Biogas Desulphurization by Iron Oxide Impregnated Coco Coir in Laboratory Scale Fixed Bed Reactor
Muchlis Hasan Research Center for Chemistry Indonesian Institute of Sciences
Bandung, Indonesia [email protected]
Agus Jatnika Effendi
Faculty of Civil and Environmental Engineering, Institut Teknologi Bandung
Bandung, Indonesia [email protected]
Abstract—Removal process for corrosive, noxious and offensive odorous hydrogen sulfide (H2S) is often needed in waste to energy conversion system based on biogas. A hydrated iron oxide based reactive adsorbent that called iron oxide impregnated coco coir (IOCC) has been prepared for supporting H2S removal from biogas. For establishing its optimized separation performance, a series of laboratory scale experiments have been carried out in 100 mm bed length of fixed bed reactors and using synthetic biogas with 1000 ppm H2S concentration under plug flow condition. It was applied two level factorial design of experiment for parameters: pH 6.5 and 9; superficial gas velocity (v) 1.0 and 2.5 m/min; and two level regeneration treatments. The main indicator of reactor performance was characterized with sulfur removal capacity (SCb) that calculated from breakthrough curves with 100 ppm maximum outlet H2S concentration. The results showed that, under the experimental conditions, the separation process was not controlled by the external diffusion mass transfer step, but controlled by the chemical reaction step. The reactor performance could be improved by increasing pH with Na2CO3 addition and increasing the contact time with decreasing superficial velocity. The optimum operating conditions are pH=9 and superficial velocity v=1.0 m/min. Besides that, the sulfur capacity of saturated adsorbent could be recovered partially by oxidation reaction with air. This study constitutes the basis data for the next step of the scale-up experimentation at pilot scale fixed bed reactor.
Keywords—biogas; hydrogen sulfide; iron oxid; fixed bed reactor; coco coir
I. INTRODUCTION
Biogas is a source of renewable energy that can be generated by anaerobic digestion process from organic waste at industrial or municipal waste treatment facilities [1,2].
Biogas that produced from anaerobic digestion is primarily composed of methane (CH4, 60 – 70%) and carbon dioxide (CO2, 30 – 40%) under saturated water condition. Trace amounts of hydrogen sulfide (H2S, 10 – 10,000 ppm), ammonia (NH3, < 1%), siloxanes (< 0.02%), and other impurities can present and might be inconvenient when not removed immediately [3,4]. Among the impurities, H2S is the most harmful substance because of its toxic, odorous and corrosive properties that are harmful to both human and environment. In addition, the presence of too high H2S concentration with its corrosive effect has limited the biogas
utilization as a source of renewable energy by more efficient power generator technologies, such as internal combustion engine, micro turbine and fuel cells [4,5,6].
There are many choices of technologies to clean up biogas from hydrogen sulphide such as absorption, adsorption (either physical or chemical), bio filtration, membrane filtration and various direct treatment to the anaerob digester [7]. Among these options, a chemical adsorption process with iron oxide based reactive adsorbent is the most reliable and efficient technology for supporting waste to energy conversion system.
This technology is well known as iron sponge process and have many advantages such as simple process with high selectivity towards H2S, low investment cost, optimal at relatively low temperatures process (25-50°C) and very efficient for low sulfur removal capacity that less than 200 kg sulfur/day. The basic principle of this separation technology is a chemisorption process, where hydrogen sulphide reacts with iron oxide based adsorbent to form iron sulphide. Saturated adsorbent can be reused several times with simple regeneration process to optimize the adsorbent capacity and minimize the amount of spent adsorbent at final disposal. The chemical reactions at sorption and regeneration process are shown by (1) and (2) [8].
Sorption: Fe2O3.H2O + 3H2S Æ Fe2S3.H2O + 3H2O (1)
Regeneration Fe2S3.H2O + 3/2O2 Æ Fe2O3.H2O + 3S (2) Generally, the iron sponge process is operated continuously in fixed bed reactor for easier technical operation. But the operating cost associated with adsorbent handling and consumption often dominates the cost of this process [7]. Thus, higher capacity of adsorbent with longer operating life span of the fixed bed reactor is an important objective for reducing the operating cost.
