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Simultaneous dewatering and recovering oil from highviscosity oily sludge through the combination process of demulsification, viscosity reduction and centrifugation Xiaoyuan Zheng, Zhi Ying, Jie Cui, Bo Wang, Jian Chen, and Qi Zhang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02481 • Publication Date (Web): 28 Nov 2017 Downloaded from http://pubs.acs.org on November 29, 2017
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duration of 120min to reduce the interfacial tension and thus increased the dewatering
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efficiency by 12.01%. The highest value of recovered oil quality and dewatering
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efficiency could be achieved at the demulsifier concentration of 0.3% with the
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increasing softness of asphaltenes films. As the dosage of demulsifier solution rose,
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the dewatering efficiency increased and the recovered oil quality get better. In general,
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the combination process is effective for dewatering and oil recovery from
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high-viscosity oily sludge.
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Keywords: oily sludge, oil recovery, dewatering, demulsification, viscosity reduction,
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centrifugation
35 36
1 Introduction
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With the continuing depletion of energy resources and concerns over secondary
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pollution, oil recovery from oily sludge generated in the petroleum processing
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operations including production, transportation, storage and refining has received
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more and more attentions [1,2]. In China, the production of oily sludge is estimated at
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3 million ton per annum [3]. The sludge contains high concentrations of petroleum
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hydrocarbons and various recalcitrant components [4,5]. Therefore, it has been
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recognized as a hazardous waste in many countries. Its improper disposal has serious
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threats to the surrounding environments and human health [6]. However, it also
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represents a considerable energy resource due to its relatively high concentrations of
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petroleum hydrocarbons, whose recovery could be the most desirable management
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option with the advantages of generating profits and reducing waste volume and
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pollutant concentration [2,7]. Currently, numerous treatment approaches have been
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proposed, such as thermal cracking, electrokinetic method, microbial treatment,
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chemical extraction, and mechanical centrifugation [2, 8-13]. Thermal cracking is
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widely under investigations. But precise operations of the reaction conditions are
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required to maintain safety. The process is costly and composed of complex facilities.
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Furthermore, secondary pollution may occur because of the used catalysts [14,15].
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Microbial treatment is an environmentally friendly way but gives low oil recovery
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efficiency [16]. Microwave treatment has been tried in the fields of bitumen
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extraction, heavy oil upgrading and viscosity reduction but has existed at the
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laboratory scale due to its high setup costs and uncertain potential [17]. Chemical
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extraction can recover the oil efficiently but requires a large volume of chemical
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reagents [18]. Centrifugation can provide a basic mechanical separation solution for
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both solid particles and water droplets removal from oily sludge with the advantages
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of simplicity, low industrial implementation costs, and low environmental impact [19,
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20]. Ideally, the formation of three layers can be observed after centrifugation, which
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consists of oil in the upper layer, water in the middle layer and solid residues in the
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bottom layer. The oil can be recovered from the upper layer. However, due to the
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presence of intrinsic surface-active surfactants (asphaltenes, resins, etc), the added
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surfactants and the natural solid particles (clay and waxes), oily sludge is usually a
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complex mixture of water/oil emulsion and solid particles [1,2]. Furthermore, the
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solid particles size ranges from micrometers to millimeters [21]. The size distribution
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of emulsified water lies in the ranges of a few micrometers [22]. Small particles may
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remain in the upper oil layer after centrifugation, having a negative impact on the
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recovered oil quality, while it is very challenging for the separation of the diverse
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sizes of water droplets in the concentrated and opaque emulsions. They are stabilized
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by rigid film at water/oil interface, which prevents the coalescence of water droplets.
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The fine demulsified water retained in the recovered oil may have significant
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influence on its further utilization. When investigating the solid particle behavior and
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the migration of emulsified water droplets in petroleum sludge during centrifugation,
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Huang et al. found that the sludge viscosity had a great influence on the water and
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solid particles removal, and both of them decreased with a rising viscosity [23,24].
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Therefore, prior to centrifugation, viscosity reduction and demulsification are required
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for high-quality oil recovery.
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Viscosity reduction is always found efficient by changing the transfer behavior
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of solid particles [24]. Hence, it is conducive to remove solid particles and is likely to
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improve the recovered oil quality. Viscosity reduction is achieved by preheating or
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adding solvent, which are used in the transportation of heavy crude oils [25,26]. High
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capital and operational cost limit the use of preheating [24,27]. Hasan et al. found that
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the blending of heavy crude oil with a limited amount of lighter crude oil provided
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better performance than the other alternatives, such as forming oil-aqueous solution of
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surfactant emulsion or blending oil with alcohol [28]. Different from heavy crude oils,
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the solid particles and highly stable water/oil emulsions existed in the oily sludge had
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the significant impact on its high viscosity [29]. Therefore, further investigations on
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viscosity reduction of oily sludge are required, in particular the effect of the process
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coupling with viscosity reduction on the oil recovery and recovered oil quality.
