A large-scale experimental study on CO2 capture utilizing slurry-based ab-adsorption approach

Shuren Yan,Peng Xiao,Ding Zhu,Hai Li,Guangjin Chen,Bei Liu*

State Key Laboratory of Heavy Oil Processing,China University of Petroleum,Beijing 102249,China

Keywords:CO2 capture Zeolitic imidazolate framework-8 Slurry Bubble column Scale-up

ABSTRACT The increasing concentration of CO2 in atmosphere is deemed the main reason of global warming.Therefore,efficiently capturing CO2 from various sources with energy conservation is of great significance.Herein,a series of experiments were carried out to successfully test the slurry-based ab-adsorption method for continuously capturing CO2 in the large-scale cycled separation unit with cost-effect taking into account the scale-up criteria.A bubble column (with height 4900 mm and inner diameter 376 mm) and a desorption tank (with volume 310 L) are the essential components of the separation unit.The novel slurry used in this study was formed with zeolitic imidazolate framework-8 and 2-methylimidazole-water solution.The influence of operation conditions was investigated systematically.The results show that increasing sorption pressure and slurry height level,decreasing gas volume flow and sorption temperature are beneficial for separation processes.The volume fraction of CO2 in the feed gas was also studied.Although the scale-up effect had been observed and it was found that it exerted a negative effect on CO2 capture,depending on experimental conditions,CO2 removal efficiency could still reach 85%-95% and the maximum CO2 loading in the recycled slurry could be up to 0.007 mol·L-1·kPa-1.Furthermore,the slurry-based method could be operated well even under very moderate regeneration conditions (333 K and 0.05 MPa),which means that the novel approach shows greater energy conservation than traditional amine absorption methods.

1.Introduction

It is widely believed that the global warming is mainly attributed to the CO2emissions from human activities.Different regions of the world have been suffering from the extreme climate caused by global warming.A number of insightful governments and organizations are dealing with the CO2emission and setting the reduction target.Therefore,capturing CO2from various sources are significant because they are responding for reducing the amount of CO2in the atmosphere.The applications of CO2capture are widely involved in various processes in the chemical and electricity industries,including purification of natural gas or biogas,and hydrogen purification in steam reforming from coal or natural gas [1,2].Researchers have been developing various technologies of CO2capture and separation,mainly including membrane separation,chemical absorption,and physical adsorption.Membrane technology have been developed and investigated by scientists.According to the reports of Lv et al.[3],the performance of CO2capture deteriorating problem could be solved by depositing a rough layer on the surface of polypropylene fiber membrane.Although the membrane approaches show high selectivity for specific gas sources,the unsatisfactory robustness withstanding harsh operation conditions considerably hinder their commercial scale applications [4,5].Various CO2capture approaches based on chemical absorption have been widely applied in the commercial industry.Damartzis et al.[6]reported that the economic benefit of capturing CO2from the flue gas stream was increased by 15%–35%due to the combinations of amine solvent and flowsheet structure.The aminbased amino acid-functionalized ionic liquid with high capacity was used to capture CO2efficiently,reported by Lv et al.[7].Barzagli et al.[8]described encouraging carbon capture performance of aqueous solutions based on pure alkanolamines.Recently,the promising approaches for CO2capture based on physical adsorption have attracted great attentions.A study was systematically carried out by Wang et al.[9]to investigate the physical adsorption.In the work,the amin-impregnated solid adsorbents were synthesized to capture CO2directly from atmospheric air.The CO2capture performance of Zeolite 13X was studied by Konduru et al.[10].The results indicated that even after fifth adsorption-regeneration cycles,the recovery efficiency still reaches 82–93%.Currently,the approaches widely applied in commercial industry for carbon capture mainly depend on chemical absorption and physical adsorption.However,both chemical absorption and physical adsorption have the intrinsic drawbacks.In terms of chemical absorption,in order to meet the requirement of the regeneration of absorbents,a large amount of energy would be consumed because of the new formed covalent bonds.In contrast,physical adsorption displays a great advantage in the energy conservation because of principle of weak van der Walls forces.Frustratingly,the intrinsic drawbacks of some famous materials(such as molecular sieves,zeolites,and activated carbons) have been proven that it is difficult to overcome.Depending on several studies,some intrinsic drawbacks include undesirable CO2/N2selectivity [11],low CO2capture capacity [12],and deteriorating carbon capture performance after recycled processes [13].

