Du Weihua

  Introduction

  Nitrogen trifluoride is a colorless, odorless, non-toxic, and non-flammable gas under normal temperature and pressure. It decomposes into nitrogen difluoride and fluorine gas at around 350°C, with reaction properties similar to fluorine. When used as an oxidizer, nitrogen trifluoride can serve as a source of nitrogen difluoride radicals. It is also used as a fluorine source for high-energy chemical lasers and high-energy fuels.

  Currently, there are two main methods for preparing nitrogen trifluoride: one is the direct combination of fluorine and ammonia, and the other is the electrolysis of ammonium hydrogen fluoride in molten salt. Regardless of the method used to produce nitrogen trifluoride, the content of CF4 impurities is always a major factor affecting the purity of NF3. Impurities in the crude gas can typically be removed using methods such as adsorption, membrane separation, and low-temperature distillation [1]. In recent years, significant work has been done both domestically and internationally on the production of NF3 using nickel anode electrolysis, with the purity of NF3 after conventional purification reaching over 99.999%. With the development of the microelectronics industry, the requirements for NF3 purity and impurity content are extremely high. Given the promising application prospects of nitrogen trifluoride, further research on NF3 purification technology to improve product purity is essential.

  1 Composition and Physicochemical Properties of Nitrogen Trifluoride Crude Gas

  1.1 Crude Gas Composition

  The main components of nitrogen trifluoride crude gas include NF3, F2, HF, CF4, N2, O2, CO, CO2, N2F2, N2F4, and N2O. Based on the composition of NF3 crude gas and the properties of its impurities, methods such as absorption, adsorption, and low-temperature distillation can be used to remove impurities like F2, HF, CO2, N2, O2, CO, N2O, and N2F2. However, these methods have limited separation capabilities for CF4, and the product purity mainly depends on the CF4 content in the crude gas. Even with methods like adsorption and membrane separation [2], it is difficult to completely remove CF4.

  1.12 Physical Properties of Main Components

  The physical properties of the main components are shown in Table 1.

  1.3 Product Specifications of Nitrogen Trifluoride

  The product specifications of nitrogen trifluoride used domestically and internationally are shown in Table 2.

  2 Research on NF3 Extraction Distillation Purification Process

  2.1 Principle of Nitrogen Trifluoride Extraction Distillation Process

  Impurities such as F2, HF, CF4, N2, O2, CO, CO2, N2F2, N2F4, and N2O in nitrogen trifluoride crude gas can be removed using methods like absorption, adsorption, and low-temperature distillation (except for carbon tetrafluoride) [3]. Since the standard boiling points of pure CF4 and NF3 are -129.11°C and -128.11°C, respectively, separating CF4 and NF3 using conventional distillation methods is extremely difficult. The relative volatility of CF4 and NF3 is close to 1.10, making conventional distillation methods economically unsuitable. Additionally, CF4 and NF3 mixtures form azeotropes or near-azeotropes over a wide range of temperatures and pressures, making complete separation using conventional distillation methods impossible.

  Since CF4 or NF3 can be separated from various fluorides and from each other, adding an extractant with a much higher boiling point than the components in the raw material (the extractant does not form azeotropes with the components) can increase the relative volatility between CF4 or NF3 products and their respective fluoride impurities. This allows for the effective separation of fluoride impurities from the desired CF4 or NF3 products through distillation of the CF4 and NF3 mixture. This special distillation is known as extractive distillation. Since the mixture to be separated is a non-ideal solution, the addition of the extractant changes the interactions between the original components, thereby altering their activity coefficients and relative volatilities.

  2.2 Selection of Extractant

  In the extractive distillation of CF4 and NF3 mixtures, the extractant must be able to change the relative volatility of CF4 and NF3, have high selectivity, be easily recoverable from the separated mixture, not react chemically with the original components, not form azeotropes, and have a certain boiling point difference from the original components. Additionally, the extractant should be safe, non-toxic, thermally stable, and cost-effective.

  In NF3 extractive distillation, the normal boiling point of the extractant should be between -110°C and -25°C. Typically, suitable extractants for separating CF4 and NF3 include nitrous oxide (N2O), chlorodifluoromethane (HCFC-22), difluoromethane (HFC-32), and fluoromethane (HFC-41) [4]. Considering the characteristics of the system to be separated and the principle of not introducing new impurities, nitrous oxide is the most suitable extractant for separating CF4 and NF3. The addition of N2O extractant increases the relative volatility between NF3 and CF4 from 1.10 to 5.10, making the separation of NF3 and CF4 easier.

