THE DESIGN PROCESS
Ref: Seider, Seader and Lewin (2004), Chapter 1
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The Design Process
Schedule - The Design Process
 Primitive Design Problems
– Example
 Steps in Designing and Retrofitting Chemical Processes
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Assess Primitive Problem
Process Creation
Development of Base Case
Detailed Process Synthesis - Algorithmic Methods
Process Controllability Assessment
Detailed Design, Sizing, Cost Estimation, Optimization
Construction, Start-up and Operation
 Environmental Protection
 Safety Considerations
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The Design Process
Primitive Design Problems
 The design or retrofit of chemical processes begins
with the desire to produce profitably chemicals that
satisfy societal needs that arise in the broad spectrum
of industries that employ chemical engineers:
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petrochemicals,
petroleum products
industrial gases
foods
pharmaceuticals
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polymers
coatings
electronic materials
bio-chemicals
 Partly due to the growing awareness of the public, many
design projects involve the redesign, or retrofitting, of
existing chemical processes to solve environmental
problems and to adhere to stricter standards of safety
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The Design Process
Origins of Design Problems
 Often, design problems result from the explorations of
chemists, biochemists, and engineers in research labs to
satisfy the desires of customers to obtain chemicals with
improved properties for many applications
 However, several well-known products, like Teflon (polytetrafluoroethylene), were discovered by accident.
 In other cases, an inexpensive source of a raw material(s)
becomes available
 Other design problems originate when new markets are
discovered, especially in developing countries
 Yet another source of design projects is the engineer
himself, who often has a strong inclination that a new
chemical or route to produce an existing chemical can be
very profitable.
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The Design Process
Typical Primitive Design Problem
Consider, the need to manufacture vinyl chloride (VC),
H
H
C
C
Cl
H
A typical primitive problem statement is as follows:
“An opportunity has arisen to satisfy a new demand for VC
monomer (VCM), on the order of 800 million pounds per
year, in a petrochemical complex on the Gulf Coast, given
that an existing plant owned by the company produces onebillion pounds per year of this commodity chemical. Since
VCM is an extremely toxic substance, it is recommended
that all new facilities be designed carefully to satisfy
governmental health and safety regulations.”
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The Design Process
Steps in Process Design and Retrofit
Assess Primitive
Problem
Detailed Process
Synthesis Algorithmic
Methods
Development
of Base-case
Plant-wide
Controllability
Assessment
Detailed Design,
Equipment sizing, Cap.
Cost Estimation,
Profitability Analysis,
Optimization
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The Design Process
Steps in Process Design and Retrofit
SECTION A
Detailed Process
Synthesis Algorithmic
Methods
Development
of Base-case
Assess Primitive
Problem
Plant-wide
Controllability
Assessment
Detailed Design,
Equipment sizing, Cap.
Cost Estimation,
Profitability Analysis,
Optimization
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The Design Process
Steps in Process Design and Retrofit
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The Design Process
Assess Primitive Problem
 Process design begins with a primitive design problem that
expresses the current situation and provides an
opportunity to satisfy a societal need.
 Normally, the primitive problem is examined by a small
design team, who begins to assess its possibilities, to
refine the problem statement, and to generate more
specific problems:
– Raw materials - available in-house, can be purchased or need
to be manufactured?
– Scale of the process (based upon a preliminary assessment of
the current production, projected market demand, and
current and projected selling prices)
– Location for the plant
 Refined through meetings with engineering technical
management, business and marketing.
 Brainstorming to generate alternatives
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The Design Process
Example: VC Manufacture
 To satisfy the need for an additional 800 MMlb/yr of VCM,
the following plausible alternatives might be generated:
Alternative 1. A competitor’s plant, which produces 2 MMM
lb/yr of VCM and is located about 100 miles away, might be
expanded to produce the required amount, which would be
shipped. In this case, the design team projects the purchase
price and designs storage facilities.
Alternative 2. Purchase and ship, by pipeline from a nearby
plant, chlorine from the electrolysis of NaCl solution. React
the chlorine with ethylene to produce the monomer and HCl as
a byproduct.
Alternative 3. Since the existing company produces HCl as a
byproduct in large quantities are produced, HCl is normally
available at low prices. Reactions of HCl with acetylene, or
ethylene and oxygen, could produce 1,2-dichloroethane, an
intermediate that can be cracked to produce vinyl chloride.
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The Design Process
Survey Literature Sources
 Stanford Research Institute (SRI) Design Reports
 Encyclopedias
– Kirk-Othmer Encyclopedia of Chemical Technology (1991)
– Ullman’s Encyclopedia of Industrial Chemistry (1988)
– Encyclopedia of Chemical Processing and Design (McKetta and
Cunningham, 1976)
– ...
 Handbooks and Reference Books
– Perry’s Chemical Engineers Handbook (1997)
– CRC Handbook of Chemistry and Physics (1997)
– ...
 Indexes
– See Ferdowsi Library
 Patents (see web for example www.uspto.gov/patft)
 Internet
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The Design Process
Steps in Process Design and Retrofit
Assess Primitive
Problem
Detailed Process
Synthesis Algorithmic
Methods
Development
of Base-case
Plant-wide
Controllability
Assessment
SECTION B
Detailed Design,
Equipment sizing, Cap.
Cost Estimation,
Profitability Analysis,
Optimization
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The Design Process
Steps in Process Design and Retrofit
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The Design Process
Steps in Process Design and Retrofit
Assess Primitive
Problem
Detailed Process
Synthesis Algorithmic
Methods
Development
of Base-case
Detailed Design,
Equipment sizing, Cap.
Cost Estimation,
Profitability Analysis,
Optimization
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Plant-wide
Controllability
Assessment
SECTION C
The Design Process
Steps in Process Design and Retrofit
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The Design Process
Environmental Issues in Design
 Handling of toxic wastes
– 97% of hazardous waste generation by the chemicals and
nuclear industry is wastewater (1988 data).
– In process design, it is essential that facilities be included to
remove pollutants from waste-water streams.
 Reaction pathways to reduce by-product toxicity
– As the reaction operations are determined, the toxicity of all
of the chemicals, especially those recovered as byproducts,
needs to be evaluated.
– Pathways involving large quantities of toxic chemicals should be
replaced by alternatives, except under unusual circumstances.
 Reducing and reusing wastes
– Environmental concerns place even greater emphasis on
recycling, not only for unreacted chemicals, but for product
and by-product chemicals, as well. (i.e., production of
segregated wastes - e.g., production of composite materials
and polymers).
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The Design Process
Environmental Issues in Design (Cont’d)
 Avoiding non-routine events
– Reduce the likelihood of accidents and spills through the
reduction of transient phenomena, relying on operation at the
nominal steady-state, with reliable controllers and faultdetection systems.
 Design objectives, constraints and optimization
– Environmental goals often not well defined because economic
objective functions involve profitability measures, whereas the
value of reduced pollution is often not easily quntified
economically.
– Solutions: mixed objective function (“price of reduced
pollution”), or express environmental goal as “soft” or “hard”
constraints.
– Environmental regulations = constraints
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The Design Process
Safety Considerations
 Example Disaster 1 – Flixborough: 1st June 1974
http://www.hse.gov.uk/hid/land/comah/level3/5a591f6.htm
– 50 tons of cyclohexane were released from Nypro’s KA plant
(oxidation of cyclohexane) leading to release of vapor cloud
and its detonation. Total loss of plant and death of 28 plant
personnel.
– Highly reactive system - conversions low, with large inventory
in plant. Process involved six, 20 ton stirred-tank reactors.
– Discharge caused by
failure of temporary pipe
installed to replace
cracked reactor.
– The so-called “dog-leg”
was not able to contain
the operating conditions
of the process (10 bar,
150 oC)
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The Design Process
Safety Considerations
 Flixborough - What can we learn?
– Develop processes with low inventory, especially of flashing
fluids (“what you don’t have, can’t leak”)
– Before modifying process, carry out a systematic search for
possible cause of problem.
– Carry out HAZOP analysis
– Construct modifications to same standard as original plant.
– Use blast-resistant control rooms and buildings
T. Kletz, “Learning from Accidents”, 2nd Ed. (1994)
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The Design Process
Safety Considerations (Cont’d)
 Example Disaster 2 – Bhopal: 3rd December 1984
http://www.bhopal.com/chrono.htm
– Water leakage into MIC (Methyl isocyanate) storage tank
leading to boiling and release of 25 tons of toxic MIC vapor,
killing more than 3,800 civilians, and injuring tens of
thousands more.
– MIC vapor released because the refrigeration system
intended to cool the storage tank holding 100 tons of MIC
had been shut down, the scrubber was not immediately
available, and the flare was not in operation.
 Bhopal - What can we learn?
– Avoid use of hazardous materials. Minimize stocks of
hazardous materials (“what you don’t have, can’t leak”).
– Carry out HAZOP analysis.
– Train operators not to ignore unusual readings.
– Keep protective equipment in working order.
– Control building near major hazards.
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The Design Process
Safety Issues: Fires and Explosions
Flammability Limits of Liquids and Gases
LFL and UFL (vol %) in Air at 25 oC and 1 Atm
Compound
LFL (%) UFL (%)
Acetylene
2.5
100
Cyclohexane
1.3
8
Ethylene
2.7
36
Gasoline
1.4
7.6
Hydrogen
4.0
75
These limits can be extended for mixtures, and for elevated
temperatures and pressures (see Seider et al, 2004).
With this kind of information, the process designer makes sure
that flammable mixtures do not exist in the process during
startup, steady-state operation, or shut-down.
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The Design Process
The Design Process - Summary
 Steps in Designing and Retrofitting Chemical Processes
– Assess
Assess Primitive
Problem
Primitive
Problem - covered today
– Process
Process Creation
Creation
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- next week
Development of Base Case
Detailed Process Synthesis - Algorithmic Methods
Process Controllability Assessment
Detailed Design, Sizing, Cost Estimation, Optimization
Construction, Start-up and Operation
 Environmental Protection
– Environmental regulations  design constraints
 Safety Considerations
– Should strive to design for “inherently safe plants”
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The Design Process