The characteristic of bed material and fluid flow in the fixed bed reactor have significant effect to the fixed bed reactor performance [9]. As in [10], the separation process in a fixed bed column adsorption, either physical or chemical, is not a steady state process but more determined by the behavior of the overall system dynamics. In order to understand the 2015 International Conference on Sustainable Energy Engineering and Application (ICSEEA)
process performance of the fixed bed column adsorption, breakthrough curves analyses are common method to be used [11, 12]. Due to the high cost and technical difficulties associated with pilot scale experiments, it is advisable to run laboratory scale column experiments before carrying out the high-cost pilot scale column experiments.
This study is an attempt to investigate the performance of a new developed Fe2O3 based reactive adsorbent that called iron oxide impregnated coco coir (IOCC), which prepared for supporting biogas desulphurization process in waste to energy conversion system. The purpose of this study was to understand the effect of process parameters such as superficial gas velocity, pH and regeneration treatment to the performance of the fixed bed reactor containing IOCC adsorbent. The target of the experimentation was to trace the breakthrough curves and to determine the limiting step of the phenomenon (external diffusion or chemical reaction at the surface of the adsorbent).
II. METHODOLOGY A. Materials
This study used Fe2O3 based reactive adsorbent that called iron oxide impregnated coco coir (IOCC). The adsorbent has homogeneous basic composition that consists of coco coir (coco peat and coco fiber), iron (III) oxide (Fe2O3) and water.
The coco coir was obtained from Tasikmalaya county, while Fe2O3 were obtained from Alpha Chemicals, USA. Briefly, 14 – 30 mesh size of coco peat was impregnated in ultrasonic bath (Branson 8210) for one hour with 1%-w Fe2O3 solution in water. Subsequently, the material was filtered and dried in oven at 60oC for three hours. Then, it was mixed homogeneously with 14-30 mesh size of coco fiber at 1:1 volume ratio and adjusted its Fe2O3 content to 260 kg Fe2O3/m3 bed. Without addition of Na2CO3, the IOCC adsorbent has pH 6.5. While with the addition of Na2CO3 at 16 mg / mL adsorbent, the IOCC has pH 9. The IOCC adsorbent in this experiment has 26% water content and 46% bed porosity.
The IOCC adsorbent performance was evaluated by using synthetic biogas with composition that similar to biogas from anaerobic digester at waste treatment facility. The composition of the synthetic biogas is 60% nitrogen N2 (instead of methane CH4), 40% carbondioxide CO2, and 1000 ppmv H2S. The synthetic biogas was prepared in laboratory by continuous mixing of N2, CO2 and H2S in a polyethylene bag with a capacity of 0.8 m3. Nitrogen and carbondioxide gases were obtained in pressurized gas cylinders from industrial gas distributor in the Bandung city. Whereas, H2S gas was directly synthesized in the laboratory through chemical reaction method by using paraffin and sulfur. The usage of nitrogen gas is intended as replacement for methane in order to reduce the risk of fire or explosion during the experiments. According to [11], both of nitrogen and methane are equally inert to the active component of iron oxide.
B. Methods of Experiment
The experiments were conducted in a laboratory scale of fixed bed reactor, as shown in Fig. 1. Reactor was made of Plexiglas cylindrical tubing with 16 mm inside diameter and supported by a stainless steel wire mesh. Reactor was filled with 9.56 grams of IOCC adsorbent to form a 100 mm bed height. Before application, the fixed bed reactor was humidified by water saturated air at 30 oC with flow rate 500 mL/min for 30 minutes. The synthetic biogas was fed continuously to the reactor in an up flow mode by using air pump and flow meter (Dwyer, RMA-1). Before entering the reactor, the synthetic biogas was humidified with H2S saturated water in a 30 mL glass impinger at room temperature (26±1 ◦C). The saturation is confirmed by analyzing the inlet and outlet of the H2S stream by using precision H2S detector tube (colorimetric method) from MSA-Auer.
