Key Equipment Design Principles in Evaporation and Crystallization Systems for Zero Liquid Discharge (ZLD)

EVAPORATOR SOLIDEST

Zero Liquid Discharge (ZLD) has become a critical objective in industrial wastewater treatment, especially in sectors like chemical manufacturing, electroplating, and textiles. Evaporation and crystallization systems play a core role in achieving ZLD, using advanced thermal separation techniques to recover clean water and solid salts.

This article focuses on the design principles of major equipment components in evaporation-crystallization systems, especially those used in multi-effect evaporation and Mechanical Vapor Recompression (MVR) setups.

Forced Circulation Heater Design

1. What is a Forced Circulation Heater?

In ZLD systems, forced circulation heaters are typically shell-and-tube heat exchangers designed to handle highly concentrated and scaling-prone wastewater.

2. Heat Transfer Area Calculation

The heat transfer area AAA is calculated using the formula:

A = Q / (K × Δtm)

• Q = Heat load, calculated via energy balance

• K = Overall heat transfer coefficient (typically 800–1200 W/m²·°C)

• Δtm = Temperature difference between heating steam and boiling wastewater

  
  

  Boiling Point Elevation (BPE): Due to solutes like sodium sulfate and sodium chloride, wastewater boiling points can rise by 4–10°C depending on concentration and pressure.

3. Tube Specifications & Layout

• Tube outer diameter (do): 25–32 mm

• Tube length (l): 6–12 m

• Tube arrangement: Equilateral triangular pattern

• Shell diameter (D): D = 1.6 × do × √n

4. Design Enhancements

• Steam Inlet: Include a baffle or deflector to prevent direct impact on tubes.

• Non-condensable Gas Outlet: Located at the far end to ensure efficient removal.

• Double Heating Chamber: Optional design to reduce energy consumption.

Vapor-Liquid Separator Design

1. Purpose of the Separator

This unit separates secondary steam from the concentrated liquid to ensure efficient vapor reuse and clear condensate.

2. Diameter and Height Calculations

   Diameter (D):D = 1.128 × √(Vg / ug)

   • Vg = Vapor volumetric flow

   • ug = Vapor velocity (depends on liquid and vapor density)

   Vapor Space Height: Typically 1.5–3.0 m, based on a recommended evaporation volume intensity of 1.1–1.5 m³/(m³·s)

3. Defoamer and Inlet Design

   
   

   Install mist eliminators like wire mesh or baffle demisters on the top to improve condensate quality.

   Recommended Feed Direction: Axial feed is preferred in ZLD applications for better flow stability.

Forced Circulation Pump Design

1. Pump Type

High-flow, low-head axial flow pumps are used for forced circulation in evaporators.

2. Flow Rate (V) Calculation

V = 0.25 × π × di² × n × u

• di: Inner tube diameter

• u: Velocity of liquid inside the tube (1.5–2.5 m/s)

• n: Number of tubes

3. Head and Power Calculation

   - Head (H): 3–5 m, determined via Bernoulli’s equation

   - Power (N):N = V × H × ρ / (102 × η)

   • ρ: Liquid density

   • η: Pump efficiency

4. Cavitation Prevention

   Avoid Cavitation: Always maintain at least 0.5 m positive Net Positive Suction Head (NPSH) margin between pump and system design to prevent impeller damage and ensure stable operation.

Steam Compressor Selection for MVR Systems

Steam compressors are the heart of MVR systems, boosting vapor energy and enabling reuse.

Compressor Types in ZLD

1. Roots Blower

• Speed: 750–1650 rpm

• Temperature Rise: 22–25°C

• Pros: Simple structure, stable

• Cons: Low efficiency, max 5 t/h throughput

2. Standard Centrifugal Compressor

• Speed: 6000–9500 rpm

• Temperature Rise: 6–9°C

• Commonly Used: In pairs for higher evaporation loads

3. High-Speed Single-Stage Centrifugal Compressor

• Speed: Up to 33,000 rpm

• Temperature Rise: 20–24°C

• Pros: High processing capacity

• Cons: More complex control system

Compressor Selection Guidelines

• Evaporation Load > 3 t/h: Use centrifugal compressors

• Parameters Required: Flow rate, inlet pressure/temp, ΔT, material

• Temperature Rise: Typically 16–20°C

• Material Choice:

  • Duplex stainless steel for Roots and standard centrifugal

  • Titanium alloy for high-speed centrifugal (due to high stress)

Anti-Surge Protection

Install sensors for flow, temp, pressure, and vibration. Implement automatic control systems to avoid surge conditions.

Conclusion

Designing a ZLD evaporation and crystallization system requires precision engineering and thorough understanding of thermodynamics, material compatibility, and hydraulic principles. Each component, whether it’s a heater, separator, pump, or compressor, plays a vital role in maintaining the system’s efficiency and reliability. By following these design principles, industries can ensure longer equipment life, lower energy consumption, and achieve environmental compliance.

FAQs

1. What is the role of a forced circulation heater in ZLD systems?

It provides the thermal energy needed to evaporate high-salinity wastewater, ensuring uniform heat distribution and preventing scaling.

2. Why is boiling point elevation important in evaporator design?

Because it affects heat transfer calculations and determines how much temperature difference is needed for effective evaporation.

3. What type of pumps are best for evaporation systems?

Axial flow pumps with high flow and low head, designed to avoid cavitation and withstand high salt concentrations.

4. Which steam compressor is best for high-load MVR evaporators?

Single-stage high-speed centrifugal compressors are ideal for systems with evaporation loads above 3 t/h.

5. How do vapor-liquid separators enhance condensate quality?

They remove entrained droplets and foam, often using mist eliminators, ensuring clean steam for reuse and reducing fouling risks.

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