Spray drying apparatus

Abstract: A nozzle unit comprises a pressure nozzle (6) for spraying feed liquid and a cylindrical outer tube (3) disposed around said pressure nozzle for high-speed gas blowing. The tip (4) of the nozzle unit is of converging construction. When water is sprayed at low pressure in the nozzle unit, water is atomized to fine droplets.

Description: Field of the Invention The present invention relates to a spray nozzle unit which can function satisfactory even when low pressure is applied during a period of start-up, and a spray drying apparatus equipped with the nozzle unit. Background of the Invention In general, it is necessary that temperatures in a spray drying chamber should be stabilized in a start-up period to avoid product overheating and to provide thermal protection on equipment downstream of the spray drying chamber. Therefore, water is usually injected through the same pressure nozzle as used for a feed liquid. The rate of water to be sprayed must be equivalent to the water content of the feed liquid, which is usually in the range of 30 - 80 % by weight. Thus, the rate of water to be sprayed is also 30 - 80 % by weight of that of the feed liquid. Because of pressure nozzle characteristics, the spray nozzle pressure decreases to 10 - 80 %, depending on the liquid viscosity when this low rate of water is fed. Water droplets thus produced are likely to be so coarse that they may adhere to the surface inside the drying chamber. Subsequent spraying of the feed liquid causes dried powder to adhere to the wet surface to form deposits. According to the prior art, devices as shown in Figs. 6 and 7 are used to cope with this situation. In Fig. 6, a plurality of spray nozzles 10 are disposed at the top of the inside of the spray drying chamber. When water is injected for spraying, the number of spray nozzles used is limited to avoid low pressure spraying. On the other hand, Fig. 7 illustrates an example in which a water spray nozzle 11 is disposed separately from a feed liquid spray nozzle 12. However, the device of Fig. 6 has disadvantages of uneven liquid droplet dispersion and ununiform temperature distribution because of a longer distance between the nozzles and the very existence of unused spray nozzles. In addition, plugging problems are likely to occur around the nozzles left unused when water is injected. On the other hand, the device of Fig. 7 has disadvantage of feed liquid clogging inside the feed liquid spray nozzle since no cooling or flushing cannot be conducted through the feed nozzle. FR-A 938 920 shows an apparatus for spray drying a liquid, comprising a drying chamber which has in its upper portion a nozzle unit. The nozzle unit of this prior art apparatus is constructed of two substantial component parts, the first of them being an inner feed liquid nozzle and the second being an outer air conducting unit. Both of the above described component parts are fed with either liquid from a liquid conduit 6 or air from a pressure air conduit 8. The air and feed liquid arrive at the upper portions of the respective outer or inner unit and are fed thereto tangentially so that the air and the liquid are swirled in counter-current flow in their respective nozzle parts. While the air stream is supplied to the outer nozzle component part under pressure, the liquid stream of this prior art nozzle is not pressurized. When the compressed swirled air arrives at the outlet 12 of the outer component part 11 of the nozzle, it is output there aspirating the unpressurized liquid leaving the inner nozzle at its outlet 10. The outlets 10, 12 of the nozzle unit are located at lower converging end portions of those components parts 9, 11. The apparatus as described above has, however, certain disadvantages. The main disadvantage is that by feeding the inner nozzle with a liquid which is not pressurized and leaves the nozzle at is outlet only because it is aspirated by the surrounding air flow, only small flow rates can be realized. Another disadvantage of the above described nozzle unit is that resulting from its specific construction the outer air conducting component part covers only a portion of the inner liquid conducting nozzle part. As in most cases the liquid fed to the inner component part is hot said component part becomes heated by the liquid. Thus temperature regulation of the inner part is achieved by the air passing over the circumference of the inner nozzle. The part of the inner nozzle that is not covered by the outer component part is, however, not regulated in temperature the way the inner nozzle is. Additionally the tangentially injected air and liquid stream may lose a substantial part of its energy by friction and alteration of direction. A final disadvantage of this prior art apparatus lies in the flow of the gas supplied for heating the drying chamber 1. As there is no gas outlet provided in the lower portion of said drying chamber, this heating gas flow is not