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

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 liquid and by the absorbing capacity of the amine.

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, 1996; Hvitved, 2002).

$$P\_A = \mathcal{y}\_A \cdot P = H\_A \cdot \mathfrak{x}\_A \tag{1}$$

Where:

*PA* Partial pressure of component A in gas phase *P* Total pressure

Removal of H2S and CO2 from Biogas by Amine Absorption 141

Temperature inside and outside of the bubbler 0.1oC 0-50 oC Volumetric gas flow 0.1 slpm 0-2 slpm Time 0.1 s N/A

gas-phase components by liquid phase absorbers in the bubbling method.

remove 100% of the CO2 biogas content in this first stage.

Table 4. Variables to be monitored during the determination of the absorbing capacity of

Figure 2 shows that any of the amines solutions can remove 100% of the H2S biogas content in the initial part of the test. However it is required at least 50% of amine concentration to

 Fig. 2. Evolution of the H2S and CO2 concentration during bubbling tests with MEA (left) and H2S and CO2 absorbing capacity of MEA and DEA as function of their concentration in

MEA

*Ac,CO2*

**(g CO2/kg amine)**

0 5 10 15 20 25 30

*Ca* **(%v)**

DEA

*Variable Resolution Range*

CO2 ±3% CH4 ±3% O2 ±1% H2S 35ppm CO2 0-100% CH4 0-100% O2 0-25% H2S 0-5000ppm

Where

(N/A Not applies)

water (right).

*yi* H2S molar concentration at the inlet *yo* H2S molar concentration at the outlet

Molar concentration at the inlet and outlet

*HA* Henry's constant of component A

*yA* Molar concentration of component A in gas phase

*xA* Mass concentration of component A in liquid phase

It is determined in a temperature and pressure controlled close box by measuring the equilibrium concentration of the component in both gas and liquid phase. Therefore, it requires spectrophotometric or chromatographic analysis to determine component concentration in the liquid phase (Wark, 2000). It has been observed that H2S concentrations in amines solutions are highly sensible to pressure and temperature, making spectrophotometric or chromatographic analysis hardly suitable for this application. For this reason literature does not report amines H2S and CO2 absorbing capacity.

As an alternative it was proposed to determine the H2S and CO2 absorbing capacity of the amines by using the gas bubbler setup illustrated in figure 1. This set up looks for a full interaction of the gas stream with the absorbing substance such that it can be assumed thermodynamic equilibrium at the liquid-gas inter phase. Experiments are conducted under standard conditions of pressure and temperature (101 kPa, 25oC). To ensure constant temperature for exothermic or endothermic reactions the set up is placed inside a controlled temperature water bath.

Temperature, pressure, gas flow and degree of water dilution of the absorbing substance are measured. The amount of solution in the bubbler is kept constant in 0.5 L. Table 4 describes the variables measured and their requirements in terms of resolution and range.

Fig. 1. Setup to determine the absorbing capacity of gas-phase components by liquid phase absorbers in the bubbling method.

Several tests were conducted to verify reproducibility of the method. Figure 2 shows the results obtained in terms of absorbing efficiency vs. time. Absorbing efficiency (*<sup>f</sup>*) is defined as:

$$
\pi \eta\_f = \frac{y\_i - y\_o}{y\_i} \tag{2}
$$

### Where

140 Mass Transfer in Chemical Engineering Processes

It is determined in a temperature and pressure controlled close box by measuring the equilibrium concentration of the component in both gas and liquid phase. Therefore, it requires spectrophotometric or chromatographic analysis to determine component concentration in the liquid phase (Wark, 2000). It has been observed that H2S concentrations in amines solutions are highly sensible to pressure and temperature, making spectrophotometric or chromatographic analysis hardly suitable for this application. For this

As an alternative it was proposed to determine the H2S and CO2 absorbing capacity of the amines by using the gas bubbler setup illustrated in figure 1. This set up looks for a full interaction of the gas stream with the absorbing substance such that it can be assumed thermodynamic equilibrium at the liquid-gas inter phase. Experiments are conducted under standard conditions of pressure and temperature (101 kPa, 25oC). To ensure constant temperature for exothermic or endothermic reactions the set up is placed inside a controlled

Temperature, pressure, gas flow and degree of water dilution of the absorbing substance are measured. The amount of solution in the bubbler is kept constant in 0.5 L. Table 4 describes

Fig. 1. Setup to determine the absorbing capacity of gas-phase components by liquid phase

Several tests were conducted to verify reproducibility of the method. Figure 2 shows the

*i o*

*i y y y*

(2)

*<sup>f</sup>*) is defined

results obtained in terms of absorbing efficiency vs. time. Absorbing efficiency (

*f*

the variables measured and their requirements in terms of resolution and range.

