Microsoft Word - Emas Report Water PTTEP October D3TECHNICAL
REPORT
EMAS FPSO Mercury and Arsenic Removal from Arthit Produced Water
Process Description Prepared for: Mr. Roel Jager Manager Topside
Arthit Project Emas Offshore 15 Hoe Chiang Road #16-01, tower 15
Singapore E-mail:
[email protected] Phone: +65 63498535
ext 589 H/P: +65 97889276 Prepared by: Mercury Technology Services
23014 Lutheran Church Rd. Tomball, TX 77375 USA Dr. S. Mark Wilhelm
[email protected] The information in this report is furnished under a
confidentiality agreement. Copies of this report cannot be
distributed beyond the signatory parties without written
authorization of Mercury Technology Services.
October 28, 2007
2 (© Mercury Technology Services 2007)
Table of Contents Page 1.0 INTRODUCTION 3 2.0 GENERAL PROCESS
DESCRIPTION 4 3.0 WATER TREATMENT PROCESS 6 4.0 SOLIDS VOLUMES 14
Appendix A – MSDS for Chemicals A1 Appendix B - Nalco Proposal B1
Appendix C - Chevron Patent C1 Appendix B - Chevron/Unocal Process
Description D1
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3 (© Mercury Technology Services 2007)
EMAS FPSO Mercury Removal from Produced Water Process Description
1.0 INTRODUCTION PTT Exploration and Production (PTTEP) will start
up production at the Arthit Project in the second quarter of 2008.
PTTEP plans to increment the production rate of Arthit (Figure 1)
by using temporary processing facilities to accelerate the gas
delivery. The objective is to achieve 120 - 150 MMSCFD for a
minimum of 3 years starting from the second quarter of 2008. Emas
Offshore (EMAS) will modify an existing tanker to become a Floating
Production, Storage and Offloading vessel (FPSO), which EMAS will
operate for PTTEP at the Arthit field. Produced fluids from North
Arthit will be transferred to an FPSO for separation, treatment and
export of gas. Relatively high levels of H2S and CO2 are expected
in the feed gas along with heavy metals (mercury and arsenic) in
all the well fluids. After processing, sales gas will be exported
to via a pipeline. The condensate will be temporarily stored in the
cargo tanks of the vessel and regularly offloaded into a shuttle
tanker. Produced water will be treated onboard for discharge to the
ocean. This document describes the water treatment system to be
used on the FPSO.
Figure 1 – Location of Arthit Field
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4 (© Mercury Technology Services 2007)
2.0 GENERAL PROCESS DESCRIPTION A simplified process flow diagram
for the EMAS FPSO is shown in Figure 2. Produced fluids are
received via pipeline. Gas, condensate and water are separated and
treated separately. Gas is treated to remove mercury, dehydrated,
compressed and sent to pipeline. Condensate is stabilized and sent
to tanks. Water is treated to remove hydrocarbon, mercury and
arsenic and discharged to the ocean.
• Inlet fluid pressure: 315 psig temp. max 50C
• Gas export pressure: 2015 psig, temp max 50 C
• Gas in: 200 MMscfd
• Gas feed: Hg: max 1,500 µg/Nm3
• Gas out: Hg: max. 50 µg/Nm3
• Condensate feed: 6,500 b/d; Hg: 4,000 - 8,600 ppb,
• Produced water feed:
o 8,700 b/d
• Produced water discharge:
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Figure 2 – Simplified Process Flow Diagram, EMAS FPSO
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3.0 WATER TREATMENT PROCESS Chemistry - The process selected to
treat water to remove mercury and arsenic is patterned after that
employed by Unocal, now Chevron, to treat water on gas production
platforms in the Gulf of Thailand. The process flow diagram (PFD)
is shown in Figure 3 and process and instrument diagrams (P&ID)
of key equipment are shown in Figures 4 -6. Chevron’s patent and
process description are provided in Appendices C and D,
respectively. The water treatment process (equipment) is
commercially supplied by Natco under license from Chevron. Nalco
will supply chemicals and technical support. The water treatment
process is the only documented commercial application in which
mercury and arsenic have been successfully removed from produced
water containing 40 ppm hydrocarbon. The water treatment process,
in a form similar to that to be used by EMAS and described herein,
is currently operated by Chevron on Gulf of Thailand gas production
platforms. The process is reported by Chevron to treat water for
ocean discharge having required concentration limits for mercury,
arsenic and hydrocarbon. The limit on mercury in water discharged
from the FPSO is set at 5 µg/L (ppb) total mercury. Total mercury
means that the measured amount includes dissolved and suspended
forms. Suspended forms of mercury are postulated to constitute the
majority of the measured total mercury concentration in Arthit
water. The limit on total arsenic in water discharged to the ocean
is 250 µg/L. Chemical analysis data for produced water indicate
elevated concentrations of mercury and arsenic in water. The
measured concentrations reflect mercury and arsenic in suspended
solids and in suspended hydrocarbon obtained from tests on water
produced during early production tests. The actual concentrations
of mercury and arsenic in produced water will not be known with
certainty until production is initiated but actual concentrations
are expected to be much lower than those obtained in the early
tests. The primary target for mercury and arsenic removal from
produced water includes the dissolved species, which are not
separately distinguished in the water analysis. In the case of
arsenic, it is postulated that both arsenate and arsenite species
are present. The two inorganic redox states of As in water are
As(III) and As(V). At normal pH, arsenite exists in solution as
H3AsO3
and H2AsO - 3 (pK1a = 9.2 and pK2a = 12.7). Arsenate is present as
H2AsO
- 4 and HAsO2-
4 (pK1a = 2.3, pK2a = 6.8, pK3a = 11.6). In the case of mercury,
most of the mercury is either suspended or in the elemental state
(Hg0). The water treatment process (Figure 3) consists of four
steps:
1. Sodium hypochlorite (NaOCl, common bleach or hyter) is added to
oxidize elemental mercury to ionic mercury and arsenite to
arsenate:
Hg0 + OCl- → Hg2+ + OH- + Cl-
H2AsO - 3 + OCl- → H2AsO
- 4 + Cl-
The oxidation reactions require some time to complete due to the
dilute concentrations and thus the chemical in injected ahead of
the degasser. The purpose of the degasser is to assist removal of
volatile hydrocarbons in the water stream but some volatile mercury
Hg0 may be removed as well. The water residence time in the
degasser allows the chemical reactions to complete.
