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Chemical Engineering Apparatus Design
11/10/2022 CHE. Apparatus Design

Introduction
Almost all chemical processes contain three operations.
1) upstream
process
(pretreatment )
2) Chemical process
(reaction )
3) downstream
process
(separation )
Raw
material
Target
product
By products

Chemical Engineering Equipment
 Chemical engineering apparatuses are equipment used in the chemical
processes, and they can be divided into two classes:
 Proprietary, such as pumps, compressors, filters, centrifuges and
dryers, is designed and manufactured by specialist firms. They are
designed by the manufacturer to meet standard performance
specifications.
 Nonproprietary/custom designed equipment is designed as special,
one-off, items for particular processes; for example, reactors, distillation
columns and heat exchangers.
 Unless employed by one of the specialist equipment manufacturers, the
chemical engineer is not normally involved in the detailed design of
proprietary equipment.
11/10/2022 CHE. Apparatus Design 2

Cont.
 The Chemical engineering’s job will be to select and specify the
equipment needed for a particular duty; consulting with the vendors to
ensure that the equipment supplied is suitable.
 He/she may be involved with the vendor’s designers in modifying
standard equipment for particular applications; for example, a standard
tunnel dryer designed to handle particulate solids may be adapted to dry
synthetic fibers.
 Reactors, columns and other vessels are usually designed as special
items for a given project. In particular, reactor designs are usually unique,
except where more or less standard equipment is used; such as an
agitated, jacketed, vessel.

Cont.
 Distillation columns, vessels and tubular heat exchangers, though non-
proprietary items, will be designed to conform to recognized standards
and codes; this reduces the amount of design work involved.
The chemical engineer’s part in the design of “non-proprietary”
equipment is usually limited to selecting and “sizing” the equipment(they
design the aspects that are significant from process point of view).

Cont.
 For example, in the design of a distillation column, the work will
typically be to determine the number of plates; the type and design of
plate; diameter of the column; and the position of the inlet, outlet and
instrument nozzles.
The information is then transmitted, in the form of sketches and
specification sheets, to the specialist mechanical design group, or the
fabricator’s design team, for detailed design.

Apparatus design
 Determining the aspects of an apparatus such as size and thickness, which
meets requirements (like production rate, safety, etc.) at lowest cost is called
apparatus design.
 Process aspect: what should be the size of the equipment according to the
capacity
 Mechanical aspect /mechanical design: is required for the fabrication,
erection, installation, and commissioning of the equipment. In mechanical
design, the thickness of the material is calculated. No process related
parameters are considered except the process conditions such as pressure
and temperature.
 This course covers more of the mechanical design of chemical engineering
apparatus.

Cont.
 Chemical engineering apparatus is designed to meet national or
international codes and standards, and it involves selection of
appropriate material of construction.
 On march 20, 1905 in Masachusset, there was a boiler explosion, and
58 were died and 117 injured. This catastrophe brought attention to the
need to protect the public against such accidents with pressure-retaining
equipment.
 As a result, A large body of rules has been developed over the years to
provide protection of life and property while assuring a long and useful
service life.

Codes and standards
 A standard is a document that provides engineering or technical
requirements for products, practices, methods or operations for different
fields.
 The purpose of developing and adhering to standards is to ensure
minimum performance, meet safety requirements, make sure that the
product/system/process is consistent and repeatable, and provide for
interfacing with other standard-compliant equipment.
 Since standards are easy to recognize and reference, they help
organizations ensure that their products or services can be manufactured,
implemented and sold around the world.
 serves as a language to give a common understanding about design of
standard equipment.

Cont.
 A code is a standard that has been enacted into law by local, regional,
or national authorities having jurisdiction so that people like engineers or
building contractors are legally compelled to comply with it.
 The code may be an industry or government based standard.
 The main purpose of codes is to protect the public by setting up the
minimum acceptable level of safety for products and processes.

