Pr
o c e s s i n g
Changing feed operating conditions at Khalda
Petroleum Co.’s Salam
gas processing plant in
Egypt’s Western Desert
led the company to make Gas Processing
changes to its membrane
system for removing
CO2.
The plant’s feed operating conditions
had changed over time. An
increase in feed flow rate
and feed CO2 content combined with membrane permeability variations—due to
natural membrane aging and
use—required adjustment in
system operating conditions
to fulfill KPC targets of maximizing system hydrocarbon recovery and meeting
acceptable CO2 sales-gas specification.
This article will present the flexibility of membrane systems when
such changes occur and describe how
performance of a two-membrane-stage
system was optimized.
industry are under development.2 3
Membranes can be used to separate
other gaseous components such as H2S,
N2, He, and water vapor from nonassociated or associated gas stream under
a partial pressure differential.4 Table 1
shows typical membrane materials that
can be used to separate the impurities
from natural gas.2
Removal of CO2 is the only
Changing feed conditions push Egyptian
gas plant to upgrade CO2 membrane system
Membrane concept
membrane-based natural gas separation process currently practiced on a
large scale; more than 200 plants have
been installed. The membrane system
operates on the principles of selective
permeation.5 6
Certain gases permeate or pass
through the membrane more easily
than others (Fig. 1). This allows the
more rapidly permeating (fast) components to be collected in one stream
and the slower permeating (slow)
Total worldwide production of
natural gas is about 103.8 tcf/year.1 All
of this gas requires
some treatment,
Membrane material for separating
Table 1
and about 20%
impurities
from
natural
gas
requires extensive
Category of
Component
treatment before
preferred polymer to be
Typical
polymer
used
material
permeated
it can be delivered
to the pipeline.
Cellulose acetate, polyimide
Glass
CO
Cellulose acetate, ether-amide block
As a result, sev copolymer
Glass and rubber
HS
Polyimide, perfluor polymers
Glass
N
eral billion dollars’
Many
Glass and rubber
Water vapor
worth of natural
gas separation
equipment is
T HIN SEMIPERMEABLE BARRIERS
installed annually
worldwide. The
membrane market
share is about 2%,
essentially all for
CO2 removal.2
Slow
Fast
This fraction
Membrane
will likely increase
because other applications of memAr, CO, N2, CH4
H2O H2 He
CO2, O2, H2S
branes in the natuHydrocarbons
ral gas processing
Mahmoud Abu El Ela Mohamed
Cairo University
Giza, Egypt
Mostafa Nabawi
Khalda Petroleum Co.
Cairo
2
2
2
Fig. 1
C1, C2, C3
C1, C2, C3
CO2
H2S
H2O
Reprinted with revisions to format, from the July 14,
2008
2.5
xedition
1.5 of OIL & GAS JOURNAL
Copyright 2008 by PennWell Corporation
No. z080714OGJpel01
CO2
H2S
H2O
o c e s s i n g
S ALAM PROCESS FLOW
Fig. 2
High-press.
flare
To fuel-gas system
Train A
Inlet separator
From
wells
Shams
High-press. permeate
compressor
Turboexpander
Export-gas
compressor A
Glycol
contactor
TUT
Turboexpander
Inlet separator
Salam
South
Export-gas
compressor B
Second-stage
membrane
To flare
First-stage
membrane
pretreat.
First-stage
membrane
First-stage
membrane
pretreat.
First-stage
membrane
Glycol
contactor
Train B
NGL flush
drum
Hg-removal
unit
From South
Umbarka gas plant
Second-stage
membrane
pretreatment
Gas to
Matrouth
Slug catcher
Second-stage
separator
Condensate
stabilization
Inlet
manifold
Low-press. Low-press. permeate
flare
compressor
Export-gas
metering
Storage
tank
Recycle
compressor
From existing TEG assoc. gas
From existing high-press. gas
From existing flare assoc. gas
Sales gas
Condensate to
export pipeline
Meleiha/El-Hamra
Flare assoc. gas
compressor
components to be collected in a second
stream. In natural gas treating, CO2,
hydrogen, helium, H2S, and water vapor
are highly permeable gases, sometimes
characterized as “fast” gas molecules
that permeate to a greater extent across
the membrane to the lower pressure
side than do the “slower” gases (i.e.,
hydrocarbons).
