Allunitoperations 131201070542-phpapp01 (1)

An Overview of Chemical
Process Technology

Chemical Industry
1.

Structure of the Chemical Industry

2.

Raw Materials and Energy

3.

Chemical Processes

Unit 2: An Overview of Process Technology

Raw materials are converted into products for
other industries and consumers.
 Basic raw materials can be divided into:
Organic
Inorganic
 Inorganic raw materials include:
Air
Water
Minerals.
 Fossil fuels and biomass belong to the class of
organic raw materials.



About 85% of chemicals are produced from
~ 20 simple chemicals called base chemicals.
 Base chemicals produced from ~ 10 raw
materials.
 Base
chemicals converted to ~ 300
intermediates.
 About
30,000 consumer products are
produced from intermediates.



Where these chemicals go..






12 % of the cost of a car
polyurethane seat cushions;
neoprene hoses and belts;
airbags and nylon seat restraints
10 % of the cost of a house
including the cost of important insulation
pipes
electrical wiring
10 % of what the average household consumer buys
and uses every day
food products
clothing
health and personal care products
household cleansers

First step in petrochemical industry is
conversion of raw materials into base
chemicals.
 Synthesis gas (H and CO) through steam
2
reforming of NG → ammonia or methanol.
 Lower alkenes through steam cracking of
ethane or naphtha: ethene, propene,
butadiene.
 Aromatics through steam cracking of ethane
or naphtha or the catalytic reforming process:
benzene, toluene, xylenes (‘BTX’).



The second step involves a variety of
chemical processes often aimed at
introducing various hetero-atoms (O, Cl, S
etc.) into the molecule.
 This leads to formation of intermediates such
as: acetic acid, formaldehyde, acetaldehyde
and monomers like acrylonitrile etc.
 The third step yields consumer products.



Lower Alkenes from oil


Lower Alkenes from NG


Aromatics production


Ammonia and methanol production


Making Sense of Process Technology


Unit Operations or “Unit ops” Concept




Each chemical process can be broken down
into a series of steps (operations)
Individual operations have common
techniques – based on the same scientific
principles


Behind the Complex Appearance,
Chemical Manufacturing is Simple
 Raw

materials are mixed and/or reacted to
create useful products
 These products are separated in one or
more steps
 Between each step, process streams may be
heated or cooled to optimum temperatures
 In some cases, products may be
mechanically processed to convenient form
for transport and use

Introduction to Chemical Reactors


Reactor basics
A + B  C (+ byproducts) (+ unreacted A & B)
 Every reaction is governed by:
reaction stoichiometry
reaction equilibrium (maximum conversion)
rate of reaction



Some Common Reactor Types
 Batch
 Semi-batch
 Continuous

Packed bed
Fluidized bed
 Membrane Reactors
 Bioreactors


Heat Transfer Equipment


Heat Transfer Operations






Needed to heat or cool reactants and/or products
control of process conditions
recovery of process heat
cooling (“quenching”) of product
to effect phase change
Can be stand-alone or integrated with other unit
operation
reactor heat/cooling
distillation reboiler/condenser
May use either radiative or convective heat
exchange

Blast furnace: air is
blowing into the furnace
to maintain temp. that is
higher than 800°C.
Melting iron flows into
ladle.
Continuous casting can
be achieved by cooling
the molten iron in a
convey system. Precise
temperature control is
critical to the quality of
the final product.

Heat exchanger description
 Device

that facilitate the exchange of
heat between two fluids that are at
different temperature without allowing
them to mix.


Heat exchanger Types


Most heat exchangers are classified in one of
several categories on the basis of configuration of
the fluid flow path through heat exchanger.

 Double-Pipe Exchangers
 Compact Exchangers
 Shell and Tube Exchangers
 Plate and Frame Exchangers


Shell and tube heat exchanger


 The

advantage of this type are:

 The

configuration gives a large surface area
in small volume.
 Easily cleaned.
 Can be constructed from a wide range of
materials.


