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.

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.

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

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.

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

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

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: jplotkin@nexant.com or Heidi Junker Coleman, phone: 1-914-609-0381, e-mail address: hcoleman@nexant.com

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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.

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).

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.

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.

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

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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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.

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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.

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.

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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