RU2202507C2 - Method of pneumatic transportation of power-like oxidizer with additives - Google Patents

Abstract

FIELD: materials handling facilities. SUBSTANCE: invention relates to pneumatic transportation of loose materials, including powderlike fire and explosion-hazard mixtures, namely, to method of pneumatic transportation of powder-like oxidizer with additives. Proposed method includes discharge of powder-like oxidizer with additives from container delivery of said oxidizer into aerochamber for fluidization, transportation of oxidizer by air along pipeline, settling in unloader and entrapping of particles of oxidizer. Discharge of powder-like oxidizer with additives from container is carried out after breaking of material bridges at vibration. Said oxidizer is passed through vibroscreen, is rubber on rubbing drum and is brought into fluidized state by using porous partition provided in said aerochamber. When compressed dehydrated air with moisture content not exceeding 0.8 g per cu. M and controlled temperature and pressure is passed through oxidizer, particles of oxidizer are entrapped in cyclone and continuously returned and mixed with flow of said oxidizer getting into and settled in unloader. EFFECT: provision of pneumatic transportation on loose material without changing its granulometric content and physical and chemical properties. 7 cl, 2 dwg

Description

 The invention relates to the field of pneumatic transportation of bulk materials, including powdered fire and explosive mixtures.

 Due to a number of advantages compared to other types of transportation of bulk materials, pneumatic transportation is widely used in various industries. Pneumatic transportation has found application as a safe and easy way, with interphase and interoperational transfer of fire and explosive mixtures in the production of mixed solid rocket fuel.

 A detailed description of the applied schemes, the main components of pneumatic conveying systems and the calculation of the parameters of pneumatic transportation of bulk materials is given in the book by V. A. Uspensky "Pneumatic transport", Metallurgizdat, 1959. Similar elements to the claimed object are contained in the pneumatic installation of the combined system, page 10, eleven.

 However, the known methods and devices for pneumatic transportation of bulk materials do not fully satisfy the requirements when using them for a powdery oxidizing agent with additives (AML) in the production of mixed solid rocket fuel (STRT). So, in the device described in SU 117964 A and adopted as a prototype, there is no preliminary preparation of the transported powder, necessary for better aeration with air and mandatory for prone to caking materials.

 To perform these tasks in existing methods, it is necessary to additionally provide for certain technical solutions.

 Powdered ammonium perchlorate is mainly used as an oxidizing agent in the manufacture of STRT. Moreover, it is used as a mixture of several fractions to ensure maximum particle packing.

 Ammonium perchlorate, especially its small particles, can actively adsorb moisture from the environment. This property leads to caking and aggregation of particles with subsequent change in their initial size.

 Due to the tendency to caking and aggregation of particles, especially fine fractions, to prevent these phenomena, hydrophobic agents are treated or powdered additives (aerosil, calcium phosphate) are introduced. In some cases, special-purpose additives, for example, powder hardeners, are introduced into the oxidizing agent in a small amount.

 In the manufacture of STRT, certain requirements are imposed on the rheological properties of the fuel mass, on the physicochemical and ballistic characteristics of the finished product, which in turn depend on the particle size distribution and physicochemical properties of AML. To produce a STRT that meets the specified requirements, it is necessary to ensure the safety of the initial properties throughout the whole range of characteristics (particle size distribution, humidity, flowability, component content) during pneumatic transportation.

 The following types of pneumatic conveying systems are known: pressure, suction and combined.

 In pressure installations, the material is fed into the transport pipeline by vortex-type chamber pumps with tagnant nozzles. The main disadvantage of pressure installations is the uneven and uncontrolled flow of material from the chamber pump into the pipeline during one cycle and between cycles of the pump, caused by a variable level of material in the pump and the variability of its flow. As a result, transportation is carried out in uneven portions and the concentration of the material in the air, which leads to a significant and unstable grinding of its large particles, i.e., to a violation of the initial particle size distribution. An increase in the ability to grind particles of a powdery material with a decrease in concentration in air is well known.

