Resins for Condensate Polishing Plants

Historical Use of Macroporous Resins
Companies operating a condensate polishing plant (CPP), particularly those operating older systems, have historically and exclusively used macroporous cation and anion resins in a mixed bed.

The perfect CPP mixed bed resins require the following characteristics:

• Excellent kinetics to cope with the high operating velocities through the bed.

• Robust structure that can cope with the physical attrition associated with a resin transfer. The service units are typically located in the heart of the power station and regeneration system in the water treatment plant. Some power stations transfer resins hundreds of meters (yards) to and from these locations using demineralized water as the transfer media.

• Osmotically stable to cope with vigorous regenerations required to remove suspended solids (crud) and the high ionic loading per cycle that requires high regeneration levels.

• Perfect separation of the cation and anion components on backwashing and isolation of the two components for separate regeneration. This ensures minimal cross-contamination, which affects treated water quality.

• Good regeneration efficiency in obtaining the extremely high treated water quality required for modern, high-pressure boilers.

• Some resistance to surface fouling material, such as dissolved organics, suspended solids and, in some cases, oil.

• High working capacity.

When condensate polishing was introduced in the last century, the obvious resins of choice were macroporous as they best met the above criteria. Higher divinylbenzene content makes them robust — not only regarding resin transfer — but concerning resistance to osmotic shock attrition (OSA). These, along with greater surface area, are critical to helping delay the effect of surface fouling. The importance of having good resistance to OSA is best shown clearly by the early testing of resins for CPP, devised by companies like the CEGB in the UK. Testing and approval of CPP resins are very concentrated on physical strength and testing the resistance to OSA. The initial test protocol for macroporous resins was 500 cycles with no significant breakdown. Gel resins were only tested over 100 cycles due to their lower resistance to OSA.

As the initial CPPs were successful and improved power station reliability, particularly when condenser leaks occurred, this application quickly established macroporous resins. The only significant early change to these resins was the introduction of graded versions. The cation was made up of coarser beads and the anion finer beads to improve separation on backwashing, reducing cross-contamination and improving treated water quality.

At this stage, gel resins were absent from the application, despite their much higher breaking weight (Chatillon test comparison) and higher working capacity due to being easier to regenerate than higher cross-linked macroporous resins.

Now, over 50 years later, gel resins are considered for this duty in mixed beds, particularly for the cation component in many mixed bed systems and lead cation units before the mixed bed. 

This change was possible because leading resin companies — like Purolite — made great strides in developing a new type of gel cation resin described as “super gel.” These resins have modified DVB cross-linking, more evenly spaced within the beads. Super gel resins, such as Purolite SGC650H (super gel 10% DVB cation), retain all the attributes of conventional gel resins, but have the added advantage of improved resistance to osmotic shock attrition, thus making them a viable option. 

Additional factors for the use of gel resins
Early condensate systems used ammonia to elevate the pH to protect the system from corrosion. There was a gradual worldwide trend to operate at a higher pH to provide even greater protection. However, increasing dosing of ammonia to achieve a higher pH significantly increased cation loading when operating CPPs in H-OH cycle. It quickly became evident that a higher cation capacity was the only way of maintaining cycle lengths between regeneration (unless ammonia cycle operation is possible, which will be covered elsewhere).

Macroporous resins are lighter than gel resins, because of the pores. A gel cation will separate more easily from a macroporous anion in CPP mixed beds, giving better performance from the system with less cross-contamination.

The main ionic load on a well-operated CPP system is cationic in nature. Hence there is often more cation resin in CPP designs. Many CPPs use twice as much cation resin as anion resin in each column. However, under condenser leakage conditions, the anionic load can be significant — regardless of the cooling media — whether it be seawater, freshwater or air. Unfortunately, early anion resins had much lower ion exchange capacities than cation resins, and with less resin anion present, anion capacity is an issue on some plants. Some old sites and new installations have subsequently looked to employ “super gel” anion resins like Purolite SGA550 with the “super gel” cation.

There is some good news for operators. Condensers today are far more robust and reliable. Seawater condensers are now often made from titanium, so many sites do not encounter leaks, unlike the old admiralty brass condensers, which inevitably developed leaks.

With anion capacity issues becoming less important, using a heavier gel cation with a macroporous strong base anion to achieve complete separation on backwashing has become more common in many CPP mixed beds.

Conclusion
In studying this advice and knowing the performance and limitations of your own CPP, it should be possible to decide what type of resins are best suited for your site. If you use an existing resin combination — whether a macroporous-based mixed bed system or gel cation with macroporous anion — and it works well, giving a good performance and long life in the duty, then it is suggested to stay with that generic resin combination.

Finally, a macroporous cation is never used with a gel anion in a mixed bed. This is explained by the fact that gel anion resins are heavier and complete separation from the macroporous cation is considered impossible using conventional mixed bed gradings. It would likely lead to unacceptable cross-contamination.