Oily Sludges Thermal Treatment

Crude oil tank bottoms and other petroleum oily sludges and emulsions containing paraffins and volatile hydrocarbons can be economically reprocessed with heavy-oil dehydration facilities to recover residual hydrocarbons and to achieve volume reductions.

The main factors affecting the use of this alternative are:

  • The characteristics of the sludges requiring treatment.
  • The availability of waste heat or existing high temperature (>350°F) dehydration facilities.
  • Air emissions from the process.
  • Effluent criteria for treated residues.

The process performance was quantified in terms of general operating parameters (flash point and paraffin, oil, water, and solids content); specific constituent analyses for benzene, toluene, ethylbenzene, and xylene (BTEX); and analyses for total petroleum hydrocarbons (TPH) content. Information on the percent removal of these parameters, characteristics of the treated residues, and the hydrocarbon recovery efficiency of the process are presented.

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Introduce

Several residual materials generated by the petroleum E&P industry are candidates for reprocessing in high-temperature dehydration systems. Examples of such materials include tank bottoms, skim oils, and slop-oil emulsion and solids. These E&P materials, if disposed of as wastes, are exempt from regulation as hazardous wastes under the Resource Conservation and Recovery Act (RCRA). However, some states have recently adopted regulations that supersede the E&P RCRA exemption, generally increasing the costs for disposing of E&P residuals.

In these areas, High-temperature Reprocessing (HTR) can be a low-cost alternative to conventional treatment and disposal methods. In other areas, HTR can be a proactive tool for achieving waste minimization goals and limiting the liability of disposing of these E&P residuals. It is generally cost-effective to maximize hydrocarbon yield and reduce the residual volume of oily materials in any regulatory environment.

HTR is a process in which influent emulsion or sludge is heated above the boiling point of water and then allowed to flash in a separation tower where steam and light hydrocarbons are subsequently extracted. Heavier hydrocarbons and inorganic material are removed from the separation tower as a slurry phase. Light hydrocarbons and water can be recovered by condensation; heavier hydrocarbons can be recovered after liquid/solids separation of the slurry phase. This process is similar to conventional oil and gas production dehydration processes but functions at much higher temperatures.

HTR provides improved conditions for separation and recovery of hydrocarbons from emulsions and sludges because the oily materials are initially heated to temperatures high enough to ensure vaporization of water from the material in the separation tower. Furthermore, at high temperatures, mass transfer rates of hydrocarbons from the solid inorganic phase increase because the viscosities of hydrocarbons are decreased. Hydrocarbons desorbed from the solid phase can then be recovered and sold.

Similar processes have been tested for separation and hydrocarbon recovery for petroleum refining wastes, but these processes use pressurized separation instead of a flash tower. Higher temperatures and pressures are also used; materials are heated to 575 to 650 F at 2,000 psia before entering the pressurized separator. These processes have been successfully demonstrated for such refinery wastes as separator sludge, dissolved-air-flotation float, slop-oil emulsion solids, and heat-exchanger sludge, but they have been ineffective for treating water-in-oil emulsions generated by the petroleum E&P industry.

HTR offers several potential advantages over conventional types of sludge treatment: filter pressing, centrifugation, and biological land treatment. Because HTR facilities are similar to other types of production facilities, they can be considered part of the production process and can be incorporated into existing permits to operate. Furthermore, as part of the production process, the function of HTR facilities is to “enhance process efficiency” through increased oil recovery rather than to “treat residuals.” This may result in fewer permitting requirements, particularly in states where “treatment” of E&P residuals can be regulated.

A second advantage is that the area requirements are relatively low. Thus, HTR could find application in such areas as offshore E&P operations. Operation of HTR units at temperatures above the melting point of paraffins also allows treatment of materials higher in paraffin content ( 1%) than those treated in processes that use pressurized filtration and centrifugation. Volatile organic compounds, which may cause air permitting issues with conventional processes, can be recovered by condensation or vapor recovery during HTR. Another advantage is the reduced risk of off-site contamination because materials are processed in a closed environment.

Screening studies can be conducted to determine whether HTR is viable for an oily sludge. Materials should be screened for applicable parameters, which may include paraffin, oil, water, and solids content and flash point, to determine whether the material requires processing and whether the material has sufficient hydrocarbon content to merit HTR. In addition, oily sludges to be reprocessed should be tested for heavy metals, such as lead and zinc, because metals cannot be addressed through HTR. However, HTR can be used to minimize the volume of heavy metal-contaminated material that must be disposed of if precautions are taken to prevent contamination of other production facilities.