*HA* Henry's constant of component A

temperature water bath.

absorbers in the bubbling method.

as:

*yA* Molar concentration of component A in gas phase *xA* Mass concentration of component A in liquid phase

reason literature does not report amines H2S and CO2 absorbing capacity.



### (N/A Not applies)

Table 4. Variables to be monitored during the determination of the absorbing capacity of gas-phase components by liquid phase absorbers in the bubbling method.

Figure 2 shows that any of the amines solutions can remove 100% of the H2S biogas content in the initial part of the test. However it is required at least 50% of amine concentration to remove 100% of the CO2 biogas content in this first stage.

Fig. 2. Evolution of the H2S and CO2 concentration during bubbling tests with MEA (left) and H2S and CO2 absorbing capacity of MEA and DEA as function of their concentration in water (right).

Removal of H2S and CO2 from Biogas by Amine Absorption 143

the saturated amine passes through the desorption column where it is regenerated. A heat exchanger is used to cool the regenerated amine before it re-enters the absorption column.

A H2S and CO2 amine wash biogas scrubber was designed to meet the design parameters specified in section 1 (final H2S and CO2 concentration lower than 100 ppm and 10%, respectively, 60 m3/s of biogas flow and minimum pressure drop). It is a counter flow column where amine solution fall down due to gravity and raw biogas flows from the bottom towards the top of the column due to pressure difference. The column is fully packed with inert polyetilene jacks to enhance the contact area between the gas and liquid phases. In addition several disks are incorporated to ensure the uniform distribution of both

The length of the column is designed to obtain the specified final H2S and CO2 concentration and the diameter is designed to meet a minimum pressure drop with the specified gas flow. This procedure is well established and reported in references (Wiley, 2000; Wark, 2000). It requires as data input the results reported in section 3. Table 5 shows the technical

**Absorption Desorption**

**Parameter Column**

Material PVC SS Gas flow [m3/h] 7.6 8.25 Liquid flow [l/h] 33.3 69

Diámeter [cm] 6.7 6.7 Height [cm] 240 240 Pressuere drop [in.c.a] 0.28 0.2-3 Working reagent MEA at 10% H2O Qr 230 N/A H2S 98% N/A CO2 75% N/A YH2S start >5000 ppm N/A YH2S final <100 ppm N/A YCO2 start >40% N/A YCO2 final <10% N/A

Packing material Jacks SS rasching rings

Table 5. Technical characteristics of the columns used in the amine based biogas scrubber The absorption column was instrumented with temperature and pressure sensors at the inlet and outlet. Flow meters were used for both the biogas and the liquid phase absorbing substance. Biogas CH4, CO2, O2, and H2S concentration were measured at the inlet and outlet of the column by gas detector tubes and electro chemical cells with the technical

The absorption column was evaluated with MEA, DEA, and MDEA. Initially all amines were diluted at 30% (*Ca*=30%) in water as recommended by manufacturer (Romeo et al, 2006). However, later on, results from section 3 were incorporated and therefore it was used

**4.1 Absorption column** 

flows through the column.

(N/A Not applies)

characteristic specified in Table 4.

7.5% and several other levels of dilution.

characteristics of the absorption column.

Figure 2 also shows that absorbing efficiencies depend on the degree of saturation of the absorbing substance and on the ratio of the gas flow and the mass of absorbing substance in the bubbler. Additionally, this figure shows that the saturation profiles are similar and have an S type shape. The absorbing capacity under quasi-equilibrium conditions (*Ac,e*) is defined as:

$$A\_{c,e} = \frac{M}{R^o T} \int\_0^{t\_s} (y\_o - y\_i) Q \, dt \tag{3}$$

Where:

*M* H2S or CO2 molecular weight

*Ro* Universal gas constant

*T* Absolute temperature

*m* Mass of the absorbing substance within the bubbler

*Q* Gas volumetric flow measured at standard conditions

Figure 2 shows that MEA and DEA exhibit similar H2S and CO2 absorbing capacities and that they depend on their concentration in water. They exhibit a minimum around 20% and a maximum around 7.5% of volumetric concentration. These results indicate that scrubbing systems should work around 7.5% for applications where H2S removal is the main concern or higher than 50% where CO2 removal is the main objective. However at this high concentration it was observed that amines traces cause corrosion on metallic components, especially when they are made of bronze. Finally, figure 2 shows that on average at 7.5% of MEA or DEA concentration in water their absorbing capacity is of 5.37 and 410.1 g of H2S and CO2, respectively, per Kg of MEA or DEA.
