WO1998013134A1

WO1998013134A1 - Method and apparatus for generating an aerosol

Info

Publication number: WO1998013134A1
Application number: PCT/SE1997/001581
Authority: WO
Inventors: Darko Marijan
Original assignee: Darko Marijan
Priority date: 1996-09-24
Filing date: 1997-09-19
Publication date: 1998-04-02
Also published as: SE9603488D0; SE507132C2; SE9603488L; AU4477497A; EP0942779A1

Abstract

A method and apparatus for partially fluidizing an enclosed fluidized bed by means of a fluidizing fluid flow that is directed onto a bottom (LB) of the bed (B) from above. The partially fluidized bed is transported out of the enclosure (10) as an aerosol, by means of the fluid flow.

Description

METHOD AND APPARATUS FOR GENERATING AN AEROSOL

The present invention relates to a method and to apparatus for generating an aerosol from a fluidized bed in an enclosure.

When generating an aerosol of this kind, the aerosol is normally used as a medium for transporting material in a fluid state to a point at which the material shall be treated in different ways, e.g. packaged or mixed with other materials .

In the case of typical aerosol generators, the solid particles to be fluidized are placed in the form of a bed on a fluid-permeable grate in a vertical column (FIG. 1) and pressurised fluid is caused to flow through the material from beneath and therewith set the solid particles in motion.

The fluid looses energy as it passes through the channels and passageways formed between the particles. The pressure drop in a stationary bed of solid particles can be calculated with Ergun's equation. If the rate at which the fluid moves is constantly increased, there is finally reached a point at which the particles no longer remain stationary and fluidized under the effect of the fluid.

If the fluid flow velocity is allowed to increase uniformly, its pressure drop will also increase as it moves through the bed. The drop in pressure will finally be as great as and exceed the resultant force acting on the particles as a result of gravity and net lift in the fluid, whereupon the particles will begin to move. This takes place at point A in FIG. 2 (see also the column in FIG. 2 representing bed volume). The bed first expands to an insignificant extent, with the particles still in contact with one another. The porosity ε then increases and the drop in pressure increases at a much slower rate than was earlier the case. The bed is in its loosest state at point B, with the particles still in contact with one another. As the speed further increases, the particles begin to separate from one another, wherewith a state of genuine fluidization is introduced. The drop in pressure sometimes decreases to an insignificant extent from point B to point F. The particles progressively move more violently and in random directions from point F and onwards. In the case of a conventional fluidized bed, the linear velocity of the fluid between the particles is much higher than the velocity of the fluid in the space above the bed. Even with violent fluidization only the smallest particles (particles with the highest fluidizing capacity) are entrained by the fluid and carried away.

When the speed of the fluid is further increased, the porosity t: of the bed will also increase and the bed therewith expand and its density decrease. The extent to which the particles are entrained does^not become noticeable until point P has been transgressed, whereafter entrainment is considerable and finally complete. The porosity obtains the value 1 at the point Q, wherewith the bed ceases to exist and obtains a state with simultaneous flow in two phases. The pressure drop is almost constant from point F to point P.

The particles at the bottom of the column require more energy to achieve the fluidized state than particles located on the upper side of the solid bed. This is the reason why it has not hitherto been possible to fluidize a bed partially or to only a limited extent. In conventional fluidizing processes, the bed can thus either be fluidized completely or not at all.

When producing aerosols with the aid of a conventional fluidizing method according to the aforegoing, the fluid will move at a speed which is equal to or greater than the speed at which permanent entrainment of the particles is achieved. The particles introduced into the fluidizing column with the aid of suitable means will therefore be fluidized immediately and carried away. When the fluid is a gas, there is formed an aerosol which consists of gas and solid particles, the mass flow of the solid particles being determined by the mass flow input of the solid particles in the column.

Consequently, when the aforedescribed method is used, for instance, as an aerosol generating method for transportation of solid particles, the aerosol flow exiting from the column can only be controlled by the supply of solid particle material, since this material is fluidized immediately upon entry into the column.

