Fundamentals of petroleum processing_ lecture5.pdf

SCE 4106 :Fundamentals of Petroleum Processing
INSTRUCTOR: OGENE FORTUNATE [BSc(MUK), M.TECH (IIT GUWAHATI, INDIA)]
Email: ogene.fortunate@gmail.com
Tel: +256(0) 759 682273,+256(0) 771 641557
SCE 4106: Fundamentals of Petroleum Processing
5/27/2021
1

Fuel Refining Processes
5/27/2021
2

6/1/2021
3

5/27/2021
4
 Catalytic cracking
 Reforming
 Isomerization
 Alkylation
Polymerization
Delayed Coking
Vis-breaking

Catalytic Cracking
6/1/2021
5

Catalytic Cracking
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6
Originally cracking was accomplished thermally but the catalytic process
has almost completely replaced thermal cracking because more gasoline
having a higher octane and Less heavy fuel oils and light gases are
produced;
Typically cracking involves the thermal or catalytic decomposition of
petroleum fractions having huge quantities of higher molecular weight
compounds;
In catalytic crackers, the action of heat is reinforced by use of a catalyst.
Catalytic cracking ; is the most important and widely used refinery process
for converting heavy oils into more valuable gasoline and lighter products;

Catalytic Cracking
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Since heat is required,
typically cracking reactions
are carried out in furnaces
that are supplied with either
fuel oil or fuel gas or natural
gas or electricity as heat
source;
The light gases produced by
catalytic cracking contain
more olefins than those
produced by thermal
cracking.
Thermal Versus Catalytic Cracking Yields on Similar Topped Crude
Feed

Catalytic Cracking
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8
The cracking process produces carbon (coke) which remains on the
catalyst particle and rapidly lowers its activity;
To maintain catalyst activity at a useful level, it is necessary to regenerate
the catalyst by burning off this coke with air. As a result, the catalyst is
continuously moved from reactor to regenerator and back to reactor;
o The cracking reaction is endothermic and the regeneration reaction
exothermic.
The catalytic-cracking processes in use today can all be classified as
either moving-bed;
i. Thermafor catalytic cracking process, TCC or .
ii. Fluidized-bed units (fluid catalytic cracker (FCC)).

Catalytic Cracking
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FCC process employs a catalyst in
the form of very fine particles (70
µm) which fluidise when aerated with
vapour;
FCCU 35%
Reformer 30%
Alkylation 20%
Isomerization 15%
Most modern units now in operation use a fluid catalytic cracking (FCCU)
process, in which the feed is vaporised and makes the catalyst behave as
a fluid;
FCC is “heart” of a modern refinery. Nearly every major fuels refinery has
an FCCU;
Contributes the highest volume to the
gasoline pool (usually between 35%-
40%);

Catalytic Cracking
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10
Feedstock
 FCC Processes primarily gas oils i.e (both atmospheric and vacuum
coker gasoil) using catalysts to crack the carbon-carbon bonds;
o Cracking lowers the average molecular weight & produces higher yields of
fuel products.
 Attractive feed for catalytic cracking should have the following
characteristics:
i. Small concentrations of contaminants
o Poisons the catalyst
ii. Small concentrations of heavy aromatics
o Cracks & deposit coke on catalyst
 Products may be further alkylated to improve gasoline anti knock
properties;

Catalytic Cracking
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11
 Feed not often used include:
i. Visbreaker gas oils
o Contains aromatics
ii. Delayed coker gas oils
o Contains aromatics, olefins, & sulfur
iii. Vacuum resides
o High aromatics; could be diluted with gas oils
 Feeds normally limited to CCR(continuous catalyst regeneration) of
approx. 3-7 wt.% (why?);
Feedstock

Catalytic Cracking
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12
 Primary goal of FCC is to make gasoline(“Cat gasoline”) & diesel and
o Minimize the production of heavy fuel oil
 Coke production small but very important
o Burned in regenerator & provides heat for process
 Light ends contain large amounts of olefins
o Good for petrochemical feedstock
o Can recover chemical grade propylene & ethylene
o Propylene, butylene, & C5 olefins can be alkylated for higher yields of
high- octane gasoline
Products from the Catalytic Cracker

