Jim Norton and Chris Steves (Norton Engineering)
The optimum pH for a wet gas scrubber treating FCC flue gas is 6.9 to 7.1. This pH is maintained by the controlled addition of sodium hydroxide. The flow rate of sodium hydroxide is usually set up with a cascaded set point from a pair of redundant pH probes. This optimum pH is based on operating with bisulfite buffering. At higher pH, caustic consumption will increase as CO2 will be neutralized in addition to SO2. It should be noted that sometimes it is necessary to operate at the high pH of the bicarbonate buffering point if there is high excess O2 in the FCC flue gas. Under oxidizing conditions, the bisulfite species is not stable. If on the other hand, the pH decreases, SO2 removal will decrease. The SO2 slip starts to increase significantly below a pH of 6.0. If this pH continues to drop, it will also contribute to accelerated corrosion.
Dennis Haynes (NALCO Champion)
Slurry oil catalyst fines Settling Aid chemistries have been used for many years in this type of application. Electrostatic precipitators and filtration equipment is also available for minimization of ash content in slurry oil. These technologies have demonstrated cases of reduction to below 50 ppm ash for FCC slurry oil product.
Greg Savage (NALCO Champion)
Ash is a particular problem for slurry oils, especially those that are heavy and viscous needing long residence time to allow for catalyst settling. The source of ash in the slurry oil stream is composed of catalyst fines carried over into the fractionator from the reactor section of the FCC. The reactor cyclones are the first point where catalyst fines are removed from the reactor vapors going to the fractionator. Although highly efficient at separation of the vapors and catalyst, a certain level of fines is always present in the slurry oil stream. FCC hardware manufacturers are continuing to make improvements in cyclone design to remove a greater fraction of the fines.
To obtain low ash, special techniques such as heating, chemical additives, filters, electrostatic precipitators, centrifuges, and cyclones might be used. Catalyst selection may help reduce attrition to a great extent.
Many refiners make use of slurry filtration devices to remove as much of the fines as possible, and recycle the fines back to the FCC reactor by backwashing the filters. These devices can reduce ash inslurry but may require frequent maintenance and a significant capital investment for the equipment andinstallation. Tank settling is the most common means of reducing the ash content of slurry oil. Often, limited tankcapacity reduces the residence time available for settling of the ash. Many refiners “de-ash” with chemical settling aids which speed up ash settling in storage. These chemicals are polymeric compounds which adhere to the catalyst surface causing agglomeration of the fine particles in order to accelerate separation. In order to consistently meet very low ash content, a combination of the options described above may be required.
