Tray Column Design
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Transcript of Tray Column Design
LKMT
Industry Academia Workshop on “Design of Refinery Distillation Columns“
October 23rd – 24th 2013
Di till ti E i t D i M th d l d P tiDistillation Equipment Design : Methodology and Practices
S K Saxena AGMS.K. Saxena, AGMEIL, New Delhi
Selection of contacting device for Columns
The selection process for tower internals can beThe selection process for tower internals can bestraightforward for some designs, but it could bedifficult for others as It depends on variousfactors like whether it’s a Grass root design or arevamp activity.
21 October 2013 2
Choices Available (Contacting Devices)
There are two broad categories of contacting devices:
• Trays (Vales/Sieves/Bubble Caps etc.)• Packed Beds (Structured or Random Packing)
21 October 2013 3
Contacting Devices:
PackingPacking TraysTrays
21 October 2013 4
Selection of contacting device depends upon
• Operating pressure
• Turndown Ratio• Turndown Ratio
• Foaming Tendency
• Solids present, dirty or polymerized solution handling
• Heat removal requirementsq
21 October 2013 5
A Typical Tray column :
21 October 2013 6
Types of Trays
ValveSieve
Trays with Down comer
SieveBubble Cap
Shed- DeckTrays without Down comer
Dual FlowBaffleDisc & DonutRi l
Shed Deck
Ripple
Multi down comer traysHigh capacity Trays (Proprietary Design)
Non Fractionation TraysCollector/ Chimney Trays
Pro valve, Triton, Nye, SuperFrac, MVG
21 October 2013 7
Non- Fractionation Trays
Valve Trays:
A li ti
Vapor Vapor and Liquid Contacting and Liquid Contacting Mechanism in Valve TraysMechanism in Valve Trays
Applications:
• Offer high capacity & good efficiency• Good turndown characteristics• Good turndown characteristics• Maximum application-widely used in almost all applications.• Cheaper and simple in fabrication.
21 October 2013 8
Sieve Trays:
VaporVapor and Liquid Contactingand Liquid Contacting Mechanism in SievesMechanism in SievesVapor Vapor and Liquid Contacting and Liquid Contacting Mechanism in SievesMechanism in Sieves
Applications:
• System where high capacity near design are to be maintained incontinuous service.
• Poor turndown characteristics as compared to valve trays• Suitable for Liquid-liquid extraction applications
21 October 2013 9
q q pp
Bubble Caps:
Vapor Vapor and Liquid Contacting and Liquid Contacting Mechanism in Bubble Mechanism in Bubble CapsCaps
A li tiApplications:
• Suitable for all services, except extremely coking, polymer formationor other high fouling conditionsor other high fouling conditions.
• Used for extremely low flow conditions i.e. good turndown characteristics& offer maximum flexibility in flow range.
21 October 2013 10
Advantages of Trays
• Tray columns exhibit wider operating range than packed columns i.e. having Potential to handle vapor & liquid loadingsliquid loadings
• Very low liquid rates leads to incomplete wetting ofVery low liquid rates leads to incomplete wetting of packing as compared to trays thus having higher contacting efficiency
• High residence time provided for liquid on trays
• If solids are present in the fluid, tray columns can be designed to permit easier cleaning.
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g p g
Comparison of Commonly Used Trays
Parameters Bubble* Sieve ValveCapacity % 100 120-150 120-150
Efficiency % 100 105 115 105 115Efficiency % 100 105-115 105-115
Flexibility or turndown
10:120:1 (possible)
2:13:1 ( possible)
4:16:1 (possible)
Entrainment 300 100 Least
Cost % 100 70-85 70
Fouling service suitability
Fair Good (for large holes)
Good
Tray spacing 24”-36” 20”-30” 18”-30”
*With bubble cap trays as base case*With bubble cap trays as base case
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OTHER TYPE OF OTHER TYPE OF TRAYS:TRAYS:
1. Baffle Trays / Shed deck
2. Ripple Tray
3 Disc and Donut tray3. Disc and Donut tray
4. Dual Flow tray
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Applications Other Types of Trays
Type Applications
Dual flow /Ripple •Same as for sieve traysDual flow /Ripple trays
Same as for sieve trays.•Higher capacity but low efficiency•Poor turndown characteristics
Baffle trays / shed deck/ Disc & Donut
•Good for heat transfer applications•Can handle coke & solids.
