Comparison Of Water Scrubbing And Propylene Carbonate

Published Date: 02 Nov 2017

Hong Nie,† Hao Jiang,† Daowen Chong,† Quangui Wu,‡ Chunming Xu,† Hongjun Zhou*,†

†New Energy Research Institute, State Key Laboratory of Heavy Oil Processing, China University of Petroleum (Beijing), Beijing, China 102249

‡Unconventional Oil and Gas Technology Corporation Limited, Beijing, China 102200

KEYWORDS

Biogas upgrading, water scrubbing, propylene carbonate, carbon dioxide, hydrogen sulfide

ABSTRACT

Biogas upgrading by physical absorbent is a simple and efficient technology with low energy requirements for regeneration. Owing to the good performance of carbon dioxide removal, propylene carbonate absorption is widely utilized in the purification of natural gas and synthesis gas. In this work, the feasibility of biogas upgrading with propylene carbonate as absorbent was studied by the comparative tests of water scrubbing and propylene carbonate absorption. The influence factors of absorption gas/liquid ratio, air stripping gas/liquid ratio and hydrogen sulfide content in the feed gas were investigated respectively. The capacity of biogas treated by propylene carbonate was 4–5 times of that by water. The propylene carbonate absorption showed better tolerance to the existence of hydrogen sulfide. When the concentration of hydrogen sulfide in feed gas increased to 4000 ppm, the methane content in product gas decreased by 5.09% with water as absorbent, while decreased only 1.68% in propylene carbonate absorption test. The results showed that the propylene carbonate absorption had high efficiency and obvious advantage of energy conservation when applied for biogas upgrading. The drawbacks of low methane recovery and high solvent loss should be avoided through flash operation and solvent recovery.

1. INTRODUCTION

Renewable energy sources are of great potential to replace fossil fuels and to mitigate climate change.1 Biogas is an important source of renewable methane.2 It derives from anaerobic digestion of biomass such as industrial organic wastewater, municipal wastewater, agricultural and forestry wastes, municipal solid wastes, livestock manure and so on.3-9 The composition of biogas is determined by the raw source and fermentation condition. Typically, biogas from anaerobic digestion is primarily composed of methane (CH4, 53–70 vol.%) and carbon dioxide (CO2, 30–47 vol.%) with trace compounds like hydrogen sulfide (H2S, 0–10000 ppm), ammonia (NH3, <100 ppm), water vapor (H2O), hydrogen (H2), nitrogen (N2), oxygen (O2), etc.10 Biogas can be used as a kind of fuel, while the presence of CO2 significantly reduces the calorific value and lowers the efficiency in the transportation as well as storage.11 After removal of CO2 and other impurities, the final product, biomethane, typically containing 97–99% CH4 and 1–3% CO2, can be used as an alternative for natural gas.9, 12-15

Many technologies have been taken into practice to separate CO2 from biogas. The operations mainly include absorption, adsorption, membrane separation and cryogenic separation.16 Absorption means a solvent is used to absorb CO2 from biogas with small loss of CH4 on the basis of physical effect or chemical action or both. Slight amount of H2S can also be removed at the same time, especially when the absorbent is organic. Water and propylene carbonate are two representative physical absorbents applied in CO2 removal process.

Water scrubbing is the most widely utilized biogas upgrading technology in European countries.17, 18 Furthermore, it is supposed to be one of the easiest and cheapest methods.19 The main process of water scrubbing is the raw biogas and water contacting in a countercurrent way, and CO2 as well as H2S dissolve in water at the same time. Water scrubbing can get high efficiency ( > 97% CH4) and less CH4 loss ( < 2%).15 But the drawbacks are microbial growth on the surface of packing and low flexibility toward variation of input gas.15, 20 In addition, it is more appropriate for the places with sufficient water.

Propylene carbonate absorption was first applied to remove CO2 and H2S from pressured natural gas. It is also called the Fluor solvent process, which is licensed by the Fluor Corporation of Los Angeles.21 Propylene carbonate, a kind of organic solvent, has the advantages of higher absorbing capacity for CO2, no corrosion to carbon steel, low vapor pressure and good chemical stability. Consequently, propylene carbonate absorption was successively applied in the process of oil recovery and purification of the synthesis gas in ammonia plants.22 Chemical additives, such as triethanolamine (TEA) and methyldiethanolamine (MDEA), were tried to add into propylene carbonate to improve the absorption capacity, absorption rate and purity of product gas.22, 23 The process of propylene carbonate absorption is similar to that of water scrubbing, and due to the higher absorbing capacity for CO2 and H2S, propylene carbonate absorption needs less investment and operation cost. Also, there will not be clogging or foaming caused by bacterial growth. And comparing to absorption with polyethylene glycol or chemical absorption with amines, propylene carbonate is easy to regenerate completely. But the application of propylene carbonate in the biogas decarburization has been rarely reported.24, 25

