Ion Exchange

Ion Exchange Aim : To obtain a breakthrough curve, degree of saturation, number of transfer units, and the volumetric mass transfer co-efficient of a given ion exchange resin. Apparatus and Chemicals: Glass column packed with resin, stopwatch, measuring cylinder, rotameter, flasks, burette, Ion exchange resin (Zeo Karb-225, cationic), NaOH solution, HCl solution, Oxalic acid, phenolphthalein, deionised water. Theory: Ion exchange- a special case of adsorption: Ion exchange can be considered as a special case of adsorption. Adsorption uses the ability of certain solids to preferentially concentrate specific substances from fluids in contact onto their surfaces. A large number of industrially important examples utilizing adsorption can be cited where fluid involved is a liquid or a gaseous mixture. The adsorption phenomena: Adsorption of a substance can either be physical or chemical. The latter, referred to as chemisorption is of relevance because of its strong reversibility.Physical adsorption, or Vander Wall’s adsorption, is the result of intermolecular attraction forces between the molecules of gas/liquid itself, the solute condenses on the solid surface. This condensation is different than the conventional in the sense that the solute partial pressure need not exceed the prevailing vapour pressure. Adsorption is an exothermic process. In the case of adsorption of gaseous solutes, the heat of adsorption is of the same order of magnitude as the heat of sublimation of a gas.

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The adsoorption wav ve:

Fig 1 : An Adsorption Wave. Considerr a binary sollution, eitherr a gas or liqquid, containning a solute of concentrration Co The fluidd is to be passsed continuoously down through a reelatively deep bed of adsorbennt/exchanger initially freee of adsorbaate. The uppeermost layerr of the solidd, in contact with w the strongg entering, at a first adsorbbs the solutee rapidly andd effectively, and what liittle solute iss left in the sollution is substantially rem moved by thhe layers of solid s in the lower part off the bed. Thhe effluent from f the botttom of the bed b is practiccally solute free, f as at Caa. The distribbution of thee adsorbatee in the solid d bed is indiccated in the sketch in thee upper part of the figuree at a, wheree the relative density d of thee horizontal lines in the bed indicatees the relativve concentrattion of adsorrbate. The uppeermost layer of the bed is practicallyy saturated, and a a bulk off adsorption takes place over a relativeely narrow ad dsorption zoone in whichh the concenttration changges rapidly. The solutionn continuess to move do ownwards ass a wave, at a rate ordinaarily very muuch slower than t the lineaar velocity of o the fluid through t the bed. b Later, as a at x, roughhly half of thhe bed is satuurated with the t solute, efffluent conceentration is still s substanttially zeroed. At b the low wer portion of the adsorrption zone has just reached d the bottom m of the bed, and the conccentration off the solute in i the effluennt has suddeenly risen to o an appreciaable value Cb for the firstt time. The system s is saiid to have

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reached the t breakpoiint. The soluute concentraation now risses rapidly as the adsorpttion zone paasses through the t bottom of o the bed annd e has subsstantially reaached the inittial value C0. 0 The portiion of the efffluent conceentration currve between positions b and a e is term med as the breakthroough curve. If the solutioon continuess to flow, litttle adsorptioon takes place, since the bed b is for all practical purrpose entirelly in equilibrrium with thhe feed soluttion. The shappe and time of o appearancce of the breaakthrough cuurve greatly influence thhe method off operatingg the fixed bed adsorbennt. The curvees generally have h a S shaape but they may be steep or relativelyy flat.

Figg 2: A typicaal Break-Through Curve Factors affecting a th he breakpoin nt and the breakthroug b gh curve: Factors contributing c to the shapee of the curvee produced for f any systeem are: 1) The actual a rate an nd mechanissm of the adssorption proccess 2) Naturre of adsorpttion equilibrrium 3) Fluidd velocity 4) Conccentration off solute in thee feed 5) Lenggth of the adssorbed bed

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Generally, the breakpoint time decreases with: 1) Decreased bed height 2) Increased particle size of the exchanger/adsorbent 3) Increased rate of flow through the bed height below which the solute concentration in the effluent will rise rapidly from the first appearance of the effluent. Adsorption equilibria: Just as in heterogeneous system the equilibrium considerations are of great importance. The equilibrium is affected by the temperature of the system and concentration of the solute for a given solute-adsorbent system. In general, the equilibrium data are reported as plots of the fluid phase (Y) and solid phase (X) concentration of the solute with temperature as a parameter. A constant temperatire X-Y plot is the adsorption isotherm. Various relationships for correlating adsorption equilibrium have been proposed. Amongst these, the Freundlich equation is the most widely used relation in the laboratory and the industry. The Freundlich equation is particularly suited for waste treatment applications involving dilute solutions and is mathematically simple. The usual relationship for liquid phase applications is given by the following equation Y*=m(X*)1/n Y* and X* are normally expressed as weight fractions on solute free basis in the liquid and solid respectively. An ideal adsorbent in terms of equilibrium is that which can be loaded to high concentration (high X) at relatively low concentration of solute in the fluid (low Y). Considering the above that both X and Y are less than unity. Thus a high index value (n<1) indicates high equilibrium capacity of the adsorbent.

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Adsorption operations: Both stage wise and differential contact equipments can be used for adsorption operations 1) Stage-wise Operation: In this case, the adsorbent is brought in intimate contact with the liquid for a sufficient time to allow close approach to equilibrium. The treated solution is filtered to remove the adsorbent and the liquid is treated with fresh adsorbent in the next stage. The adsorbent used is generally in the powdered form than in granular form. 2) Differential Contact Operations: In this type the fluid and solid are in continuous contact throughout the entire apparatus without periodic separation of phases. The operation can be carried out in strictly continuous, steady state manner. The fluid and solid continuously flow in and out of the apparatus. In addition, the semibatch mode can be advantageously used here since the rigid adsorbent particles can be fixed adsorbent particles. The operation proceeds under unsteady state conditions. In the case of solids the granular shape is more amenable to flow. Similarly, for fixed bed applications the granular type yields much more porous bed (lower pressure drop) than the powdered type. Therefore, for differential contact, granular adsorbent is preferred over powdered adsorbent. Fixed bed unsteady state mode is more frequently used because of the equipment simplicity.

