3.1: Compound Property Data
Other sections in Chapter 3:
One of the most important procedures involved with the solution of engineering problems is finding the data required to determine relations between variables. In chemical systems much of the data is associated with properties of compounds. Properties such as the boiling point and latent heat are obviously important in establishing how a compound would behave when heated. Individual numbers that specify such properties may be found in Tables in sources like Perry's Chemical Engineers' Handbook, Lange's Handbook of Chemistry, and The Handbook of Chemistry and Physics as well as the Appendices in various Chemical Engineering texts. Looking up such information can be quite tedious, but it is a procedure that all engineers have to become proficient in. Even more important than the fact that it is tedious is that looking up data in a printed table can lead to errors if the data is stored in units that are not consistent with what you need in solving a problem or if you simply enter the data incorrectly into your solution algorithm. Many of the physical properties that we need in our solution are not simply fixed numbers, but depend on state variables such as temperature and pressure. Thus we need more than just individual numbers to determine these properties.
In order to expedite the solution of problems that require physical property data, we have collected data for most of the compounds that you will encounter in your homework and exams. This data has been converted to a consistent set of units (SI) and stored in files in two MATLAB data structures. stores much more data used in Mass & Energy balances for almost 150 compounds. Note that all compounds in struct4.mat are also in struct3.mat, but not vice versa.
If you would like to read more on structures, the way they store data and the way they are created, go to the CENG 303 pages.
Several variables' "vectors" of data can be used to approximate properties that depend on temperature. The specific heat data specify polynomial approximations exactly as in the Reklaitis text. Thus the specific heat of water vapor is approximately given as a function of temperature by the fourth order polynomial:
Cp(T) = 34.04 - 9.65*10-3*T + 3.299*10-5*T2 - 2.044*10-8*T3 +4.302*10-12*T4
Of course any program that uses this data must know that it gives the coefficients in a fourth order polynomial and that the data is stored in ascending order. The units for both Cp and T must also be known. In the functions developed for this course, the units for Cp are J/mol*K and T is assumed to be in K. Polynomials approximating the specific heat of our compounds in the liquid and solid states are assumed to be cubic polynomials with the coefficients in the table arranged in the same way as for the vapor.
The Antoine constants give the vapor pressure as a function of temperature so that the approximate value for water vapor pressure is:
P*(T) = exp(16.5362 - 3985.44/(T - 38.997))
Again, our programs can only make use of this expression if units for the vapor pressure: P* and T are known. We have used the pressure units kPa and temperature again in K for our data base.
For many of the compounds, only a limited amount of data is present. In some cases much of the data is not meaningful: e. g. the specific heat of most compounds is known only for one or two states, but not all three.
Users who need compound data for the modules that are not contained in either of the two structures, may easily set up their own compound data. Currently, we are in the process of making a way to add to the Mass and Energy Balance structure. However, there already exists a simple way to add to the Mass Balance Only structure. We will assume that you have the formula and common name of the compound that you wish to add. If you only have the common name check the ChemFinder web page to look up the formula. Then we recommend that you run the UNIX program fwt to calculate the molecular weight. Say we want to add naphthalene (C10H8), it would look like this:
vermiculated% fwt
For each Formula you Give, fwt will report:
1) the number of atoms of each element in it,
2) its molecular weight and
3) its name if it is in the chemical engineering
compound list: namfor.t
Give the formula or stop to STOP
C10H8
Element Number: 6 1
Number of Atoms: 10 8
Molecular Weight= 128.173 Name:
Give the formula or stop to STOP
stop
Formula Molecular Weight Name
C10H8 128.1732
If you want the mol wts. printed for copying to MATLAB, reply: y
n <-- Answering "y" here just prints a vector that is
useless for this purpose. Simply select and paste
the molecular weight in a MATLAB session
Now that we have all the information necessary, we now want to start a MATLAB session and use the create function to add our compound. When you add a compound, you will create a new structure that will be saved in your own directory. Thus, it is important that as you continue to add compounds, that you add them to Your Previous Database, since they are rather large and will use up your quota space.
>> help addcomp Add a compound to the data base function st=addcomp(st0) You can choose to load 301's Mass or Mass & Energy Bal. structure or you can add to the structure st0. The compound in the initial structure's first position is used to set the type and size of the data added to each field. The structure with data for the added compound is returned. >> addcomp Choose a database to edit
|
|
<-- Select Mass Balances since that will give fewer items to add. The program will allow any number of items, but this will do for now. |
After choosing the 301 Mass Balance structure to edit the following menu will be displayed to allow you to enter all the data you know about the compound. In all cases you will want to give the compound a name and in most cases you will want to give its formula and molecular weight. If you start with the 301 Mass Balance structure these three items are the only ones that can be set for your new compound.
|
|
<-- This menu will appear as many times as you wnat.Select cnms, then form, then mw. Then select Finished to end this part of the session. |
You are given the option in start301 to use either CENG 301's database or Your own database. So, if you are using a compound that you have created, use your own database. Otherwise, use the CENG 301 database. From time to time, compounds will be added to the CENG 301 databases, so be conscious of which databases contain which compounds, and decide carefully which database you use in start301.
