a greater scale is limited by its low density. It is known that activated carbons incorporating metal or metal oxides have enhanced their H₂S removal efficiencies ^[22][36]. Several investigations ^[23][24][37,38] have surveyed the life-cycle cost and the environmental footprints/impact of these hybrid materials. Biochars derived from banana peel, rice hull, and sawdust by pyrolysis are used as adsorbents of H₂S from municipal solid waste ^[25][39]. The order for removal efficiency of H₂S (>94%) was banana peel > saw dust > rice hull. The use of a goethite-based material to adsorb H₂S from a mimic biogas mixture (H₂S and N₂) was described in ^[26][40]. In the process, crystalline FeS was formed. With respect to pristine s-C3N6 (−0.33 eV to −0.45 eV), Fe-, Pt-, and Ti-modified s-C3N6 structures (−1.06 eV to −2.66 eV) presented larger adsorption energies for the removal of H₂S on their surfaces ^[27][41]. s-C3N6-Fe has semiconducting properties after adsorbing this harmful gas. Pristine s-C3N6 and its modified derivatives represented a series of smart adsorbents and sensing media for toxic gases. The defective GaSe monolayer material having selenium vacancies and being doped with oxygen and nitrogen is another material to be considered in the removal of toxic gases from gaseous streams ^[28][42]. After gas adsorption, the distance between the defective GaSe monolayer and four toxic molecules is reduced. An activated carbon-fiber mat containing both metal chlorides and metal oxides is used to investigate H₂S (and ammonia) removal ^[29][43]. The adsorbent, containing NiCl₂ and MgO, has a hierarchical porous structure, large surface area, and a good dispersion of bimetallic active sites. Metal oxides promote the chemical adsorption of H₂S. The use of these ternary hybrid materials increases gas uptake with breakthrough capacities up to 209.2 mg/g. Adsorbent materials from banana empty fruit bunch biochar (BEFBB) and banana peel biochar (BPB) wastes are used to remove H₂S from biogas ^[30][44]. Both types of adsorbents have good characteristics to remove H₂S (low performance in the case of CO₂ and methane), with breakthrough capacities of 7.65 mg/g and 5.85 mg/g, respectively. Pellet-sized adsorbent (0.5 cm) has a better gas removal efficiency than larger pellets (1.5 cm), this being attributable to its larger surface area. Both hydroxide and carboxylic groups present in the BEFBB material are responsible for the removal of H₂S from the gas stream.

3. Membranes and Membrane Contactors to Remove Hazardous H

₂

S from Gas Streams

The membranes as a separation technology are widely used in a variety of industries, and this wide usefulness is attributable to the properties or characteristics of the technology: modular aspects, easiness of operation, low environmental impact, etc., that in many aspects surpassed the offer presented by other separation technologies, this being specially noted when the operation, i.e., H₂S removal, is accomplished in areas or locations where communications are not easy ^[31][47]. There are some recent publications reviewing the use of these membrane technologies on the treatment of gases ^{[32][33][34][35]}[48,49,50,51], but surprisingly, they are dedicated to industrial gases instead of H₂S; some of the gases mentioned in these reviews are: nitrogen, hydrogen, oxygen, CO₂, etc. In the case of H₂S, polymeric membranes are the candidates to resolve this important industrial issue. The permeation of gas across these types of membranes is ruled by diffusion, and a series of investigations ^[36][37][52,53] have mentioned that plasticization of the membrane performs well in the removal of H₂S when the gas in the stream is present at high concentrations; moreover, this plasticization improves the selective separation of H₂S over methane. This selective separation is attributed to the transport mechanism, which in the case of H₂S is of sorptive in nature, and this separation is negatively influenced by diffusion. The transport ruled by solubility is particularly important in the case of rubbery polymers, and this is a key rule in the separation of H₂S from methane. It is reported ^[38][54] that the variation of the cross-linking density tuned the usefulness of polyethylene glycol-based membranes. This change in the membrane properties significantly improves the selectivity in the pairs H₂S/methane and CO₂/methane. Computational methods have developed a series of tools to improve the knowledge and optimize the use of these membrane technologies in the capture of gases ^[39][57]. Another implication of membrane technologies on the removal of gases is their use to act as an interface between the different streams, liquid and gases, feeding the removal process. These membranes, preferably with high porosity, increased the gas flow through the membrane structure. In practice, hollow fiber modules are the most used devices; under operation, the gas flowing in the shell side diffuses across the membrane’s pores to the tube side; here, the gas is conveniently absorbed by a solution, whereas these modules can be operated in co-current and counter-current form, and the latter is usually preferred since it gives a better contact between both operating phases. This counter-current configuration is shown in Figure 1. In these membrane modules, the gas flux is controlled by its concentration gradient ^[40][58]. Mass transfer in the module is conveniently represented in Figure 2. Some parameters to be controlled in these hollow fiber operations are pore size and its distribution ^[41][59], and these assured the best transfer across the module fibers. Some improvements in the technology are described in the literature ^[42][60].