Gas cyclone-liquid jet absorption separator used for treatment of tail gas containing HCl in titanium dioxide industry

Liwang Wang,Hualin Wang,Liang Ma,Zhanghuang Yang,Erwen Chen

School of Mechanical and Power Engineering,East China University of Science and Technology,Shanghai 200237,China

Keywords:Wet dechlorination HCl Tail gas Gas cyclone-liquid jet absorption Separator Titanium dioxide industry

ABSTRACT In the titanium dioxide industry,there is a lack of a low-cost and high-efficiency treatment method for chloride containing tail gas.In this paper,the removal of HCl from the titanium dioxide industry by gas cyclone-liquid jet separator was studied,while Ca(OH)2,Na2CO3,NaOH solution,and water were used as absorbents.This paper investigated the influence of gas cyclone-liquid jet separator’s various process parameters on the removal rate of hydrogen chloride gas.The mechanism of mass transfer in the process of removing hydrogen chloride was discussed,and the effect and feasibility of HCl gas removal in the gas cyclone-liquid jet absorption separator were studied.The results showd that the removal efficiency of hydrogen chloride maintained above 95%,up to 99.9%,and the total mass transfer coefficient reached 0.28 mol?m-3?s-1?kPa-1.Under the same conditions,the absorption effect and total mass transfer coefficient of weak basic absorption liquid can be greatly improved by increasing the flow rate of absorption liquid,but the absorption effect and total mass transfer coefficient of strong alkaline absorption liquid can’t be improved obviously.The larger the inlet gas volume,the higher the gas concentration,the lower the absorption efficiency and the lower the total volumetric mass transfer coefficient.

1.Introduction

In 2019,the annual production of titanium dioxide in the world will reached more than 7 million tons,among which the annual production of titanium dioxide by Chemours and Tronox Incorporated in the United States exceeded 1 million tons.In the titanium dioxide industry,the most widely used preparation method of titanium dioxide is chlorination.The main components of tail gas discharged by this method include HCl,TiCl4,Cl2,etc.,and the content of HCl accounts for the absolute majority.Since 2010,the European Union has required that the average emission concentration of HCl should be less than 10 mg?m-3.

For tail gas containing hydrogen chloride,calcined limestone[1]is the most commonly used treatment method for removing HCl from flue gas of coal-fired power plants,and the absorption effect is the best when the temperature is 500–600°C.Kamedaet al.[2]used Mg-Al oxide slurry to treat HCl at 500°C and achieved 99%treatment effect.Nunokawaet al.[3] used sodium aluminate to absorb HCl to below 1.5 mg?m-3,but it could only maintain a high absorption effect for a short time.Wuet al.[4] found that 600°C was the most suitable temperature for Ca-Mg-Al layered double hydroxide.Although the absorption effect of dry treatment of HCl gas is excellent,the requirements for ambient temperature or inlet air temperature are extremely high,meanwhile,there are some shortcomings such as limited occasions and high energy consumption,so it is not suitable for the titanium dioxide industry.

Lokhandwalaet al.[5]proposed the membrane process for chlorine removal.After Lindbr?then [6] discussed the feasibility of membrane method for chlorine removal,Sanet al.[7]used a membrane method to recover HCl from devil liquor.At present,membrane process is mainly used in the treatment of HCl in the liquid phase,rather than in exhaust gas.Zhuet al.[8] proposed to prepare high-purity titanium dioxide from titanium slag to avoid HCl emission,but this process is far from industrialization.Kurellaet al.[9] achieved 93.8% absorption using a multistage dual-flow sieve plate wet scraper.Balet al.[10] used selfpriming venture scraper to treat HCl gas,and the absorption effect reached 92.54%.Kimet al.[11] installed a practical scale reactor downstream of the existing semi dry reactor (SDR) to reduce the HCl emitted from a municipal waste incinerator.Although the operation cost was low,the absorption effect of these two methods was general.

From the above studies,existing treatment technologies are either too expensive or inefficient.Therefore,the recent researches on HCl treatment methods mainly focuses on reducing the cost and maintaining a high treatment effect.At present,in the chlor-alkali industry,2–3 water washing spray towers and 1–2 alkali washing spray towers are usually used in series to treat the chlorinated tail gas.Due to the low absorption efficiency of a single spray tower,the total cost of this treatment method is very high and the floor area is quite large.Liuet al.[12] improved the SO2removal efficiency of spray tower by about 3%by using CFD method and experimental research.Ochowiaket al.[13] modified the spray tower interface,which can improve the efficiency of the spray tower to a certain extent.At present,the research on spray tower was limited to improving the absorption efficiency,but failed to further improve the tower equipment in terms of cost and floor area.Therefore,there is an urgent need for a low-cost and highefficiency way to deal with the chlorination tail gas treatment of the titanium dioxide industry.

