Hello, and welcome to this lecture on the characteristics of block flow diagrams.
My name is Marina Miletic.
Let's look at this ethylene oxide production process.
This process takes ethylene and oxygen and converts them via a catalyzed process to ethylene
oxide.
In addition to this main reaction, we also have two side reactions creating carbon dioxide
and water.
The main reaction does not feature 100% conversion of the reactants, so this means that ethylene
oxide, along with unreacted ethylene, oxygen, water and carbon dioxide come out of the reactor
and first go into a flash separator, where the oxygen and carbon dioxide are separated
from the heavier components.
The ethylene and ethylene oxide and water are sent to a distillation column, where the
ethylene comes out of the distillate because it is more volatile than the ethylene oxide
and water, which are sent to the bottoms.
Ethylene is recycled back to the reactor to help increase the overall conversion.
In the second distillation column, ethylene oxide is separated in the distillate, while
water is removed from the bottoms.
Notice that oxygen is not recycled because to do so would involve separating it from
carbon dioxide, and that would probably involve very high pressures and low temperatures.
It would simply be too expensive.
Let's look at some characteristics of block flow diagrams.
First, notice that flow through the process should be from left to right.
There should be a clear horizontal path of flow, except for recycle loops, which have
to return to a process unit at the beginning.
This is perfectly fine, it is actually unavoidable.
Notice that equipment is represented by blocks.
Be careful not to overcomplicate a diagram like this with detailed shapes.
Notice also that only critical pieces of equipment are shown.
I would say at the very least, reactors and separators need to be shown on a diagram like
this.
Reactors and separators are instrumental in chemical engineering processes.
I would consider that pumps, most heat exchangers, storage tanks, valves, and similar pieces
of equipment should probably not be put on a block flow diagram.
These are unnecessary to show.
Next, notice major flow lines are shown with arrows.
I would say besides blocks and some text, arrows are the only other thing allowed on
a diagram like this.
Note that in no way does this process only have 9 streams.
There are obviously far more streams than the ones shown here, including complicated
bypass streams around control valves and around units.
This level of detail is not shown on a diagram like this.
Note also that if lines must cross, horizontal lines take precedent over vertical lines going
up or down.
The vertical line must be broken, and the horizontal line stays solid.
A block flow diagram would not be complete without mass flow rates of all streams coming
in and going out of the process.
You have no idea of the scale of the process or the amount of product being made per day
unless you provide this mass balance information.
Make sure that the units are in pounds per hour, kilograms per hour, tons per day, or
in some usable and relevant flowrate which gives you about 2 to 4 digits.
Not, say, grams per day, which would probably give you numbers like 70 million.
Notice that light components like oxygen and carbon dioxide are shown coming out of the
process higher up on the diagram than less volatile components like ethylene oxide or
water.
This helps organize the diagram, and also gives you a qualitative understanding of the
relative volatilities of each stream.
Notice that when I say lighter, I mean in terms of volatility, not molecular weight.
For example, even though ethanol has a higher molecular weight than water, it should probably
be shown higher up on a diagram, since it's more volatile.
Some people drawing block flow diagrams don't follow this "lightest component above heavier"
for reactants, and instead they list what is considered the most important reactant
first.
For example, since ethylene is generally considered more important to oxygen for this reaction
system, some would say it should be shown first.
This is technically OK because they're being mixed in the same inlet stream.
So it probably doesn't make a difference what order they're shown on the left side.
The last point about this diagram is that it must include a figure number and caption.
A diagram this important needs to be referred to in reports, and you can't do that if there
isn't a figure number.
notice also that the caption for the figure provides a good description, and it's not
filled with acronyms.
Also, if possible, include any or all relevant reactions.
In the case of a process such as a refinery, this is absolutely not possible, because there
are dozens of reactions occurring in some cases.
But in the case of this ethylene oxide process, there are only three reactions, and that's
why they're included.
To recap, remember that on a block flow diagram, flow is from left to right, equipment is represented
by blocks, and only main pieces of equipment are shown.
If lines must cross, horizontal lines stay solid and vertical lines are broken.
Make sure flow rates are shown, show more volatile components higher up on the diagram
than heavier components, and include a figure number, caption, and list of reactions.