Liangliang Zhang *,Shuying Wu Zuozhong Liang Hong Zhao 2,Haikui Zou Guangwen Chu

1 Research Center of the Ministry of Education for High Gravity Engineering and Technology,Beijing University of Chemical Technology,Beijing 100029,China

2 Suzhou Research Institute of Beijing University of Chemical Technology,Suzhou 215100,China

1.Introduction

As environmental pollution is becoming more and more serious,replacing coal by natural gas is gradually developing in China.Natural gas is a competitive energy alternative in both its energy efficiency and environmental impact,and it can be used for heat and electricity production as well as vehicle fuel[1].However,the production of onshore natural gas cannot meet the huge consumption demand.Thus,the exploitation of offshore gas fields becomes very important for our country.In order to transport natural gas from offshore platform to the land,a cleaning process to remove the harmful components in natural gas is needed.Among the harmful impurities,hydrogen sul fide(H2S)is extremely troublesome because it causes corrosion in piping,compressors and gas storage tanks.Moreover,hydrogen sul fide is highly toxic,which endangers lives of human people once emission occurs[2,3].Therefore,H2S removal is generally the preceding procedure of natural gas purification and strict standard is put forward for H2S removal.

So far,a wide range of approaches,including liquid absorption[4,5],solid adsorption[6–8],biological degradation[9–11],membrane separation[12]and so on,have been developed for H2S removal.Among these approaches,absorption ofH2S in liquids is a kind ofwidely used desulfurization method for mixture gas containing H2S,in which H2S is chemically absorbed by different liquids such as NaOH,FeCl3,and Fe(OH)3solution according to different reaction mechanisms[13].Compared with other liquid absorption methods,catalytic oxidative absorption of H2S using a ferric chelate solution has attractive advantages.Because this method not only transforms H2S into the desirable product of elemental sulfur directly,which could be recovered to bring considerable economic benefits,but also easily realize the regeneration of absorbent[14].Therefore,this desulfurization method is a prior choice in many H2S removal projects.

Usually,conducting of this scrubbing process in traditional packed tower is economical and feasible[15].But this method is hard to be accepted on offshore platform.Firstly,construction cost of offshore platform is very expensive(cost more than 1 million dollars per square meter),and traditional packed tower occupies a large number of platform space,which means the construction cost of offshore platform will significantly increase.Secondly,in order to achieve a certain removal efficiency,the height of the traditional packed tower is generally very high,which causes the potential threat to the safety of the offshore platform.Therefore,it is significant to develop a new technology with high process intensification efficiency for such system and gain the acceptance from an industrial viewpoint.

Various research groups have studied the reaction of H2S with ferric chelate on different experimental conditions[16,17].The studies indicated that the reaction rate of catalytic oxidative absorbent with H2S is quite fast and the absorption process is strongly affected by gas–liquid mass transfer limitation in traditional reactors.Thus,more efficient reactors are required to intensify the mass transfer of the reaction,thereby improving the absorption rate and H2S removal efficiency.

Rotating packed bed(RPB)as a novel and high effective contactor and reactor has been utilized to intensify the mass transfer[18].The basic principle of RPB is to create a high-gravity environmentviathe action of centrifugal force as so called “Higee”.The fluids going through the packing are spread or split into very fine droplets,threads,and thin films by the strong shearforce,resulting in a significant intensification of micro mixing and mass transfer between the fluid elements and gas[19,20].According to previous studies,the RPB has a higher gas–liquid mass transfer efficiency which is evidenced by the fact that the volumetric mass transfer coefficient in a RPB is 1 to 3 orders of magnitude higher than that in a conventional packed tower[21].Consequently,a smaller equipment size may be achieved,thereby benefiting a reduction of capital and operating costs.Based on the above advantages,RPB has been successfully applied in distillation,absorption,stripping and reactive crystallization,exhibiting prominent process intensification characteristics[22–27].

