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Fractionation of oily sludges produced in the treatment of hydrocarbon wastes


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Oily sludges are hazardous wastes that contain large amounts of hydrocarbons, water and dissolved solids. A simple distillation procedure was used to fractionate this sludge and isolate a gas phase (10%), and an organic liquid phase (30%) that can be used as fuels. This procedure also separates the water phase from the dissolved solids enabling the independent management of those phases with more appropriated methods
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The cleaning operations of oil tanks and ships or the recycling of used mineral oils produce large
amounts of oily sludges that can be classified as hazardous wastes. The oily sludge is a complex
emulsion of hydrocarbons, water and mineral components, classified with code 05 01 (Sludge and
Solid Residues with hydrocarbon components) in the List of Waste (LoW) (EU 2000) and Annex
III to Directive 2008/98/ EC on waste (EU 2008), and take place in the list of hazardous waste,
which have special and specific manipulation many of which are toxic, so there is an increasing
interest in the development of new technologies for remediation or stabilization of this sludge
(Mazlova 1999),(Hu et al. 2013).
In Portugal these sludges have been, so far, co-incinerated or landfilled. Nevertheless, their
high contents in polyaromatic hydrocarbons and in some cases heavy metals result in high envi-
ronmental pressures associated with these disposal methods. The aim of this work is to explore
the oil sludge fractionation by distillation as a methodology for the isolation of homogeneous
fractions that can be further remediated or valorized by different technologies. The components
of this sludge are toxic, and when landfilled, the soil will be contaminated and it is a dangerous
situation for the humanity and the environment.
The contaminated soils are a huge preoccupation for the governments worldwide. This concern
is justified because of the polycyclic aromatics hydrocarbons (PAH), an oil contaminant (Reddy
et al. 2011).
According to INE (National Statistical Institute) and APA (Environmental Portuguese
Agency), Portugal in 2012, have produced ca 29 809 ton of used mineral oil residues, from which
23 110 ton were recovered. These numbers are according to the yields of oil recover from sludge
that are around 80%. Nevertheless to recover the sludge are also part of the recover process (sub
product) (Jean et al. 1999), (API 2010).
The Portuguese law allows that some enterprises can treat this kind of sludge, but there are
limits for the incorporation in cement industry and also for the amount to landfill. So there is
always the question: what to do with the remaining wastes?
The physical and chemical properties of the sludge may vary with the nature of the oils, the
operating parameters of the recycling process and the time and conditions of storage (Jean et al.
Fractionation of oily sludges produced in the treatment of
hydrocarbon wastes
A. P. Oliveira, M. Gonçalves, C. Nobre & B. Mendes
Mechanical Engineering and Resources Sustainability Center, Department of Science and Technology of
the Biomass, Faculty of Science and Technology, New University of Lisbon, Caparica, Portugal
M. C. Vilarinho & F. Castro
Mechanical Engineering and Resources Sustainability Center, Mechanical Engineering Department,
University of Minho, Guimarães, Portugal
ABSTRACT: Oily sludges are hazardous wastes that contain large amounts of hydrocarbons,
water and dissolved solids. A simple distillation procedure was used to fractionate this sludge and
isolate a gas phase (10%), and an organic liquid phase (30%) that can be used as fuels. This
procedure also separates the water phase from the dissolved solids enabling the independent man-
agement of those phases with more appropriated methods.
Nevertheless there is still a fraction of the hydrocarbon components that are retained in this
sludge and that have a potential for energetic valorization.
In this work oil sludges from pre-treatment units were fractionated by distillation to yield ho-
mogeneous fractions that were characterized in order to evaluate possible valorization processes.
2.1 Samples
The raw oily sludge used is this work was supplied by a local waste management industry (Car-
mona-Sociedade de Limpeza e Tratamento de Combustíveis, S.A.). The sludge was stored in large
plastic bottles, and kept in the dark, at room temperature, until analysis.
