Southern Methodist University School of Engineering and Applied Science SMU SSH 8321 SMU ME 5315

1. Southern Methodist University School of Engineering and Applied Science SMU SSH 8321 SMU ME 5315 NTU HW 741-N Treatment Technology I - Physical and Chemical Methods April 25, 2000 Dr. Roger Dickey

2. Column Adsorption Tests - Continuous flow bench-scale or pilot-scale column adsorption tests are often performed to establish design parameter values. Data from these tests are usually developed in the form of a “Breakthrough Curve.” An idealized breakthrough curve for a single column adsorber is shown in Figure 6.1.4 on page 6.11 in the Freeman textbook.

3. The “Breakthrough Point” is defined as the location on the breakthrough curve where the pollutant concentration in the column effluent rises to the allowable discharge concentration, CA.

4. The total mass of activated carbon is often divided among two or more identical column reactors that are then operated in series when conducting these column adsorption tests. Test columns are typically in the range of 1 to 6 inches in diameter and the cumulative carbon bed depth for the columns operated in series is often in the range of 3 to 12 ft.

5. Design Scale-up Approach - Full-scale column adsorbers are often designed using a simplified scale-up approach based on breakthrough curves and design parameter values evaluated during bench-scale or pilot-scale column adsorption tests. Design parameters commonly used include:

6. (1) Hydraulic Loading Rate, RH , where, Q = volumetric flow rate [L3/T] AP = cross-sectional area of the column [L2]

7. (2) Empty Bed Contact Time, EBCT [T], where, BV = bed volume, i.e., bulk volume of GAC in the column [L3]

8. Consider a typical cylindrical adsorption column: Side View Plan View GAC Z GAC D AP = (D2/4) [L2] D = column diameter [L] BV = APZ [L3] Z = carbon bed depth [L]

9. (3) Number of Bed Volumes per Unit Time, QB [1/T], QB is the applied volumetric flow rate in terms of the number of bed volume equivalents processed per unit time. Notice that,

10. (4) Actual Full-Scale Surface Concentration, (x/m)A [M/M], at Breakthrough

11. where, m = mass of GAC in the column [M] C0 = influent pollutant concentration [M/L3] C1 = average pollutant concentration in the column effluent over the length of the run until breakthrough occurs [M/L3]

12. VB = breakthrough volume, that is, the volume of wastewater processed over the length of the run until breakthrough occurs [L3] In full scale column reactors operated either singly or in parallel, (x/m)A is typically 15% to 50% of the equilibrium (x/m) value obtained in batch adsorption tests at the design pollutant concentration.

13. (5) Carbon Usage, [M/M], at Breakthrough Notice that,

14. C1 can be approximated from the breakthrough curve: C0 Pollutant Concentration CA T Time

15. where, T = time to breakthrough [T] CA = allowable pollutant concentration at breakthrough [M/L3]

16. Approximate the initial portion of the breakthrough curve as a straight line where the column effluent pollutant concentration rises from zero to CA at the breakthrough point. For any straight line over a range of x-values from x1 to x2, the average y-value is . Thus, from the breakthrough curve, or

17. Substituting for C1 in the equation for

18. (6) Carbon Dosage, UC [M/L3], at Breakthrough

19. Notice the relationship between UC and : Also notice that, where is the volume of waste treated per unit mass of carbon as defined in equation 12.6 on page 360 in the text.

20. (7) Time to Breakthrough, T [T], Consider Equation 12.9, page 361 in the text, Substitute (QT) for VB in the equation for and solve for the time to breakthrough or allowable run length, T,

21. Alternatively, substitute (QT) for VB in the equation for UC and solve for T,

22. Columns in Series - In this arrangement, the run length is extended for the first column until all of the GAC is at equilibrium with C0, i.e., to the point of carbon exhaustion. This maximizes use of the adsorption capacity of the GAC. Consider two columns in series:

23. Influent Q Spent GAC Spent GAC Adsorp. Zone Fresh GAC Effluent Q

24. Theoretically, the batch adsorption isotherm can be used to estimate the surface concentration at exhaustion, (x/m)EX, for the GAC at equilibrium with C0,

25. The carbon usage at exhaustion is then given by, and the time to exhaustion for the first column reactor can then be estimated from,

26. Typical Carbon Column Design Criteria - Parameter Typical Range RH 2 - 10 gpm/ft2 EBCT 10 - 50 min Carbon Usage, 3 - 10 lb GAC/lb COD Vessel Diameter, D 2 - 12 ft Bed Depth, Z 5 - 30 ft Z:D Ratio 1.5 - 5.0 Backwash Rate (if used) 12 - 20 gpm/ft2

27. Regeneration - In large wastewater applications, activated carbon is typically regenerated thermally using either a rotary kiln or a multiple hearth furnace. During the regeneration process, 5% to 10% of the carbon is lost due to abrasion and loss of fines or due to combustion. The lost carbon is replaced with fresh carbon.

