EXPERIMENTAL STUDIES ON DETERMINING MASS TRANSFER BETWEEN GAS AND LIQUID IN A BUBBLE ABSORPTION APPARATUS

ABSTRACT

This article presents the results of experimental studies on the mass transfer process in a bubble absorption unit. The research focused on the transition of H₂S from the gas phase to the liquid phase. The mass transfer coefficient was determined through theoretical calculations, and the results were compared with experimental data obtained under laboratory conditions. The calculations took into account the specific contact surface of ceramic packings and the gas-liquid interaction time. The correspondence between experimental results and theoretical predictions indicates that an accurate model of the mass transfer process was selected. The effectiveness of ceramic packings is confirmed by a significant reduction in H₂S content when they are used. The findings of this study have important practical implications for the design and optimization of bubble absorption units.

АННОТАЦИЯ

В данной статье изложены результаты экспериментальных исследований процесса массообмена в барботажной абсорбционной установке. В ходе исследований изучался процесс перехода вещества H₂S из газовой фазы в жидкую. Коэффициент массопередачи определялся с помощью теоретических расчетов, а полученные результаты сравнивались с экспериментальными данными в лабораторных условиях. При расчетах учитывались удельная контактная поверхность керамических насадок и время взаимодействия газа и жидкости. Соответствие экспериментальных результатов теоретическим прогнозам указывает на то, что была выбрана корректная модель процесса массообмена. Эффективность работы керамических насадок подтверждается значительным снижением содержания H₂S при их использовании. Результаты данной работы имеют важное практическое значение при проектировании и оптимизации барботажных абсорбционных установок.

Keywords: bubble absorption, mass transfer coefficient, H₂S absorption, contact surface, active working volume, concentration difference, contact time, gas velocity, liquid flow rate.

Ключевые слова: барботажная абсорбция, коэффициент массопередачи, поглощение H₂S, контактная поверхность, активный рабочий объем, разность концентраций, контактное время, скорость газа, расход жидкости.

Introduction.

This article presents the results of experimental studies on the mass transfer process in a bubble absorption device. Currently, washing columns, plate absorbers, and packed absorbers are widely used in oil refineries. In these devices, gas and liquid flows move in opposite directions, facilitating the mass transfer process. However, in such systems, the contact efficiency between gas and liquid is limited, and there are drawbacks, especially for pollutant gases at low concentrations. The bubble absorption device we studied provides a high degree of mixing by passing gas bubbles through the liquid. Due to the large contact surface area and effective structure of the ceramic nozzles, the transfer of H₂S into the liquid phase is carried out efficiently. The mass transfer coefficient was determined through both theoretical calculations and experimental methods, which laid the foundation for creating a reliable model of the device. This apparatus is not only highly effective but also offers technological and economic advantages. This article presents the results of experimental studies on substance exchange in a bubbling absorption unit.

Materials and methods:

The object of research was an experimental bubble absorber developed at the Department of "Technological Machines and Equipment" of Fergana State Technical University, based on the Far No patent. Experimental studies were conducted in block 49 of the 1st section of the Fergana Oil Refinery. When evaluating mass transfer processes, a calculation formula based on the results of theoretical studies was used to determine the mass transfer coefficient. [2,3].

Results and discussions

The following main parameters were selected for conducting experimental studies. Gas flow rates Q_g ranged from 0.15 to 0.55 m³/hour (with increments of 0.10 m³/hour) at constant values, and correspondingly, the gas velocities were ω_g = 0.017, 0.028, 0.04, 0.05, and 0.062 m/s.  The liquid supplied to the mixing zone had velocities ω_l ranging from 0.021 to 0.081 m/s (with increments of 0.03 m/s). The dimensions of the ceramic packing installed in the mixing zone were d = 9 to 15 mm (with increments of 3 mm). The volume of the support mesh for placing ceramic packing was V = 0.000369 m³, the total height of the apparatus was H_total = 1080 mm, and the gas cushion height at constant gas velocities was h = 12, 20, 24, 40, and 55 mm [4, 5]. In our apparatus, the gas and the absorbent flow in the same direction. Therefore, a schematic of an elementary mass transfer unit, in which mass transfer occurs between co-current phases, is considered.

Figure 1. Schematic diagram of the experimental setup	Figure 2. General view

The mass flow rates of gas and liquid relative to the interface separating the phases are denoted by G and L (kg/hour), respectively, while the concentrations of the dispersing substance are represented by y and x (kg/kg), respectively (Figure 3).[7]

Figure 3. Related to the equation of the material balance and the working line.

To calculate the overall mass transfer coefficient in the apparatus, we need to determine the total contact time of the gas and liquid phases and the amount of H₂S transferred from the gas phase to the liquid phase during a specific time. These indicators serve to evaluate the effectiveness of the mass transfer process and experimentally validate theoretical models. We began our preliminary research by installing a 9mm ceramic packing and determining the material balance.

                                                    (1)

When we conducted these processes using 12mm and 15mm expanded clay packing, the material balance (M) was equal to  and , respectively.

The efficiency of mass transfer between gas and liquid largely depends on the residence time (contact time) of the gas flow in the apparatus. Therefore, it is crucial to determine the contact time for various gas velocities in the apparatus. The contact time was calculated using the following formula:

                                                                         (2)

here:

t - contact time (in seconds),

H - apparatus height (m),

ω_g - gas velocity (m/s).

When we substituted the given gas velocity values into formula (2) to calculate the contact time between the gas and the liquid, we obtained the following results.

Table 1.

Contact times are determined depending on gas velocity

Figure 4. Graphical representation of the relationship between gas velocity and contact time

Based on the table results, a graph depicting the relationship between gas velocity and contact time was constructed (Figure 4). The graph clearly shows that as gas velocity increases, contact time decreases. This process demonstrates that the gas retention time in the apparatus is inversely proportional to the gas velocity.

In the next stage, we examined the change in purification efficiency about gas flow rate (Q_g). To determine the purification efficiency, the concentration of H₂S gas at the inlet and outlet of the apparatus was measured in laboratory No. 3 of the FOR according to GOST 22985-2017 standards. Under experimental conditions, the H₂S concentration in the gas stream entering the apparatus was X_in = 38,500 mg/m³, which constituted 2.5% of the total gas composition. The H₂S content in the exhaust gas was recorded as X_out = 0, 0.00025, 0.005, and 0.05 mg/m³, corresponding to different gas flow rate regimes. These data were processed using formula (3) to calculate the efficiency of the purification process. 0.005 and 0.05 mg/m³ were taken as values.

                                                              (3)

As a result of the experiments, with the installation of a ceramic nozzle with a diameter of 9 mm, the gas flow rate Q_g = 0.15; 0.25; 0.35; 0.45 and 0.55 m³/hour, respectively, the cleaning efficiency of the unit η = 100; 100; 99.9; 99.8 and 98%, respectively. These results showed a relative decrease in cleaning efficiency with an increase in gas consumption. When conducting experiments with a ceramic nozzle with a diameter of 12 mm, η = 100; 100; 98.6; 98 and 97.6%, respectively. Compared to a 9 mm nozzle, the efficiency was slightly lower, and at the maximum gas consumption value, η was approximately 97.6%. When removing a 12 mm diameter nozzle from the apparatus and replacing it with a 15 mm diameter ceramic nozzle, the efficiency decreases further with increasing gas consumption, and η = 100; 100; 97.6; 96.4 and 95.2%, respectively. All experiments were repeated five times for each gas flow rate value using nozzles with dimensions of d_n = 9, 12, and 15 mm, and the average results were recorded. In general, it was determined that an increase in the nozzle diameter negatively affects the cleaning efficiency of the apparatus, with this difference becoming particularly evident in high gas flow rate modes.

Figure 5. A graph showing the relationship between cleaning efficiency and gas consumption

The obtained regression equations

y=-4,2x+101,03      R² = 0,5759

y=-6,8x+101,22      R² = 0,9263

y=-13,2x+102,46    R² = 0,9453

Conclusion

The results of experiments conducted on a bubbling absorption unit installed in block 49 of the 1st section of the Fergana Oil Refinery showed that an increase in the diameter of the nozzle and an increase in gas consumption lead to a decrease in cleaning efficiency. This is explained by a reduction in contact time. The small-diameter nozzle provided the highest efficiency and the optimal gas-liquid contact time. In medium and large-diameter nozzles, the cleaning efficiency decreased significantly. The experimental results proved the necessity of choosing the optimal gas flow rate and nozzle diameter.

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