Shuo Yang,Jilong Zhang,Jiaxing Xue,Qingpeng Wu,Qunsheng Li,Hongkang Zhao,*,Liqun Zhang

1 State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China

2 China Nuclear Power Engineering Co., Ltd, Beijing 100840, China

3 College of Mining Engineering, Taiyuan University of Technology, Taiyuan 030024, China

4 Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, China

Keywords:Flow-guided hole Spray mode Hydrodynamics Mass transfer Calculation model

A B S T R A C T Distillation is the most common separation technology utilized in the petroleum and chemistry industries. Due to the wide usage of the distillation column, even a small improvement in performance may result in significant energy cost savings.Aiming to improve the hydrodynamics and mass transfer performance, the flow-guided trapezoid spray-packing tray (FTS-PT) was designed by combining flow-guided holes and trapezoidal caps with structured packing. And the experimental measurements of the FTSPT, including pressure drop, clear liquid height, weeping, entrainment, and tray efficiency, were conducted in a 500 mm diameter plexiglass column with the air-water-oxygen system. Moreover, the performance of the FTS-PT was compared with that of new vertical sieve tray (New VST) and F1 valve tray.The results show that FTS-PT has a significant advantage in pressure drop,entrainment,and capacity.Furthermore,the calculation model of the pressure drop was derived and used for the FTS-PT with a relative deviation of less than 5%.

1. Introduction

Distillation, as the most common separation technology, takes up a considerable proportion of the industrial separation process[1-3]. The packed column and tray column are the two main equipment in the distillation process and have been the focus of research. Considering the wide usage and high throughput of column setup, even a small optimization or improvement in design can bring great significant energy cost savings. Therefore, the column internals, packing and tray, have been the key and focus of research [4].

Tray column has been widely used in petroleum,chemical,biological, pharmaceutical, and other fields because of its simple structure,anti-blocking,high flexibility in operation,simple installation[5].The tray is the mass transfer internal of the tray column whose technical level directly affects the production capacity,production cost,efficiency,and energy consumption of the separation system.On the other hand,column packing also has its own advantages. For instance, packing can offer a high surface and void fraction, which is beneficial for better mass transfer efficiency.Therefore,a combination of tray and packing is an important route to design a new column internal with higher efficiency and larger capacity.For instance, combined packing-trays were designed and utilized in commercialized columns of absorber and stabilizer for high efficiency [6]. Kachuret al. [7] also proposed the structured packing tray, which alternately installed a thin slice of structured packing and sieve tray plate throughout the whole column. Furthermore, Gaoet al. [8,9] proposed the SiC Foam Monolithic Tray,which is made of Silicon carbide (SiC) foam material similar to packing, and is also a novel tray with excellent performances.

As a typical vertical tray,new vertical sieve tray(New VST)was developed firstly by Mitsui Engineering & Shipbuilding in the 1960s[10].The standard New VST uses cylindrical caps with circular sieve holes of the same size distributed on the cap. It has a higher mass transfer efficiency and greater operating flexibility.However, its cap structure will cause pressure holding and back mixing when the gas velocity is high. Then, based on the New VST, the combined trapezoid spray tray (CTST) was designed and used in the sour water stripper in three refineries. After the replacement of valve trays by the CTST trays, the capacity was double and the steam consumption was reduced by more than 11% [11].

Compared with conventional trays, such as bubble cap tray,valve tray, sieve tray and so on, New VST makes full use of the space above the tray, and enhances the two-phase contact and mass transfer so that it has higher efficiency and larger capacity[12].Some researchers in China also designed a series of new trays based on the New VST. However, their comprehensive performance had not been greatly improved [13-16].

In this work, based on the New VST, the flow-guided trapezoid spray-packing tray (FTS-PT) was designed by combining flowguided holes, trapezoidal caps with structured packing. Here,structure packing was introduced into the vertical tray in order to improve its hydrodynamics and mass transfer performance[17,18]. In a 500 mm diameter plexiglass column with an airwater-oxygen system, the hydrodynamics and mass transfer performance of FTS-PT was explored and then compared with that of New VST and F1 valve tray. Furthermore, the calculation model of the pressure drop for FTS-PT was derived and verified using the above experiment data.

