This document discusses the importance of instrument air quality for automation plant control systems. It describes how instrument air is used to operate pneumatic valves, pumps and other devices to keep the plant running properly. The key parameters for instrument air quality are dew point, oil content, particulates and temperature. It then examines different methods for drying instrument air including chemical drying, refrigeration and adsorption. Adsorption is identified as the most common method used, and it details how adsorption driers work using desiccants like activated alumina, silica gel and molecular sieves. The document presents the objectives and plan to design an air drier package to meet industrial specifications for instrument air quality in a plant.

1. ESSENTIAL FOR CONTROL SYSTEM
OF AN AUTOMATION PLANT
PRESENTED BY – PRESENTED BY -
TAPAS DAS ADHIKARI DIPANKAR MAITY
ROLL NO : 91/PPR/091031 ROLL NO : 91/PPR/091026
PETROLEUM & PETROCHEMICALS REFINARY ENGG.
SECTION
& PETROLEUM & PETROCHEMICALS REFINARY ENGG.
SECTION
DEPARTMENT OF CHEMICAL TECHNOLOGY DEPARTMENT OF CHEMICAL TECHNOLOGY
UNIVERSITY OF CALCUTTA UNIVERSITY OF CALCUTTA

2. INTRODUCTION :
Instrument air is the mixture of N2, O2 & very small amount of dust
(below the required limit) used in the instrument to operate pneumatic valves, certain
types of pumps, fans, some blowing down hoses. The Instrument air in a plant is used
to supply motive force for control valves & that keeps the plant in control and running.
Instrument air is often specially dried to reduce the risk of condensation freezing-out
in the small-bore piping. To maintain the above situation in the plant Instrument air
must be dried to remove any moisture and/or condensate for:
 Protecting the instruments and control system from damage.
 Obtaining exact readings through these instruments and control system.
So the instrument air supply should be of highest quality.

3. Instrument Air Quality:
The quality of instrument air is what distinguishes it from a compressed or
service air system. The quality of the air is important to ensure that instrumentation
will function properly and reliably. The most important parameters in specifying air
quality are:
 Dew Point
 Oil Content
 Particulates
 Temperature
The Instrument Society of America sets quality standards for instrument
air in ISA S7.3.

4. Different pieces of equipment consume different amounts of air. For example, a
shutdown valve will consume air when it is being actuated. A throttling valve will
have a constant bleed rate with additional consumption when the valve is
modulating. A diaphragm pump consumes air when it is being actuated.
Instrument air is provided by a compressor and requires minimal treatment to
ensure that the air is free of oil, water or particulate matter. This is usually
accomplished with some type of filter regulator on the compressor outlet and a dryer.

5. There are three general methods of drying air:
 Chemical drying
 Refrigeration.
 Adsorption
Selection of air drying equipment is based upon required dew point, quantity of air
to be dried, pressure of the incoming air, excess air capacity of compressor station,
and utility for electricity, steam & water.

6. Adsorption:
Adsorption is the property of certain porous materials to hold vapours in the pores until the
desiccant is either heated or exposed to a drier gas. Adsorption drier is the common type
used in instrument air system. The material is solid & operates alternately through drying &
regeneration of the deactivated catalyst bed with no change in composition. Adsorption
materials in use are Activated alumina, Silica gel, Molecular sieves. By adsorption method -
400C to -600C dew point of air can be easily achieved.
A typical instrumental air drier package unit includes the following components:
 Air Receiver
 Compressor
 Pre-filter
 Two Air Dryer
 Post-filter
 Local Control Panels

7. The air to be dried alternately passes through one air drier & then to the other. While
one air drier is being used to dehydrate, the other is regenerated. The whole operating cycle
of a drier is equally divided between regeneration & drying steps & is automatically
controlled.
Adsorption method can be classified into two categories on the basis of regeneration
mechanism.
1) Temperature Swing Adsorption
2) pressure swing adsorption
Temperature Swing Adsorption:
In Temperature swing adsorption the adsorbate-loaded bed may be regenerated by hot
purge gas, relatively an inert gas (steam & air are common) to remove adsorbed substances.
The regeneration temperature is selected on the basis adsorption equilibrium or isotherm at
different temperature & on the stability & characteristics of the adsorbent & adsorbate.

