Pneumatic conveying plug flow

A. Levy, Two-fluid approach for plug flow simulations in horizontal pneumatic conveying, Powder Technol. 112 (2000) 263-272. 

Incremental models track the local conditions of the gas and particles through the dryer, mainly in one dimension. They are especially suit le for cocurrent and countercurrent dryers, e.g., flash (pneumatic conveying) and rotary dryers. The air conditions are usually treated as uniform across the cross-section and dependent only on axial position. This method can also be used to determine local conditions (e.g., temperature) where a simpler model has been used to find the overall drying rate. A two- or three-dimensional grid can also be used, e.g., modeling vertical and horizontal variations in a band dryer or plug-flow fluidized bed. 

The model is effective and appropriate for dryers where both solids and gas are approximately in axial plug flow, such as pneumatic conveying and cascading rotary dryers. However, it runs into difficulties where there is recirculation or radial flow. 

For pneumatic conveying all the particles are evenly dispersed in the gas. This makes contacting ideal or close to ideal. The plug flow model is thus well suited for the dilute transport reactors, but has also been used for the denser fast fluidization regime neglecting gradients in the solids distribution. For first order reactions the model can be written as ... 

Pneumatic conveying dense phase No jJow or flow < design plugged line/mal-function of line boosters because of stuck check valve/high humidity. Solids fed to conveying line < design ratio of air to fluidize in the blow tank relative to convey is too small/fault in control system. 

The layout of the piping systems has many important factors in pneumatic conveying. One should keep the flow path as the most direct between two points. Bends should be eliminated as much as possible. Care should be taken in the design that the distance after a feed point before the first bend is inserted in a minimum of 3 meters (10 feet) when two-phase conditions are present. If the flow is dilute or dense, this distance is not crucial. The two-phase condition tends to cause a sloshing of the solids in the bend in an unsteady condition. This sloshing behavior causes plugging and other upsets in the operation of the pneumatic conveying systems. As noted before, one should at all costs avoid more than two bends in quick succession. 

The first section was a 238 m horizontal pipeline from the ice plant to the top of the vertical mineshaft, within which pneumatic conveying was employed to transport the ice. Depending on the superficial air velocity, the flow could be Mly suspended dilute-phase (above 40 m/s) stratified dilute-phase (30-40 m/s) sliding clusters (27-30 m/s) or plug flow with nearly fiill-bore plugs (20-27 m/s). Similar successive flow regimes have been described frequently for a variety of conveyed materials [e.g. 6,7]. In this case the saltation point... 

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