SPRAY DRYING PROCESS AND APPARATUS THEREFOR

Abstract: The spray drying process and the apparatus there­ for, of the present invention use a spray drying chamber (1) formed by a heat-resistant porous membrane (15). Said porous membrane (15) enables rapid separation of a discharge gas and a produced powder and accordingly total recovery of the produced powder from the spray drying chamber. Therefore, there occurs no nonuniformity in product composition, cleaning and blowing-off of adhered fine powder can be conducted rapidly and easily even in frequent product change inevitable with a less grade larger quantity production system; as a result, a fine powder of higher quality and higher purity can be produced.

Description: The present invention relates to a spray drying process and an apparatus therefor. Spray drying processes and apparatus therefor basically have three essential functions, namely, (1) spraying of a feed liquid to form microdrops, (2) drying of the microdrops to produce a powder and (3) separation and recovery of the powder product (see US-A-1512776, FR-A-441742 and GB-A-382129). These spray drying apparatus are ordinarily equipped with an atomizer, a drying chamber and a fine powder collector to perform each of said functions. Examples of conventional spray drying apparatus are shown in Fig. 9 and Fig. 10. (Reference is made to JP-B-32601/1983 and JP-A-26950/1983). In the spray drying apparatus of Fig. 9, in a drying chamber 1, a feed liquid is sprayed from a nozzle 5 and momentarily heated by hot gas (heated air) blown from a hot gas inlet 2, whereby the liquid component of the feed liquid is evaporated and the solid component is converted to a fine (dried) Powder. Most of the fine powder is taken out as a product through a rotary valve 6, but part of the fine powder is carried by an exhaust gas into a cyclone collector 7 (hereinafter referred to as "a cyclone") through a discharge gas outlet 3 and recovered from a rotary valve 8. The spray drying apparatus of Fig. 10 is same as that of Fig. 9 except that as the atomizer, a rotary spraying disc 10 is used in place of the nozzle 5. In the spray drying apparatus of the type as shown in Fig. 9 and Fig. 10, a fine powder as a product is recovered in two portions from the apparatus main body (drying chamber) and the cyclone, respectively. In this case, product particles having a heavier weight and larger particle diameters are taken out from the apparatus main body (drying chamber) and product particles having a lighter weight and smaller particle diameters from the cyclone. Hence, there occurs a slight nonuniformity in product composition. This nonuniformity in product composition has hitherto presented no serious problem, depending upon the kind of the product. There is now a demand for spray drying systems which produce a variety of products in small quantities, especially in the production of medicines and fine ceramics. With such systems a new problem has arisen in addition to the slight nonuniformity in product composition. This is that the systems require more frequent product changes and each change requires rapid and complete cleaning of solid which has adhered to the inner wall of the drying chamber. Such cleaning is necessary because (1) efficient operation of the systems requires a cleaning time as short as possible and (2) production of high purity and high quality products requires complete cleaning of an adhered solid at each product change. In actuality, complete cleaning is extremely difficult. Moreover, when the product is harmful to human bodies, direct contact with the product must be avoided making the cleaning very difficult. Further, the slight nonuniformity in product composition has become a more serious problem in connection with the increased requirement for higher purity products. The present invention seeks to provide a spray drying process suited for producing a wide variety of products in small quantities, especially fine powder products of high purity, as well as an apparatus therefor. The invention provides in one aspect a process for producing a powder from a feed liquid, which process comprises spraying the feed liquid into a spray drying chamber formed by a single easily-detachable heat-resistant porous membrane having a funnel-shaped lower portion to form microdrops, introducing a hot gas into said chamber to dry said microdrops and form powder particles, withdrawing gas from said chamber through said porous membrane thereby to separate said powder particles from said gas, and removing said separated powder particles from said chamber through said funnel-shaped lower portion. The invention also provides a spray drying apparatus for use in producing a powder from a feed liquid, which apparatus comprises a spray drying chamber having an inlet for hot