METHODS TO CLEAN PRODUCED WATER

tickbitewryMechanics

Oct 30, 2013 (3 years and 11 months ago)

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TPG 4510

Petroleum Production Specialization Project


Department of
P
etroleum

Engineering and Applied G
eophysics


Supervisor
: Jon Steinar Gudmun
d
sson


METHODS TO CLEAN PRODUCED
WATER














Carlos Arribas Miranda

Trondheim, Norway

June
2013

i

Abstract


Produced water is the largest oilfield waste; the total amount rounds
250 Mbbl/day. It is a mixture of
inorganic and organic compounds,
including oil, metals, chemicals, gases, microorganisms, etc.

This report
is an overview of

different methods to treat the components
and contaminants of produced water and the technologies applicable for
this purpose reduc
ing the environmental impact of oil and gas industry
.

Methods explained are physical, chemical and biological and the
facilities where those technologies could be applied.
There are several
technologies described and compared according to the particles the
y
separate, size, applications, advantages and disadvantages, etc.

The limits of discharge and disposal are becoming more restrictive so
water treatment companies have to keep on researching and developing
new technologies in order to achieve those specifi
cations.




ii

Index

LIST OF TABLES

................................
................................
................................
........

IV

LIST OF FIGURES
................................
................................
................................
.........
V

1

INTRODUCTION

................................
................................
................................
...

1

2

DISPOSAL STANDARDS

................................
................................
.....................

2

3

CHARACTERISTICS OF P
RODUCED WATER

................................
...............

3

3.1

P
RODUCTION AND
S
USPENDED
S
OLIDS

................................
................................
.

3

3.2

D
ISSOLVED SOLIDS

................................
................................
................................

4

3.3

D
ISSOLVED AND
D
ISPERSED
O
IL

................................
................................
...........

5

3.4

P
RODUCTION CHEMICAL C
OMPOUNDS

................................
................................
..

6

3.5

D
ISSOLVED
G
ASES

................................
................................
................................
.

6

3.6

W
ATER IN OIL
E
MULSIONS

................................
................................
.....................

7

3.7

N
ATURALLY
O
CCURRING
R
ADIOACTIVE
M
ATERIALS
(NORM)

.............................

8

4

THEORY OF SEPARATION
................................
................................
.................

9

4.1

P
HYSICAL
T
REATMENT

................................
................................
..........................

9

4.1.1

Gravity Separation
................................
................................
...................

9

4.1.2

Coalescence and Dispersion

................................
..............................

10

4.1.3

Flotation

................................
................................
................................
...

10

4.1.4

Membrane Treatment

................................
................................
..........

11

4.2

E
VAPORATION

................................
................................
................................
....

15

4.3

A
DSORPTION

................................
................................
................................
......

15

4.4

C
HEMICAL TREATMENT

................................
................................
......................

17

4.4.1

Ion exchange process

................................
................................
...........

1
7

4.4.2

Electrodialysis (ED)

................................
................................
..............

18

4.4.3

Chemical Oxidation and Ozonation
................................
.................

18

4.4.4

Flocculants and Coagulants
................................
...............................

19

4.5

B
IOLOGICAL TREATMENT

................................
................................
....................

20

5

BEST AVAILABLE TECHN
IQUES (BAT)

................................
.......................

21

5.1

S
KIM
T
ANKS

................................
................................
................................
.......

21

5.2

C
ORRUGATED
P
LATE
I
NTERCEPTOR
(CPI)
................................
..........................

22

5.3

UF

WITH
C
ERAMIC MEMBRANES

................................
................................
.........

24


iii

5.4

D
ISK
S
TACK
C
ENTRIFUGES

................................
................................
.................

25

5.5

H
YDROCYCLONES
................................
................................
...............................

26

5.6

IGF

................................
................................
................................
....................

28

5.7

C
OMPACT
F
LOTATION
U
NIT
(CFU)

................................
................................
...

29

5.8

S
AND CYCLONES

................................
................................
................................

30

5.9

C
-
T
OUR

................................
................................
................................
.............

31

5.10

MPPE
................................
................................
................................
...............

32

5.11

W
ALNUT SHELL FILTERS

................................
................................
....................

34

5.12

M
ARES
T
AIL

................................
................................
................................
.....

35

5.13

BAF

................................
................................
................................
.................

35

5.14

A
CTIVATED SLUDGE

................................
................................
.........................

36

5.15

MBR

................................
................................
................................
................

37

6

FLOW DIAGRAM
................................
................................
................................
.

37

7

NEW CHALLENGES

................................
................................
............................

38

8

CONCLUSIONS
................................
................................
................................
....

40

FIGURES

................................
................................
................................
......................

48

TABLES

................................
................................
................................
........................

42

REFERENCES
................................
................................
................................
..............

48





iv

List of Tables


Table 1
:
Worldwide produce discharges
(Steward, 2008)

............................

42

Table 2
: Worldwide produce discharges (Neff, 2011)

................................
...

42

Table 3
: Constituents summarize from gas fields (Ahmadun, 2009)

......

43

Table 4
: Constituents summarize from oil fields (Ahmadun, 2009)

........

44

Table 5
:

Chemical compounds in gas
-
oil fields (Steward, 2008)

...............

43

Table 6
:

Ceramical membranes characteristics (USBR)

................................
.

45

Table 7
: Ekofisk C
-
Tour performance (Philli
ps)

................................
...............

45

Table 8
: BAT comparison

................................
................................
........................

46




v

List of F
igures


Figure 1
: Water/Oil production profile (Ebenezer, 2012)

.............................

48

Figure 2
: Drag force, Stoke´s law (Fluids Mechanics UPM, 2010)

.............

48

Figure 3
: Membrane sketch (Cheryan 1998)

................................
....................

49

Figure 4
: ED sketch (
EET corporation)

................................
................................

49

Figure 5:
Skim Tank configurations (Steward 2008)
................................
......

50

Figure 6
:
Down
-
flow CPI (ESI)
................................
................................
.................

50

Figure 7
: Up
-
flow CPI (ESI)

................................
................................
......................

51

Figure 8
: Oil coalescence and solids settling (ESI)
................................
..........

51

Figure 9
: Disk Stack Centrifuge (Veolia)

................................
............................

52

Figure 10
: Liner (Aker, Prosep)

................................
................................
............

52

Figure 11
: Hydrocyclone (NETL)
................................
................................
............

53

Figure 12
: Micron size separati
on Vs Oil viscosity (Cyclotech)

..................

53

Figure 13
: Max and Min pressure drop operation Vs eff (Cyclotech)

.......

53

Figure 14
: Micron separation Vs Droplet inlet size (Cyclotech)

.................

54

Figure 15
: Hydraulical IGF (Unidro)

................................
................................
.....

54

Figure 16
: Mechanical IGF (Unidro)
................................
................................
......

55

Figure 17
: CFU sketch (Statoil, 2010)

................................
................................
.

55

Figure 18
: Sand Cyclone (Veolia)

................................
................................
..........

56

Figure 19
: C
-
Tour flow diagram (Statoil, 2010)

................................
...............

56

Figure 20
: MPPE flow diagram (Veolia)

................................
...............................

57

Figure 21
: MPPE and flotation Comparison (Meijer 2010)

...........................

57

Figure 22
: Walnut shell filter sketch (Siemens)
................................
................

57

Figure 23
: Activated Sludge (Pipeline, vol 14,
2003)

................................
.....

58

Figure 24
: ubsea Separation sketch, Marlim Project (FMC, Orlowski)

......

58

Figure 25
: Flow Diagram Example

................................
................................
.......

59



1

1

Introduction



Oil and gas industry is one of the most

important industries nowadays.
S
ince 1850 when Edwin Drake drilled the first oil well, oil demand has

been increasing

thus,

the
oil and gas production. One of the problems
of this production

increase

is the big amount of water it is produced
with it, prod
uced water is the largest byproduct stream associated with
oil and gas produc
tion
[
Duho
n
, 2012]
.
So that
, produced water can be
defined,
as the water that comes with oil and gas in the production
facilities and it needs to be treated for different purpose
s

such as
reinjection or disposal
.


