micro bubble flotation technology in secondary tertiary produced water treatment english a4

Upload: deepakkumargorai

Post on 03-Jun-2018

224 views

Category:

Documents


2 download

TRANSCRIPT

  • 8/11/2019 Micro Bubble Flotation Technology in Secondary Tertiary Produced Water Treatment English A4

    1/19

    1www.exterran.com

    1 ABSTRACT

    Over the years a variety of oil/water separation methods have been developed

    throughout the world, including gravity separation, corrugated plate interceptors,

    centrifugal separation, hydrocyclones, and induced gas flotation. With increasingly

    tight legislative limits on OIW (Oil in Water) discharges, it is important that

    operators have an effective produced water treatment system which enables

    compliance with regulatory overboard discharge limits, meets specifications for

    re-injection, steam generation, irrigation and any other downstream process.

    This paper makes a comparison between various oil water separation

    technologies commonly used for secondary and tertiary treatment of produced

    water. It draws on Computation Fluid Dynamic CFD modeling, trial work and case

    studies. It also describes an overview of each separation method along with

    generalized configuration and internal designs of the applicable technology.

    Also this paper examines the current state of gas flotation in the oil and gas

    industry, and provides an in-depth look at the differences between flotation

    technologies including their strengths, weaknesses, and their niche in the

    contemporary marketplace. Different ways currently used to separate oil and

    solids from produced water will be compared and an explanation offered for

    why gas flotation technologies incorporating micro-bubbles remain the highest

    performance option within Secondary water treatment equipment for many

    companies around the globe.

    2 INTRODUCTION

    Many aging oil and gas production fields are experiencing rising water cuts which

    have increased the necessity for the handling of greater volumes of produced

    water. The need for more efficient treatment of produced water is exacerbated by

    the ever tightening discharge regulations, and the want of increased production

    given the current high price of oil. As a result of this the demand for more

    cost effective and efficient oil and water separation technologies has greatly

    increased, and will continue to do so in the future.

    For previously acceptable oil and grease effluent limits there are a number of

    widely accepted conventional separation methods that have been in use for a

    long time, and with great success. However, despite many companies allocating

    large sums of money to research and development of new products there have

    been relatively few genuinely new technologies that have emerged over the past

    few years. Many newer designs have relied on modifications of old designs to

    improve efficiencies, or different combinations of the same equipment.

    TECHNICAL PAPER

    Nicholas Owens

    Douglas W. Lee

    1GLR Solutions Ltd.

    Suite S, 1338 36th Ave. NE

    Calgary, AB

    Canada T2E 6T6.Tel: +1.403.219.2210

    The Use of Micro-Bubble Flotation Technology inSecondary & Tertiary Produced Water Treatment

    A Technical Comparison With Other Separation TechnologiesReprint of paper presented at the Produced Water Workshop - Aberdeen, Scotland - May 2007

  • 8/11/2019 Micro Bubble Flotation Technology in Secondary Tertiary Produced Water Treatment English A4

    2/19

    2www.exterran.com

    In more recent years some separation technologies have received more attention

    than others and there have been a number of advancements that have helped

    greatly increase the efficiencies of them. It is some of these on which this paper

    will focus. This paper will compare and contrast differences in these approaches

    to advancing the art of gas flotation.

    3 SEPARATION TECHNOLOGIES

    Oil water separation technologies can be broadly separated into two main

    types, namely gravity, and non-gravity based. The following sections present a

    summary of some of the main conventionally available oil and water separation

    methods that are available in both of these categories, most of which havebeen successfully used for many years and have thousands of commercial

    installations world wide.

    3.1 Non-Gravity Based Separation Technologies

    3.1.1 Hydro-cyclones

    Hydro-cyclones work by generating centrifugal

    forces on a stream of liquid (produced

    water). The difference in specific gravity

    between oil and the water causes the oil to

    migrate towards the centre of the vortex that

    is created in the cyclone, thus forcing it to

    one end of the cyclone and the water passing

    out of the opposite end. Figure 1 shows a

    typical hydro-cyclone in operation.

    Hydro-cyclones have been in use for

    many years and have a number of distinct

    advantages, their biggest one perhaps being

    their size and weight. They are compact

    pieces of equipment making them very

    attractive for offshore application where deck

    area and mass loading is always a premium.

    They are also capable of handling relatively

    large inlet oil concentrations (upwards of

    2000 ppm), making them highly versatile in

    production facilities.

