1. Introduction to Chemical Process & Product
Design
Objectives
After completing this chapter, students should
be able to
 appreciate the importance of chemical process and plant design
 understand the natures of chemical process and plant design
 know the sequence of process and plant
design
 have a basic understanding of the organisation of a chemical engineering project
 have a basic knowledge on how to produce
a project documentation
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 know how to specify/identify codes & standards, design factors, and systems of units
used in the chemical engineering project
 understand how to set the design objecttive(s) and that the design always has a
limitation/constraints
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1.1 Nature of Chemical Process and Plant Design
Chemical products are essential to modern
society. High living standards depend heavily on
the chemical products
Almost all aspects of our everyday life are
supported by chemical products in one way or
another [1]
Examples of chemical products widely used in
our daily life are illustrated in Figures 1.1-1.6
3
Figure 1.1 Products from poly-ethylene (PE)
(from http://www.ineos.com & http://www.freelin-wade.com)
Figure 1.2 Pharmaceutical products
(from http://www.rc-globalholding.com &
http://www.spotoncoating.com)
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Figure 1.3 Luxury products from PVC
(from http://www.lyst.com & http://www.ebay.com)
5
Figure 1.4 Automobile parts made of plastic
products
(http://www.myplaticmold.com)
Figure 1.5 Petroleum products
(http://www.bloomberg.com)
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Figure 1.6 Fragrances
Chemical products can be divided into 3 categories [1]:
1) Commodity or bulb chemicals:
 Produced in large volumes
 Purchased on the basis of chemical composition, purity, and price
 Examples are sulphuric acid, nitrogen,
and oxygen
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2) Fine chemicals:
 Produced in small volumes
 Purchased on the basis of chemical composition, purity, and price; but with
higher purity
 Examples include
o chloropropylene oxide (used for the
manufacture of epoxy resins and ionexchange resins)
o dimethyl formamide (used as an intermediate in the manufacture of
pharmaceutical products)
o n-butyric acid (used in the production of beverages, flavourings, and
fragrances)
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3) Specialty or functional chemicals:
 Purchased of their function
 High value-added and sold at a very
high margin (or profit)
 Short lifetime
 Examples are
o pharmaceutical products
o flavourings
o perfumes
These chemical products (of any category) are
produced in chemical plants, which were operated
mainly by chemical engineers
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The creation of plans & specifications and the
analysis of financial feasibility/profitability for
the construction/modification/operation of chemical processes/plants to produce chemical products is the activity of chemical engineering design
Process and plant designs are the focal point
of chemical engineering practice [2]
The development of chemical processes and
plant products is the creative activity [2-3]
The designers of either processes or plants
normally start their designs from specific objecttives or customers’ needs and arrive at the best
way to achieve such objectives/needs
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1.2 Basic Components of Chemical Processes
It is necessary for a design engineer to understand the basic components of each chemical process, so that he/she can select and specify equipment for each stage efficiently
The basic components of a chemical process
comprise:
1. Raw material storage
Storage of raw materials are necessary
to avoid the fluctuations of the production
and of the product quality or to avoid the
interruptions of the production
The amount of raw materials to be
stored depends on, e.g.,
 the nature of raw materials
 the method of delivery
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Storing too high amounts of raw materials leads to higher operating costs; on the
other hand, if the amount of raw materials
stored is not sufficient, it can result in the
interruption of the process
2. Feed preparation
This stage is required to prepare the
raw materials to be, e.g.,
 at the appropriate purity
 in the right form/size
 free of contaminants that can be
poison to the catalysts
3. Reaction
This stage is the most important
stage (or the heart) of a chemical process.
The design engineer must design the
reactor such that the desired product(s)
is(are) produced at the desired amount
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4. Separation
In this stage, the desired product(s) are
separated from the by-product(s) and the
un-reacted reactant(s)
At times, the un-reacted reactant(s)
is(are) recycled to the reaction or the feed
preparation stage
5. Purification
In this stage, the main product(s)
is(are) purified using various kinds of techniques, in order to meet the standard(s) or
market/customer need(s)
6. Product storage and sales
The amount of product(s) to be stocked
before sales depends on the nature of the
product(s) and/or the market/customer
demand
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In addition to the basic components of chemical processes, ancillary processes producing and
supplying such services or utilities as
 process water
 cooling water
 air/process gas (e.g., nitrogen, oxygen)
 steam
are also needed for each component of a chemical
process
The design engineer must not overlook these
services/utilities
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1.3 Choice of Continuous vs Batch Production
As students have already learned from the
course of Material & Energy Balances,
 Continuous processes are designed to operate 24 hours a day, 7 days a week, throughout a long period of time (e.g., a year)
The operating rate (commonly called
the attainment percentage) of the continuous process can be determined by the
following equation:
éNumber of hours ù
ê
ú
êactually operatedú
ê
úû
% Attainment = ë
´ 100
8, 760
(1.1)
Generally, % attainment of continuous
processes ranges from 90-95% on annual
basis
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 Batch processes are designed to run inter-
mittently (for a certain period of time; e.g.,
10 hours, depending on the nature of the
chemical process)
It should be noted that the combination of
continuous and batch operations is common for
chemical processes; for example, a batch reactor
is employed to produce the mixture of ethanol +
water at a low concentration of ethanol through
fermentation, and this mixture of ethanol + water
is used as a feed to a continuous distillation
column to produce ethanol with a higher purity
Continuous processes are usually more economical than batch processes, especially for large-
scale production, as their capital/fixed costs are
much lower (for a high-volume production)
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However, batch processes are more flexible, as
they allow the production of multiple products
with different grades/purities in the same equipment; additionally, they are easier to clean and
maintain sterile operation
Accordingly,
 the continuous processes are the best
choice for producing commodity or bulb
chemical products
 the batch processes are highly recommended
for specialty or functional chemical products
Fine chemical products can be produced by
either continuous or batch processes, depending
on the quantity produced.
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1.4 Organisation of a Chemical Engineering
Project
The structure of a chemical engineering project can be listed sequentially as follows
1. Project specification (setting the design objectives or determining customers’ needs)
In this stage, the designer should obtain
as clear and unambiguous requirements as
possible
The needs can be categorised into
 Must-have: cannot be compromised
during the design
 Should-have: can be relaxed during
the design
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Additionally, in this stage, the design
basis is set; the design basis is a more pre-
cise statement of the design problem, e.g.,
production rate and purity specifications,
along with constraints that will affect the
design, such as
 the international, national, local, or
company’s standards/codes
 the details of raw materials avail-
able
 information regarding the possible
plant location(s), e.g., climate data,
seismic condition, infra-structure
availability
 information concerning the condi-
tions, availability, and price of utilities (e.g., electricity, water supply,
fuels)
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It is necessary to have a clearly defined
design basis before detailed design can be
started
2. Determination of possible designs/choices
In this stage, possible solutions to the
design problem are to be analysed, evaluated, and selected
For example, the possible ways of increasing the plant capacity for producing a
higher amount of polymer according to an
increase of the market need are [1]
 Choice 1: 10% increase, with a modest capital cost
 Choice 2: 20% increase, with a significant capital cost
 Choice 3: 30% increase, with an extremely huge capital cost
 Choice 4: Build a new plant
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When the choice is selected, the next
steps (i.e. the economic evaluation and the
detailed design & equipment selection) will
be proceeded according to the selected
choice
Generally, chemical engineering projects can be categorised into 3 types:
1) Modifications to the existing plant,
to, e.g., increase the purity of the
product, or to lower the emissions
of pollutants
2) Expansion of the existing plant, to
meet the growing demand
3) Development of the new process/
plant
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The next step is to examine the fitness of
the designs/choices, which includes the selection of the process and the sketch of flow
diagrams
In this stage, the designer must evaluate each design/choice to see how well it
fit the purpose (objective/need)
Process simulation software package
(e.g., Aspen Plus®) are to be employed to
test the choices
3. Performing material & energy balances
4. Preliminarily selecting & designing process
equipment
In this stage, the detailed specifications
of equipment in the chosen process, e.g.,
vessels, heat exchangers, pumps, reactors,
and distillation columns, are specified (by
chemical & mechanical engineers)
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Control systems are also examined and
selected (by chemical & electrical engineers)
Additionally, plant site preparation for
further construction is to be made (by civil
engineers)
At times, these tasks are carried out by
an Engineering, Procurement, and Construction (EPC) company (or a contractor).
5. Formulating the process flow diagram
(PFD); an example of PFD is depicted in
Figure 1.7
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Figure 1.7 An example of a process flow diagram
(PFD)
(from http://chemengineering.wikispaces.com)
6. Preliminarily estimating process/plant costs
and acquiring the source of funds
After selected design(s)/choice(s) can
be chosen from Stage 2 & 4, economic perspective of each design/choice will be analysed
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In addition to economic analysis, effects
on human’s health/hazards and the environment will also be determined
7. Piping & instrumentation design [an exam-
ple of piping & instrumentation diagram
(P & ID) is as shown in Figure 1.8] and
detailed process design, which includes
 selecting/designing chemical engineering
equipment
 selecting/specifying instrument & con-
trol systems
 selecting/specifying pumps & compres-
sors
 a reactor design
 a heat exchanger design
 selecting/specifying/designing separa-
tion equipment
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Figure 1.8 An example of piping & instrumentation diagram (P & ID)
(from http://www.creativeengineers.com)
 a piping design
 designing/specifying utilities & other
services
 selecting/specifying electrical motors,
switch gear, and sub-stations
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8. Structural & plant layout design, which
comprise
 a structural design
 a plant layout design
 designs of general civil works, foun-
dations, drainage systems, and roads
 designs of offices, laboratories, and con-
trol rooms
9. Project cost estimation & fund authorisation
10. Procurement/purchasing
11. Construction
12. Start-up (or commission)
In this stage, even though the plant is
fully operational, it is not for commercialisation yet; the purpose of this stage is to
examine whether or not the plant is ready
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During this start-up period, the design
engineer must be ready to be called to
resolve the start-up and operating problem
13. Operation
Up to this point, the plant is ready for
commercial operation
14. Sales (including Marketing)
1.5 Project Documentation
Since the chemical engineering project is very
complicated and requires the co-operation of
several groups, it is necessary to have an effective
and well-organised documentation, which include
 Correspondence within the design group
and with, e.g.,
o government departments
o the client
o vendors
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 Calculation sheets, which comprise, e.g.,
o material & energy balances
o design calculations
o a cost estimation
 Drawings, e.g.,
o process flow sheets/diagrams (PFSs/
PFDs)
o P & IDs
o a plant layout
o architectural drawings
o electricity drawings
o mechanical drawings
o detailed drawings of each equipment
 Specification sheets, e.g.,
o the design basis
o feed & product specifications
o equipment list
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o detailed specifications of each equip-
ment
 Information on health, safety, and the en-
vironment, e.g.,
o material safety data sheets (MSDSs);
an example of MSDS is as illustrated in
Figure 1.9
o HAZOP or HAZAN documentation (will
be discussed in detail later)
o documents concerning emission assess-
ments
 Purchase orders, e.g.,
o quotations
o invoices
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Figure 1.9 An example of a material safety data
sheet (MSDS)
(from http://www.zeofill.com)
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 Process manuals:
describing the process
and the basis of the design, which provide
a complete technical description of the process; generally, process manuals are accompanied by PFDs/PFSs and P & IDs
 Operating manuals: the detailed, step by
step, instructions for the operation of the
whole process and of each equipment
1.6 Codes and Standards
Nowadays, the standardisation is needed;
thus, all the design of chemical processes must
follow codes and standards strictly
There are several codes and standards to be
complied with, e.g.,
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 International Organisation for Standardi-
sation (ISO): http://www.iso.org/
 British Standards (BS):
http://www.bsigroup.com/en-GB/
 German Institute for Standardisation or
Deutsches Institut für Normung:
http://www.din.de/
 American National Standards Institute
(ANSI): http://www.ansi.org/
 American Society for Testing and Mate-
rials (ASTM): http://www.astm.org/
 Japanese Industrial Standards (JIS):
http://www.jisc.go.jp/eng/
 Thai Industrial Standards (TIS:
http://www.tisi.go.th/eng/
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มอก):
1.7 Design Factors
Since errors and uncertainties arising from the
data and approximation are unavoidable, it is
common for the designer to include some degrees
of over-design, commonly known as design factor or safety factor [1-2]
Concerning the design/safety factor, the designer should keep in mind that
 if design/safety factor is too low, the pro-
cess might not work or it may run at a high
risk
 on the contrary, if the design/safety factor
is too high, it would cause the process to
be unnecessarily expensive or less efficient
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Thus, a balance must be made between these
two extremes [1]
1.8 Systems of Units
Even though modern engineering design is
based on SI units, traditional scientific (i.e. metric
system) and engineering [i.e. American Engineering (AE) system] are still widely employed
Additionally, some useful data are also available in metric and AE systems
Accordingly, design engineers must be familiar
with other unit systems (as mentioned above), in
addition to SI units, and must be able to make a
conversion between the unit systems fluently
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The following are the examples of the units
commonly used in chemical process design:
 Temperature is presented in oC or oF, ra-
ther than K or R
 Pressure is commonly given in bar, rather
than Pa (N/m2)
 Volume or volumetric flow rate is provided
in L or L/time, rather than m3 or m3/time,
which gives too small values
 kg or tonnes (103 kg) used normally em-
ployed to describe plant capacities; g gives
too high values and Gg (gigagramme) is
rarely used
 In the USA, M is used for 103, and MM is
used for 106, which can be confusing to
those familiar with SI or metric units
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It is highly recommended that design engineers clearly specify the unit systems used in the
project before the start of the design process
1.9 Design Objectives and Constraints
In the design process, it must have the design
objective(s) for the whole process or for each subprocess
The design objective is to either minimise or
maximise a specific quantity
For example, to
 maximise a profit
 minimise a cost or emissions
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However, the design objective(s) always has
(have) a limitation (limitations), which is generally
called constraints
Examples of constraints are as follows
 Product purity ≥ 99.99 wt%
 Production rate ≤ 500,000 tonnes/year
 NOx emissions ≤ 200 ppm
In order to satisfy the design objective(s), a
design engineer must be able to translate the
design objective(s) and constraint(s) into equation, in order to be able to solve for the suitable
values of variables related to the design objective(s) and constraint(s) that lead to the
satisfaction of the design objective(s), which is
commonly called optimisation
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An example of an optimisation problem is as
illustrated below:
Maximise:
z = 3x 1 + 5x 22
Constraints: x 1 + x 2 = 10
x1 ³ 4
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References
[1]
R. Smith, Chemical Process: Design and Integration,
Wiley, 2005.
[2]
R. Sinnot and G. Towler, Chemical Engineering
Design: Principles, Practice, and Economics of Plant
Design, 2nd ed., Elsevier, 2013.
[3]
R. Turton, R.C. Bailie, W.B. Whiting, J.A. Shaeiwitz,
and D. Bhattacharyya, Analysis, Synthesis, and Design
of Chemical Processes, 4th ed., Pearson, 2013.
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