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Principles of ion exchange: An ion exchange reaction may be defined as the reversible exchange of ions between a solid phase (the ion exchanger) and a solution phase, the ion exchanger being insoluble in the medium in which the exchange is carried out. If an ion exchanger M-A+, carrying cations A+ as the exchanger ions, is placed in an aqueous solution phase containing B+ cations, the ion exchange reaction taking place is represented by the following equation:-

M-A+ +

M-B+ + A+

B+

The equilibrium represented above is an example of cation exchange, where M- is the insoluble anionic fixed complement of the ion exchanger M-A+, often called simply the fixed anion. The cations A+ and B+ are referred to as counter ions, whilst ions in the solution which bear the same charge as the fixed anion of the exchanger are called co-ions. Similarly, anions can be exchanged provided that an anion receptive medium is employed. Most ion exchangers in large-scale use are based on synthetic resins—either preformed and then chemically reacted, as for polystyrene, or formed from active monomers (olefinic acids, amines, or phenols). Natural zeolites were the first ion exchangers, and both natural and synthetic zeolites are in use today. When NaCl solution is passed through the packed bed containing the above resin, Na+ ions are replaced by H+ ions and outlet stream contains only HCl. As the process continues, the adsorption zone travels downward till appoint where the entire bed is saturated by Na+ ions and outlet stream sharply shows an increase in NaCl content. When the bed is completely exhausted, outlet NaCl concentration becomes same as inlet NaCl concentration.

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Design of Unsteady state adsorbers based on Mass Transfer Zone or Adsorption concept: Height of the adsorption zone is Za and θa is the time required for the formation of this zone. If θa is the time required for travel of this zone from one end to the other end of the column and θb is the time required for the zone to move its own height. Then, θa = Wa/Ga where Wa=We-Wb, and Gs is the mass velocity of the effluent, kmol/m2s θe = We/Gs Fractional ability of the adsorbent zone to adsorb solute f =

(Y0-Y)dW Y0Wa

Where numeratorrepresents quantity of solute removed from the liquid upto the breakpoint θf = (1-f) θa Za

=

Zθa θe-(1-f) θa

S= degree of saturation at the breakpoint S

=

Z-Zaf Z

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Procedure: 1. Regenerate the column by passing 3N HCl solution. 2. Pass deionised water through the resin bed till the excess acid is washed off (this can be checked using an AgNO3 solution) 3. Now adjust the flowrate of the NaCl solution, depending upon the length of the ion exchange column. 4. Allow the NaCl to flow through the column, adjusting the overflow level such that the liquid level in the column is always above the glass wool. 5. Collect effluent samples every 5 mins, after passing the NaCl solution through the column for about 15-20 minutes. (Collect enough sample so that an equal quantity can be titrated every time) 6. Continue till the concentration of HCl in the effluent is almost zero. 7. Collect samples after every 2 minutes if possible once the breakthrough point has been achieved, that is once there is a drastic decrease in the titre value towards the end. 8. Make a note of the volumetric flowrate of NaCl and the height of the bed. Observations: 1. Height of Resin column (Z) =

m

2. Inner radius of Glass Column (R) =

m

3. Intial concentration of the NaCl solution (Co)=

mol/lit

Observation Table : Time (t) (mins)

Burette Reading (ml)

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Calculations: 1. Calculation of Y Amount (in moles) of HCl coming out of the column (a) = Titre reading x molarity of NaOH Vol of sample titrated Therefore, amount of NaCl adsorbed = a mol/lit Amount of NaCl in the effluent = Co-a mol/lit Y= kg NaCl/kg water Y= (Co-a)*(MW of NaCl)/1000

2. Area of cross section of the column (A) = πR2 3. Mass Flow Rate of NaCl (M) = Volumetric Flow rate(Q) x Density of the Solution (ρ) 4. Mass Velocity of NaCl (Gs) = Q/A 5. W = Gs x t 6. Plot Y vs W and get the values of YE, YB, WE, Wb. 7. f =

= Area over the breakthrough curve Ye*W

Total Area

8. Calulation of Number of Transfer Units We know Yin = inlet concentration of NaCl solution. Find Xin using the equilibrium relation Y*=m(X*)1/n (n=1.71, m=22) The equation of the operating line is given by Y= (Yin/Xin) *X Now, using the operating line find the values of X corresponding to values of Y obtained from the experiment.

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Find the equilibrium concentration (Y*) corresponding to X obtained above. Now plot 1/(Y-Y*) vs Y and find the area under the curve from YB to YE. This area equals the NTU. 9. Calculation of Height of a Transfer Unit tE = WE/GS

tB = WB/GS tA = tE - tB

tF = (1-f)* tA Height of Adsorption zone (ZA) = Z x tA tE - tF Height of Transfer Unit = ZA/NTU 10. Volumetric mass Transfer co-efficient (Ky-pa) = NTU*Gs/Za

Results: 1. S

=

2. NTU = 3. HTU = 4. Ky-pa = Conclusion: Comments: Refrences: 1. Mass-Transfer Operations, Robert E. Treybal, Third Edition, Chapter 11. 2. Unit Operations of Chemical Engineering, McCabe & Smith, Seventh Edition, Chapter 25

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