Steam Applications in the OPC Sector

Agenda

1. Steam used around the distillation column:

Furnaces

Stripping

Re-boilers

2. Tank Farms

3. Flares

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Steam applications around the distillation column:3

HP steam

MP steam

LP steam

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Fired Heaters / Furnaces

Furnaces are used to heat the product to high temperatures before entering the distillation column.

Found throughout the refining / petrochemical process. Examples include:• Crude Distillation Unit (CDU)• Vacuum Distillation• Visbreaking• Hydrodesulpherisation & fractionation• Hydrotreating• Hydrocracking

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Fired Heaters / Furnaces

Steam usage around furnaces

Key steam applications include:

• Purging the combustion chamber

• Fuel atomisation

• Snuffing in case of fire

• Soot blowing

• Hydrocarbon dilution

• Air pre-heaters (covered earlier – boiler house)

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Furnaces – Steam Applications

Steam Purging:

Eliminates risk of explosion when lighting furnaces due to potential build up of volatile gases

Typical steam loads: 130 kg/h per m3 of furnace volume

(Visbreaker (5,000kg/h - 2” line) : Hydrocracker (20,000kg/h - 4” line))

Pressures: Typically MP steam (<42 barg)

Customer issues:

Slow start up due to build up of layer of condensate in furnace, affecting the photocells used as part of burner start up

SxS Solution:

Separator & trap set upstream of control valve

Trap set or APT downstream of control valve

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Furnaces – Steam Applications

Atomising Steam:

Used to atomise fuel increasing combustion efficiency.

Flow rate: 0.15 – 0.35 kg / kg oil

Pressure: approx 8 bar g

Temperature: 30°C superheat

Customer issues:

Steam must be dry

SxS Solution:

Separator and trap set upstream of control valve – removes condensate formed during shutdown

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Furnaces – Steam Applications

Snuffing Steam:

Prevents or snuffs out fire if process line ruptures, there is an electrical failure, or a flame-out situation.

Flow rate: 130 kg/h per m3 of furnace volume 3,000 to 20,000 kg/h – dependent on application

Pressure: Typically MP / HP steam (<42 barg) Preferably with some superheat

Customer issues:

Potential water hammer in the combustion chamber (rapid opening of valves)

SxS Solution:

Separator and trap set upstream of valve to allow dry steam when activated.

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Furnaces – Steam Applications

Soot Blowing:

Removes residual oil and soot forming on heating surfaces impeding heat transfer to product

Flow rate: 10,000 to 20,000 kg/h

Pressure: Typically MP / HP steam (<42 barg)

Customer requirement:

Steam should be dry to limit erosion problems

SxS Solutions:

Separator and trap set upstream of valve to allow dry steam when activated.

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Furnaces – Steam Applications

Dilution of hydrocarbons:

Prevents coking so increasing the yield.

Increases heat transfer in the heating coils, by reducing fouling on heat transfer surfaces.

Steam is injected directly in with the hydrocarbon prior to entering the furnace.

Customer issues:

Steam should be dry

SxS Solutions:

Separators and trap sets

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Furnaces – Steam Applications

Air Pre-heaters:

Pre-heating combustion air increases heat output from furnace.

Pre-heaters use either steam or waste heat from process

Flow rate: 2,000 to 5,000 kg/h

Pressure: LP steam (approx 3.5 bar g)

Customer issue:

Poor temperature control due to heat exchanger stalling.

If temperature too low can lead to SOx condensing out producing sulphuric acid in flue – corrosion issues. (Covered in steam generation presentation)

SxS Solutions:

APT or pump trap combinations.

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Steam Stripping

Feed stream to stripping column Cooling

Water out

Water cooled condenser

Condensed steam and volatile components to gravity separator

Stripping SteamDirect steam injector

Stripped product stream

Side draw

Cooling Water in

Steam stripping enables higher recovery efficiency of volatile components to be achieved.

Steam desorbs and entrains the dissolved volatiles lifting them up the distillation column.

Steam Stripping - Applications

Steam Stripping is through direct steam injection.

Typical Applications include:

Side columns - e.g. atmospheric crude distillation columns

Bottom of a distillation tower – aimed at stripping hydrocarbons from heavy residue

In the riser of the reactor in a catalytic cracker – steam introduced with the catalyst giving “lift” to the mixture and to separate out the catalyst from the hydrocarbon in the higher disengagement chamber

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Catalytic cracker15

Steam Stripping – Steam requirements16

Typical steam pressures: • 3.5 bar g – naphthas, kerosene, & diesel side strippers• 10 bar g – gas oil side strippers and bottom stripper• 8.6 – 10 bar g – reactor in the cat cracker

Typical steam loads:

0.1 – 0.2 kg/h per US gallon (3.8 litres)

Requirements:

Dry or slightly superheated steam.

SxS Solutions:

Steam conditioning and condensate removal upstream of stripper

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Reboilers:Typical distillation column

Reboilers take a portion of the distillate, adding energy, typically before returning it to the distillation column.

