Instructions

1. This article contains a series of steps to feed the data to the flowsheet, run the simulation, interpret the results
   
2. User can either follow the series of steps mentioned in the article or download the ProTreat file attached to this article, review the inputs and run the simulation  
   

Simulation Set Up

Objective

Inputs

1. Components>> Carbon dioxide, Hydrogen sulphide, Methane, Ethane, MDEA, MEA, Hydrogen, Water

Stream Information (Feed Gas)

1. Set up a conventional absorber & regenerator loop in ProTreat as shown in the flowsheet
2. Inputs taken for the exercise 
1. Gas flow rate>> 2,165 m³/hr
2. Temperature>> 46 °C
3. Pressure>> 23 kg/cm²
4. Component composition>> (CO₂>> 2%, H₂S>> 2.5%, CH₄>> 91.3%, C₂H₆>>0.8%, H₂>> 3.4%, Saturation>> 100%,  All in mol%) 
   

Absorber Inputs

1. Configuration
1. Select "1-Generic valve trays"
   
1. Data
   
1. Number of trays>> 20
   
2. Tray spacing>> 0.6 meters
   
3. Number of passes>>1
   
4. Weir height>> 0.05 meters
5. Foam derating factor>>0.8
   
2. Size
   
1. Select "Diameter calculated"
   
1.  Vapor flood & Downcomer flood>>70%
3. Pressure
   
1. Enable "Set pressure by feed stream" and select " feed stream>>2"
   
2. Enable "Calculate pressure drop"
2. Models
   
1. Thermodynamic Package
   
1. OGT Gas Treating
   
2. Reaction kinetic model for CO2 and COS
   
1. Select " ProTreat default (most columns)"
   
3. Streams
   
1. Feed & Draw streams
1. Connect stream 1 >>select "connects above top tray"
   
2. Connect stream 2 >> select "connects below bottom tray"
2. Top vapor stream
   
1. Select "Rate stream"
   
3. Bottom liquid stream
   
1. Select " Rate stream"
4. Options
   
1. Enable "Calculate approach to equilibrium" and " Use automatic low-weir-load correction"

Stream Information (Lean amine)

1. Inputs taken for the lean amine
2. Lean amine flow rate>> 28 m³/hr
3. Temperature>> 49 ° C
4. Pressure>>24 kg/cm²
5. Component composition>> (CO₂>> 0.003 loading,  H₂S>> 0.001 loading, MDEA>> 40 wt. %) 
6. Select the water specification as “Is Remainder(Solvent streams only)”

Run Simulation

1. Run simulation at base case i.e., MDEA concentration is 40 wt. %
2. CO₂ concentration in the treated gas for the base case is 1.55 mol%.
3. H₂S concentration in the treated gas is 3750.4 ppmv.
   
4. CO₂ in treated gas met the treating objective whereas, the concentration of H₂S in the treated gas did not met the objective 
5. So, let us switch to 30 wt.% MEA instead of 40 wt.% MDEA and run the simulation 
6. CO₂ concentration in the treated gas at 30 wt.% MEA is 0.001 mol%.
7. H₂S concentration in the treated gas at 30 wt.% MEA is 2645.1 ppmv.
8. Here CO₂ met the objective whereas H₂S didn't, then how can we meet H₂S spec?
   
9. Increase the circulation rate in MDEA case to 48 m³/hr and run the simulation
   
10. CO₂ concentration in the treated gas for MDEA case is 1.22 mol%.
11. H₂S concentration in the treated gas for MDEA case is 4.822 ppmv.
12. Even there is a drastic decrease in H₂S concentration from the previous case of circulation rate 28 m³/hr, Concentration of H₂S in treated gas is yet to meet the objective
    
13. Try the same case for 30 wt.% MEA (Run the simulation) 
    
14. CO₂ concentration in the treated gas for MEA case (circulation rate: 48 m³/hr) is 0.00003 mol%.
15. H₂S concentration in the treated gas for MEA case is 0.085 ppmv.

Results & Discussion

Output

1. The concentrations of the preferred components in the treated gas met the objective at the last case i.e., at a solvent circulation rate of 48 m³/hr and amine concentration>> 30 wt.% MEA
   
2. Let us interpret the absorber profiles for a solvent circulation rate of 28 m³/hr for MDEA and MEA
   

Temperature Profiles

1. These types of temperature profiles can be observed when the loop absorption regeneration loop is under circulated.
   
2. If you observe MEA temperature profile the curve at top of the absorber is somewhat sharp and has high temperature when compared to that of MDEA
3. This is because MDEA is a tertiary amine and its reactivity towards CO2 is low when compared to MEA
4. MEA is a primary amine and technically 20 wt.% MEA is 40 wt.% MDEA, It’s reactivity towards CO₂ is very high compared to MDEA.
5. By increasing the circulation rate you can observe the change in the temperature profile
   
6. Let us interpret the absorber profiles for a solvent circulation rate of 48 m³/hr for MDEA and MEA
   

1. The profile transformed in such a way that the peak temperature in the entire column is recorded in the 15th tray for MDEA and 19th tray for MEA
   

Concentration Profiles of H₂S

Concentration Profiles of CO₂

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Concentration profiles of CO₂ at 40 wt.% MDEA and circulation rate>> 28 m³/hr

Concentration profiles of CO₂ at 30 wt.% MEA and circulation rate>> 28 m³/hr

Interpretation

2. In a similar way if you observe concentration profiles of CO2, maximum of the absorption takes place between 12-20 trays in the case of MEA.
3. In the case of MDEA there exists a continuous driving force for absorption. 

Concentration profiles of CO₂ at 40 wt.% MDEA and circulation rate>> 48 m³/hr

Concentration profiles of CO₂ at 30 wt.% MEA and circulation rate>> 48 m³/hr

Interpretation

1. On comparing the modified concentration profiles with the under circulated column profiles we can observe the change on both MDEA and MEA cases.
2. In the MDEA case bulge is at the 17th tray and on comparing the profiles we can say that this column is not lean end pinched with respect to CO₂ 

Effect of increasing no of trays on treating

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1. Run “Absorber parameters_Bulge.ptd” at 20 trays and analyze the temperature profile
2. Now review inputs and run the simulation increasing the number of trays and what can be the effect on treating?
3. Does the feed gas get overtreated or undertreated or remains the same as in the case of 20 trays.
   

Graphical representation of absorber temperature profiles at varying no of trays

Interpretation

1. If you observe the combined graph of temperature profiles, you can see that there is no significant change in the degree of treating by just increasing the number of trays in the absorber.
2. That can be a significant result for the internals manufacturers who come across some customers asking for increasing the number of trays for efficient mass transfer inside the column.

Something to think about

1. Is there any possibility for a situation to happen like an absorber is both lean and rich end pinched at the same time?
2. For more detailed information about the temperature and mass transfer pinches visit ___________________