Patent Application
20120104111
The invention relates to the field of drying fluids.
Drying a fluid means that droplets of the fluid are dried to become particles. In the remainder, the terms particles and droplets generally identify the same entity, depending on a drying stage. Drying of fluids is e.g. applied in the food industry for drying a nutrient or an ingredient therefore, like dairy, proteins, carbon hydrates, fats, or combinations thereof. Spray drying may further be applied for drying fluids such as, but not limited to, detergents, pigments, catalysts, pharmaceuticals, cosmetics, polymeric resins, ceramic powders, powder coating materials, adhesives, gypsum, cement, metal powders, etc.
In a conventional type of spray drying, the fluid is fed to a nozzle, which produces a mist of droplets of the fluid in a vessel. The droplets are subsequently dried, e.g. in a stream of air in a drying tower. A disadvantage of this method is that it takes a relatively large amount of energy for drying the droplets to become particles.
It is an object of the present invention to provide a system and method for reducing the amount of energy required for spray drying.
In one aspect, the invention provides a system for spray drying a fluid, the system comprising a fluid reservoir and a spraying device. The spraying device comprises at least one outflow opening for projecting droplets of fluid from the reservoir out of the at least one outflow opening and at least one energy source for at least partially drying the droplets. The spraying device is arranged to project the droplets into a determinable droplet trajectory and the at least one energy source is arranged for providing energy focused substantially on the droplet trajectory. By focusing, embodiments are meant to be encompassed wherein the energy is confined in a limited space around the droplet trajectory for example, by a heated gas flow guide. By a ‘determinable trajectory’ it is meant that a trajectory of individual droplets is largely determined by predefined course, for example a jet trajectory of jetted droplets, or a flow trajectory of a carrier flow.
In another aspect, the invention provides a method for spray drying a fluid. The method comprises the steps of projecting droplets of fluid from a reservoir out of at least one outflow opening and at least partially drying the droplets with energy provided by at least one energy source. The droplets are projected into a determinable droplet trajectory and the energy is focused substantially on the droplet trajectory.
In the embodiment shown in
To provide a determinable droplet trajectory, the embodiment of the spraying device 200 illustrated in
In another embodiment of the spraying device according to the present invention 201, shown in
In another embodiment, the pump may be arranged so as to keep the flow rate constant, which results in a certain pressure. An example of such a pump is a constant flow pump.
Another embodiment of the system for spray drying a fluid 300 is shown in
In the embodiment illustrated in
If the spraying device 3 comprises a plurality of outflow openings 4, a single control element 11 may be used for varying the pressure of substantially all fluid jets generated by the plurality of outflow openings 4.
In another embodiment similar to the embodiment shown in
When the fluid is substantially homogeneous, substantially equal sized droplets will have substantially equal mass.
This is only one possible way of providing monodisperse droplets, other ways may be possible.
In case of a plurality of outflow openings 4, the outflow openings 4 may have substantially identical dimensions, to allow droplets of substantially identical dimensions to be ejected from each separate outflow opening 4, hence allowing the droplets to be produced with the relatively narrow size distribution.
An advantage of providing monodisperse droplets may be that the amount of drying energy can be related to the size of the droplets and the desired degree of drying. Because the droplets have substantially the same size, the drying result of the droplets will be substantially the same. This may reduce the amount of energy required for drying the droplets because it is no longer necessary to choose the amount of energy such that even the largest droplets will reach the preferred degree of drying. Also, because the droplets substantially have the same size, formation of particles may be prevented that are smaller than the desired droplet size. Smaller droplets could become too dry and even burn. Although the burning of droplets could be useful in some situations, it is typically unwanted. In contrast with conventional atomizing systems, so called fines (ultra fine particles) are not formed, which is beneficial for efficient reuse of energy since fines can cause problems and pollution in heat exchanger systems.
Preferably, the amount of energy will be such that the droplets will completely and efficiently dry without forming a hard crust around the surface of the droplet. Such a crust may prevent water in the inside of the droplet to diffuse to the surface and may therefore disturb the drying process. There may however also be situations in which it is desired to only partially dry the droplets. In yet other circumstances, it may be preferred to provide an amount of energy such that pyrolysis of droplets comprising an organometallic compound occurs. This may e.g. be useful in the field of droplet printing electrical components like insulators and semiconductors on substrates.
The present invention wherein monodisperse droplets are dried over a predeterminable trajectory further enables to optimize the drying process by selectively providing energy along the trajectory. This means that the amount of energy supplied to the droplet may vary depending on the position of the droplet in the trajectory and therefore depending on the drying degree of the droplet. Examples of energy sources arranged for selectively providing energy along the droplet trajectory are shown in
Coagulation of droplets may be harmful for a narrow droplet distribution. Several precautions may be taken to prevent coagulation.
