Investigating the rheological properties of light crude ... Madji · Investigating the rheological
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Fuel 95 (2012) 97107
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Investigating the rheological properties of light crude oil and the characteristicsof its emulsions in order to improve pipeline flow
Madjid Meriem-Benziane a, Sabah A. Abdul-Wahab b,, Mohamed Benaicha c, Mansour Belhadri ca Mechanical Engineering Department, University of Chlef, P.B. 151, Chlef 02000, Algeriab College of Engineering, PO Box 33, Sultan Qaboos University, Al-Khod 123, Omanc Laboratory of Rheology, Transport and Treatment of the Complex Fluids, University of Science and Technology, Mohamed Boudiaf, B.P. 1505, El MNaour, Oran 31000, Algeria
a r t i c l e i n f o a b s t r a c t
Article history:Received 10 June 2011Received in revised form 10 September 2011Accepted 6 October 2011Available online 25 October 2011
Keywords:Non-NewtonianStressViscosity complexStorage modulusLoss modulus
0016-2361/$ - see front matter 2011 Elsevier Ltd. Adoi:10.1016/j.fuel.2011.10.007
Corresponding author.E-mail address: firstname.lastname@example.org (S.A. Abdul-W
The objective of this study was to investigate the rheological properties of the light crude oil and its emul-sions in order to obtain more knowledge about the rheological behavior of oil flow in pipelines. Theexperiments were carried out at a temperature of 20 C by using the RS600 RheoStress (ThermoHaake,Germany). The results showed that the viscosity of the prepared emulsions varied with their water con-tents. In the case of 100% light crude oil, the study of the functional relationship demonstrated the quasi-Newtonian behavior with a moderate constant viscosity. However, for emulsions with different waterconcentrations, their rheological behaviors were described in better way by the Ostwald de Waele andthe HerschelBulkley models. The stability of emulsions was identified by measuring the rheologicalproperties including non-Newtonian viscosity, the elastic modulus, (G0), the loss modulus, (G00), the phaseangle (d) and the complex viscosity (g). The results indicated that the rheological properties and thephysical stability of emulsions were significantly influenced by the water contents and the nature ofcrude oils.
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It is known that crude oil plays an essential role in giving theenergy supply of the world among different sources of energy. Fur-thermore, in the hydrocarbon industry, the existence of stablewater-in-crude oil emulsion is considered to be undesirable aswater should be separated from oil. In general, the main parame-ters for identifying the crude oil are specific gravity (API), density(d) and sulfur content (S). According to their respective mean val-ues (i.e., 38 < API < 44, 0806 < d < 0830 and 0.2 < %S < 0.3), theAlgerian crude oil is classified as a light one. Therefore, it is neces-sary to transport the Algerian crude oil in complex installationspipelines in order to reduce its corrosive power and to avoid itsinteraction with the metal of the pipelines.
Apart from this, the high viscosity of heavy crude oils and emul-sions undoubtedly contribute to the increase of the transport billby the pipelines. The analysis of the influence of the viscosity ofoilwater emulsions is considered essential in the field of rheologyto find out ways for the development and transportation of oil[1,2]. Often, crude oil is found in mixed state in which the concen-tration of water is very consistent. Due to the complex behavior of
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the crude oil, it is subjected to numerous difficulties during variousprocesses such as production, separation, transportation, and refin-ing. Indeed, the high concentrations of sulfur and acids in the hea-vy petroleum feed stocks has a negative impact on their transportthrough pipelines. It should be noted that the water contents in thelight oilwater mixture has a great impact on its rheologicalbehavior and its type .
With respect to energy point of view, the presence of water inthe oil is a factor which affects its quality and therefore the elimi-nation of water improves the calorific value of the light crude oil. In addition, research on the rheological behavior of the lightcrude oil is very important, especially if someone takes into ac-count the presence of surfactants.
The characteristics of emulsions with respect to their concen-trations indicate the possibility of their structural changes. Accord-ing to Wang et al. , these structural changes can be addressed byintroducing the concepts of viscous modulus, elastic modulus, andrelaxation time (i.e., viscoelastic analysis). The stability of an emul-sion depends on the presence of solid particles in the films andtheir interfacial elasticity.
