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INSTRUMENT AIR PACKAGE
PRESENTED BY -DIPANKAR MAITYROLL NO : 91/PPR/091026 PETROLEUM & PETROCHEMICALS REFINARY ENGG. SECTIONDEPARTMENT OF CHEMICAL TECHNOLOGYUNIVERSITY OF CALCUTTA
PRESENTED BY –TAPAS DAS ADHIKARIROLL NO : 91/PPR/091031PETROLEUM & PETROCHEMICALS REFINARY ENGG. SECTIONDEPARTMENT OF CHEMICAL TECHNOLOGYUNIVERSITY OF CALCUTTA
&
ESSENTIAL FOR CONTROL SYSTEM OF AN AUTOMATION PLANT
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.
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.
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.
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.
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
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 adsorptionTemperature 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.
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.
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.
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).
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.
Physical properties1/8” (2-5mm) 3/16”(4-8mm) 1/4” (5-10mm)
Color and form White bead White bead White beadBulk density 48 lbs/ft3 48 lbs/ft3 48 lbs/ft3
Crush strength 17-30 lbs 45-60 lbs 50-70 lbsSurface area 1.74 x 106 sq ft/lb 1.65 x 106 sq ft/lb 1.59 x 106 sq ft/lb
Static AdsorptionHumidity
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%
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.
SILICA GEL
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.
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
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.
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.
1) Process Selection
PROCESS DETAILS:
b) Chemical Drying
: .: . . ..
Advantages Disadvantages
i. Low installation cost, low operating cost & high reliability
i. Moderate drying
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
Advantages Disadvantages
i. Very low dew point (-400C to -600C) can be achieved
i. Regeneration temperature is very high (~1900C).High electricity consumption
ii. High capacity, large cycle length usually 6-10 hrs
ii. There are some troubles with burnout & heating elements hot spots
iii. Air consumption is low, generally 2-5% of incoming air
iii. If any oil vapour adsorbed with water regeneration becomes difficult as coking may occur
c) Adsorptioni) Temperature swing adsorption :
Advantages Disadvantagesi. Very low dew point (-400C to -600C) can be achieved
i. Large purge air quantity (10-20% of 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
ii) Pressure swing adsorption :
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.
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 condition (indicating type only)
iii. Low abrasive & mechanical/crush strength
Activated Alumina
Advantages Disadvantagesi. 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
Molecular sieve
Advantages Disadvantages
i. High selectivity and can achieve very low dew point
i. High initial cost, high operating cost
ii. Uniform retention capacity ii. Vulnerable to oil
iii. Round bead shape
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
3) Process Flow Diagram:
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.
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),
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.
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.
EXAMPLES :
Size - HP Air CompressorOptions
Air Dryer Air Receiver (gallon) Air Filtration
2 - 5 Reciprocating Heatless Regenerative Air
Dryer
60 - 120 Oil Coalescing & Particulate Removal
5 - 30 Reciprocating, Rotary Vane, Rotary
Screw
Heatless Regenerative Air
Dryer
120 - 400 Oil Coalescing & Particulate Removal
30 - 300 Reciprocating, Rotary Vane, Rotary
Screw
Heatless Regenerative Air
Dryer
240 - 2000 Oil Coalescing & Particulate Removal
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
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
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.
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.
CONCLUSION:
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, [email protected], 2009. Air driers- [email protected] , 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 [email protected] Swing Adsorption, [email protected], june-2010, England.Activated Alumina & Molecular Sieves, Axens, Procatalysts & Adsorbents- [email protected] A BeaconMedæs Continuing Education Publication On Instrumental Air [email protected]
BIBLIOGRAPHY
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