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.

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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.


Sunday, 19 April 2015

Did you know series: How to come up with a working distillation column model for a vaguely specified separation task

This post was inspired by a post on LinkedIn about converging a column. When you have a distillation problem that is fully defined, you usually do not need to resort to the below. But sometimes the problem specification is partial or ambiguous. For modelling a column, HYSYS has two simplified models that come in handy when you are having issues. One is the shortcut column and an even simpler model is the component splitter. The aim of using these simplified models is ultimately to get the more rigorous stage by stage calculation to converge, the simplified models help to understand better the nature of the problem you are trying to solve.


Although this description starts from the simplest approach, quite often you'll end up going from distillation to short cut to component splitter and then back to short cut and then distillation. At times "regressing" to the short cut may be enough, sometimes it is not. In the picture to the left you can see that you can use the same feed for all three unit operations. You will have to set your preferences to allow multiple stream connections!

I'll use the general description of the problem discussed on LinkedIn:
Feed is a mixture of 10% mass Oil and 90% n-Hexane
The aim is to recover 99% mass of the n-Hexane as a distillate

No info was given about the composition of the oil, so I assumed in my test case a mix of C7 to C16 (10% mass of each).

When you model this using a component splitter, it is pretty easy to specify the 99% recovery of the n-Hexane. But, it will (should ...) be quite clear that you need to have some info about what needs to happen with the oil components. In the description above, there is no indication of that info. Preferably you'd find out from the person/company that posed the problem what the specifications are concerning recovery of oil or tolerated amount of oil in the n-Hexane. If no info can be obtained, you would have to come up with something reasonable yourself. In this case, I assumed that 0.1% mass of C7 was acceptable in the n-Hexane. I assumed the other components would not be present in the n-Hexane. You MUST have a non-zero spec for one of the components! The component splitter would allow a zero spec, but the short cut and stage by stage model would not work with a zero spec.

The results of the component splitter will help you specify the short cut distillation. You need the desired mole fraction of the light key (n-Hexane) in the bottoms product and the desired mole fraction of the heavy key in the distillate. These cannot directly be deduced from the original specifications. But once you have specified the component splitter, the required inputs for the short cut distillation are readily available. You might be surprised that in the example case the mole fraction of n-Hexane in the oil that corresponds to a 99% recovery is 13.3% mole .
When you have specified the key component mole fractions and the operating pressure, the short cut model will report a minimum reflux ratio. The actual reflux ratio you will use is an economical decision. Factors that will have a high impact are the cost of the cooling and heating medium you need to use and the operating pressure and the metal of construction. I have seen cases where the selected reflux ratio was as low as 15% above the minimum reflux ratio and sometimes it is more like 50% above the minimum reflux ratio.

Once you have the short cut column working, it is easy to derive the stage by stage model. You could use the same specifications as in the short cut model to start with. Once the column is converged, you can switch to specifications that make more sense to you.

If you would like to download the file I made, use this link: https://dl.dropboxusercontent.com/u/302086/Setting%20up%20a%20distillation.hsc
The case in in Aspen HYSYS V7,3 format

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