In order to realize the industrial application on offshore platform,a new approach is proposed here for large scale H2S removal in con fined space.This hybrid approach combines the advantage of RPB as a high effective reactor and catalytic oxidative absorbent as environmentally benign H2S removal media with high thermal stability and regeneration efficiency.Catalytic oxidative absorbent,which contains ferric chelate and other auxiliary components such as defoaming agent,crystallizing agent and alkali,is adopted to validate the feasibility and efficiency of H2S removal in RPB.The effects of absorbent pH value,absorbent temperature,RPB rotating speed,gas–liquid ratio and different type packing in RPB on H2S removal efficiency are studied.The absorption/desorption process is continuously conducted for 72 h to investigate the regeneration ability of the absorbent and the stability of the process.

2.Experimental

2.1.Materials

Analytical reagent grade potassium hydrate(KOH),mercuric oxide(HgO)and hydrogen nitrate(HNO3),and diphenyl thiocarbazone(C6H5NHNHCSN=NC6H5)are purchased from Beijing Reagent Factory of China.Deionized water is obtained from a water purification system(RO-DI plus,Hitech,PRC).The gaseous mixture(N2mixed with 2.5 g·m?3H2S),which is used to simulate natural gas,is purchased from Beijing Ruyuanruquan Technology Co.,LTD,China.Catalytic oxidative absorbent(ARI340 and ARI350),which contains ferric chelate and other auxiliary components such as defoaming agent,crystallizing agent and alkali,is provided by China National Offshore Oil Corporation.

2.2.Experimental setup and procedure

The experimental set-up for H2S absorption was schematically shown in Fig.1.

The H2S removal experiment is performed as follows.Firstly,the pipelines and equipment are swept by pure N2prior to use to remove the moisture and air.The RPB reactor and the absorbent are preheated to the set temperature by water bath.Then the gas mixture stream from the gas cylinder as the continuous phase flows inward from the outer edge of the RPB by a pressure-driving force,while the catalytic oxidative absorbent as the dispersed phase is sprayed through 3 holes(the diameter is 1 mm)in the liquid distributor at the center of the RPB.The absorbent moves outward and leaves from the outer edge of the RPB through a centrifugal force.Gas and liquid streams have a counter current contact in the RPB.Consequently,H2S in the gas stream dissolves in the liquid stream,and reacts with the absorbent before the gas and liquid streams leave the RPB from gas outlet and liquid outlet,respectively.Rich absorbent containing elemental sulfur solid,which leaves from the outlet of the RPB, flows into the regenerator tank and then is oxidized by bubbled air,forming lean absorbent.The whole process combining absorption and desorption can realize continuous operation by well controlling the process condition.

In our study,the pressure of inlet gas is maintained at0.10 MPa.Based on previous study,the pressure drop from the inlet to the outlet is lower than 5 kPa.Hence,the pressure drop of the gas phase is ignored.All the data are obtained until a steady-state operation is maintained for 5 min.

The schematic diagram of the RPB for H2S removal experiment is shown in Fig.2.The RPB reactor mainly consists of a packed rotator,a fixed mesh casing,a liquid inlet,and a gas inlet.The bed is packed with stainless wire meshes.The inner and outer diameters of the packing are 50 mm and 140 mm,respectively,and the axial length of the packing is 35 mm.And two types of packing,type I,whose porosity and wire diameter are 0.95 and 1 mm,and type II,whose porosity and wire diameter are 0.9 and 0.5 mm,are used here.Type I packing is used in most of the experiments unless otherwise specified.The rotator is installed inside the fixed casing and rotated at an adjustable speed.The H2S concentration in the outlet is measured using the mercurimetric titration method,which is reported in the literature[28].

Fig.1.Experimental set-up.

Fig.2.Schematic diagram of the RPB for H2S removal experiment.(1)Liquid inlet,(2)gas outlet,(3)gas inlet(4)liquid outlet,(5)liquid distributor,(6)mesh packing.

2.3.Mechanism of desulfurization reaction

The mechanism of catalytic oxidative absorption desulfurization is described as Eqs.(1)–(5)[16,17].

