Evaporation is one of the modern chemical unit operations, which uses heating to vaporize and remove part of the solvent in the solution to increase the concentration of the solution or create conditions for solute precipitation. The triple-effect evaporator desalination method is a method that uses a concentration crystallization system to remove inorganic salts from wastewater through evaporation.

A triple-effect evaporator consists of three evaporators connected in series. Low-temperature (around 90℃) heating steam is introduced into the first effect to heat the wastewater. The generated steam is introduced into the second effect as heating steam, causing the wastewater in the second effect to evaporate at a lower temperature than the first effect. This process is repeated until the last effect. The condensate from the first effect returns to the heat source, and the condensate from the other effects is collected and output as desalinated water. One part of steam input can evaporate multiple times the amount of water.

At the same time, the high-salt wastewater is successively concentrated from the first effect to the last effect, reaching supersaturation and crystallizing out in the last effect, thus achieving solid-liquid separation of salt and wastewater.

In the treatment of saline wastewater, the saline wastewater enters a triple-effect concentration crystallization device. Through the triple-effect evaporation and condensation concentration crystallization process, it is separated into desalinated water (desalinated water may contain trace amounts of low-boiling organic matter) and concentrated slurry wastewater; inorganic salts and some organic matter can be crystallized and separated, and incinerated to produce inorganic salt waste residue; organic matter that cannot be crystallized can be treated using a rotary evaporator to form solid waste residue, which is then incinerated; desalinated water can be returned to the production system to replace softened water.

The triple-effect evaporator desalination method has the advantages of mature technology, wide range of wastewater treatment, small footprint, fast processing speed, and energy saving. With the development of the chemical industry, more and more high-salt wastewater needs to be treated, and the application of the triple-effect evaporator desalination method will become increasingly widespread.

1. Application range of triple-effect evaporators

Triple-effect evaporators can be used to treat high-salt wastewater produced in the process production of chemical production, food processing plants, pharmaceutical production, oil and natural gas collection and processing enterprises. Suitable wastewater has a salt content of 3.5%～25% (mass percentage) and a COD concentration of 2000～10,000ppm.

2. Composition and principle of triple-effect evaporator

The triple-effect evaporator mainly consists of three sets of evaporators connected in series, a condenser, a salt separator, and auxiliary equipment (as shown in the figure). The three sets of evaporators operate in series to form a triple-effect evaporator. The entire evaporation system adopts a continuous feeding and continuous discharging production method.

High-salt wastewater first enters a single-effect forced circulation crystallization evaporator. The crystallization evaporator is equipped with a circulation pump that pumps the wastewater into the evaporation heat exchange chamber. In the evaporation heat exchange chamber, externally connected steam liquefies to generate vaporization latent heat, heating the wastewater. Because the pressure in the evaporation heat exchange chamber is relatively high, the wastewater is heated to superheat in the evaporation heat exchange chamber at a pressure higher than the normal boiling point of the liquid.

After the heated liquid enters the crystallization evaporation chamber, the pressure of the wastewater rapidly decreases, causing part of the wastewater to flash evaporate or boil rapidly. The steam generated after wastewater evaporation enters the second-effect forced circulation evaporator as power steam to heat the second-effect evaporator. The unevaporated wastewater and salt are temporarily stored in the crystallization evaporation chamber.

The first, second, and third-effect forced circulation evaporators are connected by balance pipes. Under the action of negative pressure, the high-salt wastewater flows from the first effect to the second effect and then to the third effect. The wastewater is continuously evaporated, and the salt concentration in the wastewater increases. When the salt content in the wastewater exceeds the saturation state, the salt in the water will continuously precipitate and enter the salt collecting chamber at the bottom of the evaporation crystallization chamber.

A salt suction pump continuously sends the salt-containing wastewater to a vortex salt separator. In the vortex salt separator, the solid salt is separated and enters the salt storage pool. The separated wastewater enters the second-effect forced circulation evaporator for heating. The entire process is repeated to achieve the final separation of water and salt.

