8 TPH Sodium Sulfate Multi-Effect Evaporator: Scaling Up Recovery for a Dye Intermediates Plant

Dye and pigment manufacturing generates some of the most challenging industrial wastewater streams, particularly when sulfonation reactions produce concentrated sodium sulfate solutions that cannot be discharged without treatment. For a major dye intermediates producer in India, the bottleneck was not chemistry—it was evaporation capacity. Their existing two-effect thermal system, installed in 2015, could process only 4.5 tons per hour of sulfate-laden brine. With production targets rising 40% year-over-year, the plant urgently needed an 8 TPH sodium sulfate multi-effect evaporator that would double throughput while maintaining the crystal purity required by their captive sodium sulfate crystallization unit. CONQINPHI’s proposal for a four-effect forced-circulation sodium sulfate evaporator won the contract based on thermal efficiency, proven crystallization performance, and a delivery schedule that matched the client’s expansion timeline.

The project presented genuine process engineering challenges. Sodium sulfate exhibits inverse solubility—its solubility decreases as temperature increases—which means conventional cooling crystallization is often preferred. However, the client’s existing infrastructure was built around hot crystallization at 105°C, and they wanted to preserve this approach to avoid reconfiguring downstream centrifuges and dryers. The sodium sulfate multi-effect evaporator had to deliver 8 tons per hour of water evaporation while concentrating feed from 12% to 38% sodium sulfate, then feeding a dedicated crystallizer at precisely controlled temperature and solids concentration. Any deviation in thermal balance would throw off the entire downstream train.

Thermal Design: Four Effects for Maximum Steam Economy

H2: Effect Configuration and Temperature Profile

CONQINPHI designed the sodium sulfate multi-effect evaporator as a four-effect forced-circulation system operating in forward-feed arrangement. This configuration was selected for two reasons: the feed is relatively dilute at 12% Na₂SO₄, minimizing scaling risk in early effects, and forward feed allows gradual concentration buildup that matches the crystallizer’s appetite for hot, dense mother liquor.

The temperature cascade was optimized for sodium sulfate’s thermal properties:

• First effect: 128°C (0.25 MPa g steam), concentrating to 18% solids

• Second effect: 105°C, concentrating to 25% solids

• Third effect: 82°C, concentrating to 32% solids

• Fourth effect: 58°C, final concentration to 38% solids

Live steam enters only the first effect; vapor from each subsequent effect becomes the heating medium for the next. This cascading arrangement gives the sodium sulfate evaporator a steam economy of 3.8 kg water evaporated per kg steam consumed—approaching the theoretical maximum for a four-effect system.

H2: Forced-Circulation Design for Scaling Resistance

Sodium sulfate brines are notorious for rapid scaling on heat transfer surfaces, particularly in the transition zone where solids begin to precipitate. The sodium sulfate multi-effect evaporator addresses this through several engineering measures:

• High circulation velocities: 2.2 m/s tube-side velocity in all effects, maintained by vertical axial-flow pumps with titanium impellers

• Submerged inlet design: Feed enters below the liquid level to prevent flash evaporation and salt plating at the tube sheet

• Oversized tubes: 38 mm OD tubes with 2.5 mm wall thickness in 2205 duplex stainless steel, providing extended service life against chloride stress corrosion

• Online cleaning capability: Each effect includes isolation valves and quick-connect chemical injection points for scheduled acid washing without full shutdown

Integration and Commissioning: Matching the Existing Train

H2: Crystallizer Interface and Control

The sodium sulfate evaporator does not produce crystals directly—it feeds a separate DTB crystallizer operating at 105°C. This interface demanded precise control. CONQINPHI installed a density-controlled transfer system: when fourth-effect liquor reaches 1.32 specific gravity, it is pumped to the crystallizer surge tank. If density drops (indicating dilution from upstream), the transfer valve modulates closed to protect crystallizer stability.

A shared DCS links the sodium sulfate multi-effect evaporator with the crystallizer, allowing operators to view integrated trends. Key automated sequences include:

• Feed-forward steam control based on inlet flow rate and temperature

• Vacuum regulation in the fourth effect using a two-stage steam ejector with barometric condenser

• Level interlocks preventing pump cavitation during circulation upsets

H2: Performance Validation

After 60 days of continuous operation, performance testing confirmed the sodium sulfate multi-effect evaporator met all design guarantees:

The plant has since reported that the sodium sulfate evaporator operates with only one scheduled cleaning per month, compared to weekly shutdowns required by the old two-effect unit. Annual steam savings alone are estimated at $156,000 based on current natural gas pricing.

If your facility is constrained by aging evaporation capacity or rising steam costs, a modern sodium sulfate multi-effect evaporator may deliver the fastest payback in your capital program. CONQINPHI offers no-cost preliminary assessments.

Submit your feed data—flow rate, Na₂SO₄ concentration, temperature, available steam pressure, and utility costs. Our engineers will model optimized effect configurations and return a budget proposal with guaranteed performance parameters within 72 hours.