**6. Conclusions**

146 Mass Transfer in Chemical Engineering Processes

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

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

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

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

*CO2* **(%)**

0 50 100 150

2nd pass Recovered

1st pass

*Qr*

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.

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

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

0 50 100 150

2nd pass Recovered 1st pass

*Qr*

Literature reports that amines can be regenerated 25 times before being degraded.

sequestrated and H2S recovered for industrial applications.

technical specification.

atmospheric pressure of 85 KPa).

volumetric concentration.

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

*H2S* **(%)**

**5. Economical evaluation** 

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

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 reported.

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

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that they depend on their concentration in water. They exhibit a minimum around 20% 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. 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.

Using this information, it was designed an absorbing gas-liquid column to reduce the H2S and CO2 content to 100 ppm and 10%, respectively, from ~60 m3/hr biogas streams, with negligible pressures drop. The manufactured column was tested with three different types of amines: MEA, DEA, and MDMEA. Results permitted to identify the ratio of amines to biogas flow (*Qr*=230) required to obtain the highest H2S and CO2 removing efficiencies ( 98% and 75% respectively) along with the highest mass transfer in the column (86%) when it is used MEA at 9%.

Then, an amine regenerative system was designed, manufactured and tested. Exhaust hot gases from the engine were used to heat the diluted amine up to 95ºC. Tests showed 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 efficiencies.

Finally, an economical analysis was performed assuming 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 found that under these circumstances, electric power generation from biogas has a cost of 0.024 USD/kW-h. This cost can be reduced up to 61% (0.015 USD/kW-h0 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.
