External Regeneration in Condensate Polishing Plants (CPP)

There are three principal reasons why condensate polishing regeneration of mixed bed units is routinely performed outside of the main units and completed in a separate regeneration station. These are:

• Flow rates

• The need to both cope with suspended solids as well as the ionic loading

• Extremely high-treated water quality required on modern, high-pressure boiler 

Flow Rate
There are large differences in flow rates between the service operation and regeneration. For example, on a 500MW power station, the circulating condensate flow rate may be approximately 1000 m3h per hour, while regeneration flow rates (caustic injection) could be less than 10 m3h per hour on some designs.

The designs of distribution and collection systems are critical in obtaining the best performance from ion exchange resins. No one can design a system that gives uniform plug flow across a bed with a high turndown (100:1 ratio) between the largest and smallest flows. A separate regeneration system with resin transfer from the service vessels to the regeneration units is essential. Typical standard systems used in columns with in situ regeneration achieve (at best) a 10:1 ratio between flow rates and cannot be employed in high flow rate CPP.

Crud
Condensate systems not only have ionic contamination but suspended solids contamination captured by the resin bed during normal operation. This is often referred to as crud and can be present at high levels after startup if a vigorous cleaning regimen is not adopted to remove the crud before regenerating the resin.

Such a cleaning regimen cannot be applied to the high flow rate service unit. This is a major reason that a separate regeneration system is required, where vigorous air scouring and backwashing can be employed to remove the crud with sufficient freeboard space.   

Treated Water Quality
Since condensate polishing was introduced over 50 years ago, the design of boilers and turbines has become more advanced. Higher operating pressures and new plant designs have meant the treated water quality required from a CPP has become more difficult to achieve. It has reached almost the theoretical conductivity of pure water, which is historically associated with the electronics industry. As the systems are so pure, it has been shown that regenerating the cation and anion resins can be the biggest source of system contamination via the acid or caustic soda used to regenerate the resins.

In situ regeneration of such modern systems would never achieve the treated water qualities due to cross-contamination of the cation/anion resins and regenerant chemicals. Modern CPP resins require excellent kinetic performance and top-quality regeneration. The two mixed bed components must achieve "perfect” separation. Considerable work has been carried out over the past thirty years and has resulted in improved resin products and their performance in CPP. The advancements of special external regeneration systems (e.g., CONESEP® technology from Ovivo) and the introduction of special regeneration techniques ensure resin cross-contamination is subsequently been kept to a minimum.