THERMAL DESORPTION MEETS BDAT STANDARDS AT LOUISIANA REFINERY

Patricia Broussard-Welther
GDC Engineering Inc.
Baton Rouge, La.

On June 18, 1992, GDC Engineering Inc. (GDC) demonstrated its new, innovative thermal desorption process for petroleum refinery K-wastes at a Louisiana refinery.

The process proved to be capable of reducing the volume of K-wastes to as little as 10% of their original volumes and thermally treating the residue solids to meet land disposal treatment standards.

The refiner will recover almost 300 tons/year of oil for recycling from the K-wastes. This refinery waste management system (RWMS) will produce an annual savings in processing and disposal of K-wastes of 37% when compared to the fuels-blending program which will be described later.

Representatives of the Louisiana Department of Environmental Quality (Ladeq) observed the demonstration process-one step in obtaining best demonstrated available technology (BDAT) status for the RWMS. The RWMS met BDAT status by reducing the K-waste contaminants to below U.S. Environmental Protection Agency's (EPA) land disposal treatment standards.

The RWMS utilizes a horizontal decanter centrifuge to dewater the refinery waste streams and a thermal sludge drying unit (TSDU) to process the dewatered waste (cake) at temperatures of up to 1,400 F. to achieve maximum volume reduction and produce a BDAT residue.

The organics and water are recovered from the centrifuge and TSDU for recycle, utilizing a liquid/vapor recovery system. The overall process has maintained a recovery of over 75% of the hydrocarbons and 100% of the water in the original sludge.

The demonstration was monitored by Ladeq representatives from the hazardous waste division, who verified that it was conducted according to the demonstration plan submitted to the Ladeq.

The tightly controlled demonstration proved that the new thermal desorption process for petroleum refinery K-wastes qualifies as a BDAT by consistently and economically reducing the organic contaminants in the K-wastes to below EPA's land disposal treatment standards.

BDAT FOR K-WASTE

Complex waste streams result from refinery operations such as crude desalting, fractionation, steam/sour water stripping, condensing/accumulating, water/steam blowdowns, and general spills and leaks.1

The solids and colloidal organics (sludge) derived from these waste streams are hazardous wastes listed as follows in the U.S. Code of Federal Regulations (CFR):

K048-DAF float
K049-Slop oil emulsion solids
K050-Heat exchanger bundle cleaning sludge
K051-API separator sludge
K052-Leaded tank bottoms. 2

Table 1 presents detailed definitions of these wastes. Concentrations of water, solids, and oil in these sludges vary depending on the type of waste and the individual refinery. 3

This RWMS evolved from the volume reduction and resource recovery services provided by GDC. These services were provided to the refining industry for many years prior to the implementation of the requirements of the 1984 Hazardous and Solid Waste Amendment (HSWA) to the Resource Conservation and Recovery Act (RCRA).4

The HSWA established requirements for the treatment of listed hazardous wastes to specific treatment standards prior to land disposal. Refinery wastes (K048-K052) became subject to the HSWA requirements on Nov. 8, 1990.

The first application of the RWMS became a reality in the fall of 1990 when the refiner decided to expand its ongoing volume reduction/resource recovery efforts to include the processing of solid residues to meet the treatment standards for landfill disposal as promulgated by the HSWA to RCRA.

Ladeq was encouraging and supportive of the efforts of both the refiner and GDC in developing and implementing this new process. The process was designed to further reduce the volume of K-wastes and produce a residue that qualifies as BDAT, thereby permitting disposal in a landfill.

Consistent with the goal of the refiner and Ladeq to achieve cost-effective waste reduction and resource recovery, the RWMS was developed to provide a significant reduction in the disposable waste while meeting the EPA treatment standards for organics prior to landfilling.

Fig. 1 demonstrates the success of the RWMS and its qualification as a BDAT process. In Fig. 1, the treatment standards are compared to the average residue concentrations of approximately 473 tons processed during the optimization period.5

The RWMS succeeded in producing a residue that met all organic treatment standards for landfilling on 39 of 43 batches processed. The four failed batches, produced while testing the productivity limits of the equipment, were reprocessed to ensure that all residue would meet organic treatment standards prior to shipment to the landfill.

It should be noted that no thermal treatment (including incineration) will be able to guarantee meeting the treatment standards for metals. Any residues that fail to meet metals treatment standards are further processed by stabilization/ fixation prior to landfilling.

