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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 control devices. 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. As pointed out in our comments on the 

proposal, adding a thermal oxidizer/scrubber for handling Group 1 continuous, 

halogenated front-end process vents will introduce a significant additional maintenance 

concern that may result in additional startup/shutdowns of the entire front-end or in 

extending startup/shutdowns that occur primarily for other reasons.1

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

source category units the proposed requirements for Group 1 continuous, halogenated 

front-end process vents to achieve at least a 98% organic HAP reduction and 99% HCl 

reduction cannot be met because the minimum thermal oxidizer temperature cannot be 

maintained. That situation persists until some point in the startup sequence. On 

shutdown, where the equipment is to be gas-freed, these instances occur after process 

fluid is no longer entering the equipment and the vent streams contain only the organic

compounds and organic HAPs that were trapped in the process equipment being vented

and the nitrogen being used to displace those organics. As the nitrogen purging 

proceeds, the organic content of the vent streams rapidly diminishes. Typically, nitrogen 

purging is done by pressuring the equipment with nitrogen and then rapidly depressuring 

to a low pressure. This approach is much more efficient that attempting to purge the 

equipment using a nitrogen flow-through approach. On startup of equipment that has 

been gas-freed, flow-through or pressuring and depressuring using process gas must 

occur to displace the nitrogen in the equipment that was used to displace the air that 

entered when the equipment was opened. It may take several equipment volumes of 

nitrogen or process gas to complete the purging cycles.

 

1 ExxonMobil Comments to Docket ID EPA-HQ-OAR-2010-0600-0265.1, 12-6-2010, Page 20 “Thermal oxidizers and 

halogen scrubbers, on the other hand, due to more moving parts and the severity of service, require more maintenance 

and typically are not spared (and the cost of sparing was not included in the cost or cost-effectiveness analyses). In this 

particular service, corrosion is expected to be a particular service factor issue.”
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Possible Approaches to and Considerations for Standards during Startup and 

Shutdown

There are two potential approaches to imposing standards on Group 1 continuous, 

halogenated front-end process vents during those portions of a start-up and shutdown 

when the normal organic HAP and halogen reduction requirements cannot be met. One 

possibility is to establish a lower organic HAP reduction requirement for such periods. 

The second, if the requirements of CAA section 112(h) are met, is to impose a work 

practice requirement. We discuss the feasibility of these two approaches in detail below.

Lower Organic HAP Reduction Standard

Imposing an organic HAP reduction standard lower than the normal 98% reduction is a 

possible approach given that the scrubber system would still be capable of removing at 

least 99% of any HCl formed in the combustion process. (Because less organic halide is 

being converted to HCl by the thermal oxidizer during startup and shutdown, less than 

the normal mass of chlorine will be removed from the vent stream by the scrubber under 

these conditions.). However, there are two problems with this approach that make it 

impractical. First, the organic HAP reduction capability is constantly changing. Second,

there is no way to demonstrate compliance with the lower reduction standard.

As nitrogen displaces organics in the process equipment being shut down, the thermal 

oxidizer temperature will fall2. At some point, the temperature will drop below the point 

that indicates 98% organic HAP reduction (and 98% conversion of organic halide to 

hydrogen halide). As the vent stream continues to be diluted with nitrogen, the thermal 

oxidizer temperature will continue to fall as will the amount of organic HAP reduction. 

On startup, the temperature in the thermal oxidizer will drop as nitrogen is displaced to 

the thermal oxidizer by organics entering the equipment and then, as the organic 

concentration in the vent increases, the temperature will increase until it is above the 

minimum temperature required to achieve 98% organic HAP reduction (and 98% 

conversion of organic halide to HCl). It is these time periods during which the thermal 

oxidizer minimum firebox temperature cannot be maintained where an alternative 

standard is required. Since the thermal oxidizer reduction capability is constantly 

changing during this time and varies with each startup/shutdown permutation, it is 

impossible to establish a single numerical standard for these situations.

If a percent organic HAP reduction requirement were established for startup and 

shutdown periods when the normal standards cannot be met, performance tests would 

be needed to establish the reduction that corresponds to particular firebox temperatures, 

as the basis for continuously demonstrating compliance. Performance tests require at 

least three one-hour test runs3and stable operation for that time at the particular set of 

conditions being tested. But vent composition and flow are constantly and rapidly 

changing during startup and shutdown. Additionally, the entire period of concern is likely 

to be less than 3 hours for some startups and shutdowns and only somewhat longer for 

most. Finally, there are a large number of permutations of startup/shutdown situations. 

Thus, it is difficult, if not impossible; to develop a sound correlation between firebox 

 

2 Temperature is the specified continuous compliance monitoring parameter for thermal oxidizers and is an indicator of the 

organic HAP reduction achieved.

3

It is likely that more than 3 hours of operation would be needed to measure the very low levels of organic HAP in the 

thermal oxidizer outlet that would occur in these situations.
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temperature, vent conditions and organic HAP reduction. Attempting to run performance 

tests at simulated startup and shutdown conditions is impractical, because the constantly 

changing conditions and the multitude of potential situations cannot be simulated. 

