topic 3 1 class notes separators jan2011 edited

Facilities EngineeringTransportation and Storage

EMB 5443

Mohd Shiraz Aris

Department of Mechanical Engineering,

Universiti Teknology PEtronas.

Separators and Filters

Acknowledgement: Pn. Putri Nazirah, Dpt of Chemcial Engineering, UTP

SeparatorsWeek Date Topics Lecturer Assessment

1& 2 24/01-04/01 Introduction:

E&P business, PSC, Project Life Cycle Concept

MSA/AL

3 & 4 07/02-18/02 Field Development Concept:

Fixed Platform, Manned and Unmanned Platform,

Minimum manning, Jackets, Tripod & Monopod,

Subsea Facilities Concept, Guyed Tower System,

Light Weight and Concrete Gravity Structures,

FPSO and FSO System, Integrated Development

Systems

MSA/AL

Group Project

Start (MSA)

&

Lab Work

(MSA)

5 21/02-25/02 Oil and Gas Production Processes:

Oil Production Process, Gas and Production

Process

MSA/AL

Lab Work

(MSA)

11 21/3-1/04 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 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

• 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

reservoir

well

wellhead

Wellhead manifold FIRST STAGE

SECOND STAGE

To export

Disposal

Storage tank – final oil treatment

Water treatmentWater

Gas to gas scrubber and gas compression module

Oil

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

– MomentumFluid phases at different densities have different momentumChanges in fluid direction will separate fluids at different momentum

– GravityLiquid phase separated from gas due to difference in weight of droplets

– CoalescenceSmall droplets coalesced when “combined” togetherCoalescing 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• Corrosive tendencies

Must know and 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 (effect of gravity is enhanced by spinning the fluids at a high velocity)

Selection of separators is based on obtaining the desired results at the lowest cost

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

The drag coefficient C’ is found to be a function of the

particle shape and Re of the flowing gas

drag

'

)(2

CA

gmVt

pgl

glp

'3

)(4

CA

gD

pg

glp

gravity

Gas velocity

Liquid /solid

droplet

Governing Laws

• Particles are assumed to be a solid sphere

• 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:

gtpVD1488Re

2

382' )()10)(95.0(

Re

glpgDC

Estimation of Particle Size

Estimation of Particle Distribution in a

Separator

Governing Laws

• Gravity settling for larger particlesfor particles 1000 microns or larger, Newton’s Law with a limiting drag coefficient of 0.44 (Re >500). Substituting for C’ = 0.44

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

g

glp

t

gDV

)(74.1

33.02

)(

glg

crp

gKD

Governing Laws

• Stokes Law

for low Re (less than 2), a linear relationship exists between Drag and Re

Dp for Re less than 2 is found using Kcr = 0.008 in,

The lower limit of Stokes Law is for a droplet diameter of approx. 3 microns.

18

)(1488 2glp

t

gDV

33.02

)(

glg

crp

gKD

Alternative Generic Terminal Velocity Formulae

• Particles falling through a fluid by the pull of gravity:

Where,

A and N are constants related to the flow regime and the drag coefficient as

determined by

3/1

2

)(

lpl

p

gDK

)2

1(

)1(

1

3

)(4 N

Nt

A

gDV

Nl

lpN

p

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 Sections:

A – primary

B – secondary

C – coalescing

D – sump


• 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


• 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


• 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 400-600psig and 65-105 oF would require the seprator manufacturer to offer a unitsized for the worst conditions, ie. 150 MMscfd at 600 psig and 105 oF


• Basic design equations for

separators with mist

extractors (vertical):

critical velocity (max)

correlation

by Sounders and Brown

Gm – maximum allowable gas mass-velocity

necessary for particles of size Dp to drop or

settle out of gas

sec)/()(

ftKVg

gl

t

)./()( 2fthrlbCG glgm

Separator TypeK 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

Vertical or Horizontal (w/horiz. Pad)

@atm pressure

@300 psig

@600 psig

@900 psig

@1500 psig

0.18 to 0.36

0.36

0.33

0.30

0.27

0.21

648 to 1260

1260

1188

1080

972

756

Wet Steam 0.25 900

Most Vapor under vacuum 0.20 720

Salt and Caustic Evaporators 0.15 540

Note:

(1) K = 0.35 @100 psig – subtract 0.01 for every 100 psi above 100 psig

(2) For glycol and amine solutions, multiply K by 0.6 –0.8

(3) Typically use one half of the above K or C values for approximate sizing of vertical separators without woven demisters

(4) For compressor suction scrubbers and expander inlet separators multiply K by 0.7-0.8


• Horizontal separators with mist extractors are sized using similar equations + additional factors for length, L.

