There are three primary sources of aromatics:
- Catalytic reformate
- Pyrolysis gasoline
- Coal tar
The production of aromatics from these sources involves two types of processes: those which separate contained benzene, toluene and xylenes (BTX) and those which convert one form to another. One example of the latter, hydrodealkylation of toluene, is used to adjust the relative proportions of benzene and toluene as demand and economics dictate.While reformate is the major source of aromatics in most of the world, in Western Europe pyrolysis gasoline is also a significant source of total aromatics (47 percent). Both pyrolysis gasoline and reformate are mixtures of aromatic and aliphatic hydrocarbons, with the proportions of each and the composition of the aromatic portion varying according to the processing conditions applied during their production. They differ, however, in the following important respects:
- Pyrolysis gasoline is a by-product from olefins manufacture and its yield and composition are determined primarily by conditions fixed by the needs of the olefins producer. Reformate on the other hand is produced deliberately from naphtha and, within the technical limits available, its production and composition can be set by the needs of the aromatics producer.
- In general, pyrolysis gasoline tends to be richer in total BTX than reformate, although an overlap occurs under extreme conditions.
- Within the BTX portion, pygas is normally much richer in benzene than reformate and considerably less rich in xylenes. This can be seen in the figure below. The benzene content of the pygas for standard and medium cracking severities is well over 40 percent while the benzene content of reformate from continuous (CCR) and semiregenerative (SR) reformers is below 20 percent. On the other hand the mixed xylenes content of the reformate is over 40 percent in each case.
The processes applied for the extraction and subsequent separation of the aromatics contained in the two feedstocks are considerably influenced by this difference in composition, with pygas treatment geared primarily to benzene recovery and reformate treatment geared to the extraction of the whole BTX, with particular emphasis on xylenes. Typical BTX Composition
 Reformers were primarily added to refineries to upgrade naphtha to high octane gasoline blendstocks. The introduction of benzene and aromatics limits in gasoline now limits the use of reformers for the gasoline pool. Consequently, reformate must principally be regarded as a source of toluene and xylenes, particularly the latter. Indeed, reformers associated with chemicals operations are increasingly being regarded as a source of mixed xylenes for para-xylene production, with benzene and toluene credited as co-products. Moreover, the more efficient continuous catalytic reformers (CCR) are increasingly used for this purpose rather than the older semi-regenerative (SR) reformers. From the same feedstock a CCR type of reformer produces 22 percent more benzene, 30 percent more toluene and over 50 percent more mixed xylenes. Pyrolysis gasoline is produced as a by-product of olefins production by the steam cracking of naphtha or gas oil feedstocks. NGL feedstocks do not produce significant amounts of BTX, making the recovery of the aromatics uneconomical in most situations. Pyrolysis gasoline contains a high proportion of aromatics, primarily benzene and toluene, and a smaller amount of mixed xylenes which themselves can contain up to 40 percent ethylbenzene. Pyrolysis gasoline also contains significant quantities of diolefinic materials, which tend to form gum on standing for any period of time, even at ambient conditions. These diolefinic materials are removed (to olefins and paraffins) by a first stage hydrotreatment. A second stage of hydrotreatment is necessary before the pygas is suitable for aromatics extraction. Yields of pyrolysis gasoline are determined by the severity of the cracking operations, and the composition of the feedstocks. Both of these factors are themselves determined by the overall economics of the plant with aromatics production of secondary importance. The effect of increasing severity of cracker operation is to increase the yield of aromatics on feed. As the yield of ethylene is also increasing, initially at a rapid rate in response to higher furnace temperatures, the ratio of aromatics to ethylene falls and then rises. Nonconventional routes to BTX production include Asahi Chemicals' Alpha process, BP/UOP's Cyclar process, CP Chem's Aromax process, and UOP's RZ Platforming process. Benzene production from the Aromax process is significantly more than from the Alpha process (i.e. 65 percent versus 15 percent). The benzene production from the Cyclar and RZ processes fall between these two processes. Compared to a conventional reformate distribution of BTX, the Alpha process produces a mixture of BTX that is most similar to the conventional composition. The Aromax process produces a BTX stream that is high in benzene when compared to the other nonconventional and conventional routes. It is necessary to use a solvent extraction technique to recover BTX products of commercial quality, since aromatics and nonaromatics 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 distillation, which uses a low boiling point solvent with an affinity for nonaromatics. The solvent is distilled overhead with the nonaromatic raffinate.
- Extractive distillation (ED), which employs a high boiling point solvent with an affinity for aromatics. The effect of the aromatics is significant. For benzene the boiling temperature increases by almost 100°F, whereas for cyclohexane the delta is about 13°F. Thus, the nonaromatics can be distilled overhead, whereas the bottoms product will consist of solvent loaded with aromatics. The aromatics can then be separated from the solvent by distillation. ED is normally used when the feed is a heart cut of the appropriate boiling range.
- Liquid/liquid solvent extraction (LLE), which uses solvents that form a separate liquid phase. Aromatics are considerably more soluble than nonaromatics in the solvents employed. The aromatics are extracted from the feed stream in a liquid/liquid contactor, and the extract stream is stripped to recover the aromatic product and the solvent. In this case, the aromatic product would tend to be more contaminated by the light nonaromatics that are preferentially stripped off in the second stage. For liquid/liquid extraction, a low light/heavy selectivity is required (i.e. fewer light nonaromatics pass into the solvent).
In general LLE processes are more capital intense and have higher utility requirements, but produce a higher product yield and are suitable for BTX separation. ED processes have lower capital requirements and lower utility requirements. The ED processes tend to give a lower product yield and are suitable for benzene separation. LLE is normally selected where there is a need for high quality, high yield of toluene, particularly for subsequent conversion. Dealkylation of toluene (THDA) to benzene is the most common dealkylation route. The use of toluene dealkylation is sensitive to the producer's alternative value for toluene (i.e., gasoline or chemicals), the hydrogen value (on-purpose or by-product), as well as the benzene market price. This report presents process details for the commercial processes mentioned above and also discusses developing technologies for BTX production. Production cost estimates are included for reformate via the conventional CCR process as well as the non-conventional Aromax, Cyclar, RZ Platforming, and Alpha processes. Benzene production cost is estimated from the Sulfolane extraction of reformate; from pyrolysis gasoline as a feedstock using solvent extraction, extractive distillation or bulk distillation; and from coke oven light oil via the Litol process. Economic estimates are also provided for the conversion of toluene to benzene by hydrodealkylation and by disproportionation. Market analysis covers the various end uses for benzene, as well as the global demand, supply (including producer capacities) and trade picture by ten world regions. Toluene uses are discussed, and capacities for the United States and Western Europe are detailed. |
