WO2014132031A1

WO2014132031A1 - Drying apparatus

Info

Publication number: WO2014132031A1
Application number: PCT/GB2014/050421
Authority: WO
Inventors: Thomas Henry White
Original assignee: Dickinson Legg Ltd
Priority date: 2013-02-28
Filing date: 2014-02-13
Publication date: 2014-09-04
Also published as: JP2016508596A; HK1218153A1; CN105264319A; GB2511331A; GB201303570D0

Abstract

A drying apparatus for drying a fibrous material is described. The apparatus provides a drying apparatus for drying a fibrous or granular material comprising, a heater arranged to heat a first gas stream and a fibrous material, the first gas stream arranged to enter the drying apparatus at an inlet, the gas stream including steam. A separator is provided for separating a said fibrous material from the first gas stream to form a second gas stream, and a fan and a duct are arranged to recirculate the second gas stream to the process heater steam injection apparatus. A flow variation apparatus is arranged to vary the flow of gas in a first gas stream and improve the drying function due to increased heat and mass transfer between the gas stream and the tobacco product.

Description

DRYING APPARATUS

The present invention relates to drying apparatus and a method for drying a fibrous or granular material, particularly, but not exclusively to a drying apparatus and a method of drying a fibrous material such as tobacco leaves or portions of tobacco leaves.

Tobacco leaves are processed for cigarette manufacture in a primary tobacco process. Laminate portions of tobacco leaves are compressed and then cut in a cutting machine to provide tobacco particles suitable for cigarette manufacture. Moisture is removed from the fibrous tobacco material typically to improve the handling and filling properties of the tobacco material.

It is known, within the tobacco industry, to employ pneumatic conveying dryers, otherwise known as flash dryers, for reducing the moisture content of cigarette filler material to a level such as is suitable for cigarette manufacture and packing. The general principle in such dryers is that the tobacco product is conveyed along a passageway by a flow of hot gas. The gas flow provides an environment in which the tobacco product is dispersed and dried within the passageway. The tobacco product is then separated by some means from the gas stream. Product drying takes place due to heat and mass transfer between the gas stream and the tobacco product. It is known to use superheated steam, air, a mixture of air and carbon dioxide or air with other combustion components as the conveying gas.

Tobacco is the most expensive component of cigarettes, therefore manufacturers have a strong incentive to process it in such a way as to maximise the number of cigarettes that can be produced from a given weight of tobacco. The generation of a lower density of cut tobacco product provides an improved yield from a given weight of starting product. The present drying with a pneumatic conveying dryer, also known as a flash dryer, and superheated steam as the drying medium, does provide an improved yield with a lower density of cut tobacco than results from other drying methods.

Figure 1 illustrates a conventional drying apparatus 100, such as a pneumatic conveying dryer comprising a process heater 10, a product feeder device 1 , apparatus for providing controlled discharge and release of material, in this example the release apparatus comprises a rotary air lock 2, further comprising a drying duct 3, a separator and another rotary air lock and discharge apparatus 5. Typically the separator is a cyclonic separator 4. Stored tobacco product is passed to the drying apparatus and a flow of moist tobacco product is generated by the feeder device 1 . The moist tobacco product then passes through rotary air lock 2, the product is dispersed into the duct 3 and a mixture of gas and solids is conveyed along the duct 3 to the cyclonic separator 4 by a conveying flow of hot gas. The hot gas gives up heat to the tobacco product, causing some of moisture content of the tobacco to evaporate. The cyclonic separator 4 separates dried solids of tobacco product from the gas stream. The dried tobacco product is discharged through air lock 5 and may then be conveyed onto the next stage of the cigarette manufacturing process as required. The conventional drying apparatus illustrated in figure 1 further comprises components to capture and recirculate the conveying gas stream. The drying apparatus further comprises a fan 6, outlet 7, collection duct 8 and steam injection apparatus. Gas flow exiting the cyclonic separator 4 is conveyed to the fan 6 for recirculation towards the process heater 10 or other heat source (heat exchanger) and again to the drying duct 3. Steam is injected into the system via a suitable control valve 9 and flow metering system. The fan 6 acts to generate sufficient pressure to convey the gas stream along duct 8. Surplus gas (a proportion of the returned gas) is exhausted or vented at outlet 7 to maintain the mass balance of the system. Surplus gas results from the inevitable inward leakage of air through the air locks, the injection of steam and from the evaporation of moisture from the originally moist tobacco product. In the example illustrated in Figure 1 , the surplus gas is shown exhausted to atmosphere the gas may however, be treated or purified before release instead. Examples include condensing or scrubbing, for example chemical scrubbing, to remove any of impurities, odours and pollutants from the vapour emission.

