JP7322367B2 - Powder drying apparatus and powder drying method - Google Patents

Description

The present invention relates to powder dryers such as flash dryers, ring dryers, and spray dryers, and more particularly to device configurations and operating methods for preventing fire accidents that have been reported in many cases in these dryers.

These drying devices are devices for rapidly drying the wet powder by throwing it into a rapidly flowing high-temperature medium, colliding the wet powder with the high-temperature medium, and transporting the wet powder in a floating state within the apparatus. Examples of powder include wheat flour, powder medicine, toner, fertilizer, titanium oxide, activated carbon, cement, alumina, and powdered metal, and are applied in various industrial fields. It is also used to dry substances such as ores, which have a larger particle size than powders, and fluff-like substances, such as wood pulp, which are slightly different in shape from powders. Furthermore, there are also cases where a substance dissolved in a liquid as an emulsion that is finer than powder, such as milk, and a substance completely dissolved in a liquid, such as salt water, are dried and taken out as powder. These granular, flocculate, emulsion or solution forms are hereinafter collectively referred to as "powder", but the object of the present invention is not limited to the substances exemplified here.

In these powder dryers, there have been many reports of fire accidents and explosion accidents if the temperature is raised too high, especially when drying powder that is easily oxidized. There are some cases where fire extinguishers and fire extinguishers are installed. Alternatively, safety is ensured by setting an upper limit on the operating temperature, accepting that the drying efficiency and processing capacity will be lowered so that the temperature falls below the combustion limit temperature, which is the temperature that causes spontaneous ignition.

In addition, Patent Document 1 (Japanese Patent Application Laid-Open No. 9-48612) deals with drying of substances in which alcohols are by-produced during drying. To achieve this, the oxygen concentration in the gas used for drying should be kept at 10% or less, preferably 5% or less.” It is better to use a heating medium gas that does not react with organic solvents such as,” and shows one part of the solution to suppress fire accidents and explosion accidents. Although this example is a case of a special condition in which by-product alcohol is generated during drying, it can also be applied to ordinary drying of combustible substances in which combustible by-products are not generated.

JP-A-9-48612

The method according to Patent Document 1 is an effective method for truly safely drying substances with a combustible oxygen concentration exceeding 10% to 5%. However, from a cost perspective, it is also possible to adopt a closed system in which the water and organic solvent evaporated from the medium gas used for drying is recovered by means of cooling, etc., and the medium gas is recycled.” The cost of the medium gas is a problem, and even if the medium gas is recycled in a closed system, equipment for cooling and recovering the evaporated water is essential. Equipment costs and operating costs were a problem, and it was difficult to say that the equipment was widely suitable for industrialization.

Thus, Patent Document 1 is a truly safe method that does not, in principle, cause fires or explosions, and should be widely applied. was difficult from an economic point of view.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a powder drying apparatus and a method of operating the apparatus that can substantially reduce the risk of fire using an economically low-cost medium.

The inventor proposes to use superheated steam as the medium gas as a means of solving this cost problem. Water also behaves as a gas above 100°C, and like nitrogen, it does not support combustion. In Patent Document 1, it is natural to send a dehumidified medium to the drying cylinder, so it is selected to remove not only alcohols but also water from the medium. It is presumed that he did not realize the truth that it could be used as a medium.

However, if superheated steam exceeding 100° C. is generated from water, a heat source corresponding to the latent heat of vaporization is required in addition to the sensible heat, and it is clear that the cost becomes a new issue. Therefore, in the first place, the basic function of the drying apparatus is to evaporate water and exhaust it, and especially when the exhaust temperature exceeds 100 ° C, the moisture in the exhaust becomes superheated steam. Therefore, the inventors have found that the problem of the production cost of the latent heat of vaporization can be solved by recycling the superheated steam as a source of supply of the medium as necessary, and have completed the present invention.

A first aspect of the present invention relates to a powder drying apparatus that dries powder by causing a high-temperature medium and wet powder to collide with each other. In the drying apparatus according to the present invention, part of the steam that is evaporated from the powder and exhausted is heated to form superheated steam, and the superheated steam is returned to the drying apparatus as a high-temperature medium. The concentration is increased, thereby keeping the oxygen concentration in the dryer below the flammable limit oxygen concentration of the powder. In the present invention, the internal pressure of the high-temperature medium is preferably kept within ±10% of the ambient pressure of the device.

