Applications of high-gravity technologies in gas purifications:A review☆

Jing Guo,Weizhou Jiao*,Guisheng Qi,Zhiguo Yuan,Youzhi Liu*

Shanxi Province Key Laboratory of Higee-Oriented Chemical Engineering,School of Chemical Engineering and Technology,North University of China,Taiyuan 030051,

China National Demonstration Center for Experimental Comprehensive Chemical Engineering Education,North University of China,China

Keywords:High gravity technology Rotating packed bed(RPB)Gas purification Desulfurization Pollution

A B S T R A C T The traditional gas purification techniques such as wet gas desulfurization,with their advantages of large-scale implementation and maturity,have still been widely used.However,the main drawback of these techniques is the low transfer efficiency,which normally needs towers as tall as tens of meters to remove the pollutants.Therefore,new technologies which could enhance the mass transfer efficiency and are less energy-intensive are highly desirable.As a process intensification technology,high-gravity technology,which is carried out in a rotating packed bed(RPB),has recently demonstrated great potential for industrial applications due to its high mass transfer efficiency,energy-saving,and smaller volume.This consequently provides higher efficiency in toxic gas removal,and can significantly reduce the investment and operation costs.In this review,the mechanism,characteristics,recent developments,and the industry applications of high-gravity technologies in gas purifications,such as hydrogen sulfide,nitrogen oxide,carbon dioxide,sulfur dioxide,volatile organic compounds and nanoparticle removal are discussed,most of the demonstration projects and practical application examples in gas purification come from China.The perspective and prospective of this technology in gas purification and other fields are also briefly discussed.

1.Introduction

Rapid industrial development brings social progress and economic growth,however,it also leads to many serious environmental problems.In China,nearly 70%of the pollution come from industry plants,and one of the major pollutants in industry is harmful gas,which contains hydrogen sulfide(H2S),sulfur oxides(SOx),nitrogen oxides(NOx),volatile organic compounds(VOCs),carbon dioxide(CO2)and particulate matter(PM)[1,2].The acidic gas such as SO2and NOxwould cause corrosion to tubes,equipment and constructions,as well as result in the catalyst poisoning and acid rain.Furthermore,both of the acidic gas and fine particulates are harmful to the respiratory system and human health.Therefore,effective air pollution control in industries is greatly necessary.

The traditional wet processes,such as fixed beds with packings,falling film column[3],spray towers[4],and rotating-stream tray scrubber[5],are widely used for flue gas desulfurization due to their high removal efficiency,low operating cost and stable operation.For instance,in the US,approximately 85%of the flue gas desulfurization units installed are wet scrubbers.However,these traditional wet processes have several shortcomings,such as limited mass transfer rate and large volume of equipment,thereby leading to a huge reactor size and high capital costs[1].Therefore,new technologies which could enhance the mass transfer efficiency and are less energy-intensive are highly desirable.

As one of the hottest topics in chemical process intensification technologies,high-gravity(HiGee)technology,which is carried out in a rotating packed bed(RPB),has recently demonstrated great potential for industrial applications due to its high mass transfer efficiency,energysaving,and smaller volume[6].This consequently provides higher efficiency in toxic gas removal compared to the traditional technologies.Especially,it can significantly reduce the investment and operation costs.

Except working on the theoretical,mechanism and packing structures of the high gravity processes[7-14],researchers in high gravity technology field also focus on the practical applications of this technique[15,16],which contains gas purification [17-20], nanomaterial preparation[21-27],rectification[28-31],absorption[32,33],wastewater treatment[34-38]and so on.In this review,the mechanism,the characteristics of the RPB, and the recent developments and practical applications of high-gravity technologies in gas purifications, such as H2S, SO2, NOx,CO2,NH3,VOCs and nanoparticle removal are described.This article provides a pioneering review on the practical applications of high gravity technology in gas purifications;the perspective and prospective of this technology in gas purification and other fields are also briefly discussed.

2.The High Gravity Technology

High gravity technology was originally invented by Ramshaw[39].The rotating packed bed,which rotates a doughnut-shaped packing element to generate a centrifugal acceleration as high as several hundred folds of gravity to intensify mass transfer efficiency,plays an important role in process intensification[40].In RPB,liquid can be spread and split into liquid elements such as micro-scale droplet,thread or film by the high-speed rotating packing, the flow rate of these liquid elements may be up to ten times higher than that in the field of gravity, then the high-speed liquid elements contact counter-currently with the flowing gas,which intensifies the interphase mass transfer greatly,the mass transfer efficiency may be up to 1-3 orders of magnitudes higher than that in a conventional packed bed,and the size of the reactor is dramatically reduced compared with the conventional reactor.This leads to a significant reduction in the capital and operating costs. Therefore,nowadays the RPB has been applied in various gas-liquid contacting processes,such as material and medicine synthesis,petroleum production,and pollution control[40,41].In this section,the features and principles of high gravity technology and the types of the RPB reactors are discussed.

2.1.The types of RPB reactors

In an RPB,liquid is normally injected from the liquid distributor into the inner periphery of the rotor,flows uniformly through the packing,and leaves the outer edge of the rotor into the machine case,as show in Fig.1.Visual studies for the high gravity reactors show that,the diameters of liquid droplets at the outer periphery of the RPB decreased from 1200 μm to 400 μm with the increase of the rotational speed from 300 to 1100 r·min-1[10,12].The gas can be forced to flow through the packing zone in various directions to contact with the liquid in the rotor,containing counter-current flow (i.e., radially inwards and leave via the rotor center),co-current flow(i.e.,radially outwards and leave via the outer rotor) and cross flows (i.e., upwards and leave via the top of rotor).For all of these types,the packing characteristics are generally uniform throughout the packing zone[42].

2.2.Features and characteristics

The high gravity technology possesses a number of benefits,such as:

1) High micro-mixing efficiency and mass transfer efficiency,the mass transfer coefficient is 1-3 orders of magnitudes higher than that in a conventional packed bed.

Fig.1.A schematic diagram of the RPB with cross flow.

2) High hydraulic and flooding capacity.

3) Low liquid hold-up, which is suitable for treating the expensive,toxic,or inflammable materials.

4) Small reactor size, which is cost-saving, space-saving, and easy installation.

5) Better self-cleaning of the rotor, which is difficult for scaling and blocking.

6) Ultra-short staying time,which is suitable for the fast reaction and mixture processes.