capable of contacting uniformly the already dried media disposed in the lower portion of the chamber. It is the object of the invention to propose a spray drying apparatus which overcomes the disadvantages of the prior art solution. This object is attained by the features defined in the characterizing portion of the single patent claim. According to the present invention, a gas is blown at high speed through the annulus formed between the pressure nozzle and the cylindrical outer tube so as to atomize water into very fine droplets even when only low pressure, which would otherwise produce coarse droplets, is applied in the pressure nozzle. Therefore, complete drying is carried out, and no water droplets adhere to the inside wall of the spray drying apparatus. Brief Description of the drawings Fig. 1 illustrates a preferred embodiment of the nozzle unit of the present invention. Fig. 2 shows a partially cross sectional view of the end portion of the nozzle unit illustrated in Fig. 1. Figs. 3 (a), (b) and (c) depict an example of a slit used for the nozzle unit of the invention. Figs. 3 (a), (b) and (c) are a top plan view, a bottom plan view, and a side view, respectively. Fig. 5 is a schematic illustration of an embodiment of the spray drying apparatus equipped with the spray nozzle unit of the present invention. Figs. 6 and 7 show conventional nozzle units. Detailed Description of the Invention In the following examples are described preferred embodiments to illustrate the present invention with particular reference to the drawings. However, it is to be understood that the invention is not intended to be limited to the specific embodiments. Fig. 1 illustrates a preferred embodiment of the nozzle unit of the present invention. Fig. 2 shows a partially cross-sectional view of the end portion of the nozzle unit illustrated in Fig. 1. Referring now to the drawings, there are shown a feed liquid (or water) pump 1, a Roots blower 2, a jacket pipe 3, an air nozzle 4, a feed liquid (or water) pipe 5, and a pressure nozzle 6 for discharging feed liquid (or water). The jacket pipe 3 is disposed around the feed liquid (or water) pipe 5. The end portions of the pressure nozzle 6 and the air nozzle 4 are of converging construction or so shaped that their diameters diminish as they near the tip of the nozzle as shown in Figs. 1 and 2. For the purpose of increasing spray angles of feed liquid or water, it is preferable that a gas slit 7 is provided between the pressure nozzle 6 and the air nozzle 4 or on the outer area of the pressure nozzle 6 to give swirling motion to the discharging air stream as shown in Figs. 3 (a), (b) and (c) which are a top plan view, a bottom plan view, and a side view, respectively. The spraying pressure of feed liquid or water required for the pressure nozzle 6 is appropriately determined using the following Equation I and Equation II. The former is the general equation expressing flow characteristics of a pressure nozzle while the latter expresses droplet diameters for a specific pressure nozzle used, which is an SX nozzle manufactured by Spraying Co. for one preferred embodiment of this invention. W = K ₁ · D ² · P 0.6 where W is flow rate (kg/h), K ₁ is coefficient, D is orifice diameter (mm), and P is pressure (kg/cm²). D p = K₂ · W -0.44 · µ 0.16 · D 1.52 where W is flow rate (kg/h), D p is liquid droplet diameter (µm), K₂ is coefficient, and µ is liquid viscosity (cp). The air nozzle 4 disposed around the pressure nozzle 6 has an air velocity of 80 m/s or higher, preferably 100 m/s or higher, and generally has an air pressure of 0.1 kg/cm² or higher, preferably 0.2 kg/cm² or higher, but both air velocities and air pressures are not limited to these values. Other values beyond the above ranges may be used depending on the construction of the nozzle used. Referring now to Fig. 5 which is a schematic illustration of an embodiment of the spray drying apparatus equipped with the spray nozzle unit of the present invention, it will be described how the spray drying apparatus is operated. For start-up, a feed liquid pump 1 discharges water via a feed liquid pipe 5 to a pressure nozzle 6 for spraying. Spraying of water is carried out at significantly low pressure. However, since air is blown off at high speed around the pressure nozzle 6 and swirling motion is formed in the air stream, preferably with the use of a slit 7, water is atomized into fine droplets of the desired particle size even at low pressure. With fine water droplets of the desired particle size, every water droplet is dried with hot air as referred to as A which is blown off into a drying chamber 8 of the spray drying apparatus. Thus, no undried water is present in the drying chamber 8 and almost no temperature distribution is found in the chamber. In other words, the temperature in the drying chamber 8 is maintained to be constant. Then, a feed liquid to meet a specific objective is blown off into the drying chamber 8 of the