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Energy xxx (2010) 1e5

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Energy

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Biogas quality upgrade by simultaneous removal of CO2 and H2S in a packedcolumn reactor

N. Tippayawong*, P. ThanompongchartDepartment of Mechanical Engineering, Faculty of Engineering, Chiang Mai University, Chiang Mai 50200, Thailand

a r t i c l e i n f o

Article history:Received 13 October 2009Received in revised form5 April 2010Accepted 7 April 2010Available online xxx

Keywords:BiogasChemical absorptionFuel upgradeGas scrubbingRenewable energy

* Corresponding author.E-mail address: [email protected] (N. Tip

0360-5442/$ e see front matter � 2010 Elsevier Ltd.doi:10.1016/j.energy.2010.04.014

Please cite this article in press as: TippayawEnergy (2010), doi:10.1016/j.energy.2010.04.014

a b s t r a c t

Biogas from anaerobic digestion of biological wastes is a renewable energy resource. It has been used toprovide heat, shaft power and electricity. Typical biogas contains 50e65% methane (CH4), 30e45%carbon dioxide (CO2), moisture and traces of hydrogen sulphide (H2S). Presence of CO2 and H2S in biogasaffects engine performance adversely. Reducing CO2 and H2S content will significantly improve quality ofbiogas. In this work, a method for biogas scrubbing and CH4 enrichment is presented. Chemicalabsorption of CO2 and H2S by aqueous solutions in a packed column was experimentally investigated.The aqueous solutions employed were sodium hydroxide (NaOH), calcium hydroxide (Ca(OH)2) andmono-ethanolamine (MEA). Liquid solvents were circulated through the column, contacting the biogas incountercurrent flow. Absorption characteristics were examined. Test results revealed that the aqueoussolutions used were effective in reacting with CO2 in biogas (over 90% removal efficiency), creating CH4

enriched fuel. H2S was removed to below the detection limit. Absorption capability was transient innature. Saturation was reached in about 50 min for Ca(OH)2, and 100 min for NaOH and MEA, respec-tively. With regular replacement or regeneration of used solutions, upgraded biogas can be maintained.This technique proved to be promising in upgrading biogas quality.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Renewable energy deriving from biomass sources has greatpotential for growth to meet our future energy demands. Biogas isa very important source of renewable methane. It is produced fromanaerobic biodegradation of biomass in the absence of oxygen andthe presence of anaerobic microorganisms. Anaerobic digestion isthe consequence of a series of metabolic interactions amongvarious groups of microorganisms. The process is carried out indigesters that are maintained at temperatures ranging from 30 to65 �C. Biogas is rich in CH4 with typical range between 40 and 70%,and its lower heating value is between 15 and 30 MJ/Nm3. InThailand, biogas resources are from industrial wastewater and livestock manure, which have potential of 7800 and 13,000 TJ/year,respectively [1]. Conversion of chemical energy in biogas to heat orelectricity is possible via combustion. Apart from direct combustionin burners or boiler units, gas engines are usually employed asprime movers in utilization of biogas [2e4]. There is even greaterpotential for biogas if it can be made viable as a transport vehicle

payawong).

All rights reserved.

ong N, Thanompongchart P,

fuel. Biogas may be made transportable via pipelines, or by com-pressing in gas cylinders. This is possible only after removing CO2,H2S andwater vapour. In all cases, quality of biogas is crucial in bothits CH4 content and purity. CO2 is present in large quantities inbiogas and is inert in terms of combustion. Its presence thereforedecreases the energetic content of biogas. The CH4 content inbiogas will be increased if CO2 is removed. The most commoncontaminant in biogas is H2S and other S-containing compoundsthat come from S-bearing organic matters [5]. Depending on thecomposition of the organic matter, the H2S content in biogas canvary from 100 to 10,000 ppm. This contaminant is highly undesir-able in combustion systems due to its conversion to highly corro-sive and environmentally hazardous compounds. Its removal isessential before any eventual utilization of biogas.

