Selected Process Options for FCC
A modern U.O.P designed Fluidized Catalytic cracking Unit is having four sections.1) Feed preheated section
2) Catalyst Section
3) Fractionation Section
4) Gas concentration Section
These four sections operate together in an integrated manner.
Feed Preheating Section:
Hot vacuum gas oil from new Vacuum Distillation Unit or from F.C.C.U feed tanks 1,2,3,4 of capacity 23000MT (EACH) received in raw oil surge drum. A Disulphide line from Merox also joins at the top of raw oil surge tank. Level controller is provided on the line coming from booster pumps taking suction feed tanks for controlling the surge drum level.The raw oil from surge drum is pumped by raw oil charge pumps and passes through the main column bottoms raw oil exchanger for preheating. BY-passing facility for heat exchanger provided. This feed flow is recorded by the flow controller. A temperature controller is also provided for the inlet and outlet of the heat exchanger. Raw oil connection is also provided for flushing oil, Torch oil, and slurry settler diluting lines. The Raw oil from heat exchanger outlet goes to charge heater.
A 4” Circulating oil recycle line from discharge pump joins the feed line at the inlet of charge heater under flow control. Another 4” line from downstream of flow controller joins the feed line at the outlet of charge heater bypassing heater, with isolating block valves. Combined feed to furnace enters in three passes under the flow controller which resets the control valves on fuel oil or fuel gas supply lines. Fuel gas flow is indicated by a controller and fuel oil supply and return flows are indicated by fuel oil controllers. Hot slurry from slurry settler is recycled under flow control to feed on the heater and this combined feed goes to Receiver Riser. Charge heater outlet is provided with a safety valve which releases the excess pressure to the Main column below the tray.
Furnace convection section heat is used for superheating the H.P.and M.P steam produced by two heat exchangers.
The superheated steam coming out from furnace section i.e. H.P and M.P steams are provided with each with a silencer for start-up purpose, and heater to regulate steam temperature with water injection. Both these coils are designed for no flow. H.P and M.P super heater outlets are provided with a set pressure of 41.5KG/CM2.
Catalyst Section:
ReactorThe combined feed contacts with hot catalyst from regenerator at the bottom of reactor riser. The resulting catalyst-oil vapours mixture is raised to the designed reaction temperature by using the sensible heat of catalyst, Most of the cracking reaction takes in the Riser. The oil vapours disengage from the catalyst in the reactor dense phase and leave the reactor through single stage cyclone separator, suspended from the reactor top head. The catalyst from the cyclone returns to the dense phase of the reactor.
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| Fluid Catalytic Cracking Plant Flow Diagram |
The spent catalyst leaves the stripper through reactor standpipe to the regenerator. A slide valve, which is located on this standpipe and is reset by a level controller which controls the level of the catalyst in the reactor.
The head of the Catalyst standing in the reactor standpipe produces enough pressure to overcome the differential pressure between the reactor and regenerator and causes flow through the slide valve. An expansion joint on the standpipe accommodates the relative expansion of reactor and regenerator, to ensure free movement and prevent Catalyst from packing. Expansion joints are provided with aeration points.
In case of Emergency when Regenerator Temperature Becomes uncontrollable, LPD and LCO will be used as quench to Reactor feed. For this purpose, a 2” line from LPD Net over Head and LCO are provided at the feed inlet to Riser.
Regenerator
The regenerator has a lining of insulating refractory gunnies on a reinforcing support. This lining is necessary to protect the metal wall of the vessel from the high temperature at which the regenerator operates and should keep the outer shell of the regenerator below 220 degrees at all times. Coke deposition on Catalyst surface takes pale in the reactor during the process of cracking, due to this active surface of the catalyst reduces. To regenerate the Spent Catalyst the deposited coke is burnt off by air in the regenerator. The atmospheric air is taken through a Blower, an air is discharged to the regenerator through a control valve which is reset by temperature control which controls the differential temperature between the regenerator dilute phase and dense phase or plenum chamber.
The regenerated Catalyst leaves the regenerator through the regenerator standpipe. A slide valve which is reset by controller controls the temperature of the reactor. An expansion joint is provided on this stand-pipe. The regenerated catalyst goes to the bottom of the reactor riser and mixes with the incoming feed. Thus a catalyst circulation is established between Reactor and Regenerator
Orifice Chamber
The coal is burnt in the regenerator to form a mixture of CO, C02,. The gases mostly CO and CO2 come out of 2 Double stage cyclones in Regenerator and leaves at the top. Entrained Catalyst is separated in the cyclone. Fines pass out through a Double pipe slide valve which is reset by the controller, which controls the differential pressure between the Reactor and regenerator. Gases are passed through an orifice chamber where a series of restriction orifices reduce the pressures of gases so that these can be burnt in CO boiler. The two port slide valve located at the bottom of orifice chamber can divert the gases either to CO Boiler or to the stack by passing CO boiler.
