Slide 1 - Texas Tech University by wanghonghx


									Ch. E. 4232                 Fall 2010                                 Dr. Hedden

Pre-Lab Notes: Ion Exchange

• Ion exchange is a separation technique based upon transfer of ions from solution onto a solid support called
  an ion exchange resin, which is usually present as a packed bed of near-spherical particles.

• The ion exchange resin is a solid, porous polymer that contains a high concentration of immobilized ionic
functional groups (e.g., acids, bases, salts) on its surface.

• The key principle underlying IE is that the ions we want to remove have a great affinity for the surface sites on
the resin, allowing them to displace the original counter-ions in the resin and "stick" to the beads.

• The most familiar application of IE is the removal of minerals from tap water to make "soft" water or "deionized"

• There are many different types of ion exchange resins that perform different exchange reactions
Tap Water Softening

Question: what is meant by "hard" and "soft" tap water if both contain dissolved ions?

Answer: Hard water contains multivalent Ca2+ and Mg2+, whereas "soft" water contains only Na+

Question: what are some examples of industrial process operations in which "hard" water can
produce serious problems?

Answer: Any process where water is continuously boiled or evaporated, can have problems
with precipitation of minerals in solid forms. Good examples are boilers, cooling towers, and

Question: How high is the concentration of "minerals" in typical tap water?

              Soft:                   0-60 mg/L
              Moderately hard:        61-120 mg/L
              Hard:                   121-180 mg/L
              Very hard:              >181 mg/L

     Sources: 1. USGS Water-Quality Information: Water Hardness and Alkalinity
Water Deionization

Deionization of water utilizes a different type of resin, which replaces Ca2+ and Mg2+ with H+, and the
counter-ions (such as carbonate, CO32- or sulfate, SO42-) with OH- ions, meaning the dissolved
minerals are replaced with H+OH-.

                                 A common material for the cation exchange resin beads is sulfonated
                                 polystyrene, a polymer that contains numerous surface sulfonate
                                 (RSO3-) groups, which are capable of binding many types of cations.
                                 The polymer is chemically crosslinked to keep it from dissolving.

                                 Whether the cation exch. column releases Na+ or H+ depends on the
                                 "regeneration" procedure used. For deionization, we obviously want
                                 to avoid releasing Na+ into solution.

                                 Anion exch. beads are also a crosslinked polymer, which has bound
                                 Lewis Acids (cationic) surface sites that can bind anions.
  Continuous-Flow Water Softening

   It is possible to soften water by a batch process, by soaking the ion-exchange beads in the water
   for some time. However, as with many processing operations, continuous flow is preferred.
   Thus, mineralized water is pumped through a cation-exchange column, which gradually becomes
   depleted as it fills up with hard cations:

                                                               In addition, the water may be passed through
                                                               a second column that exchanges the anions,
                                                               replacing them with OH-. This column also
                                                               gradually becomes depleted, or filled with the
                                                               undesirable anions.

                                                               When the mineralized water reaches the
                                                               bottom of the column because the resin is
                                                               depleted, we observe breakthrough.

                                                               Therefore, it is necessary to periodically
                                                               regenerate both cation and anion columns by
                                                               a process that removes the hard cations and
                                                               undesirable anions, renewing the resin for
                                                               further use.

Some ion exchangers actually have a mixed bed full of both cation-exchanging beads and anion-
exchanging beads.
The Regeneration Process

Once the cation and anion-exchange beads are fouled with undesirable ions, they may be cleansed
and re-used. The process of cleansing the unwanted ions is called regeneration.

Quantities of ions are often discussed in terms of gram equivalents, or millequivalents, so let's define
these quantities up front.

gram equivalent: The weight of a substance, usually in grams, that combines or reacts with a
standard weight of a reference element or compound.

millequivalent: One thousandth (10-3) of a gram equivalent of a chemical element, an ion, a
radical, or a compound. See examples on following page.

For water softening, we are replacing each millequivalent of mineral salt (like calcium carbonate)
with one millequivalent of NaCl. The Na+ ions go onto the cation exchange resin beads, and the Cl-
ions go onto the anion exchange resin beads.

For deionization, we are replacing each millequivalent of mineral salt (like calcium carbonate)
with one millequivalent of H2O. The H+ ions go onto the cation exchange resin beads, and the OH-
ions go onto the anion exchange resin beads.

