August 2017

Water Management

Filter process streams to protect downstream equipment and improve product quality

Throughout the petroleum industry, most discussions of filtration focus on the oil itself as it is extracted from the wellhead and refined into saleable products.

Clements, M., Eaton Corp.

Throughout the petroleum industry, most discussions of filtration focus on the oil itself as it is extracted from the wellhead and refined into saleable products. However, throughout this process, a critical and consistent component is the use of water. Although water quality requirements depend upon the end use, without filtration, unfiltered water can foul downstream equipment and contaminate catalysts. The result is increased maintenance and repair costs, a potentially downed system, and lower conversion rates and process yields.

Due to its influence on process flow, its ability to protect downstream equipment and piping, and its significant role in the quality—and hence value—of the finished product, filtering process water can play a critical role in optimizing the refining process. The right filtration equipment can affect a company’s environmental impact through the reduction of emissions and waste generation. It can also safeguard employees by minimizing their exposure to hazardous materials. These factors, in turn, impact a company’s productivity and bottom line.

Despite its significance, many refineries have not realized the benefits of optimized filtration for process water. Installing a system where one did not previously exist can be difficult to justify with tight capital budgets. Decision-makers face the same challenge when a filtration system is in place and operating. However, a careful look at key cost factors can quickly justify an investment that will generate a significant return. These factors include minimizing overall maintenance costs, labor costs, the potential costs of lost production, impact to the environment, and the conversion and recovery of petroleum products during scheduled and unscheduled downtime.

With an increased focus on reducing environmental impact, greater emphasis is being placed on reducing the amount of water used for industrial processes—particularly freshwater. One method is to use equipment that requires less freshwater. When the amount of water used is mandated by process requirements, another method is water reuse. This trend is fueled by several economic benefits that can be broken down into four separate and specific areas of costs savings:

  • Reduced cost for purchase and treatment of freshwater
  • Reduced cost for heating process streams, or money saved through energy recovery
  • Reduced process losses of expensive and unspent catalyst fines that can be reintroduced and reused
  • Reduced waste treatment costs.

Any decision regarding water filtration should be weighed against the relative importance of each of these factors. This article examines specific areas for water treatment within the petroleum industry, and potential solutions will be suggested.

Water is used for downhole injection, process cooling, steam generation, dilution fluids, gas sweetening reactions, quench operations, heat transfer and as a universal hydrophilic solvent. Each of these applications is improved with cleaner water. The cost of dirty water is seen in the short term as a process slowdown, and in the long term as equipment plugging, under deposit corrosion, fouled resin and equipment erosion.

Oilfield production site

At production sites, water is often injected into the producing formation to help extract oil. This water is generally recycled water that has been previously pumped out of the formation with the oil. The water is separated from the oil and prepared for reinjection, and contains scale and iron (Fe) particles. Solids in the injection water degrade high-pressure equipment due to line corrosion and plugging—most significantly, plugging the formation.

FIG. 1. An automatic self-cleaning strainer is used to protect downstream heat exchangers in a polysilicon production facility’s cooling water system.
FIG. 1. An automatic self-cleaning strainer is used to protect downstream heat exchangers in a polysilicon production facility’s cooling water system.

Alternatively, the filtration of produced water at the injection site keeps formation flow high and maintains secondary recovery. Clean water also extends the life of high-pressure injection pumps and other critical system equipment. Ultimately, corrosion in the distribution lines is reduced, positively affecting the overall system.

To optimize equipment protection, throughput and system performance, the injection water should be progressively filtered from gross strainers at the intake down to 10 microns–40 microns to protect the surface equipment, and then filtered again to 0.5 microns–2 microns prior to the injection wellhead (FIG 1).

Prior to being sent to the wells, water is often filtered through a large sand filter. This works reasonably well, but requires a large footprint—high air and water volumes are required to backwash and maintain the media. As an alternative, barrier filtration can eliminate the need for air scrubbing, greatly reducing backwash volume and requiring little maintenance.

Traditional cartridge filtration remains a viable solution for the filtration of injection water. Absolute rated media guarantees 99.98% filtration to levels as low as 0.5 microns. For continuous throughput without the need to change out bags or cartridges, tubular filter systems offer an alternative with cleanable, reusable media at retentions similar to those available with cartridge systems. In fact, 98% efficiency down to 2 microns can be achieved with some models. This type of filter system eliminates media and disposal costs, as well as operator time spent changing cartridges.