Fractionation Section:
The cracked vapours from the reactor top are fractionated into different cuts in this section. Vapours enter to the main fractionating column at the bottom. The column has 41 trays,
Overhead section
Vapors from the column top are condensed in overhead condensers and further cooled in trim condensers. Condensed liquid and uncondensed gases are received in main column receiver. A part of this condensate is put back as reflux to main column top with the help of pumps. Under temperature control valve which controls main column top temperature, the remaining liquid is pumped by the main column near overhead pumps to primary absorber under level controller which controls level. A 4-inch tie line is provided between the discharge off and discharge on so that in case of reflux pump failure it can be used for reflux service. 4 inch take off from this, joins downstream of stripper level control valve to facilitate bypassing primary absorber sponge absorber and stripper and route to debutanizer.
Gases from main column receiver go to gas compressor in G.C.U. Main column receiver pressure is controlled by releasing the excess pressure to flare.Water accumulated in the boot of the main column is pumped out by the sour water pump to sour water stripping unit located in the merox unit and can be routed to sewer below overhead drum platform or at owes during outage of sour water stripping unit.
There are three side streams from the main column as Light cycle oil. Heavy cycle oil and circulating oil.
Light Cycle Oil
L.C.O is drawn from 30th tray of the main column and goes to L.C.O stripper on the 6th tray under the level controller. Medium pressure stripping steam is introduced at the bottom of the stripper under flow control. The vapours from the stripper top return back to the main column below 31st tray. Stripped L.C.O is taken to pumps and is pumped to stripped light cycle oil cooler. A 3 inch line take off from upstream goes to additional cooler outlet, which joins back to the downstream after the cooler, L.C.O gaps to total cycle oil headers to storage under flow control.
Circulating Oil
The outer part of L.C.O drawn from 30th tray of the main column goes to LCO circulation pumps and then it is pumped to naphtha stripper reboiler. After exchanging heat with naphtha column on 33rd tray, the L.C.O flow control can be cascaded with stripper top vapour flow controller.
Heavy Cycle Oil
H.C.O is drawn from 16th tray of the main column and send to the stripper under the level controller. A 2inch take-off joins the downstream of HCO to route circulating oil to maximize diesel production. Local flow controller provided on this take off. H.C.O is introduced on the 6th tray of the H.C.O stripper under flow control. The vapours from the stripper top return back to the main column below the 17th tray. Stripped H.C.O, feed water exchanger and then downstream goes to additional cooler outlet which joins back to he downstream, facilitate bypassing. After the cooler it goes underflow controller to T.C.O to storage and part of it can be routed to H.C.O under flow control.
Circulating Oil
The other part of H.C.O drawn from 16th tray of the main column goes to H.C.O circulating pumps and then it is pumped to debutanizer reboiler after exchanging heat with debutanizer bottoms H.C.O goes back under flow control to the main column on 20th tray. One 4 inch line from outlet of heat exchanger upstream goes to sponge absorber through flow control. Rich oil from the bottom of sponge absorber again joins back to the downstream of the control valve.
Column Bottoms
The slurry collected from the bottom is pumped by turbine driven pumps to heat exchangers. It goes to steam generator underflow controller and to main column bottoms under control. After passing through slurry oil it goes back to the main column above the 5th tray along with main. The slurry oil partly goes to column bottom as quench. This quench flow is indicated by quench flows cascaded with column bottom temperature recorder and controller.
The remaining slurry oil goes to slurry settler where the catalyst settles down at the conical bottom of the vessel. Bandoleer oil free from catalyst particles comes out from the top. This is called clarified oil. A part of this clarified oil returns back to the main column on the 1st tray through CLO and clarified oil coolers and then it is pumped to storage by pump under flow control which is controlled with column level controller. Slurry settler bottom rich in catalyst joins the feed to the reactor after the furnace under the flow control.
GAS CONCENTRATION UNIT (GCU)
Function
The main duty of GCU is to Separate Lean Gas, LPG and Stabilized Gasoline from Wet Gases and unsterilized Gasoline obtained from main fractionator Overhead Drum. The GCU mainly consists of WGC, Primary, Sponge Absorbers, Stripper, debutanizer, Associated Pumps, Reboiler etc. for the above purpose.
