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Adsorption DOI 10.1007/s10450-016-9790-0 Experimental and modeling study on room-temperature removal of hydrogen sulfide using a low-cost extruded Fe2O3-based adsorbent Nguyen Quang Long1 • Tran Xuan Loc1 Received: February 2016 / Revised: 26 February 2016 / Accepted: 11 March 2016 Ó Springer Science+Business Media New York 2016 & Nguyen Quang Long nqlong@hcmut.edu.vn Faculty of Chemical Engineering, Ho Chi Minh City University of Technology, 268 – Ly Thuong Kiet St., Dist 10, Ho Chi Minh City, Vietnam Graphical Abstract 0.3 Experimental data Thomas model 0.2 C/C0 Abstract In order to prevent corrosion and catalyst deactivation, commercial adsorption processes for hydrogen sulfide (H2S) removal operate at relatively high temperature (350–450 °C) by ZnO adsorption This paper reports firstly data from experimental and modeling studies of the dynamic performance of a low-cost extruded Fe2O3based adsorbent for H2S removal at room temperature The H2S adsorbent containing iron (III) oxide (Fe2O3) and bentonite was prepared by hydrothermal-precipitation method and the material has been characterized by several techniques including scanning electron microscopy (SEM), X-ray diffraction (XRD), and low temperature N2 physical adsorption The Fe2O3-based extruded adsorbent was used in a continuous fixed-bed column experiments to evaluate the efficiency for removal of H2S under the effect of various process parameters including the bed depth, the flow rate and the initial H2S concentration The results showed that the total H2S uptake slightly increased with increasing the bed depth and the initial H2S concentration, and decreased with increasing the flow rate In the modeling part, the dynamics of the adsorption process was modeled by two adsorption models namely, Thomas model and BDST (bed depth service time) model The kinetic parameters obtained from the models were used to predict the breakthrough time for a larger column which contained the adsorbent about 40 times more than that in the mini column experiments The Thomas model was the suitable model for prediction of 10 % C0 breakthrough time with an error about % BDST model 0.1 0 300 600 900 1200 1500 1800 Time (minutes) Keywords H2S removal Á Fe2O3 Á Extruded adsorbent Á Modeling Á Breakthrough time List of symbols Ct (ppm, mg/L) C0 (ppm, mg/L) Cb (ppm, mg/L) D (cm) F (L min-1) h (cm) kTh (L mg-1 min-1) K (L mg-1 min-1) N0 (mg L-1, mg g-1) m (g) t (min) tb (min) Effluent H2S concentration Feed H2S concentration Breakthrough H2S concentration Internal diameter Volumetric flow rate Bed depth Thomas rate constant (Thomas model) Adsorption rate constant (BDST model) Adsorption capacity (BDST model) Mass of adsorbent Adsorption time Breakthrough time 123 Adsorption t0.1, t0.5 q0 (mg g-1) Breakthrough time corresponding to Ct = 10 % C0, Ct = 50 % C0 Maximum adsorption capacity (Thomas model) Introduction The removal of toxic components from gaseous streams is currently one of the most important environmental issues being researched (Karmakar et al 2015; Nor et al 2013; Nguyen et al 2008, 2010) Biogas, a potential sustainable energy fuel and feed-stock for chemical productions, is a product of anaerobic degradation of organic substrates The conversion of the chemical energy contained in biogas, which is rich in CH4, into electricity is possible through combustion in internal combustion engines (Abatzoglou and Boivin 2009) These units can be seriously damaged by the hydrogen sulfide (H2S), because it can cause corrosion Moreover, hydrogen sulfide is a well-known poison for metallic catalysts Thus, H2S concentration in feedstocks should be decreased to parts per million levels before their use, for example in fuel cell application (Liyu and King 2006) Therefore, removal of H2S is important for protection of pipe-line systems as well as catalysts in chemical processes Desulfurization of gaseous stream can be done on various adsorbents depending on the temperature of the feed gas Most gas phase desulfurization units employ metal oxides, such as zinc oxide (Liyu and King 2006; Novochinskii et al 2004; Sasaoka et al 1994, 2000), iron oxide (Xie et al 2010; Wang et al 2011; Ren et al 2010; Najjar and Jung 1995), copper oxide (Abbasian and Slimane 1998; Slimane and Abbasian 2000), and manganese oxide (Zeng et al 2015; Cheah et al 2011) as active sorbents Among them, while ZnO is a well-known and commercial adsorbent used to capture H2S from fuel gas streams in a moderate temperature range (300–500 °C), Fe2O3 is a potential candidate for lowtemperature H2S removal It is because of the fact that iron oxide exhibits very favorable thermodynamics in reaction with H2S at low temperature The mechanism of H2S adsorption on Fe2O3-based adsorbent at low temperature consists of several steps (Eqs 1–3) (Davydov et al 1998) Fe2 O3 ỵ 2H2 S ! Fe3ỵ H2 S O3 1ị 3ỵ 3ỵ Fe H2 S O3 ! H2 O Fe S OHị2 fastị 2ị H2 O Fe3ỵ S OHị2 !H2 O ỵ HO Fe2ỵ S Fe2ỵ OH ỵS 3ị Moreover, the used Fe2O3-based adsorbent can be regenerated by reaction with oxygen according to the following equations (Davydov et al 1998; Wie¸ckowska 1998) 123 Fe2 S3 ỵ 3=2O2 ! Fe2 O3 ỵ 3S 4ị Fe2 S3 ỵ 3=2O2 ỵ 3H2 O ! 