Column Design Flashcards
What is the maximum height for a plate distillation column?
30-50m
What is the maximum diameter range for a plate distillation column?
1-9m
What is the problem with a plate distillation column having a diameter less than 1m?
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.
What is the average range for plate tray spacing?
0.45-0.6m
How do you calculate the height of the top part of a column?
Top Space = 1 tray space + 0.15m + dome height (approx 0.3-0.4m)
How do you calculate the height of the centre section of a distillation column?
Centre space = (n-1) trays x tray spacing
How do you calculate the height of the bottom part of a distillation column?
Bottom space = 1 tray + 0.15m + 10 mins of liquid.
How do you calculate reflux ratio, R?
R = Liquid back in column/Distillate off R = L/D
Define ‘total reflux’.
The minimum number of plates required for separation if no product is being withdrawn from the column (D=0).
What is the effect on the cost of a plate column when the reflux ratio increases?
Reflux ratio increases, number of stages decreases, shorter column, capital cost decreases, operating cost increases
Describe the differences between having a plate column that is pressurised or vacuumed.
Pressurised - T increases, low-boiling material, varying alpha.
Vacuum - T decreases, high-boiling material, no thermal breakdown.
Describe 3 features of atmospheric distillation.
1) Cheap construction
2) Materials above atm at top of column prevent air in
3) Pressure decreases down column due to liquid on plates.
When distilling to separate 3 components, which component should be separated first? Why?
Remove the largest % component first. This reduces costs as the 2nd column can be made shorter than the first as there is less in.
What direction does the ‘q-line’ go in on a McCabe Thiele curve for a system at bubble point?
Vertical
What is the gradient of the top & bottom operating lines on a McCabe Thiele curve?
TOL = R/R+1 = L(RT)/V(T) BOL= L(B)/V(RB)
What letters represent the light & heavy component in a column?
A = light component B = heavy component
Define Fenske’s equation. What does it assume?
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
How do you calculate the number of theoretical plates from the Fenske equation?
Nm = n + 1
Define the Underwood equation.
Rmin = (1/alphaF - 1) x [(D/F)A - alphaF (D/F)B]
What are the standard reflux ratio values for a refrigerated & hot service column system?
Refrigerated, R = 1.2Rmin
Hot service, R = 1.35Rmin
Give the x and y axis equations for the Guilliand Correlation.
x: R - Rmin/R + 1
y: N - Nm/N + 1
What is the reflux ratio for a reboil ratio of 95kW?
R = 1.57
What are the 4 types of tray?
1) Sieve
2) Bubble cap
3) Valve tray
4) Disc & donut tray (heavy fouling fluids)
What is the standard hole size for a column plate?
= 5mm
Range 2.5-12mm
How do you calculate plate efficiency?
E = no. ideal stages/no. real stages
Draw a column plate & label the following features.
- Downcomer & weir
- Manway
- Plate support ring
- Major beam
- Peripheral ring clamps
See notes
What is the standard weir height for a atmospheric and vacuum plate column?
ATM = 40-90mm Vac = 6-12mm
What is the standard weir length & range for a plate column?
L = 0.77 (12% downcomer)
Range 0.6-0.85 x D
What is the purpose of the weir?
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.
What is weeping? What is it a result of?
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.
What is coning? What is it a result of?
Coning is when vapour flow rate exceeds the liquid flow rate so separation is lost. It is a result of low liquid flow rate.
What is entrainment? What is it a result of?
When vapour carries liquid onto higher up trays, effecting purity & causing flooding. It is a result of high vapour flow rate.
What is downcomer backup flooding? What is it a result of?
Liquid overflows weir & goes through downcomer where vapour bubbles up causing foaming & flooding. It is a result of high liquid flow rate.
How do you calculate F(LV) to use the flooding correlation?
F(LV) = L/V x sqrt(vapour density/liquid density)
How do you calculate column diameter for a plate column?
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
How do you calculate % flooding for a packed column?
flooding % = sqrt (D/K) x 100
loading % = (actual A/theoretical A) x flooding %
When may a packed column be chosen for distillation as oppose to a plate column?
- Lower pressure drop
- Smaller diameter
- Allow corrosive materials
- Allow foaming liquids
What is the standard diameter for a packed column?
15 x packing diameter
What are the 3 main materials used in random packing? Give 2 examples for each.
1) Metal - raschig ring, pall ring
2) Plastic - cascade ring, teller rosette ring
3) Ceramic - raschig ring, berl saddle
What are the benefits to having structured packing rather than random packing?
Packing fits the column perfectly.
Draw a diagram and label a packing column with 6 features.
- *See notes**
1. Demister
2. Distributor
3. Hold-down plate
4. Packing (eg. raschig rings)
5. Support
6. Collector
How do you calculate the equivalent height of the packing from the number plates.
Height Equivalent Theoretical Plate (HETP) index directly correlates this so;
Height packing = plate no. x HETP
What is the loading point?
The point where all surfaces are covered in liquid when having increased the liquid volume from 0.
How does a packed column become flooded?
Liquid volume increases past loading point making it harder for the gas to push through the packing until its flooded.
What is the standard pressure drop for vacuum distillation in a packed column?
10mm water/m packing
What is the standard pressure drop for a gas absorber packed column?
15-50mm water/m packing
What is the standard pressure drop for atmospheric distillation in a packed bed column?
40-80mm water/m packing
When using pressure drop correlations for a packed column, what does V*w denote?
Gas mass flow rate per unit area (kg/m2 s)
How do you calculate relative volatility? (alpha)
alpha = (yA/xA) / (yB/xB)
What percentage of the flooding gas flow rate is the loading gas flow rate?
Approx 70%.
