MVR evaporator for evaporation and concentration to recover waste sulfuric acid

MVR evaporators offer significant energy-saving advantages in the field of waste sulfuric acid concentration and recovery. However, the strong corrosiveness and high boiling point elevation of waste sulfuric acid place stringent requirements on equipment materials and process design. The following is a systematic analysis based on sulfate treatment experience and the principles of MVR technology:

I. Technical Feasibility Analysis

Waste Sulfuric Acid Characteristics:

Strong Corrosivity: Concentrated sulfuric acid is extremely corrosive to metals, requiring special materials.

Significant Boiling Point Elevation: Sulfuric acid solutions have a large boiling point elevation (e.g., 50% H₂SO₄ boiling point is approximately 130℃), increasing compressor power consumption.

Acid Mist Evaporation: Sulfuric acid mist is generated during the evaporation process, requiring specialized collection and treatment.

Applicability: MVR is suitable for concentrating dilute sulfuric acid (e.g., 20-60% H₂SO₄). For concentrated sulfuric acid (>70%), due to its strong oxidizing properties and high viscosity, a special design is required.

II. Core Process Flow

1. Pretreatment System

Impurity Removal: Quartz sand filtration removes suspended solids, ion exchange removes heavy metal ions (such as Fe³⁺, Cu²⁺), ensuring turbidity <5 NTU.

Corrosion Protection: Add corrosion inhibitors to control Cl⁻ content (as Cl⁻ exacerbates corrosion).

pH Adjustment: If the waste acid contains other acidic impurities, neutralization pretreatment followed by salt separation is required.

2. Evaporation and Concentration Stage

Evaporator Selection:

Forced Circulation Type: Primarily made of titanium or Hastelloy C-276, flow rate 1.5-3 m/s, preventing calcium sulfate and other crystal deposits from clogging the pipes.

Falling Film + Forced Circulation Combination: First-effect falling film evaporation to 30-40% concentration, second and third-effect forced circulation evaporation to the target concentration (e.g., 60%), balancing efficiency. Efficiency and Scale Prevention

Compressor Configuration:

Centrifugal Compressor: Temperature rise needs to reach 12-18℃, compression ratio 1.5-2.5, matched with secondary steam volume (e.g., 10-30t/h), COP = 15-20

Material: Compressor flow parts are made of titanium alloy or lined with tantalum, resistant to sulfuric acid mist corrosion.

3. Acid Mist Treatment System

Wire Mesh Demister + Wet Electrostatic Precipitator: Installed at the top of the evaporator, collection rate >99.5%, preventing acid mist corrosion of the compressor.

Alkali Absorption Tower: Uncollected trace amounts of SO₃ gas are absorbed with NaOH solution, generating sodium sulfate as a byproduct.

4. Product Recovery

Concentrated Sulfuric Acid: Concentration can reach 60-70%, recycled for production processes.

Miscellaneous Salt Treatment: Sulfates generated in the pretreatment stage (e.g., sodium sulfate) Ammonium sulfate can be recovered through MVR evaporation and crystallization.

III. Key Equipment and Material Selection

IV. Operating Parameter Control

Temperature and Pressure:

Evaporation Temperature: 40-80℃ 0℃ (vacuum 0.04-0.06MPa) to avoid localized overheating (>100℃) that could cause sulfuric acid decomposition.

Compressor temperature rise: Adjusted according to concentration; 8-12℃ for dilute acid (<30%), 15-20℃ for concentrated acid (>50%).

Concentration control: Real-time monitoring with an online density meter to prevent over-concentration leading to crystallization and blockage by calcium sulfate, etc.

Energy consumption indicators:

Electricity consumption per ton of water: 40-55 kWh/ton for materials with moderate boiling point elevation (e.g., sodium sulfate); 50-70 kWh/ton of water for sulfuric acid due to its higher boiling point.

Energy saving rate: Compared to traditional triple-effect evaporators (consuming 0.3t/ton of steam), MVR saves 60-80% on energy, resulting in annual steam cost savings of 2-5 million RMB (based on a processing capacity of 10t/h). (Calculation)

V. Technical Advantages

1. Extreme Energy Saving: Secondary steam heat recovery rate >90%, requiring only a small amount of live steam for startup, reducing operating costs by 60-70%.

