Measurement and correlation of solubility of carbon dioxide in triglycerides
Graphical abstract
Comparison of experimental results with correlation for solubility of CO2 in triglycerides as a function pressure at two different temperatures of 289.15 and 303.15 K, respectively.
Introduction
In the recent years, there has been intense research focused on developing efficient and cost effective technologies for transportation biofuels for securing energy, reducing environmental concerns and lessening foreign oil dependency [1], [2], [3]. One of the key components in a renewable energy portfolio is biodiesel, which is biodegradable, less toxic and has energy density similar to conventional petroleum diesel [4], [5], [6]. Oleaginous microorganisms are considered as one of the possible alternative feedstocks for biodiesel as they can accumulate more than 20% lipid on a dry weight basis [7]. To use these lipids as biodiesel feedstock, they must be first extracted from the microorganisms. The traditional Bligh and Dyer [8] method used to extract the lipid from the oleaginous yeast in the laboratory setting would be uneconomical on a commercial scale because of high extraction cost. The conventional oil extraction method [9] for oleaginous algae requires lipid source to be nearly free of water with solid content more than 90%. Since biomass drying accounts for more than 75% total energy consumption, the process becomes economically unfeasible [10]. However, the lipid recovery can be increased if the cell is disrupted prior to extraction, potentially decreasing the need to dry to (⩾90%) solids [11], and thus reducing the energy requirement for the process [12]. Efficient and economical cell lysing results in enhanced lipid extraction due to the increased mass transfer rates [13].
Some of the commonly available methods for microbial cell disruption are mechanical, physical, chemical, and enzymatic cell disruptions [14]. Cell lysing using chemical solvent is a useful technique that is able to disrupt the microbial cell wall by reacting with lipophilic tail, but seems to be unfeasible due to the cost of solvent recovery. Different solvents were tested to evaluate the feasibility of cell inactivation, and it was found that carbon dioxide is superior to other chemicals such as nitrogen, argon, tetrafluoroethane, etc. [15]. Lin et al. reviewed the microbial cell disruption using supercritical and subcritical CO2 [16]. The lipid extraction cost using supercritical CO2 (pressure and temperature above 7380 kPa and 304.5 K) is significantly higher; thus, a process utilizing subcritical CO2 can potentially reduce the extraction cost. If the cell is disrupted using pressurized CO2 prior to lipid extraction, the extraction kinetics enhances due to the reduced mass transfer limitation since carbon dioxide can penetrate through the phospholipid membrane of the cell [15]. CO2 facilitates the disruption due to the high solubility of carbon dioxide in lipid. Furthermore, CO2 is cheap, nontoxic, nonflammable, and naturally abundant [17]. A thorough understanding of solubility of carbon dioxide in triglycerides is essential for designing efficient cell disruption processes. Although there is a large number of studies [18], [19], [20], [21], [22] conducted on solubility of triglycerides in super critical carbon dioxide for oil extraction, there is a lack of data on low pressure CO2 solubility in triglycerides.
This paper presents an experimental design based on pressure drop gas apparatus that not only allows for the measurement of gas solubility but also cell disruption by applying pressure and vacuum in a cyclic manner. The solubility of CO2 in canola oil, a triglyceride consisting primarily of oleic, linoleic, and alpha linoleic acid was measured at different temperatures and pressures. We selected canola oil because its fatty acid profile is very similar to the triglycerides produced by the microbes [7]. The experimental solubility was correlated with three thermodynamic models. Thermodynamic properties such as enthalpy of dissolution, entropy of dissolution and Gibbs energy of dissolution were determined using van’t Hoff’s plot.
Section snippets
Materials
Refined canola oil was purchased from Fisher Scientific, USA. The iodine value, saponification value, acid value, and peroxide value of canola oil were 111, 190.3, 0.01, and 1.67 as described by the supplier. For the characterization of canola oil, its fatty acid compositions were determined, and compared with the available literature [23]. For fatty acid methyl ester (FAME) analysis, the refined canola oil was transesterified, and analyzed using Agilent 6890N gas chromatograph equipped with a
Data validation
To validate the experimental set-up, the solubility of CO2 in water was determined at three different temperatures of 288.2, 293.2 and 298.2 K, respectively at different pressures and compared with the data available in the literature [30], [31], [32], [33]. The experiments were conducted in triplicate to ensure the accuracy of the data obtained from this work. From Table 3, it is evident that there is a good agreement with the previous experimental data as the deviation was small, which
Conclusions
A new pressure drop gas apparatus was developed to measure the solubility of gases in liquids. The apparatus was validated using both solubility of carbon dioxide in water as well as CO2 solubility in tetradecane. The solubility of carbon dioxide in triglycerides was measured at different temperatures and pressures, and it was found that solubility of CO2 in triglycerides is significantly higher than that of pure water. The solubility was correlated using Krichevsky–Kasarnovsky (KK), Mather-Jou
Acknowledgements
Md Howlader is thankful to the Dave C. Swalm School of Chemical Engineering, Mississippi State University for the financial support. He is grateful to Mrs. Sara Shields-Menard for helping in GC analysis, Mr. Peter Alzobaidi for his help setting up the experiment, Mrs. Magan Green for her help in FAME analysis and Dr. Bill Elmore for his general help during the research. The author is cordially acknowledges the help of Dr. Keisha B. Waters for providing the Karl-Fischer instrument and Mrs.
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