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In order to improve the recovered oil quality further, demulsification was of
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great importance for the fine emulsified water removal, which could be accomplished
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by physical, thermal, or chemical methods [30]. The chemical method played the
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most important role in breaking water/oil emulsions by modifying the interfacial
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properties and displaced the asphaltenic-stabilized film from water/oil interface.
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Amphiphilic block copolymers with hydrophilic ethylene oxide (EO) and
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hydrophobic propylene oxide (PO) blocks were commercially available and widely
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used as demulsifiers in demulsification of water in crude oil emulsions [31-33]. They
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could destabilize emulsions. The water could be separated from oil by flocculation
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and/or coalescence of the water droplets. Then larger water droplets formed, which
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were readily separated from the oil. The destabilizing mechanisms involved: (1) the
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demulsifiers exhibited greater interfacial activity in comparison with the stabilizing
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species and could thus penetrate stabilizing films at the oil-water interface; (2) the
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demulsifiers disrupted and softened the stabilizing interfacial films; and (3) the
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demulsifiers suppressed interfacial tension gradients which were responsible for
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emulsion stabilization by the Marangoni effect [34].
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Therefore, the application of EO/PO copolymers to the oily sludge coupling with
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viscosity reduction and centrifugation was proposed. The objective of this study was
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to investigate the effect of the combination process on oil recovery and water removal
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from high-viscosity oily sludge. The study could be divided into three sections. The
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first one was to examine the rheological properties of the oily sludge sample and the
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solvent selection according to the degree of viscosity reduction. The second section
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was to screen the optimum demulsifier in terms of the dewatering efficiency. The
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third section was to address the impact of the combination process on the dewatering
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efficiency and deoiling efficiency. The process conditions considered in this section
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included centrifugation speed, centrifugation duration, demulsifier concentration,
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dosage of demulsifier solution and demulsification duration. The recovered oil quality
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was determined as well. The results presented here could provide a reference for
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high-viscosity oily sludge disposal.
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2 Materials and Methods
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2.1 Materials
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Two oily sludge samples were collected from a petroleum refinery (PR) and a
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solid waste disposal company (SWDC) in Zhejiang Province. They were stored in a
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sealed glass jar at 25oC. Before use, they were homogenized by mechanical stirrer to
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keep an evenly mixing. The compositions are listed in Table 1. The water content and
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solid particle content were determined according to azeotropic distillation by
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Dean-Stark method, whereas the oil content was calculated by difference [35,36]. The
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extracted oil compositions based on the above extraction method are listed in Table 2.
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The SARA analysis was conducted following the Chinese standard method SY/T
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5119-2008. The heteroatoms content was determined using VARIO EL III elemental
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analyzer. The metal elements were analyzed using Inductively Coupled Plasma Mass
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Spectrometry (ICP-MS).
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Three demulsifiers (AR grade) named P9393, P9935 and L62 were purchased
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from Wuhan, China. The resulting molecular structures provided by their supplier are
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presented in Fig.1. They were polyether demulsifiers using amines, phenolic resins
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and alcohols as starting substances, respectively. The RSN (Relative solubility
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number) value of P9393, P9935 and L65 were 5~7, 17~20 and 16.6.
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Two solvents (AR grade) were selected for viscosity reduction experiments,
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including the naphtha named 120# solvent oil and 1-pentanol. 120# solvent oil was
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produced from petroleum distillates, which was naphtha in nature and usually used as
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solvent. Its fraction boiling was between 80oC and 120 oC. It consisted of molecules
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with 6-9 carbon atoms.
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Fig.1 Molecular structure of demulsifiers (a) P9935, (b) P9393, (c) L62
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Table 1 Composition of selected sludge samples Item
PR
SWDC
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Water/wt (%) Oil/wt (%) Solid particles/wt (%) 148
24.90 30.14 44.96
35.09 47.15 17.76
Table 2 Composition of extracted oil from the samples Items SARA analysis/wt (%) Saturates Aromatics Resins Asphaltenes Heteroatoms content/wt% O N S Heavy metal concentration/mg·kg-1 Ni Fe V
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2.2 Methods
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2.2.1 Viscosity measurements
PR
SWDC
50.00 22.88 19.60 7.52
44.85 32.31 6.69 16.16
3.34 0.72 2.86
26.70 0.29 1.82
28.17 170.48 386.90 47380.07 66.72 98.43
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The viscosity measurements were conducted by HAAKE VT550 viscometer
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using a coaxial cylinder geometry with a shear rate range of 0-600s-1. The viscometer
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had several operating test modes including a universal controlled rate (CR) mode, a
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controlled stress (CS) mode, and an oscillation (OSC) test mode. In this study, the CR
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mode was adopted, which could be controlled by the software OS 550. The
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temperature for viscosity measurement was set at 25oC.
