Ion Exchange Columns. Expt. MT 404. Aim. Theory

Transcription

1 Expt. MT 404 Ion Exchange Columns Aim To plot the breakthrough curve of strong acid cation exchange Amberlyst resins, and determine their capacity by batch and continuous flow processes. Theory Ion Exchangers Of all different natural and synthetic products which show ion exchange properties, the most important are ion-exchange resins, ion-exchange coals, mineral ion exchangers, and synthetic inorganic exchangers. Ion exchangers owe their characteristic properties to a peculiar feature of their structure. They consist of a framework held together by chemical bonds or lattice energy which carries a positive or negative surplus charge. Counter ions of opposite charge move throughout the framework and can be replaced by other ions of same sign. For example, the framework of a cation exchanger can be regarded as a macromolecular or crystalline polyanion, while the framework of an anion exchanger can be regarded as a polycation. Ion Exchange Resins These constitute the most important class of ion exchangers. Their frame-work called the matrix, consists of an irregular, macromolecular, 3-D network of hydrocarbon chains. The matrix carries ionic groups such as SO 3, COO in cation exchangers and NH + 3, NH+ 2 in anion exchangers. Ion exchange resins are thus cross linked polyelectrolytes. The matrix of the resins is hydrophobic. However, hydrophilic components are introduced by the incorporation of ionic groups such as SO 3 H. Linear hydrocarbon macromolecules with such molecules are soluble in water. So ion exchange resins are made insoluble by introduction of cross-links which Interconnect the various hydrocarbon chains. An ion-exchange resin is practically one single macromolecule. Its dissolution would require rupture of C-C bonds. Thus resins are insoluble in all solvents by which they are not destroyed. The matrix is elastic and can swell by taking up solvent, a fact referred to as heteroporosity or heterodictality. The chemical, physical, and mechanical stability and the ion-exchange behavior of the resins depend primarily on the structure and the degree of cross-linking of the matrix and on the nature and number of fixed ionic groups. The degree of cross-linking determines the mesh width of the matrix and thus the swelling ability of the resin and the mobility of the counter ions in the resin, which in turn determine the rates of ion-exchange in the resin. Highly cross linked resins are harder and more resistant to mechanical breakdown. MT 404-1

2 Amberlyst-15 with sulphonic acid functionality is our resin of interest. It is a highly porous, macro reticular ion-exchanger prepared by a variation of the conventional pearl-polymerization technique. In pearl polymerization, monomers are mixed, and a polymerization catalyst such as benzoyl peroxide is added. The mixture is then added to an agitated aqueous solution kept at the temperature required for poymerization. The mixture forms small droplets, which remain suspended. A suspension stabilizer is added to prevent agglomeration of droplets. In the case of Amberlyst, an organic solvent which is a good solvent for the monomer, but a poor solvent for the polymer is added to the polymerization mixture. As polymerization progresses, the solvent molecules are squeezed out by the growing copolymer regions. In this way, spherical beads with wide pores are obtained. Selectivity Ion exchangers prefer one species over another due to several causes : 1. The electrostatic interaction between the charged framework and the counter ions depend on the size and valence of the counter ion. 2. In addition to electrostatic forces, other interactions between ions and their environment are effective. 3. Large counter ions may be sterically excluded from the narrow pores of the ion exchanger All these effects depend on the nature of the counter ion and thus may lead to preferential uptake of a species by the ion exchanger. The ability of the ion-exchanger to distinguish between the various counter ion species is called selectivity. Separation Factor The preference of the ion exchanger for one of the two counter ions is often expressed by the separation factor, defined by α = m am b m a m b = C ac b C a C b = x ax b x a x b (1) The molal selectivity coefficient, which is used for theoretical studies, is defined as M Z B KB A = A M Z A B M Z A B M Z B A (2) The selectivity of the ion exchange process depends on the properties of the ion exchanger used and the composition of the aqueous phase. In the case of two ions having the same charge and very similar radii, the selectivity due to the properties of the ion exchanger (such as acidity, basicity, and the degree of cross linking ) is not sufficient for ensuring effective separation. In such a case, an appropriate complexing agent has to be added to the aqueous phase: the selectivity attained is then either due to the difference in the stability constants or to the different charges or structures of the complexes formed. Increased selectivity can be brought about in many ways. For eg., one can exploit the preference of an exchanger for highly charged ions in dilute solutions, or one can choose a chelating resin. MT 404-2