 Increased air flow increases the entrainment of small particles of material from the dust collecting devices in the unloader, and also violates the initial particle size distribution. In addition, in this case, there is a violation of the component composition due to the entrainment of some of the additives. The disadvantages of pressure installations are also dusting, the difficulty of sealing them, the bulkiness, the complexity of the organization of control during remote process control in the case of use for explosive materials. In suction pneumatic conveying systems, the material is fed into the pipeline by a manually controlled nozzle, which is dangerous when working with explosive substances. In some cases, stationary nozzles and mechanized material feeding are used.

 The disadvantage of suction systems is the conduct of the process at low concentrations of the material and the possibility of their use only for transmission over relatively small distances.

The main driving force in pneumatic transport is the pressure drop at the beginning and end of the pipeline. The suction pumps and the suction cavity of the blower machines create a maximum vacuum of 95%, i.e. the maximum difference between the atmosphere and their suction cavity is 0.095 MPa (0.95 kg / cm ² ), which limits the possibility of transporting the material in a dense layer.

 The advantage of suction pneumatic transport using liquid ring vacuum pumps is the ability to provide safe operating conditions, as excludes hit of explosive dry dust in moving mechanisms. To ensure pneumatic transportation of AML with increased concentration and safe operating conditions, it is advisable to use the pressure-suction principle.

 At the same time, maintenance of pressure-suction parameters of air within certain limits should be provided.

In known pneumatic conveying installations, the energy carrier is atmospheric air, the moisture content of which can reach up to 15 g / m ³ .

Studies show that even a short-term contact of POD with air containing more than 3 g / m ^{3 of} water vapor leads to adsorption of particles by their surface, followed by caking, loss of flowability and a negative effect on the rheological properties of the fuel mass of STRT. This circumstance requires the use of additional measures to protect AML from moisture.

 The technical task of the invention is the use of a method of pneumatic transportation of bulk materials, including feeding and transporting through a pipeline by air flow, precipitation in a precipitant (unloader) and cyclones, the catch in the filter for a powdery oxidizer with additives without changing its particle size distribution and physico-chemical properties (exception humidification and entrainment of components).

 The technical result is achieved by transporting AML with a high concentration by creating conditions for uniform supply of the pipeline, supplying and suctioning dried compressed air with the required parameters, returning the particles additionally captured in the cyclone to the unloader in the re-transported stream.

 In the process of pneumatic transportation, there is friction of particles on the walls of the pipeline, impact in curved sections, movement in the axial direction and in cross section. Under these mechanical influences, particle size reduction is possible. The greatest mechanical effects, respectively, grinding occurs at low concentrations of the material in the air, when the particles have the greatest mobility. When pneumatic conveying with a high concentration of material in the so-called dense layer, grinding is practically absent. Thus, to maintain particle size distribution in the process of pneumatic transportation, it is necessary to ensure a high concentration and its constancy.

 A stable mode of pneumatic transportation can be achieved only when each particle is surrounded by air. If agglomerated particles, lumps are present in the material, then they will lead to the formation of rifts, blockages with blockage of pipelines. To eliminate them, you will need to change the parameters of the air, leading to undesirable grinding of particles.

 A stable mode of pneumatic transportation is largely determined by the state of the transported material.

 Uniform power supply of the pipeline is ensured when the material is in a fluidized state, i.e., in a state of saturation with air, when the powdery material acquires the property of a liquid in many respects. In a different state, uniform flow into the pipeline, and accordingly pneumatic transportation, is difficult, especially in the initial section, where the air speed is minimal and there is a loss of energy to disperse the particles. In the absence of fluidized material for pneumatic transportation, a decrease in concentration will be required, which will lead to a violation of the particle size distribution and increased entrainment of components. According to its physical properties, AML is prone to particle aggregation and clumping. Therefore, to create conditions for pneumatic transportation with a high concentration, certain operations are necessary.

