U.S. patent application number 13/624845 was filed with the patent office on 2013-03-28 for apparatus and process for treatment of water.
The applicant listed for this patent is James Craig Pauley, Prakhar Prakash, De Vu. Invention is credited to James Craig Pauley, Prakhar Prakash, De Vu.
Application Number | 20130075334 13/624845 |
Document ID | / |
Family ID | 47910082 |
Filed Date | 2013-03-28 |
United States Patent
Application |
20130075334 |
Kind Code |
A1 |
Prakash; Prakhar ; et
al. |
March 28, 2013 |
Apparatus and Process For Treatment of Water
Abstract
The invention relates to the treatment of water, including for
example treatment in connection with hydrocarbon production
operations. Silica in water produces undesirable scaling in
processing equipment, which causes excess energy usage and
maintenance problems. Electrocoagulation (EC) at relatively high
water temperature may be combined with a process of ceramic
ultra-filtration (UF filtration) employed to treat water, and
optionally followed by any of membrane distillation or forward
osmosis (FO). Water to be treated may be produced water that has
been pumped from a subterranean reservoir. The treated water may be
employed to generate steam. The treatment units (e.g., EC, forward
osmosis, UF filtration, etc) can be configured into one system as
an on-site installation or a mobile unit for on-site or off-site
water treatment.
Inventors: |
Prakash; Prakhar; (Danville,
CA) ; Pauley; James Craig; (Bakersfield, CA) ;
Vu; De; (El Cerrito, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Prakash; Prakhar
Pauley; James Craig
Vu; De |
Danville
Bakersfield
El Cerrito |
CA
CA
CA |
US
US
US |
|
|
Family ID: |
47910082 |
Appl. No.: |
13/624845 |
Filed: |
September 21, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61537661 |
Sep 22, 2011 |
|
|
|
61537666 |
Sep 22, 2011 |
|
|
|
61537682 |
Sep 22, 2011 |
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Current U.S.
Class: |
210/640 ;
205/751; 210/650; 210/702; 210/703; 210/738 |
Current CPC
Class: |
C02F 9/00 20130101; C02F
1/445 20130101; Y10T 29/494 20150115; C02F 1/447 20130101; C02F
2209/10 20130101; B01D 63/02 20130101; C02F 1/463 20130101; C02F
2209/06 20130101; B01D 61/002 20130101; B01D 61/145 20130101; B01D
61/58 20130101; B01D 71/024 20130101; C02F 1/444 20130101; C02F
2209/20 20130101; B01D 61/364 20130101 |
Class at
Publication: |
210/640 ;
205/751; 210/650; 210/703; 210/738; 210/702 |
International
Class: |
C02F 5/02 20060101
C02F005/02; C02F 1/44 20060101 C02F001/44; C02F 1/66 20060101
C02F001/66; C02F 1/24 20060101 C02F001/24; C02F 1/52 20060101
C02F001/52; C02F 1/463 20060101 C02F001/463; B01D 61/36 20060101
B01D061/36 |
Claims
1. A method for treating produced water for steam generation, the
method comprising: providing a source of produced water to be
treated, the water having contaminants selected from the group of
TOC and TDS as silica and hardness ions; removing at least a
portion of the silica and hardness ions as suspended solids by
subjecting the produced water to an electrocoagulation process;
removing at least a substantial portion of the suspended solids by
at least one of floatation, sedimentation, filtering,
centrifugation, settling, hydrocyclone, gravity settling device,
and combinations thereof, generating a pre-treated water; passing
the pre-treated water to a filtration unit to remove at least 70%
of the TOC as free oil, for a treated water stream having less than
10 mg/L TOC and less than 500 ppm TDS.
2. The method of claim 1, wherein no heat energy is added to or
removed from the pre-treated water prior to passing the pre-treated
water to the filtration unit.
3. The method of claim 1, wherein the electrocoagulation process
employs sacrificial electrodes.
4. The method of claim 1, wherein the pH of the produced water is
adjusted to a pre-select pH prior to removing at least a portion of
the silica and hardness ions by the electrocoagulation process.
5. The method of claim 4, wherein the pre-select pH ranges from 7.2
to 11.5.
6. The method of claim 1, wherein the filtration unit operates at a
water temperature of at least 50.degree. C.
7. The method of claim 6, wherein the filtration unit operates at
water temperature of at least 80.degree. C.
8. The method of claim 1, wherein the filtration unit employs at
least a ceramic membrane.
9. The method of claim 1, wherein the filtration unit employs at
least a polymeric membrane, wherein the polymeric membrane
comprises a material selected from sulfonated polyether ether
ketone (PEEK), sulfonated tetrafluoroethylene based
fluoropolymer-copolymer, poly(phthalazinone ether sulfone) (PPES),
poly(phthalazinone ether ketone) (PPEK) and poly(phthalazinone
ether fulfone ketone) (PPESK).
10. The method of claim 1, further comprising: treating the treated
water stream in a direct contact membrane distillation (DCMD) unit,
thereby generating steam-generating quality water having less than
10 mg/L TOC, less than 50 ppm silica, and less than 10 ppm hardness
ions.
11. The method of claim 10, wherein the DCMD unit employs at least
a hydrophobic hollow fiber membrane.
12. The method of claim 11, wherein the membrane fibers have a
length ranging from 1 to 200'', a wall thickness ranging from 2 to
100 .mu.m.
13. The method of claim 10, wherein the DCMD unit employs at least
a composite membrane comprising a hydrophilic polymer layer and a
hydrophobic polymer layer.
14. The method of claim 13, wherein the hydrophilic polymer
comprises any of polysulfone, polyether sulfone, polyetherimide
polyvinylidenefluoride, and cellulose acetate.
15. The method of claim 13, wherein the hydrophobic polymer layer
comprises fluorinated surface-modifying macromolecule (SMM).
16. A method for treating produced water for steam generation, the
method comprising: providing a source of produced water to be
treated, the water having contaminants selected from the group of
TOC and TDS as silica and hardness ions; removing at least a
portion of the silica and hardness ions as suspended solids by
subjecting the produced water to a chemical precipitation process;
removing at least a substantial portion of the suspended solids by
at least one of floatation, sedimentation, filtering,
centrifugation, settling, hydro-cyclone, gravity-settling device,
and combinations thereof, generating a pre-treated water; passing
the pre-treated water to a filtration unit to remove at least 70%
of the TOC as free oil, for a treated water stream having less than
10 mg/L TOC and less than 500 ppm TDS. wherein no heat energy is
added to or removed from the pre-treated water prior to passing the
pre-treated water to the filtration unit.
17. The method of claim 16, further comprising passing the produced
water through a screen to remove large particulates prior to
subjecting the produced water to a chemical precipitation
process.
18. The method of claim 16, further comprising mixing a caustic
with the produced water for a pH of 7.2 to 11.5 prior to or during
the chemical precipitation process.
19. The method of claim 16, wherein passing the pre-treated water
to a filtration unit comprises directing the pre-treated water to a
ceramic membrane to filter the pre-treated water with the ceramic
membrane to produce a retentate stream and a permeate stream as the
treated water stream.
20. The method of claim 16, wherein passing the pre-treated water
to a filtration unit comprises directing the pre-treated water to a
polymeric membrane to filter the pre-treated water to produce a
retentate stream and a permeate stream as the treated water stream,
wherein the polymeric membrane comprises a material selected from
sulfonated polyether ether ketone (PEEK), sulfonated
tetrafluoroethylene based fluoropolymer-copolymer,
poly(phthalazinone ether sulfone) (PPES), poly(phthalazinone ether
ketone) (PPEK) and poly(phthalazinone ether fulfone ketone)
(PPESK).
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims benefit under 35 USC 119 of U.S.
Provisional Patent Application Nos. 61/537,661; 61/537,666; and
61/537,682, all with a filing date of Sep. 22, 2011.
FIELD OF THE INVENTION
[0002] The field of the invention relates to the treatment of
water, including for example treatment of water in connection with
hydrocarbon production operations.
BACKGROUND
[0003] For every barrel of crude oil produced, about three to ten
barrels of water is produced. In the oil and energy industry, water
that is drawn from the formation is referred to as "produced
water." The injection of steam for heavy oil recovery has become an
important enhanced oil recovery (EOR) method. In EOR, high pressure
steam is injected at a rate sufficient to heat the formation to
reduce the oil viscosity and provide pressure to drive the oil
toward the producing wells. For EOR, steam is normally produced in
steam generators, with full steam makeup of water is required to
feed the generator. The feed water should be substantially free of
hardness, e.g., calcium and magnesium to prevent scale formation in
the steam generator tubes or in the oil formation, causing plugging
of downhole injection lines, causing increased pressure drop and
increasing the power demand on pumps. Silica at high concentration
can also pose a precipitation problem with scaling in steam
generators and associated pipelines. Since fresh water is not
always available for EOR, the treatment of produced water in the
oil recovery process becomes necessary.