    Figure 1 A Typical Hydro-Cyclone

    The main drawback to this technology is based on the fact that they require

    the density of the oil to be substantially different to that of the water to work

    efficiently. Oils with degrees of API of less than 15-17 cannot be easily separated

    by this method and can be prone to causing blockages inside the device. Tight,

    small (

  • 8/11/2019 Micro Bubble Flotation Technology in Secondary Tertiary Produced Water Treatment English A4

    3/19

    3www.exterran.com

    Due to their nature a large pressure drop is often associated with the operation

    of hydro-cyclones and without sufficient inlet pressures downstream processes

    can be affected. This pressure drop will be prone to fluctuate as banks of

    individual cyclonic tubes are brought on or off line depending on fluctuations in

    the inlet flow rate, due to the fact that each individual hydro-cyclone can handle

    a limited operating range of flows. Other limitations include the hydro-cyclones

    ability to deal with process fluctuations (slugging) and solids build up which can

    blind liquid paths and create additional pressure loss.

    3.1.2 Filtration

    There are various different types of filters commercially available that are capable

    of separating oil from water. The most common types for oil removal are Sand(Upflow and Downflow), Multi-Media, and Black Walnut Shell Filters, and an

    English Walnut Pecan mix. However, virtually all filters (with the exception of the

    Upflow) have one thing in common, in that they are more of a final polishing unit

    and are not well suited to high inlet oil concentrations (>50 ppm). If higher inlet

    oil concentrations are fed into these filters they become clogged quickly, cause

    fractures in the tight media bed, which causes breakthrough and then require

    excessive amounts of backwashing (cleaning). Thus large waste streams are

    produced which in turn require treatment, which can be costly.

    3.1.2.1 Walnut Shell Filters

    One of the most effective media for oil removal is crushed black walnut shells.The reason for this is that these shells are highly hydrophilic and oleophobic,

    meaning that although they do a good job of straining off oil they do not

    bind with, or hold on to it. Due to these characteristics once the system is

    mechanically backwashed the oil

    is easily released and the filter

    bed is returned to its original

    condition. Walnut shell filters are

    very frequently applied in the heavy

    oil market as they can remove oil

    to very low concentrations and

    are not affected by the low degree

    of API of the Oil. A typical walnut

    shell filter is shown in Figure 2. In

    addition to oil these filters are also

    capable of removing up to 95% of

    suspended solids (depending on

    inlet concentrations).

    Figure 2 A SabianBlack Walnut Shell Filter

  • 8/11/2019 Micro Bubble Flotation Technology in Secondary Tertiary Produced Water Treatment English A4

    4/19

    4www.exterran.com

    3.1.2.2 Sand Filters

    Sand filters have performances very similar to those of walnut shell filters, but,

    due to a number of important differences they are becoming less common in the

    oil and gas industry. A comparison of the main differences is shown in Figure 3.

    Sand Filters Walnut Shell Filters

    Typical Filtration Flux Rate 6 usgpm/ft2 12 usgpm/ft2

    Typical Backwash Duration 12 minutes 8 minutes

    Approx. Media Life / Attrition 30% per annum 5% per annum

    Media Regeneration Solvent / Surfactant Wash Simple Backwash

    Figure 3: Comparison of a SabianWalnut shell filter and a typical sand filter.

    As can be seen from the data sand filters need to be approximately twice the

    size to deal with the same flow rates, whilst generating much greater volumes of

    backwash fluid. High media attrition

    can also lead to significantly higher running costs. Oil will also tend to coat

    the sand granules requiring a solvent and potentially a surfactant wash cycle

    periodically. Both of these negatively impact the surface charge of the sand and

    its removal performance.

    3.1.2.3 Multi-Media Filters

    Multi-media filters operate by having a bed that comprises of different gradesof media, where porosity decreases with increasing depth. The result of this is

    that smaller and smaller particles are trapped in the media as the water travels

    down through the bed. The two most common media types used are Garnet and

    Anthracite (Coal). Figure 4 shows a typical configuration of a multi-media filter.

    In addition to oil, multi-media filters

    are also very effective in removing

    suspended solids from water. Due

    to the fact that oil tends to coat

    the media in the same way it does

    with the sand filters, walnut shellfilters are more commonly used for

    oil removal. Therefore these filters

    are commonly limited to solids

    removal. They can typically remove:

    95% of 5 m particles and 90% of

    2 m particles in a produced water

    stream. Backwashing is usually

    achieved with water although

    surfactants are normally required

    periodically to attain better results.

    Figure 4 A typical multi-media filter

  • 8/11/2019 Micro Bubble Flotation Technology in Secondary Tertiary Produced Water Treatment English A4

    5/19

    5www.exterran.com

    3.1.3 Coalescing Media

    Oleophilic and hydrophobic coalescing media are becoming more widely accepted

    as a conventional technology in the separation industry. These media are

    capable of achieving low outlet oil concentrations (

  • 8/11/2019 Micro Bubble Flotation Technology in Secondary Tertiary Produced Water Treatment English A4

    6/19

    6www.exterran.com

    labour to change the media and the disposal of spent media, which often needs

    to be transported large distances for disposal, the operating costs for these

    types of technologies can be very high. There is also the associated negative

    environmental impact due to the amount of near toxic waste that is generated.