The uncondensed Wet Gases and UP Distillates (Net O/H Liquid) from the Main Column overhead receiver are fed to the gas concentration Unit. Wet Gases are compressed in Wet Gas Compressor in two stages. LPG and Stabilized- Gasoline are separated by Absorption Process from the compressed Gases and L P Distillates in the Primary absorber, Stripper and Debutanizer and off Gases from Primary Absorber pass through sponge absorber to absorb residual C3 and C4 of the off Gases with HCO. The functions and the detailed process flow description of the main equipment such as WGC, Primary Absorber, Sponge Absorber, Stripper and Debutanizer are described in detail as under.
WET GAS COMPRESSOR
Function
The main purpose of the Wet Gas Compressor is to compress the uncondensed Gas from fractionator overhead Vessel for recovery of C3, C4 and C5 Components.
Process Description:
Uncondensed vapours enter the compressor suction drum before going to the compressor. First stage suction is provided to knock out any liquid carry over along with the vapours. This drum is having stainless steel wire mesh blanket at the top which acts as a demister. From compressor suction drum, vapours free from liquid go to Wet Gas Compressor first stage suction. Compressed Gas from the first stage goes to interstage cooler. Before entering the cooler it meets with a wash water stream from condensate injection pumps. Condensate injection pumps take suction from condensate Injection tank. After Cooling, Cooled products flow to compressor interstage receiver bottom level pumped by interstage receiver condensate pumps under level controller to the inlet of High-Pressure Coolers. A 2" recycle line is also provided.
A 6" line from compressor first stage discharge joins the inlet of Main Column Overhead Condenser as a spillback under a pressure control valve which maintains the pressure. 14-PRC-01 is also cascaded with 1st stage flow. In case of low flow compressor, first stage suction gives the signal and opens the control valve.
Gas Compressor:
Gas leaving the top of reactor vessel flows to second stage suction compressor. The compressed gas from the second stage flows to the inlet of High-Pressure Cooler. Compressed gas from the second stage discharge meets the following streams at the inlet of High-Pressure Cooler.
1) Unstabilized gasoline from the bottom of the Primary Absorber pumped by Primary Absorber Rich Oil Pumps under level control.
2) A 6" line of stripper gas from the stripper.
3) Liquid from Inter-Stage Receiver pumped by a heat exchanger under level control.
Above mentioned three streams pass through the High-Pressure Cooler and then a mixture of uncondensed gases, liquid hydrocarbons and wash water from the outlet of exchanger flow to the High-Pressure Receiver.
Sour water collected in the boot of High-Pressure Receiver flows by system pressure under level control to the inlet of Main Column Overhead Trim Condensers. A 1" line from Compressor Suction Drum Pump also joins this line. A pump takes suction from Compressor first stage suction drum.
A 6" line of rich gas from reactor vessel goes to the Primary Absorber below the tray. Unstabilized gasoline from reactor vessel is pumped by Stripper Charge Pumps under flow control to the Stripper. The fractionator is cascaded with HP drum level, which maintains the level of the reactor vessel.
PRIMARY ABSORBER
Function
The main duty of the Primary Absorber is to obtain a perfect separation of propane, propylene, butane and butylene from the rich gas by the method of Absorption. The Primary Absorber is a 36 tray Column. Gas from the high-pressure receiver enters the first tray situated at the bottom section of the tower and liquid enters at the top of the tower either on 36th or 27th tray. Stabilized gasoline from debutanizer and unstabilized gasoline from the vessel are used as an absorbent,
Process Description
Stabilized gasoline from Debutanizer is pumped by Recycle Gasoline Pumps under flow control. This Stabilized gasoline enters the Primary Absorber on 36th tray. Unstabilized Gasoline from Main Column Receiver is pumped by Main Column Net Overhead Pumps under level controller to the Primary Absorber on 27th tray. The rich gases from High Pressures Receiver enter the Primary Absorber below the first tray at the bottom section of the tower. The heat of absorption is removed by circulating two streams of rich gasoline from upper and lower section of the absorber.
Upper section stream from tray 24 goes to Primary Absorber Upper Intercooler Pump and it is pumped back to the absorber above the 24th tray after cooling the stream in Primary Absorber Upper Intercooler.
Similarly lower section stream from tray 12 goes to Primary Absorber Lower Intercooler Pumps and it is returned back to the absorber above the tray 12 after cooling the stream in Primary Absorber Lower Intercooler. Upper and Lower intercooling circulation can be adjusted (For better absorption) with the globe valves provided on the outlet of both the heat exchangers respectively. The upper and lower inter cooling circulation can be maintained with the locally provided pumps.
Lighter hydrocarbon vapours coming from the top of the Primary Absorber 15-C-01, go to the Sponge Absorber below the first tray at the bottom section. A Flow transmitter has been provided on this line.
