Nasser AL-Tell
Bechtel Corp.
Houston
Richard Lueders
Chevron Corp.
Port Arthur, Tex.
Chevron Corp. has started up a new waste water treatment plant at its Port Arthur, Tex., refinery.
Although the refinery had a functioning waste water treatment unit, the need for a newer, more effective one was dictated by:
- The U.S. Environmental Protection Agency's recent ruling called "National Emission Standards for Hazardous Air Pollutants"' or Neshap, as it applies to benzene
- Resource Conservation and Recovery Act (RCRA) regulations
- Economic incentives (Operating costs will be less for the new unit. Building the new unit also was less expensive than retrofitting the existing waste water treatment plant.)
The new facility has an hydraulic capacity of 10,000 gpm and will treat process waste water, cooling tower blowdown, and contaminated storm water (Fig. 1).
The plant includes:
- A process unit for removing free and emulsified oil
- An equalization facility
- A biological system for organics biodegradation
- A volatile organic compounds (VOC) control system.
The new plant was designed by the Houston regional office of Bechtel Corp.
The final Neshap rule was promulgated in December 1992 and became effective Apr. 7, 1993. The rule sets standards for control of benzene and other VOC emissions from waste operations. The rule applies to chemical and petroleum industries that manage 10 megagrams/year (Mg/yr) or more of benzene in waste, and includes streams of 10 ppm or more benzene concentrations (annual average). RCRA regulations prohibit the collection and treatment of process waste water with benzene concentrations of 0.5 ppm or more in earthen basins.
The existing waste water treatment plant at the Port Arthur refinery included earthen basins and no VOC control mechanisms. The new plant will enable the refinery to meet Neshap, RCRA, and new source performance standards, and best available control technology (BACT) for air emissions, as mandated by the 1990 Clean Air Act Amendments.
WASTE WATER SOURCES
The refinery generates an average of 3,500 gpm of process waste water. These streams come from various operations in the refinery and chemical plant, such as desalters, sour water strippers, and quench water.
Several cooling towers generate about 2,300 gpm of blowdown, which also will be routed to the new treatment plant. Contaminated storm water will be held and treated when capacity is available.
Normal flow through the new plant is about 6,000 gpm with a biological loading of 25,000 lb of biological oxygen demand (BOD)/day. (BOD is a measure of the waste water contamination.)
The plant is designed to handle a peak flow of up to 10,000 gpm with a biological loading of 54,000 lb BOD/day. The waste water contains free and emulsified off, ammonia, and soluble contaminants which must be removed prior to discharge to a nearby bayou.
PREDESIGN STUDIES
Various studies were performed prior to the design of the new treatment plant, The studies were intended to ensure optimum design and to better understand the quality and quantity of waste water for source control purposes.
Four major areas were studied: waste water characterization, benzene reduction, oxygen transfer rate, and treatability.
During the waste water characterization study, samples and flow readings were randomly collected from the major refinery units, as well as from selected locations in the waste water collection header. The samples were analyzed for the concentration of various contaminants such as BOD, oil content, and benzene. Statistical analyses were performed on the collected data.
The study identified the major flow and contaminant load to the waste water treatment plant and marked the potential for waste minimization. Preliminary cost analyses using the characterization study results indicated that treatment at the end of the collection header ("end of pipe" treatment) would be more economical than treatment at the production units.
The refinery performed a benzene-reduction study aimed at determining the benzene biological biodegradation rate.
The study was to be used to build a benzene-reduction reactor to reduce the benzene concentration to less than 10 ppm, or to optimize the size and configuration of the biological system.
A pilot plant was put in place in the existing treatment plant and run for 3 weeks. Various design parameters such as the concentration of system microorganisms, biodegradation time, and air flow rates were measured. Although the study showed that benzene is easily biodegraded, no breakthroughs were found to drastically alter the design of the biological system.
The oxygen-transfer rate (referred to as "Alpha factor) was studied in the existing plant using the refinery's laboratory. The purpose of the study was to optimize the size of the new planes air supply (air blowers).
Waste water samples were collected daily, and analyzed for oxygen-transfer rates in a bench-scale model for 2-weeks. The results were comparable to those of other Chevron refineries.
The treatability study was crucial in determining the biological system design parameters. Waste water samples were brought daily to a Houston lab where three bench-scale model,; were operated. The models simulated what is known as an activated-sludge system, which utilizes microorganisms and oxygen to biodegrade the contaminants.
The microorganism mass is settled in clarifier, and recycled to degrade new waste. Two of the models simulated different biodegradation times, and the third simulated the powdered activated carbon treatment (PACT) system. In the PACT system, powdered carbon is added to enhance contaminant removal.
Data such as removal efficiencies, microorganism concentrations, and settling rates A,ere collected daily for 3 months. Numerous plots, calculations, and analyses were generated to produce the specific design parameters for the new treatment plant.
The study eliminated potential inaccuracies inherent in using general design values. Optimizing the size of the biological system reduced capital and operating costs.