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CLASSIFICATION OF TRAYS
Based on Liquid Flow Paths:
Single PassSingle Pass Two PassTwo Pass
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CLASSIFICATION OF TRAYS
Three PassThree Pass Four PassFour Pass
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High Capacity Trays (HCT)
HCT
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Difference in Configuration:
Major difference in Conventional & High capacity trays are due to modification in the type/ construction of the internals :
•Modification in active area
•Modification in down comer area
R d ti i T S i•Reduction in Tray Spacing
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Types of HCT :
By modifying Active Area By Modifying Down Comer Area
Bifrac & Superfrac (M/s Koch)
Provalve (M/s Norton)
Maxfrac (M/s Koch)
Triton (M/s Norton)( )
MVG (M/s Nutter)
( )
MD, ECMD & VGMD (M/s UOP)
Hi Fi (M/ S l )Hi-Fi (M/s Sulzer)
Nye (M/s Glitch)
21 October 2013 19
Claims by HCT Vendors:
Vendor Tray Capacity increase (Upto)
Koch-GlitschNorton
NyeBifracMaxfrac
15-20%25%25%
SuperfracTritonProvalve
30%25%20%Provalve 20%
UOP ECMDVGMD
20%30%VGMD 30%
Sulzer MVG 20%
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Various types of High Capacity Valves
MVG (MINI V GRID) PROVALVEMVG (MINI V-GRID) PROVALVE
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Continuation….
O Hi Fi T b S lMD Trays by UOP Hi-Fi Trays by Sulzer
21 October 2013 22
Continuation….
ECMD Trays by UOP
21 October 2013 23
Comparison between various types of Trays
Tray Type Capacity Efficiency Cost / unit area
Flexibility Remarks
Bubble Cap Med. to High except Med. To High High ( almost 3/1 to 4/1 Use for high flexibilityplow to med. @ high liquid rates
twice the cost of sieve trays)
Sieves Med. to High High, Equal to or better than others
Lowest of all trays 2/1 to 3/1 First choice for mostapplications;
Valves Med. to High High, as good as sieves
Medium (About 110% of sieve trays)
High.Possibly up to5/1.
Not recommended forfouling services.
UOP-MD, Very High, estimated Low to Medium Higher than valve Low. (< 2/1) Consider for revampsUOP MD,UOP-ECMD,Sulzer Hi-Fi
y g ,to be 30-40% than a conventional sievetrays for high liq. rates
gtrays
( ) pwhere no other device is acceptable. Low Tray Spacing, Non-fouling services
SuperFrac Very High estimated Medium More than 105% Medium Good alternative toSuperFrac,MVG, ProValve,Triton
Very High, estimated to be 10-15 % than a conventional sievetrays for high liq. rates
Medium More than 105% of sieve trays
Medium.Slightly higherthan sievetrays.
Good alternative to sievetrays at low liquid ratewhere higher capacity is needed.
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Packed Column
Liquid DistributorLiquid Distributor
Packing
Liquid Collector/ Re-distributor
Support Plate
Bed Limiter
S t Pl tSupport Plate
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Applicability of Packed Columns in a Refinery:
• For column < 900 mm I.D: Packing are usually cheaper than trays.
• Acids & many other corrosive materials can be readily handled in packed column. Because materials like ceramic, carbon or other acid resistant material can be used in packed beds.
• Hold-up of liquid can be quite low in packed column, an p q q p ,advantage when liquid is thermally sensitive.
• Liquid tending to foam may be handled more readily inLiquid tending to foam may be handled more readily in packed column because of relatively low degree of liquid agitation by gas.
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Contd….
• Potential to handle high throughput at high efficiency.
• Packing itself acts as a foam breaker.Packing itself acts as a foam breaker.
• Extensive use in revamp application. However. acceptable in grass-root design alsograss-root design also
• Offer low P/ unit theoretical stage.
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Historical Generation of Packing in Industries
Packing
1st Generation 2nd Generation 3rd Generation
g
Raschig Rings
Lessing RingsPall Rings
HyPak GemPakFlexiPak
Cross Partition RingsBerl Saddles
IMTPCMRN tt Ri
MellaPakParlPak
Nutter Rings
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Types of Random Packing
INTALOX SADDLEIMTP INTALOX SADDLE
PALL RING RASCHIG RING
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PALL RING RASCHIG RING
Structured Packings
EIL’s Parlpak
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Comparison between various types of Packings
Packing Type
Capacity Efficiency Cost / unit area
Flexibility Remarks
Random Packing(Pall ings, Metal Intalox, etc.)
