In the United States and across the globe, interest is growing in the use of ethanol for fuel, whether as an additive to traditional petroleum gasoline or as a complete substitute for gasoline, to be used in alternative-fuel vehicles. This interest has been driven by growing environmental concerns, such as air quality and global warming, as well as mounting concerns about energy independence and the non-renewability of crude oil.
The Challenges of Ethanol Fuel Production
The ethanol production process involves the fermentation of corn and other grains to produce a “beer,” a slurry of particulate solids, yeast and dissolved solids, ethanol, and trace other metabolic organic chemicals. The resulting beer from the fermenters is pumped into a large holding tank, called the beer well, and then processed to recover ethanol, distillers grains and solubles (DGS), and byproducts such as corn oil.
When dried, the DGS becomes cattle feed, referred to as dried distillers grains and solubles (DDGS); if not dried, it is still cattle feed but has a much shorter shelf life. The beer starts out at approximately 11 to 15 weight percent (wt%) ethanol; to produce fuel-grade ethanol, this must be brought up to at least 99.5 wt%. Standard distillation processes, however, can only generate ethanol with a purity of about 95 wt%.
At 95 wt%, ethanol becomes an azeotrope, a liquid mixture of two or more compounds that exist in the same concentrations in the vapor phase as the liquid phase. At the azeotropic phase, standard distillation is no longer capable of separating the compounds within a liquid mixture. To remedy this, an entrainer — or a dehydrating agent — is added, breaking down the azeotrope by adsorbing water molecules and allowing for further distillation. Requiring large amounts of energy, azeotropic distillation is not particularly cost effective, however, and has long been replaced by adsorption techniques.
The Advent of PVSA
The 1980s saw advancements in adsorption processes and the invention of molecular sieves, materials with molecular-sized pores and tremendous surface areas capable of adsorbing the component molecules of fluids. These advancements allowed for the development of the pressure swing adsorption (PSA) process and pressure vacuum swing adsorption (PVSA). For fuel, ethanol PVSA is the preferred process. Like traditional methods, the PVSA ethanol production process involves a sequence of distillations. At the end of the PVSA process, however, a 3A zeolite — rather than an entrainer — is used to break the azeotropic and recover purified ethanol.
3A zeolites are aluminosilicate minerals comprised of a crystalline compound of sodium, aluminum, silicone, and oxygen. The porous structure may include cations of sodium, potassium, calcium, and magnesium. Including such cations changes the pore size and structural strength of the zeolite crystal. They act as molecular sieves, adsorbing the water molecules from the ethanol azeotrope without excessive adsorption of the ethanol molecules. PVSA is a much more cost- and energy-efficient process than azeotropic distillation.
Improving PVSA Modelling
Despite the many advantages of PVSA, a detailed study analyzing the operation of the PVSA process and its precise performance had yet to be conducted. Most suppliers of such systems use empirical results and past installations to guide design and operation. In an actual implemented process, however, the PVSA operation is inherently transient, so ad hoc submodeling generally falls short of providing precise, true-to-life results.
Thermal Kinetics sought to change this, working with a professor from the University at Buffalo’s Department of Chemical and Biological Engineering and a team of engineers during a five-year R&D program. These efforts led to the design and implementation a unique simulation program of the PVSA process. The modeling of adsorption equilibrium, heat of adsorption, heat transfer dynamics, and kinetics of adsorption, as well as desorption, were used to create detailed PVSA models. The resulting computer simulation program was sufficiently accurate and robust to allow computational experiments to be conducted, eventually leading to advanced and improved methods of operation. To learn more, we invite you to download the full study, “Simulation of Pressure Swing Adsorption in Fuel Ethanol Production Process.”