Catalytic Cracking
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13
 Small amount of kerosene or jet fuel
o High sulphur content
 Catalytic diesel fraction
o Low centane number because of aromatics
o Lowers quality of diesel pool because of low cetane number & high
concentrations of sulphur
 Bottoms
o Contains sulphur, small ring & polynuclear aromatics, & catalyst fines
Products from the Catalytic Cracker

Catalytic Cracking
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Cracking Reactions
 Cracking facilitates initiation, propagation and termination reactions
amongst the hydrocarbon themselves;
 The primary reactions can be represented as follows:
o Paraffin → paraffin + olefin
o Alkyl naphthene → naphthene +olefin
o Alkyl aromatic → aromatic +olefin
Using paraffin cracking mechanism: Eg. n-Octane(where R =
CH3CH2CH2CH2CH2-) , the following catalytic reactions steps take
place:

Catalytic Cracking
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15
Cracking Reactions
Step 1: Mild thermal cracking initiation reaction;
Step 2: Proton shift;
Carbonium ions are formed
initially by a small amount of
thermal cracking of n-
paraffins to form olefins;
These olefins add a proton
from the catalyst to form
large carbonium ions which
decompose to form small
carbonium ions and olefins;

Catalytic Cracking
6/1/2021
16
Cracking Reactions
Step 3: Beta scission;
Step 4: Rearrangement toward more stable structure. The order of Carbonium ion
stability is tertiary > secondary> primary;
Step 5: Hydrogen ion transfer.
Large carbonium ion decomposes to form
small carbonium ions and olefins
The small carbonium ions propagate
the chain reaction by transferring a
hydrogen ion from a n-paraffin to form
a small paraffin molecule and a new
large carbonium ion.
Thus another large carbonium ion is
formed and the chain is ready to repeat
itself.

Catalytic Cracking
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17
 Commercial cracking catalysts can be divided into three classes:
1) acid-treated natural aluminosilicates;
2) amorphous synthetic silica-alumina combinations;
3) crystalline synthetic silica-alumina catalysts called zeolites or
molecular sieves;
 Most catalysts used in commercial units today are either class 3 or
mixtures of classes 2 and 3 catalysts;
 The advantages of the zeolite catalysts over the natural and synthetic
amorphous catalysts are:
Cracking Catalyst

Catalytic Cracking
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 The advantages of the zeolite catalysts over the natural and synthetic
amorphous catalysts are:
i. Higher gasoline yields at a given conversion
ii. Higher activity(permits short residence time cracking )
iii. Production of gasolines containing a larger percentage of paraffinic
and aromatic hydrocarbons
iv. Lower coke yield
v. Increased iso-butane Production
vi. Ability to go to higher conversions per pass without over cracking
Cracking Catalyst

Catalytic Cracking
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19
Cracking Catalyst
Comparison of
Amorphous and Zeolite
Catalysts:
Y Zeolite Types in
Catalyst:

Catalytic Cracking
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20
Feed is heated (260-425 oC) by
heat exchange/furnace and
injected into the reactor where it
meets hot catalyst;
The cracking reactions take place
on the catalyst as it flows up the
riser;
To prevent “over cracking” which produces excessive light ends at the
expense of gasoline yield, the contact time between the oil and catalyst
is minimized (Riser contact times in the order of 250 ms);
Catalytic Cracking Process

Catalytic Cracking
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21
Flue gas leaving the regenerator contains a
large quantity of carbon monoxide which is
burned to carbon dioxide in a CO furnace to
recover the available fuel energy;
At the top of the riser, the catalyst and product
are separated using cyclone separators;
By this stage the catalyst is covered with coke
and has lost most of its activity;
Stripper removes hydrocarbons from catalyst by
steam injection;

Catalytic Cracking
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Objective is to remove carbon deposits &
restore the activity of the catalyst to something
approaching its fresh state;
The spent catalyst is sent to a regenerator where the coke is burned off in
air;
Un-regenerated (spent) catalyst flows down
through a standpipe & lifted into regenerator
dense bed;
Regenerated catalyst overflows and flows down
through a standpipe to be lifted to the reactor by
steam & fresh feed.

