rfcc manual checked1

Khartoum Refinery Co

Khartoum Refinery Co., Ltd

Residue Fluid Catalytic Cracking Operating Manual

Written by:liu xiangpu, zhangzhouming ,yangqing

Checked by: MOTAMAN ABDALLA A.AZIZ

Technology Department:

Safety and Environment Department:

Production Department:

Approved by:

June 2004

On the basis of the relevant management regulations with respect to the technology, safety and production, and in combination with the real situation and the development need of the catalytic cracking unit of Khartoum Refinery Co. (KRC), Ltd., this Operating manuals is made through supplement, perfection and modification on the basis of the original Operating manuals of the Catalytic cracking unit. The operators and management personnel of the catalytic cracking unit and the persons who enter into the unit must follow this Operating Rules, and any person has no right to violate or makes any modification without being allowed by the relevant authorities.

This Operating Rules has made the operating instruction on the production process of the residue fluid catalytic cracking, defined the production procedures, working range and operating methods, so that the operator in each position can follow the operating rules, guaranteeing to implement in accordance with the process requirements.

This Operating Rules will be implemented on .

Residue Fluid Catalytic Cracking Unit Operating Manual

Chapter I RFCC Unit Overview

I.Design Bases

Sudan, Khartoum Refinery Design Contract (Contract No.: G297057);

Technical materials provided by Sudan party with respect to the Khartoum Refinery;

The Sudan mixed crude oil evaluation experiment and simulation calculation data provided by SINOPEC Research Institute of Petroleum Processing (RIPP);

Minutes of Meeting of Second Design Coordination Meeting of Sudan Khartoum Refinery

(97) LZ No.35

Minutes of Meeting of Third Design Coordination Meeting of Sudan Khartoum Refinery

(97) LZ No.50

Minutes of Meeting regarding discussion on major blower and power generation issues

(May 21st, 1997)

II.Overview on process technology

1. Unit components and production capacity

This complex is designed to process atmospheric residuum annually at 1.8 million tons MTA, accounting for 70% of the total crude oil processing capacity of the whole refinery, as the main production unit for LPG, gasoline and diesel oil, mainly composed of the reaction-regeneration section, fractionation section, absorption-stabilization section, desulphurization section and waste-heat boiler section.

1.1 Reaction-Regeneration Section:

Is the key section of the catalytic cracking unit, as the head of the whole unit:

(1) Riser reactor: four primary cyclone separators (coarse cyclones) are installed at the outlet of the riser, and four single-stage cyclone separators (fine cyclones) are installed inside of settler.

(2) 8 sets of two-stage cyclone separators are installed inside the first regenerator, emptiness is inside of the second regenerator, and 4 sets of two-stage cyclone separators are installed outside of the second regenerator.

(3) The high-activity super-stable molecular sieve catalyst is used in the riser reactor, and in order to recover the wasted heat, the external heat recovery device is installed at the outside of the first regenerator (external-type heat exchanger);

(4) Flue gas recovery section: in order to sufficiently recover the waste heat, decreasing the energy consumption, high-temperature flue gas cooler, flue gas expander turbine and waste-heat boiler are installed on the mixed flue line of the first regenerator and the second regenerator. In order to decrease the catalyst loss and protecting the flue gas expander turbine as well as the ecological environment, the three-stage cyclone separator is specially installed to recover the catalyst powder; in addition, the reaction-regeneration section also includes the catalyst management system (fresh and waste) and feedstock discharging system, passivator injection system, and the first regenerator auxiliary air heater (F-2101) and secondary regenerator auxiliary air heater (F-2102).

(5) Catalyst cooler is being used to ease the heat balance problem.

1.2 Fractionation Section.

The fractionation section is mainly composed of the fractionating column, diesel strippers, feedstock buffer drum, recycled oil drum, slurry oil steam generators, feed stock preheating exchangers and the intermediate heating exchangers as well as the air cooling and water cooling units.

1.3 Absorption-Stabilization section.

The absorption-stabilization section is mainly composed of the rich gas compressor set, absorption column, desorption column, stabilization column, and re-adsorption column as well as various heat exchangers and air cooling devices.

1.4 Desulphurization section:

The desulphurization section is mainly composed of the gasoline caustic pre wash drum, gasoline mercaptans-removal column, alkaline oxidation column, alkaline settling drum, disulfide settling drum, gasoline filter column, (off-gas water scrubber), mixture oxidation column, liquid hydrocarbon mercaptans -removal column, alkaline oxidation column, liquid hydrocarbon settling drum, alkaline settling drum and net gas desulphurization posts.

2. Design unit and the process

Upon the request of Khartoum Refinery Co., Ltd., the unit is designed by SINOPEC Engineering Incorporation (SEI) in accordance with the overseas mature technology of the heavy oil catalytic cracking, mainly adopting with the Two-stage complete Regeneration Process Technology.

3. Main process technical features

3.1The catalyst regeneration system has adopted the two-stage complete regeneration technology, the dilute phase with high oxygen concentration is designed for the second regenerator, and external cyclone separators are installed to guarantee the second regeneration can be performed at the high temperature, so as to make the carbon concentration of the regenerated catalyst not to exceed 0.05%, as well as solving the problem of the catalyst hydrothermal deactivation.

3.2The riser (lift-pip) reactor is used, and the high-speed separator is installed at the outlet of the riser reactor to cut off the reaction. The high-temperature short-term contact operation condition is applied for the reaction in the riser reactor, being favorable to the yield of the products.

3.3The high-efficiency atomized (spray) nozzle feed stock injection system is adopted, to atomize the feed stock into the oil particle with the diameter less than 60m, so as to decrease the coke formation and improve the yield of the liquid products.

3.4The metal passivantor is injected to prevent (suppress) the action of the heavy metal such as Ni on the catalyst, as well as to improve the product distribution.

3.5The feedstock heating-furnace has been cancelled but the slurry oil at the bottom of the distillation column is used as the heat source for the feedstock. At the beginning of the startup, the slurry oil steam generator will be used to heat up the feedstock.

3.6The external heat transfer technology and high-temperature flue gas energy recovery system are used in this unit. The high-temperature heat exchanger is set on the high-temperature flue line to produce the middle pressure steam, and the flue gas expander turbine is used to recover the excess energy of the high-temperature flue gas and directly driving a synchronized power generator unit. The motor of the flue gas expander turbine is the product of Shanghai Motor Plant, the butterfly valves at the inlet of the flue gas expander is imported from Italy, the flue gas expander is provided by the Mechanical Plant of SINOPEC Lanzhou Refinery and Petrochemical Corporation, with the model of YL-18000A, and the model of the power generator of the flue gas turbine as QF-20-2.

3.7Gas compressor set (including the turbine and compressor) is produced by France D-R Company.

3.8The major air blowers system is composed of the axial compressor (AV56-14) produced by Shanxi Blower Plant of China and the condensed steam turbines (NK-40/56/0) produced by Hangzhou Turbine Plant of China.

3.9The air booster set is composed of the prime mover produced by Shanghai Motor Plant of China and the blower (D300-12) produced by Shenyang Blower Plant of China.

3.10In order to further improve the product quality and preventing the environmental pollution, the sweetening and desulphurization section of LPG and gasoline are installed in the desulphurization section, so as to remove the sulphur components in the LPG and gasoline.

4. Construction period and startup time

The construction period of the project is from April 20, 1998 to January 23, 2000, and the project will be put into use on April 16, 2000, to hit the success in one time.

5. Feed stock and product quality indexes (see process cards)

6. Main process operational conditions (see process cards)

7. Main design indexes

Feed stock and products

(1) Materials balance

Item

Weight %

Kg/hr

Tons/day

MTA (x104)

I. Feedstock

Atmospheric residue (LR)

225000

5400

180

Total

100

225000

5400

180

II. Products

1. Dry gas

3.743

8421.75

202.122

6.7374

2. LPG

10.753

24194.25

580.662

19.3554

3. Gasoline

43

96750

2322.0

77.4

4. Diesel

25

56250

1350.0

45.0

5. Slurry

6.504

14634.0

351.216

11.7072

6. Coke

10.5

23625

567.0

18.9

7. Loss

0.5

1125.0

27.0

0.9

Total

100

225000

5400

180

(2) Chemical materials and their quality indexes

No.

Item

Feature

Manufacturer

Service life and index

Consumption

Tons/year

1

Catalyst

LV-23CS

Lanzhou Refinery Catalyst Plant

Microactivity is more than 60, specific surface area is more than 180m2/g

2700

2

Passivator

Yxm-9

Yixing Hanguang Group

Sb or Bi contents must be more than 5%

40

3

Turbine oil

20#

5

Sodium triphosphate

6

Methyl diethanolamine

Purity>99% (good-quality level)

Yixing Hanguang Group

Half year

7

Slurry anti fouling agent

Nanjing Petrochemical Plant

8

Sodium hydroxide

20%(m) liquid

9

CO Promotor

Deep blue powder

Changchun Huigong Chemical Co., Ltd.

