Installing a power recovery system on FCC or RFCC units
Full examination of the design options and operating issues related to the installation of a PRS on a FCC or RFCC unit will increase energy efficiency and unit availability
Samsung Engineering America
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Installing a power recovery system (PRS) to generate power from an existing fluid catalytic cracking unit (FCCU) or residual fluid catalytic cracking unit (RFCCU) increases the energy efficiency of the unit and effectively results in an overall CO2 emissions capture credit for the refinery. A FCCU or RFCCU converts heavy fractions of crude oil to lighter products, mainly gasoline, and contributes to more than 40% of gasoline production in North American refineries and even higher percentages in other countries. While crude oil heavy fractions fed to a FCCU are typically limited to gas oil (340°C+), a RFCCU produces gasoline from heavier feed streams including atmospheric and vacuum residue. Figures 1 and 2 show typical PRS installations in FCCUs and RFCCUs.
As Figures 1 and 2 show, a PRS typically consists of the following:
• A third-stage separator (TSS) to reduce the catalyst particulate loading in the flue gas stream to levels acceptable to the downstream expander
• A fourth-stage separator (FSS) system consisting of a fourth-stage cyclone, a cyclone hopper and a fines storage hopper to properly dispose of the particulates collected in the TSS underflow
• A power recovery train (PRT) consisting of an expander, a main air blower (MAB), a motor/generator, a start-up steam turbine and the associated accessories
• A new expander bypass system, which typically includes a butterfly valve and a downstream orifice chamber. This bypass system mainly controls the regenerator pressure when the expander is out of service, or when the flue gas flow rate exceeds the maximum capacity of the expander.
Roughly 0.3 horsepower (HP) can be generated by the expander of the PRS per daily barrel of feed to the FCCU or RFCCU. This quantity varies, depending on a number of operating parameters, including the operating pressure of the regenerator and others such as oxygen enrichment in the air supply to the regenerator. In a 100 000 bpd RFCCU, for example, installing a PRS generates about 30 000 HP net expander shaft power, which is equivalent to 22.37 MW less electric power consumed in the unit and the overall refinery. This reduces net CO2 emissions from the power-generating plant supplying electricity to the refinery by about 100 000 tonnes per year for a gas-fired combined-cycle power plant and by about 200 000 tonnes per year for a coal-fired, Rankine-cycle power plant. In addition to electrical cost savings, the CO2 capture credit makes the installation of a PRS financially more attractive.
The energy saving and economic benefits of a PRS installation can be maximised by proper integration of the PRS with the existing FCCU or RFCCU and the overall operating requirements of the refinery. The ability to operate the FCCU or RFCCU without the expander in service will maximise the unit’s availability and is often economically attractive. This article discusses the design options and process engineering issues to be considered in a PRS installation project to accomplish these objectives.
Main air blower (MAB)
The MAB supplies air to the regenerator, where coke on the catalysts is combusted to regenerate the catalysts. For a PRS installation project with no or a minor increase in the unit’s capacity, reusing the existing MAB can be a viable option to minimise the capital cost of the project. Oxygen injection to enrich the oxygen content of the comb-ustion air to the regenerator may also enable the reuse of the existing MAB for meeting the higher unit capacity requirement. In this option of reusing the existing MAB, the PRT typically consists of the expander and the generator, and the existing MAB remains separated from the new PRT.
A PRT installation where the expander and the MAB are two separate rotating systems on two separate shafts is also commonly referred to as the “Gen-Set” design (see Figure 3). When the PRT is out of service, this option enables operation of the unit at full capacity without shutting down. As such, any interrupted service of the PRT does not affect the overall avail-ability or reliability of the unit. A dedicated, fast-acting expander bypass system is typically installed for overspeed protection of the PRT, as the train’s rotating speed will suddenly become excessive in the event of a breaker trip disconnecting the generator from the power grid.
For projects where reusing the existing MAB is not viable, a new, higher capacity MAB can be installed as a component of the new PRT, as shown in Figures 1 or 2. If a future expansion plan has been decided to further increase the capacity of the FCCU or RFCCU, specifying the new MAB and the remaining new PRT components in advance to meet part or all of the future capacity requirement may be economically attractive and should be considered.
Before deciding to replace the existing MAB with a new one as a component of the new PRT, it is necessary to first assess the impact of taking the existing MAB driver out of service. If the existing driver is a steam turbine, the overall steam balance of the refinery can be greatly affected by the decommissioned MAB and should first be evaluated by the responsible engineer to minimise the refinery’s post-PRS steam consumption. For some cases, where high-pressure steam needs to be normally let down to meet process heating requirements, the new start-up steam turbine of the PRT can be specified to generate power from the letdown steam.
Start-up steam turbine
Adding a start-up steam turbine to a PRT may also be necessary when the existing electrical system does not allow the amperage surge during the start up of the PRT from a dead stop. This start-up amperage surge increases significantly, especially when the PRT motor’s power rating is specified to operate the MAB at full capacity without the expander in service. In addition to minimising the start-up amperage surge, the start-up steam turbine can efficiently recover power from the letdown steam, whenever the refinery’s steam balance requires a continuous letdown of steam from a high-pressure level to a lower level to meet process heating require-ments. This power recovery improves the refinery’s overall energy efficiency and reduces the net CO2 emissions from the power generation facilities associated with the refinery. These benefits accrued from adding a start-up steam turbine must be considered before deciding between an upgrade of the existing electrical system or the addition of a start-up steam turbine.
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