Chapter 4 b4
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Transcript of Chapter 4 b4
Learning outcome
2
Classify and differentiate the primary processing
and secondary processing in petroleum refinery
processing.
Outline and sketch the operating processes in
petroleum refinery.
- downstream processing
- cracking
- thermal cracking
Introduction
3
Primary refining process
- the distillation of the feedstock into its basic fractions,and then the re-distillation of these in separate towersinto highly concentrated intermediates.
- In a simple refinery, the split would probably consist ofsix basic cuts: petroleum gas (refinery gas and LPG),gasoline, naphtha, kerosene, gas oil and residue.
Introduction (cont.)
4
Source: http://www.galpenergia.com/EN/agalpenergia/Os-nossos-negocios/Refinacao-Distribuicao/ARL/Refinacao/Paginas/Didatico-Refinacao.aspx
Introduction (cont)
5
Primary processing unit
The purpose of primary unit is to separate the crude into
different fractions by distillation
Know as mother unit of the refinery, consist of
•CRUDE DISTILLATION UNIT (CDU)
•VACUUM DISTILLATION UNIT (VDU)
Commonly referred as Atmospheric and Vacuum Distillation
unit (AVU)
Introduction (cont.)
6
Secondary Processes (downstream)
Scenario
- When automobile industry was in its infancy, it become
obvious that then production of large quantities of
straight run gasoline from crude distillation would
involve production of much larger quantities of residual
fuels than the market could absorb.
- The demand for gasoline thus spurred on scientists to
look for means of obtaining a higher yield of this product
from crude oil.
7
Modern refining
- Essentially involves 2 categories of processing:
1) The physical separation of the raw material into
a range of homogeneous petroleum fractions,
include distillation and blending.
2) The subsequent chemical conversion of certain
fractions to alter the product yield and improve
product quality, include cracking, coking,
reforming, alkylation, polymerization,
isomerization and hydrogen treatment.
Introduction (cont.)
Physical and Chemical Processes8
PHYSICAL
Distillation
Solvent extraction
Propane deasphalting
Solvent dewaxing
Blending
THERMAL
Visbreaking
Delayed coking
Flexicoking
CATALYTIC
Hydrotreating
Catalytic reforming
Catalytic cracking
Hydrocracking
Catalytic dewaxing
Alkylation
Polymerization
Isomerization
CHEMICAL
Secondary Processing
9
Secondary processes (downstream)
After crude oil is separated into its fractions, each
stream is further converted by changing the size and
structure of molecules through cracking, reforming
and other conversion processes.
Significance of Secondary Processing
10
• The significance of secondary processing
(1) to remove any impurities and undesirable constituents from
the distilled fractions.
(2) to convert some of the distilled hydrocarbons into different
molecular forms.
(3) to improve product quality.
(4) to improve the refinery profit margins by converting low
value heavy ends to high value products like LPG/ gasoline/
kerosene/ diesel.
11
(1) to remove any impurities from the distilled fractions
- All crudes contain organic sulphur compounds (eg. H2S-
hydrogen sulfide, mercaptan) which will be carried out over from
the column into the resulting gases, distillates and residues.
- The higher the density of a crude, the greater its sulphur content.
- Sour cuts are corrosive and possess an extremely objectionable
odor.
- The secondary process for the treatment of toxic, corrosive and
evil-smelling sulphur-compound impurities is known as
“sweetening”.
- Caustic washing and hydrodesulphurization are finishing
processes designed to remove H2S and mercaptan impurities.
Significance of Secondary Processing
12
(2) to convert some of the distilled hydrocarbons into different
molecular forms.
- in conversion processes, the structures of natural hydrocarbon
molecules are changed.
- cracking process, large hydrocarbon molecules are cracked or
broken to form two or more smaller molecules.
- cracking can be done by the action of heat and pressure alone
(thermal cracking) or by heat in the presence of suitable catalyst
(catalytic cracking).
- the main purpose of cracking is to increase the yield of lighter,
more valuable fractions from medium and residual cuts.
Significance of Secondary Processing
Factors for Selecting Secondary
Processes for a Refinery14
Type of Crude
Product Slate
Product specifications
Investment cost
Operating cost
Sophistication of technology
Chemical Processing15
We can change one fraction into another by one of
three methods:
CRACKING
• Breaking large hydrocarbons into smaller pieces
UNIFICATION
• Combining smaller pieces to make larger ones
ALTERATION
• Rearranging various pieces to make desired hydrocarbons
Cracking16
Cracking takes large hydrocarbons and breaks them
into smaller ones.
After various hydrocarbons are cracked into smaller
hydrocarbons, the products go through another
fractional distillation column to separate them.
