Final Report Summary - FLOW REGIME MAPS (Identification of Various Regime Transitions in Gas-Liquid-(Solid) Bubble Columns Based On Chaos and Statistical Analyses of CARPT, CT, ...)
The identification of the boundaries of the main flow regimes in multiphase reactors is very important research goal since the degrees of mixing, mass and heat transfer depend strongly on the prevailing flow regime. The prediction of the main regime transition velocities Utrans in bubble columns is important for their design and scale-up as well as effective operation. There are limited number of criteria and correlations in the literature for predicting the Utrans values. However, most of them yield unsatisfactory results outside the range of the experimental conditions which they cover. A reliable method for prediction of the main transition velocities is needed in order to control the performance of the multiphase reactors. Such a method is not available in the literature yet.
The main originality of the project is the development of a new identification method based on the information entropy (IE) theory. The method is universal since it is applicable to different signals (photon counts, gauge pressure fluctuations, differential pressure fluctuations, etc.) measured in bubble columns, fluidised beds and spouted beds. The development of such a new method is needed since the available correlations in the literature cannot predict reliably the regime transition velocities. The signal's range is divided into different regions and then the number of signal's visits into different regions is counted. The probability of signal's visits into a particular region is a function of the number of visits into each region. The information amount and the information entropy are functions from this probability. It was found that in the case of photon counts (recorded by means of computed tomography (CT) or nuclear gauge densitometry (NGD)), the maximum IE should be used for identification of the main transition velocities in bubble columns and fluidised beds. In the case of gauge pressure fluctuations measured in two spouted beds and differential pressure fluctuations measured in a bubble column the total IE was found capable of identifying the main transition velocities. The Kolmogorov entropy (KE) was also used as a flow regime identifier. It is an intellectual challenge to match both the KE and IE profiles in order to identify the same Utrans values. In the case of photon counts recorded in a bubble column it was demonstrated that both entropies yield very similar results.
On the basis of two well-pronounced drops in the maximum information entropies IEmax, extracted from photon counts in a bubble column (0.162 m in ID) operated with an airtherminol LT system, two transition velocities Utrans (0.02 and 0.09 m·s-1) were identified at ambient pressure P. They delineated the boundaries of bubbly flow, transition and churn-turbulent regimes. The existence of two Utrans values was confirmed by the Kolmogorov entropy (KE) values. At P=0.4 MPa, the first Utrans shifted to somewhat higher value (0.04 m·s-1), whereas the second Utrans was identified earlier (at 0.07 m·s-1). At P=1.0 MPa both first and second Utrans values shifted to higher values (0.05 and 0.1 m·s-1).
It was found that in airwater bubble column (0.1 m in ID) the IEmax values (extracted from photon counts) were capable of identifying only one Utrans value (0.02 m·s-1). In a fluidised bed (0.44 m in ID) operated with airpolyethylene system, the minimum fluidisation velocity Umf was identified at 0.103 m·s-1 based on the well-pronounced drop in the IEmax values.
The IE theory was also applied to differential pressure fluctuations measured in a bubble column (0.102 m in ID) operated with both nitrogentap water and nitrogenethanol systems. In the first case, by means of total information entropies IEtotal were identified two transitions: from bubbly flow to transition flow regime and from the latter to the churn-turbulent regime. The effect of the axial position z on each of these Utrans values was studied. In the case of nitrogentap water system, it was found that both Utrans values were slightly higher in the middle of the column but they decreased in the upper part of the column. In the case of nitrogenethanol system, the first transition velocity occurred much earlier (at UG=0.0062 m/s) at all three axial positions studied. This critical velocity is in good agreement with the transition criterion Re0 (orifice Reynolds number) =2000 and the results of Lin et al. (1999).
