Low-concentration SO₂ treatment: process parameter optimization of two-stage magnesium hydroxide absorption tower
1. Special challenges of low-concentration SO₂ treatment
The treatment of industrial flue gas with low-concentration SO₂ (usually <1000mg/m³) faces three problems: slow reaction kinetics, low marginal benefits of equipment investment, and difficulty in resource utilization of by-products. Taking a petrochemical catalytic cracking unit as an example, its flue gas SO₂ concentration fluctuates between 300-800mg/m³, the desulfurization efficiency of the traditional single-stage absorption tower is only 75%-82%, and the magnesium hydroxide consumption is as high as 1.2kg/kg SO₂. The oxidation rate of the by-product magnesium sulfite is less than 40%, forming a dilemma of "treatment means loss".
The two-stage absorption tower process has become an innovative path to solve the contradiction between the efficiency and cost of low-concentration SO₂ treatment through gradient reaction design and precise parameter control. According to a refinery renovation case in Shandong, the desulfurization efficiency of the optimized two-stage system increased to 98%, the utilization rate of magnesium hydroxide increased by 35%, the purity of the byproduct magnesium sulfate reached the industrial standard (≥98%), and the annual comprehensive income exceeded 6 million yuan.
2. Optimization strategy of core process parameters
1. pH gradient control in the absorption section
Enhanced absorption in the first-stage tower: Control the pH in the range of 6.0-6.5, and use the high reactivity of magnesium hydroxide to quickly capture SO₂. Actual measurements in a project in Hebei showed that the magnesium sulfite generation rate in this pH range increased by 50% compared with the traditional process, and the reaction time was shortened to 8 seconds.
Deep purification in the second-stage tower: Maintaining pH 5.5-6.0, promote the conversion of magnesium bisulfite (Mg(HSO₃)₂) to magnesium sulfate. By injecting oxygen-enriched air (O₂ concentration ≥25%) into the bottom of the tower through an online oxidation blower, the oxidation rate jumped from 45% of the single-stage system to 92%.
2. Dynamic adaptation of liquid-gas ratio
Optimization of liquid-gas ratio of primary tower: In view of the fluctuation characteristics of inlet SO₂ concentration, a medium liquid-gas ratio of 3.5-4.5L/m³ is adopted to reduce the energy consumption of circulating pump while ensuring mass transfer efficiency. Data from a chemical plant in Jiangsu shows that the power consumption under this parameter is 28% lower than that of traditional processes.
Fine adjustment of secondary tower: reduce the liquid-gas ratio to 2.0-2.5L/m³, compensate the mass transfer efficiency by increasing the specific surface area of the packing layer (≥350m²/m³), and control the system pressure loss within 800Pa.
3. Flue gas flow rate and temperature coordination
Flow rate control: The flue gas flow rate of the primary tower is set to 3.0-3.5m/s, and the secondary tower is reduced to 2.0-2.5m/s to extend the gas-liquid contact time. A project in Fujian adopts a variable diameter tower design, and the two-stage flow rate gradient increases the SO₂ removal rate by 12%.
Temperature management: The inlet flue gas temperature is stabilized at 90-110℃, and waste heat is used to promote oxidation reactions. A thermal power plant in Liaoning recovers heat energy through a flue gas-slurry heat exchanger, saving 12,000 tons of steam consumption annually.
4. Innovation of spray layer structure
Solid cone coverage of the first-stage tower: 120° titanium alloy nozzle is used, the atomization particle size is ≤80μm, and the coverage rate is ≥180%. Actual measurements show that this design increases the contact area between droplets and flue gas by 40%.
Second-stage tower cyclone enhancement: A cyclone distributor is installed above the packing layer to form a turbulent mixing of gas and liquid, and the mass transfer coefficient is increased to 1.2×10⁻⁴m/s, which is 3 times higher than that of traditional spraying.
III. System integration and intelligent control
1. Modular design of oxidation zone
Cyclone aeration device: A honeycomb aeration pipe is arranged at the bottom of the second-stage tower, with a bubble diameter of ≤2mm and an oxygen mass transfer efficiency of 85%. After application in a project in Tangshan, the oxidation time of magnesium sulfite was shortened from 45 minutes to 12 minutes.
