Page 133 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

P1: ZCK/FLS P2: FYK/FLS QC: FYK Revised Pages
Encyclopedia of Physical Science and Technology EN002G-100 May 19, 2001 18:49

Chemical Process Design, Simulation, Optimization, and Operation 755

FIGURE 2 Sequential modular solution of a chemical process with recycle.

D. Flowsheet Simulations ulation flowsheet. Expert interfaces prompt the user for
required information until the degrees of freedom have
Computers were not commonly used for simulation in the
been completely specified.
chemical process industry until the late 1950s, and few
The solution of a chemical process simulation problem
process engineers had access to the mainframe computers
using the sequential modular technique is represented in
used at that time. Many oil and chemical companies and
Fig. 2. Here, the modeling equations can be written such
engineering firms began writing software to solve individ-
that the outlet stream from each unit is a function of the
ual unit operations, such as distillation columns. Gradu-
inlet streams to each unit:
ally, the stand-alone software codes were integrated so
unit operation simulation modules could be solved se- y 11 = g 11 (x 11 , x 12 )
quentially to simulate process flowsheets. This method,
y 21 = g 21 (x 21 , p 2 )
known as the sequential modular method, is still widely
used in commercial simulation packages to this day. An y 31 = g 31 (x 31 , p 3 )
example of an early flowsheet simulator is FLOWTRAN
y 32 = g 32 (x 31 , p 3 )
from Monsanto (the former basic chemicals division of
Monsanto is now known as Solutia). where x ij = input j to unit i, y ij = output j from unit i,
By the late 1970s most companies decided it was not and p i = parameters for unit i.
effective to continue development and support of their In Fig. 2, the recycle stream has been selected as the
own in-house packages. Widely used commercial simula- “tear stream.” A guess for x 12 must first be made, then
tion packages included PROCESS from Simulation Sci- the equations for units 1, 2, and 3 are solved. The output
ences and DESIGN from CHEMSHARE. The ASPEN of unit 3, y 31 , is then compared with the original guess
(Advanced System for Process Engineering) project at for x 12 . The problem is solved when y 31 has converged to
MIT was supported by the National Science Foundation x 12 within the desired tolerance. The nonlinear algebraic
to develop advanced computational software for chemical equations to be solved can be written as x 12 = g(x 12 )or
process simulation and design. The ASPEN package was f (x 12 ) = x 12 − g(x 12 ) = 0 and solved using the techniques
further developed by ASPEN Technology, Inc., and the discussed in Section III. Notice that the streams between
current release is known as ASPEN PLUS. units 1 and 2 or 2 and 3 could also have been chosen as
By the early 1990s, advances in user interfaces made the tear stream in Fig. 2.
the simulation packages much easier to use. HYSIM from An alternative to the sequential modular approach is to
Hyprotech was the first package to be developed specif- solve the equations modeling all of the units in a process
ically for a personal computer (PC) platform. All com- flowsheet simultaneously; this is known as the equation-
monly used process simulators (including PRO/II from based approach. Advantages to the sequential modular
Simulation Sciences) are now available on a PC plat- approach include: (1) specialized numerical techniques
form, with interfaces that allow flowsheet simulations to tailored to each unit operation can be used, and (2) the nu-
be formulated rapidly. Icons representing feed and product merical failure of one unit operation may still yield usable
streams and unit operations are placed directly on a sim- flowsheet information. Advantages to the equation-based