  2.3 Process Flow of Extractive Distillation

  The typical process flow of NF3 extractive distillation is shown in Figure 1. The main equipment includes a low-boiling point distillation column, an extractive distillation column, an NF3 product distillation column, and an N2O extractant recovery distillation column. Since the boiling point of the N2O extractant is higher than that of the components in the original solution, it is always discharged from the column bottom. To maintain a high solvent concentration on most of the column trays, the N2O extractant inlet must be above the raw material inlet. However, it generally cannot be introduced from the top of the column because a few trays above the extractant inlet are needed to form the extractant recovery section, ensuring that the extractant concentration in the distillate from the top is reduced to a negligible level. The N2O extractant and high-boiling impurities from NF3 are discharged from the bottom of the extractive distillation column and sent to the extractant recovery separation device, then returned to the extractive distillation column for recycling.

  Figure 1 illustrates the NF3 extractive distillation process. The mixture containing NF3, CF4, and other impurities is fed into the low-boiling point distillation column T1 through a pipeline. Low-boiling impurities such as oxygen and nitrogen are separated and discharged from the top, while NF3, CF4, and other high-boiling impurities flow out from the bottom and enter the extractive distillation column T2. CF4 and trace amounts of NF3 and other low-boiling impurities are discharged from the top, achieving the separation of NF3 and CF4. NF3 and high-boiling impurities flow out from the bottom of the extractive distillation column T2 and enter the product separation column T3. High-purity NF3 product flows out from the top of T3, while the extractant and other trace high-boiling impurities flow out from the bottom and enter the extractant separation column T4. Impurities are discharged from the top of T4, and the N2O extractant flows out from the bottom and is returned to the extractive distillation column T2 through a pipeline for recycling. The loss of N2O extractant in the entire process is minimal, and only a small amount of N2O extractant needs to be added periodically to compensate for the loss.

  2.4 Characteristics of the Extractive Distillation Column

  In addition to the raw material and reflux liquid, the extractant is also fed into the extractive distillation column, with its flow rate significantly exceeding that of other materials. Since the extractant is added in liquid form, there is a sudden change in the liquid flow rate at the extractant addition tray. Therefore, the temperature of the added extractant should be close to or equal to the column temperature to stabilize the vapor flow rate in the distillation and recovery sections. In the extractive distillation column, the descending liquid flow rate is much higher than the ascending vapor flow rate, resulting in less than ideal vapor-liquid contact and lower tray efficiency. Additionally, reasonably increasing the extractant flow rate helps to increase the relative volatility between components. In actual operation, an optimal reflux ratio needs to be determined, which is influenced by the matching relationship between the reflux ratio and the extractant concentration on the separation efficiency.

  2.5 Calculation of the Extractive Distillation Process

  In the extractive distillation process, the addition of a large amount of high-boiling N2O extractant results in a high concentration of N2O in the column, with the liquid flow rate significantly higher than the vapor flow rate. Therefore, the liquid heat capacity in the column is much higher than in conventional distillation, and the vapor and liquid flow rates in the column also change significantly, causing the vapor-liquid flow ratio and N2O extractant concentration to vary considerably along the column height. As a result, the operating line is no longer straight. For this reason, the RadRrac model in Aspen Plus is used for the extractive distillation calculation of NF3. RadRrac is a rigorous model for simulating all types of multi-stage vapor-liquid distillation operations, with extensive capabilities for designing and evaluating trays and packings, and is suitable for non-ideal three-phase systems with liquid phases. The main input parameters for the extractive distillation calculation are shown in Table 3.

  The main calculation results of the extractive distillation are shown in Table 4.

  2.6 Operating Results

  In the above process, we used liquid nitrogen as the coolant for the condensers of columns T1 to T4 and CaCl2 brine at 0 to -30°C as the heat source for the column reboilers. A series of process tests were conducted, and the operating results matched the design data. Typical operating results are shown in Table 5.

  3 Conclusion

  In summary, using N2O as the extractant for the separation of NF3 and CF4 mixtures is appropriate. The purity of the NF3 product reaches over 99.9999%, with a recovery rate greater than 90%, achieving efficient separation of NF3 and CF4 mixtures and solving the technical challenge of NF3 purification.