SYSTEM DESIGN

The RWMS was developed to provide the maximum volume reduction of refinery sludge while recovering the hydrocarbons for reuse. Since the major constituent of the sludge is water, the recovery and recycling of water provides for a significant reduction in the volume of the hazardous waste generated.

The system process flow diagram is shown in Fig. 2.

Prior to the feed entering the centrifuge, the sludge is heated and polymers are injected to flocculate the solids phase of the sludge, The chemical injection system utilizes a combination of chemical metering pumps, mixing chambers, and static mixers to activate the polymer prior to injection.

Inside the centrifuge, the flocculated solids are subjected to 2,100 Gs of centrifugal force. This force compresses the solids to the external bowl of the centrifuge.

During this compression phase, the solids are continually conveyed to the conical end of the bowl by the internal scroll. At the conical end of the bowl, the solids are discharged through a series of ports and dropped into a screw conveyer for delivery to the TSDU.

The liquid phase of the sludge is discharged over a series of dams at the opposite end of the bowl to a centrate receptacle. From the centrate receptacle, the liquids are pumped, using air diaphragm pumps, through a heat exchanger to reduce the temperature, then returned to the refiner's API separator.

The vapors generated during the heating and dewatering process are drafted to the liquid/vapor recovery system, virtually eliminating all fugitive emissions (Fig. 3).

The TSDU portion of the RWMS is an electrically heated infrared belt-conveyor furnace (Fig. 4). The dewatered material (about 50% of the water content is recovered by the centrifuge system) is conveyed to the feed-leveling section of the TSDU. The feed material is then conveyed through the TSDU on a woven wire mesh belt which is supported on high temperature alloy rollers.

A friction drive system is used to pull the belt through the dryer via a variable-speed hydraulic drive. This drive can be adjusted to achieve waste material residence times in the furnace from 10 to 40 min. When the residual material reaches the discharge end of the TSDU, it drops off the belt into a cooling screw auger and then through a rotary airlock into a roll-off box (Fig.

The thermal energy is provided by transversely mounted, electrically powered, silicon carbide heating elements isolated in high-temperature alloy tubes. The elements are located along the entire length of the furnace above the wire mesh conveyor belt.

The TSDU is divided into two heating zones which can be adjusted from 250 to 1,750 F. to provide precise temperature control. The TSDU, as demonstrated, operates at sufficient temperatures to maintain a peak material temperature of approximately 1,000 F. with a residence time of 30 min. A typical temperature profile o the solids during thermal desorption is illustrated in Fig. 6.

Rotary rakes (cake breakers) are used to gently turn the material on the belt to increase exposure of the entire waste mass to the high temperatures. The rotary rakes, which are transversely mounted, consist of L-shaped fingers welded to stainless steel rods.

These rakes not only increase the heat exposure of the lower layers of the bed, they also act as breakers to reduce the size of large lumps of material that may form in the drying section of the TSDU.

The vapors from the TSDU, mix tank, and centrifuge are drafted into a liquid/vapor recovery system to remove the condensable liquids for recovery and recycle, while the remaining vapors can either be used as a fuel or flared. The recovery system utilizes a simplified design to minimize fouling from the heavy hydrocarbons and to provide a more effective liquid/vapor separation.

The first stage of the system is a quench tank designed and fabricated by GDC. The quench tank cools the TSDU vapors from as high as 1,500 F. down to 150 F., and provides adequate capacity for removing the condensed liquids from the gas stream. The gas then flows into a rotary wet scrubber.

Utilizing centrifugal forces, which cause impingement of water with the fine oil and solid particles, the rotary wet scrubber removes the condensed particles from the vapors. The gas then passes through a packed-bed scrubber and is further cooled in a chiller to remove the moisture before transfer to a refinery process heater.

The residual heating value of the vent gas is utilized in the process heater. This guarantees the destruction of the volatile organic hydrocarbons, yielding only carbon dioxide, nitrogen, and water vapor.

PERMITTING

Because this first system was in Louisiana, the applicable regulations for the project were:

The Louisiana Administrative Code (LAC) 33:V, Chapter 15, which regulates facilities which treat, store, or dispose of hazardous waste (TSD facilities)
LAC 33:III, which regulates air emissions.

