Water Technologies & Solutions technical paper

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factors affecting counter flow ion exchange effluent quality Authors: W.S. Miller and M.B. Yeligar, Permutit Co.

18th Annual Liberty Bell Corrosion Course 4 - 1980

introduction Review of the available literature on the counter flow ion exchange operation reveals that there are many factors that affect the performance.1 These factors can be classified into two groups. They are:

• Equipment Parameters - directly related to the engineering design of the system, such as maintaining a compact bed and proper regenerant distribution.

• Operational and/or Process Parameters - directly related to the operational characteristics and the process expectation. These include quality of water used for acid dilution, displacement rinse, service endpoint, regenerant recovery, etc.

The following presentation will detail our findings as to the effect of each of the above factors on the quality of water produced by the counter-flow system. Much of the data was generated from tests started in 1974 and performed in our 48-inch diameter pilot unit, employing up flow regeneration with a water block flow technique. We feel the data presented here would be valid for any other method employed in achieving the basic principle, i.e. Water Block, Air Hold Down, Packed Bed, Split-Flow, and other engineering modifications.

definition and general principles In the water treatment industry, with reference to ion exchange applications, “Counter flow” is defined as service flow in the opposite direction of re-generation flow. In the conventional ion exchange unit operation, the service flow is carried out in the same direction as

that of the regeneration flow and is commonly referred to as co-flow unit operation.

During regeneration of a “co-flow” cation unit, hydrogen ions displace cations such as calcium, magnesium and sodium ions in the same direction as that of the service flow. At the normal 5-l0 lb/ft3 H2SO4 dosages used during regeneration, a small portion of unregenerated resin is left at the bottom of the unit. Since the ion exchange process is reversible, the hydrogen ions produced in the top portion of the bed during service displace the residual cations from the unregenerated portion of the resin present at the outlet end of the unit. This results in what is commonly termed as “leakage.” In the demineralization process, the leakage is mostly due to sodium because of its lower selectivity for a strong acid exchange resin compared to magnesium and calcium ions. Consequently, it is this sodium leakage that results in high anion conductivity (sodium hydroxide) during a 2-step co-flow demineralization process.

In the counter flow operated unit, whether it is up flow regeneration – down flow service or up flow service – down flow regeneration, the resin at the outlet end of the service flow is substantially regenerated to the hydrogen form. In other words, there should not be any partially regenerated resin, which can contribute to the leakage during subsequent service cycle contrary to the co-flow operated unit. Therefore, at comparable regenerant dosages, leakage is significantly lower in the counter flow operated unit resulting in improved quality of demineralized water produced compared to a co-flow operated unit.

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equipment parameters Compact Bed In the counter flow operated ion exchange unit operation, the effective resin bed must be maintained compacted at all times during regeneration and preferably also during service. In our developmental work, we observed that by deliberately fluidizing a well operated cation counter flow unit during regeneration, under the exact operating conditions, the sodium leakages from the fluidized unit were at least three times higher than those of well compacted beds. Further, such a disturbance in the bed will not only result in poor quality effluent for that particular cycle but also will continue to produce poor quality effluent for several cycles. We do want to point out, however, that keeping the bed compacted has some definitive vessel and process design requirements but is not as difficult a problem as one might be led to believe.

Proper Regenerant Distribution The regenerant introduced should come in contact with the entire volume of resin under consideration. The performance of the counter flow operated unit depends on how well the resin is regenerated and maintained at near complete conversion to the regenerated form at the outlet end of the service. If in an up flow regenerated unit, for example, the regenerant is not well distributed at the very bottom of the unit, a substantial degradation in performance will be observed immediately.

The effects of improperly distributing the regenerant are shown in Table 1.

Runs A and B were performed in the 48-inch diameter test vessel which had a hub lateral under drain. Runs C and D were duplicate tests with the only exception being the addition of a distribution media to disperse the regenerant so that it could effectively contact the resin at the outlet end of the service cycle. The sodium leakage data clearly indicates that for successful operation of the counter flow cation units the regenerant must be distributed properly throughout the entire bed.

Table 1: Effect of Proper Distribution of Regenerant in Counter Flow Systems

operational parameters

Dilution and Rinse Water Quality Requirements In order to succeed in producing the highest quality water possible from a 2-step counter flow system, it is imperative that the regenerant dilution water and displacement rinse water be of product water quality. Thus, anion product water is used for caustic dilution and displacement while either cation effluent or anion effluent can be used for acid dilution and displacement. The effects of using slightly contaminated regenerant dilution water for a counter flow cation regeneration is shown in Figure 1. The use of dilution water containing just 8 mg/l of sodium as CaCO3 to dilute the concentrated sulfuric acid regenerant to 2% strength results in a 3-fold increase in the average leakage on the very next service cycle even if deionized water is used for displacement. While admittedly the quality is still excellent compared to co-flow performance, for an equipment manufacturer who has guaranteed less than 0.2 ppm (mg/L) Na as CaCO3 leakage, it is quite unacceptable.

