ARRHENIUS EQUATION CONSTANTS AND THERMODYNAMIC ANALYSIS OF CH4 AND H2S PRODUCTION FOR THE VINASSES ANAEROBIC TREATMENT
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ARRHENIUS EQUATION CONSTANTS AND THERMODYNAMIC ANALYSIS OF CH4 AND H2S PRODUCTION FOR THE VINASSES ANAEROBIC TREATMENT Alejandra Castro-González* She obtained the degree Pharmaceutical Biological Chemist at the Facultad de Ciencias Químicas, Universidad Veracruzana in Orizaba, Veracruz, Mexico City, March 1995; the Master of Science in Chemical Engineering (Environmenttal Protection) degree was obtained at the Instituto Tecnológico de Orizaba, in Orizaba, Veracruz, Mexico, May 1997. She is presently finishing the doctoral studies in Chemical Sciences (Chemical Engineering) to obtain the degree in the National Autonomous University of Mexico in 2002. Program for Environmental Chemical Engineering and Chemistry, PECEC (Programa de Ingeniería Química Ambiental y de Química Ambiental), Facultad de Química, UNAM. Paseo de la Investigación Científica s/n, Lab. E-301. Ciudad Universitaria, 04510 México D.F. México. Tels. (+52) 55 5622-5300 to 02 and 04, Fax (+52) 55 5622-5303. alcastro@servidor.unam.mx María del Carmen Durán-Domínguez-de-Bazúa Program for Environmental Chemical Engineering and Chemistry, PECEC (Programa de Ingeniería Química Ambiental y de Química Ambiental), Facultad de Química, UNAM. Paseo de la Investigación Científica s/n, Lab. E-301. Ciudad Universitaria, 04510 México D.F. México. Tels. (+52) 55 5622-5300 to 02 and 04, Fax (+52) 55 5622-5303. mcduran@servidor.unam.mx ABSTRACT Temperature effects on the methanogenesis and sulfatoreduction was investigated in up-flow anaerobic sludge blanket reactors (UASB) at 35, 45, and 55ºC, adapted to sugarcane alcohol producing wastewaters with an initial organic load measured as soluble chemical oxygen demand (CODs) of 120,000 mg/L. The work was divided in three steps: 1) Adaptation of the anaerobic biomass to vinasses on mesophilic conditions (35ºC); 2) Adaptation of anaerobic biomass from mesophilic to intermediate mesophilic and thermophylic conditions (45 and 55ºC, respectively); and 3) Arrhenius equation application on the methanogenesis and sulfatoreduction processes of biomass adapted to vinasses on intermittent tests. The three reactors have a useful volume of 2.6 L with 17 days of hydraulic residence time. When reactors pseudo steady state conditions were reached, CODs percent removal and overall CODs removal were 62, 71, and 78%, and 0.0065, 0.0075, and 0.0082 kgCODsremoval/day at 35, 45, and 55ºC, respectively. The productivity in the gas phase with respect to methane was 0.23, 0.37, and 0.47 m3CH4(NPT)/kgCODsremoval at 35, 45, and 55ºC, respectively. The gas composition (gas-liquid equilibria) was 0.066, 0.12, and 0.165 molCH4/day, and 0.0099, 0.0100, 0.0107 mol H2S/day, at 35, 45, and 55ºC. Temperature has a positive effect on the generation of methane for the methanogenic bacteria whereas H2S generation remains constant indicating that sulfatoreducing bacteria activity is neither increased nor diminished due to temperature effects. The activation energy for the methanogenesis was 62.86 kJ/kgmol, and for sulfatoreduction, 93.09 kJ/kgmol. Key words: Vinasses, temperature, methane, H2S, anaerobic. INTRODUCTION In Mexico, each liter of the alcohol produced generates 12 liters of vinasses. A rough estimate the alcohol production is of 70,000 m3/year generating 840,000 m3/year of vinasses (Jiménez et al., 1995). An associated problem of these liquid effluents is the sulfate content due to the addition of sulfuric acid to the yeasts culture
media. The anaerobic degradation of the organics present in vinasses is not efficient enough. Some authors think that one of the causes is the sulfate ion presence that promotes the generation of H2S present in both liquid and gas phases. Serious technical problems arise with the H2S presence in the system, like pipelines corrosion as well as motors impeller damages, and other corrosion problems, as well as obnoxious odors, and the inhibition of anaerobic bacteria including methanogenic, acetogenic, and sulfatoreducing bacteria, affecting the process efficiency. Although the research has been intensive on the last decades, there are still not sufficient data to formulate practical models to reduce the apparent competition between methanogenic and sulfatoreducing bacteria (Lens and