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Gas Generation in CEDI Cells Print E-mail

Chlorine Gas Production

Normally the reaction at the anode will result in the formation of oxygen, but with sufficient voltage and with chloride ion present, chlorine gas can be formed. In particular this is an issue with electrodeionization systems which inject sodium chloride into the concentrate and electrode streams in order to reduce module resistance and improve performance. Because many EDI devices feed the reject water to the electrode chambers, concentrate salt injection increases the chloride concentration at the electrode. As shown below in Eq. 13, chloride at the anode is converted to chlorine, which is a strong oxidant and a toxic gas that can damage ion exchange membranes and create a safety hazard for those near the device.

2Cl- → Cl2 + 2e-



Eq. 13

 

To investigate the extent of chlorine formation, an E-Cell MK-1 module was operated at 600 volts DC and 3.0 amps. The temperature of the feed water was varied between 15 and 25C such that the resistance of the module changed proportionally. Then the concentration of sodium chloride was varied at the inlet of the concentrate cells to overcome this change in resistance and maintain 3.0 amps. Free chlorine was measured in the electrode product water. As shown in Figure 7, a significant amount of free chlorine was detected. There was also a large off-gassing of chlorine evidenced by the odor near the device. Concentrations in the range shown in Figure 7 can cause significant damage to ion exchange membranes or resins in contact with the electrode stream. In particular this is an issue with electrodeionization systems which inject sodium chloride into the concentrate and electrode streams in order to reduce module resistance and improve performance.

Figure 7. Chlorine Generation at the Electrodes in an E-Cell Module


Other systems do not require brine injection because they use an ion-exchange resin filler in the concentrate compartments in order to minimize the electrical resistance of the module. This in itself limits the possibility of chlorine generation at the anode by reducing the chloride concentration. Because the resin is orders of magnitude more conductive than typical RO product water, it removes the conductivity of the water as a contributing factor in the overall resistance. This alleviates the need for salt injection or recirculation. In addition, chlorine production is virtually eliminated. An additional advantage of the this design is that with elimination of salt injection and chlorine production, the EDI concentrate will generally be better quality than the raw water and can often be recycled.

To show the effect of concentrate resin filling, two modules were tested with and without salt injection under similar conditions. Shown in Figure 8 is the resistance per cell pair for a module with concentrate resin filling compared to a module without concentrate resin filling, as a function of temperature. The electrical resistance of the module with a resin filled concentrate was lower by two orders of magnitude.

Figure 8. The Effect of Concentrate Resin Filling on Module Resistance


Hydrogen Gas Production

People often express concern about the hydrogen gas produced at the cathode, since it is known that under certain conditions hydrogen can be explosive. However, the amount of gas that is produced by an EDI module is so small that it does not present a safety hazard when the EDI system is installed in an area with normal ventilation. Codes require that buildings have multiple air changes every day, one air change being a turnover of air equivalent to the building's internal volume. The number of air changes varies depending upon the building's use and the local codes, but a widely accepted value is half an air change per hour.

To show how little risk is presented by the electrode gases, let's perform an example calculation assuming that an EDI module is installed in a 4m x 4m x 4m office which has the normal ventilation of half an air change per hour. This is equivalent to an air flow of 32 m3/h. Assume the EDI module is operating with a DC current of 10 amps, therefore producing hydrogen at the rate of 74.6 ml/min (equivalent to 0.0045 m3/h). If all the hydrogen gas leaves the water and enters the room air, then the resulting concentration of hydrogen in the air would be about 0.014% (v/v), or about 141 ppm. This is well below the explosive limit of a hydrogen/air mixture, which is 4.2% v/v at STP, and also well below the concentration at which asphyxiation would occur. Even at the maximum current output of an EDI DC power supply there is no safety risk as long as the ventilation meets typical building code requirements.

 
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