H2S is usually present in biogas, and its concentration frustrates widely according to the feedstock.13 It is a typical impurity which has to be removed in order to avoid damage to pipeline and motors. Although H2S is appreciably more soluble in water and propylene carbonate than CO2, it is typically removed in an early stage of the biogas upgrading process, especially when large amounts of H2S are present in biogas. One major reason is that when the solvent is regenerated by air stripping, element sulfur will be formed and it will block the column.15 Nevertheless, small quantities of H2S are removed simultaneously with CO2 in some water scrubbing plants which are designed primarily for CO2 absorption.21

Propylene carbonate is widely used for the decarburization of shift gas in many nitrogenous fertilizer plants in China. Expanding the propylene carbonate absorption to the field of biogas has great application prospect. The objective of this study was to detect the feasibility of biogas upgrading process with propylene carbonate as absorbent. Propylene carbonate absorption was compared with water scrubbing on the same experimental facility feeding with biogas from the fermentation of cassava waste. The influence factors of absorption gas/liquid ratio, air stripping gas/liquid ratio and hydrogen sulfide content in the feed gas were investigated respectively.

2. EXPERIMENTAL SECTION

2.1. Feed Gas and Absorbents. The experiments were carried out at Golden Yimeng Group Co. Ltd., Shandong Province, China. Raw biogas was from a digester where the feedstock was the waste of cassava after ethanol fermentation. The biogas, pretreated successively by filtration, compression, buffering and desulphurization, consisted of CH4 (54.63–58.35 vol.%), CO2 (41.65–45.37 vol.%) as well as trace quantity of O2 and N2. The concentration of H2S before desulphurization was 2000–7000 ppm, changing with the fluctuation of feeding or other conditions. After desulphurization, there was no H2S detected. When studying the influence of H2S on decarburization, biogas containing different concentrations of H2S was obtained by mixing desulfurized biogas and sulfur-containing biogas (biogas after buffer tank and before desulphurization column, as Figure 1 shows). According to the concentration of H2S detected in sulfur-containing biogas, a desired H2S content in the mixed gas was obtained by adjusting volume flow rate of the two gases. The mixture was treated as feed gas to study influence of H2S on CO2 removal. Water used in the study was tap water. Propylene carbonate was purchased from Shandong Senjie Chemical Co. Ltd., China, with a specified minimum purity of 99.64% and maximum moisture content of 0.04%.

2.2. Apparatus. The experimental apparatus is shown in Figure 1. The primary parts of the upgrading device are absorption column (36 mm i.d. and 1560 mm high), desorption column (44 mm i.d. and 1400 mm high) and absorbent container (15 l). The absorption and desorption columns were randomly packed with stainless steel Dixon Rings (3×3 mm), 1.3 m and 1.0 m high respectively. The flow rate of the feed gas (biogas) was adjusted by mass flow controller (Beijing Sevenstar Huachuang Electronics Co. Ltd., China). The flow rate of the absorbent was controlled by plunger pump (Pulsafeeder, American). Compressed biogas and absorbent were cooled down respectively before entering into the absorption column. After pretreatment, the biogas was supplied from the bottom of the absorption column; meanwhile, the lean liquid solvent was sprayed from the top. After absorption, a small fraction of product gas was sent into gas chromatography (GC) for composition analysis and the flow rate was measured by soap film flowmeter. The other part of product gas was passed through the buffer tank and the flow rate was measured by a wet type gas flowmeter. Absorbent carrying dissolved compounds was regenerated by air stripping at atmospheric pressure in the desorption column, where the air entered from the bottom and solvent sprayed from the top. Next, solvent after regeneration flowed into the absorption container and a new cycle began. Exhaust gas was released from the top of the desorption column.

Figure 1. Schematic of the experimental setup.

2.3. Experimental Procedure. The experiments can be divided into three parts: investigations on absorption gas/liquid ratio, air stripping gas/liquid ratio, as well as hydrogen sulfide content.