Gas cyclone–liquid jet absorption separator can make use of the strong centrifugal force generated by the high-speed rotation of gas-phase to increase the relative velocity of gas and liquid,update the contact area between phases rapidly,and strengthen the transfer process.It also has the advantages of simple structure and no blockage and scaling.As a result of the above advantages,gas cyclone–liquid jet absorption separator may be a new method to improve the capacity of HCl waste gas treatment in the titanium dioxide industry.Chenget al.[14]studied the dust removal characteristics of WSA.Qiuet al.[15] carried out simulation research on the flow field of the cylindrical gas cyclone–liquid jet absorption separator.Wanget al.[16] applied this kind of equipment to flue gas desulfurization,and Maet al.[17–19] studied the ammonia removal performance of gas cyclone–liquid jet absorption separator and the impact of overflow on performance.

Gas cyclone–liquid jet absorption separator has great advantages in gas–liquid mass transfer compared with the traditional spray tower.However,no research team has studied the HCl removal properties of the gas cyclone–liquid jet absorption separator.By studying the process parameters and mass transfer characteristics of a gas cyclone–liquid jet absorption separator for treating HCl tail gas,it is able to expand the application of this device in this field.In this paper,the removal of hydrogen chloride was studied by adjusting the process parameters,such as gas treatment capacity,absorbent circulation flow rate,hydrogen chloride concentration,absorbent type and concentration.The mass transfer mechanism of the hydrogen chloride removal process was discussed,and the derivation formula of gas-phase total volume mass transfer coefficient was analyzed.

2.Experimental

2.1.Establishment of a mathematical model

The research on mass transfer performance of gas cyclone–liquid jet absorption separator is a key link to guide equipment design,shorten process flow,improve product quality,and reduce investment cost.At present,the description of the absorption process is usually based on three theories:double membrane theory,surface renewal theory,and solute permeation theory,among them,double membrane theory is the most influential.This paper mainly used the double membrane theory to establish the absorption reaction model.The speed of the absorption process can be expressed by the absorption rate,which refers to the amount of solute absorbed by unit mass transfer area between units.By determining the absorption rate of the equipment,the size of the device needed to complete the specified absorption task and the purification degree of the mixed gas can be obtained.Due to the special structure of gas cyclone–liquid jet absorption separator,two main mass transfer factors,high gravity mass transfer,and jet crushing and atomization mass transfer,are involved in the gas–liquid mixed impingement process.

In terms of high gravity mass transfer,Tunget al.[20] pointed out that the penetration theory of the traditional gravity process is also applicable to the high gravity process.Chenet al.[21]summarized the total mass transfer coefficient of gas phase volume in high gravity rotating bed.

Zhenget al.[22] used the NaOH-CO2chemical absorption system to analyze the mass transfer performance of the rotating packed bed with foam nickel filler before and after modification.It was considered that the Reynolds number of the gas–liquid phase had the greatest influence on the mass transfer coefficient,and Suet al.[23] held similar views.

In terms of mass transfer in spray atomization,Yoshidaet al.[24]proposed in 1965 that mass transfer between bubble and liquid phase is related to gas–liquid viscosity and velocity,but not to gas-phase resistance.Kashidet al.[25] considered that the mass transfer performance of any gas–liquid reactor depends on the two-phase flow pattern.Opletalet al.[26]proposed that after nozzle injection,the two phases are strongly mixed in the mixing impact,and the process can be considered as isothermal due to the high specific heat capacity of water.MacInneset al.[27]believed that the two basic factors affecting mass transfer are phase flow ratio and phase contact efficiency,in which the contact efficiency depends on the complexity of two-phase flow generated in a specific device.

In conclusion,the main factors affecting the mass transfer coefficient of gas cyclone–liquid jet absorption separator are phase Reynolds number,liquid phase Reynolds number,Grachev number,diffusion coefficient,contact area and Henry constant.Since the value ofaeis difficult to be measured experimentally,the productKGa(total mass transfer coefficient of gas-phase volume)ofaeandkGis taken as the parameter to characterize the mass transfer performance of the gas cyclone–liquid jet absorption separator.

In the gas cyclone–liquid jet absorption separator,the gas–liquid coupling effect is very strong.In the process of crushing atomization contact mass transfer,the mixing impact of gas–liquid two-phase is much higher than that of an ordinary nozzle,so the temperature difference between two sides of the membrane is very low,which can be approximately 0.As a result,the influence of theGrnumber can be ignored.At 25°C,the Henry constant of HCl is 0.0022×10-4mmHg?mol-1(1 mmHg=133.322 Pa),so that the influence of Henry coefficient is ignored in this analysis.The characteristic parameterKwas added to characterize the influence of gas cyclone–liquid jet absorption separator on the reaction.

In the absorption process of HCl and alkali liquor,the reaction is an indispensable part.The reaction enhancement coefficientEis added to reflect the effect of reaction.The analytical formula for the total mass transfer coefficient of gas-phase volume is as follows:

aandbare constants.

The gas-phase density is as follows:

The gas-phase viscosity is as follows:

The gas-phase velocity is as follows:

Gis the inlet volume flow.SGis the inlet area,the value is 28 mm2.