During the absorption process,H2S is dissolved in an aqueous solution which contains ferric chelate(Eq.(1)),and then dissociatesviatwo reactions(Eqs.(2)and(3)).HS?produced in Eq.(2)is oxidized by Fe3+in the solution and converts to elemental sulfur(Eq.(4)).During the desorption process,elemental sulfur is separated by a press filter and ferrous chelate is oxidized to ferric chelate by air,which is bubbled from the bottom of the regenerator.And then the ferric chelate can be used for oxidizing HS?again.The regeneration reaction is described as Eq.(5).Fe3+is the oxidizer in the desulfurization process.As it is regenerated during the circulations,it could also be regarded as a catalyst of the process.In order to maintain a high H2S absorption rate,the pH of the absorbent is controlled to the desired value by the addition of an alkali solution to neutralize the produced H+in Eq.(2).

3.Results and Discussion

The removal efficiency(η)is simply determined from the difference between the concentration of H2S entering and leaving the RPB,which is selected for evaluating the performance of the RPB,and expressed

wherec1andc2denote mass concentration of gas phase H2S entering and leaving the RPB,respectively.

3.1.Effect of absorbent pH value on H2S removal ef fi ciency

Fig.3 displays the effect of absorbent pH value on H2S removal efficiency with gas–liquid ratio 60,RPB gravity level 120gand absorbent temperature 40°C.It can be seen that the removal efficiency obviously increases with the absorbent pH value increasing from 7.9 to 8.5.As the pH value is further increased from 8.5 to 9.2,the removal efficiency has no notable change,and the removal efficiency keeps higher than 99.85%.Clearly,when H+is neutralized,Eq.(2)will be greatly promoted to the forward direction.Thus,alkaline environment is favorable to make the reactions(1)and(2)shift to the right,thereby benefiting the mass transfer of H2S from gas phase to the absorbent.When the pH value of the absorbent is further increased from8.5 to 9.2,the dissolution of the H2S is further enhanced,but the effect is not as obvious as the pH below 8.5.

In practical situation,natural gas usually contains CO2with concentration much higher than that of H2S,and CO2is also dissolved in the desulfurization solution.The dissolved CO2would be hydrolyzed and dissociated in the absorbent.High pH would cause lots of concurrent absorption ofCO2,leading to frequent supplement of alkali solution,which increases operational cost.So the pH value of the absorbent should be kept around 8.5 in the industrial H2S removal system to ensure an optimal H2S removal efficiency and a good impediment against CO2absorption.

Fig.3.Effect of absorbent pH value on H2S removal efficiency in RPB.

3.2.Effect of gas–liquid ratio on H2S removal ef fi ciency

In industrial running,the absorbent flow rate usually keeps stable and the gas flow rate always fluctuates,which causes the gas–liquid often changes.Thus,gas–liquid ratio,serving as a major concern in the design and investment of absorption process,will directly in fluence the operation flexibility of the technique.

Fig.4 shows the effect of gas–liquid ratio on the H2S removal efficiency with RPB gravity level 120g,absorbent pH 8.5 and temperature 40 °C.The gas–liquid ratio is controlled by changing the flow rate of gaseous mixture under a fixed liquid flow rate of 90 L·h?1.As a whole,H2S removal efficiency decreases with the rise of the gas–liquid ratio,but the removal efficiency can maintain higher than 99.65%.An increase of gas–liquid ratio not only affects the gas–liquid contact state but also changes the load capacity of the absorbent.The increase in the gas–liquid ratio means less absorbent per unit of gas.It is crucial for the absorption process and leads to the decrease of the H2S removal efficiency.

Fig.4.Effect of gas–liquid ratio on H2S removal efficiency in RPB.

3.3.Effect of gravity level on H2S removal ef fi ciency

Fig.5 shows the effect of gravity level on H2S removal efficiency with gas–liquid ratio 60,absorbent pH 8.5 and temperature 40°C.The gravity level means the multiple of centrifugal acceleration in RPB,which is equal to acceleration of gravity.The numerical amounts of gravity level can be calculated by the following equation:

Fig.5.Effect of gravity level on H2S removal efficiency in RPB.