gas, an outlet for discharge gas, an outlet for produced powder and spray means in said chamber for use in atomising the feed liquid into said hot gas in said drying chamber, characterised in that said chamber is formed by a single easily-detachable heat-resistant porous membrane having a funnel-shaped lower portion, said membrane being pervious to said gas but substantially impervious to said atomized feed liquid. The present invention further provides a spray drying apparatus of integration type characterised by comprising a housing having an upper room and a lower room; in said upper room, two or more units for generation of a hot gas, two or more units for distribution of hot gas, two or more units for atomisation of feed liquid and at least one unit for discharge gas; in said lower room two or more moveably arranged units for recovery of produced powder; and between each feed liquid atomization unit in the upper room and each corresponding powder recovery unit in the lower room one spray drying chamber formed by a single easily-detachable heat-resistant porous membrane having a funnel-shaped lower portion. The porous membrane used in the present invention preferably is bag-like or cylindrical. The membrane must be heat-resistant and preferably also has excellent powder releasability. Hot gas is usually introduced into the spray drying chamber at an inlet temperature of 120°C to 280°C and leaves the chamber at an outlet temperature of 70° to 150°C; therefore, the porous membrane must withstand such a high temperature. Further, since a fine powder constantly adheres to and deposits on the porous membrane, the membrane preferably has excellent powder releasability in order to allow its continuous use. The porous membrane functions to separate fine powder product from the exhaust gas. Any porous membrane having such characteristics can be used. However, there are ordinarily used fiber-made cloths, or laminates each consisting of two different such cloths. Preferably, there are used laminates of a powder-releasable, reticulate membrane and a woven or unwoven cloth as a reinforcing material. Preferable examples of the reticulate membrane are a membrane of polytetrafluoroethylene (PTFE) (brand name: Teflon) type and a polytrifluoroethylene type membrane, and preferable examples of the woven or unwoven cloth are cloths made from a polyimide, a heat-resistant nylon, a polyester or an aramid. In the present invention, the whole of the spray drying chamber is formed by a single porous membrane. This makes detaching of the chamber easy and accordingly its maintenance very simple. In a bag-like or cylindrical spray drying chamber formed by a porous membrane, a feed liquid is sprayed to form microdrops and the microdrops are momentarily dried by hot gas blown from its inlet and having an inlet temperature of 120° to 280°C to be converted into a fine powder and a gas. Some of the fine powder deposits on the bottom of the spray drying chamber under gravity, while the gas passes through the pores of the porous membrane, seperates from the rest of the fine powder, and is discharged through an outlet for exhaust gas. Hence, all of the fine powder is separated from the gas and, as a result, there occurs no nonuniformity in the composition of the fine powder product. Continuous operation of the porous membrane causes adhesion and deposition of a fine powder to and on the membrane, leading to an increase in pressure drop and finally stoppage of operation. In order to avoid this, the fine powder needs be taken off as necessary. Since the porous membrane in general has excellent powder releasability, the adhered powder can be easily removed from the porous membrane in a short time by mechanical vibration, air injection from the opposite side (pulse air or reverse jet) or the like. When complete removal (cleaning) of the powder from the membrane without causing contamination with impurities is required, it can be done by replacing the porous membrane with a new one from outside without touching the product inside the membrane. This replacement is easy when the porous membrane has a bag-like shape. Separation of a fine powder and a gas by the porous membrane is ordinarily conducted by passing a gas containing a fine powder through the meshes of the fiber constituting the porous membrane and allowing the fine powder to remain on the meshes. In actuality, however, this filtration is not simple. The meshes are usually many times as large as the particle sizes of the fine powder; accordingly, the fine powder is collected not only on the surface of the filtration cloth but also partially inside the cloth. In the present invention, such a filter cloth is used primarily for a fine powder of good solubility. Preferably, the porous membrane used in the present invention is a laminate of (1) a reticulate membrane as mentioned previously, having good powder releasability and capable of conducting surface filtration and (2) a woven or unwoven cloth as mentioned previously, of excellent heat resistance and strength as a carrier for the membrane (1). The effect of the present invention can be enhanced by the use of such a laminate. The diameters of the particles which can be captured by the porous membrane of the present invention can vary from 0.1 µm to several millimeters. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows an example of the spray drying apparatus of the present invention. Fig. 1(a) is a schematic sectional view of the whole portion of the apparatus and Fig. 1(b) is a partially enlarges sectional view of the porous membrane used in the apparatus. Fig. 2 to Fig. 5 are schematic sectional views showing other examples of the spray drying apparatus of the present invention. Fig. 6 is a perspective view showing an example of the spray drying apparatus of integration type according to the present invention wherein each unit of the apparatus is accommodated in a housing. Fig. 7 shows plan views of Fig.6, an example of the spray drying aratus of integration type according to the present invention wherein each unit of the apparatus is accommodated in a housing. Fig. 7(a) is a plan view of the upper room of the housing and Fig. 7(b) is a plan view of the lower room of the housing. Fig. 8 is a drawing explaining an example of the operation of the spray drying apparatus of integration type according to the present invention wherein each unit of the apparatus is accommodated in a housing. Fig. 9 and Fig. 10 are schematic sectional views showing conventional spray drying apparatuses. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Fig. 1 shows an embodiment of the spray drying apparatus of the present invention for carrying out the spray drying process of the present invention. Fig. 1(a) is a schematic sectional view of the whole portion of the apparatus and Fig. 1(b) is a partially enlarged sectional view of the porous membrane forming the spray drying chamber of the apparatus. In Fig. 1, a spray drying chamber 1 formed by a bag-like porous membrane 15 is arranged inside the main body 11 of the spray drying apparatus along the inner wall of the main body 11. As shown in Fig. 1(b), the porous membrane 15 is constituted by a laminate of a reticulate membrane 13 and a filter cloth 14 as a reinforcing material. Hot gas X is sent into the spray drying chamber 1 from the top of the main body 11 through an inlet 2 and is allowed to make counterflow contact with a feed liquid sprayed upward from the lower portion of the spray drying chamber 1 through a nozzle 5. A fine powder produced enters a product receiver (not shown) from an outlet 6 of seal valve type. A gas after drying is separated from a fine powder A via the porous membrane 15 forming the outer wall of the spray drying chamber 1 and is then discharged outside from a discharge gas outlet 3 installed at the lower portion of the main body 11. Fig. 2 is a schematic sectional view showing another embodiment of the spray drying apparatus of the present invention. Fig. 2 is different from Fig. 1 in that a rotary spraying disc 10 is used as an atomizer, in place of the nozzle 5 and both hot gas X and a feed liquid from the rotary spraying disc 10 are introduced downward as parallel flows. In other respects, the structure of the apparatus of Fig. 2 is approximately same as that of Fig. 1. Fig. 3 to Fig. 5 are schematic sectional views also showing other embodiments of the spray drying apparatus of the present invention. In Fig. 3, hot gas X and a feed liquid from a nozzle 5 are introduced downward as parallel flows. In Fig. 4, hot gas X and a feed liquid from a nozzle 5 are introduced upward as parallel flows. In Figs. 5(a) and (b), hot gas X and a feed liquid from a nozzle 5 are introduced from the side of the spray drying chamber 1. In each of the apparatuses of Figs. 3 to 5, there is provided a blow-off mechanism Z (reverse jet and pulse air type) for blowing off a fine powder produced. For instance, pulse air is sent into a main body 11 from the opposite direction through a line 12, whereby a fine powder A which has adhered to a porous membrane 15 is blown off. Fig. 6 and Fig. 7 each shows an embodiment of the spray drying apparatus of integration type according to the present invention wherein each unit of the apparatus is accommodated in a housing. Fig. 6 is a perspective view of the whole portion of the apparatus. Fig. 7(a) is a plan view of the upper room of the housing of the apparatus. Fig. 7(b) is a plan view of the lower room of the housing of the apparatus. In Figs. 6 and 7, a housing 16 consists of an upper room 17 and a lower room 18. In the upper