Produced water is a complex mixture of
inorganic and organic
compounds,

including oil, metals, chemicals
,

gases, microorganisms,
etc [Neff, 2011]. The total amount of produced water is estimated in
250 Mb
bl
/d being the w
ater oil ratio between 2 and 3 to 1 depending on
where
and when
the water is being produced


[
Ferro and Smith
]
.

The
old
er the production field the

bigger the WO ratio represented in figur
e
1

with an oil
-
water
vs.

time profile [Ebenezer, 2012]
.

The

produced

water can
be classified in
formation water and injected w
ater. Formation
water is the
one
trapped
with the oil in the reservoir

and since the well
starts to produce oil so it does. Injected water is the artificial via to
maintain reservoir pressure and le
ngthen the production of the field.



The motivation of

the project is to explain different methods to treat the
components

and contaminant
s

of produced water and the technologies
applicable for this purpose
reducing

the environmental
impact of oil and
gas

industry.





2

2

Disposal Standards



The environmental impacts of discharging water with
out

the appropriate
treatment are incalculable. That is why there are production water
disposal standards for produced water, both offshore and onshore,
according to the

current water separa
tion technology and the limits they
can achieve. Several techniques are being developed and investigated in
order to accomplish the zero content discharge. Both onshore and
offshore can treat the water also for water reinjection to mai
ntain
pressure reservoir and lengthen the production of the field. The
treatment of the water includes oil removal but also production
chemicals, suspended solids, bacteria, etc.


Offshore regulations require total oil and grease content of the effluent
be
low the
regulations;

they

vary from o
ne country to another. They
range between 15 mg/l) in Argentina and Venezuela, up to 50 mg/l
in
the Guinean Gulf
[Steward, 2008]
(Table

1
); fro
m 29 mg/l in U.S. up to
40 mg/l [Neff, 2011]
(Table

2
). In the North Sea is
regulated by OSPAR
commission (Oslo
-
Paris) and i
t is 30 mg/l. Despite
he existence of these
standards,
there are plenty of offshore facilities that do not achieve the
regulations registered in the OSPAR commission.


Onshore facilities normally treat onsho
re production wells and offshore
produced water that only has been treated superficially in order to be
transported to the onshore facility.

Onshore plants normally discharge

by subsurface injection into rock formations, which has more restrictive
limits t
han the onshore facilities.

The limits are higher because of the
risk of polluting fresh water in aquifers or soil pollution.

Disposal in
freshwater streams or aquifers is generally forbidden.




3

3

Characteristics of Produced Water



As it was explained, p
roduced water is a mixture of o
rganic and
inorganic materials that depends

on several

factor
s
, for instance,
geographical location of the field,
type of reservoir
,

lifetime of its
reservoir…Also type of hydrocarbons produced
affect the chemical and
physica
l properties of the produced water

[
Veil, 2004
]
. Characteristics
will vary form

oilfields, gas fields or oil and gas fields
; t
able
s 3 and 4

show a summarize list of the possible compounds that exist in the
different production fields.
All parameters will be explained in this
chapter.
It is important that the total amount of prod
uced
water in gas
fields is much

lower

than in the oilfields, mainly because there is no
water injection

in gas fields for gas recovery increase
.



3.1

Pr
oduction

an
d S
uspended
S
olids


Production and suspended solids include clays, scales, waxes, bacteria,
carbonates, sand, silt and asphaltenes [
Veil, 2004].


Concentration of
the solids varies from one field to another depending on the reservoir
initial conditions. The general amount of susp
ended solids is small
except in
wells that produce in unconsolidated formation, where large
volumes of san
d and other sus
pended solids might

be produced
. In
order to accomplish the water

disposal requirements, the solids cannot
affect oil measurement methods, and special equipment must be used.
When suspended solids are present, it is necessary to apply different
techniques
in order to remove the solids. Chemical treatment is used to
separate the oil dro
plets form the solid particles and t
he
equipment
must incorporate solids removal ports, jets and/or plates.


Precipitation solids or scales are the ions capable of reacting w
ith
temperature, pressure or composition changes. This phenomenon can

4

occur in tubing, pipelines, vessels and water treatment equipment. The
most common precipitating solids are carbonates and sulfates.


Carbonate scales can occur in all systems containin
g CO
2

and ions, for
instance Ca
2+
, which will precipitate as CaCO
3
. Carbonate scale
formation is mostly affected by changes in CO
2

pressure and
temperature, also by mixing different waters. Large pressure changes
happen in chokes or flash tanks while temp
erature variations will take
place in the heat

exchangers [Sandengen, 2012].


Sulfates form in the same parameters variations but they are more
dependent on concentration than pressure or temperature changes.
They precipitate fast and cause big problems wh
en they do it inside the
production wells. It is possible to control its formation with the
production temperature. For example, CaSO
4

reaches its highest
solubility at 38ºC (2150 mg/l), if the temperature is placed a
t 93ºC
solubility decreases until

1600g
/l.


3.2

Dissolved solids


Dissolved solids
are ino
rganic constituents

that are predominantly
so
d
ium (Na
+
)

cations and chloride anions (Cl
-
)
.
Other common cations
are potassium (K
+
), magnesium (Mg
2+
), calcium (Ca
2+
), barium (Ba
2+
),
Strontium (Sr
+
), iron (Fe
2+
),

etc
. There are also other anions such as
carbonates (HCO
3
-
,

CO
3
2
-
) and sulfates (SO
4
2
-
)

[Steward, 2008
].

Tables 3
and 4 enumerate

the metals dissolved

in both gas fields and oil fields
.


These ions affect produced water chemistry in salinity and scale
potential principally
[
Hansen 1994
].

The amount of solids dissolved in
the produced water can vary from less than 100 to over 300,000 mg/l
[
Steward
,

2008; Roach, 1994
].

I
t is important to pay

special attention
to
dissolved

solids in order to prevent scale formation in the piping
,

5

wellbore
-
bore formation, etc. It would

carry big costs in cleaning and
maintenance and the stop of the production in most cases
.


3.3

Dissolved and Dispersed Oil



Dissolved and dispersed oil components are mixture of hydrocarbons
including BTEX (benzene, toluene, ethylbenzene and xylene), PAH´s
(polyaromatic hydrocarbons) and phenols.

Dissolved oil is composed by
polar constituents distributed between low and medium carbon ranges,
meanwhile the small droplets of oil suspended in the produced water
are called dispersed oil.

The size of the oil droplets is betwe
en 0,5
μ
m
and over 200
μ
m

[
Steward, 2008
]
.


The amount of dissolved oil depends on the ty
pe o
f oil, volume of
produc
ed water and age of production
[
Ahmadun, 2009
].

The
experience from the field tells that the temperature range where the
water is treated (25
-
75ºC), does not affect
the solubility of oil.
Temperature only affects solu
bility above 75ºC.
Phenols concentrations
are low normally, in the North Sea for instance they have been never
been detected over 20mg/l [Neff, 2010
].
BTEX and phenols are the most
soluble compounds in pr
oduced water
, followed by aliphatic
hydrocarbons, carboxylic acid and low molecular weight aromatic
compounds.
Typical gravitational separation is not enough to separate
dissolve oil from the produced water. Other technologies are needed
such as adsorption
, filtration, biological treatment or membranes.


PAH´s
and heavier alkyl phenols (C6
-
C9) are related to
the
dispersed oil

because they are less soluble in produced water.
They are considered
the greatest environmental concern because of its toxicity and
p
ersistence in the environment.
The total quantity of dispersed oil is
determined by

the source of the produced water. For example, produced
water from gas/condensate fields exhibit high
er levels of dissolved oil

[Neff, 2011]
.
Oil droplets

size

distribution is the most important

6

parameter, which affects oil and water separation treatments.