The permeate gas will have a higher
concentration of the fast gases and a
lower concentration of the slow gases,
relative to the feed. The gas that remains
on the higher pressure side, also called
the residual gas, will be depleted in the
fast gas components relative to the feed
gas.
The driving force for permeation
is the difference in partial pressure of
a given component between the feed
(high pressure) side and the permeate
(low pressure) side of the membrane.
The greater the partial-pressure difference, the greater the driving force.6
A simple mathematical representation (accompanying equation box) can
illustrate gas
transport across
Fullwide
x 3a membrane.6 The permeability coefficient, K,
is a function of both the solubility and
the diffusivity of the gas in the polymer
matrix. Several mathematical models are
available that derive the permeability
coefficients.7 8
Membranes have two performance
characteristics that determine methane
product recovery and purity: permeation rate and selectivity.
Permeation rate or flux is the rate at
which a gaseous component can diffuse
through the membrane medium to the
low-pressure side. The rate of perme-
Equation
ν = KA (P1–P2)
where:
ν
Volume of permeated gas, scf/hr
K
Permeability coefficient of permeating
gas, ft/(hr-psi)
A
Membrane surface area, sq ft
P1
Partial pressure of feed stream, psi
P2
Partial pressure of permeate stream,
psi
ation for each component is determined
by the following:
• The characteristics of the component.
• The characteristics of the membrane.
• The partial pressure differential
of the gaseous component across the
membrane.
Since the permeation rate for CO2 is
greater than for methane, increased flux
through the membrane will increase the
methane product purity, but decrease
the methane recovery in the residual
gas stream. Selectivity refers to the ratio
of the permeation rates of a fast gas
(CO2) and a slow gas (methane or other
hydrocarbon). The selectivity of CO2 to
methane determines the efficiency of the
separation and the methane recovery.
Salam gas plant
Khalda Western Desert gas development project at Salam is 70 km from
Matrouh. The plant processes gas
condensate from Salam field, Qasr field,
No. z080714OGJpel06
Pr
No. z080714OGJpel03
South Umbarka, and an oil
contains a pretreatment unit
plant’s associated gas. The
and membrane skids. Each
project produces about 200
pretreatment unit consists
MMscfd of export gas at an
of a filter coalescer, a guard
export pressure of 101 bara
bed, and a particle filter. Each
and 9,000 stock tank bbl
membrane skid consists of
(stb) of condensate. The plant
membrane tubes containing
started up in July 1999.
membrane elements.
The sales gas is designed
These elements consist of
to have a maximum CO2 concellulose acetate membrane
tent of 3% mol, a maximum Salam gas plant in Egypt’s Western desert employs a membrane CO2-removal sheets that are bound onto a
system (Fig. 3).
H2S content of 4 ppm (vol),
woven cloth support. A mema gross heating value greater
brane sheet has two layers: a
than 1,040 btu/scf, a water dewpoint
relatively thick microporous layer that is
expander technology.
of less than 0.0º C. at 71 bara, and a
in contact with the cloth support and a
After dewpointing, the gas enters
cricondentherm of 5º C. The condensate the gas-sweetening system (two-stage
thin active layer on top of the micropois designed to have a maximum of 11
rous layer.
membrane package) to reduce CO2 in
the export gas. The final step is to expsi rvp.
