Chapter 4. Experimental Test 66
Figure 28 – Hematite powder has been weighted with a precision balance under a fume hood. The picture shows the instruments used in this operation: balance, inox steel spoon and a piece of paper used as funnel for inserting the powder in the pipe.
trace oxygen (trace O2) and trace moisture (trace H2O) for consistent, precise process gas measurement [51]. Because of it, it also turned out to be necessary to change the solvent gas from argon to nitrogen. In fact, argon as a gas might be interfering with the chosen analytics, while nitrogen is more suitable as it does not undermine the quality of the measurements.
After this changes to the gas tanks and metering tool, it had been necessary to calibrate the gas flow meters of the cabinet, to be sure the sample was receiving the correct quantities of gas. To do this, an external instrument was used to measure volumetric and standardized gas flow in the pipe coming out of the pipe exiting the reactor. This means the calibration was effectuated with an empty alumina tube already in place, and by checking the flow after the coming back to the cabinet. The calibration proceeded by setting some volumetric flows on the cabinet controller computer and checking what the flow measured by the other instrument effectively was. The tool used for the calibration is the Definer 220 by Mesa Labs. Its manual reports a volumetric flow accuracy of 0.75%
and standardized flow accuracy of 1% of reading [52]. After collecting different points and correlating the set mass flows with the measured ones, it has been possible to understand what was the value to be set on the cabinet controller to effectively have the desired flow output. This procedure has been repeated for both methane and nitrogen.
Next, the empty pipe was substituted with a reactor charged with hematite with an L/D ratio of 2, therefore 3.79 g. The pipe edges have been folded with teflon to ensure a better sealing during the operation and the reactor was inserted in the support. A few changes were made also in the general layout of the solar paraboloid to make it more useful for this test’s purpose. The B-type thermocouple was extracted from the inside of the pipe and was placed on the external surface, reaching with its tip the focal point. This exposed it to a much stronger heat flux and more variable conditions. This change was made because it was not possible anymore to keep it inside the pipe, due to its volume (the concerns are related to the fact it would interfere with the glass wool and reaction bed and would cause a pressure increase in the gas flow). Moreover, an external thermocouple would be able to record the effective temperature the pipe is exposed to, rather than the of course lower one that is reached inside of it. Then, an
Chapter 4. Experimental Test 68
Figure 29 – Final setup for test #1, performed on the 25th of June. The external B-type thermocouple can be seen held by a wire.
additional N-type thermocouple was inserted in the reactor until the glass wool thanks to its reduced diameter. The final setup for test #1 is reported in Fig 29.
After the setting up, the void pump was activated by the command on the controller to extract all the air in the line and check whether the sealing was effectively separating the reaction environment from the external environment. The pressure difference measured was around −70 bar, indicating a sufficient sealing of the line.
After the setup was fully completed, the solar concentrator was switched on and set on automatic mode at 13:25. The parabolic was quickly oriented towards the Sun and the temperature started growing extremely quickly. In fact, the measured temperature jumped from 3.40°C when not oriented (as the thermocouple is designed for working at high temperature, it is quite normal the value read when not operating is not accurate)
Figure 30 – Picture showing the ceramic fracture of the pipe. The edges are sharp and the pipe was divided in only two pieces that perfectly match.
to 883.65°C in a minute. After one more minute, at 13:27, the measured temperature reached even 1077.31°C.
At 13:42, gas flows started according to the calibration previously undertaken. The system started injecting 91 sccm of CH4 and 165 sccm of N2 (the flows set on the computer controller were 86 and 224 sccm respectively). Unfortunately, when the gas flows were switched on and the recording of the test started, the analytic connected to the output of the line kept on measuring air concentration of oxygen for the whole duration of the test. Moreover, no CH4 was detected. After more than one hour of running the test, at 13:57, the solar concentrator was switched to its initial position and turned off. The pipe was found broken into two by a sharp ceramic fracture, as it can be noticed from Fig. 30, explaining the reason why the analytic was measuring oxygen in the mixture.
The reason of the failure of the test was thought to be related to the extremely fast heating up of the reactor, which was not present in the literature, when the material was heated up more slowly by following a maximum of 150°C/min in the oven according to the website of an alumina producer [53]. As it can be understood in Appendix A.1, by Fig 60, the heating up ramp before reaching a more or less stable value of temperature, is around 500°C/min, with a peak of more than 800°C/min during the first minute.
Therefore, the research team decided to try solving the problem before setting up a new test.
Chapter 4. Experimental Test 70