2FeOHị3 ỵ3S 5ị 2FeS ỵ 3=2O2 ỵ 3H2 O ! 2FeOHị3 ỵ2S 6ị Although many studies on H2S adsorption by Fe2O3based adsorbent have been published, these researches mainly focused on (1) high temperature adsorption (Xie et al 2010; Wang et al 2011; Ren et al 2010; Najjar and Jung 1995) and (2) adsorbent in the form of powder/pelletized particles (Davydov et al 1998; Arcibar-Orozco et al 2015) For practical applications in fixed-bed columns, powder adsorbents must be formed in granules, spheres, or extrudes in order to reduce the pressure drop Fe2O3 supporting on montmorillonite in granule form was reported (Nguyen-Thanh et al 2005) However, H2S capacity of granuled Fe2O3/montmorillonite were only (0.57–9.65) mgS/g A study on Fe2O3-based extruded adsorbent for H2S removal at room temperature has not been reported in literature as our knowledge Additionally, the fixed-bed adsorber does not run under equilibrium conditions, so the flow conditions and masstransfer aspects throughout the column have to be considered (Serna-Guerrero and Sayari 2010) The dynamic behavior of a fixed-bed column is described in terms of the effluent concentration–time profile (the breakthrough curve) which is essential in the evaluation of the efficiency of an adsorber The time between the absorbent start-up and the appearance of the maximum concentration of impurity in the bed outlet determines the absorbent breakthrough (or service) time Correct prediction of the breakthrough time is critical for designing the fixed-bed adsorber This paper reports firstly the dynamic performance for room temperature H2S removal of an low-cost extruded Fe2O3-based adsorbent which prepared by a hydrothermal precipitation method The effect of reaction conditions and modeling approaches of the dynamic behavior of the adsorbent have been investigated The data given in this paper provide a basis for a dynamic adsorption model that could be used to design and evaluate an adsorption column using the low-cost extruded Fe2O3- based adsorbent on a larger scale Materials and methods 2.1 Sorbent preparation and characterization The Fe2O3-based adsorbent was prepared from iron (III) nitrate nonahydrate (Fe(NO3)3.9H2O, 99 %), sodium hydroxide (NaOH, 99.5 %), bentonite (Binh-Thuan Bentonite, Vietnam), starch and de-ionized water Firstly, 75 g Fe(NO3)3.9H2O and 10 g bentonite were dissolved in 300 mL distilled water under magnetic stirring, and were Adsorption continuously stirred at room temperature for minutes to obtain a homogeneous solution Then, NaOH solution was added drop-wise into the above solution until the pH was adjusted to and a reddish-brown precipitate quickly formed The solution was stirred for 30 before being transferred to an autoclave for hydrothermal treatment at temperature 120 °C for 12 h The produced precipitations were filtered and washed by de-ionized water, after being dried at 100 °C for h The collected solid was pulverized and mixed with the starch, which was % weigh of solid into a mixer for h with a speed of 200 rpm Afterwards they were extruded to particles with the same size (2-mm diameter and 5-mm length) Finally, the pellets were calcined at 500 °C for h in a static oven The final Fe2O3-based adsorbent was characterized by several bulk and surface analysis techniques Scanning electron microscopy (SEM) analysis was carried out by using Keyence VE8800 apparatus was for characterization of the structure properties of the synthesized material with different magnifications at an electron acceleration voltage of 20 kV in a high vacuum condition The crystallization phase analysis was executed by X-ray diffraction (XRD) technique using Rigaku Multiflex diffractometer operating at 40 kV, 20 mA, and CuKa ˚ ) N2 adsorption at monochromatic radiation (k = 1.54 A low temperature (77 K) was used for specific surface area of the material (BET method) using Autosorb-1 (QuantaChrome) equipped with an analysis software measured continuously