The ω is the angular speed of the RPB,theris the geometric average radius of the packing and thegis the acceleration of gravity(9.8 m·s?2).

It can be seen that H2S removal efficiency obviously increases with an increase in the gravity level from 40 to 120g.This is mainly because the liquid phase is broken into smaller liquid droplet and thinner liquid film when the gravity level of the RPB increases,which benefits intensifying the gas–liquid mass transfer.However,when the gravity level is further increased,a reverse effect on H2S removal efficiency is observed.The possible reason is that the extent of reduction in mass transfer resistance at a higher gravity level is compensated by a reduction of the retention time,which is unfavorable to chemical absorption.The phenomenon indicates that the gravity level of 120g(rotating speed of 1000 r·min?1)is optimal for our device.

3.4.Effect of absorbent temperature on H2S removal ef fi ciency

Fig.6 presents the effect of absorbent temperature on H2S removal efficiency with gas–liquid ratio 60,RPB gravity level 120gand absorbent pH 8.5.With the increasing temperature from 20 °C to 40 °C,the removal efficiency rapidly increases from 99.68%to 99.94%.As the temperature is further increased from 50 °C to 70 °C,the removal efficiency slowly decreases.Obviously,for catalytic oxidative absorption,the increase of the temperature can improve the reaction rate between HS?and ferric chelate,thereby enhancing the mass transfer of H2S from gas phase into the absorbent.Meanwhile,a higher temperature will reduce the dissolution of H2S in the absorbent,which is elucidated by Henry's Law.As the temperature increases beyond 50°C,the impediment of gaseous H2S dissolution overshadows the enhancement by increasing the reaction rate,thereby leading to a lower H2S removal efficiency at a higher temperature.So the optimum temperature is in the range of 40 °C to 50 °C for this H2S absorption system.

Fig.6.Effect of absorbent temperature on H2S removal efficiency in RPB.

3.5.Effect of different type packing on H2S removal ef fi ciency

Fig.7 presents the effect of different type packing on H2S removal efficiency with gas–liquid ratio 60,RPB gravity level 120g,absorbent pH 8.5 and absorbent temperature 40°C.Type I packing is a kind of thick mesh with the wire diameter 1 mm and porosity 0.95,and type II packing is a kind of thin mesh with the wire diameter 0.5 mm and porosity 0.9.It can be clearly seen that H2Sremoval efficiency in RPB using type Ipacking is obviously higher than using type II packing.The possible reason is that different types of packing have different effects on the flow of sulfur particles inside RPB.In the absorption process,the sulfur particle continuously forms and grows.Due to the bonding of sulfurparticle and mesh,sulfurparticle easily aggregates inside the wire mesh packing orattaches on the packing surface.When a high porosity and thick wire mesh is used,the sulfur is easily taken out from the packing due to the vigorous impingement between liquid and packing.However,when a low porosity and thin wire mesh is used,the tortuous packing channel leads to a certain amount of sulfur held in the packing,affecting the internal gas–liquid flow channel inside RPB.Thereby,this phenomenon results in a weak gas–liquid mass transfer of RPB and decreases the H2S removal efficiency.

Fig.7.Effect of different type packing on H2S removal efficiency in RPB.

3.6.Result of long-time continuous operation

The regeneration effect of the desulfurization absorbentis very important to the continuous operation of the process.In our experimental,the rich absorbentin the regeneration tank is oxidized by the airbubbled in the bottom.In order to investigate the stability of the process,a continuous experiment was carried out for 72 h.The experiment was conducted under the optimal condition for the desulfurization systemrelated to this work,which is gas–liquid ratio 60,RPB gravity level120g,absorbent pH 8.5 and absorbent temperature 40°C.The H2S removal efficiency is measured at the interval of6 h.Fig.8 presents the result of long time continuous experimental operation.