Recently,many researchers have been paying great attention to an emerging class of porous crystalline materials,so-called metalorganic frameworks (MOFs),to capture CO2[14].The outstanding features of the MOFs mainly include high uptake,ultrahigh porosity,large surface area,and thermal and chemical stability [15].However,since water exists ubiquitously in various real flue gases[16],the unsatisfactory hydrothermal stability of MOFs greatly restrict their application for CO2capture [17].That means the working conditions of MOFs need to meet the requirement of dehydrated circumstance because the porous structure of MOFs would be destroyed by water[18].One latest subclass of the MOFs to appear are zeolitic imidazolate frameworks(ZIFs)[19].Not only have ZIFs the same desirable features as MOFs,but also have distinctive properties including prominent hydrophobic characteristics as well as excellent chemical and thermal stability [20].Therefore,ZIFs have been considered as a latest generation of adsorbents with enormous potential for application in practical industry.Although wider scientific community are encouraged by above advantages of ZIFs,the engineering community are not resonated.The reason why engineers are lack of enthusiasm for applying ZIFs into commercial industry can be explained that ZIFs as physical solid adsorbents are limited in fixed-bed reactors with less-efficient batch operation.And the advanced integration heat exchangers are difficultly set up to recover numerous amounts of energy produced in the separation processes.In contrast,chemical absorption utilized in commercial industry usually achieves the continuous separation processes with effective heat exchange because of the favorable flow properties.However,to regenerate the chemical absorbents (usually aqueous amine solutions),high temperature is necessary to meet the requirements of regeneration processes.Inevitably,the water will be vaporized,thus consume a large amount of latent heat.Due to the hard-to-recycle heat,the bottleneck of energy conservation really exists in the chemical absorption processes.Therefore,a satisfactory approach should meet the needs of both high CO2capture capacity and low regeneration energy cost.In addition,the practical applications of ZIFs and other MOFs have been hindered by their high price.Recently,our group[21]reported a method which decreases the cost of ZIF-8 and makes it possible to be used in the practical applications.

A novel absorption-adsorption hybrid CO2capture approach was proposed by Liu et al.[22,23].The innovative technology is mainly based on the slurry by suspending zeolitic imidazolate frameworks (ZIF-8) in the glycol and 2-methylimidazole (mIm)mixture solution.It firstly combines both advantages of chemical absorption and physical adsorption,which means that the slurry can be operated continuously with great energy conservation.Further,it is encouraging to observe that the sorption enthalpy of slurry-based method was only-29 kJ·mol-1,resulting in the lower requirement of regeneration.Lei et al.[24]also proposed a similar absorption-adsorption approach based on ZIF-8 in the ionic liquid slurry.According to the results,the higher selectivity was obtained under low CO2partial pressures.However,the effects of water have been neglected in the previous studies.Liu et al.[25]have reported that ZIF-8 and water would undergo irreversible chemical reactions leading to the partial or complete destruction of ZIF-8 structure.But Jia et al.[26]drew the conclusions that adding the sufficient amounts of mIm into the slurry,the structure of ZIF-8 could be prevent from water destroying.The flow situations of solvents are considered as the significant factors in the practical industry.If the viscosity of the slurry exceeds 40 mPa·s,it has been considered as the main hindrances for its applications in commercial industry.Li et al.[27]reported that the existence of water in the slurry could considerably decrease the viscosity and simultaneously improve the capacity,which was great beneficial for the separation processes.Our previous work took one step further[28],i.e.a pilot-scale continuous unit for CO2capture was constructed to verify the feasibility of slurry-based method for commercial application.Meanwhile,a series of experiments were carried out to research the influence of the unit structures and the operation conditions.The results show that the CO2removal efficiency could reach as high as 92%–99% even under the atmosphere pressure.However,the scale-up effect was not researched systematically and reflected accurately.

In this study,a larger scale of experimental unit was originally constructed to make the separation conditions more similar with those in commercial industry.To our best knowledge,this is the first research of slurry-based method for CO2capture in a large scale of bubble column similar to practical industry.This work focused mainly on studying the influences of scale-up effect and operation conditions on CO2removal efficiency.The experimental results indicate that the slurry-based approach could be utilized to efficiently separate CO2with lower requirement of regeneration energy.Therefore,great cost-efficient would be achieved due to the combination of higher capture efficiency and lower energy cost.

