METHODS FOR MEASURING VISCOSITY OF PETROLEUM PRODUCTS

ABSTRACT

In this work, methods for measuring the viscosity of liquids are studied. When non-Newtonian fluids flow through a porous medium, the topology of the pore space results in a wide range of flow rates and shear rates. Existing approaches estimate the effective viscosity by first calculating the effective shear rate, mainly by adopting a power-law rheology model and incorporating an empirical correction factor [1]. The viscosities of liquids for standard solutions have been established.

АННОТАЦИЯ

В данной работе изучены методы измерения вязкости жидкостей. Когда неньютоновские жидкости текут через пористую среду, топология порового пространства приводит к широкому диапазону скоростей потока и скоростей сдвига. Существующие подходы оценивают эффективную вязкость, сначала вычисляя эффективную скорость сдвига, в основном путем принятия степенной модели реологии и включения эмпирического поправочного коэффициента [1]. Установлены вязкости жидкостей для стандартных растворов.

Keywords: viscosity, capillary, viscometer, Stabinger, traceability, metrological control, Newtonian fluid, accuracy, vibration viscometers, standard solutions.

Ключевые слова: вязкость, капиллярный, вискозиметр, Штабингера, прослеживаемость, метрологический контроль, Ньютоновская жидкость, точность, вибрационные вискозиметры, стандартные растворы.

Viscosity is the ratio of shear stress (F⁄A) to the velocity gradient (∆v_x/∆_z or 〖dv〗_x/d_z) in a fluid. The usual form of this relationship, called Newton's equation, states that the resulting displacement of a fluid is directly proportional to the applied force and inversely proportional to its viscosity. The similarity to Newton's second law of motion (F=ma) should be obvious [2].

Numerous methods for accurately measuring viscosity, as discussed earlier, are essential in many fields and have been developed over the years. Typically, those methods involve a trade-off between accuracy, ease of use, and suitability for certain types of fluid or flow. For the sake of brevity and simplicity, this paper describes and discusses various types of viscometers without resorting to mathematical complications. Further discussion of their types, advantages, operating modes and accuracy is also not sufficiently discussed [4,5].

In terms of accuracy, viscometers can be divided into primary and secondary, where the former is considered more accurate than the latter. However, the underlying equations and corrections for secondary viscometers are incomplete or not well known. Alternatively, viscometers can be classified into six types based on their geometric shape. This includes capillary, rotational, oscillatory, incident and vibrating wire viscometers [6,7].

Capillary viscometer:

Capillary viscometers were one of the first widely used methods in viscometry. They are able to obtain viscosity from two different approaches: absolute and relative. Absolute mode is used if the physical constants of the viscometer's mathematical equation can be sufficiently calculated without resorting to a calibration procedure. The relative approach is used in contrast, when these constants can only be accurately determined using liquids of known viscosity, i.e. calibration fluids.

However, viscosity can be measured when the volumetric flow rate Q of a known volume of liquid is measured by causing that quantity to travel a certain distance through a capillary tube for a known pressure drop. Its main equation is [5]:

                     (1)

Where: a is the radius of the pipe, ∆P is the pressure drop across the tubes, p is the density of the liquid, n and m are correction factors, L is the length of the tube.

The above equation was tested by Poiseuille in 1846 and the following assumptions [8]:

• the capillary is straight and has the correct cross-section (1);
• the liquid is incompressible, therefore its density is considered constant (2);
• the liquid is Newtonian (3);
• liquid temperature is stable (4);
• the type of fluid flow must be laminar (4);
• there is no slipping on the pipe walls (5).

In a real measurement, it may not be possible to verify some of the above assumptions, which is why the necessary corrections must be applied. It is interesting to note that Swindells et all adopted wastewater viscosity, the only primary standard, after the determination in an absolute capillary viscometer with an accuracy of ±0.1 to 0.25%.

