Question

Describe the process of carbon dioxide absorption by a named solvent in a plate tower.

Answer 1

Now a days, in Hydrocarbon Industry (Fertilizer, refinery etc), aMDEA is used as solvent to recover Carbon Di oxide from a gaseous stream. aMDEA stands for Activated Methyl Diethanol Amine.CO2 absorber in this process is Tray Type.Process gas enters CO2 absorber from bottom where majorityof CO2 from gas stream is removed by contact with semi lean aMDEA solution.Then remaining gas flows continue to travel upwards in CO2 absorber where majority of residual CO2 is absorbed by contact with Lean aMDEA solution. (NOTE: FOR DEFINITION OF GENERATION OF THIS LEAN/SEMI LEAN SOLUTION, KINDLY GO HTROUGH WHOLE ANSWER. YOU SHALL FIND SAME IN LAST PARA.)Feed gas flowing upwards also carries it with some amount of aMDEA solution. To recover this aMDEA solution, wash trays are placed at top of tower and a demister pad at exit. After exit, gas goes to knock out drum to remove any further liquid entrained with gas. Semi lean solution leaving tower from bottom is almost saturated with CO2 (thus rich in CO2 content). Since this solution is pressurized, its energy is recovered by letting down its pressure in hydraullic turbine of semi lean pump which is used to pump semi lean solution to CO2 absorber. This letting down of pressure serves dual purpose :

First, it allows to reduce power consumption of unit.

Secondly, this rich solution also contains some inert or other gases absorbed from feed gas,When pressure is reduced across turbine, these other gases are flashed from solution and are separated from CO2 in HP Flash drum. Flashed gases from HP flash drum are mostly other gases with some quanity of CO2. This amount of CO2 is recovered by washing these flash gases with Lean Solution in Packed Bed Column called Lean CO2 scrubber.

Solution from bottom of HP Flash Drum is sent to LP Flash drum where pressure is further reduced to such extent that most of CO2 disengages from solution. This is a packed tower. But like other towers, in this unit this column also contains water wash trays at top & knockout drum alongwith demister to recover any aMDEA entrained with CO2. Overhead from LP Flash drum is sent to LP Flash condenser where gaseous CO2 is cooled to condense any water content. Condensed water is separated from CO2 & CO2 product has been obtained which is sent to downstream units for further usage or processing.

Now, bottom aMDEA solution obtained from LP Flash Drum is called Semi Lean Solution. Thjs solution contains almost no quanity of CO2 hence it has great capacity to absorb CO2. That is why feed gas entering CO2 absorber is first gets contacted with Semi Lean Solution & it absorbs major CO2 from gas.

Major portion of this semi lean solution is sent to CO2 absorber by semi lean solution pump (which you may recall from first aragraph is driven by hydraullic turbine). Some portion of this semi lean solution is sent to CO2 stripper which recovers dissolved CO2 in HP Flash Gases. Befor entering CO2 stripper, it gets heated by bottom solution (lean solution) coming from CO2 stripper in heat exchanger called Lean Semi Lean Heat Exchanger.

Bottom solution coming out from CO2 stripper is called Lean solution. It has high content of CO2 dissolved hence it is contacted with feed gas in CO2 absorber after major portion of CO2 has been recovered by semi lean solution.

aMDEA solution has a tendency to foam therefore anti foam solution is added continuously at various points in stream.