8. This technique is called temperature swing since the bed temperature alternates
between the adsorption & regeneration temperatures. Heat requirement for the
regeneration is about 2.5 times the enthalpy of desorption. A hot regenerated bed needs
to be cooled down before it goes for the next adsorption half-cycle. Countercurrent
heating & cooling ensure a low residual solute retention in the bed.
The main disadvantages are:
I. Energy consumption for heating of the bed for the regeneration ;
II. Presence of condensate material in the feed causes various problems
during regeneration ;
III. Cooling of the bed should follow regeneration.

9. Pressure Swing Adsorption:
Pressure swing adsorption (PSA) has evolved as an alternative to the
temperature swing adsorption (TSA) process because of above disadvantages of the
latter. It may be called heat less drying because of the particular application.
Pressure swing adsorption depends on the fact that the adsorption capacity of solid
increases with increasing pressure of the solute. Thus in a pressure swing adsorption
occurs at a higher pressure of the feed gas when it is stripped off the solute. The flow
of the feed at breakthrough & the bed is regenerated by reducing the pressure. At this
stage the adsorbate is recovered in a relatively concentrated form. It is to be noted that
a packed bed of adsorbent responds more rapidly to changes in pressure than to
changes in temperature. On the whole process requires less energy & is operationally
simpler than TSA.

10. The four basic steps of the PSA process according to the Skarstrom cycle are-
 Adsorption- the feed gas (air in this case) flows at a higher pressure through the
bed, the more absorbable component (the solute) gets trapped; the purified gas is
simultaneously drawn as the product.
 Depressurization- the pressure in the vessel is reduced; the adsorbed component
& the residual gas in the bed voids are drawn out as another product (or may be
discarded if it is of no use).
 Purging – a small fraction of the product gas from the other bed is passed through
the vessel as purge gas to remove most of the residual solute from the bed.
 Depressurization – feed is supplied into the adsorption vessel to raise its pressure
to that of the feed (no product is drawn during this step).

11. Study of Adsorbent:
ACTIVATED ALUMINA
Activated alumina is a porous form of aluminum oxide. It has a high surface area which
adsorbs vapors without any change in form. Activated alumina will not soften or disintegrate
easily when immersed in water. Dew points to - 40 F and lower may be achieved with
activated alumina depending on dryer design and operating conditions.
Applications:
 Primarily it is used for air drying & in low temperature applications for getting better
dew point.
 Especially suited for compressed air drying system.
 Extremely effective for air drying gases.
 A highly efficient adsorbent with large porosity & contact surface.
 Stable with respect to physically & chemically even at high temperature & corrosive
environment.

12. Physical properties
1/8” (2-5mm) 3/16”(4-8mm) 1/4” (5-10mm)
Color and form White bead White bead White bead
Bulk density 48 lbs/ft3 48 lbs/ft3 48 lbs/ft3
Crush strength 17-30 lbs 45-60 lbs 50-70 lbs
Surface area 1.74 x 106 sq ft/lb 1.65 x 106 sq ft/lb 1.59 x 106 sq ft/lb
Static Adsorption
Humidity
100% 42.0% 40.0% 36.0%
90% 37.5% 35.0% 32.0%
60% 21.0% 21.0% 17.5%
10% 7.5% 7.0% 6.0%

13. SILICA GEL
Silica gel is a spherical bead consisting of 97-100% silica. It is available in two types:
indicating, a translucent bead impregnat-ed with blue or yellow color and non-
indicating white translucent color.

14. MOLECULAR SIEVE
Molecular sieve is a sodium alumino-silicate. It has a fixed pore size according to
the material specified, 4 Angstrom being the most common.
Because of its higher cost, it is normally used for special process applications.
Pressure dew points to -100 F may be achieved with molecular sieve depending on
the dryer design and operating conditions.

15. OBJECTIVE:
To attain the industrial specification of the instrumental air as close as
possible & design an air drier package of required capacity.
PLAN OF WORK:
Instrumental air must be freed from moisture, dust, oil, mud etc. to
make it usable in the instruments of the plant. For which naturally available
air is dried by adsorption method.
 Selection of process
 Selection of desiccant
 Process Flow Diagram preparation

16.  Process Description preparation
 Desiccant size selection
 Desiccant volume calculation
 Calculation of L/D ratio for Adsorber
 Pipe sizing
 Checking of final dew point of the dried air.

17. PROCESS DETAILS:
1) Process Selection
There are several processes available for drying of air as discussed before.
Among these which one will be the preferable that can be suggested only after
pointing out their advantage and disadvantages.
a) Refrigeration
Advantages Disadvantages
i. Very low operating cost. i. Dew point below 3.50C not possible
ii. If there is a chance of presence of
oily particles in the feed air, these oily
particles can form emulsion and plug
water removal traps.