 After manufacturing, AML is stored in containers and enters into them during the manufacturing phase of STRT. During storage, certain forces arise in the places of contact of the particles, leading to arch formation with loss of mobility. Therefore, when unloading AML from the container, mechanical destruction of the arch is required, followed by vibration exposure. After the destruction of the arch, the lumps unloaded from the AML container contain lumps that must be destroyed. This operation can be carried out by vibrating through a grate. Further destruction of the bonds between the particles can be realized by applying additional mechanical forces, which can only be done by mechanical rubbing through a grating with holes with a diameter of not more than 5 mm. Rubbed AML enters the aero chamber to the porous partition. The transfer of the rubbed AML to the fluidized state is possible by supplying compressed air through the porous partition and the AML layer, which simultaneously serves as a transporting medium when it enters the pipeline and is transported through it.

The increase in pressure drop along the length of the pipeline when using the suction system is more than 0.95 MPa (0.095 kgf / cm ² ) with an increase in the concentration of transported AML, which is necessary to prevent grinding and reduce component entrainment, can be achieved by supplying compressed air in the initial section of the pipeline in front of the zone loading into it AML.

The most acceptable option, as shown by experimental tests, is to maintain the pressure at the inlet to the pipeline in the range from 0.07 to -0.02 MPa (from 0.7 to -0.2 kgf / cm ² ). The upper limit is due to the fact that at a pressure of more than 0.07 MPa (0.7 kgf / cm ² ) dusting occurs through the sealing feeder and it becomes difficult for the AML to enter the aerial camera through it.

A larger value than -0.02 MPa (-0.2 kgf / cm ² ) of vacuum leads to a decrease in concentration during pneumatic transport, which contributes to the grinding of large particles and greater entrainment of components with a violation of chemical composition.

With the above limiting parameters, the main mode of pneumatic conveying AML is in the range of 0.05-0 MPa (0.5-0 kgf / cm ² ). Exceeding these values by 0.02 MPa (0.2 kgf / cm ² ) and -0.02 MPa (-0.2 kgf / cm ² ) has a short-term character at the beginning and end of pneumatic conveying.

 A stable mode of pneumatic transportation, which consists in maintaining the particle size distribution and physicochemical properties of AML, can be achieved only with a constant flow of conveying air, which in turn also depends on the operability of the vacuum pump and the suction of air into the system from the environment.

When implementing the method, a vacuum pump is used, which is a traction stimulator. The efficiency of the vacuum pump is determined by the level of vacuum achieved when it is a muffled suction line. Based on the operating experience, for satisfactory performance of the vacuum pump for this parameter, we can consider providing a vacuum of at least -0.093 MPa (-0.93 kgf / cm ² ). Therefore, before starting the pneumatic transport installation into operation, a vacuum is provided in front of the mentioned vacuum pump when its suction line is muffled not lower than -0.093 MPa (-0.93 kgf / cm ² ).

 An integral part of pneumatic conveying systems is a precipitator, dust collector, filter, which are interconnected by air ducts and require careful sealing in order to prevent leakage from the environment. For greater reliability, a control parameter must be set. Such a parameter can serve as a vacuum in front of the vacuum pump with a muffled inlet of the transport pipeline, characterizing the tightness of the entire system.

A stable mode of pneumatic transportation, preserving the particle size distribution and physicochemical properties of AML, is ensured in the case of vacuum at the vacuum pump when checking the tightness of the entire system not lower than -0.08 MPa (-0.8 kgf / cm ² ).

In order to eliminate the adsorption of moisture from the environment, it is necessary to ensure contact with dried air during its processing. Therefore, for pneumatic transportation, it is necessary to supply compressed, dried air with a moisture content of not more than 0.8 g / m ³ , which is provided by a compressor unit equipped with a desiccant. This norm guarantees the absence of moisture condensation at temperatures up to -20 ^o C.

 Between shipments, i.e. if the vacuum pump is not working and there is no air supply, water vapor may move from the wet filter to the pneumatic conveying system. This can lead to the filling of the system with moist air up to condensation of moisture in the pipeline under appropriate conditions - at low outdoor temperatures. This factor can be eliminated by closing the line in front of the wet filter and liquid ring vacuum pumps by installing automatically closing valves.

