TECHNICAL PAPER 192
Treatment of Coal Bed Methane Produced
Water Using Short Bed Ion Exchange
MICHAEL SHEEDY AND PAUL ROBINSON, Eco-Tec Inc., Pickering,
Ontario
IWC-08-XX
KEYWORDS: ion exchange, coal bed methane, produced water
ABSTRACT: The water produced during the recovery of natural gas from coal beds, coal bed
methane - CBM, is often a major problem in developing this resource. A pilot study of a short
bed ion exchange process treating CBM produced water is presented. The paper will discuss
background issues related to CBM produced water, alternate treatment methods, short bed
technology, pilot plant test data, and design of a full scale system.
BACKGROUND
Coal bed methane, CBM exists in coal
deposits throughout North America and at
the turn of this century was estimated to
supply 7% of US natural gas demand. The
importance of this source of gas has only
continued to grow over the subsequent
years.
Natural gas in the coal seams is trapped by
the ground water pressure. Production
wells are drilled into the coal seam and
once a sufficient amount of water is
removed the well goes into gas production.
Withdrawal of this ground water is one of
the major concerns related to utilizing this
source of energy and has been the focus of
federal and state regulators, local
landowners, and other special interest
groups.
Consequently beneficial use of this water is
of great importance. Alternate uses for this
water include the following: agriculture – for
livestock and irrigation, municipal supply,
underground injection to recharge ground
water supply, surface discharge to streams
or soil, impoundment storage and as a
SM/Technical Paper 192
supply for industrial use. Water quality
standards vary depending on the use.
CBM produced water is primarily composed
of sodium bicarbonate. A typical analysis
for produced water in the Powder River
Basin in Wyoming is shown in Table 1.
Table 1: Typical Analysis of
Produced Water
Component
Range (mg/L)
Na
470 – 1,100 as Na
Ca
8 – 41 as Ca
Mg
2 – 18 as Mg
Fe
0.1 – 0.35 as Fe
Ba
0.1 – 1 as Ba
HCO3
1,400 – 2,900 as HCO3
SO4
10 as SO4
Cl
20 as Cl
TSS
5 – 25
SAR
100 - 513
Recovered water for surface use typically
has limits placed on conductivity, pH, in
some case suspended solids and turbidity,
and the sodium absorption ratio, SAR.
SAR, also known as sodicity, is calculated
using Eq.(1).
SAR = Nameq/L/((Cameq/L + Mgmeq/L)/2)1/2
Eq.(1)
October 2008
TECHNICAL PAPER 192
Target levels for conductivity and SAR are
set to ensure water uptake by plant roots
and proper soil porosity. Jangbarwala(1)
presents a good review of this topic. An
example of a typical water quality target for
surface discharge in the Powder River
Basin is given in Table 2.
Table 2: Typical Water Quality
for Surface Discharge
Component
Range
Conductivity
1,000 – 2,000 µS/cm
SAR
3 – 7.5
pH
6.5 - 9
Na
200 – 500 mg/L as Na
Cl
<70 mg/L as Cl
Turbidity or TSS
Quantitative limits not
always defined.
Should be no visible
turbidity
counter-currently upwards into a backwash,
rinse and regeneration zone. Between
pulses the resin is regenerated using HCl.
Advantages of this system include
continuous operation, which should help to
minimize the need for large feed and
product storage tanks, counter-current
regeneration to minimize acid and rinse
water requirements, and an ability to handle
some
suspended
solids
via
the
backwashing
process.
Disadvantages
include the overall height as shown in
Figure 1, a relatively more complicated
process compared to a conventional IX
column, and resin attrition due to the
pumping and crushing in the isolating
valves between the different loop sections.
TREATMENT TECHNOLOGIES
In many cases the quality of the produced
water must be improved to permit its
beneficial use. For SAR and conductivity
control
the
most
commonly
used
technology is ion exchange, but various
other technologies exist, as do different
methods of ion exchange, IX. A partial list
of alternate technologies would include
electrodialysis reversal-EDR, distillation,
reverse
osmosis,
and
freeze-thaw
evaporation.
