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)
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.
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.
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.
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:
In addition to Catalytic Reforming, other current commercial technologies discussed in this Process Evaluation/Research Program (PERP) report include:
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.
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).
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.
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 1,000°C; 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.
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 Chemical’s Alpha Process, BP/UOP’s Cyclar TM Process, CP Chem’s Aromax ® Process, and UOP’s RZ Platforming TM Process.
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:
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.
The costs of production of benzene from various sources employing different technologies – see below - have been developed.
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.
Nexant’s 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.
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
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: firstname.lastname@example.org or Heidi Junker Coleman, phone: 1-914-609-0381, e-mail address: email@example.com
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