Rapid, Accurate Measurement of the Oil and Water Contents of Oil

Article pubs.acs.org/IECR

Rapid, Accurate Measurement of the Oil and Water Contents of Oil Sludge Using Low-Field NMR Yuqi Jin, Xiaoyuan Zheng, Yong Chi,* and MingJiang Ni State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Hangzhou 310027, China ABSTRACT: Rapid, accurate measurement of the oil and water contents of oil sludge is vital to determine technological solutions for the treatment of that oil sludge. Low-field 1H NMR as a rapid noninvasive method was used in this study. Carr− Purcell−Meiboom−Gill (CPMG) experiments were conducted to construct the transverse relaxation time (T2) distribution curves. The instrument’s ability to quantify oil sludge’s water and oil contents was verified. MnCl2·4H2O was added to the oil sludge to separate the oil and water signals. For calibration curve construction, excellent results were achieved, with correlation coefficients of 0.9996 and 0.987 for regressions between the mass and the relative peak area in the T2 distribution curve for oil and water separately. Good correlation of R2 = 0.998 was achieved between low-field NMR and azeotropic distillation for both water and oil along with standard deviations of 2.67% and 2.61% for the calibration curve method, or standard deviations of 2.81% and 1.88% for water and oil, with more or less similar correlation coefficients, after correction for the amplitude indices of both the water and the oil. good 1H sensitivity, nondestructiveness, and ease of measurement.13 Low-field 1H NMR measures the hydrogen proton responses in magnetic fields. Hydrogen protons possess a property known as spin, which aligns either parallel or antiparallel to the field lines in an external magnetic field. Protons are measured in low-field 1H NMR because they are present in oil sludge (oil and water) and give off a strong signal.14 As such, the signal comes only from the oil and the water present in oil sludge. In recent years, low-field 1H NMR has been widely used, including applications to crude oil viscosity prediction,15,16 study on water in oil emulsions,13,17−20 characterization of oil sands, ores, froths, and fine tailings,21−25 food processing and analysis,26−28 analysis of biowaste feedstocks,29 the pharmaceutical industry,30 and so on. The oil sands are unconsolidated sand deposits which consist of sand, clays, water, and bitumen, a highly viscous oil.31,32 And the water-in-oil emulsions are biphasic mixtures. These are different from oil sludge, which is a multiphase system due to the adsorption of asphaltenes to hydrophilic solid particles at the oil−water interface and also to the presence of polar macromolecules, such as asphaltenes.33,34 Therefore, it is a new objective for the use of low-field 1H NMR in an oil sludge. In addition, the deconvolution algorithm is used to separate the signals from the water and oil in the oil sands spectra.21−24 However, it is a failure for 25% of the oil sand samples with low bitumen content and large clay-boundwater fractions to use this algorithm in the relative spectra.22 Therefore, a new method is needed. In this study, considering that both oil and water are rich in hydrogen and can be distinguished by the different relaxation

1. INTRODUCTION In the process of crude oil production, transportation, storage, and refining, oil sludge is one of the main solid wastes produced: it mainly comprises petroleum, water, and solids in differing proportions according to its origins. Owing to its high concentration of petroleum hydrocarbons, it is considered to be a hazardous waste and thus detrimental to the environment and to human health if improperly discarded.1 At the same time, oil sludge with a high concentration of oil can be regarded as an energy resource provided that the oil is economically viable to extract and thus supply as a fuel. At present, effective oil recovery technologies are becoming the focus of increasing attention worldwide. A number of approaches have been suggested in the literature and in practice, such as pyrolysis,2 thermochemistry,3 solvent extraction,4 ultrasound,5 freeze/thaw cycling,6 and so on. However, in the process of recovering oil, it is vital to simultaneously determine the oil and water content of the oil sludge accurately and rapidly. In both the literature and in practice, conventional methods such as Soxhlet method,1,7−9 proximate analysis,10,11 and distillation based on the ASTM method12 are reported. The Soxhlet method is, in essence, an extraction procedure, whereby solvent is used to separate the oil from the oil sludge. Oil and solids contents are both measured. The deficiency with this method is that it requires large amounts of solvents and is time-consuming (taking over 6 h). The proximate analysis originally used in coal analysis can be inaccurate in this context because of the presence of emulsions and volatilization of smaller chain length hydrocarbons at lower boiling points. The distillation method also has the determination range limitation along with the same problems as the Soxhlet method. New methods for accurate and rapid measurements of oil and water content are needed. Low-field 1H NMR (LF-NMR) is proposed as a method for making benchtop measurements of the oil and water contents in an oil sludge because of its intrinsic characteristics such as © 2013 American Chemical Society

