TITLE OF THE INVENTION
A Contaminant Control System in an Evaporative Water Treating System
FIELD OF THE INVENTION
Recently, evaporators have been adopted as an alternate water treatment in the heavy oil industry
to treat produced water from a SAGD facility. This invention introduces means of controlling
contaminants that are introduced through and/or are specific to the SAGD process.
BACKGROUND OF THE INVENTION
Water is used in many industrial processes for a variety of applications such as steam production,
cooling, washing, diluting, scrubbing, etc. Increasing efforts have been made to conserve water
by maximizing the reuse of process water and hence reducing the amount of waste water being
discharged and fresh water make-up resulting in both economical and environmental benefits.
However, re-using process water has its own challenges since the process water generally is
contaminated in its initial use and requires additional treatment such as filtration, sedimentation,
flocculation, evaporation or chemical treatment before it can be reused. The purification of the
process water for reuse must in itself be efficient and economical, and its extent determined by
its intended use.
One such method is mechanical vapor compression (MVC) evaporation, where a compressor is
utilized to produce the pressure and temperature differential to drive the falling film exchanger.
A compressor provides the pressure and temperature differential to drive a falling film heat
exchanger to produce a high purity distilled water product and a concentrated brine product. The
schematic in Figure 14 depicts a typical MVC Evaporator System.
Evaporators have been used extensively in the mining and pulp and paper industries as means of
concentrating solids into a brine or recovering water from waste streams. In these applications,
the solid contaminants are generally soluble in water. However, the SAGD process can
introduce contaminants that are not normally present or in different concentrations as a result of
injecting steam into an underground reservoir that is recovered as hot water with the production
fluids. Oil and water soluble solids present in the reservoir may cause variances in produced
water quality at any given time, which can lead to operating problems in standard evaporator
designs.
In the SAGD industry, the produced water recovered from the SAGD production fluids and
make-up water added to account for losses must be treated to remove various contaminants to
meet the boiler feed water specification for the boilers. The contaminants include water
hardness, silica, minerals, and residual oil/bitumen. If the water hardness, silica, and minerals
are not removed from the water prior to steam generation, they will precipitate in the boiler
causing reduced heat transfer, lower capacities, higher boiler tube temperatures, and ultimately
failure of the boiler or extended boiler outages for cleaning and repairs. If the residual
oil/bitumen is not removed from the water prior to steam generation, there will be foaming and
fouling issues in the boiler drum and tubes, again leading to process upsets and shutdowns.
The majority of SAGD production facilities utilize hot or warm lime softening systems
combined with Weak Acid Cation (WAC) ion exchange systems to treat produced and make-up
water. However, this process does not produce a high quality boiler feed water and necessitates
the use of Once Through Steam Generators (OTSG) which only partially boil the feed water
(75-80%) to prevent scale deposition by maintaining solids in solution in the un-boiled water.
This leads to energy inefficiency and excessive water disposal rates. OTSGs are custom built for
the oil sands industry making them very costly compared to conventional boilers.
Recently some SAGD operators have adopted falling film evaporators that produce a high
quality distilled water for boiler feed water, which has made it possible to shift to more
conventional drum boilers. The combination of falling film evaporators and drum boilers results
much higher water recycle rates in a SAGD facility, which is becoming an increasingly critical
environmental consideration..
However, operating companies are finding that there are many shortcomings with the current
industry practice and evaporator system designs in SAGD facilities. Improvements to the
current state of falling film evaporator design for SAGD water treatment focused on the five
most problematic technical issues that have been observed in the field:
• Prevent accumulation of hydrocarbons in the evaporator sump
• Ensure silica, calcium and other water soluble contaminants are maintained in solution to
prevent scaling on the evaporator heat transfer tubes.
• Select materials of construction suitable to the service such as high levels of chlorides in
the evaporator sump due to the use of non-potable saline make-up water, pH in the sump
or the need to concentrate the brine to maximize water recycling.
• Minimize power consumption in a water treatment unit where all of the recovered water
is evaporated and re-condensed.
• Minimize the possibility of liquid carryover into the compressors of designs with
mechanical vapor compression.
Control of Hydrocarbon Accumulation
One unique shortcoming not addressed by the current designs is the tendency of residual oil
(including hydrocarbons, heavy oil and SAGD emulsifiers/reverse emulsifiers) to accumulate in
the evaporator sump. The typical designs withdraw a concentrated brine stream from the
evaporator sump at the outlet of the evaporator recirculation pumps. Owing to its lower density,
oil will tend to slowly build up on the surface of the water reservoir in the evaporator sump. To
control accumulation of contaminants in the evaporator sump, a controlled volume of water is
removed from the system at the discharge of the Brine Circulation Pumps. However, oil that
accumulates on the surface of the water in the sump cannot enter the brine recirculation pumps
since the pump suction line is drawn from the bottom of the evaporator sump. The accumulation
of oil on the surface of the sump will lead to 'foaming' events in the sump, fouling of heat
exchange surfaces and the need to shutdown the sump to withdraw accumulated oil. The need to
shutdown the evaporator to deal with foaming events reduces the overall reliability of the SAGD
plant and reduces the production volumes. One objective of the invention is to remove the oil
that accumulates on the surface of the sump on a continuous basis to prevent the foaming effect.
Control of Water Soluble Contaminants
The operation of the evaporator is a time and labor consuming operation which have to be highly
controlled both before, after, and during operation. A typical control scheme for an evaporator
consists of the following:
• The blow-down flow set-point is changed by an operator in response to a lab analysis of
the concentration of solutes in the evaporator sump, so the concentration of solutes
(silica, chloride, etc.) is controlled manually.
• The evaporator feed rate is adjusted automatically by a sump level controller in response
to changes in sump level.