In the first part, for water scrubbing trials, single pass absorption was operated with fresh water to eliminate influence of incomplete desorption. When water scrubbing tests were performed, fresh water was added into the absorbent tank at intervals to ensure the liquid level was above one third to avoid air entering plunger pump. For propylene carbonate absorption, continuous cycle experiments were taken in order to reduce waste and discharge of organic solvent. Air stripping gas/liquid ratio was fixed to 10 to achieve complete desorption with little absorbent loss. At the beginning of each run, 12 l propylene carbonate was placed in the absorbent tank.

In the second part of the experiments, upgrading efficiencies were investigated at different air stripping gas/liquid ratios. When the air stripping gas/liquid ratio was 0, additional air did not pass through the desorption column. And the solvent was regenerated only by depressurizing and warming (room temperature was around 30 oC). When the air stripping gas/liquid ratio was more than 0, solvent was regenerated by depressurizing, warming and air stripping at the same time.

In regenerative water scrubbing tests, 12 l fresh water was added into the absorbent container at the beginning of each run. Continuous test for 8 hours was performed after the system reached steady state. So was the treatment for propylene carbonate absorption.

At last, influence of H2S on decarburization efficiency was investigated by feeding biogas containing different concentrations of H2S.

Flooding velocities were calculated according to Sherwood’s correlation diagram.26 For water scrubbing, the gas/liquid ratio in the absorber at which flooding occurs is 25. For propylene carbonate absorption, the ratio is 100. Experimental operating conditions are listed in table 1.

Table 1. Experimental operating conditions

Operating condition

Water scrubbing

Propylene carbonate absorption

Feed gas flow rate (ml/min)

1800–3000

5000–6500

Solvent circulation (ml/min)

200

100

Absorption pressure (kPa)

800

800

Absorption temperature (oC)

15

15

% CH4 in the feed gas

54.63–57.29

55.25–57.60

% CO2 in the feed gas

42.71–45.37

42.40–44.75

H2S in the feed gas (ppm)

0–4000

0–4000

Desorption pressure (MPa)

0.1

0.1

Desorption temperature

Ambient temperature

Ambient temperature

Air flow rate (l/min)

0–4

0–2

2.4. Analysis. The raw biogas, product gas and exhaust gas were analyzed by gas chromatography (GC) with a thermal conductivity detector (TCD) (model GC-2000III, Shanghai Institute of Computing Technology, China) using hydrogen (H2) as the carrier gas. CH4 and CO2 were separated by a TDX-01 packed column. O2 and N2 were separated by a molecular sieve 5A column. H2S was determined by gas detection tubes (range 2–20 ppm and 200–5000 ppm, Beijing Beike Lvzhou Safety & Environmental Technology Co., Ltd., China). The raw gas was analyzed at 8:00, 13:00 and 18:00 respectively every day. If the composition of product gas did not vary obviously (concentration of each gas varied less than 0.5% in one hour), this experimental condition was regarded stable. The product gas was analyzed every 20 minutes after the system operating steadily. Three sets of data were collected in an hour, and at least three sets of data were used to determine the uncertainty at each experimental condition.

2.5. Upgrading Parameters. Upgrading efficiency was studied at different absorption gas/liquid ratios, air stripping gas/liquid ratios and H2S contents. Absorption gas/liquid ratio is defined as the volume flow ratio of biogas to absorbent entering into the absorption column. The air stripping gas/liquid ratio is the volume flow ratio of air to solvent entering into the desorption column. The absorption temperature was 15 oC, determined on the basis of reference.20 The concentration of CH4 in product gas, recovery rate of CH4 and volume of CO2 dissolved in the absorbent were used as indicators of upgrading efficiency.

Recovery rate of CH4 is calculated by Eq.(1):

(1)

where is the recovery rate of CH4, is the flow rate of biogas (N ml/min), is the flow rate of product gas (N ml/min), while and denote CH4 concentration in biogas and product gas, respectively.

Volume of CO2 dissolved in absorbent is calculated by Eq.(2):

(2)

where is the volume of CO2 dissolved in unit capacity of absorbent (l CO2 per l absorbent), is the flow rate of absorbent (ml/min), and represent CO2 concentration in biogas and product gas, respectively.

