**4.1 Absorption column**

142 Mass Transfer in Chemical Engineering Processes

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:

0

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,

Figure 3 illustrates the general configuration of an amine based biogas scrubber. It consists of an absorption column, a desorption column and a water wash scrubber. Initially, raw biogas enters the absorption column where the amine solution removes H2S and CO2. Then, the biogas passes through the water wash scrubber where amines traces are removed and

*st c e o o i*

( )

*<sup>M</sup> <sup>A</sup> y y Q dt R Tm* (3)

,

*m* Mass of the absorbing substance within the bubbler *Q* Gas volumetric flow measured at standard conditions

**4. Amine based H2S and CO2 biogas scrubber** 

Fig. 3. Illustration of the amine based biogas H2S and CO2 scrubber.

Where:

*M* H2S or CO2 molecular weight *Ro* Universal gas constant *T* Absolute temperature

respectively, per Kg of MEA or DEA.

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 flows through the column.

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 characteristics of the absorption column.


(N/A Not applies)

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 characteristic specified in Table 4.

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 7.5% and several other levels of dilution.

Removal of H2S and CO2 from Biogas by Amine Absorption 145

value is acceptable. Higher mass transfer efficiencies can be obtained increasing the length of the column or using more appropriate filling materials. Table 5 summarizes the final

0

*ηCO2 (***%)** 20

40

*ηCO2 (***%)** 60

80

100

Amines desorb H2S and CO2 when they are heated up to 120oC at atmospheric pressure (Kolh & Nielsen, 1997). For the present application, this heat addition can be obtained in a counter flow heat exchanger between the amine and the engine exhaust gases. Alternatively, exhaust gases can be used to generate saturated steam and then heat the amines by direct mixing with this steam in a desorbing column. Attending literature recommendations on this matter the latest alternative was chosen (Kolh & Nielsen, 1997). A desorbing column was designed, manufactured and tested to regenerate amines solutions by mixing with steam. Figure 3 illustrates its operation. Preheated saturated amine solution fall down through the desorption column due to gravity while steam moves in counter-flow due to pressure difference. Under steady conditions the energy requirements for the

Fig. 5. H2S and CO2 removing efficiencies of the absorption column as function of

volumetric ratios of biogas to amine flows for the case of MEA.

24% MEA, 1st pass 24 %MEA, 3rd pass 24% MEA, 2nd pass

**4.2 Regenerative column** 

*m,CO2*=86% for *Qr*=230. For practical applications this

0 200 400 600 800

24% MEA, 1st pass 24 %MEA, 3rd pass 24% MEA, 2nd pass

0 200 400 600 800

*Qr*

*Qr*

24% MEA 35% MEA 9% MEA

*P* Pressure *T* Temperature

80

85

90

*ηH2S (***%)**

*ηH2S (***%)**

95

100

*<sup>a</sup>*Amine density

*Ri* Component i gas constant

Using this definition, it was found that

24% MEA 35% MEA 9% MEA

operational conditions of the absorption column.

0 200 400 600 800

0 200 400 600 800

*Qr*

*Qr*

Figure 4 shows that pressure drop along the column increases quadratically with the volumetric ratio biogas to amine solution (*Qr*). For a biogas volumetric flow of 7.6 m3/h, the pressure drop is about 3 inches of water column, which is acceptable for this application. This result implies that the final diameter of the column should be 18.8 cm to meet the condition of 60 m3/h of biogas flow.

Figure 5 shows the results obtained in terms of H2S and CO2 removing efficiencies (*H2S* and *CO2*) as function of *Qr*. It shows that the different types of amines produce similar results and that the column with all the amines is able to reach *H2S*>98% (final *YH2S*=100 ppm) for *Qr* ≤ 230 when *Ca*=9%. Under this circumstances *CO2* >75% (final *YCO2*<10%). Since MEA is the cheapest amine, it was selected as the working reagent for the absorption column.

Fig. 4. Pressure drop along the absorption column as function of *Qr*. Amine solution flow was kept constant at 26.5 L/h.

