Saturation of oil with oxygen

Having shown that the rig is air tight and that the reaction system temperature is well controlled, it was important to develop a method to saturate the oil within the reaction rig with oxygen from the surrounding air or any other oxygen/nitrogen mixture. The ability to reliably saturate the oil with oxygen provides a useful method of defining the start point for any batch oxidation and provides the first calibration point for each batch oxidation.

To reach saturation, oxygen must be supplied at a rate much faster than it is removed by oxidation. In order to be sure that oxygen can be supplied at a rate fast enough to saturate the oil, a reaction system consisting of 90 mL of fish oil at 35ºC was set up. Once preheated to the test temperature, 35ºC, the oil was placed into a reaction vessel and sparged with nitrogen at a flowrate of 0.3 L.min-1. The oxygen probe (FOXY-R- 8cm, OceanOptics, Florida, USA) was placed in a plastic tube attached to the outlet valve of the reaction chamber (see Figure 4.3 and Figure 4.4) and the probe response was recorded every two seconds (TauTheta, OceanOptics, Florida). Once all oxygen had been removed, signaled by a steady state response from the probe during sparging with nitrogen, air was sparged through the sample at a rate of 0.3 L.min-1. This sparging rate was chosen as this was deemed to be approximately the highest flowrate that could be used to sparge the sample without there being significant losses of sample with the exiting gas and because it provided significant mixing.

Figure 4.8: Oxygen probe response while in the exit gas stream above a sample of mixed fish oil at 35ºC. Initial results are during sparging with nitrogen, followed by sparging with air.

As expected, the response from the probe (lifetime) initially began at its highest point when the sample of oil was sparged with nitrogen gas. Upon commencement of sparging, the response from the probe dropped quickly and reached a steady state value after approximately 180s. At the end of the experiment, the probe was removed from the exiting gas stream and placed in 35ºC air at atmospheric pressure. The resulting probe response was equal to the steady state value given during the experiment and showed that the sample of oil must have been saturated with oxygen.

From the data used to create Figure 4.8 it is possible to determine the mass transfer coefficient, 𝑘_𝐿𝑎, for this system and shown in Equation 4.2. By estimating the magnitude of 𝑘_𝐿𝑎 it should be possible to understand how oxygen consumption rates alter the time needed for the oil to become saturated with oxygen.

𝜕𝑂2

𝜕𝑡 =𝑘𝐿𝑎(𝑂2 ∗_{− 𝑂}

2)− 𝑟 Eq. 4.2

where 𝑂₂∗is the saturated oxygen concentration, 𝑂₂is the oxygen concentration, and r is the oxygen consumption rate by oxidation reactions

0 0.2 0.4 0.6 0.8 1 1.2 1.4 0 20 40 60 80 100 120 140 160 180 200 Li fet im e res po nc e ( us ) Time (s)

As the dissolved oxygen probe gives a measure of the partial pressure of oxygen rather than the concentration in traditional concentration units, Eq. 4.2 cannot be used directly to estimate 𝑘_𝐿𝑎 without knowing the saturated oxygen concentration in oil. To estimate 𝑘_𝐿𝑎, we must first ensure that the rate of oxygen supply is significantly greater than the rate of oxygen consumption via reaction. Doing this allowed for the removal of the reaction term from Eq. 4.2. Secondly, using Henry’s Law, which suggests a linear relationship between partial pressure and concentration, it is possible to convert the units of Eq. 4.2 from mol.m-3s-1 to Pa.s-1 as shown in Eq. 4.3.

𝜕𝑃_𝑂2

𝜕𝑡 = 𝑘𝐿𝑎�𝑃𝑂2∗− 𝑃𝑂2� Eq. 4.3

where 𝑃_𝑂2∗ is the saturated partial pressure of oxygen and 𝑃_𝑂2is the partial pressure of oxygen

Integrating Eq. 4.3 gives Eq. 4.4 which can then be used to create a plot to estimate

𝑘𝐿𝑎.

𝑙𝑛 �𝑃_𝑃𝑂2∗−𝑃𝑂2𝑖

𝑂2∗−𝑃𝑂2�= 𝑘𝐿𝑎(𝑡 − 𝑡0) Eq. 4.4

where 𝑃_𝑂2𝑖is the initial partial pressure of oxygen

By collecting data during sparging of oil and plotting the logarithmic term in Eq. 4.4 versus time, it was possible to estimate 𝑘_𝐿𝑎. The results, shown in Figure 4.9, gave a

𝑘𝐿𝑎 of 8.5x10-2 s-1 for sparging of a sample of 90 mL of mixed fish oil (Bakels Ltd, NZ) at 35 ºC with an air flowrate of 0.3 L.min-1.

Figure 4.9: Plot to estimate the mass transfer coefficient kla for sparging of mixed fish oil with 0.2 L.min-1 air at 35 ºC

The above experiment suggests that it does not take long for a sample of oil to become saturated with oxygen. However, the results also show that the temperature of the sample does have an effect on the response from the probe. Allowing a preheated sample of oil to equilibrate with the incubator environment is likely to be a good practice. Therefore, a 30 minute sparging and equilibration time was used for all experiments from this point on.