Energy Fuels 2010, 24, 2213–2220 Published on Web 12/08/2009
: DOI:10.1021/ef900761t
Effect of Pressure on the Crystallization of Crude Oil Waxes. II. Evaluation of Crude Oils and Condensate† Lenise C. Vieira,*,‡,§ Maria B. Buchuid,‡ and Elizabete F. Lucas§ ‡
Petrobras Research Center, Av. Hor acio Macedo, 950 Cidade Universit aria, CEP 21941-915, Rio de Janeiro, RJ, Brazil and Macromolecules Institute, Rio de Janeiro Federal University, Av. Hor acio Macedo, 2030 Ilha do Fund~ ao, 21941-598, Rio de Janeiro, RJ, Brazil
§
Received July 21, 2009. Revised Manuscript Received November 16, 2009
Laboratory analyses of the formation of wax deposits in crude oil are generally performed at atmospheric pressure on stabilized samples without the presence of light components. Therefore, the effects of two important factors that influence the solubility of waxes, namely, light fractions and pressure, are not considered. As a consequence, the results may not reflect what really happens in production lines and equipment. In this work, we evaluated five Brazilian crude oil samples and one condensate, recombined with different gases and under varying pressures, using the high-pressure microcalorimetry technique (HPμDSC). The samples were characterized with regard to the distribution of waxes by the number or carbon atoms to correlate with the results for wax appearance temperature (WAT) and crystallization enthalpy. The results indicate that rising pressure causes the WAT to rise as well and the recombination with gases composed mainly of light hydrocarbons reduces the WAT. Methane has the greatest influence on the oils containing higher amounts of macrocrystalline waxes, an effect that is stronger at higher pressures. Nitrogen does not act as a solvent of the paraffins present, whether of the macro- or microcrystalline type. Fractions lighter than C3, contained in the saturation gas mixtures, are more efficient in the solubilization of waxes, mainly those with lower molar masses. Variations in temperature and enthalpy of crystallization depend upon the pressure, composition of the gas (or mixture of gases), and composition of each crude oil.
The level of light components in crude oil influences the solubility of the heavier components (paraffins), keeping them in solution. Under the heat and pressure conditions, in which petroleum is found in reservoirs, the waxes with high molar weight are maintained in solution by the light components, so that the fluid has low viscosity. When the oil flows through the production strings and pipelines, the pressure falls (with the loss of light hydrocarbon fractions), as well as the temperature. Therefore, the heavy fractions become less soluble, until at the wax appearance temperature (WAT), the waxes precipitate out.5 The precipitated waxes can aggregate and cause an increase in viscosity, requiring more energy for pumping. Oils with large quantities of waxes can become jellified during operational shutdowns, making it very hard to re-establish the flow.6-8 The WAT value measured in the laboratory depends upon many factors, among them the composition, thermal history, pressure, measurement technique, and cooling rate of the oil.9,10
1. Introduction The composition of crude oils is highly complex, making it difficult to estimate the number of components. Nevertheless, it is usual to consider the presence of waxes with linear chains (n-paraffins) and branched chains (isoparaffins), naphthenes (cycloparaffins), and aromatic compounds. Besides these components, there are also small quantities of compounds, such as asphaltenes and resins, both including ones containing heteroatoms (oxygen, nitrogen, and sulfur) as well as heavy metals.1,2 Waxes with low molar mass are the main constituents of natural gas, and those with high and medium molar mass predominate in the liquid phase of petroleum.3 Waxes can be divided into two groups: macro- and microcrystalline. The macrocrystalline waxes are composed mainly of paraffins with normal chains (n-alkanes), with between 18 and 30 carbon atoms and melting points between 40 and 60 °C. Microcrystalline paraffins contain a high content of branched and cyclical paraffins, with chains containing between 30 and 60 carbon atoms and melting point >60-90 °C.3,4
(5) Elsharkawy, A. M. Determination and prediction of wax deposition from Kuwaiti crude oil. Proceedings of the Society of Petroleum Engineers (SPE) Latin American and Caribbean Petroleum Engineering Conference, Caracas, Venezuela, 1999; SPE 54006, pp 1-10. (6) Sing, P.; Fogler, S.; Nagarajan, N. J. Rheol. 1999, 43 (46), 1437– 1459. (7) Ronningsen, H. P. Energy Fuels 1991, 5 (6), 895–908. (8) Pedersen, K. S.; Ronningsen, H. P. Energy Fuels 2000, 14, 43–51. (9) Hammami, A.; Raines, M. A. Paraffin deposition from crude oils: Comparison of laboratory results to field data. Proceedings of the Annual Technical Conference and Exhibition of the Society of Petroleum Engineers, San Antonio, TX, 1997; SPE 38776, pp 5-8. (10) Hammami, A.; Ratulowski, J.; Coutinho, J. A. P. Pet. Sci. Technol. 2003, 21 (3 and 4), 345–358.
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Presented at the 10th International Conference on Petroleum Phase Behavior and Fouling. *To whom correspondence should be addressed. E-mail: lenise@ petrobras.com.br. (1) Singh, P; Venkatesan, R.; Fogler, H. S.; Nagarajan, N. AIChE J. 2000, 46, 1059–1074. (2) Misra, S.; Baruah, S.; Singh, K. SPE Prod. Facil. 1995, SPE 28181, 50-54. (3) Freund, M.; Csik os, R.; Keszthelyi, S.; Mozes, G. Paraffin Products; Elsevier Scientific Publishing Company: New York, 1982; pp 13-14. (4) Dirand, M. J. Chem. Eng. Data 2002, 47, 115–143. r 2009 American Chemical Society
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Energy Fuels 2010, 24, 2213–2220
: DOI:10.1021/ef900761t
Vieira et al.
Table 1. Characteristics of the Oil and Condensate Samples19 crude oils and condensate properties
A
C
D
E
F
relative density (20 °C/4 °C) API density (60 °F/60 °F) KUOP factor water content (Karl Fisher) (wt%) pour point (°C) SARA content (wt%) saturates aromatics resins asphaltenes paraffins content asphaltenes content acidity index (TAN) (mg of KOH/g) WAT optical microscopy (°C)
0.8026 43.9 12.4 0.48 12
0.8192 41.1 12.3 0.01 -9
0.7840 48.0 12.2 0.80 36
0.9278 20.4 11.8 0.80 -27
0.7971 45.1 12.2 0.05 -6
72.9 17.0 9.7 0.4 5.4
