CC BY
36
AZERBAIJAN CHEMICAL JOURNAL № 2 2019
15
UDC 541.182
MODELING OF THE PROCESSES OF SEPARATION OIL EMULSIONS G.I.Kelbaliyev, V.LKerimli, G.N.Huseynov
M.Nagiyev Institute of Catalysis and Inorganic Chemistry, NAS of Azerbaijan
[email protected] Received 08.11.2018
This work offers the models and analytical determination of viscosity of the oil emulsions separation processes. The estimation of thickness of adsorption layer on the droplet surface has been done. A generalized viscosity equation has been proposed for structured disperse systems with their peculiar rheo-logical properties. Some characteristic features of the viscosity curve have been revealed at the beginning of structuration. Different variants of viscosity equations for structured disperse systems have been considered, and their comparison with the experimental data has been performed.
Keywords: coalescence, breaking, drop, oil emulsions, structuration, rheology, modeling, viscosity.
https://doi.org/10.32737/0005-2531-2019-2-15-21
Introduction
The processes of separation of oil emulsions are the important stage for preparation and cleaning of crude oil from water, mineral salts and different associated admixtures which oil contains. The processes of separation of oil emulsions which are aimed to complete reduction of the aggregative and kinetic stabilities of that emulsions carried out in a different ways: in the gravitational (settling systems and other modifications), in the centrifugal, electrical and magnetic fields [1-4], and also by the use of filtration through solid and liquid layers, applying microwave and membrane technologies. The oil emulsions contain enough large sizes of water drops and thereby in the majority of practical applications to reach the higher productivity of separation processes, the settling systems are used. When sizes of drops and their concentrations are small, the electrostatic field and
membrane technologies or combined methods which allow reaching the high rate of purity are applied. The small size droplets (a<200 ^m) in the settlers in accordance with terms of dynamic equilibrium create the suspended (intermediate) layer representing a filter bed, which mostly holds the fine - dispersed droplets and mechanical admixtures in its volume. The experimental and theoretical researches prove the very complex, in terms of dispersed composition and running phenomena (coalescence, deformation, and breaking), structure of intermediate layer. Thus, layer is dynamic one, as far as its structure (drops dispersion) and thickness are not constant. Such structure is determined by the entering conditions of the flow into the layer, drops sizes, their coalescence, and as a result of that the specified thickness of tight packaging of the drops in the intermediate layer occupying 10-20 % of whole volume is shown in Figure 1.
ni
Fig.1. The scheme of separation of oil emulsion with intermediate layer: I - oil emulsion, II - water, III - oil, IV- intermediate layer.
Changing of the structure, dispersed composition, and drops geometry by time favors the changing of porosity of intermediate layer and origination of turbulent flow in the porous channels. A lot of works [2-4] are dedicated to the theoretical and experimental researches of the mechanisms of formation, stabilization and destruction of oil emulsions as heterogeneous media, but many problems associated with phenomena, which occur on the interface of oil and water, with coalescence and break-up of water drops, exfoliation and sedimentation have not been correctly solved. Oil emulsions are polydisperse media with water drops sizes of 1-150 pm, although they may contain some coarsely dispersed (150-1000 pm) and colloidal particles (0.001-1 pm).
Such spread of sizes exercise a significant influence on the break-up, separation and drop sedimentation mechanisms in the oil emulsions. The mechanism of break-up and coalescence of drops in the oil emulsions can be subdivided into the set forth below stages.
a) Deformation and break-up of adsorption casings on the oil and water interface in the volume of stream using surface-active substances at defined temperatures (60-700C) and pressures.
b) Approaching and collision of the drops of different size with interfacial film formation. It should be noted that the drops transport in the polydisperse medium are basically defined by the hydrodynamic conditions and turbulence of flow.
In isotropic turbulence environment, the frequency of drops collision depends on specific dissipation energy of the turbulent flow and properties of the medium and dispersed phase [7-9]. Interfacial film of a circular section is formed as a result of collision and fixing of two drops with radius of that film can be determined as [10]:
Rk =
^ Pm ( + k2 ) ar
1/3
where RK radius of interfacial is film; Pm is maximal compression pressure; k, k2 are coefficients of elasticity of two drops; ar = axa2 / (ax + a2 ) is
an average size of the drops; a is two drops sizes.
