WO1991015129A1

WO1991015129A1 - Apparatus and method for the conditioning of particulate material

Info

Publication number: WO1991015129A1
Application number: PCT/GB1991/000448
Authority: WO
Inventors: Victor Albert Montgomery White
Original assignee: Gbe International Plc
Priority date: 1990-03-30
Filing date: 1991-03-26
Publication date: 1991-10-17
Also published as: GB9007150D0

Abstract

An apparatus for the transportation and conditioning of particulate material, comprises a chamber (11) having an inlet (11a) for receiving material to be transported, an outlet (11b) for the transported material, gas inlet zones (13) in the chamber wall for the passage of gas (air and/or steam) under pressure into the chamber and gas outlet zones (14) in the chamber wall; conditioning means (17) including gas heating/cooling means (18), and gas humidifier means (19); duct means (16) for connecting said conditioning means (17) to said chamber (11) to feed the latter with conditioning gas under pressure; and means (20) for driving or drawing the conditioning gas through the chamber via the inlet (13) and outlet (14) zones. The inlet zone (13) and said outlet zone (14) have gas guide surfaces disposed transversely of the direction of transport along said chamber which guide surfaces are so positioned in the chamber wall and so dimensioned as to cause the gas to have a transverse velocity component directed to urge the transported material away from the wall of the chamber whilst being pneumatically conveyed along the chamber.

Description

APPARATUS AND METHOD FOR THE CONDITIONING OF PARTICULATE MATERIAL

This invention relates to the conditioning of particulate material and more particularly but not solely to the conditioning of tobacco ( via heating or humidification) .

In pneumatic conveying the axial velocity of the air must be greater than the axial velocity of the product in order to provide the drag necessary to convey. However, in conventional pneumatic conveying of tobacco the product proceeds in clumps and tends to hug the wall of horizontal ducts and bends. There is a flow of air around and past the clumps rather than through them.

It is well known to dry tobacco in a pneumatic conveying duct. Air or superheated steam may be used as the conveying and drying means. Typical practical conveying capacities are 0.2 lbs (0.0S kg) of tobacco per lb (0.47 kg) of air. The thermal capacities are such that the air or steam has to be very hot (200-400^* C) to introduce sufficient heat to evaporate the moisture. Even though it is only applied for a very short time, such a high temperature can cause damage to the product, and ultimately degradation.

As explained above the product tends to convey in clumps so that the outside, which is subjected to the hot gas, is over dried whilst the inside is under dried. The variation is not only very undesirable from a process point of view, but the over dried tobacco is very vulnerable to degradation. It is also well known to cool tobacco after drying in a pneumatic conveyor. There is a thermal capacity problem as in pneumatic methods of drying. Typical air flows for conveying the tobacco have too little heat capacity to carry away the tobacco heat, without the air rising in temperature and limiting the product cooling, so pre-cooling of the air is necessary. Clumping of the product again results in non- uniform cooling.

The transport of tobacco pneumatically is not generally considered good practice, particularly with cut tobacco at low (cigarette making) moisture contents due to a degradation of the strand length at the entrance to the ducting, at bends, at discharge separators and due to the interference between strands.

Pneumatic conveying is however very convenient, particularly in the distributed feed of cut tobacco from a single source to a number of makers.

An object of the invention is to provide a process and apparatus for conditioning the material i.e. drying, cooling or moistening the material by adjusting the conveying-air condition whilst simultaneously pneumatically conveying the material in a manner such as to minimise risk of degradation of the particles.

According to the invention there is provided an apparatus for the transportation and conditioning of particulate material, comprising a chamber having an inlet for receiving material to be transported, an outlet for the transported material, gas inlet zones in the chamber wall for the passage of gas (air and/or steam) under pressure into the chamber and gas outlet zones in the chamber wall; conditioning means including gas heating/cooling means, and gas humidifier means; duct means for connecting said conditioning means to said chamber to feed the latter with conditioning gas under pressure; and means for driving or drawing the conditioning gas through the chamber via the inlet and outlet zones, characterised in that said inlet zone and said outlet zone have gas guide surfaces disposed transversely of the direction of transport along said chamber which guide surfaces are so positioned in the chamber wall and so dimensioned to cause the gas to have a transverse velocity component directed to urge the transported material away from the wall of the chamber whilst being pneumatically conveyed along the chamber.