Reducing CO2 and H2S content will significantly improve thequality of biogas. Various technologies have been developed forseparation of CO2 from gas streams in the past. These includeabsorption by chemical solvents, physical absorption, cryogenicseparation, membrane separation and CO2 fixation by biological orchemical methods [5e7]. There are also a number of techniques toremove H2S. Examples are chemical absorption in aqueous solu-tions, physical absorption on solid adsorbents and conversion tobase S or low solubility metal sulphides [8,9]. These techniques are

Biogas quality upgrade by simultaneous removal of CO2 and H2</ce:...,

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Fig. 1. Experimental apparatus.

Nomenclature

A fraction of specific gas absorbed or removed fromthe gas mixture after time t (e)

A0 fraction of specific gas absorbed or removed fromthe gas mixture after t¼ s (e)

C outlet CO2 concentration (% v/v)Ci inlet CO2 concentration (% v/v)k absorption constant (e)t absorption time (s)s characteristic absorption time (s)

N. Tippayawong, P. Thanompongchart / Energy xxx (2010) 1e52

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of significant industrial importance, generally applied duringnatural gas sweetening as well as in the removal of CO2 from fluegases of power plants. Apart from the use of gas permeationmembranes [10], applications of such technologies to biogas havebeen quite rare. To the authors’ knowledge, there were relativelyfew reports on biogas purification and upgrading, especially insmall-scale applications. The majority of previous studies werecarried out on water scrubbing systems [11,12], which is thesimplest and cheapest method involving the use of pressurisedwater as an absorbent. The process performance is dependent onfactors such as the dimensions of the scrubbing tower, gas pressure,composition of raw biogas, water flow rates and purity of waterused. It is possible to produce high quality CH4 enriched gas frombiogas by chemical absorption where a packed bed column andbubble column are normally used to provide liquid/gas contact [13].However, there remain several drawbacks of using liquid solutionsfor CO2 and H2S removal. These include high energy requirementfor regeneration, stability and selectivity of chemicals used, envi-ronmental impact from waste liquids, requirement for largeequipment sizing and high equipment corrosion rate. Even thoughthis conventional method is complex and expensive, it may beadopted and modified for employment in small-scale biogasproduction. There is a paucity of literature available on small scalesizing of biogas upgrading systems. This work is among the firstattempt to investigate the technical feasibility of this concept andits application to a small farm.

The objective of this work was therefore to determine technicalfeasibility of a chemical absorption system designed for small-scalebiogasproduction. In thiswork, absorptionbychemical solventswasused to capture CO2 and H2S in biogas. A laboratory scale, packedcolumn reactor was used to investigate absorption performance byNaOH, Ca(OH)2 and MEA solutions at different gas-to-liquid flowrates. The main objective was to evaluate chemical absorptiontechnology for biogas purification, and to generate data for scale-upcalculation and techno-economic analysis of the process.

2. Theory

Chemical absorption is an efficient technology for the removal ofCO2 and H2S from gas mixture. In an absorption column, thepollutant is transferred from the gas to the gas/liquid interface, andthen to the bulk of the liquid phase, where reactions take place.Alkaline and alkanolamines are among the popular reagents forpractical applications. In the case of CO2 absorption, the followingreactions take place [14]:

CO2þ 2OHe/ CO32eþH2O (1)

CO2þ CO32eþH2O/ 2HCO3

e (2)

CO2þ R-NH2þH2O/ R-NH3þþHCO3

e (3)

Please cite this article in press as: Tippayawong N, Thanompongchart P,Energy (2010), doi:10.1016/j.energy.2010.04.014

CO2þ RR0-NH/ RR0-NCOOeþHþ (4)

In the aqueous solution, the dissolved CO2 reacts with alkalineor amine following a complex reaction mechanism. This dependedstrongly on the pH, CO2 concentration and other factors. In thiswork, the temperature, pressure and ionic strength of solutionswere assumed to be constant. The packed column reactor wasconsidered as a continuously stirred tank reactor and the rate ofCO2 absorption can then be obtained through amaterial balance fora complete mixing reactor. Correlation may be developed forempirical modeling of absorption process. The fraction of CO2 in thegas mixture that is absorbed or removed by the solution in thepacked column at time t was denoted as;