Catalytic Cracking
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1) Increasing reactor temperature increases conversion
o Catalytic cracking is an endothermic reaction
o Heat to the reactor is controlled by: catalyst circulation rate,
regenerated catalyst temperature, & feed preheat
o Increasing feed preheat reduces the need for catalyst circulation for
the same reactor temperature
2) Catalyst/Oil ratio is a primary variable controlled by changing catalyst
circulation rate
o Increasing catalyst/oil ratio increases conversion (why?)
o Raising catalyst circulation also increases catalytic cracking activity &
increases reactor temperature
Catalytic Cracking Process variables

Catalytic Cracking
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3) Increasing catalyst activity increases conversion
o Activity is decreased by coke deposition and deposition of heavy
metals
Catalytic Cracking Process variables
4) Higher pressures generally do not affect conversion but do increase
coke production
5) Increasing space velocity decreases activity & conversion
o Increasing cat/oil residence time:
6) Increasing cat/oil residence time:
o Decreases catalyst activity
o Increases conversion, vapour velocity, coke and metal deposition on
the catalyst
o Gasoline yield increases initially and then decreases

Reforming
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Catalytic reforming is a process whereby light petroleum distillates
(naphthas) are contacted with a platinum-containing catalyst at elevated
temperatures and hydrogen pressures for the purpose of raising the
octane number of the hydrocarbon feed stream;
The low octane, paraffin-rich naphtha feed is converted to a high-octane
liquid product (reformate) that is rich in aromatic compounds;
Reforming Improve the octane of gasolines rather than yield;
In fact yield decreases due to hydrocracking reactions which take place in
the reforming operation;

Reforming
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The typical feedstock to catalytic reformers are:
a) heavy straight-run (HSR) gasolines and
naphthas [180–375°F (82–190°C)] and
b) heavy hydrocracker naphthas;
Feedstock
Naphtha feedstock to reformers typically contain
Paraffins, Olefins, Naphthenes, and Aromatics
(PONA) with 6–12 carbon atoms;
Feed naphthas often hydrotreated to remove
metals, olefins, sulphur, and nitrogen, prior to
being fed to a reforming unit;

Reforming
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Catalyst used in during reforming is noble metal (platinum) & very
sensitive to sulphur & nitrogen degradation;
o Feed stocks is often hydrotreated for sulphur removal,
o Control of chloride & water also important,
Choice of Feedstock
Reforming is not appropriate for Gas oil streams
o These tend to easily hydrocrack & deposit coke on the reforming
catalyst
Reforming is best applied to naphtha feeds without sulphur;
o However, not catalytic cracker naphtha,
 Because it contains olefins & aromatics,
o And not delayed coking naphtha
 Because it contains high levels of sulphur, olefins, & aromatics

Reforming
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29
 Catalytic reformer contributes the second highest volume to the gasoline
pool;
Reformate is desirable
component for gasoline due to :
i. High octane number,
ii. low vapour pressure,
iii. very low sulphur levels &
iv. low olefins concentration
 Almost every refinery in the
world has a reformer

Reforming
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Desirable reactions in a catalytic reformer all lead to the formation of
aromatics and isoparaffins as follows:
i. Paraffins are isomerized and to some extent converted to Naphthenes;
o The Naphthenes are subsequently converted to aromatics;
ii. Olefins are saturated (hydrogenated)to form paraffins which then react
as in (i) above ;
iii. Naphthenes are converted to aromatics;
iv. Aromatics are left essentially unchanged;
Reactions

Reforming
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There are four major reactions that take place during reforming:
i. Dehydrogenation of naphthenes to aromatics,
ii. Dehydrocyclization of paraffins to aromatics,
iii. Isomerization, and
iv. Hydrocracking,
i. Dehydrogenation of Naphthenes
Produces an aromatic from a Naphthene;
The reaction is highly endothermic (favoured by high reaction
temperature and low pressure;
Naphthene Aromatic
Reactions (cont…)

Reforming
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32
ii. Isomerization of Paraffins and Naphthenes:
The reaction involves ring re-arrangement, and thus ring opening to form
paraffin is possible;
Reactions (cont…)
The paraffin isomerization reaction occurs rapidly at commercial
operating temperatures;
Isomerization reactions are promoted by the acid function of the catalyst;

Reforming
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iii. Dehydrocyclization of Paraffins:
The most-difficult reforming reaction to promote is the
dehydrocyclization of paraffins;
This reaction consists of molecular rearrangement of paraffin to a
naphthene;
Paraffin cyclization becomes easier with increasing molecular weight of
the paraffin because the probability of ring formation increases;
Dehydrocyclization is favoured by low pressure and high temperature
and requires both the metal and acid functions of the catalyst;
Reactions (cont…)