It is
experimentally demonstrated that the bigger the droplet diameter is,
the better the equipment efficiency. The

size distribution is

influenced
by
system
shearing (pumping, pressure drop in the piping system, etc.),
oil
-
water interfacial tension, temperature, turbulence, density and other
factors.


3.4

Production chemical compounds



Chemical components are added to treat operational
problems. T
hey are
dissolve
d and used to prevent

hydrate and scale formation,

corrosion,
wax deposition, bacterial growth, gas dehydration and emulsion.

The
totality of the chemicals varies from field to field and sometimes they
appear in
insignificant

amounts.

These low concentrati
ons are explained
by the solubility of the chemicals in the oil phase, thus they are not
treated in the cleaning water systems.



Production chemicals can be very injurious in low concentration, 0.
1
ppm [Glickman 1998]. Besides th
e danger

it represents, some chemicals
like the corrosion inhibitor can reduc
e oil/water efficiency [Veil, 2004
]
.

Table 5

shows the typical chemical production in oil and gas fields.
The
most common chemicals used in oil/gas production that affect the water
facili
ties are the Scale inhibitors, scavengers, coagulants and flocculants
and finally some gas treatment
chemicals because they remain in water
phase
[Neff, 2011].


3.5

Dissolved G
a
ses


The main
gases, which are encountered in produced water, are

natural
gas (meth
ane, ethane, propane and butane)
, hydrogen sulfide,
carbon
dioxide

and oxygen
.

They are formed naturally, by chemical reactions
or bacterial activities.
Most of the gasses are saturated at reservoir
conditions but as the well starts producing, most of the

gases flash to

7

v
apor phase [Arthur, 2005]
. These gases are remove
d in separators and
stock tanks in most of the occasions.


The gas separation is influenced by the pressure and temperature in
which the process occurs. The higher the separation pressure the higher
the quantity of dissolved gasses will be. The opposite effect we get with
the separation temperature, the higher the

temperature the lower the
quantity of dissolved gasses.


Natural gas components are barely soluble in water at operation
pressures. This solubility is based on pressure,
temperature and specific
gravity

of the water.
It is important to comment that these

compounds
are attracted to the dispersed oil droplets, that attraction is taken into
account to design the flotation equipment for the water treatment

[Steward 2008]
.


Looking at the other
common gasses, hydrogen su
lfide is corrosive and
enables

iron sulf
ide scaling, besides is extremely toxic if inhaled. It is
necessary to be especially careful if the sulfide is present in the flotation
cells when maintenance and adjustments are done. Carbon dioxide

is
also corrosive and may originate

CaCO
3

scaling. When

the CO
2

and the
H
2
S are removed, pH increases so scale could also form.


It is relevant to comment the role of the oxygen. It is not found naturally
in produced water but produced water may absorb it when it comes to
surface. Water with dissolved oxygen c
auses corrosion and oil
weathering that difficult the separation.


3.6

Water in oil

Emulsions


Emulsion is a mixture of two immiscible liquids.
In the normal
emulsions, water is dispersed in small droplets from 100
μ
m to 400
μ
m in
diameter.
If the emulsion is
unstable
, the oil droplets will coalesce into

8

larger ones. This is a short time process. However a stable emulsion is a
suspension of

the two liquids with a stabilizer that maintains a film
between the phases. This film may be removed so coalescence starts

to
act. In order to break it down chemicals or heat are used.

In water in oil
emulsions, the emulsion breakers must be oil soluble, so that, they have
more time to act during the separation processes.


3.7

Naturally Occurring Radioactive Materials (
NORM
)


N
OR
M originates in geological formations and can be brought to surface
with produced water [
Veil, 2004
]. They can be found in production
wastes, equipment and solid
s at the production facilities
. The most
abundant NORM compounds are
226
Ra and
228
Ra
, the ambi
ent
concentrations are ranged between 0,3 and 1,3 Bq/L and 16 to

21 Bq/L
[Gafvert,
2006]
.

In the North Sea, Utvik confirms that the
measure

concentrations of NORM in produced water range from 0,23 to 14,7
Bq/L.

Both

compounds derive

from
uranium and thorium present in
hydrocarbon bearing formations. As the produced water approaches to
surface, temperature and pressure decrease so it may lead to a NORM
scale production. [
Veil 2004
,
Steward 2008
].

The scales and
sludge

would accumulate in wa
ter separation facilities
.


NORM regulations are more focused on the equipment accumulation
rather than
produced water limits. It has been proved than seafood
consumption from produced water disposal does not affect human
health. The specifications in NOR
M management are centered on
identificatio
n, control and volume reduction of the wastes and solids,

in
order to diminis
h human exposure to radiation [Ebenezer, 2012].






9

4

Theory of S
eparation



The
main goals for proper water t
reatment are nine

[Arthur, 2005
]
. De
-
oiling

(
removal of free dispersed oil and grease
); d
issolved organics
(bacteria and microorg
a
nisms) and gas elimination such a
s natural gas
or carbon dioxide; suspended solids removal (
mostly sand and
other
particles
); d
esalinat
ion; s
u
lfates and
scaling agents clearance;
disinfection and softening, in order t
o adjus
t water hardness and make
it available for irrigation; and finally NORM removal.

To meet this
achievements different methods can be used, mostly physical and in
less often ch
emical and biological procedures.


Physical procedures will separate contaminants and oil from water by
the application of different forces. Chemical system bases its separation
in the addition of components that will react with the contaminants
wanted to
remove. Finally biol
ogical will be focus in the use

of several
types of bacteria
and microorganisms.


4.1

Physical T
reatment


4.1.1

Gravity Separation


Gravity separation is the most usual process in water treatment. As it is
known, oil is lighter than the volu
me of water they displace so, by
Arch
imedes
principal, oil droplets experiment a buoyant force. But

the
vertical movement of the particles through the water originates a drag
force that withstands the flotation

described by the Stoke´s law and
sket
ched in
figure 2
. Droplets reach a constant veloci
ty when the to
forces are equal

[Fluid Mechanic N
otes, UPM, 2010
].





10

v
s
= particles velocity

g = gravity acceleration

r = particles radius

ρ
p
= particle density

ρ
f
= fluid density

μ

= water viscosity


With the formula we can conclude that the bigger the droplet and the
density difference the higher the vertical velocity. If temperature is
increased, viscosity will reduce so a higher velocity is also obtained.

Stokes law may be applied to droplets never

below 10
μm, but field
experience indicates that the
lowest limit applicable is 30μm

[
Devold, 2006
].


4.1.2

Coalescence

and Dispersion


Coalescence

is the process in

which two or more droplets, bubbles or
particles merge during contact to form a single daughter

droplet,
bubble or particle
. If this occurs repeatedly, a continuous liquid phase
forms [Schlumberger,

2013].

Coalescence is a time dependent process,
the
smaller de oil droplets
diluted
the

greater the

time to grow bigger
droplets
.

Dispersion is the act

of breaking up particles into smaller
ones and distributing them throughout a liquid or gaseous medium
.
This process occurs when a large amount of energy is input in the
system in a short period of time [Schlumberger, 2013]. This energy
applied minimizes
the surf
ace area between the two
fluids, which

favors
the separation between the droplets.


The coalescence and dispersion processes occur at the same time and
they are totally opposed
.

If the kinetic energy of the particles in the
system is larger than th
e difference in surface energy between the single
droplet and the two smaller droplets form
ed

from it
,

dispersion process
is happening. In the other side, the motion of the smaller droplets
causes coalescence [
Ebenezer, 2012
].

4.1.3

Flotation



11

Flotation consists

in the injection and dissolution of air in the produced
water. Then, the small air bubbles adhere

to the oil droplets increasing

its
buoyancy
, the specific gravity of the oil
-
gas bubbles combined is
significantly lower that the oil droplet alone
.
When the

oil has floated to
the surface it is
normally
skimmed and removed.
Flotation process is
really effective, over 90% of the oil is removed in short periods of time
and can

remove very small oil droplets.

