Improvements in Nitrogen Rejection Unit Performance with
Changing Gas Compositions.
Gas Processors Association – Annual Conference.
Warsaw 21st – 23rd September 2005
Katarzyna Cholast, Andrzej Kociemba
KRIO, Odolanów
Harry Isalski
Technical Director, Tractebel Gas Engineering
John Heath
Special Projects Manager, Ebara International
Corporation, Cryodynamic Division
ABSTRACT
The KRIO Nitrogen Rejection Plant in Odolanów, Poland was constructed in the 1970s to
upgrade a lean natural gas to pipeline specification and to produce liquid helium for sale. This
plant has seen changes to the gas feed composition which necessitated modifications to it in
order to continue economic performance and to reduce methane emissions. Several studies
were undertaken to examine options for modifications that would satisfy environmental needs
and changing market requirements. Whilst the feeds gas became richer in methane, the needs
to retain plant flexibility became more important.
The paper describes the history of the plant, the changes that have occurred over its years of
operation, the decisions taken to implement the modifications and the final configuration of
the cryogenic part of the process. The paper also covers the initial start-up trials, the results
and the final acceptance test data. The paper discusses the operational aspects of the facility
and the benefits in having the main modifications in the selected locations.
The paper describes the use of Variable Frequency Drive (VFD) of liquid methane pumps to
improve cold production in the process as the first modification. Also covered is the
subsequent installation of LNG expanders, now an important part of every new LNG
liquefaction plant, where expansion of methane-rich, sub-cooled liquid enters the two-phase
region. The paper reports the successful operational experience of two such turbine Expanders
implemented in the nitrogen rejection unit revamps in order to improve their performance.
The implementation of these 2-phase expanders heralds a new chapter in the use of expanders
in the LNG and general cryogenic industry. This paper also serves to update the audience with
results of a further years’ operation of these turbines.
Improvements in Nitrogen Rejection Unit Performance with Changing Gas Compositions.
GPA – Annual Conference, Warsaw, September 2005. Cholast, K, et al
1
Improvements in Nitrogen Rejection Unit Performance with
Changing Gas Compositions.
INTRODUCTION
Natural gases which contain a significant amount of nitrogen are often not saleable since they
do not meet minimum heating value requirements of the grid into which they are compressed.
As a result the feed gas will generally undergo processing, wherein heavier components such
as heavy hydrocarbons, water and carbon dioxide are initially removed and the remaining
stream containing nitrogen and methane, and also possibly containing lower boiling or more
volatile components such as helium, is usually separated cryogenically when passing through
a nitrogen rejection unit.
The nitrogen rejection unit (NRU) comprises compact heat exchangers, pumps, cryogenic
distillation columns and Joule-Thomson (J-T) valves linked together in a highly integrated
process to carry out efficient separation into the required products. J-T valves are applied to
reduce the pressure of streams entering the distillation columns in order to decrease the stream
temperature and assist in the provision of refrigeration to the process. Two such plants were
constructed in the 1970s in Poland to achieve this and subsequently, these were modified by
the application of novel turbine expanders with Variable Speed Constant Frequency Control
(VSCF) to accommodate a feed gas having a different composition to the original design
range and the application of Variable Frequency Drive ( VFD) of the methane liquid pumps.
HISTORY OF THE NITROGEN REJECTION FACILITIES IN POLAND
The nitrogen rejection facility is located in south-western Poland in Odolanów. It comprises
two trains each processing up to 136000 NCMH of raw feed gas. Figure 1 shows a block
diagram of one of the processing trains and shows how it is linked to the supply and the sales
gas compression.
Figure 1 – Block Diagram of one train of the Odolanów plant
Improvements in Nitrogen Rejection Unit Performance with Changing Gas Compositions.
GPA – Annual Conference, Warsaw, September 2005. Cholast, K, et al
2
Raw gas is admitted from the plant supply header into each train. The first processing unit in
each train is an amine unit which removes the carbon dioxide from the gas down to about
50ppm by volume. The next process block is a thermally regenerated adsorption system
which consists of two adsorbers and a regeneration system. Each of the adsorbers has both
molecular sieve to remove water to less than 0,1ppm and an activated carbon guard bed to
remove benzene and heavy hydrocarbons. The adsorbents are regenerated using hot, low
pressure gas to drive off the water and heavy hydrocarbons. The adsorption and regeneration
cycle is controlled by a pre-determined sequence using changeover valves. The dry gas is then
admitted to the cryogenic unit which separates the nitrogen from the methane, producing a
sales gas of appropriate quality for compression into a high pressure gas. In addition to
nitrogen/methane separation, crude helium is also produced in the process which is
subsequently passed to a helium purification unit and then liquefaction. Helium has always
been a valuable product from this facility and the initial concentration was as high as 0.4% in
the raw gas.
