Hi there, and welcome to this lecture on process flow diagrams: equipment symbol representations.
My name is Marina Miletic.
These symbols do not represent equipment and are not technically required on a process
flow diagram, but you can probably guess their purpose.
This is a process input, or some stream that's being introduced to the process, and this
is a process output, or some product that is leaving the process.
It's important to remember that these symbols are only used for streams involved in making
some process product, so in other words, these symbols would not be used for utilities, like
steam, cooling water, refrigerant, or for anything that's not directly mixed with the
reactants, intermediates, or products in the process.
The point of these symbols is to help the reader quickly determine where the starting
and ending points of the process are, and the number of reactant and product streams.
One of the most difficult things about reading a complicated process flow diagram is just
knowing where to start.
These symbols help bring some organization to these diagrams.
I'd like to discuss two units that are rarely labeled on a process flow diagram, yet they're
something you'll be using extensively in setting up your flowsheet in CHEMCAD or Aspen.
Can you guess what these are?
These are mixers and splitters, and you'll see these frequently in CHEMCAD and Aspen.
This is a typical stream mixer used in CHEMCAD, and this is one used in Aspen.
In both of these simulation programs, the sole purpose of a stream mixer is to combine
two or more streams before sending the components to a reactor, heat exchanger, pump, or almost
any other unit.
This is a typical stream splitter used in CHEMCAD, and this is one typically used in
Aspen.
The purpose of a splitter is to split one stream into two or more so they can be sent
to different process units.
This split does not have to be even, such as 50/50, or 33/33/33.
The strip can be split in any combination, and the outgoing streams all have the same
composition as the stream going into the splitter.
The reason why splitters and mixers are used so frequently in simulation software is because
in almost all cases, both CHEMCAD and Aspen don't allow you to send multiple streams directly
into a unit.
Similarly, you can't send multiple streams of the same composition out of a reactor,
heat exchanger, pump, or any other unit.
When you're using simulation software, in almost all cases, you must have one process
stream arrow going in, and one process stream arrow coming out.
So if two or more streams need to be sent to a process unit in CHEMCAD or Aspen, a mixer
must be used, such as in these examples.
What's critical to remember about this is, just because you're forced to use a mixer
or splitter unit in CHEMCAD or in Aspen, it doesn't necessarily mean there's an actual
tank or elaborate apparatus in place in the plant which mixes or splits process streams.
I've noticed that most students tend to assume that if they use a mixer in CHEMCAD or Aspen,
which you're forced to do if you need to combine two or more streams, it means there needs
to be a surge or accumulation tank sitting there in the plant, right before the next
process unit.
This is almost always not the case.
Let's look at how streams are actually mixed in a real process.
In order to mix two streams in an actual plant, often times you'll have two pipes coming together
in a T. Two streams meet at the T, and leave as one combined stream.
There also might be in-pipe baffles after the T, which help the liquids, gases, or vapors
combine more effectively, or there might be an orifice plate that helps inject one stream
into another, but these are not always necessary.
Not very different from mixers, a splitter in a plant is usually a pipe and valve system,
which controls the splitting of the main stream into two or more streams.
The valve and piping system are what are used to split the stream, and a tank or special
piece of process equipment is not necessary.
There are of course some situations where mixing tanks are needed in a process.
If by chance a mixing tank is being used, it's probably for mixing two or more streams
that are viscous, non-newtonian, or contain suspended solids, which you'll see alot in
the food industry.
In alot of large chemical processes, you'll be dealing with high pressure gases, which
tend to mix very easily, and liquids with low viscosities, so mixing tanks are not necessary
in almost all of these situations.
So in almost all cases, mixers and splitters are not labeled, and they do not have a letter
abbreviation.
However, when a mixing tank is needed, they should be represented as vessels.
So for example, while these may be representations of mixers in CHEMCAD or Aspen, if an actual
mixer is necessary before these reactors, they should be represented as vessels, like
this, on a process flow diagram.
I'd like to make one last point about mixing streams right before major process units.
I often times find students pointing out to me that they can in fact send more that one
stream into a reactor or heat exchanger in CHEMCAD or Aspen.
Remember that if you see extra connections like these for some process units in your
simulator, these are almost always designated for utilities, such as cooling water, steam,
or any other heating or cooling media.
Do not assume this means you can send another process stream into that piece of equipment.
Be sure you become familiar with which connection is meant for the process stream, and which
connection is meant for the utility stream.