Facilities Engineering Transportation and Storage EMB 5443
Separators and Filters Mohd Shiraz Aris Department of Mechanical Engineering, Universiti Teknology PEtronas.
Acknowledgement: Pn. Putri Nazirah, Dpt of Chemcial Engineering, UTP
Separators Week 1& 2
Date 24/01-04/01
3&4
5
11
Lecturer MSA/AL
Assessment
07/02-18/02
Field Development Concept: Fixed Platform, Manned and Unmanned Platform, MSA/AL Minimum manning, Jackets, Tripod & Monopod, Subsea Facilities Concept, Guyed Tower System, Light Weight and Concrete Gravity Structures, FPSO and FSO System, Integrated Development Systems
Group Project Start (MSA) & Lab Work (MSA)
21/02-25/02
Oil and Gas Production Processes: Oil Production Process, Gas and Production Process
21/3-1/04
Topics Introduction: E&P business, PSC, Project Life Cycle Concept
Process Equipment and Facilities: Separator Design & Stages Required, Knockout Drums & Flare System Design, Instrumentation & Electrical Power Requirement & Design, Flowline System design, Pump/Compressor Requirement, Water Injection & Gas Injection Facilities
MSA/AL MSA
Lab Work (MSA) Lab Work (MSA)
Introduction •
Main Offshore Production Facilities (key components): Wellhead Equipment
Separation
Waster Handling
Pump/Compressor
Gas utilities, flaring
Introduction •
Wellhead equipment Wellhead and Christmas tree used to maintain surface control of well
Contain key components (valves) for the safe production of crude/gas from the wells: manual master gate valve, manual wing gate valve, manual swab gate valve, automatic shutdown valve, choke valve Flowline and production manifolds send the well fluids to the production and test separators
Introduction •
Wellhead equipment
Introduction •
Wellhead equipment
Introduction •
Flowlines and production header
Introduction •
Separation System
Introduction •
Crude Separation and Export System-Overview
Introduction •
Crude/Gas Separation System-Overview
Introduction •
Gas Separation and Export System-Overview
Introduction •
Gas Separation System-Overview
Introduction •
Separation System-Focus
Separators
Separators “SEPARATORS form the HEART of the production process” SEPARATION MODULE
wellhead Wellhead manifold
FIRST STAGE
Gas to gas scrubber and gas compression module
well SECOND STAGE reservoir
Water
Water treatment Disposal
Storage tank – final oil treatment
Oil
To export
What is a separator? • A SEPARATOR is a pressure VESSEL designed to DIVIDE a combined liquid-gas system into individual COMPONENTS that are relatively free of each other for SUBSEQUENT PROCESSING or disposition
Why separator is needed? • Downstream equipment cannot handle gas-liquid mixtures
– Pumps require gas-free liquid – Compressor/ dehydration equipment require liquid-free gas
• Product specifications has limits on impurities – Oil should not contain > 1% impurities – Gas sales contract no free liquids in gas
• Measurement devices (metering) for gases/liquids highly inaccurate when the other phase is present
Principles of Separators • Principals of separation: momentum, gravity and coalescing
– Momentum Fluid phases at different densities have different momentum Changes in fluid direction will separate fluids at different momentum
– Gravity
Liquid phase separated from gas due to difference in weight of droplets
– Coalescence
Small droplets coalesced when “combined” together Coalescing devices force small droplets flowing through it to collide, form larger droplets and then settling out of the gas phase by gravity
Principles of Separators • Equipment and components involved in a separation process: – Filter separators: typically with 2 compartments (filter coalescing elements and wire mesh) – Flash tank: Separation as a result of high DP – Line drip: Removal of free liquids in a dominant gas stream (high gas/liq) – Liquid-liquid separators: Similar in design to gas/liquid separators except at much lower velocities – Scrubber/knockout: Handling of high gas/liquid stream. Liquid typically entrained as mist or free flowing along pipe walls
Principles of Separators – Separator: separation of mixed phase streams into gas and liquid phases that are relatively free from each other – Slug catcher: ability to absorb sustained in-flow of large liquid volumes at irregular intervals – 3 phase separator: separation of gas and two immiscible liquids of different densities
What properties affect separation? • • • • • •
Gas and liquid flow rates Operating & design pressures and temperatures Surging or slugging tendencies of the feed streams Fluid physical properties – density, compressibility Desired phase separation - gas-liquid or liquid-liquid Desired degree of separation - e.g. remove 100% particles >10 micron in size • Presence of impurities – paraffin, sand, scale • Foaming tendencies Must know and • Corrosive tendencies understand the characteristics of the flow stream in order to design separators!
