Zheng Li,Xiaoyan Ji,Zhuhong Yang,Xiaohua Lu,*

1 State Key Laboratory of Materials-Oriented Chemical Engineering,College of Chemical and Chemical Engineering,Nanjing Tech University,Nanjing 210009,China

2 Energy Engineering,Division of Energy Science,Lule? University of Technology,Lule? 97187,Sweden

Keywords:Air-blast atomizer CO2capture Aqueous alkali solutions

ABSTRACT In this work,an air-blast atomizing column was used to study the CO2capture performance with aqueous MEA(mono-ethanol-amine)and NaOH solutions.The effects of gas flow rate,the liquid to gas ratio(L/G),the CO2concentration on the CO2removal efficiency(η)and the volumetric overall mass transfer coefficient(KGav)were investigated.The air-blast atomizing column was also compared with the pressure spray tower on the studies of the CO2capture performance.For the aqueous MEA and NaOH solutions,the experimental results show that the η decreases with increasing gas flow rate and CO2concentration while it increases with increasing L/G.The effects on KGavare more complicated than those for η.When the CO2concentration is low(3 vol%),KGavincreases with increasing gas flow rate while decreases with increasing L/G.However,when the CO2concentration is high(9.5 vol%),as the gas flow rate and L/G increases,KGavincreases first and then decreases.The aqueous MEA solution achieves higher η and KGavthan the aqueous NaOH solution.The air-blast atomizing column shows a good performance on CO2capture.

1.Introduction

The increase of the atmospheric CO2concentration resulting from over a century of combustion of fossil fuels has led to a significant global climate change[1].CO2capture has received widespread attention[1-4],and various CO2capture technologies,including physical absorption,chemical absorption[5],adsorption[6-11],membrane[12],etc.,have been developed.Among these technologies,chemical absorption is widely used[13-18],and the process with the amine-based solvents such as mono-ethanol-amine(MEA)is adopted as the commercialized one in most post-combustion plants[17].However,for the MEA-based technology,there also exists some drawbacks,such as the solvent loss arising from its high volatility and solvent degradation that occurs in the process[19].Meanwhile,CO2capture with aqueous NaOH solutions has also been studied,showing a higher CO2absorption capacity than that of MEA[17,20].In addition,NaOH is even more abundant and cheaper than MEA.

In practice,CO2absorption can be conducted with the packed and tray columns.A lot of work has been carried out with different solvents and column internals[13-15,21,22].However,there are few works on the studies of the CO2adsorption with the spray column.Chen et al.[23]used a lab-scale spray-dryer to investigate the removal efficiency of CO2under different operational conditions,and it showed that the performance of the spray-dryer is better than that for the traditional NaOH wet-scrubber.Kuntz and Aroonwilas[24,25]studied the mass-transfer performance of a spray-column for the removal of CO2from a gas stream with the aqueous MEA solutions,and it was found that the spray-column can achieve a higher rate by a factor of 2-7 than the packed-column.Niu et al.[26,27]studied the performances of the packed reactor and spraycolumn used for the absorption of CO2into aqueous ammonia solutions,and the investigation showed that the spray-column can be used to capture CO2and high CO2removal rates can be achieved at low absorbent concentrations.Ma et al.[28]studied the performance of the spray tower in the mass transfer of CO2absorption in the aqueous ammonia solution,and Mahmud et al.[29],Chen et al.[30],Hatamipour et al.[31],Lee et al.[32]and Wu et al.[33]studied the CO2absorption performance in spray-column,showing a good performance for CO2absorption.

Meanwhile,atomization of liquids has an important impact on many industrial processes,such as internal combustion engines,spray drying,fire-fighting,thermal spraying,and painting[34-47].The most important types are the pressure and twin-fluid atomizers[47].Pressure atomizers discharge the liquid with a relatively high velocity to the gaseous atmosphere.An alternative approach of atomization is to inject a slow-moving liquid into a high velocity gas stream,i.e.twin-fluid or air-blast atomizers.These alternative atomizers have many advantages over the pressure atomizers,such as low operational pressure and high quality of sprays generated[34,35,42,43,46,48].In particular,all these atomizers can be used for the fluids with high viscosity.Recently,ionic liquids(ILs)have been proposed as promising absorbents for CO2capture[49-54];however,the absorption rate is too low due to the high viscosity,limiting IL-based technology industrialization.The use of atomizers can be a promising option to enhance CO2absorption rate in ILs.However,to the best of our knowledge,no work has been conducted in using atomizers in the area of CO2absorption with ILs.

