María J. Fernández Torres

Due to the increase in global energy demand, as well as environmental concerns, hydrogen presents itself as a promising energy source. Liquid hydrogen is more suited for long-distance transportation, but hydrogen liquefaction is an energy-intensive process, and many studies have been published proposing more efficient liquefaction cycles. In this study, simple hydrogen liquefaction cycles like Claude, pre-cooled Linde–Hampson (PLH), single mixed refrigerant (SMR), and dual mixed refrigerant (DMR) were assessed regarding the influence of the cycle’s high pressure on energy efficiency, exergy destruction, and its distribution along the equipment. Among the main results, Claude presented the best specific energy consumption (SEC) of 16.47 kWh/kg_LH, followed by DMR with an SEC of 17.30 kWh/kg_LH, SMR with 17.58 kWh/kg_LH, and finally PLH, with an SEC of 45.07 kWh/kg_LH. The exergy efficiency followed the same pattern as the SEC, with Claude having the lowest exergy destruction, followed by DMR and SMR with close exergy destruction, and finally PLH. Nonetheless, although cycles were not optimized in evaluating the effect of increasing the high pressure, which constrains the direct applicability of the result found, especially in the pre-cooled cycles, the analysis provides valuable insights into the sensitivity of cycle performance. The method and its findings provide the basis for further studies, including optimization steps.

Due to the increase in global energy demand, as well as environmental concerns, hydrogen presents itself as a promising energy source. Nevertheless, a hydrogen energy chain depends on its storage and transportation. Liquid hydrogen is more suited for long-distance transportation and is a lower volume-consuming form. Hydrogen liquefaction, however, is an energy-intensive process, and many studies have been published proposing more efficient liquefaction cycles. These studies focus only on the optimum cycle’s operation conditions and do not assess fully the characteristics of the cycles. In this study, simple hydrogen liquefaction cycles were assessed, regarding the influence of the cycle’s high pressure on the energy efficiency, exergy destruction, and its distribution along the equipment. Among the main results, Claude and Precooled Linde-Hampson cycles presented a specific energy consumption (SEC) of 16.47 kWh/kg_LH and 45.07 kWh/kg_LH, respectively.

With over 20 billion gallons of bio-ethanol produced annually, optimization of bio-ethanol production processes is a major priority for sustainability research. Recent research has made great strides toward improving the efficiency of bio-ethanol production through the development of gasoline pre-blending processes which use liquid–liquid phase separation to eliminate excess water with minimal energy input. This paper investigates the effects of process parameters on the performance and efficiency of this class of processes, offering a design basis for engineers developing new processes along with a broader understanding of their potential performance and economic value. Also explored are a range of process modifications capable of improving process performance. It has been found that blending ratio and initial alcohol concentration are the key parameters in determining ethanol recovery, with the number of liquid–liquid contact stages and the temperature also being significant. It has also been shown that temperature-swing decanting can significantly improve alcohol recovery, reducing ethanol losses by as much as 33% in a typical gasoline pre-blending setup.

Transport emissions account for around a quarter of all CO₂ emissions and so, automotive fuels are a key area of research for combating climate change. Widespread adoption of bio-ethanol fuel blends has been the first step in this direction. However, the economic and environmental viability of bio-ethanol usage is hampered by inefficiencies in its production, resulting in a need for significant improvements. The gasoline pre-blending method is a recent advancement that shows great promise in minimizing the separation energy required for bio-ethanol recovery by circumventing full purification and using gasoline’s phase equilibrium properties to eliminate water from a partially purified fermentation product. This paper models all possible gasoline pre-blending processes incorporating ethanol, butanol or both. It is found that butanol is highly suitable for gasoline pre-blending and that high recovery can be achieved even when starting with dilute concentrations. In addition, the presence of even small amounts of butanol is effective at drawing ethanol into the fuel phase, improving the overall alcohol recovery. Energy savings of up to 19% have been shown using a case study while a graphical methodology for preliminary design and synthesis of any gasoline pre-blending process has been developed, allowing for rapid flowsheet development and performance prediction.

Currently, membrane gas separation systems enjoy widespread acceptance in industry as multistage systems are needed to achieve high recovery and high product purity simultaneously, many such configurations are possible. These designs rely on the process engineer's experience and therefore suboptimal configurations are often the result. This article proposes a systematic methodology for obtaining the optimal multistage membrane flow sheet and corresponding operating conditions. The new approach is applied to cross‐flow membrane modules that separate CO₂ from CH₄, for which the optimization of the proposed superstructure has been achieved via a mixed‐integer nonlinear programming model, with the gas processing cost as objective function. The novelty of this work resides in the large number of possible interconnections between each membrane module, the energy recovery from the high pressure outlet stream and allowing for nonisothermal conditions. The results presented in this work comprise the optimal flow sheet and operating conditions of two case studies. © 2017 American Institute of Chemical Engineers AIChE J, 63: 1989–2006, 2017