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Introduction
Benzene, or benzol, is an organic chemical compound and a known
carcinogen with the molecular formula C6H6.
Benzene is a colorless and highly flammable liquid with a sweet
smell and a relatively high melting point.
Benzene is a natural constituent of crude oil, but it is usually
synthesized from other compounds present in petroleum.
Benzene is a natural component of crude oil, and petrol contains
15% by volume.
Manufacturing Process
Benzene is formed from natural processes, such as volcanoes and
forest fires, as well as from human activities.
Four chemical processes contribute to industrial benzene
production: catalytic reforming, toluene hydrodealkylation, toluene
disproportionation, and steam cracking.
The traditional method of manufacturing benzene from the
distillation of light oils produced during the manufacture of coke
has been overtaken by a number of processes.
A growing source of benzene is by the selective
disproportionation of toluene where benzene is coproduced in the
manufacture of a paraxylene-rich xylenes stream.
Uses
Today benzene is mainly used as an intermediate to make other
chemicals. Smaller amounts of benzene are used to make some types
of rubbers, lubricants,
dyes, detergents, drugs, explosives, napalm and pesticides.
Benzene has been used as a basic research tool in a variety of
experiments including
analysis of a two-dimensional gas. Benzene is primarily used as
a solvent, as a starting material for the synthesis of
other chemicals and as a gasoline additive. Benzene is produced
in large quantities from petroleum sources and is used for the
chemical synthesis of ethyl benzene, phenol, cyclohexane and
other substituted aromatic hydrocarbons.
It is used to make styrene, which is used to make plastics and
polymers, and in the manufacturing process of nylon.
Market
Chemical industry is the main consumer of pure benzene,
accounting for 71 per cent of the total consumption; light industry
accounts for 2 per cent of the total consumption; pharmaceutical
industry, 0.5 per cent; and others, 26.5 per cent.
Ethylbenzene is the largest chemical outlet for benzene at
around 52% and nearly all is consumed in the production of
styrene.
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The second largest outlet for benzene, accounting for 19% of
demand, is cumene which is nearly all consumed in phenol production
with acetone formed as a coproduct.
Benzene demand throughout the world is dominated by the
production of three derivatives: ethylbenzene, cumene and
cyclohexane.
Dana K. SullivanUOP LLCDes Plaines, Illinois
The introduction of reformulated gasoline with mandated limits
on benzene content has caused many refiners to take steps to reduce
the benzene in their gasoline products. The major source of benzene
in most refineries is the catalytic reformer. Reformate typically
contributes 50 to 75 percent of the benzene in the gasoline
pool.
The two basic approaches to benzene reduction involve
prefractionation of the benzene and benzene precursors in a naphtha
splitter before reforming, postfractionation in a reformate
splitter of the benzene after it is formed, or a combination of the
two (Fig. 9.1.1). The benzene-rich stream must then be treated to
eliminate the benzene by using extraction, alkylation,
isomerization, or saturation (Figs. 9.1.2 and 9.1.3).
If the refiner has an available benzene market, the benzene-rich
stream can be sent to an extraction unit to produce
petrochemical-grade benzene. Alkylation of the benzene may also be
an attractive option if propylene is available, as in a fluid
catalytic cracking (FCC) refinery. An isomerization unit saturates
the benzene and also increases the octane of the stream by
isomerizing the paraffins to a higher-octane mixture. Saturation in
a stand-alone unit is a simple, low-cost option.
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PROCESS DISCUSSION
The UOP* BenSat* process was developed to treat C5-C6 feedstocks
with high benzene levels. Because almost all the benzene is
saturated to cyclohexane over a fixed bed of catalyst, no
measurable side reactions take place. Process conditions are
moderate, and only a slight excess of hydrogen above the
stoichiometric level is required. The high heat of reaction
associated with benzene saturation is carefully managed to control
the temperature rise across the reactor. Product yield is greater
than 100 liquid volume percent (LV %), given the volumetric
expansion associated with saturating benzene and the lack of any
yield losses from cracking to light ends.
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The product has a lower octane than the feed as a result of the
conversion of the highoctane benzene into lower-octane cyclohexane.
However, the octane can be increased by further processing the
BenSat product in an isomerization unit, such as a UOP Penex
unit.
PROCESS FLOW
The BenSat process flow is shown in Fig. 9.1.4. The liquid feed
stream is pumped to the feed-effluent exchanger and to a preheater,
which is used only during start-up. Once the unit is on-line, the
heat of reaction provides the required heat input to the feed via
the feedeffluent exchanger. Makeup hydrogen is combined with the
liquid feed, and flow continues into the reactor. The reactor
effluent is exchanged against fresh feed and then sent to a
stabilizer for removal of light ends.
CATALYST AND CHEMISTRY
Saturating benzene with hydrogen is a common practice in the
chemical industry for the production of cyclohexane. Three moles of
hydrogen are required for each mole of benzene saturated. The
saturation reaction is highly exothermic: the heat of reaction is
1100 Btu per pound of benzene saturated. Because the
benzene-cyclohexane equilibrium is strongly influenced by
temperature and pressure, reaction conditions must be chosen
carefully.