Thermal Treatment Principles

Thermally enhanced separation of produced water-in-oil and oil-in-water emulsions is widely applied in petroleum production facilities. Some examples are indirectly heated separators, gun barrels, and heater/treaters. Thermally enhanced separation has been particularly useful in processing oil-in-water emulsions produced from heavy-oil reservoirs under steamflood. Emulsions are heterogeneous liquid systems consisting of two immiscible liquids (water and oil) with water intimately dispersed in the form of fine droplets (diameter <1 mm) in oil, or vice versa.

The goal of HTR is to use heat for recovery of hydrocarbons from influent oily emulsions and sludges. Temperature influences the physical nature of hydrocarbons and water; hence, temperature affects the separation of oil-in-water emulsions. The fundamental principle of thermally enhanced separation of emulsions is to create conditions where four main benefits of heat are used:

  1. Reduction of oil viscosity; 
  2. Increased molecular movement of droplets, which aids coalescence through increased collision frequency of the dispersed-phase droplets;
  3. Heat deactivation of the emulsifier, such as the dissolution of paraffins;
  4. Increased difference in density between oil and water.

It has been demonstrated that emulsions can be separated into oil, water, and solid phases at high temperatures and pressures; however, the phases tend to re-emulsify as temperatures and pressures are reduced. In HTR, the water phase is flashed and removed from the oil/solids slurry in the separation tower, preventing re-emulsification.

Crude oils may contain high concentrations of paraffinic compounds. In addition to other problems associated with production of paraffinic crude oils, such as wellbore and flowline plugging, solid paraffinic material may accumulate in tank-bottom sludges. Paraffins have low solubilities in water, ranging from 0.66 to 62.4 mg/L at 77°F, but they become soluble in crude oil as temperature increases Thus, paraffins in influent emulsions and sludges can be solubilized into the oil phase and recovered through HTR.

Volatile organic compounds (vapor pressure >5 mm Hg at 77°F), such as BTEX, may also be present in petroleum sludges. At high temperatures, as the viscosities of hydrocarbons decrease, the mass transfer rates ofthese compounds from the liquid phase to the vapor phase increases. Recovery of BTEX compounds through the vapor phase is desirable because these compounds may be of regulatory concern for effluent residual materials. Prevention of volatile emissions from HTR facilities can be ensured by cooling and condensing off-gases and installing and maintaining vapor recovery systems on tanks handling HTR effluent streams.

Additionally, removal of volatile organic compounds may correlate to increases in the flash point of residual materials. In areas where E&P residuals are not exempt from regulation as hazardous wastes, flash point is the criterion used to determine whether materials are hazardous by the ignitability characteristic. For example, under Title 22 of the California Code of Regulations, residuals with a flash point of <140°F are classified as ignitable. Therefore, reductions in the concentrations of volatile organic compounds and corresponding increases in flash point in residual materials not only will reduce the potential risks of handling the residual materials but also may result in classification of these residuals as nonhazardous.

HTR Performance Parameters

Chemical and physical parameters can provide an assessment of process performance by quantifying the removal of hydrocarbons during thermal treatment of oily sludges.

TPH

The hydrocarbon content of influent and effluent sludge can be monitored by measuring the TPH concentration. High TPH concentrations (>500,000 mg/kg) indicate that an influent material has substantial hydrocarbon content and that the costs of reprocessing may be offset by the sale of recovered hydrocarbons. Although hydrocarbons are removed during processing, TPH concentrations in the residual material may increase as the water fraction has been removed, potentially concentrating the hydrocarbons that remain. The standard method for measuring total petroleum hydrocarbons is EPA Method 418.1.

Paraffin Content

Paraffins are a common constituent of crude oil tank-bottom sludges. Theoretically, because of the high temperatures used in HTR, paraffin content would be expected to decrease during treatment as these compounds are solubilized in oil and removed. Paraffin removal can be important for processing of sludges because volatile organic compounds may also be liberated and recovered as the paraffins are solubilized. Paraffin content can be measured by ether/acetone extraction.

