Optik 167 (2018) 37–41
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Original research article
Optical constants of oilfield wastewater Da Zhao a , Lu Yang b , Xinyan Zhang a,∗ , Dong Li b a b
College of Science, Heilongjiang Bayi Agricultural University, Daqing 163319, China School of Architecture and Civil Engineering, Northeast Petroleum University, Daqing 163318, China
a r t i c l e
i n f o
Article history: Received 17 January 2018 Accepted 9 February 2018 Keywords: Oilfield wastewater Transmittance spectra Optical constants
a b s t r a c t The optical properties of oilfield wastewater play an important role in the study of oilfield wastewater. Optical constants are the important parameters of optical properties, which are usually obtained by measuring transmittance spectra. Transmittance spectra of oilfield wastewater in the wavelength range of 190–900 nm at normal incidence were measured by a TU-1900 double beam ultraviolet visible spectrophotometer. The optical constants of oilfield wastewater were calculated by the double thickness method. The results show that the optical constants of oilfield wastewater with different oil content are different. The refractive index of oilfield wastewater increases with oil content increasing, and the values of them are in the range of 1.06–1.17. The absorption index of oilfield wastewater increases with the increase of oil content, and the values of them are in the range of 2.70 × 10−7 7.60 × 10−7 . © 2018 Published by Elsevier GmbH.
1. Introduction Oilfield wastewater is a multi-component mixture produced in oilfield production, and it contains a large amount of water and complex composition, such as suspended matter, petroleum and polymer [1–5]. Based on the oilfield wastewater has seriously endangered our survival and health [6,7], it is important to accurately measure the oil content of oilfield wastewater. The optical properties of oilfield wastewater are the basis on the oil content of optical measurement. However, there are limited researches about the optical properties of oilfield wastewater. The optical constants are important parameters for determining the optical properties of oilfield wastewater. The present methods to calculate the optical constants of liquid materials contain the reflectance and transmittance spectra combination method [8–10], the reflectance method [11], the transmittance method [12,13], and transmittance combine with KramersKronig method [14]. Among these methods, the double thickness transmittance method proposed by Tuntomo and Tien [15] is a common method for calculating the optical constants of liquid materials [16,17]. For example, Li et al. [18] measured the transmittance spectra of nine different samples of edible oils in the infrared spectral range, and calculated the optical constants of different edible oils with different temperature by the double optical path length transmission method. Li et al. [19] measured the transmittance spectra of sodium chloride (NaCl) solution with different NaCl concentration, and calculated the optical constants of sodium chloride solution in the range of 300–2500 nm by the double optical path length transmission method. Ai et al. [20] improved the double-thickness method by combining with genetic algorithm, and obtained the optical constants of distilled water and Chinese No.-35 diesel fuel. The double thickness transmittance method considered the surface reflectance at the interface of the liquid sample and the optical window.
∗ Corresponding author. E-mail address:
[email protected] (X. Zhang). https://doi.org/10.1016/j.ijleo.2018.02.036 0030-4026/© 2018 Published by Elsevier GmbH.
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Z. Da et al. / Optik 167 (2018) 37–41
Table 1 Stirring process of oilfield wastewater. Qualification
First paragraph
Second paragraph
Third paragraph
Speed (r/min) Time (min)
100 1
500 20
50 0.5
In this paper, the transmittance spectra of oilfield wastewater were measured with different oil content, and the optical constants of oilfield wastewater were calculated for the first time by the double thickness method. 2. Experimental method 2.1. Model to determine absorption coefficient The optical constants of oilfield wastewater contain refractive index n andabsorption index k. They need to be indirectly derived from the transmittance spectra of oilfield wastewater. The result experimentally measured is the total transmittance of optical cell filled with oilfield wastewater [21]. In order to eliminate the multiple reflection effects between optical window and oilfield wastewater, the concept of the equivalent transmittance is proposed, that is, the transmittance of oilfield wastewater is the ratio of the transmittance of the optical cell filled with oilfield wastewater under two different optical path lengths. The total transmittance ratio of the optical cell filled with oilfield wastewater with thickness of L1 –L3 is T1 –T3 , respectively. L’ is less than L". Then the transmittance T’ and T" of the oilfield wastewater with thickness of L’ and L" are given as T =
T2 T1
(1)
T =
T3 T1
(2)
L = L2 − L1
(3)
L = L3 − L1
(4)
So the reflectivity of the oilfield wastewater can be calculated as follows
4kL
Te
=1−
⁄ +
T e
4kL
⁄ (5)
2
The absorption index k and refractive index n of oilfield wastewater are equal to
k=−
ln
T T
4 L − L
n = (1 + ) +
(6)
2
(1 + ) − (1 − )
2
1 + k2
/ (1 − )
(7)
2.2. Experimental instruments and samples preparation The experimental samples are including oilfield wastewater and distilled water. The oilfield wastewater was brought from the New Apricot nine joint station of the fourth oilfield of the four oil production plant in Daqing Oilfield. The experimental instruments are including double beam ultraviolet visible spectrophotometer (TU-1900), Experimental agitator (TA6-1), and Quartzcell (5, 10 and 20 mm). Taking appropriate amount of oilfield wastewater and different volumes of distilled water, and then mixing and stirring them by the experimental agitator to prepare oilfield wastewater samples, and oil content are 7.05, 4.70 and 3.53 mg/L, respectively. The stirring process is shown in Table 1. The oil content of oilfield wastewater is equaled to C=
A*V2 0.0005V1
(8)
Where, C is the oil contentof oilfield wastewater (mg/L). A is the absorbance of oilfield wastewater at 430 nm. V2 is the volume of the extracted liquid (mL), and V1 is the volume of oilfield wastewater (mL). Finally, the transmittance spectra of oilfield wastewater with different oil content were measured by the TU-1900 double beam ultraviolet visible spectrophotometer. The experimental system for measuring transmittance spectra is shown in Fig. 1. First, turning on the spectrometer, then correcting the baseline, and then the optical cell filled with oilfield wastewater was
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Fig. 1. Oilfield wastewater optical detection principle.
putted in the sample pool. At this time, the light passed through the optical cell was received by the photo detector. Finally, the signal was transmitted to the fiber spectrometer, and then the transmittance spectra of oilfield wastewater were displayed on PC.