Rich gasoline from the bottom of the absorber is pumped by Primary Absorber Rich Oil Pumps under level control to the inlet of High-Pressure Cooler.
SPONGE ABSORBER
Function
The Main duty of the Sponge Absorber is to recover the valuable components from the gas which could not be recovered in Primary Absorber. H.C.O. from the main fractionating column is used as an absorbent. Sponge Absorber has 20 trays with a Monel wire mesh blanket at the top (above the tray 20) which acts as a demister to eliminate any liquid carry over from the fuel gas leaving the top of the Sponge Absorber.
Process Description
Gases from primary absorber enter the bottom of sponge absorber below the tray l. Circulating H.C.O. pumped under flow control goes back to the main column after exchanging heat with Debutanizer Reboiler. One 4" line from outlet of the heat exchanger, upstream of the control valve, goes to Sponge Oil Exchanger. After exchanging heat in Sponge Oil Exchanger with Sponge Absorber bottom oil it is cooled In Sponge Oil Cooler and then it goes to the suction of Sponge Absorber Lean Oil Pumps. HCO Is pumped to the top of Sponge Absorber by pump under flow control. The unabsorbed lean gas from the top of sponge absorber flows under pressure control to Refinery Fuel Gas Header through the Gas Knock out Drum.
The HCO entering Sponge Absorber at the top comes down to the bottom of the absorber, absorbing the gases travelling up in the countercurrent direction. The rich sponge oil from the bottom of sponge absorber goes under level controller through sponge oil exchanger and joins at the downstream of Control Valve on HCO circulating line which is going back to the main fractionating column on the 20th tray.
STRIPPER
Function
The main duty of stripper is to strip-off the light ends (lighter than propane and propylene) and also the bulk of H2S that is present in the unstabilized gasoline which is coming from H.P. receiver. The stripper is a 36 tray column.
Process Description
Unstabilized gasoline from H.P. receiver is pumped by Stripper charge Pumps enters stripper on the 36th tray. Before entering the stripper, unstabilized gasoline is heated in stripper Charge Preheated with stabilized gasoline from Debutanizer. Unstabilized gasoline temperature is controlled by a three-way temperature Control Valve. Preheated unstabilized gasoline enters the stripper at the top above tray 36.
The Stripper bottom temperature is controlled by supplying heat from stripper Reboiler and stripper Bottoms Preheater. In stripper Reboiler, Stripper bottom liquid is heated by Circulating LCO. In stripper Bottoms, Preheated Stripper bottom liquid is preheated by stabilized gasoline from Debutanizer.
Stripped vapours from the top of the stripper go to the inlet of High-Pressure Cooler. The unstabilized gasoline from the bottom of the stripper goes to Debutanizer under level controller which controls the level. A flow transmitter has been provided on this line. Unstabilized gasoline from stripper goes to Debutanizer under system pressure.
DEBUTANIZER
Function
The Main purpose of the Debutanizer is to separate mainly propane, butane, propylene and butylene as overhead products to produce stabilized gasoline of required RVP. It has 40 trays
Process Description
Unstabilized gasoline from stripper enters debutanizer on the 20th tray. Debutanizer bottom temperature is brought up by supplying heat from Debutanizer Reboiler. The heating medium in the reboiler is circulating HCO from primary absorber heat exchanger. After exchanging heat with gasoline, HCO goes back to the main column on the 20th tray. Vapour coming from the top of debutanizer are condensed. Debutanizer Receiver receives the condensate and gases from the exchanger.
Debutanizer top temperature is controlled by reflux from Debutanizer Pumps. The pumps take suction from MC Receiver and pump a part of the overhead liquid from MC Receiver as reflux under flow control. Reflux goes to Debutanizer 40th tray at the top.
The remaining liquid from MC Receiver is pumped to LPG Merox Unit after cooling in Debutanizer Overhead Cooler. This stream of LPG from MC Receiver goes under control which is also cascaded. Gasoline from Debutanizer bottom is cooled in the exchanger, Stripper Bottom Preheated, Stripper Charge Preheated and Debutanizer Bottoms Cooler and then it goes to the suction of Debutanizer Bottoms Pumps and Recycles Gasoline Pumps. Gasoline from Debutanizer is pumped to Gasoline Merox. Gasoline from pump discharge goes to Primary Absorber top.