• The compressor speed and/or guide vane position is adjusted in response to the level of
water in the distillate tank.
• The production rate of distillate from the evaporator is changed slowly in response to the
level of the downstream tank, and in extremes, in response to the level of the feed tank.
• Startup and shutdown of the evaporator is done manually, with significant time pressures
on the operator; the mode changes, and especially startups and the response time
immediately after a trip/malfunction are the most dangerous times in a process plant.
• Operators make manual adjustments to rates to manage the inventories in the upstream
produced water tank that feeds the evaporator and the downstream boiler feed-water tank
that holds the evaporator product distillate.
An objective of this invention is to provide a process control scheme that provides system
control across a broad range of operating conditions with minimal need for operator intervention.
The typical control scheme requires significant operator intervention, both during changes in
operating mode (startup, shutdown, etc.) and periodically during operation, with manual
adjustments to both feed flow and blow-down flow. Improved control will reduce the staffing
requirements without affecting risk or operating costs, and in fact can simultaneously reduce risk
and staffing costs and increase operating efficiency.
Automatic control of sump composition allows the evaporator to maximize efficiency of water
use or power, depending on which is the most effective constraint.
Yet another objective of the invention is to remove the oil that accumulates on the surface of the
sump on a continuous basis to prevent the foaming effect.
Another objective of this invention is the coordinated control of the different processing units
which will eliminate the need for online surge tanks, yielding a reduction in capital and operating
costs compared to other processes.
Another objective of this invention is to reduce risk of damage, injury, production loss or
environmental incident by reducing the operator workload at the most critical time.
Further and other objects of the invention will become apparent to one skilled in the art when
considering the following summary of the invention and the more detailed description of the
preferred embodiments illustrated herein.
SUMMARY OF THE INVENTION
The control schemes and equipment designs included in this invention can be readily adapted to
a variety of MVC Evaporator designs available on the market. The invention are specifically
designed to improve performance of this technology in a SAGD process facility.
The solution for foaming is to remove the oil that accumulates on the surface of the sump on a
continuous basis. To achieve this, the brine removal point is moved from the outlet of the brine
recirculation pumps to a blowdown sump incorporated into the evaporator sump. A vertical
partition plate, or weir, is added to the bottom sump section of the evaporator separating it into
an evaporator or main sump and a smaller blowdown sump. The key feature of the invention is
the ability to continuously remove oil from the evaporator via the blowdown sump and thus
prevent foaming events.
According to a primary aspect of the invention, there is provided a water purification process in a
steam assisted gravity drainage system for a heavy oil recovery facility, the process comprising
an evaporator and a set of controllers. The evaporator having a bottom with a sump provided at
the bottom thereof and including an oil skimming weir dividing the sump into a main sump and a
blowdown sump, wherein the water containing impurities flows over the weir from the main
sump to the blowdown sump. In a normal operation mode, the evaporator receives water from
the process and discharges distilled water; while discharging waste brine from the blowdown
sump.
According to yet another aspect of the invention the weir separating the main sump and the
blowdown sump shall have a v-notch at the top of the weir to allow variations in the main sump
level to vary within the height of the v-notch with varying but continuous flow of brine to the
blowdown sump. The location of the overflow weir ensuring that any hydrocarbon floating on
top of the water in the main sump will be continuously removed from the evaporator with the
waste brine and elimination of the hydrocarbon related foaming problem and unit shutdowns
associated with the foaming problem.
The set of controllers includes:
• a distilled water flow meter provided at the discharge of evaporator;
• a blowdown flow meter measuring the flow from the blowdown pump discharge;
• a cycle calculator for calculating the ratio between the distilled water flow and
blowdown flow and a total flow controller.
In this case, the cycle calculator provides a set point to the total flow controller, thus the flow of
the water into the evaporator does not directly depend on the level of the liquid in the main
sump. Further the operation of the evaporator functions in a contained closed loop environment.
According to another aspect of the invention the process includes a distillate tank, receiving
distilled water, the distillate tank having a level controller for controlling distilled water
production. This distillate tank is the only major surge capacity in the process.
According to still another aspect of the invention the set of controllers further comprises a
hardness controller, a silica controller and a pH controller. The hardness controller manipulates
the set point of the cycle controller affecting the flow of make-up water into the process to
maintain a target hardness concentration in the evaporator sump and is adjusted to account for
variations in hardness concentration in the evaporator feed. The pH controller adjusts the flow
of caustic to the evaporator feed to achieve the target pH specification. The target pH
specification is calculated based on the silica concentration in the controller, which determines
the set point for the pH controller and is adjusted to account for the concentration in the sump
due to removal of distillate. The objective is to keep the sump pH at a level that provides a safe
margin from the point where silica will precipitate from solution. The Figure 13 illustrates the
dependence of silica solubility on pH.
Preferably the evaporator receives the upstream water from a de-oiling or Inert Gas Floatation
(IGF) unit, wherein the flow of make-up water is set providing the desired flow into and out of
the IGF unit.
According to yet another aspect of the invention the evaporator has two additional modes of
operation:
1) an idle mode; and
2) a recycle mode.
During the idle mode there is no flow in or out of the evaporator, and during the recycle mode
the distilled water is recycled to the inlet of the evaporator with these modes being initiated when
starting up the evaporator, when shutting down the evaporator, when there is a loss of flow, and
in response to a malfunction in the system such as a compressor malfunction, a pump
malfunction or a boiler malfunction. The evaporator can be operated in said modes automatically
or be initiated by an operator.
Preferably the recycle mode is used during the shut down procedure and after a malfunction such
as a boiler malfunction, and the idle mode is used after a compressor malfunction.
According to still another aspect of the invention the start up of the system includes the
following steps; when shut down the system is switched to idle mode, then to recycle mode and
finally to normal operation mode. This way the careful selection of modes is reducing operating
risks to the system and to the personnel and providing smooth operation compared to single
mode evaporators.