The IE theory was applied also to gas holdup time series measured by means of wire-mesh sensor in a bubble column (0.4 m in ID) operated with an airwater system. A wide range of superficial gas velocities UG was studied. The gas holdup range was divided into different regions and on the basis of the maximum number of signal visits into each region two transition velocities were identified: at UG=0.034 m/s the bubbly flow regime transformed itself into a transition flow regime, whereas at UG=0.078 m/s the onset of the churn-turbulent flow regime was identified. In the case of a smaller bubble column (0.15 m in ID) equipped with a similar type of perforated plate gas distributor and operated also with airwater system, two transition velocities were also identified: at 0.034 and 0.067 m·s-1. It is worth noting that both the transition and churn-turbulent flow regimes start somewhat earlier in a smaller bubble column. A comparison between the experimental transition velocities and the predictions based on the criteria of Miller (1974) and Reilly et al. (1994) were performed. It was found that both criteria yield unsatisfactory results and they should be improved. A new correlation is to be developed.
In two spouted beds (0.076 and 0.152 m in ID) operated with an airglass beads system, the IEtotal values were extracted from gauge pressure fluctuations. It was found that the minimum spouting velocity Ums in the smaller spouted bed was equal to 0.785 m·s-1, while Ums decreased down to 0.58 m·s-1 in the bigger spouted bed. The stable spouting regime in the smaller spouted bed covered wider range of gas velocities.
The effect of scintillation detector position on the transition velocities Utrans in a bubble column and fluidised bed was also studied. It was found that the IEmax profiles identify different Utrans values depending on the position of the scintillation detector. In this regard, it was concluded that the results from the centrally positioned detector (directly opposite to the radioactive source) should be considered as the most reliable.
By means of KE values extracted from gauge pressure fluctuations measured in two bubble columns (0.14 and 0.44 m in ID) equipped with perforated plate distributors and operated with airdeionised water system was detected the range of superficial gas velocities in which the gas maldistribution zone was formed. So, a new method for detection of gas maldistribution zone was proposed. In the bigger bubble column (0.44 m in ID, perforated plate sparger, 241 holes, Ø 3 × 10-3 m) the KE profile exhibited a well-pronounced minimum at UG=0.018 m/s. The clear liquid height H0 was set at 1.91 m. At higher H0 value (2.67 m) the end of the gas maldistribution zone shifted to higher UG value (0.026 m/s). In the smaller bubble column (0.14 m in ID, perforated plate sparger, 121 holes, Ø 1.32×10-3 m) the onset of the bubbly flow regime is identified at almost the same UG value (0.016 m/s) as in the bigger column provided that the clear liquid height is kept the same (H0=1.91 m).
The documented hydrodynamic analogies in the behavior of churn-turbulent bubble columns and bubbling fluidised beds were further supported by the existence of so-called chaotic analogies. It was found that the KE values monotonously decreased in both reactors (operated under highly turbulent conditions), corresponding to the growth of large bubbles. The onset of the monotonous KE decrease identifies the beginning of the fully developed churn-turbulent regime in bubble columns and bubbling fluidisation regime in fluidised beds. The KE values were extracted from the absolute pressure fluctuations measured in three different bubble columns (0.19 0.38 and 0.8 m ID) as well as gauge pressure fluctuations measured in a large bubble column (0.44 m ID). In addition, the same declining KE trend has been derived from NGD scans performed in a churn-turbulent bubble column (0.1 m ID). An air-water system was always used. Decreasing KE values derived from CT scans in an air-therminol BC (0.162 m ID) were also extracted. Gauge pressure fluctuations were also performed in a bubbling fluidised bed (0.14 m ID) operated with an air-glass beads system. The same declining KE trend was obtained. Such a trend was also extracted from NGD scans performed in a bubbling fluidised bed (0.44 m ID) operated with an air-polyethylene system and gas-solids riser (0.152 m ID) operated with an air-glass beads system.
The man flow transition velocities were also identified on the basis of gauge pressure fluctuations measured in two fluidised beds (0.14 and 0.44 m in ID) operated with air-glass beads system. The effect of the axial position on the transition velocities was studied. Both the information entropies and Kolmogorov entropies were extracted from optical probe measurements in a conical bubble column (0.14 m in ID) operated with air-water system. Based on these parameters the main transition velocities were identified for the first time in a conical bubble column.
References
Lin, T.-J. Tsuchiya, K., Fan, L.-S. 1999. On the measurements of regime transition in high-pressure bubble columns. The Canadian journal of chemical engineering 77, 370-374.
