Hongkang Zhao ,Lun Li,Baohua Wang ,Dan Yu ,Qunsheng Li,*

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

2 Sinopec Engineering Incorporation(SEI),Beijing 100101,China

3 College of Chinese Pharmacy,Beijing University of Chinese Medicine and Pharmacy,Beijing 100102,China

Keywords:Packing tray Hydrodynamics Tray efficiency High capacity

A B S T R A C T Column setup has been widely utilized in the petroleum and chemistry industries.However,with the fast progress of industries and the increasingly serious energy shortage,designing a new column setup with better performance and higher capacity becomes more urgent.In order to improve column's capacity and expand operating condition,a new type of column tray named novel vertical spray packing(NVSP)tray was designed and experimented.The performances of the novel tray,including pressure drop,weeping,entrainment and tray efficiency,were tested in a plexiglass column.In addition,performances of the novel tray were compared with that of the sieve tray and the Glitsch V1 valve tray.Based on the experiment data,the mathematical correlations of pressure drop,weeping and entrainment for the novel tray were established by regression analysis method.A fundamental model of dry pressure drop of the novel tray was promoted at the form of sum mode.The results indicate that the novel tray has wider operating condition and better performance.

1.Introduction

Column setup is the most widely used separation device in the petroleum and chemistry industries[1].Due to the wide usage of column,even a small improvement in design and application can lead to significant energy and cost savings[2].Therefore,the new column setup with better performance has become the goal of many efforts[3].

Tray and column packing have their own advantages.Combination of the two column internals is an important route to design a new column tray[4].Many efforts have been made for new column internals based on the design idea although the combination may increase the manufacturing costs.For example,combined packing-trays were designed and utilized in commercialized columns of absorber and stabilizer for high efficiency[5].Kachur et al.also proposed the structured packing tray which alternately installed a thin slice of structured packing and sieve tray plate throughout the whole column[6].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%[7,8].

In this work,we proposed the novel vertical spray packing tray based on the same idea.However,the combination method of the tray and structured packing was different from the previous attempts.Structured packing was introduced into this novel tray,just as part of the rectangular cap,in order to optimize the two-phase contact and improve the mass transfer efficiency.

The sieve tray and the Glitsch V1 valve tray have a long history and represent the majority of tray types in industrial application.Both of them are still widely utilized in the chemical,petrochemical,and environmental technology industry.The hydrodynamics performances and mass transfer efficiency of the novel tray were compared with that of the sieve tray and the Glitsch V1 valve tray.The column setup size is an important condition and has a great impact on the hydrodynamics performance[9].Therefore,the novel tray was experimented in a plant scale column in this study.And the column is made of plexiglass which is convenient to observe the phenomena occurring inside the column.The hydrodynamics performance data was obtained using a water–air system and the tray efficiency was obtained using a water–air–oxygen system.In the work,extensive experimental conditions were employed with the liquid loads including 3.18×10-3,4.76× 10-3,6.35 × 10-3and 7.94 × 10-3m3·m-1·s-1and the superficial gas range of 0.53 to 3.21 m·s-1.The pressure drop,weeping,entrainment and tray efficiency of the three kinds of column trays were experimented and compared[10].The results show that the novel vertical spray packing tray has better performance and higher capacity compared to the other two trays.The dry pressure drop of the novel tray is slightly less than that of the sieve tray but significantly less than that of the Glitsch V1 valve tray.In terms of entrainment and weeping,the novel tray has better performance especially at high superficial gas velocity.As a whole,the tray efficiency of the novel tray is stable at wider operating range.What's more,the fundamental model of dry pressure drop of the novel tray is in good agreement with the experiment data.The mathematical correlations by regression analysis method can well predict the pressure drop,weeping and entrainment of the novel tray.