3.Practical Applications

After 30 years of rapid development,the mode of economic growth of China is going to change from high pollution,high energy consumption and high emission to low pollution,energy saving,and low toxic emission.Therefore,the chemical engineering industry is under mounting pressure,the sustainable development becomes a hot issue of the chemical engineering. Chemical process intensification technologies are aiming at energy conservation and emission reduction,by means of developing new equipment and innovative techniques,which has shown excellent potential in the sustainable development of chemical engineering[43].The ultimate purpose of all the technological innovation is industrial applications, only realizing practical applications could promote the industrial progress.As a process intensification technology,high gravity technique has been applied in industry since the 1990s, especially in the gas purifications [1]. The following part is going to describe the recent developments and practical applications of high-gravity technologies in gas purifications,such as H2S,SO2,NOx,CO2,NH3,VOCs and nanoparticle removal.

3.1.H2S emission control using high-gravity technology

In coal chemical industry, coke-oven gas can be used to produce many kinds of chemical products,such as,methyl alcohol,coal oil,olefins, chemical fertilizer, and natural gas. However, the utilization of the coke-oven gas has been greatly limited due to the low concentration of H2S in it.The H2S will lead to the corrosion of the equipment,and the poisoning of the catalyst,moreover,the emission of H2S will cause serious air pollution which is harmful to the human health.Therefore,the control of the H2S emission in chemical industry is highly significant.

In traditional H2S desulfurization process,alkali liquor is the most commonly used solution.This desulfurization process is a fast chemical reaction process, and the mass transfer process of the gas-film is the rate-controlling step.For this kind of mass transfer process,it is better to choose packings with high surface area or equipment which has high mass transfer coefficient to increase the reaction efficiency,thereby to decrease the volume of the reactor.In RPB,the mass transfer efficiency may be up to 1-3 orders of magnitudes higher than that in a conventional reactor,which is an excellent candidate for desulfurization[40].

As a chemical process intensification technology,high-gravity technology combined with conventional wet process can significantly reduce the investment and operation costs of H2S removal, which has been used in many industrial H2S containing gas,such as natural gas,refinery gas,coal-bed gas,synthesis gas,and coal-oven gas[17-19,44].The wet catalytic oxidation desulfurization method using alkali liquor as absorbent(i.e.Na2CO3,ammonia water,organic amine)is normally consisted of catalytic chemical absorption and catalytic oxidation two processes,and the H2S is catalytically oxidized to sulfur[18].The main difference of these processes is the type of catalyst; the commonly used catalysts are dinuclear cobalt-phthalocyanine sulfonate (PDS),chelated iron,CoS,888,etc.[45].

Table 1 presents the performance evaluation of the H2S removal processes using the high gravity technology in practical applications.The results indicate that the H2S removal ratio using high gravity can reach 90.0%-99.9%,depending on the type of the absorbents and the type of the equipment [55]. In 2008,Qi et al. presented the industry H2S removal by applying high gravity technology to replace conventional packed tower as desulfurization equipment,in one of the chemical plants in Shanxi province of China[46].In this process,the Na2CO3solution was used for absorption of H2S in synthesis gas,the dinuclear cobalt-phthalocyanine sulfonate(PDS)was used as catalyst.The reactions of this process are as follows:

Catalytic chemical absorption of H2S:

Table 1Performance evaluation of the H2S removal and selective removal processes using the high gravity technology

Catalytic oxidation(regeneration):

In this case,the packing was horizontal type with counter-flow,the inlet H2S concentration was 2000 mg·m-3,the Na2CO3concentration was from 6 to 14 g·L-1, and the PDS concentration was from 5 to 20 mg·L-1. The gas flow was 340 m3·h-1, and the liquid flow was from 1.7 to 4.5 m3·h-1. During the 7 days of continuous operation,the average removal efficiency was 92.4%. In this condition, 4 kg of Na2CO3needs to be added every 8 h,which means 800 kg of Na2CO3needs to be consumed to remove 1 t of H2S.The experimental results showed that the removal efficiency of the desulfurization could reach 99.0%under proper conditions.Compared with tower equipment,the high gravity method had many advantages,including higher desulfurization, lower liquid-gas ratio, and smaller equipment size. Fig. 2 shows an on-site photograph of the RPB desulfurization process.

In 2008,Han et al.presented the use of high gravity technology in shift gas desulfurization in one Fertilizer Company of China.Hindered amine was used as adsorption solution with the pressure of 3.2 MPa,when the flow rate of the shift gas was 65000 m3·h-1,and the concentration of inlet H2S was 1200 mg·m-3,the outlet concentration of the H2S was 20 mg·m-3.The process could run stably,and the steam consumption of the rich solution regeneration was 4 t·h-1,less than the desulfurizing tower,which showed considerable economic benefit[47].

In 2010,RPB was used for the removal of H2S from associated gas in the oilfield production by choosing chelated iron as desulfurizer[48].In this process,alkali liquor is also the mainly absorbent to adsorb H2S,the function of the iron ion is to convert HS-into sulfur,as shown in the following equations:

1) The adsorption of H2S:

2) The oxidation of HS-:

Fig.2.A desulfurization site.

3) The regeneration of desulfurization liquid:

4) Total reaction:

In this case,the RPB had been operated smoothly more than two years,the gas handling capacity was 6250 m3·h-1,the inlet concentration of H2S was 2500 mg·m-3,and the design temperature and design pressure were 1.6 MPa and 80°C,respectively.The industrial research results showed that after desulfurization the concentration of H2S in cleaned gas was less than 20 mg·m-3,and normally,the monitoring value was less than 10 mg·m-3.

Qi et al.reported that,an industrial unit of H2S removal from coke oven gas in a rotating packed bed using wet oxidation process was built at a plant site,and the gas flow rate was 10000 m3·h-1.In this process,Na2CO3was used as alkaline source,CoS was used as catalyst,and the high gravity technology equipment was built behind the previous desulfurization tower.During the 40 day continuous monitoring,the inlet concentration of H2S was from 22.2 mg·m-3to 3781.3 mg·m-3,the average inlet concentration was 592.0 mg·m-3,and the lowest outlet concentration of H2S was 5.01 mg·m-3, the average outlet concentration was 45.3 mg·m-3.The H2S removal rate could reach as high as 99.8%,and the average removal rate was 90.15%. When the inlet concentration was lower than 100 mg·m-3, although the desulfurization efficiency was low,the outlet H2S concentration could still be controlled at lower than 50 mg·m-3.So the authors claimed that,the high gravity technology for desulfurization has high operating flexibility,it can be used for both bulk removal and fine removal of H2S. If one RPB setup cannot satisfy the desulfurization concentration requirement, two or more high gravity set-ups in series can be used to get higher desulfurization efficiency[18,49].

As the volume of the RPBs is much smaller than the traditional desulfurization towers,the RPB could be qualified for desulfurization in various situations,such as onshore/offshore platform and frontier area in oil field,.Zou et al.reported that,the RPB had been applied in the offshore operating platform to remove H2S from the natural gas.The H2S concentration of the inlet gas was 7589 mg·m-3,after desulfurization the concentration of the H2S was as low as 4.5 mg·m-3, which showed excellent removal efficiency.Moreover,compared with the commercial desulfurization towers,the volume of the high gravity equipment was only 1/10 of the commercial desulfurization towers,which shows high flexibility for various locations[50].Moreover,to overcome the limitation of the traditional desulfurization methods in the confined space,Zhang et al.reported using ferric chelate absorbent in alkaline environment to remove H2S.The N2and H2S mixture was used to simulate natural gas;the initial H2S concentration was 2500 mg·m-3.During the 72 h continuous experiment, the H2S removal efficiency was higher than 99.6%, and in the optimal condition it could reach more than 99.9%[44].

As mentioned above,high gravity technology can be applied for H2S removal in many different conditions,such as,industrial off-gases,cokeoven gas, natural gas, especially in the confined space like offshore operating platform,and existing factories.The advantages of high desulfurization efficiency,excellent operating flexibility,small unit size,and low investment and operation cost,show the bright prospect of this technology.Furthermore,high gravity technology could also be applied for selective removal of H2S from mixture gases.

3.2.Selective H2S removal

There is an increasing interest in the selective removal of H2S from gas streams with a high ratio of carbon dioxide(CO2)to H2S.Both H2S and CO2are acidic and similar in many physical and chemical aspects,and therefore in the traditional wet oxidation desulfurization process,a considerable quantity of CO2can also be absorbed simultaneously,which would increase the circulation volume of the absorption liquid and increase the energy load of the solution regeneration system[56-58]. Accordingly, it is necessary to selectively absorb H2S in the presence of CO2,which will greatly improve the process efficiency and bring significant social and economic benefits.In 2005,the high gravity technology was applied for the selective absorption of H2S with high concentration CO2in an ammonia synthesis plant by Prof.Liu and his co-workers[51,52].Fig.3 is the actual view of the selectivity desulfurization RPB equipment.The inlet gas flow rate was 21000 m3·h-1,the volume fraction of CO2was 98.97%and the concentration of H2S was 10600 mg·m-3,the PDS was used as the catalyst.The operation result shown that both of the desulfurization rate and the selectivity were more than 90%.Removing 1 t H2S,the consumption of alkali was 0.5 t,and the emission reduction of the H2S was about 700 t·a-1.The good H2S selectivity absorption had overcome the shortcomings of the large scale synthesis ammonia system, which has shown excellent energy conservation and emission reduction.

Fig.3.A selectivity desulfurization RPB equipment.

In 2012,an industrial test for methyldiethanolamine(MDEA)selective absorption of H2S was performed by Prof.Chen's group to solve the problem that a large amount of CO2accompanying H2S is simultaneously removed in the conventional desulfurization process.In this case,an RPB highlighted that MDEA was kinetically selective toward H2S and restricted its thermodynamic selectivity toward CO2.The flow rates of refinery dry gas ranged between 8000 and 13000 m3·h-1,the liquid flow rate was 21 t·h-1, H2S content in feed gas ranged from 10000 to 15000 mg·m-3, and CO2content in feed gas was approximately 4%.After desulfurization by RPB,the H2S content in sweet gas was less than 20 mg·m-3and the coabsorption rate of CO2was approximately 8.9%,which decreased nearly 90%compared with the conventional tower desulfurization process[53].

The latest work using RPB for selective absorption of H2S with high concentration of CO2by Jiao et al.showed that the removal rates of H2S and CO2could reach 99.13%and ＜1%,respectively,which indicated excellent H2S removal efficiency and selectivity.In this study,the Na2CO3was used as alkali source and the dinuclear cobalt-phthalocyanine sulfonate(PDS)was the catalyst[54].The authors claimed that,the most important function of RPB in selective absorption of H2S was to restrain the CO2removal efficiency and sharply intensify the absorption of H2S.Although the mass transfer of CO2absorption was also enhanced by the RPB,the total amount of CO2mass transfer was low.It was due to the short gas-liquid contact time.In this study,the H2S removal efficiency increased with the increase of liquid-gas ratio,rotating speed,Na2CO3concentration,and PDS concentration.The desulfurization efficiency and selectivity as the function of rotation speed are shown in Fig.4.Those experimental results testified the advantages of RPB over traditional tower equipment in selective H2S absorption.

Fig.4.Effects of rotation speed on desulfurization efficiency and selectivity[54].

Both the above experimental and industrial works indicated that high gravity technology had great benefits for desulfurization,such as,higher efficiency,higher selectivity,smaller packing and equipment volumes,lower investment,and minor energy consumption.Comparing with traditional towers,the volume mass transfer coefficient within RPBs increases 1-2 orders of magnitude,and the volume and the weight of the equipment are much less than the towers.

3.3.SO2 emission control using high-gravity technology

SO2is considered to be one of the serious air pollution gases,which is the major source of haze.A large amount of the SO2emission is from the combustion of coal and other fossil fuels.To control the SO2emission,a variety of flue gas desulfurization(FGD)schemes have been developed,containing wet,dry,and semidry processes.Wet processes are the most popular methods for SO2emission control and share 84%of the total desulfurization market,with its advantages of high efficiency,low cost,and stable operation [59]. In wet processes, the limestone or limebased scrubbing shares 70% of the market. However, wet limestone technology is not regenerative which brings the problem of gypsum fouling.Moreover,the mass transfer efficiency of the traditional SO2absorption process is poor,it normally needs a large column packing to realize the removal efficiency which leads to high investment and operating costs.Therefore,researchers have been focused on developing regenerative techniques with process intensification to decrease the investment and operation cost.As one of the typical process intensification technologies,high gravity technology has been widely applied in SO2removal processes[60,61].

3.3.1.The mechanism of SO2removal

The mechanism of SO2removal in RPB is generally chemical absorption between the gas and liquid phases.With its great micro-mixing ability,SO2in the flue gas can be transferred from the gas phase to liquid phase within ultra-short time.Various aqueous solutions can be used as the liquid phase,such as sodium citrate buffer solution,double-alkali solution,phosphate buffer solution,and ammonium buffer solution,as described below:

3.3.1.1.Double-alkali method.Double-alkali method is a commonly used method to remove SO2from industrial waste gases.In this method,first SO2is absorbed by NaOH solution to form Na2SO3or/and Na2SO4,then the Na2SO3/Na2SO4solution reacts with lime slurry to obtain CaSO3and CaSO4sediment.The absorption process can be expressed as the following equations[62]:

The equations of regenerative process are as follows:

In this process,NaOH can be regenerated to go back to the system,the consumable is the cheap Ca(OH)2.The regeneration reactions can be held in a sedimentation basin to prevent the fouling and blockage of the tubes and the desulfurization equipment.

