The ability to recycle byproducts as feedstocks, rather than discard them as wastes, helps optimize refining efficiency and presents the potential for substantial environmental and economic benefits. Robust recycling solutions, particularly in the face of diversified crude slates, will help refineries keep pace with shifting production and refining requirements as unconventional resources experience heavy growth in North America. This alternative thinking has resulted in economic advantages gained by efficiently processing millions of barrels containing oily residuals each year, while reducing chemical, maintenance, disposal and transportation costs.
Along with an increase in a more variable crude slate come new and special challenges to the management of residuals from refining process operations. The various byproduct streams from crude processing and related activities have historically yielded challenging economics compared to the demand for sustainable and environmentally sound solutions.
While most refinery process designs offer significant latitude in the handling of crude, they can be negatively impacted by the production of oil-bearing secondary materials (such as problematic desalter emulsions and oily residuals from various refining units). In particular, oil-water-solid emulsions that are difficult to separate can create issues in the biological wastewater treatment unit, negatively impacting the performance of unit components like flotation separators. The result is typically higher chemical, operation and maintenance costs. These emulsions can also negatively affect refinery production. Moreover, the costs associated with discarding such streams may be significant, not only because such streams are hazardous waste when discarded, but also because they represent lost production value that must be subsequently replaced by additional virgin feedstocks. Consequently, refineries generally prefer to process the difficult and voluminous emulsion, which requires storage of the streams, typically in tanks originally intended to store crude or other products.
While expansions and upgrades can help to sustain a robust treatment infrastructure, the design of older systems can sometimes impose practical limitations on such corrections. For example, as the hydraulic residence times decrease in a floatation cell, this will typically result in a commensurate increase in treatment chemical dosage as operations struggle to combat the performance decline. Another outcome can be higher sludge blowdown volumes intended to mitigate potential downstream unit operation upsets. To keep these operations moving while still being able to contend with the increased blowdown volumes, partnerships with service providers that provide recycling solutions (ancillary separation and recovery) have helped refineries mitigate these issues, regain the lost efficiencies, and further increase overall recovery of hydrocarbons.
Sustainable solutions that maximize resource recycling into ongoing production processes in a cost-effective manner while minimizing environmental impact have become a key value proposition for refinery industry leaders.
Oil-bearing secondary material
Every refinery generates oil-bearing secondary materials as part of normal operations. These materials (mixtures of oil, water and solids) cover a broad composition range. For example, in desalter, API and related separations equipment, the predominant phase is water, while in tank farms and fluid catalytic cracking unit (FCCU) slurries, the oil and solids phases are relatively high with little to no water present. The source operation also factors heavily into how much volume is generated. While API separators can generate thousands of barrels per month of blowdown, sludge accumulated in tank farms grows at a comparatively slower and almost surreptitious rate. Regardless of source or composition, refineries must generally take one of two actions to facilitate turn rates associated with the tanks utilized for storage and handling of these oil-bearing materials:
- Process them into the refining process, or
- Discard themdisposing them as costly hazardous wastes imposes long-term risks and responsibilities on the refinery.
Clearly, processing into the refining process is the ideal solution.
Several physicochemical processes can be used to recycle oil-bearing materials within a refinery. Often, they are paired with leveraging the refinerys coking operation (where available) to provide a tightly integrated solution that maximizes material recovery into an ongoing production process while at the same time helping to minimize waste generation.
The most fundamental level of available alternatives involves simple filtration/dewatering. This process can be applied to reduce the stream volume, typically leaving an oily cake (which, if not recycled into the refining process must be disposed as a hazardous waste) and a water phase, which is transferred to the refinerys wastewater treatment unit. The efficacy of this approach is largely contingent upon the oil and solids content and total composition of the material stream. Unfortunately, it is often inefficient, operator intensive, and susceptible to changes in feed composition.
Moving to the next level, a three-phase, high-speed decanter centrifuge can be used. This will generally afford relatively clean oil that is suitable for introduction into the ongoing refinery production processes, and water streams (less than 1,500 ppm total oil and solids) that are suitable for transfer to the refinerys wastewater treatment plant. When conducted with the proper process, this approach will also generate an oil-bearing material solids phase conducive to further processing and introduction into the refinerys ongoing production processes.