2.Geometry of the Novel Vertical Spray Packing Tray

The detailed geometry and dimension of the NVSP tray tested in this study are shown in Fig.1 and Table 1.And Fig.2 shows the photography of the novel tray.Three rectangular caps are established on the tray deck.The perforated deck with vertical caps over the deck holes is the main structure of the novel tray and there is a structured packing installed above each vertical cap.There is also a larger disengaging slat on the top of the structured packing to reduce the formation of entrainment.Besides sieve holes on the wallof caps allow the fluid to spray out at the relatively low position,which is good for improving the tray efficiency.The structured packing sheets are made of carbon steel.In order to improve the packing surface wet tability,the surface is treated with potassium permanganate.

3.Experiment Setup

Fig.3 illustrates the schematic representation of the experiment setup.It mainly contains three sections:(a)measurement system,(b)oxygen saturation setup and(c)a cold model column setup.The experimental operating conditions:USchanged from 0.53 to 3.21 m·s-1and QL/LW=3.18 × 10-3m3·m-1·s-1,4.76 × 10-3m3·m-1·s-1,6.35 × 10-3m3·m-1·s-1,7.94 × 10-3m3·m-1·s-1.Constant weir height of 0.05 m,tray spacing of 0.45 m and downcomer clearance of 0.02 m were utilized in the experiment.Inside the column,three trays were established:the central one was the novel vertical spray packingtray,the bottom and top ones were a gas distribution board and a foam-captured board respectively.The gas distribution board was a sieve tray,and the uniform sieve holes were good for the even distribution of gas phase[11].

Table 1The detailed dimension of experiment setups

During the experiment,the blower brought air into the bottom of the column and gas velocity was measured by an orifice meter.A centrifugal pump sent water into the top of the column setup from a storage tank and a calibrated flow meter was used to measure the water flow rate[12].In the tray efficiency measurement,the oxygen saturation setup produced oxygen-rich water replacing the water.The gas and liquid came into contact on the tray inside the column setup.The exiting liquid finally left from the bottom of the column and the exiting gas was discharged out from the top.

Fig.1.The novel vertical spray packing tray tested in the experiment and figures(b),(c),(d)represent the front,side,top views of the vertical cap respectively.

Fig.2.The photography of the novel vertical spray packing tray.

The pressure drop across the tested tray was measured using a U-shaped pressure manometer.Two caps installed above and below the tested tray and were linked to the U-shaped pressure manometer[13].

where ΔP is the pressure drop,Ptand Pbare the pressures of the top and bottom sides of the tested tray respectively.

When the weeping reached steady state,weeping liquid flowing out from the column was collected over a certain period of time.The entrained liquid came through the foam-captured board and was captured by a 150 mm thick demister.The flow rates of both entrainment and weeping were determined by measuring the volume of the liquid in a vessel over a period of time.

In the study of tray efficiency,an(Dissolved Oxygen Apparatus YSI 550A)oxygen gauge was used to obtain the data.The Murphree tray efficiency EMLis shown as follows[14].

where Xo*is the oxygen concentration on the tested tray in equilibrium with the leaving gas;Xoand Xiare the oxygen concentrations of liquid at the outlet and inlet respectively.

4.Results and Discussion

4.1.Pressure drop

The pressure drop,including dry tray pressure drop and total pressure drop,signifies the energy loss of fluid flowing through the tray[15].The pressure drop has a significant effect on the fluid distribution,especially under low pressures[16].What's more,it is the basic item for the study of new trays[17].Therefore,accurate prediction and good understanding of the dry tray pressure drop are the first steps in the research of the novel vertical spray packing tray[10].

4.1.1.Dry pressure drop

In the absence of liquid phase,the dry pressure drop means the energy loss of gas phase and is caused by the obstruction of tray[18].Accurate prediction of the dry pressure drop is necessary to investigate other hydrodynamics parameters as well as tray efficiency[3].An important parameter that affects the pressure drop is the opening area.In the study,the opening areas of the novel tray,the sieve tray and the Glitsch V1 valve tray were 20.03%,19%and 19.5%respectively.Fig.4 shows the comparison of dry pressure drop among the three trays.It is clear that the dry tray pressure drop of the novel vertical spray packing tray is slightly lower than that of the sieve tray but much significantly less than that of the Glitsch V1 valve tray.

