Conclusión: enseñanza y unión

The power consumed by the air blower is a significant part of an FCC unit's operating expense. Utility costs continue to rise, only partially offset by more efficient motors and turbines. Recovery of the energy in the hot flue gas from the regenerator can increase the overall efficiency of the unit. This was first done with steam generation systems. The flue gas exchanges heat with a circulating water stream. A 30,000 BPD (195 m³/hr) FCCU without a CO boiler can produce 40-70 M-lb/hr (18-32 t/hr) of 600 psig (42 kg/cm²) steam. Heat recovery in this scheme is somewhat limited by a minimum allowable flue gas temperature. Sulfur oxides and water vapor in the stack gas can cause corrosion of the equipment if they condense in the flue gas duct. The temperature at which the condensation occurs is known as the acid gas condensation point, which shifts depending on the concentration and distribution between the different oxides. The acid gas condensation point is typically in the range of 400-600°F (200-315°C), although it may be higher for some units. The maximum temperature limit of the flue gas is typically a function of metallurgical design limits for downstream equipment, which may include an electro-static precipitator, flue gas scrubber, and/or stack.

The major disadvantage of a straight steam generation energy recovery scheme is that no power is recovered from gas pressure, normally 10-40 psi (0.7-2.81 kg/cm²) above atmospheric pressure at the regenerator outlet. Another approach to recovering energy from the flue gas was tried in 1950. This was a turbo expander, driven directly by hot flue gas. Initial results were unsatisfactory; after only 750 hours of operation catalyst fines in the flue gas had substantially eroded away the turbine blades and casing. The fines problem was solved by placing an additional catalyst separator, known as a Third Stage Separator (TSS) outside of the regenerator.

In the TSS, flue gas moves through a large number of small cyclone assemblies in which the catalyst is centrifugally separated from the flowing gas stream. To remove the separated catalyst fines from the TSS, a small amount of gas, typically 3% of the regenerator flue gas, is used to pneumatically sweep the catalyst fines

out the bottom of the vessel. The clean flue gas is then directed to the inlet of the power recovery expander.

UOP has been designing power recovery systems since 1973. Between 1973 and 2004, UOP licensed 33 TSS’s with 22 placed into operation. The original units were designed by UOP under license from Shell. Over the years, UOP improved upon the original design by implementing several modifications. Even with these modifications incorporated into the base design, very little had actually changed in the overall design of the TSS in 25 years. These TSS designs still suffered from the limitations imposed by radial flow gas distribution and reverse flow in the cyclone elements.

In 1996, UOP launched a development program to design and offer a smaller, more economic, high efficiency TSS that could not only be utilized in power recovery installations, but also be a viable alternative to electrostatic precipitators and wet gas scrubbers for environmental applications.

The cold flow modeling (CFM) test program extended over 2 years, during which both dimensional variables and process flow variables were studied. Based on a thorough understanding of cyclone theory, and drawing on other sources of cyclone expertise, the UOP program investigated the contribution of many variables on catalyst separation efficiency. These variables included:

 Cyclone diameter and geometry

 Inlet velocity

 Length to diameter ratio

 Outlet velocity

 Catalyst loading

 Gas distribution

Over 200 individual tests were conducted on single and multiple cyclone models to determine the highest efficiency and highest capacity design cyclone. The tests were conducted with commercial FCC catalyst fines. Computational fluid dynamic (CFD) computer modeling was used to validate and benchmark the CFM work, and to quickly investigate potential improvements and guide the physical modeling program.

The development work culminated in the new UOP TSS design (see Figure 9). The most significant improvement in the design is that the UOP TSS utilizes axial flow for catalyst/gas separation. The flue gas flow is maintained essentially in one direction - in the top and out the bottom of the unit. Axial flow distribution minimizes the potential for solids re-entrainment resulting from gas flow direction change and resultant eddy current formation. The older style TSS utilizes radial flow distribution in which the flue gas is distributed from the centerline of the TSS, radially outward between the two tube-sheets. As such, the inner tubes see a higher gas and dust loading than the outer tubes. The mal-distribution of flue gas and fines inherent in this design results in varying efficiency across the older style TSS.

