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11/15/13 Gibbs vs. Equilibrium Reactor

www.owlnet.rice.edu/~ceng403/hysys/gibbs.html

Gibbs vs. Equilibrium Reactor

In order to save time and make it possible for you to campare the results of the various reactors, this example

will begin by using the same components and reactions that were employed in the Plug Flow Reactor Example. It i

recommended, but not necessary, that you work through that example first (or at least glance at the component list

and reactions). If you do not have a version of your own of that example from which to start this example, I have

made available a completed version of that example entitled PlugFlowEx.hsclocated in the

\\Hartsook\Hysys\SAMP403 directory. There is also a file containing this and all other completed reactor

examples called AllReactors.hscin the same directory.

The Gibbs Reactor

We will begin by installing the Gibbs Reactor. Set up your Process Flow Diagram (PFD) like you see below

(remember the button is great for quick connecting as it toggles you in and out of Connect Mode). The Gibbs

Reactor is available from the Unit Ops menu or from the Object Palette by pressing the GeneralReactors button

which opens this menu . Press for the Gibbs Reactor.
http://www.owlnet.rice.edu/~ceng403/hysys/pfex.htmhttp://www.owlnet.rice.edu/~ceng403/hysys/pfex.htmhttp://www.owlnet.rice.edu/~ceng403/hysys/pfex.htm

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www.owlnet.rice.edu/~ceng403/hysys/gibbs.html 2

Note that it is not necessary to attach a liquid product stream to the reactor. All of the reactors may be run with

only one product stream (vapour or liquid). Should both phases be present in the product and only one or the other

product stream available, HYSYS will tell you it needs another product stream. At that point, you must add anothestream, but do not be dismayed. Remember, you can always mix them back together again. Also note that you may

add an energy stream to the reactor. That we have not indicates that we intend the reactor to be adiabatic.

Since we desire the GibbsFeed stream to be the same as the Plug Flow Feed stream, the easiest method is to

double-click on the Gibbs Feed, opening the property view, and switch to the UtilitiesPage. There, press the

button marked "Copy Stream Specs From ..." and choose the Plug Flow Feed stream.

You'll find that this is all that is required for the Gibbs reactor to work. If you so wished, you could now open

the reactor's property view and add a pressure drop on the ParametersPage. You can also switch to the

ReactionsPage where you will see a couple more options. Currently selected should be Gibbs Reactions Only

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This is the choice that gives total equilibrium of all components (excluding those labeled Inert, or otherwise

specified, on the CompositionPage). Below that is Specify Equilibrium Reactions, the option that allows you t

attach an equilibrium reaction set. We shall return to this option later in the example. The last option is to have No

Reactionoccur at all. The reactor then would simply act as a separator (provided both vapour and liquid product

streams were connected).

Changing nothing, however, we find our product stream looks like this

Note these are extremely different results than we obtained in the Plug Flow Example. The main difference is that

where in the PFR the reforming reaction was not occuring, here it is occuring in great quantity, backwards. At this

temperature, the equilibrium is to the left (something we did not even allow for in the kinetic equations we defined

for the reforming reaction), and methane is formed in what is called the Sabatier reaction (same reaction, two

names, go figure. We are ready to start putting in the Equilibrium Reactor.

The Equilibrium Reactor

We cannot install a working equilibrium reactor without first creating the reactions it will use. Therefore the first

thing you must do is enter the basis environment. On the Simulation Basis Manager, switch to the Reactions

Page (close any other windows that might have been left open there). Before continuing any further, if they are not

already, I suggest that you change the names of the existing two reactions to ReformingLHand WGSkinfor

consistency with my examples and to prevent confusion later (you will begin to collect many very similar reactions)

You should also probably change the name of the reaction set containing those reactions to Kinetic. That

completed, your window should look something like this
http://www.owlnet.rice.edu/~ceng403/hysys/pfex.htm#results

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Now, in the Reactions Sub-Group, press the Add Rxn ...button. Choose Equilibrium Reactionand name th

reaction that appears ReformormingEq. Since this is a very common reaction of interest, before beginning to fill

out coefficients we will check the Library Page to see if it is already stored in HYSYS. Luckily for us, it is, as noone was kind enough to provide us with any equilibrium data on this reaction. As you can see it is second on the lis

and the Water Gas Shift reaction that we will be adding next is first.