Flash Point

Flash point may be a limiting factor in testing to determine whether such materials as petroleum sludges are hazardous for the ignitability characteristic. Where applicable, tank-bottom sludges being reprocessed should be tested for flash point before and after treatment by use of the American Society for Testing and Materials Method D 93

Specific Organic Constituent Analyses

Removal of organic constituents to a specified regulatory limit may be the goal of an HTR application. For example, local regulations may require grab samples of treated effluent to contain not more than X mg/kg of benzene or Y mg/kg of total xylenes before the effluent can be beneficially reused. Hence, analyses may be conducted to determine the total extent of the losses of specific organic constituents. One important group of organic constituents are BTEX. BTEX’s are volatile, nonpolar compounds consisting of a single unsubstituted aromatic ring (benzene) and its ethyl and methyl substituted derivatives (toluene, ethylbenzene, and xylenes). There has been considerable interest in the BTEX’s because of the inclusion of these compounds on the U.S. Environmental Protection Agency’s (EPA’s) list of priority pollutants. These compounds may also provide an assessment of the removal efficiency of the process. Concentrations of BTEX compounds can be measured using EPA Method 8020.

Screening Parameters

The oil, water, and solids content of the oily sludge indicates whether the material is treatable by HTR. Ideally, the sludge will have a high concentration (>50%) of oil and a relatively low concentration «30%) of solids. Oil, water, and solids content can be determined by distillation and extraction. The solvent-soluble, acid-soluble, and acid-insoluble percentages of the solids in the oily sludge are also important for determining the applicablility of HTR. Solvent soluble solids are generally paraffinic solids that can be solubilized into oil at high temperatures. Acid-soluble solids are generally thought to be iron sulfide scale from piping.

These solids will remain in the residual sludge. Acid-insoluble solids are silica and other produced mineral solids that will also be present in the residual sludge from HTR. The respective fractions can be determined by solubilizing samples in toluene followed by HCI. The materials should also be screened for heavy metals, such as lead and zinc, which can be present in crude oil tank bottom sludges and cannot be removed through HTR.

Field Scale Demonstration on HTR

Treatment Processes

Field use of HTR was successfully demonstrated for several tankbottom sludges and skim oils generated at light- (>30° API) oil-producing leases in Kern County. Kern County is an integrated oil-producing area, yielding both light and heavy oil. Tank bottoms from light-oil leases are typically hazardous under California statutes by the ignitability characteristic (flash point <140°F).

For some of these sludges, volatile hydrocarbons bound in the paraffinic fraction of the sludge are thought to cause low flash points. This hypothesis is based on low flash points measured for weathered sludges. Conventionalliquidlsolids separation of these tank bottoms through filter pressing and centrifugation is not cost-effective because of the high paraffin content. Consequently, the lowest-cost existing alternative for treatment/disposal of the tank bottoms was determined to be stabilization, followed by disposal at a hazardous waste landfill at costs of approximately $401bbl.

The application of HTR through heavy-oil dehydration facilities was chosen to increase product recovery from the light-oil tank bottoms, thereby producing a smaller volume of nonhazardous residue. Steam dehydration facilities at heavy-oil-producing leases, such as those in the Midway-Sunset field, use high temperatures to maximize oil production.

These facilities, which are the last step in the separation process, use surplus steam generated for steamflooding operations to heat indirectly the recalcitrant emulsions pumped from the lease reject tank. Material is recirculated from the reject tank to the steam dehydration facility until the basic sediment and water content of the oil phase in the reject tank is below the prescribed limit and the oil can be pumped to the sales tank. Fig. 1 is a schematic of the steam dehydration facility used to demonstrate HTR.

The material treated in the first test phase consisted of Sludges I, 2, and 3 with TPH concentrations of 529,000,446,000, and 526,000mg/kg, respectively (Fig. 2). The average oil, water, and solids composition of these sludges (Fig. 3) indicated that the oil content of the materials was approximately 50%. During the initial phase of testing, approximately 1,100 bbl of these sludges was treated through HTR.

For this phase, a temporary storage tank was placed near the reject tank and a line was constructed to connect the temporary tank with the line from the reject tank to the steam dehydration facility. After removal from tanks at the originating leases, the tank bottoms were transported to the reprocessing facility by vacuum truck and loaded into the temporary storage tank.

During reprocessing, material was fed from the temporary tank to the steam dehydration unit at approximately 32 gal/min. The steamlheat exchanger in the facility was operated at 350°F. After heating, the material flowed into the flash tower, where the pressure was maintained at 25 psig.

Water and volatile hydrocarbons were flashed and removed overhead through the off-gas line before being cooled, condensed, and processed through the produced-water handling facilities. The hot oil/solids slurry was pumped from the flash tower through an exchanger for preheating influent material and returned to the reject tank for separation.