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The solvents with mass ratio of 0%, 2.5%, 5% and 10% were added into the
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homogenized samples. They were mixed by stirring at 1000 r·min-1 for 30min until
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homogenization, then transferred to the sample cup with setting temperature until it
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was flush with the top of the rotator. Each measurement repeated three times and their 8
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average value was used. To evaluated the effect of solvent addition and its dosage on
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the extent of the viscosity reduction, the degree of viscosity reduction rate (D) was
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defined as follows,
= 164
− × 100%
Where is the reference viscosity at 300s-1 shear rate, Pa·s, and is the
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corresponding viscosity at 300s-1 shear rate, Pa·s.
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2.2.2 Oil recovery by the combination process
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In this process (shown in Fig.2), the factors including solvent type and its dosage
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for viscosity reduction, demulsifier type, demulsifier concentration, dosage of
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demulsifier solution, centrifugation speed, centrifugation duration and demulsification
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duration were investigated. The oily sludge samples were homogenized thoroughly
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before commencement of the experiments.
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For each experiment, different concentrations of demulsifier solutions were
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prepared using toluene as solvent. About 30g oily sludge sample was placed into a
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100-ml centrifuge tube, a given dosage of demulsifier solution and 120# solvent oil
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were added in the sequence as demulsifier solution, oily sludge sample and 120#
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solvent oil. Then the centrifuge tube was sealed and placed on a machinery shaker for
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shaking demulsification at 200rpm under ambient conditions. After shaking for
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certain duration, the tube transferred to a centrifuge at specified speed for setting
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duration. After centrifuging, the sample in the centrifuge tube was settled for 24h at
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room temperature. Three layers were observed in the tube including a sludge oil layer
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on the top, an aqueous layer in the middle and the sediment at the bottom. The top
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(sludge oil) layer was collected to a 100-ml beaker. The beaker was put in an oven at
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40oC, dried to remove any residual solvent, and weighed. The final product in the
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beaker after subtracting the mass of remained solids and water was considered as the
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recovered oil (mo). The middle (water) aqueous layer was removed to a 100-ml beaker
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using a pipette and then weighed (mw). The remaining semi-solid residue in the tube
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dried in the oven for 24h at 40oC to remove any residue solvent.
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The deoiling efficiency and dewatering efficiency were calculated as follows, × 100% × 100% Dewatering efficiency = Deoiling efficiency =
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Where and
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sample and was the mass of original oily sludge sample.
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The quality of the recovered oil was assessed as follows,
were the oil content and water content in the original oily sludge
!"#$%"&"' $() *+,)(-. =
/ 0 / × 100% /
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Where / , /
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recovered oil, respectively. They were determined according to azeotropic distillation
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by Dean-Stark method [29,30]. Therefore, the smaller the value was, the better the
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quality of recovered oil.
and / were the oil content, water content and solid content of
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Fig.2 Schematic view of the combination process
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3 Results and Discussion
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3.1 Rheological characteristics of oily sludge and its viscosity reduction
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Fig.3 shows the relationship between shear rate and viscosity of oily sludge
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sample PR at 20oC. The viscosity η of the sample PR decreased as the shear rate rose
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up to 300s-1. The phenomenon of shear thinning occurred. Then the sample turned to
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be Newton fluids by further increasing the shear rate. It could attribute to its high
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solid content of 44.96% (Table 1) [29]. High viscosity for the sample PR was
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observed in Fig. 3 because of the high content of heavy components including resins
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and asphaltenes in the extracted oil [37]. Their total content was higher than 20%, as
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listed in Table 2. Furthermore, according to the findings reported by Chen et al., the
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heteroatoms such as Ni, V, O and N, associated within the asphaltenes or other heavy
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components had great impacts on the high viscosity as well [38]. As listed in Table 2,
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high concentration of heteroatoms and heavy metals in the extracted oil was
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determined. 30 o
T=20 C
25
Viscosity/Pa.s
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0
100
200
300 400 -1 Shear rate/s
500
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Fig.3 Rheological curve of oily sludge sample PR
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High viscosity was detrimental to the water coalescence and solid removal on the
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oil phase separation, which caused the reduction of recovered oil quality. Therefore,
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viscosity reduction was the key process for the oil recovery from high-viscosity oily
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sludge. Fig.4 shows the effect of solvents and their dosage on the viscosity reduction.