The RWMS meets the exclusions allowed under these regulations, and therefore, is exempt from permitting requirements as a TSD facility.

According to the Air Quality Division of Ladeq, the RWMS is exempt from air regulations under LAC 33:V, Chapter 17, because the system generates an "unconfined vent gas" which is recovered for reuse and not emitted into the atmosphere.

The Hazardous Waste Division of Ladeq has ruled that the "unconfined vent gases" from the vapor recovery system are not subject to hazardous waste management based on Louisiana statute LSA-R.S. 30:2173(B). Because the RWMS meets these exemptions and qualifies as a small source under LAC 33:III.505.B, the system was granted a small source permit exemption.

The RWMS was developed as an extension of the refiner's existing waste water treatment system, and therefore Ladeq granted approval for its operation under the regulations that provide for a permit exemption for waste water treatment systems.

It has long been accepted that the waste water treatment system in a refinery includes various collection basins-the API separator, DAF/IAF units, storage tanks, and ponds or impoundments, for settling and equipment dewatering. These system components either receive, treat, or store the refinery liquid hazardous waste streams and thus form a "waste water treatment unit" under LAC 33:V. 109.

Because the TSDU is an extension of the refinery waste water treatment system, Ladeq granted an exemption from the thermal treatment permitting requirements under the provisions of LAC 33:V.1501.C.3.

The RWMS also qualifies for an EPA exemption for solid wastes because the process recovers resources for recycling consistent with the exclusion found in 40 CFR 261.4(a)(8). The exclusion states that "secondary materials (sic., hydrocarbons and water) that are reclaimed and returned to the original process or processes in which they were generated where they are reused in the production process...," are not solid wastes, and are therefore exempt. The standards set by the regulation for exemption require that:

The system "...is closed by being entirely connected with pipes or other comparable enclosed means of conveyance;"
The system ". ..does not involve controlled flame combustion ... ;"
"...the secondary materials are never accumulated ... for over twelve months without being reclaimed;" and
"...the reclaimed material (i.e., oil) is not used to produce a fuel..."

An additional EPA regulation applicable to the exemption of the RWMS is found in 40 CFR 261.6(a)(3)(vi). The regulation states that if oil is "...reclaimed from hazardous waste resulting from normal petroleum refining, production, and transportation practices...... then the system making the reclamation is exempt.

The RWMS met all the requirements for exemption under these EPA regulations.

OPERATIONS

As with the development of any new technology, the optimization period allowed the system to be thoroughly tested to identify productivity limits, maintenance difficulties, and ideal equipment settings to promote optimum functioning of the RWMS.

Some of the typical operation problems that required innovative solutions were:

Rapid fouling of heat exchangers by heavy hydrocarbons
Identifying and sealing of air leaks
Increasing the efficiency of the glycol system to improve heat exchanger performance
Dust control to handle dried solids
An occasional pyrophoric reaction in the dried and cooled solids while waiting to be shipped.

In the search for solutions to these problems, modifications were made which improved the efficiency of the RWMS. The improvements made during optimization further reduced the organic content in the solids, eliminated fugitive vapors emitted to the atmosphere, and maximized productivity.

Composite samples of the solids residue in each roll-off box are submitted to a third-party laboratory and to the disposal facility (landfill) for analysis prior to shipping the material to the landfill. Even though the analytical results from the laboratory proved that the solids residue from the first batch of sludge processed did meet the treatment standards, the acceptance of the material by the disposal facility took significantly longer than anticipated.

After 2 months of evaluation, the disposal facility verified the third-party laboratory results and began accepting the solids residue for landfilling. Now that the RWMS has a long history of consistently meeting BDAT treatment standards and has gained Ladeq approval for BDAT status, the disposal facility evaluation period for other applications will be limited.

ECONOMICS

Even though the cost for treating the waste at each refinery will vary according to the volume of material processed, utility costs, and residue disposal cost, the following economic discussion demonstrates that there are major potential savings associated with on site thermal treatment.

The numbers shown are site-specific to the Louisiana refinery and should not necessarily be construed as expected costs at other refineries. The actual costs are confidential, so for the purpose of this article, a cost index comparison is used, with before-landban costs being the base index.

The cost index is based on annual dollars spent, so the changes in the index reflect the combined effect of the waste minimization program, the increased cost due to the landban, and the decrease in cost due to the implementation and refinement of the RWMS.

