Environmentally Benign Synthesis Of Sodium Hydroxide And Hardness Removal Using Ion Exchange Fibers PDF Download

Two environmentally sustainable processes using polymeric ion exchange fibers (IX-fibers) are detailed in this investigation. Ion exchange softening with fibers used only harvested snowmelt (or rainwater) as the regenerant chemical along with sparged carbon dioxide or sequestered flue gas. Consequently, the spent regenerant does not contain a high concentration of aggressive chemicals like sodium chloride or acid like traditional ion-exchange processes nor does the process produce voluminous sludges similar to lime softening. The bulk of carbon dioxide consumed during regeneration remains sequestered in the aqueous phase as alkalinity. Low intraparticle diffusional resistance is the underlying reason why IX-fibers are amenable to efficient regeneration using snowmelt sparged with carbon dioxide. By increasing the carbon dioxide partial pressure to 6.8 atmosphere, over ninety percent calcium desorption efficiency is obtained. On the contrary, commercial weak-acid ion exchange resins in spherical bead forms are ineffective for regeneration with carbon-dioxide-sparged snowmelt due to extremely slow ion-exchange kinetics involving counter-transport of Ca2+ and H+. In the second inquiry, fibers were used to generate sodium hydroxide (caustic soda) in an environmentally benign fashion from calcium hydroxide (lime) and seawater. This simple fixed-bed process required no regulated chemicals nor did it result in the co-production of chlorine. Although chlorine has many beneficial applications, its bulk usage continues to have long-term adverse impacts on the environment. Currently, the production of sodium hydroxide (NaOH) and chlorine (Cl2) are closely linked and they are produced universally as co-products of electrolysis processes. Thus, it is nearly impossible to promulgate regulations banning or reducing production of various chlorinated compounds and enforcing them globally. The proposed process may offer an economic and ecologically clean route to independently synthesize sodium hydroxide without coproduction of excess chlorine. The unique properties of IX-fibers that make the proposed process both environmentally sustainable and economically attractive are: (i) its amenability to efficient regeneration with carbon dioxide and harvested snowmelt/rainwater; (ii) its suitability to handle dilute suspensions in fixed-bed configurations; (iii) its superior sorption/desorption kinetics compared to commercial zeolites or ion exchange resins.