The aim of our work was to study the CO2capture performance in solvents with the air-blast atomizer.In this work,the aqueous MEA and NaOH solutions were chosen as two typical solvents to study the performance of the air-blast atomizer as well as the optimal operational conditions.The work was conducted experimentally under various conditions to investigate the effects of process parameters,including the CO2concentration in the gas phase,the gas flow rate,and the L/G on the performance of spray-column.It is expected that the research results will provide knowledge for CO2absorption with the viscous absorbents,such as ILs,conducted in the future work.

2.Theory

2.1.The mechanism of CO2absorption

The mechanism of the overall CO2absorption in the aqueous MEA solutions can be explained as follows:

For the aqueous NaOH solutions,as a strong electrolyte,the NaOH is completely dissociated into Na+and OH-in water.The CO2absorption in the aqueous NaOH solutions can be explained as follows:

In the aqueous solutions,when CO2is excessive,also can react with CO2as described in Eq.(4),and the whole reaction between aqueous NaOH solution and CO2can be shown in Eq.(5):

2.2.CO2removal efficiency(η)and overall volumetric mass transfer coeffciient(KGav)

The CO2removal efficiency η was defined as the percentage of CO2in the gas stream that is removed during the absorption process,and it can be simply determined by the difference between the amounts of CO2entering and leaving the spray-column.The η can be expressed by the following equation[33]:

where y1and y2represent the mole fractions of CO2in the gas-phase at the inlet and outlet of tower,respectively.They can be obtained from the experiment measurements.

The calculation of KGav[28]was based on the double film theory combined with the mass balance.An infinitesimal height dh is taken from any location of the spray-column,and the KGavfor the absorption of CO2with solvents can be expressed as:where dY represents the mole ratio of CO2to N2at the infinitesimal height dh;G1is the flow rate of N2in kmol·m-2·h-1;p is the total gas pressure in kPa;y represents the mole fraction of CO2in the gasphase and y=and y*is the mole fraction of CO2at equilibrium corresponding with the CO2concentration in the liquid.

Due to the rapid chemical reaction between CO2and MEA or NaOH,y*will be extremely low,and thus y-y*can be approximately equal to y.Conducting integral of Eq.(7)from the bottom of the column to the top,we can get

where Y1and Y2represent the mole ratios of CO2to N2at the inlet and outlet of tower,respectively,and h represents the effective height of the spray-column in m.

Based on the double film theory,the relationship between KG,kGandis:

where kGandare the individual mass transfer coefficients in the gas and liquid film,respectively.E is the enhancement factor of the chemical reaction of CO2in the liquid phase.H is the Henry's law constant.

3.Materials and Setup

3.1.Materials

Reagent-grade NaOH and MEA with purities of≥99%were produced by Chinese Huixing Medicine Corporation Ltd.A fixed ratio of gaseous CO2(with a purity of≥99%)and N2(with a purity of≥99.9%)was supplied by Nanjing Tianhong Gas Ltd.,China.The purified water was prepared in our laboratory through a reverse osmosis membrane.All the purchased chemicals were used as received.

3.2.Experimental setup

Fig.1.The diagram of the experimental system.(1.N2/CO2mix gas cylinder;2.U-shaped manometers;3.Rotameter 4.Spray nozzle;5.Spray-column;6.Drying tube filled with anhydrous calcium chloride;7.Pump;8.Gas chromatography;9.Liquid storage tank;10.Gas-liquid separation tank;V1,V2.Valve.)

The setups used for gas absorption in this work are shown in Fig.1.The main equipment,the spray-column with one spray nozzle(purchased from Dongguan Bomei Spray System Ltd.,China),is in the upper part.The spray-column was constructed of acrylic plastic with the height of 1.5 m and an inside diameter of 0.1 m.The mixed gas with a fixed ratio of CO2to N2from the gas cylinders entered the spray-nozzle after depressurization by the reducing-valve,the pressure of the gas was measured by the U-shaped manometers,and the gas flow rate was measured by the rotameter.During the experiment,the absorbent entered the spray nozzle from the liquid storage tank with a metering pump.The concentration of CO2in the exhaust gas was measured with a gas chromatography after dehydration using a drying-tube filled with anhydrous calcium chloride.The exhaust gas was released from the top(open the valve V1 and close the valve V2)or the bottom(close the valve V1 and open the valve V2)of the column as depicted in Fig.1,respectively.

