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 from
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 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 removal 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 successful 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
244 M.J. Lewis Table 9.1. Summary of mechanical solid separation techniques Solids from liquids Principles: gravity, centrifugal, electrostatic, magnetic centrifugation Examples: gravity settlers, centrifugal clarifiers, hydrocyclones; use of chemical floc Filtration:(see also Chapter 8; fat fractionation) Principles: gravity, vacuum, pressure and centrifugal Examples: sand and cake filters, rotary vacuum filters, cartridge and plate and frame filters, microfilters( Chapter 5): use of filter aids Solids from gases Principles: sedimentation and filtration Examples: cyclones, bag filters, electrostatic precipitators 200 mg m-to below 10 mg m air. The powder can be recovered from the bags and the clean air'can be used for heat exchange. Further details are provided by Knipschildt (1986) However, rather than removing all the particles, there may be a requirement to fractionate the powder, based on particle size(see Sections 9.3 and 9.4). This example illustrates the theme for this chapter, where the main emphasis is placed on the separation of components from within a solid matrix. Solids come in many forms, shapes and sizes, so the first part of the chapter will be devoted to discussion of the main properties of solid foods which will influence the different types of separation processes 9.2 PHYSICAL PROPERTIES OF SOLIDS Solids come in a wide variety of shapes and sizes. All solid foods are particulate in nature and there are a wide range of sizes and shapes to contend with. Some examples are illustrated from the different food sectors in Table 9. 2. It should be noted that although all these foods are regarded as solids, their moisture content may range from less than 10% to greater than 90%. Their moisture content and chemical composition can be found from foods composition tables, for example Paul and Southgate (1978)(see also Chapter 2 Indeed, one of the main objectives is often to remove selected components from the food Some operations where separations from solids is involved and constitutes an impor- tant part of the process are cleaning of agricultural produce(see Section 9.6.3); sorting and size grading, particularly for quality grading of fruit and vegetables peeling of vegetables, dehulling of cereals and legumes and deboning or shelling of meat and fish fractionation or recovery of the main components within the foods, e.g. proteins, fat, carbohydrates and minerals
244 M. J. Lewis Table 9.1. Summary of mechanical solid separation techniques Solids from liquids Sedimentation: Principles: gravity, centrifugal, electrostatic, magnetic centrifugation Examples: gravity settlers, centrifugal clarifiers, hydrocyclones; use of chemical flocFiltration: (see also Chapter 8; fat fractionation) Principles: gravity, vacuum, pressure and centrifugal Examples: sand and cake filters, rotary vacuum filters, cartridge and plate and frame filters, microfilters (Chapter 5); use of filter aids culants or air flotation Solids from gases Principles: sedimentation and filtration Examples: cyclones, bag filters, electrostatic precipitators 200 mg m-3 to below 10 mg m-3 air. The powder can be recovered from the bags and the ‘clean air’ can be used for heat exchange. Further details are provided by Knipschildt (1986). However, rather than removing all the particles, there may be a requirement to fractionate the powder, based on particle size (see Sections 9.3 and 9.4). This example illustrates the theme for this chapter, where the main emphasis is placed on the separation of components from within a solid matrix. Solids come in many forms, shapes and sizes, so the first part of the chapter will be devoted to discussion of the main properties of solid foods which will influence the different types of separation processes. 9.2 PHYSICAL PROPERTIES OF SOLIDS Solids come in a wide variety of shapes and sizes. All solid foods are particulate in nature and there are a wide range of sizes and shapes to contend with. Some examples are illustrated from the different food sectors in Table 9.2. It should be noted that although all these foods are regarded as solids, their moisture content may range from less than 10% to greater than 90%. Their moisture content and chemical composition can be found from foods composition tables, for example Paul and Southgate (1978) (see also Chapter 2). Indeed, one of the main objectives is often to remove selected components from the food. Some operations where separations from solids is involved and constitutes an important part of the process are: cleaning of agricultural produce (see Section 9.6.3); sorting and size grading, particularly for quality grading of fruit and vegetables; peeling of vegetables, dehulling of cereals and legumes and deboning or shelling of meat and fish; fractionation or recovery of the main components within the foods, e.g. proteins, fat, carbohydrates and minerals
