Recovery of high-purity NO2 and SO2 products from iron-ore sintering flue gas by distillation: process design, optimization and analysis
Graphical abstract
Introduction
Industrial flue gas emissions include large amounts of NOx and SO2 [1]. China's total emissions of NOx and SO2 were about 12.6 and 8.8 million tons [2], accounting for 34% and 22% in the world [3], [4], respectively. To control the flue gas emissions, technologies of digesting or converting the pollutants into other clean wastes through physical and chemical processes have been widely developed, such as calcium desulfurization [1], [5], selective catalytic reduction denitrification [6], adsorption, electron beam [7], [8], [9], pulse corona discharge [10], [11], [12], membrane absorption [13], [14], [15], [16] and microbial removal methods [17], [18]. Although some of these technologies succeed in reducing SO2 and NOx emissions, they require additional operating costs without yielding any economic benefits during the pure environmental protection processes, which pose extra burden on enterprises.
As a matter of fact, high-purity SO2 and NO2 gases are important chemicals in metallurgy, chemical [19] and medicine industries [20]. NO2 is widely used as the rocket fuel propellant, the germicide and disinfectant in industrial wastewater treatment, the catalyst in the production of sulfuric acid, and the key ingredient of concentrated nitric acid. SO2 is a main raw material for producing insurance powders, sodium sulfite and sodium hydrogen sulfite, a bleaching agent in the paper industry, and a solvent and scouring agent in the petrochemical industry. The current market prices of high-purity NO2 and SO2 products in China are around 42 RMB/kg (~6.0 U.S. dollars (USD)/kg) and 2.5 RMB/kg (~0.36 USD/kg), respectively. The large emissions of NO2 and SO2 from flue gases cause not only environmental issues but also significant resource waste. If NOx and SO2 with respective concentrations of 175 and 350 ppm from a typical scale (1 million m3/h) of iron-ore sintering flue gas could be recovered and upgraded, 2011 tons of NO2 and 6244 tons of SO2 contributing to a total annual revenue of 14.33 million USD could be achieved literally. However, this remains challenging due to technical difficulties as: (1) non-destructive recovery of these pollutant gases that maintain the original molecular structures; (2) upgrading the gases from trace concentrations to high-purity products.
Adsorption technology has been well applied in removing SO2 and NOx from flue gases, such as the activated-carbon adsorption [21], [22], [23], NOXSO (a dry, regenerable flue gas treatment system that simultaneously removes SO2 and NOx from flue gas produced by the combustion of coal) [24], [25], and LILAC (a flue gas desulfurization system developed by HEPCO and Mitsubishi Heavy Industries Ltd. (MHI)) [26]. In these processes based on specific adsorbents [1] (activated carbons, zeolites, etc.) [27], [28], [29], concentrations of SO2 and NOx can be significantly enriched in the desorbed gas where NO-NO2 conversion plays a significant role in the oxidative adsorption process based on active sorbents and adequate oxygen in flue gas [30], [31], [32], [33], [34]. Although the enhanced concentrations of SO2 and NO2 are far below their high-purity product requirements, the adsorption–desorption process can be taken as the first step for efficient capture and pre-upgrading of two gases for subsequent refining processes [34].
Considering the boiling point as the major difference between NO2 (21 °C) and SO2 (−10 °C) physical properties, low-temperature distillation ought to be a favorable approach for NO2/SO2 separation [35]. Previous studies on cryogenic distillation mainly focused on large-scale productions of high-purity nitrogen, oxygen and argon in air separation [36]. The distillation separation of NO2/SO2 has not been reported. One-step distillation process recovering NO2 and SO2 from ppm level concentrations to high-purity products would be impractical due to great energy consumption and operating cost. However, if the preliminary enhancements of NO2 and SO2 concentrations through existing adsorption purification process are utilized, the threshold concentrations for an energy-saving distillation process (e.g. operating temperature above −50.0 °C) could be readily obtained. For instance, under a practical operating condition (15 bar and −45.0 °C), >90% NO2 liquefaction ratio can be achieved based on 2.03% NO2 feeding concentration [37]. The concept of successive process of adsorption capture of flue gas followed by a distillation recovery unit is depicted as shown in Fig. 1. The flue gas is firstly cleaned with a dust removal device, dehydrated with cooling and pressure or temperature swing adsorption (PSA or TSA) process, and purified with TSA, PSA or other hybrid adsorption processes. The desorbed gas with enriched SO2 and NO2 is introduced to the distillation unit for SO2/NO2 separation and upgrading.