The list of ion exchange methods includes
the following: short bed IX (described in a
separate section), continuous fluidized IX,
conventional deep bed IX, and a novel
fluidized IX process for selective Na
removal.
CONTINUOUS FLUIDIZED IX - The well
known Higgins Loop(2,3) technology is
used for Na removal from CBM water and is
shown in Figure 1. In this system the
produced water is pumped downward
through an adsorption bed of SAC resin,
while the resin is periodically pulsed
SM/Technical Paper 192
Figure 1: Higgins Loop
CONVENTIONAL IX - In this process a
standard fixed bed IX column is used to
exchange the Na from produced water.
Normally these processes are regenerated
co-currently and are regenerated with either
H2SO4 or HCl. The advantages of this
process are its simplicity. However,
regenerant operating costs will be higher
and larger volumes of waste will be
produced. The Catalyx(4) process appears
to be an example of a conventional system
with a slight modification to the valving and
process steps that permit some waste
minimization.
October 2008
TECHNICAL PAPER 192
Generally SAC resin is used for Na
removal, but at least one reference(5) was
found that describes the use of WAC resin.
Normally WAC resins will not work in an
acid cycle for Na removal because the low
pH that results from the Na exchange
forces the weak carboxylic group to remain
in the acid form. However, the very high
levels of bicarbonate in CBM water buffer
the pH and permit the exchange to occur.
The benefit of using a WAC resin is a
reduction in regenerant consumption to
almost the stoichiometric amount due the
ease of converting the carboxylic group
back to the acid form. Higher levels of Cl
and SO4, while not expected, will result in
Na leakage when using a WAC resin.
Furthermore,
if
a
counter-currently
regenerated SAC resin is used in a packed
or short bed type system there should be
no difference between the acid consumed
by either resin. Also, the SAC resin will be
less expensive and will not be as sensitive
to increases in Cl and SO4.
Na SELECTIVE IX - This continuous IX
process is shown in Figure 2 was
developed with Montana State University(6)
and adjusts an IX reactor volume,
residence time, and the amount and state
of the IX resin to control the rates of
exchange such that the higher Na
concentration increases the Na removal
rate and leaves more Ca in the withdrawn
reactor solution than would result if the
mixture was allowed to come to equilibrium.
Normally when using SAC resin in a more
conventional IX process Ca and Mg
removal are strongly preferred relative to
Na and the product water will contain no
hardness. Leaving Ca and Mg in solution
helps to achieve the SAR target level
without the need to replace hardness in the
final product water. This appears to be the
principal benefit of this process.
Unfortunately, no case study data or
examples in the patent application could be
found to support the claims of this process.
Also, the patent application describes a
relatively complex process consisting of a
fluidized reactor, fluid distributor, elutriation
lines, resin separators (possibly a
hydrocyclone), rotary valves, a separate
regeneration system, and provision for
make-up of fresh resin. All of these
components must then be operated to
carefully balance resin regeneration level,
water and resin mix ratio, resin withdrawal
rates and other parameters to ensure the
relative exchange rates favourable to
selective divalent exchange are achieved.
SPECIAL PROCESS CONCERNS
Compared to oil field produced water, CBM
water is more easily treated since it is not
necessary to separate oil, control silica, or
achieve low hardness levels from high TDS
waters. However, CBM water does require
special attention with regards to the
following: Ba control, Fe control, waste
disposal, and remote location operation.
Ba CONTROL - Table 1 indicates that Ba
levels ranging from 0.1 – 1 ppm are typical.
The solubility of BaSO4 is approximately 1
ppm as Ba. Such a low level can pose a
problem when the treatment technology
being used concentrates the Ba. Cation
exchange resins have a high selectivity for
Ba, which readily concentrates on the resin.
Figure 2: Continuous IX Process
SM/Technical Paper 192
The lower cost of sulphuric acid makes it an
attractive IX regenerant chemical however,
October 2008
TECHNICAL PAPER 192
when used with a Ba loaded resin insoluble
BaSO4 is almost immediately produced.