After AML pneumatic transportation, before turning off the vacuum pump, due to the resistance of the system, a vacuum is maintained in it, the value of which decreases along the line from the vacuum pump to the initial section of the pipeline. For example, in the precipitator (unloader), the vacuum is from -0.03 to -0.04 MPa (from -0.3 to -0.4 kgf / cm ² ). When the vacuum pump is turned off, the vacuum at the pump drops faster than in the precipitator (unloader). For this reason, the volume of the precipitant (unloader) is spontaneously filled with moist air from a wet filter, as well as through leaks in the system. The influence of this factor can be avoided if compressed air is introduced into the system before stopping the vacuum pump, providing pressure and vacuum in the precipitator (unloader) in the range (from 0.002 to -0.012) MPa, i.e. (from 0.02 to -0.12) kgf / cm ² .

Pressure above 0.002 MPa (0.02 kgf / cm ² ) leads to dusting through leaks of the sealing shutter and, as a consequence, to disruption of the metering device located under the feeder. This level can be limited by installing a check valve on the air exhaust line from the unloader. The lower limit for vacuum is limited by the need to reduce air leakage from the room.

 After turning off the vacuum pump when additional air is supplied at the inlet to the pipeline, the vacuum in the whole system simultaneously decreases, including in front of the vacuum pump, which excludes, as shown by experimental data, the reverse flow of air flow. Data for several pneumatic conveying lines are given below in table 1.

As can be seen from the table, the vacuum drops to "0" faster in the unloader than the vacuum pump. In this case, the vacuum at the vacuum pump should not be higher than -0.03 MPa (-0.3 kgf / cm ² ).

The moisture absorption of AML begins upon contact with air in conditions of relative humidity of 60% or more. Therefore, in the phase of preparation and processing of AML in a production environment with direct contact with ambient air, relative humidity of not more than 60% and a temperature in the range of 15-35 ^o C. are set.

When using AML pneumatic transportation with compressed compressed air with a moisture content of 0.8 g / m ³ at the indicated temperatures, the relative humidity will be in the range of 2-8%. In the unloader, the moisture content and relative humidity will increase due to suction from the room.

In operating mode, with a maximum possible suction of 20% of the room, the moisture content will be:
0.8 • 0.8 + 0.2 • 13 = 0.64 + 2.6 = 3.24 g / m ³ ,
where 0.8 and 13 g / m ³ is the moisture content in the compressed, dried air and the room (the maximum recorded value);
0.8 and 0.2 - the proportion of compressed dried air and air sucked from the premises into the unloader.

At a temperature in the range of 15-35 ^o C, the relative humidity in the unloader will be in the range of 8-32%. The heat-insulated pipeline between the buildings runs along the street, so in winter conditions the air in the pipeline cools. Measurements of the air temperature at the outlet of the pipe showed a decrease to -30 ^o C at an outdoor temperature of -40 ^o C, this leads to an increase in relative humidity. In this case, the AML temperature decreases by 3 ^o C.

When cooling the compressed dried air to -30 ^o With the temperature in the unloader will be in the range:
0.8 • (-30) +0.2 (15 - 35) = -21 - (-17 ^o С).

At a maximum temperature of -21 ^o With moisture content in the air will be about 1 g / m ³ . Excess moisture 3.24-1.0 = 2.24 g / m ³ falls into the condensate, which is not permissible.

Permissible relative humidity of not more than 60% with a moisture content of 3.24 g / m ^{3 is} provided at an air temperature in the unloader of 3 ^o C.

где t_x - температура воздуха на выходе из трубопровода.This air temperature in the unloader will be at 20% air intake from a room with a temperature at the lowest permissible level of 15 ^o C, if the temperature of the compressed dried air at the end of the pipeline is not lower:
0.2 • 15 + 0.8 • t _x = 3

where t _x is the air temperature at the outlet of the pipeline.

