(617cf) Process Intensification of Ethanol Dehydration Under Liquid Phase Conditions

Replacement of petroleum-derived
chemicals with renewable alternatives has potential to improve the
sustainability of the chemical industries, mitigate global CO₂
emissions, and provide a bridge to an economy featuring cost-effective
biofuels.  Because most of the needed infrastructure is already in place, chemical
upgrading of inexpensive fermentation products is especially attractive.  Specifically,
acid-catalyzed dehydration of ethanol to produce ethylene is a commercial
process which has been demonstrated using a variety of acidic catalysts. One
common upgrading catalyst is ZSM-5 zeolite, a three dimensional silica-alumina
microporous framework made of internal channels with Brønsted acid sites. ZSM-5
catalyzed ethanol dehydration is highly selective for the desired ethylene
product at lower operating temperatures relative to Syndol®, the catalyst currently
used commercially for ethanol dehydration. In this work, we have examined
continuous ethanol dehydration in the liquid phase, exploring a region of
thermodynamic phase space that has received scant attention compared to the
vapor phase. 

Operating under liquid phase
conditions has many potential benefits, including manipulation of product
distribution, reduced separations burden, increased process intensity,
decreased coke formation rates, and superior heat management for the highly
endothermic reaction. As seen in Figure 1, the favored is butanol, a potential
drop-in biofuel which is more than twice as valuable as ethylene according to
current commodity prices. On the other hand, ethanol conversion does not suffer
under liquid phase conditions compared to the vapor phase. Because the density
of liquid reaction mixture is more than 100-times greater than the vapor phase
at reaction temperatures, the result is an improvement in process intensity –
that is moles of ethylene produced by gram of catalyst – or nearly 2 orders of
magnitude.  In terms of coke formation, operation under liquid phase conditions
shows a clear reduction in catalyst coking.  Finally, engineering calculations
show superior heat transfer to the scaled-up liquid-phase reactor compared to
the vapor-phase reactor.  Based on this analysis, we conclude that liquid-phase
ethanol upgrading has commercial promise and that liquid-phase reactions in
general deserve increased attention.

Figure 1: Product selectivity from ethanol
dehydration operated under liquid phase conditions at 390^OC