In this post I want to share how to size amine system for natural gas sweetening, especially for MEA and DEA. For other amine systems, the licensee should be contacted for detailed design information. In the end of this post, I share simple spreadsheet on how to estimate amine circulation rate, minimum contactor diameter, and reboiler duty of amine system.
Amine absorbers provide intimate mixing between amine solution and sour gas. Typically, small diameter towers use stainless steel packing, while larger towers use stainless steel trays.
For preliminary design, a tray spacing of 24 in and a minimum diameter of capable of separating 150 to 200 micron droplets can be assumed. The size of packed towers must be obtained from vendor.
Commonly, amine absorbers are equipped with integral gas scrubber section in the bottom of the tower. The function of the scrubber is to remove entrained water and hydrocarbon liquids from the gas, therefore it protects amine solution from contamination.
Separate scrubber vessel can also be provided so that the tower height can be decreased. The vessel should be design using two-phase separators.
For MEA systems with large gas flow rate, a scrubber should be considered for the outlet sweet gas. It is because it will be helpful to reduce MEA losses in the overhead sweet gas. DEA systems do not require scrubber because vapor pressure of DEA is very low.
Amine Circulation Rate
Amine circulation rate can be determined from acid gas flow rates, amine solution concentration, and acid gas loading.
Table below provides recommendation of solution strength and acid gas loading without an excess of corrosion:
|Solution Type||Recommended amine strength (in weight percent)||Recommended acid gas loading (mole acid gas/mole amine||Densities at 60oF (lb/gal)|
It is recommended to increase amine circulation rate by 10%-15% to supply an excess of amine.
Typically flash drums are designed for two to three minutes of retention time for the amine solution while operating half full.
Reboiler provides heat to amine stripper, which drives off the acid gas.
The heat duty of amine reboiler varies with the system design. The higher the reboiler duty, the higher the overhead condenser duty, the higher the reflux ratio, thus the lower the number of trays required.
For design, reboiler temperatures in a stripper operating at 10 psig can be assumed to be 245oF for 20% MEA and 250oF for 35% DEA.
Amine strippers use heat and steam to reverse chemical reaction with CO2 and H2S. The steam acts as a stripping gas to remove the CO2 and H2S from the liquid solution and to carry these gases to the overhead.
For most field gas units, it is not necessary to specify a stripper size. Vendor have standard design amine circulation packages for a given amine circulation rate, acid gas loading, and reboiler.
Calculation Spreadsheet of Amine Design
You can download a simple spreadsheet to estimate amine absorber diameter, amine circulation rate, and reboiler duty here.
Please be noted that there are several variables that need to be checked from graphs, such as compressibility factor and gas viscosity.
To estimate minimum absorber diameter, we also need to estimate drag coefficient (CD) first. In this spreadsheet I use goal-seek to find correct drag coefficient.
I know there are many aspects that can be improved in this spreadsheet. But I always hope you find this post useful.
Arnold, Ken and Maurice Stewart. Surface Production Operations Volume 2. Elsevier Science: 1999