Equinox® Stabilizer a Highly Effective, Halogen Stabilizer for Wet End Paper Applications

Introduction

The proper selection and use of a wet end biocidal treatment program is a key element in a paper mill’s effort to optimize efficiencies in the face of technological changes.  With increased emphasis on the use of recycled paper, changes in papermaking chemistry from acidic to alkaline, efforts to reduce water use and consumption, and the ability to control microorganisms and associated problems has become even more critical.  Modern biocidal treatment programs seek to achieve the performance of the ideal biocide that is cost effective, fast acting, broad spectrum in efficacy and applicability, environmentally friendly, and low in toxicity.  Oxidizing biocides meet these requirements.

Hydantoin stabilization technology was developed to achieve the advantages of oxidants in the high oxidant demand environment of papermaking applications.  Specifically, it was discovered that 5, 5-dimethylhydantoin (Equinox® Stabilizer, DMH) dramatically improves bactericidal and slimicidal efficacy of NaOCI solutions in pulp slurries, significantly reducing the amount of NaOCI required to achieve biological control.  As a result, a safe and non-toxic formulation of Equinox® Stabilizer was developed which, when combined with NaOCl, reduces chlorine’s normal tendency towards unselective oxidation and, thus, increases its slimicidal efficiency.  This technology is described in detail in US Patents 5.565.109. 6.429.181. and 7.407.590 [1 – 3]

The benefits of hydantoin technology results from the conversion of free chlorine to stabilized chlorine, which effectively increases the lifetime of active chlorine in the presence of organic components and contaminants.

The results of several laboratory and field evaluations of Equinox® Stabilizer technologies in papermaking and cooling water applications were previously published in technical literature and demonstrated the following benefits [4 – 11]:

  • stabilization and efficacy enhancement
  • biofilm efficacy
  • photostabilization
  • compatibility with additives
  • absorbable organic halides (AOX) reduction
  • benign toxicological profile
  • excellent cost performance and
  • cost-effective field performance.
Figure 1 - Equinox Chemical Structure
Figure 1. Dimethylydantoin (Equinox® Stabilizer)

Further, the following additional properties of hydantoin technology are currently reviewed and compared with other chlorine stabilizing chemistries:

  • versatility and handling
  • microbiological efficacy at various NaOCI: Equinox ratios
  • biofilm efficacy and
  • liquid and vapor phase corrosion properties.

Results & Discussion

Reactivity Evaluation

Treatment program versatility is a highly desired property of any performance chemical application. The greater the program flexibility the more applicable and effective the treatment program is to the variety of paper grades and chemistries employed in modern papermaking operations. This allows for enhanced prevention of, and greater ease of recovery from, system upsets. Specifically, flexibility in dosing ranges and blending ratios of oxidizing biocides and stabilizer systems provides a significant advantage in the implementation of a mill’s chemical oxidant treatment program. Reactivity evaluation studies were initiated to demonstrate the versatility of common oxidizing technologies via neat blending at various NaOCI to active ingredient mole ratios (e.g., NaOCl:NH3 and NaOCl: DMH). In order to quantify the degree of flexibility for comparative purposes, NaOCl was combined at various concentrations and mole ratios with a range of concentrations of both ammonium salts in a continuous flow chamber. As shown in Table 1, ammonium salts produce significant exotherms as high as 114oF (46oC) and pH reductions up to 10 pH units in the absence of specialized systems and adjustments to the NaOCI, over a wide range of NaOCl:NH3 mole ratios, even at 1:1 ratios [12]. This is likely due to the decomposition of formed halamines during mixing [13]. In contrast, Equinox was compatible with NaOCl, with little or no change in either temperature or pH, over an equivalently broad range of ratios using the highest concentrations of NaOCl solution and Equinox.

Table 1

Sample
% NaOCI Used
% Active
NaOCI: Active Mole Ratio
ΔT
ΔpH
Ammonium Sulfate
9.1%
40.0%
1.0
27
-10.8
12.2%
20.0%
4.0
28
-4.8
9.1%
20.0%
2.0
26
-10.8
4.6%
20.0%
1.0
13
-7.5
12.2%
15.0%
4.0
32
-4.7
12.2%
15.0%
2.0
23
-10.4
12.2%
15.0%
1.0
23
-10.5
9.1%
10.0%
4.0
21
-5.3
4.6%
10.0%
2.0
38
-9.8
2.3%
10.0%
1.0
5
-3.1