by a H2S sensor system purchased from Alphasense, England and be calibrated by Gastec H2S-test kit (Japan) Prior to the adsorption test, the sample was dried at 100 °C overnight The effects of various process parameters: (1) bed depth (h = 3–6 cm), (2) inlet H2S concentration (C0 = 600–1200 ppm), and (3) flow rate (F = 3.3–8.3 10-2 L/min) were investigated to evaluate the performance of breakthrough on H2S adsorption by Fe2O3 and the breakthrough of H2S was 10 % of the inlet H2S concentration All the experiments were carried out at room temperature (30 ± °C), atmospheric pressure The H2S adsorption capacity of the material was calculated by the following equation: 2.2 Hydrogen sulfide removal test Detailed information on the composition, structure, physical and chemical properties of the stationary phase is important for appropriate understanding and description of surface diffusion The surface diffusion therefore is complicated by a complex nature of interactions between the adsorbate and the surface adsorbent as well as by the complexity of the surface The mass transfer coefficient The adsorption test system for H2S is described in Fig The system consists of gas cylinders for production of H2S/ N2 model mixture The adsorbent was packed in a Pyrex U-tube reactor (ID = 0.8 cm) which located in a temperature controllable furnace H2S concentrations were CS ¼ 10À3 Â F:P tb r ðCSin À CSout Þdt madsorbent R:T MS ð7Þ where Cs—H2S adsorption capacity (mgS/g); Ms—molacular weight of S (=32); msorbent—mass of the adsorbent; CSin—H2S concentration of the input stream; CSout—H2S concentration of the output stream; F—total gas flow rate; tb—time of the adsorption until the concentration of the output stream higher than the breakthrough point (breakthrough time) 2.3 Modeling 2.3.1 Mathematical analysis Fig Hydrogen sulfide adsorption test system 123 Adsorption (km) was calculated by the Ranz-Marshall correlation (Gutierrez et al 2014) Sh ẳ 2:0 ỵ 1:8Re1=2 Sc1=3 (Kundu and Gupta 2007; Thomas 1944; Bharathi and Ramesh 2013): 8ị qtotal ẳ F where: Sh ẳ Zttotal Cads dt ¼ F udp qg lg k m dp ; Re ẳ ; Sc ẳ Dm eịlg qg Dm C0 ð11Þ The total amount of H2S fed to the column (X, mg) is as follows: mtotal ¼ C0 Fttotal ð12Þ The total percent removal of H2S by the column can be calculated from the following equation: q total H2 S removal %ị ẳ total 100 13ị mtotal 2.3.2 Modeling of breakthrough curves Prediction of the breakthrough curve for the effluent is the predominant factor for the prosperous design of a column adsorption process However, the process does not operate in a steady state as the concentration in gas phase changes through the time and space as the feed moves through a fixed-bed, so it is quite difficult to develop a model which accurately describes the dynamic behavior of adsorption in a fixed-bed system Therefore, this study is starting from the mass balance between the solid and gas The variation of this balance during the reaction can be illustrated by Fig 2b The equation of mass balance material can be stated as: input flow = output flow ? flow inside pore ? matter adsorbed onto the bed (Taty-Costodes et al C0 C0 Mass transfer zone FCt Ct = Ct = 10ị Veff ẳ Fttotal where Dm, dp, qg, lg, e, M and V are the molecular diffusivity, particle diameter, gas density, gas viscosity and bed void fraction, molecular weight, the diffusion volume, respectively The loading behavior of H2S to be removed from biogas containing the Fe2O3 media in a fixed-bed column is shown by breakthrough curves The shape of the concentration– time profile of breakthrough curves is the important characteristics for determining the operation and the dynamic response of an adsorption column A typical breakthrough curve is illustrated in Fig 2a When the volume of the mixture gas begins to flow through the column, the mass transfer zone varies from % of the inlet concentration (corresponding to the free adsorbent) to 100 % of the inlet concentration (corresponding to the total saturation) (Calero et al 2009; Taty-Costodes et al 2005) The breakthrough plot is usually expressed in terms of adsorbed H2S concentration (Cads (ppm) = inlet H2S concentration (C0)—effluent H2S concentration (Ct)) Thus, the total adsorbed H2S quantity (maximum column capacity) (qtotal, mg) in the column for a given feed concentration and flow rate can be estimated as follows C0 ðC0 À Ct Þdt where F is the volumetric flow rate of the mixture gas (L min-1), ttotal is the total time of flow (min) and the volume of the effluent (Veff) can be calculated from the following equation: If Re [ 100, the flow rate of gas through the porous media is turbulent flow 0:5 103 T 1:75 M1gas ỵ MH1 S Dm ẳ 9ị 2 1=3 1=3 P Vgas þ VH2 S Fig Representation of a typical breakthrough curve (a) and schema of bed depth for modeling (b) (Taty-Costodes et al 2005) Zttotal Ct = Cb Saturation point Ct = C0 ε : bed porosity 1.0 Ct/C0 dh Breakthrough curve Vp h Break point 0.1 tb Time (a) 123 FC0 (b) Adsorption 2005; Kundu and Gupta 2007) For this system, the balance can be expressed mathematically as: FC0 ẳ FCt ỵ Vp dC dq ỵm dt dt 14ị where FC0 is the inlet flow of H2S in the column (mg min-1), FCt is the outlet flow of H2S leaving the column (mg min-1), Vp the porous volume (Vp = (1/ (1-e))V where V is the bulk volume and e is the void fraction in the bed), Vp(dC/dt) is the flow rate through the column bed depth (mg min-1) and m(dq/dt) is the amount of H2S adsorbed onto Fe2O3 (mg min-1) where m is the mass of Fe2O3 in the bed (g) and dq/dt is the adsorption rate (mg g-1 min-1) According to Eq (14), it is obvious that the linear flow rate (u = F/Sc, where Sc is the column section, m2), the initial concentration, the adsorption potential and the porous volume are the determining factors of the balance for a given column bed depth Thus, in order to optimize the fixed-bed column adsorption process, it is necessary to examine the parameters and to estimate their influence (Taty-Costodes et al 2005) However, these equations derived to model the fixed-bed adsorption system with theoretical power are differential in natural and usually require complex numerical methods to solve them Therefore, various simple numerical models have been developed to predict the dynamic behavior of the hydrogen sulfide on Fe2O3, the mathematical models must include the adsorption isotherm, the mass-energy balance inside the adsorbent particle and the gaseous phase in the bed Some of these models have been discussed here 2.3.3 The Thomas model The Thomas model is one of the most general and widely used models to describe the behavior of adsorption process in fixed-bed column The model was based on the hypothesis that (1) the process follows Langmuir isotherms and second-order kinetics of sorption–desorption with no axial dispersion, (2) the adsorption is not limited by the chemical reaction, but controlled by the mass transfer at the interface (Kundu and Gupta 2007; Thomas 1944; Bharathi and Ramesh 2013) The linearized form of the model is given as: C0 kTh q0 m ln kTh C0 t ẳ 15ị F Ct where kTh is the Thomas rate constant (L mg-1 min-1), q0 is the maximum adsorption capacity (mg g-1), m is the mass of adsorbent in the column (g) The kinetic coefficient kTh and the adsorption capacity of the column q0 can be determined from a plot of ln((C0/ Ct)-1) against t (=Veff/F) at a given flow rate 2.3.4 The bed depth service time (BDST) model The BDST model (Hutchins 1973), proposed by Hutchins, describes a relation between the service time and the packed-bed depth of column This model was derived based on the assumption that the diffusion steps (external and internal) are very fast, and the surface reaction step is rate-controlling The model has the following form: N0 h C0 ln 16ị tẳ uC0 KC0 Ct where C0 is the influent concentration of H2S (ppm), Ct is the effluent concentration of H2S at time t (ppm), K is the adsorption rate constant (L mg-1 min-1), N0 is the adsorption capacity (mg L-1), h is the bed depth (cm), u is the linear flow rate (cm min-1) and t is the service time to breakthrough (min) Experimental data obtained are used to plot BDST curves and estimate the characteristic parameters K and N0 from the slope and intercept of the plots Results and discussion 3.1 Characterization of the Fe2O3-based extruded adsorbent The results of XRD analysis as shown in the Fig indicates that the crystalline structure of the Fe2O3-based extruded adsorbent consists of Fe2O3 and bentonite structure This means that the calcination temperature (500 °C) was successfully converted ferric hydroxide (Fe(OH)3) into Fe2O3 and no other new crystalline structure was formed during the heat treatment Figure reports also SEM image of the inner part of the extruded adsorbent which was obtained by breaking the sample before SEM analysis Moreover, the specific surface area, which was analyzed by N2 adsorption at 77 K using BET method, of the Fe2O3based extruded adsorbent was 60 m2/g The average pore Fig XRD pattern of the Fe2O3-based extruded adsorbent 123 Adsorption Fig SEM images of the Fe2O3-based extruded adsorbent Table Properties of the Fe2O3-based extruded adsorbent by N2 