Figure 1. Glass capillary viscometers and thermostatic bath

This class of viscometers can also be used in relatively extreme conditions, such as high pressure and high temperature, provided that special additional instruments to those used in normal conditions and the necessary precautions are considered. Moreover, its availability, low cost, ease of use and structure are other advantages. However, they do have some disadvantages; for example, large quantities of liquid are required, time consuming, a number of calibration liquids are required to cover the desired viscosity range, measurements cannot be taken continuously and are difficult to automate. Its applicability is limited to Newtonian fluids. Sometimes it is desirable for online viscosity data to be obtained, for example during pipeline operation, which a capillary viscometer obviously cannot support.

Rotational viscometer

The operating principle of a rotational viscometer is based on the fact that when a rotor (such as a cylinder, disk or cone) is immersed in a liquid and rotates at a constant speed, the surrounding liquid causes a torque on its surface. If such a torque can produce steady rotational motion, the viscosity of the fluid can be considered to be directly proportional to the torque. The operating equation that relates torque to viscosity depends primarily on the geometry of the viscometer.

Figure 2. Brookfield RVDV-E rotational viscometer

There are many types of rotational viscometers. However, they are most suitable for studying non-Newtonian fluids, since the shear stress can be constant throughout the fluid sample. This consistency in shear stress is a major advantage over capillary viscometers. Rotational viscometers require a small sample and do not require large volumes [4-8].

Viscometer with oscillating body

The basic idea of ​​an oscillating body viscometer is that when a suspended axisymmetric body is immersed in a fluid medium and then put into free harmonic oscillatory motion, the body will exhibit a torque on its surface because the fluid resists its motion. As a sequence of this resistance, a series of oscillations of decreasing amplitude will be observed, where the amplitude of each complete oscillation will be less, but a constant part of the previous one. Therefore, the logarithm of the period will differ from each other by a constant amount, known as the logarithmic increase.

Figure 3. SVM 3001 Stabinger viscometer

This effect can be used as a measure of viscosity. In other words, viscosity can be related to both the period of oscillation and the logarithmic decrement and can be calculated using reliable working equations. This type of viscometer has been successfully used in both gases and oils, under various temperature and pressure conditions. Although this type of viscometer is very easy to construct mechanically and its accuracy is difficult to select mathematical processing. However, an approximate mathematical interpretation can be obtained, but it is necessary to take into account some correction conditions, such as, for example, correction of the outer boundary, second flow and variations in fluid compressibility due to pressure changes [7, 8].

Falling ball viscometer:

A falling body viscometer can be used to measure the viscosity of a liquid. By determining the time of free fall of a shaped body, travel over a certain distance, where this time is directly related to the viscosity of the liquid. In other words, the more viscous the fluid, the longer it takes the body to travel that distance. A simple schematic illustration is shown in Figure 4.

Figure 4. Simple sketch illustrating a falling ball viscometer

According to Stokes' law, if a ball falls slowly through an incompressible Newtonian fluid of infinite extent and constant speed v, the viscosity η of the fluid can be found from the following equation:

                             (2)

Where g is the constant of free fall, R is the radius of the body, t is the time of fall over a distance L and ρ_s is the density of the sphere ρ is the density of the liquid.

An additional requirement is that the Reynolds number, , is much less than unity [5] and which are two practical obstacles preventing the applicability of the above equation and hence the use of this type of viscometer [9]. The first obstacle is that it is difficult to obtain a Reynolds number less than one unless small spheres are used in a highly viscous environment, meaning that viscosity measurements will be limited to a small range. Although the second obstacle is satisfactorily solved by considering the cylindrical configuration, the correction factor associated with the operating equations is a major disadvantage [8]. Other disadvantages are that a falling body in a viscometer cannot continuously measure the viscosity of a liquid, T, such as when the viscosity changes due to changes in temperature or pressure [4] and is not generally recommended for non-Newtonian fluids [7]. However, it is generally most suitable for high pressure measurements and fairly viscous materials [8].