The rate of liquid phase velocity is as follows:

Lis the volume flow rate of the absorption solution.SLis the area of small hole,approximately 0.5 mm2.

The hydraulic radius of the inlet is taken as the gas phase characteristic size,the numerical value is 8.062 mm,and the liquid phase characteristic size is the small hole diameter,the numerical value is 0.8 mm.The diffusion coefficient (DG) is 2.29×10-9m2?s-1.

Only using the above analytical formula is not enough to obtainKGathrough experiments,therefore,another calculation formula is needed.

According to the double membrane theory,the total absorption rate equation of the absorption reaction process can be divided into the absorption process driven by pressure difference and the absorption process driven by a concentration difference.

Hydrogen chloride has high solubility,and HCl will react with water by proton transfer:

The reaction is quite thorough and almost completely ionized.Therefore,the reaction is mainly a gas-film controlled absorption process.The total absorption coefficientKGis as follows:

Since Henry’s coefficient is very small,KGcan be approximated tokG.

The spray holes in the gas cyclone–liquid jet absorption separator are uniformly arranged,and it can be approximately considered that the mole fraction of HCl in the gas phase decreases with the decrease of height.The mass transfer differential equation of the whole absorption reaction process can be expressed as follows:

Since the reaction between HCl and lye is fast reaction,the concentration of HCl in the liquid phase is extremely low.It can be considered that the equilibrium partial pressure of HCl in the liquid phase is 0,the apparent gas velocityuGand the total gas volume mass transfer coefficientKGaare fixed values.The expression ofKGais obtained by integrating the whole range of gas cyclone–liquid jet absorption separator,

In the gas cyclone–liquid jet absorption separator,it is difficult to accurately define the specific gas–liquid contact time due to the huge breakup and atomization scattering during gas–liquid coupling.In this experiment,the estimated value of gas–liquid contact time (t) is 0.01 s.

In this experiment,the control variable method was used to study the influence of various factors on the absorption efficiency by changing the absorption liquid flow rate,inlet flow rate,and hydrogen chloride concentration at 25°C ambient temperature and absorption liquid temperature.When changing the absorption liquid flow rate,the fixed inlet gas concentration was 1% and the air inlet rate was 4.175 m3?s-1(25 L?min-1);when the inlet flow rate was changed,the fixed absorption liquid flow rate was 11.12 m3?s-1(40 L?h-1) and the HCl concentration was 1%;when the hydrogen chloride concentration was changed,the fixed absorption liquid flow rate was 11.12 m3?s-1(40 L?h-1) and the inlet flow rate was 4.175 m3?s-1(25 L?min-1).

2.2.Experimental equipment and process

From the above study,it can be seen that the flow rate and concentration of gas phase and liquid phase are the main influencing factors of the mass transfer coefficient,so it is necessary to design relevant experiments to study these variables.

Hydrogen chloride gas with a purity of 99.9% was used in the experiment.Different concentrations of hydrogen chloride gas were obtained by controlling the amount of hydrogen chloride and air intake and mixing.The absorbents were studied with 1.5%,3% and 5% NaOH solution,1.5% Ca(OH)2solution,3% and 5%Na2CO3solution and water.Water was used as the absorption solution to study the absorption when there was no reaction enhancement coefficient.

The whole set of experimental equipment flow consists of the gas cyclone–liquid jet absorption separator,liquid storage tank,water pump,fan,gas–liquid separation device,liquid,gas flow meter,rotameter,pressure gauge,the cylinder containing 99.9%HCl gas,etc.,as shown in Fig.1.

The gas flow rate was the sum of the values shown by the two gas flow meters,and the liquid flow rate was the sum of the values shown by the two liquid flow meters.The dehumidifier was used to separate the wet purified gas,in order to avoid the influence of large entrained droplets on the detection of HCl content at the outlet.Specific equipment parameters are shown in Table 1.

The structure design of gas cyclone–liquid jet absorption separator used in the system is shown in Fig.2.

The cylinder inner diameter of the gas cyclone–liquid jet absorption separator was 16 mm,and the overall height was 77 mm.Small holes were set in the upper half of the cylinder for hydraulic injection.The total length of the area with holes was 15 mm,the aperture was 0.8 mm,and the axial distance between the two holes was 3 mm.Eight rows of holes were evenly distributed on the sidewall of the absorber.The number of holes in each row was 6 and the total number of holes was 48.The outer jacket of the area with holes on the cylinder body was the liquid division area,which was mainly used for the liquid jet to ensure that the liquid was evenly sprayed into the cylinder and had enough pressure to make it atomized.The gas inlet was a rectangular tube with a length of 9 mm and a width of 6 mm.The inner diameter of the middle exhaust pipe of the device was 5 mm,and the gas inlet of the exhaust pipe was in the lower part of the liquid jet area.The bottom of the separator was a conical connecting circular pipe,with a total length of 12 mm and an outlet pipe inner diameter of 6 mm.The overall wall thickness of the device was 1 mm,and the material is plexiglass,in order to observe the gas–liquid coupling process and the intensity of the reaction.Specific equipment parameters are shown in Table 2.