An object of the present invention is to provide a method and an apparatus of the kind described in the introduction which will enable a fluidizable bed to be fluidized partially or to a limited extent with lower energy consumption and more accurate control of an aerosol flow from the bed enclosure.

This object is achieved with a method and apparatus having the features set forth in the following Claims. Thus, when fluid is introduced into the enclosure with a downwardly directed flow component at a level above the enclosed bed of solid particles in accordance with the present invention, solely those particles that are located in a layer on the upper side of the otherwise solid bed will be fluidized to an extent that is dependent on the energy of the downwardly acting flow above a threshold value. This downwardly acting flow need not, of course, be vertically directed, but may define an angle of between 0 and 180° with a horizontal bottom of the enclosure.

Other features of the invention and advantages afforded thereby will be evident from the Claims and from the following detailed description.

An exemplifying embodiment of the invention will now be described in more detail with reference to the accompanying drawings, in which FIG. 1 is a schematic illustration of a known aerosol generator; FIG. 2 is a diagram illustrating fluid pressure drop on the one hand and the bed porosity and volume as a function of the fluid flow rate during fluidizing of a bed of solid particles with the aid of an aerosol generator according to FIG. 1 on the '-ether hand; FIGS. 3A-C are respective sectional views of a laboratory arrangement according to the invention during different stages of partially fluidizing a bed of solid particles, with parts of the arrangement being partially broken away; FIG. 4 is a diagram that shows the mass flow of solid particles from an arrangement according to FIG. 3 as a function of the time at varying inlet flows; FIG. 5 is a part sectional view of an arrangement corresponding to Fig. 3, where the vehicle fluid inlet to the enclosure opens out at out at a level beneath the upper side of the bed of solid particles; and FIG. 6 illustrates schematically an inventive aerosol generator which includes a system for controlling or regulating the exiting aerosol flow.

Shown in FIG. 3A is an enclosure in the form of an E-flask 10 which is closed with a stopper 12 through which there extends an inlet conduit 16 that includes a pair of inlet pipes which are connected commonly to a source of pressurised fluid, in this case air, via lines external of the flask 10, and an outlet conduit 18 in the form of an angled pipe through which aerosol is discharged from the enclosure. The flask 10 is filled partially with a solid particle material, which forms a bed B on the bottom of the flask.

In this example, the pressurised fluid, hereinafter referred to alternately as the vehicle fluid as a result of its particle transporting ability, is introduced into the enclosure through the ends of the inlet pipes at a level LA above the bed B, although in certain instances, e.g. for fluidizing a bed of agglomerated material, the fluid may alternatively be introduced at a level beneath the level LT of the upper surface of the bed, as illtietrated in FIG. 5.

At very low rates of fluid inflow qi, the fluid causes no motion in the particles in the bed B. When the energy supplied is less than the lowest energy required to fluidize particles in the illustrated system, the vehicle fluid will pass through the enclosure without affecting the bed of solid particles. On the other hand, however, when the vehicle fluid has a sufficiently high energy, the whole of the bed of solid particles can be fluidized instantaneously, wherewith the system in this respect is equivalent to the aforedescribed known system. However, the bed will be fluidized only partially at each other energy level between these two points .

When the low flow rate is slowly increased, the energy level required to fluidize the particles on the upper side of the bed B is reached, whereby a primary aerosol A will be formed in the space above the bed. By fluidizing the particles at the upper side of the bed B, the energy of the vehicle fluid will be lowered to the level at which it enters the system and leaves the system, together with the particles present in the primary aerosol A. When a determined amount of the primary aerosol A has left the system through the outlet conduit 18, the energy in the vehicle fluid will increase momentarily by the amount required to hold together the number of particles that were earlier in a fluidized state in the primary aerosol. When the flow rate is held at a constant level, this process will continue for as long as no fluidized particles are present in the system. The concentration of the solid phase is constant in the vehicle fluid during the process, as is also the mass flow of solid particles in the outfeed qo . The energy of the vehicle *luid is no longer used to bring particles to a fluidized state, when the last particle in the bed of solid particles has been fluidized. The mass flow rate increases momentarily and then ceases as a result of the reduction in the concentration of solid particles in the primary aerosol.