Specifically, the drying apparatus according to the present invention includes a drying tube (column) for drying the wet powder by contacting the powder with a heating medium (for example, superheated steam), and a heating medium supplied to the drying tube. , a separation device (cyclone) that separates the powder from the heating medium that has passed through the drying tube, and a circulation line that supplies the heating medium that has passed through the separation device to the heating device. It is preferable that the oxygen concentration in at least the powder transport line from the drying tube to the separation device is 16% or less and the temperature of the heating medium is controlled to over 100°C.

In particular, it is preferable to prevent air (make-up air, etc.) from the outside of the apparatus from flowing into the circulation line. Specifically, it is preferable that there is no mechanism for introducing external air into the circulation line. The inventors noticed that superheated steam (heating medium) exceeding 100° C. at normal pressure is a physically perfect gas, and obtained the knowledge that the drying heat energy can be greatly reduced by using this fact. That is, the superheated steam can be treated as an ideal gas in terms of its physical properties, similar to well-known gases in the atmosphere such as nitrogen, oxygen, argon, and carbon dioxide. Therefore, superheated steam and other gases can be mixed with each other in any ratio between 0 and 100%. In other words, as long as the temperature exceeds 100°C at normal pressure, there is no need to be bound by the concept of humidity, which means that it is carried by air. You can. Therefore, if superheated steam is used as a heating medium for drying powder, it is not necessary to supply make-up air into the circulation path. It can be said that it is possible to prevent the temperature from decreasing, which leads to an improvement in thermal efficiency and a reduction in drying energy. For this reason, it is preferable not to supply atmospheric air, called make-up air, into the circulation path other than the air necessary for combustion in the heating device.

A second aspect of the present invention relates to a method for drying powder in which a high-temperature medium and wet powder collide to dry the powder. In the drying method according to the present invention, part of the steam that is evaporated from the powder in the drying device and exhausted is heated to form superheated steam, and the superheated steam is returned to the drying device as a high-temperature medium. By increasing the water vapor concentration in the dryer, the oxygen concentration in the dryer is controlled below the combustion limit oxygen concentration of the powder.

Specifically, the drying method according to the present invention includes a step of heating a medium supplied to a drying tube with a heating device, and a heating medium (e.g., superheated steam) in which the wet powder is heated in the drying tube (column). a step of drying the powder by bringing it into contact with the powder, a step of separating the heating medium and the powder that have passed through the drying tube by a separation device (cyclone), and transferring the heating medium that has passed through the separation device to the heating device and supplying. It is preferable to control the oxygen concentration in the powder transport line from at least the drying tube to the separator at 16% or less and the temperature of the heating medium at 100°C or higher. In particular, it is preferable to prevent the air from the outside of the device from flowing into the circulation line.

According to the present invention, it is possible to provide a powder drying apparatus and a method of operating the same, which can substantially reduce the risk of fire using an economically low-cost medium.

FIG. 1 shows a conventional example in which the spray dryer described in Patent Document 1 is applied to a single-column flash dryer. FIG. 2 shows a first embodiment of the present invention, in which the present invention is applied to a single-column flash dryer. FIG. 3 shows a second embodiment of the present invention, in which the present invention is applied to a double-column flash dryer. FIG. 4 shows a third embodiment of the present invention, in which the present invention is applied to a single column flash dryer equipped with a direct heat exchange type heating device.

Although Patent Document 1 has a spray dryer in mind, the principle of drying is the same for a flash dryer. FIG. 1 shows an example in which the apparatus configuration disclosed in Patent Document 1 is applied to a single-column flash dryer having one drying tube (column).

As shown in FIG. 1, the medium dehumidified in the cooling tower is heated by the heating device and encounters wet powder at the bottom of the column. The wet powder is dried by receiving heat from the medium in the column and then sent to a cyclone to separate it from the medium and remove it as a dry powder. At that time some medium also exits, so N ₂ (nitrogen) gas is replenished into the cyclone. In addition, the powder is decomposed during the drying process to produce alcohol as a by-product. The medium containing water and alcohol separated from the dry powder by the cyclone is sent to the cooling tower by a circulation fan. In the cooling tower, water and alcohols in the medium are condensed by the cooling water, separated from the medium as drain water, and the medium is regenerated.