Table 1 Parameters of experimental equipment.

Table 2 Dimensional parameters of gas cyclone–liquid jet absorption separator

The purpose of this experiment was not only to determine the total mass transfer coefficientKGaof gas phase volume under different working conditions,but also to determine whether the change of different parameters would have an obvious impact onKGaand the magnitude of the change.According to the above analysis,Reynolds number was simplified as gas-phase velocity and liquid phase velocity,and the influence of the change of twophase velocity was observed.In addition,the influence of the change of gas–liquid concentration on the results and the change of the temperature of absorption liquid were also observed.There were three groups of experiments altogether.

During the experiment,the prepared absorption liquid was added in advance and the water pump was opened to make the inlet rate of the absorption liquid reach a large value,let absorbent fill the experimental equipment,to seal the liquid outlet liquid to prevent the gas from ejecting from the liquid outlet.Then,the whole equipment was started,it extracted gas from the hydrogen chloride and the atmosphere respectively.The inlet concentration was determined according to the flowmeter.After waiting for about 2 minutes,when the gas mixture became stable,the inlet rate of the absorption liquid was reduced to make the flow rate reach the preset value.Continue to wait for about 3 minutes,until the injection column of absorption liquid was stable again.The absorption solution was recycled.By detecting the PH value of the absorption solution,the concentration of the absorption solution was ensured to be near the predetermined value.By observing the thermometer,the temperature of the absorption liquid was ensured to be around 25°C.In the case of deviation,the valve needed to be opened to discharge part of the liquid.Then alkali liquor or purified water was added to adjust the liquid concentration and temperature to avoid overheating.The external circulation pump was kept open and the liquid phase was kept at the initial state as far as possible.

The absorption effect of hydrogen chloride and each parameter was measured every 10 minutes.Measurement for each group of data was repeated for three times.Taking the average value of the three values as the experimental result.If there was a parameter deviation that was too large,the measurement should be repeated.Each change of experimental variables is regarded as a cycle,and the above operation is repeated in each cycle.The hydrogen chloride absorption rate η was defined by the following formula:

2.3.Experimental principle

The traditional wet absorption method of hydrogen chloride gas is to directly pass the hydrogen chloride gas into the absorbent,but this simple way seriously restricts the maximum effect of the absorbent as a result of low mass transfer area and mass transfer rate.

Fig.1.Experimental installation.

Fig.2.Gas cyclone–liquid jet absorption separator.

Gas cyclone–liquid jet absorption separator can spray the prepared absorbent in the form of fine liquid column or mist in the equipment,and then,absorbent contact with the gas with swirling trajectory.As shown in Figs.2 and 3,the mixed gas enters the gas cyclone–liquid jet absorption separator tangentially from the air inlet,and produces a strong swirling mixed airflow field.At the same time,the absorption liquid in the liquid storage tank is sucked out by the water pump,enters the purifier,and is dispersed by the small holes on the pipe wall to form a small water column.The surface layer of columnar or foggy absorbent droplets is subjected to shear force by the gas to be absorbed along with the swirling flow at a very high speed,resulting in the continuous breaking and reorganization of the absorbent droplets,forming numerous absorption solution droplets with a diameter of 200–700 μm,which rotate with the high-speed turbulent flow field in the equipment,so that the surface always has the best absorption capacity After the interaction,the mixed gas is absorbed by several absorption solution droplets.The specific absorption process is shown in Fig.3.

This process not only increases the total contact area between the treated gas and the absorption liquid,but also reduces the mass transfer resistance on the gas film side,which greatly improves the removal efficiency of hydrogen chloride from the treated gas.The performance form of absorbent is related to the power of the water pump and the diameter of the small hole on the gas cyclone–liquid jet absorption separator.Different jet forms are used for different types of treated gas.Because of the centrifugal effect,in the process of gas–liquid fusion and absorption,there is a high gravity effect,which can get the best absorption results.

Since hydrogen chloride gas is easily soluble in water,its aqueous solution is strongly acidic.Therefore,the use of alkaline solution is beneficial to the removal of gas containing hydrogen chloride.In this experiment,Ca(OH)2solution,Na2CO3solution,and NaOH solution were selected for the removal of hydrogen chloride-containing gas.In the comparative experiment,water was used as the absorption liquid.

When strong basic Ca(OH)2solution is used as absorbent,neutralization reaction will occur:

The calcium chloride obtained by the reaction is easily soluble in water and its solution is a neutral solution.

When the weak basic Na2CO3solution is used,a double decomposition reaction will occur:

The result of the reaction is similar to that of Ca(OH)2solution,but bubbles(carbon dioxide)will be produced during the reaction.

When strong alkaline NaOH solution is used as absorbent,neutralization reaction will occur:

The sodium chloride solution obtained by the reaction is easily soluble in water and its solution is a neutral solution.

