Page 14 - Process simulation and control using Aspen

INTRODUCTION AND STKPWISK ASPEN PI.US SIMULATION 7

To develop a flowsheet, first choose a unit operation available in the Model Library.
Proprietary models can also be included in the flowsheet window using User Models
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option. Excel workbook or Fortran subroutine is required to deine the user model. In
the subsequent step, using Material STREAMS icon, connect the inlet and outlet streams
with the process. A process is called as a block in Aspen terminology. Notice that clicking
on Material STREAMS, when we move the cursor into the flowsheet area red and blue
arrows appear around the model block. These arrows indicate places to attach streams
to the block. Red arrows indicate required streams and blue arrows are optional.
When the flowsheet is completed, the status message changes from Flowsheet Not
Complete to Required Input Incomplete. After providing all required input data using
input forms, the status bar shows Required Input Complete and then only the simulation
results are obtained. In the Data Browsery we have to enter information at locations
where there are red semicircles. When one has finished a section, a blue checkmark
appears. In subsection 1.3.2. a simple problem has been solved, presenting a detailed
stepwise simulation procedure in Aspen Plus. In addition, three more problems have
also been discussed with their solution approaches subsequently.

1 . 3 STEPWISE ASPEN PLUS SIMULATION OF FLASH DRUMS
1 .3 . 1 Built-in Flash Drum Models

In the Model Library, there are five built-in separators. A brief description of these
models is given below.
Flash 2: It is used for equilibrium calculations of two-phase (vapour-liquid) and three-
phase (vapour-liquid-liquid) systems. In addition to inlet stream(s), this separator can
include three product streams: one liquid stream, one vapour stream and an optional
water decant stream. It can be used to model evaporators, flash chambers and other
single-stage separation columns.
Flash 3: It is used for equilibrium calculations of a three-phase (vapour-liquid-liquid)
system. This separator can handle maximum three outlet streams: two liquid streams
and one vapour stream. It can be used to model single-stage separation columns.
Decanter: It is typically used for liquid-liquid distribution coeicient calculations of a
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two-phase (liquid-liquid) system. This separator includes two outlet liquid streams along
with inlet stream(s). It can be used as the separation columns. If there is any tendency
of vapour formation with two liquid phases, it is recommended to use Flash3 instead of
Decanter.
Sep 1: It is a multi-outlet component separator since two or more outlet streams can
be produced from this process unit. It can be used as the component separation columns.
Sep 2: It is a two-outlet component separator since two outlet streams can be
withdrawn from this process unit. It is also used as the component separation columns.
At this point it is important to mention that for additional information regarding a
built-in model, select that model icon in the Model Library toolbar and then press Fl
on the keyboard.