DESCRIPTION OF FLOW WITH CONTROLS

This section outlines the logic requirements for the operation of the Lock Hopper Sequence, the Make up Valve, the isolation Systems, the catalyst Flow Interrupt Systems, the Regeneration Systems, the Valve verification System, and the Bypass Switches. This explanation has been simplified to include only those items that the operator can check for proper operation. For a complete explanation of the logic, refer to the UOP Process and Information Control Equipment and Instructions Databook, chapter 2.

1.1 CATALYST FLOW CONTROL

1.1.1 Catalyst Flow Pushbutton

The catalyst Flow Pushbutton starts and stops the transfer of regenerated catalyst from the Lock Hopper to the Reactors.

The Catalyst Flow Pushbutton has only two positions: ON and OFF. An indicator on the control panel shows the position of the pushbutton. The catalyst Flow Pushbutton can be turned on or off using either the DCS or the Operator Interface. Initially, the catalyst Flow Pushbutton is in the OFF mode.

ON

To start transferring catalyst, the following condition must be met:

1. The regenerator is not in a Hot Shutdown.

Then, when the operator depresses the Catalyst Flow Pushbutton, the catalyst transfer starts. The indicator changes from OFF to ON and the catalyst regeneration control system (CRCS) starts the following systems:

♦ Lock Hopper Cycle

♦ Makeup Valve Control

♦ Catalyst Flow Control

♦ Spent catalyst Lift Rate Limiter

OFF

The Lock Hopper stops automatically when a Hot Shutdown occurs.

The operator can manually stop the catalyst transfer at any time in order to begin the Hot Shutdown Procedure. If the Catalyst Flow Pushbutton is depressed while catalyst is transferring, then the transfer stops. The indicator changes from ON to OFF and the systems listed above are all stopped.

1.1.2 Catalyst Circulation Rate Setpoint

The catalyst Circulation rate Setpoint is used by the Regenerator Control System to control the catalyst circulation rate. The operator sets the setpoint in three steps.

First, the operator decides on the desired catalyst circulation rate as required by the Reactor Section and Regeneration Section operating conditions. This catalyst circulation rate should not be above the design catalyst circulation rate.

Next, the operator computes the Catalyst Circulation Rate Setpoint using the formula below:

Catalyst Circulation Rate, % =

(pounds/h)

The result should be a number between 0 and 100%.

Finally, the operator enters the Catalyst Circulation Rate Setpoint into the Regenerator Control System (CRCS). The operator enters the setpoint at the control panel. Even though the setpoint can be adjusted up to 100%, the operator should not enter a setpoint that results in a catalyst circulation rate above the allowable catalyst circulation rate. The General Operating Curve (see Figure 3.1) may limit the circulation rate to less than 100%. And even though the desired setpoint may be less than 25% the setpoint can only be adjusted down to a minimum of 25%

The CRCS utilizes the Catalyst Circulation Rate Setpoint to generate the setpoint for the Regenerated Catalyst Lift Iine Differential Pressure Controller 0.13-PDIC-531. When catalyst flow is turned ON, the regenerated catalyst lift line ∆P controller setpoint is initially zero. The CRCS then ramps the setpoint to a value that is estimated to yield a circulation rate half that of the target catalyst circulation rate setpoint. If the Lock Hopper Surge Zone level was above 40% when Catalyst Flow was turned ON, the CRCS uses the Regenerated Catalyst Lift Line ∆P ramp rate entered on the Catalyst Flow Setup Screen. If the level was below 40%, the CRCS uses a 2% per minute ramp until the level rises above 40% for the first time.

The Regenerated Catalyst Lift line ∆P controller setpoint is held at the estimated value until sufficient Lock Hopper Cycles occur for the calculation of the actual catalyst circulation rate. The CRCS then adjusts the setpoint, using the ramp rate from the Catalyst Flow Setup Screen, to achieve the target catalyst circulation rate.

The catalyst circulation rate is achieved by adjusting the lift line ∆P controller setpoint.

The CRCS compares the Catalyst Circulation Rate Setpoint to the actual Averaged Circulation Rate as determined by the Lock Hopper cycle time (see section on the Lock Hopper system later in this chapter). The lift line ∆P controller setpoint is adjusted up or down by the CRCS until the setpoint and actual catalyst circulation are in agreement.

