A novel method was developed for measuring gas solubility in organic solvents using a semipermeable Teflon AF-2400 tube-in-tube membrane contactor. This membrane tube ensures the gas saturation of liquids that flow continuously at specific pressures and temperatures. After the liquid is depressurized, the amount of gas emitted is measured with a bubble meter and used for solubility calculations. This method is suitable for the measurement of the solubility of oxygen in toluene and benzyl alcohol. Validation experiments were initially performed by comparing the solubility of the obtained oxygen in toluene with the literature data. As the temperature increases, the solubility of oxygen in benzyl alcohol increases, indicating that the oxygen dissolution process is endothermic. Finally, the relationship between Henry's law constant and temperature is determined.
Gas solubility data in organic solvents is important in the chemical industry for the design of various unit operations such as separators and reactors, and the solubility of oxygen is probably one of the most important. In general, the solubility of oxygen in liquids can be measured experimentally by analytical or synthetic methods. The two main methods are classified according to whether the equilibrium composition is determined (analysis) and whether the total mixture is prepared using a predetermined amount of components (synthesis). For example, the solubility of oxygen in ionic liquids was measured using analytical gravimetry. The composition is calculated based on the change in the weight of the liquid on the weight microbalance before and after gas adsorption. The solubility of oxygen in CO2 expanded liquids was assessed by analyzing gas and liquid samples in equilibrium using a gas chromatograph. The solubility of oxygen in organic solvents was determined by a static synthesis method, in which the amount of pure components was precisely known, and changes in temperature and pressure during the experiment were monitored.
The oxidation of alcohols is one of the most basic organic transformations and is of great significance in chemical synthesis. Traditionally, such reactions involve stoichiometric inorganic oxidants (hexavalent chromium or permanganate), leading to serious environmental problems. From the perspective of green chemistry, there is a growing demand for catalyzing the oxidation of alcohols using molecular oxygen as an oxidizing agent. The amount of oxygen available affects the degree of oxidation reaction. On the one hand, local oxygen shortages due to low solubility of oxygen in solvents and poor mass transfer may limit the reaction rate. On the other hand, excess oxygen may cause excessive oxidation of the product or catalyst. Mixtures of oxygen and organic solvents at high temperatures and pressures can also raise serious safety concerns. Therefore, there is a strong need for more data on the solubility of oxygen at reaction pressure and temperature.
Recent advances in membrane tube science provide an opportunity to develop new methods for measuring oxygen solubility. In particular, Teflon AF-2400 uniquely combines excellent chemical resistance, thermal stability, and mechanical properties with a high free volume fraction. Its permeation properties for gases and liquids ensure high flux of gases passing through the membrane tube and low permeability of liquids at the same time.
The researchers first developed a tube-in-tube contactor, which consists of a Teflon AF-2400 inner tube and a polytetrafluoroethylene (PTFE) outer tube, and applied it to various reactions such as ozone decomposition and hydrogenation. As the liquid flows through the inner tube, the pressurized gas can pass through the semi-permeable tube and dissolve in the liquid until equilibrium is reached. It has been observed that the saturation concentration of gases in the liquid inside the tube-in-tube contactor roughly follows Henry's law. In this study, the Teflon AF-2400 tube-in-tube contactor was used as a novel device to measure the solubility of gases in liquids, using the case of oxygen in toluene and benzyl alcohol as an example.
2.1 Materials
Oxygen used in this study (99995 %) from BOC. Toluene ( 995%) and benzyl alcohol (99%) from Sigma-Aldrich. All materials can be used without further purification.
2.2 Instruments and equipment
A schematic diagram of the tube-in-tube membrane contactor device is shown in Figure 1. The membrane tube contactor is defined by an inner diameter (id) of 08 mm, outer diameter (OD) is 1The 0 mm Teflon AF-2400 inner tube consists of (to purchase the Teflon AF-2400 inner tube, please contact Fulin Plastics) and the inner diameter is 24 mm, outer diameter of 32 mm PTFE outer tube. Unless otherwise noted, perfluoroalkoxy (PFA) tubing (I.D. 1.) is used for all parts of the unit0 mm, outer diameter 16 mm) connection. The total length of the tube-in-tube membrane contactor is 100 cm. The liquid, i.e., toluene or benzyl alcohol, is pumped into the inner tube, while oxygen flows in the annular space between the inner and outer tubes. The liquid flow rate is controlled using an HPLC pump and the fluid pressure is maintained by a back pressure regulator. The oxygen flow is controlled by a mass flow controller and the oxygen pressure (PO2) is maintained by another back pressure regulator. The actual pressure of the gas and liquid is measured by two pressure sensors.
Immerse the tube-in-tube membrane contactor in a stirred oil bath. The temperature of the oil bath is controlled by a hot plate fitted with thermocouples. Use a gas sparger to pre-bubble the liquid with oxygen at room temperature and atmospheric pressure 05 hours to remove other dissolved gases. When the oil bath temperature and oxygen pressure are stable, the liquid is pumped into the contactor. Oxygen passes through the semi-permeable membrane and dissolves into the liquid, reaching equilibrium in the contactor when the liquid stays long enough. As the liquid flows out of the contactor and passes through the back pressure regulator, the dissolved oxygen is degassed from the liquid due to decompression. The mixture of gases and liquids is directed into a 250 ml glass container separator, pre-filled with pure oxygen, and placed in a water bath maintained at 298 K, in which the gases and liquids are separated. Thus, the liquid collected in the separator is saturated with oxygen at a pressure of 1 bar and 298 K.