Removing efficiency is a metric to evaluate the performance of the column reaching the final specified concentration. It evaluates under which conditions of *Qr* and *Ca* the biogas exits with the final specified concentration. However it does not evaluate the performance of the column in terms of mass transfer. In other words, it does not evaluate the column length (*L*). Amine solution can leave the absorption column unsaturated, which is an undesirable condition since it will increase the total amount of amine required, and therefore the operational costs of the system. Figure 5 shows this effect as a high removing efficiency obtained when the amine solution is passed for a second time along the same column. To quantify this effect, here, it is proposed to define the mass transfer efficiency of the column for component i (*m,i*) as:

$$
\mathfrak{m}\_{m,i} = \frac{A\_{cr,i}}{A\_{c,i}} \tag{4}
$$

$$A\_{cr,i} = \frac{P}{R\_i T} \frac{Y\_{i,o} \eta\_i}{\rho\_a \ C\_a} Q\_r \tag{5}$$

Where:


*P* Pressure

144 Mass Transfer in Chemical Engineering Processes

Figure 4 shows that pressure drop along the column increases quadratically with the volumetric ratio biogas to amine solution (*Qr*). For a biogas volumetric flow of 7.6 m3/h, the pressure drop is about 3 inches of water column, which is acceptable for this application. This result implies that the final diameter of the column should be 18.8 cm to meet the

*CO2*) as function of *Qr*. It shows that the different types of amines produce similar results

<sup>2</sup> - 0.001*Qr* + 0.498

R² = 0.895

*H2S* and

*H2S*>98% (final *YH2S*=100 ppm) for

*CO2* >75% (final *YCO2*<10%). Since MEA is

Figure 5 shows the results obtained in terms of H2S and CO2 removing efficiencies (

the cheapest amine, it was selected as the working reagent for the absorption column.

Fig. 4. Pressure drop along the absorption column as function of *Qr*. Amine solution flow

*P* = 5E-05*Qr*

100 150 200 250

*Qr*

Removing efficiency is a metric to evaluate the performance of the column reaching the final specified concentration. It evaluates under which conditions of *Qr* and *Ca* the biogas exits with the final specified concentration. However it does not evaluate the performance of the column in terms of mass transfer. In other words, it does not evaluate the column length (*L*). Amine solution can leave the absorption column unsaturated, which is an undesirable condition since it will increase the total amount of amine required, and therefore the operational costs of the system. Figure 5 shows this effect as a high removing efficiency obtained when the amine solution is passed for a second time along the same column. To quantify this effect, here, it is proposed to define the mass transfer efficiency of the column

,

*<sup>A</sup>* (4)

(5)

*A*

, *cr i*

*c i*

,

*io i cr i r i aa P Y A Q RT C* 

,

*m i*

,

*Ac,i* Component *i* amine absorbing capacity as reported in section 3.

*Acr,i* Component *i* real absorbing capacity of the column

condition of 60 m3/h of biogas flow.

was kept constant at 26.5 L/h.

*m,i*) as:

for component i (

Where:

and that the column with all the amines is able to reach

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

*P* **[inches H O] 2**

*Qr* ≤ 230 when *Ca*=9%. Under this circumstances


Using this definition, it was found that *m,CO2*=86% for *Qr*=230. For practical applications this value is acceptable. Higher mass transfer efficiencies can be obtained increasing the length of the column or using more appropriate filling materials. Table 5 summarizes the final operational conditions of the absorption column.

Fig. 5. H2S and CO2 removing efficiencies of the absorption column as function of volumetric ratios of biogas to amine flows for the case of MEA.

### **4.2 Regenerative column**

Amines desorb H2S and CO2 when they are heated up to 120oC at atmospheric pressure (Kolh & Nielsen, 1997). For the present application, this heat addition can be obtained in a counter flow heat exchanger between the amine and the engine exhaust gases. Alternatively, exhaust gases can be used to generate saturated steam and then heat the amines by direct mixing with this steam in a desorbing column. Attending literature recommendations on this matter the latest alternative was chosen (Kolh & Nielsen, 1997).

A desorbing column was designed, manufactured and tested to regenerate amines solutions by mixing with steam. Figure 3 illustrates its operation. Preheated saturated amine solution fall down through the desorption column due to gravity while steam moves in counter-flow due to pressure difference. Under steady conditions the energy requirements for the

Removal of H2S and CO2 from Biogas by Amine Absorption 147

horizon time of 10 years and a scale of power generation of 1 kW in a typical farm in Mexico without any governmental subsidy or benefits from green bonuses. It was also assumed an annual interest rate of 5%. From the engine manufacturer experience it is known that oil change period is reduced from 1000 to 250 hr and that overhaul maintenance is reduced from 84000 hr to 24000 hr when using biogas without any treatment. Additionally it was considered in the analysis that power output increases ≈30% when using the amine treatment system. Under these circumstances it was found that electric power generation from biogas currently has a cost of 0.024 USD/kW-h and that this cost can be reduced up to 61% (0.015 USD/kW-h) when the amine based H2S and CO2 biogas scrubber is included.