According to still another aspect of the invention the system presented above can be installed in
a modular portable/mobile SAGD system for heavy oil recovery.
Direct manipulation of the water balance by the cycles controller is unique. It provides very fast
integral control of the water purification area of the plant. Conventional mass-balance control
would result in more level fluctuations and would reduce the overall response time of the
process.
Recycle mode is new and unique. It reduces operating risk during startups, shutdowns and after a
trip. After a significant event elsewhere in the plant, such as a boiler trip, the evaporator can be
placed in recycle mode. Following a compressor trip the evaporator can be placed into hot idle
mode. Similarly, a startup would take the evaporator from shutdown to hot idle, then to recycle,
then to normal operation. These intermediate modes reduce the time pressures on the operator
and allow for smoother startups.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic flow diagram of the water in the water treatment unit.
Figure 1A is a schematic view of the evaporator.
Figure IB is a schematic view of a weir with v-notch and without v-notch.
Figure 2 is a scheme for the evaporator sump level control.
Figure 3 is a scheme for hardness control.
Figure 4 is a scheme for pH control.
Figure 5 is a scheme for distillate tank level control.
Figure 6 is a scheme for boiler feed water tank level control.
Figures 7-10 are logic diagrams for boiler feed water control.
Figure 11 is a logic diagram for blowdown control.
Figure 12 is a logic diagram for evaporator sump silica control.
Figure 13 is a graph illustrating the dependence of silica solubility on pH.
Figure 14 is a schematic view of a typical MVC Evaporator System as found in a prior art.
DETAILED DESCRIPTION OF THE DRAWINGS
The solution to the oil foaming in the sump is to remove the oil that accumulates on the surface
of the sump on a continuous basis. To achieve this, the brine removal point is moved from the
outlet of the brine recirculation pumps to a blowdown sump incorporated into the evaporator
sump. A vertical partition plate, or weir, is added to the bottom surge section of the evaporator
separating it into an evaporator or main sump and a smaller blowdown sump.
Preferably the weir also has a v-notch cut-out see Figure IB. Intention is to have flow of brine
and oil skim continually overflow from main sump to blowdown sump. The v-notch cut-out on
the weir provides smoother control of the blowdown sump level as the overflow rate will vary as
level moves up and down the V-notches, the level in the main sump can vary slightly while
maintaining flow to the blowdown sump.
The continuous removal of oil will prevent 'foaming' events in the evaporator. When oil
accumulates on the surface of the sump, it acts like a surfactant, and can lead to hydrocarbon
fouling of the heat exchanger tubes or foaming on the water surface. Foaming increases the
potential for carry over of liquid droplets into the evaporator vapour compressor suction and this
could trip or damage the vapour compressor. When foaming occurs, operators will add extra
antifoam chemicals. However, this is at best a temporary fix and eventually the evaporator must
be shutdown in order to remove the accumulated oil and/or the evaporator cleaned. A regular
need to shutdown the evaporator reduces the reliability of the SAGD facility and reduces
production volumes. By implementing a continuous overflow of oil and brine to the blowdown
sump, the current design will prevent a build up of oil in the main sump. The continuous
overflow will deal with oil that accumulates slowly over time resulting from the small amount of
residual oil in the evaporator feed. It will also deal immediately with larger amounts of oil in the
evaporator feed that may be present when there are upsets in the upstream produced water deoiling
system.
As noted above the current design adds a vertical partition plate or weir as illustrated in Figure
1A, to the bottom surge section of the evaporator separating it into an evaporator or main sump
and a smaller blowdown sump. Oil that accumulates on the surface of the main sump
continuously overflows into the blowdown sump and is removed from the system as part of the
blowdown stream. The main sump level is not controlled at a fixed point and "floats" slightly
within the range of the V-notch height based on the evaporator inlet and brine outlet flows. The
blowdown sump level is controlled very tightly so as to provide direct indication of flow from
the main evaporator sump into the blowdown sump.
Evaporator controllers:
The control of the evaporation rate is relatively conventional. The compressor duty is
manipulated by the distillate tank level controller. The distillate tank level is the major surge
capacity in the process, and so must be managed carefully to provide maximum damping
between the evaporator and boiler(s). The level controller will not be a conventional PID
algorithm, but will be a nonlinear controller with rate limiting. Feed control is unconventional.
The evaporator sump level does not directly manipulate feed rate. In fact, the sump is not
controlled directly or even necessarily measured. Instead, the evaporator sump level is the result
of the control scheme described below. The blowdown sump level is controlled very tightly. This
provides direct indication of flow from the main evaporator sump into the blowdown sump.
The flow of distillate to the distillate tank is also measured. These two flows allow for direct
calculation of evaporator cycles, which is the ratio of the distillate flow to the blowdown flow.
The sump level can be inferred indirectly from the blowdown flow, since the flow into the
blowdown sump from the main sump is a function of the main sump level.
The calculation of evaporator cycles is used by the soft sensors for sump pH, sump silica and
sump hardness. The soft sensors are corrected by lab results when available.
The sump hardness calculation is used by the sump hardness controller to manipulate the setpoint
of the cycles controller. Fewer cycles will result in a lower concentration of hardness in the
sump, while more cycles will raise the level of hardness.
The sump silica concentration will be used to determine the set-point of the sump pH controller.
The sump pH controller will in turn determine the inlet pH controller set-point, which will adjust
the flow of caustic to the evaporator sump.
The cycles controller will directly manipulate the plant water balance by setting the set-point for
the total flow into the water purification section of the plant. The total flow controller will adjust
the makeup water flow to the upstream IGF directly under normal conditions, and only under
upset conditions will direct water to or from the produced water tank. This configuration
eliminates the need for online surge capacity.