Work Practice Standard

As this discussion demonstrates, it is technically and economically impractical to 

measure organic HAP reduction during startup and shutdown situations or to 

demonstrate compliance with an organic HAP reduction requirement, particularly in light 

of the insignificant amount of organic HAP and HCl emission potential during this time. 

Even if a mass emissions limit were established instead of a reduction requirement for 

such periods, it would be impossible to measure emissions for the same reasons - the 

period when the normal reduction requirements cannot be met is of very short duration, 

with constantly changing vent flows and emissions. In summary, it is not technically

feasible to complete stack testing - in particular, to repeat the multiple required test runs 

- during periods of startup and shutdown due to physical limitations and the short 

duration of startup and shutdown periods. Therefore, establishment of a separate work 

practice standard for periods of startup and shutdown is authorized by CAA section 

112(h) and the approach we recommend.

Work Practice Recommendation

A work practice requirement would provide an emission standard for startup and 

shutdown periods, while minimizing emissions during periods when the thermal 

oxidizer/scrubber system cannot achieve the specified emission reduction requirements 

and while facilitating the rapid and efficient removal of hydrocarbon from process 

equipment. 

In order to quickly and efficiently clear hydrocarbon from process equipment, the 

equipment is isolated from process fluids, drained of liquids, pressured with nitrogen and 

then depressured to a low pressure. Depressuring to higher pressures is ineffective,

and flow-through approaches are slow and use inordinate amounts of nitrogen, 

extending downtimes and thermal oxidizer criteria pollutant emissions. For these 

operations, the pressure needed to get the Group 1 continuous, halogenated front-end 

process vents into the thermal oxidizer/scrubber system is anticipated to be relatively 

high compared to the low pressures needed for optimal removal of the organics from the 

process equipment.

Because flare gas collection systems operate at low pressure, diverting the Group 1 

continuous, halogenated process vents to the flare when they can no longer be handled 

in the thermal oxidizer or when it is necessary to allow efficient hydrocarbon clearing of 

the process equipment makes sense. Use of the flare is an allowed compliance option 

for non-halogenated streams and thus will allow compliance with the organic HAP 

reduction requirement for most of the startup/shutdown time period, while continued use 

of the thermal oxidizer system would not. It is true some HCl emissions will result from 

diverting these vents from the thermal oxidizer/scrubber to the flare, but the amount of 

HCl will be small since the vents will contain only residual amounts of organic halide and 

this situation will only occur for short periods. This small amount of HCl emission is 

preferable to the release of organic halide from a thermal oxidizer/scrubber system

operating at lower rates of reduction and the emissions associated with slow, inefficient 

removal of hydrocarbon from the process equipment.
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It is difficult to identify just what conditions would require relief from the normal 

requirements prior to designing and testing the thermal oxidizer/scrubber system. Thus, 

we recommend the parameter values that would trigger diversion from the thermal 

oxidizer to the flare and then from the flare to the atmosphere be established following 

equipment design and initial system testing and then communicated in the Notice of 

Compliance Status (NOCS).

At some point, the vent stream pressure will not allow routing these streams or Group 1 

continuous, non-halogenated front-end process vents to the flare and they will have to 

be routed to the atmosphere. We believe being unable to send the vent to the flare due 

to low pressure 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, 

halogenated front-end process vents aligned to the thermal oxidizer/scrubber until 

equipment to be shutdown is isolated (i.e., no additional hydrocarbon or halogen is 

entering the equipment) and the minimum thermal oxidizer firebox temperature can no 

longer be maintained. Then, either route the vent to the flare or demonstrate by

engineering analysis that the vent contains <50 ppmv organic HAP, in which case it may 

be routed to the atmosphere. Group 1 continuous, halogenated front-end process vents 

may be routed to the atmosphere when 1) the vent pressure is too low to go to the flare, 

or 2) the vent contains <50 ppmv organic HAP as demonstrated by engineering analysis.

Proposed startup work practice procedure (from point where equipment is under 

nitrogen pressure): Align the Group 1 continuous front-end process vent to the flare 

and begin displacing nitrogen to the flare with process gas. For Group 1 continuous, 

halogenated front-end process vents, route the vent to the thermal oxidizer/scrubber 

when the vent composition is acceptable for maintaining the required minimum firebox 

temperature, based on engineering assessment.

Suggested Regulatory Language

§63.485(w) Startup and shutdown

(1) During periods of shutdown, when the requirements of §63.113(a)(2) for a Group 1 

halogenated continuous front-end process vent cannot be met, a Group 1 halogenated 

continuous front-end process vent is not subject to the requirements of §63.113(c) and 

may be routed to a flare, if process fluid is not entering the process vessel that is the 

source of the vent. The 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).

(2) During periods of startup, when the requirements of §63.113(a)(2) for a Group 1 

halogenated continuous front-end process vent cannot be met, a Group 1 halogenated 

continuous front-end process vent is not subject to the requirements of §63.113(c) and 

may be routed to a flare, until the stream characteristics allow the requirements of 

§63.485(a)(2) to be met.
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(3) The owner/operator shall identify the parameter(s) and parameter values that 

indicate the requirements of §63.113(a)(2) and (c) cannot be met for Group 1 

halogenated continuous front-end process vents and report those parameters in the 

Notice of Compliance Status required by §63.506(e)(5)