(1) definition, composition and classification of cement bulk carrier
Cement bulk carrier refers to a special tank car that transports bulk powder such as cement, coal powder, fly ash, talcum powder, flour and other powders. Powder bulk transportation can improve transportation efficiency, save transportation costs, reduce product cost, and at the same time realize mechanization of loading, transportation, unloading and storage. The cement bulk carrier used in recent years is a pneumatic bulk carrier that uses pneumatic unloading. It consists of six parts, namely, automobile chassis, tank assembly, air compressor and air pipeline, discharge piping system, and Force transmission device, monitoring instrument and safety device, etc., pneumatic discharge is to pass compressed air with a certain pressure through the fluidization device at the bottom of the tank into the powder in the tank, so that the powder and air are mixed and present in a flowing state. Then, the discharge valve is opened, and the powder and air mixture are discharged under the pressure difference between the inside and the outside of the tank, and flow into the ground container through the pipeline.
The cement bulk carriers can be classified into the following four types according to their tank types:
1. Vertical cement bulk carrier
The vertical cement bulk carrier has a vertical centerline and the vehicle can carry one or more vertical tanks. Vertical cement bulk carrier has a wide range of applications and can be used for bulk transportation of powders, pellets and other powder and granule materials. However, the vehicle has a high center of mass, and the structure is complicated when using multiple tanks, and the manufacturing cost is also high.
2. Horizontal cement bulk carrier
The centerline of the tank is horizontal, and the tank can be a single tank or two compartments. If the fluid bed in the tank is at an angle to the horizontal plane, it is called an introverted horizontal tanker. If the centerline of the tank has a small inclination angle with the horizontal plane, it is a horizontally inclined cement bulk carrier. The horizontal cement bulk carrier has the advantages of simple structure, convenient operation, stable discharge performance and low mass center, but the applicability is limited, and generally only used for bulk transportation of powder with better fluidization performance.
3. Lift type cement bulk carrier
The lifting cement bulk carrier is in a horizontal position when loading and driving, and the lifting mechanism lifts the front end of the tank into a tilted state. The bottom of the tank of the lift type cement carrier is usually only provided with a fluidized bed at the discharge port, and the unloading into the tank body is inclined, and the powder particles automatically slide down under the action of gravity, and are concentrated to the discharge port. Unloading. Therefore, the internal structure of the tank body is simple, the volumetric efficiency is high, and the application range is wide, and the powder material with poor fluidization performance is often used for shipment. However, due to the addition of the lifting mechanism, the use and maintenance are complicated.
4. Bucket type cement bulk carrier
The tank of the bucket type cement bulk carrier is composed of a straight cylinder or a rectangular tube whose horizontal position is horizontal and a cone which is perpendicular to the center line, as shown in Figure 3-56. The bucket type cement bulk carrier usually does not have a fluidized bed, and is automatically discharged by the gravity of the powder. Therefore, it has a simple structure and a wide application range. The remaining amount is small, and the inside of the can is easy to clean.
Basic knowledge of powder fluidization
1. powder characteristics
The transportation of bulk powders, handling equipment and powder properties are closely related. The main characteristics of the powder are as follows:
(1) Particle size, the shape of the powder particles is irregular, and there are grooves, pits or holes on the surface, and the particle size is also different. Generally, the particles having a diameter within a certain range are defined as a particle group, and the average particle diameter of the particle group is referred to as a particle size. The particle size distribution in the powder directly affects the properties of the powder.
(2) Density density refers to the mass of powder per unit volume, and the unit is commonly used in kg/L3 or t/m3. Due to the difference in the void ratio of the powder (the ratio of the air volume between the particles to the entire volume, the cement is 0.66), the particle size distribution, etc., it can be further divided into:
a. True density. Refers to the ratio of the mass of the particles to the true volume of all the particles that do not include the voids on the surface of the particles and the voids before the particles.
b. Apparent density. Refers to the ratio of the mass of the particles to the volume of all particles that do not include the gap between the particles.
c. Particle density. Refers to the ratio of the mass of the particles to the total volume of the particles including the voids between the particles on the surface of the particles and the particles.
d. Bulk density. Also referred to as loose density, refers to the ratio of the mass of the particles to the volume of the container when the particles are filled in the container. Commonly used are particle density and bulk density.
(3) The angle between the cone busbar and the bottom surface formed by the repose angle in the powder pile is called the static angle of repose; the angle of repose formed by vibration when stacked is called the dynamic angle of repose. The size of the angle of repose is related to the particle size, internal friction angle and adhesion of the powder.
(4) Friction angle The friction angle represents the coefficient of friction between the material and the solid wall. The magnitude of the friction angle is related to the particle size, adhesion, and material, shape, and surface roughness of the wall.