3. RESULTS AND DISCUSSION

3.1. Effect of absorption gas/liquid ratio. Figure 2a shows that CH4 content in the product gas decreased obviously with the increase of gas/liquid ratio both in water scrubbing and propylene carbonate absorption tests. Higher gas feed rate will improve the interphase surface, which is helpful to gas-liquid mass transfer.27, 28 But it also indicates that more CO2 need to be treated. Because of the limit solvent flow rate, the absorbent will easily reach saturation, leaving more CO2 unabsorbed. Although the trend could be expected, propylene carbonate decarburization had a significant advantage concerning the processing capacity from the results here. For example, when the gas/liquid ratio was 11 in water scrubbing trials, CH4 content in the product gas was 93.66 ± 0.40 vol.%, whereas the ratio could get as high as 55 in propylene carbonate decarburization tests to obtain a similar CH4 purity. At a certain pressure and temperature condition, unit volume of the absorbent can absorb a certain quantity of CO2. Consequently, with the increase of the gas flow, the absorbent will gradually reach saturation and the amount of CO2 dissolved by unit volume of the solvent will achieve stability by degrees, as shown in Figure 2b.

Figure 2c depicts that in propylene carbonate absorption tests, the recovery rates of CH4 varied from 83.84% to 89.63%, all below 90%. There are two major reasons resulting in the great loss of CH4. One is the solvent nature. Under the same conditions (e.g. 25 oC and 101.3 kPa), the solubility of CH4 in propylene carbonate is 7 times of that in water.21 The other reason is there was no flash operation in this study due to the condition limit. Table 2 lists the concentrations of CH4 and CO2 in exhaust gas at different gas/liquid ratios in propylene carbonate absorption process. The concentration ratio of CO2 to CH4 in exhaust gas can reflect the selective absorbability of propylene carbonate indirectly. Considering the CH4 loss, flash gas should be sent back to compressors, so as to reduce the loss rate of CH4.

As is known that large gas/liquid is helpful to mass transfer, but too large absorption gas/liquid ratio is harmful to the gas-liquid countercurrent contact. For one thing, higher gas rate will cause higher pressure drop, even flooding.28 For another, absorbent will partly be taken away by fast-moving gas, reducing the actual solvent flow rate in the absorber. Since the solvent will easily reach saturation with the increasing absorption gas/liquid ratio, the liquid-phase driving force for mass transfer will be decreased. However, if the gas/liquid ratio is too small, though the desired degree of purification can be achieved, processing capacity of the device will be reduced and energy consumption per unit volume of product gas will be increased. Therefore, after comprehensive consideration, for the next water scrubbing and propylene carbonate absorption experiments, the absorption gas/liquid ratios were controlled at 13 and 55, respectively. Because these two ratios were the largest ones at which CH4 content in product gas could reach or near 90 vol.%. Meanwhile, they were the lowest ones at which the solvents got saturated, allowing to use all the absorbents’ loading capacity and reducing CH4 loss in the product gas as more as possible.

Figure 2. Concentration of CH4 in product gas, recovery rate of CH4, volume of CO2 dissolved in absorbent at different absorption gas/liquid ratios during tests of single pass water scrubbing and regenerative propylene carbonate absorption.

Table 2. Concentrations of CH4 and CO2 in exhaust gas at different absorption gas/liquid ratios in propylene carbonate absorption

Absorption gas/liquid ratio

Exhaust gas

CH4 (vol.%)

CO2 (vol.%)

50

3.75 ± 0.01

61.38 ± 0.15

55

3.96 ± 0.00

62.93 ± 0.10

60

3.61 ± 0.02

62.17 ± 0.50

65

3.69 ± 0.01

63.58 ± 0.23

3.2. Effect of air stripping gas/liquid ratio. Figure 3 illustrates the decarburization effect increased with the increase of air flow rate. In water scrubbing tests, when the absorbent was regenerated without air stripping (air stripping gas/liquid ratio was 0), the average concentration of CH4 in product gas was 77.93 ± 0.31 vol.%, whereas CH4 content reached 91.22 ± 0.29 vol.% when the ratio was 20. The volume of CO2 absorbed had the same trend. In propylene carbonate absorption trials, the variation tendency was similar to that of water scrubbing. But when the air stripping gas/liquid ratio was 0, due to the incomplete desorption, the decarburization effect was not satisfactory and it decreased gradually as time goes on. The complete desorption was closed to with the increasing air stripping gas/liquid ratio. For example, if the ratio raised from 10 to 20, the concentration of CH4 in product gas increased by 1.24 vol.% when water was used as absorbent, and only increased by 0.69 vol.% when propylene carbonate was used.

