Estimation of Freezing Point of Ternary Coolant Mixture

Aqueous glycol solutions are widely used as coolant in secondary refrigeration systems. Freezing point is one of the crucial properties used to characterize the performance of coolant. Instrumental methods are often complex and require expensive equipment. For selection of right coolant mixture there was a need for systematic study of aqueous-glycol-salt solution over a wide range of composition. The research work in this paper presents a novel and rapid way for freezing point characterization oftheternarycoolant mixture.The freezing behavior of ternary mixture was studied for different blends of ethylene glycol (EG), propylene glycol (PG), sodium chloride (NaCl) and water. Two sets of ternary mixture TM1 (EG-NaCl-water) and TM2 (PG-NaCl-water) were evaluated experimentally at various concentrations of glycol and NaCl. Effect of glycol and salt concentration on freezing point depression was analyzed. The results demonstrated that ternary mixtures required less EG/PG for cost effective formulation of secondary coolant for different cooling application in dairy and food processing industry.


Introduction
The temperature at which the first ice crystal can form and grow is the initial freezing temperature of a mixture (Stefl and George, 2000). The freezing point of water drops when a solute is added to it. Freezing point depression is the term used to describe the difference between the freezing temperature of pure water and that of a solution (Lamas et al., 2022). One of the most crucial measurements to determine the eutectic point for various aqueous solutions depends on the freezing point. The eutectic point of the aqueous is the lowest temperature at which a liquid solidifies completely under normal atmospheric pressure, therefore antifreeze solutions have a freezing range rather than a single freezing point (Ibrahim et al., 2019). When one or more compounds functioning as solutes are mixed with one or more solvents in such amounts as to yield lower freezing point of the mixture, the resulting aqueous solution is called as a eutectic mixture (Echlin, 2013). According to several researchers, the freezing point of the solution is the primary factor that directly affects the effectiveness of the cooling process and other significant parameters can also be determined using the freezing point (Auledaet al., 2011).
Only pure water can create a solid phase. Antifreeze liquids don't become solid at temperatures below their freezing point. Antifreeze chemicals create ice crystals in their solution and turn into slush when exposed to temperatures 10 °C degrees below their freezing point. At very low temperatures more ice crystals will form, and the antifreeze substance will turn into a stiff mush rather than a crystalline solid. As freezing proceeds the water portion decreases while the concentration of remaining antifreeze increases and the freezing point depression is increased further (Knight and DeVries, 2009).
There are different methods for determination of freezing point like cooling curve method, extrapolation method, cryoscope, and differential scanning calorimetry (DSC). The most widely employed instruments for freezing point determination are DSC, osmometer and cryoscope. The benefits of DSC include its quick and easy operation and the ability to derive useful information from thermograms. However, because these tools are expensive and very sophisticated, they cannot always be used to determine the freezing point. The DSC method's primary drawback is that it cannot precisely pinpoint the freezing point as shown by the cooling curve method (Rahmanet al., 2009). DSC analysis requires a small sample size (10-15 ml) and therefore it is challenging to generate a representative sample for a multi-component, heterogeneous, and complicated mixture (Rahman et al., 2002;Boonsupthip and Heldman, 2007). To produce meaningful outcomes, the tools require expertise. Therefore, it is imperative to create a quick and precise procedure for determining the freezing point.
Freezing point is one of the important properties of the secondary coolant. Therefore its appropriate determination has a vital role in the cooling application (Bainy, 2015). There are many literatures available on different aqueous-glycol-salt solution. But these are mostly isolated studies and do not provide wide range of data. For selection of right coolant mixture there was a need for systematic study of aqueous-glycol-salt solution over a wide range of composition. A novel and rapid four step methods has also been proposed to determine freezing point from the experimental data. In this study, the freezing behavior was studied for two set of ternary mixture TM1 (EG-NaCl-water) and TM2 (PG-NaCl-water) at various concentrations of glycol and NaCl.

Formulation of Ternary Mixture
There are basically two different types of antifreeze i.e. salt which is often soluble only to a certain amount and liquid that is soluble over the entire range of concentration with water. The freezing point of mixture will drop when antifreeze chemical is added to a water-based fluid as an additive (Stefl and George, 2000). In this study NaCl was used as salt component. For liquid antifreeze component ethylene glycol (EG) and propylene glycol (PG) were used. Analytical grade chemicals EG (Cat no. 47471, Sisco Research Laboratories Pvt. Ltd., India) and PG (Cat no. 95961, Sisco Research Laboratories Pvt. Ltd., India) were procured from authorized chemical suppliers. Different compounds viz. NaCl, EG and PG at different concentration were used along with reverse osmosis (RO) water for the formulation of ternary mixture.The objective of developing ternary component coolant was for cooling application in dairy and food processing industry.