A membrane element is a spiralport the gas via the export compressors. wound assembly with a perforated
The flow diagram of the Salam gas
Condensate collected from the
plant (Fig. 2) shows gas flowing from
permeate tube at its center (Fig. 4). One
the wells into two parallel trains. First it various processing steps moves to
or more membrane leaves are wrapped
enters a three-phase separator where the stabilization before being stored in the
around the permeate tube. Each leaf
three storage tanks. The stabilizer tower contains two membrane-cloth composmain water-condensate-gas separation
removes the light hydrocarbons to avoid ite layers that are separated by a rigid,
takes place. Gas from the three-phase
separator goes to the mercury-removal release in the tanks and to achieve the
porous, fluid-conductive permeate
unit. Then it flows to the glycol contac- rvp specification. Condensate is then
channel spacer. These leaves are separatshipped from the storage tanks via ship- ed from each other by a high-pressure
tors to remove water from the gas to
avoid hydrate formation and to achieve ping pumps to El-Hamra.
channel spacer. The membrane leaves
water-content specifications.
are sealed with an adhesive on three
Membrane system
Gas is then diverted to the dewsides; the fourth side is open to the
The membrane system at the Salam
pointing package, whose function is to
permeate tube.6
During the sweetening process,
separate entrained traces of condensate gas plant had been started up in July
the feed gas from the turboexpander
and heavier hydrocarbons that condense 1999 (Fig. 3) and has operated sucis compressed through the first-stage
as liquids from the gas at lower temper- cessfully and without major upsets or
export compressors. The feed gas is then
membrane replacement. It includes
atures. This step achieves hydrocarbon
delivered to the pretreatment section of
two membrane stages. Each stage
dewpoint specifications using turbothe membrane unit that provides key
protection for the membrane elements
S PIRAL-WOUND MEMBRANE ELEMENTS*
Fig. 4
and keeps membrane feed dry and free
Feed
of contaminants. The gas first passes
through a high-efficiency filter coalescPermeate
er for removal of entrained contaminants such as sand, pipe scale, lubricating oil, and hydrocarbon, or water
condensate. The filtered gas then passes
through a guard bed to remove trace
contaminants and is then sent through
Residual
Feed spacer
a particle filter to remove any entrained
Membrane
dust or particulates from the guard bed.
Permeate spacer
After exiting the pretreatment secMembrane
tion, the feed gas combines with the
Feed spacer
recycle gas from the second-stage
Residual
membrane before entering the first*Two membrane sheets with permeate spacer between; leaves are separated by feed spacers and wrapped around a
stage membrane skids (Figs. 2 and 5).
permeate tube facing it with three open ends.
2x2
Pr
o c e s s i n g
I MPACT OF OPERATING PARAMETERS ON MEMBRANE PERFORMANCE
No. z080714OGJpel05
Membrane replacement
Permeate pressure
Feed gas pressure
Feed gas temperature
Membrane areas
normal membrane aging can result in
a CO2 concentration increase in the
First or second stage*
sales gas. Membranes are subjected
to a lifetime that varies with feed-gas
conditions, membrane pretreatment
Operating parameters change
design, and operator skills. Salam gas
plant has shown excellent performance
with membrane lifetime of more than
8 years.
Design of a membrane system takes
into account the natural performance
Impact on membrane performance:
decline (membrane aging) by sizing
the system for end-of-life conditions,
Sales gas CO2 content
so that the system will always reach the
Hydrocarbon recovery
required specifications. During the life(assuming an unlimited permeate compressor capacity)
time of the membrane, the system will
*Increase = j decrease = .
require minor operational adjustments
as the membrane properties (selectivity
permeability) vary.
x 2 from 206 MMscfd up to 235 andThis
This combined stream has a design CO2 2 creased
article will further describe how
content of 6.34% mol and is the feed
MMscfd.
the Salam gas plant has been operated
gas to the first-stage membrane skids.