Received: Revised: Accepted: Published: 2228

November 15, 2012 January 17, 2013 January 20, 2013 January 21, 2013 dx.doi.org/10.1021/ie303143g | Ind. Eng. Chem. Res. 2013, 52, 2228−2233

Industrial & Engineering Chemistry Research

Article

behavior of 1H nuclei, the objective of this study is concerned with the quantification of the water and oil contents manifest in an oil sludge by low-field 1H NMR. MnCl2·4H2O is used to separate signals from the water and oil in an oil sludge. Sample behaviors and analytical procedures are investigated to increase the knowledge and potential industrial application for this rapid, noninvasive technique.

2. MATERIALS AND METHODS 2.1. Oil Sludge Sample. The oil sludge used in these experiments was sampled from Hangzhou Petroleum Refinery Plant and Zhoushan Nahai Solid Waste Central Disposal Co., Ltd., China. According to the major sources of oil sludge, the samples can be divided into four types with different oil and water contents including the crude oil storage tank sludge from Zhoushan, the dissolved air flotation (DAF) scum, the biological sludge, and the oil storage tank sludge (HZ) from Hangzhou. Before NMR measurement, each sample was homogenized thoroughly and no water layer could be observed with the naked eye. Each sample was then subdivided into two subsamples. A certain amount of 2.8% MnCl2·4H2O aqueous solution, prepared using MnCl2·4H2O and deionized water, was thoroughly mixed with one subsample. Both subsamples were then placed into 2.0 mL glass vials. 2.2. Experimental Design. 2.2.1. Azeotropic Distillation by the Dean−Stark Method. Azeotropic distillation, which was regarded as the standard reference method, was conducted on the oil sludge (±8 g thereof) in a 500 mL flask using 200 mL of toluene (AR grade, Sinopharm Chemical Reagent Co. Ltd., Beijing, China) as a solvent. The water−toluene mixture was boiled-over azeotropically into a graduated Dean−Stark reservoir and separated spontaneously after water cooling. The amount of water was recorded, and the water content of the oil sludge was thus calculated. The solids remaining in the filter cartridge were then oven-dried at 105 °C until the toluene was removed. The filter cartridge and solids were weighed, and the solid content of the oil sludge was calculated. The oil content was obtained by the difference between the previous results. 2.2.2. LF-NMR Analysis. All NMR measurements were performed on an NMI 20 NMR spectrometer (Niumag Corporation Ltd., Shanghai, China), equipped with a permanent magnet and a 15 mm diameter probe, operating at 21.960 MHz. The Carr−Purcell−Meiboom−Gill (CPMG) experiments were then conducted, with 5500 echoes recorded for each transient, an inter-echo interval of 200 μs, a recycle time of 1500 ms, and a total of 16 added transients. The pulse durations used were 13.5 and 27.00 μs for the 90° and 180° pulses, respectively. The T2 distribution curves were computed using the iterating optimal method with the echo decay data, using Win-MRIXP software (Version 1.0, Niumag Corporation Ltd., Shanghai, China). The T2 distribution curves were constructed following a logarithmic selection of the points in the CPMG data.

Figure 1. Representative spectra of bulk oil and water.

However, the spectra from the actual oil sludge, denoted by the black blocks in Figure 2, were different from those in Figure

Figure 2. T2 distribution curves: oil sludge sample with, and without, paramagnetic ions.

