George P. Rasmussen
Waste Tech Services Inc.
Golden, Colo.
A new desorption process for treating refinery wastes has been proven in pilot demonstrations at Amoco Oil Co.'s Texas City, Tex., refinery.
The process Waste Tech Services Inc's desorption and recovery unit (DRU) - treats petroleum contaminated refinery wastes and recovers oil and water suitable for recycling to the refinery.
The DRU meets Resource Conservation and Recovery Act (RCRA) recycle exemptions and produces solids that satisfy U.S. Environmental Protection Agency (EPA) land disposal restrictions (LDRs).
RCRA WASTES
U.S. petroleum refineries generate some 500,000-800,000 tons/year of dewatered, oily RCRA hazardous wastes. These wastes designated K048, K049, K050, K051, K052, F037, and F038 include API separator sludge, dissolved air flotation (DAF) float, slop oil, heat exchanger solids, tank bottoms, and other primary and secondary treatment sludges.
Before K and F wastes can be disposed of on land, the must be treated to meet LDR regulations established under EPA hazardous waste regulations (40 CFR, Part 268).
Refiners are spending considerable money to meet these LDR best demonstrated available technology (BDAT) treatment standards. In most cases, on site treatment requires generators to withstand the difficult and time consuming process of obtaining a RCRA Part B treatment permit.
Regulations in 40 CFR Part 261.6 establish "requirements for recyclable materials." These requirements allow refiners to reclaim and recycle oil from hazardous wastes, or use the wastes as fuel, without requiring a Part B treatment permit. This stipulation can be superseded, however, if individual state requirements are more stringent.
The DRU process can manage refinery wastes by reclaiming normally lost waste oil for recycle, in accordance with these recycling exemptions. In mid-1992, the Texas Water Commission now part of the Texas Natural Resource Conservation Commission authorized Waste Tech Services to install and operate a 1.8 million lb/year (900 tons/year) pilot scale DRU at Amoco's Texas City refinery.
PROCESS DESCRIPTION
The DRU process uses two stage indirect heating to reduce waste volumes (Fig. 1). The first heating stage dries the sludge or soil by volatilizing and removing water and low boiling organic compounds, which are subsequently condensed. The second heating stage increases the solids temperature to about 925 F. (496 C.), which volatilizes the remaining organic compounds, including EPA regulated semivolatile organic compounds.
Oil from each condensing system is separated individually and returned to the refinery for refining. The water condensate is processed through the refinery's waste water treatment system.
Uncondensed organic vapors, consisting of light hydrocarbons, are oxidized for process heat or vented through an approved air pollution control device. Dry solids exiting the second heating stage are cooled before discharge and disposal.
DEMO UNIT
The Texas City DRU is shown in Fig. 2.
Sludge and cake feeds were received and fed to the process by an active bottom, variable speed screw feeder and lock valve arrangement. The first stage heater is a thermal screw processor, which indirectly heated the sludge by hot heat transfer fluid circulated in the double screw mechanism and the processor jacket.
Oxygen levels in the processor and offgas condensing system were maintained at less than the autoignition concentration by introducing nitrogen at points where there is the potential for air to leak in during operation.
Water and organics volatilized by the processor were drawn off, cooled, and condensed in a vapor scrubber/condenser tower. Condensation was achieved by direct contact with a condensed oil and water recirculating stream maintained at temperature by indirect contact in an air cooled heat exchanger system.
The condensed oil and water were pumped to a gravity separator. Recovered oil was recycled to the refinery for processing. Recovered water was treated in the refinery waste water treatment system.
Dry, partially deoiled solids at 550 600 F. (287 316 C.) were gravity discharged from the first stage heater directly into the second-stage, paddle type mixer with extended surface area. In this mixer, the solids were heated to no more than 950 F. (510 C.) by electric heaters in the sealed, molten salt filled, hollow shaft and paddles.
Volatilized oil was partially condensed by direct contact with a recirculating oil stream in a quench tower maintained at 250 350 F. (121 177 C.). The recirculating quench tower oil temperature was maintained by indirect contact in a glycol/water, air cooled heat exchanger system.
Uncondensed water and lower boiling hydrocarbons passed through the condenser, where the remaining water and hydrocarbons were cooled by indirect contact in a glycol/water heat exchanger.