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Figure 1: Effect of Sodium in Dilution Water

Effect of Overrunning the Cation Unit Unlike co-flow regenerated cation units, which are often rated for capacity based on an endpoint considerably above the average leakage during the run (e.g. 10 mg/l above lowest leakage), counter flow regenerated cation units should never be operated to an endpoint leakage substantially above the leakage experienced during the service cycle. Despite some technical literature references indicating positive capacity corrections for operating to higher sodium endpoints, we and others2 have found by experimentation that very inconsistent results are obtained regarding both leakage and capacity in subsequent service cycles.

For example, Figure 2 shows the effects of overrunning a counter flow regenerated cation unit to a 5 mg/l sodium as Na endpoint following four runs terminated on a volume throughput basis where the endpoint leakage did not exceed 0.7 mg/l sodium. Whereas, a substantial increase in capacity was noted

on the first overrun cycle, no such capacity increase was observed for the next 3 cycles which were terminated at the pre-overrun endpoint. In fact, capacity was actually less and leakage much higher than before the attempt to overrun. Even after a total of 8 cycles, pre-overrun equilibrium had not yet been established. This work was performed in 8 inches and 48-inch (20 cm to 123 cm) diameter units. Figure 3 shows test results from the in 8 inches and 48-inch (20 cm to 123 cm) diameter units, indicating the number of cycles required to re-establish equilibrium conditions after a service cycle overrun of a counter flow regenerated cation unit.

Figure 2: Effect of Counter Flow Cation Overrun

Figure 3: Cycles Producing Inferior Quality Water After

Overrun of Counter Flow Cation

Effect of Overrunning the Anion Unit Results similar to overrun counter flow cation units are obtained with counter flow regenerated anion units which are allowed to operate to a substantial silica endpoint leakage. Figure 4 illustrates the erratic behavior obtained when trying to run a counter flow regenerated anion column to a definite silica endpoint. In this case, we tried for a 200-400 ppb Si02 endpoint.

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Review of the data shows that capacity and average silica leakage vary from cycle to cycle.

The results of substantially overrunning the counter flow anion unit is depicted in Figure 5. The extended service cycle represents an overrun of 25% beyond the pre-established volume endpoint at which a minimum of 4 cycles were completed. As can be seen from the graph, it takes six additional non-overrun cycles before silica leakages return to pre-overrun conditions. Note that these results were obtained with a regenerant dosage of only 2lb NaOH/ft3. It is quite possible that less than 6 cycles would be necessary if the regeneration dosage was 3lb or 4lb/ft3 but such higher dosages are not required for the 2.5% silica test water used in this particular experiment which was conducted in an 8-inch diameter column.

Figure 4: Effect of Operating to a Substantial Silica Leakage

Endpoint

Figure 5: Effect of Counter flow Anion Overrun

Regenerant Recovery in Counter Flow Systems In co-flow regenerated 2-step demineralization systems where relatively high dosages of regenerant chemicals are used to obtain acceptable leakages, reclamation of a portion of the excess regenerants is sometimes possible. However, two factors adversely affect regenerant recovery in counter flow systems:

1. At the “relatively low” regenerant dosages employed in counter-flow systems, there is not much excess regenerant available to recover.

2. The elution characteristics of counter flow regenerant systems are such that sodium and silica are present in the latter portion of the regenerant effluents.

To illustrate these points, counter flow regenerated cation and anion elution curves are shown in Figures 6, 7, and 8.

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Figure 6: Counter Flow Cation Elution Curve 48” Diameter Unit

Figure 7: Counter flow Cation Elution Curve 48” Diameter Unit

Figure 8: Counter flow Anion Elution Curve 8” Diameter Unit Type 1 Strong Base Resin

The elution curves shown in Figures 6 and 7 were obtained from a 48-inch (1.2 m) diameter strong acid cation unit counter flow regenerated with 4lb and 6lb/ft3 H2SO4. Quite notable is the elution of calcium and magnesium before sodium. At the 4lb acid regeneration level, any attempts at acid reclaim would result in acid substantially contaminated with both sodium, and hardness ions while, at the 6lb level, substantial sodium contamination of the reclaimable acid is still prevalent in counter flow regenerated systems.

