ESSENTIAL FOR CONTROL SYSTEM OF AN AUTOMATION PLANTPRESENTED BY – PRESENTED BY -TAPAS DAS ADHIKARI DIPANKAR MAITYROLL NO : 91/PPR/091031 ROLL NO : 91/PPR/091026PETROLEUM & PETROCHEMICALS REFINARY ENGG.SECTION & PETROLEUM & PETROCHEMICALS REFINARY ENGG. SECTIONDEPARTMENT OF CHEMICAL TECHNOLOGY DEPARTMENT OF CHEMICAL TECHNOLOGYUNIVERSITY OF CALCUTTA UNIVERSITY OF CALCUTTA
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, certaintypes of pumps, fans, some blowing down hoses. The Instrument air in a plant is usedto 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-outin the small-bore piping. To maintain the above situation in the plant Instrument airmust 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 orservice air system. The quality of the air is important to ensure that instrumentationwill function properly and reliably. The most important parameters in specifying airquality 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, ashutdown valve will consume air when it is being actuated. A throttling valve willhave a constant bleed rate with additional consumption when the valve ismodulating. A diaphragm pump consumes air when it is being actuated. Instrument air is provided by a compressor and requires minimal treatment toensure that the air is free of oil, water or particulate matter. This is usuallyaccomplished with some type of filter regulator on the compressor outlet and a dryer.
There are three general methods of drying air: Chemical drying Refrigeration. AdsorptionSelection of air drying equipment is based upon required dew point, quantity of airto 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 thedesiccant is either heated or exposed to a drier gas. Adsorption drier is the common typeused in instrument air system. The material is solid & operates alternately through drying ®eneration of the deactivated catalyst bed with no change in composition. Adsorptionmaterials 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. Whileone air drier is being used to dehydrate, the other is regenerated. The whole operating cycleof a drier is equally divided between regeneration & drying steps & is automaticallycontrolled. Adsorption method can be classified into two categories on the basis of regenerationmechanism. 1) Temperature Swing Adsorption 2) pressure swing adsorptionTemperature Swing Adsorption: In Temperature swing adsorption the adsorbate-loaded bed may be regenerated by hotpurge 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 atdifferent temperature & on the stability & characteristics of the adsorbent & adsorbate.
This technique is called temperature swing since the bed temperature alternatesbetween the adsorption & regeneration temperatures. Heat requirement for theregeneration is about 2.5 times the enthalpy of desorption. A hot regenerated bed needsto be cooled down before it goes for the next adsorption half-cycle. Countercurrentheating & 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 thetemperature swing adsorption (TSA) process because of above disadvantages of thelatter. It may be called heat less drying because of the particular application.Pressure swing adsorption depends on the fact that the adsorption capacity of solidincreases with increasing pressure of the solute. Thus in a pressure swing adsorptionoccurs at a higher pressure of the feed gas when it is stripped off the solute. The flowof the feed at breakthrough & the bed is regenerated by reducing the pressure. At thisstage the adsorbate is recovered in a relatively concentrated form. It is to be noted thata packed bed of adsorbent responds more rapidly to changes in pressure than tochanges in temperature. On the whole process requires less energy & is operationallysimpler 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 thebed, the more absorbable component (the solute) gets trapped; the purified gas issimultaneously 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 bediscarded if it is of no use). Purging – a small fraction of the product gas from the other bed is passed throughthe 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 pressureto that of the feed (no product is drawn during this step).
Study of Adsorbent: ACTIVATED ALUMINAActivated alumina is a porous form of aluminum oxide. It has a high surface area whichadsorbs vapors without any change in form. Activated alumina will not soften or disintegrateeasily when immersed in water. Dew points to - 40 F and lower may be achieved withactivated 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 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/lbStatic 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%
SILICA GELSilica 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.
MOLECULAR SIEVEMolecular sieve is a sodium alumino-silicate. It has a fixed pore size according tothe 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 onthe dryer design and operating conditions.
OBJECTIVE: To attain the industrial specification of the instrumental air as close aspossible & design an air drier package of required capacity.PLAN OF WORK: Instrumental air must be freed from moisture, dust, oil, mud etc. tomake it usable in the instruments of the plant. For which naturally availableair 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.
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 afterpointing out their advantage and disadvantages. a) Refrigeration Advantages Disadvantagesi. 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.
b) Chemical Drying Advantages Disadvantagesi. Low installation cost, low i. Moderate dryingoperating 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
c) Adsorptioni) Temperature swing adsorption : Advantages Disadvantagesi. Very low dew point (-400C to -600C) can i. Regeneration temperature is verybe achieved high (~1900C).High electricity consumptionii. High capacity, large cycle length ii. There are some troubles withusually 6-10 hrs burnout & heating elements hot spotsiii. Air consumption is low, generally 2- iii. If any oil vapour adsorbed with5% of incoming air water regeneration becomes difficult as coking may occur
ii) Pressure swing adsorption : Advantages Disadvantagesi. Very low dew point (-400C to -600C) i. Large purge air quantity (10-20% ofcan be achieved total incoming air) requires a constant compressor over-sizingii. Heatless process. ii. Moderate capacityiii. No electricity consumption iii. Cannot be used if system pressure is below 5 kg/cm2iv. Any oil vapour adsorbed is easilyremoved during regeneration, with nochance of coking
From the above discussion it is clear that Refrigeration and Chemical dryingare 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 temperatureswing adsorption is not accepted and pressure swing adsorption is the mostfavorable 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 toproduce instrument air.