Cont.
 All of the developed countries and many of the developing countries have
national standard organizations, which are responsible for the issue and
maintenance of standards for the manufacturing industries and for the
protection of consumers.
The principal ones of interest to chemical engineers are
 The American Petroleum Institute (API),
 The American Society for Testing Materials (ASTM),
 The American Society of Mechanical Engineers (ASME) (pressure vessels and pipes)
 The National Fire Protection Association (NFPA; safety)

 The safe design and operationof facilities have paramount importance to every company that is
involved in the manufacture of fuels, chemicals, equipment, and pharmaceutical products.
 All manufacturing processes are to some extent hazardous, but in chemical processes there are
additional, special, hazards associated with the chemicals used and the process conditions.
 The designer must be aware of these hazards and ensure, through the application of sound
engineering practice, that the risks are reduced to acceptable levels.
 Safety in process design can be considered
1. Identification and assessment of the hazards.
2. Control of the hazards: for example, by containment of flammable and toxic materials.
3. Control of the process: preventionof hazardous deviations in process variables (pressure, temperature, flow) by
provision of automaticcontrol systems,interlocks, alarms,and trips, together with good operating practices and
management
11
Safety and integrity of equipment

 The basic process control system should be designed to maintain the plant under safe conditions
of temperature, pressure, flow rates, levels, and compositions.
 In most continuous plants, the process control system will attempt to maintain the process within
reasonable bounds of a steady-state condition.
 If a process variable falls outside of the safe operating range, this should trigger an automatic alarm in
the plant control room. In the event that process operators are unable to bring the process back into
control when there is a significant deviation of a variable that indicates a hazardous condition, an
automatic shutdown of the process (also known as a “trip”) should be activated.
 Most of the materials used in the manufacture of chemicals are poisonous, to some extent. The
potential hazard will depend on the inherent toxicity of the material and the frequency and duration
of any exposure
 The toxicity, flammability and corrosivity of a material has to be considered while the process and
equipment are designed.
12

 Over-pressure, a pressure exceeding the system design pressure, is one of the most
serious hazards in chemical plant operation. Failure of a vessel, or the associated
piping, can precipitate a sequence of events that culminate in a disaster.
 Pressure vessels are invariably fitted with some form of pressure- relief device, set at
the design pressure, so that (in theory) potential over-pressure is relieved in a
controlled manner.
 Excessively high temperature, over and above that for which the equipment was
designed, can cause structural failure and initiate a disaster.
 High temperatures can arise from loss of control of reactors and heaters; and,
externally, from open fires.
13

• In the design of processes where high temperatures are a hazard,
protection against high temperatures is provided by:
1.Provision of high-temperature alarms and interlocks to shut down reactor
feeds, or heating systems, if the temperature exceeds critical limits.
2. Provision of emergency cooling systems for reactors, where heat continues to
be generated after shut-down.
3. Structural design of equipment to withstand the worst possible temperature
excursion.
4. The selection of intrinsically safe heating systems for hazardous materials
14

Selection of materials of construction
 The process designer will be responsible for recommending materials that will be
suitable for the process conditions and must also consider the requirements of
the mechanical design engineer
 The most economical material that satisfies both process and mechanical
requirements should be selected; this will be the material that gives the lowest
cost over the working life of the plant.

Cont.
 The most important characteristics to be considered when selecting a material of
construction are:
1. Mechanical properties
2. Effect of Temperature on mechanical properties
3. Corrosion resistance
4. Special properties required; such as thermal conductivity, electrical resistance,
magnetic properties etc.
5. Ease of fabrication
6. Availability in standard sizes- tubes, sections, plates
7. Cost
8. Contamination

Mechanical properties
a) Tensile stress/strength: the measure of the basic strength of a material.
It is the maximum stress that a material can withstand before breaking,
measured by a standard tensile test.
b) Stiffness: the ability to resist bending and buckling
c) Toughness: this is associated with tensile strength, and is a measure of
material’s resistance to crack propagation/fracture.
d) Hardness: a measure of material’s ability to resist wear. This will be an
important property if the equipment is being designed to handle
abrasive solids, or liquids containing suspended solids which are likely
to cause erosion.
e) Fatigue: Weakness in materials caused by repeated variations of stress.
Fatigue failure is likely to occur in equipment subject to cyclic loading;
for example, rotating equipment, such as pumps and compressors
11/10/2022 Chemical Engineering apparatus Design 17

Cont.
f. Creep: Creep is the gradual extension/deformation of a material under
a steady tensile stress, over a prolonged period of time.
It is usually only important at high temperatures; for instance, with steam
and gas turbine blades.
For a few materials, notably lead, the rate of creep is significant at
moderate temperatures. Lead will creep under its own weight at room
temperature, and lead linings must be supported at frequent intervals.
 The creep strength of a material is usually reported as the stress to
cause rupture in 100,000 hours, at the test temperature.