Figure 4 shows the average composition of product gas at different air stripping gas/liquid ratios in 8 hours. The composition of product gas when air stripping gas/liquid ratio was 0 in propylene carbonate absorption was not shown in Figure 4 for the large decline in the concentration of CH4. It can be seen that air stripping introduced more O2 and N2 into product gas. Not only will the air dissolved in the solvent desorb in the pressurized absorption process, but a small quantity of air mechanically entrained owing to the gas-liquid contact on packing surface in the desorption column is another source. Utilizing water as absorbent, the average total amount of O2 and N2 in product gas during 8 hours was 0.19 vol.% when the ratio was 0, whereas it reached 0.47 vol.% when the ratio was 20. But the total contents did not change obviously when the ratio increased from 10 to 20, indicating that in the desorption column, air dissolved in the water reached saturation and the amount of entrained air had a limitation. In general, air stripping contributed about half of the total amount of O2 and N2 in product gas when the air stripping gas/liquid ratio was more than 5. Utilizing propylene carbonate as absorbent, the total content of O2 and N2 in product gas also increased with the air flow and the total amount raised from 0.36 vol.% to 0.50 vol.%. Comfortingly, the concentrations of O2 in product gas were all below 0.5 vol.% both in water scrubbing and propylene carbonate absorption process, which can meet the O2 limit requirements in the vehicle fuel and grid injection standards 10.

Desorption is the inverse process of absorption, and the mass transfer is also completed by the contact of gas and liquid. Increasing the air stripping gas/liquid ratio is good for the solvent regeneration, but larger gas flow can also cause great amounts of solvent loss. Thus, considering upgrading efficiency, energy consumption as well as absorbent loss, the proper ratios for water scrubbing and propylene carbonate decarburization were respectively fixed at 15 and 10 for further trials.

Figure 3. Concentration of CH4 in product gas, volume of CO2 dissolved in absorbent at different air stripping gas/liquid ratios (ASGLR) during 8 hours utilizing water and propylene carbonate as absorbent, respectively.

Figure 4. Average composition of product gas at different air stripping gas/liquid ratios during 8 hours utilizing water and propylene carbonate as absorbent, respectively.

3.3. Effect of hydrogen sulfide. Table 3 indicates that water scrubbing process was influenced a lot by H2S. When the concentration of H2S reached 4000 ppm, in water scrubbing test, CH4 content in product gas decreased by 5.09 vol.%, whereas in propylene carbonate decarburization process, it reduced by 1.68 vol.%. Additionally, the recovery rate of CH4 and the volume of CO2 absorbed also fluctuated more by the influence of H2S when water was used as absorbent. In conclusion, propylene carbonate absorption showed higher tolerance and flexibility to the existence of H2S comparing with water scrubbing. Meanwhile, it presented obvious economic advantage for its good performance at a lower stripping gas/liquid ratio.

Decline of CH4 content in the product gas means more CO2 was unabsorbed. It is particularly recognizable when H2S was 4000 ppm in water scrubbing process as table 3 lists. There may be two reasons why the upgrading efficiency was reduced by the absence of H2S. One is the decline of mass transfer coefficient for CO2. The other is reduction of absorption capability for the solvent due to cycling operation.

Pohorecki and Możeński22 investigated the vapor-liquid equilibrium (VLE) of CO2 and H2S absorption in pure propylene carbonate. The results of their investigations show that the absorption rate of H2S does not influence that of CO2. Furthermore, simultaneous presence of CO2 and H2S does not affect solubilities of these gases in their absorption process. For water scrubbing, although both hydrosulphuric acid and carbonic acid are binary weak acids, pKa1 of H2S is markedly more than that of CO2.29-31 Thus, decarburization effect influenced by hydrogen ions ionized by hydrosulphuric acid can be ignored. Table 4 lists upgrading performance at different concentration of H2S in single pass water scrubbing. It is clear that H2S had no obvious impacts on decarburization effect. It can be seen that the efficiency decline with the appearance of H2S in regenerative experiments was caused by the cycling operation. Because H2S is a typical soluble gas in water, less H2S will desorb in the desorption column and more H2S will accumulate in the solvent. It will lead to pH reduction of the barren liquor, which is against to CO2 absorption. Owing to the higher solubility of H2S in propylene carbonate and nature of organic solvent, minor accumulation of H2S will not lead to obvious variation of decarburization effect.

Due to the high solubility of H2S in the two solvents, the concentrations of H2S in product gas were all bellow the detection limit of 2 ppm in all conditions studied above.

