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
248 M.J. Lewis Fig 9.1.(a)Frequency distribution(F), (b)cumulative distribution(C): see also data in Table 9.5 Table 9. 5. Frequency distribution Size range Mean diameter Number Frequency Cumulative Cumulative (um) in range distributiondistribution volume 0to10 18 1.8 10to20 20to30 129 20.3 2.0 30to4035 18.4 38.7 7.0 40to5045 20.3 0.0 6055 18.5 41.0 640 70to8075 74 96.7 850 80to9085 98.0 90to10095 0.4 100.0 0 mean diameter =45.96 um; d2/ 1=53. 29 um; d3/2=58.86 Am Cumulative volume represents the percentage of the total volume less than the mean diameter of the range Other values which may be calculated from the distribution include the mean diameter nd the median diameter and the standard deviation, which gives an indication of the The simplest is the mean diameter, defined ∑n4/∑
248 M. J. Lewis 100 80 60 10 40 20 0 LL *OB 0 0 20 Size 40 (prn) 60 80 100 0 Fig. 9.1. (a) Frequency distribution (F), (b) cumulative distribution (0: see also data in Table 9.5. Table 9.5. Frequency distribution Size range Mean diameter Number Frequency Cumulative Cumulative OLm) of range (pm) in range distribution distribution volume distribution 0 to 10 5 5 1.8 1.8 0 10 to 20 15 15 5.6 7.4 0 20to30 25 35 12.9 20.3 2.0 30to40 35 50 18.4 38.7 7.0 40to50 45 55 20.3 59.0 20.0 50to60 55 50 18.5 77.5 41 .O 60to70 65 32 11.8 89.3 64.0 70to 80 75 20 7.4 96.7 85.0 80to90 85 8 2.9 99.6 98.0 90to 100 95 1 0.4 100.0 100.0 >loo 0 27 1 - mean diameter = 45.96 pm; d2/1 = 53.29 pm; d3/2 = 58.86 pm Cumulative number frequency indicates the percentage of the total number less than the mean diameter of the range. Cumulative volume represents the percentage of the total volume less than the mean diameter of the range. Other values which may be calculated from the distribution include the mean diameter and the median diameter and the standard deviation, which gives an indication of the spread. The simplest is the mean diameter, defined as C nidi /C ni 7
Solids separation processes 249 where ni is the number of particles in class i and d; is the mean diameter of class i. The median diameter is the diameter which cuts the cumulative distribution in half. The d2/1 ratios and d3/2 ratios are also calculated However, one widely used characteristic is the Sauter mean particle diameter(d3/2) This is calculated from This gives the diameter of the particle having the same surface area to volume ratio as the entire dispersion The surface area/volume ratio =6/d3/2 Rates of heat transfer and mass transfer are proportional to the surface area to volume ratio. Therefore the surface area exposed has a big influence on physical properties, e ettability, dispersion, dissolution and chemical reactions, such as oxidation, as we the forces acting at the surface of powders. Equation(9. 2) demonstrates that decreasing 3/2 will increase the surface area to volume ratio Such data can be converted to frequency or cumulative distribution based on surface area or volume, by calculating the surface area and volume of each range. These cumula tive distributions based on numbers and volume are compared in Fig. 9. 2. This distinction is made because the shape of a numbers distribution and a mass or volume distribution is quite different because the area and volume distributions are most influenced by the larger diameter particles, since the volume ==r. For example, it can be seen that only 10.7% of the particles are greater than 65 um, whereas on a volume basis, 36% by olume are greater than 65 um(Fig. 9.2). The weight fraction distribution would be similar to the volume fraction distribution, provided that the solid density is independent of particle size. The volume distribution is a common form of presentation in emulsion science, since it is often the larger particles which are likely to cause separation problems Therefore it can be very informative to know what fractions by volume are bigger than a particular size. For example, in cream separation in milk, problems may arise from a relatively small number of large fat globules g. 9.2. Comparison of volume distribution(V and cumulative number distribution(M). St also data in table 9.5
Solids separation processes 249 where ni is the number of particles in class i and di is the mean diameter of class i. The median diameter is the diameter which cuts the cumulative distribution in half. The d2/1 ratios and d3/2 ratios are also calculated. However, one widely used characteristic is the Sauter mean particle diameter (d& This is calculated from d3p = xnid:/xnid,? (9.1) This gives the diameter of the particle having the same surface area to volume ratio as the entire dispersion. The surface area/volume ratio = 6/d3/2. (9.2) Rates of heat transfer and mass transfer are proportional to the surface area to volume ratio. Therefore the surface area exposed has a big influence on physical properties, e.g. wettability, dispersion, dissolution and chemical reactions, such as oxidation, as well as the forces acting at the surface of powders. Equation (9.2) demonstrates that decreasing d312 will increase the surface area to volume ratio. Such data can be converted to frequency or cumulative distribution based on surface area or volume, by calculating the surface area and volume of each range. These cumulative distributions based on numbers and volume are compared in Fig. 9.2. This distinction is made because the shape of a numbers distribution and a mass or volume distribution is quite different because the area and volume distributions are most influenced by the larger diameter particles, since the volume = 4 m3. For example, it can be seen that only 10.7% of the particles are greater than 65 pm, whereas on a volume basis, 36% by volume are greater than 65 pm (Fig. 9.2). The weight fraction distribution would be similar to the volume fraction distribution, provided that the solid density is independent of particle size. The volume distribution is a common form of presentation in emulsion science, since it is often the larger particles which are likely to cause separation problems. Therefore it can be very informative to know what fractions by volume are bigger than a particular size. For example, in cream separation in milk, problems may arise from a relatively small number of large fat globules. 100 40 0 liL 20 0 20 40 60 80 100 Size (pm) Fig. 9.2. Comparison of volume distribution (V) and cumulative number distribution (N). See also data in Table 9.5
