A pharmaceutical company's 3t/h triple-effect evaporation and crystallization system for mixed salts

I. Project Overview 

This comprehensive pharmaceutical company generates 60 m³ of high-chloride mixed salt mother liquor daily during the production of antiviral and cardiovascular raw materials. This mother liquor contains 6-8% NaCl, 4-5% Na₂SO₄, 1.2% organic solvents, and a COD of 18000-28000 mg/L, exhibiting high color and strong corrosiveness. The original outsourced disposal cost was 450 RMB/ton, resulting in annual expenditures of nearly 9 million RMB. In 2022, the company initiated a project to construct a 3t/h mixed salt triple-effect evaporation and crystallization system. Design requirements: steam consumption ≤0.30 kg/kg water, by-product NaCl ≥97%, Na₂SO₄ ≥98%, mother liquor volume ≤1.5%, and overall skid-mounted delivery, with commissioning within 3 months.

II. Process Route and Equipment Configuration

2.1 Overall Flowchart

The process consists of five stages: neutralization and degassing → triple-effect cross-flow falling film concentration → salt-nitrate separation crystallization → centrifugal drying → mother liquor drying. The main body is made of 2205+2507 duplex stainless steel, and the contact surfaces with materials are electrolytically polished to Ra≤0.4μm, meeting GMP cleaning requirements.

• Pretreatment: The mother liquor is adjusted to pH 6.5 with CaOH₂, and acetone/ethanol is recovered under vacuum at 50℃, reducing VOCs by 92%; turbidity is removed using a ceramic membrane 0.1μm, achieving SDI≤3.

• Triple-effect cross-flow: Live steam 0.6MPa 158℃ → shell side of effect I, boiling point 125℃; secondary steam is diverted to effect II 105℃ and the nitrate crystallizer, while effect III 65℃, -0.078MPa is dedicated to salt concentration, with a total specific heat transfer coefficient of 1900W/m²·K. 

• Salt-nitrate separation: Utilizing the NaCl-Na₂SO₄-H₂O phase diagram, the crystallization sequence is controlled within the 55-80℃ range: anhydrous Na₂SO₄ 90% whiteness precipitates first in the high-temperature section, and NaCl 97% purity precipitates in the low-temperature section. After centrifugation, both are dried separately, achieving "one system, two commercial salts".

• Mother liquor drying: The centrifuged mother liquor has a COD of 100,000 mg/L. It is dried using a low-temperature vacuum paddle dryer to obtain 0.25 t/h of mixed salts with a water content ≤4%, which is then sent for hazardous waste incineration.

2.2 Key Parameters

• Evaporation Capacity: 3t/h inlet water 10% saline

• Heat Exchange Area: Effect I 95m², Effect II 85m², Effect III 75m²

• Steam Consumption: 0.28kg/kg water including autoclave pump

• Cooling Water: 35m³/h 28→35℃

• Vacuum System: Roots + water ring, limit -0.082MPa

• Footage: 12m×4.5m×5m two-layer skid-mounted

III. Technical Innovations

3.1 Salt-Nitrogenate Co-production

Through coupled control of "temperature-vacuum-residence time", a single unit can simultaneously produce two types of commercial salts, with NaCl recovery rate of 88% and Na₂SO₄ recovery rate of 85%, saving 30% of investment compared to the traditional "mixed salt + post-treatment" process. 

3.2 Thermal Steam Recompression (TVR)

The secondary steam from the first-effect stage, initially at 130℃, is pressurized to 145℃ by a 0.7MPa power steam jet pump and reused in the shell side of the first-effect stage. This results in a 20% steam saving, reducing overall steam consumption to 0.28 kg/kg water.

3.3 Scale Prevention and CIP

Falling film tubes with Φ32×1.2mm internal grooves are used, with a flow rate of 1.8 m/s. Online CIP employs alternating cleaning with 2% NaOH + 1% HEDP chelating agent at 80℃, with a cycle of 14 days and a cleaning time of 3 hours.

IV. Operational Results

• Successful start-up on October 2022. 72-hour performance evaluation:

• Evaporation rate: 3.15 t/h

• Steam consumption: 0.275 kg/kg water

• NaCl production: 0.18 t/h, purity 97.2%

• Na₂SO₄ production: 0.12 t/h, purity 98.5%

• Unit availability: >98%

V. Economic and Environmental Benefits

5.1 Economic Indicators

• Annual operating cost: RMB 650,000 (steam, electricity, reagents).

• Annual savings in outsourced disposal fees: RMB 5.4 million;

• By-product salt sales revenue: RMB 850,000;

• Annual net profit: RMB 5.6 million.

5.2 Environmental Indicators

Annual reduction of high-salinity wastewater discharge: 18,000 t; COD emission reduction: 540 t; CO₂ emission reduction: 1,400 t/year;

150 t of hazardous waste salts incinerated in compliance with regulations, achieving "reduction, resource recovery, and commercialization".