In the research [9] equation for the hy-drodynamic compression pressure in the turbulent flow is defined as:
_ Pm=™2rPcU\
where U2 means square fluctuation rate of the turbulent stream, pc is density.
c) Thinning-down and rupture of the interfacial film with the following coalescence and agglomeration of the drops. Rupture of interface film helps to junction of the smaller droplets to the droplets of bigger size. It is important to note, that as a result of transport of the oil emulsion in the pipes the rate of breaking the droplets becomes much higher than the coalescence rate, so the oil emulsion is characterized by higher spread of droplets' sizes and by poly-dispersity of medium.
d) The sedimentation of the droplets and extraction of dispersed phase as a continuous phase.
Another important factor affecting the effectiveness of separation of oil emulsions are conditions of thinning and rupture of interface film [7] and the rate of coalescence associated with break-up of adsorbed film on the surface of the drops with demulsifies participation.
Therefore, the purpose of this work is researching formulas for calculating the effective viscosity of oil emulsions and affection of asphaltens on the formation of adsorptive films on the surface of water droplets.
Rheological properties of oil emulsions
The rheological properties of oil emulsions are determined by the effective viscosity, depending on the content of water droplets and other particles in it, as well as shear stress. Effective viscosity is the main indicator of rheo-logical fluids that determines their mobility, and in dispersed systems, it depends on shear stress t| = t/y, concentration, size and shape of particles. Where x is shear stress, 9 is volume fractions, y is velocity gradient, ^ is viscosity. The simplest expression, which reflects the dependence of viscosity on the particle content for unstructured laminar flow, is the Einstein equa-
tion = 1 + 2.59, suitable for extremely dilute media (9 < 0.01) with fine spherical solid
particles. In the literature [1-3] one can find many empirical formulas describing the dependence of the effective viscosity of dispersed systems on the content of particles in the stream, the enumeration of which would make up a whole bibliography. Many of these formulas take into account the concentration of particles corresponding to their dense packing. In the work [11] for calculating the viscosity of suspensions, the Barney and Mazrachi equations are proposed, which describe the experimental data quite well in a wide range of values 9:
= exp
1 - k2 9
(1)
Many of these formulas do not take into account the structure formation, which significantly distorts the viscosity curve. From the analysis of many experimental studies, it follows that the viscosity in the region of the onset of structure formation obeys a semi-empirical equation of the form
(2)
Here is the viscosity of the system without structure formation, — is the maximum deviation of the viscosity of the system. In the region of the beginning of structure formation, k is the coefficient. The solution to this equation will be presented as
ln - = - k (9-9, )2 + Q,
where the integration is constant determined from the initial conditions in the form C1= ln(-0--s). The final solution to this equation will be presented as
-0 -n
■exp
- k (9-9,)2
(3)
where ^s dynamic viscosity of the medium is, is dynamic viscosity of the drop. This expression determines the nature of the curve of
the dependence of viscosity on the volume concentration in the region of its deformation and further structure formation. Here 9ro is the volume fraction of particles corresponding to their dense packing, 9 the fraction of particles corresponding to the beginning of the formation of aggregates or the inflection point of a curve. The region of onset of structure formation is characterized by thixotropic properties, since, if the concentration of particles does not increase, the aggregates are destroyed. Below in Figure 2 the dependence of viscosity on the content of water droplets in the oil emulsion is shown [12] and it is described by the expression [13]:
- = (-11.6 + O.69) exp
- 7exp[-0.01(9- 60)2
2.89
(9-100 )2
(4)
From this formula it is possible to calculate the relative viscosity of the dispersion medium in the form ^ / ^ where the dynamic viscosity of the oil is ^ «14Pas. On these curves
areas of structure formation and intensive growth of viscosity are clearly visible.