8. Energy Consumption

Reaction and distillation part:

No.

Item Name

Unit

Designed annual consumption

Designed unit consumption (water, steam) kg/t power KWH/t air Nm3/t)

Mark

Steam

1

1.0Mpa steam

Tons

91680

50.93

2

3.9Mpa steam

Tons

-426960

-237.2

3

0.35Mpa steam

Tons

/

/

Water

4

Circulating water

Tons

54694400

30385.8

5

Fresh water

Tons

16000

8.9

6

Sewage with oil (discharged)

Tons

80000

44.5

7

Desalted water

Tons

696000

386.7

8

Desalted water (wet air cooling use)

Tons

748800

416.0

9

Purified water recycled use

Tons

120000

66.7

10

Sour water

Tons

248000

137.8

11

Living water

Tons

8000

4.4

*10

12

Electricity

kWh

-110133440

61.2

Air

13

Non-purified air

Nm3

1200000

0.7

14

Purified air

Nm3

31032000

17.2

15

Coke

Tons

189000

105.0

Fuel

16

Fuel (oil)

Tons

3.2t/h

/

For startup

17

Fuel (gas)

Tons

3.2t/h

/

For startup

18

Fuel (boiler)

Tons

0-1.63t/h

/

Not indispensable

Chemical materials

19

Fresh catalyst

Tons

2160-2700

1.2-1.5

20

Balance catalyst

Tons

300-400

/

For startup

21

Passivator

Tons

107-150

0.06-0.08

25%Sb

22

22# turbine oil

Tons

30

0.0167

23

Nitrogen

Nm3/h

90-158

/

For startup

24

Sodium Triphosphate

Tons

4

0.0022

Desulphurization part:

No.

Item Name

Unit

Annual Consumption

Designed unit consumption (water, steam) kg/t power KWH/t air Nm3/t)

Mark

Designed

Designed

Steam

1

1.0MPa steam

Ton

68000

61.3

Water

2

Circulating water

Ton

2656000

2414.5

3

Fresh water

Ton

184000

167.3

4

Desalted water

Ton

696000

386.7

5

Electricity

kWh

1593600

1.45

6

Nitrogen

Nm3

240000

0.22

7

MDEA solvent

Ton

18

0.016

8

30% concentration of alkaline liquid

Ton

1000

0.91

9

CoPc

Ton

0.3

/

9. Production Control

Steam control analysis

Steam

Sample taking place

Analysis item

Quality index

Analysis frequency

D-2701/1,2 D-2218

D-2220

D-2221

D-2806

Over-heated steam

Na (g/L)

15

1 time/4h

SiO2 (g/L)

20

Boiler water

D-2701/1,,2

D-2218

D-2806

D-2220

D-2221

PO43-(mg/L)

PO43- (mg/L)

1 time/4h

pH value(25)

9~11

SiO2 (mg/L)

3~5

Water supply

High-temperature heat boiler steam drum

Hardness (mol/L)

1.5

1 time /4h

pH value(25)

8.5~9.2

1 time /4h

Decreator

Hardness (mol/L)

1.5

1 time /4h

pH value (25)

8.5~9.2

1 time /4h

* dissolved oxygen (g/L)

15

1 time /24h

Oil products quality control analysis

No.

Sample name

Analysis items

Quality index

Analysis frequency

Mark

1

Feedstock

Density (150C), kg/m3

Reported

2 times/week

8:00

Monday Thursday

Carbon residue CCR , %(m/m)

Reported

Distillation range at reduced pressure:

Reported

initial boiling point, 0C

Reported

10% recovery temperature, 0C

Reported


Reported

3500C concentration %(m/m)

Reported

4500C concentration %(m/m)

Reported

5000C concentration (m/m)

Reported

2

Gasoline stabilized

Density (150C), kg/m3

1 time/24h

Distillation range:

1 time/8h

initial boiling point, 0C


68


Reported


Reported

final boiling point,

202

Copper sheet corrosion (500C ,3h), grade

1

1 time/8h

Doctor

Passed

1 time/8h

Gum

0.0009

1 time/8h

Colloid, mg/100mL

Reported

2 times/week

8:00 Tuesday, Saturday

Vapor pressure

Reported

periodical

Octane rating

Reported

periodical

3

Diesel

Density (150C )kg/m3

Reported

1 time/24h

Distillation range:

1 time/8h

Initial boiling point, 0C

Reported


Reported


298


353


363

cloud point, 0C

Reported

1 time/8h

Acidity, mgKOH/100mL

Reported

1 time/24h

Flash point, 0C

67

1time24h

Chroma

Reported

1time8h

Copper sheet corrosion, grade

1

1time8h

4

Slurry oil

Density (150C )kg/m3

Reported

1time24h

Distillation range at reduced pressure:

Reported

2 times/week


3500C percent recovered %(m/m)

Solid concentration,

0.8%V/Vmax.

1time24h

5

Dry gas

Components

Reported

1time24h

H2Sppm

Reported

1time24h

6

LPG

Components

Reported

1time8h

7

Rich gas

Components

Reported

2 times/week


8

Un stabilized gasoline

Density, kg/m3

Reported

1 time/24h

Distillation range:

1 time/4h

Initial boiling point, 0C

Reported


68


120


180

Final boiling point, 0C

202

9

Lubricant oil

kinematic viscosity (400C )mm2/s

Reported

1time/week

16:00 Tuesday

Impurity, %

Reported

1timeweek

Flash point, 0C

Reported

1timeweek

Acid value

Reported

1timeweek

Water concentration, %

Reported

1timeweek

10

Regenerated catalyst

Carbon concentration, %

Reported

1 time/day

8:00 Tuesday

Bulk density

Reported

1timeweek

Microactivity

Reported

1timeweek

Sieve analysis

Reported

1timeweek

11

Semi-regenerated catalyst

Carbon concentration, %

Reported

1 time/24h

12

Purified Dry gas

H2S, ppm

Reported

1time24h

13

Flue gas of first and second regenerator

CO concentration

Reported

CO2 concentration

Flue gas dust

(mg/Nm3)

200

13

Purified LPG

H2Sppm

Reported

1 time/24h

14

Acid gas (sour gas)

H2Sppm

Reported

periodical

CO2

Reported

periodical

Components

Reported

periodical

15

Lean amine

Amine concentration

Reported

1 time/24h

H2S

Reported

1 time/24h

CO2

Reported

1 time/24h

16

Rich amine

H2S

Reported

1 time/24h

CO2

Reported

1 time/24h

17

Gasoline caustic pre wash alkaline liquid

NaOH concentration, %

Reported

1 time/24h

18

LPG caustic pre wash alkaline liquid

NaOH concentration, %

Reported

1 time/24h

Chapter IIProcess Description

Section IReaction -Regeneration Process

RFCC, the residue fluid catalytic unit operates at high temperature and low pressure and is used to convert heavy oils and residues of low valuable price into light transporting oils of higher value.The unit is facilitated with two kinds of feeding systems, hot feeding and cold feeding. Hot feedstock from the atmospheric distillation unit (CDU) and the delayed coking unit is directly fed to the feedstock surge (buffer) drum (D-2203) within the plant, while in cold feeding, the cold feedstock is drawn using the raw oil pump (P-2202/1, 2, 3)from the unit tank farm (unit 17) to the surge (buffer) drum (D-2203) and then drawn out using the feed stock pump (P-2201/1, 2), or directly from the unit tank farm with out passing through the surge drum and then heated to 150-200OC in the slurry/feed heat exchangers (E221/1-4) prior to being fed into the lower section of the riser. Antimony passivator solution is injected into the inlet header of feedstock using the pumps (P-2101/1, 2) to suppress the action of the heavy metal contaminants.

The feed stock header is divided into 4 flow control valves and then injected (atomized) into the riser and converted into fine droplets through 8 nozzles using atomizing steam at two locations above each other where only 2 nozzles below and 6 nozzles above. The atomized feed stock mixes with the ascending hot regenerated catalyst from the 2nd regenerator (C2103), it vaporizes and the catalyst and oil are accelerated in the upward direction then the cracking reactions take place during the residence time in the riser (2 4 sec) producing the cracked oil vapor which is a mix light products as LPG, gasoline and diesel as well as dry gas, slurry and coke that deposited on the catalyst. The hot regenerated catalyst enters the riser (C2101) at the WYE section below the feed injection points and pushed up by lift steam ring located at the bottom of the riser then 4 nozzles of lift dry gas. .The specially designed feedstock injection nozzles guarantees the maximum conversion of stock oil to light-end products, and minimizes the coke formation. coker gasoline nozzles were added 2 nozzles below and 2 above the upper feedstock nozzles f. Through 2 control valve 4 nozzles, recycle oil is injected in the riser above the feed injection point to increase the conversion per cycle. Nozzles for recycle slurry and light oils are installed on the upper part of the riser. The riser is terminated by 4 quick cyclone separators where spent catalyst is separated from the hydrocarbon vapors and dropped down through the dip legs to the stripping section, the dip legs of the cyclone are submerged inside the catalyst bed that is kept in fluidized state by the steam via steam rings. The hydrocarbons allied with the spent catalyst are stripped off by steam flow controlled and the catalyst is trickled through the stripper (settler C2101) via several baffle plates. A pre stripping steam ring is installed at the upper section of the stripper to improve the stripping efficiency and negate the secondary reactions and a decoking steam ring at the top of the stripper just below the plenum chamber to prevent coke deposition and build up. The cracked oil vapors and the steam are separated from the entrained catalyst by another 4 single stage cyclones that mounted high at the top of the stripper which direct the hydrocarbons vapor to be collected at the plenum chamber at the top. The hydrocarbons vapor, steam and the fine catalyst enter the bottom of the fractionator (C2201) through the reactor vapor line. The stripped spent catalyst is passed to the 1st regenerator (C2102) via short stand pipe equipped with a slide valve that control the level of the settler catalyst fluidized bed.

