The need to transport solids has always existed, from the first industrial revolution, when coal was transported to boilers in trucks, until today, when the same coal is extracted from the mines and transported to the plant for processing. More than two centuries have passed since the first industrial revolution (1760-1840) and the manner in which solids are transported has changed radically. Below, we will discuss the transportation of solids by means of pneumatic conveying systems.
Transportation of solids by means of pneumatic conveying systems
The objective of a pneumatic conveying system is the transportation of solid materials from one point to another by means of a flow of pressurised gas, either positive or negative, via a pipeline. One can transport materials with a particle size measuring from microns up to 20mm horizontally and/or vertically to distances of up to 2 kilometres and with capacities of up to 1000T/h through pipes with a diameter of up to 500mm.
What are the advantages and disadvantages of the Pneumatic Conveying System?
The main advantage of pneumatic conveying is that the systems are closed and therefore not pollutant. The material transported is enclosed within the pipe which protects the product from the environment and vice versa. In addition, it is a system which allows the product to be transported from one point to several unloading points, simplifying the transport route per plant.
Among the disadvantages it is important to note that not all materials can be transported pneumatically, but rather materials which are dry, non-cohesive and relatively fine. Fragile materials can suffer undue damage, and abrasive materials can cause wear and tear of the equipment and installation. Other limitations are the maximum particle size, the maximum transport capacity and the transport distance.
Systems for the pneumatic conveying of solids
There are several systems for the pneumatic conveying of solids. Below, we look at the most common:
- Most direct method of transport.
- The particles are transported diluted with the flow of air.
- Possible thermal processing of the material such as drying, heating and cooling.
- Method that uses the force of the air for transportation.
- It is used for liquefiable products.
- The product mixes homogeneously with the air.
Low pressure and dilute phase system
Of the different systems that exist for the transportation of solids, at the moment positive and negative low pressure, continuous, high velocity and dilute phase systems are the ones most commonly used in the industry. This is due to their greater transport capacity in terms of flow, greater transport distances and ease of regulation and monitoring.
Positive pressure dilute phase
In positive pressure pneumatic conveying, the motor component is normally a blower placed in the product starting point and it “blows” it with gas until it reaches the destination point. The pressure generated at the blower outlet can vary between 0 and 1000 mBar.
Negative pressure dilute phase
In negative pressure pneumatic conveying, the motor component is normally a blower or a vacuum pump. The suction element is placed at the product destination point and it “sucks” it together with the gas introduced at the starting point. The pressure generated at the blower input can vary between 0 and -500 mBar.
A major advantage of this conveying system is that it is cleaner than the propulsion system because in the event of leakage in the pipeline, the product will most likely remain inside the pipes.
Size and design of pneumatic conveying systems
To outline the operation of a pneumatic conveying system, we will use a state diagram. For a particular system, we plot the pressure drop per unit length of pipe, ΔP / L, as a function of the conveying gas velocity, 𝑈𝑔, for constant solid curves 𝑊𝑠.
The pressure drop depends on the velocity of the conveying gas and the flow of solids. In the case of dilute phase systems, the pressure drop increases when the gas velocity increases, whereas in the case of dense phase systems, the pressure drop increases when the gas velocity decreases due to the higher friction of the solid with the wall and the smaller effective area of the pipe.
There is an unstable area between both and an area in which it is not possible to convey the material.
For the design and calculation of a new pneumatic conveying system or to check whether an existing system is suitable, the physical characteristics as well as the fluidity of the material to be conveyed must be determined.
The nature of the material can significantly restrict the choice of pneumatic conveying system.
It is vital that you are familiar with the following properties:
- Size of the particles: maximum, minimum.
- Density and granulometry.
- Fluidity of the material.
- Others: abrasivity, toxicity, fragility, hardness, etc..
The conveying velocity is one of the most important parameters for the efficient design of dilute phase pneumatic conveying systems. Conveying systems designed to work at high velocities involve a high energy consumption, possible degradation of the material and excessive wear and tear of the installation. On the other hand, systems designed to work at low velocities or high solid flow rates can experience damage due to the depositing of particles in the pipes, unstable flow and can even lead to the pipe collapsing, stopping the system completely.
The wide variety of materials, the integration between both phases (solid-gas) and the large number of variables involved make theoretical modeling of these kinds of flow very complicated. Therefore, experimental studies are of vital importance in order to produce a database that will enable us to understand their workings and improve the design of systems.