Case Study: 35 t/h Sodium Chloride Wastewater MVR Evaporation and Crystallization System

Project Background

A chlor-alkali chemical enterprise generates large quantities of high-concentration sodium chloride wastewater during the production of caustic soda and downstream derivatives. The peak treatment requirement is 35 tons per hour, with sodium chloride content consistently ranging from 18% to 22%. Direct discharge of this wastewater would not only pose a severe risk of soil salinization but also result in the loss of significant recoverable salt resources. The client previously utilized a traditional multi-effect evaporation process, which incurred massive steam consumption and high operating costs; consequently, there was an urgent need for a highly efficient, energy-saving alternative.

Solution

Following multiple rounds of technical evaluation, we designed and delivered an MVR (Mechanical Vapor Recompression) evaporation and crystallization system with a processing capacity of 35 tons per hour. Centered on a centrifugal steam compressor, the system compresses and raises the temperature of the secondary steam generated by the evaporator, reusing it as a heat source; this fundamentally transforms the traditional evaporation model that relies on external live steam for energy. To address industry-wide challenges such as scaling and uneven crystal size distribution during sodium chloride crystallization, we employed a combined process featuring an Oslo-type crystallizer and a forced-circulation evaporator. This setup, integrated with seed crystal dosing and an online back-flushing system, ensures stable, long-term operation.

Core Equipment and Process Highlights

The complete unit comprises a preheater, a falling-film evaporation section, a forced-circulation crystallization section, a steam compressor, a thickener, a centrifuge, and a mother liquor recycle system. The main equipment utilizes a composite structure of titanium alloy and duplex stainless steel, effectively resisting pitting and stress corrosion in the high-temperature, high-chloride environment. The MVR compressor features a variable frequency drive (VFD), allowing it to automatically adjust rotational speed based on feed load to maintain optimal energy efficiency. The system operates at an evaporation temperature of 75°C to 85°C; this low-temperature operation significantly reduces scaling rates and the risk of heat-sensitive side reactions. The crystallization section is equipped with precision density monitoring and an automatic seed crystal dosing unit, yielding sodium chloride crystals with uniform size and high purity, while maintaining a moisture content below 2%. Project Outcomes

Following the system's commissioning, the design processing capacity of 35 tons per hour was fully validated, with actual operating loads flexibly adjustable between 30 and 38 tons. By recycling the thermal energy of secondary steam through MVR technology, the system reduced overall energy consumption by approximately 65% compared to traditional triple-effect evaporation; steam consumption dropped to below 0.05 tons per ton of water, resulting in annual savings of over 12,000 tons of standard coal. The purity of the separated solid sodium chloride meets the standards for premium-grade industrial salt, allowing for direct reuse in electrolysis processes or external sales, thereby achieving the dual goals of zero wastewater discharge and complete resource recovery. The client achieved a return on investment in less than two years, realizing significant economic and environmental benefits.

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