- Email: [email protected]
Intensified biogas purification in a stirred tank
Chemical Engineering and Processing 86 (2014) 1–8
Contents lists available at ScienceDirect
Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep
Intensified biogas purification in a stirred tank T.N. Wedraogo a, *, S. Poncin a , J. Wu b , Huai Z. Li a a b
Laboratoire Réactions et Génie des Procédés, Université de Lorraine, CNRS, 1 Rue Grandville, BP 20451, 54001 Nancy, France State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 5 February 2014 Received in revised form 19 August 2014 Accepted 6 October 2014 Available online 8 October 2014
The present study aims at intensifying the absorption of carbon dioxide in aqueous phases. For this purpose, a second immiscible liquid phase is used to improve the mass transfer of carbon dioxide in water. Absorption measurements are carried out in a stirred cell by online infrared monitoring of the output carbon dioxide concentration. As dispersed phase, 1-octanol shows an improvement in mass transfer whereas colza oil retards it. Toluene has no significant effect in the range of concentrations investigated. Mass transfer coefficients are determined using a chemical method. Nevertheless, the variety of behaviors could not be explained simply by solubility or variation of mass transfer coefficients. In order to understand the complex gas–liquid–liquid interactions and deduce the mass transfer mechanism between these three phases, surface and interface tension measurements and direct observation of bubbles and droplets are also carried out. ã 2014 Published by Elsevier B.V.
Keywords: Biogas purification Intensified CO2 absorption Liquid–liquid dispersion Reversible emulsion
1. Introduction Biogas produced during the anaerobic treatment of wastewater is an interesting energy resource from an environmental and economic point of view. The process of methanisation yields a biogas composed of 55–70% of methane and 30–45% of carbon dioxide along with impurities such as hydrogen sulfide. It can be burned on site to provide heating or electricity, but there are other value adding uses. For example, biogas can be injected into a grid of natural gas provided that the methane content reaches 95%. Numerous separation methods can be used to eliminate the carbon dioxide, the most common of which is absorption in aqueous amine solutions. This process exhibits high absorption rates but is rather energy intensive because of the heat needed for desorption. An alternative for carbon dioxide separation is physical absorption. The physical solvents require less energy for desorption but suffer from a slower absorption rate. In the present study, the emphasis is given to the intensified absorption of carbon dioxide in water. Intensification happens through the use of a second liquid organic phase to form a reversible emulsion, dispersed droplets can enhance the gas–liquid transfer conditions [9]. This kind of gas–liquid–liquid systems has attracted only few attention despite it is quite common in the industry. Among
* Corresponding author. Tel.: +33 662685345. E-mail address: [email protected] (T.N. Wedraogo). http://dx.doi.org/10.1016/j.cep.2014.10.004 0255-2701/ ã 2014 Published by Elsevier B.V.
the existing applications, there is the dispersion of hydrocarbons in water in order to give a substrate to aerobic bacteria. Yoshida et al. [22] noticed that the presence of toluene of oleic acid enhances the oxygen transfer to water, while the presence of paraffin retards it. The authors attribute these changes to the position of the organic phase around gas bubbles. Later, Linek and Beneš [13] investigated two possible transfer mechanisms: transfer in parallel where direct gas– oil contact is possible and gas is absorbed in both water and oil simultaneously; and transfer in series for which there is no direct gas–oil contact. They concluded that the pathway for gas absorption is strongly affected by interfacial properties. Bruining et al. [5] introduced a concept firstly developed for the study of gas–liquid– solid systems. In this “shuttle mechanism”, elements of the dispersed phase move towards the gas–liquid interface, uptake the solute and release it in the bulk of the liquid phase. Furthermore, these authors remark that mass transfer is not enhanced unless the gas is highly soluble in the dispersed phase. According to the same reference, droplets of dispersed phase also need to be small enough to accommodate in the thin mass transfer zone around bubbles. Rols et al. [19] proposed a mechanism where the dispersed liquid phase spreading over a gas bubble absorbs the solute and redisperses when the bubble explodes. Although there are numerous attempts to understand the underlying processes of absorption in liquid–liquid systems, no convincing representation can currently explain the large variety of behaviors observed. The modification of the volumetric mass transfer coefficient kL and the interfacial area a was also extensively studied but contradictory results were
2
T.N. Wedraogo et al. / Chemical Engineering and Processing 86 (2014) 1–8
Nomenclature a A C D Ech Eph F Ha J kL kapp m N Q R VL
specific gas–liquid interfacial area (m2 m3) total gas–liquid interfacial area (m2) molar concentration (mol m3) diffusivity of CO2 in the solution (m2 s1) chemical enhancement factor physical enhancement factor enhancement factor due to the presence of an emulsion Hatta number specific absorption rate (mol m2 s1) liquid side mass transfer coefficient (m s1) apparent rate constant for the absorption reaction (s1) ratio of the solubility in the dispersed phase to the solubility in the continuous phase impeller rotation speed (rpm) volumetric gas flowrate (m3 s1) volumetric absorption rate (mol m3 s1) volume of liquid in the reactor (m3)
Exponents 1 refers to 2 refers to * refers to i refers to
the the the the
inlet of the reactor outlet of the reactor equilibrium state gas–liquid interfacial conditions
of 1300 rpm which allows a good dispersion of the oil phase while keeping a relatively flat surface. The reactor is operated at 273 K (0.5 K) and 1.1 bar (0.1 bar). A flowmeter measures the outlet gas flow rate. The outlet gas is cooled down to 4 C in order to remove the water vapors and its composition is determined by a GAS3230R infrared online analyzer (GEIT, Belgium). A by-pass can be used to lead the initial gas mixture directly to the analyzer and check the composition of the inlet gas mixture. The molar fraction of methane and carbon dioxide are equal to the volume fraction measured by the infrared analyzer. The mass balance on CO2 between the inlet and outlet of the reactor can be expressed as follows: Q 1 C 1CO2 ¼ Q 2 C 2CO2 þ R V L
(1)
Once the steady state is reached, the carbon dioxide concentration and the gas outlet flowrate are stable. Then the carbon dioxide absorption rate is obtained from Eq. (1). The Danckwerts plot technique is used to obtain both the mass transfer coefficient kL and the interfacial area a. Every absorption experiment is conducted with a 0.5 M K2CO3/0.5 M KHCO3 buffer in which the following global reaction takes place: K2 CO3 þ CO2 þ H2 O ¼ 2KHCO3
(2)
Three reactions are involved: Subscripts c
relative to the catalyst R the value is relative to the continuous phase DR = D(oil phase)/D (continuous phase) Greek letters d diffusion film thickness (m) e dispersed phase volumetric fraction () f absorption rate (mol/s) t contact time according to penetration model (s)
þ CO2 þ H2 O ¼ HCO 3 þ H k3
(3)
CO2 þ HO ¼ HCO 3 k4
(4)
2 þ HCO 3 ¼ CO3 þ H k5
(5)
The rate of reaction (3) is very slow but can be catalyzed by a number of agents, among which sodium hypochlorite (the catalytic rate constant is written kc). Its rate is given by: r3 ¼ ðk3 þ kC C C Þ C CO2
obtained [9]. These authors reported a dramatic decrease of kLa at low fraction of dispersed phase which is attributed to initial changes in the surface tension. Cents [6] used a chemical method for the measurement of kL and a. The results showed that some substances enhance the transfer whereas others retard it, this could be related to the presence or absence of dispersed phase droplets in the mass transfer zone around bubbles according to the author. 2. Experimental
(6)
Reaction (4) is a second order reaction. Its rate is given by: r4 ¼ k4 cOH cCO2
(7)
Eq. (5) allows the calculation of the hydroxyl anions concentration: C HO ¼
K W C CO2 3
K 5 C HCO3
¼
KW K5
(8)
2.1. Mass transfer Fig. 1 shows a schematic representation of the set-up. It comprises a feed section, a stirred cell and an analysis section. The first part is designed to mix pure methane and carbon dioxide (from two 3.5 grade gas bottles) of desired composition by means of two SLA5850 mass flow regulators (Brooks, USA) and thus produce a synthetic biogas. The gas mixture is then led into a 1 L tank (10 cm internal diameter) through a sintered metal distributor. A pitched blade turbine with four blades at a 45 angle is used as an impeller and is located just above the gas inlet in order to disperse the bubbles properly. The reactor is equipped with four baffles to prevent the formation of a vortex. All experiments are conducted at a fixed impeller speed
The overall reaction rate can therefore be calculated as follows: KW cCO2 ¼ kapp cCO2 (9) rCO2 ¼ k3 þ kc cc þ k4 K5 In this equation cCO2 represents the carbon dioxide concentration. Danckwerts also derived a criterion which can be used to assess if the reaction is of pseudo first order: the concentration of species HCO3, CO32 and OH must be constant and uniform from the bulk to the interface. The validity of this criterion has been discussed by Cents et al. [7] who concluded that in order to reduce the error in the calculation to less than 3%, the following condition should be fulfilled:
T.N. Wedraogo et al. / Chemical Engineering and Processing 86 (2014) 1–8
3
Fig. 1. Experimental set-up.