18. b) Chemical Drying
Advantages Disadvantages
i. Low installation cost, low i. Moderate drying
operating cost & high reliability
ii. Desiccant must be replaced periodically
:
. iii. Maximum operating temperature is
: limited to 380C; in fact some desiccant
. materials melt or fuse together at 35-400C.
. iv. Oil must be kept out of the desiccant bed
. v. Most of the chemical absorbents are ruled
.
out for use on instrument air drying system
because of toxicity, corrosiveness, and cost
of regeneration systems

19. c) Adsorption
i) Temperature swing adsorption :
Advantages Disadvantages
i. Very low dew point (-400C to -600C) can i. Regeneration temperature is very
be achieved high (~1900C).High electricity
consumption
ii. High capacity, large cycle length ii. There are some troubles with
usually 6-10 hrs burnout & heating elements hot spots
iii. Air consumption is low, generally 2- iii. If any oil vapour adsorbed with
5% of incoming air water regeneration becomes difficult as
coking may occur

20. ii) Pressure swing adsorption :
Advantages Disadvantages
i. Very low dew point (-400C to -600C) i. Large purge air quantity (10-20% of
can be achieved total incoming air) requires a constant
compressor over-sizing
ii. Heatless process. ii. Moderate capacity
iii. No electricity consumption iii. Cannot be used if system pressure is
below 5 kg/cm2
iv. Any oil vapour adsorbed is easily
removed during regeneration, with no
chance of coking

21. From the above discussion it is clear that Refrigeration and Chemical drying
are not suitable for instrument grade air (at least -400C dew point)
production. Also chemical drying cannot operate above 380C.
To avoid the difficulties due to high temperature application temperature
swing adsorption is not accepted and pressure swing adsorption is the most
favorable choice for moisture removal to produce instrument grade air.
Though its capacity is low, up to a certain limit it is easy to operate.
So pressure swing or heatless adsorption process is selected for air drying to
produce instrument air.

22. 2)Desiccant or Catalyst Selection:
Useful adsorbents/desiccants for water removal are Silica Gel, Activated
Alumina and Molecular Sieve. At first discussion about the advantages and
disadvantages of all these three adsorbents is necessary.
Silica Gel
Advantages Disadvantages
i. The fastest and cheapest i. Silica gel fractures in the presence of
liquid water. It must be protected by a
10% to 15% (by weight) layer of
activated alumina as a water buffer on
the inlet side of the desiccant bed
ii. Uniform bead shape ii. Its selectivity is very low
iii. Provides visual check of desiccant iii. Low abrasive & mechanical/crush
condition (indicating type only) strength

23. Activated Alumina
Advantages Disadvantages
i. High adsorption capacity i. Cannot dry up to very low dew point
as compare to molecular sieve
ii. Low abrasion ii. Its selectivity is moderate. At low
humidity its capacity becomes very low
iii. Resists liquid water iii. Costly than silica gel
iv. High crush strength
v. Uniform bead sizes
vi. Moderate cost

24. Molecular sieve
Advantages Disadvantages
i. High selectivity and can achieve very i. High initial cost, high operating cost
low dew point
ii. Uniform retention capacity ii. Vulnerable to oil
iii. Round bead shape

25. From the above study Activated Alumina is the most suitable
adsorbent for pressure swing adsorption in respect to selectivity, capacity,
compatibility, regenerability, cost & kinetics. Liquid water will not
fracture the alumina bed; it has high crush & abrasive strength which
prevent dust formation, also invulnerable to oil, initial & operating cost is
very small compare to molecular sieve. It can easily attain dew point
required for instrument grade air.
So Activated Alumina is selected as desiccant medium for air drying to
produce instrument grade air

26. 3) Process Flow Diagram:

27. 4) Process Description:
Air is dried by means of adsorption of moisture on Activated alumina bed.
There are two such beds, at any time one adsorbs moisture from saturated
air and another is regenerated (desorbs moisture) by a part of the dry air
using pressure swing method.
At first saturated air is collected from the receiver & compressed in a
compressor at room temperature. Then the compressed stream pass
through a coalescer type pre-filter (001-PG-001A/B) to remove oil &
condensate and send directly through a control valve to the dryer where
adsorption of moisture is done.