Miller, D., 1974. Scale-up of agitated vessels gas-liquid mass transfer. AIChE Journal 20, 445-453.
Reilly, I. G., Scott, D. S., De Bruijn, T. J. W., MacIntyre, D., 1994. The role of gas phase momentum in determining gas holdup and hydrodynamic flow regimes in bubble column operations. The Canadian journal of chemical engineering 72, 3-12.
The main originality of the project is the development of a new identification method based on the information entropy (IE) theory. The method is universal since it is applicable to different signals (photon counts, gauge pressure fluctuations, differential pressure fluctuations, etc.) measured in bubble columns, fluidised beds and spouted beds. The development of such a new method is needed since the available correlations in the literature cannot predict reliably the regime transition velocities. The signal's range is divided into different regions and then the number of signal's visits into different regions is counted. The probability of signal's visits into a particular region is a function of the number of visits into each region. The information amount and the information entropy are functions from this probability. It was found that in the case of photon counts (recorded by means of computed tomography (CT) or nuclear gauge densitometry (NGD)), the maximum IE should be used for identification of the main transition velocities in bubble columns and fluidised beds. In the case of gauge pressure fluctuations measured in two spouted beds and differential pressure fluctuations measured in a bubble column the total IE was found capable of identifying the main transition velocities. The Kolmogorov entropy (KE) was also used as a flow regime identifier. It is an intellectual challenge to match both the KE and IE profiles in order to identify the same Utrans values. In the case of photon counts recorded in a bubble column it was demonstrated that both entropies yield very similar results.
On the basis of two well-pronounced drops in the maximum information entropies IEmax, extracted from photon counts in a bubble column (0.162 m in ID) operated with an airtherminol LT system, two transition velocities Utrans (0.02 and 0.09 m·s-1) were identified at ambient pressure P. They delineated the boundaries of bubbly flow, transition and churn-turbulent regimes. The existence of two Utrans values was confirmed by the Kolmogorov entropy (KE) values. At P=0.4 MPa, the first Utrans shifted to somewhat higher value (0.04 m·s-1), whereas the second Utrans was identified earlier (at 0.07 m·s-1). At P=1.0 MPa both first and second Utrans values shifted to higher values (0.05 and 0.1 m·s-1).
It was found that in airwater bubble column (0.1 m in ID) the IEmax values (extracted from photon counts) were capable of identifying only one Utrans value (0.02 m·s-1). In a fluidised bed (0.44 m in ID) operated with airpolyethylene system, the minimum fluidisation velocity Umf was identified at 0.103 m·s-1 based on the well-pronounced drop in the IEmax values.
The IE theory was also applied to differential pressure fluctuations measured in a bubble column (0.102 m in ID) operated with both nitrogentap water and nitrogenethanol systems. In the first case, by means of total information entropies IEtotal were identified two transitions: from bubbly flow to transition flow regime and from the latter to the churn-turbulent regime. The effect of the axial position z on each of these Utrans values was studied. In the case of nitrogentap water system, it was found that both Utrans values were slightly higher in the middle of the column but they decreased in the upper part of the column. In the case of nitrogenethanol system, the first transition velocity occurred much earlier (at UG=0.0062 m/s) at all three axial positions studied. This critical velocity is in good agreement with the transition criterion Re0 (orifice Reynolds number) =2000 and the results of Lin et al. (1999).
The IE theory was applied also to gas holdup time series measured by means of wire-mesh sensor in a bubble column (0.4 m in ID) operated with an airwater system. A wide range of superficial gas velocities UG was studied. The gas holdup range was divided into different regions and on the basis of the maximum number of signal visits into each region two transition velocities were identified: at UG=0.034 m/s the bubbly flow regime transformed itself into a transition flow regime, whereas at UG=0.078 m/s the onset of the churn-turbulent flow regime was identified. In the case of a smaller bubble column (0.15 m in ID) equipped with a similar type of perforated plate gas distributor and operated also with airwater system, two transition velocities were also identified: at 0.034 and 0.067 m·s-1. It is worth noting that both the transition and churn-turbulent flow regimes start somewhat earlier in a smaller bubble column. A comparison between the experimental transition velocities and the predictions based on the criteria of Miller (1974) and Reilly et al. (1994) were performed. It was found that both criteria yield unsatisfactory results and they should be improved. A new correlation is to be developed.
