### Page 1

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, t he frequency of planned start ups/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 hurricane s . For the three ExxonMobil units i mpacted 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 shutd own. 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 tha t 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 propo sed requirements for Group 1 continuous, halogenated front

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

re duction 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 HAP s that wer e 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 eq uipment 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 pr essuring 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 an alyses). In this particular service, corrosion is expected to be a particular service factor issue.”

### Page 2

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 re duction 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 im pose a work practice requirement. We discuss the feasibility of these two approaches in detail below.

Lower Organic HAP Reduction Standard

Imposing a n organic HAP reduction standard lower than the normal 98% reduction is a possible approach given that t he 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 chlori ne 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 re duction 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 fall 2

. At some point, the temperature will drop below the point that indicates 98% o rganic HAP reduction (and 98% conversion of organic halide to hydrogen halide) . A s 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 increase s , the temperature will increase until it is above the minimum temp erature required to achieve 98% organic HAP reduction (and 98% conversion of organic halide to HCl)

. It is the se time period s during which the thermal oxidizer minimum firebox temperature cannot be maintained where an alternative standard is required. Si nce 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 req uirement 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 temperature s , as the basis for continuously demonstratin g compliance . Performance tests require at least three one

- hour test runs 3 and 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 perm ut ations 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 m ore than 3 hours of operation would be needed to measur e the very low levels of organic HAP in the thermal oxidizer outlet that would occur in these situations.

### Page 3

temperature, vent conditions and organic HAP reduction

. A ttempting to run performance tests at simulated startup and shutdown conditions is impractical, because the constantly changing conditi ons and the multitude of potential situations cannot be simulated.

Work Practice Standard

As this discuss ion demonstrates , it is technically and economically impractical to measure organic HAP re duction during startup and shutdown situations or to demon strate 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 re duction requirement f or such periods, it would be impossible to measure emissions for the same reasons

- the period when the normal re duction requirements cannot be met is of very short duration, with constantly changing vent flow s and emissions. In summary, it is not technic ally 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, establish ment of a s eparate 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 per iods, while minimiz ing emissions during periods when the thermal oxidizer/scrubber system cannot achieve the specified emission reduction requirements and while facilitat ing 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 f low

- through approaches are slow and use inordinate amounts of nitrogen

, extending downtimes and thermal oxidizer criteria pollutant emissions

. For these operations, t he pressure needed to get the Group 1 continuous, halogenated front

- end process vents int o 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 Gr oup 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 clear ing of the process equipment makes sense. Use of the flare is an allowed compli ance option for non

- halogenated streams and thus will allow compliance with the organic HAP reduction requirement for most of the startup/shutdown time period

, wh ile continued use of the thermal oxidizer system would not

. I t is true some HCl emissions wil l 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 resi d ual amounts of organic halide and this situation will only occur for short periods. T his small amount of HC l emission is preferable to the release of organic halide from a thermal oxidizer/scrubber system operating at lower rates of re duction and the emissions associated with slow, inefficient removal of hydrocarbon from the process equipment

.

### Page 4

I t 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 oxid izer 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 routin g 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 5 0 ppmv organic HAP cont e nt (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 c ontinuous front

- end vents from the flare to the atmosphere.

Proposed shutdown work practice proced ure: 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 f irebox temperature can no longer be maintained . Then

, either route the vent to the flare or demonstrate by engineering analysis that the vent contains < 5 0 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 < 5 0 ppmv organic HAP as demonstrated by engineering analysis.

Proposed startup work practice procedure (from point wh ere 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 th ermal 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, whe n the req uirements 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 demonstra ted 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 r equirements 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.

### Page 5

( 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 vent s and report those parameters in the Notice of Compliance Status required by §63.506(e)(5)