Power generation from waste heat: Ionic liquid‐based absorption cycle versus organic Rankine cycle

Abstract

Aspen Plus ® simulations using the Peng-Robinson (PR-EOS) and the COSMO-SAC models were performed to study absorption power cycles (APCs) using mixtures of R-134a with two ionic liquids, [C 2 C 1 im][Tf 2 N] or [C 6 C 1 im][Tf 2 N], and compared against an R-134a organic Rankine cycle (ORC) operating under similar conditions. The PR-EOS results were in agreement with calculations from a PR model fitted to the R-134a + IL experimental phase equilibrium data. The APCs have similar efficiencies and outperform the ORC by 3%-46%, with the largest differences observed when operating with lower grade (lower T H) heating sources, lower quality cooling (higher T L), and lower subcooling in the pump inlet stream. The PR-EOS and the Conductor-like Screening Model Segment Activity Coefficient (COSMO-SAC) results follow similar trends, but numerical discrepancies are observed in the cycle efficiencies and stream flow rates and compositions due to differences in solubilities and enthalpy changes between both models, suggesting that improvements are needed to increase the accuracy of COSMO-SAC for these systems.

Process Simulation and Optimization on Ionic Liquids

“…The use of other refrigerants (water, R-134a, pentane, etc.) in IL-based absorption refrigeration cycles has also been reported in the literature. ,, Again, Yokozeki et al initially published the use of ILs in water absorption refrigeration cycles compared with the traditional water/[Li]­[Br] system and found that some water/IL pairs may compete with the reference, with the [emim]­[MePO 4 ] IL having the highest COP (0.691). The proper IL selection and optimization of the cycle’s operating variables enhanced the performance of the traditional water/[Li]­[Br] system.…”

“…46 NTf 2based ILs have also been evaluated for an R-134a system under different conditions and compared with the traditional organic Rankine cycle (ORC) for cooling production. 111 Without optimization, the ORC achieved better efficiencies than absorption refrigeration cycles using NTf 2 -based ILs. This was especially clear when operating with lower-grade heating sources.…”

“…Thus, an increase in the NH 3 (or refrigerant) concentration (weak IL solution) led to a higher COP and a reduced f ratio, which enhanced the cycle performance , (see Figure ). The enhancement of temperature generator led to lower cycle efficiencies (COPs) even though the f ratio decreased and, thus, the absorption operation was improved. ,,,, Regarding “cold production,” Figure shows COP plotted as functions of the evaporator temperature for different ILs and NH 3 concentrations. Notably, although the four ILs could produce the same evaporator temperatures (up to −6 °C), they obtained different thermodynamic efficiencies and increased the COP while obtaining less “cold production” (up to 9 °C).…”

“…in IL-based absorption refrigeration cycles has also been reported in the literature. 46,111,309 those of previously evaluated ILs and [Li][Br]. Ammonia-and water-refrigerant systems are the most extended industrial absorption refrigeration cycles.…”

“…The multiscale methodology concept emerged as a flexible and multilevel strategy that combined computer-aided product (IL) design and (IL-based process design) simulations to improve the IL features and, thus, key performance indicators (KPIs) within an experimentally validated model that linked the molecular and process scales. This solution was widely addressed by several chemical engineering research groups in many different IL application fields using a wide variety of computational strategies involving differently formulated predictive thermodynamic models, such as the predictive COSMO-SAC/RS ,,− and UNIFAC − methods, and those based on equation of state, such as PRK − and PC-SAFT. ,− …”