(7a) Exergetic Efficient Distillation in Microchannel Architecture

The technology of separating liquid-liquid mixtures via distillation has been practiced for literally thousands of years, yet it remains a highly energy-intensive, although ubiquitous unit operation for performing chemical separations. Modern distillation consumes over 5 quadrillion BTUs annually, in the U.S. alone. Over the past several decades, new distillation column, tray, and packing designs have yielded incrementally improved efficiency, while larger improvements have remained elusive. Microchannel architecture permits a radically new approach to distillation; one that has the potential to provide a step change improvement in energy consumption as well as lower capital costs and other process advantages.

Exergy is the maximum theoretical amount of work that can be extracted from a physical system by exchanging matter and energy with large reservoirs in a reference state. The exergetic efficiency of a process is an objective performance measure of a real system as compared to a reversible system with no lost work. While energy is conserved, exergy can be destroyed. Using this definition and understanding that distillation consists of heat transfer for vaporizing and condensing, and mass transfer for the separation of the mixture; a distillation unit can be described as an exergy converter where thermal exergy is converted into chemical exergy. Exergetic inefficiencies in conventional distillation result from both technological and capital cost constraints. For example, increased efficiency in conventional distillation towers can be gained by incorporating side reboilers at multiple stages, which utilize waste heat mediums at temperatures lower than the bottom reboiler heat medium. However, the capital cost of adding multiple reboilers and any associated equipment to conventional distillation towers can make this option cost prohibitive. During times of cheap energy, this trade-off between energy cost and capital cost almost always favors the solution with lower capital cost.

Microchannel distillation technology enhances mass transfer by contacting thin vapor and liquid films in very small channels, dramatically reducing the Height-to-an-Equivalent Theoretical Plate (HETP), from more than 12 inches to less than 2 inches. Microchannel architecture also allows temperature profiles to be tightly controlled along the length of the channels using adjacent heat exchange channels integrated in the separation hardware. Enhancements in both mass transfer and heat integration enable dramatically improved performance and new process flow sheet options which were previously seen as commercially unattractive. Microchannel technology is scaled to commercial plant capacities by increasing the numbers of parallel channels conducting identical processes. Techniques for design and fabrication of scaled-up microchannel processing hardware have been developed by Velocys for a range of large-scale micro reactor applications.

One commercially important distillation application is the C2 splitter, which separates ethane from ethylene. In this application alone, the microchannel distillation process could reduce energy consumption by up to 20% for the entire process, which translates to over 40 trillion BTU/year by 2020, in the U.S. alone. This presentation will cover the technical approach, benefits, and recent accomplishments in the development of the microchannel distillation process, with a specific focus on the C2 splitter.