Buell Third Stage Separator Collectors of FCC Regenerator off-gases

The term Third Stage Separator refers to a cyclonic collection device or system installed following the two stages of cyclones within the Fluid Cat Cracker (FCC) Regenerator in the gas outlet line. It is in this context that we refer to this device as a Third Stage Separator or TSS.

The function of a third stage separator is to capture catalyst escaping from the Regenerator to protect downstream equipment and/or reduce particulate emissions to the atmosphere. Third Stage Separators are installed before the Expander Turbine primarily to remove large catalyst particles (>10) from the gas stream before the gas enters the turbine; thereby avoiding excessive blade erosion, frequent blade replacements and downtime. In the USA most third stage separators applications were directly associated with power recovery applications. In other parts of the world the third stage separator has been recognized as a cost-effective means to control particulate emissions. Buell has significant experience in the application of third stage technology for both applications.

The Buell TSS utilizes high efficiency cyclones configured to meet the customer’s specific requirements for the project. The cyclone inlets are connected directly to a common inlet manifold to assure good distribution of gas and catalyst. The first Buell Third Stage Cyclone was installed in 1962 in the Sinclair Hartford (IL) Refinery for the protection of an expander turbine as part of a pilot program designed to determine if it was practical to produce energy from regenerator off-gases. In 1978 Buell supplied its first full-scale commercial unit for power recovery to Ingersol-Rand as part of cyclone Separator- Expander Turbine package for a Chinese refinery. Buell has designed over 30 units for power recovery and emission control applications all over the world (See Figure 1).

Third Stage Separator Configurations

The most common configuration of a Third Stage Separator consists of a single vessel containing many cyclones. The vessel can be designed to operate at high temperature and pressure making this configuration the appropriate choice for application immediately downstream of the Regenerator before an Expander Turbine. Because Buell’s third stage vessels and cyclones are custom designed, upgrades to improve performance and increase production can be accommodated.

Figure 2 illustrates a Buell Third Stage Separator that was designed as a replacement of an existing Shell unit. The new Buell unit was designed to accommodate substantially increased volume while utilizing the existing foundations and minimizing rework required to the overhead line, gas outlet to the expander, resulting in a minimum of downtime during the turnaround.

When small gas volumes require only a few cyclones it may be appropriate to consider manifolding several external cyclones together. In this case, the cyclones must be designed as pressure vessels to withstand operating temperature and pressure. Compact and cost effective solutions can be provide even for small gas volumes. This type of installation is ideal for application with high pressure and low temperature, for particulate emissions control. An example of one such arrangement is shown in Figure 3

Buell TSS systems can be designed for operation either continuous or batch disposal of the collected catalyst. Continuous removal of a small amount of gas with the collected catalyst (referred to as underflow) is the most common method of transporting the catalyst. Often the underflow is ducted to a 4th Stage Cyclone where the catalyst is again collected and outlet gases returned to the system, such a system is illustrated on the front cover of this brochure. Underflow is most often used for power recovery application because of the beneficial effect on the performance of the TSS. Greater volumes of underflow are often used on competing designs in an effort overcome inherent gas and catalyst distribution problems associated with some design. Buell Third Stage Separators utilize minimum underflow, maximizing the potential for power recovery and the return on the Buyers capital investment. This factor is very significant when evaluated over the life of the equipment.

The use of underflow gas as a means of conveying the collected catalyst provides the flexibility of arranging the 4th Stage Cyclone beneath the TSS in a Stacked configuration, when the plant area is restricted in plan, or next to the TSS in the more cost effective Side-By-Side configuration.

When a 4th Stage is utilized in an underflow system, It is important to remember that the losses from the 3rd and 4th Stage Cyclones are additive when evaluating losses from the system. System losses can be greatly affected by the performance of the 4th stage cyclone, Buell designs and supplies high efficiency cyclones for this critical component.

Buell Third Stage Separators can also be designed to operate without underflow. This is accomplished by equipping Third Stage Cyclones with trickle valves to minimize counter flow of gasses and re-entrainment of collected catalyst. The catalyst is evacuated by gravity from hopper of the TSS into a bin commonly referred to as a Fines Received. Multiple bins have been used to achieve more cooling of the collected catalyst prior to disposal, an example is shown in Figure 4.

Systems, Components and Design Services
Buell designs and supplies all the individual components associated with Third Stage Separator systems, including underflow piping, valves, expansion joints, 4th stage cyclone and fines hoppers. Buell provides components and design engineering services to customers looking to add underflow, upgrade TSS performance or evaluate structural problems. Collector performance is not always the cause of higher than expected losses from a TSS. Structural problems and cracks resulting from high stress concentrations can also cause unexpected losses. A single crack allowing dirty gas to flow to the outlet plenum will result in a large increase in losses across the full spectrum of particle sizes. Since cracks are often associated with thermal stress which are magnified during startup and shutdown it is not improbable that a unit may be inspected and repaired during shutdown and develop a crack on startup. If cracking is experienced on a routine basis, the condition of the material, magnitude and the nature of the stresses should be identified and addressed. Buell uses computer generated Finite Element Analysis (FEA) to identify and deal with temperature gradients and high stress areas.

Figure 5 reflects an example of a FEA temperature model of an existing third sage vessel head developed as part of a paid engineering study. Buell provided an analysis of mechanical loads (including nozzle loads), and anticipated thermal stresses. This service is available to evaluate structural problems one might encounter. An underflow system is also subject to significant expansion and thermal stress. Analysis is performed on underflow piping using CAESAR piping software to establish pipe stress, movements and loads on equipment interfacing with the underflow line.

Other Considerations
Third Stage Separators often are often located immediately downstream of the Regenerator and are designed for similar conditions. This is appropriate when power recovery is the main consideration. However, alternate location should always be evaluated when control of particulate emissions is the prime objective. Cyclone efficiency is a function of Gas Temperature and Viscosity. Better efficiency and lower losses can be achieved from the same cyclone operating at lower temperatures. The ideal location of a TSS for control of particulate emissions is down stream of a Waste Heat Boiler, which is designed to operate at full Regenerator pressure, thus providing the least volume of gas to be treated and the highest efficiency practical. The most difficult application for a TSS is immediately after the regenerator.

Careful consideration should be given to installing a TSS upstream of Electrostatic Precipitators. Although the TSS will reduce the total loading of catalyst to the precipitator the reduced particle size of the effluent can negatively effect precipitator operation. Contact your supplier and explore the effects of altering the precipitator inlet particle size, i.e. decreasing the mean particle size.

Third Stage collectors collect only particulate matter, which exists as particulate as it passes through cyclones. As gaseous products of combustion are cooled, certain chemical compounds are precipitated from the gasses to form that which is referred to as condensables or pseudo particulate. Condensables will appear during testing unless specific testing techniques are used to exclude condensables, i.e. EPA Method 17. The plant permit may or may not exclude this material.

Advantages

• Immediate access to literally hundreds of years of experience in the industry with over 35 TSS applications.
• Buell can optimize performance of your Regenerator Cyclones to minimize losses downstream.
• Buell systems are custom designed to meet the customer’s specific needs, providing the best value.
• Each third stage cyclone is connected to Inlet plenum to assured proper distribution of gas and particulate providing dependable performance.
• Cyclones and diplegs are sized to handle large amounts of catalyst without plugging in the event of a Regenerator upset.
• Pressure losses across a Buell TSS are low in comparison to competing designs resulting in quicker payback when power recovery is being considered.

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