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PETROLEUM REFINING

THERMAL PROCESSES

Dr. Sahar El-Marsafy

Professor – Chem. Eng.

Cairo University – Faculty of Engineering

Chemical Engineering Department

Fourth Year

INTRODUCTION

The typical fuels refinery has as a goal: the

conversion of as much of the barrel of crude oil into

transportation fuels as is economically practical.

Although refineries produce many profitable

products, the high-volume profitable products are

the transportation fuels gasoline, diesel and turbine

(jet) fuels, and the light heating oils. These

transportation fuels have boiling points between 0

and 345°C (30 to 650°F).

Although products such as lubricating oils,

refrigeration and transformer oils, and

petrochemical feed stocks are profitable, they

amount to less than 5 percent of the total crude oil

charged to refineries.Chemical Engineering Department - Petroleum Refining - Dr. Sahar El-Marsafy

o The optimum flow pattern for any refinery is dictated

by economic considerations and no two refineries

are identical in their operations.

o Each refinery has its own unique processing

scheme which is determined by:

the process equipment available

crude oil characteristics

operating costs

product demand

INTRODUCTION (CONT.)

Chemical Engineering Department - Petroleum Refining - Dr. Sahar El-Marsafy

Atmospheric Distillation: Fractionation of crude oil feedstock to intermediate distillates fuels and atmospheric residuals.

Vacuum distillation: Low pressure conversion of heavy gas oils and atmospheric residuals to lighter gas oil feed to Fluid Catalytic Cracking Unit (FCCU), lube and delayed coking units.

Hydrotreating: Treatment of middle distillates, naphtha, and gas oils to remove contaminants such as compounds of sulfur, oxygen, nitrogen and metals before being fed to FCCU, catalytic reformers.

TYPICAL REFINING PROCESSES


Catalytic reforming: Improving the combustion quality of

atmospheric naphtha stream by

increasing its octane number before

blending to gasoline.

Isomerization: Conversion of paraffin-rich C5/C6 streams to

high octane product for blending with straight

run naphtha to gasoline

Hydrocracking: Upgrade of vacuum gas oils alone or

blended with various feedstock such as light

cycle oil, de-asphalted oil and coker gas oil to

lighter hydrocarbon for use as petrochemical

feedstock, low sulphur fuel oil and feed to FCCU,

isomerization and alkylation units.


(CONT.)


Catalytic cracking: The selective conversion of atmospheric gas oils and residual feedstock to higher value cracked products for use as petrochemical feedstock, transportation fuels, middle distillates, and light cycle oil.

Coking: Conversion of hydro-treated and virgin vacuum residues, various petroleum tars and coal tar pitch through delayed coking to yield fuel gas, LPG, naphtha, gas oils, and coke.

Solvent De-Asphalting: Prepare quality feed for FCC units (de-asphalted oil) and hydro-crackers from vacuum residues and blending stocks for lube oil and asphalt manufacturing.


(CONT.)


Gasoline blending: The physical mixture of a number of different hydrocarbons i.e. alkylate, isomerate, crackate and light straight run naphtha to produce a finished gasoline product with the desired characteristics

Sulfur recovery: Removal of dissolved H2S from produced liquid sulphur or converts hydrogen sulfide in sour gases and hydrocarbon streams to elemental sulfur


(CONT.)


Hydrogen production: Production of hydrogen from refinery processes principally from catalytic reformer product gases and steam reforming

Liquefied petroleum gas (LPG) production: Gas plant separate refinery gas components including butanes for alkylation, pentanes for gasoline blending, LPG's for fuel, and ethane for petrochemicals.


(CONT.)


DIAGRAMFLOWEFINERYR


Isomerization

Vapor recovery

Alkylation

Unsat. HC

OVERALL REFINERY FLOW

PROCESS DESCRIPTION

1. The crude oil is heated in a furnace and

charged to an atmospheric distillation

tower, where it is separated into: butanes and lighter wet gas,

unstabilized light naphtha,

heavy naphtha,

kerosene,

atmospheric gas oil,

atmospheric reduced crude (ARC).

2. The topped crude is sent to the vacuum

distillation tower and separated into: vacuum gas oil stream

vacuum reduced crude bottoms (VRC).Chemical Engineering Department - Petroleum Refining - Dr. Sahar El-Marsafy

PROCESS DESCRIPTION (CONT.)

3. The wet gas streams from the crude unit, coker, and cracking units are separated in the vapor recovery section (gas plant) into: fuel gas,

liquefied petroleum gas (LPG),

unsaturated hydrocarbons (propylene, butylenes, and pentenes),

normal butane, and isobutane.

o The fuel gas is burned as a fuel in refinery furnaces and the normal butane is blended into gasoline or LPG.

o The unsaturated hydrocarbons and isobutaneare sent to the alkylation unit for processing.


4. The light naphtha streams from the crude

tower, coker and cracking units are sent to

an isomerization unit to convert straight-

chain paraffins into isomers that have higher

octane numbers.