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2. Ferric chloride (FeCl3) is then added ahead of a retention tank.
Ferric chloride generates positively charged ions in solution that
cause dispersed solids to coagulate. Ferric chloride is typically
sold in solution form (27-43% FeCl3 in water). The retention tank
allows time for the coagulation to take place. The coagulated
material is a complex mixture of suspended and ionic mercury,
arsenic and iron. A thiol complexing agent (thio-carbonate and/or
dithiocarbamate, Nalco proprietary chemical) is also added in this
step to form an insoluble metal complex with ionic mercury and
arsenic.
3. A polymer flocculating agent (copolymer of acrylate and
acrylamide) is added
downstream of the retention tank to provide a suspension of
insoluble (in water) material containing mercury and arsenic (and
hydrocarbon). The flocculating agent is an ionic polymer used to
form bridges between individual charged particles in the water
solution thus forming a suspension of solids in condensate. The
suspended material is the floated to the surface using injected air
and skimmed off in the induced air flotation unit.
4. The skimmed oil and associated solids combined with a clarifying
aid and sent to the
clarifier where the majority of water is separated from solids.
Water is recycled and condensate plus suspended solids are sent to
the condensate tanks.
5. Some portion of the floated solids is soluble in condensate and
thus dissolves in the
condensate. Some portion of solids is insoluble in condensate and
will accumulate on the bottom of the tanks.
Chemical Injection - MSDS sheets for chemicals are compiled in
Appendix A. A description of the Chemical Injection system, as
described by Nalco, is presented in Appendix B. Chemical injection
is accomplished using manually controlled pumps to achieve 200 ppm
each (hypochlorite, ferric chloride, thiol, polymer) in the water
stream at the point of injection. Hypochlorite reacts with both
hydrocarbon and metals. The injection rate of the hypochlorite
depends on hydrocarbon level and must be optimized to achieve the
mercury and arsenic discharge limits.
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Figure 3 - Water Treatment Process Diagram
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Figure 4 - Retention Tank
Figure 5 – Induced Gas Flotation Unit
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Figure 6 – Water Clarifier (Oil/Solids Recovery) Unit
CONFIDENTIAL DRAFT
IGF Design Philosophy
Design case Minutes ppm ppb ppb
Water from separator 3000 2,000 300
After Cyclone 500 500 300
After Degasser 5 200 500 300
After Retention Tank 2 200 500 300
IGF Tank (solids to clarifier) 10 high high high
Clarifier (to IGF) 10 <30 <5 <250
Discharged water <30 <5 <250
Water flow 8700 bpd
ppm ppm ppm ppm
200 (before)
200 (after)
Clarifier (water to IGF) 10 10 1
Discharge (from IGF) 10 10 1
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14 (© Mercury Technology Services 2007)
4.0 RECOVERED OIL and SOLIDS VOLUMES The most likely case, 2000 ppb
Hg, 300 ppb As, 8,700 bpd water, was considered to calculate
volumes of recovered condensate and associated suspended solids.
The maximum allowable concentrations allowed for discharge of
produced water overboard was 5 ppb Hg, 250 ppb As.
• Residual condensate will be the largest contributor to the volume
of recovered (recycled) material. Assuming an oil concentration
into the IGF water treatment system of 200 ppm and 30 ppm on the
discharge, approximately 250 kg/d (0.3 m3) of condensate will be
removed and consolidated with the solids.
• Next most significant inorganic contribution to the solids will
be the ferric chloride and thiol (Nalmet). Ferric chloride and
thiol will add 150 kg/d to the solids volumes, based on estimated
injection rates and assuming high recovery rates of solids with
oil.
• Mercury and arsenic in the solids contribute relatively minor
amounts. Approximately 2.8 kg/d of mercury and 0.3 kg/d of arsenic
are estimated to be recovered in the oil stream.
• Water in the solids matrix will be significant but is accounted
for in volume calculations (see below).
• The total solids volumes for will be approximately in the range
of 400 - 450 kg/d.
The actual volumes of material recovered from the system are
calculated as follows:
• It is assumed IGF will carry-over approx 3% of total produced
water throughput = 261
bbl/d (41 m3) into the clarifier.
• The skimmed oil/solids volume will greatly expanded by water tied
up in the solids/oil matrix. If the oil and solids are comprised of
75% water and 25% condensate and solids, the total skimmed material
volume will be 1.2 m3/d.
• Thus the ratio is assumed to be 41 m3/day of PW and 1.2 m3/day of
oil/solids material (i.e. oil and solids comprise 2.8% by volume of
the skimmed fluids).
• It is assumed that the clarifier will achieve 99% separation of
oil/solids and water. 40 m3/day of water will be returned to the
IGF
• The amount of oil plus solids recycled into condensate is
estimated to be 1 to 1.5 m3/day.
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Figure 5 – Oil/Solids Disposal
