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Chapter 1

Introduction

The basic purpose of chemical industry is to convert raw materials into useful products for the

society. For example, crude oil sourced from oil reservoirs cannot be used as it is. Refineries

convert the crude oil, through several chemical and physical processes, into products that we use

in our daily life. Petrol, diesel, lube oils, LPG are a few products of the refineries. There are

about 10 basic raw materials such as oil, natural gas, coal, available in nature. These are

converted base chemicals, which are then transformed to intermediate chemicals. Final products

are made from the based and intermediate chemicals. Below Figure shows chemical industry

“tree.”

Adapted: Chemical Process Technology, Moulijn et al., 2nd

ed, 2013

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Bulk chemicals are manufactured by continuous processes; fine chemicals, specialty chemicals

and pharmaceuticals are often made by batch processes due to low volumes of production,

seasonal requirements and flexibility to make several products. The continuous processes are

easier to control than batch processes because the former operate at steady state; the former are

more economical than the latter due to economies of scale.

Table below shows the bulk and fine chemicals manufactured (in 103 tons),

Bulk chemicals Fine and specialty chemicals

pharmaceuticals

Chemical Production in kg/year Chemical Production in kg/year

Sulfuric Acid 3.25 x 1011

α-interferon ~10

Ethane 2.4 x 1011

Streptokinase ~10

Propane 1.4 x 1011

Insulin ~104

Chlorine 9.7 x 1010

Penicillin 5 x 107

Chemical engineering/technology is orchestration of science [chemistry, physics and

mathematics], engineering, economics, safety, environment to make a product from a raw

material. As engineers, we need to ensure that raw materials and energy resources are conserved

and not wasted (excessive use of raw materials and energy); pollution standards of the time are

adhered to; quality of the products is maintained; safety of equipment and personnel is given due

importance.

You have taken several core courses in chemical engineering curriculum such as material and

energy balances, fluid mechanics, thermodynamics, reaction engineering, separation processes

and economics. This course (CL 408: Chemical Processes) unifies all the previous courses,

draws upon the fundamentals and applications learnt in those courses to develop a flowsheet to

make a chemical from a given raw material. Figure in the next page shows connection between

the core courses and CL 408.

Goal of this course is to study several processes for manufacturing organic and inorganic

chemicals. At the end of this course, you will be able to

Develop a flowsheet and/or a block diagram for the production of a chemical given

necessary information.

Choose/select reactor configuration for a given process; select appropriate reaction

conditions.

Identify potential areas of a plant to improve productivity.

Identify safety measures for a given process.

Develop separation schemes, upstream and downstream of reactor.

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P, PI, PID

Instrumentation

Control valves

…

Costing, taxes

Rate of return

Pay back period

Select from

alternatives

…

Overall balances

Raw material

needs

Energy needs

…

Heat exchangers

Distillation

Absorption

Extraction

…

Equilibrium

Efficiencies

Sizing of

compressor

Refrigeration

Equilibrium

constant

…

Pumps/fans/blowers

Pressure drop

Piping design

Instrumentation

…

Reactor selection

Reactor design

RTD

…

CHEMICAL

PROCESSES

Material and

energy balances

Fluid mechanics

Thermodynamics

Heat and mass transfer

Reaction

engineering

Economics

Process control

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General scheme for manufacture of a chemical

Production process for any bulk chemical (it is applicable for fine chemicals also) can be

roughly divided into three sequential steps: raw material preparation, reaction and

downstream separation. Usually raw materials are physically processed in one or several unit

operations to make them suitable for reaction. Examples of physical treatment of raw

material

Coal needs to be pulverized for burning/ gasification

Limestone is crushed and ground in cement manufacture

H2S is removed from natural gas

Benzene is dehydrated before further processing

The raw material is converted into product in a reactor. The reaction could be

heterogeneous or homogeneous; exothermic or endothermic; liquid phase or vapour phase.

