DI WATER/ DEMI WATER -- CAPT AJIT VADAKAYIL
DI water is used to flush out the chlorides from CHEMICAL TANKER tank bulkheads prior sensitive wall wash cargoes .
Methanol cannot dissolve chlorides. Water is a good solvent for salt . It has a high di-electric constant or relative permitivity. ( 81 --it has no units ). This means the force of attraction between ions is 81 times less in water than in a vacuum and this allows them to separate. High di-electric constant is equivalent to high insulating properties to lower the attraction between the oppositely charged ions once they are solvated. Water solvates both cations and anions. When you do a chloride wall wash test rememer that a pinch of salt put in methanol remains there , and water is the best solvent.
BASICS: A positive ion is called a cation and negative ion is called anion. The protons and neutrons are held together very tightly in the nuclues. But the outer orbital electrons are held very loosely. They can jump from one atom to another. It is also possible to shift electrons from one atom to another. The atom which loses one electron has one excess proton as a result in the tight nucleus and thus gets unbalanced. Such an atom is called a positive ion.
Ion exchange units are insoluble, cross linked, long chain organic polymers with microporous structure and the functional groups attached to the chain are responsible for ion exchanging properties.
Resins containing acidic functional groups are capable of exchanging H+ ions with other cations.
Resins containing basic functional groups are capable of exchanging anions with other anions.
Ion Exchangers are thus insoluble granular substances which have in their molecular structure acidic or basic radicals that can exchange. The positive or negative ions fixed on these radicals are replaced by ions of the same sign in solution in the liquid in contact with them.
CATION EXCHANGE RESINS: Copolymers which on sulphonation and carboxylation become capable to exchange their hydrogen ions / H+ with cations in water.
ANION EXCHANGE RESINS: Copolymers which after treating with NAOH solution become capable to exchange their OH- ions with anions in water.
First filter the water to reduce turbidity to less than 10 ppm.
Then this filtered water is first passed through cation exchange cylinder which removes CA2+/ MG2+ ( water hardness ions ) cations from it and an equivalent amount of H+ ions are released into the water. The water is now softened. (THIS ACIDIC WATER CAN DESTROY YOUR ZINC SILICATE COATING WHICH CANNOT TAKE pH LESS THAN 6.)
Then this water is passed through an anion exchange cylinder which removes SO42- CL- ( chlorides ) anions from it and an equivalent amount of OH- ions are released into the water. The chlorides are now removed. The water is now called de ionized water or de mineralized water.
The ion exchange is complete without deterioration or changing the total number of ions in the liquid before the exchange
H+ cations and OH- anions get combined to form a water molecule H2O.
When the capacity of cation and anion exchangers to exchange H+ and OH- ions is lost they are said to be exhausted. Regeneration of cation cylinder is done by dilute Hydrochloric acid or Sulphuric acid .
Anion cylinder is regenerated by dilute Sodium Hydroxide. Then both the cylinders are washed with DI water to wash of the unwanted Ca2+, Mg2+, SO42, CL- ions.
Regardless of brand, the core principles of ion exhange resin remain the same. There are 4 main types of resin, each with their own function: Strong acid cation (SAC), Weak acid cation (WAC) , Strong base anion (SBA), Weak base anion (WBA).
MIXED BED :
Mixed Bed resins, sometimes referred to as mix bed resin, are a combination of cation and anion resins. Usually the ratio for mixed bed resin is 1:1 or 1:2 depending on the application.
Single cylinder containing intimate mixture of cation/ anion exchange resins. When the mixed bed is exhausted it is backwashed by by forcing water in the reverse upward direction. The lighter anion resins gets displaced on top and the heavier cation resins remain at the bottom. Then regenerate anion resin beads with NAOH and cation beads with Sulphuric acid.
AMBERLITE marketed by Rohm and Haas is usually used on chemical tankers as they are available in major ports.
Amberlite MB20---- Ionically equilibrated mixed bed resin. Mixture of strongly acidic cation exchanger with a strongly basic type anion exchanger.($)
Amberlite UP6150--- Fully regenerated mixed bed of cation and anion exchange resins.($$)
Amberlite IRN150--- Gelular polystyrene cation and anion exchange resin ($$$--expensive)
Water softening is done by cation exchange using Alumino silicate gel.
De ionizing and demineralising involves two steps. Soluble salts are converted to acids by passing through a Hydrogen exchanger. Secondly they are removed by an acid adsorbent or synthetic resin.