Separator Design Checklist • A primary separation section to remove the bulk of the liquid from the gas • Sufficient liquid capacity to handle surges of liquid from the line
• Sufficient length of height to allow small droplets to settle out by gravity • A means of reducing turbulence in the main body to ensure proper settling • A mist extractor to capture entrained droplets • Back pressure and liquid level controls
Separator classification and types • Classification – Two-phase separation (gas-liquid) – Three-phase separation (liquid-liquid i.e. water/oil/gas separation)
• Types – Gravity separators • Horizontal • Vertical • Spherical
– Centrifugal separators
Selection of separators is based on obtaining the desired results at the lowest cost
(effect of gravity is enhanced by spinning the fluids at a high velocity)
Governing Laws •
Momentum Fluid phases at different densities will have different momentum Change in fluid flow direction will separate fluids at different momentum Momentum separation method applied for bulk separation of 2 phases in a stream
•
Gravity settling Liquid phase separated due to difference in weight of droplets
Vt
2 gmp ( l g )
lgApC '
drag
4 gDp ( l g ) 3gApC '
The drag coefficient C’ is found to be a function of the particle shape and Re of the flowing gas
Gas velocity Liquid /solid droplet
gravity
Governing Laws • Particles are assumed to be a solid sphere Re
1488DpVtg
• Solving the equation requires the elimination of either variables, Vt or Dp. The use of specific drag coefficient charts together with C’Re2, enables the particle diameter and eventually the terminal velocity to be solved: C Re '
2
(0.95)(108 ) gDp 3 ( l g )
2
Estimation of Particle Size
Estimation of Particle Distribution in a Separator
Governing Laws • Gravity settling for larger particles for particles 1000 microns or larger, Newton’s Law with a limiting drag coefficient of 0.44 (Re >500). Substituting for C’ = 0.44
Vt 1.74
gDp ( l g )
g
and for the upper limit of the Newton’s law, the maximum droplet size is estimated from,
where for, Re = 200,000 Kcr = 18.13
2 Dp Kcr g g ( l g )
0.33
Governing Laws • Stokes Law for low Re (less than 2), a linear relationship exists between Drag and Re
1488 gDp 2 ( l g ) Vt 18 Dp for Re less than 2 is found using Kcr = 0.008 in,
0.33
2 Dp Kcr diameter of approx. 3 The lower limit of Stokes Law is for a droplet g g ( l g ) microns.
Alternative Generic Terminal Velocity Formulae • Particles falling through a fluid by the pull of gravity:
4 gDp N 1 ( p l ) Vt (1 N ) N 3l A
(
1 ) 2 N
Where, A and N are constants related to the flow regime and the drag coefficient as determined by
gl ( p l ) K Dp 2
1/ 3
Law
K
A
N
Stokes
K<3.3
24
1
Intermediate
3.3 ≤ K ≤43.6
18.5
0.6
Newton’s
K > 43.6
0.44
0
Governing Laws • Coalescing Small droplets coalesced and separated by gravity. Coalescing devices like wire mesh screens, vane elements, and filter cartridges force small particles flowing through it to collide, forming larger droplets and then settling out of the gas phase through gravity
Other Separation Techniques • Cyclone Separator concept of inertia separation is employed where the different speeds of gas and solid particles would cause separation to occur. Baffles are use to recover/capture the solid particles
• Floating Separators Removal of solid objects in a solid-liquid phase through the use of bubbles. Horizontal vessels are used and fluid directed through the chamber would be fed by bubbles from underneath. The bubbles would tend to float the solid particles and this would captured at the upper portion of the vessel with the aid of baffles. Utilized in a Deinking process in the pulp and paper industry.
Separator Design and Construction • Usually characterized as vertical, horizontal or spherical • Parts of a separator 4 major sections: primary separation, gravity (secondary), coalescing, sump • Primary section separates main portion of free liquid through inertial effects or abrupt change in direction. • Gravity section utilizes gravitational force for enhanced separation and entrainment of droplets • Coalescing section uses a mist extractor to remove very small droplets of liquid from gas. • The sump section is basically a collector of all liquid from the gas stream
Separator Design and Construction • Separator Sections:
A – primary B – secondary C – coalescing D – sump
Separator Design and Construction • Separator Configurations: Factors to consider in separator selection: handling of extraneous material available floor space transportation and handling issues spacing for interfacing room for additional features, ie heat coils surface area for degassing of separated liquid handling of surge liquid necessary for large liquid retention volume?