Medium Medium Medium to Low > 3/1 - Good efficiency perunit of pressure drop.
- Mainly used in Absorbers where liquid loads are highloads are high
Structured Packing(FlexiPac
High to Very High High Medium(Varies with Metallurgy)
>3/1 Best efficiency per unit of pressure drop.
(FlexiPac,GemPak, MellaPak etc.)
eta u gy)
Grid Very High Good in Medium to High < 2/1 -Good for high vapor &Grid(FLexiGrid, Intalox Grid, etc.)
Very High Good in Entrainment
removal & Heat Transfer
applications only.
Medium to High 2/1 Good for high vapor & low liquid service .
- Used in wash zones of HC fractionators where Coking formation
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predominates
Criteria for Internal selection:
Following criterions help the designer in selecting the best internal for a given application depends upon the variousinternal for a given application, depends upon the various factors but primarily on whether it’s a :
- A Grass Root Design, or - A Revamp Activityp y
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Internal Selection for New tower (Table-1):
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Internal Selection for New tower (Table-2) :
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Internal Selection for a Revamp service (Table -3):
21 October 2013 35
Internal Selection for a Revamp service (Table-4):
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Comparison between Trays & PackingParametersParameters TraysTrays Packed columnPacked columnParametersParameters TraysTrays Packed columnPacked column
RandomRandom StructuredStructuredService -Process with exothermic
reactions- HP AbsorbersP A d (HT)
-Vacuum serviceRreactions
- Fouled service- High no. of liquid withdrawals- High turndown & low liquid rates
- Pump Around (HT)- Vacuum services
-Revamps-High no. Of stages.-H.P absorbers
Capacity
Pressure Drop
rates.
1.0 1.0-1.15
1/3-1/5
1.2-1.3
1/5-1/10Pressure Drop (P)
Column size
1.0
1.0
1/3-1/5
0.9
1/5-1/10
0.8
Cost Internals Columns
1.01.0 1.2
0.91.5*0.9
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Focus on Design of Trays
Before proceeding for the design of a tray tower, the Design Specialist needs the following data from his process counterpart:
a. Vapor Loads entering the trays and its properties viz. temperature, pressure, mol. wt. & compressibility factor.
b. Liquid Loads leaving the tray and its properties viz. density, q g y p p y,viscosity, surface tension etc.
c. Allowable pressure drop per tray.d. Turn down/ turn up requiremente. Foaming tendencyf. Material of constructiong. Fouling Characteristicsg gh. Any special requirements like heat transfer etc.
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Tray Geometry: Major Components
Following are the major components of the tray :
• No of PassesNo. of Passes• Active Area • Down comers• Number of valves• Number of valves• Pitch/Layout• Calming zone• Inlet & Outlet Weir• Inlet & Outlet Weir• Tray Spacing• Under Down comer Flow Clearance (UDFC)• Column Diameter• Column Diameter• Column Height• Other additional features like chimney tray, distributors etc.
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Typical Tray Layout
Plan View
Elevation View
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Tray design :
Following are the 3 broad categories that influence the design of the tray:design of the tray:
a.a. Parameters affecting vapor capacityParameters affecting vapor capacityg p p yg p p yb.b. Parameters affecting liquid capacityParameters affecting liquid capacityc.c. Other Important parametersOther Important parameters
The same can be shown clearly from this graph…..
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Typical Performance Diagram
Area of Normal Operationapor
Rat
e
Area of Normal Operation
Moderate Weeping
Va
Heavy Weeping
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Liquid Rate
Tray design : Effect of Vapor Loadings
There are certain limitations on the vapor handling capacity of the column about which the designer shall be fully aware of before finalizing the configuration ofbe fully aware of before finalizing the configuration of the Column:
/ fThe main controlling/limiting factors are :
•Jet Flood•Flow Regimes•Entrainment
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Parameters affecting Vapor Loadings: Jet flood
Jet Flood is the state of entrainment when the down comers can't handle adequate liquid flow and liquid backs up and fills the trays & the vapor-liquid contacting mass of a tray reaches the one tray p q g y yabove, resulting in massive entrainment and liquid recycling which eventually affects the distillation and overloads the down comers.
Effect of Jet Flood on Column performance:
• This can result in loss of bottoms level low tray efficiency high tower pressure• This can result in loss of bottoms level, low tray efficiency, high tower pressure drop, control problems, etc.
