Tobacco – Tobacco treatment – Puffing
Patent
1990-05-18
1992-11-10
Millin, Vincent
Tobacco
Tobacco treatment
Puffing
131304, 131306, A24B 304, A24B 312
Patent
active
051615487
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention concerns the conditioning of tobacco products, in particular the conditioning of cut lamina and cut mid rib (known as cut stem) by the introduction of steam to a vibratory conveyor wherein the steam passing by way of the perforations includes a component of flow parallel with the supporting surface of the conveyor.
It is well known to subject tobacco products to steam at atmospheric pressure after cutting and before drying in order to expand or puff the tobacco.
This can be achieved by any means which transports the tobacco in a given direction whilst subjecting it to a transverse flow of steam, but with varying effectiveness.
One means is a rotary cylinder with axis slightly inclined to the horizontal to transport the tobacco, enclosing a stationary pipe parallel with the axis carrying a number of steam jets which direct steam onto and at right angles to the moving tobacco.
Another means is a vertical metering tube or column with axial perforated steam tube which directs steam transversely to the tobacco flowing down the tube.
Another means is an enclosed rotary screw conveyor with steam jets arranged in the trough and/or lid which are directed at right angles to the transported tobacco.
Another means is a simple horizontal gauze band conveyor with the upper strand conveying tobacco over an open topped plenum chamber fed with steam which passes through the tobacco at right angles to its motion.
To achieve expansion or puffing of the tobacco it is necessary to heat the tobacco near to the boiling point of the moisture within the tobacco, in order to create the conditions for expansion.
Tobacco is a hygroscopic material and below a critical moisture, which for tobacco is around 40 to 50%, the moisture is "bound" and exerts a vapour pressure below that of free water, e.g. it can be held in capillaries where the vapour pressure is lowered by the concave water surface. Above the critical moisture there is also free "unbound" moisture on the surface of the tobacco or held in voids which exerts the full vapour pressure. (See Elements of Chemical Engineering by Badger and McCabe page 299).
In general the tobacco has to be heated above 100 degrees C. to achieve boiling point. In fact the elevation can be deduced from the equilibration moisture curves. For example for a typical grade of cut stem at a cutting moisture of 33% the elevation is 2 degrees C. and at 27% the elevation is 4 degrees C., whilst at the critical moisture content of 46% the boiling point is that of free water. In practice there is a compensation factor:
When a hygroscopic material, like tobacco, below the critical moisture content is heated by saturated steam it will first absorb the condensation moisture (typically 5% to raise it from 20 to 100 degrees C.) and then continue to absorb moisture at a much slower rate by a reverse wet bulb process, driven by the vapour pressure difference between the steam and the tobacco. But in this case the tobacco rises in temperature above the steam in order to transfer the latent heat. Like the wet bulb an equilibrium temperature difference is established at which the flow of heat from the tobacco to the steam equals the latent heat of condensation. In fact the elevation in temperature is very similar to the elevation in boiling point in the example above.
The heating ability of steam is dependent on it being 100% saturated steam; it is reduced by two factors: superheat and air dilution.
Saturated steam is a vapour and transfers heat by condensation. Very high transfer rates are possible, because as the steam condenses to water it releases a large latent heat and also reduces to 0.06% of the volume, so that further steam flows in to fill the void.
Superheated steam on the other hand behaves as a gas and transfers heat by conduction, with correspondingly low heat transfer rates, only around 1% of the rate by condensation. To compensate high temperature differences must be used.
In addition the heat available from the superheat is very small compared with the latent
REFERENCES:
patent: 3978867 (1976-09-01), Wochnowski
patent: 4195647 (1980-04-01), Wochnowski et al.
patent: 4932424 (1990-06-01), Liebe et al.
Doyle J.
GBE International plc
Millin Vincent
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