In Patent Document 1, the powder decomposes to generate combustible alcohols, but the medium is mainly composed of N ₂ gas and evaporated water vapor, and the medium does not support combustion. Although there may be some oxygen entering from the feeder, it is monitored with an oxygen concentration meter installed in the route, and by monitoring that it is 10% to 5% or less, the combustion of alcohol It guarantees that the oxygen concentration is below the critical oxygen concentration and that there is no possibility of combustion or explosion.

A first embodiment of the present invention is shown in FIG. 2 and will also be described with a single column type flash dryer. As shown in Figure 2, the first embodiment of the present invention comprises a feeder, a column, a heating device, a cyclone, an exhaust fan, a circulation line, a blower fan and an oximeter. A feeder supplies the column with wet powder to be dried. The column dries the wet powder by contacting it with a heating medium. The heating device generates a heating medium (superheated steam) by heating the medium, and supplies this heating medium to the column. A cyclone separates the powder from the heating medium that has passed through the column. Furthermore, in the cyclone, powder may be separated into dry powder that has been completely dried and wet powder that has not been dried sufficiently, and this wet powder may be returned to the feeder. The dry powder is collected as a product. An exhaust fan exhausts a portion of the heating medium separated in the cyclone. The exhaust fan can adjust the exhaust amount of the heating medium. It is also possible to stop the exhaust fan so that the exhaust amount of the heating medium is zero, or not to provide the exhaust fan itself. A circulation line supplies all or part of the heating medium separated in the cyclone to the heating device. A blower fan is arranged in the circulation line and regulates the amount of heating medium supplied to the heating device. The entire amount of heating medium separated in the cyclone can also be supplied to the heating device via a circulation line. Oxygen meters can be provided, for example, on both or one of the inlet side and outlet side of the column, and measure the oxygen concentration in the medium supplied to the column or the oxygen concentration in the medium discharged from the column. The amount of air discharged by the exhaust fan or the amount of air blown by the blower fan can be adjusted according to the oxygen concentration measured here.

In the present invention, the steam discharged from the cyclone is not cooled and separated, but is actively used as a heating medium. , it can be seen that the equipment and operating costs are reduced. As in Patent Document 1, even if combustible gases such as alcohols are generated, there is no risk of fire or explosion because there is no combustible gas in the system. In addition, the installation position of the oxygen concentration meter is not limited to the embodiment, and it is preferable to select a place where air can easily enter when the equipment is damaged.

When the drying process according to the present invention progresses beyond a certain level, all the air in the drying apparatus at the initial stage is replaced with superheated steam, which is a heating medium. From then on, the discharged medium consists entirely of superheated steam, the amount of which is equal to the amount of water evaporated from the wet powder. This is because, if the amount of exhaust gas is greater than the amount of water evaporated from the wet powder, the negative pressure inside the apparatus increases and the amount of exhaust gas decreases, eventually becoming equal to the amount of water evaporated from the wet powder and balanced. If it becomes less, the opposite effect will occur, and it will also become equal and balance.

In this way, since the operating pressure of the device forms a stable system, the device can be operated even if it is left to nature. is also possible. For example, if the pressure inside the device is kept higher than the surroundings of the device, even if the device cracks or ruptures, outside air will not immediately enter through that part, and the structure will be resistant to fire and explosion. can. The exhaust fan shown in FIG. 1 can function as pressure regulation means. Moreover, as a pressure detection mechanism, a pressure gauge may be provided at an arbitrary location in the pipe through which the medium circulates.

Conversely, if the internal pressure is set to a negative pressure, the medium can be prevented from leaking from small gaps in the device or poorly sealed portions of the feeder. It becomes easy to control the outflow to the outside.

However, in the range in which the inventor operated the drying apparatus, the pressure should be kept within ±10% of the ambient pressure, and a pressure difference greater than that would cause damage due to expansion and contraction of the apparatus, and should be avoided. preferable.

FIG. 3 shows a second embodiment of the present invention, in which the present invention is applied to a double-column flash dryer. In the second embodiment, as in the first embodiment described above, the outlet of the exhaust fan and the inlet of the blower fan are connected to circulate most of the medium, and no air is taken in.