Vibration viscometers:

Vibrating wire viscometers are different from oscillating body viscometers. Viscometers in that their oscillatory motion is transient, while the oscillatory motion of a body is rotational [10]. When a tight wire immersed in a liquid medium is set in motion by an electromagnetic field. At its first resonant frequency, the width of its resonance curve may be related to the viscosity of the liquid [4,11]. If a buoyant mass is attached to one end of a wire, the fluid density can be calculated using theoretical operating equations [4,12,13]. Thus, such a device can be used to measure viscosity and density simultaneously. An important feature of this device is that its operating equations are complete under certain achievable assumptions due to the fact that these equations were strictly developed [11]. Thus, such a device, unlike other types of viscometers, does not require boundary corrections. It also does not require volumetric movement of a body or liquid and can be used for various 24 liquid phases, such as mixtures and gases [4]. In addition, it is a simple mechanical design and can be automated and operated under extreme temperature and pressure conditions [4]. However, the vibrating wire viscometer is commercial as it is a very specialized instrument and hence is used in several research institutes around the world [9]. Electromagnetic coupling is used to drive a wire into transient oscillatory motion and simultaneously record the induced voltage due to its motion.

Figure 5. Vibrating viscometer (viscosity analyzer) SV-10 AND

This kind of coupling can be achieved by placing a wire perpendicularly in a uniform magnetic field and then applying alternating current to the wire. Thus, the electromotive force due to the interaction of the current in the magnetic field moves the wire in a transverse direction around its rest. If current is continuously applied to the wire with a constant amplitude but at different frequencies, the viscometer operates in the frequency domain [4]. Alternatively, another mode of operation, transient mode, can be used where current flows briefly and then switches off. Consequently, the damping time of oscillations due to the viscosity of the liquid is equal to the 25th measure of viscosity, where the more viscous the liquid, the shorter the damping time [4,14].

Table 1.1.

Comparative data of the main types of viscometers

*K.V - capillary viscometer;

*R.V - rotational viscometer;

*V.Ko - viscometer with an oscillating body;

*V.P.Sh - Viscometer with a falling ball;

*V.V - Vibration viscometers.

According to Article 11 of the Law “On Metrology” [15] and the Presidential Decree [16] the Uzbek National Institute of Metrology carries out scientific research in the field of metrological activities and has a sufficient research and material and technical base for the creation of standard solutions of established viscosity in order to create analogues of standard solutions in the world. As a result, in recent years, we have created standard solutions that meet the properties of standard solutions and international analogues. For this purpose, well-known oils were selected, which are produced in the Republic of Uzbekistan, containing mainly paraffin hydrocarbons above C20, and thus, based on motor oil and gas condensate products, liquid samples were produced that comply with ISO 17034 [17], meeting the requirements of international standards. At the same time, based on experiments carried out using the above viscometers, it was established that the resulting solutions are completely suitable for establishing viscosity and for comparative results of the existing more than 200 chemical laboratories involved in measuring the viscosity of oils produced in the Republic of Uzbekistan. Experiments have shown that such oils are completely produced in the Republic of Uzbekistan and the resulting compounds can be produced in large quantities and meet consumer requirements. In this case, capillary glass viscometers of the Ubbelohde type, Stabinger viscometer SVM 3000 and others were used. It was shown mainly the presence of functional groups, for example, carboxyl, carbonyl and hydroxyl groups interfere with the establishment of viscosity and this was more overestimated for such compounds. Therefore, the main emphasis was placed only on hydrocarbons, since, based on such compounds, it is easy to obtain compositions with the same viscosity, and they are fast-flowing. The research results are shown in Table 1.2.

Table 1.2

Характеристики полученных стандартных растворов

CONCLUSION

Analyzing the above information, we can conclude that the criteria by which the viscosity device, among those discussed earlier, should be selected depending on several factors (Table 1.1). Although the accuracy of the instrument was not considered among the above criteria due to its dependence on other factors, vibrating and oscillating are assessed by precision viscometers, but capillary viscometers are considered to be the most accurate than other viscometers [18-20]. It has been shown that standard solutions can be obtained based on hydrocarbons; due to their rapid fluidity, it is easy to establish their equal viscosity.

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