This can be expressed in other words by saying that the vehicle fluid energy available for limited fluidization of the particles varies in a given system. The total energy of a vehicle fluid is determined by the inlet pressure and will not change during the limited fluidizing process when the inlet pressure is constant. Distribution of the total energy of the vehicle fluid is changed during the process, meaning that the more energy that is consumed in fluidizing the bed, the less energy that will available for transporting the vehicle fluid and fluidized particles out of the system. This can be described by the relationship:

Total energy = limited fluidization energy + transport energy.

The aforedescribed system can be considered as being analogous with a liquid-vapour system in a closed vessel. The partial pressure of the vapour above the liquid phase in a closed system is a function solely of temperature. Should the concentration (partial pressure) of the gas phase decrease in some way, the molecules will leave the liquid phase from its upper surface and keep the partial pressure constant if the temperature is constant.

Aerosol generators can be formed for different purposes, by varying the distribution of the totaV energy of a vehicle fluid, e.g. by varying the flow rate in different ways, including the choice of outflow nozzles of different sizes and shapes and different nozzle distances from the bed.

The aforedescribed laboratory arrangement was tested by charging an aerosol generator according to FIG. 3 with 0.24 kg of A1₂0₃ in powder form. At an airflow into the vessel 10 of slightly above about 5^'10^~4 m³/3 (30 1/min.) through four inlet pipes having an inner diameter of 3 mm, the vessel was emptied in 100 min., corresponding to a particle mass flow qs of slightly more than 3.33^'10^"5 kg/s (2 g/min.). The concave surface' (c.f. FIG. 3B) of the bed B was formed at the beginning of the process and causes minimum resistance to fluid flow. The mass velocity increased significantly (c.f. FIG. 4) during this period, which never exceeded 5 min. (typically about 2.5 min.). This period, however, is extremely short in comparison with conventional aerosol generators, whose calibrating period may be as long as one hour. The mass flow remains constant even when the bottom of the vessel 10 was exposed (FIG. 3C), since the flow of vehicle fluid is then reflected by the smooth glass surface and deflected to the remaining bed of solid particles, which is the principle absorbent of the energy available in the vehicle fluid. At the end of the process, the particle mass flow increased for a period of about 0.5 min. and then stopped rapidly with a residue of about 3% of the original material in the vessel. This residue comprised particles of very low fluidizing ability, i.e. agglomerates or impurities. The inventive method of limiting fluidization thus also solves the problem of avoiding the presence of agglomerates in the exiting aerosol, because agglomerates do not allow themselves to be fluidized by the limited energy of the vehicle fluid.

The described aerosol generator based on restricted fluidization exhibited calibration times within two seconds (the time required to achieve 99% of the nominal mass flow after changing the flow rate of the vehicle fluid). This enables the aerosol generator to be regulated very easily, by restoring regulation via sensing with the aid of flow rate sensing or weighing.

FIG. 6 is a schematic illustration of an inventive aerosol generator provided with a regulating system which is constructed to control the exiting aerosol flow qo in a relatively simple fashion. Similar to that described with reference to FIG. 2 above, a bed B of solid particle material is provided in the enclosure 10. This material is to be fluidized with the aid of a vehicle fluid to form an aerosol A which is transported by the fluid to a consumer point (not shown) such as a mixing or packaging station. Many other applications are possible, however. For instance, a flow regulated device in accordance with the invention can be used to meter a predetermined quantity of airborne particles into a ventilation system with a high degree of accuracy, to enable the particle concentration to be later measured at different measuring points in the system and therewith obtain information relating to the flow through the system and therewith evaluate the efficiency and effectiveness of the system.