Thus, to eliminate the sorption of moisture AML, it is necessary to ensure the air temperature at the outlet of the pipeline is not lower than 0 ^o C.

The increase in air temperature at the inlet from the pipeline from -30 to 0 ^o With can be achieved by supplying hot air. To assess the required temperature level by experimentally calculated method, the heat transfer coefficient between air on the street and in the pipeline was determined, which for different lines was in the range of 0.5-0.9 kcal / m ² • min • ^o C.

According to the heat transfer coefficient, the geometric dimensions of the pipeline and the parameters of the air using the well-known heat transfer formula, it is easy to determine the temperature of the air with which it is necessary to supply it to the pipeline so that it is not lower than 0 ^o C. at the outlet

 For convenience, setting the temperature regime, you can build a graphical dependence of the temperature of the air supplied to the pipeline, on the temperature of the outside air. An example for one of the installations is shown in figure 1.

 The process of pneumatic transportation of a powdery oxidizing agent with additives is shown in figure 2. AML enters the container 2. Using the unloading device 1 produce the destruction of the code AML in the container. Unloading AML from the container is facilitated by vibration induced by the vibrator. Lumps are destroyed by sifting on a vibrating grate 3 and wiping in a wiping drum 6. For continuous AML supply from the loader 4 to the wiping apparatus, a feeder with flexible blades is used 5. The AML entering the porous partition of the air chamber 7 is saturated with it and acquires a fluidized state. To create an air flow before starting transportation include a vacuum pump 15.

The pressure at the inlet to the pipeline is maintained in the range from 0.07 MPa (0.7 kgf / cm ² ) to -0.02 MPa (0.2 kgf / cm ² ) by supplying compressed, dried air along the main line through the pressure regulator. In the cold season, compressed, dried air for heating is supplied through a heat exchanger. The temperature is controlled so that it is at the outlet of the pipeline was not lower than 0 ^o C.

 The AML mixture with air through pipeline 8 enters the unloader 9. It separates from the air and precipitates the bulk of the product due to centrifugal forces and a sharp drop in the flow rate. Small particles carried away from the air from the unloader are deposited in a cyclone 10, from which a lock gate 11 is continuously returned and mixed with the AML flow entering and deposited in the unloader.

 It was established that small particles of AML are prone to sticking with a gradual increase on the walls of the cyclone. This leads to a violation of aerodynamic regimes in the cyclone, and the catch efficiency decreases. This leads to a violation of both the particle size distribution and the chemical composition of AML. In addition, this leads to an additional emission of harmful substances into the atmosphere. In order to eliminate this factor, a device is mounted on the cyclone, which has a shock-pulse effect on the walls of the cyclone. To increase operational reliability, reduce wear of the working bodies of the specified device, it is advisable to work in a cyclic mode.

 It was established by experiments that in order to completely remove the sticking of small fractions and additives from the walls of cyclones, an impact pulse is necessary for 20 s with a break of 70% of the working time, i.e.

что составляет верхнюю границу.

which makes up the upper bound.

Так, при продолжительности работы по пневмотранспортированию в течение 20-25 сут через 5-7 сут наблюдалась забивка циклона. По этой причине вместе с отработанным воздухом ежесменно уносилась мелкая фракция ПОД в количестве 10-25 кг за каждые 6 ч в контрольный циклон и направлялась на уничтожение. При апробировании ударно-импульсного воздействия получен положительный эффект, что видно из приведенных в табл.2 данных.Depending on the length of the pneumatic conveying line and the properties of the AML, the productivity changes by about 1.5 times, hence the lower limit for the break will be:

So, with the duration of the pneumatic transportation work for 20-25 days, after 5-7 days, cyclone clogging was observed. For this reason, together with the exhaust air, a small fraction of AML in the amount of 10–25 kg for every 6 hours was taken away to the control cyclone and sent for destruction. When testing the shock-pulse impact, a positive effect was obtained, which can be seen from the data given in table 2.