Hulshoff Pol, 2000). The sulfatoreducing species activity range and its capacity as competitors for the carbon sources could be limited by temperature increases. In contrast, several methanogenic bacteria are active to higher temperatures. Thus, the hypothesis of this research is that temperature increase has a positive effect on the generation of methane by methanogenic bacteria and that there is an inhibition of the sulfatoreducing bacteria proliferation. To prove it the following experimental work was conducted. MATERIALS AND METHODS The adaptation of the anaerobic biomass coming from a beer factory to vinasses at mesophilic conditions (35ºC) was performed in a 10 Liter up flow anaerobic sludge blanket (UASB) reactor that, after 600 days of operation, became stable (Castro-González and Durán-de-Bazúa, 2001a). Vinasses came from an ethyl alcohol producing plant with an average concentration of dissolved organic and inorganic compounds measured as chemical oxygen demand of 120,000 mgCODs/L. With the adapted biomass, three 2.6 Liter UASB reactors were inoculated. Each anaerobic reactor was set to a different temperature (35, 45, and 55ºC), to have now the biomass adapted to these temperatures (Castro-González and Durán-de-Bazúa, 2001b). Experiments were then carried out in 30 mL vials inoculated with the temperature adapted biomass to obtain the constants of the Arrhenius equation (activation energy and rate constant). A schematic diagram of the vials is shown in Figure 1. Figure 1: Schematic diagrams of the vials for the determination of the Arrhenius equation constants
Figure 2 shows the experimental design including the proposed degradation routes considered in this study
and the chemical species evaluated (APHA, 1985) techniques. The CODs was evaluated using the APHA
technique adapted by Oaxaca-Grande (1997). Biogas analyses were carried out in a Perkin Elmer Autosytem
gas chromatograph (GC), using thermal conductivity and flame ionization detectors, designing a complete
methodology to separate the peaks for CH4, H2S, NH3, and CO2 (Castro-González, 2002). To evaluate the
elemental analysis considering C, N, and S, a Carlo Erba elemental analyzer Model EA 1110 was used. The
gas-liquid equilibria was applied by using the Henry’s law, considering Mexico City total pressure, and the
constants found in literature (HCP, 1991; Sander, 1999). All parameters were evaluated by triplicate. All data
were calculated at normal pressure and temperature (NPT).
The Arrhenius equation was based on the production rate of CH4 and H2S as the products of the methanogenic
and sulfatoreducing reactions, respectively. For finding the constants in the Arrhenius equation, first the
reaction order was evaluated from the experimental data gathered in the vials experiments. Results rendered a
first order kinetics. Drury (1999) did a work on the modeling of sulfatoreduction on anaerobic reactors for the
treatment of mine waste. He did an equivalence to the equation 1 for the sulfatoreduction. Thus, the equations
used for methanogenesis and sulfatoreduction were:
CH4(t)=CH4,max [1-exp(-KCH4t)] ecuación (1)
where
CH4(t) is the cumulative methane production during time t (in mL at NTP)
CH4,max is the maximum methane production (in mL at NTP)
KCH4 is the first order reaction rate constant for methanogenesis (d-1)
t is the time (d)
and
H2S(t)= H2S,max [1-exp(-KH2S t)] ecuación (2)
where
H2S (t) represents the H2S cumulative production during time t (in mL at NTP)
H2Smax is the maximum H2S production (in mL at NTP)
KH2S is the first order reaction rate constant for sulfato-reduction (d-1)
t is the time (d)
The rates of methanogenesis and sulfato-reduction are represented as a function of temperature by the
Arrhenius equation:
Kv=koexp-Ea/RT ecuación (3)
where:
Ea= reaction activation energy, cal/mol
R= gases universal constant, cal/mol K=1.987
Kv=KCH4 o KH2S reaction rate constant for methanogenesis or sulfato-reduction, respectively (d-1)
T=temperature, K
ko= Constant or Frequency factor (d-1)
The parameters values for CH4,max and k were estimated by the minimum square method using the methane
cumulative production (Doucet y Sloep, 1992). Linearizing equation 3 renders a simple way for calculating
the constants, as shown in Figure 3 and equation 4:
Ea 1
lnKv = − + ln kO ecuación (4)
R TInfluent Effluent
COD, SO4-2, C, N y S
COD, Methanogenic route
CH4(gas) CH4(gas) ÙCH4(liq.)
