Department of Chemical Engineering, Northeastern University , Boston, Massachusetts 02115, United States.
Cain Department of Chemical Engineering, Louisiana State University , Baton Rouge, Louisiana 70803, United States.
Langmuir. 2017 Oct 24;33(42):11611-11625. doi: 10.1021/acs.langmuir.7b02003. Epub 2017 Aug 24.
We used computational tools to evaluate three working fluid mixtures for single-effect absorption refrigeration systems, where the generator (desorber) is powered by waste or solar heat. The mixtures studied here resulted from combining a widely used hydrofluorocarbon (HFC) refrigerant, R134a, with three common deep eutectic solvents (DESs) formed by mixing choline chloride (hydrogen bond acceptor, HBA) with urea, glycerol, or ethylene glycol as the hydrogen bond donor (HBD) species. The COSMOtherm/TmoleX software package was used in combination with refrigerant data from NIST/REFPROP, to perform a thermodynamic evaluation of absorption refrigeration cycles using the proposed working fluid mixtures. Afterward, classical MD simulations of the three mixtures were performed to gain insight on these systems at the molecular level. Larger cycle efficiencies are obtained when R134a is combined with choline chloride and ethylene glycol, followed by the system where glycerol is the HBD, and finally that where the HBD is urea. MD simulations indicate that the local density profiles of all species exhibit very sharp variations in systems containing glycerol or urea; furthermore, the Henry's law constants of R134a in these two systems are larger than those observed for the HFC in choline chloride and ethylene glycol, indicating that R134a is more soluble in the latter DES. Interaction energies indicate that the R134a-R134a interactions are weaker in the system where ethylene glycol is the HBD, as compared to in the other DES. Radial distribution functions confirm that in all systems, the DES species do not form strong directional interactions (e.g., hydrogen bonds) with the R134a molecules. Relatively strong interactions are observed between the Cl anions and the hydrogen atoms in R134a; however, the atom-atom interactions between R134a and the cation and HBD species are weaker and do not play a significant role in the solvation of the refrigerant. In all systems, R134a has the largest diffusion coefficients, followed by the HBD, the anion and the cation; the diffusion coefficients are the largest in the systems containing ethylene glycol, followed by those having glycerol and urea. This work is our first step toward our long-term goal of designing and demonstrating optimal working fluid mixtures for use in absorption refrigeration systems. Our results suggest that COSMO-RS can be used to perform a rapid screening of a large number of working fluid mixtures, and select a few candidates for further exploration using molecular simulations and experiments. These latter approaches can be used to refine the accuracy of the COSMO-RS predictions, and to optimize the selection of optimal working fluid mixtures for demonstration in absorption refrigeration systems powered by solar or waste heat sources.
我们使用计算工具评估了三种用于单效吸收式制冷系统的工作流体混合物，其中发生器（解吸器）由废热或太阳能加热。这里研究的混合物是由广泛使用的氢氟碳化合物（HFC）制冷剂 R134a 与三种常见的深共晶溶剂（DES）组合而成，这些 DES 是由氯化胆碱（氢键受体，HBA）与尿素、甘油或乙二醇混合而成，作为氢键供体（HBD）物质。使用 COSMOtherm/TmoleX 软件包结合 NIST/REFPROP 中的制冷剂数据，对使用建议工作流体混合物的吸收式制冷循环进行热力学评估。之后，对三种混合物进行了经典的 MD 模拟，以在分子水平上深入了解这些系统。当 R134a 与氯化胆碱和乙二醇结合时，循环效率更高，其次是甘油作为 HBD 的系统，最后是 HBD 为尿素的系统。MD 模拟表明，在含有甘油或尿素的系统中，所有物质的局部密度分布都有非常明显的变化；此外，这两个系统中 R134a 的亨利定律常数大于在氯化胆碱和乙二醇中观察到的 HFC 值，表明 R134a 在后者的 DES 中更易溶解。相互作用能表明，与其他 DES 相比，在 HBD 为乙二醇的系统中，R134a-R134a 相互作用较弱。径向分布函数证实，在所有系统中，DES 物质与 R134a 分子之间没有形成强的方向性相互作用（例如氢键）。在所有系统中，观察到 Cl 阴离子和 R134a 中的氢原子之间存在相对较强的相互作用；然而，R134a 与阳离子和 HBD 物质之间的原子-原子相互作用较弱，并且在制冷剂的溶解中不起重要作用。在所有系统中，R134a 的扩散系数最大，其次是 HBD、阴离子和阳离子；在含有乙二醇的系统中，扩散系数最大，其次是含有甘油和尿素的系统。这项工作是我们朝着设计和演示用于吸收式制冷系统的最佳工作流体混合物这一长期目标迈出的第一步。我们的结果表明，COSMO-RS 可用于对大量工作流体混合物进行快速筛选，并使用分子模拟和实验选择少数候选物进行进一步探索。这些后续方法可用于提高 COSMO-RS 预测的准确性，并优化用于太阳能或废热源驱动的吸收式制冷系统的最佳工作流体混合物的选择。