physical adsorption at 77 K Property Value BET surface area (m2/g) 60.6 Langmuir surface area (m /g) 72.1 Average pore diameter (nm) 99.0 Total pore volume (cm3/g) 13.9 size was 99 nm which was large enough for the gas diffusion Thus, it can be concluded that the adsorbent was relatively porous and potential for sorption application (Table 1) 3.2 Experimental results of H2S removal The shape of the breakthrough curve and the time for the breakthrough appearance are the predominant factors for determining the operation and the dynamic response of an adsorption column (Kundu and Gupta 2007; Mahmoud 2016) The general position of the breakthrough curve along the volume/time axis depends on the capacity of the adsorbent with respect to bed height, the feed concentration and flow rate The dynamic behaviors for hydrogen sulfide removal of the Fe2O3-based extruded adsorbent at different operating conditions are reported in the Fig The detail adsorption data are given in the Table It can be seen from Fig 5a and Table that when the bed depth of the adsorbent increased from cm to cm the breakthrough time at 10 %C0 (t0.1) was increased from 228 to 438 The adsorption capacities at t0.1, however, were almost unchanged, in the range 17.1–17.4 mg/g The maximum adsorption capacities (when C = C0) were from 22.2 to 24.5 mg/g It means that about 70–80 % of the adsorption sites had been used before t0.1 On the other hand, the change of volume flow-rate led to significant changes of 123 the maximum adsorption capacity and slightly changes of the adsorption capacity at t0.1 as revealed from Table The adsorption capacities at t0.1 and the maximum adsorption capacities were decreased from 17.1 to 16.0 and 25.1 to 18.5 mg/g when the volume flow-rate was raised from 3.3 10-2 L/min to 8.3 10-2 L/min These results can be explained by the shorter contact time at the higher flow-rate Figure 5c and Table show that the initial H2S concentration slightly affected the adsorption capacities at t0.1 and the maximum adsorption capacities The adsorption capacity at t0.1 was 16.0 and 17.1 mg/g at C0 = 600 and 1200 ppm, respectively while the maximum adsorption capacity was 22.4 and 25.1 mg/g at C0 = 600 and 1200 ppm, respectively Additionally, according to the adsorption mechanism (Eqs 1–3) capacity of the Fe2O3-based extruded adsorbent could be about 226 mgS/g if all Fe3? ions were exposed to the surface and the reaction time was long enough to obtain equilibrium state Since the estimation of total O2- on Fe2O3 material is (1–2) 1019 atoms/m2 (Davydov et al 1998), total Fe3? ions present on surface of the Fe2O3-based extruded adsorbent is approximately (24–48) 1019 atoms/g As a result, the H2S capacity of the adsorbent may reach (77–154) mgS/g The capacity of the Fe2O3-based extruded adsorbent at Cb = C0 as shown in Table varies in the range 18.5–25.1 mgS/g Therefore, the interaction is possibly in monolayer Even though the capacity in the dynamic conditions of the Fe2O3-based extruded adsorbent is lower than the equilibrium capacity due to the short reaction time and mass transfer limitation, the capacity of this material is much larger than those of the granule Fe2 O3/montmorillonite which were (0.57–9.65) mgS/g (Nguyen-Thanh et al 2005) Furthermore, the capacity of the Fe2O3-based extruded adsorbent (per gram of Fe2O3) is much higher than that of the powder pure Fe2O3 which capacity was (7.9–13.0) mgS/g (Davydov et al 1998) The difference is possibly from the specific surface area Adsorption (a) 1 (c) 0.8 0.8 0.8 0.6 0.6 0.2 0.050 L/min 0.4 0.067 L/min 0.083 L/min 0.2 0 500 1000 0.6 C/C0 cm cm cm cm 0.4 0.033 L/min C/C0 C/C0 (b) 600 ppm 800 ppm 1000 ppm 1200 ppm 0.4 0.2 0 500 t (min) 1000 1500 Fig Breakthrough curves of H2S adsorption onto the Fe2O3-based extruded adsorbent: a at different bed depths (F = 0.05 L/min, C0 = 500 1000 t (min) t (min) 1000 ppm), b at different flow rates (h = cm, C0 = 1000 ppm) and c at different initial H2S concentrations (F = 0.05 L/min, h = cm) Table Adsorption data of H2S removal by the Fe2O3-based extruded adsorbent at different process parameters Process parameters Breakthrough time (Cb = 0.1 C0) (min)a Treated volume (L)b H2S adsorption capacity (Cb = 0.1 C0) (mg/g)c Total H2S removal (%)d H2S maximum adsorption capacity (Cb = C0) (mg/g)c Bed depth, h (cm)e (m = 0.85 g) 228 11.40 17.1 99.18 22.2 (m = 1.13 g) 303 15.15 17.1 99.36 24.8 (m = 1.41 g) 380 19.00 17.2 99.27 24.3 (m = 1.69 g) 438 21.90 17.4 99.51 24.5 3.3 10-2 455 15.17 17.1 99.50 25.1 5.0 10-2 303 15.15 17.1 99.36 24.8 6.7 10-2 215 14.33 16.2 99.08 