New Solvent Systems for Distillation and Absorption Intensification

Solvents with an improved price/performance ratio allow for processes with less operating and/or capital expenditure. Therefore, from an industrial perspective, it is worth reviewing the R&D progress in designing new competitive solvents on a regular basis. This paper aims at discussing the potential of new solvent systems applicable as absorbents or entrainers for boosting the performance of absorption and distillation processes. In this context, special focus will be dedicated to solvents like ionic liquids, amines and hyperbranched polymers. In particular, it will be demonstrated how these solvents can contribute to improve the performance of absorption processes such as absorption chillers and CO2 capture processes as well as distillation processes like the separation of azeotropic mixtures by extractive distillation. To illustrate the potential value of new solvent systems for the distillation and absorption community, the use of ionic liquids for absorption chillers is worth mentioning here. These sorption systems transform latent heat of the phase change liquid-vapor between different temperature levels by means of two working fluids, the refrigerant and the absorbent (working pair). The systems find use in different areas of application such as chiller applications (gas cooling, solar cooling, waste heat cooling), refrigeration, heat pumping and industrial heat recovery. One of the most prominent areas in which these absorption systems are applied is providing chilled water for air-conditioning by H2O/LiBr absorption chillers. In this context, the development has led from single-effect to double-effect systems and from steam-fired to direct-fired generators. The main freedom in designing and improving these kinds of absorption processes lies in the choice of the working pair and in the principle of multistaging. Thus, a crucial fraction of fixed and/or variable costs for absorption chillers are defined by absorbent and refrigerant properties. Although many innovative working pairs have been suggested over the last decades, only two meet the prevailing industrial requirements: (a) H2O/LiBr for water chillers and (b) NH3/H2O for refrigeration. However, these state-of-the-art working pairs show a number of drawbacks, including corrosiveness, a restricted temperature lift due to crystallization (H2O/LiBr), toxicity, high working pressure and/or the need for rectification (NH3/H2O). Especially for multi-effect cycles with their potential for primary energy savings, selected ionic liquid-based absorbents combining a high capacity with adequate solution viscosities, thermal and chemical stability and a moderate corrosiveness at high process temperatures can substantially contribute to improve the performance of absorption chillers. This example indicates the potential of new solvents and underlines why this contribution will be dedicated to new solvent systems for distillation and absorption intensification. Apart from the use of suitable ionic liquids in absorption chillers, new solvent systems for carbon dioxide capture and extractive distillation will be presented and discussed from an industry perspective.