| An improved heat removal of ETFE polymerization vessel was acomplished
by designing and developing a new mixing system using the improved
Double Helical Ribbon impeller (DHR). And the fluidization in the
polymerization vessels were calculated by Computational Fluid Dynamics
(CFD) to evaluate the efficiency of new mixing system. Fluon(R) ETFE (Ethylenetetrafluoroethylene copolymer) is a melt - processable fluoropolymer and is applied to various devices. Fluon ETFE(R) is usually polymerized by suspension polymerization with fluorinated solvents. The polymerization solvents were changed into HFCs (Hydrofluorocarbons) and HCFCs (Hydrochlorofluorocarbons) from ban on CFCs (Chlorofluorocarbons). ETFE polymerization in HFCs and HCFCs was examined and it was found that the rheologic properties of ETFE slurry in polymerization was changed from a Newtonian fluid to more thixotropic fluid. As a result of examination, we found that it is difficult to remove the heat of polymerization and mix the fluid well using original agitator and polymerization in non - CFCs. *Research Center **Chemical Research &Development Division |
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| 目次 | |
| 1. Introduction | |
| 2. Experimental | |
| 3. Results | |
| 4. Discussions | |
| 5. Conclusion | |
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| 1.Introduction |
| Fluon(R) ETFE (Ethylenetetrafluoroethylene
copolymer)is a melt - processable fluoropolymer
and is applied to various devices, such as electrical insulators,
flexible windows in greenhouses, lining (reactors, pipes etc. ),
fuel tubes. |
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| 2. Experimental |
| 2.1 Measurement of the flow curve diagram1)
The flow curve diagrams of ETFE slurry were measured by using the Vibro viscometer (A&D CO., LTD.). Vibro viscometer is similar to a tuning fork. Vibrators are submerged into a fluid to check the viscosity and the amplitude value of vibration is measured. The amplitude value is inversely proportional to the viscosity. Therefore, it is possible to calculate the viscosity of the fluid by an inverse proportion expression. In addition, changing the electric current of the vibrators and using Equation 1, the rheology curve can be obtained. |
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| 2.2 Measurement of the heat conductivity of ETFE slurry2)
The heat conductivity of ETFE slurry was measured by using the needle probe method. A theory of measurement is as follows. Supposing that an infinite long tin wire is the isotropic infinite fluid where the temperature is fixed at T0, and the generation of heat value is q. The wire is then heated by the surrounding fluid to a surface temperature θt where θt is time dependent and t is the heating time. From this, Equation 2 is formed. |
Heat conductivity is calculated with Equation 2. |
| 2.3 Mixing test The flow patterns of the DHR impeller with vertical PTFE scrapers and the DHR impeller with helical PTFE scrapers were examined. Each impeller mixed two ETFE slurry, one polymerized with a CFC solvent and the other with a non - CFC solvent. A 10 - L glass vessel was used, and plastic pellets were placed inside to easily see the flow pattern. 2.4 Measure the overall heat transfer coefficient A 10 - L stainless steel autoclave was used for the experiments of the ETFE polymerization. ETFE was polymerized by feeding tetrafluoroethylene (TFE)and ethylene continuously until a predetermined polymer concentration. Six thermocouples were set in the reactor and a thermocouple was set in the cooling water jacket to monitor each temperature (Fig.2). The temperature data was calculated by Equation 3 to find the overall heat transfer coefficient. Overall heat transfer coefficients of the vertical PTFE scrapers were compared with that of the helical PTFE scrapers. |
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| 2.5 Computational Fluid Dynamics (CFD) The fluidization in the polymerization vessel was calculated by Computational Fluid Dynamics (CFD) to evaluate the efficiency of the impellers in actual polymerization autoclave. The fluidization parameters were flow velocity, fluid pressure, shear stress, temperature and so on. CFD software, STAR - CD (CD - Adapco Japan Co. , LTD. )was used, and finite volume method was chosen to calculate the fluidization parameters. The finite volume grids are shown in Fig.3. The number of cells is 101840. At first, the flow pattern of the 10 - L vessel was determined by calculating velocity vectors of each impeller. Using this data, the heat transfer ability of each impeller was then evaluated. Heat transfer calculations were carried out to obtain temperature distribution diagrams in the polymerization vessel. Also, the flow pattern of a 1m3 vessel was calculated to estimate the efficiency of an impeller after scale up. |
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| 3. Results |
| 3.1 Rheologic properties of Fluon(R)
ETFE slurry Figures 4-1 and 4-2 show flow curve diagram of each ETFE slurry at a polymer concentration of 9wt%(polymer weight / solvent weight). Initially, CFCs had been used as the solvent of Fluon(R) ETFE polymerization. This ETFE slurry was comparatively close to a Newtonian fluid (Fig. 4-1), because the hysterisis curve did not have a large difference between increasing shear rate and decreasing shear rate. However, after the ban on CFCs, HFCs and HCFCs replaced CFCs as the polymerization solvent. In the case of these HFCs and HCFCs solvents, the rheologic properties of slurry were changed into more thixotropic fluid (Fig.4-2). The difference between CFCs and non- CFCs depended upon the swelling of the polymers. ETFE swelled well in CFCs rather than HFC and HCFC. 3.2 ETFE slurry heat conductivity and formation of polymer layer During ETFE polymerization, an ETFE layer was generated on the reactor wall as shown in Fig. 5.This ETFE layer acted as a heat insulator. The heat conductivity of the ETFE slurry was 0.063kW/m3, and was determined the needle probe method. If the ETFE polymer layer thickness was 1mm, theoretical temperature difference between the cooling water and the reactor inside was about 100Celsius (Fig.6). Therefore, it is difficult to remove the heat of polymerization and to control polymerization temperature. This polymer layer was observed by conducting a mixing test with a glass vessel as follows. 