Coke is one of the reaction products that deposited on the catalyst and composed mainly of carbon and hydrogen with traces (depending on the feed properties) of sulfur, nitrogen and metals compounds. The deposited coke deactivate the catalyst by blocking the active centers of the catalyst pores and upon combustion in the regenerators, the coke is converted into flue gas that can be easily removed from the catalyst pores.

The heat liberated by the coke combustion is needed for the endothermic cracking reactions and the high temperature regenerated catalyst provides the heat necessary for feed vaporization and for the reaction, i.e., the riser outlet temperature is controlled by the opening of regenerated catalyst slide valve. The coke combustion distribution -in the two regenerators, the feed preheat temperature , and the riser outlet temperature must be adjusted to keep the difference of carbon on spent and regenerated catalyst below 1.2 wt %, i.e., to keep the catalyst/oil ratio within the range of 6-8 during operation.

The first regenerator (C2102) is operated under the mild conditions, partial combustion, at the pressure of 0.39Mpa (absolute) and fluidized catalyst dense phase temperature of 635-690OC, most of hydrogen and part of carbon in the total coke are burned out, the quantity of the burned coke can be determined by the feedstock, the combustion amount of the carbon and the temperature of the regenerator are controlled by the air quantity that is allowed to enter into the first regeneration, so as to maintain the flexible operation conditions. The air needed is distributed through two rings big and small that provide the combustion air and fluidization air. Due to the existence of the water vapor, the temperature of the first regenerator should be controlled relatively low, so as to control the hydrothermal deactivation of the catalyst at the minimum. The catalyst carried by the flue gas is separated from the flue gas rich CO through 8 (pairs) sets of cyclone separators and the separated flue gas is collected in the 1st regenerator plenum chamber and the discharge rate of the flue gas from 1st regenerator is controlled by the duel slide valves that and actuated by the differential pressure between the two regenerators..

The flue gas out from the 1st regenerator mixes with the flue gas rich of O2 out from the second regenerator where CO reacts with O2 and at this point air is injected to ensure that all of the CO is converted to CO2, that is why the flue gas temperature increases to 900-11000C, then, enters into two parallel high-temperature heat exchangers (flue gas coolers outlet D2701\1,2) to cool down the flue gas temperature to lower than 7400C and generate middle-pressure steam, then enters into the energy and heat recovery system. The flue gas coolers outlet temperature and load is controlled by the flue gas cooler outlet butterfly valves,

The semi-regenerated catalyst from the first regenerator goes through the semi-regenerated catalyst standpipe, semi-regenerated slide valve that control the 1st regenerator level to enter into air lift-pipe, the pressure of the main air is increased by the air booster (K2102/1-2) as the pressure boosted air (its amount accounts for approximate 1/3 of the combusted air of the second regenerator), the catalyst is lifted up to the second regenerator, the operating pressure of the second regenerator is 0.38MPa (absolute), and the temperature of the dense phase bed is 700-740OC.

The catalyst enters into the bottom through the center of the single air distribution ring inside the second regenerator, and distributed evenly, the rest carbon will be totally burnt off, the flue gas out of the 2nd regenerator will contain no CO as it is operated in complete combustion mode with excess O2, since almost all the hydrogen is burnt out in the first regenerator, the partial pressure of the steam is decreased in the second regenerator, so that the second regenerator can be operated under the high temperature, without leading to the hydrothermal deactivation of the catalyst.

Only air distribution ring and secondary cyclones dip legs are placed inside the 2nd regenerator. 4 sets of two-stage external cyclone separators are installed with the abrasion proof and heat proof lining, so as to resist the high temperature flue gas effect.

The hot regenerated catalyst comes out of the second regenerator to enter into the degassing tank, and then enters into the regenerated catalyst standpipe, in order to maintain the stable and even flow in the standpipe, a number of loosing air injection points are set along the flow direction of the catalyst, to make the pressure in front of the regenerated catalyst slide valve stable within range, then, enters into the riser bottom through the regenerated catalyst slide valve, and complete the circulation of the catalyst.

Two direct fire air heater placed at the inlet of the regenerators and only used during the unit start up to dry out the system lining and heating up the catalyst before injecting the feed

In order to maintain the heat balance and increase the operation flexibility and control the regenerated catalyst temperature, an external heat remover (catalyst cooler C2105) that can adjust and control the 2nd regenerator dense phase temperature is placed at the side of the first regenerator. Part of the semi regenerated catalyst(635-690OC) passed to catalyst cooler via slide valve that control the catalyst cooler level after the hot semi regenerated catalyst (635-690OC) flows downwards (shell side) through the space of the vertical pipes of the heat exchanger, the heat withdrawn tubes are immersed inside the catalyst fluidized bed with the boiling water flowing in the tubes, the countercurrent fluidizing air is introduced into the bottom of the catalyst cooler to maintain the good fluidization, leading to the good heat transfer between the fluidized catalyst bed and the vertical tubes by creating turbulence and mixing, the temperature of the catalyst after heat exchange is decreased to approximate 100OC. The catalyst flows via the cooled standpipe through slide valve that control the 2nd regenerator bed temperature and which will be lifted up and accompanied by the hot semi regenerated catalyst to the second regenerator via the air riser.

The de aerated water used for catalyst cooling comes from the wasted heat boiler section, enters into the steam drum (D-2118).In the catalyst cooler tube side a middle pressure saturated steam is generated. Large amount of water from the bottom of the steam drum (D-2118) is forced circulated by the hot water pump (P-2103/1, 2, 3) to protect the cooler tubes from the thermal damage by insuring that the tube walls remain wet all the time. A mixture of steam and water in the cooler tubes flow upward back to the steam drum (D2118) where steam and water are separated. The middle pressure saturated steam generated leaves (D2118) and goes to the waste heat boiler.

The air needed for the coke combustion in the regenerators is fed to the regenerators in flow control and provided by the two axial compressors (air blowers K2101/1-2). The air boosters (K2102/1-2) pressurized part of the main air to be used in the air riser to transport the catalyst to the 2nd regenerator and small part of the boosted air is used as a fluidizing air in the catalyst cooler and for the distribution ring of the degassing tank of the second regenerator.

The fresh and balance catalysts for the startup are sent to the unit by the vehicle. The fresh catalyst drum is to be loaded in (D-2101) and balance catalyst in drum (D-2102), which will be sent to the first regenerator and the second regenerator by the plant air (at pressure 0.7MPa and temperature 400C). During normal operation in order to compensate the catalyst losses as well as maintaining uniform and desired system inventory catalyst activity, fresh catalyst can be fed into the 1st regenerator either by using the automatic loading machine or by the plant air though the small scale feeding line. In order to guarantee the activity of the catalyst and the concentration of the heavy metal not over the allowed range, the catalyst is to be unloaded from the second regenerator periodically to the waste catalyst drum (D-2103). The passivator from the barrels is pumped to the passivator drum (D-2104) by the passivator air pump (P-2102), and then, the passivator is injected into the feed stock header near the riser by the pump (P-2101/1,2) to be mix with the feed , and sent to the feeding nozzle of the riser. Diesel (Seal oil) is injected in the passivator drum (D-2104) to dilute the passivator at the proportion of 10:1.

Section IIFractionation Section

The fractionator column (C-2201) has 32 trays (Tongue type) and is divided into two main sections that cracked oil vapors cooling and washing section and the actual fractionator section. At the bottom of the column there are 10 layers of baffles. The high-temperature cracked oil vapors from the settler top enters into the bottom of the fractionator underneath the baffles to come in contact with the countercurrent circulating slurry of 275OC returned from the top of the baffles, so that the oil vapors can be sub cooled from bottom to top. After the cracked oil vapors are fractionated, dry gas, LPG and raw gasoline (over head products), light and heavy diesel, recycled oil, and slurry are obtained. In order insure proper heat and mass balance sufficient internal reflux and evenly distribute the load of the column, four circulated refluxes are set for the fractionator column.

