PROPANE EXTRACTION TREATS REFINERY WASTES TO BDAT STANDARDS

Richard de Filippi, John Markiewicz CF Systems Woburn, Mass. The first commercial solvent-extraction unit for treatment of refinery hazardous wastes has been operating successfully since Mar. 17. The full-scale unit is continuously treating hazardous-waste sludges to meet U.S. Environmental Protection Agency (EPA) specifications for land-disposable wastes at a major Gulf Coast refinery at Port Arthur, Tex. (Fig. 1). The unit, based on solvent extraction, uses liquid propane as the solvent. The

Sept. 9, 1991

6 min read

Richard de Filippi, John Markiewicz
CF Systems
Woburn, Mass.

The first commercial solvent-extraction unit for treatment of refinery hazardous wastes has been operating successfully since Mar. 17.

The full-scale unit is continuously treating hazardous-waste sludges to meet U.S. Environmental Protection Agency (EPA) specifications for land-disposable wastes at a major Gulf Coast refinery at Port Arthur, Tex. (Fig. 1). The unit, based on solvent extraction, uses liquid propane as the solvent.

The propane extraction unit was built and is operated by CF Systems, a Morrison Knudsen company.

Large quantities of hazardous-waste sludges are generated each year from petroleum refining operations. In the past, disposal by landfarming and landfilling have been acceptable options.

However, concerns about the environmental impact of contaminant migration have prompted the EPA to enact a "land ban" under the Resource Conservation and Recovery Act (RCRA), restricting land disposal of many industrial wastes.

Under the land ban, refinery waste sludges, known by their EPA designations KO48-KO52, must be treated to reduce certain contaminants to specified levels before land disposal of any kind. The final revised treatment standards were published in The Federal Register, Vol. 55, No. 106, June 1, 1990.

Sludges requiring treatment can also come from old waste impoundments. Before EPA began to regulate the land disposal of KO48-KO52 wastes (or "K" wastes), substantial quantities were placed directly in large on site waste pits.

EPA has ruled that these sites must be cleaned up by reducing the levels of certain hydrocarbons and metals, as established under the RCRA and Superfund programs.

From a technical viewpoint, incineration provides an effective means of disposal: destruction.

However, the lack of sufficient incineration capacity and the high costs (significantly greater than landfilling costs) will place severe restrictions on the effective use of that technology.

In contrast to incineration, solvent extraction equipment can be made readily available.

Moreover, solvent extraction provides an alternative process to refiners concerned with waste minimization and site remediation, offering treatment levels similar to incineration, but at substantially lower cost.

In addition to treating hazardous wastes to EPA-required levels, the propane extraction process provides the following benefits:

The volume of material requiring disposal is significantly reduced.
Oil from the sludge is recovered and can be recycled.
Treated water discharged from the process meets the criteria for disposal to a waste water treatment system.
The extraction unit is a closed system and does not produce unacceptable emissions to the atmosphere.

Because the oil is recyclable and the unit is a closed system, solvent extraction requires no RCRA permits when operating on refinery sites.

Incinerators require RCRA permits which take 3-5 years to obtain.

PROCESS DESCRIPTION

The unit operates in seven basic steps encompassing extraction, phase separation, and solvent recovery. In the current commercial configuration, liquefied propane, at low temperature (90-120 F.)

and moderate pressure (about 300 psig), is used as the solvent.

Fig. 2 shows a simplified diagram of this configuration.

In Step 1, slurried sludge is fed to a stirred-tank extractor. The raw sludge may require pretreatment to remove oversized material and to adjust chemical characteristics to optimum levels.

Concurrently, in Step 2, propane is compressed to operating pressure, condensed, and allowed to flow into the extractor, where nonreactive contact is made with the feed slurry.

Oil contained in the sludge is dissolved in the liquefied propane.

In Step 3, the extractor contents flow to a decanter, where gravity phase separation occurs.

The water/solids fraction settles to the bottom, and the lighter propane/oil fraction rises to the top.

The raffinate product, containing the water and treated solids, is collected from the bottom of the decanter in Step 4, and then dewatered to obtain a filter cake for final disposal in a landfill.

In Step 5, the mixture of solvent and oil constituents leaves the top of the decanter, and flows to a solvent-recovery still (consisting of a tower and a reboiler).

The combination of reduced pressure in the tower and heat provided by the recompressed vapor through the reboiler (Step 2) causes the solvent to vaporize in the still.

The oil stream that remains is collected from the bottom of this vessel in Step 6 and recycled to the refinery.

The vaporized propane leaves the top of the tower, is combined with makeup propane as required, and recycled through the compressor as fresh solvent in Step 7.

The extractor/decanter block shown in Fig. 2 actually represents multiple extractor/decanter stages, which achieve extraction efficiencies approaching 100%.

The type and quantity of oil constituents in the feed stream are not critical design parameters.

Therefore, a wide range of wastes can be treated using this process-from soils containing low concentrations of organics to heavy sludges with virtually any concentration of liquids, solids, and oil.

It is this feature that allows the unit to serve as both waste treatment and site remediation facilities. The process does not extract heavy metals and other inorganic compounds from the feed. These concentrate in the treated solids fraction where they are immobilized using conventional chemical fixation.

OPERATIONS

Prior to commercial operations, the process had been demonstrated by the operation of a mobile demonstration unit (MDU) for 3 years.

During that period, the MDU operated at 13 locations. Most were refineries, but there were also commercial hazardous-waste treatment facilities and contaminated Superfund sites. The refineries were located mainly on the West and Gulf Coasts and in eastern Canada.

Table 1 presents data obtained with the MDU, demonstrating the effectiveness of the process for treating refinery wastes, including:

API separator sludge
Dissolved air flotation sludge
Slop oil emulsion solids
Impoundment wastes.

The MDU was tested by EPA under a standard protocol.

It achieved the EPA designation, "Best Demonstrated Available Technology," or BDAT, for refinery hazardous wastes KO48-KO52 (sludge provided for the EPA study is presented in Table 1 as Location 1).

In June 1990, CF Systems began testing a commercial-scale unit at the Port Arthur refinery.

Early testing was performed, in part, under contract to the Petroleum Environmental Research Forum (PERF), and was closely monitored by representatives of eight participating petroleum companies.

From the test results, PERF concluded that organic contaminants in refinery sludges were reduced to BDAT standards.

Table 2 shows that analytical data for raffinates generated by the commercial process compare favorably to MDU results, and conform to BDAT standards for volatile and semivolatile components.

Since the completion of the early test phase, CF Systems has modified the unit to convert it to an automated commercial system.

The modifications were completed in March 1991, and the plant is now operating on a commercial basis, continuously treating the refinery's hazardous-waste sludges to EPA land-disposal standards.