Condensed water and oil from the heat exchanger were discharged to a gravity oil/water separator before being recycled to the refinery. Uncondensed vapors (produced mainly by hydrocarbon cracking at the high processing temperatures) were directed to the heat-transfer fluid heater, where the vapors were mixed with propane and thermally oxidized to provide heat for the process.
Higher boiling hydrocarbons condensed in the quench tower were discharged periodically to the refinery for recycle. The deoiled solids were cooled to less than 200 F. (93 C.) before being discharged via a glycol/water cooled solids cooler.
OPERATING CONDITIONS
Table 1 shows the wide range of regulated wastes processed at Texas City, their designations, EPA classifications, general descriptions, and approximate compositions. Table 2 identifies the range of operating conditions during the demonstration.
While processing each stream, solids samples were taken from both the first and second stage desorption and recovery units. The samples were analyzed for both the regulated Toxicity Characteristic Leaching Procedure (TCLP) metallic constituents and the total organic constituents.
Recovered water and organic products also were analyzed for compatibility with both treatment in the waste water treatment system and recycling to refining processes.
ANALYSES
The residual solids from the first and second stages of the DRU were analyzed for physical and chemical properties. The results of these analyses were compared with TCLP, metals limits, and the LDR regulated total organic constituents for K048 K051 wastes.
Although waste stream samples that are characteristically hazardous (as defined by the EPA) are not subject to K048 K051 LDR treatment standards, these samples also were evaluated against LDR criteria.
Tables 3 and 4 compare the semivolatile and volatile organic compound constituents in the feed and residual solids with TCLP and LDR treatment standards. Metals in the feed and residual solids are compared to TCLP and LDR treatment standards in Table 5.
SEMIVOLATILES
The residual solids generated by desorbing each waste stream were analyzed for the semivolatile organic compounds listed in Table 3. Only those compounds with detection limits greater than 25 mg/kg in the raw waste, or greater than 0.2 mg/kg in the residual solids, were tracked for removal.
The data in Table 3 show that, at 840 F. (449 C.), and sometimes as low as 600 F. (315 C.), the semivolatile organic compounds remaining in the wastes were less than LDR treatment standards.
VOLATILES
Table 4 lists only those volatile organic compounds detected in the raw wastes at greater than the 2 mg/kg detection limit. Although the range of concentrations was wide (particularly for the stormwater tank emulsion), the volatilization profile was consistent for all waste streams. TCLP and LDR treatment standards were met at a solids temperature of 548 F. (287 C.).
Although the detection limit for the volatile organic compounds in the treated solids also was 2 mg/kg, no LDR treatment standards were exceeded. And because the TCLP test involves a 20-to 1 dilution factor, no TCLP limits would have been exceeded, even if an organic compound present at 2 mg/ kg were totally soluble, because the maximum possible concentration in the leachate would be 0.1 mg/1.
LEACHABLE METALS
Residual solids from the desorption and recovery system were subjected to TCLP testing (Table 5). Cyanide was not detected in any raw waste streams or residual solids at concentrations greater than 1 mg/kg detectability limit. Regardless of the concentrations of regulated metals in the deoiled solids, all TCLP and LDR treatment standards for metals were were achieved.
Eight additional metals (beryllium, cobalt, copper, antimony, tin, thallium, vanadium, and zinc) on EPA's hazardous substance list were monitored and included in the evaluation. Of these metals, only trace quantities of copper and zinc were detected in the leachate from several of the wastes.
RECOVERED OIL AND WATER
The physical characteristics of the od recovered from the samples during the demonstration were consistent, regardless of the type of oil in the waste. This consistency is attributable to the cracking that occurred at the high treatment temperatures.
Recovered oil characteristics are shown in Table 6.
The physical characteristics of the water recovered from the desorption and recovery process are shown in Table 7. This table also shows the concentration ranges of regulated metals and organic compounds.
Like the recovered oil, the recovered water composition was consistent, regardless of the waste stream source. The recovered water was comparable to the water already present in the API separator system; thus the recovered water could be returned to the separator without further treatment.
Desorption and recovery can process a wide range of refinery generated wastes and recover recyclable oil and water. Waste volume reduction, and thus cost savings, are proportional to the oil and water content in the wastes.
Copyright 1994 Oil & Gas Journal. All Rights Reserved.
