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Preparation and characterization of porous cordierite pellets and use as a diesel particulate filter
Separation and Purification Technology 55 (2007) 321–326
Preparation and characterization of porous cordierite pellets and use as a diesel particulate filter Jai-Koo Park a,∗ , Jay-Hyun Park a , Jung-Wook Park a , Hong-Seok Kim b , Young-Il Jeong b a
Department of Geoenvironmental System Engineering, Hanyang University, 17, Heangdang-dong, Seongdong-gu, Seoul 133-791, Republic of Korea b Engine R&D group, Korea Institute of Machinery and Materials, 171 Jang-dong, Yuseong-gu, Daejeon 305-343, Republic of Korea Received 3 April 2006; received in revised form 8 November 2006; accepted 10 November 2006
Abstract This paper discusses the preparation and characterization of porous pellet filters for a diesel particulate filter (DPF). The process for creating the porous ceramic pellets was developed using simple gelcasting and a bubble-in-water-in-oil type of pseudo-double-emulsion. The pellets, consisting of a foamed cordierite suspension, were dispersed and consolidated in spherical shapes in liquid paraffin as gelation progressed, and were prepared by a conventional mechanical foaming method combined with gelcasting. In comparison with a wall-flow filter, the pellet filter has the advantages of being free of cracks during regeneration and having a flexible shape. Experiments were conducted in a test simulation of diesel engine exhaust conditions. Pressure drop and particle loading rate were compared using two pellet filters having porosities of 70% and 0%, respectively. Regeneration capability was also tested. The filter which mixed 1 and 2 mm pellets demonstrated feasibility as a DPF. © 2007 Published by Elsevier B.V. Keywords: Diesel vehicle; Particulate matter; Porous pellet; DPF
1. Introduction Reduction of emissions from diesel vehicles has become a more critical issue in recent years and research has resulted in improvements to engine technology and fuels and in wider use of after treatment devices. One of the most promising technologies is the diesel particulate filter [1–3]. The technology of DPF is largely divided into reduction and regeneration of particulate matter (PM) and this system is classified into a filter, a regeneration unit and a control unit. Filter configurations and materials are important factors in determining the performance of the DPF system. Typically, a wall-flow monolithic cell (honeycomb type) is used for configuration and the filter is composed of Cordierite (2MgO·2Al2 O3 ·5SiO2 ) or Silicon carbide (SiC). However, these types of filters are very expensive and may break due to thermal shock under an uncontrolled regeneration condition called stochastic regeneration. Metal filters and pellet filters have been studied as possible solutions to this defect [4–7]. Properties of thermal shock resistance and thermal conductivity of metal filters are superior to
∗
Corresponding author. E-mail address: [email protected] (J.-K. Park).
1383-5866/$ – see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.seppur.2006.11.009
those of ceramic monoliths. But, adhesion of ceramic based washcoat to metal filter surface, corrosion of metal in high temperature and high cost are problems [8]. Compared to a wall flow monolith filter, a pellet filter has the advantage of not cracking during regeneration and flexion. A pellet filter will reduce production costs because of its simplicity, ease of fabrication and installation. Pellets also are replaceable by refilling the casing. However, it is generally known that the pellet filter is heavier and generates higher backpressure than monolith filter [8]. The use of porous pellet might be solve this problems since porous pellets are light and many internal pores of pellets could help to reduce the backpressure of filter. Though high porosity of pellets aggravates the problem of pellet attrition when the pellets rubbed against one another in filter casing, porous pellets are still necessary to be studied because their mass transfer rate for soot regeneration can be improved by enlarged surface area and turbulent flow through pellets bed. Actually, Muto koichi in japan had developed the irregular porous pellets DPF due to the catalytic regeneration performance of porous pellets [9]. This paper presents a novel fabrication process for a porous cordierite ceramic pellet filter using mechanical foaming and considers the feasibility of porous cordierite ceramic pellets as a diesel particulate filter.
