Regenerator pressure

In the modern unit design, the main vessel elevations and catalyst transfer lines are typically set to achieve optimum pressure differentials because the process favors high regenerator pressure, to enhance power recovery from the flue gas and coke-burning kinetics, and low reactor pressure to enhance product yields and selectivities. 

In most units, the flue gas pressure is reduced to atmospheric pressure across an orifice chamber. The orifice chamber is a vessel containing a series of perforated plates designed to maintain a given back-pressure upstream of the regenerator pressure control valve. 

Flue gas line hydraulics to the expander, including the regenerator pressure control valve... 

Instead of using the expander main bypass valve for regenerator pressure eontrol, it is also possible to install a small bypass valve, say 30%, downstream of the expander inlet butterfly valve, bypassing to tlie expander exhaust line as shown in Figure 4-73. Although tills valve may not appear neeessary, it ean provide additional flexibility and proteetion. 

The expander inlet valves are modeled using the exaet Cg versus position eharaeteristies supplied by the valve vendor. The speeified valve stroking times ean be varied as part of the analysis. A eontroller positions the valves to hold eonstant regenerator pressure. 

An example is shown in Figure 4-84, whieh represents the regenerator pressure following a malfunetion of the expander bypass valve. The bypass valve trips open in 1 see while the expander inlet valve maintains eontrol. Tlie regenerator pressure drops approximately 2.0 psi before starting to reeover. The inlet valve stroking time was 10 see and the eontroller settings were 5% proportional band and 30 see reset. 

Figure 4-84. Regenerator pressure deviation during expander bypass valve malfunction.

The eontrol of the proeess is based on the reaetor-to-regenerator pressure differential. The pressure differential signal will be transmitted to the expander inlet butterfly eontrol valve and expander bypass eontrol valve, whieh will operate on split range eontrol. 

The addition of a seeond expander main bypass valve (Figure 6-38) in parallel with the initial valve ean provide eloser proeess eontrol and flexibility. Both valves may be identieal 50% eapaeity valves or, one valve may be a 100% eapaeity main bypass valve and the other a smaller 30% eapaeity valve. Either approaeh inereases flexibility and provides more preeise regenerator pressure eontrol. 

A typieal TPG valve arrangement is the five valve system (Figure 6-41). The two expander inlet valves provide regenerator pressure eontrol, overspeed proteetion, and flue gas shut-off to the expander. The expander exhaust valve enables expander isolation. The full and partial expander bypass valves permit aeeurate eontrol of the FCC proeess when the expander is not in operation. 

The strict requirements placed on the quality of the regenerator pressure control system necessitate complex control strategies that can only be achieved using modern freely programmable control systems, while the short actuating times of the control valves require controllers with ultrafast response. 

If, however, the FCC unit should be operated at a different duty point, whether this is due to a different flue gas flowrate or a different regenerator pressure, the bypass valves would either open too wide or not wide enough. The result is fluetuation in the regenerator outlet pressure. 

Additional non-linearities arise from the faet that valves of different nominal sizes are operated in sequenee. An initial improvement in the eontrol response was aehieved in that the steady-state duty point eharaeteristies for operation with and without the expander were stored in funetion generators in the eontroller. Depending on the operational state, the output of the proeess eontroller (regenerator pressure, or differential pressure, between the regenerator and the reaetor) is applied to one or the other of these eharaeteristies. In the event of an expander trip, the system immediately switehes from one eharaeteristie to the other. This results in linearization of the eharaeteristie profile. 

In the event of an expander trip, the regenerator pressure decreases by 46 mbar, and then increases to overshoot the steady-state value by 16 mbar. This constitutes a substantial improvement over the previously described process, but still did not meet the specifications of the end-user. 

In the dynamic simulation run, the pressures and flowrates at the input and output of each module are known. It is, therefore, possible to perform non-linear correction of the control mode, such that the changes in regenerator pressure in the event of load shedding are minimized. In a test performed with a correspondingly corrected controller structure, the pressure drop after load shedding was reduced from 46 mbar to 19 mbar. The subsequent pressure rise of 27 mbar is just below the specified threshold. 

A further substantial improvement in control response was achieved by including the expander inlet pressure into the control algorithm. With this change, the regenerator pressure falls by only 5 mbar, thereafter rising by 9 mbar, representing a fluctuation that is substantially below the contractual agreement. 

The regenerator pressure fell by 28 mbar. This value was within the eontraetually agreed limits. Nevertheless, two further tests were performed with modified parameters. Five hours after beginning the optimization proeess, a test was performed in whieh the regenerator pressure dropped by just 8 mbar. This value is within the normal operating spread of the regenerator output pressure. 

Further, the absolute regenerator pressure needs to be limited to a maximum value. This is aehieved by a eontroller PIC that manipulates the expander bypass valve using a high signal seleetor (HSS). 

Step 7 Calculate the differential in Ae One PRT eontrol objeetive is to maintain the differential pressure between the regenerator and the reaetor stripper. At the time of the breaker opening, it is assumed that the reaetor stripper pressure will not vary. Therefore, to keep the differential pressure eonstant, the regenerator pressure needs to also remain eonstant. Eor the expander, this means that must remain eonstant. To keep P eonstant, the mass flow before and after the breaker opening must remain eonstant (Equations 7-7 and 7-8). This implies that whatever mass flow is redueed on the inlet valve must be rerouted over the bypass valve. 

These trimming faetors ean, for instanee, be used to reduee the bypass valve step in eomparison to that ealeulated. Opening the bypass valve too mueh ean eause the regenerator pressure to drop, eausing eatalyst to enter the expander and exit the staek. This should be avoided, whieh ean be aehieved by the trimming faetors. 

However, when an expander has to operate at mismatehed eondi-tions, the aetual mismateh usually oeeurs in the inlet butterfly valve, whieh attempts to maintain regenerator pressure. Fortunately, proeessing parameters ean also be modulated under different produetion eapaeities. In sueh eireumstanees, it is important to keep the volume flow in the normal range to maintain system effieieney. For an ideal gas, the eonditional equation is ... 

In the regenerated catalyst standpipe, a 40 Ib/ft (640 kg/m ) catalyst density versus a 25.4 Ib/ft (407 kg/m ) density produces 3 psi (20,7 Kj,) more pressure head, again allowing an increase in circulation or a reduction in the regenerator pressure (gaining more combustion airi... 

The reactor pressure is not directly controlled instead, it floats on the main column overhead receiver, A pressure controller on the overhead receiver controls the wet gas compressor and indirectly controls the reactor pressure. The regenerator pressure is often controlled directly by regulating the flue gas slide or butterfly valve. In some cases, the flue gas slide or butterfly valve is used to control the differential pressure between the regenerator and reactor. 

Increase in Regenerator pressure Decrease in 0-40 microns fractions of E Cat Increase In eiH microns fractions of E-Cst Change in Catalyst Average Particle Size (APS)... 

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