First, the predetermined pressure varying frequency of the pressure varying means 10 may be chosen such that a distance between two droplets consecutively ejected at the outflow opening 4 is greater than or equal to two times the droplet diameter, preferably greater than or equal to three times the droplet diameter.
Second, in case of a plurality of outflow openings 4, the nozzles 4 may be arranged for generating a plurality of mutually divergent jets of the fluid. When the jets are mutually divergent, the risk of coagulation of droplets ejected by mutually different nozzles 4 is reduced, since the distance between the droplets ejected by the mutually different nozzles 4 increases during flight.
Third, the distance between two adjacent nozzles 4 of the plurality of nozzles 4 may be larger than 1.5 times a transverse dimension, such as a diameter, of at least one of two adjacent nozzles 4, more preferably larger than 2 times, most preferably larger than or equal to 2.5 times the transverse dimension. This also reduces the risk of coagulation of droplets.
Also, referring to the
The system for spray drying a fluid according to the present invention may be used with a variety of droplet generating systems. The droplets may e.g. be generated continuously or non-continuously, e.g. on a drop-on-demand basis. Further, droplets of low viscosity fluids may be generated, e.g. droplets of saline solutions with a viscosity of 2 mPa·s and also high viscosity droplets may be generated, e.g. with a viscosity of 250 mPa·s.
Spray drying high viscosity fluids is desired for example in the food processing industry, e.g. for drying milk. In conventional milk drying, the milk may be atomized and dried in a drying tower. Milk comprises a substantial amount of water and removing such an amount of water in a drying tower may not be very energy efficient. Using high viscosity printing systems enables to first extract water from the milk in an energy efficient way and subsequently dry the extracted high viscosity of milk droplets. An exemplary viscosity of extracted high viscosity milk droplets is 250 mPa·s. Drying extracted milk droplets may be much more energy efficient than the conventional milk drying process.
Projecting the droplets into a determinable droplet trajectory and arrange an energy source for providing energy focused substantially on the droplet trajectory, as in the present invention, may even further increases the energy efficiency of the drying process.
To print high viscosity fluids, in another embodiment the spraying device 3 is further arranged to cause a pressure drop in the fluid across the outflow opening 4 which is larger than 15 bar, so as to enable projecting droplets of fluid with a viscosity of at least 100 mPa·s. Projecting droplets of high viscosity fluids enables to spray dray fluids with a larger concentration of dry matter. This means that less fluid has to be removed while drying the droplets. This may increase the energy efficiency.
For printing high viscosity fluids, the pressure drop in the fluid across the outflow opening 4 is preferably between 50 bar and 400 bar and more preferably between 100 and 200 bar.
The viscosity of the fluid is higher than 10 mPa·s, preferably higher than 25 mPa·s, more preferably higher than 50 mPa·s even more preferably higher than 100 mPa·s, and most preferably higher than 200 mPa·s, determined at the temperature which in use prevails in the outflow opening 4. The temperature of the material to be printed is preferably between −50 and 300° C. and more preferably between 40 and 100° C. The shear rate as in use present in the outflow opening 4 is preferably between 1·104 and 1·106 s−4 and more preferably 5·105 s−1, using a capillary viscosity meter.
The smallest transverse dimension, such as a diameter, of the outflow opening 4 may be smaller than or equal to 150 micrometer, preferably smaller than or equal to 100 micrometer, more preferably smaller than or equal to 80 micrometer and most preferably smaller than or equal to 60 micrometer.
The combination of the pressure drop, high viscosity and dimension of the nozzle 4 provide that droplets of the high-viscosity fluid can be produced having a desired size of preferably smaller than or equal to 250 micrometer in average and more preferably smaller than or equal to 100 micrometer in average.
Spray drying a high viscosity fluid desires the feed pressure in the interval of 15-3000 bar and preferably in the interval of 15-600 bar.
Spray drying a high viscosity fluid desires the control element (11) being positioned at a predetermined distance between 2 and 1500 micrometer and preferably between 15 and 500 micrometer to the outflow opening (4). This provides the advantage that a pressure variation exerted to the high-viscosity fluid is prevented from being damped by the high-viscosity fluid to an extent that the jet projected from the outflow opening 4 does not experience a pressure variation with large enough amplitude to effectively break up into droplets.