    3.2 Gravity Based Separation Technologies

    Gravity separation technologies rely on the fact that the specific gravity of oil is

    less than that of water. If oily water is left to stand the oil will rise to the surface of

    the water where it can be skimmed off. Gravity separation technologies can broadly

    be divided into two main categories, those that operate with, and those that

    operate without the assistance of gas in the flotation process. Non gas assisted

    flotation includes gravity separation tanks and corrugated plate interceptors.

    3.2.1 Gravity Separation Tanks

    Gravity separation tanks are the simplest form of separation technology, and rely

    on diverting flow into a large settling tank and allowing gravity alone to act on

    the oil and water. They perform best with lighter oil as performance drops with

    the oil degree of API. A large amount of time is required for separation to occur

    and the tanks need to be very large. This makes them poorly suited to offshore

    applications where space is at a premium. Once the oil is on the surface there

    are a wide variety of different skimming methods to remove it. Depending on inlet

    condition the best achievable outlet oil in water concentrations is 50-100 ppm,

    which today is unacceptable. A further separation stage is required downstreamof such tanks.

    3.2.2 Corrugated Plate Interceptors

    In 1962 the first corrugated plate interceptors (CPIs) began to appear. The

    principle of a CPI is similar to that of a gravity separation tank, but inside are

    inclined flat plates that act as a coalescing surface for oil droplets. This enables

    small oil droplets to merge and the larger coalesced oil droplets are therefore

    able to float to the surface more rapidly. With the same inlet condition, similar

    outlet performance to gravity separation tanks can be achieved, but the main

    advantage is that the size of the vessel is greatly reduced, thus leading to large

    cost savings. To improve performance modern CPIs typically use a series of

    plates, often in combination with one of the coalescing media discussed above.

    The disadvantage of CPIs is that they are generally restricted to the removal of

    oil droplets of greater than 50 microns, with large amounts of chemicals being

    required to remove lower particle sizes. They are sensitive to fluctuations in flow

    and perform poorly in upset conditions, and with heavier or emulsified oils, and

    can become blocked with high solids loading. Due to these limitations onshore

    facilities typically pair CPIs with gravity tanks.

    3.2.3 Gas Assisted Flotation Technologies

    The use of gas bubbles to assist with flotation is not a new idea and has anumber of distinct advantages. Developed and first used in the mining industry

    for many years, gas flotation units have become the preferred method of

    secondary oil and water separation for many oil producers today. Gas flotation

  • 8/11/2019 Micro Bubble Flotation Technology in Secondary Tertiary Produced Water Treatment English A4

    7/19

    7www.exterran.com

    relies on introducing gas bubbles into a produced water stream to assist with

    separation. The gas bubbles grow and join with the oil droplets and greatly

    accelerate the separation rate in terms of both time and efficiency. The versatility

    of flotation technology and the ability to handle upset conditions has helped

    define their niche in the market. Gas flotation equipment comes in a variety

    of types and configurations, with it most commonly occurring in horizontal and

    vertical pressure vessels and lately in larger specially designed API type skim

    tanks. When compared to other gravity separation technologies gas flotation

    units are typically smaller per unit of flow treated, and are therefore well suited to

    both onshore and offshore applications.

    4 PRINCIPLES OF EFFECTIVE FLOTATION

    Before a successful comparison of the various gas flotation technologies can be

    made it is useful to have a basic understanding of the principles of gas flotation,

    and what the parameters are that affect the efficiency of the process. This

    subject has been discussed at length in other papers [1], but a brief summary of

    the main factors is given below.

    4.1 Contact of Gas Bubbles and Oil Droplets

    For any type of gas flotation to occur the first thing that must happen is for a gas

    bubble to come into intimate contact with an oil droplet. To ensure that there is

    a high probability of contact between these two particles it is therefore desirableto have any bubble release/mixing points located at or near to points where the

    whole of the flow passes. This prevents only very localised mixing from occurring.

    The amount of time during which the oil droplet and gas bubble are in close

    proximity is critical, and it is for this reason that bubble size is important. If

    the gas bubbles rise too fast they will be less likely to come into contact with

    an oil droplet. Small gas bubbles rise more slowly than big ones and improve

    contact time. Even distribution within the vessel ensures that the oil droplets

    of the same size and larger than the gas bubbles will effectively be removed.

