Separator FundamentalsProcess Engineering Lunch and Learn Session (1 April 2008)

OverviewSafety Moment: Aerosol Cans in CarsBasic Principles of SeparationSeparator ComponentsTypes of SeparatorSeparator SpecificationsSeparator Sizing will be discussed in another session

Safety Moment: Aerosol Cans in CarsThis information has been reprinted from a published newsletter on August 12, 2003 for the San Antonio Works EHS Department:Do you keep WD-40, hair spray, etc. in your vehicle? If so, you might want to reconsider. The incident pictured happened at a refinery in Beaumont, Texas. A deodorant spray can was left in the back of the vehicle that was parked in an open space in the middle of a hot, sunny day.Without warning, the can exploded inside the car. Fortunately, no one was inside or near the car when it happened.

Safety Moment: Aerosol Cans in Cars

Basic Principles of SeparationThree basic principles are used to achieve physical separation of gas, liquids and solids:MomentumGravityCoalescing (a subset of momentum)Any separator may employ one or more of these principles to achieve separation.Fluid phases must be immiscible and have different densities for separation to occur.

MomentumFluid phases with different densities will have different momentum.If a two phase stream changes direction sharply, greater momentum will not allow particles of the heavier phase to turn as rapidly as the lighter fluid, so separation occurs.Momentum is usually employed for bulk separation of the two phases in a stream.

GravityLiquid droplets will settle out if the gravitational force acting on the droplet is greater than the drag force of the gas flowing around the droplet.Droplet attains terminal velocity (Vt) when gravitational force = drag forceVt is directly proportional to DpFigure 7.2 GPSA Databook (11th Edition)

CoalescingVery small droplets such as fog or mists cannot be separated practically by gravity.These droplets can be coalesced to form large droplets that will settle by gravity.Coalescing/demister devices in separator force droplets to follow a tortuous path.Droplets collide with other droplets on the coalescing device, forming larger droplets which can then settle out by gravity.

Components of SeparatorInlet Piping (outside of separator)Primary Separation (A)Gravity Settling (B)Coalescing/Demister (C)Liquid Collecting (D)Figure 7.5 GPSA Databook (11th Edition)

Separator Inlet PipingMinimises turbulence and velocity of fluid flowing into separatorCommon specifications:Straight length of pipe for 10 pipe diameters from separatorAvoid horizontal bends, elbows and control valves on inlet line immediately upstream of separator (source of shear, turbulence and causes small droplets)Larger size of pipe to match separator inlet nozzle size for 10 pipe diameters from separator If inlet piping size cannot be modified (e.g. retrofit, space restriction):Install guiding vanes or cyclone in inlet pipingAbove specifications can minimise the amount of entrained liquid (mist) therefore assisting separationBy providing piping data and process data, vendor can quantify amount of bulk liquid, entrained liquid (mist) and particle size distribution to greatly improve separator design

Primary SeparationSeparates bulk liquid from gasUses inlet device to break the momentum and distribute the flow

Figure 11.5 Campbells Volume 2

Half Pipe Inlet DeviceHalf Pipe Inlet Device

Gas shoots towards the back of the wallMaldistribution of gasSends both gas and liquid downwardsLiquid entrainment in vapour flowing onto demister www.cdsengineering.com

Vane Inlet DeviceDistributes the fluid along the vanesFluid (gas and liquid) changes direction Liquid follows the vane wall and drops offGas follows the vane wall and flows up

www.prosep.comwww.prosep.com

Cyclone Inlet DeviceFluid is directed by a swirl element to spin along the cyclone wallLiquid flows downwards along the cyclone wallGas flows downwards together with liquid then upwardsLiquid outlet can be submerged in liquid phase to minimise foaming

www.prosep.comwww.prosep.com

Gravity SettlingUtilises the force of gravity to enhance the separation of entrained droplets.Gas moves through the barrel at low velocity

Gravity SettlingVertical vessel: specify diameter so that Vgas < VterminalHorizontal vessel: specify length of vessel to allow droplet to settleStraightening vanes are sometimes used to reduce turbulence

Demister SectionRemoves small droplets by coalescing small droplets to form larger droplets so they can settle by gravity

Vane Pack and Wire MeshVane Pack

Gas and liquid droplets forced to change directionDroplets absorbed by wet surface and collected in pocketsCan handle higher flow than wire mesh

Mist Mat/Wire Mesh

Gas and liquid droplets forced to change directionDroplets strike surface then flow downwardsMore effective at separating smaller particles than vane pack

Figure 7-13 GPSA Databook 11th Editionwww.natcogroup.com

Cyclone Demister DeviceUses centrifugal forces to separate liquid droplets from a gas stream. Gas and liquid enter the cyclone tube and is set into rotation by vanes mounted on a central body.Gas flows axially along the tube. The heavier liquid droplets are thrown to the wall due to the centrifugal action. The liquid is then transported through slits at the cyclone wall into a liquid collecting chamber and drained back to the vessel through downcomers.Not effective below certain velocity.

www.prosep.com

Liquid CollectionProvides enough residence time:to allow vapour to be released from the liquid.to allow 2 liquid phase to separateContains volume to handle sudden surge in inlet liquid flow.One of liquid collection issues is foaming.Presence of chemicals such as corrosion inhibitor and glycol can increase the likelihood of foaming.Foaming can be minimised by reducing shearing, turbulence and velocity of fluids into separator.Foaming can be dealt with by increasing the height of the separator to allow foams to settle.

Liquid Levels Definition

Liquid Collection DevicesCalming baffle reduces turbulence, creates uniform velocity.Coalescing pack causes discontinuous phase to form larger droplets.Weir separates different liquid phases.

Liquid Collection DevicesVortex breaker prevents vortex formation and gas carryunder.Figure 11.10 Campbells 7th Edition Volume 2.

Separator TypesBasic types: Vertical Separator and Horizontal Separator

Separator TypesBasic types: Vertical Separator and Horizontal Separator

Separator SpecificationsWhat separator vendor requires:Process data:Max. liquid, max vapour and max Rho V (momentum) casesInlet flow rate, density and viscosityLiquid surface tensionPresence of solidsPresence of corrosive materialsRequired separation efficiency carryover, particle size removed

Separator SpecificationsWhat separator vendor requires:Inlet piping data (optional):Length of straight run into vesselPipe sizePresence of bends, elbows, control valvesSpecifications:Applicable industry standardsApplicable client standards

Separator SpecificationsWhat performance guarantee is required from vendor:Vapour and liquid flow capacityLiquid carryover: total liquid carryover (e.g. 0.1 US gallon liquid carryover/MMSCF gas)particle size carryover (e.g. 99.9% of 30 microns and above removed) Beware of particle size carryover, this does not reflect the total amount of liquid carryover.On some separators, particle size carryover is important (e.g. flare scrubber, compressor scrubber and filter coalescer).On some separators, particle size carryover is not important (e.g. production separators). In this case, total liquid carryover is more important.

Future Topic: Separator SizingSome of the topics which can be covered:Separator Sizing Formula (K-Factor)Cautions about using K-Factor (GPSA extract)Nozzle sizingWorleyParsons Standard SpreadsheetDesign Guides and StandardsCase Study

Question TimeQuestions?