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Modelling of a Reactor Using Simulink

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Note that the feed-, vapour-, and liquid side streams are all moved a stage down. This is due to the total condenser that is considered a stagfe on its own (and it’s above all the other stages). It can be seen (from figure 1) that the feed stream, liquid stream and vapour stream are linked to stage 15, 11 and 25, respectively. It can also be seen that the pressure difference in the overall distillation clumn (from stage 1 [the condenser] to stage 30 [the reboiler]) is 5 psia. This will be the pressure drop that will be specified in Aspen (and the condenser pressure will be specified). It will be assumed that the feed stream is fed on stage. The previous mentioned information together with data from figure 1 will now be used in Aspen to model this distillation column. a)

It can be seen that the temperature is highest at stage 30, and lowers as the stage lowers. This make sense, since stage 30 is the reboiler (where heat is added to the distillation column). b) It was found that the minimum reflux ratio is around 25.709. This value is very high, since the equilibrium data for these different components are reletively close to one another. The following figures contain the different compositions in the four product streams at different reflux ratios:

The bottoms product in a distillation column will have most of the lowest volatility components. This is part of the function of a distillation column (to create seperation, where the lowest volatility components will be found in the bottoms). Note that the trend of all the components cannot be seen, since some of them are overrun by other components (in the above figure). The following figures show the different components in the bottoms prouct stream:

It can be see n that the lowest volatility component (octane) increases as the reflux ratio is increasing, while the lower volatility components are decreasing (such as butane, pentane and hexane). This indicates that the distillation columns’ efficiency in seperation is increasing as the reflux ratio increases, since he bottoms contains more octane. This will also be verified when looking at the distillate stream:

As mentioned before, the trend of each component cannot be properly seen. The following figures shows the different components trend when the reflux ratio is adjusted (in the distillate product stream):

It can clearly be seen that the butane (lowest volatility) composition increases as the reflux ratio increases, as expected. This supports the previous mentioned statement, that better seperation is found when the reflux ration is increased. Better seperation is found for the distillate and bottoms streams (highest and lowest volatility, respectively). Let’s consider the compositions of the side streams. The following figure contains the side stream that is removed at stage 11:

A better trend of the above figure can be seen when considering the components apart. The following figures contains different component compositions in the side stream at stage 11:

It can be seen that the butane, hexane and octane compositions in this side stream decreases, while the pentane (second most volatile component) composition increases, as the reflux ratio is increased. This indicates that side stream 11 becomes pentane rich as the reflux ration is increased. This is good, since less pentane will be found in the distillate and bottoms. This also indicates better seperation. Consider the side stream at stage 25:

The following graphs, as before, contains the different component compositions on a single graph, for side stream that is removed from stage 25:

The compostitons of butane, pentane and octane decreases as the reflux ratio increases, while the compostions of hexane increases (the second least volatile component). Thus, better seperation occurs as the reflux ratio is increased, where side stream 25 becomes rich in hexane. Throughout these conclusions, it can be seen that the highest product stream (the distillate) is rich in the highest volatile component (butane). As one goes down the distillation column, the second product stream (side stream 15) is rich in the second highest volatile component (pentane). The third product stream (side stream 25) is rich in the second lowest volatile component (hexane). The last product stream (bottoms) is rich in the lowest volatile component (octane). It was also seen that the seperation becomes better as the reflux ratio is increased (the previously mentioned streams becomes more rich with their associating component). The increase of the reflux ratio also means that more flow will be found inside the distillation column, this could mean that a large distillation column might be needed, or a high amount of energy is required for the condenser and reboiler. This will be verified by considering the duty of the condenser and reboiler as a function of the reflux ratio:

The graph above shows that more energy needs to be removed as the reflux ratio is increased (note that energy removed has a negative sign).
It can also be seen that more energy needs to be added to the reboiler as the reflux ratio is increased. This is discribed by the amount of mixture in the distillation column as the reflux ratio is increased. A larger flow rate is found in the intersages as the reflux ratio is increased. More energy is required for condensation and reboiling if the flowrate of these streams are higher. An operation point will be considered where the wanted composition is produced, and also where the energy requirements, as well as the size of the distillation column (since a larger distillation column will cost more) isn’t so high that a loss in profit is induced.

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