Design problem: Absorption column
An industrial gas stream is available @ 2 kg/sfrom a cracking operation of NH3 containing 72% H2, 24% N2 and 4% NH3 by mole, at 202.65 kPa and 35°C. You have been asked to design a multistage countercurrent bubble cap absorber to remove NH3 from the above stream with water as the scrubbing liquid. The liquid mass flow rate is limited to be 2 to 3.5 times of gas mass rate. NH3 concentration should not be greater than 0.003 mg per m3 of the exit gas.
Assumptions/ design considerations :
• Lean water-NH3 system follows Henry's law and the corresponding equilibrium relation: y* = 0.85 x @30°C
• Isothermal gas absorption at room temperature (~30°C)
• Optimum adsorption factor (A) = 1.2 to 2
• Overall column efficiency=70%
• Pressure drop per plate= 1 kPa
• Minimum liquid loading=70% of expected maximum loading
Hints: The flow rates of liquid and gas entering and leaving the absorber is almost constant thought out the column if a small amount of the solute gas is absorbed. This is a typical case, also common in practice when the solute gas concentration in the feed stream is low (dilute gas absorption). For such operations, the variation of temperature between column top and bottom trays is insignificant (isothermal operation). The pressure drop, if the column is not too tall, has minor effect on the physical properties of process fluids that could influence the column design. The section-wise determination of number trays and design are performed if the gas stream and/ or the solvent liquid (usually makeup solvent) are introduced at any intermediate point of the column. The number of theoretical trays can be estimated using either Kremser equation or graphical technique for multistage counter current lean gas absorption ([1] page 290). The vapor loading is the highest at the bottom tray even though its variation is not appreciable. For the safe side, the design is usually performed at the bottom tray.
The mole fractions (x, y) between the phases are plotted in McCabe Thiele method of distillation calculation. In case of absorption, the mole ratios (
) are used for the determination of number of trays in graphical method instead of mole fractions.
References
[1]. Robert E. Treybal, Mass Transfer Operations, McGraw-Hill, Inc., 3 rd ed. 1981.
[2]. Perry's Chemical Engineers' Handbook, McGraw-Hill, Inc., 8 th ed. 1997.
[3]. R. K. Sinnott, Coulson & Richardson's Chemical Engineering: Chemical Engineering Design (vol. 6), Butterworth-Heinemann, 3 rd ed. 1999.
[4]. Perry's Chemical Engineers' Handbook, McGraw-Hill Companies, 7 th ed. 1997.
[5]. Henry Z. Kister, Distillation Design, McGraw-Hill, Inc., 1st ed. 1992.