Cont.
The effect of temperature on mechanical properties
Tensile strength and elastic modulus of a material decreases with increasing
temperature.
 For example, the tensile strength of mild steel (low carbon steel, C < 0.25
per cent) is 450 N/mm2 at 25 ºC falling to 210 at 500 ºC.
If an equipment is being designed to operate at high temperatures, materials
that retain their strength at high temperature are selected. The stainless steels
are superior in this respect to plain carbon steels.

Cont.
 Creep resistance is important if the material is subjected to high stresses
at elevated temperatures. Special alloys, such as Inconel (International
Nickel Co.), are used for high temperature equipment such as furnace
tubes.
 At low temperatures, less than 10 ºC, metals that are normally ductile
can fail in a brittle manner. Serious disasters have occurred through the
failure of welded carbon steel vessels at low temperatures.

Cont.
 The phenomenon of brittle failure is associated with the crystalline
structure of metals.
 Metals with a body-centered-cubic (bcc) lattice are more liable to brittle
failure than those with a face-centered-cubic (fcc) or hexagonal lattice.
 For low-temperature equipment, such as cryogenic plant and liquefied-gas
storages, austenitic stainless steel (fcc) or aluminum alloys (hexagonal )
should be specified, and this could be due to difference in arrangement of
atoms/packing density.

Cont.
Corrosion resistance
The conditions that cause corrosion can arise in a variety of ways.
General wastage of material (uniform corrosion)
 Galvanic corrosion dissimilar metals in contact.
 Pitting - localized attack.
 Intergranular corrosion-corrosion of material at the grain (crystal) boundaries.
 Stress corrosion – corrosion due to stress
 Erosion corrosion- increased rate of attack caused by combination of erosion
and corrosion.
 High temperature oxidation.
 Hydrogen embrittlement- loss of ductility caused by the absorption
(and reaction) of hydrogen in a metal.

Cont.
Rate of uniform corrosion
Uniform corrosion describes the more or less uniform wastage of material by
corrosion, with no pitting or other forms of local attack.
If the corrosion of a material can be considered to be uniform, the life of the
material in service can be predicted from experimentally determined corrosion
rates.
 Corrosion rates are usually expressed as a penetration rate in inches per year,
or mills per year (mpy), where 1 inch= 1000 mills.
They are also expressed as a weight loss in milligrams per square decimeter
per day (mdd).
 In corrosion testing, the corrosion rate is measured by the reduction in
weight of a specimen of known area over a fixed period of time.

Cont.
Table 3.1. acceptable corrosion rates for inexpensive alloys(for expensive
numbers, the rates should be divided by two (Coulson, volume 6)).
Corrosion units in imperial unit

Cont.
 The corrosion rate is dependent on the temperature and concentration of the
corrosive fluid.
An increase in temperature usually results in an increased rate of corrosion; though
not always.
The effect of concentration can also be complex. For example, the corrosion of
mild steel in sulfuric acid, where the rate is unacceptably high in dilute acid and at
concentrations above 70 per cent, but is acceptable at intermediate concentrations
The rate also depends on other factors that are affected by temperature, such as
oxygen solubility.

Cont.
Selection for corrosion resistance
 In order to select the correct material of construction, the process
environment to which the material will be exposed must be clearly defined.
Additional to the main corrosive chemicals present, the following factors must
be considered:
1. Temperature affects corrosion rate and mechanical properties
2. Pressure
3. pH.
4. Presence of trace impurities
5. The amount of aeration
6. Stream velocity and agitation- erosion-corrosion.
7. Heat-transfer rates
 The conditions that may arise during abnormal operation, such as at start-up
and shutdown, must be considered, in addition to normal, steady state,
operation.