The original incoming gas had a composition approximately as indicated in Table - 1 below
which compares the composition today.
Table – 1. Feed Gas Composition Changes.
Original
Current
Helium
0.4 vol%
0.2 vol%
Nitrogen
42.7 vol%
33.0 vol%
Methane
56.0 vol%
65.7 vol%
Ethane
0.5 vol%
1.0 vol%
Propane+
0.1 vol% max.
0.1 vol%
CO2
0.3 vol%
Water
Saturated at inlet conditions.
Pressure
5.6MPa
Temperature 10 – 15 oC
Flow
136000 NCMH
The sales gas required from the plant had to contain less than 4% nitrogen whilst the waste
nitrogen vented to atmosphere was required to have less that 1% methane. In addition, the
helium recovery was required to be better than 85% having a final purity of 99.999% as liquid
at -269oC. The original design required that the sales gas emanated from the cryogenic unit at
about 1.8MPa.
Over the years of operation, the raw feed gas has gradually changed and today, has about 32 –
35 % nitrogen in the feed. In addition, the helium content has decreased to a little over 0.2%.
Therefore, the refrigeration potential of the feed gas has been reduced because there is now
proportionally less nitrogen to expand to near atmospheric and by difference, more methane
that is produced at 1.8MPa. This has gradually caused more severe operational problems for
the two plants.
The plant was originally capable of producing small amounts of liquid methane or nitrogen
for sale to third parties. All of this was severely impaired, or even curtailed, with the lowering
of the nitrogen content in the feed gas. In addition, there was a further negative environmental
effect in larger than normal methane losses in the waste gas.
Improvements in Nitrogen Rejection Unit Performance with Changing Gas Compositions.
GPA – Annual Conference, Warsaw, September 2005. Cholast, K, et al
3
In summary, there are several main factors associated with the change of initial composition
of the feed gas :
- The environmental impact of increased methane content in the waste gas
- The economic loss as the consequence of methane emission
- Less stable operation of the cryogenic unit.
- Inability to produce liquid products.
As the feed gas pressure decreases, the plant may require expensive investment in a precompression step. It would also be desirable to improve the efficiency of the NRU operation
so that even with the drop of the feed gas pressure there will be still enough cold produced in
the process to be able to operate in a stable manner. The current feed gas pressure is about 5.3
MPa and is being maintained by adding new gas sources.
As a result, various studies were carried out in the late 1990s and 2000 to address the problem
and find some solutions. In addition, the helium, a high value product, has decreased in
concentration in the feed gas. This made it imperative that any changes to the process would
not have any negative effect on the helium recovery. The study work examined many
conventional, radical and novel solutions. In order to understand these better the process is
described below.
BRIEF DESCRIPTION OF THE CRYOGENIC PROCESS
In the original process, all energy needed for natural gas separation in low temperature units
was provided by pressure reduction of the natural gas across Joule-Thomson valves. The
process is shown in Figure 2.
Figure 2. Process Flowscheme for the NRU Cryogenic Section.
The dry feed gas is admitted into E1 of the cryogenic unit where it is cooled at 5.3MPa,
condensed and sub-cooled before it is passed through JT1 for expansion to the lower column
K1. The pressure in the lower column is about 2.1MPa and thus there is some flash into the
lower column. The “rich liquid” from the base of K1 is then sub-cooled in E2 against
Improvements in Nitrogen Rejection Unit Performance with Changing Gas Compositions.
GPA – Annual Conference, Warsaw, September 2005. Cholast, K, et al
4
returning colder methane liquid and gaseous nitrogen streams to about -150oC before passing
through JT3 where the sub-cooled rich liquid expands to 0.2MPa, thereby creating some flash
before entry into the upper column.
The upper column operates at about 0.15MPa and carries out the main distillation between
nitrogen and methane. Reboil to this column is provided by condensing nitrogen at 2.1MPa in
the top part of the lower column. This condensed nitrogen is very pure having less than
10ppm methane, and is drawn from a downcomer seal as a sidestream from the lower column,
sub-cooled in exchanger E3 before it is expanded across JT2 and admitted to the top of the
upper column, K2. This nitrogen stream provides the necessary reflux to purify the nitrogen
waste gas which returns through E3, E2 and finally, E1, before it is vented to atmosphere.