In Previous versions of the start301, there were two programs, one used to retrieve the data, then, another to take the data and set it into variables. These programs were written and compiled in FORTRAN to retrieve data from files. However, as a result of a new way of storing data called structures, the start301 programs have been consolidated into a single stand alone program. The program now loads a database, searches for selected compounds and then places the data in variables where they can be used.
|
Command |
Used For |
Example |
|---|---|---|
|
FORTRAN program used to list compound names, formulae, or headings. |
station% shownoh
If you want to show, reply:
compound names n
Names of headings h
Neither, STOP listing s
|
|
|
FORTRAN program that lists data stored for a compound. |
station% listcom
Compound Data Listing Program
If you want to look at the compounds'
names and/or formulae, reply: y
|
|
|
MATLAB program reads a database and sets the arrays used in mass and energy balance calculations. |
>> start301
Copyright 1998 Rice University
All rights reserved
Welcome to CENG301's start301!!
You have three choices, Please read them carefully
Then click on the appropriate choice in the menu bar
1: Click (1) to start a new session
2: Click (2) to enter the name of a datafile from
a previous session, in the current directory
3: Click (3) to use compound names not in the CENG 301 database
WARNING...if you hit (3) no compound data will be available
|
Since Section 1.3.3 showed a session
to list names, here is a Unix session used to list headings:
station%
shownoh
If you want to show, reply: compound names n Names of headings h Neither, STOP listing s h If you want to show the short (MATLAB) names, reply: y y <-- This will show both kinds of headings Antoine constants <-- Main heading Aabc <-- Name used for Antoine constants in Matlab Acentric Factor <-- Main heading AcF <-- Matlab name for Acentric Factor Acentric Factor <-- Main heading Acentf <-- Another abbreviation that is accepted ......... <-- the list goes on
Options in the use of shownoh include:
If you want to see the names you can choose from, reply y at the first prompt. See the 2nd session for a continuation of the example to list the data for water. You can see all data stored in the course data file for a compund by:
station% listcom
Compound Data Listing Program
If you want to look at the compounds' names and/or
formulae, reply: y
Give the name or formula of the compound
carbon
Known: Name, Formula, Data File
Mol. Wt. 12.01100 Formula: C
File name: /home/ceng301/sdata/carbon
Solid: Cp=a0+a1T+a2T^2+a3T^3 J/mol with T in K
a0 a1 a2 a3
-3.1776 0.53012E-01 -0.42295E-04 0.14057E-07
Heat of Form. Gibbs Free Ener Ref. State Heat of Comb.
Hi/Lo
kJ/mol at 25 C kJ/mol at 25 C 0=s,1=l,2=v kJ/mol at 25C
0=Lo,1=Hi
0.00000 0.00000 0.00000 -393.50
0.00000
This program starts by asking if you want to see the list of compound names. It uses the same listing program for the names that shownoh does and gives the same options. A carriage return bypasses the name listing program. Only the data given in the file are listed. For many compounds (such as water) much more data will be shown than for carbon. For many compounds only the name, formula and molecular weight will be given.
The MATLAB program start301 has been demonstrated in the section on the mass balance modules. To use them in problems that require information stored in the data files, you simply answer Energy & Mass Balances at the pop up menu. For example:
>> start301 Copyright 1998 Rice University All rights reserved Welcome to CENG301's start301!! You have three choices, Please read them carefully Then click on the appropriate choice in the menu bar 1: Click (1) to start a new session 2: Click (2) to enter the name of a datafile from a previous session, in the current directory 3: Click (3) to use compound names not in the CENG 301 database WARNING...if you hit (3) no compound data will be available<-- Choose (1) New Session Now choose from the above menu
<-- We want Mass & Energy Balances
Please wait while loading database... Which units do you want to use for temperature? Choose one from the menu
Input the name of your new file: test The output file name is: test Input the number of compounds: 2 The number of compounds is: 2 Enter the name of compound # 1: carbon Enter the name of compound # 2: water Enter the number of reactions: 0 Here are your compounds' formulae and names: No. Formula Name ---------------------------------------- 1 C carbon 2 H2O water Here are your reactions: ---------------------------------------- No reactions given Enter the number of streams: 2 The variables for your compounds have now been created, you may continue, or come back later and reload the same data.