Solids separation processes 245 Table 9. 2. Some examples of solid foods Fruit: apples, oranges, grapes, blackcurrants, pears, bananas Vegetables: potatoes, carrots,sprouts, peas Cereals and legumes: rice, wheat, soyabeans, cowpeas, sorghum Animal produce: large carcasses, small joints, minced meats, fish fillets, prawns, shrimps and other shellfish Beverages: coffee beans, tea leaves, instant powders and granules Other powders: milled products, powders produced by drying and grinding methods A special range of operations and an area of increasing interest is concerned with the separation or fractionation of solids, in their particulate or powder form, and their recov ery from other materials. In this chapter, emphasis will be placed on the separation of powders, based on factors such as size and shape, density differences, flow properties olour and electrostatic charge. An important pretreatment for many such operations is size reduction Methods of size reduction are discussed in Section 9. 3. 1. Size reduction increases the surface area and the surface area to volume ratio thereby enhancing rates of heat and mass transfer However, in some cases very fine powders provide processing problems, and size enlargement or agglomeration may be used to improve flow characteristics and wettability Many foods which are solid in appearance, will also flow if the shear force provided is great enough, e.g. butter, spreads and starch doughs. This behaviour is known as plasticity. The flow behaviour of powders is also important and is discussed in more detail in Section 9.2.7. Some of the important phys operties of solid foods are listed in Table 9.3. These are discussed in more detail by Lewis(1990), Jowitt et aL.(1983 1987), Mohsenin(1984, 1986)and Peleg and Bagley(1983). Many of these properties are influenced by the chemical composition of the food, and in particular its moisture content Of special interest in this context is the behaviour of particulate systems and the separation of mixtures. Many such separations are based on density differences. In some cases the powders may be subjected to various forces, gravitational, which are slow Table 9.3. Physical properties of solids Appearance, size, shape, size distribution, colour Specific gravity, particle density, bulk density, porosity, overrun(for aerated products Thermal properties; specific heat, latent heat, thermal conductivity, thermal diffusivity Rheologial properties; plasticity, elasticity, viscoelasticity, hardness Electrical conductance or resistance, electrical charge, dielectric constant, dielectric loss Diffusion and mass transfer characteristics
Solids separation processes 245 Table 9.2. Some examples of solid foods Fruit: apples, oranges, grapes, blackcurrants, pears, bananas Vegetables: potatoes, carrots, sprouts, peas Cereals and legumes: rice, wheat, soyabeans, cowpeas, sorghum Animal produce: large carcasses, small joints, minced meats, fish fillets, prawns, shrimps Beverages: coffee beans, tea leaves, instant powders and granules Other powders: milled products, powders produced by drying and grinding methods and other shellfish A special range of operations and an area of increasing interest is concerned with the separation or fractionation of solids, in their particulate or powder form, and their recovery from other materials. In this chapter, emphasis will be placed on the separation of powders, based on factors such as size and shape, density differences, flow properties, colour and electrostatic charge, An important pretreatment for many such operations is size reduction. Methods of size reduction are discussed in Section 9.3.1. Size reduction increases the surface area and the surface area to volume ratio, thereby enhancing rates of heat and mass transfer. However, in some cases very fine powders provide processing problems, and size enlargement or agglomeration may be used to improve flow characteristics and wettability. Many foods which are solid in appearance, will also flow if the shear force provided is great enough, e.g. butter, spreads and starch doughs. This behaviour is known as plasticity. The flow behaviour of powders is also important and is discussed in more detail in Section 9.2.7. Some of the important physical properties of solid foods are listed in Table 9.3. These are discussed in more detail by Lewis (1990), Jowitt et al. (1983, 1987), Mohsenin (1984, 1986) and Peleg and Bagley (1983). Many of these properties are influenced by the chemical composition of the food, and in particular its moisture content. Of special interest in this context is the behaviour of particulate systems and the separation of mixtures. Many such separations are based on density differences. In some cases the powders may be subjected to various forces, gravitational, which are slow Table 9.3. Physical properties of solids Appearance, size, shape, size distribution, colour Specific gravity, particle density, bulk density, porosity, overrun (for aerated products) Thermal properties; specific heat, latent heat, thermal conductivity, thermal diffusivity, Rheologial properties; plasticity, elasticity, viscoelasticity, hardness Electrical conductance or resistance, electrical charge, dielectric constant, dielectric loss Diffusion and mass transfer characteristics specific enthalpy factor