Therefore, the expansion valves JT1,2 & 3 provide all of the refrigeration to the process. The
methane product pump, P1, raises the pressure of the methane product from 0.2MPa to about
1.8MPa before the methane is evaporated and reheated in E2 and E1 respectively.
Over the years of operating the plant, KRIO has seen the nitrogen reduce from 42% to about
32% now. As the nitrogen in the feed decreased, the amount of this nitrogen reflux also
decreased in proportion to the methane that entered the upper column. Consequently, the
distillation process did not perform as well as before and methane content in the waste gas
increased.
CONCEPT DEVELOPMENT
Initial study work was carried out to select the most effective way of providing refrigeration
to the upper column. The analysis included the options listed in Table 2 below.
Improving the performance of the upper column was the key area which had to be addressed
to reduce methane losses. If LNG or liquid nitrogen was also required, then more cold had to
be supplied to the process. Dropping the column pressure with blowers, adding external
cycles or using liquid nitrogen (Options 1, 4 & 5) were eliminated mainly for cost reasons,
though Option 1 also had limited benefit. Simply adding distillation trays or high efficiency
packing (Option 2) would marginally improve the methane losses in the overheads, but would
have no effect on the cold production. Adding other separation equipment to condition the
feed to the lower column (Option 3) was also felt to be prohibitively expensive and time
consuming to implement. These options would be very expensive to implement and were
discarded quickly.
The use of gas expanders (Option 7) was seriously considered, but the cold production was
not in the correct location, i.e. too warm, and hence this option was not utilised. Adding gas
from high nitrogen fields was a complex issue and also a costly one. This could still be an
option in the future but will require high investment costs and a long project schedule.
Therefore, two options remained: VFD control of the methane pumps and installation of
flashing liquid expanders and these were implemented in sequence. The interesting point
about this approach was that the VFD control scheme implemented and tested on the methane
level control in the upper column applied to the methane pumps was already fully proven in
the VSCF control of the liquid expanders selected.
Improvements in Nitrogen Rejection Unit Performance with Changing Gas Compositions.
GPA – Annual Conference, Warsaw, September 2005. Cholast, K, et al
5
Table 2 – Options Studied to improve cold production to upper column.
Option Description
Number
1
2
3
4
5
6
7
8
9
Likely Cost/Benefit
Lowering Pressure in Upper Column
with Blower on the waste gas at the
outlet of the cold box.
This would increase the relative volatilities
hence needing less reflux to the column.
High Capital and Operating Cost. Some
benefit but reduces overall plant reliability.
Addition of extra trays to the upper
Limited benefit. Long duration shutdown to
column or replacing its internals with implement and very high cost to implement.
high efficiency packing.
Addition of pre-separation column or This is a complete revamp with high cost,
separator(s) to increase nitrogen
long shut-down and thus considered
content into double column.
impractical.
Adding a separate cycle to provide
This is potentially a good solution since it
refrigeration. This could be a nitrogen gives the ability to produce large amounts of
cycle taking the waste gas as the
LNG or liquid nitrogen. Very High Capital
working fluid.
and Operating Cost. Complex solution which
requires strong market drive to achieve the
benefit of the high CAPEX. Long schedule.
Importing liquid Nitrogen to assist in High operating cost. Simple implementation
cold production.
and with good benefit.
Modification of methane pump
Low cost, with good benefits, with limited
control scheme to VFD controller. scope for LNG production. Now
operational and described in this paper.
Application of gas expanders in
Difficult to provide cold at correct
various locations.
temperature level. Quite expensive.
Application of liquid expanders in The (b) & (c) options provide cold at the
various locations: (a) feed to lower appropriate temperature level. Limited
column, (b) rich liquid & (c) poor
sources of machinery. Testing was
liquid
required. Option (b) now operational.
Bring in other gases with higher
These were not easy to implement at the time
nitrogen content.
of the study work. This option also requires a
high capital investment for pipelines &
controls.
Revamping the Methane Pump Control Scheme.
