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Modeling Continuous Steady State Distillation
Once you enter the simulation environment, you will be given a
green screen where you can start building your model.
Select The Model And Define Connecting Streams
For the
24-plate continuous distillation column case, follow these
instructions:
- In the graphic area, select the distillation column by
clicking on the icon shown by Figure 1 then clicking on the green
screen. You will obtain a graph looking like Figure 2. You are
ready to enter the different parameters for the column.
- Double click on the icon that looks like Figure 2 to enter
the different specifications.
- First, you should enter the number of stages, which is 24 for
the continuous column. To do so click on the number of stages
then go to the bar at the top of the screen then click to
edit.
- Select TOP DOWN to start numbering the stages from the
top.
 
Figure 2
- Name your feed (we will name it here F). Assume the feed to
enter at plate 10. (You can change that option later to adjust
your output from the column).
- Select TOTAL REFLUX because we will only draw liquid
from the condenser. Name your distillate stream (in this example
D) and your bottom stream (in this example B). Also name your
condenser and reboiler duties (in this example respectively Qc
and Qr). Once done, click anywhere in the screen and the
NEXT button will appear bold indicating we are ready to
move to the next step. Click on it.
- In the laboratory we operate at atmospheric pressure.
Therefore both the reboiler and the condenser will be at 1 atm.
Enter the pressure by clicking on the appropriate box then on the
bar up the screen. Enter 1 then in the small window next to the
input bar, select by clicking on the arrow the right unit: atm.
Repeat the same procedure for the other unit. We will leave the
condenser pressure drop to be 0. Click on the NEXT button
to continue.
- The next window displayed asks for estimated temperatures for
the condenser and the reboiler. These temperatures are taken to
be initial guesses when HYSYS performs iterations when finding
variables. We will leave them blank and we will move on to the
next screen.
- In the next window, we are asked to input the reflux ratio.
In our example, it will be 3. We will not specify the liquid rate
(otherwise our problem will be over specified). However, the
liquid rate can be chosen as an independent variable, but one of
the other variables we specified in the example would need to be
dropped to avoid over specification. Click on DONE to
continue. Then you will obtain a monitor window that will perform
the calculations. Close that window to now enter the feed and
other stream conditions.
Define The Feed Stream Parameters
In the laboratory as mentioned before, the feed is at room
conditions (T=25oC, P=1atm). We will assume the
concentration of the feed to be 50% ethanol, 50% isopropanol in
mass fractions. (The concentration of the feed in the laboratory
will be calculated prior to experiment). In our example we will
also assume that we are feeding at a rate of 163 lb/hr. For the
reboiler heat duty, we will assume a load of 120,000 BTU/hr.
- To input the parameters into the feed stream F, double click
on the feed stream and you will obtain a window like Figure 3. In
this window you will enter the characteristics necessary for the
feed. In the Vapour/Phase Fraction box, enter 0 since the
feed is purely liquid. Then click on the Temperature box
to enter room temperature (25oC). Then click on
pressure to enter the normal atmospheric pressure (1atm). Click
on the Mass Flow box to input 163 lb/hr. The next step is
to input the feed concentrations.
- To input the concentrations, click on COMP shown by
the arrow on Figure 3. Click on EDIT. In the
COMPOSITION BASIS section select Mass Fraction then
enter 0.5 for each of the components. Then click OK. Close
the window to go back to the main screen. You will notice that
the feed stream changed color. This is because we have specified
all the independent variables we care for on that stream and the
dependant variables were automatically calculated by HYSYS.
The next step is to enter the reboiler duty. To do so, double
click on the reboiler duty stream (Qr in our example). When the
screen appears, enter in the Heat Flow box the duty in BTU/hr (in
our example 120,000 BTU/hr). Click on CLOSE to go back to the
main environment.
- We need to inform HYSYS that the overhead and bottom
products are liquid (no vapor). To do so, double click on the
top stream (in our case D) then input 0 in Vapour/Phase Fraction
box, then click on CLOSE. Repeat the same procedure for the
bottom stream (B in our case).
We are now ready to begin performing column calculations
since all the variables have been specified.
Figure 3
We are now ready to run a column simulation to obtain all the
dependent variables such as the overhead and bottom distillate
conditions, the condenser load and so on.
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