The elution curve shown in Figure 8 was obtained from an 8-inch (20 cm) diameter strong base (Type I) anion unit counter flow regenerated with 3lb/ft3 NaOH. In this case, sulfates and chlorides appear in the effluent first, with silica peaking last, thus posing a serious technical problem for any caustic reclaim system to be used for counter flow regenerated anion units.

quality of demineralized water There is no doubt of the ability of 2-step counter flow systems to produce, at reasonable regeneration dosages, excellent quality water from influents up to 500 ppm (mg/L) TDS containing very high sodium and low alkalinity values. The question remains,

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“What is excellent quality water?” We would like to suggest that “excellent quality water” from a 2-step counter flow system is water of less than 3 micromhos conductivity. Indeed, some specifications are being written requesting 1µmho guarantees from 2-step systems. However, conductivity determinations alone do not adequately describe water quality since such measurements do not indicate the specific type and quantity of ions leaking from the individual units.

Figure 9: System: Counter flow Cation Co-Flow Anion

Counter flow Cation and Co-Flow Anion 2-Step Systems A very common 2-step demineralizing system today is the one in which the cation unit is counter flow regenerated while the anion unit is regenerated in a co-flow mode. In such a system, reasonably low cation regenerant dosages (3-5lb/ft3 H2SO4) can be used to produce very low sodium leakage cation effluent which then requires only a standard co-flow regenerated anion unit to produce water of 3 µmho or better. In such systems, however, we have found that the sodium leakage from the anion unit during the service cycle will often exceed the sodium leakage from a

“non-overrun” counter flow cation unit as shown in Figure 9. Indications are that the difference in leakage between the anion and cation effluent will vary depending on such factors as structure and age of the anion resin and the regeneration techniques employed (e.g. hot or cold caustic), but we do not have sufficient data to present such items at this time.

Figure 10: System: Counter flow Cation Co-Flow Anion

Slight Overrun Condition 5th Cycle

Figure 10 is another set of runs made by operating the cation unit under a very slight overrun condition. In this case after 75% of the run, the cation effluent sodium actually exceeds the anion sodium effluent, giving indications that the anion resin, at times, may even remove some sodium from the cation effluent during the latter portion of the service cycle after adding sodium during the first 60% to 70% of the service cycle. We have observed this phenomenon many times during our counter flow studies, but the differences are not great enough to overcome

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possible analytical precision errors. The overall effect is that the average sodium leakage measured from the anion effluent almost always exceeds the average sodium leakage measured from the cation effluent in 2-step counter flow cation, co-flow anion systems. Anion effluent conductivity generally is 2 to 3 µmho and rarely reaches 1 µmho under normal operating conditions.

Counter flow Cation and Counter Flow Anion 2-Step Systems A 2-step system where both the cation and anion units are regenerated counter flow has the potential for producing 1 µmho product water, if that is an objective. However, one must critically evaluate the projected end use of the water to determine if indeed 1 µmho water is really “better” than the 2-3 µmho water obtainable from a 2-step system where only the cation unit is counter flow regenerated.

The real advantage of employing a counter flow anion unit is the potential savings in caustic for regeneration since Type II strong base anion resins can be used to give comparable silica leakages to Type I units which are co-flow regenerated. Indeed, where 2.5-5 times stoichiometric caustic dosages are often required to obtain 20 ppb silica or less from Type I anion resins co-flow regenerated, similar effluent quality can be obtained at 1.5-2.5 times stoichiometric caustic dosage from counter flow regenerated Type II strong base resins. We emphasize that the preceding sentence is a generalized statement and is subject to many variables, which could alter the resin selection and stoichiometry in the final design. Such variables include the percentage of silica in the influent to the anion unit, inlet water temperature, neutral waste requirements, and whether or not mixed bed polishing will be provided.

The effluent quality obtainable from counter flow regenerated anion resins are presented in Figure 11. As shown, the two resins tested have slightly different effluent conductivities which are mainly the result of slightly different sodium leakages from each resin despite both having an influent cation sodium leakage of less than 10 ppb as Na.

