Problem
Many chemical and petroleum companies use batch reactors to make
their products. These reactors typically have vents which are
opened and closed during emptying, filling, mixing, heating or
cooling and other steps of the production process. The gases
coming from these vents must be controlled under most government
regulations. Generally speaking, the emissions produced are
inert (little or no oxygen) streams with relatively low flow and
high concentration of Volatile Organic Compounds (VOCs). In
this case, the company was operating several reactors that required
venting for depressurizing, filling and mixing. The process
flow rate was less than 50 SCFM and was essentially all light
hydrocarbons such as methane, ethane and propane, with some
halogenated hydrocarbons.
There are two oxidation strategies for this type of process
stream, the first is to dilute the process vent with fresh
air. This strategy provides oxygen for combustion and reduces
the Lower Explosive Limit (LEL) below 50%, using a conventional
oxidizer system. The National Fire Protection Association
(NFPA) and FM Global guidelines suggest facilities keep vent
collection systems airstreams below the 50% LEL for safety
reasons. Because of the high BTU (British Thermal Unit)
content of the process vent in this application, it would have
required a high volume of fresh air to achieve the necessary LEL
condition which would have dramatically increased operating
expenses and raised safety concerns.
Solution
While the above scheme is sometimes acceptable,
Anguil implemented a different, safer strategy for this
application. Instead of diluting the process vent with fresh
air Anguil kept the process stream inert, sending it directly
through a burner port of a multi-stage Direct Fired Thermal
Oxidizer (DFTO). Essentially this allowed the combustion
device to use the high BTU content as fuel for oxidation.
Once the DFTO is brought up to operating temperature with
natural gas, the inert process gases are directed to the
burner. During normal system operation, the VOC laden process
vent will fuel the pollution control device. During periods
of low process flow or energy content a supplemental fuel is added
to the burner, natural gas in this case, to maintain operating
temperature of 1400-1600°F (760°C-870°C) in the oxidizer combustion
chamber. A minimum of one second residence time at these
temperatures ensures a destruction efficiency of 99%+, an oxygen
meter in the exhaust stack ensures that sufficient oxygen was
present for complete destruction of the VOCs.
A soft refractory lining inside the oxidizer lets the operator
start and stop the system without risk of refractory failure that
can occur with other designs. This also allows them to shut
down the oxidizer during substantial periods of process downtime
without negatively affecting the longevity of the equipment.
For inert gases that contain no halogens or sulfur compounds the
hot, purified, gas will be released to atmosphere from the
combustion chamber or possibly sent to a heat exchanger for energy
recovery. This particular application contained halogenated
compounds, therefore the hot exhaust gases leaving the DFTO are
directed into a hastelloy quench where they are cooled down before
entering a packed bed scrubber.
The recirculation with a caustic solution inside
the scrubber removes HCl (Hydrochloric Acid) and HBr (Hydrobromic
acid). The scrubbed gases are then pulled into an Induced
Draft fan and finally out an exhaust stack. An induced draft
fan is used with halogenated streams so that there is no potential
for corrosive gases escaping to atmosphere. Any leakage of
acid gases will result in substantial risk to equipment longevity
and to personnel. These scrubbing systems provide 99%+
removal of the acid gases prior to discharge to the atmosphere.
With these acid gases there is also a potential for corrosion of
the oxidizer shell behind the insulation if the metal temperature
is below the acid dew point. To prevent that from occurring,
the oxidizer is designed with an external shroud to keep the carbon
steel shell at a temperature above the acid gas dew point, avoiding
corrosion concerns.
The Anguil System has a complete control system with
communication capability to DCS (Distributed Control Systems)
systems and modems for remote monitoring / diagnostics. These
controls provide for automatic purge, system heat-up and a wide
range of operating conditions. Magnetic driven pumps and
scrubber controls are also provided for automatic operation without
personnel adjustments.