Distillation Column Internals China

Distillation Column Internals: Design, Function, and Application

1. Introduction

Distillation column internals are the engineered components inside separation towers that facilitate mass and heat transfer between vapor and liquid phases. Their design directly determines column efficiency, capacity, energy consumption, and operational stability. Wangdu (Hebei) Chemical Engineering Co., LTD specializes in designing and manufacturing these critical components, focusing on the integration of fluid dynamics, material science, and process engineering principles to meet specific separation requirements across various industries.

2. Core Components: Trays and Packing

Distillation internals are primarily categorized into tray-type and packed-type designs, each with distinct hydrodynamic characteristics.

2.1 Tray-Type Internals
Trays create discrete stages where vapor and liquid interact. Common designs include:

• Sieve Trays: Feature perforations typically 3-12 mm in diameter, with a free area of 8-15%. They provide straightforward design, moderate efficiency (Murphree vapor efficiency often 70-85%), and relatively low cost. Pressure drop per tray ranges from 40-100 mm liquid column depending on hole velocity and weir height.

• Valve Trays: Incorporate movable valve units that adjust opening with vapor flow. This design provides operational flexibility with turndown ratios reaching 4:1, compared to sieve trays' typical 2:1. Valve trays maintain efficiency across varying loads but have higher initial cost.

• Bubble Cap Trays: Though less common due to higher cost and pressure drop, they remain relevant in applications with very low liquid rates or where positive liquid sealing is essential, such as in some reactive distillation processes.

2.2 Packed-Type Internals
Packings provide continuous vapor-liquid contact over the bed height. Key types include:

• Random Packings: Ceramic or metal pieces (Pall rings, Berl saddles) dumped into the column. Modern high-efficiency metal Pall rings (size 25-50 mm) provide surface areas of 120-220 m²/m³ and HETP values of 0.4-0.6 m for many common systems.

• Structured Packings: Corrugated sheets arranged in ordered patterns. They offer low pressure drop (1-3 mbar/m), high efficiency (HETP 0.3-0.5 m), and high capacity. Standard corrugation angles are 45° for high capacity and 60° for high efficiency.

3. Supporting Components and Distribution Systems

Auxiliary internals are equally critical for column performance.

3.1 Liquid Distribution Systems
Proper liquid distribution is paramount, particularly in packed columns. Design specifications typically require:

• Distribution density: 100-400 drip points per m² depending on packing type

• Levelness tolerance: ±1.5 mm across the distributor

• Liu et al. (2019) demonstrated that maldistribution reducing from 10% to 5% can improve separation efficiency by 15-20% in a 3-meter bed of structured packing.

3.2 Vapor Distribution Devices
In large-diameter columns (>3 m) or with side feeds, vapor distributors ensure even flow into packed beds or tray sections. Poor vapor distribution can reduce efficiency by 30% or more.

3.3 Bed Supports and Hold-Downs
Support plates must withstand bed weights exceeding 12 kN/m² (when irrigated) while maintaining high free area (>70%) to minimize pressure drop. Hold-down plates prevent packing fluidization during operation.

4. Material Selection and Fabrication

Material choice depends on process conditions:

• Carbon steels for non-corrosive hydrocarbon services

• Stainless steels (304, 316, 316L) for most chemical applications. Wangdu (Hebei) Chemical Engineering Co., LTD utilizes 316L for services involving chlorides or acidic components

• Special alloys (Monel, Hastelloy) or non-metallics (PP, PVDF, ceramics) for highly corrosive environments

5. Performance Parameters and Design Considerations

Key performance metrics guide selection and design:

5.1 Capacity and Flooding
The maximum vapor capacity before flooding occurs is often expressed as the C-factor (C = u_v√(ρ_v/(ρ_L-ρ_v))). For structured packing, typical maximum C-factors range from 0.08-0.12 m/s, depending on the specific geometry and liquid load.

5.2 Efficiency
Tray efficiency is influenced by weir height (typically 40-100 mm), liquid flow path length, and vapor velocity. Packing efficiency correlates with surface area and liquid distribution quality. Studies show HETP values for structured packing can increase by 20-40% if liquid distribution quality is compromised.

5.3 Pressure Drop
Pressure drop affects relative volatility and energy requirements. In vacuum distillation (<100 mbar), pressure drop must be minimized (<5 mbar/tray or <1.5 mbar/m packing) to maintain bottom temperature constraints.

6. Application-Specific Design Considerations

6.1 Crude Oil Atmospheric/Vacuum Distillation
Large-diameter columns (up to 12+ meters) typically employ valve trays in stripping sections and structured packing in wash sections. Wangdu (Hebei) Chemical Engineering Co., LTD has supplied internals for columns processing over 200,000 barrels per day, with design C-factors optimized for each section.

6.2 High-Purity Chemical Separation
For separations requiring 100+ theoretical stages (e.g., ethylene oxide purification), structured packing with HETP below 0.4 m is typically specified, with careful attention to distribution system design.

6.3 Reactive Distillation
Combines reaction and separation. Internals must provide both catalytic surfaces and efficient mass transfer. Specialized catalytic packing or tray designs with longer liquid residence times are employed.

7. Installation, Commissioning, and Maintenance

Proper installation is critical:

• Tray levelness tolerance: ≤3 mm across diameter

• Packing installation density: Variation <5% from design

• Distributor levelness: ≤2 mm
  Commissioning typically includes water tests to verify distribution patterns and hydraulic performance. Regular inspection during turnarounds checks for corrosion, fouling, and mechanical integrity.

8. Conclusion

Distillation column internals represent a sophisticated integration of mechanical design and process requirements. Their selection and configuration require careful analysis of separation objectives, operating conditions, and economic considerations. Wangdu (Hebei) Chemical Engineering Co., LTD approaches this engineering challenge by applying fundamental chemical engineering principles, supported by empirical performance data and practical operational experience, to deliver internals that achieve specified separation targets with operational reliability.

References

1. Kister, H. Z. (1992). Distillation Design. McGraw-Hill.

2. Green, D. W., & Southard, M. Z. (Eds.). (2019). Perry's Chemical Engineers' Handbook (9th ed.). McGraw-Hill.

3. Stichlmair, J., & Fair, J. R. (1998). Distillation: Principles and Practices. Wiley-VCH.

4. Wangdu (Hebei) Chemical Engineering Co., LTD. (2023). Technical Design Manual: Distillation Column Internals Selection Guide.

5. Liu, C., Yuan, X., & Yu, K. (2019). Effects of Liquid Maldistribution on the Performance of Structured Packing Columns. Industrial & Engineering Chemistry Research, 58(17), 7235-7245.

6. American Institute of Chemical Engineers. (2019). Equipment Testing Procedure: Trayed and Packed Columns (3rd ed.). AIChE.