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\begin_body
\begin_layout Chapter
Entering Dimensional Equations
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\begin_layout Plain Layout
equation, dimensional
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\end_inset
from Handbooks
\begin_inset CommandInset label
LatexCommand label
name "cha:dimeqns"
\end_inset
\end_layout
\begin_layout Standard
Often in creating an ASCEND model one needs to enter a correlation
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\begin_layout Plain Layout
correlation
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\end_inset
given in a handbook that is written in terms of variables expressed in
specific units.
In this chapter, we examine how to do this easily and correctly in a system
like ASCEND where all equations must be dimensionally correct.
\end_layout
\begin_layout Section
Example 1-- vapor pressure
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\begin_layout Plain Layout
pressure, vapor
\end_layout
\end_inset
\end_layout
\begin_layout Standard
Our first example is the equation to express vapor pressure using an Antoine
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\begin_layout Plain Layout
Antoine
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\end_inset
-like equation of the form:
\end_layout
\begin_layout Standard
\begin_inset Formula
\begin{equation}
\ln(P_{sat})=A-\frac{B}{T+C}\label{eqn:dimeqns.lnPsat}
\end{equation}
\end_inset
where
\begin_inset Formula $P_{sat}$
\end_inset
is in {atm} and
\begin_inset Formula $T$
\end_inset
in {R}.
When one encounters this equation in a handbook, one then finds tabulated
values for
\begin_inset Formula $A$
\end_inset
,
\begin_inset Formula $B$
\end_inset
and
\begin_inset Formula $C$
\end_inset
for different chemical species.
The question we are addressing is:
\end_layout
\begin_layout Quote
How should one enter this equation into ASCEND so one can then enter the
constants A, B, and C with the exact values given in the handbook?
\end_layout
\begin_layout Standard
ASCEND uses SI
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\begin_layout Plain Layout
SI
\end_layout
\end_inset
units internally.
Therefore, P would have the units {kg/m/s^2}, and T would have the units
{K}.
\end_layout
\begin_layout Standard
Eqn
\begin_inset CommandInset ref
LatexCommand ref
reference "eqn:dimeqns.lnPsat"
\end_inset
\noun off
is, in fact, dimensionally incorrect as written.
We know how to use this equation, but ASCEND does not as ASCEND requires
that we write dimensionally correct equations.
For one thing, we can legitimately take the natural log (ln) only of unitless
quantities.
Also, the handbook will tabulate the values for A, B and C without units.
If A is dimensionless, then B and C would require the dimensions of temperature.
\end_layout
\begin_layout Standard
The mindset we describe in this chapter is to enter such equations is to
make all quantities that must be expressed in particular units into dimensionle
ss quantities that have the correct numerical value.
\end_layout
\begin_layout Standard
We illustrate in the following subsections just how to do this conversion.
It proves to be very straight forward to do.
\end_layout
\begin_layout Subsection
Converting the ln term
\end_layout
\begin_layout Standard
Convert the quantity within the ln() term into a dimensionless number that
has the value of pressure when pressure is expressed in {atm}.
\end_layout
\begin_layout Standard
Very simply, we write
\end_layout
\begin_layout LyX-Code
P_atm = P/1{atm};
\end_layout
\begin_layout Standard
Note that P_atm has to be a dimensionless quantity here.
\end_layout
\begin_layout Standard
We then rewrite the LHS of Equation
\begin_inset CommandInset ref
LatexCommand ref
reference "eqn:dimeqns.lnPsat"
\end_inset
\noun off
as
\end_layout
\begin_layout LyX-Code
ln(P_atm)
\end_layout
\begin_layout Standard
Suppose P = 2 {atm}.
In SI units P= 202,650 {kg/m/s^2}.
In SI units, the dimensional constant 1{atm} is about 101,325 {kg/m/s^2}.
Using this definition, P_atm has the value 2 and is dimensionless.
ASCEND will not complain with P_atm as the argument of the ln
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ln
\end_layout
\end_inset
() function, as it can take the natural log of the dimensionless
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\begin_layout Plain Layout
dimensionless
\end_layout
\end_inset
quantity 2 without any difficulty.
\end_layout
\begin_layout Subsection
Converting the RHS
\end_layout
\begin_layout Standard
We next convert the RHS of Equation
\begin_inset CommandInset ref
LatexCommand ref
reference "eqn:dimeqns.lnPsat"
\end_inset
\noun off
, and it is equally as simple.