The level in the IGF will be controlled very tightly by adjusting the flow into the evaporator.
This will effectively pass through the adjustments made by the total flow controller immediately
after they are made.
Mode change
Automated procedures will be defined for starting up, shutting down, responding to compressor
trip, brine pump trip, boiler/HRSG trip and loss of plant feed. While not all steps in the
procedure can be automated, many of them will be, and others will be automatically verified by
instrumentation to ensure that the operator has performed the appropriate manual steps.
In particular, the evaporator has two operating modes that are between shutdown and normal
operation that can be sustained indefinitely. These two modes provide safe parking spots for the
evaporator if the operator needs to troubleshoot other aspects of the plant. They are hot idle and
recycle mode.
Hot idle mode
In this mode, flows in and out of the evaporator are shut, the brine is recycled, the compressor is
off, and low pressure steam is added to keep the evaporator hot. Most controls are similarly in
standby, largely in automatic, but with flows of zero.
Recycle mode
In this mode, the evaporator itself operates normally. Feed and blowdown flows are normal, the
compressor is running, and distillate is produced. The distillate is recycled from the output of the
boiler feed pumps back to the produced water tank. Evaporator feed is drawn from the produced
water tank.
The primary benefits of this control scheme are:
a) Reduced operating costs resulting from cooling and re-heating of water flowing through
surge tanks;
b) In addition, different aspects of the control scheme have additional benefits;
c) Hot idle and recycle modes of operation: risk reduction, since there is a reduced operator
workload and stress during periods of plant/process upset;
d) Automated procedures: risk reduction, for the same reason;
e) Closed loop control of concentration of solutes in sump: reliability, on-stream time and
operating cost: the plant can run closer to limit consistently without deposition of solids
on exchanger; and
f) Direct manipulation of plant water balance: operability, since the plant will not be waterlong
or water-short during normal operation. Only during upsets or abnormal operating
modes will there be a need for surge capacity. The removal of online surge capacity is
unique to this process, and the direct manipulation of the water balance is required for the
process to operate at high rates. Without it, there will be continuous fluctuations in the
levels and flows in the plant.
Control schemes (Figures 2, 3, 4)
This part of the plant receives de-oiled water from the Produced Water De-Oiling section of the
plant and fresh make-up water from water wells. The purpose of the Produced Water De-oiling
section is to:
a) Add the required amount of makeup water to the process;
b) Reduce the concentration of hydrocarbons in the produced water from 500 ppm to 30
ppm;
c) Provide the required flow rate of feed to the evaporator;
d) Treat produced water (reduce silica, hardness and hydrocarbon concentrations); and
e) Produce high-purity boiler feed water.
The main equipment:
a) Produced Water Flash Drum, 13-V-03 (preferably CS)
b) Induced Gas Flotation (IGF) Package, 13-X-01 (preferably Wetted Parts: Duplex SS
Vessel: CS
c) Produced Water Tank, 13-T-01 (preferably CS)
d) IGF Discharge Pumps, 13-P-02 A B (Casing: CS Wetted Parts: Duplex SS)
e) PW Tank Transfer Pumps, 13-P-04 (Casing: CS Wetted Parts: Duplex SS)
f) Evaporator Package, 14-X-01, which includes:
g) Inline Feed Mixer, 14-MX-01
h) Evaporator, 14-V-01 preferably made of Duplex SS or AL6XN
i) Brine Circulation Pump, 14-P-01A/B preferably made of Duplex SS or AL6XN
j ) Evaporator Vapor Compressor, 14-C-Ol
k) Distillate Tank 14-V-02
1) Water Treatment Chemicals Package, 14-X-02 (caustic, antifoam, and scale inhibitor)
m) Distillate Product Pumps, 14-P-02 A/B
n) Brine Pumps, 14-P-03 A/B preferably made of Duplex SS or AL6XN
o) Evaporator Blowdown Cooler, 14-E-01
p) Utility Water Cooler, 14-E-02
The intent of this section is to:
a) Provide a high level process overview of components of the Produced Water De-oiling
and Water Treatment systems.
b) Explain how make-up water is added to the system to maintain a constant flow of water
to the Induced Gas Flotation unit and Evaporator.
c) Explain how feed water and blowdown rates are determined, to maintain a desired
concentration of hardness in the Evaporator (which will prevent fouling of the
equipment).
d) Explain how pH will be controlled in the Evaporator to keep silica in solution.
The overall objective of this part of the plant is to maintain the plant water balance and maximize
the concentration of impurities in the evaporator water which will minimize the evaporator
blowdown flow and water make-up flows, but keep the concentrations below the point at which
salts will begin to precipitate out in the evaporator and cooler and cause fouling problems.
There are four main controllers discussed in detail in this narrative:
a) Blowdown Sump Level Control (Figure 2) - The objective of this control will be to
maintain the level in the sump by manipulating the flow of brine leaving the sump;
b) Hardness Control (Figure 3) - The objective of this control will be to maintain the brine
concentration at the correct set-point - a point low enough to prevent the precipitation of
hardness on the Evaporator internals, and yet high enough to optimize performance of the
Evaporator. The desired value for the set-point concentration will be determined during
detailed engineering and initial operation of the process;
c) Makeup water control - The objective of this control will be to maintain a steady flow of
water to the IGF and maintain the plant water balance. During abnormal operating
conditions, this controller will draw from or push water into the Produced Water Tank
13-T-01;
d) pH Control (Figure 4) - The objective of this control will be to maintain the Evaporator
brine pH at a point high enough to prevent the precipitation of silica on the Evaporator
internals.
There are other basic control loops in this section of the plant that will not be discussed.