Since hydrogen chloride is very soluble in water,the solubility of HCl is about 72 g per 100 g H2O at 20°C and standard pressure,so water can be used as absorbent.

3.Results and Discussion

3.1.Influence of an absorbent flow rate on absorption effect

Fig.4 shows the change of absorption effect under different flow rates by changing the absorption liquid flow rate when the room temperature was 25°C,the inlet air volume was constant at 4.175×10-4m3?s-1(25 L?min-1),and the hydrogen chloride concentration in the treated gas was constant at 1 %.Fig.5 shows the effect of the flow rates on the total volumetric mass transfer coefficient.When the flow rate was 8.34×10-6m3?s-1(30 L?h-1),the absorption efficiency was between 98% and 99%,and the total volumetric mass transfer coefficient was between (0.16–0.275)×10-3mol?m-3?s-1?Pa-1.When the flow rate was 16.68×10-6m3?s-1(60 L?h-1),the absorption efficiency was between 99.4%and 99.9%,and the total volumetric mass transfer coefficient was between (0.205–0.28)×10-3mol?m-3?s-1?Pa-1.When the flow rate was 25.02×10-6m3?s-1(90 L?h-1),the absorption efficiency was between 99.5%and 99.9%,and the total volumetric mass transfer coefficient was between(0.215–0.28)×10-3mol?m-3?s-1?Pa-1.If the total mass transfer coefficient of 0.25×10-3mol?m-3?s-1-?Pa-1was taken as the pass line,the two types of strongly alkaline solutions (NaOH solution and Ca(OH)2solution) with flow rate greater than 13.9×10-6m3?s-1(50 L?h-1)and 5%Na2CO3solution with a flow rate greater than 19.46×10-6m3?s-1(70 L?h-1) can meet the requirements.

As is shown in Figs.4 and 5 that no matter what kind of absorption solution was used,the greater the absorption liquid flow rate was,the larger the total volumetric mass transfer coefficient was.This was because a faster liquid phase injection rate in the gas cyclone-liquid jet absorption separator made the phenomenon of droplet breakage and recombination more severe.The gas–liquid two-phase coupling was better,which enhanced the reaction rate.Meanwhile,it also increased the contact area between absorbent and treated gas,and increased the effective mass transfer area,to better absorb HCl and increase the total volume mass transfer coefficient.

Fig.3.Purification principle of the gas cyclone–liquid jet absorption separator.

When the absorbent was water,the absorption efficiency was the lowest,ranging from 98% to 99.5%,and the total volumetric mass transfer coefficient was between (0.16–0.215)×10-3mol?m-3?s-1?Pa-1.Although water did not react with hydrogen chloride,the physical properties of hydrogen chloride,which was very soluble in water,can still absorb hydrogen chloride gas to a certain extent.In the whole absorption process,the temperature of the water had no obvious change,and there were no other byproducts.This characteristic can be used to recycle hydrogen chloride and improve economic benefits in the chlor-alkali industry.

When the absorption solution was weak alkaline solution,the absorption efficiency was between 99.1% and 99.9%,and the total volumetric mass transfer coefficient was between (0.19–0.275)×10-3mol?m-3?s-1?Pa-1.The absorption effect was better than that of water,but inferior to that of a strong alkaline solution.Sodium carbonate contacted with hydrogen chloride and reacted with hydrogen chloride,which consumed part of hydrogen chloride dissolved in water and achieved better absorption effect than water.The higher the concentration of sodium carbonate was,the more violent the reaction was,and the better the absorption effect of hydrogen chloride was.

When the absorption solution was a strong alkaline solution,the absorption efficiency was between 99.7% and 99.9%,and the total volumetric mass transfer coefficient was between (0.245–0.28)×10-3mol?m-3?s-1?Pa-1.The absorption effect was excellent and basically met the requirements of industrial application.The neutralization reaction between a strong base and hydrogen chloride (strong acid) was very fast,and a lot of heat was produced at the same time.From the experimental data,the concentration of NaOH solution had little effect on the experimental results,which may be due to the fact that the acid-base neutralization reaction rate itself was very fierce,only increasing the concentration couldn’t effectively improve the reaction rate,and the increase of flow rate could improve the absorption effect to a certain extent,but the increase was also not obvious.

It is observed from the figure that the absorption effect of Ca(OH)2solution was slightly lower than that of NaOH solution at the same concentration,but the OH-ion concentration of calcium hydroxide solution was greater than that of sodium hydroxide solution at the same concentration.It was considered that calcium hydroxide was slightly soluble in water and its solubility decreased with the increase of temperature.Although the absorption solution was recycled to ensure the state of the absorption liquid remains unchanged during the experiment,the heat released during acidbase neutralization reduced the actual concentration of Ca(OH)2during gas–liquid coupling,and the viscosity of calcium hydroxide solution increased sharply with the increase of temperature,which led to the decrease of absorption effect to a certain extent,but the level of reduction was not large.

Fig.5.Influence of the absorbent flow rate on total mass transfer coefficient of gasphase volume.

