Resin bed

Instead of using separation data may be expressed in terms of a volume distribution coefficient ) , which is defined as the amount of solution in the exchanger per cubic centimeter of resin bed divided by the amount per cubic centimeter in the liquid phase. The relation between and is given by ... 

The degree of swelling and shrinking is important for design of ion-exchange columns, especiaUy for the location of the distributors used to disperse incoming fluids, and coUect outgoing ones, evenly over the cross-sectional area of the resin bed. Once placed, these distributors are not adjustable. The upper distributor should be above (the lower one below) the resin bed, even in the bed s swoUen form. 

Semicontinuous and continuous systems are, with few exceptions, practiced in columns. Most columnar systems are semicontinuous since flow of the stream being processed must be intermpted for regeneration. Columnar installations almost always involve the process stream flowing down through a resin bed. Those that are upflow use a flow rate that either partially fluidizes the bed, or forms a packed bed against an upper porous barrier or distributor for process streams. 

The space immediately above the resin bed may or may not be filled with Hquid in downward flow systems, depending on the design. If not filled, water entering the column from the top and impinging on the upper surface of the resin bed forms hills and valleys unless the flow is dispersed over the cross-sectional area. A distributor similar to the one used to collect resin below the bed, or splash plate, is placed a short distance above the resin bed to improve the distribution of the process stream flow. 

Temperatures should not exceed 60°C for the Type I resins, and 40°C for Type II and acryflc resins. Thermal degradation and the loss of functional groups occur when these temperatures are exceeded. Elimination of siUca from the resin bed is further improved by preheating the bed with warm water before injecting the NaOH solution. 

Rinse. When transfer of the required volume of regenerating solution to the column has been completed, a small amount of regenerating solution occupies space immediately above the resin bed, between resin particles in the bed, and within the resin particles. It must be displaced with water before the column can be returned to the adsorption step. Rinsing should begin at the same flow rate as used during regeneration and continue at that rate until a volume of water equal to 1—2 bed volumes has been used. After that, the flow rate is increased to the rate normally used during the adsorption step, and continued at that rate until the effluent is of satisfactory quaHty, as deterrnined by pH, conductivity, or resistivity. The water need not be at an elevated temperature unless the process stream is above ambient temperature. 

Liquid-liquid extraction is used primarily when distillation is imprac-tic or too costly to use. It may be more practical than distillation when the relative volatility for two components falls between 1.0 and 1.2. Likewise, liquid-liquid extraction may be more economical than distillation or steam-stripping a dissolved impurity from wastewater when the relative volatility or the solute to water is less than 4. In one case discussed by Robbins [Chem. Eng. Prog., 76 (10), 58 (1980)], liquid-liquid extraction was economically more attractive than carbon-bed or resin-bed adsorption as a pretreatment process for wastewater detoxification before biotreatment. 

FIG. 16-42 loD-exchaDger regcDeratioD. a ) CoDveDtioDal. Acid is passed down-flowthrough the cation-exchange resin bed. (h) Coiinterflow. Regenerant solution is introduced iipflow with the resin bed held in place hy a dry layer of resin. 

The Asahi process (Fig. 16-63) is used principally for high-volume water treatment. The hquid to be treated is passed upward through a resin bed in the adsorption tank. The upward flow at 30-40 m/h [12-16 gal/(min ft")] keeps the bed packed against the top. After a preset time, 10 to 60 min, the flow is interrupted for about 30 s, allowing the entire bed to drop. A small portion (10 percent or less) of the ion-exchange resin is removed from the bottom of the adsorption tank and transferred hydraulically to the hopper feeding the regeneration tank. 

Use closed-loop systems for pickling regenerate and recover acids from spent pickling liquor using resin bed, retorting, or other regeneration methods such as vacuum crystallization of sulfuric acid baths. 

The regeneration of the resin bed is never complete. Some traces of calcium and magnesium remain in the bed and are present in the lower-bed level. In the service run, sodium ions exchanged from the top layers of the bed form a very dilute regenerant solution which passes through the resin bed to the lower portion of the bed. This solution tends to leach some of the hardness ions not removed by previous regeneration. 

Channeling. Cleavage and furrowing of the resin bed ean be eaused by faulty operational proeedures or a elogged bed or underdrain. This ean mean that the solution being treated follows the path of least resistanee, runs through these furrows, and fails to eontaet aetive groups in other parts of the bed. 

Mechanical strain. When broken beads and fines migrate to the top of the resin bed during serviee, mechanieal strain is eaused whieh results in ehaimeling, inereased pressure drop, or premature breakthrough. The eombination of these resulting eonditions leads to a drop in eapaeity. 

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. 

Uniformly distribute the service and regeneration flow through the resin bed... 

The resin bed is brought in con- tact with the regenerant solution. In the case of the cation resin, acid elutes the collected ions and converts the bed to the hydrogen form. A slow water rinse then removes any residual acid. 

The resin bed is subjected to a fast rinse that removes the last traces of the regenerant solution and ensures good flow characteristics. 

Attrition The rubbing of one particle against another in a resin bed frictional wear that will affect the site of resin particles. 

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. 

Breakthrough The first appearance in the solution flowing from an ion-ex- change imit of unabsorbed ions similar to those which are depleting the activity of the resin bed. Breakthrough is an indication that regeneration of the resin is necessary. 

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

Head loss The reduction in liquid pressure associated with the passage of a solution through a bed of exchange material a measure of the resistance of a resin bed to the flow of the liquid passing through it. 

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. 

Leakage The phenomenon in which some of the influent ions are not ad- sorbed and appear in the effluent when a solution is passed through an underregenerated exchange resin bed. 

The dealkalization process removes the temporary hardness in water. This uses an acid resin bed for regeneration—in this case sulfuric acid (H2SO4). 

The effluent was discarded and the resin bed was then washed with the following solutions in the specified sequence (1) 120 ml of an aqueous 0.1 N hydrochloric acid solution ... 

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