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2005 Progress Report: Environmentally Benign Synthesis Of Sodium Hydroxide Without Chlorine Using Ion Exchange Fibers
EPA Grant Number: R831433Title: Environmentally Benign Synthesis Of Sodium Hydroxide Without Chlorine Using Ion Exchange Fibers
Investigators: SenGupta, Arup K. , Munley, Vincent G. , Sengupta, Sukalyan , Warner, Steven B.
Institution: Lehigh University , University of Massachusetts - Dartmouth
EPA Project Officer: Richards, April
Project Period: October 15, 2003 through December 4, 2007
Project Period Covered by this Report: October 15, 2004 through December 4, 2005
Project Amount: $319,998
RFA: Technology for a Sustainable Environment (2003)
Research Category: Pollution Prevention/Sustainable Development
Description:
Objective:Currently, the production of sodium hydroxide (NaOH) and chlorine (Cl-) are closely linked, and they are produced universally as co-products of electrolysis processes. As long as chlorine production remains coupled with the production of NaOH, it will be nearly impossible to promulgate regulations banning or reducing the productions of various chlorinated compounds and enforcing them globally. The general objective of this research project is to synthesize NaOH without co-production of chlorine through an ecologically clean route. The specific goal of this research is to synthesize 4-6 percent NaOH using ion exchange (IX) fibers from seawater without producing chlorine.
Progress Summary:Evidence of shrinking Beads and the Subsequent Effect on Efficiency of Carbon Dioxide Regeneration
One of the striking findings of previous kinetic investigations was that although the IX fibers were amenable to efficient regeneration with carbon dioxide-sparged snowmelt, commercial weak-acid ion exchange resins (C-104) responded poorly to the same regeneration process. Spherical resin beads and IX fibers are chemically similar; both have weak-acid carboxylate functional groups covalently attached to a polymer substrate. Their equilibrium properties are thus identical, and they exhibit high calcium removal capacity in the presence of competing sodium ions at near-neutral pH. However, because of the spherical geometry of the resin beads with sizes in the range of 400-1200 μm, the sorption kinetics is intraparticle diffusion controlled.
Light microscopy was used to evaluate shrinking tests for fiber and resin samples. Light microscopy was performed on a Westover Scientific Micromaster I binocular light microscope. Objective lenses of 10X and 40X magnifications were used for resin and fibrous materials, respectively. Fiber and resin samples in calcium forms were placed on a glass slide and covered with a cover slip. A small amount of water was added at the edges. To observe the effects of swelling and shrinkage, a few drops of 0.25 M HCl was added at the edges of the cover slip. All pictures of particles and measurements of particle diameters were made using a magnetic circular dichroism digital microscope head attached to the light microscope head and a laptop computer. Digital camera measurements were calibrated using calibration slides prior to data acquisition. All calibrations and measurements were made in the units of micrometers.
During the regeneration process, a spherical bead with a 490 μm diameter shrank fairly rapidly to 402 μm under the experimental conditions in less than 10 minutes. The rate of shrinking, however, gradually slowed down, and no significant shrinking was observed after 10 minutes. For an enhanced sensitivity, the weak-acid IX fiber chosen for the test under identical conditions had a relatively high cylindrical diameter (65 μm). Note that no significant change or trend in shrinking was observed for the fiber material during the course of the experiment.
To develop a mechanistic understanding of the poor regenerability of calcium-loaded spherical resin beads with carbon dioxide, let us consider a single bead as a polyelectrolyte gel with carboxylate functional groups. The affinity sequence for weak-acid carboxylate functional groups stands as follows: H+ >> Ca2+ > Na+. Uptake of H+ during regeneration by a weak-acid carboxylate group is essentially an association reaction leading to a major decrease in its osmotic pressure, thus causing expulsion of water from the gel phase. A spherical IX resin bead, therefore, gradually shrinks with the progress of regeneration through the uptake of hydrogen ions that involves counter-transport of H+ and Ca2+. At the onset, hydrogen ions would initially displace the outermost (i.e., peripheral) calcium ions. Such an exchange would, however, dramatically decrease the water content of the regenerated portion, thus decreasing the overall intraparticle diffusivity near the outer periphery of the resin bead. The progress of the regeneration process increases the depth of the relatively impervious skin, thus further slowing down the counter-transport of H+ and Ca2+. Scientifically, this hypothesis is in agreement with the sequence of photographs taken during the light microscopy test. Previous studies with weak-acid cation exchange resins also provided optical confirmation of shrunk cores during acid regeneration. For carbon dioxide regeneration, hydrogen ion concentration in the bulk phase cannot be as high as it is normally with mineral acid regeneration. Thus, the concentration gradient across the shrunk core is too small to overcome the diffusional resistance. The poor regenerability of resin beads with carbon dioxide is thus attributed to enhanced diffusional resistance offered by the shrunk peripheral layers with very low water content.
To the contrary, the IX sites for fibers reside primarily on the surface and the phenomenon of intraparticle diffusion, as demonstrated earlier, is of lesser significance. Protonation of weak-acid functional groups has only marginal impact on diffusional resistance and, hence, the carbon dioxide regeneration is efficient for IX fibers.