PROCESS VARIABLES REACTOR
Conversion:
The purpose of cracking unit is to convert the heavy oil charge into more valuable lighter products. The change of heavy charge into lighter products is denoted by a percent conversion which is defined as the percentage of gas oil cracked to unstabilized gasoline and lighter products. For a given period of time;
Volume conversion = 100 - vol% ( cycle oils +clarified slurry)
The conversion should be corrected for any gasoline contained in the charge and it is conventional to correct the yield of cracked gas oil to that which would be produced if 90%by vol. of gasoline distilled at 1930C.
Charge Rate:
The unit well accommodates quite a wide variation in charge rate at constant conversion. Even greater variation is possible if a high conversion is desired at low charge rates since the change in the optimum combined feed ratio will tend to keep the total charge to riser more or less constant. In fact, at very low charge rates, it is generally advisable to maintain a high combined feed ratio.
Charge Temperature:
Increasing the temperature of the raw oil charge is one method of increasing the combined feed temperature, therefore the heat input to the reactor. The effects of increased combined feed temperature are given below:
1. Reduced Catalyst/Oil Ratio:
An increase in combined feed temperature will require less quantity of catalyst to heat it to reactor temperature. Hence the opening of regenerated catalyst slide valve TR-401 which maintains the reactor temperature will drop to reduce the quantity of catalyst flowing to the reactor. This will reduce catalyst/oil ratio.
2. Reduction In Conversion:
Since the quantity of catalyst is reduced the severity of reaction will go down as the amount of feed remains same. In other words, the conversion will be reduced.
3. Increased Regenerator Temperature:
Since the heat requirement from regenerated catalyst reduces (quantity of catalyst drawn to the reactor) due to increased combined feed temperature, the regenerator bed temperature will settle at a new temperature higher than the original.
REDUCE COKE YIELD:
Since the degree of severity reduces due to lower cat/oil ratio, the coke yield reduces.
Recycle Rate:
The recycle rate is generally determined by setting the combined feed ratio which is defined as:
C.F.R = ( total recycle (m3/hr.) +1.0
Raw oil (m3/hr.)
Following are the recycle streams
Slurry:
The settled slurry is recycled to return most of the catalyst that has escaped from the cyclones. The slurry recycle is about 10%of feed. The slurry is poor stock as far as cracking is concerned as it is more refractory.
Circulating Oil:
Circulating oil in large quantities is recycled so that after depletion of heavy molecules in feed, it is preferentially the HCO molecules which crack further and not the LCO molecules which have been produced by the cracking reactions.
The CFR is generally maintained at 1.4 for high conversion [C.F.R range is between 1.35 and 1.5]. Normally CFR is maintained on the lower side at the high feed rate and on the higher side at low feed rate. The optimum combined feed ratio may be expected to be affected by the quality of the raw oil charge as follows:
Conversion optimum C.F.R
55% to 60% 1.05
60% to 70% 1.05 - 1.5
70% to 80% 1.5 - 1.75
A 0.2 increase in C.F.R. will produce the following approximate effects:
1. Increase in gasoline yield if the C.F.R is below optimum
2. A 1-2%increase in conversion
3. A 0.3% increase in coke yield
4. A charge in regenerator temperature which depends on recycles composition
5. A decrease in the average boiling point of the total cracked gas oil (cycle oil).
6. An increase in the pour point of the total cracked gas oil.
7. An increase in the loading of the fractionation section which demands appropriate
Adjustments.
Hence increasing C.F.R will increase the conversion and also increase the ratio of LCO to (LCO+HCO) to an optimum value.
Recycle Composition:
The composition of the recycle is largely dependent on the type of raw oil charge and upon the conversion level. Normally sufficient thickened slurry is recycled from the bottom of the slurry settler to the reactor to return all the entrained catalyst, while the clarified slurry is routed to storage to control the temperature in the bottom of the main column (or alternatively to control the LCO endpoint).
If CLC yield is reduced and the slurry recycle is increased by a similar amount, while the other recycle streams are reduced to maintain constant combined feed ratio, the main column bottom temp. Will remain roughly constant, but since the total recycle is heavier, the regenerator dense phase temperature will tend to rise.
It should be noted that in order to change the quality of recycling, the net withdrawal rate of one recycle stream must be changed; a change in the ratios of the different recycle streams will not have the same effect.
If the unit is operating at either maximum coke burning rate or at maximum regenerator temperature, it is advisable to lighten the total recycle by maximizing the clarified oil withdrawal rate. If the unit is operating at low regenerator temperature a more profitable operation is generally obtained by reducing the clarified oil yield and increasing cracking severity.
The recycle stream is usually composed of a mixture of circulating oil and settled slurry. The relative proportion of these streams is of importance for balancing the operation of the fractionation section or in achieving maximum combined feed temperature.