The droplets separation size is
lower
if a chemical
p
retreatment is used to favor the flotation,
coagulants

and flocculants

for example
.

This process can also be used
to remove natural organic matter, volatiles, grease
, etc.



The efficiency of the flotation process depends on specific gravity
difference, droplet size and temperature.
The
y

usually work better with
low temperatures, because at high temperature, dissolving air into the
water requires more pressure.

It also depends

on the air bubbles size,
the smaller the bubble size the mo
re chances to adhere to the oil.


Flotation can operate as the principal separation force in

two kinds of air
flotation systems: Dissolved Air Flotation
(DAF)
and Induced Air
Flotation

(IAF)
,

IAF

will be explained later

but DAF is barely used in
offshore facilities for its size and weight
, operation

at high
temperatures
, etc [Unidro
, Prosep
]
.

I
t can function

also

as a secondary
force to help other separation principles to perform.


4.1.4

Membrane T
reatment


Membranes are thin films of synthetic organic or inorganic materials,
which separate a certain fluid from its components.
The separation is
achieved by diffusion through the membrane under pressure difference
.
Several processes exist for this pur
pose, microfiltration (MF),
ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO) and
polymeric or ceramic membranes. Membrane treatment is more suitable
for the stable oil water emulsions.


12


The process usually operates with a recycling water sy
stem that
maintains a constant water flow. The same amount of water introduced
in the tank at the same rate as it is withdrawn and clean. The process is
stopped when the limit of particles displaced reaches a certain level
concentration in the process tank
. When the procedure
stops
, a c
lean in
place is carried out (C
I
P),

process is sketched

in figure 3
. It is important
to

mention that membranes normally need
a pretreatment to remove
free oil and bigger particles

to lengthen the life of the membranes
.


These

types of procedures have some advantages over the traditional
methods of flotation and separation. Using membranes for the
treatment reduces the oil concentration from 1/40 to 1/200 initial feed
and the total quantity of water used can be recycled

[Madaen
i, 2003]
.
Th
ey also have some disadvantages; i
t is very expensive to install
membranes over a
certain size.
Most of the membrane processes
include chemical pretreatments; to avoid scale formation
(in
NF
and RO)
and
is common the addition of coagulants.

T
he
y also suffer high
degradation during their use so that they must be changed frequently

in
order to avoid membrane fouling
[Madaeini 2003]
. The flux varies with
the time, the longer

the time the more attached oil and sol
i
d
s stuck at
the surface of the membrane
the lower the flux
[Jiang, 2008].
The space
of installation needed is higher than the traditional methods and
b
ecause of the

chemicals used

in the pretreatments several kinds of
impacts into the environment may occur
.


4.1.4.1

Filtration: MF, UF, NF and RO


MF
has the larges
t

pore size (0,
05
μ
m to
2
μ
m)

and operating pres
sure
difference below 2 bars. MF

is mainly used to

remove suspended solids
.
UF ranges from 2
n
m to
0,05
μ
m

and operating pressure between 1
-
20
bars; it is used fo
r colloids

and
solids
separation

[Martinous, 2001
, Judd
2003
]
. Both systems
used as a pretreatment for other cleaning

13

technologies NF, RO and
electrodyalisis [Jurenka, 2010
].
MF and UF can
treat any type of produced
water;

they can operate with high TDS and
salt concentrations.


NF

is normally used for metals removal from produced water. It
has
membrane pore size between 0,5nm
and 2nm, pressure difference of
the process between 10 and 100 bars.

It is used for multivalent io
ns and
charged polar molecules [Martinous, 2001, Judd 2003].

NF membranes
hav
e negative charge at neutral pH;

it is an important key for the
separa
tion properties of the membrane

[Sutherland, 2009].

RO is
capable of
remove over 99% of the organic macromole
cules and
colloids
,

besides
inorganic

ions

are also removed

over 0,1nm

[Bilstad
1994]
.

The most important problem of RO
and NF
is the complex
pre
treatment that needs to be done;

NF and RO are mostly used for
human consumption

in desalination processes
.


Me
mbranes

can operate either cross flow separation or dead end
filtration. Cross flow separation occurs in perpendicular direction with
the flow, gravity and density difference makes the particles fall to the
bottom of
the flow and then be filtrated, o
nly part of the feed water is
treated. In dead end separation all water is treated and flow and
filtration happen in the same direction.


The membranes applicable for oil separation purposes are polymeric or
ceramic, being the second ones more expensive
but capable of treating
more water. Therefore a cost/benefit analysis must be done. Both
membranes are explained below.




4.1.4.2

Polymeric/C
eramic M
embrane
s



14

Ceramic (or inorganic) membranes have attracted interest due to their
superior mechanical, thermal, and
chemical stability. The primary
advantage of using ceramic membranes is the ability to accomplish the
current and pending regulatory treatment objectives with no chemical
pre
-
treatment

[Ebrahimi, 2010].

Ceramic membranes

are made from
alumina, titanium
,
silica

and zirconium
oxides

and carbides
. They

are
tubular and consist of a porous support material (
α
-
alumina), a
separating layer and a decreasing pore diameter layer. Different
materials applied for the different range of filtrations used by Ebrahimi.
.
UF with ceramic membranes has been shown to be very effective in
treating waste oil, grease and effluents

and can compete against
traditional separation techniques

[Fabish, 2001].


Polymeric membranes

are made from polyacrylonitrile (PAN) and
polyvinyliden
e fluoride (PVDF)
. The main problem of the stable organic
materials is th
e hydrophobicity of their basic

materials
. This results in a
low water permeation rates. PAN membranes combine chemical stability
and good aqueous filtration [
Scharnagl
, 2001].

The g
ravest problem of
the polymeric membranes is their integrity. As a consequence to that,
the integrity of the membrane must be tested to ensure the process.
This test can be done with a pressure decay test. In this test,
pressurized
air is applied to the me
mbranes

at a pressure less than
would cause the air to flow through the membrane, and the pressure
decay is measured [
Colorado School of Mines
, 2009].


Current experiments mixed both types of membranes, a PVDF
membrane with nano
-
sized aluminum particles improving antifouling
performance of the membranes. In the UF experiment the removal
efficiency of COD was 90% and in TOC 98%; oil residue was less
than

1%
[Lia, 2006]. This shows that UF process is the most competitive
compared
with the traditional systems of wastewater treatments.

The
main challenges of the membrane treatments to consolidate are the
scale forming and the clogging. On their side, they have good efficiency

15

and capacity; and also they
are flexible and can
accept
well variations in
flow and quality [Statoil, 2010].


4.2

Evaporation


The
processes for produced water cleaning, which include

steam
formation in some way
,

are also called thermal technologies.
Di
fferent

evaporation systems has been tested and proposed for wa
ter treatment.
They

use few o
r

none
chemicals so waste sludge is cleaner.

They also
reduce

eq
uipment needed for the process, t
herefore
O&M
costs
decreases
substantially.

The most applied thermal technologies used
nowadays

are multi
stage flash (MSF
), multi
effect distillation (MED), vapor
compressor distillation (VCD), AltelaRain
S M

and freeze
-
thaw evaporation
(FTE).

The thermal technologies have being applied for water
desalination

and solids removal

for human consumption sin the middle
XX century.

The application of these technologies has increased lately
for produced water with the proliferation of shale gas in the
United
States. They have high O & M

costs and energy consumption. In their
advantage they d
o not need pretreatment and

can handle over

100
.
000
of TDS [Dores, 2012].


Evaporation can also be used in traditionally in evaporation ponds where
solar energy efficiently evaporates de water placed in artificial ponds.
They have no mechanical systems, so low O&M costs. It is the cheapest
facilit
y for saline water disposal in the applicable areas. The main
disadvantages of the ponds are that they need vast extensions and dry
weather. These means they are only
suitable in dry areas with high
evaporation rates and availability of lands at low cost [
Ahmed 2000].