The action of the transverse velocity component is to keep the tobacco suspended in the air and away from the walls of the duct, whether it is horizontal, vertical or at bends. One effect of this is that product friction is less and that the axial air flow rate required is less.

The axial flow of air is preferably achieved by giving the transverse air flow an axial component. That is by entering the transverse air through perforations with an axial directional bias, the guide surfaces at least at the inlet to the chamber being set obliquely to the direction of the transported material. - A -

Alternatively, the transverse component may be fed at right angles to an axial air flow moving in the direction of transport driven or drawn by a fan.

In the apparatus of the present invention the tobacco is held in suspension or fluidised and behaves in a more open and less clumpy fashion, with the transverse and axial components of the air flowing through the product, providing a more intimate contact between the air and the product.

Considerably more air flow is used than in a purely axial air flow conveyor. Since the extra air is due to the transverse air flow it all comes into contact with the product. Whereas increased air flow in an axial flow conveyor tends to by-pass the product.

For example with a 6 in (150 mm) diameter duct the conventional conveyor axial air flow is 800 cfm (22 cu m/min) and the cut tobacco capacity about 700 Ib/h (325 kg/h). Conveying speeds are typically 55 fps (17 m/s).

With the equivalent of a 6 in (150 mm) diameter duct the conveyor with transverse air flow uses 200 cfm (5.5 cu m/min) per metre run. For the same cut tobacco capacity as above the conveying speed is 33 fps (10 m/s). So for a typical 40 run the air flow is 10 times that in a conventional conveyor.

Further according to the invention there is provided a method for the transportation and conditioning of particulate material, such as tobacco, comprising conveying the material along a chamber having gas inlet and outlet zones, feeding a conditioning gas from conditioning means including gas heating/cooling means, and gas humidifier means to said gas inlet zones, driving or drawing the conditioning gas through the chamber via the inlet and outlet zones, characterised by imparting to said conditioning gas feed to the chamber a transverse velocity component which causes the transported material to be urged away from the wall of the chamber whilst being pneumatically conveyed along the chamber.

The invention will now be described by way of example with reference to the accompanying drawings in which: FIGURE 1 is a schematic cross section taken longitudinally along line Y-Y of Figure 2,

FIGURE 2 is a schematic cross section taken along the line Z-Z of Figure 1,

FIGURE 3 is a schematic cross section taken along the line X-X of Figure 2, and

FIGURES 4 and 5 show alternative arrangements of the walls of the transport chamber by which horizontal and transverse velocity components are achieved.

A number of such units can be joined together to form a continuous conveying duct, but each section has its own recirculation fan and air conditioning unit.

The unit 10 comprises a chamber 11 having side walls 12 which diverge upward from a vertical plane. The chamber 11 is closed at the top and bottom respectively by diffusing air plates 13, 14 having air inlets formed by inclined louvres which ensure that the transporting air has horizontal and vertical components of flow. Tobacco to be conditioned enters at the open inlet end 11a of the chamber 11 and leaves at the outlet end 11b..

A plenum chamber 15 is provided beneath the bottom plate 14 and is connected via a pipe 16 to the outlet of an air conditioning unit 17.

The air conditioning unit 17 includes finned heating or cooling coils 18 which can be fed with heated or chilled water as required, or with steam. It also includes a steam or ultrasonic humidifier 19 to adjust the air humidity.

A centrifugal fan 20 connected to a plenum chamber 21 above the top plate 13 circulates the air back to the conditioning unit 17 via a pipe 22. The fan 20 provides enough pressure to overcome the resistance of the diffusion plates and remainder of the system.

As shown in Figure 1, the upper outlet plate 14 is offset from the lower inlet plate 13 whereby a cantilevered portion 14b of the upper plenum 21 overlaps the adjacent open region 14a of the upper plate 14 when several units 10 are connected with the chambers 11 in series.

An air temperature and humidity sensor are included in the system (not shown) to control the temperature and humidity of the air to desired values. Each section can be controlled to different air conditions to suit the changing tobacco conditions, and can operate in a drying, moistening or cooling mode.