The over head of the fractionator column exchanges the heat with the circulation water through heat exchanger (E-2203/1, 4), then enters into the wet air coolers (EC-2201/1, 20) to cool down to 40OC prior to enter into the oil-gas low pressure separator (Over head drum D-2201); the uncondensed gas in D-2201 (with temperature at 40 OC, pressure at 0.26- 0.16MPa) enters the rich gas compressor. The condensed raw gasoline is divided into two streams one stream is pumped back to the fractionator top as cold reflux and the second stream is pumped to the top of the adsorption column (C2301) by the raw gasoline pump P-2203/1, 2.

The sour water collected in the over head separator boot passes to the sour water drum D2207 where the entrained oil is to separated and pump back to D2201 by P2215/1-2 while the sour water is transferred out to unit 5 by P2213/1-2. Small part of the of the recovered water is to be used as washing water and injected at the inlet of the water cooler E2203/1-4 and the wet air coolers to dissolve and reduce the corrosion effect of the substances present in the over head vapors as sulfides, cyanides, ammonia, phenols and chlorides which are water soluble substances. This large amount of water comes from the steam injected in the riser and settler, fractionator bottom and diesel strippers.

Upon the requirement of the production, the -10# or 0# diesel flow to the diesel strippers (C-2202 and C-2203) from tray 17# and tray 21# of C-2201 respectively, and then is pumped out by the P-2206/1,3 after stripping by the steam (or dry gas for the heavy diesel), in which the 10# light diesel exchanges the heat with the de aerated water comes from E-2212 in E-2218/1,2 to be cooled down to 90OC, and then enters into the water cooler (E-2219) to be cooled down to 60 OC. The 0# heavy diesel exchanges the heat with the de aerated water comes from E-2218/1,2 in E-2206/1,2 to be cooled down to 95-100 OC , and then cools down to 60 OC by air cooler (EC-2202/1,2) to be sent out of the unit as intermediate product.

The lean oil is taken from tray 22# or 18# of C-2201by the pump (P-2207/1,2), firstly being as the heat load supplement (mainly for startup) for the bottom of the stripper (C2302) , cools down to 150 OC by reboiler (E-2309), and then enters heat exchanger (E-2204/1,2) to exchange the heat with the rich oil, then enters E-2205 to exchange heat with the desalted water, then goes to wet air cooler (EC-2203/1,2) to cool down, then goes into E-2207 to exchange heat with the circulation water to cool down to 40 OC to be sent back to the re absorption column (C2304) as the re absorbent. The rich oil exchanges heat with the lean oil through E-2204/1, 2 and gets heated to approximate 84 OC, to return to the top of the tray 24# of the fractionator column.

The slurry is discharged by the slurry circulation pump (P-2210/1,2) from the bottom of the fractionator column at the temperature condensed cycloparaffin > olefin > single alkyl side-chain monoaromatics > cycloparaffin > paraffin.

In a same hydrocarbon family, the large molecule has stronger adsorption ability than the small one. According the chemical reaction speed, the sequence from high to low is as the follows: olefin > large molecule of single alkyl side-chain monoaromatics > isoaparaffin and cycloparaffine > small molecule of single alkyl side-chain monoaromatics > normal paraffin > condensed aromatics. It is obviously that, the adsorption ability of the hydrocarbon has the different order than the chemical reaction speed.

The adsorbed various hydrocarbon molecules are relating to the proportions of various hydrocarbons in the feed stock, besides relating to the adsorption ability. If much aromatics present in the feed stock, especially the condensed aromatics and the small molecule of single alkyl side-chain monoaromatics (less than C9), the adsorption ability is very strong but the reaction speed is very slow, leading to long stay on the surface of the catalyst and not easy to be desorbed then condensing as the coke.

2. Catalytic Cracking Chemical Reactions

The chemical reactions of the catalytic cracking are not only the cracking reaction, but also many other chemical reactions happened at the same time. Under the condition of the catalytic cracking reaction, the chemical reactions have the different speeds and difficulties. Due to the different components of the catalytic cracking feed stock, the chemical reactions are quite complex. The main chemical reactions are as the follows:

2.1Cracking reaction

The major reaction of catalytic cracking is the cracking reaction, with higher reaction speed. The cracking reaction is the rupture of the C-C bond. As for the same family of hydrocarbons, the bigger the molecular weight, the faster the reactions speed. Generally speaking:

1) The olefin is easier than the paraffin to crack, and when the cycloparaffin is cracking, not only the side chain can break but also the paraffin ring can be broken to change as the olefin;

2) The aromatic ring is very stable hard to take reaction. As for the aromatics, the longer the side chain, and the deeper the substitution depth, faster the reaction is. The single ring aromatic cannot remove the methyl, only the side chain with more than 3 carbon atoms can participate in reaction, the breakage of the side chain of the condensed aromatic is normally at the root.

3) The sequence of the C-C bond breaking speed is: tertiary carbon > secondary carbon > primary carbon.

2.2Isomerization reaction

The isomerization reaction is the important reaction of catalytic cracking, in which under the circumstance of the constant molecular weight, the hydrocarbon molecule occurs the structural and spatial changes, normally as the followed:

1) Skeletal isomerization;

2) Olefin bond movement;

3) Olefin space location change

Due to the isomerization, the catalytic cracking contains a lot of isomerized hydrocarbons.

2.3Hydrogen transfer reaction

The hydrogen transfer reaction is also called as the hydrogen redistribution reaction, i.e., after the hydrogen removes from a hydrocarbon molecule, it is added onto another hydrocarbon molecule, so as to make this olefin saturated. If the molecule being removed the hydrogen atom is the olefin, finally an olefin is saturated, but the molecule being removed hydrogen is getting more unsaturated, as for the saturated molecule, it can be called as the saturation reaction.

The hydrogen transfer is the unique catalytic cracking reaction with the low reaction speed. The cycloparaffin with the side chain is the main resource of the hydrogen. Differentiated with the dehydrogenation and the hydrogenation participated by the hydrogen molecule, the hydrogen transfer presents that an active hydrogen atom transfers to another hydrocarbon molecule from a hydrogen molecule to saturate the olefin, so that the diolefin is changed as monoolefin or saturated hydrocarbon, the cycloparaffin is changed as cycloolefine and further as aromatic; therefore, the catalytic cracking product has a good stability.

The accompanying reactions with the hydrogen transfer are the condensation reactions of the large molecule olefin, cycloparaffin and aromatic, and the result is that these big molecules continuously give out the hydrogen atoms, and finally they are becoming as cokes accumulating at the surfaces of the catalysts, decreasing the activity of the catalyst.

2.4Aromatization reaction

The aromatization reaction is that the paraffin and olefin are cyclized as the cycloparaffin and cycloolefine, and then perform the hydrogen transfer reaction to continuously give out the hydrogen atoms, and finally form the aromatics. The aromatization reaction is the important reaction of catalytic cracking. Due to the aromatization reaction, the aromatics concentration of gasoline and diesel is much.

2.5Transmethylation

The transmethylation is to happen between two aromatics. One aromatic removes the methyl, and the other gains this methyl, called as the dimethyl product. This reaction is also called as the disproportionation reaction.

2.6Poly-reaction

The poly-reaction and the alkylation reaction are performed under the operating condition of catalytic cracking, that is, at temperature of 500OC and atmospheric pressure, the proportions of these two reactions are not much, the poly-reaction is performed between olefins, with the result of producing big molecule of olefin.

2.7Alkylation reaction

The additional reaction of the olefin and aromatics is called as the alkylation reaction, and the olefin is mainly added to the condensed aromatics, to produce coke after further dehydration.

3.Mechanism of the catalytic cracking reaction

The catalytic cracking reaction is very complex with many reaction types. In order to understand how these reactions are performed, as well as explain some phenomena, for instance, C3 and C4 are much in the gas, and the isomerized hydrocarbons are much in the gasoline, the reaction process of the hydrocarbon is need to be further discussed. At present, the carbonium ion theory is the better one for explaining the mechanism of the catalytic cracking reaction.

The so-called carbonium ion means the hydrocarbon ion formed by the carbon atom lacking of a pair of valence electron. The carbonium ion is named after the ammonium ion (NH4+), however, it doesnt like the ammonium ion that can dissociate from solution to exist as the free ion, but only adsorbs at the surface of the catalyst to perform the reaction. The essential difference is that the carbon surface of the carbonium ion has only three pairs of electrons.

Normally it is know that the carbonium ion is hard to be formed during the thermal cracking process, since the thermal cracking is the common gaseous phase thermal reaction, its result is that the hydrocarbon molecule is evenly broken as the free radicals, but only the uneven breakage can produce the carbonium ion, which needs much more energy than the even breakage; therefore, the uneven breakage is hard to occur. Under the existence of the acid catalyst, the energy for producing the carbonium ion is very less.