1. It can be seen that the T2 distribution curve with black blocks was composed of three continuous peaks, but the contributions from oil and water were superimposed. In such a case, it was impossible to distinguish the oil and water’s contributions. Now 2.8% MnCl2·4H2O aqueous solution was added to the oil sludge and mixed homogenously. Previous work shows that this concentration of paramagnetic ions in solution can accelerate the relaxation of the 1H nuclei in the water with no effect on the 1H relaxation in the oil, greatly lowering the corresponding T2 values to within a certain range, which causes a separated peak to appear in the T2 distribution curve denoted by the red diamonds in Figure 2. The curve had three separated peaks. The first peak was attributed to the relaxation of the 1H nuclei in the water, and the 1H nuclei in the oil accounted for the presence of the other two peaks. To determine the oil and water content in oil sludge, it is important to relate these quantities to the NMR data. The approach to the analysis of the data from Figure 2 was to quantify the oil and water contents by using the calibration curves.

3. RESULTS AND DISCUSSION Figure 1 shows a typical spectrum of the bulk oil extracted from the oil sludge and water. Because of the different viscosities of oil and water, the corresponding NMR spectra were distinct, and the individual oil and water amplitudes could be readily separated. 2229

dx.doi.org/10.1021/ie303143g | Ind. Eng. Chem. Res. 2013, 52, 2228−2233


Article

Figures 3 and 4 present the results of CPMG experiments on deionized water and oil extracted from the oil sludge. A narrow

Figure 5. Relationship between the water peak area in the T2 distribution curve and the actual water mass. Figure 3. T2 distribution curves for water.

Figure 6. Relationship between the oil peak area in the T2 distribution curve and the actual oil mass. Figure 4. T2 distribution curves for one oil sample extracted from oil sludge.

According to the calibration curves presented in Figures 5 and 6, both the oil and the water masses can be read out with known peak area, and thus the water and oil contents in the oil sludge are obtained. A comparison of oil and water contents determined by azeotropic distillation (AD) with those obtained by this method is illustrated in Figures 7 and 8. Again, good linear correlation was found, with adjusted correlation coefficients of 0.998 for both water and oil. The high correlation coefficients obtained suggest that LF-NMR was as successful for oil and water content determination in oil sludge as the conventional AD method, commonly used to analyze oil sands’ ore components.21,22,24 The main disadvantage of this method is, of course, the need for a calibration curve. Another approach to the analysis of these data is to directly quantify the water and oil contents by associating their masses with the relative area which is integrated by the amplitude, thus dispensing with the need for a calibration curve. A correction factor, the amplitude index (AI), was used in this case. Single pulse experiments were performed to obtain the AI for a known mass of oil and deionized water. Considering that the total mass of the oil sludge is measured, the water and oil content can be determined by eq 2:

peak associated with water appeared after 1000 ms while the oil’s contribution appeared before 100 ms in broad-band form. In both Figures 3 and 4, the same phenomenon can be seen whereby both of the peaks attributed to the water and oil increased at the same rate as the relative mass. The linear relationship between the peak area and the mass associated with oil and water is shown in Figures 5 and 6 with the linear fit serving as a calibration plot for each of the water and oil contents. High adjusted correlation coefficients of 0.9996 and 0.998 for water and oil, respectively, were obtained by this procedure; these are also indicated in Figures 5 and 6. The oil peak area (Ao) is available from the T2 distribution curve (the red diamonds in Figure 2); the water peak area is then obtained from eq 1: m A w = A t − Ao o mp (1) Where Aw is the water peak area, At is the total peak area achieved from the T2 distribution curve (see plot with black blocks in Figure 2), mo is the mass of original oil sludge, and mp is the mass of oil sludge with paramagnetic ions. 2230



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Figure 7. Oil content calculated with a calibration curve compared to that by AD.

Figure 9. Oil content estimated by AI compared with that by AD.

Figure 10. Water content estimated by AI compared with that by AD. Figure 8. Water content calculated with a calibration curve compared to that by AD.

