Column Design

Question 1

What is the maximum height for a plate distillation column?

Answer 1

30-50m

Question 2

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

Answer 2

1-9m

Question 3

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

Answer 3

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.

Question 4

What is the average range for plate tray spacing?

Answer 4

0.45-0.6m

Question 5

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

Answer 5

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

Question 6

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

Answer 6

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

Question 7

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

Answer 7

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

Question 8

How do you calculate reflux ratio, R?

Answer 8

R = Liquid back in column/Distillate off
R = L/D

Question 9

Define ‘total reflux’.

Answer 9

The minimum number of plates required for separation if no product is being withdrawn from the column (D=0).

Question 10

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

Answer 10

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

Question 11

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

Answer 11

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

Question 12

Describe 3 features of atmospheric distillation.

Answer 12

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

Question 13

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

Answer 13

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

Question 14

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

Answer 14

Vertical

Question 15

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

Answer 15

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

Question 16

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

Answer 16

A = light component
B = heavy component

Question 17

Define Fenske’s equation. What does it assume?

Answer 17

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

Question 18

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

Answer 18

Nm = n + 1

Question 19

Define the Underwood equation.

Answer 19

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

Question 20

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

Answer 20

Refrigerated, R = 1.2Rmin

Hot service, R = 1.35Rmin

Question 21

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

Answer 21

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

Question 22

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

Answer 22

R = 1.57

Question 23

What are the 4 types of tray?

Answer 23

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

Question 24

What is the standard hole size for a column plate?

Answer 24

= 5mm

Range 2.5-12mm

Question 25

How do you calculate plate efficiency?

Answer 25

E = no. ideal stages/no. real stages

Question 26

Draw a column plate & label the following features. - Downcomer & weir - Manway - Plate support ring - Major beam - Peripheral ring clamps

Answer 26

**See notes**

Question 27

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

Answer 27

``` ATM = 40-90mm Vac = 6-12mm ```

Question 28

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

Answer 28

L = 0.77 (12% downcomer) | Range 0.6-0.85 x D

Question 29

What is the purpose of the weir?

Answer 29

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.

Question 30

What is weeping? What is it a result of?

Answer 30

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.

Question 31

What is coning? What is it a result of?

Answer 31

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

Question 32

What is entrainment? What is it a result of?

Answer 32

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

Question 33

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

Answer 33

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

Question 34

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

Answer 34

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

Question 35

How do you calculate column diameter for a plate column?

Answer 35

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

Question 36

How do you calculate % flooding for a packed column?

Answer 36

flooding % = sqrt (D/K) x 100 | loading % = (actual A/theoretical A) x flooding %

Question 37

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

Answer 37

- Lower pressure drop - Smaller diameter - Allow corrosive materials - Allow foaming liquids

Question 38

What is the standard diameter for a packed column?

Answer 38

15 x packing diameter

Question 39

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

Answer 39

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

Question 40

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

Answer 40

Packing fits the column perfectly.

Question 41

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

Answer 41

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

Question 42

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

Answer 42

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

Question 43

What is the loading point?

Answer 43

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

Question 44

How does a packed column become flooded?

Answer 44

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

Question 45

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

Answer 45

10mm water/m packing

Question 46

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

Answer 46

15-50mm water/m packing

Question 47

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

Answer 47

40-80mm water/m packing

Question 48

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

Answer 48

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

Question 49

How do you calculate relative volatility? (alpha)

Answer 49

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

Question 50

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

Answer 50

Approx 70%.