Bubbling is a low energy consumption operation to enhance mass and heat transfer processes, which is widely used in many fields such as chemical industry and environmental protection.Bubble operation has simple equipment, easy control of operating conditions, convenient cleaning and replacement, and can be realizedautomationCompared with other salt drying methods, it is easier to achieve.
seawater desalination technology As an effective means to solve the shortage of freshwater resources, it has been widely used in coastal and island areas.After seawater desalination, a large amount of concentrated seawater will be produced, which contains rich magnesium, potassium, bromine, lithium, etcMineral element。The direct discharge of concentrated seawater not only has a certain impact on the marine environment, but also wastes a lot of valuable resources.Concentrated seawater is the most direct way to extract marine resources, and can reduce the adverse impact of direct discharge on the environment.Seawater salt production can be divided into electrodialysis method and beach drying method according to the principle.Seawater beach drying salt production process is mature, simple and low in operation cost, but it has the disadvantages of low land utilization rate, being greatly affected by the weather, laborious salt farmers and low profits.Strengthening the evaporation process of concentrated seawater can not only increase salt production, but also enhance the resistance to climate impact.At present, many technologies are under research, including falling film evaporation method, solar chimney method, ultrasonic method, artemia methodCarbon nanotubesLaw.However, these methods have great limitations.
Bubbling is a low energy consumption operation to enhance mass and heat transfer processes, which is widely used in many fields such as chemical industry and environmental protection.Bubbling operation has the advantages of simple equipment, easy control of operating conditions, convenient cleaning and replacement, and automation, which is easier to achieve than other salt drying methods.Tao Hengcong and others tried to soak the concentrated seawater in Liuheng Island, Zhoushan (Figure 1), and found that the daily evaporation of seawater by the bubbling method was 1.5~2.25 times that of traditional salt drying.In addition, the bubbling evaporation of concentrated seawater needs to select appropriate climatic conditions. The higher the sunshine intensity is, the lower the environmental humidity is, and the faster the evaporation rate is.Usually, the time from 12:00 to 15:00 every day is the best time for blistering and salt drying, while the effect of blistering and salt drying is poor in other times.120mtwoA maximum of 240kg of water can be evaporated per hour in the concentrated seawater bubbling salt drying experiment pool, which is about 120kg more than the evaporation volume in the traditional salt drying control pool.[1]
Figure 1 Structure of test pool
Factors influencing bubbling efficiency
Announce
edit
Bubbling process of concentrated seawater shoal is the process of bubbles from generation to rupture, which can be divided into three stages according to the movement state: bubble rising, bubble floating and bubble rupture.In the rising stage, bubbles not only take away part of the water vapor, but also affect the floating and distribution of bubbles on the liquid surface.In the floating stage, the drift of the bubble group not only increases the gas-liquid contact area, but also causes bubble coalescence and rupture.In the rupture stage, the number of small droplets has a significant impact on the evaporation process.In fact, in traditional bubbling equipment, bubble size is a key parameter that determines the magnitude of various interphase forces between water flow and bubbles. Experimental studies show that different bubble sizes will produce differences in aspects such as gas holdup distribution, turbulent structure, etc.In addition, the diameter of the bubble will affect the number of droplets generated after the break.
For this reason, the above processes and influencing factors were comprehensively studied, and the operating conditions such as liquid level, bubble opening spacing, single hole gas volume, etc. were optimized.In this paper, bubbles with a diameter greater than 5mm are called large bubbles, and those with a diameter less than 5mm are called small bubbles.In addition, this paper uses a self-made bubbling generator whose structure is similar to that of sintered plate, but the number of air outlets in a single generator cannot be determined, so it is stated in this paper that the gas volume is used instead of the apparent gas velocity.
1. Factors affecting the distribution of floating bubbles
The bubbled bubbles are hemispherical when floating on the liquid surface, and their gas-liquid mass transfer area is twice the covered area.It can be seen that the increased evaporation area of bubbles floating on the surface of the bubbling pool is one of the main factors to increase the evaporation of concentrated seawater.However, not only the bubble size will affect the evaporation area, but also the bubble depth, the interference of two adjacent bubbles and the gas distribution in the bubble array will significantly affect the bubble coverage area on the liquid surface.