Cooling systems

Cooling systems accumulate airborne dirt and other contaminants that can affect system instrumentation and efficiency. Dirt and Fe fouling can rob the system of the required heat transfer capacity and can increase frictional hydraulic losses. Cooling systems with inefficient filtration can necessitate labor-intensive tower cleanouts and heat exchanger refurbishing. Efficient systems will extend equipment life and maintain maximum heat transfer. Removal of 98% of particles of 44 microns and larger is recommended for effective cooling water filtration.

Tubular backwashing systems provide an economical method of automated solids removal with minimal backwash volumes. Clean cooling water reduces manual cleanouts and chemical consumption, helps eliminate instrument plugging and maximizes cooling efficiency. If space is at a premium, tubular systems are available in a nonlinear configuration.

Amine systems

Another process stream that benefits from filtration is the amine system. Amine systems require filtration to remove contaminates such as pipe scale, iron sulfide (FeS) and salt precipitants from the process. If the amine is dark amber to greenish in color, the stream is dirty. For maximum efficiency, the amine stream should be as clear as fresh water. Contaminate removal increases amine efficiency and reduces the system’s operating costs.

Problems created by dirty or unfiltered amine systems include iron sulfides that cause foaming in towers. Contaminants that collect on tower trays reduce efficiency and increase pressure drop across the tower. Solids buildup in the flash drum reduces the available liquid area, resulting in incomplete separation of hydrocarbons and amine. Contaminants in the surge drum introduce a constant supply of new solids into the flow. When particles settle out, they collect in piping, tanks, heat exchangers and reboilers, causing poor heat transfer and increasing corrosion, plugging and fouling of the equipment.

Typically, disposable media are used on the clean amine loop and backwashing filters on the rich amine loop. When considering disposables, proper media selection will have the most direct effect on system efficiency and, by extension, cost effectiveness. For example, nominal efficiency media are ineffective because too much dirt passes through the filter. Polypropylene media traps oils and can become blind before the full dirt holding capacity is reached.

FIG. 2. A rich amine loop shown here is used to protect the heat exchanger in an upstream exploration and production facility. This tubular filter replaced traditional cartridges/bags, eliminating operator interaction with the H2S-laden fluid. In addition, the hazardous material disposal of the cartridges/bags into the environment was eliminated, while reducing maintenance costs and improving operating margins.
FIG. 2. A rich amine loop shown here is used to protect the heat exchanger in an upstream exploration and production facility. This tubular filter replaced traditional cartridges/bags, eliminating operator interaction with the H2S-laden fluid. In addition, the hazardous material disposal of the cartridges/bags into the environment was eliminated, while reducing maintenance costs and improving operating margins.

The optimum flowrate is a compromise between cost and dirt retention. The slower the flowrate, the more dirt the media will hold. If oils are in the amine or if the temperature exceeds 180°F, cellulose or polyester media are recommended. Cellulose media generally cost less.

If the amine system has not been filtered, or if signs of high dirt loading are evident (again, a green or amber color), then filtration is highly recommended. To reduce costs, stage the cleaning process. Begin with media that will eliminate larger micron particulate, and step the process gradually into smaller retentions with each change-out, ending ultimately with retentions in the 5 microns–10 microns range. To maintain optimum performance, use high-efficiency, absolute rated media at a maximum of 10 microns.

Although more prevalent on the sulfur-rich side, backwashing or self-cleaning tubular systems are ideal for both the rich and lean sides of the amine loop, as they eliminate operator exposure and the disposal of often hazardous spent bags or cartridges (FIG. 2). The closed system benefits of a backwashing filter are especially important on the rich side of the amine loop, due to the elimination of operator exposure to the highly toxic sulfur being removed.

Backwashing filters should be sized at a flux rate of approximately 4 gpm/ft2–8 gpm/ft2. The filter media should be 5 microns–10 microns, high-efficiency (> 98%) filter elements constructed of single-layer sintered wire mesh. When the filter is installed on a dirty system, it will typically backwash continuously for the first 2 d–3 d. Then, the backwash interval will begin to increase; within a week, the amine will be clean and the backwash interval will increase to 8 hr or more.

Backwashing filters are self-correcting during and after upsets. During upset conditions, the filter will start backwashing continuously. After the process returns to normal, backwash interval times will also return to normal.