The density reduction and improved yield from pneumatic conveying dryers can be attributed to the rapidity of the drying process which is a function of the rate at which heat can be transferred from the drying gas to the product. A high rate of heat transfer will generate a higher vapour pressure within the tobacco cellular matrix compared with the vapour pressure generated by a gradual rate of heat transfer. If the internal vapour pressure is higher than that of the surroundings, the cellular matrix will tend to be inflated, thus resulting in a density reduction in the tobacco product during and after the drying process. The heat transfer between the drying gas and the tobacco product is determined by a number of factors, in particular the thermodynamic properties of the drying gas, since these affect the heat transfer characteristics of the boundary layer of gas which surrounds the surface of each particle of product as it is conveyed through the dryer.

The drying apparatus described above can be bulky. The physical size of a flash dryer and in particular its height, is closely related to the desired throughput capacity and moisture removal duty and efficiency. It is therefore an aim to improve the heat transfer between the drying gas and the product, so that the overall size and capital cost of a dryer can be reduced and to provide an alternative apparatus and method for drying a fibrous product such as tobacco.

According to a first aspect, the invention provides a drying apparatus for drying a fibrous or granular material comprising, a heater arranged to heat a first gas stream and a fibrous material, the first gas stream arranged to enter the drying apparatus at an inlet, the gas stream including steam, a separator for separating a said fibrous material from the first gas stream to form a second gas stream, a fan and a duct arranged to recirculate the second gas stream to the process heater and steam injection apparatus, and a flow variation apparatus arranged to vary the flow of gas in a first gas stream. US 8037620, US 5252061 and US 4859248 describe commercial dryers and apply gas flow pulsation to enhance heat transfer to the drying of organic materials such as grain and wood chips. In these dryers, pulsing the burner flow i.e. pulsating gas flow is generated by a pulse combustor or pulse jet engine which generates a flow of pulsating hot gases for direct heating of the organic material. In the drying of tobacco, unlike some other examples of organic material, it is not generally acceptable to allow flue gases to contact the product as the chemical composition of the flue gases is considered to affect the taste or smoking characteristics of the tobacco being processed. The design of tobacco dryers is influenced by the view that higher temperatures are considered to have a deleterious effect on the tobacco taste. High temperatures can remove flavour due to burning of sugar and starch components within the tobacco. Higher drying temperatures do tend however, to result in higher expansion of the tobacco due to the increased rate of heat transfer into the product. The present invention utilises the flow variation apparatus feature to exploit the thermodynamic and other process benefits of a pulsating gas flow without the associated use of a pulsating combustor. The present invention is described with reference to pneumatic conveying dryers or flash dryers, the invention and claimed arrangements are also suitable for other drying apparatus such as those where heat transfer to the product to be dried is convective. One example is fluid bed dryers using conveying or fluidising gas as the method of heat transfer.

In an embodiment the flow variation apparatus is arranged to vary the flow in a cyclical manner. The cycling of the gas flow has the effect of repeatedly heating the tobacco product aiding heat transfer. These variations in flow may take the form of velocity variations, or pressure variations, or a combination of the pressure and velocity.