spray drying apparatus via the pressure nozzle 6 of the above nozzle unit and is dried by a hot air blown off via an inlet 9 to obtain a powder product of specified grade.13 is an outlet for exhaust gas. When the feed liquid is actually sprayed using this nozzle unit, it is desirable to let a little air flow through the air nozzle 4 because the air can cool the nozzle unit for preventing feed liquid plugging. Now the present invention will be described in detail in connection with the following examples: Example 1 Atomization tests for a feed liquid and water were made under the conditions shown in Table 1. An SX nozzle having a hexagonal cross-section manufactured by Spraying Co. was used for a pressure nozzle 6. The circumference of the nozzle tip was covered with a cylindrical pipe having a circular cross-section to obtain an annular space used for an air nozzle 4. The distance between the SX nozzle and the cylindrical pipe were about 5 mm at their widest site, and about 3 mm at their closest site. The inner diameter of the cylindrical pipe was 7 mm at its tip. The results of these tests are shown in Table 1 below. Conventional Nozzle Nozzle of This Invention Conventional Nozzle Feed Liquid Poly vinylchloride(PVC) Water Water Orifice Dia./Core(mm) 0.787/425 0.787/425 0.787/425 Spray Pressure(kg/cm²) 23 6 6 Feed Rate (kg/h) 50 30 30 Liquid Viscosity (cp) 110 Solids Content (%) 40 Air Pressure (kg/cm²) 0.26 Air Flow Rate (kg/h) 20.5 Air Blow Speed (m/s) 127.1 Inlet Temp. (°C) 102 102 102 Outlet Temp. (°C) 55 55 58 Particle Size (µm) 91 40 120 Spray Angle (deg.) about 15 about 60 Dryness Good No Wet Material adhered Good No Wet Material adhered Poor Wet Material adhered to dry chamber cone section The pressure nozzle used was an SX nozzle manufactured by Spraying Co. In addition, the test was made for the case in which swirling motion was provided in the high-speed air stream in the nozzle unit of the present invention. The results are shown below. Nozzle According to This Invention Average Droplet Diameter (µm) 40 Spray Angle (Deg.) Approx. 30 Example 2 The same nozzle as described in Example 1 was used, but nozzle diameters were changed to obtain particles of larger sizes. A large drying chamber was used in this example. The results of these tests are shown in Table 2 below. Conventional Nozzle Nozzle of This Invention Conventional Nozzle Feed Liquid Poly vinylchloride(PVC) Water Water Orifice Dia./Core(mm) 1.067/425 1.067/425 1.067/425 Spray Pressure(kg/cm²) 7 2 2 Feed Rate (kg/h) 50 30 30 Liquid Viscosity (cp) 110 Solids Content (%) 40 Air Pressure (kg/cm²) 0.26 Air Flow Rate (kg/h) 20.5 Air Blow Speed (m/s) 127.1 Inlet Temp. (°C) 102 102 102 Outlet Temp. (°C) 55 55 65 Particle Size (µm) 150 60 640 Abnormal spraying Spray Angle (deg.) about 15 about 60 Dryness Good Good No Wet Material adhered Poor Wet Material adhered to dry chamber cone section As clearly seen from the above results, water is atomized to give fine water droplets with the nozzle according to the present invention. Dryness in the spray drying apparatus is improved since spray angles increase when swirling motion is provided in air streams discharging from nozzles. The effects of the present invention are listed in the following: 1) Even when water is sprayed at low pressure in the spray nozzle of the present invention, water is atomized to fine droplets which is then completely evaporated. 2) Feed liquid plugging can be prevented by the cooling of the spray nozzle unit, and 3) in a spray drying apparatus equipped with this spray nozzle unit, the spraying of a feed liquid can be performed effectively and stably even after water is sprayed at low pressure.

Claims: A spray drying apparatus, comprising: a) a drying chamber (8) b) a nozzle unit disposed at an upper end of said drying chamber (8) and comprising: b1) a liquid feed conduit (5); b2) a pressure nozzle (6) connected to an end of said liquid feed conduit; and b3) a tubular member (3) disposed about said liquid feed conduit and said pressure nozzle so as to form a volume therebetween and including an air nozzle (4) of converging construction at the end of the liquid feed conduit, which sprays a gas stream from said volume; c) means (7) for inducing a swirling motion in the gas stream; and d) a hot air inlet (9) disposed at said upper end of the drying chamber, characterised by , e) a liquid feed pump (1) connected to said liquid feed conduit (5) to supply said pressure nozzle with liquid; f) said means for inducing a swirling motion in the gas stream being slits (7) provided on the outside of the pressure nozzle (6); and g) a gas outlet (13) disposed at a lower portion of said drying chamber (8) for removing exhaust gas therefrom.

Inventors: AISHIMA SHIZUO, KODERA AKIRA, TANAKA TOSHIYUKI, AISHIMA, SHIZUO, KODERA, AKIRA, TANAKA, TOSHIYUKI
Applicants: OGAWARA KAKOKI KK, OHKAWARA KAKOHKI CO., LTD.
Application year: 1989
Main classification: B05B7
IPC codes: B05B7, B01D1, B05D3