C, Æ (methanogenic
CO2(gas) CO2(gas)ÙCO2(liq.)+H2OÙH2CO3ÙH++HCO3ÙH++CO3=ÙCO3
N, activity)
S,
SO4-2 Æ Sulfatoreducing route
(sulfatoreducing H2S(gas), H2S(gas) ÙH2S(liq.) ÙH++HS-ÙH++S=ÙS=
activity)
Figure 2: Diagram of the experimental design and proposed degradation routes
ln ko (ordinate to the origin)
slope= -Ea/R
ln Kv
1/T
Figure 3: Linear representation to obtain the Arrhenius equation constants
RESULTS AND DISCUSSION
Table 1 shows the results of characterization of the vinasses used in this work. Methanogenic ecosystems
adaptation to the temperature elevation may be clearly seen. As mesophylic biomass is transformed into
thermophylic biomass, at 55ºC, a reduction tendency in CO2 is observed with respect to methane.
Interestingly, H2S production by the sulfato-reducing bacteria, SRB, increases just as well as the methane
production by the methanogenic bacteria, MB. Therefore, no SRB inhibición at 55ºC is found. CODs removal
is increased since carbon sources are used by both types of communities. Table 2 presents the average values
found using the solubilities of the gases generated in the three reactors. The results obtained from the 2.6 liters
reactors concerning gas production and using the Henry’s law to evaluate the molar fractions in the liquid
phase are shown in Figures 4 and 5. From the three gases found, the most soluble in water is H2S>>CO2>CH4,
and methane is the least soluble one. This property is conserved for vinasses (66.7, 60.8, 50.8E-4 for
H2S>>20, 17, 13E-4>0.9, 0.8, 0.7E-4 moles/d, for these three gases in the liquid phase, at 35, 45, and 55°C,
respectively). In the gas phase, they are exactly opposite: H2Sphase is 0.0067 moles at 35°C and 0.0051 in the liquid phase). Thus, solubility is having an effect on the
production of H2S by the sulfatoreducing bacteria. It is not clear if an inhibitory effect is present but definitely
kinetics has an effect, although this is not as marked so for the methanogenic bacteria. An interesting future
line of research seems to lay here.
Table 1: Some physicochemical characteristics of the cold-room (4ºC) stored vinasses
PARAMETER Value
Value of pH 4.34
Total solids, g/L 58.45
Total suspended solids, g/L 4.30
Total dissolved solids, g/L 54.15
Total volatile solids, g/L 47.25
Volatile suspended solids, g/L 4.10
Volatile dissolved solids, g/L 43.15
Fixed total solids, g/L 11.20
Fixed suspended solids, g/L 0.20
Fixed dissolved solids, g/L 11.00
Chemical oxygen demand total, mg/L 124,428
Soluble chemical oxygen demand, mg/L 120,001
Nitrogen, mg/mg vinasse 0.00124
Carbon, mg/mg vinasse 0.03818
Hydrogen, mg/mg vinasse 0.04496
M o l o f C H 4 in g a s a n d liq u id p h a s e
0 .2 5
0 .2 0 A v e ra g e
0 .1 6 4 8 5 5 ºC
l H4/d a y
0 .1 5
0 .1 2 4 0 4 5 ºC
m oC
0 .1 0
0 .0 6 5 8 3 5 ºC
0 .0 5
0 .0 0
0 50 100 150 200 250 300 350
D ays
Figure 4: Representation of CH4 mol in the gas phase + liquid phase at 35, 45, and 55ºC
M o l o f H 2S in g a s a n d liq u id p h a s e
0 .0 1 4
0 .0 1 2 A v e ra g e
0 .0 1 0 7 5 5 5 ºC
0 .0 1 0 0 6 4 5 ºC
0 .0 1 0
S /d a y
0 .0 0 9 9 4
3 5 ºC
0 .0 0 8
m o lH
2
0 .0 0 6
0 .0 0 4
0 .0 0 2
0 .0 0 0
0 50 100 150 200 250 300 350
D ays
Figure 5: Representation of H2S mol in the gas phase + liquid phase at 35, 45, and 55ºCFigure 6 presents the global yield of the 2.6 L reactors in terms of generated biogas per unit CODs removed.