(1) Distribution law of single bubble group
The experiment found that the large bubbles were dispersed after a single vertical floating up to the liquid level in turn, while the small bubbles that were bubbling out at the same time were scattered like chrysanthemums in the floating stage, so there was no small bubble distribution on the liquid level directly above the small bubble generator.
With the increase of gas volume, the coverage area of large bubbles and small bubbles on the liquid surface increases under the same operating conditions.At the same time, it can be seen that the influence of depth change on coverage area is more significant than that of gas volume change.When the gas volume is the same, the coverage area of both large bubbles and small bubbles increases with the rise of the bubbling mouth depth. However, when the bubbling mouth depth of large bubbles is greater than 12 cm and the bubbling mouth depth of small bubbles is greater than 8 cm, the coverage area increases slowly.However, under the similar coverage area, the air volume required by large bubbles is almost 5 times that of small bubbles, so small bubbles are more suitable for the bubbling process from the perspective of air volume.Since the small bubbles begin to diverge from the coalescence state after the liquid level is higher than 8cm, the depth of the small bubble bubbling pool is 8cm.
(2) Distribution of adjacent bubble groups
The interaction between bubbles from two adjacent bubbles will affect the flow and rupture of bubbles, thus affecting the coverage of bubbles on the liquid surface.In this paper, two self-made small bubble generators are set at a depth of 8cm, at 0.8L · min-1Adjust the bubble opening spacing under the air volume, and set the bubble opening at a depth of 12cm, at 4L · min-1Adjust the bubble opening spacing under the air volume, and investigate the influence of the hole spacing on the bubble coverage area. When the spacing is 2cm, due to the mutual influence of the bubble groups from the two bubble openings, the convergence phenomenon occurs between the two bubble groups, and the bubble group coverage area is at the lowest value.The coverage area of large bubbles and small bubbles increases with the expansion of bubble opening spacing.When the hole spacing of large bubbles is expanded to 14cm, the bubble groups from the two bubbles are independent of each other, and the coalescence phenomenon between bubble groups no longer occurs. Later, with the expansion of the spacing, the bubble coverage area no longer increases.When the distance between two adjacent holes of small bubbles is expanded to 12cm, the coverage area of bubbles no longer increases with the increase of the distance.To sum up, large bubble double hole bubbling at 4L · min-1Under the air volume, the hole spacing should be set as 14cm;Small bubble double bubble mouth at 0.8L · min-1Under the air volume, the optimal hole spacing is 12cm.
3. Distribution of Bubbles in Bubble Array
When the depth of bubbling mouth is 8 cm and the spacing between bubbling mouths is 12 cm, the average pore gas volume of large bubbles and small bubbles is 0.1, 0.4, 1, 2 L · min-1The coverage of bubbles.When the average bubbling mouth gas volume is 0.4L · min-1The coverage of air bubbles is only 0%~60%, and there is a large gap between air bubbles.In comparison, except that the small bubbles are not covered above the bubble mouth, other areas are completely covered, with a coverage rate of 80%~90%, and the bubbles are densely arranged and stacked.When the gas volume rises to 1L · min-1After that, the coalescence and renewal rate of bubbles is accelerated, and the fluctuation of liquid level is more intense. The coverage rate of large bubbles is reduced to 30%~40%, and the diameter of small bubbles is increased, and the coverage rate is reduced to less than 80%.When the gas volume drops to 0.1 L · min-1The coalescence and renewal rate of bubbles decreases, the fluctuation of liquid level slows down, the coverage of large bubbles decreases to 20%~30%, and the coverage of small bubbles decreases to less than 70%.From the perspective of increasing the coverage rate of salt field liquid level, small bubble groups are more suitable for the bubbling process, and the gas volume is 0.4L · min-1Is most appropriate.
2. Sputtering of liquid droplets
During blistering and salt drying, rainbow phenomenon was observed from a specific angle below the level of the bubbling pool, which is less than 40cm high.This phenomenon indicates that a large number of tiny droplets are ejected from the bubble during the crushing process.During the experiment, the measured wind speed above the salt pond was basically maintained at level 3-4. By calculating the data of the reduction of the liquid level in the salt pond and the rise of Baume degree, it showed that the ejected droplets were basically not blown away by the wind.Bubbles eject a large number of small droplets, making the surface evaporation become three-dimensional evaporation.