Development of Binary Isotherm Data
Binary isotherm data for ions most significant to this process were developed through two basic methods. First, batch isotherm tests were conducted using strong-base IX fibers. These fibers are the key component in generating sodium bicarbonate from seawater. Binary data were collected regarding chloride (Cl-) and bicarbonate (HCO3-) ions. Batch tests were conducted using 200 mL containers at total aqueous phase bicarbonate concentrations of 5, 10, and 100 meq/L. Varying masses of Cl--loaded fiber (0.1-12.0 g) were placed in the batch containers and stirred for a period of 72 hours, until equilibrium had been achieved. The Cl-/HCO3- separation factor under the experimental condition is:
This separation factor in favor of chloride indicates that the regeneration of spent strong-base fibers using a bicarbonate solution is likely to be the least efficient portion of the described process. High concentrations of bicarbonate are likely to be required during for the regeneration phase to be most efficient.
The second set of binary selectivity data assessed the relative affinity of calcium (Ca2+) and hydrogen ion (H+) toward IX fibers with carboxylate functional groups, and their exchange kinetics are centrally responsible for the success of the proposed process. At near-neutral or under slightly alkaline conditions, calcium or other divalent ions are very selectively removed by IX fibers in the presence of sodium ions. Conversely, calcium ions can be efficiently eluted by increasing the hydrogen ion concentration in the aqueous phase (i.e., reducing pH). To determine relative Ca2+/H+ affinity for IX fibers, an equilibrium desorption test was carried out by sparging carbon dioxide in a batch reactor containing 10.0 gm of calcium-loaded IX fiber in 1.0 L of distilled water. The desorption process was quite rapid, and nearly 26 percent of the calcium was eluted from the fibers at an equilibrium pH of 5.6. The H+/Ca2+ separation factor under the experimental condition is:
A very high separation factor value in favor of hydrogen ion explains why a weak-acid gas, such as carbon dioxide, is an effective regenerant.
Otherwise identical experiments were carried out for ions that may enter the process stream in addition to calcium. These ions included sodium, magnesium, zinc, nickel, and copper. Selectivity data gathered during these experiments suggest the following selectivity sequence:
H+ > Cu2+ > Ni2+ > Zn2+ > Ca2+ > Mg2+ > Na+
For all cases, the hydrogen ion selectivity with respect to each of the above-mentioned ionic species was greater than unity. This indicates that hydrogen ion in the form of carbon dioxide-sparged snowmelt may serve as an efficient regenerant.
Laboratory Scale Generation of Sodium Hydroxide and Validation of Product Purity
A laboratory-scale unit was assembled to produce NaOH. This unit consisted of a 300 mm epoxy-coated glass column that contained the fiber materials. In addition, there were several vessels containing the necessary reagents. Two reagent containers (2 L volume each) were used to hold the lime slurry solution and the sodium bicarbonate solution. An additional 27-L stainless steel pressure vessel was used to contain the carbon dioxide-sparged snowmelt regenerant solution. Stainless steel pumps, Teflon tubing, and Teflon valves were used to regulate reagent flow. Dry, reagent-grade sodium bicarbonate and calcium hydroxide were used to prepare the necessary solutions. High purity carbon dioxide was used to sparge within the regenerant solution.
Difficulties Encountered
During the initial column runs, difficulties were encountered on account of two phenomenons. Initial sodium loading of the fiber material resulted in significant swelling effects within the column. These observations during operation are consistent with experimental findings discussed earlier in this report. These swelling effects resulted in a pressure drop across the ion exchange column. The subsequent pumping of a lime slurry through this column during the NaOH generation phase further exacerbated the pressure drop across the column. Additional clogging was observed within the tubing and valve apparatus.
The response to this problem included the implementation of a fluidized bed style reactor in place of the glass column, which contained the tightly packed IX fiber materials. This new reactor configuration allowed the materials to expand and contract freely without the significant constriction that was seen within a glass column. Valve and tubing apparatus was reevaluated and reconfigured to minimize the overall path length to which the lime slurry solutions would be subjected. This was intended to minimize the use of valves and long tube sections that may create additional blockages. Finally, the addition of dry lime to the slurry was regulated to optimize the thickness of the mixture. This facilitated an easier flow without significantly affecting the effectiveness of the process.
Currently, investigations into the NaOH synthesis process are continuing using the revised process configuration. To this point, experimental results suggest that the previously encountered difficulties have been largely eliminated. Concentrations up to 1 percent have been seen to this point, though a longer experimental run time is needed to ultimately determine if the specific project goal of 4 to 6 percent NaOH product stream is possible.
Journal Articles:No journal articles submitted with this report: View all 3 publications for this project
Supplemental Keywords:chlorine, sodium hydroxide, caustic soda, environmental risk reduction, ion exchange fibers, green house gas, carbon dioxide extraction, alternative chemical synthesis,
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INTERNATIONAL COOPERATION, Sustainable Industry/Business, Scientific Discipline, RFA, Technology for Sustainable Environment, Sustainable Environment, Chemical Engineering, Chemicals Management, Environmental Chemistry, chlorinated solvent reduction, environmentally-friendly chemical synthesis, green chemistry, alternative solvents, alternative chemical synthesis, alternative materials, ion exchange, carbon dioxide extraction
Progress and Final Reports:
2004 Progress Report
Original Abstract
Final Report