Drying and gas or vapor contact with solids – Apparatus – With means to treat gas or vapor
Patent
1983-04-11
1985-08-20
Schwartz, Larry I.
Drying and gas or vapor contact with solids
Apparatus
With means to treat gas or vapor
34 34, 34134, 34135, 34138, F26B 304
Patent
active
045355509
DESCRIPTION:
BRIEF SUMMARY
This invention relates to the processing of particulate materials, and to apparatus for this purpose.
The invention has particular application to the drying, heating and cooling of particulate material and the following description is directed to that application.
The drying as well as the heating and cooling of solids are important and very essential features of many commercial processes. Drying of solids can be defined as a process of simultaneous heat and mass transfer, in which the heat essential for the vaporization of the liquid in the solid phase is either obtained by convection or conduction from the drying medium. Cooling and heating are essentially heat transfer processes, although a mass transfer might also occur at the same time. Convection and conduction frequently are the dominant modes of heat transfer in industrial drying, heating and cooling processes in which particulate solids are treated by means of a hot or cold gas stream, as required. This means, therefore, that the better the particulate-gas contact, the better is the heat transfer between the gas and the solid and the greater is the mass and/or heat transfer between the solid and the gas. Thus it is not surprising that fluidized systems, in which the solids are suspended in the gas, give better heat and mass transfers than any other currently known mode of drying or cooling.
Depending upon the mode of heating or cooling a distinction can be drawn between direct and indirect apparatus and processes and, depending upon the gas-solid flow, it is normal to differentiate between concurrent and countercurrent apparatus and processes.
Rotary driers, heaters and coolers, which consist of a rotating cylindrical shell slightly inclined from the horizontal plane, are frequently used in commerce for the large scale drying, heating or cooling of such materials as mineral ores, fertilizers and chemicals. In order to give a greater gas-solid contact in one type of rotary apparatus, the shell can be equipped with flights, which lift the solid particles and allow them to fall through a stream of the gas. Such apparatus, which is normally referred to as a cascading rotary type, is ideally suited for the drying of relatively course particulate matter which does not lend itself to fluidization.
From a purely theoretical point of view, continuous rotary apparatus of the cascading type should have an excellent heat and mass transfer, since the particles fall through the gas which is passing along the shell. However, practical experience is that the heat transfer is poor; the thermal efficiency of such apparatus normally being in the order of 40% to 55%. This poor performance is entirely due to the poor contact between the solid particles and the gas. In this type of apparatus the solid particles form within the shell, curtains of falling solids which offer a greater resistance to the gas flow than the empty space between the falling curtains. The particles thus come into contact with only a very small part of the total gas flowing through the shell. In addition, the small quantity of gas coming into contact with the solids in the curtains very quickly reaches equilibrium conditions in regard to heat as well as mass transfer, which means that the rate of drying, heating or cooling within the curtain will diminish rather rapidly.
In another type of such rotary apparatus, the heating or cooling gas is passed along channels which are created by overlapping louvre plates and enters the bed of particulate solid through the louvres. This type of apparatus gives good heat and mass transfer. However, it cannot be used for sticky products since such products clog the louvres.
In contrast to these continuous rotary driers or coolers, fluid beds with their high degree of interaction between the gas and the particulate solid normally give a thermal efficiency of about 90% to 95%. Thus it is not surprising that fluid bed systems are used in the process industries for the drying, heating and cooling of particulate solids. Unfortunately the practicability of drying, he
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Adelaide & Wallaroo Fertilizers Ltd.
Schwartz Larry I.
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