This results in fouling of the resin bead
surface and the accumulation of solid
material within the resin bed. A microscopic
photograph of a Ba fouled resin bead is
shown in Figure 3. In this case the resin
had only been in service for 3 days when
treating a stream containing 1 ppm-Ba and
regenerated with sulphuric acid. It was
possible to remove some of the BaSO4 by
taking the resin out of the column and
immersing it in HCl. Other cleaning
chemicals such as EDTA and NaCl were
found to be ineffective.
The first method simply involves dosing
sulphuric acid into the feed stream. The
patent(9) indicates that the invention can be
used as pretreatment for cation exchange
or reverse osmosis to reduce fouling. As
already noted the solubility limit of BaSO4 is
1 ppm as Ba, which like Ca does not
depend on the level of SO4 present.
Therefore, adding sulphuric acid will not
drop the Ba level below 1 ppm. When using
sulphuric acid the Ba level in the feed to the
IX unit needs to be <0.1 ppm. Therefore,
this method is not a viable pretreatment
method for an IX unit regenerated with
sulphuric acid.
The patent abstract for the second method
indicates the invention is particularly useful
for BaSO4 scale control in acid cycle water
softeners. Beaker scale tests were
conducted by dosing PESA into 50 g/L
sulphuric acid and then adding BaCl2.
There was no increase in the soluble level
of Ba in any of these tests regardless of
dosage or contact time. Essentially all the
solutions became turbid immediately upon
dosing of the Ba. This indicates that this
chemistry will not prevent Ba fouling during
sulphuric acid regeneration.
Figure 3: Fouled Ba Resin Bed
Of course the most obvious way to avoid
BaSO4 fouling is to use HCl or if using
sulphuric to first remove Ba using a Na
cycle softener. The selection is governed by
the overall economics of the system.
Alternate methods that have been proposed
for Ba control when using a sulphuric acid
regenerant include a “Rapid Barium
Removal” process(7), scale inhibitors such
as polyepoxysuccinic acid, PESA(8), and
so called “trickle filters”.
SM/Technical Paper 192
The final method is reported to involve
passing the CBM stream through a channel
of particulate gypsum. Possibly the
sulphate in the gypsum or surface
exchange on the gypsum particle would be
responsible for Ba removal. Another beaker
scale test was conducted using a large
excess of freshly prepared gypsum and a
solution containing 1 ppm Ba. The liquid
was filtered after mixing for more than 24
hours. There was no change in the Ba
concentration. This suggests that this
method may also not be viable. No
information
concerning
design
or
performance of these “trickle filters” could
be found.
October 2008
TECHNICAL PAPER 192
Fe CONTROL - Fe fouling of softener
resins is a common problem. It occurs
when insoluble ferric iron is carried into the
softener bed or forms on the resin as a
result of oxidation of ferrous ion previously
exchanged onto the resin. The use of HCl
as the regenerant or periodic cleaning
solution helps to prevent Fe fouling by redissolving any colloidal ferric iron. Beaker
scale cleaning tests of Fe fouled resin have
shown that sulphuric acid is not nearly as
effective as HCl. It would also be possible
to remove Fe upstream of an IX unit by
treatment with manganese greensand
filtration. Other common practices include
adding chemicals to the regenerant salt
such as hydrosulphite, preventing solution
exposure to oxygen and using oxygen
scavengers such as ammonium bisulphite.
Figure 4 is a picture of a pilot plant filtrate
tank. The filtrate was clear upon leaving the
filter. The photograph shows the tank
contents after sitting exposed to air for only
a few hours. The stored filtrate became
very turbid and had a yellow/orange colour
characteristic of colloidal iron. Obviously
processing such a fouled solution would
result in rapid fouling of IX resin. Figure 5
shows the same filtrate solution before and
after acidification with HCl. The Fe level in
this feed was 3 – 4 ppm.
Figure 4: Pilot Plant Filtrate Tank
SM/Technical Paper 192
Figure 5: Filtrate Solution Before and After
Acidification with HCl
REMOTE LOCATION AND WASTE
DISPOSAL - In many cases the CBM well
sites are in remote locations and therefore
consideration must be given to processes
and equipment that are reliable and require
minimal operator attention.
