Demetallation and Recovery of Fuel Oil from Hazardous Waste

Nimish Dhuldhoya, A. R. Tarrer and Wen-Terng Wu Department of Chemical Engineering, Auburn University

INTRODUCTION

In the United States, automotive and other industrial sources generate about 1.2 billion gallons of used oil each year. The most pertinent features of this oil with regard to recycle and disposal problems are flash point and water, sediments, ash, nitrogen, and oxygen content. Many of the impurities are dispersed in the oil due to the very effective dispersant characteristics of the additives in a modern-day lubricant system(1).

Waste oil, although contaminated, has a high energy value, and burning as a fuel is a major outlet for used oil. Unfortunately, used oil contains high concentrations of metallic contaminants, and its combustion can cause adverse environmental effects. Some of the metallic materials in used motor oils are introduced during use. Typically, the metals introduced by means of wear or corrosion are aluminum, copper, iron, lead, silicon and tin, while sodium, barium, calcium, zinc and magnesium usually come from additives(2).

A number of re-refining processes have been used in the past (3-5). The low yield, high operating cost and generation of a large amount of hazardous sludge make these processes economically unattractive. In view of today's problem it appears that an economically viable re-refining process is needed which can:

* eliminate or minimize potential byproduct pollutants (spent acid, spent caustic, spent clay, SO2 etc.)

* produce acceptable products, and

* remain flexible enough to adapt to changing additives and/or environmental considerations.

In principle, many methods could be employed for the removal of the contaminants, but economic considerations frequently dictate using chemical treatment as a viable alternative for the production of fuel oils. Although demulsifiers, flocculating aids, contaminant oxidizer and conditioning reagents (e.g. caustics) are available commercially, chemical treatment of waste oils has not yet been demonstrated on a large scale. However, low capital and operating costs, high product yields, potential wide application, low energy treatments and especially low residue production make chemical treatment an attractive choice for waste oil processing facilities involved primarily in fuel oil production.

In this work, a chemical demetallization agent was used to convert entrained non-filterable metals into a form which could be effectively removed by filteration or sedimentation. Waste oils were demetallized by diammonium phosphate (DAP). A detailed parametric study was undertaken to map out the process variables so as to identify the most efficient demetallization conditions.

EXPERIMENTAL PROCEDURE

Materials. Used oil was collected from the Auburn University waste oil reprocessing pilot plant and from different service stations in the vicinity of Auburn.

Equipment. A three-neck distillation flask was used as a reactor. In a typical run, the flask was charged with waste oil and demetallizing reagents. The content was agitated and heated by a mechanical stirrer and heating mantle respectively. The reaction was carried out at atmospheric pressure, and water vapor and light ends were condensed and collected during the process. Oil was filtered immediately after the reaction by means of a vacuum filteration system, or allowed to settle down at constant temperature for a sedimentation study.

Analysis. Experimental results were verified in accordance with an EPA

approved quality assurance project plan. The properties of the demetallized used oil were evaluated to assess its potential as an EPA specification-grade fuel oil by using the following test methods: water content, ASTM D4006-81 (water in crude oil by distillation); ash content, a modification of ASTM D482-80 (ash from petroleum products); and lead content by standard atomic absorption analysis.

METAL REMOVAL MECHANISM

Lead and other metal contaminants are present in waste oil in organometallic form. The DAP (NH4)2HPO4) reacts with organometallic compounds to form insoluble and separable products. The reaction of this type has been discussed by Miller(6):

(RSO3) aCa]j (OH) c (CO3) J + excess (NH4)2HP04 - Ca5(PO4)3(OH) + NH4RSO3 + CO2 + H2O

Based on the above reaction, the calcium overbased sulfonate detergent reacts with DAP to form a metallic hydroxy1 apatite (Cas(PO4)3OH) , which is insoluble in aqueous and oil phase. This reaction mechanism is general for Ca, Ba, Mg, and Zn additives. Lead is present in a physically different form than additive compounds in the used oil. The lead particles present in used oil are too large in size to react with DAP, except for surface reactions.