Again, convert the temperature used in the RHS into:
\end_layout
\begin_layout LyX-Code
T_R = T/1{R};
\end_layout
\begin_layout Standard
ASCEND converts the dimensional constant 1{R} into 0.55555555...{K}.
Thus T_R is dimensionless but has the value that T would have if expressed
in {R}.
\end_layout
\begin_layout Subsection
In summary for example 1
\end_layout
\begin_layout Standard
We do not need to introduce the intermediate dimensionless variables.
Rather we can write:
\end_layout
\begin_layout LyX-Code
ln(P/1{atm}) = A - B/(T/1{R} + C);
\end_layout
\begin_layout Standard
as a correct form for the dimensional equation.
When we do it in this way, we can enter A, B and C as dimensionless quantities
with the values exactly as tabulated.
\end_layout
\begin_layout Section
Fahrenheit
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\begin_layout Plain Layout
Fahrenheit
\end_layout
\end_inset
-- variables with offset
\begin_inset CommandInset label
LatexCommand label
name "sec:dimeqns.Fahrenheit"
\end_inset
\end_layout
\begin_layout Standard
What if we write Equation
\begin_inset CommandInset ref
LatexCommand ref
reference "eqn:dimeqns.lnPsat"
\end_inset
\noun off
but the handbook says that T is in degrees Fahrenheit, i.e., in {F}? The
conversion from {K} to {F} is
\end_layout
\begin_layout LyX-Code
T{F} = T{K}*1.8 - 459.67
\end_layout
\begin_layout Standard
and the 459.67 is an offset.
ASCEND does not support an offset for units conversion.
We shall discuss the reasons for this apparent limitation in Section
\begin_inset CommandInset ref
LatexCommand ref
reference "ssec:dimeqns.handlingUnitConv"
\end_inset
.
\end_layout
\begin_layout Standard
You can readily handle temperatures in {F} if you again think as we did
above.
The rule, even for units requiring an offset for conversion, remains: convert
a dimensional variable into dimensionless one such that the dimensionless
one has the proper value.
\end_layout
\begin_layout Standard
Define a new variable
\end_layout
\begin_layout LyX-Code
T_degF = T/1{R} - 459.67;
\end_layout
\begin_layout Standard
Then code
\begin_inset CommandInset ref
LatexCommand ref
reference "eqn:dimeqns.lnPsat"
\end_inset
\noun on
Equation 7.1
\noun off
as
\end_layout
\begin_layout LyX-Code
ln(P/1{atm}) = A - B/(T_degF + C);
\end_layout
\begin_layout Standard
when entering it into ASCEND.
You will then enter constants A, B, and C as dimensionless quantities having
the values exactly as tabulated.
In this example we must create the intermediate variable T_degF.
\end_layout
\begin_layout Section
Example 3-- pressure drop
\begin_inset CommandInset label
LatexCommand label
name "ssec:dimeqns.pressure drop"
\end_inset
\end_layout
\begin_layout Standard
From the Chemical Engineering Handbook
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\begin_layout Plain Layout
Chemical Engineering Handbook
\end_layout
\end_inset
by Perry
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Perry
\end_layout
\end_inset
and Chilton
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Chilton
\end_layout
\end_inset
, Fifth Edition, McGraw-Hill, p10-33, we find the following correlation:
\end_layout
\begin_layout Standard
\begin_inset Formula
\[
\Delta P_{a}^{\prime}=\frac{y(V_{g}-V_{l})G^{2}}{144g}
\]
\end_inset
where the pressure drop on the LHS is in psi, y is the fraction vapor by
weight (i.e., dimensionless), Vg and Vl are the specific volumes of gas and
liquid respectively in ft3/lbm, G is the mass velocity in lbm/hr/ft2 and
g is the acceleration by gravity and equal to 4.18x108 ft/hr2.
\end_layout
\begin_layout Standard
We proceed by making each term dimensionless and with the right numerical
value for the units in which it is to be expressed.
The following is the result.
We do this by simply dividing each dimensional variable by the correct
unit conversion factor.
\end_layout
\begin_layout LyX-Code
delPa/1{psi} = y*(Vg-Vl)/1{ft^3/lbm}*
\end_layout
\begin_layout LyX-Code
(G/1{lbm/hr/ft^2})^2/(144*4.18e8);
\end_layout
\begin_layout Section
The difficulty of handling unit conversions defined with offset
\begin_inset CommandInset label
LatexCommand label
name "ssec:dimeqns.handlingUnitConv"
\end_inset
\end_layout
\begin_layout Standard
How do you convert temperature from Kelvin to centigrade? The ASCEND compiler
encounters the following ASCEND statement:
\end_layout
\begin_layout LyX-Code
d1T1 = d1T2 + a.Td[4];
\end_layout
\begin_layout Standard
and d1T1 is supposed to be reported in centigrade.
