(395w) Removal of Heavy Metal Ions From Lean Amine Solvent Using Chitosan Coated Ion-Exchange Resins in a Gas Sweetening Plant

1. Introduction

In natural gas sweetening plant aqueous
methyldiethanolamine (MDEA) solution is used to scrub natural gas. Finally, the
rich MDEA solution is fed to a regeneration column where heat is applied to
strip the acid gas components out of the MDEA solution. Make-up water and MDEA
are added to maintain the concentration of MDEA solution. The heavy metal ions
present as contaminant in lean amine solvent may come from make-up water.  Many
researchers identified the metal ions present in the lean amine solvents and removed
using electrodialysis and ion-exchange (IE) resins. IE process outperforms electrodialysis
technique in terms of higher energy requirements and continuous supply of huge
volumes of water [1].  IE processes are capable of achieving very low salt
concentrations [2], operating and installation costs are less and the
requirements for regeneration chemicals have reduced currently [3]. Cummings
and Smith [4] used continuous adsorption studies passing the alkanolamine
solutions over cation bed where positively charged contaminants such as ferrous
ions and sodium ions were exchanged with protons. Audeh and Yan [5] showed that
weakly acidic cationic resins with carboxylic acid functionality outperform
strongly acidic resins with a sulfonic acid groups.

The basic objective of this
research work was to remove heavy metal ions (iron, chromium, lead etc.)
accumulated in lean MDEA solution in gas sweetening process. MDEA containing
both amine and hydroxyl group chelate and adhere heavy metal ions strongly.  Thus
bare ion-exchange resins cannot always successfully remove heavy metal ions
from this lean amine solvent overcoming the chelation. The novelty of this
research work lies in the use of chitosan coated ion-exchange resin which
contains both the above groups present in the amine solvent and could hence
successfully remove heavy metal ions from MDEA.

2.
Materials and Methods

Lean MDEA solvents (45 weights %)
were obtained from the Gasco, Habshan, Abu Dhabi. Commercially available ion-exchange
resins (ZGC151) were obtained from Hangzhou Zhengguang Resin Co., Ltd., China. Chitosan
and all other chemicals used in this study were purchased from Sigma Chemical
Co., USA. At first, the bare resins were coated with chitosan. 1.0 gram
chitosan powder was added to 100ml 5% acetic acid solution and left overnight
in stirring condition to dissolve completely. 10 grams of ion-exchange resin
was added into the chitosan solution and stirred for six hours, decanted and
kept at vacuum oven at 60^oC for curing. The resin beads was then
added drop wise to 100 ml 4% sodium hydroxide solution and stirred overnight for
chitosan coating. Chitosan coated ion-exchange resins were treated to
regenerative form before their use in ion exchange batch sorption experiments.

Elemental analysis was carried
out using Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES,
Optima 8000). The FTIR studies of chitosan coated ion-exchange resins were
carried out using FTIR, Nicolet, iS10, Thermo Scientific instrument using OMNIC
software (number of scans 64, resolution 4). Scanning Electron Microscopy (SEM)
was carried out with FEI Quanta 200, The Netherland.

3.
Batch and Continuous ion exchange experiments studies

Ion exchange experiments were
conducted using mixed batch reactor technique for obtaining the equilibrium
parameters data for the removal of heavy metal ions using chitosan coated cation
exchange resins. The amount of heavy metal ions adsorbed per unit mass of the
resin (q_e, microgram/gram) at equilibrium was calculated as:

q_e = (V/m)*(C_o?C_e)                                                       (1)

Where V is the volume of the lean
amine solution, m is the mass of the ion exchanger resin; C_o and C_e
are the respective ion concentrations in solution (microgram/liter) initially
and at equilibrium. The equilibrium studies were conducted with lean amine
solvent on ion exchange resins at pH 10.6, at 296 K.

The equilibrium data were fitted
on to well-known established sorption isotherm models:

The
Langmuir isotherm

The Langmuir isotherm [6] model
and its linearized form are usually described by:

1/q_e = (1/q_maxk_L)(1/C_e) 
+ 1/q_max                                      (2)

Where, q_max is the
maximum sorption capacity of anions (microgram/gram of resin), C_e is
the equilibrium heavy metal ion concentration (microgram/liter) and k_L
is the langmuir isotherm constant (liter/microgram).

The
Freundlich isotherm

The Freundlich [7] isotherm in its
linearized form is expressed as:

ln(q_e) =  lnk_F 
+  (1/n) lnC_e                                              (3)

In this equation k_F(microgram^(1-1/n)L^(1/n)/gram)
and n are the Freundlich constants.