Laboratory scale fixed bed reactor experiments were continued with constant parameters design as shown on Table I. Whereas, the variable parameter in design of experiments consists of two level variations for each independent variable:
pH of adsorbent; superficial velocity (v), and regeneration treatment, as shown on Table II. The regeneration method was conducted by oxidation reaction with 500 mL/min air flow rate for 20 minutes. The values of pH were chosen based on neutral and alkaline condition that might alter the kinetic of chemical reaction. The values of superficial velocities were chosen based on the optimum range that common in iron sponge process as in [13] that may alter the mass transfer resistance by external diffusion.
The superficial velocities were calculated based on empty bed column and controlled by flow meter (Dwyer, RMA-1) with constant reactor inside diameter. While variations in pH conditions were controlled through the addition of sodium carbonate Na2CO3 into the IOCC adsorbent. The inlet H2S concentrations were monitored with H2S detector tube (MSA- Auer) that has concentration measurement range of 100 – 4000 ppm H2S and 15% uncertainty measurement. The outlet H2S concentrations were monitored continuously with a Toxiplus H2S monitor (Biosystems, Inc) and video camera to obtain breakthrough curve. The maximum breakthrough concentration value was decided as 100 ppm H2S, based on Fig. 1. Scheme of the experimental set-up laboratory scale fixed bed reactor
the maximum H2S concentration in biogas that allowable for internal combustion engine application [4].
The main indicator to show the process performance is the sulfur capacity, SCb, that was defined as the total milligram mass of sulfur that adsorbed per gram mass of IOCC adsorbent until the saturated condition achieved. Equation 3 was used for calculating this indicator, based on breakthrough curve data that display the profile of outlet H2S concentration CA,out (ppm) versus time t (min) until saturated time tb (min) achieved.
(3)
where Q is feed gas flow rate (m3/min), CA,in is inlet H2S concentrations (ppm), Wfb is adsorbent weight, and factor 1.32 is the conversion factor from ppm H2S into mg S/m3.
III. RESULTS AND DISCUSSION
A. Fixed Bed Reactor Length and Flow Characteristics Most of H2S removal experiments in this study were conducted in a relative short laboratory scale fixed bed reactor with 100 mm bed length. This condition gave advantages through saving of various laboratory resources that needed for
obtaining the breakthrough data. Even so, the gas flow characteristic in these lab scale fixed bed reactors was still good enough to follow the plug flow reactor criteria that common in large scale reactor. As in [13] plug-flow can be assumed when the Peclet Number (Pe) is larger than 1000.
The Peclet number indicates the ratio of the rate of transport by convection to the rate of transport by dispersion. Equation 4 was used to calculate the Peclet Number based on the lowest superficial velocity [11].
(4) As in [12], the diffusivity coefficient of H2S, DH2S , was calculated as in (5) by using Fuller’s method for H2S – N2
binary gas mixture that give lower Peclet Number than the H2S – CO2 system.
(5) where molecular weight of H2S and N2 respectively MWH2S = 34 kg/kmol, MWN2 = 28 kg/kmol, summation of diffusion volumes at temperature T = 299 K, and pressure P = 101 kPa for nitrogen = 18.5; and hydrogen sulfide = 27.52. This similarity in flow pattern is one of important similarity criteria that make the laboratory scale reactor can predict the behavior of its pilot plant scale reactor [9]
The breakthrough curves and their performances for various operating condition are shown in Fig. 2 and Table 3.
It can be seen that each of experiment gave specific breakthrough curves data that can be distinguished each other by the parameters of breakthrough time (tb) and sulfur capacity (SCb). In general, the ideal performances of reactor are characterized by a long breakthrough time and high sulfur capacity value. For the same surface loading (SL), higher sulfur capacity of a reactor can be identified by longer breakthrough time.