    As technology has improved bubble size has been reduced and Micro-Bubble

    Flotation (MBF) is the term that is typically used for flotation where gas bubblesare less than thirty microns.

    4.2 Adhesion or Encapsulation

    Once a gas bubble and oil droplet have come into contact it is critical that they

    unite, and once they are combined they stay together. There are two main ways in

    which this bonding can occur, namely adhesion and encapsulation:

    a) Adhesion is a relatively weak bond between a gas bubble and oil droplet, but is

    the most likely to occur in a gas flotation system. It occurs when there is contact

    between a small section of the surface of both the bubble and the droplet, and

    looks as if the two particles are stuck together. As the bond is weak it can be

    broken fairly easily with turbulent flow. The surface of the gas bubble can also act

    as a coalescing surface and if two oil droplets adhere to the same gas bubble

    they can combine.

  • 8/11/2019 Micro Bubble Flotation Technology in Secondary Tertiary Produced Water Treatment English A4

    8/19

    8www.exterran.com

    b) Encapsulation is a stronger bond and occurs when an oil droplet completely

    surrounds a gas bubble. This situation is more desirable for the gas flotation

    process as it is much more difficult to separate the two particles, meaning their

    chance of reaching the surface together is greatly increased. Figure 5 shows oil

    encapsulating gas bubbles.

    Figure 5 Oil encapsulation of gas bubbles

    4.3 Low Turbulence and Shear Forces

    Once adhesion or encapsulation has occurred between an oil droplet and a gas

    bubble it is desirable that the two remain in contact until they reach the surface.

    If there is premature detachment re-entrainment will occur, which will have a

    detrimental effect on performance. The most likely causes of forced separation

    are due to large shear forces and turbulent flows inside a flotation tank or vessel,

    when the two particles are effectively ripped apart from one another. There are

    two main ways that these forces are generated, the first is due to poor internal

    design inside a gas flotation vessel, and the second, and the most common is

    related to the size of the gas bubble. Large gas bubbles that rise quickly create

    large amounts of turbulent flow in their wake, and it is therefore again better to

    have small gas bubbles that create significantly lower forces.

    4.4 Fluid Dynamics

    Well designed vessel or tank internals have a huge impact on the efficiency of a

    gas flotation system. Poor designs can lead to short circuiting, where oil droplets

    are carried straight to the exit and are not acted upon by the gas bubbles.Localised high downward velocities can also lead to gas bubbles being dragged

    down and carried out of the vessel. This is highly undesirable especially if there

    are pumps downstream as the gas can cause them to cavitate. Today these

    issues are most easily addressed with computer simulations, or CFD modeling.

    4.5 Sequential Removal

    A mixture of CFD and experimental data [2] has shown that a compartmentalised

    (or multichambered) configuration inside a gas flotation vessel or tank is highly

    desirable. A multichambered approach involves a certain percentage of oil

    removal at each stage of the process, with each chamber acting as a separate

    removal cell. Upsets can also be effectively dealt with as they are held up in the

    first chamber, making their removal easier and protecting the final chambers

    from contamination.

  • 8/11/2019 Micro Bubble Flotation Technology in Secondary Tertiary Produced Water Treatment English A4

    9/19

    9www.exterran.com

    4.6 Skimming Method and Location

    When an oil droplet and gas bubble reach the surface the bubble will usually

    burst, leaving a layer of oil on the surface that needs to be removed. Having

    a large build-up of oil on the surface (a low interface) is undesirable as re-

    entrainment can easily occur, and a large amount of oil will also reduce the

    effective volume of the tank or vessel that is available for separation. There are a

    variety of different skimming methods available, but a good skimmer is generally

    considered to be one that removes oil from as large a surface are as possible,

    and does not skim unnecessarily high volumes of water.

    5 DIFFERENT FLOTATION TECHNOLOGIES5.1 Bubble Generation Methods

    There are a number of different ways of producing bubbles and introducing them

    into the flotation chamber, and different companies use one or multiple methods

    for their various products. They can however be broadly classified into the

    following sections.

    1. Sparging This is the simplest way of introducing gas into the process stream

    and was the first to be used commercially. A typical setup is shown in figure 6,

    and involves a pipe being inserted directly into the chamber, through which gas

    is forced. Sparging is simple and cheap, but has a number of problems. Early

    spargers were not easy to maintain as the diffuser heads or holes in the piped

    were prone to blockage due to solids settling, or more commonly chemical

    reactions between the gas and the liquid would occur immediately at the point

    of gas injection. However, the main problem is that the bubbles created are very

    large, meaning that their high rise velocities and low surface area make it difficult

    for them to attach to oil droplets, and if they

    manage to do so the large amount of turbulent

    flow that the mechanical sparger and bubbles

    generate makes premature separation more

    likely. The location of introduction is usually

    in the centre of the chamber meaning thatlarge parts of the flow never see any gas

    bubbles, making effective separation very

    difficult. Nowadays few companies use

    this method of gas introduction, with most

    switching to eductor based designs.