2.Experimental

2.1.Experimental units

Fig.1.Schematic diagram of pilot scale CO2 separation system.1,air compressor;2,gas mass flow controller;3,pressure regulating valve;4,mixed gas buffer tank;5,pressure gauge;6,absorption tower;7,temperature detector;8,liquid mass flow meter;9,cooler;10,heat exchanger;11,circulating pump;12,desorption tank;13.condenser;14.vacuum pump;and 15.slurry inlet;and 16,real-time gas composition detector.Black line:gas;Orange line:slurry;Green line:coolant.

Fig.2.The picture of pilot scale CO2 separation system.

Fig.3.Absorption column setup.

The schematic diagram of the pilot CO2separation system and the corresponding picture are shown in Figs.1 and 2,respectively.In the pilot CO2separation system,the bubble(absorption)column is one of the key experimental components,as shown in Fig.3.The inner diameter and effective absorption height of the absorption column,which are the key structural parameters of the system,are 376 mm and 3690 mm,respectively.According to our previous study [28],the experimental results,especially the CO2removal ratio has a great relevance with the absorption temperature.In order to maintain constant absorption temperature and prevent this study from being affected by external temperature,48 vertical heat exchange tube bundles(inner diameter,2 mm)were installed in the absorption column.Water was used as the coolant flowing in the tube bundles to maintain constant experimental temperature.Another key component of the pilot CO2separation system is the desorption tank,as shown in Fig.4.The main structural parameter of the desorption tank is the volume of 310 L.The mechanical stirrer was set up in the desorption tank in order to accelerate the process of regeneration.In this study,the main structural parameters of the pilot CO2separation system are summarized in Table 1.

Fig.4.Desorption tank setup.

Due to the large scale of the experiment,a large amount of gas is used.In order to meet the requirement of sufficient amount of feed gas,the automatic gas-intake system was designed and set up.Two parts,CO2intake unit and air compressor,were included in the automatic gas-intake system.CO2and air were mixed in the mixed gas buffer tank according to the volume fraction required by this study.They were supplied by CO2intake unit and air compressor,respectively.CO2intake unit consisted of two combining-flow pipelines and each pipeline integrated 10 CO2cylinders to supply feed gas.To safely discharge CO2from the cylinders,the pressure reducing regulators and the separation control valves were set up.Meanwhile,the check valves were used to prevent reverse flow into the cylinder.To avoid the feed gas being interrupted due to insufficient gas supply,each pipeline operated independently.When the feed gas of one combiningflow pipeline was exhausted,another one was replaced to supply feed gas continuously.

In this study,three secondary platinum resistance thermometers were set up in the absorption column to monitor the absorption temperature.The uncertainty of the thermometers is ±0.1 K.The system pressure was measured by the differential pressure transducers with uncertainty of ±0.01 MPa.Changes in temperature and pressure were recorded as functions of elapsed time via a data integration system.CO2content in mixed gas discharged from top outlet of the absorption column was detected by the real-time gas composition detector,which is shown in Fig.5.

Table 1 Summary of key structural parameters of the separation apparatus

Fig.5.The picture of the real-time gas composition detector.

2.2.Experimental procedures

Feed gas was prepared in the mixed gas buffer tank.A pressureregulating valve was set up to regulate and stabilize the intake pressure of feed gas before it was injected into the absorption column.And then,the feed gas was injected from the gas inlet at the bottom of the absorption column.In the absorption column,the feed gas moved upward and was in contact with the slurry,which flowed downward from the slurry inlet at the top of the absorption column.In this study,two phases flowing model is counter-current model,which is beneficial for gas–liquid contact to enhance and improve the degree of mass transfer.Usually the commercial scale bubble columns are equipped with vertical heat exchange tube bundles to maintain the constant temperature.Therefore 48 vertical heat exchange tube bundles with inner diameter of 2 mm were installed in the absorption column.

