J. Cent. South Univ. Technol. (2011) 18: 367-373

DOI: 10.1007/s11771-011-0705-7

Modification technology for separation of oily sludge

LI Xiao-bing(李小兵), LIU Jiong-tian(刘炯天), XIAO Yun-qi(肖云奇), XIAO Xin(肖鑫)

School of Chemical Engineering and Technology, China University of Mining and Technology,

Xuzhou 221116, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2011

Abstract: Based on the analysis of the properties of oily sludge samples, the effect of modification parameters, such as liquid to solid (L/S) ratio, agitation temperature, agitation intensity, agitation time and pH on the modification of oily sludge was investigated with the content of oil remnants in dry sludge as a reference index. Remixing experiments were carried out according to a simplex-lattice design, where Sx4056 was used as the demulsifier, petroleum sulfonate as the surfactant and sodium silicate (Na₂SiO₃) as the dispersant. The surface modification reagent formulation was optimized by a regression equation on the modified effect and based on the amounts of surface modification reagents. The results show that the content of the oil remaining in dry oily sludge is 0.28% of 10.15% oily sludge, when the reagent concentration rises to 3.5 g/L under the optimum experimental conditions.

Key words: oily sludge; surface modification; reagent; simplex-lattice design

1 Introduction

Oily sludge is a kind of mineral-oil-rich solid waste, which includes oily sands, tank bottoms and three kinds of sludge from refineries (bottom sludge of oil removed from pools, dissolved air flotation scum and excessively activated oily sludge). Oily sludge is the major source of pollution produced in the process of oilfield production and development and is a key factor in constraining the improvement of environmental quality in oilfields [1-2]. Solid particles in oily sludge are difficult to settle because of their small size, oil-rich, fully emulsified and highly viscous characteristics. One or more layers of oil-water are attached to the surface of solid particles with hydration, resulting in barriers of particle aggregates between each other [3-4]. The process of oily sludge has attracted much attention from scientists in the chemical research of the oilfield because of its complex composition and poor separation. Currently, oily sludge is commonly treated using physical, chemical and biological processes. These technologies have their own advantages and disadvantages (Table 1) [5-7].

Surface modification of oily sludge is commonly used in several conventional treatment technologies in order to destroy the stable system, change the properties of the oily sludge, such as its strong water-holding capacity, high degree of emulsion and viscosity, and determine key factors for its further treatment. A general way for oily sludge modification is to disperse the oily sludge completely by adding an appropriate amount of agents and water, followed by heating, agitation and adjusting its pH. The addition of reagents results in the modification of the hydrophilic properties on the interfaces of oil/water, oil/sludge and water/sludge [8-9]. According to the activation mechanism of reagents, the modification reagents can be classified into demulsifiers [10], surfactants [11-12] and dispersing agents [13]. Generally, a combined application of these agents may improve the effect of the modification.

LI et al [14] reported that an optimized detergent of LAS and Na₂SiO₃ with a mass ratio of 1:2 was obtained after screening and remixing. Their results indicated that the amount of residual oil was 0.8% in a sample of 21.2% oily sand when the LAS-Na₂SiO₃ composite detergent concentration was 2.8 g/L. In Ref.[15], peregal, nonylphenol ethoxylate and sodium alkyl benzene sulfonate were selected for further mixing experiments, based on qualitative comparison experiments among ten kinds of surfactants. The results showed that the efficiency of oil washing can be as high as 90.15% with a reagent concentration of 0.3 g/L. If the reagent concentration rises to 2.5 g/L, the efficiency of oil washing could be as high as 98%. LUO et al [16] used sodium dodecyl benzene sulfonate and polyoxyethylene laury lether as demulsifiers in their study in the treatment of oily soil, which had been exposed to air for a long time. Their results indicated that high oil removal efficiency could be obtained under conditions of an agitation speed of 400 r/min, an agitation time of 40 min and an agitation temperature of 70 °C. PANG et al [17] investigated the modification techniques on oily sludge from the Shengli Gudao oilfield, and it was shown that high oil removal efficiency could be obtained under conditions of a solid to liquid ratio of 1:2, an agitation temperature of 80 °C, an agitation intensity of 200-  250 r/min, an agitation time of 60 min and an alkali concentration of 0.3%. TONG et al [18] used a surface preprocessing and hot washing method to treat oily sludge. Their experiment proved that the content of residual oil in soil could be reduced to 1.2% in a sample of 25% oil-containing oily sludge under the condition of a pH value of 9, a washing temperature of 60 °C, a washing time of 20 min and a ratio of solid to liquid of 1:2.

