Carbon black has excellent reinforcing effect and is widely used in rubber products such as tires. Because the carbon black particle size is very small and easy to fly, the closed pneumatic feeding system is usually used to effectively prevent the pollution caused by the carbon black spill. The pneumatic feeding system has strict requirements on the crushing of carbon black particles and the residual amount of pipes. The reason is that the fine powder content in carbon black directly affects the mechanical properties of the rubber material, while affecting the fluidity of carbon black, extending the feeding time of carbon black, and increasing the amount of dust will also reduce the amount of carbon black actually entering the mixing machine, affecting the quality and efficiency of rubber mixing. In this paper, the selection and design considerations of the main components such as feeding device, air flow control element, feeding pipe, separation and filtration equipment in the carbon black pneumatic feeding device of rubber factory are discussed.

1 Selection of pressure feeding tank and its control components

1.1 Selection and layout of pressure feeding tank

The arrangement of pressure feeding tank includes single feeding tank, parallel type and series type. The layout requires sufficient installation space. For carbon black feeding, in order to reduce the crushing rate of carbon black, large-volume sending tanks should be used, and parallel arrangement is generally adopted. It is best to design the volume of the pressure feed tank to be equal to the volume of the corresponding space bag for carbon black packaging to improve the feeding efficiency.

The design pressure of the pressure feeding tank is generally not less than 0.45MPa, and the best design is 0.60MPa. The pressure feeding tank is a pressure vessel and must be designed, processed and inspected according to relevant national standards. The top of the pressure feed tank needs to be installed with a safety valve, and the exhaust capacity of the safety valve is calculated according to the drainage area of the drainage device in the B.8 clause of Appendix B of the national standard GB150.1-2011 Pressure Vessels Part 1: General Requirements. The size of the safety valve is related to the volume of the pressure feed tank, the flow rate of the feeding and cleaning air (referred to as air volume) and the size of the intake pipe. The safety valve specifications of the feeding system with different feeding pipe diameter should be different. Table 1 shows the safety valve specifications for different inlet pipes of the pressure feed tank when the relief pressure (absolute pressure) of the safety valve is 0.643MPa. It is recommended to select the safety valve specifications according to the parameters listed in the table.

1.2 Compressed air flow control device

Pneumatic feeding depends on compressed air, the flow of compressed air affects the feeding capacity, feeding state, feeding energy consumption and other key indicators. Reasonable feeding gas volume can reduce the air speed in the feeding pipeline (referred to as gas speed), can alleviate the wear of the pipeline, reduce the crushing rate of the feeding material particles, reduce the pressure loss and energy consumption. In the design of carbon black pneumatic feeding system, it is very important to control the air volume, especially to reduce the crushing rate of carbon black particles. At present, dense phase feeding is usually used for carbon black feeding, which can achieve a larger ratio of material to gas, but for the same system, a material under certain feeding conditions, the maximum ratio of material to gas is limited, if the ratio of material to gas is too large, there will be tube blocking phenomenon. In the design, the supply of compressed air needs to be determined according to the capacity of the feeder and the pressure change in the feeding pipe, so as to achieve the highest feeding ratio without tube blocking.

