New measurement methods for wax appearance temperature of petroleum fluids using NIRS and RI techniques

doi:10.11949/0438-1157.20191524

Abstract

Abstract:

Wax appearance temperature (WAT) is an important parameter of petroleum fluids. Obtaining accurate wax appearance temperature is a prerequisite for ensuring the safe transportation of waxy crude oil pipelines. New measurement methods were proposed in this study, including temperature scanning method using near infrared spectroscopy (NIRS) and refractive index (RI) method. Furthermore, wavelength scanning method using NIRS was improved in its testing and data analysis process. In addition, these methods were compared with two commonly used ones, polarizing microscopy (PM) and differential scanning calorimetry (DSC). The results showed that reliable WAT results could be obtained by NIRS and RI methods with high sensitivity and stability. The measurement results were less susceptible to the simple testing and analysis process, especially suitable for volatile samples. Moreover, the stirring in the testing process of NIRS methods effectively reduced the influence of supercooling degree, while it was still different from the flowing condition in real pipelines.

Key words: measurement, petroleum, wax appearance temperature, crystallization, near infrared spectrum, refractive index

摘要：

析蜡温度是重要的石油流体性质参数，得到准确的析蜡温度是保证含蜡原油管道安全输送的前提。提出了近红外光谱温度扫描测试法和折射率测试法，改进了近红外光谱波长扫描测试法，并从可靠性、灵敏性和稳定性等方面与常用的偏光显微镜法和差示扫描量热法进行比较。结果表明，近红外光谱法和折射率法可以得到可靠的测试结果，并具有较高的灵敏性和稳定性；实验操作和数据分析方法简单，测试结果不易受到制样过程影响，尤其适合挥发性较强的油样。此外，近红外光谱法测试时油样处于运动状态，可有效减小过冷度的影响，但与实际管输条件仍存在一定差别。

关键词: 测量, 石油, 析蜡温度, 结晶, 近红外光谱, 折射率

CLC Number:

TE 81

Si LI, Kaifeng FAN, Qiyu HUANG. New measurement methods for wax appearance temperature of petroleum fluids using NIRS and RI techniques[J]. CIESC Journal, 2020, 71(7): 3333-3344.

李思, 范开峰, 黄启玉. 利用NIRS和RI技术测试石油流体析蜡温度新方法[J]. 化工学报, 2020, 71(7): 3333-3344.

Figures/Tables 22

Fig.1 Carbon number distribution of the refined solid paraffin

Table 1 Basic properties of DJ crude oil

凝点/

℃

含蜡量/

%(质量)

沥青质/

%(质量)

胶质/

%(质量)

20℃时密度/

(kg·m^-3)

40℃时黏度/

(mPa·s)

Fig.2 Carbon number distribution of wax components in DJ crude oil

Fig.3 Images of the sample chamber and sample cells

Fig.4 Principle of the refractive index measurement

Fig.5 Heat flow vs temperature of crude oil in DSC method

Fig.6 Absorbance vs temperature with different cooling rates in NIRS temperature scanning method

Fig.7 Absorbance vs temperature with different wavelengths in NIRS temperature scanning method

Fig.8 Determination of WAT in thermal equilibrium by extrapolation method

Fig.9 Absorbance of model oil in spectrum mode

Fig.10 Fitted results of absorbance and temperature

Table 2 WAT results fitted at different wavelengths

波长/nm	吸光度基线值	析蜡温度/℃
600	0.0293	20.88
800	0.0113	20.82
1000	0.0194	20.79
1300	0.0474	20.78
1600	0.0825	20.77

Fig.11 Absorbance curves of 2.0% asphaltenes in toluene before and after density correction

Table 3 WAT determined by different wave lengths of model oil samples with different asphaltenes contents

沥青质含量0%		沥青质含量2.0%		沥青质含量5.0%
波长/nm	WAT/℃	波长/nm	WAT/℃	波长/nm	WAT/℃
600	21.13	1300	20.26	1000	20.23
700	21.12	1400	20.23	1050	20.07
800	21.11	1500	20.21	1100	19.94
900	21.10	1550	20.19	1150	19.82
1000	21.09	1600	20.18	1200	19.72

Fig.12 Refractive index and density vs temperature of n-heptane and model oil sample with 10.0% wax

Fig.13 Influence of temperature interval on refractive index

Fig.14 Influence of holding time on refractive index

Fig.16 Particle parameters vs temperature under different cooling rates

Fig.15 Micrographs of model oil with 10.0% wax at different cooling rates and temperatures

Table 4 WAT results of PM under different cooling rates

油样	析蜡温度/℃
油样	1℃·min^-1	5℃·min^-1	10℃·min^-1
含蜡量10.0%	22.67	22.16	22.00
含蜡量15.0%	26.33	25.58	25.17

Table 5 Comparison of WAT results of different measurement methods

油样	析蜡温度/℃
油样	NIRS温度扫描	NIRS波长扫描	RI	PM	DSC
含蜡量5.0%	14.88	15.04	16.1	17.25	14.75
含蜡量10.0%	20.15	20.26	21.0	21.93	20.10
含蜡量15.0%	23.56	23.75	24.4	25.58	23.31
DJ原油	48.63	47.71	47.9	50.33	45.95

Table 6 WAT results in repeatability test of different measurement methods

测试方法	析蜡温度/℃					平均值/℃	标准差/℃
测试方法	1	2	3	4	5	平均值/℃	标准差/℃
温度扫描法	20.05	19.98	20.14	20.22	20.34	20.15	0.1264
波长扫描法	20.24	20.11	20.25	20.29	20.42	20.26	0.0995
RI	21.0	21.1	21.0	21.0	20.9	21.0	0.0632
PM	22.16	23.83	20.33	22.00	21.33	21.93	1.1479
DSC	20.01	20.45	19.76	19.34	20.96	20.10	0.5592

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