The UOP BenSat process uses a commercially proven noble metal
catalyst, which has been used for many years for the production of
petrochemical-grade cyclohexane. The catalyst is selective and has
no measurable side reactions. Because no cracking occurs, no
appreciable coke forms on the catalyst to reduce activity. Sulfur
contamination in the feed reduces catalyst activity, but the effect
is not permanent. Catalyst activity recovers when the sulfur is
removed from the system.
FEEDSTOCK REQUIREMENTS
Light straight-run naphthas must be hydrotreated to remove
sulfur. Light reformates usually have very low sulfur contents, and
so hydrotreating may not be required. Any
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olefins and any heavier aromatics, such as toluene, in the feed
are also saturated. Table 9.1.1 shows typical feedstock sources and
compositions. The makeup hydrogen can be of any reasonable purity
and is usually provided by a catalytic reformer.
COMMERCIAL EXPERIENCE
The estimated erected cost (EEC) for a light reformate,
fresh-feed capacity of 10,000 barrels per stream day (BPSD) at a
feed benzene level of 20 percent by volume is $5.6 million.
Estimated erected costs are inside battery limits, U.S. Gulf Coast
open-shop construction (2002). The EEC consists of a materials and
labor estimate; design, engineering, and contractors fees;
overheads; and expense allowance. The quoted EEC does not include
such off-site expenses as cost and site preparation of land, power
generation, electrical substations, off-site tankage, or marine
terminals. The off-site costs vary widely with the location and
existing infrastructure at the specific site. In addition, off-site
cost depends on the process unit. A summary of utility requirements
is shown in Table 9.1.2. There are four BenSat units in operation.
BenSat catalyst and technology are also used in four additional
operating UOP Penex-Plus units.
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PERP Program - Benzene/Toluene
INTRODUCTION
Primary sources of aromatics are from refinery catalytic
reformers, pyrolysis gasoline from olefins plants, and coal tar
processing. Secondary sources include toluene disproportionation
(TDP) and toluene hydrodealkylation (THDA) units. THDA units are
the swing source and used when benzene supply is tight and prices
get high enough to justify the economics of those plants.
About 70 percent of the global production of benzene is by
extraction from either reformate or pyrolysis gasoline (pygas). The
former is produced in the catalytic reforming of naphtha, a
technology primarily directed at the production of high octane
gasoline components. The latter is a liquid byproduct formed in the
production of olefins by steam cracking liquid feeds, such as
naphtha or gas oil. Ethylene plants typically operate near full
capacity, but the feedstock slate may vary depending on market
conditions. Extraction from reformate and pygas are the most
economical sources of benzene.
The composition of BTX (benzene, toluene and xylenes) depends on
the source. The table below compares BTX from pygas and reformate.
Pygas is typically rich in benzene, whereas xylenes and toluene are
the main components of reformate.
Typical BTX Composition from Pygas and Reformate , (Weight
percent)
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The table also very roughly shows the global demand for BTX
products. In general, benzene is present in the main feedstocks in
proportions lower than market demand, whereas toluene is in
considerable excess. To some extent this imbalance is corrected by
their relative values as gasoline components because refiners have
the option of extracting BTX as chemical products or blending them
in fuel. Xylenes and toluene are more valuable as blendstocks than
benzene as the benzene content in gasoline is restricted for
environmental reasons.
CURRENT COMMERCIAL TECHNOLOGY
In this section, technologies based on extraction and
dealkylation are described, along with a discussion of each major
feedstock and estimates of reformate and benzene production costs.
A discussion of non-conventional routes to BTX is also included.
The emphasis of the economic analysis is placed on benzene because
of its importance as a chemical product.
Catalytic Reforming
Modern catalytic reforming using platinum was first
commercialized in 1949 by UOP for use by the petroleum industry;
The term "reforming" is used to designate a process by which the
molecular structure of naphtha is changed, with the intent of
lessening the knocking tendency (i.e. raising the octane number) of
naphtha intended for use in internal combustion engines.
It is important to note the simultaneous production of hydrogen
when aromatics are manufactured by catalytic reforming, as
exemplified in the reactions shown below (the dehydrogenation of
cyclohexanes to aromatics, dehydroisomerization of
alkylcyclopentanes to aromatics and dehydrocyclization of paraffins
to aromatics). This hydrogen by-product is an important source of
hydrogen within the refinery.
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The maximum potential yields of aromatics that could be obtained
from naphthenes and paraffins if hydrocracking could be suppressed
are determined by the thermodynamic equilibria for aromatization
reactions. These data show, first, that corresponding aromatic
yields from the various classes of compounds follow the order (from
highest to lowest) alkylcyclohexanes, alkylcyclopentanes,
paraffins. Second, aromatic yields increase with the number of
carbon atoms per molecule; benzene from C 6 paraffin has a lower
yield than toluene from C 7 paraffin. Third, for a given reactant,
the potential aromatics yield increases as the hydrogen partial
pressure is decreased.
As the catalyst ages, it is necessary to change the process
operating conditions to maintain the reaction severity and to
suppress undesired reactions.
This Section discusses many aspects of Catalytic Reforming, such
as the:
Chemistry of reforming processes including dehydrogenation
reactions, isomerization of paraffins and naphthenes, hydrocracking
and miscellaneous others.
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Catalysts used in these reforming processes. Process Variables
including pressure, temperature, feedstock quality, feed boiling
range,
naphtha quality (naphthenic versus paraffinic), impurities,
space velocity, hydrogen to hydrocarbon ratio.
