US 4,424,176 A · Process for granulation of molten materials

Abstract

The granulation of molten material using a heat transfer process whereby solid particles are heated, dried, or cooled by a gas: Particles are repeatedly lifted and dropped as, for example, in flighted rotary drums. By forced recirculation, gas is passed through showering particles generally perpendicular to their plane of fall. Examples show gas forced to contact particles in this manner imparts an overall volumetric heat transfer coefficient to a rotary drum superior to that expected from classical literature or presently operating rotary drum heat exchangers. Disclosure teaches gas temperature adjustment between forced recirculations. Technique usage will reduce exchanger capital cost, increase energy efficiency, and reduce pollution problems. Also, prewetting of the particles in certain instances can be utilized to effect substantially increased pseudo volumetric heat transfer coefficients when cooling non-hygroscopic materials.

In an improved process for the granulation of materials wherein same is effected in an enclosed vessel by spraying particles of molten material on rapidly moving granules of substrate such that the granules of substrate are increased in size by accretion as the sprayed particles solidify quickly on the surface of said substrate, and wherein at least a substantial amount of the heat of crystallization and associated heat from cooling the molten material and subsequently solid material is removed by evaporation of water in said enclosed vessel into air being moved through said enclosed vessel which comprises:

(a) atomizing the water to be evaporated in said enclosed vessel into finely divided droplets having a median volume diameter of less than 1.967.times.10.sup.-4 ft;
(b) spraying said water droplets with a spray angle, the arc of which is at least 30 degrees;
(c) spraying said water droplets in such a manner that the liquid water comprising same totally evaporates while in said air in a period of time of less than 2 seconds and from a predetermined juxtaposed position such that no water in the liquid phase contacts either the sprayed particles of molten material or of the substrate granules which are increasing in size by accretion from the solidification of said sprayed particles of molten material, or both;
(d) controlling the moisture content of said air below the dew point of said air everywhere in said enclosed vessel, below the concentration in said air where condensation would take place on any surface inside said enclosed vessel, and below the concentration in said air where granules of substrate of any size or at any point in said enclosed vessel would absorb water from said air, thus preventing water absorption by said granules of substrate and condensation of water in the air onto the inner surfaces of said enclosed vessel;
and
(e) removing said heat of crystallization and associated heat from cooling the molten material and subsequently solid material by evaporation of said finely divided droplets of water to give an overall heat transfer rate which is represented in Btu/hr by either the numerator or denominator of the following equation which represents the thermodynamics of the instant process, i.e. ##EQU1## where .pi. is the ratio of the circumference of a circle to its diameter and is 3.1416 and dimensionless;
r is the average radius in ft. of an evaporating droplet over its life span and is a maximum of 6.56.times.10.sup.-5 ft;
n is the number of water droplets present in atomized form in the vessel at a given instant and as such is dimensionless;
t.sub.a is the average dry bulb temperature of the air in .degree.F.;
t.sub.w is the average wet bulb temperature of the air in .degree.F.;
t.sub.g is the average temperature of granules in contact with the air in .degree.F.;
k.sub.f is the thermal conductivity of the air in Btu-ft/(hr)(ft.sup.2)(.degree.F.);
k.sub.c is the forced thermal convection of air in contact with water droplets in Btu-ft/(hr)(ft.sup.2) (.degree.F.);
W.sub.a is the mass flow rate of air through the vessel less water evaporated in the vessel in lb/hr;
.DELTA.H.sub.a is the difference in enthalpy of the air entering and leaving the vessel less water evaporated in the vessel in Btu/lb;
W.sub.v is the mass flow rate of water sprayed and evaporated in the vessel in lb/hr;
.DELTA.H.sub.v is the average change in enthalpy of the water evaporated in the vessel after it is in the vapor phase in Btu/lb;
h.sub.g is the average heat transfer coefficient between granules and air in Btu/(ft.sup.2)(hr)(.degree.F.);
N.sub.g is the number of granules exposed to the air at any one instant and is dimensionless;
A.sub.g is the average surface area of each one of those granules exposed to the air in square feet;
and q.sub.i is the rate of heat flow from granules to the air by indirect means in Btu/hr, and where the left hand term of the numerator fully describes the rate of heat transfer by evaporation of water in Btu/hr, and the middle and right hand terms of the numerator fully describe the heat absorbed by the air and the water vapor, respectively, in Btu/hr, and the denominator fully describes the rate of heat transfer from granules to air in Btu/hr;
the improvement in combination therewith for effecting improved heat transfer between the said granulated material in said enclosed vessel and the air introduced therein, which comprises the additional steps of:
(1) alternately causing at least a portion of said movement of granules of substrate in said enclosed vessel to be vertically displaced from and returned from a first zone therein, the angle of the fall line of said granules of substrate in the descending attitude of said vertical displacement for return to said first zone ranging from about 90 degrees to the horizontal to about 5 degrees greater than the angle of repose of said granules of substrate;
(2) causing at least a portion of said air in said enclosed vessel to be moved in a generally horizontal attitude in a manner so as to intersect said granules of substrate in said descending attitude of said vertical displacement for return of same to said first zone, said horizontal movement of said air being sufficient to at least partially penetrate at least a portion of that segment of said enclosed vessel occupied by said returning granules of substrate;
(3) causing at least a portion of said air in said enclosed vessel having intersected and at least partially penetrated said segment of said granules of said substrate therein to be displaced to a second zone in said enclosed vessel, said second zone being at least horizontally displaced away from said granules of substrate;
(4) adjusting the enthalpy of the air displaced into said second zone to predetermined levels to maintain a predetermined temperature differential between said granules of substrate in said enclosed vessel and said air displaced into said second zone;
and
(5) removing at least a portion of air displaced into said second zone after the enthalpy thereof has been adjusted and returning same into contact with at least a portion of that segment of said enclosed vessel occupied by said returning granular substrate to effect heat transfer therewith by means of at least partial penetration thereof in step 2 above in a manner such that the contact angle between said air and the plane of the fall line of said granular substrate ranges from between about -45 degrees to about +45 degrees from the direction perpendicular to the plane of said fall line;
said process characterized by the fact that the thermodynamic characteristics thereof as they relate to the heat transfer between said aid and said granular substrate are represented as follows:
q.sub.t =Ua V(.DELTA.t).sub.m
where
q.sub.t =total heat transferred in the enclosed vessel, Btu/hr
Ua=volumetric heat transfer coefficient Btu/(hr)(ft.sup.3 enclosed vessel)(.degree.F.)
V=volume of the enclosed vessel, ft.sup.3
(.DELTA.t).sub.m =true mean temperature difference between the air and the granular substrate, .degree.F.

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Process for granulation of molten materials

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Process for granulation of molten materials

Life of this patent

Abstract

Claims17