• If the down comer is too small or tray pressure drop is too high flooding can beIf the down comer is too small or tray pressure drop is too high, flooding can be caused by down comer backup even if entrainment is small.
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Jet Flood Vs Column Efficiency
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Parameters affecting Vapor Loadings: Flow Regimes
Different flow regimes can occur on a tray in a column.
Following are the prominent flow regimes that can be encountered in aFollowing are the prominent flow regimes that can be encountered in a column depending on Vapor-liquid flow rates:
E l i i ti i il t l l iti d• Emulsion regime operation occurs primarily at low vapor velocities and high liquid rates i.e. the liquid phase is continuous
• Froth regimes occurs primarily when vapor passes through the liquidFroth regimes occurs primarily when vapor passes through the liquid on the tray as discrete bubbles of irregular shape, formed at the tray perforations and are swept away by the froth
• Spray regime operation occurs primarily at high vapor velocities and low liquid rates i.e. In the spray regime, the vapor phase is continuous.
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Flow Regime within Normal Operating Range
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Effect on Efficiency:
From the above graph it is clear that:
•Froth regime is the most desirable operating regime for an efficient performance of a column.
•Operation in the Spray regime can be very detrimental to good tower performance as tray efficiency dropping sharplyperformance as tray efficiency dropping sharply.
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Parameters affecting Vapor Loadings: Entrainment
Entrainment occurs because of inadequate vapor space above thevapor-liquid contacting zone on a tray. Liquid doesn't fully disengagefrom the vapor, and is carried to the tray above.
Eff t f E t i t C l P fEffect of Entrainment on a Column Performance:
• Entrainment becomes more prevalent when tower vapor loads increase. p p• Massive entrainment leads to jet flooding.
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Entrainment in a Column:
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Tray design : Effect of Liquid Loadings
Similar to vapor loadings, there are certain limitations on the liquid handling capacity of the column as well:
Liquid loading effect the following parameters :
•Down comer Flood•Down comer Flood•Under Down Comer Velocity•Down Comer Back Up limits•Down Comer Back-Up limits
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Liquid Handling Limitations: Down Comer Flood
Down Comer Flood is the criterion that determines how close a tower is to flooding as a result of excessive liquid height in the down comer.
or,
Percent down comer flood represents the ratio of the actual vapor and liquid feed rates to the feed rates that would result in p q100% down comer froth backup.
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Down Comer Filling in a Column
Down comer filling (Hd) is defined as the clear liquid height in the down comer.
Hd = Ht + Hud + Hi + Hdc
And, Dry Tray Pressure drop (Ht) is given by :
Ht = Hed + Hc
where;Hd - Down Comer Filling
Ht = Hed + Hc
Ht - Tray pressure drop,Hud - Head loss under the down comer,Hi - Inlet head on the tray,Hdc - Head loss due to flow through the down ComerHdc - Head loss due to flow through the down ComerHed - Dry tray pressure drop Hc - Clear liquid height
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Each head is expressed in inches (mm) of hot clear liquid.
Typical in a down comer
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DC Back Up:
This is the measure of the aerated liquid in the down comer, calculated in terms of clear liquid height.
• Backup is a function of dry and wet tray pressure drop and head loss under the down comer.
• If the restrictions to flow are too large and/or a foamy system is involved, the aerated liquid in the down comer can backup to the level of the outlet weir and cause tower flooding.
• Calculated backup should generally not exceed about half of the tray spacing
L b k h ld b d f f t• Lower backups should be used for foamy systems
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Other Important Parameters:
There are many other important parameters which shall be satisfied or shall be taken care of in the design viz.
• Tray Area & Efficiency• Tray Spacing• Turn-down Ratio / weeping• Tray pressure Drop• F li• Fouling• Foaming• OthersOthers
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Tray Area:
Mainly 2 Components ascertain Tray Area:
1) Calming Area : This is an area preserved for vapor1) Calming Area : This is an area preserved for vapor disengagement prior to liquid entering a down comer. The un-perforated narrow strip of active area next to the outlet weir is considered to be calming area.
2) Acti e Area : This is the tray deck area enclosed by the tower2) Active Area : This is the tray deck area enclosed by the tower wall, the outlet weirs, and the edges of the inlet areas. Valves or sieve holes are located in the active area
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Tray capacity & Tray efficiency:
Tray capacity and tray efficiency are the two most important criteria in tray design:
• Diameter of a tower is mainly determined by capacityconsiderations,• Tray efficiency determines the number of actual trays and• Tray efficiency determines the number of actual trays, andtherefore the height, of the tower.