A ¼ 1� CCi

(5)

where Ci and C are, respectively, the inlet CO2 concentration andCO2 concentration in the gas exiting the column after time t. Theremoval rate is a transient effect for this particular setup. Rate ofdeclining CO2 absorption was assumed to be proportional to theCO2 fraction that was absorbed in the packed column, and theCO2 fraction that passed through. It can then be expressedas [15];

dAdt

¼ �kA ð1� AÞ (6)

Integration of the above equation yields:

ln�AA0

ð1� A0Þð1� AÞ

�¼ kðs� tÞ (7)

After rearrangement, we get:

t ¼ 1kln�

CCi � C

�þ s (8)

where k is an absorption constant and s is the characteristicabsorption time when 50% absorption of CO2 occurs. This impliesthat the solutions in the packed column should be completelysaturated after elapsed time of 2s.


Table 1Compositions of biogas immediately after the treatment with different solvents.

Inlet NaOH Ca(OH)2 MEA

CH4 (%) 53.1 95.5 95.0 98.0CO2 (%) 46.8 3.2 4.0 1.3H2S (ppm) 2150 0 0 0

1.0

N. Tippayawong, P. Thanompongchart / Energy xxx (2010) 1e5 3

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3. Experimental

The semi-batch experimental apparatus is shown in Fig. 1, with(1) gas inlet measurement point, (2) flow meter, (3) flow distrib-utor, (4) packed column, (5) packing material, (6) outlet measure-ment point, (7) flare, (8) solvent container, (9) drainage, (10) pHmeter, (11) pump, (12) flow meter, (13) pressure gauge, (14)injector. The apparatus consists of a Pyrex glass cylinder of 70 mmin diameter and 1.0 m high. The absorption column was randomlypacked with a packing material (commonly known by the tradename “plastic bioball”, having an overall spherical shape, withuniform and structured spikes around the body and high surfacearea to volume ratio of 1895 m2/m3, as shown in Fig. 2) to a heightof 700 mm, to facilitate gas/liquid interaction. These bioballs arenormally used in anaerobic filter reactors for biogas production.The packed bed was found to have a void fraction of 0.916. NaOH,CaO and MEA were employed in the present investigation. Theywere obtained from Fisher Scientific. Biogas was obtained froma local chicken farm. At the beginning of a typical run, 10 l of theliquid solvent was placed in the vessel. The biogas purificationprocess took place in the packed column reactor where the gas wascontinuously fed from the bottom of the column, and the liquidsolvent was sprayed from the top, creating counter current flow.The solvent was circulated between the column and a liquid vesselby means of a peristaltic pump. The liquid flow rate was controlledto maintain a smooth liquid film. Gas flow rates were regulatedusing a gas flow meter. The pressure was slightly above atmo-spheric level. The CO2 and H2S concentrations in the biogasentering and exiting the column were constantly monitored by anIR gas analyzer. The pH of the liquid solvent was measured usinga pH meter. All measurements were repeated at least three times.

0 20 40 60 80 100 120 0.0

0.2

0.4

0.6

0.8

NaOH Ca(OH) 2 MEA

C

/ C

0

Time (min)

12.0

13.0

4. Results and discussion

Aqueous solutions of NaOH, Ca(OH)2, and MEA were used aschemical solvents to demonstrate the experimental apparatusfunction and ability to absorb CO2 and H2S. Table 1 showscomposition of biogas downstream of the experimental setup afterabout three minutes of operation. The packed column reactor wasobserved to simultaneously remove high proportion of CO2 and H2S(over 90% removal efficiency), resulting in CH4 enriched biogas.However, this was a time dependent process. These gaseousconcentrations were found to decreasewith time. This effect will bediscussed later in the section. Initially, the liquid solvents reactedrapidly with, and almost completely absorbed CO2 and H2S. Theirconcentrations at the outlet stream were practically very smallcompared to their original values. As the absorption process

Fig. 2. Plastic bioball as packing material.


