Ps In Water.pdf

Fuel 118 (2014) 214–219

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Characterization of emulsified water in petroleum sludge Qunxing Huang ⇑, Feiyan Mao, Xu Han, Jianhua Yan, Yong Chi State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Hangzhou 310027, China

h i g h l i g h t s  DSC analysis was employed to characterize emulsified water and salt in petroleum sludge for the first time.  A new mathematic equation was proposed for obtaining the size distribution of water droplets in petroleum sludge.  Three different kinds of petroleum sludge have been studied to predict proper treatment method.

a r t i c l e

i n f o

Article history: Received 4 August 2013 Received in revised form 4 October 2013 Accepted 23 October 2013 Available online 9 November 2013 Keywords: Petroleum sludge Differential scanning calorimetry Water-in-oil emulsion Size distribution

a b s t r a c t Increasing depletion of conventional oil resources has driven interest in recovery of oil resources from petroleum sludge. To improve the quality of recovered oil, it is essential to maximize the removal of emulsified water, dissolved salts, and other impurities. Therefore, this study was conducted to characterize the emulsified water droplets and salt in petroleum sludge through differential scanning calorimetry (DSC). To accomplish this, samples of three different types were evaluated before and after centrifugation. The results indicated that with DSC water content and eutectic of salt hydrates could be identified during controlled heating from 60 °C to room temperature. Moreover, a new equation to retrieve water droplets size distribution according to the cooling phase of the DSC thermogram from 50 °C to 60 °C is proposed. Samples collected from a mixed petroleum sludge storage tank and a tank cleaning wastewater reservoir were found to contain emulsified water droplets with a broadened size distribution, and the freezing peaks shifted from 44 °C to 20 °C as the diameter of water droplets increased. Additionally, water droplets in the sample collected from the crude oil storage tank were very strongly emulsified and were relatively uniform, with a diameter of 2.7–3.3 lm. Following centrifugation, small droplets were found in the upper oily layer, suggesting that de-emulsification pre-treatment prior to water/oil/ solid phase separation is essential to obtain high-purity oil resources. This is the first time DSC is employed to characterize emulsified water and deduce water droplets size distribution in petroleum sludge. These results are essential and useful for optimization of petroleum sludge treatment. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Petroleum sludge is a major waste generated during crude oil production, transportation, storage, and refining. Petroleum sludge can generally be considered a complex emulsion of aqueous droplets dispersed in petroleum liquid that typically contains 30–80% oil, 30–50% water, and 10–20% solids by mass. Petroleum sludge is known as a potential energy material and can be used as a fuel for cement kilns, crude distiller feedstock, and coking feedstock. However, petroleum sludge contains many toxic species including benzene, toluene, ethyl-benzene, xylene, polycyclic aromatic hydrocarbons, and heavy metals. Thus, it is classified as hazardous ⇑ Corresponding author. Address: Institute for Thermal Power Engineering, Zheda Road No. 38, Zhejiang University, Hangzhou 310027, China. Tel.: +86 571 87952834; fax: +86 571 87952438. E-mail address: [email protected] (Q. Huang). 0016-2361/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2013.10.053

waste in many countries [1] and inappropriate treatment may lead to contamination of soil, groundwater, and the environment [2,3] and pose threats to the health of local residents. As the second largest consumer of crude oil in the world, more than 1,000,000 t of petroleum sludge are generated in China annually. Therefore, it is of great interest to recover oil from this waste to achieve economic and environmental benefits. In the past several years, many technologies have been developed to recover hydrocarbons from petroleum sludge before final disposal [1,4]. These technologies are based on individual or combinations of the following procedures: (1) mechanical methods (e.g., centrifugation) [1,5]; (2) chemical methods (e.g., solvent extraction) [6,7]; (3) biological methods (e.g., microbial remediation) [8,9]. For petroleum sludge, water and salt are the most unwanted components when the recovered hydrocarbons are used as chemical materials or fuel. High water content will cause explosions when the sludge is subjected to a burner or pressurized chemical

