1

Technical Memorandum

Date: March 30, 2011

Subject: Summary of U.S. EPA Discussions with ExxonMobil Representatives to Discuss the 

Polymers and Resins I Portion of the 2010 RTR Proposal/Supplemental Proposal

From: Nick Parsons, EPA/OAR/OAQPS/SPPD/RCG

The purpose of this memorandum is to summarize a series of discussions between 

representatives of the U.S. Environmental Protection Agency (EPA) and ExxonMobil. The topic 

of these discussions was the proposed revisions to the National Emission Standards for Hazard 

Air Pollutants (NESHAP) for Polymers and Resins I (40 CFR part 63, subpart U), specifically 

for the Butyl Rubber and Ethylene Propylene Rubber Production source categories.

Participants

Nick Parsons, U.S. EPA

Penny Lassiter, U.S. EPA

Bob Morehouse, ExxonMobil

Dates of Discussions

February 25, 2011

March 17, 2011

March 22, 2011

March 23, 2011

March 28, 2011

Summary of Discussions

The purpose of these discussions was to discuss ExxonMobil’s comments on the October 

21, 2010 proposal/supplemental proposal for several source categories, including the Butyl 

Rubber (BR) and Ethylene Propylene Rubber (EBR) Production source categories of the 

Polymers and Resins I MACT (75 FR 65068).

ExxonMobil representatives presented their concerns with the proposed requirements for 

the BR and EPR source categories. The key issue of concern for the company was their ability to 

be in compliance with the MACT standard during periods of startup and shutdown.

Based on these discussions, ExxonMobil submitted additional technical feasibility issues 

and proposed alternative regulatory language during periods of startup and shutdown to be 

considered, depending on whether EPA decided to set the standard at the MACT floor or
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beyond-the-floor. This information showed that it was not technically feasible for ExxonMobil to 

achieve 98% organic HAP control from a flare or 98% organic HAP control and 99% HCl

control from a thermal oxidizer/scrubber due to low pressure or low heat content in the front-end

process vents during these periods.

EPA agreed with ExxonMobil’s rationale that they could not comply with the MACT 

standard for continuous front-end process vents during periods of shutdown, and developed an 

alternative standard for continuous front-end process vents in the BR and EPR source categories

during periods of shutdown.
Appendix A

Supplemental Startup/Shutdown Information (2/25/11)


Appendix B

Supplemental Startup/Shutdown Information (3/17/11)
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.”
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 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. 

 

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.
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.
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 frontend 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 frontend 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.
(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 frontend process vents and report those parameters in the Notice of Compliance Status required by 

§63.506(e)(5) 
Appendix C

Supplemental Startup/Shutdown Information (3/23/11)
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).
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).
Appendix D

Supplemental Startup/Shutdown Information (3/28/11)