Fig.8.72 hour continuous H2S removal experiment in RPB using regeneration absorbent.

From Fig.8,it can be seen that the process can realize well simultaneous desulfurization and absorbent regeneration.The H2S removal efficiency keeps relatively stable in the whole duration and maintains higher than 99.5%using type II packing and 99.75%using type I packing respectively.The result indicates that high gravity technology desulfurization process,which is simple,high-efficiency,and space intensive,has a good prospect for industrial application ofH2S removal in con fined space.

4.Conclusions

In this article,the experiment of H2S removal from a mixture of N2and H2S was conducted in a RPB reactor with ferric chelate absorbent.The effects of absorbent pH value,gas–liquid ratio,gravity level of RPB,absorption temperature and character of the packing on the desulfurization efficiency were investigated.The results showed that H2S removal efficiency could reach above 99.6%.Optimal condition for the desulfurization system related to this work is gas–liquid ratio 60,RPB gravity level 120g,absorbent pH 8.5 and absorbent temperature 40°C,and H2S removal efficiency could reach above 99.9%under this condition.A long-time continuous experiment was conducted to investigate the stability of the whole process combining absorption and regeneration.The result shows that the process can well realize simultaneous desulfurization and absorbent regeneration,and the H2S removal efficiency keeps relatively stable in the whole duration of 72 h.In summary,high gravity technology desulfurization process,which is simple,high-efficiency,and space intensive,exhibits a good prospect for industrial application ofH2S removalin con fined space,especially for offshore platform.

Nomenclature

Ggas flow rate,L·h?1

ggravity acceleration,m·s?2

Lliquid flow rate,L·h?1

rradius of the packed rotator,m

Ttemperature,°C

η removal efficiency

ω angular velocity,rad·s?1

[1]H.Huo,Q.Zhang,F.Liu,K.He,Climate and environmental effects of electric vehicles versus compressed natural gas vehicles in China:A life-cycle analysis at provincial level,Environ.Sci.Technol.47(3)(2013)1711–1718.

[2]Y.W.Ma,Z.Z.Chen,H.J.Gong,Study on selective hydrogen sul fide removal over carbon dioxide by catalytic oxidative absorption method with chelated iron as the catalyst,Renew.Energy1(2016)1–8.

[3]A.Pieplu,O.Saur,J.C.Lavalley,Claus catalysis and H2S selective oxidation,Catal.Rev.Sci.Eng.40(4)(1998)409–450.

[4]J.Lu,Y.Zheng,D.He,Selective absorption of H2S from gas mixtures into aqueous solutions of blended amines of methyldiethanolamine and 2-tertiarybutylamino-2-ethoxyethanol in a packed column,Sep.Purif.Technol.52(2006)209–217.

[5]B.N.Markus,F.Anton,K.Ulrich,T.Thomas,Modelling selective H2S absorption and desorption in an aqueous MDEA-solution using a rate-based non-equilibrium approach,Chem.Eng.Process.43(6)(2004)701–715.

[6]D.W.Neumann,S.Lynn,Oxidative absorption of H2S and O2by iron chelate solution,AIChE J.30(1984)62–69.

[7]G.Edith,T.Turek,Removal of hydrogen sul fide by permanganate based sorbents:Experimental investigation and reactor modeling,Chem.Eng.Sci.151(2016)51–63.

[8]C.A.P.Zevenhoven,K.P.Yrjas,M.M.Hupa,Hydrogen sul fide capture by limestone and dolomite at elevated pressure.2.Sorbent particle conversion modeling,Ind.Eng.Chem.Res.35(3)(1996)943–949.

[9]A.M.Montebello,T.Bezerra,R.Rovira,L.Rago,J.Lafuente,X.Gamisans,Operational aspects,pH transition and microbial shifts of a H2S desulfurizing biotrickling filter with random packing material,Chemosphere93(2013)2675–2682.

