Report

Flow Behavior of Granular Materials and Powders Part III Asst. Prof. Dr. Muanmai Apintanapong Bin and Hopper Design Figure 1: a. pressure in a silo filled with a fluid (imaginary); b. vertical stress after filling the silo with a bulk solid; c. vertical stress after the discharge of some bulk solid Gravity flow through orifices Law of hydrodynamics do not apply to the flow of solid granular materials through orifices: Pressure is not distributed equally in all directions due to the development of arches and to frictional forces between the granules. The rate of flow is not proportional to the head, except at heads smaller than the container diameter. No provision is made in hydrodynamics for size and shape of particles, which greatly influence the flow rate. Particle history Hopper Flow Modes Mass Flow - all the material in the hopper is in motion, but not necessarily at the same velocity Funnel Flow - centrally moving core, dead or non-moving annular region Expanded Flow - mass flow cone with funnel flow above it Mass Flow all the material in the hopper is in motion at discharge, but not necessarily at the same velocity Material in motion along the walls D Does not imply plug flow with equal velocity Typically need 0.75 D to 1D to enforce mass flow Funnel Flow If a hopper wall is too flat and/or too rough, funnel flow will appear. (centrally moving core, dead or nonmoving annular region) “Dead” or nonflowing region or stagnant zone Expanded Flow mass flow cone with funnel flow above it Funnel Flow upper section Mass Flow bottom section Problems with Hoppers Ratholing/Piping and Funnel Flow Arching/Doming Insufficient Flow Irregular flow Inadequate Emptying Time Consolidation - Caking Ratholing/Piping • Occurs in case of funnel flow. • The reason for this is the strength (unconfined yield strength) of the bulk solid. • If the bulk solid consolidates increasingly with increasing period of storage at rest, the risk of ratholing increases. Stable Annular Region Funnel Flow -Segregation -Inadequate Emptying Coarse Fine Coarse -Structural Issues Segregation In case of centric filling, the larger particles accumulate close to the silo walls, while the smaller particles collect in the centre. In case of funnel flow, the finer particles, which are placed close to the centre, are discharged first while the coarser particles are discharged at the end. If such a silo is used, for example, as a buffer for a packing machine, this behaviour will yield to different particle size distributions in each packing. In case of a mass flow, the bulk solid will segregate at filling in the same manner, but it will become "remixed" when flowing downwards in the hopper. Therewith, at mass flow the segregation effect described above is reduced significantly. Arching/Doming • If a stable arch is formed above the outlet so that the flow of the bulk solid is stopped, then this situation is called arching. • In case of fine grained, cohesive bulk solid, the reason of arching is the strength (unconfined yield strength) of the bulk solid which is caused by the adhesion forces acting between the particles. • In case of coarse grained bulk solid, arching is caused by blocking of single particles. • Arching can be prevented by sufficiently Cohesive Arch preventing large outlets. material from exiting hopper Insufficient Flow - Outlet size too small - Material not sufficiently permeable to permit dilation in conical section -> “plop-plop” flow Material under compression in the cylinder section Material needs to dilate here Irregular flow Irregular flow occurs if arches and ratholes are formed and collapse alternately. Thereby fine grained bulk solids can become fluidized when falling downwards to the outlet opening, so that they flow out of the silo like a fluid. This behaviour is called flooding. Flooding can cause a lot of dust, a continuous discharge becomes impossible. Inadequate emptying Usually occurs in funnel flow silos where the cone angle is insufficient to allow self draining of the bulk solid. Remaining bulk solid Time Consolidation - Caking Many powders will tend to cake as a function of time, humidity, pressure, temperature Particularly a problem for funnel flow silos which are infrequently emptied completely What the chances for mass flow? Cone Angle Cumulative % of from horizontal hoppers with mass flow 45 0 60 25 70 50 75 70 *data from Ter Borg at Bayer Mass Flow (+/-) + flow is more consistent + reduces effects of radial segregation + stress field is more predictable + full bin capacity is utilized + first in/first out - wall wear is higher (esp. for abrasives) - higher stresses on walls - more height is required Funnel flow (+/-) + less height required - ratholing - a problem for segregating solids - first in/last out - time consolidation effects can be severe - silo collapse - flooding - reduction of effective storage capacity How is a hopper designed? Measure - powder cohesion/interparticle friction - wall friction - compressibility/permeability Calculate - outlet size - hopper angle for mass flow - discharge rates Types of Bins Conical Pyramidal Watch for inflowing valleys in these