For a number of years, sulfuric acid was used in water treatment facilities to control
alkalinity. It’s a product that works, but it also has may potential problems. Sulfuric acid
can be difficult to apply and control. It is potentially dangerous to store and handle. Safety
showers have to be installed and readily available to operating personnel who must wear
special clothing for their protection.
The extremely corrosive acid
requires special material for
equipment and piping.
Maintenance of the system
demands frequent component
repairs and replacement. Carbon
dioxide on the other hand is safe
to handle, easy to apply, efficient
and ecologically safe.
When pH control is critical to your process or effluent quality, CO2 can be a most viable
alternative.
The Chemistry
Carbon dioxide is a gas, which once dissolved into water produces as weak acid:
carbonic acid. This substance reacts immediately with alkalis such as caustic soda,
sodium carbonate and dissolved lime, turning them into neutral carbonates and
bicarbonate salts.
CO2 + H2O H2CO3
H2CO3 + 2NaOH Na2CO3 + 2H2O
H2CO3 + Na2CO3 2NaHCO3
Carbonic acid is a mild acid present in water as ions H+
and HCO3
-
, which are highly
reactive. They will immediately react with ions responsible for alkalinity of water.
PH Control System
CO2 is introduced into the high pH water by means of a diffuser, which is typically
installed in an existing pressurized pipe or at the bottom of a basin. Small CO2 bubbles
are then released by the diffusers into the high pH water. A pH probe is installed
downstream of the gas injection point, thus measuring pH in the water after absorption
and CO2 reaction. An in-line CO2 pH control installation is depicted in this document.
Porous Diffuser
pH Controller
CO2 Manifold CO2 Storage Tank
pH Probe
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A pH control manifold is used to modulate gas flow automatically and achieve high
efficiency and reliability with little supervision from plant personnel. The CO2 pH control
technology can be customized to suit your control needs. Existing pH control instruments
may be retained when sulfuric acid is replaces with carbon dioxide.
Model Of Supply
For small requirements carbon dioxide is supplied with liquid cylinders. Larger volumes
will be delivered by truck and stored on site into a bulk CO2 storage tank. The CO2
storage tank is typically supplied, installed and maintained by the supplier of the gas.
Typically CO2 is stored in pressurized vessels up to 300 psig and thus do not require feed
or transfer pumps to supply the process.
Benefits of CO2 pH Control
Safe-to-use: carbon dioxide in the absence of water is inert and non-corrosive. It does
not require mechanical transfer or handling equipment. It becomes active only when
dissolved in water. CO2 leaks dissipate safely into the atmosphere, leaving no residue to
be neutralized. Furthermore, carbon dioxide does not corrode metal equipment. No
special alloy or plastic distribution piping is required for the CO2 system.
Accuracy of Regulation: CO2 is better than strong acids for controlling pH because it
forms a mild but highly reactive acid which minimizes risks of overt acidification and
rapidly responds to any variations of the incoming pH or water flowrate. Over or under
treatment with mineral acids will often result in a pH, which rapidly deviates from the
compliance range.
Low Maintenance System: Systems are generally engineered to be pressure driven.
With a minimum number of moving parts, this system offers continuous trouble free
operation. Moreover, trained technicians can be rapidly dispatched to service the bulk
CO2 tank in the unlikely event of a problem.
Flexibility: With a turndown ratio in control of the CO2 injection rate exceeding 10:1, the
pH control system will efficiently and rapidly respond to any fluctuation of flowrate or
incoming pH.
Safe for the Environment: There is no secondary pollution introduced into the treated
water by salts such as chlorides (from HCl) or Sulfates (from (H2SO4). The introduction of
CO2 will contribute to the chemical equilibrium of water by forming neutral carbonates and
bicarbonates.