For continuous adsorption
experiment in column, a pyrex tube of 2.54 cm inner diameter and 10 cm height
was used. Thirty grams of chitosan coated pretreated cation exchange resin (ZGC
151) was randomly packed in the column to a bed volume (BV) of 30 cm³.
The lean MDEA solution was fed to the bottom of the resin column by a peristaltic
pump at a rate of 4.5 ml/min. Samples were taken periodically at the top of the
resin column for individual and total heavy metal content and measured using ICP-OES.

4. Results and
discussion

4.1
FTIR Analysis

FTIR analysis was
carried out to observe the adherence of chitosan on resin surface. Figure 1 shows
the FTIR spectra of the two structures. From the FTIR spectrum, it could be
concluded that chitosan was successfully coated on resin surface to remove heavy
metal ions from the lean amine solvent.

Figure1.
FTIR Spectra of a) bare resin b) chitosan coated resin

4.2
SEM Analysis

Modification of the chitosan
coated resin molecules followed by adsorption of MDEA was observed from the SEM
images (Figure 2). Smooth chitosan coated resin surface was observed before
adsorption (a) whereas surface becomes rough after adsorption reactions
occurring between resin molecules and MDEA (b).

Figure2. SEM analysis of a)
before and b) after adsorption studies

4.3 Heavy metal ions
present in Lean amine samples

The major heavy metal ions
present in amine samples [Chromium (479 ppb), Iron (967.1 ppb), Lead (1009
ppb), Magnesium (75.29 ppb), Manganese (111.7 ppb), Cadmium (48.32 ppb), Strontium
(26.81 ppb) and Titanium (31.77 ppb)] were detected using ICP-OES.

4.4 Equilibrium
Adsorption studies

The isotherm models gave an
indication of the interaction between the heavy metal ions from solution with
the ion exchange resin and their distribution over ion exchange surface. The corresponding
isotherm models are shown in Figure3 and parameters are shown in Table1. Based
on the data fit, major heavy metal ions follow a Langmuir type monolayer
adsorption. The Freundlich isotherm predicted the constant, n calculated from
linear fits falls in the range of favourable adsorption (n>1).

Figure3. Langmuir and Freundlich
Isotherms for Chromium, Iron and Lead

Table1. Isotherm model parameters
for the adsorption of heavy metal ions on ion exchange resin

4.5 Continuous Ion Exchange
experiment studies of heavy metal ions from lean amine solution

A practical application of the
experimental breakthrough curves is the determination of the breakthrough time,
which helps establish the optimum operating conditions of the ion exchange
process. It was observed that the breakthrough time was 8hours for same bed
volume of the packed bed.

Figure4. The breakthrough curve for
total heavy metal ions.

5. Conclusions

In this research, chitosan coated
ion exchange adsorption was employed for efficient recovery of heavy metals
from lean amine solution. Experimental tests were conducted to determine the
equilibrium ion exchange capacities of the resins. Continuous adsorption experiment
determined the breakthrough times of the ion exchange process and facilitates
the process design by optimizing the operation conditions as useful to remove
heavy metals from lean amine samples.

6. Acknowledgement

The authors are grateful to Gas
Research Centre, The Petroleum Institute, Abu Dhabi for funding the projects.
Sincere thanks to Mr. Alaa Abdul Aziz and Mr. Marwan Alawlqi, The Gasco,
Habshan for their co-operation and support.

7. References

1.  Hong, M., Shuang, Z., Chunxi,
L., Liangshi, L., Removal of heat stable salts from aqueous solutions of
N-methyldiethanolamine using a specially designed three-compartment
configuration electrodialyzer, Journal of Membrane Science, 322,
436-440, 2008.

2.  Burns, D., Gregory Jr., R.A.,
The UCARSEP: process for on-line removal of non-regenerable salts form amine
units. In: Laurence Reid Gas Conditioning Conference, Norman, OK, 1995.

3. Cummings, A.L., Smith, G.D.,
Nelsen, D.K., Advances in amine reclaiming- why there's no excuse to operate a
dirty amine system. In: Laurence Reid Gas Conditioning Conference, Dickinson
TX, USA, 2007.

4. Cummings, A.L., Smith, G.D.,
Better alkanolamine system operations through chemical analysis. In: The Sulfur
Recovery Symposium, Vail, CO, USA, 2010.

5. Audeh, C.A., Yan, T.Y.,
Clean-up of ethanolamine solution by treating with weak ion exchange resins.
United States Patent No. 802,586.  Mobil Oil Corporation, Fairfax, VA., 1994.

6. Langmuir, I., The adsorption of gases on plane surface
of glass, mica and platinum, Journal of American Chemical Society, 40,
1361, 1916.

7. Freundlich, H.M., Over the adsorption in solution, Journal
of Physical Chemistry, 57, 385-470, 1906.