proceeded with time, the CO2 and H2S were continuously accu-mulated in the solvents. Due to its much higher concentration incomparisonwith H2S, the CO2 started to evolve in the outlet streamafter a certain time. The end of each run was determined when theliquid solvent became completely saturated or neutralized (pH7e8). The corresponding breakthrough curves obtained are pre-sented in Fig. 3, showing variation of the dimensionless CO2concentration and changes in pH level with time. It was found thatNaOH, Ca(OH)2 and MEA showed similar patterns. NaOH and MEAwere found to become saturated in about 100 min. However, rate ofCa(OH)2 saturation appeared to be much faster than for the othertwo. This was observed after around 50 min, compared with about100 min observed for NaOH and MEA. It should be noted that MEApossessed lower basicity than NaOH and Ca(OH)2 but its rate ofchange in pH level was also slower. The relatively fast saturationtime realised in this investigation may be attributed to the fact thatbiogas used here contained very high concentration (47%) of CO2. Itshould be noted that each data presented was an average value.

0 20 40 60 80 100 120 6.0

7.0

8.0

9.0

10.0

11.0

NaOH Ca(OH) 2

MEA

H

p

Time (min)

Fig. 3. Variations of normalized outlet CO2 concentration and pH of the solutions withtime; solvent concentration of 0.1 M, gas to solvent flow ratio of 1.0, 47% inlet CO2

concentration and temperature of 30 �C.


-2.0 -1.0 0.0 1.0 2.0 3.00

20

40

60

80

100

120

NaOH Ca(OH)2

MEA NaOH Ca(OH)2

MEA

)nim( e

miT

ln[C/(Ci-C)]

Fig. 4. Kinetics of CO2 absorption by different solvents: solvent concentration of 0.1 M,gas to solvent flow ratio of 1.0, 47% inlet CO2 concentration and temperature of 30 �C.

Fig. 5. Total CO2 absorption and loading by different chemicals used.

Table 3Comparison of absorption performance reported in literature.

References Gas composition Aqueous solvents Capacity(mol/mol)

This work 47% CO2, 53% CH4 0.10 M NaOH 0.200.10 M Ca(OH)2 0.320.10 M MEA 0.25

N. Tippayawong, P. Thanompongchart / Energy xxx (2010) 1e54

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Their coefficient of variations, defined as a ratio between standarddeviation and mean value were found to be within about 10%.

Plots of absorption time against dimensionless biogas concen-tration for CO2 absorption by NaOH, Ca(OH)2 andMEA are shown inFig. 4. These curves may be approximated by straight lines, asshown by Eq. (6). These linear relationships were obtained withina different range of operation times for different solutions used. Theranges were 100, 50, and 110 min for NaOH, Ca(OH)2 and MEA,respectively. Table 2 lists model parameters for these aqueoussolutions, as well as correlation coefficient R2 of each linearapproximation. For NaOH and Ca(OH)2, R2 values of greater than0.96 were obtained, showing good linear relationships. However,an R2 value of only 0.84 was obtained for MEA. It appeared that theMEA curve was not exactly linear. A single linear relationship maynot be sufficient to describe the behaviour of MEA. This observationwas in line with the previous reported work [15] on absorptionwith primary and tertiary amines, where two line sections wereproposed. The reasons for the observed characteristics of MEA aswell as for the difference from NaOH and Ca(OH)2 are not yetknown. From the reaction kinetic parameters of NaOH and linearlyapproximated MEA, they were found to be similar. The saturationtime of the solutions in the packed column was predicted to be 2s.While saturation time for Ca(OH)2 observed from the experimentwas closely predicted by the theoretical model, significant theo-retical underestimations of saturation time were observed forNaOH and MEA.

With respect to absorption capacity of the solvents, Fig. 5depicts the total amount of CO2 absorbed by these solutions in kgCO2 per kg chemicals used. Error bars show standard deviations ofeach case. It was observed that the total amounts of CO2 absorbedwere in the range between 0.18 and 0.22 kg/kg, with highestloadings occurring for NaOH solution. The better loading for NaOHover MEA did not imply that NaOH was a better absorbent for CO2removal. Other properties such as cost, ability to regenerate,chemical stability, etc. must be taken into account. For a packed bed

Table 2Kinetic parameters for CO2 absorption by different solvents.