The carbonium ion is produced under the effect of the hydrocarbon molecule and catalyst. One is that the activity center of the catalyst gives out the proton, making the olefin protonation to produce the carbonium ion, for example:

R-CH=CH2+Z acidity center gives out the protonR-CH++CH3+Z-

There is a rule for the proton is adding onto the olefin, i.e., to be added onto the carbon atom with more hydrogen atoms; thus, the carbon atom with less hydrogen atoms is lacking of a pair of electrons to produce the carbonium ion, for example:

R-CH2-C3+A+ hydride is withdrawn R-CH+-C3+AH

According to the above circumstance, there must be two conditions for producing the carbonium ion, one is there must be olefin which comes from raw material or is produced by thermal cracking; the other is there must be the acid catalyst which can give out the proton.

The proton is the hydrogen atom losing the electrons, expressed as H+.

We should know that, the proton given by the catalyst hasnt broken away from catalyst to move freely, but the olefin stays at the surface of the catalyst, to produce the carbonium ion under some certain conditions.

The carbonium ion reaction is very complex, with the reaction characteristics as the follows:

3.1The formation of the carbonium ion is depending on the combination of the olefin and proton, the small carbonium ion recombines with the olefin to produce the large carbonium ion, as the followed:

3.2The carbonium ion can automatically isomerize, the primary carbon can automatically convert as secondary carbon, and the secondary carbon can convert into tertiary carbon, going towards stability. The sequence of the stabilization of the carbonium ion is as the follows: tertiary carbon > secondary carbon > primary carbon. The ion isomerization is expressed as the transference of a pair of valence electron, or the transference associating with the hydrogen and methyl.

3.3When the carbonium ion meets with hydrocarbon molecule, it captures the hydrogen of the hydrocarbon molecule to produce a new carbonium ion, namely, the formation of the carbonium ion or the substitution of the carbonium ion, to build up the chain reaction, as the followed:

3.4the carbonium ion can lose the proton to produce olefin, this proton is returned to the acidity center of the catalyst or other olefin to produce the new carbonium ion, but the carbonium ion losing the proton will become as the olefin product, such as:

3.5A large carbonium ion can be decomposed to produce an olefin and a small carbonium ion, that is, to perform the cracking reaction, as the followed:

3.6The carbonium ion reacts with itself to carry out the cyclization, such as:

CH2-CH2-CH2-CH=CH2+A+cyclohexene +AH

4.The Main Factors Impacting Reactions

4.1Feed stock properties

The feed stock properties are the most important operating condition. When choosing catalyst, making production scheme and selecting the operational conditions, the nature of the feed stock should be understood at first. During production process, we ask for the relative stability of the feed stock. Meanwhile, when several feed stocks with different properties are processed, they should be blended well in the feed stocks tank or pipeline prior to be sent to the unit. In addition, the water removal in tank yard should be highly concerned, and the water shouldnt be sent to the reactor due to the feed stock switch; otherwise the reaction temperature will be decreased sharply, and the reaction pressure will be increased fiercely owing to the vaporization of the water, leading to the heavy accident.

4.2Type of the catalyst

At present, there are several types of catalyst can be chosen. Normally the Y-type molecular sieve is taken as the host catalytic groundmass, to choose the HY or REY catalyst. Each catalytic cracking unit should choose the proper catalyst according to the feed stock properties, product scheme and the type of the unit. When the catalyst is chosen, not only the activity and the specific surface, but also the selectivity, the anti-contamination capacity and the stability of the catalyst should be concerned.

During the production process, when the catalyst needs to be changed due to the great changes of the feed stock properties and the product scheme, the gradual replacement method should be adopted; to unload the catalyst as well as load the new catalyst, and the replacement speed should be controlled not too fast; otherwise, due to large quantity of the supplement catalyst, and the high activity of the balance catalyst, the operation balance is hard to control.

4.3Reaction time:

if the reaction time is short, the conversion rate is low and the recycled oil ratio is increased; if the reaction time is long, the conversion rate is improved, but if the reaction time is too long, the conversion rate will be too high, leading to the decrease of the gasoline yield, the saturation of the olefin in the LPG, and the yield decrease of the propylene and butylenes. The reaction time for the industrial unit normally is 2-4 seconds. At present, some residual oil units have taken less reaction time, so as to decrease the coke formation rate. The reaction time cant be adjusted at random, which is determined by the dimension of the riser (diameter and length), but the reaction time is variable. The quantity of the feedstock and the conversion rate change caused by other conditions will lead to the change of the reaction time.

For a specific unit, if we need to change the reaction time at great extent, the method of injecting the diluted medium (steam or dry gas) to the riser can be chosen. But the basis for using this method is the low production capacity of the unit.

4.4Reaction temperature

If the reaction temperature is high, the cracking speed is high and the conversion rate is high. Normally the temperature between 500-530OC should be adopted. If the reaction temperature is increased, the dry gas will be increased and the gasoline yield will be decreased. Under the constant conversion rate, the coke yield will be decreased (owning to the suppression of polyreaction). With the temperature increasing, the yields of the propylene and butylenes are speeded up very fast. The increase of the temperature can increase the octane rating of the gasoline, since both the olefin and the aromatics are increased with the temperature.

The reaction temperature is the major parameter that we can control in the production process, which is also the most sensitive parameter affecting the product yield and the product quality. In the riser, the reaction temperature is controlled by the opening of the regenerated catalyst slide valve, to adjust the circulation quantity of the catalyst.

4.5Feed stock pre-heating temperature

As for the riser, the feed pre-heating temperature is no longer the major mean to control the reaction temperature. However, the different feed stocks and the different structural feed stock nozzle have the different requirements on pre-heating temperature. Normally, the pre-heating temperature has some certain impact on the atomization effect, and also has impact on the product yield and the product quality at different extents. If the atomization is good, under the condition of the same yield, the yields of the coke and the gas are decreased, but the yield of the light oil is increased. If the atomization is poor, the yield of the coke will be increased. Normally, the high the pre-heating temperature, better the atomization effect. As for the nozzle with good atomization effect, the lower pre-heating temperature can be taken according to the allowance of the heat balance; as for the nozzle with poor atomization effect, the higher pre-heating temperature should be adopted according to the allowance of the heat balance and the requirement on catalyst-oil ratio. The pre-heating temperature of the feed stock is restricted by the heat balance, process conditions, catalyst-oil ratio and the designed temperature of the regeneration equipment, etc..

4.6Catalyst-oil ratio (C/O)

The catalyst-oil ratio is the ratio of the catalyst circulation amount and the total feedstock of the reactor.

Catalyst-oil ratio = catalyst circulation amount (tons/hour)/total feedstock (tons/hour)

The catalyst-oil ratio is not an independent variable but a dependent one, and all the factors affecting the reaction temperature can also impact it, while the catalyst-oil ratio also plays the counteraction on reaction temperature. If the catalyst-oil ratio increases, the reaction temperature also increases; however, while the regeneration temperature and the reaction temperature increase, the catalyst-oil ratio decreases. The increase of the catalyst-oil ratio, the yield (conversion) also increases, the coke yield increases, and so does the octane rating of the gasoline. As for the heavy oil cracking, the catalyst-oil ratio is normally within 6-8. The high catalyst-oil ration compensates the hydrothermal inactivity of the catalyst caused by the high-temperature regeneration, to keep high dynamic activity of the catalyst. The most sensitive method to change the catalyst-oil ratio is to adjust the raw material warm-up temperature (when the reaction temperature is controlled automatically) and the regeneration temperature.

4.7Catalyst activity

The higher the catalyst balance activity, the higher the conversion yield, the concentration of the olefin in the product is decreased but the concentration of the paraffin is increased.

The catalyst with different type has the different balance activity at great extent. The catalyst with the proper activity should be chosen in the production process in accordance with the feed stock properties, the products scheme and the unit type. Besides the operational conditions, the activity of the catalyst is mainly adjusted by the replacement speed of the catalyst, and the normal fresh catalyst supplement is performed by the small feeding amount.

The heavy metals contaminants will lead to reduce the activity of the catalyst, the selectivity is getting inferior (bad) obviously, the gas yield and the coke yield are increased, the hydrogen concentration in the dry gas is obviously increased, but the yield of the gasoline is decreased. Under such circumstance, the catalyst needs to be replaced at great extent. While unloading the balance catalyst, the large quantity of fresh catalyst should be supplemented; meanwhile, the metal passivator quantity should be injected properly.

The hydrothermal deactivation is one of the factors affecting the catalyst activity. Because of the two-stage regeneration, the oxygen concentrations of the flue gases as well as the regeneration temperatures in two stages are playing the key roles on the activity of the catalyst. The low oxygen concentration and the low regeneration temperature in the first regenerator can burn out almost all the hydrogen taken by the catalyst; therefore, the high oxygen concentration and the high regeneration temperature in second regenerator can be adopted, to greatly decrease the carbon concentration of the catalyst, as well as greatly avoid the hydrothermal deactivation of the catalyst, so as to decrease the impact of the hydrothermal deactivation on the catalyst in the maximum degree.