The most aggressive approach is to forcibly desorb the volatile phases at high temperature, recovering all water and oil, and leaving a dry solid phase. The oil and water phases are then condensed, with the oil being returned to the refinerys ongoing production processes (and a potential for the water to be beneficially reused within the refinerys processes). Inherent in the thermal desorption option is the use of a centrifuge to provide a preliminary, gross separation for the phases prior to desorption. Indeed, the three-phase decanter is at the heart of most robust oil-bearing material recycling processes.
For a prominent Gulf Coast refinery in Louisiana, recycling oil-bearing secondary materials via centrifugation followed by the injection of properly prepared oil-bearing material solids into its coker is helping to improve efficiencies and also to reduce costs (Fig. 1). The process recycles the oil-bearing material streams in compliance with all applicable regulations. Leveraging the coker, the fraction of oil not recovered during centrifugation becomes part of the cokers product. The residual, non-volatile solids do not harm the coke and become a minor inert fraction. Consequently, the process helps the refinery to address considerations related to tankage availability and also assists in minimizing waste generation while increasing refining utilization. In this manner, the utilization of oil within the residuals is maximized, and the separated water phase does not represent an increased burden on the wastewater treatment unit. In other words, the recycling process deployed at the refinery helps to both reduce costs and protect the environment.
| Fig. 1. Invoking strategies that focus on recycling oil-bearing |
secondary materials can help refineries boost efficiencies
and reduce costs.
Effective oil, water and solids phase separation is critical in a successful oil-bearing secondary materials recycling process. While various processes can be used to accomplish phase separation, few offer the cost-effective and robust performance that the three-phase centrifuge does, especially when supported by experienced process and operational know-how. The process generally includes the following steps:
- Transfer the oil-bearing material stream(s) from the source operations via pipeline or vacuum trucks
- Process the material to a centrifuge, which separates the oil, water and solids
- Introduce the recovered oil into the refinerys ongoing production processes, with the water being sent to the refinerys wastewater treatment unit
- Choose one of three options: Stabilize and prepare the solids for injection into the refinerys coker unit; further process them via thermal desorption; or invoke disposal options.
The entire process is monitored from a single control room using instrumentation and control hardware and software.
Leveraged, integrated solution
In a petroleum refinery, a coker unit thermally cracks residual heavy oil from the vacuum distillation column (resids) into lower-molecular-weight products, leaving a solid carbon (coke) phase. The overhead gas and heavy oil phase is condensed and recovered, while the solid coke can be used as a fuel (low purity) or further processed into anodes (high purity). A coker unit is particularly well-suited for use in an integrated oil-bearing material recycling process because the prepared solids closely resemble numerous refinery intermediate streams that comprise typical coker feedstocks (feed-side injection), and their use in this process helps to facilitate increased materials recovery (gas overhead and coke products). The prepared solids may also be used as a quenching agent during the coke cooling period (quench-side injection), and US Environmental Protection Agency (EPA) guidance indicates that oil recovery from prepared solids during the quenching process is comparable to efficiencies associated with a coking operations feedstock side. The prepared solids can be fed during the feed, quench or both portions of a cycle.
The cokes end use (fuel or anode) and the feedstock composition injected during feed/quench may impose limits on the amount of prepared oil-bearing material solids that can be used. In particular, limits placed on sulfur, ash and other key physical/chemical properties can set quantifiable limits on the amount that can be used. A detailed mass balance is useful when assessing composition and mass-based limits. Conversely, for prepared solids fed during the cycles quench portion, the overhead setpoint temperature will typically govern the amount of such streams that can be fed; this is essentially an energy balance consideration. Given the various sizes, ages and operating conditions associated with cokers across the industry, every situation warrants a separate analysis.
No matter how or when the prepared solids are fed to the coker unit (feed or quench), the basic processing through the centrifugation operation is essentially the same. Afterward, the oil-bearing material solids may be prepared specifically to address the preferred coker injection process. A general processes summary is explored here.
Coker feed-side injection. In these applications, oil-bearing material solids produced by the centrifuge operation are prepared into slurry that will not settle or stratify. This involves a proper ratio of solids, oil and water. Improper preparation can cause problems with the coker operation, including foaming and increased water load.