In a typical chemical plant, reactor operates at high temperatures. As a result, side products

are inevitably produced. Catalyst may improve selectivity toward the desirable component

but will not eliminate side products. There are a few exceptions, however. Downstream of

reactor, therefore, product is separated from the side-products in one or many unit operations

(only physical separations happen, no chemical changes occur). Unreacted reactant(s) are

recycled to the reactor to enhance conversion. Most of the production cost is due to raw

materials.

Remember overall conversion and single pass conversion from CL 152?

Raw material

recycle

Unit operations

Unit operations

Reactor

Energy in and out

Other chemicals

Energy in

and out

Products

chemicals in

and out

Energy in

and out

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air

Molten

sulphur

air

air

Sulphuric Acid (H2SO4)

Sulphuric acid is by far the largest volume chemical produced. For this reason, it is also

called the king of chemicals. Major uses for sulphuric acid are in: manufacture of fertilizers,

alkylation step in crude oil refining, copper leaching, making of lead-acid storage battery in

motor vehicles as an electrolyte etc.

The source of sulphur in the production of sulphuric acid is crude oil. It contains about 0.05-

6 wt% of sulfur. During refining of the crude oil, sulphur (present as an element in organic

molecules) is removed as H2S to protect catalyst from poisoning. Elemental sulphur is made

from H2S by Claus process. Sulphur is one of the raw materials needed for making H2SO4.

Complete the block diagram by indicating other raw materials that are required for H2SO4

production.

Reactions: S (l) + O2 (g) SO2 (g) ∆𝐻298𝑜 = −298.3

𝑘𝐽𝑚𝑜𝑙

SO2 (g) + 0.5 O2 (g) SO3 (g) ∆𝐻298𝑜 = −98.5

𝑘𝐽𝑚𝑜𝑙

SO3 (g) + H2O (l) H2SO4 ∆𝐻298𝑜 = −130.4

𝑘𝐽𝑚𝑜𝑙

Which of the options is appropriate for burning sulphur? Think about energy requirements

and transportation issues.

a) Grind it to fine powder, mix it with air.

b) Melt it, atomize it by passing the melt through a burner and bring the fine sulphur

droplets in contact with air.

Sulphur oxidation:

H2SO4

Plant

S

S

S

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Sulphur is usually stored outdoors and dust and dirt may collect on it. Coagulating the dirt by

adding certain chemicals to molten sulphur and passing the melt through settlers and filter

cleans sulphur; the dirt particles may choke the sulphur burner.

Select an option for supply of O2 to oxidation of S. Compare pros and cons of the two

options. Hint: think about O2 separation from air, energy recovery from the effluents of the

combustor, adiabatic flame temperature.

A) O2 in air

B) Pure O2 after separating air into N2 and O2.

Moisture if present in the air supplied to the combustor forms mist of sulfuric acid during the

process. This mist corrodes the equipment and the pipes of the plant; also, the mist is difficult

to separate and escapes through the stack into ambient air. Hence, the moisture from the air

should be removed.

Moisture removal is effectively done by,

A) Adsorption onto silica gel

B) Absorption in H2SO4

C) either of the above

D) None of the above

What determined your choice?

What is a logical unit operation downstream of sulphur combustor?

SO2 oxidation:

𝑆𝑂2 𝑔 +1

2𝑂2 𝑔 ↔ 𝑆𝑂3 ∆𝐻 = −98.5 𝐾𝐽/𝑚𝑜𝑙

Challenge for engineers: conversion of SO2 > 99.7 %. This conversion is challenging because

the SO2 oxidation is equilibrium limited. The reaction occurs over a solid catalyst, vanadium

pentoxide (V2O5).

Make a sketch equilibrium conversion versus temperature for SO2 oxidation. Recall Le

Chatlier’s principle.

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Qualitatively the SO2 oxidation reaction should be carried out at: (Catalysts are usually

expensive; typical designs minimize catalyst requirement)

A) High pressure [high conversion, high cost]

B) Low pressure [low conversion, low cost]

C) High temperature [low equilibrium conversion, high rates]

D) Low temperature [high equilibrium conversion, low rates]

E) High air to SO2 ratio [high conversion, high cost]

Draw typical reactor configurations for carrying out heterogeneous reactions. Which one is a

better choice for SO2 oxidation?