The organometallic additives in waste oil react with DAP in the interfacial region as shown in figure 1. The additives are oriented with the metallic parts near the aqueous phase and the organic parts toward the oil phase. This what causes the outer surface of the droplets to bear a negative charge, which attracts the positive dipole associate with the organolead compounds. The ionic attraction effect is enhanced when water is removed from the agglomerates. The lead particles are entrained with the agglomerates of metallic hydroxyl phosphates arid then settle down.

RESULTS AND DISCUSSIONS

The proposed DAP demetallization mechanism suggest that water removal during the reaction favors the agglomeration of metallic hydroxyl apatites, which would facilitate the solid-liquid separation later on. The presence of water during the reaction was found to play an important role as a carrier for the DAP, aiding in its dispersion and reducing the importance of mass transfer. The DAP was dissolved in the water (30 wt% solution) prior to the reaction. The optimum ratio of DAP to ash content was found to be between 2 to A wt%. A higher percentage of DAP favors the ash and lead reduction, but problems associated with using a large amount of DAP are: (i) the great deal of sludge produced, which results in a solid-waste disposal problem; (ii) wear and tear of equipment: (iii) a decrease in the filteration rate; and (iv) the high cost of the reagent.

To investigate the effect of DAP on ash and lead reduction, several experiments were performed in which waste oil was treated with different amounts of DAP. The reaction was carried out at 150°C for an hour. Oil was filtered after the reaction by Whatman 3 filter paper. Table 1 shows the percentage reduction of ash and lead with respect to DAP. It should be noted that the amounc of DAP required to demetallized a waste oil depends on the ash content of the waste oil. The higher ash content requires a higher amount of DAP.

The reaction between additive metals (Ca, Ba, Mg, etc.) and DAP goes to completion and is less dependent of reaction temperature and time. Lead is present in motor oil in a form that is physically and chemically different from additive metals. The additive metals are almost exclusively group I and group II elements. Compounds of these elements tend to be more reactive than lead compounds and also are more ionic in nature than their lead counterparts. Since lead is not

OIL PHASE

AQUEOUS PHASE

METAL

HYDROXYL

PHOSPHATE

PARTICLES

FORM AT

INTERFACE

OIL PHASE

LEAD COMPOUND PARTICULATES

HEATING EVAPORATES AQUEOUS PHASE

AGGLOMERATION OF-HYDROXYL PHOSPHATES WITH ENTRAINED LEAD PARTICULATES (REMOVED BY FILTRATION)

AGGLOMERATION OF-HYDROXYL PHOSPHATES WITH ENTRAINED LEAD PARTICULATES (REMOVED BY FILTRATION)

Figure 1 Proposed Mechanisms for Additive Metal and Lead Removal(6)

TABLE 1: EFFECT OF DAP ON ASH AND LEAD REDUCTION

w.r.t. OIL CONTENT REDUCTION CONTENT REDUCTION

Reaction Temperature: 150°C, Holding Time: 1 Hr

Ash and Lead content of used oil before reaction was 0.6 wt %, 660 ppm, directly removed via chemical reaction with DAP, its removal is not independent of the temperature or the residence time. As seen in figure 2, increasing the residence time has a great effect on the removal of lead from used oil.

Table 2 shows the effect of temperature on the lead reduction. There is hardly any lead reduction when reaction temperature is less than 100°C. Presence of water during the reaction prevents agglomeration of hydroxyl phosphates, which facilitate the removal of lead particles. As discussed earlier, the removal of water during the reaction entrains the lead particles with the agglomerates of metallic hydroxyl phosphate.

SLUDGE REMOVAL

All hydrocarbon oils react with oxygen upon exposure to air at sufficiently elevated temperatures for long periods of time. Over the range of temperatures developed in engine crankcase, the rate of oil oxidation has been found to double for every 20°F rise in temperature. Although all lubricating oil hydrocarbons are susceptible to oxidation, of more importance to engine performance are the oxidation products. Paraffinic hydrocarbons apparently combine with oxygen at the carbon atoms near the end of the chains, and the mechanism of oxidation has been suggested as:

(1) paraffinic hydrocarbons ----

acids.