This resulted, in August 2007 after 8 as feed-gas conditions have changed
As the feed gas passes through the
years of operation, in a CO2 concentra- and as membranes have aged, keeping
tion increase in the sales gas up to 4.7% in mind KPC’s objectives of producmembrane tubes, the gas is separated
mol from the designed 3% mol and an ing gas with an acceptable CO2 content
into a high-pressure methane-rich
while minimizing hydrocarbon losses
increase in sales-gas flow rate to 210
gas (residual), and a low-pressure gas
that translates directly in sales gas volMMscfd from 200 MMscfd.
stream concentrated in carbon dioxide
ume and revenue.
This observation is in line with the
(permeate).
The first membrane stage is designed well-known principles of membrane
Adjustment to changes
systems. For traditional solvent-based
to produce a residual gas (sales gas)
Several operating parameters affect
CO2-removal technologies, unit size is
with 3% mol CO2 content, which is
supplied to the export compressors for mainly driven by the absolute amount
gas separation by membrane, including
gas metering. The permeate gas conof CO2 to remove. Membrane sysfeed-gas flow and composition, pressure
tems, however, are CO2 bulk-removal
taining 30% mol CO2 is compressed
differential across the membrane, gas
through the permeate compressor and
technologies for which unit size is
temperature, online membrane area, and
then directed to the second stage mem- mainly driven by the percentage of CO2 sales-gas specification. A good underremoval. A given membrane system de- standing of the effects of these process
brane package.
signed to reduce CO2 down to 3% mol parameters is important to maximize the
The second membrane stage is designed to recover most of the hydrocar- from 6% mol (=50% CO2 removal) will efficiency of the operation.
bons from the first-stage permeate gas. also be able to handle a gas containing
When conditions are changing, plant
8% and produce a gas with 4% mol, all operation needs to be reassessed in
The second membrane stage residual
other conditions remaining constant
gas is recycled back to the first memorder to find a new optimum operating
brane stage. The second-stage permeate (feed-gas flow, pressure, temperature,
mode in term of sales-gas specification
so forth).5
gas containing 71% mol CO2 is flared.
and hydrocarbon recovery (directly reMoreover, membrane systems are
lated to sales-gas volume and revenue).
Membrane operational
modular and can easily cope with
For the two-membrane-stages system at
flexibility
increase of feed flow rate. An increase
Salam gas plant, not only the two indiWith time, plant operating conof feed flow rate requires a proportional vidual membrane stages’ performance
ditions at the Salam gas plant have
increase in membrane area requirehas been reassessed, but also the overall
changed:
ments. If the membrane area is fixed,
system including recycling step from
• The feed gas CO2 concentration
an increase in feed flow will result in an the second to the first membrane stage.
increased from 6.34% mol to 8.5%
increase of CO2 in the produced gas.
The following parameters, individuNext to the changes in feed-gas
mol.
ally or combined, can be used to opticonditions (flow and composition),
• The feed flow to the plant inmize plant operation in order to cope
Table 2
First, second-stage area
With an increase in the first-membrane stage area, the unit can handle
a constant feed flow rate and produce
sales gas with a lower CO2 concentration or can handle a higher feed flow
rate producing sales gas with the same
CO2 concentration.
In the former case, the throughput
feed gas to the second membrane stage
will increase. If the permeate compressor capacity is not limited, a higher
flow richer in CO2 will be recycled
back to the first membrane stage. The
second-stage permeate gas flow rate
will slightly increase. (Hydrocarbon
losses increase as a lower sales-gas CO2
spec is reached.)
An increase in second-membrane-
M EMBRANE SYSTEM
Fig. 5
3% CO2
Residue
7% CO2
Feed
70% CO2
Permeate
30% CO2
10% CO2
x 2 area, a lower CO2 concentration
stage area will result in a second-stage 2 brane
in the sales gas/recycle stream will be
residue with a lower CO2 concentration
and thus also a lower CO2 concentration reached.
in the first membrane-stage feed gas and
in the sales gas. A higher membrane area First, second-stage
permeate pressure
will allow not only a higher CO2 perA decrease in permeate pressure
meation flow but also a higher methane
increases the driving force across the
permeation flow and thus an absolute
membrane and results in a net increase
increase in vented hydrocarbons.
in permeation through the membrane.
