Fluidizacion y transporte hidraulico de trozos en zanahorias

Solo disponible en BuenasTareas
  • Páginas : 23 (5672 palabras )
  • Descarga(s) : 0
  • Publicado : 17 de diciembre de 2011
Leer documento completo
Vista previa del texto
Journal of Food Engineering 6 (1987) 377-399

Fluidisation and Hydraulic Transport of Carrot Pieces
G. McKay, W. R. Murphy and S. Jodieri-Dabbaghzadeh
Department of Chemical Engineering, The Queen’s University of Belfast, Belfast, Bl9 lNN, N. Ireland

(Received 2 1 November 1985; revised version received 13 June 1986; accepted 3 November 1986)

ABSTRACT
The behaviour of discs andcylindersof carrot in a water-fluidised bed was studied. Pressure drop measurements and terminal (or transportation) velocityrelationshipswere establishedusing cylinders of carrot, PVC and Nylon 6.6. This enabled the effect of pa&e density to be studied. Comparisons were made with the results of previous work by McKay and McLain (1980) on spheres and cuboids. At minimum fluidisation, large cylindersdidnot behave as spherical or near-spherical solids and the equation derived by Ergun (1952) for spheres, relating friction factor to Reynolcisnumber and voidage, did not apply. However, a similar relationshiphas been established, viz:

log f, = 1.439 + 053 log--(lR2:O)
which is valid for cylinders having spheric@ factors in the range 0.73-087. The relationships derived by Wade11(1934) for spheresand discs, having sphericitiesfrom O-125to lY&Iand largest dimension 50J mm, were unsatisfactory predicting terminaljluidisation velocitiesof carrot for or plastic cylinders. The resistance coflcient was related to the spheric@ factor by: log c, = 0.77 + 4.75 log #s As the LID ratio increased, i.e. the spheric@ decreased, the drag on the particles was reduced. This is the converse of theirsettlingbehaviour and shows that a resistance coeficient measured in settling conditions cannot be used to predict the j’uidising behaviour of cylinders having large L/D ratios.
377 Journal of Food Engineering 0260-8774/87/$03.50 - 0 Elsevier Publishers Ltd, England, 1987. Printed in Great Britain Applied Science

378

G. McKay, W. R. Murphy, S. Jodieri-Dabbaghzadeh Relationships between the slopesof the plots of log U vs log E and volumetn’c shape factor, K, were investigated. The slope, n, of these plots was related to K by: log n = 0.28 - 0.226 log K

NOMENCLATURE projected area of particle in its most stable orientation (mm*) particle drag coefficient particle resistance coefficient particle diameter or characteristic linear dimension (mm) diameter of sphere having the samevolume-to-surface ratio as the actual particle (mm) true nominal diameter of non-spherical particle (mm) mean effective diameter of a sample containing a range of particle sizes (mm) pipe or column diameter (mm) friction factor gravitational constant (m s - *) volumetric shape factor length of particle slope of the plot of log Uagainst log E pressure drop across bed of particles (N mP3) pressure dropacross bed of particles at minimum fluidisation (N m-‘) particle Reynolds number, p Ud,/p particle Reynolds number, p U,d,/p particle Reynolds number, p U,d,/,u superficial flow velocity (mm s - ’ ) free falling velocity of settling particle (heavier) (mm s - ’ ) in section, Effect of Column Wall free falling velocity of fluidisation particle (lighter) (mm s - ‘) in section, Effect of Column Walisuperficial flow velocity at minimum fluidisation (mm s - I ) terminal flow velocity obtained from the intercept on the U axis at E= 1 of a plot of log U against log .5( mm s- l) height of fluidised bed (mm) height of fluidised bed at minimum fltidisation (mm) fluid density (kg m- “) particle density (kg m- “) fluid viscosity (Ns m- :) bed voidage fraction

Fluidkation and transport of carrot pieces379

bed voidage at minimum fluidisation conditions shape factor, volume/surface area ratio sphericity factor mean sphericity factor dimensionless density ratio, (,q, - p)/p

INTRODUCTION Fluidisation of small solids (d, < 1.0 mm) is now a well established technique and the theory is well documented. However, many industries handling larger solids (4 > 5-O mm) and needing to use pipeline...
tracking img