4.3

Adsorption



16

Adsorption is the
process where a special solid used for removing
substances from the water.

For oil and other non
-
polar substances
presents in the oil, BTEX and PAH´s the active carbon is the most used
solid. It

is made in order to achieve big internal surface, which improves
the adsorption process. There are two kind of active carbon, Powder
Activated Carbon (PAC) and Granular Activated Carbon (GAC). The one
employed in the oil
-
water treatment is the GAC, it nor
ma
lly gas greater
diameter than 0.
1 mm

[EPA, 2009]
.


GAC can be regenerated removing the adsorbed compounds through
steam, thermal o physical/chemical procedures. The first two are
common methods to recycle the active carbon.
Steam regeneration is
only
suitable option when the carbon has only retain volatile products.
Thermal regeneration is based in pyrolysis (burning the organic
substances). It is a very effective regeneration process but it also has
two big inconveniences, high carbon losses and cost

[lennthec library,
2010]
.

Other adsorbents have been found in order to substitute the
active carbon, specially the GAC. These

materials are the organoclays
,
hydrophobic zeolite or polymer adsorbents.


Organoclays

present several benefits in comparison with

the GAC. Th
e
y
have higher adsorption capacity of hydrocarbon
s and they are very
e
ffective in removing soluble and dispersed hydrocarbons. Organoclays
a
re used mainly in two purposes. The first one is as a
pretreatment for
membr
ane filtration systems UF /
RO and also for ion exchange resins
method. The second is a post treatment for oil and water separators
[Islam, 2006].

O
rganoclays are manufactured by
modifying bentonite
with quaternary amine. Bentonite is basically
montmorillonite;

there are
two types
, s
odium bentonite and calcium bentonite. Quarter am
ines
used as oil
-
wetting agents
, corrosion inhibitors and bactericides.


Zeolite is an
alumina
-
silicate crystal with uniformly sized pores. It is
naturally hydrophilic (affinity for polar molecules), after i
t is treated and

17

the aluminum is removed, it becomes hydrophobic (affinity for non
-
polar molecules). Polymers are manufactured with pore ranges from
macro to almost micro pores. They

worked in polymer beds and they are
proved to adsorb faster than the acti
ve carbon.



4.4

Chemical treatment


4.4.1

Ion exchange process


Ion exchange is the process where an ion replaces another one in an
aqueous solution. The synthetic materials specially designed for these
purpose are called ion exchangers or resins; resins developed

for the
water treatment purposes are IX resins

[Colorado School of Mines,
2009]
. These resins are capable of capturing the contaminant cationic
ions

dissolved in water, Calcium, Magnesium… and be substituted by
exchange cations from the resin.


Resins us
ed in produced water treatment are known as Strong Acid
Caution

(SAC) where hydrogen and sodium cations highly dissociate and
remain ready for the exchange (Eq
ua
tion 1
) [Arthur, 2005].






(





)




(






)







(

)


This process is only

applied for hardness water removal and it can also
be named as water softening. When the resin cannot exchange more
ions
it
must be regenerated. The resin is backwashed with the typical
cations that form the resins, Na cations, so that it is ready to beg
in the
process again; there are regeneration looses, around 2% [Colorado
School of Mines, 2009]. Ion exchange process is typically used for
drinking water or discharge to environment and also is a usual process
in nuclear power plants.



18

The major benefits
of the process are low energy consumption; high
efficiency in the resin regeneration process and TDS values manageable
up to 7000mg/l. Important disadvantages are the needs of pre
-
treatment and post
-

treatments that increase significantly the O&M costs
and

high sensitivity to fouling. Therefore, its main application is in the
coal bed methane produced waters because they are free of the
contaminants, which affect ion exchange performance.


4.4.2

Electrodialysis (ED)



ED and electrodialysis

reverse (EDR) are processes
where
dissolve
inorganic ions from salts are separated from the water through ion
exchange membranes. The membranes are placed in between two
electrodes and allow the ions to pass through. If the membrane is
positively charged
the negative ions will be separated and in reverse

as
shown in figure 4
. That is why several membranes are positively or
negatively charged alternately so every ion can be removed.
EDR and ED
are very similar processes; the difference lies in the
electrodes. In EDR
electrodes polarity is reversed in order to free accumulated ions in the
membrane surface.


A pretreatment is needed
for both ED and EDR,
because suspended
particles above 10 micron
s will block the membrane pores

[lenntech
treatment solu
tions]
, also potential scaling minerals must be removed
.

But they present some benefits,
low
-
pressure

req
uirements,

no chemical
addition and long membrane working life.

ED process is able to
eliminate from 59% to 94% of dissolved solids and up to 12.000 mg
/L of
TDS (normal opera
tional conditions are 1.200 mg/l
) [Jurenka, 2010].


4.4.3

Chemical O
xidation

and Ozonation




19

The main objective of the chemical oxidation is to generate a powerful
oxidizing hydroxyl (OH
-
), which reacts rapidly but non
-
selectively with
ne
arly

all

organic compounds, formatting carbon dioxide and inorganic
salts or less toxic products.
Typical chemical oxidation processes are
Advance Oxidation Processes

(AOP´s)

and Ozonation
.


AOP´
s

present some advantages; the most important benefit is its
capability of
oxidation

of organic compounds. Main disadvantage of the
process is the addition of chemicals that increases the cost. AOP´s have
been tested in labs and in fields for produced water

treatment but it is
not applied commercially [Dores, 2012]. Its application for other
wastewater treatment makes this process potentially valid for oilfield
-
produced water

when biological treatment cannot be used
.



Ozone gas is created with electrical di
scharges in the ozone generator
and then it is pumped into the tank. Inside the tank the ozone bubbles
flows into a contactor where adsorption takes place.

This process is
called Ozonation. It is more effective than chlorine destroying bacteria
and no harm
ful removal is needed (ozone decomposes rapidly). It is
complicated process that needs lots of technology to be applied. It is
also corrosive, non
-
suitable with suspended solids, possibly toxic, etc.



Both processes provide high removal efficiency of tox
ic compounds,
specially alkylated phenols but in the other hand some other toxics
forms (Chlorinated an
d brominated p
henols) in low concentrations
[Grini, 2002].

Biological treatments are preferred over chemical
oxidation because they are non
-
environmental

friendly, complex and
expensive to operate and maintain.


4.4.4

Flocculants and C
oagulants



20

Coagulation is the process in which it is reduced the electric repulsion of
particles (same electrical charge) with the addition of salts; then
particles aggreg
ate becau
se of the
remain
ing

forces that attract the
particles. Flocculation

causes the

aggregation with polymers aid.
Coagulants and flocculants are the agents that cause

respectively

both
processes. They

normally remove efficiently heavy weight organic
particles being incapable of removing low molecular weight and non
-
polar particles. Those particles can be collected by biological systems.


Flocculants and coagulants mius
t be non
-
hazardous and biodegradab
le.
They are designed to aid in the oil
-
water separation
processes;

typically
they are ammonium and acrylamide.

There are also several types of
coagulants, which can be cationic (positively charged), anionic
(negatively charged) and nonionic (neutrally cha
rged). Primary
coagulants are made to neutralize the charges while secondary or
coagulant aid mission is to maintain the flocs together so that they will
not break during the process. Some primary coagulants are aluminum
sulfate, ferrous sulfate or artific
ial polymers. Secondary coagulants can
be sodium silicate or charged polymers

[Minerallurgy

notes, UPM 2010]
.