Ammonium Bromide
12.2%
34.1%
2.0
46
-9.7
5.2%
34.1%
1.0
23
-3.8
12.2%
17.1%
4.0
34
-5.3
5.2%
17.1%
2.0
20
-9.6
5.2%
17.1%
1.0
18
-9.1
12.2%
15.0%
4.0
31
-5.3
12.2%
15.0%
2.0
37
-9.5
12.2%
15.0%
1.0
35
-9.3
4.4%
7.2%
4.0
15
-3.2
5.2%
7.2%
2.0
15
-3.9
1.1%
7.2%
1.0
7
-2.3

Equinox
12.2%
15.0%
4.0
4
0.1
12.2%
15.0%
2.0
3
0.2
12.2%
15.0%
1.0
3
0.2
The versatility of an Equinox stabilized NaOCl treatment system is further demonstrated in Figure 2. In this highlighted example, concentrated (12.2%) NaOCl was blended with ammonium salt solids at concentrations equivalent to a Equinox® Stabilizer concentration of 15% over a range of NaOCl:stabilizer ratios. Again, significant temperature increases and pH reductions, on the order of 86oF (30oC) and 10 pH units, were observed over a wide range of NaOCl:NH3 ratios.

As in the previous test, negligible ΔT’s and ΔpH’s were observed for the NaOCl:DMH blend. These results demonstrate the ease of use and flexibility of hydantoin technology relative to ammonium salts. Specifically, the wide allowable NaOCl:DMH range translates to improved system control over a broad range of conditions and paper grade. It is clear that ammonium salt technologies present less flexibility.

Figure 2. Temperature And Ph Changes Upon Reaction Of Halogen Stabilizers With 12% Naoci
Figure 2. Temperature and pH Changes upon Reaction of Halogen Stabilizers with 12% NaOCI

Microbicidal Efficacy

Microbial control is an important aspect of achieving optimum mill performance and efficiency. Bacterial growth can result in both biofilm problems (i.e., slime; vide infra) as well as microbiological corrosion on metal equipment. Correspondingly, a component of many programs that seek to maintain high mill production efficiency includes the control of planktonic bacteria levels. In this regard, the planktonic efficacies of Equinox® Stabilizer and ammonium salt based oxidizing technologies were compared. The results are demonstrated in Figures 3 and 4, and reveal that ammonium salts in conjunction with NaOCI at 1:1 molar ratios have higher efficacy than hydantoin technology at this same ratio. However, an increase in the ratio of NaOCI to Equinox® Stabilizer leads to improved bacterial kill, with ratios of 4:1 – 6:1 exhibiting planktonic efficacies equivalent to the ammonium salt technologies at both 1 and 2 ppm total halogen. At the same time, additional amounts of chlorine do not change the efficacy of ammonium salts technology (Figure 3). Hence, hydantoin stability and persistence within a mill environment can provide flexibility in a planktonic control program via adjustment of the NaOCl:DMH mole ratio if a specific treatment philosophy requires it.

Figure 3: Planktonic Efficacy Of Equinox Vs. Ammonium Sulfate At Multiple Total Halogen: Active Ingredient Ratio At 1Ppm Total Ci2
Figure 3. Planktonic P. aeruginosa Efficacy, pH = 9.0 low demand synthetic cooling water
Figure 4: Planktonic Efficacy Of Halogenated Equinox Vs. Ammonium Salts At Multiple Naoci:dmh Ratios
Figure 4. Planktonic Efficacy of Halogenated Equinox vs. Ammonium Salts at Multiple NaOCI:DMH Ratios

Biofilm Efficacy

It is well known that biofilm control represents a unique and particularly challenging form of microbial control within an operating paper mill, and can lead to a variety of mechanical issues as well as problems for the finished paper product (e.g., breaks, tears) [7, 14]. In fact, the ability of a particular biocide to kill planktonic microorganisms does not necessarily translate to good biofilm control. In the case of NaOCI, resistance of sessile bacteria is associated with neutralization of the oxidant by the biofilm components and inhibition of transport to the microbial cell [15]. Hence, a technology that can stabilize the oxidative capacity of NaOCI and improve diffusional transport through biofilm is highly desired.

Results of the biofilm efficacy testing with Equinox® Stabilizer in a dynamic system are shown in Figure 5. The laboratory system was performed with pure culture Pseudomonas aeruginosa biofilms grown in suspended coupon reactors (SCR). Equinox® Stabilizer was combined with NaOCl at a NaOCl:DMH ratio of 2:1 and a total chlorine dose of ~1 ppm. Inspection of the figure reveals that there was significant control of biofilm population over the course of the application (2.5 logs on Day 7; Figure 5). These results correspond well with field experience where excellent paper machine performance was obtained by emphasizing control of slime deposits (sessile organisms) over planktonic efficacy [16; vide infra].

