T 401: Introduction: The absorption column that will be design will be used in a carbon dioxide capture process using DGA as the main amine. Column Sizing:
Finding the flow rate of each streams:
The basis flow rate given to our group is 55000 kg/h. We will take that as the flow rate for stream 33. Doing the water mass balance, solvent mass balance and hydrogen mass balance as shown in the appendix will give the flow rate of each streams: Stream number Mass flow rate (kg/h) 34 68793.2844 36 9688.2461 16 14288.3140 17 73244.0092
Finding the number of stages required:
Using Figure 1, the number of stages required at different mG
m
/L
m
will be determine: mG
m
/L
m
0.8 1.0 1.1 1.2 1.3 N
OG
2.2 2.8 3.2 4 6 We will chose the N
OG
of 6 as this will give a higher flow rate, hence the column will not be underdesigned.
Calculating column diameter:
The physical properties of the gas as found in HYSYS: Molar flow rate (kmol/h) Mass flow rate (kg/s) Density (kg/m
3
) Viscosity (Pa.s) Gas flow 1308 3.97
12.71 1.6×10

Liquid flow 2915 19.11
1015.12 1.1×10

Pressure drop given is 25 kPa or 2549.29 mmH
2
O, the packed bed height is 6 m, so the pressure drop/ height will be 425 mmH
2
O. The highest pressure drop per height in the Figure 11.54 is around 125 mmH
2
O, so we will take that and the column will be overdesigned, which is good for safety precaution or to upscale the process in the future. 3 sizes of Pall rings packing will be considered, which are 25 mm, 32 mm and 51 mm. Pall rings packing will be used as it is the most suitable packing for absorption of hydrocarbons. Packing size (mm) F
p
(m

