flow properties of weathered crude oils and their ...

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FLOW PROPERTIES OF WEATHERED CRUDE OILS AND THEIR EMULSIONS a

b

a

Ahmed A. Elgibaly , Ibrahim S. Nashawi , Mahmoud A. Tantawy & Ali Elkamel a

b

College of Petroleum and Mining Engineering , Suez, Egypt

b

College of Engineering and Petroleum , Kuwait University , P.O. Box 5969, Safat, 13060, Kuwait Published online: 27 Apr 2007.

To cite this article: Ahmed A. Elgibaly , Ibrahim S. Nashawi , Mahmoud A. Tantawy & Ali Elkamel (1999) FLOW PROPERTIES OF WEATHERED CRUDE OILS AND THEIR EMULSIONS, Journal of Dispersion Science and Technology, 20:3, 857-882, DOI: 10.1080/01932699908943824 To link to this article: http://dx.doi.org/10.1080/01932699908943824

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J. DISPERSION SCIENCEAND TECHNOLOGY, 20(3), 857-882 (1999)

FLOW PROPERTIES OF WEATHERED CRUDE OILS AND THEIR EMULSIONS

Downloaded by [Kuwait University] at 01:50 26 June 2014

Ahmed A. Elgibaly", Ibrahim S. Nashawi", Mahmoud A. Tantawy', and Ali Elkamd 'College of Petroleum and Mining Engineering, Suez, Egypt 2College of Engineering and Petroleum, Kuwait University, P.O. Box 5969, Safat 13060, Kuwait

ABSTRACT The present paper proposes the emulsification of weathered crude oils in water as a competitive and cost effective method for reducing their viscosities. Weathered crude oil samples were collected from major Kuwaiti oil lakes. Emulsion preparation involved using, either a nonionic surfactant or alkali, as well as both alkali and fatty acid. The obtained emulsions were characterized by measuring the droplet size distribution of the dispersed phase using optical microscopy. Emulsion stability was also examined in terms of the system breakdown. The rheological properties were measured using a concentric cylinder rotary rheometer. The emulsion rheological behavior has been studied as a function of composition, temperature, and shear rate. A constitutive model was developed to characterize the pseudoplastic behavior of the crude oil and the emulsion systems. The model fitted well the experimental results with a correlation coefficient higher than 95%. Associated with the pseudoplastic behavior, viscoelastic behavior has been observed with emulsions and some oils at high shear rates. The results of this investigation indicated that the examined weathered crude oils can be transported through pipelines as emulsions of up to 80 vol. % oil concentrations. The proposed method of treatment with NaOH and oleic acid offers several advantages over the surfactant treatment. It exhibited comparable rheological behavior at lower cost and less mixing energy. It also provided higher emulsion stability, which favors oil transportation for longer distances. Keywords: weathered oil/water emulsion, rheology, NaOH, oleic acid, stability • Corresponding author, Email:[email protected]

857 Copyright

ce 1999 by Marcel Dekker, Inc.

www.dekker.com

ELGIBALY ET AL.

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INTRODUCTION The transportation of weathered crude oils (WCO) from Kuwaiti oil lakes through pipelines presents a major technical problem. The WCO viscosity is more than 100,000 cp, Various methods have been proposed for reducing oil viscosity or conditioning such oils for pipeline transportation and processing. I- 3 These methods include: (I) reduction of viscosity by heating, (2) blending with lighter


crude oils or petroleum products, (3) lubrication by introducing a water layer between the crude oil and pipeline material, and (4) oil-in-water emulsification.3 The selection of the appropriate method depends basically on the particular oil lake location and size, oil physical properties, flow rate, and the pipeline length and size. The thermal method is expensive and requires environmental precautions, as well as elaborated facilities and insulation to minimize heat loss. The improvement of the quality of the weathered crude oils by blending with an oil refinery product has been investigated by Khan et a12. A series of experiments were performed using raw kerosene in proportions up to 20 wt.%. Fresh water and chemical demulsifiers were employed in the process of desalting and dehydrating of oilwater emulsions. The dewatering ofa blend ofWCO and 20 wt.% kerosene with wash water up to 50 wt.% removed as much as 12.0 wt.% of brine from the emulsion, thus, reducing WCO density and viscosity. It appears that emulsification of oil in water offers extra advantages over the viscosity reduction by a chemical solvent. Brackish water can be used as a thinner to which a very small amount of emulsifier is added. The water also serves in washing the salty crudes. The separation process is rather inexpensive. In order to form an emulsion of oil and water, an emulsifier should be used. Emulsifiers such as asphaltenes, resins, cresols, phenols, organic acids, metallic salts, and clays, are naturally found in crude oil. These materials can be exploited to reduce the need for expensive surfactants. If an alkaline aqueous solution is added to the crude oil, it will react with the acidic compounds existing in oil and generate

WEATHERED CRUDE OILS

surface active substances or surfactants.

859

These surfactants lower the interfacial

tension between the oil and the aqueous phase and form emulsions/l- 5

The

additional operating costs required to break down these emulsions at the delivery end and the decrease in the pipeline capacity can be tolerated in comparison with the obtained profit. The success recorded in a number of field projects6- 12 demonstrates the


feasibility of using water as a substitute for solvent to reduce the viscosity of heavy oil to be transported through pipelines. The oil content in the emulsion can reach up to 60 percent with an apparent viscosity of the emulsion as much as 10,000 times less than that of crude oil even though the oil viscosity is I miIlion cp at room temperature.8,9,13 However, it appears that the rheological behaviors of the oil-lake oils and their emulsions have not been yet studied in terms of oil content, temperature, and shear rate. The main objectives of the present study include: (I) the investigation of the rheological characteristics of the WCO at various temperatures and shear rates, (2) comparison of the rheological behavior of emulsions formed by a commercial surfactant with that of emulsions prepared by less expensive chemicals such as an alkali and/or fatty acid, and (3) development of a constitutive model for o/w emulsions that can predict the emulsion viscosity as a function of oil concentration, temperature, and shear rate.

RHEOLOGICAL MODEL OF OIW EMULSIONS Laboratory results 9, 10,14 suggest that the rheological behavior of oil-inwater emulsions, within the practical range of shear rates, can be best approximated by a power-law relationship expressed as: T

= Kv"

(I-a)

The apparent viscosity is defined as:

-..£..... =Ii =Ky"-'

-y

(I-b)

860

ELGIBALY ET AL.

where K and n are constants. For a Newtonian fluid n

= 1.0 and 11 = K. For

pseudoplastic fluids, n varies from 0 to I, and hence the exponent n-I in Eq. (I-b) is negative. Therefore, a plot of the natural logarithm of ~ versus that of y usually gives a straight line with a negative slope. For highly pseudoplastic fluids, n tends to zero, and n-I approaches a value of -1.0. The relationship between the apparent viscosity of emulsion, 11, and oil


content,