[0004] It is desirable to reduce the levels of silica and hardness
to improve the efficiency of steam generators and simultaneously
reduces the carbon generation to the atmosphere by reduction of
natural gas consumption in the production of steam. Residual amount
of free oil in the produced water also causes inefficiencies in the
steam generators. Oil needs to be separated from the water for
further processing, and such separation is a major issue in
production operations. High efficiency reverse osmosis ("RO")
membranes reduce silica and hardness to negligible concentrations.
The process desalinates the produced water--which further improves
the quality of steam produced. However, reverse osmosis processes
are energy intensive and generate significant pumping costs. The
amount of free oil in water is a deterrent in steam generation
processes, as it may cause significant fouling of reverse osmosis
membranes. Materials that may undesirably serve to decrease reverse
osmosis efficiency are free oil, dissolved organics, silica,
magnesium ions and calcium ions.
[0005] There is a need for alternative and improved methods to
treat produced water to avoid undesirable scale build-up within
processing equipment.
SUMMARY
[0006] In one aspect, a system and method of treating produced
water is disclosed. The produced water is treated to remove
contaminants including but not limited to silica, hardness ions,
TDS, TOC, and COD. The produced water is subject to an
electrocoagulation process to remove at least a portion of the
silica and hardness ions as suspended solids. A substantial portion
of the suspended solids are removed by at least one of floatation,
sedimentation, filtering, centrifugation, settling, and
combinations thereof, generating a pre-treated water. The
pre-treated water is further treated in a direct contact membrane
distillation (DCMD) unit, generating treated water having less than
10 mg/L TOC, less than 50 ppm silica, and less than 10 ppm hardness
ions. In one embodiment, the DCMD unit employs cross-flow
hydrophobic hollow fiber membranes.
[0007] In one embodiment, a high-temperature filtering device is
employed to handle produced water at a temperature of at least
50.degree. C., wherein no heat exchanger is employed to remove or
add energy to the water treatment system. In one embodiment, the
high-temperature filtering device is a ceramic ultra-filtration
unit. In another embodiment, a high temperature polymeric membrane
is used in the filtering unit.
[0008] In one embodiment instead of or in addition to membrane
distillation, the water treatment process includes a forward
osmosis membrane separation process to produce high quality
desalinated water.
[0009] In another aspect, the invention relates to a system for
treating produced water containing contaminants selected from the
group of dissolved organics, free oil and grease, and TDS as silica
and hardness ions. The system comprises: an electrocoagulation unit
for treating the produced water to remove silica and hardness ions
from the produced water as suspended solids to generate a
pre-treated produced water stream having a LSI from -3 to 3; a
filtration unit employing at least a membrane to remove free oil
and grease from the pre-treated produced water at a temperature of
at least 50.degree. C., generating a filtered water stream with a
reduced concentration of free oil and grease; a membrane
distillation unit for removing at least 90% of dissolved organic
content from the filtered water stream with a reduced concentration
of free oil and grease; a forward osmosis unit for removing at
least 90% of dissolved organic content from the filtered water
stream with a reduced concentration of free oil and grease. The
units in the system are configured in a permutable fashion for the
units to be interconnected and interchangeable for the water
treatment system to operate in: a sequential mode with the
individual units running sequentially; a parallel mode with at
least two of the units running in parallel; a combination of
parallel and sequential mode; all units online; at least one unit
online and at least one unit being idle; and combinations
thereof.
BRIEF DESCRIPTION OF THE FIGURES
[0010] FIG. 1 is a block diagram of a system/process configuration
employing an EC unit in conjunction with high-temperature
filtration to treat produced water.
[0011] FIG. 2 is block diagram of another embodiment, wherein
chemical precipitation is used in conjunction with ceramic
ultra-filtration for produced water treatment.
[0012] FIG. 3 is block diagram of a third embodiment, wherein an EC
unit is used in conjunction with high temperature polymer
ultra-filtration for produced water treatment.
[0013] FIG. 4 is a block diagram showing a variation of the
system/process configuration of FIG. 1, further comprising a direct
contact distillation membrane unit for further process treatment of
the produced water.
[0014] FIG. 5 is a block diagram showing a variation of the
system/process configuration of FIG. 4, wherein forward osmosis is
used to further treat the produced water.
[0015] FIG. 6 is a block diagram showing yet another variation of
the system/process configuration of FIG. 4, wherein the produced
water stream feed is split with some untreated water being combined
with the treated water as feed to the forward osmosis unit.
[0016] FIG. 7 is a block diagram illustrating a system/process
configuration to treat a produced water stream with a relatively
low level of silica, with treatment in an ion exchange unit to
remove hardness.
[0017] FIG. 8 is a block diagram illustrating a variation of the
system/process configuration in FIG. 7, wherein an untreated
produced water stream is mixed with the treated stream for further
treatment.
DETAILED DESCRIPTION OF THE INVENTION
[0018] The following terms will be used throughout the
specification and will have the following meanings unless otherwise
indicated.
[0019] "ppm" refers to parts per million. One ppm is equivalent to
1 mg per liter.
[0020] LSI refers to the Langelier Saturation index, an equilibrium
model derived from the theoretical concept of saturation and
provides an indicator of the degree of saturation of water with
respect to calcium carbonate. It can be shown that the Langelier
saturation index (LSI) can be correlated to the base 10 logarithm
of the calcite saturation level. The Langelier saturation level
approaches the concept of saturation using pH as a main variable.
The LSI can be interpreted as the pH change required to bring water
to equilibrium. Water with a negative LSI means that there is
little or no potential for scale to form, with the water typically
dissolving CaCO3. If the LSI is positive, scale will typically form
and CaCO3 precipitation will typically occur.
[0021] "Flowback water" refers to return water from fracking
operations in shale gas plays.
[0022] "Fracking" may be used interchangeably with hydraulic
fracturing, referring to a technique used to release petroleum,
natural gas (including shale gas, tight gas and coal seam gas), or
other substances for extraction as a result of the action of a
pressurized fluid such as the injection of water into the
formation.
[0023] "Produced water" may be used interchangeably with
"production water," referring to water separated from the
production of stream and gas wells, including but not limited to
tar sand wastewater, oil shale wastewater, water from steam
assisted gravity drainage oil recovery process, and flowback
water.
[0024] "Silica" (SiO2) will be used to refer generally to
silica-based compounds.
[0025] "Absorbing" or absorption refers to a method or apparatus in
which absorbants, such as active carbon, are used to absorb
impurities in the water.
[0026] "FPSO" (floating production, storage and offloading) vessel
refers to a vessel or a platform located over or near a subsea well
site, a near-shore separation facility, or an onshore separation
facility. Synonymous terms include "production facility" or
"gathering facility."
[0027] "Steam-generation quality water" refers to water having less
than 10 mg/L TOC ("total organic carbon"), less than 50 ppm silica,
and less than 10 ppm hardness ions.
[0028] In the process of producing oil, "produced water" is
generated during oil production as a waste stream. In many
instances, this waste stream can be seven or eight times greater
than oil produced at any given oil field. Some of this water can be
re-injected to the well for pressure maintenance, some is injected
to deep well for final disposal in the case of proper aquifer
conditions, and some is reclaimed for use as oilfield steam
generator feed-water. Large amount of water is typically needed for
steam generation. Large amount of energy is needed to create steam
from water. The produced water, which is not re-injected to the
production well such as reclaimed water for steam generation, has
to be treated. Produced water has distinctive characteristics due
to organic and inorganic matters, potentially causing fouling and
limiting steam generator reliability, and ultimately oil
production. The invention relates to improved processes and systems
for the treatment of produced water, e.g., water for use in steam
generation, including of electrocoagulation pretreatment, lime
(chemical) precipitation pretreatment, ceramic ultra-filtration,
forward osmosis (FO), membrane distillation, and combinations
thereof.