246 M.J. Lewis compared to centrifugal forces, drag forces or electrical, electrostatic or magnetic forces Also, the flow characteristics and behaviour of food powders are markedly different to those of fluid Some of the physical properties of food powders will now be considered in more detail, especially those which will influence the effectiveness, quality and nature of the eparation process. 9. 2.1 Classification of powders Powders can be characterised in a large number of ways; Peleg (1983) gives some by usage: e. g flours, beverages, spices, sweeteners; by major component: e. g starchy, proteinaceous, fatty y process: e. g. ground powders, freeze-dried, agglomerated y size: e.g. fine, coarse y moisture sorption characteristics: e. g hygroscopic; by flowability: free flowing, sticky, very cohesive Further classification could be by hardness, by explosion potential or by microbial hazards. Hayes (1987)summarises a detailed system used for characterising a wide range of food powders based on density, size, flowability, abrasiveness, a range of miscellane- ous properties and hazards such as flammability, explosiveness and corrosive nature Some important physical, chemical and functional properties of powders are given in Table 9. 4. For products such as beverages, the palatability and sensory characteristics of the reconstituted products are important and may be variables considered when grading these products. Care should also be taken to ensure that the microbial count is within cceptable limits for the products Determination of some of these properties for milk powders is described in publica tions by the Society of Dairy Technology(SDT, 1980), and Schubert (1987a) Table 9. 4. Factors contributing to the quality of powd ppearance Size and shape stabilit Bulk density and particle density Nutrient content Microbiological quality
246 M. J. Lewis compared to centrifugal forces, drag forces or electrical, electrostatic or magnetic forces. Also, the flow characteristics and behaviour of food powders are markedly different to those of fluids. Some of the physical properties of food powders will now be considered in more detail, especially those which will influence the effectiveness, quality and nature of the separation process. 9.2.1 Classification of powders Powders can be characterised in a large number of ways; Peleg (1983) gives some examples: by usage: e.g. flours, beverages, spices, sweeteners; by major component: e.g. starchy, proteinaceous, fatty; by process: e.g. ground powders, freeze-dried, agglomerated; by size: e.g. fine, coarse; by moisture sorption characteristics: e.g. hygroscopic; by flowability : free flowing, sticky, very cohesive. Further classification could be by hardness, by explosion potential or by microbial hazards. Hayes (1987) summarises a detailed system used for characterising a wide range of food powders based on density, size, flowability, abrasiveness, a range of miscellaneous properties and hazards such as flammability, explosiveness and corrosive nature. Some important physical, chemical and functional properties of powders are given in Table 9.4. For products such as beverages, the palatability and sensory characteristics of the reconstituted products are important and may be variables considered when grading these products. Care should also be taken to ensure that the microbial count is within acceptable limits for the products. Determination of some of these properties for milk powders is described in publications by the Society of Dairy Technology (SDT, 1980), and Schubert (1987a). Table 9.4. Factors contributing to the quality of powders Appearance Size and shape Wettability Sinkability Solubility Dispersibility Bulk density and particle density Palatability Nutrient content Microbiological quality
Solids separation processes 247 9.2.2 Particle size and particle size distribution As mentioned earlier, food powders come in a wide range of sizes and shapes. Uniform shapes, such as spheres, can be characterised by one dimension, i. e the diameter, whereas two or more measurements may be required for more complex shapes. Whatever the shape, there are several methods available to characterise the size and particle size distri bution. Virtually all operations that result in the production of a powder, e.g. milling or spray drying, will give rise to a product with a distribution of particle sizes and this distribution is of extreme importance and will affect the bulk properties. Particle size may range over several orders of magnitude, ranging from less than I um to as large as hundreds or even thousands of microns for some large granules. Particle size can be measured in principle by measuring any physical property which correlates with the geometric dimensions of the sample. According to Schubert(1987a)the attributes used to characterise particles may be classified as follow geometric