• To achieve good efficiency, the designer must optimizeg y, g pthe weir height, open area, bubble area, liquid flow pathlength, number of liquid passes, and other variables.
21 October 2013 58
Tray Spacing:
Tray spacing along with Column Diameter is an important parameter that set the capacity of the tower.
•As the distance between trays is increased, the tower capacity will increase and the column diameter could be reduced.• Low tray spacing can also be used but this increases the column• Low tray spacing can also be used, but this increases the column diameter as to handle a given set of vapor and liquid loadings. Also, such low spacing may lead to difficult maintenance.
T i ll t i f 24 i h i f d• Typically , a tray spacing of 24 inches is preferred.
Therefore, Selection of optimum tray spacing is a function ofTherefore, Selection of optimum tray spacing is a function of column diameter and service (clean or fouled) and most importantly depends upon the experience of the equipment designer/databank
21 October 2013 59
designer/databank.
Turndown Ratio :
Turndown Ratio is the term used to define the range ofloadings over which acceptable tray performance is achieved.This usually means the range over which the tray efficiencystays at or above the design value.y g
Note : While deciding the turndown ratio, a specialist must checkthe weeping efficiency curve that how much design efficiency isobtained at turn down.
21 October 2013 60
Effect of Weeping:
This is the passage of liquid through the tray deck seams, and valve or sieve holes.
• It occurs more when vapor rates are low, and is the primary reason sieve trays lose efficiency at turndown operation.
• When 25% of the liquid load weeps through the tray, one should expect significant loss of tray efficiencyshould expect significant loss of tray efficiency
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Effect of weeping on Efficiency:
21 October 2013 62
Foaming Factor:
Foaming can occur in distillation towers via several different mechanisms including:
• Presence of surface active materials.• Presence of solids.Presence of solids.• Entrainment of HCs liquids into aqueous systems.• Condensation of HCs vapors into aqueous systems.
To design for foaming, the tray is usually oversized by using a lower percent of jet flooding and down comer flood, a low dry tray pressure drop a low down comer entrance velocity and a reduced allowabledrop, a low down comer entrance velocity, and a reduced allowable down comer filling.
21 October 2013 63
Packed Column Design :
Following are the major parameters that influence the design of the packed columns :design of the packed columns :
a.a. Vapor & Liquid Capacity limitationsVapor & Liquid Capacity limitationsp q p yp q p yb.b. Efficiency & Turn downEfficiency & Turn downc.c. Heat TransferHeat Transferd.d. Other considerationsOther considerations
21 October 2013 64
Packed Column DesignPacked Column Design: Effect of Vapor/: Effect of Vapor/LiqLiq Capacity Capacity
Phenomena of Flooding in Packed Beds
(In both Random Packing or Str ct red Packing)(In both Random Packing or Structured Packing)
Flooding occurs when liquid begins to accumulate in theFlooding occurs when liquid begins to accumulate in thepacking and the pressure drop begins to rise more sharply.With further increases in vapor rate, the pressure drop risesp p palmost vertically and liquid begins to “pile up” on the top ofthe packing.
21 October 2013 65
Packed Column Design: Packed Column Design: Efficiency & Turn DownEfficiency & Turn Down
• For most applications, random packing should beconsidered first because they are lower costconsidered first because they are lower cost.
• If a low pressure drop per theoretical stage and/or a shortcolumn height is required, then structured packing shouldg q , p gbe evaluated as an option.
• To ensure optimum efficiency at all rates, a high qualityli id di t ib t t b dliquid distributor must be used.
• The selection of a liquid distributor is critical in the design ofa packed beda packed bed
21 October 2013 66
Packed Column Design: Packed Column Design: Other FactorsOther Factors
There are various parameters that also influence the Design of the packed columns, for instance:Design of the packed columns, for instance:
a.a. Heat transfer RequirementHeat transfer Requirementqqb.b. Vapor & Liquid distributionVapor & Liquid distributionc.c. Fouling , etc.Fouling , etc.
21 October 2013 67
Future Challenge & Trends
Challenges lying ahead for this industry:
•Lower capital cost & optimized design•Energy efficient hardware•Higher continuous run length•Flexibility to handle wide variations in loads.
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