2. Low-Temperature Operation: Vacuum evaporation avoids accelerated corrosion of equipment due to high temperatures, extending equipment lifespan.

3. Environmental Protection and Emission Reduction: No waste heat emissions, condensate can be reused, reducing cooling water consumption by over 90%.

4. Stable Product Quality: Automated control system ensures stable concentration; concentrated sulfuric acid purity can reach industrial-grade standards.

VI. Technical Challenges and Solutions

Challenge 1: Strong Corrosion

Problem: Concentrated sulfuric acid corrodes metals rapidly, resulting in high equipment investment.

Solution: Use tantalum heat exchangers (optimal corrosion resistance) or glass-lined equipment; enhance acid mist capture to reduce vapor phase corrosion.

Challenge 2: Boiling Point Elevation Increased Energy Consumption

Problem: The higher the sulfuric acid concentration, the more significant the increase in boiling point, leading to an exponential increase in compressor power consumption.

Solution: Segmented concentration; the first stage uses MVR to concentrate to 40%, and the second stage uses single-effect vacuum evaporation or membrane distillation to balance energy consumption and equipment investment.

Challenge 3: Crystallization Blockage

Problem: Calcium sulfate and ferric sulfate in waste acid easily crystallize on the heating tube wall.

Solution: Forced circulation flow rate ≥2m/s + online cleaning (CIP) + ultrasonic descaling (frequency 20-40kHz).

Challenge 4: High Material Costs

Problem: Titanium and tantalum materials are 20-50 times more expensive than carbon steel, resulting in a long investment payback period.

Solution: For low-concentration waste acid (<30% H₂SO₄), PTFE-lined steel or duplex stainless steel can be used, reducing costs by 40%.

VII. Implementation International Application Reference Cases

Case 1: Lead-acid Battery Waste Acid Recovery

Processing Capacity: 5t/h (containing 15-20% H₂SO₄)

Process: MVR forced circulation evaporation and concentration to 50% H₂SO₄

Results: Annual operating cost of 949,600 RMB, saving 3,064,400 RMB/year compared to a double-effect evaporator

Material: Titanium heating tubes + glass-lined evaporation chamber

Case 2: Rare Earth Industry Waste Acid Treatment

Process: MVR evaporation and concentration to 30%, then returned to the acid leaching process for reuse

Economy: Selling miscellaneous salts as a byproduct, with annual revenue of 2-3 million RMB

VIII. Applicability and Selection Recommendations

Applicable Scenarios

Dilute Sulfuric Acid Recovery: Concentrating 10-40% waste sulfuric acid to 50-70%

Acid Pickling Wastewater: Sulfuric acid from the steel and electroplating industries Pickling Solution Regeneration

Sulfate Conversion: Recovery of by-product salts such as sodium sulfate and ammonium sulfate through MVR evaporation and crystallization

Unsuitable Scenarios

Concentrated Sulfuric Acid (>70%): High viscosity and strong corrosiveness, requiring special design.

Waste Acid Containing Large Amounts of Organic Matter: Organic matter interferes with vapor compression efficiency, requiring pretreatment.

Selection Recommendations

1. Concentration <30%: Recommended triple-effect MVR falling film + forced circulation combination, balancing efficiency and investment.

2. Concentration 30-60%: Recommended forced circulation MVR, titanium-palladium alloy material.

3. Containing Solid Impurities: Add plate and frame filter press pretreatment, suspended solids <5mg/L before entering the evaporator.

MVR vacuum evaporators are technically feasible and energy-efficient in the field of waste sulfuric acid recovery, but the issues of material corrosion resistance and boiling point elevation need to be addressed. For large-scale dilute sulfuric acid recovery projects (>10t/h), MVR is significantly more economical than traditional triple-effect evaporation; for small-scale or high-concentration waste acid, a comprehensive evaluation of the return on investment is required.