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The index of D was introduced to evaluate the extent of viscosity reduction. When the
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addition of 1-pentanol increased from 2.5% to 10%, D rose from 63% to 85%,
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indicating that 1-pentanol had significant effect on the viscosity reduction due to the
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functional group of -OH in its molecular structure. This functional group had good
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ability to form hydrogen bond with certain functional group in the heavy components
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such as asphaltenes from oily sludge, which could break their space structures and
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thus reduce its viscosity [39]. 100 1-pentanol 120# solvent oil Viscosity reduction D (%)
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90
80
70
60
50 0.0
2.5
5.0 7.5 Dosage/mass%
10.0
12.5
Fig.4 Effect of solvents and their dosage on the viscosity reduction of sample PR
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From Fig.4, the same trend could be observed for 120# solvent oil with the
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increment of its dosage. However, under the same dosage higher than 5.0%, D for
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120# solvent oil was higher than that for 1-pentanol, showing better ability of
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viscosity reduction. Reducing the heavy oil viscosity by adding light oil such as
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natural gas condensate, naphtha, solvent oil, and so on was the common method. They
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could dilute the heavy oil by weakening the molecular interaction among the heavy
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components like asphaltenes and resins, in particular. Further, they could break the
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space structure of the whole colloidal system [39]. When the dosage of 120# solvent
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oil was 10%, D of 90% could be achieved. What’s more, the increasing dosage could
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weak the non-Newton phenomenon. The flow pattern of the sample turned to be
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Newton fluid under the dosage of 10%. Therefore, 120# solvent oil with the dosage of
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10% was adopted in the future study.
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3.2 Screening of demulsifier
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In the oily sludge, it formed highly stable yet complex water-in-oil emulsions,
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which were stabilized by surface active components (asphaltenes, resins, etc), natural
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solid particles (clay and waxes), and added chemical surfactants. Demulsification was
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necessary for water and solids removal to improve the recovered oil quality. Three
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polyether demulsifiers such as P9935, P9393 and L62 were selected. According to the
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dewatering efficiency of 23%, the demulsifier P9935 had the best performance due to
246
its molecular weight and starting substance of phenolic resins, which had higher
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structure rigidity than that of alcohols and amines. It could make the demulsifier
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molecular stretch on the water/oil interface and thus a loose interfacial film was
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formed. Thus, it resulted in stronger destructive effect. At the same time, its
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condensed ring group had more strong hydrophobicity, because its structure was
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similar to that of surface active components like asphaltenes in the oil phase. This
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condensed ring group in demulsifier P9935 had good solubility and diffusivity and
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had greater impact on the water/oil interface with better advantages on dewatering.
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Furthermore, the highest molecular weight of demulsifier P9935 could be observed in
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Fig.1. The demulsification efficiency of the demulsifiers was directly proportional to
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their molecular weights [40, 41]. The higher the molecular weight was, the better the
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demulsification performance. The dewatering efficiency of a demulsifier was
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correlated with the RSN. When dehydration tests of water-in-diluted bitumen
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emulsions using centrifugation were conducted by Xu et al., an optimum RSN value
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around 20 was observed for polyoxyalkylated DETA demulsifier [32]. The observed
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RSN value was close to the RSN value of demulsifer P9935. Therefore, the
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demulsifier P9935 was determined for the future experiments.
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Note that, the errors of dewatering efficiency in the experiments came from two
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aspects including the separated water extraction method using a pipette and the
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separated water remained at the bottom layer. Therefore, to examine the feasibility of
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the combination process, more attentions were paid on the deoiling efficiency and
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recovered oil quality.
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Dewatering efficiency/%
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24
22
20 P9935
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P9393 Demulsifier
L62
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Fig.5 Effect of demulsifier on deoiling efficiency and dewatering efficiency of sample
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SWDC (experimental condition: demulsifier concentration of 0.2%, demulsifier
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solution dosage of 5%, 120# solvent oil dosage of 5%, shaking speed of 200rpm,
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centrifugation speed of 4000rpm, centrifugation duration of 30min)
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3.3 Effect of different factors on the combined process
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In this section, oily sludge sample SWDC was used. In terms of the combination
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process, Figs. 6 and 7 show the deoiling efficiency and dewatering efficiency under
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the impacts of centrifugation speed and centrifugation duration. It could be observed
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from Fig.6 that both deoiling efficiency and dewatering efficiency improved by
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increasing the centrifugation speed. They increased from 5% and 0% at 1000rpm to
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66.36% and 27.42% at 5000rpm, respectively.
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According to Huang’s results and Stokes’ principle,
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∆) = %- =
282
Where ∆l is the separation distance, v is the sedimentation velocity of a water
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droplet with diameter D, ω is the centrifugal angular speed, R is the distance between
2 3 4546 738 9:;
-
(1)
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the sample and the center of the centrifuge rotor, µ is the viscosity of continuous oil
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phase, t is the centrifugation duration, ρ and
