Electrolysis Cell Error
Simulate chemical processes using advanced thermodynamic models
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I got this error message: Error: Gas Separator: PH Flash: One or more flash specification values are invalid. Please check the associated Unit Operation or Material Stream for incorrect specifications or miscalculated values.
I have no idea what this means and I was trying to get the sample Electrolysis Cell to work...
Thank you!
- Aditi
Use this fixed XLSX file in the Excel UO. The one that ships with DWSIM is broken.
Thank you! The sample cell works really well, but I have another question. How would I go about simulating the electrolysis of carbon dioxide and what is the purpose of the Electrolysis Excel file?
Thank you!
- Aditi
This sample demonstrates the usage of an Excel-Unit-Operation which is used to create user specific units which are not available in DWSIM yet.
The Excel file contains the calculations for that Model. This special example was selected as no specific Unit Operation for an electrolysis cell is available in DWSIM and we can demonstrate how to create such user defined special unit operations.
How this is all working? Just press F1 key when this object is selected in DWSIM flowchart to get help on that object. If this is not sufficient to answer your questions don't hesitate to ask again.
Gregor
I tried to modify the file to accomodate the electrolysis of carbon dioxide as well as water and it didn't work. Is there a way to integrate both processes (both chemical reactions) in the same Excel Spreadsheet and how would I do that?
I found the Excel file template and I was also wondering how the Calculations tab works...
Thank you!
- Aditi
Hi Aditi,
did you already read the help? What exactely you didn't understand?
First of all, i reccomend not to alter the original sample in the DWSIM sample folder.
You may either create a new Excel calculation file from scratch via the dialog ()of the ExcelUO object or copy the existing file and connect.
Now you can edit the file via the edit button.
You will find 3 tabs in that Excel file: Input, Output and Calculations
When you create a new Userobject, you usually have to set up the simulation with all required components first. Then you add a new ExcelUO object, connect at least one input and one output stream and create/copy an Excel file which executes the required calculations.
Now you do an initial calculation. This step will map all components into the input and output folders of the Excel file. You also may define additional parameters required for calculations which are written from DWSIM into this Excel file during runtime.
In the Calculations folder there are all the user defined calculations for that unit operation. If you want to calculate a different unit operation like a centrifuge or an osmosis you will need completely different procedures for calculation. You also may add additional folders if you want.
As i don't know the chemistry and the other conditions of your model i can't supply any further help at that point.
Eventually you will have to write the calculated results into the predefined fields of the output folder. You also can write additional results as "Parameters" to that folder as depicted in the sample file. The data of that folder are read back into DWSIM and written to the attached output streams.
If there are any questions left, you should attach your simulation file together with your Excel file to that thread for further inspections.
Gregor
I'm trying to simulate the electrolysis of carbon dioxide and water to form ethanol and oxygen gas. I have a few more questions (sorry for the inconvenience):
My model operates at STP and at approx. 10 volts.
I am trying to test out the efficiency of a catalyst that I have developed for the sequestration of carbon.
The electrolyte is caesium carbonate, and the electrodes are made of copper and graphite, with the catayst coating the graphite.
Thank you!
- Aditi Kumar
Last edit: Aditi Kumar 2016-02-09
Hi Aditi,
I had a look into your simulation files.
I really wonder if such a reaction really will take place in reality. Most probably you only will produce hydrogen and Oxygen from water splitting. At least you will nead to add this as a side reaction depending on a new input parameter for Electrolysis efficiency. If a good efficiency is possible this certainly could be a great breakthrough. We (Daniel & me) would be glad if you would mention DWSIM in your scientific paper which you certainly are going to publish on that!
I made some changes to get this sample working and attached it.
The following modifications were applied:
User defined components were removed as they are only catalyst and dont play a role for simulation
Other components were replaced by components from ChemSep database. This database usually contains the best quality datasets.
Property package was changed to NRTL. This will allow a more realistic representation of gas solubilities in liquid phase and will allow operations like destillation of ethanol from liquid.
