Chemical Engineering October 2016 - 77

file and assigning these variables to
the simulation forms the " skeleton "
for the code that will be written.
Crude slate example
FIGURE 5. A simple flowsheet is given to demonstrate the generation of the HMB table for three
crude slates
Mistakes in programming VBA
code are also common. After all, we
are only human. An error as simple
as entering the stream temperature
values in the spreadsheet row that
is set up for pressure values is an
easy mistake to make. The process
engineer should always review the
HMB for these types of errors.
VBA will extract all properties
and conditions by the variable unit
exactly as defined by the process
simulator. For example, the Hysys
unit for pounds per hour is " lb/hr, "
not " lbs/h " and kilograms per hour
is " kg/h, " not " kg/hr " . These must
be consistent in order for the code
to function correctly.
Not only can VBA extract data
from the simulation, but it can also
write data to the simulation. This
becomes useful if the engineer
needs to modify the simulation with
data in the spreadsheet and then
extract data that results from the
user upload. For example, an engineer
may need to extract HMB data
for a process with different material
feed conditions, such as temperature,
flow or pressure. The engineer
would use the SetValue method
to change the stream properties.
Then the code would extract the
simulation data that result from the
updated feed properties.
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68
The Developer tab in Excel has
a function called " Record Macro. "
This is a very useful function that
automatically generates code for
the user. For example, if a user
clicks " Record Macro " and then
changes the color of any cell, VBA
will automatically generate the necessary
code to change the color of
a cell. To record a macro, first, set
up Excel to show the Developer tab.
The user should press the Office
button, click on Excel options and
then check, " Show Developer tab in
the Ribbon " from the Popular page.
The Object Browser (Figure 2)
from the VBA Editor View menu is
helpful to find the available properties
and methods of each object.
Many properties have their own
properties and methods. These are
easily found by placing a period
after the first property, and a dropdown
list will appear (Figure 3).
It is important to remember to
add references to your Excel file.
From the VBA editor, click the
Tools menu, click References, and
then enable the Hysys Type Library
(Figure 4) check box (or whichever
library is appropriate for your process
simulator of choice).
Another critical step to get started
is variable declaration. Declaring the
VBA variables for the Hysys HMB
CODE EXAMPLE FOR GETVALUE COMMAND
he following code presents a simple example of the GetValue command for the
given spreadsheet data:
Set HMB_stream = ProjectSimulation.Flowsheet.MaterialStreams.Item( " 100 " )
Range( " B2 " ).Value = HMB_stream.Pressure.GetValue( " kPa " )
Range( " B3 " ).Value = HMB_stream.MassFlow.GetValue( " kg/h " )
Range( " B5 " ).Value = HMB_stream.ComponentMassFraction(0)
Range( " B6 " ).Value = HMB_stream.ComponentMassFraction(1) ❑
In many cases, the owner would like
to analyze different operating cases
for the same (or similar) process configurations.
These cases could differ
with regard to feed composition, operating
temperature and pressure,
flowrate or product quality. Instead
of creating different simulation files,
VBA could be used with different parameters
to set the values within the
simulation. Then, once converged,
the spreadsheet could get the values
from the simulation to populate
the HMB.
Consider the example of a petroleum
refinery processing different
crude slates. Each crude slate assay
would have various components,
composition and true boiling-point
(TBP) data. This could consist of over
100 components once the crude
assay is input into Hysys. A simple
method would be to use VBA to assign
an assay composition to the feed
and allow the simulation to converge
for its unique material balance. Consider
this simple example of three different
crude slates, each with its own
process requirement of temperature
and pressure (Figures 5 and 6).
The user would write the VBA
code to select the assay and assign
the composition to the unit feedstream.
The code would then assign
the feed flowrate, temperature and
pressure, as well as the temperature
and pressure downstream of the
heat exchanger.
A real-world, practical example of
an application of VBA automation
to a process with several different
operating configurations is demonstrated
by a request from one of the
author's recent clients, a petroleum
refinery. The client wanted to analyze
eight different cases and determine
which configuration would provide
the greatest profit versus performance,
as well as verify the process
against different feedstocks. The
engineering team reviewed two different
feedstocks, and four different
operating conditions for each. That
is, several different operating points
(pressure at the distillation column,
temperature at the outlet of different
heat exchangers and column pumpCHEMICAL
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Chemical Engineering October 2016

Table of Contents for the Digital Edition of Chemical Engineering October 2016