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1. Introduction to Chemical Process & Product

Design

Objectives

After completing this chapter, students should

be able to

appreciate the importance of chemical pro-

cess and plant design

understand the natures of chemical pro-

cess and plant design

know the sequence of process and plant

design

have a basic understanding of the organi-

sation 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 & stan-

dards, design factors, and systems of units

used in the chemical engineering project

understand how to set the design object-

tive(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

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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)

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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 cate-

gories [1]:

1) Commodity or bulb chemicals:

Produced in large volumes

Purchased on the basis of chemical com-

position, 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 com-

position, purity, and price; but with

higher purity and price

Examples include

o chloropropylene oxide (used for the

manufacture of epoxy resins and ion-

exchange resins)

o dimethyl formamide (used as an in-

termediate in the manufacture of

pharmaceutical products)

o n-butyric acid (used in the produc-

tion 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/healthy 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 che-

mical processes/plants to produce chemical pro-

ducts is the activity of chemical engineering de-

sign

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 object-

tives 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 under-

stand the basic components of each chemical pro-

cess, so that he/she can select and specify equip-

ment 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 mate-

rials leads to higher capital and 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 tech-

niques, 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(s)

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In addition to the basic components of chemi-

cal processes, auxiliary 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 oper-

ate 24 hours a day, 7 days a week, through-out a long period of time (e.g., a year)

The operating rate (commonly called

the attainment percentage) of the conti-

nuous process can be determined by the

following equation:

Number of hours

actually operated% Attainment 100

8,760

é ùê úê úê úë û= ´

(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 econo-

mical 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 equip-

ment; 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 pro-

ject can be listed sequentially as follows

1. Project specification (setting the design ob-

jectives 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 utili-

ties (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, evalu-

ated, and selected

For example, the possible ways of in-

creasing 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 mo-

dest capital cost

Choice 2: 20% increase, with a sig-

nificant capital cost

Choice 3: 30% increase, with an ex-

tremely 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 pro-

jects 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 se-

lection of the process and the sketch of flow

diagrams

In this stage, the designer must evalu-

ate each design/choice to see how well it

fit the purpose (objective/need)

Process simulation software package

(e.g., Aspen Plus® or Aspen HYSYS® or

Pro/II®) 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,

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and distillation columns, are specified (by

chemical & mechanical engineers)

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 Con-

struction (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 per-

spective of each design/choice will be ana-

lysed

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In addition to economic analysis, effects

on human’s health/hazards and the envi-

ronment will (must) 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 & instrumenta-

tion 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 authorisa-

tion

10. Procurement/purchasing

11. Construction

12. Start-up (or commission)

In this stage, even though the plant is

fully operational, it is not for commerciali-

sation 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 pro-

cess; generally, process manuals are ac-

companied 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 (DIN):

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: มอก):

http://www.tisi.go.th/eng/

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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 fac-

tor or safety factor [1-2]

Concerning the design/safety factor, the de-

signer 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 Engi-

neering (AE) system] are still widely employed

Additionally, some useful data are also avail-

able 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) or atm

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 – i.e.

109 g) 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

(where k and M are used for 103 and 106,

respectvely)

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It is highly recommended that design engi-

neers 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 sub-

process

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 equa-

tion, in order to be able to solve for the suitable

values of variables related to the design ob-

jective(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: 2

1 23 5z x x= +

(this is an objective)

Constraints: 1 2

10x x+ =

1

4x ³

The obtained values of 1

x and 2

x must satisfy

all objectives and constraints

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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.