The condenser is connected to a vacuum system. The vacuum system removes the uncondensed gas generated in the evaporation system, maintaining a negative pressure state in the condenser and evaporator to improve the evaporation efficiency of the evaporation system. Under the action of negative pressure, the secondary steam generated by the wastewater in the triple-effect forced circulation evaporator automatically enters the condenser, and under the cooling of the circulating cooling water, the secondary steam generated by the wastewater quickly turns into condensate. The condensate can be continuously discharged and recovered to the recycled water pool.

1. Treatment object and process

The main component of high-salt wastewater is a 15% sodium chloride solution. The wastewater pH is 6～8, and the wastewater COD is 50,000ppm. The treatment capacity is 3t/h. According to the characteristics of high-salt wastewater, the process design is designed according to a triple-effect evaporator. According to the calculation, the main technical parameters of the triple-effect evaporator are determined as follows:

Evaporation amount Q=3000kg/h (3000kg of water evaporated per hour); actual steam consumption Q=1200kg/h (inlet pressure 0.3～0.4MPa); heat transfer area of the first-effect evaporator S=80m2, vacuum degree P=-0.03MPa; heat transfer area of the second-effect evaporator S=80m2, vacuum degree P=-0.06MPa; heat transfer area of the third-effect evaporator S=80m2, vacuum degree P=-0.085MPa; circulating cooling water consumption Q=40t/h; condensation cooling area A=240m2; total power of the unit P=25kW; footprint of the unit is 10m long × 5m wide × 4m high. According to the process, fully considering the corrosiveness of wastewater to the equipment, and based on the principle of saving costs while meeting usability, the system equipment materials are selected as follows:

1) The evaporator body is made of carbon steel with heavy corrosion protection, which can withstand the corrosion of acid, alkali, and salt solutions below 120℃;

2) The heater uses Ta1 titanium tubes;

3) The condenser tubes use 316L stainless steel;

4) The discharge screw pump uses 316L stainless steel;

5) The recycled water tank and flash tank use carbon steel with spray-coated anti-corrosion paint;

6) Process pipes, fittings, and valves use 316L stainless steel + PPR material;

7) Crystallization tanks are made of heavy anti-corrosion carbon steel.

2. Treatment Results and Existing Problems

After treatment with a triple-effect evaporator, high-salt wastewater produces crystalline salt, concentrated organic waste liquid, and desalinated water. The crystalline salt and concentrated organic waste liquid are sent to a hazardous waste disposal center for centralized incineration, while the desalinated water is reused in production.

The operation of this system has revealed that although the triple-effect evaporator can effectively treat high-salt wastewater, there are still some problems that need to be further overcome, mainly manifested in:

a. High wastewater treatment cost. Due to the corrosive nature of the wastewater being treated, the selection of materials for the equipment needs to consider corrosion resistance, resulting in higher costs.

b. Severe corrosion and short lifespan of the entire equipment during operation. Although corrosion-resistant materials are used as much as possible in the design of the triple-effect evaporator, corrosion cannot be completely avoided, and the equipment still has a relatively short lifespan, requiring timely replacement.

c. Treating high-salt wastewater with a triple-effect evaporator requires a large amount of steam, which is not feasible in many places.

d. High-salt wastewater treated by the triple-effect evaporator still needs to be sent to a hazardous waste disposal center for further processing.

High-salt wastewater must be properly treated before it can be returned to the environment. Practice has proven that traditional wastewater treatment methods are not suitable for treating high-salt wastewater. Among the many high-salt wastewater treatment technologies, the triple-effect evaporator desalination method has the advantages of mature technology, a wide range of wastewater that can be treated, small land occupation, fast processing speed, and energy saving, and has considerable development prospects in China. Although the triple-effect evaporator has drawbacks such as high processing costs, short equipment lifespan, and high steam requirements, with further technological development, the application of this technology in the field of high-salt wastewater treatment will further expand.