Recycle Techniques
Recycle problems can be grouped into three main categories and different methods are employed by PRO/II to solve each type.
1. Thermal recycle
These are loops in which only the stream temperatures (and therefore enthalpies) change, while the stream rates and compositions remain unchanged. This type of recycle is typically found in heat exchanger networks, and depending on how the user specifies the heat exchanger outlet conditions, this type of recycle calculation may be eliminated.
In PRO/II the Reference Stream concept may be used to supply missing data for a stream by referring it to another stream. This method can be used to enhance convergence for thermal recycle loops.
2. Mass and Energy recycles
These are loops in which stream compositions change along with stream temperature (enthalpy) with each loop iteration. For solving this type of recycle loop, PRO/II compares the current values for every stream in the recycle loop against values from the previous iteration. If all component flows, temperatures, and pressures are the same for consecutive iterations, within a specified tolerance level, the recycle loop is considered to be converged. You can optionally test only the tear streams for convergence.
In some composition recycle cases, a more precise solution can be obtained by changing the recycle streams into internal streams, which are then computed within one of the unit operations. For example, in the case of sidestrippers or siderectifiers attached to a column using the Inside-Out algorithms, PRO/II merges the units into a single column for calculation purposes, essentially eliminating the recycle. For Sure columns, PRO/II employs a special set of heuristics to take advantage of the close relationship between columns and their associated side columns.
3. Purge/ make up systems
These are a special type of composition recycle loop, where additional measures must be used to insure a flowsheet mass balance. Components must be removed (purge) if they build up in the flowsheet or added (make up) if they are lost. Without removing or adding material, the recycle tear streams will never converge. In many cases, a calculator unit may be used to calculate the appropriate values for the purge/make up streams. In general the solution method will vary from situation to situation.
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