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2. Experimental 2.1. Preparation of porous ceramic pellets Porous ceramic pellets were prepared from a 3-phase foamed suspension (3PFS), also called pseudo-double-emulsion (PDE), a schematic of which is presented in Fig. 1. The double-emulsion method has been developed in the pharmaceutical industry for preparing microspheres to impregnate medicines [10]. Foaming can be considered as a type of emulsification since the both are very similar. In order to produce a PDE, a 3PFS is re-emulsified, as in the case of an oil-in-water-in-oil type double-emulsion [11,12]. In this PDE, constituent bubbles are the first dispersed phase, globules are the second dispersed phase, a cordierite suspension is the first continuous phase, and oil is the second continuous phase. That is, the globules of 3PFS dispersed in the second continuous phase contain an even smaller dispersed phase within themselves. Fig. 2 shows a flow chart for the overall process, which consists of suspension preparation, foaming by mechanical
whisking, fabricating, drying, and sintering. Aqueous suspensions, containing 45 vol.% cordierite powder (Koyritsu Ceramic) and 3.5 wt.% dispersant of polyethyleneimine (PEI, BASF), were ground by attrition mill for 4 h. Sodium lauryl sulfate (SLS, Samchun Pure Chemical) was used as a foaming agent and added to 100 cm3 of cordierite suspension. The suspensions were whisked vigorously in order to swell 3PFS to 400 cm3 . Then, each 3PFS was whisked at 600 rpm for 30 s. Next, the gelation agent was added to the 3PFS and whisked. The reactor for the PDE was a cylinder filled with 4000 cm3 of liquid paraffin (second continuous phase). About 400 cm3 3PFS was injected into the reactor by a peristaltic pump at a constant impeller revolution rate of 12.5 cm3 /s during the fabrication process to produce a PDE. The mixture was stirred at 240 rpm for 1 h. The gelated spheres (pellet) were separated by sieving from the liquid paraffin, followed by drying in air at 80 ◦ C for 6 h. Pyrolysis of the dried pellet was carried out in an alumina crucible at 600 ◦ C in air for 2 h and sintering of the pellet was performed with an electric furnace at 1350 ◦ C for 4 h using heating rates of 100 ◦ C/h up to 600 ◦ C and then 200 ◦ C/h up to the final temperature in air. 2.2. Apparatus set-up Fig. 3 is a schematic of the experiment to measure pressure drop, particle loading rate and regeneration through the trap. Fig. 3(a) shows the test set-up to measure the pressure drop and particle loading rate of the pellet filter. It consists of a particle container, pellet filter, filtering paper to measure the loading
Fig. 1. Schematic representation of a PDE including globules of a 3PFS and the reactor.
Fig. 2. Flow chart for preparing the porous pellet according to the PDE method.
Fig. 3. Schematic diagram of experimental set-up.
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carbon black powder about 20–25 nm in primary particle size (Printex U, Degussa) [13]. These particles are treated in a vacuum oven at 250 ◦ C to remove moisture to avoid agglomeration in humid atmospheric conditions. 3. Results and discussion 3.1. Characteristics of porous pellets
Fig. 4. Pellet filters for testing.
rate, a laminar flow meter and a super charger. The exhaust flow rate is determined by a supercharger using an A/C (alternating current) motor (220 V, three phase current, Max 1720 rpm) for vehicles and controlled by an inverter. The pressure drop across the trap is measured by a differential pressure meter (Dwyer 677). Differential pressure under laminar flow between the front and rear parts of the filter paper is also measured by a differential pressure meter (Dwyer 677, 616), and the data is transmitted via an A/D board and collected by computer. Fig. 3(b) shows the equipment (laminar flow meter, heater and pellet filter covered by stainless steel) for the regeneration test of the pellet filter. The flow rate is controlled with the same conditions as the test set-up for pressure drop and particle loading. In this study, porous cordierite pellet and nonporous alumina pellet are compared. Fig. 4 is a photo of a 100 mm pellet filter. From the left side photo, in order, the filter is composed of: (a) half 1 mm and half 2 mm porous cordierite pellets, (b) 2 mm diameter porous cordierite pellets only, (c) half 1 mm and half 2 mm diameter nonporous alumina pellet, and (d) 2 mm diameter nonporous alumina pellets only. The diameter of the filter case is 30 mm after being miniaturized to a 10th of the size of a real honeycomb filter equipped in 10 l 4-cycle engine. Pressure drop and filtration efficiency are considered across three pellet filter lengths (75, 100, 125 mm). When the 2200 cm3 engine car (Santafe, Hyundai) reach a velocity of 100–120 km/h, engine rotational frequency is about 2400 rpm which is 60% of maximum rpm (about 4000 rpm) of this engine. So it was tested the face velocity of filter at 2400 rpm. Face velocity at 2400 rpm was about 285 cm/s. We determined the maximum flow rate of miniaturized filter to maintain same face velocity condition. Maximum flow rate of miniaturized filter is 2000 cm3 /s. Assuming an engine rotational frequency up to 2400 rpm, this experiment has been undertaken at flow rates of up to 2000 cm3 /s at intervals of 500 cm3 /s. When measuring the pressure drop, the carbon container is removed and the differential pressure of the front and rear parts of the filter is measured as a function of the air flow. In this study, to measure the particle loading rate, all 0.5 g carbon particles in the particle container were moved by the pressure of the particle container and then the weight of particles which weren’t collected by the filter were measured using a precision balance. At this time, the particle container is designed hydromechanically to perfectly seed carbon particles. The particles are commercial
