Measures to Increase Water Production Per Cycle

The increased ion content in the influent and the decreased working exchange capacity of the resin led to a reduction in the cycle water production of the mixed bed. Measures such as recovering urea process waste liquid and reducing the degree of resin crosslinking were taken to increase the cycle water production of the mixed bed, increasing the cycle water production from 1610 to 4294t and the water production cycle from 14h to 39.5h.

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 mixed bed's water production capacity typically falls short of the design value of 140 t/h, only maintaining around 110 t/h. Therefore, we have implemented a series of technical improvements to increase the mixed bed's cycle water production.

The main measure to reduce the ion content of the feed water for the multi-bed reactor is the recovery of process wastewater from the urea plant. Urea process wastewater is the condensate produced by the evaporation system of the urea unit, containing 5%
_NH3 , 0.8% urea, and 2.1% CO2 _by mass, with a flow rate of approximately 50 t/h and a temperature of 90–100℃. After deep hydrolysis and desorption, the NH3 _and urea concentrations are both below 5 mg/L, and it was designed for use as boiler feedwater. However, due to unstable operation of the desorption system and frequent equipment leaks in previous years, NH3 _and urea levels frequently exceeded the standards, and the urea wastewater was used for heating in the plant area, discharged on-site in summer. Now, it is no longer used for winter heating. After equipment maintenance and technical upgrades during the major overhaul in May 1998, the urea wastewater's compliance rate reached over 95%, with NH3 and urea concentrations generally _around 2 mg/L, making it suitable for recovery. Furthermore, its water quality is much better than that of the synthesis process condensate and softened water.

The recovery plan involves adding one plate heat exchanger to the urea section, and two conductivity meters to the urea and demineralized water sections respectively. The demineralized water is also equipped with a temperature monitoring meter. Circulating cooling water is used to cool the urea waste liquid to 40°C, which is then collected in the 222B softening water tank. The softened water is then pumped to the mixed bed and co-bed treatment processes before entering the 222A demineralized water tank for reuse.
After the pipeline was installed in September 1999, our plant began recovering and reusing the urea vapor condensate, reducing the mixed bed load to 2.1 mmol/L and achieving a maximum cycle water production of 644 Ot. Table 1 compares the mixed bed water production before and after urea waste liquid recovery.

Reducing the crosslinking degree of resin
The resins used in our plant's multi-bed system are all styrene and divinylbenzene. Under the action of an appropriate amount of porogen, different functional groups are introduced to form copolymer spheres with different properties. The content of divinylbenzene as a crosslinking agent in the resin determines the amount of exchangeable ions stored per unit volume of resin. Reducing the crosslinking degree of resin can objectively increase the exchange capacity of resin ^[1] , increase the water production of multi-bed system, and the lower the crosslinking degree of resin, the larger the pore diameter of resin particles, which is more conducive to the particle diffusion of ions, which can increase the ion exchange rate of resin and shorten the time required for exchange. However, if the crosslinking degree is reduced indiscriminately, the mechanical strength will decrease as the resin pore size increases, which will affect the service life of resin and restrict the water production capacity.
According to the operation of multi-bed system, it is recommended that the manufacturer appropriately reduce the crosslinking degree on the basis of ensuring the mechanical strength of resin, so that the crosslinking degree is reduced from 7% to about 5.5%, the working exchange capacity of resin is improved to a certain extent, and the water production capacity of multi-bed system is increased to 130t/h.

Improving Regeneration Efficiency
Based on the influent conditions of the multi-bed regeneration system, we conducted experiments to adjust the regeneration concentration and time. In March 2000, we determined the short regeneration conditions to be: 4% acid by mass, 25 min; 1.8% alkali by mass, 60 min, achieving good results.
Operationally, the multi-bed resin was periodically wiped and revived to restore its working exchange performance.

Transformation effect

After implementing several measures, the operation of the multi-bed desalination bed was significantly improved, the cycle water production was greatly increased, and the monthly regeneration frequency of the multi-bed desalination bed decreased from 71 times to 32 times, saving on acid, alkali, and raw water consumption. The acid and alkali consumption per ton of desalinated water decreased from 1.07 kg and 0.52 kg to 0.6 kg and 0.295 kg, respectively; the raw water consumption per ton of desalinated water decreased from 450 kg to 225 kg. 1200 tons of urea waste liquid were recovered daily, saving raw water costs of (1.56 × 1200) 1872 yuan/day, and reducing the cost of desalinated water from 3.4 yuan/ton to 2.16 yuan/ton.
The quality of the effluent from the multi-bed desalination bed was improved to a certain extent, with conductivity decreasing from 1.01 μS/cm to 0.45 μS/cm, and the number of silica leakage incidents significantly reduced, improving the effective utilization rate of the multi-bed desalination bed and stabilizing desalinated water production. The results are shown in Table 2.