An adjustable blade (not shown) at the outlet of the fan can divert a small proportion of the circulating air out of the system (typically 10%). Since this blade is near the outside of the fan casing it will also divert via a dust bleed pipe 23 the dust which is thrown to the outside by the fan. This diverted air and dust can be fed to a filter unit (not shown) via a common duct collecting dust.

The diversion of air is also necessary in the drying mode to remove moisture or in the cooling mode to remove excess heat. A fresh air inlet 24 is fitted to the suction side of the fan 20 which admits air to replace the diverted air.

For drying, additional heat can be supplied direct to the tobacco by means of dielectric heating electrodes 25 in the sides of the transport chamber 11.

The diffusing inlet and outlet air plates 13, 14 may be formed by slotted plates in which the regions between the slots have been set, by pressing, to an angle. Instead as shown in Figure 4 the top and bottom plates maybe adapted from plain sheet material which has been perforated in zones in the bottom plate offset in relation to zones in the upper plate and covered individual plenum chambers 31 - 34.

In another embodiment shown in Figure 5, the upper and lower plates are provided therealong with sets of inclining inlet and outlet guide passages 36, 37; those sets in the top plate being offset in relation to those in the bottom plate considered in the direction of transport.

By controlling the heat and humidity levels of the transporting gas (air) and by the use of additional energy sources such as dielectric heating the apparatus of the invention can perform deliberately required condition changes better than in conventional systems. It is also possible to prevent undesired changes occurring.

The invention may serve to promote or suppress drying. Furthermore, a product may be bought to a desired temperature without fluid loss or the removal of organic solvents such as sublimating carbon dioxide may be effected in a controlled manner.

The larger air flow means that for drying, lower air temperatures can be used and for cooling the tobacco is cooled nearer to the cooling air temperature. The more intimate contact between the air and the product means more uniform processing.

Apart from bringing more air into contact with the tobacco the transverse air flow permits fresh air to be introduced and then discharged at any point along the ducting.

By means of the conditioning apparatus of the present invention considerably more air or steam can be brought into contact with the product than is possible with known apparatus so that the air or steam need only be heated to relatively low temperatures in order to introduce the drying heat. Alternatively, the evaporative heat for drying can be introduced by dielectric loss heating of the tobacco or using microwave generating sections interspersed with transverse air flow ventilating sections. This enables drying to be carried out at low temperatures with the air flow only conveying and carrying off the moisture.

With measurement of the product moisture content the use of microwaves or dielectric heating to provide the evaporative heat offers a very gas response means of automatic control of the drying process.

With the apparatus of the present invention considerably more air can be brought into contact with the product for cooling with the result that the air is heated less and the product cooled more. Furthermore, with transverse air flow fresh cool air can be introduced along the length of the chamber so it is possible to cool to within a few degrees of the cooling air temperature.

The transverse air flow permits the process conditions to be varied progressively along the chamber to match the varying conditions of the product or to give a preferred moisture or temperature profile along the chamber. For example when cooling a product from a high temperature in a conventional pneumatic duct after drying it is not possible to prevent further drying taking place. This is because the cool entering air even if saturated has a very low humidity, so that as it is warmed by the tobacco its relative humidity reduces and it tends to dry the tobacco. With transverse air flow the cooling air at the feed end can be hot, though cooler than the product, and at a high humidity, so that evaporation from the product is suppressed, (or even reversed, i.e the product is moistened). The air condition of temperature and humidity can then be progressively reduced towards the discharge end to still cool the product but suppress evaporation.

It is less well known to moisten tobacco in a pneumatic conveying duct, but in principal this can be done in three ways: by condensation from a hot saturated mixture of air and vapour; by spraying fine water droplets onto the tobacco and by absorbt on from a humid atmosphere at low temperatures. The first and second tend to be messy and a heated duct is required to prevent the collection of product on the duct surface. The third is slow and only suitable for small additions.

With moistening by condensation from hot saturated air there is less of a problem due to lack of thermal capacity except at low temperatures. This is because the vapour adds considerably to the thermal capacity particularly at high temperatures. However a transverse air flow allows a more precise temperature control of the tobacco, independent of the tobacco flow rate.

For moistening by water spray the transverse air flow enables a very fine droplet airborne spray to be progressively introduced down the length of the duct, rather than the whole amount at the entrance to the duct. The finely atomised spray generated by compressed air or by ultrasonics are suitable for this purpose.