2
C CO2
3
"sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi# D kapp 1 2 4 5 iþ 1þ h 1 < 0:1 2 2 HCO kL 3 CO3
(10)
The solubility and diffusivity of carbon dioxide in the buffer were determined respectively from the work of Weisenberger and Schumpe [21] as well as Joosten and Danckwerts [12]. The physico-chemical parameters used are gathered in Table 1. Using relevant data from Table 1 and experimental values, the maximum value of the left-hand term in Eq. (10) is 0.07, showing that the criterion is satisfied in the present experiments. Kinetic constants k3, kc and k4 can be determined respectively from the work of Pinsent and Roughton [17], Benadda et al. [2] and Pohorecki and Moniuk [18]. Finally, the Danckwerts surface renewal model establishes the following equation for absorption rate for a pseudo first order reaction with negligible bulk concentration: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi kapp D i ca R ¼ kL 1 þ (11) 2 kL The value of kapp can be modified by varying the sodium hypochlorite concentration (cc). The corresponding values of R can then be used to fit kL and a from Eq. (11).
droplets can be determined from the images of the camera. It is also possible to see where droplets of the dispersed phase are located around gas bubbles. To our best knowledge, such visualization does not exist for a complex gas–liquid–liquid system in the literature. 3. Results and discussion 3.1. Mass transfer experiments Absorption rate is measured in emulsions for various organic phases: edible oil, toluene and octanol, without any surfactant to avoid the stabilization of emulsions. The results are compared to those obtained using a single liquid phase. Since the water is saturated in carbon dioxide in only a few minutes, it was decided that the liquid phase reference should be the 0.5 M K2CO3/0.5 M KHCO3 solution in which absorption rate is stable in time. Fig. 2 displays the values of absorption rate in the stirred tank for the different gas–liquid and gas–liquid–liquid systems for a given rotation speed of the impeller. These values are compared using an intensification factor Eph which is defined as the ratio of the absorption rate in the emulsion
2.4. Direct observation of bubbles and droplets The set-up illustrated in Fig. 1 is designed to observe visually a gas–liquid–liquid system from inside. A high speed camera (Dantec Dynamics, Denmark) is linked to a borescope (Olympus, Japan) which can be placed at various heights in the tank. A light source (Olympus, Japan) provides lighting in the area of focus. The emulsion is created by the rotation of a four-blade turbine. Four baffles prevent the formation of a vortex. A gas phase is then injected at the bottom of the tank. The size of the bubbles and Table 1 Physico-chemical parameters at T = 20 C. Parameter
Value
Unit
Reference
k3 kc k4 DCO2 HeCO2 VL
0.02 1.54 2.16 1.48 109 3.83 104 0.50
s1 m3 mol1 s1 m3 mol1 s1 m2 s1 mol m3 Pa1 L
[17] [2] [18] [12] [21] Fig. 2. Temporary evolution of the absorption rate. N = 1300 rpm, Q CH4 = Q CO2 = 50 mL min1.
4
T.N. Wedraogo et al. / Chemical Engineering and Processing 86 (2014) 1–8
to that in the buffer alone. This factor accounts for the enhancement due to physical absorption in the dispersed phase. The evolution of Eph with the fraction of the dispersed phase e for various organic phases is reported in Fig. 3. Three different behaviors can be observed. Octanol clearly enhances the gas–liquid transfer especially at low organic phase fraction. Similar results were obtained by Zhang et al. [23] for the absorption of carbon dioxide in a isoamyl alcohol–water emulsion with a maximum at low dispersed phase fraction. Toluene exhibits small effect on the absorption rate in the range of volume fraction investigated. Colza oil retards mass transfer whatever the involved amount. These observations allow establishing a first categorization of the oils regarding their influence on the gas– liquid mass transfer rate of carbon dioxide. Furthermore, it was decided to carry on experiments with octanol and colza oil only since toluene has limited effect on mass transfer and presents health and safety issues. It is common to represent the chemical absorption rate as: (12) R ¼ kL a C CO2 C CO2 Ech This equation is based on the hypothesis that there is no resistance to mass transfer located at the interface. It is also written for a unique liquid phase. Thus, we have to consider the emulsion as a pseudo homogeneous phase with properties stemming from those of the relative pure liquids. 3.1.1. Influence of solubility The solubility of carbon dioxide in the emulsion is calculated as the mean of the solubility in the pure phases weighted by the percentage of dispersed phase e and displayed in Table 2, the solubility in colza emulsion being unknown. The solubility of carbon dioxide in pure toluene and in pure octanol used in the calculations were respectively published in Dack [8] and Abraham et al. [1]. Obviously, these data reveal that the increase of solubility cannot explain alone the variation of the absorption rate. Indeed, Fig. 3 shows that within the investigated range, Eph does not increase with the amount of organic dispersed phase while the solubility does, at least in toluene and octanol. Moreover, despite a better solubility in toluene emulsions, carbon dioxide is absorbed more quickly when the dispersed phase is octanol.
Table 2 Solubility of carbon dioxide (mol m3) in the emulsions at T = 20 C and PCO2 = 0.5 atm.
e
Octanol
Toluene
0 0.05 0.1 0.15 0.2
12.2 12.6 13.1 14.0 14.9
12.2 12.9 14.0 15.8 17.6
3.1.2. Hydrodynamic influence The presence of a dispersed phase modifies also the global hydrodynamics which controls the mass transfer process. Fig. 4 displays the ratio between values of kL and a in emulsions and in pure buffer (kL0 and a0). Experimental a/a0 and kL/kL0 data reported in Fig. 4 show that octanol and colza oil display opposite effects on both kL and a. The presence of colza oil increases the interfacial area and reduces the mass transfer coefficient kL. On the contrary, the presence of octanol improves hydrodynamics through the increase of kL but leads to a lower interfacial area. Our results are very similar to those of Cents [6] for the system octanol–CO2 but interfacial area a is often reported to increase with the addition of an oil phase because the presence of droplets would reduce the rates of coalescence [9]. This discrepancy might be caused by the interpretation of the Danckwerts model. As mentioned before, the numerous results published about the variation of kLa in presence of a dispersed phase are contradictory. Mass transfer coefficient kL is known to increase with the diffusivity of a gas in the liquid. Since the diffusivity of carbon dioxide is approximately three times higher in octanol than in the buffer, the results in Fig. 4 are consistent with the expectations. The resulting kLa values for octanol are slightly higher than those obtained in pure buffer, while they are 20% lower in the case of colza oil. Consideration of such a point leads to attribute the low carbon dioxide absorption rate in this emulsion to the low kLa value of colza oil emulsion. For the octanol however, the classical approach to quantify gas–liquid mass transfer, which relates to gas–liquid interfacial area a and hydrodynamics of the liquids through the mass transfer coefficient kL, does not seem relevant. As above-mentioned, precise mechanisms of mass transfer should be proposed to take into account the effects not involved in Eq. (12). The mechanism of mass transfer in parallel describes a situation where carbon dioxide can be absorbed simultaneously into both liquid phases. In contrary, solute must be absorbed in the continuous phase first in the case of mass transfer in series. 3.1.3. Influence of surface tension To account for the possibility of the dispersed phase to be in direct contact with gas bubbles, Yoshida et al. [22] proposed to use the spreading coefficient S: (13) S ¼ s wg s og þ s ow
Fig. 3. Eph as a function of e. N = 1300 rpm, Q CH4 = Q CO2 = 50 mL min1.
If S is positive, direct transfer to the organic phase is possible as a parallel transfer mechanism, whereas for negative S, the gas must be absorbed in water first following a transfer in series scheme. However, contradictory results are obtained with this approach and it is difficult to draw an unequivocal conclusion [9]. The recent work of Pinho and Alves [24] shows that the value of S determines the mass transfer mechanism, except when S is close to zero. Cents [6] obtains positive S for toluene and octanol, which is confirmed by our measurements made on a tensiometer (I.T. Concept, France) (Table 3). The accuracy is 1 mN m1. The surface tension of watersimulated biogas is 70 mN m1.