4.5

Biological treatment



Biological treatment is normally used for organic material removal with
bacteria and other
microorganisms
; it is the
latest process of produced
water before discharge

or reuse
. It is very important to know the
composition of the water in order to plan a specific treatment, for
instance in oil industry there is a special high demand of oxygen from
de bacteria to process t
he water [Schultz, 2005]. There are three basic
biological treatment groups, aerobic (presence of oxygen), anoxic
(oxygen deficient) and anaerobic (lack of oxygen). This oxygen quantity
is directly linked with the type of bacteria involved in the degradat
ion of
the contaminants.


21


A
erobic treatments will take place in air presence with microorganisms,
which use the oxygen molecules to assimilate the organics creating
other compounds; they are also called aerobes. Anaerobic and anoxic
microorganisms or anae
robes will process in air absence to assimilate
the impurities. The compounds generated usually are carbon dioxide,
water and biomass

for the aerobics processes and carbon dioxide,
methane and biomass for the anaerobic

[Mittal
, 2011].


The main biological
techniques are activated sludge
, Se
quenced Batch
Reactors (SBR´s)
and Membrane B
ioreactors
(MBR´s)

and Biological
Aerated Filters (BAF´s)
.

Because of its size and time of operation (days),
they are impossible to install in offshore platforms with the usual flux of
the production offshore facilities.

They are also used in downstream oil
treatment.



5

Best Available Techniques (BAT)



This chap
ter of the
report

is a description of the best available
techniques for produced wate
r in the oil & gas applications applying the
principles

explained in chapter 4
.

There are many different technologies
to be able to cover all kind of diverse pro
duced wate
rs, varying
its
characteristics not only from on
e

field to another
but also the variation
during the production time.



5.1

Skim Tanks


Skim tanks are the simplest and primary treatment of produced water.
They are designed for long time residence
(
up t
o hours)
where
coalescence

and gravity separation occur. They can have vertical or
horizontal configuration and work at atmospheric pressure or under
pressure.

These tanks can have several purposes, dispersed oil removal

22

(Skim tanks), solids removal (Settl
ing Tank) and when oil and water ratio
is high, in order to make a bulk separation (Wash tanks).


In vertical ski
mmers oil droplets rise upward, meanwhile in the
horizontal vessels, the droplets rise in a perpendicular direction with the
water inlet flow.
In both configurations, the air released during the water
injection in the vessel help
s the droplets to float. Figure 5

show
s
both
configuration sketches. Vertical skimmers can include a spreader
that
helps the distribution of the flow.

The oil is skimmed
at the surface in
both shapes. In order to control the oil level in the weir, a water leg
could be used. Horizontal skimmers are proved to be more efficient
than vertical skimmers. But vertical skimmers present useful features
when sand and other particles

must be handled because a sand drain
can be added at the bottom. Also, vertical skimmers are less sensitive to
flow variations.


Pressure vessel
s

might be used when the water has to be pumped for
any reason or there is a gas blow that creates difficulties

in the water
injection into the system. Otherwise, atmospheric tanks should be
installed because of its lower cost.


Skimmers can remove droplet size above 150

m

and a minimum time
residence of 20 minutes [Steward, 2008]. The vessels are highly affected
by temperature and they are not suitable for cold produced water.
Horizontal baffles can be installed to perform a better separation. They
can treat high oil concentrations with s
olid contaminants.


5.2

Corrugated Plate Interceptor (CPI)


CPI´s are coalescers
,
coalescers are devices that use

gravity separation
like the skimmers but they also induce coalescence to improve the
s
eparation. CPI is a basically certain number of
parallel
-
corrugated

plates

with 2.
5cm distance between them,

where the oil water

23

separation takes place

called CPI pack

(figure 6
)
.

Figure
7
shows
a
down
-
flow through

the CPI pack, the process can happen the other way
round call up
-
flow
process

(figure 8)
.
It also exists crossed
-
flow devices
that they work under pressure. It allows bo
th horizontal and vertical
configuration systems.


P
rocess
begins when the water enters into the nozzle

(1)
, over there
biggest

solids will sink and settle for posterior collection

(2)
. Water and
oil will
pass through a perforated distribution baffle plat
e (3). The CPI
pack (4) receives the oily water, where the oil rises

to the pe
aks of the
corrugations (figure 8
)

and coalesces
(5), it keeps moving upwards
exiting the pack reaching the surface at the top of the chamber (6),
where it flows over a weir

(7)

until the oil compartment (8). Water exits
the

pack

(9
) where

the smaller solids settle and they are also removed
(10).

Water flows upward (11) into the clean water compartment (12).
There is a secondary oil outlet adjacent to the water outlet (13) and
val
ve to ensure a gas blanket in the camber (14)

[Energy Specialties
International]
.


Down
-
flow and Up
-
flow

processes have some
differences. T
he inclination
of

the pack is usually 45ºand 60º

respectively

(see figures 7and 8
)

and
the droplets size separation achieved is better in the down
-
flow system
,
around

50

m

but solid removal is not important;

meanwhile in the up
-
flow is always above 50

m

but the solids size removal cut off is up to
10

m
[
Veolia
, 2013]
. Therefore, for oil

and water separation, if solids
co
ntent is i
nsignificant
down
-
flow might be used and in the opposite
way. The
inlet oil influent accepted can be as high as 3000mg/l within a
flow rate variation

from 20

m
3
/h to 200m
3
/h

[Veolia, Paramount].


CPI exhibits ma
ny advantages,
little operation and maintenance costs, it
is simple and it has no moving parts, so that no energy requirements. It
offers a continuous processing with high oil and solids efficiency (up to
150mg/l). The main disadvantage for oil wastewater
is that this

24

technology is
inefficient

with high amount of solids and sometimes it
requires a post treatment if the disposal specifications are not reached.


5.3

UF

with Ceramic membranes


Membrane process, as it explains chapter 4.1.4 of the
report
, basically

consists in the filtration of the produced water through a membrane
with specific pore siz
e because of a pressure drop between both
sides of
it.
The application of the UF
/MF

water treatment for produced water has
become a successful discovery, which can c
ompete with traditional oil
wastewater processes.

It has been proved in various studies and field
trails [
Dores, 2010
, Szép 2010].

MF is also a possible process but
sometimes it does not reach the water disposal require
ments;

therefore
UF is more popular.
UF membranes are suitable for suspend solids oil
and grease, orga
nic carbons removal and metals; d
issolved ions and
organics will not be separated.


Ceramic membranes can have multiple pore sizes and configurations.
Table

6

Summarizes filtration range, mem
branes materials and pore
shapes

for different manufacturers
. The filtration size ranges from 5nm
to 1.4

m depending on the different technologies developed for the
companies. All the membranes are built with
alumina oxide and the
filtration
channel

can have several shapes
hexagonal, round, squared,
etc

[Benko]
.

The most important operating parameters for a ceramic
membrane process are the volumetric flow rate of
the water per
filtration area,

the trans
-
membrane pressure (average of feed and reject
p
ressure minus filtrate pressure) and the
back pulse

of the water from
the filtrate side to the feed side
.


For instance, Veolia´s CerMem technology offers two different channel
sizes 2mm and 5 mm with a di
mension of 8.64m/1.
42m. Tha
t makes a
filtration
area of 10.
7m
2

and 5

m
2
respective
ly. And the pressure drop is

25

1.3 bar in the first one and 0.
5 bar in the second.
Shows de
approximated values
of the water flow
for the membranes described.

Cross flow velocity should be between 3 to 4 m/s. Membrane
compo
nent materials determine the pH range, 0 to 14 for silica
membranes and 2 to 13 fro alumina and titania. The production rates
depends of the number of modules installed, the biggest flow available
is 170m
3
/h (30 or 52 modules installed, determined by the
channel size)
and it needs pump power up to 170 kW.


5.4

Disk Stack Centrifuges


Increasing the acceleration the droplets are subjected to can enhance
the settling velocity of oil droplets

achieving its separation

from water,
t
his c
an be realized in a centrifuge [Van den Broek, 1996]. For oil/water
separation the centrifuges used are the Disk Stack Centrifuges. They
consist in a frame, a motor with a transmission, separator bowl (double
conical shape) and the inlet feed. The bowl ha
s special inserts, the
gravity discs with conical shape that establish the oil water interface. It
is where the separation takes place; the distance in between the discs is
less than 1 mm. The centrifugal force generated ranges from 5000 to
6000 g´s.