When catalyst circulation is started from zero, the lift line controller setpoint is ramped upward from 0% at a rate determined by the Regenerated Catalyst Lift Line Differential ramp rate. This ramp rate is set so as to gradually increase the Reduction Zone catalyst level, and thus gradually increase the spent catalyst lift rate. The rate of increase of the spent catalyst lift rate must be slow so that the spent catalyst lift line ∆P generated does not cause the ∆P between the L-Valve assembly and the Catalyst Collector to fall out of its control range. This ramp rate is also used during normal operations when the actual catalyst circulation rate differs by more than 10% from the setpoint. For initial operation of the unit this ramp rate should be set slow, 2-5% / min, then adjusted higher with greater operating experience.

When the actual catalyst circulation rate and the catalyst circulation setpoint are within 10% of each other, the RCS will make incremental changes in the lift line ∆P setpoint based on the error in percent toward the desired circulation rate every time a Lock

Hopper cycle is completed. The size of the lift line ∆P setpoint changes will depend on the Catalyst Flow Rate Gain constant.

1.1.3 Spent Catalyst Lift Rate Limiter

Whenever catalyst circulation is started, the CRCS slowly ramps up the spent catalyst lift rate from 0% to 100% at a rate given by the Spent Catalyst Lift Line Differential Pressure Ramp Rate that is entered in the Catalyst Flow Setup Screen. The ramp occurs at a rate set slow enough so that the lift line ∆P generated does not cause the ∆P between the L-Valve assembly and the Catalyst Collector to become too high and prevent catalyst flow out of the reactor. When the limiter output becomes greater than the Reduction Zone level Controller output, the signal from the Reduction Zone Level controller will take control and reset the lift line ∆P controller. The limiter is also activated when the Reduction Zone level rises above 50% after a Spent Catalyst Flow Interrupt occurs.

The limiter will complete its ramps and hold at a value of 100 unless the Spent Catalyst Ratio Limit Override Switch is ON. The Override limits the ability of the Reduction Zone Level controller to set the Spent Catalyst Lift Line DP Controller setpoint to a higher value than that required for circulation at the target rate. The Override will be useful if high catalyst circulation rates through the Reduction Zone are causing high temperature excursions in the Reduction Zone. For a more detailed discussion of the Spent Catalyst Ratio Limit Override Switch, refer to the PIC Equipment and Instructions Databook, section 2.13.

The ramp rate for both the regenerated and the spent catalyst lift lines should be set the same. For initial operation of the unit this ramp rate should be set slow, 2-5% per minute, then adjusted higher with greater operating experience.

1.1.4 Catalyst Flow Interrupt Systems

The catalyst flow from the Reactor and from the Lock Hopper Surge Zone can be interrupted to prevent low levels in those vessels

The CRCS controls the operation of these systems and catalyst flow will be allowed or interrupted automatically when certain conditions exist.

During normal operation the interrupt systems have no effect on catalyst circulation.

However, during shutdowns, the systems prohibit catalyst transfer in order to maintain catalyst inventory in the Reactor and the Lock Hopper

a. Spent Catalyst Flow Interrupt

The catalyst flow from the Reactor will be interrupted in two ways depending on the level at the top of the Reactor. If the Reduction Zone LRC (Level Recorder Controller) 013-LIC-501 indicates a low level (~10%), the CRCS resets the Spent Catalyst Lift Rate Limiter controller to zero via the low signal selector. At the same time, the secondary lift gas control valve (FC) 013-FV-512 is close by de-energizing a solenoid in the instrument air line, shutting off the instrument air supply.

If the reduction Zone level continues to drop below the low level indicator switch point and reaches 0%, the spent catalyst isolation will trip. Tripping the isolation will cause the two isolation valves 013-XV-523 & 522 to close and the nitrogen pressure valve 013-XV-524 to open. This results in a Hot Shutdown.

(1) CLOSED

The following condition will cause a Spent Catalyst Flow Interrupt:

The catalyst level in the Reduction Zone falls below 10%

Then, the indicator labelled SPENT CATALYST FLOW INTERRUPTED on the control panel lights up.

