Mixed bed filter
Mixed-bed exchangers , also known as mixed-bed filters , are used in water treatment when fully demineralized water is to be generated from drinking or industrial water with ion exchangers . The mixed-bed filters remove the remaining ionogenic compounds from a water that are still present as slip after the upstream cation and anion exchangers. They also take on a protective function if ions break through if the upstream filter is disturbed. The filters are therefore also called police filters. This name is also used in plant construction for filters that do not take on any ionic residual cleaning or ionic protective cleaning function. The following is a sketch for a mixed-bed filter with the typical fittings and pipe connections.
The construction of the mixed-bed filter is largely comparable to that of direct current exchangers . While the latter contain either only cation or anion exchange resins, at least two types are used in mixed-bed filters, namely one cation and one anion exchange resin. With these two types of exchange resin, electrolytes dissolved in the water can be completely removed. Since two different ion exchange resins are present in one container, an additional drainage system must be installed in the case of mixed-bed filters that are not regenerated externally so that the resins can be regenerated spatially separately .
Development of full desalination
Until the development and industrial production of strongly basic anion exchange resins, natural water could only be fully softened or largely partially desalinated using the ion exchange technique. After strongly basic anion exchange resins became available from around the beginning of the 1950s, complete desalination using ion exchangers was possible with simple technology and a high degree of efficiency. Large amounts of such fully demineralized water are required above all for the operation of modern high-pressure boilers in power plants. Before the development of the mixed-bed filter, fully demineralized water with a conductivity of less than 1.0 µS / cm could only be generated with great effort, for example with systems with the following circuit:
- 1st stage cation exchanger with strongly acidic exchange resin
- 2nd stage anion exchanger with weakly basic exchange resin
- 3rd stage cation exchanger with strongly acidic exchange resin
- 4th stage anion exchanger with strongly basic exchange resin
Without a division into two cation and two anion exchangers, only a residual conductivity of> 1 µS / cm could be achieved due to the counterion effect. For a further improvement of the residual conductivity to clearly <1.0 µS / cm, the additional arrangement of a slip filter after a desalination line with direct current filters was necessary.
Normally the majority of the residual conductivity of over 90% is caused by the sodium slip. After a strongly basic anion exchanger, the sodium is present as dissociated sodium hydroxide . A downstream slip filter with weakly or strongly acidic cation exchange resins safely removes this slip. This significantly reduces the conductivity. The disadvantage of such slip filters is that, although they significantly reduce the conductivity by removing the sodium ions, they leave the anion slip unchanged. In addition to the chloride ions, this anion slip consists mainly of weakly dissociated anions of, for example, silica and humic acid . Removal of this anion slip could only have been carried out with a further slip filter with strongly basic anion exchange material. With the introduction of the mixed-bed filter in the treatment technology for water, the treatment to fully demineralized water became possible without higher residual ion slip and with significantly less system effort.
After the introduction of the countercurrent exchanger , however, it was already possible to generate desalinated water with an average conductivity of <0.5 µS / cm without a slip filter. Before that, mixed-bed filters were generally required, which also took over the residual desalination as the last stage . With the introduction of countercurrent technology , combined work and police filters became possible, which take on both the function of desalination and the function of the slip filter with economical chemical expenditure . In practice, special types of filters with at least 2 to 3 chambers in a filter column are successfully used for this.
The adjacent procedural sketch shows a sandwich filter . It is a multi-chamber filter that works on the countercurrent principle and contains cation and anion exchange resins in the separate chambers. Such filters require the use of an additional inert resin. A specifically light and buoyant inert resin, arranged in the area of the wastewater drainage, separates the incoming and outgoing regeneration water from the two ion exchange resins. With 3-chamber filters instead of 2-chamber filters, cation exchange resin is used again in the upper chamber. In this way, a possible, but already very small, cation slip is safely removed. The deionized water quality after such a filter corresponds completely to the values that can also be achieved with normal mixed-bed filters after a desalination line.