Porosities, mean pore sizes, pore size distribution, pore shape and pressure drop are the microstructural factors to the present application. The apparent porosity was calculated by difference between bulk density and apparent density, which was measured by liquid pycnometer technique. The porosity measured by the difference between true density and bulk density was about 70% and the porosity measured by mercury intrusion was 66% for all sintered porous pellet. The pore structure of a pellet is compared by SEM (Fig. 5). There is no pore on the surface of the nonporous pellet in Fig. 5(a). In contrast, there are many pores on surface of the porous pellet and shown some pores in the range 100–500 m in Fig. 5(b) and (c). This variety of pore size of pellet sample is mainly due to the formation and growth time difference of each bubble in foaming process. Fig. 5(d) is a micrograph of an interconnected open pore channel, which is formed by contact between two pores. The well-developed channels give porous ceramics filtration ability. Fig. 6 shows pore diameter distributions of the pellets using a mercury intrusion method and indicates that the mean pore size is around 10–20m. 3.2. Pressure drop of the pellet filter The pressure drop was tested by pressure measurement in both ends of a cylindrical filter filled with pellets at different flow rate. Fig. 7 shows the effect of flow rate on pressure drop in a porous pellet filter and a nonporous pellet filter, respectively. The length of the filter is 100 mm and the pressure drop is shown to be proportional to the flow rate. The smaller diameter pellets show higher pressure drop in the filter according to the Ergun equation [14]. The pressure drop of a 1 mm-porous pellet filter is less than that of a 1 mm nonporous pellet filter and the difference in pressure drop between these two increases as the flow rate increases. This means that the rate of gas flow through the pore channel in porous pellet increases as the flow rate increases. On the other hand, the pressure drops of the 2 mm porous pellet filter and the 2 mm nonporous pellet filter are nearly the same even the flow rates are changing up to 2000 cm3 /s, since the pressure drop of 2 mm porous pellet filter is too low to flow through the pore channels in pore of pellet. That is, interstitial space of the 2 mm pellet filter is large enough to allow 2000 cm3 /s of the gas to flow through it regardless of the porosity of pellet, unlike the case of the 1 mm pellet filter. Fig. 8 shows the pressure drop for pellet filters of different lengths and porosity. The flow rate is maintained at 2000 cm3 /s during the test sequence. The pressure drop decreases with pellet filter length because the flow through the pellet containing 70% open pores can easily pass the inside channels of each pellet
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Fig. 5. Scanning electron microscope (SEM) of the surface of the pellet; (a) non porous pellet surface, (b) porous pellet surface, (c) cross-section of porous pellet, (d) interconnected pore channel in pore.
as well as the space of pellets. A pellet filter with many open inside channels reduces the pressure drop, compared with the nonporous pellet filter. 3.3. The particle loading test of a pellet filter
Fig. 6. Pore size distribution by mercury intrusion.
Fig. 7. Pressure drop with flow rate (filter length 100 mm).
The particle loading test which is most important to filtration performance is measured and the results are shown in Figs. 9 and 11. Fig. 9 shows the pressure drop of the filter, filter paper pressure and the removal efficiency flow rate with a 100 mm filter filled with 1 mm porous pellets. The flow rate was changed from 0 to 1000 cm3 /s. The pressure drop of the filter increased rapidly to about 11 kPa at the beginning of the test due to increasing flow rate and soot collection. The pressure drop of filter paper rose to 0.9 kPa and then declined to around 0.7 kPa by decreasing the flow rate. Fig. 10 represents a mechanism of
Fig. 8. Effect of porosity on pressure drop (flow rate = 2000 cm3 /s, pellet diameter = 1 mm).
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Fig. 9. Change of pressure drop and flow rate with time.
Fig. 10. Mechanism of PM trap in the channel of a porous foam pellet.
soot filtration and shows that small size PM is collected and regenerated efficiently in the pore. Fig. 11 shows carbon particles, which collect in a pellet filter and filter paper. Fig. 11(a) shows carbon particles which were filtered by a 100 mm pellet filter composed half of 1 mm diameter and half of 2 mm diameter pellets. It indicates two things. First, most carbon particles are collected between a 2 mm diameter pellet layer and a 1 mm diameter pellet layer. Theoretical max-
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Fig. 12. Effect of porosity and particle loading rate with respect to pellet diameter (filter length, 100 mm).
