Adapting Wastewater Treatment Equipment for Today’s Oil Refineries
Although many refineries in the United States are operating under grandfathered National Pollutant Discharge Elimination System (NPDES) permits that do not require upgraded equipment, market forces are driving equipment upgrades.
Many U.S. refineries were initially designed to process light crude from the Middle East. To remain competitive, they must now have the ability to process crudes that are heavier, waxier, and oftentimes salty. The less expensive North American crudes require not only different refining processes, but also different wastewater treatment considerations.
For example, many U.S. refineries have been forced to add a desalting step. The desalting process increases process wastewater, resulting in desalter brine effluent (DBE). DBE can increase the chloride or salt content of the wastewater, which can prematurely corrode units that were not designed for a saltwater influent.
DBEs can also induce large slugs of oil that could possibly overwhelm a refinery’s existing oil water separator, dissolved air flotation/dissolved nitrogen flotation (DAF/DNF) capacity and, in some cases, even impact its secondary clarification system. In addition, DBE can potentially add caustics such as heavy metals and introduce significant amounts of suspended solids.
The following sections discuss the wastewater treatment equipment that refineries can optimize to meet the challenges that lower-quality crude sources present.
Challenges with Traditional Wastewater Treatment Equipment
Much of the industrial wastewater treatment equipment that refineries use – such as filtration, final solids separation, and sludge dewatering equipment – is not particular to treating refinery wastewater. These types of traditional equipment generally operate as they would in various other industrial wastewater treatment applications.
However, some equipment is more specific to use in the oil refining and petrochemical industries. These types of equipment are more vulnerable to wastewater changes that crude-oil characteristics introduce.
Equipment to Remove Emulsified Oil
The overflow from the oil water separator (which we will discuss in the following sections) goes to a flocculation tank, and from there to a DAF/DNF unit. With the assistance of chemicals – a coagulant, a polymer, and sometimes an acid – this unit can break up emulsions to separate oil and suspended solids, allowing plants to further reduce oil and solids in the wastewater stream.
To facilitate this process, the DAF/DNF uses a tank that saturates solids particles with micro bubbles of nitrogen (or air). This process aids particle separation through flotation.
Plants cover these separation vessels and often blanket them with an inert gas, such as nitrogen, to help address safety concerns and to prevent volatile organic compounds (VOCs) from escaping to atmosphere.
The saturation tank or a saturation pumping system allows the gas to form microbubbles that carry the lighter solids and oil particles to the top, where a skimmer mechanism removes them to a sludge box, explains WesTech Applications Engineer Barbara Mumford.
WesTech offers DAF/DNF units in both rectangular and circular designs.
Oil Water Separator Equipment for Removing Free-Floating Oil
Many oil and water separators are designed to meet American Petroleum Institute (API) standards specified in API Publication 421. These units incorporate separation principles that quantify the rate at which oil and water solutions will separate. The oil and water separator is also designed to act as a primary sedimentation device, allowing some of the solids to settle out of suspension.
Traditional API 421 code-designed separators include a rectangular tank, the exact proportions of which are based on parameters such as flow rate, specified API number, and temperature.
The tanks can be steel or concrete and above or below ground, although U.S. practices increasingly require that tanks be covered and installed above ground for leak detection.
The API specification calls for a chain-and-flight mechanism, which serves to scrape oil from the surface of the water while raking solids into a disposal sump.
Heavy solids intensify the wear that chain-and-flight mechanisms already experience. Many of these mechanisms have high-friction wear surfaces and are constructed of fiberglass reinforced plastic (FRP) or ultra-high-molecular-weight (UHMW) polyethylene to prevent damage to costlier parts. The rubbing parts can also be constructed of UHMW polyethylene on stainless steel, which is subject to galvanic corrosion from high salt levels.
Although these wear parts are not necessarily high-cost items, the cost of on-site labor, tank construction, heavy equipment, logistics for vessel entry, and safety requirements make chain-and-flight mechanisms extremely expensive to maintain, repair, and replace.
It can take three or four weeks just to get the permits – including confined space permits – to get inside the tanks, WesTech Industrial Branch Unit Leader David Romer explains.
Other time-consuming requisites include on-site safety training to address the inherent dangers associated with entering, and working within, a confined space that has been used in the refinery process. In short, plants must meet numerous safety requirements and perform job hazard analyses (JHAs) through every step of the maintenance and repair process – from taking the equipment offline, to draining and repairing it, to putting it back in service.
To make matters worse, many installations lack redundancy, which makes it difficult for plant operators or owners to schedule maintenance or repairs.
These are just a few of the challenges currently facing refinery and petrochemical facilities with traditional API 421 oil water separators. However, plants have a number of options for addressing these challenges. They can consider:
- Replacing existing rectangular tanks, often with stainless steel material that is suitable for high-salt concentrations
- Upgrading to a circular oil water separator design for higher torque applications (high solids load)
- Retrofitting existing oil water separation tanks with more efficient skimming mechanisms
- Upgrading secondary clarifiers with more efficient skimming mechanisms
- Upgrading capacity in DAF/DNF units or clarifiers
- Adding a tertiary filtration step
Replacing an Existing Rectangular Tank
WesTech still provides traditional API 421-compliant rectangular tanks for refineries that want them (using stainless steel chain-and-flight mechanisms when the wastewater contains destructive contaminants). It also offers a new tank design that is slanted at the bottom for more efficient solids collection. With a conveyor assembly that mechanically moves settled solids to waste, this new design can help refineries process the heavier solids load that can accompany processes that produce DBE or heavy coke fines.
