Chapter 7 Predicting the Thermodynamic Properties of Hydrofluoroethers and Fluorinated Mixtures with
7.1 Introduction and Background
Fluorinated molecules have received significant interest in recent years due to their wide ranging potential industrial and research applications.36,216,217 For example, perfluoroalkanes, have increased solubility in carbon dioxide compared to their hydrocarbon equivalents,37,38 giving them promising applications in removing carbon dioxide from gaseous effluents and in the design of carbon dioxide-philic surfactants. Other fluorinated molecules such as hydrofluoroethers (HFEs) also present exciting applications when mixed with carbon dioxide, notably, as a solution to replacing current industrial refrigerants, such as hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs), which are being phased out due to environmental concerns. HFEs are an ideal greener replacement for current refrigerants due to their lower global warming potential, zero ozone depletion potential, and short atmospheric lifetimes.39,40,230,238 However, due to their toxicity and flammability, HFEs alone cannot be used as refrigerants. Mixing HFEs with carbon dioxide has been shown to allow their beneficial environmental properties to remain, while helping eliminate their negative toxicity properties43,45,47,50.
Due to the high interest in binary mixtures of perfluoroalkanes and carbon dioxide, there have been multiple experimental and theoretical studies performed on these systems.
Perfluorohexane with carbon dioxide has been most widely considered both experimentally42,252,273 and using theoretical methods such as the Statistical Associating Fluid Theory,48 likely due to its high thermal stability. Limited experimental studies for perfluoroheptane43,274 and perfluororooctane41 with carbon dioxide have also been published, along with some theoretical
studies for perfluoroheptane and perfluorooctane using SAFT48,41. Published work examining perfluorononane with carbon dioxide has been limited to experimental and correlative studies using methods such as the Peng-Robinson equation and the non-random two liquid (NRTL)43 approach. However, none of the studies are comprehensive, including data from all available mixtures of perfluoroalkanes with carbon dioxide and a majority of the methods fit parameters to each molecule individually and are thus not predictive, making it difficult to expand on the current data. Therefore, a comprehensive overview of all the available experimental data for perfluoroalkane and carbon dioxide mixtures and a method to predict their phase behavior and expand upon the available data is needed.
Similarly, refrigerants and alternative refrigerants have been widely studied, but most of the studies have focused on HFCs238,275–280 and hydrofluoroolefins (HFOs)221,242,281,282. Though the experimental and theoretical studies for HFCs are quite extensive, HFCs were phased out as industrial refrigerants by 2020, making HFCs of less interest in more recent years. Due to this, HFOs and HFEs have been of higher interest, largely driven by their use as possible alternative refrigerants to the current options283,284. HFOs have been more extensively studied by theoretical approaches as there is a larger set of experimental data against which to compare, in comparison to HFEs. Therefore, many of the theoretical HFE studies are limited due to the lack of experimental data to fit to and primarily focus on correlated data for a small group of HFEs, i.e., less than 5 HFEs at a time19,46,98,220,225,285–288. Though the agreement between the theoretical calculations and experimental data for the systems studied are reasonable, the parameters were adjusted to mixture data. Utilizing a model that could predict the thermodynamic properties of HFEs without having to fit to large sets of experimental data would greatly assist in the industrial adoption of HFEs.
Finally, there have been some studies for mixtures of HFEs with other molecules44,46,220,266,289,
however these studies center around the properties at atmospheric pressure, likely due to the difficulty of experimentally studying HFEs at high temperatures and pressures, making the higher temperature and pressure regime an area of interest, as little is currently known.
Herein a method to accurately represent pure fluid properties of HFEs utilizing the group contribution statistical associating fluid theory for potentials of variable range (GC-SAFT-VR) has been developed and their mixtures with carbon dioxide and some alcohols studied. GC-SAFT-VR is also used to study mixtures of perfluoroalkanes and carbon dioxide, including perfluorohexane, perfluoroheptane, perfluorooctane, and perfluorononane. While previous work utilized GC-SAFT- VR to study perfluorohexane and carbon dioxide,47 other perfluoroalkanes with carbon dioxide were not considered.
GC-SAFT-VR combines SAFT-VR99 with a group contribution22 approach. This allows molecules to be built up from segments that have their own size and energy parameters and represent the different functional groups within each molecule. The chain retains the heterogeneity of the groups which allows for the location of association sites to be assigned to specific groups, which is not typically seen in other SAFT-based GC methods. The benefit of describing molecules by the functional groups that they are comprised is that parameters for each functional group and their cross interactions can typically be developed from a limited set of pure component data and used transferably to study other molecules containing the same functional groups, allowing the GC-SAFT-VR to be a highly predictive model. The only exception to determining model parameters from pure component data only, is for functional groups that are not contained within any pure components, such as carbon dioxide. Therefore, any cross interactions with carbon dioxide have to be fit to mixture data; however, this is typically done for one mixture at a single
temperature and used to describe the interactions with CO2 in other systems of interest and at other state points in order to maintain the predictive power of the model.
GC-SAFT-VR parameters for a wide variety of functional groups that describe molecules including alkanes, alcohols, ketones, aromatics, esters, ethers and additional fluids, both associating and non-associating are available from previous work.10,11,22,47,121,133 As a large set of functional groups have already been parameterized, here we introduce one new functional group to describe HFEs, the OCF2 group. Additionally, we adjust the cross interaction between fluorinated molecules with carbon dioxide to more accurately represent mixtures of carbon dioxide and fluorinated molecules. All of the HFEs considered in this work are listed in Table 1 and have been numbered for ease of identification.