imum particle sizes, which pass through between 1 and 2 mm diameter pellets are 0.31 and 0.157 mm, respectively; however, the carbon particles which are smaller than 0.31 and bigger than 0.157 mm pass through the 2 mm pellet layer, and these are collected above the 1 mm pellet layer. Second, carbon particles are distributed and collected evenly all over the non-porous pellet filter, while carbon particles are rare at the bottom of the 1 mm porous pellet filter. As a result, it may be inferred that many carbon particles can easily pass between the nonporous pellets, but many carbon particles are collected by the pore of the porous pellet (especially, on the upper part of a 1 mm pellet layer). Fig. 11(b) shows the carbon particles, which were collected on the filter paper. The case for using the nonporous pellet filter is that it filters more carbon particles than does the porous pellet filter. The particulate loading rate of a 100 mm filter filled with porous pellets is compared with that of one filled with nonporous pellets in Fig. 12. The 1 mm diameter porous and nonporous pellet filters have particulate loading rates of 96% and 83%, respectively. The porous pellet filter has better performance than the nonporous pellet filter. The 2 mm diameter pellet-filled filter has a low particulate loading rate compared with the 1 mm diameter pellet filter. Considering the results of the particulate trap with the pressure drop results from Fig. 7, the porous pellet filter has lower pressure drop and better trapping efficiency of carbon particulates by channel trapping in porous pellets than does the nonporous pellet filter. Fig. 12 shows the particulate
Fig. 11. Carbon particles collected in a pellet filter and filtering paper.
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higher than those of the 2 mm pellet filter, because the 1 mm pellet filter collects more soot particles than the 2 mm pellet filter. The inlet pressure of the filter decreased with the regeneration (soot oxidation) of the filter. 4. Conclusion
Fig. 13. Temperature and pressure of the porous pellet filter during regeneration.
loading rate of porous pellet filters with different porous pellet diameters (1 mm, 2 mm and mixed). The particulate loading rate in the case of the filter containing 2 mm pellets was about 70%; however, the particulate loading rates of the filters containing 1 mm pellets and those containing both 1 and 2 mm pellets are about 96% and 94%, respectively. 3.4. The regeneration test of the pellet filter The regeneration test of filters filled with pellets whose diameters are 1 and 2 mm, respectively, was investigated by measurement of temperature and pressure of heated air flowing over the filter. The air was heated at a rate of about 1.5 ◦ C/s. The filter length was 100 mm and flow rate was 2500 cm3 /s. Fig. 13 shows the results of the regeneration test for both filters. The 1 mm pellet filter temperature rose and dropped rapidly in the inlet air temperature range of 540–615 ◦ C, which is the regeneration range of soot. The 2 mm pellet filter is regenerated in the range of 530–580 ◦ C, which is narrow compared with the regeneration range of the 1 mm pellet filter. Temperatures of the 1 and 2 mm pellet filters were elevated to 830 and 590 ◦ C, respectively, due to the exothermic reaction caused by soot oxidation. The exothermic reaction durations of the 1 mm and the 2 mm pellet filters were about 30 s (540–585 ◦ C) and 15 s (530–555 ◦ C), respectively. That is, the exothermic reaction duration and the elevated temperature of the 1 mm pellet filter are longer and
The gelation with water-soluble polymers and a PDE consisting of 3PFS globules were used with a mechanical foaming method in order to fabricate porous pellets. The effects of pellet size, porosity and arrangement were investigated by measuring a pressure drop, PM loading rate and regeneration. The 70% improved porous pellet filter has higher efficiency than the nonporous pellet filter in terms of pressure drop and the effect increases with flow rate. The filtration efficiency of the porous pellet filter is superior to that of the nonporous pellet filter. This result suggests that small particles are collected inside pores of the porous pellet. When comparing the filter which mixes 1 and 2 mm pellets with that using only 1 mm pellets, the filtration efficiency is similar and the pressure drop is remarkably low. References [1] US E.P.A., Regulatory impact analysis, EPA-420-R-00-026. Office of air and radiation, 2000. December. [2] T.D. Durbin, X. Zhu, J.M. Norbeck, Atmos. Environ. 37 (2003) 2105– 2166. [3] www.dieselnet.com (web site). [4] O. Kazushige, S. Koji, T. Noriyuki, S. Hong, N. Takeshi, K. Teruo, SAE (2000), 2000-01-0185. [5] S.L. Cook, P.J. Richards, Atmos. Environ. 36 (2002) 2955–2964. [6] N. Taoka, O. Kazushige, S. Hong, S. Hiroki, Y. Yutaka, K. Teruo, SAE (2001), 2001-01-0191. [7] J.S. Lee, J.K. Park, J. Mater. Sci. Lett. 20 (2001) 205–207. [8] R.M. Heck, R.J. Farrauto, Catalytic Air Pollution Control, John Wiley & Sons, New York, 2002, pp. 132–133. [9] M. Koichi, Japan Patent 2004-316513. [10] T. Ehtezazi, C. Washington, C.D. Melia, J. Controlled Release 57 (1999) 301–314. [11] N. Garti, J. Colloid Surf. 123 (1997) 233–246. [12] J.K. Park, S.H. Lee, J. Ceram. Soc. Jpn. 109 (7) (2001) 580–586. [13] R.J. Locker, N. Gunasekaran, C. Sawyer, SAE (2002), 2002-01-1009. [14] K. Inoya, K. Gotoh, K. Higashitani, Powder Technology Handbook, Marcel Dekker, 1997, pp. 145–147.