Table 3. Decarburization effect when different concentrations of H2S were contained in the feed gas during tests of water scrubbing and propylene carbonate absorption process

Water scrubbing a

Propylene carbonate absorption b

Concentration of H2S (ppm)

0

2000

4000

0

2000

4000

Concentration of CH4 in product gas (vol.%)

91.03 ± 0.26

88.19 ± 0.27

85.94 ± 0.10

93.13 ± 0.33

91.68 ± 0.43

91.45 ± 0.36

Recovery rate of CH4 (%)

95.21 ± 0.27

95.18 ± 0.29

96.95 ± 0.16

84.55 ± 0.30

84.33 ± 0.39

84.78 ± 0.34

Volume of CO2 absorbed

(l CO2 per l absorbent)

4.89 ± 0.02

4.98 ± 0.02

4.56 ± 0.01

22.25 ± 0.09

22.14 ± 0.12

22.29 ± 0.10

a The air stripping gas/liquid ratio in water scrubbing tests here was 15.

b The air stripping gas/liquid ratio in propylene carbonate absorption trials here was 10.

Table 4. Decarburization effect when different concentrations of H2S were contained in the feed gas in single pass water scrubbing process

Single pass water scrubbing a

Concentration of H2S (ppm)

0

1000

2000

3000

4000

Concentration of CH4 in product gas (vol.%)

89.96 ± 0.26

90.31 ± 0.43

90.16 ± 0.43

90.31 ± 0.42

90.35 ± 0.37

Volume of CO2 absorbed

(l CO2 per l absorbent)

5.04 ± 0.02

5.06 ± 0.04

5.17 ± 0.08

5.04 ± 0.07

4.98 ± 0.03

a Absorption gas/liquid ratio here was 13.

3.4. Comparison of the present experiments with other absorption tests. A general understanding of biogas flow rate, air stripping gas/liquid ratio and H2S content on biogas upgrading efficiency can be derived from the experiments above, utilizing water and propylene carbonate as absorbent separately. One of the objectives of this study was to explore the feasibility of propylene carbonate absorption for biogas upgrading. Thus, a further interest was felt to compare the performance of the current experiments with other absorption tests.

Tippayawong et al. 2 and Baciocchi et al. 32 performed similar experiments of CO2 absorption by alkaline solution in packed columns. Comparing the capacity data for CO2 in physical and chemical solvents, it is evident that chemical absorbents have obvious advantage in the capacity of absorbing CO2. That is why chemical scrubbing can be widely applied for CO2 removal. Although the dissolving capacity of CO2 in physical solvent is not very satisfactory, low operation and maintenance cost is enough to catch attention.17, 33 In addition, despite the larger concentration of CO2 (42.40–44.75 vol.%) in this study, the heat effect of absorption can be ignored utilizing physical absorbent, and it is beneficial for the low temperature operation. When concerning the processing ability and H2S absorption, propylene carbonate decarburization has more advantages over water scrubbing process. However, due to the lack of flash operation, the recovery rates of CH4 for propylene carbonate absorption were all below 90%. Besides that, the absence of solvent recovery results in great loss of this organic solvent and the loss reached 0.2087 l per Nm3 product gas.

4. CONCLUSION

In this paper, the feasibility of CO2 removal from biogas by propylene carbonate was studied through the comparative tests of water scrubbing and propylene carbonate absorption. The processing capacity of propylene carbonate absorption for biogas is considerable higher than that of water scrubbing process.

In this work, special attention was paid to the concentration of O2 in product gas and decarburization tolerance to the existence of H2S. With the air stripping operation, O2 content in product gas were all below 0.5 vol.% with the range of experimental parameters. The existence of H2S affected the decarburization efficiency. However, propylene carbonate absorption showed higher tolerance and flexibility to the existence of H2S comparing with water scrubbing. Although propylene carbonate absorption shows great advantages for biogas upgrading, there are two major drawbacks. First, the solvent suffered great loss without recovery. Second, the loss rate of CH4 was more than 10%. In the practical application of propylene carbonate decarburization, the recovery rate of CH4 can be improved by adding flash process, and the loss of propylene carbonate can be decreased by reasonable and effective recovery of solvent entrained by gas. In conclusion, propylene carbonate absorption has a great development prospect in the field of biogas purification. Also, since the processes and devices of propylene carbonate absorption and water scrubbing are the same, it is possible to switch between the two techniques to achieve good operational flexibility.

AUTHOR INFORMATION

Corresponding Author

* Telephone: +86-10-89731979. Fax: +86-10-89731979. E-mail: [email protected].

Notes

The authors declare no competing financial interest.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT

This study was supported by the National High Technology Research and Development Program (863 Program) of China (2012AA063507), National Technology Support Program (2011BAD15B02) and Science Foundation of China University of Petroleum, Beijing (YJRC-2011-09).