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There are four logical unit
ops that are used primarily in steady-state mode. These are
SET, ADJUST, BALANCE, RECYCLE,
and the Spreadsheet. The rest of the logical unit
operations are used primarily in Dynamic Mode and will not
be discussed here. The Chapeter in the Reference Volumes
which covers all of logical ops is
Chapter 14 in RV2. Much of what
is said here may also be found there, as well as a more
thorough explanation of each option available.
 The
SET Operation
SET is used to set the
value of a specific process variable (PV in the manuals) in
relation to another PV. The relation must be of the form
Y = mX + b and the process variables
must be of the same type. For example, you could use the SET
to set one material streams temperature to always be 20
degrees hotter than another material stream's temperature.
SET may work both ways (i.e. if the target is known and not
the source, the target will "set" the source).
You can see an example on
SET's use that includes an example of the Spreadsheet
in my Set Example. You may also
read more about it in Section 14.7 in
RV2.
As a small aside. You can use the Spreadsheet to set the
multiplier, m, or increment, b, of the SET operation
(similar to what I mention with the ADJUST below). But I
don't see much point, since, if you're going to invoke the
Spreadsheet anyway, you might as well just import the source
variable, do your necessary calculations, and export the
result directly to the target without ever bothering with
SET.
 The
ADJUST Operation
Use the ADJUST operation
when you want HYSYS to adjust one variable until a target
variable reaches a user specified value or matches a
specification. That source of that specification can be yet
a third variable (of the same type as the target and offset
by a constant if you so wish). The Gas Processing Example in
the Tutorials
Book uses an ADJUST to vary one stream's
temperature until another stream's dew point equals a
specified value.
One irritating little feature
is that if you are using another object as the source of the
target value, the menu of objects will include objects that
you cannot actually use that way. I tried to connect the
results of a spreadsheet that way, but it did not work.
You can use the results of
Spreadsheets, however (which is good, since it greatly
increses the power and flexibility of ADJUST). It must be
done by exporting the variables from the Spreadsheet itself
to the ADJUST. You can export to the target value or to the
offset used whenthe target value is another object. If you
export to the target value, the number will appear in the
user specified box (the radio button must therefore be set
to that).
Before installing the
ADJUST module, it is often a good practice to inialtize the
independent variable, allow the flowsheet to solve, and note
the value on the dependent variable. Then self-adjust the
independent variable and make sure that the dependent
variable is actually affected by it. This should also give
you an idea of the step size you will require. The step size
is set on the Parameters Page and is the initial step
size employed until the solution is bracketed. Note that too
small a step size and you may not bracket the solution
before exceeding the maximum number of iterations (you may
change that number as well though). For a description of the
rest of the items on the Parameters Page, another example,
and a discussion on the use of multiple ADJUSTs
simultaneously (involves a change in the Solving Method),
see Section 14.1 of RV2.
 The
BALANCE Operation
This is one of the most
interesting and deceptively useful of the logical units.
Remember way back when you took CENG 301. You had to do Mass
and Energy Balances over sections of or entire plants to
solve for unknowns in composition, flow, energy, or
whatever. Well, that is what this operation will do for you.
You tell it what streams of material and/or heat are
crossing the imaginary boundary into your system of interest
and what streams are exiting. You know that HYSYS shares
information both forward and backward throughout the PFD at
all times. However that is not always enough for HYSYS to
know when the situation is entirely specified but in a more
unorthodox manner. The Mole and Heat Balance example
in Section 14.2 of RV2 shows a
single stream passed through two coolers in series. Though
you know the composition and the temperature of the stream
both before the first cooler and after the second cooler,
you do not know the flowrate or the temperature in between.
You do, however, know the duties of the two coolers, which
gives us enough information to figure out the unknowns. The
balance is set up so that the two coolers are the system,
which thus has one material inlet stream, one material
outlet stream, and two outlet heat streams. (Note: when
setting up balances, even when the flow is negative --
something you are more likely to see with energy than with
material streams -- choose which streams are inlet and which
streams are outlet from the direction of the arrows, into or
out of the system, on the PDF. HYSYS will handle the signs
correctly then.)
There are a total of 5 different types of Balance
available to you, they are:
- Mole and Heat Balance - I mentioned this one
already. It is the most commonly used and is good for
calculating ONE unknown based on the total material
balance and ONE unknown based on a total energy balance.
It should not be used in conjunction with a reactor as
the balance is on a molar basis. The material balance is
conducted first and will complete even if the energy
balance is under- (or worse, over-) specified. Though it
does not conduct individual component balances like the
General Balance, you may leave the composition of one
stream empty and HYSYS will know how to fill it.