[0029] Produced water feed: Produced water feed to the treatment
process typical contains both inorganic and organic constituents
that limit the discharge options, e.g., dispersed oil, dissolved or
soluble organics, produced solids, scales (e.g., precipitated
solids, gypsum (CaSO4), barite (BaSO4)), bacteria, metals, low pH,
sulfates, naturally occurring radioactive materials (NORM), and
chemicals added during extraction. The produced water contains at
least 1,000 mg/L TDS in one embodiment, at least 5,000 mg/L TDS in
a second embodiment, and at least 10,000 mg/L TDS in a fourth
embodiment. In some locations, the produced water may have TDS
concentrations of at least 150,000 mg/L. In terms of hardness level
(as Mg, Ca, Sr, Ba), the concentration may range from 200-2000 mg/L
Mg; from 5000 to 40,000 mg/L Ca, from 1000-10,000 mg/L Sr, and from
1000-10,000 mg/L Ba.
[0030] The oil related compounds in produced water include benzene,
xylene, ethyl benzene, toluene, and other compounds of the type
identified in the sample analysis shown in Table 1 and in other
crude oil and natural gas sources. The amount of TOC as free oil
and grease can be substantially higher as shown when there is an
occasional process upset. Normally, the production water will also
contain metals, e.g., arsenic, barium, iron, sodium and other
multivalent ions, which appear in many geological formations, as
illustrated in Table 1 for an example of produced water from
Wellington, CO after oil water separation in an .DELTA.PI
separator:
TABLE-US-00001 TABLE 1 Produced Water Quality Parameters after
separation mg/l mg/l Inorganics Total Dissolved Solids (TDS) 1200
6000 Total Hardness as CaCO.sub.3 30 300 Total Alkalinity as
CaCO.sub.3 1000 4000 Chloride (Cl) 40 1000 Fluoride <1 10
Phosphate (PO.sub.4) <0.5 30 Nitrite + Nitrate-Nitrogen
(NO.sub.2 + NO.sub.3--N)* <0.5 40 Metals Antimony (Sb) <0.005
1.00 Arsenic (As)* <0.005 1.00 Barium (Ba)* 3.00 30.00 Berylium
(Be) <0.0005 1.00 Cadmium (Cd) <0.001 1.00 Chromium (Cr)
<0.02 1.00 Copper (Cu) <0.01 1.00 Iron (Fe)* 0.10 30.00 Lead
(Pb) <0.005 5.00 Manganese (Mn)* <0.005 10.00 Mercury (Hg)
<0.0002 0.10 Nickel (Ni)* <0.05 10.00 Selenium (Se) <0.005
5.00 Silver (Ag) <0.01 5.00 Thallium (Tl)* <0.002 1.00 Zinc
(Zn) <0.005 10.00 Organics Oil and grease* 20.0 200.00 Benzene*
1.00 10.00 Toluene* 1.00 5.00 Ethylbenzene* 0.10 1.00 Xylenes,
total* 1.00 5.00 n-Butylbenzene* 0.01 0.50 sec-Butylbenzene* 0.01
0.10 tert-Butylbenzene* 0.01 0.10 Isopropylbenzene* 0.01 0.10
4-Isopropyltoluene* 0.01 0.10 0.01 0.10 Naphthalene* 0.01 0.10 0.01
0.10 n-Propylbenzene* 0.01 0.10 0.01 0.10 1,2,4-Trimethylbenzene*
0.10 1.00 1,3,5-Trimethylbenzene* 0.10 1.00 0.10 1.00 Bromoform*
<0.001 1.00
[0031] Depending on the concentration of the produced water feed,
the selected pretreatment method (e.g., chemical precipitation,
electrocoagulation, etc.), and the end-use applications, in some
embodiments, additives such as complexing agents, coagulants,
oxidizing agents (e.g., ozone, polyaluminum chloride), etc., can be
added to the produced water feed upfront prior to or during the
pretreatment step.
[0032] In one embodiment prior to water treatment, the produced
water feed may pass through a screen or strainer to capture larger
particulates, including large solids/particulates that may
potentially damage or foul the blades within the EC unit.
[0033] Chemical Precipitation Pretreatment Unit: Depending on the
properties of the produced water feed, chemical precipitation (CP)
can be used as a pretreatment step for the removal of silica and/or
hardness, with the addition of certain reagents in amounts in
excess of the silica and/or hardness ions in the produced water
feed.
[0034] In one embodiment for the removal of silica, the produced
water is dosed with a crystal forming compound such as magnesium
oxide to remove silica, converting soluble silica to insoluble
silica. The crystal forming compound forms crystals in the produced
water that adsorb silica, resulting in silica being driven or
pulled out of solution and adsorbed on the formed crystals. Various
crystal forming materials can be added, e.g., magnesium oxide or
magnesium chloride, which forms magnesium hydroxide crystals that
function to absorb silica in the produced water, resulting in the
conversion of silica from soluble to insoluble form. It should be
noted that in the case of magnesium, there is an insufficient
concentration of magnesium typically found in produced water to
yield a substantial amount of magnesium hydroxide crystals. Thus,
magnesium compounds are added to the produced water. In some cases,
the dissolved silica and the produced water can be subsequently
removed from solution by mixing the produced water with compounds
having surface active properties to draw silica out of solution.
Examples of such compounds are oxides of aluminum, silica and
titanium.
[0035] In another embodiment to soften water removing hardness
ions, lime, soda ash and/or caustic is used in the pretreatment
step. Both the lime (calcium hydroxide) and caustic are mixed with
the feed water. Lime converts carbon dioxide to bicarbonate ions
and neutralizes the bicarbonate alkalinity of the produced water
and removes calcium carbonate hardness. The caustic removes
magnesium hardness present in the feed water and raises the pH of
the produced water to a basic level. In one embodiment, the pH is
raised to above 10.5. In many cases, the pH is maintained in the
range of 10.5 to 11.5. The lime softening step can be carried out
at normal raw water temperature (cold lime process) to reduce the
hardness of the produced water down to 30-50 ppm, or at
temperatures near or above the boiling point (hot or warm lime
process) to reduce the hardness of the produced water down to 15-25
ppm.
[0036] In one embodiment, other reagents or compounds
("coagulants") can be added to the produced water instead of in
addition magnesium compounds, lime, caustic. The coagulants may act
to destabilize the solids generated during the softening process
and further facilitates or enhances the separation of solids from
the liquid in subsequent portions of the process. Examples include
but are not limited to ferric chloride, aluminum oxide, aluminum
chloride, aluminum sulphate, polyaluminum chloride, ferrous or
ferric sulfate, calcium oxide and mixtures thereof. Dosage may vary
depending on the nature and characteristics of the feed water, but
in many cases, the dosage will vary in the range of 10-50 mg/l.
[0037] The pH of the produced water is maintained in the range of
9.5 to 11.2 in one embodiment, and between 10.0 and 10.8 in a
second embodiment for optimum precipitation of silica. Some caustic
in the form of sodium hydroxide or sodium carbonate may be added to
trim the pH to a proper value.
[0038] The total hardness of the CP treated water (by lime process)
is less than 10 ppm in one embodiment; less than 5 ppm in a second
embodiment; and less than 1 ppm in a third embodiment. In one
embodiment, the LSI value of the treated water is 0. The total
(soluble) silica in the CP treated water (treatment by magnesium)
is less than 50 ppm in one embodiment, and less than 25 ppm in a
second embodiment.
[0039] After water passes through the CP unit, the precipitates can
be subsequently removed from the oily produced water stream by
means known in the art, e.g., in a solid separation unit, prior to
further treatment depending on the application.
[0040] Electrocoagulation ("EC") Pretreatment Unit: In one
embodiment, electrocoagulation (EC) is employed to remove silica
and hardness from the produced water instead of chemical
precipitation. EC refers to a process of applying electrical
current to treat and flocculate contaminants without having to add
coagulations. EC consists of pairs of metal sheets called
electrodes, arranged in pairs of two, anodes and cathodes. At least
one of the cathode and anode is sacrificial and made from materials
such as iron, aluminum, zinc, or magnesium, with the ions thereof
migrate into the electrolyte and bond with impurities to create
precipitates. In the EC, possible reactions that may occur on the
anode surface are metal dissolution and oxygen evolution. The
half-cell reactions may be any of anodic and cathodic reactions. In
an example with iron being employed for the electrode, the possible
anodic reactions are metal dissolution, oxygen evolution, and
oxidation of metal ion to higher oxidation state, as shown
below:
Fe.dbd.Fe2++2e-
4Fe.sup.2.sup.++O2+4H+=4Fe.sub.3++2H.sub.2O
4Fe.sup.2++2+4H+=4Fe.sub.3++2H.sub.2O
[0041] The primary cathodic reactions that may occur on the cathode
surface are oxygen reduction and hydrogen evolution, which may be
expressed as shown below:
O.sub.2+4H++4e-=2H.sub.2O
2H++2e-=H.sub.2
[0042] Ferric ions precipitate as ferric hydroxide. These ions
function to capture constituents in the produced water such as
silica within the ferric hydroxide complexes, generating
precipitates, as shown below.
