#LyX 1.4.1 created this file. For more info see http://www.lyx.org/ \lyxformat 245 \begin_document \begin_header \textclass book \language english \inputencoding auto \fontscheme default \graphics default \paperfontsize default \spacing single \papersize a4paper \use_geometry false \use_amsmath 2 \cite_engine basic \use_bibtopic false \paperorientation portrait \secnumdepth 3 \tocdepth 3 \paragraph_separation indent \defskip medskip \quotes_language english \papercolumns 1 \papersides 2 \paperpagestyle default \tracking_changes false \output_changes true \end_header \begin_body \begin_layout Chapter Entering Dimensional Equations \begin_inset LatexCommand \index{equation, dimensional} \end_inset from Handbooks \begin_inset LatexCommand \label{cha:dimeqns} \end_inset \end_layout \begin_layout Standard Often in creating an ASCEND model one needs to enter a correlation \begin_inset LatexCommand \index{correlation} \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 \begin_inset LatexCommand \index{pressure, vapor} \end_inset \end_layout \begin_layout Standard Our first example is the equation to express vapor pressure using an Antoine \begin_inset LatexCommand \index{Antoine} \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 \begin_inset LatexCommand \index{SI} \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 LatexCommand \ref{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 LatexCommand \ref{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 \begin_inset LatexCommand \index{ln} \end_inset () function, as it can take the natural log of the dimensionless \begin_inset LatexCommand \index{dimensionless} \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 LatexCommand \ref{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 \begin_inset LatexCommand \index{Fahrenheit} \end_inset -- variables with offset \begin_inset LatexCommand \label{sec:dimeqns.Fahrenheit} \end_inset \end_layout \begin_layout Standard What if we write Equation \begin_inset LatexCommand \ref{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 LatexCommand \ref{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 LatexCommand \ref{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 LatexCommand \label{ssec:dimeqns.pressure drop} \end_inset \end_layout \begin_layout Standard From the Chemical Engineering Handbook \begin_inset LatexCommand \index{Chemical Engineering Handbook} \end_inset by Perry \begin_inset LatexCommand \index{Perry} \end_inset and Chilton \begin_inset LatexCommand \index{Chilton} \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 LatexCommand \label{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 \begin_inset LatexCommand \index{offset} \end_inset and difference units \begin_inset LatexCommand \index{difference units} \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