Valve Selection for Ethylene Production
4 min read
Introduction
Ethylene production can be categorized into several methods based on the cracking process used: tubular furnace cracking, catalytic cracking, partial oxidation, and thermal carrier cracking. Regardless of the cracking method, the resulting cracked gas is a mixture that must be separated to obtain pure olefin products. The main methods for separating the mixtures into olefins and aromatics are deep-cooling separation, oil absorption intermediate cooling separation, and super-adsorption separation. Most ethylene units currently use the deep-cooling separation method. The basic separation processes include sequential separation, pre-ethane removal, and pre-propane removal, depending on the cracking feedstock.
Overview of Valve Applications
1. Cracking System
- Feedstock Oil System: The pressure is relatively low, and the toxicity and flammability of the medium are relatively mild. Therefore, there are no special requirements for valve selection. However, if the sulfur content in the feedstock oil is high, the valve materials in the high-temperature section must meet high-temperature sulfur corrosion requirements.
- Cracked Gas: This is a mixture of hydrocarbon mediums, hydrogen, hydrogen sulfide, and carbon dioxide. During the cooling process, consider high-temperature hydrogen sulfide corrosion and low-temperature hydrogen sulfide and carbon dioxide corrosion. Compared to hydrogenation units, the hydrogen partial pressure here is lower (0.5%-3.6% by mass), and the medium pressure is also relatively low (about 0.2 MPa). Therefore, at high temperatures, the primary requirement for valves is suitable material selection, and below the dew point temperature, consider resistance to sulfide stress corrosion cracking. Due to the presence of hydrogen (Class A flammable gas), ethylene (Class A flammable gas), and hydrogen sulfide (highly hazardous medium), it is necessary to improve internal and external leakage control indicators for the valves. Additionally, high-temperature cracked gas tends to coke, so valves with good flow characteristics should be selected. For the waste heat boiler system, the influence of high-temperature and high-pressure steam medium characteristics on valve selection should be considered. The requirements for valves in high-temperature and high-pressure environments.
2. Compression and Drying System
- Alkali Wash System: Contains cracked gas, so resistance to hydrogen sulfide and carbon dioxide corrosion is the primary consideration when selecting valves. Refer to other chapters for detailed information.
3. Ethylene Separation System
- Corrosion and Toxicity: The medium's corrosion on metals is relatively mild, and the toxicity is weak, so these factors have little impact on valve selection. However, the system experiences low-temperature conditions of -52°C (ethylene stripping column system), -110°C (methane stripping column system), and -165°C (hydrogen stripping system). Therefore, valve selection should primarily consider the impact of low-temperature conditions.You can refer to another article on this website: Valves For Cryogenic Service
4. Propylene Separation System
- Corrosion and Toxicity: Similar to the ethylene separation system, the medium's corrosion on metals is mild, and the toxicity is weak, so these factors have little impact on valve selection. Although the system also experiences low-temperature conditions of -52°C and -32°C, these are much milder compared to the ethylene separation system. Hence, this section does not require a detailed introduction.
Conclusion
Valve selection for ethylene production units involves considering various factors such as temperature, pressure, medium composition, and potential corrosion. Each system within the unit has specific requirements based on these factors, ensuring optimal performance and safety.
FAQs
- What are the primary methods for separating cracked gas mixtures in ethylene production?
- The main methods are deep-cooling separation, oil absorption intermediate cooling separation, and super-adsorption separation.
- What are the key factors to consider when selecting valves for the cracking system?
- Factors include medium toxicity, flammability, sulfur content, temperature conditions, and potential corrosion from hydrogen and sulfur compounds.
- How does the presence of hydrogen sulfide impact valve selection?
- Hydrogen sulfide requires valves to have resistance to sulfide stress corrosion cracking and suitable material selection for high temperatures.
- Why is deep-cooling separation commonly used in ethylene units?
- Deep-cooling separation effectively separates olefins and aromatics from cracked gas mixtures, providing high purity products.
- What are the typical temperature conditions that valves need to withstand in ethylene and propylene separation systems?
- Ethylene separation systems face temperatures as low as -165°C, while propylene separation systems face temperatures down to -52°C and -32°C.