Chlorine Dosing Systems Sodium Hypochlorite, Gas Chlorine

What is Chlorine?

Chlorine is a gas at room temperature and pressure and is soluble in water. Chlorine hydrolyses with water to form hypochlorous acid and hydrochloric acid according to the following equation:

Cl₂ + H₂O → HOCl + HCl

Sodium Hypochlorite reacts with water as follows:

2NaOCl + H₂O → NaOH + NaCl

This hydrolysis feature makes chlorine very suitable for water disinfection applications where turbulence is present, as chlorine is not present in water as a dissolved gas like chlorine dioxide or ozone.

How is Chlorine Available?
Chlorine is commercially available as a liquified gas in 70kg cylinders or 920 kg drums.
Sodium hypochlorite is available as a 12.5% solution or 10% solution (depending upon location).
Calcium hypochlorite tablets and granules are sold in 10kg and 50kg buckets. The tablets contain 65% available chlorine.
Chlorine can also be generated from salt using electrochlorination systems. Electrical power is used to convert NaCl to sodium hypochlorite NaOCl

Gas Chlorine Dosing
Gas chlorine is safely dosed from cylinders or drums using ProMinent Hydro remote vacuum gas chlorination equipment. The vacuum regulator (the heart of the chlorinator) is mounted directly on the cylinder isolation valve using a lead gasket. Chlorine gas under pressure is fed to the inlet pressure reducing valve, the needle of which is seated on a diaphragm. At the water end of the system, pressure differential is applied across a venturi by a boost flow which causes the venturi to suck. This vacuum is applied to the diaphragm, causing the needle to be pushed into the spring loaded valve, thereby allowing chlorine gas to flow into the chlorinator. Gas chlorine under vacuum then proceeds through a flow regulating device (needle valve V notch) and through the vacuum tube to the venturi where it is dissolved in water.

The key feature of the gas chlorination system for water disinfection is that it operates completely under vacuum and is therefore inherently safe. If the booster pump stops or the venturi becomes blocked or the vacuum tube is broken, vacuum is lost at the chlorinator. The inlet pressure reducing valve immediately closes and the cylinder or drum is completely isolated. Even if the cylinder was to fall over and the body of the chlorinator was to smash, the inlet pressure reducing valve would still stay closed and no chlorine gas would escape.

Gas chlorination can occur from 920kg drums. Chlorinators are equipped with a drop leg and heater where any liquid chlorine is vaporised. Normally, this is only required for connection of the drum immediately after transport. After a short operation time, the heater is not required but is normally left on.

Gas chlorination systems are usually supplied as duty-standby units so that chlorine gas is continuously available for water disinfection. Change-over from the duty cylinder or drum to the standby unit is via a vacuum shuttle valve. When the cylinder is empty, vacuum increases to such an extent that the valve shuttle is pulled over to the other side, allowing the standby cylinder to come on line.

Sodium Hypochlorite Dosing
Sodium Hypochlorite is supplied as a 12.5% solution and is readily available in 20L drums, 200L drums and tanker loads for larger installations. Chemical storage tanks are usually manufactured from polyethylene. Sodium hypochlorite is dosed with solenoid operation dosing pumps or motor driven metering pumps.

12.5% sodium hypochlorite undergoes gasification as it decomposes to give off oxygen gas. This gasification can cause problems with chemical metering pumps as the gas will accumulate in the metering pump suction line, eventually making its way into the liquid end where loss of prime is the result. The ProMinent Gamma/L and Beta dosing pumps have solved this problem with the specially designed auto-degassing liquid end. Gas is removed from the suction line through a vent valve and directed back to the storage tank with a small amount of liquid. The clear chemical is then dosed through the discharge valve.
Larger metering pumps (>20 L/hr) don’t usually suffer from loss of prime as the valves are much larger and the volume of chemical being pumped is also larger.

Suction connections between the bulk sodium hypochlorite storage tank and the metering pump should be designed to minimise formation of gas pockets. A calibration cylinder immediately prior to the metering pump will function as a gas accumulator and allow gas to rise and vent off. If the suction line is run downwards from the tank to the pump, gas will rise back into the tank.

One way to prevent gasification is to dilute the 12.5% solution to 1% using potable water. Although the metering pump will have to be 12.5 times larger, the 1% solution will last decay approximately 144 times slower than the concentrated solution.

