CAPITULO I: ASPECTOS GENERALES
1.2 Objetivos de la investigación
1.2.2 Objetivos específicos
In order to understand the modification happening on the material during repeated short cycles, other tests similar to those showed before have been made. In this case, however, the mass spectrometer connected at the exit of the reactor was less sensitive than the one used in the previous case, so it was not possible to obtain the trend of hydrogen. However, these new tests have been made mainly to characterize the magnetite at different stage of the cycling. Characterization through XRD, Mössbauer, spectroscopy and carbon analyses have been performed on the magnetite sample. Before reporting the results of this characterization it was important to verify the proper functioning of the cycling process and ethanol conversion.
1 2 3 4 5 6 7
a.u.
reduction step 1 2 3 4 5 6 7
a.u.
Ethanol conversion
The conversion of ethanol during the 5min step cycling process has been followed by mass spectrometry (Figure 5-16).
Figure 5-16. Trend of the ethanol consumption in the composition of the outgassed stream as observed from the mass analysis during repeated cycles of 5 minute per step.
The reduction steps are those were the ethanol is observed in higher quantities. It is possible to note that the amount of ethanol in the outgassed stream increases during about the first 120 minutes of cycling i.e. 15 steps of 5 minute before stabilization. This means that the ethanol conversion is decreasing at the beginning before reaching a state. To understand if this characteristic depends exclusively from the material or from the duration of the steps, another test switching the feeding between ethanol and water every two minutes has been performed.
0 20 40 60 80 100 120 140 160 180 200
u
.a.
Figure 5-17. Trend of the ethanol in the composition of the outgassed stream as observed from the mass analysis during repeated cycles of 2 minute per step.
As it is possible to observe in Figure 5-17, the same behavior of ethanol conversion was observed for the lower time cycling.
5.3.2.1. Characterization of the magnetite during cycling
It was important ti verify if the material was changing radically in the short cycles process as in the longer one. Characterization by XRD and Mössbauer spectroscopy have been made first at the beginning of and the end of the reduction re-oxidation step of the first short time cycle and secondly at the beginning and the end of the re-oxidation of the twentieth short time cycle, which corresponds to a total time of 3h20 (longer than the 2h hours of a 1h-step cycle).
0 20 40 60 80 100 120 140 160
u.a.
Figure 5-18. Comparison of the X-ray diffraction patterns of Fe3O4 at the beginning (in red) of and the
end (in black) of the re-oxidation step of a 5 minutes cycle
The X-ray diffraction patterns show that after 5 minutes of reduction the material is slightly reduced (Figure 5-18). In fact there is only one small peak at 45° 2 corresponding to iron. When completed the cycle this peak disappears and the pattern is the typical one of magnetite. In both the cases, the peaks are relatively narrow which indicates that sintering already took place during the reduction step of the material.
Figure 5-19. Comparison of the X-ray diffraction patterns of Fe3O4 at the beginning (in red) of and the
10 20 30 40
2-theta
50 60 70 80 after 5' reductionafter 1cycle (5+5)
10 20 30 402-Theta
50 60 70 80after 19 cycles
+ 5' reduction
after 20 cycles (5+5)
The X-ray diffraction patterns of the material at the beginning and the end of the re- oxidation of the twentieth short time cycle shows are similar to those recorded during first cycle. No coke and no carbide are detected. In order to better quantify the phases and detect the presence of minimal amounts of carbide or a coke, Mössbauer analyses and carbon analyses have been performed.
Table 5-1. Mössbauer parameters computed from the spectra of Fe3O4 after the different steps of the
repeated cycles, recorded at 25°C; IS: isomer shift; H: internal magnetic field.
SAMPLE % IS (MM/S) H (T)
Fe3O4 after 5 minutes reduction
Fe3+ 35 0.26 48.8
Fe2,5+ 63 0.65 45.8
Fe0 2 -0.16 32.5
Fe3O4 after 1 complete cycle
(5/5minutes)
Fe3+ 35 0.26 49.2
Fe2,5+ 65 0.65 46.1
Fe3O4 after 19 complete cycles + 5
minutes reduction
Fe3+ 33 0.26 49.1
Fe2,5+ 63 0.65 46.0
Fe0 4 -0.04 32.6
Fe3O4 after 20 complete cycles
Fe3+ 37 0.24 49.0
Through the Mössbauer analysis it has been possible to quantify the amount of the different species of iron at the same stages as those studied by XRD. After the first 5 min of reduction a metallic iron phase characterized by an average magnetic hyperfine field of 32.5T and an isomer shift of -0.10mm/s is formed and represents 2% of the iron species in the sample. After the re-oxidation only the sextet characteristic of magnetite are observed.
For the twentieth cycle, the Mössbauer analysis is similar except that the relative amount of iron formed is slightly higher with 4%.
The carbon content of the samples have been measured through the CHN analysis. The results are presented in Table 5-2
Table 5-2. Amount of carbon present in the sample after the various step of the repeated cycle as obtained from the CHN analysis
SAMPLE % OF CARBON IN WEIGHT
Fe3O4 after 5 minutes reduction 0.3
Fe3O4 after 1 complete cycle (5/5minutes) 0.2
Fe3O4 after 19 complete cycles + 5 minutes reduction 1.3
Fe3O4 after 20 complete cycles 1.0
At the beginning of the re-oxidation step of the first 5 min cycle, the analyses show the presence of a small amount of coke of 0.3% in weight. After the re-oxidation step the value slightly decreases to 0.2%. The coke has thus been only partially oxidized during the
At the beginning of the re-oxidation step of the twentieth 5 min cycle, the analyses show that the amount of carbon has increased with 1.3% which is a very good results considering that the overall time of cycling at this point is 195 minutes.
Finally, it has measured that after 20 complete cycles of 5 minutes (200 minutes totally) per step the C% is only 1.0%.