solvents k (min�1) s (min) R2

NaOH 23.74 42 0.962Ca(OH)2 13.00 29 0.975MEA 26.71 36 0.844


column reactor technology, comparison of CO2 absorption capacitywith data from various references are summarised in Table 3. It canbe seen that most studies were undertaken at relatively low CO2concentrations of about 15% or less, compared to that found intypical biogas (>40%). Various chemicals used include NaOH, NH3,MEA and MDEA (methyl-diethanolamine). Results obtained fromthis work appeared to be in similar range to those in the literature.With respect to MEA, our results were observed to be lower thanthose from Lin and Shyu [15] and Aroonwilas et al. [17] results. Thismay be attributable to the fact that they used higher MEAconcentrations, 0.1 M in our case compared to 1.8 M and 3 M for Linand Shyu [15] and Aroomwilas et al. [17] cases, respectively. Pub-lished data for NaOH and Ca(OH)2 was rather scarce. However, withregards to NaOH, we obtained higher absorption than that reportedby Georgiou et al. [14].

Attempts were made to generate high quality CH4 enriched gasfor a prolonged period. This was conducted by replacing used NaOHand Ca(OH)2 solutions with fresh ones at about 20% of total volumeto maintain basicity level above pH 11.9. However, preliminarytesting revealed that the addition had to be done in the first 15 min.And after 30 min, addition of fresh solutions was not able toproduce greater than 80% purity of CH4 enriched gas. Therefore,sole spent solutions had to be replaced in a 30-min interval. ForMEA, a similar procedure was also performed. Replacement ofa 20% volume fraction of usedMEA solutionwith clean solutionwascarried out at regular intervals of 15 min. This was done to mimicpartial regeneration of the spent MEA solution. Within the opera-tion time considered, the basicity level of recirculated MEA wasfound to reduce only slightly from its original value prior to whenthe first mixing commenced. Fig. 6 shows CH4 concentrationprofiles obtained, and comparison with the case where the absor-bents were employed until saturated. It was clear that all solutions

[14] 10% CO2, 90% N2 0.10e0.25 M NaOH 0.05e0.12[15] 5% CO2, 95% N2 10% wt MEA 0.44

10% wt MDEA 0.378% CO2, 92% N2 10% wt MEA 0.51

10% wt MDEA 0.4415% CO2, 85% N2 10% wt MEA 0.50

10% wt MDEA 0.46[16] 15% CO2, 85% N2 7e14% wt NH3 0.04e0.06[17] 15% CO2, 85% Air 3 M MEA 0.25e0.55


Fig. 6. Comparison of CH4 concentration profiles in the treated biogas (i) with and (ii)without fresh solvent addition.

N. Tippayawong, P. Thanompongchart / Energy xxx (2010) 1e5 5

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used may be operated in such a way that purity of greater than 80%CH4 enriched biogas could be obtained over a period of time. Interms of practical applications, these chemicals proved to bepotentially suitable absorbents. However, it should be pointed outthat although high concentrations of CH4 were obtained aftera short time (as shown in Table 1), these high concentrations werefound to decrease rapidly. To maintain high absorption rate,a substantial fraction of original volume of the solution needed tobe replaced or regenerated. Frequent replacement would lead toa fluctuation in concentrations, as observed in Fig. 6. One of thelimitations of biogas upgrading lies in economics of the bulkseparation of CO2. The main disadvantage of using alkaline solventswas that they are very difficult or impossible to regenerate forreuse. Even though they are relatively low cost materials, largeamount of chemicals would be required to satisfy purity require-ments of CH4 enriched gas. Use of MEA has the advantage of beingable to be regenerated. However, for a small scale biogas productionas considered in this study, initial investment cost for constructingthe set up, as well as energy requirement may be excessive andoutweigh the benefit of using high purity biogas. Further work maythus be required.