Simulations and experimental studies on distillation process optimization have been conducted for the purposes of feasibility analysis, energy consumption and capital investment reductions, and eco-efficiency improvement [38]. Langè et al. [39] proposed a dual-column dual-pressure distillation process to remove CO2 components from natural gases. Li et al. [36] simulated a single-column cryogenic distillation process to upgrade methane from coke oven gas using ASPEN Plus software, giving sensitivity analysis and the residual curve map indicative of the feasibility of ternary distillation. Salerno et al. [40] reduced the heat duty of condensers in separating ethylene from light gases using a dual-column cryogenic distillation process. Yousef et al. [41] proposed a biogas reforming cryogenic distillation process, and reduced energy consumption and avoided frost formation by optimizing process parameters such as distillation pressure, temperature, reflux ratio, number of total stages and biogas feed composition.
This work studied the distillation separation of SO2 and NO2 subsequent to a flue gas adsorption capture process. Note that the process in this work is defined as low-temperature distillation instead of conventional cryogenic distillation, considering the definition of the operation temperature for cryogenics being <153 °C from the International Dictionary of Refrigeration [42], [43]. The distillation processes with different feeding ways were compared, selected and validated. The process parameters including the number of total stages, the feed stage number, the mole flow rate at bottom of column and the reflux ratio were optimized. The SO2/NO2 separation performance and economic benefits were further discussed in detail.
Section snippets
Simulation work
A series of simulation work was conducted to determine the preferred distillation process and corresponding parameters. The relevant analysis of heat and mass transfer was also carried out based on the simulation results.
Experimental work
In order to validate the WILS-NTH method in the simulation, the condensation experiments for single-component SO2 and NO2 were conducted. The experimental results were compared with simulation results and theoretical calculations to confirm the accuracy. The schematic diagram of the simulation process is shown in Fig. S3 with detailed methodologies and theoretical calculation method given in Supplementary Material.
Model validation
The mole fractions of residual SO2 and NO2 at different temperatures and pressures obtained from the condensation experiments, theoretical calculations and process simulations are shown in Fig. 5. From Fig. 5(a) and (b), the mole fractions for both SO2 and NO2 from three aspects show similar upward trends with temperature. The average relative bias for SO2 and NO2 between experimental and simulation results are 5.77 and 6.77%, while those between experimental and calculation results are 3.18
Conclusion
The NO2/SO2 distillation separation subsequent to the adsorption capture process was developed for recovering high-purity NO2 and SO2 liquid products from flue gases. A dual-column liquid-phase feeding distillation process was proposed and theoretically verified, which possesses several advantages such as energy-efficient, smaller device size and easy operation in comparison with gas-phase feeding counterpart. For treating 2218 m3/h feed flow of desorbed gas originated from 1000,000 m3/h
CRediT authorship contribution statement
Yingshu Liu: Writing - original draft, Investigation, Funding acquisition. Ningqi Sun: Writing - review & editing, Methodology, Data curation. Ziyi Li: Conceptualization, Data curation, Writing - review & editing, Formal analysis, Investigation, Funding acquisition. Penny Xiao: Writing - review & editing. Yi Xing: Investigation. Xiong Yang: Supervision, Resources. Chunyu Zhao: Investigation. Chuanzhao Zhang: Investigation, Funding acquisition. Haoyu Wang: Investigation, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
The authors would like to express the gratitude to the National Natural Science Foundation of China (No. 21808012, 21676025), the National Key R&D Program of China (No. 2017YFC0210302), the Beijing Natural Science Foundation (No. 8182019), the Scientific Research Project of Beijing Educational Committee (No. KM202011417007), and the Fundamental Research Funds for the Central Universities (No. FRY-IDRY-19-025, FRY-TP-20-011A2).
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