4.3. Model Validation
5.1.1. Volume fraction contours of Pt/Al 2 O 3 Catalyst
Figures 5.1 - 5.3 denote the prototype volume fraction contours of three phases (i.e., bio- oil, hydrogen gas and the solid catalyst) in the hydrotreatment of bio-oil in the presence of Pt/Al2O3 catalyst. The color bar on the left hand side of the image denotes the range of contours of the specific phase at a particular instant. This has been mentioned similarly to the volume fraction of the gas and the oil phases as well. The contour volume fraction of the catalyst bed variation with respect to the different time periods are clearly shown in Figure 5.1. The catalyst bed tends to fluidize at the attainment of minimum fluidization velocity such that the particles tends to rise gradually.
Figure 5.1: Expansion of the Pt/Al2O3 catalyst bed volume fraction WHSV=3 h-1, T=673 K and P=8720 kPa with increasing time.
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As the velocities of the inlet feeds are continuous the bed shows a consistent rise and reaches a point where there is no further change in the height of the bed and also the minimal variations of the compositions. Here the bed has reached a maximum height of the 0.71 m and thereafter the back mixing is taking place inside the reactor. The approximate time taken for the catalyst bed to reach the steady state with the given operating conditions is t = 80sec.
Figure 5.2 illustrates the contour volume fraction changes of the bio-oil phase inside the reactor with respect to the time. There is a gradual change in the height of the bed with respect to the time through the inlets of gas and oil phases such that the complete and proper mixing of the reactants takes place inside the reactor. This is one of the advantages of the ebullated/fluidized bed reactors as compared to the fixed bed reactors.
Figure 5.2: Volume fraction of the pine pyrolytic-oil phase with increasing time at WHSV=3 h-1, T=673 K, and P=8720 kPa in the presence of Pt/Al2O3 catalyst.
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At t = 80 sec the complete mixing of the reaction is taken place and hence the contour density has been changed from the red color (at t=10sec) to mixture of green, yellow and red at t=80sec. Being a lighter medium, the gas occupies the reactor very quickly as compared to the bio-oil, pronounces the initial reaction conditions (seen in Figure 5.3).
The stage is set such that the catalyst particles are in close vicinity with the hydrogen gas and when the bio-oil enters the reactor of the solid catalyst particles tends to react with the other two phases and improves the miscibility. Here also it is seen that the gas phase height has been reached to a maximum of 0.71m. The density of the contour varies with respect to time indicates that higher dense is the presence of single phase and the lower dense is the presence of two or more phases introduced into the reactor.
Figure 5.3: Volume fraction of the H2 gas phase with increasing time at WHSV=3 h-1, T=673 K, and P=8720 kPa in the presence of Pt/Al2O3 catalyst.
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The detailed summary of the hydrodynamic effects with respect to the pertinent operating conditions are presented in Table 5.1. The steady state variation of volume fraction of processed/upgraded bio-oil with respect to temperature, pressure and WHSV are shown in Figure 5.4. The x-axis denoted the operating pressures used for the present simulation studies ranging between 6996 kPa to 10443 kPa, left y- denotes the variation of volume fractions of oil and the catalyst at the pertinent operating conditions, right y-axis denotes the volume fraction of the gas phase. The solid lines in the Figure 5.4 denotes the volume fraction of processed bio-oil at lower temperatures i.e., T=623 K, dotted lines indicate the moderate temperatures T=648 K, and dashed dot line indicates the higher temperatures T=673 K.
At a steady value of WHSV =2 h-1, the variation in the volume fraction of bio-oil remains unaffected for lower (T=623 K) and moderate temperature (T=648 K) with the increasing pressure; whereas for higher temperature (T=673 K) a decreasing trend is observed with pressure indicate that the solubility of the oil is increased at higher pressures and hence the stability after P=8720 kPa is achieved seen in Figure 5.4a (dash dotted line).
Figure 5.4: Steady volume fractions of pine oil, H2 gas and Pt/Al2O3 catalyst at different temperatures and pressures.