2. Structure Description of FTS-PT

The schematic diagram FTS-PT is shown in Fig. 1. The main structural features of the tray are summarized as follows:

(1) There is a bubble promoter[19]with a height of 6 mm at the inlet of the liquid phase. It aims to reduce the liquid level gradient between the inlet of liquid and the overflow weir so that liquid can quickly contact with the gas as soon as it enters the tray, thus eliminating the upstream non-active zone.

(2) The direction of flow-guided holes [20-22] is consistent with the liquid flow direction. The gas blown from the flow-guided holes forms concurrent flow with the liquid on the tray, pushing the liquid flow forward and reducing the liquid level gradient.Thus,the pressure drop is reduced.At the same time, the number and arrangement of these flow-guided holes can be adjusted according to the specific situation to ensure the flow velocity uniform in all areas of the tray.

(3) The cap of FTS-PT has a unique trapezoidal-rectangular vertical structure,which can effectively promote the liquid liftup and increase the gas-liquid interfacial area,therefore the weeping is greatly reduced and the mass transfer efficiency is improved.

(4) The upper part of the cap is open,which solves the problem of pressure-holding and decreases the pressure drop.Mellapak 250 structured packing is installed to increase the gasliquid contact area, reduce the entrainment and improve the mass transfer efficiency. The structure parameters of the experimental tray are listed in Table 1.

3. Experimental Setup

The hydrodynamic performance of FTS-PT, including clear liquid height,dry/wet pressure drop,weeping,and entrainment,were tested with an air-water system. And the mass transfer performance was tested with an air-water-oxygen system. The experiment was carried out in a plexiglass tower with a diameter of 500 mm, which contained three trays from top to bottom: foamcaptured tray, experimental tray and gas distribution tray. The parameters of the tower are listed in Table 2. The tray spacing and diameter of the tower is enough for the experimental tray.

Table 1 Structure parameters of experimental tray

Table 2 Parameter of the experimental column

Fig.2 depicts the process of the experiment.The water from the water storage tank enters the bottom of the oxygen absorption tower by the pump, and then fully contacts with oxygen. The air from the air blower is evenly distributed from the bottom of the tower to the middle of the tower through the gas distribution tray,while the oxygen-rich water enters from the top of the tower to the top of the experimental tray.The gas-flow rate and liquid-flow rate are regulated by orifice flowmeter(uncertainty of 0.001 m·s-1)and rotameter (uncertainty of 0.01 m3·m-2·h-1) respectively. Air is in contact with oxygen-rich water on the experimental tray, and the oxygen dissolved in water was removed by air. The pressure drop is tested by the U-tube differential manometer (uncertainty of 0.01 Pa). The height of the clear liquid layer is measured using the principle of the connector. Three graduated communicating tubes (uncertainty of 0.1 mm) are respectively connected to the liquid phase inlet,outlet and the arcuate area of the tray.The average of the height of the clear liquid layer can be obtained by reading the scale of the communicating tubes. Weeping is collected through the flexible pipe connected to the column bottom and led out to the hold-up vessel outside the column. Entrainment is collected through the flexible pipe connected below the foamcaptured tray and led out to the hold-up vessel outside the column.Both weeping and entrainment are measured by electronic scale(uncertainty of 0.001 g) and stopwatch (uncertainty of 0.001 s)using the gravimetric method. The mass transfer efficiency of the experimental tray is calculated by oxygen analysis method, and the oxygen content in the water is measured by the dissolved oxygen meter (uncertainty of 0.001 mg·L-1). More detailed information of the experiment section is provided in our previous literature [19-21].

Fig. 1. Structural representation of FTS-PT. (unit: mm)

Fig. 2. Schematic diagram of the experimental setup

4. Results and Discussion

4.1. Pressure drop

Pressure drop is one of the important hydrodynamics parameters to evaluate the performance of the tray [23,24]. It refers to the energy loss produced by the fluid passing through the tray,including dry-plate pressure drop and wet-plate pressure drop[25].