Material costs
An indication of the cost of some commonly used metals is given in
Table 3.2. The actual cost of metals and alloys will fluctuate quite widely,
depending on movements in the world metal exchanges.
Table 3.2.

Cont.
The quantity of a material used depends on the material density and strength
(design stress), and these must be taken into account when comparing material
costs.
Moore (1970), compares costs by calculating a cost rating factor defined by
the equation:
 He calculated cost ratings, relative to the rating for mild steel (low carbon).
Materials with a relatively high design stress, such as stainless and low alloy
steels, can be used more efficiently than carbon steel.

Cont.
 The relative cost of equipment made from different materials depends on the cost
of fabrication, as well as the basic cost of the material.
If the corrosion rate is uniform, then the optimum material can be selected by
calculating the annual costs for the possible candidate materials. The annual cost will
depend on the predicted life, calculated from the corrosion rate, and the purchased
cost of the equipment.
 In a given situation, it may prove more economic to install a cheaper material with a
high corrosion rate and replace it frequently; rather than selecting a more resistant
but more expensive material.

Cont.
 The statement at the end of the previous slide works for simple
equipment with low fabrication cost where premature failure does not
cause serious hazard.
 For example, carbon steel could be specified for an aqueous effluent
line in place of stainless steel, accepting the probable need for
replacement. The pipe wall thickness would be monitored in situ
frequently to determine when replacement was needed.
 The more expensive, corrosion-resistant, alloys are frequently used as a
cladding on carbon steel.
 If a thick plate is needed for structural strength, as for pressure vessels,
the use of clad materials can substantially reduce the cost.

Cont.
 The clad materials:
 increase mechanical strength
 improve resistance to cracking during increased
temperature change
 reduce water absorption
 increase resistance to sunlight
 provide resistance to air and chemical pollution
 offer protection against rain, strong winds, and molds.

Cont.
Contamination(refer Coulson and Richardson)
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Cont.

Cont.
Surface finish
 In industries such as the food, pharmaceutical, biochemical, and textile
industries, the surface finish of the material is as important as the choice
of material, to avoid contamination.
 Stainless steel is widely used, and the surfaces, inside and out, are given
a high finish by abrasive blasting and mechanical polishing.
 This is done for the purposes of hygiene; to prevent material adhering
to the surface; and to aid cleaning and sterilization.

Commonly used materials of construction
Iron and steel : Low carbon steel (mild steel) is the most commonly
used engineering material. It is cheap; is available in a wide range of
standard forms and sizes; and can be easily worked and welded.
It has good tensile strength and ductility. The carbon steels and iron are
not resistant to corrosion, except in certain specific environments, such
as concentrated sulfuric acid and the caustic alkalis.

Cont.
 Mild steel is susceptible to stress-corrosion cracking in certain
environments.
The high silicon irons (14 to 15 per cent Si) have a high resistance to
mineral acids, except hydrofluoric acid.
They are particularly suitable for use with sulfuric acid at all
concentrations and temperatures. They are, however, very brittle.

Cont.
Stainless steel: The stainless steels are the most frequently used
corrosion resistant materials in the chemical industry.
To impart corrosion resistance, the chromium content must be above 12
per cent, and the higher the chromium content, the more resistant is the
alloy to corrosion in oxidizing conditions.
Nickel is added to improve the corrosion resistance in non-oxidizing
environments.
Types:
i. Ferritic: 13 -20 per cent Cr, < 0.1 per cent C, with no nickel
ii. Austenitic: 18 -20 per cent Cr, > 7 per cent Ni
iii. Martensitic: 10 -12 per cent Cr, 0.2 to 0.4 per cent C, up to 2 per
cent Ni.

Cont.
Nickel: Nickel has good mechanical properties and is easily worked. The
pure metal (> 99 percent) is not generally used for chemical plant, but its
alloys are preferred for most applications.
 The main use is for equipment handling caustic alkalis at temperatures
above that at which carbon steel could be used; above 70 °C. Nickel is
not subjected to corrosion cracking like stainless steel.