<-- We will choose Kelvin most often for the purpose of more applications. It can be altered, so if you are unsure, choose Kelvin and then change if necessary.
Here is a list of all the variables that were set with start301 from using who:
>> who Your variables are: Aabc LhslkJ TmpK critP hcpl ne AcF StStdh cnms critT hcps ns Gibb StdhkJ cpl critZ hcpv nst LJones TbpK cps dHComb mw LhlvkJ Tdeg cpv form nc
You may find several blanks or NaNs in several variables. This
occurs because there are many compounds which don't have an entry for
every variable.
Here is a brief description of the data stored in each variable:
|
Name |
Rows |
Cols. |
Holds |
|---|---|---|---|
|
Aabc |
nc |
3 |
Antoine Constants |
|
AcF |
nc |
1 |
Acentric Factor |
|
Gibb |
nc |
1 |
Gibbs Free energy |
|
LJones |
nc |
1 |
Lennard Jones Parameter |
|
LhlvkJ |
nc |
1 |
Latent heat of Vaporization |
|
LhslkJ |
nc |
1 |
Latent Heat of Melting |
|
StStdh |
nc |
1 |
Reference State for Heat of Formation |
|
StdhkJ |
nc |
1 |
Heat of Formation |
|
TbpK |
nc |
1 |
Boiling Point |
|
Tdeg |
1 |
1 |
Temperature Unit |
|
TmpK |
nc |
1 |
Melting Point |
|
cnms |
nc |
varies |
compound names |
|
cpl |
nc |
4 |
Liquid Heat capacity coefficients |
|
cps |
nc |
4 |
Solid Heat capacity coefficients |
|
cpv |
nc |
5 |
Vapor Heat capacity coefficients |
|
critP |
nc |
1 |
Critical pressure |
|
critT |
nc |
1 |
Critical Temperature |
|
critZ |
nc |
1 |
Critical compressibility factor |
|
dHComb |
nc |
1 |
Heat of Combustion |
|
form |
nc |
varies |
Chemical formulae |
|
hcpl |
nc |
5 |
Liquid enthalpy coefficients |
|
hcps |
nc |
5 |
Solid enthalpy coefficients |
|
hcpv |
nc |
6 |
Vapor enthalpy coefficients |
|
mw |
nc |
1 |
molecular weight |
|
nc |
1 |
1 |
Number of Compounds |
|
ne |
nst |
nc+3 |
Stream data for mass & energy balances |
|
ns |
nst |
nc |
Stream data for mass balances |
|
nst |
1 |
1 |
Number of streams |
>> Aabc
Aabc =
1.0e+03 *
NaN NaN NaN <-- There are no Antoine data for C
0.0165 3.9854 -0.0390
>> cps
cps = <-- One of few variables for
-3.1776 0.0530 -0.0000 0.0000 which we know values for
-2.4167 0.2440 -0.0008 0.0000 both compounds
>> hcps
hcps =
0.0000 -0.0000 0.0000 -0.0032 -1.0629 <-- calculated from
0.0000 -0.0000 0.0001 -0.0024 -298.8616 other data
>> nst
nst =
2 <-- Number of streams
>> ne
ne =
0 0 0 0 0 <-- Initial array for storing stream data
0 0 0 0 0
The array ne is set with the proper size to store data from simulations in them, but it simply has zeros in all its elements. Note that the Aabc array has a row of NaNs in it. Many of the arrays you set will contain these numbers. They are inserted for all data that is missing in the data files for one or more compounds while others do have that data in their files.. The NaNs may prevent your doing meaningful calculations that involve missing data. If you try to get the vapor pressure of carbon, you will simply have the answer: NaN. This should be an indication that you need to gather more data for one or more compounds. Alternately, you might have to avoid calculations for the compounds that have missing data.
The start301 program searches a structure for data, sets variables, calls a program to check chemical equations to be sure they are balanced, if asked, and creates a data file for later use in start301. The structure searched through for property data are usually in the CENG 301 structure, but a user's custom database may also be used. Once a start301 file is created, it may be used several times. The user may find more recent property data than that in the department's files. If so, editing the data file to include that new data can be done with ease. The simplest way would be to run start301, then change the arrays in your Matlab session so that the new data is used.
Balancing Equations With Start301
As previously discussed is start301's ability to check the equations that you enter to see if they are balanced. If you are confident (as we are in our examples) in your reactions you can bypass this step as shown in several examples. If you check your equations, regardless of whether you find them correct or not, you are given one opportunity to change your equations and then the program moves on. Below is an example of how to use start301 to check your equations. Keep in mind that the simplest way to input the data requested is to cut and paste. This is done by highlighting the area to copy and then moving the cursor to the desired place and click the middle mouse button. This example is run using (1) New Session, Energy & Mass Balances, and CENG 301's database. The units for temperature was chosen to be Kelvin.