In two spouted beds (0.076 and 0.152 m in ID) operated with an airglass beads system, the IEtotal values were extracted from gauge pressure fluctuations. It was found that the minimum spouting velocity Ums in the smaller spouted bed was equal to 0.785 m·s-1, while Ums decreased down to 0.58 m·s-1 in the bigger spouted bed. The stable spouting regime in the smaller spouted bed covered wider range of gas velocities.
The effect of scintillation detector position on the transition velocities Utrans in a bubble column and fluidised bed was also studied. It was found that the IEmax profiles identify different Utrans values depending on the position of the scintillation detector. In this regard, it was concluded that the results from the centrally positioned detector (directly opposite to the radioactive source) should be considered as the most reliable.
By means of KE values extracted from gauge pressure fluctuations measured in two bubble columns (0.14 and 0.44 m in ID) equipped with perforated plate distributors and operated with airdeionised water system was detected the range of superficial gas velocities in which the gas maldistribution zone was formed. So, a new method for detection of gas maldistribution zone was proposed. In the bigger bubble column (0.44 m in ID, perforated plate sparger, 241 holes, Ø 3 × 10-3 m) the KE profile exhibited a well-pronounced minimum at UG=0.018 m/s. The clear liquid height H0 was set at 1.91 m. At higher H0 value (2.67 m) the end of the gas maldistribution zone shifted to higher UG value (0.026 m/s). In the smaller bubble column (0.14 m in ID, perforated plate sparger, 121 holes, Ø 1.32×10-3 m) the onset of the bubbly flow regime is identified at almost the same UG value (0.016 m/s) as in the bigger column provided that the clear liquid height is kept the same (H0=1.91 m).
The documented hydrodynamic analogies in the behavior of churn-turbulent bubble columns and bubbling fluidised beds were further supported by the existence of so-called chaotic analogies. It was found that the KE values monotonously decreased in both reactors (operated under highly turbulent conditions), corresponding to the growth of large bubbles. The onset of the monotonous KE decrease identifies the beginning of the fully developed churn-turbulent regime in bubble columns and bubbling fluidisation regime in fluidised beds. The KE values were extracted from the absolute pressure fluctuations measured in three different bubble columns (0.19 0.38 and 0.8 m ID) as well as gauge pressure fluctuations measured in a large bubble column (0.44 m ID). In addition, the same declining KE trend has been derived from NGD scans performed in a churn-turbulent bubble column (0.1 m ID). An air-water system was always used. Decreasing KE values derived from CT scans in an air-therminol BC (0.162 m ID) were also extracted. Gauge pressure fluctuations were also performed in a bubbling fluidised bed (0.14 m ID) operated with an air-glass beads system. The same declining KE trend was obtained. Such a trend was also extracted from NGD scans performed in a bubbling fluidised bed (0.44 m ID) operated with an air-polyethylene system and gas-solids riser (0.152 m ID) operated with an air-glass beads system.
The man flow transition velocities were also identified on the basis of gauge pressure fluctuations measured in two fluidised beds (0.14 and 0.44 m in ID) operated with air-glass beads system. The effect of the axial position on the transition velocities was studied. Both the information entropies and Kolmogorov entropies were extracted from optical probe measurements in a conical bubble column (0.14 m in ID) operated with air-water system. Based on these parameters the main transition velocities were identified for the first time in a conical bubble column.
References
Lin, T.-J. Tsuchiya, K., Fan, L.-S. 1999. On the measurements of regime transition in high-pressure bubble columns. The Canadian journal of chemical engineering 77, 370-374.
Miller, D., 1974. Scale-up of agitated vessels gas-liquid mass transfer. AIChE Journal 20, 445-453.
Reilly, I. G., Scott, D. S., De Bruijn, T. J. W., MacIntyre, D., 1994. The role of gas phase momentum in determining gas holdup and hydrodynamic flow regimes in bubble column operations. The Canadian journal of chemical engineering 72, 3-12.