Table 1 Comparison of oily sludge treatment processes

However, these conventional separation methods of oily sludge present certain limitations, such as poor oil removal efficiency and large dosage of reagents. In this work, the effect of parameters on the modification of oily sludge was investigated based on the analysis of their properties. As well, an optimized modification reagent was obtained after screening and remixing.

2 Experimental

2.1 Materials

Oily sludge samples used in this study were collected from the Gudao No.6 joint station in the Shengli Oilfield, China. The sample was fully mixed and sealed for using. The demulsifiers used in this experiment were Sx4056, GP-4 and NMDE, obtained from the Shengli Oilfield, China. The surfactants were sodium dodecyl benzene sulfonate, petroleum sulfonate and cetylpyridinium bromide; and the dispersants were urea, sodium silicate and calgon.

2.2 Methods

250 mL flask was filled with 30 g oily sludge samples with appropriate amounts of hot water and reagents, and the value of pH was adjusted. The samples were then agitated in a water-bath at given temperature and agitation intensity. Flotation experiments for this modified oily sludge were carried out in 1.5 L flotation cell (XFD-III) for 12 min. The residual water and sludge in the flotation cells were transferred to a 2 L beaker. A certain amount of polyacrylamide was added to the beaker. After 0.5 h standing, clear liquid at the top of the beaker was removed as far to the top water layer. The remaining sludge was filtered and dried in order to analyze the oil content by UV-spectrophotometry. The effect of modification parameters such as liquid-to-solid (L/S) ratio, agitation temperature, agitation intensity, agitation time and pH on the modification of the oily sludge was investigated. An orthogonal experiment was conducted to optimize the process conditions. Furthermore, suitable demulsifiers, surfactants and dispersants were selected. The modification reagent formulation experiments were conducted according to a simplex-lattice design.

3 Results and discussion

3.1 Properties of oily sludge samples

The oily sludge samples contained 6.61% water, 83.34% sand and 10.05% oil (mass fraction).

The distribution of the size of dry sludge particles was analyzed with a laser particle size analyzer (LS-100Q). Dry sludge particle size of 100-200 μm accounts for 39.8% (volume fraction) of particles and the size of 38.6% (volume fraction) particles is between 200 and 900 μm. About 9.52% (volume fraction) of the particles are 100 μm or smaller (Fig.1), which are difficult to modify because of their small size, high degree of emulsification and high viscosity.

Characterized by X-ray diffractometry (XRD, D/Mas-3B, Rigaku), the phases of mineral compositions of the dry sludge are shown in Fig.2.

According to Fig.2, the mineral phases present in the dry sludge are mainly quartz and feldspar, in addition to kaolinite and illite. The kaolinite is a fraction of clay minerals formed with cryptocrystalline, scattered powder and loose aggregates with high water absorbance, resulting in the formation of a stable emulsion system, which makes it difficult to modify the oily sludge.

Solutions of oil dissolved in chloroform were scanned with a UV-spectrophotometer at wavelengths ranging between 200 and 400 nm, obtained by evaporating the organic solvent from the extract of oily sludge samples. The results show two absorption peaks with wavelength varying from 210 to 230 nm and from 250 to 260 nm. The peak at 210-230 nm indicates aromatic compounds with benzene rings, while the absorption at 250-260 nm may be attributed to compounds with conjugated double bonds.

GC-MS was carried out to investigate the organic compounds in the oily sludge samples (Fig.3). The sample above the oil phase was obtained by evaporating the organic solvent from the extract of the oily sludge samples. This composition is attributed to olefin oil, consisting mainly of alkanes. Large amounts of olefin, high freezing points and little amounts of sulfur are typical characteristics of crude oil, which is difficult to treat. Furthermore, the boiling points of most types of complex compounds are higher than 105 °C, except those of cyclohexanone, 2-methyl-hexane and heptane.

3.2 Effect of parameters on modification of oily sludge

3.2.1 Effect of L/S ratio

Fig.4 shows that the amount of oil remnants in dry sludge decreases with an increase in the L/S ratio. For example, the oil remnants in dry sludge are 4.82%, 1.13% and 1.12% for L/S ratios of 4, 6 and 8, respectively. The modification efficiency decreases when the L/S ratio is too small. This occurs because the oily sludge can neither be entirely in contact with water nor realize the fluidization during preprocessing. In contrast, the consumption of water and reagents increases when the L/S ratio is too high. The amount of oil remnants in dry sludge gradually decreases when the L/S ratio is 5. This experiment shows that the L/S ratio should be maintained between 5 and 6.

Fig.1 Distribution of dry sludge particles

Fig.2 X-ray diffraction pattern of dry sludge

Fig.3 Gas chromatogram of oil phase

Fig.4 Effect of L/S ratio on modification of oily sludge

3.2.2 Effect of agitation time

According to Fig.5, the amount of oil remnants in dry sludge decreases quickly with an increase in agitation time and then starts to stabilize after a certain time (30 min). For example, the oil remnants in dry sludge are 10.05%, 1.12%, 1.00% and 1.10% for agitation times of 10, 30, 40 and 50 min, respectively. Only a small oil phase is removed from the surface of the oily sludge when the agitation time is too short. The modification efficiency is improved when the agitation time is extended, but the energy consumption also increases and an oil-in-water emulsion forms easily. These experimental results imply that the agitation time should be kept between 30 and 40 min.

3.2.3 Effect of agitation temperature

Fig.6 shows the variation in the oil remnants in dry sludge as a function of agitation temperature. For example, the oil remnants in dry sludge are 9.83%, 1.99%, 0.91% and 1.02% for agitation temperatures of 40, 60, 70 and 80 °C. The results show that the amount of oil remnants in dry sludge decreases with an increase in agitation temperature and becomes stabilized at a certain temperature (70 °C). It is mainly due to the adhesion of oily sludge, which results in oily sludge particles congealing and too close for separation when the temperature is low. As the agitation temperature increases, the input energy exceeds the adhesion energy, which causes the viscosity to decrease gradually, and the opportunity for contact between agents and oily sludge to increases. However, the energy consumption is high and the activity of reagents decreases when the agitation temperature is too high. Thus, an agitation temperature of 70 °C should be established as the most suitable temperature.

Fig.5 Effect of agitation time on modification of oily sludge

Fig.6 Effect of agitation temperature on modification of oily sludge

3.2.4 Effect of agitation intensity

Fig.7 shows that the amount of oil remnants in dry sludge decreases with an increase in agitation intensity and then starts to stabilize at the agitation intensity of   1 800 r/min. For example, the oil remnants in dry sludge are 9.68%, 1.19% and 1.12% for the agitation intensities of 1 400, 1 800 and 2 000 r/min, respectively. Oil still attaches to the surface of the oily sludge when the agitation intensity is too small, which results in poor modification efficiency. However, the consumption of energy is too high and oil-in-water emulsion also forms easily when the agitation intensity is too high. In terms of these experimental results, the agitation intensity should be kept between 1 800 and 2 000 r/min.

Fig.7 Effect of agitation intensity on modification of oily sludge

3.2.5 Effect of pH

It can be seen from Fig.8 that the modification efficiency is improved with an increase in the value of pH, but the modification efficiency is poor under acid conditions. The composition of alkaline hydrates and organic acids in oily sludge causes the reaction to form carboxylate surfactants, which is an anionic surfactant with strong elution ability. In contrast, the difficulty of circulating water treatment would increase at a high pH value. These experimental results indicate that the pH value should be adjusted between 9 and 10.

Considering the above experimental results, it can be seen that a L/S ratio of 5, an agitation temperature of 80 °C, an agitation intensity of 2 000 r/min, an agitation time of 40 min and a pH value of 10 constitute the optimal conditions.

Fig.8 Effect of pH on modification of oily sludge

3.3 Screening and remixing of oily sludge modified reagents

3.3.1 Screening of demulsifier, surfactant and dispersant

A considerable amount of hydrophilic compounds on the molecular chain in the demulsifier could be strongly adsorbed at the oil/water interface, which would replace the original emulsifier molecules. The hydrophobic organic compounds in oily sludge could be wrapped by the formation of micelles and removed from the water phase, which increases the mobility of the hydrophobic organic compounds, resulting in the demulsification efficiency of oily sludge [19-21]. The dispersant used in this experiment causes a high level of dispersion of sand particles, which prevents these particles from settlement and cohesion. The experimental conditions are L/S ratio of 5, agitation temperature of 80 °C, agitation intensity of 2 000 r/min, agitation time of 40 min and pH of 10.

1) Screening of demulsifiers

Fig.9 shows that the amount of oil remnants in dry sludge decreases with an increase in the concentration of the demulsifiers Sx4056, GP-4 and NMDE. For example, the oil remnants in dry sludge are 1.80%, 1.90% and 2.80% for the concentration of 0.5 g/L Sx4056, 2.0 g/L GP-4 and 1.0 g/L NMDE, respectively. It is found that GP-4 and NMDE prevent poor modification of oily sludge; however, Sx4056 obtains better modification efficiency.

2) Screening of surfactants

Fig.10 shows that the amount of oil remnants in dry sludge decreases with an increase in the concentration of dodecyl benzene sulfonate and petroleum sulfonate. For example, the amount of oil remnants in dry sludge is 2.21% in 2.0 g/L sodium dodecyl benzene sulfonate and 1.60% in 2.0 g/L petroleum sulfonate. However, the amount of oil remnants in dry sludge increases with an increase in the concentration of cetylpyridinium bromide, where a large amount of oil is still attached to the oily sludge surface. These experimental results suggest that the petroleum sulfonate prevents better modification efficiency.

Fig.9 Effect of demulsifier dosage on modification efficiency

Fig.10 Effect of surfactant dosage on modification efficiency

3) Screening of dispersants

Fig.11 shows that the amount of oil remnants in dry sludge decreases with an increase in the concentration of carbon amide, sodium silicate and sodium hexametapho- sphate. For example, the amounts of oil remnants in dry sludge are 4.57%, 1.15% and 3.66% when the concentrations of carbon amide, sodium silicate and sodium hexametaphosphate are 2.0, 2.0 and 2.0 g/L, respectively. The amount of oil remnants in dry sludge gradually decreases when the concentration of these dispersants is greater than 2.0 g/L. The experiment shows that sodium silicate has better modification efficiency.

3.3.2 Remixing of modification reagent

A simplex-lattice design [22-23] was used to carry out mixture experiments for the selected demulsifiers, surfactants and dispersants (Fig.12). An optimized modification reagent formula was obtained from the functional equation between modification efficiency and dosages of reagents. The concentration range of Sx4056 used was between 0.2 and 0.8 g/L, that of petroleum sulfonate was between 0.5 and 2.0 g/L and that of sodium silicate was between 1.0 and 3.0 g/L:

                              (1)

where  is the modification efficiency. x₁, x₂ and x₃ are simulated coordinates.

Fig.11 Effect of dispersant dosage on modification efficiency

Fig.12 (3,3) distribution points in simplex-lattice design experiments

The optimal value of the regression equation was obtained by programming with Lindo 9.0, given the constraints: x₁+x₂+x₃=1 and x₁≥0，x₂≥0，x₃≥0. The results show that the minimum amount of oil remnants in dry sludge is 0.274%, when x₁=0.065 7, x₂=0.231 8 and x₃= 0.703 0. The actual values of concentration corresponding to the optimal proposed component coordinates (0.041 3, 0.247 8 and 0.710 9) of the regression equation are 0.23, 0.87 and 2.42 g/L. These experimental results indicate that the optimal constituents of modification reagents is 1 (Sx4056): 4 (petroleum sulfonate): 10 (sodium silicate). The variation in the amount of oil remnants in dry sludge, given the dosage of modification reagents, is shown in Fig.13.

Fig.13 Effect of modification reagent dosage on modification efficiency

Fig.13 shows that the amount of oil remnants in dry sludge decreases with an increase in the dosage of modification reagent. For example, the amount of oil remnants in dry sludge is 0.28% of 10.15% oily sludge for a dosage of the modification reagent of 3.5 g/L. However, the ratio of residual oil to dry sludge tends to become constant in the end when the dosage of the modification reagent is greater than 3.5 g/L. Thus, the experiment shows that the optimal concentration of the modification reagent is 3.5 g/L.

4 Conclusions

1) Sx4056, petroleum sulfonate and sodium silicate were selected as demulsifier, surfactant and dispersant based on the experimental results. The optimal constituents of surface modification reagents is 1 (Sx4056): 4 (petroleum sulfonate): 10 (sodium silicate), which is obtained by using a simplex-lattice design method of analysis.

2) Under the optimum experimental conditions (L/S ratio of 5, agitation temperature of 80 °C, agitation intensity of 2 000 r/min, agitation time of 40 min and pH of 10), 0.28% of oil remnants in the dry oily sludge remains when the reagent concentration rises to 3.5 g/L.

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(Edited by YANG Bing)

Foundation item: Project(50974119) supported by the National Natural Science Foundation of China; Project(2006A019) supported by the Science and Technology Fund of China University of Mining and Technology

Received date: 2010-05-16; Accepted date: 2010-10-14

Corresponding author: LIU Jiong-tian, Professor, PhD; Tel: +86-516-83885878; E-mail: scetljt@126.com