0 25 50 75

Holding Time (Min)

Fig. 2 Effect of Holding Time on

Lead Reduction (DAP - 2 we%. Reaction Temp. - 150°C)

0 25 50 75

Holding Time (Min)

TABLE 2: EFFECT OF REACTION TEMPERATURE ON LEAD REDUCTION

REACTION TEMP. LEAD CONTENT % LEAD

oc (ppm) REDUCTION

200 20 97

150 70 97

120 150 77

80 600 9

Ash and Lead content of used oil before reaction was 0.6 wt% and 660 ppm.

(2) paraffinic hydrocarbons ----

ketonic acids.

Thus, oxidation of paraffins yields alcohols, aldehydes, ketones and acids. The acids formed are vigorously corrosive to copper, lead and cadmium engine bearings. Naphtenic hydrocarbons yield products similar to paraffins upon oxidation. Aromatic hydrocarbons are the most readily oxidized constituents of lubricating oils. The end products are very complex condensation and polymerization products and tend to be insoluble in oil. These products constitute the sludges, resins and varnishes which allegedly cause piston ring sticking in the engine. In particular, detergent additives are used to disperse oxidative products and other contaminants, and to keep them in suspension. The amount of additives and sludge present roughly corresponds to 10% of the used oil component. These impurities clog the filter media and slow down the filteration rate considerably.

Removal of carbonaceous impurities and sludge from lubricating and industrial oils generally have been achieved by acid-clay treatment. During acid-clay treatment, a substantial amount of the diaroraatics and polyaromatics-polar materials are lost. The higher molecular weight aromatics are generally associated with natural lubricity characteristics of the base oil, and therefore selective removal of these compounds by acid treatment would affect this parameter of the lubricant product. Acid treatment produced a very large amount of acid sludge which threatens to pose a larger pollution problem than that caused by the used lubricating oil. Relatively low process yield (50-60%) and almost intractable residue problems lead to a firm recommendation against its use in the re-refining industry.

In order to remove the sludge from the used oil, a large number of chemical agents were tried as sludge flocculants. Many of the reagents were ineffective in removing the sludge from the used oil. Of the most effective materials, triethanolamine (TEA) was found to be a good sludge flocculant. TEA has three hydroxyl groups and one amino group. In general, it can be considered that the hydroxyl group serves to reduce the vapor pressure and increase the water solubility, while the amino group provides the necessary alkalinity in water solutions to cause the neutralization of acids(8).

A detailed parametric study was done in order to evaluate the effectiveness of TEA and DAP on the ash and lead reduction. Prior to reaction TEA was dissolved in the DAP

solution (20 wt%) and was treated with used oil for an hour. After the reaction processed oil was allowed to settle down for a day at 80°C. The amount of oil recovered is calculated and analyzed for ash and lead content.

Table 3 shows that ash and lead content of oil is reduced significantly at low temperature (80°C.). One possible explanation of high lead reduction at low temperature is that lead, being heavier than additive metals, separates with the sludge, while reaction between additive compounds and DAP is less dependent on the temperature.

Tables 4 and 5 shows the effect of different amounts of DAP and TEA on ash and lead reduction. Significant points emerging from the data of tables 4 and 5 are: (1) without DAP, only 25% ash reduction is obtained compared to 87% with 1 wt% of DAP;

(2) ash reduction seems to increase with increasing amount of DAP, and it remains constant after further increase in percentage of DAP; and

(3) the low oil recovery at high concentration of TEA (table 5) is due to separation of heavy hydrocarbons with the sludge. The last column of these tables (vol% recovery) is based on the volume % of recovered oil, whose ash and lead content was below 0.15 wt% and 10 ppm, respectively. It can be seen from these tables that 90% of oil is recovered after the treatment, which is very high considering the fact that used motor oil contains roughly 8 to 10% of sludge and additives. Figures 3 and 4 shows the effect of settling temperature and time on oil recovery. Higher temperature facilitates the separation of sludge from the oil.