In an inkjet printing system, the transverse dimension of the droplets, e.g. the diameter depends on the flow rate and the pressure varying frequency. E.g. an outflow opening with a flow rate of 2.4 ml/min and a pressure varying frequency of 40.000 Hz obtains an average droplet diameter (d50) of 124 micrometer. The same flow rate of 2.4 ml/min and a much lower frequency of 500 Hz results in a droplet diameter of 535 micrometer. It is noted that droplet diameter refers to the diameter of the droplet directly after leaving the outflow opening. The dimensions of the dried powder particle may be smaller.
In principle, the droplet diameter does not depend on the diameter of the outflow opening. However, the diameter of the outflow opening may influence the possible flow rates. It might e.g. not be possible to stably push as little as 0.2 ml/min through the same 80 micrometer nozzle.
For a flow rate of 2.4 ml/min, the diameter of the nozzle is preferably 80 micrometer, and a flow rate of 0.6 ml/min is preferable combined with a 30 micrometer nozzle. The latter flow rate of 0.6 ml/min combined with a pressure varying frequency of 40.000 Hz results in droplets with a diameter of 78 micrometer and combined with a frequency of 500 Hz in droplets with a diameter of 337 micrometer.
To further enhance the determinability of the droplet trajectory, the system for spray drying a fluid may comprise a gas flow guide arranged for manipulating a droplet guiding gas stream to aerodynamically position droplets into the predeterminable droplet trajectory. Furthermore, the gas flow guide may form an effective means in itself to provide energy focused substantially on the droplet trajectory. The gas stream may e.g. be a stream of air. If it is desired to prevent oxidation of the droplets of fluid, nitrogen, argon or any other suitable oxidation preventing gas may be used. In some situations, a slight oxidation of the droplets may be helpful. In these cases e.g. a mixture of oxygen and argon could be used.
An example of such a gas guided drying system 400 is shown in
In more detail, the gas flow guide comprises an inlet piece 123 and an outlet piece 124 defining a generic flow direction P and a droplet trajectory T along oppositely arranged upstanding walls 120, 122, the walls defining an elongated space 122 in between the upstanding walls 120, 122 of limited width and having an axial direction aligned with the droplet trajectory T; in use arranged to provide a gradiented laminar gas flow in the gas guide 12 and having a gas flow velocity equal or larger than the particle velocity, thereby defining the particle trajectory T. Here, flow direction P and particle direction T are aligned in the same direction.
The gas stream 13 is preferably laminar and has a parabolic profile due to the tube form of the gas guide 12.
The gas stream has preferably the same velocity as the droplets of fluid 5 or a larger velocity. The droplets follow the stream of gas, but little differences in the size and shape of the droplets may cause them to deviate from their trajectory. Using the gradiented velocity profile, in particular a parabolic profile, of the gas flow, the droplets 5 are stabilized by an aerodynamic lift effect that substantially forces the droplets into a preferred trajectory through the tube 12, for example, along the centre axis of the tube. It is noted that if the droplets 5 have a higher velocity than the gas flow, this effect may be reversed and the droplet 5 will then move away from the centre of the flow, which may be undesirable.
Aerodynamically correcting the droplets may also prevent droplets from being stuck in the spray drying system and thus polluting the system.
Optionally a diffuser 15 is provided for effecting a gradual influx of the gas. Another option is to provide a plurality of gas streams 13′ and 13″ through a plurality of gas supply openings 14′ and 14″ for further stabilizing the system. The gas streams may be conditioned, in particular, dried and/or heated. Outflow openings 140 are provided to divert the outflow gas steams 13, 13′ 13″.
As further shown in
In particular
Such a flow regulating structure may be provided as a constriction 125 accelerating the flow until a Stokes number >1 so that the particles continue their trajectory and no longer follow the air stream. Distance A should be short enough so that the droplets keep a Stokes number >1 so they keep following their own trajectory. Distance B should be taken long enough the give the droplets 5 time to slow down to such an extend that the flow in the tube 12 is larger then the droplet speed, resulting in droplet stabilization. If the distance is too short, the droplets 5 may be destabilized by the airflow causing them to hit the wall of the tube 12. Accordingly, distance B functions as a deceleration structure provided between subsequent segments 12, 12′ to decelerate the droplet prior to receiving the particle in a particle inlet 123′ of a subsequent segment 12′.
It is noted that other deceleration methods may be considered, such as locally increasing a gas pressure or the like; or that a deceleration structure may be dispensed with, as illustrated in the embodiments of
One aspect of the invention is an energy source 126 providing energy focused substantially in the droplet trajectory. According to the embodiment, a heater 126 may be arranged in the walls 120 of the gas guide. It is noted that the gas guide 12 may, through proper dimensioning, have any orientation respective to the gravity direction, but is preferably held substantially horizontal. In addition, the tubes 12 may be curved to provide compact designs, for example, in the form of a coil system. In addition, the tubes 12 may be clustered. It is noted that by (thermal) conditioning of the gas flow the heater 126 may be dispensed with while providing localized evaporation energy to the droplets resulting in droplet drying.
