A Model of Foam Filters

  • PDF / 373,452 Bytes
  • 5 Pages / 593.972 x 792 pts Page_size
  • 92 Downloads / 251 Views

DOWNLOAD

REPORT


cells have been an attractive technology for power generation due to their clean and nonpolluting nature. During the past decade strong growth in demand has occurred for solar cell modules. The current dominant semiconductor material used in photovoltaics is a silicon wafer, particularly multicrystalline silicon. Multicrystalline silicon is cast in a silicon nitride (Si3N4)-coated quartz crucible and is subsequently directionally solidified. A Si3N4 coating that prevents the adhesion of the silicon ingot to the quartz crucible walls is the source of the silicon nitride inclusions in the silicon ingot. The lining refractory and graphite heating elements may be a source for silicon carbide particles.[1] After directional solidification, the parts that contain solid particles are cut off from the top of the ingot. Usually slices 10 to 20 mm deep are removed. These regions have a high concentration of impurities such as iron, aluminum, SiC, Si3N4, and so on. Although the bottom and the sides are recycled and used again in the solidification process, the top cut is sold as low-value scrap.[2] Removal of SiC and Si3N4 particles from the top-cut scrap has been accomplished successfully by filtration with foam filters. Filtration efficiency of a 30-ppi (pores per linear inch) foam filter is more than 99 pct. All the particles larger than 10 mm are removed from the melt.[3] Many authors have investigated filtration. Several researchers[4–11] approximated the geometry of the foam filter unit cell to that of two spherical pores that overlap ARJAN CIFTJA, Research Scientist, is with the Department of Metallurgy, SINTEF Materials and Chemistry, Alfred Getz Vei 2B, Trondheim NO-7465, Norway. THORVALD ABEL ENGH and MERETE TANGSTAD, Professors, are with the Department of Material Sciences and Engineering, Norwegian University of Science and Technology, Alfred Getz Vei 2B, Trondheim NO-7491, Norway. Contact e-mail: [email protected] Manuscript submitted August 31, 2009. Article published online October 8, 2009. 146—VOLUME 41B, FEBRUARY 2010

each other as shown in Figure 1. By considering the fluid flow inside these pores, the researchers attempted to predict the particle trajectory inside the filter and the sites where these particles are captured. Engh[12] used the isolated sphere model to predict filtration efficiency for a filter made of a packed bed of spheres. Tien and Ramarao[13] used the constricted-tube model to describe deep bed filtration. Here, we discuss these models, apply them to ceramic foam filters, and finally, develop a new model that should describe better the removal efficiency of foam filters.

II.

FILTRATION

A. Filtration Efficiency for Various Models Filtration in metallurgy is the process of removing inclusions by forcing molten metal through a porous material. An important characteristic of a filter is the filtration efficiency E, which is expressed as follows: cout ½1 E¼1 cin where cin is the number of inclusions per unit volume that enters the filter and cout is the number per unit volume that exit. Filtration efficiency follow

Data Loading...