Reformer Types including s emi-regenerative, continuous catalyst
regeneration (ccr), cyclic.
Feed Preparation. Reformer Operation including Gasoline Mode,
BTX Operation. Yields and Utilities . Commercial Technology.
In addition to Catalytic Reforming, other current commercial
technologies discussed in this Process Evaluation/Research Program
(PERP) report include:
Production from Pyrolysis Gasoline
Pyrolysis gasoline (pygas), a byproduct of olefins production by
steam cracking naphtha of gas oil feedstocks, contains a high
proportion of aromatics, primarily benzene and toluene, and a
smaller amount of C 8 aromatics that contain up to 40 percent
ethylbenzene.
Aromatics Extraction
It is necessary to use a solvent extraction technique to recover
BTX products of commercial quality, since aromatics and
non-aromatics may have similar boiling points and form azeotropes.
After extraction, the BTX products can be separated, if necessary,
by distillation. There are three basic types of solvent extraction
systems: Azeotropic, Extractive, liquid/Liquid solvent).
Dealkylation Processes
The market demand for benzene, as a proportion of total BTX, is
higher than the proportion of benzene in typical BTX products.
Conversion of toluene and, to a lesser extent, xylenes, is
practiced by two basic techniques: (1) Hydrodealkylation which
involves stripping the methyl groups from toluene or xylenes to
produce benzene and methane e.g. Detol, Litol and Pyrotol
processes); and (2) Toluene disproportionation - although not
purely dealkylation - is also included under this heading as a
discretionary method of producing benzene. The toluene is converted
to benzene and xylenes in this process.
Production from Coke Oven Light Oil
Light oil arises as a byproduct in the coking of coal, largely
to provide a carbon source in steel making. To make coke, coal is
pyrolyzed at around 1,000C; temperatures vary widely in practice.
About 70 percent of the product is solid coke, consisting primarily
of carbon. The remainder is volatilized, and leaves through the top
of the coke ovens. This gaseous stream is fractionated, and its
cuts are used in various ways.
Production of Aromatics via Nonconventional Routes
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There exist several nonconventional routes to convert low value
refinery byproducts to benzene, toluene, and xylenes. These have
been developed and commercialized by various companies over the
past several years and include Asahi Chemicals Alpha Process,
BP/UOPs Cyclar TM Process, CP Chems Aromax Process, and UOPs RZ
Platforming TM Process.
DEVELOPING TECHNOLOGIES
Since the last PERP report on this subject there have been
numerous patents and patent applications dealing with the
production of aromatics. A majority of these have been awarded to
the two major licensors of aromatics technology, namely UOP and IFP
(Axens). Nexant has reviewed the recent developments for the
production of benzene and toluene. The more interesting
developments are discussed in this section:
A novel catalyst combination that converts methanol (MeOH) to
aromatics (MTA) and especially xylenes.
Direct catalytic conversion of methane to higher hydrocarbons
and specifically to aromatics (e.g., such as benzene).
Axens is licensing a new technology developed and patented by SK
Corp. for upgrading pyrolysis gasoline.
Chevron Chemical Company (now Chevron Phillips Chemical Company)
has been awarded a patent in which reforming/aromatization of
hydrocarbons occurs in two parallel reformers in order to maximize
the benzene and para-xylene production.
China Petroleum and Chemical Corporation (CPCC) and Sinopec have
developed a new composite solvent for extractive distillation (ED)
of aromatics.
ExxonMobil proposes a bound zeolite catalyst for use in
alkylation, transalkylation or isomerization of aromatic
hydrocarbons.
Fina has been awarded a number of patents dealing with toluene
disproportionation and transalkylation of heavy aromatics.
IFP has discovered, among other things, a catalyst with
substantially improved properties with respect to previous
reforming catalyst.
With respect to the Cyclar TM process, SABIC has made several
improvements to the process and catalyst.
UOP has developed a new family of zeolites that can be used in
alkylation of aromatics, transalkylation of aromatics,
isomerization of aromatics and alkylation of isoparaffins.
ECONOMIC ANALYSIS
The costs of production for the various technologies for
producing reformate have been developed at a world scale plant
capacity. Of the five types of technologies reviewed - see below -
we have shown that the economics can vary widely. This range of
economic performance is clearly seen where all five processes are
viewed on a side-by-side basis.
CCR TM Reforming RZ Platforming TM Cyclar TM Aromax
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Alpha Process
The costs of production of benzene from various sources
employing different technologies see below - have been developed.
(http://www.chemsystems.com/about/cs/news/items/PERP%200607_6_BenzeneToluene.cfm)
Benzene from Reformate Extraction (Sulfolane) including BTX
Distillation Benzene Recovery from Pygas:
o Solvent Extraction (LLE) of Pygas o Extractive Distillation
(ED) of Pygas o Bulk Dealkylation of Pygas
Benzene via the Litol Process Benzene via Toluene
Hydrodealkylation (THDA) Benzene via Toluene Disproportionation
(TDP) Benzene via Selective Toluene Disproportionation (STDP)
It is important to note that the economics presented herein are
in essence a snapshot in time. Nexant have tried to mitigate this
by carrying out sensitivity analysis using five-year historical
averages for feed and product prices. The results of this
sensitivity for the reformate cases and for the benzene cases
studied in this report are discussed.