Initially, the reject tank contained produced water under an oil/emulsion layer. As the oil/solids slurry from the flash tower entered the reject tank, partial separation of oil and solids occurred, and residual oily solids settled through the produced water and accumulated at the bottom of the tank. From the 1,100 bbl reprocessed during the initial testing phase, approximately 330 bbl of residual tank bottoms was accumulated in the reject tank and 450 bbl oil was recovered and sold. The amount of oil recovered was determined by estimating the incremental increase in oil sold through the net oil tank during reprocessing and by completing a mass balance by subtracting the residual tank bottoms and vaporized water from the initial volume of material.

To determine the applicability of HTR for processing different materials, two additional sludges were tested: Sludge 4 and skim oil. The initial TPH concentrations of the two materials were 749,000 and 478,000 mg/kg, respectively (Fig. 2). Analyses to determine the solvent-soluble, acid-soluble, and acid-insoluble fractions of the skim oil (Fig. 4) showed that 72% of the material was solvent-soluble and was thus potentially recoverable. The operating conditions were identical to those for the first three materials, but a material balance was not constructed for these materials.

For all the materials tested, flash point, paraffin content, and concentrations of volatile organic compounds were measured in the raw material before reprocessing and in the residual tank-bottom material to quantify process performance. Fig. 5 shows the results of analyses for BTEX in the five different sludges and skim oils tested before reprocessing and in the residual materials. The sludges initially contained total BTEX concentrations ranging from 154 to 994 mgt kg. Total BTEX dropped an average of 94% in the materials after processing, indicating a significant reduction in volatile hydrocarbons.

The residual BTEX concentrations ranged from 82 mg/kg for Sludge 2 to 4.2 mg/kg for skim oil. Before reprocessing, each sludge also contained a significant fraction (>5%) of paraffins, with the highest concentration in Sludge 4 (10.3%). Analyses for paraffin content (Fig. 6) indicate an average 75% reduction in paraffins in the residual materials relative to the materials before reprocessing.

Reductions in paraffin content and BTEX concentrations in the HTR residuals were expected also to contribute to increases in flash point. Results of the analyses for flash point (Fig. 7) indicate that each of the five sludges had flash points well below 100°F before reprocessing. However, the residual materials in the reject tank after reprocessing were nonhazardous under California statutes, having flash points exceeding 140°F. Therefore, the residuals could potentially be beneficially recycled as berm material or road base.

In addition to eliminating the liability associated with disposal of a hazardous waste, the cost of processing tank bottoms and skim oil was reduced from >$40Ibbl to about $41bbl. Through use of this method of processing, income was also generated from the sale of the recovered oil. Using $141bbl for the price of heavy crude and 40% recovery of oil by volume from the influent materials, this results in a profit from the sale of recovered oil of more than $61bbl of sludge reprocessed. Thus, the materials were reprocessed at a net profit of approximately $21bbl.

Conclusions

It is expected that the applicability of HTR will vary from one area to the next, owing to differing types of oil production and facility configurations and to varying oily sludge composition. However, the data from these studies indicate that HTR is capable of processing petroleum sludges at initial BTEX and paraffin concentrations as high as 994 and 103,000 mg/kg, respectively. Some residual material will require proper handling as a nonhazardous waste. Significant removal (>94%) of most BTEX compounds occurred in this application.

The data from these studies demonstrated that a properly designed and operated HTR facility can be used to achieve volume reductions, to recover residual oil, to increase flash point, and to reduce the concentrations of volatile organic compounds. Consequently, this treatment method should be considered for meeting current and future performance standards with petroleum oily sludges.

Steam dehydration facility
fig 1. Steam dehydration facility
TPH concentration for unprocessed sludges and skim
oil
fig 2. TPH concentration for unprocessed sludges and skim oil
Composition of a typical light-oil tank-bottom sludge
Fig 4. Composition of a typical light-oil tank-bottom sludge
Soluble fractions for skim oil
Fig 4. Soluble fractions for skim oil
Total BTEX in sludges and skim oils before and after reprocessing.
Fig 5. Total BTEX in sludges and skim oils before and after reprocessing.
Paraffin content of sludges and skim oil before and after
treatment.
Fig 6. Paraffin content of sludges and skim oil before and after
treatment.
Flash point of sludges and skim 011 before and after reprocessing.
Fig 7. Flash point of sludges and skim 011 before and after reprocessing.

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