The flow variation apparatus is arranged to pulse the flow of process gas and act to enhance heat transfer in an embodiment. The flow variation apparatus generates pulsations in the flow of process gas which pulsations break up and agitate the boundary layer of gas surrounding the surface of each particle of product as it is conveyed through the dryer. This in turn increases the heat transfer rate between the process gas and into the tobacco product, improving product expansion at lower process gas temperatures, with the ensuing benefits described above of providing lower density tobacco with improved taste characteristics.

Thus the apparatus seeks to utilise the thermodynamic and other process benefits of a pulsating gas flow with a variation in flow rate but without the associated use of a pulsating combustor to directly heat the process gas.

In an embodiment the flow variation apparatus is arranged to vary the flow in the range of frequencies from 0.3 Hz to 200Hz and can be arranged to vary the flow in the range of frequencies from 0.3 Hz to 5Hz. Here by operating at lower frequencies typically within but not confined to the range 0.3 to 5 Hz, the tobacco flow within the drying apparatus can be made alternately to hesitate and then accelerate cyclically. This cyclic hesitation performs two functions: firstly the heat transfer is increased because the relative velocity between the tobacco product and the gas stream is increased. Secondly, the residence time of the tobacco product within the dryer can be increased, also allowing the drying process to take place at a lower temperature with consequent taste benefits. In the present invention the location of the apparatus providing the variation in flow and the cyclic variations is at an outlet of the cyclone exhaust or may be at the outlet of a heat exchanger. The variation in flow takes the form of velocity variations, or pressure variations, or a combination of the pressure and velocity.

In an embodiment the flow variation apparatus comprises a mechanical device. In the embodiment described below the first gas stream and fibrous material has a direction of process flow and wherein the flow variation apparatus comprises a rotatable disk or chopper having an axis of rotation perpendicular to the direction of process flow. This provides a straightforward flow variation apparatus and the disk comprises at least one opening for the first gas stream process flow. The disk can be retrofitted to an existing drying apparatus. The disk in an embodiment comprises a plurality of openings for the first gas stream process flow. A number of openings can assist in the passage of the gas flow and fibrous product such as tobacco and can be helpful, for example if an opening were to become clogged or blocked.

There is also described an embodiment in which the first gas stream and fibrous material has a direction of process flow and wherein the flow variation apparatus comprises a rotatable damper plate having an axis of rotation perpendicular to the direction of process flow. A damper plate is a convenient, reliable construction for a flow variation apparatus.

In an embodiment where higher frequency flow variation is required the flow variation apparatus comprises a pneumatically driven sound emitter. In another embodiment the flow variation apparatus comprises an electrically driven sound emitter.

The heat transfer between the process gas and the product is related to the conditions pertaining within the boundary layer of gas molecules immediately adjacent to and surrounding the product particles of the fibrous material. Higher frequency flow variations, for example acoustic vibration, applied in heat exchange systems results in significantly enhanced heat transfer, due to the break down or disruption of the boundary layer of gas. An electrically or pneumatically driven sound emitter or acoustic horn is used to provide higher frequency flow variations to the drying apparatus system, for example in the range 20 - 400 Hz in an embodiment.

In the drying apparatus the steps of steam generation and heating require significant energy input. In addition the influx of air at various points within the system can result in dilution of steam content of the drying gas, which is compensated for by increasing the amount of steam injected into the system. It has been suggested that heat energy from the exhaust and surplus gas given off from the drying process may be recovered using a heat exchanger. In an embodiment the drying apparatus further comprises a heat exchanger arranged to condense and separate a first steam component and a water component from the second gas stream, and further arranged to generate a second steam component at low pressure, and a compressor arranged to compress the second steam component and return the steam component to the inlet. In an embodiment the drying apparatus further comprises a compressor arranged to compress the second gas stream, a heat exchanger arranged to condense and separate a steam component and a water component from the second gas stream, and further arranged to generate a second steam component at low pressure, and return the second steam component to the inlet. This means that recovery the energy content of the exhaust from the drying apparatus or dryer is possible and is returned to the drying apparatus for re-use.

