Reference Streams
The reference stream feature is used to set the composition of a stream (target stream) to be identical to another stream (source stream). The temperature, pressure, bubble point, dew point and/or rate of the target stream may be specified to be different from the source stream. The source stream supplies any missing data necessary for the target stream and whenever the source stream composition changes, the new values immediately propagate to the target stream. The target stream is re-flashed to establish the correct enthalpy and phase condition.
A source stream can be referenced only when its composition is available in the flow sheet. The source stream itself cannot refer to another stream.
Usage
From the Stream Data Window, select the Referenced to Stream option for the stream type and push the Flowrate and Stream button.
Click on ??? to select the reference stream from the drop-down list. You may enter the fluid flow rate for the stream. This specified flow rate will override the value obtained from the reference stream.
On selecting the reference stream, you have the option to select either bubble point or dew point at the temperature or pressure of the reference stream. To do this from the drop-down list select the first specification as temperature or pressure and the second specification as bubble point or dew point. For example, if the first specification is temperature and second specification is bubble point, then the temperature is taken from the source stream and bubble point pressure is calculated for the referenced stream. If the first specification is pressure and second specification is bubble point, then the pressure is taken from source stream and bubble point temperature is calculated for the referenced stream. Similarly select pressure as first specification and bubble point/dew point as the second specification to calculate bubble point temperature and dew point temperature respectively.
For referenced stream user can provide a value for temperature or pressure. If one of these values is provided as first specification and second specification is either bubble or dew point. These values are calculated at respective temperature or pressure values supplied.
Application Examples
This type of referencing is useful in a number of situations, such as breaking thermal recycles in heat exchanger network calculations. It is most useful where stream temperature, rate, and/or pressure changes, but the composition remains the same. Examples include:
Back exchange cold separator gas with the warm feed to the chiller in natural gas processing.
A preheat exchanger for a distillation column might exchange the bottoms product with the cold feed.
Guidelines
Reference streams can be a powerful tool when used by careful and knowledgable users. They also can compromise a simulation when used inappropriately. Users should exercise caution when using reference streams. The following points should be considered.
A referencing stream is one that obtains data from another stream.
A referenced (or source) stream is one that provides the data to use.
A referencing stream is updated every time the referenced stream changes. This usually ensures the referencing stream obtains the most current data from the referenced stream. It also can lead to unexpected results, because a referencing stream cannot be set to update once, then not update again.
A referencing stream usually should be located downstream from the referenced stream in a flow sheet. This sometimes is called "forward referencing". One of the safest forms of referencing is when the referenced stream is a source feed into the flow sheet. This is safe because the referenced stream properties never change after initialization. In effect, the referencing stream is updated only once.
Forward referencing frequently is used to provide an initial estimate for a recycle stream in a down-stream recycle loop. Such a reference can accelerate or even eliminate iterative calculations to significantly enhance convergence.
Problems still may occur with forward referencing when the upstream portion of the flow sheet is in a separate recycle. In some configurations, the sequencer makes a final pass to solve the upstream portion of the flow sheet after the final iteration through the down stream portion. If the (upstream) referenced stream is recomputed in this final pass, it may result in a discontinuity from the solved ( downstream) referencing stream.
Back referencing refers to a situation where the referencing stream is upstream from the referenced stream. It usually should be avoided. Back referencing usually changes the referenced stream after the referencing stream extracts the reference data.
Back referencing may be appropriate in certain situations, as when the upstream portion of the simulation never executes after the (downstream) referenced stream is calculated. It also may be useful in delicate situations to force iterative convergence. Such applications must ensure the referencing creates an appropriate implicit recycle. Caution must be exercised, because this routinely results in the solved referencing stream not agreeing with the solved referenced stream. Proper use of back referencing includes ensuring agreement between the referenced and referencing streams in the final solution.
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