Dr. Hairul Nazirah Abdul Halim School of Bioprocess Eng.

Transcription

1 Dr. Hairul Nazirah Abdul Halim School of Bioprocess Eng.

2 Introduction to particulate solids Characterization of solid particles Particle sizes Screen analysis Tyler standard screen analysis

3 Large quantities of particles handled on the industrial scale Silica gel Amberlite Thus, it is necessary to know: - the distribution of particle sizes in the mixture - the mean size of particles It is frequently necessary to reduce the size of particles or form them into aggregates or sinters to ease the handling process

4 Individual solid particles are characterized by: (a) Shape regular e.g. spherical or cubical irregular e.g. a piece of broken glass (b) Size influence the properties such as : the surface per unit volume the settling rate of particle in fluid (c) Composition - determine properties such as density and conductivity

5 The shape of an individual particle is expressed in terms of sphericity, which is independent of particle size - For spherical particle of diameter, D p ; = 1 - For nonspherical particle; (1) D p = nominal diameter of particle S p = surface area of a particle ʋ p = volume of a particle

6 - For most of crush materials; 0.6 < < For particles rounded by abrasion; > For a cube and cylinder; = 1 Source ; Unit Operation of Chemical Engineering (McCabe,Smith & Harriot) ( Table 7.1 page 164)

7 Mixed particle sizes In a sample of uniform particles of diameter D p, the number of particles in a sample, N is given by: m = total mass of the sample (2) (2) From Eq. (1) and (2), the total surface area of particle sample, A can be computed by: (3) These equation applicable to a mixture of particles having various sizes and densities, the mixture is sorted into fractions, each of constant density and approximately constant size.

8 Specific surface of mixture The specific surface, A w (the total surface area of a unit mass of particles) if and are constant is given by; (4)

9 Average particle sizes Volume-surface mean diameter ( ) The volume-surface mean diameter, the A w is given by: is related to (5) By substituting Eq. (4) in Eq. (5); (6)

10 Effective mean diameter, D pm : Arithmetic mean diameter ( ) where is the number of particles in the entire sample Mass mean diameter ( )

11 Volume mean diameter ( ) Average volume of a particle = Divide the total volume of the sample with the number of particles in the mixture The diameter of such a particle is the volume mean diameter, For samples consisting of uniform particles, these average diameters are all the same. For mixtures containing particles of various sizes, however the several average diameters may differ widely from one another.

12 Number of particles in mixture For a given particle shape, the volume of any particle is: where is the volume shape factor. It is different for various regular solids which are; (1) for a sphere (2) for a short cylinder (height = diameter) (3) 1.0 for a cube Assuming is independent of size:

13 A method of separating a mixture or grains or particles into 2 or more size fractions. The over sized materials are trapped above the screen, while undersized materials can pass through the screen. Sieves can be used in stacks, to divide samples up into various size fractions and hence determine particle size distributions. Sieves and screen are usually used for larger particle sized materials i.e., greater than approximately 50µm (0.050mm).

14 SIEVE TRAYS Pan (the finest particles retained here) Sieve (different mesh size) Sieve trays were put on a stack (shaker)

15 Used to measure the size of particles in the size range between in.(76 mm and 38 µm) A set of standard screens is arranged serially in a stack, with the smallest mesh at the bottom and the largest at the top The sample is placed on the top screen and the stack shaken mechanically for 20 min The particles retained on each screen are removed and weighed, and the masses of the individual screen increments are converted to mass fractions or mass percentages of the total sample Any particles that pass the finest screen are caught in a pan at the bottom of the stack

16 Tyler Standard Screen based on mesh opening sizes (200 mesh-screen). The mesh number system is a measure of how many openings there are per linear inch in a screen. Sizes vary by a factor of 2. Refer Appendix 5 (McCabe) for the Tyler standard screen series.

17 Table 28.1 Screen analysis Mesh Screen opening, D pi, mm Mass fraction retained, x i Average particle diameter in increment, D pi, mm Cumulative fraction smaller than D pi Pan

18 Column 1 mesh size Column 2 width of opening of the screens Column 3 the mass fraction of the total sample that is retained on the designated screen x i is the number of the screen starting at the bottom of the stack; thus; i = 1 for the pan, and screen i + 1 is the screen immediately above screen i Column 4 average particle diameter D pi in each increment (particle diameter equal to the mesh opening of screen i) Column 5 cumulative fraction smaller than each value of D pi In screen analysis, cumulative fractions are sometimes written starting at the top of the stack and express as the fraction larger than a given size

19 The screen analysis shown in Table 28.1 applies to a sample of crushed quartz. The density of the particles is 2,650 kg/m 3 ( g/mm 3 ), and the shape factors are a = 0.8 and Ф s = For the material between 4-mesh and 200-mesh in particle size, calculate: (a) (b) (c) (d) (e) (f) (g) Individual average particle diameter in increment total surface area of a unit mass of particles (in square millimeters per gram) and number of particles per gram Volume mean diameter Volume-surface mean diameter Mass mean diameter Number of particles for the 150/200-mesh increment The fraction of the total number of particles in the 150/200-mesh increment

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21 Mesh D pi x i Avg D pi x i / Avg. D pi x i / (Avg. D pi )^ Sum: