Analysis of Degassing Efficiency of Vacuum Deaerator

Abstract: At present, the performance test and efficiency evaluation of deaerators are performed on-site. The field test conditions are relatively limited. It is difficult to analyze the factors that affect the performance of the deaerator based on the test results. It is not accurate to use traditional theoretical analysis. To predict its outgassing performance. To this end, the CFD technology is used to study the internal flow field of the vertical vacuum deaerator, analyze the factors affecting the degassing performance, and evaluate the degassing efficiency. The results show that the inclination and area of the separation umbrella have a greater impact on the deaerator, and reasonable design of the inclination and area can not only achieve high degassing efficiency, but also reduce the volume of the deaerator; a multiphase flow mixing model is used to quantify The prediction of the degassing efficiency of the deaerator, the reasonable design can make the efficiency of the vertical vacuum deaerator reach more than 90%.

Key words vacuum degasser numerical simulation flow field degassing efficiency

Degasser is one of the key equipments in drilling fluid processing system. It is used to remove the gas from drilling fluid to stabilize the performance of drilling fluid. There are many types of degasser used in oil and gas field drilling, such as vacuum type, atmospheric type and centrifugal type. The most widely used and the best effect is the vacuum type deaerator. Among them, the vertical type vacuum deaerator is widely used at home and abroad due to its small size and small footprint. Its working process is: start the vacuum pump to generate a vacuum in the degassing tank, the drilling fluid is sucked in through the suction pipe under the action of the vacuum, and flows into the degassing tank through the umbrella piece. The gas in it is exposed to the surface of the drilling fluid under vacuum, and is pumped away by the vacuum pump through the top of the degassing tank. Drain to the system. The flow of drilling fluid in the degassing tank is relatively complicated, so that there are many factors affecting the degassing efficiency. Traditional theoretical analysis cannot accurately predict its degassing performance. Laboratory testing is also very complicated and the cost is very high. Research. At present, domestic performance testing and efficiency evaluation of deaerators are performed on-site. The field test conditions are relatively limited, and it is difficult to analyze the factors affecting the performance of the deaerator based on the test results. To this end, the research on the performance and efficiency evaluation of the vertical vacuum deaerator was cooperated. The CFD technology was used to study the internal flow field of the vertical vacuum deaerator, analyze the factors affecting the degassing performance, and evaluate the degassing efficiency. The author's research results are of great guiding significance for the design and use of vertical vacuum deaerators.

1 Model establishment

1.1 3D computing model and model discretization

The parameters of the separation umbrella determine the degassing performance of the vacuum deaerator, so the separation umbrella is the object of numerical simulation research. According to the structure of the separation umbrella, the overcurrent part is represented by the solid body, and the other parts are represented by the virtual body. The calculation model of the deaerator separation umbrella is shown in Figure 1. When simulating the flow of gas invasion drilling fluid on the separation umbrella, in order to reduce the amount of calculation, one of the umbrella pieces can be used for simulation. The model diagram is shown in Figure 2.

Calculation model of separation umbrella for vacuum deaerator

Figure 1 The calculation model of the separation umbrella

Calculation model of single umbrella piece of vacuum deaerator

Figure 2 Single Umbrella Calculation Model

In three-dimensional problems, tetrahedral mesh, hexahedral mesh, pyramid mesh, and wedge mesh, or a mixture of two meshes can be used to mesh the calculation model. Because the shape of the calculation area is more complicated, I use a hybrid grid.

1.2 Boundary conditions and physical parameters of the medium

The vacuum degasser works mainly by sucking fluid by the degree of vacuum in the degassing tank. Therefore, the inlet boundary is the pressure boundary, and the outlet boundary is also the pressure boundary, and the pressure is the degree of vacuum in the degassing tank. The condition of non-slip solid wall is adopted, and the fluid flow near the solid wall is determined by the standard wall surface function. The working medium is a drilling fluid containing a certain amount of gas, the gas integral is 18% [1], the bubble diameter is 0.8mm, the liquid dynamic viscosity is 0.06Pa · s, and the liquid density is 1 200kg / m 3.

