PARTICLE TECHNOLOGY Lecture ## - Bangladesh … · Angle of...

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PARTICLE TECHNOLOGY Lecture ## K. M. Tanvir Ahmmed Lecturer, ChE, BUET

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Transcript of PARTICLE TECHNOLOGY Lecture ## - Bangladesh … · Angle of...


    K. M. Tanvir Ahmmed

    Lecturer, ChE, BUET

  • Angle of Repose, r

    When bulk granular materials are poured onto a horizontal surface, a conical pile will form

    The internal angle between the surface of the pile and the horizontal surface is known as the angle of repose

  • Angle of Repose

    Lower the angle of repose, the more free flowing is the material

    Very free flowing granules: 25 < < 30

    Free-flowing granules: 30 < < 38

    Fair to passable flow of powders: 38 < < 45

    Cohesive solids: 45 < < 55

    Very cohesive solids: 55 < < 70

  • Angle of Repose

    Angle of repose is sensitive to the condition of the supporting surface - the smoother the surface the smaller the angle

    Angle of repose is also sensitive to moisture -it tends to increase the angle of repose

  • Angle of Repose

    Used in the design of equipment for the processing of particulate solids

    may be used to design an appropriate hopper or silo to store the material

    It can also be used to size a conveyor belt for transporting the material

    It can also be used in determining whether or not a slope will likely collapse

  • Angle of Repose

  • Angle of Repose

  • Angle of Internal Friction, m The angle of internal

    friction is the ability of particles to withstand a shear stress

    Defined as the equilibrium angle between flowing particles and stationary solids in a bin

    The tangent of this angle is the coefficient of friction,

  • Angle of Repose & Internal Friction

    If the mass are truly homogeneous both angles are equal

    In practice, the angle of repose is smaller than the angle of internal friction

    -Moisture content

    -Degree of Packing

  • Pressures in Masses of Particles

    The minimum pressure in a solid mass in the direction normal to that of the applied pressure

    Ratio of the normal pressure to the applied pressure is a constant K

    K depends on

    -Shape and interlocking tendencies of the particles

    -Stickiness of the grain surfaces

    -Degree of packing of the material

  • Storage of Solids

    Open storage areas are used for the high range because of their lower cost

    In low- and medium-capacity ranges partially or totally enclosed storage is used

  • Bulk Storage - Stockpiles

    Stacked on a concrete or earthen pad

    May be covered by a roof structure or left uncovered

    Formed using various overhead conveyors

  • Silo!

  • Bin Storage

    Cylindrical or rectangular vessels of concrete or metal

    Silo is tall and relatively small in diameter

    Bin is not so tall and usually fairly wide

    Hopper is small bin with a slopping bottom

  • Pressures in Circular Bin

  • Janssen's Equations for Bins

    One cannot solve Eq. ## directly since it contains two unknowns, the average vertical Pv and the average tangential stress PL. To obtain a solution, additional information is required as a correlation between the two stresses. Janssen proposed such a correlation in the form

    PL / Pv = K ----------1

  • Janssen's Equations for Bins

    where K is the so-called Janssen constant. Eq. (1) implies a constant relationship between the two average stresses, independent of the geometry of the slice and, while there is no physical reason that such a relationship should hold, it was found in practice that for routine industrial calculations the results are reasonably accurate.

  • Janssen's Equations for Bins

    At very small distances, close to the upper surface of the bin, one can expand the exponential function

    It yields the vertical stress Pb = ZT (g/gc)b. This is exactly the stress encountered in a static fluid of constant density as one moves down from its surface and is the so-called hydrostatic pressure.

  • Pressures in fluids and stresses in bulk solids (in principle)

  • Mass Flow & Tunnel Flow

  • Mass Flow

  • Core Flow/Tunnel Flow


    N. B. Most of the pictures are taken from English Wikipedia. This document is prepared for educational purposes only.

    For first section of this presentation the author is expressing his indebtedness to Ashfaq M. Ansery.