Performance of CO2 absorption in a diameter-varying spray tower☆

Xiaomei Wu ,Yunsong Yu ,Zhen Qin ,Zaoxiao Zhang ,*

1 School of Chemical Engineering and Technology,Xi'an Jiaotong University,Xi'an 710049,China

2 State Key Laboratory of Multiphaseflow in Power Engineering,Xi'an Jiaotong University,Xi'an 710049,China

1.Introduction

Global warming and the greenhouse effect have become a huge challenge for the sustainable development of the world[1–3].It is well known that CO2is the major greenhouse gas that contributes to global warming more than 60%,resulting in the necessity to reduce the CO2emission[4,5].Currently,post-combustion CO2capture is a promising choice in the near-to middle-term,since it can be retro fitted to the existing power plants compared to the other approaches[6,7].Among allthe technologies,chemicalabsorption is generally recognized as the most mature technology for industrial application,and monoethanolamine(MEA)is the most widely used absorbent[8,9].

Conventional MEA absorption process suffers from high energy consumption due to its immense steam consumption in the regeneration process,leading to the extremely high operating cost[3].In order to reduce the cost,apart from choosing good absorbent,it is very important to select effective reactors and proper operating conditions.The application of a spray tower instead of a packed reactor for CO2capture is a relatively recent development.Javedet al.[10]studied the low-concentration CO2spray absorption with NaOH aqueous solution and the experimental results drew a conclusion that the existing of nozzle has greatly improved the mass transfer performance.Kuntzet al.[11,12]compared the mass transfer efficiency of spray towers with packed columns for CO2absorption into MEA solvent and declared thatthe spray tower was capable ofremoving CO2from gas mixture ata higher rate than that of the packed column.Niuet al.[13,14]conducted an experiment for CO2absorption into MEA solution in a spray tower.The experimental results showed that the mole ratio of MEA to CO2was the main factor for absorption performance,and the spray tower can achieve more than 95%CO2removalrate,which verify the feasibility of the spray tower used in CO2capture.Zenget al.[15]studied the absorption of CO2into aqueous ammonia,and found that the performance of spray towers varies with the operating parameters.The overall mass transfer coefficient was measured to provide reference data for the future industrial design.Limet al.[16]studied the relationships between the capture efficiency and the operating parameters and also reported the optimum tower diameter for a given spray nozzle.However,most of the previous experiments were conducted in a cylindrical tower by using a single spray nozzle,whose con figuration differs from that used in actual industry,making the results of these studies far from application.

This paperfocuses on the enhancementofCO2absorption process by using an improved diameter-varying spray tower.As mentioned in literature,absorption in spray tower mainly occurred in the nozzle exit,hence increasing the space of nozzle exit is a feasible way to improve the absorption performance.The reaction sections of the proposed spray towerare composed oftwo parts:the cylindricalsection and the conical section.The existence of the conical section would increase the effective contacting area and gas–liquid contacting time,which will benefit the absorption performance.A new spray mode of dual-nozzle opposed impinging spray was also proposed to replace the single nozzle spray method,aiming to enhance heat and mass transfer performance.When droplets from two opposite spray nozzles impinge and exchange momentum in the center of the tower,the droplets breakup into smaller size which would cause a rapid increase of interfacial area leading to better heat and mass transfer performance.Experiments were carried out in the spray tower under a wide range of operating conditions to investigate the effects of various operating parameters,including CO2inlet concentration,total gas flow rate,liquid flow rate,MEA concentration,liquid to gas ratio and mole ratio of MEA to CO2on absorption performance.The performance of the proposed spray towerwas evaluated in terms ofthe CO2removalrate and the overall mass transfer coefficient.Additionally,empirical correlations for the mass transfer coefficient of the proposed diameter-varying spray tower absorption system were developed to predict the experimental results.