I had to add Hydrogen as component to enable your reaction and maintain massbalance.
You did the calculations of electrolysis reactions in a wrong and incomplete way
You didn't write calculation results to the output sheet to be transfered back to DWSIM
* You removed all parameters from input and output sheets. I added some properties again. You held and created all parameters only inside your Excel table. Now you can steer this model from DWSIM surface instead of opening the Excel file for modifications. This also allows to run sensitivity studies in DWSIM where you could influence the amperage for instance.
P.s. If you add or remove any property you need to save anything and load the simulation again. The existing parameters of an Excel file are only read during loading the DWSIM simulation.
p.s.s. You asked about the "initial calculation". This means, that DWSIM is writing the component names and the stream names into the Excel file at the beginning of a calculation only. Therefore the correct procedure is to connect the required streams to the Excel unit and do a calculation in order to initialise the Excel file for later editing.
p.s.s.s: The calculations in the Electrolysis Excel file of DWSIM samples is correct. Only the description of the reaction is a little "unconventional". I did some minor imporvements for the reaction formula description and the parameter labels to clarify this. The new file is commited to GitHub already.
Have fun
Gregor
Last edit: Gregor Reichert 2016-02-09
Thank you so much for your help and I'll definitely mention DWSIM in any research paper I publish! A few more questions...
Further information on my project:
- I have a standard fuel cell with a PEM made of Nafion 117
- The anode and cathode are graphite and copper with the catalyst (a derivation of meso-tetraphenylporphyrin) coating the graphite.
- The electrolyte is caesium carbonate
- The inputs to the system is energy, water, and concentrated carbon dioxide
- After the ethanol is formed, a miroemulsion is formed which flows into a second chamber
- The oxygen gas is directed to another chamber and is ignored
- The ethanol is easily separated from the caesium carbonate because it does not dissolve in the caesium carbonate
- A coalescer is present in the second chamber which then allows the ethanol to separate from the electrolyte, and the layer of ethanol is poured out as the supernatant and then can be utilised as a fuel, etc.
Equation(s):
2CO2 + 6H+ + 6e- --> CH3OH + H2O
2CO2 + 3H2O --> C2H6O (ethanol) + 3O2
I also attached a diagram of what the fuel cell does/looks like.
Thank you for all of your help!
- Aditi Kumar
Last edit: Aditi Kumar 2016-02-10
Hi Aditi,
this process layout is a little differnt to the model I sent to you.
We need a membrane cell to model instead of the single compartment cell without a membrane i asumed. Therefore we need two inlet and outlet ports for anolyte and catholyte solutions.
As i can see from the diagram and your explanations i think this process might be possible to be simulated with existing version with DWSIM. We need some more informations:
We need to create Caesiumcarbonate as a usercomponent. The way you did this (what can be seen within your simulation) is not sufficient as no component data were defined except the formula and name. I reccomend to send/attach the component definition file for inspection and correct implementation for usage. Actually i get an error when i try to load your simulation!
The other component is not necessary as this is only a cover of the electrode but not part of the mixture. The effect of this catalyst will be seen in reaction selectivity to ethanol.
As far as i understand, you use a saturated solution of caesium carbonate in water. The created ethanol is building a second liquid phase. Is this right? Do you have solubility data available? We could simulate this with the NRTL model as well. What we need for that is solubility of ethanol of carbonate solutions depending on carbonate concentration and temperature. We also need the concentration of water in ethanol in equilibrium with these carbonate solutions. In other words you need to create a triangular phase diagram for Water/Carbonate/Ethanol like a Liquid/Liquid diagram. What is solubility of carbonate in water depending on temperature?
At the anode we only have a standard water electrolysis producing oxygen and H+ ions. These ions are moving through the membran to cathode side. What is the anolyte solution composed off? Due to your diagram this should be pure water which i think is not very efficient due to a low conductivity. You should use an acid here (e.g. sulfuric acid) There is also no water fed to the anode side which is necessary to produce oxygen. Usualle water is attached to the H+ ion and moving to the cathode side as well. I think you need to add water here.
