Submicron Liquid Coated Particle Loading
3. Results and Discussion
3.3 Loading Behavior of Submicron Liquid-coated Particles Generated by Heating-Evaporation-Condensation Method
For this part of the study, two different vapor carrier gases, compressed air and nitrogen, were separately used to bubbled through the CMAG saturator. Fig. 3.7 shows the loading result using compressed air as the DEHS vapor carrier gas. Compressed air was to demonstrate the filter performance when loaded liquid-coated particles in the ambient environment, where liquid oxidation is unavoidable. Liquid-coated particle with three different liquid volume percentages, 65%, 85% and 98%, were tested in this case. Note that the outer diameters of the test particles were kept nearly the same size, around 400 nm. As seen in Fig. 3.7, the characteristic shape of the loading curves for liquid-
initially increases relatively slowly in a linear manner and then sharply increases when a certain critical loading volume is reached. This similar behavior is because the liquid volume percentage of the test particle is high.
Loaded Particle Volume per Unit Filtration Area [cm3/m2]
0.1 1 10 100 P /P 0 [ -- ] 1.0 1.2 1.4 1.6 1.8 2.0 0% DEHS 65% DEHS 85% DEHS 98% DEHS 100% DEHS
Figure 3.7 Loading curves for liquid-coated particles generated with compressed air
carrier gas
In addition, the loading curves of different liquid volume percentages gradually shift toward the right-hand side of the plot as the liquid volume percentage increases. This shift can be explained by the increasing fluidity with increases in the liquid volume percentage of the liquid-coated particle. However, the curve of the 98% liquid volume fraction still deviates far from that of the pure liquid particle loading. In other words, the lifetime of the filters is dramatically reduced when they are loaded with liquid-coated particles. For the 85% loading curve, it initially located below the solid particle loading curve and later crosses the curve around a loaded volume of 7 cm3/m2. Moreover, the
loading curve of 65% liquid volume percentage is completely to the left side of the pure solid particle loading curve. One implication is that the loading curves could shift further toward the left when the liquid percentage of the particle varies from 0% to 65%, which is opposite to the rightward general trend of the loading curves as the liquid percentage increases from 65% to 100%.
Loaded Particle Volume per Unit Filtration Area [cm3/m2]
0.1 1 10 100 P /P 0 [ -- ] 1.0 1.2 1.4 1.6 1.8 2.0 0% DEHS 25% DEHS 35% DEHS 45% DEHS 65% DEHS 100% DEHS
Figure 3.8 Loading curves for liquid-coated particles generated with nitrogen vapor
carrier gas
We do not have a clear understanding about the factor resulting in such a leftward loading behaviour transition. It was, however, found that the viscosity of bulk DEHS liquid in the saturator was slightly increased after long term loading tests. At the same time, the other liquid properties, such as surface tension, may have changed, too. It is thus suspected that the change in liquid properties due to the liquid oxidation at the elevated
temperature of the saturator in the CMAG was always around the boiling point of the coating liquid. Contal et al. (2004) [2] have shown that the clogging of a filter loaded with glycerol is much faster than one loaded with DEHS. They attributed their observation to the higher surface tension of glycerol. Therefore, an increase in the viscosity or surface tension of the coating liquid due to oxidation in our loading experiment might move the loading curves to the left-hand side of where they otherwise would be. In addition, the transparent DEHS liquid became to brown after continuous heating in the saturator. Therefore, additional particles might have been generated by the reaction between the coating liquid and the compressed air, and contaminated the liquid- coated particles.
In the original CMAG manual, nitrogen was recommended as both the vapor carrier gas and the air source for the atomizer generating nuclei. However, for our filter loading test, the experimental time was normally more than 10 hours for each run, and the total flow rate was in the range of few liters per minute. Therefore, it was not economically feasible to perform the experiment completely with nitrogen. In this part of the study, only the vapor carrier gas was replaced with nitrogen to reduce changes in the coating liquid properties due to oxidation. After the loading tests, the viscosity of the bulk coating liquid was measured, and it was found that the variation of the viscosity before and after the tests was less than 3%. In addition, the color change of the bulk DEHS liquid in the saturator was less pronounced than before. Consequently, the liquid properties are believed to be more stable in the case of a nitrogen carrier, and the characteristics of the loading curves for liquid-coated particles are better controlled.
Five different liquid volume percentages, 25%, 35%, 45%, 65% and 90%, were tested in the case of nitrogen. As seen in Fig. 3.8, the characteristics of these loading curves are in general similar to that for pure liquid particle loading, except for the curve with a liquid volume fraction less than 35%. At 35%, it is observed that the liquid coated particle loading curves switch from linearly increasing to exponential growth. In other words, when the solid volume fraction is greater than 65%, the liquid-coated particle loading behaves more like that of solid particle loading. For curves where the liquid volume fraction was greater than 35%, their characteristics are similar to that of pure liquid particle loading. As the liquid percentage increases, the curves approach that of pure liquid particle loading. The trend was already observed for the cases using compressed air as the carrier gas, but the initial slopes of the liquid-coated particle loading curves obtained in this part of the study were all less than that of the pure solid particle loading curve. Compared to the cases using compressed air as carrier, more particle volume can be loaded on the filter for liquid-coated particles. In summary, the leftward and rightward transition behavior seen in the case of compressed air was not observed with nitrogen, and the loading curves continuously approach that of pure liquid particle loading as the liquid percentage increases. This experimental observation indirectly evidences the importance of the liquid properties on filter performance under liquid-coated particle loading. On the other hand, it also suggests the complexities of the loading behavior of liquid-coated particles in the real world.