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reactor. Additionally, salt components, especially chloride, will cause serious corrosion of industrial facilities and may deactivate oil refinery catalysts. Therefore, the properties of water droplets and salt are extremely important to the choice of sludge treatment solution and improvement of the quality of recovered hydrocarbon resources. Currently, methods of analyzing water content in petroleum sludge include proximate analysis [10,11], the Karl Fischer reagent method [12], and the chemical distillation [7]. Salt content can be measured through the ASTM method D3230 [7]. However, the understanding of water droplets and salt components contained in this complex multiphase waste is very limited. Most previous studies concerned with de-emulsification and desalting of petroleum sludge have lacked effective characterization methods because of its high viscosity and opacity and the coexistence of fine solid particles with water droplets. Thermal techniques are widely used as reliable and powerful analytical tools for characterization of degradation, evaporation, oxidation, and combustion of petroleum products and their derivatives [13–16]. Differential scanning calorimetry (DSC) is a very suitable technique for investigating the behavior of emulsions submitted to environments with changing temperatures. Clausse et al. [17–20] and Avendano-Gomez et al. [21] have deduced the quantitative properties of water-in-oil (W/O) emulsions, including water droplets size distribution and heat flux during freezing and melting, and their research has indicated that DSC is a promising tool for characterization of emulsions. This study was conducted to investigate the characteristics of W/O emulsions in petroleum sludge and to estimate the water droplets size distribution after centrifugation treatment. To accomplish this, three different types of petroleum sludge were evaluated before and after centrifugation through DSC and the results were compared with those of chemically extracted samples. This is the first time DSC is employed to characterize solidification and melting features of emulsified water droplets and eutectic of salt water in petroleum sludge. A fitting equation was also proposed to deduce the water droplets size distribution from the heat release profile. The results of this study will be useful for determining proper petroleum sludge treatment methods to recovery energy resources while alleviating environmental burdens.

2. Materials and methods 2.1. Materials Three different petroleum sludge samples were evaluated. ZS (Zhoushan Sludge) was collected directly from the bottom of a crude oil storage tank in the port of Zhoushan. NS (Nahai Sludge) was obtained from the bottom of tanks containing used oil tank cleaning water at Nahai Solid Waste Central Disposal Co., Ltd. During collection of the sludge, oily water in the upper layer of the same reservoir was also sampled. YS (Yimin Sludge) was collected from a sludge tank storing mixed oily sludge with a complex composition in the Zhoushan Yimin Waste Recovery Plant. All samples were subjected to a series of analyses to determine their primary components. Water content in petroleum sludge was determined by the ASTM-D95-05 procedure and total hydrocarbon content was evaluated by soxhlet extraction using toluene as solvent. SARA (saturates, aromatics, resins, asphaltenes) were first separated by precipitation of asphaltene with n-hexane and maltene, followed by further separation through a chromatographic column according to ASTM-D2007-02. Analytical grade chemical reagents were purchased from Sinopharm Chemical Reagent Shanghai Co., Ltd.