4.8 Carbon concentration in the regenerated catalyst

The molecular sieve catalyst is very sensitive to the carbon concentration of the regenerated catalyst. The carbon concentration of the regenerated catalyst means the remaining coke content on the catalyst after regeneration. If the carbon content of the regeneration catalyst is too high, the activity and the selectivity of the molecular sieve catalyst will be decreased; therefore, the conversion yield will be decreased greatly, and the gasoline yield will also decrease, the bromine value will increase and the induction period will decrease. Adopting high-temperature regeneration can decrease the carbon concentration of the catalyst, normally, 0.1% carbon content decreasing, 2-3 units of the activity will be increased.

4.9Reactor pressure

To increase the reactor pressure is to improve the hydrocarbon partial pressure inside the reactor. Increases of the hydrocarbon partial pressure means increases of the reactant concentration resulted in speeding up of the reaction, so as to improve the conversion yield.

To increase the reaction pressure, the coke formation will be slightly increased, the yield of dry gas will also increase, the yield of gasoline will decrease slightly, but not very obviously.

As for a riser reactor, to increase the reaction pressure means to decrease the bulk flow rate of the reacting materials inside the reactor. Under the circumstance of the constant feeding rate, the reaction time is delayed. If the reaction time is kept constant, the production capacity is increased. The reactor pressure normally varies very little with the feed rate; higher reactor pressure reduces the rich gas compressor horsepower required and in turn increases the main air blower horsepower.

4.10Stripping steam quantity

To strip out the hydrocarbons absorbed by the catalyst particles, the steam quantity needed is depending on the catalyst circulation rate. The quantity of the stripping steam is normally 1-2Kg/ton circulated catalyst, or through slowly decreasing the stripping steam quantity during the operation, observe the change of the temperature, when the quantity of the stripping steam is decreased to a certain value, the temperature of the regenerator will increase sharply, and the hydrocarbon not stripped out starts to burn, the quantity of stripping steam at this time is getting the minimum, and the 1.1 time of such quantity should be adopted in the normal operation. Overmuch stripping steam quantity is disadvantageous to the load of the main fractionator column top, which will be increased and resulted in the hydrothermal deactivation of the catalyst as well.

4.11Atomizing steam

The atomizing steam is playing the key role on the feed stock reaction if the flow rate of the atomizing steam is proper, the atomization effect of the feedstock is good and the vapors partial pressure can be decreased as well, to avoid the coking. Once the feedstock is broken off, the atomizing steam prevents the nozzles from being plugged.

4.12Pre-lifting steam

The pre-lifting steam can decrease the vapors partial pressure inside the riser, and decrease the delta coke (coke weight/catalyst weight), decrease the temperature of the regenerator and decrease the formation of the light gas.

4.13recycled oil ratio

The recycled oil ratio is the ratio between the recycled oils (including the slurry to riser) and the fresh feed stock. The increase of the recycled oil ratio will cause: the increase of the coke yield, the increase of the regeneration temperature, the increase of the diesel oil yield, the decrease of the gasoline yield, increase the gas make and the increase of the total yield of the light oils.

4.14Heavy metals Contents

It is inevitable that the heavy metals are present in the feed stock. Among the common metals, the iron, nickel, vanadium, sodium, and copper are especially dangerous. During the catalytic cracking reaction, they will permanently settle down on the surface of the catalyst, changing the products distribution through facilitating the increase of the hydrogen yield. The existence of sodium will decrease the melting point of the catalyst and damage the structure of the catalyst (active sites), so deactivating the catalyst. Under the catalytic cracking conditions, the nickel will boost up the dehydrogenation reactions of the hydrocarbon, reducing the selectivity of the catalyst.

The metal contaminants of the feedstock are weighted with the metal factor Fm.

FmFe+V+10(Ni+Cu)

Where Fe, V, Ni, Cu means the concentrations of iron, vanadium, nickel, and copper in the feedstock, expressed with the weight concentration of ppm. Over 3 Fm indicates a danger poisoning and about 1 is considered to be safe. Two main results of metal contaminant which are the increase in the quantity of hydrogen in the dry gas and that can be detected through the hydrogen methane ratio of the dry gas and the increase in the olefin content of the LPG.

4.15Metal passivator

There are many types of metal passivators; normally, the compounds of the antimony and the bismuth are taken as the metal passivators at concentration of 20-40%. Through adding the metal passivators, the heavy metals in the feedstock and the compound of the antimony and the bismuth in the passivators are combined to form low-melting point alloy, to avoid the heavy metals settling down on the surface of the catalyst so as to decrease the activity and the selectivity of the catalyst. The injection amount should be adjusted according to the contents of the heavy metals and the ratio of H2/CH4.

4.16Fluidization, feedstock atomization and cyclone separation effect

These factors have the effect on the yield and quality of the products, but they are mainly decided by the internal structures of the equipments, being adjustable in minimum during production. What need to highlight is these factors are not taken as the adjusting means during normal operation, but weather the operational status is normal is depending on the measurements for some parameters and the observance as well. Weather these factors are in normal or not are quite important, which deserves being highly concerned during production.

For example, the fluidization status of the riser can be observed based on the pressure difference, temperature distribution and the density distribution of the riser reactor. The situation of the cyclone separator can rely on the pressure difference of the prompt separator and the concentration of the feedstock at the inlet of the cyclone separator. If there are some problems, we should take the proper measures to minimum the disadvantageous impact (adjusting the parameters).

II.Catalyst Regeneration and Its Influential Factors

The regeneration of the catalyst can produce a large quantity of heat, which is sufficient to provide the necessary energy for the reaction. Under some conditions, large quantity of over heat can be generated. Therefore, the regeneration of the catalyst has two functions, to restore the activity of the catalyst, and to provide the energy for reaction.

1. Regeneration reactions, and regeneration heat of reaction

The coke burnt out in the regenerators includes four parts:

Catalytic carbon the polymerized product of the cracking reaction;

Additional carbon the carbon converted by the allied (inherent) residual carbon of the feedstock CCR.

Strippable carbon the residual hydrocarbons inside the catalyst pores due to the incomplete stripping;

Polluted carbon the coke formed due to the impact of the heavy metals contaminants.

Since the coke is mainly composed of the carbon and hydrogen plus very small amount of sulphur and nitrogen depending on the feed properties the chemical reactions of the regeneration and the reaction heat are mainly focusing on the reactions of carbon, hydrogen with the oxygen contained in the air, and the produced heat of the reactions.

The main reactions of the coke combustion reaction are as the follows:

C+O2CO2 (exothermic): +33913KJ (8100kcal)/kg carbon

C+1/2O2CO (exothermic): +10258 KJ (2450kcal)/kg carbon

H2+1/2O2H2O (exothermic): +119742 KJ (28600kcal)/kg carbon

In the catalyst regeneration process, and due to the oxygen deficiency (lean) operation in the first regenerator, CO will be produced in the flue gas, so the concentration of CO2 in the flue gas of the first regenerator is low; however, due to the rich oxygen operation in the second regenerator, the concentration of CO2 in the flue gas of the second regenerator is extremely high and no CO present in it's flue gas, with the increase of the oxygen concentration, after two streams of flue gas are mixed in the flue, the CO will react with the rich oxygen to produce the CO2, and release large quantity of heat.

2. Regeneration speed

There are many factors influencing the regeneration speed, in which mainly there are: the accumulated carbon on the regeneration catalyst; regeneration temperature, the oxygen concentration of the flue gas; the regeneration pressure, the structures of the catalyst and the regenerator, and the fluidization status, etc..

2.1 Regeneration temperature

The regeneration temperature is one of the factors impacting the coking speed. Increasing the regeneration temperature can greatly increase the coke formation speed (provided no change in other parameters). At the temperature about 600OC, every 10 OC increase will increase the coke formation speed at about 20%. However, the increase of the reaction temperature is restricted by the hydrothermal stability of the catalyst, the equipment structure and materials.

As for the general regeneration, if the aluminum silicate catalyst is used, the regeneration temperature is controlled at 580-600 OC normally. If the molecular sieve catalyst is used, it can be operated at 650 OC, but the secondary combustion restriction should be considered. As for the complete regeneration, it can be operated at the temperature of 680-700 OC. As for the regeneration unit with two-stage regenerator, the hydrogen is almost burnt out in the first stage, and there is almost no water in the second stage; therefore the regeneration temperature can be increased higher.

2.2 Oxygen partial pressure

The coke formation speed is proportional to the oxygen partial pressure of the flue gas. The oxygen partial pressure is the product of the regeneration pressure and the molecular concentration of the oxygen of the flue gas; therefore, to increase the regeneration pressure or increase the oxygen concentration of the flue gas can increase the coke formation speed.

To increase the regeneration pressure, the pressure at the outlet of the key compressor should be increased. The regeneration pressure of a unit is restricted by the main air blower.

2.3 Regenerator inventory

The inventory of the regenerator determines the residence time of the catalyst in regenerator.

Residence time = regenerator inventory /catalyst circulation rate

Increasing the inventory can prolong the combustion time, and increases the coke formation and decrease the carbons of the regenerated catalyst. However, the volume of the regenerator needs to be increased, and the prolonged residence time will speed up the catalyst deactivation; thus, to increase the coke formation amount by means of increasing the residence is not a good method.

