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PROCESS DESIGN > Column Design > Flashcards

Column Design Flashcards

1
Q

What is the maximum height for a plate distillation column?

A

30-50m

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2
Q

What is the maximum diameter range for a plate distillation column?

A

1-9m

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3
Q

What is the problem with a plate distillation column having a diameter less than 1m?

A

Surface area to volume ratio is too high so there will be lots of heat loss through the walls & a high heat duty at the bottom of the column.

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4
Q

What is the average range for plate tray spacing?

A

0.45-0.6m

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5
Q

How do you calculate the height of the top part of a column?

A

Top Space = 1 tray space + 0.15m + dome height (approx 0.3-0.4m)

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6
Q

How do you calculate the height of the centre section of a distillation column?

A

Centre space = (n-1) trays x tray spacing

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7
Q

How do you calculate the height of the bottom part of a distillation column?

A

Bottom space = 1 tray + 0.15m + 10 mins of liquid.

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8
Q

How do you calculate reflux ratio, R?

A 
R = Liquid back in column/Distillate off
R = L/D
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9
Q

Define ‘total reflux’.

A

The minimum number of plates required for separation if no product is being withdrawn from the column (D=0).

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10
Q

What is the effect on the cost of a plate column when the reflux ratio increases?

A

Reflux ratio increases, number of stages decreases, shorter column, capital cost decreases, operating cost increases

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11
Q

Describe the differences between having a plate column that is pressurised or vacuumed.

A

Pressurised - T increases, low-boiling material, varying alpha.
Vacuum - T decreases, high-boiling material, no thermal breakdown.

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12
Q

Describe 3 features of atmospheric distillation.

A

1) Cheap construction
2) Materials above atm at top of column prevent air in
3) Pressure decreases down column due to liquid on plates.

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13
Q

When distilling to separate 3 components, which component should be separated first? Why?

A

Remove the largest % component first. This reduces costs as the 2nd column can be made shorter than the first as there is less in.

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14
Q

What direction does the ‘q-line’ go in on a McCabe Thiele curve for a system at bubble point?

A

Vertical

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15
Q

What is the gradient of the top & bottom operating lines on a McCabe Thiele curve?

A 
TOL = R/R+1 = L(RT)/V(T) 
BOL= L(B)/V(RB)
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16
Q

What letters represent the light & heavy component in a column?

A 
A = light component
B = heavy component
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17
Q

Define Fenske’s equation. What does it assume?

A

Assumes total reflux.
Nm (top) = log[(A/B)T x (B/A)F] / log(alpha)T/F
Nm (bottom) = log[(A/B)F x (B/A)B]/log(alpha)F/B

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18
Q

How do you calculate the number of theoretical plates from the Fenske equation?

A

Nm = n + 1

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19
Q

Define the Underwood equation.

A

Rmin = (1/alphaF - 1) x [(D/F)A - alphaF (D/F)B]

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20
Q

What are the standard reflux ratio values for a refrigerated & hot service column system?

A

Refrigerated, R = 1.2Rmin

Hot service, R = 1.35Rmin

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21
Q

Give the x and y axis equations for the Guilliand Correlation.

A

x: R - Rmin/R + 1
y: N - Nm/N + 1

22
Q

What is the reflux ratio for a reboil ratio of 95kW?

A

R = 1.57

23
Q

What are the 4 types of tray?

A

1) Sieve
2) Bubble cap
3) Valve tray
4) Disc & donut tray (heavy fouling fluids)

24
Q

What is the standard hole size for a column plate?

A

= 5mm

Range 2.5-12mm

25
Q

How do you calculate plate efficiency?

A

E = no. ideal stages/no. real stages

26
Q

Draw a column plate & label the following features.

  • Downcomer & weir
  • Manway
  • Plate support ring
  • Major beam
  • Peripheral ring clamps
A

See notes

27
Q

What is the standard weir height for a atmospheric and vacuum plate column?

A 
ATM = 40-90mm
Vac = 6-12mm
28
Q

What is the standard weir length & range for a plate column?

A

L = 0.77 (12% downcomer)

Range 0.6-0.85 x D

29
Q

What is the purpose of the weir?

A

Maintains a liquid level on the plate at one time. The higher the weir, the greater mass transfer takes place but causes larger pressure drop.

30
Q

What is weeping? What is it a result of?

A

When liquid is seeping through plate perforations. It is a result of low vapour flow rate which doesn’t exert enough pressure to hold up the tray.

31
Q

What is coning? What is it a result of?

A

Coning is when vapour flow rate exceeds the liquid flow rate so separation is lost. It is a result of low liquid flow rate.

32
Q

What is entrainment? What is it a result of?

A

When vapour carries liquid onto higher up trays, effecting purity & causing flooding. It is a result of high vapour flow rate.

33
Q

What is downcomer backup flooding? What is it a result of?

A

Liquid overflows weir & goes through downcomer where vapour bubbles up causing foaming & flooding. It is a result of high liquid flow rate.

34
Q

How do you calculate F(LV) to use the flooding correlation?

A

F(LV) = L/V x sqrt(vapour density/liquid density)

35
Q

How do you calculate column diameter for a plate column?

A

1) Use flooding correlation for F(LV) to find K1 (2dp)
2) uf = K1 x sqrt[(density liquid - vapour)/density vapour]
3) active area = vapour volumetric flow rate/(uf x 80%)
4) Assume 12% is downcomer so inactive
Total A = Active A/(1-0.12)
5) A = pi/4 x D^2

36
Q

How do you calculate % flooding for a packed column?

A

flooding % = sqrt (D/K) x 100

loading % = (actual A/theoretical A) x flooding %

37
Q

When may a packed column be chosen for distillation as oppose to a plate column?

A
  • Lower pressure drop
  • Smaller diameter
  • Allow corrosive materials
  • Allow foaming liquids
38
Q

What is the standard diameter for a packed column?

A

15 x packing diameter

39
Q

What are the 3 main materials used in random packing? Give 2 examples for each.

A

1) Metal - raschig ring, pall ring
2) Plastic - cascade ring, teller rosette ring
3) Ceramic - raschig ring, berl saddle

40
Q

What are the benefits to having structured packing rather than random packing?

A

Packing fits the column perfectly.

41
Q

Draw a diagram and label a packing column with 6 features.

A
  • *See notes**
    1. Demister
    2. Distributor
    3. Hold-down plate
    4. Packing (eg. raschig rings)
    5. Support
    6. Collector
42
Q

How do you calculate the equivalent height of the packing from the number plates.

A

Height Equivalent Theoretical Plate (HETP) index directly correlates this so;
Height packing = plate no. x HETP

43
Q

What is the loading point?

A

The point where all surfaces are covered in liquid when having increased the liquid volume from 0.

44
Q

How does a packed column become flooded?

A

Liquid volume increases past loading point making it harder for the gas to push through the packing until its flooded.

45
Q

What is the standard pressure drop for vacuum distillation in a packed column?

A

10mm water/m packing

46
Q

What is the standard pressure drop for a gas absorber packed column?

A

15-50mm water/m packing

47
Q

What is the standard pressure drop for atmospheric distillation in a packed bed column?

A

40-80mm water/m packing

48
Q

When using pressure drop correlations for a packed column, what does V*w denote?

A

Gas mass flow rate per unit area (kg/m2 s)

49
Q

How do you calculate relative volatility? (alpha)

A

alpha = (yA/xA) / (yB/xB)

50
Q

What percentage of the flooding gas flow rate is the loading gas flow rate?

A

Approx 70%.

PROCESS DESIGN (6 decks)

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