bins! Chisel Types of Bins Wedge/Plane Flow L B L>3B Design diagram for mass flow (wedgeshaped hopper) w (angle of wall friction) (slope of hopper wall) = effective angle of internal friction Design diagram for mass flow (conical hopper) w (angle of wall friction) (slope of hopper wall) = effective angle of internal friction Stress conditions in the hopper (emptying) c < 1 : flow c > 1 : arching is stable, no flow 1 = bearing stress, 1 = major principal stress c = unconfined yield strength Flow function and time flow function Hopper forms Summary The design of silos in order to obtain reliable flow is possible on the basis of measured material properties and calculation methods. Because badly designed silos can yield operational problems and a decrease of the product quality, the geometry of silos should be determined always on the basis of the material properties. The expenses for testing and silo design are small compared to the costs of loss of production, quality problems and retrofits. Critical dimensions of hopper openings To determine critical dimension, failure conditions must be established for two basic obstructions; arching (no flow) and piping (flow may be reduced or limited). Consider that the strongest possible arch may form, the critical opening dimension (B) becomes: B c/w (for slot opening) B 2c/w (for circular opening) Where w = bulk density T = thickness B = opening dimension 1 Flow factor (ff) depends upon: effective angle of internal friction) w (angle of wall friction) (slope of hopper wall) w funnel flow Mass flow ff Example Calculate the critical width B for arching of the slot opening of a wedge shaped, mild steel hopper with = 30C 55 For mild steel hopper with wall friction angle = 35, the maximaum effective angle of friction () = 55 ff = 1.25 ff = 1.25 No intersection of ff and this FF, there is no arching problem ff = 1.25 There is an intersection of ff and this FF, there is arching problem c = 50 1 = 65 From 1 = 65 lb/ft2, c = 50 lb/ft2, w = 90 lb/ft3 and = 55, therefore B 50/90 0.6 ft or critical slot with for arching is about 7 inches. Determination of Outlet Size B = c,i H()/W H() is a constant which is a function of hopper angle Bulk density = W H() Function H() 3 2 1 10 20 30 40 50 Cone angle from vertical 60 Example: Calculation of a Hopper Geometry for Mass Flow An organic solid powder has a bulk density of 22 lb/cu ft. Jenike shear testing has determined the following characteristics given below. The hopper to be designed is conical. Wall friction angle (against SS plate) = w = 25º Bulk density = W = 22 lb/cu ft Angle of internal friction = = 50º Flow function c = 0.3 1 + 4.3 Using the design chart for conical hoppers, at w = 25º c = 17º with 3º safety factor & ff = 1.27 Example: Calculation of a Hopper Geometry for Mass Flow ff = /a or a = (1/ff) Condition for no arching => (1/ff) = 0.3 1 + 4.3 a > c (1/1.27) = 0.3 1 + 4.3 1 = 8.82 c = 8.82/1.27 = 6.95 B = 2.2 x 6.95/22 = 0.69 ft = 8.33 in Discharge Rates (Q) Numerous methods to predict discharge rates from silos or hopper For coarse particles (>500 microns) Beverloo equation - funnel flow Johanson equation - mass flow For fine particles - one must consider influence of air upon discharge rate Beverloo equation Q = 0.58 b g0.5 (B - kdp)2.5 where Q is the discharge rate (kg/sec) b is the bulk density (kg/m3) g is the gravitational constant B is the outlet size (m) k is a constant (typically 1.4) dp is the particle size (m) Note: Units must be SI Johanson Equation Equation is derived from fundamental principles - not empirical Q = b (/4) B2 (gB/4 tan c)0.5 where c is the angle of hopper from vertical This equation applies to circular outlets Units can be any dimensionally consistent set Note that both Beverloo and Johanson show that Q B2.5! Silo Discharging Devices Slide valve/Slide gate Rotary valve Vibrating Bin Bottoms Vibrating Grates others Rotary Valves Quite commonly used to discharge materials from bins. Screw Feeders Dead Region Better Solution Discharge Aids Air cannons Pneumatic Hammers Vibrators These devices should not be used in place of a properly designed hopper! They can be used to break up the effects of time consolidation. Flow rate equations From Ewalt and Buelow (1963), measuring flow of shell corn from straight-sided wooden bins equipped with test orifices: Horizontal openings, circular orifice (8.4% MC db) Horizontal openings, rectangular orifice (12.1% MC db) Q = 0.153 W1.62 L1.4 Vertical openings, circular orifice (12.7% MC db) Q = 0.1196 B3.1 Q = 0.0351 B3.3 Vertical openings, rectangular orifice (12.4% MC db) Q = 0.0573 W1.75 L1.5 Q = KWn K and n are two constants which can be found either by substituting experimental data from two sets of tests and solving the two equations simultaneously or by determination them directly from the slope and yintercepts of the straight line plot of Q versus one of the dimensions on log-log graph paper. Q = f(i, r, d/D, D, bulk density and etc.) There is no single parameter satisfactory relationship for estimating Q. Most important parameter is the opening diameter (greatly affect on flow rate) Q D3 Log Q Slope ~ 2.8-3.2 Log D