It was observed that the content of HCl in the purified gas could hardly be reduced to less than 15 mg?m-3.Although there were occasional experimental values of 13–14.5 mg?m-3,the average value of multiple detections was always close to 16 mg?m-3.It can be considered that the maximum volume total mass transfer coefficient of the equipment was about 0.28×10-3mol?m-3?s-1-?Pa-1,and the maximum absorption efficiency was 99.9%.The measured inlet pressure was about 10×103Pa,and the pressure drop was less than 0.4×103Pa,which was less than the error range of the pressure gauge.It showed that gas cyclone–liquid jet absorption separator has excellent pressure drop characteristics.

3.2.Influence of intake flow rate on absorption effect

Fig.4.Influence of the absorbent flow rate on the absorption efficiency.

Fig.6 shows the influence of inlet flow on the absorption effect when the room temperature was 25°C,the absorption liquid flow rate was 11.12×10-6m3?s-1(40 L?h-1),and the HCl concentration was kept at 1%.Fig.7 shows the effect of the inlet flow on the total volumetric mass transfer coefficient.The results showed that the absorption efficiency was between 99.4% and 99.9%,and the total volumetric mass transfer coefficient was between (0.21–0.28)×10-3mol?m-3?s-1?Pa-1when the inlet air volume was 2.505×10-4m3?s-1(15 L?min-1).The absorption efficiency was between 99.1% and 99.9% and the total volumetric mass transfer coefficient was between (0.19–0.28)×10-3mol?m-3?s-1?Pa-1when the air inlet volume was 3.758×10-4m3?s-1(22.5 L?min-1).The absorption efficiency was between 98%and 99.9%and the total volumetric mass transfer coefficient was between (0.16–0.28)×10-3mol?m-3?s-1?Pa-1when the inlet air volume was 5.01×10-4m3?s-1(30 L?min-1).If the total mass transfer coefficient of 0.25×10-3mol?m-3?s-1?Pa-1is taken as the pass line,then the two kinds of strong alkaline solutions can meet the requirements of use.

It can be seen from the Figs.6 and 7 that with the increase of air intake,the overall absorption efficiency presented a downward trend.With the increase of intake rate,more hydrogen chloride gas entered into the gas-phase cyclone liquid jet purifier,which improved the intensity of the strong swirl mixed airflow field in the equipment,made the breakup and reorganization of the absorption liquid droplets become more severe,so that the gas–liquid two-phase can be better coupled with each other.However,more treated gas also brought greater purification load to the equipment,resulting in the decline of the overall purification effect.Meanwhile,the partial pressure of gas–liquid coupling was increased.When the total mass transfer coefficient decreased,the total absorption rate may not necessarily decrease.By calculating the concentration difference between import and export hydrogen chloride,the total absorption rate was determined to be improved.

The absorption efficiency of water is between 98% and 99.5%,and the total volumetric mass transfer coefficient is between(0.19–0.16)×10-3mol?m-3?s-1?Pa-1,and the decreasing trend of total volumetric mass transfer coefficient was the most significant among all kinds of absorbents.Since water mainly depends on the extremely high solubility of hydrogen chloride for purification,the absorption effect per unit volume of water hadn’t been greatly increased in the face of a larger intake air volume,as a result,the overall absorption efficiency has dropped significantly.

Fig.7.Influence of intake change on total mass transfer coefficient of gas phase volume.

The absorption efficiency of sodium carbonate solutions with different concentrations is between 99.2%and 99.8%,and the total volumetric mass transfer coefficient is between(0.20–0.24)×10-3mol?m-3?s-1?Pa-1.The overall absorption efficiency was still higher than that of water but lower than that of a strong alkaline solution.The higher the concentration of sodium carbonate solution,the higher the absorption efficiency.The absorption effect of the three concentrations had a similar downward trend,all of which showed a smooth decrease.It is worth noting that,the absorption effect of the 5% sodium carbonate solution is about 2% lower than that of the strong alkaline solution,and the total volumetric mass transfer coefficient is about(0.2–0.3)×10-3mol?m-3?s-1?Pa-1.There is still a certain gap between the absorption efficiency of the weak base and strong base.

The absorption efficiency of sodium hydroxide solution and calcium hydroxide solution was between 99.8% and 99.9%,the total volumetric mass transfer coefficient was between (0.25–0.28)×10-3mol?m-3?s-1?Pa-1.The absorption efficiency of a strong alkaline solution was still high.Among them,the absorption efficiency of 1.5%NaOH solution and 1.5%Ca(OH)2solution showed a downward trend,but the absorption efficiency of 3%NaOH solution and 5%NaOH solution was not greatly affected.Although the acid-base neutralization reaction rate was extremely high,but more gas made these two kinds of alkaline solutions of general concentration too late to absorb all hydrogen chloride,causing part of the hydrogen chloride gas to escape,resulting in a decrease in absorption efficiency.The NaOH solution with higher concentration reacted more intensely and could absorb hydrogen chloride gas more effectively,so it was not greatly affected.The 3%NaOH solution was sufficient to complete the absorption process,so the total mass transfer coefficients of the 3% and 5% NaOH solutions were similar.