2.4The air flow rate

The air flow rate to the first regenerator should be controlled with the purpose of no oxygen in the outlet flue gas; and of the second regenerator should be controlled with the purpose of the oxygen concentration at the inlet of the third stage cyclone separator at 3-5%(V). If the combustion air is not sufficient for the combustion of the coke formed, the Carbon build up will be caused the reason that the coke burning rate lower than the coke forming rate during the cracking reaction process, as the essential reason.

III. Material, heat and pressure balances in the Reaction-Regeneration post:

As for the catalytic cracking unit, to realize the stable operation is to control three balances: material, pressure and heat balance.

During production, the parameters affecting the balances are not constant, but change with some factors, so the balances are broken. The main task of the operators is to make analysis on the reasons for imbalance and timely adjust the corresponding parameters, to make the system restore balance or reach the balance status.

1. Material balance

Material balance means the balance of the materials entering and discharging the reaction-regeneration system. For instance: feed stock and products, single pass conversion and recycled oil rate, coke burning and coke formation, oxygen supply and oxygen demand, catalyst loss and supplement, air yield and compressor capacity, etc.. The above balances which playing key roles on operation are shown as the followed:

1.1The balance between the single pass conversion and recycled oil rate

The relation between the single pass conversion and the recycled oil rate actually is the relation between the unit capacity and the products yield. During production, the operator normally takes the level of the recycle oil drum (D2202) as the eyes to observe the reaction depth. That is to say, the level of the recycle oil drum is the sign to weigh the conversion of the feed stock and the recycled refining. The increase or decrease of the level of the recycle oil drum indicates the damage to the balance, and the key reason for damaging the balance is relying on the conversion depth or the change of the single pass conversion. Therefore, firstly, the reaction depth should be adjusted. The adjusting methods are as the follows: to increase or decrease the reaction temperature, or speed up or slower down or stop the fresh catalyst injection rate (changing the activity of the catalyst); as for some units with less catalyst loss, due to the restriction of the system inventory, these methods are not flexible; so the regular method is to increase and decrease the reaction temperature and adjust the catalyst-oil ratio.

The method to only adjust the recycle oil flow rate directly can also be adopted, but it should be as less as possible, since to increase the recycle oil to riser flow rate will shorten the reaction time of the riser, decrease the conversion rate and increase the recycle oil. Therefore, only when the level of the recycle oil drum fluctuates severely, this method can be adopted to avoid the exhaustion of the recycle oil pump (P2209), or it can be used as the emergency measure for the recycle oil drum being full. At the meantime of adjusting the reaction depth, the flow rate of the recycle oil amount should be adjusted to keep at a proper recycle oil ratio. In the unit to produce the heavy diesel, sometimes decreasing the heavy diesel out flow for short time is used to balance the recycle oil drum level, since this method has small influence on the operation, but the quality of the heavy diesel should be concerned.

1.2 Coke formation Oxygen demand Balance

These two pairs are closely related. The coke formed during the reaction must be burned out in the regenerators, so as to keep the carbon concentration in the regenerated catalyst within the specification. To burn out the coke, sufficient oxygen must be supplied by the main air blower (compressor). Normally, the oxygen supply should be slightly higher than the oxygen demand.

If the air flow rate is not sufficient, the oxygen for coke burning is not sufficient, the oxygen concentration of the regeneration flue gas will decrease to zero, the color of the regenerated catalyst will turn to black and the carbon concentration will greatly increase, the activity and the selectivity of the catalyst will decrease, the coke formation quantity will increase, and the coke burning is not sufficient, leading to serious offset. The more the accumulated carbon on catalyst, the less the activity of the catalyst, which will lead to the decrease of the gas and oil gas yields, and the increase of the recycled oil and slurry, such phenomenon is called as Carbon build up. If serious, the stock feeding has to be cut or minimized for fluidized coke burning. If the oxygen is too much, the concentration of the oxygen in flue gas will get too high, and the regular regeneration will occur secondary combustion. Meanwhile, the process air will be wasted and the energy consumption will be increased.

During production, there must be two reasons causing the carbon build up: one is the increase of the coke formation on the basis of the original balance between coke formation and coke burning; the other is the process air is not sufficient, for examples: feed stock is getting heavier, the reaction temperature is too low, the catalyst for regeneration has oil; the flow rate of the small feeding supplement is too fast; the proportion of the fresh catalyst loaded at the very beginning of startup is too high; the activity of the catalyst is too high; and the failure of the oxygen concentration gauge, indicating too high oxygen concentration. All these above factors can cause carbon build up.

As for the high-efficiency regeneration of the two-stage regenerator, under the normal conditions, at the outlet of the second regenerator, the higher oxygen concentration will be adopted, normally at 3-5% (V). Since the coke burning temperature of the second regenerator is high, and the coke burning speed is fast, the possibility of carbon build up is less than that of the regular regeneration. Once the coke formation is increased due to some reasons, normally the temperature of the regeneration system will be increased, and then, the oxygen concentration will get back to zero, and the temperature difference between the dilute phase and the dense phase will also get back to zero. The phenomenon after oxygen concentration getting back to zero is as same as that of the regular regeneration.

The treatment for the carbon build up is as the follows:

(1) Very fast decrease the coke formation, stop recycling the slurry, greatly decrease the feeding rate and the recycled oil to riser rate, if necessary, the stock feeding has to be cut off, and the burning coke should be fluidized.

(2) During the process of coke burning, the heat balance should be controlled well to avoid the over temperature. During the time of decreasing the feeding rate, the temperature of the reactor should be observed, and the heat reduction media (normally gasoline) should be injected through the riser lower feed nozzles. If the injected gasoline cant take away the heat, leading the temperature increase of the regenerator, the process air flow rate should be decreased, so as to decrease the coke burning speed. That is to say, the heat balance should be better controlled during the process of coke burning.

1.3 The balance of the catalyst loss and fresh catalyst loading

For the catalyst loss, it can be distinguished two cases, the normal loss, and the up normal loss. What so-called normal loss means a certain balance catalyst activity should be maintained in order to guarantee the required conversion rate. Therefore, we should replace (unloading) some catalyst of low activity caused by aging, metal contamination and abrasion from the circulated catalyst in the reaction-regeneration at a certain speed. Catalyst will pass to the fractionator bottom with the reactor vapor effluent which can be found in the slurry oil by checking its solid content that is to be kept in its minimum value, another source of loss is the stack of the unit where the flue gas contains the catalyst fine particles that not recovered by the system cyclones. In order to maintain the system inventory, the small scale feeding of the fresh catalyst can meet the above-mentioned requirement. As for the unit to process the feed stock with high concentrations of heavy metals, since the concentrations of the heavy metals on the surface need to be maintained at a certain level. A certain quantity of catalyst needs to be unloaded from the regenerator, and the fresh catalyst needs to be loaded in. Close monitoring of the parameters as well as keeping the system pressure balance will minimize the catalyst losses.

1.4The Rich Gas Compressor Role and compression capacity

The rich gas compressor seems like the control valve of the reaction pressure during operation, playing direct impact on the pressure balance of two-stage regenerator. Its capacity has a certain restriction; therefore, the rich gas flow rate cant exceed the capacity of the compressor, so that the rich gas compressor can be kept stable and the system pressure can be kept balanced. The rich gas flow rate can be decided by two factors, one is the cracked oil vapor rate, and the other is the cooling effect of the coolers at the top of the fractionator column. If the cooling effect is not good, there must be a stream of light gasoline component going into compressor with the rich gas. Not only is the quantity of the rich gas increased, but also the increase of the rich gas molecular weight which increases the load on the compressor.

In addition, the components of the rich gas should be concerned. If the heavy metal contaminants of the catalyst are too high, the hydrogen concentration of the rich gas will be too high, and the molecular weight of the rich gas will greatly lowered than the designed parameter, then, under a certain rotating speed, the pressure at the outlet of the compressor cant reach the rating. At this time, if the pressure of the absorption column (C2301) is maintained high (higher than that of the compressor outlet), the compressed gas cant be discharged from the compressor, and the surge will take place in the compressor, leading to the temperature increase of the compressor; at the meantime, the reaction pressure will be affected. At this moment, one method is to properly increase the compressor rotating speed to improve the outlet pressure; the other is to decrease the operating pressure of the absorption section, to make the compressor discharge normally, so as to maintain the normal pressure of the reaction system. If under the emergency situation, the outlet of the compressor has to be connected to the flare to make its discharge normal, and then to be reconnected with the absorption section, but the pressure of the absorption section still has to be lowered a bit. When the temperature is low then the temperature of the vapor at the top of the fractionator column after condensing and the gas components are impacted as well as lower molecular weight rich gas is obtained, affecting the normal operation of the compressor. At this time, the temperature of the oil-gas separator can be adjusted. There are two adjusting methods: one is to adjust the air flow rate of the air cooler (no. of fans) and the rate of the cooling water; the other is to adjust the opening of the valve on the pipeline of the oil-gas separator prior to air cooling, to change the temperature of the oil-gas separator, so as to change the compositions of the gas. The former method is more reasonable, but the latter one can solve the problem faster than the previous one.