T.N. Wedraogo et al. / Chemical Engineering and Processing 86 (2014) 1–8
5
Fig. 4. Relative values of a and kL, N = 1000 rpm, Q CH4 = Q CO2 = 50 mL min1. Table 3 Surface tension and interfacial tension at T = 20 C. Organic phase
s o g (mN m1)
s o w (mN m1)
S (mN m1)
Octanol Colza oil Toluene
24 22 26
9 58 34
37 10 10
For colza oil, the negative spreading coefficient would indicate that there is no direct contact between the gas bubbles and the dispersed liquid phase. The transfer could then be in series and the assumption of a pseudo-homogeneous liquid phase to represent the emulsion would be valid. In the case of toluene and octanol, the spreading coefficient S is positive, which means that in an emulsion, these liquids can be in direct contact with the gas. However this observation does not explain the difference of intensification observed between toluene and octanol. So far, there is still no study on the physical meaning of the precise value of S. In the work of Ngo and Schumpe [16], n-heptane (S = 11.8 mN m1) always enhances kLa while n-hexadecane (S = 2.2 mN m1) and ndodecane (S = 2.2 mN m1) enhance kLa up to e = 0.3 but have a negative effect on kLa when the concentration is higher. In our work, colza oil (S = 10 mN m1) always reduces kLa. This needs to be investigated further but it seems that the spreading coefficient is an indicator for the mass transfer mechanism. The difference in Eph between octanol and toluene can be interpreted under the angle of another mass transfer mechanism known as the shuttle effect. In this case, droplets of dispersed phase travel to the diffusion film around the bubbles where they uptake the solute to release in the bulk of the continuous liquid phase. By simulation, Brilman et al. [4] determined that a droplet can enhance the mass transfer only if it is very close to the interface. Another result of their work reveals that the enhancement factor is inversely proportional to the droplet diameter. In our case, the lower value of the interfacial tension between octanol and water could make droplets small enough to accommodate in the
thin mass transfer film around the bubbles since a low value of s o w indicates that a small amount of energy is needed to create liquid–liquid interface. In order to further investigate this issue, measurements are performed in the presence of surfactants. The results of these experiments are gathered in Table 4. Due to foaming properties of the surfactant, we are unable to measure the absorption rate when there is no dispersed phase or when the dispersed phase is colza oil. With these liquids, the stirred tank rapidly fills up with foam, leading to a complete absorption of carbon dioxide. This is thought to be due to an extremely high value of the interfacial area created in the foam. Some previous works report either no significant influence of the presence of surfactants on the absorption rate [10] or a decrease of this rate due to the reduction of the interfacial area and mass transfer coefficient kL [11]. The factor Eph is calculated as the ratio between the absorption rate in the stabilized emulsion and the absorption with no surfactant and no dispersed phase, it might therefore be underestimated. It can be seen from Fig. 5 that the stabilized emulsion enhances the mass transfer more than the emulsions without surfactant. Since the presence of a surfactant usually prevents the droplets and the bubbles from coalescing and reduces droplets’ size due to a lower interfacial tension, the shuttle effect and the interfacial area
Table 4 Mean droplet diameter d (mm).
e
Octanol
Octanol + Tween80
Colza oil
Colza oil + Tween80
0.02 0.05 0.1
– 119 131
– 55 61
137 172 195
100 133 177
Fig. 5. Intensification factor Eph in presence of Tween 80. N = 1300 rpm, Q CH4 = Q CO2 = 50 mL min1.
6
T.N. Wedraogo et al. / Chemical Engineering and Processing 86 (2014) 1–8
Fig. 6. Illustration of the shuttle mechanism in the system biogas–water–octanol (5%v).
are logically enhanced with respect to reversible emulsion without surfactant. 3.1.4. Influence of viscosity None of the previously discussed models takes into account the viscosity of the emulsion although it can affect significantly the mass transfer rate. The viscosity of the emulsion has a huge effect on the global hydrodynamics in the reactor. All other things being equal, an increase in viscosity will reduce the turbulence and the gas–liquid interface renewal because of higher energy dissipation. This is often mentioned as the reason why the enhancement factor levels of at high fraction of dispersed phase [19,20]. 3.2. Direct observation of the gas–liquid–liquid system Even though it cannot be used as a method to determine absolute droplet and bubble sizes, we can use the set-up described above to compare the behavior of our various systems. Due to the opacity of the emulsions, the light source must be as close as possible to the borescope. It results that acceptable images can be obtained only in the bottom of the tank. The size distribution measured is therefore shifted towards the small diameters
(because the big droplets and bubbles ascend more quickly). Since the high speed camera allows very short exposure time (down to 1 ms) we can obtain very clean images of quick moving objects. As shown in Table 4 droplets are smaller in octanol emulsions than in colza oil emulsions. This is in agreement with the interfacial tension measurements. Moreover, the mean droplet diameter increases with the dispersed phase fraction in each set of data. Results in Table 4 also confirm that the presence of surfactants (Tween80) is responsible for a diminution of the droplet size. The visual observation of gas–liquid–liquid systems reveals that octanol droplets are attached to almost every gas bubble whereas colza oil droplets only adhere to very small gas bubbles. Fig. 6 is an illustration of the shuttle mechanism: droplet 1 (D1) initially sticks to the gas bubble (B) and is released on the third image. On the contrary, droplet 2 (D2) is initially free. It comes into contact with the bubble on the second image and stays attached. Oil droplets in contact with a gas bubble can take various positions. Most of the time, two spherical droplets are close to each other. In this configuration, they are in contact but the contact area is limited. From this configuration it is however possible to switch to another position where the droplet is no longer spherical but
Fig. 7. Illustration of the spreading of a droplet of octanol on a bubble.
T.N. Wedraogo et al. / Chemical Engineering and Processing 86 (2014) 1–8
Eph
Table 5 Expression of J and Eph according to the mass transfer model. Film model
d ðC C Þ
J
D
Eph
1 þ eðmR DR 1Þ
Higbie penetration theory qffiffiffiffiffi D ðC C Þ 2 pt pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1 þ e mR DR 1
adopts the curvature of the bubble and therefore covers a large part of its surface. The transition from the first configuration to the second one is illustrated in Fig. 7. The reverse phenomenon, i.e., the detachment of the droplet from the bubble was not observed so it is difficult to specify the threshold conditions that for this kind of structural organization. 3.3. Modeling 3.3.1. Shuttle effect The most extensively used model to describe the enhancement factor is firstly developed by Bruining et al. [5]. It is based on the shuttle effect and therefore it assumes that direct contact between the gas and the organic phase does not happen. Other hypotheses include liquid–liquid equilibrium and the presence of droplets of organic phase in the diffusion zone in the same proportion as in the rest of the liquid. The mass balance with a first order reaction in the mass transfer zone is (C denotes the CO2 concentration):
@2 C @C ¼ kapp ð1 eÞC þ ð1 þ eðmR 1ÞÞ @t @x2
D
(14)
with the following initial and boundary conditions: t ¼ 0; x > 0 : C ¼ C 0 t > 0; x ¼ 0 : C ¼ C i t > 0; x ! 1 : C ¼ C 0
(15)
When the capacity of the bulk is sufficient, we can assume that C0 = 0 mol m3. Eq. (14) can be solved analytically to obtain the absorption rate: pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Nabs ¼ kL 1 þ eðmR 1Þ þ ð1 eÞHa2 C i aV L (16) The enhancement factor Eph can then be calculated as follows: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Nabs ðe 6¼ 0Þ 1 þ eðmR 1Þ þ ð1 eÞHa2 ¼ (17) Eph ¼ Nabs ðe ¼ 0Þ 1 þ Ha2 Since Ha = 3.5 103 in the absence of catalyst, we can simplify Eq. (17) to:
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ 1 þ eðmR 1Þ
7
(18)
A more recent contribution by Brilman et al. [3] tried to improve this model by taking into account the distribution of droplets near the interface and liquid–liquid mass transfer. These authors showed that only the droplets very close to the gas–liquid interface determine mass transfer enhancement. Their model fails in describing the enhancement factor levelling off at high dispersed phase fraction but a good agreement is obtained with the data of Littel et al. [14]. The simple model of Bruining generally underestimates the enhancement factor by approximately 20% [9]. Moreover, the estimated thickness of the mass diffusion film in our work never exceeds 5 mm while droplet sizes are never smaller than 20 mm. Even though the droplet size distribution may not be precise due to both the emulsion opacity and small droplets’ size, it does not seem to satisfy the major hypothesis of this model. Nagy [15] proposed a one-dimensional model to calculate the enhancement factor whatever the size of the droplet. However, the values of relative solubility m for our systems are so low that even for very small droplets very close to the interface the enhancement factor predicted by a shuttle effect model would be lower compared to our experimental results. 3.3.2. Transfer in parallel This mass transfer mechanism was not often discussed in the literature. The first model was proposed by Van Ede et al. [20]. When the solute gas can be absorbed in both liquid phases, the absorption rate consists of two terms: transfer from gas to the continuous phase (’GC) and transfer from gas to the dispersed phase (wGD). The enhancement factor Eph: Eph ¼
’GC þ ’GD JGC AGC þ JGD AGD ¼ ’GC JGC AGC
(19)
It is then assumed that: 1. Liquid phases are at equilibrium with each other. 2. The fraction of gas–liquid interface occupied by the dispersed
phase ADC is equal to eA and the fraction of gas–liquid interface occupied by the continuous phase AGC is equal to (1 e)A. 3. Enhancement due to chemical reaction is negligible. Finally, the specific absorption rate J is given by different expressions depending on the model used for the behavior of the liquid film (Table 5). As shown in Fig. 8, Bruining’s model for the shuttle effect can be used to estimate Eph in the case of toluene. However, in the case of octanol this model underestimates significantly the value of Eph since it is of the same order of magnitude as the maximum
Fig. 8. Experimental and model based values of Eph. Dispersed phase: (a) octanol, (b) toluene.