2.2 Sludge distillation
Sludge subsamples (~200g) were subject to simple distillation in an apparatus equipped with a
vacuum receiver adapter that enabled the collection of gas products in a Tedlar bag. The distil-
late was collected from 98ºC to 130ºC and was composed by an aqueous phase and an organic
immiscible phase although some degree of emulsification occurs between these phases.
The product yield was evaluated for all liquid products and for the solid residue and expressed
in wt. %. The gas yield was evaluated by the weight difference between the initial crude sludge
and the liquid and solid products from distillation.
2.3 Product characterization
The oily sludge, the distilled products and the solid residue were characterized by determination
of their proximate analysis and elemental analysis.
The proximate analysis was performed according to standards BS EN 14474-2: 2009, BS EN
15148: 2009 and BS EN 14775: 2009. Fixed carbon was evaluated by difference.
The elemental analysis (N, C, H and S) was performed in duplicate, using an Elemental Ana-
lyzer (Thermo Finnigan-EC Instruments, Flash template and CHNS 112 series). Oxygen was
evaluated by difference.
The gas phase composition (H2, N2, O2, CH4, CO and CO2) was determined using GC-TCD
(Thermo Electron Corporation) equipped with a Supelco-25467, Carboxen®-1010 PLOT capil-
lary column.
The molecular weight distribution of the liquid products from the oily phase was evaluated by
GC-FID (Trace GC, Thermounicam) and their qualitative composition was studied using GC-MS
(Focus GC-Polaris Q, Thermounicam). The main hydrocarbon components were identified by co-
injection of the corresponding standards. The aqueous phase organic components were sequen-
tially extracted with petroleum ether and ethyl acetate; both extracts were dried and analyzed by
3.1 Characterization of the oily sludges
The characteristics of the crude oily sludge (proximate analysis) were evaluated in three subsam-
ples collected after homogenization of the sludge and are presented in Table 1.
The sludge included in this study presents high contents of moisture and volatile matter when
compared with similar wastes studied by other authors (Table 1), but a separate aqueous phase is
not visible.
Table 1. Proximate analysis of oily sludge.
(%, a.r.)
Volatile matter
(%, d.b.)
Fixed carbon
(%, d.b.)
(%, d.b.) Reference
84.8 ± 1.8 86.8 ± 0.1 0.1 ± 0.8 13.1 ± 0.2 This wor
27.7 ± 6.3 - - 11.7 ± 1.0 (Al-Futaisi et al. 2007)
16.95 44.73 3.28 51.99 (Zhou et al. 2009)
29.5 - - 50.1 (Li et al. 1995)
32.64 73.98 9.35 16.67 (Qin et al. 2015)
29.26 - - - (Jing et al. 2011)
59.86 88.09 3.30 8.61 (Xu et al. 2014)
26.3 31.7 - 31.6 (Karayildirim et al. 2006)
24.0 - - - (Jasmine & Mukherji 2015)
- - - 35.0 (Conesa et al. 2014)
The ash content although moderate is also a factor that limits some management/remediation so-
lutions and contributes for the hazardous nature of those sludges.
The ultimate composition of the dried sludge (C, N, H, S, O) was evaluated and results are
presented in Table 2.
The dried sludge has a high carbon content (44.9 wt. %) but also a high oxygen content that
indicates the presence of a high concentration of oxygenated organic compounds. The fractiona-
tion of the crude sludge in different fractions with different composition and properties is there-
fore an interesting approach to valorize some of its components and reduce the volume of land-
filled waste.
Table 2. Ultimate analysis of dried oily sludge.
