**2.1.4 Membranes**

A membrane is a layer of material which serves as a selective barrier between two phases and remains impermeable to specific particles, molecules, or substances when exposed to the action of a driving force. The driving force is the pressure difference between both sides of the membrane. Gas permeability through a membrane is a function of the solubility and diffusivity of the gas into the material of the membrane. Membranes are expensive and their separation efficiencies are low (Ramírez, 2007).

### **2.2 H2S removal from gas streams**

Table 3 compares the different alternatives reported for H2S removal from gas streams (Walsh et al, 1988).

### **2.2.1 Regenerative processes**

It refers to processes where the cleaning reagent, once it becomes saturated, regains its removal capacity through a change in the external conditions.

Removal of H2S and CO2 from Biogas by Amine Absorption 137

Table 3. Alternatives for H2S removal from gas streams (EPRI, 1992; Freira, 2000;

Ryckebosch et al, 2011).



Table 3. Alternatives for H2S removal from gas streams (EPRI, 1992; Freira, 2000; Ryckebosch et al, 2011).

**Method Option/Alternative Advantages Disadvantages** 

High efficiency ( >97% CH4), Simultaneous removal of H2S when H2S < 300 cm3 /m3, Capacity is adjustable by

High efficiency ( >97% CH4), Simultaneous removal of organic S components, H2S, NH3, HCN and H2O, Energetic more favorable than water, Regenerative, low CH4 losses

impurities

changing pressure or temperature, Low CH4 losses (<2%), tolerant to

High efficiency (>99% CH4), cheap operation, Regenerative, More CO2 dissolved per unit of volume (compared to water), very low

Highly efficient (95-98% CH4), H2S is removed, low energy use: high pressure, compact technique, also for small capacities, tolerant to

H2S and H2O are removed, simple construction, Simple operation, high reliability, small gas flows treated without proportional

 Gas/gas: removal efficiency: <92% CH4 (1 step) or > 96% CH4, H2O is removed Gas/liquid: Removal

> efficiency: > 96% CH4, cheap investment and operation, Pure CO2 can be obtained

90-98% CH4 can be reached, CO2 and CH4 in high purity, low extra energy cost to reach liquid biomethane (LBM)

Removal of H2S and CO2, enrichment of CH4, no unwanted

end products

Table 2. Alternatives to remove CO2 from gas streams (Ryckebosch et al, 2011).

CH4 losses (<0.1%)

impurities

increase of costs

Expensive investment and operation, clogging due to bacterial growth, possible foaming, low flexibility toward

Expensive investment and operation, difficult operation, Incomplete regeneration when stripping/vacuum (boiling required), reduced operation when dilution of glycol with

Expensive investment, heat required for regeneration, corrosion, decomposition and poisoning of the amines by O2

Precipitation of salts, possible

Expensive investment and operation, extensive process control needed, CH4 losses when malfunctioning of valves

Low membrane selectivity: compromise between purity of CH4 and amount of upgraded biogas, multiple steps required (modular system) to reach high

Expensive investment and operation. CO2 can remain in

Addition of H2, experimental -

purity, CH4 losses.

the CH4

not at large scale

or other chemicals

variation of input gas

water

foaming

Absorption with water

Absorption with polyethylene glycol

Chemical absorption with amines

Carbon molecular

Zeolites Molecular

Alumina silicates

sieves

sieves

Gas/gas Gas/liquid

PSA/VSA

Membrane technology

Cryogenic separation

Biological removal

Removal of H2S and CO2 from Biogas by Amine Absorption 139

It uses microorganisms under controlled ambient conditions (humidity, oxygen presence, H2S presence and liquid bacteria carrier) (Fernández & Montalvo, 1998). Microorganisms are highly sensitivity to changes in pressure, temperature, PH and certain compounds. It

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

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.

Several works have been developed to model mass transfer in gas-liquid chemical absorbing systems and especially for simultaneous amine H2S and CO2 absorption (Little et al, 1991; Mackowiak et al, 2009; Hoffmann et al, 2007). It has been concluded that the reaction of H2S with amines is essentially instantaneous, and that of CO2 with amine is slow relatively (Qian et al, 2010). Therefore, for amine H2S and CO2 absorption in packed columns mass transfer is not limited by chemical reaction but by the mechanical diffusion or mixing of the gas with

The Henry's constant defines the capacity of a solvent to absorb physically gas phase components. Under these circumstances of instantaneous reaction it can be extended to chemical absorption. The Henry´s law states than under equilibrium conditions (Treybal,

*P y PH x <sup>A</sup> A AA* (1)

 The amount of H2S and CO2 to be removed and their desired final concentrations Availability of environmentally safe disposal methods for the saturated reagents

Requirements regarding the recovery of valuable components such as S

However in this document, results only for the case of amines are reported.

**3. Determination of the amines H2S and CO2 absorbing capacity** 

the liquid and by the absorbing capacity of the amine.

*PA* Partial pressure of component A in gas phase

1996; Hvitved, 2002).

*P* Total pressure

Where:

**2.2.3 Biological methods** 

**2.3 Selection** 

1996):

Cost

requires moderate investments.

The volumetric flow of biogas