Mineral contaminants are usually present in ionized form.
In general cation resin for water softening and demineralization may last 5 to 10 years. Anion resins used in demineralization last anywhere from 3 to 5 years and are dependent on operational conditions. Like every other resin, the life of mixed bed resin is dependent on the application in which it is being used.
Resin life is difficult to predict; in critical applications it’s best to start analyzing and benchmarking the resin at least once per year.
Exchange capacity: This is the weight of ions that can be retained per unit volume ( or sometime per unit weight ) of the exchange material concerned
Bed volume: volume per hours of liquid to be treated / volume of resin
Ion flux: Bed volume * salinity of water
Regeneration level: weight of reagent used / volume of ion exchange material
Regeneration rate: 100* (grams-equivalents of regenerating reagent)/ (grams-equivalents of the eluted ions)
Regeneration efficiency: This is the opposite ratio above
Ion leakage: This is the concentration of unwanted ion left in the treated liquid
In European ports it is cheaper to get a truck of DI water which is wasted from nuclear plants.
Use Graco pumps ( President and Monark models ) for spraying DI water.
For the uninitiated after reading the above---
Check the taste of the DI water you generate before spraying it on a zinc bulkhead— if the taste is acidic sour , check the pH. As you already know zinc silicate lining can take only 6 to 9 pH.
SINGLE FILTER DEMINERALISER.
The practical aspects of a common place DI water single cylinder mixed bed filter , available on board Che,ical tankers is described herein.
The cylinder is mounted vertically and has a working pressure of 8 bars. The max pressure should be 2 bars higher. Install a pressure relief valve in your system if there is no effective control of max line pressure.
The mixed bed resin used here is a polishing resin, to produce ultra pure water from FWG water feed. Don’t try to use shore water as you will exhaust the resin fast. You can measure the out put by chloride PPM or by using a conductivity meter—the max allowed is 750 uS/ cm
Pureness of the DI water depends on flow velocity through filter. Ensure the water flows in the correct direction. After filling the mixed bed resin, fill the water slowly at first.
After use the bed must be dry after draining out all water. Filter capacity for this model is about 250 to 900 litres per hour
Replace the resin when the conductivity becomes high eg- 10 uS/cm or in case the resin if it has a coloring agent, gets discoloured.
You can regenerate the exhausted resin, if you land it ashore. To remove old resin put the filter horizontal and flush out with water.
Take the riser pipe with bottom distributor out of the filter.
Close the top end of the riser pipe with some tape
Make sure that the riser pipe and the bottom distributor is not filled with resin. Put the riser pipe with bottom distributor into the filter
Put funnel on top of the filter and fill the resin
Take care during filling that the riser pipe is situated in the middle of the filling hole
After filling , take the funnel off
Take the tape off the riser pipe.
Clean the internal thread of the filter carefully.
Clean the top distributor and mount it on the adaptor
Mount this assembly on the riser pipe and filter.
Screw the adaptor clockwise hand tight
Connect the FW inlet and outlet pipes
Filter is now ready for use. Water enters from top and exits from below.
Proforma: Conductivity / PPM chlorides/ Production in tons
1 / 0.3 / 750
2 / 0.6 / 375
5 / 1.5 / 150
10 / 3 / 75
20 / 6 / 37
In case the conductivity is above the desired limits , reduce the flow through the filter. If this does not give results, renew the resin. You can convert conductivity into ppm , by using a graph provided.
For best results the filter must be properly installed and operated as per the instruction manual.
After the end of DM water production, if the filter will not be used for a period longer than one week, drain all water from the filer, leaving it dry. Close both inlet and outlet valves.
Protect the system and the resin from frost. No make of resin in the filter can stand frost.
Maintain a log, with columns for date, FW quality from FWG in pp, flow rate in litres/ hour, DI water out put ppm and DI water quality in litres.
WATER HARDNESS is defined as concentration of Cal/ Mag cations in terms of Calcium Carbonate. Concentration is measured in degrees hardness. There are many units like American/ English/ French/ German hardness etc.
Hardness units: 1 ppm = 1 mg/l = 0.1 Fr = 0.07 Cl = 0.02 Meq/l
Fr = French Cl = Clarke Meq = milli equivalent
Good fresh water has a hardness of 2 while seawater has a hardness of 6800 mg/l