Mixed Bed Demineralisation Plant
Derwent Water Systems design and build bespoke mixed bed polishing units to suit all of our customers requirements.
Cation and Anion resins are combined in a GRP or stainless steel vessel to produce ultra pure water.
Usually a mixed bed polishing unit contains strong acid Cation (SAC) resin and strong base Anion (SBA) resins combined to a ratio of approximately 40% Cation resin to 60% Anion resin. (All resin volumes are dependant upon the incoming water analysis and the required quantity of treated water).
Mixed bed polishers are typically installed when a water quality of less than 1 micro ohm conductivity is required. Derwent Water Systems can install mixed bed polishers after a standard Reverse Osmosis or Demineraliser, to produce ultra high quality water.
The Cation resins in our mixed bed polishers are of a higher specific gravity and settle at the bottom of the vessel whilst the Anion resin will automatically float to the top.
After a service cycle, each resin is regenerated as listed:
(SAC) : Strong Acid Cation resin is regenerated with hydrochloric or sulphuric acid.
(SBA) : Strong Base Anion is regenerated with caustic soda.
Please note: The above regenerate strengths refer to the values before being diluted by the mixed bed polisher educator systems.
Please contact us for further details.
This process differs essentially from the separate bed system in that the two strong resins, the Cation and the Anion, are combined in a single vessel. The two resins are intimately mixed by agitation with compressed air. The two resins form a homogenous mixture. The whole bed behaves like an infinite number of Anion and Cation exchangers in series.
To carry out regeneration, the two resins are separated hydraulically during a precisely controlled backwash phase. As the Anion resin is the lighter, it rises to the top, while the heavier Cation resin falls to the bottom.
When the resins have been separated, each is regenerated in turn, with caustic soda and a strong acid. Any excess regenerant is displaced by rinsing each bed separately. After partial emptying of the vessel, the two resins are remixed with compressed air. Rinsing is completed and the vessel is then ready for a fresh cycle.
The advantages of mixed bed systems as compared with separate bed systems are as follows:
- The water obtained is of very high purity and its quality remains constant throughout the cycle (its conductivity is below 0.2µs/cm, its silica level is less than 20mg/l),
- The pH is almost neutral
- Rinse water requirements are very low
The disadvantages of mixed bed systems are:
- A lower exchange capacity and
- A more complicated operating procedure as the separation and remixing processes must be carried out precisely.
Correctly designed mixed bed exchangers can be used directly on low tds raw waters, or water which has under gone prior treatment such as Reverse Osmosis, distillation or condensed water. A complex system of ion exchangers can be replaced by a single mixed bed.
Special configurations have also been used as follows:
SCR + | CO2 removal | + MB
Softener + MB
SCR + WBR + | CO2 removal | + MB: useful arrangement for a water containing many strong Anions.
However, the mixed bed exchangers are most often used in polishing treatment.
Installation equipped with a polishing system
The quality of water flowing out of a primary system, whatever its composition, is determined by the ion leakage from the Cation exchanger. This ion leakage varies, depending upon the properties of the raw water and the rate of regeneration.The quality of the demineralized water obtained is not sufficient for certain uses such as that of feedwater for very high pressure boilers and various applications in the chemical, nuclear or electronics industries.Therefore, it has to be further treated in a system known as a polishing plant.
The ion leakage from the Cation exchanger is converted to a free base on the Anion exchanger, which therefore entails a silica leakage from the latter. As a result, a polishing system must necessarily contain a strong acid Cation exchanger and a strong base Anion exchanger.
The polishing system may be arranged so that there are two columns in series with regeneration taking place in the direction SBR2 SBR1 and SCR2 SCR1. In these arrangements the polishing exchangers are perfectly regenerated and the quality of the demineralized water is excellent (the conductivity is less than 1µs/cm-1, its silica level ranges between 5 and 20mg/l at 25°C, and have silica leakage’s that are considerably below 10mg/l, are preferred. A polishing mixed bed, due to the low volume of inflowing ions, is regenerated only every 5 to 10 cycles of the primary system. In some applications, it may be sufficient to have as a polisher a weak or a strong acid Cation exchanger designed to neutralise the caustic soda leakage from the Anion exchanger in the primary system. By using the polisher exchanger, known as a “buffer filter”, it is possible to obtain water that is virtually free of Cations (with a conductivity below 1µs/cm-1) and has a pH value of between 6 and 7.