Because modifications to coke drums are costly and can only be completed during unit turnarounds, the use of existing nozzles and piping is generally desired. Possible injection points include the residual feed line or any existing nozzle large enough to accommodate the necessary flow. Use of the antifoam injection line is not recommended, as this is critical and best not compounded with other feedstocks.
Coker quench-side injection. Quench-side injection typically requires slurries with a different ratio than feed-side injection slurries. Because this material is introduced to the coker after the coke bed has formed, the material stream must be processed in a manner that facilitates optimal permeability through the coke bed. Compared to feed-side injection, quench-side injection is generally more attractive because it does not require additional equipment or energy to remove water prior to injection. Also, quench-side injection does not impact the coker feed rates because the aqueous slurry simply replaces quench water that would otherwise be used after coking is completed anyway.
As an alternative to coker injection, thermal desorption also helps to increase oil recovery and water phases for introduction into the refining process. There are, however, two key disadvantages associated with this option as compared to using a coker:
- Where a coker exists, there is no need to install the additional equipment required for thermal desorption. Thus, capital costs are much lower when using coker injection.
- Despite the increased materials recovery associated with thermal desorption, the operations produce a solids phase that must be disposed (because no further hydrocarbons can be recovered from the material).
In a typical thermal desorption process, the oil-bearing material solids produced by the centrifuge operation (referred to as cake) are recycled via two processing stages. The first stage removes the water and a significant fraction of the organics from the cake. This low-temperature stage utilizes an indirect heat source (gas or electric) and the equipment operates under an inert atmosphere (typically nitrogen). In the next stage, the dried cake is processed in a high-temperature compartment that completes removal of organics, leaving clean, dry solids. The vapors produced by each stage are condensed, facilitating the recovery of oil and water phases, which are separated and returned to the refinerys ongoing production process.
The resulting solids need to be characterized as hazardous waste unless a delisting exemption is obtained. Delisting is a regulatory authorization that allows specific wastes from a particular generating facility to be conditionally removed from the hazardous waste list. Successful petitions for delisting authorization include economic benefits for the petitioner and significant benefits for the environment as well.
Recycling via thermal desorption presents significant benefits to the refinery by recovering all oil, minimizing the amount of solids requiring disposal (which also has a positive impact on disposal-related expenses), and, in the case of successful delisting petitions, helping to diminish the costs and liabilities associated with hazardous waste disposal. For instance, a major refinery in Louisiana utilizes thermal desorption to recover an estimated 31,000 barrels of oil per year, and, as a result of its successful delisting petition, the resulting solids that comply with the delisting conditions may be characterized as a nonhazardous-waste stream when disposed.
General end results associated with the oil-bearing secondary material recycling processes can be illustrated by a hypothetical comparison, which assumes a refinery produces 1,000 barrels of an oil-bearing material stream (with 5%wt solids, 15%wt oil, and 80%wt water) that is recycled and produces cake that is 40%wt solids, 10%wt oil and 50%wt water. A block diagram of the various options is shown in Fig. 2. Table 1 summarizes the general disposition of oil, water and solids for each approach.
| Fig. 2. Various options for oil-bearing secondary material |
Some key conclusions from the table include:
- Dewatering is the least attractive approach, as the solids generated would be hazardous
- Coker quench or feed yield complete recovery for all water and oil with no residual solids phase
- While coker quench may initially appear to recover more oil, solids prepared for coker feed have a much larger percentage of oil than solids prepared for coker quench; however, both operations maximize the oil recovery that is introduced into ongoing refinery production processes
- Thermal desorption affords complete recovery for all oil and water phases but generates solids that still must be disposed; these solids will generally be classified as hazardous waste unless a successful delisting petition is secured (and its conditions are met for each applicable batch of solids).
Further to the environmental and economic benefits, an effective oil-bearing secondary materials recycling/management program provides a more robust operation that can also help refineries improve tankage utilization (freeing space occupied by such materials). It also provides additional latitude in the operation of units that produce such material streams, since operators have additional confidence that the blowdown will not be as likely to create process upsets downstream.
As the oil and gas industry continues on a projected path of strong growth, the technologies and services applied to enhance recovery and improve the operation will become increasingly important. Successful partnerships that leverage integrated, holistic solutions are a key to success. HP
Steve Hopper is executive vice president of Veolia Water Americas Industrial Business Group, a part of Veolia Water North America. The company provides water and wastewater partnership services to industrial and municipal customers.