Modeling LPG Transfer and Vapor Recovery
By Andrew Nathan - VMG USA
The Dynamics engine in VMGSim enables users to model a variety of transient and non-steady state behavior. Below is a specific example of utilizing VMGSim Dynamics to model the Liquefied Petroleum Gas (LPG) liquid transfer and subsequent vapor recovery from a railcar to a storage tank.
Liquefied petroleum gas (LPG) typically encompasses several hydrocarbon species, but usually refers to propane and butane mixtures. LPG can be transported and stored as a liquid, and when liquefied, LP gases are always at their boiling point at normal temperatures.
A slight drop in pressure will cause the LPG to boil and release vapor/gas. This feature plays a critical role in the transfer of liquefied gases from
railcars (a typical way in which LPG is transported) to storage tanks. LPG is stored in an enclosed vessel under pressure, where the liquid is in equilibrium with the gas on top of the fluid.
This equilibrium in a closed container provides the necessary pressure to keep the liquid from boiling.
The typical method of unloading LPG is to use a reciprocating compressor with a four-way valve. The compressor will draw vapor from the storage tank, where it will be compressed slightly, and injected into the railcar through a vapor line. This compressed gas will slightly reduce the pressure in the storage tank while raising the pressure in the railcar. This pressure difference will cause the liquid to be transferred from the railcar to the storage tank through a liquid line running from the top of the railcar to the bottom of the storage tank. Please see the diagram below for a schematic of this configuration:
The transfer process switches from liquid transfer to vapor recovery once all the liquid is transferred from the railcar to the storage tank. This is accomplished by rotating the four-way valve, causing the compressor to draw vapor from the railcar and insert it into the storage tank. This is opposite to what was done during liquid transfer. In addition, the liquid line is shut. The compressor will withdraw vapor from the top of the railcar, compress it slightly, and discharge the gas into the liquid section of the storage tank. The liquid already present in the storage tank will condense these vapors back into liquid. This causes the railcar pressure to gradually drop, while increasing the storage tank pressure incrementally. This operation is stopped when the current pressure in the railcar is approximately 25-30% of the original railcar pressure. Going beyond this threshold is not economical due to the additional time and energy required, when compared with the very small amount of product recovered at this stage. Please see the diagram below for a schematic of this configuration:
We can model this LPG transfer and vapor recovery in VMGSim. The specifications/input used in this example are typical settings for this process, based on standard equipment sizes/geometry. We first specify the composition of the LPG, setting the ambient temperature and calculating the vapor pressure by setting the vapor fraction to 0 (we also set an arbitrary flow rate for the stream) in Steady State.
Next, we use the Depressuring Assistant to initialize the inventory in the railcar as per below (we set the liquid level to 90%). A typical 33,600 gallon railcar has a diameter of 118.75 inches.
We leave the Valve and Heat Loss parameters at their default values. Based on the process description given in the previous sections, we modify the resulting PFD and build the following flowsheet in VMGSim Dynamics.
We model the compressor as a reciprocating compressor with the following specifications:
We model this compressor using 2 identical compressor unit operations, because the first step (liquid transfer) involves vapor going from the storage tank to the railcar, while the next step involves vapor going from the railcar to the storage tank. We also model the liquid line at the top of the railcar using a pipe segment with the following specifications, simulating a vertical pipe:
We then set the dimensions/geometry of the storage tank and set the liquid level to 0. The typical storage tank volume is 90,000 gallons, with a diameter of 131 inches.
Next, we set the size and Cv of the four-way valve (each should have the same size/Cv):
Finally, in order to run all the various sequences/steps in order, we utilize the Scheduler block to create Events and run Actions in the case:
The first step is LPG transfer, which once activated using the Start/Stop button at the top, will run the following list of actions simultaneously:
This is done until the liquid level in the railcar drops to less than or equal to 1% (a small amount of liquid heel is left in the railcar). Once this condition is reached, vapor recovery begins, which involves the following list of actions:
This is performed until the railcar pressure drops to 35 psia, which is 25% of the original railcar pressure. After this, vapor recovery is halted, and the following list of actions are performed:
We can track the progress of the liquid transfer and vapor recovery by utilizing a stripchart. We add the Railcar Liquid Level, Railcar Pressure, Storage Tank Liquid Level and Storage Tank Pressure to the stripchart. We then start the Sequence in the Scheduler by clicking on the Start/Stop button, and run the Dynamic Integrator. We can then track the variables on the stripchart as per below:
The entire process of liquid transfer and vapor recovery takes approximately 6.5 hours, with the specifications and equipment sizes/geometry entered.
After a benchmark case is developed, one can look at the various options/specifications for compressors, valves and piping to speed up this process. In addition, with this case already set up, it is also possible to look at other sequences or to study equipment failure scenarios.
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