The dry pressure drop model of the sieve tray is written as follows:

Fig.3.Schematic of the experiment setup:(1)the air blower,(2)orifice meter,(3)U-shaped pressure manometer,(4)column setup,(5)pump,(6)tested tray,(7)gas distribution board,(8)communicating vessel with scale,(9)rotameter,(10)liquid storage vessel,(11)saturation column setup,(12)buffer tank,(13)oxygen vessel.

Fig.4.Dry pressure drop of the novel vertical spray packing tray,the sieve tray and the Glitsch V1 valve tray vs the superficial gas velocity U S.

where C0is the orifice coefficient,αsis the percentage of sieve hole area based on the effective area,and USis superficial gas velocity.

The form of kinetic energy was used to illustrate the dry pressure drop of the Glitsch V1 valve tray when the float valves were closed or fully opened.

When the float valves were closed:

When the float valves were opened:

where KOand KCrepresent the loss coefficients for the opened and closed valves respectively,and Klein's model could determine their values.When the valves were partly opened,Bolles'model was expressed as follows:

where Rvwis the mass ratio of the whole float valve to the valve lid without legs,the coefficient Cvwis imported to consider the eddy losses;and Hvis the thickness of the valve lid.

The dry pressure drop of vertical cap is mainly the energy costing of gas shrinkage and expansion while flowing through the opening holes.According to the fluid mechanism,Lai wenheng et al.proposed the dry pressure drop model of vertical cap is shown as follows[19]:

where Δhdis the dry pressure drop,Pa;ε is the resistance coefficient of the cap and uhis the hole velocity of gas,m·s-1.

In order to simplify the study of structured packing,it was treated as a tubular channel with resistance,and it's dry pressure drop could be expressed as the pressure loss of the gas as follows:

where Z is the height of structured packing,m;Leis the equivalent length per unit length,m;deis the equivalent diameter,m;and λ is the resistance factor.

For the certain structure,the parameters Leand deare constant.Therefore,the correlation can be rewritten as

The metal structured packing(Mellapak 250Y)is installed at the top of the vertical cap.As one part above the vertical cap,we assume that the air velocity inside the vertical caps is equal to that of the structured packing.The pressure drop of the NVSP tray is approximately seen as the resistance sum of the cap and the Mellapak structured packing,so that the problem is simplified.

where hpis the height of the structured packing.

In addition,for the structured packing Mellapak 250Y,the parameters are α =7.52 and β =1.62.Therefore,the dry pressure drop of the novel vertical spray packing tray can be rewritten as

4.1.2.Total pressure drop

Fig.5 shows that the total pressure drop of the novel vertical spray packing tray changed versus superficial gas velocity.The experiment data was measured at liquid loads of 3.18 × 10-3m3·m-1·s-1,4.76 × 10-3m3·m-1·s-1,6.35 × 10-3m3·m-1·s-1and 7.94 ×10-3m3·m-1·s-1.In general,the total pressure drop of the novel tray increased as the superficial gas velocity increased.On the other hand,the increase of the liquid load also led to the increase of the total pressure drop.At about US=1.75 m·s-1,there was a turning point where the total pressure drop began to decrease with the continuous increase of the superficial gas velocity.At the turning point,a fraction of gas escaped from the cap clearance and new pass way of gas took shape in the clear liquid.Therefore,the increasing section area of the gas pass way might be the cause of the turning point.In Fig.5,it was evident that the superficial gas velocity corresponding to the turning point became bigger with the increase of the liquid load.The phenomenon signified that it became more difficult for gas to escape from the cap clearance when the liquid load increased.

Fig.5.Total pressure drop of the novel vertical spray packing tray vs the superficial gas velocity U S at different liquid loads.