The sensitivity of the costs of production to feed price for the
costs of production of reformate and for the costs of production of
benzene, for the cases studied are also discussed.
Nexants view with respect to some of the strategic issues
(Access to feedstock, Outlet for by-products, Investment
requirements, Revamp and integration potential or strategy,
Feedstock/product price fluctuations/forecasts, Technology
availability/licensing terms, Technology risk, Security of
supply/strategic importance) for the reformate processes considered
is given.
MARKET ANALYSIS
Benzene has many uses, and demand continues to grow despite
increasing restrictions and environmental regulations. These uses -
including, Styrene/Ethylbenzene, Cumene/Phenol, Cyclohexane,
Nitrobenzene, Chlorobenzene, Alkylbenzene, Maleic Anhydride and
others - are discussed in this section.
Regional benzene consumption for the United States is shown in
the figure below. Just under half of the benzene in the United
States is consumed in the production of ethylbenzene for styrene.
Its growth is modest due to low polystyrene production growth and a
projected reduction in styrene exports. Cumene is the next largest
benzene derivative in this region and makes up just over
one-quarter of the total consumption. Cyclohexane, nitrobenzene and
LAB consume most of the rest of the benzene within the region.
U.S. Benzene End-Use Pattern
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Toluene is primarily used as a component in gasoline, and is
extracted from reformate or other sources. Controls on the total
aromatics content in gasoline will be less stringent than those
relating to benzene; the blending value of toluene is around 10
percent higher than benzene's.
Of the toluene extracted or otherwise produced, the largest
single use is for the production of benzene by dealkylation or the
production of both benzene and xylenes by disproportionation. The
other toluene applications are outlined.
Consumption, Supply/Demand and Trade data for the USA, Western
Europe, and Asia Pacific is discussed. This includes:
An extensive listing of Benzene and Toluene plant capacity for
each of the three regions: Details of company, plant site, Benzene
and Toluene capacity at the specified plant, the process used and
the country where the plant is located are given.
These reports are for the exclusive use of the purchasing
company or its subsidiaries, from Nexant, Inc., 44 South Broadway,
White Plains, New York 10601-4425 U.S.A. For further information
about these reports contact Dr. Jeffrey S. Plotkin, Vice President
and Global Director, PERP Program, phone: 1-914-609-0315; fax:
1-914-609-0399; e-mail: [email protected] or Heidi Junker
Coleman, phone: 1-914-609-0381, e-mail address:
[email protected]
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AROMATICS COMPLEXES
James A. Johnson
INTRODUCTION
An aromatics complex is a combination of process units that can
be used to convert petroleum naphtha and pyrolysis gasoline (pygas)
into the basic petrochemical intermediates: benzene, toluene, and
xylenes (BTX). Benzene is a versatile petrochemical building block
used in the production of more than 250 different products. The
most important benzene derivatives are ethylbenzene, cumene, and
cyclohexane (Fig. 2.1.1). The xylenes product, also known as mixed
xylenes, contains four different C8 aromatic isomers: para-xylene,
ortho-xylene, meta-xylene, and ethylbenzene. Small amounts of mixed
xylenes are used for solvent applications, but most xylenes are
processed further within the complex to produce one or more of the
individual isomers. The most important C8 aromatic isomer is
para-xylene, which is used almost exclusively for the production of
polyester fibers, resins, and films (Fig. 2.1.1). In recent years,
polyester fibers have shown growth rates of 5 to 6 percent per year
as synthetics are substituted for cotton. Resins have shown growth
rates of 10 to 15 percent per year, corresponding to the emergence
of PET (polyethylene terephthalate) containers. Note that benzene
can be a significant by-product of para-xylene production,
depending on the type of technology being used. A small amount of
toluene is recovered for use in solvent applications and
derivatives, but most toluene is used to produce benzene and
xylenes. Toluene is becoming increasingly important for the
production of xylenes through toluene disproportionation and
transalkylation with C9 aromatics.
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CONFIGURATIONS
Aromatics complexes can have many different configurations. The
simplest complex produces only benzene, toluene, and mixed xylenes
(Fig. 2.1.3) and consists of the following major process units:
Naphtha hydrotreating for the removal of sulfur and nitrogen
contaminants Catalytic reforming for the production of aromatics
from naphtha Aromatics extraction for the extraction of BTX
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Most new aromatics complexes are designed to maximize the yield
of benzene and para-xylene and sometimes ortho-xylene. The
configuration of a modern, integrated UOP* aromatics complex is
shown in Fig. 2.1.4. This complex has been configured for maximum
yield of benzene and para-xylene and includes the following UOP
process technologies:
CCR Platforming* for the production of aromatics from naphtha at
high severity Sulfolane,* Carom, on extractive distillation for the
recovery of benzene and toluene
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Parex* for the recovery of para-xylene by continuous adsorptive
separation Isomar* for the isomerization of xylenes and the
conversion of ethylbenzene Tatoray for the conversion of toluene
and heavy aromatics to xylenes and benzene
The Tatoray process is used to produce additional xylenes and
benzene by toluene isproportionation and transalkylation of toluene
plus C9 aromatics. The incorporation of a Tatoray unit into an
aromatics complex can more than double the yield of para-xylene
from a given amount of naphtha feedstock. Thus, the Tatoray process
is used when paraxylene is the principal product. If there is
significant need for benzene production, it can be achieved by
adjusting the boiling range of the naphtha feed to include more
benzene and toluene precursors. In such cases, technologies such as
PX-Plus* or even thermal hydrodealkylation (THDA) can be used to
maximize benzene production. The cost of production is highest for
THDA, so it is being used only in situations where benzene supply
is scarce.