Highlight the reforming reaction and press the Add Library Reactionbutton. You will find the reaction is now

completely defined. HYSYS inputed for you a table of Keqvs. Temp values (from which it also fitted the equation

uses to interpolate and extrapolate -- speaking of which, be aware of the temperature range of the table from whic

HYSYS will draw its information). To learn more about ways of inputing your own data into the form read the

section on Equilibrium Reactionsback on my Reactionspage or Section 11.3.3 of RV1.

Now that we are done with the reforming reaction, close this window and add a new equilibrium reaction called

WGSeq. Again, go to the Library Page and choose this time the first reaction on the list. Close the window of the

completed second equilibrium reaction.
http://www.owlnet.rice.edu/~ceng403/hysys/reactions.htmlhttp://www.owlnet.rice.edu/~ceng403/hysys/reactions.html#equilreaction

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Our next job is to create a reaction set for our two new reactions. Though, HYSYS will allow you to put kineti

and equilibrium equations in the same reaction set (see the General Reactor), you could not put these in the same

reaction set with the others because they refer to the same two reactions (evaluated in different ways). Create a

new reaction set from the reactions page of the Simulation Basis Manager and call it Equilibrium. Add the two

reactions you just created to the active list. It should now look like this:

Close the window. Back again on the Simulation Basis Manager, highlight the reaction set you just created and pus

the Add to FPbutton as we must associate the reaction with our fluid package before it can be used in the

flowsheet. There should be only on fluid package listed (if there is more than one, pick the one that is used on your

main flowsheet). Select it, then push Add Set to Fluid Package. The completed Reactions Page of the Simulation

Basis Manager should look like this:
http://www.owlnet.rice.edu/~ceng403/hysys/reactions.html#general

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Note the Assoc. Fluid Package box only shows those Fluid Packages associated with the highlighted reaction set.

You may now return to the simulation environment as we are ready to install the equilibrium reactor.

The Process "Switch"

Rather than simply create another feed stream for the Equilibrium Reactor copied from the PFR or Gibbs

Reactor, let us assume that you wish to compare the results of the two different reactors on the rest of an involved

process. You could simply use Save Asto make a copy of the file and substitute one reactor for the other, but tha

makes comparison of results difficult and cumbersome. You could also use the Logical Operation Balanceto

dynamically copy the feed to one reactor to another stream which would then become the feed to the other reactor

That way you can have the upstream of the plant duplicated to both reactors on the same flowsheet. Unfortunately,

to get the downstream part of the plant, you would have to copy every stream and unit, a difficult prospect in itself

which would clutter your PFD, and then you would have to make every future adjustment to the downstream

section twice, a tedious business at best and heaven help you if you have a downstream stream affecting anupstream stream. Basically, this option is out for any but the simplest of flowsheets or unless you only care about th

immediate output after the divergence.

Your last, and I think best option (though there are times when a Save As might well be the most expedient

solution), is a technique which I find HYSYS well suited to take advantage of, but which I found curiously missing

from the manuals. I call it the Process "Switch". It takes advantage of a lesser used option in the preferences

allowing you to make multiple connections to the same stream and the ability of HYSYS to "ignore" unit operations

without you deleting them.

Therefore to start, open the Preferences from under the Toolsmenu. On the first page, marked Simulation,check the box marked Multiple Stream Connections. You may then close the Preferences window.
http://www.owlnet.rice.edu/~ceng403/hysys/logicals.html#balance

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Since there is no point tempting fate by upsetting HYSYSand to make things clearer to you, we will go ahead

and make HYSYS ignore the Gibbs Reactor. Open that reactor's property view and switch to the Parameters

Page. Check the box that says Ignore This Unit Op During Calculations. The reactor should now turn yellow

and the product stream to a pale blue, indicating it is not solved.

It is time to place the Equilibrium Reactoron the PFD. Either do so from the Unit Ops Menuor as follows

from the Object Palette.

Push which opens this menu . Press for the Equilibrium Reactor.

Your PFD should look something like this:

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Now from the Equilibrium Reactor's Property View, go to the ConnectionsPage and for the Feed stream, go to

the drop down menu and choose FeedGor whatever you called the feed to the Gibbs Reactor (if you had not

already changed the multiple streams option in the preferences, that feed stream would not be a listed option). Nowif you were to go right to the Vapour Outletand attempt to connect ProductG, you would get this error:

I believe this is due to the similarity of the two units you are trying to "switch" out. When I tried this first with aGibbs Reactor and a PFR, I did not get the error. There is an easy way around it, however. Simply attach a valve

with zero pressure drop (don't forget this) to the Equilibrium's Vapour Outlet, then in the valve's

property view, set the outlet to be ProductGor whatever you called the Gibbs Outlet.

