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Recommended Procedures for Handling Group 1 Front-end Process Vents

From Butyl and Ethylene Propylene Rubber Source Categories

During Startup and Shutdown

Background: 

For Butyl and Ethylene Propylene Rubber operations, the frequency of planned 

startups/shutdowns is a function of product demand and periodic maintenance or 

construction needs (e.g., a startup/shutdown may be needed to install the facilities 

required to comply with this proposal). Unplanned, but controlled, shutdowns and the 

subsequent startups can also occur in anticipation of major weather events such as 

hurricanes. For the three ExxonMobil units impacted by this rule, we typically have one 

to three startups/shutdowns per year, with major turnarounds every 5-10 years. Most 

shutdowns do not involve the entire process unit, which means that only some vents 

would need to be routed away from the flare. A shutdown may take 24 to 48 hours with 

process equipment clearing and depressuring occurring during the last part of the 

shutdown. Startups typically take 12 hours.

Recommended Approach for Standards during Startup and Shutdown of Vents 

Normally Routed to the Flare.

At some point during the shutdown of Butyl Rubber and Ethylene Propylene Rubber 

source category front-end equipment for the purpose of opening that equipment the 

proposed requirements for Group 1 continuous front-end process vents to route the vent 

to a flare or to remove 98% of the organic HAP can no longer be met. These instances 

occur during shutdown after process fluid is no longer entering the equipment and the 

vent streams contain only small amounts of residual organic compounds and organic 

HAPs. Typically, depressuring to the flare and nitrogen purging is used to displace the 

bulk of the hydrocarbon in the equipment. Since flare pressure is low, this quickly 

reduces the residual organic HAP to insignificant levels. The final step in removing any 

residual HAP and opening the equipment is to route the vent to the atmosphere.

It is technically and economically impractical to measure organic HAP reduction during 

this portion of the shutdown period or to demonstrate compliance with an organic HAP

reduction or mass requirement, because the vent flow, pressure and HAP concentration 

is constantly changing and the HAP concentration and vent pressure are quickly 

reduced to zero. It is not technically feasible to complete stack testing - in particular, to 

repeat the multiple required test runs - during these portions of shutdown due to physical 

limitations, methodology limitations and the short duration of the final purging steps.

Therefore, establishment of a separate work practice standard for these periods of 

startup and shutdown is authorized by CAA section 112(h).
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Work Practice Recommendation

A work practice requirement would provide an emission standard for these shutdown 

periods when the vent streams can no longer be routed to the flare and facilitate the 

rapid and efficient removal of hydrocarbon from process equipment. We believe being 

unable to send the vent to the flare due to low pressure or low heating value or achieving 

50 ppmv organic HAP content (the criterion for a vent being a Group 1 vent, as 

demonstrated through engineering analysis) is the appropriate point at which to allow 

routing Group 1 continuous front-end vents from the flare to the atmosphere.

Proposed shutdown work practice procedure: Keep Group 1 continuous front-end 

process vents aligned to the flare until 1) the vent pressure or heating value is too low to 

go to the flare, or 2) the vent contains <50 ppmv organic HAP as demonstrated by 

engineering analysis. 

Suggested Regulatory Language

§63.485(w) Shutdown

(1) During periods of shutdown, a Group 1 continuous front-end process vent may be 

routed to the atmosphere when the vent characteristics no longer allow routing it to the 

flare or the organic HAP concentration in the vent is <50 ppmv (as demonstrated by 

engineering analysis).