    Figure 6 A typical sparging tube

    2. Eductors An eductor, sometimes called a venturi, is a device that contains a

    throat through which a fluid (water) passes (Figure 7). The higher fluid velocities

    in the throat create a partial vacuum which is used to draw in a second stream(gas) into the fluid. The magnitude of P and the flow rate through the device

    determines the amount of gas that is drawn into the main flow. The advantage of

    these devices is that they are fairly simple to operate and maintain and are very

  • 8/11/2019 Micro Bubble Flotation Technology in Secondary Tertiary Produced Water Treatment English A4

    10/19

    10www.exterran.com

    reliable. They have two main disadvantages however, the fist being that a large

    pressure drop is generated across them, which, especially at larger flows requires

    a larger pump. The second issue is with regard to bubble size. The size of the

    gas bubbles that are generate is determined by the width of the throat inside

    the eductor. This can be relatively large,

    meaning only large bubbles are generated,

    which, as noted above is not desirable.

    Almost all gas flotation vendors currently

    use eductors in various products, but many

    designs have some sort of diffuser type

    nozzle to try and reduce bubble size. Some

    systems use a striker plate to divert/diffuse the flow while others use in-line

    static mixers to create shear to try and

    break up the bubble. This goes some way

    to addressing the problem of bubble size

    but larger bubbles are still present.

    Figure 7 A typical Eductor

    3. Pumps Several types of specialist pump exist that will introduce small

    gas bubbles into a stream of produced water. These pumps operate in slightly

    different ways, and a summary of them is given below. Regardless of the typepumps are advantageous for their small footprint, making them ideal for offshore

    applications. Their disadvantage is that they do not perform as well when

    operated at very high temperatures.

    DGF pumps:Dissolved Gas Flotation (DGF) pumps work by water and gas being

    drawn into the eye of the pump suction, where there is a natural vacuum due to

    a specially designed back-vane impellor. Once gas reaches the forward edges

    of the impellor it is sheared, and there is subsequently a high pressure region

    which causes gas to dissolve into the water stream. Once a pressure drop is

    encountered downstream (usually across a globe valve) the bubbles break out

    of solution, and can then be used for flotation. The big advantage of this type of

    pump is that the bubbles produced are typically much smaller than those created

    with eductors, and therefore more effective for flotation. The main problem with

    DGF pumps is their limitations for use with insoluble gases or in high temperature

    water conditions. These conditions limit the ability of the pump to dissolve or

    vacuum induce gas from a vessel headspace without cavitation. Due to the single

    stage pump design additional challenges can be had due to lower discharge

    pressures. This is especially important in 50 Hz applications where even less

    pressure and shear is generated due to slower impeller rotation speeds.

    ONYX Micro-bubble pumps:Like the DGF pump, the ONYXpump (Figure 8)

    takes gas and water into its suction. Inside it utilizes a multi-stage centrifugal

    design to impart impact and shear on the large gas bubbles to shatter them

    and generate the small micro-bubbles (circa 30 micron). This physical method of

    bubble generation (the gas is not dissolved) entrains approximately 40% more

    gas than a DGF pump, and is the only micro-bubble generating pump that utilizes

  • 8/11/2019 Micro Bubble Flotation Technology in Secondary Tertiary Produced Water Treatment English A4

    11/19

    11www.exterran.com

    ANSI, or optionally API, components (couplings, bearings, seals, shafts etc.),

    giving a high degree of suitability for the oilfield environment. Use of the pump

    for MBF allows for smaller footprints and weights which are critical in offshore

    applications. Due to its multistage design high outlet pressures can be achieved

    at 50 or 60 Hz.

    Figure 8 The ONYX Micro-bubble pump

    4. GLR Reactors Gas Liquid Reactors (GLR) are specialist bubble making

    devices supplied uniquely by GLR Solutions Ltd. They consist of a small vertical

    pressure vessel which contains a special internal nozzle. Fluid containing large

    gas bubbles passes into the GLR unit where impact and shear forces shatter

    the large gas bubbles and create m icro-bubbles. These are typically in the

    range of 5-50 micros, which are some of the smallest bubbles produced by any

    commercial technology and therefore produce some of the highest oil water

    separation rates in the industry. There are no moving parts, or parts requiring

    special maintenance, inside making operating cost very low. A typical GLR reactor

    is show in Figure 9.