 Fluid








location: shell or tube

Corrosive fluid
Fouling fluid
Higher temperature
Higher pressure
More viscous
Low Flow rate

Tube
Tube
Tube
Tube
Shell
Shell




Shell and tube fluid velocities
For Tube (1-2) m/s
For Shell (0.3-1) m/s

 The

closer the approach temperature
used, the larger will be the heat transfer
area required.
Minimum approach temperature = 10oC



Calculating the ΔTlm

( T1 − t 2 ) − ( T2 − t1 )
∆Tlm =
( T1 − t 2 )
ln
( T2 − t1 )




Correction to LMTD

(T1 −T2 )
R=
(t 2 −t1 )

(t 2 −t1 )
S =
(T2 −t1 )

 (1 − S ) 
( R 2 + 1) Ln 
(1 − RS ) 


Ft =
 2 − S R + 1 − ( R 2 + 1)
( R − 1) Ln 
 2 − S R + 1 + ( R 2 + 1)


(
(

) 
) 






Calculating the ΔTlm

∆Tm = Ft × ∆Tlm

Selecting a trial value for the overall heat
transfer coefficient U

 Calculating

the heat transfer area A

Q
A=
U × ∆Tm


 Compute

film coefficients and clean over-all
coefficient:
1
1 Do ln( Do Di ) Do
= +
+
U oc ho
2k m
Di hi

 Calculating

coefficient

tube

side

heat

transfer

K
hi =
jh Re Pr 0.33
di






Calculating
coefficient

shell side heat transfer
K
ho =
jh Re Pr 0.33
de

If fouling resistance is large enough, the
exchanger is suitable for the service
1
1
rf =
−
U oD U oc











Pressure drop
Tube-side

Shell-side

∆P
t


= N 8× j
f
p



 ρu 2
L
t
+ 2.5

di
 2


2
D
s )( L )( ρu s )
∆P = 8 × j (
s
f d
l
2
e
B

L = tube length, m
LB= baffle spacing
Np =number of tube-side passes
u = tube-side velocity

Fouling In Heat Exchangers
 Fouling

will reduce heat transfer and
increase the pressure drop across the heat
exchanger.


 Crystallization

is one of the most common
type of fouling. Certain salts commonly
present in natural waters have a lower
solubility in warm water than cold.
 Sedimentation, the depositing of dirt, sand,
rust, and other small matter is also common
when fresh water is used.
 Biological Organic growth material occurs
from chemical reactions, and can cause
considerable damage when built up.

 Chemical

Reaction Coking appears where
hydrocarbon deposits in a high temperature
application.
 Corrosion can destroy surface areas of the
heat exchangers, creating costly damage.
 Freezing Fouling results from overcooling
at the heat transfer surface causing
solidification of some of the fluid stream
components.

Summary
 Shell-and-tube

heat exchangers provide
large heat transfer area in a compact
space
 Fluid velocities are controlled by the
number of tube passes and baffle
spacing
 LMTD must be corrected for the
parallel/countercurrent patterns
 Select
and rate exchangers by
determining the available fouling
resistance

Separations Equipment


Separations






Most chemical reactions are not complete (some
unreacted inputs remain)
Side reactions may result in one or more
unwanted (or desired) byproducts
Separations needed to obtain purified product to
be used by
customers or
downstream
manufacturers


Mechanism of Separation
Some properties of importance are:
1. Molecular properties
Molecular weight
van der Waals volume
van der Waals area
Molecular shape

Polarizability
Dielectric constant
Electric charge
Dipole moment

2. Thermodynamic and transport properties
Vapor pressure
Adsorptivity
Solubility
Diffusivity

Separations Processes






Gas-Liquid
Distillation (single stage=“flash”)
Evaporation
Gas Absorption or Stripping
Liquid-Liquid
Liquid extraction
Supercritical fluid
Solid-fluid
Filtration
Adsorption and ion exchange
Crystallization
Drying
Leaching


Flash Separation


Gas, oil and water separation was
achieved by the difference in gravity, or
weight, of each fluid.


Three general types of separators
 Horizontal

separator for high-pressure and
medium-pressure service

 Vertical

separator for low-pressure service
(generally)



Spherical separator more compact and
cheaper limited separation space and liquid
surge capacity
for low-volume remote
platforms


Three general types of separators


Separator Sizing
 Fluid

physical properties required for

sizing:
Density for liquid and vapor phases
Operating pressure
Volumetric flow rate of vapor and
liquid phases


Separator Sizing


The settling velocity of liquid droplets
 ρ L − ρ v 

ut = 0.07 
 ρ

v





1/ 2

Relation between operating pressure and
Lv/Dv
Operating
L /D
pressure (bar)
0-20
20-35
>35

v

v

3
4
5


Distillation
 Separates

liquids having differing
boiling points
 Can separate solutions where all
components are appreciably volatile
(fractionation)
 Mixture heated to boiling of most
volatile component (i.e. lowest boiling
point), compound becomes gaseous,
then condensed again in attached
vessel.


