Keywords

1 Introduction

Methane hydrates are inclusion compounds formed by water and methane at high pressure and low temperature (Sloan and Koh 2007). They are present in sedimentary deposits in permafrost regions and beneath the sea in outer continental margins (Kvenvolden 1993; Paul and Dillon 2000; Hester and Brewer 2009). One volume of gas hydrates releases about 160 volumes of methane at standard temperature and pressure (STP) (Kvenvolden 1993). It is estimated that carbon bound in methane hydrates is twice the carbon content in fossil fuel reserve, and hence these are supposed to be the future potential energy resource. The worldwide assessment of (Kvenvolden 1998a) methane hydrate is equivalent to 250 trillion cubic meters of methane gas. The amount of hydrate bound methane gas is approximately 100 times as large as that of conventional natural gas resource. As per the USA department of energy, if only very small amount of the methane stored in these hydrates could be exploited, it would be almost twice the current domestic supply of natural gas (Haq 1998; Holder et al. 1987). They are also seen as future technology for storage and transportation of gas.

1.1 Energy Potential of Gas Hydrates

The amount of methane caught in gas hydrate is not certain. Various groups have given various amounts of methane trapped in global gas hydrate deposit ranging from ~1017 ft3 or 105 trillion cubic feet (TCF) (Mciver 1981) to 108 TCF (Trofimuk et al. 1977; Kvenvolden 1988, 1998b; Gornitz and Fung 1994; Harvey and Huang 1995). Recently in review on the concerned subject by (Boswell and Collett 2011), the amount of methane caught in gas hydrates has been estimated around 105 TCF (Boswell and Collett 2011). As per the idea of USA department of energy, even negligible amount of methane stored in hydrates can be exploited. It is more than the current supply of natural gas in the country (Holder et al. 1987; Haq and Boulder 1998). Methane hydrates can be assumed upcoming source of hydrocarbon energy and will be a future fuel (Paul and Dillon 2000; Kvenvolden and McMenamin 1980).

2 Present Study

This paper investigates the effect of a biosurfactant rhamnolipid on methane hydrate formation kinetics in laboratory. Rhamnolipid was obtained from P. aeruginosa strain A11. The presence of P. aeruginosa has been reported in Gulf of Mexico gas hydrate samples (Lanoil et al. 2001) which helped in generating rhamnolipids at the site. Methane hydrate formation experiments were performed with 90 % saturation of distilled water (and water–surfactant solution) in porous C-type silica gel (pore volume 0.90 cm3 g−1). The study is carried out in high-pressure vessel maintained at low temperature.

3 Experimental Setup, Materials, Procedure

3.1 Apparatus

Video hydrate cell was used to study the gas hydrate formation which is a mercury-free cell. It can measure the induction time for formation of hydrates and monitor the pressure drop as a function of time during hydrate formation. The system consists of constant volume hydrate cell having capacity of 250 cm3 and pressure rating up to 3000 psi. The temperature is controlled by the thermostatic bath. A computer is attached for data acquisition of temperature and pressure versus time. The diagram of the gas hydrate cell is shown in Fig. 1.

Fig. 1
figure 1

Experimental setup

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3.2 Materials

Pure methane with 99.99 % pure methane (Chemtron Science Laboratory, Navi Mumbai, India), reverse osmosis water (Millipore SA, 67,120 Molshein, France), C-type silica gel (Merk Merck Specialities Pvt. Ltd., Mumbai, India), and rhamnolipid synthesized in laboratory from strain A11 were used for the experiment.

3.3 Procedure

First, 90 % saturated C-type silica gel was used for experiment; then 90 % saturated C-type silica gel with 100 ppm rhamnolipid aqueous solution and then 1000 ppm rhamnolipid aqueous solution was used for the experiment. The test sample was placed in hydrate cell and constant temperature was maintained immersing the cell into a temperature-controlled bath. A mixture of water and ethylene glycol (25 %) was used as a liquid for bath. A vacuum pump was used to remove the air from the cell before pressurizing methane gas in the cell. The cell was pressurized up to the desired value with methane gas. Sudden pressure decline was observed for the hydrate formation and online video picture was also seen for identifying hydrate formation. When noticeable pressure drop is not observed, it signifies the completion of process of formation of hydrates. The induction time, moles of gas consumed, and rate of hydrate formation were calculated from the obtained pressure temperature date.

4 Results and Discussion

The results obtained in the present study are discussed as follows.

4.1 Induction Time

The induction time, t i, is the passed from the beginning of the experiment at t s to the onset of hydrate formation t o and it is one of the kinetic parameters of hydrate formation. The induction time for various experimental conditions is given in Table 1, and the moles of methane consumed are shown in Fig. 1.

Table 1 Methane hydrate formation parameters for various types of samples
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It can be concluded from Table 1 that the induction time in the presence of 100 ppm rhamnolipid has been reduced which states that rhamnolipid is acting as promoter for the formation of methane gas hydrates.