  • 8/11/2019 Micro Bubble Flotation Technology in Secondary Tertiary Produced Water Treatment English A4

    12/19

    12www.exterran.com

    The main disadvantage of GLR

    Reactors is their size. For large flow

    rates these vessels can become

    rather large, meaning that if space

    is an issue, i.e. offshore, they are

    not always suitable.

    Figure 9 A Gas Liquid Reactor

    5.2 Tanks and Vessels

    When gas flotation is carried out

    inside a horizontal or vertical vessel

    the process is typically called

    Induced Gas Flotation (IGF). In more

    recent times gas flotation is also

    carried our inside large API type

    tanks. This could still be considered

    to be Induced Gas Flotation, but

    is more commonly referred to as

    Tank Based Flotation. It is also increasingly common to retrofit many existing

    horizontal and vertical vessels such as degassers into flotation units, giving

    significant footprint and weight advantages.

    5.2.1 Horizontal Vessels

    Horizontal IGF vessels are most often found on shore as they tend to be larger

    than vertical units, and offer better performance. Depending on the manufacturer

    horizontal IGF vessels differ in design considerably, with some vessels contain

    single cells (or chambers) and others contain multiple chambers. Some vessels

    contain partial inter-chamber walls, other have full length walls. Despite these

    difference there are

    also various features

    that most IGF units

    have in common.

    Figure 10 shows a

    horizontal IGF designed

    by GLR Solutions Ltd.

    This is followed by a

    comparison of various

    common similarities and

    difference that there are

    between different IGF

    units on the market.

    Figure 10 A Horizontal Revolift

  • 8/11/2019 Micro Bubble Flotation Technology in Secondary Tertiary Produced Water Treatment English A4

    13/19

    13www.exterran.com

    5.2.2 Chamber Dividing Walls

    The majority of vessels on the market contain multiple internal chambers for

    oil removal. Separated by dividing walls the aim of each chamber is to act as a

    separate oil removal cell with the walls helping by trying to reduce short circuiting

    from the main inlet

    directly to the main

    outlet. The majority

    of the units on the

    market contain partial

    walls with a gap, hole

    or baffle that is open

    at the bottom, withthis void allowing

    water to flow freely

    from one chamber to

    the next. Figure 11

    shows a unit, with the

    underflow baffle (gap) marked on it.

    Figure 11 A unit with a partial dividing wall (underflow baffle)

    These partially enclosed walls are becoming less common on new installations

    as although they go some way to reducing short circuiting it still tends to occur

    along the bottom of the vessel. Oil that passes under one on the baffles has

    a chance of passing under the remaining baffles and making its way to the

    exit of the vessel. More modern designs tend to eliminate this problem by

    completely sealing off this wall and channelling the flow to where it is desired.

    Figure 12 shows an example of a chamber wall that is completely sealed off.

    Flow passes under a baffle or weir that is parallel to the length of the vessel and

    exits the chamber through a small hole further up the vessel. As the flow has

    to pass through a much more complicated route short circuiting can be almost

    completely eliminated.

    Interconnecting

    Hole

    Outlet Weir

  • 8/11/2019 Micro Bubble Flotation Technology in Secondary Tertiary Produced Water Treatment English A4

    14/19

    14www.exterran.com

    5.2.3 Water Weirs / Water Introduction

    The location and method that water is introduced into a chamber is important

    to the process as it can be used to enhanced separation performance. As

    mentioned previously, older designs that have partial dividing walls use these

    gaps in the walls to introduce the flow from one chamber to the next. To prevent

    short circuiting more modern designs tend to introduce the flow higher up the

    chamber, meaning that any oil droplets have a more challenging path to get to

    the outlet. Some do this partially by adding a second baffle next to the partial

    wall; others divert the flow by adding an elbow type an arrangement to send the

    flow upwards (Figure 12). Some of the newest designs take this one step further

    by having weir structures inside each of the chambers. These can be used to

    optimise flow patterns, minimise short circuiting and maximise performance. CFDhas played a large part in this development.

    Figure 12 Methods of Introducing Bubble

    5.2.4 Oil Skimming

    Once oil has been floated to the surface the location and method that it is

    removed is critical for successful operation. Skimming methods can be most

    easily divided into two categories,

    mechanical and non-mechanical.

    Mechanical skimmers typically rely

    on rotating paddle-wheels or discs

    to remove oil from the surface of the

    water. These can be highly effective if

    the oil is concentrated in a compact

    region, and skimming volumes can be

    easily controlled by adjusting the speed

    of rotation. Figure 13 shows a typical

    paddle wheel in operation. The main

    disadvantage of mechanical skimmers

    is that they have lots of moving parts

    and therefore require a relatively large

    amount of maintenance.Figure 13 A Mechanical Skimmer

  • 8/11/2019 Micro Bubble Flotation Technology in Secondary Tertiary Produced Water Treatment English A4

    15/19

    15www.exterran.com

    Non-mechanical skimmers can be further subdivided into two main categories;

    floating and fixed. Floating skimmers are buoyant and float on the surface of the

    water, whereas fixed skimmers, or troughs, are static and do not move. Floating

    skimmers are well suited to applications where water levels vary e.g. offshore.