2 Numerical simulation analysis of gas-liquid two-phase flow in vacuum deaerator

The fluid flow on the separation umbrella can be considered as turbulent flow, and the turbulence model adopts the RNG κ-ε model. This model is derived from strict statistical techniques.It is similar to the standard κ-ε model, but has the following improvements [2]: The RNG model adds a condition to the ε equation, which effectively improves the accuracy; the RNG model considers turbulence Vortex also improves the accuracy in this respect; RNG theory provides an analytical formula for the turbulent Prandtl number, while the standard κ-ε model uses a user-provided constant; the standard κ-ε model is a high Reynolds number model The RNG theory provides an analytical formula that takes into account the flow viscosity of low Reynolds numbers, and can calculate the effects of low Reynolds numbers.
The fluid in the deaerator is a liquid-gas two-phase flow, and the multi-phase flow model uses a mixed model. The mixed model allows phases to penetrate each other. The volume fraction of a control volume depends on the space occupied by the phases. The concept of slip velocity is used to allow phases to move at different speeds. The mixed model solves the continuity equation of the mixed phase, the momentum equation and energy equation of the mixed phase, the volume fraction equation of the second phase, and the algebraic expression of the relative velocity. Applications of mixed models include multi-phase flows such as low-load particle loading, bubble flow, sedimentation, and cyclonic separation. The numerical calculation uses a separate implicit solution method. The momentum equation is discretized using a second-order upwind difference format, the volume fraction calculation is discretized using the Quick format, and the SIMPLE algorithm is used to couple speed and pressure.
The flow field in the deaerator mainly includes the velocity field, the pressure field, and the distribution of the volume fraction of each phase. At the same time, the degassing efficiency can be calculated by counting the density values of the inlet and outlet. The author takes the separation umbrella flow field distribution of the deaeration tank vacuum of 0.03MPa as an example to introduce its characteristics.
(1) Pressure distribution performance: There is a significant pressure difference between the inside and outside of the slurry tube, so that the liquid flows through the slurry holes to the separation umbrella under the effect of the pressure difference. The larger the vacuum degree in the deaerator, the larger the pressure difference, and the larger the flow rate when the slurry pore size is constant.
(2) Velocity distribution performance: The velocity near the outlet of the slurry separation hole is relatively large, and the liquid on the separation umbrella sheet flows downward along the inclined separation umbrella under the effect of gravity, so the speed is very small. In the work, it is hoped that the liquid flowing out from the slurry separation hole first fills the space at the top of the separation umbrella, and the separation umbrella separation holes are multiple small holes.
(3) The distribution of gas-liquid integral numbers in the separation umbrella: In the space between the two layers of separation umbrellas, the uppermost part is gas, the middle part is gas-liquid mixture, and the bottom part is liquid with little gas content. It can be seen that under the action of the separation umbrella, most of the gas has been removed from the liquid film, and the remaining liquid film contains very little gas, thereby achieving the purpose of degassing. Further research found that the liquid flowing out of the slurry separation hole near the slurry separation pipe directly flushed to the upper back of the upper separation umbrella, so the gas-liquid interface formed in the separation umbrella space facing the slurry separation hole was not Obviously, in other areas where the slurry holes are not opposite, a good gas-liquid interface is formed, and the two phases of gas and liquid are well separated.

3 Analysis of factors affecting degassing efficiency

The statistics of outgassing efficiency can be calculated according to formula (1) [3], that is, η = ρl-ρiρ0-ρi × 100% (1)
Where ρ0 --- the original liquid density;
ρi --- inlet gas invasion density;
ρl ———— the medium density of the liquid outlet.

3.1 Influence of the angle and diameter of the deaerator separation umbrella on the degassing efficiency

The angle and diameter of the separation umbrella determine the area of the separation umbrella. Numerical simulation analysis shows that under the same area of the separation umbrella, the smaller the separation umbrella inclination angle, the more obvious the degassing effect. The author takes a smaller inclination angle (20 °) to analyze the influence of the separation umbrella area on the degassing efficiency. Fig. 3 is the relation curve between the separation umbrella area and the degassing efficiency. It can be seen from Figure 3 that as the area increases, the outgassing efficiency continues to increase. When the area of the separation umbrella is increased from 2.00m 2 to 2.50m 2, the degassing efficiency is significantly improved; when the area of the separation umbrella is increased from 2.50m 2 to 3.00m 2, the gas removal efficiency is improved, but it is not obvious. From this, it can be judged as follows: when the area of the separation umbrella is less than 2.50m 2, the bubbles cannot completely escape from the liquid film, so the degassing efficiency is very low; when the area of the separation umbrella is greater than 2.50m 2, the bubbles in the liquid film have ample time Overflow, most of the gas can be removed at this time, so the degassing efficiency is relatively high. When the area of the separating umbrella is further increased, the degassing efficiency will not change significantly, and the larger the area of the separating umbrella will increase its diameter, which will cause the volume of the deaerator to be too large. The inclination angle of the separation umbrella also has a large effect on the separation efficiency. When the area of the separation umbrella is constant, the larger the inclination angle is, the larger the flow velocity of the liquid on the separation umbrella is and the smaller the diameter is.

