Valve Selection in Ethylene Oxide and Glycol Plants

7 min read

Introduction

Ethylene oxide (EO) and ethylene glycol (EG) are important basic chemical raw materials and petrochemical products, and have a wide range of applications. At present, the main production process of EO/EG in the world is ethylene oxidation. This involves the direct oxidation of ethylene and oxygen to ethylene oxide (EO) in the presence of a silver catalyst, and then the hydration of ethylene oxide to ethylene glycol (EG) in the absence of a catalyst.

Diagram Of A Ethylene Oxide and Ethylene Glycol Plants

Overview of Valve Applications

For utility systems, auxiliary production systems, and other medium and low-pressure environments in EO/EG plants, the application of valves is generally similar to that in other plants. The following sections will focus on some distinctive valves used in EO/EG plants.

1. Valves for Ethylene Oxide Reaction System

The typical valve applications in the ethylene oxide reaction system include oxygen systems and systems containing ethylene oxide reactants. Since the working conditions of the ethylene oxide-containing reactant systems are similar to those in the subsequent EO recovery system, this section will only discuss the selection of valves for high-pressure oxygen pipelines.

Operating Conditions for Ethylene Oxidation Reaction System:

  • Pressure: 2.7–6.6 MPa
  • Temperature: 75–204°C
  • Medium: Medium-pressure oxygen

Key Considerations for Valve Selection:

  1. Valve Type:
    • Isolation Valves: Different patent holders have different interpretations of valve selection. A common principle is that isolation valves in oxygen systems should not be used for regulation. The flow path should transition smoothly when the valve is fully open, making ball valves the first choice, followed by Y-type globe valves. Gate valves are not recommended.
    • Low Resistance and Reliable Sealing: Due to the explosive nature of oxygen, incidents of valve burnouts have occurred due to improper selection and operation. For temperature and pressure-permissible conditions, fire-safe soft-sealed ball valves are preferred. Globe valves should not be used for sizes larger than DN200 due to the difficulty in operation.
    • Check Valves: Prefer flange-connected check valves over wafer-type check valves because flange connections offer better rigidity and sealing reliability. Wafer-type check valves have central valve discs that increase resistance and are prone to "flutter" effects. Generally, use Y-type lift check valves for diameters smaller than DN50 and swing check valves for DN50 and larger. For DN200 and larger, inclined plate check valves can be used. Swing Check Valve
  2. Valve Structure:
    • External Leakage: Mostly occurs at the sealing pairs comprising the valve stem, packing, and packing box, and the valve cover, gasket, and clamping bolts. Bellows-sealed valve covers can effectively solve external leakage issues, but it is not mandatory to use them.
  3. Material Selection:
    • For medium-pressure, high-purity oxygen systems, materials should be non-oxidizing and corrosion-resistant, with low carbon content to reduce the risk of spark generation. Common choices include low-carbon or ultra-low-carbon austenitic stainless steels like ASTM A351 CF8, CF3, CF8M, CF3M, or ASTM A182 F304, F304L, F316, and F316L. Stainless steel castings must be pickled to remove sand and oxides. For high-purity, high-pressure (CLASS600) conditions, nickel-based alloys such as Monel or Inconel are preferred for their superior fire resistance.
  4. Post-Processing:
    • Degreasing, Oil-Free, and Anti-Static: Oxygen valves must be free from oil contamination, as contact between oxygen and oil can cause fire or explosion. All parts should undergo degreasing before assembly, and the valve passage should be degreased again after pressure testing. Anti-static devices are essential to prevent sparks during valve operation.
  5. Other Requirements:
    • These include the selection of drive mechanisms, bypass applications, heat treatment requirements, and non-destructive testing. These requirements are usually specified in the project procurement technical specifications.

2. EO Recovery and CO2 Removal Systems

EO Stripping System Valves:

  • Medium: Ethylene oxide and water, with CO2 and trace organic acids.
  • Pressure: ~3.6 MPa
  • Temperature: Up to 232°C

EO is highly flammable, explosive, and hazardous. Special design considerations are needed for transportation and storage, including water seals or covers for tanks and loading systems to prevent leaks. Safety equipment such as eye wash stations and emergency showers must be nearby.

Valve Requirements:

  • Sealing Performance: Ball valves or plug valves are recommended due to stringent sealing requirements to control both external and internal leaks. Check valves should be flange-connected rather than wafer-type for better sealing reliability.
  • Cleaning and Degreasing: Valves must be cleaned and degreased thoroughly. Full bore valves are preferred to reduce medium flow resistance and CO2 corrosion.
  • Other Requirements: These include drive mechanisms, low leakage requirements, bypass applications, heat treatment, and non-destructive testing as specified in the project procurement technical specifications.

CO2 Removal System Valves:

  • Absorber Operating Pressure: 2.0 MPa
  • Absorber Operating Temperature: 105–115°C
  • Regenerator Operating Pressure: ~0.2 MPa
  • Regenerator Operating Temperature: ~100°C

Key Considerations:

  • CO2 Corrosion and Alkaline Stress Corrosion Cracking: Austenitic stainless steel is recommended for longer life. Carbon steel can be used but has a shorter lifespan. Valves with good flow characteristics are beneficial in reducing corrosion but are not critical at low flow speeds.
  • Valve Types: Ball valves or plug valves are preferred, but globe valves and gate valves can also be used.

3. Light Component Removal and EO Refining System Valves

EO Refining Tower:

  • Operating Pressure: ~0.4 MPa
  • Top Temperature: ~50°C
  • Bottom Temperature: ~150°C
  • Medium: Mainly ethylene oxide with trace formaldehyde.

Valve selection criteria are similar to the EO recovery system, with lower temperature and pressure requirements and minimal CO2 corrosion. Austenitic stainless steel is still preferred to avoid iron ion contamination.

4. EG Reaction and Multi-Effect Evaporation and Drying System Valves

EG Generation Reaction (Hydration Reaction):

  • Temperature: 150–220°C
  • Pressure: 1.0–2.5 MPa
  • Byproducts: DEG, TEG, acetaldehyde, ethyl acetate, etc.

In the EG evaporation system, acetaldehyde, ethyl acetate, and residual oxygen can severely corrode carbon steel. Hence, austenitic stainless steel is generally used for valves. Ethylene glycol is relatively mild, so no special valve requirements are needed.

Conclusion

Valve selection for EO and EG plants requires careful consideration of operating conditions, materials and safety requirements. The main focus is to ensure a reliable seal, minimize corrosion, and prevent leaks and explosions. Correct material selection and post-treatment such as degreasing and anti-static measures are essential to achieving these goals.

FAQs

  1. What are the primary factors to consider when selecting valves for EO and EG plants?
    • Consider operating pressure and temperature, medium properties, sealing performance, material compatibility, and safety requirements.
  2. Why are ball valves preferred for oxygen systems in EO plants?
    • Ball valves provide smooth flow transition, low resistance, and reliable sealing, essential for high-pressure oxygen systems.
  3. How is valve corrosion in CO2 removal systems mitigated?
    • Using austenitic stainless steel or carbon steel with proper flow characteristics to reduce medium flow speed and corrosion impact.
  4. What post-processing treatments are necessary for valves in oxygen systems?
    • Degreasing, oil-free handling, and anti-static measures to prevent fire hazards.
  5. Why is austenitic stainless steel commonly used in EO and EG plant valves?
    • It offers excellent corrosion resistance, durability, and reduces the risk of iron ion contamination.

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