Design of resin polymers for the adsorption of metal ions using computer aided molecular design (CAMD)
AIChE Annual Meeting, Salt Lake City, Utah, Nov. 8-13, 2015
For submission to "Adsorbent Materials"
Rajib Mukherjee and Urmila M. Diwekar
Vishwamitra Research Institute, Center for Uncertain Systems:
Tools for Optimization and Management (VRI-CUSTOM),
2714 Crystal Way, Crystal Lake, IL 60012
The advancement and economic viability of vertical drilling technology has lead to the expansion of gas production with hydraulic fracturing of sedimentary rocks known as shale formations. A huge amount of water is used in the process. The flowback water from the process of hydraulic fracturing may contain a variety of formation materials, including brines, heavy metals, natural occurring radioactive material (NORM), and organics, which can make wastewater treatment difficult and expensive. One of the easiest and common practices for the removal of heavy metals and radionuclides from water is the use of ion exchange resin polymers.
<> Commonly occurring NORM in hydraulic fracking includes radium and barium. NORM and other heavy metals can be removed using cation exchange resins by adsorption. The adsorption mechanism depends on the difference in activities of NORM and other heavy metals in adsorbents and the bulk fluid phase. The activity coefficients can be obtained using the group contribution methods. In the present work, the activity coefficient in the adsorbent resin polymers is obtained using group contribution methods. For example, polystyrene sulfonate is a cation exchange resin. It contains sulphonic acid functional group as cation exchanger. Other functional groups can also act as cation exchangers includes carboxylic acid, phosphonic acid, phosphinic acid, phenolic, arsonic acid, silinonic acid etc. that can act as a strong acid or weak acid cation exchangers. Keeping the same polymer backbone, different functional group for cation exchange shows different adsorption capacity of the resin polymer. For the backbone polymer, other than polystyrene, different poly venyl aryl compounds can also be used. The monomer units of the polymers can also be divided into structural and functional groups with varied interaction parameters with NORM and other heavy metals. Thus, the activity coefficient in a complex adsorbent can be obtained using group contribution method.
The UNIFAC group contribution is generally used for different compounds. In case of polymer, a free volume of polymer has been added. The modified UNIFAC method is used for estimating activities of adsorbents in the polymer phase. In our research, the interaction parameters of NORM and other heavy metals with structural and functional groups obtained from different resin polymers used as cation exchange resins are obtained using the modified UNIFAC method. The interaction parameters are found by solving an optimization problem where the goal is to minimize the percentage error in the theoretical estimation of adsorption with the experimental results obtained from adsorption isotherm.
The interaction parameters and other group properties obtained with modified UNIFAC group contribution method can be used in a computer-aided molecular design (CAMD) methodology for optimal design of novel resin polymers for NORM and other heavy metals removal. In this work, optimal design of novel polymer is solved as a mixed integer nonlinear programming problem. A new algorithm called efficient ant colony optimization (EACO) is used to solve the problem. The algorithm for CAMD of adsorption resin polymer design maximizes the adsorption capacity of the adsorbent subject to structural feasibility, thermodynamic property correlations, process conditions and process constraints.
In this work we have developed a technology for the design of novel resin polymers useful for the removal of heavy metals and NORM from water. Polymers can be divided into structural and functional groups. The structural and functional property of a polymer depends on these groups. We have used the adsorption theory and develop new group contribution methods (GCMs) to predict interaction properties of each group present in the adsorbents based on their thermodynamics. We have also used a novel CAMD framework to design adsorbent polymers with improved properties using properties of the structural and functional groups that have been developed by the group contribution methods.
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