An unconverged column can cause serious problems — especially for the people tasked with troubleshooting it! Continuing from my previous blog post, here are some insights on how to troubleshoot the ratebased calculation in the acid gas columns and specific strategies for the common solvents, such as MDEA and MDEA + PZ.
If you don’t know how to determine where the issue is with your acid gas column, or if you don’t know how to troubleshoot the initialization calculation, please check out the previous post. If you are trying to get your ratebased calculation to converge, you are in the right spot.
The second major step of the calculation in a ratebased column is the ratebased calculation. If the initialization calculation succeeds but the ratebased calculations fail, you can modify some of the ratebased parameters to get to a converged solution. You can access all of the variables by going to Advanced Modeling Convergence Parameter, and by clicking the View button next to the Convergence.
In the example below, the ratebased calculation failed after reaching iteration limit and is not close to convergence:
1. Continuation/Homotopy Parameters
The first method is to modify the Continuation/Homotopy parameters. This method could help to switch smoothly from equilibrium to ratebased solution by solving a series of subproblems at the intermediate states. The subproblems are the intermediate steps used to switch from equilibrium model to ratebased model.
a) Iterations
The Continuation/Homotopy Iterations controls the number of subproblems to be solved. The default value is 0, meaning no subproblems to be solved. The recommended value for iterations is 2 – 5, because larger iteration numbers will increase the computation time .
b) Parameter
The Continuation/Homotopy Parameter dictates how close the subproblem is to the equilibrium solution. The default value is 1, meaning a direct jump to ratebased solution without solving the intermediate states. An increasing value represents the subproblems being closer to the equilibrium solution. When the parameters are in use, HYSYS runs a series of simulations to solve the subproblem by decreasing the Continuation/Homotopy parameters. With large enough parameter, the first subproblem will be very close to equilibrium solution and should converge easily. The solution is then used to initialize the next subproblem, until ratebased solution converges. The recommended value for parameter is 2 – 5 as well.
While the larger parameters will smooth the convergence, they will increase the computation time.
2. Stabilization Method
The second method is to modify the Stabilization Method. The two available methods are DogLeg and LineSearch. The DogLeg is the default selection. If the convergence failed with the default method, you can try the LineSearch, which is more robust but slower. If you observe the error/tolerance value in the ratebased iterations oscillates with a large variation, AspenTech recommends changing the method back to DogLeg.
Now you have learned different strategies and methods to troubleshoot a ratebased column for acid gas systems. The tips vary depending on the solvent used. The table below provides some common examples for different solvents.
In the example of MDEA, common failures can be found in both initialization calculation and ratebased calculation. You can employ the temperature estimate strategy and specify a top stage temperature that is 20 K below that of the top stage feed. If the ratebased calculations fail, try to modify the stabilization method to LineSearch.
Solvent  Problem  Solution 
MDEA 

1. Provide temperature estimates
2. Set the Stabilization Method to LineSearch 
DEA 

1. Provide temperature estimates
2. Modify the Liquid Holdup Estimate Field to 1e7 m3 3. Set the Damping Level to Mild 
DGA 

The DGA reaction occurs very quickly. 1. Modify the Liquid Holdup Estimate Field to 1e9 m3 2. Provide temperature estimates

MEA 

1. Provide temperature estimates

MDEA + MEA 

The reaction rate is very quick because of the existence of MEA.
1. Modify the Liquid Holdup Estimate Field to 1e5 ft3 2. Provide temperature estimates

MDEA + PZ 

The PZ+MDEA reactions occur very quickly. 1. Modify the Liquid Holdup Estimate Field to 1e4 ft3 2. Modify the Continuation/Homotopy Parameter to 5 3. Modify the Continuation/Homotopy Iterations to 3 
You should definitely try out some of these strategies if you have an unconverged ratebased column in front of you. Solving the column allows you to take a step closer to a better design and return in capital by optimizing the entire gas plant within Aspen HYSYS^{®}. You can also view the top 10 questions about our acid gas technology here.
If all of the strategies fail, please contact our amazing customer service support team: they are always happy to help with issues like these.
Happy column troubleshooting!
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