Instrument air package

it about the feed of an automation plant

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

2. INTRODUCTION : Instrument air is the mixture of N 2 , O 2 & 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 S 7.3.

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 - 40 C to - 60 C 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: Energy consumption for heating of the bed for the regeneration ; Presence of condensate material in the feed causes various problems during regeneration ; 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 is a spherical bead consisting of 97-100% silica. It is available in two types: indicating, a translucent bead impregnat­ed with blue or yellow color and non-indicating white translucent color. SILICA GEL

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.   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 . Refrigeration Advantages Disadvantages Very low operating cost. Dew point below 3.5 C 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 :

18. 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 38 C ; in fact some desiccant materials melt or fuse together at 35-40 C. 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. Advantages Disadvantages i. Very low dew point (- 40 C to - 60 C) can be achieved i. Regeneration temperature is very high (~ 190 C).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) Adsorption i ) Temperature swing adsorption :

20. Advantages Disadvantages i. Very low dew point ( -40 C to - 60 C) 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/cm 2 iv. Any oil vapour adsorbed is easily removed during regeneration, with no chance of coking ii) Pressure swing adsorption :

21. From the above discussion it is clear that Refrigeration and Chemical drying are not suitable for instrument grade air (at least -40 C dew point) production. Also chemical drying cannot operate above 38 C. 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 condition (indicating type only) iii. Low abrasive & mechanical/crush 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 low dew point i. High initial cost, high operating cost 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. 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 Compressor Options 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

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. Instrument grade air should contain very low amount of water (below -40 C 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 5 kg f /cm 2 to perform the pressure swing adsorption method. Also the total pressure drop across the equipments should not exceed 0.5-0.7 kg f /cm 2 . CONCLUSION :

35. 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, 3 rd Edition, McGraw Hill Book Company. Atkins, Physical Chemistry, 8 th Edtion-2004, Elsevier Science & Technology Books. Activated alumina balls- SORBED [email protected] Pressure 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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