First, second-stage feed temp. The membrane area requirement thereThe permeability of gases generally
fore drops or, for a fixed membrane
increases with increasing temperature.
area, a lower CO2 concentration in the
sales gas will be reached. On the other
The permeability for carbon dioxide,
hand, a lower amount of hydrocarbons
however, increases at a lower rate than
will permeate, which translates in a
methane and other hydrocarbons with
higher hydrocarbon recovery.
an increasing temperature. This means
A decrease of the second-membranethat selectivity for carbon dioxide over
stage permeate pressure will result in
methane decreases as the operating
a decrease in amount of hydrocarbon
temperature increases. Consequently,
permeating through the second memmethane-product recovery (hydrobrane stage and thus in a better overall
carbon recovery) will decrease, while
hydrocarbon recovery.
purity (sales-gas CO2 content) will
increase as the operating temperature
increases, all other operating conditions First, second-stage
replacement
being constant.
An aging membrane element can
First, second-stage
show declining selectivity. This means
feed pressure
that more methane permeates at a fixed
The increased pressure creates a
CO2 permeation. Consequently, more
greater driving force across the memhydrocarbons are lost, and the load on
brane and results in a net increase in
the permeate compressor (in the case of
permeation through the membrane.6
a first-membrane-stage decline) increasThe membrane area requirement
es. Therefore, a membrane at its end of
therefore drops or, for a fixed memlife will need to be replaced.
No. z080714OGJpel04
with the changes in feed conditions and
the natural membrane aging:
• Membrane area of first and second
membrane stage.
• Temperature of first and second
membrane stage feed gas.
• Pressure of first and second membrane stage feed gas.
• Pressure of first and second membrane stage permeate gas.
• Replacement of those membranes
that have reached the ends of lives, in
first or second membrane stage.
Table 2 illustrates the qualitative
impact of the operating parameters on
the overall two-stage membrane system.
The impact, in absolute terms, of each
of these parameters will vary. In combinations of different parameters, therefore, each of them will weigh more or
less on the overall performance. Note
that other factors, such as permeate
compressor capacity limitations, need
to be taken into account as well.
Table 2 shows that the following
parameter adjustments always have a
positive effect, both on sales-gas CO2
content (decreased) and on hydrocarbon recovery (increase): feed pressure
increase, permeate pressure decrease, as
well as membrane replacement.
The following describes the impact
of the operating parameters, independently.
Pr
o c e s s i n g
Membrane systems are built modularly and systems can thus be easily and
partially isolated. After replacement of
a high number of membrane elements,
a step change in plant performance
becomes apparent:
• An increased selectivity of the firststage membranes translates in a lower
permeate compressor load.
• An increased CO2 permeation rate
permits reduction in the first-stage
membrane area or an increase in the
first-stage permeate pressure.
• An increased selectivity of the
second membrane stage translates into
a lower hydrocarbon content in the
vented second-stage permeate gas and
thus a higher hydrocarbon recovery. It
also leads to a lower CO2 concentration
in the recycle stream; this also results in
a lower CO2 content in the sales gas.
In conclusion, a (partial) membrane
replacement effort is completed only
after the operating parameters of the
membrane system are fine tuned in
order to reach the correct sales-gas
specification, while maximizing the
hydrocarbon recovery.
Adjustment to operational
changes
The following parameters have been
tested at Salam gas plant in order to
cope with the changing feed conditions (CO2 content and flow increase)
as well as with the normal aging of the
membranes:
• In August 2006, the secondmembrane-stage area was increased by
20%. This resulted in a decrease of the
CO2 content in the sales gas. In the first
membrane stage, an increase of membrane area has not been envisioned.
• To determine the optimum
second-membrane-stage operating temperature, operators gradually increased
the feed temperature of the second
membrane stage with continual monitoring of CO2 content in the secondstage permeate gas and in the sales gas.