28. This compressed air stream is the feed stream which can be fed to either
Adsorber-1(001-C-001) or Adsorber-2(001-C-002). These two dryers with
packed beds of adsorbent are commonly used to serve the purpose of
adsorption of moisture. The adsorbers operate in cycles. when Adsorber-1
receives the feed & operate in adsorption mode, Adsorber-2 receives purge
gas & operates in the regeneration mode, the air after removal of moisture in
Adsorber-1& divided into two streams, one is going through a non returning
valve by means of which dry air from any of these two dryers can be collected
and send to the after-filter (001-AG-001A/B),

29. where final filtration is done and the product air may be send to a
secondary dryer or to the instrumental air header (if the moisture content is
satisfactory) & flow of the other stream is controlled in such a way that 10-
20% passes through the Adsorber-2 at atmospheric pressure when it is in
regeneration mode, after regeneration the stream is vented to the atmosphere
through a flow control valve (001-FC-02). After half cycle, the operation is just
switch over i.e. Adsorber-1 will be in regeneration mode & Adsorber -2 in
adsorption mode.
Total system is controlled by a sequence controller (001-SC-001) with the
help of some flow control valve in each inlet & outlet line of two adsorbers.

30. EXAMPLES :
Instrument Air Packages
Northwest Equipment Ltd. has extensive experience in the
design, construction and commissioning of Instrument Air Packages for
Canadian as well as international installations.
Size - HP Air Compressor Air Dryer Air Receiver (gallon) Air Filtration
Options
2-5 Reciprocating Heatless 60 - 120 Oil Coalescing &
Regenerative Air Particulate Removal
Dryer
5 - 30 Reciprocating, Heatless 120 - 400 Oil Coalescing &
Rotary Vane, Rotary Regenerative Air Particulate Removal
Screw Dryer
30 - 300 Reciprocating, Heatless 240 - 2000 Oil Coalescing &
Rotary Vane, Rotary Regenerative Air Particulate Removal
Screw Dryer

31. 5-30 HP EXAMPLE
TYPICAL PACKAGE
 Duplex rotary vane air compressors
 15 hp TEFC motors
 Local control panel
 120 gallon horizontal air receiver
 Heatless regenerative air dryer

32. 30 - 300 HP Examples
TYPICAL PACKAGE
 Duplex reciprocating air compressors
 TEFC motors
 Local control panel
 Duplex regenerative air dryers
 Sun shade for extreme climate conditions

33. Complete Building Package
A completely self-sufficient instrument air package can be packaged within a
enclosure and shipped to the site for simplified installation and
commissioning.
Even the smallest of air compressor packages can be provided as a complete,
pre-engineered and ready-to-operate system.

34. CONCLUSION:
Instrument grade air should contain very low amount of water (below -400C at
atmospheric pressure) and should not contain any oil & dust particles. From
the above study to ensure the quality of instrument grade air following
sequence is follows
 Pre-filtration to remove oil mists or other condensate by using a
coalescer type filter.
 Drying of air using activated alumina as desiccant by pressure
swing adsorption method.
 After filtration to remove any dust particles.
It is important to maintain the inlet air pressure above 5kgf/cm2 to perform the
pressure swing adsorption method. Also the total pressure drop across the
equipments should not exceed 0.5-0.7 kgf/cm2.

35. BIBLIOGRAPHY
Ullmann’s Encyclopedia of Industrial Chemistry, volume-1, sixth edition.
Kirk Othmer, Encyclopedia of Chemical Technology, volume-1, 18, third edition.
http://www.sulphuric-acid.com/techmanual/Utilities/instrair.htm
Foster wheeler, Off-site manual, plant & instrument air section, August-1983
Binay K. Dutta, Principle of mass transfer & separation processes, fourth printing-2010, PHI-learning
pvt. Ltd., New Delhi.
Adsorbent Desiccants, Air & Vacuum Process, Inc. VAN AIR SYSTEM, info@vanairsystems.com,
2009.
Air driers- info@www.nwequipltd.com , Penton Media, Inc, 2011.
Kent S. Knaebel, Adsorbent Selection, Adsorption Research, Inc, Dublin, Ohio-43016.
Robert E. Treybal, Mass Transfer operations, 3rd Edition, McGraw Hill Book Company.
Atkins, Physical Chemistry, 8th Edtion-2004, Elsevier Science & Technology Books.
Activated alumina balls-SORBED INDIA-info@www.sorbedindia.com
Pressure Swing Adsorption, info@www.pall.com, june-2010, England.
Activated Alumina & Molecular Sieves, Axens, Procatalysts & Adsorbents- info@www.axens.net
 A BeaconMedæs Continuing Education Publication On Instrumental Air mallen@beaconmedaes.com

36. THANK YOU