Fluid Catalytic Cracking (FCC) Processes
From Powder Recovery
In Fluid Catalytic Cracking (FCC) processes, widely used in oil refineries, the separation and recovery of powder catalysts is a major concern. Particularly, the concentration of catalyst after the FCC regenerator is usually over 150 mg/Nm3. Since these levels of emissions didn’t represent any environmental harm in the past, they were not controlled by any separator or filter. Plant managers main concern was to clean a fraction of the gas to produce energy in a Turbo Expander, or similar, while concentrating the rest of the pollutant in a dirty gas fraction and releasing it to the atmosphere.
With stricter emission limits, plant managers have to control catalysts emissions and therefore need a very robust particle collection system, capable of withstanding temperatures of many hundreds of degrees, while being more cost effective than expensive sinterized filters. The system shall be placed immediately after the regenerator and replace the separator (option A), or after the particle separator in the dirty fraction gas stream for emission control before the flow is combined to go the WHB (Waste Heat Boiler) and then through the stack (option B).
Client's needs include a low maintenance cyclone system efficient enough to comply with regulatory PM limits, thus avoiding the use of a filter.
Black Powder is a solid contaminant found in natural gas transmission and distribution systems throughout the world. It consists primarily of iron oxides, iron sulfides and some silica which results from internal erosion and corrosion of carbon steel pipelines.
Either when it is attached to the pipe wall or collected in the pipe bottom, black powder increases roughness, decreases flow area and increases pressure drop. It can cause a wide array of problems, including product contamination, erosion wear in compressors, instrument clogging, erosion and sealing problems for valves as well. Damages in equipment range from large plant burners to residential home appliances.
An adequate solution has eluded suppliers this far. Available commercial cyclones are only efficient for larger sized particles and cartridge filters clog continuously, sometimes multiple times each day. Magnetic separation technology results in very high investments. Maintenance operations require inordinate amounts of manpower and pigging and other maintenance operations entail a lot of downtime.
Clients' needs include a very high efficient final dedusting system, to apply upstream or instead of cartridge filters, which can ensure particle collection for the whole spectrum of particle sizes, and simultaneously reduce maintenance and operation costs.
Numerically optimized cyclones, considering particle agglomeration
Hurricane geometries maximize powder collection for each different application, while minimizing reentrainment and keeping pressure drop at reasonable levels.
These high efficiency cyclones are the output of nonconvex nonlinear problems formulated and solved after years of work in partnership with the Faculty of Engineering of Porto.
A single Hurricane is more efficient than any other known cyclone available in the market for the same pressure drop, usually resulting in less than half of the emissions of other high efficiency cyclones available.
- Air and Dust from Dryer or Boiler
- Hurricane Cyclone
- Collected Particles
- Clean Air
Optimized cyclones with mechanical recirculation
In order to obtain very high efficiencies and compliance with emission limits, an improve recirculation system has been designed to take advantage of the Hurricane cyclones. A mechanical ReCyclone
(ReCyclone MH) is made up of a Hurricane with a particle separator (straight-through cyclone), placed downstream of the cyclone - called the 'recirculator'.
The main purpose of the recirculator is to reintroduce the fine non captured particles into the cyclone after those have been driven to the outer walls of the recirculator by centrifugal forces. While this tangential gas stream is enriched in particles, the axial gas stream exhaust to the stack is clean of particles. Recirculation is achieved through an additional fan.
Since the recirculation system only serves the purpose of dust separation (and not collection), the particles are exclusively collected in the cyclone and the need of rapping mechanisms is thus avoided.
Efficiency increases due to recirculation and agglomeration of very small particles with larger ones coming directly from the process. A ReCyclone MH decreases emissions of Hurricane cyclones alone by 40 to 60 %. As with cyclone systems, ReCyclone MH systems are very robust, do not have temperature limitations and do not have moving parts to be frequently changed. Finally, recirculation control provides the benefit of handling variable process flow rates.
The next figure exemplifies a ReCyclone MH in stainless steel AISI 304L for particle recovery in drying process. Recirculators can be disposed vertically, as shown, or horizontally.
- Air and dust from boiler or dryer
- Hurricane cyclone
- Discharge valve
- Collected Particles
- Clean air