Activated carbon is successfully used in a host of applications in a typical petroleum refinery (see Sidebar).  Recent increases in costs for virgin activated carbons, however, have created a renewed interest in a “green” solution for many carbon applications: the replacement of virgin activated carbons with reactivated carbons. Reactivated carbon is spent carbon that is recycled by being regenerated with very high temperatures.  In petroleum refineries, reactivated carbons can be used for VOC abatement in vapor phase applications, in wastewater treatment, and groundwater remediation.

The most important benefit of using reactivated carbon is cost savings.  The process of carbon recycling reduces operating costs since the cost of reactivated carbon is typically 20-40% less than the cost of virgin carbon.  In addition, some facilities also receive environmental credits issued by regulatory agencies for waste minimization.  The reactivation process ends the chain of custody for adsorbed contaminants, thereby eliminating the liabilities associated with handling and disposal of spent carbons.  In fact, reactivated carbon is considered to be a recovered resource.

The Reactivation Process

Thermal reactivation is the process by which spent activated carbon is regenerated using steam and elevated temperatures to remove and destroy the organic compounds that have been adsorbed onto the carbon. A typical reactivation flow process is shown in Figure 1.  The reactivation process has three goals: restore the spent carbon’s activity level to as near as original as possible; maintain the internal pore structure of the media; and minimize product losses through gasification and attrition. 

During reactivation, residence time in the furnace, reactivation temperature and gas composition must be carefully controlled to obtain the desired reactivated carbon quality.  The reactivation process typically has three stages:  in the first stage (temperatures of 200-500°F), the carbon is dried and more volatile / low boiling point organics volatilize off the spent carbon.  In the second stage (400-1200°F), pyrolysis of higher boiling point organics occurs on the carbon surface.  In the final stage (1350-1800°F), gasification of the pyrolysis residues occur.  The reactivation gases, containing volatilized organics and residues, then exit the furnace, where they pass through an afterburner to mineralize any remaining organic compounds.  Inorganic gases are removed in a wet scrubber.

There are two main furnace types that are utilized to thermally reactivate carbon:  the multiple hearth furnace and the rotary kiln.  Each kiln has distinct advantages and disadvantages:

Multiple Hearth Furnace (MHF)

• High degree of control over reactivation process, especially the furnace atmosphere for steam ratio as well as gas use
• Typically results in lower losses due to gasification
• Typically results in lower losses due to physical attrition
• Typically consumes less energy per unit of product
• Offers better carbon/gas contact

Rotary Kiln

• Provides flexible operation
• Creates clean breaks between product runs
• Operation and maintenance requires less operator skill than MHF
• Typically lower labor and equipment maintenance cost than MHF

The decision about which furnace type to use is determined by the spent carbon that will be processed at the facility: the more complex the contaminants in the spent carbon, the greater likelihood that MHF will be used; however, fuel sources, requirements for operating flexibility, local technology sources, and emission constraints are also factored into the decision.

Evaluating Reactivated Carbon Performance

The performance of reactivated carbon can be measured by a variety of methods including Iodine and Butane Numbers.  These standard adsorption capacity tests are used as an initial basis for comparing reactivated with virgin carbon, and while these tests are sufficient for most applications, some applications require comparing the removal of targeted compounds.  For these applications, bottle point isotherms or the Rapid Small Scale Column Test (RSSCT) should be used, which compares the adsorption performance of reactivated vs. virgin carbons with a sample of the influent water stream.

Who Provides the Reactivation Service?

A number of vendors provide this service, and offer different programs to suit the client’s application.  These include:

“React and Return” (offered by Siemens Water Technologies, for example) is a highly controlled program where activated carbons are removed, reactivated, and returned for reuse to the same client, and is most commonly applied to potable water applications.  The carbon is segregated from other carbons during reactivation and storage.  Virgin carbon is used to offset normal losses that occur during handling and reactivation to make sure 100% of the original carbon is returned to the customer.