8
T.N. Wedraogo et al. / Chemical Engineering and Processing 86 (2014) 1–8
enhancement factor. This evidence, along with the proof that octanol can spread over a bubble (Fig. 7), is a reliable indication that mass transfer could be in parallel for the system octanol– carbonate buffer-CO2. When a surfactant is added into the liquid, the mass transfer in the system toluene–carbonate buffer-CO2 seems to switch from a transfer in series to a transfer in parallel. This is confirmed by the fact that liquid–liquid interfacial tension should drop in presence of Tween 80, increasing the value of the spreading coefficient. This point would however need further experimental validation since, S could not be measured and Eph cannot be accurately estimated in presence of a surfactant.
[3]
[4]
[5]
[6] [7]
4. Conclusion A novel approach for the upgrading of biogas was developed in this work. The influence of various organic phases on the absorption efficiency of carbon dioxide in an aqueous solution was investigated. With octanol as dispersed phase, an enhancement of 20% was observed, whereas absorption rate decreased when colza oil was used. The presence of toluene induced a small enhancement of the absorption rate of carbon dioxide. Experimental evidence such as a large spreading coefficient and visual observation of a droplet spreading over a bubble indicates that mass transfer might be in parallel in the system octanol– carbonate–CO2. This mechanism allows much higher enhancement factors than the shuttle effect mentioned in the literature. The measurement of kL and a in the gas–liquid–liquid systems was also performed. The negative impact of the viscosity on the mass transfer results in the enhancement factor leveling off at high holdup, unfortunately it is difficult to measure the viscosity of reversible emulsions. The solubility of methane in the liquids could also be a source of error for the accurate measurement of the absorption rate. Methane is 15 times less soluble in water than carbon dioxide but its solubility in organic phases is not reported in the literature. Further investigations are still required to take into account a detailed mass balance. Acknowledgments The financial assistance provided by the French Agence Nationale de la Recherche (ANR PROMET) and by the Natural Science Foundation of China (51061130555) to this joint project is gratefully acknowledged.
[8] [9]
[10]
[11]
[12]
[13]
[14]
[15]
[16] [17]
[18]
[19]
[20]
[21]
[22]
References [1] M.H. Abraham, J. Le, W.E. Acree Jr., P.W. Carr, A.J. Dallas, The solubility of gases and vapours in dry octan-1-ol at 298 K, Chemosphere 44 (2001) 855–863, doi: http://dx.doi.org/10.1016/S0045-6535(00)288-5. [2] B. Benadda, M. Prost, S. Ismaily, R. Bressat, M. Otterbein, Validation of the gaslift capillary bubble column as a simulation device for a reactor by the study of CO2 absorption in Na2CO3/NaHCO3 solutions, Chem. Eng. Process.: Process
[23]
[24]
Intensif. 33 (1994) 55–59, doi:http://dx.doi.org/10.1016/0255-2701(94) 85003-8. D.W.F. Brilman, M.J.V. Goldschmidt, G.F. Versteeg, W.P.M. van Swaaij, Heterogeneous mass transfer models for gas absorption in multiphase systems, Chem. Eng. Sci. 55 (2000) 2793–2812, doi:http://dx.doi.org/ 10.1016/S0009-2509(99)491-1. D.W.F. Brilman, W.P.M. van Swaaij, G.F. Versteeg, A one-dimensional instationary heterogeneous mass transfer model for gas absorption in multiphase systems, Chem. Eng. Process.: Process Intensif. 37 (1998) 471–488, doi:http:// dx.doi.org/10.1016/S0255–2701(98)55-5. W.J. Bruining, G.E.H. Joosten, A.A.C.M. Beenackers, H. Hofman, Enhancement of gas–liquid mass transfer by a dispersed second liquid phae, Chem. Eng. Sci. 41 (1986) 1873–1877, doi:http://dx.doi.org/10.1016/0009-2509(86)87066-X. A.H.G. Cents, 2003. Mass transfer and hydrodynamics in stirred gas–liquid– liquid contactors. A.H.G. Cents, D.W.F. Brilman, G.F. Versteeg, CO2 absorption in carbonate/ bicarbonate solutions: the Danckwerts-criterion revisited, Chem. Eng. Sci. 60 (2005) 5830–5835, doi:http://dx.doi.org/10.1016/j.ces.2005.05.020. M.R.J. Dack, Solutions and Solubilities, Part I 386, Wiley, 1975. E. Dumont, H. Delmas, Mass transfer enhancement of gas absorption in oil-inwater systems: a review, Chem. Eng. Process.: Process Intensif. 42 (2003) 419–438, doi:http://dx.doi.org/10.1016/S0255-2701(02)67-3. R. Farajzadeh, A. Barati, H.A. Delil, J. Bruining, P.L.J. Zitha, Mass transfer of CO2 into water and surfactant solutions, Pet. Sci. Technol. 25 (2007) 1493–1511, doi:http://dx.doi.org/10.1080/10916460701429498. D. Gómez-Díaz, N. Gomes, J.A. Teixeira, I. Belo, Oxygen mass transfer to emulsions in a bubble column contactor, Chem. Eng. J. 152 (2009) 354–360, doi:http://dx.doi.org/10.1016/j.cej.2009.04.059. G.E.H. Joosten, P.V. Danckwerts, Solubility and diffusivity of nitrous oxide in equimolar potassium carbonate-potassium bicarbonate solutions at 25 and 1 atm, J. Chem. Eng. Data 17 (1972) 452–454, doi:http://dx.doi.org/10.1021/ je60055a016. V. Linek, P. Beneš, A study of the mechanism of gas absorption into oil–water emulsions, Chem. Eng. Sci. 31 (1976) 1037–1046, doi:http://dx.doi.org/ 10.1016/0009-2509(76)87024-8. R.J. Littel, G.F. Versteeg, W.P.M. Van Swaaij, Physical absorption of CO2 and propene into toluene/water emulsions, AIChE J. 40 (1994) 1629–1638, doi: http://dx.doi.org/10.1002/aic.690401005. E. Nagy, Three-phase mass transfer: one-dimensional heterogeneous model, Chem. Eng. Sci. 50 (1995) 827–836, doi:http://dx.doi.org/10.1016/0009-2509 (94)441-S. T.H. Ngo, A. Schumpe, Absorption of CO2 into alkane/water emulsions in a stirred tank, J. Chem. Eng. Japan 45 (2012) 737–741. B.R.W. Pinsent, F.J.W. Roughton, The kinetics of combination of carbon dioxide with water and hydroxide ions, Trans. Faraday Soc. 47 (1951) 263–269, doi: http://dx.doi.org/10.1039/TF9514700263. R. Pohorecki, W. Moniuk, Kinetics of reaction between carbon dioxide and hydroxyl ions in aqueous electrolyte solutions, Chem. Eng. Sci. 43 (1988) 1677–1684, doi:http://dx.doi.org/10.1016/0009-2509(88)85159-5. J.L. Rols, J.S. Condoret, C. Fonade, G. Goma, Modeling of oxygen transfer in water through emulsified organic liquids, Chem. Eng. Sci. 46 (1991) 1869–1873, doi:http://dx.doi.org/10.1016/0009-2509(91)87034-A. C.J. Van Ede, R. Van Houten, A.A.C.M. Beenackers, Enhancement of gas to water mass transfer rates by a dispersed organic phase, Chem. Eng. Sci. 50 (1995) 2911–2922, doi:http://dx.doi.org/10.1016/0009-2509(95)133-P. S. Weisenberger, A. Schumpe, Estimation of gas solubilities in salt solutions at temperatures from 273 K to 363 K, AIChE J. 42 (1996) 298–300, doi:http://dx. doi.org/10.1002/aic.690420130. F. Yoshida, T. Yamane, Y. Miyamoto, Oxygen absorption into oil-in-water emulsions. A study on hydrocarbon fermentors, Ind. Eng. Chem. Proc. Des. Dev. 9 (1970) 570–577, doi:http://dx.doi.org/10.1021/i260036a013. Z. Zhang, W. Chen, S. Yu, W. Li, Enhancement of gas-to-water mass-transfer rates by the second liquid phase, Ind. Eng. Chem. Res. 49 (2010) 3223–3227, doi:http://dx.doi.org/10.1021/ie9018808. H.J.O. Pinho, S.S. Alves, Effect of spreading coefficient on gas-liquid mass transfer in gas-liquid-liquid dispersions in a stirred tank, Chem. Eng. Comm. 197 (2010) 1515–1526, doi:http://dx.doi.org/10.1080/00986445.2010.485011.