In the case of the FIG. 6 embodiment, the vehicle fluid, e.g. air, is fed into the enclosure 10 by means of a compressor 14 having a variable, adjustable displacement. The flow of vehicle fluid may, however, be adjusted via other means (not shown), such as a pressure limiting valve. The vehicle fluid is introduced into the enclosure 10 in the earlier described manner at the flow rate qi, via a plurality of inlet openings in the enclosure 10 and comes into contact with the bed B, where the downwardly acting vehicle fluid flow is reflected at the surface of the solid bed B and fluidizes particles at this surface, these particles being entrained by the reflected flow and form the fluidized part of a partially fluidized bed, or form an aerosol A above the bed. The aerosol A is discharged through the outlet line 18 for transportation to said consumer point.

The outlet line 18 incorporates a sensor 20 which senses the outflow qo. Although the sensor 20 used to achieve desired adjustment of the outflow may be any type of flow sensor, the sensor 20 of the illustrated embodiment is an optical reflection sensor which continuously delivers a signal a in accordance with the percentage of light that is reflected from particles in the aerosol flow. Thus, in this case, the signal a is a measurement of the quantity of particles that pass a section of the line 18 at each point in time. The signal a is then compared with a preset control value or reference value r in a comparator 22, to form a difference value therebetween. The difference value is then delivered to an appropriate converter or amplifier 24, such as a PID regulator, from which a signal is- sent for adjusting displacement of the compressor 14. This thus enables the mass outflow of solid particles from the partially fluidized bed AB to be controlled accurately in a manner which is simple from a regulating/technical aspect, solely by varying the vehicle fluid inflow qi.

It will be immediately apparent to one skilled in this art that the invention can be modified in many ways within the scope of the following Claims. For instance, the flow of fluidizing fluid may alternatively be obtained by a fan mounted inside the enclosure.

Claims

1. A method of generating an aerosol from a fluidizable bed in an enclosure, characterized by directing a fluid flow (qi) downwardly onto an upper side (LT) of the enclosed bed, such that part of the bed on the upper side thereof will be fluidized and form the aerosol (A) in the enclosure above a residual part (B) of the bed.

2. A method according to Claim 1, characterized by causing the fluid to flow into the enclosure from outside said enclosure; and by causing an aerosol flow (qo) of the aerosol (A) to flow out of the enclosure in accordance with the fluid (qi) flowing into the enclosure from outside said enclosure.

3. A method according to Claim 2, characterized by varying the shape, size and position of a fluid flow inlet nozzle and therewith vary the aerosol flow (qo).

4. A method according to Claim 2 or 3, characterized by varying the fluid flow (qi) on the basis of sensing the aerosol flow (qo), therewith to vary the aerosol flow (qo) in a regulatory/technical fashion.

5. A method according to any one of Claims 2-4, characterized in that the aerosol flow (qo) is varied in a regulatory/technical manner, by varying the fluid flow (qi) on the basis of sensing the concentration of material leaving the bed (B) in the aerosol flow (qo).

6. Apparatus for carrying out the method according to Claim 1, comprising an enclosure (10) that encloses a fluidized bed (B), and means for generating a fluidizing airflow in the enclosure, characterized in that said means are arranged to direct the fluid flow onto an upper side (LT) of the bed (B) from above, such as to partially fluidize the bed at its upper side (LT).

7. Apparatus according to Claim 6, characterized in that said means for generating a fluid flow in the enclosure (10) include a pressurised fluid source (14) having at least one inlet (16) in the enclosure; and in that the enclosure (10) has at least one outlet (18) for an aerosol flow containing bed material fluidized by the pressurised fluid.

v 8. Apparatus according to Claim 6 or 7 , characterized by a regulating system which includes a flow sensor (20) that functions to sense an actual aerosol flow (qo) at the outlet and deliver signals to regulating means (14, 22, 24) which regulate the fluid flow (qi) in accordance with the actual aerosol flow, such as to obtain an adjusted aerosol flow at said outlet.

9. Apparatus according to Claim 8, characterized in that the flow sensor (20) is an optical mass flow sensor.

10. Apparatus according to Claim 9, characterized in that the mass flow sensor is a particle sensor.