(5) Adhesive adhesion refers to the adhesion between the powder and the wall between the powder. According to the analysis, the adhesion between the particles and the interaction between the particles and the wall are mainly the attraction between the molecules, the electrostatic force and the capillary force of the water. Adhesion is related to the characteristics of the powder, moisture and wall materials, and surface roughness.
(6) Moisture content The moisture of the material includes free water attached to the surface of the particles and chemical water incorporated inside the particles. Chemical water is a constituent of the particles. Free water is the water content of the powder, expressed by the water content w:
M1—-the mass before the sample is dried (kg);
M2—-The mass (kg) of the sample after drying at a temperature of 105 ° C for 2-4 h.
2. Suspension speed and sedimentation speed of materials
The levitation speed is the velocity of the airflow when the material particles are in suspension in the plumbing pipe. Suspension speed is the main kinetic performance of the powder, indicating the difficulty of pneumatic conveying. The suspension speed of the material is generally determined experimentally.
The sedimentation velocity means that when the material particles fall freely in the still air, the descending speed gradually increases due to gravity, and the air resistance of the particles also increases. When the air resistance increases to be equal to the floating weight of the particles (the difference between gravity and air buoyancy), the material particles drop at the same speed at the maximum speed at this time. This constant falling speed is called the sedimentation velocity of the material particles. The suspension speed of the material is equal to the sedimentation velocity and the direction is opposite.
3. Fluidization of materials
Fluidization is the process of imparting certain similar liquid characteristics to a bed after it has been introduced into the bed of powdered granules. The container is filled with the granular material, and a gas permeable member is arranged at the lower part of the material to support the powder. The flow rate of the gas through the cross section of the container is u, and the actual flow rate of the gas passing through the gap of the powder layer is obviously, < u. As u changes, various phenomena will occur in the granular bed.
(1) Fixed bed When the air velocity is small, the airflow passes through the gap between the particles, the particles are stationary, the void ratio E of the bed is constant, the bed maintains the original height, and the velocity of the airflow through the bed And the pressure loss segment. This state of the powder is referred to as a fixed bed.
(2) After the critical fluidized bed gas velocity u increases to a certain value, the resistance of the gas flow through the bed is just equal to the bulk strength of the powder on the bed, the bed begins to expand, and the void optimization method increases with increasing u. As shown in the middle section. As a result of the increase, the area of the intergranular channel also increases, so the actual flow rate u0 of the gas stream passing through the bed does not increase. Therefore, the pressure loss of the gas flow does not increase due to the increase of u. At this time, the gravity of the powder and granules is no longer directly supported by the gas permeable member, but by the frictional state between the gas and the powder and granules. For each particle, it is no longer maintained by the contact of adjacent particles, and they are free to move in the bed. The pressure drop across any of the sections of the bed is approximately equal to the gravitational force of the powder body on the section, and the height of the bed increases as the bed increases, but has a distinct upper interface. The bed begins to produce this varying airflow rate.
(3) In the fluidized bed and the fluidized bed, if the airflow velocity ratio is not large and the particle size of the powder is small, the bed of the granular body expands continuously, and the average distance between the particles increases. Uniform flow in the bed, becoming a fluidized bed, or a first-class bed; if the gas flow rate is much larger, the excess gas will accumulate in the form of bubbles and flow through the bed, then become a poly flow A fluidized bed or a bubbling fluidized bed, a non-uniform fluidized bed. The bed heights of the two fluidized beds are increased, but there is still a distinct upper interface.
(4) Dilute phase fluidized bed, when the gas filling speed in the bed increases to the material suspension speed, the powder and granules begin to fly out of the upper interface and enter the upper space. When the air velocity exceeds, the particles will be taken out of the container. Forming a dilute phase fluidized bed. At this time, the void ratio sharply increases, and the pressure drop caused by the friction loss between the particles sharply decreases, and the solid-gas two-phase–dilute phase pneumatic conveying state in which the powder and the gas are composed is actually formed. If this state occurs in the bed, the discharge cannot be completed. It can be seen that the gas flow rate of the fluidized bed can only be between and between.
4. Liquid-like properties of fluidized bed powder
The powder of the fluidized bed has some liquid-like properties. When the large and light particles have the bouncing property, the particles are easily returned to the bed when the particles are pressed; after the pressure is released, they bouncing again into the upper space. When the bed has fluidity, when the container is tilted, the upper interface can still be level and the particles can be ejected from the side wall apertures. When the two fluidized beds are connected, the particles can flow from the high bed to the low bed, automatically balancing the bed height, and the pressure difference at any two points in the bed is substantially equal to the static pressure at these two points. The pneumatic unloading of the cement bulk carrier utilizes these characteristics of the fluidized powder to achieve pneumatic conveying.