A low conversion is achieved in one catalyst bed. Given this, how do we meet the challenge

of 99.7% conversion?

A) Inter-stage cooling

BED 1

Catalyst

BED 2 BED 3 Cooling Cooling

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H2SO4

Recycle Purge

B) Effluents of reactor bed are separated, SO3 is absorbed in water to form H2SO4,

and unconverted SO2 is recycled. To operate the process continuously a portion of

the recycle stream is purged because the fresh feed stream contains N2, an inert

gas. Think: fraction of N2 in the fresh feed stream and the fraction of recycle that

needs to be purged. Can the challenge of 99.7% conversion be achieved with the

purge?

Consider one bed of catalyst for SO2 oxidation. Plot the SO2 conversion versus inlet

temperature of reactants for this bed? Based on the plot, decide the inlet temperature. What

information is needed to calculate this temperature?

Reproduce equilibrium conversion versus temperature plot below and, in the plot, indicate

how high conversion is achieved.

After few stages of cooling, the incremental conversion achieved in every subsequent bed is

marginal. Removal of SO3 from the effluent stream leaving a series of beds by absorbing it

Reactor bed

Absorption

H2O

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710 K

770 K

695 K

700 K

85%

99.7%

5%

710 K

865 K

705 K

720 K 94%

in H2SO4 and recycling the SO3-free stream to another catalyst bed achieves the challenge of

high conversion. This removal is typically done from the exit gases of third bed.

Indicate equilibrium curve after absorption in the below plot.

Benefits of removing SO3 are:

1.

2.

A typical temperature profile in a four-bed SO2 converter is shown below. Inter-stage

cooling is not shown here.

90

91

92

93

94

95

96

97

98

99

100

660 680 700 720 740 760 780

Equ

ilib

riu

m c

on

vers

ion

Temperature, K

SO2, O2, N2

SO3, SO2, O2, N2

Conversion = 65%

Record below your

observations/interpretations of the data:

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Now that we discussed several aspects of H2SO4 production, let’s draw a block diagram for

it. This diagram should include interconnected boxes with each box representing a

reactor/unit operation. The boxes are to be connected by arrows and they need to be labeled

(indicate compounds).

Study the flow sheet of sulfuric acid plant in the next page. Discuss energy integration of the

plant.

Adapted: Chemical Process Technology, Moulijn et al., 2

nd ed, 2013

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Because of two columns are used for SO3 absorption, the process for H2SO4 manufacture is

called double-contact-double-absorption process. The plant generates 1.3 tons steam per ton

of H2SO4 produced.

Air is fed to the process at the dryer inlet at sufficient flow rate and pressure such that it

moves through all units and exits from the stack. As the pressures needed are ~ 2 bar, a

blower is used for moving the air.

How do you estimate power consumption of the blower for, say, 300 ton/day production of

H2SO4?

How much power is saved if pressure drop across the plant is reduced by 0.1 bar? That is,

the blower develops 1.9 bar pressure.

The blower in a sulphuric acid plant is found to operate at 60% of its rated capacity for most

of the time. That is, only 60% of the rate volume is delivered by the blower. How could this

reduction be achieved? Choose among the three options.

a. Inlet damper

b. Outlet damper

c. Variable speed drive

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In an old sulfuric acid plant, the operator discovered that catalyst dust accumulates over

catalyst beds of SO2 converter. The dust is generated due to wear and tear of the catalyst.

What is (are) the implications of the accumulation?

A) Pressure drop across the bed rises

B) Power consumption rises (of blower)

C) SO2 conversion drops

D) All of the above

You are working as a process engineer in a sulphuric acid plant. Management of your

company decides to raise the production by 15% based on market demand. You are

commissioned to check the feasibility of the increase in the plant. How would you go about

doing it?