Design considerations when flaring ethylene oxide
The flaring of gases released from normal process vents and safety valve discharges following an overpressure scenario is widely practiced in refineries, petrochemical and chemical plants.
The flaring of gases released from normal process vents and safety valve discharges following an overpressure scenario is widely practiced in refineries, petrochemical and chemical plants. In past projects, ground flares, elevated flare stacks or a combination of these two systems have been successfully used for the flaring of pure ethylene oxide (EO) or EO-rich streams. However, the flaring of pure EO or EO-rich streams requires certain additional precautions, as well as several stringent design and safety considerations due to it being unstable, toxic, highly reactive and flammable. In addition, EO was often vented off from high point vents directly to the atmosphere in many older plants. Since EO is toxic and a known carcinogen, the disposal of large quantities of EO or EO-rich streams from process vents and safety valve discharges by direct venting to the atmosphere has raised environmental concerns in recent years.
EO is the simplest cyclo-ether. It is a colorless gas at room temperature, with a sweet etheric odor and is prepared by reacting ethylene with air or oxygen over a silver oxide catalyst. EO is a good sterilizing agent and is also used to treat foodstuff. However, EO is generally further reacted with other chemicals to produce EO derivatives, the most important being ethylene glycol which is used to manufacture polyester and automotive antifreeze. EO is an important raw material in the manufacture of ethanolamines (used in the production of soaps, detergents and textile chemicals), ethyleneamines, glycol ethers (e.g., solvents for surface coatings) and polyurethanes.
The two reactions of EO of special note are the following:
- Decomposition of EO: EO vapor or EO vapor mixed with air can decompose explosively, generating carbon monoxide and methane. This exothermic reaction is represented in Eq. 1:
C2H4O t CO + CH4 (1)
- Disproportionation of EO: Disproportionation of EO—which consists of a reduction-oxidation reaction—can result in the production of ethylene and carbon dioxide. It is typically represented by Eq. 2:
4C2H4O t 3C2H4 + 2CO2 + 2H2 (2)
Design considerations
Owing to its unstable, toxic, highly reactive and flammable nature, a standalone EO flare system (piping, knockout drum, liquid seal drum and flare stack)—used for the disposal of vapors containing EO or EO-rich streams—should take into account special design and safety requirements. It must be emphasized that a detailed design and safety review should be performed on the piping and instrumentation diagrams of the EO flare system before it is engineered. Furthermore, strict operational and safety considerations need to be enforced during the operation of the system. The following are the main design considerations that should be considered when designing an EO flare system.
Rupture disc upstream of the pressure relief valve (PRV). The two most used relief devices in the process industry are rupture discs and PRVs. Owing to their non-closing nature, rupture discs should not be used in EO service. When relieving EO, a rupture disc should be installed upstream of the PRV to prevent the build-up of solids or blockage at the inlet to the PRV. Solid deposits at the safety valve inlets could form as a result of EO polymerization. All PRVs used in EO service should conform to the requirements of API 520 and API 521. Furthermore, the PRV should be de-rated due to the upstream rupture disc and a capacity correction factor of 0.9 should be used.1
Minimization of the relief device inlet pipe length. The inlet pipe length from the source (i.e., vessel or column shell) to the relief device should be minimized, as pockets of stagnant EO vapor in a long inlet line could lead to EO polymerization. This can result in a build-up of solids, which, if unchecked, could ultimately lead to a blockage of the line, leading to a hazardous situation in the plant during a major relief scenario.
Purging requirements. As practiced in a typical hydrocarbon flare network, a normal fuel gas or natural gas purge must be provided at all flare header and sub-header dead ends to maintain a small positive velocity in the header or sub-header.2 This should be backed up by nitrogen to increase the reliability of the purge.
However, in addition to the normal purge, an emergency purge must be provided for the EO flare. The main function of the emergency purge (natural gas or nitrogen) is to sufficiently dilute the EO-rich stream to make it non-explosive. It must be ensured that the emergency purge is always available.
The availability of the normal and emergency EO flare purges is one of the most critical considerations for an EO flare and must be closely monitored. These purges are essential for the uninterrupted operation of the connected plant. It is highly risky to operate the plant if there is a failure of either one of these purges and strict operational procedures should be enforced to monitor the normal and emergency purges on a routine basis.
The concentration of EO diluents is a function of the pressure and temperature of the system. In the absence of air within the system, the concentration of diluents required to keep the system non-explosive must be more than 15% methane (considering a binary mixture of EO and methane) or 40% nitrogen (considering a binary mixture of EO and nitrogen).3 However, it is recommended that an appropriate factor of safety (around 2-3) should be imposed on these limits, owing to the limited availability of data at higher temperatures.
To improve the reliability of the EO flare system, the emergency natural gas purge should be automatically backed up by nitrogen using a SIL-rated interlock. A pressure sensing system—consisting of two or more pressure transmitters placed between the rupture disc and the PRV inlet—should be used for all PRVs, which could potentially release pure EO or EO-rich streams to the EO flare. During an overpressure scenario, the rupture disc would rupture and the high pressure at the PRV inlet would be used to trigger the emergency purge. One or more additional pressure transmitters can be located at each PRV discharge to further increase the reliability of the system.
Flare gas analyzer. A flare gas analyzer (e.g., a gas chromatograph-based analyzer that is sensitive to 1 ppm of EO) located on the main flare header can be programmed to trigger the emergency flare purge in case the concentration of EO or oxygen exceeds a certain fixed value.
Materials of construction. Any piping and/or equipment that can come into contact with the EO-rich stream must be made of stainless steel (SS). The use of SS minimizes the potential for rust formation. The 300 series austenitic SS has been widely used in EO service. Type 304L has been successfully used for the EO flare headers and sub-headers, while Type 304 and Type 316 SS have been used for small tubing, which cannot be cleaned of rust. Austenitic SS can be used in those areas where EO liquid is likely to remain for long periods of time (suction and discharge piping of flare knockout drum pumps, low point drains, etc).
Traces of rust on the internals of carbon steel piping or equipment would catalyze the disproportion of EO, which would further raise the local temperature above the EO decomposition temperature, leading to a hazardous situation. Furthermore, even clean carbon steel could catalyze the polymerization of EO but at lesser rates than rusted carbon steel. Therefore, the use of carbon steel piping and equipment in an EO flare network should be prohibited.
Since EO attacks several non-metallic materials, including several types of polymers and elastomers, proper care should be taken to select a proper material of construction for gaskets, O-rings, packing, etc. This would include rigorous monitoring and inspection programs before a material is deemed fit for use in EO service. Polytetrafluoroethane (PTFE) is resistant to EO even up to 260°C and has been used successfully in such applications.3
Grounding requirements. EO liquid is conductive. If EO is stored in a metallic container that is grounded, the static charge cannot accumulate. However, if the system is not properly grounded, a static charge can be generated and lead to ignition—owing to the low value of the minimum ignition energy of EO, which is even lower than gasoline vapor.
Therefore, all EO flare system components (including piping and equipment) must be properly grounded to prevent the build-up of static electricity, which could ignite EO and start a fire or explosion.
Prevention of flashback. A flare system is usually equipped with a liquid seal drum to prevent flashback. However, this method suffers from some drawbacks due to the possibility of losing the liquid seal (e.g., if the seal gets blown out following a peak release or if there are issues in establishing and maintaining the required liquid level). The use of two liquid seal drums in a series—one located at the base of the flare stack and another located between the outside battery limits flare knockout drum and the stack—can further enhance the reliability of the EO flare liquid seal system.
In the case of EO flares, the flare tip contains an anti-flashback device (velocity section), which must be designed to minimize possible flame flashback initiated at the flare tip by ensuring that the forward velocity of the flared gases exceeds the flash-back velocity.3 Furthermore, an appropriate velocity seal would need to be provided to prevent air ingress and conserve purge gas. Close follow-up with the flare vendor is recommended at every stage during the design of an EO flare system to increase the reliability of the system in view of the hazards associated with EO. Per flare design regulations followed in some countries, (e.g., Russia), the possibility of including a spare flare stack, liquid seal drum system and knockout drum may also be considered to further increase the availability of the flare system; thus, ensuring uninterrupted operation of the connected units.4
Sampling of flare condensate. Flare condensate collected in the flare knockout drum must be periodically sampled. Any EO-containing flare condensate is required to be routed to the reabsorber column or elsewhere inside the EO unit for further recovery of EO. However, if the flare condensate does not contain EO, it may be routed to wastewater treatment. This can be accomplished through an interlock based on the EO concentration as measured by an on-line analyzer. HP
REFERENCES
- API, “Sizing, Selection and Installation of Pressure-relieving Devices, Part 1—Sizing and Selection,” API Standard 520, December 2013
- API, “Pressure-relieving and Depressuring Systems,” 6th Ed., January 2014
- American Chemistry Council, “Ethylene Oxide Products Stewardship Manual,” 3rd Ed., online: https://www.americanchemistry.com/EO-Product-Stewardship-Manual-3rd-edition/
- Russian standard PB 03-591-03, “Regulations for the Design and Safe Operation of Flare Systems,” 1992
The Author
Suares, D. A. G. - Gas Processing Consultant, India
David A.G. Suares is a process engineer with 29 yr of experience. He earned an MS degree in chemical engineering from the Indian Institute of Science, Bangalore, India. He started his career at Engineers India Limited and later worked for Bechtel (India) Pvt. Ltd. and Fluor Daniel (India) Pvt. Ltd. In the past, he has worked on a variety of flare and relief system projects and has authored several articles based on the same. He is now functioning in the capacity of a freelance consultant in India.
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