Separation Process Synthesis for High-GWP Refrigerant Mixtures: Extractive Distillation using Ionic Liquids

Abstract

Due to high global warming potential (GWP) of hydrofluorocarbons (HFCs), the separation and recovery of HFCs from different refrigerant mixtures is an important issue. Most HFC mixtures are azeotropic in nature, thereby rendering the conventional distillation-based separation difficult and energy intensive. Extractive distillation (ED) with ionic liquid (IL) as solvent provides an attractive strategy for selective separation of HFC mixtures. However, systematic design and optimization of ED-based separation processes is nontrivial. In this work, we present SPICE_ED which is a software framework for the detailed design, synthesis, and techno-economic analysis of ED-based separation processes. The framework employs a building block representation followed by superstructure optimization that is able to automatically generate numerous design solutions and screen the best without requiring prior expert knowledge of candidate configurations. For a given IL as solvent and a set of design specifications, one can automatically determine the feasibility of the solvent and obtain the optimal process flowsheets that correspond to minimum energy consumption, minimum separation cost, or minimum emission/waste. We demonstrate the capability of SPICE_ED for the separation of R-410A (50 wt % R-32 and 50 wt % R-125) using [bmim][PF 6 ], a commonly used IL. Our optimized design requires an equivalent work of only 338.2 kJ/kg R-410A, which is about 48% less than the previously reported value of 656 kJ/kg. The newly identified design also achieves more than 47% and 27% reductions in CO 2 -equivalent emission (sustainability) and cost, respectively. Through multiobjective optimization, we further identify an operating regime to separate R-410A at a near-minimum cost without significantly increasing the energy consumption and emission. The processes obtained from SPICE_ED show excellent agreement with the key performance metrics when simulated in Aspen Plus, thereby establishing confidence in our designs as realistic and implementable.

An Overview of Computer‐aided Molecular and Process Design

“…The reason is that intensified equipment can perform multitasking by exploiting the synergy between multiple phenomena. Indicative examples of intensified equipment include task-integrated column [108][109][110], membrane reactor [111], reactive distillation column [112], dividing wall column [113], and extractive distillation [6,114]. To harness the synergy between multiple phenomena and to come up with novel process alternatives, one has to design a processing unit from the fundamental physicochemical level of a chemical process.…”

“…However, if the mixture is azeotropic in nature, then distillation alone may not achieve the required separation. In such a case, one may opt for pressure or concentration-based driving forces using membrane, or temperature-based driving forces using reactive or extractive distillation-based separation schemes [5,6]. The next question is: which class of membrane or solvent should one pick?…”

Understanding the Molecular Features Controlling the Solubility Differences of R-134a, R-1234ze(E), and R-1234yf in 1-Alkyl-3-methylimidazolium Tricyanomethanide Ionic Liquids

“…In this sense, new F-gas legislation established a new paradigm in the RAC sector by including the term “reclamation”, defined as the reprocessing of a fluorinated greenhouse gas recovered during maintenance or prior to disposal, to match the equivalent performance of a virgin substance . At present, different technologies have been proposed for the separation of close-boiling and azeotropic refrigerant mixtures, such as membrane separation, − adsorption on porous materials, − or absorption-based extractive distillation processes. ,− This last application is where ionic liquids (ILs) have attracted the most attention because of their unique properties, among which their nonvolatility and the ability to solubilize large amounts of F-gases stand out, related to the objective of the present study. , …”

What Data Are Most Valuable to Screen Ionic Liquid Entrainers for Hydrofluorocarbon Refrigerant Reuse and Recycling?

“…Ionic liquid (IL) entrainers can effectively separate HFC mixtures, overcoming the energy costs of traditional separation technologies. − Over the past 18 years, IL and HFC systems have been studied in a variety of contexts including the characterization of their individual and mixture physical properties − and their use in extractive distillation, membrane, and adsorption separation processes . More recent work has used data generated from these studies to regress thermodynamic models and then perform process design, optimization, technoeconomic analyses, and life cycle assessments for IL-enabled HFC separation schemes. ,− However, because millions of theoretical ILs are available, each with unique properties, trial-and-error molecular and process design is intractable since each HFC within a refrigerant blend exhibits a different boiling point and solubility with an IL . This necessitates a framework which integrates experiments, mathematical models, and computational optimization to concurrently design ILs and separation processes for azeotropic HFC refrigerant mixtures.…”