Fe.sup.3++3OH--.dbd.Fe(OH).sub.3
[0043] As it passes through the EC cell, the coagulants introduced
by the passage of electric currents through iron or aluminum
electrodes in the EC chamber help reduce the concentration of
silica to a low value with the formation of precipitates. The EC
process is tunable, meaning that variations may be introduced to
adapt to slightly changed conditions. In one embodiment by changing
the amperage in the process, it is possible to manipulate/vary the
amount of silica removed.
[0044] Depending on the composition of the produced water to be
treated, additives may be used if needed during the
electrocoagulation. For example, when non-sacrificial cathodes and
anodes are used, the additives may be used to form ions to interact
with solutes and particulate matter in coagulating the impurities
out of suspension and solution. When sacrificial cathodes and
anodes are used, additives may be used to increase the conductivity
of the water stream to enhance electrocoagulation processes. The
additives may be later removed, or involved in the chemical
processes to form precipitates. In addition, to improve
flocculation, flocculants can also be added to the
electrocoagulation. In one embodiment, with the addition of an
oxidizing agent such as Fenton's reagent to the EC step, the
dissolved organic carbon content may be further reduced. Fenton's
reagent is a commercially available solution of hydrogen peroxide
and an iron catalyst that is used to oxidize contaminants or waste
waters.
[0045] The EC pretreatment process is quite efficient in treating
produced water from fields which have a large amount of TDS. In one
embodiment with the use of iron as one of the electrodes, as
coagulation is governed by the amount of ferric ions released, the
dosage is dependent on the amount of current in the system based on
the following equation: Fe generated (mg/s)=I*M/Fn.(1000 mg/g);
wherein M=Molecular weight of iron; F=Faraday's Constant
(96,485C/mol); I=Applied current (Amps/s); n=number of electrons
transferred in the reaction.
[0046] As shown, when the conductivity of the solution is high, its
resistance `R` is low. For a lower voltage, the same current can be
generated. The power consumption, defined as I 2*R is therefore
significantly reduced for the same amount of coagulant generated.
This makes the process very efficient for certain applications
where the hardness and silica are to be removed from sea water type
salinity. Such high TDS water is commonly seen in many carbonate
type subterranean reservoirs.
[0047] In some applications, EC reduces silica at least 75% of the
silica in one embodiment and as much as over 90% in another
embodiment, reduce hardness by about 60% to 90%, reduce dissolved
organic carbon content by about 25%-50%. Additionally, depending on
the composition of the produced water, the pH of the feed water to
the EC unit can be optionally adjusted to a pre-select pH to
optimize its operation to maximize the removal of both the silica
and the hardness level. The removal of hardness materials such as
calcium carbonate helps reduce scaling of further treatment units
downstream, e.g., filtration membranes.
[0048] In one embodiment with the use of sacrificial electrodes,
some caustic in the form of sodium hydroxide or sodium carbonate
can be added to the produced water feed to adjust the pH. By
changing the pH conditions of the produced water to a pre-select
basic pH, at least 90% of the hardness is removed in one
embodiment, at least 95% removal in a second embodiment, and at
least 99% removal in a third embodiment. This pre-select pH is at
least 9 in one embodiment; at least 9.5 in a second embodiment, at
least 10 in a third embodiment, and at least 10.5 in a fourth
embodiment for the removal of at least 90% of the silica and
hardness. In yet another embodiment, the pre-select pH is
maintained in the range of 7.2 to 11.5; and between 10.0 and 10.8
in another embodiment for optimum precipitation of silica. The EC
treated water has a silica concentration of less than 50ppm in one
embodiment; less than 25 ppm in a second embodiment. The total
hardness of the EC treated water is less than 10 ppm in one
embodiment; less than 5 ppm in a second embodiment; and less than 1
ppm in a third embodiment. The LSI value of the EC treated water
ranges from 3 to 3 in one embodiment; a value of 0 in a second
embodiment; and a value of -2 in a third embodiment.
[0049] In one embodiment with an ultra-filtration step or a
membrane distillation step downstream from the EC unit, EC treated
water generates a cake layer on the membranes that is more easily
cleaned than with treated water via other methods, e.g., using
conventional coagulants. It is hypothesized that the cake layer
formed on the membranes downstream of the EC unit are less
compressible, as evolution of hydrogen gas during the EC process
makes the flocs less dense and thus easier cleaning.
[0050] Besides the easier maintenance work downstream and desirable
end result of high quality treated water for steam generation, the
use of the EC unit in one embodiment results in an incremental
increase in water temperature. The produced water enters the system
is at a temperature as much as 50.degree. C. in one embodiment, at
least 70.degree. C. in a second embodiment, and in the range of
80-90.degree. C. in a third embodiment. As the high temperature is
maintained in the EC step, then a limited amount of heat may be
needed to boil the water to create steam. Additionally, the current
increases the temperature of the produced water. This additional
heat aids the thermal driving force downstream desalination forward
osmosis/membrane distillation step.
[0051] After water passes through the EC unit, the precipitates can
be subsequently removed from the oily produced water stream in a
solid separation unit, prior to further treatment depending on the
application.
[0052] Solid Separation Unit: In the solid separation unit, a
substantial portion of the precipitates are removed using means
known in the art, e.g., floatation, sedimentation, filtering, and
the like, using any of an incline plate settler, settling tank,
centrifuge, hydrocyclones, or enhanced gravity separation device,
or a combination thereof.
[0053] In one embodiment, the treated water is passed through a
clarifier to remove precipitates, sludge, etc. In one embodiment,
the clarifier comprises a settling tank, formed in the bottom of
the settling tank is a sludge scraper. Once the feed water reaches
the setting tank of the clarifier, solids in the form of
precipitants and suspended solids will settle to the bottom of the
settling tank to form sludge. Sludge is pumped from the bottom of
the settling tank. The characteristics of the produced sludge is
dependent on the characteristics of the feed water being treated,
such as hardness, the metals contained in the feed water, and the
alkalinity of the feed water. Typically in a process treating
produced water, the sludge comprises predominantly iron hydroxide
(60% to 70%) if iron electrodes are used, with the balance
comprising insoluble compounds derived from hardeness causing
ions.
[0054] After the solid separation step to remove a substantial
portion of the suspended solids, the treated water may further pass
through any of a filtration unit, a membrane distillation unit, a
forward osmosis unit, or combinations thereof for further
treatment. The removal rate (of suspended solids) is at least 80%
in one embodiment, at least 90% in a second embodiment, and at
least 95% in a third embodiment.
[0055] Adsorbing Unit: In one embodiment wherein the removal of
dissolved organic carbon removal is insufficient in the
pretreatment step, e.g., EC or CP process, an absorbing medium such
as activated carbon can be employed for the removal for on shore
applications for at least 95% removal in one embodiment, and at
least 99% removal in a second embodiment. In some applications,
advanced oxidation using ozone generators/UV-peroxide
(H.sub.2O.sub.2) may be used instead of activated carbon, such as
in some offshore applications. After the optional step to remove
dissolved organics, the treated water may further pass through any
of a filtration unit, a membrane distillation unit, a forward
osmosis unit, or combinations thereof for further treatment.
[0056] In one embodiment, the absorbing unit employs walnut shell,
wherein water flows down a bed of walnut shells where oil is
adsorbed and suspended solids are filtered. Black walnut shells
have a unique property in that they have an equal affinity for oil
and water. Since the walnut shell filters are hydrophilic, the
loosely bond residual oil can be easily separated using low
pressure backwashing. The unit can be pressurized to force water
through the adsorbing bed to get the desired performance of at
least 99% removal.
[0057] Filtration Unit: Depending on the particular
application/end-use of the treated produced water, after the CP
and/or EC pretreatment and after an optional clarification step,
there may still be some residual free oil and particulates
(residual suspended solids) in the treated water. In one
embodiment, filtration is employed to remove the free oil content
("polishing" or "polishing de-oiling") before the water can be
further treated if needed, e.g., in a membrane distillation unit.
In other embodiments, filtration may be bypassed if a downstream
treatment step, e.g., membrane distillation unit, is tolerant to
residual suspended solids and residual dissolved organics after the
electrocoagulation treatment.