4.Results and Discussion

The effects of the operational parameters,such as the total gas flow rate,the liquid flow rate,and the CO2inlet concentration on the CO2removal efficiency(η)and the overall volumetric mass transfer coefficient(KGav)were studied systematically.The gas flow rate was set to be 0.2-0.8 m3·h-1,and the inlet gas pressures were 30 to 65 kPa which was detected with the U-shaped manometers.The experiment was conducted at the room temperature.All the operational parameters are listed in Table 1.

Table 1 Experimental operational parameters

4.1.The effect of gas flow rate on η and KGav

The effect of the gas flow rate on η and KGavwas studied under the condition at CO2concentration=9.5 vol% and liquid flow rate=0.5 L·h-1.As the results shown in Fig.2,the η decreases with the increase of the gas flow rate.For the aqueous 3 mol·L-1MEA solution(i.e.3 mol·L-1MEA),when the gas flow rates are 0.2 and 0.8 m3·h-1,the values of η are 48.1% and 19.2%,respectively.For the 3 mol·L-1NaOH,under the same conditions,the values of η are 35.4%and 0.66%,respectively.

Fig.2.Effect of gas flow rate on η.(CO2concentration=9.5 vol%,liquid flow rate=0.5 L·h-1)

The influence of the gas flow rate on KGavwas investigated with the results illustrated in Fig.3.For the 3 mol·L-1MEA,as the gas flow rate increases from 0.2 to 0.8 m3·h-1,KGavincreases first and then decreases,and the maximum KGavis 0.082 kmol·m-3·h-1·kPa-1at 0.6 m3·h-1;For the 3 mol·L-1NaOH,similar observation can be observed,and the maximum KGavis 0.06 kmol·m-3·h-1·kPa-1at 0.4 m3·h-1.These results on η and KGavindicated that the MEA has a better CO2capture performance than that of NaOH under the same experimental conditions.

Fig.3.Effect of the gas flow rate on KGav.(CO2concentration=9.5 vol%,liquid flow rate=0.5 L·h-1).

For the air-blast atomizers,the diameter of the sprays decreases with the increase of the gas flow rate[40],as reported in our previous work[55],the decrease in the mass transfer distance is beneficial to enhance the absorption process.It also means that a larger gas-liquid interfacial area can be achieved at a higher gas flow rate,according to the double film theory,which is in favor of KGav.In addition,as reported by Ma et al.[28],with increasing flow rate,kGincreases,improving the mass transfer between the gas and liquid phases and thereby increasing the value of KGav.However,the increase of the gas flow rate will short the residence time of the gas in the spray-column and increase the CO2amount in the gas phase,resulting in the decrease of η.For 3 mol·L-1MEA,the increase in CO2amount in the gas phase makes the CO2absorption of MEA tend to be saturated(Fig.4);this subsequently declines the chemical reaction enhancement factor,and thereby the mass transfer coefficient in the liquid phase reduces,hindering mass transfer between gas and liquid.For 3 mol·L-1NaOH,there is no enough resident time for NaOH to absorb CO2,and this characteristic is also used by some researchers to separate gases selectively[56].Due to the above reason,the KGavincreases first and then decreases.Therefore,the gas flow rate should be kept with a suitable value.

Fig.4.Effect of the gas flow rate on CO2loading.(CO2concentration=9.5 vol%,liquid flow rate=0.5 L·h-1).

Based on the discussion and analysis above,in addition to the gas flow rate,the KGavis affected by three factors:the liquid flow rate,the CO2concentration,and the resident time.A lower CO2concentration,a higher liquid flow rate,and the different options of the exhaust gas outlets were further studied.