The new UOP TSS is about 40% smaller than other TSS offerings for the same capacity; making it less expensive to fabricate, easier to install, and better suited where plot space is a premium.

The first UOP TSS was commercialized in April 2002. Performance testing on the unit was performed twice in 2002, following the unit startup in April and again in December. The initial test showed that the UOP TSS discharged between 36-50 mg/Nm³ of particulates, depending on flue gas rate. The NSPS compliance testing resulted in a particulate matter emission of 0.6 lbs/1000 lbs of coke burn, only 67%

of that allowed by NSPS standards. This performance showed that the UOP TSS could not only provide power recovery expander erosion, but could also be used as in the refiners particulate emission control strategy, by replacing more traditional, costly, and hazardous means (electrostatic precipitators and wet gas scrubbers) of controlling particulates exiting the flue gas stack.

A comparison of the older style TSS and newer style TSS is shown in Figure 4.

Both vessels are carbon steel vessel with 4" (100 mm) of refractory lining and stainless steel internals. The cold-wall construction is more effective on both erosion and cost basis than the early hot-wall stainless steel separators. A coarse screen, or grate, covers the flue gas outlet entrance to trap large chunks of refractory or other debris.

The overall efficiency of the separator depends on the efficiency of the regenerator cyclones and the quantity of catalyst fines being generated in the reactor-regenerator system. The separator should remove >70-90% of the particles for high

and low regenerator cyclone efficiency, respectively. Most of the fines which pass through the separator are smaller than ten (10) microns. These small particles do not cause much erosion to the expander blades but the smallest particles can deposit on the expander blades and casing, causing vibration problems.

The pressure drop across the expander is on the order of 10-30 psi (0.7-2.1 kg/cm²), with a temperature drop of 200°-250°F (110°-140°C). After driving the turbine, the flue gas goes to a steam generator for further energy recovery.

The majority of the catalyst is removed from the flue gas with the underflow from the third stage separator which is typically routed back into the flue gas downstream of the expander. If required, an electrostatic precipitator or flue gas scrubber may be placed downstream of the steam generator to remove any remaining catalyst fines before the flue gas is exhausted to atmosphere. Alternatively the underflow may be filtered to achieve ~99.99% removal of the catalyst fines, or routed to a 4^th stage cyclone separator to achieve ~60-90% removal of the catalyst fines from the underflow stream, depending on local environmental restrictions.

The power recovery train usually consists of five parts; the expander turbine, motor/generator, air blower, and a steam turbine, and is commonly referred to as a

“5-Body Train”, see Figure 5. In this arrangement the expander turbine is coupled to the main air blower shaft to directly supplement the power requirement of the blower. The 5-body train requires a steam turbine or motor to get it started; in some cases only one of them is provided.

The expander, shown in Figures 6 and 7, is a single stage machine because of the low pressures involved. The gas to the expander is accelerated over a parabolic nose cone. Pressure energy is converted to velocity energy, and the high velocity gas drives the turbine.

Expander turbines designed in the past were generally limited to an inlet temperature of 1200-1250°F (650-675°C) to prevent heat damage. This generation of expanders however, still required quench injection systems in the regenerator plenum chamber to protect the expanders in the event of a regenerator temperature

excursion. Quench systems essentially dumped steam and water into the regenerator plenum to cool the flue gas. While this provided for thermal protection of the expander, the increased steam and water in the flue gas often resulted in

“sticky” catalyst that agglomerated in cement-like deposits that increased blade fouling on the expander. Newer expander turbines normally have a design temperature in excess of 1375°F (750°C) and do not require a quench control system.

For units with a power recovery system, butterfly valves in the flue gas line control the differential pressure between the reactor and regenerator. The PDIC sends a signal to the large butterfly valve which is located at the inlet to the expander. A smaller butterfly valve will allow flue gas to bypass the expander when the large butterfly valve is fully open because of an excessive flue gas rate or when the expander is off line. This prevents over pressuring the regenerator.

In the traditional five piece power recovery train, the motor/generator is usually a constant speed induction type machine that provides extra power to the blower shaft when needed. If the expander produces more energy than is required by the blower, the machine will act as a generator and feed power into the electrical grid.

This acts as a braking mechanism and provides some over-speed protection for the machine.

Figure 4