characteristics, such as linear dimensions, areas or volumes mass; settling rates interference techniques such as electrical field interference and light or laser scattering or diffracti Based on these attributes, the following methods have been used for food materials microscopy or other image scanning techniques; wet and dry sieving methods electrical impedance methods such as the Coulter counter; laser diffraction patterns, such as the Malvern, Northrup and cilas instruments Since particles can vary in both shape and size, different methods of particle size analysis do not always give consistent results, both because of the different physical principles being exploited, but also because size and shape are interrelated. Sampling is alse important to ensure that a representative sample is taken, usually by the method of Whatever method of measurement is used, a large number of particles must be meas ured in order to ascertain the particle size distribution. It has been suggested for light microscopy that 200 measurements are made on each of three separate slides(Clout, 1983); this makes the method very tedious. The simplest way to present such results is the form of a distribution curve, the two most common being in the form of either a frequency distribution(histogram) or a cumulative distribution(see Fig. 9.1). The cumu- lative distribution can be based on percentage oversize or percentage undersize. Percent age undersize is used more often. The method used for data collection may give a distribution in terms of number of particles(for example by counting)or the mass (weight)of particles(for example by sieving) If the number of particles is known, the distribution can be represented by a frequency distribution. Table 9.5 gives some typical figures for the number of particles collected by microscopical examination, arranged into numbers falling within different size ranges(0 10 um) etc, together with the frequency distribution and cumulative number distribution undersize
Solids separation processes 247 9.2.2 Particle size and particle size distribution As mentioned earlier, food powders come in a wide range of sizes and shapes. Uniform shapes, such as spheres, can be characterised by one dimension, i.e. the diameter, whereas two or more measurements may be required for more complex shapes. Whatever the shape, there are several methods available to characterise the size and particle size distribution. Virtually all operations that result in the production of a powder, e.g. milling or spray drying, will give rise to a product with a distribution of particle sizes and this distribution is of extreme importance and will affect the bulk properties. Particle size may range over several orders of magnitude, ranging from less than 1 pm to as large as hundreds or even thousands of microns for some large granules. Particle size can be measured in principle by measuring any physical property which correlates with the geometric dimensions of the sample. According to Schubert (1987a) the attributes used to characterise particles may be classified as follows: geometric characteristics, such as linear dimensions, areas or volumes; mass; settling rates; interference techniques such as electrical field interference and light or laser scattering or diffraction. Based on these attributes, the following methods have been used for food materials: microscopy or other image scanning techniques; wet and dry sieving methods; electrical impedance methods such as the Coulter counter; laser diffraction patterns, such as the Malvern, Northrup and Cilas instruments. Since particles can vary in both shape and size, different methods of particle size analysis do not always give consistent results, both because of the different physical principles being exploited, but also because size and shape are interrelated. Sampling is also important to ensure that a representative sample is taken, usually by the method of quartering. Whatever method of measurement is used, a large number of particles must be measured in order to ascertain the particle size distribution. It has been suggested for light microscopy that 200 measurements are made on each of three separate slides (Cloutt, 1983); this makes the method very tedious. The simplest way to present such results is in the form of a distribution curve, the two most common being in the form of either a frequency distribution (histogram) or a cumulative distribution (see Fig. 9.1). The cumulative distribution can be based on percentage oversize or percentage undersize. Percentage undersize is used more often. The method used for data collection may give a distribution in terms of number of particles (for example by counting) or the mass (weight) of particles (for example by sieving). If the number of particles is known, the distribution can be represented by a frequency distribution. Table 9.5 gives some typical figures for the number of particles collected by microscopical examination, arranged into numbers falling within different size ranges (0- 10 pm) etc., together with the frequency distribution and cumulative number distribution undersize