• Gas capacity is calculated by subtracting the cross sectional area occupied by the liquid from the vessel cross section

• Common for horizontal separators to maintain its seam-seam length to its diameter ratio of between 2:1 to 4:1

56.0

10

)(

LKV

g

gl

t

56.0

10)(

LCG glgm

Gas


• 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.


• 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)

The fraction of the total area available for gas flow

can be found using the following table

gtVM 3600

FMdm 2785.0

h/D F

0 1

0.05 0.981

0.1 0.948

0.15 0.906

0.2 0.858

0.25 0.804

h/D F

0.30 0.748

0.35 0.688

0.40 0.626

0.45 0.564

0.50 0.5

0.55 0.436

D

h


• 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 gas to flow from inlet to outlet is the same as thetime for the liquid droplet of size Dm to fall from the to pof the vessel to the liquidsurface vt

a

DV

QL

4

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 = P (MW) / RTZ

= (514.7)(21.72) / ( 10.73)(560)(0.90)

= 2.07 Ib/ft3

Liquid Density l = 0.5 (62.4)

= 31.2 Ib/ft3

Mass flow rate m = 60 x 106 ( 21.72) / ( 379)(24)(3600)

= 39.8 Ib/sec

Particle Diameter Dp = (150)(0.00003937) / (12)

= 0.000492 ft

C’Re2 = (0.95)x108 gDp3 (l-g) /

2

= 4738

Drag Coefficient, C’ = 1.40

Terminal Velocity =

= 0.46 ft/sec

Gas Flow rate = m/g

= 19.2 ft3/sec

'3

)(4

C

gDV

g

glp

t

Example 1

Solution

Assume a diameter, Dv = 3.5 ft

Vessel Length, L = 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)

•


•


• 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

07.2

)07.22.31(31.0

tV

sec16.1

ftVt

tV

QA

16.1

2.19A


A = 16.5 ft2

Vessel ID = 60 in

ftDv 59.4

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


• 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

A = QA/Vt = 19.2/4.88

= 3.93 ft2

Dv = 2.2 ft

= 26.9 in. min.

Select a 30” ID separator

07.2

)07.22.31(3.1

tV

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


• Vertical vessels

• Wcl – flowrate of light condensate liquid (bbl/day)

• Shl – specific gravity of heavy liquid

• Sll – specific gravity of light liquid

• Horizontal vessels

• 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

2* )785.0()(

v

llhl

cl DSS

CW

ll

llhl

cl HLSS

CW )785.0()(*

1440

)(tWU


• 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


• Typical retention time for liquid-liquid separation

Type of Separation Retention time (min)

Hydrocarbon/water Separators

Above 35o API HC

Below 35o API HC

100oF and above

80oF

60oF

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


• Example 5

Determine the size of a vertical separator to handle 600 bpd of 55o 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 2* )785.0()(

v

llhl

cl DSS

CW


• Example 5

Using manufacturer’s std size vessels might result in specifying a 20” OD

separator

2)785.0(55.0

)76.001.1(1100/600 vDdaybbl

ftDv 24.1

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


• Vessel Shell Thickness

as specified by ASME VIII, Div 1 (sect UT-27)

PSE

PRt

i

6.0

t - thicknessRi - internal radius of shell (exc. Corrosion allowance)Ro - external radius of shellP - working pressure S - maximum allowable stressE - joint efficiency

Double Welded Butt JointFully radiographed 1.0Spot radiographed 0.85No radiographed 0.70

Single Welded Butt JointFully radiographed 0.9Spot radiographed 0.80No radiographed 0.65

PSE

PRt

o

4.0

PSE

PRt

i

2.0

Spheres:


• Weight and Deck Area calculations

The weight of the internals (Wi) may be estimated from the following table:

dtWb 15 Wb - mass per unit length (Ibm/ft)d - internal diameter, int - wall thickness (inc. corrosion allowance), in

For skidded equipment the following factors have been

found satisfactory for preliminary estimates:

Piping, Wp – 40% of Wv

Electrical and Instumentation, We – 8% of Wv

Skid Steel, Ws – 10% of Wv

Wskid = Wv+ Wp + We + Ws


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

topic 3 1 class notes separators jan2011 edited

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