Grid holes

Gas flowing from the grid holes can either take the form of a series of bubbles or a permanent jet, depending on system parameters and operating conditions. However, a permanent jet prevails for most industrial conditions. Jet penetration is one of the most important design parameters since ithelpsin ... 

Figure 1. Jet penetrations at grid holes for different orientations.

Therefore, the lowest grid hole has the lowest pressure drop, and hence for pressure drop, the criterion must apply with respect to the lowest hole on the grid. 

The gas velocity through the grid hole (orifice equation) ... 

Figure 3. Grid hole discharge coefficient design chart.

Hole Size. To increase the gas residence time in the bed, it is desirable to introduce the greatest number of small gas bubbles as possible into the bed. This can be achieved by maximizing N at the expense of dh in Eq. (10) (within the limits of mechanical, cost, and scale-up constraints). To minimize stagnant zones, the number of grid holes per m2 should be >10. In practice, the number of grid holes per square meter is generally about 2 to 3. 

The relationship between the grid hole pitch, Lh, and the number hole density (holes per unit area of the bed), Nd, depends on whether the holes are laid out in triangular or square pitch. 

Figure 4. The relationship between hole density and grid hole pitch for triangular and square pitch.

Shrouds are generally placed around grid holes to reduce the velocity at the gas-solids interface and reduce particle attrition. Shrouds simply consist of short pipes centered over the smaller grid holes which have been selected in size and number to operate at a hole velocity defined by Eq. (9). 

If excessive particle attrition is expected, it is a common practice to place a shroud/nozzle around a grid hole as discussed in Sec. 3.5. For properly sized nozzles, one can derive from Eq. (17), particle attrition is reduced by a factor ... 

LI low = elevation of lowest grid hole for curved grid, m... 

Pg,h = density of gas entering the grid hole (plenum conditions), kg/m3... 

Due to the pressure drop requirements across the gas distributor for good gas distribution, the velocity through the grid hole may be higher than desired in order to minimize or limit particle attrition. Therefore, it is common industrial practice to place a length of pipe (called a... 

Eroded grid holes] hole velocity too high/materials of construction/contaminants in fluid. [Excessive backmixing] [maldistribution] /[poor bubbling hydrodynamics]. ... 

Unstable catalyst bed] airflow too low/grid holes eroded/faulty grid design. 

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