Mixed bed is the abbreviation for mixed ion exchange column, which is a device designed for ion exchange technology.

The so-called mixed bed refers to the mixing and filling of a certain proportion of cation and anion exchange resins in the same exchange device to exchange and remove ions from the fluid. Due to the higher specific gravity of the cation resin compared to the anion resin, the anion resin is on top and the cation resin is on the bottom in the mixed bed. The general ratio of resin filling for yang and yin is 1:2, and there are also filling ratios of 1:1.5, which can be selected according to different resins. Mixed beds are also divided into in vivo synchronous regeneration mixed beds and in vitro regeneration mixed beds. The synchronous regeneration mixed bed operates and the entire regeneration process is carried out inside the mixed bed. During regeneration, the resin does not move out of the equipment, and the cation and anion resins are regenerated simultaneously. Therefore, fewer auxiliary equipment are required and the operation is simple.

The equipment for mixed bed treatment process includes mixed ion exchangers and in vitro regeneration equipment. The in vitro regeneration equipment mainly includes resin separators, anion (cation) resin regenerators, resin storage towers, hybrid resin towers, and acid-base regeneration equipment. The main characteristics of domestic mixed bed treatment technology are reflected in the resin separation and regeneration process. There are three types of resin separation and regeneration processes.

1. The effluent quality is excellent, and the pH value of the effluent is close to neutral.

2. The effluent quality is stable, and short-term changes in operating conditions (such as inlet water quality or composition, operating flow rate, etc.) have little impact on the effluent quality of the mixed bed.

3. Intermittent operation has little impact on the effluent quality, and the time required to restore the water quality to before shutdown is relatively short.

4. Recovery rate reaches 100%

The materials used to make the mixed bed shell include fiberglass, organic glass, stainless steel, carbon steel, anti-corrosion, etc. The outer shape is cylindrical, with a diameter of Φ 200-2500mm and a water production rate ranging from 0.5t/h to 98t/h. The cation bed is loaded with strong acid cation exchange resin, and the anion bed is loaded with strong alkali anion exchange resin. The loading height is always between 1000-2400 mm. There is no support layer at the bottom of small filter cap equipment, and there are multi-level quartz sand support layers with different particle sizes at the bottom of medium and large equipment (this is not recommended now because quartz sand and acid will react chemically during acid washing, which will affect water quality). The top layer of the counter current regeneration fixed bed resin has a 200 mm thick resin layer (special resin is used to cover the resin below). The operating pressure of organic glass columns is ≤ 0.15MPa, and the operating pressure of equipment made of other materials is ≤ 0.6MPa. The cation bed is equipped with an acid tank and acid pump regeneration system, while the anion bed is equipped with an alkali tank and alkali pump regeneration system.

Run

There are two ways of water inlet in this system: softening (water treated by softener) inlet and initial desalination (water treated by reverse osmosis) inlet, which are controlled by their respective control valves.

During operation, the desalination inlet control valve, inlet valve, and production valve should be opened, and all other valves should be closed!

　　Backwash

Close the inlet valve and production valve; Open the backwash inlet valve and backwash discharge valve, and backwash at 10m/h for 15 minutes. Then, close the backwash inlet valve and backwash discharge valve. Let it settle for 5-10 minutes. Open the exhaust valve and middle exhaust valve, partially drain water to about 10cm above the surface of the resin layer, and close the exhaust valve and middle exhaust valve.

　　regeneration

Open the inlet valve, acid pump, acid inlet valve, and intermediate discharge valve to regenerate the cation resin at 5m/s and 200L/h. Clean the anion resin with reverse osmosis water to maintain the liquid level in the column 10cm above the surface of the resin layer. After regenerating the cation resin for 30 minutes, close the inlet valve, acid pump, and acid inlet valve, open the backwash inlet valve, alkali pump, and alkali inlet valve to regenerate the anion resin at 5m/s and 200L/h. Clean the cation resin with reverse osmosis water to maintain the liquid level in the column 10cm above the surface of the resin layer and regenerate for 30 minutes.

　　Replacement, mixing, and rinsing

Turn off the alkali pump and alkali inlet valve, open the inlet valve, and simultaneously introduce water up and down to replace and clean the resin. After 30 minutes, close the inlet valve, backwash inlet valve, and intermediate exhaust valve, open the backwash discharge valve, inlet valve, and exhaust valve, and mix the resin for 0.5-5 minutes at a pressure of 0.1-0.15 MPa and an air volume of 2-3m3/(m2 · min). Close the backwash discharge valve and intake valve, and let it settle for 1-2 minutes. Open the inlet valve and the positive wash discharge valve, adjust the exhaust valve, fill the column with water until there is no air, then close the exhaust valve and flush the resin. When the conductivity meets the requirements, open the water production valve, close the positive wash discharge valve, and start water production.

