Countercurrent exchanger
Countercurrent exchangers , also known as countercurrent filters, are used to exchange unwanted ions in liquids for bound ions and enable very good and economical full desalination. In water technology, this designation indicates the design of the ion exchange columns. Since the 1970s, this new development has largely replaced the direct-current exchangers as working filters (cation and anion exchangers) in a full desalination plant. For more information on the theory and application of ion exchange, see Ion exchangers and ion exchange columns .
Procedure
In order to improve the economic efficiency of ion exchangers, various techniques have been developed, namely:
- Compound regeneration (series connection of 2 filters either with weakly and strongly dissociated ion exchangers or only strongly dissociated ion exchangers that are regenerated together)
- Layer bed exchanger (use of 2 types of resins in a filter, for example, weak cation exchangers and strongly acidic)
- Countercurrent exchanger
However, optimal improvements without other disadvantages could only be achieved with countercurrent exchangers. In the following, therefore, only the countercurrent processes are discussed in more detail.
Basics of the countercurrent process
Co-current exchangers are flowed through in the same direction during operation and regeneration , counter-current exchangers in opposite directions. The main advantage is that during regeneration, the resin layers that were last flowed through during operation only come into contact with a pure regeneration solution. In contrast, during the regeneration of the cocurrent filters, these resin layers are only treated with a contaminated regeneration solution. This contamination occurs when the ions absorbed during loading - cations or anions, depending on the type of resin - diffuse into the regeneration solution. Complete regeneration of the resins is only possible with a pure solution. Incompletely regenerated resins cause a counter ion effect and thereby a deterioration in the quality of the pure water during operation. In order to mitigate and compensate for these disadvantages with direct current filters, significantly higher amounts of regenerant are required. Contact of the last resin that flowed through during operation with the contaminated regeneration solution must therefore be avoided. This is achieved if the exchange resins are flowed through in opposite directions during operation and regeneration. Two different directions are possible for the flow: water flow from top to bottom or from bottom to top with the respective opposite flow direction during regeneration.
Top-to-bottom operation
One of the first countercurrent processes successfully introduced in practice in the 1960s was the Permutit process . The flow through the exchanger is from top to bottom during operation and from bottom to top during regeneration. Since the free space above the resins was severely limited in order to avoid shifts during regeneration, the backwashing in the working tank that was required before regeneration could not take place. For this purpose, a hydraulically connected and lockable backwash tank was installed above the working tank. This method was very complex and therefore expensive in terms of apparatus. The procedure was therefore not able to establish itself in practice and was replaced by the subsequent procedures.
Bottom-up operation
Floating bed process
Another process was the floating bed process developed and patented by Bayer AG (Leverkusen) in the early 1960s . In this process, the container is only filled to about 70% with resins and the water flows through it from bottom to top. Depending on the flow rate, both a fixed bed - the uppermost resin layer that presses against the upper nozzle base - and a lower floating bed formed during operation. In this floating bed, resins are not tightly packed, but in motion. When the filter is switched off, the suspended resins start to move and settle on the lower nozzle base. Backwashing for loosening up before regeneration is therefore not necessary. Disadvantages are the lack of stratification in the floating bed and the possible rearrangement of the resins when the system is shut down. The compact layer structure of the resins required for a good countercurrent effect is partially canceled out by these rearrangements. Frequent start-up and shutdown of the exchanger before regeneration is not permitted, since the sodium slip in the cation resins then increases and the pure water quality is deteriorated. These disadvantages can be reduced by modifying the method, such as using a circulation pump for operation and / or increasing the resin volume in the filter. This floating bed process in the originally patented version is hardly used any more.
Fixed bed process in countercurrent regeneration
The disadvantages of the fluidized bed process are avoided in the further development of the countercurrent fixed bed process . The following is a sketch of this exchanger with the internals and pipe connections necessary for the process.
These fixed bed processes with countercurrent regeneration must not be confused with the classic fixed bed process, which is regenerated in cocurrent. Exchangers designed according to the countercurrent fixed bed process are almost completely filled with resins, only their swelling capacity is taken into account. The change in volume of a strongly acidic cation resin, for example, is about 5% between the loading and regeneration form. The changes in volume of the anion exchangers are, at approx. 15-25%, significantly greater, but minor shifts in these resins do not cause as high counterion effects as with strongly acidic cation resins. Shifts in the cation resins, which lead to a significant deterioration in the operating values, cannot occur when these fixed-bed filters are switched on and off. The necessary loosening of the resin bed before regeneration is achieved anyway. The use of monodisperse resins has an advantageous effect. The use of inert resins, which otherwise serve as a floating drainage layer between the upper resin distribution system and the resin bed, can be dispensed with with these resins. In practice, more frequent external backwashing and cleaning of the resins is only necessary for applications with post-precipitation in the resin bed or inadequate prefiltration. As a rule, it is sufficient to compensate for the resin loss that occurs due to mechanical wear (approx. 2 - 7% per year) by adding new resin (every 1 - 3 years).