4.2 Moles of Gas Consumed

While the hydrate is formed, the drop of pressure was observed, which is due to the consumption of gas of methane. The methane takes over the unoccupied cavities of water while hydrate formation. The amount of gas consumed during hydrate formation can be calculated by real gas equation:

$$n = n_{\text{i}} - n_{\text{f}} = \frac{V}{R}\left( {\frac{{P_{\text{i}} }}{{z_{\text{i}} T_{\text{i}} }} - \frac{{P_{\text{f}} }}{{z_{\text{f}} T_{\text{f}} }}} \right)$$
(1)

where n is the amount of gas consumed when hydrates form, V is the gas volume, P i, T i and P f, T f are the pressure and temperature at initial and final conditions, and R is the universal gas constant. Compressibility factors (z i, z f) are calculated at the corresponding pressure and temperature, respectively. The pattern of gas consumed during formation of hydrate is shown in Fig. 2.

Fig. 2
figure 2

Moles of methane gas consumed

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As it is evident from Fig. 2 that maximum numbers of moles are consumed for 1000 ppm rhamnolipid, at lower dose, i.e., at 100 ppm, the number of moles of methane gas consumed is lesser than without rhamnolipid but as the concentration of rhamnolipid is increased than amount of methane gas consumed is more than that of without rhamnolipid.

Figure 3 shows the consumption of gas and pressure drops as a function of time. The growth and gas consumption regions show the two symmetric plots that could be explained as conservation of mass during the hydrate growth. Pressure drop shows the number of methane molecules leaving the gas phase to hydrate phase drop, whereas gas consumption plot shows the number of methane molecules entering into the hydrate cages. Mass remains conserved in an isochoric system, which is shown by the symmetry of the two plots about an axis passing through the intersection of the two plots and parallel to the time axis. As shown in Fig. 3, the process is splitted into three regions. First region starts at time zero where the consumption of the gas is less in the system and pressure slightly decreases because of dissolution of gas. Second region is after the induction time where gas consumption is increased; this happens because of the hydrate growth. In third region as the hydrate is formed, gas consumption is as shown in Fig. 3.

Fig. 3
figure 3

Growth curve of methane hydrate formation in C-type silica gel in the presence of 100 ppm rhamnolipid

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4.3 The Rate of Methane Hydrate Formation

As seen from Fig. 4, the slope of the curves after nucleation indicates an exponential behavior from which we can assume a first-order reaction. Equations 2 and 3 can be used for a first-order reaction which is defined as follows:

Fig. 4
figure 4

N/N0 versus time plot for methane hydrate formation in the presence of C-type silica gel containing 100 ppm rhamnolipid

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$$N = N_{o} e^{ - kt}$$
(2)
$$\ln \frac{N}{{N_{o} }} = - kt$$
(3)

where N is the total number of moles at time t, N 0 is the initial number of moles, k is the rate constant (min−1), and t is the time in minute. The rate constant (k) of hydrate formation can be calculated from the slope of the curve of ln (N/N 0) versus time. The hydrate formation from region 2 onward Fig. 3 was divided into five zones of time as shown in Fig. 5 (0–20, 20–40, 40–80, 80–160, 160–260 min) after nucleation. The data of these five zones are used to determine the rate constant of hydrate formation in each zone. Hydrate formation rate is determined by putting the values of slopes, i.e., rate constant k calculated from Fig. 4 in the following equation:

Fig. 5
figure 5

Semi-logarithmic plot of change of moles of gas while methane hydrate formation in the presence of C-type silica gel containing 100 ppm rhamnolipid

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$$\frac{{{\text{d}}N}}{{{\text{d}}t}} = - N_{o} ke^{ - kt}$$
(4)

The values of rate constant and concerned rates obtained in present study in the presence of 100 ppm rhamnolipid are shown in Table 2.

Table 2 The rate of methane gas hydrate formation for 100 ppm rhamnolipid
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The above results indicate that rhamnolipid acts as promoter for the methane hydrate formation. It is also observed that the induction time of hydrate formation is reduced in the presence of rhamnolipid (Table 1). The rate of hydrate formation is found to increase many time (Table 2).

The zeta potential of rhamnolipid synthesized in the present study from strain A11 was found to be negative; hence, it is anionic surfactant and much of work in literature on anionic surfactants such as SDS, etc. on gas hydrate formation has been reported. These results are in agreement with various studies reported on synthetic anionic surfactants like SDBS (Dai et al. 2014) ans LABS (Ganji 2007; Fazlali et al. 2013; Kumar et al. 2014).

The rhamnolipid acts as a promoter as the biosurfactant spherical micelles are formed by long-chain carbon alkyl groups which solubilize hydrocarbon gases (MacKerell 1995).

The above results indicate that rhamnolipids act as promoter which can help in synthesizing gas hydrates. Industries looking upon gas hydrates not because of their energy potentials rather it is becoming a technology these days because of their other applications such as transportation and storage of gas, desalination of water, etc. The present study clarifies the role of rhamnolipid a biosurfactant as promoter in natural sites and giving a green biodegradable promoter as much of work as promoter of gas hydrates is reported on synthetic surfactants; however, the study has given a replacement of synthetic surfactant by a green biosurfactant.

5 Conclusion

The above study clearly indicates the role of rhamnolipid in the formation of methane gas hydrate formation as promoter; thus small dosages of rhamnolipids produced by P. aeruginosa strain A11 must clearly enhance the gas hydrate formation kinetics in natural sites (as in Gulf of Mexico). Rhamnolipid is a green biodegradable promoter for gas hydrates formation, and is a green bio-surfactant which can substitute the synthetic surfactant which is used presently as promoter for the formation of gas hydrates.