Download - 62620_Chap 3B Petro
-
8/21/2019 62620_Chap 3B Petro
1/39
Flow diagram of a delayed coking unit:5 (1) coker fractionator, (2)
coker heater, (3) coke drum, (4) vapor recovery column.
-
8/21/2019 62620_Chap 3B Petro
2/39
-
8/21/2019 62620_Chap 3B Petro
3/39
Fluid Coking Heated by the produced coke
Cracking reactions occur inside the heater and the fluidized-bed reactor.
The fluid coke is partially formed in the heater.
Hot coke slurry from the heater is recycled to the fluid reactorto provide the heat required for the cracking reactions.
Fluid coke is formed by spraying the hot feed on the already-formed coke particles. Reactor temperature
is about 520C, and the conversion into coke is immediate,with complete disorientation of the crystallites of product
coke. The burning process in fluid coking tends to concentrate the
metals, but it does not reduce the sulfur content of the coke.
-
8/21/2019 62620_Chap 3B Petro
4/39
Characteristics of fluid coke:
high sulfur content,
low volatility, poor crystalline structure, and lowgrindability index.
Flexicoking, integrates fluid coking with cokegasification.
Most of the coke is gasified. Flexicoking gasificationproduces a substantial concentration of the metals in
the coke product.
-
8/21/2019 62620_Chap 3B Petro
5/39
Flow diagram of an Exxon flexicoking unit:5 (1) reactor, (2)
scrubber, (3) heater, (4) gasifier, (5) coke fines removal, (6)
H2S removal.
-
8/21/2019 62620_Chap 3B Petro
6/39
CATALYTIC CONVERSION PROCESSESCatalytic Reforming
To improve the octane number of a naphtha.
Aromatics and branched paraffins have high octane ratingsthan paraffins and cycloparaffins.
Many reactions: e.g. dehydrogenation of naphthenes and thedehydrocyclization of paraffins to aromatics.
Catalytic reforming is the key process for obtaining benzene,
toluene, and xylenes (BTX).
These aromatics are important intermediates for the productionof many chemicals.
-
8/21/2019 62620_Chap 3B Petro
7/39
Reformer Feeds heavy naphtha fraction produced from atmospheric distillation
units.
Naphtha from other sources such as those produced from
cracking and delayed coking may also be used.
Before using naphtha as feed for a catalytic reforming unit, it
must be hydrotreated to saturate the olefins and to
hydrodesulfurize and hydrodenitrogenate sulfur and nitrogen
compounds.
Olefinic compounds are undesirable because they are precursors
for coke, which deactivates the catalyst.
Sulfur and nitrogen compounds poison the reforming catalyst.
The reducing atmosphere in catalytic reforming promotes forming
of hydrogen sulfide and ammonia. Ammonia reduces the acid
sites of the catalyst, while platinum becomes sulfided with H2S.
-
8/21/2019 62620_Chap 3B Petro
8/39
Importantis :
Types of hydrocarbons in the feed.
Naphthene content
The boiling range of the feeds
Feeds with higher end points (200C) are favorable because some
of the long-chain molecules are hydrocracked to molecules in the
gasoline range. These molecules can isomerize and dehydrocyclize
to branched paraffins and to aromatics, respectively.
-
8/21/2019 62620_Chap 3B Petro
9/39
Reforming Catalysts
Bi-functional to provide two types of catalytic
sites, hydrogenation-dehydrogenation sitesand acid sites.
platinum, is the best known hydrogenation-
dehydrogenation catalyst Alumina, (acid sites) promote carbonium ion
formation
The two types of sites are necessary foraromatization and isomerization reactions.
-
8/21/2019 62620_Chap 3B Petro
10/39
Pt/Re catalysts are very stable, active, and selective.
Trimetallic catalysts of noble metal alloys are also used for thesame purpose.
The increased stability of these catalysts allowed operation atlower pressures.
Reforming Reactions
Reforming Catalysts
Aromatization
-
8/21/2019 62620_Chap 3B Petro
11/39
The reaction is endothermic i.e. favoured @ higher temp and
lower pressures.
Effect of temp on the conversion and selectivity:
-
8/21/2019 62620_Chap 3B Petro
12/39
Catalytic Cracking Catalytic cracking (Cat-cracking): To crack lower-value stocks
and produce higher-value light and middle distillates. To produce light hydrocarbon gases, which are important
feedstocks for petrochemicals.
To produce more gasoline of higher octane than thermal
cracking. This is due to the effect of the catalyst, whichpromotes isomerization and dehydrocyclization reactions.
Feedsvary from gas oils to crude residues
Polycyclic aromatics and asphaltenes peoduce coke.
-
8/21/2019 62620_Chap 3B Petro
13/39
Catalytic Catalysts Acid-treated clays were the first catalysts used.
Replaced by synthetic amorphous silica-alumina, which ismore active and stable.