2. Reaction heat balance

The heat balance is the balance between the heat demand and the heat supply of the reaction. The heat needed by the reaction is mainly provided by the heat released by coke combustion in the regenerator, which will be transferred to the reactor though the circulation of the catalyst; therefore the heat demand and supply balance among two regenerators and reactor should be kept, so as to keep some certain temperatures of the reactor and regenerators.

The reaction temperature and the regeneration temperature are mainly relying on the nature of the feed stock, the formation scheme, the required conversion ratio, the coke burning speed and the regeneration mode. Normally during the normal operation, the reaction temperature will be controlled. The regeneration temperature is the consequence of the heat balance self-adaptation under the circumstance without the heat- removing device. If there is heat-removing device for regenerator, the heat removing can be controlled for better heat balance. The abnormal heat balance such as over heat is the breakage to the heat balance between two regenerators and the reactor, which should be highly concerned. For instance, the feed stock is getting heavier, and the stripping effect is inferior (bad), the abnormal operation may be due to the catalyst containing oil, so the combustion inside the regenerator will be increased suddenly, leading to overheat. Since such over heat is hard to control, the regeneration temperature will be increased, the equipment will be damaged, and the activity and the specific surface area of the catalyst will be decreased extremely.

3. Pressure balance

The so-called pressure balance means the pressure balance in the reaction-regeneration system. The pressure balance is the key (driving force) to maintain the normal circulation of the catalyst, and also the key to guarantee the safe and reliable operation of the unit.

In the normal operation of production, the pressure balance between the reactor and the two regenerators is maintained through adjusting the pressure differences of the reactor and regenerators, then the normal circulation of the catalyst can be maintained. If the pressure difference of the reactor or regenerator exceeds a certain range, the reverse flow of the catalyst will be resulted, if serious, the air will go into the reactor or the hydrocarbon will go into the regenerators, so as to lead to the violent events.

Section II Products

I.Fractionation Philosophy and Process Characteristics

1. Fractionation philosophy

The fractionation of the catalytic cracking is based on the distillation philosophy. The distillation is a physical process to separate the mixture of completely dissolved liquids at different boiling points, namely, a method to separate the components with different volatilities or boiling points in the vapor-phase or liquid-phase mixture. The distillation is realized inside the distillation or fractionator column, in which the distillate with high boiling points is separated at the bottom of the column, and the distillate with low boiling points is separated at the top of the column. The distillate that is withdrawn from the middle section of the column with the boiling points between those at the top and the bottom is called as side-cut distillate. The condensed liquid of distillation is called as the distillate, to respectively collect the distillate according to the different boiling range is called as fractionation, and the collected distillate is called as the distillation cut. The distillation process is to partially vaporize the liquid mixture and export the vapor of the light components (with low boiling points), so as to be separated from the heavy components, and then, the vapor is condensed as liquid, so as to make the original mixture be separated as two parts with light and heavy components respectively. Therefore, it is obvious that the distillation process includes the heat-up, vaporization, separation and condensation, while the majors are the vaporization and condensation.

2.Process characteristics of the fractionation post

The feedstock of the fractionator column is normally the super heated vapors with traces of catalyst particles at the temperature of approximately 500OC, and the heavy components must be condensed in the reacted oil gas. As the liquid-phase product from the bottom of the column, in order to prevent the side-cut product from pollution, the oil gas must be condensed to saturation status, and the powder involved in the oil gas must be purged, so as to perform the fractionation and avoid plugging the column trays. Therefore, many layers of gable baffles are installed at the bottom of the fractionator, called as purging and cooling section. Since all the feedstock of the column is the vapor-phase stock, the inlet is located at the bottom of the column (the bottom of the gable baffle). The stripping section hasnt been set, and the reacted cracked oil vapor is condensed by the circulated slurry; meanwhile, most of heat is used for pre-heat the feedstock or generating the steam by means of heat exchanger, so as to reach the purpose of recovering the heat. In order to guarantee the solid concentration in the slurry at a certain value (less than 5g/l), some slurry oil needs to be recycled or discharged out of the unit after being cooled.

II.Absorption-Desorption Philosophy

Absorption is a process to separate the gas mixture, making use of the liquid solvent to deal with the gas mixture, to make one or several components of gas dissolved in the solvent, so as to reach the purpose of separating the mixture. The absorbed gas component is called as the solute or absorbate, the unabsorbed component is called as the inert gas, and the used liquid is called as the solvent or absorbent. The absorbent absorbed the absorbate is called as the solution or saturated absorption solution.

The substance of gas absorption is the based on the solubility of the gas in liquid. When the gas contacts with the liquid, the gas dissolves in the liquid to reach certain solubility. The absorption degree depends on the gas-liquid balance (equilibrium) under the condition of the absorption.

The gas absorption is the transfer of the substance from the gas phase to the liquid phase, also a process of mass transfer. During this process, the absorbate of the gas phase (gas molecules) will go into the liquid phase through the interface, while some of the gas molecules going into liquid phase will also return to gas phase. The more the absorbate gas dissolved in the liquid, the faster of the gas molecules running over the liquid phase. When the rate of the gas molecules going into liquid phase from gas phase is equivalent to the rate of the gas molecules returning gas phase from liquid phase, the dynamic balance between the liquid phase and gas phase is reached, the concentration of the solution will not make change any longer, that is to say, the solution has reached saturation, namely, reached the solubility at a certain conditions. At this time, the absorbate gas on the top of the solution will produce a certain partial pressure.

The absorption degree of each component in the mixed gas is depending on the partial pressure of this component in the gas phase, and also depending on the balance pressure of this component in the solution. The driving force of the gas absorption is the difference of the above partial pressures in the gas and liquid phases. The direction of the mass transfer is depending on the partial pressure of this component in the gas phase and the balance pressure in the solution. As long as the partial pressure in the gas phase is bigger than the balance pressure in the solution, the absorption process will be continued until the dynamic balance between the gas phase and the liquid phase is reached; vice versa, if the balance pressure in the solution is higher than the partial pressure in the gas phase, the mass transfer will be performed on the opposite direction, and this component will transfer to gas phase from the liquid phase, called as desorption.

Normally according to weather the chemical reaction occurs, the absorption is differentiated as the chemical and physical absorption. In the refining industry, there is no chemical reaction when the oil absorbs the gas hydrocarbon, which can be taken as the physical process that only the gas dissolves in the liquid. When the gas dissolves in the liquid, the dissolution heat will be release; however, the concentration of the absorbed component is very low, and the absorbent has a large quantity, so the increase of the temperature is not obvious, normally taken as the isothermal absorption.

It is well known that, under the condition of the pressure not too high, the hydrocarbon mixture is taken as the ideal solution; therefore, the philosophies of the ideal solution can be taken as the theoretical basis for separating the hydrocarbon mixture. Under a certain temperature, and not too high pressure, the absorbate in the diluted solution has the liquid-gas balance relation subject to Henrys Law, when the absorption reaches to the gas-liquid phase balance, the concentration of the gas component in the liquid phase is proportional to its partial pressure in the gas phase, and the gas phase is subject to the Daltons Law of Partial Pressures. Therefore, of the hydrocarbon absorption, when the absorption reaches to balance, the gas components in the gas phase and liquid phase are subject to the phase balance formula as the followed:

Yi=KXi

Where:Yithe gas-phase balance molecular concentration of the absorbed component;

Xithe liquid-phase balance molecular concentration of the absorbed component, namely, the solubility of this gas component in the solvent;

Kthe phase balance constant, to express the distribution proportions of the absorbate in the gas and liquid phases; it depends on the nature of the solution, the temperature and the pressure.

Under a certain temperature and pressure, the bigger the molecular weight of the hydrocarbon, the less the K value, for increasing the recovery rate, the less the K value, the better the recovery. The less K value means the concentration of the gas-phase component is low in the gas phase, but high in the liquid phase, the hydrocarbon with high molecular weight is easer to be absorbed. As for a hydrocarbon, lower the temperature and higher the pressure, the less is the K value, which is favorable to absorption.

As for the desorption process, it is on the contrary, the hydrocarbon with low molecular weight is easier to be desorbed, so the high temperature and low pressure are favorable to desorption.

The absorption and desorption process of the catalytic cracking compressed rich gas is performed in the column of trays. The purpose is to separate the dry gas and the LPG, and the key components of separation are C2 and C3. it is required that the dry gas after absorption should contain the C3 as less as possible, and deethanized gasoline after desorption shouldnt contain the C2 as possible as it can, so as to separate the key components of C2 and C3 accordingly.

In the absorption column, the lean absorption oil goes downwards to countercurrent contact with the uprising hydrocarbon mixed gas that enters the column at the bottom tray to perform multiple gas-liquid converse contacts to finish the absorption

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