CO2was then absorbed by the slurry in the counter-current of the bubble column.After the CO2absorption process,the CO2-rich slurry flowed out from the slurry outlet at the bottom of the absorption column.And then the CO2-rich slurry was injected into the desorption tank to regenerate.In order to improve the regeneration process,the mechanical stirrer was installed in the desorption tank.When the CO2-rich slurry entered into the desorption tank,it was quickly stirred by the mechanical stirrer,which serve to increase the surface area of slurry and accelerate the regeneration rate.At the same time,the desorption tank was heated to the desirable temperature,which is beneficial for the regeneration process.CO2was released from the CO2-rich slurry and pumped out through vacuum exhaust pipe after the regeneration process finished.And the slurry,which was desorbed to release CO2,was so-called CO2-lean slurry in this study.It was important to note that a small amount of slurry was vaporized because of the regeneration conditions in the desorption tank.Therefore,a small part of slurry was inevitably entrained by the released CO2escapeing from the desorption tank,which would result in a reduction of the amount of slurry and decreasing the CO2removal efficiency.To avoid this adverse effect,a condenser (see Fig.1) was set up on the gas outlet of the desorption tank to recover the gaseous slurry from the released CO2.Table 2 summarizes the main operating conditions of this study.

In this study,great care was also taken to exclude negative effect caused by the contaminants.Clean water was used to wash the units before every experiment was carried out.

2.3.Materials

In this study,the ZIF-8 material used in the slurry (ZIF-8/mImwater slurry) was synthesized by our group.Pan et al.[29]have reported the method of ZIF-8 synthesis in detail.To verify the structural consistency of the synthesized ZIF-8 in our laboratory,the standard samples of ZIF-8 were compared with the synthesized ZIF-8.The results of x-ray diffraction (XRD) patterns show an acceptable agreement,as shown in Fig.6.All standard materials,ZIF-8 and mIm,were purchased from Sigma-Aldrich company.The feed gas was gas mixture of CO2and air,which was supplied by automatic gas-intake system (shown in Fig.1).CO2was purchased from the Fulite Gas Industry Company.

In this study,the ZIF-8 materials were suspended in the solution of mIm and water as the medium capturing CO2.The detailed information about the work medium is shown in Table 3.

2.4.Data processing

In this study,mass balance is the fundamental principle guiding and calculating the experimental results.Reference to theindustrial CO2separation processes,the CO2removal ratio is defined as a key indicator to estimate the efficiency of CO2capture.It is calculated by:

Table 2 Summary of key operating conditions

Fig.6.XRD patterns of (a) commercially available ZIF-8 and (b) synthesized ZIF-8 used in this work.

Table 3 Mass of each component in ZIF-8/mIm-water slurry used in this study

In above formulation,V0represents the average volume flow of the feed gas from the gas inlet of the absorption column at experimental temperature and pressure.V1represents the average volume flow of the emission gas from the gas outlet of the absorption column at experimental temperature and pressure.Correspondingly,y0and y1are the mole fractions of CO2in the feed gas and the emission gas,respectively.In this study,y0and y1were measured by the real-time gas composition detector.

The amount of CO2included cyclic slurry is defined as GL,calculated by:

where L represents the volumetric flow rate of cyclic slurry.

Scis defined as the absorption coefficient of CO2included in the cyclic slurry in this study.It is calculated by:

where P represents the pressure at the bottom of absorption column.

3.Results and Discussion

3.1.Effect of absorption pressure

The experiments were carried out to test and verify the feasibility of industrial application of the absorption-adsorption hybrid separation method.The operating conditions including gas flow,height of slurry level,and absorption temperature and pressure were adjusted to try to improve and increase the CO2removal efficiency.Firstly,we found the CO2removal efficiency is almost less than 80% under normal pressure,as listed in Tables 4 and 5.This is different from our previous study[28],in which the CO2removal efficiency could exceed 90% under normal pressure.Therefore,we focused on researching the influence of absorption pressure on CO2removal ratio at first in this study.The absorption pressure was increased to 0.2–0.4 MPa then to try to increase the CO2capture efficiency.

Table 4 The influence of gas flow and height of slurry level on CO2 removal ratio under the normal pressure

Table 5 The influence of gas flow and absorption temperature on CO2 removal ratio under the normal pressure

Fig.7 clearly shows that increasing the absorption pressure was able to remarkably increase the CO2removal ratio.When the absorption pressure increased from normal pressure to 0.4 MPa,the CO2removal efficiency increased from 78.63% to about 94%.