4.2.The effect of CO2concentration on η and KGav

As mentioned in Section 4.1,a lower CO2concentration(i.e.3 vol%),compared to 9.5 vol%,was chosen to study the effect of CO2concentration on η and KGav;the results are shown in Figs.5 and 6;the values of KGavand η are apparently affected by the CO2concentration in the gas stream.It can be seen from Fig.5 that the η decreased with the increase of the CO2concentration under the same gas flow rate.The 3-mol·L-1MEA solution also shows a better performance on the CO2removal compared to the 3-mol·L-1NaOH solution.

As the results shown in Fig.6,the gas flow rate affects KGavdifferently when the CO2concentration decreases from 9.5 to 3 vol%.The KGavincreases with the increase of the gas flow rate at the low CO2concentrations(3 vol%)for both MEA and NaOH and decreases with the increase of CO2concentration.When the gas flow rate increases from 0.2 to 0.8 m3·h-1,for the 3 mol·L-1 MEA solution,KGavincreases about 3 times,while it increases 2 times for the 3-mol·L-1NaOH solution.The KGavfor 3 vol% is much greater than that for the 9.5 vol%.The 3 mol·L-1MEA solution achieves a greater KGavthan that of the 3 mol·L-1NaOH solution.

CO2concentration reflects the amount of CO2molecules in the gas phase.Based on the double film theory,an increase in CO2concentration results in an increase in the mass transfer of the gas phase.However,it should be noted that,the mass transfer process is controlled by the resistance of the liquid phase due to the diffusion in the liquid phase is restricted as compared with that of CO2from the gas phase to the gasliquid phase.As discussed in Section 4.1,the amount of CO2in the gas phase increases with the increase of the gas flow rate,and also increases with the increase of the CO2concentration,and thus the concentration of absorbent in the liquid phase decreases significantly.The same phenomenon can be observed for the lower CO2concentration(3 vol%),as shown in Fig.7.Therefore,KGavdecreases with increasing CO2concentration,the CO2concentration is another key parameter to design the spray-column.

As described in the above Sections 4.1 and 4.2,the 3 mol·L-1MEA solution always shows a better performance on the CO2removal compared to the 3 mol·L-1NaOH solution.However,at a comparable temperature of 293 K,the value of the second-order reaction rate constant(k2)for aqueous MEA[57]is smaller than that for NaOH[58].This indicates that the CO2absorption rate into aqueous MEA solution is smaller than that into NaOH due to both lower operating temperatures.In this work,the opposite results were obtained,which means that the chemical reaction resistance is not the restricted as compared with the diffusion in the liquid phase,the same results as this work was obtained in the packed column[59].In the spray column,the spray dimeter increases with the increasing of the surface tension of the absorbent[60].The surface tension of 3 mol·L-1MEA is lower than that of 3 mol·L-1NaOH,which means the smaller spray dimeter under the same operating condition in this work.As reported in our previous work[55],the decrease in the mass transfer distance is beneficial to enhance the absorption process.Therefore,the 3 mol·L-1MEA solution always shows a better performance on the CO2removal compared to the 3 mol·L-1NaOH solution.

Fig.5.Effect of CO2concentration on η.(a),3 mol·L-1 MEA;(b),3 mol·L-1 NaOH.(CO2concentration=3 and 9.5 vol%,liquid flow rate=0.5 L·h-1)

Fig.6.Effect of CO2concentration on KGav.(a),3 mol·L-1 MEA;(b),3 mol·L-1 NaOH.(CO2concentration=3 and 9.5 vol%,liquid flow rate=0.5 L·h-1).

Fig.7.Effect of CO2concentration on CO2loading.(a),3 mol·L-1 MEA;(b),3 mol·L-1 NaOH.(CO2concentration=3 and 9.5 vol%,liquid flow rate=0.5 L·h-1).

4.3.The effect of L/G on η and KGav

The L/G is a very important parameter in the CO2absorption process.To investigate the influence of L/G on η and KGavin the air-blast atomizing column,the 3 mol·L-1NaOH solution was selected in this section,and the following experimental conditions were set:the inlet gas flow rate of 0.2 to 0.8 m3·h-1,the CO2concentration of 9.5 vol%,and the liquid flow rates of 0.5 and 9 L·h-1.The results are shown in Figs.8 and 9.

