As it may be just a numerical problem, did you try to give various initial values to your streams to 'help' it converge? It would help also  if you had a reference case where the stream flow values are known to just check the flow diagram and then try to change inputs very slightly. 

1- Export the simulation in Aspen Dynamics, and do your simulation in that. Aspen dynamics has stronger solvers.

2- Place a heater (DeltaT=0 and DeltaP=0) to hold an initial value for the recycle stream.

You can switch from steady state to Dynamic mode in Aspen Plus and run the simulation , crosschecking all inputs.

Thank you all for your answers. I tried to change into dynamic mode, but still the problem arises. I used the tear stream and no results. Maybe the way I started the simulation is wrong. So what is the most suitable way to start the simulation. Do I have to simulate unit by unit and check if they converge and connect everything together and then use tear stream and balance block

How do you set the MEA flow?  If you try to totally recycle a stream the round-off error tends  to send the flow of the recycle to zero or infinity.  You must, at some point in the recycle break the stream and set the flowrate of the recycle stream.  By mass balance the return flow will be the same as the fixed flow.  Since the MEA stream will always include water and some CO2, which are fixed by equilibrium, fix only the MEA component flow and you can let the other component flows be recycled without additional constraints.

I did not connect the recycled stream directly to the absorber. I copied the properties of this stream and connected to the absorber and it did converge. However, if I connected to the absorber It wont converged. It is a mass balance problem indeed. I need to use the calculator block but I dont know how. Can you show me the steps please. 

I'm having licensing problems and don't have access to Aspen right now but basically this is how to do it.  First, do you loose any MEA through solubility in an exit stream?  You will do things slightly different if you have MEA losses. (1) Without MEA losses:  Set up so that you do connect the recycle back to the process.  Use a SEQUENCE block to define where the tear stream occurs in the process.  If you let Aspen define the location of the tear stream it can make a really bad choice so it is best for you to choose he best place.  Provide initial estimates of component flow of all components in the tear stream, including the MEA flow rate.  Now make a calculator block to  set the MEA flow in the tear stream and use the SEQUENCE block to activate the calculator block before the block fed by the tear stream.  You may have to sequence most blocks in the recycle loop.  It should now converge.  If not, examine the calculation sequence chosen by Aspen for the blocks you did not include in your sequence block.  If Aspen makes bad choices you may have to include additional sequence blocks to make the simulation well behaved. (2) If you do have MEA losses, its similar except add a make-up stream with input MEA just before the tear stream and, instead of a calculator block, use a design spec to control the flow rate of the make-up stream so as to  have the desired value of MEA flow at the tear stream.

Thanks for showing me the steps. My case is the second one. But one thing I couldn't get. How to define the tear stream using the sequence block. 

The initial stream composition and rate are entered in the stream block. Like I say, I don't currently have access to Aspen so I can't check things out.  Maybe it is a convergence block where you define which streams to tear and the sequence block that defines in which order blocks are to be calculated.

Thanks Dr.Lloyd. I did what you have told me, but I am getting this message

" ERROR DURING FLOWSHEET ANALYSIS THE FOLLOWING POSSIBLE TEAR STREAM(S) IN SEQUENCE SQ-1 ARE NOT BEING CONVERGED AND COULD LEAD TO ERROR INFORMATION STREAM FROM MEAMIXER TO C-1 CHANGE CHECK SEQUENCE ON CONV-OPTIONS.MAIN IF THIS IS OK. "

If you can send me an old project, I will be grateful. 

Here is my email: [email protected]

Thanks in advance.

I haven't actually done MEA but I have done other systems with high recycle.  You are very close.  You may be updating the MEA flow after the simulation thinks it is done.  You may actually have the result you want.  Check through to find out what order Aspen is actually calculating the blocks in the whole simulation.  Think through how what order you would calculate the blocks if you were doing it by hand.  You may have to add additional blocks to your sequence.  Another approach is to cut the tear stream completely.  Make it an input stream and an output stream.  After converging the simulation, manually copy the results from the output stream to the input stream, except for the MEA flow that you are holding constant.  Do this several times and see what is happening.  Are the component flows converging or oscillating?  If oscillating you will want to change the method of converging the tear stream.  I believe that the Aspen default is simple substitution.  You can change the convergence method for the tear stream, I think you can even use the Newton method, but it takes much longer.

Thanks for your collaboration. I really appreciate that. I managed to reduce the error of convergence from 200 to only 6. Now the problem lays on the calculation sequence. If I send you a picture of the process, would you be willing to have a look at it and give an estimate about the calculation sequence? 

I'll try.  Send it to lloyd.dot brown at tcheme dot com.  Also send the output results text file.  I don't remember what the suffix is, maybe *.out or *.res.

I sent you a picture of the process with the output results. your help is much appreciated  

Can you tell me please where I can find courses that show how to deal with calculation sequence in Aspen plus?

Mr. Lloyd. I am facing another problem. I am trying to regress experimental data using the ELEC-NRTL equation. The NRTL equation has 6 elements (A, B, C, D, F). SO when I include these 6 elements in the parameters for regression, I won’t get result and the reason being is that element 3 and 4 are always symmetric and the rest are un-symmetric. Do you have any clues?