3. Results and discussion 3.1. Transmittance spectra The transmittance spectra of oilfield wastewater were measured in the range of 190–900 nm, and the spectral bandwidth is 2 nm. The transmittance spectra measured at the room temperature (23 ◦ ) are shown in Fig. 2. Fig. 2 shows that the transmittance spectra with different oil content are similar in the range of 190–900 nm. The transmittancespectra include theabsorption region and the high transmittance area. The wavelength range of absorption region is 200–400 nm, and the wavelength range of high transmittance region is 400–850 nm. In high transmittance region, the change of oil content has little effect on the transmittance spectra of oilfield wastewater. In the absorption region, the transmittance spectra of oilfield wastewater decrease with the oil content increasing, and the change of the transmittance is greatly influenced by the thickness of the oilfield wastewater.That is because the C C conjugated double bond and aromatic compound containing benzene ring of the oilfield wastewater have strong absorption in the range of 215–225 nm and 250–260 nm, respectively.
3.2. Optical constants of oilfield wastewater The optical constants of oilfield wastewater are calculated by the double thickness method based on the measured transmittance spectra of oilfield wastewater. The refractive index n and absorption index k in the wavelength range of 238–304 nm are shown in Fig. 2 and Table 2. Fig. 2 shows the refractive index n and absorption index k in the wavelength range of 238–304 nm.The refractive index spectra of oilfield wastewater are increased with oil content increasing. The refractive index of oilfield wastewater decreased and then increased with wavelength.The peak point of the refractive index spectra shifted with the change of oil content. The minimum values of refractive index of oilfield wastewater at 7.05, 4.70 and 3.53 mg/L are 1.11, 1.07 and 1.06, respectively. The shapes of absorption index spectra of oilfield wastewater are similar, but the values of absorption index with different oil content are different. The absorption index of oilfield wastewater decreased with wavelength increasing. The absorption index spectra of oilfield wastewater are increased with oil content increasing. The absorption index of oilfield wastewater at 7.05, 4.70 and 3.53 mg/L are in the range of 4.80 × 10−7 –1.07 × 10−6 , 3.70 × 10−7 –17.60 × 10−7 and 2.70 × 10−7 –5.60 × 10−7 , respectively.
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Fig. 2. Transmittance spectra of oilfield wastewater with different oil content of (a) the thicknessis 5 mm, (b) the thickness is 10 mm, and (c) the thickness is 20 mm.
Table 2 Optical constants of oilfield wastewater with different oil content. Wavelength (nm) 304 298 294 288 284 278 274 268 264 258 254 248 244 238
k
n 7.05 mg/L
4.70 mg/L
3.53 mg/L
7.05 mg/L
4.70 mg/L
3.53 mg/L
1.16568 1.15632 1.14701 1.15234 1.14088 1.14621 1.13421 1.11774 1.12561 1.12237 1.11677 1.108 1.114 1.13064
1.14171 1.13776 1.13946 1.14243 1.141 1.11935 1.1239 1.10252 1.10284 1.07244 1.0799 1.09569 1.11326 1.12134
1.12615 1.12318 1.11066 1.12676 1.12365 1.10501 1.10556 1.10983 1.0836 1.06344 1.07672 1.08931 1.09362 1.09931
4.80E-07 5.10E-07 5.40E-07 5.80E-07 6.10E-07 6.50E-07 6.80E-07 7.00E-07 7.10E-07 7.20E-07 7.40E-07 7.90E-07 8.70E-07 1.07E-06
3.70E-07 3.90E-07 4.10E-07 4.40E-07 4.60E-07 4.90E-07 5.00E-07 5.20E-07 5.30E-07 5.30E-07 5.40E-07 5.80E-07 6.30E-07 7.60E-07
2.70E-07 2.90E-07 3.10E-07 3.20E-07 3.40E-07 3.60E-07 3.70E-07 3.80E-07 3.90E-07 4.00E-07 4.00E-07 4.20E-07 4.60E-07 5.60E-07
4. Conclusion In this work, the transmittance spectra of oilfield wastewater at 7.05, 4.70 and 3.53 mg/L were measured. The transmittance spectra contain the absorption region and the high transmittance region. In the absorption region, the transmittance spectra of oilfield wastewater decrease with the oil content increasing. The refractive index spectra and absorption index spectra of oilfield wastewater are all increased with oil content increasing. The refractive index of oilfield wastewater decreased and then increased with wavelength. The minimum values of refractive index of oilfield wastewater at 7.05, 4.70 and 3.53 mg/L are 1.11, 1.07 and 1.06, respectively. The shapes of absorption index spectra of oilfield wastewater are similar and the absorption index of oilfield wastewater decreased with wavelength increasing. The absorption index of oilfield wastewater at 7.05, 4.70 and 3.53 mg/L are in the range of 4.80 × 10−7 –1.07 × 10−6 , 3.70 × 10−7 –17.60 × 10−7 and 2.70 × 10−7 –5.60 × 10−7 , respectively.
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