    In these environments fixed skimmers can flood, and then be followed by periods

    of no flow. The main disadvantage of floating skimmers is their area of influence.

    Floating skimmers are typically round, with the area that is available for skimming

    being limited to the circumference of the oil skimmer. Once a region is skimmed

    the proportion of water that is skimmed tends to increase, and areas that are not

    close to the skimmer tend to build up with oil. Fixed skimmers tend to be much

    simpler in design, are usually longitudinal troughs (often containing v-notches)

    that have a much greater skimmer area. This means that more even skimmingoccurs, and the skim is of a greater quality. Skimming volumes are controlled by

    raising or lowering the water level inside the vessel. Due to their simplicity they

    usually require little or no maintenance, significantly reducing operating costs.

    Figure 14 shows a comparison of the two types.

    Figure 14 F ixed and Floating skimmers

    5.2.5 Bubble Injection

    Except for systems utilising spargers, where gas is directly injected into the

    process vessel, the majority of flotation units operate by taking a slipstream of

    the clean outlet water. This slipstream, typically 20-30% of the inlet flow, is then

    utilised as the steam to which bubbles are added using one of the methods

    described above. This bubble rich stream is then recycled back into the IGF

    vessel, and flotation is therefore able to occur. Figure 15 shows a typical process

    flow for a For the flotation process to occur efficiently the release point where

    the bubble stream and water stream mix is critical. Some systems which utilise

    larger bubbles release the gas stream near the bottom of a chamber, with theaim being that as the bubbles rise like a blanket they come into contact with oil

    droplets. This method is not particularly efficient as the bubbles rise fast, and

  • 8/11/2019 Micro Bubble Flotation Technology in Secondary Tertiary Produced Water Treatment English A4

    16/19

    16www.exterran.com

    usually spread our in a cone shape and miss part of the flow. It is not possible

    to do this with smaller gas bubbles as their lower rise velocities mean they would

    be drawn down. Most modern systems mix the bubble and water streams to the

    inlet piping, before the flow even enters the vessel. This has several advantages

    as there is an increased chance of intimate contact between oil and gas

    particles, and there is a lesser change of gas bubbles missing any part of the

    flow. A boost of additional bubbles is typically needed in each chamber, with ideal

    locations being ones where contact with the entire flow is most likely.

    Figure 15 A Typical IGF Process Flow

    5.2.6 Flow Patterns

    Historically flotation systems have evolved on a trial and error basis. Engineering

    judgement, pilot testing and data collected from commercial installations were

    used to modify existing designs to try and improve performance. Today, advances

    in computer technology have lead to the development of highly efficient CFD

    modeling packages that enable new designs to be tested, analysed and modified

    before a single pilot unit is built. This has drastically reduced research and

    development costs and has been one of the main driving forces behind the

    higher optimisation and efficiency of many of the newer designs available on the

    market. Flow patterns can be easily monitored

    and any undesirable characteristics or shortcircuits easily identified. Figure 16 shows

    a plot of the surface of the water for

    one chamber of an IGF generated

    using CFD modelling. The different

    Figure 16 A Typical CFD

    surface plot colours represent

    different fluid velocities on

    different parts of the surface.

    These techniques are continually

    pushing the limits of IGF efficiencies

    to their maximum.

    Figure 16 A Typical CFD surface plot

  • 8/11/2019 Micro Bubble Flotation Technology in Secondary Tertiary Produced Water Treatment English A4

    17/19

    17www.exterran.com

    5.2.7 Vertical Vessels

    Vertical IGF vessels are relatively

    new when compared with the

    original horizontal versions, and their

    development has been largely driven

    by the offshore market where space

    is a premium. The reduced footprint

    that vertical vessels offer is highly

    attractive in freeing up deck space,

    although the height of these vessels

    tends to increase and are larger than

    horizontal vessels of the same flowrate. Internally the process performs

    in much the same way, although there

    are several significant differences in

    the overall design on these vessels.

    Vertical vessels do not lend

    themselves to multi-chambering as

    well as horizontal ones. Since the number of flotation chambers affects the

    performance of the separation process designers are faced with one of two

    options, they must either have multiple single chambered vessels in series to

    achieve the desired performance, or come up with a way of multi-chamberinga single vertical vessel. Both types of design are commercially available, with

    multichambered vessels (Figure 17) being slightly squatter than the singe

    chambered versions. More than two chambers, either inside one vessel or two

    separate single chambered vessels, are unusual in the field, meaning that the

    performance of vertical IGF systems will not be as good as horizontal ones.