Do you have data of the cell voltage depending on current density (A/cm²)? If this is available it can be implemented into the model as well.
I think you give me what is available on data and i will try to come up with a simulation representing your process. But this will take some days.
Some answers to your questions above:
Parameters given in the input tab can be edited in DWSIM as they are listed here as write-parameter. You may add more by adding more lines with new parameters.
Parameters in the output tab will be reported in DWSIM in the results section and can only be read here.
About efficiency: There are two kinds playing a role here - chemical and energetic efficiency.
The electrolysis efficiency actually is a selectivity and is calculated as the fraction of electrons used for the desired reaction for production of ethanol instead of hydrogen at the kathode.
The second type of efficiency is an energetic efficiency. This is calculated by the relation of KJ/mol used for production via electrolysis compared to an imaginary reaction of Ethanol->H2O+CO2
Ok, thats enough for now. Just give me some more realistic data as requested above and i will try my best. It's always sort of an adventure to move into new unexplored fields with DWSIM to develop it's capabilities further.
Regards Gregor
Thank you for your help!
More questions!
- what does the dT stand for in the calculations tab?
- do I need to input my catalyst as a usercomponent
- the equation used should be 2CO2 + 3H2O + 12e- --> C2H6O + O2
Last edit: Aditi Kumar 2016-02-11
Thank you so much for your help!
- Aditi
Hi Aditi,
give me a while and i will come up with an improved model, which should be able to calculate the phase split.
Could you deliver some more required information to get this as realistic as possible?
Gregor
Hi Aditi,
as promised, i created a preliminary model for you.
You will see a membrane cell with anolyte and catholyte streams. You also will find that ethanol is splitting into a second liquid phase.
I also created a new user component to enable this phase split. In order to get realistic properties like density, heat capacity and so on, you will have to add experimental data to this component definition and run a regression for each property.At the moment these are all a mere estimation.
From Wikipedia i found that CsCO3 is soluble in ethanol as well. Therefore i adapted some NRTL parameters for the pair Caesiumcarbonate/Ethanol which gives such a behavior. If you have some realistic data available you should optimise these parameters further.
All concentrations and component flows are only initial values from me in order to demonstrate the working of this model. You need to adapt these to represent your actual setup.
During creating this sample i had to do some small improvements to the PH-flash algorithm. You will have to fetch the actual version from GitHub to run this sample completely. This new version is required for calculating the flash vesels. The electrolysis model will run with actual version as well.
The Excel model of the electrolysis is still to be seen as preliminary. Neither any efficiencies nor a diffusion of ethanol or Caesium ions via the membrane are considered yet. You may adapt this to represent you experimental results.
regards
Gregor
Last edit: Gregor Reichert 2016-02-13
Water flow across membrane: How much water is attached to H+ ion crossing the membrane? I would expect 1 - 2 mol of H2O for one mole of H+
No water, I think, at the anode the Half reaction 2 H2O --> 4 H+ + O2 (g) + 4 e- takes place and at the cathode 4 H2O + 4 e- --> 2 H2 (g) + 4 OH- takes place. The electrons go through the wires and the ions move in the solution. If the membrane were not there, Cs+ and H+ ions generated at the anode would move toward the cathode and CO3 2- and OH- ions would move toward the anode. I have a proton selective membrane separating the two chambers. Only H+ moves across the membrane and reacts with OH- on the other side to form water.
Is there any ethanol crossing the membrane?
I think not. It is proton selective.
Is there any Caesium crossing the membrane?
Ditto.
What is the ratio of Cs+/H+ crossing the membrane?
Zero.
How much H2 is produced at the cathode? I can't imagine a 100% selectivity for ethanol production for the cathodic reaction.