As shown in FIG. 3, the second embodiment of the present invention comprises a feeder, a first column, a first cyclone, a second column, a second cyclone, a boost fan, a heating device, an exhaust fan, a circulation line, and a blower fan. , and an oximeter. A feeder supplies the first column with wet powder to be dried. The first column dries the wet powder by contacting it with a heating medium. Heating medium is supplied to the first column by a boost fan. A first cyclone separates the powder from the heating medium that has passed through the first column. The powder separated by the first cyclone is fed to the second column. The second column further dries the powder separated by the first cyclone by contacting the powder with a heating medium. In the second cyclone, the powder is separated into dry powder that has been completely dried and wet powder that has not been dried enough, and this wet powder is returned to the first column together with the heating medium via the boost fan. be The dry powder is collected as a product. The heating device generates a heating medium (superheated steam) by heating the medium, and supplies this heating medium to the second column. An exhaust fan exhausts a portion of the heating medium separated in the first cyclone. The exhaust fan can adjust the exhaust amount of the heating medium. It is also possible to stop the exhaust fan so that the exhaust amount of the heating medium is zero, or not to provide the exhaust fan itself. The circulation line supplies all or part of the heating medium separated in the first cyclone to the heating device. A blower fan is arranged in the circulation line and regulates the amount of heating medium supplied to the heating device. The entire amount of heating medium separated in the first cyclone can also be supplied to the heating device via a circulation line. Oxygen concentration meters can be provided, for example, on both or one of the inlet side and outlet side of the first column, and measure the oxygen concentration in the medium supplied to the first column or the oxygen concentration in the medium discharged from the first column. to measure. The amount of air discharged by the exhaust fan or the amount of air blown by the blower fan can be adjusted according to the oxygen concentration measured here.

Two column devices are arranged so that the powder and the medium are dried countercurrently, but the functions and features are the same as in the first embodiment. In addition, as a result of the increase in the size of the equipment, the blower resistance is increased, so the blower is appropriately divided and arranged.

FIG. 4 shows a third embodiment in which a direct heat exchange type heating device is used. In the third embodiment, the heating device employs a direct-fire system in which thermal energy obtained by burning fuel is used to heat combustion air to generate hot air. It is the same as the first embodiment shown in FIG.

In direct-fired systems, gaseous fossil fuels such as city gas, propane gas and methane gas, and liquid fossil fuels such as kerosene and heavy oil are commonly used. Since air is mainly used to burn these, the medium contains oxygen (residual O ₂ ) that has not been used for combustion, as well as gases such as nitrogen and carbon dioxide. If excessive combustion air is supplied, the residual _O2 will increase, and as a result, the oxygen concentration of the medium may become higher than the combustion limit oxygen concentration of powder, so it is necessary to limit the amount of combustion air supplied. be. In addition, since an excessive amount of air means heat loss due to heat taken out by the exhaust gas, it is necessary to limit the combustion air flow rate with respect to the fuel flow rate in this sense as well.

For example, when methane gas is used as fuel, the stoichiometric air-fuel ratio is 10 parts air to 10 parts methane gas as a volume flow ratio. However, it is said that the air-fuel ratio that maximizes thermal efficiency is about 1:12, considering that no unburned gas remains. Guidelines for ideal air-fuel ratios are generally known for other fuels as well. It is also known that the residual O ₂ concentration in the combustion gas is about 4 to 6% when the fuel is burned according to the air-fuel ratio. Therefore, the oxygen concentration in the medium in which the combustion gas and the superheated steam are mixed is below 4-6%.

Regarding the combustion limit oxygen concentration of combustible powder, there is data that when the gas coexisting with oxygen is nitrogen, it is 11% for wheat flour and 10% for sawdust. In conventional flash dryers that use air as a medium, as a result of mixing a medium with an oxygen concentration of 21% and a combustion gas with an oxygen concentration of 4-6%, the oxygen concentration in the medium can reach the combustion limit oxygen concentration depending on the mixture ratio. there were. This is the reason why so many fires occurred. However, in the present invention, the medium is a mixture of combustion gas and superheated steam, the oxygen concentration of which does not exceed 4-6%, which is the oxygen concentration of the combustion gas. In that sense, it is an essentially fire-free drying method for drying many combustible powders, including flour and sawdust.