The absorbtion of moisture from humid air is a slow process and limited by the small amount of moisture in the air at low temperatures. The increased air flow associated with the transverse air flow mode of conveying increases the rate of absorbtion and the amount of moisture available, but this is still relatively small around 0.5%.

A particular application of the conditioning of tobacco by absorbtion with simultaneous pneumatic transport is in the distribution of cut tobacco to cigarette makers.

The ultimate aim of a cigarette making machine is to produce a cigarette o constant dimensions, weight and firmness. The machine is essentially volumetric in that it produces a constant column stream of cut tobacco which is first trimmed according to two measurements and then formed into the cigarette rod.

The first measurement is one of air pressure drop across the stream, which is related to its bulk density, and this provides a short term adjustment of the trimming wheel. The second measurement is a radiation gauge measurement of the cigarette rod, which is related to the total mass of the cigarette and this provides a longer term adjustment of the trimming wheel. The total mass includes the mass of tobacco and of moisture within the tobacco. Hence a change in total mass may be signalled when in reality the mass of the tobacco only is correct and the quantity of moisture is incorrect. The moisture content and temperature of the strands effect their flexibility and hence bulk density. The first measurement is an attempt to compensate for variations in density due to these factors, by trimming the depth of the stream inversely to its pressure drop.

However, pressure drop can be effected by other factors and is not precisely related to bulk density, also the trimming wheel has to move very fast to keep pace with variation in the fast moving tobacco stream. It is clearly preferable to keep the tobacco moisture content and temperature constant rather than trying to correct for variations.

By conditioning the tobacco during pneumatic transport to the making machine a final adjustment of moisture content and temperature may be effected.

The moisture content of tobacco along with other hygroscopic materials reaches an equilibrium with the humidity of the surrounding air. This is achieved when the vapour pressure of the moisture in the tobacco equals the vapour pressure of the moisture in the air.

Below a critical moisture content the moisture in hygroscopic materials is said to be "bound" that is held physically in capillaries or chemically bonded. This bound moisture exerts a vapour pressure below that of a free water surface and an equilibrium curve can be drawn relating the equilibrium moisture content of the material to the relative humidity of the air. The curve can be drawn for various temperatures. But because the vapour pressure of both the material and the air are altered by temperature the effect of temperature is not as pronounced as the effect of relative humidity on the material moisture content.

Above a critical moisture there is "unbound" or free moisture which exerts the same pressure as saturated air, that is 100% relative humidity. This is generally above 40% moisture content and does not concern us here.

The simplest approach to final correction of the cut tobacco moisture and temperature is to feed the pneumatic conveyor with air at a temperature and equilibrium relative humidity corresponding to the desired tobacco temperature and moisture content. Tobacco which is above the desired moisture content would tend to dry and tobacco which is below would tend to moisten.

However, this equi libration process (absorbtion/desorption) tends to be slow and does not have an appreciable effect in the few seconds it takes to convey the tobacco. The amount of adjustment is also limited to a fraction of one percent by the moisture capacity of the humid ai r.

It is more effective to be either desorbing or absorbing, but preferably to be drying rather than moistening as the tobacco then enters the duct slightly above the desired moisture content and is thereby less vulnerable to degradation. By drying only the air condition can be slightly below equilibrium to provide a bigger vapour pressure difference between the tobacco and air and more drying "drive". The air temperature can also be slightly above the desired tobacco temperature to provide evaporative heat to the tobacco to prevent evaporative cooling.

The product moisture range is reduced by approaching an equilibrium condition. A factor of improvement of 2 or 3 to 1 is possible and is reflected in a reduction of product moisture standard deviation.

Alternatively the evaporative heat can be supplied directly to the tobacco by dielectric heating, and controlled from a continuous moisture meter as discussed above.

Pneumatic conveying with transverse air flow provides additional air with a larger moisture capacity and a better contact between tobacco and air.

Moistening of the tobacco can be achieved by setting the air condition slightly above the equilibrium relative humidity condition to provide moistening "drive" and slightly below the desired tobacco temperature to absorb the heat of condensation from the tobacco to prevent it rising in temperature.

With control from a continuous moisture meter the conveying air condition and temperature can be swung from a moisture absorbing to a moisture desorbing condition as the need arises.