About one-half of the existing UOP aromatics complexes are
configured for the production of both para-xylene and ortho-xylene.
Figure 2.1.4 shows an ortho-Xylene (o-X) column for recovery of
ortho-xylene by fractionation. If ortho-xylene production is not
required, the o-X column is deleted from the configuration, and all
the C8 aromatic isomers are recycled through the Isomar unit until
they are recovered as para-xylene. In those complexes that do
produce ortho-xylene, the ratio of ortho-xylene to para-xylene
production is usually in the range of 0.2 to 0.6.
The meta-xylene market is currently small but is growing
rapidly. The meta-xylene is converted to isophthalic acid and,
along with terephthalic acid derived from para-xylene, is converted
into PET resin blends for solid-state polymerization (SSP). The
demand for PET resin blends has grown significantly during the last
decade, as new food and beverage bottling and packaging
applications have been developed. In 1995, UOP licensed the first
MX Sorbex* unit for the production of meta-xylene by continuous
adsorptive separation.
Although similar in concept and operation to the Parex process,
the MX Sorbex process selectively recovers the meta rather than the
para isomer from a stream of mixed xylenes. An MX Sorbex unit can
be used alone, or it can be incorporated into an aromatics complex
that also produces para-xylene and ortho-xylene.
An aromatics complex may be configured in many different ways,
depending on the available feedstocks, the desired products, and
the amount of investment capital available. This range of design
configurations is illustrated in Fig. 2.1.5. Each set of bars in
Fig. 2.1.5 represents a different configuration of an aromatics
complex processing the same fullrange blend of straight-run and
hydrocracked naphtha. The configuration options include whether a
Tatoray or THDA unit is included in the complex, whether C9
aromatics are recycled for conversion to benzene or xylenes, and
what type of Isomar catalyst is used.
The xylene/benzene ratio can also be manipulated by
prefractionating the naphtha to remove benzene or C9_ aromatic
precursors (see the section of this chapter on feedstock
considerations).
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Because of this wide flexibility in the design of an aromatics
complex, the product slate can be varied to match downstream
processing requirements. By the proper choice of configuration, the
xylene/benzene product ratio from an aromatics complex can be
varied from about 0.6 to 3.8.
DESCRIPTION OF THE PROCESS FLOW
The principal products from the aromatics complex illustrated in
Fig. 2.1.4 are benzene, para-xylene, and ortho-xylene. If desired,
a fraction of the toluene and C9 aromatics may be taken as
products, or some of the reformate may be used as a high-octane
gasoline blending component. The naphtha is first hydrotreated to
remove sulfur and nitrogen compounds and then sent to a CCR
Platforming unit, where paraffins and naphthenes are converted to
aromatics. This unit is the only one in the complex that actually
creates aromatic rings. The other units in the complex separate the
various aromatic components into individual products and convert
undesired aromatics into additional high-value products. The CCR
Platforming unit is designed to run at high severity, 104 to 106
research octane number, clear (RONC), to maximize the production of
aromatics. This high-severity operation also extinguishes virtually
all nonaromatic impurities in the C8_ fraction of the reformate,
thus eliminating the need for extraction of the C8 and C9
aromatics. The reformate product from the CCR Platforming unit is
sent to a debutanizer column within the Platforming unit to strip
off the light ends. The reformate from the CCR Platforming unit is
sent to a reformate splitter column. The C7_ fraction from the
overhead is sent to the Sulfolane unit for extraction of benzene
and toluene. The C8_ fraction from the bottom of the reformate
splitter is clay-treated and then sent directly to the xylene
recovery section of the complex.
The Sulfolane unit extracts the aromatics from the reformate
splitter overhead and rejects a paraffinic raffinate stream. The
aromatic extract is clay-treated to remove trace olefins. Then
individual high-purity benzene and toluene products are recovered
in the benzene-toluene (BT) fractionation section of the complex.
The C8_ material from the bottom of the toluene column is sent to
the xylene recovery section of the complex. The raffinate from the
Sulfolane unit may be
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further refined into paraffinic solvents, blended into gasoline,
used as feedstock for an ethylene plant, or converted to additional
benzene by an RZ-100* Platforming unit.
Toluene is usually blended with C9 and C10 aromatics (A9_) from
the overhead of the A9 column and charged to a Tatoray unit for the
production of additional xylenes and benzene. The effluent from the
Tatoray unit is sent to a stripper column within the Tatoray unit
to remove light ends. After the effluent is clay-treated, it is
sent to the BT fractionation section, where the benzene product is
recovered and the xylenes are fractionated out and sent to the
xylene recovery section. The overhead material from the Tatoray
stripper or THDA stripper column is separated into gas and liquid
products. The overhead gas is exported to the fuel gas system, and
the overhead liquid is normally recycled to the CCR Platforming
debutanizer for recovery of residual benzene.