Reactor Temperature:
The reactor temperature is determined by the heat content of the combined feed and by the quality of catalyst circulated through the riser along with the combined feed (cat/oil ratio). Increasing the reactor temperature will, of course, increase conversion but the same charge can be cracked to the same conversion at fixed reactor temperatures if the combined feed temperature or ratio is changed. In fact, changes in reactor temperature are generally used to trim the conversion level after the other the variables have bee set.
An increase in the reactor temperature (at constant conversion) will produce the following effects:
1. An increase in the lean gas yield (C2 & lighter).
2. An increase in total C3 – C4 yields.
3. A decrease in gasoline yield (the increase in reactor temperature with constant conversion helps in cracking the gasoline further into lighter products and C2, C3, C4 yields increase at the cost of gasoline).
4. An increase in the olefin content of all liquid products.
5. An increase in the clear octane number of the gasoline but almost no effect on the leaded octane.
Reactor Pressure:
The reactor pressure is not an independent variable in the sense that it can’t be varied be at will. The reactor pressure will vary as gas compressor suction conditions are changed and with the change in the pressure drop through the main fractionator.
The effect of the change in reactor pressure is quite small compared with the effects of other variables.
An increase in the pressure will produce the following effects:
A decrease in space velocity in the reactor i.e. increases in residence time of the catalyst and hence increases in conversion.
1. A decrease in reactor velocity and in turn cyclone inlet velocity reduces the cyclone loading but simultaneously decreases cyclone efficiency.
2. With the increase in conversion more coke is formed and hence increases in regenerator temperature.
Stripping Steam Rate:
The quantity of the steam required to strip the oil vapours from the spaces between the catalyst particles is determined principally by the cat. Circulation rate and is generally about 1-2 kg. Per MT of catalyst circulated. If the quantity of steam used exceeds that necessary to displace all the oil vapours between the catalyst particles no additional benefit is realized.
Generally, the unit is started up using a high stripping steam rate. When the unit is lined out, the operator will slowly systematically reduce the stripping steam. At first no effect on regenerator temperature will be observed, but finally, a relatively large increase in regenerator temperature will occur when the stripping steam rate is reduced below the minimum. For routine operation, the steam rate should be increased by about 10% above this minimum.
1. An increase in the fresh feed charge rate.
2. An increase in recycles rate.
3. A decrease in combined feed temperature.
The effects of an increase in stripping steam rate are as follows:
1. An increase in reactor pressure.
2. A decrease in reactor temperature.
3. Deterioration in catalyst particle size distribution.
Catalyst/Oil Ratio:
Reference is frequently made to the cat/oil ratio which is defined as
The weight of catalyst Circulated per hour
Catalyst/oil ratio = Weight of fresh feed or combined feed per hour
The cat/oil is not an independent variable. It will increase with an increase in reactor temperature and with a decrease in regenerator and combined feed temperature.
When process conditions are changed so that an increase in cat/oil ratio occurs, an increase in conversion and in coke yield will probably also observe.
Riser Steam:
It is occasionally desirable to inject some steam directly into the charge immediately ahead of the riser. To avoid catalyst attrition, it is important that this steam is injected through the oil nozzles and not into the annulus around the nozzle.
REGENERATOR:
The function of the regenerator is to burn the coke off on the catalyst returned from the reactor and in doing so, provide the heat necessary for the operation of the unit. Regenerator temperature influences reactor performance. If the catalyst is inadequately regenerated, product distribution will be affected, and increased production of light gases will probably observe. More important is the effect, that variations in regenerator dense phase temperature have on the cat/oil ratio. A low regenerator dense phase temperature results in high catalyst circulation rate, which increases both conversion and the coke yield, but which could also limit the capacity of the unit.
After Burning:
Coke mainly consists of carbon and hydrogen and is oxidized during regeneration according to the following exothermic reactions:
C + O2 => CO2 + Heat
C + 1/2O2 => CO + Heat
H2 + 1/2O2 => H2O + Heat
All above reactions are exothermic. Carbon can be oxidized to eighter CO or CO2. Also, CO can be further oxide as per the following reaction:
CO + 1/2O2 => CO2 + Heat
Thus is CO if exposed to more oxygen it will be oxidized to CO2 and this reaction being more exothermic liberates more heat than the combustion of carbon to CO only. This is after burning reaction which takes place above the regenerator bed and in the cyclones and flue gas lines if sufficient oxygen is present to support the reaction. The occurrence of after burning is readily observed with increase in regenerator temperature.
When this heat is released in dense phase in the presence of the considerable quantity of catalyst, this catalyst absorbs the heat and limits the temperature rise. When afterburning occurs in the second stage of the cyclones and in the flue gas line, the same amount of catalyst is not present to absorb the heat and much larger temperature rise is observed.