28. Regeneration can be accomplished on-site or off-site. Manufacturers of GAC will usually take back spent carbon for regeneration at large regional or national facilities.

29. For small applications, pre-packaged GAC containers including small canisters, 55-gallon drums, or moderate sized columns can be obtained commercially. The supplier delivers fresh containers and picks-up spent containers by truck in a “milk delivery” fashion.

30. (B) Ion Exchange The ion exchange process depends upon the ability of certain solid substances (called exchange media) to exchange ions bound to the surface of the solid with inorganic ions of another species dissolved in water. To be practical and useful for waste treatment, the process must be both selective for the ions to be removed and reversible.

31. Once the ion exchange sites on the solid medium have been exhausted, the medium is removed from service and chemically regenerated. Although various naturally occurring materials have ion exchange properties, most ion exchange media now in use for waste treatment are synthetically produced resins.

32. Ion exchange media have a finite number of exchange sites available and the total solid-phase concentration of ions is called the ion exchange capacity of the medium (meq ions/gm of medium).

33. Typical ion exchange capacities of various materials: Exchange Ion Exchange Medium Capacity (meq/gm) Clay 0.03 - 1.1 Glauconite (green sand) 0.09 - 0.18 Zeolite (aluminosilicates) ~ 1.4 Sulfonated Coal ~ 1.7 Synthetic Resins 3 - 10

34. Synthetic resins are polymers that are manufactured in the form of pellets or beads with a size of approximately 0.5 mm.

35. Common applications of ion exchange include: • water softening • demineralization • deoxygenation • desalination • recovery of valuable metals • recovery of acids • chemical manufacture • treatment of haz. and radioactive wastes

36. There are two general types of ion exchange resins: • cation exchangers • anion exchangers

37. General ion exchange reactions and regeneration reactions: (1) Cation Exchange Cation exchange is employed in two modes, (a) Hydrogen cycle exchange The exchange reaction is,

38. where, M2+ = metal cation (also M3+or M+) Ex = exchanger solid matrix

39. The regeneration reaction is, • A large excess of H+ ions are provided for regeneration by a strong acid solution (2% to 10% by weight) usually H2SO4 or HCl.

40. (b) Sodium Cycle Exchange • Commonly used for water softening. • 2 Na+ ions are exchanged for each metal M2+ ion in solution (or 3 Na+ exchanged for each M3+, and etc.). The exchange and regeneration reactions can be represented together as,

41. Exchange Regeneration • An excess of Na+ ions are provided for regeneration via a concentrated NaCl brine (2% to 10% by weight).

42. (2) Anion exchange • Anion exchange usually involves exchange of OH- ions in two modes: • (a) Salt Splitting • The exchange and regeneration reactions can be represented together as,

43. Exchange Regeneration • A large excess of OH- ions are provided for regeneration using a concentrated NaOH solution (2% to 10% by weight).

44. Exchange Regeneration • (b) Acid Removal • The exchange and regeneration reactions are, • A large excess of OH- ions are provided for regeneration using a concentrated NaOH solution (2% to 10% by weight).

45. Reactor Configurations - Most ion exchange units are cylindrical steel pressure vessels with a fixed bed of ion exchange resin (i.e., column reactors) operated in a downflow mode with a cyclic operation that allows for alternation between exchange and regeneration reactions as follows:

46. The column is operated to a predetermined leakage or breakthrough concentration of contaminant when the exchange capacity of the resin bed is considered exhausted.

47. After the exchange capacity of the resin bed is exhausted, the unit is taken off-line and backwashed to clean and rinse the bed, the chemical regenerate solution is then added downflow, and finally the bed is rinsed downflow to flush excess regenerate from the bed.

48. This is called a cocurrent operation. Ion exchange system designs are sometimes based on a countercurrent fixed bed or countercurrent moving bed mode. Conceptually, contacting techniques for ion exchange column reactors are virtually identical in configuration to GAC column reactors.

49. Conceptually, contacting techniques for ion exchange media are virtually identical to those used for GAC: • both employ column reactors that inherently operate under nonsteady-state conditions • batch isotherms and continuous flow breakthrough curves are often used for developing design data

50. the column reactors must be removed from service when breakthrough occurs and the medium regenerated • the processes are reversible and the pollutant is removed and concentrated when the medium is regenerated