Preparation and characterization of porous cordierite pellets and use as a diesel particulate filter Jai-Koo Park a,∗ , Jay-Hyun Park a , Jung-Wook Park a , Hong-Seok Kim b , Young-Il Jeong b a
Department of Geoenvironmental System Engineering, Hanyang University, 17, Heangdang-dong, Seongdong-gu, Seoul 133-791, Republic of Korea b Engine R&D group, Korea Institute of Machinery and Materials, 171 Jang-dong, Yuseong-gu, Daejeon 305-343, Republic of Korea Received 3 April 2006; received in revised form 8 November 2006; accepted 10 November 2006
Abstract This paper discusses the preparation and characterization of porous pellet filters for a diesel particulate filter (DPF). The process for creating the porous ceramic pellets was developed using simple gelcasting and a bubble-in-water-in-oil type of pseudo-double-emulsion. The pellets, consisting of a foamed cordierite suspension, were dispersed and consolidated in spherical shapes in liquid paraffin as gelation progressed, and were prepared by a conventional mechanical foaming method combined with gelcasting. In comparison with a wall-flow filter, the pellet filter has the advantages of being free of cracks during regeneration and having a flexible shape. Experiments were conducted in a test simulation of diesel engine exhaust conditions. Pressure drop and particle loading rate were compared using two pellet filters having porosities of 70% and 0%, respectively. Regeneration capability was also tested. The filter which mixed 1 and 2 mm pellets demonstrated feasibility as a DPF. © 2007 Published by Elsevier B.V. Keywords: Diesel vehicle; Particulate matter; Porous pellet; DPF
1. Introduction Reduction of emissions from diesel vehicles has become a more critical issue in recent years and research has resulted in improvements to engine technology and fuels and in wider use of after treatment devices. One of the most promising technologies is the diesel particulate filter [1–3]. The technology of DPF is largely divided into reduction and regeneration of particulate matter (PM) and this system is classified into a filter, a regeneration unit and a control unit. Filter configurations and materials are important factors in determining the performance of the DPF system. Typically, a wall-flow monolithic cell (honeycomb type) is used for configuration and the filter is composed of Cordierite (2MgO·2Al2 O3 ·5SiO2 ) or Silicon carbide (SiC). However, these types of filters are very expensive and may break due to thermal shock under an uncontrolled regeneration condition called stochastic regeneration. Metal filters and pellet filters have been studied as possible solutions to this defect [4–7]. Properties of thermal shock resistance and thermal conductivity of metal filters are superior to
∗
Corresponding author. E-mail address: [email protected] (J.-K. Park).
1383-5866/$ – see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.seppur.2006.11.009
those of ceramic monoliths. But, adhesion of ceramic based washcoat to metal filter surface, corrosion of metal in high temperature and high cost are problems [8]. Compared to a wall flow monolith filter, a pellet filter has the advantage of not cracking during regeneration and flexion. A pellet filter will reduce production costs because of its simplicity, ease of fabrication and installation. Pellets also are replaceable by refilling the casing. However, it is generally known that the pellet filter is heavier and generates higher backpressure than monolith filter [8]. The use of porous pellet might be solve this problems since porous pellets are light and many internal pores of pellets could help to reduce the backpressure of filter. Though high porosity of pellets aggravates the problem of pellet attrition when the pellets rubbed against one another in filter casing, porous pellets are still necessary to be studied because their mass transfer rate for soot regeneration can be improved by enlarged surface area and turbulent flow through pellets bed. Actually, Muto koichi in japan had developed the irregular porous pellets DPF due to the catalytic regeneration performance of porous pellets [9]. This paper presents a novel fabrication process for a porous cordierite ceramic pellet filter using mechanical foaming and considers the feasibility of porous cordierite ceramic pellets as a diesel particulate filter.
