Process for improving power recovery from regeneration gas under turndown conditions

In a process for recovering power from motor-generator/expander power recovery turbine/axial flow air compressor set, wherein the expander power recovery turbine drives the axial flow air compressor and either the motor-generator supplements the drive of the axial flow air compressor or the expander power recovery turbine drives the motor-generator;

• and the axial flow air compressor compresses and supplies air to a lower portion of a regenerator wherein coke contaminated, partially deactivated catalyst is regenerated in a fluidized catalytic cracking process wherein in the regenerator the catalyst is fluidized into a dense phase and a dilute phase by the upward flow of air from the lower portion of the regenerator and the air oxidizes the coke in the dense phase and the dilute phase to oxides of carbon and thereby regenerates the deactivated catalyst and produces a hot flue gas which hot flue gas drives the power recovery turbine which power recovery turbine drives the axial flow air compressor;
• wherein the fluidized catalytic cracking process is under a turndown condition wherein insufficient flow of coke on partially deactivated catalyst is provided to the regenerator, insufficient in that the air rate required to oxidize essentially all the coke is less than the air rate necessary to avoid axial flow air compressor surge and wherein axial flow air compressor air rate set point is adjusted to an air rate above the rate at which compressor surge occurs, thereby driving the air compressor to compress more air than necessary to oxidize all coke and wherein to avoid afterburn in the dilute phase, excess air is vented to the atmosphere wherein the improvement comprises:
• (a) adjusting axial flow air compressor air rate set point to about 10% above surge;
• thereby
• (b) compressing an amount of air in excess of regeneration requirements,
• (c) flowing all air compressed by the axial flow air compressor to the regenerator, and
• (d) using complete combustion catalyst in the fluidized catalytic cracking process thereby with all the air compressed and flowed, oxidizing essentially all coke on the coke contaminated catalyst to carbon dioxide in the dense phase of the regenerator and thereby producing a hot flue gas essentially free of carbon monoxide, and
• (e) driving the expander power recovery turbine with all the hot flue gas produced, and thereby
• (f) driving the motor-generator and generating electric power.

In a process which produces flue gas contaminants when spent hydrocarbon cracking catalysts having coke deposited thereon are regenerated, comprising

• (a) burning the coke on the spent catalyst in a regenerator to produce a catalyst having a coke on regenerated catalyst level less than 0.2 wt % and a flue gas containing less than 2.0 vol % carbon monoxide;
• (b) expanding the flue gas from the regenerator through an expander power recovery turbine to drive an axial flow air compressor of a motor-generator/expander power recovery turbine/axial flow air compressor set to reduce the pressure thereof;
• (c) recovering the energy produced from the expansion;
• wherein insufficient coke is supplied to the regenerator to produce sufficient flue gas to expand through the expander and to drive the axial flow air compressor at other than surge, wherein the improvement comprises:
• (1) adjusting axial flow air compressor air rate to about 10% above surge;
• thereby
• (2) compressing an amount of air in excess of regeneration requirements,
• (3) flowing all air compressed by the axial flow air compressor to the regenerator, and
• (4) using complete combustion catalyst with the spent catalyst in the fluidized catalytic cracking process thereby with all the air compressed, oxidizing essentially all coke to carbon dioxide and thereby producing a flue gas reduced in carbon monoxide, and
• (5) driving the expander power recovery turbine with all the flue gas produced and thereby
• (6) driving the motor-generator and generating electric power.