Objective 09: Demineralization Systems

Question 1

Demineralization

Answer 1

Removal of all mineral salts

Question 2

Demineralization System

Answer 2

An arrangement of cation and anion exchange beds, usually in series, which replaces cations with hydrogen ions (H⁺) and anions with hydroxyl (OH^-) ions.

Question 3

What factors determine the specific Demineralization System arrangement used?

Answer 3

1. The properties of the raw water
2. The desired properties of the treated water
3. Equipment costs
4. Regeneration costs
5. Ease of operation and control

Question 4

What are the differences in different Demineralization System Arrangements?

Answer 4

Different combinations of cation and anion exchangers but the anion exchanger always follows the cation exchanger.

Question 5

How do systems with both weak and strong cation exchangers require more equipment but yet reduce chemical costs as opposed to a single strong cation unit?

Answer 5

The weak cation exchanger has a higher regeneration efficiency and requires a lower concentration of acid regenerant. If it precedes the strong acid cation exchanger, it will remove most of the calcium and magnesium ions from the water and only the few that are left must be removed in the strong cation exchanger. Therefore, the strong cation exchanger requires less frequent regenerations, with a corresponding saving in chemical costs.

Question 6

Does the practice of combining weak and strong cation exchangers to reduce chemical costs also apply to anion exchangers?

Answer 6

Yes

Question 7

What function do strong anion exchangers perform?

Answer 7

Silica removal

Question 8

What is a benefit of having multiple ion exchangers in a system?

Answer 8

Individual exchangers can be removed from service for regeneration, without interrupting system production.

Question 9

How is carbonic acid produced in Degasification Units?

Answer 9

It is produced by some of the reactions in the cation exchangers.

Question 10

Can weak anion exchangers remove carbonic acid downstream from cation exchangers?

Answer 10

No, only strong anion exchangers

Question 11

What is the danger of allowing carbonic acid to remain in the water?

Answer 11

It can break down at reduced pressure into carbon dioxide which can cause corrosion.

Question 12

Where are degassifiers typically placed in large demineralization systems?

Answer 12

Between Cation and Anion Exchangers

Question 13

What are the 4 steps in a typical Regeneration of a Demineralizer Exchanger?

Answer 13

1. Remove from Service
2. Backwash
3. Regeneration
4. Rinsing

Question 14

What determines the specific procedures and parameters for regeneration of demineralization systems?

Answer 14

The particular arrangement and established operating guidelines.

Question 15

What are 2 instances where it is determined that Regeneration should be performed on a Demineralization System?

Answer 15

1. A predetermined throughput, regardless of whether or not the resin is fully exhausted
2. Breakthrough of undesirable ions which occurs when the resin is nearing exhaustion

Question 16

How does one determine when breakthrough occurs in a Demineralization System?

Answer 16

Testing at the exchanger outlet either through automatic test equipment or manually by an operator

Question 17

What are some typical examples of parameters that indicate breakthrough has occurred?

Answer 17

1. Weak acid cations show an increase in alkalinity (e.g. 40 ppm) and pH (e.g. 5.5)
2. Strong acid cations show a sharp drop in free mineral acidity and/or a rise in hardness
3. Strong anion exchangers will show a sharp rise in conductivity and/or silica

Question 18

What is the purpose of backwash in the Regeneration of a Demineralization System?

Answer 18

To loosen and fluff the settled resin and to remove any deposits.

Question 19

What caution must be taken when backwashing a Demineralization System?

Answer 19

The backwash flow rate must be kept within reason, to avoid loss of resin from the exchanger.

Question 20

What 3 factors must be controlled in putting regenerate solution through a Demineralization System?

Answer 20

1. Concentration
2. Flow Rate
3. Total Quantity

Question 21

What is a method to control concentration in the regenerant?

Answer 21

To mix it with dilution water

Question 22

What are the typical concentrations that Demineralization Systems are regenerated with?

Answer 22

1. Weak acid cations use a solution of <0.5% sulphuric acid
2. Strong acid cations use a solution of <2% to 8% sulphuric acid
3. Weak base anions use a solution <1% caustic soda
4. Strong base anions use a solution of about 4% caustic soda

Question 23

What is the danger of using excessive regenerant?

Answer 23

Damage and fouling of the resin

Question 24

What are the 2 stages of rinsing performed on a Demineralization System during Regeneration?

Answer 24

1. Slow Rinse

2. Fast Rinse

Question 25

Describe the Slow Rinse Stage performed during Regeneration

Answer 25

A small flow gradually pushes all regenerant down through the bed, thus making use of all regenerant.

Question 26

Describe the Fast Rinse Stage performed during Regeneration

Answer 26

A high flow of water displaces all regenerant and undesirable ions from the bed, to the sewer until tests prove the water is suitable for the exchanger to be returned to service.

Question 27

What generally produces the highest quality of treated water?

Answer 27

Mixed Bed Demineralizers

Question 28

What happens financially if a Mixed Bed Demineralizer is only used?

Answer 28

1. Equipment cost is low

2. Regeneration Chemical Costs are high

Question 29

How are Mixed Bed Demineralizers typically used?

Answer 29

Downstream of all other exchangers as a final treatment of the treated water.

Question 30

Describe the makeup of Mixed Bed Demineralizers

Answer 30

The exchanger bed is a mixture of strong acid cation and strong base anions resins. As the water flows through the bed, there is and exchange of both cations and anions. When exhausted, the two resins must be regenerated separately.

Question 31

What is the Interface Collector used for in Mixed Bed Demineralizers?

Answer 31

It is located at the interface of the 2 resins and anion regenerant enters it from the top and cation regenerant enters it from the bottom and through it they leave.

Question 32

Describe the Regeneration process for Mixed-Bed Demineralizers

Answer 32

1. Taken out of service
2. The mixed bed is backwashed causing the differently sized anion and cation resin beads to separate into 2 distinct layers
3. The cation resin is regenerated with acid using reverse flow from below the bed and up through until it exits to drain at the interface collector
4. The anion resin is then regenerated with caustic which flows downward through the anion and out through the interface collector to drain
5. The resins are rinsed
6. The vessel is drained
7. Air is injected from below the bed to remix the resins
8. Refill is done
9. Final rinse performed to remove lingering regenerant and undesirable ions.

Question 33

Do the separate resins in Mixed-Bed Demineralizers have to be regenerated separately?

Answer 33

No, it can be also be done simultaneously depending on the particular unit

Question 34

What is the regeneration time for a Mixed-Bed Demineralizer?

Answer 34

2-3 hours

Question 35

What is a solution to the long regeneration times of Mixed-Bed Demineralizers?

Answer 35

External Regeneration

Question 36

External Regeneration

Answer 36

The resins are sluiced from the exchangers and sent to a separate regeneration facility

Question 37

What are 2 advantages of External Regeneration?

Answer 37

1. Standby resin can replace the spent resin immediately with the exchanger out of service for a relatively short time
2. The chances of contaminating the treated water with acid or caustic soda are reduced.