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Water/Waste Water Treatment > Chap 8 > Flashcards

Chap 8 Flashcards

1
Q

8.1a. Capex

A

Capital expense

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2
Q

8.1a. Opex

A

operational expense

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

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3
Q

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

A

-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)

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4
Q

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

A

Lime is $70/T

Caustic Soda is $450/T

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5
Q

8.3b II. reaction shows Adding lime removing calcium hardness

A

CaO + H2O

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

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6
Q

8.3j. Figure from Betz

Why water recirculated

A 
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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7
Q

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

A

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

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8
Q

8.4c. ii. WAC regenerated with HCL then NAOH

A

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

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9
Q

8.2b 2 ways of additives used to prevent the scaling

A

Crystal Modifiers

chelating agents

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10
Q

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

A

Cold lime 35 - 50 mg/l CaCO3

HLS: 10 -20 mg/l CaCO3

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11
Q

8.3c. reactions show how slaked lime reacts with CO2

Ca(HCO3)2 Mg(HCO3)2

A

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

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12
Q

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

A

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

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13
Q
  1. 3d Reactions: Lime soda softening involved in
    a. Na2CO3 is used to remove Permanent Ca Hardness
    b. Na2CO3 used to remove permanent Mg Hardness
A

-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

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14
Q

8.3e. factors that affect the solubility of hardness ions

A
  • the ions present
  • the pH
  • the temp
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15
Q

8.3f. Four ways along w chem used to keep opex minimum

E + Ca2 + HCO3 + OH- = CaCO3 + H2O

A
  • 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
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16
Q

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

A
  • 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
17
Q

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

A
  • the crystals that do not settle out
18
Q

8.2b. 2 types of additives to prevent scaling

A
  • Crystal Modifier

- Chelating agents

19
Q

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

A
  • Lime softening
  • Ion exchange
  • Nanofiltration
20
Q

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

A
  • Evaporation
  • Cooling tower and cooling water
  • membranes and reverse osmosis
  • boilers
21
Q

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

A
  • cold lime: Ca hardness reduced to 35-50ppm

- Hot lime:10-20ppm

22
Q

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

23
Q

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

24
Q

8.3h. 3 options for concentrating the sludge

A
  • Settling tanks (100x50x10m)
  • Centrifuges
  • belt processes
25
Q

8.4a. HLS and SAC

A

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

26
Q

8.4a. HLS and SAC

A

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

27
Q

8.4c Regeneration by HCl

A

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

step 2:

28
Q

8.4c Regeneration by HCl

A

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
Q

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

A

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

30
Q

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

A

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

Water/Waste Water Treatment (9 decks)

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