DESIGN
A task force of Chevron and Bechtel engineers evaluated the various technologies applicable to each unit's operation.
The evaluation began with a brainstorming session to fist the various equipment alternatives and plant overall configuration. The team used 10 criteria to select equipment and meet requirements for safety, regulations, total cost, operation, reliability, constructability, maintainability, schedule, appearance, engineering practice, and Chevron's needs.
The technical evaluation team, which met for 2 days, recommended the use of the following units:
- API Separators for free oil removal
- An equalization tank to dampen the effect of load surges
- Dissolved nitrogen flotation for emulsified oil removal
- An activated sludge system for contaminant biodegradation
- A regenerative-type thermal oxidizer for VOC heat destruction.
The choice of VOC control system is of special interest. Numerous alternatives were discussed and evaluated. The first set of alternatives focused on reduction of VOC in the liquid phase prior to entering the treatment plant. The second set focused on collecting and treating VOC offgases from the applicable units at the plant.
VOC CONTROL
The first group of treatment alternatives included evaluation of air or steam stripping of the VOC, solvent extraction, refrigerated condensers, and chemical oxidation using ultraviolet light in conjunction with hydrogen peroxide or ozone.
Air or steam stripping was found to be impractical because of the high cost. The VOC-rich air or steam also would require further treatment such as condensation or incineration.
Solvent extraction was rejected because of cost and operability problems. Also, no known industry application exist for benzene extraction from waste water.
Refrigerated condensers were rejected because of cost and potential maintenance and operability problems.
The technical team was not confident of the safety, operability, reliability and maintainability of chemical oxidation. It was found to be expensive, and its selection could have resulted in schedule problems.
The use of ozone for oxidation was also studied. A preliminary academic analysis and a brief pilot study showed that a sizeable ozone-generation plant would be needed with an immense electricity consumption.
The following VOC offgas-control systems were evaluated: Carbon adsorpion, flare/afterburner, catalytic oxidation, and regenerative and recuperative" thermal oxidizers.
The flare system was not chosen because of its fuel consumption. Flares are more effective for low flows less than 500 scfm).
Catalytic oxidation uses a catalyst to reduce the required combustion temperature of the VOC offgases. High cost, potential media fouling, and maintainability made this option unsuitable for this application.
Thermal oxidation was determined to be the most appropriate technology for VOC control at the new plant. The regenerative-type oxidizer was selected because of its good operating history, low fuel requirements, and built-in safety measures.
FINAL DESIGN
The final configuration of the plant included the following units, in processing order.
- Three API separators in parallel (Fig. 1)
- Skimmed oil recovery system
- An equalization tank
- A flocculation tank
- Two dissolved nitrogen flotation (DNF units in parallel (Fig. 2)
- Two aeration tanks in series
- Three clarifiers in parallel
- A regenerative-type thermal oxidizer
- Chemical feed systems.
Fig. 3 is an illustration of the new waste water treatment plant.
The process waste water is split equally into the three API separators, where quiescent conditions promote settling of suspended solids and the separation of free oil.
Each separator includes a chain-drive mechanism to collect the settled solids in three hoppers at one end of the separator and oil at the other end. Oil is collected with a continuously rotating skimmer and a slotted pipe.
Each separator includes a floating roof to prevent emission of VOC to the atmosphere. The front and back end of each separator has a fixed cover to facilitate inspection of unit operation. Offgases from the end are collected to a nitrogenrich header and sent to the thermal oxidizer. The end section of each separator is kept under a slight vacuum.
The collected oil from the API separators is transferred to a gravity, separation tank to remove excess water before pumping the oil to the refinery's oil recovery system.
The equalization tank includes two side-mounted mixers to fully mix the waste water and keep solids in suspension. This is known as load equalization. The process helps contain spills and dilutes toxin concentrations.
Bechtel devised a model to size the equalization tank and to predict its effect on the waste water concentration.
The flocculation tank receives flows from the equalization tank and could receive the cooling tower blowdown, as well as contaminated storm water, depending on the quality.
The flocculation tank is used to control the pH of the waste water and for polymer addition. Polymer is needed for removal of emulsified off at the dissolved nitrogen flotation units. The tank includes a vertically mounted mixer and a fixed roof. The tank is kept under a slight vacuum to collect any offgases.
Waste water from the flocculation tank splits equally into both DNF units. The DNF influent waste water mixes with a pressurized (40-60 psig) nitrogen-saturated, recycled stream before entering the DNF tanks. When the mixed streams are released to the atmospheric DNF tanks, "microbubbles" form and carry the flocculated emulsified oil particles to the surface, where they are skimmed and collected.
This "float" is sent to the refinery's oil-recovery unit and the waste water flows into the aeration tanks. Each DNF includes a skimmer/rake assembly to collect the float and any settled solids.