3.3 Mixing test Diagrams of the mixing test results are shown in Figure 7. In the case of the CFC slurry, each impeller worked well. However, in the case of the non - CFC slurry, DHR with vertical PTFE scrapers did not work well. The slurry was mixed well close to the impeller blades, but not in the center of the vessel, near the mixing shaft. It was observed, from an aerial view of the reactor, a whirlpool fluid motion with blade, while the vertical circulation flow (as in Fig.7-1)became weaker with a change in rheologic properties. On the other hand, DHR with helical PTFE scrapers worked well even if the slurry contained non - CFCs. 3.4 Overall heat transfer coefficient It is important to check the heat transfer ability of each mixing impeller to polymerize ETFE safely (Fig.8) shows the comparison of the overall heat transfer coefficients between helical PTFE scrapers and vertical PTFE scrapers. The overall heat transfer coefficient of DHR with helical PTFE scrapers was found to be 2 - 3times as large as that of DHR with vertical PTFE scrapers. Therefore DHR with helical PTFE scrapers was suitable impeller to polymerize ETFE in non - CFCs. 3.5 Computational Fluid Dynamics (CFD) We calculated the fluidization in the polymerization vessel by Computational Fluid Dynamics (CFD)to evaluate the efficiency of the impellers. Mixing Reynolds number of the 10 - L vessel was 20. Figures 9-1 and 9-2 show the results of DHR with vertical PTFE scrapers. The revolution flow was observed in the velocity vector diagram of the vertical PTFE scrapers (Fig.9-1). In the temperature contour diagram (Fig.9-2)varying temperature distribution in the vessel was found. Figures 10-1 and 10-2 show the result of DHR with helical PTFE scrapers. We found that the big vertical circulation flow in the vessel was formed and that there was the strong up - flow in the vicinity of reactor - wall and the strong down - flow near the mixing shaft in the velocity vectors diagram (Fig.10-1). A uniform temperature distribution was found in the temperature contour diagram (Figure 10-2). Through calculations for asup 1m3 scale up of the autoclave, it was found that thesup DHR with helical PTFE scrapers will work wellsup and the flow pattern will be sufficient to mix thesup entire contents of the autoclave. Figure 11 showssup the calculated result of 1m3 sup . In the case of 1m3 thesup mixing Reynolds number was 580. |
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| 4. Discussions |
| When CFCs were used as the polymerizationsup solvents, we used
the Double Helical Ribbon (DHR)sup impeller with vertical PTFE scrapers
showed insup Fig.7-1.
PTFE scrapers were attached verticallysup for scraping off the polymer
layer on the reactorsup wall and to remove the heat of polymerizationsup
easily. Formerly we found that this method wassup very effective to
increase the heat transfersup coefficient at the reactor inner wall.
Without thissup scraper, it was very difficult to control thesup polymerization
temperature. It was necessary tosup scrape off the ETFE layer on the
reactor wall insup order to remove the heat of polymerization easily.
sup However after changing the solvents into non -
sup CFCs it became difficult to remove the heat ofsup polymerization
using this conventional DHRsup impeller with vertical PTFE scrapers.
It was foundsup that by mixing the thixotorpic fluid (such as non
- sup CFC polymer slurry)with conventional impellerssup uneven
mixing occurred. The slurry under highsup shear rate stirring zone
was well mixed, but thesup slurry under low shear rate zone was not
wellsup mixed. Therefore, it was observed that thesup revolution flow
promoted and the verticalsup circulation flow became weaker in the
vesselsup according to the change of the rheologic propertiessup during
the polymerization (Fig.4-1
and 4-2).
Consequently, we had to improve conventional impeller. In order to improve the heat transfer ability, wesup designed the helical PTFE scrapers attached tosup mixing helical impeller (Fig.7-3). Using this newsup impeller, the strong vertical circulation flow wassup generated to mix the thixotropic fluid and the heatsup transfer ability was improved. In the terms of heat transfer ability, by using DHR with helical scrapers, the overall heat transfer coefficient was improved to be 2 - 3 times as large as that of conventional DHR. Therefore, it became easy to control the temperature of ETFE polymerization even if ETFE was polymerized in non - CFCs solvents. The efficiency of the improved DHR with helical PTFE scrapers was found by calculating the flow pattern and temperature distribution using CFD. |
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| 5. Conclusion | ||
| It was necessary to change the polymerization solvents of ETFE
to non - CFCs after the ban on CFCs. At that
time, although we had some problems with the rheologic properties
change of ETFE slurry, an improved heat removal and accelerated vertical
circulation flow was accomplished by designing and developing the
new mixing system. Measuring the overall heat transfer coefficient
showed that the coefficient was improved 2 -
3 times, and it became easy to control ETFE polymerization temperature.
It is possible to apply this type of impeller to other similar polymerization processes, with the use of high viscosity non - Newtonian fluids. The efficiency of the new mixing system was evaluated by calculating the fluidization in the polymerization vessel with CFD. CFD is a useful tool to be able to confirm the flow pattern and temperature distribution in the actual vessel, which under normal circumstances is impossible to see. −References − |
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