Separator Design and Construction •
Vertical Separators high gas-liquid ratios low total gas volume handling capacity increases with increase in height level controls not critical use of mist extractors to reduce vessel diameter example: compressor suction scrubber
Separator Design and Construction •
Horizontal Separators high total fluid volume large amounts of dissolved gas provides for larger liquid surface area increased capacities through shorter retention time and increased liquid levels example: rich amine flash tank
Separator Design and Construction • Spherical Separators high pressure service compactness low liquid volumes
Specifying Separators • Basic parameters: temperature, pressure, flow rates, physical properties of the fluids as well as degree of separation • Define time frame of separation occurrence
• For known fluids, specify type and amount, also state ie. mist, free liquid or sludge • Select worst case scenario and apply safety factors: “safer to be wrong on the right side” A compressor suction scrubber desgined for 70-150 MMscfd gas at 400-600 psig and 65-105 oF would require the seprator manufacturer to offer a unit sized for the worst conditions, ie. 150 MMscfd at 600 psig and 105 oF
Specifying Separators
K factor (ft/sec)
C factor (ft/hr)
Horizontal (w/vertical pad)
0.4 50 0.5
1440 to 1800
Spherical
0.2 to 0.35
720 to 1260
0.18 to 0.36
648 to 1260
0.36 0.33 0.30 0.27 0.21
1260 1188 1080 972 756
Wet Steam
0.25
900
Most Vapor under vacuum
0.20
720
Salt and Caustic Evaporators
0.15
540
Separator Type
• Basic design equations for separators with mist extractors (vertical): critical velocity (max) Vt K
correlation by Sounders and Brown
( l g )
g
( ft
Gm – maximum allowable gas mass-velocity necessary for particles of size Dp to drop or settle out of gas
Gm C
g ( l g ) (lb / hr. ft 2 )
Vertical or Horizontal (w/horiz. Pad) @atm pressure @300 psig @600 psig @900 psig / sec) @1500 psig
Note: (1) (2) (3)
(4)
K = 0.35 @100 psig – subtract 0.01 for every 100 psi above 100 psig For glycol and amine solutions, multiply K by 0.6 – 0.8 Typically use one half of the above K or C values for approximate sizing of vertical separators without woven demisters For compressor suction scrubbers and expander inlet separators multiply K by 0.7-0.8
Specifying Separators • Horizontal separators with mist extractors are sized using similar equations + additional factors for length, L. Vt K
( l g ) L 10 g
0.56
L Gm C g ( l g ) 10
0.56
• Gas capacity is calculated by subtracting the cross sectional area occupied by the liquid from the vessel cross section Gas
• Common for horizontal separators to maintain its seam-seam length to its diameter ratio of between 2:1 to 4:1
Specifying Separators • Important note: The separator sizing equations given are used in the sizing of the separation elements. It is common for the separation elements to be placed in a larger vessel ie. For surging purposes.
Specifying Separators • Mass flow rates: In most instances it is convenient to use mass flow rate for sizing purposes. When handling gas flows, the flow is given in volume flow rate (MMSCFD)
M 3600Vtg
m 0.785Md 2 F The fraction of the total area available for gas flow can be found using the following table
D
h/D
F
h/D
F
0
1
0.30
0.748
0.05
0.981
0.35
0.688
0.1
0.948
0.40
0.626
0.15
0.906
0.45
0.564
0.2
0.858
0.50
0.5
0.25
0.804
0.55
0.436
h
Specifying Separators •
Horizontal separators without mist extractors are dependent of gravity as its sole mechanism for separation.
