Closed loop pneumatic conveying
Conveying solids in a closed loop
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| Section summary |
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| 1. Definition of closed loop pneumatic conveying systems |
| 2. Design of a closed loop pneumatic conveying system |
Closed loop pneumatic conveying is a specific application of pneumatic conveying for which the gas is recirculated. It is used in special applications where there is an interest to keep the conveying gas within the system and minimize its loss.
1. Definition of closed loop pneumatic
conveying systems
The large majority of pneumatic conveying systems are open, which means that the gas is released in the environment after conveying and separation with the conveyed material. This is applied typically with air, which is a free conveying media.
However some applications may not accept such an operation, typically if the conveying gas is not air, for example nitrogen, for very specific process reasons such as dust explosion protection, or prevention of oxydation of the conveyed solid. The solid might be also radioactive or toxic thus the air would be contaminated and should remained confined.
For such specific applications, letting the gas escape to environment after conveying will be uneconomical as huge quantities of gas, and thus high costs, will be required to supply the conveying line in nitrogen. It becomes then crucial to re-use the gas after solid-gas separation to convey again materials by recycling it to the beginning of the line. The gas is thus circulating constantly in the conveying system which is a closed loop from the gas perspective.
The solids load ratio is defined as the ratio of the mass of solids transported over the mass of air - or other gas - transporting the solids. Along with the air conveying velocity, the solids load ratio is very important parameter to characterize the flow of the solids in the conveying pipe and classify it as dilute phase, dense phase or intermediary.
2. Design of a closed loop pneumatic conveying system
Figure 1 : typical design of a closed loop pneumatic conveying system
A closed loop pneumatic conveying system retains some commonalities with a regular system. There is always :
- A gas mover / blower
- A conveying pipe
- A solids feeding system
- A gas- separation system at the end of the conveying system
However, recirculating the gas is bringing a certain number of challenges that must be adressed :
- Through recirculation and compression in the blower, the temperature of the gas inside the loop will increase significantly
- There are always losses of gas in the system, if not compensated, there may be a vacuum created in the system, particularly at the blower suction
- The gas may be loaded in fines which can lead to a damage of the blower
As a consequence, to address those points, the design must present the following features :
- An heat exchanger should be installed in the closed pneumatic conveying loop, either before or after the blower in order to maintain the temperature constant over time
- There should be a point, before or after the blower at atmospheric pressure that will serve as a reference to admit or purge gas. The system will operate in pressure or in vacuum depending on the location of the reference point (before or after the blower). The gas and purge can be used at start-up to fill up the line with the conveying gas. The use of an online analyser (for example to measure oxygen %) may also be necessary.
- Efficient filters must be installed at the gas-solid separation receiver, and a safety filter should be position after to catch dust in case of failure of the main filter which will avoid the dust to reach the blower. A way to detect a leak of the main filter, for example by having a pressure difference measurement on the safety filter should be implemented.
- Proper instrumentation (pressure sensors, temperature sensors, pressure differential sensors on filters)
The points above are only the key features of a closed loop conveying system and are not exhaustive or sufficient to design and build such a system. One should get in touch with reputable companies having experience of those designs for an actual project.
Source
Bulk Solids Handling, Equipment Selection and Operation, Don McGlinchey et al, Blackwell Publishing, 2008