>> start301
Copyright 1998 Rice University
All rights reserved
Welcome to CENG301's start301!!
You have three choices, Please read them carefully
Then click on the appropriate choice in the menu bar
1: Click (1) to start a new session
2: Click (2) to enter the name of a datafile from
a previous session, in the current directory
3: Click (3) to use compound names not in the CENG 301 database
WARNING...if you hit (3) no compound data will be available
Now choose from the above menu
Please wait while loading database...
Which units do you want to use for temperature?
Choose one from the menu
Input the name of your new file: test
The output file name is: test
Input the number of compounds: 5
The number of compounds is: 5
Enter the name of compound # 1: CO2
Enter the name of compound # 2: CO
Enter the name of compound # 3: H2O
Enter the name of compound # 4: O2
Enter the name of compound # 5: H2
Enter the number of reactions: 3
Enter the coefficients for each compound in the same order that
the compounds are listed. Coefficients for reactants should be
Negative, and coefficients for products should be positive
Enter the coefficients for each compound in reaction # 1
CO2 CO H2O O2 H2
0 0 -2 1 2
Enter the coefficients for each compound in the same order that
the compounds are listed. Coefficients for reactants should be
Negative, and coefficients for products should be positive
Enter the coefficients for each compound in reaction # 2
CO2 CO H2O O2 H2
-1 1 0 1 0
Enter the coefficients for each compound in the same order that
the compounds are listed. Coefficients for reactants should be
Negative, and coefficients for products should be positive
Enter the coefficients for each compound in reaction # 3
CO2 CO H2O O2 H2
-1 1 -1 1 1
Do you want to check to see if the coefficients are correct?
type "y" for yes or simply press enter to move on: y
number of compounds = 5
number of reactions = 3
Here are your formulae...
CO2
CO
H2O <-- Highlight these and copy them below
O2
H2
Give the number of compounds and number of reactions
5, 3
Give the formulae for each of the compounds
CO2
CO
H2O <-- Enter these by the copy method
O2
H2
Give the stoichiometric coefficients for reaction # 1
0 0 -2 1 2 <-- Enter these by the copy method
0. 0. -2.00000 1.00000 2.00000
the equation is balanced
Give the stoichiometric coefficients for reaction # 2
-1 1 0 1 0 <-- Enter these by the copy method
-1.00000 1.00000 0. 1.00000 0.
WARNING ! The element O
is not balanced in that equation.
Give the stoichiometric coefficients for reaction # 3
-1 1 -1 1 1 <-- Enter these by the copy method
-1.00000 1.00000 -1.00000 1.00000 1.00000
the equation is balanced
Your reactions are currently as follows:
1) 2H2O --> O2 + 2H2
2) CO2 --> CO + O2
3) CO2 + H2O --> CO + O2 + H2
Type "y" to change an equation, or press "enter" to continue: y
How many equations do you want to change: 1
Enter the equation number you wish to change: 2
<-- We only need to change 1 equation and it is
equation number 2
Enter the coefficients for each compound in the same order that
the compounds are listed. Coefficients for reactants should be
Negative, and coefficients for products should be positive
Enter the coefficients for each compound in reaction # 2
CO2 CO H2O O2 H2
1 -1 0 -.5 0
<-- Correcting the Equation
Do you want to check to see if the coefficients are correct?
type "y" for yes or simply press enter to move on: y
number of compounds = 5
number of reactions = 3
Here are your formulae...
CO2
CO
H2O
O2
H2
Give the number of compounds and number of reactions
5, 1 <-- We only need to check one equation this time
Give the formulae for each of the compounds
CO2
CO
H2O <-- Enter these by the copy method
O2
H2
Give the stoichiometric coefficients for reaction # 1
1 -1 0 -.5 0 <-- Enter these by the copy method
1.00000 -1.00000 0. -0.500000 0.
the equation is balanced
<-- Note no additional opportunity to alter the reactions
Here are your compounds' formulae and names:
No. Formula Name
----------------------------------------
1 CO2 carbon dioxide
2 CO carbon monoxide
3 H2O water
4 O2 oxygen
5 H2 hydrogen
Here are your reactions:
----------------------------------------
1) 2H2O --> O2 + 2H2
2) CO + 0.5O2 --> CO2
3) CO2 + H2O --> CO + O2 + H2
Enter the number of streams: 3
The variables for your compounds have now been created,
you may continue, or come back later and reload the same data.