Grantee Research Project Results
2004 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. , Warner, Steven B. , Munley, Vincent G. , Sengupta, Sukalyan
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, 2003 through December 4, 2004
Project Amount: $319,998
RFA: Technology for a Sustainable Environment (2003) RFA Text | Recipients Lists
Research Category: Nanotechnology , Pollution Prevention/Sustainable Development , Sustainable and Healthy Communities
Objective:
Currently, the production of sodium hydroxide (NaOH) and chlorine (Cl-2) 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 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 objective of this research is to synthesize 4-6 percent NaOH using ion exchange (IX) fibers from sea water without producing chlorine.
Progress Summary:
Characterization of the Fiber Material
Commercially available IX fibers have been procured from three separate sources (Actilex Fibers from Focus Polymers, United Kingdom; Smopex fibers from Johnston Matthey, Illinois; and Fiban Fibers from Unitechprom, Belarus). We currently have adequate amounts of IX fibers in our laboratory and can purchase more as necessary. Initial investigations focused on a characterization of the fiber material. To this end, a sample of the weak acid IX fiber material was titrated with the strong base NaOH in an attempt to verify the overall capacity of the fiber. These findings were used to validate the manufacturer-suggested capacity of the material (4.5 meq/g) and are comparable to commercially available polymeric resins. Microscopic analysis was also used to determine the approximate dimensions of the material. Fiber materials were found to have a cylindrical shape with a diameter ranging from 1 to 50 μm and an average length of 3.7 cm. It should be noted that commercially available polymeric resins are spherical in form and have a significantly larger diameter (500 to 1000 μm).
Regeneration of the Fiber Material Using Carbon Dioxide
This research concurrently examined the properties of both traditional and fibrous IX materials as exhibited during the regeneration cycle. During experimental regeneration cycles, the fiber and resin materials were placed in an epoxy coated 8 mm x 250 mm glass column. Carbon dioxide-sparged snowmelt served as the regenerant solution and was pumped from a pressurized reactor through the column using a liquid chromatography pump. Components were connected to the ion exchange column by 2.0 mm (inside diameter) Teflon tubing. For each regeneration cycle column run, 1 g of the fibers were weighed and packed into the column to a uniform height of 3.5 cm, thus ensuring a uniform bed resistance for each individual column run. The regeneration cycle was examined for the fiber material and compared to a similar column run using 0.45 g of the commercially available resin. The quantities of each material used ensured a similar overall exchange capacity for both materials. It should be noted that although physically dissimilar, chemically, the two ion exchange materials used were identical (e.g., they both contained carboxylic functional groups). Prior to use in the regeneration experiment, fiber and resin materials were placed in an ionic form containing only calcium. The effluent concentration was analyzed for calcium using a Perkin Elmer Atomic Absorption Spectrophotometer (Model AAnalyst 200) and a hollow cathode lamp. The effect of an increase in carbon dioxide partial pressure (above the regenerant solution) was examined for both materials. Partial pressures ranging from 1 psi to 8 psi were employed. Experimental data suggest that fiber materials, unlike resin materials, could be efficiently regenerated (> 95% recovery) using an insignificant amount of regenerant solution (< 10% of recovered product water). Additionally, this regeneration efficiency was seen to increase with corresponding increases in carbon dioxide partial pressure. The identical experimental configurations also suggest that fiber materials present a suitable alternative to traditional resin ion exchange materials without significantly altering or compromising the operational characteristics associated with these traditional ion exchange processes.