The original plant used a pump spillback line to control level in the upper column. The pump
basically operated at its full speed/load and the surplus flow was returned back to the upper
column at 0.15MPa having been pumped to 2.1MPa. The original design of the pumps were
such that the power absorbed was about 130kW. With a significant portion of the pumped
liquid being returned via the level controller, about 30 – 40 kW of power was wasted across
the level control valve and heat input to the process. The level control valve expanded this
liquid back into the upper column from 2.1MPa to 0.15MPa. By adding the VFD control to
adjust the speed of the motor, the correct amount of liquid methane could be withdrawn from
the column without wasting power. The pumps themselves were capable of delivering a
Improvements in Nitrogen Rejection Unit Performance with Changing Gas Compositions.
GPA – Annual Conference, Warsaw, September 2005. Cholast, K, et al
6
higher than required head and it was possible to remove one of the stage impellers and still
meet the demand. These modifications were successfully implemented in 2002. The benefits
immediately showed with either lower methane losses in the waste gas or LNG production in
small quantities. Other benefits observed were greatly improved plant stability and an
immediate power saving of 30kW on each of the two plants. This system has been operational
since early 2003. This gave the plant operations staff the confidence that this kind of control
would operate successfully before utilising it on a more radical revamp i.e. the liquid turbine
expanders.
Original Scheme
New Scheme
L
L
M
M
VFD Control
L – Level Controller; M – Electric Motor; VFD – Variable Frequency Drive
FIGURE (3) – Pump Control scheme modification, before and after.
Rich Liquid Expanders.
There were three possibilities for adding liquid expanders in parallel with JT1, JT2 or JT3.
Adding an expander at JT1 location would only be of partial benefit since the cold was not
produced for the upper column (i.e. it was at too warm a temperature to be fully useful in
reducing the methane level in the waste gas or producing cold for LNG production). The
addition of an expander in parallel with JT2 would be of benefit, but the flow was not as great
as the rich liquid flow so the location in parallel to JT3 was considered to have the best
potential.
Therefore a liquid expander which produced two-phases at its outlet was located in parallel
with Joule-Thomson valve, JT3. Expansion turbines are inevitably more efficient since they
carry out “isentropic” depressurisation which generates work (which can be extracted as drive
power or electricity) instead of isenthalpic depressurisation across a Joule-Thomson valve,
which generates no work..
The choice of this location for an expander meant that the liquid entering the expander was
well sub-cooled ensuring a good quality single phase at the turbine inlet since it exits
exchanger, E3, at below -150oC at that point, and is at a pressure of 2.1 – 2.4 MPa. The outlet
produces about 80 – 95% by volume of vapour. This issue represented a significant challenge
from two aspects. The first is the two-phase flow stability for a fluid rising to the upper
column entry point for all the cases. The second, and by far the most challenging, was the
Improvements in Nitrogen Rejection Unit Performance with Changing Gas Compositions.
GPA – Annual Conference, Warsaw, September 2005. Cholast, K, et al
7
design of an efficient turbine which converted not only the hydraulic energy into useful work,
but also the gas expansion energy.
The first issue was successfully addressed by correct selection of the fluid regime for all the
practically expected cases, use of specially design piping and entry systems to prevent surges
and vortices causing problems with pressure stability at the turbine outlet and flow stability at
the upper column inlet.
The second issue of two phases within the turbine itself had already been proven on several
test programs with various companies dating as far back as the 1980s. However, no sustained
commercial application of a similar nature was found. There was confidence in the success of
the flashing flow from the turbine based on some tests done over the past 20 years in several
cases with success over short periods of time but with less vapour generated at the turbine
outlet.
It was decided that the Ebara turbine lent itself well to producing power for export, since the
generator is located inside the vessel that houses the turbine. The control of the unit was
facilitated by Variable Speed, Constant Frequency (VSCF) control of the turbine speed
making it act like a control valve. The level signal from the lower column, K1, was
successfully used for the control of the turbine speed. With the rich liquid cooled down by up
to 6 deg C more than from the control valve, JT3, more cold is routed to the upper column
and the separation of methane from the nitrogen waste gas stream is considerably more
effective.
FIGURE 4) – Picture of the VSCF control cabinet.
Improvements in Nitrogen Rejection Unit Performance with Changing Gas Compositions.
GPA – Annual Conference, Warsaw, September 2005. Cholast, K, et al
8
TWO-PHASE EXPANDER DESIGN CONCEPT
Two-phase expander design concepts essentially follow existing single-phase turbine and
expander technology. The hydraulic energy of the pressurized fluid is converted by first
transforming it into kinetic energy, then into mechanical shaft power and finally to electrical
energy through the use of an electrical power generator.
