Expander Unit Operation - Technical Information
The expander unit simulates a single stage isentropic expansion in PRO/II. The operating specifications for the expander unit are the outlet conditions (outlet pressure, pressure drop, pressure ratio, work) and the expander efficiency. If the outlet pressure is specified, the power generated and the outlet temperature are calculated. If work is specified, the corresponding letdown pressure is calculated.
Calculation Method
PRO/II performs expander calculations by simulating the Mollier diagram. The point corresponding to the inlet condition is determined by calculating the enthalpy and entropy at the inlet pressure and temperature. A constant entropy path is then followed until the lower outlet pressure is reached. The resulting adiabatic work is determined through the enthalpy difference between the initial and final conditions.
If the adiabatic efficiency is not 100%, the actual enthalpy change is determined by multiplying the specified efficiency with the adiabatic enthalpy change. In this case, PRO/II calculates the polytropic coefficient, the polytropic efficiency, and the polytropic head and then reports the polytropic gas horsepower as work.
Quite often it is desired to transfer the work generated by an expander to a compressor. This is easily achieved using the Define system.
The expander unit supports both VLE and VLLE methods to determine the individual phase compositions. See the VLE Model and the VLLE Model for more details.
Feed and Product Streams
The expander unit can have any number of feed streams. The inlet pressure is taken to be the lowest pressure of all the feed streams.
The expander unit can have up to four product streams with different phases in each stream. The possible product phases are vapor, liquid, decanted water / second liquid phase, a mixture of vapor and liquid, and solids. If there are multiple product streams leaving the expander unit, the phase condition for each stream must be specified using the Product Phases Window.
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