In addition, a new design option uses skimmers – such as drum and disc skimmers – to improve the rate of oil removal while possibly avoiding the need for a costly chain-and-flight mechanism.
Because WesTech’s new rectangular unit’s footprint fits into existing rectangular spaces, it is an excellent choice for refineries that are interested in updating their traditional oil water separator but have limited space.
Refineries can also opt to have their new rectangular oil water separators completely assembled off-site. This option is available for units that are up to 500 square feet (ft2) – or 46 square meters (m2).
Factors such as regulations and safety considerations can make on-site construction up to 10 times more expensive than off-site construction, which means that upgrading to a newer rectangular design that is built in a fabrication facility can significantly reduce overall project costs.
Upgrade to a Circular Oil Water Separator
WesTech developed its circular oil water separator to address multiple issues that its customers were experiencing with traditional API 421-code designed units. Circular units are based on the same physical principles that govern API 421-compliant vessels. However, circular units offer some distinct advantages for plants that have the necessary space for them.
- Employ a single drive with a center shaft or cage that moves skimmer arms and scrapers together; a single drive unit means fewer moving parts and fewer tank penetrations (which require seals)
- Can be designed with much higher torque capacity than their rectangular counterparts; WesTech uses the same technology in circular oil water separator drive designs that it uses in other industrial wastewater treatment – such as equipment for heavy mining or power applications, where raking capacity and drive torque are of primary concern
- Convey oil and solids radially instead of horizontally, which leads to better efficiency
- Eliminate the need for chain-and-flight mechanisms, which are designed to break before damage can occur to chains, sprockets, and motors; operators have a difficult time maintaining chain-and-flight mechanisms in covered units – indeed, it can be very difficult to even detect failures when they occur
- Help refineries address problems such as short-circuiting or inefficient separation
In addition, plants can retrofit an existing circular tank to function as a circular oil water separator, a potentially cost-saving alternative to building a new vessel and the infrastructure that goes with it.
Retrofit Traditional Tanks with More Efficient Skimming Mechanisms
Plant owners who do not want to invest in a new circular or rectangular tank design may continue to experience overwhelming amounts of oil in their oil water separators. Depending on a plant’s available assets and specific geometries, it can, however, at least reduce operational costs by replacing its chain-and-flight mechanisms with more efficient and less maintenance-intensive rope or pipe skimmers. (WesTech offers both automated and manual pipe skimmers.) Pipe skimmers can be particularly effective for refineries with traditional API 421 separators that incorporate concrete basins.
Free-Oil and Bulking Solids Removal in the Clarifier
Oil that is not removed in the API-designed oil water separator will pass into the DAF/DGF. If there is no flotation step, or if oil removal is insufficient, this can result in free oil accumulating downstream. Bulk biological solids can also accumulate in secondary clarifier applications. Because most conventional systems include biological treatment, this means oil and solids can accumulate in aeration basins and clarifiers.
When free oil or solids begin accumulating in the biological process, it can be very difficult to remove them because many clarification systems are antiquated or in disrepair and were never designed to remove oil and solids in significant quantities. In such instances, an effective solution can be to upgrade skimmer systems with new technology.
For example, WesTech added slotted pipe (nicknamed “ducking”) skimmers to a secondary clarifier at a large refinery located in the Gulf Coast. Operators can adjust this new skimmer design during operation, allowing them to remove a tremendous amount of surface material in a short time when necessary. This adjustable design can be useful when a large quantity of surface material accumulates in the wastewater plant, overwhelming its removal capacity. Such could be the case in a desalting operation.
One of the things the plant likes about these skimmers, says WesTech Industrial Branch Unit Leader Floyd Griffiths,
is that it’s not an all or nothing proposition. They can adjust the ducking skimmer. They can vary the depth using the controls we gave them when they’ve got a lot of oil or solids and turn it back when they get smaller amounts. These skimmers are capable of handling large volumes of oil and solids that would otherwise float on the surface of the clarifier for days.
Process Engineering Makes the Most of Upgrade and Retrofit Projects
As environmental regulations and crude oil sources evolve, it will be essential for refineries and chemical plants to adapt processes and equipment to keep up with the new challenges. Which adaptations will offer the greatest value? Consulting process engineers can help answer this question.
I don’t think I’ve ever seen two configurations that were exactly the same, observes Mumford.
Many factors in addition to crude quality will affect the answer.
The factors that influence decisions at the plant level are as varied as the plants themselves. Regardless of which factors govern, the services of a qualified team of process engineers can help reduce costs. For example, a process engineer can recommend an elevated unit that enables the refinery to implement a gravity-fed process design that eliminates the need for pumping equipment. Such a design also offers the benefit of secondary leak containment.
A process engineering team from a company that can offer all or most of the equipment a refinery could need to treat its wastewater may be able to facilitate process audits, process support, mobile/rental equipment, laboratory testing, and more. WesTech’s process engineers can also provide assistance with operations for plants that are experiencing a shortage of qualified labor or experienced operators.