[0058] The filtration can be in succession with the treated
produced water (from any of EC, CP, solid separation unit, carbon
absorber) is directed through a number of filters in series. The
filters can be staged in successive filtration sizes and capacity,
from filters to ultra-filters or membranes. The filters can be of
the same of different types, e.g., ceramic filtration followed by
high-temperature polymeric membrane filtrations or vice versa. In
one embodiment, an ultra-filtration ("UF") unit is employed. In
some applications, polymeric ultra-filtration UF membranes may be
employed. These UF membranes may be comprised of polyethersulfone
(PES), polyacrylonitrile (PAN) or polyvinylidene difluoride (PVDF),
which may operate up to a maximum of about 40-45.degree. C. For
produced water feed with at a temperature above 50.degree. C.,
e.g., at about 80-90.degree. C., the water temperature will need to
be lowered before subsequent treatment in the UF step using these
membrane types.
[0059] In applications handling produced water at 80.degree. C. or
more, special high-temperature polymeric membranes are employed.
Examples include sulfonated polyether ether ketone (PEEK) membranes
as disclosed in US Patent Publication No. 20100319535, incorporated
herein by reference, Nafion membranes (sulfonated
tetrafluoroethylene based fluoropolymer-copolymer), and membranes
constructed out of poly(phthalazinone ether sulfone) (PPES),
poly(phthalazinone ether ketone) (PPEK) or poly(phthalazinone ether
fulfone ketone) (PPESK).
[0060] In another embodiment for handling produced water at
80.degree. C. or more, ceramic UF membranes are employed, wherein
cross-flow filtration is carried out with the high-velocity
produced water "crossflows" across the face of the ceramic
membrane. Suitable ceramic membrane materials include titanium,
alumina, zirconia, and combinations thereof (e.g., alumina membrane
with zirconia coating, etc.). Depending on the produced water to be
processed, and the end use application, the membranes can be
micro-rated and of different sizes, e.g., ranging from 0.005 mm to
about 0.2 mm in one embodiment, and from 0.005 .mu.m to 0.02 .mu.m
in another embodiment, from 0.05-10 .mu.m in a third embodiment,
from 0.5 to 2 .mu.m in a fourth embodiment.
[0061] The oil-free water passes through the ceramic membrane (as
permeate or filtrate) while the oily waste is concentrated in a
process reservoir or retained on the feed side of the membrane as
retentate. With crossflow filtration, the tangential motion of the
bulk of the fluid across the membrane causes trapped particles on
the filter surface to be rubbed off Thus, one advantage of
cross-flow filtration is that the filter cake (which can blind the
filter) is substantially washed away during the filtration process,
increasing the length of time that a filter unit may be
operational. Cross-flow filtration can be a continuous process,
operating continuously at relatively high solids loads without
blinding, unlike batch-wise dead-end filtration.
[0062] In one embodiment, the ceramic UF unit comprises a stainless
steel housing containing ceramic membrane elements constructed from
aluminum oxide and tubular in shape. Water passes along the
parallel tubes from the feed inlet to the outlet. The surfaces of
the tubes are coated with a ceramic membrane material that has a
uniform pore size to provide microfiltration or ultra-filtration.
The feed stream may be introduced under pressure at the inlet and
is withdrawn as retentate at the downstream end. Permeate passes
through the membrane into the porous ceramic structure. The
combined permeate from all of the tubular passageways flows through
the monolith support to permeate conduits within the monolith that
transport the permeate through slots to an external collection
zone.
[0063] The use of ceramic membranes for the UF unit is advantageous
as ceramic membranes, being inorganic, are not as prone to fouling
as some of the polymeric membranes and requiring less cleaning as
compared to a polymeric membrane, thereby reducing the amount of
downtime and backwash cycles during the operation. In cleaning
operations, ceramic membranes may withstand aggressive cleaning
with sodium hydroxide, unlike most polymeric membranes, which
cannot withstand a cleaning solution pH of greater than 11. The
abrasive resistance of ceramic membranes makes them suitable for
high total dissolved solids ("TDS") in water, when compared to the
polymeric UF membranes. Ceramic membranes may be used for the
entire pH range (0-14), thereby facilitating high pH treatment of
water in the EC process for hardness reduction.
[0064] The use of ceramic membranes or high-temperature polymeric
membranes that can withstand water high temperatures up to
130.degree. C. (as opposed to the maximum operating temperature of
about 45.degree. C. with some polymeric UF membranes) allows the
handling of produced water as is. Produced water that is very hot
need not undergo any temperature reduction before entering the
membrane module when using ceramic membranes, reducing overall
energy consumption. In one embodiment, the use of ceramic membranes
may avoid the cost of heat exchangers employed for heat integration
and reduces associated energy losses in heat integration. With the
use of ceramic or high-temperature polymeric membranes in the UF
unit, generating a relatively high temperature water with sensible
heat that may be gainfully utilized downstream of the UF unit.
[0065] The filtration unit can remove at least 70% of the TOC as
free oil in one embodiment; at least 90% of the TOC as free oil in
a second embodiment; and at least 95% in a third embodiment, for an
final free oil level of less than 10 ppm in one embodiment and less
than 100 ppm in a second embodiment. In some embodiments, treated
water from the ultra-filtration unit may be fed into other units in
the water treatment process, e.g., a forward osmosis unit or a
membrane distillation unit. In yet another embodiment, the outflow
from the filtration unit is first treated in a gas floating unit,
with the addition of an agent to help float the oil/particles to
the top of the tank for removal prior to optional treatment
downstream.
[0066] Forward Osmosis Unit: Osmosis is the molecular diffusion of
solvent across a semi-permeable membrane, which rejects the solute.
Osmosis is driven by a chemical potential gradient. This gradient
is caused by differences in component concentration, pressure
and/or temperature across the membrane. In the non-ideal case, the
use of solvent activity in lieu of the concentration accounts for
the solvent-solute interactions. At a constant temperature, the
chemical potential may be defined by:
.mu..sub.i=.mu..sub.i.sup.o+RT ln a.sub.i+V.sub.iP, where is the
chemical potential of 1 mol of pure substance at a pressure P and
temperature T, a.sub.i is the activity of component i (1 for pure
substances), R is the gas constant and V.sub.i is the molar volume
.sub.of component i.
[0067] The driving force is defined as the osmotic pressure of the
concentrated solution. The membrane permeable species (solvent)
diffuses from the region of higher activity to a region of lower
activity. The osmotic pressure is the pressure that must be applied
to a concentrated solution to prevent the migration of solvent from
a dilute solution across a semi-permeable membrane. A common
application of this phenomenon is the desalination of seawater
using "reverse osmosis (RO)" using hydraulic pressure to overcome
the osmotic pressure, (also, known as hyperfiltration). It is used
to reverse the flow of the solvent (water) from a concentrated
solution (e.g. seawater) to obtain potable water.
[0068] Osmotic pressure can be calculated from the activity (the
product of the mole fraction (x) and activity coefficient
(.gamma.)) of the solvent in the two solutions. The relationship is
as follows:
.DELTA. .pi. = R T V i ln [ x 1 .gamma. 1 x 2 .gamma. 2 ]
##EQU00001##
[0069] wherein R is the gas constant, T is the temperature, V.sub.i
is the molar volume of the solvent (water), x1 and .gamma..sup.1,
x.sup.2 and .gamma..sup.2 refer to the water mole fraction and
activity coefficients in the higher activity and lower activity
solutions respectively.
[0070] In the absence of the hydraulic pressure for reverse
osmosis, the solvent flow will continue until the chemical
potential equalizes in both the feed and the draw solution. This
`natural` flow of solvent is called forward osmosis.
[0071] In one embodiment, forward osmosis (FO) is employed for the
removal of dissolved organic content in the water. In an example
illustrating the difference among FO, PRO ("pressure retarded
osmosis") and RO for the same solvent flows of a feed (dilute
solution) and brine (concentrated solution). For FO, .DELTA.P is
approximately zero and water diffuses to the more saline side of
the membrane. For PRO, water diffuses to the more saline liquid
that is under positive pressure (.DELTA..pi.>.DELTA.P). For RO,
water diffuses to the less saline side due to hydraulic pressure
(.DELTA.P>.DELTA..pi.). For FO, .DELTA.P is zero; for RO,
.DELTA.P>.DELTA..pi. (osmotic pressure); and for PRO,
.DELTA..pi.>.DELTA.P. A general flux relationship for FO, PRO
and RO for water flux from higher activity to lower activity (i.e.
