Xylenes production figures

The process consists of a reactor section, continuous catalyst regeneration unit (CCR), and product recovery section. Stacked radial-flow reactors are used to minimize pressure drop and to facilitate catalyst recirculation to and from the CCR. The reactor feed consists solely of LPG plus the recycle of unconverted feed components no hydrogen is recycled. The liquid product contains about 92 wt% benzene, toluene, and xylenes (BTX) (Figure 6-7), with a balance of Cg aromatics and a low nonaromatic content. Therefore, the product could be used directly for the recovery of benzene by fractional distillation (without the extraction step needed in catalytic reforming). 

The raffinate and extract streams leave the UOP Parex unit adsorption section via the UOP rotary valve and are respectively routed to the raffinate and extract columns for separation of the mixed xylene components from the PDEB as shown in Figure 7.4. Because the desorbent has a higher boiling point than the mixed xylenes, the desorbent exits the bottom of the distillation column and is pumped back to the adsorbent chamber section. The mixed xylene raffinate stream is taken as a side cut from the raffinate column to remove water. The extract p-xylene stream is taken from the top of the extract column and routed to a finishing column where any toluene that was in the Parex feed is removed. The p-xylene product exits the bottom of the finishing column. The adsorbent has some selectivity for toluene as well as p-xylene. 

The industrial production of m-xylene is very similar to that of p-xylene. In fact, most of the production of m-xylene is done in facilities where a much larger quantity of p-xylene is produced. Figure 7.5 is a typical flow diagram for an aromatics complex where m-xylene is produced. It is quite like the flow diagram for the production of p-xylene except that a fraction of the Parex unit raffinate, containing typically over 60% m-xylene, is used as fresh feed to the MX Sorbex unit for m-xylene extraction. Because the required m-xylene production is typically much lower than that of p-xylene and the MX Sorbex fresh feed stream is three times more concentrated than the Parex unit fresh feed stream, the feed stream to the... 

Another thing to consider when designing a reaction is the conditions. For example, are you promoting the formation of the kinetic or thermodynamic product Figure 8-5 illustrates this concern. Figure 8-6 illustrates the arrangement of one of the kinetic products (o-xylene) to the thermodynamic product (m-xylene). 

A relatively small number of chemicals form the basis of the petrochemical industry. These are methane, ethylene, propylene, butylenes, benzene, toluene, and xylenes. These chemicals are used to derive thousands of other chemicals that are used to produce countless products. Figure 19.2 lists some of the principal chemicals and products derived from these seven basic chemicals. 

Figure 4.11 shows an example of how ZSM-5 is applied as a catalyst for xylene production. The zeolite has two channel types - vertical and horizontal - which form a zigzag 3D connected structure [62,63]. Methanol and toluene react in the presence of the Bronsted acid sites, giving a mixture of xylenes inside the zeolite cages. However, while benzene, toluene, and p-xylene can easily diffuse in and out of the channels, the bulkier m- and o-xylene remain trapped inside the cages, and eventually isomerize (the disproportionation of o-xylene to trimethylbenzene and toluene involves a bulky biaryl transition structure, which does not fit in the zeolite cage). For more information on zeolite studies using computer simulations, see Chapter 6. 

Upon heating, the loaded HZSM5 catalyst in a gas stream containing both reactants, the coadsorption complex was observed up to temperatures of 453 K. The decrease in its concentration occurred in parallel with the appearance of the first reaction products (xylenes)(see Figure 3). Thus, we concluded that it is likely to be a possible precursor to the transition state in the methylation reaction. This is supported by the fact that under reaction conditions the rate of methylation of toluene was found to be directly proportional to the surface concentration of the activated methanol species [23,10]. We think that during the reaction only a small concentration of the bimolecular complex exists which can not be monitored by IR spectroscopy. Its abundance should, however, depend upon the concentration of chemisorbed methanol. 

The Bureau of Mines is a source of many chemical statistics. The monthly Coke and Coal Chemicals report, part of the bureau s Mineral Industry Surveys, contains, in addition to data on oven and beehive coke production, figures on production of ammonium sulfate, ammonia liquor, naphthalene, benzene, toluene, xylene, solvent naphtha, pyridine, crude coal tar, and cresote oil. Sales and end-of-month stock figures are also shown in the report. A useful feature of the report is the year-end supplement which gives year s totals by months. 

The growth in consumption of p- and o-xylene has been particularly spectacular, as can be seen from U.S. production figures shown in Figure... 

In 1968 U.S. production of p- and o-xylene was estimated at 1.3 billion and 0.97 billion pounds per year respectively. Production figures for m-xylene have never been published by the Tariff Commission. Its use has, however, remained quite small in relation to p- and o-xylene. It is expected that domestic demand for both p- and o-xylene will continue to increase. Terephthalic acid is the key component required for production of polyester film and fibers and is presently produced only from p-xylene. Phthalic anhydride is produced from both naphthalene and o-xylene. Although o-xylene is not expected to replace naphthalene entirely, its use for phthalic anhydride manufacture is expected to increase. 

Pressure crystallizer 2 Vacuum crystallizer 3 Mixing vessel 4 Melting vessel Figure 4.17 Flow diagram for p-xylene production by crystallization using the Chevron process... 