•
Important to set minimum droplet diameter to be removed
•
Typical range of droplet diameters 150 – 2000 microns
•
Vessel length can be calculated using,
Assuming the time taken for the 4 gas Qto a flow from inlet to outlet is the same as the Lofsize Dm to fall from the to pof the vessel to the liquid time for the liquid droplet VtDv surface
Example 1 A horizontal gravity separator ( without mist extractor) is required to handle 60 MMscfd (39.8 Ib/s) of 0.75 specific gravity gas (MW = 21.72) at a pressure of 500 psig and a temperature of 100 F. compressibility is 0.9, viscosity is 0.012 cp and liquid specific gravity is 0.50. It is desired to remove all entrainment greater than 150 microns in diameter. No liquid surge is required. Note: 1 micron = 0.00003937 in MMscfd = 1000000 ft3/day
Example 1 Solution Gas Density
g
Liquid Density
l
Mass flow rate
m
Particle Diameter
Dp
C’Re2 Drag Coefficient, C’ Terminal Velocity
Gas Flow rate
= P (MW) / RTZ = (514.7)(21.72) / ( 10.73)(560)(0.90) = 2.07 Ib/ft3 = 0.5 (62.4) = 31.2 Ib/ft3 = 60 x 106 ( 21.72) / ( 379)(24)(3600) = 39.8 Ib/sec = (150)(0.00003937) / (12) = 0.000492 ft = (0.95)x108 gDp3 (l-g) / 2 = 4738 = 1.40 4 gDp ( l g ) = Vt g 3C ' = 0.46 ft/sec = m/g = 19.2 ft3/sec
Example 1 Solution Assume a diameter, Dv Vessel Length, L
= 3.5 ft = 4Qa / Vt Dv = (4)(19.2)/(3.14)(0.46)(3.5) = 15.2 ft
Varying diameters, appropriate lengths
=Diameter, ft
Length, ft
3.5
15.2
4
13.3
4.5
11.8
5
10.6
Example 2 What size vertical separator without mist extractor is required to meet the conditions in example 1
Solution Area
= Q / Vt = 19.2/0.46 = 41.7 ft2
Dv
= 7.29 ft (minimum) = 90” ID selected
Separators with Wire Mesh Mist Extractors • Frequently used as entrainment separators for the removal of very small liquid droplets ( less then 10 microns) • Horizontally located and perpendicular to gas flow • Should be within 0-30o flat • Sizing is conducted using the previous terminal velocity equations for horizontal and vertical vessels ( K value also obtained from same table) •
Separators with Wire Mesh Mist Extractors
•
Separators with Wire Mesh Mist Extractors • Example 3 What size of vertical separator equipped with a wire mesh mist extractor is required for conditions used in the previous examples From table for K values: K = 0.31 ft/sec Vt 0.31
(31.2 2.07) 2.07
Vt 1.16 A
Q Vt
A
19.2 1.16
ft sec
Separators with Wire Mesh Mist Extractors
Dv 4.59 ft
A = 16.5 ft2 Vessel ID = 60 in
Separators with Vane Type Mist Extractors •
No draining back through rising gas stream
•
A downcomer is used to routes liquid out to drain
•
Inertia forces liquid droplets against the vane walls
•
Offer similar separation performance to wire mesh with the added advantage of higher resistance to plugging and cane be easily installed in smaller vessels
•
The dependence on inertial forces can be a disadvantage at reduced production rates
Retention Time in Separators • Liquid retention time – Retention time is average time a liquid molecule is retained in vessel – To ensure liquid and gas reach equilibrium so that gas molecule can evolve from liquid phase – Retention time = Volume of liquid storage in vessel Liquid flow rate – Usually 1 to 3 minutes
Retention Time in Separators • Oil/water retention time – Need certain amount of oil storage so that oil reaches equilibrium, entrained gas liberated, and ‘free’ water coalesced to fall into water storage – Need certain amount of water storage for entrained large droplets of oil have time to coalesce and rise to oil-water interface – Retention time 3 – 30 minutes
Separators with Centrifugal Elements • Separation of solids and liquids from a gas stream • Advantage over filter separators is lesser maintenance
• The disadvantage include : – Lower efficiency compared to other separator designs – Higher pressure drops compared to mist extractors – Narrow operating flow range to achieve higher efficiencies
Filter Separators • Higher separation efficiency compared to centrifugal separator • Periodic replacement of filter can be seen as a disadvantage • Solid particles are filtered out and the liquid phase is separated through coalecing small droplets