FO) is as follows: J.sub.w=A(.sigma..DELTA..pi.-.DELTA.P), wherein
A is the water permeability constant of the membrane, .sigma. the
reflection coefficient, and .DELTA.P is the applied pressure
difference. The reflection coefficient accounts for the imperfect
nature (solute rejection less than 100%) of the membrane. The
reflection coefficient is 1 for complete solute rejection.
[0072] By choosing an appropriate salt in the draw solution in the
FO unit, it is possible to pull water from a feed solution of
produced water. In the FO method, produced water is introduced into
the FO feed chamber, wherein it is separated into a retentate
stream containing contaminants in the feed chamber and a permeate
stream (depleted in contaminants such as dissolved organics, TDS,
free oil, etc.) in the FO draw chamber which is mixed with the draw
solution to form an outlet draw solution. In one embodiment, the
draw solution comprises polyvalent osmotic ions or monovalent
osmotic ions. In another embodiment, the draw solution comprises an
alkaline earth metal salt solution with a halide. Examples include
but are not limited to NaCl, Na.sub.2SO.sub.4, AlCl.sub.3,
MgSO.sub.4, NH.sub.4HCO.sub.3, MgCl.sub.2 and mixtures thereof.
[0073] The FO process has several potential benefits over RO
including but not limited to: less membrane fouling tendencies;
less membrane support and equipment used; less energy intensive
process via efficient heat integration, by treating the draw
solution at a high temperature (>50.degree. C.) to recover
desalinated water; and lessening the need for several unit
operations. FO treatment is efficient in removing particulate
matters and almost all dissolved constituents for greater than 90%
removal of TDS in one embodiment, and greater than 95% removal in a
second embodiment. Commercial forward osmosis units are available
from various vendors, such as Hydration Technology Innovations of
Albany, Oreg. and Oasys of Boston, Mass. Forward osmosis units may
employ various membranes. Generally speaking, forward osmosis units
are less prone to fouling than a conventional reverse osmosis
unit.
[0074] Membrane Distillation ("MD") Unit: In embodiments wherein
the reduction in the hardness and silica level are not necessary,
the produced water can be fed directly to the MD unit without the
pretreatment step (e.g., via EC unit, CP unit, or filtration unit).
In another embodiment, membrane distillation is employed as one of
the steps after the solid separation step. In another embodiment,
membrane distillation is employed as one of the steps after the
filtration step. The MD process is a thermally driven transport of
vapor, typically through a non-wetted porous hydrophobic membrane,
suitable for applications in which water is the major component
present in the feed solution.
[0075] In one embodiment, "direct contact" membrane distillation
(DCMD) is used to remove the total dissolved solids and salinity in
the water. In DCMD, both the warm vaporizing feed stream and the
cold condensate stream (treated produced water feed) are in direct
contact with the membrane distillation apparatus. The driving force
for membrane distillation is the partial pressure differential
between each side of the membrane pores. Both the feed and permeate
aqueous solutions may be circulated tangentially to the membrane
surfaces by means of circulating pumps. Alternatively, the solution
may be stirred inside the membrane cell by means of a magnetic
stirrer. The trans-membrane temperature difference induces a vapor
pressure differential. Volatile molecules evaporate at the hot
liquid-vapor interface, cross the membrane pores in vapor phase,
and condense in the cold liquid-vapor interface inside the membrane
module. The liquid feed water to be treated by DCMD is maintained
in direct contact with one side of the membrane without penetrating
the dry pores unless a trans-membrane pressure higher than the
membrane liquid entry pressure is applied. The hydrophobic nature
of the membrane usually prevents liquid solutions from entering
membrane pores due to surface tension forces. Liquid-vapor
interfaces are formed at the entrances of the membrane pores.
[0076] In one embodiment, the DCMD unit employs membrane of the
type as disclosed in U.S. Pat. No. 8,167,143 and PCT patent
publication WO 2012/097279, the disclosures of which are
incorporated herein by reference. The membrane system employs
hydrophobic hollow fiber membranes in a shell casing, with the
fiber comprising any of regenerated cellulose (RC), cellulose
acetate, and cellulose triacetate (CTA). In one embodiment the
membrane module is configured and dimensioned to permit cross flow
of the produced water (to be treated) relative to the hollow
fibers. The hollow fiber module includes a central feed distributor
tube, hollow fiber membranes positioned around the central feed
distributor tube, end caps with ports for the flow of sweep air,
and optionally a shell casing. The central feed distributor tube
includes small holes to allow the removed oil to flow out radially
on the shell side. Sweep air may be introduced into the bore of the
hollow fibers in the tube side to remove permeated water vapor.
Each membrane unit includes about 5,000 to 200,000 hollow fiber
membranes in one embodiment; from 10,000 to 100,000 fiber membranes
in a second embodiment. The membrane fibers have a length of 1 to
200 inches in one embodiment; from 5 to 100 inches in a second
embodiment. The membranes have a wall thickness ranging from 2 to
100 .mu.m in one embodiment; from 5 to 75 .mu.m in a second
embodiment; and from 10 to 50 .mu.m in a third embodiment. The
membranes have a surface area of about 100 cm.sup.2 to about 2.0
m.sup.2.
[0077] In yet another embodiment, the DCMD unit employs membranes
of the type as disclosed in US Patent Publication No.
US20110031100A1, the disclosure of which is incorporated herein by
reference. The membrane is of a composite hydrophilic/hydrophobic
type having a high vapor flux, comprising a hydrophilic polymer
layer and a hydrophobic polymer layer. In one embodiment, the
membrane has a vapor flux of at least about 50 kg/m.sup.2-hr. The
hydrophilic polymer layer comprises any of polysulfone, polyether
sulfone, polyetherimide polyvinylidenefluoride, cellulose acetate,
or combinations thereof. The hydrophobic polymer layer comprising a
fluorinated surface-modifying macromolecule (SMM). In one
embodiment, the SMM is poly(urethane propylene glycol) or poly(urea
dimethylsiloxane urethane).
[0078] In one embodiment, after passing through the DCMD unit, the
pretreated water contains as little as less than 5% of the original
silica concentration; less than 25% of the original hardness
causing ions such as calcium and magnesium; less than about 5 ppm
oil; and less than 50% of the original dissolved organic carbon
content.
[0079] The membrane distillation apparatus may be washed in
different ways using different washing agents known to one of
ordinary skill in the art. For example, a sodium hydroxide (NaOH)
aqueous solution and deionized water may be used sequentially to
wash the membrane when needed. In another example, dilute
hydrochloric acid can be used to wash the membrane when needed.
[0080] Membrane distillation may be very useful for desalination of
produced water. Membrane modules are modular and compact. The
impact of salinity on water flux is minimal, since the vapor
pressure decline for even a 10% brine solution is only 5% of pure
water vapor pressure. The produced water may be pumped from the
reservoir at temperatures of greater than about 50.degree. C., and
in some applications, at greater than about 70.degree. C. If
relatively cold water is run on the permeate side at about
25.degree. C., a temperature differential of about 45.degree. C.
may be used to create a driving force for generating considerable
water flux across the membrane. In one embodiment, the treated
water on one side of the membrane is at a temperature of at least
5.degree. C. less than the temperature of the pre-treated water. In
another embodiment, the treated water is at a temperature at least
65.degree. C. less than the temperature of the pre-treated
water.
[0081] The DCMD unit removes at least 50% of residual silica and
hardness in the water in one embodiment; at least 80% removal in a
second embodiment, and at least 90% removal of residual silica and
hardness in a third embodiment, for a steam-generation quality
water or boiler quality. The chemical oxygen demand (COD) level,
which is an indication of organic levels in the water, of the water
after treatment in the DCMD unit is less than 10% of the initial
level before treatment in the DCMD unit. The total COD level is
less than 25 mg/L in one embodiment, and less than 20 mg/L in a
second embodiment. Another indication of total organics removal
efficiency is TOC, which is expected to be less than 10 mg/L after
the DCMD treatment in one embodiment, and less than 5 mg/L in a
second embodiment.
[0082] Ion Exchange (IE) Unit: In some applications wherein the
primary water treatment objective is removing the hardness and with
a silica level of less than 100 ppm rendered acceptable, an
ion-exchange unit is employed to capture the hardness in the
produced water. In the IE unit, the hardness ions are "exchanged"
and bound onto the resin, thus effectively removed from the water
for least 95% of hardness removal in one embodiment; at least 98%
of hardness removal in a second embodiment; and at least 99%
hardness removal in a third embodiment.