These Ybiogás are 0.38, 0.51, and 0.57 m3 biogas NPT/kg CODs removed, at 35, 45, and 55°C, respectively.
Gases tend to desorb from liquids at higher rates as temperature increases.
Table 3 shows the results of the data used for the application of the Arrhenius equation, considering methane
generation of the vials experiments. Table 4 presents the same information but for sulfatoreduction. From the
drawn graphics the values for the constants were obtained (with r2 = 0.9641 and 0.9718 for methanogenic data
and 0.9891 and 0.9643 for sulfatoreduction data). The results indicate that activation energy required for the
methanogenic reaction (62.865 kJ/mol) is lower than for the sulfatoreduction reaction (93.088 kJ/mol),
corroborating the methane production enhancement over H2S production.
Biogas production at NPT
0.70
Average
m biogas/ kgCOD removed
0.60 0.57
55ºC
0.50 0.51
45ºC
0.40
0.38 35ºC
0.30
0.20
0.10
3
0.00
0 50 100 150 200 250 300 350
Days
Figure 6: Biogas yields for the 2.6 L reactors at 35, 45, and 55ºC
CONCLUSIONS
Considering the results obtained in this research, the hypotesis stated that temperature has a positive effect on
the generation of methane by the methanogenic bacteria in the temperature range studied (35-55°C) was true.
The solubility of the methane in the liquid phase is very low, and due to this, the methane is desorbed as soon
as it is generated by the methanogenic bacteria allowing an excelent recovery in biogas.
On the other hand, kinetics of the H2S production by sulfatoreducing bacteria, although is also favored by
temperature in a global basis, its higher solubility in the liquid phase do not allow its desorption. This
phenomenon probably could be acting as an inhibition factor for H2S generation by the SR biocommunities
that grow in the reactors and its competitivity might be considered somewhat limited within the temperature
range studied.
Thus, although competition between sulfatoreducing bacteria and methagenic bacteria for the substrate is not
apparent as temperature increases, methanogenic bacteria definitely are favored as temperature increases from
35 to 55°C. Activation energy found demonstrates this.
The activity of SRB is also favored by temperature increase but due to the higher solubility of H2S, its
desorption does not occurred as readily as for methane, and its presence in the liquid phase might be
inhibiting further production as temperature arises, maintaining an almost constant production of H2S ,
independent of temperature.Tabla 2: Average values for gas-liquid equilibria in the 2.6 L reactors at the three working temperatures
Parameter Effluent
35ºC 45ºC 55ºC
Gas flow, L/d 3.33 5.82 7.29
Gas flow, L/d (NTP) 2.27 3.85 4.67
% CH4 66 67 85
% CO2 28.65 29.12 10.26
% H2S 3.42 3.52 4.13
LCH4/d 1.47 2.78 3.32
LCO2/d 0.65 0.90 1.13
LH2S/d 0.08 0.13 0.20
CH4
moleCH4/d (gas phase) 0.066 0.124 0.148
mole fraction CH4 dissolved 0.00001949 0.000016507 0.000014043
moleCH4/L in solution (liquid phase) 0.00108311 0.0009171 0.00078022
moleCH4/d (liquid phase) 0.00008663 0.00008049 0.000067345
moleCH4/d (gas phase + líquid phase) 0.06608663 0.12408049 0.148067345
CO2
MoleCO2/d (gas phase) 0.01981 0.026627 0.032318
mole fraction CO2 disolved 0.00045084 0.00035094 0.00027073
MoleCO2/L solution (liquid phase) 0.025048 0.019498 0.0150418
mole CO2/d (liquid phase) 0.002003 0.001711 0.00129834
moleCO2/d (gas phase + liquid phase) 0.0218113 0.028338 0.03361634
mole H2CO3/L solution 0.00008517 0.00006629 0.00005114
mole H2CO3/d 0.00000681 0.00000582 0.00000441
mole HCO3-/L solution 0.000081 0.000069 0.000052
mole HCO3-/d 0.00000663 0.00000605 0.00000451
-2
