Calculating the filling capacity of propane tanks with the help of Aspen HYSYS simulation
Propane Tanks
As
a chemical engineer, understanding the characteristics of objects is something
that must be known. Things that can cause the object to change shape due to
external and internal influences must also be known. For example, in the case
of thermal expansion, the shape of objects due to expansion can change. In
industrial cases, when we design a tank for a particular fluid we must assess
what things can change the condition of the fluid in the tank at any time. As
always, before going further to simulate with Aspen HYSYS we must get
acquainted with the processes that occur there.
Introduction
Thermal
expansion is a phenomenon that can occur in solids, liquids, and gases. In thermal
expansion, an object can expand due to an increase in heat (temperature).
Thermal expansion is defined as the tendency of an object to change its
dimensions either in length, density, area, or volume due to an increase in temperature. When
a substance is heated, its kinetic energy increases. Three types of thermal
expansion are:
-
Linear Expansion
Linear
expansion occurs due to an increase in temperature which has an impact on
changes in the length of the object.
-
Volume Expansion
Volume
expansion occurs due to an increase in temperature which causes a volume change. Generally, this expansion occurs in fluids.
-
Area Expansion
Occurs due to an increase in temperature so that changes in the area occur in objects
When
a substance is heated, the heat causes the molecules to move faster (heat
energy is converted into kinetic energy) so that the force of attraction
between the molecules of the substance weakens. Liquid fluids expand more
easily than solids, this is because the bonds between the molecules are less
tight. Gases are more easily expanded than liquids and solids, but gases at a
fixed volume cannot be expanded but gases whose temperature increases cause an
increase in pressure.
On this occasion, we will simulate a liquid fluid, namely Propane. We will calculate the maximum capacity of a tank that can be filled with propane. In running the simulation, chemical engineering colleagues must remember that a liquid fluid whose temperature is increased causes the density of a substance to decrease, this has an impact on increasing the volume of the substance. For more information, let's simulate the case example below.
Case example
When
a propane tank is filled at 25°C, we need to leave enough volume for liquid
propane expansion due to the temperature increase. The hottest weather ever
recorded is around 58°C. In real-life practice, propane tanks are only filled
to 80-85% of the tank volume. Why? We know that propane will expand when
heated. But why 80-85%?
Aspen HYSYS Completion
Create
a new simulation in Aspen HYSYS
Enter
the component list. In the Component List folder select Add. Add Propane
to the component list
Define
the Fluid packagers used. Click the Fluid Packagers folder, and click Add.
Select Peng-Robinson as the package property
Next,
enter the simulation by clicking Simulation on the bottom left screen.
Add
Heater to flowsheet
Double-click heater (E-10).
Define Inlet
flow as Feed, Outlet Stream as Out-Het, and Energy Stream
as Q-Het.
Go
to the Worksheet tab. In the Feed Stream enter the Vapor Fraction, Temperature, and Molar Flow
values of 1, 25oC, and 1 kgmole/h respectively. in the Composition
section enter the Mole Fraction value of propane = 1.
Next, define the Out-Het
condition. Enter the value of Vapor Fraction 0 and Temperature 58oC.
Add
a Spreadsheet to the flowsheet to calculate the maximum capacity of the
propane tank to allow for expansion when heated.
Double-click on the
spreadsheet (SPRDSHT-1). Enter the Spreadsheet in cells A1, A2, A3 follow
as below
Now
we will enter the variable values into the spreadsheet. Right-click on cell B1
and select Import Variable. On
Object click Feed, on Variable
click Molar Density, and click OK when done.
Right-click
on cell B2 and select Import Variable to import the Molar
Density of the Out-Het Stream. Click OK when finished.
The
spreadsheet will look like this
Next
right click on B3 enter " =(B2/B1)*100". This is
intended to divide the molar density value at 58oC by the molar
density at 25oC. The resulting number is the percentage of the
propane tank that must be filled at 25°C to allow thermal expansion to 58°C.
The maximum fill % of propane at 25°C is 87.86%, as seen in the spreadsheet
Conclusions
Calculations
show that the maximum fill is 87.8% if the ambient temperature does not exceed
58°C. To accommodate special cases, it is common for propane tanks to be filled
to 80-85%. After completing this exercise, you should be familiar with how to
create a spreadsheet to perform specialized calculations. It is important to
note that, in real life, the contents in a filled propane tank are usually a
mixture, not pure propane. Apart from propane, the mixture also has some other
light components such as methane and ethane. Therefore, to perform calculations
for real projects, we need to know the composition of the mixture we are
dealing with.
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