Measurement of Cooling Temperature Profile
The 100 ml of ternary mixture was prepared by mixing salt, liquid component antifreeze chemical, and RO water using magnetic stirrer. Prepared ternary mixture was poured into temperature monitoring module (TMM) fitted with pt 100 type temperature sensor. The sensors were interfaced to data logger (Model: CT708U, Countronics, India) using 3 wire connection. TMM was kept in a deep freezer maintained at -20 °C for 12 hours. The cooling temperature profile was recorded using the data logger. The temperature data collected in form of CSV file format was imported into MS-Excel software as raw data.
Estimation of freezing point using four step method Cooling curve or temperature vs. time plot was drawn using raw temperature data in MS-Excel. The plot was used to visualize the freezing characteristics. Temperature spike was observed at the commencement of ice formation due to the liberation of heat of fusion. This spike in the curve helps to determine the initial freezing point. The initial or equilibrium freezing point is defined as the maximum temperature after ice nucleation (Rahman et al., 2010).
For analysis, two segments were identified on the freezing curve (Fig. 1). Segment A denotesliquid being cooled to the freezing point. In segment B,phase change occurs. As freezing process proceeds there is gradual increase in solute concentration. Drop in freezing point can be seen as a gradual slope in freezing curve. If the liquid being cooled is not a pure solvent, then the segment B will not be perfectlyhorizontal, but a tilted line. Initial freezing point was calculated at the intersection point of trend lines drawn on segments A and B (Ribero et al., 2007). Step 1 In MS-Excel temperature versus time was plotted using raw cooling data from temperature data logger (Fig. 2a).
Step 2 Data was selected to split the graph in two segments based on the vertical and horizontal trend. Vertical segment A was plotted (Fig. 2b).
Step 3: On the same graph, horizontal segment B was plotted (Fig. 2c).
Step 4: Trend lines were added to segments A and B. Initial freezing point was calculated at the intersection of trend lines drawn on segments A and B (Fig. 2d).

Results and Discussion
The typical freezing profile of ternary mixture was indicated by the presence of dip in the cooling curve below 0 °C was due to super cooling that took place till the formation of ice crystals (Fig. 2a) There may be risk of inaccuracy due to error in data selection. Such study often requires an expert with curve fitting skills.  observed that the TM of ratio 1:1 and 1:2 (EG/PG: RO water) with a different concentration of NaCl was not frozen completely during the freezing at -20 °C but it was in an ice slurry form after 12 h of freezing due to the lower eutectic temperature of the mixture which is also similar to the findings of Gupta and Ramachandran (2018) and Al-Zubadi, 2007.

Effect of Salt Concentration on Freezing Point of Ternary Mixture
In the present study, NaCl act as a nucleating agent and it ends super cooling state and initiate freezing. When NaCl concentration was increased from 0% to 5%, the freezing point decreased at the rate of 2.5°C (Table 4 and 5) and the trend was similar to the findings of Stefl and George (2000). The freezing point depression of 5% KCl was 2.32 °C (Robert, 1994) which is lower than NaCl. Molecular weights of both the compounds are comparable (62.07 and 58.44 for EG and NaCl, respectively).
In terms percentage weight basis, NaCl is almost twice as effective as EG as a freezing point depressant because NaCl dissociates into two species while EG does not. The best way to understand and compare this phenomena is by adding 20 wt% NaCl to water. It will depress the freezing point by 16.5°C, whereas a 20% (wt basis) ethylene glycol solution in water will only lower the freezing point by approximately 8°C (Woods et al., 1999). At 5% NaCl concentration of TM, the rate of freezing was slow and consumed more time as compared to lower concentration of NaCl. It could be due to higher latent heat storage and freezing point depression at higher concentration of NaCl which is also similar to the finding of Li et al. (2013) and .    In the present study Freezing point depression was maximum (-16.5 °C) in case of ratio 1:1 (EG: RO water) with 5% NaClsolution. While, it was -15 °C for TM2 having ratio 1:1 (PG: RO water) with 5% NaCl (Table 4 and 5). It may be because EG has a higher thermal conductivity than PG (Dynalene, 2020, Engineering Tool Box, 2019).Freezing point depression was minimum (-5°C) in case of ratio 1:8 (PG: RO water) with 0 % NaClternary mixture.It was -6 °C for the ratio 1:8(EG: RO water) with 5% NaCl mixture.According to Guilpartet al. (2006), the preservation of refrigerated food at temperature between 2 to 5 °C would require a coolant temperature at around -5 °C and the freezing point of formulated TM is in the range of -5 to -16.5 °C which is extremely efficient for the subzero temperature cooling application.

Conclusion
Systematic study of aqueous-glycol-salt solution over a wide range of composition will help in selection and formulation of ternarycoolant.
The freezing point of ternary mixtures (EG, NaCl and RO water; PG, NaCl and RO water) at the different concentration was successfully determined using novel four step method. The results demonstrated that ternary mixtures required less EG/PG for cost effective formulation of secondary coolant for different cooling application in food industry.