Bottom Pump

Reflux Drum (Accumulator)

Overhead Line

Distillation Column

Steam

Reflux Return

Preheater Feed Riser

Reboiler

DowncomerStraight Run Residue (Liquid)

Gas

To Liquid Storage

Steam

Overhead Condenser

Reflux Pump

Typical re-boiler applications

• Gas plants • Light-ends unit• Vapour recovery unit• Alkylation units (HF and sulphuric)• Sulphur recovery units• Aromatics recovery units• BTX (benzene, toluene, xylenes)• Benzene/cumene• Aromatics extraction process• Catalytic reformers.

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Types of re-boilers

Kettle Re-boilerInternal Re-boiler

Horizontal Thermo-syphon

Forced CirculationVertical

Thermo-syphon

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Kettle Type Reboiler

(Courtesy BSI . BS3274)

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Typical kettle reboiler

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Thermo-syphon re-boiler

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Plate reboiler

Plate HTX are:

• Compact

• Creates high turbulence increasing heat transfer

• Typically requires between only 25% to 50% heat transfer area compared to shell and tube.

• High turbulence reduces fouling.

• Easily cleaned by removing end plates

• Costs 30% to 40% less than shell and tube

• Reduced footprint by a factor of 5

Process liquid/Vapour

CondensateProcess liquid

Steam

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Plate re-boiler on stripper tower

These re-boilers operate at 125°C and 2.3 bara

Re-boiler

Steam conditions:• Typically between 3 bar g to 14 bar g • Saturated steam• Flow rates vary from process to process

Customer issues:

Poor temperature control / corrosion / erosion – Stall (discuss later)

Poor steam quality – no steam conditioning

Reduced life of control valves – poor installation practices

SxS Solutions:

Steam conditioning – separators / desuperheaters

Traps / pump traps around control valves

Efficient condensate removal from re-boiler (discussed later)

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Typical control valve station

Resulting Customer Issues:

• Standing corrosion to the isolation valve and associated piping

• HTX temperature fluctuations

• Water hammer in the coils

• Product losses through poor repeatability

What is wrong with this installation?

Valve at low point with no condensate drainage.

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Re-boilers – typical control systems

Steam flow control

Cascade control with steam flow control (Slave) and temperature control of process (Master).

Sometimes use feed forward control based on the process flow rate.

FC

TC

Re-boiler summary:

Common customer issues caused by:• Poorly conditioned steam

• Poor installation practices

• Poor condensate removal (Stall – discuss later)

SxS Solutions:• Steam conditioning upstream of re-boiler

• Condensate removal from the re-boiler

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Steam usage in tank farms

Tank heating

Objectives: • Prevent solidification of stored product• Allow contents to be pumped to where they are required.

Steam is used in:• Heating coils within the tanks (Steam pressure typically < 3 bar g)• Outflow heaters (Steam pressures typically < 10 bar g)• Both heating coils and outflow heaters

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Calculating steam / condensate loads

Using:Q = U.A.(Tt – Ta) = m.hfg

Steam / condensate loads can now be calculated.Allows sizing of SA controls and steam traps.

Loads are typically based on heat losses from tanks:

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Tank Coils – Condensate RemovalUsual Method

What is the main problem with this design?

Group trapping

Coils B & C are likely to flood

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Tank Coils – Condensate RemovalPreferred Method

Coils individually trapped

Typically a SA control is used, leading to possible stall issues.Potential for APT14.

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Tank Coils - Preferred Traps

• First choice FT or IB

• Bimetallic can be used on excessively long coils (poor temperature control)

• TD’s can be used on short coils with a thermostatic air vent

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Outflow Heaters

Purpose:

Heat process fluid to an elevated temperature for transportation. This allows fluid in tank to be stored at a lower temperature, so saving energy.

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Temperature controls

Self acting controls preferred on tank farms.

Why? • Remote location of tank farms

• Intrinsically safe

• Minimum commissioning

• Minimum maintenance

• Perfect for steady loads

Pilot operated temperature control

Direct acting self-powered control.

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Tank farms – other opportunities

PPP

Discharge to drain

Motive Steam

Exhaust

Drainage of tank sump pit:PPP’s ideal for this application

Light Hydrocarbons

Tank Roof

Tank roof "tilt" alarmsColima level alarms positioned around roof circumference.

Flare stacks

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Typical Flare Gas System

Schematic of typical flare gas system with integrated flare gas recovery unit

LCLC

PI

TIC

LC

PI

LC

PI

FRC

Flare Gas Recovered Flare Gas

Recycle control valve

Three phase separator

Purge Gas

Igniter Line

Sealant Liquid Cooler

Steam to nozzle manifold for Smokeless burning

Fuel Gas to pilots

Flame Front Generator

MOL Seal

Air Supply

Steam

Liquid-ring Compressor

PI

Steam

Ratio

Flare StackWater

Water seal

To Sewer

To oil recovery or Slops

Knock-out Drum

From Relief or vent header system

Flow measuring element

TI

Steam

Recovered hydrogen liquid

Steam trap typically missing

NextCondensate Removal