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Jun-2016

Rationalisation of and mitigation options for proposed EPA-HQ-OAR-2010-0682

Part 1: Regulatory background and impact on delayed coking and catalytic reforming units.

Aaron Wright, Robert Ohmes, Joni Lipkowitz
KBC Advanced Technologies
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Article Summary
In May 2014, the EPA issued EPA-HQ-OAR-2010-0682, FRL-9720-4, RIN 2060-AQ75 for review and comment. The rule closes emissions limits under all of the major air emission categories such as NSPS, NESHAP and MACT. In addition, new requirements are added for periods of startup, shutdown, and malfunction. Specifically, the proposed new regulations address emissions associated with the following:
• Adding MACT standards for Delayed Coking Unit (DCU) decoking operations
• Revising the Reformer Catalyst Regeneration Unit (CRU) purge vent pressure exemption
• Adding operational requirements for flares used as air pollution control devices (APCD) in Refinery MACT 1 and 2
• Adding requirements and clarifications for vent control bypasses in Refinery MACT 1

Hence, many non-steady state emissions incidents, or malfunctions, will now be treated in the same manner as steady state emissions instances, including legal action. Though these are just proposed regulations, the historical tendency is that proposed actions become required actions, with minimal modifications during the review process. Therefore, refiners should begin addressing how these requirements will impact unit and refinery operations during normal and abnormal operations.

The focus of this two part paper will be to summarise the key areas of concern for refiners from this ~800 page document and offer potential solutions and mitigation options for refiners to consider and evaluate regarding FCCU, DCU, and CRU issues. Part 1 of the paper will focus on background of the regulations and impacts on the Delayed Coker (DCU) and Catalytic Reformer (CRU) units, whereas Part 2 will focus on the FCC.  The impact of these regulations on the other refining areas is left for future discussion. Options will be based on industry experience and practical thinking, as well as offering novel alternatives. 

Background
This regulation applies to startup, shutdown and malfunctions. These changes are comprehensive and will bring the entire US refinery system to a lower emission level standard than current levels. The work is a result of the Consent Decrees issued in the late 1990s to 2005 timeframe. Data were collected by refineries and these were used to support the proposed changes. The following is subject to change as the document has gone out for public comment, and the EPA is currently evaluating the industry comments.

The federal emission standards that are the primary subject of this proposed rulemaking are:
• National Emission Standards for Hazardous Air Pollutants from Petroleum Refineries 
(40 CFR part 63, subpart CC)  - (Refinery MACT 1)
• National Emission Standards for Hazardous Air Pollutants for Petroleum Refineries: Catalytic Cracking Units, Catalytic Reforming Units, and Sulfur Recovery Units
(40 CFR part 63, subpart UUU)   -   (Refinery MACT 2)
• Standards of Performance for Petroleum Refineries
(40 CFR part 60, subpart J  -  (Refinery NSPS J)
• Standards of Performance for Petroleum Refineries for which Construction, Reconstruction, or Modification Commenced After May 14, 2007
(40 CFR part 60, subpart Ja)  -  (Refinery NSPS Ja)

DCU Proposed Rule
EPA regulation Refinery NSPS Ja had previously established a pressure limit of 5 psig prior to allowing the coke drum to be vented to the atmosphere. Based on a review of permit limits and consent decrees, the EPA found that coke drum vessel pressure limits in industry have been regularly established (and achieved) as low as 2 psig.1

After review of the operating DCU information, the EPA considered setting additional work practice standards regarding draining, deheading, and coke cutting.1 However, it was determined  that, due to the nature of the operation during these phases of the coker cycle, reliable measurements of hazardous air pollutants could not be made and a reasonable standard could not be set. The proposed coke drum pressure limit will therefore cover all phases of the drum operation where the process is exposed to the atmosphere.

Therefore, the EPA is proposing the coke drum must reach 2 psig or less prior to any decoking operations, which includes atmospheric venting, draining, deheading, and coke cutting.1  DCUs that cannot currently meet the 2 psig pressure limit would be expected to modify their operation or arrangement.  The EPA assumed operators would install and operate a steam ejector system to meet the requirement.1

Impact
Most North American DCUs are in compliance or actually vent at lower pressures. Some older designs have blowdown system limitations that make it more difficult to reach this limit.

At least six different options to address the 2 psig limit for depressuring exist:2
1.    Connect to flare gas recovery compressor
2.    Add off-gas compressor on the blowdown settling drum
3.    Add steam ejector on the blowdown settling drum
4.    Add steam ejector on the coke drums
5.    Upgrading the closed vent system to reduce pressure losses or to increase steam condensing capacity
6.    Water fill and overflow drums

It is important to understand what occurs in the drums when they are quenched, as water will not cool all material present and will not reach equilibrium. It is possible to fill the drum with water very fast, although, it is not good to operate the drum in this way, as well as being time consuming and costly. The hot coke will continue to generate steam. Thus, it is possible to pull a vacuum with an eductor. There does not seem to be the necessity for a timeframe to hold at 2 psig before just venting to the atmosphere so that cycle time is reduced.

The answer is to put in steam eductors or compressors, depending on the availability of steam or electricity in the refinery. The overhead accumulator of the blowdown system is the easiest place to introduce steam.

An additional option is to upgrading the closed vent system.  Making modifications would allow the DCU to reduce pressure losses or to increase steam condensing capacity. This change may allow the DCU to depressurise more quickly while emissions are still vented to the closed blowdown system.

Older DCU’s that are operating at high pressures of ~25-40 psig to push throughput cause pressure to have to be raised in the whole system (including coke drums) to keep drum overhead velocities down and minimise entrainment. Therefore, the blowdown systems are operating at a higher pressure differential to the drum. If a steam eductor is then added to try to pull vacuum, the unit could have a problem due to the pressure difference with steam. Thus, these high pressure units will need a different solution, such as a flare recovery system, and require another option besides trying to recover in the fractionator of the DCU.  New DCU’s should have this technology in place.
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