Reforming
6/4/2021
34
Reactions
iv. Hydrocracking
 Paraffin hydrocracking is favoured by high temperature and high
pressure;
 Relatively slow reaction (so it occurs in the last section of the reactor);
 Hydrogen is consumed.
v. Dealkylation
 Dealkylation of aromatics includes both making the alkyl group (a side
chain on the aromatic ring) smaller and removing the alkyl group
completely;

Reforming
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iv. Dealkylation (cont…):
 Examples of dealkylation reactions include:
i. conversion of ethylbenzene to toluene and
ii. converting toluene to benzene, respectively;
 if the alkyl side chain is large enough, the reaction is similar to
paraffin cracking;
 Dealkylation is favoured by high temperature and high pressure;
 NOTE: The last two reactions; dealkylation and hydrocracking are
undesirable reaction during reforming;
Reactions (cont…)

Alkylation
6/1/2021
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Purpose of Alkylation:
o To make high octane gasoline from
materials that are too light;
A major constituent of alkylate is 2,2,4-
trimethyl pentane ; which is defined as 100
on the octane scale;
Motor fuel alkylation in the petroleum processing industry refers to the
acid catalysed conversion of C3-C5 olefins with alkyl group( preferably
isobutene) into highly branched C5-C12 isoparaffins collectively called
alkylate;

Alkylation
5/27/2021
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 Alkylation contributes 20 % of the gasoline

Alkylation
5/27/2021
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In the 1920s & 1930s gasoline octane was improved by:
o Tetra Ethyl Lead in Straight Run Gasoline,TEL;
o Thermal reforming of naphtha;
o Thermal polymerization of Olefinic light ends;
alkylation of olefins was later developed in early 1940s to improve the
octane of aviation gasoline;
o Vladimir Ipatieff had discovered aluminium chloride Catalysis in 1932;
Catalytic cracking significantly increased the production of light ends
which formed feedstock for alkylation unit;
High concentration of the C3, C4, & C5 isomers, both olefinic & paraffinic;
led to development of both catalytic polymerization & alkylation;

Sulphuric Acid Alkylation
6/1/2021
40
A group of major refiners & contractors consisting of Anglo-Iranian Oil, Humble
Oil & Refining, Shell Development, Standard Oil Development, & the Texas
Company developed process with sulphuric acid as the catalyst;
The first alkylation unit was placed on stream at the Humble Baytown Refinery
in 1938;
Many alkylation plants were built at the same time as the catalytic cracking
units;
o Operated during World War II for aviation gasoline production.
Sulphuric acid alkylation required access to acid regeneration on a large scale;
o Most sulphuric acid alkylation plants were located on deep water for barge
transport of spent acid to regeneration at acid plants & return of fresh acid;

Hydrofluoric Acid (HF) Alkylation
6/1/2021
41
Later Phillips Petroleum & UOP developed a process using hydrofluoric acid as
a catalyst
o HF could be readily regenerated in alkylation plant facilities;
o No need to transport catalyst in large quantities for regeneration;
Feed Stocks
Olefins and isobutane are used as alkylation unit feedstock;
Butenes (Butylene) and propene (propylene);are common olefins used, but
pentenes (amylenes) are included in some cases.
Butylene is the preferred olefin since it produces the highest octane number &
yields; Iso-butane & iso-pentane can be reacted with the olefin;
Iso-pentane not usually used since it is a good gasoline blend stock (high
octane number & low RVP):

Alkylation Products
5/27/2021
42
The product streams leaving an alkylation unit are:
o LPG grade propane liquid
o Normal butane liquid
o C5+ alkylate gasoline
o Tar
 Alkylate gasoline is the most desirable product for high performance
automotive fuels
o Very high octane index (R+M)/2 of 95
o Low vapour pressure
 Essentially no olefins, benzene or aromatics are produced from
alkylation;

Process Chemistry
6/2/2021
43
 Propylene, butylene, & pentenes
are olefins used (butylene most
preferred)
o High octane isooctane alkylate
produced;
o Lower reactant consumption
Acid catalyzed alkylation combines isoparaffins & olefins to form alkylate,
highly branched alkanes;
 The reaction is carried out in the liquid phase with an acid/reactant
emulsion maintained at moderate temperatures and high pressure;