The generator is submerged in the cryogenic liquid and mounted integrally with the expander
on a common shaft. The cryogenic induction generator uses insulation systems specifically
developed for cryogenic service giving submerged windings significantly superior dielectric
and life properties. Thrust balancing and lubrication for the bearings is provided by an
internally designed system which takes a small portion of the liquid passing into the turbine
and routing it to the bearings. This produces some inefficiency, but the design of this system
is far simpler than a typical external oil lubrication system. This was another attractive feature
of the “canned” turbine design since the revamp required a smaller plot area, less connections
to the main process (no needs for seal gas etc).
Figure – 5 Ebara Two-Phase Expander Cross Section.
Figure 5 shows the cross section of a typical Ebara International Corporation cryogenic twophase submerged expander. The expander consists of a nozzle ring generating the rotational
fluid flow, a radial inflow reaction turbine runner and a two-phase jet exducer. Symmetrical
flow is achieved in the two-phase expander by utilising a vertical rotational axis to stabilize
the flow and to minimize flow induced vibrations, with the direction of flow being upward to
take advantage of the buoyant forces of the vapour bubbles. (Expanders with horizontal
rotational axis generate asymmetric flow conditions which can result in higher vibration
levels.) The hydraulic assembly is designed for continuously decreasing pressure to avoid any
cavitation along the two-phase flow passage. The low vibration levels observed on site with
the turbines operating at design flows support the design thinking very well.
Improvements in Nitrogen Rejection Unit Performance with Changing Gas Compositions.
GPA – Annual Conference, Warsaw, September 2005. Cholast, K, et al
9
FIELD EXPERIENCE USING TWO-PHASE EXPANDERS
To upgrade low-methane natural gas by extracting undesired nitrogen, two Ebara two-phase
expanders (each located in parallel to JT3, see Figure 2) were installed. During the early
running, optimisation of the turbine runners and exducers was carried out in order to improve
their power output and hence cold production giving successive improvements. The
operations and maintenance staff gained excellent confidence in the operational and
maintenance monitoring issues of the turbine as well as a good impression of the cryogenic
unit behaviour and hence were able to contribute considerable guidance in process and turbine
improvements. This gave an optimum process operating regime, a sound mechanical design
and very high plant overall availability.
Briefly, based upon the operational experience the following statements can be made:
-
The expanders required limited modification of the existing equipment and consequently
their installation is normally very easy and retrofitting can be done very quickly.
- The two-phase expanders have been in stable operation for more than 18500 hours now.
Throughout that period regular inspections have shown no incipient failures in bearings or
materials, vibration levels have been less than 20% of API 610 allowable limits.
- The expanders operate surprisingly quietly; not being audible alongside neighbouring
equipment of average noise level below 80dB.
- The employed expanders have made the process really flexible in terms of its adjustment
to changing mass flows, varying even by 100% . Even with such considerable changes
they assure easy and precise regulation of level in the lower columns of both trains, which
is of fundamental value for stable running of the process.
- Due to the greater temperature difference achieved by the expander, the heat exchangers
(particularly E3) operate in a more efficient and flexible way.
- The use of this two-phase expander in place of JT3 allows the sales gas product from the
NRU to be at higher outlet pressure, increasing by approximately 0.2MPa debottlenecking the compression.
- The NRU has continued to produce either liquid nitrogen or LNG for sale to third parties.
The “Holy Grail” in such duties is to extract more than just the hydraulic power from the
expansion. For the duty in question, the hydraulic power that could be extracted by the turbine
was estimated to be about 65kW. The actual power extracted from the turbine generator into
the grid as export was between 80 and 85kW clearly demonstrating that gas expansion energy
was being utilised effectively. When calculating the overall expansion, thermodynamic
efficiency, the value was found to be relatively low (below 60%) compared with large gas
turbines of today which regularly achieve over 80%. However, this is a relatively small
machine, with an onerous duty which is also using the process fluid as bearing and generator
coolant.
BENEFITS TO THE PROCESS MODIFICATION
During the modifications to the expander runners, the efficiency of the turbine rose to a high
enough level that gas expansion energy was being utilised, and subsequently a test run was
carried out on both the trains during the 4th quarter of 2004. Table 3 below also shows recent
plant data which demonstrates that there is no deterioration in turbine and hence plant
performance after a further year or operation.
Improvements in Nitrogen Rejection Unit Performance with Changing Gas Compositions.
GPA – Annual Conference, Warsaw, September 2005. Cholast, K, et al
10
Table 3 – Examples of plant data taken during the test runs and current Operation.