Nitrogen was selected, instead of air, to render the DNF atmosphere inert. Each DNF is coupled with a nitrogen-saturation tank where about 40 scfm of nitrogen is dissolved into a recycled stream. Each DNF system functions independently of the other. The DNFs have fixed roofs and operate at a slight vacuum to collect the nitrogen-rich offgases.
The DNF effluent is pumped into the first aeration tank, where it is mixed with a recycled sludge stream from the clarifiers. The aeration tanks operate in series. Air blowers supply the oxygen required for the microorganisms to biodegrade the contaminants and to keep the waste water and biological mass in suspension. A network of diffusers, known as coarse-bubble diffusers, at the bottom of each tank distribute the air through the tank.
Offgases from the first aeration tank are collected in a header and sent to the thermal oxidizer.
Most of the VOC win be removed in the first aeration tank through biodegradation and the stripping action of the supplied air, consequently, no offgases are collected from the second aeration tank.
The pH and dissolved oxygen levels are monitored and controlled closely in the tanks to ensure proper contaminant removal.
The influent waste water remains in the aeration tanks for about 18 hr before it flows into the clarifiers.
Effluent from the aeration tanks is split and flows by gravity into three clarifiers. One of the three clarifiers was part of the existing waste water treatment plant.
At the clarifiers, biological solids are settled and recycled to the first aeration tanks, and clear, treated water flows to a polishing pond. Each clarifier includes a skimmer/rake mechanism to facilitate removal of any floating material or sludge. The new clarifiers utilize polymer and flocculation mechanisms to enhance the settling of solids.
An off site gravity thickener is designed to remove excess water from the API separator bottom sludge and DNF oily solids. The thickened solids are then pumped to a solids-handling facility for treatment.
The plant includes two headers-a small nitrogen-rich header and a large air-rich header-to collect offgases from different units and send them to the thermal oxidizer. The nitrogen-rich header collects offgases from units that generate relatively VOC-rich offgases: API separators, flocculation tanks, and DNF tanks.
Nitrogen was chosen to maintain an inert atmosphere.
The air header collects the diluted offgases from the first aeration tank. Both headers discharge into a water-seal drum to prevent any flashback from the oxidizer.
The combined offgases from the water-seal drum discharge into the thermal oxidizer. The thermal oxidizer is sized for 14,500 scfm. At the oxidizer, the VOCs are thermally destructed with at least 98% efficiency.
The oxidizer recovers approximately 95% of the heat utilizing three ceramic saddle beds and valve-sequencing action. Occasionally, natural gas may be added.
The treated offgases flow through a stack and are analyzed to ensure the proper operation of the oxidizer. An induced-draft fan creates the necessary vacuum to collect the offgases at both headers in the plant.
Various safety measures are incorporated in the oxidizer and the collection headers.
Chevron and Bechtel conducted safety reviews of the new plant design and produced a design risk assessment to evaluate possible future new conditions on the design of the new unit.
The new plant was designed to have flexibility.
The plot space requirements are 1,400 ft x 900 ft, in an L-shaped configuration. The plant's modular design allows taking some of the tanks out of service without interrupting the operations of the plant.
All rotating equipment is spared, and the electrical feed circuit is redundant with automatic switch over.
COST SAVINGS
Innovative cost-saving measures were implemented, such as the use of floating roofs, even in very small tanks. This alleviates the need to collect offgas from many tanks.
The equitation model optimized the cost of the equalization and aeration tanks.
A study was conducted to optimize the height of the aeration tank, which affects the oxygen-transfer efficiency and blower size.
The design of the oily solids thickener eliminated the cost of a separate VOC control system. A number of other cost savings were considered during the detailed design, such as using specially designed sample valves and using pipe reducers, to allow the use of smaller valves.
PROCESS CONTROL
A distributed control system (DCS) will be used in the new plant. The DCS monitors use 25 screens to monitor and control most of the plant operations.
DCS alarm messages will alert the operator to any discrepancies or abnormalities.
The are transmitted to a pager system in case the operator is away from the DCS console.
OTHER ACTIVITIES
Bechtel prepared the operating standards manuals for the new plant. The Chevron-Bechtel team prepared the procedures manual, duty book, and the daily log sheets.
The team also planned two, 2-week training sessions for the plant operators. The training sessions covered a wide range of subject matter such as the plant operation and the required safety training.
Bechtel produced a video tape to illustrate the rudimentary plant operation to the construction staff and operators. Bechtel is currently developing a simulation model, using the DCS, to assist the new plant operation staff in understanding both the DCS and plant operations.
CONSTRUCTION
Bechtel's engineering activities spanned a 2-year period. Manpower requirements during the engineering phase peaked at 120 people.
Construction began in the fourth quarter of 1992. The construction team constructed the plant without hindering the refinery's activities, especially in the nearby units.
The peak manpower requirements reached 600 people during construction. Chevron, in conjunction with the design and construction teams, is coordinating the commissioning and start-up of the plant.
The engineering and construction were ahead of schedule and 10% under budget.
This project has been an example of team work, dedication, and value engineering.
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