Process Chemistry(cont…)
5/27/2021
44
 The reaction is carried out using acid catalyst at low temperature and
high enough pressure to keep the hydrocarbon in the liquid state;
 The quality of the product depends on the olefin feedstock i.e Isobutylene
> butenes > propene> pentene;
 High isoparaffin/Olefin ratios (4:1 to 15:1) are used to minimize
polymerization and to increase product octane;
 Efficient agitation (thorough mixing)to promote contact between the acid
and hydrocarbon phases is essential to high product quality and yields;

Primary Reactions
5/27/2021
45
 The principal reactions which occur during alkylation are the
combinations of olefins with iso-paraffins as follows;
OR

Choosing between HF and H2SO4 systems
5/27/2021
46
 HF process involves both low capital and total operating costs are less
compared to H2SO4 process because of the following reasons:
o Smaller, simpler reactor design;
o Use cooling water not refrigeration;
o Smaller settler devices needed for emulsion;
o Essentially complete regeneration of HF, no disposal of spent acids;
o Flexibility of operation regarding temperature and ratio of isobutane
to olefin;

Choosing between HF and H2SO4 systems
5/27/2021
47
 H2SO4 processes counter the above arguments for HF processes with
the following:
o HF require additional equipment (HF stripper tower, HF
regeneration tower, etc.) to recover or neutralize HF in various
streams, while H2SO4, the entire effluent hydrocarbon stream is
neutralized.
o Equipment is required to dry the feed streams to a few ppm water
in HF processes which is not required for H2SO4 processes.
o HF can form a toxic, ground-hugging vapour cloud (additives can
reduce volatility).

Process variables and their effects
6/2/2021
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1. Type of Olefin
 Propylene is worse
 Octane numbers are low (89-92 RON)
 Propylene & acid consumption are high
 Butylene is preferred
 Produces the highest isooctane
o Resulting Research Octane Numbers of 93-95 (with isobutane)
o RON and MON are about equal for alkylate (sensitivity of the alkylate
is very low)
 Amounts of butylene consumed per alkylate produced is the lowest
 Pentene results are mixed; Side reactions frequent.

6/2/2021
49
3. Isobutane/Olefin Injection & Mixing
 This is more relevant in sulphuric acid systems
 Provide optimal reaction conditions for the very fast reaction
 Inject olefin feedstock in incremental fashion to increase
isobutane/olefin ratios;
 Newer plants have multi-injection & vigorous mixing systems.
2. Isobutane concentration
Excess isobutene required;
High i-C4/olefin ratio increases octane number and yield, reduces side
reactions and acid consumption
Olefins need to be surrounded by isobutene exposed to acid –if not
,olefins will polymerise instead of alkylate

6/2/2021
50
4. Reaction Temperature
Temperature is the most noticeable variable in both reaction systems (HF
&H2SO4).
Reaction temperature has a greater effect in sulphuric acid processes than
those using HF acid;
 Therefore sulfuric acid systems run at 45 °F (max: 70 oF; min: 30 oF;
above 70 oF, significant polymerization occurs);
 In sulphuric acid system, at low temperature, acid viscosity increases
thus good of the reactants and subsequent separation of emulsion
becomes difficult.
However, in HF systems increasing temperature reduces octane number;
 HF systems run at 95oF (increasing temperature from 60 to 125oF
degrades the alkylate quality about three octane)

6/2/2021
51
 Acid strength has varying effects on alkylate quality depending on the
effectiveness of Reactor mixing and the water content of the acid;
5. Acid Type & Strength
 The water concentration in the acid lowers its catalytic activity about
3 to 5 times as much as hydrocarbon diluents, thus an 88% acid
containing 5% water is a much less effective catalyst than the same
strength acid containing 2% water.
In sulfuric acid alkylation, the best quality and highest yields are obtained
with acid strengths of 93 to 95% by weight of acid, 1 to 2% water and the
remainder hydrocarbon diluents.
Increasing acid strength from 89 to 93% increases alkylate quality by 1-2
octane numbers;

6/2/2021
52
 The poorer the mixing in a reactor, the higher the acid strength
necessary to keep acid dilution down.
In hydrofluoric acid alkylation the highest octane number alkylate is
attained in the 86 to 90% by weight acidity range. Commercial operations
usually have acid concentrations between 83 and 92% hydrofluoric acid
and contain less than 1% water.
Generally only strong acids can catalyse the alkylation reaction but
weaker acids can cause polymerization to take place;
5. Acid Type & Strength

5/27/2021
53
END

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