Pool reactivation is a program whereby spent carbons are segregated and then pooled according to application type (vapor phase/liquid phase) and mesh size.  These pooled carbons can then be sold into many applications as a substitute for virgin carbon to lower the operating costs.

Field services associated with reactivation programs include spent carbon profiling, spent carbon removal and packaging, non-hazardous and/or hazardous waste handling and transportation to the reactivation plant or a hazardous waste reactivation plant, 

carbon vessel inspection with minor repair, and vessel reloading with reactivated carbon.  Vendors performing reactivation services can offer a certificate of reactivation for each shipment, confirming that the spent carbon has been recycled in a manner that meets or exceeds all applicable RCRA and Benzene NESHAP regulations.

Prior to reactivation, spent carbon must be profiled and tested to ensure that it can be safely and efficiently reactivated within the permit limits of the reactivation facility.  Typically, a profile form on the spent carbon is completed and a small sample of spent carbon provided for testing.  Some contaminants, such as PCB’s for example, may not be accepted at certain vendors’ plants.  Determination of the spent carbon’s regulatory status (RCRA hazardous, state hazardous, non hazardous, or falling under the sludge exemption – see next section) is the responsibility of the generator of the spent carbon.

The Green Advantage

Besides the fact that it recycles the spent carbon and thus eliminates waste to the environment, reactivation may have other environmental advantages.  A refinery that reactivates its spent carbon may be eligible for environmental credits from regulatory agencies for waste minimization.  One example is the sludge exemption, which allows the spent carbon to be handled as a non-hazardous waste, and is exempt from the regulations for hazardous waste as described in 40 CFR 262.  The sludge exemption only applies to spent carbon that is not exposed to any wastes specifically listed in 40 CFR 262, and is only used when the generator of the spent carbon intends to send it to a facility where the material will be reactivated for reuse.  As defined in 40 CFR 260:10, “sludge means any solid, semi-solid, or liquid waste generated from a municipal commercial or industrial wastewater treatment plant, water supply treatment plant, or air pollution control facility exclusive of the treated effluent from a wastewater treatment plant.”

The EPA further provides that a spent carbon may be classified as a sludge when it contains no listed hazardous waste and is returned to a reactivation facility where the spent carbon is reclaimed.  Since the spent carbon in these applications meets the definition of a sludge, it is not considered a solid waste and therefore not considered a hazardous waste.

The sludge exemption must be accepted by the governing state environmental regulations in both the generator’s home state and the state in which the reactivation facility resides. To claim the exemption, the refinery must contact its state environmental regulatory agency to obtain its approval.

Spent carbon profiled as RCRA sludge exempt can be transported either as a DOT-regulated hazardous material or as a non-DOT-regulated material.  The choice is determined by the quantity, in one container, of the hazardous substance on the spent carbon. If that quantity equals or exceeds the Reportable Quantity (RQ) for the hazardous substance, the spent carbon is a DOT-regulated Class 9 hazardous material.  If that quantity is less than the hazardous substance’s RQ, the spent carbon is not DOT regulated.

Case Study:  A U.S. West Coast Refinery

Challenge

A refinery in Washington state is using reactivated carbon to meet environmental compliance for VOC emissions. The refinery was using virgin carbon in numerous carbon adsorbers for VOC emissions at its sewer vents and effluent plant, to meet the requirements of a consent decree associated with the Benzene NESHAPS rule.  This rule requires a 95% reduction of VOC’s (or 98% benzene reduction) from vapor emissions at various points throughout the refinery.  The refinery was also using significant manpower from its personnel and an onsite contractor to service these carbon filters.  In addition, the refinery was shipping approximately 200,000 pounds annually of hazardous spent carbon to a disposal facility for landfill or incineration.

The refinery wanted to reduce costs, manpower requirements, and hazardous waste and safety risk/liability while ensuring continued compliance with the consent decree requirements.

Solution

After contacting a water treatment systems/services vendor, the refinery chose a turnkey solution that would standardize their activated carbon equipment, significantly reduce their cost, reduce the manpower requirement, reduce overall safety risks/liability, and eliminate onsite hazardous waste.