[0083] In one embodiment, the feed to the IE unit can be
desalinated water feed stream from the MD unit (or the FO unit). In
another embodiment, the feed to the IE unit is a combination stream
with a first portion being untreated produced water feed and a
second portion being desalinated water feed stream from the MD unit
(or the FO unit) for a combined TDS of less than 10,000 ppm. In a
third embodiment, the feed to the IE unit is a combination stream
with a first portion being untreated produced water feed and a
second portion being desalinated water feed stream from the MD unit
(or the FO unit) for a combined silica level of less than 150 ppm.
In a fourth embodiment, the feed to the IE unit is a combination
stream with a first portion being untreated produced water feed and
a second portion being desalinated water feed stream from the MD
unit (or the FO unit) for a combined silica level of less than 100
ppm. In a fifth embodiment, the feed to the IE unit is a
combination stream with a first portion being untreated produced
water feed and a second portion being desalinated water feed stream
from the MD unit (or the FO unit) for a combined silica level of
less than 100 ppm and a combined TDS of less than 10,000 ppm.
[0084] In one embodiment for treating oil free water with TDS of
less than 5000 ppm, the IE unit comprises two beds of strong acid
IE resin in series with the first bed removing the bulk of the
hardness, and the second bed acting as a polisher to remove the
last traces of calcium and magnesium. In one embodiment, the IE
resin is a sulfonated copolymer of styrene and divinylbenzene,
which functions by exchanging sodium ions for calcium and magnesium
ions. The resin can be regenerated with sodium chloride brine.
[0085] In another embodiment for treating produced water with TDS
between 5000-8000 ppm, the IE unit employs two beds of IE resin in
series, with a strong acid followed by a weak acid. The strong acid
as the primary softener to remove majority of the hardness,
followed by the weak acid to ensure the final softness of the water
meets spec. In one embodiment, the weak acid resin is a carboxylic
acid group within an acrylic divinylbenzene matrix with a strong
selectivity for calcium and magnesium. The resin can be regenerated
by treatment with HCl to remove the calcium and magnesium, then
with caustic soda to convert the resin back to the sodium form.
[0086] In one embodiment with the produced water having TDS of
>8000 ppm, the system comprises two beds in series with a weak
acid followed by a weak acid bed to reduce the hardness to a level
meeting spec, e.g., to less than 1 ppm.
[0087] System Configurations & Applications: The produced water
for treatment in the system can be from different sources with
different compositions/properties with different treatment
requirements. The feed from different sources can be combined for a
particular suitable treatment. The feed stream can also be split as
feedstock to different treatment units in the system, e.g., in one
embodiment, some of the water is treated in one of the treatment
units such as the EC unit, and some remains untreated for
combination with the treated water stream from the EC unit as a new
feed stream for subsequent treatment such as in an IE unit for
hardness removal. In another embodiment wherein the final TDS is
not an important quality factor, produced water containing a high
level of hardness is treated in a warm lime process, with the
treated stream being combined with untreated produced water from
another source such that the combined TDS is less than 15,000 ppm.
The combined stream can be treated in an IE unit for hardness
removal to less than 1 ppm. In another embodiment, the combined
stream for subsequent hardness treatment has a total TDS of
<5,000 ppm.
[0088] The above described water treatment units (e.g., EC unit,
solid separation unit, filtration unit, DCMD unit, etc.) can be
configured into one system as an on-site installation, or as a
mobile unit for use on-site or off-site. Mobile herein means that
the water treatment system can be moved from one location to
another, e.g. distances of at least 0.1 miles between the
locations. In one embodiment, the units are installed on a
converted tanker that can move (sail) from one FPSO or off-shore
production unit to another for the treatment of produced water. In
another embodiment, each unit is designed and configured for easy
access on a trailer (or box trailer, or a cargo trailer) as a
modular unit that can be mobile (transportable), reused,
interchangeable for assembly according to various configurations,
forming a complete "water treatment plant" or deployed as
stand-alone units.
[0089] For remote onshore drilling or production environment,
offshore production facilities, or smaller sized production
facilities such as single service processing of parcels of less
than 300,000 barrels of fluids (produced water) per day, routine
service or the building of a tailored produced water treatment
system can be expensive and may not economically feasible. The
system employing the modular units can be scaled to the appropriate
size, transported from one facility to another, and at the
destination, constructed (interconnected) per a particular modular
design so as to be functional and particularly suitable for these
facilities.
[0090] The modular systems can be assembled with the individual
units being interconnected according to tailored configurations
suitable for the treatment of the produced water at the facility,
e.g., a modular system including EC unit, a modular system without
EC unit, a modular system using CP unit (instead of EC), a system
employing ceramic and polymeric membranes, multiple modular systems
running in parallel or multiple units within a system running in
parallel to handle larger treatment loads. In one embodiment, the
modular unit contains at least one of each: EC unit, solid
separation unit, high-temperature UF unit (employing ceramic or
high-temperature polymeric membrane), membrane distillation unit
(e.g., DCMD, VMD or vacuum membrane distillation, etc.), and
forward osmosis unit.
[0091] The modular system can be configured to run in serial
(sequential) mode in one embodiment, e.g., the individual units run
sequentially with output (effluent stream) from one unit being
passed on to another unit for further treatment, e.g., filtrate
from the UF unit being further treated in the DCMD, dilute draw
from a FO unit being further treated in a DMCD unit to produce
desalinated water and regenerate the draw for re-use in the FO
unit, etc. In another embodiment the system is configured to run in
parallel mode, e.g., filtrate from the UF unit is split for
processing in both the DCMD and FO units. In yet another
embodiment, the system is configured for some of the units to be
online (e.g., the DCMD unit running) and some units being off-line
not in use (e.g., FO unit not being used). The system can also be
configured to be running in both parallel and sequential modes,
e.g., multiple UF units being employed in series and both DCMD and
FO units being used for the removal of dissolved organics, etc.
[0092] Reference will be made to the figures that schematically
illustrate various embodiments of different configurations for the
treatment of produced water, particularly for steam generation.
[0093] FIG. 1 illustrates an embodiment of a process and a system
that employs electrocoagulation to treat water to produce steam at
significant energy savings. In the system, produced water feed 11
passes through screen 10 for the removal of large particles that
may damage downstream equipment. The screened feed stream 21 is
treated in EC unit 20, for the removal of at least 95% of the
hardness and 90% of the silica respectively. In one embodiment,
additional additives 22 such as flocculants, oxidizing agents and
the like can also be added to the EC unit 20. In another
embodiment, the pH of the screened feed stream 21 is adjusted to a
pre-select pH with the addition of at least a base 12 prior to
treatment in the EC unit. A clarifier 30 (or other applicable solid
removal units) is used for the removal of any flocs or precipitates
as wastestream 32. The clarified water 41 is sent to a
high-temperature filtration unit 42 for the removal of any free oil
42, generating a treated water stream meeting specs for hardness
and silica for use in steam generation.
[0094] FIG. 2 is an alternative configuration, wherein instead of
an EC unit, a chemical precipitation (CP) unit 25 is employed with
the addition of any of lime 12, caustic, and/or magnesium compounds
26, and wherein a ceramic UF unit 45 is used for high-temperature
filtration to remove at least 90% of the free oil in the treated
water stream 41.
[0095] FIG. 3 is yet another variation of the configuration in FIG.
1, wherein a high temperature polymer UF unit 47 is employed to
remove free oil in the treated water stream 41 prior to steam
generation.
[0096] FIG. 4 is a variation of the configuration in FIG. 3,
wherein the treated stream 43 is further processed in a DCMD unit
50 for the removal of dissolved organic constituents, generating a
high quality treated stream 51 for steam generation.
[0097] FIG. 5 is yet another variation of the configuration in FIG.
3, wherein the treated stream 43 is sent to a FO unit 55 (instead
of a DCMD unit) for the removal of dissolved organic constituents,
generating a high quality treated stream 51 for steam
generation.
[0098] FIG. 6 employs the same system configuration in FIG. 5 for
the treatment of produced water. However in this embodiment, an
untreated produced water stream 52 having a low concentration of
TDS can be combined with stream 43 and fed directly to the FO unit
for further treatment. In one embodiment, the ratio of untreated
water stream 52 to treated water stream 43 is controlled such that
the final treated water stream 51 remains within spec.
[0099] In the configuration of FIG. 7, the produced water has a
sufficiently low silica level to start and only hardness removal is
needed. After pretreatment with screening in unit 10 and ceramic UF
unit 45, the treated water 43 is desalinated in DCMD unit 50,
generating a treated stream 43. An untreated produced water stream
52 having a low concentration of TDS can be combined with treated
stream 431 for further treatment in IE unit 60 to remove hardness
ions to a concentration of less than 1 ppm.