Figure 5: Reactor Biofilm Concentration
Figure 5. Reactor Biofilm Concentration

Corrosion Studies

Liquid Phase Corrosion

An understanding of the corrosion properties of any oxidative chemical treatment program is important part of maintaining machine cleanliness. In the current study, the relative corrosion properties of three halogen stabilizing technologies, ammonium sulfate, ammonium bromide, and Equinox® Stabilizer, were compared. Figure 6 demonstrates that each halogen stabilizer results in significantly less liquid phase corrosion than free chlorine. This is a major advantage of oxidant stabilized technologies. Further, it can be seen that overall corrosion rate of all 3 technologies are similar. This is primarily due to the reduced oxidation potentials of the stabilizer systems relative to NaOCl.

Figure 6. Liquid Phase Corrosion At 4Ppm Total Halogen
Figure 6. Liquid Phase Corrosion at 4ppm Total Halogen

Vapor Phase Corrosion

In addition to liquid phase corrosion, corrosion due to condensed vapor, particularly in the dryer section of a paper mill, is an important concern. Minimizing or eliminating corrosion will not only extend the lifetime of the equipment in use, but improve runnability via the prevention of breaks. Consequently, reducing the potential for oxidant volatization is an important step towards this goal.

Result of vapor phase corrosion experiments are shown in Figures 7 and 8. In Figure 7 it can be seen that Equinox® Stabilizer exhibits less corrosion than ammonia-based technologies, comparable to NaOCI.

Figure 7. Vapor Phase Corrosion At 10 Ppm Total Halogen
Figure 7. Vapor Phase Corrosion at 10 ppm Total Halogen

Interestingly, ammonium bromide exhibited less surface oxidation at NaOCl:NH3 levels of 2:1 than at 1:1, as shown in Figure 8, whereas the amount of visual corrosion was essentially the same for ammonium sulfate at both ratios. This is potentially due to a lower ammonia concentration at the 2:1 ratio relative to the 1:1 ratio at the same total halogen concentration (0.24 ppm vs. 0.48 ppm), thereby reducing the impact of halamine vaporization. Further, the differences between ammonium bromide and ammonium sulfate can be attributed to the anion impact on the halamine chemistry.

Over a total halogen range of 2 – 10 ppm and pH range of 7 – 8, ammonium sulfate based systems exhibited the highest level of surface oxidation on 1010 carbon steel. Equinox and NaOCI exhibited the lowest level, whereas ammonium bromide was intermediate. The difference between hydantoin and NaOCI and the ammonium salts results primarily from the fact that Equinox® Stabilizer and the -OCl anion are not significantly volatile, whereas ammonia and its corresponding halamines are gases. These data are in agreement with literature data, which reports that:

  1. Ammonia increased the volatilization of hypochlorous and hypobromous acid on the order of 10-100 times [17 – 19] .
  2. Equinox significantly decreases volatilization rates, on the order of 10 times [18].
  3. Ammonia dramatically increases the vapor phase corrosivity of hypochlorous acid solutions while Equinox suppresses corrosivity [20].

In summary, the addition of hydantoin (Equinox® Stabilizer) to active chlorine solutions minimizes vapor phase corrosion relative to other technologies.

Figure 8. Vapor Phase Corrosion At 2Ppm Total Halogen
Figure 8. Vapor Phase Corrosion at 2ppm Total Halogen

Field Trials

Published studies have documented that the use of halogenated hydantoins provide several key advantages [2,6,9,10]:

  • Straightforward handling of hydantoin technology components
  • Ability to safely apply neat blending without complex pre-dilution equipment
  • Flexibility to use the technology with various qualities of NaOCI and in conditions of gradual decay of NaOCI quality
  • Flexibility to change NaOCl:stabilizer (e.g., Equinox® Stabilizer) ratios to address changes in application parameters
  • Ability to maintain plant material integrity during application
  • Provide long-term machine cleanliness due to technology stability and persistence
  • Ability to adjust/optimize application parameters on-site

Equinox® Stabilizer technology was applied at an 80t/day mill (site undisclosed)[16]. The mill primarily produces high bulk book paper on a Fourdrinier machine with Dandy Roll in a grammage range going from 60 to 240 g/m². In conjunction with an overall treatment program, Equinox® Stabilizer technology provided several key benefits within this application.