The
feed is introduced in the bowl and is accelerated to maximum
rotational speed. The discs distribute the water due to the centrifugal
force and separate oil, water and solids. The oil flows towards the center
of the bowl to the upper side of the discs; mean
while the water and
sediments flow in the opposite direction. The liquids are led to the neck
of the b
owl where they are removed
.

At the bottom of the bowl, in it
widest point,

some solids discharge ports

are installed
. A pis
ton moves
these ports
, when the

piston is at its lower position sediments are
released [Faucher and Sellman, 1998].




26

An example of this system is the X20 developed by Alfa Laval. It is a
special centrifuge system adapted to the oil and gas separation industry.
It can process 170 m
3
/h a
nd its energy consumption is 1
50kw/h. The
small dimensions (3.15 m tall, 2.34 m long and 1.
53 m wide) make it
suitable for offs
hore purposes. Represented in figure 9

Disk stack
centrifuges are capable of separate droplets with an approximately size
of 5 to

15 microns and solids from 3 to 10 microns and above. In the
solids removal, density is an i
mportant factor and it may be 1.
4 g/ml or
higher in order to have a proper separation [Miedek and Fislage].


Centrifuges system present some benefits, the most imp
ortant ones are
its efficient removal of smaller oil particles and solids and its application
fo
r heavy oil de
-
oiling (up to 11.
5 API) [Alfa Laval]. Centrifuges do not
need demulsifiers and also the rag layers found in traditional vessels are
eliminated. B
ut they have high maintenance and operational cost
because of

the rotation parts and also higher energy consumption. T
hey
are meant for small water streams [Statoil, 2010].


5.5

Hydrocyclones


H
y
drocyclone
vessels are
unit
s

formed with
conical device
s

where
c
entrifugal force
and the specific gravity difference separate
oil and
water.
Individual hydrocyclone

conical devices are

called liners
(figure
10
)
. The quantity of liners varies depending on the produced water
characteristics and the water amount that need
s to be treated.

Figure 11
shows a hydrocyclone vessel with the liners inside.


Produced water is introduced under pressure into the hydrocyclone
vessel, and makes its way to the water/oil inlet ports; place
d

at the
larger

diameter end of each liner. P
ressure drop between the inlet port
s

and the outlet ports of the liner ensures the flow path. A swirl
positioned axially in the liner induces a rotation flow

throughout it
. The

27

conical shape of the liner increases de fluid speed rotation. As the
diameter o
f the liner gets narrower the speed increases. Therefore the
centrifugal forces also augment resulting in the separation of light oil
and gas and heavy water and solids. The heavier materials move to the
walls of the liner towards the outer port. Meanwhile
, the oil moves in a
closer vortex to the axis moving in the opposite direction towards the
inlet port
[FMC, Veolia brochures, 2012]. The functioning of a liner is

sketched also in figure 10
.



There are different factors that influence the separation perf
ormance,
such as the operating temperature that affects the viscosity, usually the
higher the temperature the lower the viscosity. The decrease in water
viscosity favors the droplet settling velocity and the coalescence activity.
Figure

12

shows a comparis
on between different oil at constant
efficiency, as the viscosity increases, the droplet size of the separation is
also bigger. It can vary from 30

m to 10

m from heavy oil to light oil.
The pressure drop in the liners is a very important factor. The hig
her the
pressure drop the higher the tangential velocity is; and the hydrocyclone
performance is better. But if the hydrocyclone operates at maximum
flow rate, some turbulence may appear and it lows the efficiency of

the
process as it shows figure

13
. Anot
her important agent is the droplet
inlet size. Cyclotech Technologies

affirms that
there is a critical droplet
size around 10

m
to 15

m
where the efficien
cy drops notably (figure
14
).


Several oil & gas companies have elaborated different designs, Sieme
ns,
Veolia, FMC… because of the advantages they provide. They are
compact modules with high efficiency and the can reduce oil
concentration to 10 ppm. They do not need any pre treatment and
energy consumption is very low, they only use energy to pump the w
ater
into the vessel. The main disadvantages of this system are that the
solids can block the inlet and scale formation could happen increasing
the maintenance cost. Besides they can treat any kind of produced

28

water, a post
-
treatment may be needed in order

to remove other
dissolved
components
to achieve the disposal standards.


5.6

IGF


In the IGF units, the water is injected into the floatation tank but the
bubbles are generated by physical procedures
.it can be several gas
injected such as nitrogen, natural gas, carbon dioxide or air (it is
necessary to be extremely careful with the air ant its oxygen content for
its explosion potential).
There are two types of IGF, Hydraulic and
Mechanical.

In both of them a coagulant pretreatment could be use to
favor the flocculation.


Hydraulic

typical

system is showed in Figure
15
. It shows a flotation unit
with thr
ee cells. The recycled water
flows through the venture eductors,
where the gas is sucked a
nd the mixture is released into the chamber
where flotation occurs. Then the oil is skimmed and removed and the
water is pumped into the recycling system [Natco]. Normal
manufacturers design each cell with around 50% efficiency that makes
not cost
-
effectiv
e to in
stall more three or four cells
because more cells

efficiency increase is too low [Steward, 2008]
.



Mechanical system

includes

a rotating impeller, driven by an electric
motor

that
creates a vortex, which introduces de gas into the vessel.
The gas m
ixes with the water and originates the bubbles. F
igure
16

sketches the process.
Hydraulic system is less expensive and involves
less maintena
nce than the mechanical system bec
ause of the rotating
parts. In the other hand mechanical procedure allows to cont
rol bubble
size and usually they are more efficient.


IGF can handl
e low oil concentrations from 15mg/l up to 500 mg/l
,
mechanical IGF are more efficient with lower concentrations (belo
w
150mg/l) [Aker, Unidro]
. It offers really high efficiency, over 98% i
n the

29

separation sizes
over 15

m. They are not affected by

flow rate

variations
and they

can operate heavy oils with not big density difference

achieving
good separation
results
. They have high power consumption and a
proper chemical pretreatment can increase their efficiency. They can
operate onshore and offshore but in no floating facilities, as a
consequence of skimming over a weir.



5.7

Compact Flotation Unit

(CFU)


The CFU is
a vertical separator vessel, which separates the three phases
oil/water/gas by using centrifugal force and gas flotation.

It has no
moving parts and is capable of achieving high standards of oil removal.

It has smaller volume and shorter retention than tra
ditional f
lotation
units (Statoil, 2010). That is why CFU suits ideally offshore application
s
,
it reduces the size and weight

of the oil/water separation facilities

compared old systems [
EPCO
N, Siemens]
.
CFU was developed by EPCON
that nowadays is owned by

MISwaco (Schlumberger)
, other companies
have built

similar systems such

as

Siemens with its Vorsep technology
or OPUS
.

CFU systems are capable of reducin
g the oil content below 10
ppm [
EPC
ON, Opus, VWS westgarth]
, and if two CFU systems work
together, th
is content can be reduced to 5 ppm (EPCON).



Small oi
l droplets are made to coalesce, creating
larger droplets, which
are easier to remove. The droplets because of specific gravity

difference

form a continuous layer at the top of the vessel. Oil water separation is
helped with a simultaneous flotation effect, caused by the release of
residual gas from the produced water. In some occasions the gas
flotation is increased with external gas inject
ion and flocculants.