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2. Experimental 2.1. Preparation of porous ceramic pellets Porous ceramic pellets were prepared from a 3-phase foamed suspension (3PFS), also called pseudo-double-emulsion (PDE), a schematic of which is presented in Fig. 1. The double-emulsion method has been developed in the pharmaceutical industry for preparing microspheres to impregnate medicines [10]. Foaming can be considered as a type of emulsification since the both are very similar. In order to produce a PDE, a 3PFS is re-emulsified, as in the case of an oil-in-water-in-oil type double-emulsion [11,12]. In this PDE, constituent bubbles are the first dispersed phase, globules are the second dispersed phase, a cordierite suspension is the first continuous phase, and oil is the second continuous phase. That is, the globules of 3PFS dispersed in the second continuous phase contain an even smaller dispersed phase within themselves. Fig. 2 shows a flow chart for the overall process, which consists of suspension preparation, foaming by mechanical
whisking, fabricating, drying, and sintering. Aqueous suspensions, containing 45 vol.% cordierite powder (Koyritsu Ceramic) and 3.5 wt.% dispersant of polyethyleneimine (PEI, BASF), were ground by attrition mill for 4 h. Sodium lauryl sulfate (SLS, Samchun Pure Chemical) was used as a foaming agent and added to 100 cm3 of cordierite suspension. The suspensions were whisked vigorously in order to swell 3PFS to 400 cm3 . Then, each 3PFS was whisked at 600 rpm for 30 s. Next, the gelation agent was added to the 3PFS and whisked. The reactor for the PDE was a cylinder filled with 4000 cm3 of liquid paraffin (second continuous phase). About 400 cm3 3PFS was injected into the reactor by a peristaltic pump at a constant impeller revolution rate of 12.5 cm3 /s during the fabrication process to produce a PDE. The mixture was stirred at 240 rpm for 1 h. The gelated spheres (pellet) were separated by sieving from the liquid paraffin, followed by drying in air at 80 ◦ C for 6 h. Pyrolysis of the dried pellet was carried out in an alumina crucible at 600 ◦ C in air for 2 h and sintering of the pellet was performed with an electric furnace at 1350 ◦ C for 4 h using heating rates of 100 ◦ C/h up to 600 ◦ C and then 200 ◦ C/h up to the final temperature in air. 2.2. Apparatus set-up Fig. 3 is a schematic of the experiment to measure pressure drop, particle loading rate and regeneration through the trap. Fig. 3(a) shows the test set-up to measure the pressure drop and particle loading rate of the pellet filter. It consists of a particle container, pellet filter, filtering paper to measure the loading
Fig. 1. Schematic representation of a PDE including globules of a 3PFS and the reactor.
Fig. 2. Flow chart for preparing the porous pellet according to the PDE method.
Fig. 3. Schematic diagram of experimental set-up.
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carbon black powder about 20–25 nm in primary particle size (Printex U, Degussa) [13]. These particles are treated in a vacuum oven at 250 ◦ C to remove moisture to avoid agglomeration in humid atmospheric conditions. 3. Results and discussion 3.1. Characteristics of porous pellets
Fig. 4. Pellet filters for testing.
rate, a laminar flow meter and a super charger. The exhaust flow rate is determined by a supercharger using an A/C (alternating current) motor (220 V, three phase current, Max 1720 rpm) for vehicles and controlled by an inverter. The pressure drop across the trap is measured by a differential pressure meter (Dwyer 677). Differential pressure under laminar flow between the front and rear parts of the filter paper is also measured by a differential pressure meter (Dwyer 677, 616), and the data is transmitted via an A/D board and collected by computer. Fig. 3(b) shows the equipment (laminar flow meter, heater and pellet filter covered by stainless steel) for the regeneration test of the pellet filter. The flow rate is controlled with the same conditions as the test set-up for pressure drop and particle loading. In this study, porous cordierite pellet and nonporous alumina pellet are compared. Fig. 4 is a photo of a 100 mm pellet filter. From the left side photo, in order, the filter is composed of: (a) half 1 mm and half 2 mm porous cordierite pellets, (b) 2 mm diameter porous cordierite pellets only, (c) half 1 mm and half 2 mm diameter nonporous alumina pellet, and (d) 2 mm diameter nonporous alumina pellets only. The diameter of the filter case is 30 mm after being miniaturized to a 10th of the size of a real honeycomb filter equipped in 10 l 4-cycle engine. Pressure drop and filtration efficiency are considered across three pellet filter lengths (75, 100, 125 mm). When the 2200 cm3 engine car (Santafe, Hyundai) reach a velocity of 100–120 km/h, engine rotational frequency is about 2400 rpm which is 60% of maximum rpm (about 4000 rpm) of this engine. So it was tested the face velocity of filter at 2400 rpm. Face velocity at 2400 rpm was about 285 cm/s. We determined the maximum flow rate of miniaturized filter to maintain same face velocity condition. Maximum flow rate of miniaturized filter is 2000 cm3 /s. Assuming an engine rotational frequency up to 2400 rpm, this experiment has been undertaken at flow rates of up to 2000 cm3 /s at intervals of 500 cm3 /s. When measuring the pressure drop, the carbon container is removed and the differential pressure of the front and rear parts of the filter is measured as a function of the air flow. In this study, to measure the particle loading rate, all 0.5 g carbon particles in the particle container were moved by the pressure of the particle container and then the weight of particles which weren’t collected by the filter were measured using a precision balance. At this time, the particle container is designed hydromechanically to perfectly seed carbon particles. The particles are commercial
