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Chapter 8 – Egg-shell catalysts

8.4 Catalytic Tests

Catalytic tests have been carried out in a fixed bed reactor. 4 g of each catalysts were charged into the reactor. Before starting the tests, the catalysts were activated for 4h at 400°C under hydrogen flow. For the tests, a mixture of H2 and CO, with molar ratio 2:1, was fed to the reactor. The total feeding flow is 12 l/h: 8 l/h of hydrogen and 4 l/h of CO. The tests are carried out under a pressure of 20 bar.

The data of test of FT08 are collected between 180°C and 250°C. As it can be seen by observing the Graph 8.2, the conversion increases rapidly with temperature up to 100% at 240°C, then it begins to decrease.

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Graph 8.2: Conversion vs temperature for FT08

Another test with this catalysts was carried out at the same conditions, but by maintaining the temperature constant at 220°C, in order to evaluate the behaviour of the catalyst during the time (Graph. 8.3).

Graph 8.3: Conversion vs time for FT08

By regarding the data, a little decrease of the conversion is observed. It passes from around 50%

to a little less than 40%. To this trend, due to an incipient deactivation, an increase is associated of the production of methane (from 20 to 30%) and of C2 – C4 hydrocarbons (from 10 to around 20%), while the CO2 production remains unaltered. The liquid phase discharged at the end of the test shows a good agreement with the ASF theory (Graph 8.4), with an α value of 0.79.

0 20 40 60 80 100

150 170 190 210 230 250 270

Conversion (%)

Temperature (°C)

FT08

0 10 20 30 40 50 60

0 1 2 3 4 5 6

Conversion/Selectivity (%)

Days of test

FT08

Conversion

Methane selectivity CO2 selectivity C2 - C4 selectivity

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Graph 8.4: linearized production of condensed products for FT08

By regarding the results of the characterization of the FT09 catalysts, we decided to catalytically test the FT09-A which seems the better one. The condition of catalytic tests are those already seen for FT08.

The conversion at 190°C was 60% (Graph 8.5) and the catalyst reaches a total conversion at 220°C, before starting to decrease.

Graph 8.5: Conversion vs temperature for FT09-A

Also in this case, a test at constant temperature at 220°C was carried out (Graph 8.6). By regarding the graph of this test, a rapid deactivation of the catalyst is observed. The conversion

-12 -10 -8 -6 -4 -2 0

0 5 10 15 20 25 30

ln(Wn/n)

n° of carbons

FT08

0 20 40 60 80 100

180 190 200 210 220 230 240

Conversion (%)

Temperature (°C)

FT09-A

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decreases from 100% to 40%. However, the selectivity to methane and to C2-C4 hydrocarbons remains almost unaltered at 20 and 10% respectively. The CO2 production, instead, is present also in the first moments of the test, then it decreases rapidly until it disappears.

Graph 8.6: Conversion vs time for FT09-A

The hydrocarbon liquid phase collected at the end of the test was analyzed and the results are shown in the graph: this graph shows that it results an α value of 0.72. However, the decrease of conversion during the test let us think about a deactivation due to the formation of waxes. Then, a procedure of extraction was carried out over the discharged catalysts. More than 6 g of solid hydrocarbons were found on the catalysts. That means around 1.5 g of waxes for each gram of catalyst. The deposition of waxes on the catalyst surface causes an occlusion of the pores and the resulting decrease of the conversion. These waxes are not included in the calculation of ASF distribution, so the resulting α value is underestimated.

0 20 40 60 80 100

0 1 2 3 4 5

Conversion/Selectivity (%)

Days of test

FT09-A

Conversion

Methane selectivity CO2 selectivity C2 - C4 selectivity

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Graph 8.7: linearized production of condensed products for FT09-A

The catalyst FT10 was tested between 180°C and 220°C. It shows a small increase of conversion at increasing temperatures (Graph 8.8). The conversion increases from 45% to 65%.

Graph 8.8: Conversion vs temperature for FT10

Also in this case the test at constant temperature was carried out at 220°C, to compare the results with the others obtained at the same temperature.

By regarding the graph, the conversion decreases rapidly and linearly during the test, from 65% to 35%. The selectivity to methane, and to lighter (C2-C4) hydrocarbons remain almost unaltered

-14 -13 -12 -11 -10 -9 -8 -7 -6

0 10 20 30

ln(Wn/n)

n° of carbons

FT09-A

0 20 40 60 80 100

170 180 190 200 210 220 230

Conversion (%)

Temperature (°C)

FT10

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during the test and always at low values. This means that, as already shown in the case of FT09-A, there is an high formation of solid products that deactivates the catalyst.

It is interesting to note that there is not production of CO2.

Graph 8.9: Conversion vs time for FT10

The results of the analysis of liquid phase collected are plotted in the graph, with an α value of 0.70, but also in this case this value is underestimated due to the exclusion of solid products.

Graph 8.10: linearized production of condensed products for FT10

0 20 40 60 80 100

0 1 2 3 4 5

Conversion/Selectivity (%)

Days of test

FT10

Conversion

Methane selectivity CO2 selectivity C2-C4 selectivity

-16 -14 -12 -10 -8 -6 -4 -2 0

0 5 10 15 20 25 30

ln (Wn/n)

n° of carbons

FT10

117 8.5 Conclusions

By this part of work, it was possible to optimize the parameters for the preparation of egg-shell catalysts supported over silica by using the sol-gel technique. The optimal amount of oxalic acid, used as precursor of CO2 that gives an high porosity during the gelification, was determined by B.E.T. surface area.

The next step was to found the optimal ratio alkoxide/solvent in order to form the best upper layer in the “egg-shell” catalyst. The study was made with three different molar ratios of TMOS/ethanol: .,3, 0.5, and 0.6. From IR, XRD and electronic microscopy analyses, no differences between samples were observed. Otherwise, the TPR, TPO, H2-TPD, and surface area analyses showed that, in the case of 0.3 molar ratio (FT09-A), the resulting catalyst with the smaller uplayer, was the best one between the three catalysts. This catalyst was tested in the plant and compared with a reference catalyst prepared by the incipient wetness impregnation method.

The results showed that the catalyst was highly active and selective to the desired products, but the formation of waxes during the test deactivated rapidly the catalyst itself. This deactivation was then due to the formation of desired products and, however, was reversibly; further procedures of reactivation gave back the catalyst newly active. Similar results were obtained for an analogue egg-shell catalyst, again with a shell of silica, supported over a core of alumina.

118 Bibliography

[1] A. Di Michele; PhD Thesis: “Sonochemical synthesis of metal nanoclusters and their application in the Fischer-Tropsch process”; 2007

[2] C. Perego, P. Villa; Catalysis Today. 1997. 34, 281 [3] T. Lopez; React. Kinet. Catal. Lett. 1992. 46, 42

[4] A. Y. Khodakov, W. Chu, P. Fongarland; Chem. Rev. 2007. 107, 1692 [5] M. Voß, D. Borgmann, G.Wedler; Journal of Catalysis 2002. 212, 10 [6] A. M. Saib, M. Claeys, E.V. Steen; Catal. Today 2002. 71,395 [7] C. Chen, H. Yuuda, X. Li; Appl. Catal. A 2011. 396, 116

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