Instead of feeding the toluene to Tatoray, another processing
strategy for toluene is to feed it to a para-selective catalytic
process such as PX-Plux, where the para-xylene in the xylene
product is enriched to _85% and cyclohexane-grade benzene is
coproduced. The concentrated para-xylene product could then be
easily recovered in a single-stage crystallization unit. In such a
case, the C9_ aromatics could be fed to a Toray TAC9 unit and
converted predominantly to mixed xylenes.
The C8_ fraction from the bottom of the reformate splitter is
clay-treated and then charged to a xylene splitter column. The
xylene splitter is designed to rerun the mixed xylenes feed to the
Parex unit down to very low levels of A9 concentration. The A9
builds up in the desorbent circulation loop within the Parex unit,
and removing this material upstream in the xylene splitter is more
efficient. The overhead from the xylene splitter is charged
directly to the Parex unit. The bottoms are sent to the A9 column,
where the A9 fraction is rerun and then recycled to the Tatoray or
THDA unit. If the complex has no Tatoray or THDA unit, the A9_
material is usually blended into gasoline or fuel oil.
If ortho-xylene is to be produced in the complex, the xylene
splitter is designed to make a split between meta- and ortho-xylene
and drop a targeted amount of ortho-xylene to the bottoms. The
xylene splitter bottoms are then sent to an o-X column where
high-purity ortho-xylene product is recovered overhead. The bottoms
from the o-X column are then sent to the A9 column.
The xylene splitter overhead is sent directly to the Parex unit,
where 99.9 wt % pure paraxylene is recovered by adsorptive
separation at 97 wt % recovery per pass. Any residual toluene in
the Parex feed is extracted along with the para-xylene,
fractionated out in the finishing column within the Parex unit, and
then recycled to the Tatoray or THDA unit. The raffinate from the
Parex unit is almost entirely depleted of para-xylene, to a level
of less than 1 wt %. The raffinate is sent to the Isomar unit,
where additional para-xylene is produced by reestablishing an
equilibrium distribution of xylene isomers. Any ethylbenzene in the
Parex raffinate is either converted to additional xylenes or
dealkylated to benzene, depending on the type of Isomar catalyst
used. The effluent from the Isomar unit is sent to a deheptanizer
column.
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The bottoms from the deheptanizer are clay-treated and recycled
back to the xylene splitter. In this way, all the C8 aromatics are
continually recycled within the xylene recovery section of the
complex until they exit the aromatics complex as para-xylene,
ortho-xylene, or benzene. The overhead from the deheptanizer is
split into gas and liquid products. The overhead gas is exported to
the fuel gas system, and the overhead liquid is normally recycled
to the CCR Platforming debutanizer for recovery of residual
benzene.
Within the aromatics complex, numerous opportunities exist to
reduce overall utility consumption through heat integration.
Because distillation is the major source of energy consumption in
the complex, the use of cross-reboiling is especially effective.
This technique involves raising the operating pressure of one
distillation column until the condensing distillate is hot enough
to serve as the heat source for the reboiler of another column. In
most aromatics complexes, the overhead vapors from the xylene
splitter are used to reboil the desorbent recovery columns in the
Parex unit. The xylene splitter bottoms are often used as a hot-oil
belt to reboil either the Isomar deheptanizer or the Tatoray
stripper column. If desired, the convection section of many fired
heaters can be used to generate steam.
FEEDSTOCK CONSIDERATIONS
Any of the following streams may be used as feedstock to an
aromatics complex:
Straight-run naphtha Hydrocracked naphtha Mixed xylenes
Pyrolysis gasoline (pygas) Coke-oven light oil Condensate Liquid
petroleum gas (LPG)
Petroleum naphtha is by far the most popular feedstock for
aromatics production. Reformed naphtha, or reformate, accounts for
70 percent of total world BTX supply. The pygas by-product from
ethylene plants is the next-largest source at 23 percent. Coal
liquids from coke ovens account for the remaining 7 percent. Pygas
and coal liquids are important sources of benzene that may be used
only for benzene production or may be combined with reformate and
fed to an integrated aromatics complex. Mixed xylenes are also
actively traded and can be used to feed a stand-alone Parex-Isomar
loop or to provide supplemental feedstock for an integrated
complex.
Condensate is a large source of potential feedstock for
aromatics production. Although most condensate is currently used as
cracker feedstock to produce ethylene, condensate will likely play
an increasingly important role in aromatics production in the
future. Many regions of the world have a surplus of low-priced LPG
that could be transformed into aromatics by using the new UOP-BP
Cyclar* process. In 1999 the first Cyclar-based aromatics complex
started up in Saudi Arabia.
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This Cyclar unit is integrated with a downstream aromatics
complex to produce para-xylene, ortho-xylene, and benzene.
Pygas composition varies widely with the type of feedstock being
cracked in an ethylene plant. Light cracker feeds such as liquefied
natural gas (LNG) produce a pygas that is rich in benzene but
contains almost no C8 aromatics. Substantial amounts of C8
aromatics are found only in pygas from ethylene plants cracking
naphtha and heavier feedstocks. All pygas contains significant
amounts of sulfur, nitrogen, and dienes that must be removed by
two-stage hydrotreating before being processed in an aromatics
complex.
Because reformate is much richer in xylenes than pygas, most
para-xylene capacity is based on reforming petroleum naphtha.