Actually, I am not
absolutely sure about this type's limitations. It seems
to be able to handle more than the manuals say it should
(I suspect that it really does do some sort of individual
component balance). It was able to handle the example I
made up for the General Balance below, but it couldn't
handle the example for the General Balance contained in
the Reference Manuals.
- Mole Balance - There is another way to use the
Balance Operation. The "system" for which you have inlets
and outlets need not be part of the process at all. You
can use the balance to create new streams with
characteristics of streams in your process. In the HYSYS'
Gas
Processing Tutorial, they use the Mole
Balance to create a stream with the same composition
and flow as a process stream, but then they give the new
stream a different pressure and a vapor fraction of 1 to
see what the stream's dew point would be (remember, when
you specify vapor fraction of 1 or 0, HYSYS assumes you
want saturated vapor or liquid and solves for the
corresponding temperature or pressure).
- Mass Balance - The Reference Manual says that
this is good for non-stoichiometric reactors like
alkylation units and hydrotreaters. The compositions of
all streams must be specified and all but one flowrate.
Be aware that the only thing this balance passes is a
Mass Flowrate. Note that there is no sort of elemental
balance. You could have five streams of pure nitrogen as
inlets and one stream of CO2 as the outlet, and
HYSYS will gleefully pass the sum of the mass
flowrates of the nitrogen to the CO2. So, be careful with
this one.
- Heat Balance - You know, I'll be honest. I
cannot think of a good use for this one, but hey, I can't
do all your thinking for you.
- General Balance - This is the really good one.
In addition to conducting individual component balances,
and thus being able to solve a greater variety of
problems that way, you can specify ratios between
components within a stream (unfortunately not between
streams). It could even handle inputing flowrates for
some of the components but leaving the rest of the
composition <empty>, this was a little tricky
though. Take a look at my neat, little example
and you'll see what I mean. As with the Mole and Heat
Balance, the material balance is conducted independently
of the energy balance and will complete even if the
energy balance is under- (or worse, over-) specified. A
warning, though. Either balance may surprise you by
making the other balance overspecified, if you are
not watching for that possibility when inputting
data.
For all Balances, the balance will not start to calculate
until the Auto Calculation box is checked, but
don't check it until all is ready.
 The
RECYCLE Operation
Use this operation every
time you need to recycle a stream. The logical block
connects the two streams around the tear (remember the tear
does not have to be the official "recycle" stream itself,
but instead should be the best place in the loop to make the
break for convergence purposes). Before you can install the
RECYCLE the flowsheet must have completed. That means there
need to be values for both the assumed stream and the
calculated stream. Once the Recycle is attached and
running, HYSYS compares the two values, adjusts the
assumed stream, and runs the flowsheet again.
HYSYS repeats this process until the two streams match
within specified tolerances.
Those tolerances are set on
the Parameters Page. There are tolerances for Vapour
Fraction, Temperature, Pressure, Flow, Enthalpy, and
Composition. The tolerances you enter are not
absolute. They are actually multipliers for HYSYS'
internal convergence tolerances. For example, the
internal value for Temperature is .01 degrees (note that is
in Kelvin, because HYSYS does all of its calculations in an
internal unit set), so a multiplier of ten means the two
streams must be within a tenth of a degree of each
other.
On the Numerical Page,
among other things, you may set the RECYCLE to either
Nested (the Op is called whenever it is encountered
in the flowsheet) or Simultaneous (all of the
RECYCLEs are invoked at the same time -- use with multiple
inter-connected recycles).
There is far more to this
Operation. I cannot really do it justice. See
Section 14.5 in RV2 for much
more information on the options available to you as well as
two excellent examples which include strategies on reducing
the number of recycles and convergence time.
 The
Spreadsheet
You must have noticed me
raving about this operation on occaision. This is by far one
of the most powerful and flexible of the tools
available to you. You may import almost any variable to the
Spreadsheet, do a variety of calculations there (including
boolean logical operations like if ... then
statements), and then export results to any specifiable
field in your simulation. It serves the same basic Function
as the FORTRAN blocks do in Aspen, but again, no surprise,
it's a heck of a lot more user friendly an interface.
Besides reading about in Section 14.8
in RV2 and looking at their example which shows how
you may use the Spreadsheet to calculate the pressure drop
across a heat exchanger (at least they calculate the
Reynolds Number), I have made two different examples showing
further uses for the Spreadsheet.
One example was mentioned earlier
in this page, SetEx. The other
example shows how the Spreadsheet
may be used to calculate reaction conversion for use with a
Case Study.
In the first example, I import
and export via the Connections Page. In the second
example, I do it by directly dragging the variables into and
out of the spreadsheet.
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