Online crystallization inhibition: Add 0.05%-0.1% polycarboxylate dispersant to the circulating slurry to prevent magnesium sulfate crystals from clogging the pipeline, and the maintenance cycle is extended to 6 months.
2. DCS intelligent control system
Dynamic parameter coupling: Establish a multivariable control model of SO₂ concentration, pH, liquid level, and pressure loss, with a response time of ≤5 seconds. After application in a refining and chemical enterprise in Qingdao, the fluctuation range of outlet SO₂ concentration was narrowed from ±15% to ±3%.
Energy efficiency optimization algorithm: Based on historical data training machine learning model, real-time recommendation of the best liquid-gas ratio and pH combination, annual power saving rate of 18%.
3. By-product quality improvement path
Membrane separation and purification: Nanofiltration + reverse osmosis double membrane system is used to increase the concentration of magnesium sulfate solution from 15% to 30%, meeting the crystallization conditions of magnesium sulfate heptahydrate.
Low-temperature evaporation crystallization: Using flue gas waste heat to drive the triple-effect evaporator, the energy consumption per ton of magnesium sulfate crystallization is reduced from 1.2 tons of steam to 0.6 tons.
IV. Engineering verification and economic benefits
Case 1: Huadong Petrochemical catalytic cracking unit
Process configuration: two-stage countercurrent absorption tower + cyclone oxidation zone + double membrane purification system
Operation data:
Inlet SO₂ concentration: 450-780mg/m³
Outlet SO₂ concentration: ≤35mg/m³
Magnesium hydroxide consumption: 0.78kg/kg SO₂
Byproduct income: Annual sales of magnesium sulfate heptahydrate of 12 million yuan
Return on investment: Total investment in transformation is 32 million yuan, and a payback period of 2.1 years is achieved through cost savings + revenue generation
Case 2: South China Waste Incineration Power Plant
Special challenges: Flue gas humidity > 12%, Cl⁻ content > 500mg/m³
Innovative solution:
First-stage tower is equipped with a demister to reduce humidity to below 8%
Second-stage tower uses glass fiber reinforced plastic anti-corrosion lining to withstand Cl⁻ corrosion
Byproduct magnesium sulfate is used for landfill leachate treatment, saving 3 million yuan in reagent costs annually
V. Technology evolution direction
1. Material science breakthrough
Biological-based corrosion inhibitor: Extract natural corrosion inhibitors from marine microorganisms to ｒｅｐｌａｃｅ chemical additives
Self-cleaning filler: Develop TiO₂ photocatalytic coating fillers to decompose adherent organic matter
2. Energy synergy network
Photovoltaic driven circulation pump: flexible photovoltaic modules are installed on the top of the tower to meet 20% of the system's electricity demand
Hydrogen energy co-production: electrolysis of magnesium sulfate solution to produce green hydrogen, 62m³ H₂ per ton of byproduct
3. Digital twin operation and maintenance
Three-dimensional flow field simulation: real-time simulation of gas-liquid distribution in the tower, predicting efficiency attenuation trend
Blockchain certification: byproduct carbon footprint data is uploaded to the chain to meet international green supply chain standards
When the pH gradient of the two-stage absorption tower is precisely matched with the cyclonic field, and when nano-scale magnesium-based particles are directed to snipe SO₂ molecules in turbulence, this technological innovation for low-concentration pollutants has gone beyond the scope of environmental governance and evolved into the value reconstruction of industrial ecology. From the refining cluster in Bohai Bay to the environmentally friendly power plant at the mouth of the Pearl River, the art of parameter optimization is redefining the boundaries of clean production - here, every micron of atomization accuracy is the ultimate pursuit of efficiency, and every gram of regenerated magnesium sulfate is writing a new paradigm for the circular economy. Driven by the "dual carbon" goals, only by breaking through with technological innovation and weaving a network with systematic thinking can we open up a new blue ocean of high-quality development in the segmented track of low-concentration governance.