Produced Water Flash Drum
Produced water from the Desand/FWKO/Treater Vessel enters the Produced Water Deoiling
System. This stream mixes with the make-up water from the Make-up Water Pumps (42-P-
01/02) and the produced gas liquid from the Dump Condenser (15-E-02). This mixed stream then
enters the Produced Water Flash Drum (13-V-03), along with the boiler blowdown from the
Package Boiler (15-B-01) and the blowdown from the HRSG Steam Drum (40-V-01). These
streams are flashed at a constant pressure of -5 kPag to cool the stream and ensure no flashing
occurs in downstream equipment.
Induced Gas Floatation
The liquid stream then enters the IGF Package. The purpose of the IGF is to reduce the oil
concentration in the produced water from 500 ppm (coming from the Inlet Cooling and
Separation system) to 30 ppm (needed for the Water Treatment System). Inlet oil concentrations
are expected to fluctuate within the range of 500 - 1,150 ppm. An inlet oil concentration of 500
ppm will signify normal operation where an oil concentration of 1,150 ppm will be the design
upset case. The IGF works by using micro-bubbles of natural gas to float oil to the top of the
chambers in the IGF Vessel 13-V-01, where it is skimmed off and sent to the Skimmed Oil
Drum 13-V-02. The skimmed oil is recycled back to the inlet of the Desand/FWKO/Treater
Vessel using the Oil/Water Pumps 13-P-03 A/B. De-oiling is done in several consecutive
chambers inside the IGF.
If additional sub-cooling is required, there is the ability to add make-up water to the IGF inlet. If
make-up water is added to this stream, the equivalent amount of make-up water will be reduced
to the feed to the Produced Water Flash Drum to keep the mass balance constant.
Evaporator Feed
The clean, de-oiled water leaving the IGF splits into two streams: the main flow and a 35%
recycle stream. The recycle stream is used to provide the source of bubbles for the IGF and is
recycled back to the IGF inlet and chambers.
The de-oiled produced water is sent to the IGF Discharge Pumps 13-P-02 A/B. The de-oiled
produced water then flows to the Evaporator package 14- X-01. The Produced Water Tank
(PWT) 13-T-01 is used for surge capacity only during abnormal operating conditions. The signal
to transfer water into or out of the PWT will come from the water balance controller. Controls in
the PWT will reduce production rates if the level in the tank gets too high.
Evaporator
The evaporator package processes a feed water stream containing dissolved solids and produces
distillate of sufficient quality to be used in the package boiler. The unit also produces a
concentrated wastewater stream. Produced water is supplied at the package boundary by the IGF
Discharge pumps 13-P-02 A B. Various chemicals are added to the evaporator feed water.
Caustic ( aOH) is added to control the pH to ensure silica species remain in solution. Caustic is
fed from the Caustic Storage Tank 14-T-01 using the Caustic Pumps 14-P-04A/B/C. Antifoam is
added to reduce foaming in the Evaporator and reduce liquid carryover to the vapour
compressor. Antifoam is fed from the Antifoam Tank 14-T-02 using the Antifoam Pumps 14-P-
05A/B. Scale inhibitor can be added to minimize scale formation in the Evaporator exchanger.
Scale inhibitor is stored in the Scale Inhibitor Tank 14-T-03 and fed using the Scale Inhibitor
Pumps 14-P-06A/B. The Antifoam and Scale Inhibitor Tanks are expected to be removable totes.
An Inline Feed Mixer 14-MX-Ol is used to ensure the chemicals are adequately mixed into the
feed water.
Feed water is then sent to the Evaporator sump. The Brine Circulation Pumps 14-P-01A/B
circulate brine from the Evaporator sump to the top of the Evaporator exchanger. Brine flows
through the tube side of the Evaporator exchanger as a falling film where a small mass fraction is
evaporated. The remaining liquid drains back into the sump. Vapours that disengage from the
falling brine are directed to the Evaporator Vapour Compressor 14-C-Ol. The Evaporator Vapour
Compressor increases the temperature and pressure of the vapour, and will require a Variable
Frequency Drive (VFD) motor. The compressed vapour condenses on the shell side of the
Evaporator exchanger while vapour is generated from the falling film on the tube side. Distillate
from the shell side of the Evaporator exchanger is drained by gravity to the Distillate Tank 14-V-
02, distillate will also be used for wash water for mist eliminator. A slip stream of distillate will
be cooled to ~48°C in the Utility Water Cooler 14-E-02 and sent to the utility water header. The
temperature will be controlled by a temperature loop that controls the amount of cooling glycol
flowing through the exchanger.
Concentrated brine will overflow a weir to the blowdown sump. The blowdown material is
pumped by the Brine Pumps 14-P-03 A/B through the Evaporator Blowdown Cooler 14-E-01
and transferred to the Disposal Water Tank 43-T-01. A temperature controller will maintain the
temperature of the disposal water by regulating the glycol flow through the cooler. The process
is designed such that the Evaporator has one hot idle mode and one recycle mode. The hot idle
mode involves re-circulating the brine through the Evaporator while the Evaporator Vapor
Compressor is not operating. Utility steam is used to offset any heat losses in order to keep the
Evaporator warm. Water from the condensed utility steam will go to the Distillate Tank. The
recycle mode involves producing distillate with the Evaporator Vapor Compressor in operation
and routing all of the distillate back to the Evaporator sump. Distillate will be routed from the
outlet of the HP BFW Pumps 15-P-01 A/B to the Produced Water Tank and back to the sump
through the Evaporator feed line. A vent is required from the Produced Water tank and/or the
Evaporator Compressor discharge to prevent an increase in Evaporator temperature and pressure.
Saline makeup water will be added to compensate for the water lost to the vent stream.