All processes that are used to improve the
quality of CBM water result in the
production of waste. For example, this
would be the spent regenerant and rinse
streams in the case of ion exchange, the
reject stream from an RO, or the
concentrate stream from an EDR process.
Generally this waste is stored onsite and
evaporated in lined holding ponds that must
ultimately be remediated or it is shipped offsite for deep well injection. Both options are
costly and therefore consideration must be
given to minimizing the volume of waste
produced.
In the case of IX regenerant streams the
recovery of the waste salts (NaCl or
Na2SO4) have been proposed. In most
cases this requires additional processing to
purify and crystallize the material and then
establishment of a market. The additional
capital equipment, the logistics of shipment,
and the difficulty in marketing relatively
small quantities of salt makes waste
recovery uneconomic relative to the current
practices.
October 2008
TECHNICAL PAPER 192
SHORT BED ION EXCHANGE
Short bed ion exchange is a packed bed
system that can be described as a
compressed, short bed (CSB) ion exchange
process. An overview of packed bed
technology and some of the theory of short
bed systems is presented in reference (10).
It is characterized by the combination of a
number of design features and benefits:
Fine Mesh Resins (Fig 6 – Using resin
beads having a mean diameter of 20%
of that commonly used in ion exchange
processes (Figure 7), results in a very
high surface area. This means that a
greater proportion of exchange sites are
at the surface of the resin particles.
Since the rate of ion exchange is
dependant on the rate of diffusion of
ions to exchange sites, a greater
proportion of surface exchange sites
can be accessed rapidly through
diffusion of ions through the liquid film
around the resin particles rather than
through the much slower process of
intra-particle diffusion to access internal
resin sites. Consequently, contact time
required in the resin bed is short. This
allows flow velocities of 30 - 50 gpm/ft2
(73 – 122 m/hr).
Figure 7: Conventional Ion Exchange Resin
Low Resin Loading Depending on the
application only 10% - 40% of the resin
capacity is used in the process. This
favours use of the surface exchange
sites, which have the highest kinetics.
Short Beds of Compressed Resin The
high ion exchange kinetics of fine mesh
resins, allows resin bed heights of only
3-6 inches (75 –150 mm). Resin is
packed into resin columns under slight
compression with no freeboard. This
provides excellent flow distribution and
eliminates any possibility of channeling.
Low resin loading minimizes resin
shrinking and swelling.
Countercurrent, Short Regeneration
Since the resin beds are fully packed,
with no freeboard, countercurrent
regeneration, which is the most
chemically
efficient
approach,
is
employed using simple equipment
designs. The short resin beds, and low
loading permits regeneration of a resin
bed in 30-90 seconds.
Figure 6: Short Bed Ion Exchange Resin
SM/Technical Paper 192
While these features offer many benefits
the technology does require special
attention with regards to pretreatment for
the removal of suspended solids. Any solids
that pass into the resin bed are readily
trapped by the fine mesh resin and since no
freeboard space exists they cannot be
October 2008
TECHNICAL PAPER 192
removed via backwashing within the
column. The accumulation of solids results
in a steady increase in pressure drop and a
corresponding drop in flowrate and loss of
system capacity.
In practice, suspended solids should be
removed from all streams being treated by
ion exchange processes in order to prevent
resin fouling and to address product water
turbidity and TSS target limits. The greater
sensitivity of packed bed systems
necessitates higher levels of filtration
efficiency. One of the most economic
methods for removing suspended solids is
via dual media filtration. A high efficiency
filter of this type has been developed for
use with short bed IX systems(10). A
specialized polishing filtration media is used
to achieve filtrate turbidity levels of 0.1
NTU. Pretreatment to this level prevents
fouling of short bed systems.
SHORT BED CBM PRODUCED WATER
PROCESS FLOWSHEET
Short bed technology is now been applied
to the treatment of a CBM produced water
stream in the Powder River Basin in
Wyoming.