2.2. Sample preparation prior to DSC analysis For comparison, the samples of oil recovered from sludge through Soxhlet extraction and centrifugation were also analyzed. During Soxhlet extraction, 5 g of each sludge sample were washed with 200 mL of toluene for 6 h, after which the weight of extracted oil was measured gravimetrically following removal of the extracting solvent through evaporation at 65 °C. For centrifuged sludge, 10 g of each sample were placed in a 120-mL centrifugal tube and centrifuged at 2500 rpm/min, using a Thermo Scientific ST40 centrifuge for 20 min, after which the upper oil layer was collected. For analysis, a 1.0-mL syringe was used to collect samples of the petroleum sludge and recovered oil after mixing well, and one droplet of the sample was gently deposited on a clean aluminum crucible. 2.3. DSC analysis The original petroleum sludge and the oil phase recovered from Soxhlet extraction and centrifugation were submitted to a regular heating and cooling cycle within a temperature range of 50 °C to 60 °C. A Netzsch DSC 200F3 thermo-analyzer with nitrogen carrier gas at a flow rate of 60 mL/min was used to detect the absorbed or released heat flux during the phase change of W/O emulsions or salts contained in the original petroleum sludge and recovered oil samples. A hermetically sealed aluminum crucible containing 9 mg of the sample and a sealed empty crucible as the reference cell were subjected to analysis. All heating and cooling rates were set to 5 °C per min according to the recommendations of Clausse et al. [20]. Samples were heated to 50 °C, where they were held for 5 min to enable better contact with the crucible and ensure a uniform temperature distribution within the sample. The sample was then cooled down with liquid nitrogen coolant to 60 °C and reheated to 20 °C. 3. Results and discussion 3.1. Primary properties of the sludge samples The primary compositions of three different petroleum sludge samples are shown in Table 1. The compositions differed according to the production source and storage conditions. All samples contained water/oil/solid phases with different ratios. Petroleum sludge ZS was a typical water-in-oil emulsion containing very small portions of solid particles. Sample NS contained 20.7% solids as a result of mixing with mechanical impurities (such as sand, scrap iron, and soil particles) during cleaning of the oil tank. Sample YS contained the highest content of solids (52.4%), which was mainly attributed to the complicated composition of the stored sludge. Additionally, the heating value was found to be proportional to the oil content. Finally, SARA analysis indicated that ZS had the highest concentration of resins and asphaltenes.

Table 1 Main properties of the studied sludge samples.

Heating value (kJ/kg) Water by distillation/wt (%) Oil by solvent extraction/wt (%) Solid residues/wt (%) SARA analysis/wt (%) Saturates Aromatics Resins Asphaltenes

ZS

NS

YS

28194.6 33.5 65.0 1.5

24360.0 20.2 59.1 20.7

16186.8 16.3 31.3 52.4

23.8 25.0 32.6 17.8

42.1 30.9 12.7 14.3

37.2 35.7 19.0 8.1

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3.2. Freezing behavior of emulsified water in petroleum sludge

Fig. 1. DSC thermogram of original ZS, oil extracted from ZS, and centrifugationtreated ZS with a cooling/heating rate of 5 °C/min.

Fig. 2. DSC thermogram of original NS, oil extracted from NS, and centrifugationtreated NS with a cooling/heating rate of 5 °C/min.

DSC thermograms of ZS, NS, and YS petroleum sludge samples are shown in Figs. 1–3, respectively. The main melting and solidification peak temperatures for each sample are also indicated in the corresponding figures. The DSC curve of the original ZS sample overlaps well with that of the centrifuged sample. As shown in Fig. 1, there was only one exothermic signal throughout the entire cooling period, and the sole Gaussian shaped peak located at 43.5 °C accounts for solidification of the emulsified water droplets in ZS. The nucleation temperature was much lower than the ice-water equilibrium temperature because overcooling is required for emulsified water droplets to form ice particles [18]. Because the profile of this peak is very similar to that of other reported W/O emulsions with uniform water droplets, it is likely that the size distribution of the water droplets in the ZS sample are relatively narrow [19,20,22]. The cooling part of the NS DSC curve was completely different from that of the ZS sample. As shown in Fig. 2, crystallization of the NS sample starts at around 13 °C and is followed by several small peaks and one sharp large peak at 23.6 °C. When the sample was decreased to 29.3 °C, another peak appeared. Previous investigations of W/O emulsions have revealed that smaller water droplets are associated with lower solidification temperatures [18]. Accordingly, these asymmetrical exothermic peaks can be considered the product of several individual Gaussian-shaped peaks that each reflects water droplets with a similar size. The peak at around 44.0 °C is consistent with the same peak in Fig. 1, accounting for the crystallization of fine water droplets. After centrifugation, the overall specific heat-release rate near 13 °C increased significantly. These findings indicated that centrifugation caused some small droplets to merge together to form large droplets. The heat-releasing nature of the YS sludge sample when it was cooled to 60 °C was similar to that of NS, except that only two smooth peaks appeared and the shape of these peaks were broader than those of NS owing to crystallization of the different- sized droplets. It should also be noted that, after centrifugation, the heat flux during the phase change became much stronger, especially for the YS sample. This occurred because removal of most solid particles to the bottom layer and increased water content in the upper layer will increase the specific heat-releasing rate. This phenomenon explains the almost overlapped DSC curves for the ZS sample before and after centrifugation treatment due to the relatively low solid particle concentration. Overall, these findings indicate that, for petroleum sludge YS, mechanical centrifugation is essential to remove solid particles before further treatment because of the high concentration of solid particles. Moreover, the DSC curves for chemically extracted oil samples shown in Figs. 1–3 demonstrate that, after Soxhlet extraction, water droplets were removed completely, making the recovered hydrocarbons very suitable for chemical use. 3.3. Water content deduced from DSC analysis Rapid and accurate measurement of the water content and its properties is of great importance to determining the optimal petroleum sludge treatment process. According to the DSC principle, the total heat absorbed by frozen water during melting within emul (%): [23] sion can be used to deduce the total water content w

 ¼ w Fig. 3. DSC thermogram of original YS, oil extracted from YS, and centrifugationtreated YS with a cooling/heating rate of 5 °C/min.

mw ; ms

ð1Þ

where mw (g) is the weight of the water in the sludge sample, ms (g) is the weight of the sludge sample, and mw is obtained from:

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mw ¼

Qw ; Hf

ð2Þ

where Qw (J) is the total heat absorbed by the sample during ice melting and Hf(J/g) is the specific melting enthalpy of frozen water. Hf is considered to be unchanged with temperature; therefore, Qw and Hf can be obtained from the results of DSC by numerical integration as follows:

Q w ¼ ms  qw ¼ ms 

Hf ¼

Z

tb

Z

tb

q_  dt ¼ ms 

ta

ð3Þ

i¼1

ð4Þ

i¼1

mw qw ¼ ms Hf

ð5Þ

Based on experimental calculation, the fusion enthalpy Hf was determined and set as 233 J/g. The water contents deduced from Eq. (5) for the three different samples are given in Table 2. The water content values in Table 2 agree well with the data in Table 1, which were determined by the standard method. The relative deviation between both methods was 1.94% for NS, 5.07% for YS, and 8.89% for ZS. Based on these findings, when compared with distillation, which is time consuming and involves toxic aromatic solvents, DSC analysis provides a rapid, clean, and efficient method to determine the water content from petroleum sludge directly. Moreover, distillation and chemical methods are incapable of retrieving other information that can be determined by DSC, such as the size distribution of the emulsified water droplets. 3.4. Determination of water droplets size distribution from DSC analysis According to previous studies, the water droplets in W/O emulsions usually have different sizes. Based on the thermal equilibrium principle, Clausse et al. [19] suggested an exponential equation to describe the relationship between droplet radiuses based on crystallization temperature. Their fitting method was employed in present study as shown in Eq. (6):

ð8Þ

ð9Þ

n(Di) can be obtained from

6  DT i  qi  ms

q  Hf  k  p  ½exp35=ðT i þ10Þ 3

;

ð10Þ

where, k is the cooling rate. Figs. 4–6 illustrate the water size distribution of water droplets in three sludge samples determined according to Eqs. (8) and (10). Petroleum sludge ZS was found to have relatively uniform and small water droplets with a diameter of 2.7–3.3 lm. Sample NS showed bimodal distribution with two main groups centered at 3 lm and 6 lm, respectively, according to their number density. The size distribution profile for the droplets in sample YS was similar to that of NS, except for the larger density of small droplets near 3 lm. The water content in recovered hydrocarbon resources is very important to their reuse. For W/O emulsions, smaller water droplets result in a more difficult dewatering process. To assess the effects of centrifugation on water droplets, the water droplet size before and after centrifugation treatment with respect to the weight fraction was evaluated (Table 3). After centrifugation, the mean size of the water droplets in NS and YS increased. This occurred because large water droplets will be dragged into the sludge, and small droplets in the path of the larger droplets will be merged during centrifugation. However, the water droplets in the original ZS samples were so small that the centrifugal force was not sufficient to overcome the oil–water interfacial tension; therefore, the particles size distribution of ZS was almost completely unchanged after centrifugation. Petroleum sludge ZS is a typical tank bottom sludge with a high ratio of asphaltenes, which are considered to be natural emulsifiers. The emulsification effect makes water droplets in the sludge very stable; accordingly, de-emulsification pre-treatment by methods such as ultrasonic irradiation [12,24], freeze/thawing [25], or electrical treatment [26] is required to decrease the water content in hydrocarbon resources recovered from ZS.

