**2.3 Selection**

138 Mass Transfer in Chemical Engineering Processes

 Amines: Monoethanolamine (MEA), Diethanolamine (DEA) and Methildiethanolamine (MDEA) are organic chemical compounds derived from ammonia as a result of the exchange of one hydrogen molecule by an alkyl radical (Kohl & Nielsen, 1997). The

 Redox process: Through this process H2S is physically absorbed in water and then, by the use of a chelating ferric solution, elemental sulphur is formed. After saturation, the reagent is regenerated in air (Horikawa & Rossi, 2004). It can be obtained more than

 Activated carbon: Activated carbon, also called activated charcoal or activated coal, is a form of carbon that has been processed to make it extremely porous and thus to have a very large surface area available for adsorption or chemical reactions (Horikawa &Rossi, 2004). It shows affinity to polar substances such as H2O, H2S, SO2 among many others. In the case of H2S, activated carbon absorbs and decomposes it to elemental sulphur (Garetto, 2000). It can be regenerated by temperature at around 400oC. The main disadvantage of this alternative is its affinity for no polar substances such as methane, which makes the alternative inappropriate in pre-combustion processes

 Zinc oxides: It is based on the reaction of a metal oxide with H2S to form the corresponding metal sulfide. Unlike iron oxides, zinc oxides treatment process is irreversible. Absorption reaction occurs at temperatures between 200ºC and 400ºC

 Iron oxides: It is based on the reaction of a ferric oxide and a triferric oxide with H2S to form iron sulfide, sulphur and water. The absorption reaction occurs at temperatures

 Sodium nitrite: It is based in the reaction of H2S with a solution of sodium nitrite. It produces a high percentage of H2S removal. Its main drawback is the environmentally

 Caustic wash: It is an effective method to remove H2S y CO2 from gas streams. Generally, it uses sodium hydroxide and calcium oxide (slaked lime) solutions to promote the chemical reactions showed in Table 3. Disposition of the saturated solutions should be performed according to environmental regulations (Zapata, 1998). Permanganate solutions: Potassium permanganate absorbs H2S according to the reaction shown in Table 3. It has high removal efficiency but it is costly and requires

 Water: It can be used to remove H2S y CO2 by physical adsorption. It is rarely used because water consumption is high and removal efficiency is low for large volumes of

between 30oC to 60oC (Svard, 2004; Steinfeld & Sanderson, 1998).

safe disposition of the saturated solution (Ramírez, 2007).

special treatment of the saturated solutions (Ramírez, 2007).

99% of H2S absorption. Its main advantage is that it uses low toxicity solutions. Ferric oxide: Absorbent material must contain iron in form of oxides, hydrate oxides or hydroxides (Muche & Zimmermann, 1985). Reagent regeneration occurs by exposition to open atmosphere It is one of the most used methods in biogas treatment. It is very efficient at low scale. However, in high and medium scale applications this method becomes inefficient due to the labor costs involved. Reagent disposal is a serious

chemical reactions involved in the absorption process of H2S are exothermic.

environmental issue (Ramírez, 2007).

(Ramírez, 2007).

(Mabres et al, 2008).

biogas (Kapdi et al, 2007).

**2.2.2 Non regenerative processes** 

To select a methodology for H2S and CO2 removal it should be taken into account (Treybal, 1996):


Table 2 and table 3 show that most of the existing methods for H2S and CO2 removal are appropriate for either small scale with low H2S and CO2 concentration or large scale with high pressure drops. Applications with intermediate volumetric flows, high H2S and CO2 content and minimum pressure drop, as in the present case, are atypical. Table 3 shows that for the case of H2S, in the present application, the most appropriate methods are amines and iron oxides, which also absorb CO2. Iron oxides are meant for small to medium scale applications while amines are meant for large scale applications. Amines have higher H2S and CO2 absorbing efficiency than iron oxides. Both methods have problems with disposition of saturated reagents. Even though amines are costly, they can be regenerated, and depending on the size of the application they could become economically more attractive than iron oxides. Both methods were selected for the present applications. However in this document, results only for the case of amines are reported.