4.1.1. Dry-plate pressure drop

When only gas flows through the tray,the energy loss caused by the structure of the tray is called dry-plate pressure drop[26].The trapezoidal cap of FTS-PT consists of a trapezoidal gas cylinder and structured packing. The dry-plate pressure drop of FTS-PT mainly comes from rectangular plate holes, sieve holes, the interface between the packing and gas cylinder, and the packing.

Lai [27]studied the dry-plate pressure drop of New VST on the basis of the fundamental of hydrodynamics. The parameter model is expressed as follows:

where ζ is the resistance coefficient,u0is the velocity of gas passing through holes in the absence of liquid.

Whether the gas passes through a rectangular plate hole or sieve hole,the dry-plate pressure drop is generated by the condensation and expansion of the gas passage. Therefore, both of them can be combined to make a derivation.At the same time,the interface between the packing and gas cylinder is regarded as many sieve holes to calculate the dry-plate pressure drop. Thus, the dry-plate pressure drop of the rectangular plate hole, sieve hole of the cap and the interface between the packing and gas cylinder can be written as:

where subscript t indicates the trapezoidal cap of FTS-PT,Atis the area of the rectangular plate holes,AS+Pis the sum of the areas of the sieve holes and the interface between the packing and gas ascending cylinder.

The packing used on the cap of FTS-PT is Mellapak 250Y,whose dry-plate pressure drop is generated by the gas passing through the corrugated passage and can be written as [28]:

Since the interface between the packing and the gas ascending cylinder is regarded as sieve holes, the gas velocity through the packing can be expressed as:

The dry-plate pressure drop for the trapezoidal cap of FTS-PT can be obtained as follows:

In addition, FTS-PT is composed of caps and flow-guided holes,so the dry-plate pressure of flow-guided holes should be considered.Hugh mark-O’Connell correlation was often used to calculate the dry-plate pressure drop of the sieve tray [29]:

where φ is the area fraction of sieve holes in the opening area of a tray,C0is the orifice flow coefficient. If the calculation formula of the dry-plate pressure drop for the sieve tray is used to calculate the dry-plate pressure drop of the flow-guided sieve tray,there will be a deviation.Zhouet al.[30]proposed a correction coefficientkto correct the deviation based on the principle of equal pressure drop.The formula forkis as follows:

where φ′is the ratio of the opening area of the flow-guided holes to the total opening area. When only flow-guided holes exist, there is φ′=1. The dry-plate pressure drop formula of the flow-guided holes can be expressed as:

whereC0is the orifice coefficient, which can be obtained by Ref.[29].

The flow-guided holes and the trapezoidal caps on the FTS-PT are in parallel. In the case of ignoring the gravitational potential energy of air, the dry-plate pressure drop of flow-guided holes is equal to the dry-plate pressure drop of trapezoidal caps according to the Bernoulli equation. Thus

Therefore, the dry-plate pressure drop of FTS-PT cannot be calculated by the sum of the two parts. We can try to calculate the dry-plate pressure drop of FTS-PT by coupling the dry-plate pressure drop of the two single mass transfer element trays. The dryplate pressure drop of the packing on the upper part of the caps is smaller than the total dry-plate pressure drop of the caps. It can be approximated that the dry-plate pressure drop of the caps is proportional to the square of the pore gas velocity.The following results can be obtained:

whereK0,Kfg,Ktis the total resistance coefficient of FTS-PT, flowguided hole and trapezoidal cap respectively.

According to the law of conservation of mass, the Eqs.(11) and(12) is expressed.

whereA,Afg,Atrepresents the total opening area,the plate hole area of the caps and the flow-guided holes respectively.The Eq.(13)can be obtained by simultaneous Eqs. (10)-(12).

where αfgis the ratio of the opening area of flow-guided holes to the total opening area of the FTS-PT,αtis the ratio of the opening area of plate hole of the caps to the total opening area of the FTS-PT.

According to the Eq.(13),the dry-plate pressure drop of FTS-PT can be written as:

whereuTis the empty tower gas velocity of the FTS-PT, φ is the opening ratio of FTS-PT. When the size of the trays is same, the empty tower gas velocity of the tray with a single mass transfer element and the composite tray is equal, so there are:

whereu0,FGandu0,Trespectively represent the pore gas velocity of the tray consisting only of flow-guided holes and the pore gas velocity of the tray consisting only of trapezoidal caps.φFGand φTrepresent the opening ratios of the above two types of trays,respectively.For the tray with a single mass transfer element, the dry-plate pressure drop is also proportional to the square of the pore gas velocity and the total resistance coefficient remains the same.