 Di ¼ exp

 35 ; T i þ 10

4pðDi =2Þ3 ms  Dt i  qi ¼ : 3 Hf

Additionally, with

nðDi Þ ¼

where qw (J/g) listed in Table 2 is the specific heat absorbed during melting, ta and tb are the beginning and ending time of melting during DSC analysis with a known heating rate, Dt i is the numerical integration interval, i.e., i = 1 ti = ta and i = N t = tb, qi (W/g) is the  of the petroleum heat flux at time ti. Thus, the water content w sludge is described as:

 ¼ w

mðDi Þ ¼ nðDi Þ  q 

DT i ¼ k  Dt i ;

i¼N X Dt i  q_ i

i¼N X q_  dt ¼ Dti  q_ i

ta

where n(Di) is the number of water droplets with diameter Di and q is the density of water. According to Eqs. (3), (5), and (7), m(Di), the weight of the water droplet with diameter Di, can be formulated as:

ð6Þ

where the droplet diameter D was determined from the droplet freezing temperature T, which can be obtained by DSC analysis at a known cooling rate. When the water droplet size is known, the weight of a spherical water drop can be determined as follows:

" # i¼N X 4pðDi =2Þ3 mw ¼ nðDi Þ  q  ; 3 i¼1

ð7Þ

Table 2 Water content deduced from DSC. Melting peak

ZS-‘b’

NS-‘e’

YS-‘d’

Peak area qw (J/g) Water content (wt%)

85 36.48

48 20.60

40 17.17

Fig. 4. Weight and number percentage of water content in ZS with respect to droplet size.

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Fig. 5. Weight and number percentage of water content in NS with respect to droplet size.

Fig. 6. Weight and number percentage of water content in YS with respect to droplet size.

Table 3 Size distribution of water droplets before and after centrifugation.

<3.5 (lm/wt%) 3.5–10.0 (lm/wt%) >10.0 (lm/wt%) a b

ZSa

ZSb

NSa

NSb

YSa

YSb

100.0 0 0

100.0 0 0

20.0 72.1 7.9

14.9 75.1 10.0

42.5 50.0 7.5

34.6 54.7 10.7

Original sample. Centrifugation-treated sample.

Fig. 7. DSC thermogram of deionized water and wastewater from the same reservoir of NS with a cooling/heating rate of 5 °C/min.

and 4.6 °C, respectively, and no other heat flux peaks were observed. However, the presence of dissolved salt in the wastewater sample inhibited the crystallization of water, which caused the heat flux-releasing peaks for freezing and melting to shift to 21.1 °C and 1.7 °C, respectively. Clausse et al. [17,18] reported that melting temperature can be used as a qualitative estimator of the concentration of dissolved salt in W/O emulsions, with higher salt ratios being associated with lower melting points. Based on his argument and considering the heat absorption flux peaks shown in Figs. 1(b), 2(e), and 3(d), the salt content of sample NS is larger than that for YS, while ZS has the lowest salt content. Similar to the heat absorbing peaks observed for NS and YS, a small heat absorbing peak was observed at around 23 °C for the wastewater sample. These peaks are believed to be associated with the melting process of eutectic salt-water mixtures, namely H2O– NaCl and H2O–NaCl–KCl. These peaks provide further evidence of the existence of salt in petroleum sludge. Moreover, the value of the specific heat released during eutectic melting is proportional to shifts in temperature, with higher heat peaks being associated with larger temperature shifts or higher salt content. The hydrocarbon resources recovered from petroleum sludge are expected to be used as chemical material or combustion fuel. To eliminate the unfavorable effects caused by salt species, desalting treatment is required. DSC analysis provides a simple method to obtain salt information that can be used to guide the desalination process. For example, sludge NS contains a large aqueous phase, yet relatively low levels of emulsified water droplets; therefore, desalting and dewatering of this sludge can be achieved at the same time through ultrasonic irradiation [20,24]. 4. Conclusion

