Analysis of the Reasons for the Decrease in Water Production During the Re-Bed Cycle

The desalination plant consists of two parts: a mixed bed (primary desalination) and a double-bed (secondary desalination). As operating time increases, the water production cycle gradually shortens, the mixed bed regeneration becomes more frequent, the self-consumption rate increases significantly, and acid and alkali consumption also rises considerably. The water production capacity of the mixed bed typically does not reach the design value of 140 t/h, but only maintains around 110 t/h. Therefore, we analyzed and studied the reasons for the decrease in the cycle water production of the mixed bed in order to improve the cycle water production capacity.

According to the principle of ion exchange equilibrium, the desalination production process is calculated as follows:

N=V·E0/(Q_{T ·} ∑c) (1)

Where: N – water production per cycle, m3;

V – volume of ion exchange resin, m3;

E0 – working exchange capacity of ion exchange resin, mmol/L;

QT – water treatment capacity per exchanger, m3/h;

∑c – total amount of ions in water, mmol/L.

As can be seen from equation (1), when the water production and resin volume are constant, an increase in the ion content in the mixed bed feedwater and a decrease in the working exchange capacity of the ion exchange resin will both lead to a decrease in the cycle water production of the mixed bed.

Therefore, we conducted a survey and statistics on the operating conditions of the mixed bed and the performance of the resin, as shown in Table 1. Table 1 shows that the ion content in the mixed bed feedwater increases significantly every year, and the working exchange capacity of the resin also decreases to a certain extent.

Table 1. Statistics on water production in multi-bed systems

Analysis of the Reasons for the Increase in Ion Content in the Influent of the Multi-bed Desalination

System The influent to our plant's desalination system is softened water from the Yellow River, which has undergone coagulation, filtration, and dealkali removal via a weak acid cation exchange bed. Historical data on Yellow River water quality show significant changes, as illustrated in Table 2.

Table 2. Changes in the total amount of ions in the Yellow River water

Due to the continuous deterioration of the Yellow River's water quality, the ion content in the raw water has been increasing year by year, resulting in a reduction in the water production during the combined bed cycle.

Analysis of the Reasons for the Decrease in the Working Exchange Capacity

of Ion Exchange Resins The working exchange capacity of ion exchange resins is related not only to their inherent process characteristics but also to operating conditions, such as regeneration method, influent ion concentration, control indicators at the exchange endpoint, resin bed height, and water flow velocity. Whether the exchanger can be completely regenerated also significantly affects the working exchange capacity.
When the regeneration method and control indicators at the exchange endpoint remain unchanged, the reasons for the decrease in working exchange capacity should be investigated from the following aspects: ① Insufficient strength of domestically produced weak-base resins, leading to breakage and delamination during operation, clogging the water cap and causing a decrease in the working resin bed height. ② Increased pressure differential in the mixed bed restricts water flow velocity, reducing the rate of membrane diffusion during exchange and affecting the full utilization of the ion exchange resin's exchange capacity. ③ Changes in the influent ion concentration cause a mismatch between the resin regeneration time/concentration and operating conditions, resulting in poor regeneration effect. ④
Excessive carbon black content in the process condensate recovered from the synthesis section, which then enters the mixed bed, causing the influent to become black and sticky, clogging the resin mesh and affecting resin adsorption and ion exchange, thus causing a decrease in working exchange capacity.