2.Experimental Method

2.1.System description

The CO2absorption experimental setup is shown in Fig.1(a).It mainly comprises the spray tower,the absorbent distribution system,the flue gas distribution system and the gas analyzing system.The diameter-varying spray tower is uniquely fabricated with two spray nozzles locating on the opposite side and at the upper part.Fig.1(a)only shows one side ofthe gas and liquid inlets for clearprocess description.The detailed structure of the diameter-varying spray tower is shown in Fig.1(b).During the experiment,aqueous MEA solution is pumped through the spray nozzles(0.5 mm orifice diameter,60 deg.spray angle)to form droplets,then droplets from two opposite spray nozzles impinge and exchange momentum in the center of the tower.The droplets would breakup into smaller size,then contact with the gas mixture entered from the bottom of the tower.MEA solution is piped to the spray nozzles using a plunger metering pump(0.8–1.0 MPa),and the flow rate is measured with a calibrated rotameter.CO2and N2are mixed to certain concentrations(8 vol%,12 vol%,16 vol%or 18 vol%)before entering the absorber to act as the flue gas.The flow rate of gas mixture is controlled by a mass flow controller,and the gas mixture is fed through a gas mixing tank to ensure a uniform distribution of species in the gas.Then the gas is introduced into the tower from two bottom inlets and reacts with MEA solution.After absorption,the vent gas from the top of the absorber is dried throw a drying tower with anhydrous silica gel.Then the CO2concentration in the gas mixture is continuously measured atboth gas inlets and gas outlets,using an infrared gas analyzer(model IRME-S,Xi'an Weichuang Instrument Inc.).The reading range of the analyzer is 0–20.0%of CO2by volume with the accuracy of 0.1%of the full-scale reading.Experiments are repeated to validate the reproducibility of the results.

2.2.Experimental conditions

The geometry of the diameter-varying dual-nozzle opposed impinging spray tower and the operating parameters in experiments are listed in Table 1.

Table 1Geometry and operating parameters of the proposed spray tower

2.3.Mass transfer model

The CO2removal rate and the overall mass transfer coefficient are chosen to evaluate the absorption performance of the improved spray tower.

Fig.1.The schematic of experimental setup of CO2 absorption by aqueous MEA in the diameter-varying dual-nozzle opposed impinging spray tower(a),the geometry of proposed spray tower(b).(1.N2 cylinder;2.CO2 cylinder;3.Gas mixing tank;4,8,12.Pressure gauge;5,13.Gas flowmeter;6.dual-nozzle opposed impinging spray tower;7.Liquid receiver;9.Liquid flow meter;10.Pump;11.Feed receiver;14.Drying tower;15.CO2 analyzer;16.Computer.)

2.3.1.The CO2 removal rate

The removal rate(η)defines the percentage of CO2in the gas stream that is removed during absorption process,and it is simply determined by the difference between the amounts of CO2entering and leaving the spray tower,which can be expressed by the following equation:

Fig.2.Effect of liquid flow rate on the CO2 removal rate and the overall mass transfer coefficient.(G L=30 wt%,G=3 m3·h-1,T=20 °C,C G=8 vol%).

2.3.2.The overall mass transfer coefficient

The overall mass transfer coefficient(KGae)is a lumped parameter that represents the absorption performance per unit volume of reactor.It is a combination of thermodynamics,kinetics,and hydrodynamics of CO2absorption system.Thus,itis really necessary to introduce the overall mass transfer coefficient to qualify the mass transfer performance of the improved spray tower.The material balance of the spray tower can be expressed as

whereGIis the inert gas flow rate,P(yCO2,G-y?CO2)is the mass transfer driving force of gas phase,Zis the height of the tower,andYCO2,Gis the mole ratio of CO2in gas phase.

According to Eq.(4),the overall mass transfer coefficient can be expressed as

where λ1,λ2are proportionality coefficient,d1is the diameter of cylindrical absorption section,andd2is the equivalent diameter of the conical absorption section.