3.2 Distillation Yields
The fractionation of the oily sludge by simple distillation yielded the following products (n=4):
gas components (8.5 ± 5.4 wt %), an oily phase (28.0 ± 3.1 wt %) and an aqueous phase (26.4 ±
4.2 wt %). An additional liquid fraction was collected in the form of an oil:water emulsion (18.3
± 8.7 wt %), that could not be separated. The solid residue left in the distillation flask corre-
sponded to around 18.8 (wt %) of the original sludge (Fig. 1). The gas and liquid products were
collected at temperatures from 40ºC up to 130 ºC, and correspond to around 81.2 wt. % of the
initial mass.
(%, d.b.)
(%, d.b.)
(%, d.b.)
(%, d.b.)
(%, d.b.) Reference
44.9 ± 1 6.4 ± 0.0 1.04 ± 0.0 1.7 ± 0.0 45,9 This wor
58.0 ± 2.4 9.8 ± 2.4 0.1 ± 0.0 2.1 ± 0.1 30.0 ± 2.6 (Al-Futaisi et al. 2007)
20.85 2.7 1.4 0.11 6.0 (Zhou et al. 2009)
64.1 10.6 - - 4.0 (Li et al. 1995)
72.72 5.20 4.05 2.07 14.96 (Qin et al. 2015)
33.16 7.18 0.45 0.68 (Xu et al. 2014)
16.0 - 0.3 2.3 13.0 (Jasmine & Mukherji 2015)
51.2 7.54 0.52 1.69 4.05 (Conesa et al. 2014)
Figure 1. Product yield (% w/w) for the distillation of the oily sludge.
The distillation yields showed a high variability due to the heterogeneous nature of this sludge
especially in what concerns the water content of the sample.
The aqueous phase which is the effluent to be treated corresponds to 37.4% wt of the original
mass thus allowing a significant reduction of the waste volume. A considerable amount of solid
components (~20% w/w) is isolated in this process reducing the soluble solids and ash content of
the liquid phases.
3.3 Product characterization
The gas phase shows high concentrations of hydrogen and some methane that together account
for 34.5% vol; the permanent gases included in this analysis correspond to around 50% vol of the
gas phase and the remaining components are probably low molecular weight hydrocarbons that
will certainly contribute to the gas heating value (Fig. 2).
Figure 2. Gas products collected during the distillation of the oily sludge.
Further analysis of hydrocarbon gases from the gas phase can eventually reinforce the conclusion
that the distillation gas products can be used as a gaseous fuel to ensure the energetic requirements
of the process. The oil phase was analyzed by GC-FID to evaluate its carbon number distribution
and the boiling point range of their components (Fig. 3).
H2 O2 N2 CH4 CO CO2
Concentration (% v/v)
Gas Aqueous
Oil Water/oil
Concentration (% wt)
Figure 3. GC-FID of the oil phase composition.
The bio-oil chromatogram was compared to the chromatogram of a hydrocarbon standard mix
from C8 to C20 to determine the carbon number distribution. The main components of the oily
phase are volatile or semi-volatile organic compounds (~ 89.8 wt%) with carbon numbers from
C5 to C20 to which correspond boiling points from 36ºC to 343ºC. The lighter components of
this phase with boiling points lower than 250ºC account for 67.2% of the oil phase.
Figure 4. Hydrocarbon distribution of the oily products collected during distillation.
The analysis of the oil phase by gas chromatography and mass spectrometry provided some struc-
tural information concerning its components and confirmed that the oily phase is rich in saturated
hydrocarbons, from C8 to C25 (Fig. 4), and also contains alkenes, cyclic hydrocarbons and aro-
matic hydrocarbons. The analysis of the organic components present in the water phase revealed
high concentrations of aromatic hydrocarbons and oxygenated compounds both aliphatic and ar-
omatic, confirming that this phase constitutes a highly contaminated effluent that must be subject
to a remediation process adequate to its hazardous nature.
The oily phase and the solid residue were also subject to elemental analysis in order to evaluate
the fractionation of the different elements between dried sludges and distillation products.
The oil product is a carbon-rich liquid with a low content of oxygen, nitrogen or sulfur that
could be reintegrated in the refinery raw materials or used as raw fuel for industrial boilers.