You may notice that the effluent conductivities are somewhat higher than the rule of thumb 5 µmho conductivity per 1 mg/l Na as CaCO3 leakage for a 2-step demineralizing system. This has been a consistent finding in all our counter flow test work. The only significant point that we would like to make

about this is that you cannot apply the same rule of thumb guidelines to predicting the effluent conductivity

Figure 11: System: Counter flow Cation Counter Flow

Anion

from a “low” sodium leakage counter flow regenerated 2-step demineralizing system as reliably as from the “higher” sodium leakage co-flow systems. Examples of actual measured vs. “rule of thumb” conductivities are summarized in Table 2 below:

Table 2: Actual measured vs. “rule of thumb” conductivities

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Similar findings of effluent conductivity from 2-step counter flow systems being higher than that predicted solely from sodium leakage can be seen by examining data presented elsewhere for operating 2-step counter flow systems.3 This anion test work was carried out in an 8-inch (20 cm) diameter unit which was fed by a 16-inch (41 cm) diameter counter flow regenerated cation unit. All conductivity measurements were by in-line probe while sodium analysis was by atomic adsorption. The following section discusses operating data obtained from full-scale installations.

operating data

Case I: Paper Industry in Gulf States The demineralization system consists of three trains: counter flow regenerated cation units, one degasifier and co-flow operated anion units. The system was designed to operate at a normal flow of 400 gpm (1.5 m3/h), maximum 600 gpm (2.3 m3/h). The cation units are 7' (2 m) diameter and contain 270 ft3 (7.7 m3) of resin. The cation resin is regenerated with 4% sulfuric acid at 6 lbs./cu.ft. dosage. The following is the raw water analysis: Table 3: Raw Water Analysis

The demineralization system is operated at the designed flow of 400 gpm (1.5 m3/h). During the service run, the Na leakage from the cation unit was monitored and found to be 0.17 to 0.20 ppm (mg/L) as CaCO3. The corresponding anion effluent conductivity was 0.7 to 1.0 µmho/cm. The service cycle is terminated on the basis of gallonage of decationized water produced and the system has been running satisfactorily since February of 1979.

Case II: Electronic Industry - South Eastern Region The demineralization system consists of two trains. The strong acid cation is operated counter flow and a weak base anion is operated co-flow. Each train is designed to operate at an average flow of 200 gpm (0.8 m3/h). The system treats variable raw water containing 500 to 700 ppm (mg/L) TDS as CaCO3 with 90% sodium and 45% alkalinity. During the service cycle, the sodium leakage in the cation effluent were monitored and found to be 60-70ppb as CaCO3. The cation unit is regenerated with sulfuric acid at 5.5 lbs./cu.ft. This installation has been running satisfactorily for the past 8 months.

Case III: Refinery in Gulf States The demineralization system consists of four trains: counter flow operated strong cation unit, co-flow operated weak base, strong base and conventional mixed bed polisher. Each train is designed to operate at a normal flow of 750 gpm (2.8 m3/h) and a maximum flow of 1000 gpm (3.8 m3/h).

The cation unit is 12'6" (4 m) in diameter whereas the weak base and strong base units are 12' (3.6 m) diameter. The cation unit contains 950 cu.ft. of resin and is regenerated with 2% and 4% sulfuric acid at 4 lbs./cu.ft. dosage. The demineralization system treats clarified and filtered Mississippi River water with the analysis shown in Table 4.

Table 4: Demineralization System Treats Clarified and Filtered Mississippi River Water Analysis

The demineralization system has been in operation for 8 months and has been producing an effluent

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quality of 0.6 to 0.8 µmho prior to mixed bed polishing. Samples analyzed for sodium leakage during the latter part of the service cycle from the strong base anion unit are shown in Table 5.

Table 5: Samples Analyzed for Sodium Leakage During the Latter Part of the Service Cycle from the Strong Base Anion Unit

The system has been operating very satisfactorily in producing both the guaranteed effluent quality and quantity of demineralized water.

summary In consideration of the foregoing discussion, it is clear that for successful operation of the counter flow ion exchange system there are certain rules one must follow. These are: compacted bed, proper distribution of regenerant, product water quality for regenerant make-up and displacement rinse, termination of service cycle based on predetermined volume throughput, and deep beds. With the counter flow system, the recovery of uncontaminated regenerants is difficult to achieve. However, the superior quality of water produced by utilizing fresh regenerants at considerably lower chemical dosages than co-flow requirements will likely negate the cost savings of reclaim regenerant systems. The quality of demineralized water produced by the counter flow system cannot be equated solely to the sodium leakage for the cation unit. There are several factors that affect final effluent quality including sodium throw from the anion unit, which may vary with resin type, age, and regeneration procedure.

references 1. Abrams, I. M., “Counter-Current Ion Exchange

with Fixed Beds,” 10th Annual Liberty Bell Corrosion Course, 1972.

2. Jackson, E. W. and Smith, J. H., “Make-up Treatment-Counter Current Regeneration Experience in the United Kingdom,” 38th International Water Conference, Pittsburgh, Pa.

3. Howard, J. R. and Morgan, V. G., “Countercurrent Demineralizer at Ohio Edison Sammis Station,” Proc. 37th International Water Conference, p. 191, 1976