Failure Control Case Analysis of Ion Exchange Demineralized Water Processor

Taking the pure water station of the protein separation workshop of Chengdu Institute of Biological Products as an example, this system is a main pipe water treatment system . The system structure is: sand filter - activated carbon filter - coarse filter - cation bed - primary anion - secondary anion - mixed bed - fine filter - pure water tank. The system has a water production capacity of 5 t/h. In the failure control research of the system, we proposed the concept of unit failure control, which is to make full use of the advantages of the main pipe water control system to control the failure of the system.

The following is partial data from one operating cycle of the first water treatment unit (i.e., unit 1.1) in this workshop:

Table 1. Partial data from the one-day operation cycle.

The above data are all actual operation records of the workshop. The conductivity of the effluent after the supply of 1.1 anion water to 1.2 anion water is guaranteed to be below 3μS/cm. Combined with the final stage mixed bed, the final effluent water quality is always maintained at 0.1μS/cm - 0.5μS/cm.

(1) The removal rate of RO for each organic solute is greater than that of NF membrane. (2) The removal rates of different organic solutes are different, and some even differ greatly (for example, the absorbance removal rates of RO and NF membranes for acetic acid are 95.34% and 81.45%, respectively, while the absorbance removal rates for aniline are 61.50% and 46.82%, respectively).

3. Effluent water quality

After primary desalination, the conductivity (25℃) of the raw water is less than 10μS/cm, and the silicon content in the water is less than 100μg/L.

3.1 Effluent quality from the cation exchange bed

In Figure 1, the three curves before point b all drop rapidly, indicating that after resin regeneration in the ion exchanger, the content of various impurities (acidity, sodium ion concentration, and hardness) in the effluent during the forward wash decreases rapidly. When the effluent quality reaches a certain standard (such as point b), it can be put into operation. Therefore, during the operation of section ba, the effluent from the cation exchange bed is acidic, and the acidity changes slowly. When the operation reaches point a, cations begin to penetrate the cation exchange bed. According to the ion exchange activity, sodium ions are the first to leak out. In the desalination system, in order to remove all cations other than H⁺ from the water, the strongly acidic ion exchanger must stop operating when sodium leakage occurs (generally, the acidity of the effluent is close to neutral at this time) and undergo regeneration, as shown at point a in Figure 1, which is the sodium ion penetration point.

3.2 Effluent quality from the anion bed

Since the anion exchange bed is always the downstream stage of the cation exchange bed in an ion exchange desalination system, the water quality changes in the anion exchanger can be categorized into two situations:

① When the cation exchange bed is operating normally, the effluent quality of the anion exchange bed when the anion exchange bed fails first.

The effluent quality of the anion exchange bed is shown in Figure 2. Before point b, the curves show a rapid decline, indicating that during the backwash after regeneration, impurities in the water decrease rapidly until the effluent quality meets the operational standards. The ba range represents the stable exchange operation period, with an effluent pH of 7-9, conductivity <5 μS/cm, and silicon content (as SiO2 ₎ of 20-50 μg/L. After reaching point a, the anion exchange bed begins to fail, but the cation exchange bed continues to operate normally. At this time, the pH of the anion exchange bed decreases due to acid leakage; simultaneously, the silicon content and conductivity in the anion exchange bed effluent increase.

② When the cation exchange bed fails first, the effluent quality of the anion exchange bed will be affected.

As shown in Figure 3, when the multi-bed system reaches point a, the cation bed begins to fail, but the anion bed continues to operate normally. At this time, the Na+ leaking from the cation bed flows through the anion bed, resulting in NaOH in the anion bed effluent. This increases the pH value of the anion bed effluent and interferes with the adsorption of HSiO3- by the strongly alkaline anion resin ^, _increasing the silicon content of the effluent. The reaction is as follows:

RHSiO₃ ₊ NaOH = ROH + NaHSiO₃

4. Conclusions and Improvement Methods

(1) When the cation exchanger is nearing failure, the Na ⁺ content in the water gradually increases, thus affecting the anion exchanger. When the requirements for purified water quality are not very high (such as in the pharmaceutical industry), the failure of the cation exchanger can be controlled by controlling the acidity of the effluent from the cation exchanger (mother exchanger) and the conductivity of the effluent from the anion exchanger (primary mixed exchanger). Since CO2 is dissolved in the cation exchanger product water _, it enters the anion exchanger without being removed and reacts with OH- in the anion exchanger, affecting the treatment capacity of the anion exchanger. Therefore, a carbon removal device can be installed between the cation exchanger and the anion exchanger.

(2) Since HCO3- ^is generally removed in the cation bed, and the conductivity is more sensitive to OH-, the failure of the anion bed can be controlled by detecting the effluent conductivity of the anion bed in industries that do not use silicon as a detection item (such as the pharmaceutical industry).

(3) Silicon leakage in ion exchange is a global problem. Practice has shown that using a dual-cathode bed + mixed bed ion exchange system is very effective in controlling silicon leakage.