Rubber factory carbon black pneumatic conveyor feeding geometric distance is generally 30~200m, a feeding system can feed a variety of carbon black, and can provide multiple sets of auxiliary system carbon black dosage, to meet the different feeding distance. According to different materials and different feeding distances, it is very important to use different feeding air flow rates. At present, there are the following air flow control methods: 1) Combination of manual pressure reducing valve and fixed Laval tube. The air flow can not be automatically adjusted, once the debugging is completed, the air flow is unchanged, for short distances, a set of auxiliary machinery system (feeding distance changes little) is more economical. 2) Combination of proportional valve and fixed Laval tube. The air flow is controllable within a certain range, and the inlet pressure of the Laval tube is changed by automatically changing the proportional valve setting pressure, thus regulating the flow rate. According to the pressure requirements of the feeding resistance and the critical flow of the Laval tube, the set pressure of the proportional valve is generally not less than 0.25mpa, generally 0.25~0.35MPa, and too much pressure will affect the service life of the aluminum alloy lined rubber tube. The resistance of the feeding pipe is preferably less than 0.20MPa, generally 0.10~0.15MPa, especially for the system where the feeding distance of different target tanks is more than 100m, it is very difficult to control the gas speed of different carbon black and different target tanks in a reasonable range. 3) Pressure reducing valve and automatic adjustable Laval tube volume automatic control system. The feeding method is a gas control method commonly used by internationally renowned pneumatic feeding equipment companies, most of which belong to patented products. The gas control range is wide, especially suitable for different materials and different feeding distances, and the gas control is stable and accurate. 4) Combination of pressure reducing valve and flow regulating valve. At present, the German Zeppelin company introduced a feeding method that consists of 1 pressure reducing valve and 2 flow regulating valves to automatically control the pressure feed tank and the auxiliary pipe air flow, the auxiliary pipe air flow accounts for about 50% of the total feeding volume, the feeding distance is less than 100m, and the air flow rate at the end of the feeding pipe is less than 7.5m/s. The pressure of the feeding main and auxiliary pipes is controlled by a pressure reducing valve, which needs to use a large size pressure reducing valve, such as the diameter of the feeding pipe is 225mm, the inner diameter of the air supply main pipe is 100mm, and the diameter of the pressure reducing valve is 100mm. No matter what kind of control method is used, pressure reducing valves, proportional valves, and air filters are generally essential, and their selection has a very important impact on the performance of the pneumatic feeding system.

1.2.1 Selection of pressure reducing valve and proportional valve

The role of the pressure reducing valve is to control the change of air pressure to achieve the required pressure of the system feeding. By correctly selecting the pressure reducing valve, the gas flow required by the entire system can be ensured under a small air source pressure, to avoid the situation of pipeline blockage when the air source pressure is reduced, to ensure that there will be no residual materials when the pipeline is cleaned, and to reduce the energy consumption (the air source pressure is reduced from 0.7MPa to 0.6MPa, which can save about 12%). Most carbon black pneumatic feeding systems require the air source pressure to be greater than 0.6MPa, and the gas tank pressure to be no less than 0.55MPa. According to experience, when the pressure of the gas storage tank is less than 0.55MPa, the probability of pipeline blockage increases significantly. The reason is that there are problems in the configuration of pressure reducing valves, filters and air pipelines, and the air source system can not provide the amount of gas required for the feeding process, and the pipeline is blocked and the cleaning is not clean.

The pressure characteristic curve of the pressure reducing valve is shown in Figure 1 ". The vent of VEX193-20 proportional valve of SMC(Guangzhou) Pneumatic Components Co., Ltd. is set as R, the pressure of inlet P is 1.0MPa, and the pressure of outlet A is 0.1-0.9mpa. The corresponding air flow characteristic curve is shown in Figure 2[2]. The model of the reducing valve can be selected according to the pressure, flow characteristics and operating conditions of the reducing valve. At present, large air pressure reducing valves are divided into direct acting and pilot operated. Direct acting pressure reducing valve is divided into non-balanced and balanced type 2, of which non-balanced pressure reducing valve structure is simple, low price, but its pressure characteristics are not good, generally can not be used for pneumatic feeding system. As can be seen from Figure 1, the pressure characteristics of the balanced pressure reducing valve are very good, and the outlet pressure is very stable at different input pressures.

It must be noted that the flow characteristics of different brands of pressure reducing valves of the same inlet and outlet sizes are different, such as the diameter of 50mm SMC(AR925 type pressure reducing valve of Guangzhou Steam Components Co., LTD.) is not only different from the maximum allowable flow of VEX193-20 type proportional valve, but also different from the flow of the same caliber pressure reducing valve of the British Norguan brand. Therefore, it is necessary to select according to the characteristic curve of the feeding air flow and the cleaning flow. When the inlet pressure of the reducing valve is fixed, the flow rate will change with the change of the set outlet pressure. As can be seen from Figure 2, the greater the set pressure of the pressure reducing valve, the greater the flow rate. For example, the VEX193-20 proportional valve has a diameter of 50mm. When the outlet pressure is set to 0.2MPa, the maximum volume flow rate is 30000L/min. When the pressure is set to 0.3MPa, the maximum volume flow rate reaches 38000L/min, that is, the mass flow rate is 0.76kg/s. The mass flow of the pressure reducing valve at different pressures can be calculated by the formula [3].