• Body size estimates for a horizontal filter separator uses a K value of 1.3 • Units designed for water will be smaller than units sized to remove light hydrocarbons
Filter Separators
Separators with Centrifugal Elements •
Example 4 A filter separator is required to handle a flow of 60 MMscfd at the similar conditions found in previous examples. Estimate the diameter of a filter separator
and
Vt 1.3
(31.2 2.07) 2.07
A = QA/Vt
= 19.2/4.88 = 3.93 ft2 Dv = 2.2 ft = 26.9 in. min. Select a 30” ID separator
Liquid-Liquid Separators • Divided into 2 broad separation categories: gravity and coalescing • Horizontal and vertical separators share the same principles of separation; horizontal separators have the advantage of a larger surface area •
2 factors affecting gravity separation in the liquid phase: – extra fine particles with random movement – electric charge from dissolved ions (repelling instead of coalescing)
• Separator sizing is based on Stokes’ Law
Liquid-Liquid Separators • Vertical vessels Wcl C * • • •
( Shl Sll )
(0.785) Dv 2
Wcl – flowrate of light condensate liquid (bbl/day) Shl – specific gravity of heavy liquid Sll – specific gravity of light liquid
• Horizontal vessels Wcl C * • •
( Shl Sll )
(0.785) LlHl
Ll - length of liquid interface area, ft Hl – width of liquid interface area, ft
• For unknown droplet sizes liquid-liquid separator sizing can be done through retention time, U– volume of settling section, bbl W – total liquid flow rate, bbl/day
U
W (t ) 1440
Liquid-Liquid Separators • Values of C*
Emulsion Charactersitics
Droplet diameter (microns)
C*
Free liquids
200
1100
Loose emulsion
150
619
Moderate emulsion
100
275
Tight emulsion
60
99
Liquid-Liquid Separators • Typical retention time for liquid-liquid separation Type of Separation
Hydrocarbon/water Separators Above 35o API HC Below 35o API HC 100oF and above 80oF 60oF
Retention time (min)
3-5 5-10 10-20 20-30
Ethylene Glycol/HC separators
20-60
Amine/HC separators
20-30
Coalescers, HC/Water separators 100oF and above 80oF 60oF
5-10 10-20 20-30
Caustic/Propane
30-40
Caustic/Heavy Gasoline
30-90
Separators with Centrifugal Elements •
Example 5 Determine the size of a vertical separator to handle 600 bpd of 55 o API condensate and 50 bpd of produced water. Assume the water particle size is 200 microns. Other operating conditions are as follows: Operating temperature = 80 F Operating pressure = 1000 psig Water specific gravity = 1.01 Condensate viscosity = 0.55 cp @ 80 F Condensate specific gravity for 55o API = 0.76
For 200 microns, C* =*1100 ( Shl Sll )
Wcl C
(0.785) Dv 2
Separators with Centrifugal Elements • Example 5 600bbl / day 1100
(1.01 0.76) (0.785) Dv 2 0.55
Dv 1.24 ft
Using manufacturer’s std size vessels might result in specifying a 20” OD separator
Separators: Construction Aspects • Fabrication specifications: governed by specific codes and standards ASME pressure vessel code ( the most widely used: Div 1 and 2)
BS/EC JIS DIN
Separators: Construction Aspects • Vessel Shell Thickness as specified by ASME VIII, Div 1 (sect UT-27)
PRi t SE 0.6 P t
PRo SE 0.4 P
Spheres:
t
PRi SE 0.2 P
t Ri Ro P S E
-
thickness internal radius of shell (exc. Corrosion allowance) external radius of shell working pressure maximum allowable stress joint efficiency
Double Welded Butt Joint Fully radiographed 1.0 Spot radiographed 0.85 No radiographed 0.70 Single Welded Butt Fully radiographed Spot radiographed No radiographed
Joint 0.9 0.80 0.65
Separators: Construction Aspects • Weight and Deck Area calculations
Wb 15dt
Wb d t
- mass per unit length (Ibm/ft) - internal diameter, in - wall thickness (inc. corrosion allowance), in
The weight of the internals (Wi) may be estimated from the following table:
For skidded equipment the following factors have been found satisfactory for preliminary estimates: Piping, W p – 40% of W v Electrical and Instumentation, W e – 8% of W v Skid Steel, W s – 10% of W v Wskid = W v+ W p + We + W s
Separators: Construction Aspects The total weight of the vessel can now be estimated using: Wv = WbL + WI + WN
Separators: Instrumentation and Controls Split range level control
Level control with for pumping
Separators: Instrumentation and Controls Liquid residence time and control
