Chemical Engineering July 2012 - 34

Feature Report
Engineering Practice
Evaluating Green Projects:
Modeling Improves Economic Benefits
Allen Williams, Private Consultant
Ken Dunwoody, Retired
T
he ability to model a process to
see if it meets project requirements
and is economically viable
enhances the conceptual design.
Modeling gives engineers a means
to identify the most promising processes,
in terms of capital investment
and return, and this can be particularly
important when the project is
environmentally oriented (a so-called
green project).
Did you ever consider producing a
new product from your existing plant
inventory, or implementing a particular
green project that could help to reduce
your tax liability but for which
high fuel costs ultimately killed the
overall economics of the project?
It is important to evaluate green
ideas that could potentially improve
your bottom line, but how do you sort
through a myriad of computations to
screen all the options associated with
such tasks, and evaluate the pros and
cons of each proposed idea?
The answer is simpler than you
might think. The trick is to use a dynamic
process model that is equipment-specific
but still generic in the
way that it handles mass and energy
computations. Today, while there are a
number of commercial models available,
there are many benefits that
come from building your own realtime
dynamic model - one that can
allow you to assess your process and
its associated controls.
Dynamic modeling platforms are
available with most distributed control
system (DCS) hardware, but
in many cases, a PC-based system
model provides the greatest versatility
and flexibility. Specifically,
building your own computer process
allows you to customize the functions
and tailor the model to fit your
site-specific application and this
provides a smoother way to manage
project change.
The use of dynamic modeling can help to identify
the engineering and economic feasibility of any
proposed project. This sample project, involving the
conversion of solid waste to synfuel, shows how
SUPH
575°F
Drum
486°F. 600 psi
CH4 in at
1,150°F
H2O
Separator
Conveyor
Furnace
1,500 -
1,600°F
Methanol reaction
100°F, 200 - 250 psi
Makeup H2O
CH3OH
75,000
lb/h
Steam
Economizer
CH4 Feed
1,480 lb/h
DEA
298°F
FIGURE 1. Shown here is a typical process sketch for a solid-fuel-fed steam reformer
and methanol reactor. The hydrogen and carbon monoxide blend are reacted
to produce methanol (SUPH = super heater; DEA = deaerator)
Developing your own process
model is not as daunting a task as it
may first seem, especially if you define
the modeling system and identify
what is required before tackling
the process. And, your time investment
will be paid off by increased
analysis proficiency, speed and capability.
A model with greater analytical
capabilities allows users to
explore more alternative scenarios,
in greater detail.
Operating data and physical constants
define the boundary conditions
that determine how well a model
represents an actual process. But,
don't expect extraordinary or threedecimal-place
accuracy in computations
- rather, such models are useful
for identifying trends and direction of
changes. Most rudimentary modeling
efforts will fall within ±10%. Morethoroughly-developed
algorithms can
attain accuracy of ±1-3%.
34 CHEMICAL ENGINEERING WWW.CHE.COM JULY 2012
The process
To begin, make a short sketch of the
selected process (such as that shown
in Figure 1). Identify the temperatures,
pressures and flowrates for each
segment that is to be modeled. Such a
conceptualization sketch must include
key state variables, such as component
concentration, moisture content,
levels of excess oxygen in the furnace
and so on.
The process model profiled here emulates
a 100- to 125-million-Btu/h industrial
combustor with feed preheating,
a deaerator, an economizer and
a solids-conveying system. A green
project idea proposes to use the system
to produce synthetic fuel from
municipal waste.
The process is represented by a set
of ordinary differential equations that
conserve mass and energy, and utilize
thermodynamic relationships, reaction
kinetics and authentic equipment
Reformer
300 -
350 psi
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Chemical Engineering July 2012

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