mole CO3 /L solution as CaCO3 0.000000456 0.000000426 0.000000401
mole CO3-2/d as CaCO3 0.000000040 0.000000036 0.0000000023
H2S
mole H2S/d (gas phase) 0.0023755 0.0039822 0.0056814
mole fraction H2S disolved 0.001499901 0.0012466 0.0010584
mole H2S/L solution (liquid phase) 0.0833334 0.069266 0.058807
mole H2S/d (liquid phase) 0.0066658 0.0060794 0.0050759
mole H2S/día (gas phase + liquid phase) 0.0090413 0.0100616 0.0107573
mole HS-/d (liquid phase) 0.006065 0.005532 0.004619
mole S-2/d (liquid phase) 0.000521 0.000457 0.000298
mole SO4-2/d (liquid phase) 0.0042 0.0033 0.0017
Table 3: Arrhenius constant obtained for the production of methane in vials
Average 1 Average 2
Temperature (TºC) 35 45 55 35 45 55
Reaction velocity KCH4 (d-1) 0.0313 0.0528 0.1397 0.0305 0.0533 0.1380
Arrhenius constant, ko 1.179x109 1.464x109
Average 1.321x109
Activation energy (kJ/mol) 62.56546 63.1647
Average 62.865 kJ/molTable 4: Arrhenius constant obtained for the H2S production in vials
Average 1 Average 2
Temperature (TºC) 35 45 55 35 45 55
Reaction velocity KH2S (d-1) 0.0040 0.0179 0.0476 0.0097 0.0191 0.0679
Gas constant 0.00813141 kJ/mol
Arrhenius constant 2.342x1015 5.945x1011
Average 1.171x1015
Activation energy (kJ/mol) 104.6018 81.57391
Average 93.088 kJ/mol
ACKNOWLEDGEMENTS
First author acknowledge Conacyt doctoral scholarship. Helpful advise from Dr. Enrique Bazúa on the
thermodynamic analysis is also acknowledged by authors.
REFERENCES
APHA, 1985. Standard Methods for the Examination of Water and Wastewater. 16th Ed. American Public
Health Association. Washington, DC, USA.
Castro-González A. 2002. Estudio del efecto de la temperatura en la actividad metanogénica y sulfato-reductora
de consorcios microbianos en condiciones anaerobias. Doctoral Thesis (in review). Programa de Maestría y Doctorado en
Ingeniería, UNAM. Mexico City, Mexico.
Castro-González A. and Durán-de-Bazúa C. 2001a. Adaptation of anaerobic biomass to a new substrate in an
upflow anaerobic sludge blanket reactor. Tecnol. Ciencia Ed. (IMIQ, Mexico), 16(1):49-55.
Castro-González A. and Durán-de-Bazúa C. 2001b. Temperature effects on the microbial activity in an
anaerobic upflow reactor during the treatment of sugarcane ethyl alcohol wastewaters treatment. Presented at IWA
Congress 2001. Berlin, German Federal Republic.
Doucet, P. and Sloep, P.B. 1992. Mathematical modelling in the life sciences. The Ellis Horwood Ltd.
Chichester, England.
HCP. 1991. Handbook of Chemistry and Physics. 42th Ed. Chemical Rubber Pub. Co. Cleveland, Ohio, USA.
Jiménez R., Martínez M., Espinoza A., Noyola A., and Durán-de-Bazúa C. 1995. La caña de azúcar, su entorno
ambiental. Parte II. Tratamiento de vinazas en una planta piloto en México en un reactor anaerobio de lecho de lodos.
Informe técnico de proyecto VIN-01-95. GEPLACEA, CNIIAA, PIQAYQA-FQ-UNAM. Pub. Facultad de Química,
UNAM. Mexico City,. Mexico.
Lens P. y Hulshoff Pol L. 2000. Environmental Technologies to Treat Sulfur Pollution: Principles and
Engineering. IWA Edition, London, England.
Oaxaca-Grande A. 1997. Estudio comparativo para la determinación de la demanda química de oxígeno entre el
método estándar de reflujo abierto y el método colorimétrico (rápido) de reflujo cerrado. Professional Thesis. Instituto
Tecnológico de Orizaba. Orizaba, Veracruz, Mexico.
Sander, R. 1999. Non commercial reproduction. Air Chemistry Dept. Max Planck Institute of Chemistry. Mainz,
German Federal Republic. www.mpch-mainz.mpg.de/ sander.
Veeken A. and Hamelers B. 1999. Effect of temperature on hydrolysis rates of selected biowastes components.
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