A pharmaceutical factory's 12.5t/h triple-effect evaporation and crystallization system for vitamin wastewater

I. Project Background 

A vitamin B5 production base produces 8000t of calcium pantothenate annually, discharging 300m³ of process mother liquor daily. This mother liquor contains 10-12% Na₂SO₄, trace amounts of pantothenic acid, COD 25000-35000mg/L, and pH 4.5. The original triple-effect evaporation + centrifugal process consumed 0.42kg/kg of water steam, and the sodium sulfate purity was only 93%, failing to meet usage standards. Furthermore, outsourcing the centrifugation mother liquor cost as high as 420 RMB/t. In 2024, the company plans to construct a new 12.5t/h triple-effect evaporation crystallization system with the following targets: steam consumption ≤0.28kg/kg water, anhydrous sodium sulfate byproduct ≥99%, and mother liquor volume ≤2%.

II. Process Route

2.1 Process Overview

The process consists of five stages: neutralization and degassing → triple-effect co-current falling film concentration → OSLO forced circulation crystallization → thickening centrifugation → mother liquor drying. The main body is made of 2205 duplex stainless steel, and the surfaces in contact with the materials are electrolytically polished to Ra≤0.4μm.

• Pretreatment: Wastewater is adjusted to pH 7.0 with NaOH, and vacuum degassed at 50℃ to recover CO₂ and light COD components. Secondary steam undergoes acid washing and activated carbon fiber adsorption, reducing VOCs by 90%.

• Triple-effect co-current: Live steam at 0.5MPa 152℃ enters the shell side of effect I, with a boiling point of 120℃; secondary steam sequentially enters effect II 95℃ and effect III 68℃, -0.073MPa, with an overall specific heat transfer coefficient of 1850W/m²·K. 

• Crystallization Separation: The output concentration of the III-effect crystallizer is 28%, pumped into an OSLO-type forced circulation crystallizer at an operating temperature of 80℃, with a crystal slurry density of 25%. After washing and classification, particles with a D500.45mm diameter are obtained. A two-stage piston pusher centrifuge has a processing capacity of 6 t/h of salt, with a wet salt moisture content ≤3%.

•  Mother Liquor Drying: The centrifuged mother liquor has a COD of 120,000 mg/L. After passing through a paddle dryer, 0.8 t/h of mixed salt with a moisture content of 5% is obtained and sent to hazardous waste incineration.

2.2 Key Equipment Parameters

Evaporation Capacity: 12.5 t/h --inlet water 11% salt content

Heat Exchange Area: Effect I 420 m², Effect II 380 m², Effect III 340 m²

Steam Consumption: 0.27 kg/kg water --including thermocompressor pump

Cooling Water Circulation: 140 m³/h 28→36℃

Vacuum System: Three-stage Roots + water ring, ultimate vacuum -0.085 MPa

III. Technological Innovations

3.1 Thermocompressor Steam Recompression TVR

Secondary steam from Effect I at 120℃ is pressurized to 135℃ by a 0.6 MPa power steam jet pump and reused in the shell side of Effect I, saving 22% of steam.

3.2 Sodium Sulfate Crystallization Control

Utilizing "80℃ high-temperature crystallization + graded washing" technology, the formation of decahydrate salt is suppressed, resulting in dense crystals with a whiteness of 88% and a purity of 99.2%. 

3.3 Scale Prevention and CIP: The falling film pipe uses a Φ32×1.2mm internal groove pipe with a flow velocity of 2.0m/s, and the shear force inhibits calcium sulfate scale. Online CIP uses alternating cleaning with 3% NaOH + 2% HEDP chelating agent at 75℃, with a cycle of 12 days and a cleaning time of 4 hours.

3.4 Condensate Deep Treatment: Secondary condensate with a COD of 400~600mg/L is treated with catalytic ozone + BAF, resulting in an effluent COD ≤30mg/L and conductivity ≤80μS/cm. This effluent is then reused for boiler soft water makeup, achieving a reuse rate of 96%.

IV. Operational Data

• Successful start-up on June 2024. 72-hour performance evaluation:

• Evaporation rate: 12.7 t/h

• Steam consumption: 0.265 kg/kg water

• Anhydrous sodium sulfate production: 1.35 t/h

• Unit availability: >99%

V. Economic and Environmental Benefits

5.1 Economic Indicators

• Annual operating cost: RMB 2.85 million steam, electricity, chemicals.

• Annual savings in outsourced disposal fees: RMB 16.8 million;

• Sales revenue from by-product sodium sulfate: RMB 5.2 million;

• Annual net profit: RMB 19.15 million.

5.2 Environmental Indicators

• Annual reduction of high-salinity wastewater discharge: 90,000 t; COD emission reduction: 2,700 t;

• CO₂ emission reduction: 6,200 t/year;

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