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There are two kinds of column, tray or packed, to choose from. In this project, the type of column is chosen after taking a few considerations into account (Peters, Timmerhaus, and West 2004, 772), for instance, the flow rate, the diameter and pressure drop. The flow rate of the feed is quite large; therefore, a tray column is preferred. When the flow rate is large, the diameter of the column will increase too. Normally, a tray column is not smaller than 0.67 meters and random packed column is not larger than 1.5 meters (Peters, Timmerhaus, and West 2004, 777). Thus, tray column is preferred. As for the pressure drop, it would be best that it is as low as possible. From the book, Plant Design and Economics for Chemical Engineers, it is preferred that a structured packed column is used to reduce the pressure drop (Peters, Timmerhaus, and West 2004, 772). As a conclusion, tray column is chosen due to the high flow rate and the large diameter of the column.
Determination of Actual Number of Stages
The number of equilibrium stage is taken from Hysys. There is other method to determine the number of equilibrium stages such as Kremser Equation. To use this method, the equilibrium line must be known. The equilibrium line can be obtained using the equation 1.1. The equilibrium line cannot be obtained due to the Henry's constant is not obtainable; therefore, the number of equilibrium stage is decided to be taken from the Hysys simulation. From Hysys, the actual number of equilibrium stage is 6.
YB = mole ratio of solute B in gas phase
XB = mole ratio of solute B in liquid phase
HB = Henry's constant for solute B
PT = total pressure
Determine the Column Efficiency
In real industrial practice, it is impossible for the mass transfer through absorption to have 100% efficiency. From Hysys, the actual number of equilibrium stage is 6.25. By using the equation suggested by Sinnott and Towler the efficiency of the column is calculated. Therefore, the equation is shown in equation 1.2.
The calculated efficiency is 80.0 %.
Selection of Plate Type
Main factors such as cost, performance, and operating range are considered in selecting plate type, which are between sieve plate, bubble-cap, and valve plate. Sieve plate is chosen as it is relatively cheaper than bubble caps and valve plate since sieve plate does not have moving part. Besides, sieve plate gives lowest pressure drop among the three plate types. Therefore, sieve plate is selected as it is suitable for most application and cheapest in cost (Sinnott and Towler 2009, 732).
Approximate Column Sizing
Selection of Tray Spacing
The normal spacing for a tray column is in between of 0.15 m to 1 m. Therefore the chosen spacing is the 0.5m spacing. The plate spacing will be the determination of the overall height of the column. Close spacing is only used when the diameter of the column is small (Sinnott and Towler 2009, 730).
Determination of Column Diameter
The key to the column diameter is the vapour flow-rate. This flow-rate should not be too small or too large. If the vapour flow-rate is too small, it will cause excessive liquid entrainment, as for too large, it will cause high-pressure drop across the column. By using the Souders and Brown equation (Sinnott and Towler 2009, 730), the superficial velocity can be found thus the area and diameter can be found. Below is the equation for Souders and Brown.
Where, ûv = maximum allowable vapour velocity, m/s
lt = plate spacing, m
ρL and ρv = density of liquid and vapour respectively, kg/m3
Before the net vapour velocity can be calculated, the liquid-vapour flow factor should be calculated as in equation 1.4, so that the Souders and Brown factor can be obtained from Figure 15-5 of Peters, Timmerhaus and West (2009, 778).
Next the actual vapour velocity need to be calculated. The actual vapour velocity is calculated by assuming it is 80% of net vapour velocity. Then the net column area is calculated using equation 1.5. The cross-sectional area of the column is the summation of the net column area and downcomer area. Downcomer area is assumed that it occupies 15% of the cross-sectional area as can be calculated using equation 1.6.
Where, m'v = volumetric flow rate of the vapour, m3/s
Vn = actual vapour velocity, m/s
Where, An = net column area, m2
With the known cross-sectional area of the column, Ac, the column diameter, DC can be calculated using the formula below.
Where, Vw = maximum vapour rate, kg/s
The calculated value of diameter is 1.322 m.
Determination of Column Height
With the known plate spacing, the column height can be calculated. The column height is important to be calculated as it is needed in the mechanical design of the column. The column height is calculated as below based on the equation from Plant Design and Economics for Chemical Engineers (2009, 779).
