Introduction

Treating of natural gas with Methyldiethanolamine (MDEA, 50 weight% used by Gasco Habshan, Abu Dhabi) is the best industrial method used for effective removal of H2S and CO2 in natural gas sweetening unit. The removal of heavy metal ions and heat stable salt (HSS) contaminants from lean MDEA is always a challenge to the gas industry. Natural biopolymers such as alginate are considering increasing attention due to their extraordinary affinity towards adsorption. Alginates (Alg) are hydrophilic natural polysaccharides obtained from brown seaweed algae, consisting of different proportions and sequential arrangements of a-L-guluronic acid (G-block) and ß-D-mannuronic acid (M-block) units in linear chain. Researchers have demonstrated that calcium-alginate gel beads can remove heavy metals from industrial pollutants [1, 2].

In the present work, bio-polymeric adsorbent calcium alginate (Ca-Alg) was used for the first time to remove total contaminants present in lean MDEA [3]. The main aim of this work was to optimize the preparation procedure of calcium alginate hydrogel for removing both total organic acid anions as HSS and heavy metal ions like iron and chromium present in the industrial lean MDEA at varying temperature conditions observed in middle-east (23°C-53°C). The kinetic studies, two parameter equilibrium isotherm models like Langmuir and Freundlich and thermodynamic parameters like Gibb’s free energy, enthalpy and entropy of adsorption were determined to check the feasibility of adsorption process.

Experimental

Materials

Alginic acid sodium salt of 91% food grade was obtained from Loba Chemie Pvt. Ltd, India and calcium chloride dihydrate was obtained from Merck KGaA, Germany. Lean amine containing 50 weight % MDEA was obtained from Gasco, Habshan, Abu Dhabi for the analysis.

Methods

The Ca-Alg gel bead preparation procedure was optimized using various conditions by dropping different weight % sodium alginate into calcium chloride solution. The concentration of sodium alginate and calcium chloride was optimized in terms of removal efficiency of total contaminants present in lean MDEA. The optimized Ca-Alg beads were prepared by drop wise addition of 1.0 weight % sodium alginate (Na-Alg) into 1.0 (M) CaCl2 solution under constant stirring to prevent agglomeration. The beads were kept in CaCl2 solution for 24 hours for complete polymerization and finally, washed with
water at 60°C under constant stirring to remove excess CaCl2. The produced beads were further dried
into a hard mass by keeping the beads in an oven at 60°C overnight [2]. The dry beads were used in the
batch experimental studies as it can be easily handled and stored; making it as a good commercial adsorbent.

Instrumentation

The thermal degradation property of the calcium alginate beads were measured using a TGA, NetzschSta 409 PC/PG, Germany. The SEM and FTIR images were recorded and analyzed using FEI

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Quanta 200, Netherland and FTIR, Nicolet iS10, Thermo Scientific instrument with OMNIC Software respectively. The concentration of total organic acid anions in the lean amine solution was determined using UV-VIS Spectrophotometer (DR5000, Hach Lange; Test kit 365). Inductively coupled Plasma Optical spectroscopy (ICP-OES, Optima 8000; Perkin Elmer) was used for the elemental analysis of metal ions.

Batch Adsorption Studies

The kinetics and equilibrium parameters for the removal of total organic acid anions and heavy metal ions were studied from batch adsorption experiments using mixed batch reactor technique. Varying amount of Ca-Alg beads (0.2 – 5 gm) were added into 10 ml lean MDEA solution taken in a
150 ml conical flask and allowed to equilibrate in a water bath shaker (Dihan, Kora) at 110 rpm at different temperatures ranging from 23°C-53°C for 4 hours. Table 1 shows the equations used for the adsorption studies.

Table 1 Equations used for adsorption studies

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Thermodynamic parameters

= Change in Gibbs free energy

= Change in Enthalpy

= Entropy change

Results and Discussions

Thermo Gravimetric Analysis (TGA)

Figure1 shows the thermal degradation of calcium alginate using TGA. The weight loss observed up to 200°C was caused by the dehydration of calcium alginate beads. In order to explain the thermal degradation process, it was observed that calcium carbonate was formed at 150°C and as temperature was increased calcium oxide and calcium hydroxide were found when heated to 700°C [4].

Figure1. Thermal degradation curve of calcium alginate beads

Fourier Transform Infrared (FTIR) Spectroscopy

The difference between the pure Ca-Alg hydrogel beads and adsorbed with MDEA were studied using FTIR analysis and shown in Figure2. A consistent shift in % of transmittance for Ca-Alg with pure and adsorbed with MDEA was due to adsorption of organic acid anions on the adsorbent surface [5].

