Ca 2 + in water Ca 2+ & Na + Na + in water Soft water CIVL 375: Experiment 7 SOFTENING BY ION EXCHANGE The most usual ion exchange material employed in water softening is a sulphonated
Trang 1Ca 2 + in water
Ca 2+ & Na +
Na + in water
Soft water
CIVL 375: Experiment 7 SOFTENING BY ION EXCHANGE
The most usual ion exchange material employed in water softening is a sulphonated styrene based resin, supplied by the makers in the sodium form The resin has a strong affinity for calcium and magnesium ions and will also remove ferrous ions after nearly the complete removal of calcium and magnesium
Softening can be carried out as a batch process However, it is more convenient to operate a continuous flow process by passing the water slowly downwards through a column of resin beads The exchange reaction takes place rapidly enough for the upper layers of the bed to approach exhaustion before the lower layers being able to exchange ions
There is thus, a zone of active exchange which move down the column until the resin at all depths becomes exhausted The position at an intermediate stage can be illustrated as shown below
When the zone of active exchange reaches at the bottom of the column, the emerging water starts to show an increasing hardness This is the breakthrough, when it becomes necessary to regenerate the resin with a strong sodium chloride solution
Objective
To determine the exchange capacity of a cationic resin in the softening of hard water
Procedure
1 Take up to 10 liters of hard water (ground water)
2 Determine the hardness of water and place it in the test water reservoir
Original hard water
Hardness
Trang 23 Make up 200 ml of 10% NaCl solution by dissolving 20 g NaCl in distilled water and then place this solution in regenerate tank B
Backwashing
Fill the cation exchanger with cation resin to a depth of 300 mm Select tank D open valve 3 and 6 and back wash for five minutes Gradually turn off and measure the final depth
Regenerate
Select tank B, open valves 2, 12 (and 10 for sampling) Set flow meter to 10 ml/min and continue flow for 15 minutes
Softening
Select tank C and open valves 2 and 10 Set the flow meter to 50-70 ml/min Collect samples
at five minutes intervals Determine the hardness of each sample Continue until the hardness reaches more than 50% of the value for the influent (input) water
Results and Calculations
1 Fill in the following table
6 5
4 3
2 1
Sample
30 25
20 15
10 5
Time
Final depth
Volume of water treated (ml)
Hardness mg/l
2 Calculate the exchange capacity using
depth final bed
re of volume
4
) 10 15 ( sin
2 3
bed wet of Volume
CaCO as l mg water original of
hardness liters
Volume capacity
where Volumebreakthrough is the volume of water treated until the breakthrough point
Requirements:
1 Fill out the provided experiment data sheet and carry out the required calculations
2 If you are requested to submit a report of this experiment then your report should contain (a) an introduction that ends with the objective (b) methodology including sample
identification (c) results and discussion (d) conclusion (e) references and (f) an appendix that contains the filled data sheet
Trang 3CIVL 375: Experiment 7 Data Sheet SOFTENING BY ION EXCHANGE
Experimental Conditions
Experiment date
Sample identification
Initial concentration of
hardness (mg/l as CaCO3)
Resin name
Column diameter (cm)
Length of packed resin (cm)
Flow rate (ml/min)
Effluent Hardness
Time
(min)
Hardness (mg/l CaCO3)
Time (min)
Hardness (mg/l CaCO3)
Required Calculations
1 Plot hardness versus time
2 Calculate the exchange capacity