28. If the separation were operated in a tray column, with material flow conditi
ID: 109321 • Letter: 2
Question
28. If the separation were operated in a tray column, with material flow conditions and operating conditions the same, how many trays would be required? Assume average tray efficiency = 0.58.
A. 7
B. 5
C. 8
D. 3
For questions 24 - 28: so, is to be removed froma 3 mol % SO2-air mixture by scrubbing with water in a tower packed with 1.5-in plastic Nor-Pac. It is desired that 98% of entering SO2 be removed. Entering gas rate is 800 lbh and the water rate is 5000 Ib/h. The gas mixture behaves ideally Data: average density of liquid stream average molecular weight of gas stream operating temperature operating pressure equilibrium relation 62.1 Ib/ft 30.5 20°C 101.325 kPa y = 3.05x Estimate NTU at No A. 2.67 B. 3.86 C. 4.08 D. 4.99 If Ka 2.86 x 10 Ibmol/s (ft/ft), estimate the packed height. A. 15.9 ft B. 12.7 ft C. 10.4 ft D. 16.7 ft Estimate the flooding velocity A. 4.4 ft/s B. 6.1 fts C. 7.4 ft/s D. 8.1 fus The absorption factor is approximately A. 3.5 B. 3.1 C. 2.7 D. 4.0Explanation / Answer
The reflux ratio, R, is normally defined as:
R = flow returned as reflux / flow of top product taken off
The number of stages required for a given separation will be dependent on the reflux ratio used.
In an operating column the effective reflux ratio will be increased by vapor condensed within the column due to heat leakage through the walls. With a well-lagged column the heat loss will be small and no allowance is normally made for this increased flow in design calculations. If a column is poorly insulated, changes in the internal reflux due to sudden changes in the external conditions, such as a sudden rain storm, can have a noticeable effect on the column operation and control.
Total reflux is the condition when all the condensate is returned to the column as reflux: no product is taken off and there is no feed. At total reflux the number of stages required for a given separation is the minimum at which it is theoretically possible to achieve the separation. Though not a practical operating condition, it is a useful guide to the likely number of stages that will be needed. Columns are often started up with no product take-off and operated at total reflux till steady conditions are attained. The testing of columns is also conveniently carried out at total reflux.
As the reflux ratio is reduced a pinch point will occur at which the separation can only be achieved with an infinite number of stages. This sets the minimum possible reflux ratio for the specified separation.
Q = heat to vaporize 1 mol of feed / molar latent heat of feed.
The feed point should be located on the stage closest to the intersection of the operating lines.
The re-boiler, and a partial condenser if used, act as equilibrium stages. However, when designing a column there is little point in reducing the estimated number of stages to account for this; they can be considered additional factors of safety. The following guide rules can be used to estimate the efficiencies for a multicomponent system from binary data:
1. If the components are similar, the multicomponent efficiencies will be similar to the binary efficiency.
2. If the predicted efficiencies for the binary pairs are high, the multicomponent efficiency will be high.
3. If the resistance to mass transfer is mainly in the liquid phase, the difference between the binary and multicomponent efficiencies will be small.
4. If the resistance is mainly in the vapor phase, as it normally will be, the difference between the binary and multicomponent efficiencies can be substantial.
Column-top temperature 65 0C, bottom temperature 120 0C.
Average relative volatility light key = 2.0
Take the viscosity at the average column temperature, 93 0C,
viscosities, SO2 = 0.03 mNs/m2
butane = 0.12 mNs/m2
pentane = 0.14 mNs/m2
For feed composition, molar average viscosity = 0.03 * 0.05 + 0.12*(0.15 + 0.25)+ 0.14*(0.20+ 0.35) = 0.13 mNs/m2 = 0.26
Therefore the NTU = 2.67
Packed height = 12.7 ft
Flooding velocity = 6.1ft/s
Absorption factor = 3.1.
Assuming average tray efficiency = 0.58, 7 trays would be needed.
The method is well established, and in the absence of experimental values, or proprietary prediction methods, should be used when more than a rough estimate of efficiency is needed. The approach taken is semi-empirical.
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