I tried regressing some data many years ago but was unsuccessful.  My company ended up contracting with Aspen to develop a model for the system of interest.  I do know that there are both symmetric and asymmetric electrolyte models.  I don't know exactly what that means but I do know that the ELEC-NRTL model is symmetric.  If the model you developed fits the data to your satisfaction then I would use the model.

Yeah I am using ELEC-NRTL  and I got good results. But one thing I don't understand. Do you have any idea about these parameters [ A, B, C, D, E and F] for NRTL equation and what they represents. I found on some papers that they only use the two parameters for regression 

thanks 

Look in the Aspen online manuals.  They have explanations for all the models they use.  I think you want the Aspen Properties Manual.

 When you Data regression system in Aspen plus and you received this message "  CONSTRAINTS CANNOT BE SATISFIED FOR THIS CASE ". What does that mean exactly?

Thank you

I'm not sure.  It might be that the model requires some parameters be either positive or negative and the regression tries to make them the wrong sign.  You should probably contact Aspen Tech for an answer.  It might be that you can change your initial guesses so that the solution approaches the optimal values from a different direction.

Thank you. That was really helpful

anorher question. Is there any method to calculate the K equilibrium for a certain rection? I need to know the K equilibrium for the reacton of L- Arginine with water but it seems that no one has done an experimental work on this.

Thanks 

If you know the Delta G of formation of the reactants and products you can calculate the equilibrium constant from -DG/RT=ln(K).  If you don't know the DGs you can estimate them by group contribution methods. 

Thank you. But. can I use group contribution methods on ions or aqueous like HCO3- 

I don't know.  What is the reaction of interest?  I'll look in HSC Chemistry and see if it has any thermo data on your reactants and products.  It does have room temperature data for solid D-Arginine, which should have the same thermo as L-Arginine: Hf(25°C)=-149.020 kcal/mol, Sf(25°C)=59.895 cal/mol*K, Cp(25°C)=55.449 cal/mol*K.  But, I doubt that your reactant is a solid.  Your best bet may be to do a literature search for thermo on related species and assume a similar equilibrium constant.  You may have to resort to an experiment.

It is L-Arginine diluted with water. These are the following reactions:

1. ARG+ + H2O ------> L-ARG + H3O+

2. ARGCOO- + H2O ...........> L-ARG + HCO3- 

These two are the reactions of L-Arginine with water. 

By similar species you mean species with the same MW?

I don't have those species in my database.  By similar species I mean species with very similar chemistry.  Your dealing with amino acids and this is not at all my area of expertise but I think that you should be able to use equilibrium constants from other amino acids that differ only in the hydrocarbon part far from the acid groups.  Maybe you can find a series of amino acids and plot K vs some parameter and see where your compound would fall.  The simplest thing might be to measure the pH to determine the acid constants.  OK, I just did a Google search and found this http://www.cem.msu.edu/~cem252/sp97/ch24/ch24aa.html which gives the pKa for both functional groups of your molecule.  Those should be the equilibrium constants that you are looking for.

Increase the number of flowsheet calculations (.g like from 20 to 200) in the convergence option then run the simulation

The suggestion of Gman Gura did really work, increasing the number of iterations, completely solves the problem.

Hello Sadah, I have the same problem. How did you solve it? Even if I increase the number of iterations to 200 the problem is still. Also, which tear stream did you choose? in my case Aspen just allows me to use one of the exits of the stripper. Thank you.

To converge such simulations I initialize the lean solvent stream already with some CO2 content. For MEA, you want to use something from 0.1-0.3 molCO2/molMEA. Then I use the design-spec tool in order to fix the conditions at the stripper's bottom to the same desired CO2 loading by manipulating the reboiler duty. Depending on your operating pressure, you should use a balance block to replace your water and MEA losses by adding a make-up stream. When your recycle stream is similar enough to your lean solvent stream (temperature, pressure, composition), you can use a transfer block to copy the recycle stream into the lean stream on every iteration, virtually closing the recycle. Try changing the tear converging method from Wegstein to Broyden as well. I've found this to be a quite robust approach. If you are interested, a detailed explanation is also given in my linked publication:

I hope that helps,

Best regards,

Alberto

http://pubs.acs.org/doi/abs/10.1021/acs.iecr.5b04910

@Sadah Ahmed . You can increase either no. of iterations or check whether the property model selected is accurate or not.

You should initially introduce a certain amount of CO2 in the lean amine. Also, it is necessary define the balance of the loop by using balance manipulator and makeup water, also it should be reconciled the bottoms product of the desorber (right click and reconcile).

Here I attached two models, open and closed loop converged for DEA.

Regards

Daniel.

Hi:

I need VLE data for HCl-H2O system at ordinary pressure. However when i plot the binary the Txy, it gives the VLE but with errors in the control panel, stating that Flash calculations didn't converge in 30 iterations. I increased the number of Iterations but still same problem persists. Can someone help?