Standpipe aeration

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. 

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. 

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. 

Overaeration of an unstable standpipe is a common response to process or catalyst changes not easily recognized. Factors influencing standpipe operation such as catalyst mass flux rate, catalyst density and PSD, standpipe aeration and pressure profile, and unit configuration should be thoroughly evaluated before making large adjustments to the aeration. 

Figure 6. Calculation of the dimensionless solids throughput V as a function of the aeration rate for sand of 0,15 mm diameter in a standpipe aerated about midway down and operated with a modest backpressure. Note the existence of two steady values of V for some ranges of Ua the dashed line represents an unstable portion of the curve. From Mountziaris [36],...

For long standpipes aeration gas will need to be added at several levels in order to keep the voidage within the required range (see the worked example on standpipe aeration). 

With the fluidized underflow standpipe, aeration gas is added to the standpipe to maintain the solids in a fluidized state as they flow down the standpipe. As the solids flow down the fluidized underflow standpipe from a low pressure to a higher pressure, the gas in the standpipe is compressed, which causes the solids to move closer together. When the standpipe is operating at low pressures, the percentage change in gas density from the top of the standpipe to the bottom can be significant. If aeration is not added to the standpipe to prevent this, the solids can defluidize near the bottom of the standpipe (Fig. 11 A). Defluidization of solids in the standpipe results in less pressure buildup in the standpipe and a reduction in the solids flow rate around the loop. 

Table 4 lists the regimes in different parts of the FCC along with typical densities. The regime achieved is a direct result of the typical operating velocities quoted in Table 1. Of all the FCC components, the standpipes can be the most fickle. Depending on the relative velocity of gas to flowing catalyst, standpipe densities can vary from 0 to 40 Lb/FF if the standpipe is underaerated and back down to 0 Lb/FF again if there is overaeration. This explains why so many catalyst circulation problems in FCCs emanate from standpipe aeration problems.

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. 

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. 

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... 

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... 

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