Contents lists available at ScienceDirect
Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep
Intensified biogas purification in a stirred tank T.N. Wedraogo a, *, S. Poncin a , J. Wu b , Huai Z. Li a a b
Laboratoire Réactions et Génie des Procédés, Université de Lorraine, CNRS, 1 Rue Grandville, BP 20451, 54001 Nancy, France State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 5 February 2014 Received in revised form 19 August 2014 Accepted 6 October 2014 Available online 8 October 2014
The present study aims at intensifying the absorption of carbon dioxide in aqueous phases. For this purpose, a second immiscible liquid phase is used to improve the mass transfer of carbon dioxide in water. Absorption measurements are carried out in a stirred cell by online infrared monitoring of the output carbon dioxide concentration. As dispersed phase, 1-octanol shows an improvement in mass transfer whereas colza oil retards it. Toluene has no significant effect in the range of concentrations investigated. Mass transfer coefficients are determined using a chemical method. Nevertheless, the variety of behaviors could not be explained simply by solubility or variation of mass transfer coefficients. In order to understand the complex gas–liquid–liquid interactions and deduce the mass transfer mechanism between these three phases, surface and interface tension measurements and direct observation of bubbles and droplets are also carried out. ã 2014 Published by Elsevier B.V.
Keywords: Biogas purification Intensified CO2 absorption Liquid–liquid dispersion Reversible emulsion
1. Introduction Biogas produced during the anaerobic treatment of wastewater is an interesting energy resource from an environmental and economic point of view. The process of methanisation yields a biogas composed of 55–70% of methane and 30–45% of carbon dioxide along with impurities such as hydrogen sulfide. It can be burned on site to provide heating or electricity, but there are other value adding uses. For example, biogas can be injected into a grid of natural gas provided that the methane content reaches 95%. Numerous separation methods can be used to eliminate the carbon dioxide, the most common of which is absorption in aqueous amine solutions. This process exhibits high absorption rates but is rather energy intensive because of the heat needed for desorption. An alternative for carbon dioxide separation is physical absorption. The physical solvents require less energy for desorption but suffer from a slower absorption rate. In the present study, the emphasis is given to the intensified absorption of carbon dioxide in water. Intensification happens through the use of a second liquid organic phase to form a reversible emulsion, dispersed droplets can enhance the gas–liquid transfer conditions [9]. This kind of gas–liquid–liquid systems has attracted only few attention despite it is quite common in the industry. Among
* Corresponding author. Tel.: +33 662685345. E-mail address: [email protected] (T.N. Wedraogo). http://dx.doi.org/10.1016/j.cep.2014.10.004 0255-2701/ ã 2014 Published by Elsevier B.V.
the existing applications, there is the dispersion of hydrocarbons in water in order to give a substrate to aerobic bacteria. Yoshida et al. [22] noticed that the presence of toluene of oleic acid enhances the oxygen transfer to water, while the presence of paraffin retards it. The authors attribute these changes to the position of the organic phase around gas bubbles. Later, Linek and Beneš [13] investigated two possible transfer mechanisms: transfer in parallel where direct gas– oil contact is possible and gas is absorbed in both water and oil simultaneously; and transfer in series for which there is no direct gas–oil contact. They concluded that the pathway for gas absorption is strongly affected by interfacial properties. Bruining et al. [5] introduced a concept firstly developed for the study of gas–liquid– solid systems. In this “shuttle mechanism”, elements of the dispersed phase move towards the gas–liquid interface, uptake the solute and release it in the bulk of the liquid phase. Furthermore, these authors remark that mass transfer is not enhanced unless the gas is highly soluble in the dispersed phase. According to the same reference, droplets of dispersed phase also need to be small enough to accommodate in the thin mass transfer zone around bubbles. Rols et al. [19] proposed a mechanism where the dispersed liquid phase spreading over a gas bubble absorbs the solute and redisperses when the bubble explodes. Although there are numerous attempts to understand the underlying processes of absorption in liquid–liquid systems, no convincing representation can currently explain the large variety of behaviors observed. The modification of the volumetric mass transfer coefficient kL and the interfacial area a was also extensively studied but contradictory results were
2
T.N. Wedraogo et al. / Chemical Engineering and Processing 86 (2014) 1–8
Nomenclature a A C D Ech Eph F Ha J kL kapp m N Q R VL
specific gas–liquid interfacial area (m2 m3) total gas–liquid interfacial area (m2) molar concentration (mol m3) diffusivity of CO2 in the solution (m2 s1) chemical enhancement factor physical enhancement factor enhancement factor due to the presence of an emulsion Hatta number specific absorption rate (mol m2 s1) liquid side mass transfer coefficient (m s1) apparent rate constant for the absorption reaction (s1) ratio of the solubility in the dispersed phase to the solubility in the continuous phase impeller rotation speed (rpm) volumetric gas flowrate (m3 s1) volumetric absorption rate (mol m3 s1) volume of liquid in the reactor (m3)
Exponents 1 refers to 2 refers to * refers to i refers to
the the the the
inlet of the reactor outlet of the reactor equilibrium state gas–liquid interfacial conditions
of 1300 rpm which allows a good dispersion of the oil phase while keeping a relatively flat surface. The reactor is operated at 273 K (0.5 K) and 1.1 bar (0.1 bar). A flowmeter measures the outlet gas flow rate. The outlet gas is cooled down to 4 C in order to remove the water vapors and its composition is determined by a GAS3230R infrared online analyzer (GEIT, Belgium). A by-pass can be used to lead the initial gas mixture directly to the analyzer and check the composition of the inlet gas mixture. The molar fraction of methane and carbon dioxide are equal to the volume fraction measured by the infrared analyzer. The mass balance on CO2 between the inlet and outlet of the reactor can be expressed as follows: Q 1 C 1CO2 ¼ Q 2 C 2CO2 þ R V L
(1)
Once the steady state is reached, the carbon dioxide concentration and the gas outlet flowrate are stable. Then the carbon dioxide absorption rate is obtained from Eq. (1). The Danckwerts plot technique is used to obtain both the mass transfer coefficient kL and the interfacial area a. Every absorption experiment is conducted with a 0.5 M K2CO3/0.5 M KHCO3 buffer in which the following global reaction takes place: K2 CO3 þ CO2 þ H2 O ¼ 2KHCO3
(2)
Three reactions are involved: Subscripts c
relative to the catalyst R the value is relative to the continuous phase DR = D(oil phase)/D (continuous phase) Greek letters d diffusion film thickness (m) e dispersed phase volumetric fraction () f absorption rate (mol/s) t contact time according to penetration model (s)
þ CO2 þ H2 O ¼ HCO 3 þ H k3
(3)
CO2 þ HO ¼ HCO 3 k4
(4)
2 þ HCO 3 ¼ CO3 þ H k5
(5)
The rate of reaction (3) is very slow but can be catalyzed by a number of agents, among which sodium hypochlorite (the catalytic rate constant is written kc). Its rate is given by: r3 ¼ ðk3 þ kC C C Þ C CO2
obtained [9]. These authors reported a dramatic decrease of kLa at low fraction of dispersed phase which is attributed to initial changes in the surface tension. Cents [6] used a chemical method for the measurement of kL and a. The results showed that some substances enhance the transfer whereas others retard it, this could be related to the presence or absence of dispersed phase droplets in the mass transfer zone around bubbles according to the author. 2. Experimental
(6)
Reaction (4) is a second order reaction. Its rate is given by: r4 ¼ k4 cOH cCO2
(7)
Eq. (5) allows the calculation of the hydroxyl anions concentration: C HO ¼
K W C CO2 3
K 5 C HCO3
¼
KW K5
(8)
2.1. Mass transfer Fig. 1 shows a schematic representation of the set-up. It comprises a feed section, a stirred cell and an analysis section. The first part is designed to mix pure methane and carbon dioxide (from two 3.5 grade gas bottles) of desired composition by means of two SLA5850 mass flow regulators (Brooks, USA) and thus produce a synthetic biogas. The gas mixture is then led into a 1 L tank (10 cm internal diameter) through a sintered metal distributor. A pitched blade turbine with four blades at a 45 angle is used as an impeller and is located just above the gas inlet in order to disperse the bubbles properly. The reactor is equipped with four baffles to prevent the formation of a vortex. All experiments are conducted at a fixed impeller speed
The overall reaction rate can therefore be calculated as follows: KW cCO2 ¼ kapp cCO2 (9) rCO2 ¼ k3 þ kc cc þ k4 K5 In this equation cCO2 represents the carbon dioxide concentration. Danckwerts also derived a criterion which can be used to assess if the reaction is of pseudo first order: the concentration of species HCO3, CO32 and OH must be constant and uniform from the bulk to the interface. The validity of this criterion has been discussed by Cents et al. [7] who concluded that in order to reduce the error in the calculation to less than 3%, the following condition should be fulfilled:
T.N. Wedraogo et al. / Chemical Engineering and Processing 86 (2014) 1–8
3
Fig. 1. Experimental set-up.