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The increase in the solubility ensures the higher availability of hydrogen on the catalyst surface for the reaction to take place. This increases the reaction rate of the catalyst and decreases oil volume fraction in the reactor. With the increase in WHSV the steady state volume fractions of the upgraded bio-oil phase shows a constant trend with no significant deviations irrespective of temperatures, and pressures as seen in Figure 5.4b. The reason could be as the WHSV is increased the residence time for the reaction inside the reactor is decreased and hence the conversion rate of the reactants to products is unchanged. Hence there is no change observed in the volume fraction of the oil phase at WHSV=3 h-1 and continued the same trend for WHSV=4 h-1 as shown in Figures 5.4b and 5.4c. It is also observed that there is no change noted for the increment in pressure and temperature with the increasing WHSV values.
Figure 5.4b denotes the steady state volume fraction of the gas phase at WHSV ranging between 2 h-1 and 4 h-1, temperature vary between 623 K-676 K and the extent of pressure 6996 kPa - 10443 kPa. At constant WHSV =2 h-1 the volume fraction of the gas shows a slight variation with respect to operating pressure and temperature. For lower temperatures T=623 K the deviation due to pressure is compensated by the decrement of the oil volume fraction at T=623 K shown in Figure 5.4a. At moderate temperature T=648 K (Figure 5.4a) the trend is constant without any deviation with respect to the varying pressure. At higher temperature T=673 K the volume fraction of gas is slightly increased with the pressure resulting due to the increased solubility of the liquid phase ascribed in Figure 5.4a (T=673 K). This increment in the gaseous fractions are due to the unidentified carbonaceous gases resulted from unconverted oxygenates and insoluble tars of the bio-oil. Further, varying the WHSV to higher values the volume fraction of gases follow a stable trend without much significant deviation to the pertinent operating conditions. This can be clearly seen from Figure 5.4b and Figure 5.4c. Whereas, for the
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higher temperatures the volume fraction of catalyst shows a increasing trend with respect to pressure. This increment is resulted due to the transient dispersion of the catalyst to occupy the volume of the reactor. Further, enhancing the WHSV values resulting the steady state values pointing that the equal dispersion of the catalyst throughout the reactor volume is attained with no further deviations as seen in Figure 5.4c. Hence, the effect of the WHSV is majorly attributed to the residence time, reaction rate and higher pressures to the solubility of bio-oil in the vicinity of the catalyst. Table 5.1 presents the numerical values at the pertinent conditions in details.
Table 5.1: Volume fractions of product phases with respect to different operating conditions in the presence of (Pt/Al2O3) catalyst
Phase WHSV=2h-1 WHSV=3h-1 WHSV=4h-1
P = 6996 kPa
T=623K T=648K T=673K T=623K T=648K T=673K T=623K T=648K T=673K
Catalyst 0.0206 0.0211 0.0300 0.0210 0.0210 0.0300 0.0209 0.0207 0.0200
Oil 0.0539 0.0537 0.1100 0.0535 0.0535 0.1100 0.0532 0.0527 0.0950 Gas 0.9180
. 0.9250 0.9250 0.9254 0.9254 0.9250 0.9258 0.9265 0.8440 P = 8720 kPa
Catalyst 0.0204 0.0206 0.0500 0.0208 0.0206 0.0240 0.0204 0.0204 0.0200
Oil 0.0525 0.0502 0.0486 0.0528 0.0522 0.0950 0.0518 0.0518 0.1000 Gas 0.9180 0.9270 0.9315 0.9262 0.9270 0.9200 0.9277 0.9277 0.9200
P =10443 kPa
Catalyst 0.0201 0.0197 0.0710 0.0204 0.0197 0.0205 0.0198 0.0201 0.0198
Oil 0.0505 0.0476 0.0354 0.0517 0.0513 0.0934 0.0501 0.0509 0.0998 Gas 0.9007 0.9282 0.9516 0.9273 0.9282 0.9156 0.9289 0.9285 0.9562
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