FEED WATER AND TARGET VALUES The composition of the CBM produced
water stream is demonstrated in Table 3.
The effluent target values that must be
achieved are shown in Table 4.
SHORT BED CBM PRODUCED WATER
SYSTEM - The process flowsheet is shown
in Figure 8. The feed is collected in a
holding pond and then pumped to the
process. Given the feed composition and
the final water quality target values only 721
gpm needs to be treated. The remaining
329 gpm bypasses the system and is
recombined with the treated water, checked
to confirm that the target values have been
achieved and is then discharged to a river.
SM/Technical Paper 192
After splitting the flows the first step in the
treatment process is the removal of
suspended solids using high efficiency dual
media filters(10) (Spectrum filters). A
coagulant (normally PAC) is used to ensure
a low turbidity value in the filtrate. The filter
backwash waste is recycled to the feed
pond.
Table 3: CBM Produced Water
Stream Composition
Value or
Range
Parameter
Unit
Flowrate
gpm
1,050
Alkalinity
mg/L as
2,010
HCO3
Sodium
mg/L as Na
785
Chloride
mg/L as Cl
17
Sulphate
mg/L as SO4 11
Calcium
mg/L as Ca
12
Magnesium mg/L as Mg 5
Conductivity µS/cm
2,800
pH
--8.5
Iron
mg/L as Fe
0.350
Barium
mg/L as Ba
0.722
TSS
mg/L
<5
DOC
mg/L as C
2
o
Temperature C
2 – 21.5
Table 4: Target Values
Parameter
Units
Value
SAR
--≤6
Chlorides
mg/L as Cl
70
Dissolved
µg/L as Fe
320
Iron
pH
pH
--6.5 – 9.0
Specific
µS/cm
1,215
Conductance
Dissolved
mg/L as Na 420
Sodium
mg/L
810
Total
Dissolved
Solids
October 2008
TECHNICAL PAPER 192
Figure 8: Process Flowsheet for Short Bed CBM Produced Water System
The filters are followed by a short bed Na
cycle Softener, Figure 9, that is used for Ba
removal. The softener has a bed diameter
of 5.5 ft, a depth of 0.5 ft and utilizes SAC
resin. To avoid Fe fouling this softener is
periodically rinsed with HCl. The softened
water is then fed to another short bed
system (decationizer) where Na is removed
and the resin is regenerated using sulphuric
acid.
Figure 9: Na Cycle Softener
SM/Technical Paper 192
The decationizer bed is 6 ft in diameter and
1 ft in depth and also uses SAC resin.
Sulphuric acid was selected because it
resulted in a lower overall cost per barrel of
water (relative to the use of HCl) even when
the additional softener capital and
regenerant salt were included. During pilot
testing regenerant consumption for this
system was only 1.15x the stoichiometric
minimum. Figure 10 shows the decationizer
in the foreground with the inline softener of
Figure 9 in the background. The spent
regenerant and rinse solutions from both
units are directed to an evaporation pond.
The effluent stream leaving the Na removal
unit is fed to a decarbonator for the removal
of carbon dioxide. This raises the pH and
minimizes the amount of lime that must be
used to adjust the final pH and hardness
level. This short bed process typically
results in ≤ 2% of the stream being lost
through operation of the softener and
decationizer units.
October 2008
TECHNICAL PAPER 192
PERFORMANCE DATA
At the time of writing this paper the plant
was still in the process of being
commissioned and no performance data
were available. The design of the ion
exchange system was based on pilot plant
operation and the results of these tests are
shown below in Table 6.
Table 6: Short Bed Pilot Plant
Softener Data
Cycle Time (min): 58
Figure 10: Decationizer
The final step in the process is the
recombination of the processed and
bypassed streams, the inline addition of
lime (or in some cases CaCl2) for pH and
SAR adjustment and measurement of pH,
hardness
and
conductivity
before
discharge. Out of compliance water is
recycled back to the feed holding pond.