3.3.1.2.Sodium citrate buffer solution.Our group first proposed RPB com

bined with sodium citrate buffer solution to treat the SO2pollution.SO2removal by absorption into sodium citrate(Na3Ci)buffer solution with RPB is generally considered as a fast, safe, green, and economical method.The advantages of this process are simple,non-toxic,insignificant losses,no fouling problem,and low oxidation of SO2.In these processes,SO2in flue gas is absorbed by Na3Ci solution,and the dissolved SO2can be regenerated by steam stripping or other regenerative method. The absorption solution can also react with H2S to obtain sulfur,which can be separated by flotation[63,64].The desulfurization of SO2with absorption and desorption in sodium citrate buffer solution is

3.3.1.3.Ammonium buffer solution.In ammonia-based wet desulfurization process,the(NH4)2SO3and NH4HSO3containing solution was normally used as the absorbent to react with SO2,which come from the flue gas.The following reactions(18)-(20)mainly occur during the absorption process,and the reactions(21)and(22)generate(NH4)2SO4byproducts,which can be used as fertilizer[65].

3.3.1.4.NaOH-based solution.It is reported that the size of the desulfurization equipment can be reduced greatly if high gravity technology could be employed for the sulfur removal with NaOH-based solution.A commercial RPB was applied on one offshore platform to remove H2S from natural gas[51].Thus,the use of an RPB may be a better choice and more likely to be accepted by ship owners.Chu et al.proposed using sodium sulfite(Na2SO3)solution in an RPB to remove SO2which displays a bright future for the offshore application of SO2removal.The mainly occurring equations during the absorption of SO2into Na2SO3solution are as follows[66]:

3.3.1.5.Phosphate buffer solution.The upper wet regenerative flue gas desulfurization methods have been applied in RPB and have shown excellent SO2removal efficiency,however they still have some shortcomings,such as,Na2SO3has high reactivity and small liquid-gas ratio,but during the regeneration process the Na2SO3is easily oxidized into Na2SO4to decrease the SO2recycle rate;sodium citrate has high sulfur capacity and inoxidizability, but it easily loses efficacy; ammoniabased solution has not only high sulfur capacity and high reactivity,but also high evaporability.Based on these problems,phosphate buffer solution has attracted our attention.The use of phosphate buffer solution such as alkali metal phosphate(Na2HPO4)could significantly increase the capacity of SO2absorption,and the reaction between SO2and Na2HPO4solution is reversible and instantaneous,which is easy to regenerate and has no second pollution. Moreover, the property of Na2HPO4is stable which can be used for a long time.

The process is shown in Eqs.(26)and(27)[67]:

During this process the absorption process is the forward reaction,the desorption process is the reverse reaction.At first,the SO2in the flue gas transfers into liquid phase to form H+and HSO3ˉ, and then the H+will combine with HPO42ˉto promote the forward reaction of Eqs.(26)and(27)to absorb more SO2into liquid phase.The rich liquor can be regenerated by steam stripping or other regenerative method to release SO2. These SO2can be used for producing H2SO4or other products.

3.3.2.Practical applications

In the sulfuric acid industry, the tail gas normally containing 3000 ppm SO2,the traditional wet desulfurization method was usually carried out in columns packed with various packings or a spray tower,a rotating stream tray scrubber,and so on[1].To meet the ultra-low emission limitation control of those sour gaseous pollutants in China,it needs two or three towers in series.However,if using high gravity technology to treat SO2containing inlet gas,even for 5000 mg·L-1SO2,only one setup is enough to meet the emission limitation.Moreover,it is difficult to upgrade the existing desulfurization system by adding new columns due to the limitation of the area and space in the running chemical factories.Therefore,the high gravity technology is a promising pathway to solve the above problem.In 1994,Chen et al.applied RPB at one sulfur acid company of China as a plant test,the treatment capacity of the gas was 3000 m3·h-1,after desulfurization the concentration of SO2was lower than 100 mg·L-1,which was much lower than the 300 ppm emission limitation at that time[68].

Table 2 presents the performance evaluation of the SO2removal processes using the high gravity technology in practical applications.Jiang et al.reported[40]using a laboratory scale RPB to absorb SO2from SO2/air mixture with sodium citrate buffer solution(Na3Ci).Sodium citrate buffer solution SO2removal is generally considered as a fast,safe,green,and economical method with the advantages of non-toxic reagent,simple process, and no gypsum fouling. In this procedure, SO2in flue gas is absorbed by aqueous sodium citrate solution,and the dissolved SO2is subsequently recovered from the solution by steam stripping or other regenerative method,or react with H2S to obtain sulfur,and then to be separated by flotation. In this work, the inlet concentration of SO2was 5085 mg·m-3,and the desulfurization efficiency was as high as 97.3%.

Due to the high desulfurization efficiency and useful byproducts,ammonia-based solution has also drawn great attention to absorb SO2.In the desulfurization process,(NH4)2SO3and NH4HSO3coexisting solution is used as the ammonia-based.The sulfuric acid industry can gain profit from the(NH4)2SO4byproduct,which is a favorable resource of sulfur fertilizer.A cocurrent rotating packed bed(CO-RPB)with novel SiC structured packing was employed to upgrade existing desulfurization systems using ammonia-based solution.The experimental results showed that in order to meet the demand of SO2emission standard,which is 400 mg·m-3for the sulfuric acid industry, the suitable range of SO2inlet concentration should be less than 5000 mg·m-3[59].An RPB with SiC structured packing and plastic packing was investigated to absorb SO2from a gas mixture with an ammonia-based solution.Experimental results showed that,the plastic packing had higher desulfurization efficiency than the SiC packing under the same operation conditions[65].

In 2010,two RPBs were designed by Prof.Chen's group and applied in Zhejiang Juhua Group Co.of China to remove SO2from the tail gas of sulfuric acid production lines with a capacity of 150000 t·a-1and 200000 t·a-1,respectively.By using the ammonia-based solution as the absorbent,the industrial running data showed that the average desulfurization efficiency was higher than 97%and the concentration of SO2in the outlet tail gas was less than 300 mg·m-3,which was below the emission limit of 400 mg·m-3at that time[68].

Table 2The performance evaluation of the SO2 removal processes using the high gravity technology

Xu reported that,in 2011 the RPB was used in Anhui Tongling Huaxing Co.of China,the ammonium buffer solution was used for absorption of SO2in the flue gas while producing ammonium sulfate precipitates utilized as fertilizers.In this case,the removal efficiency of SO2could reach 98.8%with a gas flow rate of 80000 m3·h-1.The inlet SO2concentration was 5000 mg·m-3, and the average outlet concentration of SO2was 360.63 mg·m-3.In addition,the high gravity process could operate at a relatively higher gas-to-liquid(G/L)ratio,i.e.,140-10000(v/v),than the conventional packed bed,i.e.,～10(v/v).As a result,the treatment capacity of flue gas using the high gravity technique would be greater than that of the same size of conventional packed bed reactor[69].

