HYDRODYNAMICS OF A BUBBLE EXTRACTOR

ABSTRACT

The article proposes a new construction of a multistage bubble extractor working in complex heterogeneous systems. The structural structure and principle of operation of the device are given. The prospects of its industrial application are highlighted. As a result of theoretical studies, an equation was derived that calculates the value of the gas cushion, which is important for the equal distribution of inert gas to the device stages. Because the value of the gas cushion is important to ensure that the mixing zones in the contact devices located in the apparatus steps work in a hydrodynamic process of equal speed. This, in turn, ensures the efficient implementation of mass exchange processes at the hardware stages.

АННОТАЦИЯ

В статье предложена новая конструкция барботажного экстрактора работающего в сложных гетерогенных системах. Приведена конструкционная структура и принцип работы аппарата. Освещены перспективы его промышленного применения. В результате теоретических исследований получено уравнение, позволяющее рассчитать величину газовой подушки, которая важна для равномерного распределения инертного газа по ступеням аппарата. Потому что величина газовой подушки важна для обеспечения того, чтобы зоны смешения в контактных устройствах, расположенных в ступенях аппарата, работали в гидродинамическом процессе с равной интенсивностью. Это, в свою очередь, обеспечивает эффективную реализацию массообменных процессов на ступянях аппарата.

Keywords: bubbler, extractor, mixing zones, stage, inert gas, extraction, gas cushion, gas velocity, mass transfer.

Ключевые слова: барботаж, экстрактор, зоны смешения, ступень, инертный газ, экстракция, газовая подушка, скорость газа, массообмен.

Introduction

Today, liquid extraction processes are widely used in the chemical, oil refining, food, hydrometallurgical, pharmaceutical and other industries worldwide. Therefore, by applying the flow of inert gases to the mixing of liquid phases, it is important to create high-performance, compact, energy-efficient devices with high performance and extraction capacity [1,2].

Scientific research is being conducted in Japan to create new designs of highly efficient extractors for liquid extraction processes, to increase the contact surface of liquid phases and to accelerate the mixing process. In this regard, the use of compressed gas energy, which is chemically inert to liquids, the improvement of drop crushing and mass exchange models according to the physico-chemical properties of liquid phases, the reduction of extractant consumption and stability in the device stages, the reduction of the number of stages, high-performance metal and energy-saving, compact, various special attention is being paid to the creation of a new category of extractors capable of extracting liquids [1,2].

The main advantage of bubble extractors is the simplicity of its construction, as well as high technological reliability due to the absence of internal moving devices when mixing liquids [1,2].

Firstly, these devices can be prepared in the conditions of repair shops of enterprises, and secondly, they can be used in the extraction of extremely chemically toxic and dangerous liquids. In addition, due to the small dimensions of the device, scarce, alloyed metals are used less in its preparation.

The use of bubble extractors is relevant and can be easily achieved by changing the amount of inert gas supplied to the extraction process in multi-ton production [1,2].

Research object

Based on the above requirements, in the following years, we created a new construction of a bubble extractor with inert gas mixing [3,4]. Figure 1 shows the construction structure of the extractor [4].

The structure of the extractor is as follows. The extractor body 1 is divided into separate mixing and grinding steps by means of barriers 2. Mixing devices are made of inner 3 and outer 4 concentric nozzles, and the barrier is installed in 2. The nozzles 3 are installed on the barriers 2 in such a way that their lower parts 5 protrude from under the barriers 2 and serve as gas distribution nozzles through the holes 6 on the side wall. Also, heavy liquid discharge pipes 7 are installed on the barrier 2. Holes 8 are opened in the lower parts of these pipes. The upper part of the pipe 7 is closed with a cap 9. The cap 9 has a lower 10 and an upper hole 11. Gas pipelines 12 are installed on barrier 2. The top of these pipes is closed, and holes 13 are opened on its side walls for releasing inert gas, through these holes, the inert gas exits into the annular channel between the nozzles 3 and 4 of mixing devices. A special hole 14 is opened along the diameter from the lower part of the center of the hole 13, which is opened in the pipelines 4, which transmits gas to the ring channel, and a cover (filter) 15 made of fiber material, which absorbs heavy liquid well, is installed in it. Slots 16 are formed in order to ensure the flow of the heavy liquid, which has been quenched, to the next step when the pipes 4 are installed on the barriers.