As shown in Fig.8,the η increases initially very sharply with the increase of L/G and thereafter reaches almost a constant value beyond a L/G ratio of 0.0225 m3·m-3.The liquid flow rate has a great impact on the overall mass-transfer performance of the spray-column.Fig.9 shows that the KGavincreases with increasing of the liquid flow rate.When the gas flow rate is 0.2 m3·h-1,the CO2in the gas phase is completely absorbed,and no KGavcan be obtained.

Fig.8.Effect of L/G on η.(CO2concentration=9.5 vol%,liquid flow rate=0.5 L·h-1).

As the liquid flow increases,larger effective interfacial area(av)can be formed between the liquid and gas phases[61],leading to a higher overall mass transfer.As discussed in Section 4.1,with increasing the gas flow rate,KGavincreases first and then decreases.However,in case that the absorbent is enough(Fig.10),KGavalways increases with increasing the gas flow rate.With the increase of L/G,the effective mass transfer cross-sectional area,the gas phase mass transfer coefficient,and the liquid phase mass transfer coefficient all increase,and thus KGavincreases apparently.

Fig.9.Effect of liquid flow rate on KGav.(CO2concentration=9.5 vol%,liquid flow rate=0.5 L·h-1)

Fig.10.Effect of the liquid flow rate on CO2absorption loading.(CO2concentration=9.5 vol%,liquid flow rate=0.5 and 9 L·h-1)

Fig.11.Effect of gas flow rate on η.(CO2concentration=9.5 vol%,liquid flow rate=0.5 L·h-1).

Fig.12.Effect of gas flow rate on KGav.(CO2concentration=9.5 vol%,liquid flow rate=0.5 L·h-1).

4.4.The effect of the exhaust options on η and KGav

Two gas outlets(top and bottom of the column,as shown in Fig.1,named top discharge and bottom discharge)were set up to control different residence times under the same operational conditions for single factor experiments.The 3-mol·L-1NaOH solutions was also selected in this section.Figs.11 and 12 show the effect of the gas flow rate on η and KGavin these two options.In the case of the top discharge,as the gas flow rate increases,both η and KGavdecreases,except at the gas flow rate of 0.2 m3·h-1.In the case of the bottom discharge,η and KGavdecrease with the increase of the gas flow rate,but the values are greater than those of the top discharge under the same experimental conditions.This may be due to the bottom discharge has a longer gas-liquid contact time than that of the top discharge.

4.5.Comparison with the performance(KGav)of other columns in the literature

In Table 2,the CO2capture performances(KGav)obtained in this work and those from literatures with other columns under similar experimental conditions are listed for comparison.The KGavobtained in this work is lower than that in literature when the liquid flow rate is 0.5 L·h-1.However,when the liquid flow rate increases to 9 L·h-1,the KGavin this work is much higher than that in the literature,which means that the air-blast atomizing column has a better performance on CO2capture.

For aqueous MEA and NaOH solutions,the CO2absorption rate with the common tower is already acceptable,and the use of the air-blast atomizing column will enhance the absorption rate.While for the new developed absorbents,such as ionic liquids,the viscosity is relatively high,leading to low absorption rate,and there is a need to enhance the absorption rate.The use of air-blast atomizing column can be an option to solve the problem,and this work will provide knowledge on how to choose suitable operational conditions for the air-blast atomizing column.In this future,the new developed ILs will be used as absorbents to study the performance of air-blast atomizing column for CO2capture.

5.Conclusions

To improve the CO2absorption rate,the performance of spraycolumn on the CO2absorption with aqueous MEA and NaOH solutions was studied,and several important factors,such as the CO2removal efficiency and the overall volumetric mass transfer coefficient,were concerned.The effects of the gas flow rate,L/G,CO2concentration on η and KGavwere studied systematically.The results show that for the aqueous MEA and NaOH solutions,η decreases with increasing the gas flow rate and CO2concentration and it increases with increasing L/G.For KGav,when the CO2concentration is low(3 vol%),it increases with increasing the gas flow rate and decreases with increasing L/G.When the CO2concentration is high(9.5 vol%),it increases first and then decreases with increasing the gas flow rate and L/G.The aqueous MEA solution achieves a higher η and KGavcompared to the aqueous NaOH solution.The air-blast atomizing column shows a good performance on CO2capture.This work will provide knowledge on how to choose suitable operational conditions for the air-blast atomizing column.

Table 2 Comparison with the performance(KGav)of other columns in the literature①