Sep-2016

Troubleshooting standpipe aeration

Standpipe aeration plays a critical role in smooth catalyst circulation in the riser and reactor of a FCC unit. A refiner details guidelines for effective aeration.

RAHUL PATIL, AJAY GUPTA and ASIT DAS
Reliance Industries Limited

Article Summary

Fluidisation of catalyst is most important in catalytic cracking units since the rate at which catalyst is circulated in the riser reactor configuration decides the overall performance of the unit. Difficulty in catalyst circulation can lead to a lower than desired gas to oil ratio, resulting in less conversion. Besides lower conversion, improper aeration can lead to vibrations in the riser-reactor structure, which is cause for concern from the point of view of safety and reliability of operation. Standpipes play a major role in establishing the smooth circulation of catalyst from the FCC riser-
reactors to the combustor-regenerators and vice versa.

A standpipe is essentially a pipe connecting the reactor to the combustor and the regenerator to the riser through which solids flow. The solids in standpipe flow downward due to gravity. The riser-reactor and combustor-regenerator system needs to be in perfect pressure balance to get smooth circulation of catalyst in the system. The solids in the standpipe must therefore be well fluidised to ensure a high pressure build-up between the two ends of the standpipe.

The fluidised FCC catalyst emulsion descends from the top of the standpipe and undergoes a form of compression due to the increasing pressure head seen at any given depth in the standpipe. This increasing pressure compresses interstitial gas as well as gas entrapped in the pores of the catalyst. The net result is a reduction in the volume of fluidising gas surrounding catalyst particles, resulting in an increase in emulsion density. Aeration along the length of the standpipe is thus essential to keep the solids in a minimum fluidisation state. Failing to maintain the proper aeration rate to the standpipe may lead to irregular flow and poor pressure build-up. Inadequate or excessive aeration are both undesirable for fluidisation in the standpipe. Inadequate aeration may lead to a packed bed regime of catalyst due to loss of fluidising gas below the incipient fluidisation regime, and can cause non-uniform catalyst flow through the standpipe. Excessive aeration may lead to large bubbles inside the standpipe, which can cause obstruction to catalyst flow, resulting in non-uniform catalyst flow. Optimum aeration and aeration locations are therefore critical for operation of the FCC unit. The aeration rate should be just enough to restore the catalyst emulsion to its desired volume at each location in the standpipe (see Figure 1).

There are methods available in the literature to calculate the required aeration rate. These require separate calculation for each small section of the standpipe to obtain the total aeration rate by adding the results of each section. We could obtain an accurate, unified, integrated expression for the whole length of the standpipe to calculate the aeration rate in one go.

Method from literature
The simple method given in the literature can calculate the aeration rate in the standpipe.¹ The following steps are involved:
Calculation of the volume of catalyst emulsion travelling down the standpipe per minute:

                                   (1)

Calculation of the total volume of interstitial and inter particle gas that is circulated with the catalyst:

                                   (2)

Calculation of the absolute pressure at the outlet aeration tap (see Figure 2):
    
                                   (3)
 
Calculation of gas volume change due to pressure increase across the aeration tap:
            
                                  (4)

If the distance between each aeration tap, the catalyst circulation rate, the catalyst skeleton and the minimum fluidisation density are known, one can easily calculate the aeration rate at each tap.

Calculation of aeration rate
Consider a small element of height dZ at height Z from the inlet of the standpipe, as shown in Figure 3.

The total pressure at the inlet is PT (kg/cm2) and H (m) is the total elevation of the standpipe up to the slide valve. Catalyst is entering the standpipe at a circulation rate of MC (kg/h) with a flowing density of ρT (kg/m3). The stripper is operated such that the catalyst enters the standpipe at minimum fluidisation condition i.e. with minimum fluidisation density ρmf (kg/m3). So, ρT = ρmf (normally in the range of 550-600 kg/m3). Pi is the pressure at height Z and Pi+1 is at height Z+dZ. It is required to calculate the compression of the gas and thus the additional aeration rate needed to maintain constant density of bed throughout the standpipe. dQa is the aeration rate required at height Z+dZ to maintain element of height dZ 
at a desired density of ρmf. Once the requirement of the aeration rate for a small element is determined, integration along the height of the standpipe will give total aeration rate Qa for smooth circulation of catalyst from the standpipe.

Aeration for a small element of standpipe
The volume of catalyst emulsion at height Z in the standpipe can be calculated easily by the following relationship:
        
                                 (5)

The volume of gas at height Z in the standpipe can then be calculated by the following relationship:

                                 (6)

where, ρsk is the skeletal density of the catalyst.
The volume of gas at height Z+dZ in the standpipe can be calculated by:

                                 (7)

Thus, the change in the volume of gas (dQa) across height dZ of the standpipe can be calculated by subtraction of Vgas,Z+dZ from Vgas,Z:

                                 (8)

The pressure at height Z and Z+dZ can be calculated by: