Chemical Engineering December 2017 - 41

and the reclaiming of spent catalyst
* A function for reactor cooling.
* A function for slurrying catalyst in
feed oil
* A function for cooling the oil to
storage temperature
The TFF is not a stand-alone document.
Also needed are material and
energy balances. When synthesizing
a block flow diagram, Murphy proposes
using a Degree of Freedom
analysis when doing the material balance
[9]. In addition, each flowsheet
function requires added definition.
This is done via a document called
the Technical Function Definition
(TFD). It details the following:
* The purpose of each function
* Important variables and operating
conditions
* The basis for the variables and for
any unit operation that was specified
(see Table 3 for the TFD that
accompanies the final issue of the
flowsheet). The TFD is the forerunner
of the process definition,
which is a detailed prose description
of the process
Option creation tools. As already
mentioned, generating a high quality,
comprehensive option list can be
difficult. Most of the difficulties are
linked to the engineers' mindset and
to organizational pressures. Generally,
engineers have good problemsolving
skills, which enable them to
quickly solve technical questions;
but in terms of option generation,
this gets in the way. Once there is a
solution, there is no need to look at
other options. Engineers also tend
to have a production focus, which
creates internal pressures to " get on
with the job " and not take time to explore
multiple options.
The main organizational issues are
schedule pressures and standardized
designs. Schedule pressures
push the engineer to quickly make
design decisions, not spend time
looking at options. I have found,
however, that more fully examining
options often simplifies execution
and may shorten the schedule.
Standardization
conflicts
with
the
philosophy of considering options. If
thorough option analysis was done
prior to setting the standard and
freezing the design, standardization
could yield big economic benefits.
On the other hand, standards do
periodically need updating, and the
standardization
policy makes this
hard to do.
There are forces that hinder option
list development. To counter
these, this section of the article
presents tools that make the option
creation task easier and faster. Two
are useful in all situations, brainstorming,
and a variation, the " 63-5 "
method [10]. Both are small
group tools that foster creativity by
allowing group members to build
upon each other's ideas. The Process
Synthesis Model also uses two
other tools, Probing Questions and
Unit Operations Guides. Both lead
to more thorough lists.
Probing Questions. These questions
will help the synthesis team
better understand their process and
uncover more options. The questions
are arranged as follows: general, feed
preparation, reactor, separations, recycle/purge,
HSE and references.
General:
* Could the product be purchased?
If so, why not buy it rather than
make it?
* Should the process be batch or
continuous?
* Does the process require materials
of construction more expensive
than carbon steel? If so, how
might the corrosive streams be
eliminated or their concentrations
be changed to reduce corrosion
problems and to lower costs?
* Does the proposed process require
operation outside the ranges of 0
and 150 psig or 100 and 650°F?
If so, how might the process be
changed to permit operation within
these ranges, lowering costs?
* Should there be surge between
unit operations (UOs)? How much?
* What are the technical and economic
tradeoffs among the main
operations on the flow sheet - reactor,
recycle and purge streams,
separation systems and heat recovery
system? In general, the
reactor design determines how
the separator will be designed and
what recycle or purge streams will
be required. These, in turn, affect
the heat-recovery plan and systems.
All have an effect upon the
utility needs [11]
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Chemical Engineering December 2017

Table of Contents for the Digital Edition of Chemical Engineering December 2017