The function of the multi-step filters is comparable to the sandwich filters . However, these are designed according to the criteria of the floating bed process . With both filter types in the 2-chamber design, low-electrolyte pure water can be generated with little chemical expenditure, even without an additional mixed-bed filter. Residual conductivities of less than 0.06 µS / cm at 20 ° C are achieved.
Mixed bed filter basics
If strongly acidic and strongly basic ion exchange resins are mixed in a filter bed, then this corresponds to a series connection of very many cation and anion exchangers for the ion exchange. This inevitably minimizes the ion slip and achieves a complete ion exchange. The following requirements must be met for this maximum exchange function:
- extensive regeneration of the two types of resin
- optimal mixture of the two resins
Requirements for an optimal mix are:
- a sufficient amount of air is used
- the mixing time is sufficient
- the proportion of water in the resin bed during mixing is limited
The above-mentioned requirement of extensive regeneration of the ion exchange resins inevitably also limits the permissible loading condition. The ion exchange is an equilibrium reaction. According to the following equation for a strongly acidic cation resin in the regenerated H-form and the loaded Na-form:
If the content of free Na ions in pure, neutral water (pH = 7.0) should be a maximum of 1 ppb, the following values are specified for the loading and regeneration status:
- Concentration of hydrogen ions = 1 · 10 −7 mol / l
- Concentration of sodium ions = 4.3 · 10 −8 mol / l
The sodium concentration in the water depends on the equilibrium coefficient K of the cation exchange resin used. For a normal cation exchange resin, for example, this is 1.5. This stated value for the equilibrium coefficient K is valid for a polystyrene sulfonic acid resin with 8% divinylbenzene as a crosslinking agent. The necessary degree of regeneration for such an ion exchanger is calculated as follows:
- K = (R * Na) * (H + ) / (R * H) * (Na + ) = 1.5
- (H + ) = 1 · 10 −7 mol / l
- (R Na) = (1 - R H) mol%
- (R · H) / (1 - R · H) = 1 / 1.5 · 1 · 10 −7 / 4.3 · 10 −8 = 1.53
- (R · H) = 1.53 (1 - R · H)
- (R · H) = 0.606, in mol% this corresponds to approx. 61% of the total capacity of the exchanger
At least 61% of the exchanger must be in the H form so that a value of 1 ppb for the sodium ions in the water is not exceeded via the equilibrium. The same requirements must be taken into account for the anion exchanger and the equilibrium for OH and Cl ions according to the following equation:
The given high degree of regeneration of the exchange resins limits the use of a mixed-bed filter as a working stage , since water can only be desalinated with a significantly higher amount of chemicals .
Quality of the ion exchange resins
Both gel resins and the mechanically stronger macroporous types are suitable for use in mixed-bed filters. Basically only strongly acidic cation exchangers and strongly basic anion exchangers type I are used. The operating temperature, the quality of the inlet water as well as the resin bed height and filter speed are decisive for the choice of types. The correct grain size distribution and a clear difference in spec. Weights, so that a problem-free separation can be achieved by backwashing. In order to be able to observe the separating layer more clearly, lighter-colored cation exchange resins with darker anion exchange resins are usually used. However, cation exchange resins in particular become markedly darker after a long period of operation due to the absorption of impurities. This will later reduce the original color difference of new resins.
Regeneration technology
In order to achieve a largely separate regeneration of the two replacement resins, two processes are common. In the first process, which is more complex but seldom used, the entire resin is hydraulically rinsed out of the mixed-bed filter in an external regeneration station, where it is spatially separated before regeneration. An external regeneration station usually consists of 3, sometimes only 2 containers. With the 3 container variant, these are: 1. Separation and cation resin regeneration container, 2. Regeneration container for anion resin and 3. Resin mixing and storage container. Due to the extensive spatial separation of the two types of exchangers prior to regeneration, some of the problems, such as heavy loading with the wrong chemical in each case, are avoided with this technology. This can easily occur in the internal process if the required pressure and flow conditions in the resin bed are not optimally maintained.
In the second method, which is more favorable in terms of investment costs and is used much more frequently, the resins are regenerated in the mixed-bed filter. The resins are backwashed before regeneration. In addition to removing solid particles taken up during operation, the filter bed is loosened up and separated into the heavier lower layer of the cation resins and the upper, lighter layer of the anion resins. Only after this separation can the actual regeneration be carried out by adding the chemical solutions. Normally, 4-6% dilute hydrochloric acid is used for the cation resin and 2.5-4% dilute sodium hydroxide is used for the anion resin. Sulfuric acid is used more frequently instead of hydrochloric acid, especially in systems for condensate treatment. This is to prevent chloride ions from getting into the pure condensate via regeneration.
Two different methods are also used to carry out the internal regeneration. In the first method, the chemicals flow one after the other through the respective resin layer. First, the lye is passed over the top anion resin layer. It leaves the container either via the drainage system arranged in the separating layer between the anion and cation resins or via the lower container outlet. With the latter technique, the cation resin is inevitably completely converted into the sodium form before regeneration with acid.
With the other method, alkali and acid are passed over the resins at the same time. Both alkali and acid leave the container together via the drainage system. The advantage of this regeneration technique is the shorter regeneration time and the reduced possibility of alkali or acid getting into the wrong resin layer.
After the chemical treatment has ended and the regeneration solutions have been sufficiently washed out of the resins, the water level in the container is lowered to a few centimeters above the resin bed. Then both resin layers are mixed intensively with air for a sufficiently long time. After filling and a sufficient run-in period before operation, fully demineralized water can be generated again with the mixed-bed filter.
The trio bed is a special form of the mixed-bed filter . This system is used for both internally and externally regenerated mixed-bed filters. The use of an inert 3rd resin component was already described in a German patent at the beginning of the 1950s. It was first used successfully in practice in 1976 in a nuclear power plant in Belgium. In terms of design, this type is comparable to normal mixed-bed filters. The additional inert resin for the resin filling has a specific gravity which is higher than that of the anion exchange resin but lower than that of the cation exchange resin. After backwashing, the inert resin forms a separating layer between the two other types of resin in the area of the drainage system . During regeneration, this largely prevents contact with the wrong regeneration solution.
Individual evidence
- ↑ a b H. R. Brost, F. Martinola: Exchange processes in mixed bed filters ; In: VGB Kraftwerkstechnik 60, 1980 January, issue 1, p. 60.
- ^ H. Brost, F. Martinola: Process engineering of the fine purification of water by ion exchange ; In: VGB Kraftwerkstechnik 66, 1986 February, issue 2, p. 167.
- ^ H. Brost, F. Martinola: Process engineering of the fine purification of water by ion exchange ; In: VGB Kraftwerkstechnik 66, 1986 February, issue 2, p. 168.
- ^ RR Harries, NJ Ray: The Mixing Process in Ion Exchange Mixing Beds ; In: VGB Kraftwerkstechnik 60, 1980 September, issue 10, p. 722.
- ↑ HR Brost, F. Martinola: Exchange processes in mixed bed filters ; In: VGB Kraftwerkstechnik 60, 1980 January, issue 1, p. 53.
- ↑ HR Brost, F. Martinola: Exchange processes in mixed bed filters ; In: VGB Kraftwerkstechnik 60, 1980 January, issue 1, p. 54.
- ↑ HR Brost, F. Martinola: Exchange processes in mixed bed filters ; In: VGB Kraftwerkstechnik 60, 1980 January, issue 1, p. 57.
- ^ Gary D. Jones, Roger L. Long, An Improved Condensate Polishing Plant ; In: Combustion, 1972 July, p. 18.
- ^ MA Sadler, JC Bates, GR Mills: Use of a tri-bed condensate cleaning system ; In: VGB Kraftwerkstechnik 61, 1981 March, issue 13, pp. 221-233.
- ^ Permutit AG, Berlin, Germany ; In: Patent DBP 971 771, 1952.
- ↑ Product brochure from Duolite International; In: Triobed ; PWT 8202 A Oct. 82.