Hydrocarbon Processing - April 2021 - 59

Process
Engineering
D. A. G. SUARES, Gas Processing Consultant, India

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. 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:
1. 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)

2. 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
Hydrocarbon Processing | APRIL 2021

Hydrocarbon Processing - April 2021

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