Straight-run naphtha is the material that is recovered directly
from crude oil by simple distillation. Hydrocracked naphtha, which
is produced in the refinery by cracking heavier streams in the
presence of hydrogen, is rich in naphthenes and makes an excellent
reforming feedstock but is seldom sold on the merchant market.
Straight-run naphthas are widely available in the market, but the
composition varies with the source of the crude oil. Straight-run
naphthas must be thoroughly hydrotreated before being sent to the
aromatics complex, but this pretreatment is not as severe as that
required for pygas. The CCR Platforming units used in BTX service
are run at a high-octane severity, typically 104 to 106 RONC, to
maximize the yield of aromatics and eliminate the nonaromatic
impurities in the C8_ fraction of the reformate.
Naphtha is characterized by its distillation curve. The cut of
the naphtha describes which components are included in the material
and is defined by the initial boiling point (IBP) and endpoint (EP)
of the distillation curve. A typical BTX cut has an IBP of 75C
(165F) and an EP of 150C (300F). However, many aromatics complexes
tailor the cut of the naphtha to fit their particular processing
requirements. An IBP of 75 to 80C (165 to 175F) maximizes benzene
production by including all the precursors that form benzene in the
reforming unit. Prefractionating the naphtha to an IBP of 100 to
105C (210 to 220F) minimizes the production of benzene by removing
the benzene precursors from the naphtha.
If a UOP Tatoray unit is incorporated into the aromatics
complex, C9 aromatics become a valuable source of additional
xylenes. A heavier naphtha with an EP of 165 to 170C (330 to 340F)
maximizes the C9 aromatic precursors in the feed to the reforming
unit and results in a substantially higher yield of xylenes or
para-xylene from the complex. Without a UOP Tatoray unit, C9
aromatics are a low-value by-product from the aromatics complex
that must be blended into gasoline or fuel oil. In this case, a
naphtha EP of 150 to 155C (300 to 310F) is optimum because it
minimizes the C9 aromatic precursors in the reforming unit feed. If
mixed xylenes are purchased as feedstock for the aromatics complex,
they must be stripped, clay-treated, and rerun prior to being
processed in the Parex-Isomar loop.
CASE STUDY
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An overall material balance for a typical aromatics complex is
shown in Table 2.1.1 along with the properties of the naphtha
feedstock used to prepare the case. The feedstock is a common
straight-run naphtha derived from Arabian Light crude. The
configuration of the aromatics complex for this case is the same as
that shown in Fig. 2.1.4 except that the o-X column has been
omitted from the complex to maximize the production of para-xylene.
The naphtha has been cut at an endpoint of 165C (330F) to include
all the C9 aromatic precursors in the feed to the Platforming
unit.
A summary of the investment cost and utility consumption for
this complex is shown in Table 2.1.2. The estimated erected cost
for the complex assumes construction on a U.S. Gulf Coast site in
1995. The scope of the estimate is limited to equipment inside the
battery limits of each process unit and includes engineering,
procurement, erection of equipment on the site, and the cost of
initial catalyst and chemical inventories. The light-ends
by-product from the aromatics complex has been shown in the overall
material balance. The fuel value of these light ends has not been
credited against the fuel requirement for the complex.
COMMERCIAL EXPERIENCE
UOP is the worlds leading licenser of aromatics technology. By
2002, UOP had licensed nearly 600 separate process units for
aromatics production, including 168 CCR Platformers, 215 extraction
units (Udex,* Sulfolane, Tetra,* and Carom*), 78 Parex units, 6 MX
Sorbex units, 52 Isomar units, 41 Tatoray units, 38 THDA units, and
1 Cyclar unit. UOP has designed over 60 integrated aromatics
complexes, which produce both benzene and para-xylene. These
complexes range in size from 21,000 to 1,200,000 MTA (46 to 2646
million lb) of para-xylene.
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ebooks.lib.unair.ac.id/download.php?id=2868
Benzene (C6H6, boiling point: 80o C, density: 0.8789, flash
point: 11 o C, ignition temperature: 538o C), is a volatile,
colorless, and flammable liquid aromatic hydrocarbon possessing a
distinct, characteristic odor.
Benzene is practically insoluble in water (0.07 part in 100
parts at 22C); and fully miscible with alcohol, ether, and numerous
organic liquids.
For many years benzene (benzol) was made from coal tar, but new
processes that consist of catalytic reforming of naphtha and
hydrodealkylation of toluene are more appropriate. Benzene is a
natural component of petroleum, but it cannot be separated from
crude oil by simple distillation because of azeotrope formation
with various other hydrocarbons. Recovery is more economical if
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the petroleum fraction is subjected to a thermal or catalytic
process that increases the concentration of benzene.
Petroleum-derived benzene is commercially produced by reforming
and separation, thermal or catalytic dealkylation of toluene, and
disproportionation.
Benzene is also obtained from pyrolysis gasoline formed in the
steam cracking of olefins.If benzene is the main product desired, a
narrow light naphtha fraction boiling over the range 70 to 104C is
fed to the reformer, which contains a noble metal catalyst
consisting of, for example, platinum-rhenium on a
high-surface-area alumina support. The reformer operating
conditions and type of feedstock determine the amount of benzene
that can be produced. The benzene product is most often recovered
from the reformate by solvent extraction techniques.