According to Xia et al. , the measurements of the dynamicviscoelasticity of expansion can be used to study the chemicalcomposition of the emulsions in order to provide more informationon the dynamics of polymer chains and their interactions with thesurfactant molecules. Also the study revealed that the compositionand the structure of the rigid layer around droplets of water in the
98 M. Meriem-Benziane et al. / Fuel 95 (2012) 97107
emulsions are responsible for the stability and for the preventionof coalescence phenomenon . According to White et al. , inmost of the emulsions which are stabilized by proteins, the inter-actions between the colloids present in the droplets and electro-static interactions influence the state of aggregation of droplets.According to Dan and Jing , the rheological behavior of heavyoilwater emulsion is non-Newtonian.
During the transport of oil via pipeline, the stability of crude oilis particularly favored by the concentration of chemical additivessuch as surfactants which contribute to decrease the interfacialtension between the crude oil and the water .
The water content in the oil emulsions can be high (about 60%by volume). But, this quantity is not very important for refiningoperations. However, the effective viscosity of the emulsion de-pends largely on the volume fraction of water and the temperaturephases in thermodynamic equilibrium. In this connection, the pre-vious research work guides in developing correlations to predictthe actual values of viscosities . With increasing tempera-ture, the stability of the oilwater emulsion decreases. For differentlevels of oil, pour points of oilwater emulsions are always lowerthan those of crude oil from the reservoir.
According to Hasan et al. , the modeling analysis can beused to find out certain rheological models that fit the measure-ments. Three rheological models of Newton, HershelBulkly, andCasson models can be used to determine the rheologicalcharacteristics.
The objective of this work was to study the rheological proper-ties of light crude oil and the characteristics of its emulsions. Thestudy was divided into four parts. The first part was devoted to-wards experimental analysis to establish the viscosity rheogramas a function of the composition of the emulsions with differentwater concentrations (i.e., 30%, 50% and 70%) at a temperature of20 C. The second part investigated the yield stress of the emul-sions. In the third part, different rheological models were used(e.g., Newton law, HershelBulkly law and Casson law) in orderto simulate the flow behavior of the emulsions. The fourth partinvestigated the impact of the stability on the rheology ofemulsions.
Fig. 1. Samples of light crude oil after the agitation.
In this study four different samples of light crude oil were usedfor the preparation of emulsions. These samples of light crude oilwere taken from different oil fields in Algeria. The origin of thesesamples was recognized by identifying them with letters A, B, Cand D. At temperature of 20 C, the samples were characterizedby a density of 806 kg/m3 and a specific gravity (API) between 36and 44. The emulsions were prepared with different concentra-tions of water (i.e., 30%, 50% and 70%). The perfect homogenizationwas achieved by stirring the solution for 10 min with a magneticbar. The desired temperature for each test was maintained byusing a temperature controller (type DC30). The PH of the emul-sions was determined by a PH meter electrode calomel type HannaInstruments-213.
The rheological behavior of different samples was studied byexploiting the performance of the rheometer (RheoStress 600, typeZ40 DIN, Germany) which was operated at a pressure of 2.5 bar.Once, the thermodynamically stabled emulsion was obtained,and then different rheological performance of the samples wasmeasured by exploiting the rheometer as mentioned earlier. Thestudy of liquidliquid stability and the influence of temperatureon the partitioning of coexisting phases were essential in choosingthe appropriate solvent (liquidliquid extraction). Thus for
analyzing this physicochemical aspect, the emulsion was kept fora period of 24 h in order to settle in vials after stirring.
It was noted that the formation of emulsions light crudeoilwater (O/W) was an alternative for improving its flow in pipe-lines. Therefore, the study of their rheological properties was ofgreat importance in the petrochemical industry. These werediscussed by analyzing their stability on the basis of the volumeconcentration in water.
2.2. Rheological measurements
The samples were tested with a modular rheometer (Rheostressof Rs. RS600) that was manufactured by ThermoHaake in Karlsrule,Germany. The meter was equip