As expected, it was found that with
an increase in the feed temperature of
the second membrane stage, the CO2
concentration in the second stage resi-
due gas decreased and with it the CO2
concentration in the feed gas to the first
membrane stage. But also observed was
that the amount of hydrocarbons lost
through the second membrane stage
permeate increased.
Increasing feed-gas temperature in
the first membrane stage hasn’t been
implemented at Salam gas plant due to
permeate compressor capacity limitation and the objective to maximize
hydrocarbon recovery.
• In 2007-08, membrane elements
have been replaced in both the first
and second stages. Replacement of the
membrane elements in the second stage
did result in a significant decrease of
the CO2 content in the sales gas to 3.8%
mol from 4.7% mol. First stage membrane replacement is still under evaluation.
• The deeper CO2 cut (to 3.8% from
8.5% compared with design of 3%
from 6.34%) has resulted in a higher
load on the permeate compressor.
Excess first-stage permeate (exceeding
the compressor capacity) is flared and
results in additional hydrocarbon losses.
• In terms of permeate pressure,
design plant operating conditions are
based on the lowest possible permeate pressure. A temporary increase of
80% of the permeate pressure has been
tested on the second stage, resulting in
a significant increase in hydrocarbon
losses.
• Feed pressure increase has not
been tested at Salam gas plant.
Acknowledgment
The authors thank Khalda Petroleum Co. for permission to publish this
article. The authors are also grateful to
the gas operations department and field
staff at Khalda Petroleum Co. and to the
technical staff at UOP whose ideas and
discussions largely formed the basis of
this article. ✦
References
1. BP Statistical Review of World
Energy, June 2008.
2. Baker, R.W., “Membrane Technology and Applications,” Second Edition,
Chapter 8, London: John Wiley & Sons
Ltd., 2004.
3. Spillman, R.W., “Economics of
Gas Separation by Membranes,” Chem.
Eng. Prog, 85, 41 (1989).
4. Fournie, F.J.C., and Agostini, J.P.,
“Permeation Membranes Can Efficiently
Replace Conventional Gas Treatment
Processes,” SPE paper 13729 , Journal of
Petroleum Technology, June 1987, pp.
707-712.
5. Dortmundt, D., Schott, M., and
Cnop, T., “Sour Gas Processing Applications using Separex Membrane Technology,” UOP LLC, Des Plaines, Ill., 2007.
6. Dortmundt, D., and Doshi, K.,
“Recent Developments in CO2 removal
membrane technology,” UOP LLC, Des
Plaines, Ill., 1999.
7. Koros, W.J., “Membrane Based
Gas Separations; Data Base and Models
for Glassy Polymers,” presented at the
Sunriver Membrane Conference, Sept.
19, 1983, Sunriver, Ore.
8. Dinello, M.S., Narayan, R.S.,
and Patton, C.J., “Bulk CO2 Removal
Achieved through Membrane Separation,” SPE paper 13281, SPE Production
Engineering, February 1989, pp. 88-92.
The authors
Mahmoud Abu El Ela
([email protected])
is an assistant professor in
the Petroleum Engineering
Department, Cairo University,
and also works as a petroleum
process consulting engineer
at Khalda Petroleum Co. He
previously worked as a research
engineer at Woodside Research
Foundation, Curtin University of TechnologyAustralia. Abu El Ela holds a BSc and MSc in
petroleum engineering from Cairo University
(Egypt) and a PhD from Curtin University of
Technology (Australia). He is a member of the
Egyptian Engineers Syndicate and SPE.
Mostafa Nabawi is a general
manager in the gas operations
department at Khalda Petroleum. He previously worked as
assistant general manager in
the gas operations department
at Khalda Petroleum. Nabawi
holds a BSc in mechanical engineering from Assuit
University, Egypt. He is a member in the Egyptian
Engineers Syndicate and SPE.