Claims

CLAIMS :

1. An apparatus for the transportation and conditioning of particulate material, comprising a chamber

(11) having an inlet (11a) for receiving material to be transported, an outlet (11b) for the transported material, gas inlet zones (13) in the chamber wall for the passage of gas (air and/or steam) under pressure into the chamber and gas outlet zones (14) in the chamber wall; conditioning means

(17) including gas heating/cooling means (18), and gas humidifier means (19); duct means (16) for connecting said conditioning means (17) to said chamber (11) to feed the latter with conditioning gas under pressure; and means (20) for driving or drawing the conditioning gas through the chamber via the inlet (13) and outlet (14) zones, characterised in that said inlet zone (13) and said outlet zone (14) have gas guide surfaces disposed transversely of the direction of transport along said chamber which guide surfaces are so positioned in the chamber wall and so dimensioned to cause the gas to have a transverse velocity component directed to urge the transported material away from the wall of the chamber whilst being pneumatically conveyed along the chamber.

2. An apparatus as claimed in claim 1, characterised in that said inlet and outlet zones are formed by lower and upper apertured plates (13, 14) which define the lower and upper end of the chamber (11).

3. An apparatus as claimed in claim 1 or 2, characterised in that the gas guide surfaces at least at the inlet zone (13) are set obliquely to the direction of the transported material.

4. An apparatus as claimed in claim 3, characterised in that said gas guide surfaces are formed by inclined louvres extending along the length of the lower plate (13).

5. An apparatus as claimed in claim 2 or 3, characterised in that the upper plate is provided with inclined louvres extending along the length of the upper plate and serving as the outlet zone.

6. An apparatus as claimed in claim 3, characterised in that the inlet and outlet zones are formed by sets of inlet and outlet guide passages, the sets in the upper plate being offset in relation to the sets in the lower plate considered in the direction of transport.

7. An apparatus as claimed in claim 2, characterised in that the gas guide surfaces at least in the lower plate (13) are arranged at right angles to the direction of transport, means being provided to provide an axial flow of air along the chamber.

8. An apparatus as claimed in claim 7, characterised in that the apertures in the upper plate (14) are also arranged at right angles to the direction of transport.

9. An apparatus as claimed in claim 8, characterised in that the inlet and outlet zones are formed by sets of inlet and outlet guide passages, the sets in the upper plate being offset in relation to the sets in the lower plate.

10. An apparatus as claimed in any one of claims 2 to 9, characterised in that respectively below and above the lower and upper plates are provided plenum chambers (15, 21) and in that the upper plate (14) is offset from the lower plate (13) whereby a canti levered portion (14b) of the upper plenum (21) overlaps an adjacent open region ( 14a) of the upper plate when adjacent units are serially arranged.

11. An apparatus as claimed in claim 10, characterised in that the side walls of the chamber (11) diverge upward from a vertical plane.

12. An apparatus as claimed in claim 10 or 11, characterised in that said gas heating/cooling means (18) comprises finned heating/cooling coils for feeding with heated or chilled water as required or with steam and said gas humidifier means (19) comprises a steam or ultrasonic humidifier.

13. An apparatus as claimed in claim 10, 11 or 12, characterised in that said means (20) for driving or drawing the conditioning gas comprises a centrifugal fan connected between the upper plenum (21) and the conditioning unit (17), the latter being connected at its downstream end with the lower plenum ( 15) .

14. A method for the transportation and conditioning of particulate material, such as tobacco, comprising conveying the material along a chamber having gas inlet and outlet zones, feeding a conditioning gas from conditioning means including gas heating/cooling means, and gas humidifier means to said gas inlet zones, driving or drawing the conditioning gas through the chamber via the inlet and outlet zones, characterised by imparting to said conditioning gas feed to the chamber a transverse velocity component which causes the transported material to be urged away from the wall of the chamber wh lst being pneumatically conveyed along the chamber.

15. A method as claimed in claim 14, characterised in that the conditioning gas is fed into said chamber obliquely of the direction of material transportation.

16. A method as claimed in claim 14, characterised in that the material is transported along the chamber by an axial flow along said chamber and in that the conditioning gas is fed into said chamber at right angles to the direction of material transportation.