The following sections focus on the controls associated with the Evaporator:
a) Blowdown Sump Level Control (Figure 11)
b) Hardness Control
c) Makeup Water Control
d) pH Control
Evaporator feed control is unconventional. The evaporator sump level does not directly
manipulate feed rate. In fact, the sump is not controlled directly or even necessarily measured.
Instead, the evaporator sump level is the result of the following control scheme.
Blowdown Sump Level Control (Figure 2)
In the Evaporator sump, liquid will overflow a weir from the circulating section to the blowdown
section of the Evaporator. The liquid height to flow ratio over a straight edge weir is very narrow
and difficult to control. Hence, a "V-notch" weir will be used to help with this control. Level will
be maintained in the blowdown sump by level controller 14-LC-001(or XXX). The output from
the controller will manipulate the level control valve on the blowdown line going to the Waste
Disposal Tank 43-T-01. The blowdown sump level is controlled very tightly. This provides an
indication of flow from the main evaporator sump into the blowdown sump, and therefore a
directional indication of the main evaporator sump level.
Start-ups, Shutdowns and Trips
Start-up of this control should be with the level controller in automatic mode. The Brine Pumps
should not be started until a level is established in the Evaporator sump. The level controller can
then stay in automatic mode for most situations.
Hardness Control (Figure 3)
The flow of distillate to the distillate tank is measured, as well as the blowdown flow, to permit
direct calculation of evaporator cycles. The calculation of evaporator cycles is used in turn to
infer sump hardness along with the feed conductivity, 14-AI-xxx. The inferred value is corrected
by lab results when available. The sump hardness calculation 14-AY-xxx is used as the process
value of the sump hardness controller 14-AC-xxx to manipulate the set-point of the cycles
controller, 14-FFC-023. Fewer cycles will result in a lower concentration of hardness in the
sump, while more cycles will raise the level of hardness. The cycles controller, 14-FFC-023, will
manipulate the set- point of the total flow controller, 13-FC-037A.
Start-ups, Shutdowns and Trips
There are a number of inputs to the blowdown ratio control scheme. Prior to putting the system
into service Operations will have to ensure that all associated instrumentation is functioning
properly. Default values will have to be programmed into the DCS to deal with instruments that
are not indicating properly. Provided all are functioning properly and the Evaporator sump is
filled, this controller can be put into automatic mode. On a shutdown or trip it is recommended
that this control scheme be taken out of automatic mode.
Makeup Water Control
The cycles controller will directly manipulate the plant water balance by setting the set-point for
the total flow into the water purification section of the plant, 13-FC-037A. The total flow
controller will adjust the makeup water flow directly under normal conditions, and only under
upset conditions will direct water to or from the produced water tank. The makeup water flow
set-point will be equal to the total flow controller set-point, less the produced water and boiler
blow-downs. The total flow controller thus controls the total flow into the water purification
section of the plant. The first water purification unit is the Induced Gas Flotation unit, or IGF.
The level in the IGF will be controlled very tightly by adjusting the flow into the evaporator.
This will effectively pass through the adjustments made by the total flow controller immediately
after they are made. There is no meaningful surge capacity in the IGF.
The Produced Water Tank (PWT) will only be used under two exceptional cases:
a) There is more produced water entering the process than is required as evaporator feed to
be purified for boiler feed water, due to a boiler or evaporator trip or other short-term
upset. In this case water will be sent to the produced water tank.
b) The water from the FW O is off spec and not enough water can be drawn from the
wells. In this case water will be drawn from the produced water tank.
If the level in the PWT gets too high, level controller 13-LC-018 will begin to reduce production
from the pads by increasing the Inlet Separator (10-V-Ol) pressure. In the event of a low level,
an alarm will notify the operator to respond accordingly by reducing boiler firing rate before the
PWTank Transfer Pump 13-P-04 trips.
pH Control (Figure 4)
Evaporator feed pH will be a critical parameter to prevent fouling of the Evaporator internals
with silica. Silica will stay in solution provided the pH in the vessel is maintained high enough.
The sump silica concentration will be used to determine the set-point of the sump pH controller.
Both sump silica concentration and sump pH will be calculated in a manner similar to the
calculation of sump hardness. The sump pH controller, 14-AC-xxx, will in turn determine the
set-point of the inlet pH controller, 14-AC-002, which will adjust the flow of caustic to the
evaporator sump.
The target pH specification is calculated based on the silica concentration in the controller,
which determines the set point for the pH controller and is adjusted to account for the
concentration in the sump due to removal of distillate. The objective is to keep the sump pH at a
level that provides a safe margin from the point where silica will precipitate from solution. The
Figure 13 illustrates the dependence of silica solubility on pH.
Start-ups, Shutdowns and Trips
As with the blowdown ratio control, there are a number of inputs to this control scheme. Prior to
putting the system into service Operations will have to ensure that all associated instrumentation
is functioning properly. Default values will have to be programmed into the DCS to deal with
instruments that are not indicating properly. Provided all are functioning properly and the
Evaporator sump is filled, this controller can be put into automatic mode. On a shutdown or trip
it is recommended that this control scheme be taken out of automatic mode.
Preferable Mode Of Operation Of The Water Treatment Unit
The equipment in these areas of the plant and in turn, covered in this control narrative includes:
(BFW stands for: Boiler Feed Water)
a) Distillate Tank 14-V-02
b) Evaporator Vapour Compressor 14-C-Ol
c) Distillate Product Pumps 14-P-02 A/B
d) Steam Generation:
e) BFW Tank 15-T-01
f) HP BFW Pumps 15-P-01 A/B
g) BFW Booster Pump 15-P-02
h) Dump Condenser 15-E-02
i) Package Boiler 15-X-01, comprised of:
j ) Steam Drum 15-V-01
k) Boiler 15-B-01
1) Air Pre-heater 15-E-01
The intent of this section is to:
a) Provide a high level process overview of the BFW supply system.
b) Explain how BFW is directly fed from the Distillate Tank to the Boiler using the BFW
Tank as a volume buffer.