3.Results and Discussions

3.1.Effect of liquid fl ow rate

The effects ofliquid flow rate on the CO2removalrate and the overall mass transfer coefficient were investigated.As can be seen from Fig.2,the CO2removal rate and the overall mass transfer coefficient increase from 62.1%to 93.1%and 0.167 to 0.452 kmol·m-3·h-1·kPa-1respectively,as the liquid flow rate increases from 40 L·h-1to 100 L·h-1.This give rise to the number of droplets produced by the spray nozzles increases and the size of droplets becomes smaller,as the liquid flow rate increases.In this sense,the interfacial area between the gas and liquid phases increases,leading to a better mass transfer performance between MEA and CO2molecules.Besides,with the increase of liquid flow rate,the droplets flow rate increased and the boundary layer of liquid phase decreased.So the resistance for gas diffusion to the liquid phase decreased and the mass transfer performance is enhanced.As has mentioned above,both the CO2removal rate and the overall mass transfer coefficient increased with the liquid flow rate.However,the increasing tendency dropped rapidly at the higher range of liquid flow rate,this is because the reduction in droplet size by the increasing of liquid flow rate becomes insignificantand the increase ofeffective interfacial area is limited.Hence,the mass transfer performance cannot be enhanced furthermore at higher liquid flow rate.

3.2.Effect of MEA concentration

Fig.3 shows the pro file of the CO2removal rate and the overall mass transfer coefficient under different MEA concentrations.As shown in Fig.3,the CO2removal rate and the overall mass transfer coefficient increase from 84.2%to 94.0%and 0.312 to 0.472 kmol·m-3·h-1·kPa-1respectively,as the MEA concentration increases from 10 wt%to 40 wt%.This is attributed to the fact that the increase of the MEA concentration yields more active MEA molecules available to diffuse toward the gas–liquid surface and then react with CO2molecules,which will enlarge the reaction enhancementfactor and lead to a better mass transfer performance.Nevertheless,from the point of industrial application,the viscosity of solution increases significantly at higher MEA concentration.As for the packed tower,the increase of liquid viscosity seriously affects the distribution of absorbents on the packing,which would block the absorption process.Forthe spray tower this side effect becomes insignificant because the absence of packing.However,the increase of liquid viscosity would do harm to the droplets distribution of spray nozzles and severe corrosion would occur in the equipment(like pipes,pumps,nozzles,and tower).These side effects would block the improvement of absorption performance and increase the capital cost for the maintenance.Hence,the absorption rate and cost should be balanced when increasing the concentration of MEA.

Fig.3.Effect of MEA concentration on the CO2 removal rate and the overall mass transfer coefficient.(L=80 L·h-1,G=3 m3·h-1,T=20 °C,C G=8 vol%).

3.3.Effect of gas fl ow rate

Fig.4 shows the effect of gas flow rate on the CO2removal rate and the overall mass transfer coefficient.The experimental results show that when the gas flow rate increases from 1.0 m3·h-1to 5.0 m3·h-1,the overall mass transfer coefficient increases from 0.150 to 0.574 kmol·m-3·h-1·kPa-1,however,the CO2removal rate decreases from 93.5%to 86.8%.According to the gas–liquid mass transfer theory,the mass transfer coefficient increases with the increase of gas flow rate.This is because as the total gas flow rate increases,the amount of CO2molecules available to contact and react with MEA molecules increased,which will lead to an increase of the overall mass transfer coefficient.However,the mole ratio of MEA to CO2decreases with the increasing total gas flow rate,which means that more CO2molecules will contact and react with limited MEA molecules bringing about a decrease of the CO2removal rate.Therefore,in order to keep the CO2removal rate at a higher value,it is important to maintain the mole ratio of MEA to CO2at a suitable point.

Fig.4.Effect of gas flow rate on the CO2 removal rate and the overall mass transfer coefficient.(L=80 L·h-1,C L=30 wt%,T=20 °C,C G=8 vol%).