Feed
Regenerant
Brine
(150-g/L)
Product
Waste
Regenerant
Wash
Ca
Mg
Na
(mg/L)
(mg/L)
(g/L)
7.6
2
2.7
0.2
1.1
57.5
703.7
0.781
1.25
1130
1
549
n/a
n/a
22
188.000
3.0
1250
305
n/a
161,800
Bed
Volume
703.7
0.78
Ba
(ug/L)
938
PROCESS ECONOMICS – Table 5
demonstrates the process economics for
the Short Bed CBM Produced Water
System.
Table 5: Short Bed CBM Produced Water Economics
Total CBM Water FeedFlowrate (gpm)
Treated Flowrate (gpm)
Feed Composition as per previous table
Unit
Units
Consumption
Price
$0.24
$/lb 100%
0.272
Sulphuric Acid
$3.54 $/gal 100%
0.0012
Coagulant (PAC)
$0.39
$/lb 100%
0.00311
Hydrochloric
Acid
$0.08
$/lb 100%
0.04399
Sodium Chloride
$0.08
$/lb
0.10305
Lime
$0.06
$/kWh
0.06534
Electricity
$0.54 $/bbl waste
0.009
Waste
Total
*bbl = 42 gal barrel of total CBM water feed
**Waste based on financed cost for evaporation pond
SM/Technical Paper 192
1,050
721
Units
lb-100%/bbl
lb-100%/bbl
lb-100%/bbl
lb-100%/bbl
lb-100%/bbl
KWh/bbl
bbl-waste/bbl
Cost/1000 gal
of total CBM
feed
$1.55
$0.01
$0.03
$0.08
$0.20
$0.09
$0.12
$2.08
October 2008
TECHNICAL PAPER 192
It should be noted that since the softener
process objective was only to remove Ba
significant amounts of Ca and Mg were
allowed to leak into the product stream.
These levels could be lowered by a
reduction of the feed volume treated. The
results
of
the
Decationizer
are
demonstrated in Table 7.
Table 7: Short Bed Na Removal
Decationizer – Pilot Plant Data
Cycle Time (min): 7.34
Feed
Conc.H2SO4
Regenerant
(96%)
Onstream
Regen
Bed
Volume
18.2
0.028
17.3
1.0
Na
Ca
Mg
H+
(mg/L)
(mg/L)
(g/L)
(eq/L)
1158
n/a
n/a
n/a
n/a
35.5
17.6
18612.9.
n/a
n/a
5.
Bob Bradely, Mark Reinsel, “An
Economical Process to Treat
Produced Water From Coal Bed
Methane Deposits in the Powder
River Basin”, Proceedings of the
2001 International Water Conference
- IWC, Paper# IWC-01-15.
6.
Ronald Neil, US Patent Application
20060261010, November 23, 2006.
7.
Applications Bulletin –
CleanSweepBarium,
http://www.tbeconline.com/id6.html
8.
J. Michael Brown, US Patent
5,486,294, January 23, 1996.
9.
Bruce W. Bandorick et al, US Patent
7,081,204, July 25, 2006.
10.
Michael Sheedy, Peter Kuzora, “The
Use of Short Bed Ion Exchange
Technology for the Production of
High Purity Water at WE Energies’
Pleasant Prairie Power Plant”,
Proceedings of the 2004
International Water Conference –
IWC.
n/a
n/a
SUMMARY
A short bed ion exchange process has
been developed for the treatment of CBM
produced water. The first system has been
installed and is currently in the process of
being commissioned. Field performance
data are not yet available and may be the
subject of a future paper or report.
REFERENCES
1.
Juzer Jangbarwala, “CBM-Produced
Water A Synopsis of Effects and
Opportunities” Water Conditioning
and Purification, December 2007.
2.
I. R. Higgins, US Patent 2,815,322,
December 3, 1957.
Irwin R. Higgins, Mark S. Denton,
“CSA Continuous Countercurrent Ion
Exchange (CCIX) Technology”
Separation Science and Technology,
v 19, March 1987 p 997 - 1015.
3.
4.
Juzer Jangbarwala, US Patent
6,776,913, August 17, 2004.
SM/Technical Paper 192
October 2008