This part of the plant will draw water from the Distillate Tank on an as needed basis with water
fed directly to the Boiler. In the event of a small, sudden increase or decrease in demand the
system will respond by adjusting the speed or the Inlet Guide Vanes (IGVs) of the Evaporator
Vapour Compressor. If the change in demand is faster than the Evaporator Compressor can
respond, the system will send or draw water from the BFW Tank until the Compressor catches
up. Controls on the tank wil eventually take its level back to set-point by slowly sending or
taking water from the system. The controls associated with equipment in and upstream of the
Evaporator 14-V-01 will adjust to maintain a reliable supply of distillate.
The overall control objective for this part of the plant is to supply the necessary BFW to the
boiler so it can produce the steam to meet the desired injection requirements.
There are two main controllers discussed in detail:
Distillate Tank Level Control and Pump Minimum Flow Control (Figure 5): The objective of
this control is to maintain the Distillate Tank at a desired set-point for all upset conditions (large
or small) while ensuring minimum flow for pumps.
BFW Tank Level Control (Figure 6): The objective of this control is to maintain the level in the
tank without impacting the rest of the system.
Process Description
Distillate is produced in the Evaporator Package 14-X-01. Condensed water gravity flows to the
Distillate Tank. Primarily, the rate of evaporation and subsequent production of distillate is
determined by the speed of the Evaporator Vapour Compressor, the position of the IGVs, and the
amount of heat added to the system. Water is pumped from the Distillate Tank to the Boiler
using two sets of pumps in series, the Distillate Product Pumps and the High Pressure (HP)
Boiler Feed Water Pumps. The HP BFW Pumps raise the BFW pressure to 5,790 kPaG and
pump the distillate through the Inlet Coolers 10-E-Ol A/B for heat recovery. The normal BFW
temperature leaving the Inlet Coolers will be approximately 145°C. The pre-heated HP BFW
will be sent to the Package Boiler directly based on a demand set by the steam drum level control
valve. As Boiler demand rises and falls relative to what is being delivered by the Evaporator,
water will be directed to and from the BFW Tank. The BFW Tank level will be maintained at
approximately 80% of level range.
The Boiler is a drum style configuration that produces steam at 4,200 kPag and superheated by
25°C to prevent condensation in the steam pipeline going to the well pads. The combustion air is
heated to 90°C by recovering heat from the glycol return stream in the Air Pre-heater 15-E-01.
The Steam Drum has an average blowdown rate of 2%, which is recycled back to the process
through the Flash Drum 13-V-03.
A portion of the 4,200 kPaG steam is let down to 500 kPaG and de-superheated with HP BFW
for use as utility steam. The majority of this steam is used in the Water Treatment System for
start-up of the Evaporator. The steam header also receives steam from the Heat Recovery Steam
Generator (HRSG) 40-X-Ol which recovers heat from the gas turbine producing the site power
requirements.
Control Philosophy
This section focuses on the following controls:
a) Distillate Tank Level and Pump Minimum Flow Control
b) BFW Tank Level
Distillate Tank Level and Pump Minimum Flow Control (Figure 5)
Boiler controls will be standard and included by the vendor with the supplied package. BFW
flow to the Steam Drum will be controlled by the drum's level valve. BFW will be directly fed
from the Distillate Tank to the Boiler through a series of pumps. Changes in Boiler demand will
have a direct impact on the level in the Distillate Tank. Level in the tank will be controlled via
two level controllers, 14-LC-042 and 14-LC-043.
The output from the first controller 14-LC-042 will be tied to the Evaporator Vapour
Compressor speed controller 14-SC-044 and 15-FV-007 (through high select 15-FY-007) on the
line from the discharge of the Distillate Product Pumps to the BFW Tank. 14-LC-042 will be
tuned aggressively since the Distillate Tank is small and response to disturbances will have to be
quick. The compressor will not be able to adjust its speed very fast. If the BFW demand
suddenly decreases the level in the Distillate Tank will begin to rise. On a rising level 14-LC-042
will call for the compressor to slow down. If the compressor does not respond fast enough the
output from 14-LC-042 will then call for 15-FV-007 to open so distillate can be sent to the BFW
Tank.
If the BFW demand suddenly increases, the level in the Distillate Tank will begin to fall. On a
falling level, 14-LC-042 will call for the compressor to speed up. If the level continues to fall, a
low-low alarm on 14-LC-042 will activate. This will signal for the BFW Booster Pump to start.
The second level controller 14-LC-043 will then start opening 15-LV-027 (through high select
15-LY-027) on the line going from the discharge of the BFW Booster Pump to the suction of the
HP BFW Pumps.
A minimum flow through the Distillate Product Pumps will be maintained by 14-FC-008. The
output from that controller will control 14-FV-008 which will allow distillate to circulate from
the discharge of the Distillate Product Pumps back to the Distillate Tank. A minimum flow
through the HP BFW Pumps will be maintained by 15-FC-007. The output from that controller
will control 15-FV-007 (through the high select block 15-FY-007) which will allow BFW
(distillate) to flow from the discharge of the HP BFW Pumps to the BFW Tank.
BFW Tank Level Control ( Figures 6, 7-10)
Level in the BFW Tank will be maintained at approximately 80%. If the level drops, the output
from the level controller will slowly increase to open the valve to send BFW to the tank. So as
not to upset the BFW system, the controller will be de-tuned to match the Evaporator Vapour
Compressor's dynamics. If the level rises above 85%, the controller will be used to start the
BFW Booster Pump to slowly draw the level down in the tank.