The breakthrough behavior of the chemisorption system in this research has showed opposite behavior to the common adsorption theory as in [14], where the reactors with lower sulfur capacity always has stepper breakthrough curves with narrower mass transfer zone (Table III and Fig. 2). However, this result is consistent with previous similar experiments as in [11] that conducted for Fe2O3 - H2S chemisorption system with a commercial adsorbent type CG-04. Thus, the behavior of this chemisorption process should be described by using heterogeneous gas-solid reactor theory, instead of the adsorption theory. Based on the shrinking core model in heterogeneous gas-solid reactor theory [16], there are three steps of mechanism that could control the rate of H2S consumption in the fixed bed reactor, i.e. external diffusion with gas-film mass transfer, ash-layer diffusion, and surface reaction. The slowest step of the mechanism is the limiting step that controls the overall mass transfer rate in the fixed bed reactor.
TABLE I. CONSTANT PARAMETERS IN LABORATORY SCALE FIXED BED
REACTOR EXPERIMENTS
Constant Parameters Value Units
Bed diameter 16 mm
Bed height 100 mm
Bed porosity 0.46 -
IOCC weight 9.56 gram
Fe2O3 content 5.22 gram
Water content 26 %
H2S inlet concentration 1000 ppm
CO2 concentration ~ 40 %
N2 concentration ~ 60 %
Temperature 25 – 27 oC
TABLE II. EXPERIMENTAL RANGE AND LEVELS OF INDEPENDENT
PROCESS VARIABLES
Independent Variable Range and Levels
Low High
pH of Adsorbent 6.5 9
Superficial Velocity, m/min 1.0 2.5 Regeneration Treatment
0 ( fresh adsorbent)
1
(regenerated adsorbent)
B. Effects of Superficial Velocity
As shown in Fig. 2 and Table III, the fixed bed with higher superficial gas velocity always gave stepper breakthrough curve, shorter breakthrough time and higher sulfur capacity.
Similar results were also found in the studies of H2S removal by another iron oxide based reactive adsorbents as in [11,12].
The superficial velocity represents the residence time of the fixed bed reactor. As shown in Table III, the fixed bed with higher superficial velocity always has lower EBCT (empty bed contact time) and lower sulfur capacity. Equation 6 shows the calculation of EBCT.
(6) At a higher superficial velocity, the surface loading of the reactor is higher with insufficient residence time of H2S molecule in the column for conducting diffusion and chemical reaction at the surface of adsorbent. These conditions also make the unreacted H2S to be swept ahead easily and the breakthrough time reaching saturation decreased significantly.
As in [17], the minimum EBCT that recommended for optimum reactor performance is 60 seconds. So that, due to very short of contact time, the sulfur capacity reported in this laboratory studies are much lower than the reported commercial adsorbent as in [11, 12].
According to refference [16], higher superficial velocity should result in higher convective mass transfer coefficient and higher external diffusion mass transfer rate. But, the results of the experiments showed that higher superficial velocity resulted in lower total sulfur capacity. When the effect of contact time is neglected, higher superficical velocity also tend to resulted in lower values of SCb/EBCT, i.e. sulfur capacity that normally to contact time. So, this negative phenomenon indicates that the external diffusion was not the limiting step of the overall mass transfer rate in the fixed bed reactor.