Uniting Eq. (14), Eq. (15), Eq. (16) and Eq. (17), we can obtain:

where φfgand φtrespectively represent the opening ratios of the flow-guided holes on FTS-PT and the opening ratio of the rectangular plate holes below the trapezoidal caps on the FTS-PT.

When φ=φFG=φT,the dry plate pressure drop of the FTS-PT can be calculated as:

ζtcan be measured by experiment and its value is 0.852.

The dry-plate pressure drops calculated by the model were compared with the experimental value, as shown in Fig.3. In general,the relative deviation between the calculated and experimental values of dry-plate pressure drop for FTS-PT was within 5%,which indicated that the derived model was credible. WhenFT＞4, the deviation increases significantly. This is because the gas distribution becomes more heterogeneous with the increase of the gas load and the influence of non-uniform velocity on the model is more obvious. The model derived in this paper is based on the average gas velocity, so the non-uniformity of gas velocity will lead to the deviation of the model to be larger under the condition of high gas velocity.But it is acceptable in industrial applications.It was feasible to calculate the dry-plate pressure drop of the composite tray by coupling the dry-plate pressure drop of trays with a single mass transfer element. The dry-plate pressure drop of FTS-PT was compared to the New VST and F1 valve tray, as shown in Fig. 4. The dry-plate pressure drop of FTS-PT was about 20% lower than that of New VST, and about 70% lower than that of F1 valve tray.

Fig.3. Comparison of dry pressure drops between experimental and model for FTSPT.

4.1.2. Wet-plate pressure drop

Fig.5(a)showed that wet-plate pressure drops of FTS-PT under different gas-liquid phase loads.To sum up,the wet-plate pressure drop of FTS-PT increased as the liquid phase load increased and presented two stages with the increase of gas phase load. When the gas load was small, the wet-plate pressure drop increased rapidly with the increase of gas load. When the gas-phase load was large,the wet-plate pressure drop in the middle section would appear a gentle or even decreasing trend when the liquid flow strength was small. The clear liquid height increased with the increase of the liquid flow strength, and the static pressure difference between the inside and outside of the trapezoidal caps also increased,which led to the increase of the liquid quantity entering the trapezoidal caps. Therefore, both the resistance of gas passing through the liquid layer on the plate and the energy consumption for lifting the liquid in the caps increased, resulting in an increase in the wet-plate pressure drop. When the gas-liquid contact state changed from bubble state to a foam state, the clear liquid height would decrease rapidly, so that the liquid layer resistance on the plate decreased greatly. At this time, if the liquid load was small,the decreased degree of the liquid layer resistance was large,which might be equal to or even greater than the increased amplitude of dry-plate pressure drop.Thus,the wet-plate pressure drop showed a mild change or a slight decrease trend.

Fig. 5(b) showed that the comparison of wet-plate pressure drop between FTS-PT,New VST and F1 valve tray.It was clear that the wet-plate pressure drop of FTS-PT was lower than that of New VST and F1 valve tray, which was more obvious when the gas phase load was larger. Pressure drop is the energy loss produced by the fluid passing through the tray and the pressure drop of FTS-PT is lower than that of New VST and F1 valve tray.Therefore,the FTS-PT has an advantage in operation consumption saving.

4.2. Clear liquid height

The clear liquid height is also named liquid holding capacity on the tray,whose change reflects the stability of the gas-liquid mass transfer process to a certain extent [31,32]. In addition, it will directly affect the pressure drop, weeping, entrainment, and tray efficiency. Studying the clear liquid height can better control the fluid flow characteristics on the tray and is helpful for the research and development of the tray [33].