However, accurate air-fuel ratio control requires correct measurement and control of the air flow rate, and when gaseous fossil fuel is used, the flow rate measurement and control are also required. However, gas flow rate measuring instruments are prone to error, and depending on the degree of error, the air flow rate may increase unintentionally and the oxygen concentration in the medium may increase. In order to prevent the occurrence of fire accidents due to this, monitoring by installing an oxygen concentration meter is recommended.

In this way, if a medium mainly composed of superheated steam is used, the oxygen concentration in the medium can be maintained below the combustion limit oxygen concentration, so that the facility can be constructed without fear of fire. This is a truly useful invention in terms of safety and disaster prevention. However, if this greatly reduces the drying capacity or significantly increases the fuel consumption for obtaining the same drying level, the industrial value is lost. Therefore, the present invention is superior to the conventional technology in terms of thermal efficiency, and as a result of improving the drying speed, it is advantageous in terms of operating costs and equipment costs, and is an industrially useful method.

Flash dryers dry powders by transferring heat from a medium to wet powders in a volume to vaporize the water content of the powders. Therefore, a medium that contains a larger amount of heat in a given volume and that has a faster transfer rate of heat to the powder is more advantageous for drying. As an example, the physical properties of superheated steam and air at 200° C. will be compared.

(1) Comparison of thermal efficiency The amount of heat required to evaporate 1 kg of water does not depend on the drying method. Thermal efficiency is determined by how much heat is lost to the outside of the system during the drying process. In the method of the present invention, only the evaporated water is discharged out of the system. On the other hand, in the conventional technology, the temperature of air as a medium is raised together with the evaporated moisture, and the air is exhausted at the same time. In this sense, the present invention is superior in terms of thermal efficiency, as there is no air going out of the system, although there is an effect of the difference in temperature at the time of discharge.

(2) Comparison of retained heat amount Drying by a flash dryer is mainly performed in a drying tube having a fixed volume. As shown in Table 1, the superheated steam has a higher specific heat per unit volume, and can hold a larger amount of heat in the same volume. In other words, equipment with a smaller volume can be operated while retaining the same amount of heat, so the equipment can be made smaller, or more powder can be dried with the same equipment.

(3) Comparison of heat transfer speed It is known that superheated steam can transfer condensation heat to water at 100°C or less, and the speed is two to three orders of magnitude faster than contact heat transfer. In other words, the speed at which heat is transferred is overwhelmingly fast. Therefore, the same amount of heat can be transferred with a smaller contact area, so the equipment can be smaller or more powder can be dried in the same equipment.

From the above, it can be said that superheated steam has physical properties more suitable than air as a medium for transferring heat.

Superheated steam exceeding 100°C is a gas that can be simulated as an ideal gas like air, and does not become a liquid, that is, it does not wet the powder, but rather can dry the powder. . And its power is superior to that of air.

In the specification of the present application, the embodiments of the present invention have been described with reference to the drawings in order to express the content of the present invention. However, the present invention is not limited to the above embodiments, and includes modifications and improvements that are obvious to those skilled in the art based on the matters described in this specification.

Claims (3)

A powder drying apparatus that dries powder by colliding a high-temperature medium supplied by a heating device with wet powder, wherein the high-temperature medium and the wet powder are brought into contact in a drying tube. A part of the water vapor evaporated from the wet powder and discharged from the drying tube is heated by the heating device to be superheated water vapor, and the superheated water vapor is passed through the drying tube as the high-temperature medium through a circulation line. By returning the powder, the water vapor concentration in the drying tube is increased, thereby keeping the oxygen concentration in the drying tube lower than the combustion limit oxygen concentration of the powder, and there is no heat source corresponding to the latent heat of vaporization other than the heating device . Body drying device. 2. A powder drying apparatus according to claim 1, wherein the internal pressure of said medium is maintained within ±10% of the pressure around the apparatus. A method for drying powder in which the powder is dried by colliding a high-temperature medium supplied by a heating device with the wet powder, wherein the high-temperature medium and the wet powder are brought into contact with each other in a drying tube. A part of the water vapor evaporated from the wet powder and discharged from the drying tube is heated by the heating device to be superheated steam, and the superheated steam is returned to the drying tube as the high-temperature medium through the circulation line . By doing so, the water vapor concentration in the drying tube is increased, so that the oxygen concentration in the drying tube is controlled to be lower than the combustion limit oxygen concentration of the powder, and latent heat of vaporization is not generated by a heat source other than the heating device . Powder drying method.

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