Train
1
1
1
2
2
2
1
1
1
2
2
2
Case
1
2
3
1
2
3
1
2
3
1
2
3
LNG T/D
21.6
0
32.9
15.7
0
22.6
0
31.6
0
34.8
Liq. N2 T/D
0
45
0
0
52
0
44
0
48
0
Measured kW
82.2
80.9
78.7
81.2
84.7
80.5
84
82
81.6
83.5
Waste CH4 %
0.93
>1
0.3
0.58
>1
1.0
>1
<1
>1
<1
Date
25 Oct 04
26 Oct 04
25 Oct 04
25 Oct 04
26 Oct 04
25 Oct 04
Aug 2005
Aug 2005
Aug 2005
Aug 2005
Aug 2005
Aug 2005
Case 1 –
Minimum/no liquid production
Case 2 –
Maximum liquid nitrogen production
Case 3 –
Maximum LNG production
It should be noted that during extraction of liquid nitrogen, reflux to the upper column is reduced, hence
the rise in methane loss from the waste gas. Also, during 2005, market reasons dictated that the plant
was not run without liquid production.
Note that the original design was for 42% nitrogen in the feed. With the composition as
tested, the waste gas methane content rose to above 3% with very poor plant stability and
frequent operator intervention. In addition, the liquid production was severely impaired, and
in some cases not possible when using just the J-T valves as cold producers. However, with
the turbine and methane pump control scheme in operation, the following benefits were
observed:
-
-
-
-
-
Because of the higher efficiency of the described process employing Ebara liquid twophase expansion turbines the upper column feed is colder which helps to further cool the
upper column top section leading to a lower methane content in the waste gas, when
processing the lower nitrogen content feed gas.
With the turbine in operation the NRU can run even with short-term carbon-dioxide
increases without shutdown. The plant can then accept short term higher carbon dioxide
concentration with no danger of plugging or consequent shut-down of the whole NRU.
This added tolerance is only temporary, since the plant would have to be thawed out later
on in any case. Whilst one would not design a new plant in this way, it gives operations
staff more flexibility and a greater on-stream time for the facility.
The higher process efficiency allows easier operation at a lower feed gas pressure, though
this is not a big issue at the moment. In case of reducing pressure of the feed gas from
depleting sources one can postpone the decision to install an expensive pre-compression
step.
Due to the high process efficiency, there is a possibility of withdrawing considerable
amounts of low-pressure LNG or a liquid nitrogen stream, running the nitrogen methane
separation in a stable manner at the same time. The possibility of producing LNG may be
useful for Peak Shaving opportunities. If liquid nitrogen production is considered, one
should be aware of the increased methane content in waste gas.
Employing liquid two-phase expansion turbines in the separation of nitrogen and methane
will allow generation of energy that can be used for export or as a drive power for another
duty.
Improvements in Nitrogen Rejection Unit Performance with Changing Gas Compositions.
GPA – Annual Conference, Warsaw, September 2005. Cholast, K, et al
11
-
One of the most significant benefits of the turbine operation has been the enormous
flexibility that the plants now have. The operators can easily switch from one mode of
operation to another within a matter of a few hours and are therefore able to rapidly
respond to market needs to achieve:
-
Maximum sales gas output.
Maximum helium recovery.
Below 1% methane in the waste gas.
Up to 50t/day liquid nitrogen.
Up to 35 t/day LNG per train.
.
CONCLUSIONS
The installation of the VFD control of the liquid methane pumps and subsequently the Ebara
expanders fulfilled and in some instances, exceeded the initial design expectations and offered
an excellent solution to ensure efficient operation when the plant is given feed gas with much
lower than design nitrogen content. Apart from reducing emissions of methane from the plant
the turbine has:
¾ Improved operational stability & availability with low nitrogen in the feed gas.
¾ Allowed greater LNG or liquid nitrogen production.
The VFD control of the methane pumps has now been operating for a total of more than
38000 hours and the turbine expanders have now been in operation for over 18500 hours
which demonstrate that the two modifications are an excellent way of adding refrigeration and
plant stability to cryogenic processes.
For more information please email
Improvements in Nitrogen Rejection Unit Performance with Changing Gas Compositions.
GPA – Annual Conference, Warsaw, September 2005. Cholast, K, et al
[email protected]
12
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Improvements in Nitrogen Rejection Unit Performance with Changing Gas Compositions.
GPA – Annual Conference, Warsaw, September 2005. Cholast, K, et al
13