If the main pipe and auxiliary pipe respectively adopt a proportional valve of this specification, the inner diameter of the feeding pipe is 250mm, and the gas velocity at the control end is 12m/s, the volume flow rate of the feeding gas is 2120m/h. For the feeding system, if two proportional valves are used, the gas mass flow rate can be 0.56kg/s, that is, the volume flow rate is 1680m/h, which cannot meet the requirements. If the feeding distance is large, the end gas velocity is greater than 12m/s, or the outlet pressure of the proportional valve is reduced during debugging, and the air source pressure is less than 0.45MPa during the feeding process (some users reach the minimum pressure of the air source 0.3MPa during the feeding process), the feeding volume can not be met. It is recommended to use a 65mm diameter pressure reducing valve or proportional valve for this feeding system, or add a pressure reducing valve to the cleaning gas path.

1.2.2 Selection of Laval tube throat diameter

Laval tube plays the core role in gas volume control, and the adjustable Laval tube has a crucial influence on the control of the whole pneumatic feeding system. When the inlet pressure of the Laval tube is stable, the gas volume can be controlled by changing the displacement of the regulating rod to adjust the flow rate. The control rod and the throat diameter of the Laval tube form a circular flow area of the adjustable Laval tube, and the sonic speed is reached at the minimum cross section of the pipe. According to the fluid mechanics continuity equation, the flow rate through the throat diameter is equal to that at any section, so the flow rate in the pipeline no longer changes, realizing the Laval pipe flow control. For the carbon black pneumatic batching system, when the adjustable Laval tube is selected, its throat diameter should be determined according to the grade of the carbon black fed, and it is generally designed according to the carbon black with larger gas volume required, such as the grade N660 carbon black and large particle white carbon black.

2 Feeding pipe

Carbon black pneumatic feeding pipeline is generally used for feeding by external bypass pipeline, referred to as double pipe feeding. The feeding pipe has two feeding forms: single auxiliary pipe and double auxiliary pipe. The single auxiliary pipe upper gas booster valve adopts series arrangement, while the double auxiliary pipe booster valve adopts parallel arrangement. The parallel arrangement of the booster valve is more conducive to pipeline dredging, because it can judge the blockage position according to the pressure sensor on the feeding pipe, and dredge the blockage position by supplementing the air separately. However, when the single auxiliary pipe is blocked, it needs to fully replenish the air between the blockage position and the pressure feed tank. For the case of serious blockage, the physical material may be pressed, so that the material can not be dredged. The feeding head has an aluminum alloy lined rubber tube, the advantage is that there is no carbon black adhesion, the residual amount of the pipeline is small, but the rubber lining is aging, so it is easy to break, and the price is higher. The structural oil absorption value and specific surface area of carbon black particles have a great influence on the adhesion of carbon black to the surface of equipment and pipelines. For example, carbon black grade N772, N774, the use of metal pipes is easy to appear pipe adhesion, blockage.

When there is a sign of blockage in the carbon black feeding pipe, the compressed air in the by-pass intake pipe enters the feeding pipe through the by-pass intake pipe, and the carbon black is cut into a short plug by the air flow, so as to achieve normal feeding. The physical properties and feeding distance of the carbon black to be fed are related to the distance between the two intake pipes on the bypass pipe. The filter nozzle and check valve are set in the bypass pipe to prevent the failure of the bypass pipe caused by the carbon black backflow to the pipe when the air flow in the bypass pipe stops.