22.4 8.3 10-2 153 12.75 16.0 98.72 18.5 Flow rate, F (L/min)f Initial H2S concentration, C0 (ppm)g a 600 468 23.40 16.0 99.56 22.4 800 361 18.05 16.3 99.26 23.1 1000 1200 303 252 15.15 12.60 17.1 17.1 99.36 99.00 24.8 25.1 Obtained from Fig b,c,d Calculated according to Eqs (7), (11) and (13), respectively e at C0 = 1000 ppm, F = L/h f at C0 = 1000 ppm, m = 1.13 g g at F = L/h, m = 1.13 g difference The specific surface area of the extruded adsorbent was 60.6 m2/g which was much higher than that of the powder pure Fe2O3 (10 m2/g) It is because the powder pure Fe2O3 was prepared by precipitation method while the extruded one was prepared from the Fe2O3 which synthesized by hydrothermal-precipitation method The capacity of the Fe2O3-based extruded adsorbent is (30.8–41.8) mgS/g_Fe2O3 which is comparable to that of the powder pure Fe(OH)3 which capacity was about (34.5–42.5) mgS/g_Fe2O3 (Davydov et al 1998) Since the 123 Adsorption surface area of the powder pure Fe(OH)3 was 100 m2/g which was higher than that of the extruded adsorbent, the surface (–OH) groups may limit the H2S to access to the Fe3? ions reducing the H2S capacity according to the adsorption mechanism (Eqs 1–3) 3.3 Modeling Two mathematic models have been used for modeling the breakthrough curves basing on the experimental data reported in Fig and Table Adsorption process by the low-cost Fe2O3-based extruded adsorbent consists of several steps However the adsorption rate usually determines by the slowest step which is called the rate-determining step The rate-determining step can be one of the followings: (1) surface reaction, and (2) mass transfer Identification of the rate-determining step is important for understanding the kinetic nature of the adsorption process, designing the adsorption column, and providing crucial information for adsorbent development The two selected models were based on various rate-determining step assumptions The Thomas model assumed that the diffusion step is the rate-determining step while the surface reaction was assumed to be the rate-determining step in the BDST model Firstly, a set of experimental data with C B 10 % C0 was used as fitting data for both models to determine the model’s parameters The effects of the process conditions on the model’s parameters were then analyzed Finally, both models were applied to predict the breakthrough curve of a pilot experiment The coefficients of the Thomas model were determined from the slope and intercepts obtained from the linear regression derived from the Eq (15) Table presents the values of kTh and q0 obtained from the model fitting and Fig shows the comparison of simulated breakthrough curves and the experimental data The fact that values of coefficient of determination (R2) were closed to unity ([0.95) revealed good agreement between the obtained models and the experimental data The errors of the predicted 10 %C0 breakthrough times were from -0.8 to 3.3 % Thus, the model can provide correct information of the t0.1 The errors of the predicted 50 %C0 breakthrough times were relatively high, from 3.3 to 18.6 % The reason is that the complicated changes of adsorption’s driving force, which is the concentration different between the concentration of H2S on the Fe2O3-based adsorbent and the H2S in the gas, leads to complicated mechanisms as increasing reaction time As can be seen from the Fig the deviation became larger when the reaction time increased for all the tested conditions Additionally, the kTh and q0 were varied when the process parameters changed While the adsorbent’s weight and the initial H2S concentration slightly affected the model’s parameters, the effect of volume flow-rate on them was significant In order to simplify the effects of process parameter and to possibly obtain the Thomas model’s coefficients for scale-up cases, the kTh and q0 are correlated to (m/F), a contact-time related parameter The results are shown in Fig together with the maximum adsorption capacity (q0 Exp) taken from Table It is seen that the relationships between the coefficients and the (m/F) were not clear at low (m/F) However, if the (m/F) is large enough ([23 g L-1 min), the constant values can be obtained for the Thomas model’s coefficients (kTh and q0) The two constant values are obtained by taking the average of the kTh and q0 with (m/F) [ 23 g L-1.min, respectively It is known that if the contact time is long enough the equilibrium state is obtained At a fixed temperature, the adsorption capacity at equilibrium state is a constant For the Fe2O3-based extruded adsorbent in this