Hi there, and welcome to this lecture on process flow diagrams: introduction to stream tables.
My name is Marina Miletic.
There are two main tables that supplement every PFD: a stream table, also called a flow
summary table, which is often accompanied by a utility stream table, and second, an
equipment summary table.
Stream or flow summary tables describe and list the physical properties of each process
stream.
Utility stream tables describe and list the properties of each utility stream.
Equipment summary tables describe the physical properties of each piece of equipment in terms
of its type, size, capacity, power, and other important aspects.
I'll be discussing stream tables.
These are stream numbers, and since they're not pieces of equipment, they do not have
a letter abbreviation.
In CHEMCAD and Aspen, stream numbers are automatically labeled for your and put in squares, and some
other diagrams may have them in diamonds.
The reason why these stream numbers are important is because the temperature, pressure, vapor
fraction, composition, and other information related to each of these streams is provided
in a stream table, which is a required supplement to every process flow diagram.
Stream numbers help organize the diagram and provide identifying information for the stream
table.
Each stream on a process flow diagram must be numbered.
All chemical simulation packages, including CHEMCAD and Aspen, keep track of the physical
properties of each of these streams, including flowrate, temperature, pressure, composition,
and other information in a stream table.
This table is a required element for every process flow diagram, since it quantitatively
describes how the physical properties and composition is changing as we move through
the process.
You might have noticed that streams are numbered based on the path of the process, from starting
materials to products.
The numbers of these streams should effectively trace the path of the process, as the reactant
streams enter and the product streams leave.
Just like with numbering equipment, there's a bit of a judgement call that must be made
when you're numbering parallel streams.
The most important thing to keep in mind when numbering streams is to make the stream table,
where all these streams are described in detail, as easy to follow as possible.
For example, it's a little challenging to be reading a table and to be switching back
and forth between two completely different streams, such as these oxygen and ethylene
paths.
If instead there's a sequential progression to the point where these paths are mixed,
it's easier to read and keep track of these streams, as well as understand the pressure
and temperature changes.
Let's next look at the stream table for the ethylene oxide process you just saw.
The process has 21 streams, and all of them are included in this table, which may take
up several pages.
Information which is necessary to include about each stream are: the stream number,
overall temperature of the stream, pressure of the stream, vapor fraction, total mass
flowrate of the stream, total molar flowrate of the stream, and individual molar flowrates
of the components in the stream.
Optional information you might want to consider including are individual component mole fractions,
individual component mass fractions, individual component mass flowrates, volumetric flowrates,
density of the stream, viscosity of the stream, overall heat capacity of the stream, enthalpy,
and a descriptive name for the stream.
Of all of these, I believe mass or mole fractions are the most common addition to the table,
since it provides the quickest and easiest way to see the concentration of each stream.
It's important to think about what management will want to see in this table, but be sure
not to overwhelm the reader with too much information, so they're not flipping through
pages of tables trying to make sense of the streams on the diagram.
Next, I'll provide some important tips in constructing a well-organized stream table.
Something that makes a stream or flow summary table easier to read is not listing the components
in alphabetical order.
It's easier to read if the components are listed as reactants first, then products.
This is because it's easier to understand what's happening in each stream, especially
before and after the reactor, if you put the components in this order.
Another thing that helps create an easy to read stream table is to choose the right units,
especially for mass and molar flowrate.
I've noticed that using metric tons per hour, US tons per hour, or kilomoles per hour, for
most large chemical processes, is a good way of making sure you have numbers that are around
one to four digits.
You want to avoid numbers that are in the tens of thousands or larger.
This crowds the table and makes it very difficult to read, and if you do have to present numbers
in the thousands range, be sure to use commas to help make them easier to read.
Also try to avoid having a table which is filled with numbers all in scientific notation.
Remember that these guidelines are much easier to follow if you just choose units which result
in smaller numbers.
In addition, frequently I've found that when people present large numbers, they like to
provide five or more significant figures.
For both clarity and accuracy, you want to avoid doing this.
Even though CHEMCAD or Aspen will technically provide you with 12 significant digits for
every number, it doesn't mean you should expect your process to function at that level of
precision.
It's very misleading to claim that you know your flowrates, temperatures, and concentrations
out to give significant figures, and that you expect to control them at those same values
continuously as the process is running.
I know that I have five significant figures displayed for these numbers.
I added a .0 to these flowrates to make them easier to read, not because they have five
significant figures.
This was a compromise I made between removing zeroes and making the table more consistent-looking.
Remember that not having more than around four significant figures shown is a guideline,
and you will not always be able to follow that guideline 100% of the time in all cases.
Another tip for creating an easy to read table is to make sure you right justify all numbers.
This means that you should line up all numbers to the right side, which in this case aligns
all of the decimal points.
Right-aligning these numbers makes them much easier to read than if they were centered
or left-aligned.
Also, if you can align most or all of your decimal points in each column, this can really
help enhance the readability of the table.
Next, let's briefly talk about labeling.
Since this is a table, it almost goes without saying that it must be numbered and accompanied
with a table caption.
Refer the reader to the main diagram from where the stream numbers came.
Regarding layout, it's alright to have a table that wraps around on the same page, but be
careful with labeling.
Notice this is labeled "Table 2," but this is unnecessary, since the entire stream table
is table 1.
So this wraparound table should remain unlabeled like this.
If the table wraps around to the next page, however, you should include a table label,
which denotes this as "Table 1 Continued."
This is not a different or separate table, it's just a continuation of the previous one,
which is why it's not labeled Table 2.
Something to keep in mind is that this might look alot like the stream table that CHEMCAD
or Aspen creates for you.
These programs are notorious for giving numbers such as 9.9 times to the -8, when in fact
the number is basically zero.
I suggest that you transfer these CHEMCAD or Aspen stream table numbers into Excel and
format them so that they display the correct number of significant figures, and are easy
to read.
Don't underestimate the amount of time it will take to properly format a table like
this, because it almost always takes longer than most people think, so never ever leave
this for the last minute.