Porosities, mean pore sizes, pore size distribution, pore shape and pressure drop are the microstructural factors to the present application. The apparent porosity was calculated by difference between bulk density and apparent density, which was measured by liquid pycnometer technique. The porosity measured by the difference between true density and bulk density was about 70% and the porosity measured by mercury intrusion was 66% for all sintered porous pellet. The pore structure of a pellet is compared by SEM (Fig. 5). There is no pore on the surface of the nonporous pellet in Fig. 5(a). In contrast, there are many pores on surface of the porous pellet and shown some pores in the range 100–500 m in Fig. 5(b) and (c). This variety of pore size of pellet sample is mainly due to the formation and growth time difference of each bubble in foaming process. Fig. 5(d) is a micrograph of an interconnected open pore channel, which is formed by contact between two pores. The well-developed channels give porous ceramics filtration ability. Fig. 6 shows pore diameter distributions of the pellets using a mercury intrusion method and indicates that the mean pore size is around 10–20m. 3.2. Pressure drop of the pellet filter The pressure drop was tested by pressure measurement in both ends of a cylindrical filter filled with pellets at different flow rate. Fig. 7 shows the effect of flow rate on pressure drop in a porous pellet filter and a nonporous pellet filter, respectively. The length of the filter is 100 mm and the pressure drop is shown to be proportional to the flow rate. The smaller diameter pellets show higher pressure drop in the filter according to the Ergun equation [14]. The pressure drop of a 1 mm-porous pellet filter is less than that of a 1 mm nonporous pellet filter and the difference in pressure drop between these two increases as the flow rate increases. This means that the rate of gas flow through the pore channel in porous pellet increases as the flow rate increases. On the other hand, the pressure drops of the 2 mm porous pellet filter and the 2 mm nonporous pellet filter are nearly the same even the flow rates are changing up to 2000 cm3 /s, since the pressure drop of 2 mm porous pellet filter is too low to flow through the pore channels in pore of pellet. That is, interstitial space of the 2 mm pellet filter is large enough to allow 2000 cm3 /s of the gas to flow through it regardless of the porosity of pellet, unlike the case of the 1 mm pellet filter. Fig. 8 shows the pressure drop for pellet filters of different lengths and porosity. The flow rate is maintained at 2000 cm3 /s during the test sequence. The pressure drop decreases with pellet filter length because the flow through the pellet containing 70% open pores can easily pass the inside channels of each pellet
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Fig. 5. Scanning electron microscope (SEM) of the surface of the pellet; (a) non porous pellet surface, (b) porous pellet surface, (c) cross-section of porous pellet, (d) interconnected pore channel in pore.
as well as the space of pellets. A pellet filter with many open inside channels reduces the pressure drop, compared with the nonporous pellet filter. 3.3. The particle loading test of a pellet filter
Fig. 6. Pore size distribution by mercury intrusion.
Fig. 7. Pressure drop with flow rate (filter length 100 mm).
The particle loading test which is most important to filtration performance is measured and the results are shown in Figs. 9 and 11. Fig. 9 shows the pressure drop of the filter, filter paper pressure and the removal efficiency flow rate with a 100 mm filter filled with 1 mm porous pellets. The flow rate was changed from 0 to 1000 cm3 /s. The pressure drop of the filter increased rapidly to about 11 kPa at the beginning of the test due to increasing flow rate and soot collection. The pressure drop of filter paper rose to 0.9 kPa and then declined to around 0.7 kPa by decreasing the flow rate. Fig. 10 represents a mechanism of
Fig. 8. Effect of porosity on pressure drop (flow rate = 2000 cm3 /s, pellet diameter = 1 mm).
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Fig. 9. Change of pressure drop and flow rate with time.
Fig. 10. Mechanism of PM trap in the channel of a porous foam pellet.
soot filtration and shows that small size PM is collected and regenerated efficiently in the pore. Fig. 11 shows carbon particles, which collect in a pellet filter and filter paper. Fig. 11(a) shows carbon particles which were filtered by a 100 mm pellet filter composed half of 1 mm diameter and half of 2 mm diameter pellets. It indicates two things. First, most carbon particles are collected between a 2 mm diameter pellet layer and a 1 mm diameter pellet layer. Theoretical max-
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Fig. 12. Effect of porosity and particle loading rate with respect to pellet diameter (filter length, 100 mm).