Fig. 6 showed that clear liquid height of FTS-PT under different gas-liquid phase loads.It can be seen that the clear liquid height of FTS-PT increased as the liquid-height load increased,and increased rapidly to the maximum value at first and then decreased slowly with the increase of the gas phase load.The larger liquid flow rate,the more liquid entering the tray, and the more liquid stayed on the tray. Therefore, the clear liquid increased with the increase of liquid flow strength. When the gas phase load was small, the kinetic energy of the gas passing through the tray was too small to compete with the gravity of the liquid.A large amount of liquid fell from the holes on the tray,resulting in a large amount of weeping. At this time, there was less liquid on the plate, and the clear liquid height was very small. With the increasing of the gas phase load,the gas kinetic energy increased gradually,the weeping began to decrease rapidly, and the clear liquid height increased rapidly.When the gas phase load was further increased, the weeping had almost disappeared,and the clear liquid height gradually increased to the maximum value and then decreased. There are several reasons accounting for the phenomenon. Firstly, the increase of gas velocity will increase the lifting amount of liquid in the caps,thereby reducing the clear liquid height. Secondly, the flowguided holes on the plate has the function of pushing the liquid forward. When the gas velocity is large, the pushing action of the flow-guided holes is enhanced, the residence time of liquid on the plate is shortened and the liquid quickly enters downcomer,thereby reducing the clear liquid height.Thirdly,with the increase of gas velocity, the gas-liquid flow regime gradually evolves from the bubbling regime to emulsion regime, foam regime and even the spray regime, the number of bubbles in the liquid layer increases, and the boundary of liquid layer is blurred. The continuous phase also changes from liquid phase to gas phase, and the foam layer becomes thicker,resulting in a decrease in the clear liquid height.

Fig. 4. Comparison of dry-plate pressure drop between FTS-PT, New VST and F1 valve tray under the same experimental measurement conditions.

Fig. 5. Wet-plate pressure drop of FTS-PT: (a) wet-plate pressure drop of FTS-PT under different gas-liquid phase load; (b) comparison of wet-plate pressure drop between FTS-PT, New VST and F1 valve tray when L = 11.43 m3·h-1·m-1.

Fig. 6. Relationship between clear liquid height and gas-liquid phase load.

Fig. 7. Weeping of FTS-PT:(a) weeping of FTS-PT under different gas-liquid phase load; (b) comparison of weeping between FTS-PT, New VST and F1 valve tray when L = 11.43 m3·h-1·m-1.

Fig. 8. Entrainment of FTS-PT: (a) entrainment of FTS-PT under different gas-liquid phase load; (b) comparison of entrainment between FTS-PT, New VST and F1 valve tray when L = 11.43 m3·h-1·m-1.

4.3. Weeping

When the power provided by the gas passing through the holes on the tray does not compete with the liquid phase gravity, the phenomenon that the liquid falls to the next tray is weeping.Serious weeping can cause abnormal operation of the tray [34,35]. In industry,the weeping rate was usually not allowed to exceed 10%.

Fig.9. Tray efficiency of FTS-PT:(a)tray efficiency of FTS-PT under different gas-liquid phase load;(b)Comparison of tray efficiency between FTS-PT,New VST and F1 valve tray when L = 8.57 m3·h-1·m-1.

Fig.7(a)showed that the weeping rate of FTS-PT under different gas-liquid phase loads. The changing trend of the weeping rate of FTS-PT with the increase of liquid phase load presented two stages.When the gas phase was small, the weeping rate decreased with the increase of liquid phase load, and the difference was great.When the gas phase load was large, the weeping rate increased with the increase of the liquid phase load, but the difference was small,and all of them were less than 10%.According to the definition of weeping rate, the weeping rate is the ratio of weeping to inflow. When the gas phase load was small, the increase of weeping with the increase of liquid phase load was less than that of the liquid phase load, which resulted in the decrease of weeping rate with an increase of liquid phase load. When the gas phase load was large, the weeping rate of different liquid phase flow rate began to show a great difference. The increase of weeping with the increase of liquid phase load exceeded the increase of the liquid phase flow rate, resulting in the increase of weeping rate with the increase of liquid phase load. But the weeping phenomenon was not so obvious at this time,so the difference was small.When the liquid phase load was constant, the weeping rate of FTS-PT decreased with the increase of gas phase load.The rate of decrease was faster at low gas velocity and slower at high gas velocity. The reason is that the kinetic energy provided by the gas phase was not enough to resist the gravitational potential energy of the falling liquid at low gas velocity, resulting in more weeping, and the weeping rate varied greatly with the gas phase load. When the gas velocity was large, the kinetic energy supplied by the gas phase increased and the weeping decreased greatly. At this time, the weeping rate had basically decreased to less than 10%, and the change of the leakage rate with the gas phase load was no longer obvious.