Fig.8 indicates the influence of the absorption pressure (ranging from 0.2 MPa to 0.4 MPa)on the CO2capture efficiency,where the gas volume flow is 50 L·min-1and the absorption temperature is 313 K.As the study went on,the CO2concentration(mol%)in the emission gas increased rapidly in the first few minutes.In the next few hours of the study,the CO2concentration maintained stable.In this study,the stabilized CO2concentration represents the CO2capture efficiency,which could be used to estimate the separation results.As shown in Fig.8(b),the CO2capture efficiency can be displayed more straightforwardly when CO2removal ratio was used.Obviously,the CO2removal ratio was strongly influenced by the absorption pressure.When the absorption pressure was set to 0.4 MPa,the CO2removal ratio reached approximately 94%.However,it decreased to about 91% when the absorption pressure was set to 0.2 MPa.The CO2removal ratio decreased along with the decrease of absorption pressure.

In our previous study[28],we found the CO2removal efficiency is desirable and satisfied (＞92%) even under the normal pressure.However,the similar results were not obtained under the normal pressure in this study.Three main reasons were expected to explain the differences:(1) The obvious scale-up effect appeared with the larger scale of experimental units.The inner diameter of the absorption column used in this study is 376 mm,which is much larger than the absorption column with inner diameter of 60 mm used in our previous study.Some researchers[30–32]have reported that the gas holdup in the bubble decreased when the inner diameter of the bubble column was increased.The larger size of absorption column played an adverse role in maintaining the homogeneous flow regime stability.(2) Youssef et al.[33]indicate that the gas–liquid contact would deteriorate rapidly with the existence of vertical tube bundle in the bubble column.Chen et al.[34]reported that the experiments were carried out in the bubble column with vertical heat exchanger tubes.Because the vertical internals decreased the length scales of turbulence physically,the turbulent stress and eddy diffusivity decreased considerably.(3) Wong et al.[35]reported that the bubble size in the bubble column was significantly determined by the distributor aperture.The small bubbles,which could be obtained from gas distributor with small openings,were believed to support the good gas–liquid contact.Under laboratory conditions,the hole diameter of the distributor is ‘‘fine holes”(hole diameter,d0＜1 mm).However,rarely distributors with ‘‘fine holes”were installed and used in the bubble column.In this study,therefore,the distributor with‘‘larger holes”(hole diameter,d0＞1 mm)was selected,resulting in the undesirable CO2removal efficiency under the normal pressure.

Fig.7.Comparison of CO2 removal ratios under different absorption pressures.The gas volume flow,liquid volume flow,absorption temperature,regeneration pressure,and regeneration temperature are 50 L·min-1,3.3 L·min-1,313 K,0.05 MPa,and 333 K,respectively.

Fig.8.Influence of absorption pressure on CO2 capture efficiency.The gas volume flow,slurry volume flow,absorption temperature,regeneration pressure,and regeneration temperature are 50 L·min-1,3.3 L·min-1,313 K,0.05 MPa,and 333 K,respectively.The height of the slurry is 3200 mm.(a) Changes of mole percent of CO2 in the emission gas in the absorption column with the elapsed time,and (b)changes of CO2 removal ratio with the elapsed time.

Fig.9.Influence of gas volume flow on CO2 capture efficiency.The slurry volume flow,absorption pressure,absorption temperature,regeneration pressure,and regeneration temperature are 3.3 L·min-1,0.2 MPa,313 K,0.05 MPa,and 333 K,respectively.The height of the slurry is 3200 mm.(a) Changes of mole percent of CO2 in the emission gas in the bubble column with the elapsed time,(b)Changes of CO2 removal ratio with the elapsed time.