The comparison of total pressure drop among the novel tray,the sieve tray and the Glitsch V1 valve tray was shown in Fig.6.There was a step-increase in the plot of the Glitsch V1 valve tray.At the beginning,the total pressure drop of the Glitsch V1 valve tray was less than that of the other two trays.However,with the increasing superficial gas velocity,the total pressure drop of the Glitsch V1 valve tray soared sharply.The total pressure drop of the novel tray was less than that of the sieve tray,even though they both had similar trend.As a whole,the novel tray performed well in the terms of total pressure drop in the whole range.Fitting the total pressure drop data of the novel tray yielded the correlation:

4.2.Weeping

The weeping,as an importanthydrodynamics parameter,has a decisive influence on the low operating limit of the column tray,which could badly affect the tray efficiency[20].In this paper,the weeping rate of the novel tray was investigated and compared with the other two trays.Fig.7 shows that the weeping rate increased with the increasing liquid loads.On the other hand,the weeping rate decreased with the increase of superficial gas velocity and finally disappeared.

In Fig.8,the comparison of weeping rate among the three trays was illustrated.It was clear that the weeping rate of the Glitsch V1 valve tray was much less than that of the other two trays in the whole range.The novel tray had high weeping rate especially at low superficial gas velocity.When the superficial gas velocity reached about 1.6 m·s-1,the weeping rate of the novel became less than the sieve tray.Based on experiment data,the weeping correlation of the novel tray was obtained by regression method.

Fig.6.The total pressure drop of the novel vertical spray packing tray,the sieve tray and the Glitsch V1 valve tray vs the superficial gas velocity U S.(At the operating conditions:Q L/L w=3.18 × 10-3 m3·m-1·s-1).

Fig.7.The weeping rate of the novel vertical spray packing tray vs the superficial gas velocity U S at different liquid loads.

4.3.Entrainment

Different from weeping,entrainment often occurs in the conditions of high superficial gas velocity and low liquid load.Serious entrainment will affect the upper operating limit[1].Therefore,excessive entrainment decreases the capacity and tray efficiency.Remedial steps must be carried out in the industrial application,when the entrainment reaches 5%of the liquid load[21].Entrainment decreases the driving force of mass transfer and is fatal for the tray efficiency.Therefore the effect of entrainment on the overall performance is an important factor in the design of column trays[22].In this study,the entrainment of the novel tray was investigated.As shown in Fig.9,the entrainment rate of the novel tray increased as the superficial gas velocity increased.On the other hand,at different liquid loads,the trend of the entrainment rate versus the superficial gas velocity was similar.The increase of liquid load also facilitated the entrainment slightly,which means the effect of liquid loads on the formation of entrainment was weaker than that of the gas.

Fig.8.The weeping ratio of the novel vertical spray packing tray,the sieve tray and the Glitsch V1 valve tray vs the superficial gas velocity U S.(At the operating conditions:Q L/L w=3.18 × 10-3 m3·m-1·s-1).

Fig.9.The entrainment rate of the novel vertical spray packing tray vs the superficial gas velocity U S at different liquid loads.

Fig.10 showed the entrainment ratio of the three trays at the liquid loading of QL/Lw=3.18 × 10-3m3·m-1·s-1.It is clear that the entrainment of the Glitsch V1 valve tray was much more than that of the other two trays.At the beginning,the value of the entrainment ratio of the three trays was small,and then the entrainment of the Glitsch V1 valve tray increased significantly.In industrial application,the entrainment ratio of 10%was the upper operating limit,while the entrainment ratio of the Glitsch V1 valve tray reached the upper operating limit at US=1.65 m·s-1.In the whole range,the entrainment ratio of the NVSP tray and the sieve tray had a similar trend,but the entrainment of the novel tray was the lowest.The entrainment correlation of the novel tray was shown as obtained by the regression method

Fig.10.The entrainment ratio of the novel vertical spray packing tray,the sieve tray and the Glitsch V1 valve tray vs the superficial gas velocity U S.(At the operating conditions:Q L/L w=3.18 × 10-3 m3·m-1·s-1).

Fig.11.The tray efficiency of the novel vertical spray packing tray vs.the superficial gas velocity U S at different liquid loads.

4.4.Tray efficiency

Tray efficiency can directly reflect the mass transfer performance.Therefore,it is an important criterion for evaluating all kinds of trays[23].High tray efficiency not only saves equipment cost,but also brings economic benefits by reducing equipment investment and re flux ratio[24].In this work,the tray efficiency of the novel tray was investigated as shown in Fig.11.Generally the tray efficiency of the novel tray was stable in the whole range,even though the superficial gas velocity reached 3.21 m·s-1.