study, when the Table Thomas model parameters for H2S adsorption by the Fe2O3-based extruded adsorbent at different bed depths, inlet H2S concentrations and flow rates C0 (ppm) m (g) F (L/min) m/F (g L-1 min) kTh (L mg-1 min-1) q0 (mg/g) R2 e0.1 e0.5 1000 1.69 5.0 10-2 33.8 0.0375 20.1 0.989 0.4 3.3 1000 1000 1.41 1.13 5.0 10-2 5.0 10-2 28.2 22.6 0.0372 0.0387 20.5 20.7 0.998 0.987 1.6 0.0 18.6 7.6 1000 0.85 5.0 10-2 17.0 0.0443 21.3 0.990 -0.2 10.3 1000 1.13 3.3 10-2 34.2 0.0293 20.2 0.974 3.3 16.0 1000 1.13 6.7 10-2 16.9 0.0532 19.8 0.975 0.0 10.2 -2 3.8 1000 1.13 8.3 10 13.6 0.0809 17.6 0.993 -0.7 600 1.13 5.0 10-2 22.6 0.0402 19.5 0.957 0.0 6.1 800 1.13 5.0 10-2 22.6 0.0418 20.0 0.950 -0.8 9.6 1200 1.13 5.0 10-2 22.6 0.0329 21.2 0.999 -0.4 13.1 e0.1 = (t0.1_exp - t0.1_cal)/t0.1_exp 100 (%), e0.5 = (t0.5_exp - t0.5_cal)/t0.5_exp 100 (%) 123 Adsorption (b) (a) (c) 0.5 0.5 0.4 0.4 0.4 0.3 0.3 0.3 C/C0 C/C0 C/C0 0.5 cm 0.2 0.033 L/min 0.2 cm cm 0.1 800 ppm 1000 ppm 0.067 L/min 0.1 cm 0.1 0.083 L/min model 0 400 200 1200 ppm model model 600 ppm 0.2 0.050 L/min 0 600 200 400 600 800 1000 Time (min) Time (min) 200 400 600 800 1000 Time (min) 30 0.15 20 0.1 q0 (Exp.) 10 0.05 q0 (Thomas) k_Th 0 10 20 m/F (g.L-1.min) 30 40 Fig Effect of (m/F) on Thomas model parameters (a) kTh (L.mg-1.min-1) q0 (mg/g) Fig Comparison between experimental data and simulated data by Thomas model: a at different bed depths, b at different flow-rates, and c at different initial H2S concentrations (m/F) is larger than 23 g L-1 min, the equilibrium state is achieved as can be reflected from the curve for experimental maximum adsorption capacity (q0.exp) in Fig Coefficients for BDST model were obtained from linear relationship plots between experimental data t and ln(C0/ Ct -1) and reported in Table This model also can be used to simulate the breakthrough curve with Ct B 0.1C0 for the mini column since the values of R2 are close to unity The small errors of the predicted t0.1 by the BDST model from -9.5 to 0.5 % were obtained It means that the prediction by this model is acceptable However, similar to (b) (c) 0.4 0.4 0.4 0.3 0.3 0.3 cm 0.2 0.1 C/C0 0.5 C/C0 0.5 C/C0 0.5 0.033 L/min 0.2 cm 0.050 L/min cm 0.067 L/min cm 0.1 0.083 L/min model 200 400 Time (min) 600 800 ppm 1000 ppm 0.1 1200 ppm model 0 600 ppm 0.2 model 0 200 400 600 800 1000 Time (min) 200 400 600 800 1000 Time (min) Fig Comparison between experimental data and simulated data by BDST model: a at different bed depths, b at different flow-rates, and c at different initial H2S concentrations 123 Adsorption Table BDST model parameters for H2S adsorption onto Fe2O3 at different bed depths, inlet H2S concentrations and flow rates C0 (ppm) h (cm) u (cm/min) h/u (min) K (L mg-1 min-1) N0 (mg/L) (mg/g) R2 e0.1 e0.5 1000 6.0 99.5 0.060 0.0710 11,229 20.0 0.989 -5.7 4.4 1000 5.0 99.5 0.050 0.0698 11,485 20.5 0.998 -1.3 18.8 1000 4.0 99.5 0.040 0.0733 11,597 20.6 0.987 0.5 8.1 1000 3.0 99.5 0.030 0.0838 11,970 21.2 0.990 -3.7 10.3 1000 4.0 66.3 0.060 0.0589 11,210 19.9 0.995 -1.5 17.1 1000 4.0 132.7 0.030 0.1014 11,101 19.7 0.982 -6.0 10.9 1000 600 4.0 4.0 165.9 99.5 0.024 0.040 0.1708 0.0283 9844 10,894 17.5 19.4 0.974 0.957 -5.9 6.4 5.5 6.1 800 4.0 99.5 0.040 0.0528 11,170 19.9 0.950 -2.8 10.7 1200 4.0 99.5 0.040 0.0889 11,946 21.2 0.999 -9.5 13.4 3.4 Scale-up assessment Two models have been analyzed and both models can simulate well the breakthrough curves with Ct B 0.1C0 of the mini column Depending on the contact-time related parameter (m/F or h/u) the model’s coefficients can be obtained For checking the scale-up, the adsorption of H2S onto the Fe2O3-based extruded adsorbent was carried out on a pilot column to demonstrate the BDST, Thomas models ability to be scale-up and used for design Parameters of the pilot column are presented in Table The results are reported in Fig 10 and Table It can be seen 123 30 20 0.4 0.3 q0 (Exp.) 0.2 N0 (BDST) 10 K 0.1 K (L.mg-1.min-1) (a) 0.02 0.04 h/u (min) 0.06 0.08 (b) 0.2 K (L.mg-1.min-1) the Thomas model, this model showed deviation in the breakthrough time (t0.5) which was obtained experimentally at Ct = 50 %C0 The differences were from 4.4 to 17.1 % Figure shows the comparison of modeled breakthrough curves and the experimental data It can be seen that the model can be applicable for simulation of the adsorption at low C/C0 region Moreover, the model’s coefficients (K and N0) varied complicatedly with the column bed depth, the linear flowrate and the initial H2S concentration It is seen from the Table that similar to previous models the contact