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Refinery Odor Control and Management Options in Wastewater: A study in water quality and public relations

By Don McClary / Cesco Solutions, Inc.

A refinery, located on an environmentally sensitive saltwater bay, was built in an agricultural setting. Decades have passed and the neighborhood has changed into an aware and sophisticated suburban community. This refinery has a crude running rate of about 150,000 barrels / day and a wastewater throughput of approximately 3,000,000 gallons / day. Regulated discharge is into a saltwater bay, which is also part of a major flyway for migratory birds.

The trend here, as for all refineries, is toward tighter controls on industrial wastewater, while at the same time processing higher sulfur feed stocks and producing lower sulfur fuels. These requirements can conflict and sometimes result in shock loading at the Effluent Plant Biotreater. The surrounding suburban area has high environmental expectations for residents and businesses that include no objectionable odors. It should be noted, in this difficult and conflicting management arena, the Effluent Plant (EP) has earned the company CEO’s Environmental Award for three continuous years. These awards were made for total compliance with all discharge parameters, without exceeding any specification.

There are times in this refinery when production in the Sulfur Recovery Unit (SRU) can have system upsets. Amine Regeneration Units (ARU’s) operate on a continuous cycle scrubbing sour gases and liquids with lean amine solution. Absorbed acid gases are released from rich amine solution and regenerated back to lean amine, which is recycled to the Functional Catalytic Cracking Unit, Poly Unit and Hydrotreater to treat more sour gas. This creates the potential of Diglycolamine’s (DGA’s) with associated Hydrogen Sulfide (H2S) getting into the wastewater, creating undesirable odors in the EP Biotreater and surrounding area/community. When DGA’s and associated sulfur compounds are added to wastewater, Chemical Oxygen Demand (COD) rises dramatically.

The Refinery also processes a portion of its spent caustic through the EP as a cost saving measure. Spent caustic sometimes contains odor causing sulfur compounds.

In the past, system upsets sending DGA’s to EP were handled by diversion to available basins and to storage tanks. This could go on for a long time. The hoped for “soft-landing” was to have enough storage to take care of the upset and then feed a small continuous stream back into the EP process which would eventually consume the storage. This was a game of who blinks first. Would the upset pass sooner than available tankage for diversion – or – would wastewater throughput have to be restricted because no more diversion was available and discharge limits could be exceeded? This situation risked good public relations with the local community.

To increase management options, Cesco Solutions, Inc., a supplier to this refinery for twenty years, diagnosed this plant’s problems and proposed a simple, as-needed solution. Hydrogen Sulfide measurements were taken over time using a Hach Hydrogen Sulfide test kit. Measurements indicated an odor threshold in the Biox about 2-PPM Sulfide in the wastewater. However, this 2-PPM threshold was not exceeded every time there was an odor attack. It was suspected other sulfur species such as disulfides, mercaptans, thiosulfates and thiosulfites contribute to odor attacks. A small (as needed) injection of Hydrogen Peroxide (H2O2) was suggested. The effluent stream remains in the pH range of 7.0 to 7.4 so odor causing sulfur species would be converted to soluble sulfate with some elemental sulfur, both being digested in the Biotreater.

There was agreement to try the idea. Cesco provided a Hydrogen Peroxide Injection Module for trial purposes. This consists of liquid storage and containment in approved material containers with a “plug-and-play” pump for metered injection prior to the Biotreater. Active strength of the H2O2 would be just below 20%, which provides a number of advantages:

Strong oxygenation for odor causing sulfur species.
Oxidation of sulfur compounds without reacting with ammonia or other reducing compounds.
Product degrades into oxygen and water without potential for producing other toxic byproducts.
As supplied at 17.5% active, the DOT corrosive designation is removed. This eliminates one more corrosion issue needing to be tracked at the refinery.
At 17.5% active, worker safety is enhanced while providing the right strength to react economically with waterborne sulfur species.

(Note: Cesco provides and maintains the injection module. Operator’s involvement is limited to plugging the pump into 110 VAC when injection is needed.)

A location was carefully chosen upstream from the Biotreater that was clean and away from hydrocarbons, alkalies and acids. A unit safety review and personnel training was completed. Initial injection of H2O2 extinguished odor outbreaks in 15 minutes. It takes twenty minutes for a complete H2O2 reaction and it was noted that odors tended to be worse in the first two rings of the Biotreater. It takes ten minutes for circulation through each of the first two rings so injection location and timing did apply H2O2 just when and where needed. The oxidation was completed in one and one half rings of circulation or in about 15 minutes.

A cause and effect decrease in COD was also noted. This was expected in theory and to have both odor elimination and decrease in COD actually happen was a double win. A COD operational limit, below maximum legal discharge, had been used as a target maximum. Exceeding this target maximum could allow a higher than normal COD situation to get out of hand, exceed the legal maximum limit and stop discharge to the bay. It was noted that injecting H2O2 eliminated odor and also reduced existing COD about 50% Temporarily using H2O2 during a COD excursion beyond the target maximum allowed continuous wastewater processing and discharge. During the trial, injections were decreased by careful optimization so that one pump setting satisfied both odor and high COD issues.

Injection for most odor and COD issues is 3 g/hour for a throughput of approximately 3,000,000 g/day. By reviewing ten months of data, the duty cycle so far is 20%. This includes long periods with no activity along with some small unit turnarounds causing known high COD, occasional SRU odor causing system upsets, and some odor outbreaks attributable to compounds in spent caustic and maintenance activities. From data accumulated, it is possible to identify sources of odor and high COD, and cost share wastewater remediation. This could work a number of ways:

Spent caustic is wasted continuously. Sometimes sulfur compounds in the caustic exceed a threshold and cause odor from the Biotreater. When this happens, and there are no other odor or high COD activities, the hourly cost could be assigned to disposing spent caustic into the wastewater stream.

When a unit goes down for repair or maintenance and effluent will be affected by high COD or odor, that unit could be assigned cost for remediation (Sulfur Recovery Unit, Catalytic Cracking Unit, etc.).

When a non-process maintenance activity creates high COD or odor, that cost center is charged for remediation (cleaning storage tanks, etc.).

This may be more trouble than it is worth, considering cost of time, paperwork and amount of back charges. However, it demonstrates that odor and high COD situations can be identified and managed —simply, safely, and economically.

Also, public odor complaints and heroic diversion activities can be avoided. Discharge to bay can be kept normal and all concerned will be able to get a better night’s sleep.

   
   
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