Incorporating zeolites (crystalline alumina-silica) with the
silica/alumina catalyst improves selectivity towards aromatics.
These catalysts have both Lewis and Bronsted acid sites thatpromote carbonium ion formation. An important structural
feature of zeolites is the presence of holes in the crystal
lattice, which are formed by the silica-alumina tetrahedra.
Each tetrahedron is made of four oxygen anions with either an
aluminum or a silicon cation in the center. Each oxygen anion
with a (II) oxidation state is shared between either two silicon,
two aluminum, or an aluminum and a silicon cation.
-
8/21/2019 62620_Chap 3B Petro
14/39
Catalytic Catalysts
Bronsted acid sites in HY-zeolites mainly originate from protonsthat neutralize the alumina tetrahedra. When HY-zeolite (X- and
Y-zeolites are cracking catalysts ) is heated to temperatures in
the range of 400500C, Lewis acid sites are formed.
-
8/21/2019 62620_Chap 3B Petro
15/39
Zeolite Catalysts Highly selective due to its smaller pores, which allow diffusion
of only smaller molecules through their pores, and to thehigher rate of hydrogen transfer reactions. However, the
silica-alumina matrix has the ability to crack larger molecules.
Deactivation of zeolite catalysts occurs due to coke formation
and to poisoning by heavy metals. Deactivation may be reversible or irreversible.
Reversible deactivation occurs due to coke deposition. This is
reversed by burning coke in the regenerator.
Irreversible deactivation results as a combination of fourseparate but interrelated mechanisms: zeolite dealumination,
zeolite decomposition, matrix surface collapse, and
contamination by metals such as vanadium and sodium.
-
8/21/2019 62620_Chap 3B Petro
16/39
Cracking Reactions A major difference between thermal and catalytic cracking is
that reactions through catalytic cracking occur via carbocationintermediate, compared to the free radical intermediate in
thermal cracking.
Carbocations are longer lived and accordingly more selective
than free radicals. Acid catalysts such as amorphous silica-alumina and
crystalline zeolites promote the formation of carbocations.
The following illustrates the different ways by which
carbocations may be generated in the reactor:
-
8/21/2019 62620_Chap 3B Petro
17/39
-
8/21/2019 62620_Chap 3B Petro
18/39
-
8/21/2019 62620_Chap 3B Petro
19/39
-
8/21/2019 62620_Chap 3B Petro
20/39
Aromatization Reactions Dehydrocyclizationreaction. Olefinic compounds formed by
the beta scission can form a carbocation intermediate withthe configuration conducive to cyclization.
Once cyclization has occurred, the formed carbocation can lose a proton,and a cyclohexene derivative is obtained. This reaction is aided by the
presence of an olefin in the vicinity (RCH=CH2).
-
8/21/2019 62620_Chap 3B Petro
21/39
-
8/21/2019 62620_Chap 3B Petro
22/39
Cracking Process Most catalytic cracking reactors are either fluid bed or moving
bed. In FCC, the catalyst is an extremely porous powder with an
average particle size of 60 microns.
Catalyst size is important, because it acts as a liquid with the
reacting hydrocarbon mixture.
In the process, the preheated feed enters the reactor section
with hot regenerated catalyst through one or more risers
where cracking occurs. A riser is a fluidized bed where a
concurrent upward flow of the reactant gases and the catalystparticles occurs.
-
8/21/2019 62620_Chap 3B Petro
23/39
The reactor temperature is usually held at about 450520C,
and the pressure is approximately 1020 psig.
Gases leave the reactor through cyclones to remove the
powdered catalyst, and pass to a fractionator for separation ofthe product streams. Catalyst regeneration occurs by
combusting carbon
deposits to carbon dioxide and the regenerated catalyst is then
returned
-
8/21/2019 62620_Chap 3B Petro
24/39
Typical FCC reactor/regenerator
-
8/21/2019 62620_Chap 3B Petro
25/39
Isomerization
Reactions leading to skeltal rearrangements over Pt catalysts
-
8/21/2019 62620_Chap 3B Petro
26/39
Hydrocracking
A hydrogen-consuming reaction that leads to higher gas
production
Hydrdealkylation
A cracking reaction of an aromatic side chain in presence of
hydrogen
-
8/21/2019 62620_Chap 3B Petro
27/39
Deep Catalytic Cracking Deep catalytic cracking (DCC) is a catalytic cracking process
which selectively cracks a wide variety of feedstocks into lightolefins.
It produces more olefines than FCC.
-
8/21/2019 62620_Chap 3B Petro
28/39
Hydrocracking Process It is a cracking process in presence of hydrogen.
The feedstocks are not suitable for catalytic cracking becauseof their high metal, sulfur, nitrogen, and asphaltene contents.
The process can also use feeds with high aromatic content.
Products from hydrocracking processes lack olefinic
hydrocarbons.
The product slate ranges from light hydrocarbon gases to
gasolines to residues.