As RPB with high mass transfer efficiency and small size,it is also an excellent choice for SO2removal from the exhaust gas in ships.Recently,Chu et al.investigated SO2removal in a lab scale and a pilot RPB with sodium sulfite(Na2SO3)solution[66].The inlet concentration of the SO2was 700 ppm,and the gas flow rate for the pilot scale was 550 m3·h-1.The experimental results showed that the SO2concentration in the outlet gas of the RPB could decrease sharply and there was no obvious scale up effect.Under the same operating conditions as those for the spray tower,the RPB had a stable desulfurization efficiency that was more than 97%.The comparison results showed that the SO2removal efficiency of the RPB(pH=6)could reach 98%,which was 6%higher than that of the spray tower(pH=10).

3.4.CO2 emission control using high-gravity technology

The emission of CO2accounts for the high proportion of the greenhouse gases, and about 82%of the CO2comes from burning coal and other fossil fuel.In other side,CO2is also a very useful industrial raw material,which is widely used in food,chemical,and other industries.So it is necessary to develop new processes which not only has high absorption efficiency,but also could utilize the CO2resource.

Carbon capture utilization and storage(CCUS)technologies are currently considered as an effective approach to reduce industrial CO2emissions.However,none of the CCUS technologies alone can provide a short-term solution to reduce CO2emissions[70].Actually,high gravity technology is also a good candidate for industrial CO2capture.

In 2007,Xing et al.applied RPB to reclaim CO2from flue gas.This process was developed to replace the traditional absorption method to treat the very low rate of CO2in some power plants.In this process,N-methyldiethanolamine(MDEA) and Triethylenetetramine(TETA)mixed solution was used to absorb CO2from the flue gas,in which the inlet CO2volume fraction was 5%-8%[71].

As the MDEA is a tertiary amine,the adsorption rate of CO2is slow,but it has high adsorption load,and the desorption energy consumption is very low.However,for the TETA,it has two primary amine nitrogenatoms and two secondary amine nitrogen-atoms,which has higher adsorption rate.In this case,the MDEA and TETA mixed liquor was used as the CO2absorbent,both has the fast adsorption rate property of primary and secondary amines,and the high adsorption load of tertiary amine,which could highly increase the CO2adsorption rate. There were cross-reactions in this mixed liquor adsorption process, the tertiary amine could react with the reaction product of the primary and secondary amines with CO2.The reaction processes are as follows:

Eq.(28)is an ultra-slow reaction which is controlled by the liquor film.Eq.(30)is the control step of the primary and secondary amines to adsorb CO2.Eqs.(29)and(31)are instantaneous reactions.Thereby,in this case,the control step of the CO2adsorption is Eqs.(28)and(30).For this adsorption process,the adsorption rate is related with the solubility coefficient,wet phase molecular diffusivity,and the concentration of the mixed mine.Therefore,in order to realize the recycling of the CO2from the flue gas,we need to increase the volume mass transfer coefficient and the mass transfer driving force to intensify the mass transfer rate.

Based on the former mechanism of the CO2adsorption with mixed organic mine in the RPB,the pilot test results indicated that the absorption rate and capacity of CO2could be remarkably enhanced by the system of TETA and MDEA,and the removal efficiency of CO2from flue gas could reach 85.81% under proper conditions, which was about 25%higher than that of the traditional absorption method[71].

In 2007,Jassim et al.[33]reported the performance of a pilot-plant scale RPB in absorption and desorption of CO2using monoethanolamine(MEA) solutions. In this process, the outside diameter of the packing was 398 mm,the inside diameter of the packing was 156 mm,and axial depth was 25 mm.They claimed using MEA concentrations above 30 wt%could achieve lower CO2penetration levels.For MEA concentration at 30 wt%,the feed CO2concentration was 3.5 vol%-4.5 vol%,after absorption the CO2concentration could reach 0.03 vol%-0.13 vol%.In a comparison with a simulated stripper showed at similar operational conditions,rotating packed bed could save size and space and has high efficiency.

In 2011,Chen et al.developed a new structured RPB which was applied in a coal-fired power plant to absorb CO2,the absorption efficiency could reach higher than 80%at the optimum condition[51].

In order to decrease the oxidative degradation caused by the dissolved oxygen(DO)in solution which has always happened in CO2capture processes,Chen and Tan et al.proposed to use sodium aliphatic diamine sulfonate (NaADS) to mix with piperazine (PZ) and diethylenetriamine(DETA)to capture CO2.Although the viscosity of the mixed absorbent solution PZ/DETA/NaADS was higher than that of PZ/DETA, more free amines presented in solution and high shear stress provided in an RPB,the similar CO2capture efficiency and CO2capture amount could still be achieved.In addition,the regeneration energy of PZ/DETA/NaADS was less than PZ/DETA and MEA at the general regeneration condition at 202.65 kPa and 120°C[72].

Chiang and his co-workers have conducted accelerated carbonation via the high gravity technology to enhance the micro-mixing efficiency and the mass transfer.The concept,known as the high-gravity carbonation (HiGCarb) process, was first proposed in 2012 by Chang et al.[73],and then successfully applied in a steel plant in Taiwan[74,75].For the HiGCarb process with a CO2removal efficiency greater than 90%, the total energy consumption was estimated to be (268.6 ±57.9)kW?h·(t CO2)-1,and the scale of this process was 170 kg CO2captured per day[74].Another application of HiGCarb process which used lime by product as absorbent was carried out in a petrochemical industry at a scale of ～600kg CO2captured per day in Taiwan since 2016[1].

In addition,researchers also worked on the modeling of RPB CO2capture system which will be helpful in scale-up,optimization,optimum design and process analysis of this system[76-78].For example,Borhani et al.applied the first principles in gPROMS?to model an RPB absorber using MEA as the solvent to capture CO2at steady state condition.The result showed good agreement with the experimental results and revealed that rotor speed has the most important effect on carbon capture level,and after that lean MEA solution flow rate has the second importance[77].

3.5.NOx emission control using high-gravity technology

NOxis a general term for the nitrogen oxides,including NO, NO2,N2O3,N2O4,and N2O5.Long-term exposure to NOxwould increase the risk of respiratory disease. NOxalso contributes to the formation of acid rain, photochemical smog, and ground level ozone. Most of the NOxemission comes from the vehicle exhaust and the stationary source emissions. The stationary source emissions can be divided into two parts, one part is from the fossil fuel combustion, the other part is mainly from the chemical process which related to HNO3,such as nitrogen fertilizer plants, gunpowder and dynamite plants and organic chemical plants. As a major pollutant flue gas, NOxemission has received great attention in the world.Various technologies have been developed to reduce NOxemission[2].