Product quality considerations have tended to dictate the use of superheated steam as the drying medium in tobacco dryers. In these dryers, a high concentration of steam in the drying gas is achieved by minimising the influx of air, and by injecting steam into the process to supplement that which is generated by evaporating the tobacco product's own moisture.

It is desirable to maintain a low oxygen level within such a dryer, as this prevents any risk of fire and improves product quality by inhibiting surface oxidation of the product. The exhaust from the drying process comprises the water evaporated from the product, plus inward air leakage and steam injection. The exhaust is at a pressure which is close to that of atmospheric, and has a large energy content due to the enthalpy of the water evaporated from the product. The exhaust condition for the drying apparatus is that it can comprise 90% steam and 10% air by volume, at a temperature of about 150 °C and a pressure of 1 bar absolute. Using a heat exchanger, heat energy can be recovered from such a gas stream but the temperature of the recovered energy is relatively low: it will be significantly below the condensation temperature of the process gas, which will be below 100°C at atmospheric pressure.

There is described a system where the gas stream includes superheated steam. It is desirable here to raise the temperature of the recovered energy to a useful level, thus it is necessary to introduce a compression stage into the process. A heat exchanger can be employed as described in co-pending patent application publication number WO 2010/094913. The heat exchanger here generates steam from water at low pressure, and then the resulting sub- atmospheric pressure steam can be compressed to a pressure which is suitable for injection into the process. In this embodiment the compressor is only required to handle clean steam which is uncontam inated with particles and dust from the process exhaust, although a larger compressor is required to accommodate the lower gas density at its inlet. The use of a heat exchanger ensures that the steam which is generated from the recovered heat is uncontam inated with air, or other substances which might affect the quality of the product being processed. In the drying apparatus described it further comprises apparatus arranged to introduce additional steam to the first gas stream. The additional steam is from an alternative source, external to the drying apparatus in an embodiment. The embodiment described includes a heat exchanger is arranged to have a pressure of the gas stream of less than 1 bar, preferably in the range from 0.3 bar to 0.7 bar pressure, for optimum operational characteristics. As described above an embodiment comprises a compressor and optimum operation occurs with the compressor arranged to compresses the gas stream to a pressure greater than 1 bar, preferably in the range from 1.1 bar to 1.5 bar pressure. In an embodiment the drying apparatus includes flow variation apparatus to vary the flow of gas in a first gas stream with an amplitude of gas velocity variation from a mean velocity in the gas stream in the range of from 0 to 20m/second.

In accordance with the present invention as seen from a second aspect, there is provided a method of drying a fibrous product using the apparatus of the present invention.

Further preferred features of the invention are defined in the accompanying claims and include a process in which the flow of process gas within a flash dryer is varied cyclically in order to enhance the heat transfer between the drying gas and the product. Also included is a process in which the flow of process gas within a flash dryer is varied cyclically such that the product residence time within the dryer is increased. Also include is a process in which the flow of process gas within a flash dryer is cyclically varied using frequencies in the range of 0.3Hz to 200Hz.

Embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1 is a schematic view of a prior art drying apparatus;

Figure 2 is a schematic view of the drying apparatus of the present invention;

Figure 3 is a schematic view of part of the drying apparatus of Figure 1 according to a first embodiment of the present invention; Figure 4 is a schematic view of part of the drying apparatus of Figure 3 parallel to the direction of process flow; Figure 5 is a schematic view of part of the drying apparatus of Figure 1 according to a second embodiment of the present invention;

Figure 6 is a schematic view of the drying apparatus of the present invention according to a third embodiment;

Figure 7 shows gas velocity and operation according to the present invention;

Figure 8 is a schematic view of the drying apparatus of the present invention in an alternative configuration; and

Figure 9 is a schematic view of the drying apparatus of the present invention in a further an alternative configuration.

A type of conventional known drying apparatus and the drying apparatus of the present invention will now be described with reference to the apparatus of Figures 1 to 9. Some details of the structure and method of operation of known drying apparatus have been set out above and summary details are as follows. Moisture is removed from moist tobacco products in a duct 3 conveying and dispersing the tobacco product in a gas stream. A cyclonic separator 4 acts to separate the tobacco product from exhaust gas which can then be processed and purified or recirculated.