2
C CO2
3
"sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi# D kapp 1 2 4 5 iþ 1þ h 1 < 0:1 2 2 HCO kL 3 CO3
(10)
The solubility and diffusivity of carbon dioxide in the buffer were determined respectively from the work of Weisenberger and Schumpe [21] as well as Joosten and Danckwerts [12]. The physico-chemical parameters used are gathered in Table 1. Using relevant data from Table 1 and experimental values, the maximum value of the left-hand term in Eq. (10) is 0.07, showing that the criterion is satisfied in the present experiments. Kinetic constants k3, kc and k4 can be determined respectively from the work of Pinsent and Roughton [17], Benadda et al. [2] and Pohorecki and Moniuk [18]. Finally, the Danckwerts surface renewal model establishes the following equation for absorption rate for a pseudo first order reaction with negligible bulk concentration: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi kapp D i ca R ¼ kL 1 þ (11) 2 kL The value of kapp can be modified by varying the sodium hypochlorite concentration (cc). The corresponding values of R can then be used to fit kL and a from Eq. (11).
droplets can be determined from the images of the camera. It is also possible to see where droplets of the dispersed phase are located around gas bubbles. To our best knowledge, such visualization does not exist for a complex gas–liquid–liquid system in the literature. 3. Results and discussion 3.1. Mass transfer experiments Absorption rate is measured in emulsions for various organic phases: edible oil, toluene and octanol, without any surfactant to avoid the stabilization of emulsions. The results are compared to those obtained using a single liquid phase. Since the water is saturated in carbon dioxide in only a few minutes, it was decided that the liquid phase reference should be the 0.5 M K2CO3/0.5 M KHCO3 solution in which absorption rate is stable in time. Fig. 2 displays the values of absorption rate in the stirred tank for the different gas–liquid and gas–liquid–liquid systems for a given rotation speed of the impeller. These values are compared using an intensification factor Eph which is defined as the ratio of the absorption rate in the emulsion
2.4. Direct observation of bubbles and droplets The set-up illustrated in Fig. 1 is designed to observe visually a gas–liquid–liquid system from inside. A high speed camera (Dantec Dynamics, Denmark) is linked to a borescope (Olympus, Japan) which can be placed at various heights in the tank. A light source (Olympus, Japan) provides lighting in the area of focus. The emulsion is created by the rotation of a four-blade turbine. Four baffles prevent the formation of a vortex. A gas phase is then injected at the bottom of the tank. The size of the bubbles and Table 1 Physico-chemical parameters at T = 20 C. Parameter
Value
Unit
Reference
k3 kc k4 DCO2 HeCO2 VL
0.02 1.54 2.16 1.48 109 3.83 104 0.50
s1 m3 mol1 s1 m3 mol1 s1 m2 s1 mol m3 Pa1 L
[17] [2] [18] [12] [21] Fig. 2. Temporary evolution of the absorption rate. N = 1300 rpm, Q CH4 = Q CO2 = 50 mL min1.
4
T.N. Wedraogo et al. / Chemical Engineering and Processing 86 (2014) 1–8
to that in the buffer alone. This factor accounts for the enhancement due to physical absorption in the dispersed phase. The evolution of Eph with the fraction of the dispersed phase e for various organic phases is reported in Fig. 3. Three different behaviors can be observed. Octanol clearly enhances the gas–liquid transfer especially at low organic phase fraction. Similar results were obtained by Zhang et al. [23] for the absorption of carbon dioxide in a isoamyl alcohol–water emulsion with a maximum at low dispersed phase fraction. Toluene exhibits small effect on the absorption rate in the range of volume fraction investigated. Colza oil retards mass transfer whatever the involved amount. These observations allow establishing a first categorization of the oils regarding their influence on the gas– liquid mass transfer rate of carbon dioxide. Furthermore, it was decided to carry on experiments with octanol and colza oil only since toluene has limited effect on mass transfer and presents health and safety issues. It is common to represent the chemical absorption rate as: (12) R ¼ kL a C CO2 C CO2 Ech This equation is based on the hypothesis that there is no resistance to mass transfer located at the interface. It is also written for a unique liquid phase. Thus, we have to consider the emulsion as a pseudo homogeneous phase with properties stemming from those of the relative pure liquids. 3.1.1. Influence of solubility The solubility of carbon dioxide in the emulsion is calculated as the mean of the solubility in the pure phases weighted by the percentage of dispersed phase e and displayed in Table 2, the solubility in colza emulsion being unknown. The solubility of carbon dioxide in pure toluene and in pure octanol used in the calculations were respectively published in Dack [8] and Abraham et al. [1]. Obviously, these data reveal that the increase of solubility cannot explain alone the variation of the absorption rate. Indeed, Fig. 3 shows that within the investigated range, Eph does not increase with the amount of organic dispersed phase while the solubility does, at least in toluene and octanol. Moreover, despite a better solubility in toluene emulsions, carbon dioxide is absorbed more quickly when the dispersed phase is octanol.
Table 2 Solubility of carbon dioxide (mol m3) in the emulsions at T = 20 C and PCO2 = 0.5 atm.
e
Octanol
Toluene
0 0.05 0.1 0.15 0.2
12.2 12.6 13.1 14.0 14.9
12.2 12.9 14.0 15.8 17.6
3.1.2. Hydrodynamic influence The presence of a dispersed phase modifies also the global hydrodynamics which controls the mass transfer process. Fig. 4 displays the ratio between values of kL and a in emulsions and in pure buffer (kL0 and a0). Experimental a/a0 and kL/kL0 data reported in Fig. 4 show that octanol and colza oil display opposite effects on both kL and a. The presence of colza oil increases the interfacial area and reduces the mass transfer coefficient kL. On the contrary, the presence of octanol improves hydrodynamics through the increase of kL but leads to a lower interfacial area. Our results are very similar to those of Cents [6] for the system octanol–CO2 but interfacial area a is often reported to increase with the addition of an oil phase because the presence of droplets would reduce the rates of coalescence [9]. This discrepancy might be caused by the interpretation of the Danckwerts model. As mentioned before, the numerous results published about the variation of kLa in presence of a dispersed phase are contradictory. Mass transfer coefficient kL is known to increase with the diffusivity of a gas in the liquid. Since the diffusivity of carbon dioxide is approximately three times higher in octanol than in the buffer, the results in Fig. 4 are consistent with the expectations. The resulting kLa values for octanol are slightly higher than those obtained in pure buffer, while they are 20% lower in the case of colza oil. Consideration of such a point leads to attribute the low carbon dioxide absorption rate in this emulsion to the low kLa value of colza oil emulsion. For the octanol however, the classical approach to quantify gas–liquid mass transfer, which relates to gas–liquid interfacial area a and hydrodynamics of the liquids through the mass transfer coefficient kL, does not seem relevant. As above-mentioned, precise mechanisms of mass transfer should be proposed to take into account the effects not involved in Eq. (12). The mechanism of mass transfer in parallel describes a situation where carbon dioxide can be absorbed simultaneously into both liquid phases. In contrary, solute must be absorbed in the continuous phase first in the case of mass transfer in series. 3.1.3. Influence of surface tension To account for the possibility of the dispersed phase to be in direct contact with gas bubbles, Yoshida et al. [22] proposed to use the spreading coefficient S: (13) S ¼ s wg s og þ s ow
Fig. 3. Eph as a function of e. N = 1300 rpm, Q CH4 = Q CO2 = 50 mL min1.
If S is positive, direct transfer to the organic phase is possible as a parallel transfer mechanism, whereas for negative S, the gas must be absorbed in water first following a transfer in series scheme. However, contradictory results are obtained with this approach and it is difficult to draw an unequivocal conclusion [9]. The recent work of Pinho and Alves [24] shows that the value of S determines the mass transfer mechanism, except when S is close to zero. Cents [6] obtains positive S for toluene and octanol, which is confirmed by our measurements made on a tensiometer (I.T. Concept, France) (Table 3). The accuracy is 1 mN m1. The surface tension of watersimulated biogas is 70 mN m1.