3 Capacity Capacity is defined as the number of counter-ion equivalents in a specified amount of material. Capacity and related data are primarily used for two reasons:- for characterizing ion-exchange materials, and for use in the numerical calculation of ion-exchange operations. Capacity can be defined in numerous ways: 1. Capacity (Maximum capacity, ion-exchange capacity) Definition : Number of inorganic groups per specified amount of ion-exchanger 2. Scientific Weight Capacity Units : meq/g dry H + or Cl form 3. Technical Volume Capacity Units: eq/liter packed bed in H + or Cl form and fully water-swollen 4. Apparent Capacity (Effective Capacity) Definition : Number of exchangeable counter ions per specified amount of ion exchanger. Units : meq/g dry H+ or Cl form (apparent weight capacity). Apparent capacity is lower than maximum capacity when inorganic groups are incompletely ionized ; depends on experimental conditions (ph, conc.,etc) 5. Sorption Capacity. Definition : Amount of solute, taken up by sorption rather than by exchange, per specified amount of ion exchanger 6. Useful Capacity Definition : Capacity utilized when equilibrium is not attained Used at low ionexchange rates Depends on experimental conditions (ion-exchange rate, etc.) 7. Breakthrough Capacity ( Dynamic Capacity) Definition : Capacity utilized in column operation, Depends on operating conditions 8. Concentration of fixed ionic groups Definition : Number of fixed ionic groups in meq/cm3 swollen resin (molarity) or per gram solvent in resin (molality) Depends on experimental conditions(swelling, etc.) Used in theoretical treatment of ion-exchange phenomena A Q v = (1 b) d (100 W) 100 Q w Q v = volume cpacity in equivalents per liter packed bed. Q w = Scientific weight capacity in milliequivalents per gram. b = fractional void volume of packing W = water content of the resin in weight percent d = density of the swollen resin in grams per ml B The molality of fixed groups in meq/g is m = The molarity of fixed groups in meq/ml is X = (100 W) Q w W (1 + Q i MQ w 10 3 ) d (100 W) Q w 100 (1 + Q i MQ w 10 3 ) MT 404-3

4 Batch Process Apparatus Stirred tank reactor with stirrer, belt, stand, pipette, resins, copper sulphate solution, test tubes ( 15), ion meter, cupric electrode, Ionic Strength Adjustor (ISA), volumetric flasks Procedure 1. Calibrate the ion meter using cupric nitrate standards of concentrations ppm, ppm, ppm, and ppm. 2. Take known weight of resins in the stirred tank reactor. Fit the reactor on the stand and attach the belt to the stirrer which is adjusted on the pulleys. Switch on the stirrer. 3. Pour quickly calculated volume of 800 ppm cupric sulphate solution into the tank and start the timer. 4. Withdraw 1 ml samples from the tank using a pipette at every 40 seconds for about 10 minutes. 5. Dilute the samples to 50 ml in volumetric flasks and measure their concentrations using ion meter. 6. Plot a graph of concentration vs. time. 7. The amount of cupric ions consumed is calculated from the initial and final concentrations. Calculations Initial concentration - C 0 ppm Final concentration - C f ppm Qty. of cupric ions used Q = (C f C 0 ) Capacity of resins = (Q/63.55) eq/g of resin Observation Table S.N. Time (min) Cu ion concentration (ppm) Graph Plot a graph of concentration versus time. MT 404-4

5 Continuous Process Procedure 1. Take 10 g resins and prepare a slurry with distilled water. Charge the column with the slurry such that there are no air bubbles trapped. 2. Keep adding cupric sulphate solution to the resin, and let it flow out at approximately 1 ml/min. 3. After every 20 ml, measure out 1 ml of effluent, dilute it to 50 ml in the volumetric flask and measure its concentration in the ion meter. 4. A graph of concentration versus volume is plotted. Observation Table S.N. Time (min) Cu ion concentration (ppm) Graph Plot a graph of concentration versus time. Results and Comments MT 404-5