Note:When I was experimenting with the "switch" and the PFR, the PFR began having trouble doing

its integration and I was forced to set the segment initialization to Re-initializefor it to work.

When you are finished connecting, your reactors should look like this:

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Now all that remains is to specify the Equilibrium Reactor. All you need to do is open the Equilibrium Reactor

Property View, move to the Reactionspage and attach the EquilibriumReaction Set. HYSYS will do the rest.

You may now switch back and forth between the two reactors. I recommend that you try to have one of the

two "ignored" at all times (though in this simple case nothing crazy happens and you should observe what happens i

you break this "rule"), thus you would start a "switch" by going to the ParametersPage of the Equilibrium

Reactorand setting it to be ignored. Then go to the Gibbs Reactor and remove the command to ignore.

If you examine the product stream as you are switching back and forth, you will see that the results of the tworeactors, as they are currently configured, are almost identical. This kind of confirmation is good to see as the

reactors go about this calculation in completely different ways. You can see this for yourself by going into the

reaction manager and changing one of the equilibrium reactions (or a copy) to have a fixed K of, say, 1. (I leave it

to you to remember how to do it). You will then find that while the results for the Equilibrium Reactor change

drastically, the Gibbs Reactor continues to give the same "correct" values. This same technique can be used later to

show that when an equilibrium reaction set is attached to the Gibbs Reactor, that it is truly only taking the

stoichiometry info.

Another important thing to note about the similar results is that the thermodynamic property set we are using is

Antoine. This is one of the thermodynamic packages that has essentially ideal vapour and liquid phases. You'll find

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that if you change your thermo package to one of the non-ideal ones, like Peng-Robinson, that the results of the

Gibbs Reactor begin to diverge from those of the Equilibrium Reactor (remember the equalibrium reactions set Ke

as a function of T only). More so in other examples than this one. (You may also have noticed that for whatever

reason, the PFRtakes a lot longer to re-calculate in Antoinethan in PR). To see a case where the difference

caused by the thermo package is more severe see the file called methpuz.hscin \\Hartsook\Hysys\SAMP403 .

Now we are ready to experiment with attaching reactions to the Gibbs Reactor. Open its property view and go

to the ReactionsPage. Switchthe Reactor Type to Specify Equilibrium Reactions. Try attaching one of the

reaction sets to the reactor. You probably were not suprised to see that you could not select the Kineticset, but

you might have been suprised to discover that you also could not attach the Equilibriumset! Located down at the

bottom, in the Trace Window, is this explanation:

"There is no need to specify reactions if all of them are equilibrium type. Gibbs Reactor will take care

of attaining equilibrium by itself"

Remember that in the standard setting on the Gibbs Reactor, the reactor will solve for the equilibrium of all possible

reactions (element balance is maintained). As it turns out, with the limited number of components in this example,

the two reactions we are exploring compose the entirety of possible independentreactions. HYSYS will not allow

you to attach a set containing both reactions as it could accomplish the same thing better by simply forcing you to

choose Gibbs Reactions Only.

Frequently, however, you do not want all possible reactions to go to equilibrium. Suppose instead you wish to

model a system that normally does not go to equilibrium quickly (there is a large activation energy). You, however,

know that there is a catalyst which will lower the activation energy for the reaction you want and onlythe reaction

that you want. You know that eventually a PFR will be used, but for whatever reason, you do not wish to model

one yet. It is sufficient to assume that the Plug Flow would sufficiently near equilibrium that you may model the

reactor as if it went all the way to equilibrium. In a case such as this, you would want to use an Equilibriumor Gibb

Reactor with only the one reaction attached. This is the time to use the Specify Equilibrium Reactionsoption in

the Gibbs Reactor.

For this example, we must first de-activate one of the reactions in the EquilibriumSet before we may use it. L

us assume that we wish to focus on the Water Gas Shift reaction and expect to see little or no Reforming (at the

temperatures we are operating at, a not unreasonable assumption). Under the Flowsheetmenu, select Reaction

Package. This will open a window showing the available reactions and reaction sets. Double-click on Equilibrium

and its window will open. Highlight ReformingEqand push the Make Inactive ->button.

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1) gibbs vs

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