C. Effects of pH
As shown in Fig 2 and Table III, the addition of Na2CO3
that raise the pH of IOCC adsorbent has significant impact on increasing reactor performance. But, as shown in Fig. 3, the performance of reactor without Fe2O3 that consists of coco coir and Na2CO3 at pH 9 didn’t give significant breakthrough time and sulfur capacity. It evinced that the Na2CO3 didn’t function as reactant that replaced Fe2O3 position, but it just played as substance that accelerate the chemical reaction between H2S and Fe2O3. As in [18], the rate of H2S removal by Fe2O3 is determined by the formation rate of anion HS- as intermediate substance. Meanwhile, the value of H2S dissociation equilibrium constant for the formation of anion HS- in alkaline conditions is higher in basic condition than in acidic conditions [19]. In this case, the effort to increase the pH by TABLE III. FIXED BED REACTOR PERFORMANCE UNDER VARIOUS OPERATING CONDITIONS
Run V
(m/min) pH Reg
EBCT (sec)
SL (gr S/m2.
sec)
tb (min)
SCb (mg S/ gr adsorbent)
SCb /EBCT (mg S/
gr adsorbent. sec.)
SCb total (mg S/ gr adsorbent)
1 1.0 6.5 0
6 2.2 2.78 0.071 0.012
0.106
2 1.0 6.5 1 1.38 0.035 0.006
3 1.0 9 0
6 2.2 33.28 0.820 0.137
1.465
4 1.0 9 1 26.03 0.645 0.108
5 2.5 6.5 0
2.4 5.5 0.67 0.025 0.010
0.046
6 2.5 6.5 1 0.62 0.022 0.009
7 2.5 9 0
2.4 5.5 4.43 0.283 0.118
0.505
8 2.5 9 1 3.47 0.222 0.093
(a)
(b)
Fig. 2. BreakthroughCurve Behavior at Various pH and Superficial Velocity for Fresh and Regenerated IOCC Adsorbent. (a) Fresh IOCC adsorbent; (b) Regenerated IOCC adsorbent
Na2CO3 addition that consist of Na+ cations and CO32- anions were estimated to play acid-base Bronsted-Lowry interactions thatcontribute in accelerating the formation of anion HS-.
Thus, as the pH increases, anion HS- formation rate increases, so does the overall mas transfer increases, which results in higher sulfur capacity in the fixed bed reactor. The significant effect that caused by pH treatment has shown that the chemical reaction at the surface of the IOCC adsorbent is the limiting step of the overall mass transfer rate.
D. Effects of Regeneration Treatment
The regeneration effort by air oxidation method has given good results, as shown by Table 3. But the performance of the regenerated IOCC adsorbent always gave shorter breakthrough time and lower sulfur capacity than the fresh IOCC adsorbent (Fig. 2 and Table 3). As in [15], molecules that can be chemisorbed only those that possessing suitable configuration with surface active site and have enough required activation energy. In addition, the configuration probability for a molecule occupying a single site will be proportional to the fraction of the surface that is unoccupied.
Besides that, the interaction forces between occupied and unoccupied site could increase the activation energy and decrease the chemical reaction rate. So that, the permanent fraction of occupied site by sulfur that generated from regeneration reaction, as in (1), could decrease the chemical adsorption reaction rate at the surface of IOCC adsorbent.
Thus, lower chemical reaction rate caused the overall mass transfer rate decreased and resulted in the regenerated adsorbent with lower sulfur capacity.
IV. CONCLUSIONS
The effects of superficial velocity, pH and regeneration treatment to the performance of lab scale fixed bed reactor containing IOCC adsorbent have been studied by using synthetic biogas with 1000 ppm inlet H2S concentration under plug flow condition. The results show that under the experimental condition, the overall mass transfer rate was not controlled by external diffusion rate, but controlled by chemical reaction rate. The reactor performance could be improved by increasing pH with Na2CO3 addition and increasing the residence time with decreasing superficial velocity. The optimum operating conditions are pH=9 and superficial velocity v=1.0 m/min. Besides that, the sulfur capacity of saturated adsorbent could be recovered partially by oxidation reaction with air.
ACKNOWLEDGMENT
The authors gratefully acknowledge to The Research Center for Chemistry LIPI and The Ministry of Research and Technology Republic Indonesia that has funded this research through RISTEK scholarship.
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Fig. 3. Effect of Dinatrium Carbonate Addition to the Breakthrough Curve (v= 1.0 m/min; fresh adsorbent)