The output from 15-LC-027 will go to 15-FV-007 and 15-LV-027 through the high select blocks
15-FY-007 and 15-LY-027 respectively. 15-LC-027 will be tuned so as not to allow a flow rate
change in or out of the BFW Tank to exceed the reaction time of the Evaporator Vapour
Compressor speed control. When the measured level is at set-point, the output from 15-LC-027
will be at 50% and therefore, will not be calling for 15-FV-007 or 15-LV-027 to be open.
If the level in the tank is more than 5% above set-point, the BFW Booster Pump will
automatically start, the output from 15-LC-027 will start to increase, and 15-LV-027 (through
high select 15-LY-027) will begin to open. This will allow BFW to flow from the BFW Tank to
the suction of the HP BFW Pumps. Once the level in the BFW Tank has decreased to within 1%
of set-point the BFW Booster Pump will automatically be turned off. If the level in the tank is
below set-point the output from the controller will
decrease and 15-FV-007 (through high select 15-FY-007) will begin to open. This will allow
BFW to flow from the discharge of the HP BFW Pumps to the BFW Tank. 15-LC-027 will then
close the valve once the level in the tank is at set-point.
Start-ups, Shutdowns and Trips/ Malfunctions
In the event of a boiler trip the following events will occur:
a) The minimum flow programming will send BFW to the BFW tank.
b) Level in the distillate tank will rise causing the Evaporator Vapour Compressor to slow
down.
c) The level in the Evaporator will rise. This will cause the system to back makeup water
out and to send produced water to the Produced Water (PW) Tank.
d) Operations will have to watch the level in the BFW Tank as it will likely be the first tank
to reach its high level limit. At that point the evaporator would have to be put into recycle
mode and produced water would be sent directly to the PW Tank.
On start-up the Evaporator would be started in recycle mode where BFW would be circulated
from the Distillate Product Pumps and HP BFW Pumps to the BFW Tank and then back to the
suction of the HP BFW Pumps. Once the Boiler has started up, the BFW Tank level should be
drawn down prior to swinging distillate to the HP BFW pumps. At that point all controllers
should be placed in automatic mode and the system will slowly ramp itself up as the Boiler's
demand increases.
Those and other benefits of the disclosed system and its parts and specifically elimination of
unnecessary heaters, coolers, and storage tanks makes it suitable for use in a mobile modular
SAGD system in which the size of the equipment is paramount.
As many changes can be made to the preferred embodiment of the invention without departing
from the scope thereof; it is intended that all matter contained herein be considered illustrative of
the invention and not in a limiting sense.
We claim:
1. A contaminant control system for an evaporative water purification process in a SAGD
(Steam Assisted Gravity Drainage) system for a heavy oil recovery facility, the system
comprising of an evaporator and a set of controllers;
the evaporator having a bottom with a sump provided at the bottom thereof and including an oil
skimming weir dividing the sump into a main sump and a blowdown sump, wherein water
containing impurities flows over the weir from the main sump to the blowdown sump;
in a normal operation mode, the evaporator receiving water from the process and discharging
distilled water from a distillate tank and discharging waste brine from the blowdown sump;
the set of controllers including:
• a distilled water flow meter provided at the discharge of evaporator,
• a blowdown flow meter measuring the flow from the main sump to the blowdown sump,
• a cycle calculator for calculating the ratio between the distilled water flow and blowdown
flow and a total flow controller;
wherein the cycle calculator provides a set point to the total flow controller, thus the flow of the
water into the evaporator does not directly depend on the level of the liquid in the main sump;
wherein the operation of the evaporator functions in a contained closed loop environment.
2. The process of claim 1 further comprising a distillate tank, receiving distilled water, the
distillate tank having a level controller for controlling the energy supplied to the evaporator and
distilled water production, wherein the distillate tank is the only major surge capacity in the
process.
3. The process of claim 1 wherein the set of controllers further comprises a hardness
controller, a silica controller and a pH controller, wherein the pH controller adjusts the flow of
caustic to the evaporator sump, the silica controller determines the set point for the pH controller
and the hardness controller manipulates the set point of the cycle controller affecting the flow of
make-up water into the process.
4. The process of claim 1 wherein the evaporator receives the upstream water from a water
purification unit, wherein the flow of make-up water is set providing the desired flow into and
out of the water purification unit.
5. The process of any one of claims 1 to 4 wherein the evaporator has two additional modes
of operation; an idle mode and a recycle mode; wherein during the idle mode there is no flow in
or out of the evaporator, and during the recycle mode the distilled water is recycled into the inlet
of the evaporator; said modes being initiated; when starting up the evaporator, when shutting
down the evaporator, when there is a loss of flow, and in response to a malfunction in the system
such as a compressor malfunction, a pump malfunction or a boiler malfunction; wherein the
evaporator can be operated in said modes automatically or by an operator.
6. The process of claim 5 wherein the recycle mode is used during the shut down procedure
and after a malfunction such as a boiler malfunction, the idle mode is used after a compressor
malfunction.
7. The process of claim 5 wherein start up of the system includes the following steps; when
shut down the system is switched to idle mode, then to recycle mode and finally to normal
operation mode, the careful selection of modes reducing operating risks to the system and to the
personnel and providing smooth operation compared to single mode evaporators.
8. The process of claim 1 wherein the weir separating the main sump and the blowdown
sump shall have a v-notch at the top of the weir to allow variations in the main sump level to
vary within the height of the v-notch with varying but continuous flow of brine to the blowdown
sump, the location of the overflow weir ensuring that any hydrocarbon floating on top of the
water in the main sump will be continuously removed from the evaporator with the waste brine
and elimination of the hydrocarbon related foaming problem and unit shutdowns associated with
the foaming problem.
9. The process of any one claims of 1 to 4 wherein the units are installed in the
mobile/portable modular SAGD system for heavy oil recovering.