250 M.J. Lewis Most of the discussion has focused upon spherical particles or those closely approxi mating to these. However, the particle shape is also very likely to be important and a ide variety of shapes are also found Irregular-shaped objects are more complicated define and a number of characteristic dimensions have been used to represent them. Some are given in Table 9.6 Table 9.6. Characteristic diameters for irregular shaped particles Surface diameter The diameter of a sphere having the same surface area Volume diameter he diameter of a sphere having the same volume as dd Drag The diameter of the particle having the same resistance to motion as the particle in a fluid of the same density and viscosity Sieve diameter The width of the minimum square aperture through which the particle will pass Other dimensions include the free-falling diameter and Stokes diameter, the projected area diameter and the specific surface diameter. In many cases the shape is more complex and a large number of dimensions would be required to describe the size and shape Image analysis methods, whereby an image of the object is transferred to a computer screen and software is available to do any number of manipulations and calculations on the shape, are useful for this The particle size and distribution has a pronounced effect on interparticle adhesion, which will affect some of the bulk properties, such as bulk density, porosity, flowability and wettability(see Section 9.2.5) 9.2.3 Particle density The density of an individual particle is important as it will determine whether the compo- nent will float or sink in water or any other solvent; the particle may or may not contain air. It can be measured using a specific gravity bottle, using a fluid in which it will not issolve, Alternatively, it may be measured by flotation principles. However, surface forces may start to predominate for fine powders In the absence of air, the particle density can be estimated from the following equation, based on the mass fractions and densities of the food ce M1/p1+M2/p2+…+M/Pn where Mi is the mass fraction of component 1, P1 is the density of component 1 and n is the number of components. Data on mass fractions can be found from the Composition of
250 M. J. Lewis Most of the discussion has focused upon spherical particles or those closely approximating to these. However, the particle shape is also very likely to be important and a wide variety of shapes are also found. Irregular-shaped objects are more complicated to define and a number of characteristic dimensions have been used to represent them. Some are given in Table 9.6. Table 9.6. Characteristic diameters for irregular shaped particles 4 Surface diameter dv Volume diameter dd Drag diameter The diameter of a sphere having the same surface area as the particle The diameter of a sphere having the same volume as the particle The diameter of the particle having the same resistance to motion as the particle in a fluid of the same density and viscosity The width of the minimum square aperture through which the particle will pass. 4 Sieve diameter Other dimensions include the free-falling diameter and Stokes diameter, the projected area diameter and the specific surface diameter. In many cases the shape is more complex and a large number of dimensions would be required to describe the size and shape. Image analysis methods, whereby an image of the object is transferred to a computer screen and software is available to do any number of manipulations and calculations on the shape, are useful for this. The particle size and distribution has a pronounced effect on interparticle adhesion, which will affect some of the bulk properties, such as bulk density, porosity, flowability and wettability (see Section 9.2.5). 9.2.3 Particle density The density of an individual particle is important as it will determine whether the component will float or sink in water or any other solvent; the particle may or may not contain air. It can be measured using a specific gravity bottle, using a fluid in which it will not dissolve. Alternatively, it may be measured by flotation principles. However, surface forces may start to predominate for fine powders. In the absence of air, the particle density can be estimated from the following equation, based on the mass fractions and densities of the food components. P = 1/ [ (M1 lP1+ M2 / P2 + * * * + Mtl lPn ,I (9.3) where Ml is the mass fraction of component 1, p1 is the density of component 1 and n is the number of components. Data on mass fractions can be found from the Composition of