Figure 2.1. A new design of the multi-stage bubble extractor

The extractor works as intended: Light liquid (L.L) enters the manifold 3 through the gas distribution nozzle 5. Heavy liquid (H.L) flows through holes 8 of pipes 7 to the same nozzles. The mixture of liquids moving together from bottom to top inside the tube 3 is intensively mixed with the help of bubbling inert gas entering through the holes 6 in the gas distribution nozzle 5. After mixing the liquids, this part of the gas is collected in the gas cushion under the barrier 2.

At the same time, the remaining part of the inert gas is discharged through the hole 13 opened in the gas transfer pipe 12 to the ring channels between the 3 and 4 pipes. This part of the inert gas moving from the bottom up in the annular channels passes through the flow of the liquid mixture moving from the top down. In this transition, it mixes the liquid flow and accumulates in the gas cushion under the barrier 2 s. In the process of the light liquid flowing through the holes 14 and the fibrous material (filter) 15 covered by the nozzle 4, the heavy phase drops with small particles are trapped in the fibrous material (filter) and as a result of their mutual addition, they become large droplets. turns and begins to sink under the influence of gravity and inertial forces. The light liquid, which is purified from the heavy liquid, continues to move to the top. Drops of heavy liquid settle in the lower part of the annular channel formed by nozzles 3 and 4 and flow out through the slits 16 in the lower part of nozzle 4 and form a homogeneous layer on the barrier 2.

Such installation of the pipe 4 on the barriers 2 ensures the maximum use of the volume of the annular channel, the passage of light liquid only through the holes 14 and the fibrous material (filter) 15 covered with them. The slots 16 in its lower part ensure the movement of heavy liquids only.

The heavy liquid that has settled in the barrier 2 flows through the hole 10 of the cap 9 to the pipes 7 and through the hole 8 falls to the lower step. The sizes of the holes 14 formed along the diameter of the pipe 4 and the fibrous material 15 covered with it are determined by the condition of ensuring a uniform flow rate of the light liquid. The dimensions of the slits 16, which ensure the flow of heavy liquid from the annular channel formed in the lower part of the nozzle 4, are also determined by the condition of flow without any resistance depending on the consumption of heavy liquid.

The results obtained:

The holes in the mixing zones, which are the contact elements of the gas cushion bubble extractor [4], depend on the gas velocities coming out of the holes 6, 13, and the diameters of the holes play a key role and are of great importance in creating a stable gas cushion. The device provides even distribution of gas to several bubble valves and multi-channels located in the tanks. This ensures the operation of the extractor stages in a normal and stable hydrodynamic mode. Gas cushion calculation equation V.N. Sokolov and Y.K. Gellis as proposed [5], Figure 2 shows the calculation scheme. Gas cushion “h” means the height from the center axis of the hole to the level occupied by the liquid.

Figure 2. Gas cushion calculation scheme

When designing the extractor, the diameter of the holes should be chosen in such a way that the inert gas supplied from the gas cushion to the internal and external mixing zones creates conditions for a hydrodynamic process of equal speed in these zones [5]. If Bernoulli's equation is applied to the sections I - I and II - II in order to find this height “h” then the pressure in the gas bag Р_с and the pressure inside the bubble tube Р_о depends on the total pressure in the sections.

              (1)

Here - the relative speed of the liquid in the section I-I  m/sec;

- the amount of gas in the liquid in the section.

 is the pressure loss at altitude, which is found using the following equation.

                                (2)

The pressure in the gas bag is represented by , and the pressure lost to overcome  the hydraulic resistance of the gas ports to the bubbler is represented by .

                                                 (3)

where  is the pressure lost to overcome the hydraulic resistance of the holes and is defined as [5].

                            (4)

Here  hole resistance coefficient;

 - static pressure of the fluid layer;

 - the radius of the gas hole in the bubbler nozzle.

If the equation (4) is put into the equation (3) and the necessary mathematical operations are performed, the following equation will appear.

                                        (5)

Putting this equation into equation (1), we can get the following equation.

                  (6)

 - the actual speed of the gas coming out of the hole, m/s;

The value of  is found by determining and  by means of experiments according to the above equations. After that, the corresponding empirical equations are determined. As a result, a calculation equation is derived that is sufficiently reliable in practice. But with such an approach, if the mechanism of gas leakage from the hole in the liquid layer is analyzed, some unreasonableness will arise [5].