5. The heavy naphtha streams from the crude

tower, coker, and cracking units are fed to

the catalytic reformer to improve their

octane numbers. The products from the

catalytic reformer are blended into regular

and premium gasolines for sale.



6. The atmospheric and vacuum crude unit gas oils and coker gas oil are used as feed stocks for the catalytic cracking or hydro-cracking units. These units crack the heavy molecules into lower

molecular weight compounds boiling in the gasoline and distillate fuel ranges.

The products from the hydrocracker are saturated.

The unsaturated catalytic cracker products are saturated and improved in quality by hydro-treating or reforming.



7. The alkylation unit uses either sulfuric or

hydrofluoric acid as catalyst to react

olefins with isobutane to form isoparaffins

boiling in the gasoline range.

The product is called alkylate, and is a high-

octane product blended into premium motor

gasoline and aviation gasoline.

8. The middle distillates from the crude unit,

coker, and cracking units are blended into

diesel and jet fuels and furnace oils.



9. The heavy vacuum gas oil and reduced crude from paraffinic or naphthenic base crude oils are processed into lubricating oils. After removing the asphaltenes in a propane deasphalting unit, the reduced crude bottoms is processed in a blocked operation with the vacuum gas oils to produce lube oil base stocks. The vacuum gas oils and deasphalted stocks are

first solvent-extracted to remove the aromatic compounds and then dewaxed to improve the pour point.

They are then treated with special clays or high-severity hydrotreating to improve their color and stability before being blended into lubricating oils.



10. The reduced crude bottoms (VRC) from

the vacuum tower is then thermally

cracked in a delayed coker to produce wet gas

coker gasoline

coker gas oil

coke

Without a coker, this heavy residue would

be sold for heavy fuel oil or (if the crude oil is

suitable) asphalt.



When a hydrocarbon is heated to a

sufficiently high temperature thermal

cracking occurs. This is sometimes referred

to as pyrolysis (especially when coal is the

feedstock). When steam is used it is called

steam cracking. When hydrogen is used it

is called hydrocracking.

We will examine two thermal processes

used in refineries.

Visbreaking

Delayed coking

THERMAL PROCESSES


OKINGC

Coking units convert heavy feedstocks into a solid

coke and lower boiling hydrocarbon products which

are suitable as feedstocks to other refinery units for

conversion into higher value transportation fuels.

From a chemical reaction viewpoint, coking can be

considered as a severe thermal cracking process in

which one of the end products is carbon (i.e., coke).

Actually the coke formed contains some volatile

matter or high-boiling hydrocarbons.

To eliminate essentially all volatile matter from

petroleum coke it must be calcined at approximately

2000 to 2300°F (1095 to 1260°C). Chemical Engineering Department - Petroleum Refining - Dr. Sahar El-Marsafy

OVERVIEW

o Coking is a thermal process for the conversion of heavy low grade oil to lighter products (mainly gasoline related). It can be considered as an improved version of thermal crackers.

o As a result of thermal cracking, coke is formed in the reactor.

o The first delayed coker was built by Standard Oil of Indiana at Whiting, Indiana in 1929.

o Removal of the coke deposit on the reactor wall was a major challenge.

o The development of hydraulic decoking was an important achievement after which a number of commercial cokers were built.


o The coke obtained is usually used as fuel or further treated to marketable products such as electrodes, metallurgical coke, etc.

o Majority of sulfur in oil is concentrated on coke, providing products with less sulfur which facilitate the downstream operations.

o The coke contains sulfur compounds and metal impurities, which must be removed before further treatment for environment consideration

OVERVIEW


o Coking is a semi-continuous operation as the solid coke should be removed from the reactor at certain times.

oCoke forms a thick solid layer over the reactor inner wall.

oCoking reactors are relatively large in size due to relatively large residence time.

o The reactor needs decoking upon completion of the process.

o Two or more reactors are used for continuous operation. One reactor is decoked while the other one is in operation.

OVERVIEW


PURPOSE

Coking was used primarily to pretreat vacuum

residuals to prepare coker gas oil streams suitable for

feed to a catalytic cracker.

Benefits:

This reduced coke formation on the cracker catalyst and

thereby allowed increased cracker throughputs.

This also reduced the net refinery yield of low-priced residual fuel.

Added benefit was obtained by reducing the metals content

of the catalytic cracker feed stocks.

In recent years coking has also been used to prepare

hydrocracker feedstocks and to produce a high

quality ‘‘needle coke’’ from stocks such as heavy

catalytic gas oils and decanted oils from the fluid

catalytic cracking unit.Chemical Engineering Department - Petroleum Refining - Dr. Sahar El-Marsafy

PRODUCTS& STOCKEEDF

The feed stock can be:

Straight run residue

Cracked residue

Vacuum distillation bottom product

The products are: Gases

Naphta

Fuel oil

Gas oil (major product, feedstock for catalytic cracking operations)

Coke


COKEPETROLEUMOFYPEST

Much of delayed coker coke is produced as hard, porous,

irregular-shaped lumps ranging in size from 20 inches (50

cm) down to fine dust. This type of coke is called sponge

coke because it looks like a black sponge.

