5t/h sodium hydroxyl triple-effect cross-flow evaporation crystallization system

I. Project Background 

This fluorosilicone new material manufacturer has an annual production capacity of 20,000 tons of fluorocarbon alcohols and fluorosilicone modified monomers. The production process generates 5 t/h of sodium hydroxide mother liquor (equivalent to water evaporation), with the following components: sodium glycolate (sodium hydroxyacetate) 220-250 g/L, NaCl 60-80 g/L, NaF 15-25 g/L, SiO₂ colloid 300 mg/L, COD 8000 mg/L, color 3000 times, and temperature 55℃. The original single-effect evaporator consumed 0.48 t/t of water steam, and the equipment suffered from severe coking and SiO₂ scaling. Furthermore, the sodium hydroxide purity was <90%, making it unsellable. In 2024, the company decided to implement a "zero-emission" technological upgrade. We delivered a 5t/h sodium hydroxyl triple-effect cross-flow evaporation crystallization system under an EPC model. It successfully started up in March 2025 and has been running continuously for over 7200 hours to date, with a steam consumption of 0.25t/t water, sodium hydroxyl purity ≥99.0%, and an annual increase in revenue of 22 million yuan.

II. Process Route

1. Influent Characteristics

Boiling point elevation Δb = 13℃ (TDS 320g/L, 70kPa·A)

Thermosensitive: Sodium hydroxyl > 100℃ easily decarboxylates to form ethylene glycol; NaF > 60℃ solubility drops sharply, easily co-deposition

High SiO₂, high COD, scaling factor RF = 0.90, highly corrosive

2. Process Overview

Mother liquor → Cooling and desiliconization (MgO + PAC, SiO₂ ≤ 50mg/L) → High-density precipitation (Ca²⁺ ≤ 30mg/L) → Tubular microfiltration (SDI ≤ 3) → Two-stage preheating (condensate + secondary steam) → Triple-effect cross-flow evaporator → DTB crystallizer A (low-temperature NaF precipitation) → DTB crystallizer B (medium-temperature sodium hydroxyl precipitation) → Centrifugation → Fluidized bed drying → Automatic packaging

Live steam operates at 0.4 MPa(g) and 152℃. The first-effect heating chamber uses 2507 + titanium composite tube sheet, while the second and third effects use 2205, ensuring a margin for both fluoride and chloride ion corrosion.

3. Process Innovation

Cross-flow + Low-temperature first effect: First effect 65℃, second effect 55℃, third effect 45℃. Low-temperature precipitation of NaF in the first effect reduces sodium hydroxyl entrainment by <2%.

Separate crystallization: Controlling the density of each effect to introduce seed crystals, NaF segment ΔC <1.5g/L, sodium hydroxyl segment ΔC <2.5g/L, salt purity ≥99% for all segments.

Vacuum flash cooling + heat pump reuse: Partial compression and pressurization of the secondary steam in the third effect to 18℃ drives the evaporation in the first effect, saving 15% of steam.

Online defluorination: An alkaline scrubbing tower is installed in the secondary steam pipeline, achieving an F⁻ absorption rate >99%, and an F⁻ concentration in the tail gas ≤0.5mg/m³.

III. Equipment Configuration

1. Evaporators: Single-effect 60m², Double-effect 50m², Triple-effect 40m², total heat exchange area 150m², design margin 25%.

2. Crystallizers: Two DTB units, material 2507 + rubber lining, design pressure -0.1MPa, slurry density 25-30%.

3. Centrifuges: LW450×1800-N, separation factor 1400, frequency converter, screen gap 0.08mm, washing section uses 3℃ chilled mother liquor.

4. Drying: Closed-loop circulating fluidized bed, nitrogen protection, oxygen content <3%, product temperature ≤50℃, to prevent sodium hydroxide thermal decomposition.

IV. Automation and Energy Saving

1. DCS+APC: Multi-variable predictive control of evaporation rate, liquid level, density, and vacuum degree, fluctuation ±3%.

2. Dual closed-loop system with steam flow feedforward + temperature feedback, live steam consumption 0.25t/t water, better than the design value of 0.30t. The secondary steam condenser uses a titanium plate heat exchanger, consuming only 2.5 t/t of cooling water, saving 15% more electricity than traditional surface condensers.

3. Dual-redundant SIS: 14 interlocks including low liquid level, high vacuum, centrifuge vibration, and excessive F⁻ concentration, with a response time of <500ms.

V. Operational Data (Performance Assessment in June 2025)

• Processing Capacity: 5t/h (Design 5)

• Water Production Rate: 80% (including condensate reuse)

• Live Steam: 0.25t/t water (Design 0.30)

• Power Consumption: 20kWh/t water (circulating pump + vacuum pump + centrifuge)

• Output: Sodium hydroxide 1.8t/d, purity 99.2%; NaF 0.3t/d, purity 98.5%; NaCl 0.4t/d, purity 98.8%, all reaching first-class industrial dry basis

• Condensate: TDS≤100mg/L, F⁻≤3mg/L, SiO₂≤5mg/L, reused in circulating cooling water, reducing overall plant water consumption by 10%

VI. Economic and Environmental Benefits

Annual steam savings of 0.6 million tons, equivalent to 700t of standard coal, reducing carbon emissions by 1800t Savings of 3 million yuan/year in hazardous waste transportation costs (originally 2500 yuan/t × 1200t). Sodium hydroxyl is sold at 12,000 yuan/t, and NaF at 6000 yuan/t, resulting in an additional 22 million yuan in annual revenue.

VII. Customer Feedback

"The 5t/h cross-flow triple-effect system has been running for six months, with the vacuum level stable at -88kPa and the sodium hydroxyl purity consistently above 99%, completely solving the scaling and energy consumption problems of fluoride-containing mother liquor."