Styrene ethylbenzene dehydrogenation

In 1869 Berthelot- reported the production of styrene by dehydrogenation of ethylbenzene. This method is the basis of present day commercial methods. Over the year many other methods were developed, such as the decarboxylation of acids, dehydration of alcohols, pyrolysis of acetylene, pyrolysis of hydrocarbons and the chlorination and dehydrogenation of ethylbenzene." ... 

Ethylbenzene is a high volume petrochemical used as the feed stock for the production of styrene via dehydrogenation. Ethylbenzene is currently made by ethylene alkylation of benzene and can be purified to 99.9%. Ethylbenzene and styrene plants are usually built in a single location. There is very little merchant sale of ethylbenzene, and styrene production is about 30x10 t/year. For selective adsorption to be economically competitive on this scale, streams with sufficiently high concentration and volume of ethylbenzene would be required. Hence, although technology has been available for ethylbenzene extraction from mixed xylenes, potential commercial opportunities are limited to niche applications. 

Experimental results on conjugated ethylbenzene dehydrogenation with hydrogen peroxide, shown in Table 4.2 (experiments 1 and 2), indicate that the use of 15% aqueous hydrogen peroxide promotes high yields of styrene for both the missed and the converted ethylbenzene 36.3% and 90%, respectively1. 

In the second series of experiments (Table 4.2, experiments 3-6) on ethylbenzene dehydrogenation by molecular oxygen, styrene yields were much lower than in the first series. On the quartz reactor walls condensation product precipitation in amounts up to 1.7% was observed. Moreover, if molecular hydrogen was absent in the products of conjugated dehydrogenation, the total amount of hydrogen equals 78-82% of the whole volume of gaseous products (2.2% in total products). 

Figure 5. The temperature dependences of benzene (curve 1) and styrene (curve 2) yields at cyclohexane and ethylbenzene dehydrogenation, respectively, on the catalyst, obtained from copolymer carbonisate, containing 7 mass per cent of Re.

Ethylbenzene (EB) is the precursor molecule for production of styrene monomer. It may be synthesized via alkylation of benzene and is also produced as a component of the C8 aromatics fractions obtained from catalytic crackers and reforming units (4). EB is converted to styrene via dehydrogenative or oxidative routes. As much as... 

Fina/Badger Styrene Ethylbenzene Two-stage adiabatic dehydrogenation yields high-purity product 50 2000... 

Styrene is at the centre of an important industry, with a value of some 66 billion euros. The styrene production capacity is ca. 20 Mt/a worldwide. Most is obtained by ethylbenzene dehydrogenation and all the production is used for the synthesis of polymers (polystyrene, styrene-acrylonitrile, styrene-butadiene) used as plastics and rubbers in the manufacture of household products packaging, tubes, tires, and endless other applications (see also Chapter 7). 

The ethylbenzene dehydrogenation (Figure 25) is a reversible endothermic reaction (AH(6oo°c) = 124.9 kJ/mol) that takes place in the gas phase. As one mole of ethylbenzene yields 2 moles of products (styrene and hydrogen), to drive the equilibrium forwards requires the use of as low a pressure as possible. 

The production of styrene by dehydrogenation of ethylbenzene is a good example (121). When rates of reaction are high, short diffusion lengths are required, suggesting structured, thin-layered catalytic reactors. When selectivity is an issue, this is even more the case. 

A fixed-bed reactor often suffers from a substantially small effectiveness factor (e.g., 10 to 10 for a fixed-bed steam reformer according to Soliman et al. [1988]) due to severe diffusional limitations unless very small particles are used. The associated high pressure drop with the use of small particles can be prohibitive. A feasible alternative is to employ a fluidized bed of catalyst powders. The effectiveness factor in the fluidized bed configuration approaches unity. The fluidization system also provides a thermally stable operation without localized hot spots. The large solid (catalyst) surface area for gas contact promotes effective catalytic reactions. For certain reactions such as ethylbenzene dehydrogenation, however, a fluidized bed operation may not be superior to a fixed bed operation. To further improve the efficiency and compactness of a fluidized-bed reactor, a permselective membrane has been introduced by Adris et al. [1991] for steam reforming of methane and Abdalla and Elnashaie [1995] for catalytic dehydrogenation of ethylbenzene to styrene. 

However, when membrane tubes are inserted in the fluidized-bed reactor, hydrogen is continuously removed from the reaction mixture thus, the main reaction of ethylbenzene dehydrogenation continues to move in the direction of forward reaction. The ethylbenzene conversion and the yield of styrene increase as a result of the selective permeation of hydrogen through the membrane. Both the conversion and the yield exceed those of the industrial fixed-bed reactors and fluidized-bed reactors without membranes. When 16 membrane tubes are used, the selectivity to styrene is expected to be almost 100% due to suppression of by-products such as toluene [Abdalla and Elnashaie, 1995]. A high ethylbenzene conversion (96.5%) along with a high styrene yield (92.4%) is possible under properly selected realistic conditions. 

Consider, for instance, ethylbenzene dehydrogenation to styrene. The traditional plant used in the process industry [32] is based on an fixed-bed catalytic reactor to which a preheated mixture of ethylbenzene and steam, which prevents coke formation, is fed. The reaction products then normally undergo a rather complex separation scheme, mostly based on distillation columns, aimed at recovering styrene (the desired product), benzene, toluene and H2 (by products), and a certain amount of unconverted ethylbenzene, which has to be recycled. The overall conversion per pass is typically around 60%, whereas selectivity is close to 90%. 

Large amounts of styrene are commercially produced by dehydrogenation of ethylbenzene (EB) in the presence of steam using iron oxide-based catalysts. Carbon dioxide, small amounts of which are formed as a by-product in the ethylbenzene dehydrogenation, was known to depress the catalytic activity of commercial catalyst [7,8]. However, it has been recently reported that several examples show the positive effect of carbon dioxide in this catalytic reaction [5,9,10]. In this study, we investigated the effect of carbon dioxide in dehydrogenation of ethylbenzene over ZSM-5 zeolite-supported iron oxide catalyst. 

Fig. 18 Typical ethylbenzene dehydrogenation unit for the production of styrene monomer. (View this art in color at h H H . dekker. com.)...

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