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Figure2. FTIR Spectra of Ca-Alg beads a) before and b) after adsorption studies

Scanning Electron Microscope (SEM)

The SEM images (Figure3) obtained for Ca-Alg beads before (a) and after adsorption (b) were showed that the change in structure of the Ca-Alg beads due to adsorption reaction occurring between functional groups of alginate and the total contaminants present in lean MDEA as organic acid anions and heavy metal ions.

Figure3. SEM analysis of Ca-Alg beads a) before and b) after adsorption studies

Analysis of total organic acid anions (HSS) and heavy metal ions

The concentration of total organic acid anions (e.g. glycolate, formate, acetate, propionate, butyrate, valerate) in the lean MDEA was found to be 5685 ppm and the major metal ions were found to be chromium (615.7 ppb) and iron (1114 ppb) respectively [3].

Kinetic studies

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To observe the kinetic behaviour of adsorption studies, 2.0 gm of Ca-Alg was taken in 10 ml lean MDEA and kept in the water bath shaker at 23°C. Sample was collected every half an hour to check the kinetic behavior. It was observed that the amount of organic acid anions adsorbed followed pseudo- second-order kinetics having rate constant 0.0124 g/mg. minute.

Adsorption Equilibrium Isotherm

The intake capacity of organic acid anions and heavy metal ions on Ca-Alg adsorbent at different equilibrium concentration was shown in Figure4. The isotherms were fitted two parameter isotherm models (Langmuir and Freundlich) to identify the interaction with the bio-polymeric sorbent. Based on the data fit, total organic acid anions follow a Langmuir type monolayer adsorption. The maximum adsorption capacity (qmax) values for total organic acid anions predicted from the Langmuir isothermal model was 909.09 mg/g at 23°C and subsequently reduced once the temperature increased to 53°C. Freundlich isotherm also indicated a favorable adsorption process. The negative values of Gibbs free energy (-12.948KJ/mol at 23°C) explained the spontaneous nature of the adsorption reaction and the positive values of enthalpy (2.596 KJ/mol) explained the endothermic nature of adsorption of total organic acid anions on Ca-Alg adsorbent.

Figure4. Intake capacity of (a) organic acid anions and (b) heavy metal ions at room temperature

Regeneration of adsorbent

The organic acid anions and metal ions adsorbed on the adsorbent were desorbed by washing thrice with distilled water. Following the recovery of the adsorbent, it was thoroughly dried back to hard
mass by keeping in oven at 60°C and was reused for further adsorption-desorption cycles. It was
observed that even after seven cycles of adsorption and desorption, the variation of residual
concentration were less than 10%, demonstrating the extended usefulness of Ca-Alg adsorbent.

Conclusions

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The batch adsorption studies using calcium alginate beads were tested to remove both total organic acid anions (55.58%) and heavy metal ions like chromium (52.82%) and iron (35.13%) from the Industrial lean amine at 23°C using 5.0 gm of Ca-Alg beads and it was observed that Langmuir isotherm best explained the adsorption process. The predicted maximum intake capacity (qmax) for the removal of organic acid anions was 909.09 ppm at

23°C and showed spontaneity of the adsorption process (?G = -12.9484kJ/mol). The adsorbent could be

regenerated using water and reused without considerable loss of its adsorption capacity up to seven subsequent adsorption-desorption cycles, making it the best natural adsorbent.

Acknowledgement

The authors are grateful to Gas Research Centre, The Petroleum Institute, Abu Dhabi for funding the project. Sincere thanks to The GASCO, Habshan unit for their continued co-operation and support.

References

[1] M. M. Araújo, J.A. Teixeira, Trivalent chromium sorption on alginate beads, J. Inter. Biodeter. & Biodeg., 40-1(1997) 63–74.
[2] Hyun Gyu Park, Tae Won Kim, Activated carbon containing alginate adsorbent for the simultaneous removal of heavy metals and toxic organics, J. Pro. Chem, 42 (2007), 1371-1377.
[3] P. Pal, F. Banat, Contaminants in Industrial Lean Amine Solvent and their removal using biopolymers–A new aspect. Journal of Physical Chemistry & Biophysics, 2014, 4:1.
[4] Qing-shan Kong, Bing-bing Wan, Thermal Degradation and Flame Retardancy of Calcium Alginate
Fibers, Chinese Journal of Polymer Science, 27-6 (2009) 807-812.
[5] M. Ruiz, C. Tobalina, H. Demey-Cedeñ, Sorption of boron on calcium alginate gel beads, J. Rea. Fun. Poly., 73-4 (2013) 653–657.

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