The purpose of condensate polishing is to remove metal corrosion products and trace organic substances from the condensate of steam turbines and various hydrophobic systems to a certain standard before reuse. Therefore, condensate polishing has become an indispensable component of large capacity units.
Due to the high temperature of condensed water and the large amount of water processed, the requirements for resin are different from those for ordinary resin used for makeup water. The basic requirements are that the particle size of the resin should be uniform, with good strength, good temperature resistance, and fast exchange speed (usually greater than 100m/h). The requirements for its physical and chemical properties can be found in relevant standards. Some condensate treatment systems are equipped with pre hydrogen treatment systems to reduce the burden on the condensate mixed bed. The pre hydrogen bed is actually a strong acid cation bed, whose main function is to remove ammonia, iron oxide, and other cations from the condensate water, and extend the operating cycle of the subsequent condensate mixed bed.
The regeneration method of the front cation bed is the same as that of the ordinary make-up water cation bed. If it can be combined with air scrubbing, the effect will be better.
The ratio of anion and cation resins in the high-speed mixed bed of condensate is determined based on the water quality pollution of the condensate and the operating conditions of the unit. In condensate systems with high ammonia content (pH>9.1), a ratio of 1:2 for anionic and cationic resins is more suitable; PH is around 7, and the salt content is not high or ammoniation is used for operation. A 1:1 ratio of anion and cation resins is more suitable; The condensate water has a high temperature, high salt content, or the cooling water is seawater. The ratio of anion to cation resin is 3:2.
There are currently two operating modes for condensate polishing mixed bed resin, namely H-OH type and NH4-OH type mixed bed. The effluent quality of the H-OH mixed bed is excellent, which can make the hydraulic conductivity of the condensed water below 0.1us/cm. However, its disadvantage is that it also removes NH4+that should not be removed. This NH4+is added to reduce the corrosion of thermal equipment, and its content is often higher than other impurities. So, the exchange capacity of the H-OH type mixed bed is mostly consumed by it. In order not to remove effective NH4+, the NH4-OH type mixed bed has emerged. It is a mixed bed water production mode that continues to operate in the NH4-OH type mode after NH4+leaks out after the H-OH type mixed bed fails.
Difference between H-OH type and NH4-OH type mixed beds: H-OH type and NH4-OH type mixed beds are very different in chemical equilibrium. Take NaCl as an example to illustrate:
When using an H-OH mixed bed, the exchange reaction is as follows:
RH+ROH+NaCl→RNa+RCl+H2O(1)
The product of this reaction is a weak electrolyte H2O, so the reaction is easy to proceed. However, when using NH4-OH mixed bed, the reaction is as follows:
RNH4+ROH+NaCl→RNa+RCl+NH4OH（2）
Although the product of this reaction is also a weak electrolyte NH4OH, it is much stronger than H2O. The reaction of (2) is much more difficult to carry out than that of (1), and there is a greater tendency for reversible reactions, making it easy for Na+and Cl - to leak out of the effluent.
From the above equation, it can be seen that in order to reduce the leakage of Na+and Cl - in the NH4-OH mixed bed, it is necessary to minimize the amount of RNa and RCl in the resin, which means that the regeneration degree of the anion and cation resins should be high.
Based on the estimation of the selectivity coefficient of the above types of resins, it was found that in order to achieve a Na+=1mg/l and Cl -=1.5mg/l effluent from the mixed bed, the regeneration degree of the cation resin in the H-OH mixed bed should be greater than 54%, and the regeneration degree of the anion resin should be greater than 11.6%; For the NH4-OH mixed bed, it is necessary to ensure that the regeneration degree of cation resin is greater than 99.5%, and the regeneration degree of anion resin is greater than 95% to meet the effluent requirements of Na+=1mg/l and Cl -=1.5mg/l. However, the amount of residual Na and Cl resins in the typical H-OH mixed bed far exceeds the above range. Therefore, when the H-OH mixed bed is converted to the NH4-OH mixed bed for operation, both Na+and Cl - are easily penetrated and leaked. Therefore, in the NH4-OH type condensate mixed bed, there is a high requirement for resin regeneration, and the corresponding quality requirements for acid and alkali are particularly high (especially for the quality of alkali solution).
The condensate mixed bed generally adopts an in vitro regeneration method, and the parameters are shown in the table below:

The exchange principle of the condensate mixed bed is completely the same as that of the make-up water treatment mixed bed. To ensure the water quality and operating time of the effluent, it is necessary to ensure the layering and regeneration degree of the anion and cation resins. Therefore, condensed water resins are generally regenerated in vitro. The current separation devices and methods for anion and cation resins are relatively advanced. Due to their ability to ensure resin separation, regeneration is relatively simple and will not be elaborated.
The effluent quality indicators of the high-speed mixed bed for condensate are as follows: