Gas-liquid absorption design problem

[Yr11Kid]

Hey everyone

i'm in the process of designing an absorption tower (packed) to remove pure CO2 from a natural gas stream flowing at 25 MMscfd.

The natural gas contains 8% CO2 initially, and must contain 3% CO2 on exiting the tower.

The feed gas inlet conditions are 298K and 6900kPa

i've decided to use a propylene carbonate solvent

The CO2 absorption into the solvent at the process conditions follows henrys law

Basically i want to know how i would go about calculating the number of overall gas transfer units required to reach the 3% CO2 specification.

 

[Achy47]

Hi there,

Calculating the number of overall gas transfer units (OGTUs) required for your absorption tower is a crucial step in the design process. OGTUs represent the number of times the gas stream needs to come into contact with the solvent in order to reach the desired CO2 concentration. Here is a step-by-step guide on how to calculate OGTUs for your specific system:

1. Determine the equilibrium CO2 loading of your solvent at the process conditions. This can be done using Henry's Law, as you mentioned, or by using a CO2 solubility table for propylene carbonate.

2. Calculate the difference in CO2 loading between the inlet and outlet gas streams. In your case, it would be 8% - 3% = 5%.

3. Convert the CO2 loading difference to a concentration difference. This can be done by multiplying the loading difference by the molecular weight of CO2 (44 g/mol) and dividing by the molecular weight of natural gas (approximately 16 g/mol). In this case, it would be (5% * 44 g/mol) / 16 g/mol = 13.75%.

4. Determine the height of your packed tower and the packing material used. This information is needed to calculate the overall mass transfer coefficient (Kya) for your system.

5. Calculate the gas phase mass transfer coefficient (Kga) using the following equation:

Kga = 0.00002 * (gas flow rate in m3/hr) / (packing height in meters)

6. Calculate the overall mass transfer coefficient (Kya) using the following equation:

Kya = (Kga * (solvent density in kg/m3)^0.5) / (solvent viscosity in kg/m*s)

7. Calculate the overall gas transfer units (OGTUs) using the following equation:

OGTUs = (packing height in meters) * (Kya / (overall mass transfer coefficient at infinite height))

8. Finally, calculate the number of theoretical stages required for your absorption tower using the following equation:

Number of theoretical stages = OGTUs / (height of a theoretical stage in meters)

I hope this helps you in your design process. Best of luck with your absorption tower!