room 17, there are arranged each three sets of units 19 for generation of hot gas, units 20 for distribution of hot gas and units 21 for atomization of feed liquid, as well as one common unit 22 for exhaust gas. In the lower room 18, there are arranged, below the feed liquid atomization units 21, the same sets (i.e. three sets) of units 23 for recovery of fine powder product. In the lower room 18, there are further arranged units 24a and 24b for blowing-off of fine powder and a unit 25 for preparation of feed liquid. Between the feed liquid atomization units 21 in the upper room and the corresponding fine powder recovery units 23 in the lower room, there are arranged the same sets (i.e. three sets) of spray drying chambers 1. A large number of bag-like porous members each to be used as a spray drying chamber 1 are stored in the lower room. Referring to Fig. 8, there is explained the operation of the spray drying apparatus of integration type according to the present invention. The apparatus of Fig. 8 accommodates two sets of spray drying chambers for producing different products G and H. Explanation is made on one set. A feed liquid G F supplied from a feed liquid preparation unit 25 through a pipe 27 is sprayed into a drying chamber 1 by a feed liquid atomization unit 21 to form microdrops, and the microdrops are momentarily dried by hot gas X blown from a hot gas generation unit 19 through a hot gas distribution unit, to be converted into a fine powder and a vapor. The fine powder is recovered as a product G₁ by a fine powder receiver 23. When the product to be produced next is G₂ or G₃ of different grade but similar to G₁, the porous membrane constituting the spray drying chamber 1 is removed and can be rapidly replaced by a porous membrane of exclusive use for G₂ or G₃ which is stored nearby. Another set is also operated in the basically same manner. Meanwhile, the gas is separated from the fine powder through the pores of the porous membrane 26 and is discharged outside from a common discharge unit 22. The spray drying apparatus and the apparatus therefor according to the present invention will be explained in more detail below by way of Examples. (Example 1) Using a spray drying apparatus of the type shown in Fig. 1, a feed liquid containing 60 % of stabilized zirconia (Ca 0.15 Zr 0.85 O 1.85 ) or 70 % of lanthanum-added barium titanate [BaTiO₃ + 0.01(La₂O₃)] was sprayed at a rate of 150 kg/hr. The drying chamber of the apparatus had dimensions of 2,500 mm in diameter, 2,500 mm in straight body height and 4,300 mm in total height, and had a cylindrical shape at the upper portion and a cone shape at the lower portion. The porous membrane forming the drying chamber was a laminate of a heat-resistant unwoven nylon cloth and a polytetrafluoroethylene. The inlet temperature of heated air was 220°C and the exhaust gas temperature was 120°C. Under the above conditions, the spray drying operation of each feed liquid was conducted. As a result, the fine powder products obtained had the following properties. (1) (2) Average particle diameter 50 µm 60 µm Water content 0.3 % 0.7 % Nonuniformity in composition None None Yield after 4 hours of operation 99 % 99 % In the above, (1) refers to stabilized zirconia and (2) refers to lanthanum-added barium titanate. The time required for product change was as short as about 40 minutes. (Example 2) Using a spray drying apparatus of the type shown in Fig. 8, a feed liquid containing 70 % of lanthanum-added barium titanate [BaTiO₃ + 0.01, 0.015 or 0.02 (La₂O₃)] was sprayed at a rate of 100 kg/hr. The drying apparatus was of G type. The drying chamber of the apparatus had dimensions of 2,000 mm in diameter, 500 mm in straight body height and 3,500 mm in total height, and had a cylindrical shape in the upper portion and a cone shape in the lower portion. The porous membrane forming the drying chamber was a laminate of a heat-resistant unwoven nylon cloth and a polytetrafluoroethylene. Each one membrane was used for each feed liquid. The inlet temperature of heated air was about 220°C and the exhaust gas temperature was 120°C. Under the above conditions, the spray drying operation of each feed liquid was conducted. As a result, the fine powder products had the following properties. (1) (2) (3) Average particle diameter 50 µm 25 µm 50 µm Water content 0.5 % 0.4 % 0.5 % Nonuniformity in composition None None None Yield after 4 hours of operation 99 % 99 % 99 % In the above, (1) refers to the case of 0.01% lanthanum addition; (2) refers to the case of 0.015% lanthanum addition; and (3) refers to the case of 0.02% lanthanum addition. The time required for product change was as short as about 30 minutes. In this Example, cleaning at the time of product change was conducted only for the atomizer. The drying chamber was