GC-FID profile of the oil phase composition
Figure 5. Elemental analysis of the dried sludge and the oily phase obtained by distillation.
The dried sludge and the solid product have high oxygen content indicating the presence of polar
functional groups. The solid product still represents a significant fraction from the total sludge so
its material valorization as adsorbent or raw material should be exploited (Fig. 5).
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sludge in Oman. Journal of hazardous materials, 141(3), pp.557–64.
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Jean, D.S., Lee, D.J. & Wu, J.C.S., 1999. Separation of oil from oily sludge by freezing and thawing. Water
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Concentration (%wt)
Dried Sludge
Oily Phase
Solid Product
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Oil sludges vary widely in composition. They are complex systems consisting of oily matter, water, and mineral components (sand, clay, silt, etc.). The ratio of these components varies over extremely wide limits, depending on the type of crude oil, the processing scheme, and the equipment and reagents used in the waste water treatment. In general, sludges may be defined as heavy oily residues with average contents (in wt. % ) of 10 - 56% oily matter, 30 - 85% water, and 13 - 46% particulates. In Table 1 we show the phase composition of sludges from various refineries [ 1 ]. In this group of refineries, more than 7 million tones of oil sludge has accumulated, derived in part from wastewater treatment; other sources are the recirculating water supply system, equipment maintenance operations, and tank cleaning. The problem of processing the sludge from slime pits has not yet been solved, as this type of sludge is hard to break down, and its composition and properties are constantly changing as a result of exposure to the atmosphere during storage in open slime pits. As time passes, the emulsion "ages" through the evaporation of light fractions, oxidation and tar formation in the oil, the formation of colloidally dispersed micellar conglomerates, and the entry of additional amounts of inorganic particulates into the sludge. In determining the class of hazards presented by sludge, no consideration is generally given to the many toxic and carcinogenic elements that are present in this material. Here we may mention heavy-metal ions, polycyclic aromatic hydrocarbons, sulfur compounds, light aromatic fractions, naphthenic hydrocarbons, mineral and organic salts, and surfactants. In considering the possibility of burying these wastes, the hazard class of the sludge is generally established on the basis of one integral index, the content of oily matter, without any regard for the contents of individual toxins. When oil gets into the soil as a result of sludge burial, wastewater discharge, or accidental spills, it seeps downward under the influence of gravitational forces and spreads laterally under the influence of surface and capillary forces. When the oil moves vertically along the soil profile, a chromatographic effect is created, leading to differentiation of the oil composition: in the upper humus layer, high-molecular-weight components containing large amounts of resins and asphaltenes and cyclic compounds are sorbed; the material penetrating into the lower levels consists mostly of lowmolecular-weight compounds. In comparison with the macromolecular materials, these light materials have higher water solubility and diffusivity. Light hydrocarbons are highly toxic. Microorganisms poorly assimilate them, and hence they are retained for long periods in the lower parts of the soil profile, under anaerobic conditions. The light fractions consist mostly of C 5- C,
Oily sludge obtained from a refinery in India contained 10–11% oil associated with fine particulates. Along with Fe, Ca and Mg various toxic elements were associated with the sludge solids (Pb, Mn, Cu, Zn, As, Bi, Cd, Cr, Co, Ni and V). The oil contained 41–56% asphaltenes and the maltenes comprised of 49 ± 4%, 42 ± 2% and 4 ± 2%, aliphatic, aromatic and polar fractions, respectively. Biodegradation studies with the maltene fraction of oil provided as sole substrate revealed higher degradation by various 3-5 membered reconstituted consortia compared to pure bacterial strains and up to 42 ± 8% degradation could be achieved over 30 days. In contrast, over the same period up to 71.5 ± 2% oil degradation could be achieved using dried oily sludge (15% w/v) as sole substrate. Significant biodegradation observed in the un-inoculated controls indicated the presence of indigenous microorganisms in oily sludge. However, large variability in oil degradation was observed in the un-inoculated controls. Greater biodegradation of the maltene fraction led to significant enrichment of asphaltenes in residual oil associated with the sludge.