According to Laplace's law, the pressure in a gas bubble with a spherical structure of radius  increases the pressure of the surrounding liquid by .

When the gas is sprayed through the hole, the maximum value of  is reached when a hemispherical bubble with a small radius  equal to the hole radius  is formed in front of the hole. In addition,  in all phases. Therefore, the ratio in equation (4) should be taken  into account only in the case of individual bursting of bubbles. For the case under consideration,  can be ignored in the expression consisting of  [5].

In the jet regime of the gas coming out of the hole, Laplace's law (4) states that  in the equation should not be taken into account.

From the studies carried out in the model of a multi-tube reactor, it is known that as the number of bubble tubes increases, so does the gas cushion. It can be explained like this. The pulsating passage of gas through different pipe openings is due to the vibrations of the upper level of the gas-liquid mixture in the apparatus.

In multi-pipe bubblers, the average actual velocity of the gas in the orifice, which determines the value of the gas cushion, is higher than the velocity calculated by the gas consumption. Research shows that such a difference depends on the number of bubble tubes in the hardware stages. For a high level of liquid in the apparatus, the law of averaging dynamic fluctuations comes into force (Fig. 3).

From this graph, it is possible to see the relationship between the pulping coefficient  and the number of bubble tubes  [5].

(6) - from the equation, the height of the gas cushion  is obtained by performing the necessary mathematical operations.

                                     (7)

Here  - the resistance coefficient of the hole in the bubble tube;  - consumption speed of gas passing through the hole, m/sec;  - gas density kg/m³;  - pulpation coefficient,  - liquid density, kg/m³; g - free fall acceleration, (9.8 m/sec²); - density of the mixture, kg/m³.

Figure 2.2. Pulpsation coefficient change graph

This equation is the gas cushion calculation equation proposed by V.N.Sokolov and Yu.K.Gellis and can be applied to the device we are examining. For this, it is necessary to take into account the speed of the gas passing through the hole, which is distributed only to the internal mixing zone of the apparatus, and the geometric pressures in the center of the holes that distribute gas to the internal and external mixing zones. The geometrical pressures acting on the centers of the gas transfer holes to the mixing zones of the extractor vary with height  (Fig. 2).

If it's , it's . Accordingly, geometric pressures are also  [1]. (fig. 2).

Since the mixing zones of the apparatus are in the form of a contiguous container, taking into account the ratio of the geometric pressures in the centers of the holes and the distribution of gas on both sides, multiplying the two into the equation (7), through the speed of the gas passing through the hole supplying gas to the internal mixing zone of the apparatus, the total value of gas consumption is given only to the internal mixing zone the value of the gas bag can be calculated.

That is, the ratio of pressures in the mixing zones of the apparatus.

                          (8)

If we multiply this ratio and two to the equation (7), the equation will look like this.

  (m)                    (9)

Here - the amount of gas in the fluid moving in the external mixing zone;  - height of external mixing zone, (m);  - the amount of gas in the fluid moving in the internal mixing zone;  - internal mixing zone height.

Using this equation, it is possible to determine the gas velocity passing through the gas transfer hole to the internal mixing zone in the gas distribution elements of the bubble extractor. In the tested apparatus, there is an additional mixing zone, and it is not possible to determine the speed of the gas passing through the gas transfer hole to this zone. Experimental studies are conducted to determine this. In this case, separate experiments are carried out in the holes that transmit gas to the internal and external mixing zones, and the values of the gas content in the gas cushion and the mixing zone are determined. From the difference of the gas cushion, the gas costs distributed to both sides are determined. Depending on the gas consumption, the gas velocities coming out of the holes are calculated.

Conclusion

The constructional structure and principle of operation of the device were presented in the article. The prospects of its application in industry were highlighted. As a result of theoretical studies, the equation for calculating the value of the gas cushion, which is important for the equal distribution of inert gas to the equipment stages, was derived based on certain laws. As a result, an opportunity was created to determine the value of the gas cushion to ensure that the mixing zones in the contact devices located in the apparatus stages work in a hydrodynamic process of equal speed. This, in turn, ensures the efficient implementation of mass exchange processes at the hardware stages.

References:

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