micro bubble flotation technology in secondary tertiary produced water treatment english a4
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
Canada Sales
1721 27th Ave NE
Calgary, Alberta Canada
T2E 7E1
Latin America Sales
Talcahuano 833 piso 11A
C1013AAQ
Buenos Aires, Argentina
Middle East Sales
P.O. Box 293509
East Wing 5B, 4th Floor
Dubai Airport Free Zone
Dubai, UAE