Page 139 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology EN002G-100 May 19, 2001 18:49

Chemical Process Design, Simulation, Optimization, and Operation 761

FIGURE 6 Two possible distillation sequences for a ternary feedstream.

energy integration was the energy crisis of the 1970s. So- nomic impact. Different process designs can result in large
called pinch technology was developed to determine the differences in the project payout periods. It is particularly
minimum consumption of utilities for a heat-exchanger important for designs to have good flexibility (able to han-
network. One of the first commercially available codes dle variations in process parameters, feed conditions, etc.)
was HEXTRAN from Simulation Sciences. andoperability(abletohandledisturbancesandbedynam-
ically controllable). Dynamic simulation is being used to
determine if a steady-state design is dynamically control-
3. Task-Integrated Process Synthesis
lable or to test process safety in the event of equipment
An understanding of combined reaction and separations failure. Additionally, three-dimensional computer-aided
processes has led to process designs with significant cap- design (CAD) is being used to enable the visualization of
ital and operation cost savings compared to more tradi- equipment layout (placement of valves, pipe racks, etc.)
tional process synthesis approaches. A prime example is for ergonomic and safety reasons.
the methyl acetate process patented by Eastman Kodak. A There has been active work in the development of pro-
traditional methyl acetate process involves a reactor and cesses that are safer and have less potentially damaging
nine separation vessels (including a mix of distillation and environmental impacts. These “environmentally benign”
extraction operations). A revolutionary design resulted in or “green” process designs typically include a life-cycle
a reactive/extractive distillation process with a single col- analysis to account for the long-term environmental (and
umn. The resulting design yielded savings in both capital economic) impact of a product or design.
and operating costs of over 80%.

VI. DESIGN FOR OPERABILITY
B. The Design Project AND FLEXIBILITY
The focus of process engineers handling a chemical pro-
A. Introduction
cess design project is naturally on development of the
process flowsheet. The process design team, using Increased process integration due to increases in energy
computer-aided simulations, hand calculations, and pre- and feedstock costs has made processes more difficult
vious experience, specify the basic flows, heat duties, and to operate. These operational difficulties have led to the
separation stages and create the process flow diagram need for integration of process design and process control.
(PFD). Other engineers are often involved in the detailed A number of techniques to address design for flexibil-
equipment design. For example, the process team may ity and operability have been proposed. These techniques
specify an exchanger heat duty. A project engineer will include flexibility analysis (based on nonlinear program-
perform the detailed exchanger equipment design, includ- ming and steady-state models), dynamic resiliency (based
ing size and number of tubes, materials of construction, on linear, multivariable models), and steady-state and dy-
etc. A good depiction of the total design project is shown in namic “back-off” analysis (where the actual operating
Fig. 7. Although the process design phase is less than 15% point is chosen by “backing off” from the optimum point
of the total project cost, it can result in a tremendous eco- which lies at the intersection of constraints). Frequently,