Solids separation processes Foods Tables(Paul and Southgate, 1978). A simple two-component model can be used (n= 2; water and solids) or a multicomponent system. The density of the major ce nts are given as(kg m -)(Peleg, 1983) 1000 salt 2160 900950 citric acid 154 protein 1400 cellulose 1270-1610 sucrose 1500 glucose It is noteworthy that all solid components except fat are substantially more dense than water. However the differences between protein and the various types of carbohydrates are less marked, although minerals are much higher. In comparison air has a density of 1.27 kg m This equation is not applicable where there is a substantial volume fraction of air in the particle. Any deviation between the experimentally determined value and the value calculated from the above equation may mean that there is substantial air within the solid. An estimate of the volume fraction of air( va) can be made from p=VaPa +vsPs= vaPa+(l-va)Ps where Pa= density of air; Ps =density of solid(estimated using eq (9.2))and p=true lid density, measured experimentally. This volume fraction (Va) of air is sometimes known as the intenal porosity Many other foods contain substantial amounts of air, for example mechanically worked doughs. One solution to determine the unaerated density is to measure the dough density at different pressures and extrapolate back to zero pressure(absolute) to obtain the unaerated density. This methodology could then be used to determine the extent of aeration during the mixing process Note that from the compositional data, the calculated particle density of an apple is about 1064 kg m, Most apples float in water, indicating a density less than 1000 kg m-3 Mohsenin(1986)quotes a value of 846 kg m-3, suggesting an air content of about 20%. One important objective of blanching is to remove as much air as possible from fruit and vegetables prior to heat-treatment in sealed containers, to prevent exces- sive pressure development during their thermal processing. Data on the amount of air in fruits and vegetables are scarce in the food literature. There is evidence that this air is quickly displaced by water during soaking Data on particle densities are provided by Lewis(1990), Mohsenin(1986), and Hayes (916 kg m3 at 0 C). However, not all the water is likely to be frozen, even at-300 ted (1987). Note that if the food is frozen, the density of ice should be substi The particle density of dehydrated powders is considerably affected by the conditions of spray drying. Increasing the solids content of the feed to the drier will result in higher particle densities and bulk densities. High particle densities will enhance sinkability and ing and separation techniques, e.g. flotation, sedimentation and air classification ]l clean reconstitution properties. Differences in particle densities are exploited for seve
Solids separation processes 25 1 Foods Tables (Paul and Southgate, 1978). A simple two-component model can be used (n = 2; water and solids) or a multicomponent system. The density of the major components are given as (kg m-3) (Peleg, 1983): water 1000 salt 2160 fat 900-950 citric acid 1540 protein 1400 cellulose 1270-1 6 10 sucrose 1590 starch 1500 glucose 1560 It is noteworthy that all solid components except fat are substantially more dense than water. However the differences between protein and the various types of carbohydrates are less marked, although minerals are much higher. In comparison air has a density of 1.27 kg m-3. This equation is not applicable where there is a substantial volume fraction of air in the particle. Any deviation between the experimentally determined value and the value calculated from the above equation may mean that there is substantial air within the solid. An estimate of the volume fraction of air (V,) can be made from P= &pa + V,P~ =VaPa +(l-va)Ps (9.4) where pa = density of air; ps = density of solid (estimated using eq. (9.2)) and p = true solid density, measured experimentally. This volume fraction (V,) of air is sometimes known as the internal porosity. Many other foods contain substantial amounts of air, for example mechanically worked doughs. One solution to determine the unaerated density is to measure the dough density at different pressures and extrapolate back to zero pressure (absolute) to obtain the unaerated density. This methodology could then be used to determine the extent of aeration during the mixing process. Note that from the compositional data, the calculated particle density of an apple is about 1064 kg m-3. Most apples float in water, indicating a density less than 1000 kg m-3, Mohsenin (1986) quotes a value of 846 kg m-3, suggesting an air content of about 20%. One important objective of blanching is to remove as much air as possible from fruit and vegetables prior to heat-treatment in sealed containers, to prevent excessive pressure development during their thermal processing. Data on the amount of air in fruits and vegetables are scarce in the food literature. There is evidence that this air is quickly displaced by water during soaking. Data on particle densities are provided by Lewis (1990), Mohsenin (1986), and Hayes (1987). Note that if the food is frozen, the density of ice should be substituted (916 kg m-3 at 0°C). However, not all the water is likely to be frozen, even at -30°C. The particle density of dehydrated powders is considerably affected by the conditions of spray drying. Increasing the solids content of the feed to the drier will result in higher particle densities and bulk densities. High particle densities will enhance sinkability and reconstitution properties. Differences in particle densities are exploited for several cleaning and separation techniques, e.g. flotation, sedimentation and air classification