imum particle sizes, which pass through between 1 and 2 mm diameter pellets are 0.31 and 0.157 mm, respectively; however, the carbon particles which are smaller than 0.31 and bigger than 0.157 mm pass through the 2 mm pellet layer, and these are collected above the 1 mm pellet layer. Second, carbon particles are distributed and collected evenly all over the non-porous pellet filter, while carbon particles are rare at the bottom of the 1 mm porous pellet filter. As a result, it may be inferred that many carbon particles can easily pass between the nonporous pellets, but many carbon particles are collected by the pore of the porous pellet (especially, on the upper part of a 1 mm pellet layer). Fig. 11(b) shows the carbon particles, which were collected on the filter paper. The case for using the nonporous pellet filter is that it filters more carbon particles than does the porous pellet filter. The particulate loading rate of a 100 mm filter filled with porous pellets is compared with that of one filled with nonporous pellets in Fig. 12. The 1 mm diameter porous and nonporous pellet filters have particulate loading rates of 96% and 83%, respectively. The porous pellet filter has better performance than the nonporous pellet filter. The 2 mm diameter pellet-filled filter has a low particulate loading rate compared with the 1 mm diameter pellet filter. Considering the results of the particulate trap with the pressure drop results from Fig. 7, the porous pellet filter has lower pressure drop and better trapping efficiency of carbon particulates by channel trapping in porous pellets than does the nonporous pellet filter. Fig. 12 shows the particulate
Fig. 11. Carbon particles collected in a pellet filter and filtering paper.
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higher than those of the 2 mm pellet filter, because the 1 mm pellet filter collects more soot particles than the 2 mm pellet filter. The inlet pressure of the filter decreased with the regeneration (soot oxidation) of the filter. 4. Conclusion
Fig. 13. Temperature and pressure of the porous pellet filter during regeneration.
loading rate of porous pellet filters with different porous pellet diameters (1 mm, 2 mm and mixed). The particulate loading rate in the case of the filter containing 2 mm pellets was about 70%; however, the particulate loading rates of the filters containing 1 mm pellets and those containing both 1 and 2 mm pellets are about 96% and 94%, respectively. 3.4. The regeneration test of the pellet filter The regeneration test of filters filled with pellets whose diameters are 1 and 2 mm, respectively, was investigated by measurement of temperature and pressure of heated air flowing over the filter. The air was heated at a rate of about 1.5 ◦ C/s. The filter length was 100 mm and flow rate was 2500 cm3 /s. Fig. 13 shows the results of the regeneration test for both filters. The 1 mm pellet filter temperature rose and dropped rapidly in the inlet air temperature range of 540–615 ◦ C, which is the regeneration range of soot. The 2 mm pellet filter is regenerated in the range of 530–580 ◦ C, which is narrow compared with the regeneration range of the 1 mm pellet filter. Temperatures of the 1 and 2 mm pellet filters were elevated to 830 and 590 ◦ C, respectively, due to the exothermic reaction caused by soot oxidation. The exothermic reaction durations of the 1 mm and the 2 mm pellet filters were about 30 s (540–585 ◦ C) and 15 s (530–555 ◦ C), respectively. That is, the exothermic reaction duration and the elevated temperature of the 1 mm pellet filter are longer and
The gelation with water-soluble polymers and a PDE consisting of 3PFS globules were used with a mechanical foaming method in order to fabricate porous pellets. The effects of pellet size, porosity and arrangement were investigated by measuring a pressure drop, PM loading rate and regeneration. The 70% improved porous pellet filter has higher efficiency than the nonporous pellet filter in terms of pressure drop and the effect increases with flow rate. The filtration efficiency of the porous pellet filter is superior to that of the nonporous pellet filter. This result suggests that small particles are collected inside pores of the porous pellet. When comparing the filter which mixes 1 and 2 mm pellets with that using only 1 mm pellets, the filtration efficiency is similar and the pressure drop is remarkably low. References [1] US E.P.A., Regulatory impact analysis, EPA-420-R-00-026. Office of air and radiation, 2000. December. [2] T.D. Durbin, X. Zhu, J.M. Norbeck, Atmos. Environ. 37 (2003) 2105– 2166. [3] www.dieselnet.com (web site). [4] O. Kazushige, S. Koji, T. Noriyuki, S. Hong, N. Takeshi, K. Teruo, SAE (2000), 2000-01-0185. [5] S.L. Cook, P.J. Richards, Atmos. Environ. 36 (2002) 2955–2964. [6] N. Taoka, O. Kazushige, S. Hong, S. Hiroki, Y. Yutaka, K. Teruo, SAE (2001), 2001-01-0191. [7] J.S. Lee, J.K. Park, J. Mater. Sci. Lett. 20 (2001) 205–207. [8] R.M. Heck, R.J. Farrauto, Catalytic Air Pollution Control, John Wiley & Sons, New York, 2002, pp. 132–133. [9] M. Koichi, Japan Patent 2004-316513. [10] T. Ehtezazi, C. Washington, C.D. Melia, J. Controlled Release 57 (1999) 301–314. [11] N. Garti, J. Colloid Surf. 123 (1997) 233–246. [12] J.K. Park, S.H. Lee, J. Ceram. Soc. Jpn. 109 (7) (2001) 580–586. [13] R.J. Locker, N. Gunasekaran, C. Sawyer, SAE (2002), 2002-01-1009. [14] K. Inoya, K. Gotoh, K. Higashitani, Powder Technology Handbook, Marcel Dekker, 1997, pp. 145–147.