In one of our FCC units (Kellog Orthoflow model), we are suffering severe problems of fouling (fines deposition) in the turboexpander. The scheme of the flue gas circuit is: two stage cyclones in the regenerator + Shell Third Stage Separator before turboexpander + 4th Stage Separator (cyclon) to recover flue gas from fines coming from TSS.
We have also observed high level of moisture in the fines from 4th Stage Separator (10-15%wt). So we suspect that the fouling of the expander is due a cold point in the flue gas circuit (where flue gas humidity is condensed) or an uncontrolled inlet of water / steam.
Has anyone experienced this kind of problems in an FCCU? What could be the potential causes of the severe fouling of the expander?
Ralph Ragsdale, Ragsdale Refining Courses, firstname.lastname@example.org
It's no consolation, but your experience does not appear to be that unusual. Here is an excerpt from my course manual
"Power recovery trains typically run two years before requiring repairs. To make such repairs, it is necessary to shut down the FCC, decouple the turbine, start up the FCC, repair the turbine, shut down the FCC, recouple, and start up. To improve the service factor of the power recovery turbine, regularly scheduled online walnut hulling is often practiced.
A third stage separator is always used ahead of the power recovery turbine to reduce blade erosion. An efficiency of 90% removal of catalyst fines 2.8 microns and larger is achieved.
Some refiners have selected a power recovery train configuration that features two shafts, avoiding the need for decoupling in the event of a problem with the recovery turbine (expander). The expander and generator are on one shaft, and the blower and motor on the other shaft. The typical target run length for the FCC unit is five to six years."
Lindsay McRae, Pall Corporation, Lindsay_McRae@pall.com
Turboexpander (TX) fouling / erosion is more likely due to FCC catalayst fines passing though primary & secondary cyclones and TSS rather than condensed moisture. The moisture you are seeing is likely due to cooling and condensation on water in bottom of TSS or in the spent cat hopper. Regarding the moisture, make sure the air you are using for fluidising in bottom of TSS hopper is dry air and consider heat tracing on hopper cone as well as that is often a cold spot as there is little or no gas flow. In an case, the flue gas stream from the FSS is going back into the main FCC flue gas line after the TX generally so that is not going to impact on the TX I don't think.
Regarding the TX fouling, Cyclone technology is not so good for removing the small FCC fines, and their efficiency is also sensitive to upsets such as changes in flue gas flow rate, particle size distribution, and the solids loading. So if there has been upsets such as an attrition source, or increase on FCC flue gas flow rate, and also wearing of cyclones towards the end of the FCC campaign then the cyclone efficiency drops and the solids pass through into the TX. Even if the solids is below 100mg/Nm3 it can cause reliability / maintenance problems in the downstream TX. Can I ask you, has the FCC stack particulate emission level increased also and is this still within your emission licence limit?
Are you seeing signs of erosion or fouling on your Critical Flow Nozzle? CFN fouling can result in carryover from the TSS to the TX sometimes and besides TX erosion/fouling can also result in off spec emissions to the environment. A fourth stage filter (FSF) has been proven to provide good protection to CFN and also reduce particulate emissions at the stack.