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Submitted to: Journal of Powder Technology
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 3/9/2009 Publication Date: 3/25/2009 Citation: Liu, K.S. 2009. Some factors affecting sieving efficiency and performance. Powder Technology. 193:208-213. Interpretive Summary: The size distribution of particulate matter is very important in determining its physicochemical properties in a large number of processes of various industries (e.g. production of food powders, chemicals, colorants, paints, and pharmaceuticals). The sieves/screens are the oldest and most widely used working elements for the separation of solid particles by size. They are used both industrially and at a laboratory scale for the classification of particulate material. During sieving, a stack of sieves of decreasing mesh size is most commonly used. Alternatively, particles can be sifted in a fine to coarse order by multiple sieving steps with each step using a single sieve. This method is defined as the reverse sieve method. The objectives of the present study were: (1) to make a systematic comparison between the stacked and reverse sieve methods for separation of flour, and (2) determine the effects of flour type, milling method, sieving duration and sieve percussion on sieving performance using both the stacked and reverse sieving methods. Since hundreds of millions of tones of particulate material are subjected to industrial sieving/screening each year, an understanding of the factors affecting sieving efficiency and performance has great economic significance. This study has demonstrated how the selected variables can affect sieving efficiency and performance. It is also the first to report the decisive effect of reserve sieve method over the conventional stacked sieve method on improving sieving efficiency under all the conditions of this study. The observed difference between the two sieving procedures can be explained by the beneficial effect of oversized particles on reducing sieve blinding by near or sub-sieve sized particles. Furthermore, by using the reverse sieve procedure, not only the mass frequency of finer particle classes was significantly increased but also the difference in protein content among sieved fractions was enlarged. Technical Abstract: Sieving or screening has been the oldest yet important unit operation for industrial separation of solid particles or as a laboratory method in size analysis. Oftentimes a stack of sieves with decreasing mesh size is used. Alternatively, particles can be sifted in a fine to coarse order by multiple sieving steps with each step using a single sieve. This method is referred to as reverse sieve method. This study compared the two methods for sieving performance and efficiency of flour made from soft and white wheat, hulless barley and medium grain rice. Additional factors, such as milling method (Cyclone milling or scarifying), flour moisture (7% or 11%), duration of sieving (60 or 120 min), and tapping (percussion during sieving) or no-tapping were also investigated. Mass frequency and protein content of oversize fractions were measured. Results show that all the variables and their interactions had significant effects on sieving performance and efficiency. Among them, tapping was most important one, followed by sieving duration, sieving method, milling method, flour type, and flour moisture. When other conditions were kept same, the reverse sieve method always gave improved sieving efficiency over the stacked sieve method. The observation can be attributed to the beneficial effect of oversized particles on reducing sieve blinding by near or sub-sieve sized particles. Furthermore, the reverse sieve method also enlarged the difference in protein content among sieved fractions. Because of its practical significance, this so far unreported effect would bear further confirmation of other sieving and screening conditions in general. |