This is the heart of the research problem and why the big research institutions are doing research and writing papers about this. H2 is a good product that can be transformed into other things, like fuel for hydrogen cars. But the goal here is to make a commercially viable product by reducing CO2 to CO or a liquid fuel like ethanol. Some have pretty good selectivity for ethanol, but my goal is to create a catalyst that limits excess H2 production and maximizes ethanol production. There is still a market for the other products that are formed. Just like crude oil, all the components eventually get used. So in conclusion... I have no idea and that's really the problem I'm trying to address with my catalyst.
Any other organic byproducts?
Yep.
Do you have the data for dependency of cell voltage over a wider range?
A wider range of what?
Where does the water for anode reaction to O2 is coming from? Do you feed any water here or is it crossing through the mebrane?
The cell has to have a water source, just like it has a CO2 source. A nice benefit of the waste heat is that it can be used to distill polluted water that could not normally be used for commercial or residential use. And yes, the water is fed into the system.
Do you have data of water/Carbonate concentrations in ethanol phase?
In pure water/ethanol, 95% ethanol, 5% water is a low boiling point azeotrope. The azeotrope boils at a lower temperature than pure ethanol, so you cannot make 100% ethanol by simple distillation. For motor fuel the 5% water must be removed by another means. Olympia brewery - who used to make fuel grade ethanol, used a molecular sieve to remove the excess water.
Do you have data of ethanol concentration in anodic carbonate phase?
Zero. I think.
NOTES:
The carbonate phase gets sent back to the electrolysis cell and is reused.
Thank you for all of your help this is awesome!!
- Aditi
Last edit: Aditi Kumar 2016-02-13
One last question, where does the catalyst information get input? If I were to test the efficiency of a TPP catalyst, where would I input the data for the catalyst in the simulation??
You don't need to add this catalyst as a component to the simulation. If you would have a perfect catalyst you will only get pure ethanol as product. A bad catalyst will result in hydrogen only. Therefore you will have to add a input parameter "selectivity" which defines the efficiency for calculation. This parameter is to be measured by your experiments. DWSIM is not calculating this efficiency! You rather must measure this and put it into the simulation to enable DWSIM to reproduce the experimental results.
Did you have a look into the latest samples? Doese it work? Are the concentrations of ethanol and carbonate phase nearly realistic?
p.s.: You say there is no transport of water, ethanoly or caesium across the membrane. I think this is pure theory. You will have to measure this in your experiments!
Due to my experience with another type of electrolysis (NaOH/H2SO4) there is always some kind of transport. For instance we observed a ratio of Na+/H+ of nearly 70% for ionic transport across the membrane. There was also a transport of water which is attached to the ions during transport. This will be depending on concentrations on both sides, temperature, type of membrane, current density and other factors.
Gregor
I saw the latest samples and they work really well and I modified the concentrations of the ethanol and the carbonate phase to suit my purposes, but I was confused on what you meant when you said that I have to add the input parameter of selectivity...
Thank you!
- Aditi
Last edit: Aditi Kumar 2016-02-16
I was trying to add the "specficity" parameter but I'm not sure how to...
Thank you!
- Aditi
Hi Aditi,
i modified the Excel table and the simulation in order to integrate the electrolysis efficiency.
If you want to add a new parameter you just append a new line at the parameter section of the input tab of the excel file. After saving the Excel file and closing the DWSIM file you open this DWSIM file again. Now you will see this parameter in DWSIM as well. I admit this procedure is a little awkward and need some improvement.
If you enter an efficiency below 100% hydrogen is produced instead of ethanol. I had to add hydrogen as new component to the simulation which shuffled the order of the components inside the Excel table. I had to reconnect the internal formulas to suit the new order. This issue needs some improvement in a futur release of DWSIM to make editing more convenient.
As soon as i had hydrogen in the catholyte i encountered calculation problems in the phase separator which caused me to remove these objects until a solution will be found.
Did this answer your questions?
Gregor