Waste Reduction Sheet Metal

Fig. 3 Effect of Settling

Temperature on Oil Recovery (DAP - 1 wt%. TEA -0.5 wt% Settling Time - 24 hrs)

Fig. 3 Effect of Settling

Temperature on Oil Recovery (DAP - 1 wt%. TEA -0.5 wt% Settling Time - 24 hrs)

Fig. 4 Effect of Settling Time on Oil Recovery (DAP - 1 wt%, TEA - 0.5 wt%, Settling Temperature - 80°C)

TABLE 3: EFFECT OF REACTION TEMPERATURE ON ASH AND LEAD REDUCTION

ASH CONTENT RED. LEAD CONTENT RED.

Before After Before After

TABLE 4: EFFECT OF DAP ON ASH AND LEAD REDUCTION

DAP ASH CONTENT RED. LEAD CONTENT RED. RECOVERY

Before After Before After

0.

.0

0.

,86

0.65

25

114

50

56

0.0

0.

.5

0,

.89

0.47

47

91

25

73

0.0

1.

.0

0.

.89

0.11

87

109

6

93

89

1.

,5

0.

.80

0.10

86

89

4

95

92

2.

,0

0.

,82

0.11

86

69

2

97

92

TABLE 5: EFFECT OF TEA ON ASH AND LEAD REDUCTION

TABLE 5: EFFECT OF TEA ON ASH AND LEAD REDUCTION

TEA ASH CONTENT RED. LEAD CONTENT RED

Before After Before After

TEMP OC.

RECOVERY vol%

FLOW SHEET FOR WASTE OIL RECOVERY PLANT

OIL FROM VENDORS

OIL FROM VENDORS

Dap Process Diagram Pic

Based on this work, the Chemical Engineering Department of Auburn University is developing a pilot plant for waste oil recovery (see flow diagram). The pilot plant will have a capacity to treat 5 gallons\min of waste oil. Further research work is still being carried out in this field.

CONCLUSIONS

(1) The organometallic additives in waste oil react with DAP in the interfacial region and form metal hydroxyl phosphate.

(2) TEA was very effective in removing sludge and lead from the waste oil.

(3) Reaction conditions of this process were very mild compared to other waste oil recovery processes.

(4) Oil recovery depends on settling time and temperature and is greater at higher temperature.

(5) More than 85% ash and lead reduction is obtained by treating waste oil with 1 wt% DAP and 0.5 wt% TEA for an hour at 80°C under atmospheric pressure.

REFERENCES

(1) Vaughn S. Kimball, Waste Oil Recovery and Disposal; New Jersey: Noyes Data Corporation (1975).

(2) T. D. Coyle and A. R. Siedle, "Metals in Oil Occurrence and Significance for Reuse of Spent Automotive Lubrication Oil." NBS special publication 556-559 (1979).

(3) Cotton, F. 0., "Waste Lubricating Oil. An Annotated Review," BETC / IC-

(4) Stubbs, Overbeck and Associates, "Engineering Design of a Solvent Treatment / Distillation Used Lubrication Oil Re-Refinery," DOE / BC / 1009-9 (1980).

and F. 0. Cotton, "Waste Lubricating Oil Research: An Investigation of Several Re-Refining Methods," Bumines RI 7884 (1974).

(6) Miller, T. M., "An Investigation of the Demetallization Chemistry Associated with the Re-Refining of Used Motor Oil," M. S. thesis, North Carolina State University, Raleigh, N.C. (1983).

(7) Comeford, J. J., "Test Procedures for Recycled Oil Program," NBS special publication 556:133-137 (1979).

(8) Arthur Kohl, Fred Riesenfeld, Gas Purification; Gulf Publishing Company (1985) .

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