Resin secondary separation and regeneration process
It is to transport the failed mixed bed resin to the resin separator (cation resin regenerator), complete hydraulic separation, and transfer the upper anion exchange resin to the mixed bed to the anion resin regenerator. The mixed resin near the separation surface of the yin-yang resin is transported to the mixed resin tower, and then the yin-yang resin is regenerated separately. For the resin in the mixed resin tower, it will be sent back to the resin separator (cation resin regenerator) for secondary separation during the next regeneration. Here, the resin separator also serves as a cation resin regenerator.
　　Cone separation regeneration process
The cone separation regeneration process is to make the bottom of the resin separator into a cone. This device serves as both a negative resin regenerator and not a positive resin regenerator. After the failed resin is transported from the mixed bed to the resin separator for hydraulic stratification, the cation resin located at the lower part of the resin separator is transported from the bottom of the cone to the cation resin regenerator. Due to the conical shape at the bottom of the resin separator, there is very little resin at the resin separation interface, which reduces the amount of mixed resin in the middle and improves the separation effect. When transporting cation resin, the automatic detection of the separation interface during automatic control often uses photoelectric or conductivity methods. The so-called photoelectric method refers to using a photoelectric meter to detect the depth of the color of the yin-yang resin; The conductivity law measures the change in conductivity of the water transported by the Yin Yang resin. When the resin transported by W2 gas changes from the Yang resin to the Yin resin, its conductivity will decrease. Use the signal changes generated by two methods to control the boundary between yin and yang resins.
　　Regeneration process of yin-yang resin
It is to transport the failed resin from the mixed bed to the resin separator, and after hydraulic stratification of the failed resin, the anode and cathode resins are regenerated simultaneously in the separator. The resin separator also serves as a resin regenerator. This method is completely the same as the desalination mixed bed for supplementary water treatment.

ion exchanger

The ion exchange process requires the exchange agent to be placed inside an ion exchanger (or bed), and the ion exchange capacity is restored through regeneration after the ion exchange agent fails. In order to improve the economy and technical applicability of ion exchange processes, different combinations of resins, different bed types, and various ion exchange systems have been developed. There are two common types of ion exchangers: fixed bed (ion exchanger) and continuous bed.

The working principle is ion exchange.

Run time: Resin (H-R)+(M+) → (M-R)+(H+)

Negative resin (OH-R)+(X -) → (X-R)+(OH -)

M+is a metal ion and X - is an anion. The regeneration process is its reverse process.

Failure control of ion exchangers

The simpler process for treating desalinated water through ion exchange is a first stage complex bed desalination system consisting of a cation bed and a anion bed. Some primary complex bed desalination systems adopt a unit system, which means that each set of primary complex bed desalination system includes one cation bed, one carbon remover, and one anion bed. During the operation of ion exchange desalination, whether the cation bed or anion bed fails first, they are regenerated simultaneously; Some first level complex bed desalination systems use a mother control system, which means that the cation bed and cation bed or anion bed and anion bed operate in parallel, and the one that fails is regenerated.

　　Principle of Detection and Control

The adsorption sequence of various cations in water by strong acid cation resin is:

Fe3+>Al3+>Ca2+>Mg2+>Na+>H+； From this, it can be seen that the ability of metal ions Na+in water to be adsorbed is weak. Therefore, when ion exchange occurs, the various ion adsorption layers of the resin layer gradually move downwards, and H+is finally replaced by other cations. When the protective layer penetrates, the first to leak is the lowest layer of Na+; Therefore, monitoring the failure of cation exchangers is based on sodium leakage as the standard; The reaction equation is (A represents metal cation, R is resin group):

An++nRH=RnA+nH+

HCO3-+H+=H2O+CO2 ↑

The adsorption sequence of various anions in water by strong alkaline anion resin is:

SO42->NO3->Cl->OH->HCO3->HSiO3- 。 From this, it can be seen that the adsorption capacity of HSiO3- is weak. Therefore, when ion exchange occurs, the various ion adsorption layers of the resin layer gradually move downwards, and OH - is replaced by other anions. When the protective layer penetrates, the first to leak is the bottom layer of HSiO3-; Therefore, monitoring the failure of anion exchangers is based on silicon leakage as the standard; The reaction equation is (B represents acid anion, R is resin group):

Bm -+mROH=RmB+mOH-

　　2 Equipment advantages

(1) The effluent quality is excellent, and the pH value of the effluent is close to neutral.

(2) The effluent quality is stable, and short-term changes in operating conditions (such as inlet water quality or composition, operating flow rate, etc.) have little impact on the effluent quality of the mixed bed.

(3) Intermittent operation has little impact on the effluent quality, and the time required to restore the water quality to before shutdown is relatively short.

(4) The quality of the produced water is excellent, with a conductivity of ≤ 0.2us/cm

Ion exchangers are mainly used for the preparation of pure water and high-purity water, and are mainly used for medium and high pressure boiler feedwater; Production process water for new materials, new energy, new chemical materials, light metal materials, nanomaterials, fiberglass graphite, composite materials, etc.