The exhausted gas of the gunpowder plant containing a great amount of NOx,the traditional denitrification process with absorption tower using water as absorbent, which need large area,high investment,high operation cost.Moreover,the removal efficiency is low,normally after denitrification the NOxemission concentration is still higher than 5000 mg·m-3,which is much higher than the emission limitation.In order to overcome those shortcomings,based on the experience of the desulfurization process,Prof.Liu and his colleagues had developed the high gravity denitrification technique.

In 2007,nitration tail gas mainly of NOxwas treated by an RPB.In this process,the carbamide was used as the absorbent,also containing some additive to adjust the pH of the solution.The mechanism of this process is as follows:

After being adsorbed into the solution,the NOxwill form HNO3and HNO2,and ionized into NO3-and NO2-respectively.The NO2-ions will react with (NH2)2CO to form N2and CO2. The operation conditions and running stability were examined during the pilot test based on our earlier research.The gas flow was 100 m3·h-1,and the inlet NOxconcentration was from 18000 to 20000 mg·m-3and used 20 wt%of carbamide as absorbent. With an excellent micro-mixing efficiency under such gravitation,a NOxremoval ratio of greater than 96%can be achieved by the high gravity technique with two RPBs in series, the NOxconcentration in the outlet gas was less than 240 mg·m-3,which was lower than the Chinese national emission limitation at that time.Compared with the traditional towers, the RPB could decrease the equipment investment nearly 75%, reduce the operation cost about 79%[79]Fig.5 presents a denitrification RPB equipment.

In 2010,Li et al.applied the RPB in industry denitrification[80].NO,NO2,N2O3,N2O4,O2,and other inert gas species are in the gas phase.The gas phase reactions in this study are:Then the NOxin the gas phase will transfer into the liquid phase,and for pure water or nitric acid solution,the adsorption reactions are as follows:

Fig.5.A denitrification RPB equipment.

In this case,the nitric acid solution was used as absorbent,ozone was introduced to oxidize HNO2to HNO3to prevent the decomposition of HNO2in the liquid phase and therefore to improve the NOxabsorption.In an acidic system at ambient temperature,the reaction is.

Reaction(40)is a fast reaction and the mass transfer of ozone is the controlling step.In this process,it was intensified by the RPB,so that the absorption amount of NOxis increased.The mechanism of NOxabsorption into nitric acid solutions in the presence of ozone is represented in Fig.6.

Fig.6.Mechanism of NOx absorption into acid solution in the presence of ozone[80].

In this process,the NOxtail gas was from the nitration reaction device of one Chemical Industry Co.in Gansu,China,and the NOxconcentration in the inlet gas streams was 200-240 g·m-3,the gas flow rate was 1700 m3·m-2·h-1.Under the optimized conditions,the removal efficiency of NOxcould be higher than 90%and the concentration of nitric acid byproduct was higher than 45%.With the great potential industry applications,these high gravity denitrification techniques have been used in many chemical factories in China by our group as demonstration projects.

High gravity technology has been proved as a great technique for denitrification process which can use different absorbents.Zhang et al.[81]used RPB as a gas-liquid reactor to enhance the NO removal efficiency by FeII(EDTA)solution.Under proper conditions,the removal efficiency could reach 87%. Because NO is the major component of the NOxemission and is less soluble in water than NO2,Sun et al.recently did an experimental work to absorb NOxin to NaOH solution in an RPB with preoxidation by O3[32].In this work,before inletting into the RPB,the NO was first oxidized by the O3to increase the denitrification efficiency.At optimized conditions,the denitrification efficiency could get around 80%.This work indicates that the hydrolysis reactions of NOxare still the rate-determining steps in the NOxabsorption process and are the main obstacles hindering NOxremoval by wet scrubbing processes,so RPB has great potential in this kind of conditions.

3.6.NH3 emission control using high-gravity technology

Ammonia(NH3)is a colorless air pollutant with a strong and repellent smell.Most of the NH3emission comes from petrochemical,agriculture and chemical industries,such as synthetic ammonia industry,ammonium carbonate industry, ammonium nitrate industry and nitrophosphate fertilizer industry.The composition of these kinds of NH3containing flue gases is very complicated, which contain NH3,NOx, water and particulate. In commercial treatment, particles and water vapor during the condensation process will form a slurry which easily blocks the towers,and then decreases the recycling amount of the NH3.Moreover,the process of the traditional method is long and with high operation cost.In order to solve the above industry problems,Prof.Liu and his co-workers developed a new type of RPB with radial corrugated packing,which could realize deamination and dehumidification at the same time [82].The deamination rate is higher than 90%,both the ammonia and water can realize recycling utilization.If using acidic waste water as adsorbent,the adsorption process is chemical adsorption,the reactions are as follows:

The adsorption efficiency is based on the acid content of the solution,if the acidic solution is sufficient,the NH3could be fully adsorbed,otherwise the adsorption process of the remaining NH3will convert into water adsorption,which is controlled by the gas-liquid equilibrium,and will decrease the adsorption efficient.

In 2006, this technique had been applied in the update of the nitrophosphate fertilizer flue gas treatment process in one fertilizer company of China. The RPB setup was installed in a small platform with the height of 39 m.The gas flow rate was 55000 m3·h-1,the concentration of NH3was 8.3 g·m-3, the volume content of water was 20.6%,and the concentration of particulates was 3 g·m-3.The absorbent in this case was the waste HNO3solution from its own process,and the NH4NO3byproduct could recycle into the same process.The deamination rate of this technique could reach as high as 92%, the recycling amount of NH3was 2940 t·a-1, and the dehumidification rate was 60.8%,the recycling amount of water was 3.86 t·a-1,which could decrease operation cost about 8.82 million CNY·a-1.This technique was also applied in another company, which was the biggest slow controlled release fertilizer company of Asia.In that case it could recycle NH32576 t·a-1,recycle water 17.8 kt·a-1,and decrease the emission of NOx126 t·a-1[82].This technique has made a big contribution to the energy conservation and emission reduction of fertilizer industry and has also been applied in many other chemical companies in China.

In 2008,Meng et al.[83]aimed at the problems of treating ammonia in phosphate tail gas, an RPB was used to replace the conventional towers, with acid waste water as absorbent. Pilot scale experiment was made to determine the capacity of ammonia removal from phosphate tail gas,as show in Fig.7.The result showed that,when the gas flow was 500 m3·h-1,gas-liquid ratio was 1000 m3·m-3,high gravity factor was 90,and the absorptivity of ammonia could reach 90%.After 15 days of continuous running, the system was stable and after treatment the content of the ammonia in phosphate tail gas could reach the state discharge standards,about 739 mg·m-3.