In this embodiment, the energy source 600 is positioned in a first focal point 16 of at least part of an elliptical mirror 17 so as to focus at least part of the radiated energy in a second focal point 18. Focusing the energy may increase the drying efficiency and/or drying speed as described above. In this embodiment, the energy source 600 in the first focal point 16 may e.g. comprise a heater with a glow spiral. However, other energy sources like infrared light sources are also possible.
In conventional spray drying techniques, heated air is applied, e.g. convective or direct to the droplets. Air heating reduces air relative humidity, which is the driving force for drying. Besides, higher temperatures speed up diffusion of water inside the solids, so drying is faster. However, quality considerations limit the applicable rise to air temperature. Too hot air may lead e.g. to crust formation or ‘case hardening’.
When applying radiation driven drying, e.g. dielectric drying, droplets are heated and dried by means of radiofrequency or microwaves being absorbed inside the material. Radiation driven drying may be more efficient and/or faster than air drying techniques. First, the wavelength of the radiation source, e.g. an infrared source may be matched to the absorption characteristics of the material. Other radiation sources are possible, e.g. light sources in another than the infrared spectrum. Second, it may be possible to apply more power to the droplets via radiation than via heat drying without burning the droplets, forming a crust or causing the occurrence of case hardening effects'. Preliminary experiments have shown that droplets may be spray dried eventually over a trajectory between 50 centimetres and one meter. This is very advantageous compared to the big drying towers in use for applying conventional spray drying methods.
This is advantageous because preliminary experimental results show that the energy needed to dry a typical droplet train may be between 0.1 and 10 Watt. The power of an infrared (IR) energy source 6 is typically at least 1000 Watt, which may be too high and burn the droplets instead of drying them with the risk to cause an explosion.
In
The energy source 604 shown in
An alternative way to distribute the energy of a single energy source over a plurality of droplet trains is to position a plurality of outflow openings above a single beam of energy. This may e.g. be done in the configuration shown in
In the above it was discussed that the drying process may be optimized by selectively supplying energy along the droplet trajectory.
Other energy sources combining elements of the energy sources as described above may be used without departing from the broader spirit and scope of the invention as set forth in the appended claims.
An alternative way to provide focused energy to the droplets may be to project the droplets into streams of drying air.
In particular,
The segmented gas guide structure of
The flow provides the effect of an effective balancing height where droplets of various sizes and weights can be balanced to a substantially constant flotation height. A wedge shaped boxed design 115 with converging flow 118 may accelerate the gas flow in the gas guide, and enhance the forming of the planar flow profile 116 in a direction away from the generic flow profile 118. Gas inlet zone 130 may have a shielding part 131 provided with a particle inlet opening. The inlet shield 131 prevents cross flow between two connected segments by providing an inlet flow zone 130 separate from the generic flow 118. This inlet flow zone 130 forms the beginning of the planar flow 116, extending towards an outlet zone shaped with a corresponding outlet zone 132 protruding from the bottom wall 129.
It is noted that although in the above the invention is mainly positioned in the field of spray drying liquids for providing powders, the invention may also be applied in other fields, e.g. the field of droplet printing electrical components like insulators and semiconductors. In this particular field, metal salts are dissolved, preferably in water but also in organic solvents and printed subsequently.
Providing energy along the droplet trajectory causes pyrolysis and melting of an organometallic compound and allows a metal layer to be printed on the surface of a substrate. To print ceramic elements, ceramic suspensions are printed, dried and sintered.
The detailed drawings, specific examples and particular formulations given serve the purpose of illustration only. The embodiments described show vertical, line shaped trajectories. It is however also possible to use other, e.g. horizontal or curved trajectory forms. Furthermore, other substitutions, modifications, changes, and omissions may be made in the design, operating conditions, and arrangement of the exemplary embodiments without departing from the scope of the invention as expressed in the appended claims.
Unless physically impossible, any feature developed in the embodiments is deemed to be disclosed in combination with any other feature of other embodiments, specifically as elaborated in the subsequent claims.
In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word ‘comprising’ does not exclude the presence of other features or steps then those listed in a claim. Furthermore, the words ‘a’ and ‘an’ shall not be construed as limited to ‘only one’, but instead are used to mean ‘at least one’, and do not exclude a plurality. The mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to advantage.
| Number | Date | Country | Kind |
|---|---|---|---|
| 09151348.1 | Jan 2009 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/NL2010/050006 | 1/8/2010 | WO | 00 | 9/26/2011 |