Cont.
Monel: Monel, the classic nickel-copper alloy with the metals in the ratio 2 : 1,
is probably, after the stainless steels, the most commonly used alloy for chemical
plant. It is easily worked and has good mechanical properties up to 500 ºC.
 It is more expensive than stainless steel, but is not susceptible to stress-
corrosion cracking in chloride solutions.
 Monel has good resistance to dilute mineral acids and can be used in reducing
conditions, where the stainless steels would be unsuitable. It may be used for
equipment handling, alkalis, organic acids and salts, and sea water.
Inconel: Inconel (typically 76 per cent Ni, 7 per cent Fe, 15 per cent Cr) is used
primarily for acid resistance at high temperatures. It maintains its strength at
elevated temperature, and is resistant to furnace gases, if sulfur free.

Cont.
Copper and copper alloys
 Pure copper is not widely used for chemical equipment. It has been used
traditionally in the food industry, particularly in brewing. Copper is a relatively soft,
very easily worked metal, and is used extensively for small-bore pipes and tubes.
 The main alloys of copper are the brasses, alloyed with zinc, and the bronzes, alloyed
with tin. Other, so-called bronzes are the aluminum bronzes and the silicon bronzes.
 Copper is attacked by mineral acids, except cold, dilute, unaerated sulfuric acid. It is
resistant to caustic alkalis, except ammonia, and to many organic acids and salts.
 The brasses and bronzes have a similar corrosion resistance to the pure metal. Their
main use in the chemical industry is for valves and other small fittings, and for heat-
exchanger tubes and tube sheets.
 The cupro-nickel alloys (70 per cent Cu) have a good resistance to corrosion-erosion
and are used for heat-exchanger tubes, particularly where sea water is used as a
coolant.

Cont.
Aluminum and its alloys
 Pure aluminum lacks mechanical strength, but has higher resistance to corrosion than its
alloys.
 The main structural alloys used are the Duralumin (Dural) range of aluminum copper
alloys (typical composition 4 per cent Cu, with 0.5 per cent Mg) which have a tensile strength
equivalent to that of mild steel.
The pure metal can be used as a cladding on Dural plates, to combine the corrosion
resistance of the pure metal with the strength of the alloy.
 The corrosion resistance of aluminum is due to the formation of a thin oxide film (as with
the stainless steels). It is therefore most suitable for use in strong oxidizing conditions.
 It is attacked by mineral acids, and by alkalis; but is suitable for concentrated nitric acid,
greater than 80 per cent. It is widely used in the textile and food industries, where the use
of mild steel would cause contamination. It is also used for the storage and distribution of
demineralized water.

Plastics as a materials of construction
Plastics are being increasingly used as corrosion-resistant materials for
chemical plant construction. They can be divided into two broad classes:
1. Thermoplastic materials, which soften with increasing temperature; for
example, polyvinyl chloride (PVC) and polyethylene.
2. Thermosetting materials, which have a rigid, cross-linked structure; for
example, the polyester and epoxy resins.
 The mechanical strength and operating temperature of plastics are low
compared with that of metals. The mechanical strength, and other properties,
can be modified by the addition of fillers and plasticizers.

CERAMIC MATERIALS (SILICATE MATERIALS)
 Ceramics are compounds of non-metallic elements and include the following
materials used for chemical plant:
Glass, the borosilicate glasses (hard glass).
Stoneware.
 Acid-resistant bricks and tiles
 Refractory materials
 Cements and concrete
 Ceramic materials have a cross-linked structure and are therefore
brittle.
Ceramic materials have a cross-linked structure and are therefore brittle.

Cont.
 other commonly used materials include:
 lead
 titanium
 tantalum
 zirconium
 silver
Gold (costly and rarely used as material of construction)
 platinum etc.
 There are so many materials for construction, so the work of a designer
resides on selecting the one that is relatively cheap, corrosion resistant, and
having strong mechanical properties if possible, depending on the intended
application, and the environment in which the material is exposed to.

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