Aerated standpipe

As the fluid column in the annulus is aerated, standpipe pressure will drop. Additional compressors (i.e., increased air volume) can then be added to further lighten the fluid column and unload the hole. 

Aerating standpipes is sometimes necessary in order to achieve optimum solids flow through them. However, too much aeration is just as detrimental as too little aeration. Too much aeration leads to bubble formation and slugging. Often seal pots operate satisfactorily with no additional aeration added to the standpipe, especially with group B solids. However, it is usually wise to add aeration taps into the standpipe discharging into the seal pot. They will be there if needed and do not have to be used if they are not. An aeration point at the bottom of the standpipe and aeration points approximately every 2 to 3 m of solid seal height should be sufficient. It is also best to use a separate aeration control rotameter for each aeration point, especially in research units. In a commercial unit, restriction orifices are generally sufficient to ensure equal flows to each aeration location. 

Note that the soHds density used ia this equation should be the tme soHds, ie, skeletal, density, because the gas ia the pores is also compressed. For Group A soHds the aeration gas should also be added evenly along the standpipe. 

Group B soHds have higher minimum fluidization velocities than Group A soHds. For best results for Group B soHds flowing ia standpipes, standpipe aeration should be added at the bottom of the standpipe, not uniformly along the standpipe. 

Like the regenerated catalyst standpipe, the spent catalyst standpipe may require supplemental aeration to obtain optimum flow chin acteristics. Dry steam is the usual aeration medium. 

Every 5-8 ft (1.5-2.5 m) along the standpipe use rotameters to regulate aeration flow... 

Too little, too much, or no aeration gas either with the catalyst entering the standpipe or along the standpipe... 

To retain fluidity of the catalyst and to maintain catalyst densities in the 35 to 45 Ib/ft (560-720 kg/m ) range (the fluid range), many standpipes require external aeration gas to be injected into the down-flowing... 

Ensure that the correct amount of aeration gas is injected along the standpipes. One procedure is to vary the aeration flow until the maximum slide valve differential is observed. 

Restriction orifices with upstream pressure regulators are frequently employed to distribute aeration gas into the standpipes. The orifices... 

If the unit pressure balance indicates that either the pressure gain in the standpipes is inadequate or the delta P across the slide valves is erratic, standpipe aeration and instrumentation should be examined. Redesigning the aeration systems or replacing the standpipes can gain valuable pressure drop. Proper instrumentation can include independent aeration flow to each tap, flow indicators/controllers on each, and differential pressure indicators between the taps. 

As the fluidized catalyst descends the standpipe, the increasing pressure compresses the fluidizing gas resulting in a decrease in the gas volume. If allowed to continue without adding aeration, the flowing catalyst will defluidize leading to unstable flow and potential loss of catalyst circulation. This is particularly true... 

Assuming a catalyst density at flowing conditions in the standpipe of about 90% of the catalyst bulk density, the amount of excess gas above minimum fluidization that is entrained with the catalyst into the standpipe may be calculated. Sufficient aeration should be added to sustain minimum fluidization along the length of the standpipe. 

The choice and properties of the aeration gas are important factors for maintaining stable standpipe operation. The condensate source for steam aeration can cause several problems. If the steam is not kept dry, the condensate can lead to stress cracking of the tap piping, plugging of the tap nozzle with mud, erratic aeration rates, orifice erosion, and potentially catalyst attrition. Similar problems can occur with wet fuel gas as an aeration source. When possible, dry air and/or nitrogen are preferred rather than steam as aeration media for standpipes. However, in actual... 

The circulating catalyst physical properties have a direct impact on fluidization and stable standpipe operation. Mechanical problems may cause a loss of catalyst fines, or a change in catalyst density both of which will impact fluidization and may require adjustment to the standpipe aeration. 

---