Chemical Engineering December 2010 - 24

Department Editor: Scott Jenkins
Project design
decision-making:
Option lists
ost of the more than $100 billion in
capital spending and $1.2 trillion in
prodution costs spent annually by the
chemical process industries (CPI) is committed
by decisions made in the early phases of
process development and plant design. A more
disciplined approach to decision-making that
fosters early attention to costs and more thorough
consideration of available options has the
potential to lower costs for most projects and
better balance capital and production costs.
The Economic Design Model is a method
M
Feed
oil
to formalize this disciplined approach to
cost considerations [1]. The model has three
phases: setting project objectives; creating a
thorough and comprehensive list of options;
and conducting an economic analysis of
the technically workable options. The model
is helpful for early-stage decision-making
(product and process development, feasibility
evaluation and conceptual phases). Large
amounts of money are at stake in these early
phases because the decisions made there set
the future economic framework.
Creating OPtiOns Lists
The heart of the Economic Design Model in
a process- or plant-design project involves
creating a full list of options for completing the
project. To identify the best of the available
options, engineers must diverge early in a
project to consider multiple options before
converging on a decision. Doing so requires
carefully and deliberately constructing an
option list that is more comprehensive than a
typical one. Experience indicates that more
than one workable technical option usually exists,
so the decision comes down to economics.
Economics decisions, in turn, are best made on
the basis of balancing capital and production
costs - evaluating whether it makes sense to
spend additional capital to reduce production
costs, or if it is better to reduce capital outlay
but be left with higher production costs.
Reasons why option creation falls short
Three main reasons exist for why the consideration
of options in a design project often
falls short. One involves the heavy schedule
demands placed on engineers. These demands
can create the perception among engineers
that there is not sufficient time for anything
other than to find a workable solution to the
design problem. Another can be viewed as
a consequence of standardized plant design,
which generates a reluctance to make changes
that would alter the standardized designs
(even when improved technology is available
or economic conditions have changed). The
third reason for why option lists often fall short
of the mark is that creating effective ones can
simply be hard to do.
Option creation tools
The need for increased rigor in creating options
lists may be more difficult, but there are several
tools that can help greatly. While brainstorming
and its variations can be used for generating
options, there are additional tools that bring
discipline to the option creation process and
help overcome the factors that could cause
Catalyst
220°F
150 psig
steam
120°F 250°F
350°F
H2O
H2
Cooling
jacket
Figure 1. An engineer might
develop a flowsheet like this
one to meet business and technical
objectives for a hypothetical
oil hydrogenation example
the option lists to fall short. The
tools promote divergence before
design answers were selected,
promote higher levels of understanding
of the design and help
spark creativity.
Flowsheet development. The
first tool is an alternate methodology
for development of the
design flowsheet. The concept
is to focus on the technical
purpose of each step in the
process, rather than specific
Technical Function Flowsheet, Oil Hydrogenation
(with permission from CRC Press)
Heat oil to
reaction
temperature
Heat oil to
reaction
temperature
Feed oil
Hydrogenate oil
to convert the
tri-unsaturates
to di-unsaturates
H2
Catalyst
Figure 2. A TFF for this example could
look like this. It focuses on the technical
purposes of each step, rather than on unit
operations or specific equipment
unit operations or equipment types. The resulting
block diagrams, called technical function
flowsheets (TFFs), help to slow convergence
to design specifics until all possible options
have been thoroughly considered. Technical
function descriptions are verb-object
combinations, rather than nouns usually used
to describe unit operations. For example, in
the case of a hypothetical oil hydrogentation
process, technical function phrases could be:
" separate solids from the oil, " " heat oil " or
" react oil with hydrogen. " These descriptions
are intentionally less specific and more generalized
than the descriptions of unit operations
that could fulfill those technical functions,
such as pressure filter, shell-and-tube heat
exchanger, and multistage stirred-column
reactor (Figures 1 and 2). TFFs are backed up
by material and energy balances (often based
on pilot plant data) and a detailed table that
explains the purpose for each function, lists
important operating conditions (temperature,
pressure, flow, and so on) and describes the
basis for selecting operating conditions.
Unit operations guides. Use of the TFF methodology
sets the stage for the option list creation.
When it comes time to select unit operations
for the different technical functions, the Unit
Operations Guides, as published in [2], will
help engineers define most of the potential options.
Eight guides are available, covering the
following topics: blending and mixing; drying
(water removal only); heat transfer (inlcuding
evaporation); mass transfer (including
crystallization); material transport; mechanical
separation; reactions; and size modification.
Question lists. Intended to increase the depth
of understanding about a process, the question
list is a series of process-related questions that
will lead to more design options and higher
quality designs. It is often the case that general
Remove
catalyst so
oil is clear
Cool oil to
storage
temperature
1st issue
Catalyst
surge tank
Catalyst
Catalyst
300°F
H2O
250°F
H2O
220°F 120°F
Filter press
Hydrogenated
oil
questions that lay the foundation for a project
are not addressed well, either because the
answer appears at first glance to be obvious,
or because of the perception that there is not
enough time to answer them sufficiently, or
because no one thought to ask the question in
the first place.
The general process and process-interaction
questions address broad project issues, such
as the following examples: Should a product
intermediate be bought or synthesized? Should
the process be a batch or continuous one?
What grade of a raw material should be purchased?
Should recycle or purge streams be
used, and do those streams need to be treated
before reuse or disposal? How does this unit
operation affect the downstream process?
using the MOdeL tO FinaLize
Once a comprehensive list of options exists,
project engineers can move on to the next stage
of the Economic Design Model, in which each
option is analyzed to determine whether the option
can meet the technical needs of the project,
as well as to determine which of the technically
feasible is the most economical option.
References
1. Brown, Thane, Capital and Production Costs:
Improving the Bottom Line, Chem. Eng., January
2010, pp. 26-33.
2. Brown, Thane, " Engineering Economics and
Economic Design for Process Engineers, " CRC
Press, Atlanta, Ga., 2006.
Note: This edition of " Facts at Your Fingertips "
was adapted from Capital and Production Costs:
Improving the Bottom Line, an article authored by
Thane Brown that appears in the January 2010
issue of Chemical Engineering.
Chemical Engineering December 2010

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