3.2.Effect of gas volume flow

In order to investigate the effect of the gas volume flow on the separation process,the slurry volume flow was fixed.We found the CO2capture performance of the slurry is sensitive to the gas volume flow,as shown in Fig.9.The CO2concentration in the emission gas decreased from about 3.5% to 2.3% when the gas volume flow decreased from 100 L·min-1to 25 L·min-1.Correspondingly the CO2removal ratio increased from about 90%to 93%.These findings can be attributed to three reasons.Firstly,Besagni et al.[36]reported that the bubble size in the column would increase with gas volume flow linearly,which means that the smaller bubble size was caused by the lower gas volume flow.Therefore,it was beneficial for the good gas-slurry contact and efficient mass transfer in the bubble column.Secondly,the residence time of CO2was extended by decreasing the gas volume flow in the bubble column.Obviously,it would improve the CO2capture performance of the slurry.Thirdly,with the constant slurry volume flow,the gas-slurry flow ratio decreased when the gas volume flow decreased.The lower CO2uptake was absorbed by the slurry,implying the driving force for the gas-slurry mass transfer would be stronger.However,the throughput is considered as a significant factor in the commercial scale separation units.In fact,the captured and separated amount of CO2during the separation processes were represented by the throughput in a certain period of time.To a certain extent,the economic value of the commercial separation units mainly consists of the removal ratio and the throughput.Therefore,the throughput should not be ignored.In this study,the CO2concentration in the emission gas was decreased and the corresponding CO2removal ratio was increased by decreasing the gas volume flow.However,the maximum throughput and processing capacity of the units were also decreased simultaneously.Therefore,it is significant to individually evaluate the specific applications for optimally balancing these two contradictory and counteracting effects.

3.3.Effect of height level of slurry

Increasing the height levels of both the bubble column and the slurry is considered as an effective method to increase the throughput without the lower CO2removal ratio.In the study,the variations of CO2removal ratio with slurry height in the absorption columns were studied and shown in Fig.10.The CO2removal ratio was deeply affected by the slurry height.The reason is that the longer gas-slurry contact time was expected by increasing the height level of slurry in the bubble column.However,the higher slurry height increased the amount of slurry in a specific absorption column.The high price of the ZIF-8 material hindered its commercial application in treatment of flue gas from various sources.To our best knowledge,10,000 USD·t-1or so might be the acceptable cost in the commercial separation units if the inner structure of materials could remain stable during the absorption processes over a long period of time.Although the method of large scale synthesizing ZIF-8 has been developed by Pan et al.[29],our team have devoted ourselves to further improve the synthesis method at a large scale in the past five years.At present,the cost of ZIF-8 can be controlled at 50,000 CNY·t-1and the slurry at 14,000 CNY·t-1.

Fig.10.Variations of CO2 removal ratio with the slurry height in bubble columns.The gas volume flow,slurry volume flow,absorption pressure,absorption temperature,regeneration pressure,and regeneration temperature are 50 L·min-1,3.3 L·min-1,0.2 MPa,313 K,0.05 MPa,and 333 K,respectively.

3.4.Effect of absorption temperature

Fig.11.Effect of absorption temperature on the CO2 capture efficiency.The gas volume flow,slurry volume flow,absorption pressure,regeneration pressure,and regeneration temperature are 50 L·min-1,3.3 L·min-1,0.2 MPa,0.05 MPa,and 333 K,respectively.The height of the slurry is 3200 mm.(a) Changes of mole percent of CO2 in the emission gas in the bubble column with the elapsed time,(b)Changes of CO2 removal ratio with the elapsed time.

The absorption temperature is a critical factor for affecting the efficiency of an absorption separation process.Fig.11 shows the effect of absorption temperature on the CO2capture performance of the slurry in the bubble column.Generally,decreasing the temperature improved the carbon capture efficiency.It also can be seen that the variations of CO2capture efficiency is sensitive to the absorption temperature when the absorption temperature ranged from 303.15 K to 308.15 K.The CO2concentration in the emission gas decreased from approximately 2.5% to 1.6% when the absorption temperature decreased from 308.15 K to 303.15 K.Correspondingly,the CO2removal ratio increased from about 92.5%to 94.5%.It was not relatively sensitive when the absorption temperature is from 308.15 K to 313.15 K.The CO2concentration in the emission gas decreased from approximately 2.7%to 2.5%when the absorption temperature decreased from 313.15 K to 308.15 K.Correspondingly,the CO2removal ratio slightly increased from about 91.5% to 92.5%.The phenomena were expected by the following reasons.Firstly,decreasing temperatures would increase the equilibrium absorption capability of the slurry because that the CO2capture processes of the slurry were dominated by the physical absorption.Obviously,the lower absorption temperature was beneficial for the CO2capture.On the other hand,the viscosity of the slurry was increased rapidly when the temperature was increased.Therefore,the rising of the bubbles in the column was slowed down by the higher viscosity of the slurry [37].The bubbles were compressed and compacted by the countercurrent slurry with high viscosity.It means that the diameters of the bubbles were increased,leading to the poor gas-slurry contact.Meanwhile,the coalescence of bubbles is more likely to happen due to the smaller mean distance between bubbles.In summary,the influence of the absorption temperature on the CO2capture performance is mainly believed to be determined by the above mentioned two contradictory effects.In this study,decreasing the absorption temperature would improve the CO2capture performance because the positive effect played a more important role than the negative effect.