The drying apparatus 200, 300, 400, 500 of the embodiments of the present invention include some components substantially similar to that of the apparatus 100 illustrated in figure 1 and so similar features have been referenced using the same reference numerals. Referring to figure 2 of the drawings there is shown a drying apparatus 200 according to a first embodiment of the present invention having a flow variation apparatus 20 located within the process gas duct 3 prior to the rotary air lock 2. The flow variation apparatus may be located after the cyclonic separator

The flow variation apparatus 20 is shown in the embodiment illustrated in Figure 3 as comprising the mechanical means of a rotatable disk 21. The disk 21 is rotatably mounted at the apparatus having an axis 22 parallel to the flow direction 40 of process gas. The disk is centrally mounted at the axis 22. A number of holes 21 are located around the circumference of the disk in between the holes are a number of land portions or solid regions 24. The process gas duct 3 accommodates the disk 21 and the mounting axis 22.

Figure 4 shows a sectional view of the flow variation apparatus and the disk 21 with drive means 50 for driving the disk 21 and causing it to rotate about axis 22. In this embodiment the drive means is a variable speed motor 50. The disk

21 is directly mounted to the shaft of the variable speed motor 50. A static housing 60 is arranged to enclose the disk 21 and acts to connect the disk 21 is to the process gas duct 3 and prevents leakage of gases into or from the flow variation apparatus unit.

In use the disk 21 is driven by the motor 50 and transversely intersects the process gas duct 3. The process gas travelling in direction 40 can pass through the holes 23 when the holes 23 are aligned with the duct 3, or if a solid land part

22 of the disk 21 is aligned with the duct 3, the flow will be interrupted. The waveform of the flow can be changed according to the shape or pattern of holes

23 on the disk 21. The frequency of the variation in the flow can be increased or decreased according to the speed of rotation of the disk 21 . An alternative embodiment of the flow variation apparatus is shown in Figure 5. The flow variation apparatus is a damper plate 25 arranged in the process gas duct 3 prior to the product infeed point or after the cyclonic separator and having an axis of rotation 26 perpendicular to the process gas flow direction 40. The damper plate 25 is a rectangular shaped plate having a long side L and a short side H. The shape and size are chosen to be of complementary, but not identical shape to the internal dimensions of height and width of the cross section of the process duct 3. The process gas duct 3 here is shown as cuboid shape. The damper plate 25 is mounted and connected to a drive motor or device for driving the damper plate 25 and causing it to rotate.

In use, as the plate rotates, it alternately blocks and allows the gas flow through the duct 3. If the area of the plate 25 is equal to or close to that of the cross sectional area of the duct 3, the minimum flow will be practically zero. If the area of the plate 25 is somewhat less than that of the duct 3, the minimum flow will be proportionally closer to the maximum flow.

An illustration is provided in Figure 7 of the effect of changing the width dimension W relative to the height or diameter of the duct HH. The velocity graph 27 represents a ratio of plate width H to duct height HH of 66%, graph 28 represents the effect of a plate width H equal to 50% of the duct height HH, and graph 29 shows the effect of a plate width H which is 22% of the duct height HH. In these graphs it is assumed that the process gas fan speed is adjusted to provide the same mean gas flow throughout. A similar effect can be obtained with varying the ratio of open area to total area of the embodiment with the disk 21 shown in Figure 3. Velocity and time figures in the graphs are provided for illustrative purposes and are not limited to these values.

