Dehydrogenation ethane

Membrane Reactor. Another area of current activity uses membranes in ethane dehydrogenation to shift the ethane to ethylene equiUbrium. The use of membranes is not new, and has been used in many separation processes. However, these membranes, which are mostly biomembranes, are not suitable for dehydrogenation reactions that require high temperatures. Technology has improved to produce ceramic and other inorganic (90) membranes that can be used at high temperatures (600°C and above). In addition, the suitable catalysts can be coated without blocking the pores of the membrane. Therefore, catalyst-coated membranes can be used for reaction and separation. 

As an example the use of ceramic membranes for ethane dehydrogenation has been discussed (91). The constmction of a commercial reactor, however, is difficult, and a sweep gas is requited to shift the product composition away from equiUbrium values. The achievable conversion also depends on the permeabihty of the membrane. Figure 7 shows the equiUbrium conversion and the conversion that can be obtained from a membrane reactor by selectively removing 80% of the hydrogen produced. Another way to use membranes is only for separation and not for reaction. In this method, a conventional, multiple, fixed-bed catalytic reactor is used for the dehydrogenation. After each bed, the hydrogen is partially separated using membranes to shift the equihbrium. Since separation is independent of reaction, reaction temperature can be optimized for superior performance. Both concepts have been proven in bench-scale units, but are yet to be demonstrated in commercial reactors. 

Dehydrogenation. The dehydrogenation of paraffins is equihbrium-limited and hence requites high temperatures. Using this approach and conventional separation methods, both Houdry and UOP have commercialized the dehydrogenation of propane to propylene (92). A similar concept is possible for ethane dehydrogenation, but an economically attractive commercial reactor has not been built. 

Oxydehydrogenation. Because of the limitations of ethane dehydrogenation equihbrium, research has focused on ways to remove one of the products, namely hydrogen, by chemical methods. In this way, hydrogen is oxidized to water and there is no equihbrium limitation. 

The importance of an energized reaction complex in bimolecular reactions is illustrated by considering in more detail the termination step in the ethane dehydrogenation mechanism of Section 6.1.2 ... 

Carbon monoxide oxidation, ethane dehydrogenation, ethane hydrogenolysis, ethene hydrogenation. Pt, Mg, Zn catalysts placed either in the pores of the membrane or at the entrance of the membrane pores. 

Another route to ethylbenzene is available for those remote places where olefin plants or refinery crackers are not nearby but a supply of ethane is— catalytic dehydrogenation of ethane to ethylene followed by its reaction with benzene to produce EB. The first of two steps in Figure 8-4 use a gallium zinc zeolyte catalyst that promotes ethane dehydrogenation to ethylerie at 86% selectivity and up to 50% conversion per pass. 

The low conversion rates for both the ethane dehydrogenation and the ethylene-to-EB steps result in high capital costs for a world-scale plant. That limits the potential application of this process to boutique sites. 

You wish to design a plant to produce 100 tons/day of ethylene glycol from ethane, air, and water. The plant has three reactor stages, ethane dehydrogenation, ethylene oxidation, and ethylene oxide hydration. 

This process is much faster than the one depicted in Eq. (2.28). Since aromatization of ethylene is exothermic and ethane dehydrogenation and overall aromatization are endothermic, the coupling of the two processes would be energy-efficient. 

In 1998 scientists at Hoechst reported that the addition of Pd to the MoVNb ethane dehydrogenation catalyst enabled the efficient production of acetic acid from ethane [7]. Doping of this known ethane dehydrogenation catalyst with Pd was probably not random, but predicted on the basis of the classical Wacker catalysis. 

Champagnie, A.M., T.T. Tsotsis, R.G. Minet and I.A. Webster, 1990b, Studies of ethane dehydrogenation in a ceramic membrane reactor, presented at Int. Congr. Membr. Membr. Proc., Chicago, IL, USA. 

The macroscopic mass balance model by Tsotsis et al. [1992], when applied to the reaction of ethane dehydrogenation, compares well with experimental data and both show higher conversions than the corresponding equilibrium values based on either tube-or shell-side conditions (pressure and temperature). This is clearly a result of the equilibrium displacement due to the permselective membrane. The conversion, as expected, increases with increasing temperature. 

PF/PF Isothermal Tube side Ethane dehydrogenation Tsotsis et al., 1993a... 

Champagnie et al. [1992] adopted the aforementioned model to describe the performance of an isothermal shell-and-tube membrane reactor for ethane dehydrogenation in a co-current flow mode. Using Equation (10-8la) to represent the reaction kinetics and assuming no reactions and pressure drops on both the tube and shell sides, they were... 

In the ethane dehydrogenation process, the two unknown flow rates will be determined from balances on atomic carbon and atomic hydrogen. 

E. Gobina and R. Hughes, Ethane dehydrogenation using a high>temperature catalytic membrane reactor, J. Membr. ScL 90 11 (1994). 

Ethane dehydrogenation/ Pt-deposited asymmetric 7-AI2O3 membrane... 

Fig. 3 shows the temperature dependence of ethane dehydrogenation in the presence or absence of CO2. Ethylene yield in uncatalyzed runs did not depend on the atmosphere. Over oxidized diamond-supported Cr203 catalyst, in the presence of CO2, the ethylene yield increased linearly with an increase of the reaction temperature, and reached 40% at 700 °C. 

The effect of the partial pressure of carbon dioxide on the ethane dehydrogenation over oxidized diamond-supported Cr203 catalyst was examined. The results are shown in Fig. 4. Ethane conversion and ethylene yield markedly increased at a low partial pressure of CO2, and increased with increasing partial pressure. However, the ethylene selectivity slightly decreased with increasing partial pressure. The following reaction with CO2, in the dehydrogenation of ethane, would be possible ... 

The kinetic aspect common to all the topics discussed in this chapter is the pyrolysis reactions. The same kinetic approach and similar lumping techniques are conveniently applied moving from the simpler system of ethane dehydrogenation to produce ethylene, up to the coke formation in delayed coking processes or to soot formation in combustion environments. The principles of reliable kinetic models are then presented to simulate pyrolysis of hydrocarbon mixtures in gas and condensed phase. The thermal degradation of plastics is a further example of these kinetic schemes. Furthermore, mechanistic models are also available for the formation and progressive evolution of both carbon deposits in pyrolysis units and soot particles in diffusion flames. 

---