[0100] As shown in some figures, pretreatment in the EC unit 20
and/or UF unit is desirable before employing a MD step to prevent
flux decline and scale build-up. The EC unit can improve the
efficiency of the membrane distillation flux by reducing the
scalants (silica and hardness) and foulants (free oil and dissolved
organics). In embodiments with minimum gain/loss of energy in steam
generation, the use of EC is particularly suitable. When operated
at reasonably high temperatures, the EC process desirably adds
energy to the produced water. The EC process raises the temperature
of the water, which increases the driving force in the subsequent
membrane distillation step. Thus, there is a synergistic
combination in providing a pretreatment step of EC, followed by an
MD process. The energy the EC process adds to the water assists in
"driving" the membrane distillation process.
[0101] The use of high-temperature ultra-filtration is also
desirable for embodiments with minimum gain/loss of energy in steam
generation. In one embodiment with the use of ceramic or
high-temperature polymer materials for membranes, the clarification
step can be eliminated with the solid removals being carried out in
the ultra-filtration step. In other embodiments, ultra-filtration
may be bypassed if the membrane distillation unit is tolerant to
residual suspended solids and residual dissolved organics after the
EC treatment.
[0102] It should be noted that separate water treatment units are
configured in a permutable fashion for the system to be operating
according to any of the configurations described above depending on
the properties of the produced water source, with some of treatment
units to be online, some on stand-by mode, parallel mode with water
treatment by both EC and FO units, series mode with ceramic UF
filtration prior to treatment by the DCMD, split feed treatment
with some of the produced feed by-passing one or more of the
treatment units, etc.
EXAMPLES
[0103] The invention is shown by example in the illustrated
embodiments. However, it is recognized that other embodiments of
the invention having a different configuration but achieving the
same or similar result are within the scope and spirit of the
claimed invention.
Example 1
Pretreatment by Electrocoagulation (EC)--Produced Water
[0104] Produced water from a oil producing field was collected and
analyzed for its water quality shown below:
TABLE-US-00002 mg/L Anions Bicarbonate, HCO.sub.3.sup.-1 1395
Carbonate, CO.sub.3.sup.-2 0.0 Chloride, Cl.sup.-1 3940 Hydroxide,
OH.sup.-1 0.0 Sulfate, SO.sub.4.sup.-2 177 Sulfide, S.sup.-2 0.0
Sulfite, SO.sub.3.sup.-2 0.0 Cations Ammonium, NH4.sup.+1 0.00
Barium, Ba.sup.+2 0.76 Boron, B.sup.+3 92.8 Calcium, Ca.sup.+2 50.9
Iron, Fe.sup.+3 0.00 Magnesium, Mg.sup.+2 16.4 Potassium, K.sup.+1
115 Sodium, Na.sup.+1 2540 Strontium, Sr.sup.+2 0.00 Silica, as
SiO2 293.0 Sodium, Na.sup.+1 (Calc.) 2541.0 Chloride, Cl.sup.-1
(Calc.) 4132.0
[0105] This water is high in silica and hardness. It was treated in
a EC reactor at an applied current of 10-15 A and a pH range of
7.5-9.5. The results are shown in Table 1, showing a substantial
reduction in silica, calcium, and magnesium.
TABLE-US-00003 TABLE 1 Untreated Produced Treated Produced Water %
Contaminant Water (ppm) (ppm) Reduction Silica (SiO2) 293 15 95
Calcium 50.9 4.6 91 Magnesium 25.9 2.1 87
Example 2
EC Pretreatment--Tar Sand Wastewater
[0106] EC process has been shown in sources to treat tar sand
wastewater as the feed. Results are shown in Table 2, showing a
large decrease in total suspended solids and total organic
carbon.
TABLE-US-00004 TABLE 2 Contaminant % Removal Total suspended solids
99 Total organic carbon 50-95
Example 3
EC Pretreatment--Oil Shale Wastewater
[0107] Example 1 was repeated but with oil shale wastewater as the
feed. The contaminant removal percentage of dissolved organic
carbon was about 17-36 percent removal.
Example 4
Effect of Temperature
[0108] The produced water sample from Example 1 was run through the
EC bench scale unit at a feed water temperature of 16.degree. C.
The temperature of the treated water increased due to energy input
from the EC process as shown in Table 3.
TABLE-US-00005 TABLE 3 Power Input Impact on Treated Water
Temperature Treated Water Increase in Run Current (A) Power (W)
Temperature .degree. C. temperature, .degree. C. 1 11 660 28 12 2 9
495 27 11 3 7 343 23 7 4 5 125 22 6 5 3 60 21 5 6 1 11 19 3
Example 5
Flux Performance in DCMD
[0109] From various literature sources, as shown in Table 4, high
water flux can be achieved. This table shows water flux as a
function of the TDS and driving force. It is shown to be high for
membrane distillation, and desirably can be applied with very high
TDS water where reverse osmosis may not be applicable. It also
shows that the flux is independent of the feed water TDS. The
results further indicate that the process can be applied for many
applications in the shale gas reservoir production industry such as
in desalinating flowback water which have a salt concentration of
at least 6 wt % or more.
TABLE-US-00006 TABLE 4 Water Source Driving Force (Delta C) Flux
(gfd) City Water (low TDS) 70 28.6 Brine (6 wt %) 65 11.5 Brine (10
wt %) 65 11.5 Brine (6 wt %) 45 6.2
Example 6
FO Performance
[0110] In another example, produced water from Example 1 was
treated by FO membrane for 24 hours. 1 L of 1.25 M NaCl was used as
draw solution to recover over 45% of produced water at an average
flux of 8.1 LMH. For two consecutive experiments, no flux drop was
observed, indicating low or no fouling. The results in Table 5 show
significant reduction in total hardness by FO treatment. With
respect to removal of dissolved organic carbon, the feed water
shows an initial concentration of 441 mg/L, the concentrated feed
water has a concentration of 780 mg/L, and the feed water shows a
final concentration of 2.35 mg/L. Thus forward osmosis membranes
can be seen to reduce hardness and scale causing constituents of
produced water. In some applications, extensive pretreatment may
not be required. FO osmosis can useful in applications for
desalinated water wherein boron reduction is desired.
TABLE-US-00007 TABLE 5 Concentration Feed water (initial mg/L) Draw
water (final mg/L) Ca 50.9 1.1 Mg 25.9 2.1 Silica 293 9.4 Boron
92.8 8.8
Example 7
[0111] In an example of a produced water flow rate of 150,000 BWPD
(barrels of water per day) for steam generation purpose, a
polymeric UF membrane is used for the removal of dissolved
organics. The produced water temperature is reduced from 75.degree.
C. to 45.degree. C. before being introduced into the UF unit,
wherein the heat content is transferred to another medium via heat
exchanger and transferred back to the water after the dissolved
organics are separated/removed. For a typical heat exchanger
efficiency of 85%, it is estimated that about 320,000 MM BTU is
lost, or an operational loss of about $1.6 million a year at a
natural gas price of $5/MM BTU.
Example 8
[0112] Example 7 is repeated but with the use of ceramic membranes
for the removal of organics from a produced water flow rate of
150,000 BWPD (barrels of water per day) for steam generation
purpose. Produced water at .about.75.degree. C. can be fed directly
into the UF unit for oil removal for subsequent steam generation
purposes, for a saving of at least $1.6 million a year as there is
no need for heat exchanger systems.
[0113] For the purposes of this specification and appended claims,
unless otherwise indicated, all numbers expressing quantities,
percentages or proportions, and other numerical values used in the
specification and claims, are to be understood as being modified in
all instances by the term "about." Accordingly, unless indicated to
the contrary, the numerical parameters set forth in the following
specification and attached claims are approximations that can vary
depending upon the desired properties sought to be obtained by the
present invention. It is noted that, as used in this specification
and the appended claims, the singular forms "a," "an," and "the,"
include plural references unless expressly and unequivocally
limited to one referent. As used herein, the term "include" and its
grammatical variants are intended to be non-limiting, such that
recitation of items in a list is not to the exclusion of other like
items that can be substituted or added to the listed items.
[0114] This written description uses examples to disclose the
invention, including the best mode, and also to enable any person
skilled in the art to make and use the invention. The patentable
scope is defined by the claims, and can include other examples that
occur to those skilled in the art. Such other examples are intended
to be within the scope of the claims if they have structural
elements that do not differ from the literal language of the
claims, or if they include equivalent structural elements with
insubstantial differences from the literal languages of the claims.
All citations referred herein are expressly incorporated herein by
reference.
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