Dense Phase Temperature:
The regenerator dense phase temperature is not under direct control, but is dependent upon reactor conditions and charge stock quality. The changes in process conditions which tend to produce more coke also cause an increase in the regenerator, which via a reduction in catalyst/oil ratio reduces the coke yield and the balance.
The process changes which will cause an increase in regenerator temperature are:
1. An increase in charge specific gravity or in the average boiling point of the charge.
2. A decrease in charge quality (Higher carbon residue or lower UPO K).
3. An increase in total recycles specific gravity (produced by an increase in slurry recycle rate or a decrease in CLO withdrawal rate).
4. An increase in combined feed temperature. (This will reduce heat removal from catalyst by feed.)
5. An increase in reactor level.
6. An increase in reactor temperature.
7. An increase in reactor pressure (increase in reactor pressure will reduce the reactor velocity and hence the contact time of catalyst with vapor will increase which in turn will increase the conversion and coke yield).
Of more importance is the increased tendency of the unit to after burning at high temperatures. Whereas at 6000C about 0.5% oxygen may have to be present before after burning commences, only about 0.2% is necessary at 6500C because of the greater quantity of oxygen available at low temperature, a flash off after burning starting at low temperatures may result in a greater temperature rise, and thus be more difficult to control than one that stared at higher temperature. The maximum permissible regenerator temperature is indicated by the mechanical construction of the regenerator.
Regenerator Pressure:
The regenerator pressure is not controlled directly but is equal to the main fractionator overhead receiver pressure plus the pressure drop through the fractionators and the controlled differential pressure between the rector and the regenerator. The regenerator pressure can be varied independently by varying the reactor-regenerator differential pressure within the limits imposed by maintaining satisfactory differential pressure across both catalyst slide valves.
An increase in regenerator pressure will improve regeneration, though this variable is almost never used for that purpose. The effect of regenerator pressure on slide valve differentials, blower power consumption, and catalyst entertainment and on cyclone efficiency is more important.
Lowering regenerator pressure will:
1. Increase the spent catalyst slide valve, 14-LRC-01, differential pressure.
2. Decrease the regenerated catalyst slide valve, 14-TRC-01, differential pressure.
3. Decrease blower power consumption.
4. Slightly improve air distribution.
5. Increase catalyst entrainment to cyclones.
6. Increase cyclone efficiency
Regenerator Level:
The regenerator level will be observed to vary slightly with operating conditions, but the only real control is the relative rate of catalyst addition and withdrawal. A higher regenerator level will increase the catalyst residence time in the vessel, and thus improve regeneration. However, as the regenerator level increase the effective cyclone dip-pipe length decreases. This makes the return of catalyst from the cyclones to the regenerator bed more difficult. The above-mentioned limitations imposed a definite upper limit on the regenerator level.
Since the catalyst in any process will deactivate at certain minimum rate regardless of charge rate and composition, a certain minimum catalyst make-up trite has to be maintained, as a percentage of inventory, regardless of rate. Also if catalyst activity will tend to be less if the catalyst inventory is small. These considerations suggest that regenerator level should be as low practically possible.
One disadvantage of low regenerator level is decreased unit stability which arises from the two effects;
1. A large regenerator inventory will absorb the effects of minor upsets in the operating conditions.
2. A shallow regenerator bed results in reduced oxygen utilization. If bubbles of air pass through the regenerator bed before their oxygen content is consumed after burning will occur and the dilute phase and flue gas temperature will be erratic. In extreme cases after burning could be so severe that in order to control it, the air rate would have to be reduced below that necessary for satisfactory regeneration. However, such a situation is uncommon and generally only arises when caused by either a damaged grid or poor catalyst condition.
Catalyst Circulation Rate:
An increase in the catalyst circulation rate reduces the catalyst residence time in the regenerator and has an adverse effect on regeneration.
Regeneration Air Rate:
The most important regeneration variable is the air rate which must be matched exactly to the coke rate. If insufficient air is supplied, the coke on the catalyst will increase and the unit will get behind in burning. If too much air is too much air is supplied, there will be excess oxygen in the flue gas and after burning will occur.
Since the afterburning releases heat and results in an increase in the temperature in the flue gas rising through the regenerator, this temperature rise is an excellent measure of the amount of excess oxygen present in the regenerator. In order to provide the fine control on the temperature the main air rate is frequently held constant and adjustments are made on a small quantity of air which is vented directly to the atmosphere through 14-TDRC-01.
While the dense phase temperature is always one of the reference temperatures, the other one is varied as per the experience. This operating technique is known as the use of controlled after burning and is done automatically with the help of a differential temperature recorder controller, 14-TDRC-01. This operating technique can only be used if there is no external interference.
