Mass and Energy Balance
Navigation: User Guide ➔ Mass and Energy Balance
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Stream Properties Calculations
SysCAD will always try to maintain a mass and energy balance. For information on how SysCAD calculates its stream properties, please see the following links:
- Stream Properties using Standard Method
- Stream Density Calculations
- Calculating Energy change around a unit
- Reaction Block Energy Balance
Overall Mass Balance
When building a SysCAD project, material can be defined to enter or leave SysCAD in a number of ways:
- Through the Feeder/Cross Page unit operation
- Through a makeup source unit operation connected to makeup blocks
- Through the Source/Sink options via a reaction block
- Through vent or overflow in some unit operations
- For dynamic projects only, through spills or ventspills in some unit operations
When completing a mass balance, all input and output streams together with any mass additions or sinks in unit operations need to be included. When getting flow information, it is important to remember in a steady state balance, due to no accumulation will occur in the individual unit operations, all flow information must be obtained from the pipe model.
Some useful Excel report format that will aid in creating a Overall Mass Balance are given in SysCAD Tag Select Reports .
Water / Steam Balance
The Water and Steam balance can be easily produced by using information given in the unit operation's Access Window - Links Tab Nett.Qm.H2O(l) (t/h) and Nett.Qm.H2O(g) (t/h) values. As water or steam enter and leave a process unit, its change of value is report there, this includes changes due to reactions or phase change.
A useful Excel report for creating the water/steam Mass Balance is given in SysCAD Tag Select Reports.
Similar to the Water / steam balance, other compound of interest can be added to the Links table to keep track of change. To add extra compound to the Links table, go to View - Plant Model - Settings Tab, heading Link Balance Species.
Overall Energy Balance
Heat can be added and removed in unit operation and reaction files. When completing an Energy balance, it is necessary to look at all the inputs and output streams as well as all sources of heat transfer. These may include any unit operations (including pipes) that contain heat loss and any unit operations that need external heat input. A simple way to complete an energy balance check using numbers from SysCAD is via the Total Enthalpy @ Temperature (Hf@T) numbers. Simply sum up this variable for all input and output streams, the difference should be equal to the total energy input (or heat loss) for the project.
Mass and Energy Balance around a Unit Operation
Related Topic Audit
- For each individual unit operation, on the first tab page of the access window, SysCAD displays the total mass and energy in / out of the unit operation. The unit operation will be in the error status if a balance if not obtained.
- All Pipes around the unit operation. For steady state material and energy balance, due to no accumulation will occur in the individual unit operations, all flow information must be obtained from the pipe model. Users can obtain information such as flowrate, enthalpy, specific heat, temperature and pressure and so on for mass and energy balance calculations. Note: SysCAD has made a lot of information available to users, to see all information, make sure the Display All fields option is selected on the Access Window.
A Mass and Energy Balance is shown on the Audit tab on the Access window for each unit or pipe. The Audit tab variables will update once the project has been solved. The variables calculated and shown (if any) depend on the selection for AuditLevel in the PlantModel access window. The user can copy the balance information onto Excel using the buttons on the bottom of the Audit Tab.
Quick Hints On Completing An Energy Balance Around The Unit Operation:
SysCAD provides a considerable amount of information in terms of numbers that can be used in the energy balance. Three such ways are discussed briefly below: (Note: You will have to switch on the AllFields option in the access window to see some of the variables mentioned).
- Total Enthalpy - Hf@T: Each stream has a sum of Total Enthalpy at temperature, (Qi.totHf@T), if a unit operation is balanced, the sum of Total Enthalpy @ T in = sum of Total Enthalpy @ T out. Note: if external heat is added or heat is removed from unit operation, user must add on this to obtain an energy balance.
- Sensible Energy - Hs@T: This is basically the Q = m∫CpdT equation. Now, if a reaction takes place, you will have to add on the heat of reaction. Note: SysCAD has a global reference point at 0 °C @ atmospheric pressure. Therefore, if there is a reaction in the unit operation, the number to use from the Reaction block is RB.dH@0.Nett.
- Energy including phase change - Hz@T: This is the Q = mdH equation.
For streams containing pure steam and water, the Hz@T value will match the enthalpy values from the steam tables, thus, Hz for steam will match Hg and Hz for water will match Hl. Note the values are only correct up to 1000K.
This method is best for balances around unit operations that contain only steam / water reaction, such as the flash tank and direct contact heater. Since heat of evaporation/condensation is already included, users do not need to add on the heat of reaction when using this method. However, if other reactions are involved, users must add on the heat of reactions produced by those reactions.
Note: For more information and examples on the mass and energy balance, please see Model Examples for example calculations and sample excel spreadsheets provided with some of the distributed example projects.