We know that ASCEND stores termperatures in Kelvin {K}.
We also know that one converts {K} to {C} with the following relationshipT{C}
= T{K} - 273.15.
\end_layout
\begin_layout Standard
Now suppose d1T2 has the value 173.15 {K} and a.Td{4} has the value 500 {K}.
What is d1T1 in {C}? It would appear to have the value 173.15+500-273.15
= 400 {C}.
But what if the three variables here are really temperature differences?
Then the conversion should be T{dC} = T{dK}, where we use the notation
{dC} to be the units for temperature difference in centigrade and {dK}
for differences in Kelvin.
Then the correct answer is 173.15+500=673.15 {dC}.
\end_layout
\begin_layout Standard
Suppose d1T1 is a temperature and d1T2 is a temperature difference (which
would indicate an unfortunate but allowable naming scheme by the creator
of this statement).
It turns out that a.Td[4] is then required to be a temperature and not a
temperature difference for this equation to make sense.
We discover that an equation written to have a right-hand-side of zero
and that involves the sums and differences of temperature and temperature
difference variables will have to have an equal number of positive and
negative temperatures in it to make sense, with the remaining having to
be temperature differences.
Of course if the equation is a correlation, such may not be the case, as
the person deriving the correlation is free to create an equation that
"fits" the data without requiring the equation to be dimensionally (and
physically) reasonable.
\end_layout
\begin_layout Standard
We could create the above discussion just as easily in terms of pressure
where we distinguish absolute from gauge pressures (e.g., {psia} vs.
{psig}).
We would find the need to introduce units {dpisa} and {dpsig} also.
\end_layout
\begin_layout Subsection
General offset
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offset
\end_layout
\end_inset
and difference units
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difference units
\end_layout
\end_inset
\end_layout
\begin_layout Standard
Unfortunately, we find we have to think much more generally than the above.
Any unit conversion can be introduced both with and without offset.
Suppose we have an equation which involves the sums and diffences of terms
t1 to t4:
\end_layout
\begin_layout Standard
\begin_inset Formula
\begin{equation}
t_{1}+t_{2}-(t+t_{4})=0\label{eqn:t1+t2}
\end{equation}
\end_inset
where the units for each term is some combination of basic units, e.g., {ft/s^2/R}.
Let us call this combination {X} and add it to our set of allowable units,
i.e., we define
\emph on
{X} = {ft/s^2/R}.
\emph default
\end_layout
\begin_layout Standard
Suppose we define units {Xoffset} to satisfy: {Xoffset} = {X} - 10 as another
set of units for our system.
We will also have to introduce the concept of {dX} and and should probably
introduce also {dXoffset} to our system, with these two obeying{dXoffset}
= {Xoffset}.
\end_layout
\begin_layout Standard
For what we might call a "well-posed" equation, we can argue that the coefficien
t of variables in units such as {Xoffset} have to add to zero with the remaining
being in units of {dX} and {dXoffset}.
Unfortunately, the authors of correlation equations are not forced to follow
any such rule, so you can find many published correlations that make the
most awful (and often unstated) assumptions about the units of the variables
being correlated.
\end_layout
\begin_layout Standard
Will the typical modeler get this right? We suspect not.
We would need a very large number of unit conversion combinations in both
absolute, offset and relative units to accomodate this approach.
\end_layout
\begin_layout Standard
We suggest that our approach to use only absolute units with no offset is
the least confusing for a user.
Units conversion is then just multiplication by a factor both for absolute
{X} and difference {dX} units-- we do not have to introduce difference
variables because we do not introduce offset units.
\end_layout
\begin_layout Standard
When users want offset units such as gauge pressure or Fahrenheit for temperatur
e, they can use the conversion to dimensionless variables having the right
value, using the style we introduced above, i.e., T_defF = T/1{R} - 459.67
and P_psig = P/1{psi} - 14.696 as needed.
\end_layout
\begin_layout Standard
Both approaches to handling offset introduce undesirable and desirable character
istics to a modeling system.
Neither allow the user to use units without thinking carefully.
We voted for this form because of its much lower complexity.
\end_layout
\end_body
\end_document