As described above by operating at lower frequencies typically within but not confined to the range 0.3 to 5 Hz, the tobacco flow within the dryer apparatus can be made alternately to hesitate and then accelerate cyclically. This cyclic hesitation performs two functions: firstly the heat transfer is increased because the relative velocity between the tobacco and the gas stream is increased. Secondly, the residence time of the tobacco within the dryer can be increased, also allowing the drying process to take place at a lower temperature with consequent taste benefits. As shown in Figure 6 an alternative drying apparatus 300 is shown with a flow variation apparatus 20 arranged in the duct 3. Here the flow variation apparatus comprises a sound emitter 90 such as a pneumatically driven sound emitter such as the IKT 150/360 manufactured by Kockum Sonics of Malmo, Sweden. Such a sound emitter can be connected to the dryer ducting system by means of a branch duct prior to the product infeed as shown in Figure 6. In use the sound emitter will generate high frequency pulses of flow that will vary the process gas flow travelling along the duct 3. It is envisaged that one or more of the above described flow variation devices 20 could be used in one drying system and apparatus. In this way low frequency variation of flow is generated by the mechanical means of the rotating disk 21 or damper 25, and higher frequency acoustic vibration is generated by the pneumatically driven sound emitter 90.

In an alternative embodiment the flow variation apparatus 20 described as the disk 21 , plate 25 and sound emitter 90 may also be used in a drying apparatus 400, 500 illustrated in Figures 8 and 9 and further comprising a heat exchanger 12 for condensing the exhaust gas stream, and having an access inlet 1 1 and outlets 13, 14, a compressor 16 and a steam monitoring device 17. In a preferred embodiment the steam monitoring device is a steam flowmeter and the compressor is a rotary lobe compressor. The apparatus 200 further comprises a separate, independent steam supply 19 and steam injection system for providing additional steam into the process steam injection line 15. The steam injection system comprises a flowmeter 18 and a flow control valve 9 responsive to the steam monitoring device 17. Steam is injected into the system via the suitable control valve and flow metering system in order to maintain the required low oxygen concentration in the dryer, examples of which will be known to those of skill in the art.

The apparatus 400, 500 operates in the manner described above for conventional apparatus 100. In contrast to drying apparatus 100, the exhaust gas at 7 from drying apparatus 400 is directed to the heat exchanger 12 where it provides heat to evaporate water 1 1 which is supplied to an inlet 11 at the shell side of the heat exchanger 12. Level sensor controls (not shown) maintain the water level within the heat exchanger 12 so that the heat transfer surfaces are covered but there is no possibility of water passing out from the heat exchanger 12 into the process steam injection line 15. The pressure within the heat exchanger 12 is maintained below atmospheric pressure, at 0.5 bar absolute in the preferred embodiment. The negative pressure is created by the compressor 16. At this low pressure, steam is generated by the heat exchanger at a temperature of between 65 to 75 °C, around 70 °C, thus recovering the latent heat content of the steam in the exhaust gas. In other words the heat energy in the drying process exhaust gas mixture is used to generate steam at low pressure by means of the heat exchanger 12.

This low pressure steam, without water or other content, is then is compressed mechanically by the compressor 16 and injected into the process gas flow to maintain the desired steam concentration. The low pressure stream is compressed by the compressor 16 to a pressure greater than 1 bar absolute. Pressure above 1 bar has been found to be suitable for injection of the steam back into the process line and the drying process and process heater stage 10. The flow of steam injected into the duct 3 and separator 4 is controlled by varying the speed of the compressor 16 according to feedback from a steam flowmeter 17. If the steam flow from the compressor 16 is less than the desired steam flow, then an independent steam injection system consisting of a flowmeter 18 and flow control valve 9 can be used to supply the appropriate flow of additional steam from a separate source 19. In this preferred embodiment the separate source 19 is a central boiler plant and provides steam to bring the total steam injection flow to a predetermined desired level. Not all the exhaust flow is condensed by the heat exchanger. In particular the air content of the exhaust gas is discharged via outlet 14 from the condenser of the heat exchanger 12. The exhaust and excess gas at outlet 7 and 14 includes steam components due to the output from the heat exchanger 12 being in a saturated state. As mentioned above the exhaust gas may require to additional treatment in order to purify the gas and/or remove odourous constituents. Water will also be discharged at outlet 13 from the heat exchanger 12 where it has condensed while giving up its latent heat of condensation. By gas stream we mean the fluid flow in the drying apparatus that conveys the tobacco product, gas, steam, impurities and in some cases water through the system.