3.5.Effect of volume fraction of CO2 in the feed gas

In the commercial application,different volume fraction of CO2from various sources are the challenges must be faced and solved efficiently.Therefore,the influence of the volume fraction of CO2in the feed gas on the CO2capture performance was investigated and shown in Fig.12.The CO2concentration in the emission gas increased and the corresponding CO2removal ratio decreased when the CO2volume fraction in the feed gas increased.As Fig.12 shows,the CO2concentration in the feed gas was only approximately 2% and the corresponding CO2removal ratio was approximately 94% when the CO2volume fraction in the feed gas was 15%.However,the CO2concentration in the emission gas rapidly increased to about 5% and the corresponding CO2removal ratio decreased to about 85% when the CO2volume fraction in the feed gas increased to 35%.The higher CO2volume fraction in the feed gas increased the driving force for gas-slurry mass transfer,which accelerated the mass transfer rate and improved CO2capture capacity.However,the residence time of CO2in the bubble column was insufficient so that CO2could not be absorbed completely by the slurry because of the high rising speed.Therefore,the lower CO2removal efficiency with the larger amount of CO2in the feed gas was the inevitable consequence.In addition,increasing the CO2volume fraction in the feed gas obviously increased the amount of CO2in the bubble column with the same amount of feed gas.This increased the CO2-slurry flow ratio with a certain amount of both feed gas and cyclic slurry.The unit amount of slurry faced absorbing and separating more CO2,which might approach to the ultimate CO2capture capacity of the slurry under the experimental conditions.It is the fact that the slurry could not be saturated completely under the actual operating conditions.The rest amount of CO2in the bubble column might not be absorbed by the slurry and emitted through the gas outlet,which increased the CO2concentration in the emission gas and decreased the corresponding CO2removal ratio.

3.6.Comparison of different carbon capture methods

Fig.12.Effect of volume fraction of CO2 in the feed gas on the CO2 capture efficiency.The gas volume flow,slurry volume flow,absorption pressure,absorption temperature,regeneration pressure,and regeneration temperature are 50 L·min-1,3.3 L·min-1,313 K,0.2 MPa,0.05 MPa,and 333 K,respectively.The height of the slurry is 3200 mm.(a)Changes of mole percent of CO2 in the emission gas in the bubble column with the elapsed time,(b) Changes of CO2 removal ratio with the elapsed time.

The CO2loading in the cyclic slurry and the absorption coefficient are the typical representatives expressing the CO2capture capacity of the slurry.According to the previous study [38],the cyclic absorption coefficient is only 0.0003 mol·L-1·kPa-1at 303.15 K in water.In this study,it is encouraging to see that the maximum CO2loading of the recycled slurry reached 0.35 mol/L,as listed in Table 6.And the corresponding cyclic absorption coefficient is 0.007 mol·L-1·kPa-1at 313 K,which is more than 23 times of that in water.Meanwhile,the CO2selectivity of the slurry is much higher than that of water.Therefore,the slurry-based approach is superior to water washing approach to some extent for capturing CO2from like biogas.

It is well believed that the larger quantities energy cost in both physical adsorption and chemical absorption processes are attributed to the energy consumption of the regeneration.The same rule is also applied to the slurry-based method for capturing CO2in this study.In order to completely regenerate the rich-solvent,both higher desorption temperature and pressure are set frequently.In this study,the generation conditions are more moderate compared with other traditional methods.The desorption temperature was just 20 K higher than the absorption temperature selected,and the desorption pressure was fixed to 0.05 MPa.It is encouraging to see that the CO2concentration in the emission gas could reachapproximate 2.5 mol% and the corresponding removal ratio is about 92%.