    Several companies have tried to mitigate this problem by developing vertical

    units that utilise a number of different processes inside the same vessel. One

    design uses centrifugal inertia forces as well as single chamber gas flotation to

    aid separation. This goes some way to improving performance and making up for

    the loss of chambers.

    5.2.8 Tank Based Flotation

    Until very recently a traditional process flow for a produced water treatment

    stream would go from a large gravity skim tank or CPI to an IGF (figure 18).

    Depending on operational parameters a skim tank typically reduces oil

    concentrations from a few thousand to a few hundred parts per million (ppm) of

    free oil, with the IGF then reducing concentrations down to tens of ppm levels.

    It has been shown [2] that it is possible to successfully combine these two

    stages together, with the result being a drastic reduction in costs and an

    increase in space and efficiency. The combined process works by combining the

    benefits of a vessel based IGF system with a gravity separation tank, with theresultant design being a multi-chambered skim tank, typically much smaller than

    a gravity separation tank to treat the same volume would be.

    Figure 17 A two chambered vertical IGF

  • 8/11/2019 Micro Bubble Flotation Technology in Secondary Tertiary Produced Water Treatment English A4

    18/19

    18www.exterran.com

    Figure 18 Tank Based Flotation Layout

    Depending on a streams process conditions and specifications these multi-

    chambered skim tanks typically contain two or four chambers (Figure 19), and

    typically have residence times of 60 to 90 minutes. The internal baffles and walls

    can either be installed into existing skim tanks or built from new, depending on the

    application. Refitting older vessels is usually significantly cheaper that building new

    tanks or IGF vessels and is in part what has driven the success of this technology.

    CFD modelling is the reason behind the successful development of this

    technology, with optimal designs being achieved before any expensive pilot

    testing is required. Figure 19 shows a typical CFD output showing particle traces

    for packets of water molecules flowing through one of the four chambers. Thesetraces can be used to identify possible short circuits and design inefficiencies

    that would otherwise remain invisible.

    Figure 19

    A Four Chambered

    Flotation Tank

    & CFD Particle Trace

  • 8/11/2019 Micro Bubble Flotation Technology in Secondary Tertiary Produced Water Treatment English A4

    19/19

    CONCLUSION

    1) In recent years the mechanisms of gas flotation have become better

    understood and as a result there has been significant development and

    evolution of flotation technology

    2) Gas flotation is best utilised for secondary and tertiary oil removal, especially

    when inlet concentrations are below 1,000 ppm. This has lead to them

    replacing many other technologies such as corrugated plate interceptors.

    3) Hydrocyclones remain a proven and cost effective method for bulk removal

    of oil from water and are most suitable for application in front of flotation. 4)

    Filtration units are most suitable for polishing applications downstream of

    flotation equipment.

    5) Computational Fluid Dynamic Modelling has played a large part in the

    development of gas flotation equipment. Recent work in this area has exposed

    several design flaws in many conventional flotation units on the market

    6) Vertical Gas Flotation vessels can provide improved performance through

    additional chambers.

    7) Tank Based Flotation is becoming increasingly popular and is likely to continue

    to do so in the future. It has proven to be a viable technical alternative to

    conventional flotation units with the added benefits of buffering of surges and

    upsets in an overall smaller footprint.

    REFERENCES

    [1] DOUGLAS W. LEE, DR WILLIAM .J.D BATEMAN, NICHOLAS OWENS, Efficiency of Oil / Water Separation

    Controlled by Gas Bubble Size and Fluid Dynamics within the Separation Vessel, Produced Water Society

    Seminar 2007

    [2] FELISE MAN, NICHOLAS OWENS, DOUGLAS W. LEE, Induced Gas Flotation (IGF) within an API Skim Tank

    A Case Study of Design Approach and Results, Produced Water Society Seminar 2006

    1. GLR Solutions Ltd. was acquired by Exterran Holdings Inc. in January, 2008

    Global Sales

    16666 Northchase DriveHouston, Texas 77060

    [email protected]

    Canada Sales

    1721 27th Ave NE

    Calgary, Alberta Canada

    T2E 7E1

    [email protected]

    Latin America Sales

    Talcahuano 833 piso 11A

    C1013AAQ

    Buenos Aires, Argentina

    [email protected]

    Middle East Sales

    P.O. Box 293509

    East Wing 5B, 4th Floor

    Dubai Airport Free Zone

    Dubai, UAE

    [email protected]