However, regeneration is not accomplished by treating all of the beads with pure H2O, of course.
Rather, an acid wash followed by a water wash is used to add H+ ions to the cation exchange resin,
and a base wash followed by water wash is used to add OH- ions to the anion resin.
Example. Show that 1.0 g of sodium chloride contains 17 meq of Na+ and 17 meq of Cl-

Formula weight of NaCl: 58.44 g

{(1.0g NaCl)/(58.44 g/mol)} (1 mol Na+/ mol NaCl) (1 eq Na+ / mol Na+) / * (1000 meq/eq) = 17

{(1.0g NaCl)/(58.44 g/mol)} (1 mol Na+/ mol NaCl) (1 eq Na+ / mol Na+) / * (1000 meq/eq) = 17

Example 2. Show that 1.0 g of calcium chloride (anhydrous) contains 18 meq of Ca2+ and 18 meq of Cl-

Formula weight of CaCl2: 110.98 g

{(1.0g CaCl2)/(110.98 g/mol)} (1 mol Ca2+/ mol CaCl2) (2 eq Ca2+/ mol Ca2+) * (1000 meq/eq) = 18

{(1.0g CaCl2)/(110.98 g/mol)} (2 mol Cl-/ mol CaCl2) (1 eq Cl-/ mol Cl-) * (1000 meq/eq) = 18

 Note: the number of meq/g for CaCl22H2O would be a different number!
The Regeneration Process
How does the regeneration process work in the columns we are using?

                                   cation column regeneration
        mineral H2O                       HCl (aq)                        DI H2O (long time)

          depleted column
                    Ca2+                                            HCl                H+
           Ca2+                                     H+                                           H+
                                               H+                                                     H+
                                                                   H+        H+
             Ca2+           Ca2+          H+                                           H+
                                                    H+                                       H+
                                                          H+                                           H+
                                                                    H+      H+
                     Ca2+                H+                                        H+
                                                 H+                                         H+
         Ca2+                                            H+                                           H+
                     Ca2+                      H+                                           H+

                                     anion column regeneration
                                        NaOH (aq)                           DI H2O (long time)

                    Cl-                                        NaOH
                            Cl-            OH-                                    OH-
                                                         OH-                                 OH-
                          Cl-           OH-                                 OH-
                                               OH-                                 OH-
                    Cl-                                       OH-                                     OH-
                                           OH-                                    OH-
                                                      OH-                                   OH-
The Demineralization Process

This figure illustrates our demineralization experiment in the lab. When the system is fully
charged and working properly, meaning neither resin is depleted, H2O is the only product.

      "Mineralized" Water                                            Question 1: what comes out each
                                      2 Anion Exch. Column           column when the cation resin is
         H2O + CaCl2                                                 depleted, but the anion resin is still
                                                                     working properly? What is the final
                                                                     pH of the "deionized" water?


                                                                     Question 2: what comes out each
                                                                     column when the anion resin is
                                                                     depleted, but the cation resin is still
                                                                     working properly? What is the final
      HCl                                                            pH of the "deionized" water?

     pH~ 2.0

  1 Cation Exch. Column
Additional Facts:

Question: why does the "deionized" water that comes of our faucets have a pH of ~ 6.0 !?
 Packed/Fluidized Beds
Another aspect of our ion-exchange experiment is the study of a packed bed process. Packed beds
are important in numerous chemical engineering unit operations, especially in heterogeneous catalytic
chemical reactors. If the flow rate of gas or liquid becomes high enough in a packed bed reactor, the
packing material can become fluidized. Fluidization is characterized by fluid-like behavior of the entire
solid-liquid-mixture, with "dancing" of individual particles. At lower liquid flow rates, particles remain
stationary, and we say we are dealing with a packed bed rather than a fluidized bed. Fluidization
promotes contact between liquid and solid, which can be beneficial in many chemical process
operations. However, to properly fluidize a bed, a minimum liquid velocity is necessary. The minimum
velocity can be estimated from the classical Ergun equation for packed beds (usually given in English
units). The fluid flow rate must be fast enough to generate enough hydrodynamic force to overcome
the hydrostatic pressure due to gravity. In other words, the particles are denser than water and want to
sink, so fast enough upward fluid flow is needed to overcome gravity.

                            P       G  1   2        G2 1  
                                     kgD 2   3   1.75 kgD   3    p  l g
                                150               
                             L                                     
 P is the pressure drop in psi./ft.                                     p is the density of a swollen
 L is the length of the reactor in ft.                                   resin bead.
  is the viscosity of the fluid medium taken as 2.42 lb./ft.
     hr. for water
 k is a conversion factor equal to 144 (in.2/ft.2)                       l is the density of the liquid
 g is the gravitational constant (4.17108 lb.ft. /
 D is the granule diameter in ft.
  is the aqueous phase (inter-granule) void fraction
  is the density of the aqueous medium, taken as 62.3 lb./ft.3
 G is the mass velocity of the aqueous medium in lb./ft.2hr.,
     equal to the product of fluid density and average velocity.

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