Power recovery train controls

Power recovery trains (PRTs) are common to industrial processes, such as fluidized-bed catalytic cracking units (FCCUs) in refineries and nitric acid sections of ammonia fertilizer plants. Their purpose is to recover energy from a process regenerator’s flue gases and convert it into mechanical energy to produce power; subsequently reducing the consumption of traditional energy sources.

A typical PRT includes a flue-gas expander, a main air blower, and a synchronous or inductive motor/generator. Some also include a steam turbine (ST) or an auxiliary motor-driven air blower for starting the unit. A PRT integrated control system provides automatic control during various operating conditions.

PRTs are commonly employed in FCCUs, in which:

• Catalytic pellets circulate between a cracking reactor and a regenerator, where the carbon deposits that form during cracking are burned off.
• Compressed air from the blower passes to the reactor and regenerator, where it oxidizes the carbon deposits on the catalyst pellets and maintains the fluidization of the bed.
• Hot flue gases from the regenerator pass to the expander, which converts the energy into mechanical power to drive the train.
• If the expander delivers more rotating energy than is needed to drive the air blower, the excess is converted to electrical energy and delivered to the local electrical bus or public utility grid. If not, the generator will motor (draw power from the grid).
• The optional ST is used to drive the air blower when insufficient flue gas is available, and possibly to convert excess process steam to rotating energy that the generator will then convert to electricity and deliver to the grid

INTEGRATED CONTROL SYSTEM

An integrated control system has been developed to drive, protect, and sequence the entire PRT. Speed, performance, expander, and anti-surge control applications run continuously to coordinate the control of the expander inlet and bypass valves to maintain the proper flow and pressure for a catalyst regeneration process. Excess energy developed by the system is applied to an electrical grid through the hot gas expander and the generator.

PRT control system should regulate the air flow and pressure in the regenerator by manipulating the air blower inlet guide vanes (IGVs) and expander inlet and bypass valves. It should also manipulate the blow-off valve to protect the air blower from surge and control the rotational speed when the generator is disconnected from the utility grid by manipulating the turbine governor and expander inlet valves. Additionally, it should adjust the expander or turbine power output if the generator breaker unexpectedly trips to reduce the power and prevent the unit from over-speeding. However, it must also maintain an adequate air flow to the reactor and regenerator to prevent the catalyst bed from collapsing.

Here is how it works: The regenerator pressure is controlled by an interacting pair of PRT expander control applications that manipulate the expander inlet and bypass valves. The power recovered by the expander is regulated by the main control loop (primary or alternate), which manipulates the expander inlet valve to keep the expander flow equal to the process fluid production rate. The expander inlet and bypass control applications use the same process setpoint (SP) for both loops. Typically, if the inlet valve control application uses a local SP, it passes that SP to the bypass valve control application. The bypass valve is kept closed as much as possible to maximize energy recovery. Flow is shifted from the inlet to the bypass valve in certain circumstances including when a limit loop is beyond its acceptable range.

The purpose of power recovery train is to recover energy from a process regenerator’s flue gases and convert it into mechanical energy to produce power.

Control system stability is essential in maintaining a constant delta-pressure between the reactor and regenerator, as well as a proper flow of air through the catalyst. This prevents hydrocarbon vapors or liquids from entering the regenerator and causing an explosion and, at the same time, avoiding the ingress of air into the reactor which might lead to a major equipment damage.

PRT START-UP USING AN ST

PRT start-ups using an ST are performed using the speed control application automatic start-up and warm-up sequences avoiding critical speed ranges. This ramps the turbine up to the synchronous speed with the breaker open and no load.

When the breaker is closed to connect the generator to the electrical grid, the steam flow control application typically steps the governor valve open by a configurable amount to immediately put some load on the generator. Control of the turbine governor valve then switches from the speed to the steam flow control application, which maintains the flow of steam to the turbine blades. The air blower is also loaded to begin supplying compressed air to the reactor and regenerator. The performance control application controls the mass air flow to the air blower by modulating the IGVs. The anti-surge control application modulates the blow-off valve as necessary to protect the air blower from surge.

Once the air blower is supplying compressed air to the reactor and regenerator and the process is stabilized, the expander can be loaded. This initiates the flow of hot flue gases through the inlet valve and transitions the control of the main process variable from the bypass valve to the inlet valve. As the expander is loaded, the steam flow to the turbine is reduced, and the PRT will shift from motoring to generating mode.

If the electrical breaker trips during normal operation, the PRT control system will react quickly to either shed the excess power (to reduce the load on the train) if the PRT was generating power, or begin producing enough power to continue driving the train if the generator was motoring. The system will also begin controlling the speed of the train, which is no longer fixed by the grid frequency.

SUMMARY

PRTs require precise and reliable control over a broad range of operating conditions to maximize FCCU availability. FCCUs are operated continuously for long periods, with the main goal of maximizing feed cracking. Occasionally, refineries are forced to operate production units at reduced rates because of machinery or process issues, downstream bottlenecks, upstream process unit trips, seasonal factors, market economics, and feed availability.

This integrated control system can run a PRT in an FCCU, safely overcoming breaker trip incidents by utilizing the load-shedding algorithm. The control system also maintains a proper flow and pressure for a catalyst regeneration process under different conditions. Other features like loop decoupling can help to eliminate cyclic interactions between controller applications. The technology described is equally applicable to PRTs found in other processes.

Author: Nabil Abu-Khader, Ph.D. Training Manager and Global Training Specialist at Compressor Controls Corporation (CCC), Abu-Dhabi, UAE.
For more detailed information about

the CCC control system developed for
the PRT, nabu-khader@cccglobal.com