Equipment

tray tower centrifugal contactor
bubble column

packed column
spray tower


Tray Towers


Tray Towers

(a) perforation

(b) valve cap

(c) bubble cap

(d) Tray with valve caps


Tray Towers

(a) Spray

Liquid carries no vapor bubbles
Froth
to the tray below
Vapor carries no liquid droplets
to the tray above
No weeping of liquid through the
openings of the tray
(b) Froth (c) Emulsion (d) Bubble (e)Cellular Foam
Equilibrium between the exiting
vapor and liquid phases
is approached on each tray.


Tray efficiency
stage efficiency is the performance of a
practical contacting stage to the theoretical
equilibrium stage.
Murphree plate
efficiency is the ratio of the actual
separation achieved to that which would be
achieved in an equilibrium stage
EO =

number of ideal stages
number of real stages

Packed Columns

Figure 6.6 Details of internals
used in a packed column


Packing Materials

More surface area for mass transfer
Higher flow capacity
Lower pressure drop

(a) Random Packing
(b) Structured Packing
Materials
Materials
•Expensive
•Far less pressure drop
•Higher efficiency and capacity


Absorption


General Design Considerations
 Design

or analysis of an absorber (or stripper)
requires consideration of a number of factors,
including:

1.

Entering gas flow rate, composition,
temperature, and pressure
2. Desired degree of recovery of one or more
solutes
3. Choice of absorbent (stripping agent)
4. Operating pressure and temperature, and
allowable gas pressure drop

General Design Considerations
5. Minimum absorbent (stripping agent) flow rate
and actual absorbent (stripping agent) flow rate
as a multiple of the minimum rate needed to
make the separation
6. Number of equilibrium stages
7. Heat effects and need for cooling (heating)
8. Type of absorber (stripper) equipment
9. Height of absorber (stripper)
10. Diameter of absorber (stripper)

Liquid-Liquid Extraction
Definition
-

Separation of one or more component in a
mixture by contacting with another phase
which is liquid

-

Such a process is also referred to as liquid
extraction or solvent extraction

Liquid-Liquid Extraction


3 stages are involved in the process i.e.:
i) bringing the feed mixture and the solvent
into intimate contact
ii) separation of the resulting two phases,
and
iii) removal and recovery of the solvent from
each phase

Equipment
A) Mixer settlers for extraction








Mechanical mixer is often used to provide efficient
mass transfer between two liquid phases.
One phase is usually dispersed into the other in the
form of small droplets.
Small droplets provide large interfacial areas and
faster extraction.
The droplets must not be too small as larger settling
time will be required.

Equipment


Equipment
B) Spray columns
Either the light or the heavy phase may be dispersed.
 In (a), the light phase enters from a distributor at the
bottom of the column, rises through a heavier phase
and finally coalesces to form liquid interface at the top
of column.
 In (b) the heavier phase is dispersed, where the
interface is held at the bottom of the tower.



Equipment
C) Perforated plate/ sieve tray extraction tower
The rising droplets of the light solvent liquid are
dispersed
 The dispersed droplets coalesce below each tray and
are then returned on each tray by passing through the
perforations
 Heavy aqueous liquid flows across each plate, where
it is contacted by the rising droplets and then passes
through the downcomer to the plate below



Equipment
D) Packed column
Packing such as Raschig rings or Berl saddles cause
droplets to coalesce and redisperse at frequent
intervals throughout tower

Packing increases the interfacial area and
considerably increases mass transfer rates compared to
spray columns
 Unsuitable for use with suspensions or high viscosity
liquids



Example of extraction process50/50 acetone/water mixture


Liquid-Liquid Extraction Unit


Supercritical Fluid Extraction
 Lower

viscosity than liquid
 Gas like permeation of solid structures
 Higher density than gas
 Higher solubility than gas


Supercritical Fluid Extraction with CO2
 Replace

Organic Solvents with CO2

 Substances

easily degraded by heat can
be extracted
 Non-toxic for use in food products Nonreactive and Non-flammable Easy
processing
 Environmentally safe

Example Process Flow


After Extraction Process
 Change

Conditions

to

alter

Phase

Behavior
 Reduce Solubility by lowering the
pressure
 Equilibrium Phase comes out of solution


Filtration
 The

separation of solids from a
suspension in a liquid by means of a
porous medium or screen which retains
the solids and allows the liquid to pass is
termed filtration.


Filtration


Filtration
Filter Cake
The particles suspended in the fluid, which will not
pass through the apertures, are retained and build
up into what is called a filter cake..
 Pre-Coat
Thin preliminary coat of cake, or of other fine
particles, is put on the cloth prior to the main
filtration process. This preliminary coating is put
on in order to have sufficiently fine pores on the
filter and it is known as a pre-coat.



Classification of Filtration
Filtration can be classified
1) Based on Operating cycle
(a) Batch
(b) Continuous
2) Based on pressure drop
(a) Constant Rate Filtration
(b) Constant Pressure Filtration

Factors Considered while
Selecting Filtration Equipment
(a) The properties of the fluid, particularly
its viscosity, density and corrosive
properties.
(b) The nature of the solid, its particle size
and shape, size distribution, and packing
characteristics.
(c) The concentration of solids in
suspension.

Factors Considered while
Selecting Filtration Equipment
(d) Whether the valuable product is the
solid, the fluid, or both.
(e) Whether it is necessary to wash the
filtered solids.
(f) Whether the feed liquor may be
heated.
(g) Whether any form of pre-treatment
might be helpful.

Factors Affecting Rate of filtration
(a) The drop in pressure from the feed to
the far side of the filter medium.
(b) The area of the filtering surface.
(c) The viscosity of the filtrate.
(d) The resistance of the filter cake.
(e) The resistance of the filter medium and
initial layers of cake.

Rotary Drum Filter
 In

rotary filters, the flow passes through a
rotating cylindrical cloth from which the
filter cake can be continuously scraped.
Either pressure or vacuum can provide the
driving force, but a particularly useful form
is the rotary vacuum filter. In this, the cloth
is supported on the periphery of a
horizontal cylindrical drum that dips into a
bath of the slurry.

Rotary Drum Filter
 Vacuum

is drawn in those segments of the
drum surface on which the cake is building
up. A suitable bearing applies the vacuum
at the stage where the actual filtration
commences and breaks the vacuum at the
stage where the cake is being scraped off
after filtration. Filtrate is removed through
trunnion bearings.
 Rotary vacuum filters are expensive.

Air Filters


Air Filters are used quite extensively to remove
suspended dust or particles from air streams. The
air or gas moves through a fabric and the dust is left
behind. These filters are particularly useful for the
removal of fine particles. One type of bag filter
consists of a number of vertical cylindrical cloth
bags 15-30 cm in diameter, the air passing through
the bags in parallel. Air bearing the dust enters the
bags, usually at the bottom and the air passes out
through the cloth. A familiar example of a bag filter
for dust is to be found in the domestic vacuum
cleaner.


Biotechnological Application
 Removal

of particles less than 5 mm
diameter in modern air sterilization units

(1) Paper filters
(2) Packed tubular filters


These cover the range of sizes of
bacterial cells.

Adsorption
 Dehydration

of natural gas

 Dehydration

of natural gas is the
removal of the water that is associated
with natural gases in vapor form.
 Natural gas in transit to market should
be dehydrated to a controlled water
content to avoid hydrate as well as to
minimize the corrosion problems.

Adsorption
 Adsorption

(or solid bed) dehydration is
the process where a solid desiccant is
used for the removal of water vapor
from a gas stream. The solid desiccants
commonly used for gas dehydration are
those that can be regenerated and,
consequently, used over several
adsorption-desorption cycles.


Adsorption


95


Adsorbent materials
1. Large surface area for high capacity.
Commercial adsorbents have a surface
area of 500-800 m2/g.
2. High mass transfer rate, i.e., a high rate
of removal.
3. Easy, economic regeneration.
4. Small resistance to gas flow, so that the
pressure drop through the dehydration
system is small.

Adsorbent materials
5. High mechanical strength to resist
crushing and dust formation. The
adsorbent also must retain enough
strength when "wet".
6. Cheap, non-corrosive, non-toxic,
chemically inert, high bulk density, and
small volume changes upon adsorption
and desorption of water.


Adsorbent materials

1.
2.

3.
4.

5.

6.

Silica gel is a widely used desiccant.
Best suited for normal dehydration of natural gas
Excellent for hydrocarbon liquid recovery such as
recovery of pentanes and heavier hydrocarbons
from a sweet lean gas stream
Easily regenerated
Has higher capacity than activated alumina, but
lower than molecular sieve
Costs more than alumina, but less than molecular
sieve
Capable of dew points to –100°F.

Adsorbent materials


1.

2.

3.

4.

5.

Molecular sieve is the most versatile adsorbent
because it can be manufactured for a specific pore size,
depending on the application.
Capable of dehydration to less than 1 ppm water
content
The overwhelming choice for dehydration prior to
cryogenic processes
Excellent for H2S removal, CO2, dehydration, high
temperature dehydration, heavy hydrocarbon liquids,
and highly selective removal
More expensive than silica gel and alumina, but offers
greater dehydration
Requires higher temperatures for regeneration


Adsorbent materials


1.

2.

3.

Activated alumina dehydrates natural gas or
liquids to dew points of –100°F or lower.
Ideal for sweet, lean hydrocarbon gas drying or
LPG drying
Less expensive than silica gel or molecular sieve,
but has a higher bulk density and lower capacity
for water
Usually the most economical desiccant within its
range of application


Crystallization


What's Gas Hydrate?

A

gas hydrate is a crystalline solid; its
building blocks consist of a gas
molecule surrounded by a cage of water
molecules. Thus it is similar to ice,
except that the crystalline structure is
stabilized by the guest gas molecule
within the cage of water molecules.

Crystallization


Crystallization
 Inhibitors

Methanol
Ethylene glycol
 Hydrate Applications
 Gas storage: In situations where gas
storage is required, natural gas can be
converted to hydrates and stored at
atmospheric pressure and refrigerated.

Crystallization
Natural gas processing: Natural gas and
associated gas contain a lot of nitrogen, carbon
dioxide and hydrogen sulphide, hydrate
technology can potentially be used to separate
these gases from the source gas.
 Desalination and water treatment: In situations
where saline and brackish water need to be
cleaned, gas hydrates can be produced and
separated from the concentrated solution. This
because gas hydrates consume just water and
gas, not other constituents such as dissolved
salts and biological materials.


Drying Process
 In

drying process heat removes from
solid by heat.

 Spray

dryer
 Rotary dryer
 Tray dryer
 Freeze dryer


Solid-Liquid Extraction
Definition
-

In order to separate the desired solute
constituent or remove an undesirable solute
component form the solid phase, the solid is
contacted with liquid phase

-

Such a process is also referred to as liquid solid
leaching or simply leaching

- In leaching when an undesirable component is
removed from a solid with water, the process is
called washing

Basic Concept


3 distinct processes usually involved in
leaching operations:
dissolving the soluble constituent
 separating the solution , so formed, from
the insoluble solid residue

washing the solid residue in order to
free it of unwanted soluble matter or to
obtain, as much of the soluble material as
possible



Equipment
A) Batch plant for extraction of oil from
seeds
Consists of a vertical cylindrical vessel
divided into two sections by slanting partition
 Upper section is filled with the charge of seeds
which is sprayed with fresh solvent via a
distributor
 Solvent percolates into the bed of solids and
drains into the lower compartment



Equipment
Extraction from cellular materials;
B) Bollman extractor








Series of perforated baskets, arranged as in a bucket
elevator
Solid is fed into top basket on the downward side and
is discharged from the top basket on the upward side
Solvent sprayed on to the solid which is about to be
discarded, and passes downwards
Solvent is finally allowed to flow down through the
remaining baskets in co-current flow

Equipment
C) Dorr rake classifier






Solid is introduced near the bottom of a sloping tank
and is gradually moved up by means of a rake
Solvent enters at the top and flows in the opposite
direction to the solid, and passes under a baffle and
finally discharged over a weir
Operates satisfactorily, provided the solid does not
disintegrate & the solids are given an ample time to
drain before discharged

Materials Handling Equipment


Materials Handling
Pipes, Valves and Connection
 Pumps, compressors
 Storage tanks, containers, and vessels
 Blending and milling (e.g., mix tanks,
grinders)
 Product preparation
(e.g. Packaging stations)



Pump
 Pump

is Machine that uses energy to
raise, transport, or compress fluids.
 Pumps are classified by how they
transfer energy to the fluid.
Centrifugal pump
Positive Displacement Pump
Gear pump
Vacuum pump

Centrifugal Pump
 The

centrifugal pump, the most common
kind, consists basically of a rotating
device, called an impeller, inside a
casing.
 The fluid to be pumped enters the casing
near the shaft of the impeller. Vanes
attached to the spinning impeller give
the fluid a high velocity so that it can
move through an outlet.

Positive Displacement Pump
A

Positive Displacement Pump consisting
of a chamber containing gears, cams,
screws, vanes, plungers or similar elements
actuated by relative rotation of the drive
shaft to the casing and are characterized by
their close-running clearances.
 The reciprocating pump moves a fluid by
using a piston that travels back and forth in
a cylinder with valves to help control the
flow direction.

Gear Pump


Unit Summary
 Despite

diversity
of
processes,
underlying
equipment
and
phenomenology is relatively simple
 “Unit Ops” paradigm helps provide
unifying framework for understanding
process technology
 Each process unit has characteristic
waste and emission sources/causes
 Emissions stem from both intrinsic and
extrinsic causes

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