chemical engineering research and design 9 1 ( 2 0 1 3 ) 348–360 Contents lists available at SciVerse ScienceDirect Chemical Engineering Research and Design journa l h o me pa ge: www.elsevier.com/locate/cherd Design guidelines for granular particles in a conical centrifugal filter A.F.M. Bizard a , D.D. Symons a,∗ , N.A. Fleck a , G.C. Grimwood b a Cambridge University Engineering Department, Trumpington Street, Cambridge CB2 1PZ, UK b Thomas Broadbent and Sons Ltd., Queen Street South, Huddersfield HD1 3EA, UK a b s t r a c t Essential design criteria for successful drying of granular particles in a conical continuous centrifugal filter are developed in a dimensionless fashion. Four criteria are considered: minimum flow thickness (to ensure sliding bulk flow rather than particulate flow), desaturation position, output dryness and basket failure. The criteria are based on idealised physical models of the machine operation and are written explicitly as functions of the basket size l out , spin velocity ˝ and input flow rate of powder Q p . The separation of sucrose crystals from liquid molasses is taken as a case study and the successful regime of potential operating points (l out , ˝) is plotted for a wide range of selected values of flow rate Q p . Analytical expressions are given for minimum and maximum values of the three independent parameters (l out , ˝, Q p ) as a function of the slurry and basket properties. The viable operating regime for a conical centrifugal filter is thereby obtained as a function of the slurry and basket properties. © 2012 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Design; Filtration; Centrifugation; Powders; Separation; Drying 1. Introduction 1.1. Centrifugal filters Centrifugal filters are commonly used in the food processing and chemical industries in order to separate the liquid and solid phases of a mixture. There exist two main types of cen- trifugal filter: batch machines with a cylindrical basket and continuous machines with a conical basket. The present study focuses on continuous conical centrifuges, which are most commonly used in the sucrose industry to separate sucrose crystals from molasses. Swindells (1982) and Greig (1995) stud- ied the functioning of these machines in a semi-empirical fashion. While their work provides valuable insight into the operation of conical centrifuges in the sucrose industry it does not fully address the underlying mechanics. Consequently, only a limited number of operating parameters have been used to optimize the design and operation of the sucrose machine. Application of the results for sucrose to pharmaceu- tical, chemical or other food products has also proved difficult. ∗ Corresponding author. Tel.: +44 1223 760502; fax: +44 1223 332662. E-mail address: digby.symons@eng.cam.ac.uk (D.D. Symons). URL: http://www-edc.eng.cam.ac.uk/people/dds11.html (D.D. Symons). Received 6 December 2011; Received in revised form 6 July 2012; Accepted 19 July 2012 This study aims to provide fundamental guidelines for the design of a conical centrifugal filter, based upon idealised physical models of the machine. 1.2. Typical operation of a continuous centrifuge The operation of a continuous conical centrifuge is now described through the example of a typical sucrose industry machine. The rotating conical basket of the machine, sketched in Fig. 1, has a jump in cone angle along its length: a lower impervious cone has a semi-angle of ˛ = 15 ◦ whereas the upper perforated cone has a semi-angle of ˛ = 30 ◦ . The basket is about 1m in diameter at outlet and spins at 1800 RPM to provide a maximum centripetal acceleration of 2000 × g. The inside wall of the upper, perforated cone is fitted with a slot- ted screen, thereby allowing for fluid drainage but preventing powder losses, see Fig. 1. The feedstock, in the form of a sucrose/molasses slurry (massecuite) of mass moisture fraction M in ≈50% and temperature 60 ◦ C, is introduced along the spin axis into the lower impervious cone at a constant mass flow 0263-8762/$ – see front matter © 2012 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cherd.2012.07.008