Research on the Green Environmental Protection Method of Automatic Potentiometric Titration of Organic Heat Transfer Fluids Acid Number

This paper presents the concept and importance of acid number. Improvement method is based on the GB / T 7304-2014 “Standard test method for acid number of petroleum products by Potential Titration”. The detection and study of the acid number are obtained by halving sample volume in two categories—used and unused heat transfer fluids. This article studies and establishes a green and environmentally friendly method for automatic potentiometric titration of organic heat carrier acid value, which uses a smaller sample volume and titration solution volume to achieve fast, environmentally friendly, intelligent, and efficient results. The experimental results show that after halving the sample volume, the data reproducibility is relatively high, but still within the standard reproducibility range, and data of the heat transfer fluids is far less than the standard reproducibility range. Through this paper, the acid number of heat transfer fluids is able to be strictly controlled and timely monitored to ensure the safety, green environment development and operation of boilers and systems.


Introduction
In modern industry, service safety of boilers is closely related to people's lives.Due to the high thermal efficiency, excellent thermal stability, and easy temperature control characteristics of organic heat carriers, the use of organic heat carrier boilers has become increasingly widespread in recent years, with an increasing number of applications year by year.The application of organic heat carriers has become widespread [1].Although the working pressure of organic heat carrier furnaces is relatively low, the temperature of the heat transfer liquid inside the furnace is relatively high, and most of them are flammable and explosive.Once a leak occurs during operation, it can lead to accidents such as fires, explosions, and even cause casualties and property damage.Therefore, we must attach great importance to the safe operation and management of organic heat carrier furnaces [2].
In high temperature environments, the heat transfer fluids are easily to oxidation and deterioration.The heat transfer fluids increase the acid number, greatly shorten the service life and effect the safe operation of the boilers [3].The acid number of the heat transfer fluid is an important indicator of their degradation degree: generally expressed in milligrams of potassium hydroxide required to neutralize the acidic components in1g heat transfer fluid [4].Common heat transfer fluids are separated into unused and in-use heat transfer fluid according to the use state.For the used heat transfer fluid, in addition to the acidic substances brought by the heat transfer fluids, there are also some organic acids, phenols and esters produced by reactions such as oxidation, decomposition and rearrangement.etc in the process of use, and there may also be mixed acid substances.Acid number is a significant indicator for evaluating the degree of oxidation degradation and retesting performance of heat transfer fluids [5].According to the GB 23971 "heat transfer fluids", the acid number index in the unused heat transfer fluid is not more than 0.05mgKOH / g [6], once the acid number index in the heat transfer fluids higher than 1.5mg KOH / g, it should be stopped [7].Therefore, strict control and timely monitoring of the acid number of heat transfer fluid is very important for the safe operation of heat transfer fluid boiler.
In the detection of acid number of heat transfer fluid, the commonly used detection methods include GB / T 7304-2014 Standard test method for acid number of petroleum products by Potential Titration, GB / T 4945-2002 Standard test method for acid and base number of petroleum products and lubricants by--colour-indicator titration, and GB / T 24747-2023 Safety technology conditions for organic heat transfer fluid Appendix A, Determination of acid number of heat transfer fluid (Potential titration method).This paper mainly focuses on the most commonly used under the instruction GB / T 7304.Because the sample volume determination of acid number of heat transfer fluid is required around 10~20g, the accuracy cannot be ensured under the relevant scientific research and detection requirements when the sample volume is small, so this paper mainly focuses on the sample volume reduction.

Reagents
The reagents required for the study include the following: water: secondary water; reference substance: potassium hydrogen phthalate; anhydrous lithium chloride, potassium hydroxide, anhydrous isopropanol, toluene (above reagents are pure); buffer solution (pH value of buffer solutions are 4,7 and 11); slightly acidic water (with 1:4 HCl pH of aqueous solution is 4.5-5.5); 10 unused heat transfer fluids; 10 heat transfer fluids.

Instruments
The instrument required for the study is automatic potential titrator (Metrohm 905), it includes host, burette piston driver, burette, acid-base electrode, electrode cable, spiral stirrer, titration table, anti diffusion valve and computer operation terminal.The automatic potentiometric titrator is shown in the following figure 1.

Experimental Procedures
According to GB/T 7304-2014 Standard test method for acid number of petroleum products by potential titration [8], we conduct electrode preparation, electrode potential detection, instrument calibration, and solution preparation, including potassium hydroxide isopropanol standard solution (0.05mol/L~ 0.10mol/L), titration solvent (water+anhydrous isopropypanol+toluene), KOH standard solution concentration calibration, blank determination.And then we conduct sample determination.This paper compares and analyzes the detection data of unused heat transfer fluids and in-use heat transfer fluids under different standard requirements of weight and half volum (5-8g).
The acid value of the sample (calculated in KOH, unit: mg/g) is calculated according to equation 1: V--When titrating the sample to the inflection point closest to the potential value of the aqueous buffer solution with pH=11, the volume of potassium hydroxide isopropanol solution used, or when the inflection point is not obvious or there is no inflection point, the volume of potassium hydroxide isopropanol solution used when titrating to the potential value of the pH=ll buffer solution, in milliliters (mL).For additives, V is the volume of potassium hydroxide isopropanol solution consumed at the last displayed inflection point; V0--The blank value relative to A, in milliliters (mL); M--The concentration of potassium hydroxide isopropanol solution, in moles per liter (mol/L); W--The mass of the sample, in grams (g); 56.

Results Analysis
The graph of automatic potentiometric titration acid value is shown in figure 1.The horizontal axis represents the volume (mL) of KOH isopropanol solution used for titration, and the vertical axis represents the electrode potential (mV) in the titration solution.Only when the inflection point is obvious and very close to the potential value obtained from aqueous acid or alkali buffer solution, can this inflection point be used as the titration endpoint, i.e. the EP point in figure 2.
It can be seen from the data above, in the improvement of the potential titration method for the determination of the acid number of the unused heat transfer fluids, the reproducibility results of the acid number data are relatively high, but still within the standard reproducibility range, and the reproducibility results of the heat transfer fluids are very low, far less than the standard reproducibility range.

Conclusions
This paper introduces the important role of acid number detection of the key index of heat transfer fluid, and improves the sample volume commonly used in GB / T 7304 for acid number detection.The results show that after halving the sample volume, the acid number test results of the standard reproducibility are met for both the unused heat transfer fluid and the used heat transfer fluid.Therefore, in the future, in the relevant testing and scientific research, for example, unused heat transfer fluids provide with very few samples, or using precision instruments for heat transfer fluids acid number detection after aging test, or in the case of online monitoring and testing of heat transfer fluid boilers, for small sample volume, study could follow GB / T 7304 and use halved sample.Through this paper, the acid number of heat transfer fluids can be strictly controlled and timely monitored to ensure the safety, environmental protection and operation of heat transfer fluid boiler and system.

Figure 2 .
Figure 2. Typical titration curve of organic heat transfer fluid acid value.

Table 1 .
Acid number data of unused heat transfer fluids for different weight.Data of Used Heat Transfer Fluids.Through the experimental process of 2.3, we conducted a comparison of acid value testing on 10 used heat transfer fluids with different quantities.The relevant data is shown in table 2.
1--KOH molar mass, in grams per mole (g/mol); 2.4.Research Data 2.4.1.Data of Unused Heat Transfer Fluids.Through the experimental process of 2.3, we conducted a comparison of acid value testing on 10 unused heat transfer fluids with different quantities.The relevant data is shown in table 1.

Table 2 .
Acid number data of used heat transfer fluids for different weight.