Chapter 9 Solids separation processes M.J. LEWIS, Department of Food Science and Technology, The University of Reading RG6 6AP 9.1 INTRODUCTION This chapter will cover the separations involving solid foods, together with the propertie of those solids which will influence that separation. Some mention will also be made of handling and transporting solids and preparatory processes, such as size reduction. The separation of solids from liquids and solids from gases is not covered in detail in this chapter, although a summary of the methods based on sedimentation and filtration is given in Table 9. 1. In these applications, the term solids refers to discrete particles suspended within the fluid and not those dissolved or in the colloidal form, for which a range of other operations for their removal or separation is available. The objective may be to recover the solid for further processing or to remove the solid which may be contaminating the liquid or gas. The method selected also depends upon whether the solid is to be retained or discarded To illustrate some of the difficulties in selecting solids separation methods, the re moval of solids from gases will be further illustrated. This can be achieved by classifiers cyclones, bag filters or electrostatic precipitators. In cyclones on milk powder plant, particles less than 5-10 um may be lost. Cyclone losses of 0.35-1.0% of total production have been cited for dairy products. Such losses are now unacceptable for environmental reasons. High-efficiency cyclones have been used, whereby secondary air is introduced into the cyclone to increase the efficiency. However, these cyclones are not very success ful with powders containing fat, as considerable free fat is generated and the powder sticks to the interior surface of the drier. Therefore it is not possible to install a milk drier where the powder recovery system consists of cyclones alone. Wet systems such as scrubbers have been installed, using the pasteurised milk, prior to evaporation, as th scrubbing liquid, thereby recovering the fines and heat. From a recovery standpoint, thi would seem an excellent solution. However, from a hygiene and quality standpoint, these proved almost impossible to operate without bacteriological contamination. Most of these have now been removed(Knipschildt, 1986). The solution to the problem has been provided by bag filters, which are capable of reducing the particle concentration fromChapter 9 Solids separation processes M. J. LEWIS, Department of Food Science and Technology, The University of Reading, RG6 6AP 9.1 INTRODUCTION This chapter will cover the separations involving solid foods, together with the properties of those solids which will influence that separation. Some mention will also be made of handling and transporting solids and preparatory processes, such as size reduction. The separation of solids from liquids and solids from gases is not covered in detail in this chapter, although a summary of the methods based on sedimentation and filtration is given in Table 9.1. In these applications, the term solids refers to discrete particles suspended within the fluid and not those dissolved or in the colloidal form, for which a range of other operations for their removal or separation is available. The objective may be to recover the solid for further processing or to remove the solid which may be contaminating the liquid or gas. The method selected also depends upon whether the solid is to be retained or discarded. To illustrate some of the difficulties in selecting solids separation methods, the re￾moval of solids from gases will be further illustrated. This can be achieved by classifiers, cyclones, bag filters or electrostatic precipitators. In cyclones on milk powder plant, particles less than 5-10 pm may be lost. Cyclone losses of 0.35-1.0% of total production have been cited for dairy products. Such losses are now unacceptable for environmental reasons. High-efficiency cyclones have been used, whereby secondary air is introduced into the cyclone to increase the efficiency. However, these cyclones are not very success￾ful with powders containing fat, as considerable free fat is generated and the powder sticks to the interior surface of the drier. Therefore it is not possible to install a milk drier where the powder recovery system consists of cyclones alone. Wet systems such as scrubbers have been installed, using the pasteurised milk, prior to evaporation, as the scrubbing liquid, thereby recovering the fines and heat. From a recovery standpoint, this would seem an excellent solution. However, from a hygiene and quality standpoint, these proved almost impossible to operate without bacteriological contamination. Most of these have now been removed (Knipschildt, 1986). The solution to the problem has been provided by bag filters, which are capable of reducing the particle concentration from