[10]I.Diaz,S.I.Perez,E.M.Ferrero,M.Fdz-Polanco,Effect of oxygen dosing point and mixing on the microaerobic removal of hydrogen sulphide in sludge digesters,Bioresour.Technol.102(2011)3768–3775.

[11]A.M.Montebello,M.Fernandez,F.Almenglo,M.Ramirez,D.Cantero,M.Baeza,Simultaneous methylmercaptan and hydrogen sul fide removal in the desulfurization of biogas in aerobic and anoxic biotrickling filters,Chem.Eng.J.200(2012)237–246.

[12]W.B.Richard,L.Kaaeid,Natural gas processing with membranes:An overview,Ind.Eng.Chem.Res.44(2005)3348–3354.

[13]D.McManus,A.E.Martell,The evolution,chemistry and applications of chelated iron hydrogen sul fide removal and oxidation processes,J.Mol.Catal.A:Chem.117(1997)289–297.

[14]A.S.Michael,I.D.Norman,D.C.Peter,Catalytic H2S conversion and SO2production over iron oxide and iron oxide/γ-Al2O3in liquid sulfur,Ind.Eng.Chem.Res.46(2007)7721–7728.

[15]M.Miltner,A.Makaruk,J.Krischan,M.Harasek,Chemical-oxidative scrubbing for the removal of hydrogen sulphide from raw biogas:Potentials and economics,Water Sci.Technol.66(2012)1354–1360.

[16]D.Chen,A.E.Martell,D.Mcmanus,Studies on the mechanism ofchelate degradation in ironbased,liquid redox H2S removal process,Can.J.Chem.Eng.73(1995)264–274.

[17]H.J.Wubs,A.Beenackers,Kinetics of H2S absorption into aqueous ferric solution of EDTA and HEDTA,AIChE J.40(1994)433–444.

[18]S.Munjal,M.P.Dudukovic,P.Ramachandran,Mass transfer in rotating packed beds:I.Development of gas–liquid and liquid–solid mass-transfer coefficients,Chem.Eng.Sci.44(1989)2245–2256.

[19]F.Guo,C.Zheng,K.Guo,Y.D.Feng,N.C.Gardner,Hydrodynamics and mass transfer in cross- flow rotating packed bed,Chem.Eng.Sci.52(1997)3853–3859.

[20]J.R.Burns,C.Ramshaw,Process intensification:Visual study of liquid maldistribution in rotating packed beds,Chem.Eng.Sci.51(1996)1347–1352.

[21]H.J.Yang,G.W.Chu,Y.Xiang,J.F.Chen,Characterization of micromixing efficiency in rotating packed beds by chemical methods,Chem.Eng.J.121(2006)147–152.

[22]F.Yi,H.K.Zou,G.W.Chu,L.Shao,J.F.Chen,Modeling and experimental studies on absorption of CO2by Ben field solution in rotating packed bed,Chem.Eng.J.145(2009)377–384.

[23]H.Zhao,L.Shao,J.F.Chen,High-gravity process intensification technology and application,Chem.Eng.J.156(2010)588–593.

[24]J.F.Chen,C.Zheng,G.T.Chen,Interaction of macro-and micromixing on particlesize distribution in reactive precipitation,Chem.Eng.Sci.51(1996)1957–1966.

[25]C.C.Lin,Y.R.Su,Performance of rotating packed beds in removing ozone from gaseous streams,Sep.Purif.Technol.61(2008)311–316.

[26]J.F.Chen,L.Shao,Mass production of nanoparticles by high gravity reactive precipitation technology with low cost,China Particuol.1(2003)64–69.

[27]Y.Luo,G.W.Chu,L.Sang,H.K.Zou,Y.Xiang,J.F.Chen,A two-stage blade-packing rotating packed bed for intensification of continuous distillation,Chin.J.Chem.Eng.24(1)(2016)109–115.

[28]Y.P.Feng,Requirements and analysis methods of sul fides content in ammonia synthesis feed gas,J.Chem.Ind.Eng.(China)24(1)(2003)26–30.