5. Conclusions

Removal of CO2 and H2S from biogas by aqueous solutions ina packed column was investigated. NaOH, Ca(OH)2 and MEA wereemployed in the present study and their absorption characteristicswere examined. A simple empirical model was also adopted for CO2absorption rate prediction. Test results indicated similar absorptionpatterns between the chemical solvents used. Chemical absorptionby solvents in a packed column was an effective technique forremoving CO2 and H2S over a short operation time, but theirabsorption capability declined rapidly with time. Ca(OH)2 appearedto become saturated more rapidly than the other solvents. The CO2loading ranged between 0.18 and 0.22 kg CO2 per kg chemicals


used. Chemical absorption by alkali aqueous solutions did notappear to be promising for biogas quality upgrade due to the non-regenerable nature, requirement of large liquid solvents volumeand the environmental impact. However, amine solutions may beworth exploring further, due to the regeneration capability. Addi-tional experimental results are required to provide the basis forsystem selection and design, as well as subsequent up-scaling. Thenext phase of experimental work will involve setting up a regen-eration system for amine absorbent, evaluating its long termperformance, and undertaking cost analysis of the system, as wellas a possible scale up study and field test in a biogas farm.

Acknowledgements

Support from the Commission on Higher Education, Ministry ofEducation, and the Fund for Energy Conservation Promotion,Energy Policy and Planning Office, Ministry of Energy are highlyappreciated. The authors wish to thank Drs P. Polchan and M. O. T.Cole for their valuable technical discussions, and staff at the EnergyResearch and Development Institute, Chiang Mai University fortechnical and laboratory assistance.

References

[1] Prasertsan S, Sajjakulnukit B. Biomass and biogas energy in Thailand: poten-tial, opportunity and barriers. Renewable Energy 2006;31:599e610.

[2] Borjesson P, Mattiasson B. Biogas as a resource-efficient vehicle fuel. TrendsBiotechnol 2007;26:7e13.

[3] Tippayawong N, Promwungkwa A, Rerkkriangkrai P. Long-term operation ofa small biogas/diesel dual-fuel engine for on-farm electricity generation.Biosyst Eng 2007;98:26e32.

[4] Tippayawong N, Promwungkwa A, Rerkkriangkrai P. Durability of a smallagricultural engine on biogas/diesel dual fuel operation. Iran J Sci TechnolTrans B Eng 2010;34(B2):167e77.

[5] Abatzoglou N, Boivin S. A review of biogas purification processes. BiofuelsBioprod Biorefin 2009;3:42e71.

[6] Yang H, Xu Z, Fan M, Gupta R, Slimane RB, Bland AE, et al. Progress in carbondioxide separation and capture: a review. J Environ Sci 2008;20:14e27.

[7] Granite EJ, O’Brien T. Review of novel methods for carbon dioxide separationfrom flue and fuel gases. Fuel Process Technol 2005;86:1423e34.

[8] Horikawa MS, Rossi F, Gimenes ML, Costa CMM, da Silva MJC. Chemicalabsorption of H2S for biogas purification. Brazil J Chem Eng 2004;21:415e22.

[9] Osorio J, Torres JC. Biogas purification from anaerobic digestion in a waste-water treatment plant for biofuel production. Renewable Energy2009;34:2164e71.

[10] Favre E, Bounaceur R, Roizard D. Biogas, membranes, and carbon dioxidecapture. J Membr Sci 2009;328:11e4.

[11] Kapdi SS, Vijay VK, Rajesh SK, Prasad R. Biogas scrubbing, compression andstorage: perspective and prospectus in Indian context. Renewable Energy2005;30:1195e202.

[12] Rasi S, Lantela J, Veijanen A, Rintala J. Landfill gas upgrading with counter-current water wash. Waste Manage 2008;28:1528e34.

[13] Krumdieck S, Wallace J, Curnow O. Compact low energy CO2 managementusing amine solution in a packed bubble column. Chem Eng J 2008;135:3e9.

[14] Georgiou D, Petrolekas PD, Hatzixanthis S, Aivasidis A. Absorption of carbondioxide by raw and treated dye-bath effluents. J Hazardous Mater2007;144:369e76.

[15] Lin SH, Shyu CT. Performance characteristics and modeling of carbon dioxideabsorption by amines in a packed column. Waste Manage 1999;19:255e62.

[16] Yeh JT, Resnik KP, Rygle K, Pennline HW. Semi-batch absorption and regen-eration studies for CO2 capture by aqueous ammonia. Fuel Process Technol2005;86:1533e46.

[17] Aroonwilas A, Tontiwachwuthikul P, Chakma A. Effects of operating anddesign parameters on CO2 absorption in columns with structured packings.Separ Purif Technol 2001;24:403e11.


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