The high cost of equipment, low temperature levels, high energy consumption and limited yield of the crystallization processes has led to an increase in importance for adsorptive p-xylene production in recent years. The Parex process, first introduced by UOP in 1971, is currently the most common method of this type (Figure 4.21). 

Figure 4.20 Flow diagram of p-xylene production by countercurrent crystallization using the Phillips process...

One example of producf selectivity in a zeolite system is catalysed xylene formation from toluene and methanol. An electrophilic substitution of the aromatic ring occurs within the pores of the zeolite ZSM-5, which gives a crude mixture of the three xylene isomers ortho, meta and para). Due to the narrow, linear channels within this zeolite, it is only the para-isomer that is able to diffuse out of the zeolite and be isolated (Figure 4.4). The other ortho- and mefa-isomers can diffuse through the pores but at much slower rates (14 and 1000 times slower, respectively). Due to these slow rates of diffusion, it is more likely that the undesirable isomers will remain within the zeolite long enough to isomerise to the p-xylene product. 

The principal chemical uses of BTX are illustrated in Figure 1 and Hsted in Table 1 (2). A very wide range of consumer products from solvents to fibers, films, and plastics are based on BTX. The consumption of BTX is approximately in the proportions of 67 5 28, respectively. However, no BTX process gives BTX in these proportions. The economic value of benzene and xylenes (especially -xylene) is normally higher than that of toluene. Because of this, processes that convert toluene to benzene by hydrodealkylation (3) and disproportionate toluene to benzene and xylenes (4) have been commercialized. In addition, reforming processes that emphasize production of either benzene or -xylene [106 2-3] have been described (5). Since these are not classified as BTX processes they are not discussed in detail here. 

Performance information for the purification of p-xylene indicates that nearly 100 percent of the ciystals in the feed stream are removed as produc t. This suggests that the liquid which is refluxed from the melting section is effectively refrozen oy the countercurrent stream of subcooled crystals. A high-meltingproduct of 99.0 to 99.8 weight percent p-xylene has been obtained from a 65 weight percent p-xyfene feed. The major impurity was m-xylene. Figure 22-12 illustrates the column-cross-section-area-capacity relationship for various product purities. 

The main product, benzene, is represented by solute (B), and the high boiling aromatics are represented by solute (C) (toluene and xylenes). The analysis of the products they obtained are shown in Figure 12. The material stripped form the top section (section (1)) is seen to contain the alkanes, alkenes and naphthenes and very little benzene. The product stripped from the center section appears to be virtually pure benzene. The product from section (3) contained toluene, the xylenes and thiophen which elutes close to benzene. The thiophen, however, was only eliminated at the expense of some loss of benzene to the lower stripping section. Although the system works well it proved experimentally difficult to set up and maintain under constant operating conditions. The problems arose largely from the need to adjust the pressures that must prevent cross-flow. The system as described would be virtually impossible to operate with a liquid mobile phase. 

Simple aromatic hydrocarbons come from two main sources coal and petroleum. Coal is an enormously complex mixture made up primarily of large arrays of benzene-like rings joined together. Thermal breakdown of coal occurs when it is heated to 1000 °C in the absence of air, and a mixture of volatile products called coal for boils off. Fractional distillation of coal tar yields benzene, toluene, xylene (dimethylbenzene), naphthalene, and a host of other aromatic compounds (Figure 15.1). 

In Figure 3 the merits of the two processes for p-xylene oxidation are compared. The main disadvantages of the Eastman Kodak/Toray cooxidation method are the need for a cosubstrate (acetaldehyde of methylethylketone) with concomitant formation of a coproduct (0.21 ton of acetic acid per ton product) and high catalyst concentration. The Amoco MC process, on the other hand, has no coproduct and much lower catalyst concentrations but has the disadvantage that the bromide-containing reaction mixture is highly corrosive, necessitating the use of a titanium-lined reactor. 

Since their development in 1974 ZSM-5 zeolites have had considerable commercial success. ZSM-5 has a 10-membered ring-pore aperture of 0.55 nm (hence the 5 in ZSM-5), which is an ideal dimension for carrying out selective transformations on small aromatic substrates. Being the feedstock for PET, / -xylene is the most useful of the xylene isomers. The Bronsted acid form of ZSM-5, H-ZSM-5, is used to produce p-xylene selectively through toluene alkylation with methanol, xylene isomerization and toluene disproportionation (Figure 4.4). This is an example of a product selective reaction in which the reactant (toluene) is small enough to enter the pore but some of the initial products formed (o and w-xylene) are too large to diffuse rapidly out of the pore. /7-Xylene can, however. 

Figure 5.76. The fixed-bed reactor and the reaction scheme. Symbols A = o-xylene, B phthalic anhydride, C = waste gaseous products (CO2 and CO).

When ethylene is reacted at 573 K in the presence of water in static conditions, oligomerization and conjunct polymerization give rise to paraffinic, olefinic and aromatic products (8). Nevertheless, the distribution of the aliphatics and aromatics is quite different from that of the steam-cracking products. In the former a great variety of products is formed they include propane, n-butane, isobutane and isopentane as aliphatics, and toluene, xylenes and ethylbenzene as aromatics (Figure 6B). 

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