The solution consists of two types of standardized carbon adsorbers to replace the various types of adsorbers they were using before, and a service contract that provides offsite carbon change-out for the new adsorbers.  The adsorbers are staged in a separate area and shipped to the vendor’s facility in another part of Washington, where the carbon is replaced; the fresh filters are then shipped back to the refinery.   The vendor provides reactivation services for the spent carbon, and provides fresh reactivated carbon for refilling the vessels.

Results

The new system/service combination will allow more efficient exchange of adsorber vessels under the consent decree timeline constraints [8 hours or 24 hours] and will reduce manpower requirements at the refinery.  Since the spent carbon is being reactivated, it is considered an exempt waste, not a hazardous waste; thus, the refinery will completely eliminate the transport and disposal of hazardous waste from their process, and meet environmental compliance for handling of the spent carbon, as the carbon will be reactivated in lieu of landfilling or incineration. The refinery will also reduce safety liability for its workers.  Because of all these factors, the refinery has projected a cost savings of approximately $300,000 to $500,000 per year. 

Achieving regulatory compliance while operating profitably and producing quality products via the petroleum refining process can be challenging when considering the operation of a typical refinery.  Fortunately, for the removal of organic contaminants, activated carbon remains a tested, reliable technology to help achieve regulatory compliance and product purification goals.  Furthermore, use of reactivated carbon provides an even more cost effective and environmentally beneficial solution for refineries to consider for many applications.

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SIDEBAR: Using Activated Carbon in Refineries

Activated carbon is a versatile adsorbent material that has several common applications in a typical petroleum refinery.  Regulatory compliance and improvement in product quality are the two main drivers for the use of activated carbon. 

Common applications include:

• Vapor Phase VOC Treatment and Control:  compliance with the Benzene National Emission Standard for Hazardous Air Pollutants (NESHAP) or local air emission regulations drives the need for VOC control at sewer sumps, covered API separators and DAF units, storage tanks, and other sources.  Adsorbers utilizing either virgin (pelletized coal-based or granular coconut-shell-based) or reactivated carbons can be used.
• Vapor / Solvent Recovery:  economically valuable products at refineries and terminals can be recovered via pressure swing or temperature swing systems that utilize activated carbon.  The selection of carbon type is driven by the boiling point of the solvent; volatile solvents often require microporous carbons (e.g. coconut-shell-based), while higher boiling point solvents require macroporous (coal-based) carbons.  Carbons with high working capacities (allowing for effective adsorption/desorption cycles) are desired.
• Hydrogen Sulfide Removal:  sour crude oils often are the source of hydrogen sulfide (H2S) emissions.  Specialty grades of carbon (impregnated or catalytic grades) can remove it.
• Wastewater Treatment:  Activated carbon can help refineries meet wastewater discharge permits (COD, BOD, TOC, biotoxicity).  Reactivated carbons are most commonly used in this application, which often requires upfront testing or piloting to properly size the system.
• Groundwater Remediation:  Organic compounds (BTEX, MTBE) often migrate into groundwater supplies from leaking underground storage tanks or holding ponds.  Reactivated or virgin coal based carbons are often used to remediate BTEX, while coconut-shell-based carbons (with higher adsorptive capacity for trace removal) are often selected for MTBE removal.
• Boiler Feedwater Treatment:  Organic impurities in boiler feed water can cause scaling, corrosion, foaming or other problems.  A low silica coconut-based carbon (to prevent silica leaching into the feed water stream) can successfully adsorb the organic contaminants.
• Amine Purification:  Various alkanolamines are used in refineries to purify gas streams, during which the amine solution picks up hydrocarbons and organic acids. A slipstream of the amine solution is passed through a carbon adsorption system to prevent a buildup of these hydrocarbons.  Virgin coal-based carbons are the best fit for this application.
• Decolorization:  Activated carbon can be used for color, odor, or contaminant removal from desired end products such as jet fuel, kerosene, gasoline, and lube oil.  Powdered carbons can be used in batch treatment processes, or granular carbons can be utilized in continuous processes, for this application.

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