Prior to the restrictions imposed by the landban, the refiner utilized the centrifuge to dewater the K-waste to achieve a volume reduction before sending the material to a landfill. The total annual expenditures associated with this process, prior to the landban, is equivalent to a cost index of 1,000 as illustrated in Table 2.

The comparison of costs detailed in Table 2 includes processing, utilities, chemicals, transportation, and disposal expenses. The table illustrates the effect of the implementation of the landban as well as the savings provided by the RWMS over a fuels blending program.

After the landban, a fuels blending program was implemented to blend some of the recovered oil into the dewatered cake to enhance the BTU value to the required 6,000 BTU/lb for use in an EPA-approved cement kiln as supplemental fuel.

The HSWA landban requirements increased annual expenditures by 175%. Blending the recovered oil back into the solids increased both the volume of waste and the cost of transportation and disposal.

With implementation of the thermal desorption technology and continued waste minimization, the refiner will generate only one third the volume of waste shipped off site prior to the landban. This is a sharp decline in the generated K-waste and an increase in the recovered and recycled hydrocarbons.

Fig. 7 illustrates the estimated mass balance of a K-waste processing system for the Louisiana refinery generating only 1,400 tons/year of dewatered cake. At this refinery, the RWMS recovers annually almost 300 tons of hydrocarbons and 120 tons of combustible gases, such as methane, hydrogen, and carbon dioxide, for recycle.

During the optimization period, the refinery achieved an annual cost savings of 17%. Now that the process is operating at full capacity and many cost-saving steps have been taken, the expected 1993 expenditures for K-waste management will be reduced to near prelandban cost, providing an additional 26% savings.

The 1993 cost of K-waste management and disposal is projected to be only 63% of the fuels blending program cost. The reduction in K-waste shipped offsite, as well as the significant decrease in the cost of processing and disposal, are illustrated in Fig. 8.

The BDAT system provides total removal of the water and maximum recovery (75%) of the hydrocarbons from the K-waste for recycle. After implementation of the complete RWMS, the refiner is landfilling a BDAT residue consisting of slightly over 10% of the K-waste stream originating from the waste water treatment system.

GDC and the refiner have demonstrated that the RWMS can produce solids from the K048-K051 refinery sludge that will meet BDAT organic treatment standards for acceptance in a hazardous waste landfill. Significant resources are recovered for recycle using a technology that maximizes protection of the environment.

The reduction in treatment and disposal costs provides a strong economic benefit over alternate treatment and disposal options.

THE NEXT GENERATION

The infrared thermal treatment unit utilized at the refinery has proven to be effective in consistently producing a residue that meets BDAT standards. However, in an effort to provide flexibility in utility requirements and permit criteria, GDC has developed an indirect gas-fired TSDU.

This flexibility will allow refineries to choose the thermal treatment unit most compatible with their operation.

The second-generation unit utilizes natural gas to heat the waste material as it moves through a rotary tube furnace. This furnace will be able to process the waste at a higher throughput for refineries with larger annual volumes.

For refineries that may not be able to accept noncondensable organic vapors back into their plant systems, the gas-fired unit will provide the capability of recycling the vapors as fuel to the TSDU firing chamber without the need to discharge to the plant vapor header.

With the vapors recycled as fuel within this closed loop system, exemptions from permitting requirements can be maintained.

REFERENCES

Arcuri, Kym B., "Identification and Economic Evaluation of Reprocessing Options for Select Louisiana Waste Materials," The Environmental Excellence Group, 1991.
United States Code of Federal Regulations, 40-CFR 268, 1990.
Elnaggar, Hameed, Bordelon, Neil, and Elnaggar, Terry, "Evaluation at Waste Treatment Technologies for the Oil Refining and Related Industries," presented at Third International Symposium on Industry and Environment in the Developing in World, Alexandria, Egypt, May 27-29, 1992.
United States Code of Federal Regulations, 40-CFR 261.10, 1984.
"The RCRA Land Disposal Restrictions, A Guide to Compliance," McCoy & Associates Inc., 1990, P. 21.
Welther, Patricia, and Bordelon, Neil, "Refinery K-waste Management Utilizing Volume Reduction and Thermal Treatment," presented at Haztec International Conference, Houston, Feb. 6.