3.4.Effect of CO2 concentration

The effect of CO2concentration on the CO2removal rate and the overall mass transfer coefficient was shown in Fig.5.Experimental results show that the CO2removal rate and the overall mass transfer coefficient decrease from 92.2%to 84.0%and 0.427 to 0.292 kmol·m-3·h-1·kPa-1respectively,as the CO2concentration increases from 8 vol%to 18 vol%in a fixed 80 L·h-1liquid flow rate.In general,according to the two- film theory,the gas phase driving force and gas phase mass transfer coefficient increase with the increase of CO2concentration,which will enhance the absorption process.Whereas,the mole ratio of MEA to CO2decreased with the increasing CO2inlet concentration,which means more CO2molecules react with limited active MEA molecules and this will cause the reduction of CO2removal rate.Thus,the CO2removal rate decreased slightly with the increasing of CO2inlet concentration in the spray tower.Moreover,the gas phase driving forceP(yCO2,G-y?CO2)increased with the increase of CO2concentration,which willlead to the decrease ofthe overall mass transfer coefficient.

3.5.Effect of liquid to gas ratio

As have been mentioned above,the liquid to gas ratio affects the absorption performance to some extent.The effect of liquid flow rate and inlet gas flow rate discussed above can be summarized as the effect of liquid to gas ratio.As is depicted in Fig.6,the CO2removal rate and the overall mass transfer coefficient increase from 62.1%to 93.1%and 0.167 to 0.452 kmol·m-3·h-1·kPa-1respectively,as the liquid to gas ratio increases from 0.0136 to 0.0335.Due to the increase of liquid to gas ratio,the thinner boundary layer of liquid phase and larger contacting area would decrease the mass transfer resistance and accelerate the reaction process.However,under the larger values of liquid to gas ratio,the grow tendency becomes slow.

3.6.Effect of mole ratio of MEA to C O2

Fig.5.Effect of CO2 concentration on the CO2 removal rate and the overall mass transfer coefficient.(G L=30 wt%,G=3 m3·h-1,T=20 °C,L=80 L·h-1,C G=8 vol%).

Fig.6.Effect of liquid to gas ratio on the CO2 removal rate and the overall mass transfer coefficient.(C L=30 wt%,T=20°C,C G=8 vol%).

As have been mentioned above,the mole ratio of MEA to CO2also affects the absorption performance obviously.The effect of MEA concentration and CO2concentration discussed above can be summarized as the effect of mole ratio of MEA to CO2.Fig.7 shows that in a fixed liquid to gas ratio of 0.0267,the CO2removal rate and the overall mass transfer coefficient increase from 81.7%to 89.9%and 0.278 to 0.371 kmol·m-3·h-1·kPa-1respectively,as the mole ratio of MEA to CO2increases from 6.39 mol·mol-1to 25.5 mol·mol-1.At the same liquid to gas ratio,the increase of MEA to CO2mole ratio allows more active MEA molecules to contact and react with CO2molecules,causing the increase of CO2removal rate and the overall mass transfer coef ficient.It can be concluded that both the liquid to gas ratio and mole ratio of MEA to CO2are key factors,which affect the performance of CO2absorption process.

4.Mass Transfer Correlations

Mass transfer coefficient correlation is considered to be a very important parameter for the absorption column design,as well as for effectively operating and the prediction of experimental results.The equation ofKGaeused in this paper has been widely accepted and applied in both packed columns and spray towers[17–21].However,KGaevaries with the types of absorber,types of packing,and operating conditions.Therefore,it is necessary to develop an effective predictive correlation ofKGaefor the improved spray tower.

4.1.Development of correlations for the improved spray tower

Many researchers have developed correlations to predict the mass transfer performance for different packings and systems.The total resistance of absorption process consists of gas phase resistance and liquid phase resistance,which can be presented as

The right-hand side term of the equation represents the gas and liquid film resistance,respectively.When the system is controlled by the resistance in the liquid phase,the equation can be simplified as

Furthermore,according to Astarita[22],the enhancement factor of chemical reaction β can be expressed as

where αeqrepresents CO2loading of solution in equilibrium withPCO2,α represents the CO2loading in solution.

