A chemical plant's 1200t/d sodium sulfate double-effect MVR evaporator

I. Project Overview

In 2019, our company designed and constructed a 1200t/d sodium sulfate double-effect MVR evaporator for this client, based on evaporation water volume. It successfully started up on its first attempt in March 2020, reducing steam consumption to 0.08t/t water and electricity consumption to ≤22kWh/t water. The overall energy cost decreased by 62%, saving 11,000 tons of standard coal annually.

II. Process Route

1. Material Characteristics

• Boiling point elevation Δb = 11℃, 25% Na₂SO₄, 65kPa·A

• Critical supersaturation ΔCmax = 6g/L, prone to nucleation at 45–50℃

• High Ca²⁺ and SiO₂, scaling factor RF = 0.85, indicating a strong scaling brine

2. Process Summary

Raw material → High-density clarifier → Add NaOH + Na₂CO₃ for hardening removal, Ca²⁺ ≤ 30mg/L → Plate and frame clarifier → Sludge with 60% moisture content transported → Buffer tank → Primary preheating → Condensate → Secondary preheating → Live steam →Dual-Effect MVR

First Effect: Falling film + forced circulation composite, 65℃, concentrated to 18%

Second Effect: Forced circulation FC + salt leg, 48℃, concentrated to 28% → Oslo crystallizer → Centrifugation → Airflow drying → Ton bag packaging

The secondary steam experiences a 20℃ temperature rise after passing through a single-stage high-speed centrifugal compressor, with a compression ratio of 1.8 and an adiabatic efficiency of 83%.

3. Innovative Highlights

Dual-Effect MVR: The first effect uses falling film to handle 80% of the evaporation, while the second effect uses low-temperature crystallization, balancing energy consumption and crystal particle size.

Online Silicon Removal: MgO + flocculant is added in the preheating section, SiO₂ ≤ 25 mg/L, reducing scaling rate by 70%.

Partial Compression: All secondary steam in the first effect is compressed, while the second effect partially compresses secondary steam + supplements with live steam, achieving stable operation under -95 kPa vacuum without capacity reduction in summer.

Salt leg washing + Oslo grading ensures crystal residence time ≥ 60 min, d50 = 0.95 mm, C.V. ≤ 6%. Intelligent Cleaning: CIP is triggered when the pipe wall temperature gradient exceeds 3℃ or the vacuum level drops by 3kPa. Alternating acid/alkali/chelating agent action extends the cleaning cycle from 7 days to 20 days.

III. Core Equipment

Evaporator: The first-effect falling film evaporator uses duplex steel; the second-effect forced circulation evaporator uses 2205 stainless steel + titanium composite tubing with a 20% design margin to ensure five years of leak-free operation.

Compressor: One-piece milled titanium impeller, 1.4m diameter, linear velocity ≤285m/s, magnetically levitated bearings, designed life 100,000 hours, vibration <1.5mm/s.

Centrifuge: Two-stage piston pusher centrifuge, 0.15mm screen gap, washing section with 2℃ chilled mother liquor spray to reduce NaCl entrainment to 0.2%.

Drying: Closed-loop airflow drying, nitrogen protection, oxygen content <3%, product temperature ≤55℃ to prevent decahydrate weathering.

IV. Operational Data (Performance Assessment, June 2022)

Evaporation Rate: 50,000 kg/h (Design: 50,000 kg/h)

Live Steam: 0.08 t/t water (Design: 0.10 t/t)

Power Consumption: 22 kWh/t water (Compressor + Circulating Pump + Vacuum Pump)

Cooling Water: 1.5 t/t water (32℃→38℃)

Anhydrous Na₂SO₄ Production: 18 t/d, Whiteness ≥88%, Purity 99.2%, Cl⁻≤300ppm, Ca²⁺≤200ppm

V. Economic and Environmental Benefits

Annual Steam Savings: 108,000 tons (equivalent to 11,000 tons of standard coal), Carbon Reduction: 29,000 tons

Operating Cost Savings: 26 million RMB/year (Steam saving 17,000 RMB/day + Reduced Power Consumption) The external sales price of high-purity sodium sulfate has increased from 350 yuan/ton to 550 yuan/ton, generating an additional annual revenue of 13 million yuan. The investment payback period is 1.8 years.

VI. Customer Feedback

"The dual-effect MVR system has been running for four months with zero compressor failures. The vacuum level remains at -96 kPa even in summer. The crystal particle size is uniform, and there is no clumping after drying. The project has truly achieved the dual goals of zero discharge of high-salt chemical wastewater and high-value utilization of sodium sulfate."