 From table 2 it is seen that with the use of pulsed shock on the cyclone of the return device, the entrainment of fine fractions and additives decreased from 0.81 to 0.14-0.17%.

 Additional trapping of AML particles from the air is carried out in a cyclone 12 and a wet filter 18. Exhausted and purified air is discharged into the atmosphere through a water ring vacuum pump 15. When the vacuum pumps are idle, the air duct 13 from the wet filter and vacuum pumps is closed by check valves 14, which excludes the movement of moist air through the system.

After pneumatic transportation, the loads of one container include an additional supply of compressed dried air along the bypass line in an amount that allows reducing the vacuum at the vacuum pump to a value of no more than -0.03 MPa (-0.3 kgf / cm ² ). Turn off the vacuum pump and compressed compressed air. With these parameters, the return stroke of moist air to the unloader is excluded.

Unloading AML from the unloader is carried out through the lock gate of the drum type 16. The continuous receipt of AML from the unloader to the lock gate is carried out with constant tedding. Tedding is carried out cyclically by successively supplying compressed, dried air with a pressure of 0.05 MPa (0.5 kgf / cm ² ) under perforated rubber in three sections. The duration of the air supply in the section is 1-3 s, the time between air supply in the section is 5-15 s.

 The pressure level and the duration of the compressed air supply under the perforated rubber are set based on the achievement of a maximum deflection of not more than 200 mm and sufficient for the passage and aeration of the AML located above the rubber.

 The time between the air supply in the section within 5-15 s is set depending on the flowability of the AML. Moreover, the maximum cycle of 15 s is set for well-flowing, and the minimum 5 s for poorly flowing AML.

 The specified method of tedding without the use of mechanical elements ensures the safety of the process. Air from the unloader is removed through the valve and cyclone 17 into the atmosphere. The valve is closed in the absence of air supply and in vacuum in the unloader.

Claims (7)

1. A method of pneumatically transporting a powdery oxidizing agent with additives, including unloading the powdery oxidizing agent with additives from a container, supplying said oxidizing agent to an aerial chamber to bring it into a fluidized state, transporting it through a pipeline by air, settling in a unloader and collecting particles of this oxidizing agent, characterized in that the unloading of the powder of the oxidizing powder with additives is carried out after the destruction of the arches during vibration, while the said oxidizing agent b is passed through a vibrating grate, wiped on a wiper drum and brought into a fluidized state by using the porous septum, which is supplied with the aforementioned air chamber, when compressed compressed air is supplied through an oxidizer with a moisture content of not more than 0.8 g / m ³ and adjustable temperature and pressure, and particles this oxidizing agent is captured in a cyclone and continuously returned and mixed with the flow of said oxidizing agent entering and deposited in the unloader.  2. The method according to claim 1, characterized in that they use a vacuum pump and before starting the pneumatic transport installation into operation, provide a vacuum in front of the pump when its suction line is muffled not lower than -0,093 MPa, and when the muffled inlet to the pipeline is not lower than -0, 08 MPa. 3. The method according to claim 2, characterized in that during pneumatic conveying the pressure at the inlet to the pipeline is maintained from 0.07 to -0.02 MPa, and the air temperature at the outlet of the pipeline is not lower than 0 ^o .  4. The method according to claim 2, characterized in that they use a wet filter for additional trapping of particles, and the air ducts in front of the vacuum pump and the wet filter between the pneumatic conveying cycles are closed with valves.  5. The method according to claim 1, characterized in that after the transportation of the powdery oxidizing agent with additives along the bypass line, compressed compressed air is additionally supplied with a decrease in vacuum in front of the vacuum pump of not more than 0.03 MPa.  6. The method according to claim 1, characterized in that the walls of the cyclone are subjected to shock-pulse action for 20 s with an interval of 30 to 46 s.  7. The method according to claim 1, characterized in that when unloading the powdery oxidizing agent with additives from the unloader, tedding is carried out by using a rubber partition divided into three sections and supplying compressed, dried air to each of the sections for 1-3 s with a break between air supply in the section from 5 to 15 s.

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