The process could be adapted for maximizing gasoline, jet
fuel, or diesel production.
-
8/21/2019 62620_Chap 3B Petro
29/39
Hydrocracking Catalysts and Reactions
Bifunctional noble metal containing zeolites are used.
This promote carbonium ion formation. Catalysts with strong acidic activity promote isomerization.
The hydrogenation-dehydrogenation is promoted by catalysts
such as cobalt, molybdenum, tungsten, vanadium, palladium,
or rare earth elements. As with catalytic cracking, the main reactions occur by carbonium ion
and beta scission, yielding two fragments that could be
hydrogenated on the catalyst surface.
The first-step is formation of a carbocation over the catalystsurface:
-
8/21/2019 62620_Chap 3B Petro
30/39
The carbocation rearrange, eliminate a proton to produce an
olefin, or crack at a beta position to yield an olefin and a new
carbocation.
-Products from hydrocracking are saturated. i.e. gasolines from
hydrocracking units have lower octane ratings. They have a
lower aromatic content due to high hydrogenation activity.
- Products from hydrocracking units are suitable for jet fuel use.
Hydrocracking also produces light hydrocarbon gases (LPG)
suitable as petrochemical feedstocks.
-
8/21/2019 62620_Chap 3B Petro
31/39
Hydrocracking Process Mostly single stage, with the possibility of two operation
modes. Once-through and a total conversion of thefractionator bottoms by recyling.
In once-though operation, low sulfur fuels are produced and
the fractionator bottom is not recycled.
In the total conversion mode the fractionator bottom isrecylced to the inlet of the reactor.
In the two-stage operation, the feed is hydrodesulfurized in
the first reactor with partial hydrocracking. Reactor effluent
goes to a high-pressure separator to separate the hydrogen-rich gas, which is recycled and mixed with the fresh feed. The
liquid portion from the separator is fractionated, and the
bottoms of the fractionator are sent to the second stage
reactor.
-
8/21/2019 62620_Chap 3B Petro
32/39
Hydrocracking reaction conditions vary widely, depending on
the feed and the required products. Temperature and
pressure range from 400 to 480C and 35 to 170 atmospheres.
Space velocities in the range of 0.5 to 2.0 hr-1 are applied.
Flow diagram of a Cheveron hydocracking unit:29 (1,4) reactors, (2,5)
HP separators, (3) recycle scrubber (optional), (6) LP separator, (7) fractionator.
-
8/21/2019 62620_Chap 3B Petro
33/39
Hydrodealkylation Process Designed to hydrodealkylate methylbenzenes, ethylbenzene
and C9+aromatics to benzene. The petrochemical demand for
benzene is greater than for toluene and xylenes.
After separating benzene from the reformate, the higher
aromatics are charged to a hydrodealkylation unit.
The reaction is a hydrocracking one, where the alkyl side
chain breaks and is simultaneously hydrogenated.
-
8/21/2019 62620_Chap 3B Petro
34/39
Consuming hydrogen is mainly a function of the number of
benzene substituents.
Dealkylation of polysubstituted benzene increases hydrogen
consumption and gas production (methane).
-
8/21/2019 62620_Chap 3B Petro
35/39
Hydrotreatment Processes Hydrotreating is a hydrogen-consuming process to reduce or
remove impurities such as sulfur, nitrogen, and some tracemetals from the feeds.
It also stabilizes the feed by saturating olefinic compounds.
Feeds could be any petroleum fraction, from naphtha to cruderesidues.
The feed is mixed with hydrogen, heated to the proper
temperature, and introduced to the reactor containing thecatalyst.
H d C l d R i
-
8/21/2019 62620_Chap 3B Petro
36/39
Hydrotreatment Catalysts and Reactions
The same as those developed in Germany for coal hydrogenation.
The cobalt-molybdenum/alumina is an effective catalyst.
hydrodenitrogenation
Alkylation Process
-
8/21/2019 62620_Chap 3B Petro
37/39
Alkylation Process To produce large hydrocarbon molecules in the gasoline fraction
from small moleucles. (branched hydrocarbons).
Normally acid catalyzed using H2SO4or abhydrous HF.
The product is known as the alkylate.
-
8/21/2019 62620_Chap 3B Petro
38/39
Some recent research has been devoted to replace the
homogeneous acid catalysts by heterogeneous solid
catalysts employing zeolites and alumina, or zirconia.
I i i
-
8/21/2019 62620_Chap 3B Petro
39/39
Isomerization process Small volume but important refinery process.
Acid catalyzed. To produce branched alkanes.
Bifunctional catalysts activated by inorganic chelorides are
used.
Pt/zeolite is a typical isomerization catalyst.
Oligomerization of Olefines (Dimerization)
To produce polymer gasoline with high octane number.
Acid catalyzed. By phosphoric or sulfuric acid.
The feed is Propylne-propane or propykene-butane mixture.
The alkane is used as diluent.