Fig.6.Influence of intake change on absorption efficiency.

3.3.Effect of hydrogen chloride concentration on absorption

Fig.8 shows the effect of the gas phase concentration on the absorption effect by adjusting the HCl inlet gas concentration when the room temperature was 25°C,the absorption liquid flow rate was 11.12×10-4m3?s-1(40 L?h-1),and the inlet air flow rate was 4.175×10-4m3?s-1(25 L?min-1).Fig.9 shows the effect of the gas phase concentration on the total volumetric mass transfer coefficient.When the intake air concentration was 0.8%,the absorption efficiency was between 99% and 99.9%,and the total volumetric mass transfer coefficient is between(0.185–0.28)×10-3mol?m-3-?s-1?Pa-1.When the intake air concentration was 1.4%,the absorption efficiency was between 98.5% and 99.85%,the volume total mass transfer coefficient was between (0.18–0.27)×10-3mol?m-3?s-1?Pa-1.When the intake air concentration was 2%,The absorption efficiency was between 95%and 99.8%,and the volume total mass transfer coefficient was between (0.12–0.215)×10-3mol?m-3?s-1?Pa-1.If the total mass transfer coefficient of 0.25×10-3mol?m-3?s-1?Pa-1is taken as the pass line,then two types of strong alkaline solutions (NaOH solution and Ca(OH)2solution) met the use requirements when the HCl concentration was less than 1.8%.

It can be seen from Figs.8 and 9 that with the increase of the concentration of HCl in the gas phase,the absorption effect and the value of the total volumetric mass transfer coefficient had greatly decreased.The changing trend is similar to that of increasing the gas flow rate,but the reduction rate of the total volume mass transfer coefficient is much higher than that situation.Simply increasing the concentration of HCl without increasing the gas phase rate can’t strengthen the strong cyclonic gas field,nor can it increase the strength and intensity of the gas–liquid coupling,resulting in a decrease in the absorption effect.However,as the concentration of HCl increased,the partial pressure of the gas phase increased,and the total absorption rate was judged to be improved in the same manner as above.

The absorption efficiency of water was between 95% and 99%,the total volumetric mass transfer coefficient was between(0.12–0.19)×10-3mol?m-3?s-1?Pa-1.The downward trend of the total volumetric mass transfer coefficient was quite significant.Since water mainly depended on the extremely high solubility of hydrogen chloride for purification,it was also unable to face greater concentrations.

The absorption efficiency of sodium carbonate solutions of different concentrations was between 97.5%and 99.7%,the total volumetric mass transfer coefficient was between (0.15–0.235)×10-3mol?m-3?s-1?Pa-1.The overall absorption efficiency and total volume mass transfer coefficient were still higher than that of water but lower than that of a strong alkaline solution.The changing trend was like the experiment of changing the gas flow rate,but the change was larger.

The absorption efficiency of sodium hydroxide solution and calcium hydroxide solution was between 99.5% and 99.9%,and the total volumetric mass transfer coefficient is between (0.215–0.28)×10-3mol?m-3?s-1?Pa-1.The total volumetric mass transfer coefficient was still very high and can effectively handle these conditions.

When the intake air concentration was 2% and the absorption liquid was water,the hydrogen chloride concentration detector once burst the meter during the experiment,and a large amount of white mist was emitted.During the experiment,the white mist did not cause obvious damage to the human body or the equipment,but after a week of this phenomenon,the equipment in the laboratory showed different degrees of corrosion,and the corrosion of the equipment in this experiment was the most significant.It was believed that a large amount of HCl gas escaped,which caused corrosion effects.Other research groups should avoid this situation when conducting similar experiments.

3.4.Effect of temperature change on absorption

Fig.10 shows the effect of temperature of absorption liquid on absorption efficiency when the room temperature was 25°C,the hydrogen chloride concentration in the treated gas was constant at 1%,the inlet air flow rate was 4.175×10-4m3?s-1(25 L?min-1),and the absorption liquid flow rate was 11.12×10-4m3?s-1(40 L?h-1).Fig.11 shows the effect of temperature of absorption liquid on total mass transfer coefficient of gas phase volume.The absorption efficiency was between 98.64%and 99.91%and the total volumetric mass transfer coefficient was between (0.173–0.283)×10-3mol?m-3?s-1?Pa-1.

Fig.8.Influence of hydrogen chloride concentration on absorption efficiency.

Fig.9.Influence of hydrogen chloride concentration on total mass transfer coefficient of gas phase volume.