For the traditional Selective Catalytic Reduction(SCR)denitration technology, the setup of the liquid ammonia gasification process is large, and the volatilization of NH3is harmful to the health of the workers and may cause explosion at certain conditions. In order to solve these problems,our group presented using ammonia water stripping instead of the liquid ammonia gasification process.High gravity technology was used for the ammonia water stripping process, the free ammonia in the ammonia water was stripped into gas phase,and transferred to a denitration furnace.This technique can highly increase the denitration rate,and the volume of the setup is much smaller than the traditional ones which could highly decrease the investment and operation cost,and is much more safe.This technique has been applied in one coal power company of China,as shown in Fig.8.The ammonia water treatment amount was 0.1-0.8 m3·h-1for one RPB setup,the dosage of dilution air was 1000-2400 m3·h-1, the denitration rate was higher than 95%,the emission concentration of NOxwas less than 100 mg·m-3, and the concentration of escape NH3was less than 8 mg·m-3.

Fig.7.An RPB for NH3 emission control.

3.7.VOC emission control using high-gravity technology

Volatile organic compounds(VOCs)normally originate from motor vehicle exhaust,gasoline vapors,industrial facilities,electric utilities and chemical solvents,such as-trichloroethylene(TEC),isoprene(C3H8),benzene(C6H6),gasoline(C8H18),and vinyl chloride(C2H3Cl).Traditional VOC purification technologies include adsorption,absorption,condensation and membrane separation,however there are still several shortcomings like long-term process,high energy consumption,and secondary pollutions.Therefore,it is necessary to develop new technologies to overcome the above problems.Considering the characteristics of high gravity technology,such as small pressure drop,high transportation efficiency,small equipment volume,and flexible operation,our group applied RPB in the VOC purification. Based on the physicochemical properties of VOCs,new packing and high gravity set-ups have been developed.The VOC absorption rate is higher than 90%,which is positive to reduce the following workload.For instance,in acetic acid(CH3COOH)gas purification process,the gas flow rate was 4500 m3·h-1,water was used as absorbent,when the CH3COOH concentration was higher than 70%in the absorbent,it was transferred into rectification tower,after rectification the purity of CH3COOH was higher than 99%,the recycling amount was 527 t·a-1[41].

This technology could also be used to recycle acetone(CH3COCH3),ethyl acetate(CH3COOC2H5),etc.For instance,in one explosives production process,CH3COCH3,CH3COOC2H5,and water mixed solution was used to purify the explosives products.With the temperature growing,the amounts of the CH3COCH3and CH3COOC2H5were volatilized into the air, which causes environmental problems and the wasting of resources. In order to solve this problem, RPB was applied to recycle CH3COCH3and CH3COOC2H5,and water was chosen as absorbent.The packing rotation speed was 1000 r·min-1,the recycling time was 12 h,and the flow rate was 2 m3·h-1.The recycling solution was returned back to the previous process which could not only reduce pollution,but also decrease the operation cost[84].In 2009,this technique was also applied for the absorption of CH3COOH,the gas flow rate was 450 m3·h-1,the maximum liquid circulating volume was 0.3 m3,the liquid flow rate was 0.5 m3·h-1to 1.5 m3·h-1, after 12 h circulating, the CH3COOH mass fraction could reach higher than 60%.The recycling of acetic acid could decrease the operation cost about 1.35 million CNY·a-1[85].

Fig.8.An RPB set up for NOx and NH3 control.

3.8.Particulate matter control using high-gravity technology

The types of particulate matter (PM) in the atmosphere include suspended particulates,inhalable particulate matter(PM10),fine particulate matter(PM2.5),and ultrafine particulate matter[1].The particulate pollutants released from the industrial tail or flue gas are the key sources leading to serious haze pollution. New national regulation of the ultra-low emission limitation control of those particulate pollutants in China is lower than 5 mg·m-3or lower than 20 mg·m-3in some special cases. However, the conventional particulate collection processes like bag house filter,wet scrubber,and electrostatic precipitator are difficult to meet the ultra-low emission limitation.RPB wet particulate collection technique combined the characteristics of cyclone,filter,mechanical rotation collector,and water dust scrubber together,which can meet the ultra-low emission control.For PM 1.0,it still can achieve high purification efficiency more than 90%,and is suitable to put in series after the original wet dust collector.

A flow diagram of one of the RPB particulate removal processes is shown in Fig.9.First the particles containing gas flow through the induced draft fan(1)to get higher pressure,then flow through the vortex flow meter(2)into RPB(4),the adsorbent is pumped into RPB by the liquid pump(6)and is torn into liquid elements such as droplet,thread or film by the high-speed rotating packing to trap the particulates.After trapping,the adsorbent is flow into reservoir(5),and the clean gas discharge from the top of the RPB.The particulate concentration of the inlet and outlet gas is measured by a dust detector(3).Here,based on the different conditions and requirements,the particulate adsorbent can be circulating water,alkali liquor,etc.

Fig.9.Process flow diagram of RPB particulate removal.

This technique has been applied in many chemical factories.Table 3 presents the operating conditions and performance of PM removal using high gravity techniques. In one fertilizer company in Henan,China,the blow-air after burning,heat exchanger,and water dust scrubber still contained particulates about 100 mg·m-3.An RPB was used to solve this emission problem by Prof.Liu and his co-workers in 2016.In this case,the diameter of the RPB was 2200 mm(Fig.10),the gas flow rate was 80000 m3·h-1,the circulating water was used as purification medium, and the flow rate of the circulating water was 26 m3·h-1.After purification the concentration of the dust was lower than 12 mg·m-3, the pressure drop was only 800 Pa, which realized the ultra-low emission with low investment and operation cost.

This technique was also applied in one chemical company of China, the particulate concentration in the flue gas after burning and desulfurization process was 600 mg·m-3. The gas flow rate was 30000 m3·h-1-40000 m3·h-1,after RPB purification,the particulate concentration in the outlet was less than 50 mg·m-3.

In one fertilizer company of China,a rotary fluidized bed was used to prill CaNH4NO3particles. However, there was 3000 mg·m-3of CaNH4NO3contained in the off-gas,which not only wastes the product but also results in air pollution.In this process,the gas flow rate was as high as 43000 m3·h-1,and the particle size was small,therefore traditional particulate collection method could not get good purification efficiency.The equipment would easily be blocked and could not run well for a long time.Moreover,the CaNH4NO3production equipment was settled in a platform with the height of 22 m,not enough space for the traditional purification set-ups.In order to overcome those problems,our RPB was used to collect the CaNH4NO3particles,the diameter of the RPB was only 1600 mm,and the height of the RPB was 3500 mm,which was settled in the platform and suitable for the layout of the plant.The running results showed that,the purification efficiency was more than 99%, and the outlet particulate concentration was only 5 mg·m-3.The flow rate of the circulating water was only 12 t·h-1,which could go back to the production process.