Relation curve between separation umbrella area and degassing efficiency of vacuum deaerator
Figure 3 Curve of the relationship between the separation umbrella area of the vacuum deaerator and the degassing efficiency

However, the residence time of the fluid on the separation umbrella is shortened, and the outgassing efficiency is also reduced. Under the condition that the area of the separation umbrella is constant (2.50m 2), the influence curve of the inclination angle on the degassing efficiency is obtained (see Figure 4).

Curve of the relationship between the inclination of the separation umbrella of the vacuum deaerator and the degassing efficiency

Figure 4: Curve of the relationship between the inclination of the separation umbrella of the vacuum deaerator and the degassing efficiency

From Figure 4, it can be seen that under the condition of a certain separation umbrella area, as the inclination angle increases, the degassing efficiency gradually decreases. When the inclination angle increases from 25 ° to 30 °, the degassing efficiency decreases rapidly. Smaller inclination angles ensure higher outgassing efficiency. Of course, decreasing the inclination angle will increase the diameter of the separation umbrella and increase the volume of the deaerator.

3.2 Influence of vacuum degree of vacuum deaerator on degassing efficiency

When the diameter of the slurry hole is constant, the greater the degree of vacuum, the greater the flow through the slurry hole, which affects the degassing efficiency. The author analyzed the influence of vacuum degree on the throughput and separation efficiency under the conditions of a certain separation umbrella area (2.50m 2) and inclination angle (25 °) (see Figure 5 and Figure 6).

Relation curve between vacuum degree and throughput of vacuum deaerator

Figure 5 Relationship between vacuum and processing capacity

As the treatment volume increases, the outgassing efficiency will decrease. When the degree of vacuum is 20.0 ~ 25.0kPa, the degassing efficiency is higher; when the degree of vacuum is 25.0 ~ 30.0kPa, the degassing efficiency decreases rapidly. From Fig. 5 and Fig. 6, it can be judged that the maximum processing capacity of this model is more suitable when the maximum processing capacity is 4.7 m 3 / min. This processing capacity can meet the displacement requirements of most drilling pumps [1], and the corresponding vacuum is 25.0 kPa. When the throughput is less than or equal to 4.7m 3 / min, the degassing efficiency can be more than 85%.

Relationship between vacuum degree of vacuum deaerator and degassing efficiency

Figure 6 Relationship between vacuum degree and degassing efficiency

3.3 Influence of slurry distribution holes on degassing efficiency

Because the slurry separation tube has a certain height, the height of the slurry separation holes in each layer is different. Under the same vacuum degree, it is necessary to ensure that the processing amount of each layer is the same, and the pore size of each layer is different. The simulation calculation compares the case where the pore diameter of each layer is different and the same. When the diameter of the slurry hole in each layer is different, the degassing efficiency fluctuations between the layers are small, and the total efficiency is 3.5% higher than that in the same layer, but the difference is not large.

4 Conclusion

(1) The author uses numerical simulation methods to study the performance of vacuum deaerator, including the influence of structural parameters and the quantitative prediction of degassing efficiency. The inclination and area of the separation umbrella have a greater impact on the deaerator. Reasonably designing the inclination and area can not only achieve high degassing efficiency, but also reduce the volume of the deaerator.
(2) With the pressure condition as the boundary, the influence of the vacuum degree on the degassing efficiency is simulated. The higher the vacuum degree, the better, which is consistent with the actual conclusion of field use.
(3) The multi-phase flow mixing model is used to quantitatively predict the degassing efficiency of the deaerator. A reasonable design can make the efficiency of the vertical vacuum deaerator reach more than 90%.
references
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