The comparison of tray efficiency among the novel tray,the sieve tray and the Glitsch V1 valve tray was shown in Fig.12.As a whole,the novel tray retained high tray efficiency in the whole range.While the tray efficiency of the sieve tray and the Glitsch V1 valve tray appeared to decrease significantly at lower and higher superficial gas velocity.It is clear that the novel tray had wider operating condition.

Fig.12.The tray efficiency of the novel vertical spray packing tray,the sieve tray and the Glitsch V1 valve tray vs the superficial gas velocity U S.(At the operating conditions:Q L/L w=3.18 × 10-3 m3·m-1·s-1).

5.Conclusions

In this paper,the NVSP tray was designed by combining the tray with structured packing.Although the manufacturing cost was increased by about 30%,the novel tray achieved better performance and higher capacity.The novel tray was compared with the representative trays including the sieve tray and the Glitsch V1 valve tray.The conclusions are as follows:

1)The novel vertical spray packing tray performed well in terms of pressure drop.The dry pressure drop of the novel tray was lower than that of the sieve tray by about 35%and much significantly less than that of the Glitsch V1 valve tray.The novel tray also performed well in the total pressure drop especially at higher superficial gas velocity.

2)The weeping of the novel vertical spray packing tray was similar with that of the sieve tray and was heavier than that of the Glitsch V1 valve tray.The cause of weeping is the huge holes on the deck.

3)The entrainment of the novel vertical spray packing tray was lower than that of the sieve tray by approximately 30%and much less than that of the Glitsch V1 valve tray.

4)The superficial gas velocity of the novel vertical spray packing tray can reach as much as 3.21 m·s-1.The operation conditions were widened by about 30%and its tray efficiency remained stable at wider range of superficial gas velocity.

5)The fundamental model of dry tray pressure drop for the novel vertical spray packing tray was established.

The calculated results by this model are in good agreement with the experiment data with R2=0.997.

As a whole,the good hydrodynamics performance and mass transfer efficiency make the novel vertical spray packing tray a good choice.Especially with higher upper operating limit,the novel tray is proper in dealing with huge gas loading.

Nomenclature

ATthe total active area,m2

A

0the total opening area,m2

Cvwthe coefficient introduced to account for eddy losses

C0the orifice coefficient

EMLthe Murphree liquid efficiency,%

G referring to gas phase

g acceleration due to gravity(9.81 m·s-2)

Hvthe thickness of the valve lid,m

hCLclear liquid height,m

hdheight of downcomer clearance,m

hwweir height,m

KO,KCthe coefficient proposed by Klein et al.

L referring to liquid phase

Ldlength of downcomer clearance,m

Lwweir length,m

Pbthe pressure of bottom side of the tested tray,Pa

Ptthe pressure of top side of the tested tray,Pa

ΔP the pressure drop across the tray,Pa

ΔPDthe dry tray pressure drop across the tray,Pa

ΔPD,Cthe dry tray pressure drop of valve tray for closed valves,Pa

ΔPD,Othe dry tray pressure drop of valve tray for open valves,Pa

ΔPTthe total pressure drop across the tray,Pa

p pressure,Pa

QLliquid flow rate across tray,m3·s-1

REthe entrainment rate,m3·h-1

Rvwthe ratio of the mass of the whole valve to the mass of valve lid only

Rwthe weeping rate,m3·h-1

USsuperficial gas velocity,m·s-1

uhhole gas velocity,m·s-1

Xithe oxygen concentrations in the liquid phase at the inlet,g·ml-1

Xothe oxygen concentrations in the liquid phase at the outlet,g·ml-1

Xo*the oxygen concentration of liquid phase in equilibrium with gas,g·ml-1

αsthe area percentage of sieve holes based on the active area

αs′ the area percentage of sieve holes based on the active area for closed valves

μ viscosity of phase,Pa·s

ρ density of phase,kg·m-3