The gas flowing out of the separator includes the contribution of degassed oxygen saturated with steam and the contribution of the change in gas volume in the separator due to liquid collection (see Figure 1). Measurements were made using a 1 ml bubble flow meter connected to the outlet of the separator via a silicone rubber tubing (about 3 mm ID). The bubble meter contains about 1 ml of soapy water, assuming it is saturated with oxygen. Check the complete sealing of the separator to avoid measurement errors due to oxygen loss to the surrounding environment.
Figure 1Schematic diagram of a tube-in-tube membrane contactor device for measuring Henry's law constant. MFC: Mass Flow Controller;pump: HPLC pump;p: pressure sensor;t: thermocouple for temperature control;BPR: Back Pressure Regulator.
The experiment was performed by changing the oxygen pressure from 1 bar to 10 bar while keeping the liquid pressure at 10 bar. All stress in this study is absolute. The experimental temperature range is 298 K to 393 K. At a given oxygen pressure, three different liquid flow rates (025–0.45 ml min (1)) and measure the gas flow rate from the separator at least three times at each liquid flow rate.
2.3 Calculations
In the membrane tube contactor, the dissolution of the liquid in the membrane tube and further penetration into the gas phase are ignored. In a separator, it is assumed that the gas phase is saturated with organic vapors, which must be subtracted from the measured gas flow rate. However, the loss of organic solvents into the gas phase is negligible because of their small amounts, i.e., 01%。The constant (h) of Henry's law at a given temperature t is defined as:
where po2 is the oxygen pressure and x is the molar fraction of oxygen in the liquid at a given temperature t. The amount of oxygen released per volume of liquid (fo2) can be expressed as:
where T is the experimental temperature and VO2 is the molar volume of oxygen at STP (22400 ml mol (1));rl is the density of the liquid (g ml (1));ml is the molar mass of the liquid (g mol (1)), po2 is the oxygen pressure (bar), patm is the atmospheric pressure (1 bar), h is the constant of Henry's law at the experimental temperature t, and H298K is the constant of Henry's law at 298 k. By plotting FO2 versus PO2, one can calculate the Henry constant at a given temperature.
Initially, the method was validated by measuring the solubility of oxygen in toluene. Measurements were taken at 298 K and 348 K with oxygen pressures up to 10 bar. Experimental data for FO2-PO2 are plotted in Figure 2. The slope of the fitted line is 0 for 298 k20001 ml mlliquid (1) bar (1), 0. for 348 k21922 ml mlliquid^(–1) bar^(–1)。
Figure 2Oxygen-toluene degassed from the liquid as a function of oxygen pressure (Po2) at (a) 298 K and (b) 348 K.
The Henry's Law constants were calculated at 1057 bar and 965 bar, respectively, and compared with the literature data obtained by the static synthesis apparatus, as shown in Figure 3. The values in this work are very close to the literature data, and the difference is within 5% at the same temperature (DH H·100%), which proves the reliability of the method.
Figure 3Comparison of the Henry's Law constant of oxygen in toluene found in this study with the literature.
After method validation, the solubility of oxygen in benzyl alcohol was measured at temperatures from 298 K to 393 K and oxygen pressures up to 10 bar. Experimental O2-PO2 data are shown in Figure 4. The slope of the fitting line, as well as the calculated Henry's Law constant and the molar fraction (x) of oxygen in benzyl alcohol at 1 bar oxygen pressure, are summarized in Table 1. To provide oxygen solubility data, the saturation concentration of oxygen (CO2) in benzyl alcohol at a pressure of 1 bar of oxygen at each temperature is also provided.
Figure 4Oxygen-benzyl alcohol is a function of oxygen pressure (Po2) at (a) 298 K, (b) 313 K, (c) 333 K, (d) 353 K, (e) 373 K, (f) 393 K.
As shown in Table 1, the solubility of oxygen in benzyl alcohol increases with increasing temperature. This trend is consistent with the trend of oxygen in toluene, indicating that solubilization is endothermic. The Henry's Law constant for oxygen in benzyl alcohol at 298 K is 3462 bar, which is three times higher than the Henry's Law constant in toluene (1057 bar). This may be due to the lower enthalpy of solvation of oxy-benzyl alcohol, suggesting that the interaction between oxygen and benzyl alcohol is not as good as that between oxygen and toluene.
Table 1Henry's Law Constant and solubility of oxygen in benzyl alcohol at 1 bar pressure.
At low pressure, the constants of Henry's law can be considered independent of pressure, so they can be expressed as a function of temperature:
where a and b are the fitting parameters. The experimental data in Table 1 are plotted in Figure 5 as ln(h) vs. 1t plots. A straight line is obtained with a correlation coefficient r = 0.987。Thus, the correlation between the Henry's Law constant and temperature of the oxygen benzyl alcohol system can be described as:
Figure 5Henry's Law Constant (bar) of oxygen in benzyl alcohol as a function of temperature.
A novel device for measuring the solubility of oxygen in toluene and benzyl alcohol by means of a semipermeable Teflon AF-2400 tube casing membrane contactor is presented. The measured solubility of oxygen in toluene was consistent with the literature data, demonstrating the reliability of the method. The solubility of oxygen in both toluene and benzyl alcohol was found to increase with increasing temperature. However, the solubility of oxygen in benzyl alcohol is much lower than that in toluene, which is of great significance for the precise design of alcohol aerobic oxidation reactors.
It is important to measure the solubility of the solvent substrate used in the actual reaction, as the solubility of using similar but not identical components can lead to reactor design errors. Due to the high permeability of Teflon AF-2400 tubing to other gases such as carbon monoxide and hydrogen, the method is not limited to oxygen and organic solvents, and is generally suitable for measuring the solubility of other gases in a wide range of liquids.
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