A second form of petroleum coke being produced in

increasing quantities is needle coke. Needle coke derives its

name from its microscopic elongated crystalline structure.

Needle coke is produced from highly aromatic feed stocks

when a coking unit is operated at high pressures and high

recycle ratios (1: 1).

Needle coke is preferred over sponge coke for use in

electrode manufacture because of its lower electrical

resistivity and lower coefficient of thermal expansion.


Needle Coke


http://www.jectcorp.com/cms/en/?lm=41

COKEPETROLEUMOFSESU

The main uses of petroleum coke are as follows:

1. Fuel

2. Manufacture of anodes for electrolytic cell

reduction of alumina

3. Direct use as chemical carbon source for

manufacture of elemental phosphorus, calcium

carbide, and silicon carbide

4. Manufacture of electrodes for use in electric

furnace production of elemental phosphorus,

titanium dioxide, calcium carbide, and silicon

carbide

5. Manufacture of graphite


COKINGELAYEDD

Delayed coking is a process in which the feed is sent

through a fired heater (480 – 515 C) with horizontal

tubes.

With short residence time in the furnace, coking of the feed is thereby delayed until it reaches large coking drums which provides long residence time to allow the cracking reactions to proceed to completion.

The cracked products leave as overheads and coke deposits form on the inner surface of the drum.

The temperature in the drum ranges from 415 to 450 C at pressures from 15 – 90 psig.


UNITCOKINGELAYEDD


The delayed coking process was developed to minimize refinery yields of residual fuel oil by severe thermal cracking of stocks such as vacuum residuals, aromatic gas oils, and thermal tars.

In early refineries, severe thermal cracking of such stocks resulted in unwanted deposition of coke in the heaters.

By gradual evolution of the art it was found that heaters could be designed to raise residual stock temperatures above the coking point without significant coke formation in the heaters. This required high velocities (minimum retention time) in the heaters.

DELAYED COKING


Typically furnace outlet temperatures range from

900–930°F (482–500°C).

The higher the outlet temperature, the greater the

tendency to produce shot coke and the shorter the

time before the furnace tubes have to be

decoked. Usually furnace tubes have to be

decoked every three to five months.

DELAYED COKING


PROCESS DESCRIPTION

1. Hot fresh liquid feed is charged to the

fractionator two to four trays above the bottom

vapor zone. This accomplishes the following:

The hot vapors from the coke drum are quenched by

the cooler feed liquid thus preventing any significant

amount of coke formation in the fractionator and

simultaneously condensing a portion of the heavy

ends which are recycled.

Any remaining material lighter than the desired coke

drum feed is stripped (vaporized) from the fresh liquid

feed.

The fresh feed liquid is further preheated making the

process more energy efficient.


2. The coke drum is usually on stream for 24 hours before becoming filled with coke.

Coke removal and cleaning operations usually takes less than 24 hours.

Coke is removed in three steps

Cooling the coke with water spray

Drilling a hole through the center of the deposit (opening the central top section of the drum)

Cutting the deposit with hydraulic devices (high pressure water jets)




The drum should be heated before receiving the

hot feed. Hot vapors from the online drum are

recirculated in the cold cleaned drum, where the

drum is heated (to about 350 C) and the vapors are

condensed and removed continuously.

During the operation, vapors are continuously

condensed, cracked and coke is gradually built up

on the inner wall of the drum.

Different types of coke may be formed depending

on the type of the feed and the operating

conditions.


3. Vapors from the top of the coke drum return to the base of the fractionator. These vapors consist of steam and the products

of the thermal cracking reaction: gas, naphtha, and gas oils. The vapors flow up through the quench trays. Above the fresh feed entry in the fractionator there are usually two or three additional trays below the gas oil drawoff tray.

These trays are refluxed with partially cooled gas oil in order to provide fine trim control of the gas oil end point and to minimize entrainment of any fresh feed liquid or recycle liquid into the gas oil product.



4. The gas oil side draw is a conventional configuration employing a six- to eight-tray stripper with steam introduced under the bottom tray for vaporization of light ends to control the initial boiling point (IBP) of the gas oil. Steam and vaporized light ends are returned from the

top of the gas oil stripper to the fractionator one or two trays above the draw tray.

A pump-around reflux system is provided at the draw tray to recover heat at a high temperature level and minimize the low-temperature-level heat removed by the overhead condenser.

This low-temperature-level heat cannot normally be recovered by heat exchange and is rejected to the atmosphere through a water cooling tower.



5. The bottom product of the fractionator is recycled back to the reactor after being

combined with the preheated feed.

Steam must be used before the switch and

immediately after the switch for stripping the

unconverted liquid feed to avoid feed solidification

during the cooling process which creates major

problems in the operation.

Steam stripping also serves to transfer heat from the

hot bottom to the unconverted feed on top of the

coke drum.



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