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The pyrolysis of a sludge produced in the waste water treatment plant of an oil refinery was studied in a pilot plant reactor provided with a system for condensation of semivolatile matter. The study comprises experiments at 350, 400, 470 and 530 °C in nitrogen atmosphere. Analysis of all the products obtained (gases, liquids and chars) are presented, with a thermogravimetric study of the char produced and analysis of main components of the liquid. In the temperature range studied, the composition of the gas fraction does not appreciably vary. In the liquids, the light hidrocarbon yield increases with increasing temperature, whereas the aromatic compounds diminish. The decomposition of the solid fraction has been analysed, finding a material that reacts rapidly with oxygen regardless of the conditions it is formed.
This communication reports for the first time the feasibility of employing freeze/thaw method to separate oil from an oily sludge. The freeze/thawed sludge comprises three distinct layers, an oil layer at the top, a sediment layer at the bottom and a water layer locating in between, whose compositions were identified with the help of GC–MS. Freezing and thawing can separate over 50% of its oil content. However, ultra-fast freezing is not beneficial for oil separation.
A batch-type, controlled-air incinerator was used for the treatment of oily sludge and polyethylene (PE) plastic mixtures. The concentration and composition of 21 individual PAHs (polycyclic aromatic hydrocarbons) in the raw wastes, flue gas (gas and particle phases) and ash were determined. Stack flue-gas samples were collected by a PAH stack-sampling system. Twenty one individual PAHs were analyzed primarily by a gas chromatograph and a gas chromatography/mass spectrometer. Due to incomplete combustion, PAH content in the feeding wastes have a strong influence on PAH emission in both stack flue gas and ash residue. With the oily sludge in the feeding waste mixtures, the input mass of lower molecular weight PAHs — Nap, AcPy, Acp, Flu, PA, Ant and FL — was contributed mainly by liquid diesel, while the input mass of higher molecular weight PAHs — Pyr, CYC, CHR, BbF, BkF, BeP, BaP, PER, IND, DBA, BbC, BghiP and COR — was primarily contributed by the oily sludge. For the distribution of individual PAH mean output mass, lower molecular weight PAHs — Nap, AcPy, Acp and Flu — have > 87% of their mass discharged by the stack flue gas. However, the higher molecular weight PAHs — Ant, FL, CHR, BbF, BeP, BaP, PER, IND, DBA, BbC, BghiP and COR — have significant mass fractions (>18%) discharged by the ash residue. The total-PAH output/input mass ratios were between 0.00103 and 0.00360 and averaged 0.00203. This result indicated that the depletion of PAH mass in the combustion process was very significant. The PAH content in the fuel during the combustion process is the control factor of PAH emission. The co-combustion of oily sludge with plastic is a potential method of reducing the PAH emission and of saving the consumption of auxiliary fuel.
The pyrolysis of waste sludges was investigated using thermogravimetry/mass spectrometry (TG/MS) and a fixed-bed reactor. Two types of sludge were used, namely mixed sludge and oil sludge. In TGA/MS measurements, two degradation steps were observed. Degradation of organic structures, in sludge took place in the first step, while inorganic materials in sludge were mainly decomposed in a second step (above 500 °C). In a fixed-bed reactor, the catalytic effect of inorganic matter in addition to organic matter was monitored the quality and yield of products from pyrolysis. Pyrolysis of oil sludge produced a larger amount of oil containing more aliphatic compounds and a high calorific value. On the other hand, pyrolysis of mixed sludge gave a smaller amount of oil being rich in polar compounds. The gaseous products from pyrolysis consist of high amount of combustable gases. Landfilling was found to be the best alternative to dispose off the pyrolytic char obtained from pyrolysis.