A fine chemical plant's 30t/h ammonium sulfate and ammonium chloride triple-effect evaporation crystallization process

Process Scheme

1. Material Characteristics Reassessment

Laboratory tests on the mixed brine wastewater at 180℃ confirmed that ammonium sulfate begins to decompose at ≥95℃, and ammonium chloride sublimates more rapidly at ≥85℃. Combined with XRD patterns, the crystal precipitation sequence was determined: Na₂SO₄·10H₂O precipitates first, followed by (NH₄)₂SO₄, and finally NH₄Cl. Based on this pattern, a combination of "triple-effect co-current flow + two-stage crystallization + cryogenic denitrification" was adopted to avoid co-crystallization of ammonium sulfate and ammonium chloride.

2. Overall Flow Description

Raw material tank area → pH fine-tuning (adding 20ppm H₂SO₄ to suppress ammonia escape) → Plate preheating (recovering sensible heat from condensate to 62℃) → Effect I (heating chamber φ38×3mm, 316L tube side, Q345R shell side) → Effect II → Effect III → First-stage Oslo crystallizer (ammonium sulfate) → Thickener → WH-800 horizontal centrifuge → Airflow drying (120℃, residence time 3s) → Ton bag packaging.

The mother liquor from the third-effect evaporator undergoes plate and frame filter press purification and freeze crystallization (-5℃, vacuum 0.09MPa) to remove Na₂SO₄·10H₂O before entering the secondary DTB crystallizer (ammonium chloride). It then undergoes centrifugation, fluidized bed drying, and automatic packaging.

3. Main parameters of the triple-effect evaporator:

• Rated evaporation capacity:30 t/h; 

• Feed concentration: 15%; 

• Discharge solid content: 38%–40%.

• First effect: Temperature 88℃, Vacuum -0.02MPa, Evaporation capacity: 11.5 t/h;

• Second effect: Temperature 72℃, Vacuum -0.06MPa, Evaporation capacity: 10.0 t/h;

• Third effect: Temperature 58℃, Vacuum -0.085MPa, Evaporation capacity: 8.5 t/h.

• Overall heat transfer coefficient: ≥3800 W·m⁻²·K⁻¹ for single-effect cooling, ≥2200 W·m⁻²·K⁻¹ for triple-effect cooling; steam consumption: 11 t/h (0.45 t/t water); cooling water circulation rate: 520 m³/h (Δt=8℃).

Equipment Selection and Materials

1. Heater: The first effect uses 316L/carbon steel composite plate; the second and third effects are all 316L to avoid chloride ion pitting corrosion.

2. Evaporation Chamber: φ3600×8000mm, top equipped with a double-layer wire mesh demister (316L, 99.5% retention rate), bottom with a 60° cone angle + side-mounted forced circulation pump (flow rate 6000m³/h, head 6m, frequency conversion), ensuring a crystal slurry flow rate of 2.8m/s and a scaling cycle ≥12 days.

3. Condenser: Mixing + surface type dual-stage condenser, secondary steam condensate TDS ≤50mg/L, reused for boiler feedwater.

4. Vacuum System: Three-stage Roots + liquid ring pump combination, installed power 45kW, can establish a -0.09MPa vacuum within 5 minutes.

5. Online Cleaning: Equipped with CIP acid cleaning (5%HNO3) + alkaline cleaning (3%NaOH) automatic switching, automatically starting every 10 days of operation or when the system pressure drop >30kPa.

Energy Consumption and Economy

Compared to the previous single-effect system, the system saves approximately 72,000 tons of steam annually, equivalent to 15.84 million yuan (based on a price of 220 yuan/ton); 180 MWh of electricity and 240,000 tons of water are saved.

Environmental Protection and Safety

1. Exhaust Gas: After condensation, the non-condensable gas (NH3 ≤ 5 mg/m3) from the triple-effect secondary steam enters a two-stage pickling tower (dilute sulfuric acid circulation) + activated carbon adsorption, resulting in an ammonia emission concentration < 1.3 mg/m3 at the discharge outlet.

2. Mother Liquor: The ammonium sulfate mother liquor circulation ratio is 4.5, and the ammonium chloride mother liquor circulation ratio is 3.8, resulting in no high-salt emissions from the system; the sodium sulfate obtained from cryogenic denitrification is sold to glass manufacturers.

3. Dust: The drying and packaging sections are equipped with sealed hoods and pulse-jet bag filters (membrane filter bags, 500 g/m2), resulting in particulate matter ≤ 10 mg/m3. 4. Safety: The unit area is equipped with 12 GDS ammonia leak detection points, interlocked with the spray system; the evaporator top is equipped with a double pressure relief system of rupture disc and safety valve; the SIS system achieves triple interlocking to cut off steam in case of over-temperature, over-pressure, and low liquid level.

Manpower Saving and Convenient Operation: Production, energy consumption, and CIP plans can be remotely viewed through the operating system, achieving an "unmanned operation + minimally staffed inspection" mode, reducing the number of personnel per shift from 12 to 4.

Operating Results:

1. Production Capacity: The actual evaporation capacity of the unit is 31.2 t/h, exceeding the design capacity by 4%; ammonium sulfate production is 15.8 t/h, and ammonium chloride production is 6.5 t/h, equivalent to an annual production capacity of 126,000 tons of ammonium sulfate and 52,000 tons of ammonium chloride.

2. Quality: Ammonium sulfate N≥21.1%, H2O≤0.3%; Ammonium chloride N≥25.4%, Na≤0.25%. 3. Reliability: The uptime rate is 96%, the planned cleaning cycle is 12 days, and the actual operating time can reach 18 days; the equipment failure rate is 0.42 times/month, far lower than the industry average of 1.5 times/month.

Customer Value: Through the implementation of this project, the company not only completely solved the problem of high-salinity wastewater discharge, but also transformed the by-products ammonium sulfate and ammonium chloride into new profit growth points, generating an additional 180 million yuan in annual sales revenue and 42 million yuan in profit; at the same time, it established an industry benchmark for "green chemical industry and intelligent manufacturing," providing strong technical and data support for subsequent IPO fundraising projects.