Professionally I work at BPT and focus on creating value adding App for Process Simulation. You can find more info on those on the BPT website. Below are posts that should help HYSYS, PetroSIM and UNISIM users alike in their day to day challenges to produce accurate yet fast models efficiently.

Release the true potential of your staff Apps for Process Simulation

Monday, 11 May 2015

Did you know series: You can set a component ratio using the balance block

Sometimes you need to mix two fluids to obtain a required component ratio in the outlet stream. For feed streams with a fixed composition, this can be a one time manual exercise to calculate the required flow ratio and then you use a SET operation to maintain your flow ratio. However, if at least one of your streams is in the middle of your flowsheet, you need to recalculate your ratio all the time. Here is the list of solutions in order of increasing efficiency:
- Use an Adjust to change the flow rate of Flow-2 until you get the desired component ratio
- Use a SET operation and Adjust the flow ratio until you get the desired component ratio
- Use a spreadsheet to calculate the required flow ratio
- Use a balance block to obtain the required flow ratio

The last option might be an unfamiliar one.
You would probably put the balance block in parallel with a normal mixer as shown below:
Figure 1: Balance block and mixer in parallel

The goal is to make the normal mixer do what you'd normally expect from it and let the balance block do the component ratio calculation. In the above case, the composition of the two feed streams is known and the flow rate of one of them is also known. The requirement you have could be to have a ratio of 1 / 4 between methane and n-Butane. On the parameters page of the Balance block you can select "General" as the type. Now you can add a ratio between two or more components.


The Ratio definition screen looks like the below picture:

You can choose if you want a mass or mole or volume basis.

The key advantage is that you do not have to work out how to impose this ratio and there are no iterative calculations (outside the balance block anyways).


Sunday, 26 April 2015

Did you know series: Modelling a burner, it can be simple or quite complex

If you just need a very simple model of the combustion of natural gas

Make sure you have oxygen, nitrogen, CO2 and water in your components list
Add CO to the component list for more accuracy
Create an air stream
Feed the air and the fuel streams into a Gibbs reactor
If there is no duty stream or if you set the duty to zero, you will get the adiabatic flame temperature
If you have a duty stream and you set an outlet temperature, then you get the heat you can recover

From here you can make things more complex, it all depends on your needs

If any of your compounds have other elements than carbon and hydrogen, you also need to include the combustion products of those other elements. For sulphur for example, the simple way out is SO2, but you might want to add SO3 and H2S and even the various forms of elemental sulphur.

Sometimes reaction kinetics come into play. For example, if the oxygen excess is low, it may be prudent not to specify too low an outlet temperature for the Gibbs reactor. The issue is the kinetics of the reaction between CO and CO2. The Gibbs reactor calculates chemical equilibrium, so if you were to cool the flue gas to ambient temperature, you'd get what the chemical equilibrium composition would be. In some cases, reaction kinetics will prevent the reactions from reaching equilibrium. For the CO/CO2 equilibrium, this will be quite noticeable, but not dramatic. A more dramatic example is nitrogen oxidation. If you were to add the various nitrogen oxides to the component list, you'd end up with a lot of nitrogen oxide in the model outlet, but in real life this doesn't happen because the reaction isn't fast enough.

All the above will work as long as you use HYSYS library components. With Aspen Properties library components things will usually work, but not always. The chemical formula and the heat of formation of the components needs to be known and that is not always the case for Aspen Properties components.

And then you get to the cases where your fuel is not so nicely defined. It doesn't have to be anything exotic, just imagine the case of a fuel oil and you'd typically no longer have a feed stream defined in terms of library components. You'd have hypo components and those do not have a chemical formula nor a heat of formation (or heart of combustion). Usually you would be able to obtain a lower heating value and an elemental composition for a fuel oil. Usually the elemental composition is given on a mass basis, you'll have to convert it to a mole basis to be able to create a chemical formula. You will have to manufacture a component with a high molecular weight to reasonably match your elemental composition. I on of the elements is only present in small amounts and you want to include it, the MW will have to be really high. Once you have settled on a formula, you'll have to use this information to create a combustion stoichiometry. Then you can us this, the heat of combustion and the heats of formation of all components except the fuel to calculate the required heat of formation of the fuel. Sounds tedious? It is ... But, if you need to do this on a regular basis, I can provide you with a small program that will do all the work for you. Just send an Email to wim@bpt.no.

This hasn't touched yet on anything that really requires combustion kinetics! This comes into play in the combustion of H2S for sulphur production for example. I can go into this in another post if lots of people ask for it.


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