Where, N = number of trays
Hs = plate spacing, m
ΔH = additional height required for column, m, assume dome heads are 20% higher than the plate spacing.
Based on Dutta (2009, 205), it specifies the additional height required for the column as below:
Table . Additional Height Specification
Extra space at feed tray
Extra space at tray manholes
Total additional height, ΔH
Using equation 1.8, the calculated height is 7.5m.
Table : Summary of Column Design
Plate spacing (m)
Internal Column Design
Flooding Fraction Calculation
In any column of a plant, the flooding percentage must be known. Therefore, the flooding percentage of the absorber must be known with the operating conditions and flow-rate. The equation to use for the calculation is shown in equation 1.9.
Where, un = actual velocity based on the net area, m/s
ufa = a percentage of flooding velocity, m/s
From equation 1.9, the actual velocity based on the net area and the flooding velocity must be found before the flooding percentage can be found. The equation used for flooding velocity is also the same as the equation for the net vapour velocity.
The value of FLV is 0.0033 thus the K is 0.065. With all the values known, the flooding velocity can be calculated. The calculated value for the flooding velocity is 1.747 m/s. For the design purpose, a 80% of the flooding velocity, ufa is considered at 1.398 m/s. Next the cross-sectional area of the column which will also be the area of tray is to be calculated with just using the area formula. With the tray area of 1.373m2, the actual velocity is 1.188 m/s as calculated below.
Where, vi = inlet vapour rate, m3/s
Ac = area of tray/column, m2
The downcomer area is to be assumed of 15% of the cross sectional area of the column at Ad = 0.206 m2. The downcomer area is the area needed for the liquid to flow downwards to the last stage where it flows to the regenerator column.
The net are for the plate must be determined as the values will be needed in the following calculation.
Active area is the area where the mass transfer happens on the plate. Active area is also known as bubbling area.
Hole area is the area where the vapour pass through the holes to allow mass transfer and heat transfer. By assuming the hole area is 6% of active area, the hole area is 0.058 m2.
In this sub chapter, the weir height needs to be chosen. Based on Sinnott and Towler (2009, 747), it is recommended that 50 mm weir height to be chosen. In choosing a weir height, the efficiency and pressure needs to be considered. The higher the weir, the better efficiency of the tray, but it will also increase the pressure drop and vice versa. Thus 50 mm is chosen due to in the process, the pressure drop must be low and it is operating at high pressure. It is not possible to choose the weir height of 6 to 12 mm as the column is not operating at vacuum condition.
Based on the Figure 11.39 in Sinnott and Towler (2009, 748), the weir length can be determined. Since the assume downcomer area is 15% of the area tray, thus the lw/Dc is 0.81 from Figure 11.13. Therefore, the weir length, lw is 1.071 m.
Weir Liquid Crest
After the weir length is calculated, weir liquid crest can be estimated using the Francis formula in equation 1.10
Where, lw = weir length, m
how = weir crest, mm liquid
Lw = liquid flow rate, kg/s
The estimated weir crest is 24.41 mm liquid.
Weep point is the point where the liquid leaks through the plate holes excessively. This happens when the column operates at the lowest range of operating range. During the weep point, the vapour velocity is the minimum of the stable operation. Therefore, weep point must be determined to avoid weeping occurs in the column. The following equation 1.10 is the equation to determine the minimum vapour velocity at weep point.
Where, uh = minimum vapour velocity through the holes (based on the hole area), m/s
dh = hole diameter, mm
K2 = a constant, dependent on the depth of clear liquid on the plate
Based on Figure 11.37 (Sinnott and Towler 2009, 746), K2 is 28.6. The depth of the clear liquid is 100 mm. For the hole size, there is a range of 2.5 to 19 mm to be chosen from. Based on Sinnott and Towler (2009, 748), the recommended hole size is 5 mm. With the known values, the minimum vapour velocity is calculated as below.
The next step is to check whether the absorber column working above or below the weep point. To do that, just simply divide the volumetric flowrate of vapour with the hole area. The calculation is shown below.
Where, Vi = volumetric flowrate of vapour, m3/s
Ah = hole are, m2
This proves that the absorber column is operating above the weep point.