Catalytic Cracking
I. Brief introduction on the process flow
The catalytic cracking is developed on basis ofthermal cracking technology, which is an important process operation to improvethe working depth of crude oil and produce high-quality gasoline and dieseloil. The raw material is mainly heavy oil from crude oil distillation orfraction of other refining equipments at 350～540℃. Thereare three main parts for catalytic cracking unit, namely, reaction andregeneration, fractional distillation and absorption and stabilization. The gasphase oil, gasoline, diesel oil and heavy distillates are obtained fromproducts of catalytic cracking through fractionation.
The pressure of reactor is achieved byselecting switches to control three different valves according to differentstages of working conditions:
Two devices oven and fluidization stage: thevent valve and control valve on the outlet oil and gas pipeline installed atthe top of the depressor shall be adopted for regulating.
Gland sealing prior to oil-taking of reactor totwo devices fluidization and temperature rising stage: the pressure of the twodevices shall be controlled by regulating the opening of the oil and gasbutterfly valve at the top of the tower outlet with pressure regulator with itspressure measurement point set at the top of the catalytic fractional column.
Oil-taking of reactor to the stage prior to thestarting of rich gas compressor: the stable reaction pressure at the oil-takingstage shall be ensured through regulating the opening of small flare valve ofrich gas at the inlet of aerostatic press, as well as remote control of bigdiameter valve in parallel with small flare valve.
Normal production stage: after application ofrich gas compressor, the pressure regulator at the top of catalytic fractionalcolumn controls the speed controller of turbine and the stability of reactionpressure is ensured through control of rotate speed of turbine. Meanwhile, theanti-surge operates automatically and the pressure regulator at the inlet ofrich air compressor controls the big flare valve of rich air at the inlet ofcompressor to be automatic.
Ø Differential pressure between depressor andregenerator, pressure control of regenerator and rotate speed control of smokegas machine
The smoke gas machine is generally equipped forlarge-scale catalytic device. The flue gas produced from regenerator drives thesmoke gas machine and recovers the pressure energy at the same time. It isimportant to keep the pressure control of regenerator, differential pressurebetween depressor and regenerator and rotate speed control of smoke gasmachine, which is also the most difficult point in the whole control scheme.
The differential pressure between depressor andregenerator is controlled by differential pressure regulator with positiveacting. When the reactor is abnormal (depressor), the pressure decreases, andthe differential pressure between depressor and regenerator becomes smaller.When the differential pressure is below the given safety range, the output ofdifferential pressure regulator decreases rapidly (with smaller proportionalband P and integration time constant T). The low selector 1 chooses the outputsignal of differential pressure regulator between depressor and regenerator,and replaces the output signal of pressure regulator of regenerator withoutinterference. Increase the opening of double slide valve and high-temperaturebutterfly valve with the purpose to reduce the pressure of regenerator, andthus to maintain differential pressure between depressor and regenerator withinthe set value range. Therefore, the differential pressure signal of depressorand regenerator functions serve as overrode and soft limitation protection.When the pressure of reactor recovers, the system turns automatically withoutinterference into regenerator with pressure controlling the high-temperaturebutterfly valve and double slide valve at the inlet of smoke gas machine.
Ø Temperature control of fractional column
1) Thereare four circulating reflux within column at different places in order todischarge excessive heat and maintain uniform distribution of gas and liquidload within the column, namely, overhead circulating reflux, first middle-stagereflux, second middle-stage reflux and slurry circulating reflux. The refluxrate shall be regulated with set value.
2) Thetemperature control at the top of fractional column adopts cascade controlscheme. The main circuit is the temperature control at the top of fractionalcolumn and the auxiliary circuit is the flow rate regulating of crude gasolinereflux (overhead circulating reflux).
Ø Control of stabilizer
1) The stripping section of stabilizer isequipped with sensitive plate temperature (including column top) controlcircuit and liquefied petroleum gas (C5 and C2) control circuit as indirect anddirect index for the quality of liquefied petroleum gas. The temperaturecontrol and reflux flow of the column top are equipped with cascade regulatingsystem, which can also be connected with extracted flow of liquefied petroleumgas products to form a cascade control system.
2) To connect the product extracted flow andreflux flow at the top of column, cascade control system shall be providedbetween the liquid level of the overhead reflux of stabilizer and flow rate atthe inlet of overhead reflux pump.
3) The overhead pressure control of stabilizershall adopt hot vapor bypass control (split range control). The non-condensinggas exhausting valve shall be applied for discharging to ensure the applicationof hot vapor bypass control when there is too much non-condensing gas with thepurpose to recover the normal operation of hot vapor bypass control.
Meanwhile, the pressure regulator ofregenerator forms override (low selection) control circuit with speed regulatorof the smoke gas machine to achieve overspeed and soft limitation protection.The control flow shall refer to Fig.4.
II. Control strategy of catalytic cracking unit
2.1 Control solution
In oil refinery, the catalytic cracking unit isone of the devices with complex technology and big difficulty in operation.Many countries in the world regard the catalytic cracking process as animportant means for deep processing of crude oil and improving the economicbenefit. Thus, small improvement in such aspects as technology, device andcontrol will bring large benefit.
The final targets for catalytic cracking unitare stable operation on one hand and optimal product distribution on the otherhand.
Ø Temperature control of reactor
The temperature of reactor is the key parameterimpacting the product rate and distribution of catalytic cracking unit, whichinfluences directly the quality and quantity of products, and could becontrolled by regulating the cycle capacity of regenerated catalyst. The schemeintroduces the low value selection control composed by riser temperature TI01and differential pressure of regenerative slide valve, in which differentialpressure of regenerative slide valve serves as override control to achieve thelow differential pressure and soft limitation protection of regenerative slidevalve and to prevent back flow of catalyst .
1) There is higher differential pressure on theregenerative plug valve TV101 (fail close valve FC) under normal workingcondition to maintain normal flow of catalyst, thus the measured value PV ofdifferential pressure PDI105 of the regenerative slide valve is higher than setvalue (SV). The configuration of regulator PDICA 105 is positive with smallerproportional band P and integration time constant T, therefore, the outputvalue is rather big, which is bigger than the output value of temperatureregulator TICA101 under normal condition. The output of TICA101 is generallyselected by low selector ORST101 to control the motion of regenerative plugvalve TV101 to achieve the purpose of temperature control. When the risertemperature TI101decreases, the configuration of regulator TICA101 is negativeacting with output increasing. Control the opening of the plug valve TV101, andincrease the cycle capacity of catalyst to rise the temperature.
Under abnormal conditions, the differentialpressure on the valve will decrease when the differential pressure between thetwo systems decreases or the opening of regenerative plug valve TV101 is toobig. When the differential pressure is less than the set value (SV), the outputof regulator PDICA105 reduces dramatically and when it is lower than the outputof temperature TICA101, it will be selected to control the regenerative plugvalve TV101. The opening of the valve will be decreased to maintain thedifferential pressure of valve, and to ensure normal circle of oil and gascatalyst without backflow.
It is the core of this control scheme toachieve control right conversion of plug valve TV101 under different workingconditions.
ØOverhead pressure control of reaction depressor—fractional distillation