Project Scope

A gulf coast refinery was experiencing unacceptable hydrogen sulfide (H2S) releases into occupied work areas in the Coker unit during cutting & quenching operations. Refinery Coker units are used to process residual oil or ‘heavy ends’. A part of the coking process requires quenching the hot coke with water, resulting in latent hydrogen sulfide being transferred to the water stream and stripping into the vapor phase creating potential exposure risks to operations personnel. In order to protect worker s, hydrogen sulfides in the process water stream must be eliminated.

In late 2016, USP Technologies (USP), leaders in peroxygen-based technologies for industrial water and wastewater treatment applications, was contacted by the refinery in search of a viable solution for its H2S issues. USP recommended a full-service chemical treatment program which included 27% refinery grade hydrogen peroxide, chemical storage, a dosing equipment system, inventory analysis, logistics management, safety training and ongoing field and technical support.

Full-Service Hydrogen Peroxide Treatment Program

The refinery recycles coker quench water in a closed loop, where any chemical used for treatment cannot negatively impact the coking process. Under near neutral pH conditions, the theoretical weight ratio of H2O2:H2S is 1:1, and proceeds according to the following reaction:

H2O2 + H2S → S0 + 2 H2O

Since the products of the above reaction are water and inactive elemental sulfur, the chemistry does not negatively impact the batch coking process.

Project Scope

A sugar processing facility was experiencing hydrogen sulfide (H2S) odor issues in their process water stabilization ponds due to anaerobic conditions. The facility’s four identical ponds are 2,500 feet long, 400 feet wide and up to 12 feet deep, with a finger dike down the middle making each pond C-shaped. Each pond can hold approximately 80 million gallons when completely full. Anaerobic conditions develop in the lower layers of these ponds, leading to H2S generation and volatilization. The facility faces stringent seasonal perimeter ambient air quality standards for H2S concentration and is subject to escalating fines with every violation.

In addition, the pond treatment needs are highly variable as high chemical oxygen demand (COD) slugs and/or factory upsets mean that treatment decisions need to be made in real-time, based on measured and expected levels of sulfide, dissolved oxygen, ORP, COD, pH and temperature. Previously, the facility had been utilizing various chemicals, including peracetic acid and catalyzed hydrogen peroxide (H2O2) in totes to mitigate H2S, however, the facility was experiencing difficulty maintaining control of the H2S and was concerned with violating their permit. Given the large surface area and volume of the ponds it was essential that a cost-effective control method be identified and implemented. USP Technologies (USP) was contacted to provide a long term, viable solution.

Technology

Hydrogen sulfide (H2S) is a colorless gas with the characteristic foul odor of rotten eggs; it is heavier than air, poisonous, corrosive, flammable, and explosive. Many facilities, such as this sugar processor, have limits imposed on H2S emissions. Sulfide is generated by sulfate reducing bacteria (SRB’s) in the anaerobic layers of the ponds. H2O2 treats H2S by creating a zone of oxidation in which sulfides are oxidized before being released to the atmosphere. The reaction is as follows:

H2O2 + H2S → S0 + 2H2O

Sufficient dosing of H2O2 also prevents the formation of H2S by boosting dissolved oxygen and creating aerobic conditions hostile towards SRB’s per the following reaction:

2 H2O2 → O2 + 2 H2O

Solution

The sugar facility conducted an extensive survey of possible control methods and evaluated options on a lab scale prior to pilot testing. H2O2 was found to be the most cost-effective sulfide control agent in the ponds because of its low H2S treatment cost, ability to preferentially oxidize sulfide, including iron sulfide (FeS)

present in the water, permitting long-acting sulfide control by regenerating free ferrous iron (Fe+2) to bind more H2S.

USP provided a program demonstration in 2014, which first involved bringing in tanker truck quantities of H2O2 and pumping this directly into the ponds through fabricated floating distribution nozzles. The desired amount of H2O2 was pumped into specific sections of the ponds, which had anaerobic conditions prevailing, and then the truck would move on to the next section requiring treatment. This provided improved mixing, a lower cost of treatment per gallon, better sulfide control and lowered staff labor costs.

Building off of the success of the tanker treatment method, USP developed an engineered storage and dosing equipment system for H2O2 on each of the four ponds. This allows for treatment to be conducted whenever and wherever conditions warranted. Each of the systems have two floating manifold dosing lines, dubbed PeroxidonsTM, into which H2O2 and pond water are pumped. These lines stretch along the length of each half of the C-shaped ponds, with outlets spaced to distribute H2O2 across the surface of the entire pond. In addition, on each pond 6 – 8 additional dosing lines were installed that allow for concentrated treatment of hotspots including dead zones and the incoming flows. As a result, performance, ease of operation and cost-effectiveness could be improved with the ability to target treatment to specific sections of the pond.

Overall, USP’s full-service hydrogen peroxide program has saved the plant a significant amount of money in fines and operations. USP also maintains a consistent presence on-site, aiding in the daily collection of water and air data, which unburdens plant staff and improves the sulfide control decision-making process. In a collaborative and results-focused approach, USP continues to work closely with plant staff to find ways to improve the program’s effectiveness and to continue lowering the overall cost of treatment.

Treatment results include significant reductions in H2S, increased dissolved oxygen, cost savings compared to previous year’s solutions, reduced in-house labor associated with pond treatment and improved safety over tote handling.