) V
w
* (kg/m
2
s) Diameter (m) Percentage flooding (%) Packing size to column diameter ratio 25 160 3.63 2.6 91 104 32 92 4.79 2.3 88 72 51 66 5.65 2.1 90 41 The lowest percentage flooding is 88%. It is high, but still satisfactory. Furthermore, the column is already deliberately overdesigned to counter this type of problem. The diameter of the column will be 2.3 meter.
H
OG
estimation:
Cera
mic packing will be used as it is the first choice for corrosive liquid. The Cornell’s method and the Onda’s method will be used to estimate the H
OG
. These two methods have been found to be reliable for preliminary design work and with the absence of practical values can be used for the final design with a suitable factor of safety. The result are:
Method H
L
H
G
H
OG
Cornell’s
0.18 1.23 1.46
Onda’s
0.56 0.05 0.78
The shown that Cornell’s method will give a higher value of H
OG
and this value will be taken to size the height of the packed bed. With the value of H
OG
and N
OG
that are found, the height of the packed bed will be 8.8 meters. Stress Calculation:
Parameters:
Height Diameter Hemispherical Head Skirt Support ,height Corrosion allowance Insulation, Mineral Wool Material of construction, carbon steel Operating pressure Vessel to be fully radiographed 8.8m 2.3m 1.38m 2mm 70mm thick 88.9N/mm
2
at design temperature 200 degrees 30barg E=1 Justification of materials
Based on preliminary calculations, the thickness was calculated to be 45.35mm thick and rounded to 46mm. The inclusion of the allowance for corrosion gives an overall thickness of 48mm. It was found in accordance to the Clause 1.6 and 1.7 of the AS1210 that the vessel design could be considered as a class 1 pressure vessel due to the presence of highly volatile and flammable compounds within the vessel and that it is a major component for the process plant. The material selected for the design of the vessel should meet all the criteria in accordance to the 2010 version of the Australian Standards AS1210 and the ASME. The metal was chosen based on its strength, ductility, corrosion resistance and maintenance frequency. As the working fluid within the vessel is not corrosive, carbon steel was chosen as the material for construction instead of stainless steel. Although stainless steel has a slightly higher allowable stress of 135N/mm
2
, it is less economical to use stainless steel as it is much more expensive as compared to carbon steel. Carbon steel has a high allowable stress of 88.9N/mm
2
and is able to withstand the calculated stresses, it is also more economical. A corrosion allowance of 2mm was allowed to ensure that the functionality of the vessel does not degrade over time. Although the working fluid is not corrosive, it is typically safer to allow a few millimetres to ensure safe and long term operation.
The type of head chosen was the hemispherical head as it is the strongest shape, capable resisting about twice the pressure of a torispherical head of the same thickness. It is used when operating pressures are high. The thickness required for the head and closure were made in accordance to the ASME BPV Code Sec. VIII D.1. The skirt support is chosen as the column is vertical and they do not impose concentrated loads on the vessel shell with conformity to the clause 3.24 in the AS1210. As the vessel if 8.8m high, it is subject to wind loading and skirt supports are more suitable as compared to saddle supports. Operating temperatures range from 45degres Celsius to 75 degrees Celsius, insulation is required. A thickness of 70mm was chosen as the operating temperatures are relatively low. Mineral wool is chosen as it provides good insulation by trapping air within the small pores present in the wool. It has the capability of withstanding up to 800 degrees Celsius and has a low thermal conductivity of 0.08W/mK at 298K. It also acts as a flame retardant in the event of a breakout of fire. A double welded butt joint is required for a class 1 pressure vessel with a maximum efficiency of 1.0 as stated by the clause 3.5 of the AS1210. 2 platforms with an outer radius of 1m and a ladder is included in the calculations for dead weights. The dead weights before and after hydro testing are tabulated in table 1.1 Dead Weights Value Vessel and Head 302.73kN Ladder 0.42kN Platforms 17.62kN Insulation 9.57kN Hydro Testing (water filled in column 358.67kN Total (Before Test) 330.34kN Total (After Test) 689.01kN
Table 1.1: Dead weights
Wind loading was calculated using a wind speed of 160km/h and a drag coefficient of 0.07 to account for attachments such as ladders or platforms. Wind Loading 5627N/m Bending Moment 219818.1654Nm
Table 1.2: Wind Loading & Bending Moment
The pressure stresses, dead weight stress and bending stresses before hydrostatic testing were then calculated and tabulated in table 1.3 Longitudinal Stress
47.437
Hoop Stress
94.875
Dead Weight Stress
0.974
Bending Stress
0.129
Resultant Longitudinal Stress
( upwind) 45.541
Resultant Longitudinal Stress
(downwind) 48.282
Elasticity Stability
384.615
Maximum compression Stress
1.103
Table 1.3: Stresses on vessel before hydrostatic testing
The greatest difference is the difference between Hoop Stress and Resultant downwind longitudinal stress. The difference gives a value of 46.59
which is much lower than the maximum allowable stress of 88.9
which means that copper steel at this thickness is able to withstand external and internal stresses before hydrostatic testing is carried out. The maximum compression stress is also much lower than the elasticity stability, signifying the strength and stability of the vessel. 45.541
48.282
94.875
85.56 MPa 94.875
Upwind Downwind Resultant longitudinal stresses Hydrostatic testing is carried out to test the integrity of the vessel under high stresses and pressures to ensure the safe operation of the vessel even under extreme conditions.The stresses were then recalculated after pressure testing and tabulated into table 1.4 Longitudinal Stress
64.47
Hoop Stress
128.94
Dead Weight Stress
2.032
Bending Stress
0.1294
Resultant Longitudinal Stress
( upwind) 66.633
Resultant Longitudinal Stress
(downwind) 63.374
Elasticity Stability
384.6154
Maximum compression Stress
2.1617
Table 1.4 Stresses after hydrostatic testing
Even after hydro testing was carried out, the vessel was much able to withstand the stresses with no problem. The greatest difference between Hoop Stress and Resultant downwind longitudinal stress gives a value of 62.569
, which is much lower than the allowable stress of 88.9
. The vessel is fit and strong to be used even under extreme conditions as there is a large margin between the allowable stress and the greatest difference between stresses. The maximum compressive stress is only a mere 2.16188
, which is much lower than the elastic stability of 384.6154
. 66.633
63.374
128.94
85.56 MPa 128.94
Upwind Downwind
Skirting
A straight cylindrical skirt of plain carbon steel with maximum allowable stress of 89
and a
young’s modulus of 200,000
was used. Through careful iteration, the optimal thickness of the skirt was selected to be 20mm. The skirt height of 1.38m was carefully chosen based on the diameter of the vessel. Skirt Height 1.38m Skirt Thickness 20mm Bending moment at base of skirt 291.56kNm Dead weight on skirt 671.4kN Bending Stress
3.4485
Dead Weight Stress
(test) 4.605
Dead Weight Stress
(normal) 2.342