(2) OPEN

When the following condition is met, the Spent Catalyst Flow Interrupt will clear:

The Reduction Zone level rises above 50%

Then, the indicator labelled SPENT CATALYST FLOW INTERRUPTED on the control panel turns off. Then the spent Catalyst Secondary Lift Gas Valve’s solenoid is energized (valve opens) and the Spent Catalyst Lift Rate Limiter begins ramping up, allowing lifting to resume.

b. Regenerated Catalyst Flow Interrupt

The catalyst flow from the Lock Hopper Surge Zone will be interrupted if the catalyst level in the Surge Zone falls below 10%. If a low-low level 013-LALL-507 is indicated by the Surge Zone LR (Level Recorder) the CRCS resets the Regenerated Catalyst Lift Line ∆P controller to zero catalyst flow. At the same time, the secondary lift gas control valve 013-FV-535 (FC) is closed by de-energizing a solenoid in the instrument air line, shutting off the instrument air supply.

(1) CLOSED

The following condition will cause a Regenerated Catalyst Flow Interrupt:

The catalyst level in the Surge Zone falls below 10% (013-LSL-507)

Then, the indicator labelled REGENERATED CATALYST FLOW INTERRUPTED on the control panel lights up.

(2) OPEN

When the following condition is met, the Regenerated Catalyst Flow Interrupt will clear:

The Catalyst level in the Surge Zone level rises above 40%

Then, the indicator labelled REGENERATED CATALYST FLOW INTERRUPTED on the control panel turns off. Then the Regenerated Secondary Catalyst Lift Gas solenoid is energized (valve opens) and the Regenerated Catalyst Lift Line ∆P Controller begins ramping up, allowing lifting to resume.

When there is a low-low level in the Lock Hopper surge zone (0%), only the Regenerated Catalyst Flow Interrupt is activated, this does not result in any shutdown of the closing of the Regenerated Catalyst Isolation system.

1.1.5 Regenerated Catalyst Lift Line Valve

The valve 013-XV-018 in the regenerated catalyst lift line at the top of the reactor serves to isolate the Platforming Unit reactors from the Regeneration Section under certain circumstances. It is also useful in preventing gases from the reactor from flowing back into the lift line. The valve may be closed by the process operator from the control room, or it may be closed automatically by the CRCS.

There are two conditions that will automatically signal the CRCS to close the lift line valve:

1. The temperature of regenerated catalyst lift line, as measured by the lift line skin 013-TC-507, is above the high temperature monitor switch of the temperature indicator. This indicates a back flow of Reduction Zone gas or Reactor gas as a result of a lift line rupture.

2. The flow of gas to the Reduction gas heater n°1 (013-FI-549) AND regenerated catalyst lift gas are below the low-low level monitor switches of both indicators

1.2 LOCK HOPPER SEQUENCE

The Lock Hopper System regulates the catalyst circulation rate through the Lock Hopper D-1358 and consequently throughout the entire Reactor and Regenerator system. A description of the catalyst flow control functionality is given in chapter 2.

The catalyst is transferred trough the Lock Hopper Zone of the Lock Hopper D-1358 in small batches. The cycle time of this batch operation is used to determine the catalyst circulation rate.

The Lock Hopper System has four functions:

- Determines the actual catalyst circulation rate and adjusts the setpoint to the Regenerated Catalyst Lift Line Differential pressure Controller (See

“catalyst Flow Control” later in this chapter)

- Enables the Spent catalyst Lift Rate limiter (See “Spent catalyst Lift Rate limiter” later in this chapter)

- Ramps the two equalization valves open and closed over a predetermined pattern (see “lock Hopper cycle” later in this chapter)

- Controls the makeup gas valve using Feedthrough, Ramp, or Adaptation modes (See “ Makeup Valve Control” later in this chapter)

The lock Hopper D-1358 transfers catalyst from the Regeneration Tower T-1351 into the Regenerated Catalyst L-Valve Assembly. The catalyst is moved through the Lock Hopper Zone of the Lock Hopper in small batches. The Regenerator Control system controls the sequencing of this batch operation.

The cycle time of this batch operation indicates the catalyst circulation rate. The more often the Lock Hopper Zone cycles, the faster the catalyst circulation rate. In this way, the flow of catalyst through the Lock Hopper monitors the catalyst circulation rate throughout the entire Reactor and Regenerator System.

The Lock Hopper Cycles through five basic steps in a sequence. Two of the steps, PRESSURE and DEPRESSURE, have two sub-steps. The Five basic steps in one cycle of the Lock Hopper are: