PTQ Q1 2025 Issue

FCC unit stripper design and troubleshooting

Efficient removal of hydrocarbons from spent catalyst depends on multiple variables and is influenced by stripper design, as shown in the following discussion

Warren Letzsch FCC Consultant

T he purpose of the fl uid catalytic cracker (FCC) strip- per is to remove hydrocarbons from the spent cata- lyst before it enters the regenerator or the catalyst transfer line, which takes the spent catalyst to the regen- erator. The catalyst leaves the reactor and flows as a mix - ture of spent catalyst, hydrocarbons and dispersion steam. Steam is injected into the bottom of the stripper to push the hydrocarbons that are in the interstitial space between the catalyst particles back into the reactor, where they are recovered in the FCC gas plant. Hydrocarbons can also be desorbed from the surface of the catalyst and some of the pores. The stripper also promotes further cracking reac- tions, both thermal and catalytic. The void space in the stripper can be determined from the density of the catalyst bed, the skeletal density of the catalyst, and the total pore volume (PV) of the catalyst. If a cubic foot of catalyst in the stripper is considered, and the weight of the gases is assumed to be negligible compared to the weight of the catalyst, then:

Steam is added to displace the hydrocarbons in the strip- per. A pound-mole of steam has a volume of 379 SCF. If the stripper is at 1,000ºF and 22 psig, the volume becomes 425.5 ACF. Each pound of steam would have a volume of 23.6 ACF. These calculations can be made for any catalyst pore volume and density. A stripper is really a series of mixing stages where the catalyst and hydrocarbons flow down. A tracer such as helium injected into the top of the stripper can be measured as the percentage going out the top and bottom (H O and H i ). This is a measure of mixing efficiency but does not reflect the desorption of the hydrocarbons from the catalyst. A model of the stripper was constructed and showed that adding all the steam to the bottom of the stripper gave better performance than splitting the steam with 50% to the bottom and 50% added at the middle. Perfect mixing is assumed for each stage. The overall efficiency (E o ) is: E o = (1- (He o /He i )) (4)

(1)

while each stage has an efficiency of:

Catalyst density = Volume of catalyst x Skeletal density

E s = (1-E o ) 1

/n / 100

(5)

Volume of the catalyst pores = Weight of the catalyst x Total pore volume (2)

Stripper performance can be calculated from the stage efficiency and the number of stages from Table 1 . Eight

If the catalyst is not fluidised, then Equation 1 becomes:

Catalyst apparent bulk density = Volume of catalyst x Skeletal density

(3)

3.40

3.20

The skeletal density is the density of the solid portion of the catalyst (see Figure 1 ). Therefore, the density of the cata- lyst divided by the skeletal density is the fraction of solids in a cubic foot, and the rest is the total void space. From Equation 2 , the total catalyst pore volume is calculated, and subtract- ing from the total void volume gives the interstitial volume. As an example, a catalyst with a density of 45 lb/ft3 and a skeletal density of 150 lb/ft3 has a total void of 70%. The total pore volume is 0.30 gm/cm3, yielding 0.485 ft3 of pore volume and 0.215 ft3 of interstitial volume. For every 1,000 lb of catalyst circulated, the void space is:

3.00

2.80

2.60

2.40

Most catalysts

2.20

2.00

100

Wt. % Alumina

Figure 1 Skeletal densities of silica-alumina cracking catalysts

1,000 lb x 0.70 void space 45 lb/ft3

= 15.55 ACF

PTQ Q1 2025