Best Valves for Ethylbenzene Operations
6 min read
Introduction
Styrene (SM) is an important raw material in petrochemical industry, which is widely used in the production of styrene-based resins and styrene-butadiene rubber. It is also necessary for the manufacture of ion exchange resins and pharmaceuticals. In addition, styrene is used in the pharmaceutical, dye, pesticide and mineral processing industries. Globally, styrene resins rank third in synthetic resin production, after PE and PVC. Polystyrene (PS) is a styrene homopolymer, one of the five general purpose thermoplastic synthetic resins, widely used in injection molding, extrusion and foam products. The demand for these products has been rising significantly in recent years.
Styrene-butadiene-acrylonitrile copolymer (ABS) is the most widely used thermoplastic engineering plastic, widely used in electronics, automobile manufacturing, home appliances, toys, construction and other fields. Butadiene rubber (SBR), a copolymer of butadiene and styrene, is the most common synthetic rubber and one of the first industrially produced rubbers. SBR is the highest consumption of synthetic rubber. Styrene is also a raw material for the production of acrylonitrile-styrene copolymer (SAN), unsaturated polyester resin (UPR), SBR, styrene-butadiene latex (SBL) and thermoplastic styrene-butadiene rubber (SBS). Therefore, styrene is one of the most important monomers in the chemical industry.
Summary of Valve Applications
In general utility systems, auxiliary production systems, and upstream raw material feed systems operating under medium to low-pressure conditions, the valve applications are similar to those described earlier. Below are the key valve applications in ethylbenzene and styrene units.
1. Catalyst Preparation System in Ethylbenzene Unit
Catalysts include hydrogen chloride and aluminum chloride, which are highly corrosive to standard metal materials due to the presence of chloride in the liquid phase. Non-metal lined pipe assemblies are preferred in view of the lower operating temperature (ambient temperature). Applicable valves include ball valves, plug valves and butterfly valves, which are internally made of Hastelloy B2.
2. Dehydrogenation Reaction Area in Styrene Unit
The high-temperature piping system in the dehydrogenation reaction area includes the superheated steam furnace and piping between the first and second dehydrogenation reactors. The valves used here must handle ultra-high temperatures and vacuum conditions, with pressures around 60 kPa and temperatures between 530-890°C.
Key considerations for selecting these valves include:
- Valve Type: Valves in ultra-high temperature and vacuum environments must be highly stable, have good flow characteristics, and reliable sealing. For temperatures below 815°C (530-640°C as required), the valve body material should be 304H. For temperatures above 815°C (815-890°C), 800HT material is preferred. INCOLOY 800HT, a nickel-based alloy, has excellent creep and oxidation resistance, essential for such applications.
- Stem Packing: Low-leakage packing or packing structures are required for vacuum environments. Enhancements like higher precision in stem and packing box machining, pre-compression of packing, use of lantern rings, and live-loaded gland bolts can significantly reduce leakage.
- Sealing Surface Precision: The sealing surfaces, including the valve seat/disc, bonnet flange/gasket, and packing box/stem, require high precision machining to ensure effective sealing under high-temperature and vacuum conditions.
- Connection Method: Butt-welded connections are preferred for all valves.
- Other Requirements: Considerations include valve actuation mechanisms, bypass applications, heat treatment requirements, and non-destructive testing standards.
3. Vacuum System in the Styrene Unit Distillation Area
The vacuum system handles media like benzene, ethylbenzene, and styrene, all operating under vacuum negative pressure conditions, typically between 40-150°C. The primary requirement for valves in this system is to prevent external leakage to maintain vacuum integrity, especially given benzene's hazardous nature.
Valve requirements include:
- Shut-off Valves: Gate valves and globe valves are commonly used, with butterfly valves preferred for large diameters (≥DN600).
- Stem Packing: Low-leakage packing or packing structures are required.
- Sealing Surface Precision: High precision machining is needed for sealing surfaces.
- Leakage Control: Ensuring zero external leakage is critical due to the presence of hazardous media like benzene.
4. Benzene Media System in Ethylbenzene Unit
Given benzene's hazardous nature, the design for its transportation and storage requires special considerations, such as water seals for storage tanks to prevent leaks and emergency showers and eye wash stations for operators. Valves must ensure strict control of internal and external leakage, operating over a broad temperature range (40-350°C).
Valve requirements include:
- Shut-off Valves: Gate valves and globe valves are typically used.
- Stem Packing: Low-leakage packing or packing structures are required.
- Sealing Surface Precision: High precision machining is necessary.
- Leakage Standards: Valves must meet low-leakage standards specified by engineering firms or users.
- Bellows Sealed Valves: These valves can provide zero external leakage but are costly and impractical for large diameters (DN>250).
Conclusion
In ethylbenzene and styrene units, the selection of valves requires careful consideration of the specific operating conditions, including temperature, pressure, and the nature of the media. Ensuring high precision in manufacturing and choosing the appropriate materials are crucial for maintaining operational efficiency and safety.
FAQs
- What materials are suitable for valves in ultra-high temperature conditions?
- For temperatures below 815°C, 304H is suitable. For temperatures between 815-890°C, INCOLOY 800HT is recommended.
- Why is low-leakage packing important in vacuum systems?
- Low-leakage packing ensures that the vacuum integrity is maintained, preventing loss of vacuum and potential contamination.
- What are the main challenges in processing INCOLOY 800HT?
- The challenges include high purity requirements, a narrow temperature range for hot processing, high heat treatment temperatures, and tendencies for weld and reheat cracking.
- What types of valves are typically used in benzene media systems?
- Gate valves and globe valves are commonly used due to their effective shut-off capabilities.
- How can valve leakage be minimized in hazardous media systems?
- Using high precision machining for sealing surfaces, low-leakage packing, and possibly bellows sealed valves can minimize leakage.