Evidence of a Difference in the Kinetic Properties of Fiber and Resin Ion Exchange Materials
Evidence supporting a difference in desorption kinetics could be seen in two distinct ways during the preceding experiments. First, kinetic differences are suggested in the high regeneration efficiency of fiber materials as compared to their resin counterparts. Second, although ion exchange materials were being placed in the initial ionic form (calcium) prior to regeneration, evidence of kinetic differences were suggested by the resulting effluent histories. The two materials were placed in the desired initial ionic form by passing a calcium solution (25 mg/L) through the glass column containing the ion exchange materials. The column effluent was examined to determine the extent of conversion. During this process involving each of the two materials, as the calcium gradually started exiting the column, the influent flow was deliberately discontinued for 24 hours. When the flow was resumed, the effluent of each column was examined for changes in the calcium concentration. Little change could be seen in the calcium concentration exiting the fiber column. For the resin, however, a significant drop (> 20%) in the calcium effluent concentration could be observed. Following the passage of several hundred-bed volumes of influent solution after this restart, calcium eventually reached the concentration prior to interruption. For intraparticle diffusion controlled processes, the concentration gradient within the sorbent particle serves as the driving force and governs the overall rate. With the progress of any column run, this concentration gradient attenuates. The interruption allows the sorbed calcium to spread out evenly within the spherical bead. As a result, the concentration gradient, and thus the uptake rate immediately after the column restart, is greater than the uptake rate prior to the interruption. In other words, a faster uptake and a consequent drop in the aqueous-phase exit concentration of the solute is (as evidenced by experimental data) confirmatory evidence in support of intraparticle diffusion as the primary rate-limiting step for resin materials. For the fiber materials, this effect was absent, indicating that intraparticle diffusion is not the primary rate-limiting step.
Intraparticle Versus Film Diffusion as the Rate-Limiting Step for Fiber and Resin Ion Exchange Materials
Experimental data were collected using a shallow bed column with effluent recycle to evaluate the kinetic properties of the two materials. A solution of dilute hydrochloric acid (0.01 M) was passed through fixed bed containing 0.2 g of each material in a calcium-loaded form. The effluent resivor was analyzed for calcium at various time intervals over a 30-minute time period. The resulting experimental data plot was given as the fractional attainment of equilibrium as a function of time. The predicted desorption profiles for both materials were then obtained using a film diffusion control (FDC) model and plotted for comparison. Pertinent hydrodynamic conditions used for both of the materials were identical. Comparison of predicted and experimental data indicates that the FDC model can accurately predict the desorption kinetics of the fiber material given different hydrodynamic conditions. Alternatively, it was seen that the FDC model cannot be used to predict the desorption kinetics of the resin materials. These results provide confirmatory evidence illustrating the differences between the rate-limiting steps governing each material. Because it is well recognized that sorption/desorption kinetics of spherical ion exchanger resin (beads) are often limited by intraparticle diffusion, a second model was developed to explain the desorption kinetics of the resin material. The predicted desorption profile for the resin material was obtained a second time using a Intraparticle Diffusion model and plotted. Comparison of predicted and experimental data indicates that the Intraparticle Diffusion model can accurately predict the desorption kinetics of the resin material given different hydrodynamic conditions.
Desorption Kinetics of Carbon Dioxide Regeneration
Because the preferred regenerant for the proposed processes is carbon dioxide as opposed to dilute hydrochloric acid, kinetic tests were next performed using carbon dioxide (100 psi) as the regenerant under similar experimental conditions. Desorption experiments were conducted using both fiber and resin materials under various hydrodynamic conditions. Experimentally, this was nearly identical to the previous kinetic experiments with the use of carbon dioxide being the only significant difference. Experimental results were recorded and modeled according to the proposed kinetic mechanisms. Comparison of predicted and experimental data indicates again that intraparticle diffusion is the rate-limiting step for resin materials, whereas film diffusion is the rate-limiting step for fiber materials.
Future Activities:
Development of Binary Isotherm Data
Selectivity data will be gathered and analyzed regarding the ions most significant to this process.
Laboratory Scale Generation of Sodium Hydroxide and Validation of Product Purity
A laboratory-scale unit will be assembled and used in the preparation of NaOH. The prepared materials will then be analyzed for purity regarding specific ionic constituents.
Journal Articles:
No journal articles submitted with this report: View all 6 publications for this projectSupplemental Keywords:
chlorine, sodium hydroxide, caustic soda, environmental risk reduction, ion exchange fibers, green house gas, carbon dioxide extraction, alternative chemical synthesis,, RFA, Scientific Discipline, Sustainable Industry/Business, Sustainable Environment, Environmental Chemistry, Technology for Sustainable Environment, alternative materials, alternative solvents, carbon dioxide extraction, chlorinated solvent reduction, environmentally benign alternative, alternative chemical synthesis, environmentally-friendly chemical synthesis, green chemistryProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.