As figure

17

sketches
, the
produced
water en
ters the CFU tank
horizontally, in a tangential direction.
The distributor situated at the top
chamber disperse
d the water. The majority of

any entrained gas is
released at this point. Produ
ced water makes its way under
gravity

30

troug
h the eductors towards the bottom cha
m
ber
. The design of these
eductors ensures that the gas from the upper chamber is drowning
down into the eductors where it mixes with the water. One of the
important features o
f the CFU is the perfect mix between gas and
produced water. The shape of the eductors creates a vortex in the lower
chamber, which favors the coalescence of the oil droplets, and a toroidal
flow is created. The oil floats with the help of the gas and it
moves to
the top where it is removed with a skimmer. The water
exits

the vessel
t
hrough the bottom [Veolia
]
.


CFU system has many advantages; most of the companies confirm that
it is a robust system with small footprint and low weight. Because they
have no

rotating parts, it is easy to operate, no energy is required, and
maintenance
costs
a
re

also lower. It has a high flow capacity with low
volume, for instance, an EPCON CFU system can operate a water flow up
to 220 m
3
/h with a vessel volume of 2.
4

m
3
.



5.8

Sand cyclones


The de
sander vessel is ideal for the i
nline desanding of produced water
and is the most important element in the sand management system
[Aker, 2012]
. The vessel consists in two sections, the upper section
where the separation occurs and the
bottom section where the san
d is
removed. The principle of operation is the same as in every
hydrocyclone. Separation happens due to the pressure drop in the liners
inlet and outlet ports that creates two different vortexes. In the sand
cyclones the solid
particles move to the walls of the liner and the water
flows in the smaller vortex. The sand will be accumulated in the
catchment chamber at the bottom

and discharged intermittently,
m
eanwhile the desanded water discharge continuously.



31

T
he liners, very si
milar to de
-
oiling hydrocyclones
are placed in the
upper section

between two support plates.

They must be manufactured
with special ceramic materials such as alumina ceramic (standard) or
bonded silicon carbide [Cyclotech] in or
der to resist the erosion
pr
ovok
ed by the solid particles [Aker, Veolia].
A sand cyclone sketch is
shown in figure

18;
they

can have different diameter size depending on
the size of the particles that need to be removed.

For instance, Ake
r
suggest 1.
5
-
inch diameter for 4 mm particles

and a separation size
ranged from 10 to 20 microns. For bigger particles, around 6 mm they
put forward a
3
-
inch

liner that can separate up to 40 microns.


5.9

C
-
Tour


C
-
tour sy
stem technology developed to extract dispersed and dissolved
and dispersed oil, reducing the environmental impact in the North Sea.
The participants
were Statoil, Norsk Hydro, BP, Shell, etc.

It was
conducted at the Rogaland Research I Institute and Norsk
Hydro
research center. The name comes from the French scientist who
discovered the

phenomena of

super critical flui
ds in XIX century,
Cagniard de la Tour in 1822
[Voldum and Garpestad, 2008].


The principle of the process is to use nat
ural gas liquid
-
conde
nsate

as a
solvent to extract the hydrocarbons contaminants in produced water
[Descousse, 2004]. The process includes several steps; the first is to
collect the condensate from the production extreme. This can be done
in the gas compression train scrubber
s

[Grini, 2002
]. Then the collected
condensate is injected at

small rate into the produce water line

(0,3
-
2%
volume/volume)
. The second is the extraction of the hydrocarbons from
the water into condensate
phase; this process might take couple of
seconds
. An
d the last step is the separation of the condensate from the
water in a hydrocyclone system.

Recycling of the rejected water must be
done.

Figure 19
is a diagram of the Ctour process.



32

The condensate should accomplish some features in order to achieve
good

results in the process. The condensate must remain in liquid
phase during the injection and following extraction.
C
omposition of the
condensate is a very important factor

[Voldum, 2008]
. The condensate
may contain some aromatic components, which, could be

present in
higher concentrations than in the produced water. It can end in an
increase of the heavy aromatic compounds in the produced water. If the
condensate does not reach the needed characteristics some
pretreatments can be used. Some of the
m

are incr
easing the processing
pressure to match the liquid phase or flashing the condensate to reduce
bubble point

[Voldum and Garspetad, 2008]
.

Condensate injection and
mixture is another key element for the process. The system must ensure
a homogeneous dispersio
n throughout the produced water stream,
providing the highest possible surface favoring coalescence process.
Besides the dispersion, the higher the turbulence the better the mass
transfer will be, which also helps the performance of the system.


Table 7

pr
esents the results of the Ctour system in the Ekofisk
, offshore

oilfield in the North Sea. It is important to notice the low efficiency of
the process in the C4
-
C5 phenols.

Voldum explains this phenomenon
affirming that those phenols are highly soluble in
water with low
bioaccumulation. The aver
age of oil in water discharge r
anges 1
-
2 pp
m
and it is never higher than 2.
2 ppm.

Statoil confirms that the removal
average of the Ctour system is:
95%
for
Dispersed oil
,
92%

in

Naphthalene
,
97%
in
PAH
,
0% C0
-
C3
phenols

(very important, it meets
the zero content discharge),

50% C4
-
C5 phenols
,
97% C6
-
C9 phenols

and 10% to
80% BTEX
.


5.10

MPPE


MPPE or macro porous polymer extraction is an
Akzo Nobel

techn
o
logy
elaborated in the 1990s. It is capable to withdraw dispersed

and
dissolved hydrocarbons to very low levels

with flow rates from 200

m
3

to

33

250m
3
.

This technology is a liquid
-
liquid extraction performed by a
macro

porous polymer particle.

MPPE system is commonly placed after
the first separation processes, in gas/con
densate fields after
degasser/skimmer and in oil fields after
hydrocyclones
.


The MPPE usually consist in two columns that ensure a continuous
operation, one is destined to extraction and the second for
regeneration. T
he contaminated water
passes through
the first

column
packed with MPPE particles that contain specific extraction liquid.
The
particles have a diameter of 1

m with pore size range 0,1 to 10

m.
The
hydrocarbons with high affinity for the liquid
are removed. In order to
clean
the extraction li
quid
,

low
-
pressure

steam strips the hydrocarbons
that are

condensed and separated late
r in the second column (figure
20
).
It is a long process that takes usually one hour for each column
[
Meijer, 2004]
.

Most of the oil/gas field components can be removed
w
ith a very high efficiency, BTEX and PAH`s, reach 99´999% removal

from 2000 to 3000ppm
concentrations.
It has been proved that
chemicals such as scale corrosion inhibitors, demulsifiers or H
2
S
scavenger have no negative effect on the performance.

To improv
e the
MPPE process it is necessary to optimize de steam consumption used in
the regeneration tower and some solids pre
-
filter can be added to avoid
blocks.


MPPE presents many advantages to become an even more important
technology. It is a robust system with long life
and flexible, it
is capable
of treating dif
ferent kinds of water (oilfield/
gas
-
field) with high
efficiency removal
, it presents 84% in EIF

(Environmental Impact Factor)
analysis [Meijer, 2004]
(almost reaching the zero discharge goal)
.
It
has
been demonstrated in
Kvitebj
ørn that bioactivi
t
y in the field stopped
during year 2005,
wh
e
n the MPPE system was installed;

bioactivity of the
field wa
s restored in less than 3 months because of the high
hydrocarbon elimination
. Figure 21 presents a
comparison

between
flotation and MPPE process
.

The worst disadvantage of this unit is its

34

high price. Other disadvantages are the relatively high
-
energy
consumption and the cost o
f the pre
-
treatment in the oilfield produced
water
.


5.11

Walnut shell filters


Walnut shell filters consist in filtration and scrubbing processes in the
same vessel
. Filtration usually occurs down flow, as the liquid passes
through th
e media, oil and

solids are efficiently attached

(coalesced)
in
bed.

This process is based on time or pressure difference.

Often, air or
gas is added to create an airlift pump.


Then the scrubbing system starts, the scrub pump is opened and the
media star
ts circulating
in the scrubbing system. During this circulation
the media is positively cleaned
because of the turbulence of the
backwash water and the air (if added)
[Siemens, Cameron]. For the