time (h/ u) was strongly influenced the BDST model’s adsorption capacity (N0) and the adsorption rate constant (K) Unlike the Thomas model, K was changed significantly with initial H2S concentration (C0) The correlations are shown in Fig It can be seen from Fig 9a that if the contact time is longer than 0.04 min, K and NB0 can be set at 0.0623 L mg-1 min-1 and 20.4 mg g-1 (or 11,441 (mg L-1) at C0 = 1000 ppm, respectively The relationship of K and C0 is a linear relation as observed from Fig 9b: K = 1.01 10-4C0 [ppm] - 3.03 10-2 -1 -1 [L mg ] q0, NB0 (mg/g) e0.1 = (t0.1_exp - t0.1_cal)/t0.1_exp 100 (%), e0.5 = (t0.5_exp - t0.5_cal)/t0.5_exp 100 (%) K = 1.01×10-4C0 - 3.03×10-2 R² = 0.99 0.15 0.1 0.05 400 600 800 1000 C0 (ppm) 1200 1400 Fig Effect of (h/u) on BDST model parameters at C0 = 1000 ppm (a), and the effect of initial H2S concentration on the rate constant K at h/u = 0.04 (b) that the two models can simulate the breakthrough curve of the pilot test However, the breakthrough times for 10 %C0 predicted by BDST model were shorter than the actual breakthrough time obtained by the experiment The Thomas model gave better estimation with about % error There are two possible reasons for the results Firstly, even though both models predicted well the t0.1 in the mini column tests, the errors from the Thomas model were much lower than those from the BDST model as shown in Tables 3, and Secondly, Additional axial mass transfer resistance of the pilot column, which cross section area was 10 times larger than that of the mini column, makes the mass transfer resistance become the actual rate- Adsorption Table Data of parameters used for mini and pilot experiments Operating parameters Mini column Pilot column Inlet H2S concentration (ppm) (C0) 600–1200 1000 Linear flow rate (cm/min) (u) 66.3–165.9 98.7 Height of adsorbent (cm) (h) 3–6 12.5 Adsorbent’s mass (g) (m) 0.85–1.69 45.65 Volumetric flow (L/min) (F) 3.3 10-2–8.3 10-2 48.3 10-2 Cross section area (cm ) 0.50 4.91 m/F (g L-1) 13.6–33.9 94.4 h/u (min) 0.024–0.060 0.126 0.3 Experimental data Thomas model C/C0 0.2 BDST model 0.1 0 300 600 900 1200 1500 1800 Time (minutes) Fig 10 Comparison of experimental and calculation data for the scale-up test Table Comparison of the dynamic data predicted from kinetic models with those obtained experimentally Breakthrough time (t0.1) (h) Treated volume (L) Deviation (%) Experiment 23.7 687 – Thomas model 22.8 661 3.8 BDST model 17.6 510 25.7 determining step Therefore the Thomas model, which assumed the adsorption controlled by the mass transfer rate, can simulate the breakthrough curve better than the BDST model Conclusions The dynamic adsorption of H2S by the low-cost Fe2O3based extruded adsorbent, which was prepared by hydrothermal precipitation method, was studied The breakthrough curves were measured at different bed depths, inlet H2S concentrations and flow rates Two kinetic models which are Thomas model and BDST model were applied to the experimental data to determine the kinetic parameters and predict the breakthrough time of the pilot column Conclusions are as follows A low-cost Fe2O3-based extruded adsorbent with relatively high surface area and large pore structure was obtained by the hydrothermal precipitation method The adsorbent can be used for H2S adsorption at room temperature with adsorption capacity about 18–25 mg/g depending on the adsorption condition The capacity of the extruded adsorbent was higher than that of several Fe2O3based adsorbent reported in literature in the dynamic condition test With the low-cost synthesis, regenerability and relatively high capacity, the Fe2O3-based extruded adsorbent can be suitable for practical applications Application of Thomas model and BDST model for the mini column revealed that the models can simulated well the breakthrough curves below 10 %C0 The predicted 10 %C0 breakthrough times were in good agreement with the experimental data 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Comparison between experimental data and simulated data by BDST model: a at different bed depths, b at different flow-rates, and c at different initial H2S concentrations 123 Adsorption Table... significant changes of 123 the maximum adsorption capacity and slightly changes of the adsorption capacity at t0.1 as revealed from Table The adsorption capacities at t0.1 and the maximum adsorption... characterization of the structure properties of the synthesized material with different magnifications at an electron acceleration voltage of 20 kV in a high vacuum condition The crystallization