Sx =

A 1 × x × 100 ms AIx

Table 1. Oil Sludge Water and Oil Prediction Errors water 1. Method with Calibration Curve mean error/ % 2.15 standard deviation error/ % 2.67 average relative deviation/% 5.62 maximum error/ % 4.14 2. Method with AI mean error/ % 2.35 standard deviation error/ % 2.81 average relative deviation/% 5.46 maximum error/ % 3.77

(2)

where Sx is the water or oil content, ms is the total mass of the oil sludge and AIx is the AI of oil or water, and Ax is the peak area of oil and water. With eq 2, the need for the previous calibration curve can be avoided and the relative area of each contribution to the T2 distribution curves can be directly converted into the respective water and oil contents. Comparison of the oil and water contents obtained by this method to those obtained by AD is shown in Figures 9 and 10. Good correlations were observed again (R2 = 0.999 and R2 = 0.997 for the oil and water contents respectively): errors between this method and AD for water and oil content determination are also shown in Table 1. The standard deviations of 2.81% and 1.88% for water and oil with this method were comparable with the standard deviations of 2.67% and 2.61% achieved for the previous one. The errors in water content determination were larger because they inherit the uncertainties of the oil content determination.

oil 1.97 2.61 3.13 6.25 1.30 1.88 2.39 3.84

4. QUALITY ASSESSMENT OF RESULTS 4.1. Reliability of NMR Measurements. To test the NMR measurements’ stability, two samples have been chosen to run in triplicate using the same parameters. Figure 11 presents results that show the ratios of the average amplitude magnitude of the three values to every other test’s amplitude. The values were steady at around 1.0, which confirmed the repeatability of the NMR measurements and the results’ reliability. 4.2. Effectiveness of Azeotropic Distillation. A synthetic sample, made of known amounts of quartz, crude oil, and water, 2231



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oil sludge could be realized by low-field NMR techniques. Thus, the low-field NMR is offered as an especially promising application in oil sludge analysis.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-571-87952687. Fax: +86-571-87952438. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the National Key Technology Research and Development Program of China (no. 2012BAB09B00), National High Technology Research and Development Program of China (no. 2009AA064704), National Basic Research Program of China (2011CB201500), Research Project of Environmental Protection Commonweal Industry (201209023-4) and Important Project on Science and Technology of Zhejiang Province of China (2008C13024-1). The authors also would like to thank Niumag Corporation Ltd (Shanghai, China) for their support with the NMR analysis.

Figure 11. NMR measurement repeatability (suffixes 1−3 denote the sample without paramagnetic ions, and suffixes 4−6 denote the opposite).

was directly added into the filter cartridge to ensure consistency of the masses. Figure 12 shows the mass comparison of each

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REFERENCES

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Figure 12. Mass comparison before and after azeotropic distillation.

component before and after azeotropic distillation. The efficiency of this azeotropic distillation is expressed by the fraction of the mass from azeotropic distillation extraction relative to the original mass. An efficiency factor of about 0.966 was achieved, which was deemed acceptable for this experiment. The majority of the mass lost was attributed to the volatilization of smaller chain length hydrocarbons, with their lower boiling points, in the oven.

5. CONCLUSIONS In this study, low-field 1H NMR was used to quantify the water and oil contents present in an oil sludge: CPMG experiments were performed to construct the relevant T2 distribution curves. To separate oil and water signals from the oil sludge, MnCl2·4H2O was used. Good correlations with conventional methods, namely, azeotropic distillation, were obtained as follows: R2 = 0.998 for both water and oil with a calibration curve, or correlation coefficients of 0.997 and 0.999 for water and oil, corrected by their respective amplitude indices. Standard deviations below 3% were found for both methods. In comparison with the conventional method, it took less time (under 5 min) for low-field NMR to provide information about the oil and water contents in an oil sludge. These results revealed that quantification of the water and oil contents in an 2232



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Rapid, Accurate Measurement of the Oil and Water Contents of Oil

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