According to the analysis of the carbon black pneumatic feeding system of Zeppelin, Germany, the inner diameter of the main pipe is 225mm, the inner diameter of the auxiliary pipe is 100mm, the cross-sectional area ratio of the auxiliary pipe and the main pipe is 5:1, and the air volume entered by the main pipe and the auxiliary pipe during the feeding process accounts for 50% of the total feeding air volume (the sum of the main air volume and the auxiliary pipe air volume). In the feeding system of a company in Liaoning Province of the United States Cyclonaire Company, the inner diameter of the feeding pipe is 150mm, the inner diameter of the auxiliary pipe is 50mm, and the cross-sectional area ratio of the auxiliary pipe and main pipe is 9:1. The maximum gas volume of the auxiliary pipe can reach 70% of the total feeding gas volume, and there is no replenishing gas at the top of the pressure feeding tank during the feeding process. In the feeding system of a company in Guangdong Province, the inner diameter of the feeding pipe is 250mm, the inner diameter of the auxiliary pipe is 80mm, and the cross-sectional area ratio of the two is 9.8:1. The cross-sectional area ratio of the feeding pipe and auxiliary pipe used in the feeding system is related to the air speed and area in the pipeline, and the air speed reflects the feeding state. The above three cases are very different from the results of some domestic studies that the diameter ratio of main and auxiliary pipes is 4:1 (the cross-sectional area ratio is 16:1), which may be due to the fact that these studies mainly focus on fly ash feeding. Because the fluidization and gas storage performance of the fly ash is very good, the supplementary gas volume of the auxiliary pipe is small. For carbon black feeding, considering the strict gas volume control, the end gas velocity must be controlled within a reasonable range. The auxiliary pipe plays a very important role in the feeding process, mainly through the auxiliary pipe boost to make the carbon black flow to the front of the feeding with short plugs, and no long plugs are allowed. Therefore, the opening pump pressure of the pressure feed tank is required to be less than 0.3MPa. During the feeding process, the air volume at the top of the pressure feeding tank and the feeding speed should not be too large.

The auxiliary pipe diameter is designed according to the total feeding and cleaning air volume, and the auxiliary pipe air volume accounts for about 50% of the total air volume. The auxiliary pipe pressure is 0.05~0.1MPa larger than the main pipe pressure. The gas velocity in the auxiliary pipe is less than 10~15m/s, and the maximum allowable is 20m/s during cleaning. It is recommended that the carbon black feeding boost distance should be 0.5~1.5m, and a boost device consisting of a filter nozzle and a check valve with a wire connection size of 6.35mm should be adopted. When the inner diameter of the feeding tube is less than or equal to 125mm, the boost distance can be determined to be 1.0m, and the boost distance of the feeding tube with an inner diameter of 150mm should be 0.75m. The boost distance of the feed tube with an inner diameter of 175mm is preferably 0.5m. The larger the inner diameter of the feeding pipe, the larger the supplementary air volume of the auxiliary pipe is required. Reducing the boost distance can increase the supplementary air volume, and can also increase the boost valve specifications, such as using a nozzle and check valve with a wire size of 0.95mm, or using other large flow anti-pollution check valves, you can appropriately increase the boost distance, but the maximum should not exceed 1.5m.

With the continuous development of polymer science, new breakthroughs have been made in the materials of feeding pipes. Now many domestic and foreign enterprises in the production of feeding pipes, the application of a new polymer material - ultra high molecular polyethylene. The special arrangement of the molecules inside ultra-high molecular polyethylene makes it have unique properties, and the friction coefficient is close to 1/6 of the metal material, which can prevent material adhesion, especially for the feeding of carbon black, which can greatly reduce the crushing rate and grinding rate of the particles, and the feeding resistance is small.

3 Target tank and dust removal equipment

It is very important to select high quality pulse valve and pulse control device in the air feeding separation and filtering equipment of carbon black. For outdoor installation also need to consider antifreeze pulse valve. Once the dust collector cleaning device fails, the feeding pressure is generally 0.25~0.35MPa, and the gas into the target tank can not be discharged, and it will be guided