Referring to figure 9 of the drawings there is shown a drying apparatus 500 with flow variation apparatus 20 according to a further embodiment of the present invention. The drying apparatus 400, 500 of the embodiments of the present invention are substantially similar and so similar features have been referenced using the same reference numerals. The arrangement of the drying apparatus 500 differs from that of drying apparatus 400 in that compressor 16 is located upstream of heat exchanger 12. The compressor 16 and thus the compression stage of the drying take places before the heat exchanger 12. In this embodiment the exhaust gas is recompressed directly before being re-injected as steam into the drying process, with means such as the heat exchanger 12 of this preferred embodiment to remove air from the steam. Thus the temperature of the recovered gas and energy is raised to a useful level for use in the drying process.

Various modifications may be made to the described embodiments without departing from the scope of the present invention. There may be a different number of drying stages or compressors or heat exchanger means. There may be more than one separator. In addition other types of independent steam source may be used, such as a canister or supply entirely external to the operating plant. The efficiency of the energy recovery can be optimised as desired and depends on many detailed design considerations. The material to be dried may be fibrous or granular and may comprise organic material. The ducts here are shown as circular. Other shapes of duct may be used, they may be of any convenient section, e.g. round or rectangular. A pneumatically driven sound emitter is described, this could also be driven electrically or by other means or with other energy supplies and with a range of frequencies.

Claims

1. A drying apparatus for drying a fibrous or granular material comprising, a heater arranged to heat a first gas stream and a fibrous material, the first gas stream arranged to enter the drying apparatus at an inlet, the gas stream including steam,

a separator for separating a said fibrous material from the first gas stream to form a second gas stream,

a fan and a duct arranged to recirculate the second gas stream to the process heater and steam injection apparatus, and

a flow variation apparatus arranged to vary the flow of gas in a first gas stream.

2. A drying apparatus according to claim 1 , wherein the flow variation apparatus is arranged to vary the flow in a cyclical manner.

3. A drying apparatus according to claim 1 or claim 2, wherein the flow variation apparatus is arranged to pulse the flow of process gas and act to enhance heat transfer.

4. A drying apparatus according to claim 1 , claim 2 or claim 3, wherein the flow variation apparatus is arranged to vary the flow in the range of frequencies from 0.3 Hz to 200Hz.

5. A drying apparatus according to any preceding claim, wherein the flow variation apparatus is arranged to vary the flow in the range of frequencies from 0.3 Hz to 5Hz.

6. A drying apparatus according to any preceding claim, wherein the flow variation apparatus comprises a mechanical device.

7. A drying apparatus according to claim 6, wherein the first gas stream and fibrous material has a direction of process flow and wherein the flow variation apparatus comprises a rotatable disk or chopper having an axis of rotation perpendicular to the direction of process flow.

8. A drying apparatus according to claim 7, wherein the disk comprises at least one opening for the first gas stream process flow.

9. A drying apparatus according to claim 7 or claim 8, wherein the disk comprises a plurality of openings for the first gas stream process flow.

10. A drying apparatus according to claim 6, the first gas stream and fibrous material has a direction of process flow and wherein the flow variation apparatus comprises a rotatable damper plate having an axis of rotation perpendicular to the direction of process flow.

1 1 . A drying apparatus according to claim 6, wherein the flow variation apparatus comprises a pneumatically driven sound emitter.

12. A drying apparatus according to claim 6, wherein the flow variation apparatus comprises an electrically driven sound emitter.

13. A drying apparatus according to any preceding claim, wherein the flow variation apparatus acts to vary the flow of gas in a first gas stream with an amplitude of gas velocity variation from a mean velocity in the gas stream in the range of from 0 to 20m/second.

14. A method of drying a fibrous material using apparatus according to any preceding claim.

15. A drying apparatus substantially as herein described with reference to the accompanying drawings.