Selecting the correct filter

No filter is designed to remove all particulate from a fluid stream. The size of particulate to be removed should be determined by the quality objective or system protection requirements. Removing particles below the level identified for a specific system is a costly and unnecessary exercise with little or no return on investment (ROI).

Will the stream be reintroduced into the process? If so, what contaminate levels will yield cost-effective efficiency? The residual level of solids in the product filtrate will vary for each application. Understanding the limitations that varying solids levels will impose on the process is essential to determining the cost benefits of filtration systems.

Several variables must be considered when protecting a process through filtration. Fundamental questions include: What size particle can cause erosion in downstream equipment, and at what level of solids will deposits build up in the low, slow-flow areas?

Two other important factors to consider when designing or selecting a filtration system are environmental/safety concerns and waste disposal costs. Environmental regulations governing fugitive emissions have become increasingly stringent in recent decades, and will be even more rigorous in the future. Companies are also faced with increasing worries over lawsuits from workers exposed to hazardous materials and volatile organic compounds (VOCs). Waste disposal costs have risen dramatically in recent years, partially as a result of stricter environmental regulations.

Disposable filter media

When final product clarification is a key process objective, a general standard of particle removal is retention in the 0.2 micron–200 microns range. Disposable filter media, typically bags or cartridges, come in a wide range of micron ratings and fabrics, and many meet the retention criterion.

Two types of efficiency ratings are typically used for disposable media: nominal and absolute. Nominal ratings can vary from 50%–90% removal efficiency, depending on the product and the manufacturer. Absolute ratings imply 100% removal of particles at a set rating, which actually means 98.7%–99.99%, depending on the product and the vendor.

Bag fabrication has advanced over the past few years, improving the filtration capacity of bags and making them more efficient. Multi-layer bags increase the solids holding capacity and provide a larger surface area for filtration. These higher efficiency bags normally last longer, remove a higher percentage of contaminants (up to 99.9%) and can be rated as low-micron.

Even with new designs in bag construction, cartridge filters trap particulate that a simple bag cannot, such as soft particles, which can be extruded. The “depth” design of cartridges—layers of rigid construction—have more surface area to trap the dirt and significantly increase dirt holding capacity when compared to a similar-size bag. This makes cartridge filters the choice for absolute filtration.

While disposable media such as bags and cartridges usually have a relatively low initial cost, operating costs can increase if changeout is frequent. Media replacement and waste disposal costs can quickly outweigh any savings from the lower acquisition cost. Conversely, for applications with low processing volumes, or where media replacement is infrequent, bag or cartridge filtration may be the best choice.

Cleanable filter media

Several different types of cleanable filter media are available as alternatives to disposable media, or as a pre-filter in staged filtration systems. They include wire mesh, wedge wire, defined pore, perforated and sintered metal filters. Cleanable media can often be used in the same applications as disposable bags or cartridges, sometimes with significant labor and cost savings. Several applications make cleanable filter media a better choice, due to pressure or flow requirements. When comparing purchase price to operational expenditures, a typical payback can range from 6 mos to 1 yr.

Cleaning of this media type may be done manually, hydraulically or mechanically. Manual cleaning often requires the use of expensive cleaning compounds and can carry the risk of damage to the filter media. Additionally, labor is required for manual cleaning, which raises worker safety/exposure issues. Hydraulic cleaning involves using either the process stream or another compatible source of liquid to backwash the filter media. This may be cost prohibitive if the liquid being filtered is expensive, hazardous and/or non-compatible with an outside source of liquid.

The three main classes of cleanable filters include: vibrating screens, backwashing filters and mechanically cleaned filters. Of these three, only vibrating screens require manual cleaning, but they have limited use in the petroleum process, especially for water treatment. Mechanically cleaned filters are ideal for highly viscous liquids. Again, this does not apply to most water treatment applications.

Backwashing filters work well in high-volume applications, typically ranging from 100 gpm and upwards. A minimum pressure of 45 psi and a small volume of liquid are required for backwashing. For these reasons, backwashing filters are often found throughout the petroleum industry.

Ultimately, whatever the process stream application, careful consideration and selection of filtration equipment can significantly improve overall system performance. Although most of the attention for filtration in the petroleum industry traditionally focuses on refining crude, water is a key process component and can help drive optimization and reduce maintenance costs, repair costs and labor requirements. Water filtration will also extend the life of expensive and valuable equipment, improve a plant’s competitive position and help drive profits. HP

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