Resin backwashing

Figure 8.8 Asahi moving packed bed. A, adsorption section B, elution section C, fluidized resin backwash D, resin collection hoppers with screen top and non-return valve outlet for resin E, transfer and backwash water overflow F, feed G, barren effluent H, eluant I, eluate product J, backwash supply K, resin flow [1]. Copyright 1987 John WUey Sons, Inc. This material is used by permission of John Wiley Sons, Inc.

Elegant slide valves are used to separate the adsorption, regeneration and resin backwash stages. The contactor operates in predetermined cycles and is an ideal process for feedstreams with no suspended solids. 

Next, backwashing the bed removes fractured beads and any solids deposited by the brine. Expansion and resettling of the bed destroy channels that may have formed and reclassify the resin. Backwashing should expand the bed 50-75% for about 30 min. Depending on the temperature of the water, this requires a flow rate somewhere between... 

Separate ketdes and backwash towers are frequendy used to convert ion-exchange resins from one ionic form to another prior to packaging, and to cleanse the resin of chemicals used in the functionalization reactions. Excess water is removed from the resin prior to packaging by a vacuum drain. Both straight line filters and towers or columns are used for this purpose. 

A distributor is frequently installed at the top of the column for use during backwash. It collects water evenly and prevents resin from escaping the column should unexpected surges develop in the water flow during backwash. Columns lacking an upper distributor or screen to prevent loss of resin should have an external system to prevent resin from being lost to the drain. It is referred to as a resin trap and may consist of a porous bag that fits over the outlet pipe or a tank designed to lower the linear velocity. Resin drops to the bottom of the tank and is returned to the column when convenient. 

At times, the process stream flow must be increased after the initial installation to satisfy production demands. Depending on the magnitude of the increase, the existing system may or may not be able to handle the added flow. If it can, regeneration frequency must increase. Adding more resin to the column is often considered an alternative to installing another column. Resin addition lessens the space for backwash and may be a cause for poor column performance. 

Ion Exchange. Ion-exchange treatment (215,216) entails elution of wastewater through a suitable resin until the available sites for ion exchange become fully occupied and the contaminated ions appear in the outflow. Treatment is then stopped. The bed is backwashed and then regenerated using an... 

Improper backwash. Blowoff of resin from the vessel during the backwash step can occur if too high a backwash flow rate is used. This flow rate is temperature dependent and must be regulated accordingly. Also, adequate time must be allotted for backwashing to insure a clean bed prior to chemical injection. 

Backwash Cycle - Prior to regeneration, the cation and the anion resins are separated by backwashing at a flow rate of 3.0 to 3.5 gpm/ft. The separation occurs because of the difference in the density of the two types of resin. The cation resin, being heavier, settles on the bottom, while the anion resin, being lighter, settles on top of the cation resin. After backwashing, the bed is allowed to settle down for 5 to 10 minutes and two clearly distinct layers are formed. After separation, the two resins are independently regenerated. 

Backwashing - After exhaustion, the bed is backwashed to effect a 50 percent minimum bed expansion to release any trapped air from the air pockets, minimize the compacmess of the bed, reclassify the resin particles, and purge the bed of any suspended insoluble material. Backwashing is normally carried out at 5-6 gpm/ft. However, the backwash flow rates are directly proportional to the temperature of water. 

Brine Injection - After backwashing, a 5 percent to 10 percent brine solution is injected during a 30-minute period. The maximum exchange capacity of the resin is restored with 10 percent strength of brine solution. The brine is injected through a separate distributor placed slightly above the resin bed. 

Provide space to fluidize the resin during backwash... 

The column is backwashed to remove suspended solids collected by the bed during the service cycle and to eliminate chaimels that may have formed during this cycle. The back- wash flow fluidizes the bed, releases trapped particles, and reorients the resin particles according to size. 

Backwash The countercurrent flow of water through a resin bed (that is, in at the bottom of the exchange unit, out at the top) to clean and regenerate the bed after exhaustion. 

Bed A mass of ion-exchange resin particles contained in a column. Bed depth The height of the resinous material in the column after the exchanger has been properly conditioned for effective operation. Bed expansion The effect produced during backwashing when the resin particles become separated and rise in the column. The... 

Cycle A complete course of ion-exchange operation. For instance, a complete cycle of cation exchange would involve regeneration of the resin with acid, rinse to remove excess acid, exhaustion, backwash, and finally regeneration. 

Freeboard The space provided above the resin bed in an ion-exchange column to allow for expansion of the bed during backwashing. 

Hydraulic classification The rearrangement of resin particles in an ion-ex- change unit. As the backwash water flows up through the resin bed, the particles are placed in a mobile condition wherein the larger particles settle and the smaller particles rise to the top of the bed. 

If the unit becomes badly fouled with suspended matter (for example, after a pipe brake has introduced excessive suspended matter into the system) it must be taken out of service and cleaned. This is done with an extended backwash, possibly at higher flow rates. If this does not remove the dirt, the manhole should be opened and the resin agitated with an air lance. Non-ionic detergents can be used, but not at the same time as the air lance or the resulting froth will be impossible to control. 

Most types of ion-exchange resin suffer some breakdown and volume loss over time because of attrition, excessive heat, or other factors. Water softeners should be inspected annually, and a double backwash procedure should be provided. This generally lifts the broken resin ( fines ) to the top of the bed, where it can be removed and replaced to restore capacity. Allow for 5 to 10% resin operating capacity loss per year because of physical breakdown. At many sites the resin is unfortunately not inspected regularly but merely replaced when a serious decline in operating capacity is noticed. Here a resin life expectancy of, say, 6 to 8 years probably is the norm. 

Bed expansion (freeboard) is 50% minimum (thus, the resin tank must be at least double the volume of the resin requirement). Resin bed expansion is a function of backwash rates and temperature. 

NOTE Before use for the first time, IX resins must be conditioned by soaking in water for approximately 1 hour. Resins should then be backwashed to reclassify the bed, so that the fine particles are on the top and the coarse particles are on the bottom. After backwashing, the bed should be allowed to settle and the water drained to 1 inch above the top of the bed. Regeneration should then take place. 

Hold-down beds This design uses the traditional 30 to 50% freeboard space for internal backwashing, but retains or holdsdown the resin in a... 

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