3.5. Identification of the salt content As discussed above, salt present in recovered oil may decrease the efficiency of catalysts and cause facility corrosion and secondary pollution. However, there is currently no rapid or efficient method to identify the presence of salt in petroleum sludge. As shown in Figs. 1–3, when the samples were reheated to room temperature, their melting temperatures were found to vary slightly. Therefore, two other samples, one of deionized bulk water and another of NS petroleum tank wastewater (which contained dissolved alkali salts), were subjected to DSC analysis under the same conditions for comparison. As shown in Fig. 7, the deionized water showed heat flux peaks for freezing and melting at 18.8 °C

Characterization of emulsified water and identification of salt in petroleum sludge is very important and useful for guiding and optimizing treatment processes. In this study, three different types of petroleum sludge before and after centrifugation were evaluated by DSC, and a new mathematic equation was proposed to relate water droplet size to freezing temperature. The results indicate that different water droplets contained in the sludge have different freezing temperatures, with smaller sizes being associated with lower freezing temperatures. For petroleum sludge collected directly from a crude oil tank, the natural emulsification effect of asphaltenes causes the size of the water droplets in the sludge to be uniform and small. The size distributions of the water droplets in sludge samples collected from mixtures of petroleum wastewa-

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ter and sludge were much broader, and the droplets merged to form larger ones during centrifugation. The small heat flux absorbing peaks near 23 °C generated during the heating phase along with the decreased melting temperature of frozen water droplets could be used to identify the existence of salt in the sludge. DSC enabled qualitative and quantitative analysis of the emulsified water in petroleum sludge. This technique has the advantage of providing important information within a single measurement, including the emulsion droplets size distribution, stability of W/O emulsion in oily sludge, total amount of water, presence of salt, and the removal rate of solids after treatment. Acknowledgments We are grateful to the National Basic Research Program of China (973 Program) (Grant No. 2011CB201500), National Science & Technology Pillar Program (No. 2012BAB09B03), National High Technology Research and Development Program (No. 2012AA063505), and the Creative Team Project of Solid Waste Treatment of Zhejiang Province (A2009R50049) for their financial support. References [1] da Silva LJ, Alves FC, de Franca FP. A review of the technological solutions for the treatment of oily sludges from petroleum refineries. Waste Manag Res 2012;30:1016–30. [2] Wang SJ, Yan ZG, Guo GL, Lu G, Wang QH, Li FS. Ecotoxicity assessment of aged petroleum sludge using a suite of effects-based end points in earthworm Eisenia fetida. Environ Monit Assess 2010;169:417–28. [3] Salanitro JP, Dorn PB, Huesemann MH, Moore KO, Rhodes IA, Jackson LMR, et al. Crude oil hydrocarbon bioremediation and soil ecotoxicity assessment. Environ Sci Technol 1997;31:1769–76. [4] Diya’uddeen BH, Daud WMAW, Abdul Aziz AR. Treatment technologies for petroleum refinery effluents: a review. Process Saf Environ Prot 2011;89:95–105. [5] Elasheva OM, Lubsandorzhieva LK, Smirnov IN, Fedorova EV. Entrainment of drainage emulsions and petroleum sludges in stock crude oil. Chem Technol Fuels Oils 2003;39:151–4. [6] Ávila-Chávez MA, Eustaquio-Rincón R, Reza J, Trejo A. Extraction of hydrocarbons from crude oil tank bottom sludges using supercritical ethane. Sep Sci Technol 2007;42:2327–45. [7] Zubaidy EAH, Abouelnasr DM. Fuel recovery from waste oily sludge using solvent extraction. Process Saf Environ Prot 2010;88:318–26.

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