The possibility of arranging two or more chambers in one container is particularly advantageous. In this way, for example, weakly and strongly acidic cation resins can be filled into a container with 2 chambers. Mixing is prevented by a separating nozzle base. A comparable result is achieved by switching cation exchangers in series or in series in two separate columns, but the investment costs for this are significantly higher. With 2-chamber exchangers, an ion exchange can be achieved with almost theoretical chemical expenditure, little washing water expenditure and, at the same time, low ion slip .
By operating from the bottom up, solutions with a higher density can also be desalinated or re-salted without any problems by redeployment. For example, this makes the softening or salting of sugar solutions much easier and, depending on the concentration, only possible.
Further developments of the floating bed process are lift bed and rinsing bed processes . With these processes there is also a fixed bed in each filter during operation. 2-chamber filters are used in the lift bed process. As with the fixed bed process, the upper chamber is filled with resin as much as possible, taking into account the resin swelling. A fixed bed is therefore available during operation. The lower chamber contains only about 70% resin, comparable to the floating bed. A complete fixed bed is therefore not formed during operation.
With the rinsing bed method, both a perforated base and a drainage system are placed in a 1-chamber filter at a height of about two thirds. Resin is only filled in up to a little above the perforated base. The upper third of the chamber is therefore free of resin. During operation, the treated water exits the filter via the drainage. The resin forms a fixed bed up to the outlet drainage.
Depending on the manufacturer of the system and / or the supplier of the ion exchange resins, different sales names are used for such systems based on the fixed bed process.
Here are some of them (the list is not complete, some are no longer available):
- Amberpack (TM) process
- Econex process
- Lift bed method
- Rinsed bed method
- Mannesmann countercurrent process
- Press bed process
- ISEP (TM) process
regeneration
For regeneration, the same dilute solutions - acids / bases / salts - are used for the countercurrent as for the cocurrent processes. Only the concentrations of the regeneration solutions are slightly different. For hydrochloric acid, significantly higher concentrations of up to 10% are used. In the case of sulfuric acid, the concentration is also slightly higher. However, progressive dilution almost always has to be used in order to avoid precipitation of calcium sulfate. The upper resin layers are to be regenerated with at least 7-10% acid towards the end of the regeneration. A particularly effective use of regenerant can be achieved with an additional intermediate dilution in the resin bed.
While higher concentrations are used with the cation resins, with the anion resins, especially for the lower layers, max. Regeneration liquor concentrations of <2.8% required. The reason for this is the silicon dioxide (SiO 2 ) absorbed during operation . This reacts with the regeneration liquor to form sodium hydrogen silicate according to the following equation:
- Anion resin loaded with silica reacts with caustic soda to form regenerated resin + sodium hydrogen silicate; R = basic structure of the ion exchanger
The alkalinity of the sodium hydrogen silicate formed is strong enough to regenerate loaded weakly basic anion exchangers. In the deeper resin layers, these resins are converted into the amine form with regression of SiO 2 . The equation for this is:
- Sodium hydrogen silicate regenerates anion resin loaded with chloride to the base form, sodium chloride, silicon dioxide + water
The greater dilution of the regeneration liquor increases the flow rate in the resin bed. Precipitation of a SiO 2 gel that adsorbs Na ions and makes the washing out behavior of the regeneration liquor difficult is avoided. In addition to increasing the flow, the regeneration liquor is also heated. This also counteracts precipitation of the SiO 2 .
While countercurrent flow technology is much more economical with full demineralisation, improvements in the regenerant requirement are achieved with softening, but the advantages are not comparable.
See also
Individual evidence
- ↑ G.Siegers, G.Wuttke; Lift bed and rinsing bed processes, novel ion exchange technologies; in: VGB Kraftwerktechnik , 63, issue 1, January 1982, p. 42
- ↑ G.Siegers, G.Wuttke; Lift bed and rinsing bed processes, novel ion exchange technologies; in: VGB Kraftwerktechnik , 63, issue 1, January 1982, p. 45
literature
- Power Engineering, Nov. 1971, pp. 44-47
- E. Tscherning , VGB Kraftwerkstechnik, year 1961, issue 4, pp. 340-348
- G. Siegers et al. G. Wuttke , VGB Kraftwerkstechnik, year 1982, issue 1, pp. 42-48
- Bernard Causse , L'EAU, L'Industrie, Les Nuisances, No. 89 Jan. – Fev. 1985, pp. 37-40
- Amberpack (TM) brochure , Rohm and Haas Comp., Vol. 2000