T.N. Wedraogo et al. / Chemical Engineering and Processing 86 (2014) 1–8
5
Fig. 4. Relative values of a and kL, N = 1000 rpm, Q CH4 = Q CO2 = 50 mL min1. Table 3 Surface tension and interfacial tension at T = 20 C. Organic phase
s o g (mN m1)
s o w (mN m1)
S (mN m1)
Octanol Colza oil Toluene
24 22 26
9 58 34
37 10 10
For colza oil, the negative spreading coefficient would indicate that there is no direct contact between the gas bubbles and the dispersed liquid phase. The transfer could then be in series and the assumption of a pseudo-homogeneous liquid phase to represent the emulsion would be valid. In the case of toluene and octanol, the spreading coefficient S is positive, which means that in an emulsion, these liquids can be in direct contact with the gas. However this observation does not explain the difference of intensification observed between toluene and octanol. So far, there is still no study on the physical meaning of the precise value of S. In the work of Ngo and Schumpe [16], n-heptane (S = 11.8 mN m1) always enhances kLa while n-hexadecane (S = 2.2 mN m1) and ndodecane (S = 2.2 mN m1) enhance kLa up to e = 0.3 but have a negative effect on kLa when the concentration is higher. In our work, colza oil (S = 10 mN m1) always reduces kLa. This needs to be investigated further but it seems that the spreading coefficient is an indicator for the mass transfer mechanism. The difference in Eph between octanol and toluene can be interpreted under the angle of another mass transfer mechanism known as the shuttle effect. In this case, droplets of dispersed phase travel to the diffusion film around the bubbles where they uptake the solute to release in the bulk of the continuous liquid phase. By simulation, Brilman et al. [4] determined that a droplet can enhance the mass transfer only if it is very close to the interface. Another result of their work reveals that the enhancement factor is inversely proportional to the droplet diameter. In our case, the lower value of the interfacial tension between octanol and water could make droplets small enough to accommodate in the
thin mass transfer film around the bubbles since a low value of s o w indicates that a small amount of energy is needed to create liquid–liquid interface. In order to further investigate this issue, measurements are performed in the presence of surfactants. The results of these experiments are gathered in Table 4. Due to foaming properties of the surfactant, we are unable to measure the absorption rate when there is no dispersed phase or when the dispersed phase is colza oil. With these liquids, the stirred tank rapidly fills up with foam, leading to a complete absorption of carbon dioxide. This is thought to be due to an extremely high value of the interfacial area created in the foam. Some previous works report either no significant influence of the presence of surfactants on the absorption rate [10] or a decrease of this rate due to the reduction of the interfacial area and mass transfer coefficient kL [11]. The factor Eph is calculated as the ratio between the absorption rate in the stabilized emulsion and the absorption with no surfactant and no dispersed phase, it might therefore be underestimated. It can be seen from Fig. 5 that the stabilized emulsion enhances the mass transfer more than the emulsions without surfactant. Since the presence of a surfactant usually prevents the droplets and the bubbles from coalescing and reduces droplets’ size due to a lower interfacial tension, the shuttle effect and the interfacial area
Table 4 Mean droplet diameter d (mm).
e
Octanol
Octanol + Tween80
Colza oil
Colza oil + Tween80
0.02 0.05 0.1
– 119 131
– 55 61
137 172 195
100 133 177
Fig. 5. Intensification factor Eph in presence of Tween 80. N = 1300 rpm, Q CH4 = Q CO2 = 50 mL min1.
6
T.N. Wedraogo et al. / Chemical Engineering and Processing 86 (2014) 1–8
Fig. 6. Illustration of the shuttle mechanism in the system biogas–water–octanol (5%v).
are logically enhanced with respect to reversible emulsion without surfactant. 3.1.4. Influence of viscosity None of the previously discussed models takes into account the viscosity of the emulsion although it can affect significantly the mass transfer rate. The viscosity of the emulsion has a huge effect on the global hydrodynamics in the reactor. All other things being equal, an increase in viscosity will reduce the turbulence and the gas–liquid interface renewal because of higher energy dissipation. This is often mentioned as the reason why the enhancement factor levels of at high fraction of dispersed phase [19,20]. 3.2. Direct observation of the gas–liquid–liquid system Even though it cannot be used as a method to determine absolute droplet and bubble sizes, we can use the set-up described above to compare the behavior of our various systems. Due to the opacity of the emulsions, the light source must be as close as possible to the borescope. It results that acceptable images can be obtained only in the bottom of the tank. The size distribution measured is therefore shifted towards the small diameters
(because the big droplets and bubbles ascend more quickly). Since the high speed camera allows very short exposure time (down to 1 ms) we can obtain very clean images of quick moving objects. As shown in Table 4 droplets are smaller in octanol emulsions than in colza oil emulsions. This is in agreement with the interfacial tension measurements. Moreover, the mean droplet diameter increases with the dispersed phase fraction in each set of data. Results in Table 4 also confirm that the presence of surfactants (Tween80) is responsible for a diminution of the droplet size. The visual observation of gas–liquid–liquid systems reveals that octanol droplets are attached to almost every gas bubble whereas colza oil droplets only adhere to very small gas bubbles. Fig. 6 is an illustration of the shuttle mechanism: droplet 1 (D1) initially sticks to the gas bubble (B) and is released on the third image. On the contrary, droplet 2 (D2) is initially free. It comes into contact with the bubble on the second image and stays attached. Oil droplets in contact with a gas bubble can take various positions. Most of the time, two spherical droplets are close to each other. In this configuration, they are in contact but the contact area is limited. From this configuration it is however possible to switch to another position where the droplet is no longer spherical but
Fig. 7. Illustration of the spreading of a droplet of octanol on a bubble.
T.N. Wedraogo et al. / Chemical Engineering and Processing 86 (2014) 1–8
Eph
Table 5 Expression of J and Eph according to the mass transfer model. Film model
d ðC C Þ
J
D
Eph
1 þ eðmR DR 1Þ
Higbie penetration theory qffiffiffiffiffi D ðC C Þ 2 pt pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1 þ e mR DR 1
adopts the curvature of the bubble and therefore covers a large part of its surface. The transition from the first configuration to the second one is illustrated in Fig. 7. The reverse phenomenon, i.e., the detachment of the droplet from the bubble was not observed so it is difficult to specify the threshold conditions that for this kind of structural organization. 3.3. Modeling 3.3.1. Shuttle effect The most extensively used model to describe the enhancement factor is firstly developed by Bruining et al. [5]. It is based on the shuttle effect and therefore it assumes that direct contact between the gas and the organic phase does not happen. Other hypotheses include liquid–liquid equilibrium and the presence of droplets of organic phase in the diffusion zone in the same proportion as in the rest of the liquid. The mass balance with a first order reaction in the mass transfer zone is (C denotes the CO2 concentration):
@2 C @C ¼ kapp ð1 eÞC þ ð1 þ eðmR 1ÞÞ @t @x2
D
(14)
with the following initial and boundary conditions: t ¼ 0; x > 0 : C ¼ C 0 t > 0; x ¼ 0 : C ¼ C i t > 0; x ! 1 : C ¼ C 0
(15)
When the capacity of the bulk is sufficient, we can assume that C0 = 0 mol m3. Eq. (14) can be solved analytically to obtain the absorption rate: pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Nabs ¼ kL 1 þ eðmR 1Þ þ ð1 eÞHa2 C i aV L (16) The enhancement factor Eph can then be calculated as follows: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Nabs ðe 6¼ 0Þ 1 þ eðmR 1Þ þ ð1 eÞHa2 ¼ (17) Eph ¼ Nabs ðe ¼ 0Þ 1 þ Ha2 Since Ha = 3.5 103 in the absence of catalyst, we can simplify Eq. (17) to:
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ 1 þ eðmR 1Þ
7
(18)
A more recent contribution by Brilman et al. [3] tried to improve this model by taking into account the distribution of droplets near the interface and liquid–liquid mass transfer. These authors showed that only the droplets very close to the gas–liquid interface determine mass transfer enhancement. Their model fails in describing the enhancement factor levelling off at high dispersed phase fraction but a good agreement is obtained with the data of Littel et al. [14]. The simple model of Bruining generally underestimates the enhancement factor by approximately 20% [9]. Moreover, the estimated thickness of the mass diffusion film in our work never exceeds 5 mm while droplet sizes are never smaller than 20 mm. Even though the droplet size distribution may not be precise due to both the emulsion opacity and small droplets’ size, it does not seem to satisfy the major hypothesis of this model. Nagy [15] proposed a one-dimensional model to calculate the enhancement factor whatever the size of the droplet. However, the values of relative solubility m for our systems are so low that even for very small droplets very close to the interface the enhancement factor predicted by a shuttle effect model would be lower compared to our experimental results. 3.3.2. Transfer in parallel This mass transfer mechanism was not often discussed in the literature. The first model was proposed by Van Ede et al. [20]. When the solute gas can be absorbed in both liquid phases, the absorption rate consists of two terms: transfer from gas to the continuous phase (’GC) and transfer from gas to the dispersed phase (wGD). The enhancement factor Eph: Eph ¼
’GC þ ’GD JGC AGC þ JGD AGD ¼ ’GC JGC AGC
(19)
It is then assumed that: 1. Liquid phases are at equilibrium with each other. 2. The fraction of gas–liquid interface occupied by the dispersed
phase ADC is equal to eA and the fraction of gas–liquid interface occupied by the continuous phase AGC is equal to (1 e)A. 3. Enhancement due to chemical reaction is negligible. Finally, the specific absorption rate J is given by different expressions depending on the model used for the behavior of the liquid film (Table 5). As shown in Fig. 8, Bruining’s model for the shuttle effect can be used to estimate Eph in the case of toluene. However, in the case of octanol this model underestimates significantly the value of Eph since it is of the same order of magnitude as the maximum
Fig. 8. Experimental and model based values of Eph. Dispersed phase: (a) octanol, (b) toluene.
8
T.N. Wedraogo et al. / Chemical Engineering and Processing 86 (2014) 1–8
enhancement factor. This evidence, along with the proof that octanol can spread over a bubble (Fig. 7), is a reliable indication that mass transfer could be in parallel for the system octanol– carbonate buffer-CO2. When a surfactant is added into the liquid, the mass transfer in the system toluene–carbonate buffer-CO2 seems to switch from a transfer in series to a transfer in parallel. This is confirmed by the fact that liquid–liquid interfacial tension should drop in presence of Tween 80, increasing the value of the spreading coefficient. This point would however need further experimental validation since, S could not be measured and Eph cannot be accurately estimated in presence of a surfactant.
[3]
[4]
[5]
[6] [7]
4. Conclusion A novel approach for the upgrading of biogas was developed in this work. The influence of various organic phases on the absorption efficiency of carbon dioxide in an aqueous solution was investigated. With octanol as dispersed phase, an enhancement of 20% was observed, whereas absorption rate decreased when colza oil was used. The presence of toluene induced a small enhancement of the absorption rate of carbon dioxide. Experimental evidence such as a large spreading coefficient and visual observation of a droplet spreading over a bubble indicates that mass transfer might be in parallel in the system octanol– carbonate–CO2. This mechanism allows much higher enhancement factors than the shuttle effect mentioned in the literature. The measurement of kL and a in the gas–liquid–liquid systems was also performed. The negative impact of the viscosity on the mass transfer results in the enhancement factor leveling off at high holdup, unfortunately it is difficult to measure the viscosity of reversible emulsions. The solubility of methane in the liquids could also be a source of error for the accurate measurement of the absorption rate. Methane is 15 times less soluble in water than carbon dioxide but its solubility in organic phases is not reported in the literature. Further investigations are still required to take into account a detailed mass balance. Acknowledgments The financial assistance provided by the French Agence Nationale de la Recherche (ANR PROMET) and by the Natural Science Foundation of China (51061130555) to this joint project is gratefully acknowledged.
[8] [9]
[10]
[11]
[12]
[13]
[14]
[15]
[16] [17]
[18]
[19]
[20]
[21]
[22]
References [1] M.H. Abraham, J. Le, W.E. Acree Jr., P.W. Carr, A.J. Dallas, The solubility of gases and vapours in dry octan-1-ol at 298 K, Chemosphere 44 (2001) 855–863, doi: http://dx.doi.org/10.1016/S0045-6535(00)288-5. [2] B. Benadda, M. Prost, S. Ismaily, R. Bressat, M. Otterbein, Validation of the gaslift capillary bubble column as a simulation device for a reactor by the study of CO2 absorption in Na2CO3/NaHCO3 solutions, Chem. Eng. Process.: Process
[23]
[24]
Intensif. 33 (1994) 55–59, doi:http://dx.doi.org/10.1016/0255-2701(94) 85003-8. D.W.F. Brilman, M.J.V. Goldschmidt, G.F. Versteeg, W.P.M. van Swaaij, Heterogeneous mass transfer models for gas absorption in multiphase systems, Chem. Eng. Sci. 55 (2000) 2793–2812, doi:http://dx.doi.org/ 10.1016/S0009-2509(99)491-1. D.W.F. Brilman, W.P.M. van Swaaij, G.F. Versteeg, A one-dimensional instationary heterogeneous mass transfer model for gas absorption in multiphase systems, Chem. Eng. Process.: Process Intensif. 37 (1998) 471–488, doi:http:// dx.doi.org/10.1016/S0255–2701(98)55-5. W.J. Bruining, G.E.H. Joosten, A.A.C.M. Beenackers, H. Hofman, Enhancement of gas–liquid mass transfer by a dispersed second liquid phae, Chem. Eng. Sci. 41 (1986) 1873–1877, doi:http://dx.doi.org/10.1016/0009-2509(86)87066-X. A.H.G. Cents, 2003. Mass transfer and hydrodynamics in stirred gas–liquid– liquid contactors. A.H.G. Cents, D.W.F. Brilman, G.F. Versteeg, CO2 absorption in carbonate/ bicarbonate solutions: the Danckwerts-criterion revisited, Chem. Eng. Sci. 60 (2005) 5830–5835, doi:http://dx.doi.org/10.1016/j.ces.2005.05.020. M.R.J. Dack, Solutions and Solubilities, Part I 386, Wiley, 1975. E. Dumont, H. Delmas, Mass transfer enhancement of gas absorption in oil-inwater systems: a review, Chem. Eng. Process.: Process Intensif. 42 (2003) 419–438, doi:http://dx.doi.org/10.1016/S0255-2701(02)67-3. R. Farajzadeh, A. Barati, H.A. Delil, J. Bruining, P.L.J. Zitha, Mass transfer of CO2 into water and surfactant solutions, Pet. Sci. Technol. 25 (2007) 1493–1511, doi:http://dx.doi.org/10.1080/10916460701429498. D. Gómez-Díaz, N. Gomes, J.A. Teixeira, I. Belo, Oxygen mass transfer to emulsions in a bubble column contactor, Chem. Eng. J. 152 (2009) 354–360, doi:http://dx.doi.org/10.1016/j.cej.2009.04.059. G.E.H. Joosten, P.V. Danckwerts, Solubility and diffusivity of nitrous oxide in equimolar potassium carbonate-potassium bicarbonate solutions at 25 and 1 atm, J. Chem. Eng. Data 17 (1972) 452–454, doi:http://dx.doi.org/10.1021/ je60055a016. V. Linek, P. Beneš, A study of the mechanism of gas absorption into oil–water emulsions, Chem. Eng. Sci. 31 (1976) 1037–1046, doi:http://dx.doi.org/ 10.1016/0009-2509(76)87024-8. R.J. Littel, G.F. Versteeg, W.P.M. Van Swaaij, Physical absorption of CO2 and propene into toluene/water emulsions, AIChE J. 40 (1994) 1629–1638, doi: http://dx.doi.org/10.1002/aic.690401005. E. Nagy, Three-phase mass transfer: one-dimensional heterogeneous model, Chem. Eng. Sci. 50 (1995) 827–836, doi:http://dx.doi.org/10.1016/0009-2509 (94)441-S. T.H. Ngo, A. Schumpe, Absorption of CO2 into alkane/water emulsions in a stirred tank, J. Chem. Eng. Japan 45 (2012) 737–741. B.R.W. Pinsent, F.J.W. Roughton, The kinetics of combination of carbon dioxide with water and hydroxide ions, Trans. Faraday Soc. 47 (1951) 263–269, doi: http://dx.doi.org/10.1039/TF9514700263. R. Pohorecki, W. Moniuk, Kinetics of reaction between carbon dioxide and hydroxyl ions in aqueous electrolyte solutions, Chem. Eng. Sci. 43 (1988) 1677–1684, doi:http://dx.doi.org/10.1016/0009-2509(88)85159-5. J.L. Rols, J.S. Condoret, C. Fonade, G. Goma, Modeling of oxygen transfer in water through emulsified organic liquids, Chem. Eng. Sci. 46 (1991) 1869–1873, doi:http://dx.doi.org/10.1016/0009-2509(91)87034-A. C.J. Van Ede, R. Van Houten, A.A.C.M. Beenackers, Enhancement of gas to water mass transfer rates by a dispersed organic phase, Chem. Eng. Sci. 50 (1995) 2911–2922, doi:http://dx.doi.org/10.1016/0009-2509(95)133-P. S. Weisenberger, A. Schumpe, Estimation of gas solubilities in salt solutions at temperatures from 273 K to 363 K, AIChE J. 42 (1996) 298–300, doi:http://dx. doi.org/10.1002/aic.690420130. F. Yoshida, T. Yamane, Y. Miyamoto, Oxygen absorption into oil-in-water emulsions. A study on hydrocarbon fermentors, Ind. Eng. Chem. Proc. Des. Dev. 9 (1970) 570–577, doi:http://dx.doi.org/10.1021/i260036a013. Z. Zhang, W. Chen, S. Yu, W. Li, Enhancement of gas-to-water mass-transfer rates by the second liquid phase, Ind. Eng. Chem. Res. 49 (2010) 3223–3227, doi:http://dx.doi.org/10.1021/ie9018808. H.J.O. Pinho, S.S. Alves, Effect of spreading coefficient on gas-liquid mass transfer in gas-liquid-liquid dispersions in a stirred tank, Chem. Eng. Comm. 197 (2010) 1515–1526, doi:http://dx.doi.org/10.1080/00986445.2010.485011.