CRACKING
THERMAL CRACKING
CATALYTIC CRACKING
Chemical Processing - Cracking17
There are several types of cracking:
(1) THERMAL – heat large hydrocarbons at high temperature
(sometimes high pressure as well) until they break apart.
Steam – high temperature steam is used to break ethane, butane and
naptha into ethylene and benzene, which are used to manufacture
chemicals.
Visbreaking – residual from distillation tower is heated, cooled with gas
oil and rapidly flashed in a distillation tower. This process reduces the
viscosity of heavy weight oils and produces tar.
Coking – residual from the distillation tower is heated to temperatures
above 482 degree C until it cracks into heavy oil, gasoline and naphtha.
When the process is completed, a heavy and almost pure carbon residue is
left (coke).
18
Chemical Processing – Cracking
(cont.)
(2) CATALYTIC – use catalyst to speed up the cracking reaction.
Catalysts include zeolite, aluminium hydrosilicate, bauxite and silicaalumina.
Fluid catalytic cracking – a hot, fluid catalyst at 538 degree C cracksheavy oil into diesel oils and gasoline.
Hydrocracking – similar to fluid catalytic cracking, but uses a differentcatalyst, lower temperature, high pressure and hydrogen gas. It takes heavyoil and cracks it into gasoline and kerosene (jet fuel).
19
Fluidized Catalytic Cracking Unit
Objective: to convert heavy vacuum gas oil to valuable distillates like LPG, Gasoline,Diesel by catalytic cracking in fluidized bed.
Hydro Cracker Unit
Objective: to convert heavy vacuum gas oil to valuable distillates like LPG, Naptha,Automatic transmission fluid (ATF), Kerosene and Diesel
Visbreaker Unit
Objective: to reduce viscosity of heavy ends i.e. RCO/ Vacuum residue by thermalcracking.
Coking Unit
Objective: to produce valuable distillate from heavy ends by thermal cracking
Cracking Process Units
20
Combine smaller hydrocarbons to make larger ones
Major unification process is called catalytic
reforming, and catalyst as platinum, platinum-
rhenium mix to combine low weight naptha into
aromatics.
A significant by-product of this reaction is
hydrogen gas, which is then either used for
hydrocracking or sold.
Chemical Processing - Unification
21
The structures of molecules in one fraction are
rearranged to produce another.
Alkylation – The process combines an unsaturated
light hydrocarbon (such as olefins, propylene or
butene) with isobutane to produce alkylate in the
presence of a catalyst such as hydrofluoric acid or
sulfuric acid ( a by product from removing
impurities from many oil products).
Product of alkylation are high octane hydrocarbons.
Chemical Processing - Alteration
THERMAL PROCESSES
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. We will examine two thermal processes used in refineries.
• Visbreaking
• Delayed coking
Visbreaking
• Visbreaking is a mild form of thermal cracking that lowers the
viscosity of heavy crude-oil residues without affecting the boiling
point range.
• Residuum from the atmospheric distillation tower is heated (425-
510ºC) at atmospheric pressure and mildly cracked in a heater.
• It is then quenched with cool gas oil to control over-cracking, and
flashed in a distillation tower.
• Visbreaking is used to reduce the pour point of waxy residues and
reduce the viscosity of residues used for blending with lighter fuel oils.
Middle distillates may also be produced, depending on product
demand.
• The thermally cracked residue tar, which accumulates in the bottom
of the fractionation tower, is vacuum-flashed in a stripper and the
distillate recycled.
Alternatively, vacuum residue can be cracked. The severity of the
visbreaking depends upon temperature and reaction time (1‐8 min).
• Usually < 10 wt% of gasoline and lighter products are produced.
The objective is to reduce the viscosity as much as possible without
significantly affecting the fuel stability.
Soaker Visbreaking Process
The furnace operates at a lower outlet temperature and a soaker drum is
provided at the outlet of the furnace to give adequate residence time to
obtain the desired conversion while producing a stable residue product,
thereby increasing the heater run and reducing the frequency of unit shut
down for heater decoking . The products from soaker drum are quenched and
distilled in the downstream fractionator.
Delayed Coking
• Coking is a severe method of thermal cracking used to upgrade heavy
residuals into lighter products or distillates.
• Coking produces straight‐run gasoline (Coker naphtha) and various
middle‐distillate fractions used as catalytic cracking feedstock.
• The process completely reduces hydrogen so that the residue is a form
of carbon called "coke."
• Three typical types of coke are obtained (sponge coke, honeycomb
coke, and needle coke) depending upon the reaction mechanism, time,
temperature, and the crude feedstock.
• In delayed coking the heated charge (typically residuum from
atmospheric distillation towers) is transferred to large coke drums which
provide the long residence time needed to allow the cracking reactions to
proceed to completion.