When the gas film controls the system,the equation can be simplified as

The correlation for overall mass transfer coefficientKGaeis expressed as

whereLrepresents liquid flow rate,Grepresents gas flow rate,bandcare the coefficients.When the liquid film controls the system,the value ofbis 0.3–0.7 and the value ofcis only 0.06–0.08.However,when the gas film controls the system,the value ofcincreases to 0.67–0.80[23].

Based on the correlations discussed above,theKGaecan be expressed as

The relationship can be expressed by plotting the term ofKGae/LbGcagainst the term of(αeq-α)C/PCO2.By trial and error,the optimum values ofbandcwere found to beb=0.68 andc=0.075,and the plotis shown in Fig.8.Based on linearregression analysis,the predictive correlation forKGaefor CO2absorption into aqueous MEA in the diameter-varying dual-nozzle opposed impinging spray tower is expressed as

The predictedKGaeare in good agreement with the experimental results under varies CO2concentrations and the corresponding values ofAandBare shown in Table 2.However,this model is provided only for the purpose of predicting unknownKGaevalues based on the experimental conditions as shown in Table 1 and only for this type of spray tower.

Fig.8.Relationship between K G a e/LbGc and(αeq-α)C/P CO2 for the diameter-varying dualnozzle opposed impinging spray tower.

Table 2Calculated coefficient for predictive mass transfer correlation

5.Conclusions

A diameter-varying spray tower and a new spray mode of dualnozzle opposed impinging spray have been developed to enhance the performance of CO2absorption process.Experiments were performed to validate its mass transfer performance,in terms of the CO2removal rate(η)and the overallmass transfer coefficient(KGae).The experimental results indicate that the liquid to gas ratio and mole ratio of MEA to CO2are major factors affecting the absorption performance.Both the CO2removal rate and the overall mass transfer coefficient increase with the liquid flow rate,MEA concentration,liquid to gas ratio and mole ratio ofMEA to CO2and decrease with the inlet CO2concentration.However,with the increase of total gas flow rate,the overall mass transfer coefficient increases,but the CO2removal rate decreases.Under the experimental conditions,the maximums of η andKGaeare 94.0%and 0.574 kmol·m-3·h-1·kPa-1respectively.Furthermore,new correlations were developed to predict the overall mass transfer coefficient under different CO2concentrations for the diametervarying dual-nozzle opposed impinging spray tower absorption system in this study.The predicted results are in good agreement with the experimental results,which can be used in the tower design,effectively operating and the prediction of experimental results.

Nomenclature

Acoefficient

aeeffective contacting area,m2·m-3

Bcoefficient

bcoefficient

Camine concentration,kmol·m-3

ccoefficient

d1the diameter of cylindrical absorption section,m

d2the equivalent diameter of the conical absorption section,m

Ggas flow rate,m3·m-2·h-1

GIinert gas flow rate,kmol·m-2·h-1

HHenry's coefficient,kPa·m3·kmol-1

KGoverall mass transfer coefficient of gas phase,kmol·m-2·h-1

KGaevolumetric overall mass transfer coefficient,

kmol·m-3·h-1·kPa-1

kGgas phase mass transfer coefficient,kmol·m-2·h-1

kL0liquid phase mass transfer coefficient,kmol·m-2·h-1

Lliquid flow rate,m3·m-2·h-1

Psystem pressure,kPa

PCO2CO2partial pressure,kPa

S1the cross-sectional area of cylindrical absorption section,m2

S2the equivalent cross-sectional area of the conical absorption section,m2

YCO2,GCO2mole ratio in gas phase

Y1,Y2inlet and outlet mole ratio in gas phase

y?CO2equilibrium mole fraction of CO2

yCO2,GCO2mole fraction in gas phase

y1,y2inlet and outlet mole fraction in gas phase

Zheight of the spray tower,m

α solution CO2loading,mol CO2·mol amine-1

αeqCO2loading of solution in equilibrium withPCO2,mol CO2·mol amine-1

β enhancement factor of chemical reaction

η CO2removal rate

λ1proportionality coefficient

λ2proportionality coefficient

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