252 M.J. Lewis 9. 2. 4 Forces of adhesion There will be interactions between particles, known as forces of adhesion and also be- tween particles and the walls of containing vessels. These forces of attraction will influ ence how the material packs and how it will flow. Some of the mechanisms for adhesive forces have been described as liquid bridging by surface moisture or melted fat: electrostatic charges molecular forces, such as van der Waals and electrostatic forces crystalline surface energy. Schubert(1987a) describes some of the models that have been used to quantify these forces, and the limitations of such models There is some indication that interparticle adhesion increases with time, as the material onsolidates. Flowability may be time-dependent and decrease with time 9. 2.5 Bulk properties Although the discussion so far has focused on individual particles, the behaviour of the collective mass of particles or bulk is very important in most operations. The bulk properties of fine powders are dependent upon geometry, size, surface characteristics, chemical composition, moisture content and processing history. Therefore it is difficult to put precise values on them and any cited values should be regarded as applying only to that specific circumstance, Peleg(1983) The term cohesive is used to describe the behaviour of powders, as they are influenced by forces of attraction(or repulsion) between particles. For powders that are cohesive, the ratio of the interparticle forces to the particles'own weight is large. This ratio is alse inversely proportional to the square of the particle size, which explains why small articles adhere to each other more strongly than large particles. Schubert(1987a)states that the majority of food particles are non-cohesive(and thus free flowing)only when the particle size exceeds 100 um. Increase in moisture content makes powders more cohesive and increases the size at which the transition from cohesive to non-cohesive takes place Some of the bulk properties will be considered in more detail 9.2.6 Bulk density and porosity The bulk density is an important property, especially for storage and transportation, rather than separation processes. It is defined as the mass divided by the total volume occupi by the material. This total volume includes air trapped between the particles. The volume fraction trapped between the particles is known as the porosity (e), where where ps and pb are measured solid and bulk densities. Methods for determining bulk density are described by the Society of Dairy Technology(1980)and Niro(1978). Terms used depend upon the method of determination and include loose bulk density and com- Some bulk densities of powders are given in Table 9.7. Further values are given by Peleg(1983), Hayes(1987)and Schubert(1987a). Peleg(1983)argues that the relatively
252 M. J. Lewis 9.2.4 Forces of adhesion There will be interactions between particles, known as forces of adhesion and also between particles and the walls of containing vessels. These forces of attraction will influence how the material packs and how it will flow. Some of the mechanisms for adhesive forces have been described as liquid bridging by surface moisture or melted fat; electrostatic charges; molecular forces, such as Van der Waals and electrostatic forces; crystalline surface energy. Schubert (1987a) describes some of the models that have been used to quantify these forces, and the limitations of such models. There is some indication that interparticle adhesion increases with time, as the material consolidates. Flowability may be time-dependent and decrease with time. 9.2.5 Bulk properties Although the discussion so far has focused on individual particles, the behaviour of the collective mass of particles or bulk is very important in most operations. The bulk properties of fine powders are dependent upon geometry, size, surface characteristics, chemical composition, moisture content and processing history. Therefore it is difficult to put precise values on them and any cited values should be regarded as applying only to that specific circumstance, Peleg (1983). The term cohesive is used to describe the behaviour of powders, as they are influenced by forces of attraction (or repulsion) between particles. For powders that are cohesive, the ratio of the interparticle forces to the particles’ own weight is large. This ratio is also inversely proportional to the square of the particle size, which explains why small particles adhere to each other more strongly than large particles. Schubert (1987a) states that the majority of food particles are non-cohesive (and thus free flowing) only when the particle size exceeds 100 pm. Increase in moisture content makes powders more cohesive and increases the size at which the transition from cohesive to non-cohesive takes place. Some of the bulk properties will be considered in more detail. 9.2.6 Bulk density and porosity The bulk density is an important property, especially for storage and transportation, rather than separation processes. It is defined as the mass divided by the total volume occupied by the material. This total volume includes air trapped between the particles. The volume fraction trapped between the particles is known as the porosity (E), where E = PS - pb/ps (9.5) where ps and pb are measured solid and bulk densities. Methods for determining bulk density are described by the Society of Dairy Technology (1980) and Niro (1978). Terms used depend upon the method of determination and include loose bulk density and compacted and compressed bulk densities. Some bulk densities of powders are given in Table 9.7. Further values are given by Peleg (19831, Hayes (1987) and Schubert (1987a). Peleg (1983) argues that the relatively