Table 6 The CO2 loading in cyclic slurry and sorption coefficient under different operating conditions,where the desorption pressure and the slurry volume flow were set to 0.05 MPa and 3.3 L·min-1,respectively

Table 7 Comparison of the boiling temperatures of ZIF-8 slurry and the desorption temperatures in this study

Another very important issue need to be considered is the boiling temperature of the slurry under the desorption pressure.In this study,it was evaluated and compared with the desorption temperature,as listed in Table 7.It is found the desorption temperature is far away from the boiling point of the slurry.To our best knowledge,in the traditional chemical CO2capture approaches,the large energy consumption and the high cost are majorly attributed to the vaporization of water involved in the alkylol amine solution during the regeneration processes.According to the report,the desorption temperatures are usually 383–403 K[39].Herein,the slurry-based method to capture CO2was utilized lest solvent evaporation to the greatest extent.Therefore,the significant amounts of energy would be saved.With no doubt,a huge amount of heat produced during the separation process can be easily recovered by the advanced heat integration units.If the desorption pressure is strictly kept higher than the bubble point pressure of the slurry,appropriately increasing the desorption temperature and pressure would be beneficial for both regeneration process and energy conservation.It should be pointed out that the desorption temperature was set lower than the boiling temperatures of the solvent,which could save significant amounts of energy.The comparison of different CO2capture approaches was carried out,as shown in Table 8.

It should be noted that the desorption heat listed in Table 8 is the theoretical minimum energy consumption.The advantage of slurry-based method was observed.According to Chu et al.[40],the thermodynamic heat of CO2desorption and the vaporization latent heat of the liquid phase are mainly contribute to the difference of the heat consumption between the slurry method adopted in this work and the traditional amine absorption.In this study,the mechanical stirrer was set up in the desorption tank to accelerate the process of regeneration.It also consumes some energy,which is about 2.2 kW.It should be noted that the use of mechanical stirrer is not compulsory.For example,in our previous study[28],we did not use the mechanical stirrer.The slanted downward deflectors,rather than the mechanical stirrer,were installed in the desorption tank to increase the flow distance and the residence time of slurry.Overall,as discussed above,the heat is recovered difficultly by the advanced heat integration units when the solid adsorbents are applied in the fixed-bed reactors.This problem seems to be solved easily by utilizing the methods based on chemical absorption because those separation processes could be operated continuously with advanced heat exchanged and integrated.But some problems,such as higher energy consumption,higher amin degradation rate,and serious corrosive problems,are also caused by the higher desorption temperature of chemical absorption.The slurry-based approach used in this study combines the virtues of both physical adsorption and chemical absorption.Furthermore,the critical problems of degradation and corrosion could be avoided effectively due to the moderate desorption temperature.

4.Conclusions

In this work,the absorption-adsorption hybrid separation method was studied for its commercial application to capture CO2.A large-scale separation system was constructed to study the CO2capture performance of the slurry-based method for taking the scale-effect into account.The separation system mainly consisted of a bubble column with a height of 4.9 m and an inner diameter of 376 mm for absorption and a desorption tank with the volume of 310 L.To our best knowledge,this study is the first time the commercial application of the slurry-based method utilizing a large-scale bubble column.

Table 8 Comparison of operating conditions of different carbon capture approaches

The results indicate that the scale-up effect of the separation units cannot be ignored.Considering the scale of the units in this study,increasing the absorption pressure over 0.2 MPa could effectively improve the performance of slurry for CO2capture.On this basis,the influences of some other key factors,including the absorption pressure,the gas volume flow,the slurry height,the absorption temperature,and the volume fraction of CO2in the feed gas,were investigated systematically.Depending on different experimental conditions,the CO2removal efficiency could reach 85%-95%.The maximum CO2loading in the recycled slurry was up to 0.007 mol·L-1·kPa-1under the operation conditions.More importantly,the above results could be obtained under the moderate regeneration conditions.The desorption temperature and pressure are approximately 333 K and 0.05 MPa,which are far from the boiling conditions of the slurry.Therefore,the energy can be significantly conserved by utilizing the slurry-based method compared to the traditional amine-based methods.

This study indicated that the slurry-based approach for capturing CO2might be straightly applied for the practical industry.Meanwhile,the experimental results obtained in this work could be used to guide the further effective carbon capture processes based on physical absorption.

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

The authors gratefully acknowledge the financial supports received from the National Natural Science Foundation of China(21776301,21636009) and the Science Foundation of China University of Petroleum,Beijing (2462018BJC004).