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Flow Modelling of a FCC Slide Valve
for Oil Refinery Systems - 1

Fluidized Catalytic Cracking ( FCC ) is widely used in Oil Refinery Systems in order to break down large molecules of crude oils down into smaller ones. In the reactor the catalyst is fluidized in a hydrocarbon vapour. The catalyst is continuously circulated between the FCC reactor and the regenerator where it is regenerated with combustion in sub-stoichiometric air. The regenerated catalyst is then sent to the reactor where it again meets a new feed of crude oil. The flow of spent and regenerated catalyst between reactor and regenerator is controlled by slide valves.

The need to investigate the flow inside the valves arises from the fact that the internal parts have been found seriously damaged due to erosion [photo: severe erosion on a stem bushing - backseat]. Experimental measurements of fluid composition and flow field are very difficult in the harsh operating environment of the FCC reactor, regenerator and risers, so a computational flow dynamic study can be a solution for understanding the flow field better.

Calculation of the Valve Throat and Port Opening

Two different approaches have been used to calculate the valve throat and port opening in order to achieve the required pressure discharge. The simpler one (Shingles, 1986) is based on the theory of two-phase flow; a flow coefficient has been assumed.

The other approach which has been implemented is based on the principle of the energy conservation equation for single phase flow. After some simplifications it reduces to

where: = mass flow rate; = corrective coefficient; = flow coefficient; = density; = differential pressure across the valve.

The flow coefficient C is a function of the Reynolds number and the throttling diameter ratio and is extrapolated from previous studies. A corrective factor K has been introduced thanks to program calibration with real cases.

Results of both methods have been compared with existing valves data and the one based on the calculated flow coefficient has shown a lower error. The estimated error was calculated to be 3.5% with the following formula:

where = throat and full port area.

Calculation of Restriction Orifice on Purging System

Because of the valve pressure, a steam purging system is needed to prevent the catalyst particles from passing through the stuffing box. The service steam pressure is always higher than the required pressure and it must be reduced in order to produce a flow velocity at the valve inlet no larger than 15 m/s. For larger values, experience has shown a dangerous erosion on the disc stem due to the energized particles [photo]. This is why calibrated orifice plates must be introduced in the piping.

Program Calculation

In order to calculate the required orifice plates the total pressure drop in the stuffing box has to be calculated first. The remaining pressure drop required to obtain the right pressure at the stuffing box inlet is used to calculate the orifice hole. The pressure drop due to the friction in the piping has been calculated with the equation (Perry, 1984):

where = pressure; = dimensional constant; = fanning friction coefficient:

where = piping length; = mass velocity; = piping diameter; = absolute roughness of the piping; = temperature; = hydraulic radius; = molecular weight; = Reynolds number; = gas constant.

For annuli piping is assumed 7% larger.

For a sudden change in section the pressure drop has been calculated with the equations (Perry, 1984):

for sudden restriction

for sudden enlargement

where = coefficient depending on the sections ratio; = average velocity in the smaller and larger section; = area of the smaller and larger section.

To calculate the diameter of a sonic orifice, which is able to produce the largest pressure drop, the equation from thermodynamic calculations has been used.

go to CFD part 2

Copyright © REMOSA spa 1998-2001 Last updated on May 18, 1998