  • The versatility in combining DMH and NaOCl (vide supra) in various ratios, and the overall stability of the combined mixture allowed for two dosing points to be sufficient to treat the whole paper machine, including the broke circuit
  • The elimination of frequent chemical system cleanings, such that they are only performed 2 -3 times per year when the forming fabric is changed
  • The flexibility to optimize the DMH:NaOCl ratio in response to variable operating conditions

This example shows how the use of a hydantoin-based technology can optimize mill performance by maintaining machine cleanliness with a biocidal program which focuses on control of biofilm formation rather than planktonic bacteria.

Experimental

Reactivity Evaluation

Ammonium salt solutions were prepared by addition of the salt to water and adjusting the pH and concentration with concentrated NaOH and water to the desired levels (pH = 9.5 – 10, 10 – 40% w/w for Ammonium Sulfate and 7 – 34% w/w for Ammonium Bromide). Equinox stabilizer was used as provided.

NaOCI solutions were used over a range of 1.1 – 12.2% and prepared via dilution with deionized water. The pH of the NaOCI solutions were measured as is and used as the initial value for the ΔpH determination.

A continuous flow system was constructed consisting of a PVDF static mixing T, stopcocks, and tubing adapters (available from Cole-Parmer) connected to 2 Masterflex pumps for addition of the NaOCI solution and corresponding stabilizer. For each data point, the combined flow rates of the stabilizer and NaOCI solutions were adjusted in order to maintain: 1) the desired molar ratio of chlorine to active ingredient (A.I.), and 2) a residence time of 5 seconds in the flow system. The solution was passed over a thermometer at the outlet of the flow cell and collected in an insulated beaker. The temperature at the outlet was recorded before the onset of flow and after 1 min of continuous flow across the thermometer. The final pH of the resulting collected solution was recorded after a 2 min interval of continuous flow.

Microbicidal Efficacy

Efficacy testing was conducted according the following standard procedure:

Fine paper alkaline furnish was prepared via an adaptation of the method described in ASTM E: 1839-07. A 1% pulp slurry was prepared via shredding white laser printer paper and adding to an appropriate amount of 400 ppm alkalinity (as CaCO3) water solution. The slurry was rapidly mixed while heating to 98 (+ 2)oC, followed by cooling to room temperature. The slurry pH was adjusted to 8.5 with 1N H2SO4 and diluted to 0.5% with DI water. A 98 (+ 1 mL) aliquot was transferred to a 125 mL sample flask and steam sterilized and cooled to RT prior to use.

A stock solution of NaOCl at 400 ppm as Cl2 was freshly prepared prior to the organism challenge test. Dilutions of the test samples were made in order to prepare a 1:1 mixture of NaOCl and sample solution at the desired Cl2:Active Ingredient molar ratio and to provide a 1 mL dose to the pulp slurry. The diluted sample solutions were prepared at pH = 9.5 – 10.0 prior to use.

Twenty four hour cultures of Enterobacter aerogenes (ATCC 13048) and Pseudomonas aeruginosa (ATCC 15442) were used for the testing. Each organism was grown on TSA slants (Tryptic Soy Agar) at 34oC. Each slant was washed with 9 mL of phosphate buffered water. The organisms were pooled in a sterile container. Final bacterial concentration of the inoculum was between 2 x 106 and 1 x 107 bacteria/mL. 1 mL of the inoculum was then placed into each of the sample to be tested and samples are then placed onto a shaker for 3 hours at 34oC. After the 3 hour contact time, 1 mL of each sample was placed in 9 mL of D/E (neutralizer) and dilutions were plated through 106. TSA was used to pour each plate and all plates were placed in 34oC for 48 hours. After 48 hours, plates were counted and results were reported.

Biofilm Efficacy

Biofilm studies were conducted in suspended coupon reactors (SCRs) containing a residence volume of 11.8-13.5 oz (350-400 mL) of autoclaved and cooled fine furnish pulp slurry (0.5% w/w) in sterile tryptic soy broth. The SCR was fitted with 18 polycarbonate coupons for biofilm measurement and evaluation. The reactors were inoculated with 0.3 oz (1 mL)Pseudomonas aeruginosa from log-phase batch culture at an approximate concentration of 106 CFU/mL. The reactors were initially operated in batch mode (no inflow or outflow) for 24 hours with stirring, followed by continuous flow of nutrient at 21oC. Test sample and control reactors were run simultaneously for comparative purposes. Stock halogen stabilizer and NaOCI solutions were kept separate until introduced into the reactor via a “Y”-joint just before the reactor inlet, and were pumped into the reactor concurrently with the growth media for a target total chlorine concentration of ~ 1 ppm and a NaOCl:DMH mole ratio of 2:1. The growth media and combined halogen were added at a total flow rate of 13 mL/min (reactor residence time: 30 min) and the experiment ran for 7 days. After each 24 hr period during the course of the experiment, three coupons were harvested from both the test and control reactors and the attached biomass quantified according to ASTM Standard Method ASTM E-2196-02. Planktonic microbial concentrations were also quantified from the effluent of each reactor at each sample period. Biofilm results are reported as LOG(CFU/cm2) and planktonic microbial concentrations as LOG(CFU/mL).

