I.FLUID CATALYTIC CRACKING/A.PROCESS(Contd.)
Q-21:
What minimum nozzle velocities are required in air and steam distributors to prevent
Catalyst back flow and subsequent erosion? Please consider both upward and downward pointing nozzles?
A-21:
The FCCU has many distributors,including combustion air,stripping steam,various fluffing distributors,and even the feed nozzle system.
Good distributor and nozzle design is about half the battle in good FCC operation. Many
Routine FCC problems are the result of poor distributor operation.This can include poor yields due to poor oil/catalyst contacting,poor stripping due to lack of catalyst/steam contacting,and excessive catalyst attrition due to distributor damage.
Unfortunately distributor design is challenging in many FCC services,with too many design requirements and not enough degrees of freedom to satisfy all of them.
Distributor designs differ from designer to designer,but there are some common points:
1.The standard for FCC design is a system that incorporates dual diameter nozzles,meaning an orifice controlling the pressure drop to ensure even distribution and no catalyst back flow,and a nozzle diameter that sets an acceptable outlet velocity.
2.Sufficient distance from the orifice to the nozzle such that there is fully developed flow in the nozzle before it exits the nozzle.
3.Sufficient coverage of the process area to ensure good vapour/catalyst contacting,but this is done in two different ways.The more common manner is to have a lot of nozzles,as many as 1 per square foot of process area coverage,with appropriate sized nozzles and orifices to ensure even flow.The second manner is to have nozzles with high outlet velocity and rely on jet penetration for good process coverage.Typical distributor and nozzle designs include the following parameters:
--orifices sized to give pressure drop equal to 30% of the bed static height above the distributor.
--nozzle outlet velocity of 90 to 150 fps.
--minimum nozzle length of 5.2 times the difference between the nozzle diameter and orifice diameter,with some safety factor applied.Engineering firms use a factor of 1.5-3 times this length to set an acceptable safety factor.
--Drains to allow wet media to discharge freely.
In terms of keeping catalyst from back flowing into nozzles,there are two separate answers,but they are related.
In the event of a single nozzle(like an emergency steam nozzle),the industry rule of thumb is to have 25 fps velocity in the orifice area of the nozzle.This results in low velocity,and should keep the orifice area of the nozzle clear,but catalyst will likely to be back mixing into the nozzle diameter area downstream of the orifice.This means the nozzle must be designed for catalyst temperature.
If there is a distributor (ring,pipe,grid,horse-shoe type designs),the minimum parameter used to prevent catalyst back flow is distributor delta P.Industry guidelines vary from a minimum of 10% bed static height upwards to 30% bed static height.A common industry belief is that 10% minimum bed static height for distributors with downward pointing nozzles,and 30% minimum bed static height for upward flowing nozzles.
The most likely way that we have direct experience with catalyst back flowing into distributors is by loss of pressure of the flowing media into the distributor.This can result in an instantaneous back flow into the distributor.If this happens,it may well plug nozzles until the next scheduled maintenance turnaround on the unit allows entry to the vessel to clear the distributor.
Q-21:
What minimum nozzle velocities are required in air and steam distributors to prevent
Catalyst back flow and subsequent erosion? Please consider both upward and downward pointing nozzles?
A-21:
The FCCU has many distributors,including combustion air,stripping steam,various fluffing distributors,and even the feed nozzle system.
Good distributor and nozzle design is about half the battle in good FCC operation. Many
Routine FCC problems are the result of poor distributor operation.This can include poor yields due to poor oil/catalyst contacting,poor stripping due to lack of catalyst/steam contacting,and excessive catalyst attrition due to distributor damage.
Unfortunately distributor design is challenging in many FCC services,with too many design requirements and not enough degrees of freedom to satisfy all of them.
Distributor designs differ from designer to designer,but there are some common points:
1.The standard for FCC design is a system that incorporates dual diameter nozzles,meaning an orifice controlling the pressure drop to ensure even distribution and no catalyst back flow,and a nozzle diameter that sets an acceptable outlet velocity.
2.Sufficient distance from the orifice to the nozzle such that there is fully developed flow in the nozzle before it exits the nozzle.
3.Sufficient coverage of the process area to ensure good vapour/catalyst contacting,but this is done in two different ways.The more common manner is to have a lot of nozzles,as many as 1 per square foot of process area coverage,with appropriate sized nozzles and orifices to ensure even flow.The second manner is to have nozzles with high outlet velocity and rely on jet penetration for good process coverage.Typical distributor and nozzle designs include the following parameters:
--orifices sized to give pressure drop equal to 30% of the bed static height above the distributor.
--nozzle outlet velocity of 90 to 150 fps.
--minimum nozzle length of 5.2 times the difference between the nozzle diameter and orifice diameter,with some safety factor applied.Engineering firms use a factor of 1.5-3 times this length to set an acceptable safety factor.
--Drains to allow wet media to discharge freely.
In terms of keeping catalyst from back flowing into nozzles,there are two separate answers,but they are related.
In the event of a single nozzle(like an emergency steam nozzle),the industry rule of thumb is to have 25 fps velocity in the orifice area of the nozzle.This results in low velocity,and should keep the orifice area of the nozzle clear,but catalyst will likely to be back mixing into the nozzle diameter area downstream of the orifice.This means the nozzle must be designed for catalyst temperature.
If there is a distributor (ring,pipe,grid,horse-shoe type designs),the minimum parameter used to prevent catalyst back flow is distributor delta P.Industry guidelines vary from a minimum of 10% bed static height upwards to 30% bed static height.A common industry belief is that 10% minimum bed static height for distributors with downward pointing nozzles,and 30% minimum bed static height for upward flowing nozzles.
The most likely way that we have direct experience with catalyst back flowing into distributors is by loss of pressure of the flowing media into the distributor.This can result in an instantaneous back flow into the distributor.If this happens,it may well plug nozzles until the next scheduled maintenance turnaround on the unit allows entry to the vessel to clear the distributor.
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