2)Desiccant or Catalyst Selection:Useful adsorbents/desiccants for water removal are Silica Gel, ActivatedAlumina and Molecular Sieve. At first discussion about the advantages anddisadvantages of all these three adsorbents is necessary. Silica Gel Advantages Disadvantagesi. 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 bedii. Uniform bead shape ii. Its selectivity is very lowiii. Provides visual check of desiccant iii. Low abrasive & mechanical/crushcondition (indicating type only) strength
Activated Alumina Advantages Disadvantagesi. High adsorption capacity i. Cannot dry up to very low dew point as compare to molecular sieveii. Low abrasion ii. Its selectivity is moderate. At low humidity its capacity becomes very lowiii. Resists liquid water iii. Costly than silica geliv. High crush strengthv. Uniform bead sizesvi. Moderate cost
Molecular sieve Advantages Disadvantagesi. High selectivity and can achieve very i. High initial cost, high operating costlow dew pointii. Uniform retention capacity ii. Vulnerable to oiliii. Round bead shape
From the above study Activated Alumina is the most suitableadsorbent for pressure swing adsorption in respect to selectivity, capacity,compatibility, regenerability, cost & kinetics. Liquid water will notfracture the alumina bed; it has high crush & abrasive strength whichprevent dust formation, also invulnerable to oil, initial & operating cost isvery small compare to molecular sieve. It can easily attain dew pointrequired for instrument grade air. So Activated Alumina is selected as desiccant medium for air drying toproduce instrument grade air
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 saturatedair and another is regenerated (desorbs moisture) by a part of the dry airusing pressure swing method.At first saturated air is collected from the receiver & compressed in acompressor at room temperature. Then the compressed stream passthrough a coalescer type pre-filter (001-PG-001A/B) to remove oil &condensate and send directly through a control valve to the dryer whereadsorption of moisture is done.
This compressed air stream is the feed stream which can be fed to eitherAdsorber-1(001-C-001) or Adsorber-2(001-C-002). These two dryers withpacked beds of adsorbent are commonly used to serve the purpose ofadsorption of moisture. The adsorbers operate in cycles. when Adsorber-1receives the feed & operate in adsorption mode, Adsorber-2 receives purgegas & operates in the regeneration mode, the air after removal of moisture inAdsorber-1& divided into two streams, one is going through a non returningvalve by means of which dry air from any of these two dryers can be collectedand send to the after-filter (001-AG-001A/B),
where final filtration is done and the product air may be send to asecondary dryer or to the instrumental air header (if the moisture content issatisfactory) & 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 inregeneration mode, after regeneration the stream is vented to the atmospherethrough a flow control valve (001-FC-02). After half cycle, the operation is justswitch over i.e. Adsorber-1 will be in regeneration mode & Adsorber -2 inadsorption mode. Total system is controlled by a sequence controller (001-SC-001) with thehelp of some flow control valve in each inlet & outlet line of two adsorbers.
EXAMPLES :Instrument Air Packages Northwest Equipment Ltd. has extensive experience in thedesign, construction and commissioning of Instrument Air Packages forCanadian 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
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 PackageA completely self-sufficient instrument air package can be packaged within aenclosure and shipped to the site for simplified installation andcommissioning.Even the smallest of air compressor packages can be provided as a complete,pre-engineered and ready-to-operate system.
CONCLUSION: Instrument grade air should contain very low amount of water (below -400C atatmospheric pressure) and should not contain any oil & dust particles. Fromthe above study to ensure the quality of instrument grade air followingsequence 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 thepressure swing adsorption method. Also the total pressure drop across theequipments should not exceed 0.5-0.7 kgf/cm2.
BIBLIOGRAPHYUllmann’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.htmFoster wheeler, Off-site manual, plant & instrument air section, August-1983Binay K. Dutta, Principle of mass transfer & separation processes, fourth printing-2010, PHI-learningpvt. Ltd., New Delhi.Adsorbent Desiccants, Air & Vacuum Process, Inc. VAN AIR SYSTEM, firstname.lastname@example.org,2009.Air driers- email@example.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 INDIAfirstname.lastname@example.orgPressure Swing Adsorption, email@example.com, june-2010, England.Activated Alumina & Molecular Sieves, Axens, Procatalysts & Adsorbents- firstname.lastname@example.org A BeaconMedæs Continuing Education Publication On Instrumental Air email@example.com