In general, the generalized view of the dependence of viscosity on the volume fraction of particles can be represented as
f
m.9
\
(9-9œ)
- / — = 1 + 2.59 + A 9 exp
- A1exp (k (9-9, )2 ). (6)
The last term in this equation determines the start of structure formation in a dispersed system, i.e. deformation of the viscosity curve in the field of structure formation. If A1 = 0 in
equation (6), then the AC site is absent, what is typical for dispersed systems in the presence of a solid phase (Figure 2).
The value 9œ determines the condition for creating a dense packing of particles, and the value A1 is determined by the difference in dynamic viscosity with and without structure formati°n: A1 = (—- - ) / - .
Fig. 2. The dependence of the viscosity of oil on watering.
The value ^ corresponds to the concentration of particles at which structure formation begins. The region of the onset of structure formation in some cases is quite small, which is not visible on the viscosity curve. Then the last term in equation (6) can be omitted and the expression for viscosity is simplified. Viscosity of the dispersed system is very sensitive to the size and shape of particles.
Influence of asphaltenes on the separation of oil emulsions
Structurally-mechanical stability of emulsion systems is associated with the formation of adsorbed layers on the oil and water interface. These layers consist of asphalting, gums, paraffin's, mineral salts and solid particles i.e. of natural surface active substances [1-4]. It has been determined that metal-paraffin complexes lead to the formation of membrane itself and solid particles (sand, clay, limestone etc.), favor increase in the strength of membranes [3, 4]. Analysis of composition of these membranes on the surface of water drops in crude oil from different oilfields shows that the basic stabilizations are the asphalting and gums which consist of high-melting paraffin's and inorganic mechanical dirt. The structure, composition and physico-chemical properties of asphaltenes representing very complex compounds are described in works [14-16]. Initiation and formation of adsorption layer on the water droplets surface with elastic and vis-
cous properties lead to stabilizing of oil emulsions [16]. Therefore the stability of oil emulsions is the result of physical barrier which hampers to breaking of the film when the collision energy between droplets is not enough for the destruction of the adsorption layer. The mechanism of formation of the adsorptive films on the surface is determined by the set forth below stages.
a) Diffusive transfer of the substance mass (ashaltens) from the volume of oil to the surface of the water drops. In the work [17] the mass stream to the surface of moving drop in
unit time for small values Re =
Ua
v„
<< 1 is de-
termined as
Î4n ' D Л, "
и _ ar Л, + _
1/2
a? AC - U,
(7)
where is viscosity of the medium and
drop, D is coefficient of molecular diffusion, U is average flow velocity AC = C0 - C5, C0, C5 is the content of asphaltens and gums in volume and on the surface U is rate of the movement of droplet. Assuming that change of mass of spherical drop as a result of the formation of ad-sorptive layer is determined as dm/dt = I and
4%
m=Y%Pa
then thickness of the layer will be determined as
(R + A)3 - R3 « 4%R2Ape, A << R,
= 0.652
a„
I__1_
Pe 1 + y
1/2
A C
St,
(8)
where thickness of adsorption layer у = л^/л* ,
Pe = is Peclet number, St = U is Strouhal D ar
number, ar is an average droplet diameter, pa is
density of adsorptive layer, R is radius of droplets. From equation (2) it follows that thickness of adsorptive layer depends on diffusion of particles to the drop surface, size and mobility of drops surface as well as on the concentration of asphaltens in the stream volume. For the values Pe = 104 -105 (D *10-10-109) m2/s, у = 0.8, ACp * 10~5, St = 102 -103 from the equation we'll estimate A/a = 0.1 -0.15 . Large values
of the Pe number which are the result of the small values of the diffusion coefficient of the particles in the liquid, in some cases determine predominance of convective transport of the substance over diffusive transport. Further compaction of the adsorption layer under the influence of external disturbances and chemical transformations promote the layer's density increasing and ageing of the emulsions. In spite of the insignificant thickness of the adsorptive layer in comparison with the drop size, the strength
Г-108. g/cnr и r
of this layer on the surface of the drops for the different oils ranges within 0.5-1.1 N/m2 [3].
b) Substance adsorption on the surface of the drops.
c) Desorption and disruption of the ad-sorptive layer with surface-active substances participation. If adsorption and desorption rate is lover in comparison with a feed rate of the substance to the surface of the droplet then the adsorptive lower the formation process is limited by the adsorption and desorption processes. Assuming that concentration of the adsorbed substance in volume C0 and on the surface is r.
