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How to improve steam cracking furnace productivity and emissions control
How to improve steam cracking furnace productivity and emissions control
Hydrocarbon streams feeding ethylene steam cracking furnaces often contain significant levels of corrosion products, water, and salts.Contaminants in the inlet hydrocarbons can adversely affect ethylene production in a number of ways.
Sodium and iron oxides are known to be coke promoters, and their presence can reduce the run time of the ethylene furnaces before decoking is required, and in some instances, reduce the life of the furnace tubes by as much as one third. Unscheduled or frequent decoking cycles lead to a loss in ethylene production, shortened furnace tube life, and create higher maintenance costs. Frequent decoking will also result in increased particulate release to the atmosphere and can create environmental concerns over excessive emissions.
The naphtha filter rating: finding the best compromise to optimize furnace protection
IFT between water & naphtha at the cracking plants and in refineries range from 1 to 20 dyne/cm and is very stable. This also requires the usage of high efficiency liquid-liquid coalescers for separation.
Naphtha feed stocks can contain significant solid particulate in the form of corrosion products and both aqueous and solid contaminants need to be removed. the feed stock may require stand-alone particulate filtration, and in others cases, incorporate a coalescer system with pre-filters. In order to protect the steam cracker, a filter with an absolute rating of 10 μm should be used.
Several types of filters are available, including string wound, pleated, depth, and advanced high flow capacity designs that use laid over, crescent- shaped pleats. When the solids loading is high, backwash type filters become more economical than disposable type.
Here, the initial capital investment is higher, but the operating costs are much lower.
High Efficiency Liquid-Liquid Coalescers:
The separation of emulsified water in the feed hydrocarbon to the steam cracker can be a difficult proposition. Pall Corporation has developed and patented new non-disarming coalescer media that is constructed with novel, formulated polymers and fluoropolymers. These high efficiency coalescers are effective for emulsions having interfacial tensions as low as 0.5 dyne/cm.
High efficiency coalescer system with coalescer elements stacked on top of separator elements.
Effective prefiltration to remove solids is an important consideration for a coalescer system. The coalescer system also provided the following additional process benefits:
- Reduced fouling and plugging in the cracking coils
- Lowered fuel gas consumption due to less fouling
- Improved control of the naphtha to steam ratio due to reduced fouling of control valves and flow meters
- Reduced maintenance costs associated with mechanical cleaning of the transfer line exchangers
- Lengthened furnace tube life due to the use of liquid-liquid coalescers, which helped eliminate the sodium attacks that drastically reduce the lifespan of the tubes
- Reduced CO formation and concentration in the cracked gas
- Lowered particle and CO2 emissions due to less decoking operations in one year.
High efficiency liquid-liquid coalescers and filters used to treat liquid or gas hydrocarbon feeds in ethylene plants were found to increase the steam cracker productivity and lower emissions by decreasing the decoking frequency.