Compared with the New VST and F1 valve tray, as shown in Fig. 7(b), the weeping rate of FTS-PT was lower than New VST,and higher than F1 valve tray but quickly decreased below 10%.

4.4. Entrainment

When the gas phase load was large,the gas would wrap the droplets up to the upper tray, which was named entrainment. When the entrainment was too serious, it would also affect the tray efficiency and normal operation of the column[36,37].In industry,the entrainment rate was not allowed to exceed 10%.Fig. 8(a)showed the entrainment rate of FTS-PT under different gas-liquid phase loads. It was apparently that entrainment rate of FTS-PT was mostly below 10%,indicating that FTS-PT has a large upper operating limit.For the FTS-PT,entrainment comes from droplets carried up by the gas in the flow-guided holes and trapezoidal caps. The structured packing and separation plate on the upper part of caps could effectively intercept some droplets, thus reducing the entrainment. Compared with the New VST and F1 valve tray, as shown in Fig. 8(b), FTS-PT had a much lower entrainment rate.

4.5. Tray efficiency

Tray efficiency,namely Murphree efficiency,reflects the performance of gas-liquid mass transfer on the tray [38]. It was determined by the oxygen desorption method, as expressed in Eq. (20).

whereEMLis tray efficiency, %;Xiis the oxygen content in the import solution, mg·L-1;Xois the oxygen content in the outlet solution, mg·L-1;Xo* is the dissolved oxygen saturation concentration in the water at the measured temperature, mg·L-1.

Fig. 9(a) showed the tray efficiency of FTS-PT under different gas-liquid phase loads. The tray efficiency decreased as the liquid phase load increased because the oxygen desorption process was controlled by liquid phase. Compared with the New VST and F1 valve tray, as shown in Fig. 9(b), the tray efficiency of FTS-PT was close to that of new VST and was more than 20% higher than F1 valve tray.

5. Conclusions

In this work,the flow-guided trapezoid spray-packing tray(FTSPT) was designed by combining flow-guided holes, trapezoidal caps with structured packing. And then its performance was systematically investigated. Compared with New VST and F1 valve tray, the newly designed tray has better performance. The conclusions were obtained as follows:

(1) In terms of pressure drop,the FTS-PT performed better than New VST and F1 valve tray. The dry-plate pressure drop of FTS-PT is lower than that of New VST by 20.85% and significantly less than that of the F1 valve tray. In addition, the wet-plate pressure drop of the FTS-PT has a more obvious advantage over that of New VST and F1 valve tray,especially at high gas load.

(2) According to the mechanism of the dry-plate pressure drop,a model of dry-plate pressure drop for combined tray was proposed by coupling the dry-plate pressure drop of trays with a single mass transfer element. The model was used for the calculation of dry-plate pressure drop of FTS-PT, the relative deviation between the calculated and experimental values was within 5%.

(3) The weeping of FTS-PT is close to that of New VST, while slightly heavier than that of F1 valve tray. The huge holes on the deck of FTS-PT and New VST are the main cause of weeping.

(4) The entrainment of the FTS-PT is much lower than that of New VST and F1 valve tray.The result indicates that the separation plate and structure packing can effectively reduce entrainment.

(5) The tray efficiency of FTS-PT is close to that of new VST and higher than F1 valve tray by about 20%.

In general, in the design of FTS-PT, the combination of a flowguided hole,trapezoidal cap and structure packing not only solved the problem of high pressure drop common in the spray trays but also reduced the weeping and entrainment, which greatly increased the operational performance.

Acknowledgements

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by National Natural Science Foundation of China (22008004) and Fundamental Research Funds for the Central Universities (ZY2017).

Nomenclature

Subscript

fg flow-guided hole

t the trapezoidal cap of FTS-PT