Definition of Strength
Five common definitions of sodium hypochlorite solution strength are as follows:

Grams per litre of available chlorine = The weight of available chlorine in grams in 1L of sodium hypochlorite solution. This is determined by analysis.
Grams per litre of sodium hypochlorite = 1.05 x grams per litre available chlorine.
Trade percent available chlorine = grams per litre available chlorine/10
Weight percent available chlorine = grams per litre available chlorine/ (10 x sg)
Weight percent sodium hypochlorite = weight percent available chlorine x 1.05

Sodium Hypochlorite Decomposition
The dominant pathway for decomposition of sodium hypochlorite is to chlorate as follows:
3NaOCl → 2NaCl + NaClO₃

Calcium Hypochlorite Dosing
Calcium hypochlorite is available as granules or tablets. The tablets can be used safely in the ProMinent ProCal erosion feeder. Tablets are placed into a hopper where they are stored dry. Water is allowed to run past the bottom layer of tablets where slow erosion will occur. The chlorinated water solution then drops into a buffer storage tank where flushing and dilution occurs. The dilute (<500ppm) solution is continually pumped back to the water main. Control of chlorination is by a ProMinent D1C chlorine residual controller. This controller gives a pulse duration control signal to the ProCal feeder and the feeder automatically actuates the tablet erosion water solenoid.

The advantage of the ProCal feeder is that the tablets are dissolved using immersion erosion. This keeps the system free of blockages. Other systems use sprays and venturis which are continually blocking and causing maintenance problems.

The new ProMinent ProCalG can be used with granular calcium hypochlorite, dramatically reducing the operating costs involved with tablets.

Electrochlorination

The ProMinent electrochlorination system produces chlorine from salt using electrical power. The advantage of electrochlorination is a saving in running costs (usually very significant for sodium hypochlorite) as salt, water and power are usually cheap. Also, dangerous chemicals such as chlorine gas and sodium hypochlorite can be removed from site in favour of operators only handling bags of salt.

If you have a salt water pool, you already have a salt source available and don’t need to add any additional salt. The water can be simply treated to produce chlorine as required.

Measurement of Chlorine Residual - Chlorine Residual Analysers
ProMinent is the largest manufacturer of chlorine residual measurement probes in the world. Chlorine us measured using the amperometric principle with a closed cell utilising electrolyte and a membrane cap. The membrane cap selectively allows chlorine species through to be measured. There are different probes for measurement of either free chlorine or total chlorine. All probes do not require continuous addition of buffer, are simple to operate and easy to calibrate. The zero point of each probe is electrically stable at 4mA due to the constant pH and conductivity of the electrolyte solution which encapsulates the electrodes. The probe electrodes don’t come into contact with the water so only the membrane cap needs to be replaced if it becomes fouled. Other competitor units have open electrodes which contact the water. These electrodes become fouled and the zero point changes as the pH and conductivity of the water changes.

Typical chlorine residual analysers are supplied on wall mount backboards, pre-wired and pre-plumbed. The controller will feedback control any system such as a gas chlorinator, dosing pump or cal hypo feeder.

Safety Issues
Gas chlorine has very stringent storage and handling requirements in all countries as chlorine gas is toxic. A chlorine gas leak is to be avoided at all costs. Usually trained operators are comfortable handling gas chlorine and there have been comparatively few incidents, considering how widespread it is used. Cylinders or drums are to be stored in a lockable area, and unauthorised access prevented. The room and the dosing area should have a chlorine gas leak detector installed which will activate an alarm and shut the system down. The ProMinent ProGuard automatic cylinder shut-off device will immediately turn the cylinders off if a leak is detected.

Sodium Hypochlorite is a dangerous good and has the issue that a splash can cause blindness. Transfer of chemical from bulk tankers is to be done without operator intervention using appropriately designed systems. Transfer of small quantities (20L or 200L) can be done with a Dioxide chemical transfer system and there will be no operator contact.

Calcium hypochlorite tablets are not a major safety concern except that they should not be heated or put in contact with oxidisable substances.

Electrochlorination produces hydrogen gas which must be safely vented outside the plant room.

Please contact us for further information on chlorine water disinfection systems.