Air Distribution:
Even air distribution is essential for good regenerator operation. If more air is passing through one section of the bed than another catalyst regeneration will not be so complete, but before this occurs, it will be apparent that unconsumed oxygen is passing that section of bed with a higher air rate and causing abnormal after burning. Poor air distribution can be caused by a damaged grid or due to operation at an air rate substantially lower than the designed air rate for the grid.
Torch Oil:
The torch oil nozzles permit oil to be sprayed into the regenerator bed when additional coke is needed to satisfy the heat requirement of the unit.
Torch oil can be used on such occasions as:
1. Interruption to charge or recycle.
2. When regenerator temperature tends to fall.
3. To burn-off, the excess coke accumulated when the unit has been behind in burning. (Torch oil is used to prevent after burning that may result)
Torch oil should not be used during normal operations.
Spray Water:
Steam atomized water sprays are provided close to the inlet of first stage cyclone. These sprays are used to protect the cyclones from excessive temperature and are used only during upset conditions. In order to obtain good dispersion of water, the atomizing steam pressure should be raised to 9Kg/Cm2 before water is injected. Often steam injection alone achieves the desired effect. During normal operation, the steam pressure should be reduced to a value which is 0.3 to 0.7 Kg/Cm2 greater than the regenerator pressure so that the steam flow is just sufficient to keep the nozzles clear of catalyst. The water sprays are strictly for emergency use. It can be used briefly to control after burning resulting from an upset, until the reduction in air rate, addition of torch oil or other corrective actions are taken.
The continued water spray may cause:
1. The decrease in dense phase temperature.
2. Reduction in coke burning rate and hence liberating more oxygen which permits after burning.
The use of water sprays in normal operation can cause:
1. Obliteration of normal spread in regenerator temperature, therefore, is misguiding the operator about the indication of excess oxygen.
2. High oxygen content in cyclones so that the rate of oxidation and scaling of metal will increase.
3. If water sprayed is not the condensate, the salts in water can cause sintering of catalyst.
Catalyst Condition:
Regeneration operation is not greatly affected by normal changes in catalyst properties but abnormal changes can be significant.
If substantial loss of fines from catalyst inventory is where it will result in:
1. Poor fluidization in the regenerator.
2. The high carbon content of regenerated cat.
3. Oxygen passing through the bed making unit prone to after burning.
Occasionally catalyst becomes sintered by exposure to high temperatures or gets softened through sodium contamination. Sintering occurs when the catalyst melts just sufficiently to close some pores. If the pores contain coke, this coke cannot be regenerated since it is shielded.
The type of catalyst will have some effect on the performance of regenerator. Silica-alumina catalyst regenerates more completely than, for example, the natural catalyst. In addition, flue gas over silica-alumina catalyst has a lower CO2/CO ratio than that on the natural catalyst.
Behind In Burning
When coke is burned-off the catalyst at a slower rate than it is laid down in the reactor, the carbon content of the catalyst increases and the unit is behind in burning.
This can be avoided broadly by:
1. Always maintaining the normal regenerator temperature pattern.
2. Periodically checking the colour of the regenerated catalyst.
3. Anticipating those changes in the process condition which increase the coke yield, i.e. increase in charge rate and recycle rate, decrease in combined feed temperature.
4. Observing change over of charge tanks closely.
5. Avoiding use of spray water and torch oil.
Fluid catalytic cracking is the technology used for cracking of heavy molecular weight hydrocarbons to lower molecular weight hydrocarbons.Heavy refinery cured oil feedstock are commonly used as feed to fluid catalytic cracking operations.The main process objective is to maximize the middle distillates and convert to molecules which are in the range of LPG and gasoline. Continuous operation design has the facility of moving bed of catalyst where the regeneration of the catalyst is done and recycled to the reactor. when compared to fixed bed catalyst, moving the state of the catalyst will ensure many lifetimes and activeness characteristics where in case of a fixed bed or packed bed catalyst it has to be replaced with new catalyst and in most cases, it cannot be regenerated.In 1922 FCC is developed in France by Eugene Houdry. In ideal FCC irrespective of the designs developed two compartments are necessary they are
1. Regenerator
2. Reactor
These two are operation area in FCC where the reaction is carried in reactor and regeneration of the spent catalyst is done in a regenerator. A number of techniques were developed for example ESSO model IV design, Kellog Ortho flow design, thermofor catalytic cracking and many more.modern refinery is well equipped with a FCC which can handle most of the feedstocks.Steam and air are the utility accessories required for a FCC.