Through the estimation of the experimental data of Liuheng Island, it is considered that the contribution of air blowing volume to the evaporation volume is less than 10%, and the contribution of bubble floating volume to the evaporation volume is about 50%. It is speculated that the remaining 40% includes the contribution of droplet sputtering.
(1) Sputtering Phenomenon of Liquid Droplets
Figure 2 shows that the gas flow is controlled at 2L · min-1The image of small droplets splashed out by bubbles from a 1mm single bubble mouth floating after the liquid surface bubble is broken.
Fig. 2 Image of sputtered small droplets
It can be seen from the figure that the trajectory of the liquid drop in the air is parabolic. The length of the parabola can be seen that the sputtering speed of the liquid drop is obviously different. The longer the parabola is, the faster the speed is, and vice versa.In addition, the sputtering angles of droplets are different, which are affected by the bubble breaking point, liquid level disturbance, wind force, wind direction and other factors.It can be seen from Figures (b) and (c) that the thickness of the moving track line of the droplet varies greatly, indicating that the diameter of the ejected droplet is different.The droplet sputtering height is mainly concentrated in the range of 10-20 cm, and very few of them reach more than 20 cm.The rainbow phenomenon observed during salt drying appears below 40 cm, because the bubble diameter can reach 20 cm during salt drying, and the maximum bubble diameter in a small bubble tank does not exceed 5 cm.It is inferred that the sputtering height may be related to the bubble size.
(2) Influence of bubble size on the number of droplets
Obviously, the more droplets sputtered in the bubbling pool per unit area, the more conducive to improving the evaporation rate. In order to investigate the impact of bubble size on the number of droplets, a bubbling mouth is set 10 cm below the liquid level of the bubbling pool. By changing the structure of the bubbling mouth, the average diameters of bubbles are adjusted to 2, 10, and 15 mm, respectively,And adjust the air volume to control the coverage area of bubbles on the liquid level to about 100 cm2.When the bubble diameter is 2 mm, the maximum number of small bubbles is generated. The number of droplets ejected from the burst of these small bubbles is far more than the number of droplets ejected when the average diameter of the bubble is 10 mm and more than 15 mm. The number of droplets ejected from the burst of the bubble when the average diameter of the bubble is 10 mm is more than the number of droplets ejected when the average diameter is more than 15 mm.Under the same coverage area, the number of small bubbles is far more than that of large bubbles, and the life of small bubbles is short, and the renewal rate is faster than that of large bubbles.At the same time, small bubbles cover 100cmtwoThe air volume required for the liquid level of is 0.4L · min-1, while large bubbles cover 100cmtwo2 L · min-1。Therefore, small bubbles can significantly increase the number of droplets sprayed and improve the evaporation rate.In addition, the size of the bubble also affects the diameter of the liquid drop.
(3) Analysis of the Influence of Small Bubbles on Evaporation Process
Compared with atmospheric bubble groups, small bubble groups require higher pressure, lower air volume, wider distribution of bubbles on the liquid surface, shorter life of small bubbles, and significantly increase the number of small droplets released, which will have a reasonable effect: small bubble groups can more drive the circulation of surrounding liquid, because the liquid level depth of the salt field is generally not more than 10cm,The height diameter ratio is very different from that of the traditional bubbling device. At present, there is little research on the circulation in the shallow pool bubbling process at home and abroad. Through experimental observation, it is found that although the liquid level in the bubbling process is only 8cm, the small bubbles can appear chrysanthemum shaped distribution in the rising phase, and the small bubbles are more widely distributed on the liquid surface,It shows that the small bubbles group obviously drives the circulation of the surrounding liquid during the rising process, and the circulation promotes the expansion of the small bubbles to a wider range. The generation of the circulation can strengthen the convective heat transfer process for the bubble evaporation process of concentrated seawater shoals.
During the sputtering process, the small droplets exchange material and energy rapidly with the air, absorb a lot of heat from the air, and release a lot of water vapor, which will lead to the heat absorbed in the evaporation process of concentrated seawater mainly comes from the heat of high-temperature ground air rather than direct solar energy.The influence of ambient air temperature, humidity and circulation speed on the bubble evaporation process of concentrated seawater will become the main influencing factors.[1-2]
