Catalyst Circulation Control

The catalyst circulation is shown in Figure 2.2.7. The catalyst circulation rate for the entire system is set by the Regenerator Control System (RCS) and its direct control of the regenerated catalyst lift rate. The desired catalyst circulation rate is entered into the RCS and it generates an output signal that adjusts the catalyst circulation as described below. The lift line pressure drop is used as a control parameter because it varies directly with catalyst flux (catalyst flow rate) in the lift line. The rate of catalyst lifting, as performed by the L-Valve Assembly, is controlled by splitting a constant lift gas flow between the primary and secondary addition points. In practice, this is achieved by keeping the total lift gas flow constant and varying only the secondary lift gas rate. In addition to the above, the rate of change of the catalyst lift rate must be slow and controlled to maintain the stability of the process environment isolation systems utilized in the CycleMax design. Large or rapid changes in catalyst lift rate could result in the contamination of the oxygen atmosphere in the Regeneration Tower T-1351 by hydrogen gas, or interruption of catalyst flow to the L-Valve Assemblies.

Regenerated (oxidized) catalyst flows from the Regeneration Tower T-1351, through the Nitrogen Seal Drum D-1357, into the Lock Hopper by gravity. In the Lock Hopper, small batches are transferred from the Regeneration Tower to the Regenerated Catalyst L-Valve Assembly. Hydrogen – rich gas engages the catalyst and lifts it through the catalyst lift line to the Reduction Zone above the Platforming Reactors. The flow rate of the regenerated catalyst, and thus the entire system is set by an output signal from the RCS to the Regenerated Catalyst Lift line 013-PDIC-531. The RCS output signal is determined from the desired catalyst flow setpoint (circulation rate) entered. The RCS sets the 013-PDIC-531 setpoint to a value stored in the RCS memory commensurate with the flow setpoint entered. The ouptput signal from the regenerated catalyst lift line 013-PDRC-531 resets the regenerated catalyst secondary lift gas 013-FRC-535 (Flow Recorder Controller) setpoint directly. The flow of secondary lift gas as set by the FRC controls the catalyst lifting via the L-Valve Assembly to the Reduction Zone. As lifted catalyst is replaced by catalyst from the Lock Hopper Surge Zone, the level in that zone falls. Once the low level setting on level indicator is reached, the RCS initiates the transfer of one batch of catalyst from the Regeneration Tower to the Surge Zone via cycling of the Lock Hopper Zone. The Lock Hopper Zone load size is a known weight of catalyst, calibrated during the initial startup of the unit. The actual circulation rate is determined based on a running average of the frequency of Lock Hopper loads transferred. The regenerated catalyst lift line 013-PDRC-531 setpoint is then ramped up or down by the RCS to reach the point where the actual circulation matches the desired circulation rate entered into the RCS.

Spent catalyst flows by gravity from the bottom of the last reactor R-1304 to the Catalyst Collector. Catalyst flows downward, against a low, upward flow of nitrogen, into the Spent Catalyst L-Valve Assembly. Circulating nitrogen form the Disengaging Hopper D-1353 engages the catalyst and lifts it, via the catalyst lift line, to the Disengaging Hopper above the Regeneration Tower T-1351. Since regenerated catalyst is being lifted to the reduction zone, spent catalyst is removed from the Platforming Reactors so as to maintain a level in the upper bed of the Reduction Zone atop the reactor stack. The LRC (Level Recorder Controller) 013-LIC-501 at the Reduction Zone sends a signal resetting the spent catalyst lift line PDRC (Pressure Differential Recorder Controller) 013-PDIC-510A setpoint via a signal selector. The signal selector also receives a signal from the Regenerator Control System. The output signal from the spent catalyst lift line PDRC resets the spent catalyst secondary lift gas FRC (Flow Recorder Controller) 013-FIC-512 setpoint via a signal selector. The selector also receives an output signal from the Reactor/Spent Catalyst Lift line PDRC 013-PDIC-510B. The flow of secondary lift gas as set by the FRC controls catalyst lifting rate via the L-Valve Assembly to the Disengaging Hopper D-1353.

The signal selectors, and the secondary inputs they incorporate, are required to maintain the stability of the process environment isolation systems utilized in the CycleMax design. A large or rapid increase in catalyst lift rate could interrupt catalyst flow from the reactor. The low signal selector receiving input from the Reduction Zone LRC 013-LIC-501B also receives a signal form the Regenerator Control System. The latter signal is an adjustable ramping function designed to slowly increase the catalyst flow rate from 0% to 100% of the design catalyst circulation rate. This ramping is used by the Catalyst Flow Control in the RCS only when catalyst circulation is restarted from zero. At some point during the catalyst circulation ramp, the signal from the LRC will be less than that of the ramp function. At that point, the low signal selector will use the LRC signal as its output signal to spent catalyst lift line PDRC.

The signal selector that receives input from the lift line PDRC also receives a signal from the Reactor/Spent Catalyst Lift Line (R/SCLL) PDRC. The latter signal serves to limit the magnitude and speed of catalyst lift rate changes for system stability. If the catalyst lift rate increases rapidly, the differential pressure between the lift pipe and the Reactor will

increase (the bottom of the lift line being maintained at a higher pressure than the reactor) and the high upward flow of gas will impede catalyst flow downwards to the L assembly. Once the pressure differential increases to near that which will impede catalyst flow, the R/SCLL PDRC output will limit the catalyst lift rate by limiting the secondary lift gas flow.

Figure 2.2.7