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Flashcards in Chap 8 Deck (30):
1

8.1a. Capex

Capital expense

2

8.1a. Opex

operational expense
choice of operations is based on the smallest (capex + opex) per L of water produced

3

8.2c. 3 methods used to manage scale by removal of ions

-IX (ion exchange)
-NF (Nano filtration) high capital cost, used with space restricted applications (off shore rigs)
-LS (lime softening) - first line of attack, depending on temps of feedwater
+Cold (cold river water)
+Warm(70 - 90C)
+Hot( above 100C)

4

8.3b. Lime is prefer than caustic soda (NaOH)

Lime is $70/T
Caustic Soda is $450/T

5

8.3b II. reaction shows Adding lime removing calcium hardness

CaO + H2O
Ca(OH)2 + Ca(HCO3)2 =CaCO3 + H2O

6

8.3j. Figure from Betz
Why water recirculated

Kinetic Energy ( parameters: Temps, [reactants], catalyst, surface)
Improve the contact of sludge btw unprecipitated and precipitated carbonates so as to speed up the preciitaition process.

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8.4c. i. WAC can be regenerated with acid but a SAC can not

WAC: weak acid cation exchange columns
SAC: Strong acid cation exchange columns
When pH

8

8.4c. ii. WAC regenerated with HCL then NAOH

HCl is corrosive and so NaOH is used to remove in this controlled way

9

8.2b 2 ways of additives used to prevent the scaling

Crystal Modifiers
chelating agents

10

8.3a. Equilibrium concentrations of Ca2+ as hardness that can be expected with hot and cold lime softening

Cold lime 35 - 50 mg/l CaCO3
HLS: 10 -20 mg/l CaCO3

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8.3c. reactions show how slaked lime reacts with CO2
Ca(HCO3)2 Mg(HCO3)2

+ CO2 = CaCO3 + H2O
+ Ca(HCO3)2 = 2CaCO3 + H2O
+ Mg(HCO3)2 = Mg(OH)2 + 2CaCO3 + H2O

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8.3d. Reactions:Ca(HCO3)2 + Ca(OH)2 -> 2CaCO3 + H2O

slaked lime does not precipitate the hardness (Ca2+). the OH- from the slaked lime convert HCO3 (organic) to CO3 (inorganic)

13

8.3d Reactions: Lime soda softening involved in
a. Na2CO3 is used to remove Permanent Ca Hardness
b. Na2CO3 used to remove permanent Mg Hardness

-CaSO4 + Na2CO3 = CaCO3 + Na2SO4
CaCl2 + Na2CO3 = CaCO3 + 2NaCl
-MgSO4 + Ca(OH)2 + Na2CO3 = Mg(OH)2 + CaCO3 + 2Na2SO4
MgCl2 + Ca(OH)2 + Na2CO3 = Mg(OH)2 + CaCO3 + 2NaCl

14

8.3e. factors that affect the solubility of hardness ions

- the ions present
- the pH
- the temp

15

8.3f. Four ways along w chem used to keep opex minimum
E + Ca2 + HCO3 + OH- = CaCO3 + H2O

- used Slaked lime (Ca(OH)2
- reuse waste heat
- keep pH high to prevent producing HCO3
-Source of OH-: Na2AL2O4 -> 2NaOH + 2 Al(OH)3

16

8.3g. Silica removal
Why Mg2+ is used
MAGOX process

- high [Si] in cooling towers, volatile at high temps -> redeposits on turbine blades which required perfectly balanced at high speed. precipitate throws off this balance and needed to be removed
- Hydroxide Mg more stable than carbonates
- MAGOX: 2 steps a) Magox + H2O = Mg(OH)2
b) introduce this slaked magox and allow the slow precipitation to occur to coprecipitate the silica

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8.3j. pin floc and output stream of a HLS must be filtered

- the crystals that do not settle out

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8.2b. 2 types of additives to prevent scaling

-Crystal Modifier
-Chelating agents

19

8.2c. 3 methods used to manage scale by removal of ions

-Lime softening
-Ion exchange
-Nanofiltration

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8.2d. 4 methods to manage scale by removal of water

-Evaporation
-Cooling tower and cooling water
-membranes and reverse osmosis
-boilers

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8.3a. [Ca] as hardness that expected w cold and hot lime softening

-cold lime: Ca hardness reduced to 35-50ppm
-Hot lime:10-20ppm

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8.3c. slaked lime Ca(OH)2 react with CO2, Ca(HCO)3, Mg(HCO3)2

see note

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8.3c. slaked lime Ca(OH)2 react with CO2, Ca(HCO)3, Mg(HCO3)2

see note

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8.3h. 3 options for concentrating the sludge

-Settling tanks (100x50x10m)
-Centrifuges
-belt processes

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8.4a. HLS and SAC

SAC: 1ppm hardness
HLS: 20ppm hardness
-> SAC must be downstream of LS

26

8.4a. HLS and SAC

SAC: 1ppm hardness
HLS: 20ppm hardness
-> SAC must be downstream of LS

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8.4c Regeneration by HCl

step 1: use HCl to replace Na and Ca w H+
step 2:

28

8.4c Regeneration by HCl

step 1: use HCl ($320/T) to replace Na and Ca w H+
step 2: Use NaOH($380/T)to replace H+ with Na (to prevent low pH in boiler -> prevent corrosion

29

8.4c Capex for WAC is increased bc of the need to contain HCl

iron must be coated with rubber or epoxy -> increase CAPEX

30

8.4c Capex for WAC is increased bc of the need to contain HCl

iron must be coated with rubber or epoxy -> increase CAPEX