By analogy with a derivation of a Langmuir equation, assuming that adsorption rate of the substance on the surface of the droplet is equal to WA =PC0 (1 - T/r^) and desorption rate is equal to W = aT, then in the equilibrium, condition (W = W ) we have
T = KCo , (9)
1 + K oCo
where a, P are some constants which depend on temperature K = P/a , K0 =P/ar,, ^ is maximum saturation of the droplet surface. Equation (1) matches very good to many experimental data for the oils from the different oilfields.
cr%
Fig.3. The adsorption isotherm of asphaltenes on the surface of a water droplet.
Figure 3 shows the adsorption isotherms of asphaltenes (7=400C) on the surface of the water drops for North-Caucasian oils [16] and data calculated by the equation (9), where K=55, K°=0.5. For the adsorbed films break-up in the flow volume the different demulsifies (surface-active substances) which are characterized by high surface activity during adsorption are used. Adsorbed films break-up mechanism consists of diffusion transfer of demulsified to the film surface with further adsorption and penetration to the film volume, defects and cracks formation in its structure, change in surface tension and reduction of the strength properties which qualitatively changes the rheologi-cal properties of the films on the oil and water interface. Further oil emulsions separation is determined by the collision frequency of the drops, their fixing on the surface, thinning and break-up of the interfacial film.
Discussion of results and resume
This paper presents some aspects of modeling the separation of oil emulsions. The main phenomena in the processes of coalescence of water drops in the oil emulsions are the destruction of adsorption film on the surface, thinning and rupture of interface film. Highly concentrated disperse systems are classified as structured with typical rheological properties. The existence of various rheological models does not provide the possibility to select an unambiguous practical description of disperse systems, though this selection can be performed via the appropriate approximation of experimental data. It should be noted that the presented formulas of viscosity are not phenomenological expressions, but rather represent semi-empirical equations describing experimental data. Initiation and formation of adsorption layer is described by equation (8), comparison of which with experimental data gives satisfactory results.
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NEFT EMULSiYASININ AYRILMA PROSESiNiN MODELLO^DiRiLMOSi
Q.LKalbaliyev, V.LKarimli, Q.N.Hüseynov
Neft emulsiyalannin aynlmasi proseslarinin modellari va özlülüyün tayini ügün analitik ifadalar taklif edilir. Damla sathinda adsorbsiya qatinin qalinliginin qiymatlandirilmasi apanlmi§dir. Strukturla§dirilmi§ dispers sistemlar ügün özal reoloji xüsusiyyatlari ila ümumila§dirilmi§ özlülük tanliyi taklif edilmi§dir. Yapi§qanligin ba§langicinda özlülük ayrisinin bazi xüsusiyyatlari a§kar edilmi§dir. Strukturla§dirilmi§ dispers sistemlar ügün müxtalif özlülük tanliklarin variantlari nazardan kegirilmi§ va onlarin eksperimental malumatlarla müqayisasi apanlmi§dir.
Agar sözlar: birls§ms, qirilma, cökm3, yag emulsiyalari, strukturla§ma, reoloji, modelh§dirm3, özlülük.
МОДЕЛИРОВАНИЕ ПРОЦЕССОВ РАЗДЕЛЕНИЯ НЕФТЯНЫХ ЭМУЛЬСИЙ
Г.И.Келбалиев, В.И.Керимли, Г.Н.Гусейнов
Предлагаются модели и аналитические выражения для определения вязкости процессов разделения нефтяных эмульсий. Проведена оценка толщины адсорбционного слоя на поверхности капель воды. Предложено обобщенное уравнение вязкости для структурированных дисперсных систем с их своеобразными реологическими свойствами. Выявлены некоторые характерные особенности кривой вязкости в начале процесса структурирования дисперсной системы. Рассмотрены различные варианты уравнений вязкости для структурированных дисперсных систем и проведено их сравнение с экспериментальными данными.
Ключевые слова: слияние, разрушение, осаждение, масляные эмульсии, структурирование, реология, моделирование, вязкость.