In the platforming process (Fig. 1), the feedstock is usually a
straightrun, thermally cracked, catalytically cracked, or
hydrocracked C6 to 200o C naphtha. The feed is first hydrotreated
to remove sulfur, nitrogen, or oxygen compounds that would foul the
catalyst, and also to remove olefins present in cracked naphthas.
The hydrotreated feed is then mixed with recycled hydrogen and
preheated to 495 to 525o C at pressures of 116 to 725 psi (0.8 to 5
MPa ). Typical hydrogen charge ratios of 4000 to 8000 standard
cubic feet per barrel (scf/bbl) of feed are necessary.
The feed is then passed through a stacked series of three or
four reactors containing the catalyst (platinum chloride or rhenium
chloride supported on silica or silica-alumina). The catalyst
pellets are generally supported on a bed of ceramic spheres.
The product coming out of the reactor consists of excess
hydrogen and a reformate rich in aromatics. The liquid product from
the separator goes to a stabilizer where light hydrocarbons are
removed and sent to a debutanizer.
http://en.wikipedia.org/wiki/Pascal_(unit)
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The debutanized platformate is then sent to a splitter where C8
and C9 aromatics are removed. The platformate splitter overhead,
consisting of benzene, toluene, and nonaromatics, is then solvent
extracted.
Solvents used to extract the benzene include tetramethylene
sulfone (Fig. 2), diethylene glycol, N-methylpyrrolidinone process,
dimethylformamide, liquid sulfur dioxide, and tetraethylene
glycol.
Benzene is also produced by the hydrodemethylation of toluene
under catalytic or thermal conditions.
In the catalytic hydrodealkylation of toluene (Fig. 3):
C6H5CH3 + H2 C6H6 + CH4
toluene is mixed with a hydrogen stream and passed through a
vessel packed with a catalyst, usually supported chromium or
molybdenum oxides, platinum or platinum oxides, on silica or
alumina. The operating temperatures range from 500 to 595o C and
pressures are usually 580 to 870 psi (4 to 6 MPa). The reaction is
exothermic and temperature control (by injection of quench
hydrogen) is necessary at several places along the reaction
sequence. Conversions per pass typically reach 90 percent and
selectivity to benzene is often greater than 95 percent. The
catalytic process occurs at lower temperatures and offers higher
selectivity but requires frequent regeneration of the catalyst.
Products leaving the reactor pass through a separator
where unreacted hydrogen is removed and recycled to the
feed.
Further fractionation separates methane from the benzene
product.
Benzene is also produced by the transalkylation of toluene in
which two molecules of toluene are converted into one molecule of
benzene and one molecule of mixed xylene isomers.
In the process (Fig. 4), toluene and C9 aromatics are mixed with
liquid recycle and recycle hydrogen, heated to 350 to 530o C at 150
to 737 psi (1 to 5 MPa), and charged to a reactor containing a
fixed bed of noble
metal or rare earth catalyst with hydrogen-to-feedstock mole
ratios of 5:1 to 12:1. Following removal of gases, the separator
liquid is freed of light ends and the bottoms are then clay treated
and fractionated to produce high-purity benzene and xylenes. The
yield of benzene and xylene obtained from this procedure is about
92 percent of the theoretical.
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Other sources of benzene include processes for steam cracking
heavy naphtha or light hydrocarbons such as propane or butane to
produce a liquid product (pyrolysis gasoline) rich in aromatics
that contains up to about
65 percent aromatics, about 50 percent of which is benzene.
Benzene can be recovered by solvent extraction and subsequent
distillation.
Benzene can also be recovered from coal tar. The lowest-boiling
fraction of the tar is extracted with caustic soda to remove tar
acids, and the base oil is then distilled and further purified by
hydrodealkylation.
Benzene is used as a chemical intermediate for the production of
many important industrial compounds, such as styrene (polystyrene
and synthetic rubber), phenol (phenolic resins), cyclohexane
(nylon), aniline (dyes), alkylbenzenes (detergents), and
chlorobenzenes. These intermedi-ates, in turn, supply numerous
sectors of the chemical industry producing pharmaceuticals,
specialty chemicals, plastics, resins, dyes, and pesticides.
In the past, benzene has been used in the shoe and garment
industry as a solvent for natural rubber. Benzene has also found
limited application in medicine for the treatment of certain blood
disorders and in veterinary medicine as a disinfectant.
Benzene, along with other light high-octane aromatic
hydrocarbons such as toluene and xylene, is used as a component of
motor gasoline. Benzene is used in the manufacture of styrene,
ethylbenzene, cumene, phenol, cyclohexane, nitrobenzene, and
aniline. It is no longer used in appreciable quantity as a solvent
because of the health hazards associated with it.
Ethylbenzene is made from ethylene and benzene and then
dehydrogenated to styrene, which is polymerized for various
plastics applications. Cumene is manufactured from propylene and
benzene and then made into
phenol and acetone. Cyclohexane, a starting material for some
nylon, is made by hydrogenation of benzene. Nitration of benzene
followed by reduction gives aniline, important in the manufacture
of polyurethanes.
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FIGURE 1 Benzene manufacture by the platforming process.
FIGURE 2 Benzene manufacture by sulfolane extraction.
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FIGURE 3 Benzene manufacture by toluene hydrodealkylation.
FIGURE 4 Benzene manufacture by the transalkylation of
toluene.
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