folded up and hanged down from a hook of exclusive use, so as to be able to be used in next drying. As appreciated from the foregoing, the spray drying apparatus of the present invention uses a spray drying chamber formed by a porous membrane; as a result, there takes place no discharge, out of the apparatus, of the portion of a powder product having smaller particle diameters together with an exhaust gas, as experienced with the conventional spray drying apparatuses. Therefore, there is no nonuniformity in product composition; cleaning and blowing-off of adhered fine powder can be conducted rapidly and easily even in frequent product change inevitable with a more grade smaller quantity production system; as a result, a fine powder of higher quality and higher purity can be produced. In the spray drying apparatus of integration type according to the present invention wherein several spray drying chambers are accommodated in a housing, there are also arranged in the housing each several sets (each set has about the same size) of other necessary units such as hot gas generation units, hot gas distribution units and feed liquid atomization units and there are further stored in the housing a large number of porous membranes for each different product. Such a spray drying apparatus can provide the same performance as several tens of conventional spray drying apparatus, with a far smaller land area and a far lower facility cost. As stated above, the spray drying process and apparatus of the present invention can produce a fine powder of high purity, is suited for use in producing a variety of products in small quantities, and accordingly can be utilized in the production of high purity products such as medicines and fine ceramics.

Claims: A spray drying process for producing a powder from a feed liquid, which process comprises spraying the feed liquid into a spray drying chamber (1) formed by a single easily-detachable heat-resistant porous membrane (15) having a funnel-shaped lower portion to form microdrops, introducing a hot gas (X) into said chamber to dry said microdrops and form powder particles, withdrawing gas from said chamber (1) through said porous membrane (15) thereby to separate said powder particles from said gas, and removing said separated powder particles from said chamber (1) through said funnel-shaped lower portion. A spray drying process according to Claim 1, wherein the porous membrane (15) is bag-like or cylindrical. A spray drying process according to Claim 1 or 2, wherein the porous membrane (15) comprises a laminate of (1) a powder-releasable, reticulate membrane (13) of surface filtration type and (2) a heat-resistant filter cloth (14) as a reinforcing material. A spray drying apparatus for use in producing a powder from a feed liquid, which apparatus comprises a spray drying chamber (1) having an inlet (2) for hot gas (X), an outlet (3) for discharge gas, an outlet (6) for produced powder and spray means (5) in said chamber for use in atomizing the feed liquid into said hot gas in said drying chamber (1), characterized in that said chamber (1) is formed by a single easily-detachable heat-resistant porous membrane (15) having a funnel-shaped lower portion, said membrane being pervious to said gas but substantially impervious to said atomized feed liquid. A spray drying apparatus according to Claim 4, wherein the porous membrane (15) is bag-like or cylindrical. A spray drying apparatus according to Claim 4 or 5 wherein the porous membrane (15) comprises a laminate of (1) a powder-releasable, reticulate membrane (13) or surface filtration type and (2) a heat-resistant filter cloth (14) as a reinforcing material. A spraying drying apparatus for use in producing a powder from a feed liquid, characterized in that the apparatus comprises a housing (16) having an upper room (17) and a lower room (18); in said upper room (17), two or more units (19) for generation of a hot gas, two or more units (20) for distribution of hot gas, two or more units (21) for atomization of feed liquid and at least one unit (22) for discharge gas; in said lower room (18) two or more moveably arranged units (23) for recovery of produced powder; and between each feed liquid atomization unit (21) in the upper room (17) and each corresponding powder recovery unit (21) in the lower room (18) one spray drying chamber (1) formed by a single easily-detachable heat-resistant porous membrane having a funnel-shaped lower portion. A spray drying apparatus according to Claim 7, wherein the porous membrane is bag-like or cylindrical.

Inventors: ITO, TAKASHI, IWAYA, SHOICHI, KOBAYASHI, KATSUMI, MASUMURA, HITOSHI, OHKAWARA, MASAAKI, TAKAHASHI, HIROKI
Applicants: OHKAWARA KAKOHKI CO., LTD., TDK CORPORATION
Application year: 1986
Main classification: B01D1
IPC codes: B01D1