Recently, a new approach for electric power generation has been emerging as a consequence of the need of replacing traditional hydrocarbon fuels by renewable energies. It consists of inter-connecting thousands of small and medium scale electric plants powered by renewable energy sources to the national or regional electric grid. In this case, typical small scale (0.1 to 1 MW) plants consisting of internal combustion engines coupled to electric generator and fueled by biogas become as one of the most attractive alternatives because of its very low cost, high benefit-cost ratio and very high positive impact on the environment. However, the use of biogas to generate electricity has been limited by its high content of H2S (1800-5000 ppm) and CO2 (~40%). The high content of H2S corrodes important components of the engine like the combustion chamber, bronze gears and the exhaust system. CO2 presence reduces the energy density of the fuel and therefore the power output of the system. Therefore there is a need for a system to reduce H2S and CO2 biogas content to less

To address this need, several existing alternatives to remove H2S and CO2 content from gaseous streams were compared in terms of their range of applicability, removing efficiency, pressure drop across the system, feasibility of reagent regeneration and availability of methods environmentally safe for final disposal of saturated reagents. It was found that the existing methods are appropriate for either small scale applications 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 required in the present case, are atypical. It was also found that the most appropriate methods for the present application are amines and iron oxides, which absorb both H2S and 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 efficiencies 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, only results for the case of amines were

To design the scrubbing system based on amines it is necessary to know its H2S and CO2 absorbing capacity. Since there is not reported data on this regard, it was proposed a method to measure it by means of a bubbler. It is an experimental setup where the gas stream passes through a fixed amount of the absorbing substance until it becomes saturated. Results showed that MEA and DEA exhibit similar H2S and CO2 absorbing capacities and

Then, it was found that the turnover of the initial investment is of about 1 year.

than 100 ppm and 10%, respectively, from 60 to 600 m3/hr biogas streams.

**6. Conclusions** 

reported.

desorption column are the heats of desorption, sensible and latent for the amine solution and for the steam. They are influenced by pressure and flow rates (Chakravarti et al, 2001).

For larger scale applications the CO2 and H2S -rich vapor stream that leaves the desorption column can be passed through a reflux condenser where H2O is partially condensed, CO2 sequestrated and H2S recovered for industrial applications.

On the other side, regenerated amine solutions should be cooled before reentering the absorption column because temperature reduces the amine absorbing capacity. For this purpose it is used a heat exchanger between regenerated amine and saturated amine coming out of the absorption column. The regenerative column was made of 2.5 inches stainless steel pipe to avoid corrosive problems. It was fully packed with stainless steel rashing rings to increase the contact area between the amine solution and the steam. Additionally it was thermally isolated with a heavy layer of fiberglass to avoid heat losses. Table 5 shows its technical specification.

It was instrumented with temperature and pressure sensors at the inlet, middle and outlet of the column. Amines solution flow rate was measured. Steam flow was adjusted to obtain maximum temperature. However, since the column is an open atmosphere system, the maximum temperature that can be reached is the water boiling temperature (98oC for atmospheric pressure of 85 KPa).

Fig. 6. H2S and CO2 removing efficiencies of the absorption column as function of volumetric ratios of biogas to amine flows for the case of regenerated MEA at 15% of volumetric concentration.

Fully saturated amines solutions were passed through the desorption column and collected at the bottom. Then they were cooled and used again in the absorption column under the same conditions as they were initially saturated (*Qr*=230). Figure 6 shows results obtained in terms of removing efficiency. It shows that the H2S removing efficiencies change from 98% to 95% when the amine is regenerated. Similarly, it changes from 87% to 50% for the case of CO2. Even though these results are encouraging, they are still partial results in the sense that further work is required to ensure maximum amines regeneration before evaluating its removing efficiency. Literature reports that amines can be regenerated 25 times before being degraded.

### **5. Economical evaluation**

An economical analysis was performed to evaluate the economical feasibility of implementing this type of amine based H2S and CO2 biogas scrubber. It was assumed a horizon time of 10 years and a scale of power generation of 1 kW in a typical farm in Mexico without any governmental subsidy or benefits from green bonuses. It was also assumed an annual interest rate of 5%. From the engine manufacturer experience it is known that oil change period is reduced from 1000 to 250 hr and that overhaul maintenance is reduced from 84000 hr to 24000 hr when using biogas without any treatment. Additionally it was considered in the analysis that power output increases ≈30% when using the amine treatment system. Under these circumstances it was found that electric power generation from biogas currently has a cost of 0.024 USD/kW-h and that this cost can be reduced up to 61% (0.015 USD/kW-h) when the amine based H2S and CO2 biogas scrubber is included. Then, it was found that the turnover of the initial investment is of about 1 year.