It can be seen from the Figs.10 and 11 that with the increase of the temperature of absorption liquid,the absorption efficiency of H2O and weak basic absorption solution presented a downward trend,while the absorption efficiency of the strong alkaline absorption solution didn’t show any obvious change.The absorption efficiency of water was between 98.64% and 99.11%,which was the lowest among all absorption solutions.The process of HCl dissolving in water will release the heat of dissolution,and the high temperature of the absorption solution hinders the dissolution process.Therefore,with the increase of temperature of absorption liquid,the solubility of HCl in water will continue to decrease,resulting in a significant decrease in the absorption effect.The absorption efficiency of weak basic absorption solution was between 99.21%and 99.70%.A large amount of heat will be produced in the reaction of weakly basic solution with HCl gas.If the temperature of the absorption solution is too high,the reaction process will be hindered and the reaction will be inhibited.As a result,the absorption efficiency will decrease with the increase of temperature of absorption liquid.When the type of the absorption solution was strong alkaline solution,the absorption efficiency almost didn’t change.The reaction between strong acid and strong base is very violent and will release a lot of heat.Although the increase of the temperature of the absorption liquid will hinder the reaction to a certain extent,it can’t play a decisive role in the extremely severe acid-base neutralization reaction.As a result,the absorption efficiency was always around 99.88%.

Fig.10.Influence of temperature of absorption liquid on absorption efficiency.

Fig.11.Influence of temperature of absorption liquid on total mass transfer coefficient of gas phase volume.

3.5.Absorption efficiency comparison

It can be seen from Table 3 that the gas cyclone-liquid jet absorption separator has a huge advantage in the working condition of HCl absorption in the exhaust gas.Compared with the dry method of dechlorination,the gas cyclone-liquid jet absorption separator does not require extremely high temperatures.Being compared with similar wet removal equipment,the absorption effect of gas cyclone-liquid jet absorption separator has absolute advantages and a similar cost.Comparing with traditional spray tower equipment,the use of gas cyclone-liquid jet absorption separators can greatly reduce the floor space and improve the absorption effect.

Table 3 Comparison of each equipment’s absorption efficiency

4.Conclusions

The gas-phase cyclone-liquid-phase jet purifier can effectively purify chlorine-containing tail gas.When using strong alkaline absorption liquid,the absorption efficiency was always maintained above 98%.When the room temperature was 25°C,the intake air volume was constant at 4.175×10-4m3?s-1(25 L?min-1),thehydrogen chloride volume percentage was 1%,and the absorption liquid flow rate was 16.68×10-6m3?s-1(60 L?h-1),the absorption efficiency can reach 99.9%,and the total mass transfer coefficient can reach 0.28×10-3mol?m-3?s-1?Pa-1.

Under the same conditions,increasing the flow or concentration of the absorption liquid can greatly improve the HCl absorption effect and total mass transfer coefficient of the weakly alkaline absorption liquid,but can’t improve the HCl absorption effect and total mass transfer coefficient of the strong alkaline absorption liquid obviously.

Under the same conditions,the larger the inlet gas volume,the higher the gas concentration,the worse the absorption effect,and the lower the total volumetric mass transfer coefficient.However,the strong alkaline absorption liquid can still maintain a high mass transfer coefficient in a certain gas inlet volume range and a certain concentration range,which can meet most working conditions.Gas cyclone-liquid jet absorption separator has great advantages in absorbing HCl in the exhaust gas and has high industrial application value,especially in the titanium dioxide industry.

Nomenclature

aeeffective mass transfer area per unit volume,m2?m-3

atspecific surface area of filler,m2

cLconcentration of HCl in the solution at the gas–liquid interface,mol?m-3

c* equilibrium concentration of HCl in the solution at the gas–liquid interface,mol?m-3

DGdiffusion coefficient of HCl in gas,m2?s-1

dcharacteristic length,m

dGgas characteristic length,m

dLliquid characteristic length,m

Ereaction enhancement factor

Gintake volume flow,m3?s-1

GrGrashov number

HHenry coefficients,Pa?m3?mol-1

Kequipment parameters,m

KGoverall mass-transfer coefficient,mol?m-2?s-1?Pa-1

KGaoverall mass transfer coefficient of gas phase volume,mol?m-3?s-1?Pa-1

kGgas mass-transfer parameter,mol?m-2?s-1?Pa-1

(physical) liquid-side mass-transfer coefficient,m?s-1

Labsorption liquid volume flow,m3?s-1

NAoverall absorption rate,mol?m-2?s-1

pGpartial pressure of HCl at gas–liquid interface,Pa

p* equilibrium partial pressure of HCl,Pa

Rideal gas constant,8.314 J?mol-1?K-1

ReReynolds number

ReGgas Reynolds number

ReLliquid Reynolds number

SGair inlet area,m2

SLjet hole area,m2

Ttemperature,K

tgas-liquid contact time,s

uGgas velocity,m?s-1

uLliquid velocity,m?s-1

yHCl mole fraction in the vapor phase

zdrop in height from the inlet in gas cyclone-liquid jet seperator,m

ρGgas density,kg?m3

ρLliquid density,kg?m3

μGgas viscosity,Pa?s

μLliquid viscosity,Pa?s

Declaration of Competing Interest

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

We would like to express our thanks for the sponsorship of National Natural Science Foundation of China (21878099),and Science and Technology Commission of Shanghai Municipality(19DZ1208000).