3 replies to this topic

[jknight291]

Hello

 

I am very new to Aspen and one of my first Aspen Projects is to simulate a single pass, shell and tube heat exchanger. On the process side (tubes), there is a mixture of Cl2, O2, CO2, H2, and H2O at 200F and about 2 in-water-vac and the utility side is just cooling tower water. The objective is to cool the process down to approximately 100F while condensing a good chunk of the water out of the vapor side.

 

I am using a HeatX shell and tube in AspenPLUS 2006.5 and here is my issue / unfamiliarity:

 

The decant stream from the hot water side requires "free water" - will not let me select dirty water, but my hand calculations and evaluation of the actual running process indicates that there is residual chlorine in the condensate. So I am having difficulty modeling the condensation of the water taking some entrained chlorine with it. It is important that I am able to properly simulate this as I need to be able to further simulate the dechlorination of that condensate stream in Aspen. Has anyone had a similar issue or does anyone know another way around this? I've spent several days trying to get this right, including changing the built-in VLE and solubility data as well as attempting to model as a flash, etc. I think the problem truly is in the "freewater" requirement.

 

Process Fluid In: 200F; Out 100F

 

CTW In: 90F; out 115F

[PaoloPemi]

I am not familiar with ASPEN (I do use a different software , PRODE PROPERTIES)

but couldn't you simulate the exchanger as a series of H-P flash operations ?

Do you suppose it as a VLE convergence problem or a problenm with that (exchanger) unit operation ?

[Pilesar]

You cannot rely on modeling "entrained chlorine" in your water phase. Entrainment is not the mechanism! You have electrolytic components which will dissociate into ionic form in the aqueous phase. In other words, if you mix Cl2 and H2O, you no longer have just those two components, you have hydrochloric acid! Your Aspen model does not allow for that and sees the Cl2 as a gas above its dew point which does not condense into the liquid.

 

As you seem to know what will be removed with the water, I suggest you use that information to adjust the simulation results to conform to your hand calculations. My guess is that heat transfer and exchanger design should not change appreciably -- you will just need to adjust the exchanger effluent compositions for modeling any downstream unit operations. Depending on your required fidelity, you may need to use electrolytic components and their much more complex thermodynamic system in your model. In my opinion, modeling electrolytes is much too ambitious for a new Aspen user. Consider if you will be able to deliver your project without a rigorous model. Can you instead make conservative assumptions for equipment design? Simulation is great but sometimes engineering answers can be achieved easier some other way.

[jknight291]

I agree in regard to using an electrolyte thermo model, but the problem is actually with Aspen. I've been in contact with their support group and we are currently working on the problem. The heat exchanger models in my version will not allow anything to decant except for pure water and will not allow the user to select the option for dirty water to be used with a HeatX model. I can succesfully model my system with a simple "cooler" in Aspen, but I do not get all of the benefits of having Aspen do the grunt work for HE design.

 

I'm working so hard to get an Aspen simulation as this is not a "from scratch" design project. This is a "we already have this process in use, what happens if we do this" kind of project, so Aspen will make it easier for me to play with variables  - once I get a working model.

 

Thanks for your help, but this absolutely seems like an Aspen bug.