Understanding and Reducing the Entropy Production in Membrane Systems From Gas Separation to Natural Systems

Abstract

The purpose of this thesis was to understand how energy is dissipated in various systems, in order to find general rules that can help increasing the energy efficiency of such systems.We mainly focused on membranes for separation of CO2 from natural gas. Indeed, CO2 has to be removed from the gas, because it might cause problems, but also to prevent the release of CO2 into the atmosphere.

A first important step was to gain a better understanding of how the CO2 permeates through the membrane. In particular, we explored the possibility to use a heat source to improve the membrane separation performances. This possibility is quite attractive, since large amounts of “free” waste heat are available at the sites of extraction of natural gas.

A second important step was to identify the operation of membrane systems that dissipates the least energy (i.e. operation that gives the minimum entropy production). By studying the characteristics of the optimal systems, we tried to identify operating and design guidelines that can lead to more energy efficient membrane separation processes.

A second and wider objective of the project aimed at increasing the energy efficiency of engineering designs by gathering knowledge from natural systems that are very energy efficient.

Reindeer are known to be able to survive under very difficult conditions. In winter, temperatures can be as low as -40C, food is scarce, and water is available only in the form of snow. Therefore, they need to lose as little energy as possible, and their special breathing system helps them in that. Further knowledge on this system might guide the development of new and more efficient Nature-inspired processes. As an example, recovery ventilators in buildings have very similar tasks and operate under similar conditions. Thus, knowledge from the first system can be used to improve the second.