Table 3Performance evaluation of the particulate removal processes using the high gravity technology

In coal chemical industry,the purification of the coke oven gas,producer gas,water gas and semi-water gas is very important in the whole process.With the development of the pressure gasification technology,there are more particulates and tar than before in the gases.Even after traditional purification processes,there still contained particulates and tar from tens to hundreds of mg·m-3,which will accumulate in the process to affect the production.In order to improve the quality of the products and the running stability,new purification technology was highly desired.In this kind of case,two serious challenges lead to traditional methods not meeting the purification requirement,one is the low particulate concentration,the other one is the small size of the particulate.In order to solve the above problems, and based on the previous research work, high gravity wet purification coal gas technology has been developed by our group.This technique could both purify particulates and tar.Moreover,this high gravity technology and the equipment have been acceded to the chemical design specification of Sedin Engineering CO.LTD of China.

In 2013,our technique was applied in one coal chemical company in Xinjiang,China.The flow rate of the coal gas was 20700 m3·h-1,before our technological reformation,after traditional purification,there still contained 900 mg·m-3tar and 300 mg·m-3dust to block the inlet of the compressor,and the process needs to stopped to remove the particulate and tar once a week.In order to solve the blocking,the coal gas had to be burned before going into the compressor,which not only wastes tar,but also causes environmental pollution.Based on this technological problem,new high gravity technology had been developed,the diameter of the high gravity equipment was 1600 mm,and the height of the equipment was 4100 mm.Under the proper conditions,the tar and particulate removal efficiency was higher than 90%,the concentration of the outlet particulate was less than 10 mg·m-3.After this technological reformation, all of the coal gas could be recycled(2.3 hundred million m3·a-1),and the compressor could run well for more than half year.This technique has been applied in more than 10 companies of China,even in high pressure(4.5 MPa)coal gas purification,which has highly application prospects.

In many chemical cases,there are both particulates and SO2in the flue gases,so it is highly desired to develop a new technology which can purify particulates and SO2simultaneously.Our North University of China first presented the idea,using RPB with 1200 mm diameter to combine desulfurization and particulate purification together.This technology was first applied in one chemical company of Shanxi,China and one power plant of Taiyuan,China.Mg(OH)2and Ca(OH)2solution was used as absorbent,there were 50 g·m-3particulates and 1200 mg·L-1SO2in the flue gas.The liquid-gas ratio of our RPB wet desulfurization and dust removal technology was 0.21 L·m-3,which was much lower than the most traditional wet purification methods.Both of the desulfurization and dust removal rate could get 99%,the SO2concentration in the outlet gas was only 8 mg·L-1,at that time the emission limitation in the world was less than 300 mg·L-1[41].

Fig.10.An RPB equipment for particulate removal.

Our group reported that,the RPB could remove both hydrophilic and hydrophobic particulates,the purification capacity was higher than that of an electrostatic precipitator,and the volume of the equipment was only 1/4 of the electrostatic precipitator, which could decrease the equipment investment and the capital expenditure.The removal efficiency was equal to that of the venturi scrubber,but the pressure drop was only 1/6 of the venturi scrubber, and the energy consumption was only 17%of the venturi scrubber.In 2000, this technique passed the Shanxi province of China science and technology achievement appraisal,and in 2002 awarded second prize of Science and technology progress in Shanxi province[16].

In 2004,multi-spraying high gravity technology had been applied in one company in Guangdong to treat the boiler fuel gas.The gas flow rate was 4000 m3·h-1,the SO2concentration in the inlet gas was 3 g·m-3,double-alkali method was used for desulfurization.At first,a water dust scrubber was used to purify the dust,the diameter was 1500 mm and the height was 8000 mm, the desulfurization rate was 70%, and the dust removal efficiency was 80%. After applying the new multispraying RPB, the desulfurization rate was increased to more than 94%,and the particulate removal was up to 99%.And the diameter of the RPB was only 600 mm,the height was only 1500 mm,the volume of the RPB was only 3.6%to the volume of the water dust scrubber[62].

The upper practical application results indicate that,the removal ratio of fine particulate (～mm) by the RPB normally is significantly higher than that by conventional processes.In particular,fine particles larger than 3.5 μm could be almost completely removed by the high gravity technology [86].It is noted that, the high gravity technology could efficiently remove particulate in a wide range of particle sizes between 0.01 and 100 μm.The cutting particle sizes(the most difficult to collect)for the RPB system are typically in the range of 0.02-0.3 μm,which is quite competitive with the wet scrubber system(the critical particle sizes of 0.1-0.5 μm),but the pressure drop in the RPB system is less than that in the wet scrubber system[1].

4.Perspective and Prospective

High gravity technology is one of the process intensification technologies,which is a new branch of chemical engineering.The upper experimental and industry results demonstrated that high gravity technology is highly competitive in gas pollutant emission control for removing H2S,SO2,CO2,NOx,NH3,VOCs,and particulate from tail gas,petroleum or natural gases,and flue gas.In order to meet the new national regulation of the ultra-low emission limitation of China, industries in the chemical,power generation,and oil field need to upgrade their environmental protection equipment.The high gravity technology is highly efficient, energy-saving and small in size, which is one of the best techniques for the equipment update,the gas purification in the ship and the marine drilling platform,and has received considerable attention.In addition,high gravity technology is highly appropriate to be applied in processes which are controlled by the mass transfer and/or the molecule mixture,and other specific material industry processes,such as the treatment of high viscosity,thermal sensitivity,and expensive materials. Therefore, high gravity could be applied in the unit applications like absorption,desorption,rectification,devolatilization processes; the preparation of nano-materials; and the reaction processes such as,sulfonate,polymerization and condensation.

The high gravity technology can offer great benefits for chemical engineering processes due to its advantages of intensifying mass transfer by orders of magnitude, ultra-short residence time, as well as the small size, low cost, energy saving, and environmental friendliness.However,comprehensive commercial-scale applications of high gravity technology about the scale-up,stability and flexibility of the equipment are still needed to be undertaken.Moreover,the market reception of high gravity technology for the new design plants needs to be improved.Therefore,numerous continuously long-period operations of large-scale RPBs are necessary to gain more commercial data and experiences for ensuring the market acceptance of high gravity technology, such as the packing type and structures for the different systems and the mass transfer and micromixing properties for different systems and RPB types. In addition, scientific experiments should be coupled with a mechanism model at microscale and computational fluid dynamic(CFD)simulation at macroscale to precisely predict the behaviors of mass/heat transfer and hydraulic properties of the RPB.

As one of the process intensification technologies,high gravity technique has achieved industry applications in many fields, which has shown excellent energy conservation and emission reduction.In order to meet the fierce market competition,the process intensification technology will continue to improve and develop,and the high gravity technology will also keep playing a very important role in the development of the chemical engineering.