Corrosion Studies

Liquid Phase Corrosion

Corrosion studies were conducted via measurement of general corrosion rate, conductivity, imbalance, and free and total halogen in two 3.5 L jars filled with model water in which corrosion probes with metal electrodes were immersed for a period of 24-90 hours. The readings were taken after 1, 2, 17-20, 23-25, and 42-90 hours after the start of the test. The addition of chemicals was made following immersion of the electrodes in the model water for 1 hour. Corrosion was measured via Rohrback Cosasco Systems Inc. Corrosion Monitor “Aquamate” with Corrater corrosions probes; Halogen was measured via HACH DR 2800. 1010 Carbon steel electrodes (060814-K03005) were used.

The model water was prepared by addition of 0.44 NaHCO3 to 3.5 L of DI water, which corresponds to 150 ppm alkalinity as CaCO3. Range-finding experiments of general corrosion in model water with addition of CaCl2 in parallel to NaHCO3 demonstrated high corrosion with significant decreasing in conductivity during experiment. It was decided to limit model water additives to NaHCO3 only. The following conditions were applied: Total alkalinity 150 ppm as CaCO3 ; T= 22oC; pH = 8.0- 8.2; Solution volume 3.5 L; Stirring Rate: 300 RPM. The following halogen technologies were used: NaOCI; Equinox® Stabilizer/NaOCl 1:1 mole ratio; Ammonium bromide/NaOCl 1:1 mole ratio; Ammonium sulfate/NaOCl 1:1 mole ratio. Oxidizers were tested at 4 ppm as Cl2.

Vapor Phase Corrosion

Q-panel coupons of 4“x 6“ 1010 carbon steel were rinsed with DI water, dried, and placed into 4 L beakers. Test solution was prepared with DI water + 400 ppm alkalinity as CaCO3 with NaHCO3 and adjusted to either pH 7.0 or 8.0. 250 mL of this solution was dosed with ratios of 1:1 to 2:1 molar NaOCl as Cl2 to prototype at both 2 and 10 ppm total Cl2. One small beaker of solution was added to each of the large coupon-containing beakers which were then covered with plastic and placed in a 37oC oven. The solutions at 10 ppm (pH = 7.0) were evaluated after a 24 hr period, whereas the solutions at 2 ppm (pH = 8.0) were replaced with freshly prepared ones on day 2 of the study. Coupon corrosion was assessed visually and photographed at 2 days and at 5 days and compared.

Conclusions

In summary, ammonia and hydantoin-based halogen stabilizers have been compared under a variety of conditions, including: i) reactivity via direct mixing with NaOCI, ii) microbial efficacy, and iii) liquid and vapor phase corrosion. In the absence of extraordinarily specialized handling equipment, the data indicate the greater application flexibility of hydantoin systems relative to ammonium salt systems. In general, Equinox® Stabilizer exhibits mild reactivity at high active and chlorine concentrations relative to ammonium salts. Equinox®  (DMH) stabilized NaOCI systems demonstrate wide flexibility to vary the stabilizer to chlorine ratio, resulting in a simple, easy to handle, broad spectrum wet-end slime control technology. This is a key characteristic of any treatment program intended to achieve the best practices of an optimized paper mill operation. More specifically, the DMH:NaOCl treatment system shows:

  • Equinox® Stabilizer (DMH) is especially effective against sessile bacteria, thus providing excellent machine cleanliness
  • The inherent properties of hydantoin-based technologies (operational simplicity, chemical stability of neat blending, compatibility with wet end additives, and negligible corrosion) improve plant integrity and extend machine runnability
  • Where desired, planktonic efficacy can be optimized by increasing the Equinox® Stabilizer ratio

Acknowledgments

Equinox® Stabilizer a Highly Effective, Halogen Stabilizer for Wet End and Paper Applications is a reproduction of work written by: Sarah Kopecky, Kevin Janak, Philip Sweeny, and Michael Ludensky. Ellen Meyer revised and updated the document to include new findings.

References

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